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CHAPTER SIX



G                              LASS


economy, employing more than 150,000 people in
skilled jobs and generating more than 21 million
metric tons of consumer products each year with
                                                   INDUSTRY
                               by Douglas W. Freitag


    Glass is an important component of the U.S.         • product development of innovative new uses
                                                           for glass.
                                                            The following sections describe current glass
                                                        manufacturing processes and how ceramic-based
an estimated value of $22 billion. Glass production     materials might contribute to achieving the glass
is energy intensive and accounts for 12% of the         industry vision. Materials commonly used in
total cost of sales. In theory, about 2.2 million Btu   glass-processing equipment include fused silica,
of energy are required to melt a ton of glass. In       graphite, precious metals, and water-cooled fer-
reality, it takes twice as much energy because of       rous alloys. Ceramics are primarily used as refrac-
inefficiencies and loses. The glass industry consists   tories, with ceramic coatings increasingly being
of four major segments: container glass (bottles,       used for wear resistance. Advanced ceramics are
jars, etc.); flat glass (windows, windshields,          rarely used because of their high cost. Moreover,
mirrors, etc.); fiberglass (building insulation and     owing to the lack of suitable higher-temperature-
textile fibers); and specialty glass (cookware, flat    capable materials, fluxes are commonly added to
panel displays, light bulbs, fiber optics, medical      reduce glass-processing temperatures and permit
equipment, etc.). Flat glass accounts for 17% of U.S.   the use of conventional materials.
production by weight, container glass 60%,
fiberglass 9% and specialty glass 4%. While the
container, fiber, and flat glass industries focus on
producing soda-lime glass for large, broad-based
markets, the specialty glass industry focuses on
higher temperature glasses and produces over
60,000 products. Examples of products
manufactured by the glass industry are shown in
Fig. 6.1. Key drivers for investment in the glass
industry include cost, quality, and increased
productivity.
    The desired state of the glass industry in twenty
years has been formulated in a vision document
prepared cooperatively with the U.S. Department
of Energy (DOE). Future technology challenges
and research opportunities were identified by
comparing the vision of the future with the cur-
rent state of the glass industry. Technology chal-
lenges are broadly categorized into four areas:
• advances in melting and refining processes and
    in fabricating/forming;
• technology development of new glass-making
    techniques, processing controls, and computer
    simulations to model new processes;                   Fig. 6.1. Examples of glass industry products.
• systems improvements in emission controls,            Source: Marketing brochure from Praxair, Inc.,
    recycling methods, and solid waste manage-          Danbury, Conn.
    ment; and

                                                                                                   Page 6-1
Chapter 6


    The discussion is organized according to the        consumption, and lower temperatures above the
four primary operations for glass manufacturing:        melt for reduced emissions.
batching, melting, refining, and forming. Batching,
melting, and refining operations are common to          Combustion-Heated Furnaces
all glass manufacturing processes with some varia-
tion in furnace type. Various postprocesses are             Combustion-heated furnaces can be further
used, depending on the end product.                     divided according to the method used to recover
                                                        exhaust waste heat and the way the fuel is burned
                                                        (with air or pure oxygen). Heat recovery is impor-
6.1 BATCHING                                            tant because only 10% of the heat generated is ac-
                                                        tually used to melt the glass, whereas 67% is lost
    Raw material selection is based on chemistry,       through the exhaust. Recovery of exhaust waste
purity, uniformity, and particle size. Additional       heat is performed with a regenerator or
organic, metallic, and ceramic contaminants can         recuperator. Regenerator furnace designs repre-
be introduced throughout the delivery, storage,         sent one of the oldest and most universal devices
mixing, and sizing processes, as well as through        for saving energy in the glass industry.
the use of cullet (recycled glass). Because of the          An example of a regenerative furnace is shown
effects of contamination, the use of cullet varies      in Fig. 6.2. The batch introduced at one end
within the industry according to the desired end        immediately begins to melt. As glass is extracted
product. Flat glass manufacturers recycle up to         from the opposite end, the newly introduced glass
30% of their own cullet. Ceramic contaminants           flows along the melt zone where it becomes com-
undergo little reaction with the glass melt and re-     positionally homogeneous and bubble-free. After
side as inclusions (stones) in the finished product.    leaving the melt zone, the molten glass enters the
Metal and organic contaminants create instabili-        refining zone where it becomes thermally homo-
ties during glass processing (through reduction/        geneous. After leaving the refining zone, the glass
oxidation reactions) and degrade the quality of the     enters the forehearth, where it can be distributed
glass. Organics present in the batch are a source       to one or more forming machines.
of increased emissions and added cost for flue gas          Refractories commonly used in regenerative
cleanup. Delivery, mixing, and sizing processes are     glass melt furnaces are shown in Fig. 6.3. Alumina-
highly abrasive, and equipment used commonly            zirconia-silica (AZS) is the material of choice for
contains ceramic-coated wear surfaces or ceramic        the sidewall, with the zirconia content ranging
liners manufactured from alumina, silicon carbide,      from 33 to 41%. Various chrome-containing refrac-
or tungsten carbide. While generally cost-effective     tories are commonly used in the fiberglass indus-
and with adequate performance, lower cost solu-         try because of their superior erosion resistance but
tions offering lower risk of contamination continue     are gradually being replaced because of environ-
to be sought.                                           mental concerns about chrome. Even though su-
                                                        perior in performance, chrome-containing refrac-
                                                        tories are not widely used in the flat, container,
                                                        and specialty glass industries because chrome im-
6.2 MELTING
                                                        purities result in glass discoloration. At the front
                                                        of the furnace where the charge is introduced, ero-
    There are about 600 glass melt furnaces in
                                                        sion and thermally induced damage are the most
North America. The distribution of these furnaces
                                                        severe and limit furnace life. Materials that are
is as follows: container, 210; fiberglass, 110; flat
                                                        more corrosion resistant are sought for the
glass, 45; and specialty, 235; The life of a glass-
                                                        sidewalls. Cladding refractories with platinum or
melting furnace between rebuilds varies, but 7–8
                                                        molybdenum is considered too expensive. Not
years is not uncommon. The cost of rebuilding a
                                                        shown are the AZS electrode and burner blocks
furnace can easily exceed $1 million.
                                                        located on the sidewall below the melt line. These
    Glass furnaces can be divided into those heated
                                                        blocks support the electric boost heat electrodes
electrically and those heated primarily by
                                                        and burner nozzles, both of which experience high
combustion. Often electrical heating of the melt is
                                                        local temperatures and thermal stresses that re-
used in combination with fuel firing (electric boost)
                                                        sult in reduced life.
to improve heating uniformity, provide
                                                            The main source of a problem is above the glass
intermittent increases in melt capacity at minimum
                                                        melt where temperatures are the hottest and
cost, increase melt efficiency, reduce gas


Page 6-2
Glass Industry




   Fig. 6.2. Typical side-fired flat glass melt furnace. Source: “Development of Improved Refractories for
 Glass Industries,” U.S. Department of Energy fact sheet.




                   Fig. 6.3. Refractories commonly used in a soda-lime glass melt furnace.



 products of combustion reside. Superstructure               ($10,764/m2) and is much heavier. Because of a
 refractories can experience temperatures                    chemical incompatibility between the silica crown
 approaching 1600°C when air-fuel fired, and much            and the AZS sidewall, a buffer layer of zircon
 hotter when oxy-fuel fired. Silica is preferred for         (ZrSiO4) is used.
 the crown because of its low cost ($646m 2) and its             Water-cooled, stainless steel burners are located
 ability to be dissolved into the melt if dislodged.         on the sidewall of the furnace above the melt. In a
 However, silica does not perform well under the             regenerative furnace, firing alternates from side
 increasingly harsh environment of oxy-fuel firing.          to side, and intermittent firing is used to heat the
 Mullite has been used for the crown in processing           regenerator brickwork, which then preheats the
 higher temperature glasses but can create                   cold combustion air when firing is reversed. The
 inclusions in the glass. AZS, which has been                stainless steel burner orifice can be either water-
 considered for the crown, costs much more                   or air-cooled and frequently overheats. When


                                                                                                          Page 6-3
Chapter 6


firing, the burner orifice is cooled by the flow of
fuel and air. During the off cycle, the burner orifice
is cooled by air alone with temperatures
approaching 1040°C. The size and geometry of the
nozzle varies, but a typical orifice is 3.17 cm in
diameter and 10.16 cm long. A longer life ceramic
orifice is sought, but it cannot cost more than $200
to remain competitive with a metal orifice cost of
$150. A typical furnace has 20ioveter a61se
of a gas-fired burner with a rectangular shaped
nozzle is shown in Fig. 6.4.
    Stirring and bubbling are used in the melt zone
to improve homogeneity. Stirring is typically done
with a water-cooled stainless steel or uncooled
platinum paddle operating at glass melt tempera-
tures of 1425°C. The lifetime of water-cooled
paddles can be indefinite. While no cooling is re-
quired, platinum paddles are costly and life lim-
ited. Bubbling is accomplished with water-cooled,
high-carbon-steel nozzles located on the floor. Bub-
bler nozzle life is several years; however, when
replacement is necessary, it is easily accomplished
by drilling a sidewall, plugging the slow glass leak,
and then redrilling the sidewall for insertion of
the new bubbler tube that contains the nozzles.
Molybdenum disilicide bubblers have recently




Page 6-4
Glass Industry


     A cost of $400 for a 6.4-cm-diam, 15.2-cm-long      One approach to overcoming these limitations is to
 tube with a 1.3-cm-thick wall is desired.               preheat the batch/cullet mixture. As shown in
     In addition to those for furnace temperature,       Fig.6.5, the hot flue gas passes on one side of the
 sensors are being developed to measure refractory       heat exchanger while the batch powders fall counter
 thickness during furnace operation. These sensors       to the flue gas stream. Preheating can be provided
 are based on acoustic, eddy current, or electrical      by using a separate burner or using the heat
 capacitance measurements that change over time.         available in the exhaust stream of the furnace. In
 Sensors imbedded directly into the refractory are       both cases, a fan is used to pull the hot gases through
 also being considered. Ceramic shields are desired      the heat exchanger. Operating conditions of the
 to protect the sensors from the harsh environment.      preheater include inlet gas temperatures of up to
     Particulate emissions contained in the flue gas     982°C and exposure to the by-products of
 are removed by either cloth filters or an electro-      combustion, which with the flue gas can contain
 static precipitator. A conventionally fired regen-      high levels of moisture and corrosive vapors.
 erative furnace can generate particulate emissions      Metallic designs currently used require frequent
 approaching 9kg/h at the particle separator . Fans      maintenance and are temperature limited. By using
 are commonly used to maintain the desired level         a ceramic heat exchanger, life could be extended,
 of gas velocity and thus ensure the particles re-       weight reduced, and temperature capability
 main in the gas stream and avoid fallout. While         increased. Similar benefits exist with a ceramic fan.
 this system is adequate for current furnace designs     One benefit of this approach is the ease of retrofitting
 and regulatory guidelines, future furnace designs       existing furnaces.
 and increasingly stringent guidelines provide an            A second approach to increasing furnace effi-
 opportunity for advanced ceramics with the              ciency is to introduce oxygen for firing. The
 higher temperature capability and improved cor-         improvement in furnace efficiency is a result of
 rosion resistance sought for the filter media, fans,    the higher flame temperature and diminished ex-
 and ducts leading to the particle separator.            haust losses caused by the reduction of nitrogen
     The maximum production output of a glass            flow through the combustion system. The oxygen
 furnace is limited by the amount of energy that can     for firing can be introduced locally through lances
 be supplied inside the furnace to the melt. The limit   (tubes) immersed in the melt or by substituting
 is reached when the burners are at maximum heat         air at the burners for oxygen. Lances are con-
 output and electrical boost is at maximum power.        structed from water-cooled metals, precious met-
 Further increases in heat input at the melt furnace     als, or, more recently, molybdenum disilicide.
 are limited by the temperature capability of the        While the least expensive and simplest to setup,
 materials used to construct the furnace and the         oxygen injection can result in local hot spots and
 increased emissions (NOx) that would be generated.      damage to the furnace walls, burners, etc.




                                                                  Fig. 6.5. Batch and cullet preheating
                                                                system. Source: “Integrated Batch and Cullet
                                                                Preheater System,” U.S.Department of
                                                                Energy fact sheet.




                                                                                                        Page 6-5
Chapter 6


    Pure oxygen–based combustion introduced at              Electrically Heated Furnaces
the burners is gaining growing acceptance by the
glass industry, with 87 furnaces, mostly in the                 Electrical heating is used exclusively in many
specialty glass and fiberglass industry, currently          smaller specialty and fiberglass melting furnaces
in operation. Benefits include greater flame                because of its lower initial cost and low emissions,
stability, greater glass throughput, reduced fuel           even though energy costs remain high. Key
consumption, reduced furnace cost, reduced NOx              disadvantages of all electric furnaces are the high
and particulate emissions, and high efficiencies            energy cost and short furnace life. Electric furnaces
without the use of costly regenerators,                     are rebuilt as often as every six months, but
recuperators, or electric boosts. An important              because of their small size, down time is limited
disadvantage is the change in characteristics of the        to only about two days. An example of a bottom-
exhaust gases. A typical regenerative furnace has           entry electrode furnace with a refractory-lined,
flue gas temperatures of 315–482°C at the exhaust           water-cooled wall is shown in Fig.6.6. T op-entry
takeoff, with moisture content of 8–10%.                    electrodes are commonly used to simplify
Recuperated furnaces are slightly higher in both            maintenance because of the short life of materials
exhaust temperature and moisture content. Oxy-              currently used. Where bottom or sidewall
fuel-fired furnaces exhaust gas at 1204–1426°C and          penetration is not desired, boost elements are
have a moisture content of 55–67%. Because of the           simply placed over the side of the tank, when
high flame temperatures, corrosive volatiles (e.g.,         accessible. In the furnace example shown in
boron) are present in the exhaust gases. The high           Fig.6.6, the melt exits thr ough a submerged throat
firing temperatures and harsh exhaust gases                 into a separate temperature-conditioning chamber.
present with oxy-fuel furnaces provide a number             Bubblers and stirrers are frequently used to
of opportunities for ceramic materials.                     improve homogeneity of the melt and eliminate
    A number of alternate combustion cycles are             bubbles. Materials used in the construction of
being developed in unison with oxy-fuel firing.             small specialty glass and fiberglass furnaces
The goal is to arrive at alternatives that are less         include precious metals (for bubblers, stirrers,
difficult and costly to integrate into existing fur-        plungers) and molybdenum (for electrodes and
naces while still achieving reduced emissions and           flow-control hardware). Replacement materials are
increased efficiency. Many of these concepts re-            sought that are lower in cost and provide longer
quire advanced ceramics for burners capable of              life. Replacement materials are also sought for
higher temperatures, heat exchangers for preheat-           electrode holders that do not require water cooling.
ing combustion gases, particle separators for emis-         Natural-gas-fired immersion heaters, which could
sion control, and fans to move flue gases.




                     Fig. 6.6. Electric melt furnace for producing chopped fiber. Source:
                   “Advanced High-Temperature Material for Glass Applications,” U.S.
                   Department of Energy fact sheet.


Page 6-6
Glass Industry


 ultimately replace electrical immersion heaters,                in the melt section. Plungers and nozzles used to
 will require advanced ceramic outer liners.                     distribute the molten glass are refractory ceramics
                                                                 or molybdenum, but they experience high wear
                                                                 and erosion resulting from glass flow and
 6.3 REFINING                                                    corrosion. For lower temperature glasses, Inconel
                                                                 600 is commonly used, with similar problems
    The glass-conditioning operation (refining)                  reported. At higher temperatures, water cooling
 occurs in the forehearth and provides molten glass              can also be considered.
 that is uniform in temperature. The forehearth is                   A number of opportunities for advanced ce-
 generally natural gas fired. Electric boosting can              ramics were identified within the melting and re-
 be used to increase efficiency and improve                      fining operations of glass manufacturing and are
 temperature uniformity. Water-cooled metal heat                 summarized in Table 6.1. Many of these opportu-
 exchangers are used to help regulate temperature                nities directly relate to oxy-fuel-fired furnaces. Al-
 and would benefit from advanced ceramics.                       ternative combustion cycles are also being devel-
 Temperature sensing is critical in the forehearth               oped in which advanced ceramics are considered
 and shares problems similar to those encountered                enabling.


                           Table 6.1. Opportunities for advanced ceramics in glass melt furnaces
                  Application                            Industry need                 Opportunities for ceramics
   Refractories                            More corrosion resistance materials     Incremental improvements
                                                                                      potentially through graded
                                                                                      designs
   Flue gas ducts                          High-temperature-capable,               Fiber-reinforced ceramic composite
                                             corrosion-resistant materials            to accommodate thin wall designs
   Particle separators                     High-temperature-capable bag filters    Ceramic hot-gas filter
                                           Greater corrosion resistance
   Burners                                 Higher-temperature-capable burner       SiC or MoSi2 ceramic matrix
                                             nozzle with longer life                 composite, thermal barrier
                                                                                     coatings
   Sensor shields                          Uncooled thermocouple and IR            Engineered material to
                                             camera shields with longer life         accommodate unique
                                                                                     requirements; SiC (above the
                                                                                     melt) or MoSi2 (below the melt)
                                                                                     ceramic matrix composite
                                                                                     potential candidates
   Immersion heaters                       Change from electric boost to           Engineered material with a Mo outer
                                             natural gas fired                       liner below the melt


   Electrode supports                      Longer life blocks, uncooled holders    Incremental improvements
                                                                                      potentially through graded
                                                                                      designs
   Stirrers                                Lower cost, uncooled materials          MoSi2, ceramic matrix composite or
                                                                                    coating
   Plungers/melt flow control              Lower cost, longer life materials       Engineering ceramic matrix
                                                                                     composite. MoSi2 and mullite
                                                                                     potential candidates
   Bubblers/lances                         Lower cost, uncooled materials          Engineering ceramic matrix
                                                                                     composite; MoSi2 and mullite
                                                                                     potential candidates
   Glass stop insulation                   Longer life                             Incremental improvements
                                                                                      potentially through graded
                                                                                      designs
   Heat exchangers                         Higher temperature capable, longer      SiC or SiC ceramic matrix
                                             life materials, uncooled materials      composite tubular structures
   Fans for air flow control               High-temperature-capable fan            SiC ceramic matrix composite with
                                             materials                               engineered attachment




                                                                                                               Page 6-7
Chapter 6


6.4 FLAT GLASS                                                use the float process to form molten soda-lime-silica
                                                              glass into a flat sheet. No applications for advanced
                                                              ceramics were identified within the flat glass coating
    The U.S. flat glass industry comprises six ma-
                                                              operation, and that operation will not be discussed
jor flat glass producers operating 28 furnaces in
                                                              further. Unlike the melting furnaces used in other
16states. These plants, which employ about 12,000
                                                              glass manufacturing processes, those used for
skilled workers, produce 3.9 million tons of glass
                                                              producing flat glass are generally very large. The
per year and achieve sales of $2.1 billion. Plants
                                                              refiner is equally large because of the need to
are mainly located near sources of low-cost energy.
                                                              eliminate seeds resulting from gas bubbles and other
In 1991 the industry used 55.2 trillion Btu of en-
                                                              defects that degrade optical clarity.
ergy derived primarily from natural gas and elec-
                                                                  Two float glass forming processes are in use, one
tricity. Partly because of competitive pressures
                                                              developed by Pilkington Brothers (PB) and one
from developing countries, increases in produc-
                                                              developed by PPG Industries, Inc. Major differences
tion efficiency and energy efficiency are continu-
                                                              between the two processes involve how the glass is
ally being sought. Over the past 25 years, energy
                                                              introduced into the furnace. Essential features of the
efficiency has improved by over 50%, with much
                                                              PPG process are shown in Fig.6.7. A typical float
of the energy savings resulting from the use of im-
                                                              zone furnace is 49 m long and 9 m wide and can
proved refractories. The typical flat glass plant
                                                              hold 909 metric tons of glass. In the PPG process,
costs $100 million to build and has a campaign
                                                              the refined glass flows in a continuous, full-width
life of 12 years.
                                                              ribbon onto a molten bath of tin contained in an air-
    The typical flat glass manufacturing plant
                                                              cooled, refractory-lined steel shell. In the PB process,
consists of upstream operations for batching,
                                                              the molten glass enters a much narrower region onto
melting, refining, forming, and annealing.
                                                              a molten bath of tin and takes a very complicated
Downstream operations include reheating,
                                                              flow path before it reaches its full width. In both
reforming, coating, tempering, and laminating.
                                                              processes, glass enters at a temperature of 1040°C
Downstream operations can be performed on-site
                                                              and exits at a temperature of 600°C. The tin bath
or off-site. Upstream operations are virtually
                                                              is maintained at a temperature of 815°C, whereas
identical for all manufacturing plants because all




       Fig. 6.7. PPG float glass process. Source: Adapted from F. V. Tooley, Handbook of Glass Manufacture, 3rd
   ed., Vol.2, Books of the Glass Industry Div ., Ashlee Publishing Co., New York, 1984, pp. 714–15.


Page 6-8
Glass Industry


 the steel shell air is cooled to 100°C. An atmosphere   densates of tin sulfide vapor can dislodge and fall
 of N2plus 5–8% H 2 is used to prevent tin oxidation.    on the glass surface. Current procedures call for
 Alternate materials considered for replacement of       intermittent cleaning of the furnace roof and dis-
 the refractory-lined steel/tin bath shell (e.g.,        posal of the contaminated glass. Globars create a
 tungsten and graphite) are either costly, difficult     similar problem, but a potential solution is to re-
 to fabricate into the desired shape, or lack            place them with radiant burners.
 oxidation resistance. Few material candidates exist         From the float zone furnace, the glass sheet
 because of the highly corrosive nature of tin.          enters the annealing furnace, more commonly re-
     The flow of glass into the float zone furnace       ferred to as the lehr, to remove internal stresses
 from the refiner is regulated by a fused-silica gate    resulting from thermal gradients generated dur-
 called a tweel. The tweel meters the glass flowing      ing the plate-forming process. The lehr operates
 into the float zone furnace and thereby helps con-      at temperatures of 200°C, with heating provided
 trol the finished product thickness. The finished       by gas combustion or electricity. In an electrically
 product thickness is also varied by the glass vis-      heated lehr, the atmosphere is air; in a gas-fired
 cosity, surface tension, and, most important, the       lehr, the atmosphere contains by-products of com-
 traction forces applied to the edge of the glass rib-   bustion. The glass is transported through the lehr
 bon by water-cooled stainless steel gears (stretch-     on water-cooled stainless steel, fused-silica, or as-
 ing drives). Thicker glass can be formed by flow-       bestos-containing rollers with dimensions of up
 ing the glass against nonwetting, water-cooled          to 5 cm in diameter by 2.5 m long (as illustrated in
 graphite barriers at the edges.                         Fig. 6.8). Problems associated with supporting the
     Because the life of the tweel is only about ten     glass sheet in the lehr include roller surface dam-
 months, alternate materials are being sought to         age, roller marking, sag between rollers, roller
 increase life. In additional to molten glass com-       wave distortion, and roller contact heat transfer.
 patibility, requirements for the tweel include high-    Uniform size and speed of the rollers is critical to
 thermal-shock resistance and load-carrying capa-        avoid scratching the glass surface. Radiant burn-
 bility at molten glass temperatures. Replacements       ers are used in many gas-fired lehrs and can be a
 for water-cooled materials are also sought because      source of high maintenance. Upon cooling to room
 of the high level of maintenance and added cost         temperature, the glass is cut and packaged for
 from corrosion. More corrosion-resistant metal al-      downstream processing.
 loys (e.g., stainless steel) have been used but are         Downstream operations can include a number
 undesirable because nickel gets into the recycled       of reheating, reforming, tempering, and coating
 glass and forms nickel sulfide during annealing.        steps. Reheating and tempering occur in furnaces
 Conditioned water is commonly used throughout           similar to the lehr and only vary in temperature.
 a glass manufacturing plant to minimize corrosion.      Large volumes of heated air are frequently circu-
     The glass temperature and tin melt are main-        lated in the lehr to help reduce thermal gradients.
 tained by several thousand silicon carbide globars      The fans are produced from high-temperature steel
 located along the length of the furnace. As the glass   with water-cooled drives. Fan distortion reduces
 moves down the tin bath, it cools and increases in      life at the higher operating temperatures. After the
 viscosity, thus allowing lifting from the tin bath      glass has been reheated it can be reformed by
 by transport rolls. The transport rolls that carry      bending on cast fused-silica molds with integral
 the glass to the annealing furnace are engineered       heating elements. Because of their poor structural
 to avoid deforming the glass under its own weight.      properties, molds are easily damaged. While they
 The rolls, about 30cm in diameter and 4.3 m long,       can be repaired, repairs are time consuming and
 are constructed of asbestos or water-cooled metal.      can be costly, both in down time and the actual
     Instrumentation used to monitor the process         maintenance. Release coatings or barriers of ce-
 includes optical pyrometers for measuring surface       ramic or metal films are used to inhibit sticking of
 temperature of the glass and tin bath and shielded      the glass to the mold. Because the glass relaxes
 thermocouples for measuring the subsurface tem-         after molding, die design is an iterative process
 perature of the tin bath. Measurements of local ve-     that can occur over an extended period and be
 locity in the tin and glass and of glass thickness,     costly.
 as well as visualization of surface defects and             A summary of opportunities for advanced ce-
 stresses, are desirable but have met with limited       ramics in flat glass manufacturing is shown in
 success. Cooled instrumentation used in the roof        Table 6.2. Opportunities for the batch, melt, and
 is commonly a source of glass defects because con-      refinement operations were previously described.



                                                                                                      Page 6-9
Chapter 6




                  Fig. 6.8. Fused silica rollers used in a lehr. Source: Marketing brochure from Air
                Liquide America Corp., Walnut Creek, Calif.


                       Table 6.2. Opportunities for advanced ceramics in flat glass manufacturing

              Operation                          Industry need                      Opportunities for ceramics
  Float zone furnace            1.   Longer life tweel                      1.   Incremental improvements
                                2.   Replacement of asbestos transport      2.   Ceramic matrix composite or a
                                     rolls                                       metal/ceramic hybrid
                                3.   Improved materials for stretching      3.   SiC or Si3N4 monolithic ceramic
                                     drives                                 4.   Engineered material with
                                4.   Replacement of refractory lined tin         anticipated bath redesign
                                     bath                                   5.   Ceramic matrix composite or Si3N4
                                5.   Uncooled sensors                       6.   SiC or MoSi2 ceramic matrix
                                6.   More efficient gas-fired radiant            composite. Thermal barrier
                                     burners                                     coatings.

 Annealing, reheating,          1.   Longer life air circulation fans       1.   SiC ceramic matrix composite with
   tempering                    2.   Improved hot handling materials             engineered attachment
                                3.   High-temperature-capable               2.   Ceramic matrix composite or
                                     noncontact sensors                          cermet
                                4.   Low-maintenance radiant burners        3.   Si3N4 or ceramic matrix composite
                                                                            4.   SiC or SiC ceramic matrix
                                                                                 composite

  Reforming                     1.   Improved mold materials and            1.   Engineered materials manufactured
                                     manufacturing methods                       using 3-D rapid prototyping
                                2.   Improved mold release coatings or           technology
                                     film barriers                          2.   Ceramic-coated metal or ceramic
                                3.   High-temperature-capable                    cloth
                                     noncontact sensors                     3.   Si3N4 or ceramic matrix composite

Page 6-10
Glass Industry


 6.5 CONTAINER GLASS                                          Downstream operations can be performed on-site
                                                              or off-site. The batching, melting, conditioning,
     The container glass industry, which is made up           and annealing operations are similar to those dis-
 of 64 plants in 25 states, employs 30,000 skilled            cussed previously. No applications for advanced
 workers, produces 12.3 million metric tons of glass          ceramics were identified within the coating opera-
 per year, and achieves sales amounting to $5 bil-            tion, and thus that operation will not be discussed
 lion annually. Plants are located mainly near prod-          further.
 uct markets. In 1992 the industry used 119 trillion
 Btu of energy, 79% of which was derived from                 Forming
 natural gas. Competitive pressures occur from
 market preferences for lighter weight container                  After conditioning to the desired temperature
 materials (e.g., plastic and aluminum), the high             and uniformity, the molten glass is transferred to
 cost of end product distribution resulting from the          the forming operation through the gob feeder. The
 weight of glass containers, and the lower cost of            gob feeder is an integral part of the forehearth and
 labor in developing countries. The market share              includes a mixer, plunger, orifice, and shears, as
 lost to alternate materials is being reduced by regu-        shown in Fig 6.9. The gob feeder controls the
 latory concerns and the higher cost of “product              weight, temperature, and shape of the gob—all of
 stewardship.” The distribution cost of glass con-            which are critical to container quality. Gob-form-
 tainers is benefiting from the development of                ing rates can easily approach 300 gobs per minute.
 higher strength glass and improved manufactur-               The materials of construction for the gob feeder
 ing methods developed to produce thin-wall,                  include water-cooled metals (cut-off blades), pre-
 light-weight containers. Increased production and            cious metals (stirrer), and refractories (plunger) to
 energy efficiencies are continually sought to effec-         accommodate glass gob temperatures approach-
 tively compete against developing countries. The             ing 1100°C.
 typical container glass plant costs $70 million to               The gob next enters the forming operation
 build. The last container plant was constructed in           where the heat is extracted from the glass in a con-
 1981. Because of their high level of maturity, manu-         trolled manner during shaping. Several processes
 facturing processes are essentially the same in all          are available to form the gob; the process selected
 plants.                                                      is based on composition, container shape, size, and
     The typical glass container manufacturing                production rate. The press-and-blow process com-
 plant consists of upstream operations for batching,          monly used to produce large-mouth containers is
 melting, conditioning, and forming and down-                 illustrated in Fig.6.10.
 stream operations for coating and annealing.




       Fig. 6.9. Single gob feed cycle. Source: Reprinted from F. V. Tooley, Handbook of Glass Manufacture, 3rd
     ed., Vol.2, Books of the Glass Industry Div ., Ashlee Publishing Co., New York, 1984, p. 601.


                                                                                                          Page 6-11
Chapter 6




   Fig. 6.10. Container press-and-blow forming operation. Source: Reprinted from F. V. Tooley, Handbook of
Glass Manufacture, 3rd ed., Vol.2, Books of the Glass Industry Div ., Ashlee Publishing Co., New York, 1984,
pp.616–17.


    In the press-and-blow forming operation, the            generates a high level of noise, liquid cooling may
gob enters the mold and is first pressed into a             be employed as an alternative. A disadvantage of
blank. During the pressing operation, plunger               liquid cooling is the increased maintenance needed
temperature approaches 550°C. Because the                   for leakage and corrosion. To increase cycle times,
plunger material is not as hard at these                    mold temperature can be further reduced by
temperatures, the plunger surface is easily scored          precooling the air or liquid. Mold materials having
by the chilled glass at the interface during                higher thermal diffusivity have also been used (e.g,
extraction. The blank is removed from the press             copper-based alloys in neck rings). To reduce
mold, reheated, and then placed into a second               mold-to-glass adhesion, glass marking, and
mold for blowing into final shape. When removed             oxidation of the metal mold, the mold is swabbed
from the blow mold, the glass cools to 780°C.               with a lubricant between cycles. Because mold
Production rate is limited by the rate of heat              temperatures approach 500°C, the lubricants used
extraction and can approach 100 bottles per minute          generate smoke and vapor and sometimes ignite
when multiple molds are used with a single gob              spontaneously. Alternate approaches available
feed system. The forming operation is the single            include nonmetallic solid film lubricants or the
largest contributor to both the cost and quality of         injection of acetylene into the hot mold between
the finished glass container, and molds represent           cycles, which cracks and forms a thin deposit of
a large part of that contribution.                          carbon throughout the mold interior.
    Requirements for mold materials include low                 Alternate materials of construction for molds
cost, capability for producing complex shapes, low          currently being considered include nickel-based
distortion in use, ability to take and retain a fine        superalloys, graphite, and ceramic coatings.
surface finish, low thermal expansion, adequate             Asbestos-containing materials were used for a
hot structural properties, scale and oxidation              period but have been replaced by graphite for
resistance, high thermal diffusivity, and high              reasons of regulatory compliance. Nickel-based
resistance to thermal cycling. The primary mold             superalloys are being used in areas where higher
material is cast iron. Low-pressure air-cooling is          hot hardness and strength is desired (e.g.,
used to accelerate heat extraction and improve              plungers, baffles, funnels, blow heads, bottom
temperature control. However, because air cooling           plates, and plugs). As with flat glass, nickel


Page 6-12
Glass Industry


 contamination can be a problem with container                   bide) onto existing hot handling surfaces, espe-
 glass.                                                          cially the plunger, by a variety of processes. While
     Graphite is a preferred material for hot han-               more difficult and costly to machine, resistance to
 dling because it does not wet, mark, or adhere to               oxidation and scaling occurs when applied to
 glass; provides inherent lubricity; does not scale;             graphite and cast iron, respectively. Spallation of
 retains its strength at temperature; and is low cost,           the coating can also occur, thus increasing the risk
 easily machined, and environmentally safe. Ap-                  of glass contamination.
 plications include blow-head inserts, take-out                      After molding, the containers are placed into
 tongue inserts, sweep-outs, lehr-loading bars, gob              the lehr for removing stresses (see Fig. 6.11). The
 deflectors, and conveyor guides. Grades of graph-               lehr is similar to that previously discussed for flat
 ite used are selected based on their application.               glass; the primary difference is the use of a con-
 Porous graphite is desirable in the lehr to avoid               veyor belt to move batches of the container glass
 thermal checking, which is caused by rapid heat                 through the hot zone in a controlled manner. The
 transfer from glass containers. Higher density                  conveyor is fully contained within the furnace to
 graphite is used in molds where increased strength              reduce heat loss and increase energy efficiency.
 and hardness are desired. While graphite offers                     With the exception of coatings, no evidence was
 many advantages, a key disadvantage is its lim-                 found of advanced ceramic materials usage in the
 ited life due to poor oxidation resistance and low              forming of glass containers. Opportunities exist,
 strength. A greater number of glass defects have                however, because of the increasing emphasis on
 been seen when ceramics have been used.                         lightweight glass containers, continuous produc-
     As containers become thinner, higher hardness               tion, and improved energy efficiency as competi-
 materials are desired for mold surfaces to reduce               tion increases from abroad. Specific opportunities
 contamination and maintain surface finish and                   include replacement of graphite in areas of hot
 critical dimensions. Higher hardness materials also             handling, with alternate materials providing
 provide reduced die wear in machines where pro-                 higher toughness, greater hardness, and improved
 duction rate is being increased in place of added               oxidative stability. Lower cost materials are sought
 capacity. One solution is the application of hard               to replace precious metals used in the forehearth
 coatings (e.g., tungsten carbide and titanium car-              and gob feeder. Materials with higher thermal




                    Fig. 6.11. Gas-fired glass annealing furnace with an internal belt return. Source: Re-
                 printed from F.V .T ooley, Handbook of Glass Manufacture, 3rd ed., Vol.2, Books of the Glass
                 Industry Div., Ashlee Publishing Co., New York, 1984, p. 682.


                                                                                                                Page 6-13
Chapter 6


diffusivity and hot hardness are sought for molds             wool. Total energy consumption for the fiber glass
to extend life and reduce cycle time without ex-              industry in 1991 was 59.5 trillion Btu, a value
ternal cooling. Higher-temperature-capable mate-              exceeded only by the container industry. Unlike
rials are desired throughout the mold machine for             container or flat glass, a large amount of energy
reduced maintenance and improved quality. New                 consumed is used to fabricate fiber from molten
sensors and sensor shields are sought to measure              glass. Whereas both markets are capital intensive,
gob temperature, temperature uniformity, and                  the textile fibers industry is more so because of its
heat extraction during shaping. In all cases, ad-             greater dependence on precious metals. Glass
vanced ceramics are central to the solution. Be-              quality requirements also vary, but textile fibers
cause they are compatible with molten glass, ad-              production requires much higher quality glass and
vanced ceramics of choice are oxide-based or ce-              greater use of process control. Key drivers for the
ramic composites having predominate oxide                     use of new technology in the fiber glass industry
phases. Leading candidates are alumina, mullite,              include reduced cost, increased throughput,
and silicon carbide–alumina composites. Molyb-                reduced emissions, and a competitive advantage.
denum disilicide–based composites are also good               Competitive pressures primarily exist from organic-
candidates for many of these applications. Silicon-           based products. Processes used to manufacture fiber
based ceramics are candidates for hot applications            glass can be divided into batching, melting, and
requiring optimum structural properties that are              fiberization. Each process and opportunities for
not in contact with molten glass.                             advanced ceramics is discussed in detail in the
    A summary of opportunities for advanced ce-               following sections.
ramics in container glass manufacturing is shown
in Table 6.3.                                                 Batching

                                                                 The batching operation is similar to that used
6.6 FIBER GLASS                                               for producing other glass products with the
                                                              exception of different compositions and finer raw
    The fiber glass industry is made up of two                material particle size. Because particle size is finer,
primary markets: wool insulation and textile                  emissions control and contamination by ceramics,
fibers. Wool insulation is a short-fiber product used         metals, organics, and alloys are of greater concern.
by the construction industry. Textile fibers are a            A key ingredient for manufacturing fiber glass is
continuous-fiber product principally used as a                boron, which provides reduced melt viscosity and
polymer matrix composite reinforcement.                       improved glass durability but is volatile and
Together, the four major producers of wool and                condenses at undesirable locations. The content
the six major producers of textile fibers employ              of boron in glass fiber ranges from 5 to 10 w %
about 16,000 workers. A typical producer can                  and represents 50–75% of the total cost. A high
operate as many as 27 glass melt furnaces. In 1991,           percentage of recycled container and flat glass is
fiber glass made up 9% of all glass produced and              used to produce wool fiber glass, for which quality
accounted for 21% of the total market value. In               requirements are less stringent. Recycled fiber
1981, 71% by weight of all glass fiber shipped was            glass is not currently used because of the boron
                                                              present.

                    Table 6.3. Opportunities for advanced ceramics in container glass manufacturing

               Application                         Industry need                     Opportunities for ceramics
  Radiant burners                       Longer life burners                     SiC or SiC ceramic matrix
                                                                                  composite
  Hot handling                          Higher toughness materials with         Ceramic matrix composite, cermet,
                                          improved oxidation resistance           or carbide coatings
  Gob feeder                            Lower cost stirrer materials, reduced   Engineered ceramic matrix
                                          erosion plunger materials, and          composite plunger, cutter, and
                                          uncooled cutter materials               stirrer; mullite ceramic composite
                                                                                  plunger
  Molds                                 Higher hardness materials, higher       Engineered material or coating
                                          thermal diffusivity
  Sensors                               Longer life thermocouple shields        Si3N4 or ceramic matrix composite



Page 6-14
Glass Industry


 Melting                                                    homogenization section followed by a hearth for
                                                            distribution to the different fiber-forming
     In addition to batching, the other major process       operations. Conversion to oxy-fuel firing is also
 that fiber glass shares in common with other glass         occurring as a result of demonstrated improvements
 industries is the melting operation. The differences       in energy efficiency and emissions.
 are limited to processing challenges resulting from            A number of opportunities exist for advanced
 glass compositions and an overall smaller furnace          ceramics within the melter. Higher temperatures
 size. The low alkali content of textile fibers requires    of the melt in combination with the low alkali
 higher melting temperatures and greater fiber              content force the use of environmentally sensitive
 particle size to ensure homogeneity without the            chrome-containing refractories. Chrome-
 benefit of fluxing additives. Both textile and wool        containing refractories are also electrically
 products include boron, which can foul furnace             conducting, drawing current away from the glass
 hardware and increase particulate emissions. Glass         and reducing refractory life. The by-products of
 melters include regenerative, direct-fired, and            combustion, and higher temperatures of
 recuperative furnaces. Large recuperative gas-fired        combustion with oxy-fuel firing, also limit refractory
 furnaces are still the primary source of melters. The      life. Improved glass stop refractories are sought for
 campaign life of gas-fired furnaces is 7–10 years;         reduced maintenance. Precious metals (platinum/
 rebuilds are costly and time consuming. Some               rhodium) are used throughout the melter (gas
 conversion to electric and electric-boosted gas-fired      bubblers, plungers, thermocouple shields) with a
 furnaces has occurred to reduce gas consumption,           per furnace cost approaching $10 million. Platinum/
 increase melt efficiency, and reduce emissions             rhodium is used because of its high-temperature
 (SOx, NOx, particulate). Electric furnaces use top-        capability and stability when in contact with glass
 or bottom-entry electrodes, the former being easier        at temperatures exceeding 1371°C. Disadvantages
 to maintain. The campaign life of a electric furnace       include high cost, low creep strength, and uncertain
 is six months; atypical r ebuild takes two days to         supply. Alternatives to platinum/rhodium
 complete. An example of a cooled metal wall,               evaluated and deemed unsuccessful include
 bottom-entry electrode melter is shown in Fig. 6.12.       germanium-doped molybdenum disilicide and
 The melt exits through the submerged throat into a         Inconel. Ceramic-based material options include




              Fig. 6.12. Example of a bottom entry electrode electric glass melter. Source: “Advanced
           High-Temperature Material for Glass Applications,” U.S. Department of Energy fact sheet.



                                                                                                        Page 6-15
Chapter 6


high alumina or zirconia composites with high                shown in Fig. 6.13, a single stream of molten glass
purity and density. Ternary phase oxides could               falls into a rapidly rotating cylinder (centrifuge)
also be considered, but phase instability is of              made of a high-temperature base metal alloy. The
concern. Another option is higher creep resistant            cylinder has a 12- to 36-in. diameter and contains
nonoxide composites (i.e., silicon carbide with a            10,000–30,000 laser-drilled holes having diameters
platinum/rhodium coating for molten glass                    of 0.1–0.015in. Operating conditions include
compatibility).                                              steady state temperatures of 2000°F while exposed
    Molybdenum is also a primary material used in            to air. Typical lifetime is 40–100 h. The base metal
fiber glass melters. Applications include electrodes         alloys used are generally proprietary, investment
and glass flow control devices. For lower                    cast, and extensively recycled to reduce cost.
temperature applications (< 1315    °C), Inconel 600         Alternate materials are sought with increased
has been used. In both cases, premature failures             creep strength, resistance to corrosion (sulfidation)
have occurred due to excessive corrosion and wear.           and oxidation, and reduced cost. As the molten
Advanced ceramics are sought that can operate                glass is driven through the perforated cylinder,
uncooled, above or below the melt line, and exhibit          downwardly directed high-velocity gas jets
high-temperature creep resistance. Opportunities             attenuate the flow and form fibers of varying
for advanced ceramics also exist for oxy-fuel-fired          lengths. The fibers are sprayed with organic
burners and in shields for sensors being developed           binders and collected onto a moving belt where
to detect refractory wear.                                   they are transported to a curing oven. Alternate
                                                             approaches to the rotary process for forming wool
Fiberization                                                 include attenuation of a molten stream passed
                                                             through sieve-like platinum alloy bushings and
   Molten glass leaves the hearth and enters the             remelting of drawn fiber by a high-velocity burner.
fiberization process directly or after an                        In the continuous textile forming process
intermediate marblizing process. Marblizing is               shown in Fig. 6.14, glass marbles are fed through
commonly used for textile fiber production when              orifices in an electrically heated platinum alloy
the forming process is not located in close                  bushing. A typical bushing has a cross-section of
proximity to the melter. Fiberization occurs by              25–50 cm by 18–20cm with a depth of 5 cm. About
attenuating a melt steam with air, steam, or                 100 dies are used with a single melter, and a typical
combustion gas (for wool) or by drawing molten               production site contains four melters. Each
glass through precious metal bushings (for                   production unit processes 400–3,000 filaments
24textiles). For the wool pr ocessing example                simultaneously, with fiber diameters ranging from




               Fig. 6.13. Example of a wool forming process. Source: Reprinted from F. V. Tooley,
            Handbook of Glass Manufacture, 3rd ed., Vol.2, Books of the Glass Industry Div ., Ashlee
            Publishing Co., New York, 1984, p. 721.

Page 6-16
Glass Industry


                                                          reliability. Lower cost solutions are also sought for
                                                          the platinum sieves and bushings. Oxides
                                                          dispersion strengthened with yttria have been
                                                          evaluated as a precious metal replacement with
                                                          promising results. Platinum-coated molybdenum
                                                          was also considered but failed during testing.
                                                              A summary of opportunities for advanced ce-
                                                          ramics in fiber glass manufacturing is shown in
                                                          Table6.4. Opportunities for the batch operation ar e
                                                          similar to those discussed for container and flat
                                                          glass.




                                                             Fig. 6.15. Ceramic textile fiber handling
                                                          guides. Source: Reproduced from Kyocera’s Web site
                                                          at http://www.kyocera.com.

   Fig. 6.14. Example of a textile forming process.
 Source: Reprinted from F.V . Tooley, Handbook of
 Glass Manufacture, 3rd ed., Vol.2, Books of the Glass    6.7 SPECIALTY GLASS
 Industry Div., Ashlee Publishing Co., New York,
 1984, p.725.
                                                              This market segment is a catchall for glass
                                                          products not contained in the flat glass, container
 3.5 to 20 µm. Bushing life is about six months, at       glass, and fiber glass market segment. An impor-
 which time the bushing is recycled because of the        tant difference is that the specialty glass produc-
 high cost of raw material. Life is limited by hole       ers, unlike the other glass producers, work with
 wear and sag. Operating conditions include steady        higher temperature glasses, aluminosilicates, and
 state temperatures of 1200°C in air. Forced-             borosilicates, as opposed to soda lime glass. While
 convection air cooling is used prior to the sizing       compositions vary, processing methods are simi-
 application to increase productivity. A winding          lar to those previously discussed. The markets
 drum collects the filaments and applies a traction       served are very broad, with as many as 60,000 dif-
 force to stretch the fibers as they are pulled through   ferent products, but volumes are small. A large
 a mechanical device that applies an organic sizing       number of specialty glass producers are small,
 and twists multiple filaments into a single strand.      owner-managed, and labor intensive. Because of
 Depending on the application, additional                 the nature of the business, the technology being
 processing is performed on the multiple strands.         used is frequently developed in-house and re-
 Materials commonly used to handle and guide hot          mains highly proprietary.
 filaments are carbon based. Alumina and silicon              Specialty glasses have very specific chemical
 nitride guides are used for sizing application and       compositions with clearly defined physical and
 twisting where high wear can occur. Typical fiber        chemical properties. Furnaces are generally special
 handling guides are shown in Fig. 6.15. Alternate        designs and principally oxy-fuel fired. To further
 materials are sought for fiber handling that exhibit     improve energy efficiency, cullet preheaters are
 many of the characteristics of monolithic ceramics       being evaluated. Melter capacities of 45 metric tons
 but have improved toughness for increased                are considered large. Because of the high melting

                                                                                                      Page 6-17
Chapter 6


temperatures, refractories and thermocouple                    product. During the forming operation, molds are
sheaths are a problem. Infrared cameras are                    generally superalloys. Ceramic molds were
frequently used in place of short-lived                        considered but shown to increase the presence of
thermocouples for temperature control, and fiber               stones in the finished glass. Natural-gas-fired
optic systems are being developed. Because of                  metal radiant burners, which are commonly used
their small size, convection currents are used in              during forming and annealing, continually require
place of bubblers to homogenize the glass. A range             a high degree of maintenance.
of high-temperature base metal alloys and                         A number of niche opportunities exist within
precious metals are used throughout the melt and               the specialty glass industry. Opportunities
forming steps (e.g., platinum paddles used in the              common among glass market segments also exist,
melter for homogenization). After the melting                  but operating conditions both above and below
process, the glass goes in several different                   the melt are much harsher in specialty glass.
directions for forming, depending on the final


                      Table 6.4. Opportunities for advanced ceramics in fiber glass manufacturing
              Operation                        Industry need                     Opportunities for ceramics
               Melting             1. Improve refractories                 1. Incremental improvements
                                   2. Precious metal replacement           2. Mo based engineered material
                                   3. Longer life electrodes               3. Mo based engineered material
                                   4. Longer life flow control materials   4. Mo based engineered material
                                   5. Sensor shields                       5. Mullite or Mo based engineered
                                   6. Oxy-fuel-fired burner nozzles           material
                                                                           6. SiC or MoSi2 ceramic matrix
                                                                              composite; thermal barrier coating
            Wool forming           1. Low-cost centrifuge material         1. Mo based engineered material
                                   2. Precious metal replacement for       2. Mo based engineered material
                                      sieves and bushings
            Textile forming        1. Precious metal replacement for       1. Mo based engineered material
                                      bushings
                                   2. Improved fiber-handling materals     2. Ceramic matrix composite




Page 6-18
Glass Industry


 6.8 REFERENCES                                     Pfendler, R. D. 1995. “Glassmakers Face New
                                                        Emissions Concerns,” Ceramic Bull. 74(4),
 Publications                                           95–99.
                                                    Tooley, F. V. 1984. The Handbook of Glass Manufac-
 DOE/Conf-970292 December 16, 1997. Oxy-Fuel            ture, 3rd ed., Vols 1–2, Books for the Glass
    Issues for Glassmaking in the 90s: Workshop         Industry Div., Ashlee Publishing Co., New
    Proceedings, documenting a workshop                 York.
    sponsored by the U.S. Department of             U.S. Department of Energy, Office of Industrial
    Energy’s Office of Industrial Technologies          Technologies January 1995. A Technology
    and cosponsored by 22 companies in the              Vision and Research Agenda for America’s Glass
    U.S. glass industry, held in Arlington, Va.,        Industry.
    February 27, 1997.
                                                    U.S. Department of Energy, Office of Industrial
 Energetics, Incorporated September 8, 1994.            Technologies January 1996. Glass: A Clear
    “Materials Needs and Opportunities in the           Vision for a Bright Future.
    Glass Industry: Schuller International, Inc.”
 Energetics, Incorporated October 27, 1994.
                                                    Information Supplied by Academia and the
     “Materials Needs and Opportunities in the
                                                    Glass Industry
     Glass Industry: Libby Owens Ford Com-
     pany.”
                                                    Scott Anderson, Combustion-Tec, Inc.
 Ensor, T. F. June 1990. “Mould Materials,” Glass   Bernard Conner, Accutrue International
     Technol. 31(3), 85–88.                             Corporation
 Ensor, T. F. October 1978. “Mould Materials,”      Peter Gerhardinger, Libby Owens Ford
     Glass Technol. 19(5), 113–19.                      Company
 Geiger, G. 1990. “Recent Advances in Glass         Dr. Foster Harding, Schuller International
     Manufacturing,” Ceramic Bull. 69(3), 341–44.   Vince Henry, Ford Motor Company
 Geiger, G. 1993. “Recent Advances in Glass         Theodore Johnson, U. S. Department of Energy
     Manufacturing,” Ceramic Bull. 72(2), 27–33.    Dr. Walter Johnson, Schuller International
                                                    Dan Labelski, Libby Owens Ford Company
 Glass Technology Roadmap Workshop September
                                                    Pat Moore, Combustion-Tec, Inc.
     1997. Proceedings of the Glass Technology
                                                    Dr. Robert E. Moore, University of Missouri-
     Roadmap Workshop held in Alexandria, Va.,
                                                        Rolla
     April 24–25, 1997, prepared by Energetics,
                                                    Frederic Quan, Corning, Inc.
     Incorporated, Columbia, Md.
                                                    Phil Ross, Consultant
 Gridley, M. May 1997. “Philosophy, Design and      Dr. Robert Speyer, Georgia Institute of
     Performance of Oxy-fuel Fired Furnaces,”           Technology
     Ceramic Bull. 76(5).                           Greg White, Praxair




                                                                                              Page 6-19

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Glass industry process

  • 1. CHAPTER SIX G LASS economy, employing more than 150,000 people in skilled jobs and generating more than 21 million metric tons of consumer products each year with INDUSTRY by Douglas W. Freitag Glass is an important component of the U.S. • product development of innovative new uses for glass. The following sections describe current glass manufacturing processes and how ceramic-based an estimated value of $22 billion. Glass production materials might contribute to achieving the glass is energy intensive and accounts for 12% of the industry vision. Materials commonly used in total cost of sales. In theory, about 2.2 million Btu glass-processing equipment include fused silica, of energy are required to melt a ton of glass. In graphite, precious metals, and water-cooled fer- reality, it takes twice as much energy because of rous alloys. Ceramics are primarily used as refrac- inefficiencies and loses. The glass industry consists tories, with ceramic coatings increasingly being of four major segments: container glass (bottles, used for wear resistance. Advanced ceramics are jars, etc.); flat glass (windows, windshields, rarely used because of their high cost. Moreover, mirrors, etc.); fiberglass (building insulation and owing to the lack of suitable higher-temperature- textile fibers); and specialty glass (cookware, flat capable materials, fluxes are commonly added to panel displays, light bulbs, fiber optics, medical reduce glass-processing temperatures and permit equipment, etc.). Flat glass accounts for 17% of U.S. the use of conventional materials. production by weight, container glass 60%, fiberglass 9% and specialty glass 4%. While the container, fiber, and flat glass industries focus on producing soda-lime glass for large, broad-based markets, the specialty glass industry focuses on higher temperature glasses and produces over 60,000 products. Examples of products manufactured by the glass industry are shown in Fig. 6.1. Key drivers for investment in the glass industry include cost, quality, and increased productivity. The desired state of the glass industry in twenty years has been formulated in a vision document prepared cooperatively with the U.S. Department of Energy (DOE). Future technology challenges and research opportunities were identified by comparing the vision of the future with the cur- rent state of the glass industry. Technology chal- lenges are broadly categorized into four areas: • advances in melting and refining processes and in fabricating/forming; • technology development of new glass-making techniques, processing controls, and computer simulations to model new processes; Fig. 6.1. Examples of glass industry products. • systems improvements in emission controls, Source: Marketing brochure from Praxair, Inc., recycling methods, and solid waste manage- Danbury, Conn. ment; and Page 6-1
  • 2. Chapter 6 The discussion is organized according to the consumption, and lower temperatures above the four primary operations for glass manufacturing: melt for reduced emissions. batching, melting, refining, and forming. Batching, melting, and refining operations are common to Combustion-Heated Furnaces all glass manufacturing processes with some varia- tion in furnace type. Various postprocesses are Combustion-heated furnaces can be further used, depending on the end product. divided according to the method used to recover exhaust waste heat and the way the fuel is burned (with air or pure oxygen). Heat recovery is impor- 6.1 BATCHING tant because only 10% of the heat generated is ac- tually used to melt the glass, whereas 67% is lost Raw material selection is based on chemistry, through the exhaust. Recovery of exhaust waste purity, uniformity, and particle size. Additional heat is performed with a regenerator or organic, metallic, and ceramic contaminants can recuperator. Regenerator furnace designs repre- be introduced throughout the delivery, storage, sent one of the oldest and most universal devices mixing, and sizing processes, as well as through for saving energy in the glass industry. the use of cullet (recycled glass). Because of the An example of a regenerative furnace is shown effects of contamination, the use of cullet varies in Fig. 6.2. The batch introduced at one end within the industry according to the desired end immediately begins to melt. As glass is extracted product. Flat glass manufacturers recycle up to from the opposite end, the newly introduced glass 30% of their own cullet. Ceramic contaminants flows along the melt zone where it becomes com- undergo little reaction with the glass melt and re- positionally homogeneous and bubble-free. After side as inclusions (stones) in the finished product. leaving the melt zone, the molten glass enters the Metal and organic contaminants create instabili- refining zone where it becomes thermally homo- ties during glass processing (through reduction/ geneous. After leaving the refining zone, the glass oxidation reactions) and degrade the quality of the enters the forehearth, where it can be distributed glass. Organics present in the batch are a source to one or more forming machines. of increased emissions and added cost for flue gas Refractories commonly used in regenerative cleanup. Delivery, mixing, and sizing processes are glass melt furnaces are shown in Fig. 6.3. Alumina- highly abrasive, and equipment used commonly zirconia-silica (AZS) is the material of choice for contains ceramic-coated wear surfaces or ceramic the sidewall, with the zirconia content ranging liners manufactured from alumina, silicon carbide, from 33 to 41%. Various chrome-containing refrac- or tungsten carbide. While generally cost-effective tories are commonly used in the fiberglass indus- and with adequate performance, lower cost solu- try because of their superior erosion resistance but tions offering lower risk of contamination continue are gradually being replaced because of environ- to be sought. mental concerns about chrome. Even though su- perior in performance, chrome-containing refrac- tories are not widely used in the flat, container, and specialty glass industries because chrome im- 6.2 MELTING purities result in glass discoloration. At the front of the furnace where the charge is introduced, ero- There are about 600 glass melt furnaces in sion and thermally induced damage are the most North America. The distribution of these furnaces severe and limit furnace life. Materials that are is as follows: container, 210; fiberglass, 110; flat more corrosion resistant are sought for the glass, 45; and specialty, 235; The life of a glass- sidewalls. Cladding refractories with platinum or melting furnace between rebuilds varies, but 7–8 molybdenum is considered too expensive. Not years is not uncommon. The cost of rebuilding a shown are the AZS electrode and burner blocks furnace can easily exceed $1 million. located on the sidewall below the melt line. These Glass furnaces can be divided into those heated blocks support the electric boost heat electrodes electrically and those heated primarily by and burner nozzles, both of which experience high combustion. Often electrical heating of the melt is local temperatures and thermal stresses that re- used in combination with fuel firing (electric boost) sult in reduced life. to improve heating uniformity, provide The main source of a problem is above the glass intermittent increases in melt capacity at minimum melt where temperatures are the hottest and cost, increase melt efficiency, reduce gas Page 6-2
  • 3. Glass Industry Fig. 6.2. Typical side-fired flat glass melt furnace. Source: “Development of Improved Refractories for Glass Industries,” U.S. Department of Energy fact sheet. Fig. 6.3. Refractories commonly used in a soda-lime glass melt furnace. products of combustion reside. Superstructure ($10,764/m2) and is much heavier. Because of a refractories can experience temperatures chemical incompatibility between the silica crown approaching 1600°C when air-fuel fired, and much and the AZS sidewall, a buffer layer of zircon hotter when oxy-fuel fired. Silica is preferred for (ZrSiO4) is used. the crown because of its low cost ($646m 2) and its Water-cooled, stainless steel burners are located ability to be dissolved into the melt if dislodged. on the sidewall of the furnace above the melt. In a However, silica does not perform well under the regenerative furnace, firing alternates from side increasingly harsh environment of oxy-fuel firing. to side, and intermittent firing is used to heat the Mullite has been used for the crown in processing regenerator brickwork, which then preheats the higher temperature glasses but can create cold combustion air when firing is reversed. The inclusions in the glass. AZS, which has been stainless steel burner orifice can be either water- considered for the crown, costs much more or air-cooled and frequently overheats. When Page 6-3
  • 4. Chapter 6 firing, the burner orifice is cooled by the flow of fuel and air. During the off cycle, the burner orifice is cooled by air alone with temperatures approaching 1040°C. The size and geometry of the nozzle varies, but a typical orifice is 3.17 cm in diameter and 10.16 cm long. A longer life ceramic orifice is sought, but it cannot cost more than $200 to remain competitive with a metal orifice cost of $150. A typical furnace has 20ioveter a61se of a gas-fired burner with a rectangular shaped nozzle is shown in Fig. 6.4. Stirring and bubbling are used in the melt zone to improve homogeneity. Stirring is typically done with a water-cooled stainless steel or uncooled platinum paddle operating at glass melt tempera- tures of 1425°C. The lifetime of water-cooled paddles can be indefinite. While no cooling is re- quired, platinum paddles are costly and life lim- ited. Bubbling is accomplished with water-cooled, high-carbon-steel nozzles located on the floor. Bub- bler nozzle life is several years; however, when replacement is necessary, it is easily accomplished by drilling a sidewall, plugging the slow glass leak, and then redrilling the sidewall for insertion of the new bubbler tube that contains the nozzles. Molybdenum disilicide bubblers have recently Page 6-4
  • 5. Glass Industry A cost of $400 for a 6.4-cm-diam, 15.2-cm-long One approach to overcoming these limitations is to tube with a 1.3-cm-thick wall is desired. preheat the batch/cullet mixture. As shown in In addition to those for furnace temperature, Fig.6.5, the hot flue gas passes on one side of the sensors are being developed to measure refractory heat exchanger while the batch powders fall counter thickness during furnace operation. These sensors to the flue gas stream. Preheating can be provided are based on acoustic, eddy current, or electrical by using a separate burner or using the heat capacitance measurements that change over time. available in the exhaust stream of the furnace. In Sensors imbedded directly into the refractory are both cases, a fan is used to pull the hot gases through also being considered. Ceramic shields are desired the heat exchanger. Operating conditions of the to protect the sensors from the harsh environment. preheater include inlet gas temperatures of up to Particulate emissions contained in the flue gas 982°C and exposure to the by-products of are removed by either cloth filters or an electro- combustion, which with the flue gas can contain static precipitator. A conventionally fired regen- high levels of moisture and corrosive vapors. erative furnace can generate particulate emissions Metallic designs currently used require frequent approaching 9kg/h at the particle separator . Fans maintenance and are temperature limited. By using are commonly used to maintain the desired level a ceramic heat exchanger, life could be extended, of gas velocity and thus ensure the particles re- weight reduced, and temperature capability main in the gas stream and avoid fallout. While increased. Similar benefits exist with a ceramic fan. this system is adequate for current furnace designs One benefit of this approach is the ease of retrofitting and regulatory guidelines, future furnace designs existing furnaces. and increasingly stringent guidelines provide an A second approach to increasing furnace effi- opportunity for advanced ceramics with the ciency is to introduce oxygen for firing. The higher temperature capability and improved cor- improvement in furnace efficiency is a result of rosion resistance sought for the filter media, fans, the higher flame temperature and diminished ex- and ducts leading to the particle separator. haust losses caused by the reduction of nitrogen The maximum production output of a glass flow through the combustion system. The oxygen furnace is limited by the amount of energy that can for firing can be introduced locally through lances be supplied inside the furnace to the melt. The limit (tubes) immersed in the melt or by substituting is reached when the burners are at maximum heat air at the burners for oxygen. Lances are con- output and electrical boost is at maximum power. structed from water-cooled metals, precious met- Further increases in heat input at the melt furnace als, or, more recently, molybdenum disilicide. are limited by the temperature capability of the While the least expensive and simplest to setup, materials used to construct the furnace and the oxygen injection can result in local hot spots and increased emissions (NOx) that would be generated. damage to the furnace walls, burners, etc. Fig. 6.5. Batch and cullet preheating system. Source: “Integrated Batch and Cullet Preheater System,” U.S.Department of Energy fact sheet. Page 6-5
  • 6. Chapter 6 Pure oxygen–based combustion introduced at Electrically Heated Furnaces the burners is gaining growing acceptance by the glass industry, with 87 furnaces, mostly in the Electrical heating is used exclusively in many specialty glass and fiberglass industry, currently smaller specialty and fiberglass melting furnaces in operation. Benefits include greater flame because of its lower initial cost and low emissions, stability, greater glass throughput, reduced fuel even though energy costs remain high. Key consumption, reduced furnace cost, reduced NOx disadvantages of all electric furnaces are the high and particulate emissions, and high efficiencies energy cost and short furnace life. Electric furnaces without the use of costly regenerators, are rebuilt as often as every six months, but recuperators, or electric boosts. An important because of their small size, down time is limited disadvantage is the change in characteristics of the to only about two days. An example of a bottom- exhaust gases. A typical regenerative furnace has entry electrode furnace with a refractory-lined, flue gas temperatures of 315–482°C at the exhaust water-cooled wall is shown in Fig.6.6. T op-entry takeoff, with moisture content of 8–10%. electrodes are commonly used to simplify Recuperated furnaces are slightly higher in both maintenance because of the short life of materials exhaust temperature and moisture content. Oxy- currently used. Where bottom or sidewall fuel-fired furnaces exhaust gas at 1204–1426°C and penetration is not desired, boost elements are have a moisture content of 55–67%. Because of the simply placed over the side of the tank, when high flame temperatures, corrosive volatiles (e.g., accessible. In the furnace example shown in boron) are present in the exhaust gases. The high Fig.6.6, the melt exits thr ough a submerged throat firing temperatures and harsh exhaust gases into a separate temperature-conditioning chamber. present with oxy-fuel furnaces provide a number Bubblers and stirrers are frequently used to of opportunities for ceramic materials. improve homogeneity of the melt and eliminate A number of alternate combustion cycles are bubbles. Materials used in the construction of being developed in unison with oxy-fuel firing. small specialty glass and fiberglass furnaces The goal is to arrive at alternatives that are less include precious metals (for bubblers, stirrers, difficult and costly to integrate into existing fur- plungers) and molybdenum (for electrodes and naces while still achieving reduced emissions and flow-control hardware). Replacement materials are increased efficiency. Many of these concepts re- sought that are lower in cost and provide longer quire advanced ceramics for burners capable of life. Replacement materials are also sought for higher temperatures, heat exchangers for preheat- electrode holders that do not require water cooling. ing combustion gases, particle separators for emis- Natural-gas-fired immersion heaters, which could sion control, and fans to move flue gases. Fig. 6.6. Electric melt furnace for producing chopped fiber. Source: “Advanced High-Temperature Material for Glass Applications,” U.S. Department of Energy fact sheet. Page 6-6
  • 7. Glass Industry ultimately replace electrical immersion heaters, in the melt section. Plungers and nozzles used to will require advanced ceramic outer liners. distribute the molten glass are refractory ceramics or molybdenum, but they experience high wear and erosion resulting from glass flow and 6.3 REFINING corrosion. For lower temperature glasses, Inconel 600 is commonly used, with similar problems The glass-conditioning operation (refining) reported. At higher temperatures, water cooling occurs in the forehearth and provides molten glass can also be considered. that is uniform in temperature. The forehearth is A number of opportunities for advanced ce- generally natural gas fired. Electric boosting can ramics were identified within the melting and re- be used to increase efficiency and improve fining operations of glass manufacturing and are temperature uniformity. Water-cooled metal heat summarized in Table 6.1. Many of these opportu- exchangers are used to help regulate temperature nities directly relate to oxy-fuel-fired furnaces. Al- and would benefit from advanced ceramics. ternative combustion cycles are also being devel- Temperature sensing is critical in the forehearth oped in which advanced ceramics are considered and shares problems similar to those encountered enabling. Table 6.1. Opportunities for advanced ceramics in glass melt furnaces Application Industry need Opportunities for ceramics Refractories More corrosion resistance materials Incremental improvements potentially through graded designs Flue gas ducts High-temperature-capable, Fiber-reinforced ceramic composite corrosion-resistant materials to accommodate thin wall designs Particle separators High-temperature-capable bag filters Ceramic hot-gas filter Greater corrosion resistance Burners Higher-temperature-capable burner SiC or MoSi2 ceramic matrix nozzle with longer life composite, thermal barrier coatings Sensor shields Uncooled thermocouple and IR Engineered material to camera shields with longer life accommodate unique requirements; SiC (above the melt) or MoSi2 (below the melt) ceramic matrix composite potential candidates Immersion heaters Change from electric boost to Engineered material with a Mo outer natural gas fired liner below the melt Electrode supports Longer life blocks, uncooled holders Incremental improvements potentially through graded designs Stirrers Lower cost, uncooled materials MoSi2, ceramic matrix composite or coating Plungers/melt flow control Lower cost, longer life materials Engineering ceramic matrix composite. MoSi2 and mullite potential candidates Bubblers/lances Lower cost, uncooled materials Engineering ceramic matrix composite; MoSi2 and mullite potential candidates Glass stop insulation Longer life Incremental improvements potentially through graded designs Heat exchangers Higher temperature capable, longer SiC or SiC ceramic matrix life materials, uncooled materials composite tubular structures Fans for air flow control High-temperature-capable fan SiC ceramic matrix composite with materials engineered attachment Page 6-7
  • 8. Chapter 6 6.4 FLAT GLASS use the float process to form molten soda-lime-silica glass into a flat sheet. No applications for advanced ceramics were identified within the flat glass coating The U.S. flat glass industry comprises six ma- operation, and that operation will not be discussed jor flat glass producers operating 28 furnaces in further. Unlike the melting furnaces used in other 16states. These plants, which employ about 12,000 glass manufacturing processes, those used for skilled workers, produce 3.9 million tons of glass producing flat glass are generally very large. The per year and achieve sales of $2.1 billion. Plants refiner is equally large because of the need to are mainly located near sources of low-cost energy. eliminate seeds resulting from gas bubbles and other In 1991 the industry used 55.2 trillion Btu of en- defects that degrade optical clarity. ergy derived primarily from natural gas and elec- Two float glass forming processes are in use, one tricity. Partly because of competitive pressures developed by Pilkington Brothers (PB) and one from developing countries, increases in produc- developed by PPG Industries, Inc. Major differences tion efficiency and energy efficiency are continu- between the two processes involve how the glass is ally being sought. Over the past 25 years, energy introduced into the furnace. Essential features of the efficiency has improved by over 50%, with much PPG process are shown in Fig.6.7. A typical float of the energy savings resulting from the use of im- zone furnace is 49 m long and 9 m wide and can proved refractories. The typical flat glass plant hold 909 metric tons of glass. In the PPG process, costs $100 million to build and has a campaign the refined glass flows in a continuous, full-width life of 12 years. ribbon onto a molten bath of tin contained in an air- The typical flat glass manufacturing plant cooled, refractory-lined steel shell. In the PB process, consists of upstream operations for batching, the molten glass enters a much narrower region onto melting, refining, forming, and annealing. a molten bath of tin and takes a very complicated Downstream operations include reheating, flow path before it reaches its full width. In both reforming, coating, tempering, and laminating. processes, glass enters at a temperature of 1040°C Downstream operations can be performed on-site and exits at a temperature of 600°C. The tin bath or off-site. Upstream operations are virtually is maintained at a temperature of 815°C, whereas identical for all manufacturing plants because all Fig. 6.7. PPG float glass process. Source: Adapted from F. V. Tooley, Handbook of Glass Manufacture, 3rd ed., Vol.2, Books of the Glass Industry Div ., Ashlee Publishing Co., New York, 1984, pp. 714–15. Page 6-8
  • 9. Glass Industry the steel shell air is cooled to 100°C. An atmosphere densates of tin sulfide vapor can dislodge and fall of N2plus 5–8% H 2 is used to prevent tin oxidation. on the glass surface. Current procedures call for Alternate materials considered for replacement of intermittent cleaning of the furnace roof and dis- the refractory-lined steel/tin bath shell (e.g., posal of the contaminated glass. Globars create a tungsten and graphite) are either costly, difficult similar problem, but a potential solution is to re- to fabricate into the desired shape, or lack place them with radiant burners. oxidation resistance. Few material candidates exist From the float zone furnace, the glass sheet because of the highly corrosive nature of tin. enters the annealing furnace, more commonly re- The flow of glass into the float zone furnace ferred to as the lehr, to remove internal stresses from the refiner is regulated by a fused-silica gate resulting from thermal gradients generated dur- called a tweel. The tweel meters the glass flowing ing the plate-forming process. The lehr operates into the float zone furnace and thereby helps con- at temperatures of 200°C, with heating provided trol the finished product thickness. The finished by gas combustion or electricity. In an electrically product thickness is also varied by the glass vis- heated lehr, the atmosphere is air; in a gas-fired cosity, surface tension, and, most important, the lehr, the atmosphere contains by-products of com- traction forces applied to the edge of the glass rib- bustion. The glass is transported through the lehr bon by water-cooled stainless steel gears (stretch- on water-cooled stainless steel, fused-silica, or as- ing drives). Thicker glass can be formed by flow- bestos-containing rollers with dimensions of up ing the glass against nonwetting, water-cooled to 5 cm in diameter by 2.5 m long (as illustrated in graphite barriers at the edges. Fig. 6.8). Problems associated with supporting the Because the life of the tweel is only about ten glass sheet in the lehr include roller surface dam- months, alternate materials are being sought to age, roller marking, sag between rollers, roller increase life. In additional to molten glass com- wave distortion, and roller contact heat transfer. patibility, requirements for the tweel include high- Uniform size and speed of the rollers is critical to thermal-shock resistance and load-carrying capa- avoid scratching the glass surface. Radiant burn- bility at molten glass temperatures. Replacements ers are used in many gas-fired lehrs and can be a for water-cooled materials are also sought because source of high maintenance. Upon cooling to room of the high level of maintenance and added cost temperature, the glass is cut and packaged for from corrosion. More corrosion-resistant metal al- downstream processing. loys (e.g., stainless steel) have been used but are Downstream operations can include a number undesirable because nickel gets into the recycled of reheating, reforming, tempering, and coating glass and forms nickel sulfide during annealing. steps. Reheating and tempering occur in furnaces Conditioned water is commonly used throughout similar to the lehr and only vary in temperature. a glass manufacturing plant to minimize corrosion. Large volumes of heated air are frequently circu- The glass temperature and tin melt are main- lated in the lehr to help reduce thermal gradients. tained by several thousand silicon carbide globars The fans are produced from high-temperature steel located along the length of the furnace. As the glass with water-cooled drives. Fan distortion reduces moves down the tin bath, it cools and increases in life at the higher operating temperatures. After the viscosity, thus allowing lifting from the tin bath glass has been reheated it can be reformed by by transport rolls. The transport rolls that carry bending on cast fused-silica molds with integral the glass to the annealing furnace are engineered heating elements. Because of their poor structural to avoid deforming the glass under its own weight. properties, molds are easily damaged. While they The rolls, about 30cm in diameter and 4.3 m long, can be repaired, repairs are time consuming and are constructed of asbestos or water-cooled metal. can be costly, both in down time and the actual Instrumentation used to monitor the process maintenance. Release coatings or barriers of ce- includes optical pyrometers for measuring surface ramic or metal films are used to inhibit sticking of temperature of the glass and tin bath and shielded the glass to the mold. Because the glass relaxes thermocouples for measuring the subsurface tem- after molding, die design is an iterative process perature of the tin bath. Measurements of local ve- that can occur over an extended period and be locity in the tin and glass and of glass thickness, costly. as well as visualization of surface defects and A summary of opportunities for advanced ce- stresses, are desirable but have met with limited ramics in flat glass manufacturing is shown in success. Cooled instrumentation used in the roof Table 6.2. Opportunities for the batch, melt, and is commonly a source of glass defects because con- refinement operations were previously described. Page 6-9
  • 10. Chapter 6 Fig. 6.8. Fused silica rollers used in a lehr. Source: Marketing brochure from Air Liquide America Corp., Walnut Creek, Calif. Table 6.2. Opportunities for advanced ceramics in flat glass manufacturing Operation Industry need Opportunities for ceramics Float zone furnace 1. Longer life tweel 1. Incremental improvements 2. Replacement of asbestos transport 2. Ceramic matrix composite or a rolls metal/ceramic hybrid 3. Improved materials for stretching 3. SiC or Si3N4 monolithic ceramic drives 4. Engineered material with 4. Replacement of refractory lined tin anticipated bath redesign bath 5. Ceramic matrix composite or Si3N4 5. Uncooled sensors 6. SiC or MoSi2 ceramic matrix 6. More efficient gas-fired radiant composite. Thermal barrier burners coatings. Annealing, reheating, 1. Longer life air circulation fans 1. SiC ceramic matrix composite with tempering 2. Improved hot handling materials engineered attachment 3. High-temperature-capable 2. Ceramic matrix composite or noncontact sensors cermet 4. Low-maintenance radiant burners 3. Si3N4 or ceramic matrix composite 4. SiC or SiC ceramic matrix composite Reforming 1. Improved mold materials and 1. Engineered materials manufactured manufacturing methods using 3-D rapid prototyping 2. Improved mold release coatings or technology film barriers 2. Ceramic-coated metal or ceramic 3. High-temperature-capable cloth noncontact sensors 3. Si3N4 or ceramic matrix composite Page 6-10
  • 11. Glass Industry 6.5 CONTAINER GLASS Downstream operations can be performed on-site or off-site. The batching, melting, conditioning, The container glass industry, which is made up and annealing operations are similar to those dis- of 64 plants in 25 states, employs 30,000 skilled cussed previously. No applications for advanced workers, produces 12.3 million metric tons of glass ceramics were identified within the coating opera- per year, and achieves sales amounting to $5 bil- tion, and thus that operation will not be discussed lion annually. Plants are located mainly near prod- further. uct markets. In 1992 the industry used 119 trillion Btu of energy, 79% of which was derived from Forming natural gas. Competitive pressures occur from market preferences for lighter weight container After conditioning to the desired temperature materials (e.g., plastic and aluminum), the high and uniformity, the molten glass is transferred to cost of end product distribution resulting from the the forming operation through the gob feeder. The weight of glass containers, and the lower cost of gob feeder is an integral part of the forehearth and labor in developing countries. The market share includes a mixer, plunger, orifice, and shears, as lost to alternate materials is being reduced by regu- shown in Fig 6.9. The gob feeder controls the latory concerns and the higher cost of “product weight, temperature, and shape of the gob—all of stewardship.” The distribution cost of glass con- which are critical to container quality. Gob-form- tainers is benefiting from the development of ing rates can easily approach 300 gobs per minute. higher strength glass and improved manufactur- The materials of construction for the gob feeder ing methods developed to produce thin-wall, include water-cooled metals (cut-off blades), pre- light-weight containers. Increased production and cious metals (stirrer), and refractories (plunger) to energy efficiencies are continually sought to effec- accommodate glass gob temperatures approach- tively compete against developing countries. The ing 1100°C. typical container glass plant costs $70 million to The gob next enters the forming operation build. The last container plant was constructed in where the heat is extracted from the glass in a con- 1981. Because of their high level of maturity, manu- trolled manner during shaping. Several processes facturing processes are essentially the same in all are available to form the gob; the process selected plants. is based on composition, container shape, size, and The typical glass container manufacturing production rate. The press-and-blow process com- plant consists of upstream operations for batching, monly used to produce large-mouth containers is melting, conditioning, and forming and down- illustrated in Fig.6.10. stream operations for coating and annealing. Fig. 6.9. Single gob feed cycle. Source: Reprinted from F. V. Tooley, Handbook of Glass Manufacture, 3rd ed., Vol.2, Books of the Glass Industry Div ., Ashlee Publishing Co., New York, 1984, p. 601. Page 6-11
  • 12. Chapter 6 Fig. 6.10. Container press-and-blow forming operation. Source: Reprinted from F. V. Tooley, Handbook of Glass Manufacture, 3rd ed., Vol.2, Books of the Glass Industry Div ., Ashlee Publishing Co., New York, 1984, pp.616–17. In the press-and-blow forming operation, the generates a high level of noise, liquid cooling may gob enters the mold and is first pressed into a be employed as an alternative. A disadvantage of blank. During the pressing operation, plunger liquid cooling is the increased maintenance needed temperature approaches 550°C. Because the for leakage and corrosion. To increase cycle times, plunger material is not as hard at these mold temperature can be further reduced by temperatures, the plunger surface is easily scored precooling the air or liquid. Mold materials having by the chilled glass at the interface during higher thermal diffusivity have also been used (e.g, extraction. The blank is removed from the press copper-based alloys in neck rings). To reduce mold, reheated, and then placed into a second mold-to-glass adhesion, glass marking, and mold for blowing into final shape. When removed oxidation of the metal mold, the mold is swabbed from the blow mold, the glass cools to 780°C. with a lubricant between cycles. Because mold Production rate is limited by the rate of heat temperatures approach 500°C, the lubricants used extraction and can approach 100 bottles per minute generate smoke and vapor and sometimes ignite when multiple molds are used with a single gob spontaneously. Alternate approaches available feed system. The forming operation is the single include nonmetallic solid film lubricants or the largest contributor to both the cost and quality of injection of acetylene into the hot mold between the finished glass container, and molds represent cycles, which cracks and forms a thin deposit of a large part of that contribution. carbon throughout the mold interior. Requirements for mold materials include low Alternate materials of construction for molds cost, capability for producing complex shapes, low currently being considered include nickel-based distortion in use, ability to take and retain a fine superalloys, graphite, and ceramic coatings. surface finish, low thermal expansion, adequate Asbestos-containing materials were used for a hot structural properties, scale and oxidation period but have been replaced by graphite for resistance, high thermal diffusivity, and high reasons of regulatory compliance. Nickel-based resistance to thermal cycling. The primary mold superalloys are being used in areas where higher material is cast iron. Low-pressure air-cooling is hot hardness and strength is desired (e.g., used to accelerate heat extraction and improve plungers, baffles, funnels, blow heads, bottom temperature control. However, because air cooling plates, and plugs). As with flat glass, nickel Page 6-12
  • 13. Glass Industry contamination can be a problem with container bide) onto existing hot handling surfaces, espe- glass. cially the plunger, by a variety of processes. While Graphite is a preferred material for hot han- more difficult and costly to machine, resistance to dling because it does not wet, mark, or adhere to oxidation and scaling occurs when applied to glass; provides inherent lubricity; does not scale; graphite and cast iron, respectively. Spallation of retains its strength at temperature; and is low cost, the coating can also occur, thus increasing the risk easily machined, and environmentally safe. Ap- of glass contamination. plications include blow-head inserts, take-out After molding, the containers are placed into tongue inserts, sweep-outs, lehr-loading bars, gob the lehr for removing stresses (see Fig. 6.11). The deflectors, and conveyor guides. Grades of graph- lehr is similar to that previously discussed for flat ite used are selected based on their application. glass; the primary difference is the use of a con- Porous graphite is desirable in the lehr to avoid veyor belt to move batches of the container glass thermal checking, which is caused by rapid heat through the hot zone in a controlled manner. The transfer from glass containers. Higher density conveyor is fully contained within the furnace to graphite is used in molds where increased strength reduce heat loss and increase energy efficiency. and hardness are desired. While graphite offers With the exception of coatings, no evidence was many advantages, a key disadvantage is its lim- found of advanced ceramic materials usage in the ited life due to poor oxidation resistance and low forming of glass containers. Opportunities exist, strength. A greater number of glass defects have however, because of the increasing emphasis on been seen when ceramics have been used. lightweight glass containers, continuous produc- As containers become thinner, higher hardness tion, and improved energy efficiency as competi- materials are desired for mold surfaces to reduce tion increases from abroad. Specific opportunities contamination and maintain surface finish and include replacement of graphite in areas of hot critical dimensions. Higher hardness materials also handling, with alternate materials providing provide reduced die wear in machines where pro- higher toughness, greater hardness, and improved duction rate is being increased in place of added oxidative stability. Lower cost materials are sought capacity. One solution is the application of hard to replace precious metals used in the forehearth coatings (e.g., tungsten carbide and titanium car- and gob feeder. Materials with higher thermal Fig. 6.11. Gas-fired glass annealing furnace with an internal belt return. Source: Re- printed from F.V .T ooley, Handbook of Glass Manufacture, 3rd ed., Vol.2, Books of the Glass Industry Div., Ashlee Publishing Co., New York, 1984, p. 682. Page 6-13
  • 14. Chapter 6 diffusivity and hot hardness are sought for molds wool. Total energy consumption for the fiber glass to extend life and reduce cycle time without ex- industry in 1991 was 59.5 trillion Btu, a value ternal cooling. Higher-temperature-capable mate- exceeded only by the container industry. Unlike rials are desired throughout the mold machine for container or flat glass, a large amount of energy reduced maintenance and improved quality. New consumed is used to fabricate fiber from molten sensors and sensor shields are sought to measure glass. Whereas both markets are capital intensive, gob temperature, temperature uniformity, and the textile fibers industry is more so because of its heat extraction during shaping. In all cases, ad- greater dependence on precious metals. Glass vanced ceramics are central to the solution. Be- quality requirements also vary, but textile fibers cause they are compatible with molten glass, ad- production requires much higher quality glass and vanced ceramics of choice are oxide-based or ce- greater use of process control. Key drivers for the ramic composites having predominate oxide use of new technology in the fiber glass industry phases. Leading candidates are alumina, mullite, include reduced cost, increased throughput, and silicon carbide–alumina composites. Molyb- reduced emissions, and a competitive advantage. denum disilicide–based composites are also good Competitive pressures primarily exist from organic- candidates for many of these applications. Silicon- based products. Processes used to manufacture fiber based ceramics are candidates for hot applications glass can be divided into batching, melting, and requiring optimum structural properties that are fiberization. Each process and opportunities for not in contact with molten glass. advanced ceramics is discussed in detail in the A summary of opportunities for advanced ce- following sections. ramics in container glass manufacturing is shown in Table 6.3. Batching The batching operation is similar to that used 6.6 FIBER GLASS for producing other glass products with the exception of different compositions and finer raw The fiber glass industry is made up of two material particle size. Because particle size is finer, primary markets: wool insulation and textile emissions control and contamination by ceramics, fibers. Wool insulation is a short-fiber product used metals, organics, and alloys are of greater concern. by the construction industry. Textile fibers are a A key ingredient for manufacturing fiber glass is continuous-fiber product principally used as a boron, which provides reduced melt viscosity and polymer matrix composite reinforcement. improved glass durability but is volatile and Together, the four major producers of wool and condenses at undesirable locations. The content the six major producers of textile fibers employ of boron in glass fiber ranges from 5 to 10 w % about 16,000 workers. A typical producer can and represents 50–75% of the total cost. A high operate as many as 27 glass melt furnaces. In 1991, percentage of recycled container and flat glass is fiber glass made up 9% of all glass produced and used to produce wool fiber glass, for which quality accounted for 21% of the total market value. In requirements are less stringent. Recycled fiber 1981, 71% by weight of all glass fiber shipped was glass is not currently used because of the boron present. Table 6.3. Opportunities for advanced ceramics in container glass manufacturing Application Industry need Opportunities for ceramics Radiant burners Longer life burners SiC or SiC ceramic matrix composite Hot handling Higher toughness materials with Ceramic matrix composite, cermet, improved oxidation resistance or carbide coatings Gob feeder Lower cost stirrer materials, reduced Engineered ceramic matrix erosion plunger materials, and composite plunger, cutter, and uncooled cutter materials stirrer; mullite ceramic composite plunger Molds Higher hardness materials, higher Engineered material or coating thermal diffusivity Sensors Longer life thermocouple shields Si3N4 or ceramic matrix composite Page 6-14
  • 15. Glass Industry Melting homogenization section followed by a hearth for distribution to the different fiber-forming In addition to batching, the other major process operations. Conversion to oxy-fuel firing is also that fiber glass shares in common with other glass occurring as a result of demonstrated improvements industries is the melting operation. The differences in energy efficiency and emissions. are limited to processing challenges resulting from A number of opportunities exist for advanced glass compositions and an overall smaller furnace ceramics within the melter. Higher temperatures size. The low alkali content of textile fibers requires of the melt in combination with the low alkali higher melting temperatures and greater fiber content force the use of environmentally sensitive particle size to ensure homogeneity without the chrome-containing refractories. Chrome- benefit of fluxing additives. Both textile and wool containing refractories are also electrically products include boron, which can foul furnace conducting, drawing current away from the glass hardware and increase particulate emissions. Glass and reducing refractory life. The by-products of melters include regenerative, direct-fired, and combustion, and higher temperatures of recuperative furnaces. Large recuperative gas-fired combustion with oxy-fuel firing, also limit refractory furnaces are still the primary source of melters. The life. Improved glass stop refractories are sought for campaign life of gas-fired furnaces is 7–10 years; reduced maintenance. Precious metals (platinum/ rebuilds are costly and time consuming. Some rhodium) are used throughout the melter (gas conversion to electric and electric-boosted gas-fired bubblers, plungers, thermocouple shields) with a furnaces has occurred to reduce gas consumption, per furnace cost approaching $10 million. Platinum/ increase melt efficiency, and reduce emissions rhodium is used because of its high-temperature (SOx, NOx, particulate). Electric furnaces use top- capability and stability when in contact with glass or bottom-entry electrodes, the former being easier at temperatures exceeding 1371°C. Disadvantages to maintain. The campaign life of a electric furnace include high cost, low creep strength, and uncertain is six months; atypical r ebuild takes two days to supply. Alternatives to platinum/rhodium complete. An example of a cooled metal wall, evaluated and deemed unsuccessful include bottom-entry electrode melter is shown in Fig. 6.12. germanium-doped molybdenum disilicide and The melt exits through the submerged throat into a Inconel. Ceramic-based material options include Fig. 6.12. Example of a bottom entry electrode electric glass melter. Source: “Advanced High-Temperature Material for Glass Applications,” U.S. Department of Energy fact sheet. Page 6-15
  • 16. Chapter 6 high alumina or zirconia composites with high shown in Fig. 6.13, a single stream of molten glass purity and density. Ternary phase oxides could falls into a rapidly rotating cylinder (centrifuge) also be considered, but phase instability is of made of a high-temperature base metal alloy. The concern. Another option is higher creep resistant cylinder has a 12- to 36-in. diameter and contains nonoxide composites (i.e., silicon carbide with a 10,000–30,000 laser-drilled holes having diameters platinum/rhodium coating for molten glass of 0.1–0.015in. Operating conditions include compatibility). steady state temperatures of 2000°F while exposed Molybdenum is also a primary material used in to air. Typical lifetime is 40–100 h. The base metal fiber glass melters. Applications include electrodes alloys used are generally proprietary, investment and glass flow control devices. For lower cast, and extensively recycled to reduce cost. temperature applications (< 1315 °C), Inconel 600 Alternate materials are sought with increased has been used. In both cases, premature failures creep strength, resistance to corrosion (sulfidation) have occurred due to excessive corrosion and wear. and oxidation, and reduced cost. As the molten Advanced ceramics are sought that can operate glass is driven through the perforated cylinder, uncooled, above or below the melt line, and exhibit downwardly directed high-velocity gas jets high-temperature creep resistance. Opportunities attenuate the flow and form fibers of varying for advanced ceramics also exist for oxy-fuel-fired lengths. The fibers are sprayed with organic burners and in shields for sensors being developed binders and collected onto a moving belt where to detect refractory wear. they are transported to a curing oven. Alternate approaches to the rotary process for forming wool Fiberization include attenuation of a molten stream passed through sieve-like platinum alloy bushings and Molten glass leaves the hearth and enters the remelting of drawn fiber by a high-velocity burner. fiberization process directly or after an In the continuous textile forming process intermediate marblizing process. Marblizing is shown in Fig. 6.14, glass marbles are fed through commonly used for textile fiber production when orifices in an electrically heated platinum alloy the forming process is not located in close bushing. A typical bushing has a cross-section of proximity to the melter. Fiberization occurs by 25–50 cm by 18–20cm with a depth of 5 cm. About attenuating a melt steam with air, steam, or 100 dies are used with a single melter, and a typical combustion gas (for wool) or by drawing molten production site contains four melters. Each glass through precious metal bushings (for production unit processes 400–3,000 filaments 24textiles). For the wool pr ocessing example simultaneously, with fiber diameters ranging from Fig. 6.13. Example of a wool forming process. Source: Reprinted from F. V. Tooley, Handbook of Glass Manufacture, 3rd ed., Vol.2, Books of the Glass Industry Div ., Ashlee Publishing Co., New York, 1984, p. 721. Page 6-16
  • 17. Glass Industry reliability. Lower cost solutions are also sought for the platinum sieves and bushings. Oxides dispersion strengthened with yttria have been evaluated as a precious metal replacement with promising results. Platinum-coated molybdenum was also considered but failed during testing. A summary of opportunities for advanced ce- ramics in fiber glass manufacturing is shown in Table6.4. Opportunities for the batch operation ar e similar to those discussed for container and flat glass. Fig. 6.15. Ceramic textile fiber handling guides. Source: Reproduced from Kyocera’s Web site at http://www.kyocera.com. Fig. 6.14. Example of a textile forming process. Source: Reprinted from F.V . Tooley, Handbook of Glass Manufacture, 3rd ed., Vol.2, Books of the Glass 6.7 SPECIALTY GLASS Industry Div., Ashlee Publishing Co., New York, 1984, p.725. This market segment is a catchall for glass products not contained in the flat glass, container 3.5 to 20 µm. Bushing life is about six months, at glass, and fiber glass market segment. An impor- which time the bushing is recycled because of the tant difference is that the specialty glass produc- high cost of raw material. Life is limited by hole ers, unlike the other glass producers, work with wear and sag. Operating conditions include steady higher temperature glasses, aluminosilicates, and state temperatures of 1200°C in air. Forced- borosilicates, as opposed to soda lime glass. While convection air cooling is used prior to the sizing compositions vary, processing methods are simi- application to increase productivity. A winding lar to those previously discussed. The markets drum collects the filaments and applies a traction served are very broad, with as many as 60,000 dif- force to stretch the fibers as they are pulled through ferent products, but volumes are small. A large a mechanical device that applies an organic sizing number of specialty glass producers are small, and twists multiple filaments into a single strand. owner-managed, and labor intensive. Because of Depending on the application, additional the nature of the business, the technology being processing is performed on the multiple strands. used is frequently developed in-house and re- Materials commonly used to handle and guide hot mains highly proprietary. filaments are carbon based. Alumina and silicon Specialty glasses have very specific chemical nitride guides are used for sizing application and compositions with clearly defined physical and twisting where high wear can occur. Typical fiber chemical properties. Furnaces are generally special handling guides are shown in Fig. 6.15. Alternate designs and principally oxy-fuel fired. To further materials are sought for fiber handling that exhibit improve energy efficiency, cullet preheaters are many of the characteristics of monolithic ceramics being evaluated. Melter capacities of 45 metric tons but have improved toughness for increased are considered large. Because of the high melting Page 6-17
  • 18. Chapter 6 temperatures, refractories and thermocouple product. During the forming operation, molds are sheaths are a problem. Infrared cameras are generally superalloys. Ceramic molds were frequently used in place of short-lived considered but shown to increase the presence of thermocouples for temperature control, and fiber stones in the finished glass. Natural-gas-fired optic systems are being developed. Because of metal radiant burners, which are commonly used their small size, convection currents are used in during forming and annealing, continually require place of bubblers to homogenize the glass. A range a high degree of maintenance. of high-temperature base metal alloys and A number of niche opportunities exist within precious metals are used throughout the melt and the specialty glass industry. Opportunities forming steps (e.g., platinum paddles used in the common among glass market segments also exist, melter for homogenization). After the melting but operating conditions both above and below process, the glass goes in several different the melt are much harsher in specialty glass. directions for forming, depending on the final Table 6.4. Opportunities for advanced ceramics in fiber glass manufacturing Operation Industry need Opportunities for ceramics Melting 1. Improve refractories 1. Incremental improvements 2. Precious metal replacement 2. Mo based engineered material 3. Longer life electrodes 3. Mo based engineered material 4. Longer life flow control materials 4. Mo based engineered material 5. Sensor shields 5. Mullite or Mo based engineered 6. Oxy-fuel-fired burner nozzles material 6. SiC or MoSi2 ceramic matrix composite; thermal barrier coating Wool forming 1. Low-cost centrifuge material 1. Mo based engineered material 2. Precious metal replacement for 2. Mo based engineered material sieves and bushings Textile forming 1. Precious metal replacement for 1. Mo based engineered material bushings 2. Improved fiber-handling materals 2. Ceramic matrix composite Page 6-18
  • 19. Glass Industry 6.8 REFERENCES Pfendler, R. D. 1995. “Glassmakers Face New Emissions Concerns,” Ceramic Bull. 74(4), Publications 95–99. Tooley, F. V. 1984. The Handbook of Glass Manufac- DOE/Conf-970292 December 16, 1997. Oxy-Fuel ture, 3rd ed., Vols 1–2, Books for the Glass Issues for Glassmaking in the 90s: Workshop Industry Div., Ashlee Publishing Co., New Proceedings, documenting a workshop York. sponsored by the U.S. Department of U.S. Department of Energy, Office of Industrial Energy’s Office of Industrial Technologies Technologies January 1995. A Technology and cosponsored by 22 companies in the Vision and Research Agenda for America’s Glass U.S. glass industry, held in Arlington, Va., Industry. February 27, 1997. U.S. Department of Energy, Office of Industrial Energetics, Incorporated September 8, 1994. Technologies January 1996. Glass: A Clear “Materials Needs and Opportunities in the Vision for a Bright Future. Glass Industry: Schuller International, Inc.” Energetics, Incorporated October 27, 1994. Information Supplied by Academia and the “Materials Needs and Opportunities in the Glass Industry Glass Industry: Libby Owens Ford Com- pany.” Scott Anderson, Combustion-Tec, Inc. Ensor, T. F. June 1990. “Mould Materials,” Glass Bernard Conner, Accutrue International Technol. 31(3), 85–88. Corporation Ensor, T. F. October 1978. “Mould Materials,” Peter Gerhardinger, Libby Owens Ford Glass Technol. 19(5), 113–19. Company Geiger, G. 1990. “Recent Advances in Glass Dr. Foster Harding, Schuller International Manufacturing,” Ceramic Bull. 69(3), 341–44. Vince Henry, Ford Motor Company Geiger, G. 1993. “Recent Advances in Glass Theodore Johnson, U. S. Department of Energy Manufacturing,” Ceramic Bull. 72(2), 27–33. Dr. Walter Johnson, Schuller International Dan Labelski, Libby Owens Ford Company Glass Technology Roadmap Workshop September Pat Moore, Combustion-Tec, Inc. 1997. Proceedings of the Glass Technology Dr. Robert E. Moore, University of Missouri- Roadmap Workshop held in Alexandria, Va., Rolla April 24–25, 1997, prepared by Energetics, Frederic Quan, Corning, Inc. Incorporated, Columbia, Md. Phil Ross, Consultant Gridley, M. May 1997. “Philosophy, Design and Dr. Robert Speyer, Georgia Institute of Performance of Oxy-fuel Fired Furnaces,” Technology Ceramic Bull. 76(5). Greg White, Praxair Page 6-19