1. BAUXITE GRINDING PRACTICES AND OPTIONS
John Hadaway1
, Anthony Filidore2
1
FLSmidth Minerals Pty Ltd.; 24 to 28 Marshall Road, Airport West, Victoria, 3042, Australia
2
FLSmidth Minerals; 2040 Avenue C, Bethlehem, PA, 18017, USA
Keywords: Grinding, Bauxite, SAG, Rod, Ball, Scrubber
Abstract
Grinding of bauxite as feed to alumina refineries is undertaken in
a variety of grinding circuit types utilizing a range of mill types
such as Rod Mills, Rod/Ball Mills, Ball Mills, Rod Mill & Ball
Mill circuits, AG/SAG Circuits and SAG/Ball Mill circuits both in
open circuit and closed circuit. The paper considers the selection
criteria in terms of the objectives of the grinding process and
reviews the range of options available particularly those used in
recent installations. The advantages and disadvantages of these
options are discussed. Important evaluation criteria such as
product size distribution, specific power consumption and cost are
considered, summarized and compared with the design objectives.
Finally this paper will discuss some less common applications for
rotary drum scrubbers in the alumina industry.
Introduction
Feed preparation to alumina refineries is a critical first step in the
alumina production process. There are a full range of options
available an each has its advantages and disadvantages. The final
choice of which option best suits the application is influenced by a
number of variables. These can be broadly categorized into the
feed characteristics and the target product requirements.
Some of the feed variables are.
• Feed size distribution and feed top size.
• Grinding and breakage characteristics of the feed i.e.
Bond Work Index.
Some of the common target product requirement that are
considered in the final selection are:
• Product particle size distribution. In this respect the
main objective is to limit both the presence of course
material and very fine material. Typically specifications
will place limits on the percentage of particles
o > 1.0mm to 1.7mm
o < 45 µm
• Overall grind size is typically expressed as the size at
which 80% of particles will pass a sieve aperture of the
nominated size (P80).
These target product requirements are generally common to all
applications; however it is also common that one or two of the
criteria will take precedence. Which criteria take precedence will
have a major influence in the final grinding circuit choice.
In addition to the above criteria there are number other criteria
that will influence the final circuit design such as.
• Specific Power consumption expressed in kWh/t.
• Circuit simplicity
• Grinding plant footprint
• Ease of operation
• Health and safety considerations
• Availability
• Capital cost
• Operating cost.
In this paper we will review the various grinding circuit options
and consider the strengths and weakness of those circuits against
the criteria outlined above.
We do not expect to come to definitive conclusions here but the
objective of the paper is to explore the options and highlight
where each circuit is most appropriate. Inevitably the final choice
will be a compromise between competing criteria and the best
outcome is an optimum solution that meets best the primary
criteria specified at the project inception while not building in
excessive on going problems due to other criteria that are less
fulfilled.
In the experience of the authors’ the following product particle
size criteria are most common.
• % passing 1.7mm to 1.3mm, 98% or greater
• % passing 45µm,
• P80, 250 to 700µm
In the experience of the authors’ typical feed size distribution
specifications are
• Maximum particle size, 20 to 40mm
• 80% passing, 8 to 20mm
These feed and product size parameters are generally the primary
drivers in the selection of the grinding circuit, followed closely by
Specific Power Consumption, capital cost and operating cost.
It is very important that the key criteria are clearly defined at the
outset of the grinding circuit selection and design process.
Grinding Circuit Options
The following bauxite grinding circuits are the main circuits
employed in the alumina industry today.
Rod/Ball Mill – Open Circuit
This is probably the most common grinding mill type and has
been used for over 40 years. It is often the base case considered

2. for bauxite grinding. The mill is operated in open circuit and
comprises two compartments. The first compartment is charged
with rods to handle the course grinding of the primary feed while
the second compartment is charged with steel balls to perform the
finer, finish grinding of the final product.
Figure 1 is a diagrammatic illustration of this option.
This option has the following characteristics:
The rod compartment ensures that all the course particles are
ground to a size suitable for the subsequent grinding in the ball
compartment. This helps to minimize undesirable oversize
particles while the second, ball charged compartment produces a
reliable finished product with the desired P80. Ball-milling
however will produce a higher proportion of finer particles due to
the high surface area of the grinding media. It is also difficult to
control the shape of the product particle size distribution in open
circuit, i.e. without some form of classification.
While this solution has been successfully employed in many
bauxite grinding circuits it has a number disadvantages that are
considered important, particularly in modern plants. The mill
incorporates a central diaphragm to separate the two grinding
compartments and allow the material to be moved from the first to
the second compartment. This requires ongoing and relatively
regular maintenance which requires personnel to access the
interior of the mill and given that bauxite grinding is undertaken
in hot, caustic liquor, it is desirable to limit personnel access to the
interior for safety and health reasons. Modern safety procedures
make this a lengthy exercise. Regular entry into the mill will
therefore affect the operating availability of the mill.
This circuit also requires a relatively large foot print when
compared to other solutions. Not only does the space occupied by
the mill need to be considered but access at the feed end for rod
charging equipment is required. This can be space hungry and
difficult to arrange at the mill feed platform; particularly where
there are multiple mills. In contrast, single compartment rod mills
are usually charged from the discharge end where more space is
available.
Open Circuit Rod Mill followed by Closed Circuit Ball Mill
In this case the grinding circuit comprises an open circuit rod mill
followed by a closed circuit ball mill. The ball mill is most often
closed over a DSM style screen with a fine cut of 1 to 1.5 mm. the
course material is returned to the ball mill feed and the fine screen
underflow reports to the downstream process.
Figure 2 is a diagrammatic illustration of this option.
The primary advantages of this circuit is the ability of the rods
mill to handle course and variable feed sizes while the closed
circuit secondary grinding helps to minimize the presence of
undesirably course particles in the final product. By minimizing
the residence time in the secondary circuit it is also possible to
reduce the proportion of very fine particles in the product.
Some other operating and maintainability advantages with this
option are that both mills are overflow discharge and as such there
are no diaphragms or discharge grates to maintain. By careful
layout of the circuit it is possible to arrange the rod charge at the
discharge end of the mill thus improving the layout at the feed
end.
This option does however require a significant foot print
comprises more mechanical components, particularly in the mill
drives, bearings and lubrications systems which leads to higher
maintenance requirements. The classification screen also requires
significant maintenance and can be problematic from an
operational point of view. Overall availability can be impacted by
the availability of the screen and as such it may be necessary to
add redundant capacity in the form of standby screens; thus
adding to the complexity of this option.
Single Stage Closed Circuit Ball Mill.
This circuit has been recently applied in Australia and India. It is
essentially a development of the rod/ball mill circuit described
earlier except that the particle size reduction occurs in one
compartment. The rod mill is eliminated and the fresh feed is fed
directly to the ball mill. Because ball mills have limited ability to
handle course feed it is necessary to ensure that the feed is not too
course and the mill is required to operate in closed circuit, usually
with fine aperture screens in order to control the proportion course
particles in the final product.
This option has the advantage that the handling and management
of rods is eliminated along with the attendant availability and
safety issues. The need to enter the mill for maintenance of the

3. charge is limited too however the screens require regular
maintenance and can limit the overall availability of the grinding
circuit. As such redundant screen capacity is often employed.
Figure 3 is a diagrammatic illustration of this option.
Figure 4. 17’ x 25’, 3500 kW single stage, closed
circuit ball mill installation.
This option offers a relatively small foot print and medium capital
cost.
Single Stage Open Circuit Rod Mill
This circuit comprises a single stage rod mill operating in open
circuit. Of the options under consideration it is the simplest and
has been employed successfully on recent alumina refinery
expansion projects. Careful and comprehensive test work and
modeling was undertaken before the decision to use this option
was finalized.
The primary advantage of this circuit is its ability to handle course
and variable feed to the grinding circuit while at the same time
limiting the proportion of course particles and very fine particles
in the final product. It employs large rods (~100m diameter) in
order to ensure that there is sufficient impact energy available to
break to course particles. Characteristically, rod mills produce less
fines than other mill types.
Figure 5 is a diagrammatic illustration of this option.
Figure 6. 15’ x 21.5’, 1800 kW single stage,
open circuit rod mill installation.
The footprint for this option is small. In a recent project in
Australia three (3) 15’ x 21.5’ rod mills mill replaced three (3)
rod/ball mills and were incorporated in to the same foot print
while simultaneously increasing production by a factor of two.
The foundations of the original mills were partially reused and, by
innovative design of the modified foundations disruption to the
existing operation was kept to an absolute minimum during the
decommissioning of the old mills and installation and
commissioning of the new rod mills.
While simple, economical and compact, this option has a number
of characteristics that should be recognized, understood and
considered at the selection stage.

4. While being very effective at controlling the size and proportion
of top size particles in the product it must be recognized that this
option produces a generally courser grind in terms of the product
P80. Typically grinding circuits that employ ball mills will
produce a P80 in the range 200 to 400 micron while the rod mill
typically produces a P80 in the range 500 to 800 microns. At the
top end of the product size distribution this rod mill circuit
produces a P98/99 of approximately 1,700 microns.
Comparative size distributions are discussed later.
This circuit is also demands less energy than ball mill circuits and
typical specific energy consumption is in the range 4.0 to 6.0
kWh/t. The primary reason for the lower energy requirement is
the generally courser grind. It is therefore very important to verify
that this courser size distribution is suitable to the downstream
processes in the alumina refinery.
This circuit offers a very simple and economical plant in the right
application. With the proper management of the rod charge and
liners, high availability is achievable with limited need for
personnel to enter the grinding chamber for maintenance
purposes.
Closed circuit single stage SAG mills.
The advantage of the SAG mill is that is can handle very course
primary crusher feed and achieve very high reduction ratios. As
such secondary crushing is not required in this option and the
SAG mill can produce final product in a single stage. High
throughputs are also achievable in single line plants.
Figure 7 is a diagrammatic illustration of this option
This option requires the use of classification devices in the circuit
in order to control the top size. It is capable of high availability
provided the classification stage is adequately sized and
incorporates necessary redundancy.
It terms of capital and operating cost this option tends towards the
high end due to the relatively higher cost of SAG mills in
comparison to other grinding mill types.
Due to high reduction ratios involved and the predominance of
attrition grinding of small particles in this circuit the circulating
loads may be relatively high. As such the SAG mill will produce a
higher proportion of fine particles which may be undesirable in
subsequent processes in the refinery.
The primary advantage of this circuit is the ability the handle high
throughputs in single line plants and the simplification of the feed
preparation area.
Open Circuit SAG Mill following by Closed Circuit Ball Mill
An extension of the previous circuit is the SAG/Ball Mill circuit.
This option comprises an open circuit SAG mill followed by a
closed circuit ball mill. Due to the high throughputs associated
with this circuit, the ball mill is most often closed with a
hydrocyclone classifier. The course material is returned to the
ball mill feed and the fine overflow product reports to the
downstream process.
The primary advantage of this circuit is the ability to handle high
throughputs in a single line plant. The SAG mill can handle very
course and variable feed sizes while the closed circuit ball mill
helps to minimize the presence of undesirable course particles in
the final product.
The capital cost is high with this option as is the maintenance and
foot print requirements. It is slightly more energy efficient due to
the use of a ball mill as the secondary grinding machine.
Figure 8 is a diagrammatic illustration of this option

5. Rotary Scrubber Applications
In addition to the use of grinding mills in Bauxite preparation
circuits, the use of rotary drum scrubbers are finding increasing
application. Figure 10 and 11 illustrate the use of a scrubber in an
application where the bauxite is very sticky and contains rock
intrusions that are of no economic value. The scrubber pulps the
bauxite into a slurry which then passes over a trommel screen at
the scrubber discharge. The screen separates the fine bauxite from
the intrusions and the trommel underflow is then pumped to the
downstream process plant. The intrusions are rejected as trommel
overflow and conveyed to a waste dump.
It is apparent from Figure 10 that the scrubber is preceded by a
low speed sizer. The Run of Mine (ROM) feed is fed to the sizer
which is mounted directly above the scrubber feed chute, thus
minimizing mechanical handing of the sticky feed material.
Process water is introduced with the feed and acts to flush the
material thought the sizer and directly to the scrubbing chamber.
Feed to the scrubber is nominally 250mm top size and very little
breakage occurs in the scrubber. It is thus necessary to install
“rock lifters” at the discharge end of the scrubbing chamber to
remove the rock from the scrubber and assist with the feeding the
trommel screen. Build up of the lump material in the scrubbing
chamber is thus prevented.
Scrubbers have also been proposed to slurry the feed material
prior to the grinding circuit. In such applications this allows the
pre-classification and removal of fine material that is naturally
occurring in the feed and is of a partial size suitable for
subsequent processing without grinding. This option reduces the
occurrence of very fine particle in the plant feed that occur due to
over-grinding in the grinding circuit. Such options have been
considered for SAG and SAG/Ball circuits.
Figure 9. 26’ x 16’, 4850 kW
SAG mill in bauxite grinding Figure 11. Typical 5m x 10.4m, 1300 kW
Drum Scrubber Application.
Figure 12. Typical 3m x 8m, 200 kW
Drum Scrubber Application

6. Process Comparisons
While the type of grinding circuit selected for a given application
will be based on a wide range of criteria, the primary criteria for
an application are generally the process parameters that the
grinding circuit has to achieve. The various mills types are best
suited to different ranges of feed size and product size
distributions.
The following is intended as a general comparison of the various
circuits with respect to their optimum ranges of application. The
final selection will always depend upon a thorough
characterization of the Bauxite feed to the grinding circuit. Key
parameters to be established are
• Bond Work Index
• General breakage characteristics such as the JK
breakage parameters.
Comparison of Feed Size Distributions
The following Table 1 shows the range of application for each
circuit type described above.
Table 1. Feed Size Comparison
Circuit F80 (mm) F98 (mm)
Rod/ Ball 10 to 25 25 to 40
Rod and ball mill in
closed circuit
10 to 25 25 to 40
Single stage closed
circuit ball mill
4 to 10 8 to 16
Open Circuit rod
mill
10 to 25 25 to 40
SAG Mill 100 to 175 150 to 300
SAG/Ball Circuit 100 to 175 150 to 300
Comparison of Product Size Distributions
The following Table 2 shows the range of application for each
circuit type described above.
Table 2. Product Size Comparisons.
Circuit P80 (micron) P98 (micron)
Rod/ Ball in open
circuit
200 to 400 1000 to 1500
Rod and ball mill in
closed circuit
200 to 400 1000 to 1300
Single stage closed
circuit ball mill
200 to 400 1000 to 1300
Open Circuit rod
mill
500 to 800 1600 to 2000
SAG 250 to 500 1000 to 1500
SAG/Ball Circuit 200 to 400 1000 to 1300
Comparison of fine particles in the final product
Fine particles in the final product (i.e. % < 45 micron) is an
important consideration for the down stream processes in an
alumina refinery. It is difficult to quantify this criterion other than
to make some general remarks about the relative performance of
each circuit under discussion here.
Fines generation is largely a function of the breakage
characteristics of the given feed material but the selection of mill
type and circuit configuration will influence the final product in
terms of fines generation. The following comments are intended
as a general guide only as to the influence that the mill/circuit will
have. We have simply ascribed the descriptions, “more”, “less” or
“neutral” to describe the effect on the generation of fines in the
grinding process.
The following Table 3 given a guide to relative propensity fines
generation for each circuit.

7. Table 3. Comparison of fine particle generation
Circuit Contribution to fines generation
Rod/Ball in open
circuit
neutral
Rod and ball mill in
closed circuit
More
Single stage closed
circuit ball mill
More
Open Circuit rod
mill
Less
SAG More
SAG/ Ball Circuit More
Comparison of Specific Power Consumption
The final specific power consumption of a given circuit will
depend largely on the grindability (I.e. Work Index) of the
Bauxite and the feed and product size distributions. Each grinding
option will, however, influence the power required to meet the
required throughput under a given set of feed and product
conditions. For example if the mill is operating in open circuit the
main criteria is the minimization of the course material in the
product, then the mill power will be derated to ensure a longer
residence time to reducer the probability of course particles “short
circuiting” the grinding chamber.
The following Table 4 shows the range of application for each
circuit type described above.
Table 4. Specific Power Comparison
Circuit Specific Power Consumption (kWh/t)
Rod/Ball in open
circuit
8.0 to 12.0
Rod and ball mill in
closed circuit
6.0 to 9.0
Single stage closed
circuit ball mill
5.5 to 8.5
Open Circuit rod
mill
4.0 to 6.0
SAG 8.5 to 13.0
SAG/ Ball Circuit 8.0 to 12.0
Comparison of Other Criteria
In the final selection of the most appropriate grinding circuit for a
given application there are many criteria to be considered apart
from the process parameters. The most important of these are as
follows.
• Capital Cost
• Plant Footprint
• Availability
• Maintenance and other operating costs
• Health and safety of personnel
The priority given to each criterion will vary from project to
project and it is not the intension here to deal with these in any
particular ranking but simply to give a general idea of the relative
merits of each option.
In order to do this the simple notation is ascribed to each option;
“High”, “Medium” or “Low”.
The following Table 5 is a summary of the comparative strengths
of each option with respect to the other criteria.
Table 5. Comparison of other criteria.
CapitalCost
Footprint
Availability
Easeof
maintenance
Riskto
personnel
Rod/Ball in open
circuit
Med Med Low Low High
Rod and ball mill
in closed circuit
High High Low Low Med
Single stage
closed circuit ball
mill
Med Low Med Med Low
Open Circuit rod
mill
Low Low High Med Med
SAG High Med High Med Low
SAG/ Ball Circuit High High High High Low

8. The above assessment is, of course, subjective and open to debate
however this is considered a reasonable guide to the relative
merits of the various options.
Conclusion
It is clear from the above that the final selection of grinding mill
type and circuit is a complex balance of many competing criteria.
While the process criteria will always take precedence there are
many other important issues to consider. There is no universal
solution.
Careful definition of the most important criteria is essential.
Thorough characterization of the Bauxite from a breakage
perspective must be completed. Finally each circuit option must
be modeled and evaluated against the key criteria and other
important criteria that will effect the long term success of the final
installation.