Zinc Processes of Extraction

1. Zinc Metallurgy
ZINC EXTRACTION PROCESSES
T he most important raw material for production of Zinc is Zinc Sulphide (ZnS) ore
commonly known as Sphalerite. Next in the importance are Zinc Carbonates
(Calamine/Smithsonite) and Zinc Silicate. Most of the zinc production worldwide originates
from sulphide ores.
The principal processes by which zinc is extracted from its ores can be categorised under pyro-
metallurgical processes and hydro-metallurgical processes.
A. Pyrometallurgical Processes:
i) Horizontal Retort Process
ii) Vertical Retort Process
iii) Electro Thermal Process
iv) Imperial Smelting Process
B. Hydrometallurgical Processes:
i) Roast Leach Electrowin Process
ii) Pressure Leaching Process

2. PYROMETALLURGICAL PROCESSESS
P resently about 15 - 20% of the world's zinc production comes from pyrometallurgical route.
The horizontal and vertical retort processes and electrothermal process were used in the past for
zinc production but have become obsolete due to high power consumption and low recovery.
The only pyrometallurgical process of importance presently is Imperial Smelting Process (ISP).
Imperial Smelting Process (ISP):
Currently about 8 -10% of the world's primary - zinc production are through the Imperial
Smelting Process. The process co-produces lead bullion in addition to zinc metal using a mix of
lead and zinc concentrates or complex lead-zinc concentrates as raw material. The Imperial
Smelting Process is similar to blast furnace processes except that it is operated with hot top
whereby preventing reoxidation of zinc vapours. The process consists of basic two operations
namely; sintering and blast furnace smelting of sintered lumps to extract lead and zinc
simultaneously. The Imperial Smelting Process has the following demerits:
• Process requires mix of zinc and lead concentrates.
• It is a labour intensive process.
• Present demand scenario does not call for addition of lead smelting capacity in the
country.
• Because of high temperature involved in maintainability of the plant.
• Low plant availability.
HYDRO-METALLURGICAL PROCESSESS
A bout 80% of world’s total zinc output is produced through conventional hydrometallurgical
route i.e. Roast-leach-electrowin (RLE) route. The three out of the four plants installed in India

3. are operating on hydrometallurgical process route, whereas the fourth one at Chittaurgarh,
Rajasthan, which was commissioned in the year 1991 is based on imperial smelting process route
for simultaneous extraction of lead and zinc.
The pre-requisite condition for zinc metal extraction from sulphide concentrate through a hydro-
metallurgical route is the elimination of its sulphur content in order to make it amenable to
further treatment by leaching, i.e., the zinc mineral into dissolved state in solution. Since zinc
sulphide mineral is not easily leached, it is first converted into zinc oxide, which is easily
leached. This is predominantly accomplished by roasting of zinc concentrate in fluid bed roasters
and fixation of SO2 bearing off gases thus generated as sulphuric acid by contact process. In the
recent past, an alternative technology to conventional roasting followed by leaching, has been
developed by Sherritt-Gordon of Canada, commonly known as “Pressure Leach Process” which
eliminates the need of roasting step prior to leaching. The process fixes the sulphide content of
concentrate as elemental sulphur, thus eliminates the need for a separate sulphuric acid
production facility, whereas the zinc metal content of the concentrate is converted into a zinc
sulphate solution, thus combining both roast-leach steps of conventional process into a single
unit operation.Thus there are two-process route for extraction of zinc hydrometallurgically:
• PRESSURE LEACH ELECTROWIN PROCESS
• ROAST–LEACH–ELECTROWIN (RLE) PROCESS
ROAST LEACH ELECTROWIN (RLE) PROCESS:
The main purpose of roasting of zinc sulphide concentrate is to convert it into a product, which is
amenable to further treatment through hydrometallurgical process for extraction of zinc.
Secondly, to fix the sulphide contents into sulphur dioxide gases for subsequent economical
recovery as sulphuric acid.The principal reaction during fluid bed roasting is:
2 ZnS + 3O2 = 2ZnO + 2SO2
These rich gases are cleaned and cooled to recover dust content as zinc calcine and to remove the
various harmful impurities such as Hg, Se, F, Cl, As, etc., before being led to double contact and

4. double conversion process (DCDA) plant for manufacture of sulphuric acid.
The dead roasted product, zinc calcine, is subjected to leaching with recycled electrolyte to
extract zinc content. The enriched zinc sulphate solution is further subjected to purification with
zinc dust to eliminate impurities like copper, cadmium, cobalt, nickel etc. before being subjected
to elecrolysis.
The zinc sulphate solution is fortified with return electrolyte solution, undergoes electrolysis in
lined concrete cells for deposition of zinc on aluminium cathodes. The lead- silver plates are
used as anodes. The deposited zinc on the cathodes are periodically stripped by automatic
stripping machines, melted in induction furnace and cast into saleable zinc ingots man automatic
casting and stacking machine. In order to maximise zinc extractions the leach residues mainly
zinc- ferrites, a bye product of leaching process, are subjected to treatment to hydrolyse its iron
content into disposable Jarosite / goethite / hematite. The zinc plants are also integrated with bye-
product cadmium recovery unit in order to maximise sales realization.
The standard zinc product is Special High Grade zinc (normally obtained from RLE process
route), with an assay of 99.995% zinc, i.e. it can contain a maximum of 50 parts per million of
impurities. There is also a grade of 98.5% zinc, the main impurity being lead, and this used to be
the standard grade, called GOB (Good Ordinary Brand) or PW (Prime Western), normally a
product from the Imperial smelting process (ISP). A SHG product from the ISP can also be
produced by distillation process of refining.
PRESSURE LEACH PROCESS:
The pressure leach technique was first successfully commercially applied for zinc extraction with
the commissioning of first plant in 1981 at Cominco, Trail, Canada.
There are presently three electrolytic zinc plants in the world where this technique has been
integrated into the existing facilities. A fourth green field plant with two-stage counter-current
pressure leach-electrowin is under consideration.
In this process zinc sulphide or bulk zinc concentrates are oxidized under oxygen overpressures

5. of 1200 kpa abs. at a temperature of 1500C in sulphuric acid medium to produce zinc sulphate
solution directly and the sulphide content is precipitated as elemental sulphur according to the
following overall reaction:
ZnS + H2S04 + 0.5 02 = ZnSO4 + H20 + Sº
The various factors influencing the kinetics of above reaction are the particle size, mineralogy,
surface active additives, acidities, reaction time, temperature and oxygen over-pressures where
by maximising metal extraction and fixation of lead and iron constituents into disposable Jarosite
and other types of residues. The zinc sulphate solution thus produced is amenable to further
processing for final zinc extraction through conventional leach-electrowin units.