Zinc MetallurgyZINC EXTRACTION PROCESSEST he most important raw material for production of Zinc is Zinc Sulphide (ZnS) orecommonly known as Sphalerite. Next in the importance are Zinc Carbonates(Calamine/Smithsonite) and Zinc Silicate. Most of the zinc production worldwide originatesfrom 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 Processii) Vertical Retort Processiii) Electro Thermal Processiv) Imperial Smelting ProcessB. Hydrometallurgical Processes:i) Roast Leach Electrowin Processii) Pressure Leaching Process
PYROMETALLURGICAL PROCESSESSP resently about 15 - 20% of the worlds zinc production comes from pyrometallurgical route.The horizontal and vertical retort processes and electrothermal process were used in the past forzinc 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 worlds primary - zinc production are through the ImperialSmelting Process. The process co-produces lead bullion in addition to zinc metal using a mix oflead and zinc concentrates or complex lead-zinc concentrates as raw material. The ImperialSmelting Process is similar to blast furnace processes except that it is operated with hot topwhereby preventing reoxidation of zinc vapours. The process consists of basic two operationsnamely; sintering and blast furnace smelting of sintered lumps to extract lead and zincsimultaneously. 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 PROCESSESSA bout 80% of world’s total zinc output is produced through conventional hydrometallurgicalroute i.e. Roast-leach-electrowin (RLE) route. The three out of the four plants installed in India
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 routefor 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 tofurther treatment by leaching, i.e., the zinc mineral into dissolved state in solution. Since zincsulphide mineral is not easily leached, it is first converted into zinc oxide, which is easilyleached. This is predominantly accomplished by roasting of zinc concentrate in fluid bed roastersand fixation of SO2 bearing off gases thus generated as sulphuric acid by contact process. In therecent past, an alternative technology to conventional roasting followed by leaching, has beendeveloped by Sherritt-Gordon of Canada, commonly known as “Pressure Leach Process” whicheliminates the need of roasting step prior to leaching. The process fixes the sulphide content ofconcentrate as elemental sulphur, thus eliminates the need for a separate sulphuric acidproduction facility, whereas the zinc metal content of the concentrate is converted into a zincsulphate solution, thus combining both roast-leach steps of conventional process into a singleunit operation.Thus there are two-process route for extraction of zinc hydrometallurgically: • PRESSURE LEACH ELECTROWIN PROCESS • ROAST–LEACH–ELECTROWIN (RLE) PROCESSROAST LEACH ELECTROWIN (RLE) PROCESS:The main purpose of roasting of zinc sulphide concentrate is to convert it into a product, which isamenable to further treatment through hydrometallurgical process for extraction of zinc.Secondly, to fix the sulphide contents into sulphur dioxide gases for subsequent economicalrecovery as sulphuric acid.The principal reaction during fluid bed roasting is: 2 ZnS + 3O2 = 2ZnO + 2SO2These rich gases are cleaned and cooled to recover dust content as zinc calcine and to remove thevarious harmful impurities such as Hg, Se, F, Cl, As, etc., before being led to double contact and
double conversion process (DCDA) plant for manufacture of sulphuric acid.The dead roasted product, zinc calcine, is subjected to leaching with recycled electrolyte toextract zinc content. The enriched zinc sulphate solution is further subjected to purification withzinc dust to eliminate impurities like copper, cadmium, cobalt, nickel etc. before being subjectedto elecrolysis.The zinc sulphate solution is fortified with return electrolyte solution, undergoes electrolysis inlined concrete cells for deposition of zinc on aluminium cathodes. The lead- silver plates areused as anodes. The deposited zinc on the cathodes are periodically stripped by automaticstripping machines, melted in induction furnace and cast into saleable zinc ingots man automaticcasting and stacking machine. In order to maximise zinc extractions the leach residues mainlyzinc- ferrites, a bye product of leaching process, are subjected to treatment to hydrolyse its ironcontent 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 processroute), with an assay of 99.995% zinc, i.e. it can contain a maximum of 50 parts per million ofimpurities. There is also a grade of 98.5% zinc, the main impurity being lead, and this used to bethe standard grade, called GOB (Good Ordinary Brand) or PW (Prime Western), normally aproduct from the Imperial smelting process (ISP). A SHG product from the ISP can also beproduced by distillation process of refining.PRESSURE LEACH PROCESS:The pressure leach technique was first successfully commercially applied for zinc extraction withthe commissioning of first plant in 1981 at Cominco, Trail, Canada.There are presently three electrolytic zinc plants in the world where this technique has beenintegrated into the existing facilities. A fourth green field plant with two-stage counter-currentpressure leach-electrowin is under consideration.In this process zinc sulphide or bulk zinc concentrates are oxidized under oxygen overpressures
of 1200 kpa abs. at a temperature of 1500C in sulphuric acid medium to produce zinc sulphatesolution directly and the sulphide content is precipitated as elemental sulphur according to thefollowing overall reaction: ZnS + H2S04 + 0.5 02 = ZnSO4 + H20 + SºThe various factors influencing the kinetics of above reaction are the particle size, mineralogy,surface active additives, acidities, reaction time, temperature and oxygen over-pressures whereby maximising metal extraction and fixation of lead and iron constituents into disposable Jarositeand other types of residues. The zinc sulphate solution thus produced is amenable to furtherprocessing for final zinc extraction through conventional leach-electrowin units.