Zinc Metallurgy

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

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 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.

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 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