ELECTROLYTIC ZINC PRODUCTION
85-90% of the world’s zinc is produced by the electrolytic method. The first stage is to roast the zinc sulphide (ZnS) concentrate to zinc oxide (ZnO) in the presence of oxygen (i.e. in air), the sulphur being converted to sulphur dioxide (SO2) gas. The sulphur dioxide is then further oxidised to sulphur trioxide (SO3) which is dissolved in strong sulphuric acid. The strong sulphuric acid is then diluted with water for reuse, the surplus representing production of sulphuric acid for sale or for use in leaching (see below). Some impurities in the original concentrate, e.g. mercury, pass with the SO2 gas and have to be removed in order not to contaminate the sulphuric acid. Sulphuric acid is a major by-product of zinc smelting, up to 2 tonnes being produced for every 1 tonne of zinc in some smelters.
The impure zinc oxide, called calcine, is then leached (i.e. reacted with) sulphuric acid to dissolve the zinc, producing impure zinc sulphate (ZnSO4). Unfortunately this presents a dilemma. If the conditions are too weakly acidic then a substantial part of the zinc (that which is combined with iron in a zinc-iron compound) will not be dissolved. On the other hand, if strong and hot sulphuric acid is used the zinc will be almost completely dissolved but so will the iron, and it then becomes a problem to separate the large quantity of dissolved iron. In practice the leaching process is carried out in several steps, the first step invariably being only very weakly acid. The zinc sulphate so produced is separated from the remaining solids and passes to the next step, which is purification. (We have not forgotten the solids and will deal with them later!)
The purpose of purification is to remove the impurity metals that have dissolved with the zinc. The principal ones are cadmium, copper, cobalt and nickel. Because zinc is a highly reactive metal this property can be used to remove them. By adding powdered zinc to the solution the zinc is oxidised and dissolves, with the opposite side of the reaction being the reduction of the other dissolved metals back to their metallic form. This is analogous to the schoolboy trick of putting brass coins into silver nitrate – because silver is a noble (un-reactive) metal the coins become silver-plated with, behind the scenes, a small amount of the more reactive copper dissolving. Removal of impurities from the zinc sulphate is most important. If they remain, not only will the zinc produced from the subsequent electrolysis be insufficiently pure, but their presence will interfere with the smooth running of the electrolysis process itself. The minor elements such as cadmium and copper can be subsequently recovered by other processes to produce by-products for sale.
The purified zinc sulphate is now ready for electrolysis. In this step the zinc sulphate is passed continuously through tanks containing a series of alternate aluminium cathode (blanks) and lead anodes. Current is passed between the anodes and cathodes and metallic zinc is plated onto the cathodes by a reduction reaction; in electrolysis there always has to be an opposing oxidising reaction at the anode and, in this case, it is the liberation of oxygen gas. Periodically the cathodes are removed and the metallic zinc is harvested i.e. peeled away from the aluminium blanks, which are then replaced in the tanks for another cycle. For electrolysis alone the electric power requirement is approximately 3300kWh per tonne of zinc produced for sale and including other electrical requirements of the smelter the total requirement is approximately 4000kWh per tonne. This is substantially less than that required to produce aluminium but still represents a very large amount of energy, for example it is equivalent to a 100W electric light bulb running for 4.5 years.
The metallic sheets peeled from the aluminium cathode blanks are finally melted in a furnace and then cast into ingots for sale.
We still have to deal with the question of the solid residues separated from the leaching process. These solids still contain substantial amounts of zinc, together with the gangue components, and lead, silver and other impurities. Most commonly the solids are leached with strong, often hot, sulphuric acid, dissolving most of the remaining zinc together with iron and other impurities, leaving a lead-silver residue that may be further treated for recovery of these metals. The dissolved iron is then brought out of solution to make a solid iron residue for dumping and the remaining solution joins the main production stream for zinc. There are many variations within this “residue treatment” process. A substantially different treatment method involves the thermal treatment of the residue to produce a zinc oxide fume from which zinc can then be recovered by aforementioned means. This thermal route is a particular feature of zinc smelters located in Former Eastern Bloc countries, e.g. China, Russia, Kazakhstan, Poland, Bulgaria.