RETORT ZINC PRODUCTION
The main feature of thermal zinc smelters, including retort smelters, is that zinc is produced by the use of heat and carbon. Zinc was first produced by this method. As for electrolytic production of zinc 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. 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 reduced to metallic zinc using heat and carbon. In the early methods using horizontal retorts, small open-ended tubular retorts were used, the closed ends being exposed to the inside of a heated furnace. These retorts were charged with a mixture of calcine and a source of carbon, sometimes anthracite and sometimes charcoal. Under the influence of heat the zinc oxide in the calcine is reduced to metallic zinc by the carbon, metallic zinc being produced as a vapour. As this vapour passes from the closed heated end of the retort into the cooler outer regions, the zinc vapour condenses to liquid zinc, which can be collected and tapped off. It was important to prevent air from entering the open end of the retort for this would allow the zinc to oxidise back to zinc oxide, defeating the object of the process, and several devices such as plugs or U-bends were used to prevent this happening. The horizontal retort process was always inefficient and is now outdated, only being used at some small unofficial zinc smelters in China.
The vertical retort process was a significant advance on the horizontal retort, since it was continuous and used a larger unit. The first step in the process is the same as for the horizontal retort, namely roasting concentrates to calcine, but in this case the calcine is mixed with coal and anthracite and formed into briquettes, which are then heated in an oven to “coke” or carbonise them and, in effect, to glue the particles together. The briquettes are then treated in a vertical heated shaft or retort, as in the horizontal retort the zinc oxide being reduced to zinc vapour, this exiting the shaft at the top and being condensed to liquid zinc. In the Western World this process has not been operated since the early 1970s, but it is still actively used in China, where the lower cost of labour and the low cost of coal relative to electricity mean that it is still economic.
A major difference between the thermal and the electrolytic processes for making zinc is that, whilst the latter produces very pure zinc directly because the removal of impurities has taken place before the reduction step, all the thermal processes produce a lower grade zinc that still contains significant impurities, in particular lead, cadmium, iron, copper and tin. Whilst some of these elements can be reduced to lower levels by simple means, and the zinc may then be useable for general galvanising purposes, to achieve the highest purity the zinc must be purified by distillation. Distillation can achieve as high purity as obtainable for electrolytic zinc, but the additional cost of treatment is very high, in particular for energy requirements.