An important thermal process for producing zinc is the Imperial Smelting Process and around 9% of the world’s zinc is made by this process. The main difference between this and the other retort thermal processes is that the smelter produces lead as well as zinc, roughly one tonne of lead for every 2 tonnes of zinc. The roasting step in this case is carried out on a sinter machine, a long slowly-moving grate on which the feed material, which includes lead concentrates as well as zinc concentrates, and sometimes mixed lead-zinc concentrates, travels and through which air is blown to burn the sulphur. The reasons for using a sinter machine, apart from the burning of sulphur, are firstly that a normal zinc concentrate roasting furnace is not capable of handling significant amounts of lead and secondly that hard lumps of sintered charge are required for the blast furnace, and a sinter machine is a better way to produce them than briquetting.


The sulphur dioxide (SO2) gas produced from roasting concentrates on the sinter machine is then, after cleaning, 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 charge to the blast furnace is lump sinter and coke, the coke burning in the lower part of the shaft and the heat from this and the carbon monoxide gas produced providing the means to reduce the zinc and lead oxides to metallic zinc and lead. The lead, which is below its boiling point, flows from the bottom of the blast furnace, carrying copper, silver and gold with it. The zinc vapour passes out of the furnace near the top and is rapidly quenched and dissolved into a spray of molten lead so that the zinc vapour has insufficient time to oxidise back to zinc oxide. Due to the special relationship between lead and zinc, by cooling the lead, crude zinc is released and is separated, and the lead returns to the “condensing” process for another cycle of dissolving and then releasing more zinc.


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, including the Imperial Smelting Process, 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. Many Imperial Smelting smelters sell as much zinc without further refining as possible in order to keep overall costs down.


For a “not quite so simple” Imperial Smelting (ISP) zinc flowsheet please click on the red square >