Refining Process | Properties of Zinc | Quality Control     Zinc concentrates are received by both rail and truck transport on site. Approximately 420,000 tonnes/year of zinc concentrates are processed. The various types of concentrates are blended to provide a consistent chemical composition to the process. Typically over 95% of concentrate consists of zinc, sulphur and iron. the blended concentrate enters the process as feed to the roasting stage.   Concentrate is unloaded at the Tippler Concentrate is transported via conveyor belt to the Concentrate Storage Shed     concentrate is removed from the storage shed by two 20 tonne grab cranes Concentrate is then transported via pipe onveyor to the Concentrate daily bin     The concentrates are fed into the fluidised bed roaster and roasted with oxygen at 950°C. The objective of the roasting process is to convert zinc sulphide to acid soluble zinc oxide and sulphur dioxide gas, utilising the following reaction: 2 ZnS + 3O2 = 2ZnO + 2SO2 The solid zinc oxide (ZnO, Calcine) is separated from the gaseous sulphur dioxide (SO2) utilising various techniques. The collected zinc oxide is sent to the Leaching Plant and the sulphur dioxide goes to the Sulphuric Acid Plant via the gas cleaning plant.   Gas Cleaning The sulphur dioxide gas produced in the roaster is cleaned in the gas cleaning plant and clean gas goes to the Sulphuric Acid Plant. Sulphuric Acid The clean and dry gas enters a converter containing a catalyst.  The sulphur dioxide gas converted to sulphur trioxide (SO3) in converter by catalyst. The sulphur trioxide is then combined with water in the absorption tower to make sulphuric acid for sale H2O + SO3 =  H2SO4 Sulphuric acid is transported from site by either road or rail transport (GATX) to customers.     Leaching Zinc oxide (called calcine) is leached by electrolyte containing sulphuric acid in reactors. ZnO + H2 SO4 = ZnSO4 + H2O The solids remaining from the leaching process consist primarily of zinc iron compounds called ferrites. These solids are removed from the leaching process by filtering in a high pressure filter and pumping to the zinc ferrite pond. The solution containing the zinc sulphate is pumped to the Purif   Purification The purification process removes minor elements from the zinc sulphate solution prior to electrolysis.  The purification is achieved by the addition of zinc dust (produced in the melting and casting plant), promoting cementation reactions such as: M2+(liquid) + Zn (solid) = M (solid) + Zn2+ (liquid). As a result, impure metals are converted to the solid form and separated from the zinc sulphate solution by filtering. Solution is cooled in cooling towers to precipitate gypsum and the clear purified solution is then pumped to the electrolysis plant.     Zinc is recovered from purified solution in the electrolysis cells by an electro winning process in which zinc is deposited on aluminum cathodes while oxygen is evolved at lead anodes. ZnSO4 + H2O = Zn + H2SO4 + 1/2O2 The zinc deposits are stripped off from aluminium cathodes and transferred to the melting and casting plant. The sulphuric acid produced during the electrowinning process is returned to the leaching plant to leach zinc oxide (calcine).   The zinc metal from electrolysis is melted in an electric furnace, then cast into various shapes for sale. Several other metals such as aluminum and antimony can be added to produce alloys to meet specific customer requirements. Zinc dust is also produced and this is used in the purification plant as a reagent.