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What is the function of sodium cyanide in separation of ZnS and PbS ?

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In the froth flotation process we want some substances to settle down and some substances to float. Now naturally, the sulphide of the substance we want to extract will float and the "gangue" or useless material will sink. However, sometimes the ore we are using also has other sulphides present in it as impurities. For example, lead sulphide generally has zinc sulphide also present as impurity which will also float in the froth floatation process along with the lead sulphide. To make sure that lead sulphide floats but zinc sulphide do not float, we add depressants. Depressants prevent certain type of particles from forming froth during froth floatation process. A common depressant used in the froth floatation of lead sulphide is sodium cyanide. When the ore containing ZnS and PbS is subjected to froth floatation process in the presence of NaCN, then only PbS forms the froth. This happens because NaCN reacts with ZnS to form Na2[Zn(CN)4]. 4NaCN + ZnS -----> Na2[Zn(CN)4] + Na2S

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