interest contains other metal sulphides as impurities, depressing agents such as sodium cyanide, sodium carbonate etc are used to selectively prevent other metal sulphides from coming to the froth. For example, when impurities such as ZnS is present in galena (PbS), sodium cyanide (NaCN) is added to depress the flotation property of ZnS by forming a layer of zinc complex Na [Zn(CN) ] on the surface of zinc sulphide. . .
Leaching This method is based on the solubility of the ore in a suitable solvent and the reactions in aqueous solution. In this method, the crushed ore is allowed to dissolve in a suitable solvent, the metal present in the ore is converted to its soluble salt or complex while the gangue remains insoluble. The following examples illustrate the leaching processes. Cyanide leaching Let us consider the concentration of gold ore as an example.
The crushed ore of gold is leached with aerated dilute solution of sodium cyanide. Gold is converted into a soluble cyanide complex. The gangue, aluminosilicate remains insoluble. 4Au (s) + 8CN - (aq) + O (g) + 2H O (l) [Au(CN) ] - (aq) + 4OH - (aq) controller motor air supply froth layer concen- trate valve sensor tailings feed stirrer Figure .
Froth Flotation XII U1 Metallurgy - XII U1 Metallurgy - - - - - Recovery of metal of interest from the complex by reduction: Gold can be recovered by reacting the deoxygenated leached solution with zinc. In this process the gold is reduced to its elemental state (zero oxidation sate) and the process is called cementation . Zn (s) + [Au(CN) ] - (aq) [Zn(CN) ] - (aq) + 2Au (s) Ammonia leaching When a crushed ore containing nickel, copper and cobalt is treated with aqueous ammonia under suitable pressure, ammonia selectively leaches these metals by forming their soluble complexes viz. [Ni(NH ) ] + , [Cu(NH ) ] + , and [Co(NH ) H O] + respectively from the ore leaving behind the