This section of the refinery is part of the tankhouse impurities management system. Nickel is one of the major impurities in the tankhouse process and is also a valuable mineral hence the need to recover it. It is recovered as nickel sulphate hexahydrate crystals. The process of its recovery involves firstly the electrowinning of copper in the liberators, followed by Acid retardation, copper (Cu) and nickel (Ni) ion exchange, crystallisation, solid liquid separation and finally drying of the product.
The liberator feed is Pynne green solution that is a bleed stream from two sections in the tankhouse. This solution has a lower Cu concentration hence the liberators can remove almost all the copper. If the liberators overflow is not below 100 ppm Cu concentration it will be recycled back to the commercial cells in the tankhouse instead of being fed to the liberator storage tank. The process taking place in the liberators is electrowinning where copper from the electrolyte will be electroplated on copper cathodes in the presence of inert lead anodes.
The remaining copper in the electrolyte is removed using an ion exchange (IX) process. However the electrolyte from the liberators storage tank has a lot of acid hence it goes through an acid retardation process using the Scanacon unit. The recovered acid is returned back to the tankhouse while the electrolyte stripped of most of the acid goes through the Cu ion exchange process. From the copper ion exchange process the electrolyte goes through a nuetralisation process, followed by the Ni ion exchange process. The eluate from the Ni ion exchange process then goes through yet another ion exchange process to remove the iron (Fe) before it goes to the crystalliser.
In the crystalliser, evaporative crystallisation is used to form the NiSO4.6H2O crystals which will then pass through a centrifuge for solid-liquid separation process. The wet nickel sulphate crystals are then dried in a fluidised bed air drier. The final NiSO4.6H2O is discharged from the drier using a rotary valve, is then cooled and conveyed to a product bin. The product is bagged in 25kg plastic bags and then packaged per ton.