DESIGN OF BATTERY GRADE PURITY NICKEL AND COBALT SULPHATE CRYSTALLISATION PLANTS

摘要

The safety and performance of lithium ion batteries is greatly affected by impurities in the chemical precursors. It is imperative that these chemicals are produced to the highest purity, exceeding standards which were previously deemed acceptable. Novel flowsheets are being developed incorporating purification steps (solvent extraction & ion exchange) to produce liquors which are sufficiently pure that the crystals which may be recovered from them will meet the current stringent specifications. Historically crystallisation has at times been regarded as a unit operation which recovers a product of a purity which matches the feed solution. This assumption overlooks the potential of the crystallisation process to selectively recover a species with minimal entrainment of impure mother liquor. Consequently, the importance of crystallisation as a final purification step in producing highly pure crystals has often been understated. A properly designed crystalliser producing large, well-formed crystals enables less entrainment of liquor on the crystal surface and allows for improved washing to enable the achievement of the highest purities possible. The lower levels of moisture entering the dryer that are achieved significantly reduces the overall energy requirements of the plant. A further technical innovation is the incorporation of mechanical vapour recompression (MVR) technology into nickel sulphate crystallisers, a manifestation of the wholesale improvements in the design and development of the turbo fan. This halves the energy input per ton of nickel sulphate produced.This paper describes novel crystalliser design features for the production of battery grade nickel sulphate hexahydrate and cobalt sulphate heptahydrate products at minimum energy consumption. It will compare the operating conditions, crystalliser types, product dryer types, preferred energy source and the control of product purity.