Developing a solvent extraction process for the separation of cobalt and iron from nickel sulphate solutions

[摘要] ENGLISH ABSTRACT: Crude NiSO4 solutions are often produced as a product of Sherrit based matte leach processes leading to iron and cobalt contaminated solutions of NiSO4. To upgrade the quality of these solutions for either, the production of NiSO4 crystals or cathode/precipitated nickel, the iron and cobalt must be removed.Conventional processes use either pure or saponified Cyanex 272 in solvent extraction to extract iron and cobalt from pregnant nickel leach solutions. These processes require the addition of an alkali like NaOH to neutralise the protons being exchanged for the different metal species since extraction is a strong function of pH. Hence, while removing iron and cobalt from the solution, sodium is added instead. This limitation can be dealt with by pre-loading of Cyanex 272 with a portion of the purified nickel product prior to impurity extraction. During the extraction stage nickel is then exchanged for the impurities instead of hydrogen and no NaOH addition is necessary, resulting in a pure nickel product. The extracted metals are recovered from the organic phase by stripping it with H2SO4 and the organic phase is recycled.A solvent extraction process that purifies an 80 g/l nickel sulfate solution from 1 g/l cobalt and 3 g/l iron was developed. The key variables include pH, organic/aqueous (O/A) mixing ratio, nickel loading, H2SO4 concentration, Cyanex 272 concentration, various organic diluents and temperature. These variables were investigated via preliminary and three 24 factorial design batch experiments which reduced the variables to pH/NaOH addition and O/A ratio for the pre-loading section, nickel loading and O/A ratio for the extraction section and H2SO4 concentration and A/O ratio for the stripping section. The reduced variables were considered in further batch experiments from which the data was used to develop models and design a simulation sheet in Microsoft Excel of the extraction circuit. These final batch tests revealed the conditions that resulted in the purest aqueous nickel product after extraction, where nickel and cobalt can selectively be stripped from iron and what H2SO4 concentration and A/O ratio is necessary to finally strip iron and regenerate the organic phase. Ultimately four tests were conducted on a mixer-settler setup to test the validity of the simulation sheet.The proposed process conditions for the solvent extraction circuit are to use a 20V% Cyanex 272 solution in Shellsol D70 as diluent that are pre-loaded with an 80 g/l purified nickel product solution at an O/A ratio of 25 and a pH of 7 generating a nickel loading of 3.2 g/l for the extraction section. This loading at an O/A ratio of 1.5 produces a high pure nickel aqueous product (0.05 g/l Co, 0.01 g/l Fe). From the generated organic phase co-extracted nickel and cobalt can selectively be stripped with 0.1 M H2SO4 at an A/O ratio of 2.25 followed by an 1 M H2SO4 solution at an A/O ratio of 0.75 to strip the remaining iron. The temperature at all the stages should be controlled between 40-50°C to ensure phase separation.

[发布机构] Stellenbosch University