(464b) Direct Solvent Extraction with Cyanex 272 As Purifying Step for Nitric Acid Leach Solution of Nickel Laterite Ores

The use of nitric acid in the leaching of nickel laterite ores by the so-called Direct Nickel process is attractive as it can be used to treat the full laterite ore profile and allows the reuse of the excess acid. It also allows the regeneration and reuse of the excess magnesia as its neutralisation agent. Clearly these are considerable distinct advantages over the conventional sulphuric acid leaching of these ores. As the use of nitric acid in leaching nickel ores is new, however, no method of separating nickel and cobalt from nitric acid solutions exists.  This paper discusses the results of the work on developing a direct solvent extraction technique to separately recover cobalt and nickel directly from nitric acid solutions after the iron and aluminium precipitation steps.

The proposed technique involves two sequential solvent extraction circuits using bis(2,4,4-trimethylpentyl) phosphinic acid (Cyanex 272) in both; the first is to  remove aluminium and zinc, and the second is to remove cobalt together with most of the remaining impurities. The bench-scale results showed that >95% aluminium and zinc removal can be achieved at optimised condition but significant amount of cobalt and manganese were also co-extracted. The cobalt and manganese, however, can be easily scrubbed out with acidified water and recycled back into the process. Aluminium and zinc can also be completely stripped from the loaded organic with only dilute sulphuric acid at pH 1. The bench-scale work showed that dilution of the first circuit raffinate, the addition of a modifier in the organic phase, or a combination of both is required in the cobalt and nickel separation circuit as, otherwise, significant precipitation occurs. At optimum conditions, devised as a result of this investigation, efficient cobalt and nickel separation was achieved with complete manganese co-extraction leaving only nickel, calcium, and magnesium in the raffinate. Subsequently, both are completely separated from their respective impurities with ease.