Separation of Nickel and Zinc from Sulphate Solution by Co-extraction and Selective Stripping Using D2EHPA

Annually 20000 t of Ni-Cd filter cake is produced in Iran zinc processing plants. For recovery of remained metals, filter cake is leached with sulphuric acid, and cadmium is cemented by zinc powder, thus it is separated from the solution easily. The remained purified solution contains about 20-25 g/L Zn, 1-1.5 g/L Ni and minor amounts of other impurities with pH 3. Separation of Ni and Zn separation is not easy due to very similar properties. The most important factor in solvent extraction is extrctant type. In alkyl phosphoric acid types, nickel transportation mechanism to organic phase is cationic exchange. In this study, the possibility of Ni and Zn separation from real and synthetic solutions by co-extraction of Ni and Zn and selective stripping of Ni by D2EHPA extractant was investigated and co-extraction of Ni and Zn results was considered. By using D2EHPA that was diluted with Kerosene in different volume ratios of 20, 30 and 40%, Zn and Ni were co-extracted in pH at 5.2, and then Ni was stripped at pH 2.8 - 3. By increasing the D2EHPA concentration from 20% to 40%, maximum Ni extraction increased from 85% to 93%. Phase ratio (A/O) and extractant concentration were optimized as 1.1 and 40% (v/v).The results showed that with D2EHPA 40%, the maximum A/O ratio for co-extraction of Zn and Ni was 1.25/1 and in higher ratios, the organic phase was destroyed. A full fractional design of the experiments was used to study the effects of variables such as pH, phase ratio and extractant concentration in the process. The optimum values of O/A and stripping pH were obtained as 3/1 and 2.8, respectively. It was found that in the optimum conditions, extraction and stripping recovery were 93% and 95%, respectively.