Beneficiation of a Nigerian sphalerite mineral: Solvent extraction of zinc by Cyanex®272 in hydrochloric acid

Abstract

The extraction of Zn(II) from aqueous chloride sphalerite leached liquor using bis(2,4,4-trimethylpentyl)phosphinic acid or Cyanex ®272 in kerosene has been studied. The results of fundamental studies on solvent extraction of synthetic solutions of Zn(II) showed that extraction of metal ions increased with increasing pH, extractant concentration and temperature. The stoichiometry of the extracted metal species by Cyanex ®272 with Zn(II) was 1:1. The apparent standard molar enthalpy (∆H°), molar entropy(∆S°) and Gibb's free energy(∆G°) of 26.81 ± 0.11 kJ/mol, 107.63 ± 0.05 J mol –1 K –1 mol –1 and − 5.48 ± 0.13 kJ/mol were calculated for the process respectively. These values showed that Zn(II) extraction by Cyanex ®272 is thermodynamically favourable. The number of the theoretical stages for this process evaluated by the McCabe-Thiele diagram was six. An extraction efficiency of 95% Zn(II) was obtained with 0.047 mol/L Cyanex ®272 in kerosene from an initial sphalerite leach liquor containing mainly 603.4 mg/L Zn, 121.4 mg/L Fe and 16.3 mg/L Pb. The Pb(II), Ag(I), Cu, Sn, and Al (less than 5 mg/L) were firstly separated by cementation with Zn granules and this was followed by Iron removal by precipitating with 4 mol/L ammoniacal solution to a pH of 3.5 at 25 °C ± 2 °C. A 0.1 mol/L HCl was found to be adequate for the stripping of about 95% of Zn from the organic phase. The stripped Zn(II) solution was recovered as zinc oxide (ZnO) via precipitation with sodium hydroxide followed by calcination at 600 °C during 120 min. A practicable hydrometallurgical scheme summarising the operational procedures used for the extraction of Zn(II) and Pb(II) from the sphalerite ore was presented.