Recovery of Secondary Materials from Waste Sludges in Zinc Extraction Plant

Abstract

Leaching of valuable metals from the filtercake generating from a cold-purification stage in zinc electrowinning plant by aqueous sulphuric acid solution was studied. Chemical and mineralogical composition of the as-receive filtercake was determined by inductively couple plasma – optical emission spectroscopy (ICP-OES), X-rays diffractometry (XRD) and scanning electron microscopy – energy dispersive X-rays spectrometry (SEM-EDS). Parameters including leaching temperature, leaching time, solid-to-liquid ratio and stirring speed have been investigated in leaching experiment and ICP-OES was used to determine the metal content in the leachate. Dissolution kinetics was studied using 1 M aqueous sulphuric acid solution, the temperature range of 40-80 ºC, and the time up to 360 min and it was found to be controlled by physicochemical processes. The optimum collective leaching temperature is 80 ºC for Cd, Co, Fe, Ga, Ni and Zn, and 40 ºC for the case of In. For selective leaching of Cd, Ga, In and Zn from Co, Fe and Ni at T = 40 ºC, the optimum condition is 90 min. The solid-to-liquid ratio higher than 1/4 g/ml is ineffective and the solid-to-liquid ratio of 1/12 g/ml is chosen as the optimum. Increasing the stirring speed over 700 rpm had little effects on metal dissolution, hence the stirring speed of 700 rpm was considered as the optimum. Cu and Pb were hardly leached out by sulphuric acid solution. Finally, semi-empirical expressions for dissolution of Cd, Co, Fe, Ga, In, Ni and Zn have been given based on the shrinking sphere or shrinking-core model. A synergistic solvent extraction (SSX) system consisting of LIX 63 and Ionquest 801 was identified and developed as a novel SSX system to recover germanium from a synthetic leach solution of zinc residues. The SSX system consisting of 0.13 M anti-oxime from LIX 63 and 0.13 M Ionquest 801 in ShellSol 2046 is an optimised composition for the extraction of germanium from the synthetic leach solution containing 1 g/L Ge(IV) and 1.0 M H2SO4. The germanium(IV) extraction kinetics was slow with the equilibrium reached after 20 min of mixing at 40 °C. The stripping kinetics of germanium(IV) with strip solution containing 0.5 M NaOH and 1.0 M Na2SO4 was very fast with the equilibrium reached in 1 min of mixing. Slope analysis showed that, most possibly, the extracted Ge(IV)-organic species is 2Ge(SO4)2•(HA)•(HB)2•H2SO4 via a solvating mechanism with the SSX system consisting of Ionquest 801 (HA) and LIX 63 (HB). A novel synergistic solvent extraction (SSX) system consisting of commercially available LIX 63 and Versatic 10 was selected to recover both gallium(III) and indium(III) from synthetic leach solutions of zinc residues in one circuit. With the optimum composition of the SSX system, 86% In(III) and 92% Ga(III) were extracted in a single contact at pH 3.0. The extraction and stripping kinetics of indium(III) and gallium(III) was fast at 40 °C. Indium(III) and gallium(III) were stripped using solution with low acidity of 0.05 M H2SO4. Based on the McCabe-Thiele diagrams constructed, 4 stages of extraction at an A/O ratio of 1.2 and 3 stages of stripping at an O/A ratio of 5 are required for the extraction and stripping of both indium(III) and gallium(III). The synergistic effect of LIX 63 and Versatic 10 on indium(III) extraction was significant but that on gallium(III) extraction was insignificant. Slope analysis showed that, most possibly, the extracted metal-organic species are Ga(HA2)•B2 and In(HA2)2B via a cation exchange mechanism with the SSX system consisting of Versatic 10 (HA) and LIX 63 (HB). Finally, a conceptual flowsheet is proposed including the novel SSX system which consisting of all commercial available reagents and offers the advantages of recovering both indium(III) and gallium(III) in one circuit and stripping with low acidity solutions.

82% gallium was recovered from the synthetic stripping solution containing 5 g/L Ga by precipitation using Na2CO3 at pH 2.6 and temperature of 25 °C for 24 h. The precipitated gallium products include Ga2O3 and Ga2(SO4)3. 73% indium was recovered from synthetic stripping solution containing 15 g/L In by cementation on pure zinc plate at pH 3 and temperature of 65 °C for 6 h. The cemented indium products include In2O3 and In2(SO4)3.