Study On The Leaching Of Gallium And Germanium From Zinc Powder Replacement Residues

Abstract:Zinc sulphide concentrate produced from a lead&zinc mine in China contains high contents of gallium and germanium, and was treated desigedly by high-acid oxygen pressure leaching process, resulting in gallium and germanium dissolving into leaching solution. Then gallium and germanium were replaced from the leachate with zinc powders, and enrichied into so called zinc powder replacement residues, of which the respective content of gallinm and germanium reaches0.2%-0.3%.The zinc powder replacement residues were leached by sulphic acid solution in order to recover gallium, germanium and other valuable metals. Nervetheless, the leaching efficiencies of gallium and germanium were unusually low owing to two reasons:firstly, the forms of gallium and germanium in the residures were very complicated, existing as elementary substances, oxides, sulfides, and oxysalt; secondly, the residues also contain high contents of other ingredients, such as Zn, Cu, Si, Fe, and Pb, et al., which may impede the leaching of gallium and germanium in varing ways.In this dissertation, the leaching of gallium and germanium from the zinc powder replacement residues was investigated theorically and experimentally, aiming to clarify the reasons of difficult to leaching of gallium and germanium in detail, and improve the leaching process together with the conditions, thereby, achive the effective leaching of gallium and germanium from the residues.In this dissertation, the literatures on the leaching of the materials containing gallium and germanium were reviewed; the zinc powder replacement residues obtained from an oxygen pressure leaching plant in China were characterized comprehensively; the E-pH diagrams of Ga-H2O system, Ge-H2O system, Zn-H2O system, Cu-H2O system, and Fe-H2O system, the lg[Me]T-pH diagram of MexOy-H2O system, and the component-pH diagrams of Ga2O3-H2O system, and GeO2-H2O system, were calculated and plotted. Based on a large number of exploratory experiments, normal and high pressure acid leaching processes were studied in detail, respectively; the main conclusions obtained are as follows.(1) The chemical phase analysis results show that gallium and germanium in the zinc powder replacement residues are mainly in the forms of Ga2O3and MeO.GeO2, with a small amount as elementary substances and sulfides present; in EDS analyzes, independent phases of gallium and germanium were not found, indicating that most of them exist dispersively in the phases containing such elements as Fe, Si and so on; the thermodynamical analyzes demonstrate that the leaching in acid solutions is preferable, and adding moderate oxidant accelerate the leaching; the leachabilities of metallic oxides of the residues in acid solutions, in the order from easy to hard, are as follows:ZnO, FeO, CuO, Ga2O3, Fe2O3, GeO2;gallium and germanium in highly acidic solutions are in the forms of Ga3+and H2GeO3, respectively.(2) The study on the acid leaching at normal pressure showed that by adding moderate NaNO3as leach-assist agent, the elementary substances and sulfides of Gallium and germanium in the residues are oxidized into dissolvable oxides, thus benefiting the leaching and resulting in that the leaching efficiencies of gallium and germanium reach99.56%and76.42%respectively. However, when the NaNO3concentrate exceedes52.29g/L, Fe2+ions in the solution would be oxidized and then hydrolyzed to precipitate as ferrihydrite and therefore impede the leaching. In the leaching, adding surfactant sodium dodecyl sulfate (SDS) not only accelerates the leaching, but also improves the filterability of the leaching slurries. The effective mechanism of SDS is to promote silica gel in the leachate coagulation, thus reducing its adsorption to Ga and Ge ions and improving the slurry filteration. The optimum conditions obtained are as follows:temperature90℃, liquid to solid mass ratio10, stirring speed300rpm, leaching time4h, initial sulfuric acid concentration156g/L, and SDS concentration10g/L. Under the optimum conditions, the leaching efficiencies of Ga and Ge reach98.56%and84.37, respectively; the filteration rate of the slurry reaches28.67mL/min, indicating good filterability. (3) The study on the acid leaching under high pressure showed that increasing sulfuric acid concentration up to156g/L benefits the leaching; but hereafter, the leaching rate of germanium would decrease. Increasing leaching time and temperature both has little influences on the leaching efficiencies of gallium and germanium, but improves the filterabilities of the leaching slurries obviously. Adding cadmium nitrate or sodium nitrate promotes the leaching of gallium and germanium, moreover, adding cadmium nitrate also improves the filterability of the leaching slurry sharply. Under the optiumal conditions of temperature150℃, initial sulfuric acid concentration156g/L, liquid to solid mass ratio8, stirring speed300rpm, and leaching time4h, the leaching efficiencies of Ga and Ge reach98.50%and94.85%respectively with the addition of60g/L cadmium nitrate, as well as98.24%and96.45%respectively with the addition of25g/L sodium nitrate. Although the addition of cadmium nitrate improves the filterability of the leaching slurry sharply, the amount of leaching residues will increase twice inevitably owing to the formation of cadmium sulfate in the leaching. So in overall consideration, adding sodium nitrate in the high pressure acid leaching is more preferable.