Recovery Of Gallium And Germanium From Zinc Residue

In the hydrometallurgical zinc process to treat zinc concentrate, most of gallium and germanium come into the leach residue. In the leaching residue, a significant part of zinc remains in the form of zinc ferrite (ZnO·Fe2O3), most of gallium and germanium present in the form of isomorphism in zinc ferrite. In order to recover gallium and germanium in zinc residue produced during zinc hydrometallurgical process effectively, the process of reduction leaching-lime precipitation-extraction was presented. This process has many advantages including adaption to chemical composition change of the residue, short process, simpley operation, high gallium and germanium recovery ratio and stable product quality.In this paper, a systematic experimental study on the recovery of gallium and germanium from zinc residue was carried out and coclusions are drawn as follows:(1)X-ray diffraction analysis of the zinc residue identified zinc ferrite (ZnFe2O4), gypsum (CaSO4·2H2O), hemihydrate gypsum (CaSO4·0.5H2O), anglesite (PbSO4), sphalerite ((Zn,Fe) S) and quartz (SiO2) as the main mineral components. The independent gallium and germanium compounds were not found.(2) To recover the zinc, gallium and germanium, iron must be into the solution. There are two main ways to make iron into solution. One is hot acid leaching which needs high temperature and high acid concentration, the other is reductive leaching using sulphur dioxide as reductant in sulfuric acid solution. In the reductive leaching, the majority of the iron (Ⅲ) present is converted to the iron (Ⅱ) state. More than95%of the iron is reduced by sulphur dioxide into the solution.(3) Extraction of gallium and germanium from zinc residue was carried out by hot-acid leaching and sulphur dioxide reductive leaching. Hot-acid leaching results showed that the optimal leaching conditions were as follows:sulfuric acid concentration153g/L, leaching temperature95℃, leaching time180min, liquid-to-solid ratio6mL/g, and agitation speed400r/min. Under these conditions, the leaching rates of zinc, iron, gallium and germanium were88%,93%,88%,68%, respectively. In the presence of SO2, the extraction of gallium was maximum of90%while the extraction of zinc, iron and germanium were90%,96%and70%under the optimal condition which was determined as sulfuric acid concentration of56g/L, sulphur dioxide partial pressure of200kPa, liquid-to-solid ratio of7mL/g, agitation speed of400r/min, leaching temperature of100℃for120min. comparing the leaching rate, sulphur dioxide reductive leaching is better than hot-acid leaching.(4) The main minerals of the reductive leaching residue were sodium iron sulfate tetrahydrate, gypsum, hemihydrate gypsum, anglesite, sphalerite and quartz. Compared with zinc leaching residue, the surface of reductive leaching residue changed obviously. A fine acicular crystal was produced. It is sodium iron sulfate tetrahydrate. Zinc ferrite was almost entirely leached.(5) Potential-pH diagrams of ZnO-Fe2O3-H2O and Fe(Ⅲ)/Fe(Ⅱ)/SO2/SO42-system were drawn, which provides thermodynamical basis for the hot leaching and sulphur dioxide reductive leaching of zinc residue. The kinetics on the hot leaching of zinc residue with high concentration acid was investigated. The results showed the leaehing reactions followed the shrinking core model. The apparent activation energy was42.16kJ/mol. The leaching rate was controlled by the chemical reaction. The following expressions were the kinetic equation of leaching reaction:1-(1-x)1/3=0.12[H+]0.94[Fe3+]0.09d0-1e-42160/RTt+A(6) Gallium ion and germanium ion in sulphur dioxide reductive leaching were enriched by lime precipitation. After lime precipitating and sulfuric acid leaching, the concentration of gallium was0.58g/L and the concentration of germanium was0.28g/L. The concentration of gallium and germanium could be enriched ten times above.(7) After precipitating gallium and germanium, the solution contained zinc and iron. The iron must be removed before the solution returned to the zinc system. The advantages and disadvantages of the three iron removal methods, that are goethite method, hematite method and sodium jarosite method, were compared. The goethite method was recommended to remove iron.(8) The sulfuric acid concentration of the solution with rich gallium ion and germanium ion was controled of40g/L or so. The extraction of germanium was more than99%and the stripping rate was more than99%under the optimal condition which was determined as9.5%G315and5%isooctyl alcohol system, three stage extraction, two level stripping iron, back extraction germanium with250g/L NaOH aqueous solution in two-stage cross-flows. Gallium and other impurities such as lead, zinc were not be extracted basically. Part of iron was extracted together with germanium. The iron concentration of the back extraction liquid could be effectively controled by stripping iron from loaded organic phase.(9) The sulfuric acid concentration of germanium raffinate was controled of from5g/L to10g/L. The extraction of gallium was more than96%and the stripping rate was more than97%under the optimal condition which was determined as10%G315-5%P204and5%isooctyl alcohol system, four stage extraction, three level stripping iron, three level back extraction. Other impurities such as lead, zinc were not be extracted basically. Part of iron was extracted together with gallium. The iron concentration of the back extraction liquid could be effectively controled by stripping iron from loaded organic phase.