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Leaching Mechanism of Zinc Oxide Ores in Ammonia-ammonium Aqueous Solution

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Tutor: YangTianZu
School: Central South University
Course: Non-ferrous metallurgy
Keywords: Hemimorphite,Willemite,NH3-(NH4)2SO4-H2O system,Leaching mechanism and kinetics
Type: PhD thesis
Year:  2012
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Low grade zinc oxide ores containing high content of alkaline gangues, which are difficult to concentrate by minerals dressing, are inadequate to be treated by pyro-or acidic hydro-metallurgical process due to its high energy consumption, low recovery and large waste disposal. The combination of ammonia and ammonium salt solutions is a powerful lixiviant to treat this kind of ores. As the high selectivity, as well as the coordination of ammonia with valuable metallic ions is helpful to encourage the leaching process. In this paper, leaching mechanisms in ammoniacal solutions of zinc silicate minerals, such as hemimorphite and willemite, were investigated in NH3-(NH4)2SO4-H2O system, and the results obtained could not only fill the research gaps, but also provide theoretical instructions and basic data for the development of practical leaching processes.Comprehensively using XRD, FT-IR, XPS, SEM, EDS, TGA-DSC and ICP-AES, dissolution equilibrium, leaching mechanism and rates of pure natural hemimorphite and synthetic willemite minerals in NH3-(NH4)2SO4-H2O solution were investigated. The main conclusions were as follows:(1) The thermodynamic model of Zn2SiO4-NH3-NH4+-H2O system was established based on the laws of mass conservation, charge balance and simultaneous equilibrium principle. Then the thermodynamic diagrams regarding concentrate variations of different species in the system were plotted. The results showed that the highest [Zn]T in the system was obtained under the condition of [NH3]/[NH3]t molar ratio equal to0.5, where the higher the [NH3]T, the higher the [Zn]T. Under the conditions of [NH3]T5mol/L,[NH3]/[NH3]T molar ratio0.5, the solubility of amorphous SiO2in the system was as low as0.11g/L, and with the dissolution of zinc silicate, amorphous SiO2would soon supersaturate and precipitate from the solutions in the pH range of7.16~12.68. Owing to the limitation of low solubility of amorphous SiO2,[Zn]T under the conditions mentioned above was merely27.88g/L, which was far bellow that of ZnO in the system.(2) Hemimorphite can leach in NH3-(NH4)2SO4-H2O system. During the leaching process, the Zn-O-Zn bond broke and the Si-O double tetrahedron would detach from the hemimorphite crystal structure to form amorphous SiO2-Amorphous SiO2forms nuclei independently and would not coat the surfaces of hemimorphite.(3) Experimental results showed that the highest leaching rate of hemimorphite was obtained at [NH3]/[NH3]T molar ratio equal to0.5. Under the conditions of [NH3]/[NH3]T molar ratio0.5,[NH3]T5mol/L, S/L ratio20g/L, temperature of35℃and stirring rate350r/min, the leaching efficiency of zinc could reach as high as95%, while that is only2%of leaching hemimorphite in pure ammonia or ammonium salt systems under the same conditions. Zinc leaching efficiency of hemimorphite decreased sharply at high S/L ratio, due to [Zn]T in the system reaching saturation.(4) The leaching kinetics of hemimorphite in NH3-(NH4)2SO4-H2O system was studied under the conditions of S/L20g/L,[NH3]/[NH3]T molar ration0.5,[NH3]T of mol/L and stirring rate350r/min. The results showed that the leaching kinetics can be described by Elovich equation and the activation energy of hemimorphite of-100~+160mesh is55.42kJ/mol, where the controlling step was surface reaction in accordance with the conclusion obtained in the leaching mechanism study.(5) Experimental results showed that both crystal structure and crystallinity have significant effects on the leaching of willemite in NH3-(NH4)2SO4-H2O system. Both hemimorphite and the chemically synthesized willemite can turn to β-Zn2SiO4and a-Zn2Si04with different crystallinity after calcinations for2h at different temperatures. The leaching efficiencies of zinc for β-Zn2Si04with the crystallinity of29.88%and60.11%were86.66%and42.11%, respectively, while the leaching efficiencies of zinc for a-Zn2Si04with the crystallinity of99%and100%were10.13%and6.4%, individually, under the conditions of temperature35℃, S/L ratio50g/L,[NH3]/[NH3]T molar ratio0.5,[NH3]T5mol/L, stirring rate350r/min and leaching time120min. The leaching of β-Zn2SiO4was easier than that of a-Zn2Si04regardless of crystallinity.(6) During the leaching process of a-Zn2SiO4, zinc and silica in willemite were found to be dissolved simultaneously into the solution, which was followed by the precipitation of amorphous silica. The solubility of silica in the NH3-(NH4)2SO4-H2O system was very low with a saturated concentration of0.3g/L. In addition, the precipitation rate of dissolved silica in the leaching solution was found to be significantly slow, which lead to the difficulty of willemite leaching in the NH3-(NH4)2SO4-H2O system, especially at a low liquid/solid ratio.(7) The leaching kinetics of a-Zn2Si04of-100+160mesh with complete crystallinity in NH3-(NH4)2SO4-H2O system was studied under the conditions of S/L ratio5g/L,[NH3]/[NH3]T0.5,[NH3]T5mol/L and stirring rate350r/min. The results showed that the leaching kinetics can be described by porous particle model and the activation energy is67.93kJ/mol, which shows the controlling step is pore diffusion, and this agreed with the experimental phenomenon.
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