| There is a large amount of low grade refractory zinc oxide ore exiting in our country, in which the gangue is alkaline. It is very difficult to treat this kind of resource economically and efficiently by acid leaching. In order to extract zinc and avoid the alkali gangue being dissolved from this kind of zinc oxide ore, the method of ammonia leaching and alkaline leaching had been studied widely due to the correspond lixivants can complex with Zn(Ⅱ) and the results revealed that it's available to extract zinc selectively by ammonia leaching and alkaline leaching processes. However, it is to some extent restricted the development of separating Zn selectively from low grade zinc oxide ore, because of the ligand used in the above two complex-leaching processes are only NH3, Cl-and OH-. Therefore, the object of this work is to select and develop new ligands for zinc selective leaching from this low grade refractory zinc oxide ore, which will provide a new idea for the zinc hydrometallurgy process. The main contents and conclusions were drawn as follow:(1) Ligand selection for complex-leaching valuable metals in alkaline system was investigated, which indicated evaluation rules of the binding capacity of ligand (L) with metallic ion (M). Thus, a two-stage ligand selection method composed of a primary selection and a critical selection was determined. The applications of this method showed that iminodiacetate (Ida2-) can be used as a complex agent for complex-leaching smithsonite (ZnCO3). Moreover, the ligand selected (Ida2-) also had large binding capacity with Zn(Ⅱ) of willemite (Zn2SiO4), hemimorphite (Zn4Si2O7(OH)2·H2O), hydrozincite (Zn5(OH)6(CO3)2) and zincite (ZnO). More importantly, impurities such as Ca, Mg, Fe and Si which were abundant in low grade zinc oxide ore can not be dissolved in alkaline iminodiacetate aqueous solution, it's said that Ida2-had high selectivity with impurities of quasi-gangue during leaching process. Through these results, the scheme for treating low grade zinc oxide ore in alkaline iminodiacetate aqueous solution was proposed.(2) Thermodynamics of Zn2+-Ida2--CO32--H2O system with different equilibrium solid phases including ZnCO3, Zn5(OH)6(CO3)2, Zn3(OH)4CO3·H2O and ZnO were studied respectively at298K of pH value6-12. Subsequently, the equilibrium solid phase converting rules and its stable area of all the above probable solid phases in Zn2+-Ida2--C032--H2O system were determined. The results showed that ZnCO3, Zn5(OH)6(CO3)2and ZnO can exist stably as the equilibrium solid phase in a certain area in Zn2+-Ida2--CO32--H2O system, while Zn3(OH)4CO3·H2O can not. In Zn2+-Ida2--CO32--H2O system, the equilibrium solid phase can converted from ZnCO3into Zn5(OH)6(CO3)2and finally into ZnO with the increase of pH. It is inadequate to characterize the dissolution process of Zn(Ⅱ) in alkaline iminodiacetate aqueous solution by using the thermodynamics results of Zn2+-Ida2--CO32--H2O system with only a single equilibrium solid phase, for there is great difference of dissolution characteristics among the different equilibrium solid phases. Ideas for Zn2+precipitation from leaching liquor were proposed by means of the solubility of Zn(II) changes while equilibrium solid phase converted in Zn2+-Ida2--CO32--H2O system. Zn2+would be precipitated as ZnCO3when the pH of Zn2+-Ida2--CO32---H2O system was below8by adding with acid, and adding with alkali to adjust the system of pH>11, Zn2+would be precipitated as ZnO.(3) The process of leaching low grade zinc oxide ore in Ida2--H2O system was investigated. It showed that temperature and pH had great effect on Zn(Ⅱ) extraction and the dissolution of impurities. The optimum conditions for leaching low grade zinc oxide ore were determined as follows, total concentration of Ida2-0.9mol·L-1, temperature70℃, pH=8, liquid to solid ratio5:1and leaching time4h. As a result, the extractions of Zn are above76%, concentration of impurities Ca, Mg, Fe and Si are about340mg·L-1,6mg·L-1,16mg·L-1and25mg·L-1respectively in leaching liquor. It's certained that the Ida2--H2O system had high selectivity with impurities of quasi-gangue.(4) The process of "Zn2+precipitation-Ca2+precipitation-releaching" was adopted to treat the leaching liquor with low concentration of Zn2+, in which CaO was directly added to adjust the alkalinity and make Zn2+be precipitated as ZnO for recovery. Then the acidity of the residual solution was adjusted by CO2to remove the accumulated Ca2+as CaCO3, and the lixiviant can be regenerated simultaneously for recycling use. It showed that the recovery of Zn was influenced by the factors of temperature, the final pH value and aging time of precipitation, while the precipitation of Ca was affected by temperature. The optimal conditions for Zn2+precipitation were determined as follows:temperature of85℃, final pH of10and aging time of60min. And the optimal conditions for Ca2+precipitation were as follows:temperature of70℃, final pH of8and flow rate of CO2gas of0.2L/min. The results show that when leaching liquor is treated under above optimal conditions for5times cycle operation, the precipitations of and Ca are over90%and93%respectively, and the extractions of Zn by releaching with regenerated lixiviant are above75%. ZnO obtained from Zn2+precipitation process contains60.07%Zn and3.84%Pb (average value), while CaCO3obtained from Ca2+precipitation process has purity more than97%.(5) The pH potentiometry was used to determine the protonation constants of Ida2-and stability constants of Ida2" with Zn2+at different temperature (25℃,40℃,55℃,70℃), and the effect of temperature on Zn2+-Ida2--H2O complex medium was investigated subsequently. The results show that the protonation of Ida2-with H+and the complexation of Ida2-with Zn2+become weaker and weaker as the temperature increase. According to the thermodynamic data determined, the effect of temperature on dissolution and precipitation processes of Zn(Ⅱ) in Zn2+-Ida2--CO32--H2O systems were analyzed by theoretical calculation. The results show that the solubility of Zn(Ⅱ) in Zn2+-Ida2--CO32--H2O system declines with the increase of temperature, and the graphs of the solubility of Zn(Ⅱ) against pH on which the solubility of Zn(Ⅱ) curves move ruly with the increase of temperature to the direction of the decrease of pH value. The reason for causing these two phenomena is both due to the effect of temperature on the protonation of Ida2-with and the complexation of Ida2-with Zn2+...
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