| Consumption of zinc products has been increased each year with the wide application of zinc. As zinc sulphide is in a short supply, exploitation and utilization of zinc oxide ore have attracted more attention. There are some shortcomings in both hydrometallurgical and pyrometallurgical process when zinc oxide ore with clay and high content of silica is used as the starting material. The extraction process of zinc oxide ore was studied thermodynamically and dynamically by the vacuum carbothermic reduction with classification condensation in order to increase the energy efficiency, improve the recovery of metallic zinc, shorten the process flow, attain metal zinc with higher quality, meet the environmentally-friendly and resource-saving requirements. Two representative oxide ores were selected as the starting materials, one is smithsonite, the other is hemimorphite. The main contents studied and the important conclusions are as follows:(1)Thermodynamic studies indicate that there are specific advantages in vacuum carbothermic reduction. The critical temperature of zinc oxide or zinc silicate by carbothermic reduction is decreasing obviously, when the pressure of system is dropping, but that of zinc silicate is higher than that of zinc oxide. Cd can not be completely separated from Zn by vacuum distillation, the content of Cd in metal zinc will directly affact the quality of the zinc product.(2)The vacuum furnace with classification condensation was designed as experimental facility according to the characteristics of vacuum carbothermic reduction of zinc oxide ore and utilization of the waste heat for the condensation classification of Vapours. The vapour of different metallic products was condensed gradually at different position of the classification condenser with the decrease of temperature due to their different condensing temperature, and the products were therefore separated and purified. The experimental results show that the reaction temperature is decreased, the reduction time is shortened, and the quality of products are greatly improved by vacuum carbothermic reduction with classification condensation.(3) Smithsonite was processed by the vacuum carbothermic reduction with coal as the reductant. The effects of sintering time, sintering temperature on the strength of pellet samples and the quality of metallic zinc were investigated. The effects of the reduction temperature, reduction time, initial moles ratio of C/Zntotai and the pressure of system on the zinc yield and the quality of metallic zinc were also studied. It is found that the drop number of the pellets is greater when the suitable sintering temperature is from 823K to 873K and the sintering time is not shorter than 30 min. The optimal process condition for the vacuum carbothermic reduction is obtained as follows:the molar ratio of C/ZnTotal 3, the reaction temperature 1173K, the pressure of system 50?2000Pa, the reaction time around 60 min. At the optimal condition, the zinc yield is about 97%from smithsonite. Pb, As, Cd can be well separated from Zn, but the separation between Cd and Zn is not ideal enough. The content of Cd in metallic zinc could be decreased when the sintering conditions were changed. At suitable experimental conditions, high-purity zinc can be obtained with a purity of 99.995%.(4) The vacuum carbothermic reduction of hemimorphite was studied. The effects of sintering time, sintering temperature on the strength of pellet samples and on the quality of metallic zinc were investigated. The effects of reduction temperature, reduction time, initial moles ratio of C/Zntotal, the pressure of system, the mass of CaF2 additive on the zinc yield and the quality of metallic zinc were also studied. Results show that the drop number of the pellets is greater when the suitable sintering temperature is 823-873K and the sintering time is not shorter than 30 mins. CaF2 can catalyze the carbothermic reduction of zinc silicate. The optimal process condition is obtained as follows:the amount of CaF210%, the reaction temperature 1373K, the molar ratio of C/ZnTotal 2.5, the pressure of system lower than 20kPa, the reaction time about 40 min. At the optimal condition, the zinc yield is about 93%from hemimorphite. The content of impurity Cd in metallic zinc could be decreased when the sintering conditions were changed. The content of impurity Pb could be reduced as the pressure of system is raised within certain range. At suitable experimental conditions, the high-purity zinc can be abtained with a purity of 99.995%.(5) The vacuum furnace was improved to make the experiment conditions more accurately controlled for the kinetic study of zinc oxide ore by carbothermic reduction. The main designs are the sample transfer device and the collection equipment of the product to maintain a fairly constant temperature and pressure of system at the right reaction time.(6) The kinetics of the vacuum reduction of ZnO-C system was investigated. Experimental results indicate that the vacuum carbothermic reduction of zinc oxide from smithsonite is a first-order reaction. The diffusion is the rate-determining step of the overall reduction process, where the activation energy is estimated to be 177.72?-191.31 kJ-mol-1.(7) The kinetics of the vacuum reduction of Zn2SiO4-C without CaF2 at temperature 1323?1473K was investigated. Experimental results showed that the vacuum carbothermic reduction of zinc silicate from hemimorphite is a first-order reaction. The interface-chemical reaction is the rate-determining step of the overall reduction process, where the activation energy is estimated to be 246.16 kJ·mol-1. The kinetics of Zn2Si04-C with 10%CaF2 at reduction temperature 1273?1423K was also studied. It was found that the process is controlled by interface-chemical reaction, but the activation energy is decreased to 164.82kJ-mol-1. As F-ion can improve the activity of zinc silicate, the Zn-O bond is broken more easily and the activation energy is decreased. Therefore CaF2 could catalyze the carbothermic reduction of zinc silicate. |
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