| Indium is a kind of rare metal with unique physical and chemical properties, and it palys an important role in the national economy. With the surging demand for indium in market and the overexploitation of indium mine, rich indium resources are drying up gradually, and the utilization of poor indium resources is becoming particularly important. Indium is commonly found in zinc, tin and other non-ferrous ores. At present, most zinc refining plants operate a roasting-leaching-electrowinning process. In the course of roasting, Fe and Zn in zinc concentrate ore will inevitably produce zinc ferrite (ZnFe2O4). Meanwhile, indium will enter into zinc ferrite lattice by replacement or gap filling to produce indium-bearing zinc ferrite (IBZF). Indium-bearing zinc ferrite like zinc ferrite belongs to the spinel compounds, which has perfect octahedral structure. Hence, indium-bearing zinc ferrite has has excellent chemical stability, and it is insoluble in common acid solution. Hence, Indium-bearing zinc ferrite is a kind of refractory indium-bearing solid. Zinc leach residues, produced from indium-bearing zinc ores in zinc hydrometallurgy, always contain indium-bearing zinc ferrite and belong to the poor indium resource which is up for the utilization of indium.Microwave has some unique features such as selective heating and volume heating, and it can create internal friction heat in the interior of heated materials by the high speed rotation of material molecular. The internal friction heat can activate the heated moleculars in materials. Thus, microwave has important applications in mineral processing. In hydrometallurgy fields, the microwave applications in mineral leaching technology is a promising new technology to treat refractory or low grade ores. In order to leach indium and zinc from zinc leach residues containing indium-bearing zinc ferrite, and to avoid the disadvantages of the traditional leaching method, the microwave was chosen to intensify the leaching process for zinc leach residues.Artificially synthesized indium-bearing zinc ferrite was used as the research material in leaching experiments. The intensifying mechanism for leaching of indium and zinc by microwave was studied. Meanwhile, the kinetic model of indium and zinc leaching was explored. And on this basis, the microwave intensification of leaching indium and zinc from zinc leach residues was studied, and the kinetics for steady and unsteady state leaching were also studied. The optimum technology conditions of indium and zinc leaching from zinc leach residues under microwave heating was explored. The research contents and conclusions were as follows:(1) The temperature increasing behavior of sulfuric acid solution and zinc leach residues under microwave heating was studied. The temperature increasing behavior of sulfuric had no phenomenon of out of control of temperature. As the microwave power increased, the temperature increasing rate was increased. With the time going, the temperature of sulfuric acid tended to a constant value, and the bigger the microwave power was, the shorter the value of the heating time for sulfuric acid would be. In addition, as the sulfuric acid concentration increased, the temperature increasing rate of sulfuric acid decreased. As the microwave power increased, the temperature increasing rate of zinc leach residues was also increased. With the time going, the temperature of zinc leach residues tended to a constant value. The bigger the microwave power was, the bigger the the temperature constant value of zinc leach residues would be. The temperature increasing behavior of zinc leach residues had no phenomenon of out of control of temperature. As the material quantity of zinc leach residues increased, the temperature was firstly increased and then decreased. The maximum of temperature existed at a certain value of material quantity.(2) At constant temperature, the leaching of indium and zinc from indium-bearing zinc ferrite under microwave heating was studied. The feasibility of indium and zinc leaching from indium-bearing zinc ferrite at constant temperature under microwave heating was discussed. The dynamics of indium and zinc leaching from indium-bearing zinc ferrite at constant temperature under microwave heating were studied, and then the dynamics were compared with those at the same temperature under conventional heating. The indium-bearing zinc ferrite samples were teseted by specific surface area analyzer, XRD and scanning electron microscopy (SEM) to analyze the specific surface area, the crystalline structure and the particle morphology, respectively. The intensifying mechanism of the leaching of indium-bearing zinc ferrite under microwave heating was explored. It was showed that the leaching of indium-bearing zinc ferrite under microwave heating was obviously superior to that under conventional heating. The leaching of indium-bearing zinc ferrite by microwave intensification was feasible. When the stirring speed was over400r/min, the effect of external diffusion was eliminated completely. The apparent particle granularity has little effect on the indium and zinc leaching. The leaching efficiences of indium and zinc were both increased with the increase of temperature and sulfuric acid concentration under microwave heating, while the effect of temperature and sulfuric acid concentration on leaching reaction weaken when the temperature and the sulfuric acid concentration increased certain values. Unreacted shrinking core model controlled by surface chemical reaction could primely fit the leaching data of indium and zinc. The activation energy of indium leaching under microwave heating (Ea,In,MH,77.374kJ/mol) was bigger than that under conventional heating (Ea,In,CH,53.555kJ/mol). The value of AK0,In,MH under microwave heating was much bigger than AK0,In,cH under conventional heating. The ratio of frequency factor A0,In,MH/K0,In,cH was reached up to10818.36. Similarly, the activation energy of zinc leaching under microwave heating (Ea,Zn,MH,73.747kJ/mol) was bigger than that under conventional heating (Ea,Zn,CH,57.467kJ/mol), while the value of AK0,Zn,MH under microwave heating was much bigger than AK0,Zn,CH under conventional heating. The Ratio of frequency factor K0,Zn,MH/K0,Zn,cH was600.00. In a word, the drastic increase of the frequency factor K0,MH under microwave heating was the most important factor to intensify the leaching of indium and zinc under microwave heating. The increase of effective collision between H2SO4and ZnFe2O4, ZnFe2-mInmO4in microwave field was the results of non-thermal effect of microwave. The selective leaching of indium from indium-bearing zinc ferrite can be achieved by microwave. (3) At constant temperature, the leaching of indium and zinc from zinc leach residues, which contain indium-bearing zinc ferrite, under microwave heating was studied. The feasibility of indium and zinc leaching from zinc leach residues at constant temperature under microwave heating was discussed. The dynamics of indium and zinc leaching from zinc leach residues under microwave heating were studied, and then the dynamics were compared with those at the same temperature under conventional heating. It was showed that the leaching of indium and zinc from zinc leach residues under microwave heating was obviously superior to that under conventional heating. The leaching of zinc leaching residues by microwave intensification was feasible. When the stirring speed was over500r/min, the effect of external diffusion was eliminated completely. The leaching efficiences of indium and zinc from zinc leaching residues were both increased with the increase of temperature and sulfuric acid concentration under microwave heating, while the effect of temperature and sulfuric acid concentration on leaching reaction weaken when the temperature and the sulfuric acid concentration increased certain values. Unreacted shrinking core model controlled by surface chemical reaction could primely fit the leaching data of indium and zinc from zinc leaching residues. The activation energy of indium leaching under microwave heating (Ea,In,MH,63.532kJ/mol) was bigger than that under conventional heating (Ea,In,CH,54.536kJ/mol), while the value of AK0,In,MH(1.57x107) under microwave heating was much bigger than the value of AK0,In,CH (5.33×105) under conventional heating. Similarly, The activation energy of zinc leaching under microwave heating (Ea,Zn,MH,59.249kJ/mol) was bigger than that under conventional heating (Ea,Zn,CH,52.666kJ/mol),while the value of AK0,Zn,MH (3.36×106)under microwave heating was bigger than the value of AK0,Zn,CH (2.74×105) under conventional heating. The increase of effective collision between H2SO4and ZnFe2O4, ZnFe2-mInmO4in microwave field was the most important factor to intensify the leaching of indium and zinc from zinc leaching residues.(4) With constant microwave power, the leaching of indium and zinc from zinc leach residues, which contain indium-bearing zinc ferrite, under microwave heating was studied. The leaching behaviours of indium and zinc from zinc leach residues under microwave heating with constant microwave power were summarized, and the unsteady state dynamics of indium and zinc leaching under microwave heating with constant microwave power were also studied. As the increase of the concentration of sulfuric acid, both the leaching efficiencies of indium and zinc were increased. When the concentration of sulfuric acid was more than1.5mol/L, the leaching efficiencies of indium and zinc increased quite slowly with the increase of the concentration of sulfuric acid. When the stirring speed was over400r/min, the liquid membrane resistance was eliminated completely. As the increase of microwave power and leaching time, the temperature was increased. The relation of temperature to leaching time was non-linear. The change of microwave power had an influence on the activation energy and the frequency factor for the leaching of indium and zinc from zinc leaching residues. The relation between the microwave power and the activation energy was non-linear, and the relation between the microwave power and the frequency factor was also non-linear. With the increase of the microwave power, both the activation energy and the frequency factor firstly decreased, and then increased.(5) Leaching of indium and zinc form zinc leach residues, which contain indium-bearing zinc ferrite, was studied using different leaching processes. The processes were as follows:conventional acid leaching (conventional heating), microwave pretreatment (heating zinc leach residues with microwave) plus conventional acid leaching, microwave acid leaching (microwave heating), and microwave pretreatment plus microwave acid leaching. The optimal leaching process was chosen in view of the leaching of zinc leach residues, and the leaching conditions of the optimal leaching process were optimized. The results showed that in the four leaching processes, the process of microwave acid leaching was most effective for the indium leaching efficiency. Compared with the process of conventional acid leaching, the process of microwave acid leaching made the influence of leaching temperature and sulfuric acid concentration on leaching weaken. Moreover, the process of microwave acid leaching shortened the leaching time, and intensified the leaching of indium and zinc from zinc leach residues. Considering the unique advantage of microwave radiation and the component characteristics of the zinc leach residues, the optimum conditions for leaching of zinc leach residues by microwave acid leaching under microwave heating were as follows:stirring speed550r/min, initial concentration of sulfuric acid1.5mol/L, ratio of liquid to solid10, leaching temperature75℃, and leaching time90min. Under the optimum conditions of microwave acid leaching, the leaching efficiencies of indium and zinc which were only57.1%and54.2%in process of conventional acid leaching increased to77.0%and74.2%, respectively.Microwave heating had significant effect both on the leaching behaviours and the leaching dynamics for indium-bearing zinc ferrite and zinc leach residues containing indium-bearing zinc ferrite. Due to its unique heating properties, microwave greatly intensified the leaching of indium and zinc. The leaching of indium and zinc from the refractory material of indium-bearing zinc ferrite was efficiently finished under microwave heating. |
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