Investigations On The Mechanism Of Indium And Zinc Leaching From Indium-Bearing Zinc Ferrite Enhanced By Mechanical Activation

Indium is a representative rare scattered metal with many unique physicochemical properties. Indium is produced mainly from zinc concentrate with the presence of iron. After zinc calcine is leached using neutral solution and acid solution, about20%of zinc remains in the leaching residue, and exists in the form of zinc ferrite (ZnFe2O4). The indium enters the crystal lattice of ZnFe2O4through substituting cation ion to form indium-bearing zinc ferrite. Because zinc ferrite is very stable, it is difficult to leach indium and zinc from zinc ferrite under normal conditions. In order to improve the leaching efficiency of indium and zinc, mechanical activation was used to pretreat indium-bearing zinc ferrite, and the mechanism of indium and zinc leaching from indium-bearing zinc ferrite enhanced by mechanical activation was studied.In this paper, the effect of mechanical activation conditions (rotation speed, activation time, media filling, ball to material ratio and species of media) on the physicochemical properties and leaching behavior of synthetic indium-bearing zinc ferrite activated by tumbling mill and planetary mill were studied by means of X-ray diffraction (XRD) analysis, particle size analysis, BET specific surface area analysis, scanning electron microscopy (SEM) analysis, Fourier transform infrared spectra (FT-IR) analysis, Mossbauer spectrometry analysis, and leaching tests. The results showed that mechanical activation by tumbing mill and planetary mill led to the decrease of particle size, the increase of specific surface area and lattice defect, and the inversion between Zn2+and Fe3+. Mechanical activation improved efficiently the leaching of indium-bearing zinc ferrite in sulfuric acid solutions.For indium-bearing zinc ferrite activated by tumbling mill, the improvement of physicochemical properties and leaching behavior became more obvious with the increase of rotation speed, then was not obvious due to the critical rotation speed. The effect of activation time on particle size mainly ocurred in the initial30min of tumbing mill activtion. Prolonging activation time, the damages of crystal structure aggravated and the leaching efficiencies were improved. When the media filling was30%-50%, the changes of physicochemical properties and leaching behavior were very similar. When the media filling was bigger than50%, the improvement of physicochemical properties and leaching behavior decreased with the increase of media filling. The improvement of physicochemical properties and leaching behavior became more significient with the increase of ball to material ratio. The effect of species of media on physicochemical properties and leaching behavior was determined as zirconium oxide> stainless steel> corundum.For indium-bearing zinc ferrite activated by planetary mill, the changes of physicochemical properties and leaching behavior became more notable with the increase of rotation speed, activation time and ball to material ratio, and the decrease of media filling. The effect of species of media on physicochemical properties and leaching behavior was determined as stainless steel> zirconium oxide> corundum.Kinetic study showed that mechanical activation decreased the dependency of leaching process of indium-bearing zinc ferrite on reaction temperature and H2SO4concentration. The apparent activation energies of indium leaching from the unmilled, activated for30min by tumbling mill and activated for15min by planetary mill indium-bearing zinc ferrite were determined as78.1kJ/mol,73.5kJ/mol and49.1kJ/mol, respectively; the apparent activation energies of zinc leaching were73.9kJ/mol,70.3kJ/mol and57.2kJ/mol, respectively. The apparent reaction orders of indium leaching were0.82,0.69and0.57, respectively; the apparent reaction orders of zinc leaching were0.72,0.70and0.61, respectively. The determined kinetics models can describe the leaching process of indium-bearing zinc ferrite well.Base on the above study, the deactivation conditions and deactivation mechanism of indium-bearing zinc ferrite activated for15min by planetary mill were investigated using the method of annealing treatment. The results showed that there were two obvious deactivation phenomenon when the activated sample was treated under different anealing temperatures. The fist one occurred in anealing treatment at450℃for4h. In this deactivation stage, the decrease of activity was mainly caused by particles agglomeration. Comparing the leaching data of annealed sample to those of unilled and activated samples, it can be concluded that the decrease of particle size was not the most important cause in the improvement of leaching process of indium-bearing zinc ferrite. The effect of the decrease of particle size was less than25%. The second one occurred in anealing treatment at1000℃for4h. In this stage, the decrease of activity was mainly caused by lattice defect restoration, but the activated indium-bearing zinc ferrite can not lose activity fully. The study on the leaching kinetics of the sampe treated by anealing at450℃showed that the decrease of particle size induced by planetary mill activation had a positive effect on the improvement of the leaching kinetics of indium-bearing zinc ferrite, but it was less than the effect of the damages of crystal structure. Mechanical activation led to the damages of crystal stucture and caused various lattice defects. However, lattice defects have an important effect on the geometric stucture and electronic structure of crystal materials so that it will affect the reaction activity. In order to analysis the essential effect of the lattice defects induced by mechanical activation on the microstructure and reaction activity of zinc ferrite, the effect of the vacancy defects and the inversion defect between Zn2+and Fe3+on the geometric stucture and electronic structure of zinc ferrite was investigated using the first principle calculation based on density function theory (DFT) in this study. The results showed that zinc ferrite was a semiconductor with direct band gap; the Zn-O bonds in ZnFe2O4had sp3covalent bond character with the Zn at the center and the agnle between any two Zn-O valencies was109°28’; the high chemical stability of ZnFe2O4could be attributed to the existence of high percent Zn-O covalent bonds. Both Zn-vacancy and Fe-vacancy increased the lattice parameters and caused lattice deformation; both O-vacancy and inversion defect decreased lattice parameters and also caused lattice deformation. The formation of different lattice defects had different difficult levels. Among them, the inversion defect was the easiest one to form, then Zn-vacancy, Fe-vacancy and O-vacancy. The vacancy defects and inversion defect changed the spinel zinc ferrite from semiconductor properties into metallic properties. Zn-vacancy, Fe-vacancy and O-vacancy decreased the charge of atoms around vacancy defect. Zn-vacancy and Fe-vacancy increased slightly the bond strength around vacancy defect, and O-vacancy increased slightly the strength of Fe-O bonds and decreased significantly the strength of Zn-O bonds around vacancy defect. For zinc ferrite with inversion defect, the charge of O atoms attated with Fe atom located at the tetrahedral (A) site decreased, the strength of Fe-O bonds increased and the bond lengths became short; the charge of O atoms attated with Zn atom located at the octahedral [B] site increased, the strength of Zn-O bonds decreased and the bond lengths became long. In addition, the results of theoretical calculation have confirmed that indium entered into the crystal lattice of zinc ferrite through substituting zinc ions for the industrial indium-bearing zinc ferrite due to the lower indium concentration.In addition, the geometric and electronic structures of In-substituting ZnFe2O4were also studied by first-priciples DFT calculations. The results showed that In substitution induced the increase of lattice parameters and slight deformation of lattice cell.In substituting Zn in the ZnFe2O4had high stability. For ZnFe2O4with In substituting Zn, the bond strengths of In-O and Zn-O were similar and showed covalent character. Moreover, the strength of Fe-O around In-atom decreased. For ZnFe2O4with In substituting Fe, the bond strength of In-O was lower than those of Zn-O and Fe-O. In addition, the bond strength of Zn-O around In-atom decreased and the bond strength of Fe-O around In-atom increased.This study can perfect the mechanism of enhancing leaching indium and zinc from indium-bearing zinc ferrite by mechanical activation and kinetics theory. It can provide theoretical basis and design model for the indium and zinc recovery from hydrometallurgical zinc residue contains indium-bearing zinc ferrite. It owns high academic value and practical guiding significance.