Mechanism For Oxidative Leaching Of Chalcopyrite In Ammonia-ammonium Chloride Solution

In recent years,the demand for economic and efficient treatment of low-grade mixed minerals is increasing with the continuous consumption of high grade minerals.Due to the prominent challenges of the pyrometallurgical processes such as efficiency and pollution,hydrometallurgical processes have attracted much more attention.Ammonia leaching process has been widely used in non-ferrous metals hydrometallurgy because of its high leaching rate,selective leaching of non-ferrous metals,low corrosion of leaching solution,high purity of products and recyclability of leaching solution.The main problem of ammonia leaching of chalcopyrite is the aggregation of the product on the mineral surface during the leaching process,which limits the leaching process in terms of kinetics.In this paper,the leaching mechanism of high-purity chalcopyrite and concentrate in ammonia solution is studied,and the mechanism of solid-liquid interface oxidation and passivation is also investigated.This work aims to provide a fundamental theory for ammonia leaching of low--grade copper sulfide ore.The main results are shown as follows:When water and oxygen were adsorbed on the surface of the chalcopyrite,the Fe in the chalcopyrite migrated to the surface preferentially and formed a relatively dense iron oxide layer,which left the CuFe1-xS2 at the original position.The results show that the chalcopyrite is easier to be oxidized with the synergistic action of water and oxygen.It is found that oxidation degree of the mineral surface can be deduced from its optical microscope analysis.XPS depth analysis of the passivation layer was also performed.The surface of the sample is a dense FeOOH layer with the thickness of less than 51 nm,a mixed layer of Fe2O3 and CuS2 with a thickness ranging from 115 to 179 nm is underneath.Hydroxyl only exists on the surface,and the content of O decreases with the increase of depth.Through the thermodynamic calculation,the phases of the passivation layers on the chalcopyrite surface changes with pH and Eh values.The passivation layer is iron oxide/CuS2 or iron oxide/S0 in weak alkaline solution.Linear polarization,cyclic voltammetry,potentiodynamic/potentiostatic polarization,electrochemical impedance spectroscopy(EIS)and Mott-schottky analysis were performed to investigate the interfacial reactions of chalcopyrite electrode in ammonia solution.In the passive region of chalcopyrite from OCP to 0.35 V(vs.SCE)of anodic oxidation potential,Fe will migrate preferentially to the surface of chalcopyrite,forming iron oxides and Fe-deficient Cu polysulfide layers.When the potential reaches 0.35 V,CuFe1-xS2 begins to be decomposed,meanwhile,the surface of chalcopyrite is passivated and form S0.When the potential goes over 0.45 V,S0 can be evolved to the product with higher valence.The cell volume of the chalcopyrite remains same even if some iron migrates to the surface at the beginning of the reaction.However,the lattice parameter becomes smaller once CuS2 was formed.Subsequently,the bonding force between passivation layer and the substrate decreases,resulting in a peeling phenomenon,EIS analysis explained the electrochemical oxidation diffusion process.Warburg impedance appeared on the surface at the potential region of 0-0.1 V,which indicated it was semi-infinite diffusion model.When the potential reached 0.2 V,the electrode was passivated and the resistance for charge transfer was increased.When the potential reached 0.3 V,the inductance semi-circular circle appeared,which meant the passivation phenomenon was more obvious,and the charge transfer resistance presented the maximum value.The chalcopyrite concentrate is mainly composed of chalcopyrite,pentlandite,pyrrhotite and magnetite.The interfacial reaction is slow in the ammonia environment at 25?.Fe migrates to the surface preferentially,forming a dense oxide layer,and copper sulfide with less iron is left at the bottom.The passivation layer is stable at relatively lower leaching temperature.But the formed iron oxide on the surface was gradually stripped at the temperature of 35? in ammonia environment.XPS results show that,in the ammonia solution,the bond between the iron oxides/copper-rich sulfides and the substrate were weaken,and the passivation layer can be detached spontaneously.And,the theory of electrochemical interaction of minerals was discussed to explain why the reaction rate in the interface of minerals was different,and analyze the interaction between different mineral phases during the leaching process.