A Study On Oxidation Mechanism Of Copper-nickel Sulfides Using First-principles Calculations

As an important mineral resource of copper and nickel metals,copper-nickel sulfides can be effectively dissolved and separated through the oxidative leaching process.The difference of the oxidation activity for the sulfides makes them dissolved subsequently and separate them each other.The oxidative activity of the sulfides is mainly determined by their outer electronic structure,so the finding of the relationship between the characteristics of the electronic structure and oxidative activity,and the preference of the oxidation is very important for the establishment of the oxidation leaching process in the hydro-metallurgical industry.In this thesis,the band structure,total and partial density of states（DOS）,electron density,difference electron density,and Mulliken population of copper-nickel sulfides were systematically investigated using density functional theory（DFT）based first-principles.The relationship between the macroscopic oxidation mechanism of matter and outer electronic structure was established.In addition,the oxidation ability of various elements in copper-nickel sulfide was characterized by using the electronic descriptors of the band centers（d-band center,p-band center）and the highest peak position of partial DOS.Finally,the correctness of the calculation results is verified by combining them with the experimental results.The main conclusions are drawn as follows:（1）The electronic structure of Cu_xS_y and Ni_xS_y in high-grade nickel matte shows that the oxidation of these sulfides is a gradual oxidation process,and the Cu-S and Ni-S bonds are of polar covalent character.According to the values of their Cu,Ni-3d band centers,the oxidation activity decreases in the order Cu_1.96S>Ni₃S₂>Ni₇S₆>Ni S>Ni₃S₄>Ni S₂>Cu S.This oxidation sequence provides a theoretical basis for the substitution reaction of nickel sulfide with Cu²⁺and obtaining more stable Cu S,which is the theoretical basis of Sherritt Gordon’s selective leaching process.（2）The electronic structure of the bornite Cu₅FeS₄)and pentlandite（Fe₅Ni₄S₈）in low nickel matte shows that Fe is preferentially oxidized in both materials,and the Cu-S and Fe-S bonds are of polar covalent character.According to the value of band centers and the highest peak positions of partial DOS,the oxidation activity decreases in the order Cu₅FeS₄-（Fe）>Fe₅Ni₄S₈-（Fe）>Fe₅Ni₄S₈-（Ni）>Cu₅FeS₄-（Cu）>Fe₅Ni₄S₈-（S）>Cu₅FeS₄-（S）.This oxidation sequence explained the experimental results that Cu₅FeS₄ oxidized preferentially over Fe₅Ni₄S₈ in the oxidation leaching process of low matte nickel.（3）The electronic structure of chalcopyrite（CuFeS₂）and pyrite（FeS₂）shows that Fe is preferentially oxidized in both materials,and the Cu-S and Fe-S bonds are of polar covalent character.According to the value of the band center and the highest peak positions of partial DOS,the oxidation activity decreases in the order FeS₂-（Fe）>CuFeS₂-（Fe）>Fe Cu S₂-（Cu）>FeS₂-（S）>CuFeS₂-（S）.This oxidation sequence explained the experimental result that FeS₂ is oxidized before to CuFeS₂in electrochemical experiments.（4）The band centers（d-band center,p-band center）and the highest peak positions of partial DOS are electronic descriptors related to the energy characteristics of the outer electrons for the sulfides.The value of band center is related directly to the oxidation activity of copper-nickel sulfides,and it can be used to evaluate the oxidation activity of the copper-nickel sulfides mentioned in this study.However,the highest peak position of partial DOS can describe more exactly the oxidation activity of the copper-nickel sulfides with a narrow bandwidth of the DOS.