Density Functional Theory Calculations On The Surface Reaction Mechanism Of Nickel/Copper Sulfide

As one of the important strategic metals in China,nickel plays a supporting role in the development of national defense and green energy.Nickel ore in China is dominated by nickel sulfide,but the low-grade refractory sulfide ore occupies the vast majority.The chemical reactions involved in the smelting process are complex.It is a new challenge to analyze the complex chemical reactions so as to build the theoretical basis for the actual production.The density functional theory can analysis the complex interface reaction from the microscopic point of view by constructing the reaction interface.This paper by sulfide as the research object,by using density functional theory calculations respectively explores the mechanism of double metal sulphide oxidation,the single metal sulfide sulfate mechanism and the mechanism of metal sulfides chloride,finally analyzes the single metal sulfide sulfate and chloride mechanism between each other,in order to realize the separation of Fe and Ni/Cu and Ni and Cu separation.Firstly,the oxidation mechanism of bimetallic sulfide,pentlandite and chalcopyrite were studied.For pentlandite,the adsorption,dissociation and migration of O2 on(001)and(010)surfaces with different Fe/Ni ratios were studied.The results showe that compared with the vertical adsorption of O2,the parallel adsorption of O2 made the configuration more stable.However,the dissociation energy barrier of O2 in the process of dissociation from parallel adsorption is very low,indicating that it is easy to dissociate on both surfaces of O2.Compared with the Ni-Ni bridge position and the ni-fe bridge position,the dissociated O atom also tends to act on the Fe-Fe bridge position,when more electric charge will be transferred from the Fe atom to the O atom.The(001)surface was chosen for chalcopyrite,and the interaction of O2,H2O and O2/H2O on the surface was discussed respectively.Compared with the Cu site,the surface configuration of O2 after adsorption and dissociation at the Fe site was more stable.After the adsorption of H2O at Fe sites on the chalcopyrite surface,the relative energy would increase during the dissociation,indicating that H2O could not decompose directly on the chalcopyrite surface.However,when O2 is decomposed on the surface in advance,it can not only promote the dissociation and adsorption of H2O on the surface,but also reduce the energy barrier of H2O in the process of dissociation,indicating that O2 can promote the reaction between H2O and chalcopyrite surface.The Fe atoms for bimetallic sulfide are oxidized preferentially leading the formation of iron oxide and corresponding metallic sulfide such as nickel sulfide(NiS)and cuprous sulfide(Cu2S)and SO2 is also generated.Based on the previous studies,the oxidation and sulfation mechanisms for NiS and Cu2S were studied respectively.For NiS surface,the oxidation mechansim and the sulfation by diret way and indirect way were all performed.The indirect sulfation is studied through the sulfation for NiO generated from the completely oxidation of NiS.During the oxidation process,the energy barrier for O2 dissociation was very low,indicating that the reaction can be achieved easily.In addition,with the increase coverage of O atoms for NiS,the relative energy decreases all the time.In this process,S atoms on the second layer will migrate to the first layer to form SO2 molecules with the adsorbed O atom,demonstrating the interaction was strengthened between O atom and the NiS surface.And the desorption of SO2 leads a more stable configuration indicating that this process is favored thermodynamically.Two pathways were considered to the study the sulfation of NiS which based on the clean surface.After the O2 dissociative adsorption on the NiS surface,SO2 reacts with O atom of the surface leading the formation of SO3.The sulfation of NiS was achieved by the SO3 desorps and reacts with O atom.For the second path,SO2 reacting with two O atoms leads the sulfation of NiS directly.However,both paths lead the same configuration which can lead the formtion of pyrosulfuric acid(S2O72-)with the adsorption of SO3.The sulfation mechanism for Cu2S is similar with the NiS.Firstly,O2 adsorbs at the Cu-Cu bridge site of the Cu2S surface and the dissociative adsorption of O2 leads two oxygen atom interacting with two Cu-Cu bridges.With the increase of oxygen coverage on the Cu2S surface,S atom on the second layer also migrates to the top layer to form SO2.During this process,the relative energy continues to drop.The results show that the re-adsorption of SO2 on the Cu2S surface leads more stable configuration than the direct desorption.The sulfation of Cu2S surface was also performed for two paths.In path 1,the SO3 generated by the interaction between SO2 and an O atom,the desorption and re-adsorption of SO3 can lead the sulfation of Cu2S surface.In path 2,when SO2 interacts with two O atoms,the sulfation of Cu2S surface can be direatly achieved.The adsorption of SO3 can also causes the formation of pyrosulfate on the surface.This indicated that the surface sulfation mechanisms for NiS and Cu2S were consistent.In addition to separate NiS and Cu2S through sulfation process,chlorination agents(Cl2 and NH4Cl)can also be used to achieve it.The chlorination mechanisms of NiS and Cu2S were studied respectively.For the chlorination process of NiS,three paths were studied,namely,Cl2,NH4CI/O2,and NiO+NH4Cl.All three paths are thermodynamically feasible.First,the dissociation energy barrier of Cl2 on NiS surface is very low,indicating that Cl2 is easy to dissociate on the surface.For path 2,O2 can provides the adsorption sites for H atom from the NH4Cl dissociative adsorption,so NH4Cl can only react with NiS surface in the presence of O2.For path 3,NiO can directly react with NH4CI to achieve the surface chlorination.The chlorination mechanism of Cu2S surface is similar to that of NiS surface,and the dissociation barrier of Cl2 is also very low.In the presence of O2,NH4Cl can dissociate on the Cu2S surface leading the formation of CuCl2 species,achieving the chlorination of Cu2S.Based on the study of the mechanism of sulfation and chlorination,the interaction between the two mechanisms was explored.For NiS surface,the interaction between SO2 and the NiS surface gradually decreased with the increase of Cl atom coverage.When the Cl atom coverage is low,the effect on the formation of SO42-is weak.When Cl atom coverage is high,the NiS surface can still lead the formation of SO42-,but the interaction between SO42-and NiS surface is weak.These results suggest that chlorination of the NiS surface may affect or even weaken sulfation.For Cu2S surface,with the increase of Cl coverage on the surface,SO2 adsorption on the surface will be weakedned.It indicates that when sufficent chlorination agent is provided in the roasint process of NiS and Cu2S,the main product should be chloride.Therefore,the basic model of surface reaction of sulfides can be constructed by density functional theory calculations:after the preferential oxidation of Fe atom in bimetallic sulfides,on the one hand,the sulfation for mono-sulfides was achieved under the action of SO2/O2.The chlorination of NH4Cl can be directly achieved by NH4Cl dissociative adsorption in the presence of O2.Most of the products during the chlorination roasting process are metal chloride.