Physicochemical Mechanism and Migration of Iron in the Oxidation of Iron-rich Nickel Slag

Author:Ma Yong Bo

Supervisor:du xue yan


Degree Year:2019





The total iron(TFe)content in iron-rich nickel slag is up to 40%,and it mainly exists as fayalite(Fe2SiO4)with stable structure.Therefore,the effective extraction of Fe resources is crucial to the comprehensive utilization of nickel slag.In this work,focusing on the difficult separation of Fe component and Si component in fayalite,the physicochemical mechanism and migration behavior of Fe component during the molten oxidation of iron-rich nickel slag are investigated.The recovery of Fe component has been realized via structural reconstruction,enrichment and magnetic separation,which breaks through the bottleneck problem of comprehensive utilization of nickel slag.(1)Thermodynamic calculation shows that:the stable temperature of Fe 2SiO4 in FeO-SiO2-MgO-CaO quaternary system below 1873K;Fe2SiO4 can directly react with O2 to generate Fe3O4,and the reaction tendency can be greatly improved by the addition of CaO;The further oxidation of Fe3O4 into Fe2O3 will attenuate with the increased temperature,and it cannot occur above 1623K,hence Fe3O4 becomes the most stable iron oxide in this condition;The FeO-SiO2-MgO-CaO quaternary phase diagram shows that the major iron-bearing phase is spinel with constituents of Fe3O4and MgFe2O4,and its phase area increases firstly and then decreases,as w(CaO)increases in the range of 0-30%;The composition of silicate phase also changes significantly,and the low temperature area tends to move toward the region where w(SiO2)increases and w(MgO)decreases with the increasing w(CaO).This indicates that the addition of CaO can destroy the stable structure of Fe 2SiO4 and beneficial for the conversion of Fe2SiO4 into Fe3O4,which is conducive to solve the separation problem of Fe-Si in Fe2SiO4.(2)The research of melting characteristics shows that:FeO/SiO2,MgO/SiO2 and w(CaO)can affect differently the softening temperature(Ts),hemispheric temperature(Th)and flow temperature(Tf)of FeO-SiO2-MgO-CaO synthetic slag system;When FeO/SiO2 and MgO/SiO2 is given,Ts、Th and Tf all decrease significantly firstly and then increase slightly with the increase of w(CaO);Without CaO,Ts decreases significantly with increasing FeO/SiO2,and the influence of MgO/SiO2 is slight,while Th and Tf increase firstly and then decreased with the increase of FeO/SiO2 and decrease with the increase of MgO/SiO2;When w(CaO)is 6%,Ts increases firstly and then decreases with the increase of FeO/SiO2,and the influence of MgO/SiO2 is slight,while Th and Tf increase significantly with the increase of FeO/SiO2 and decrease significantly with the increase of MgO/SiO2;When w(CaO)is 12%or more,the melting characteristics are slightly affected by FeO/SiO2 and MgO/SiO2;The determined melting temperature of modified iron-rich nickel slag should above 1646K according to the results of melting characteristics.These studies can clarify the influence of silicate composition on the melting characteristics of nickel slag,supplement the thermodynamic data of FeO-SiO2-MgO-CaO system as well,which provides a basis for the selection of temperature in the molten oxidation of nickel slag.(3)The results of phase evolution show that:Fe component migrate from Fe2SiO4to Fe3O4 during molten oxidation in nickel slag,and MgFe2O4 forms by solid solution of Mg2+into Fe3O4 lattice,thus Fe3O4 and MgFe2O4 become the main iron-bearing phases;Most SiO44-forms Ca(Mg,Fe)Si2O6 by polymerization and peritectic reaction in the oxidized nickel slag,except a small amount exists as Mg2SiO4 and CaMgSiO4;These results are consistent well with the calculated results of FactSage 7.1 software;The oxidation conditions of Fe2SiO4 in iron-rich nickel slag are determined as300mL/min of air flow rate,0.90 of basicity,1658 K of oxidation temperature,and 80min of oxidation time,which causes the w(Fe2+)in the oxidized slag to be reduced to about 7%.This means the migration behavior of Fe component and the phase evolution of silicate phases have been clarified,and the problem of Fe-Si separation in iron-rich nickel slag has been solved.(4)The studies of oxidation kinetics show that:the isotherm oxidation of Fe2SiO4in iron-rich nickel slag is a first-order reaction,and its apparent activation energy is315.16kJ/mol;The changes of molten oxidation rate of Fe2SiO4 in iron-rich nickel slag have been determined by thermogravimetric method,and the mechanism of Fe2SiO4 molten oxidation has been explored based on the double-film theory of gas-liquid reaction;The molten oxidation process of Fe2SiO4 may be divided into external diffusion period,chemical reaction period,the mixed rate control period of chemical reaction and internal diffusion,and internal diffusion period;The corresponding rate control step gradually changes from the initial O2 diffusion rate control to the oxidation reaction rate control,and then to the oxidation reaction and the Fe2+internal diffusion mixing rate control,and finally to the Fe2+internal diffusion rate control.(5)The investigation of the microstructure and magnetic separation of Fe3O4 in the oxidized nickel slag show that:the microstructure of oxidized nickel slag is affected significantly by oxidation time and oxidation temperature;Longer oxidation time is favorable for Fe3O4 crystals to grow into full granular particles,while higher oxidation temperature causes Fe3O4 crystals to grow into skeletal particles;Granular magnetite embedded into the silicate matrix composed of augite and olivine;After three times of crushing and magnetic separation,the effective recovery of Fe component has been realized,and the magnetite particles with total iron grade of54.08%,iron recovery rate of 75.99%and yield rate of 50.67%have been obtained.