Study On Melting Characteristics And Viscosity Evolution Law Of Modified Nickel Slag During Melten Oxidation

The water quenched nickel slag,as a large amount of solid waste,is produced in nickel flash-furnace smelting,in which the content of total iron（TFe）can reach 40wt%.Therefore,it is great significance to study the recovery of iron resources for comprehensive utilization of nickel slag.The melting characteristics and viscosity are major effects on the formation of magnetite phase during the oxidation process of nickel slag because they can directly affect the chemical reaction rate,mass transfer rate,precipitation and growth rate of crystals.Nickel slag mainly contains CaO-SiO₂-FeO-MgO quaternary slag system despite of its complex composition and various elements.Therefore,based on the theory studying this quaternary slag system,we investigate the variation law and main control factors of melting characteristics and viscosity of the actual nickel slag.This will provide theoretical basis and experimental support for the extraction of iron resources from nickel slag via modified molten oxidation process.In this paper,the effects of basicity and FeO content on the phase diagram,melting characteristics and viscosity of CaO-SiO₂-FeO-MgO slag system have been investigated.The variation law of viscosity was illuminated from the view of microstructure evolution in molten slags.The influence of basicity on the melting temperature and viscosity of nickel slag was explored,and the oxidation optimal conditions were obtained.Finally,the enrichment behavior of iron in nickel slag by modified molten oxidation process was investigated.The specific conclusions are given as follows:（1）The quaternary phase diagram of CaO-SiO₂-FeO-MgO was calculated by using of FactSage software.The results show that:the composition point is located in the primary crystallization region（PCR）of spinel phase as the basicity is in the range of0.38¹.50 in air atmosphere,and the PCR area of spinel phase increases first and then decreases with the increase of basicity;the composition point is located in melilite phase when the content of FeO is in the range of 6.00¹6.00 wt%,while in the PCR of spinel phase corresponding FeO content of 26.00 wt%;the PCR area of spinel phase increases with the increase of FeO content in the range of 26.00⁴6.07 wt%.（2）The study on the melting characteristics and viscosity of CaO-SiO₂-FeO-MgO quaternary slag shows that:the liquidus temperature,melting temperature and critical viscosity temperature of the slag system decrease first and then increase with the increase of basicity in the range of 0.38¹.50;the liquidus temperature,softening temperature,hemispheric temperature,flow temperature and critical viscosity temperature of the slag system reach the lowest values at the basicity of 0.90,i.e.1297℃,1244℃,1256℃,1274℃and 1263℃,respectively;the liquidus temperature,melting temperature and critical viscosity temperature are all decreasing with the increase of FeO content in the range of 6.00⁴6.07 wt%.The viscosity of slag decreases with increasing basicity and FeO content,which is probably attributed to the transition of slag structure from layered or network structure to dimer or monomer structure resulting in the decrease of polymerization degree of silicate.（3）The study on the melting temperature and viscosity of the actual nickel slag shows that:the melting temperature and critical viscosity temperature of the slag system decrease first and then increase with the increase of basicity in the range of0.38¹.50;the softening temperature,hemispheric temperature,flow temperature and critical viscosity temperature of the slag system reach the lowest values at the basicity of 0.90,i.e.1275℃,1313℃,1406℃and 1294℃,respectively;these results are almost consistent with the results of above（2）;as to the modified molten oxidation of nickel slag,the oxidation optimal conditions are the temperature of 1400¹500℃and the basicity of 0.90.（4）The study on the precipitation behavior of the magnetite phase in the actual nickel slag shows that:the addition of the modifier CaO is conducive to the decomposition of fayalite and hortonolite in the nickel slag;the migration and enrichment of iron components to form magnetite crystals in the nickel slag can be realized under the condition of CaO addition and oxygen existence simultaneously;the main phase in spinel phases is magnetite（Fe₃O₄）;during the process of non-equilibrium solidification in the modified molten nickel slag at basicity of 0.90,the magnetite crystals begin to precipitate at 1455℃,and their growth rate is 1.20μm/s in0¹0 s,which is significantly higher than 0.16μm/s in 10²2 s;the magnetite grains remain almost invariable size and granular shapes after 22 s.