Research On The Foundamentals And Process For Ferronickel Production From Laterite Based On Semi-mloten Smelting

Stainless steel has a widespread application in military,industrial and civilian for its excellent corrosion resistance.Nickel-based stainless steel is an important component of stainless steel,accounting for more than 60%of the total output of stainless steel.In recent years,the output of stainless steel has increased rapidly,this has increased the demand for nickel resources.With the depletion of nickel sulfide ore and high grade nickel laterite ore resources,the use of low and middle grade nickel laterite is regarded as a main solution of the production of stainless steel.Direct reduction-magnetic separation process can be divided into semi-molten smelting process and solid smelting process according to the state of material in the reduction process,semi-molten smelting process adjusts the properties of slag and metal by adding flux,makes slag or metal partially melt,promotes the aggregation and growth of the reduced metal,and forms ferronickel alloy.Through cooling,crushing and separation,ferronickel alloy products can be obtained directly.Semi-molten smelting process has the advantages of short process,low energy consumption and strong adaptability of raw materials.80-85%of total energy consumption is provided by coal.It is a relatively economical method for smelting nickel laterite ore at present.This paper focuses on semi-molten smelting process,which studied the properties of nickel laterite ore,smelting thermodynamics,reduction kinetics,ferronickel grain growth behavior,slag-metal separation behavior and smelting process by theoretical and experimental research.The chemical analysis,X-ray diffraction analysis and thermogravimetry differential thermal analysis of laterite ore were carried out.The results showed that the nickel laterite ore used in this paper is a typical silica-magnesia laterite.The crystal water in the laterite mainly exists in gismondine,which is removed at about 292^o C during drying process.The hydroxyl group mainly exists in antigorite and lizardite,which are removed at about 627^o C during drying process.Based on the thermodynamics calculation of the reduction of iron and nickel oxides,and the phase transformation rule of nickel laterite under sodium or sodium carbonate was analyzed.The results show that Na₂SO₄ is easily reduced to Na₂S after adding sodium sulfate.When iron oxide is reduced to FeO,FeO can react with Na₂S to form FeS and different silicates in the presence of SiO₂,FeS can form low melting point compounds with Fe,which can decrease the melting point of ferronickel alloy,thus promoting the aggregation and growth of ferronickel alloy particles.Sodium carbonate is difficult to be reduced during carbothermal reduction of nickel laterite ore.Compared with Mg₂SiO₄,MgSiO₃ reacts more easily with Na₂SO₄ or Na₂CO₃.In the semi-molten process of nickel laterite ore,the addition of FeO and sodium salt can decrease the slag melting temperature and viscosity,which is beneficial to the aggregation and growth of ferronickel aloy particles.The reduction kinetics of nickel laterite ore under sodium or sodium carbonate was studied by thermal analysis kinetics theory.The results showed that the starting temperature of Boudouard reaction was also about 50^o C earily than that without additives.In the first stage of carbothermal reduction process,Na₂SO₄ was reduced to Na₂S.In the second stage,Boudouard reaction occurred,the solid-solid reduction reaction was transformed into the gas-solid reduction reaction.In the third stage,the main reaction was the reduction reaction of Fe₂SiO₄.The starting temperature of Boudouard reaction was about 200^o C earily than that without additives,and about150^o C earily than that with sodium sulfate.In the first stage of carbothermal reduction process,Na₂CO₃ reacted with MgSiO₃ to form Na₂Mg₂Si₂O₇,this destroied the structure of enstatite and was conducive to the reduction of nickel and iron oxides adhering to them.In the second stage,Boudouard reaction occurred,the solid-solid reduction reaction was transformed into the gas-solid reduction reaction.In the third stage,the main reactions were the reduction reaction of FeO and Fe₂SiO₄.When sodium carbonate is added,the activation energy of chemical reaction is lower than that when sodium sulfate is added,and the catalytic effect of sodium carbonate is stronger than that of sodium sulfate in promoting carbothermal reduction of laterite nickel ore.The growth behavior of ferronickel grain and metal-slag separation behavior was investigated,the results showed that when carbon or ferrous sulfide is added,the melting point of ferronickel alloy in pellets decreases obviously,and when sodium carbonate,sodium sulfide and ferrous oxide are added,the slag melting temperature in pellets decreases obviously.The decrease of melting temperature of slag phase or metal phase can produce more liquid phase during roasting,thus improving the aggregation and growth of metal particles.Adding carbon,sodium sulfide,ferrous sulfide,sodium carbonate and ferrous oxide can decrease the slag and metal starting separation temperature.When sodium sulfide or sodium carbonate is added,the separation behavior of slag and metal is mainly affected by the melting of slag;when carbon,ferrous sulfide or ferrous oxide are added,the quality of liquid slag decreases at the slag and metal starting separation temperature,the metal particles partial melted,and the separation behavior of slag and metal are affected by both the melting of slag and the melting of metal.The paper focuses on semi-molten smelting process for nickel laterite ore by simulating the heating system of rotary hearth furnace.The effects on reduction temperature,the content of additive,the content of limonitic laterite and carbon content on the ferronickel preparation were investigated.The results showed that the addition of sodium carbonate,sodium sulfate and limonitic laterite can improve the nickel content in magnetic concentrate.However,excessive addition of limonitic laterite will increase the reduction of iron oxides,resulting in a decrease in nickel content in magnetic concentrate.In the presence of sodium sulfate,Na₂S reacted with FeO to form FeS.Then,Fe and FeS formed a low melting point phase,which decreased the melting point of ferronickel alloy,and this is conducive to the aggregation and growth of metal particles and the separation of slag and metal.Reducing the carbon content in a certain range is helpful to improve the nickel grade in magnetic concentrate,but too much reduction in the proportion of carbon content will reduce the carburization amount in metal particles,which is not conducive to the aggregation and growth,thus leading to the decrease of nickel grade in magnetic concentrate.