Basic Investigation On Effect Of FeO On The Precipitation Behavior Of Spinel In CaO-SiO₂-MgO-Al₂O₃-Cr₂O₃ System

Stainless steel slag is the by-product of stainless steelmaking.It can be used as road base,building materials and ironmaking-sintering.There is a risk of Cr⁶⁺dissolution for the use of stainless steel slag,which limits its reuse of the slag.Therefore,in order to obtain good economic,social and environmental benefits by comprehensive utilization of stainless steel slag,the dissolution of Cr⁶⁺in stainless steel slag must be solved.So it is important to study the precipitation of chromium spinel and the stability of chromium in CaO-SiO₂-MgO-Al₂O₃-Cr₂O₃ slag containing FeO,as well as the harmless treatment and utilization of the slag.Based on the principle of metallurgical thermodynamics and the thermodynamic database of FactSage 7.0,the theoretical basis for the formation of chromium spinel in stainless steel slag was studied by simulation of the solidification process of the CaO-MgO-SiO₂-Al₂O₃-Cr₂O₃ based system.Scanning electron microscope（SEM）was used to observe the microscopic morphology and Energy Dispersive Spectrometer（EDS）was used to analyse the micro-composition,the linear and surface distribution of elements,and the mineral composition was analysed by XRD.Statistics of crystal particle size in samples was made via IPP Image Analysis Software.Leaching toxicity test of stainless steel slag was carried out based on standard leaching procedure HJ/T 299-2007 to verify the stability of chromium.Firstly,based on theoretical analysis and FactSage software simulation,the basic theory of spinel crystal regulated by FeO formation in CaO-SiO₂-MgO-Al₂O₃-Cr₂O₃system was studied.The results show that the precipitated crystals in synthetic stainless steel slag samples mainly contain spinel crystals,dicalcium silicate,merwinite,melilite and okenite.The spinel crystals are precipitated at high temperature.The precipitation is divided into two sections:the precipitation of the spinel crystals increased with the increasing of FeO.The precipitation amount gradually decreases at high temperature and gradually increases at low temperature with the increasing of FeO.The precipitation temperature decreases with the addition of FeO.As the FeO addition increased from 0to 20wt%,the spinel precipitation temperature decreased from 1910°C to 1800°C.The spinel solid solution mainly consists of MgCr₂O₄ and FeCr₂O₄.On the basis of theoretical analysis,the effect of FeO on the amount of precipitated spinel crystals,grain size,composition,etc were studied in CaO-SiO₂-MgO-Al₂O₃-Cr₂O₃ system by preparing synthetic slag in the laboratory.The effect of FeO on the stability of chromium in steel slag containing Cr₂O₃ was investigated by the leaching test.The results show that with the increase of FeO content,the precipitation of spinel increased,the precipitation temperature gradually decreased.FeO can effectively promote the growth of spinel crystal in CaO-SiO₂-MgO-Al₂O₃-Cr₂O₃-FeO system.The content of Fe in spinel increased,while Ca,Si,Mg and Al decreasd,the content of chromium in the matrix decreased.The increase of FeO addition effectively improves the stability of chromium in CaO-SiO₂-MgO-Al₂O₃-Cr₂O₃ system.When the FeO increased from 0 to 20wt%,the dissolution amount of Cr⁶⁺decreased from 0.1434mg/L to 0.0021mg/L.The holding time and cooling rate are important parameters in the solidification process,but they have little effect on the precipitation behavior of spinel crystals in the CaO-SiO₂-MgO-Al₂O₃-Cr₂O₃-FeO system.Under the condition of 1350¹450°C for a long time（≤120min）,the mineral phase structure of CaO-SiO₂-MgO-Al₂O₃-Cr₂O₃-FeO system unchanged,and the size of the spinel crystal increased slightly.The size increases at 1350°C,1400°C and 1450°C were 17.04%,23.11%,and 13.91%,respectively.The cooling rate has little effect on the formation and growth of spinel crystals either.The effect of cooling rate on spinel size at high temperature（>1500℃）is slightly stronger than that at low temperature（≤1500℃）.When temperature is 1580℃,the size of spinel crystal cooled by 1℃/min increases by 4.02%compared with that cooled at rate of 12℃/min.When temperature is 1400℃,the size of spinel crystal obtained at cooling rate of 1℃/min increased only 0.62%compared with that after the cooling process of 12℃/min.The composited effect of FeO and Fe₂O₃ on the precipitation behavior of spinel crystals in CaO-SiO₂-MgO-Al₂O₃-Cr₂O₃ system was studied using synthetic slag system.The experimental results show that the increase of Fe₂O₃ content in CaO-SiO₂-MgO-Al₂O₃-Cr₂O₃ system not only promotes the precipitation of spinel crystals,but also inhibits the formation and precipitation ofα-C₂S,obviously.Fe₂O₃ promotes the precipitation amount and the growth of spinel crystals.When Fe₂O₃ addition increased from 2wt%to 20wt%,the average size of the spinel crystals increased from 2.74μm to8.10μm.When the addition of Fe₂O₃ is low（≤16%）,the increase rate of spinel crystal size is large with the increase of Fe₂O₃.However,when the addition of Fe₂O₃ is high（≥16%）,the increase rate of spinel crystal size is lower,the increase amount is small.With the increase of Fe₂O₃,the contents of Cr,Ca and Si in spinel crystals decrease gradually,while the content of Fe element increases.When the addition of Fe₂O₃ in the CaO-SiO₂-MgO-Al₂O₃-Cr₂O₃-8wt%FeO system increased from 0 to 12wt%,the contents of Cr,Ca and Si decreased from 21.71at%,3.65at%and 3.02at%to 12.79at%,1.39at%and 1.15at%,respectively,while Fe increased from 6.48at%to 16.99at%.Compared with Fe O or Fe₂O₃,the coupling of Fe₂O₃ and FeO is more conducive to the growth of spinel crystals.Combine the effects of slag basicity and oxidizability on the precipitation behavior of spinel crystals in CaO-SiO₂-MgO-Al₂O₃-Cr₂O₃ system,the growth and chemical composition of chromium spinel in slag containing Cr₂O₃ and the effect of chromium stability on the growth and chemical composition of chromium spinel were investigated under the complexed reaction of FeO and basicity.It was found that the size of spinel crystal decreased with the increase of slag basicity.When the basicity increased from0.6 to 1.0,the spinel crystal size sharply decreased from 12.30μm to 6.22μm,a drop rate up to 49.43%.When the basicity increases from 1.0 to 2.2,the spinel crystal size decreases from 6.22μm to 4.47μm,and the trend is relatively gentle.The increase of basicity has an important effect on the occurrence of chromium.When the basicity is lower than 1.4,chromium is mainly present as spinel crystals.When the basicity is higher than 1.4,the content of chromium in the non-spinel phase would gradually increase.In the sample with basicity of 3.0,the chromium is mainly in the form of calcium chromate,which poses the threat to the stability of chromium in the CaO-SiO₂-MgO-Al₂O₃-Cr₂O₃-FeO system.