CaO-SiO2-FeO-MgO体系钢渣固相改质过程中的镁铁尖晶石生长机理

Growth Mechanism of Magnesioferrite Spinel in Solid Phase Modification of CaO-SiO2-FeO-MgO System

钢渣是炼钢过程产生的副产品,其排放量约为钢产量的15%,钢渣的循环利用是钢铁企业亟需解决的关键问题之一。利用其成分特点,已有少量钢渣应用于冶金、工程回填、筑路、污水处理和制备微晶玻璃等领域。除以上应用外,将钢渣掺入水泥中制备钢渣水泥是一种有效且高效的制备方式,然而钢渣中难以分离的FeO相限制了其在建材领域的应用。通过氧化改质的方式可以使钢渣中FeO向强磁性Fe 3O 4发生转变,后者可进行磁选分离,且改质过程中无温室气体排放。针对CaO-SiO 2-FeO体系钢渣的氧化改质工艺已有前人进行了相关研究,然而对更接近于实际钢渣成分的CaO-SiO 2-FeO-MgO体系的研究较少。此外,钢渣氧化改质过程中的镁铁尖晶石的形核、长大机理仍需要进一步探索研究。本工作参照某钢厂转炉钢渣的实际成分,通过氧化气氛(合成空气)下煅烧的方式对钢渣进行固相改质。借助X射线衍射(XRD)仪、背散射扫描电镜(BEI-SEM)和X射线能谱仪(EDS)对CaO-SiO 2-FeO-MgO体系合成钢渣的矿物相产物进行了分析,并结合热力学计算软件(FactSage)对主要产物相生成的热力学趋势进行了研究。此外,利用综合热分析仪(TG-DSC)对镁铁尖晶石群形成的动力学机理进行了推演,并建立了对应的动力学模型。研究结果表明,随固相改质温度由1 000℃上升至1 150℃,镁铁尖晶石产量呈现先增加后减少的趋势,且在改质温度为1 100℃时达到极大值;镁铁尖晶石相在(311)晶面对应的衍射角度随改质温度变化发生如下变化:2θ=35.44°(1 000℃)→2θ=35.49°(1 050℃)→2θ=35.49°(1 100℃)→2θ=35.43°(1 150℃);当氧化温度由1 000℃升高到1 150℃,氧化600 s后体系中质量由351.273×10^-3 mg增加至499.077×10^-3 mg,氧化1 800 s后体系中质量由364.390×10^-3 mg增加至535.341×10^-3 mg;参照动力学机理可以将CaO-SiO 2-FeO-MgO四元钢渣体系的固相改质过程分为三个阶段,即孕育阶段、化学反应阶段和扩散阶段。由理论计算所得动力学模型与TG实验结果的热重变化趋势较为一致,借助动力学模型能够较准确地描述钢渣固相改质过程中镁铁尖晶石的形核和长大过程。

Steel slag is a by-product of the steelmaking process,and its yield are about 15%of steel production.The recycling of steel slag is one of the key issues that steel companies need to solve.With its composition characteristics,a small amount of steel slag has been used in metallurgy,engineering backfilling,road construction,sewage treatment and preparation of glass-ceramics.Besides,the incorporation of steel slag into cement to prepare steel slag cement is an effective and efficient way to use.However,the existence of wustite phase,which is difficult to separate in steel slag,limits its application in the field of building materials.The oxidative modification can transform the wustite in the steel slag to the ferromagnetic magnetite,and there is no greenhouse gas emission during the upgrading process.The oxidative upgrading process of CaO-SiO 2-FeO system steel slag has been studied in the past,but the CaO-SiO 2-FeO-MgO system which is closer to the actual steel slag composition has relatively less research.In addition,the nucleation and growth mechanism of the magnesioferrite spinel in the oxidation process still needs further exploration.This work refers to the actual composition of the converter steel slag of a steel plant,and the solidification of the steel slag is carried out by calcination under an oxidizing atmosphere(Synthetic air).X-ray diffraction(XRD)analysis,backscattered scanning electron microscopy(BEI-SEM)and X-ray energy dispersive spectroscopy(EDS)were used to analyze the mineral phase products of CaO-SiO 2-FeO-MgO system synthesis slag,combined with thermodynamics.The calculation software(FactSage 7.0)was used to study the thermodynamic trends of the main product phase formation.In addition,simultaneous thermal analyzer(TG-DSC)was carried out to study the kinetic mechanism of the formation of magnesioferrite spinel,and the corresponding kinetic model was also established.The results show that with the solid phase reforming temperature rising from 1 000℃to 1 150℃,the yield of magnesioferrite spinel increases first and then decreases,and reaches a maximum value when the upgrading temperature is 1 100℃.The magnesioferrite spinel changes in the(311)crystal plane corresponding to the diffraction angle as follows:2θ=35.44°(1 000℃)→2θ=35.49°(1 050℃)→2θ=35.49°(1 100℃)→2θ=35.43°(1 150℃).With the oxidation temperature increa-sing from 1 000℃to 1 150℃,the weight gain of the 600 s oxidation system increases from 351.273×10^-3 mg to 499.077×10^-3 mg,and the weight gain of the 1 800 s oxidation system increases from 364.390×10^-3 mg to 535.341×10^-3 mg.According to the kinetic mechanism,the solid phase modification process of CaO-SiO 2-FeO-MgO quaternary system can be divided into three stages:initial stage,chemical reaction phase and diffusion phase.The theoretically calculated kinetic model is well consistent with the thermogravimetric change trend of the TG experimental results.The kine-tic model can accurately describe the nucleation and growth process of the magnesium iron spinel during the solid phase reforming of steel slag.