Study On Metal Droplet Migration And Growth Mechanism In The Process Of Semi-melted Reduction Of Stainless Steel Pickling Sludge

Stainless steel pickling sludge contains a lot of metal elements,such as nickel,chromium and so on.At present,the main way to deal with these sludge in domestic stainless steel plants is landfill,which not only wastes a lot of precious metal resources,but also causes serious damage to the environment and ecology during the process of sludge stacking,and finally causes harm to human health.Therefore,it is urgent to find efficient and energy-saving sludge treatment methods.The purpose of this thesis is to reduce the metal elements in the sludge under semi-melted conditions,and to migrate and aggregate them to form metal particles.Then,the metal particles are separated from the slag by magnetic separation,and finally the metal particles and tailings that meet the national chromium-containing solid waste discharge standards are obtained to achieve the harmless treatment and resource utilization of sludge.Based on the composition and phase analysis of stainless steel pickling sludge,a systematic sludge reduction experiment was carried out.The composition and phase of sludge and its reduction samples were analyzed by X-ray diffraction,Differential scanning calorimeter and X-ray fluorescence spectrometer.The phase transformation and interaction mechanism of the phases containing Cr,Fe,Ni and other metal elements in the sludge during reduction were explored.By means of metallographic microscope and scanning electron microscope,the morphology of the reduction products under different conditions was observed,and its composition was analyzed by XRF and EDS.The forming conditions of Ni-Fe-Cr-C alloy and the migration trends and kinetic conditions of each metal element after melting were revealed.On this basis,the factors affecting the migration process of metal droplets were analyzed,and the reduction parameters were optimized accordingly,so as to find the effective method of recovering metal elements in sludge.The experimental results showed that during the carbon reduction process of stainless steel pickling sludge,the reaction of the main metal elements Fe,Cr,and Ni in the sludge could be divided into three stages:?1?in the temperature range of 700??800?,a reduction reaction of Fe₂O₃ occured,and the reduction products were mainly Fe and Fe O.Fe O further combined with Cr₂O₃ to form Fe Cr₂O₄,and at the same time,Ni O combined with Fe₂O₃ to form Ni Fe₂O₄ spinel phase;?2?in the temperature range of 800??900?,the reduction reaction of the Ni Fe₂O₄ spinel phase occured,and the reduction product was the Ni-Fe metal phase;?3?in the temperature range of 1050??1100?,Fe Cr₂O₄ and C underwent a solid phase reduction reaction.The final product of the reduction was mainly Fe-Cr-C metal solid solution.The products in the above reduction process underwent carburizing and growth,and finally formed four kinds of metal solid solution particles of Fe-C,Ni-Fe-C,Fe-Cr-C and Ni-Cr-Fe-C,which were evenly distributed in the sludge among.With the increase of temperature,the metal phases in the sludge began to melt and grow up.The process was mainly divided into two stages:?1?when the temperature was higher than 1150?,under the action of driving force such as gravity and surface tension,Fe-C and Fe-Ni-C metal particles flowed in the solid slag phase and converged to the surface of the unmelted Fe-Cr-C alloy particles,resulted in the increase of Fe and Ni content and the decrease of Cr content in the alloy particles;?2?when the temperature was above 1243?,the alloy particles such as Ni-Fe-Cr-C melted too,formed relatively stable droplets,and began to aggregate and grow.At the same time,due to the consumption of carbon and the formation of CO,CO₂ and other gases in the solid slag phase,micro channels and pores were formed,which provided conditions for the flow,migration and aggregation growth of metal droplets.Based on the analysis of the movement and force of metal droplets under semi-melted conditions?metal melts while slag phase remains solid?,a kinetic model of metal droplets convergent growth was established.It was concluded that the size and number of micro channels and holes in the solid slag phase were the main factors affecting the size of metal droplets when the temperature and other conditions remained unchanged.The experimental statistical analysis results showed that the trend of the size of metal particles with the number of microchannels in the slag phase was basically consistent with the model prediction,which proved that the model is correct.The effect of sample density on the size of metal particles finally obtained was investigated experimentally.The experimental results showed that with the decrease of the density of the carbon-sludge sample,not only the size of the metal particles obtained by the reduction gradually increased,but also the recovery rate of the metal elements also increased.The optimal density condition was 0.53.On this basis,by further improving the experimental conditions such as reduction temperature,it was found that at a semi-melt condition of 1300?,the recovery rates of Fe,Cr and Ni reached 93.6%,84.2%and 91.8%respectively.At the same time,the Cr content in the tailings after magnetic separation was 0.04%,which was in line with the national emission standards for chromium-containing solid waste.