| During the production processing of stainless steel,a black oxide layer will be formed on the surface of stainless steel.It is necessary to remove the oxide layer on the surface of stainless steel by pickling.After pickling,the surface of stainless steel will be washed,pickling wastewater was produced.Global stainless steel production in 2018 reached 50.72 million tons(from International Stainless Steel Forum,ISSF),per ton of pickling stainless steel can produce 1～3 cubic meters(m3)stainless steel pickling waste water,is expected to reach 50.5～153 million cubic meters of pickling waste water.Pickling wastewater is usually treated by calcium neutralization precipitation method,the pickling sludge is hazardous waste(HW17).The neutralization sludge treatment technology mainly includes solidification,landfill,heat treatment and so on.High temperature reduction has the problems of high reaction temperature and high energy consumption.The purpose of this work is to develop a new process of harmless treatment and recourse utilization of stainless steel pickling wastewater.Based on the technologies of calcium and sodium synergistic neutralization and precipitation,direct reduction and magnetic separation,this process provides a new way for the efficient utilization of metallic elements and fluorine in the stainless steel pickling wastewater.The main work is as follows:(1)The process and mechanism of calcium-sodium synergistic neutralization was systematically studied.The relationship between CaO content and fluorine ion removal rate and pH,as well as the relationship between metal ion removal and pH were explored,the reason for the large production of neutralizing sludge was founded.Developed a new process of treatment of pickling wastewater by calcium and sodium co-precipitation,reduced the sludge production by 14.79%.and realized the standard discharge of wastewater.(2)The direct reduction and magnetic separation process of coal-based medium was adopted to realize the one-step harmlessness and resource recovery of stainless steel pickling sludge,Iron,chromium and nickel alloy powder and regenerated fluorite ore were obtained by reduction,fine grinding and magnetic separation of sludge produced through the new process of calcium and sodium Co-precipitation.The effects of dosage of reductant,reduction temperature,reduction time and grinding fineness were studied.The results show that the metallization and reduction of Fe-Cr-Ni at 1200℃ was carried out,with 3h reductant time and 15%dosage of reductant.The alloy powders which with the metal grade of 93.62%(mass fraction)and the regenerated fluorite were obtained.The grades of Fe、Cr、Ni in the alloy powders reached 69.31%(mass fraction),7.6%(mass fraction)and 16.71%(mass fraction),respectively.The recovery rates were as high as 95.30%,88.7%and 97.53%,respectively.(3)The factors affecting the growth of metal alloy particles were systematically studied,including the melting effect of flux Na2CO3,the pressure of building blocks,the dosage of reductant,the reduction temperature,the reduction time and so on.The Results showed that sludge briquetting,adding flux,increasing dosage of reductant,increasing reducing temperature and prolonging reducing time can promote the growth of metal particles,thus contributing to the subsequent grinding separation of metallic phase and non-metallic phase,among which the reduction temperature and the amount of flux have the most obvious influence on the growth of metal particles.from 6 μm at lower temperature to about 200μm at the optimal condition.(4)The regularities and mechanism of Cr reduction in sludge were found out.Through the reduction thermodynamics analysis of Fe,Cr and Ni,the corresponding test results were compared,and it was found that the complete metallization temperature of Cr in sludge was lower than the reduction temperature reported in theory and literature.The synergistic promotion effect of Fe and Cr in the reduction process was determined,which can reduce the reduction temperature of chromium and realize the low-temperature complete metallization and reduction of Cr elements.(5)The whole process was analyzed.The results showed that the neutralization water was standard discharge and the leaching toxicity of fluorine-containing tailings,the whole process is energy-saving,safe,green,which is an effective new process to utilize stainless steel pickling sludge. |
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