Separation And Recovery Of Cr From Typical Solid Waste Of Stainless Steel Industry And Control Of Reduction Reaction End Point

China's stainless steel industry is facing two major problems on its sustainable development:(1)the most important alloy element chromium resources are basically dependent on imports,so it is necessary to recover the chromium scattered in typical solid wastes from stainless steel industry;(2)environmental protection policies strictly restrict the leaching of carcinogenic heavy metal ions Cr6+ into the environment from solid wastes of stainless steel industry.High temperature reduction technology of Cr-containing solid wastes self-reduction briquette is a powerful method for their large-scale comprehensive treatment in stainless steel enterprises.It fully separates chromium in solid wastes in the form of ferroalloy from low chromium slag at the end of reduction,thus realizing the recovery of secondary resources.However,the large-scale application of efficient and clean solid wastes treatment process for Cr-containing wastes has not yet been achieved in China.Some imported and self-developed high-temperature treatment technologies still have problems such as unstable recovery rate of Cr,high residual Cr in final slag,and inability to flexibly take into account the composition requirements of Cr-containing slag when making materials.This is attributed to the inability to fully grasp the separation behavior of Cr and the precise control of the reaction end point in high temperature reduction of Cr-containing solid wastes.Around the "efficient separation of chromium" and "control of reaction end point",the author first analyzed the properties of Cr-containing metals and slags by means of thermochemical software FactSage,and calculated the nephogram of the lowest chromium content in slag under different reaction conditions based on a new approach.At the same time,the viscosity of Cr-containing slag at higher temperature(1540-1680?)was measured by a new high temperature viscometer,and the properties of Cr2O3 and CrO in the slag were thoroughly understood.Based on the above end point control objectives,the key parameters and related mechanisms affecting the separation and recovery of Cr from Cr-containing solid wastes self-reduction briquettes were studied.Finally,the experiment of recovery of Cr from solid wastes on Kg scale was completed and the technical treatment route of Cr-containing solid wastes was preliminarily designed.The main conclusions are as follows:(1)The increase of[C]content in Fe-Cr melt can not only significantly reduce the activity coefficient of Cr,but also change the reaction atmosphere into strong reducibility,which is very conducive to the recovery of Cr from Cr-containing solid wastes.In oxidizing atmosphere(PO2=10-4 Pa),Cr in slag is mainly Cr3+;in reducing atmosphere(PO2=10-10 Pa),Cr in slag is mainly Cr2+;compared with MgO and Al2O3,basicity(R=(%CaO)/(%SiO2))has significant effect on liquid phase ratio of slag and activity of chromium oxide.(2)Thermodynamically,compared with temperature,MgO and Al2O3 content,metal composition and slag basicity significantly affect the lowest Cr content in slag.When Fe-xCr-4.0C([Cr]:6-24%)and CaO-SiO2-8%Mg0-CrOx slag(R:1.0-1.8)were balanced at 1600?,the lowest Cr content in slag was less than 0.71%,and the lowest Cr content in slag was less than 0.3%when the basicity was greater than 1.4.When Fe-xCr-yC([C]?4%,[Cr]<24%)was balanced with CaO-SiO2-8%MgO-5%Al2O3-CrOx(R=1.4)at 1600?,the lowest Cr content in slag was less than 0.23%.(3)The viscosity of CaO-SiO2-10%Al2O3-based slag decreased with the increase of Cr2O3 content at 1540-1680?,and the effect of Cr2O3 on reducing viscosity was more obvious in acidic slag(R=0.8).Cr2O3 shows basic in dissociated state(Cr3++O2-).With the increase of Cr2O3 content,BO/Si decreased and ? increased respectively,and the variation of degree of polymerization of slag was consistent with that of viscosity.The viscosity of CaO-SiO2-CrO(R=0.5)slag decreased with the increase of CrO content,while NBO/Si and A increased with the increase of CrO content at 1550-1675?.The variation of degree of polymerization of slag was also consistent with that of viscosity.CrO has strong basicity.The slag exhibited Newtonian fluid characteristics at high temperature,and the viscosity-temperature relationship conformed to the Arrhenius equation.(4)The reduction rate of Fe-xC metal to stainless steel slag was less than 40%at 1550?.When self-reduction(adding C,Si and Al)was introduced into stainless steel slag,the reduction rate of Cr increased to more than 90%,and the reduction ability of C,Si and Al increased in turn.When C was used as the main reducing agent,the reduction of chromium could be promoted by adding a small amount of SiFe.The residual Cr in the slag was significantly affected by basicity,but not Al2O3 content.The optimum basicity of the final slag was 1.2.The residual Cr in the slag decreased with the increasing temperature,which mainly improved the reaction kinetics.(5)The amount of C in the Cr-containing solid waste self-reduction briquettes should be excessive on the basis of nc/no=1 according to the combined oxygen of reducible oxides such as FeOx,Cr2O3,MnO and so on;when the amount of C was appropriate and the basicity of the self-reduction briquettes was 1.05-1.20,it was beneficial to the full reduction of Cr and the efficient separation of slag and metal.The higher the amount of Fe2O3 added in the self-reduction briquettes resulted in the lower Cr/(Fe+Cr)and higher recovery rate of Cr.At 1550?,the Cr/(Fe+Cr)of self-reduction briquettes should not be too high and kept below 0.23.The metal bath was helpful for the recovery of Cr from the self-reduction briquettes with higher Cr/(Fe+Cr).The reason that Fe2O3 can improve the recovery efficiency of Cr from Cr-containing solid wastes was that a large number of spherical Fe-C alloy droplets formed by Fe2O3 at relatively low temperature could in situ dilute the reduction products(Cr or CrxCy),reduce the activity of Cr and promote the reduction reaction of chromium oxide.(6)The Cr recovery experiments at Kg scale showed that when the amount of C was sufficient and the basicity was 1.0-1.2 for the Cr-containing solid waste self-reduction briquettes at 1650?,SiFe was used to reduce the slag in the heat preservation stage,and CaO and CaF2 were combined to adjust the final slag basicity to 1.8 with good fluidity,the recovery rate of Cr was over 96%,and Cr2O3 in final slag was as low as 0.11%.(7)Based on the above results,the technical route of recovering Cr resources from typical solid waste of stainless steel industry by high temperature reduction method was designed,which can be used to guide production practice.