| The physical properties of stainless steel slag from a domestic steel paint were analyzed.The results showed that the slag is mainly composed of dicalcium silicate,merwinite and matrix phase.And it also contains low content of periclase,magnesium chromium spinel and calcium aluminate,etc.The chromium content in steel slag was 3.9%(Cr2O3).Chromium mainly exists in magnesia chromium spinel phase,metal phase,periclase phase and part of silicate phase.The results of TCLP toxicity leaching experiment showed that the leaching concentration of chromium in stainless steel slag was higher than the upper limit value of environmental requirement.Thus,it has environmental risk.The powder roasting followed by aqueous leaching method was used to extract chromium from stainless steel slag.The results showed that chromium extraction by NaOH roasting was significantly higher than that by Na2CO3 roasting under the same roasting temperature,sodium-to-chromium mole ratio and roasting time.When roasting temperature was 800℃,mass ratio of base-to-slag was 0.6,roasting time was 30 min,the chromium extraction by NaOH powder roasting reached 76.5%.Chromium extraction by pellet roasting increased to 85.7%under the same reaction conditions.The optimal conditions for chromium extraction from stainless steel slag by pellet roasting were temperature of 800℃,base-to-slag mass ratio of 0.6 and roasting time of 120min,and chromium extraction was 96.0%under these conditions.When the pellet consisted of stainless steel slag and NaOH was roasted under 500-700℃.the phase containing chromium was Na3CrO4 in the products.While the pellet was roasted under 800 ℃,the phase containing chromium was CaCrO3,After sufficient extraction.chromium in aqueous leaching residue mainly distributed among Fe-Cr-O phase.The results of TCLP toxicity leaching experiment showed that the residue was friendly to the environment.Compared with the original stainless slag,the silicon content in the residue decreased,but the available silicon increased.The leaching process resulted in silicon loss.The vanadium slag are mainly consisted of(Mn,Fe)(V,Cr)2O4,(Fe,Mn)2SiO4 andFe2.5Ti0.5O4 which wraps each other.Meanwhile,the slag also contains low content of Fe3O4,Ca2Si04 and CaAl2Si2O8.Vanadium and chromium mainly exsited in the spinel phase,(Mn,Fe)(V,Cr)2O4.The optimal conditions for extracting vanadium from vanadium slag by the three-phase roasting method:roasting temperature 700℃,roasting time 15min,base-to-slag mass ratio 0.5.Under these conditions,the vanadium extraction rate was 99.2%.The optimal conditions for chromium extraction:roasting temperature 700 ℃,roasting time 120min,base-to-slag mass ratio 0.5.Under these conditions,the chromium extraction rate was 97.5%.When the pellet was roasted under 600℃ for 5min,the products containing vanadium were(Cr0.15V0.85)2O3 and NaVO2 and chromium mainly exsited in trivalent oxide solid solution and sodium slat.When the pellet was roasted under 700℃ for 120min,NaV02 was trandformed into Na3VO4 and chromium exsited in Na3CrO4.The pellet roasting process involves a gas-liquid-solid three-phase reaction.Mesoscale porous structure created by briquetting not only narrows the distance between solid and liquid phases,but also improves the transfer conditions of the two phases.And the extractions of chromium and vanadium were strengthened.The NaOH transfer approaches include percolating thourgh the interspace between slag particles and diffusing in the slag particles.An unreated core model containing gas-liquid-solid three phases was created in this study.The experimental kinetic datum were fitted and the roasted process samples were analyzed.It was found that the three-phase reaction rate was controlled by NaOH percolating thourgh the interspace when the roasting temperature was in the range of 400-600 ℃,while the rate was controlled by NaOH diffusion in the slag particles when the temperature was 600 ℃.Vanadium oxidation was prior to chromium during the extraction from vanadium slag by the three-phase roasting method.And this gave rise to the extraction difference between vanadium and chromium under the same conditions. |
