| Stainless steel slag,as the main by-product of stainless steel smelting process,is produced in six million tons every year in China.However,due to containing certain amount of chromium oxide in slag,the recycling level of the stainless steel slag in the industry is severely limited.At present,the main treatment method for stainless steel slag is piling up or landfill,which not only occupied the land,but also wasted resources and caused pollution to the environment.Therefore,the development of an effective technology for the stabilization of chromium in stainless steel slag will be of great importance.Meanwhile,it is of great significance to improve the level of resource utilization of stainless steel slag and promote the reform of green manufacturing in the metallurgical industry.Based on this aspect,the study takes the stainless steel slag as research object,and the stability of typical chromium-containing phase in the slag was analyzed systematically.After that,the modification treatment and controlled cooling condition were used to achieve the enrichment of chromium in stable phase and reduce the chromium leaching.Moreover,the stirring force was adopted in the molten slag to improve crystallization kinetics condition and enhance the growth behavior of chromium-enriched phase.Main experimental results and conclusions have been obtained as follows in the study:(1)The existential state and leaching behavior of chromium in stainless steel slag were investigated.When the basicity was 1.0,chromium mainly existed in spinel and glass phase.When the basicity increased to 1.5,chromium could be presented in spinel,glass and dicalcium silicate phase.As the basicity increased to 2.0,besides spinel phase,chromium could also dissolve in periclase phase.Meanwhile,the leaching amount of chromium reached the minimum value of 0.62mg/L with basicity of 1.5.In addition,when unmelted lime existed in stainless steel slag,the chromium-containing periclase and CaCr2O4 phase with weak stability in acidic solution could precipitate at the CaO/slag interface zone,which could become the leaching source of chromium in stainless steel slag.(2)The erosion experiment of stainless steel slag in the sulfuric acid and nitric acid solution with pH value of 3.2 was conducted,and-the leaching mechanism of chromium was revealed combining with thermodynamic calculation.The results indicated that the erosion of phases in stainless steel slag in acid solution was consisted of dicalcium silicate,merwinite,akermanite,periclase,and the erosion reaction was obviously observed on the slag surface.Moreover,the glass phase showed no obvious inhibiting effect on the leaching of chromium.Spinel phase had strong stability in the leaching solution and it was considered as the stable phase for chromium.(3)The influence of modifiers(MnO?Al2O3)on the existing state and stabilbity of chromium in stainless steel slag was investigated.The addition of MnO and Al2O3 could significantly improve the enrichment of chromium in spinel phase and reduce the leaching of chromium.As the quenching temperature is 1600?,when the mass fraction of MnO increased from 0 to 8%in slag,the enrichment degree of chromium in spinel phase could increase from 57.0%to 93.8%.After cooling to 1300?,the chromium was mainly enriched in spinel phase.Meanwhile,the leaching amount of chromium in stainless steel slag dramatically decreased to 0.12mg/L.In addition,when the mass fraction of Al2O3 increased from 4%to 12%,the enrichment degree of chromium in spinel phase could increase from 57.0%to 89.9%with quenching at 1600?.After cooling to 1300?,almost all of chromium was enriched in spinel phase similarly and the leaching amount of chromium decreased to less than 0.01 mg/L.(4)The modification mechanism of MnO and Al2O3 on the stainless steel slag was revealed.It was found that MnO changed into droplets and dissolved into slag after it was added in molten slag,and meanwhile the chromium and other metallic elements migrated from slag into the MnO,accompanied with dissolution and diffusion in slag gradually.The Mn-bearing spinel precipitated at the interface zone between the MnO and slag.When Al2O3 was added in slag,a high Al2O3 concentration zone could form in the region between Al2O3 and slag,and large amount of chromium-contained magnesia-alumina spinel precipitated in this region.In the meantime,at the boundary of intermediate reaction zone in the bulk slag,Cr2O3 together with Al2O3 and MgO formed magnesia-chrome spinel.(5)The effects of cooling condition on the stabilization of chromium and growth behavior of spinel crystal in Al2O3 modified stainless steel slag were investigated.The results indicated that the enrichment degree of chromium in spinel phase increased with the decrease of cooling temperature.When the quenching temperature decreased from 1500?to less than 1300?,the leaching amount of chromium in the corresponding slag decreased from 0.70mg/L to less than 0.01 mg/L.In addition,the cooling condition had a great effect on the growth behavior of spinel crystal.There was a rapid growth trend at the temperature range of 1400?1200? in the cooling process,and the mean diameter of spinel increased from 8.5?m to 27.9?m.Outside the temperature range,the growth of spinel was relatively slow.(6)The influence of stirring on the crystallization behavior of spinel in stainless steel slag was investigated.The results indicated that,when holding 60min at 1500?,the introduction of stirring in stainless steel slag could accelerate the diffusion of basic elements and reduce the number density of spinel crystal comparing with the static condition,which promoted the formation of large size of spinel.As a stirring rate of 50rpm was added in the slag,the mean diameter of spinel could increase from 8.4?m to 11.7?m.However,when the stirring rate was too high,it promoted secondary nucleation and inhibit the growth of the spinel crystal. |
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