| Cr-bearing steel slag,as the main by-product of the stainless steel electric furnace smelting process,contains 5-10 wt.%of Cr2O3 and a small number of metal oxides including FeO and MnO,which is a kind of solid waste with great recycling value.Moreover,the hexavalent Cr in Cr-bearing steel slag is very easy to dissolve in the environment,which can cause serious harm to the environment and human health.Therefore,the recycling and utilization of Cr-bearing steel slag have faced great difficulties.At present,the traditional recovery methods face problems such as low separation efficiency,uncontrollable reaction endpoints,and Cr in the residue still needs to be solidified,which limits the comprehensive utilization of Crbearing steel slag.Therefore,the efficient solidification and enhanced separation of Cr in Cr-bearing steel slag is the key issue to perform efficient recovery and harmless utilization of Cr resources in the slag,which is of great value to the sustainable development of stainless steel production in China.Based on the key issues of efficient solidification and enhanced separation of Cr in Cr-bearing steel slag,this thesis focuses on scientific issues such as the selective precipitation of Cr.the super-gravity separation of Cr-spinel,the crystal characterization of Cr-spinel,and the dissolution of Cr in Cr-spinel and residue phases to conduct a systematic study and obtain the data on the precipitation,separation,crystal characterization,and dissolution of Cr in Cr-bearing steel slag.In addition,the tentative engineering application for the separation of Cr-spinel in Cr-bearing steel slag was further explored to provide a new idea for the harmless utilization of slag.And the following research results were obtained.Firstly,the phase equilibrium and growth kinetics of different Cr-spinels in Crbearing steel slag were clarified to provide basic data for the separation and recovery of Cr resources from Cr-bearing steel slag.In addition,the precipitation growth law,crystal growth mechanism,and the solidification law of Cr element in Cr-bearing slag with various MgO,FeO,and MnO contents for different types of Cr-spinel crystals were compared.It was found that decreasing the basicity,increasing the MgO.FeO.and MnO contents,and decreasing the temperature of the Cr-bearing slag were all beneficial to the precipitation and solidification of Cr elements in the Cr-bearing steel slag.The growth process of MgCr2O4 is divided into two stage,which lacked the interfacial reaction in non-isothermal condition,and the crystal size of MgCr2O4 is smaller.However,the growth process of MgCr2O4 is controlled by interfacial reaction and cannot be further coarsed at isothermal temperature.In general,the solidification ability order of Cr elements in various Cr-spinel was MgCr2O4>FeCr2O4>MnCr2O4.On this basis,the high-purity Cr-spinel crystals was obtained from the molten Cr-bearing steel slag by using super-gravity,and the chemical composition,crystal structure,and cell parameters of high-purity Cr-spinel crystals were further characterized using Rietveld refinement to clarify the effect of Fe-Mn and Fe-MnMg substitution in Cr-spinel crystals on the solidification and recovery of Cr in Crbearing steel slag and further verify the solidification mechanism of Cr in Cr-spinel crystals.It was found that the substitution of Fe and Mn in Cr-spinel did not affect the occupation of Cr,and the crystal size of Cr-spinel and the recovery ratio of Cr were higher when the ratio of Fe and Mn in Cr-spinel was close to 1.In addition,the substitution of Cr in Cr-spinel by Al were promoted when Fe and Mn were substituted by Mg,and the smaller the crystal size of Cr-spinel and the higher the recovery of Cr with the increase of Mg replacement.In addition,the experimental studies on the selective precipitation and supergravity separation of Cr in Cr-bearing steel slag were carried out to realize the efficient enrichment and separation of Cr in Cr-spinel and the stable discharge of low Cr residue.The effects of slag alkalinity,separation temperature and gravity coefficient on solidification of Cr and separation of Cr-spinel in Cr-bearing steel slag were investigated.It was found that the super-gravity separation of the Crspinel phase was fully achieved at R=1.00,T=1300℃,and G=600,and the mass fraction of Cr2O3 in the residue phase was 0.48wt.%,while that in the Crspinel phase was as high as 19.96 wt.%,and the recovery rate reached 95.24%.Moreover,the leaching amount of Cr elements in both the Cr-spinel and the residue phases were only 0.67 mg/L and 0.13 mg/L,and that in Cr-bearing steel slag without super-gravity treatment was up to 3.92 mg/L which indicated that the full solidification,effective recovery,and harmless treatment of Cr element in Crbearing steel slag were achieved after super-gravity treatment.Finally,based on the research idea of selective precipitation and super-gravity separation of Cr elements from Cr-bearing steel slag,a scaled-up experimental study for separation of Cr-spinel from Cr-bearing steel slag was conducted to verify the feasibility of the engineering application of superg-ravity separation of Crspinel from Cr-bearing steel slag,which provides a new idea to realize the resource utilization and harmless treatment of Cr-bearing steel slag. |
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