Applied Basic Research On Recycling And Harmlessization Of Aluminum-chrome Slag

Alumina-chrome slag is a solid slag produced by the aluminothermic smelting of metal chromium.China produces 100,000 tons of alumina-chrome slag each year,and this is a typical source of Cr?VI?-containing solid waste.With the ongoing development of industrial technology,alumina-chrome slag has been gradually applied in the field of refractories since the 1970s.However,an enhanced awareness of environmental protection has led to the understanding that the Cr?VI?present in alumina-chrome slag poses a threat to the environment,thereby restricting development of the alumina-chrome slag recycling process.A suitable and harmless recycling procedure for alumina-chrome slag is therefore required.For the purpose of this study,the basic properties of alumina-chrome slag were studied in detail,and the relationship between the components and properties of alumina-chrome slag was explored.More specifically,the effect of the Cr₂O₃ content in the Al₂O₃-Cr₂O₃ solid solution on the crystal structure was studied,and a relationship between the?104?diffraction angle and the Cr₂O₃ content in the Al₂O₃-Cr₂O₃ solid solution was obtained.In addition,the states and transformation mechanisms of the chromium ions in alumina-chrome slag were explored,and the harmless application of alumina-chrome slag was proposed.Using the standard reaction thermal effect method,we propose and explore the idea of smelting,carbonization,and the reduction of alumina-chrome slag,revealing the thermodynamic principle of chromium separation and impurity removal,and realizing the industrial treatment of alumina-chrome slag resources.Studies into the thermodynamic changes taking place during the Cr₂O₃ carbonization reaction in aluminum chromium slag are also carried out,and the mechanism of Cr7C3 preparation is determined through the melting,carbonization,and reduction of alumina-chrome slag on the basis and process principle.Through application and industrial practice,the contradiction between the utilization of alumina-chrome slag and environmental pollution is addressed,the construction and production of a harmless production line of alumina-chrome slag are completed,and good economic and social benefits are achieved.Overall,the following results were obtained:1.The main chemical composition of alumina-chrome slag is Al₂O₃ and Cr₂O₃,and the main phase composition is an alumina-chrome solid solution,among which the quantities of the CaO and Cr impurities fluctuate greatly.The average bulk den sity of alumina-chrome slag was found to be 3.29g·cm^-3,the average apparent porosity was4.7%,the average true density was 3.81g·cm^-3,the average compressive strength at high temperature?1400??was 83.61 MPa,the average compressive strength at cold temperature was 112.11 MPa,the average thermal conductivity was 4.67 W·m^-1·k^-1,and the average linear expansion coefficient at 1400°C was 9.50×10^-6°C^-1.The properties of the alumina-chrome slag varied depending on the Ca O content of the material.2.Through the use of diphenylcarbazide spectrophotometry,the contents of hexavalent chromium in the alumina-chrome slag,fused chromium corundum,and fused corundum were determined,and the transformation mechanism of Cr?III?to Cr?VI?was analyzed.It was found that the content of hexavalent chromium in the leaching solution of the alumina-chrome slag was greatly affected by different heat treatment temperatures and extractant pH value.In the case of alumina-chrome slag and basic oxides?e.g.,Ca O,MgO,etc.?,heat treatment after mixing can promote the conversion of Cr?III?to Cr?VI?;Al₂O₃ can inhibit the conversion of Cr?III?to Cr?VI?by the solid solution reaction;and TiO₂,Si O₂,and Fe₂O₃ can transform Cr?VI?to Cr?III?by heat treatment after mixing with alumina-chrome slag.Overall,the results showed that the conversion of Cr?VI?to Cr?III?can be achieved by adding an acid oxide or chromite,or by using a reducing atmosphere.The harmless application of Al Cr slag can be realized by selecting a suitable service environment.Indeed,it should be noted that the corrosion resistances of Al Cr refractories tend to be superior to those of of Mg Cr refractories in a copper smelting furnace.3.The thermodynamic behavior of the Al₂O₃-Cr₂O₃-CaO ternary system was studied using thermodynamic simulation software?factsage?.As a result,it was demonstrated theoretically that the reduction and carbonization of chromium oxide and metal chromium took place in the molten solution,and that the regulation of the solid solution of chromium oxide in alumina and the removal of chromium inclusion in the alumina-chrome solid solution were feasible thermodynamically.Through altering the concentration of the reactant"carbon,"the content of chromium oxide in the product of the alumina-chrome slag purification process was regulated,so as to obtain an alumina-chrome solid solution and fused corundum with different chromium oxide contents.In large-scale production,from a kinetic viewpoint,the melting carbonization reduction method can be considered reliable.4.Finally,the industrial experiment showed that the alumina-chrome slag and carbon particles are added in a three-phase arc furnace,and are subsequently carbonized and reduced in the molten state.Separation and enrichment were then achieved based on the density of the reactants and products in the molten liquid.The chromium oxide and metal chromium present in the alumina-chrome slag were carbonized to give a precipitate of"Cr₇C_3,"which dropped to the bottom of the furnace.As a result,the proportion of chromic oxide in the molten solution can be adjusted,and high value-added fused corundum and chromic corundum materials can be obtained.