Study On The Reduction Of Depleted Oxygen-rich Top-blown Smelting Copper Slag From Kitchen Waste Oil

The top-blown copper smelting technology sprays oxygen-enriched air into the melt layer to increase the driving force of sulfur-oxygen transfer,which makes the solid raw concentrates react strongly with gas-liquid phase for realizing the efficient production of matte.This process is in line with the development direction of modern non-ferrous metal smelting due to its simplicity and high efficiency.In the process of high oxygen potential smelting,iron is removed by oxidation to enhance the matte grade.The excessive oxidation of partial iron phase leads to the precipitation of Fe₃O₄ phase（high melting point phase）in the melt,and the viscosity of copper slag increases as a result.Furthermore,the top-blown copper smelting process taps the mixture of matte and slag periodically.Therefore,copper slag cleaning in an electric furnace becomes the critical technology,aiming at reducing Fe₃O₄ and copper content in the slag.Reduction is the most direct way to decrease Fe₃O₄in the slag,which can be transformed into FeO phase and then combined with SiO₂ to form fayalite phase[Fe₃O₄→FeO（+SiO₂）→Fe₂SiO₄]for promoting matte separation from slag.Aiming at the issues of non-renewable reductant resources and carbon neutralization,using waste cooking oil（WCO）as a green reductant was proposed in present.The effect of iron phase evolution on copper slag cleaning,high temperature cracking and reduction behavior of WCO,and the mechanism of copper slag cleaning by using WCO as a reductant are systematically investigated.The main innovative research results are as follows:The process of copper slag cleaning via a reduction method in an electric furnace was analyzed theoretically,and the fundamental reason why the matte/copper sedimentation result changes with the iron phase evolution is clarified.As a matter of fact,matte/copper particles can be naturally settled by the surface force and volume force in the molten slag,achieving the copper slag cleaning.Copper slag contained high Fe₃O₄ content however would form foaming slag,which prevents the sedimentation of matte/copper particles.Commonly,transforming the Fe₃O₄ phase into Fe₂SiO₄ can reduce the viscosity of the copper slag,which accelerates the terminal settling rate of particles thereby promoting matte sedimentation.When Fe₃O₄ is over reduced to form metallic iron,a high melting point Cu-Fe alloy（Fe>91.56 wt.%）withγ-Fe as the substrate would be precipitated,resulting in the formation of foaming slag again;and the sedimentation condition of matte/copper particles deteriorates as a result.The synergistic actions of cracking and reduction of the WCO in copper smelting slag system have been proved.Small molecular substances,such as H₂,CO,CH₄,CO₂ and C,are the main cracking produces of WCO at cleaning temperature.The results show that the conversion rates of H and C elements to H₂,CO,CH₄ and CO₂ are 88.1%,11.1%,8.0%and 0.6%,respectively,at 1250oC.Fe₃O₄ can catalyze the WCO cracking and provide oxygen for the cracking system simultaneously,thereby reducing the carbon deposition rate.In addition,carbon produced by cracking has a good reducibility;it could participate in the reduction of Fe₃O₄ in the reaction system,and then the conversions of CO and CO₂ increase.The research on the reduction behavior of WCO indicates that WCO participates in the reduction process indirectly.It mainly provides the reductants for the reaction system,namely,small molecule reductive products（H₂,CO,CH₄ and C）which produced by cracking are the direct reductants to reduce the metal oxides.Copper smelting slag cleaning is preliminarily realized through systematic researches,and the corresponding mechanism of the cleaning process by using WCO was revealed.In the slag cleaning process,the WCO cracking occurs to produce H₂,CO,CH₄,C firstly,and then these products are used as direct reductants to reduce Fe₃O₄ in the slag.It was found that the majority of matte particles would settle to the bottom of the molten pool during the reduction stage;while the rest of the small particles separated at the sedimentation stage due to its slow terminal sedimentation rate.When the WCO is injected into the internal slag layer,the reaction kinetics conditions are strengthened by enlarging the reaction interface between the reductants and components within the slag.Therefore,the efficiency of copper slag cleaning and the utilization ratio of the WCO can be improved.After reduction,the copper in the slag distributes unevenly,which increased gradually from top to the bottom slag layer.The magnetite content in the 300 g slag can be reduced to approximately 3.2 wt.%by injecting 2.2 mL of WCO at 1250oC.After sedimentation for 60 min followed by slow cooling,the copper contents in the upper,middle and lower slag layer are 0.55 wt.%,1.68wt.%and 10.89 wt.%,respectively.Slow cooling is conductive to the precipitation of spinel phase（Fe₃O₄/FeAl₂O₄）in the slag.Increase in spinel content makes the wetting phenomenon occur easily as the spinel contacts with matte particles,leading to the increase in copper content in the slag.Copper slag can maintain its melting structure by rapid water-cooling,which can avoid the precipitation of the spinel phase effectively.Under the above-mentioned experimental parameters,the results demonstrate that the copper content in the upper,middle and lower slag layers upon water-cooling decreases to 0.56 wt.%,0.55 wt.%and 1.21 wt.%,respectively.The copper contents in the middle and upper layers of the slag reach the level of the industrial slag discarding with a value of less than 0.57 wt.%.Thus,the copper slag cleaning can be preliminarily realized based on the technical route proposed in present.The critical scientific issues in the process of copper slag cleaning by using WCO as a reductant are investigated systematically.The results obtained in present provide a theoretical support for the cleaner production of copper slag cleaning;and it also offers a new approach for the utilization of WCO.