| The rapid development of electrolytic aluminum industry has led to the continuous increasing of prebaked anode production.Hazardous solid waste,more than3%of prebaked anode production,was generated during the production of prebaked anode.It is easy to produce dust discharge in the open storage or low price export,which will not only cause secondary pollution on the environment but also make it difficult to utilize the secondary resources.In this paper,the dust powder collected from the carbon block of a carbon company roasting workshop was used as the experimental raw material.The technical ideas were proposed:the powder is firstly purified by mixing acid or alkali acid,and then the high purity of qualified materials could be returned into the raw materials,ingredients and filling materials,production process.The composition and phase analysis showed that in the carbon block,the carbon content is 75.17%,which is in the form of graphite;the ash content is 14.20%with the main elements of silicon,aluminum,calcium and iron oxide.The occurrence phases are mullite,quartz,mica and hematite.Graphite distributed as a scale-like superposition and ash was dispersed between graphite particles with a small amount embedding in graphite matrix.Hydrofluoric acid and hydrochloric acid are selected as leaching agents by mixed acid method.Based on the analysis of experimental principles and combination of single factor and orthogonal experiment,the optimum combination of experiment is:the ratio of the mixed acid to the liquid is 4.5m L/g,the volume of HF is 40%,the leaching temperature is 40~oC,and the stirring time is 30min.The fixed carbon content is increased from 75.17%to 97.13%,the ash content decreased from 14.20%to 0.49%,and the microstructure of graphite in the raw material is not destroyed during the experiment.Through the range analysis,it is determined that the primary and secondary order of the effects of various factors on the purification effect is as follows:liquid-solid ratio>HF volume content>leaching temperature>stirring time.The optimal experiment was repeated and the content of the purified carbon was more than96.4%,which met the requirement ofthe composition range of the raw material powder produced by prebaked anode.The leaching process of NaOH activated roasting-water-hydrochloric acid leaching and washing are selected in the alkali acid method.In the condition of obtaining better hydrochloric acid leaching parameters in mixed acid method,the study of thermodynamics and process parameters of the roasting process are focused on emphatically.The various reactions that may occur in the process of alkali roasting and Gibbs free energy and standard enthalpy change were analyzed as well.The equilibrium product of the roasting process was predicted by Fact Sage.Single factor experiments were carried out to investigate the effects of Na OH content,calcination temperature and baking time on ash removal.The orthogonal design was applied to optimize the technological conditions.The results showed that the carbon content increases from 75.17%to 95.81%,while the ash content can be reduced from 14.20%to 2.68%at the ratio of solid to liquid 4.5mL/g,acid leaching temperature of 50~o C,acid leaching time of 30min,the concentration of HCl of 0.5mol/L,the quality of Na OH/raw materials of 25%,calcination temperature of 800~oC and the roasting time of 60min.The purification material is equipped with the return of prebaked anode production as filling materials or ingredients.It seems that Both the mixed acid method and alkali acid methods work on the purification of dust.However,the mixed acid method is much better as the process flow is shorter in comparison of alkali acid method.The purifying material can be used as the raw material,filling material or ingredients reusing of the prebaked anode,high carbon graphite for solid lubricants,refractory materials and other fields,which explores the way out for the reutilization of the powder solid waste after the purification. |
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