| At present,the Na3AlF6-Al2O3 molten salt electrolys method is the only industrial method for preparing metal aluminum in modern times.However,in the aluminum electrolysis production process,due to the selective electrochemical oxidation of the carbon anode,side residue,carbon cathode spalling and secondary occurrence in the electrolyte Reaction,carbon residue in the aluminum electrolytic cell will inevitably be produced.In 2019,our country’s primary aluminum output was as high as 35.04 million tons,so its carbon residue production was about 350,000 tons.The carbon residue of aluminum electrolytic cell is a kind of hazardous waste produced in the smelting and production process of aluminum industry.Large piles of carbon residue waste electrolyte resources,but also cause pollution of the atmosphere,soil,and water bodies,and even threaten people’s health.A lot of research work has been carried out on the recovery of carbon residue.The publicly reported carbon residue recovery methods mainly include flotation method,vacuum smelting method and roasting method,all of which have not been industrialized due to various restrictive factors.In this study,the feasibility and process of recovering carbon powder and cryolite by using carbon residue as raw material and sodium carbonate as additives were studied.The main research results are as follows:Firstly,the properties of carbon residue and the effect of salt-free roasting of carbon residue were studied by means of XRD,ICP,SEM-EDS and TG-DSC analysis.The results show that the main phase composition of carbon residue is C,Na3AIF6,NasAl3F14 and a small amount of Al2O3.Among them,the mass fraction of Na,Al,and F elements accounted for about 62%,and the mass fraction of C element accounted for about 30%.When using the salt-free roasting method to treat the carbon residue,the carbon has a limited degree of oxidation at a roasting temperature lower than 1000℃ and cannot be completely separated from the electrolyte.Secondly,the thermodynamics of the main reaction Na2CO3-Na3AIF6 system of carbon residue roasting with salt was studied.The results show that Na2CO3 and Na3AIF6 completely react to produce NaAIO2.The reaction process is carried out in two steps.The first step:Na2CO3 and Na3AlF6 produce Al2O3;the second step:Al2O3 continues to react with Na2CO3 to produce NaAlO2.According to thermodynamic analysis,the second step reaction requires the highest temperature,which is the control step of the Na2CO3-Na3AlF6 reaction.The apparent activation energy of the reaction is 795kJ/mol,and the reaction order is 0.095.Mix sodium carbonate with a mass ratio of 2.5:1 with carbon residue.After roasting at 950℃ for 2h,the alumina,cryolite and cryolite in the carbon residue are consumed by sodium carbonate.After roasting,the mixture consists of C,Na2CO3,NaF,NaAlO2 composition.After roasting,the mixture is leached for 1 hour under the conditions of pH 13 and leaching temperature of 25℃.The leaching residue after solid-liquid separation is washed with water and dried to obtain carbon powder with a purity of 89%.Properly increasing the roasting temperature and extending the holding time can improve the separation efficiency of carbon and electrolyte.Finally,in order to more conveniently explore the influence of the leaching experiment parameters on the F removal rate of fluorine-containing wastewater,a mixture of hydrofluoric acid,sodium hydroxide,sodium aluminate and deionized water was used to simulate alkaline fluorine-containing wastewater.The optimal process parameters for the recovery of F from alkaline fluorine-containing wastewater are:the solution Al/F ratio is 1:6,the initial pH of the solution is 6.2,and the crystallization temperature is 20℃.Under these conditions,the recovery rate of F can reach 98%.Using carbonation method to recover F-in the leaching solution,the cryolite produced is composed of cubic-shaped particles,the surface of the particles is smooth and irregular;most of the particle size is less than 1μm,and there is obvious inter-particle agglomeration. |
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