Study On The Separation Of Carbon And Electrolyte By Vacuum Distillation Of Electrolytic Aluminum Waste Cathode Carbon Block

The output of electrolytic aluminum in China in 2019 is 35.04 million tons,which will generate about 1.4 million tons of waste cathode carbon blocks.At present,domestic and foreign methods for processing waste cathode carbon blocks can be divided into three categories:harmless treatment,harmless utilization and comprehensive utilization,but these methods do not make full use of valuable substances and HF and other harmful gases are generated at high temperatures.In view of the above problems,this paper explores a method of harmlessly comprehensively utilizing the valuable components of the waste cathode carbon block by vacuum distillation,which provides a new idea for the treatment of waste cathode materials in aluminum electrolytic cells.In this paper,the waste cathode carbon block of an aluminum electrolysis production enterprise of Yunnan Aluminum Co.,Ltd.was used as a raw material.X-ray diffraction analysis and element analysis found that it contained carbon,sodium fluoride,cryolite,calcium fluoride,alumina and Silica and other substances.According to the calculation of its nature and saturation vapor pressure,it is determined to use vacuum distillation to separate the carbonaceous material and the electrolyte material and use them separately.(1)Vacuum distillation experiments have shown that:after vacuum distillation,only carbon and calcium fluoride remain in the residue;the condensed substance contains cryolite,sodium fluoride,aluminum and silicon,and their oxides.The removal of sodium and fluorine is due to the volatilization of sodium fluoride and cryolite.When distilled at 1350?for 2h,the removal rate of sodium has reached more than 99%,and the removal rate of fluorine has reached more than 97%.The temperature and prolonged time of sodium and fluorine removal rate increase slowly,but the calcium removal rate has been below 0.5%.The residue contains a large amount of carbon,which can be added as a raw material to the production process of the cathode carbon block;the volatile condensate is mainly Na F,Na₃Al F₆,Al and Al₂O₃,which can be added as an electrolyte back to the electrolytic cell.(2)The removal of aluminum below 1350?is due to the volatilization of cryolite;at1350?and 1400?,Aluminum oxide reacts with carbon or aluminum carbide and silicon dioxide to form Al₂O gas,thereby removing aluminum.When distilled at 1350°C for 5 h,the removal rate of aluminum can reach 74.9%,and the removal rate does not increase significantly when the distillation temperature is increased.The removal of silicon is due to the reaction of SiO₂with C to form SiO gas,which removes silicon.The initial temperature at which SiO₂reacts with C to form SiO gas is only 1183°C,which is below 1200°C,so the silicon removal rate is above 99%at all temperatures throughout the experiment.(3)In order to meet the recycling requirements,the aluminum in the waste cathode needs to be reduced to below 0.6%,which cannot be achieved only by vacuum distillation,and additives are added to remove aluminum.When silica is used as an additive and the addition amount is6wt%,the removal rate of aluminum distilled at 1350?for 5h is increased to 94.5%,and the aluminum content in the residue is 0.4%.When using ammonium bicarbonate and glucose as the pore-forming agent,when the addition amount is 6wt%,the aluminum removal rate is increased to96%and 91.39%after distillation at 1350?for 5h,and the aluminum content in the residue is0.3%and 0.55%,respectively.The carbon content in the residue increased to 89.68%and 89.43%,respectively.By comparison,it is found that the pore-forming effect and the dealumination effect of adding ammonium bicarbonate are better than adding glucose.The best experimental conditions obtained through research are:adding 6wt%ammonium bicarbonate,the distillation temperature is 1350?,the distillation time is 5h,the system pressure is 10-30Pa,and the removal rate of sodium,fluorine and silicon is 99.9%,the content of sodium,fluorine and silicon in the residue is less than 0.1%;the removal rate of aluminum reaches 96%,the content of aluminum in the residue is 0.3%;the removal rate of calcium is less than 0.5%,and89.68%can be obtained of carbon.In this study,the carbon and electrolyte in the waste cathode carbon block were effectively separated and can be used separately,achieving the purpose of comprehensive recycling.