Research On The Preparation Of Aluminum Fluoride From Hazardous Waste Carbon Slag From Aluminum Electrolysi

Spent carbon anode is a hazardous waste produced during the electrolysis of aluminum.In 2022,China’s electrolytic aluminum production reached 40.214million tons,resulting in direct economic losses of over 100 million yuan due to carbon slag.However,the country’s requirements for industrial hazardous waste treatment have increased,making the recycling of carbon slag even more urgent.If the high value-added harmless recycling of valuable fluoride in carbon residue can be realized,it is of great significance to the sustainable development of China’s aluminum industry.This article proposes a new approach to the preparation of aluminum fluoride using carbon slag,which converts the electrolyte in carbon slag into Al F₃,thereby achieving high value-added recycling of carbon slag.The specific research results are as follows:（1）Thermodynamic calculations were conducted on the reaction occurring in the wet leaching of carbon slag based on the property that aluminum ions in the solution can leach electrolytes.The electrolyte in the carbon slag was leached to obtain the optimal leaching conditions.The leaching rate reached 95.42%with an aluminum ion concentration of 2 mol/L,leaching temperature of 90℃,stirring speed of 600 r/min,liquid-solid ratio of 20:1,leaching pH=0.5,and leaching time of 2 hours.Adjust the pH of the leaching solution to obtain the product hydroxyaluminum fluoride monohydrate,and prepare aluminum fluoride through calcination.The purity of the obtained product aluminum fluoride is 63.23%,and the alumina content is 36.61%.It can be directly returned to the electrolytic cell for use,with a fluorine recovery rate of 86.21%.（2）The one-roasting method adopts the method of mixed oxygen roasting of carbon slag and aluminum sulfate,which can effectively convert the electrolyte into aluminum fluoride.In the research process,it was found that when the temperature exceeded 670°C during the roasting process,the fluorine volatilization rate in the carbon slag increased,and when the oxygen content was insufficient,aluminum sulfate reacted with carbon to form alumina.The optimal roasting conditions are:670°C,roasting time 30 min,mass ratio of aluminum sulfate/carbon slag is 3,and incoming oxygen flow rate is 7 g/min.The best leaching conditions were as follows:leaching temperature 35°C,leaching time 15min,liquid-solid ratio 3 m L/g.The purity of aluminum fluoride is 52.49%,the alumina content is 57.12%,and the recovery rate of fluorine is 90.94%,and the product can be directly returned to the electrobath for use.（3）The two-roasting methods are first decarburized and roasted by oxygen,and then converted roasted by adding aluminum salt,which can reduce the formation of alumina from the source,so as to prepare aluminum fluoride with higher purity.The optimal reaction conditions were as follows:one roasting temperature 670°C,roasting time 80 min,oxygen flow rate 7 g/min.The secondary roasting temperature was 670°C,the roasting was 5 min,and the mass ratio of aluminum sulfate to roasted residue was 1.The optimal leaching conditions were 35°C,liquid-solid ratio 6 m L/g,leaching for 15 min.Aluminum fluoride with a purity of 94.28%and alumina of 4.38%is obtained,which meets the national standard GB/T 4292-2017 for industrial aluminum fluoride.（4）Comparing the three preparation methods,the wet leaching method has low experimental temperature and is energy-saving and environmentally friendly.However,during the leaching process,there are still some electrolytes that cannot be leached,and the alumina content in the aluminum fluoride obtained after calcination is relatively high;Although the one-time roasting method can directly remove carbon and recover refractory calcium fluoride during the roasting process,with a short process and low energy consumption,it will generate sulfur dioxide and sulfur trioxide gases during the roasting process,which will cause significant corrosion to the equipment,and the aluminum fluoride obtained has a high content of aluminum oxide.The twice roasting method can prepare aluminum fluoride with a purity of over 94%,which is higher in purity compared to other methods,but with relatively higher energy consumption.This paper provides theoretical guidance and technical support for the recycling and utilization of carbon slag.