Comprehensive Recovery And Utilization Of Electrolytes In Aluminum Electrolysis

In aluminum industrial electrolyzer,Al2O3 as the raw material of aluminum electrolysis contained Na2O,and Na2O reacted with AlF3 to form excess electrolyte,named cryolite(Na3AlF6).The fluoride in excess electrolytes was extremely harmful to the atmosphere,water,and soil.Traditional treatment methods of stack and landfill could not meet the request of environmental protection.At present,no suitable technique to treat the excess electrolyte was existed,which was an urgent issue for the sustainable development of the aluminum industry.In this paper,a new method was proposed for leaching cryolite to recycle fluorine by sulfuric acid-aluminum sulfate solution.Several impact factors,including leaching temperature,time,pH value of acid solution,liquid-solid ratio,and fluorine-aluminum ratio,were systematically investigated.The compositions of solid product and solution after leaching were analyzed to determine the dissolution and leaching behavior of cryolite.A controlled process of leaching was achieved under the guidance of reaction mechanism.The leaching process includes two steps:dissolution and precipitation.The solubility of cryolite in acid solution increases with temperature.Petal-like chiolite(Na5Al3F14)and spherical aluminum hydroxide fluoride(Al2(OH)3F3·(H2O)0.375)are obtained at 40℃ and 80℃,respectively.Under the condition that leaching temperature is 95℃,leaching time is 4 h,pH is 3,liquid-solid ratio is 8,and aluminum-fluoride ratio is 3,the precipitated phase is pure aluminum hydroxide fluoride with leaching rate of 62.3%.In case of the concentration of aluminum ions in the solution is high enough,the binding capacity of aluminum fluoride formed under acidic conditions to hydroxyl group is higher than that of hydrogen ions,so fluorine is recycled in the form of aluminum hydroxide fluoride.After leaching,the fluorine is secondary recycled via a neutralization reaction of the filtrate and NaOH solution(1 mol/L).Aluminum hydroxide fluoride begins to precipitate when the pH of solution reaches 3.5.The grain size of aluminum hydroxide fluoride obtained by neutralization reaction is about 200 nm.The leaching technology by sulfuric acid-aluminum sulfate solution can effectively treat the electrolyte of industrial aluminum electrolysis.The recovery rate of single industrial electrolyte mainly consisting of cryolite is 60%.For the complex industrial electrolyte consisting of alumina,cryolite and calcium fluoride,the process of fluorine recovery includes two steps:dissolution at 60℃ and leaching at 95℃.Aluminum hydroxide fluoride is treated at high temperature to obtain alumina and aluminum fluoride,which can be again used in industrial aluminum electrolysis.This study provides a new technology to avoid the harmful accumulation of excessive electrolyte,and realize the comprehensive recycling of cryolite.