Preparation of cryolite from overhaul slag by low-temperature sulfuric acid roasting and water leaching

Resource disposal of overhaul slag is an urgent problem in aluminum industry. This paper innovatively proposed the synthesis of cryolite by low temperature H₂SO₄ roasting and water leaching process. Low temperature H₂SO₄ roasting can effectively volatilize fluoride such as Na₃AlF₆, CaF₂ and NaF into the flue gas in the form of HF for recovery, and aluminum compounds (Al₂O₃, Na₃AlF₆) are converted into NaAl(SO₄)₂ (except corundum). The water leaching can effectively extraction the aluminum and sodium compounds from the roasting clinker into the solution. Under the optimal roasting and leaching conditions, the volatilization rate of fluorine, the leaching rate of aluminum and sodium were 99.21 %, 73.95 % and 95.01 %, respectively. Then, the purified HF is passed into the solution to synthesize cryolite. The recovery and grade of cryolite were 98.69 % and 99.34 %, respectively. The technique opens up a new way for the resource disposal of hazardous waste of electrolytic aluminum.