Preparation of Aluminum Fluoride from Carbon Residue in Aluminum Electrolysis Cell by Roasting-Leaching Method

The carbon residue is a hazardous waste generated by the aluminum electrolysis production process, which contains a large amount of electrolyte fluoride salts. In order to recover the electrolyte components in carbon residue, this paper proposes a new method of using carbon residue to prepare aluminum fluoride by converting Na5Al3F14 and Na3AlF6 in carbon residue into aluminum fluoride to realize the recycling of fluoride in carbon residue. In the initial step, optimal parameters for carbon removal during roasting have been determined, including an oxygen flow rate of 7 g/min, a roasting temperature of 670 °C, and a roasting time of 80 min. The roasted clinker is obtained, and the removal rate of carbon reaches 99.95% while the loss rate of fluorine is 0.48%. In the second step, the mix anhydrous Al2(SO4)3 with roasted clinker for roasting. The optimal roasting temperature of 670°C, the time of 10 min, and the mass ratio of anhydrous Al2(SO4)3 to roasted clinker of 5:5 could convert fluoride to AlF3 products, with a conversion rate of 99.76% and a loss rate of fluorine of 1.05%. Finally, by removing sulfate impurities through water leaching for 15 min at 35°C and with a liquid-solid ratio of 6 ml/g, the loss rate of fluorine is 3.98%. This process yields AlF3 with a purity of 94.30%. The recovery rate of fluorine in the whole process is 94.56%, which successfully achieves the recycling of fluorine in the carbon residue to obtain AlF3 products.