Bench Scale Electrolysis of Alumina in Sodium Fluoride - Aluminum Fluoride Melts Below 900°C

Abstract

Current efficiency as high as 92% was obtained in bench scale electrolysis of alumina in sodium fluoride - aluminum fluoride bath with weight ratios from 0.65 to 0.80 at temperatures from about 730 to 900°C. Special aluminas calcined at low temperatures had to be used to obtain rapid solution of the alumina at these temperatures. A solid cryolite layer slowly deposited on the cathode unless the bath temperature was maintained about 100°C above the liquidus temperature of the bulk sodium fluoride - aluminum fluoride composition. The carbon anodes were attacked during anode effect, sometimes swelling, dusting, and evolving larger than normal quantities of carbon tetrafluoride. Addition of 5 wt. % LiF to the bath gave more normal anode effects. Carbon consumption approached the theoretical 0.33 g C/g Al when the alumina content was maintained at high levels and moisture additions were made to the cell atmosphere.