(1) dry method. Alumina is used as adsorbent to produce fluorine-containing alumina, which is directly returned to the electrolytic cell for use. This method is mostly used in pre-baking tank and up-inserted self-baking tank.
(2) Wet method. There are many methods of wet purification and recovery, such as clean water washing, alkali washing and seawater washing. The washing liquid is recovered by alkali method, ammonia method and acid method to prepare cryolite, sodium fluoride and aluminum fluoride. Usually used to make cryolite. Although water washing is the simplest and most economical way to purify flue gas, it is the easiest to corrode equipment because water dissolves hydrogen fluoride into hydrofluoric acid, so many countries adopt alkaline purification and recovery at present.