Aluminium smelting process. Schematic diagram of the production of aluminium metal from aluminium ore (bauxite). The first stage (left) is the Bayer Process to produce alumina (aluminium oxide) from the raw bauxite. Down left from top to bottom: the bauxite is crushed (top); chemically digested in sodium hydroxide (caustic soda, red area); alumina hydrate is precipitated (brown) and then heated to form the pure alumina (yellow). The conveyor belt deposits the alumina in molten cryolite (sodium aluminium fluoride, green, reduces melting point to 1000 degrees Celsius). Electrolysis (right, the Hall-Heroult Process) is used to obtain molten aluminium metal (grey, lower right pipe). The positive and negative electrodes are labelled with electrical symbols.