Bayer process

The Bayer process is also the main source of gallium as a byproduct despite low extraction yields.

Process

Bauxite ore is a mixture of hydrated aluminium oxides and compounds of other elements such as iron. The aluminium compounds in the bauxite may be present as gibbsite (Al(OH)₃), böhmite (γ-AlO(OH)) or diaspore (α-AlO(OH)); the different forms of the aluminium component and the impurities dictate the extraction conditions.

Aluminium oxides and hydroxides are amphoteric, meaning that they are both acidic and basic. The solubility of Al(III) in water is very low but increases substantially at either high or low pH. In the Bayer process, bauxite ore is heated in a pressure vessel along with a sodium hydroxide solution (caustic soda) at a temperature of 150 to 200 °C (302 to 392 °F). At these temperatures, the aluminium is dissolved as sodium aluminate (primarily [Al(OH)₄]⁻) in an extraction process.

After separation of the residue by filtering, gibbsite is precipitated when the liquid is cooled and then seeded with fine-grained aluminium hydroxide crystals from previous extractions. The precipitation may take several days without addition of seed crystals.

The extraction process (digestion) converts the aluminium oxide in the ore to soluble sodium aluminate, NaAlO₂, according to the chemical equation:

Al₂O₃•2H₂O+ 2NaOH → 2NaAlO₂ + 3H₂O

This treatment also dissolves silica, forming sodium silicate :

2 NaOH + SiO₂ → Na₂SiO₃ + H₂O

Alternate processes

The other components of Bauxite, however, do not dissolve. Sometimes lime is added at this stage to precipitate the silica as calcium silicate. The solution is clarified by filtering off the solid impurities, commonly with a rotary sand trap and with the aid of a flocculant such as starch, to remove the fine particles. The undissolved waste after the aluminium compounds are extracted, bauxite tailings, contains iron oxides, silica, calcia, titania and some unreacted alumina. Originally, the alkaline solution was cooled and treated by bubbling carbon dioxide through it, precipitating aluminium hydroxide:

2 NaAlO₂ + 3 H₂O + CO₂ → 2 Al(OH)₃ + Na₂CO₃

But later, this gave way to seeding the supersaturated solution with high-purity aluminium hydroxide (Al(OH)₃) crystal, which eliminated the need for cooling the liquid and was more economically feasible:

2 H₂O + NaAlO₂ → Al(OH)₃ + NaOH

Some of the aluminium hydroxide produced is used in the manufacture of water treatment chemicals such as aluminium sulfate, PAC (Polyaluminium chloride) or sodium aluminate; a significant amount is also used as a filler in rubber and plastics as a fire retardant. Some 90% of the gibbsite produced is converted into aluminium oxide, Al₂O₃, by heating in rotary kilns or fluid flash calciners to a temperature of about 1,470 K (1,200 °C; 2,190 °F).

2 Al(OH)₃ → Al₂O₃ + 3 H₂O

The left-over, 'spent' sodium aluminate solution is then recycled. Apart from improving the economy of the process, recycling accumulates gallium and vanadium impurities in the liquors, so that they can be extracted profitably.

Organic impurities that accumulate during the precipitation of gibbsite may cause various problems, for example high levels of undesirable materials in the gibbsite, discoloration of the liquor and of the gibbsite, losses of the caustic material, and increased viscosity and density of the working fluid.

For bauxites having more than 10% silica, the Bayer process becomes uneconomic because of the formation of insoluble sodium aluminium silicate, which reduces yield, so another process must be chosen.

1.7–3.3 tonnes (3,800–7,200 lb) of bauxite (corresponding to about 90% of the alumina content of the bauxite) is required to produce 0.91 tonnes (2,000 lb) of aluminium oxide. This is due to a majority of the aluminium in the ore being dissolved in the process. Energy consumption is between 7 to 21 gigajoules per tonne (0.88 to 2.65 kWh/lb) (depending on process), of which most is thermal energy. Over 90% (95-96%) of the aluminium oxide produced is used in the Hall–Héroult process to produce aluminium.