There are many commercially important impurities in bauxites, however of these silica (which may be present in various forms) is the most important.
The rule of thumb is that, on a weight basis, any mass of silica that reacts in the refining process will cause an equal loss of caustic soda, and an equal loss of alumina to the red mud in the De-Silication Product (DSP). Thus silica is a key determinant of any bauxite’s economic value.
Important impurities and their effects listed opposite. Some impurities, for example gallium, build up in the refinery liquor and are potentially an economic by-product. However due to a very small market for such metals very few refineries extract and sell them.
(Kaolin, China Clay; Al2Si2O5(OH)4 or Al2O3.2SiO2.2H2O):
- A key source of reacting silica; reactive under all refining conditions.
Quartz (Silicon Dioxide; SiO2):
- An additional source of reacting silica in high temperature refineries.
Hematite (Iron Oxide; : Fe2O3):
- Essentially passes through the refining process unchanged.
- Chemical Formula: Fe2O3
Goethite (Iron Hydroxy-oxide; FeOOH):
- Often fine grained: associated with slow or poor settling of red muds.
- Due to aluminium ion substitution and solid solution into the goethite lattice it can be a minor contributor of alumina; accessible in high temperature refining.
Titanium Dioxide (Anatase, Rutile; TiO2):
- Associated with tenacious scales inside refining equipment. These reduce refinery efficiency and increase maintenance costs.
- Entrained vegetative materials in the bauxite partially decompose to form humic acids in the refinery liquor. These interfere with alumina precipitation and final product mechanical properties.
- Costly and difficult to remove organic carbon causes problems in some important bauxites (eg those in the Darling Range, Western Australia).
- Causes iron contamination and poor mechanical properties in the product alumina, extra equipment corrosion, reduced liquor C/S.
- Costly and difficult to remove; originates from sulphur minerals such as pyrite or organic materials in the bauxite.
Source: World Bauxite Resources, USGS, CMGroup