The inorganic constituents of low-rank Victorian brown coal, which are mainly present as inherent inorganic combinations attached to the coal molecule, are different from the minerals present in higher-rank coals. Changes in the state of combination of the inorganics in the chars of these coals have been studied by determining the minerals formed when chars are prepared at various temperatures. It is shown that when brown coals containing inorganic carboxylates are carbonized, the reactive functional groups which are present will begin to decompose below 4OO'C and their decomposition is completed by 600ยฐC.
The inorganic elements released will form mineral combinations depending on the carbonizing temperature and the reduction potential of the metal. Sodium carbonate, calcium oxide, magnesium oxide (periclase) and iron oxide (magnetite) are usually the chief minerals that may be formed. Mineral constituents present in the original coal may also change during carbonization.
As the temperature increases, hydrated oxides of iron, clay and aluminium will lose water of crystallization and become dehydrated, forming magnetite, dried clay and alumina. Pyrite will lose up to half its sulphur at 400-45O'C.
If the carbonization temperature is over 600ยฐC. metallic iron may be formed from the magnetite, and sodium chloride (and later sodium carbonate) will volatilize. Quartz remains unaltered.