Oxidation of pyrite in an anoxic atmosphere
Pyrite (FeS 2 ) inclusions in coal, when heated in an oxygen deficient atmosphere (approximately 1% oxygen), become coated with magnetic Fe 3 O 4 due to oxidation. Most of the FeS 2 can thus be removed from the coal by magnetic separation to reduce the sulphur concentration. The oxidation products have been studied in greater detail by measuring the SO 2 and O 2 in the effluent gas during the heating process and by performing further magnetic measurements. At 582 K, the pyrite surface was oxidized to FeSO 4 . Significant oxidation of FeSO 4 and FeS 2 to Fe 3 O 4 was observed starting at 677 K. At about 681 K, the Fe 3 O 4 is further oxidized to α-Fe 2 O 3 . At 681 K, under isothermal conditions, the oxidation is impeded by the α-Fe 2 O 3 formed on the surfaces of the grains. If the temperature is rapidly increased, the oxygen penetrates the α-Fe 2 O 3 veneer to the FeS 2 core of the pyrite grains and oxidizes essentially the whole pyrite mass to Fe 3 O 4 before α-Fe 2 O 3 can be formed.