Oxidation of pyrite in an anoxic atmosphere

Pyrite (FeS₂) inclusions in coal, when heated in an oxygen deficient atmosphere (approximately 1% oxygen), become coated with magnetic Fe₃O₄ due to oxidation. Most of the FeS₂ 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 SO₂ and O₂ in the effluent gas during the heating process and by performing further magnetic measurements. At 582 K, the pyrite surface was oxidized to FeSO₄. Significant oxidation of FeSO₄ and FeS₂ to Fe₃O₄ was observed starting at 677 K. At about 681 K, the Fe₃O₄ is further oxidized to α-Fe₂O₃. At 681 K, under isothermal conditions, the oxidation is impeded by the α-Fe₂O₃ formed on the surfaces of the grains. If the temperature is rapidly increased, the oxygen penetrates the α-Fe₂O₃ veneer to the FeS₂ core of the pyrite grains and oxidizes essentially the whole pyrite mass to Fe₃O₄ before α-Fe₂O₃ can be formed.