Pyrite oxidation inhibition in coal waste by PO4and H2O2pH-buffered pretreatment

Abstract

Pyrite is coal waste is initially oxidized by atmospheric O₂, releasing acidity and Fe²⁺. At pH around 3, Fe²⁺ is rapidly oxidized by T. ferrooxidans to Fe³⁺ which oxidizes pyrite at a much faster rate than O₂. Commonly, the approach used to prevent pyrite oxidation in the field involves the use of limestone. This approach, however, has a short span of effectiveness. The objective of this study was to demonstrate that pyrite oxidation inhibition (in the absence or presence of coal waste) may take place when a pyritic sample is pre-treated with a solution of H₂O₂plus sodium acetate (NaAQ) with or without phosphate (PO₄ buffered at pH around 5. This objective was tested by pre-treating pyrite (with or without coal waste) with the above pretreatment solution in a chromatographic column with a 1 cm i.d. and a flow rate of 0.5 ml min⁻¹. When the pretreatment solution reached pyrite surfaces, H₂O₂ oxidized the surface portion of pyrite and released Fe³⁺ so that FePO₄ or Fe-OOH precipitated and formed a coating on pyritic surfaces. This laboratory-scale study demonstrated that the iron phosphate or Fe-OOH coating could be established with a solution composed of 10⁻⁴ to 10⁻² mol L⁻¹ KH₂PO₄ (or without phosphate in the case of Fe-OOH coating), 0.159 mol L⁻¹ H₂O₂, and greater than 0.01 mol L⁻¹ NaAC at approximately pH 5. The percent of pyrite consumed for creating these coatings ranged from 5-10%. The FePO₄ or Fe-OOH coating on pyrite surfaces was shown to inhibit pyrite oxidation in the presence/absence of coal waste. However, formation of Fe-OOH coating required a greater portion of pyrite to be consumed and appeared to be less effective in inhibiting pyrite oxidation than the phosphate coating. The process requires testing under field conditions.