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A kinetic study of the oxidation of pyrite in aqueous suspension. McKay, Donald Roderick


This thesis describes a kinetib study of the oxidation of pyrite in aqueous suspension by molecular oxygen. The reaction is found to proceed by several simultaneous paths, the direct dissociation by molecular oxygen being predominant, i.e. , FeS₂ + O₂ (aq) fast → FeS₂. O₂ FeS₂. O₂ + O₂ (aq) slow →FeS₂. 2O₂→ FeO₄+ S° In accord with this mechanism the overall rate of oxidation is found to be proportional to the pyrite surface area and to the oxygen partial pressure and independent of the solution composition. The rate Constant k₃ = 0.125 exp -13,300/RT m.cm.¯² atm.¯¹ min.¯¹ . The products of oxidation were found to be both ferrous and ferric sulphate, sulphuric acid and elemental sulphur. Solution composition did affect the distribution of products. High temperatures and low acidities favoured the formation of sulphuric acid while the opposite conditions favoured the formation of elemental sulphur. A separate study of the aqueous oxidation of ferrous sulphate by molecular oxygen revealed a second order dependence on ferrous ion and a first order dependence on molecular oxygen, i.e., (d[Fe⁺⁺]/dt) = k[Fe⁺⁺]²[O₂] k = 6 x 10⁹ exp -16,500/RT 1. m.¯¹ atm.¯¹ min.¯¹ This reaction was found to be catalyzed by small amounts of cupric Ion according to the following observed kinetics -(d[Fe⁺⁺]/dt) = k[Fe⁺⁺]²[O₂] +k₂ Fe+ k2[Fe⁺⁺]²[O₂] [Cu⁺⁺]½ k₂= [equation omitted] Although ferric sulphate was found to be an adequate oxidizing agent for pyrite, the contribution of this reaction to the over-all dissociation of pyrite in the presence of molecular oxygen was not significant since the rate of reoxidation of ferrous sulphate was found to be comparatively slow.

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