Studies on the Oxidation of Pyrite : (Part 2) Kinetics of the Oxidation of Pyrite

Abstract

The kinetics of the oxidation of pyrite in the atmosphere of the mixed gas of O₂, SO₂ and N₂ was studied. The results obtained were summarized as follows : 1. At 700°〜900°C where the thermal decomposition of pyrite proceeds quickly, the reaction occurs in two stages, the initial one is of the thermal decomposition of pyrite and the final one is of the oxidation of the thermal decomposition product, FeS₁₊ₓ. This phenomenon is more obvious at lower partial pressure of oxygen. The rate constant of the oxidation of FeS₁₊ₓ was found to be proportional to the partial pressure of oxygen, and the activation energy of this reaction was determined to be about 8.3 kcal. 2. At 550°〜650°C, the thermal decomposition of pyrite proceeds slowly and, as a result, the following three reactions occur simultaneously, FeS₂ = FeS₁₊ₓ+(1-x)/2S₂(g), FeS₁₊ₓ+(7/2)+2x)O₂ = 1/2 Fe₂O₃+(1+x)S₂, FeS₂+11/4 O₂ = 1/2 Fe₂O₃+2SO₂. For the mol fraction of x, y and z respectively for FeS₂, FeS₁₊ₓ and Fe₂O₃, the simultaneous differential equations were brought out and they were solved approximately. We defined also the ratio of weight decrease w and it was shown as a function of time t by applying the solutions of the above equations. From the expression of this w, the rate constant of the direct oxidation of pyrite, k₃, was obtained. This rate constant, k₃, was also found to be proportional to the partial pressure of oxygen and its activation energy to be about 11.9 kcal. 3. The change of mol fraction x, y and z along the oxidation was computed. From this computation, the amount of FeS₁₊ₓ formed as an intermediate product of the reaction was found to be dependent on the temperature of oxidation and the partial pressure of oxygen. 4. The calculated ratio of weight decrease w coincides considerably with the observed values at lower temperatures of the oxidation, but the coincidence is not good at higher temperatures. Since the thermal decomposition proceeds very quickly at higher temperatures, the following assumptions on the reactions fit better than the simultaneous progress of the three reactions mentioned above ; a) at the initial stage, the thermal decomposition proceeds preferentially and FeS₁₊ₓ is formed, and b) toward the end of the thermal decomposition, the oxidation of FeS₁₊ₓ commences, followed by the decomposition.