Studies on the Oxidation of Pyrite : (Part 1) Thermal Decomposition of Pyrite

Abstract

The kinetics of the thermal decomposition of pyrite and the properties of the thermal decomposition products were studied and the following results were obtained. 1. Thermal decomposition of pyrite is not followed by the first order reaction. In the (-log x)-t diagram (where x is mol fraction of pyrite), a knick point was observed at x≒0.3. From the linear parts before and after this knick point, an activation energy of 50.9kcal is obtained for the initial period of the reaction and that of 40.3kcal. for the final period of the reaction. 2. The composition of the thermal decomposition products can be represented by FeS₁₊ₓ, and this coefficient of the formula becomes smaller with an increase in the temperature of decomposition. Of these decomposition products, FeS₁.₁₃ and FeS₁.₁₁ are ferromagnetics whose Curie point is 295°C, and FeS₁.₀₉ and FeS₁.₀₈ are antiferromagnetics whose Curie point is about 220°C. And, FeS₁.₀₆, Fes₁.₀₅ and FeS₁.₀₀ are paramagnetics. 3. The thermomagnetic properties of antiferromagnetic or paramagnetic FeS₁₊ₓ remain unchanged when they are heated to 300°C in vacuum sealed capsules or in high vacuum ; on the other hand, they change into the ferromagnetic ones when they are heated in low vacuum or in the air. By this change, their lattice parameter and axial ratio also change to those of the ferromagnetic ones. From these results, it may be said that this change is due to the initial stage of oxidation of FeS₁₊ₓ and that, by this change, ferromagnetic FeS₁₊ₓ, whose composition is near the upper limit of solubility of sulphur in FeS₁₊ₓ, is formed.