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

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

Studies on the Nickel Matte : I. Nickel-Iron-Sulphur System

Main raw material for metallic nickel in Japan is garniellite, and the matte-smelting process is usually adopted here. The nickel matte produced in this process is, therefore, a ternary mixture of nickel, iron and sulphur, and it is usually ferromagnetic. The relationship of the magnetic properties of the nickel matte to its composition is considered to be important both from fundamental and practical points of view. The authors conducted measurements of the intensity of magnetization, thermomagnetic analyses and X-ray studies on prepared specimens of the binary system Ni-S and of the ternary system Ni-Fe-S. In the system Ni-S and Ni-Fe-S, ferromagnetism was found in a limited region, and it is due to the metallic nickel phase in Ni-S and the nickel-iron alloy phase in Ni-Fe-S. The hexagonal phase NiS and the cubic ternary compound (Ni, Fe)₉S₈ were investigated with X-ray, and their lattice parameters were determined. Furthermore, the range in which (Ni, Fe)₉S₈ exists was also fixed. They are as follows : a=3.436 kX, c=5.351 kX and c/a=1.557 in NiS, and the range of the existence of (Ni, Fe)₉S₈ ; 22.2 atom.% Ni and 30.7 atom.% Ni in the section of 47.1 atom.% S, and a=10.129 kX in (Ni, Fe)₉S₈ saturated with FeS a=10.095 kX in (Ni, Fe)₉S₈ saturated with Ni₃S₂. The continuous solid solution of NiS and FeS₁₊ₓ, the hexagonal phase η, was also investigated with X-ray. The Curie temperature of the specimens in the ternary system showed rather peculiar behaviors in relation to their compositions. This peculiarity of the Curie temperature seems to be due to the deviation in the content of nickel in the alloy phase of matte. Other two magnetic transformations were also observed ; one was found with the specimens in the region γ+δ+π (see Figs. 1 and 9) at about 480°C in heating and at about 420°C in cooling. The authors considered this transformation to be related to the order-disorder transformation in the Ni₃Fe lattice. The other was found in the region γ+ε and in the iron side of the region γ+ε+π. This transformation was found to be due to the α⇄γ transformation in the nickel-iron alloy phase. The properties of the nickel matte taken from the converter of a nickel smelter were also examined, and the results were in good agreement with those of the prepared specimens

Studies on the Oxidation of Pyrite III : The Intermediate Products in the Oxidation of Pyrite

The intermediate products in the oxidation of pyrite were investigated by measurement of the intensity of magnetization, thermomagnetic analysis and X-ray diffraction. In the course of the oxidation, the intensity of magnetization of the sample increases, reaches a maximum, and then decreases. The maximum value of the sample oxidized at low partial pressures of oxygen was found to be much higher than that oxidized at high partial pressures of oxygen. FeS₁₊ₓ and Fe₃O₄ were the main products of oxidation at low partial pressures of oxygen, and Fe₃O₄ changed into Fe₂O₃ immediately before the end of the oxidation. On the other hand, Fe₂O₃ was formed in addition to Fe₃O₄ from the early stages of the oxidation at high partial pressures of oxygen. The FeS₁₊ₓ formed in the intermediate stage was ferromagnetic, and its composition was near the upper limit of the solubility of sulphur in FeS₁₊ₓ. Samples taken from the roasting hearths of the Herreshoff furnace were investigated by measurement of the intensity of magnetization and thermomagnetic analysis. Pyrite decomposes into FeS₁₊ₓ in the upper hearths and, in the middle hearths, FeS₁₊ₓ is oxidized mainly to Fe₃O₄, and finally, Fe₃O₄ changes into Fe₂O₃ in the lower hearths. The intensity of magnetization reaches the maximum in the middle hearths of the furnace. The formation of a large amount of Fe₃O₄ in the middle hearths indicates that the partial pressure of oxygen in the layer of the charge in the furnace was remarkably low

Internal Stress in Electrodeposited Metals

This paper is concerned with the mechanism responsible for the internal stress in the electrodeposited metals. Metals such as nickel, chromium, copper, etc., exhibit tensile stress during their electrodeposition, whereas zinc and cadmium show compressive stress. When the electrodeposited metals are kept in air by removal from their electrolytes after electrodeposition, they release a certain amount of hydrogen and the stress in them becomes more tensile. When hydrogen is electrodeposited on the surface of electrodeposited metals and hydrogen occlusion takes place in the electrodeposits, stress in metals is changed to compressive. From the results obtained, the authors have arrived at a conclusion that one of the factors responsible for creating internal stress in the electrodeposited metals is the release or introduction of hydrogen in the electrodeposited metals

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

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

The Electrical Conductivity and Activation Energy for Ionic Conductance of the Fused Salts Mixtures

The electrical conductivity of the fused salt systems KCl-LiCl, KCl-NaCl, KCl-KBr and NaCl-NaBr have been determined as functions of both temperature and molar fractions, and the activation energy for ionic conductance is calculated with Frenkel's theory. The isotherms of equivalent conductivity against molar fractions for system KCl-LiCl showed negative deviation from additivity and a minimum value was found at 20 mol % LiCl. The activation energy showed positive deviation from additivity and a maximum value was found at 40 mol % LiCl. In the KCl-NaCl, KCl-KBr and NaCl-NaBr systems, the isotherms of equivalent conductivity showed negative deviation from additivity and the activation energy showed positive deviation. The deviation of the activation energy from additivity are caused by the interactions between different types of cation and cation hole or anion and anion hole in the liquid mixture

The Viscosity of the Fused Salts Mixtures : KCl-LiCl and KCl-NaCl Systems

Isotherms of viscosity against molar compositions have been investigated over extended temperature ranges for the systems KCl-LiCl and KCl-NaCl and the activation energy for viscous flow are calculated with Eyring's theory. The activation energy showed negative deviation from additivity in KCl-LiCl system, but in KCl-NaCl system the activation energy showed positive deviation from additivity in the range of 0〜40 mol % NaCl and showed negative deviation in the range of 40〜100 mol % NaCl. These deviations are caused by the interaction between different types of molecules or ion pairs in the liquid mixture

Internal Stress in Electrodeposited PbO₂

The change of internal stress in the electrodeposited PbC₂ from a lead nitrate bath was observed by measuring the deflection of a thin platinum anode during electrodeposition of PbC₂ on one side of it. The crystal structure of the deposits was determined by X-ray diffraction. After the electrodeposition, the change of deflection of the deposited Pb₂ was observed under the following conditions : a) Keep the deposited PbO₂ in air or in a lead nitrate bath at constant temperatures. b) Discharge the cell, Pt/PbNO₃ electrolyte/PbO₂ deposit, by connecting it to an external circuit with some resistance. The results obtained are summarized as follows : 1) The crystal structure of α-PbO₂ was observed in the deposits which had deflected in the direction of contraction during electrodeposition, and β-PbO₂ was observed in the deposits which had deflected in the direction of expansion during electrodeposition. 2) After the electrodeposition, the deposits deflected gradually in the direction of expansion when they were kept in air or left in the bath. The amount of deflection of the deposits of α-PbO₂ was larger than that of the deposits of β-PbO₂ within the same time duration. 3) The electrodeposited PbO₂ deflected in the direction of expansion by discharging the cell, and the amount of deflection of the deposits of α-PbO₂ was larger than that of the deposits of β-PbO₂