Zenjiro Asaki

Oxidation of mixed copper-iron sulfide

.A rectangular plate of mixed copper-iron sulfide composed of bornite (Cu5FeS4) and troilite (FeS) was oxidized in an O2-Ar mixed gas stream at 1023 to 1123 K. At the start of the oxidation, iron was preferentially oxidized with the rapid formation of a dense Fe3O4 layer of about 10 μm thickness on the sample surface, without the evolution of SO2 gas. Following this reaction, layers of both Fe3O4 and Fe2O3 grew on the sulfide surface in accordance with the parabolic rate law. The diffusion of iron through the oxide layers was assumed to control the oxidation rate during this stage. The effect of oxygen partial pressure on the parabolic rate constants was minor and an apparent activation energy of 126 kJ/mol was obtained. During the later stages of the reaction, when the sulfur activity in the inner sulfide core increased, the oxidation proceeded irregularly to the interior of the remaining sulfide with the formation of a porous oxide and the evolution of gaseous SO2. The remaining sulfide core was found to be a mixture of bornite (Cu5FeS4) and djurleite (Cu1.96S).

Oxidation of pyrrhotite particles falling through a vertical tube

The oxidation of pyrrhotite particles of 51 and 88 Μm size was studied during falling in a downward stream of O2-N2 gas mixture. At the temperature of reaction tube lower than 825 K, the oxidation rate of particles of 51 Μm size was higher than that of 88 Μm. At higher temperature, on the other hand, the oxidation rate was virtually unvaried with the particle size, and the fractional reaction of oxidized particles increased in proportion to the oxygen partial pressure. Progress of oxidation and fusion of the particles was calculated based on a mathematical model. According to the calculation, when the particle temperature was attained at about 1100 K, it increased instantaneously and the particles were melted, because the rate of heat loss from the particles was overbalanced by the rate of heat generation due to oxidation.

Oxidation of zinc sulfide in a fluidized bed

Kinetics of oxidation of ZnS particles in a batch-type fluidized bed were studied at temperatures between 800 and 910°C. A two-phase model was employed for the fluidized bed, and the partial pressure of oxygen and the gas-film mass transfer coefficient on the particle surface were separately evaluated in gas bubbles and in the emulsion phase. The calculated fractional reaction coincided well with the experimental results. The difference in O2 partial pressure between gas bubbles and emulsion phase was found to be fairly large especially under the vigorous fluidizing condition. Furthermore, it was shown from the mathematical model that the reaction of ZnS particles in the gas bubbles is negligible because of the extremely low solid concentration and that the overall rate of reaction in the emulsion phase is virtually controlled by the rate of gas-film mass transfer at higher temperature. The resistance of interfacial reaction within the particle also becomes significant when the temperature is lowered.

Desulfurization kinetics of molten copper by gas bubbling

Molten copper with 0.74 wt pct sulfur content was desulfurized at 1523 K by bubbling Ar-O2 gas through a submerged nozzle. The reaction rate was significantly influenced not only by the oxygen partial pressure but also by the gas flow rate. Little evolution of SO2 gas was observed in the initial 10 seconds of the oxidation; however, this was followed by a period of high evolution rate of SO2 gas. The partial pressure of SO2 gas decreased with further progress of the desulfurization. The effect of the immersion depth of the submerged nozzle was negligible. The overall reaction is decomposed to two elementary reactions: the desulfurization and the dissolution rate of oxygen. The assumptions were made that these reactions are at equilibrium and that the reaction rates are controlled by mass transfer rates within and around the gas bubble. The time variations of sulfur and oxygen contents in the melt and the SO2 partial pressure in the off-gas under various bubbling conditions were well explained by the mathematical model combined with the reported thermodynamic data of these reactions. Based on the present model, it was anticipated that the oxidation rate around a single gas bubble was mainly determined by the rate of gas-phase mass transfer, but all oxygen gas blown into the melt was virtually consumed to the desulfurization and dissolution reactions before it escaped from the melt surface.

Thermal decomposition of limestone in a fluidized bed

Particles of limestone of 16 to 28 and 60 to 100 mesh sizes were decomposed in a fluidized bed. A mathematical model for the thermal decomposition was proposed comprising the thermal decomposition at the interface within particles and the related heat and mass transfer steps. It was assumed in this model that the particles are completely mixed within the fluidized bed and that gas is in upward plug flow. Fractional decomposition of limestone particles and the bed temperature during thermal decomposition calculated from this model coincide very well with the experimental results. It was further revealed that the overall reaction rate of 60 to 100 mesh size particles is virtually determined by the rate of heat transfer from the reactor wall to the fluidized bed, and that both rates of interfacial reaction and heat transfer from the wall to the bed contribute to the overall decomposition rate of 16 to 28 mesh size particles.

Oxidation of nickel sulfide

The oxidation of nickel sulfide whose atomic fraction of sulfur,xs, is 0.40 to 0.44 was studied in a mixed O2-N2 gas stream at 923, 973, and 1023 K. The oxygen partial pressure was maintained at 2.0 x 104 Pa. In the oxidation of nickel sulfide ofxs = 0.40 and 0.41, a dense NiO layer was formed on the sulfide surface without the evolution of SO2 gas, because of the low sulfur activity. Diffusion of nickel within the inner sulfide core toward the surface controlled the oxidation rate during the first one minute of oxidation. Subsequently, the oxidation rate was controlled by the diffusion of nickel through the formed NiO layer. In the oxidation of nickel sulfide ofxs = 0.44 at 973 and 1023 K, the reaction proceeded irregularly to the interior of the sulfide core with the evolution of SO2 gas, and a porous oxide layer was formed, due to the high sulfur activity of nickel sulfide. For the same reason, this oxidation was also accompanied by the dissociation of nickel sulfide. Under the experimental conditions ofxs = 0.42, 1023 K and xs = 0.44,923 K, the oxidation started with weight increase and without the evolution of SO2 gas, and in the subsequent stage the weight decreased and SO2 gas was evolved.

Oxidation of molten ferrous sulphide

Molten ferrous sulfide of sulfur content lower than stoichiometric FeS was oxidized at 1200 and 1230°C in a stream of oxygen and argon gas mixture under conditions where the overall reaction rate was controlled by the diffusion rate of the gaseous components. The weight change of the sample during the oxidation experiment was recorded continuously. Initially, the melt absorbed a certain amount of oxygen and reached a composition very close to the FeS-FeO pseudo-binary system. Following this, the FeS oxidized to produce FeO and the sample weight decreased. As the mole fraction of FeO increased, the activity of magnetite was found to increase rapidly and the formation of appreciable amounts of magnetite occurred.

Oxidation of molten copper matte

An amount of 80 mg of molten copper matte of a pseudo-ternary Cu2S-FeS-Fe system contained in a slender alumina sample tube was oxidized at 1503 and 1533 K in a mixed O2-Ar gas stream and the oxidation path was followed, comparing the overall rate of oxidation with the gaseous diffusion in the sample tube. The following successive reactions were found to be controlled by gas diffusion. Initially, Fe was oxidized to form FeO. After the melt composition reached a pseudo-ternary Cu2S-FeS-FeO system, FeS was oxidized to form FeO. As the amount of FeO increased, Fe3O4 was also formed and subsequently Cu was produced by the oxidation of Cu2S. In the latter stage, the Cu was oxidized, and the final product under the condition of gas diffusion control was composed of Cu2O, Fe3O4, and CuFeO2. On the other hand, the rate of formation of Fe2O3, CuO, and CuFe2O4 was much slower and they were not formed during the oxidation duration where the overall rate of oxidation was controlled by gas diffusion.

Oxidation of dense iron sulfide

Oxidation of stoichiometric iron sulfide was investigated. Rectangular plates of dense FeS were oxidized in an Ar-O2 gas mixture at 1023 to 1123 K. Oxygen partial pressure was varied between 1.01 × 103 and 2.03 × 104 Pa. During the initial five minutes of oxidation, a magnetite layer of about 10 µm in thickness was formed on the surface without the evolution of SO2 gas. Diffusion of iron from the interior of the sulfide to the sulfide/magnetite interface controlled the oxidation rate. Mass transfer through the gaseous boundary layer at the sample surface also affects the oxidation rate at lower oxygen partial pressures. Following this rapid formation of magnetite, the magnetite layer continued to grow for several hours in accordance with the parabolic rate law. Diffusion of iron through the magnetite layer controlled the oxidation rate during this stage. A thin layer of hematite was also formed on the outer surface of magnetite. When the composition of the inner sulfide core reached Fe0.9S, the oxidation proceeded irregularly into the interior of the remaining sulfide. Porous oxide was formed and SO2 gas was evolved.

Kinetic study on the reaction of solid silica with molten matte

Quartz glass rods, 5.5 mm in diameter, were immersed in ternary Fe-S-O melts and quaternary Cu-Fe-S-O melts at 1493 K, and the decrease in rod diameter was measured. The rate of slag formation increased with the rotating speed of the rod and the FeO activity of the molten matte. A thin film of slag was formed on the rod surface when the FeO activity of the molten matte was higher than that of silica-saturated slag. The mass transfer coefficient of the FeO component in the boundary layer of the molten matte on the slag film was calculated on the assumption that the rate of slag formation was controlled by the mass transfer, and the results showed a dependency on the rotating speed of the rod close to the reported value. The rate of slag formation also increased with the Cu content of the molten matte, probably due to an accelerated upward movement of slag along the rod surface. At the time of lower FeO activity of the matte, the SiO2 rod was thought to dissolve in the matte, and the rate of dissolution increased with the FeO activity and Cu content of the matte.