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Oxidation kinetics of molten copper sulphide Alyaser, Abdelmonem H.


The oxidation kinetics of molten copper sulphide were investigated by quantitative measurements and qualitative observations. Off-gas analyses for SO2 and 02 were conducted to determine the oxidation rates of approximately 200-gram samples, ofmolten 99.5% Cu2S. The molten sulphide was held in alumina crucibles (approximately 44 mm diameter and 75 mm height), and top-lanced with Ar-02 gas mixtures in the hotzone of a vertical tube furnace. During the oxidation reaction, gravimetric measurements of the copper sulphide baths were also conducted to further support the results of the gas analysis measurements. A series of laboratory tests, involving reaction gas ranging in composition from 20-78% 02, was conducted to determine the effect of oxygen concentration on the kinetics of the oxidation reaction. To determine the influence of volumetric gas flow rate on the kinetics of the oxidation reaction and to study the gas phase mass transfer, a series of laboratory tests was carried out utilizing a gas flow rate range of 1-4 liters/min. The effects of other operating conditions, on the oxidation rates, such as bath mixing, and reaction temperature (1200-1300 °C), were also determined. The effect of surface-tension driven flow (the Marangoni effect) on the reaction kinetics was also investigated via surface observation and photography. The overall surface behavior was monitored for spontaneous motion, and the eruption of gas bubbles from the melt.The quantitative analysis of reaction rates was also aided by the micro-examination of quenched bath samples via optical microscopy. Approximately 4-gram samples were extracted at specific reaction times, using U-shaped quartz tubes. The samples were examined microscopically to determine the reaction progress based on the characteristics of gas bubbles, copper droplets and the phases present. The oxidation reaction of molten copper sulphide was found to take place in two distinct kinetic stages. During the primary stage, simultaneous partial desulphurization andoxygen saturation of the melt, via liquid-gas reaction at the melt surface, takes place. Upon saturation of the melt with oxygen, the secondary stage immediately commences. Throughout the secondary stage, the sulphide phase remains at a constant composition (approximately 80.83 wt% Cu, 17.7 wt% S and 1.47 wt% 0 at 1200 °C and 1 atm), due to simultaneous surface and melt reactions, until the overall reaction is complete. Three simultaneous melt reactions occur within the sulphide phase which are responsible for the formation of the metal phase (approximately 98.89 wt% Cu, 0.95 wt% S and 0.16 wt% 0at 1200 °C and 1 atm). As a result of settling oxygen- and sulphur-saturated copper droplets, the metal phase accumulates at the bottom of the bath. The experimental results revealed that the rate of reaction is controlled by the gas phase mass transfer of oxygen to the melt surface; the liquid phase mass transfer resistance and chemical reaction resistance are negligible. The bath was found to be vigorously mixed, primarily due to the effect of the Marangoni effect although the degree of mixing isslightly enhanced during the secondary stage as a result of rising SO2 gas bubbles and falling copper droplets. Based on the electrochemical behavior of the sulphide melt and the experimental revelations, a mathematical model was constructed to carry out a fundamental study of the problem and provide an overall analysis extending beyond the experimental conditions. The model predictions were found to be in good agreement with the observed results. The practical implications of this work are: the copper-making reaction in copper converting is limited by gas phase mass transfer; in the Peirce-Smith converter, one of the factors for the high degree of mass transfer in the bath is the effect of surface-tension driven flows. It is also suggested that the ionic nature of the sulphide bath is another factor for the low liquid phase mass transfer resistance.

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