The influence of interfacial turbulence on the rate of oxidation and deoxidation of molten copper and silver using low-momentum vertical gas jets
Creators: name: Barton, Robert Glen
Publisher: name: University of British Columbia
Year: 2010

description: The rate of oxidation of 99.99% and 99.999% pure copper samples at 1220°C by low-momentum jets of pure oxygen has been studied at gas flow rates of from 500 to 2000 cm³ min⁻¹. Oxidation rates at a given gas flow rate were found to be constant and were governed by starvation mass transfer kinetics. Factors studied for the reaction include: effect of lance height, effect of small additions of silicon and sulphur to the melt prior to oxidation, and effect of oxide patch area. Interfacial tension-generated flow, radially outward from the point of jet impingement, was observed during oxidation and surface velocity studies showed that such flow had a mean value of 26.1 ± 5.5 cm sec⁻¹ for all of the experiments and was independent of oxygen gas flow rate and copper bath oxygen concentration. Surface-blockage studies indicated that the bulk of the oxygen transfer to the copper occurred over the area described by the oxide patch. Liquid-phase oxygen mass transfer coefficients were calculated using the oxidation rates and oxide patch areas, and a mean value was found to be 0.104 ± 0.012 cm sec⁻¹ independent of oxygen flow rate, bath oxygen content, and dissolved sulphur and silicon contents. The rate of oxidation of 99.995% pure silver at 1100°C was studied using low-momentum jets of pure oxygen at flow rates of 1500 and 2000 cm³ min⁻¹ and was found not to be governed by starvation mass transfer kinetics. The oxidation rate was not dependent on oxygen gas flow rate and was found to be a factor of about 50 times less than those observed for copper. No spontaneous interfacial tension-generated flow was observed during oxidation of the molten silver and a possible explanation was postulated. Liquid-phase oxygen mass transfer coefficients were found to have a mean value of 2.88 ± .41 (10⁻³) cm sec⁻¹, independent of gas flow rate and bath oxygen content. The effect of interfacial turbulence on liquid-phase oxygen mass transfer coefficients in molten copper was to enhance the value by about 40 times over that observed in molten silver, where interfacial turbulence does not occur on oxidation. Copper deoxidation at 1220°C using low-momentum jets of pure hydrogen at flow rates of 1500 and 3000 cm³ min⁻¹ was studied, and was found not to depend on hydrogen flow rate, lance height, and starting oxygen concentration. The rate-controlling step was found to be the gas-phase mass transfer of hydrogen to the liquid surface for the first 3000 sec. of deoxidation. After this, liquid-phase oxygen mass transport control predominated. Dissolved silicon was found to retard the deoxidation rate, while dissolved sulphur was found to enhance the deoxidation rate through continued SO2 elimination. Interfacial tension-generated flow was observed during deoxidation and approximate surface velocities of 10 to 15 cm sec⁻¹ towards the point of jet impingement were observed. A mechanism for this flow was postulated. The gas-phase mass transfer coefficient was found to be 1.28 ± 0.25 cm sec⁻¹ for copper-oxygen alloys, and was 0.89 cm sec⁻¹ in the 1.28 ± 0.25 cm sec⁻¹ for copper-oxygen alloys, and was 0.89 cm sec⁻¹ in the presence of dissolved silicon and 2.68 cm sec⁻¹ in the presence of dissolved sulphur. An approximate value for the liquid-phase oxygen mass transfer coefficient in the liquid-phase control region was found to be 4.9 (10⁻³) cm sec⁻¹, and was found to be influenced by the presence of bubbling during this phase of deoxidation. The rate of deoxidation of molten silver at 1100°C by low-momentum hydrogen jets was studied at hydrogen flow rates of 1500 and 2000 cm³ min⁻¹ The rate was found not to depend upon hydrogen flow rate, but was found to decrease with decreasing starting bath oxygen concentration. Interfacial tension-generated flow was observed, during silver deoxidation, with approximate surface velocities of 10 to 15 cm sec⁻¹ towards the point of jet impingement. The rate-controlling step was found to be liquid-phase mass transfer of oxygen, and liquid-phase oxygen mass transfer coefficients were found to decrease with decreasing initial oxygen content. These values were enhanced by the presence of bubbling during deoxidation. Interfacial turbulence during the dissolution of solid CU₂S CU₂O, Se, and Te in molten copper was shown to occur. Values calculated for the spreading coefficient S, indicated that the spreading of these materials on molten copper was predictable.