Kinetics of hydrothermal oxidation of granular Pb metal to PbO powder in sodium hydroxide solutions
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I.
INTRODUCTION
AT the present time, the major methods for preparing PbO powder from Pb metal are the milling and Barton methods, which have gained growing concern about the adverse effects on the environment associated with Pb-bearing dusts. The hydrothermal oxidation process would be an alternative to these conventional processes. The oxidation of granular lead metal can be carried out more quickly at elevated temperatures by using proper media. The hydrothermal oxidation process can likely overcome the in-plant hygiene problems by using the hermetically sealed autoclave. In practice, the pipeline reactor may be adapted to produce PbO powder from granular Pb metal continuously. Much work has been done on the potential-pH diagram for the Pb-H2O system, and the results published show that the PbO phase is stable in weak acidic, neutral, and alkaline conditions.[1,2] For many decades, the oxidation of lead to lead oxide has been studied in order to understand and control the reaction in the various stages of PbO preparation.[3] However, the majority of these studies were conducted in acidic or neutral systems, limited information is available on the reaction kinetics in alkaline solutions, and kinetic parameters, such as activation energies and controlling mechanism, have not been established. The present work was aimed at studying the kinetics of Pb oxidation in sodium hydroxide solution, under well-defined and controlled experimental conditions. The effects of various parameters such as temperature, pH, particle size, oxygen pressure, NaOH concentration, pulp density, and stirring method on the reaction rate of the complex heterogeneous phase system—solid, liquid, and gas—have been experimentally evaluated. Based on the comparisons of various models with the experimental data, the controlling N. YAMASAKI, Professor and Director, and QI FENG, Research Associate, are with the Research Laboratory of Hydrothermal Chemistry, Faculty of Science, Kochi University, Kochi-shi 780, Japan. XIANG LAN, Researcher, is with the Institute of Chemical Metallurgy, Chinese Academy of Sciences, Beijing, People’s Republic of China. Manuscript submitted November 27, 1995. METALLURGICAL AND MATERIALS TRANSACTIONS B
mechanism is suggested. Meanwhile, the effect of Pb species in solution on the formation of PbO has been studied by calculating the equilibrium distribution of various Pb complexes under experimental conditions.
II.
EXPERIMENTAL
A. Preparation and Characterization of Granular Pb Samples The samples used in the experiments consisted of large Pb particles (0.52 to 5 mm). Prior to the oxidation experiments, the granular Pb samples were soaked in dilute HNO3 solution (0.01 pct) for 0.5 hours to dissolve away oxides already presented on the surface of raw material; they were then filtered, rinsed with distilled water, and dried in a vacuum desiccator. The total content of impurities in the treated sample was below 0.01 pct, according to wet chemical analysis. B. Hydrothermal Oxidation Experiments A microautoclave made of car
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