The effect of concentration and temperature on diffusivity of metal compounds
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I.
INTRODUCTION
S U C C E S S F U L hydrometallurgical processes are frequently found to be controlled by mass-transfer process. The dissolution of numerous metals from their respective minerals or ores has been reported to be controlled by diffusion of reactants through either diffusion boundary layer or porous product layer produced by leaching reactions. For example, the leaching rates of nickel and cobalt metals and oxides in acid and in ammoniacal solutions have been found to be limited by the mass transfer of a limiting reactant. [1-5] The extraction of gold and silver was concluded to be diffusion-controlled. [6-91 The rate of dissolution of copper from chalcopyrite and lead from galena in acidic ferric sulfate solutions was found to be controlled by ferric ion diffusion through the elemental sulfur product layer. [1~ The mass-transfer aspect in hydrometallurgy has also found its importance in such areas as solvent extraction and chemical precipitation. Correct analysis on rate processes of such hydrometallurgical systems is impossible without reliable information on diffusivity relevant to physicochemical conditions. Very limited information on diffusivity of ionic species as a function of concentration is available in the literature. [17] Furthermore, these diffusivity data were measured at limited conditions, such as at room temperature and in low concentrations. Such data are not very useful in numerous hydrometallurgical processes where high ionic strength and temperature are frequently encountered. The measurement of diffusivity has been known to be very tedious, and the reproducibility and accuracy of the measurement are very questionable due to the various
T. RUBCUMINTARA, formerly Research Fellow, Department of Metallurgical Engineering, South Dakota School of Mines and Technology, is Scientist with the Metallurgical Division, Department of Mineral Resources, RAMA 6 Rd, Bangkok 10400, Thailand. K.N. HAN, Professor and Head, is with the Department of Metallurgical Engineering, South Dakota School of Mines and Technology, Rapid City, SD 57701. Manuscript submitted August 14, 1989. METALLURGICAL TRANSACTIONS B
techniques used. [18] Kang [18] searched for the most suitable method of obtaining diffusivity values of cobalt and nickel at high concentration and temperature using three different types of diffusion cells. The types used were capillary cell, fritted disc cell, and diaphragm cell. Kang, t18[ Han and Kang, [191 Rubcumintara, L2~ and Rubcumintara and Han t211 have indicated that the porous diaphragm method using hydrophilic membrane was most suitable in terms of accuracy and convenience. In the current investigation, a study of diffusivity of numerous metal compounds in aqueous media has been conducted. Diffusivity of selected metal ions, including nickel, cobalt, iron, copper, gold, and silver, was measured as a function of ionic strength and temperature using a diaphragm cell method. The effect of metallurgically impoi~nt species, namely, ammonium ion, sulfate, chloride, nitrate, and cyanid
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