Determination of the diffusion coefficients of CuSO 4 , ZnSO 4 , and NiSO 4 in aqueous solution

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I.

INTRODUCTION

THEefficiency of electrorefming or electrowinning of metals is greatly influenced by the transport process of metal ions through the diffusion layer near both the cathode and anode. The diffusion coefficient of a metal electrolyte in an aqueous solution is a basic parameter for the analysis of the process of mass transfer. Generally, the diffusion coefficients of aqueous electrolytes vary with the electrolyte concentration, and the values of diffusion coefficients of some electrolytes in aqueous solution have been reported as a function of concentration.1 Except for values obtained at infinite dilution, however, the diffusion coefficients of some common metal sulfates are still unknown although these metal electrolytes have often been used in various hydrometallurgical processes. In this study, the diffusion coefficients of metal sulfates in the aqueous solution systems of MSO4 and MSO4-H2SOa (M2+: Cu 2+, Zn 2+, and Ni 2+) were measured by a porous diaphragm method. At the same time, the effect of concentration on the diffusion coefficient was analyzed in terms of the concentration dependency of the chemical potential of the metal sulfate and of the viscosity of the electrolyte solution.

II.

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EXPERIMENTAL PROCEDURES

Several methods, such as the porous diaphragm method,1 optical methods utilizing the interferometric technique or Moir6 pattern, 2 and the electrochemical method using a rotating disc electrode, 3 have been employed to determine the diffusion coefficient of electrolytes in an aqueous solution. In this study a porous diaphragm method, which is applicable to the determination of diffusion coefficients in a solution system of mixed electrolytes, was used for the determination of the diffusion coefficients of CuSO4, ZnSO+, and NiSO+ at 298 K. Figure 1 schematically shows the arrangement of a porous diaphragm cell which is essentially the same as that proYASUHIRO AWAKURA, Lecturer, and HIROSHI MAJIMA, Professor, are with the Department of Metallurgy, Kyoto University, Kyoto 606, Japan. TOSHIYA DOI, formerly Graduate Student, Department of Metallurgy, Kyoto University, Kyoto, Japan, is with Hitachi, Ltd., Hitachi-city 316, Japan. Manuscript submitted May 7, 1987. METALLURGICALTRANSACTIONS B

Fig. 1 - - Magnetically stirred diaphragm cell used for the determination of the diffusion coefficients of MSO4. A: upper compartment, B: lower compartment, C: porous diaphragm, D: magnet, E: glass stirrer enclosing iron wire (d > 1), F: glass stirrer enclosing iron wire (d < 1).

posed by Stokes. 4 The cells are made of Pyrex, each end having a volume of about 50 cm 3. The diaphragms are of No. 4 porosity (average pore size = 15 p,m), 40 mm in diameter and 3 mm thick. It is essential in this method that the diffusion process is confined entirely to the pores of the diaphragm; this means that the solution in the compartment on either side must be maintained at a uniform concentration right up to the surface of the diaphragm. Each compartment contains a glass tubed sti

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