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I.

INTRODUCTION

T H E efficiency of the electrorefining or electrowinning process is greatly influenced by the transport process o f metal ions through the boundary layer adjacent to the cathode. The analysis of the mass transfer process requires that the diffusion coefficient of metal electrolyte in aqueous solutions is known under various conditions. However, only a limited amount of diffusion coefficient data of electrolytes is available at the present time. Awakura et al. ~ previously reported the diffusion coefficient data of some metal sulfates in both aqueous MSO4 and MSO4-H2SO4 solutions at 298 K. In this study, the diffusion coefficients of metal chlorides in the aqueous solution systems of MCI2 and MCI2-HCI (M2+: Ni 2+, Zn 2+, and Cd 2§ have been measured at the same temperature by a diaphragm-cell method. Also, theoretical and experimental discussions are given on the concentration dependencies of metal chloride diffusion coefficients. II.

EXPERIMENTAL PROCEDURES

A diaphragm-cell method, [2al which is applicable for the determination of diffusion coefficients in a solution system of mixed electrolytes, was employed to determine the diffusion coefficients in this work, T h e diffusion coefficients of NiC12, ZnC12, and CdC12 were measured at 298 + 0.1 K in both aqueous MC12 and aqueous MC12-HC1 solutions, where the concentration of

ii

HCI was kept constant. The experimental apparatus and procedures are the same as those described in a previous paper, m The kinematic viscosity and density of the solutions involved in this work were also measured at 298 K using a Cannon-Fenske viscometer and a pycnometer, respectively. Some diffusion experiments were done in aqueous MC12-HCI solutions under the condition of constant HCI activity to examine the effect of HC1 on the diffusion coefficient of chloride. The equi-activi.ty line of HC1 was determined from the activity data of acidic chloride solutions, which were reported in a previous paper, t3J The concentrations of NiC12, ZnC12, and CdC12 in aqueous solutions were determined using a chelate titration method with EDTA. All of the chemicals used in this study were of reagent grade. The solutions were prepared with deionized water with a resistivity of more than 5 • 106 ohm cm.

III.

EXPERIMENTAL RESULTS

Figure 1 shows a t__ypical example of the integral diffusion coefficients, D, of NiC12, which is directly determined at 298 K by a diaphragm method, in aqueous NiC12-HCI solutions. The experimental error involved in D values is around 2 pct. The integral diffusion coefficient is defined as follows: m --

D

dC

[1]

Cm MASAMI AOKI, formerly Graduate Student, Department of Metallurgy, Kyoto University, is with ULSI Research Center, Toshiba Corporation, Toshiba-cho, Saiwai-ku, Kawasaki 210, Japan. YASUHIRO AWAKURA, Associate Professor, ATSUSHI TSUCHIYA, Graduate Student, and HIROSHI MAJIMA, Professor, are with the Department of Metallurgy, Kyoto University, Sakuo-ku, Kyoto 606, Japan. Manuscript submitted September 9, 1988. METALLURGICALTRANSACTIONS B

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