Fluidized bed electrowinning of copper; experiments using 150 ampere and 1,000 ampere cells and some mathematical modeli
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INTRODUCTION
SINCE their invention in the 1960's, 1.2there has been much interest in the application of fluidized bed electrodes to the electrowinning of metals. It appears that such electrodes have a very large surface area per unit volume and high mass transfer coefficients, compared to conventional (slab-like) electrodes, both of which are distinct advantages in the electrowinning of metals, as discussed further below. The present paper is part of a continuing investigation into the use of cells equipped with fluidized cathodes for the electrowinning of metals from aqueous solutions.
II.
PREVIOUS INVESTIGATIONS
Experimental investigations wherein metals have been deposited in cells equipped with fluidized cathodes are summarized in Table I. The "plane parallel design" of the cell is a configuration wherein the flow of current is parallel to the flow of electrolyte (i.e., vertical). Such a configuration is convenient for laboratory experiments but unlikely on a commercial scale, since scale-up of a cell in the direction of current flow is difficult.3'4 In the "side-by-side" design the current flows in a horizontal direction (i.e., perpendicular to electrolyte flow), and the fluidized cathode must be separated from the adjacent anode by a diaphragm. With this design scale-up in the vertical direction is an easier task. Electrical connection between the particles and the external power supply is achieved by means of "current feeders" which are rods or slabs of metal (or other conductor) projecting into the bed. Most previous investigators have located the current feeders in the region of the bed most remote from the diaphragm. In a typical cell equipped with a fluidized cathode, the current feeder to anode distance is comparable to the cathode to anode distance in a conventional cell. Consequently, the current per unit area of cell diaphragm I. F. MASTERSON, formerly with the Department of Materials Science and Mineral Engineering, University of California, Berkeley, CA, is now with Gas Products Development, Linde Division, Union Carbide Corporation, Old Saw Mill River Road, Tarrytown, NY 10591. J. W. EVANS is with Materials Science and Mineral Engineering Department, University of California, Berkeley, CA 94720. Manuscript submitted September 12, 1980. METALLURGICAL TRANSACTIONS B
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(henceforth the "superficial current density") can be compared with the current density of a conventional cell to give a first impression of the compactness of fluidized bed electrowinning. For example, the superficial current density figures in Table I for copper electrowinning should be contrasted with current densities in the range of 100 to 300 A/m 2 for conventional electrowinning. It is important to recognize that there are two different concepts of how a fluidized bed electrode might be used in a hydrometallurgical extraction scheme. The first concept is one where high superficial current densities are used, but the "actual current density" (defined as the current per unit area of particle surface) is comparable with that
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