Copper electrowinning using spouted-bed electrodes: Part II. Copper electrowinning with ferrous ion oxidation as the ano
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I. INTRODUCTION AND PREVIOUS INVESTIGATIONS
THERE have been several investigations of anodes that are alternatives to the usual copper-electrowinning anodes of lead-silver evolving oxygen. Interest in improving the electrowinning of copper resulted in several modifications, including alternative anodic reactions intended to reduce the electrical-energy requirements. Conventional electrowinning of copper is energy intensive mainly because 70 pct of the total voltage is due to the anode potential.[1] It is possible to reduce the overall cell voltage in the copper-electrowinning process by substituting an anodic reaction that occurs at lower potential than the oxygen evolution. The oxidation of sulfite has been extensively studied for this reason. The oxidation occurs at a potential more than 1 V less than the oxygen evolution. These savings on the anodic potential results in a nearly 50 pct reduction in electric-energy consumption. For example, Spring and Evans[2] examined the oxidation of sulfite, as an alternative to oxygen evolution in the fluidized-bed electrowinning of copper. A survey of the extensive research carried out on this oxidation reaction is published in the work of Subbajah et al.[3] The oxidation of sulfur dioxide has also been used, in conjunction with ferrous/ferric ions in solution,[4,5] as an alternative anodic reaction. Numerous theoretical and experimental studies have been carried out on the ferrous/ferric redox electrode.[6,7] Generally, these investigation were limited in the range of parameters investigated, particularly in terms of concentration, temperature, and mass-transfer conditions. Cooke et al.[4] reported on the behavior of this redox system under widely varying conditions. They found that the limiting current increases rapidly with increasing temperature, or increasing concentration of ferrous ion, and was about twice as high with gas sparging as with mechanical agitation. The concept behind the present investigation on copper electrowinning, with ferrous ion oxidation as the anodic reaction, is illustrated schematically in Figure 1. High-grade matte is leached by a recycle stream from electrowinning that is rich in ferric ion but low in ferrous ion and copper.
The output from leaching is a solution high in copper and ferrous ion but low in ferric ion. After solid-liquid separation to remove solid residue (and possibly solution purification), the solution may be electrolyzed to produce copper. In Figure 1, this electrowinning is done in cells with spouted cathodes. The solution from leaching is first fed to the cathode sides of the cells where the reaction is the deposition of copper. Cu2⫹ ⫹ 2e⫺ ⫽ Cu A side reaction that is the reduction of ferric ion Fe3⫹ ⫹ e⫺ ⫽ Fe2⫹ is minimized by the low content of ferric ion in the solution from leaching; there is little transfer of ferric ion from the anode sides of the cells because of the microporous diaphragm. From the cathode sides of the cells, the solution flows to the anode sides where the anodic reaction is the oxidation of ferrou
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