An investigation of carbonaceous materials reducing ferric ions in aqueous solution
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I.
INTRODUCTION
To reduce the overall cell voltage for an electrowinning cell, it has been suggested that the ferrous/ferric reaction can be substituted for the usual oxygen evolution reaction. fl-51 Under standard conditions, this occurs at 0.77 V compared to 1.23 V for the oxygen evolution reaction, but the difference is increased to almost 1 V at about 200 Am -2 due to the much lower polarization associated with the ferrous/ ferric oxidation. [6'7j However, it is important to be able to regenerate the ferrous ion from the ferric ion product; otherwise, unwanted quantities of ferric ion will accumulate and exceed the values of 5 to 10 g1-1 which can normally be tolerated in a copper electrowinning bath. Another related issue is the electrochemical generation of hydrogen ions to produce acid that is generally used for stripping in copper hydrometallurgy. During conventional electrowinning, with the evolution of oxygen, two moles of protons are generated at the anode for each mole of cathodic copper produced. If, instead of oxygen evolution, the anode process becomes the oxidation of ferrous ion to ferric, there is no net change in the proton concentration during electrolysis and the electrolyte cannot be used for stripping or oxide leaching. One way of generating the necessary acid is to reduce the ferric ion with dissolved sulfur dioxide, but the reaction Fe2(SO4) 3 -I- S O 2 -t- 2H20
~2FeSO4
where two major costs are the energy for electrowinning and the expense of the sulfuric acid. N Net acid consumption arises only from the dissolution of waste minerals, so it would be appropriate to use SO2 reduction only to the extent that it makes up for this loss. However, in processes where acid is not consumed, a chemical reduction process is preferred which does not generate excess acid. Carbon, in the form of coal or lignite, would be a suitable inexpensive reductant with the overall reaction being: Fe2(SO4)3 + H20 + C
> 2FeSO4 + H2SO4
+ carbon/organic oxides
II.
[2]
EXPERIMENTAL PROCEDURES
A. Continuous In Situ Regeneration of Ferrous Ion during Electrolysis The concurrent electrowinning/chemical reduction experiments were performed in the apparatus shown in Figure 1 comprising two modified 250 ml beakers. The anode
-I- 2H2SO 4
[1] generates twice as much acid as theoretically required for leaching. Robinson [5] has pointed out that the excess acid production could actually be useful and conserve energy if the ferrous/ferric/SO2 electrowinning scheme was integrated into certain hydrometallurgical flow assets. For example, in the Sherritt-Cominco process, strong sulfuric acid is introduced into an acid leach step and the sulfate is essentially consumed as a jarosite residue. N This net loss of acid could be supplied by the ferrous/ferric/SO2 process. Another possible route for the incorporation of this process is in the leach/solvent-extraction/electrowinning route A.V. COOKE is Senior Scientist with Martin Marietta Laboratories, Baltimore, MD 21227-3898. J. P. CHILTON and D. J. FRAY are University Lecture
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