Oxidation of Fe (II) in sulfuric acid solutions with dissolved molecular oxygen
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INTRODUCTION
T H E oxidation of Fe (II) in acidic aqueous sulfate solutions with dissolved molecular oxygen is commonly employed in many hydrometallurgical processes such as the purification of leach solutions or the regeneration of Fe (III) leachants. The mechanism of this reaction has been investigated by many researchers, ~-6 among whom Mathews and Robins reviewed the previous papers on the oxidation of Fe (II) in sulfuric acid solutions. 7 Also, they studied this reaction and proposed a reaction mechanism in which the Fe (II) hydrocomplex is a principal reacting species at a low hydrogen ion concentration. Some inconsistencies still remain in previous works in relation to the roles of hydrogen ions, sulfate ions, and ferric ions in the oxidation reaction. Since sulfuric acid is biprotic, it is important to control carefully the experimental conditions, particularly in regard to pH in order to obtain a better understanding of the reaction. This paper deals with the experimental results obtained for the oxidation reaction of Fe (II) in acidic sulfate solutions, in which the hydrogen ion and sulfate ion concentrations ranged from 2.0 • 10 -4 to 3.0 mol" dm -3 and 7.5 x 10-4 to 0.8 mol 9 -3, respectively.
The analysis of ferrous ion in solution was carried out by titration with potassium dichromate. The measurement of the redox potential of Fe (III)/Fe (II) was made by using a Pt wire electrode and an Ag-AgC1 electrode through a 3.3N KC1 agar salt bridge.
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EXPERIMENTAL RESULTS AND DISCUSSION
Figure 1 shows typical extent of reaction curves for ferrous sulfate oxidation with dissolved molecular oxygen in sulfuric acid solutions containing 0.2 mol" dm -3 FeSO4. This figure also illustrates the effect of the sulfuric acid concentration on the reaction rate, which decreases with an increase in H 2 5 0 4 concentration. However, the oxidation rate curves are almost independent of H2SO4 concentration above 1 mol 9 3. Figure 2 shows a second order plot for reaction. When 0.2 mol 9 -3 FeSO4 was subjected to the oxidation reaction,
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EXPERIMENTAL PROCEDURES
The experimental apparatus and procedures employed in the present study were essentially the same as those for the high temperature experiments described in a previous paper. 8 A 0.5 dm 3 of a ferrous sulfate solution prepared under oxygen-free conditions containing a predetermined amount of sulfuric acid was charged into a 1 dm 3 capacity glass autoclave and heated to a predetermined temperature. Oxygen gas was introduced into the autoclave, and the reaction was initiated by stirring the solution. The oxygen partial pressure was kept constant during the progress of the reaction, and unless otherwise stated, the stirring condition was kept constant at a speed of 1100 rpm to ensure chemical control of the reaction. MASAO IWAI, Lecturer, is with the Department of Metallurgy, Toyama Technical College, Toyama 930, Japan. HIROSHI MAJIMA, Professor, and YASUHIRO AWAKURA, Instructor, are both with the Department of Metallurgy, Kyoto University, Sakyo-ku, Kyot
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