evaluation of iron(ii) oxidation reactions in chloride leach solutions

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EVALUATION OF IRON(II) OXIDATION REACTIONS IN CHLORIDE LEACH SOLUTIONS
Saviour Masambi1, Christie Dorfling2,*, Steven Bradshaw3
1
Department of Process Engineering, Stellenbosch University, Private Bag X1, Matieland, 7602, South
Africa, Email: [email protected]
2
Department of Process Engineering, Stellenbosch University, Private Bag X1, Matieland, 7602, South
Africa, Email: [email protected]
3
Department of Process Engineering, Stellenbosch University, Private Bag X1, Matieland, 7602, South
Africa, [email protected]
* Corresponding author, Telephone number: +27 21 808 3674
ABSTRACT
The presence of iron in base metal leach solutions complicates the nickel and copper recovery processes.
Leach solutions are commonly purified by precipitating iron as hematite, jarosite or goethite. Fast
oxidation of ferrous to ferric is an important step to remove iron by precipitation at low pH values. This
study reviews the ferrous oxidation mechanism and reaction rates in chloride media using molecular
oxygen. This work contributes to the development of a better understanding of iron precipitation
processing steps, which continues to be an important matter in hydrometallurgy.
The specific objectives were to investigate (1) the effect of hydrochloric acid concentration on the rate and
mechanism of oxidation of ferrous by dissolved oxygen and (2) the effect of copper concentration on the
ferrous oxidation rate in chloride media. Batch oxidation tests were performed using synthetic chloride
solutions containing 45 g/L Fe and 3 g/L Ni at various temperatures (50 – 80°C), dissolved Cu
concentrations (0.3 – 3 g/L) and HCl concentrations (2 – 4.6 mol/L).
Regression analysis indicated that the significance of the effect of different variables on the extent of
oxidation achieved after five hours decreased in the following order: HCl concentration > Cu concentration
> temperature. Increasing the HCl concentration from 4 M to 4.6 M caused the degree of oxidation
achieved after two hours to increase from 58.3% to 98.1%. The catalytic effect of dissolved copper on
ferrous oxidation was more significant at low acid concentrations.
Experimental data and equilibrium calculations suggest that rapid oxidation of ferrous is possible at high
HCl concentrations for two main reasons. First, the formation of ferrous chloride complexes allows ferrous
oxidation to proceed without the formation of superoxide as an intermediate product (the rate limiting step
in dilute acid solutions). Second, the formation of ferric chloride complexes stabilises the oxidation product.
KEYWORDS
Chloride solution, iron precipitation, oxidation
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