Abstract
A systematic programme of synthesis, characterisation and solubility testing of solids precipitated from solutions containing various ratios of Fe3+, ASO4 3- and SO4 2- has enabled the relative stabilities of the various arsenical phases to be assessed empirically.
Over 90% of the arsenic can be precipitated from solutions of pH<l, Fe:As ≥1, at temperatures above 150°C; the solids formed are crystalline and usually consist of well-defined compounds or mixtures of compounds. While the crystallinities of the precipitates increase with temperature of synthesis, their stabilities do not always increase correspondingly due to the formation of different suites of compounds. The arsenical compounds formed include scorodite and two unknown compounds that have been designated Type-1 (Fe2(HAsO4)3-zH2O) and Type-2 (Fe4(As04)3(OH)x(SO4)y). Crystalline ferrihydrite can also be formed from solutions containing high concentrations of iron and this can readily incorporate substantial amounts of arsenic.
The relative order of stability (based on arsenic solubility) of the compounds synthesised at elevated temperatures is: Type-2 003D; Scorodite > Arsenical Ferrihydrite > Type-1. When a comparison is made of the stability of these high temperature crystalline solids with amorphous iron arsenates (Fe:As >3: 1) precipitated at ambient temperature and at pH 5, the crystalline solids are marginally less stable. However, the crystalline compounds contain considerably more arsenic (∼Fe:As 1: 1) than the amorphous precipitates (Fe:As >3: 1). These latter compounds consist mainly of amorphous ferrihydrite which on ageing can dehydrate and recrystallise and may release adsorbed arsenic due to reductions in surface area and active sites. Therefore the formation of stable crystalline compounds may be a more attractive disposal option for arsenic.
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© 1994 Springer Science+Business Media Dordrecht
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Swash, P.M., Monhemius, A.J. (1994). Hydrothermal precipitation from aqueous solutions containing iron(III), arsenate and sulphate. In: Hydrometallurgy ’94. Springer, Dordrecht. https://doi.org/10.1007/978-94-011-1214-7_10
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DOI: https://doi.org/10.1007/978-94-011-1214-7_10
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