Aqueous processing of arsenic trioxide to crystalline scorodite
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This article describes the conversion of arsenic trioxide into crystalline scorodite (FeAsO4 • 2H2O) using acidic nitrate solutions at temperatures ranging from 140 °C to 160 °C. A schematic process flow sheet is proposed. The reasons for processing arsenic trioxide, especially baghouse dust from metallurgical operations, lie in its toxic character and the fact that market demand for it is decreasing. An environmentally friendly way to dispose of this roaster by-product is as crystalline ferric arsenate (scorodite), since the latter has been shown to be the least soluble “host” mineral for arsenic. To produce crystalline scorodite in bulk has to date not been possible, however. This article describes a new hydrometallurgical approach for converting arsenic trioxide rapidly and at a high yield into crystalline scorodite. It is based on the fact that at 160 °C, ferric does not precipitate as an oxide from solutions that contain 2 M HNO3. By adding arsenic trioxide to such a solution, ferric precipitates as crystalline scorodite, without other iron species (e.g., hematite) precipitating. This means that a stoichiometric Fe:As ratio is sufficient. When starting with arsenic trioxide, trivalent arsenic will be oxidized before 160 °C is reached due to the strong oxidizing power of nitric acid.
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