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Reductive dissolution of goethite and pyrolusite in alkaline solution Devuyst, Eric

Abstract

The selective dissolution of pyrolusite (MnO₂) in presence of goethite (FeO.OH) in aqueous ammoniacal ammonium carbamate solution has been investigated; various reducing agents were studied, in particular sodium sulfite (Na₂SO₃) and hydrazine hydrate (N₂H₄.H₂O). Sodium sulfite was found to be an unsatisfactory reducing agent for both goethite and pyrolusite, for the leaching rate decreased steadily with time to an impractical level because of the limited solubility of the reactions products. Hydrazine hydrate was an effective reducing agent for pyrolusite even at low temperature, whereas goethite required much higher temperatures for any appreciable dissolution. A one hour leach at 35°C dissolved 90 per cent of the pyrolusite but only 0.1 per cent of the goethite. Additions of ammonium phosphate had no effect on the rates, there being no phosphate detected in the leach liquor. An activation energy of 17.0 K cal/mole was found for the dissolution of pyrolusite with hydrazine. A reaction mechanism for the leaching of pyrolusite with hydrazine was proposed in which a reduction reaction on the surface of the mineral was the rate-determining step. It was possible to recover manganese from solution with oxygen as an amorphous MnO₂ precipitate.

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