Precipitation of crystalline scorodite (FeAsO 4 · 2H 2O) from chloride solutions

Abstract

Crystalline scorodite was produced under ambient pressure at temperatures in the range 80–95°C, employing a supersaturation-controlled precipitation procedure. Failure to control supersaturation led to the production of amorphous precipitate. The procedure involved step-wise neutralization under a low supersaturation environment in the presence of seed. Acidic chloride solutions (1.0–6.0 M Cl − total), containing 2 g/1 As(V) and a variable Fe(III) / As(V) molar ratio ( 1 1 , 2 1 and 4 1 were used. In the absence of seed, crystalline scorodite was produced at 95°C only on the walls of the precipitation reactor. Use of seed made the precipitation of crystalline scorodite possible at even lower temperatures (i.e. 80°C), although a lower supersaturation level had also to be maintained. The presence of SO 4 2− in the chloride solutions was found to have an inhibitory effect on the crystallization of scorodite, resulting in a slowing down of the precipitation process. Neutralization of high Fe/As ratio solutions was found to go through sequential precipitation of crystalline scorodite (up to pH ≈1.1), followed by precipitation of β-goethite.