PRECIPITATION OF CRYSTALLINE SCORODITE (FEASO4-CENTER-DOT-2H(2)O) FROM CHLORIDE SOLUTIONS

Crystalline scorodite was produced under ambient pressure at temperatures in the range 80-95 degrees 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/l 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 degrees 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 degrees C), although a lower supersaturation level had also to be maintained. The presence of SO42- 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 approximate to 1.1), followed by precipitation of beta-goethite.