Precipitação de escorodita a partir de soluções industriais contendo arsênio

Abstract

Scorodite (FeAsO4*2H2O) has been considered as a preferable form for arsenic disposal. Compared to the AsFH, the crystalline scorodite offers the advantages of combining a relatively high arsenic content and solubility in water of less than 1mg As/L at pH 5. The present work investigated the removal of arsenic from a diluted solution (1 g/L As) generated in the washing gas tower during the roasting process. Itwas demonstrated that industrial solutions with low arsenic concentrations (1.1 0.1 g/L) could be treated in one stage of scorodite precipitation under atmospheric pressure conditions, with a removal in a range of 80.5 to 94.6%. Precipitation was carried out at 95oC. In order to control supersaturation and to avoid homogeneous nucleation that yields amorphous ferric arsenate, pH was adjusted according to the initial arsenic concentration. The removal increased with the increase of the scorodite seed concentration until 20g/L. It was shown that a surface area higher than 270 m2/g As in solution was necessary to promote an arsenic removal of approximately 85%. The performance of gypsum seeds was compared with that of scorodite and the results showed that the gypsum was a good seed only with concentrated solutions (10g/L). However, even under this level of concentration, arsenic removal was smaller (61%) than that obtained with scorodite seeds (91%). A procedure to achieve high yields of arsenic removal in continuous system was established. The recycle of seeds was required. The Toxicity Characterization Leach Procedure tests suggested that ageing plays an important role on scorodite dissolution. Scorodite was the only phaseidentified by micro-Raman and X-Ray diffraction analyses of the precipitates.