Arsenic stabilization of calcium arsenate waste by hydrothermal precipitation of arsenical natroalunite

Abstract The stabilisation of calcium arsenate waste from a copper smelter by precipitation of arsenical natroalunite has been investigated. This procedure could solve the problem of arsenical gypsum production because it is transformed into arsenic-free anhydrite. Natroalunite precipitation was studied at 180-200 °C from the slurry obtained after H 2 SO 4 -leaching and ozonation of the original waste - using sodium and aluminum sulfates as reagents. Calcium arsenate waste and final precipitates were characterized by chemical analysis (ICP), SEM-EDS and XRD. Solubility tests were also performed on original waste and selected precipitates. The effects studied in the hydrothermal treatment were: initial pH, Al/As molar ratio, As concentration, reaction time and prior gypsum removal. For (Al/As) aq  = 4.5, a complex natroalunite (~(Na,Ca)(Al,Fe) 3 ((S,As,P)O 4 ) 2 (OH) 6 )) was extensively formed. Under these conditions, As-substitution in TO 4 was 7-8% molar. Decreasing (Al/As) aq increased As-substitution in natroalunite (up to ~ 14% molar) but mansfieldite ((Al,Fe)(As,P)O 4 .2H 2 O) co-precipitated. Other effects such as the arsenic concentration in the range 3.5-7.0 g/L and prior gypsum removal did not significantly alter the arsenic phase ratio and the composition of the arsenic phases. However, treatment at 180 °C increased mansfieldite precipitation. Calcium incorporation in natroalunite was small (~ 0.04 in formula) and for initial pH = 1, precipitation of Cu, Ni and Zn was insignificant. Arsenical natroalunite can be effective for long-term storage. At its natural pH (4-5), arsenic solubility remained stabilized at ~ 0.1 mg/L in 6 months. Mansfieldite was not stable under the same conditions with arsenic solubility between 5-10 mg/L. The major disadvantage of natroalunite is the limited arsenic partition between solid and aqueous phases (~ 0.45 molar). However, as the precipitation rate is fast (