Reclamation of an arsenic-bearing gypsum via acid washing and CaO-As stabilization involving svabite formation in thermal treatment

Abstract

As one of the biggest hazardous solid waste in the metallurgical industry, arsenic-bearing gypsum (ABG) is a great threat to environmental safety owing to possible leakage and diffusion contamination. However, the reclamation of ABG suffers great challenges due to its worthless constituents and high arsenic leaching. In this study, an ABG was reclaimed as more valuable bassanite (CaSO₄∙0.5H₂O) and anhydrite (CaSO₄). Firstly, the overmuch arsenic in ABG was removed to below 150 mg/kg (Japan standard value) using acid extraction. Then, the sludge was mixed with a small amount of CaO and heated at 150 °C and 900 °C to produce bassanite and anhydrite, respectively. In this calcination, gypsum dehydration and arsenic stabilization were combined. In Japan standard leaching test (JSLT), the arsenic leaching concentrations from bassanite (5.1 μg/L) and anhydrite (1.3 μg/L) satisfied the environmental criteria (<10 μg/L) set by Japan government and remained stable for three months. In toxicity characteristic leaching procedure (TCLP), arsenic leaching concentrations remained at 1.61 and 0.61 mg/L, much lower than the limitation (<5 mg/L). Moreover, the arsenic leaching behavior implied that higher temperature was benefited to arsenic stability, which indicated that the arsenic stabilization process was temperature dependent. By XRD analyses, it was found that heating (over 100 °C) could effectively promote arsenic immobilization through reducing the carbonation influence. And the temperature dependence was attributed to the formation of insoluble svabite (Ca₅(AsO₄)₃OH) at high temperature (300-900 °C).

Keywords: Arsenic stabilization; Gypsum reclamation; Svabite; Thermal treatment.