Alternative Method for the Treatment of Hydrometallurgical Arsenic–Calcium Residues: The Immobilization of Arsenic as Scorodite

Arsenic–calcium residue (ACR) is one of the major hazardous solid wastes produced by the metallurgical industry that poses a serious threat to the environment. However, a suitable method for the effective treatment of ACR is still lacking. In this study, an alternative treatment method for ACRs via the immobilization of As as scorodite was proposed with the use of two types of ACRs (ACR(real) directly collected from a Pb refinery and ACR(lab) precipitated from waste sulfuric acid in the lab). The treatment of ACR included preparing the As-enriched solution via H(2)SO(4) dissolution–neutralization of ACR at pH < 2, As(III) was oxidized by H(2)O(2), and As(V) was immobilized as scorodite. The results showed that gypsum produced from ACR(lab) in the dissolution–neutralization process contained 68 mg/kg of As, far below the Chinese national standard for hazardous solid wastes (<0.1 wt %, GB5085.62007). The gypsum produced from ACR(real) contained 5400 mg/kg of As due to the presence of original high-As gypsum (1.6 wt %) in ACR(real). These results showed that the preliminary removal of SO(4)(2–) from waste sulfuric acid by lime neutralization–precipitation at pH ∼ 2 could produce pure-phase gypsum by avoiding the HAsO(4)(2–) isomorphic substitution for SO(4)(2–). The scorodite produced from both ACRs displayed good As stability at pH 4.95 (0.9 and 0.5 mg/L) via the toxicity characteristic leaching procedure (TCLP) method and at pH 3–7 (0.4–3.0 mg/L) via a 15 day short-term stability test.