Cotreatment strategy for hazardous arsenic-calcium residue and siderite tailings via arsenic fixation as scorodite.

Siderite tailings is a potentially cost-free iron (Fe) source for arsenic (As) fixation in hazardous arsenic-calcium residues (ACR) as stable scorodite. In this study, a pure siderite reagent was employed to investigate the mechanism and optimal conditions for As fixation in ACR via scorodite formation, while the waste siderite tailings were used to further demonstrate the cotreatment method. The cotreatment method starts with an introduction of sulfuric acid to the ACR for As extraction and gypsum precipitation, and is followed by the addition of HO to oxidize As(III) in the extraction solutions and finalized by adding siderite with continuous air injection for scorodite formation. The dissolution-oxidation of siderite can slowly produce Fe(III) to control aqueous As(V)-Fe(III) precipitation supersaturation for continuous scorodite crystallization. Chemical analyses show that the extraction efficiency of As from the ACR reaches 94.55%, while the precipitation yield of extracted As via scorodite formation arrives at 99.63% and 99.47%, leading to fixation efficiency of 94.20% and 94.04% in terms of the total As in the ACR by using siderite reagent and tailings, respectively. The final solid products show desirable TCLP stability and long-term stability, meeting the requirement for safe storage (GB 5085.3-2007). XRD, FTIR, and TEM results reveal that such high stability is attributable to the formation of scorodite and the surface adsorption of As on the raw siderite and secondary maghemite. This innovative and economical application of siderite tailings for the treatment of hazardous ACR can be extended to the management of hydrometallurgical wastes.