Investigation of Fe(III)-As(III) bearing phases and their potential for arsenic disposal
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Fe(III)-As(III) bearing precipitates have been successfully used for arsenic immobilization in copper smelter weak acid effluents. However, knowledge on their precise characteristics is very limited compared to ferric arsenate precipitates which are the preferred disposal option. As(III) is the dominant arsenic species in the weak acid effluents, and high costs are incurred in oxidizing As(III) to As(V) prior to ferric arsenate precipitation. Detailed characterization of Fe(III)-As(III) bearing residues is fundamental for accurate prediction of their long term stability. Synchrotron-based analysis of the Fe(III)-As(III) bearing precipitates from the effluent treatment plant (ETP) at Xstrata’s Horne Copper Smelter in Quebec identified ferric arsenate and gypsum as the major phases, and other minor phases including zinc hydroxide and franklinite. The predominant As species was As(V) which accounted for 49 to 84% of the total As in the Horne ETP sample. The high As(V) levels detected in the Horne ETP co-precipitates may be due to the partial oxidation of As(III) during prolonged storage, prior to synchrotron analysis. Tooeleite was investigated as an alternative potential disposal option for As(III) immobilization from copper smelter weak acid effluents. Lime neutralization of an equimolar Fe(III)-As(III) bearing weak acid solution resulted in about 85% As being removed at pH 2.7 and the formation of gypsum-bearing tooeleite at pH 2 to 3.5. At >pH 4, the rapid transformation of tooeleite occurred to form a poorly crystalline equimolar ferric arsenite which was stable at pH 6 to 10. Co-precipitation tests also showed that equimolar ferric arsenite was precipitated at pH>4. US EPA Toxicity Characteristic Leaching Procedure (TCLP) analysis indicated that both tooeleite and ferric arsenite have relatively high As solubilities. Calcination of tooeleite was explored as a method for improving the stability of tooeleite, and calcination of tooeleite in air at 600˚C produced a ferric arsenate calcine with a TCLP solubility of <5 mg/L As. The calcine iii produced from the lime-precipitated tooeleite at 600 ˚C was found to contain an iron arsenate (Fe7As6O24) which had a slightly higher TCLP As solubility of 13.1 mg/L. The precipitation of tooeleite from an As(III)-bearing weak acid and calcination of the resultant precipitate may offer a new process for As(III) fixation from copper smelter weak acid effluents.