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The large volumes of sludge produced through the active treatment of acid mine drainage (AMD) require further processing and final disposal. AMD sludge typically contains a heterogeneous mixture of iron, aluminum, magnesium, and calcium oxides/hydroxides that are of little to no practical value. However, purified iron and aluminum hydroxides have potential commercial value. Based on the solubility of the major dissolved metals, a two-step selective precipitation process was developed to recover high purity iron and aluminum as separate hydroxide products through the manipulation of current AMD treatment operations. The recommended pH for iron precipitation was pH 3.5–4.0 with precipitate purity >93.4% and iron recovery >98.6%. AMD water after iron removal was used as source water for aluminum recovery. Aluminum precipitated best at pH 6.0–7.0 with aluminum recovery >97.2% and precipitate purity >92.1%.
Wei et al. (2005) studied this question.