Recovery of Fe, Al and Mn in acid coal mine drainage by sequential selective precipitation with control of pH

• Purified Fe, Al and Mn hydroxides would be potential resources.
• Sequence of metal removal was Al > Fe > Mn by only neutralization.
• Sequence of metal removal was Fe > Al > Mn by oxidation before neutralization.
• Recovery of Fe increased from 45.8–70.3% to 99.2–99.3% at pH 4.5.
• Recovery of Al reached 70.4%–82.2% at pH 5.5, when the purity was 94.5–99.9%.

This study was carried out to develop a process for recovering dissolved metals from acid mine drainages (AMD). AMD sludge normally consists of a heterogeneous mixture of many elements that are thrown out due to a lack of low purity. However, purified Fe, Al and Mn hydroxides would be potential resources. The AMD from Samma-Taejeong coal mine, Samcheok, Gangwon province in Korea was neutralized by adding neutralizing agents and oxidant, hydrogen peroxide to evaluate recovery of the metals and purity of their precipitates. The metals in AMD were recovered individually according to two process schemes: (i) only neutralization, and (ii) neutralization after addition of hydrogen peroxide. The result shows that using only neutralizing agents, both dissolved Al and Fe were concurrently precipitated around at pH 4.5 so that caused the purity of each precipitate to be lowered. In the case of oxidation and then neutralization of AMD, sequence of metal removal was Fe > Al > Mn in order. Recovery ratios of dissolved Fe and Al reached 99.2–99.3% at pH 4.5 and 70.4%–82.2% at pH 5.5, respectively. Recovery of Mn reached 37.8–87.5% at pH 8.5. Based on the results, oxidation before neutralization increased the recovery of dissolved Fe and Al in AMD with high purity of precipitates.