Recovery of valuable metals from acid mine drainage by selective titration

Acid solutions associated with the non-ferrous metal mining industry contain economically significant amounts of heavy metals including Al, Cu, Zn, Mn and Mg. These metals are usually rendered non-recoverable by the conventional neutralization treatment technologies used to create environmentally acceptable discharge. Acid solutions which are generated as the result of the mining of a porphyry Cu deposit were subjected to laboratory scale titration sequences. When these base titrations involve appropriate initial additions of an oxidizing agent and the sulfide ion, better than 85% of the dissolved metals are sequentially separated from the solution in distinct pH regimes. These precipitates include the sulfides of Cu and Zn and the hydroxides of Fe, Al, Mn and Mg. The possibility of using specific conductance and ionic strength of the solution as a means of determining appropriate endpoints in the titration sequence is discussed. These sequential metal separations provide a means for the recovery of a substantial amount of the cost of generating an environmentally acceptable discharge.