Experimental investigations were conducted to determine the prospect of recovering valuable metals from acid mine drainage (AMD) from a Canadian source. In particular, potential for the recovery of zinc as sulfide for recycling to a zinc roaster has been examined. Several chemical methods were evaluated to selectively precipitate and recover metal ions, and a conceptual flowsheet, consisting of a three-step process, was developed. The process comprised oxidation of Fe(II) and precipitation of iron as ferric hydroxide in the first step, precipitation of Zn and Cu as sulfides in the second step, and the removal of the residual metal ions by lime neutralization in the third step. Alternative methods for selective precipitation that were evaluated included a two-step process in which ferric •iron was precipitated with limestone, followed by precipitation of zinc and copper hydroxides and other metals using sodium hydroxide. The precipitation of sulfides was studied using sodium sulfide, sodium hydrosulfide and hydrogen sulfide. The results to date showed that almost 100% iron is precipitated in step 1, more than 90% zinc recovery with greater than 50% Zn grade can be obtained in step 2, and a discharge eflluent with less than 1 mg/L heavy metals (Fe, Cu, Mn, Al) is obtained in step 3. The zinc grade was superior to that obtained by other methods evaluated which produced only 30% Zn grade. Economical and technical constraints of the three step process are reagent costs and solid-liquid separation. Acceptability of zinc hydroxide for recycling is uncertain. Alternative approaches for metal recovery based on these findings are suggested.

By a new synthetic technique, macroporous crosslinked polystyrene resin was functionalized with ligands containing sulfur, nitrogen, and/or oxygen atoms. Preliminary studies indicate that when added to a synthetic acid mine drainage (AMD) solution, some of these functional polymers were capable of selectively binding zinc without becoming "fouled" by iron. Elution of the loaded resins with acid to recover the zinc was also possible. The greatest selectivity for zinc was obtained with functional groups that contained only electronically "soft" nucleophilic centers such as sulfur and nitrogen, and not oxygen atoms, which are electronically "hard" (and prefer such "hard" electrophiles as magnesium, aluminum, or iron cations). The perfo1mance of some of these new materials exceeded that of the commercial ion-exchange resins tested with respect to capacity and selectivity.