Mine Drainage and Surface Mine Reclamation. Volume I: Mine Water and Mine Waste. Vol. I. Pittsburgh, PA: U.S. Bureau of Mines, 1988 Apr; :83-90
Federal regulations require mine operators to reduce the average concentration of manganese in the effluent to 2 mg/L. To meet this standard, the majority of mine operators add an alkaline material, typically lime or sodium hydroxide, to raise the pH to about 10.0. Our laboratory tests using actual acid mine drainage (AMD) containing iron, manganese, nickel, copper, zinc, and chromium, have indicated that the high-pH precipitation method is effective at removing these metals. However, manganese, nickel, zinc, and copper in the precipitated sludge were susceptible to dissolution upon subsequent depression of pH. Up to 30 percent of the original manganese in the high-pH precipitated sludge dissolved at pH 7.5. At pH 6.0, this figure increased to 78 pct. Additionally, 30 percent of nickel, zinc, and copper redissolved at pH values of 6.8, 5.7, and 4.7, respectively. Iron and chromium were stable down to pH 3.5. No differences were found between the use of lime or sodium hydroxide regarding the stability of the precipitated sludge. In mine waters containing high concentrations of iron in relation to manganese and other metals, manganese and other metals were removed at lower pH values than in mine waters with less iron. However, manganese in the sludge precipitated from the high iron water was less stable. Aging also affected the stability of manganese in AMD sludge. After sludge had aged for three months, 50 percent less manganese was released upon depression of pH. The problem of manganese redissolution can be avoided by using an oxidizer such as sodium hypochlorite or potassium permanganate; however, these chemicals were ineffective at removing copper, nickel, and zinc. To reduce manganese below 2 mg/L, the use of sodium hypochlorite or potassium permanganate increased chemical treatment costs by factors of 2.3 and 2.6, respectively, over the chemical costs for sodium hydroxide treatment.