GENERATION OF ALKALINITY IN AN ANOXIC LIMESTONE DRAIN

Abstract

The rate of limestone dissolution and alkalinity generation in an anoxic limestone drain is determined by many factors, including the quality of the limestone used and the partial pressure of carbon dioxide within the system. Carbon dioxide concentrations greater than 600 times atmospheric levels have been found within an anoxic limestone drain located in northwestern Pennsylvania. This situation greatly increases the solubility of limestone, making elevated alkalinity concentrations possible. Before construction of the drain, the mine drainage contained over 400 mg/L acidity as CaC03 equivalent but alkalinity concentrations of more than 300 mg/L have been found at the exit of the drain. Alkalinity generation rates of meter-3 of drain (bulk volume) day-1 approximately 117 grams of alkalinity (as CaC03 eq.) have been determined for this system. The anoxic limestone drain discharges the mine water into a settling pond-constructed wetland system where metal oxidation, hydrolysis and precipitation occurs in a strongly buffered, alkaline solution. The use of the anoxic limestone drain resulted in substantial cost savings compared to conventional chemical treatment of this drainage.