Passive Treatment of Neutral Mine Drainage at a Metal Mine in New Zealand Using an Oxidizing System and Slag Leaching Bed

Abstract

Rehabilitation at a metal mine in New Zealand is complete with the exception of a 22 L/s discharge pumped from underground. The discharge has a pH of ≈6, alkalinity of ≈150 mg/L, dissolved oxygen (DO) <1 mg/L, elevated Fe and Mn, as well as elevated concentrations of Zn and As. Planning passive treatment was difficult because historical total Fe ranged from 20 to 200 mg/L, and Mn (11–22 mg/L) is soluble in the circumneutral pH range, due to conditions in the workings. Speciation analysis and modelling of the mine drainage chemistry indicated four factors were important for passive treatment design: (1) dissolved Fe is ≈20 mg/L and all is dissolved Fe(II); (2) Fe(II) concentration is stable because it is limited by saturation with respect to siderite (FeCO3); (3) the remaining Fe is colloidal Fe(OH)3 with a variable concentration; and (4) Mn is limited by saturation with respect to rhodochrosite (MnCO3). Equilibrium of Fe and Mn with minerals means that dissolved concentrations have an upper limit and are relatively stable, allowing an optimized treatment system. A pilot-scale passive treatment system was installed that included an oxygenation cascade of drops through V-notch weirs, settling ponds, and a slag leaching bed. Oxidation of Fe(II) to Fe(III) was followed by precipitation and settling of Fe(OH)3. Bicarbonate alkalinity in the mine drainage prevented acidification, and release of dissolved CO2 caused the pH to increase slightly. Manganese was removed by precipitation (of carbonates, oxides or oxy-hydroxides) in the slag leaching bed at elevated pH and high DO. Zinc and As were removed by adsorption onto Fe(OH)3. The oxygenation system removed 82–96 % of the Fe and 10 % of the Mn. The slag leaching bed removed 99 % of the remaining Mn.