LIMESTONE NEUTRALISATION OF IRON(II)-RICH ACID WATER

pH often is the single most important parameter to be adjusted during industrial effluent treatment. Waters with pH levels below 5.5 can be toxic to plant and fish life. Acid mine waters contain high concentrations of dissolved heavy metals and sulphate, and can have pH values as low as 2.5. Acid water is produced biologically when pyrites in coal waste is oxidised biologically. Unless treated, such waters can not be discharged into public streams. Lime/limestone neutralisation Both limestone and lime are needed during treatment of acid mine waters. Benefits associated with limestone are its low cost compared to other alkalies (Table 1), automatic dosage control (in the case of a fluidised-bed reactor) as limestone only dissolves at pH-values below 7.5, and limestone is safe to handle. Benefits associated with lime compared to limestone are its ability to remove metal and lower transport cost. Metals like manganese and magnesium can be removed with lime due to its ability to raise the pH of water to 12. A pH of 9.3 is needed for manganese removal and 11.2 for magnesium removal. Through high lime treatment sulphate can also be removed to low levels of 1 200 mg/o (as SO4). This is due to removal of magnesium and high calcium concentration which reduces the equilibrium sulphate concentration as a result of the solubility product of gypsum. The transport cost of lime is lower than that of limestone due to its lower molecular mass (56 for CaO versus 100 for CaCO3). Table 1. Neutralisation cost with various alkalies