Sludge Generation in the Treatment of Acid Mine Drainage (AMD) by High-Density Sludge (HDS) Recycling Method-Optimum Neutralization Process of Horobetsu AMD (First Paper)

Calcium hydroxide (Ca(OH)2), a combination of calcium carbonate (CaCO3) and Ca(OH)2 (two step) and magnesium oxide (MgO) were used as a neutralizer in the treatment of synthetic Horobetsu Acid Mine Drainage (AMD) by High-Density Sludge (HDS) recycling method to obtain the appropriate guidelines of optimum AMD treatment. The result during the initial neutralization demonstrated that at pH 7, arsenic, either As(III) or As(V) in solution, could be removed by adsorption to the sludge with a concentration in the supernatant solution below the Japanese regulated environmental effluent standard. The sludge generated in HDS recycling method with Ca(OH)2 neutralization was composed of Fe(III) hydroxide combined with a high amount of gypsum (CaSO4·2H2O), the same as in the two step neutralization in which undissolved CaCO3 was included. The sludge generated in HDS recycling with MgO neutralization was composed of basic ferric sulphate combined with a low content of undissolved MgO. The sludge-settling rate generally decreased as the recycling process proceeded. The settling rate of the sludge from MgO neutralization was faster than that of the sludge from the Ca(OH)2 and two step neutralization. The Fe density of the sludge by MgO neutralization was highest compared to the other neutralization.