The application of coal discards for acid mine drainage neutralization

Abstract

The neutralization of acid mine drainage (AMD) with coal discards in percolating columns was investigated as a potential precursor to lime neutralization. The neutralizing capacity of three coal samples, A (70% ash), B (25.3% ash), and C (28.9% ash, estimated), sourced from three South African coal mines, was determined at different crush sizes (–40 mm, –12.5 mm and –6.3 mm). AMD solution obtained from another local coal mine was percolated over the coal samples packed in 1 m and 6 m columns, until the pH of the accumulated drainage solution measured approximately pH 7. Samples B and C, with an alkalinity content equivalent to 2.3% CaCO3, achieved neutralizing capacities of 2112 L AMD and 929 L AMD per ton coal respectively, at a –6.3 mm crush size. Sample A, with an alkalinity content equivalent to 0.48% CaCO3, neutralized only 282 L AMD per ton coal at the same crush size. An economic analysis was performed to compare neutralization with waste coal against lime neutralization in tanks. The analysis was based on a rate of AMD generation of 1750 m3/d, a neutralizing capacity of 1.4 m3 AMD per ton coal, with capital costs estimated at R18 million for lime neutralization and R27.6 million for coal neutralization. Operating costs were estimated at R24 million for lime neutralization and R9 million for coal neutralization. AMD neutralization with suitable waste coal may therefore be less expensive than neutralization with lime.