Presented paper

IMWA2019 Students work

Hydro-Geochemical Properties Of Class F Fly Ash When Leached With Acid Mine Drainage

Johnson, Angelo Gerald (1); Nel, Jacobus Marthinus (1); Reynolds-Clausen, Kelley (2)
1: The Institute of Water Studies, University of the Western Cape, Private Bag X17, Bellville 7535, South Africa; 2: Eskom Holdings SOC, Research, Testing and Demonstration, Private Bag 40175, Cleveland, 2022, South Africa

Several coal-fired power stations in South Africa are running out of storage space at their coal ash landfill sites and the electricity provider is therefore researching alternative use options for their coal ash. This study focus on the feasibility of using coal ash to backfill, historical and future, acid producing coal mines. In aims to firstly dispose of coal ash and secondly not negatively influence the decant mine water quality. However, there is limited knowledge of how South African ash would behave in acid producing mines. Therefore the aim of the project is to improve the understanding of the hydro-geochemical properties of coal ash backfill with reference to acid mine drainage (AMD).

Darcy up-flow column tests were conducted, in the laboratory, to assess the change in hydraulic conductivity (K) of ash, when leached with natural AMD. The influent and effluent from the column testing was monitored for pH and heavy metal concentrations to investigate the chemical changes in the AMD, flowing through ash.

During the placement of coal ash, the moisture allows pozzolanic reactions to solidify the coal ash and lowers the K, by an order of magnitude, relative to fresh ash. Secondary mineralization of calcium minerals, in the coal ash contributes to a further decrease in the K, by another order of magnitude. Sulphate and iron minerals from the AMD also played a major role in the decreasing K as they accumulate in void spaces and having a clogging effect. From the chemical analysis of the leachate samples, it was observed that iron and chromium do not leach. The alkaline nature of the coal ash initially neutralizes the acidic levels of AMD from an inflow pH = 2.5 to an outflow pH = 11. Acidification of the outflow towards a pH = 4 was observed, due to large volumes of AMD flowing through short coal ash columns. The K decreased 3 orders of magnitude, from an initial 10-1 m/d to 10-3 m/d, with the AMD iron concentration above 200 mg/l playing the dominant role in reducing the hydraulic conductivity.

The placement of a coal ash monolith into a coal mine would not negatively influence the current decanting AMD water quality. The water table within the mine spoil backfill can potentially be increased by the placement of a coal ash monolith, regulating the decant elevation within the mine. Ultimately, reducing AMD decant volumes and improving water quality from coal mines.