Ash Leachability

A realistic measure of trace element leaching from fly ash dams?

An understanding of the factors influencing trace element leaching will allow strategies for limiting specific element leaching to be developed leading to optimisation of ash dam operation.

Figure 1: Experimental leaching columns

The CCSD is providing a much needed database on the trace element leachability of Australian coals.

Such a database will:

• Aid in the marketing of Australian export thermal coals,
• Provide an insight into managing and minimising the environmental impact of ash dams.

Standard methods of measuring trace element leaching from fly ash samples bear little relation to actual ash dam conditions and add little to the understanding of the chemistry involved. The Centre’s work investigates trace element leaching in a way which will provide results of relevance and practical use to ash dam operators.

Fly ashes produced from nine Australian black coals have been placed in 1 m columns (Figure 1) and slowly leached over fifteen months. The trace elements (As, Hg, Se, Cd, Cu, Ca, Ba, Pb and others) washed out of the column have been measured, to provide an extensive database from which correlations and understanding can be drawn. On completion of the tests it will be possible to compare the mineralogical and chemical composition of the leached fly ash to its parent ash.

The analysis of the data has begun to show interesting trends which may influence the future operation of ash dams. It has been found that many of the more toxic trace elements are leached within 1-2 volume ratios (volume of water passed through column : volume of ash in column) which suggests that most trace elements are washed from the ash during sluicing. Others, such as barium, may have a delayed release dependent on their solubility and interaction with other dissolved species. The pH of the liquor exiting some ash columns has been found to rise initially and then fall again. These observations are not possible in standard shake tests.

Selenium leaching has been shown to depend on the iron content of the ash. This indicates that Se leaches quickly, but then re-adsorbs onto iron oxides. Substantial variations in leaching have also been found between ash from different coals (Figure 2).

As data analysis continues, additional columns have been commissioned to investigate the influence of coal combustion conditions on ash leachability. This study includes column leach tests on ash produced from three Australian coals, at two temperatures, in a pilot scale test facility (ACIRL).

An understanding of the factors influencing trace element leaching will allow strategies for limiting specific element leaching to be developed leading to optimisation of ash dam operation.

Figure 2: Variation in Selenium leaching for a range of Australian coals

Interim Findings

• Standard batch leach tests grossly over-estimate trace element release.
• Column leaching better simulates actual leaching from storage dams and provides time dependent data.
• Selenium leaching appears inversely proportional to iron content.

The information gained from this project will compliment work on "Trace Element Partitioning in Australian Coals", which has, thus far, established the principal residences for trace elements in coal. When combined with a knowledge of the deportment mechanisms for coal utilisation processes reliable models can be developed. These models will provide an early warning system for environmental impacts based on the trace element content and distribution in a coal.

For more information contact:

Prof Colin Ward
Professor of Geology
University of NSW

Mr Ken Riley
Research Scientist
CSIRO DET