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Prediction of Long-term Salt Generation from Coal Spoils

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Published: January 19Project Number: C25039

Get ReportAuthor: Mansour Edraki, Neil McIntyre, Thomas Baumgartl and Melinda Hilton | University of Queensland

The release of salts from spoil piles has the potential to affect surface and groundwater quality particularly the quality of water in final voids. Current hydro-geochemical salt balance models either assume that the total amount of inherent salts in spoils will be released over time or often predict increasing project water salinity into the future based on site monitoring data from electrical conductivity measurements. Approaches taken to the closure and rehabilitation of spoil piles and final voids, and evaluation of residual risk can be affected by the uncertainty in such model outputs.

The overall aim of this project was to develop a process for estimating long-term salinity generation rates from different classes of mine spoil and spoil pile configurations that can be used in conjunction with water balance models to predict long-term final void salinity levels or the residual risk to receiving surface water or groundwater environments.

Representative samples were collected from three mine sites in Queensland. Samples were thoroughly characterised and grouped into three major rock types based on lithology, mineralogy and texture; 1) medium to coarse grained sandstones with a grain-supported texture, 2) fine- to medium- grained sandstones with abundant authigenic minerals, and 3) mudrocks consisting of siltstone and mudstone.  The spoil samples were further classified into seven classes with respect to the degree of potential salt release using results of 'first flush' and 'degradation' tests.

Following the classification, selected spoils were subjected to a series of funnel leaching experiments to assess the role of particle size and water/rock ratios in the long-term release of salts. The funnel experiments confirmed the importance of the spoil classification to salt release, but also showed the importance of the particle size and hydrological conditions (water/rock ratio). The results of funnel tests were used to calculate average salt release rates and salt decay rates for different spoil types and size fractions and to derive the time-dependence of salt decay values (K values).

The funnel tests proved to be a practical and easy to use test to assess the salt release rates of coal spoils. It was particularly useful to collect multiple aliquots of leachate at each cycle of leaching to see the change of salinity in between leaching cycles. However, it is important to adjust the leaching cycles according to the initial classification of spoils. In other words, the design of bench-top leaching tests should be tailored to spoil classes.

With support from ACARP and in-kind contributions from The University of Queensland, a medium scale (1-1.5 tonnes) spoil leaching test facility was set up in 2016 at UQ's property in Pinjarra Hills to bridge the gap between small scale laboratory tests and field monitoring of real size spoil piles.

Fresh and typical Permian Rock-like and a soil-like spoil samples were transferred directly from site to IBC tanks. Thus the IBC samples represent the water/rock ratios, preferential flow paths, and governing solute transport processes in real-sized spoil. Furthermore, the size of the IBC samples means that they are not subject to the rapid desiccation that is often the case in bench-top leaching tests.

Salt release curves from typical soil-like and rock-like spoils were generated under unsaturated and saturated conditions, with the saturated conditions releasing higher amounts of salts in shorter periods of time due to physical disintegration and better leaching conditions.

The results of the tracer tests indicated that both the soil-like and rock-like spoils were hydrologically comparable to rapidly draining sands, but the soil-like spoil results indicated significant pockets of less permeable material that retains and releases a large amount of solute. The IBC tests produced a set of solute release functions (or spoil salinity decay curves). The reliability of these functions for predicting long-term salt release will increase as more measurements are obtained in a follow-up project.

The numerical modelling focussed on estimating long-term salt leaching rates, consistent with a generally recognised conceptual model of a spoil pile, consistent with observed salt loads from 11 cycles of the unsaturated IBCs and consistent with decay rates derived from the funnel experiments. Extrapolation to a 128-year period of observed climate conditions demonstrated possible long-term behaviour of the spoils. There is a need to further optimise and test the models as new measurements become available, and to integrate the models with hydrological models of real-size spoil piles.

The classification scheme, bench-top funnel leaching tests, medium scale IBC leaching procedures, and the numerical model developed in this project can help the mine sites to assess the potential release of salts from spoil piles. The outcomes of the study are expected to be particularly useful for the development of Progressive Rehabilitation and Closure (PRC) plans.

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