Testing of Interactions between Coal Ash Leachates and Rock Materials for Mine Backfill Evaluations

A series of laboratory-based leaching tests has been developed to evaluate the chemical interaction between ash, water and different mine-site rock materials, as a basis for assessing the potential environmental impact of using coal ash as backfill in coal mines. Two different test protocols were used: a two-stage serial batch testing program in which leachate from representative ash samples were allowed to interact separately with relevant rock materials under laboratory conditions, and a single-stage mixed batch testing program in which 50:50 mixtures of the same ashes and rock samples were extracted with water following a similar procedure to leaching tests used for the individual ash and rock samples. The concentrations of major and trace elements released in both studies were compared to the concentrations expected in the relevant experiments from the leaching characteristics of the individual ash and rock samples when tested alone. In both the serial tests and the mixed-batch tests these showed markedly lower concentrations of several key elements in solution (e.g. Mo, As, Se) after interaction of the ash leachate with the rock samples. The results indicate that some elements released from the ash in a mine backfill may be attenuated to a significant extent when the leachate comes into contact with other rock materials. The extent of attenuation was greatest for rocks with high proportions of clay minerals (e.g. shales) or Fe oxyhydroxides (e.g. iron-stained sandstones). Rocks with lower clay or Fe oxy-hydroxide contents, such as white quartz sandstones, apparently have lesser attenuating effects. The attenuation properties appear to be related to the nature of the rock in question, and test programs of this type may provide a useful basis for assessing the potential impacts of placing different ashes in close association with relevant rock strata in mine backfill and similar applications. Although testing for any individual site should also consider the design of the proposed backfill, the regional hydrogeology, and the results of other site investigations, collection, and evaluation of data from one or both procedures may be of value in more fully understanding and predicting the environmental impact of ash-based mine backfill programs.