DETERMINING GEOCHEMICAL EQUILIBRIUM IN AN ACIDIC MINE PIT LAKE RECENTLY RECLAIMED WITH FBC ASH

Since August 1997, nearly two million tonnes of dry co-generation derived fluidized bed combustion (FBC) ash has been placed into a former acid mine lake. The lake was formed as a result of surface mining of the Mammoth coal seam in the Eastern Middle anthracite field of Pennsylvania. The goals of filling the 511,000 m³ lake with alkaline FBC ash were to: 1) remove a surface hazard, 2) decrease the flow of surface water entering the deep mines, 3) provide a source of alkalinity to the basin, and 4) build a structurally stable fill so land reclamation may take place.

Now that the demonstration project is nearing completion, and the ash pore water can be sampled, new insight has been gained into the long-term geochemical processes taking place. A model of the ash fill chemistry has been developed using the pore fluid of Portland cement as an analog. The presence of 6.5 wt% portlandite in the ash altered the mine pit water from an initial pH of 3.6 to 12. The pH value has remained relatively constant, but alkalinity has varied with ash input. Both hydroxide and silicic acid contribute to alkalinity, in addition to the carbonate component, which dominates in most natural waters. At a pH of 12, the aluminum was expected to be mobilized and the concentration in the pit lake water should have been approximately 5 to 6 orders of magnitude larger than at pH 6.6. However, the observed concentrations remained at levels expected for neutral waters. Geochemical modeling of the water suggests that the mineral phase ettringite is responsible for suppressing dissolved aluminum. Ettringite formation requires a substantial sulfate activity, but this is easily provided by the CaSO₄ present in the ash. Tobermorite, a common phase in the hydration of Portland cement, also likely plays a role in the long-term strength development of the ash fill.