BEHAVIOR OF MERCURY IN FGD BYPRODUCTS

Wet flue gas desulfurization technologies (FGD) used for the removal of SOx can result in the co-removal of mercury (Hg). Subsequently, a large portion of this Hg can be incorporated into the FGD slurry and its solid byproducts, including synthetic gypsum. FGD gypsum is commonly used for the manufacturing of wallboard but other uses include cement/concrete formulations and soil amendments. The amount of Hg in FGD products may increase in the future if these units are optimized for co-capture. Among the issues that arise are the potential for atmospheric and groundwater releases of Hg during the manufacturing processes, releases from the manufactured products, and post-disposal mobilization from the wallboard or other product.

Leaching studies of coal utilization byproducts (CUB) are often performed to determine the compatibility of the material with a particular end-use or disposal environment. These studies typically employ either a batch or a fixed-bed column technique. Column techniques provide leach data over extended periods of time, at progressively increasing liquid-to-solid ratios, and often over a range of pH values. Unfortunately, clogs can form in fixed-bed leaching columns, either because of cementitious properties of the material itself, such as is seen for fluidized-bed combustion (FBC) fly ash, or because of precipitate formation, such as can occur when a high-calcium ash is subjected to sulfate-containing leachates. Also, very fine-grained materials, such as gypsum, do not provide sufficient permeability for study in a fixed-bed column. A continuous, stirred-tank extractor (CSTX) is an alternative technique that can provide the elution profile of column leaching but without the permeability problems. The CSTX has been successfully employed in the leaching of FGD products that would not be sufficiently permeable under traditional column leaching conditions. The results indicate that the leaching behavior depends on a number of factors, including (but not limited to) solubility and neutralization capacity of the mineral phases present, sorption properties of these phases and behavior of the solubilized material in the tank. Leaching to near-exhaustion of a FGD-derived wallboard allowed the isolation of a highly adsorptive phase. This phase is present in at least one FGD gypsum and accounts for the immobilization of trace metals including Hg.