LEACHING OF TRACE METALS FROM FGD GYPSUM

A continuous, stirred-tank extractor (CSTX) is an effective technique for evaluating the leachability of contaminants from flue gas desulfurization (FGD) products. It allows the chemistry of the leaching process to be studied in more detail than achievable with more traditional batch and column techniques. The continuous stirring provides constant mixing as occurs in more traditional batch-leaching tests while the continuous flow provides data over a wide range of pH values and liquid/solid ratios such as those seen in column leaching studies. Metal release patterns were examined in detail over a range of pH values extending from the material's natural, slightly alkaline pH to acidic pH conditions. The leaching behavior of individual elements depends on several factors including, but not limited to, the solubility of the mineral phases present, the sorption properties of the remaining phases and the neutralization capacity of the minerals. Bulk FGD gypsum is moderately soluble; dissolution is controlled by its solubility product and hydration reactions rather than pH. Elution and pH profiles indicate the presence of alkaline material(s) that buffers the system during the initial leaching. Iron and aluminum are not leached until the buffering capacity is exhausted. Elements bound to these phases can be mobilized during the low-pH dissolution. Metals not released during leaching become concentrated in a minor, insoluble residue remaining at the end of each experiment. Understanding the fundamental mechanisms operating during the leaching process provides a basis for evaluating the safety of FGD byproducts and ensuring these materials are used and disposed of appropriately. This information may also provide a basis for capture and sequestration technologies for mercury and other metals.