Production Development and Utilization of Zimmer Station Wet FGD By-Products: Volume 5, A Laboratory Greenhouse Study Conducted in Fulfillment of Phase 2, Objective 2 Titled <em>Use of FGD By-Product Gypsum Enriched with Magnesium Hydroxide as a Soil Amendment</em>

Abstract

The Clean Air Act, as revised in 1992, has spurred the development of flue gas desulfurization (FGD) technologies that have resulted in large volumes of wet scrubber sludges. In general, these sludges must be dewatered, chemically treated, and disposed of in landfills. Disposal is an expensive and environmentally questionable process for which suitable alternatives must be found. Wet scrubbing with magnesium (Mg)-enhanced lime has emerged as an efficient, cost effective technology for SO2 removal. When combined with an appropriate oxidation system, the wet scrubber sludge can be used to produce gypsum (CaSO4-2H2O) and magnesium hydroxide [Mg(OH)2] of sufficient purity for beneficial re-use. Product value generally increases with purity of the by-product(s). The pilot plant at the CINERGY Zimmer Station near Cincinnati produces gypsum by products that can be formulated to contain varying amounts of Mg(OH)2. Such materials may have agricultural value as soil conditioners, liming agents and sources of plant nutrients (Ca, Mg, S). This report describes a greenhouse study designed to evaluate by-product gypsum and Mg gypsum from the Zimmer Station pilot plant as amendments for improving the quality of agricultural soils and mine spoils that are currently unproductive because of phytotoxic conditions related to acidity and high levels of toxic dissolved aluminum (Al). In particular, the technical literature contains evidence to suggest that gypsum may be more effective than agricultural limestone in modifying soil chemical conditions below the immediate zone of application. Representative samples of by-product gypsum and Mg(OH)2 from the Zimmer Station were initially characterized. The gypsum was of high chemical purity and consisted of well crystalline, lath-shaped particles of low specific surface area. By contrast, the by-product Mg(OH)2 was a high surface area material (50 m2 g-1) that contained 20% CaSO4 with variable hydration state. Artificial blends of these materials containing 4% and 8% Mg(OH)2 were prepared for comparison with other liming agents in the form of agricultural limestone and gypsum amended with laboratory Ca(OH)2.