The Dewatering and Agglomeration of FGD Gypsum into Micropellets for Use as a Soil Amendment

摘要

The Thiosorbic magnesium-enhanced lime flue gas desulfurization (FGD)scrubbing process has been advanced, called ThioClear~®, that enables recovery of a very bright and pure gypsum byproduct. Co-sponsored by the Ohio Coal Development Office and Cinergy, the Dravo Lime Company evaluated dewatering, material handling, and agglomeration processes to convert a 25% solids gypsum slurry stream into a user friendly product. Vacuum filtration, belt press, and centrifuge processes were evaluated on bench and pilot scales. The dewatered gypsum using a pilot scale decanter centrifuge ranged from 60-85% solids and was thixotropic above 80% solids due to the large needle-like crystal shapes and presence of 1-5% magnesium hydroxide. Due to the use of lime instead of limestone, the synthetic gypsum had a high brightness color.rnA pilot scale solid bowl decanter centrifuge was tested to dewater the 25% solids slurry. It produced a 75-82% solids cake which was initially sticky and thixotropic. In an outdoor stockpile the gypsum wicked and lost enough moisture eventually to be agglomerated. This self-hardening behavior was believed to be caused by a shearing of the needle shaped crystals that with evaporation of excess moisture allows the crystals to re-knit. Cube strengths of about 500 psi were obtained after 28 days of curing.rnOver two million tons of gypsum are used in the US as a soil amendment but mostly in the South and West. Little is used on Ohio Valley and Appalachian area soils. Recent studies by The Ohio State University and U.S. Department of Agriculture Appalachian Soil Laboratory have shown the benefits of magnesium-enhanced FGD gypsum as a soil amendment in agriculture and a soil substitute for mineland reclamation in the above geographical area. The challenge of economically producing a 6 x 16 mesh micropellet from a high moisture FGD gypsum is addressed in this study. Test results are described on using a high rpm pin mixer and water soluble lignin binder to condition 15% moisture gypsum with backmixing dry fines to feed a conventional disk or drum pelletizing process followed by a pellet curing and sizing process. A 20 ton/hr production demonstration was successful and economical using a drum pelletizer.