MARKETABLE AMMONIUM SULFATE FERTILIZER AND FINE CALCIUM CARBONATE FROM FGD-GYPSUM

The overall goal of this project is to assess the technical feasibility of a process to produce two marketable products, ammonium sulfate fertilizer and precipitated calcium carbonate (PCC), from wet limestone flue gas desulfurization (FGD) by-product gypsum. The wet FGD process, although effectively removing S02 by using limestone, produces a large amount of solid gypsum. Power plants burning high-sulfur coal and using the FGD technologies, in addition to installation and operation expenses, are facing expensive landfill disposal costs for the gypsum. Also, the CAA Amendments of 1990 required S02 emissions be further reduced from 2.5 to 1.2 lb/MBtu by the year 2000, which will further decrease the sulfur compounds from air deposition and result in a growing demand for sulfur as a plant nutrient.  Our previous study (1994-1995) focused on a process that converted gypsum to ammonium sulfate fertilizer with a byproduct PCC. The cost estimates suggested that the process is economically feasible when granular size ammonium sulfate crystals are produced, however, the process conditions to produce an acceptable PCC for commercial utilization were not taken into consideration, which is the main objective of this phase of the investigation.

In the current study, various specifications for commercial applications of calcium carbonate, both ground and precipitated, were reviewed. The most sig:q.ificant attributes of carbonate fillers that determine their usefulness in industry are particle size (i.e. fineness), whiteness (brightness), and mineralogical and chemical purity. Reaction conditions are used to control the size and shape of the PCC particles produced, and removal of colored impurities in the gypsum increased the whiteness of the PCC products. The impurities of the gypsum from Abbott power plant were recognized and their removal methods were developed. The results suggested that either limestone with minimum colored impurities should be used during FGD processes, or purification procedures for FGD-gypsum could be used to produce a high whiteness of PCC for higher value commercial applications. The research effort is being continued in modifying reaction conditions and purification procedures for a production of a high quality PCC adequate for paper applications.