FINAL TECHNICAL REPORT

September 1, 1996, through August 31, 1997

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

ICCI Project Number: 96-1/3.2A-2

Principal Investigator: M.-I.M. Chou, Illinois State Geological Survey (ISGS)

Other Investigators: J.M. Lytle, S.J. Chou, K. Ghiassi, and M. Luo (ISGS); Sherman Cox and G.R. Jividen, Specialty Minerals (SM); S. Dewey, AlliedSignal-Chemicals, J. Randall, City Water Light and Power Co. (CWLP), J. Hill, Agrium U.S. Inc. (AG); D. Fortik, Abbott Power Plant, University of Illinois (UIUC); R. Desollar, Central Illinois Public Service Co. (CIPS)

Project Manager: D. D. Banerjee, ICCI

ABSTRACT

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 SO2 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 SO2 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 by product 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 significant 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.

EXECUTIVE SUMMARY

Wet flue gas desulfurization (FGD) processes that use limestone as a scrubber for SO2 and produce gypsum as a by-product have a considerable level of commercial development and demonstrated operational experience. These pollution control technologies will remain preferred choices for Phase-II compliance if successful commercial utilization of FGD by-products is developed to offset the cost of equipment installation, operation, and by-product disposal. A FGD system installed on a 500-MW plant burning 3.5% sulfur coal, with a desulfurization efficiency of 95% and a load factor of 65%, generates about 31 tons of gypsum per hour. From an environmental and economic standpoint, it is desirable to use this by-product as a feed material to produce a salable product. The goal of this project is to assess the technical feasibility for producing the dual commercial products, ammonium sulfate fertilizer and precipitated calcium carbonate, from this gypsum.

Ammonium sulfate is a valuable nutrient source for providing both nitrogen and sulfur to growing plants. There is a growing demand for sulfur in the sulfate form as a plant nutrient because of decreased sulfur compounds in atmospheric deposition. More stringent regulations to reduce sulfur dioxide emissions by the year 2000 (from 2.5 lbs SO2/106 Btu to 1.2 lbs SO2/106 Btu) suggest a growing demand for sulfur nutrients in the soil by the first decade of the 21st century. Also, the trend toward using high nitrogen content fertilizers has pressed incidental sulfur compounds out of traditional fertilizer. The current market for ammonium sulfate in the United States is about two million tons per year. It is anticipated that 5 to 10 million tons of new ammonium sulfate production may be required annually in fertilizer markets as a result of the acid-rain control program. The fertilizer industry is seeking a greatly increased source of such product to supply sulfur in nitrogen-phosphorus-potassium (N-P-K) fertilizer blends.

Production of fine calcium carbonate, by either wet-grinding natural high-calcium limestone (GCC) or by chemical precipitation (PCC), is becoming one of the most competitive industrial minerals markets. In traditional acid paper making, wood fiber is used as a filler and titanium dioxide (TiO2) is used as an additive to enhance the paper's whiteness. In alkaline paper making, PCC (at $100 to $300 per ton) can be used as a filler, which costs less than wood fiber (at $500 to $600 per ton). Some PCC can also offer such good light scattering quality that it can replace the much more expensive TiO2 ($2,000 per ton). Specialty Minerals Inc. has patented technologies to chemically manipulate the morphology of PCC crystals and currently offers its customers 12 different types of PCC. Particle sizes of these products range from 10 m to 0.01m with shapes including spherical, scalenohedral, and rombohedral. Due to the increasing use of alkaline paper making, PCC sales accounted for about 40% of the Specialty Minerals' $428 million in sales for 1993, up $46 million (30%) compared to 1992. Specialty Minerals is currently focusing on the paper industry, but in the future the company hopes to bring its PCC to new markets in the paint and plastic manufacturing industries.

In our previous study (1994-1995) on the current process to make ammonium sulfate from FGD-gypsum, the potential commercial applications for the by-product precipitated calcium carbonate (PCC) was not taken into consideration. The results from previous cost estimates, without considering the sale of the PCC produced, indicated that there was a healthy profit margin for converting FGD-gypsum to ammonium sulfate if a granule size of 1.2 to 3.3 millimeters could be produced. The results suggested that considering the potential markets of the PCC produced by the process would further improve the economics. Also, the average sale price of ammonium sulfate has been increasing. For example, for the years 1993, 1994, and 1995, the average price per ton was $129, $138, and $146, respectively. With the sale of ammonium sulfate at these prices, plus the possible sale of PCC by-product, the conversion of FGD-gypsum to ammonium sulfate and PCC becomes an attractive process that could improve the economics of FGD systems in Illinois.

The overall goal of this project is to assess the technical feasibility for producing the dual commercial products, ammonium sulfate fertilizer and PCC, from FGD-gypsum. Specific objectives of this phase of study were:

I. Assess knowledge on the current uses of fine calcium carbonate and their specifications for various commercial applications

II. Obtain and analyze a FGD-gypsum sample from an Illinois power plant for the work proposed in the project.

III. Refine the process engineering data, process conditions, and/or the process flow diagram which are required for process scale-up and technical and economical feasibility studies.

IV. Determine the influence of the properties of the FGD-gypsum and process conditions, if any, on the quality of the ammonium sulfate and calcium carbonate produced.

V. Evaluate the market potential of the by-product PCC made from conversion of FGD-gypsum to ammonium sulfate fertilizer.

VI. Prepare progress and final reports, presentations, and publications of the results.

A survey of literature to determine the current application, specification, and price of fine calcium carbonate and to define possible process limitations that need further examination was conducted. The most significant attributes of carbonate fillers, that help determine their usefulness in industry application, are particle size (i.e. fineness) and shape, whiteness (brightness), and mineralogical and chemical purity.

The particle size of commercial grades of carbonate fillers covers the range from 149 microns (100 mesh) to less than 5 microns. Some producers offer as many as twenty products each with a different particle size distribution. In many instances the products were designed to meet the special requirements of a certain application. Such specifications are normally the result of development work, which identifies superior performance characteristics for such "non standard" products. Changes in particle size distribution can cause a measurable change in product performance in some applications. The oil absorption or water demand of a filler is a function of its surface area. Packing density of the particles affects oil absorption, resin or water demand, and also the viscosity of aqueous and organic systems. Particle size is therefore rigidly controlled at all times during production.

A ground calcium carbonate normally has a rhombic morphology, since this is the natural shape of calcite fragments. With PCC the possibilities are more varied. The most popular PCC filler has the scalenohedral morphology, which very easily combines a suitable specific surface with any particle size wanted. Rhombohedral PCC plays a role in paper filler but is of more interest as a coating PCC. The aragonite is used for special purposes where more exotic properties of the paper are sought. The pseudo-amorphous PCC is really agglomerates of very fine calcite crystals. No practical use for these products is known, but they are of interest to the PCC producer since they tend to show up if control over the process is lost.

Whiteness of a carbonate filler is an important property of PCC for applications in paint, plastics, and paper industries. Other applications are less demanding, allowing the use of off-white to gray products. The use of off-white carbonate fillers for asphaltic roofing, asphaltic sealers, carpet backing, jointing and caulking compounds as well as some plastic applications are dependent upon filler properties such as particle size, freedom from abrasive components, and other performance related characteristics (oil absorption, bulk density, etc.).

Research, by PCC producers and users, continues to develop new products and applications for carbonate fillers. Finer products, and coated or surface modified products, have been introduced in recent years for special uses. In the past 12 years, there have been a period of dynamic growth in the use of calcium carbonate fillers. New applications, particularly in the plastics and paper industry, have necessitated expansion and upgrading of existing plants and the development of new facilities. This trend is expected to continue.

Chemical composition and particle size distribution of the FGD-gypsum sample collected from the University of Illinois Abbott Plant, Champaign, IL, were analyzed. The results showed that the sample contained 98.60% gypsum with impurities of limestone (CaCO3) 0.82%, MgCO3 0.25%, silica 0.40% in SiO2, iron 0.04% in Fe2O3, and magnesium 0.12% in MgO. The analysis of the four size-separated fractions showed that the most coarse, >100 mesh, fraction was less than 9 wt % of the total gypsum, but it contained mainly colored particles and this fraction was rejected from any feed application in this investigation. The PCC products from reagent grade gypsum, or purified FGD-gypsum, have a whiteness greater than 97% which is good for any application. However, the particle size of the PCC products is good for some applications. The results of this study suggested that in order to decrease the colored impurities such as iron and organic carbon in the gypsum, to produce a high whiteness PCC, limestone with minimum colored impurities should be used during the FGD process, or purification procedures for FGD-gypsum are needed to produce a high whiteness of PCC for high value commercial applications. Further improvement of the overall qualities of the PCC products should lead to a product that is adequate for any application.