FINAL TECHNICAL REPORTSeptember 1, 1996, through August 31, 1997

Project Title: NOVEL STRUCTURAL MATERIALS FROM ILLINOIS COAL SCRUBBER SLUDGE

ICCI Project Number: 96-1/3.2A-4

Principal Investigator: Vivak M. Malhotra, Southern Illinois University at Carbondale

Other Investigators: P. S. Valimbe, Southern Illinois University at Carbondale

Project Manager: Dr. Dan Banerjee, ICCI

ABSTRACT

The main goal of our project was to explore the feasibility of converting scrubber sludges, obtained from power plants combusting Illinois coals, into novel structural materials. In pursuit of this: (a) we conducted experimentation to alter the crystal growth habit of the sulfate-rich scrubber sludge so that structural composites could be formed from it, and (b) using the modified crystal growth properties successfully, we demonstrated the potential of forming table top and vanity top materials from the sludge. We subjected the as-received scrubber sludge (CWLP, Springfield, Illinois) and the sludge which had been dried at 180^OC to various formation pressures and temperatures to form 1.25 inch and 2.25 inch diameter cylinders. The chemical and physical structures of the pressurized materials were probed using scanning electron microscopy (SEM), transmission-Fourier transform infrared (FTIR), and differential scanning calorimetry (DSC) techniques. The formation temperature (20^OC < T < 220^OC) and pressure (400 psi < P < 5500 psi) were variables. Our previous characterization results had suggested that while the as-received sludge was largely gypsum phase, the hemihydrate phase was obtained when the sludge was dried at 180^OC. The formation pressure did alter the morphology of the crystallites formed from the sludge-derived hemihydrate-water slurry. The crystallite size was limited to 2 m to 40 m. However, the formation temperature not only controlled the size of the crystallites, but it also influenced the chemical structure of the fabricated materials. In fact at T > 100^OC, large interlocked crystallites were formed. The size of the crystallites was 20 m to 240 m in length and 5m to 50 m in thickness. Two models of composites, i.e., SEAN and ASV series, were fabricated from Illinois scrubber sludges. The type of fiber added to our composite strongly affected the flexural properties of our composites. It is worth noting that the cheapest fibers gave the best results. As expected the ingredients used to form ASV and SEAN series composites did affect the mechanical properties of the composites. However, what was interesting was that the mode of mixing of the ingredients, with ingredients being the same, very strongly affected the modulus of rapture and storage modulus of the formed composites.

Pages 1 to 22 contain proprietary information

EXECUTIVE SUMMARY

Objectives: The main goal of this project is to convert coal combustion residues, specifically scrubber sludge, into structural composite materials, e.g., decorative floor tiles, tabletops, and composite lumber. The proposed structural material should be machineable, could be formed into different shapes, and should be easy to handle.

Introduction and Background: Presently, more than 80 million tons of coal combustion residues are generated in the USA alone when coal is burned or gasified and when flue gases are scrubbed of sulfur dioxide. It is believed that the electric utility industry produces about 48 million tons of fly ash, 14 million tons of bottom ash, and 6 million tons of boiler slag. In addition to the aforementioned ashes, approximately 20 million tons of flue gas desulfurization (FGD) residues are produced in the USA every year. The Federal Clean Air Act Amendments of 1990 mandate that sulfur emissions by coal-fired industry be significantly reduced by the year 2000. This mandate has started to have and will have a serious impact on Illinois coal utilization since Illinois coal tends to be high in sulfur. The sulfur is in the form of finely dispersed inorganic minerals (chiefly pyrite) and is also organically bound. If Illinois coal's share of the market is to be maintained and expanded, then scrubbing the flue gases will play an even more important role in Illinois coal utilization. Unfortunately, the additional scrubbing will add to the 20 million tons of FGD residue already being produced in the USA.

Out of 68 million tons of coal combustion residues, i.e., fly ash, bottom ash, and slag, produced annually in the US, only about 18 million tons (27%) of it are currently utilized. Unfortunately, the utilization of FGD residue is about 6% with the rest, i.e., 18.8 million tons, going to landfills. With the current cost of residue disposal expected to rapidly escalate from $10/ton to $30/ton by the year 2000, the economic stakes for the coal producers and coal utilization industries are substantial.

It has been proposed that the gypsum produced as a by-product by the plants using FGD technology can be used in a variety of applications, e.g., road-based construction, wallboard manufacturing, and agriculture. However, due to the fluctuations in properties and variations in demand in the market, most of the scrubber sludge produced in the U.S. is presently used only in landfill applications. Therefore, technologies are needed which will convert scrubber sludge into valuable, but marketable, materials. This will not only make FGD sludge into useful products but will also reduce the costs associated with the dewatering of the sludge and its disposal for the electric utilities.

Results: In the first year of the project, we mainly focused on developing protocols for the fabrication of composites from CWLP scrubber sludge as well as on enhancing the mechanical strength of our composites. Specifically, we probed: (a) how the fabrication parameters of our scrubber sludge derived composite affect its physical and chemical structure, (b) how the curing time alters the structure as well as the thermomechanical properties of the sludge-based composite, (c) how the type of fibers controls the amount of static stress required to break the sample, and (d) how the fiber constituent alters the storage modulus of our composite.

To answer the above-mentioned questions, we conducted scanning electron microscopy (SEM), transmission-Fourier transform infrared (transmission-FTIR), differential scanning calorimetry (DSC), mechanical tests, and dynamic mechanical analyzer (DMA) measurements on composites fabricated in our laboratory. The following was concluded: (1) On hot pressing the as-received scrubber sludge, i.e., the sludge which contains no other ingredients, a primary skeleton structure was formed. However, this structure had poor mechanical strength, and the structure had a strong tendency to dust. (2) On heating the CWLP scrubber sludge at 180^oC for 2 hours, the sludge got converted from gypsum to hemihydrate phase. When materials were formed from hemihydrate-water slurry at 400 psi < pressure < 5500 psi, the crystallites formed in the material had a needle-like shape and were very different from the morphology of crystallites of gypsum in as-received scrubber sludge. The pressure variation did not alter the needle-like morphology, and the spectroscopic results revealed these needles to have a gypsum-like structure with small hemihydrate impurity phase. Generally, the crystallites were in the range of 2 um to 40 um. (3) The material formed from hemihydrate-water slurry was profoundly affected by the formation temperature. As the formation temperature increased from 60^oC, the size of crystallites formed in our material also increased. In fact at T > 100^oC, large interlocked crystallites were formed in our material. The size of the crystallites was 20 um to 240 um in length and 5 um to 50 um in thickness. The formation temperature also strongly influenced the phase of the calcium sulfate formed, but this is to be expected. (4) On forming the scrubber sludge based composites in which we used various types of biers along with an appropriate binder, the resultant composite did not display any dusting. (5) SEAN series of composites were formed at 95^oC. The formed composite largely contained gypsum and hemihydrate crystallites. The SEM results revealed that under our condition of fabrication, the crystal growth habit of the gypsum and hemihydrate was drastically altered in our composite. In fact, the crystallites formed "structural domains" in our composite material. We believe this is one of the most important characteristics of our material which imports mechanical strength to it and results in a well defined, smooth structure. (6) The curing of our composite at 200^oC dramatically altered its thermomechanical properties as shown in Figs. 1 and 2. As the curing time increased, more and more of gypsum crystallites converted into hemihydrate crystallites. However, the resultant orientation of structural domains remained intact. The variation in the storage modulus of the composite decreased as a function of curing time, suggesting that the length of curing time will have a beneficial effect on the thermomechanical properties of the material. (7) The ASV series composites, which were formed at 140^oC, showed that the ingredients were uniformly mixed throughout our sample. This was ascertained by conducting FTIR measurements on the samples obtained from different depths of the composite. (8) The crystal growth habit of "structural domains" was not altered by the incorporation of various types of fibers in our ASV series. (9) The type of fiber added to our composite strongly affected the flexural properties of ASV series composites. It is worth noting that the cheapest fibers gave the best results. (10) As expected the ingredients used to form ASV and SEAN series composites did affect the mechanical Properties of the composites. However, what was interesting was that the mode of mixing ingredients, with ingredients being the same, very strongly affected the modulus of rapture and storage modulus of the formed composites. This can be seen from Figs. 3 and 4.

The remainder of this report contains proprietary information and is not available for distribution except to the sponsor of this project.