TECHNICAL REPORT

September 1, 1995, through August 31, 1996

Project Title: DEVELOPMENT AND DEMONSTRATION OF A NEW APPROACH FOR WASTE COAL SLURRY MANAGEMENT USING NATURAL RESOURCE UTILIZATION BY-PRODUCTS

Principal Investigator: Dr. Y. P. Chugh, Southern Illinois University at Carbondale

Other Investigators: Dr. D. Dutta, Dr. S. Esling, and Dr. B. Paul, Southern Illinois University at Carbondale

Project Manager: Mr. Richard Shockley, ICCI

ABSTRACT

Dr. Chugh recently proposed an alternative approach for management of fine coal waste slurry (-28 mesh) involving natural resources utilization (coal burning power plants, limestone processing plants, cement plants, etc.) by-products. The approach involves formulating environmentally-benign products with characteristics suitable for mine reclamation. The proposed approach has significant merit over the current practice and can result in considerable savings for coal and electric utility companies. This two-year (Phase I) cooperative project, between coal companies, electric utilities, and the Coal Combustion Residues Management Program (CCRM) at SIUC, is developing the approach scientifically and will perform small scale (100-200 ton) demonstrations for six (6) mixtures at two mine sites in Illinois.

Raw materials were procured from two coal companies (coal slurries A and B), four utility companies (fly ashes A, B, C, D and ponded fly ash A, and scrubber sludge A, B) and one limestone processing plant (lime waste). Particle sizes, Atterberg's limits, paste pH, calcium carbonate equivalent (CCE) and the ASTM shake tests were conducted on all the raw materials. In addition to those tests, SEM/XRD, sulfur content, and density fractionation of slurries were also done.

Eight types of mixes involving 43 mix combinations were investigated and seven final mixes were identified for placement in the demonstration cells. The recommended mixes have 28-day compressive strength in 170 to 400 psi range and they attain a strength of more than 60 psi in one to three days. The calcium carbonate equivalent (CCE) of mixes vary from 26 to 53% and the long term (six months) pH appears to be in the range of 7.0 to 8.0. The leachates from the mixes are environmentally more benign than the individual component of the mixes and conform to Illinois water quality standards. Mixing and pumping equipment are currently being fabricated and field demonstrations are planned in early Fall.

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EXECUTIVE SUMMARY

Fine coal slurry waste (-28 mesh) from processing plants constitutes about ten to twenty percent (10-20%) of the run-of-mine coal. About seven million tons of waste coal slurry was produced in Illinois in 1994. It is estimated that coal operators currently spend between five and eight dollars per ton of slurry waste, or between seventy-five and ninety cents per ton ($0.75-0.90/t) of run-of-mine coal, for this waste disposal. The current disposal technology of pumping a relatively-low-solids-content (5-15%) slurry into leveed ponds presents long-term environmental problems related to ground water contamination, acid drainage and the stability of impounding structures.

The coal burning power plants and other natural resource utilization industries, such as limestone processing plants and cement plants, similarly produce low or zero value byproducts which are alkaline and pose long-term environmental problems similar to slurry waste disposal. The disposal cost for these by-products currently varies between five and ten dollars per ton and is expected to increase further over the next decade.

The Principal Investigator (PI) recently proposed an approach to develop environmentally benign mixtures involving coal slurry waste and natural resource utilization by-products. The developed mixes may be used in conjunction with disposal of coarse coal refuse or deposited in coal refuse embankment ponds. A preliminary economic evaluation of this alternative estimated the cost to be $2.27/ton which is significantly lower than disposal cost of slurry alone ($5-8/t) or CCB alone ($8-10/t).

This cooperative research and demonstration project between coal companies, electric utilities, and CCRM program at SIUC was initiated to further develop and demonstrate the concept. During Phase I (1995-1997), six different mixtures will be developed and 100- to 200-ton demonstrations are proposed at two mines.

In the first year (September 1, 1995, to August, 1996) of the project life, extensive laboratory studies were conducted to identify seven mixes suitable for surface demonstration. The raw materials for the mixes were coal slurries, fresh F-type fly ash (PCC and cyclone boilers), ponded F-type fly ash, fly ash stabilized and forced oxidation scrubber sludge, FBC fly ash, and lime waste.

Coal slurries were obtained from two Illinois coal processing plants. Three Illinois power plants supplied F type fly ashes (two from cyclone boilers and one from a PCC boiler) and one power plant in Illinois provided FBC fly ash. Two of the power plants also provided scrubber sludge (a fly ash stabilized dual alkali scrubber sludge and a forced-oxidation scrubber sludge). A limestone processing plant furnished lime wastes. Particle size analyses and Atterberg's limits tests were conducted on all the raw materials. Environmental characterizations of the raw materials involved determination of paste pH, calcium carbonate equivalent (CCE), total sulfur content and mineral phases (by scanning electron microscope/X-ray diffraction and dense medium centrifuging) of the slurries, response to ASTM shake and TCLP tests, and long-term pH stability.

The CCE data show that neither F type fly ash nor forced-oxidation scrubber sludge are powerful acid neutralizers. The high CCE materials also have high initial pH values around 12. However, these lime based phases do not persist in a wet low temperature environment and soon return to carbonate and sulfate phases which have pH readings in the 7.5 to 8.5 range.

Forty three (43) combinations of different by-products were investigated for their strength, flow, and environmental properties. Various geotechnical tests conducted on the freshly prepared and cured mixes are: flowability of the freshly prepared mixes measured by a flow table and a half-size ASTM slump cone, density of freshly prepared grouts, short-term strength measured by a penetrometer, 28-day uniaxial compressive strength, elastic modulus, and dry density of 28-day cured samples. Also, the bleeding test was conducted for the seven final mixes. Environmental studies of the cured mixes involved the determination of paste pH, CCE, long-term pH stability, and elemental analyses of ASTM shake test leachates.

The seven recommended mixes have 28-day compressive strength between 170 to 400 psi. Three mixes develop more than 60 psi strength in 24 to 48 hours; while the other four mixes take three to five days to attain more than 60 psi strength. For six inches slump of freshly prepared grouts, the final mixes do not show any bleeding. The CCE of the mixes vary from 26 to 53 % and the mixes appear to leach long-term pH in the range of 7.0-8.0.

One of the central tenants of this project is the claim that several byproduct streams can be mixed to yield a material that is environmentally more friendly than any of the products individually. The point is well illustrated by the mixtures involving various by-products. In individual materials leaching tests, there were indications of problems with Arsenic, Boron, Chromium, and Selenium. In the recommended mixes, the leaching of these elements were either completely eliminated or suppressed a great deal.

Preliminary hydrogeologic studies were initiated by visiting two sites (sites A and B) at two mines and collecting geologic maps of the sites. Ten groundwater monitoring wells have been installed at site A and three wells have been installed at site B. Six more groundwater wells are planned for site B. Well completion diagrams for the installed wells have been prepared. Cooperation of all parties involved in this project has been exceptional.

Different mixing and pumping systems were investigated for field demonstration and a tentative system was finalized. Fabrication of pumping system has already underway and the demonstration at site B is planned for the next quarter.

"The remainder of this report contains propriety information and is not available for distribution except to the sponsors"