INTERIM FINAL TECHNICAL REPORT

September 1, 1997, through August 31, 1998

Project Title: UNDERGROUND PLACEMENT OF COAL PROCESSING WASTE AND COAL COMBUSTION BY-PRODUCTS BASED PASTE BACKFILL FOR ENHANCED MINING ECONOMICS

ICCI Project Number: 97US-1

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

Other Investigators: D. Dutta and L. Buliang,

Southern Illinois University at Carbondale

Project Manager: Richard Shockley, ICCI

ABSTRACT

The concept being investigated in this project is that the extraction ratio in a room-and-pillar panel at the cooperating mine can be increased from about current values of 56% to about 64% and backfilling can be done from the surface, upon completion of all mining activities in a set of rooms and withdrawal of all equipment, without significant ground control problems due to increased extraction ratio. The pillars will be designed for short-term stability of 1-2 years. The mined-out areas will be backfilled from the surface with gob-, coal combustion by-products (CCBs)-, and fine coal processing waste (FCPW)- based backfills containing 65%-70% solids to minimize short-term and long-term surface movement risk. This concept has the potential to increase mine productivity, reduce mining costs, manage large volume beneficial use of CCBs, and improve mine health and safety, and environment.

For the purpose of the demonstration, Crown III mine of Freeman United coal company developed a small panel (hereafter will be called the backfilling panel) with eight entries and 80 ft by 60 ft (center-to-center) pillar sizes, and 20 ft entry width. Secondary mining was done in this panel to increase the extraction ratio to 65% from 50% to 55%.

Visits to underground areas, seven months after completion of secondary mining (Secondary mining was completed in November, 1997), indicated stable conditions throughout. Underground roof-to-floor convergence in the backfilling panel in May, 1998 showed an average closure of 0.5 inches. Even after the pillar extraction (secondary mining), the pillar and floor safety factors at the center of the panel, calculated using numerical model SIUPANREL.3D, were 1.34 and 1.56, respectively. The rock mechanics studies concluded that the short-term stability of openings with secondary mining was achievable. Backfilling materials were developed using gob, fluidized bed combustion (FBC), and F-type fly ash. Eighteen (18) preliminary mixes were studied for their flowability, bleeding characteristics, and cured strength to select five (5) final mixes. Leachate characteristics of the selected mixes were superior to the mix components. Large-scale demonstration (backfilling using 20,000 tons of developed materials) of the concept is tentatively scheduled to start in September, 1998.

EXECUTIVE SUMMARY

In order to maintain a healthy high sulfur coal industry in the U.S. and the Illinois coal basin, production costs must be reduced and economically viable management technologies for coal combustion by-products (CCBs), fine coal processing waste (FCPW) and coarse coal processing waste (gob) must be developed. Over the past decade considerable research has been done in Illinois on high volume, low value disposal/utilization technologies (disposal in surface mines, reclamation, disposal in abandoned underground mine workings). However, little or no work has been done to beneficially use these by-products in large volumes to enhance economics of mining coal and power generation.

About 70% of the underground mined coal (35 to 40 m tons) in Illinois is extracted using the room-and-pillar mining method which permits extraction of only about 50% of coal. The remaining coal is left behind in the form of support pillars to control surface and subsurface movements. Typically, power plants in Illinois in rural settings are presently spending $5 - $10/ton to dispose of CCBs in on-site and this cost is expected to grow rapidly in light of new requirements for landfill sites. If coal companies could negotiate coal contracts with electric utility companies which will reduce their CCBs management costs and cover the cost of underground backfilling and transportation, the hypothesis for partial extraction mining with backfilling is economically feasible. Implementation of this technology will result in strengthening the high sulfur Illinois coal industry and keeping the coal industry jobs in Illinois, while providing a secure source of coal supply to power plants from their backyards.

The concepts being investigated in this project are that 1) the extraction ratio in a room-and-pillar geometry at the demonstration mine can be increased from current values of about 56% to about 64%, 2) mining areas can be left open for 6-8 months until the completion of all mining activities in a set of rooms and withdrawal of all equipment, and 3) the mined-out areas can be backfilled from the surface with FCPW-, gob-, and CCBs- based backfills containing 65%-70% solids that will minimize short-term and long-term surface movement risk.

Crown III mine of Freeman United coal company is currently mining 600 ft wide panels with 11 entries on 60 ft centers with 20 ft wide entries, and extraction ratio of 50% to 55%. Coal is extracted from No. 6 coal seam at a depth of 300 to 350 ft. The panels vary in length from 3,000 ft to 5,000 ft. Seam height is seven feet and the floor is one foot thick weak claystone.

The mining company developed a small panel (hereafter will be called the backfilling panel) with eight entries and 80 ft by 60 ft pillar sizes (center-to-center). The entry width in the backfilling panels was 20 ft. Secondary mining was done in this panel to increase the extraction ratio to 65% from 50% to 55%. In November, 1997, three rows of pillars in the backfilling panel were extracted to a depth of 20 ft by two cuts of 18 ft wide in each pillar.

Rock mechanics studies were conducted both in the backfilling and the regular panels and consisted of underground visits prior to and after secondary mining in the backfilling panel, plate loading tests in the regular panel, installation of underground convergence points and surface movement monitoring stations, data collection from the monitoring stations, floor and pillar safety factor analyses for different pillar geometries and numerical modeling for predicting surface movements and pillar and floor stability.

In June, 1998, seven months after the completion of secondary mining, all the entries and crosscuts were found to be stable. However, roof falls were observed in the unbolted pillar cuts. Rib sloughing in the backfilling panel was no different than in the areas where no pillar extraction was done (in the regular panel). Roof bolts in general did not indicate signs of significant loading and all intersections were found stable. No floor heave was observed in any area.

Initial underground data showed high divergence values (0.4 to 0.9 inches) at panel edges due to lateral movements of roof beams. At the center of the panel, convergence (closure) was approximately 0.1 inches in the regular panel and the backfilling panel before the secondary mining was started. Measurements taken on May, 1998, in the backfilling panel indicated an average convergence of 0.5 inches.

Floor and pillar safety factors, calculated using the classical empirical methods (non-numerical), were more than 2.0. Numerical modeling using SIUPANREL.3D program was conducted for the regular and backfilling panels with no secondary mining and the backfilling panel with secondary mining. Even after the pillar extraction (secondary mining), the pillar and floor safety factors at the center of the panel were 1.34 and 1.56, respectively. The modeling results concluded that the short-term stability of openings with secondary mining was achievable.

To develop a backfilling material, gob and fluidized bed combustion (FBC) fly ash were selected as primary components because 80% of coal processing rejects at Crown III mine is coarse coal refuse (-4-inch to +28 mesh). Gob was crushed to sizes less than 0.25 inch. Eighteen (18) preliminary mixes were made using adequate water to make a grout of slump between 9.0 to 10.0 inches. Proportions of gob and FBC were varied from 25% to 75%. The ratio of F-type fly ash to FBC fly ash was kept in the range of 0.0 to 1.2. The F-type fly ash was provided by Coffeen power plant of CIPS and Meredosia power plant of AMEREN/CIPS. It was found that a stable paste of 10-inch slump could be prepared using different proportions of gob, FBC fly ash, and F-type fly ash. Twenty eight-day cured strength of grout was as high as 500 psi. For the strength in the range of 400 to 500 psi, elastic modulus values in the range of 20,000 to 30,000 psi were obtained. Five final mixes containing gob in 25 to 50% range were selected. Leachate characteristics of the selected mixes were superior to the mix components. One of the final mixes will be used for a large scale demonstration at Crown III mine, scheduled to commence in September, 1998.