INTERIM FINAL TECHNICAL REPORT

September 1, 1997, through October 31, 2000

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:                   Dr. D. Deb and Mr. Greg Deaton, SIUC

Project Manager:                      Ronald H. Carty, 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 current values of about 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 are designed for short-term stability of 1-2 years.  The mined-out areas are 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, provide a beneficial use for large volume CCBs, and improve the environment and mine health and safety.

For the purpose of the demonstration, Crown III mine of Freeman United Coal Company developed a small panel (hereafter called the backfilling panel) with eight entries with 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 from 50-55% to 65%. 

Two injection holes were drilled over the study area to inject coal processing waste and coal combustion by-products into the panel.  For this purpose, a plant was built to mix about 14% F-ash, 53% FBC and 34% gob with water for pumping underground.  Underground backfilling was started on August 11, 1999 through the primary borehole and subsequently on October 13, 1999 through the secondary borehole.  Altogether 9,293 tons of material were injected underground and it flowed in all directions from the point of discharge.  A maximum flow distance of 300 ft was observed underground.

Borehole camera survey of the second hole conducted on September 18, 2000 showed that no new roof falls occurred in the cross section.  On September 27, 2000, backfill operation resumed through that borehole with F ash and FBC ash are solid components.  A high-speed auger mixer is being used to mix solids with water.  As of October 30, 2000, about 6,000 tons of grout was injected underground.  It is estimated that a total of about 900~1,000 ft of mine voids could be filled during this phase of backfilling operation.  This estimate will increase as more grout is injected underground.

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 the economics of mining coal and power generation.

About 70% of the underground mined coal (30 to 35 million tons) in Illinois is mined using the room-and-pillar mining method which permits extraction of only about 50% of the 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 on-site in ponds 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%, and 2) 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 and acid-mine drainage potential.

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 weak floor is composed of 2 to 4.5 ft weak claystone. The dip of the floor is 1.6% in the South East direction of the panel.

The mining company developed a small panel (hereafter 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.

Roof-to-floor convergence and surface subsidence data were collected periodically.  Measurements taken on March 23, 1999, in the backfilling panel indicated about 1.8 inches of convergence at the center of the panel.  Roof falls were observed in a few intersections and as a result some of the measuring stations were destroyed.  Due to this reason and also due to safety concerns, no underground measurements were taken after that date.  However, surface deformation was measured periodically and it was found that, on an average, surface deformations were about 1.16 inch over the last one year.

Two injection holes were drilled to inject backfill material in the panel.  A mixing plant was built to mix gob, FBC, and F-type ash with water.  Several preliminary mixes were developed using gob and fluidized bed combustion (FBC) fly ash and F-ash.  Their engineering properties were documented in a previous report (Chugh et al, 1998).

Due to the strike at Crown III mine, no progress was made in the field demonstration of underground backfilling until February 1999. However, the Steering Committee met on April 15, 1999 at Crown III mine and finalized the work plans for this year. Two mixes were selected for underground demonstration purpose, one having 25% gob and the other having 40% gob in the mix.

Underground observation in March and borehole camera survey by OSM on July 7, 1999 showed that both the holes were open for backfilling.  In the primary hole, the camera was lowered to the mine floor level and the distances of coal pillars from the borehole were measured.  There was a roof fall in the secondary (alternate) borehole area.  However, it was found that the entries in three directions were open from that borehole.

In order to demonstrate the flow characteristics of mixes a trench was dug on the surface with two perpendicular crosscuts.  The trench was about 100 ft long, 9 ft wide and 6-10 ft deep.  On August 9, 1999, the mix with 40% gob was pumped into this trench to observe the flow behavior.  The mix flowed in all directions after discharged with little separation of water and solid components.  It was also found that the mix flowed under the water without much separation.

After two days of preparation, Phase I underground placement was started at 7:00 a.m. on August 11, 1999 through the primary hole and the operation ended on September 8, 1999.  About 8,159 ton of mix was pumped underground through the primary hole (5,873 ton of solid and 2,286 ton of water).  The daily average backfilling rate of mix was 627 tons (452 ton of solid and 175 ton of water).  It was found that the average water to powder ratio is about 40%.  With this ratio, 11-inch slump was achieved.  The average hourly rate of mix was 117.1 tons/hour (83.5 ton/hour of solid and 33.6 ton/hour of water).  On October 13, 1999 backfilling operation resumed through the secondary borehole.  A concrete pump was used to pump this mix from the plant site to the hole, a distance of about 250 ft.  After four days of operation, 1134 ton of solid and water (773 ton solid and 361 ton of water) was dumped underground.  Altogether using both boreholes, 9,293 ton of material was injected underground.

On August 24, 1999, an underground visit of the backfilling panel revealed that the mix had flowed a considerable distance (about 120 ft) as expected.  It was found that the flow pattern was sheet-like and uniform in all directions.  The gradient of backfilled material underground was 1 ft from the roof in all directions 30-ft from the point of discharge.  In addition, the backfilling operation was continued after that period.  Mining Company staff visited the backfilled area again and found that the backfill had flowed 300 ft from the primary borehole.  During the early part of October the backfilling panel was sealed off as per the instruction by MSHA and another underground visit was not possible.

On September 18, 2000, a borehole camera survey was conducted again in cooperation with OSM for the second hole to observe the underground conditions in the vicinity of the borehole.  It was found that entries in the south, west and east directions are open.  No new roof falls had occurred in that cross section.  Thus, Phase II backfilling operation resumed on September 27, 2000.  This time, a high-speed auger mixer was used to mix solids and water and then inject them underground.  This mix is composed of F ash and FBC ash.  These two ashes were premixed with 1 ratio 2 (F to FBC ash) by weight and dumped into a hopper using a front-end-loader.  Water was added at the rear end of the mixer and grout mix came out from the front end (borehole side).  As of October 30, 2000, about 6,000 tons of grout were injected through the secondary hole with an water to powder ratio of 0.47.  It is estimated that injected grout can fill over 140,000 cft of underground void.  This means that grout may fill 900~1,000 ft of mine voids assuming average entry width and opening height of 20 ft and 7 ft, respectively.  During this operation several 3x6 inch cylindrical samples were prepared for obtaining compressive strength and elastic modulus of injected grout.

During Phase I backfilling operation, many samples were prepared for testing of compressive strength, elastic modulus, slake durability, swelling strain and hydraulic conductivity of the mix.  In order to perform a sensitivity analysis of these results, five new mixes similar to field mix were prepared in the laboratory by slightly varying the proportion of each mix component.  For each mix, three (3) samples were prepared to obtain an average result.  It is found that the average strength and elastic modulus after 28-day curing are 190 psi and 17,960 psi, and those of after 90-day curing are 334 and 40,445 psi, respectively.  For the field samples, slake durability index ranges from 75 to 89% and for similar samples prepared in the laboratory, it varies from 79 to 92%.  Swelling strain for the field samples is slightly lower and ranges from 6 to 10% while that of similar samples varies from 7 to 15%.  For testing hydraulic conductivity, samples could not be prepared at Crown III mine site but the similar samples were made in the laboratory.  It shows that for pressure head between 30 to 50 psi, hydraulic conductivity varies from 0.01 to 0.06 inch/day.  It means that the backfilled material underground is not a permeable layer.

ASTM shake test was performed for the field mix and found that the mix was environmentally benign.  The pH of the mix is 11.23 with a Ca concentration of 669 ppm in 1540 mg/l of dissolved solid.  The concentration of most of the heavy trace elements is below the Class I ground water (GW) standard.

Strength and elastic modulus data from laboratory and field samples were analyzed using linear regression models.  It is found that the ratio of proportion between FBC ash and water content is the most important parameter for determining 7-day and 28-day cured strength and elastic modulus of mixes.  The ratio between F ash and FBC ash also plays an important role in estimating 7-day compressive strength.  These relationships are verified with the samples (similar to field sample) prepared in the laboratory.  This analysis provides a mathematical foundation for forecasting strength and elastic modulus of samples composed of FBC ash, F-ash, gob and water.