Excess water in mines utilizing slope belt conveyors creates significant spillage problems as coal is conveyed to the surface.  The water causes material to slide back toward the pocket, which results in material accumulating along the slope floor. This material must be shoveled manually to remove it.

This case study involves a mine experiencing slide-back problems along an 1800-foot-long, 16° slope conveyor. Unwanted water from various sources would accumulate on the belt system whenever the longwall encountered rock and during periods of down time. At a normal accumulation rate of 250 gal/min from cooling water and dust suppression sprays, any additional water on the belt system quickly created a problem. It was not uncommon for 2000 to 3000 gallons of water to be on the belt system at one time. This excess water eventually traveled the twelve-plus miles of belt system to the pocket and slope and set the stage for slide-backs.

Impact of the Problem

Washing and shoveling at the base of the slope was a daily activity. Typically, two miners per shift were required to maintain the pocket area. Following one slide-back, the amount of material on the floor along the slope was calculated to be 150 to 160 tons.  The slide-backs made it very difficult to comply with MSHA Standard 75.400 (Accumulation of Combustible Materials). On average, the mine would have to be idle one shift per month to remove spillage from large slide-backs.

Following a large slide-back, the only alternative for resuming production involved 30 to 40 miners manually digging out the material along the belt. The water made the material heavier and the 16° angle of the slope meant that the coal had to be shoveled with one foot higher than the other at all times. This position compounded the potential safety problems inherent in shoveling.

Solution to the Problem

Removing excess water from the belt before it got to the slope was the obvious solution, but accomplishing it was a challenge. Various dewatering technologies used in washing plants had been tried. However, these systems were too labor intensive and had met with limited success.

The final solution involved separating fine coal and water (underflow) from course coal using a chain deflector at the point where it comes off the main conveyor at the top of the pocket. A grizzly screen is positioned below the conveyor underflow and set at a 45° angle, allowing the less than ¼ inch fine coal and water to pass through a series of sumps and cyclones. The underflow from the primary cyclone (¼ inch by 100 mesh) and from the secondary cyclone (100 by 325 mesh) is discharged onto a high-frequency screen. The coal retained on the high-frequency screen (½ inch by 325 mesh) is fed to a surge bin that discharges to the slope conveyor at the bottom of the pocket. The water and minus 325 mesh material passing through the screen is re-circulated to the secondary cyclone. The overflow from the secondary cyclone (mostly water and a small amount of very fine coal) is discharged to the gob. Total cost of the system:  $250,000.

Impact of the Solution

The mine is now cleaner, safer, and more productive. No slide-backs have occurred since the system was installed.  The two miners per shift previously required to clean up the pocket and slope has been reduced to one miner for 20 minutes a week to hose down float dust and flakes, which translates into the elimination of 12,000 worker hr/yr of manual materials handling.

Since 85% of belt maintenance may be attributed to fugitive material, belt maintenance cost savings should be substantial as well.

The most immediate savings has been the elimination of downtime because of slide-backs. Capital expenditures were recovered within 3 months, and raw tonnage produced is up by more than 30%. No MSHA citations for accumulation of combustible materials along the slope have been issued since the dewatering process was implemented.

An additional fringe benefit is that the ability to remove unwanted water allows the operator to add water where needed. Thus, additional sprays may be added at transfer points to reduce dust.