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The processing of raw coal to a saleable clean coal requires that mine operators wash the run-of-mine product using a processing plant. The raw coal contains impurities composed of rock and fireclay. Two waste streams are created by this process: coarse rock and fine rock particles.
At the North River Mine in Berry, Ala., the coarse rock is transported to a refuse disposal area by conveyor. The fine rock particles leave the processing plant suspended in water to form slurry. Slurry is normally disposed of via surface impoundments or injected into abandoned underground mine workings.
The volume of this waste stream is significant and disposal can be expensive. At the North River Mine, the waste stream generated approximately 1,000 gallons per minute 24 hours a day.
The North River facility is an underground coal mine that produces 7 million tons of raw coal per year. The raw coal is processed to yield 3.5 million tons of clean saleable coal.
Raw coal is processed at the mine preparation plant at approximately 1,000 tons per hour. This yields about 550 tons of clean coal per hour. Thus, refuse is 450 tons per hour, with coarse rock the primary byproduct.
At normal operating levels, fireclay and fine rock particles suspended in water at a rate of 70 tons per hour in dry weight is also a waste by-product. The slurry waste stream reports to a 3 million gallon concrete thickener tank adjacent to the preparation plant.
Solids in the thickener underflow are increased with the addition of polymers to produce a waste stream of about 1,000 gallons per minute. Solids in this pump discharge vary between 25–35%. A total of approximately 1.5 million gallons of slurry is produced per day.
Particle size analysis of the fireclay and rock slurry revealed that 80% are 400 mesh or smaller. The ultra fine particles tend to stay suspended in the water.
With a possible interruption of the primary disposal methods due to available area and construction scheduling, a third method of slurry handling was sought for the interim. The mine needed to continue processing coal for shipment to meet customer commitments. This required slurry disposal.
Using geotextile containers for dewatering the slurry waste from the processing plant solved the problem. After a successful test was conducted, and permits obtained, the mine began using the geotextile containers to dewater and contain the solids from the waste stream.
Once the bags were full and dewatered, the site was ready for reclamation. The geotextile container technology worked well and provided an alternate method of coal mine slurry waste disposal for North River Mine. This process allowed the mine to operate under its normal schedule without any downtime or loss of production.