Disposal of Coal Mine Slurry Waste using Geotextile Containers

TenCate Geosynthetics, Pendergrass, Ga., United States

The processing of raw coal to a saleable clean coal requires many mine operators to wash the run of mine product using a processing plant. The raw coal contains impurities composed of rock and fire clay. Two waste streams are created by this process: coarse rock and fine rock particles. At the North River Mine in Alabama, 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 expensive to dispose of. In this case approximately 1,000 gallons per minute is created on a 24-hour basis.

North River Mine is an underground coal mine producing more than seven 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. Refuse is 450 tons per hour of which coarse rock is the primary by-product. 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 three 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 and 35 percent. A total of approximately 1.5 million gallons of slurry is produced per day. Particle size analysis of the fireclay and rock slurry reveal that 80 percent are 400 mesh or smaller. 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 be able to meet customer commitments. This required slurry disposal. Using Geotube® 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. Chevron Mining Inc. solicited the help of the Alabama Surface Mine Commission, the Office of Surface Mining, TenCate Geosynthetics, J.F. Brennan Co. Inc., Whittemore Farms Excavation and PERC Engineering Co. Inc. to develop a unique and successful method to solve the problem.

Geotube containers allowed Chevron to continue operation without curtailing production. Northern Alabama is hilly country without much flat land. The engineer designed areas that were cut back into the hillside and the Geotube containers were custom fabricated to fit the designed areas. In addition, the specially engineered high strength textile allowed the tubes to be stacked in layers five high, allowing a much larger volume of material in a smaller footprint.

Once all the bags were full and dewatered, the site was ready for reclamation. The Geotube technology worked well and provided an alternate method of coal mine slurry waste disposal for North River Mine, one that allowed the mine to operate without any downtime or loss of production.