Slurry is the mixture of solid particles and a liquid, usually water. The particles can range in size from greater than four inches in equivalent diameter to less than one-thousandth of an inch. When the solid particles in the liquid are small and finely ground, the mixture is called fine slurry, and when the particles are larger, it is called coarse slurry. Traditionally, the mining industry has employed pipelines to transport mine wastes and tailings in slurry form to disposal sites, using water as the fluid. Dredging also uses slurry pipeline. The sand, gravel, or soil dredged from a river is often pumped with water through a pipeline to a construction site for a distance of up to a few miles.
In general, when pipelines are used to transport coarse slurry, the slurry velocity must be relatively high in order to suspend the solids. Such slurry transport is very abrasive to the pipe and the pump, and the power consumed is high. Consequently, coarse-slurry pipelines are economical only over relatively short distances, normally not more than a few miles. An important application of coarse-slurry pipeline is “concrete pumping,” in which concrete is pumped from a parked truck through a portable steel pipe attached to a side boom to reach rooftops and bridge decks. It is a method of conveying and laying concrete employed increasingly in construction.
Long-distance transport of solids by slurry pipeline must use relatively fine slurry. Existing coal-slurry pipelines carry fine slurry consisting of about 50 percent coal and 50 percent water by weight. The solid is first pulverized and mixed with water to form a paste. The slurry then enters a mixing tank, which contains one or more large rotating wheels or propellers that keep the particles uniformly mixed. Next, the slurry enters the pipeline. Special plunger or piston pumps are used to pump the slurry over long distances. The United States pioneered the coal-slurry pipeline technology. The first long-distance coal-slurry pipeline was constructed in Ohio in 1957. The line was discontinued later when the competing railroad agreed to lower its freight rate. The pipeline was then mothballed for years and used as a leverage against rail rate increases. It was said to have prompted railroads to modernize and become more competitive, introducing the concept of the unit train, which employs about 100 cars to haul coal nonstop from mines to power plants.
The world’s longest coal-slurry pipeline is the Black Mesa pipeline in the United States. Built in 1970, this 18-inch pipeline transports 4.8 million tons of coal per year from Black Mesa, Ariz., to southern Nevada, over a distance of 273 miles. This coal pipeline has been highly successful. Many other long-distance slurry pipelines exist in the world to transport coal and other minerals such as iron concentrate and copper ore.
