Source: World Dredging & Marine Construction
Ellicott® custom-built hydraulic pipeline cutterhead dredges are being used increasingly in mining and aggregate operations ranging from coal to tin to aggregates.
In the coal fields of eastern Pennsylvania, coal fines once considered impractical to salvage were recovered by an Ellicott® suction cutterhead dredge. Because of the operations, up to 30 railroad cars a day of coal was recovered, enabling a power utility to have a ready supply of low-cost fuel for its steam generators. The dredge worked from a cut more than 185 feet deep where it was estimated that 1,200,000 tons of coal waste was deposited. The steep sides of the depressions, its depth, and the irregular nature of the terrain made recovery of the fines potentially difficult and time consuming. Materials were dredged out through the pipeline, relayed through a system of rehandling pumps to an overall height of 2000 feet, and then deposited in settling basins.
Water was drained off into a nearby river, while the remaining solids, including the coal fines, were conveyed through various devices to a final disposal tower. Finally, the solids were dumped into reserve piles for shipment to the power utility.
In France, hydraulic dredging turned a marginal mineral extraction operation into a highly successful one by solving production problems in an alluvial tin mining operation. The dredge proved the most suitable method of excavating and transporting the material over a considerable distance to a shore beneficiation plant. The processing plant was redesigned in relation to the dredge’s capacity and additional equipment was installed to handle the increased flow of material. The entire complex was designed to handle a constant 150 cubic meters per hour of alluvial depositions. When the distance of the dredge from the plant exceeded a practical limit, operating engineers determined that the plant would follow the dredge.
A suction cutterhead dredge replaced a dragline in alumina clay dredging operations for a cement plant in Winnipeg, Canada. Officials report a 15 to 20 percent increase in production of clay slurry through the washmill resulting from the small ball-sized clay nodules delivered to the soaking pit of the dredge. Since the dredge cutterhead and pump rejected material larger than 5 or 6 inches in diameter, time loss due to repairs in the washmill was reduced.
In Parkersburg, West Virginia, a sand and gravel corporation expanded its aggregate production facilities to meet increased demands by using an Ellicott® dredge. The hull was designed to carry the entire plant. The dredge was outfitted with three barge loading conveyors and one sand chute allowing three sizes of gravel and one size of sand to be loaded simultaneously. The beneficiation and screening plants were installed on the hull.
A suction cutterhead dredge was also used to increase the efficiency and production at a masonry sand plant. The dredge pumped 120 to 125 tons per hour of raw materials directly to the plant. The dredge has a cleaning action as it pumps the masonry sand to the plant. Prior to the addition of the dredge, the operation required two 10-wheel trucks and sometimes two cranes.
In Calvert City, Kentucky, a dredge was used in acetylene processing. Slaked lime was reused to make acetylene gas. The slaked lime was deposited from the plant into ponds. The suction cutterhead dredge removed the accumulated slaked lime from the pond and pumped it 1,200 feet to a calcining plant for recovering. Lime briquettes were combined with coke to form calcium carbide.
In Ohio, a similar dredge acted as a pumping unit to deliver calcium carbonate sludge to a recalcining plant for conversion to quick lime used in the water softening process of the municipal water supply. The dredge permitted kiln operation at a uniform and efficient rate of production, thereby reducing costs.
In some instances, the hydraulic cutterhead dredge can be used to decrease the indirect costs of mining. In Guyana, a dredge was used to deepen the waterways to allow faster movement of barges carrying bauxite ore, reducing stockpiling costs.
Ellicott® officials, whose “DRAGON®” and “SUPER-DRAGON®” dredges are used in such applications, stress the need for a suction cutterhead dredge to be dependable and easy to maintain, because it is the first and often the only, productive machine in the total chain of plant machinery.
It is good plant engineering to insure that the dredge by itself is capable of maintaining a somewhat higher rate of production than the processing plant, in order to have some reserve capacity which can be used from time to time in way of stock piling, as a hedge against periods of below average production.
Dredge production is subject to variation because of digging depth, specific gravity and compactness of material, and pumping distance between the borrow pit and stockpile or processing plant. The total variation in output may range between 0 and 200 percent, if the average production is slated at 100 percent. Since very few processing plants can be designed for a 100 percent overload capacity, it becomes a prime consideration in dredge design and construction to limit the permissible variation in output to somewhere between 50 and 150 percent.
A narrower band of hourly production variation is difficult to achieve under normal conditions. Even within this band, it is essential that the dredge be dependable to operate on a 24-hour per day basis with scheduled weekly and monthly maintenance periods.
Reprinted from World Dredging & Marine Construction