There are two largely accepted facts about mining: 1) A mine's landscape is dynamic, and 2) Within that landscape, water is a mine's worst enemy. Navigating a mine's landscape with pumps designed to move and/or remove water has long been the challenge of mine operators.
Mine plans are developed in an effort to extract the highest quantity and quality of materials at the lowest cost. Using a combination of maps, geological surveys, drilling samples and further research, a mine plan maps-to the greatest extent possible-the entire environment below the subsurface of a mine site. A mine plan can change quickly, depending on factors like the vulnerability of the landscape, and the market value and concentration of the products being mined.
It is not uncommon for a mine plan to change from mining for copper to mining for molybdenum, based on the increased market value of molybdenum. In other words, mining may move from one side of a pit because research indicates a higher molybdenum concentration exists in another ore body within the mine. As the landscape of a mine changes-whether open pit or underground-so do the requirements and space availability for dewatering equipment.
Water is a constant threat to mining, whether it enters a mine through infiltration or comes from a storm. Mine operators must be able to manage the water where it is known to exist and quickly alleviate the problems associated with it when it rises unexpectedly. Mine dewatering pumps generally need to pump water horizontally-from a smaller sump to a larger sump-or vertically from the bottom of the pit up to the elevation of the mine, and then horizontally to a discharge point. The pumps most commonly found in mine operations include vertical turbine pumps, electric and hydraulic submersible pumps, horizontal multistage centrifugal pumps and horizontal single-stage centrifugal pumps.
Vertical Turbine Pump
Vertical turbine pumps are the best candidates for pumping clean, cold water. Custom-made, vertical turbine pumps can have multistage, enclosed impellers in bowl assemblies supported by a frictionless, round driveshaft. Requiring a sump with significant depth for submergence and construction of a platform that suspends the pump, the vertical turbine pump's worst enemies are abrasive matter such as grit and sand-which destroy the wear rings and bushings-and air-which, if drawn into the pump, will immediately compromise performance.
Vertical turbine pumps also require special considerations for critical speed; sump dimension and materials of construction; multi-staging (which is limited by bowl shaft horsepower capacity and shaft stretching); pre-lubrication (the line shaft bearings need lubrication before operation via a solenoid); protection from blasted "fly-rock" and maintenance. Though they require minimal operational cost, the associated construction and maintenance costs and downtime can be detrimental to mining operations.
Electric and Hydraulic Submersible Pumps
Electric and hydraulic submersible pump ends generally require varying levels of submergence while the power packs require level and ample space on land. Electric submersible pumps can handle generous volumes (up to 6,000-gpm) or significant heads (around 375-ft), but are limited to less than 1-in solids handling at higher heads. Electric submersibles should be installed semi-permanently, moved infrequently and maintained by a certified electrician or technician.
In most cases, electric submersibles (like vertical turbines) are run at very high speeds. While the necessary heads may be achieved at speeds of roughly 3,600-rpm, the potential for wear from pumping silt-laden water at high speeds increases exponentially, decreasing pump life. Although rubber lining of wear parts is an option, it is not a long-term solution compared to the hardened wearplates found in centrifugal pumps. Electric submersible pumps can be prone to seal, motor and/or electric power failure when water seeps along the power cord and into the motor. These types of failures can result in repair costs measuring roughly half the value of the pump. It is not uncommon for one single-stage centrifugal pump to outlast three comparable electric submersible pumps in a mine.
Because of the robust construction of the pumpend, hydraulic submersible pumps are ideal for pumping lightly abrasive materials. Like electric submersibles, hydraulic submersibles are capable of either sizable flows (to 6,000-gpm) or modest heads (to 280-ft). Hydraulic submersibles are the most easily installed submersible pump because they do not require the power or switch gear that an electric submersible does. Instead, hydraulic submersibles require refueling and the preventive maintenance associated with the diesel engine on the power pack.
Horizontal Multistage Centrifugal Pumps
Horizontal centrifugal pumps can be multistage, i.e., multiple impellers on a common shaft enclosed in the same pump casing. These pumps generate increased pressure as the discharge of one enclosed impeller is routed to the suction of another enclosed impeller within the casing. Enclosed impellers limit the solids handling sizes of these pumps. In addition, and much like their vertical turbine counterparts, the efficiency and effectiveness of horizontal multistage pumps is dictated by the life of the wear rings and thin, multi-vane impellers.