Keeping the deeps dry
Carly Lovejoy explores the types and configurations of dewatering pumps used in underground mines, as well as current industry trends
Every underground mine has to deal with some degree of water ingress, and will require dewatering at one time or another during its lifecycle. How that dewatering is achieved, and the scale of the operations is largely dependent upon the mining method and design used, the depth of the mine, its location, the geology of the host rock and the type of ore being mined.
The geology of the host rock is a critical factor affecting the amount of water that needs to be removed. The porosity of the rock will determine the ability of groundwater and throughflow from natural sources such as precipitation to travel through certain layers of strata. Fractures, voids and veins can also act as conduits for water as it filters through the rock. Due to gravity, water will naturally follow through these channels, run down cave and tunnel walls and accumulate in depressions, whether natural or made-made.
Gavin Doran, general manager of sales at Sulzer Pumps (South Africa), says: “Open-pit mining sees the ingress of direct rainfall water, as well as overland or storm water flows into the operation, while underground mines see water ingress via rainfall down shafts and adits, as well as seepage of rainfall and overland flow from crack zones in geological structures, and water from pressurised aquifers that have been around for thousands of years.”
Mining in the proximity of large surface bodies of water or near coastal areas is also a concern and can lead to much greater degrees of flooding.
In addition to natural sources, water is also used at many stages in the mining process, for example, to flush cuttings from drill holes during development or production activities, and to suppress dust. This can add to the problem of naturally occurring water.
Miguel Jahncke, director of mining at Schwing Bioset, says: “Hard-rock underground mines generally have to deal with two main water sources: water that is present in the surrounding ground and enters the operation through troughs, voids, cavities, and drill holes; and water that is introduced through the mining process, through drilling, and face and muck pile hosing to suppress dust (silicosis prevention is the number one reason for keeping dust down underground).
“To prevent mining into unknown underground water bodies and ‘rivers’, development longholes are typically drilled ahead of the development headings and new mining areas to identify and eliminate, where possible, potential risks. All of the water in the mine is then collected and channelled to the lowest point through a series of channels or ditches.”
As water travels through the mine, it collects the fine particles of the rock (sometimes called fines or slimes) generated by the mining process. Downward dipping sections and working areas will collect additional water that can be pumped utilising small electric or pneumatic diaphragm pumps, or small submersible pumps. This water is pumped either to the mine dewatering channels or to one of the mine sumps. As the water reaches the lowest point of a mine, a series of settling ponds are typically used to settle the slimes, and supply cleaner water to the main sump where the main dewatering pumps are situated. These pump the water up to intermediate stages or directly up to the surface.