During the feasibility stage for any new mine, one of the fundamental questions is: How can the ground be kept dry enough for mining to be carried out efficiently and safely? The majority of mines around the world involve working below the natural water table where, if no action were taken, the areas would be completely submerged. Mines above the water table experienced the ongoing challenge of groundwater ingress from rainfall. Unless this water is removed and kept out, mining is impossible.
The importance of a mine’s dewatering system cannot be overstated. If it fails or is unable to remove enough water, the consequences can be severe—such as costly and potentially lengthy delays; compromise to critical structures; and in the worst-case scenario, risk to miners’ lives.
A large number of complex considerations must be kept in mind when designing a dewatering system, including:
- The depth of the water table
- The porosity and permeability of the ground
- The amount of surface water present
- The presence of geologic features, such as underground rivers or water pockets
- The level of precipitation and its seasonal variations
All these factors combine in a dynamic system that should be understood in detail before a dewatering system can be designed and mining can begin.
A skid-mounted dewatering systemWeather conditions can have a significant effect on the volume of water that collects in a mine. Intense rainfall can substantially increase the rate at which water must be removed from the pit and, in colder climates, the seasonal variation can be significant because melting snow in the spring can cause a high influx of water. The dewatering system must have the capacity to handle maximum volumes to avoid downtime or safety issues.
This is particularly true for open-pit mining, but to an extent, it is also the case with underground projects. However, because they are closed from the surface except for small openings and typically deeper underground, they tend to be less affected by short-term changes in weather, and the dewatering flow requirement is more stable.
Planning
In the absence of the geological surveying methods and processes available today, dewatering provisions at most mines in the past were determined by a trial-and-error process. Pumps would typically be sized and installed on approximated values and operated manually based on the water levels in the pit. If the water extraction rate was not adequate, the system would be manually adjusted by removing or adding pumps.
Modern feasibility studies for mining projects involve a lengthy and detailed modelling process. It is so thorough that pump positioning and the flow and head requirements required are commonly mapped out in advance for the duration of the project. For a large operation, this can be a period of several years.
Once the inflows have been calculated, a dewatering system is specified with a capability that exceeds the maximum requirements that could foreseeably be presented by the project, such as an extended period of unusually heavy rainfall. Unexpected changes to underground water systems may also be encountered, which could necessitate higher flow capabilities from the system.
Location
Normally, the main dewatering pump system will be positioned at the lowest point in the pit or shaft, and the water will be pumped to the surface.
This may be supplemented by a feeder system of smaller pumps at other low-level points that do not drain sufficiently to the pit bottom. These pumps usually transfer the water from these additional areas to the central point before the main dewatering pump lifts it to the surface.
Occasionally, for larger, open-pit projects, deep shafts are drilled around the pit and water is pumped out of these to lower the water table across a wider area.
As the mining develops by either lowering or widening the mined area, new stages of the dewatering system may be required. Typically, an additional pump will be added for every 70 meters descended. This multistage system can usually be replaced by a more efficient and more powerful single-stage arrangement when the mine reaches its maximum depth.
The Right Tools for the Job
The correct specification of the complete dewatering system goes beyond the selected pump types. The other design features—such as sump design, pipeline sizing and valves—are equally critical.
By far, the most commonly used dewatering pumps are horizontal centrifugal or submersible pumps because they offer a good balance of easy deployment and high performance levels. Centrifugal pumps can be single-stage units arranged in a multistage configuration, which are capable of generating pressures up to 70 bar.
Multistage centrifugal pumps can also be used, although this type may provide limited solids-handling ability. Beyond the capabilities of these pumps, positive displacement pumps that can generate significantly higher pressures but at lower flow rates would be deployed.
The equipment selected for any given situation depends on the performance required and the practical questions of space and accessibility. In some instances, the operators’ working preferences may be considered in the equipment specifications.
The pumps are either diesel or electrically driven, and the power source choice is largely based on the preference of the operator and existing practices onsite. Furthermore, for underground operation, particularly in mines, the presence of hazardous gases must be considered, which will require the use of suitably rated equipment that is certified by the relevant authority. This affects the complete pump systems and not just electrical drives and controls.
Smart Control
Pump drive and control technology advancements have helped dewatering systems become more adaptive to weather conditions. Optimum flow requirements can be achieved by employing variable frequency drives and sophisticated programmable logic controllers that can increase or decrease individual pump speeds and start up or shut down the pumps’ operation to adjust for the inflow requirement. This increase in complexity has created a demand for remote and automated functionality so that operators do not have to physically be at the pump to make system adjustments. This automated control may include inputs via flow rate, pressure or level sensors and can significantly increase operational efficiency.
Efficiency Versus Ruggedness
Historically, clear liquid-handling pumps have generally had higher hydraulic efficiency because they do not need to make allowances for handling solids. Slurry pumps, on the other hand, are designed to handle fluid that often contains large particles, so the internal designs vary significantly. Typically larger and heavier, slurry pumps have increased clearances between the impeller vanes and the volute to ensure free passage of larger particles and to reduce wear. This requirement usually means a slurry pump will deliver lower energy efficiency when pumping clear water. However, recent developments in slurry pump designs because of advanced modelling methods have allowed them to deliver efficiencies that rival some clear-water pumps.
Dewatering applications fall somewhere between the two because they require performance levels similar to clear-water pumps in terms of flow, head and efficiency but need to be able to handle some solids particles—typically not exceeding around 10 percent.
A 12-stage pump coupled to a 1,250 kilowatt turbo-charged diesel engineThe hydraulic designs and materials of clear-water pumps would not last long when faced with any significant solids content. Therefore, dewatering pumps combine some features from clear liquid and solids handling designs to ensure that reliable performance levels can be achieved. A key part of this is that the materials of construction for the parts contacting the product must provide erosion and corrosion resistance. If the solids content rises above the levels mentioned for prolonged periods of time, then conventional slurry pumps will usually be required. Mining-duty pumps are available that cover clear-water to high-solids content. These can be mounted on trailers, skids or barges and can be driven by either electric motors or diesel engines.
Vertical turbine pumps suitable for permanent pumping stations are also an option. Positive displacement pumps are often used when space is at a premium and high, single-stage heads are required.
For a mine operator specifying pumps, working with a supply partner that understands the specific requirements of the site and will get involved in planning from the feasibility stages is important because choosing the appropriate pumps makes an enormous difference to the level of fuel efficiency delivered and the ongoing cost of repair and maintenance.
The pumps must also be properly maintained so that performance levels are not allowed to fall outside the acceptable limits as a result of excessive wear. To ensure this, support services should be available—including performance monitoring, onsite maintenance and offsite pump rebuilds—to ensure that the installed equipment continues to deliver excellent performance throughout its lifespan.
When the life-cycle costs of owning a pump are considered, failing to invest in pre-planning or after-installation servicing can prove to be a significant false economy because costly interruptive maintenance will be more frequent and fuel consumption levels will be higher.
Adaptability
In some instances, the nature of the ore body could mean that mining work starts close to the surface, but as the project develops, mine water has to be pumped from increasingly deeper parts. Because of the substantial changes in the head requirement throughout projects of this scale and the high flow rates involved, no obvious single-stage solution can be adjusted to the increasing demands as the project progresses. Therefore, staged pump stations should be built as the depth of the mine increases. Each stage would include stand-alone pumps that boost the pressures as the depth increases, and a total of six separate streams will be required to handle the flows.
Many different approaches can be used for any given challenge, and no one-size-fits-all solution exists. However, the wide range of pump designs available means delivering a dewatering system for virtually any mining project is possible, no matter how extreme the dimensions or conditions.