Dewatering is a term used in many different areas of a mining environment. In all cases, it means to remove unwanted water, but it is accomplished for different reasons and with various types of equipment.
To Remove Standing Water
Dewatering can be as simple as removing standing water from a low point in the mine. Between groundwater, runoff and seeping from nearby bodies of water, a mine can encounter a number of difficulties that will hinder or stop the operations.
In the past, an area was dewatered by digging deeper dikes so that gravity would remove the water, by using simple mechanical methods or, if nothing else, by carrying water-filled buckets manually.
In today’s age, dewatering is more advanced and effective, with various styles of pumps to remove the water and transfer it to a containment area. Even underground mines miles below the surface can be successfully dewatered to allow for operation.
To Release Water Off Site
Transferring undesired water from one location to another is only the start of dewatering applications. In situations where a mine site has too much water, some of it may need to be released off site. Any time this happens, the water becomes an environmental concern and is subject to strict water quality standards.
Federal, state and local standards will set the specification to allow water to be released from a mine site even if the water has never been used as part of the process. The amount of solids allowed is normally low, so dewatering methods are employed to meet these requirements.
Some commonly used dewatering methods in this case are (but not limited to) settling ponds, Thickeners/Clarifiers and Filter Presses. Available space and release requirements can drive the choice of which method or methods to use. Monitoring and recording the quality of the release water is necessary to ensure minimal impact on the surrounding environment. Failure to comply can result in substantial fines and even mine closure.
Filter Presses can be used to eliminate settling ponds by separating the solids from the liquid in the waste stream, producing immediately reusable process water and drip-free cakes, like shown in this coal application.
To Retain Water for Reuse
The opposite side of too much water is where the process is starving for water. Much of the same dewatering equipment/methods listed above are used (settling ponds, Thickeners/Clarifiers and Filter Presses), but the priority is a focus on retaining as much water as possible from the tailings in a timely and cost-effective manner.
Without the water quality standards required for releasing water off site coming into play, the clarity of the water is less of an issue. However, there are still concerns to stop waste elements from building up in the process water. These waste elements, such as soluble clay, can have severe negative effects on the overall process and contaminate the final product. Fine particulates in the return water can also accelerate wearing in equipment and increase maintenance costs.
Once the water is in the system, it is usually more cost-effective to keep it there. Wells and other sources of water can be a financial drain and require permits, so dewatering the waste stream for water recovery and reuse can be highly beneficial.
Filter Presses (left) and Thickeners (right) recover process water that can be reused immediately in the wet processing plant.
To Feed Equipment for Optimum Operation
Separate of the water concerns in the tailings, the process itself can have significant dewatering demands. In complex processes, the percentage of water may change significantly at each stage. Every type of equipment used in a wet process has a range of water allowed for the best operational level.
An Attrition Cell works best in the range of plus 65% solids. Therefore, a Pump, which is normally operated below 35% solids to limit wear on components, cannot be used to feed an Attrition Cell.
Dewatering equipment, like a Separator™ or even a Dewatering Screen, removes the water from the solids to allow the Attrition Cells to achieve a higher level of particle-on-particle scrubbing action. After the scrubbing, the material needs to be washed to remove the waste from the product.
Dewatering concerns for a process begin as soon as water is added and should be planned for at each stage. As with any process, stability and balance is a key factor in optimizing efficiency and production.
No matter the end result — be it retaining water for reuse or feed preparation, dewatering methods may be used to separate the water from the solids.
To Reduce Hauling Costs and Draining Time
Another reason dewatering is important in the mining industry is to reduce hauling costs. A final product is normally free of water or the water is significantly removed to allow for easier and cheaper transportation. No one likes to pay to haul water, and there are environmental concerns, such as spillage, with doing so.
An important part of dewatering the final product is making sure the product is not removed with the water. A lot of time, effort and cost have been spent to create a salable product, so losing it at the final stage is like a kick in the gut.
Depending on what is to be done with the product, different types of dewatering equipment can fit the bill. When the product will be sitting around for a period of time, a Separator™ and a drainage field will do the job adequately. If the product needs to be conveyed, a Fine Material Screw Washer or Dewatering Screen will provide the reduction in moisture content necessary to avoid housekeeping issues a Separator™ will allow.
Depending on the gradation of the material, various filter equipment (such as filter tables, filter belts and Filter Presses) can be used to provide further water removal. A dryer is consider the final level, as it can produce a relatively moisture-free material. The dryer will need to work with other types dewatering equipment because it is limited in the percent moisture it can receive. The downside of a dryer is fuel costs, which can be extensive.
Fine Material Screw Washers and Dewatering Screens processing salt.
Why Dewatering Is Important in Mining
While the term dewatering may be used for different reasons and in different areas, it is an important concept across the board in every mining application that has water involved. Knowing what equipment is needed and where to use it can make a difference on how long a site continues to stay in operation.
Keeping the right balance in every stage of the operation makes for a better process. Dewatering is more about controlling the water than just removing it.