Table of Contents
Why Mining needs Water & How is Water Used
Water plays an important role in most mining and extractive processes. Understanding the main uses of water, the quantity and quality of water that arc required for each use, and the water losses associated with each use are the first steps in making decisions on how to best manage water at mining operations.
Water usage and the resulting mass balance vary depending on the type of mining and mineral processing used for the operation. For example, an underground mine will typically be required to deal with more groundwater inflows than an open pit mine. An example of varied mass balance occurs in hydrometallurgical processing, which will typically result in water laden with higher concentrations of soluble metals than an operation ending at the concentrator. The main water uses in mining operations, listed in order of typical quantities, include the following:
- Conveyance of the ore or wastes (slurries or suspensions)
- Mineral processing and separation (grinding, flotation, gravity separation, leaching, hydrometallurgy)
- Mineral extraction (hydraulic or solution mining)
- Dust suppression
- Washing of equipment
- Human use and consumption
The conveyance of ore and wastes in the form of slurries or suspensions is typically the largest water demand at a mine. Tailings disposal, in particular, can require a significant volume of water and can result in losses of large volumes of water. In conventional or slurry tailings disposal, tailings are sent to an impoundment area with approximately 50% water by volume (±10%) in the conveyed slurry. Some of the water used to transport tailings can be recycled from reclaim ponds in the railings basin. However, over time, constituents can build up within the water, resulting in decreasing water quality for the operation. Some of the water used for tailings transport can be lost to evaporation within the impoundment, infiltration or seepage through the impoundment, and permanent storage in the voids between the tailings particles. Methods of tailings disposal are available that use substantially less water, such as thickened tailings, paste tailings, dry tailings, and cemented tailings, but these methods typically have significantly different capital and operating costs that need to be considered.
Mineral processing uses water during several steps in its procedure to separate the ore from the gangue and to produce concentrates. This is typically the second highest water demand at a mine. Grinding, flotation, leaching, and hydrometallurgy are the main water users in typical mineral processing; however, the water used in these processes can often be captured and reused. The use of recycled water in mine processes results in a buildup of constituents within the water, which causes a decrease in water quality over time. Reuse of low-quality water without some treatment or dilution can lead to scaling or corrosive water. The flotation and leaching processes can be adjusted to use this lower-quality water, or other low-quality water such as seawater, brackish water, or saline groundwater, without prior treatment. However, many mineral processing steps require high-quality water, and water treatment plants are becoming a required process for mines around the world. Water losses in mineral processing can be due to evaporation from heap leach piles or ore drying, or moisture retention in the concentrate or other end products. For an in-depth description of the amount of water used for each step of the mineral processing circuit, including a breakdown of varying ore types.
Compared to mineral processing and ore or waste conveyance, there is relatively little water used in mineral extraction. Exceptions are hydraulic mining and solution mining, both of which are specialized practices for a mining operation. Hydraulic mining is the process of using jets of high-pressured water to cut through the waste rock or sediments to get to the ore body. This type of mining has been used for centuries and is still employed in underground coal mining operations.
Solution mining, also referred to as in-situ leaching or in-situ recovery, is the process of recovering soluble minerals such as uranium and sometimes copper by leaching the metals in place, typically through boreholes drilled to the mineral deposit. This type of mining is most commonly used for uranium deposits. As of 2010, solution mining accounted for 41% of the world’s uranium extraction.
Cooling, equipment washing, and human use require relatively little water (compared to uses such as railings transport and mineral processing), bur they usually result in permanent water losses because typically, no recycling options exist for these water uses. The four main losses of water at any mine are usually associated with evaporation (ponds, wetted surfaces), cooling (machines, products), storage of materials (within the tailings voids), and dust control.
Water use in coal mining and other soft-rock mining (e.g., salts, oil sands) is very different than its use in hard-rock mining. Water is often used to aid in extraction of suit-rock mining. For coal mining, water jets are used to break up a coal scam or to wash away sediments along the scam. Similarly, water is often used to wash the coal free of impurities and sulfur as part of the processing of the coal. Based on a study by the U.S. Department of Energy in 2006, water used for U.S. coal mining ranges from 270 MLD to 980 MLD (million liters per day), or 70 MGD to 260 MGD (million gallons per day), which includes the water used for cooling the drilling equipment and for coal washing.
What are the Sources of Water for Mining
Regulations at the national, state, or local level often drive the water options available for mine uses. Mining requires a significant volume of water relative to the amount of ore to be extracted. Water for mining and mineral processing can be obtained from a variety of sources, and the water source may dictate the need for treatment:
- Oceans or seas—saline water
- Lakes or rivers—typically freshwater, but may be brackish
- Groundwater may be freshwater, brackish, or saline
- Wetlands—may be freshwater, brackish, or saline
Each of these water sources has a unique set of potential associated issues, lire only source of water for which supply is not limited is oceans and seas. However, very few processes can use water with the salinity levels found in seawater, and desalination costs are very high. In addition, seawater is very corrosive, resulting in higher maintenance costs.
Fresh surface water sources, such as lakes, rivers, and wetlands, are ideal for use in a mining operation. However, competition for use of this water can be high when there are other industries or communities in the immediate vicinity. Additionally, appropriating water from these sources, wetlands in particular, may result in the draining of unique habitats, causing other environmental impacts.
Groundwater can be a convenient water source for mines operating below the water table; however, it can also be a major source of excess water that can affect the mining operation. Dewatering via extraction wells or sumps can generate significant quantities of water that need to be managed. The use of groundwater can have some of the same issues as seawater when the water is very saline, resulting in the potential need for expensive desalination treatment. Groundwater can also be very corrosive, depending on its quality, potentially causing scaling and maintenance issues. Groundwater may be the only option in arid regions. However, competition for this source of water is often very high in dry areas, creating tension with other necessary uses such as drinking water, agriculture, and other industries.
Mine Water Treatment Processes
Two factors drive the need for water treatment at mining operations, First, water treatment may be necessary to obtain the quality needed for mining or mineral processing operations. This might involve treatment of either water with naturally high salinity levels or recycled water in which solutes have been concentrated by mining processes. Second, water treatment may be required to meet objectives for water effluent to be released into the environment. Mining operations in temperate and wet regions may accumulate excess water and need to discharge to a local water body. Discharges arc typically subject to local or national water quality regulations, so water treatment may be necessary to meet the required concentration limits set by those regulations.
Most mining operations that treat their source or recycled water do so with active treatment, but passive biological or chemical treatment can also be effective under certain circumstances, especially after mine closure. Common treatment methods used in mining applications include
- Reverse osmosis/ultrafiltration or nanofiltration,
- Ion exchange,
- Active or passive biological treatment, and
- Active or passive chemical treatment.
Acid rock drainage (ARD) and heavy metal concentrations are the primary concerns associated with mine-impacted water from metal and coal mines that need to be managed. These concerns are the main drivers for water treatment in these types of mining, because ARD and heavy metal leaching can lead to long-term problems in downstream resources and very costly remediation.
Sulfate is a primary constituent associated with ARD that needs to be managed. Most metal-bearing ore bodies are found within sulfide-based minerals that oxidize into sulfate through the metal extraction processes or exposure to the natural elements and can lead to ARD. The management of sulfate has come under greater scrutiny around the world since the early 2000s, with regulations changing from being completely unregulated to being subject lo discharge limits as low as 10 mg/L in some circumstances, as listed in Table 15.1. Similar to sulfate, selenium is also a primary constituent of concern in coal mining and some hard-rock mining, which has also come under increased scrutiny in the last decade.
Increased regulatory restrictions on specific constituents such as sulfate and selenium are leading more mining companies to evaluate treatment options. The use of treatment within a mining operation can initiate evaluation of additional responsible mine water management options after assessing the comprehensive mine water and mass balance.