Water consumption in the mining industry varies significantly between Chile, the United States, Canada and Peru, especially since each country faces different environmental challenges and resource management strategies.
Due to its arid climate and prolonged droughts, Chile is one of the most water-stressed mining regions. The country’s mining sector consumes large amounts of water, particularly copper production. In 2022, water consumption in Chile’s copper mining industry was around 18 cubic meters per second (m³/s), with a significant shift towards seawater use, corresponding to 35.6% of total water consumption, up from 20% in 2018 (see Figure 1 and 2). This figure is projected to increase to almost 70% by 2034 (see Figure 2).
Reliance on desalination and seawater use is crucial as Chile faces increasing social and environmental pressures to reduce the use of continental (fresh) water sources.
Figure 1.- Percentage distribution of water consumption in copper mining by origin, 2018-2029 (Source; Cochilco December 2018)
Figure 2.- Projection of water demand in copper mining by origin 2023-2034 (Source: Cochilco June 2024)
In the Chilean copper mining industry, the concentration plant process uses the most water due to the increase in concentrate production (Cochilco June 2024).
The water use projection made by Cochilco indicates that by 2034, 82.9% of the total water consumed in the country will be used in concentrator plant processes, while 4.3% will be used in hydrometallurgy processes. The report prepared by Cochilco also indicates that concentrate production is expected to increase by 44.1% by 2034 (See Figure 3).
Figure 3.- Concentration Plant and Hydrometallurgy – Water demand in copper mining by process, Projection 2023-2034 (Source: Cochilco June 2024)
Although each country faces challenges in mining water consumption, the common trend is an increasing dependence on alternative water sources such as seawater, increased investment in desalination plants, and a shift towards more efficient water use practices to mitigate environmental impact and ensure sustainable operations.
As part of evaluating a mining process, we at PMC perform water balances to improve water resource management. This is crucial to optimizing operations, minimizing environmental impact, and ensuring compliance with regulations.
In this article, I will explain the importance of carrying out a water balance in concentration plants, its implementation, how to detect water losses in the process, and a description of the water balance in conventional tailings, pastes and filtrates.
WATER BALANCE IN THE MINING INDUSTRY
Water balance in the mining industry refers to the accounting of all water inputs, outputs (water losses), recirculation and storage within a mining operation.
In the case of a concentration plant, it involves quantifying how much water enters the system, for example, from natural sources such as rivers, wells, seawater, and external water supply.
To quantify water losses, such as water lost in the concentrate (product moisture), evaporation in the different areas of the process, and water lost in tailings deposits.
To quantify storage water in pools and ponds and in the production process.
To quantify the water that is recirculated to the process from tailings and concentrate thickeners, as well as from tailings deposits.
How can water balance be used in a Concentrator plant?
Water balance is used to manage processes and optimize water use by making systems more efficient, minimizing losses, and achieving lower freshwater consumption.
The main objective of a Concentrator plant is the separation and concentration of minerals, and water is a fundamental resource for this production process. The implementation of a water balance in this process involves the following steps:
- Quantification of water inputs:
- Fresh water supply: includes water intake from rivers, wells, groundwater, seawater or external sources (treatment plant water from nearby cities).
- Quantification of recirculated water:
- Recirculated water: water recovered from the concentrate and tailings thickening system.
- Water recovered and recirculated from the tailings storage system, such as clear water lagoon, drainage system, and hydraulic barrier wells for plume infiltration control.
- Quantification of water from the different processes:
- Mine circuit for dust control.
- Process water circuits or other parts of the plant, such as grinding and flotation.
- Fresh water addition and processed water in the reagent system water added during mineral processing, such as grinding and flotation.
- Measurement of water outputs (losses):
- Water losses in dust suppression systems for the production process in mines, plants, and tailings deposits.
- Evaporation: water loss due to evaporation from open water surfaces such as storage pools, tailings and concentrate thickeners, and tailings deposit systems.
- Water loss due to retention in tailings: water retained in the tailings that cannot be recovered for the production process and remains trapped in the tailings deposits.
- Filtration and discharge: Water is lost due to filtration in the soil when tailings infiltrate the natural ground when the tailings sent to the deposit come into contact with the natural soil.
- Product humidity: water that leaves the plant bound to the concentrated product in the order of 9% humidity.
- Waste streams: water leaving the plant or other waste products, such as wastewater treatment plants, which are treated, part of which is lost, and another part is recycled and recirculated to the production process.
- Storage monitoring:
- Tanks, reservoirs and pipes: accounting for water stored within tanks, reservoirs or pipes within the plant.
- Process equipment and circuits: water retained within the processing circuits.
WATER LEAK DETECTION
By performing a thorough water balance, water losses at a Concentrator plant can be detected by comparing measured inflows to outflows and internal storage. Unaccounted-for water can indicate losses due to leaks, unmeasured seepage, or inefficient recycling systems. Specific techniques for detecting and addressing water losses include:
- Flow meters and sensors: Install flow meters at strategic points to continuously measure water flow rates, helping to identify discrepancies that suggest leaks or unauthorized discharges.
- Tracers: Chemical or nuclear tracers are injected into the flow, where the flow rate and concentration are detected. Knowing the fluid velocity and the area of the piping can determine the flow dilution and losses along the way.
- Periodic audits and inspections: Routine inspections of pipes, tanks, and equipment can help identify physical losses, such as leaks or faulty valves.
- Water quality monitoring: By analyzing water quality at various stages, operators can detect changes that may indicate mixing with external water sources or loss of water purity, suggesting process inefficiencies.
- Modeling and simulation: Software to model the plant’s water balance based on the mine plan can help predict water use patterns and identify unexpected deviations that could indicate production losses.
WATER BALANCE IN CONVENTIONAL , PASTE AND FILTERED TAILING
Conventional tailings
The diagram below illustrates the water balance at a copper concentrator plant using conventional tailings technology (Figure 4). In this configuration, 99.5% of water consumption is attributed to losses in the tailings dam, while only 0.5% is lost through the moisture content of the final concentrate.
In iron ore concentrator plants, where the proportion of the product is higher, water consumption in the concentrate increases to 15%, with tailings accounting for 85%.
Tailings represent a significant use of water at both copper and iron ore plants. Therefore, optimizing water consumption requires a closer examination of water losses associated with tailings through a water balance.
Conventional tailings storage involves building a dam or wall, often using rejected minerals from the mine or sorted sands from the tailings flow, to close a gully and create a reservoir capable of holding millions of tons. At 50% to 60% solids, these tailings are deposited in the tailings dam, where 20% to 30% of the transported water is recovered.
For a typical conventional tailings concentrator plant in Chile (see Figure 5), with a capacity of 100 ktpd and an operational life of 20 years, the planned volume for tailings storage is approximately 720 Mt, equivalent to 480 Mm³. This volume includes a significant amount of water lost through three main pathways: 1) 15% – 20% due to evaporation from the water lagoon and wet tailings beach, 2) 10% – 15% by infiltration and 3) 65% – 75% retained in the tailings, mainly within the slimes and ultrafine material.
Figure 5.- Typical Concentrator Plant in Chile – conventional tailings – BHP Minera Escondida (September Google Maps imagery 2024)
A plant of this type, depending on the topography of the pond and the evaporation rate of the site, especially in northern Chile, can have a water consumption ranging from 0.4 m³/t for those with favourable topographic conditions to 0.8 m³/t for those that do not recirculate water. Consequently, a 100 ktpd plant uses between 400 lps and 950 lps. On average, with a water recovery of 30% from the clear water lagoon, tailings consumption is in the order of 0.60 m³/t, which corresponds to water use of 700 lps.
Given the distribution of losses indicated, studies aimed at reducing water retention in the tank by fine particles (65%-75%) and promoting the recirculation of recovered water back to the plant are essential.
Thickened tailings or paste tailings
Thickened tailings are non-segregated tailings characterized by their high viscosity and do not generate wastewater under optimal conditions (see Figure 6). These tailings, containing between 60% and 70% solids, are processed in high-density thickeners with wide-angle cones. They must handle pulps with high yield stresses ranging from 100 to 300 Pascals. They require thickeners with high torque capacities of 6.0 to 10.0 MN-m and a maximum diameter of 45 meters, which is the current industry standard for this level of development.
Figure 6.- Tailings paste concentration plant – Minera Centinela (September Google Maps imagery 2024)
Water consumption is necessary to compensate for the water discharged by the slurry, which contains 70% solids, resulting in a make-up requirement of 0.43 m³/t. Once tailings are deposited, additional water recovery is generally impossible. However, in some plants with insufficient solids content or viscosity, tailings may produce recoverable drainage of 5% to 10% of the water entering the deposit.
Filtered tailings
Filtered tailings, also known as dry stack tailings, are a modern approach to mine tailings management that involves dewatering the tailings to reduce excess moisture and stacking them in a controlled manner. They are produced using high-capacity filters, typically high-pressure filters, due to the challenges of filtering out very fine particles such as tailings. Initially, this technology was employed in small-capacity plants but is now used in plants with capacities of up to 40,000 tpd. Once the filtered tailings cake is formed, a system of belts and stackers is used to create a multi-layered tailings pile, typically 6-10 metres thick, with the potential to reach heights of up to 100 metres.
Figure 7.- Filtered tailings – Minera Mantos Blanco – Capstone Copper (September Google Maps imagery 2024)
The pulp produced contains 50% solids and is fed into filters, where its moisture content is reduced to 20%, achieving a solids content of 80%. This process significantly reduces water or make-up consumption to 0.25 m³/t, offering greater efficiency than other technologies. However, drawbacks include the high cost of filtration equipment and challenges in stacking materials that are difficult to filter.
CONCLUSION
Maintaining an accurate water balance at a concentrator plant is not just good practice; it is a crucial element in optimizing and ensuring the sustainability of mining operations. The process involves meticulous monitoring of water inputs, outputs, and losses at various plant stages. By doing so, plants can identify inefficiencies, reduce water consumption, and minimize environmental impact. In an era where water scarcity is an increasingly pressing concern, achieving an effective water balance is essential for regulatory compliance and operational excellence.
At Process Minerals Consulting, we understand the complexities involved in performing a comprehensive water balance. Our metallurgical engineering expertise and deep knowledge of concentrator plant operations allow us to provide customized solutions that address our client’s specific challenges. We employ diagnostic tools and methodologies to identify where water losses occur and how they can be mitigated. Our focus is not only on reducing water consumption but also optimizing the overall process, ensuring that every drop of water is used efficiently. Whether through filtration systems, improved water recycling practices, or improved process controls, we are committed to our clients achieving significant improvements in water use and overall plant performance. In addition, we perform dynamic water consumption and loss projections considering production scenarios and different water scenarios such as dry, medium, and wet years to manage different water scenarios in advance.
If you would like us to guide you in carrying out a detailed water balance to achieve the optimization of your process, you can contact us at info@processminerals.cl
