In part 1 of this article, we discussed the water balance in a concentrator and identified where water losses occurred in the process. We then looked in detail at the type of tailing disposal technologies as they are the process stage where the most consumption of water occur. In part 2, we will present the strategies to reduce concentrator plant water consumption.
Strategies to reduce water consumption
- Choice of tailing disposal method for new plants.
As discussed above the choice of tailings disposal method defines the water makeup in the range between 0.25 to 0.60 m3/t as is presented in figure 5 below. The selection of the best method depends on each project. Cost needs to be evaluated considering availability of fresh water, topographic conditions and impact over nearby communities.
Figure 5. Tailing technologies water make-up
- Increase in concentration of thickening solids
The optimization and modernization of tailing thickeners present an opportunity to lower the water consumption. The implementation of automatic control systems, and in-line instrumentation can improve significantly the separation process in turn increasing the water recovery and density of the slurry feeding the tailing disposal methods. Instruments such as SmartDiver (tank profiling & mud level detection system), can be used in all types of thickeners to improve control and optimisation tasks.
The use of rheological reagents that allow higher thickening rates, without increasing the viscosity of pulps for application in paste/thickened tailings also contributes to the thickener efficiency. Different tailings deposits required a different range of thickener underflow solids concentration. Paste and filtration technologies will require higher solid concentrations than the tailing dams. The higher the density the better the water savings (figure 6).
Figure 6. Tailing technologies – solids feed from thickener
- Increase of water rate recovery in conventional tailings dam
Special reagents can be used to achieve post-decanting of ultrafine tailings in the reservoir, control the lagoon volume over time and improve the measurement of evaporation rate on active beaches. In addition, satellite technology can be used to monitor the lagoon area and make changes in the discharge points to decrease the lagoon size hence decreasing water evaporation.
- Increase of filter capacity
The objective of this strategy is to increase equipment size in the filtration stage to diminish the opex and capex in bigger concentrators, such as those over 100 ktpd. However, there is limitations in terms of filtration rates and filtration area available. For example, a 100ktpd plant at the current average filtration rates of about 0.3 t/h/m2, requires 15.000 m2 filtration area.
- Replacement of fresh water with seawater
In regions where fresh water is scarce (e.g. arid regions in Chile and Peru), raw or desalinated sea water is being used as an alternative water source.
This is the case of BHP Minera Escondida in Chile where desalinated water is used to replace fresh water sources from underground wells in the copper processing facility. The desalination plant was producing 525 lps in 2006.
Another example is the use of raw sea water in the AMSA Esperanza project also in Chile built in 2010. In this year, sea water usage reached 700 lps. The water is pump lifted to the plant to an altitude of 1800 m.a.s.l. by a 150 km pipeline.
It should be noted that both projects Escondida and Esperanza also have thickeners with state-of-the-art instrumentation installed including SmartDiver to optimize control and operations and increase water recovery.
Seawater use in Chile, has increased from 0.98 m3/s in 2012 to 4.0 m3/s 2018. This correspond to 30% of total water consumption in copper mining, and is expected to continue to grow. Currently several projects under construction are to use desalinated and raw sea water to feed 500 ktpd in total processing capacity. Considering a makeup 0.6 m3/t, the projects are to supply 3,5 m3/s, nearly duplicating current sea water consumption (2).
Water consumption or make-up in a concentrator plant depends significantly on the tailing deposit technology used. The use of filtered tailings results in the lowest water consumptions (0.25 m3/t) followed by thickened or paste tailings (0.4 m3/t) and conventional dams (0.6 m3/t). However, there are other factors such the cost and availability of water, the area’s topography and the impact of the project to local communities that need to be considered during the evaluation of the tailing disposal method for a new plant.
There are other strategies that aim at reducing water consumption at different stages of the process. At the concentrator and tailings thickener, the use of automation, in-line monitoring and the latest re-agents improve the overall separation process, increasing the solids and decreasing the amount of water of the slurries feeding the tailing disposal methods.
Satellite technology for monitoring active beaches and lagoons in tailing dams can be used to measure the area of the lagoons and control the water loss in evaporation.
Replacement of fresh water with desalinated water or raw salt water from the sea is another strategy that is being used in areas where access to a secure and stable water supply is limited such as in the arid regions in Chile and Peru.
The expected water savings from different strategies range from 2% to 5% as shown in figure 6.
Figure 7. Water savings from different strategies
2. Montes, C. and Cantallopts, J., ‘Consumo de agua en la minería del cobre al 2018’, Dirección de Estudios y Políticas Públicas, Ministerio de Mineria, September 2019.
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