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Dec 19, 2017

Towards minimum impact concentrator - effects of water quality and recycled reagents on flotation performance

The quality of used process waters is crucial to the stability of highly performing flotation circuits.
Outotec Process Water Recycling plant

It may not be possible to achieve high recovery and mineral quality by using the existing methods of recycling and blending different quality waters together into one stream. Accurate information on process water qualities and quantities is needed to minimize the variations in pH, Eh, organic matter, fines, turbidity, ions, residual reagents, etc., which can be affecting the flotation performance. Significant metal recovery and concentrate grade losses can be seen, when concentrator plants close-up their water loops without considering the holistic site-wide water management. The effects of water quality variations are directly linked to decreased product value and sales return. The recovery and quality losses have started to emerge mainly due to decreased usage of fresh water, following changes in tailings management towards tailings thickening, paste or dry staking. These tailings management methods result in shorter, closed water loops with build ups of harmful water constituents from the beneficiation process point of view.

From conventional towards more advanced tailings management options

The trend for using more advanced tailings management solutions results from environmental, socio-political and water scarcity related issues, which drives the mining industry away from conventional tailings methods towards thickened tailings, paste and dry stacking options in order reduce water losses. However, operational risks and possible production losses are often not considered beforehand. These new operational risks rise from previously neglected or even misunderstood process water quality changes, resulting mainly from the changes in tailings management.

The main difference between conventional tailings management solution and tailings thickening, paste and dry stacking solutions arises from the increased retention times and water bleed volumes present at the conventional tailings management solution. When a conventional tailings dam can have a retention time from 30 to 60 days, the more advanced tailings management solutions typically have a retention time of only 4 to 8 hours, resulting in different water qualities. Within the conventional tailings dams, the long retention times results in chemi-biological reactions and degradation of especially organic molecules by UV-radiation. All this combined with long sedimentation times is expected to clean the water to required quality for reuse or discharge. However, these reactions don’t have time to take place, when using the new tailings management methods, resulting in increased fines and flotation chemistry build-ups in the process water loops.

Right water quality for the purpose

For enhanced operations, the key is to understand what kind of water qualities are needed for the flotation of specific minerals. It is important to understand the impact of different tailing management methods and local weather patterns, together with the concentrator plants’ water quality and quantity requirements, and their relation to the recovered valuable minerals. Outotec has a significant knowledge of all the above mentioned and is able to combine the knowhow into a holistic water management approach.

Figure 1, shows an example of different water qualities used in a beneficiation plant resulting in poor Ni flotation. First picture shows the quality of local river water used as a makeup water for the beneficiation plant. This raw water contains humic acids that is a natural depressant and very effective chelating agent, which blocks collector’s attachment to the mineral. Second picture is taken from beneficiation plant process water dam. It looks clean, but includes residual chemistries, unwanted organics and bacterias. The residual flotation chemicals can originate from other flotation stages, causing bulk flotation in unwanted flotation circuits. The unseen bacterias, meanwhile, simultaneously eat up the flotation chemicals and settle on the mineral’s surface inhibiting flotation chemistries attachment. Last two pictures are from concentration thickeners showing heavy overflow of fines as well as residual chemistries entering process waters.

Example of different waters
Figure 1. Example of different waters found and utilized for a beneficiation plant.

The effect of these waters on Ni recovery can be seen in Figure 2. Nickel recovery depends on the performance and selectivity of the previous Cu-circuit, and the final results are heavily depended on its performance. As can be seen in Figure 2, Ni recovery is negatively affected by the process water quality, as well as fines in comparison to the tap water.

Impact of harmful material on Ni flotation
Figure 2. Impact of harmful material on Ni flotation.

In some polymetallic cases the recycled waters from various circuits have to be separated from the main water recycling circuit in order to avoid unwanted phenomena like bulk flotation or depression of valuable minerals. These separated water circuits are suitable in restricted areas, where the accumulation of reagents, inorganic and organic matter, dissolved compounds, ions and microbial activity are not affecting the beneficiation process recovery. Cleaning process water is highly recommended if the process waters are recycled back to the main process water reservoir and generally used in all process circuits, Figures 3a and 3b.

process circuit
Figure 3a
process circuit
Figure 3b

Figure 3a and 3b. Separation of main water cycles to avoid unwanted phenomena to occur in flotation stages. Figure a shows a typical conventional process, where all different water qualities are mixed very efficient. Figure b, meanwhile, illustrates the most essential process water cleaning positions (Orange Dots) improving the flotation performances.

Water recycling plant designed for the purpose

Outotec has developed the new Process Water Recycling Plant (PWRP) to answer the customer demands for removing specific harmful components. The PWRP plant is designed for polymetallic Cu, Au and Fe reverse flotation concentrators with modularity, stable process performance and easy configuration to meet different requirements.

Outotec Process Water Recycling plant
Figure 4. An example of a Outotec Process Water Recycling plant.

In the case of selective flotation, it is recommended that the separate water circuits are implemented with Outotec’s online water quality and quantity (WQQM) control system. The WQQM solution helps to minimize discharge of process waters and thereby the environmental impact, and in addition enables chemical savings and process performance improvements. Monitoring the water quality enables quick response to process variations resulting from abundant components, chemical changes or reagent dosages in the process water circuits. A well-executed WQQM strategy enables increased water reuse and operational efficiency, and minimizes the negative effects of recycled water on flotation recoveries and grade, Figure 5.

mplemented online WQQM
Figure 5. Implemented online WQQM, for monitoring and measurement of process water qualities and quantities in order to optimize the flotation process.

Summary

All mineral flotation processes are sensitive to process water recycling issues. However, it is important to understand what kind of water qualities affect specific mineral recoveries. This knowledge is at the heart of Outotec.

Juha Saari, Kaj Jansson, Eija Saari, Piia Suvio

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