Thickeners or thickener plants – optimizing the performance and cost of thickened tailings
Mine tailings have been a hot topic in recent times as a result of both the ongoing discussion around the increasing scarcity of water and failings that have occurred at tailings facilities. The effective and safe disposal of tailings is heavily influenced by the amount of water they contain: the higher the water content, the larger the disposal site and the more important the dam wall is to ensuring its security.
Mining companies are making significant investments in studying ways to improve the security and safety of tailings depositions. Several sites have started separating acid-generating material from the bulk of their tailings in order to significantly reduce the volume of tailings that require ongoing management and to create a more environmentally sustainable tailings storage facility (TSF).
In more arid locations, paste deposition and beach disposal have proven to be effective ways to increase water recovery and reduce risk in the tailings storage area. Some sites have progressed even further to dry stacking of filtered tailings.
A holistic view to helps identify performance improvements
Thickeners are recognized as one of the more cost-effective technologies for recovering water from tailings streams. Significant energy savings can be achieved by maximizing the recovery of water close to the mill. In addition, a higher density of tailings deposited in the dam tends to reduce the footprint and produce a more stable tailings deposit.
Figure 1. Outotec Paste Thickener.
When considering a thickened tailings solution, there is the question of whether the technology to thicken tailings should be located in the thickener as a piece of equipment, or whether a complete solution is required to achieve the best performance. We typically view the components required to thicken tailings in isolation rather than considering them as a complete plant comprising a thickener, pumping system, automation systems, and the necessary ancillary equipment or systems required for operation (see Figure 2). Because all these components interact with one another, in order to identify how performance can be optimized it is necessary to look at the whole system, rather than the thickener in isolation. As the thickener produces higher degrees of separation, the underflow increases in viscosity, which in turn leads to the high frictional losses that require higher discharge pressures to pump thickened tailings to the final deposition point. Optimizing the performance and cost of a thickener plant – whether it is a self-contained plant that is separate from the main processing facility, or part of the mill – is becoming a challenge as we increase water recovery with compact equipment, lower safety factors, and increasingly simplified testing campaigns.
Figure 2. Thickener plant flow sheet.
Reducing operational variability through control system improvements
Despite the design of the thickener and installation of good instrumentation, poor overall plant design, inadequate automation, or low operator focus can be contributing factors in thickeners that fail to consistently achieve the best results.
Although the duty of a tailings thickener can be defined in quite simple terms, the thickener will experience a wide range of operational conditions during its lifespan. Mining operations will experience changes in the geology of the mine that lead to variable ore grades; the mass flow of solids will vary according to the milling rates; the concentration of solids in the feed will vary because of operational issues; and the setting rate will depend on the mineralogy and particle size. This variability needs to be compensated by the control systems and ancillaries surrounding the thickener. Improvements can be achieved by adjusting the plant’s flow sheet to suit the overall system objectives, and significant energy savings can be realized by optimizing the location of the plant.
Outotec has demonstrated that by improving control systems and replacing single-loop PI control with multivariable-model predictive control platforms, thickeners can operate with a significantly tighter distribution of underflow density and reduced operational variability. With these improvements in control logic a higher average density can be maintained, which in turn leads to a more stable tailings pump operation and higher water recovery.
Figure 3. Outotec Thickener Plant.
Improving the performance of existing plants
The benefits of good plant design are not limited to new thickener installations. Tailings dams in existing plants fill faster than is optimal, and often the technologies available when the dam was planned have evolved, providing opportunities to improve thickener performance. These improvements lead to the opportunity to improve the performance of the plant by increasing water recovery or changing the deposition characteristics of the thickened tailings. Simply upgrading the thickener to incorporate new and improved components may not lead to the plant achieving its maximum potential. By understanding the rheological properties that have an impact on the thickening and disposal methodology it is possible to realize significant improvements in TSF costs. These changes introduce all of the complexities of a new thickener installation, and consequently the entire system should be evaluated and improved as necessary.
Identifying the factors limiting performance
In order to improve the performance of a thickened tailings plant, the best approach is to identify and examine the variables that are limiting its performance. The objective of thickening is to recover water from the tailings and to produce a slurry that will optimize the costs of the tailings storage facility.
Often the tailings system is operated by several departments, with the benefits of good performance being felt in different departments to the ones that have to bear the costs, leading to poor optimization.
Pumping, or slurry transport, is the link between thickening and the tailings storage facility, and this function is often the rate-limiting variable in a tailings system. Higher densities increase the pumping pressure required or limit the flow of gravity systems and can require expensive infrastructure to operate at higher water recoveries.
The training and focus of operators can also be an important limiting factor in terms of thickener performance. The adjustable variables that control a thickener have complex interactions that are often poorly understood, and there are long lags between actions or disruptions. This can often result in conservative operation.
Water quality is often neglected when defining thickener plant performance. High water recoveries result in increased recycling of salts and process chemicals with short-term implications. Low water recoveries lead to longer-term contact between the tails and the process water in the tailings dam. Evaporation will also increase the concentration of salts, while reagents may decompose or transform in the dam. The effect of these conditions in the dam may take years to reach stability, with a limited ability to manage the outcome.
Figure 4. Paste Thickening Plant delivered by Outotec as an EPC project to Yara’s phosphate mine in Finland.
Advances in thickener design have improved the degree of separation possible from tailings streams, however limiting the selection criteria to just the thickener can constrain the operation potential. Integrating ancillaries, automation and operator training with the equipment can improve operational outcomes and decrease operating costs. By focusing on water, looking at the true costs involved and the implications of water quality on plant performance, better outcomes and safer tailings storage can be achieved.