Removal of water
Henna Punkkinen, Markku Juvankoski, Tommi Kaartinen, Jutta Laine-Ylijoki, Elina Merta, Ulla-Maija Mroueh, Jarno Mäkinen, Emma Niemeläinen & Margareta Wahlström, VTT Technical Research Centre of Finland Ltd, P.O. Box 1000, FI-02044 VTT, Finland.
In this article, three different methods to achieve a removal of water resulting in a cone or self-supporting stack of dried tailings are discussed: thickening of tailings, paste disposal, and filtering of tailings (i.e. dry stacking of tailings). Probably the main reason to choose one of these technologies as a tailings treatment method is to achieve cost savings or other benefits due to the reduced consumption of water. However, the removal of water is not very commonly used in the mining industry compared to the conventional tailings disposal methods.
Description of technology
The removal of water from tailings means the use of different kinds of dewatering processes before or at the point of tailings discharge to produce thickened slurry or paste that is led via pipelines into an impoundment. Also open pits are sometimes used as disposal areas. Alternatively a tailings-cement paste mixture that is prepared in a plant from thickened tailings can be backfilled into underground mine workings. (Lottermoser 2010) If tailings are dewatered even more, a filtered wet (saturated) cake and dry (unsaturated) cake are produced. These dry cakes, called as dry stacks, cannot be transported via pipelines like thickened tailings or paste, but must be conveyed to the tailings deposit by other means. (Engels 2012)
Thickened tailings, paste tailings and filtered tailings can all be seen as a part of the same process starting from segregated tailings with high water content, moving to non-segregated thickened tailings, then shifting to paste tailings and eventually reaching to the state of filtered tailings having a form of a solid cake. That is, during the process the water content of tailings decreases, both solids concentration and yield stress increase, and segregation on deposition ceases. (Caldwell & Charlebois 2012) (Figure 1)
Figure 1. The effects of water removal (Modified after Paterson & Cooke 2012 and Lupo (undated), cited by Caldwell & Charlebois 2012).
Thickening of tailings basically means the reduction of water content of low solids concentrated tailings slurry (Jewell & Fourie 2015). The reduction process takes place in a thickener and is done before the tailings deposition (Outotec Oyj 2012). Different techniques exist for water removal; e.g. high-density methods, gravitational methods, compression and deep cone thickeners, filtering, combination of thickeners and filtering, and manipulation of the chemical properties of the slurry (International Mining 2010, Engels 2014a, Jewell & Fourie 2015). The reuse of recovered process water is often possible, thus limiting the need of fresh water intake in the process (Outotec Oyj 2012).
As a result of the thickening process, a homogenous, non-acid generating, and little segregated tailings mass is generated, which is next pumped along pipelines to the disposal site (International Mining 2010, Diekmeyer 2011, Engels 2014a) and then stacked as a structurally stable and self-supporting conical pile (Engels 2014a). Compared to the conventional tailings disposal pond, the structure of a highly viscose low water containing tailings pile is favourable as lower embankments can be used (Outotec Oyj 2012, Engels 2014a). A pile structure also saves space, as the area required is only around a half of the area needed for conventional disposal (Outotec Oyj 2012). The height of the piles and their steepness can be controlled by solids concentration modifications (i.e. by increasing/decreasing the thickness). In the situation where water content is high beach slopes angles remain lower and vice versa. (Engels 2014a)
Thickened tailings can also be backfilled into underground mine workings as a component of tailings-cement paste containing thickened tailings, sand/crushed rock, and cement. The application requires higher backfill strengths, and its transport and disposal systems differ from the ones used in the surficial methods. (Jewell & Fourie 2015) As cement contains acid neutralizing minerals the use of this technique may limit acid generation (Lottermoser 2010).
According to different estimations, the solids concentration in thickened tailings varies between 60–72% by weight (International Mining 2010, Diekmeyer 2011, Paterson & Cooke 2012 cited by Caldwell & Charlebois 2012). The yield stress remains typically between 20 and 100 Pa (Caldwell & Charlebois 2012, Engels 2014a).
A case example of the use of thickened tailings in the site rehabilitation at Osisko’s Canadian Malarctic Mine is presented by Diekmeyer (2011).
According to Palkovits (2011) even more than 95% of the paste experiments have been proven successful. Paste tailings are produced using specialised thickeners or ultra-high-density thickeners (International Mining 2010). Also flocculants and coagulants can be used to accelerate water removal (Engels 2014b). As the paste is thicker (i.e. has lower water content) than thickened tailings, the slurry produced is very dense, viscose, and does not have a critical flow velocity (Theriault et al. 2003, Engels 2014a). The viscosity of the paste can be controlled; in an ideal situation the paste is viscose enough to form conical pile when deposited, but is still flowing when transported (Outotec Oyj 2012). In addition, according to Blodgett & Kuipers (2012) the paste tailings have the following characteristics compared to the conventional tailings:
- They do not segregate, are homogenous and more suitable to engineering design
- Pastes have low permeability, thus limiting the generation of AMD
- Shear strength in a fluid form causes paste to settle on an angle of repose. The shear strength grows when water bleeding to the surface occurs.
- Paste is able to form an angle of repose between 3-10° if the underlying paste material is stabilised. Steeper slopes save space even compared to the thickened tailings. Paste stack can be constructed in sectors or in segments to reach its final height.
- High pressure gradient compared to dilute slurries causes an increased need of energy during the pipeline transportation. (Blodgett & Kuipers 2012)
Due to its “stiff” nature, the paste is usually pumped along pipelines to the deposition site using positive displacement pumps, which increases the costs of the paste disposal compared to the thickening of tailings, and limits the distance of economically beneficial transport. (Theriault et al. 2003, Engels 2014a) After deposition, a desiccation and cracking may occur. However, as new paste material covers and fills the cracks, the layers basically become clued together thus creating a more stable pile structure (Engels 2014b).
Theriault et al. (2003) report experiences on the surface disposal of paste tailings that has been applied at Bulyanhulu Mine in Tanzania.
A suitability of the paste tailings technology to be used in Chevron Questa Mine in New Mexico, USA was evaluated by Blodgett & Kuipers (2012).
The dry stacking of tailings has recently become a more popular method to deposit tailings as nowadays many mines need to save water, limit seepage from tailings, and avoid hazards that may be caused by slope failures or pond overflows (Murphy & Caldwell 2012). After thickening, tailings are conducted to the filter (either a belt, disc, press, or vacuum filter, or their combination) (International Mining 2010, Murphy & Caldwell 2012), squeezed to a moisture content between 10 to 15% (Murphy & Caldwell 2012) and to a solids content of >85% by weight (International Mining 2010). A dry (unsaturated) cake produced is transported using conveyor or trucks to the deposition area, spread with earth movers, and left either uncompacted or compacted. Seepage waters from the stack are collected and treated. (Murphy & Caldwell 2012)
A report by AMEC (2008) presents short case examples on the application of the filtered tailings method in South and North America.
In addition, Davies et al. (2011) (cited by Blodgett & Kuipers 2012) also lists examples of Chilean and North American mines that use or have used dewatering technologies:
- La Coipa (filtered) (the case is also presented in AMEC 2008)
- Mantos Blancos (filtered)
- El Peñon (filtered)
- El Indio (mine closed; both conventional and filtered)
- Las Cenizas (paste)
- Esperanza Project (paste)
- Pogo Project, Alaska, USA (filtered dry stack) (the case is also presented in AMEC 2008)
- Raglan, Quebec, Canada (filtered dry stack) (the case is also presented in AMEC 2008)
- Kidd Creek, Ontario, Canada (thickened tailings)
- Greens Creek, Alaska, USA (paste/filtered) (the case is also presented in AMEC 2008)
- Many small scale precious and base metals mines in Canada & Alaska, USA (paste). (Davies et al. 2011 cited by Blodgett & Kuipers 2012)
According to International Mining (2010) the thickened tailings disposal method is suitable for mines having a small to moderate production rate with large and nearly flat waste area. Also in areas where the construction of an embankment is difficult (e.g. due to the weak foundation materials or the lack of them) using this method may be worth considering. However, in areas with high total rainfall, cold climate, and lack of sun light the method may not be suitable.
General advantages of thickened tailings disposal include (e.g. Dunn & Vietti (undated), International Mining 2010, Lottermoser 2010, Ulrich & Kerr 2011, Outotec Oyj 2012, Engels 2014a, Jewell & Fourie 2015):
- Small or no embankment is needed when the topography is flat. This leads to structural stability and safety, diminishing the risk of failures.
- No segregation, which improves rehabilitation potential (good surface for vegetation)
- Compared to the conventional disposal, smaller surface footprint is required for deposition
- Water savings and sustainable water use: reduced water consumption and groundwater pollution; possibility for water recovery and reuse; minimisation of seepage, spillage of process water, and evaporation losses; limited amount of free water on site etc.
- Economic advantages compared to tailings dams, for instance reclamation costs are lower; no large dams are needed (also diminishes the risk of failure); reduced impoundment needs; possible savings in water and reagent conservation; lower pumping costs
- Minimised environmental risks may help in achieving the social licence to operate
- Reduced oxygen ingress due to little or missing solid/liquid separation prevents the formation of AMD
- Processing reagents as well as soluble economic minerals may be recovered. (e.g. Dunn & Vietti (undated), International Mining 2010, Lottermoser 2010, Ulrich & Kerr 2011, Outotec Oyj 2012, Engels 2014a, Jewell & Fourie 2015)
Disadvantages of the thickened tailings disposal (International Mining 2010, Lottermoser 2010, Engels 2014a-b):
- Dust generation or liquefaction of waste are possible
- Higher operating costs
- Unproven technology if production rate exceeds moderate
- Also relatively unproven method in a global scale. However, when compared to the paste technologies, high rate thickening methods are more proven.
- Energy consumption is usually higher than in the conventional methods
- Suitable implementation is possible only in dry climates
- Changes in tailings gradations, ore characteristics, and percent solids cause operational unpredictability
- Requires flat topography
- Concerns related to the dynamic stability
- Water savings are only marginally higher than in the conventional disposal. (International Mining 2010, Lottermoser 2010, Engels 2014a, 2014b)
The paste disposal method suits best for the mines having space and water limitations, low production rate, fine grained tailings material (at least 15% of particles should be < 2µm), and inexpensive energy. The method is not suitable for the mines having moderate to high production rate or coarse grained tailings. (International Mining 2010)
Paste backfilling to the underground mine workings has been proven successful at many sites during the years, whereas surface paste disposal is quite new method with limited applications (International Mining 2010).The paste can also be mixed with the waste rock to decrease permeability of the waste rocks, and co-disposed (Blodgett & Kuipers 2012).
General advantages and disadvantages of the paste disposal are mostly the same as in thickening. However, the paste disposal saves more water compared to thickening (International Mining 2010), but its operational costs are also higher due to the higher dewatering and transportation costs (for example, the paste must be pumped using more expensive pumps) (Blodgett & Kuipers 2012, Engels 2014b).
The dry stacking of tailings is a recommended method especially in arid, cold, or highly seismic areas where space is restricted, water availability is very limited or its handling is difficult, topography hinders the dam construction, construction materials for the dam building are expensive or not available, or where the conventional methods are not able to fulfil environmental regulations (AMEC 2008, International Mining 2010). The method is not proven for mines exceeding moderate production rates (International Mining 2010) and due to its high costs and operational management requirements it is only really applicable to mines with ≤20,000 tpd throughput (Engels 2012).
These following advantages are especially connected to the dry stacking of tailings (International Mining 2010, Engels 2012):
- Considerable amount of water can be saved
- Suitable to be used in a variety of different terrains (e.g. undulating and steep), and in areas of high seismic activity
- Possibility for progressive rehabilitation causes costs to spread over a longer time period
- Higher rates of rise are possible
- In cold regions frosting and freezing of pipes may be avoided
- More efficient recovery of process chemicals and dissolved metals is possible
- Closure and rehabilitation is easier, and when compared to the other surface tailings deposition methods smaller footprint area is required. (International Mining 2010, Engels 2012)
Disadvantages according to Engels (2012):
- The dry stacking method is the most expensive of all dewatering methods. High capital as well as operating costs. However, the costs have been decreasing lately due to increased filtration capacities and operational optimisation.
- Upstream diversion systems are essential
- High concentrations (low volume) of seepage water may be generated due to sulphide oxidation
- No possibility to store water “for a rainy day”
- Dusting in arid climates due to the low moisture content of dry cakes
- High precipitation and seasonal fluctuations may cause operational problems. (Engels 2012)
Each dewatering method has varying costs, different risks, and their environmental benefits may not be similar. When evaluated, these methods often prove to be more cost-effective, are more able to fulfil regulatory compliances, have lower risks, and their use enhances mine closure and remediation actions compared to the conventional methods. (Palkovits 2011)
As already mentioned, more energy as well as more powerful equipment are needed in the dewatering than in the conventional mine waste disposal, which raises the capital and operating costs of the water removal methods. As for the energy costs, thickening is the cheapest alternative, while dry stacking of the tailings is the most expensive of the methods, a fact that strongly limits the use of the dry stacking. (International Mining 2010, Blodgett & Kuipers 2012)
It is characteristical for the mining industry that the composition and properties of the wastes generated changes, even significantly, over time for example along with the changes in operations and in the ore quality. As it is important to remember that designing thickening operations is not an exact science and the actual throughputs may differ from those achieved in a laboratory scale. Also process optimization may take time and it may be difficult to achieve designed consistencies. Currently, the only option to generate a product with a constant density is to first filter the tailings to some higher density, after which an uniform moisture content can be achieved by the tailings conditioning. (Jewell & Fourie 2015)
The thickened tailings disposal method may require a frequent flushing of the delivery pipelines. Also the thickness of the deposition layers should be carefully controlled. Air drying as well as consolidation have to be maximised to ensure the best possible efficiency. In addition, the stability of the waste piles under seismic loading conditions needs to be monitored to avoid liquefaction. (International Mining 2010)
Although the filtered tailings disposal may be more expensive in general compared to the conventional methods, it offers many benefits such as more efficient storage and reduced environmental footprint that offset higher costs. The use of the method requires a seepage control to avoid groundwater contamination. (International Mining 2010) Also, to inhibit the surface waters from ponding as well as the erosion of the waste pile through channelized runoff flow channels e.g. during rainfalls, a surface contour management is a necessity in the filtered tailings option (Engels 2012).
The dry stacking of tailings is only proven for the mines with low production rate (International Mining 2010). In addition, when compared to the conventional disposal methods, both the thickened tailings as well as the paste disposal are relatively unproven methods (Engels 2014a, 2104b). However, although the mining industry is known to be rather conservative in the implementation of new technologies (Palkovits 2011) it is assessed that the need for less intense day to day management actions of these methods will increase their popularity in the future (Engels 2014b).
At least the following design considerations should be taken into account: (Palkovits 2011, Blodgett & Kuipers 2012, Caldwell & Charlebois 2012, Outotec Oyj 2012)
- Availability of water
- Geotechnical aspects, such as stability of the slope/dam, seismicity or the area, flooding, freeze-thawing, and reduction of dusting
- Capital and operational costs, energy requirement
- Selection of the right kind of beach profile (in thickened/paste applications)
- Selection of the right kind of pump for thickened tailings/paste disposal, other equipments
- Rheology of the thickened tailings
- Material characteristics
- Chemistry of the slurry and the reagents used
- Thickening and filtration processes
- Transportation systems
- Regulatory issues
- Assessment of resources, cement and other binders (in case of paste backfilling)
- Wildlife management at the deposition area
- Expectancy of mine life
- Public acceptance and possible health effects of tailings disposal
- Availability of disposal site, right of way for a pipeline
- Are there any concurrent reclamation actions needed. (Palkovits 2011, Blodgett & Kuipers 2012, Caldwell & Charlebois 2012, Outotec Oyj 2012)
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