In-pit backfilling of waste rocks under dry cover – a case study from Kimheden Cu mine, Sweden
Henna Punkkinen, VTT Technical Research Centre of Finland Ltd, P.O. Box 1000, FI-02044 VTT, Finland, e-mail: henna.punkkinen(at)vtt.fi
The former Kimheden copper mine in Västerbotten County, northern Sweden, is a part of the Kristineberg mining area comprising of five mines, an industrial area, and a tailings area (Lindvall et al. 1999). The Kimheden mine was operated by Boliden AB from 1968 to 1974. Altogether 0.13 Mt of ore was extracted during these years from one underground shaft and two open pits (Villain 2014). The average metal concentration of the mined ore was 0.95% of copper, 0.27% of zinc, 0.44 g/t of gold, and 7 g/t of silver. The sulphur concentration of the ore was 18.4%. (Åreback et al. 2005)
The climate at the Kimheden site is continental subarctic, characterised by cold winters and short summers. The ground is usually covered with snow from October to May (Villain 2014). The local average temperature is just +0.3°C and the amount of precipitation approximately 500 mm per year (SMHI 2014a, 2014b).
The Kristineberg mining area belongs to the Palaeoproterozoic Skellefte ore district that was formed around 1.9 Ga ago. The bedrock contains deformed and metamorphosed volcanic and sedimentary rocks. The Kimheden deposit is one of the six volcanogenic massive sulphide (VMS) deposits in the area, of which the Kristineberg deposit is clearly the largest one. The VMS deposits containing mostly pyrite, chalcopyrite and sphalerite, lie within a succession of felsic meta-volcaniclastic rocks consisting of quartz–muscovite–chlorite±biotite schists. (Hannington et al. 2003) The mineralisation is sheared in the north-east direction (Villain et al. 2014a).
The Kimheden site is situated at 470-520 m altitude on a slope of Hornberget hill. The two open pits (Figure 1) were both around 20 m wide and <15 m deep. The open pit 1 was 210 m long, whereas the length of the pit 2 was 140 m. (Villain 2014) The underground mine reached to a level of approximately 400 m (Lindvall et al 1999). The blasted ore was processed in the concentrator in the Kristineberg industrial area and also the tailings disposal areas were situated there (Werner & Salmon 2001).Thus, there is no tailings pond located in the Kimheden site.
Figure 1. A map of the Kimheden site shows the locations of the former waste rock disposal areas and the two open pits (Modified after Google Earth 2015; basemap © Google Earth).
Groundwater flows in the till layer covering the bedrock and in multiple fractures within the bedrock. Fractures are mostly directed towards the open pits and also affect the pit walls. It is estimated that groundwater entering via fractures forms the clear majority of the water entering the former open pits. (Rosén & Wilske 1994, cited by Villain et al. 2014a, 2014b)
Altogether 25,000–35,000 m3 of waste rock was generated during the mining operations in Kimheden. Pyrite, chalcopyrite, and Mg and Fe-bearing aluminosilicate chlorite were the main minerals in the waste rock. (Andersson 1988, cited by Villain et al. 2013) Waste rock disposal areas were situated next to the pits. Waste rocks were piled to these areas and left uncovered, which soon led to the generation of acidic metal rich drainage waters. (Villain et al. 2014a)
Mine closure objectives
Soon after the mining was ceased in Kimheden, monitoring of the waste rock dumps revealed the formation of acidic drainage waters with high concentrations of Cu and Zn. According to the Swedish legislation, the mining company is responsible for closure and after-care measures of the mine. The remediation plan for Kimheden was created by Boliden, and the reclamation work started in the beginning of the 1980s. The target was to treat AMD (acid mine drainage) rich drainage waters until the quality of waters reached satisfactory limits. Boliden performed many kinds of remediation actions during the years aiming to prevent the formation of uncontrolled acidic discharges. The final actions were taken in 1995 and 1996. (Villain et al. 2013)
In order to direct fresh waters around the pits and to be able to collect and lead the contaminated waters for lime treatment into the tailings pond at Kristineberg, a ditch system was constructed around the site in 1982. (Figure 1) As the site is located on a slope of a hill, a pit lake establishment was not a conceivable option and a progressive backfilling of open pits with waste rock was chosen as a treatment method for remediation. In addition, a dry cover structure was constructed on top of the backfilled waste rocks. (Villain et al. 2013)
The aim of the chosen closure solutions, the backfilling of the waste and the application of a dry cover structure, was to prevent the oxidation of sulphide minerals causing the AMD. When waste rocks are placed on the bottom of the open pit they become saturated with water (so called elevated water tables option), and thus the oxidation of sulphides is inhibited. More information on backfilling of mine waste into open pits is presented here. In addition, a dry cover structure acts as a surface barrier and reduces influx of air into the underlying waste material. Different dry cover applications are evaluated under Dry cover section.
The backfilling of waste rocks was conducted between 1984 and 1995 (Villain et al. 2014b). During the process, the additions of lime were made to raise the pH of the waste and consequently delay the oxidation process. However, after the backfilling was completed, monitoring results still showed an evidence of the continued formation of the AMD. Thus it was decided to cover the backfilled waste rocks. (Werner & Salmon 2001, Breng 2015)
The cover construction took place in 1996 when a dry cover acting as an oxygen diffusion barrier was installed. 0.3 m thick sealing layer of clayey till was placed on top of the waste rocks overlain by a 1.5 m thick layer of unsorted till acting as a protective layer (Villain et al. 2014b, Breng 2015). According to Villain et al. (2014) the actual thickness of the protective layer on the pit 1 varies between ~1 and ~2.3 m, and between ~1 and ~2 m on the pit 2.
As a summary, the following remediation actions were taken (Boliden 1995, Göransson 1997; both cited by Werner & Salmon 2001):
- Surrounding ditches were sealed
- Waste rock was deposited into the eastern pit
- Sulphide containing materials from the surrounding areas (roads, industrial area) were deposited into the western pit
- Till contaminated with heavy metals was deposited into the pits
- Cover structure containing low permeability and protective layers was constructed on top of the backfilled waste rocks
- Erosion protection mechanisms were constructed in the drainage systems (Boliden 1995, cited by Werner & Salmon 2001); and
- Piezometers and oxygen probes were installed (Göransson 1997, cited by Werner & Salmon 2001).
The information on the closure actions of the underground shaft is weak (Villain et al. 2013), but at least the shaft was sealed (Lindvall et al. 1999, Villain et al. 2013) using a hydrological seal. However, it remains uncertain whether there are waste rocks backfilled in the shaft too. (Villain et al. 2013)
Also some alternative reclamation actions were suggested. For example, according to Villain et al. (2014a), Rosén and Wilske (1994) suggested that sealing of the fractures in the bedrock surrounding the pits could be used instead of dry cover option, leading to more efficient reduction of the water inflow into the open pits. However, they estimated that a dry cover application would be more effective than sealing in the prevention of AMD.
Performance and monitoring needs
According to Lindvall et al. (1999) the cost of the remediation was around SEK 2 million. The monitoring of mine waters started in 1983. The monitoring after the remediation measures showed that the metal loads were rapidly decreasing, but satisfactory concentrations have not still been reached. (Breng 2015) Especially the eastern pit is problematic (Villain et al. 2013). Due to this, the monitoring of the site is still very intensive, and the site is subjected to ongoing research activities (Breng 2015). For example, the recent doctoral thesis work by Villain (2014) contains multidisciplinary geochemical, geophysical, hydrogeological, and modelling studies aiming to evaluate the effectiveness of the reclamation measures performed in terms of mitigation of the AMD generation and identification of possible inadequacies. The investigations were carried out between 2009 and 2014. The following observations are based on Villain’s studies.
The remediation measures caused a significant reduction on copper, zinc, and sulphate concentrations downstream and a rapid stabilisation of element concentrations. These results showed reclamation to be successful, although insufficient, as the concentrations remained still too high to meet the Boliden’s remediation goals. It is also possible that the lowered concentrations were actually caused by the exhaustion of the sulphides and not the cover structure itself, but this aspect remains still unclear. (Villain et al. 2013) In 2009 and 2014 the mean dissolved metal concentrations were: copper 380–2,000 μg/l, zinc 108–450 μg/l, cadmium 0.16–1 μg/l, and aluminium 3.1–17 mg/l (Villain 2014). As concentrations were still on moderately high level, lime treatment of drainage waters was still necessary in 2014 before the waters could be safely released into the environment (Boliden personal communication 2014, cited by Villain 2014).
Also the concentration of dissolved oxygen in the groundwater was relatively high, which means that the oxidation of sulphides has continued despite the dry cover. Also the pH has remained lower than the background pH. These signs show that the production of acidic drainage waters is still possible and the mitigation actions to inhibit oxygen penetration into the waste backfill have been unsuccessful. (Villain et al. 2013)
Geophysical measures were used to find reasons for deterioration of the cover performance. No major fractures or vertical displacement in the sealing layer were detected. However, fracturing was detected from the surrounding bedrock of the pit 1. Due to this, the walls of the pit are permeable and prone to oxygen diffusion. In addition, it was noticed that the thickness of the protective layers was insufficient, i.e. <1.5 m, in some parts of the structures, which could lead to decreased performance of the cover. That is, the sealing layer might be exposed to frost or its enhanced permeability could lead to increased diffusion of oxygen into the waste. Also seepage through the dry cover was perceived that causes a risk of erosion for the sealing layer. (Villain et al. 2014a)
In addition, it seems that the ditch system is not working properly. The sealing of the ditches was not adequate and thus a considerable proportion of drainage waters leak to groundwater and the surrounding environment, whereas the waters ending up to the lime treatment are actually mostly fresh background waters. Field investigations also showed that most of the upstream ditches actually remain dry soon after snow melting, indicating that they are not capable to considerably mitigate inflow to the pits. (Villain et al. 2013)
Hydrogeological investigations of water pathways indicated that even 40% of the waste rock stays unsaturated during low-flow periods, e.g. in autumn and winter. This increases the potential of oxygen diffusion through bedrock fractures drastically. The assessed time for turnover varies from 90 days to 3 years, demonstrating that the current metal loads are caused by post-reclamation sulphide oxidation. It was also estimated that the release of metalliferous drainage waters may continue over hundred years. (Villain et al. 2014b)
The master’s thesis work by Breng (2015) focused on the hydrogeology of the eastern pit and the collection ditch. She suggested that the following additional reclamation measures could help in reducing the generation of AMD, and accelerate the performance of the ditches:
- Hydrogeological measures:
- Sealing of the bottom of the ditch sections with the highest water leakages
- Deepening of the gaining ditch to increase the amount of the drainage waters collected (only feasible to limited extent)
- Artificial raising of the water table in the pits to achieve saturation (very difficult)
- Geochemical measures:
- Construction of a reactive barrier to reduce the contamination in the off-running mine water
- Addition of liquid alkalines to raise the pH and promote the precipitation of hydroxides that can lead to the decreasing hydraulic conductivity. (Breng 2015)
The following monitoring and maintenance work are required also in the future (Breng 2015):
- Flow measurements twice a year to measure water amounts leaving off the site
- Maintenance and regular check-up of the collection ditch
- Limited geochemical sampling and analysis of water
- Surface and groundwater sampling of the area down gradient of the collection ditch. (Breng 2015)
The backfilling of mine wastes into open pits and the application of dry covers are both seen as best practice methods for the management of mining wastes (European Commission 2009). However, this does not automatically guarantee that these methods are applicable to every site. For example, at Kimheden some other measures would have probably been more efficient (Breng 2015).
Case Kimheden underlines the importance of case specific design and knowledge of characteristics of the site to achieve a sufficient end result. Especially knowledge of the prevailing hydrogeological conditions is crucial (Villain 2014). In addition to adequate design, also the fulfilment of the design objectives during the construction phase is vital. The reclamation of the Kimheden site failed in these both, as the aims for reduced contact of waste rocks with oxygen, and the construction of ditch system to lead drainage waters to treatment did not completely succeed. (Villain et al 2013)
Particularly during the low flow periods the waste rock backfill in Kimheden remains partially unsaturated, which is estimated to be the main reason for increased diffusion of oxygen into the waste rock. Also fractures in the surrounding bedrock allow excessive rates of oxygen ingress. In addition, oxygen diffusion into the waste rock material may be caused by the deficiencies in the dry cover integrity that should act as an oxygen barrier. The concentrations of dissolved sulphur in the drainage waters revealed that sulphide oxidation in the waste rocks was at least an order of magnitude higher than was originally expected during the cover design period. (Villain 2014)
Breng (2015) reminds that the pits were backfilled to the upper edge, which was a mistake as the location of the pits on the slope of a hill restrains full saturation of the backfilled waste and a great amount of oxygen-rich groundwater flows through the pits. Instead, partial backfilling below the water table level would have created a wet cover, and waste rocks would have remained saturated. Surplus of waste that did not fit into the pits could have been reclaimed by other means, e.g. by placing it to heaps under dry cover or storing/submerging it into other sites. Additionally, if these measures were not possible, a selective disposal of unreactive waste rocks on top of the reactive waste above water table or ‘pervious surround’ concept could be used instead. (Breng 2015)
Villain (2014) suggests that in similar situations than in Kimheden, the creation of alternative pathways for groundwater to flow around the waste (rather than through it) could be used. For instance, the modification of the ‘pervious surround’ (aka ‘porous envelope’) concept originally presented by MEND (2001), might lead to more successful result. The concept is normally used for the tailings management. In the modified concept, waste rocks are first solidified to lower their hydraulic conductivity. A layer of coarse sand or gravel is then placed around the waste. As there is a difference in permeability between these two materials, groundwater moves in trough the coarser zone, and the oxidation of waste as well as transport of contamination diminishes.
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