Waste management for mine closure – Aitik 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 Aitik mine, owned by Boliden AB is located outside of Gällivare near the Arctic Circle, northern Sweden. The mining started in 1968 and is planned to last at least until 2029. Aitik is currently the largest copper mine in Europe, as well as one of the largest European open pit operations. The mine has recently expanded its annual ore production from 18 Mt to 39 Mt. In 2014, more than 67,000 t of copper, almost 55 t of silver and 1.8 t of gold were produced. (Boliden AB 2015a) Boliden plans to still raise the production capacity to 45 Mt in a few years and extend the planned lifecycle to 2040 (Boliden AB 2015b).
The climate at the Aitik area is sub-arctic with an average temperature of just +0.6 °C. Winters are long and cold, whereas summers short and mild. A significant fraction of average annual precipitation of 680 mm is in the form of snow. The yearly net precipitation is approximately 500 mm (Lindvall & Eriksson 2003). Flows may occur during the snowmelt period in May-June (Lindvall 2005).
There are four different deposits in the mine area; Aitik, Salmijärvi, Aitik East, and Liikavaara, of which the Aitik deposit is clearly the largest one. Aitik, Salmijärvi, and Aitik East occur in a same mineralization zone, called “the Aitik-Salmijärvi mineralization”, which is approximately 5 km long, 500 m wide and sheared in a north-south direction. The dip of the ore body is around 50 degrees to the west. Proven ore reserves in 2013 were 762 Mt of ore, containing 0.22% of copper, 0.15 g/t of gold, 1.6 g/t of silver, and 25 g/t of molybdenum (Boliden AB 2014).
The host rock of the Aitik deposit contains metamorphosed paleoproterozoic volcanic muscovite schists, biotite gneisses, and amphibole-biotite gneisses and was formed around 1.89 Ga ago. The main value mineral is chalcopyrite, and the ore contains also other sulphides such as pyrite and pyrrhotite. Molybdenite and magnetite occur as accessory minerals. (Boliden AB 2014) The host rock is surrounded by granitic, dioritic and gabbroic intrusions, intermediate volcanic rocks, and clastic sediments (Witschard 1996, cited by Takala et al. 2001).
The industrialised area covers around 15 x 5 km of the site (Lindvall 2005). (Figure 1) Currently, the ore is mined from two open pits, Aitik and Salmijärvi (Boliden AB 2014). The Aitik pit is approximately 3 km long and 1.1 km wide. The maximum depth was 450 m in 2012 (Mueller 2012). It is assessed that the final depth of the pit may be over 600 m (Lindvall 2005). According to Boliden AB (2015a), the length of the Salmijärvi pit will be around 1 km, width 0.8 km, and depth 270 m.
Figure 1. Aitik mine site (© GoogleEarth 2015).
The ore drilling and blasting takes place in 15 m benches. The ore is crushed in the crushing stations and transported to the concentrator plant using partly underground conveyor belts of 7 km long. In the concentrator plant, the ore is first grinded and then copper concentrate is recovered by flotation. The concentrate is transported via railway to Boliden’s Rönnskär smelter at Skelleftehamn, located 400 km away from Aitik. The process water used in the concentrator plant is mainly recovered from the clarification pond. (Lindvall 2005, Boliden AB 2015a)
Description of the waste and waste areas
The ore in Aitik contains very low grades of copper, but as the quantities are enormous mining is feasible. Only around 10 kg of concentrate is produced from one tonne of ore, and as a result, 99% of the ore ends up as waste. (Lindvall & Eriksson 2003, Boliden AB 2015a) In 2011, around 30 Mt of waste rock, and 31 Mt of tailings were generated (Mueller 2012).
Waste rocks contain mostly biotite gneisses with skarn minerals, gneisses with mica schists, and pegmatites (Lindvall 2005). Altogether, more than 500 Mt of waste rock have been deposited in dumps during the whole operational life of the mine (McKeown et al. 2015), covering an area of approximately 4 km2. The waste rock disposal areas are placed above 10 m thick layer of glacial till with low permeability, and located on the both side of the Aitik pit. According to the hydrogeological investigations the areas are not connected with the Aitik open pit. Their placement was chosen from purely logistic reasons at the beginning of the mining in 1968, as the generation of acid mine drainage (AMD) was not a big issue back then. During the course of time, acidic effluent waters with high metal content have occurred. To be able to collect these waters cut-off ditches have been diked around the waste rock dumps. Basically all waters infiltrating through the waste rock dumps are collected in ditches at the toe and reused in as process water in mining operations (Lindvall et al. 1997, Lindvall 2005).
Tailings pond with a size of around 13 km2 is the largest single component on the site (Lindvall 2005), and its size is about to increase even more in the coming years. The pond is situated around 3 km west of the Aitik pit and delimited by valley-site type topography and four dams. Four different pipelines can be used to pump slurried tailings from the concentrator to the tailings pond along the upstream dam. In the pond, the tailings are distributed onto a 5 km long and 2 km wide beach. The height of the beach increases by approximately 1 m every year, and had reached the 40 m level in 2003. The volume of free water covers approximately 20% of the pond, which equals to around 2 Mm3. (Lindvall & Eriksson 2003) A spillway is used to discharge water to the 1.6 km2 wide clarification pond that is connected to the tailings pond. The clarification pond, having a holding capacity of 15 Mm3, is used for water clarification and acts as a final treatment step and a reservoir for the process waters before their reuse or periodic discharges to the receiving streams. (Göransson et al. 2001, Lindvall & Eriksson 2003)
The tailings contain 0.5-1.5% of sulphides (Mueller 2012) and 0.03% copper on average. The carbonate content is only around 0.25 %, but there are plenty of silicates available having acid buffering capacities as well (Lindvall & Eriksson 2003); as silicates such as feldspar, plagioclase, biotite and muscovite represent the main mineral type in the tailings in Aitik (Stjernman Forsberg 2008). There has not been any evidence of the generation of acid mine drainage in the tailings area yet, but the formation of hardpan has been detected (Lindvall 2005).
The Aitik mine suffered from a dam collapse in 2000. As a result of the failure, 2.5 Mm3 of water from the tailings pond discharged into the clarification pond. The rose of the water level in the clarification pond led to the need of a controlled discharge of 1.5 Mm3 clarified water to the environment. The failure did not cause any significant environmental consequences. (Göransson et al. 2001)
Mine closure objectives
During the mine operational life, no significant copper emissions to the surrounding environment exist. The amount of copper in effluent water does not exceed 50 kg in a normal year. In the internal circulation, however, the amount of Cu can be orders of magnitude larger, and thus it is vitally important to have suitable decommissioning measures to avoid the emission problems also after the mine closure. (Lindvall 2005) According to the Swedish legislation, the mining company is responsible for closure and after-care measures of the mine.
The Boliden’s closure and remediation work at Aitik aims to ”complete, final, and accurate” results that will be ”beyond any scientific doubt”. It is also important for the company that there will not be any discharges from the remediated site that cause remarkable impacts to the recipient. To fulfil these objectives, Boliden has decided to evaluate their present plan and regenerate a unified closure plan, in which the data derived from geohydrological and geochemical models, physical designs, and water balance information are utilised. Also long term rehabilitation perspective in the context of Natura 2000 is perceived. (Rönnblom Pärson 2014)
One of the most important objectives of the closure and remediation work in the Aitik has been to find a qualified waste rock cover solution. Boliden has developed a plan aiming to successful reclamation of the waste rock areas. According to the plan, the reclamation work will progress approximately 20 hectares every year after the final areas have been filled out. (Rönnblom Pärson 2014)
The work towards these goals begun already in the late eighties. During the time mining started in Aitik, no closure plan was required. It took over twenty years since the operations started, as only in 1989 Boliden was obliged to develop a first decommissioning plan covering the whole Aitik site, as a requirement in the permit for expansion. Monitoring, controlling and prevention of AMD were an essential part of the decommissioning plan. After an intensive investigation work, the first closure plan containing a method for management and decommissioning of waste rocks dumps was submitted a couple of years later, and finally approved in 1997. (Takala et al 2001, Lindvall & Eriksson 2003, 2005)
The waste rock deposition plan of the year 1999 enables a selective management of waste rocks, meaning that less extensive decommissioning actions for low sulphide content waste rocks are appropriate. The aim of the strategy is to keep the size of the area containing sulphidic waste rocks as small as possible. The selective management of waste rocks minimises the surface area of the waste rocks that are able to produce acid mine drainage thus creating environmental benefits, cost savings and even revenues (Takala et al. 2001)
In addition to the waste rock areas, the decommission plan is focused on the tailings pond and the industrial area including also the open pits. According to the existing strategy, their remediation will be performed after the mine closure. (Takala et al 2001) Extensive studies have been carried out to find a successful, cost efficient and environmentally sustainable tailings management strategy. (e.g. Lindvall & Eriksson 2003, Lindvall 2005, Mueller 2012) The current strategy for remediation is based on desulphurisation of the tailings during the last couple years of operations to minimise their sulphur and metals content. Groundwater saturation actions and construction of till cover help in minimizing oxygen penetration to the potentially acid-forming tailings and are included in the strategy as well. Also, as a part of the reclamation process, leachate waters will be collected and led to the river Leipojoki via the Aitik pit. (Rönnblom Pärson 2014)
In Aitik, selective handling and covering of waste rocks have been chosen as a closure strategy for waste rocks.
If a waste rock is characterised as amphibole-biotite gneiss or/and pegmatite and it contains <0.1% of sulphur, <0.03% of copper and its NP/AP ratio exceeds 3, it can be classified as sulphide free. These rocks can be progressively covered in their own deposition area and may be later used as a construction material for roads and railroads. The suitable cover contains a layer of 0.3 m made of till and/or other material as vegetative layer. Establishment of vegetation should be made within two years after the deposition is complete. Also requirements for water collection and reuse exist. (Takala et al. 2001) According to Lindvall & Eriksson (2005) around 65% of waste rocks generated yearly can be deposited in this manner.
In addition, to minimise oxygen penetration, potentially acid generating waste rocks are covered with an engineered cover, in which materials available at the site, such as overburden and topsoil from the pit, are used. (Lindvall & Eriksson 2005) A number of different cover alternatives were evaluated, and the reclamation work started right after the first plan was approved in 1997. A 14 ha area of the waste rock dump WRD5 was covered with 2 x 0.5 m thick individually compacted moraine layers and 0.2-0.3 m layer of topsoil that was vegetated later in the same year. There were some problems with the surface runoff water channels originally constructed from geotextile and till as erosion and snowmelt waters damaged the cover, and they were replaced using new till and erosion resistant waste rock (Takala et al 2001). Later on the WRD5 was totally covered in a similar manner, using sewage sludge transported from Stockholm as a topsoil material. Also a part of the other waste rock dump WRD2 is covered. (Sjöblom et al. 2012)
Nowadays, however, the structure of the qualified cover is different. According to the current strategy, waste rock piles are re-sloped at the beginning of the reclamation work. By cutting and filling the slopes, an overall slope angle of 1:3 (V:H) is constructed, whereas the final individual bench slopes are sloped off to 1:2.5 (V:H). Also contouring of the surface water runoff is made. After re-sloping, waste rock piles are covered with a structure containing the following layers: (Rönnblom Pärson 2014)
- 2 x 0.15 m layers of moraine, hard compacted (K< 5*10-8 m/s, packing density >95 % of proctor maximum)
- 3 x 0.50 m layers of till, compacted (K< 2 x 10-7 m/s, packing density >93 % of proctor maximum)
- 1 x 0.50 m layer of till and/or soil improvers, loosely laid
- vegetation layer. (Rönnblom Pärson 2014)
Leachate waters are collected and led to the Aitik pit. The period of active treatment lasts for 50 years. (Rönnblom Pärson 2014)
Also old waste dumps containing marginal ore have been removed and processed during the years, as they were assessed as the single most important source of possible metal release at the site (Lindvall 2005). Before their removal, the dumps released even 55 t of copper and 2,750 t of sulphate annually (Lindvall et al. 1997).
After desulphurisation, the pyrite-depleted material can be used to cover water saturated tailings with higher pyrite content (e.g. Lindvall & Eriksson 2003, Lindvall 2005, Mueller 2012), whereas the pyrite enriched material has to be deposited separately under an engineered dry cover or water cover. (Lindvall 2005) Pyrite enriched fraction containing 30-35% of sulphur is separately managed and deposited into a certain section of the tailings pond that is mainly saturated with water but could be also covered with engineered dry cover at closure (Lindvall & Eriksson 2003, Lindvall 2005). Stjernman Forsberg (2008) has studied the use of sewage sludge as fertilizer on the Aitik mine tailings cover vegetation layer. According to her studies, sewage sludge seems suitable for use to cover at least those tailings that contain less than 1% of sulphide.
According to Mueller (2012), when the mining ceases a cover constructed from depyritised tailings is placed on top of the current tailings surface. The thickness of the cover will be between 15 to 20 m. So far, flotation pilot tests have shown difficulties is achieving the target limit of <0.3% of sulphides, if only flotation is used in depyritisation. This results in the concurrent presence of both magnetite and pyrrhotite in the tailings, in addition to pyrite. Mueller (2012) suggests that a combination of flotation and magnetic separation may be the key to solve this problem.
The part of the tailings pond that contains low sulphur containing material (approximately 700 ha) will be mostly saturated with water and an unqualified cover structure (0.1 m of sludge mixed into the top 0.3 m of the tailings, vegetated layer on top) can be used to cover these areas. (Rönnblom Pärson 2014)
After the mine closure, the tailings dams will be sloped at an angle of 1:3 and vegetated. Also a wide, permanent beach at the downstream dam will be constructed. These actions increase the stability of the dams and lower the risk for erosion (Lindvall & Eriksson 2003).
The industrial buildings on the site will be dismantled after the closure, and the area will be covered with soil and vegetated (Lindvall et al. 1997, Takala et al. 2001). The open pit chemistry of the Aitik and Salmijärvi pits is under thorough investigations; especially the opportunity to purify the leachate waters originated from the waste rock area by leading them to the Aitik pit is of interest (Rönnblom Pärson 2014). After the mining ceases, the pits will be flooded up by natural water, and in about 130 yrs the Aitik open pit will become the deepest lake in Sweden (Boliden AB 2015a).
Performance and monitoring needs
The original waste rock cover structure containing 1 m of compacted till and 0.3 m topsoil was estimated to decrease oxygen inflow to 1% of the inflow before covering, enhance the establishment of vegetation, and resist the penetration of frost that could otherwise penetrate to a depth of around 0.7 m. It was estimated that also the copper load would decrease to a level below 1 t per annum. (Lindvall et al. 1997, Takala et al. 2001) However, the later modelling studies showed that predicted diffusion flux estimates were greater than the rates originally predicted in 1996. It was assessed that the increased level of water retention within the compacted till layer leading to the reduced oxygen penetration to the waste rocks could be achieved by improving the current compaction methodology (i.e. increased compaction). Both laboratory and field tests were performed to test this hypothesis. Field compaction trials in 2012 confirmed the expectations; increased compaction actually enhanced water retention capacities. Based on the results two new cover system designs were created and their performance, including the response to variable climatic conditions in terms of temperature and water storage dynamics, was tested in the field. The effects of freezing and thawing may hinder the cover performance in the long term and should be especially taken care of. However, based on the first results after one year of monitoring, freezing front did not affect the compacted till layers. Also continued meteorological monitoring and control of water balance on the site are a necessity to establish an accurate picture of cover system performance in field conditions. (McKeown et al. 2015)
Reduced environmental impacts can be achieved by selective waste rock management as AMD generating waste rocks are deposited to only a limited surface area. Selective waste rock management reduces the mobilization of contaminants, decommissioning costs, and the need for borrow material in cover construction. Another advantage is that the performance of the designed waste rock covers is well known as full-scale field tests are conducted. However, further studies are needed to achieve successful surface water management practices on the covered areas to prevent erosion. (Lindvall 2005) Also the availability of suitable cover materials as well as their price may cause challenges to the remediation work (Rönnblom Pärson 2014).
Since 1989, extensive investigations have been carried out at the Aitik mine site to find suitable technological solutions for the reclamation of waste areas after the mine closure. The enormous size of the mine and the Northern climatic conditions bring special characteristics to the closure planning. The rehabilitation of the waste rock areas has already started, whereas the tailings will be covered only after the mining ceases. As the investigations are performed in close co-operation with authorities they meet the current requirements on minimisation of waste, use of resources and reduced environmental impacts. The thorough investigations help in achieving cost savings, however, the expansion of the mine is going to increase future reclamation costs.
The basis of the Boliden’s closure and reclamation work is good as they are aiming to find holistic solutions and also the long term aspects of rehabilitation are assessed. Although there have lately been concerns about the performance of the original cover structure, corrective actions to repair the detected deficiencies are taken. The research on finding the efficient depyritisation method that can meet the requirements is still ongoing.
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