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Paper: Low depth mining in Estonian oil shale deposit-Abbau von Ölschiefer in Estland

http://www.minest.ee/miningtxt: LOW DEPHT MINING IN ESTONIAN OIL SHALE DEPOSIT ABBAU VON ÖLSCHIEFER IN ESTLAND In diesem Beitrag wird die Ölschiefergewinnung in Estland vorgestellt. Besonders wird eingegangen auf die Geologie der Lagerstätte, die Ölschiefergewinnung und -verwendung sowie auf Umweltbeeinflussungen. Weitere Schwerpunkte sind der Bergbau unter Tage und der zugehörige Versatz. The paper gives an overview of oil shale mining in Estonia. Following topics are presented: oil shale mining production, utilization, geological information, impacts to the environment, underground mining and aspects of backfilling underground mines. INTRODUCTION Oil shale is primary mineral resource in Estonia which has been mined for more than 80 years. During that period approximately 1 billion tonnes of oil shale has been extracted from estimated 5 billion tonnes of resources. Mineable reserve is currently about 1,5 billion tonnes (Valgma, 2003). Oil shale mining peaked in 1980 when more than 30 millions tonnes of oil shale were mined annually. Last few years the annual output has stayed between 11 and 14 millions tonnes per year. Estonian oil shale production makes 70% of world’s oil shale production 1 and two thirds of Estonia’s total mineral production. Oil shale consumers are primary power plants and shale oil plants what consume approximately 85% and 15% of oil shale respectively. In addition, oil shale is supplied for cement industry. Oil shale covers over half of Estonia’s primary power resources (Fig. 1). About 99% of electric power and a large share of thermal power are produced from oil shale. Therefore main goal of the oil shale industry is to preserve its competitive ability in the market of power resources. Relatively cheap and not very complicated oil shale mining with sufficient resources is capable to guarantee this competitiveness in the nearest decades. All oil shale mining enterprises have belonged to the Eesti Põlevkivi (Estonian Oil Shale Company) group which has 3 subsidiaries. In addition to surface and underground mines, Põlevkivi Raudtee Ltd. (a railway enterprise) and Mäetehnika Ltd. (Mining Machinery Enterprise) also belong to the group. However, past few years Eesti Põlevkivi does not have a monopolistic status as owner all mining fields. Shale oil and cement producers have opened new mining fields as well. Like any kind of mining activity, oil shale mining has great influence to regions environment as well. The area affected by oil shale surface and underground mining is over 400 km 2 (almost 1% of Estonia’s territory). About 220 km 2 of this mining area lies underground. On that area number of ground subsidences have occurred. The scale of subsidences depends on mining depth and used technologies. Mining influences region’s hydrology but as experienced, the influence is temporary. Besides to the mining activity, more serious impact 1 Excluding tar sands Kolloquium: Schacht, Strecke und Tunnel 2005. TU Bergakademie Freiberg, 14-15 April, Freiberg / Sachsen 2 to the environment is caused by waste deposits from oil shale processing and energy production residues as ash and semi coke. ESTONIAN OIL SHALE DEPOSIT Baltic oil shale area covers about fifty thousand square kilometres and includes the Estonia and Leningrad deposits and Tapa occurrences, of which the first two are commercially exploited. Estonia deposit is one of the largest commercially exploited oil shale deposits in the world with its total resources exceeding 5 billion tonnes of oil shale (Valgma, 1999). The oil shale bed lies in the form of a flat bed having a small inclination (2-3 m per km) in southern direction. The commercially important part of oil shale is deposited in a single mineable layer with thickness of 2,5 to 3,8 m in depth of 3 to 100 m in area of 2700 km 2 . The oil shale layers occur among the limestone interlayers in Kukruse Regional Stage of the Middle Ordovician (O2kk). It is a stratified sedimentary rock, rich in organic matter. The commercial oil shale bed and immediate roof consists of oil shale and limestone layers (Fig. 2). The main roof consists of carbonate rocks of various thicknesses. The characteristics of the individual oil shale and limestone seams are quite different. The compressive strength of oil shale is 20-40 MPa and that of limestone is 40-80 MPa. The strength of the rock increases in the southward direction. The volume density is 1,5-1,8 Mg/m 3 and 2,2-2,6 Mg/m 3 respectively. The calorific value of dry oil shale is 7,5-18,8 MJ/kg depending of the seam and the area in the deposit (Pastarus & Nikitin, 2002). OIL SHALE MINING Oil shale mining in Estonia started in 1916; it was extracted in surface mines using the opencast method. Underground mining started in 1922. Room-and-pillar mining, which is the only underground oil shale mining method today, was started in 1960. During the years 1971-2001 in some mines longwall mining with shearers was used. In total ten oil shale surface mine and thirteen underground mines have been in operation. Today two surface mines (Aidu and Narva with Sirgala) and two underground mines (Viru and Estonia) are in operation (Fig. 3) (Valgma, 2003). Surface mining is carried out in open casts with maximum overburden thickness of 30 m. Draglines with 90 m boom length and 15 m 3 bucket size are used for the overburden removal. Hard overburden consists of limestone layers and is blasted before excavation. Oil shale layers are blasted as well or broken by ripping. Excavated rock is transported with 40 and 60 tonnes trucks to the processing or crushing plant depending on opencast. Distance vary from 3 to 8 km. Open casts drainage system is based on drainage drifts which are drifted before the open cast is opened, about 3-5 m deeper from the bottom of oil shale seam. Underground oil shale mines operate in the depth from 40 to 70 m. Mining field are opened with vertical shafts and ramps (Fig. 4). For the main hoisting ramps with belt conveyors are used. In auxiliary shaft cage hoisting is used. In Viru underground mine only rail transportation is used. In Estonia underground mine rail transportation is for staff and equipment, excavated rock is transported by conveyors. In 2004 an inclined shaft was built to replace in the nearest future inefficient rail transportation in Estonia mine. The field of an underground oil shale mine is divided into panels by the panel drifts. Panels are subdivided into mining blocks, approximately 300-350 m in width and from 600 to 800 m in length each (Fig. 5). Mining blocks usually consist of two semi blocks with 6 m width collecting drift. Rooms and drifts height corresponds to the thickness of the commercial oil Kolloquium: Schacht, Strecke und Tunnel 2005. TU Bergakademie Freiberg, 14-15 April, Freiberg / Sachsen 3 shale bed, approximately 2,8 m. The width of the room is determined by the stability of the immediate roof. The latter is very stable when the room is 6-10 m wide. In this case, bolting must still support the immediate roof. The pillars are arranged in a singular grid. Actual mining practice has shown that pillars with a square cross-section suit best. The cross-sectional area of the pillars is 30-40 m 2 , depending on the mining depth. Room-and- pillar mining system gives an extraction factor of 70–80% (Pastarus & Nikitin, 2002). The main operations carried out in rooms include undercutting, drilling of blast holes, blasting, loading of excavated rock on the conveyor and roof bolting. Loading and transportation of excavated rock in face is carried out by powerful LHD machines with diesel drive like TORO and WAGNER. The average productivity of such technology is 1500 m 3 of rock mass per day. The main problems are the high number of blasting operations, low mobility and concentration of loading works due to the small entry advance rates (EAR), about 1,5-1,7 m per blasting (Nikitin & Sabanov, 2005). IMPROVED TECHNOLOGY OVERVIEW Currently tests are carried out for applying improved underground mining technology. The new mining technology is based on improved drilling-and-blasting method to move from packaged to underground bulk emulsion explosives (Nobelit 2000), from 2.0 m to 4.0 m borehole length with large boreholes to gain free space instead of undercutting (Fig. 6) and to automatization of roof drilling-bolting process with roof bolting machine. The old undercutting technology based on bottom cutting with the help of the cutter (Ural-33) which gives horizontal cut into the bottom layer A, 15 cm high and 1,7 to 1,8 m deep. The new technology is based on 6 large hole drilling with SMAG machine into the central oil shale layer C, up to 4,7 m deep with 3x280 mm diameter. Roof bolter and face drilling machines are operating with remote controls that provide higher safety conditions on a working place (Nikitin & Sabanov, 2005). GROUND SUBSIDENCE AND BACKFILLING Currently no backfilling is used in underground oil shale mines. Pillars with cross-sectional area 30-40 m 2 secure the roof stability during the excavation and are predicted to last at least 100 years. In the evolving period of room-and-pillar mining method, pillars with different dimensions were used and some of them have been collapsed causing ground subsidence. To increase the output of mines few decades ago, cross-sectional area of pillars was decreased so that pillars should last only two or five years, until a block was extracted. The ground above these areas is not stable having higher probability for ground subsidence. There have been 73 registered cases of ground subsidence caused by the collapse of pillars in the underground mining area. Most of them were test sections in the mines and were performed in the middle of low valued area (Fig. 7). In total there have been mined over 400 blocks with area more than 100 km 2 (Pastarus & Nikitin, 2002). Unexpected subsidences have occurred mostly in short period after mining. Late subsidences have been caused by breakage of drifts or adits roof (Fig. 8). One of the reasons has been low depth (5 to 8 m) of drift being close to the outcrop area of oil shale seam where both, oil shale and roof rocks have been weathered. This problem was best investigated during a railway construction on old mine area (Fig. 9). Three hectares of underground mine was cut down to the bottom of mine and backfilled with limestone. Experiences from these sites show that best solution for low depth mines is re-mining complete volume of the mine and dense backfilling. There are opposite examples where this principle has been ignored. Collapses have occurred in the areas of parking, sporting and rural squares caused by drifts Kolloquium: Schacht, Strecke und Tunnel 2005. TU Bergakademie Freiberg, 14-15 April, Freiberg / Sachsen 4 close to ground, week roof and impact from ground transport which all are results of ignoring planning rules. All undermined areas have been originally separated from building areas. About twenty years ago were investigated opportunities to evolve backfilling technology in underground mines. Suitable backfilling mixture was worked out consisting of oil shale ash and sand. Some test machines to fill rooms were put in operation. The technology was not evolved because it was not economic. More economical was to increase pillars dimensions, i.e. more losses, to secure ground stability. Today, when environmental aspects have a great impact on mining and energy industry, possibilities of backfilling should be reconsidered. The amount of oil shale ash emitted from power plants is annually 5-6 millions tonnes (0,45 tonne ash from 1 tonne of oil shale). The ash is mixed with water and pumped to landfill sites. By year 2010 these landfills must be closed and dry ash removal system evolved in power plants. Here is worth considering taking oil shale ash underground mines as backfilling material. Annually about 100 thousand tonnes of fly ash is used in construction material industry and agriculture. In oil shale preparation plants limestone is extracted from excavated rock. Approximately 0,4 tonne of limestone is extracted from 1 tonne of excavated rock. It makes 4,5-5,5 million tonnes annually. Waste limestone is suitable to use as aggregate in civil engineering but there are lack of consumers within feasible distance. A good example is a transport company who hauls wood to the furniture factory in the area and uses the same trucks to haul the limestone aggregate for road construction, keeping costs down in this way. Separation plants have crushing-screening equipment to produce aggregates but most of it is still deposited in heaps nearby preparation plants. Both, oil shale ash and waste limestone could be used as backfilling material underground mines. The distance between power plants and mines is 50 km. Today empty trains return to mine from power plant. In case of backfilling, train can transport ash on return trip. In Estonia underground mine recently new ramp was opened to replace auxiliary shaft and underground rail transportation. The auxiliary shaft can be used to take ash or prepared backfill underground and in rail drifts can be placed conveyors for backfill transportation. In mining blocks backfilling operations can start when block is nearly extracted. Bolts to support immediate roof are reusable and removed when the mining is finished in the block. It means that backfilling must be made at the same time with bolts removal. There are a number of environmental, economical and technological aspects to be considered in case of backfilling but basically mineral recourse is brought back in its native environment after the extraction of a useful matter of it. Because oil shale doesn’t have today such a high value what would make backfilling economic, needs the issue to be approached from an other side: besides depositing residues on the ground and paying pollution tax, power plants could pay for mines taking ash underground as backfill. At the same time mines can benefit from decreasing dimensions of pillars or extracting pillars entirely. CONCLUSIONS Oil shale mining continues in Estonia at least next twenty years and has therefore strategic importance in energy production in the nearest future. Therefore main goal of the oil shale industry is to preserve its competitive ability in the market of power resources. Today Kolloquium: Schacht, Strecke und Tunnel 2005. TU Bergakademie Freiberg, 14-15 April, Freiberg / Sachsen 5 relatively cheap and not very complicated oil shale mining is capable to guarantee this competitiveness but when to look forward, in the future must be paid extra to recover consequences of oil shale mining and processing. Oil shale ash landfill sites (over 10 km 2 ) need to be monitored and maintained with certain extension in the future. Pillars in underground mines are predicted to last 100 years but when they start to collapse, then expenditures must be made to recover ground subsidence in certain areas. Experiences show that best solution for activity on top of low depth mines is re-mining complete volume of the mine and dense backfilling. One possible solution to reduce post-mining impacts is backfilling underground oil shale mines with oil shale ash. This can help to eliminate two major negative impacts of oil shale mining and energy production: ground subsidence and landfill sites for ash. ACKNOWLEDGEMENTS Support for the topic was provided by: Deutsche Bundesstiftung Umwelt Estonian Science Foundation, Grant No. 5913 REFERENCES: 1. Valgma, I. 2003. Estonian oil shale resources calculated by GIS method. Oil Shale. 2003. Vol. 20. No. 3. P. 404-411. 2. Pastarus, J-R., Nikitin, O. 2002. Methods of mining block stability analysis for room- and-pillar mining with continuous miner in Estonian oil shale mines. Proc. Of the 7 th International Symposium on Environmental Issues and Waste Management in Energy and mineral Production (SWEMP 2002). Cagliari, Italy, 7-10 October 2002, P. 667-682. 3. Valgma, I. 1999. Mapping potential area of underground subsidence in Estonian underground oil shale mining district. 2 nd International Scientific Research Conference „Environment. Technology. Resources” in Rezekne, Latvia June 25-27, 1999, P. 227- 232. 4. Nikitin, O., Sabanov, S., 2005. Immediate roof stability analysis for new room-and- pillar mining technology in “Estonia” mine. In: Proc. 5th International conference “Environment. Technology. Resources", Rezekne, June 16-18, 2005 (in print). Kolloquium: Schacht, Strecke und Tunnel 2005. TU Bergakademie Freiberg, 14-15 April, Freiberg / Sachsen 6 Figure 1. Balance of Estonia’s primary power resources Figure 2. Cross-section of commercial oil shale bed. The oil shale layers of the commercial bed are marked with capital letters (from bottom upwards A, B, C, D, E, F). Kolloquium: Schacht, Strecke und Tunnel 2005. TU Bergakademie Freiberg, 14-15 April, Freiberg / Sachsen 7 Figure 3. Estonian oil shale deposit Kolloquium: Schacht, Strecke und Tunnel 2005. TU Bergakademie Freiberg, 14-15 April, Freiberg / Sachsen 8 Figure 4. Underground oil shale mine. Pillar Chain conveyor 140 -180 m 4 m Belt conveyor Collection drift Tail drift 140 -180 m 600 - 800 m LHD Figure 5. Principal layout of a room-and-pillar mining block Kolloquium: Schacht, Strecke und Tunnel 2005. TU Bergakademie Freiberg, 14-15 April, Freiberg / Sachsen 9 Figure 6. The new blasting pattern Figure 7. Typical pond in the mould of room and pillar section subsidence Figure 8. Collaps of drift close to the outcrop area Kolloquium: Schacht, Strecke und Tunnel 2005. TU Bergakademie Freiberg, 14-15 April, Freiberg / Sachsen 10 Figure 9. Opened drift in railway construction site