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Presentation: Backfilling technologies for Estonian oil shale mines


txt: 374 6th International Conference on Sustainable Development in the Minerals Industry, 30 June – 3 July 2013, Milos island, Greece Backfilling technologies for Estonian oil shale mines ABSTRACT The oil shale deposit in Estonia is located partly in a densely populated farming district of high soil fertility. Underground oil shale production is conducted using the room-and-pillar method with drilling and blasting. A large amount of neutral (limestone) and hazardous waste (ash) is generated by the oil shale industry. The use of ash and limestone as backfilling materials reduces the volume and area required for surface disposal and consequently the environmental taxes. In 1980, a preliminary investigation was started with respect to the backfilling technology for the Estonian oil shale mines. Currently, experiments continue with tests on new ashes and waste rock aggregates. The main focus of the current study is to clarify if backfilling in given conditions would be technically possible. The study includes underground and surface mining space modelling, fill material supply and sample and mixture tests, discussion of technological schemes for backfilling. Laboratory UCS tests show, that lower ash content in the mixture results in higher strength. Mine tests show the warming effect of large scale mixtures can have a positive influence to the hardening process. 1. INTRODUCTION The oil shale deposit in Estonia is located partly in a densely populated farming district of high soil fertility. Oil shale is consumed by power plants which produce about 90% of Estonian electricity, oil and large part of thermal power. Underground mining is performed for half of the Estonian oil shale mining capacity. Production is about 7 million tonnes of oil shale, not including separated limestone which amounts to an additional 40% of the produced mass per year. Currently oil shale is mined in 3 underground mines in addition to 7 surface mining fields. The maximum number of underground mines has been 13 with a total output of 17 million tonnes per year (Fig. 1, Valgma, 2000). Oil shale bedding depth reaches 80 m, while seam thickness is 2.8 m. The room and pillar mining system with drilling and blasting is used today with squared shape pillars left to support the roof. The losses in pillars increase up to 40 %. On the other hand, a large amount of neutral (limestone) and hazardous waste (ash) is generated by oil shale industry (Valgma, 2003). Using ash and limestone as backfilling materials could reduce the volume and area required for surface disposal and consequently the environmental taxes (Adamson et al., 1998). The main source of the backfill material today is Heavy Media Separation (HMS). The oil shale seam consists of up to 50% of limestone layers and pieces. This raises the question of whether to utilising the waste rock or ash in the Figure 1: Map of Estonian oil shale mining areas. 6th International Conference on Sustainable Development in the 375 Minerals Industry, 30 June – 3 July 2013, Milos island, Greece surface or underground mines or to dump them. The main focus of the current study is to clarify if backfilling under given conditions could be technically possible. 2. METHODS The study includes the following steps: 1. Underground and surface mining space modelling. 2. Tests for the fill material. 3. Determination of technological schemes for backfilling. 2.1 Underground and surface mining data collection and modeling Site analyses have been carried out for determining potential backfilling conditions. For that purpose a geological and technical model has been created (Valgma, 2002). A quasi-stable area has been detected in large areas (Fig. 2). Areas of collapse, subsidence and zones of stability have been determined. 2.2 Backfill mix components and origin The components of the backfill mix could include water, limestone (waste rock from oil shale mining), ash from the power plant, and in addition sand, fibers or cement. Concerns are related to haulage costs and other processes in the mine. One of them is the origin of limestone aggregate material. If limestone could be separated from the oil shale in situ, then a reduction in haulage costs could be achieved. For dry underground separation, tests with Bradford drums have been carried out (Fig. 3). In addition crushing buckets have been tested in several sites. The currently used impact crusher also partially works like a selective crusher, but additional HMS is needed on the surface. Sizers or other types of crushers are needed for generating the 0-15 mm oil shale fraction, and 0-45 mm limestone fraction. Since fines are difficult to handle both in the power plant and in the oil generation process, the 0-5 mm fraction should be minimal. The main problem in mines is the high percentage of the 0-25 mm fine fraction, which amounts to 30% of the total production. However, there is a possibility to use it in the power plant. If the separation process produces suitable material, the residue could be used as backfilling material (Valgma, 2009). Part of the analyses was focused towards optimizing the layout of rooms, pillars and workings. One of the options was to evaluate the possibilities of shortwall extraction with roadheaders (Hungarian F2 road header and Russian coal road header 4PP-3). This was based on the hypothesis that roadheaders or continuous miners could be used for oil shale and phosphate rock extraction (Fig. 4, Valgma et al., 2008a,b). If this could be proved, the mechanical extraction could allow decreasing the size of pillars by roughly 0.3 m from each currently blasted side without decreasing the strength of the pillars (Orru et al., 2013). In addition, further decreasing of the pillar side could be compensated with Figure 2: Spatial site model of oil shale deposit. Figure 3: Drum screener and hammer crusher for dry selective oil shale separation. 376 6th International Conference on Sustainable Development in the Minerals Industry, 30 June – 3 July 2013, Milos island, Greece side pressure by backfilled material (Zha et al., 2011). A similar method for stabilising pillar walls could be by using cutting machines. In addition to currently partially used horizontal cutters, vertical cutters could be considered (Fig. 5). 2.3 Fill material tests Industrial tests were performed with ash, aggregate and water mixture. A preliminary investigation has been started for selecting backfilling technology in Estonian oil shale mines (Valgma et al., 2012). Experiments in mines have been performed (Väizene, 2009). The main methods that have been tested were: - dry casting of waste rock to the mined out rooms and adding ash and water mixture, - pumping wet mixture with piston pump to the rooms. Currently experiments have been continued by testing new ashes (new burning and heating technologies) and waste rock aggregates. Industrial tests and laboratory tests have been performed (Fig. 6). Road stabilization tests were performed in a mined out area using a hydraulic backfilling technology (piston pump, slurry, frill hole, pumping tube). The drift was stabilised and the mixture reached stability within 2 days. Laboratory tests were performed with different ash mixtures (Fig. 7). Ash mixtures were formed in the standard concrete moulds and kept in different conditions. For simulating the mining environment, a refrigerator was built with temperature and humidity monitoring. For holding low temperature (8 degrees Celsius) an air conditioner and an air humidifier were used. In addition water circulation with wet textile was applied. The air conditioner together with the wet textile and the air humidifier guaranteed Figure 4: High-selective mining with continuous miner, has not been tested for open cast. Figure 5: Cutting in layer A. Figure 6: Hydraulic backfilling in an oil shale mine. Figure 7: Tested mixtures. 6th International Conference on Sustainable Development in the 377 Minerals Industry, 30 June – 3 July 2013, Milos island, Greece 8 degrees temperature and 90% humidity. The refrigerators stored samples for different periods of time. After each period the sample was tested for uniaxial compressive strength. In addition the sample was kept in water and leached water was analysed. 3. POTENTIAL TECHNOLOGIES Different schemes for potential technologies have been proposed and evaluated. The backfilling space between the spoils or trenches could be used for depositing ash and at the same time for stabilising spoils. Stabilised spoils could influence the maximum possible overburden thickness in the open cast mines (Fig. 8). Peat and quaternary sediments could be mixed with ash. Tests have shown that trees grow faster on this mixture than without the sediments. Open pit mines has reached old underground workings and cross sections of the subsidence occurrence have been studied (Fig. 9). Several underground schemes have been tested, including hydraulic, pneumatic and mechanical methods (Figs. 10-12). 4. RESULTS AND DISCUSSION In case of testing backfill material, the temperature, humidity and size of the sample play an important role. Several samples, that have been kept in standard conditions show good compressive strength. On the contrary, samples kept in the mine environment showed less compressive strength (Fig. 12). Tests conducted in the mine actually showed better compressive strength results: 10 MPa. This could be related to the Figure 8: Open cast scheme with backfilling. Figure 9: Subsided roof in the opened mine. Figure 10: Partial backfilling with waste rock. Figure 11: Combined room and pillar mining with partial backfilling with hardening material. Figure 12: Mining with combined pillars. 378 6th International Conference on Sustainable Development in the Minerals Industry, 30 June – 3 July 2013, Milos island, Greece warming effect of the large scale mixture which can have a positive influence to the hardening process. In addition mixture composition had influence. Highest ash content resulted in lowest compressive strength (Fig. 13). The study should be continued with larger scale tests with low ash content. ACKNOWLEDGEMENTS This research is related to the project MIN- NOVATION - mi.ttu.ee/min-novation; ETF8123 “Backfilling and waste management in Estonian oil shale industry” - mi.ttu.ee/ ETF8123; Energy Technology Program Sustainable and environmentally acceptable Oil shale mining No. 3.2.0501.11-0025 - mi.ttu.ee/ etp and Doctoral School of Energy and Geotechnology II, interdisciplinary research group “Sustainable mining” DAR8130/ 1.2.0401.09- 0082 - mi.ttu.ee/doktorikool REFERENCES Smith, J., (1980). Testing of Environmentally Friendly Materials, Proceedings, Conference on Materials, Amsterdam, May 20-22, pp. 80-85. Adamson, A., H. Hints and T. Tomberg, (1988). Rock pressure regulation with the help of filling. up the ex- cavated areas in the Rakvere Phosphorite Deposits. Transactions of Tallinn Technical University. Väizene, V., (2009). Backfilling technologies for oil shale mines. Valgma, I. (Toim.). Resource Reproducing, Low-wasted and Environmentally Protecting Technologies of Development of the Earth Interior (1 pp.). Tallinn: Department of Mining TUT; Russian University of People Friendship. Valgma, I., (2000). Oil shale mining in Estonia and Russia. Encyclopaedia of life support systems. EOLSS Publishers Co. Ltd, Oxford UK. Valgma, I., (2002). Geographical Information System for Oil Shale Mining - MGIS. (Thesis) Tallinn: Tallinn Technical University Press. Valgma, I., (2009). Oil Shale mining-related research in Estonia. Oil Shale, 26(4), 445 - 150. Valgma, I., (2003). Estonian oil shale resources calculated by GIS method // Oil Shale. 2003. Vol. 20, No.3., pp. 404-411. Valgma, I., M. Leiaru, V. Karu and R. Iskül, (2012). Sustainable mining conditions in Estonia. 11th International Symposium "Topical Problems in the Field of Electrical and Power Engineering", Doctoral School of Energy and Geotechnology, Pärnu, Estonia, 16- 21.01.2012, pp. 229 - 238. Tallinn: Elektriajam Valgma, I., T. Tammeoja, A. Anepaio, V. Karu and A. Västrik, (2008a). Underground mining challenges for Estonian oil shale deposit. Buhrow, Chr.; Zuchowski, J.; Haack, A. (Toim.). Schacht, Strecke und Tunnel (161 - 172). Freiberg : TU Bergakademie. Valgma, I., A. Västrik, V. Karu, A. Anepaio, V. Väizene and A. Adamson, (2008b). Future of oil shale mining technology. Oil Shale. 25(2S), pp. 125 - 134. Zha, J.F., G.L. Guo, W.K. Feng and W. Qiang, (2011). Mining subsidence control by solid backfilling under buildings. Source: Transactions of Nonferrous Metals Society of China, Volume: 21 Supplement: 3 Pages: S670-S674, Published: Dec. 2011. Orru, M., V. Väizene, J.R. Pastarus, Y. Sõstra and I. Valgma, (2013). Possibilities of oil shale mining under the Selisoo mire of the Estonia oil shale deposit. Environmental Earth Sciences, pp. 1-11. 0 0,5 1 1,5 2 2,5 0 10 20 30 40 50 60 UCS, Mpa Days after mixing 3 2 1 Figure 12: UCS of the mixtures.

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