Valgma backfilling

374 6th International Conference on Sustainable Development in theMinerals Industry, 30 June – 3 July 2013, Milos island, Greece

Backfilling technologies for Estonian oil shale mines

I. Valgma, M. Kolats, A. Anepaio, V. Väizene, M. Saarnak and J.-R. PastarusDepartment of Mining, Tallinn University of Technology, Estonia

ABSTRACTThe oil shale deposit in Estonia is located partlyin a densely populated farming district of highsoil fertility. Underground oil shale productionis conducted using the room-and-pillar methodwith drilling and blasting. A large amount ofneutral (limestone) and hazardous waste (ash) isgenerated by the oil shale industry. The use ofash and limestone as backfilling materials re-duces the volume and area required for surfacedisposal and consequently the environmentaltaxes. In 1980, a preliminary investigation wasstarted with respect to the backfilling technolo-gy for the Estonian oil shale mines. Currently,experiments continue with tests on new ashesand waste rock aggregates. The main focus ofthe current study is to clarify if backfilling ingiven conditions would be technically possible.The study includes underground and surfacemining space modelling, fill material supply andsample and mixture tests, discussion of techno-logical schemes for backfilling. LaboratoryUCS tests show, that lower ash content in themixture results in higher strength. Mine testsshow the warming effect of large scale mixturescan have a positive influence to the hardeningprocess.

1. INTRODUCTIONThe oil shale deposit in Estonia is located partlyin a densely populated farming district of highsoil fertility. Oil shale is consumed by powerplants which produce about 90% of Estonianelectricity, oil and large part of thermal power.

Underground mining is performed for half ofthe Estonian oil shale mining capacity. Produc-

tion is about 7 million tonnes of oil shale, notincluding separated limestone which amounts toan additional 40% of the produced mass peryear. Currently oil shale is mined in 3 under-ground mines in addition to 7 surface miningfields. The maximum number of undergroundmines has been 13 with a total output of 17 mil-lion tonnes per year (Fig. 1, Valgma, 2000). Oilshale bedding depth reaches 80 m, while seamthickness is 2.8 m. The room and pillar miningsystem with drilling and blasting is used todaywith squared shape pillars left to support theroof.

The losses in pillars increase up to 40 %. Onthe other hand, a large amount of neutral (lime-stone) and hazardous waste (ash) is generatedby oil shale industry (Valgma, 2003). Using ashand limestone as backfilling materials could re-duce the volume and area required for surfacedisposal and consequently the environmentaltaxes (Adamson et al., 1998).

The main source of the backfill material to-day is Heavy Media Separation (HMS). The oilshale seam consists of up to 50% of limestonelayers and pieces. This raises the question ofwhether 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 375Minerals 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 clar-

ify if backfilling under given conditions couldbe technically possible.

2. METHODSThe study includes the following steps:1. Underground and surface mining space mod-

elling.2. Tests for the fill material.3. Determination of technological schemes for

backfilling.

2.1 Underground and surface mining data col-lection and modelingSite analyses have been carried out for deter-mining potential backfilling conditions. For thatpurpose a geological and technical model hasbeen created (Valgma, 2002). A quasi-stable ar-ea has been detected in large areas (Fig. 2). Are-as of collapse, subsidence and zones of stabilityhave been determined.

2.2 Backfill mix components and originThe components of the backfill mix could in-clude water, limestone (waste rock from oilshale mining), ash from the power plant, and inaddition sand, fibers or cement. Concerns arerelated to haulage costs and other processes inthe mine. One of them is the origin of limestoneaggregate material. If limestone could be sepa-

rated from the oil shale in situ, then a reductionin haulage costs could be achieved. For dry un-derground separation, tests with Bradford drumshave been carried out (Fig. 3). In addition crush-ing buckets have been tested in several sites.The currently used impact crusher also partiallyworks like a selective crusher, but additionalHMS is needed on the surface.

Sizers or other types of crushers are neededfor generating the 0-15 mm oil shale fraction,and 0-45 mm limestone fraction. Since fines aredifficult to handle both in the power plant and inthe oil generation process, the 0-5 mm fractionshould be minimal. The main problem in minesis the high percentage of the 0-25 mm fine frac-tion, which amounts to 30% of the total produc-tion. However, there is a possibility to use it inthe power plant. If the separation process pro-duces suitable material, the residue could beused as backfilling material (Valgma, 2009).

Part of the analyses was focused towards op-timizing the layout of rooms, pillars and work-ings. One of the options was to evaluate thepossibilities of shortwall extraction with road-headers (Hungarian F2 road header and Russiancoal road header 4PP-3). This was based on thehypothesis that roadheaders or continuous min-ers could be used for oil shale and phosphaterock extraction (Fig. 4, Valgma et al., 2008a,b).If this could be proved, the mechanical extrac-tion could allow decreasing the size of pillars byroughly 0.3 m from each currently blasted sidewithout decreasing the strength of the pillars(Orru et al., 2013). In addition, further decreas-ing 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 se-lective oil shale separation.


side pressure by backfilled material (Zha et al.,2011).

A similar method for stabilising pillar wallscould be by using cutting machines. In additionto currently partially used horizontal cutters,vertical cutters could be considered (Fig. 5).

2.3 Fill material testsIndustrial tests were performed with ash, aggre-gate and water mixture. A preliminary investi-gation has been started for selecting backfillingtechnology in Estonian oil shale mines (Valgmaet al., 2012). Experiments in mines have beenperformed (Väizene, 2009). The main methodsthat 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 heatingtechnologies) and waste rock aggregates. Indus-trial tests and laboratory tests have been per-formed (Fig. 6).

Road stabilization tests were performed in amined out area using a hydraulic backfillingtechnology (piston pump, slurry, frill hole,pumping tube). The drift was stabilised and themixture reached stability within 2 days.

Laboratory tests were performed with differ-ent ash mixtures (Fig. 7). Ash mixtures wereformed in the standard concrete moulds andkept in different conditions. For simulating themining environment, a refrigerator was builtwith temperature and humidity monitoring. Forholding low temperature (8 degrees Celsius) anair conditioner and an air humidifier were used.In addition water circulation with wet textilewas applied. The air conditioner together withthe 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 377Minerals Industry, 30 June – 3 July 2013, Milos island, Greece

8 degrees temperature and 90% humidity. Therefrigerators stored samples for different periodsof time. After each period the sample was testedfor uniaxial compressive strength. In additionthe sample was kept in water and leached waterwas analysed.

3. POTENTIAL TECHNOLOGIESDifferent schemes for potential technologieshave been proposed and evaluated. The backfill-ing space between the spoils or trenches couldbe used for depositing ash and at the same timefor stabilising spoils. Stabilised spoils could in-fluence the maximum possible overburdenthickness in the open cast mines (Fig. 8). Peatand quaternary sediments could be mixed withash. Tests have shown that trees grow faster onthis mixture than without the sediments.

Open pit mines has reached old undergroundworkings and cross sections of the subsidenceoccurrence have been studied (Fig. 9).

Several underground schemes have beentested, including hydraulic, pneumatic and me-chanical methods (Figs. 10-12).

4. RESULTS AND DISCUSSIONIn case of testing backfill material, the tempera-ture, humidity and size of the sample play animportant role. Several samples, that have beenkept in standard conditions show good compres-sive strength. On the contrary, samples kept inthe mine environment showed less compressivestrength (Fig. 12). Tests conducted in the mineactually showed better compressive strength re-sults: 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 partialbackfilling with hardening material.

Figure 12: Mining with combined pillars.


warming effect of the large scale mixture whichcan have a positive influence to the hardeningprocess. In addition mixture composition hadinfluence. Highest ash content resulted in lowestcompressive strength (Fig. 13).

The study should be continued with largerscale tests with low ash content.

ACKNOWLEDGEMENTSThis research is related to the project MIN-NOVATION - mi.ttu.ee/min-novation;ETF8123 “Backfilling and waste managementin Estonian oil shale industry” - mi.ttu.ee/ETF8123; Energy Technology Program Sus-tainable and environmentally acceptable Oilshale mining No. 3.2.0501.11-0025 - mi.ttu.ee/etp and Doctoral School of Energy and Ge-otechnology II, interdisciplinary research group“Sustainable mining” DAR8130/ 1.2.0401.09-0082 - mi.ttu.ee/doktorikool

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Adamson, A., H. Hints and T. Tomberg, (1988). Rockpressure 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 shalemines. Valgma, I. (Toim.). Resource Reproducing,Low-wasted and Environmentally Protecting Tech-nologies of Development of the Earth Interior (1 pp.).Tallinn: Department of Mining TUT; Russian Univer-sity of People Friendship.

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Valgma, I., (2002). Geographical Information System forOil Shale Mining - MGIS. (Thesis) Tallinn: TallinnTechnical University Press.

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Valgma, I., M. Leiaru, V. Karu and R. Iskül, (2012). Sus-tainable mining conditions in Estonia. 11th Interna-tional Symposium "Topical Problems in the Field ofElectrical and Power Engineering", Doctoral Schoolof 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 forEstonian 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. Väizeneand A. Adamson, (2008b). Future of oil shale miningtechnology. 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 underbuildings. Source: Transactions of Nonferrous MetalsSociety 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 underthe Selisoo mire of the Estonia oil shale deposit. Envi-ronmental Earth Sciences, pp. 1-11.

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Figure 12: UCS of the mixtures.

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Valgma backfilling