Aggregates for Backfilling the Mined Areas in Conditions of Estonian Oil Shale Mines Julija Šommet J.-R. PASTARUS, Y. SYSTRA, I. VALGMA, V. VÄIZENE Tallinn University of Technology, Department of Mining, Estonia

Presentation Content Introduction of oil shale industry situation in Estonia Geological situation description Proposed backfill technology Geometrical and physical parameters of aggregates Results relation

Introduction Oil shale is used as a fuel for producing energy and shale oil. Oil shale mining in Estonia produce a wide assortment of waste could be considered as available fill materials. Waste rock which is not usable in civil engineering and road building may be used for backfilling underground mines. The laboratory tests and theoretical investigations were made for determination the applicability of limestone aggregates as backfill material. Underground oil shale mining is performed by using a room-and-pillar method with blasting. if the depth of excavation is over 60 m (mine Estonia) the loss in pillars increases up to 40%.

Industry Problems Loss in pillars up to 40 % Large amount of neutral (limestone) and hazardous (ash) wastes – approx. 40 % Decision – backfill technology significant impact for mined areas. The main aim - to determine the applicability of waste rock aggregates as backfill material in condition of Estonian oil shale mines.

Geological Situation Deposit is about 135 km in length and about 45 km in width in the east, narrowing gradually to 10-15 km in the western part. Oil shale bed located at deepness less than 30 m - the most of reserves are exhausted. The underground mining works are going at deepness 35-65 m, but at the southern border of deposit mining deepness is growing up to 120-130 m.

Oil Shale Parameters The commercial oil shale seam consists of six layers with 6-59 cm each, total thickness 2.06 m. The calorific value is from 10.6 MJ/kg to 20.4 MJ/kg. All layers contain limestone nodules of the thickness ~15 cm.

Backfill Technology Materials Nowadays attention has been focused on the use of combustion and mining by-products as filling materials: Separation of limestone from the raw oil shale generates waste, which consists of limestone and oil shale residues. These are stockpiled - 55 m height, and total area ~3.5 km2, is about 6.5 Mt per year. Two different oil shale combustion - pulverized firing (PF) and circulating fluidized bed combustion (CFBC), ash is very similar to cement. About 4 km2 of the landfill are occupied. Annual production is about 5 Mt.

Backfill Technology Effects The determination of different ashes parameters demands supplementary investigations and is under construction. Effect is significant: minimization of possible surface movement, improvement of safety, waste disposal, facilitation of mining operations and increase of extraction ratio.

Technology Benefits Carefully selected grain-size distribution of solid particles Ability to flow without sedimentation in pipes by low water content (10...30 %) Mixtures are able to set with lower or without presence of additional binders. Shorter binding times and better mechanical properties. Drainage and processing of bleed water eliminated.

Technology Reguirenments Depending on end uses, requirements for aggregates are specified in accordance with different standards Aggregates has low resistance to fragmentation and resistance to freezing and thawing. Waste rock is not usable in civil engineering May be used for backfilling already mined areas. Constant temperature of the oil shale underground mine is 6…8°C

Parameters of Aggregates Porosity Material have been sorted so that they are all about the same size, range of about 25.95% to 47.65%. The share of ash weight is from 23% to 43%, limestone between 57% and 77%, depending on the packing of grain used in mixture.

Parameters of Aggregates Uniformity Coefficient It is a measure of how well or poorly sorted material is. Uniformity coefficient for all aggregate sizes (4/16, 16/32 and 32/63 mm) is less than 4. Consequently, aggregates are well sorted and satisfy the requirement.

Parameters of Aggregates Shape of the Coarse Aggregate Flaky particles tend to lower the workability of concrete mix which may impair the long-term durability. Flakiness index of waste rock aggregate, average % Aggregate size, mm Mine Estonia Open cast Aidu 4/16 9 6 16/32 3 32/64 4 2 Investigation showed that the flakiness index depends on aggregate size. If the aggregate size increases, the flakiness index decreases.

Parameters of Aggregates Strength Parameters Point load testing is used to determine aggregate strength indexes. In order to estimate uniaxial compressive strength, index-to-strength conversion factors are used. Point load strength index, Mpa Uniaxial compressive strength, MPa Aggregate size, mm AVERAGE Confidence VARIATION % 4/16* >2.1 - >42 16/32 2.1 0.3 33 42 6.7 32/63 1.8 0.4 36 7.4 It is visible that the uniaxial compressive strength depends on aggregate size. Uniaxial compressive strength (35…42 MPa) of different aggregate sizes guarantees the necessary strength for backfill.

Other Parameters of Aggregates Water Absorption-from 2 to 6% Resistance to Fragmentation-LA 30…45%. Abrasivity coefficient-0.84 g/t are classified as not very abrasive rock. Content of Fines- characterized by low content of fines 1.5…4%)

Results and Conclusions Investigation presented a complex of geometrical and physical parameters of limestone aggregates, which determines the quality of fill mixtures and backfill properties after placement. Analysis showed that limestone aggregates suit best for backfill technology in conditions of Estonian oil shale mines. The choice of a proper backfill material and other mixtures parameters is essential in the control of fill costs and backfill properties after placement.

European Social Fund’s Doctoral Studies and International Program DoRa Acnowledgements Estonian Science Foundation grants: “Backfill and waste management in Estonian oil shale industry”, “Tectonics of the continental and offshore territory of Estonia and its structural evolution in Proterozoic and Phanerozoic”. SA Archimedes projects: “Sustainable and environmentally acceptable oil shale mining”, “Basics of new utilization process for oil shale combustion solid waste”. European Social Fund’s Doctoral Studies and International Program DoRa

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