AN INDUCED BLASTING TECHNIQUE IN COAL MINING

Transcription

1 AN INDUCED BLASTING TECHNIQUE IN COAL MINING Dasari Appaji ABSTRACT Induced caving by blasting during depillaring of panels in underground coal mines has received limited attention. This technique has become an integral part of a mining operation known as the blasting gallery (BG) method in India. Systematical deep hole (16-30 m) blasting has been successfully carried out from underground split galleries in BG panels. A study of drilling and blasting parameters, gas hazards, strata behaviour and ground vibration was undertaken as part of a research project for the Indian Ministry of Coal and Mines 1. INTRODUCTION Coal is the world s most abundant and safe fossil fuel. World coal reserves are estimated to last almost 200 years from now. The importance of coal as a primary source of energy is significant in India. India has a long history of commercial coal mining covering nearly 220 years from 1774 in the Ranigunj Coalfield. Sector X plan XI Plan XII Plan CAGR (%) of Actual Assessed Projected Power Utilities % Power Captive % Cement Sponge Iron & Others % Total Non-coking % Coking - Steel % Total % CAGR Compounded Annual Growth Rate Page No:393

2 Outsourcing of mining operations can lead to faster project implementation as the private parties can readily procure machinery. Creating an enabling policy environment for private participation Expediting the project approval process and faster project commissioning Private mining Higher import Overseas mining For faster execution of projects after they are approved, MoC grants advance action plan for larger projects. GoI has issued some of the coal blocks to private companies for captive mining. To enable higher imports, the GoI lowered the effective import duty on non-coking coal. CIL has formed a subsidiary, Coal Videsh Limited (CVL) to acquire coal blocks in a number of countries including Indonesia, South Africa and Australia. Enhancing producition from opencast mines Opencast mines allow higher mechanisation and can therefore ensure faster step-up in production. Increasing the extent of outsourcing in various mining operations The magnitude of coal shortage is such that active participation of private players is urgently required to broad base the country s efforts at increasing coal supply. 2. STRATEGY OF MINING IN SCCL Exploitation of coal seams from u/g mines became a major challenge for Indian Coal Mining Industry. Ever increasing demand year by year as coal is the main fossil fuel used by power sector. SCCL has initiated several steps like re-organization of existing mines to augment production. One example is mechanized open cast mining with technoeconomic viability. Introduction of mechanization for higher rate of production such as long wall system and intermediate technology etc., liquidation of long standing good quality coal pillars. About 50 % of coal reserves in India are in seams with thickness more than 4.5 m which fall under the category of thick seams. Exploitation of thick seams by u/g method poses certain difficulties / problems. Extraction of thick seams by conventional hand section is neither productive nor viable from conservation point of view. Percentage of extraction by hand section in thick seams is as low as %. However, some attempts have been made to extract thick seams with sand stowing. Sand stowing for working thick seams cannot be considered as an option because the cost is prohibitive as sand is becoming increasingly scarce commodity along with timber. At the same time, coal industry was in search of an economic method for dealing with thick / standing developed pillars. SCCL is of the opinion that blasting gallery (BG) method is the appropriate method for the extraction of thick seams up to a thickness of 8 11 metres. Mining by BG Page No:394

3 method produces about 1000 T/day with 85 % of extraction which has been in SCCL. 3. CARBONNAGE DE FRANCE (CDF) SUGGESTED BLASTING GALLERY (BG) FOR EXTRACTION OF THICK SEAMS IN INDIA The 1 st BG Panel was started in the country at East Katras colliery in Jharia Coal fields (BCCL) in The method was not successful at East Katras colliery where overriding of pillars (strata control) occurred. In Chora 10 Pit colliery in Raniganji (ECCL) in the year 1987, the method was partially successful giving encouraging results. However, it was discontinued due to spontaneous heating. Expected production and percentage of extraction could not be achieved in both the above mines. SCCL introduced BG method in collaboration with Carbonnage de France at GDK-10 Incline in the year for extraction of a coal seam with 11 mtrs. Thickness. The method was very successful resulting in 85 % of extraction with high productivity. At the same mine, where the thick seam was developed fully in top section and partially in bottom section, BG was introduced. Subsequently, SCCL has introduced BG in other mines also, namely : i. GDK-8, where bottom section was developed by bord and pillar method along the floor of the seam. ii. VK-7 Incline, Kothagudem area, where the seam was developed in two sections. During the development of BG panel, the galleries were driven in staggered manner. iii. GDK 11 A Inc., RG-I Area and later at No. 21 Area, YD Area in the year ADVANTAGES OF BLASTING GALLERY METHOD Full thickness of the seam can be extracted in a single lift. Higher percentage of extraction i.e Capital investment is nominal when compared to longwall project. Easy to train the man power and easy maintenance of the equipment. Extraction is carried under fully supported roof i.e. with remote controlled LHDs. Safety of the workmen can be fully ensured. This method can be also be adopted in virgin/developed seams. Most of the equipment and spares are indigenously available. Loss of production is minimum while shifting the equipment. This method is highly flexible as several units are in operation in a district Even if one of the units is under break down, production from the district will continue to come. The time required for preparation of panel in relation to the total life of the panel is less than other mechanised methods. LIMITATIONS OF THE METHOD This method is not suitable for gassy mines and seams with degree-i gassiness are most preferable. The method is suitable only for gradient more than 1 in 5 to allow easy movement of tyre mounted LHDs and crawler mounted electro- hydraulic jumbo drills. Though the percentage of extraction is around 75-85, still coal left in the goaf is likely to create spontaneous heating. 4. THE MANNER OF EXTRACTION Each pillar shall be split into two equal parts by level split of width not more than 4.2 m. Page No:395

4 and height not more than 3m. along bottom section. The splitting of pillars shall be restricted to one pillar from the pillar under extraction. The long hole blasting shall not be practiced at any place where two free faces are not provided. In order to create free faces in this operation / beginning of extraction in BG Method. Before practicing long holes blasting (ring holes) the operation of drilling and blasting are carried out in stages to a height of full thickness to expose roof with increasing angle and length of short holes in bye of the galleries i.e., called Potato blasting. The full thickness of the seam is extracted by blasting a ring of shot holes with about 33 shot holes. Potato Blasting Blasting of Ring holes After drilling is completed, shot holes are charged with the explosive approved by DGMS. The total number of shot holes in a ring are 33. The cartridges of explosives are distributed over the whole length of shot holes by spacers tied together by a detonating fuse called RING CORD which are initiated by No: 6 electric detonator. About in length of all holes are stemmed with clay at the end. On completion of charging and stemming of all holes, the circuit is connected in series. Before blasting, the last installed roof support at the goaf line is removed and next support line is reinforced with extra support. Specification of Explosives : The shot holes are drilled in ring pattern and sloping at an angle of about 30 0 to 40 0 from the vertical towards the goaf. The spacing between consecutive rings at shot holes shall be 1.5 m. The shot holes are drilled in a ring spaced 1.5m. apart by means of JUMBO drill from the level rooms in such a way that they cover half the pillar on rise side and half the pillar on the dip side. Extraction in level galleries shall be from in bye to out bye forming a diagonal line at an angle of about 60 0 to the level. A curtain of thickness of coal not less than 1.5m shall be left between two adjacent rooms after blasting of rings. However, it was observed that this curtain provides protection during remote operation of the LHD in the goaf. I-Stage Explosive : Uniring (IEL) (a) Weight of explosive : 200 g (b) Diameter of explosive : 32 mm (c) Length of explosive cartridge (d) II-Stage (e) b) Explosive (f) (g) Explosive type : Belgex Coal (R), Bharat Explosives Limited Weight of explosive : 185 g : 200 m Page No:396

5 (h) (i) (j) (k) (l) Diameter of explosive : 32 mm Length of explosive cartridge : 200 mm Density : g/cc VOD : 2826 M/S Composition of explosive Air gap in unconfined condn : 5 cm pass. Nitroglycerine : % Nitro-Cellulose : % Ammonium Nitrate : % Cellulose (Woodmeal) : % Salt : % Sodium Nitrate : 2 % Others : % 5. SUPPORT SYSTEM IN BG METHOD The main junction girders are secured tightly by clamps suspended by steel bolts grouted in the roof. The goaf edges and the entries of the central dip / rise galleries of panel immediately out bye of the goaf edges shall be kept supported by row cogs set an interval of 0.25 m. All roof bars set on hydraulic props shall be braced suitably. Adequate setting load of not less 6 T shall be provided. The level split and dip/rise galleries shall be kept supported by a set of ISMB girders of 150 mm x 150 mm mounted on 40 T hydraulic props at either end. The first set of cross girders shall be erected close to the goaf edges, but in no case less than 0.7 m. The distance between two such sets along the gallery shall not exceed 1.00 m/1.5 m depending on the roof conditions. The roof between the bars shall be suitably lagged by wooden sleepers. Entire roof is supported for a distance of 40 m. from the face by 250 mm x 250 mm IBMS steel bars placed on 2 No. of 40 T capacity open circuit hydraulic props. The interval between the bars being 1.00 m / 1.5 m. All junctions are supported by 6 Nos of 5.5 m long roof bars which are held in position by means of 2 Nos of 5.5 m roof bars at either end set over a cluster of 4 Nos of hydraulic props at each corner. In between the roof bars, the roof is further supported by 3 Nos of grouted wooden/bamboo roof bolts. As a precaution against the dislodging of hydraulic props accidentally by moving machinery, the hydraulic props are tied together to roof bars by means of flexible steel wires. All junctions are supported by 6 Nos of 5.5 m long roof bars which are held in position by means of 2 Nos of 5.5 m roof bars at either end set over a cluster of 4 Nos of hydraulic props at each corner. In between the roof bars, the roof is further supported by 3 Nos of grouted wooden/bamboo roof bolts. As a precaution against the dislodging of hydraulic props accidentally by moving machinery, the hydraulic props are tied together to roof bars by means of flexible steel wires. 5.1 INDUCED BLASTING When the method of extraction is to remove all the coal or as much of coal practicable and to allow the roof to cave in, the operations shall be conducted in such a way as to leave as small an area of uncollapsed roof as possible. Where possible suitable means shall be adopted to bring the goaf at regular intervals. In blasting gallery method of extraction, induced blasting has become an integral part of mining operation during depillaring of developed coal pillars. When the method of extraction is to remove all the coal or as much of coal practicable and to allow the roof to cave in, the operations shall be conducted in such a way as to leave as small an area of uncollapsed roof as possible. Where possible suitable means shall be adopted to bring the goaf at regular intervals. In blasting gallery method of extraction, induced blasting has become an integral part of mining operation during depillaring of developed coal pillars. Page No:397

6 5.2 INDUCED BLASTING IN BG PANELS In Blasting Gallery method of winning coal, usually more area of goaf is exposed after extraction of coal. If the roof does not cave by its own weight, induced blasting is generally carried out at regular interval when the roof span is about sq. mtrs. As per the DGMS strictures, explosive charge per hole is limited to 1.0 kg only. However, it is found practically difficult to conduct successful induced blasting with such limited charge. 5.3 CHARGING OF HOLES Generally, 6m long plastic pipes, 36 mm and 38 mm inner and outer diameters respectively are used for charging of holes. Where plastic pipes are not available, charging arrangement is done by cutting the blastic spacers and then making a slot where explosives are inserted and tied together with cello tape as practiced in Chora10 pit colliery of ECL. Approximately, kg of explosive is charged in each ole depending on the site condition and actual hole length. Remaining portion of the hole is stemmed with bamboo spacers. All the holes are fired in one round with miscellaneous detonators. Before firing five rows of girders are reinforced with extra props. During blasting all related measurements like gas survey, vibration, convergence and strata loads are monitored for safe and useful implementation of design and charge loading parameters. CONCLUSION Induced caving by blasting becomes more important especially, in case of winning of coal by BG method when the overhanging of roof in the goaf area is difficult to cave by it s own weight. Though, the whole operation is nonproductive work, it ensures safety to the persons and the u/g working environment. Strata monitoring through measurements of roof convergence, strata load and ground vibrations during enduced blasting are very essential for succesful implementation of induce blasting. REFERENCES 1. Banerjee S.P. Mining coal from deep horizons, the technical, economical and environmental Considerations, International Mining Engineering Journal, Volume 45(2006): pp Jayanthu S. Strata behavior observations in depillaring experimental panels vis - à- vis applicability of convergence date for working of goaf falls, International Symposium on Advances in Mining Technology and Management, (2005): pp Ray A.K. Cavability assessment of roof rocks to understand interaction of strata and support at long wall face, Minetech, Volume 26(2005): pp Satyanarayana I. Strata monitoring and analysis of blasting gallery panels - a case study, International Symposium on Advances in Mining Technology and Management, November December (2005): pp Venkatanarayana G. Measures to control/delay spontaneous heating in BG panels, JMMF, Volume 52, No. 122 (2004): pp Page No:398