Explosives and blasting in underground coal mining

Explosives and blasting in underground coal mining

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  EXPLOSIVES AND BLASTING IN UNDERGROUNDCOAL MINING M.S.S.S.SASHANK 113MN0482
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  INTRODUCTION All seams inunderground coal mines are considered gassy, the degree of gassiness may vary from I to III. Methane gas that is released from during excavation when mixed in the proportions between 5-14% forms an explosive mixture. In underground coal mines, there is also an additional risk as coal dust which undergoes a dust explosion when suitably ignited with air.
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  Methane ignition isknown to take place due to following reasons: Direct action of shock wave from the explosive.  Indirect action of the shock wave after it has been reflected from solid surfaces in the vicinity of explosives. Hot gaseous products of detonation mixing with the methane-air mixture. Hot reacting solid particles of explosives getting ejected in to the methane-air mixture.
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  Hence , speciallydesigned explosives and detonators are required for use in underground coal mines. PERMITTED EXPLOSIVES Only those explosives, which are included in the official list of authorised explosives issued by the Chief Controller of Explosives in India, may be used for blasting in mines. Only on passing the tests in an explosive approved as a “permitted” explosive. In India there are 3 types of permitted explosives, each group being designed for a particular type of operation to give maximum safety.
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  P-1Type Permitted Explosives:Theseexplosives can be used for drifting or ripping and simultaneously firing in an undercut/middlecut/overcut and in depillaring faces, in coal seams of first degree of gassiness. P-3Type Permitted Explosives:These are Equivalent-to- Sheathed Explosives used for drifting or ripping and simultaneously firing an undercut/middlecut/overcut and in depillaring faces, in coal seams of first degree of gassiness. P-5Type Permitted Explosives: These are special type of explosives with a higher degree of intrinsic safety against all types of likely hazards in delay blasting in coal and specially designed for Solid Blasting in conjunction with non- incendive copper short delay detonators.
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  NON PERMITTED EXPLOSIVES Forcertain operations such as shaft sinking and stone drifting, where there is no gas or coal dust present, explosives other than permitted explosives can be used, subject to such conditions as may be laid down by the DGMS in specific instances.
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  DETONATORS In conjunction withPermitted Explosives, only approved detonators may be used for coal blasting. Only electric detonators with copper tube are permitted for use in coal blasting. There are 2 types of detonators: => Instantaneous Copper Electric Detonators(CED) => Copper Non-Incendive Short Delay Detonators(CDD)
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  PRINCIPLES OF BLASTINGIN COAL Underground coal mining operations could be divided into two categories , namely, Development and Depillaring. Both for bord -and-pillar as well as for longwall mining, driving galleries through coal seams are done. there are two techniques employed for blasting in development galleries. =>Blasting on pre-cut face =>Solid blasting of coal
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  BLASTING ON APRE-CUT FACE In many mines coal-cutting machines are used to provide an additional free face for blasting.While this may be under , over, middle or side cut, the most common practise in Indian mines is to have undercut face. For blasting on cut faces the following conditions must be compiled with: The length of the shot hole should be at least 15cm (6 inches) less than the depth of the cut. The detonators in a round should be fired simultaneously , preferably in series. The maximum charge in hole should not exceed 800gms for PI explosives and 1000gms for P3 and P5 explosives.
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  SOLID BLASTING OFCOAL(BOS) Both in Longwall and development faces coal can beblasted without giving an undercut by a coal cutting machine. The technique of blasting of the solid is simpler, more economical and less hazardous than conventional method of breaking coal with machine cut and blasting. Only P5 Permitted Explosives (Solarcoal-5) and ‘Non- incendive” copper short delay detonators shall be used.
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  CODE OF GOODPRACTISES IN UNDERGROUND BLASTING Velocities of Air Current (DGMS stipulations): The velocity of air current measured in metres per minutes at the place shown in column (2) shall not be less than that shown in column(3) for the different seams shown in column(1) of the table
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  Degree of Gassiness Place wherethe velocity of air is to be measured Velocity of air First, Second or Third Degree Intermediate out by ventilation connection from the face 30 First, second degree (i)4.5 mts from any face whether working or discontinued 30 on the take side of the brattice or partition (ii)7.5 mts out by the discharge end of an air pipe 15 (iii) At the maximum span of a Longwall face 60 Third Degree (i)4.5 mts from any face whether working or discontinued 45 on the intake side of the brattice or partition (ii)7.5 mts out by the discharge end of an air pipe 25 (iii)At the maximum span of a Longwall face 75
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