Explosives and Blasting in UG Coal Mines

Explosives & Blasting in Underground Coal Mines Author: Partha Das Sharma (E.mail: [email protected])

Explosives and Blasting in Underground Coal Mines Author: Partha Das Sharma (E.Mail: [email protected])

Introduction All seams in underground coal mines, now a days, are considered gassy, the degree of gassiness may vary from I to III. Methane gas present in the originally formed coal seam gets released in to the mine atmosphere during excavation of coal. Methane when mixed with air in the proportions between 5-14 % forms an explosive mixture. Hence, there is possibility of a Methane-air mixture being dangerously formed in the galleries and working areas. In underground coal mines, there is also an additional risk as coal dust, when mixed with air, can undergo a dust explosion when suitably ignited. Both methane-air and coal dust explosions cause loss of life and extensive damage to the underground mines. Methane ignition is known to take place due to the following reasons when explosive is in use: a. Direct action of shock wave from the explosive. b. Indirect action of the shock wave after it has been reflected from solid surfaces in the vicinity of explosives. c. Hot gaseous products of detonation mixing with the methane-air mixture. d. Hot reacting solid particles of explosives getting ejected in to the methane-air mixture. Hence, specially designed explosives and detonators are required for use in underground coal mines. Such products come under the ‘Permitted’ category and have to pass critical tests in simulated conditions of field usage. Explosives and detonators are tested in both methane-air and coal dust-air mixtures before being certified fit for use in underground coal mines/seams of various degree of gassiness. Thus these require special properties, mainly a high degree of safety against methane-air and coal dust-air mixture ignition. Hence, they should have reduced flame temperature and duration of flame, have good continuity of detonation properties, maintain its sensitivity and ensure reliable propagation even after being subjected to the compressive shock wave generated by the earlier fired shot holes and be of nondeflagrating type. Explosives should also be water proof, generate low level of toxic post - blast fumes and be relatively insensitive to impact and friction. Risk of ignition of methane-air mixture due to blown out shots is inherent danger during blasting in underground coal mines. Blown out shots occur due to inadequate and proper stemming, excessive burden or spacing between shot holes, improper use of delay detonators and incorrect sequence of firing of shot holes. Deflagrating of explosives is more prone for risk in underground coal mines, since they burn for longer period comparative to explosive which detonate. High strength explosive readily ignites methane-air mixture in view of their high reaction temperatures. Weak explosives, inspite of having higher velocity of detonation are found to be relatively safer (present day Permitted explosives are relatively weaker), have low detonation temperature and do not contain any ingredient which continues to burn after the detonation reaction is completed.

Permitted Explosives

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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. Chemical and physical stability and reliable performance are the pre-requisites for inclusion in this list. Further testing and approval of the Director General of Mines Safety is required for an authorized explosive to be classified as a permitted explosive suitable for use in underground coal mines. Explosive to be used in coal seams must be safe for use in methane-air mixtures or coal-dust mixtures encountered in underground coal mines. To meet these requirements stipulated official tests are carried out to assess the safety of the explosives under these conditions. Only on passing these 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.

P-1 Type Permitted Explosives: These explosives can be used for drifting or ripping and simultaneous firing in an undercut / middlecut / overcut and in depillaring faces, in coal seams of first degree of gassiness.

P-3 Type Permitted Explosives: These are Equivalent-to-Sheathed Explosives (which have replaced the earlier Group P-2 Sheathed Permitted Explosives) used for drifting or ripping and simultaneous firing in an undercut / middlecut / overcut and in depillaring faces, in all coal seams.

P-5 Type Permitted Explosives: These are special type of explosives with a high 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. All Indian coal seams have been classified according to their degree of gassiness. This classification of coal seams and the explosives that can be used in different seams are summarized below. Degree of Gassiness of Coal seam I

II

III

Classification of Gassiness

< 0.1% of gas in the general body of air and rate of emission of such gas is less than 1 cu. m/t of coal production > 0.1% of gas in the general body of air and rate of emission of such gas is greater than but less than 10 cu. m/t of coal production Rate of emission of the gas is greater than 10 cu. m/t pf coal production

Type of Permitted Explosives P1 / P3 / P5

P3 / P5

P3 / P5

DGMS (India) Stipulations on Maximum Permissible Charge in a Shot Hole.

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Types of Explosives

Degree of gassiness / Type of Application

Max Permissible charge per Shot hole (gms)

P1

Degree 1 mines, Cut face, Depillaring

800

P3

Degree I, II & III mines, Cut face, Depillaring

1000

P5

Degree I-‘BOS’,

1000

P5

Degree II &III-‘BOS’

565

Non Permitted Explosives For certain 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.

Detonators In conjunction with Permitted explosives, only approved detonators may be used for coal blasting. Only electric detonators with copper tube are permitted for use in coal blasting. These are of two types (both of N0.6 strength).

1.Instantaneous Copper Electric Detonators (CED): These are Instantaneous Electric Detonators with a copper shell. P1 and P3 Explosives can only be used in conjunction with CEDs. 2. Copper Non-Incendive Short Delay Detonators (CDD): These are copper tube detonators, used for delay firing when solid blasting in coal is done. These are used along with P5 explosives. These are specially designed to meet the stringent safety requirements for solid blasting in coal, for which they have to undergo statutory tests at CMRI as stipulated by DGMS. The nominal delay of 25ms between two successive delay intervals (from 0 to6) is available to the maximum delay period of 150ms.

Principles of blasting in coal 3


From the point of view of blasting, underground coal mining operations could be divided into two categories, namely, Development and Depillaring. The techniques of blasting in each case are widely different. 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. a. Blasting on pre-cut face. b. Solid blasting of coal.

Blasting on a pre-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 practice in Indian mines is to have undercut face. For blasting on cut faces the fallowing conditions must be complied with: • • •

The length of the shot hole should be at least 15cm (6 inchs) 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 P1 explosives and 1000gms for P3 and P5 explosives.

While blasting on cut face, the following guidelines should be borne in mind for best blasting results:

•

•

•

Depth of cut: The jib should be properly ‘sumped in’, and it must be ensured that cut is of uniform depth. It should also be ensured that the cutting is done to the maximum possible depth to get the best advantage out of cut face blasting. Generally, the cut depths vary from 1.2 to 1.8 m. Cleaning of the cut: The purpose of cut is to provide an additional free face for the blast. The maximum advantage of the cut can be taken only if it is properly cleaned and all the gummings are removed. Depth and placement of shot holes: As mentioned the depth of shot holes should always be at least 15cms less than the cut depth. Holes should not be too short, otherwise un blasted coal would be left hanging. Moreover, the burden on any hole should always be less than the depth of shot hole, preferably the burden should be kept 2/3 the depth of shot hole for getting optimum results.

Some of the optimum drilling and blasting patterns with undercut are given below:

Thumb rules for u/g coal blasting in Cut Faces : 1. 2. 3. 4. 5.

No. of holes For each 1 square meter area = 1 hole Depth of hole 15cm less than cut length Charge per hole 40% of hole length Stemming 60% of hole length Explosive P1 type (Solarcoal-1) for degree 1 mines P3 type (Solarcoal-3) for degree 2/3 mines

A typical round of hole used in blasting an under cut face is shown below:

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(Numbers shown sequence of firing rounds) Depth of Hole 1.5 Meters No. of Holes 8 Charge / Holes300 g Total yield of Coal 14 tones Yield per kg 6.00 tones

Solid blasting of coal (BOS) Both in Longwall and development faces coal can be blasted without giving an under cut 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. The following points are to be remembered while doing Solid Blasting in coal: (a) To create an initial cut, holes need to be drilled in inclined position so as to provide direction and force for coal to move outwardly, to create free face for other holes. (Lever action theory) (b) Wider the opening (i.e. Free face) easier it would be for other holes to perform better. It is advisable to give zero delay for 4 to 6 cut holes in the centre. (c) Considering Beam theory, hole length beyond 70% of seam height would not bring expected result. Rather long sockets will be left and explosive energy will be wasted and P.F. will be reduced. (d) Stemming less than 60% of hole length or un proper packing may not be able to sustain the volume / pressure of gases generated during blasting and explosive energy will be wasted in form of a blown out shot. (e) Cut holes should be charged more as compared to other holes. (f) Initiation sequence should be maintained so as that cut holes give way first, nearest hole next and farthest holes last. The additional technical points are also to be adopted for solid blasting in coal

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• • • • • • • •

Only P5 Permitted Explosives (Solarcoal - 5) and 'Non-incendive' copper short delay detonators shall be used. Maximum charge per hole is 1000 gms for Degree I mine and 565 gms for Degree II & Degree III. The estimated period of Delay between the first and last shots shall not exceed 150 ms (100 ms for Degree III Mines). The estimated period of delay between any two consecutive delay shots shall not exceed 60ms. The maximum distance between the two adjacent shot holes of different delays shall not be less than 0.6m. Atleast 284 cu.m of air per minute shall be conducted up to every face where solid blasting is being done. Test for inflammable gas should be made within the radius of 20m of the blast. More than 25 shots in one round should not be blasted.

Recommended Pattern for Seam Height 2.6 m Face Size 4.6 x 2.6 Total Holes = 15 Type of Number Depth of Explosive / hole Delay Holes Holes Number Cut 6 1.7 m (5.6) 3 555 Z Holes Other 6 1.6 m (5') 3 555 I Holes 3 1.6 m (5') 3 555 II Total Explosives Expected Pull Expected Coal Powder Factor per Round (Insitu) T/Kg. 8.32 Kg. 1.27 m (4') 19.74 M. T. 2.37 Remark 1. 2-4 holes to be increased if encountered with shale /stone band. 2. Full packaging with sand clay cartridges as stemming gives 10% better

Stemming 60-65% 60% 60% Detonator Factor 1.31 results.

In stables and development headings the technique of solid blasting is done by creating initial opening either by (a) Wedge Cut Pattern, or (b) Fan Cut Pattern. In a 'Wedge Cut', two or more holes are drilled so as to converge at the rear by not less than 0.3m. This arrangement provides a concentration of charge at the back. With this pattern, the

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advance is limited. For deep pulls burden should be reduced by providing steeply inclined hole at the centre, known as 'Stab Hole' or by giving another shallow wedge cut known as 'Baby Cut'. This relieving hole should be fired prior to the main wedge cut pattern. Following some of the examples given :

No. of Holes = 14+1stab hole = 15 (Total), size of gallery = 4.3m x 2.4m Delay Number Charge (Cartridge) Explosive (gm ) 0 1 2 3 4 5

1 2 2 4 4 2

3 3 3 2 2 2

555 555 x 2 555 x 2 370 x 4 370 x 4 370 x 2

Explosives Charged (gm) 555 1110 1110 1480 1480 740

Total Charge :- 6.5 kg, Av. Pull :- 1.35m to 1.45m Blasting Ratio (PF) :- Av. 2.6 t/kg Detonator Factor :- 1.1 to 1.2 t

In Fan Cut Pattern, the initial opening is created by firing a hole drilled at an acute angle to the face. The inclination generally varies from 30 to 60 degree. The inclination of subsequent hole is gradually reduced. This pattern suitable for medium hard to hard coal seams. Some illustrations are given below :

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No. of Holes :- 14, Delay 0 1 2 3 4 5 6

Size of gallery = 4.0m x 2.40m Number Charge (Cartridge) 1 1 2 2 2 4 2

Total Charge : - 6.25kg to 6.50kg Blasting Ratio (PF) :- 2.7t/kg (Av.)

3 3 3 3 2 2 2

Explosive (gm ) 555 x 1 555 x 1 555 x 2 555 x 2 370 x 2 370 x 4 370 x 2

Explosives Charged (gm) 555 555 1110 1110 740 1480 740

Av. Pull :- 1.35m to 1.45m Detonator Factor :- 1.2 to 1.4 t

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In Long wall mining the technique of solid blasting (BOS) offer substantial overall cost advantages and higher out put and productivity as compare to use of coal-cutter in stables as this technique eliminates pre-shearing. More over, it encourages additional advantages like low capital investment, saving in man power, flexibility in deployment of drill machines, lesser stores inventory, better maintenance of Long wall face in greater gradient and uneven/disturbed seam and avoidance of frequent breakdown of coal cutting machines. A typical lay out of shot hole in long wall face is shown below.

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Code of good practices in underground blasting A. In cut faces 1. Before drilling in cut faces, all duffs / gumming must be removed from cut. 2. The cut should be carefully examined and measured. 3. Each hole should then be drilled directly above or below the cut to terminate at aleast 15cms (6 inches) short of the cut depth. If holes are drilled too deep, it is likely that the charge will be located in coal, which is not undercut. 4. The correct drilling of each hole with respect to placement, depth and angle is essential for safe and efficient breaking. 5. It is important to relate the burden to mass of explosives in the hole; overburdening can result in either inadequate fracture or blown-out shot. Under burdening will result in excessive fragmentation of the material and dangerous flying debris. Both under-and over burdening are more likely to cause an ignition.

B. Charging of shot hole 6. All drill cuttings should be removed from shot holes before charging, in order to avoid the chances of drill cuttings to remain between the cartridges, which may lead to misfires. It is recommended to place the complete charge, including primer, in the form of a ‘train’ in the mouth of the hole. This is then pushed home as a continuous charge.

C. Stemming 7. To reduce the possibility of a gas or coal dust explosion,(as per Regulations) the shot holes to be stemmed up to the collar. 8. Proper stemming material, preferably a mixture of sand and clay should be used. Effective stemming is important from a practical point view as it confines the gasses from explosion utilizing the full potential of the explosives. 9. Coal chips should not be used as stemming material, as it is a combustible material, shot may blown-out and also shot may be misfired due to short-circuit since coal chips may damage the insulation of Electric Detonator.

D. Deflagration 10. The detonator should be positioned at the toe of the hole, i.e. toe priming or inverse initiation should be adopted. This reduces the risk of deflagration and chances of blown-out. As far as possible, collar priming or direct initiation is to be avoided. 11. A distance of at least 0.6m and a maximum time interval of 60ms delay between any two adjacent shot holes should be maintained. 12. As mentioned earlier, cartridges should be loaded in such a manner to prevent air -gaps or drill cuttings coming between them. Careful loading of cartridges reduces the risk of deflagration.

E. Primers 13. The base of the detonators should point in the direction of the charge column. 14. It is important to attach the detonators lead wires correctly to prevent the detonator being pulled out of the cartridges. Care must be taken not to put strain in the lead wires where they enter the detonators.

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F. Electric Blasting & Circuit testing 15. Care must be taken when charging up and correct procedure of testing of circuit by an Ohmmeter to determine actual resistance should be adopted. In case of any fault correction to be done before firing. 16. As per the firing capacity of Exploder, the numbers of detonators are to be used in the circuit. Care should be taken to avoid having starvation of current to Electric detonators used. 17. Only approved type exploder to be used.

G. Explosives & Blasting 18. Only Permitted type explosives are to be used, as per DGMS regulations and circulars. 19. During blasting operations, proper shelter should be taken by all. 20. After the blast, only after clearing of fumes from the face persons should be allowed to enter. •

VELOCITIES OF AIR CURRENT (DGMS stipulations) : The Velocity of air current measured in meters 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 giving below :

Degree of Gassiness 1 First, Second or Third Degree

Place where velocity of air is to be measured 2 Immediate out bye ventilation connection form the face.

Velocity of air

First, Second Degree

(i) 4.5meters from any face whether working or discontinued on the in take side of the brattice or partition. (ii) 7.5 meters out bye of the discharge end of an air pipe. (iii) At the maximum span of a Longwall face.

30

(i) 4.5 meters from any face whether working or discontinued on the intake side of the brattice or partition. (ii) 7.5 meters out bye of the discharge end of an air pipe. (iii) At the maximum span of a long wall face.

45

Third Degree

3 30

15 60

25 75

[The DGMS stipulates that under no condition the presence of Carbon monoxide (CO) and oxides of Nitrogen (NOx) in blasting fumes should be more than 50 ppm and 5 ppm respectively with in a period of 5 minutes.] ------------------------------------------------------------------------------------------------------------------------

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Author’s Bio-data: Partha Das Sharma is Graduate (B.Tech – Hons.) in Mining Engineering from IIT, Kharagpur, India (1979) and was associated with number of mining and explosives organizations, namely MOIL, BALCO, Century Cement, Anil Chemicals, VBC Industries, Mah. Explosives etc., before joining the present organization, Solar Group of Explosives Industries at Nagpur (India), few years ago. Author has presented number of technical papers in many of the seminars and journals on varied topics like Overburden side casting by blasting, Blast induced Ground Vibration and its control, Tunnel blasting, Drilling & blasting in metalliferous underground mines, Controlled blasting techniques, Development of Non-primary explosive detonators (NPED), Signature hole blast analysis with Electronic detonator etc. Currently, author has following useful blogs on Web: • http://saferenvironment.wordpress.com • http://www.environmentengineering.blogspot.com • www.coalandfuel.blogspot.com Author can be contacted at E-mail: [email protected], [email protected], ---------------------------------------------------------------------------------------------------------Disclaimer: Views expressed in the article are solely of the author’s own and do not necessarily belong to any of the Company. ***

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Explosives and Blasting in UG Coal Mines

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