Because of burning of coal due to spontaneous combustion, temperatures are intense in the surrounding overburden strata and in the coal seams. Under such elevated temperature condition, it becomes more difficult to drill and charge the overburden above the burning coal as well as in the coal benches which are hot. Basically, breaking rock by explosives blasting is a hazardous job. Safety is of prime consideration while blasting and the charging blast-holes in hot strata conditions, which are to be dealt with great safety and precautions.
Charging and Blasting in Hot Strata condition Charging and Blasting in Hot Strata Condition in Opencast Coal Mines Most Crucial aspect is implementation of Effective Safety Management Author: Partha Das Sharma (B.Tech-Hons. In Mining Engg.) E.mail - email@example.com and firstname.lastname@example.org . Abstract There have been a number of incidents in the past occurred across the mining industry which have been associated with drilling and blasting in hot strata condition. Higher strata temperature affect bulk, packaged and initiating products over time, expedite the rate of degradation and potential failure which may lead to a disaster. Safety is of prime consideration while blasting and the charging blast-holes in hot strata conditions, which are to be dealt with great safety and precautions. The most important factors are the type of explosives used, the practices and procedures followed while charging and blasting of hot holes. A rigorous and fully tested set of procedures are applied for each blast, aimed at limiting the risk of an accidental detonation. Paper discusses points which should be taken care of while framing safety codes to deal with blasting in hot strata condition, also the steps undertaken to carryout charging and blasting in hot strata blast-hole in opencast coal mines. 1.0. Introduction –The increasing demand for coal everywhere for power production, has resulted in a number of old underground coal mines being re-worked by opencast coal mining methods, to mine the remaining coal reserve left in the old underground pillars and the lower-grade coal that often exists immediately above the pillars. When old coal workings are exposed to the atmosphere and water for prolonged time during the opencast mining phase, spontaneous combustion of the coal in the old workings occurs and spread into overburden rocks and strata as well. Spontaneous heating in underground coal mines involves the oxidation of coal deposits. All coals oxidize to some extent when exposed to the atmosphere. Since oxidation is an exothermic reaction between coal and the oxygen component of the atmosphere, heat is constantly being released. This reaction is directly related to temperature; if the heat released by the oxidation reaction is not dissipated, the temperature of the mass increases. In some reactive coals this oxidation can increase to the point that, if remedial control measures are not instituted, the heating can continue and the temperature will rise at an increasing rate until smoldering combustion occurs. Because of burning of coal due to spontaneous combustion, temperatures are intense in the surrounding overburden strata and in the coal seams. Under such elevated temperature condition, it becomes more difficult to drill and charge the overburden above the burning coal as well as in the coal benches. Basically, breaking rock by explosives blasting is a hazardous job. Safety is of prime consideration while blasting and the charging blast-holes in hot strata conditions, which are to be dealt with great safety and Author: Partha Das Sharma (E.mail: email@example.com) 1 Charging and Blasting in Hot Strata condition precautions. The most important factors are the type of explosives used, the practices and procedures followed while charging and blasting of hot holes. A rigorous and fully tested set of procedures are applied for each blast, aimed at limiting the risk of an accidental detonation. 2.0. Hazards associated with higher ground temperature - There have been a number of incidents in the past occurred across the mining industry which have been associated with drilling and blasting in hot strata condition. The aim of the safety codes to provide enhanced safety while handling, charging and blasting of blast-holes in hot and elevated temperature strata condition. Higher strata temperature affect bulk, packaged and initiating products over time, expedite the rate of degradation and potential failure which may lead to a disaster. Some of the major hazards associated with higher ground temperature may be: (i) Exposure of operators to high temperatures, (ii) Ignition of vapours associated with emulsion and ANFO type products, (iii) Softening of plastic components of initiating products, (iv) Melting and decomposition of bulk, packaged and initiating products, (v) Pre-mature detonation following decomposition. The main problem associate with opencast mining of fiery coal seam is drilling in hot strata, selection of suitable explosives and initiating system, charging or loading of blastholes and thereafter blasting. It is a known fact that the elements in a blast-hole that are most sensitive to temperatures above about 75 ˚C are the initiating devices, such as detonator and booster in the primer. As a general guide, the temperatures at which some common initiating explosives melt and start to be become unstable are given in the following table. Explosives HMX HNS Pentolite PETN RDX TACOT (Thermally stable high explosive, tetranitro-2,3,5,6-dibenzo-l,3a,4,6atetraazapentalene - TACOT) TATB (triamino-trinitrobenzene) Melting point 275 ˚C 318 ˚C 70 ˚C 141.3 ˚C 204 ˚C 378 ˚C 350 ˚C Thus, for blasting in hot holes, conventional shock tube and detonators are not used down-the-hole, as unexpected detonations would become a risk. 3.0. Safety Management to deal with hot and elevated temperature blast-holes (Framing of Safety codes) – In order to make the entire blasting operation safer, following points should be taken care of while framing safety codes to deal with hot strata and burning coal: Author: Partha Das Sharma (E.mail: firstname.lastname@example.org) 2 Charging and Blasting in Hot Strata condition 3.1. The nature of condition of strata which is in elevated and hot temperature is to be studied before commencing of charging and blasting operation. An outline is to be drawn regarding nature of existing temperature higher than the ambient, and also to study the reactivity of the ground, if any, in elevated temperature condition. 3.2. Estimate the hazards and risk associated with the explosives charging and blasting in the elevated temperature strata condition and also of reactive strata. Continuous evaluation is to be done of the risk assessment methods adopted. Due to prevalence of the spontaneous combustion of overburden or waste rock/ore either in dumps or in the pit, especially as it is exposed to the air; there is high degree of risk of premature blast, necessitates frequent intermittent evaluation of temperature. 3.3. Guidelines for risk management operations are to be formulated to handle explosives in elevated temperature strata conditions. Operating procedures are to be framed for explosives handling in the areas where hot holes are to be charged and blasted. These operating procedures should include: 3.3.1. Sleep time of explosives within holes – Sleep time is the time that an explosive is left in a blast hole until it is fired. Sleep time of explosives within hot-holes is to be kept minimum to avoid unnecessary heating up of explosives and initiating materials – which may result in product deterioration and premature detonation. 3.3.2. Hole-temperature monitoring – Frequent monitoring of hole-temperature is the paramount factor in the process of risk management for handling of explosives for hothole blasting. It allows hole to hole classification of temperature; as risk management varies with temperature. Selection of temperature measuring device with suitable temperature range is another important area to choose from. 3.3.3. Hole loading sequence – It is preferred that explosives charging may be started near the initiation point first, (i.e., load explosives in the sequence in which the blast will be fired first). This allows the pattern to be quickly charged, tied up and fired in the event of a change in conditions. Where possible, hottest holes to be loaded last. 3.3.4. Delineation of blasting zone - Delineation of blasting zone to be done as per temperature of hole, such as elevated temperature and non-elevated temperature zone. 3.3.5. Selection of explosives and initiation system – Effective selection of explosives and initiating systems are the better way of addressing risk management of hotter holes. It has been observed that, Bulk Emulsion explosives with Detonating cord suits hot-hole condition. 3.3.6. Minimising of spillages of explosives - While charging of blast holes there is a possibility that spilt explosives may react where ground at a higher temperature. To limit the adverse effect, procedures for minimising such spillages need to be addressed and where possible spillages to be cleaned up immediately. The procedures should also Author: Partha Das Sharma (E.mail: email@example.com) 3 Charging and Blasting in Hot Strata condition ensure that initiating systems do not come into contact with the spilt explosives as the possible decomposition of the explosive may cause the initiation system to detonate. 3.3.7. Occupational Health and Safety Considerations – Working in higher ground temperature condition may lead to exposure of personnel to scalding from hot water and steam. Sometime, toxic gases such as carbon monoxide (CO), carbon dioxide (CO2), sulpher dioxide (SO2), oxides of nitrogen (NOx), hydrogen sulphide (H2S) etc., may also be produced under the higher temperature conditions. Unburnt hydrocarbons in presence of NOx and other photosensitive oxidants cause eye irritation due to formation of smog like condition. 3.3.8. Training of personnel – Proper training of drilling and blasting crew for handling of explosives, charging and blasting at hot-hole zone and about the standard procedures are to be given. Lesson on hazards associated with the handling, charging and blasting of explosives in elevated temperature ground conditions and emergency procedure to be applied specially when explosives exhibit violent reaction under hot condition. Various case studies are to be discussed with the operating personnel. Procedure for handling misfire charges also to be discussed. 3.4. As per the situation the risk management techniques are to be reviewed and updated regularly. 3.5. Framing of guidelines and safe procedures for handling any misfire or premature detonations arising out, while charging and blasting of hot blast-holes. 4.0. Discussion on charging and blasting of hot blast-holes – As discussed, for blasting in hot holes, conventional shock tube and detonators are not used down-the-hole, as unexpected detonations would become a risk. Generally, the problem of higher blast-hole temperature is tackled by quenching with water. Proper quenching methods are to be applied to keep the temperature within 80 ˚C (as per DGMS criteria). It has been observed that, flushing hot-holes with a mixture of water, bentonite, sodium silicate and gaur gum solution help retain water to seal microfractures and cracks and bring the temperature down relatively easier. Holes with higher temperature should be identified and quenching of those hot-holes should be initiated atleast 12 hours prior to charging and blasting in order to lower down the temperature. Temperature of blast-holes is monitored and recorded soon after completion of drilling, after quenching with the help of water and just before charging of explosives. Digital temperature indicator based sensor should be used for temperature recording. Increase in temperature of charged holes, if any, also are recorded till final firing. Besides, thermal behaviour of the explosives and initiating devices to be used for hothole blasting should be tested in a laboratory in field-simulated condition, prior to any such blast. It has been observed, slurry, emulsion composition including site-mixed bulk Author: Partha Das Sharma (E.mail: firstname.lastname@example.org) 4 Charging and Blasting in Hot Strata condition explosives and detonating fuse, generally used for mining purpose, withstand hot-hole condition upto 120 ˚C for about two hours without deterioration. Punctured holes are to be plugged at bottom before charging. Air-bags may be used for the purpose of plugging hole at the bottom. It is a good practice sufficient noncombustible stemming material such as sand, crushed stone chips or drill cutting should be available near the collar of each hole prior to commencing of charging operation, in order to fast accomplishing stemming and charging operation. In some of the mines in South Africa for fiery seam blasting, the blasting engineers apply Bulk-Loading Emulsion explosives, Detonating fuse with ‘Top-Priming’ of Booster. These primers are applied shortly before blasting time, at the top of the explosives charge, where the emulsion is relatively cooler. After primers are put, stemming, tie-up of detonating fuse and firing are done as quickly as possible, without wasting any time. 5. DGMS (India) criteria for charging and blasting in hot blast-holes (Cir. Tech. 2/1985 & 2/1990) – It is recommended that while blasting in hot strata (either in OB or coal) the following precautionary measures should be adopted: 1. (a) No explosive other than slurry and emulsion explosives shall be used. (b) Blasting shall be done with detonating fuse down-the-hole. 2. Temperature inside the blast holes shall be measured (before filling with water) and if the temperature exceeds 80 ˚C, in any hole, shall not be charged. Records of measurements of temperature in each hole shall be maintained. 3. All blast holes shall be kept filled with water. When any hole is traversed by cracks or fissures, such hole shall not be charged unless it is lined with an asbestos pipe and the hole filled with water. In addition, bentonite should be used for sealing any cracks at the bottom of holes. 4. Detonating fuse shall not be laid on hot ground without taking suitable precautions which will prevent it from coming in contact with hot strata. 5. The Charging and firing of the holes in any one round shall be completed expeditiously and in any case within 2 hours. 6. Blasting operations shall be carried out under the direct supervision of an assistant manager. 6. Summary of the procedures discussed above for blasting in hot hole - General guideline of blasting in hot holes are summarised below: * Select the number of holes properly so that the total blasting operation should not exceed 2 hour from charging of first hole. * Measure the temperature of the holes almost constantly till the commencement of blasting operation. * Use water at least 12 hour before blasting to flush hot holes till the temperature comes down below 80oC. * Record the temperature of holes at a regular interval of time. Author: Partha Das Sharma (E.mail: email@example.com) 5 Charging and Blasting in Hot Strata condition * Use a mixture of Bentonite, Sodium Silicate and Water in holes which do not retain water to seal micro-fractures and cracks. Guar gum up 5 percent may be also be used for the same purpose. * Only slurry or emulsion explosives, preferably bulk explosives to be used for hot-hole blasting purpose. * Detonating fuse as initiation system only should be used. Shock tube and detonators should not be used down-the-hole. * Adequate non-combustible stemming material should be available near the collar of each hole prior to commencing of charging operation, for fast accomplishing charging operation. * Punctured holes are to be plugged at bottom before charging. Air-bags may be used for the purpose * Combination of Bulk-Loading Emulsion explosives, Detonating fuse with ‘TopPriming’ of Booster is preferred. These primers are applied shortly before blasting time, at the top of the explosives charge, where the emulsion is relatively cooler. After primers are put, stemming and blasting are done as quickly as possible, without wasting any time. References: 1. Classified DGMS (India) Circulars. 2. Blasting principles for open pit mining, By William A. Hustrulid 3. Code of Practice Elevated Temperature and Reactive Ground - Australian Explosives Industry And Safety Group Inc. (http://www.dmp.wa.gov.au/documents/Misc/code_elevated_temp_r.pdf ) 4. http://www.samining.co.za/article_august2008_1.asp 5. Improved blasting technology for coal applications - http://www.miningweekly.com/article/improvedblasting-technology-for-coal-applications-2008-09-05) 6. International Society of Explosives Engineers, 1998, Blasters’ Handbook. 7. Tripathy, D.P., Singh. Gurdeep and Panigrahi, D.C., “Environmental impacts of mine fires – An overview:(http://www.ismenvis.nic.in/My_Webs/Digital_Library/GSingh/Environmental%20Impacts%20o f%20%20Mine%20Fires%20-%20An%20overview.pdf) ----------------------------------------------------------------------------------------------------------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://miningandblasting.wordpress.com/ • http://saferenvironment.wordpress.com • http://www.environmentengineering.blogspot.com • www.coalandfuel.blogspot.com Author can be contacted at E-mail: firstname.lastname@example.org, email@example.com, *** Author: Partha Das Sharma (E.mail: firstname.lastname@example.org) 6
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