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Blasting techniques in underground metal mines

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Method of Mining in Underground Metalliferous mines mainly depend on type of Deposit, i.e., regular or irregular; Extent and depth of deposit, i.e., massive / pocket etc.; Dip and Thickness of deposit; nature of Hanging wall and Foot wall etc. Method of mining in Underground Metalliferous mines also depend on required production level to be achieved and accordingly type of mechanization adopted in relation to drilling of blast holes for stoping operation, for development operation as well as for handling and transport of mineral within and out of stope etc.

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									BLASTING TECHNIQUES IN HIGH PRODUCTION UNDERGROUND METAL MINES
By: Partha Das Sharma

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IMPROVED & ACCURATE BLASTING TECHNIQUES WITH MODERN PRECISION DRILLING - KEY TO ACHIEVE TECHNO-ECONOMICS OF HIGH PRODUCTION UNDERGROUND METAL MINES By: Partha Das Sharma, (E.mail: sharmapd1@gmail.com) Introduction: Method of Mining in Underground Metalliferous mines mainly depend on type of Deposit, i.e., regular or irregular; Extent and depth of deposit, i.e., massive / pocket etc.; Dip and Thickness of deposit; nature of Hanging wall and Foot wall etc. Method of mining in Underground Metalliferous mines also depend on required production level to be achieved and accordingly type of mechanization adopted in relation to drilling of blast holes for stoping operation, for development operation as well as for handling and transport of mineral within and out of stope etc. Of the stoping methods commonly used today for keeping high production level in Underground Metalliferous mines, vary from Sublevel and Shrinkage methods for shallow / medium depth deposit to Cut & fill methods with increasing depth. Successful efforts have also been made to extend the cheaper Open stoping methods specially Sublevel stoping; to as great a depth as possible by introducing controlled caving of walls. By implementing recent advances in blast hole drilling / blasting & stoping achieving maximum Techno-Economics in mining operation has been made possible. Sublevel stoping for high production: Sublevel stoping is one of the most important methods of choice for achieving high production rate in Underground Metalliferous mines. It is an overhand, vertical stoping, utilizing long hole drilling and blasting carried out from sublevels to break the ore. As compare to other stoping method this method uses less temporary support; no personnel including drilling and blasting crew are exposed in the stope proper; hence method is much safer. If support is needed in sublevel it is provided by rock bolt, wire meshes, cables etc. Although the stopes are unsupported, pillars are usually left between stopes and occasionally within stopes. The ore flows through the stope by gravity in the customary way and is drawn off at the haulage level. Application of this method in wide ore bodies with strong walls enable to adopt suitable mechanization in drilling & blasting, prevention of dilution and handling / transport of mineral within and out of stopes for obtaining high production rate in order to achieve effective Techno-Economics. Sublevel stoping with Long Blast Holes: Demand for increased rate of production coupled with improvement in drilling technique has lead to the use of long hole blasting for ore extraction from Underground Metalliferous mines. Considerable advances have been made by introduction of long-hole and precise drilling in stopes. Today, most of the sublevel stopes all over the world are drilled with long blast holes. This has called for radical changes in stope design as compare to conventional Sublevel stopes with increased sublevel intervals, better positioning of the sublevel in the ore body, improved method of ore handling from the stopes to the haulage level etc., for higher production. Further, by adopting improved & precise method of blasting with introduction of new

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generation Emulsion explosives and Electronic Detonators, for such Sublevel stopes the Techno-Economic level also maintained properly. It may however be noted that, large diameter holes (sometimes upto 100 mm or more) reduce drilling cost per tonne of ore broken. Moreover, blasting procedure with larger holes is safer, cheaper and less time consuming due smaller number of holes and easier charging and checking. With larger diameter of holes and by adopting improved & precise method of blasting with introduction of new generation Emulsion explosives the burden can be increased so effectively that parallel drilling can be done from sub-levels / crosscuts instead of benches. The pattern of long hole drilling can be classed under two major categories, i.e., Parallel hole drilling and Ring hole drilling. Parallel holes are preferred in many western mines for better fragmentation, easier setting up of drill rigs, greater effective drill footage and increased tonnage of broken rock per metre hole drilled as compared to Ring drilling. In many of the African Copper mines by adopting parallel holes as much as 30 percent cost reduction on explosives and drilling have been reported; which to a certain extent offset the extra cost incurred for slashing the sublevel from wall to wall. However, the greatest disadvantage with parallel hole drilling is its slowness due to extra time required for slashing the sublevel and lesser safety for drillers drilling on slashed out benches. Because of these disadvantages in some of the high production underground metalliferous mines Ring drilling is much more common than Parallel hole drilling. Fig – 1A, Fig – 1B, Fig – 2 and Fig – 3 give typical patterns of Ring and Parallel hole drilling. Rings may be drilled from a central sublevel in relatively narrow stopes (about 15 to 20 m wide) to form two or more parallel sublevels in wider stopes. It is a general practice to drill several rings of holes before blasting to make the drilling more continues process and the blasting can be restricted to week-ends only, when enough time can be allowed for clearance of post blast fumes and dust. The drills holes are made relatively smaller (40 to 50 mm dia.), bored with percussion rock drills mounted on a column and drifter with extension drill steel to a maximum length of 25 to 30 m. Hole deviation is a serious problem, however, deflection of several metres are not uncommon. The effect in blasting can be disastrous as ring drilling requires accuracy in hole placement to obtain proper fragmentation. The problem of hole deviation, now a days, have been overcome greatly because of introduction of sophisticated and accurate drilling machines; by which the deviation is bare minimum.

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Fig – 1(A)

Fig – 1(B)

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Fig – 2 As mentioned, with long hole drilling, blasting practice has advanced considerably with increasing use of stronger new generation emulsion explosives along with use of precise and accurate millisecond NONEL / Electronic delay detonators for obtaining better fragmentation during primary blasting. Judicious charging of holes in a ring using decks with suitable delay timing ensures better fragmentation, restricted ground vibration and reasonably better economy. In some of the mines where ring holes system is used, it is a common practice to charge every fifth hole to within one foot (approx. 0.5m) of the collar, every second and fourth holes to within twenty feet (approx. 6 to 7 m) and the third to within ten feet (approx. 3 to 3.5 m) of the collar. Lesser burden on rings with wider spacing of the toes of individual holes in ring accompanied by delaying the holes from centre outwards helps in good fragmentation. Where ever possible proper distribution of charge is made by using deck charges with in-hole electronic delay system; thereby fragmentation is improved and blast induced ground vibration level can also be reduced (Fig. – 7).

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Fig - 3 In most of the Indian condition holes are readily drilled to take advantage of cartridges 40 to 50 mm diameter. With 40 / 50 mm diameter cartridges burden of 2 to 2.5 m are blasted; with 32 mm cartridges burden is kept of the order of 1.25 to 1.8 m. The type of explosives used ranges from free flowing bulk loaded ANFO in dry hole condition to high strength Emulsion explosives in wet hole condition. The charge factor / explosives consumption in this type of blasting is generally kept of the order of 0.1 to 0.25 kg/tonne of blasted ore. It is a common practice to use short delay interval of 25 ms between holes in each row or ring starting from the easiest breaking section in the middle and progressing towards the walls. Under normal circumstances, it may be possible to charge holes extending to about 1.5 to 1.8 m (not more than the burden distance) of the collar without unduly overcharging, but in cases where the configuration of ore body is such that the holes are very closely spaced at the collar zone, it may be desirable to charge alternate holes only close to the collar and others upto 3 to 4.5 m from collar in order to reduce the charge factor (Fig – 4). The best pattern is decided by a series of actual trial blasts and it is always advisable to slightly overcharge the holes because a failed blast could mean considerable loss of production.

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Fig. – 4 Vertical Crater Retreat (VCR) – More advanced method of Sublevel stoping: With the advent of large diameter (100 mm to 200mm) rotary and down the hole percussion drills, it became practical to adopt modified version of large diameter parallel hole drilling and blasting for sublevel stoping. With this system hole deviation is no longer a problem (< 2%) with large, parallel holes; which can now be extended to about 90 m and accordingly sublevel spacing is increased considerably. The major innovation in Vertical Crater Retreat (VCR) method is of blasting. In this method, horizontal slice blasting of ore body with near spherical charges into the undercut is under taken. Spherical placement of explosives is the most efficient in terms of fragmentation and explosives consumption. Holes are charged from collar after plugging the opposite end. The size of charge is generally restricted to a Length-toDiameter ratio of 6:1; which suffices in the practice to simulate a spherical charge. All holes in the stope are blasted together. After the broken ore from the stope is drawn, the next slice of ore (generally about 4 to 5 m thickness in one slice) is charged and blasted ---------------------------------------------------------------------------Author: Partha Das Sharma, E.mail: sharmapd1@gmail.com

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as before. As mentioned, drilling is carried out from sublevel and is usually complete before slice blasting commences; the high level of efficiency and productivity is obtained with VCR method of sublevel stoping.

Fig. – 5 Rock mechanics, blasting and material handling considerations are the important points in deciding dimension of stopes, location of boundary pillars etc. In addition, because of the unique reliance on long hole drilling and blasting special attention in sublevel stoping must be paid to rock-breakage / blast design, hole diameter, length, burden, explosives selection, powder factor etc., to obtain desired Techno-economics of the mining system. Basting in Vertical Crater Retreat (VCR) sub-level stope: Blasting is done in stope worked by Vertical retreat method by drilling holes in slashed sublevel. The holes are drilled parallel, mostly either vertical or inclined along dip of the ore body. A few holes at both the ends of the rows near the walls are drilled inclined into the walls to prevent any ore to be left out in the vicinity of walls (Fig. – 6). Blasting is done in slices, after the holes are charged from collar by plugging at the opposite end. Extensive surveys are carried out before charging in order to know the exact extent of holes, their depth, inclination, end points etc., to ascertain correctly the placement of plugging at the bottom of holes. Delay sequence is used by using NONEL. Delays are so adjusted that, centre holes of middle rows are blasted first. Side holes are blasted subsequently.

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Fig. – 6 Drilling and blasting for Depillaring of various pillars – As the competent nature of rocks called for adoption of Sublevel open stoping, thus open stoping essentially results in leaving a network of mine pillars for maintenance of overall stability of the mine. In sublevel open stoping, as pressure abetments are quite high in the various pillars, these pillars encounter high level of lateral stresses and with prolonged working these pillars undergo continuous deterioration. One of the methods of relieving ground stresses is to mass blast these pillars regularly. Drilling activities in such high stress zone encounter following constraints. a) Failure of Drill drive and spalling from it. b) Failure or collapsing of drill holes. c) Development of extension fractures. Thus, in such a high stress zone, explosives charging into the hole and blasting thereof are important aspect, which very carefully and efficiently should be organized. With proper support spalling at the drilling site is checked. By use of non-cartridge explosives blasting operation in partially collapsed holes can be undertaken. Because of generation of extension cracks problem back-break become obvious, which require proper blast design to tackle. The adoption of following measures can effectively reduce chances of back-break. a) Use of deck charging b) As far as possible by reduction of charge density ---------------------------------------------------------------------------Author: Partha Das Sharma, E.mail: sharmapd1@gmail.com

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c) As far as possible by using de-coupled explosives. Deck charging is done to minimize the maximum charge per delay. In longer hole to restrict charge column separation of charges are done by giving deck (Fig – 7). Apart from reduction of chances of back-break, ground vibrations also are checked.

Fig. – 7 Discussion on modern precision Drilling and Electronic Delay Detonator system for enhancement of efficiency in Mines: Computerized drilling and electronic detonators add precision at the mines to lower downstream costs. Gaining a competitive edge in a mature and basic industry like underground and opencast mines / quarries, it has taken a major leap forward in enhancement of Techno-Economics advantages in working of mines. Drilling and Blasting is the most important activities in working of any mines. In particular, electronics are bringing a level of unprecedented precision to the process of ---------------------------------------------------------------------------Author: Partha Das Sharma, E.mail: sharmapd1@gmail.com

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rock breaking in the mines. Computerized drills, Measure-While-Drilling (MWD) systems, and electronic detonators are gaining wider acceptance in quarries, opencast and underground mines. Cost centers such as drilling, blasting, excavation, hauling and crushing are seen as interrelated variables in the total cost equation. As the first step in this interrelated process, improved results from drilling and blasting; even if it costs more; can significantly improve costs and productivity of downstream cost centers such as excavating, hauling, and crushing. In order for the advances in blasting technology to effectively reduce the mining industry’s cost equation, smart equipment and other innovative measurement technologies will need to be developed or advanced so that information can flow back into the process for continuous improvement. Conclusion – Therefore, with proper blast design and implementation of recent advances in blast hole drilling in open stoping methods of Sub-level inclusive of Vertical Retreat method gives a great deal of Techno-Economic advantages in order to achieve high production level. As far as possible, new techniques with improved systems are to be adopted in order to enhance over all efficiency of the mine workings.
References 1. Atlas Powder Company, Dallas, Texas, USA - “Explosive and Rock Blasting” 1987. 2. Blasters’ Handbook – E.I.du Pont de Nemours & Co. (Inc.), Wilmington, Delaware 19898. 3. Mishra, G.B. – “Recent trends in Mechanization and stope designing for higher production in underground metalliferous mines and their applicability to Indian condition”, JMMF, Proc., of symposium on Mechanisation of Mines in India at CMRS, Dhanbad (December 1961).

4. Sharma, P.D.; - ‘Improved Blasting with precision drilling patterns in Underground Metalliferous mines’; Procc. of ‘Golden Jubilee Seminar’ - present status of Mining and future Prospects, - Seminar organized by MEAI (April 2007) at Hyderabad, India. 5. Rajmeny.,P.K; Jain Suman; Rathore., S.S. – “Dealing with problems of blasting of highly stressed benches during open stoping at Mochia Mine”- IMEJ proc., of National seminar on Drilling & Blasting for next millennium; Bhubaneswar, January 2000 (Mintech Publications, Bhubaneswar). 6. Howard L. Hartman – Introductory Mining Engineering, John Wiley & Sons. 7. Product Literatures of various Drilling Machine manufacturers; such as M/s Atlas Copco, M/s Thunderbird Mining system, Aquila Mining Systems of M/s Caterpillar company etc. 8. Product literatures of various Electronic Detonator manufacturers; such as Digidet of Ensign-Bickford Co., Ikon of Orica, Daveytronic’s Digital Blasting System, Delta Caps Initiators’ Deltadet II system etc. ---------------------------------------------------------------------------Author: Partha Das Sharma, E.mail: sharmapd1@gmail.com

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9. Sharma, P.D.; - ‘Controlled Blasting Techniques – Means to mitigate adverse impact of blasting’; Procc. of 2nd Asian Mining Congress, organized by MGMI at Kolkata (India) dt. 17th – 19th January 2008 (pp: 286 – 295). 10. Langefors, U & Kihlstom, B.K. – ‘The Modern Technique of Rock Blasting’, ***
John Willy & sons, 1963.

Author’s Bio-data: Author is Graduate (B.Tech – Hons.) in Mining Engineering from IIT, Kharagpur (1979) and was associated with number of mining and explosives organizations, namely MOIL, BALCO, Century Cement, Anil Chemicals, VBC Industries, Mahashtra Explosives etc., before associating with the present organization, M/s Solar Explosives Ltd., Nagpur, 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), Electronic detonators 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: sharmapd1@rediffmail.com, sharmapd1@gmail.com 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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