PARTHA DAS SHARMA, (B.Tech-Hons.), E.mail:
[email protected]
Introduction: Method of Mining in Underground Metalliferous mines mainly depend on type of Deposit: q Regular or Irregular; q Extent and Depth of Deposit, i.e., Massive or Pocket etc.; q Dip and Thickness of Deposit; q Nature of Hanging wall and Foot wall etc. q Required production level to be achieved and q 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. 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. Although the stopes are unsupported, pillars are usually left between stopes and occasionally within stopes. The ore flows through the stope by gravity.
Sublevel stopping with Long Blast Holes: q 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. q Considerable advances have been made by introduction of long-hole and precise drilling in stopes. q Today, most of the sublevel stopes all over the world are drilled with long blast holes. q 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. q Further, by adopting improved & precise method of blasting with introduction of new generation Emulsion explosives and Electronic Detonators, for such Sublevel stopes the Techno-Economic level also maintained properly. q It may however be noted that, large diameter holes (sometimes upto 100 mm or more) reduce drilling cost per tonne of ore broken.
The Pattern of Long - Hole Drilling classed under two major categories: Parallel hole drilling. 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.
Disadvantage with Parallel Hole Drilling: 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. Ring 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 continuous 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.
Ring Hole Drilling: qThe 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. qThe 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. q Hole deviation is a serious problem, however, deflection of several metres are not uncommon. q The effect in blasting can be disastrous as ring drilling requires accuracy in hole placement to obtain proper fragmentation. q 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.
Blasting Practice with Long Hole Drilling: q 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. q Judicious charging of holes in a ring using decks with suitable delay timing ensures better fragmentation, restricted ground vibration and reasonably better economy. q 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. q 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. q 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.
Charging Long Hole with Explosives and Delays: 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. 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 are used. 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. The best pattern is decided by a series of actual trial blasts and it is always advisable to slightly overcharge the holes, as a failed blast could mean considerable loss of production.
Vertical Crater Retreat (VCR) – Advanced method of Sublevel stoping: q 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. q 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. q The major innovation in VCR method is of blasting. q In this method, horizontal slice blasting of ore body with near spherical charges into the undercut is under taken. q Spherical placement of explosives is the most efficient in terms of fragmentation and explosives consumption. q Holes are charged from collar after plugging the opposite end. q The size of charge is generally restricted to a Length-to-Diameter ratio of 6:1; which suffices in the practice to simulate a spherical charge. q All holes in the stope are blasted together.
Vertical Crater Retreat (VCR)
q 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 as before. q 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. q Rock mechanics, blasting and material handling considerations are the important points in deciding dimension of stopes, location of boundary pillars etc. q 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: q Blasting is done in stope worked by Vertical retreat method by drilling holes in slashed sublevel. q The holes are drilled parallel, mostly either vertical or inclined along dip of the ore body. q 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. q Blasting is done in slices, after the holes are charged from collar by plugging at the opposite end. q 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. q 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.
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 constraints such as (a) Failure of Drill drive and spalling from it, (b) Failure or collapsing of drill holes, (c) Development of extension fractures etc. 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. Deck charging is done to minimize the maximum charge per delay, which prevent back-break, ground vibrations etc.
Discussion on modern precision Drilling and Electronic Delay Detonator system for enhancement of efficiency in Mines: q Computerized drilling and electronic detonators add precision at the mines to lower downstream costs. q Drilling and Blasting is the most important activities in working of any mines. q Computerized drills, Measure-While-Drilling (MWD) systems, and electronic detonators are gaining wider acceptance in quarries, opencast and underground mines. q Cost centers such as drilling, blasting, excavation, hauling and crushing are seen as interrelated variables in the total cost equation. q 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. q Thus, gaining a competitive edge in a mature and basic industry, modern system of Drilling & Blasting methods have taken a major leap forward in enhancement of Techno-Economics advantages in working of mines.
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