Selection of Suitable Explosives
- 1. SELECTING SUITABLE EXPLOSIVES Presented by Aqib Ali Naich 16MN40
- 2. Selecting a proper explosive is an important part of blast design to assure a successful blasting operation. A product be selected and used in such a manner that optimum fragmentation and displacement is obtained while giving lowest overall cast with adequate safety The factors for selecting suitable explosive can be divided as; 1) Economic factors 2) Explosives 3) Rock and blasting conditions 4) Blasting results 2
- 3. 1) ECONOMIC FACTORS (a) Cost of explosive Generally ANFO has the lowest cost per unit of energy Slurry explosives range from slightly more than to about 4 times the cost of ANFO Cheaper slurries or emulsions are designed for use in large diameter blast holes and contain no high cost, high energy ingredients They are low in energy per unit weight basis The more expensive slurries are designed to be used in small diameter blast holes and are high energy products containing large amounts of aluminium or other high energy ingredients Cost of NG-based explosives range from 3 to 5 times that of ANFO depending largely on depending largely on the proportion of NG or other ingredients. 3
- 4. Despite its excellent economics, ANFO is not always the best product for the job because of its several shortcoming ANFO has no water resistance, it has low specific gravity, and under diverse conditions it tends to detonate inefficiently However site conditions may make the use of high explosives more attractive (b) Cost of drilling Under normal drilling conditions, the lowest cost explosives should be selected that will give adequate, dependable fragmentation However, cost of drilling increases typically in hard dense rock, the cost of explosives and drilling should be optimized through controlled experimentation. 4
- 5. when drilling is expensive, the blaster will want to increase energy density of explosive, even though the explosive with high energy tend to be more expensive. The energy density of explosive depends upon density and proportion of high energy ingredients such as aluminum, used in formulation. In small diameter blast hole, the density of ANFO may be increased up to 20% by high velocity pneumatic loading. The explosion energy of ANFO can be increased by the use of fine aluminum The economics of higher energy density explosives slurry or emulsion improve where the rock is more difficult to drill and blast 5
- 6. (a) Explosive energy It is the most commonly considered property when explosives are compared It is generally agreed that most explosive energy is released in the form of shock energy (S.E), bubble energy (B.E) and heat energy (H.E), of which S.E and B.E being the useful energy components for the blast process (b) Storage considerations Magazine construction is less stringent for blasting agents than for high explosives Magazine must be well ventilated, resistant to bullets, fire, weather and theft, where as blasting agents are protected from theft only. Shelf life is also important consideration. Slurries compared to NG based explosives have less shelf life 6
- 7. (c) Sensitivity considerations Sensitivity reflects on safety and dependability of an explosive More sensitive explosives like dynamite are more vulnerable to accidental initiation by impact or spark than blasting agents Slurries are generally less sensitive than NG based explosives Some explosives are affected by detonating cord used in the blast hole, therefore sensitivity to cord needs to be considered. Low energy detonating cord can be used while initiating these explosives 7
- 8. (a) Charge diameter The dependability and efficiency of ANFO are sometimes reduced at smaller diameters, specially in damp conditions or with adequate confinement. Diameter within 50mm ANFO functions best when pneumatically loaded into a dry blast hole When using charge diameter less than 50mm, many blasters prefer slurry or NG based cartridge despite the higher cost because such economy on cost by ANFO can be lost In blast hole diameter more than 100mm, a bulk loaded ANFO, slurry or emulsion should be used unless there is some compelling reason to use a cartridges explosive ANFO efficiency and dependability increases as the charge diameter increases 8
- 9. (b) Rock conditions Both drilling and fragmentation difficulties are experienced in hard, dense rock and jointed rock Explosives with high detonation velocity does help in fragmenting hard, massive rock With cartridge dynamites, the detonation velocity increases as the NG content increases It has been experienced that explosive having VOD of about 6000 m/s doesn't yield acceptable results in average conditions In general for granite, quartzite and similar rocks, explosives equivalent to 90% gelatins better results, whereas for medium soft rock ANFO type explosives are more suitable 9
- 10. (c) Water conditions ANFO has no water resistance. It may however be used in blast holes containing water if some technique is followed to overcome this drawback Emulsions are more water proof and do not depend on a package of water resistance, this is due to the protective nature of oil and wax membrane surrounding each particle. Slurries are gelled and cross linked to provide a barrier against water intrusion, and as a result exhibit good water resistance. Water resistance of any explosive can be provided by either the inherent physical and chemical properties of the explosive or it packaging. 10
- 11. (d) Temperature Until the development of slurries, atmospheric temperature were not an important factor for selecting an explosive Many NG based explosives use low freezing oils which permit their use in low temperatures. ANFO and slurries are not affected by low temp if priming is adequate. The effect of low temperature is alleviated if explosives are heated in the magazine or if they are in the borehole long enough to achieve the ambient borehole temperature Under hot storage conditions above 32C, many compounds will decompose slowly or change properties and shelf life will be decreased. 11
- 12. (e) Medical aspects Although, most explosives are oxygen balanced to maximize energy and minimize toxic detonation, some are bad from stand point of fumes. Even with oxygen balanced products, unfavorable field conditions may increase generation of toxic gases particularly when explosive without water resistance get wet. Storage and use of NG based explosives have headache producing effect (f) Explosive atmosphere Blasting in gassy atmosphere can be catastrophic if the atmosphere is ignited by the flame from the explosive. All underground coal mines are classified as gassy. In such situations permissible explosives offer protection against gas explosions Depending upon the gassiness of mines, corresponding class of explosives is to be used. 12
- 13. Production costs are directly related to the degree of fragmentation achieved. There is the evidence the matching VOD and sonic velocity of rock increase the fragmentation in hard, massive rock With NG based explosives VOD increases as the NG content increases. Thus 90% gelatin has high VOD than 60% gelatin VOD of ANFO depends on charge diameter and it particle size. In dia of 225mm, VOD of ANFO will exceed 4000m/s and it would be 5000m/s in 375mm dia. In smaller diameter below 50mm the VOD is less than 2500m/s and it may be increased to 3000m/s by high velocity pneumatic loading In many operations with expensive drilling and difficult fragmentation, it may be advantageous for blaster to compromise and use a dense, high velocity explosive in the bottom of the borehole and lower energy product as a column charge 13
- 14. THANK YOU 14