HISTORY OF DEVELOPMENTS OF EXPLOSIVES
HISTORY OF EXPLOSIVES Safe and efficient explosives of today are the result of a gradual evolution that continues still. From modest beginnings 600 years ago to today, much progress has been made. Literary sources indicate that “Greek fire” was first used about 668 A.D. by a Byzantine fleet against an Arab fleet. One authority has concluded that Greek fire was probably mixture of naphtha, quick lime and sulphur. Potassium Nitrate was unknown until about 1225. By 1250, the Arabs knew of it and it is likely that they mixed some with Greek fire to cause fiercer burning. The Chinese, as well, know about it at this fine. Infact their knowledge of it may date back as far as 1218. Roger Bacon. (1214-1292) first described the preparation of black powder, but true development had to wait for the invention of the gun by Berthold Schwartz, a legendry monk from Friedburg, Germany who lived in the 13 th or 14th century. At the beginning of the 1600s, the French physicist, mathematician, and poet, Francois Thybourel composed an epitaph for Berthold Schwartz. “Here lies Berthold the Black, the most abominable of inhumans who by his invention has brought misery to the rest of humanity”. Hence the term black powder was coined. For a period of 300 years, black powder was used solely as a rocket and gun propellant. In the beginning of 17th century, black powder was introduced and blasting replaced fire setting as the principal method of loosening rock in the mining industry in Europe. The acceptance of black powder into mining industry was relatively fast, and by the end of the 17th century most of the European miners used black powder to loosen rock. By the turn of the century, black powder became widely used in construction work. In 1802, (America), Eleuthera Irence du Pnt deNemours came to America and began the commercial manufacture of black powder at a mill situated on the banks of the Brandywine Creek near Wilmington, Delaware. As the use of black powder increased, the number of accidents also increased and safer methods of ignition were demanded. The first significantly safer method of ignition was invented by William Bickford of Cornwall, England. He devised the Miner‟s Safety Fuse wherein a continuous thin core of powder was contained in a cable of jute and string. Rinhard Bacon, Superintendent of the Copper Hill mine in Simsbury, connectient brought some Bickford fuse to America, and a factory was started in 1836. (Now Ensign – Bickford Company of Simsbury, Connectient) DYNAMITE: Three men laid the foundations for the modern high explosive industry as it exists today. All of these discoveries occurred in the first half of the 19 th century. The preparation of mercury fulminate, the first detonator ingredient, was described in a paper by Edward Howard in 1800. C.F. Schobein, a German physicist, nitrated cellulose to produce guncotton in Basel, Switzerland in 1846. Shortly thereafter, nitro-glycerine was prepared for the first time.
The discovery of (NG) nitroglyusius was the result of pioneer research by several eminent European chemists. The final step was taken in 1846 by Ascanio Sobrers (18121888) who called it piroglicerina. In 1862, Alfred Nobel, a son of Emmanuel Nobel, took over the Nobel nitroglycerin manufacturing operations for his father. The NG was shipped by Nobel as a semipure liquid in containers all over the world under the trade name of Glonion oil. NG was used as an explosive by pouring liquid from the container into the hole and then effecting a detonation. This crude method of handling and use resulted in many premature detonations, costing much loss of life and property damage. The NG liquid was packaged in containers which in turn were packaged in wooden boxes. Kieselguhr (diatomaceous earth) was added as a packing material to cushion the NG filled containers during shipment. In 1864, Alfred Nobel saw that a liquid NG filled contained had ruptured and its contents had been absorbed by the kieselguhr. Nobel found that kieselguhr absorbed three times its weight in nitroglycerin. Containers were then filled with nitroglycerin absorbed by the kieselguhr, which were handled and used much safer than containers filled with liquid NG. Later Nobel packed this mixture in paper cartridges. In cartridge form this mixture of nitroglycerin and kieselguhr was reasonably resistant to shock and was manufactured, shipped and used more safely than liquid NG. Alfred Nobel called this product dynamite (after the Careck dynamic. Meaning power). In 1867, Nobel promoted the manufacture of dynamite in the chited States through the Giant Powder company, which had the exclusive use of his patent. Forty eight years later in 1915, Atlas Powder Company acquired the Giant Powder Company. With the invention of dynamite, mankind was launched into an age of discovery. More reformulations further improved the explosive properties. Dynamite undoubtedly assisted the evolution of the Industrial Age. The first use of ammonium nitrate in explosives was patented in Sweden in 1867. It was mixed with charcoal and some nitroglycerin was added to make the mixture sensitive to detonators then in use. Nobel considered this invention of sufficient importance to buy the patent rights. Nitrocotton, first prepared in 1846, was known to Nobel during his early years of experimentation. He scarched for a substance that would retain NG in the presence of water. Guncotton, listed as an absorbent in the patent in 1863, did not perform as desired. Some time prior to 1873, an American medical student in Boston, named Mayard, found that a nitrocellulose with lower nitrogen content, when dissolved in a mixture of either and alcohol, would produce a harmless solution called collodion. Nobe studied effect of it on NG. He called the high nitrocotton content material blasting gelatine and the mixtures with active dopes, gelatine dynamite. Both were patented in 1875. A further development of the blasting gelatine concept in which the amount of soluble introcellulose was greatly increased, led to the invention of smokeless powder (ballistite) by Nobel in 1887. An explosive is a substance, which when ignited or detonated, converts itself into large volume of gas (about thousand times its original volume) at high temperature and pressure within a short period of time. The substance may be the mixture of several ingredients none of which is explosive by itself or a chemical compound. It is the quality of explosive to convert itself into large volume of gas at high pressure and temperature that is responsible for breaking or shattering the rock or coal. An explosive should have the following characteristics. 1. It should be capable of producing enough gas at high pressure in an extremely short period so as to do the required work. 2. The resultant gases should be an innocuous as possible. 3. It should be safe in gassy and dusty mines. 4. It should be stable in transport and be not liable to spontaneous decomposition.
5. 6. 7. 8.
It should resist shock and friction. It should be capable of being moulded in convenient shape. It should be safe and cheap to manufacture It should be non-hygroscopic or so contained as to prevent absorption of water.
TYPE OF EXPLOSIVES: Ι) Low explosives
ΙΙ) High explosives or detonating explosives
LOW EXPLOSIVE: It is a mechanical mixture of two or more substances none of which is explosive by itself. It comprises a fuel and supplier of oxygen. On contact with flame it burns but when confined it explodes. It burns comparatively slowly and can be exploded by direct ignition. It burns regularly from end to end, its combustion is governed by the law relating the rate of reaction to the surface area and temperature and pressure of the surrounding gases, heat being the chief medium of initiation and propagation. (if confined rate faster due to pressure and it finer particles rate faster) Commonly used low explosive is gunpowder Composition
I
Composition
II
Potassium Nitrate Charcoal Sulphur
65 – 75
Saltpetre
75
15-16 20 – 9
Charcoal Sulphur
15 10
Combustion gases of gunpowder Gases
CO2 CO N2 H2S CH4 H2
% When improper ignition takes place 32 34 19 7 3 5
% When properly ignited 48 13 32.25 0.25 4
It‟s use in U/g mines is prohibited. HIGH EXPLOSIVE A high explosive is a chemical compound and contains both fuel and oxygen atoms directly combined. They are put in 2 categories permitted and Non-permitted only permitted type of explosives can be used in gassy coal mine. Non-permitted High explosives are of two types (i) Nitroglycerine Explosives and
(ii)
TNT/Ammonium Nitrate Explosives There are two main groups of nitroglycerine explosives used in coal mines (a) Nitroglycerine gelatines (b) Nitroglycerine powder. NG gelatines have the following characteristics (i) Gelatinous consistency (ii) Water resistance and (iii) High density. The principal ingredients of explosives in this group are nitroglycerine, nitrocotton and inorganic nitrates, with a suitable combustible absorbent giving a balanced composition. Polar blasting gelatine and opencast gelignites belong to this group of explosives. Blasting gelatines are the most powerful of all industrial blasting explosives. Opencast gelignite was originally developed for blasting overburden in opencast coal mines. The NG powders are milder explosives of medium density. The principal ingreglients of these explosives are NG and ammonium nitrate with a combustible absorbent. They are not water resistant and care must be daken in their use. Nobels Explosive 695 A and Gelignol are explosives of this type. Gelignol is moderately strong and is suitable for ripping and for blasting hard coal in naked lamp mines. TNT/Ammonium Nitrate Explosves. These explosives contain ammonium nitrate sensitised with trinitrotoluene. A proportion of almumium powder or calcium silicate may be added to increase power and sensitiveness. They are not water resistant and under wet conditions they may be used in containers. Most of the modern explosives are based on cellulose (wood meal) and ammonium nitrate as the energy and gas producing base. In some of the explosives have is only about 10% of NG. In fact, they are energy producing mixtures of fuels and nitrates of which the reaction or explosions is brought about by a certain proportion of NG which acts as sensitiser. Composition of some non permitted high explosives Ingredient Polar NG % Collodion Cotton NaNO3 NH4NO3 Wood meal
Blasting Gelatin 92
Gelignite 72
Dynamite Gelignite 50
Ammonium gelignite 36
8
4
2
1
18
39
6
9
16 40 7
High explosives, depending on their composition explode at to velocities of 1500 8000m/s and produce large volumes of gases at considerable heat at extremely high pressure. High explosives can be further classified into primary and secondary. Primary explosives are characterised by their sensitiveness to stimuli like weak mechanical shock,
spark or flame the application of which will take explosive compounds from state of deflagration to detonation easily. Examples – Mercury Fulminate, Lead Azide, Lead styphnate, Tetrazene and other mixtures. Those explosives are used as initiating charges in the initiating devices such as detonators. Secondary explosives are capable of detonation only under the influence of shock wave, normally generated by the detonation of primary explosives. Military explosives – TNT, RDX, PETN, Tetryl and other combinations of these and industrial explosives like nitroglycerine, emulsion, slurries, watergels, ANFo and other powder explosives. These explosives are normally set off with suitable initiating devices like detonators or detonating coods and in some cases there is need of initiation by another high explosive. The explosive needing another high explosive is called Blasting Agent such as ANFo some slurries, some emulsions and mixtures of emulsions & ANFo.
NG based explosives. NG has been in use for a long time as the most important sensitiser for commercial explosives. It is made by reacting a mixture of glycerine glycoal and mixture of acids during which temp must be carefully controlled. It freezes at 13.2 oC. It is highly sensitive therefore converted into a more convenient gelatinous (Plastic) solid by the addition of 8% gun colon or nitro cellulose (obtained by the nitration of cotton) to form Blasting gelatine or by absorbing it in kieselybur to give straight dynamic. Due to higher cost of production and pollution control involved it is expected that the usage of NG based explosives will not increase. Wide range of NG based explosives are produced. They are packed in cylindrical cartridges 25 mm diameter and larger, with lengths ranging from 200 mm to 1000 mm. various paper shells or wrappers are used to package and protect it from moisture. Straight dynamites. Straight dynamites contain 15% to 60% explosive oil (nitro-glycerine plus ethylene glycol). In addition antacid, carbonaceous material and sodium nitrate is used. Typical % for mulations for 40% straight dynamite NG 40% Sodium Nitrate (SN) 44% Antacid 02% Carbonaceous material 14% Though they have high detonation velocities and good water resistance, they have high flammability are highly sensitive to shock & friction and produce large noxious fumes of such straight dynamite have only limited use in dictating and under water blasting. Gelatine dynamites Gelatine dynamites are similar to straight dynamites, except that the explosive oil has to be colloided with nitrocellulose to forma a gel. The result is a cohesive mixture that has better water resistance properties than straight dynamites. The material is of higher density, rubbery and much less sensitive than straight dynamites.
A typical gelatine explosive would be NG Nitro Cotton (NC) Ammonium Nitrate Sodium Nitrate Wood meal Sulphur Calcium Carbonate
-
22.5% 0.5% 15.0% 47.2% 7.2% 6.2% 1.4%
Blasting gelatine is the strongest form of explosive used industrially and serves as a standard against which explosives are graded. It consists of NG, to which a temperature depressant (ethylene glycol dinitrate) is added plus approximately 7% nitrocellulose, to form the gel. Ammonia dynamites: They are actually ammonium nitrate explosives sensitised with NG. Generally they supply same blasting strength as most straight dynamites and are less costly. Composition of 40% ammonia dynamite NG AN Sodium Nitrate Antacid Carbonaceous material
-
14% 36% 33% 01% 10%
They have heaving effect are hygroscopic and are desensitised by water and therefore their application is generally limited to soft rock and where water conditions are not a problem.
Ammonia gelatine dynamites: Ammonia gelatine dynamites are similar to the corresponding ammonia dynamites except for the addition of nitro cellulose to the explosive oil to form a gel. These semigelatinous explosives are equal in most respects to gelatine dynamites except for some what lower VOD and slightly less resistance to water, honourer, their water resistance properties are greater than conventional ammonia dynamites. A typical formulation for a 40% grade is. NG 26.2% Nitro Cellulose 0.4% Ammonium Nitrate 8.5% Sodium Nitrate 49.6% Carbonaceous fuel 8.9% Antacid 0.8%
Semigelatine and nitro-starch explosives Semigelatine dynamite is a hybrid formulation with properties between those of gelatine dynamite and granular dynamite (which contains no nitro cellulose has relatively
low density and is susceptible to the effect of water). They combine the economy of ammonia dynamites with water resistance and cohesiveness of ammonia gelatine explosives contain less explosive oil, SN and nitrocellulose but more AN. They have low cost in comparison to comparable gelatine dynamite, offer good fume characteristics and water resistance and are suitable for u/g use. Nitrostrach explosive is a formulation in which glycerine is replaced entirely by nitrostarch eliminating cartain objectionabls characteristics such as NG headache and nausea. They contain no liquid sensitisers, cannot freeze and do not exude or leak during storage Ammonium Nitrate Dry Mixes (ANFO) Prilled ammonium nitrate (AN) and fuel oil mixtures, known as ANFO, were introduced for blasting operations in mid 1995s. Ammonium Nitrate in a proper form when mixed with carbonaceous or combustible material in appropriate proportion forms a blasting agent. Although many forms of AN could be used with a solid or liquid fuel to form a blasting agent, the porous prilled forms are preferred for ANFO. Through the years, fuel oil has proved itself to be an almost ideal fuel for AN. It is readily available, is inexpensive and easily mixed with AN +. Produce a uniform mix commonly known as ANFO. Small porous spherical prills absorb oil readily giving a dry mix which has good pouring properties. The micro prills in each prill allow the AN to absorb and retain the optimum amount of fuel oil. The prills are hard anough to withstand transport shocks without broaking down, yet soft enough to breakdown and give a high loading density when blow loaded into blastholes with equipment such as pneumatic blasthole charger. AN is stable at ambient temperature but can absorb moisture from the atmosphere if the humidity is above about 60% hightly coated with anti caking agents. While it is strong supporter of combustion, AN is not flammable. However it is an oxidiser proper mixinig of AN and FO is important for predictable explosive performance. When 94.5% AN and 5.5% FO are mixed following takes place – 3NH4NO3 + CH2 = 3N2 + 7H2O + CO2 _ 930 Kcal/Kg This mixture represents the ideal reaction of an O2 balanced mixture. None of the detonation products are poisonous Maximum energy output and VoD is obtained by mixing 7.5 litres of FO with 100 kg of AN prills. With too much or too little FO energy yield falls. A mixture containing 92% AN and 8% FO 2NH4NO3 +CH2 = 5H2O + CO + 2N2 + 810 Kcal/Kg. A fuel short mixture containing 96.6% AN and 3.4% FO creates an O2 excess condition 5NH4NO3 + CH2 = 11 H2O + CO2 + 4N2 + 2NO + 600 Kcal/Kg Some of the N2 from the AN combines with this excess O2 to form nitrous oxide which, upon exposure to normal atmosphere forms NO2 an extremely toxic gas. AN O2 balanced mixture thus maximises energy realse while minimising formation of toxic gases. The apprearacnce of distinctly orange coloured fumes after an ANFO blast may indicate too littles FO in atleast some of the ANFO. These fumes can also appear where properly mixed ANFO has beame wet and has absorbed blasthole water. The field mixture commonly employed consists of 94% AN and 6% FO, which ensures a slight O 2 daficiency fuel excess as a safety measure.
[Mixing inside hand operated mixer (concrete type) (0.05-0.1 m3 cap) bowl is line with plastic or epoocy resion to prevent corrosion] (To distinguish between AN and fuel oil absorbed AN, oil soluble dye is used) The loose poured density of ANFO‟s usually about 0.80 g/cm 3. By blow loading it can be increased to 0.95 g/cm3. If ANFO is compressed beyond a density of about 1.20 g/cm3, it will not detonate as it becomes dead pressed. The VOD and the energy yield increases with the degree of confinement VOD and energy release are smallest when the charge is fired completely connefiend. The blasthole has pronounced effect on VOD of ANFO. As increases so does VOD. But the energy yield does not vary with blasthole diameter, because the initiation sensitivity of ANFO decreases with increasing blasthole diameter, a primer that is adequate far 50 mm blasthole may be inadequate for 125 mm blasthole. Well mixed loose poured ANFO can be used successfully in blasthole down to about 25 mm. Although ANFO is used predominantly as loose poured charge it is important to recognise that the available erargy increases rapidly with coupling. Where the ANFO charge does not completely fill the x section of blasthole, the charge confinement, the VOD and peak pressure acting as the walls of blasthole are reduced. Although ANFO mixtures are explosives, they are relatively insensitive and unless suitably primed, reliable detonation in large blastholes will not occur. The size and type of primer required depend upon blasthole diameter, degree of confinement, dryness of blasthole, etc. In general, the primer should have a high VOD and the maximum possible diameter. Needles to mention the primer should be in intimate contact with ANFO. Lack of water resistance is the major limitation and disadvantage of ANFO. AN is readily dissolved by water, and unfortunately the addition of 5.64% FO does little to reduce the rate of dissolution. Both the strength and VOD of ANFO are reduced by the addition of water. ANFO which contains more than about 10% water usually fails to detonate. As one would expect the longer ANFO is exposed to water the greater is damage, and lower the blast effect. Wherever there is a chance of blasthole water desensitising. ANFO and make it less effective, efforts should be made to minimise the period between charging and firing the blast. The problem of water is tackled with polyethylene line or packaged ANFO. Derdatiring of holes. The use of Heavy ANFO has allowed operators to overcome this important shortcoming. Aluminised ANFO (ALANFO) The addition of aluminium powder to ANFO increases the energy output quite considerably. The effect of adding 9.9% aluminium to ANFO mixture containing 87.6% AN and 2.5% FO mixture is 6NH4NO3 + CH2 + 2AL = Al2O3 + 6 N2 + 13 H2O + CO2 + 1320 Kcal/kg The higher heat of explosions is due to the high heat of formation of Al 2O3. The increased density and increased energy per unit weight of an aluminised blasting agent must be weighed against its higher cost. The use of aluminium with particles size in –30 + 150 mesh range ensures complete reation of the aluminium without affecting either the performance sensitivity or hazard sensitivity significantly. The water resistance of ALANFO is not better than ANFO. The most commonly used composition contain 10% & 15% Al.
Slurry Explosives These were developed as a result of attempts to water proof improve density and strength of ammonium nitrate. A slurry is a mixture of nitrates such as ammonium nitrate and sodium nitrate, a fuel sensitiser, either explosive or non-explosive and varying amounts of water. Although they contain large amounts of ammonium nitrate, slurries are made water resistant through the use of gums, waves and cross linking agents. Most commonly used fuel sensitisers are carbonaceuos fuels, aluminium and amine nitrates. They are sensitised by air bubbles which are entrapped by churning the mixture. Even when none of the ingredients are in themselves explosive substances and it is only in the final stage of production that the compositions acquire explosive characteristics. Slurry/watergel explosives are „fuel‟ sensitised AN with or without other oxidisers such as sodium nitrate or sodium perchlorate (SPC) or ammonium per chlorate (APC) in which the solid fuel and the solid part of oxide are dispersed in continuous fluid medium generally an aqueous solution with or without other polar solvents and often more or less aerated. They have been designated differentially as slurry explosives (SE) when they are sensitised with explosives (e.g. TNT) and slurry blasting agent (SBA) when the fuel is not an explosive (e.g. aluminium sulphur and sodium hydrocarbon). A further division of these water based explosives is watergels and emulsion explosives. A water gel is essentially the same as a slurry and the two terms are frequently used interchangeably. Some typical slurry composition Ingredient AN TNT H2O Gum + cross linker AN H2O Gassing agent Gum + cross linker Fuels Al AN H2O FO Gum + dispersing agent
Weight % Large diameter TNT 64% 20% 15 1 Basic Al NCN slurry 77.0 15.0 0.2 2.0 4.0 1.0 Basic FONCN slurry 72.9 20.5 0.2 0.2
Weight %
68.8 19.0 2.0 2.0 2.0 8.0
The water gel explosives have the oxygen doner in the form of nitrate salts fully or partly in a water solution which is gelled. In this gel, the fuel which is dissolved or distributed quite often consists of aluminium powder, methyl amine nitrate (MAN), glycol and urea, some of the fuel sensitises the mixture. Additional sensitising is achieved by the addition of small gas bubbles. Slurries may be classified as either explosive or blasting agents. Those that are sensitive to a blasting cap are classified as explosives, even though they are less
sensitive than NG based explosives. It is important that slurries be stored in magazines appropriate to their classification. Advantage 1) Slurries are manufactured without some of hazards usually present during explosives manufacture. 2) Use no longer suffers discomfort of headache associated with NG based explosives. Except for their excellent water resistance and higher density and bulk strength slurries are similar in many way to dry blasting agents. Good O2 balance, decreased particle size and increased density increased charge diameter good confinement and coupling, and adequate priming all increase their efficiency. Although slurry blasting agents tend to lose sensitivity as their density increases some explosive based slurries function at density upto 1.6. The effect of charge diameter on detonation is not pronounced as it is on ANFO. Most non capsensitive slurries depend on the entrapped air for their sensitivity and most cap sensitive varieties are also dependent to a lesser degree on this entrapped air. If this air is removed from a slurry through pressurisation from adjacent blast, prolonged periods of time in borehole, or prolonged storage, the slurry may become desensitised. To overcome this loss entrapper air addition of „perlite‟ or microballons is several times resorted to. Cartridge slurries for use in small diameter blastholer (50 mm or less) are normally made cap sensitive. Sensitivity and performance of some grade of slurries are adversely affected by low lemp. Slurries designed for use in large diameter blastholes (above 100 mm) are least sensitive slurries. Slurries containing neither aluminium nor explosive sensitisers are the cheapest, but they are also least dense and powerful. In wet conditions were dewatering is not practical & rock is extremely difficult to fragment, these low cost slurries offer competition to ANFO. Aluminised slurries or those containing significant amount of other high energy sensitisers, develop sufficient energy for blasting in hard, dense rock. Nitrate salts were first introduced in slurry explosives to improve the accessibility of oxidiser to fuel. However due to use of thickners and gelling agents to stabilise the fluid phase for preventing segregation of solid ingredients and to increase water resistance, it is not possible to formulate a slurry in which all the oxidisers are in solution form. Hence, much of the oxidisers must be in solid form and this reduces the intimacy of contact between oxidiser and fuel and the rate of efficiency of reactions. Emulsion Explosives: An emulsion is a two phased system in which an inner or dispersed phase is distributed in an outer or continuous phase. In simpler terms an emlsion is a mixture of two liquids that do not dissolve in one another. This unique feature couples with the fact that minute size of the nitrate solution droplets are tightly compacted within the continuous fuel phase results in good intimacy between oxidiser and fuel and increased reaction efficiency compared to other systems. The emulsion matrix is obtained by emulsification of two immiscible liquids. By the process of emulsification two types of emulsions are obtained, one is oil in water and other is water-in-oil. Water is in discrete dispersed phase in water-in-oil emulsion in the form of fine droplets dispersed in continuous phase which is oil phase. Whereas in oil-in-water emulsion the reverse is true. A typical water in oil emulsion blasting agent has the following compositon.
Wax/oil Emulsifier Water AN SN Glass bubbles
Weight % 6 2 17 17 58 2
A typical composition of oil-in-water emulsion explosive is Diesel (fuel oil) Water Sodium nitrate Calcium nitrate AN Glass bubbles Guar
6 15 5 20 51 2 1
The emulsion explosives have the O2 donor consisting of nitrates and perchlorates in an aqueous solution. The water phase is in the form of small droplets in the continuous oil phase which constitutes fuel. The fuel consist of mixtures of waxes and oils. The explosive in this case is sensitised by gas bubbles in the form of micro spheres. . Additional strength can be achieved by the addition of fuels such as aluminium powder. Three products in emulsion category 1. Straight emulsion 2. Doped emulsion 3. Repumpables emulsion Straight emulsions: They are normally hot mixed at temperature over 50 oC and a suitable chemical gassing agent is incorporated to control density. It displays exceptionally high water resistance, stability and explosive reaction efficiency leadinig to superior velocity of detonation (VOD) characteristics. Doped emulsions: It is obtained by addition of varying percentage of AN to achieve a wide range of strengths. Pumpable characteristics as in the case of emulsions are maintained at all stages and so also the superior VOD and water resistance properties of emulsions. Repumpable emulsions: These are designed for small and intermediate diameter and apart from the normal advantages of emulsions are designed for pumpability at low temperatures. Products are available for a wide range of strengths and diameters. These are low cost units for fast and accurate loading of small quantities per borehole like all bulk explosives all the components are non explosive and are mixed on the mine bench and loaded directly into the borehole.
Heavy ANFO None of the above slurry and emulsion systems have cost competitiveness with ANFO as such their usage has been limited. The exception to this is densified or Heavy ANFO. The aim in Heavy ANFO is to have advantage of both, the high density and water
resistance of slurries and emulsions plus the low cost of ANFO. In the mix of AN and FO about 50% of air in total volume exists, roughly 30% in the prills and 70% between the prills. The air in the prills provides necessary sensitivity but the air between them is a waste volume. This volume is filled with high density, waterproof product such as emulsion (15 – 40%) the resultant mix is denser and more water resistant than ANFO and with very little added cost. The water resistance and/or density of mix can be tailored by changing the emulsion level. In principle the emulsion occupies the interstics between prills of ANFO within the borehole. The denser product allows maximum use of borehole volume. Incremental amount of AN increases the density and bulk strength and a wide range of products can be obtained. However, water resistance and VOD characteristics are inferior to emulsion products due to large % of ANFO but are superior to straight ANFO. Blasting properties and economics of heavy ANFO depend upon emulsion matrix level. Aluminium is also added to increase the strength of heavy ANFO. Liquid Oxygen Explosives Explosives compositions utilising liquid on have sometimes been employed. O 2 as would be available from 4000 times its volume of atmospheric air. If a combustible ingredient, made in the form of cartridge is soacked in liquid O 2 and then subjected to combustion reaction, the rate of reaction is extremely high and a large volume of gas is instantaneously released at high temperature and pressure. The VOD under suitable confinement can be 5000 m/s. This is the principle behind use of (LOX). The fuel cartridges are dipped into the liquid oxygen immediately before being lowered into the borehole, and the cartridges are fired by the initiation system. A minimum delay is imperative, since LOX rapidly loses its power through evaporation of oxygen. Confined detonation velocities of 5700 – 5800 m/s are attained. Usage of LOX is confined to some countries only and that too in limited areas. LOX is available small 25 mm to 90 mm and for diameters above 100 mm. The basic ingredient of an absorbent cartridge is a cellulosic substance like crushed jute stalk or other agricultural product though other substances such as hydrocarbons or metallic powders which are used to impart to the soaked cartridge the properties of industrial explosive. Characteristics of LOX are not constant and it varies with delayed time between socking and firing. (within half an hour in case of large cartridge). Products for Specialised blasting operations Controlled blasting, under water blasting, blasting of high temperature explosive. In general either the available products are used or specialised products are formulated. Pipe charges : Pipe charges in rigid plastic tubes are used which can be screqed together by means of extension sleaves. These are suitable for u/g work. Having 25 mm to 50 mm Length 600 to 700 mm Explosive dia 11 to 19 mm
More decoupling and so less damage to the surrounding rock example Gurit and Nabit of Nitro Nobel of Sweden. K pipe of Finland. Powder explosives Comparatively weaker explosives are also used in many situations to blast very weak rocks. This can be done by using explosives with comparatively less energy. Polysteren granules have been mixed with ANFO to obtain reduced energy explosives this the crushing of rock and intensity and extent of cracking get reduced. Shaped charges: The term shaped charges is applied to cylindrical charges of high explosive with a cavity formed at the end opposite to the point of initiation. The effect of the cavity is to produce an intensified pressure which is projected as a jet in the direction of initiation. If the cavity is lined by metal, then this is fragmented by the detonation to produce a high velocity jet. Shaped charges have been applied to penetrate the casing of oil & gas well, to tap furnaces, to break rock underwater without carrying out drilling operations, and for penetration of fregen ground. The type of explosive depends on the usage RDX for metal cutting, slurry explosive or NG based explosives which will not get desensitised by the high hydrostatic pressure when underwater blasting is carried out. Cast boosters: Cast boosters are explosive units designed to as primers, comprising a mixture of PETN, TNT and other minor ingredients. Their high strength, high density and very high velocity of detonation (7000 m/s) make them suitable for priming ANFO mixtures and slurries. An additional advantages is their lower sensitivity to shock, friction & impact than gelignites and other NG explosives. Cast boosters are manufactured in the range of 100, 200 & 210 gm and above. Holes provided the booster permit the insertion of electric detonators and the threading of boosters on to down lines of D cord. Seismic Explosives For seismic prospecting a special range of explosives are manufactured. These are compact, flexible and high powered explosives used in seismic reflection and refraction survey work. They are cap sensitive, highly water resistant as well as not affected by long sleeping times in water filled holes. In most cases they are couplable plastic cartridges. Two component explosive Two component explosives are often called binary explosives since they are made of two separate ingradients. Neither ingredient is explosive until mixed. Binary explosives are normally not classified as explosives. Commercially available, two component explosives are a mixture of pulverized ammonium nitrate and nitromethane, which has been dyed to either a red or green color. These components are brought to the job site and only the amount needed will be mixed. Upon mixing, the material becomes cap sensitive and is ready to use. Binary explosives can be used in applications where dynamite cap sensitive slurries would be used. These binary explosives can also be used as primers for blasting agents and bulk slurries.
Permitted Explosives Developments in design of explosive has been to seek ways and mans to make shotfiring easier and safer. The design of permitted explosives is based on the fact that the inflammable mixtures of methane and air are not ignited instantaneously but need the application of heat or flame for a definite period of time which is termed as „induction period‟. The lower the temperature of source of heat or flame, the more is „induction period‟. This has been achieved by addition of cooling agents in composition of modern explosives. Ammonium oxalate and Magnesium Sulphate were perhaps the first cooling agents used, although former cannot be regarded as an inert material. (NaCl) Sodium chloride is used as cooling agent down to this date. In earlier design safety of explosives was enhanced by sheathing it with a coating of cooling agent and this class of explosives was known as sheathed explosive. The sheathing material commonly used was sodium bicarbonate regarding function of sheathing there are three mainhypothesis: i) Sheathing material is dispersed without chemical change, by pressure effects and acts as an inert blanket round the flame and hot products of explosive, so preventing their contact with an inflammable mixture, which may be present. ii) That the sheathing material during dispersion is mixed with the hot products and absorbs heat from them lowering their temperature and (possible) itself undergoing chemical change as a result of heat absorbed, the change with sodium bicarbonate being evolution of heat and CO2. iii) That the sheathing material exerts a special inhibiting effect by interrupting the chain of reaction through which methane passes during its combination with oxygen so preventing ignition. Main defects of sheathed explosive i) Defective sheathing ii) Abuse of sheathing iii) Mishandling of sheathed cartridges, which were primarily responsible for ignition of gases occurring with their use. Subsequently sheathed explosives were replaced by equivalent to sheathed explosives. “Eq.S.” Function of mixture: The particles of inert material are compressed, and suffer reduction in volume. Thus, the high pressure gases get more room to expand and cool. In practice „eq.s.‟ explosives have proved chaper and they also produce more round coal. They do not damage to roof and thereby improve roof conditions. The intrinsic safety of explosive was enhanced by design of Ultra Safe Explosive. The basic principle underlying design of explosive was inclusion in it of NaCl with fine particles having large surface area, after the researches of Bichel Dijksman and Dolan and Dempster. The experimental data shows that for complete suppression of flame in air containing 9% CH4 the quantity of NaCl used must have surface area of 1.84 m 2/cm3 of gas achieved by the use of nitrochelk as an ingredient to supplement NaCl and ultimately Nobel‟s expl. Na. 1220 & 1235 more designed. The strength was 28% of blasting gelatine, VOD 1750 m/s & density 1.1 g/cm3. P1 = Permitted explosives used in drifting, simultaneous firing in coal & ripping. P2 = Sheathed explosive used in simultaneous firing in coal ripping (replaced by P 3) P3 = Equivalent to sheathed permitted explosives used in places where g rop P2 is approved. P4 = Used in delay firing in ripping, in under cut coal and in X measure drifts.
P5 = Used in delay firing in solid coal. Name of explosive Type of Expl. Group of expl. NG Nitro Cotton TNT & DNT AN NaCl Oat husk meal China clay Moisture Woodmeal NG & Nitroglycol Nitrocellulose Sodium Nitrate NH4Cl Woodmeal Wheat flur Sodium Carbonethyl Cellulos Calcium Stearate Nitro chalk Bagasse
Unigex
Nobel‟s egl. No.
P3 11.0 – 8.0
P5 14
73.0 – 70.0 10.0 – 8.0
23 37
Polar Ajax
Polar Viking
NG type P1 27.5 – 25.5 1.25 – 0.25 2.5 – 1.0 42.0 – 29.0 15.0 – 23.0 4.5 – 2.5 3.0 – 2.0 2.0 – 0.0
NG & AN type
2.0 – 0.5 2.0 – 0.5 9.0 – 7.0
3.0 – 5.0 15.7 – 17.7 0.1 – 1.1 43.5 – 46.5 28.0 – 30.0 1.0 – 3.0 0.5 – 1.5
0.1 – 1.1 22 4
Permitted explosives are to k tested as per ISI standard No.6609. Part II – Section II. Tasts are carried out in 1.52 m x 15.24 m long gallery of which 5.48 m is the explosion chamber, explosive chamber contains 9 0.25% CH4 or 8 0.25% natural gas for safety against coal dust explosion they are find in presence of coal dust 85% of which pass through 240BSS. For testing P3& P5 2.2 Kg of coal dust is spread on the floor of explosion chamber and 2 bags of dust each containing 560 gas of dust are hung bum the top of gallery. The bags are placed 1.52 m apart, the first bag being 0.76 m away from month of gallery. For P1 expl. 560 gm of coal dust is spread on plat form placed 15 cm from the axis of the cannon & also 2.2 kg of dust is spread on the floor of gallery. The bags are fired ½ a second later than the firing of explosive using instantaneous detonators. The explosive is kept 5 cm away from month of cannon. The back of cannon is filled with sand. Cannon bore is 120 cm long x 55 cm . Break gallery test: The have safety against across a crack in the shot hole. In this test two plates are placed in the testing gallery and explosive cartridge is placed across this as though it is fired across a crack in the shothole. Characteristics of Explosive The selection of an explosive is based on three criteria: its ability to function properly in the prosed environment the performance characteristics of the explsive cost
Our selection criteria is based on obtaining the explosive which will function under the specific conditions of the blast at the lowest possible cost. Explosive differ in following ways: Minimum in which detonation will occur The ability to resist water & water pr. Generation of toxic fumes Flammability Ability to function under different temperature conditions Input energy needed to start the reaction Reaction Velocity Detonation pr. Bulk Density Strength Ability to remain in original configuration If you want to purchase an explosive that had the best performance ratings in all of the above categories, it would also be the most expensive. So for particular site some of the parameters can be left or considered as per cost to molu explosive cost effective. Sensitiveness: Sensitiveness is the characteristics of an explosive which defines its ability to propagate a stable detonation through the entire length of the change and controls the minimum for practical use. Sensitivenes is measured by determining the explosive‟s critical diameter. The term critical diameter is commonly used in industry to define minimum of explosive column which will detonate reliably. All explosive compounds have a critical diameter. For some explosive compounds it can be measured in thousand the of an inch while for other it can be few inches. Explosive should be more than critical . Water resistance: Water resistance is the ability of an explosive to withstand exposure to water without suffering determintal effects in performance. Explosive products have two types of water resistance internal & external. Internal is defined and water resistance provided by explosive composition itself. As an examples some slurries can be pumped directly into boreholes filled with water. These explosives displace water upward but are not penetrated by water and show no detrimental effects if fired within a reasonable period of time. External is provided not by explosive material itself but by packaging or cartridging into which the material is placed. Type Resistance Granular dynamite Poor to good Gelatine dynamite Good to excellent Cartridged slurry Very good Bulk slurry Very good Air emplaced ANFO Poor Poured ANFO Poor Packaged ANFO Very good* Heavy ANFO Poor to very good Becomes poor if package is broken. Water pressure tolerance:
The ability to remain unaffected by high static pressure is defined as water pressure tolerance. Some explosive compounds are densified & desensitised by hydrostatic pressures which result in deep boreholes. Problems with water pr. Tolerance most often occur with slurry and heavy ANFO. Fumes: The fume class of an explosive is a massure of the amount of toxic gases produced in the detonation process. CO & oxides of nitrogen are the primary gases what are considered in the fume class ratings, Most commercial blasting explosives are O 2 balanced to minimise fumes and optimise energy release. In u/g applications the problems that result from fumes especially with inadequate ventilation is obvious. Some conditions that can cause toxic fumes production with O2 balanced explosives are insufficient charge diameter, inadequate water resistance, inadequate priming and premature loss of confinement. CO2 is not a fume since it is not a toxic gas but death can occur if present more than 18% after blast & in ventilation restricted place is sp-gr. Is 1.53. Flammability The flammability of an explosive is defined as characteristics that deals with the case of initiation from spark, fire or flame. Some explosive compounds will explode from a spark while others can be burned and will not detonate. Flammability is an important consideration for safety in storage, transportation and use of explosives. Ex. LOX because of its flammability and inherent safety problems its use has been stopped. Temperature resistance: Explosive compounds can suffer in performance if stored under extremely hot or cold conditions. Under hot storage conditions (above 90 o F) many compounds will slowly decompose or change properties and shelf life will be decreased. Storage of AN blasting agent in temp above 90oF can result in cycling which will effect performance & safety of product. Cold resistance: Cold conditions can also effect the performance agents will not freeze under ordinary exposure to the cold temp. slurry explosives which include water get & emulsions can have serious detonation problems if stored in cold temp & not allowed to warm up before they are detonated. Performance characteristics of Explosive Environmental conditions can eliminate certain types of explosives from consideration on a particular project. Sensitivity: The sensitivity of an explosive product is defined by the amount of input energy necessary to cause the product to detonate reliably. This is sometimes called the minimum booster rating, or minimum priming requirements. Some explosives require little energy to detonate reliably. Many factors can influence sensitivity of a product. Sensitivity can be reduced by effect of water in blasthole, inadequate charge diameter and temperature extremes. It can also be expressed as maximum separation distance between a primed donor cartridge and an unprimeed receptor cartridge where reliable detonation transfer will occur.
Velocity: The detonation velocity is the speed at which the detonation moves through the column of explosive. It ranges from approx. 1524 m/s to 7620 m/s for commercially used products. Detonation velocity can be used as a tool to determine the efficiency of explosive reaction in field use. Detonation pressure: The detonation pressure is the near instantaneous pressure derived from the seek wave moving through the explosive compound when initiating are explosive with another, the shock pr. From primary explosive is used to cause initiation in the secondary explosive. Detonation pressure can be related to borehole pr. But it is not necessarily a linear relationship. Detonation pr. Is calculated mathematically. Density: The density of explosive is important because explosives are purchased, stored & used on a weight basis. Density is normally expressed in terms of sp.gr. which is the ratio of explosive weight to water weight for the same volume. The density of an explosive determines the weight of explosive that can be loaded into a specific bore hole diameter. The sp.gr. of explosive is commonly used as a tool to approximate strength and design parameters between explosives of different manufactures and different genesis families. In general terms, the higher the explosive density, the more energetic the product. Sp.gr. of commercial products range from 0.8 to 1.6. Strength : The term strength refers to the energy content of an explosive which in turn is the measures of force it can develop & its ability to do work. Weight strength & bulk strength. Cohesiveness: It is defined as the ability of explosive to maintain its original shape. There are times when explosive must maintain its original shape & others when it should flow freely. Shelf Life: The shelf life of an explosive is important because the explosive, in many instances, must be kept for a long time, often under unfavourable conditions such as heat, cold and humidity before use.
Initiation system Before the days of Alfred Nobel, when rock blasting was done with black powder, initiation was started by the flame slowly propagating through a black powder fuse. The black powder fuse consists of a narrow column of black powder lightly enclosed in a tarred hemp cover.
In 1863, Afred globel found that by enclosing a small change of finaly powdered black powder in a closed wooden capsule and leading the fuse through a tight fitting hole in the capsule plug, he could reliably ignite nitroglycerine. For patent applied 1864 & with some improvement in 1965. In the tight confinement of capsule burning rate of black powder became sufficiently high for capsule to explode and send what now know is a shock wave into the surrounding NG. The wooden capsule was an important first step toward the epochmaking invention of the detonator and marked the break through to the first use of detonating high explosives in rock blasting. Without invention of detonator, NG was useless for rock blasting. 1865, Nobel patent for the detonator made use of a small quantity of a primary explosive mercury fulminate pressed into a copper capsule which was crimped to the end of black powder fuse. About 1900, came the electrically initiated fuse head. Connected to a short length of black powder fuse in to turn connected to an ordinary detonator, this constituted the first electric delay detonator. Later, about 1920, came the instantaneous electric detonator in which a thin elelctrically heated bridge wire set on fire a minute charge of a flame producing compound. This in turn initiated a small quantity of primary explosive such as lead styphnate or lead azide. The primary explosive detonated, initiating detonation in the main charge usually about half a gram of a relatively sensitive secondary explosive such as PETN, Tetryl. Or RDX/TNT. Semiconductor Bridge Ignition Most explosive devices use small bridge wires (sometimes called hot wires) to ignite explosives, pyrotechnics or propellants pressed against the bridge. Passage of a current through the wire heats the wire and then ignites the explosive in few milli seconds. The CSB method is diffused. It uses a heavily doped polysilicon bridge which is much smaller than conventional bridge wires. Passage of a current pulse with significantly less energy than that required or hot wire ignition produces a plasma discharge in the SCB which ignition the explosive pressed against the bridge, producing an explosive output in a few microseconds.
High Precision Electric Detonating Caps (Exploding bridge wire detonator) EBW It is detonated through a capacitor discharge into a low inductance bridge wire circuit. Due to the short duration of discharge current pulse and the high.
Design of blasting rounds
Controllable variables - hole diameter - hole depth - subdrilling depth - stemming distance - stemming material - spacing - number of hole in the blast - direction of rock motion - deck loading We also have control on time - select blasthole timing to fit blast geometry - select an initiation system which will produce time sequences - plan for cap scatter in blasting pattern - design a pattern to control the vibration at protected structures - select proper time to have optimum fragmentation - select proper time to ensure minimum airblast - select the proper times for wall control - select the proper time to reduce flayrock In order to produce proper results blaster must also consider the explosives supplies available to him such as - use of an initiation system which is safe and reliable - type of expl. Which will perform properly in specific environment - explosives delivered energy - whether to bulk or cartridged expl. - How to best handle water problems. Burden: B1 D1 = B2 D2 B1 = burden successfully used on previous blasts D1 = Diameter of charge for B1 B2 = new burden D2 = new expl. Diameter Based on the characteristics of both explosive to be used and the rock mass to be shot. Allsman (1960) and Speath (1960) proposed an equation B = (K De/12) (Pe/St) 0.5 K = constant = 0.8 for most rock De = charge diameter Pe = explosion pressure St = rock tensile strength Orivken was detibatuib.exokisuib or, abd tebsuke strebgtg were readukt bit avaukavke, B = Kb De/12 ; Kb = between 25 & 35 equal to 30 in average B = burden conditions, in rock with density around 2-7 g/cc. If Kb = burden constant rock density is higher then this K would be reduced De = diameter of explosive and vice – versa. Konya (1972) proposed a burden equation similar to Ash eq. B = 3.15 De (SGe/SGr) 0.33 B = burden in ft De = Diameter of expl. In inch SGe = Sp. Gr. Of expl.
SGr = Sp. Gr. Of rock Konya (1983) proposed another burden equation, which would give similar results to those using earlier burden formula. This new eq. was simple to use, required no power functions and was suited for field use. B = [(2 S Ge/S Gr + 1.5)] De B = burden in ft S Ge = Sp. Gr. of expl. S Gr = Sp. Gr. of rock De = diameter of expl. in inch The explosive in the borehole environment may not be as efficient as would have been expected from underwater test data B = 0.67 De (Str / S Gr)0.33 B = burden in ft De = diameter of expl. in inch S tr = relative bulk strength (ANFO = 100) S Gr = Sp. Gr. of rock Correction for no. of rows Kr One or two rows of holes 1.0 Third or subsequent rows or buffer blasts 0.90 Geologic correction factors Corrections for rock deposition Bedding steeply dipping into cut Bedding steeply dipping into face Other cases of deposition
Kd 1.18 0.95 1.0
Corrections for Geologic Structures Heavily cracked, frequent weak joints, weakly cemented layers Thin well cemented layers with tight joints Massive intact rock
Ks 1.30 1.10 0.95
There are several formulae but in general for any given explosive rock combination, in mast instances, approximately same result is obtained. After analysis of 100 data of (O/C mines vutukuri & Bhandari 1973 found. B = 0.024D + 0.85 Where B is in meters, and D is the hole diameter in mm. It is generally kept 20 – 35 x explosive diameter.
Spacing As the angle of breakage can not be more than 90o spacing can not be more than 2B. Commonly suggested ratio is 1.2 to 1.3. On the basis of 100 data S = 0.9 B + 0.93 Where B & S are in waters. When adjacent charges breaking on separate delays need spacing = burden
Sub drilling; It is approximated as J = 0.3 B Sub drilling in ft. J = (0.3 to 0.5) x B B = burden in ft. Stemming: It is generally kept as 0.7 to 1.5 times of burden when expressed in m. If stemming is less than B then fly rock and premature venting may occur. Selection of blast hole & size Bench Height It should be greater than or equal to 2.5 X B. If it is greater than 4 B case should be taken due to hole deviation. Burden/Spacing ratio should be kept 1 to 2. L / B – stiffness ratio Stiffness ratio Fragmentation Air blast
Fly rock
1 2 3 4
Severe Fair Good Excellent
Poor Fair Good Excellent
Severe Fair Good Excellent
Ground vibration Sever Fair Good Excellent
Delay sequence/interval 3 to 6 ms/m of effective burden delay should be used to avoid chock blast. Angle drilling Advantages: 1) Less back break 2) Less problems at grade 3) More throw, especially on low benches 4) Better fragmentation on low benches 5) Loose rock better held on face by gravity Disadvantages of angle drilling 1) Harder to collar holes 2) Difficult to maintain accurate angle 3) More problem with geologic discontinuities 4) Easier to hang steel in holes 5) Back shatter 6) Back fly 7) Human response/human psychology. Cratering theory: Originally proposed by C.W. Livingston later modified by Lang and Bauer. It involves a spherical charge of length to diameter ratio of less than or equal to 6:1, detonated at an empirically determinal distance beneath the surface to optimice the greatest volume of permanently fragmented material between the charge and free surface. Livingston determined experimentally and theoretically that there is a constant factor between this critical burden distance and the cube root of the wt. of explosive and expressed it as: Strain energy equation: N = E x W 1/3 N = critical distance in feet W = weight of explosive
E = proportionality constant or strain energy factor, which has no units and is constant for one given explosive rock combination. E differs if same explosive used in diff. material or diff. expl. Blasted with material. When rock gets more bittle E increases and optimum crater volume occurs at higher values of depth ratio. The strain energy eq. can be written in another form that relates the charge depth from surface to the depth ratio, strain energy and explosive wt. as Upper limit of shock range dc = x E x W 1/3 dc = distance form surface to the center of gravity of the charge depth of burial = depth ratio = critical depth W = Wt. of expl. If dc is the optimum burden that yields the greatest volume of fragmented material, then it is referred to as do and the optimum depth ratio is referred to as . Exploders These are machines which provide the required electrical power to fire a series fo electrical detonators. Usually there are 2 types of exploders
Generator type Condenser discharge types Twist generator type exploder
Generator type
Hand driven generator powered capacitor discharge exploder. The rock bar exploders are operated by vigoroushy pushing down a rack bar which through a set of gears, spins the rotor in a DC generator. The electrical energy from the generator is connected t. Out put terminals when the rack bar reaches the bottom in its downward travel where it closes a switch. These are usually of large capacity capable of blasting 30 – 50 caps in single series. Another type is where a spring is first wound and with the help of key, it is suddenly released. This through a set of gears, spins the rotor in a DC generator. This type has very large capacity of 250 to 400 shots. The key twist type exploders are relatively smalls, hand held units that are operated by a quick twist of the handle with one hand while the machine is held firmly in the other hand. This type of exploders can initiated 10 to 25 shots of 4 each. The actual current output depends on the condition of machine and effort exerted by the shot firer. In some of the m/c internal mechanical or electronic switch puts off the output voltage at the end of 4 ms. There is also a buit-in provision which does not connect the voltage to the output unless it is above predetermined value. Condensor discharge type Generator m/c limited capacity and variable output has caused it to be replaced by the condenser (capacitor) discharge type machine. In these m/c a capacitor is charged to a high voltage and discharged later through the external firing circuit. CD m/c are capable of firing over 1000 caps in a parallel series circuit. A good rule is to use 1.5 to 2 times of the needed capacity. Sequential Blasting Machine
It is a unit containing 10 capacitor discharge machines that will fire upto 10 separate circuits with a preselected time interval between the individual circuits. When used in conjunction with ms delay elelctric blasting caps, the sequential m/c provides a very large number of separate delay intervals. This can be used in improving fragmentation and in controlling ground vibrations and airblast. Blasting cables: Requirements are low resistance, good waterproof insulation with suitable tensile strength and flexibility. 1.8 – 5.8 per 100m. Lightning detector: It measures atmospheric voltage gradient and operators both flashing lights and warming siren when gradient approaches the levels at which a lighting discharge could occur. Misfire meant that an explosive charge did not fire. Cut off” is a general term used to either describe a cutting of the initiator leads before it has a chance to energise, or cutting off the powder column by shifting bed rocks. Missed holes are those where the initiator was not connected into the firing circuit.
