Explosives are grouped in two types depending upon the speed with which explosive effect is produced:
1. Low explosives,
The terms "low explosives" and "high explosives" should not be confused with the terms "low density explosives" and "high density explosives".
Gunpowder is a common example of a low explosive and ammonium nitrates, nitro-glycerine, T.N.T., special gelatine, slurry explosives, etc., are high explosives. When a low explosive is blasted the process of oxidation of the constituent substances is propagated by rapid combustion from particle to particle through the mass of the explosive and the effect of explosion is relatively low. A low explosive is fired by ignition or a flame. High explosive always contains an ingredient which is explosive in itself, at least when sensitised by proper means. A high explosive explodes when a violent shock is applied to it with the help of a detonator; the process of oxidation does not proceed from particle to particle, but is instantaneous and the constituents react with high velocity. High explosives therefore produce a shattering effect.
Gunpowder: This is the earliest known explosive and contains the following constituents (by weight, approx.)
Charcoal 15%, Sulphur 10%, Potassium Nitrate 75%.
None of the constituents is explosive by itself. Gunpowder is cheap, stable, safe to handle and it does not adversely affect the roof in' underground mines as its action has a heaving effect. It is easily manufactured and is available from local supply contractors in many mining localities. It is, however, not used on a large scale as it loses its explosive power when damp and it is not as strong as other high explosives. Moreover, during blasting, it produces flame of long duration and the burning particles are liable to remain in contact with the surrounding atmosphere for some time. For these reasons gunpowder is not used in wet places and in underground coal mines. The explosive is fired by safety fuse.
Important constituents of high explosives which confer explosive properties on them are nitro-glycerine, ammonium nitrate, collodion cotton and T.N.T.
2. High explosives.
Nitro-glycerine: it is an oily fluid with sp. gr. of 1.6 and freezing point at 13°C. It is insoluble in water and is very sensitive to explosion by shock of any nature. The sensitivity to shock increases when nitro-glycerine freezes. To render it suitable for industrial use, it must either be absorbed by some inert material or be gelatinised. NG based explosives are available in 3 consistencies; gelatinous, semi-gelatinous and powdery. All explosives containing NG have a highly shattering effect and they produce fumes which cause headache after long exposure. Explosives containing NG are liable to freeze when the temperature falls to 80C or less and are then more sensitive to detonation by friction and impact. To avoid this, a low freezing agent, usually di-nitro-glycol which is itself as powerful an explosive as NG is used. Low freezing explosives are designated by such prefix as "Polar", e.g. Polar Viking, Polar special gelatine, etc.
Ammonium nitrate (NH4NO3): It is a white hygroscopic salt, very soluble in water and is comparatively very safe to handle. When it is detonated it is, however, a powerful explosive. Though ammonium nitrate is more powerful than the low explosives it is not as powerful as N.G. and it is difficult to detonate it by itself with the help of a detonator, but it can be detonated by a booster of high explosive. To use ammonium nitrate as an explosive, it should be mixed with diesel oil, N.G. or T.N.T. Ammonium nitrate is an interesting compound, in that it is a high explosive, an oxidising agent, and a cooling agent at one and the same time. Prilled ammonium nitrate of fertiliser grade mixed with diesel oil is used for large dia. holes in quarries.
Ammonium nitrate does not occur in nature and it is prepared by reacting ammonia gas with nitric acid. When detonated by extreme shock NH4N03 decomposes according to the equation:
2NH4NO3 = 4H20 + O2 + 2N2 + 346 kcal/gm.
This property comes into play in the utilisation of NH4NO3 as an explosive.
Collodion cotton: A reaction between cellulose compounds and nitric acid yields collodion cotton, a high explosive. To render it safe to handle it is gelatinised.
T N.T.: Reaction between nitric acid and benzene or toluene compounds yields Trinitrotoluene (T.N.T.) which is highly explosive.
Most of the high explosives can be blasted with the help of detonators and are said to be cap sensitive. A detonator has to be inserted into the explosive or kept in intimate contact with it. Such cap-sensitive explosive can be blasted with the help of a detonating fuse like cordtex also if the latter is in close contact. The detonating fuse itself needs to be detonated by a detonator. An explosive which is not cap-sensitive is blasted by keeping it in close contact with a booster which itself needs to be initiated by a detonator or a detonating fuse. A non cap-sensitive explosive is also known as "booster sensitive". An explosive used in underground mines has to be cap-sensitive but the explosive used in quarries may be cap-sensitive or booster sensitive.
AN-diesel explosives are not cap-sensitive and have to be blasted by keeping them in intimate contact with an explosive (booster) which itself is detonated with the help of a detonator. Some of the slurry explosives like energel, supergel and powerflo 1,2,3. are not cap-sensitive while most others are.
Explosives with NO base which have to be used in watery places should be of gelatinous or semi-gelatinous consistency but not powdery and they should sink in water .i.e. they should have sp. gr. of more than 1.0.
Booster: For effective detonation of some slurry explosives and AN-FO mixture such as GN-1, use of a high detonation-velocity booster is necessary. ICI India Ltd. manufactures a booster with the trade name "Primex" which is a mixture of PF.TN and TNT. It is water resistant, has a velocity of detonation of 7,000 m/sec. weight strength as 82 and it can be detonated by a detonating fuse or, a detonator. The booster manufactured by IDL Chemicals Ltd. is marketed by the trade name "Pentolite" which has a sp.gr. of 1.55 to 1.61- Compared with normal explosives boosters are quite costly. Prfmex is cast in cylindrical pellets provided with two longitudinal holes for threading on to a down line of detonating fuse. For priming, a detonating fuse is threaded through the two holes in the Primex pellet and a knot tied at the top. This assembly is then inserted into a cartridge of slurry and its mouth re-tied by a wire. After lowering the primer cartridge, other cartridges are freely dropped down the hole.
A cast booster is not a substitute for the explosive charge; it may be compared with a very powerful detonator of large size and is preferred for deep large dia. blast-holes in opencast mines. During use a cast booster is knotted to the detonating fuse for placement at the bottom of the blast-hole and additional boosters are threaded in the same detonating fuse so that their positions coincide with the level of hard rock when the fuse is in position in the blast-hole. After lowering the booster by the detonating fuse, AN-based site-mixed or plant-mixed slurry is poured or pumped in the hole. As the slurry is pumped in, special ingredients are added to it at intervals to cover the hard-rock portion.
Gelatinous NG-based explosives having high NG content like OCG or special gelatine can be used as a booster or primer explosive. Pentolite boosters manufactured by IDL Chemicals Ltd. do not contain NG or other headache - causing ingredients. They are equipped with 2 holes for easy initiation with detonating cord or detonators. A PHTN based booster in a shot-hole occupies only 1 to 2% of the total explosive charge but a high explosive like OCG, if used as a booster, occupies 15-20% of the total explosive charge.
AN-Fuel oil explosives (ANFO)
Ammonium nitrate, mixed with diesel oil, is used on a large scale for blasting in the quarries of coal and metal mines. The most effective and oxygen-balanced explosive mixture is one with 5 to 6% of diesel oil (by wt.) It has a sp. gr. of 0.8 to 1.0, wt. strength of 75-80 and velocity of detonation at 3500 m/sec. In the dry season, 7 litres diesel for 100 kg of AN suffice but in wet season, the quantity should be increased to 9 litres. Diesel oil in excess of 8% tends to lower the sensitivity of ANFO to initiation. The mixture causes irritation of the skin and the workers should, therefore, wear gum boots and rubber hand gloves. The mixing should be done with wooden shovels avoiding contact with iron. The mixture is safe to handle and without formation into cartridges can be mechanically loaded into blast-holes. Where ANFO consumption is heavy, stationary ANFO mixer similar to the concrete mixer may be placed at a centrally selected site. In case of a pneumatic ANFO loader, an electric detonator should not be used unless steps are taken to prevent premature initiation due to static electricity.
As the mixture cannot be initiated in the normal way by a detonator it is necessary to prime it with a small quantity of O.C.G. or a booster. It is a good practice to use high explosive primer cartridges at the top as well as at the bottom of the hole and also for the deck charge. The AN-fuel oil explosives are difficult to sink in water due to low density of AN, and should preferably not be used in watery holes. If however, AN-fuel oil mixture has to be used in .watery holes it should be packed in polythene bags and forced down the hole with the weight of a high explosive and the stemming above it. Holes of 62 mm dia. and above are considered economic for use of AN-FO explosives.
An increase in blast-hole dia. beyond 300 mm decreases the sensitivity to initiation of ANFO explosives. With above 4 percent water in ANFO, the velocity of detonation decreases sharply and the mixture with 9 percent or more water cannot be detonated. When /using ANFO, it is essential to have uniform mixing of ammonium nitrate and fuel oil. If the ammonium nitrate is not of adequate porous quality, it may separate from fuel oil resulting in inferior performance.
AN-FO explosive cannot be initiated direct by No. 6 detonator. It can, however, be blasted by a detonating fuse which needs no. 6 detonator for initiation. It may be initiated by no. 8 detonator which is not much used in mining practice.
Slurry explosives
The slurry explosives are with jelly like consistency and are water gels. (The water-gel is a mixture of an oxidiser and fuel sensitizer in an aqueous medium, thickened with a gum and gelled with cross linking agent). In the case of permitted slurry, a coolant is added to reduce incendivity.
The first commercial slurry explosive developed by Dr. Melville Cook in U.S.A. in 1957 consisted of TNT, AN and water in the ratio of 20:65:15. To this traces of chemicals for gelling and cross linking were added to stabilise the homogeneity of the mixture. In subsequent years the manufacturers developed slurries with AN as the main ingredient and using variety of sensitizers and fuels. The addition of metallic powder to the slurry enables the explosives to reach very high strength. Some of the common ingredients are:
Oxidisers: ammonium, sodium or calcium nitrates.
Cross-linking agents: potassium or sodium dichromates, antimony or boron compounds
Gelling agents: starch
Fuel sensitisers: TNT, PETN, pentolite - (all explosives). aluminium, sugar, urea, paraffin, hexamine, ethylene glycol, wood pulp - (all non-explosives)
The slurry explosive has a sp. gr. more than 1 and like ANFO can be poured directly into watery holes. They are also available in the form of cartridges with plastic or polythene wrapper and some (permitted type) can be used in underground coal mines. Such slurry explosives for use in underground coal mines have to be cap-sensitive and approved by the DGMS, The slurry explosive is highly water resistant. In the quarries holes of diameter 62 mm and above are economical for use of slurry, just as for ANFO, if it has to be poured into blast hole.
, The components required for ANFO and slurry explosives may be mixed at a plant away from the blasting site or at the blasting site itself. In the case of PMS (plant mixed slurry) system, the explosive is loaded into special tankers and from these tankers, the slurry is pumped directly into the blast-hole.
Where the volume of blasting is high enough to justify cost of transporting trucks, ICI has designed a slurry pump truck which is capable of pumping Powerflo slurry directly into, the blast holes. The system has been extensively tested in overburden strata in opencast coal mines as well as in hard iron ore formations with very satisfactory performance. The slurry remains in intimate contact with the walls of blast holes and this result, in effective utilisation of the explosive. Packaged explosive products are at least 15% less effective than the corresponding bulk explosive products due to less perfect blast hole contact.
Unlike cartridged slurries, pumpable slurries can be tailored to have the appropriate density depending on strata conditions. In the case of site mixed slurry system (SMS) only non-explosive ingredients are stored at a warehouse and transported to the blasting site in a specially designed pump-truck.
In the semi-gelled condition, pumpable slurries can be stored for about 2 weeks in the truck without any adverse effect on performance. The slurry is suitably gelled (cross linked) at the time of manufacture and incorporation of an additional gelling (cross linking) agent at the time of loading into blast-holes can prevent the possibility of explosive flowing into the cracks and fissures in the blast-hole.
In case of SMS system for blasting, one pump truck can charge nearly 25000 kgf in one shift. A small team of 5-6 professionally trained persons can load 50,000-60,000 kgf of explosives into a large number of blast holes in a single working shift.
The principal ingredients in I. D. L's permitted slurry explosives -are ammonium nitrate, water, sugar, aluminium (as fuel sensitizer), sodium chloride, sodium nitrate and gum.
Slurry explosives will not explode accidentally if dropped, or from shovel impact, or even when involved in a fire. They have low non-toxic fumes and do not cause headache. Exudation from an unstable water-gel explosive is harmless since in most cases it is nothing but water containing some dissolved inorganic salts. On the other hand NG exudation in NG-based explosives is hazardous; a spark or violent shock can set it off. Non-cap-sensitive slurry explosives can be stored in a building which is not a licensed explosives magazine, hut permitted slurry explosives have to be stored in a licensed magazine.
Shelf life of slurry explosives manufactured by most of the companies is usually one year. The best performance of slurry explosives is obtained within four months from the date of manufacture.
The standard safety tests for slurry explosives are:
(1) Burning test,
(2) Friction and impact test (Torpedo test),
(3) Impact sensitivity by hammer fall test,
(4) Rifle bullet test,
(5) Sensitivity of flame head (fuse) test, etc.
Emulsion Explosives
An emulsion is an intimate mixture of two liquids that do not dissolve in each other. In more technical terms, an emulsion is described as a two-phase system in which an inner or dispersed phase is distributed in an outer or continuous phase. Emulsions have, for many years, contributed to our daily lives in such products as insecticides, photographic films and papers and cosmetics.
The unique feature of an emulsion explosive is that both the oxidizer and the fuel are liquids. The unique properties of emulsion explosives are due to the minute size of the nitrate solution droplets and their tight compaction within the continuous fuel phase.
The emulsion slurry contains AN solution at high temperature, combined with diesel oil, and an emulsifier which is passed through a fast moving blender. The blender mixes the ingredients into pumpable emulsion slurries with grease like consistency. The resultant emulsion slurry is now made up of microscopic droplets of AN, surrounded by fuel oil film and artificially created air bubble known as micro spheres which make the emulsions detonable. There is no sensitisation by aluminium or explosive sensitizer but by mere air bubbles. Thus, there is maximum energy generation because of intimate contact of oxidiser and fuel. In small diameter products, glass micro-balloons and perlites are used to maintain sensitisation by aeration.
Emulsion explosives depend entirely on the presence of voids for initiation and propagation. A change in the amount of voids effects a change in density. It is convenient and useful to relate properties to density and to consider voids as density adjusters.
Slurry explosives require thickeners and gelling agents to prevent segregation, to provide water resistance and to control losses through cracks and fissures. Emulsion explosives cannot be gelled or cross linked. They do not have the gel structure that characterises all slurry explosives. Velocity of detonation is a good indicator of reaction efficiency and is very dependent on particle size:
Explosive
ANFO
Slurry
Emulsion
Size
2.000 mm
0.200 mm
0.001 mm
Form
All solid
Solid/liquid
Liquid
VOD (km/sec)
3.2
3.3
5.0-6.0
Emulsion explosives are highly water resistant. They are more fluid than slurry explosives and therefore create problems when loading a blast hole with fissures or cracks. They lack the strong cross-linked gel that characterizes the TNT—sensitized slurry explosives.
Emulsion slurries are claimed to have lower ingredient cost, higher density, higher VOD and higher energy conversion and resultant bore-hole pressure as compared to water gels/slurries. Therefore in low diameter sector they are fast replacing slurries and water gels in USA. The emulsions are also claimed to be sensitive even at smaller diameter holes, less than 25 mm. In addition, they are easily pumpable without affecting their quality. Thus, emulsion, slurries are hoped to be the low-cost, less hazardous replacement of NG based small diameter products.
Heavy ANFO
The latest development of 1980's had been the use of emulsion slurries mixed with different proportion of ANFO to give water resistant and higher density mixtures which are named as Heavy ANFO or HANFO. Thus, the emulsion to ANFO ratios can be from 20:80 to 50:50, depending on the severity of watery conditions and need of stronger blast energy. One of the US manufacturers has successfully developed Heavy ANFO, using NCN slurry with ANFO. Since the air spaces between AN prills are filled by emulsion, Heavy ANFO gives the advantage of lower cost like ANFO with higher density, higher energy and better water resistance than ANFO. The latest variation of this idea of HANFO is to provide colloidal or emulsion concentrates which could be mixed with ANFO or AN. Thus the mixture can have bulk density from 1.10 to 1.25 gm/cc (compared to ANFO 0.8 gm/cc) and bulk strength almost 45% more than ANFO. The high energy ANFO through HANFO system or slurry concentrates system allows expansion of drilling pattern, thereby reducing drilling costs.
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