In this article we will discuss about the explosives used in mining:- 1. Introduction to Explosives 2. Properties of Explosives 3. Transport and Storage 4. Destruction and 5. Types of Explosives.
Introduction to Explosives used in Mining:
Explosives are used in underground mines and quarries to break coal and other rocks. An explosive is a solid or a liquid substance or mixture of substances which change themselves instantaneously into a large volume of gases at high temperature and pressure when a flame, heat or sudden shock (detonation) is applied to it.
The high pressure that is built up is capable of doing the work of breaking rock. Detonation is a process of giving sufficiently violent shock to the explosive to bring about an almost instantaneous rearrangement of atoms.
The chemical energy in an explosive is released and converted suddenly into heat and mechanical energy when heat, flame or detonation is applied and this conversion is due to a chemical reaction which is essential process of oxidation. An explosive, by virtue of its constituents, contains enough oxygen necessary for complete oxidation. A commercial explosive contains, apart from the explosive substance or explosive mixture.
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The following materials:
1. Combustible matter such as wood meal, fibre, sulphur, charcoal, etc.
2. Oxidation agents, such as sodium nitrate, ammonium nitrate, potassium nitrate, etc.
3. Stabilizers such as magnesium and calcium carbonates.
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4. Anti-setting agents to prevent caking of salts.
5. Sensitizers, like metallic powders.
Properties and Types of Explosives:
The following properties of an explosive are of interest to its user:
1. Strength:
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This is a measure of the amount of energy released by an explosive during blasting and hence its ability to do useful work. The relative strength or power of an explosive is given by the term weight strength in the case of explosives manufactured by ICI India Limited and a few other explosive manufacturers.
The weight strength, in the case of ICI explosives, indicates the strength of any weight of explosive compared with the same weight of Blasting Gelatine which is taken as standard because it is the most powerful commercial explosive manufactured by ICI. The weight strength of Blasting Gelatine is 100.
At present, Blasting Gelatine is not in the regular manufacturing range of ICI India Limited. The Ballistic mortar is calibrated initially with standard Blasting Gelatine and subsequently the weight strengths of other explosives are determined with respect to the above calibration.
Indian Oxygen Ltd., and Indian Detonators Ltd. do not use the term weight strength to indicate the relative strength of their explosives.
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2. Velocity of Detonation:
It is the rate at which the detonation wave passes through a column of explosive and this is of considerable importance as the shock energy of detonation increases rapidly with this velocity. Most of the high explosives, permitted explosives and slurry explosives used in the mines have a velocity of detonation ranging between 2500 and 5000 metres per second. For high explosives which are used as boosters, the V.O.D. is high, e.g. O.C.G. – 6000 m/s; Primer – 7000 m/s.
It should be noted that the basic principle of detonation is more intimate the contact between the oxidizer and fuel, the higher is the V.O.D.
3. Density:
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The density is important when selecting an explosive for a particular use. With a high density explosive the energy of the shot is concentrated – a desirable feature in tunneling and mining operations in hard ground. On the other hand when the output of lump coal from mine is important, it is advisable to use a low density explosive, which distributes the energy along the shot hole.
4. Water Resistance:
Explosives differ widely in resistance to water and moisture penetration. Some explosives deteriorate rapidly under wet conditions, but others are designed to stand water long enough to enable the work to be done. When blasting is to be performed under wet conditions a gelatinous or slurry explosive should be used. The higher the nitroglycerine contents of an explosive, the better its water resistance properties.
5. Sensitivity:
An explosive is required to be insensitive to normal handling, shock and friction, but must remain sufficiently sensitive to be satisfactorily detonated and capable of propagating satisfactorily, cartridge to cartridge and even over short gaps such as may occur in practice.
6. Fume Characteristic:
Explosives which are to be used where ventilation is restricted must produce a minimum of harmful gases in the products of detonation. Slurry explosives and AN based explosives are preferable to the NG based ones.
7. Legal Permission:
Only permitted explosives of proper type should be used in underground coal mines.
Explosives are grouped in two types depending upon the speed with which explosive effects is produced.
1. Low explosives.
2. High 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 nitrate, nitroglycerine, T.N.T., special gelatine, slurry explosives, etc. are high explosive. When a low explosive is blasted the process of oxidation of the constituent substances is propagated by rapid combustion from 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.
Transport and Storage of Explosives:
A building where explosives and detonators are stored is called a Magazine. A magazine construction has to be approved by the Inspector of Explosives and it has to be on a site which also needs his approval. Certain safe distances are specified for the magazine site and they specify the minimum distance of residential quarters, public roads, etc. from the magazine.
These distances depend upon the capacity of the magazine; larger capacity requires longer safe distance. Small magazines in the mine premises are according to licence in Form J. Such a licence permits the magazine owner to store upto 45 kg of gun powder, 5 kg., of other NG explosives, 200 detonators, 50 kg slurry explosives (nitrate mixture) and any quantity of safety fuse. The large magazines in the mine premises are according to license in Form L for storing high explosives upto 25,000 kg and 2,00,000 detonators.
Under the Explosives Rules the following can be stored together in the same magazines:
Gunpowder (class 1), nitrate mixture (class 2), nitro compounds (class 3), chlorate mixture (class 4), safety fuse, plastic igniter chord and detonating fuse (all of class 6).
Procedure for Establishing a Magazine:
An application in form ‘C’ and ‘D’ should be made to the Regional Controller of Explosives together with six copies of site plan and magazine constructional details. He will forward all the documents to chief Controller of Explosives (CCE).
CCE will then issue a form ‘E’ plus a draft copy of the license ‘L’ and will also pass one copy of site plan to District Magistrate who will issue a ‘No Objection Certificate’. Based on this, CCE will allow the applicant to proceed ahead with the construction of the magazine and also issue a licence in form ‘L’. On completion of the building the Regional Controller will inspect the magazine and endorse the licence which must be renewed annually.
A portable magazine requires a license from the Chief Controller of Explosives. It should be located on a ground about 5 m x 2 m keeping the following safe distances:
i. From all buildings, huts, places of worship, officers, houses, schools, factories, etc…..95 m.
ii. From all roads, river walls, market, playground, etc….48 m.
iii. From overhead high tension electric line … 91 m.
Deterioration in Quality during Storage:
The chemical composition of explosives may change slightly after storage under humid and hot conditions. The explosive may become insensitive to detonation or unsafe to use if the change in chemical composition is excessive. At the mines no laboratory tests are carried out to decide the extent of deterioration, but visual examination is the only guide.
When the wrapper of a gelatinous nitroglycerine explosive is removed carefully an explosive in a good state would permit the wrapper to come off cleanly from the cartridge and there would be no evidence of discoloration, incrustation, softening or exudation of liquid in the explosive itself.
A gelatinous explosive slightly darkens during storage, particularly at the ends due to absorption of moisture from the air. Such discoloration is usually accompanied by softening and dampening of the explosives. The moisture absorbed by the explosive may cause a white incrustation of ammonium nitrate or sodium nitrate on a gelatinous explosive.
A higher degree of deterioration is indicated by exudation of nitroglycerine or aqueous solutions of sodium or ammonium nitrate and the cartridges becoming too plastic. Such explosive should not be used but destroyed.
Powdery Nitroglycerine:
Powdery nitroglycerine explosives detenorate much faster than the gelatinous quality as the former absorb moisture more readily. The deterioration is indicated by discoloration at the ends, softening and dampness. If there is any exudation of liquid the cartridges should be destroyed.
Ammonium Nitrate:
Ammonium nitrate explosives mixed with T.N.T. cake and become solid due to absorption of moisture during storage. If the cartridges become so solid that they cannot break easily between the finger and the thumb, they should not be used but destroyed.
Slurry Watergel Emulsion Explosives:
Deterioration of slurry explosives is usually indicated by a breakdown of the gel into separate liquid and solid components. It may also appeal as hardening of the explosive cartridge. Cartridges may also be swollen or distorted and salts in the composition may crystallize on the outside.
Detonators:
Detonators showing signs of corrosion of the tubes are dangerous and must not be used; this applies particularly to plain detonators.
Detonating Cords:
Almost the only cause of deterioration or desensitization of detonating cord is moisture penetration into the explosive core. Prolonged exposure to sunlight may cause the plastic outer coating to become brittle.
Destruction of Unserviceable Explosives:
Explosives which have become unserviceable for blasting should be destroyed and never dumped into a garbage heap. It is a sound policy to seek expert advice from the manufacturers for methods of destroying explosives. Always assume that burning an explosive for destruction may change to detonation.
The method adopted for destroyed are as follows:
i. Gunpowder:
It should be thrown into water, preferably hot water, which dissolves out the saltpeter and renders the explosive harmless.
ii. Ammonium Nitrate Explosives:
These are hygroscopic and may be buried in a damp ditch where they become harmless with the absorption of moisture. Or they may be destroyed by drawing in a large volume of water.
iii. Explosives with Nitroglycerine:
Not more than 25 kg of explosives should be destroyed at a time. A clear space of ground free from dry grass, about 100 m all around, should be selected and a lie of shavings of dry straw or grass lay down. On this the cartridges should be placed in a continuous line not more than two abreast with the cartridge wrappers below them and with an interval of about 25 mm between two cartridges.
The line of shavings of straw or grass should extend some distance say 6 m, beyond the explosives. Kerosene should then be sprinkled over the shavings of straw or grass. The straw should be lit with a short length of safety fuse and the operator should withdraw immediately to a safe distance. The direction of wind should be given due consideration when burning the straw.
The degree of containment caused by the cartridge wrappers in high explosives is sometimes sufficient to cause explosion. It is, therefore, essential to open the wrappers and unroll them.
iv. Safety Fuse:
It should be destroyed by burning in length in the open under suitable precautions.
v. Detonators:
They should be thrown into a deep river or alternatively, they may be soaked in mineral oil for 48 hours and then destroyed, one at a time under suitable precautions, by burning. When destroying explosives do not select a stony site where burning or detonation could create situation of flying stones during destruction.