Explosives, Theory of Breakage and Blasting Operations
Explosive is a compound or mixture which is capable of undergoing extremely rapid decomposition.
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Explosives, Theory of Breakage and Blasting Operations Author: Partha Das Sharma, B.Tech(Hons.) in Mining Engineering, E.mail: email@example.com, Website: http://miningandblasting.wordpress.com/ Introduction General types of Explosives • Commercial explosives • Military explosives 2 Explosive Ingredients and their Function Ingredient Ethylene glycol dinitrate Nitrocellulose (guncotton) Nitroglycerin Nitrostarch Trinitrotoluene (TNT) Metallic powder Black powder Pentaerythritol tetranitrate (PETN) Lead azide Mercury fulminate Ammonium nitrate Liquid oxygen Sodium nitrate Potassium nitrate Ground coal - Charcoal Paraffin Sulfur Fuel oil Wood pulp Lampblack Kieselguhr Chalk -Calcium carbonate Zinc oxide Sodium chloride Chemical formula C2H4(NO3)2 C6H7(NO3)2O2 C3H5(NO3)3 C7H5N3O6 Al NaNO3 + C+ S C3H8N4O12 Pb(N3)2 Hg(ONC)2 NH4NO3 O2 NaNO3 KNO3 C CnH2n+2 S (CH3)2(CH2) (C6H16O3)n C SiO2 CaCO3 ZnO NaCI Function Explosive base – lowers freezing point Explosive base – gelatinizing agent Explosive base Explosive base Explosive base Fuel sensitizer : used in high density slurries Explosive base Explosive base Explosive used in blasting caps Explosive used in blasting caps Explosive base : oxygen carrier Oxygen carrier Oxygen carrier – lowers freezing point Oxygen carrier Combustible, or fuel Combustible, or fuel Combustible, or fuel Combustible, or fuel Combustible, absorbent Combustible Absorbent – prevents caking Antacid Antacid Flame depressant (permissible explosives) 3 Chemical explosives • is a compound or mixture which is capable of undergoing extremely rapid decomposition. • An explosion can be broken down into four phases • • • • Release of gas Intense heat Extreme pressure, and The explosion 4 Chemical explosives When the explosive is detonated, • • • • gas is released, temperature of the gas increases, pressure also increases (Charles’ law). move and break the rock. 5 How to compare explosives • • • • • • Strength Detonating velocity Fume class Water resistance Density Physical characteristics • Storage • Freezing • • • • • Detonation pressure Energy Sensitivity Sensitiveness Flammability 6 How to compare explosives • Strength : % of active material • Velocity of Detonating (VOD): is the velocity at which the detonation wave moves through the explosive (ft/s or m/s) • Fume class : the amount of toxic fumes which determine its safety to be used in particular situation such as underground operations. 7 How to compare explosives • Detonation pressure : is the pressure behind the detonation front. • Energy • Sensitivity : the minimum energy/pressure needed for detonation. • Sensitiveness: measure of explosion wave spreading from one stick to another. • Flammability : easiness to ignite by flame or heat 8 How to compare explosives • Water resistance : is the ability to resist contamination or a reduction in strength when exposed to water. Sometimes determined by the length of time it can be submerged in water and still perform as designed. • Density : is the explosive wt per given volume. Aid in blast design. 9 How to compare explosives • Physical characteristics: commercial explosives can take three basic forms: granular, gelatin, slurry and emulsion. The choice of form depends on the usage required. 10 How to compare explosives • Storage: how explosive can be stored without affecting its safety, reliability, and performance. Early nitroglycerin (NG) dynamites were extremely poor for storing due to separation of NG from the other components and creates an extremely hazardous condition. • Freezing : important for safety and performance especially in cold climate. Anitfreezing additives may be used. 11 Drills and Drilling • The drilling system consists of the drill: the drill steel, or rod; and the bit. The bit penetrates the rock by the force it imposes on the rock. Bits are designed for percussion, rotary drilling, or both. • Hand held drills • External –percussion drills • Down-the-hole drills • Rotary drills 12 Theory of Breakage Purpose of blasting • One solid piece → smaller pieces (fragmentation) → to be moved or excavated (movement). • Underground blasting, for example, requires greater fragmentation than surface blasting because of the size of the equipment that can be used and the difficulty of access. • Get the desired results with a minimum cost 13 Theory of Breakage Involves two basic processes: • Radial cracking • Flexural rupture • Rock is stronger in compression than in tension. Therefore, the easiest way to break rock is to subject it to a tensile stress greater than its ultimate strength in tension. • Rocks are heterogeneous (contain different types of rocks). They differ in their density. 14 Theory of Breakage Free face Borehole Compression waves Radial cracking 15 Theory of Breakage • The distance from the borehole to the free face is the burden. • The denser the rock the faster the waves • Proper fragmentation when enough to travel to the face and back overcoming the tensile strength of the rock. • Along the face the outermost edge is stretched in tension which causes cracks. 16 Flexural Rupture • The second process in breaking rock by bending the rock to the point where the outside edge, the side in tension, breaks. • Caused by the rapid expansion of gases in borehole. • Analogous to the bending and breaking of a beam. • Movement or displacement are required in addition to cracking. 17 Flexural Rupture • After detonation the redial cracks expands and the gas starts to the movement by putting a CS against the borehole wall causing its bending. • The deeper the hole, the greater the burden and borehole spacing. 18 Blast Design • Is the safe and economic way to do blasting • Factors affecting blasting design • Geological factors (out of blaster’s control) • Controllable factors • • • • • Borehole dia. Burden Spacing Stemming Design of the delay firing system. 19 Burden & spacing Burden is the distance from the blast hole to the nearest perpendicular free face. Spacing Burden Free face 20 Burden & spacing determination Andersen Formula B= (dL)0.5 • B : burden, ft d : borehole dia, in • L : borehole Length, ft • Langefors’ Formula • • • • • • V= (db/33) [Ps/cf(E/V)]0.5 V : burden, m db : dia of drill bit, mm P : degree of packing = 1-1.6 kg/dm3 s : wt strength of explosives (1.3 for gelatin) c : rock constant, generally 0.45 f : 1 degree of fraction, for straight hole = 1 E/V = ratio of spacing to burden 21 Spacing determination Spacing is the distance between blast holes fired in the same row • It is necessary to complete burden calculations before determining the spacing. S= (BL)0.5 • B : burden, ft • L : borehole Length, ft 22 Controlled Blasting To control overbreak and to aid the stability of the remaining rock formation. • There are following methods: • • • • Line drilling (unloaded), Cushion blasting Smooth-wall blasting Presplitting 23 Controlled Blasting – Line drilling • Provides a plane of weakness to which the rock can break. • Helps to reflect shock waves, • Reduces the shattering effect of the rock outside the perimeter. • Do not exceed 3 in in dia and are spaced one to four diameters apart (due to cost). • Are not loaded • Requires more drilling more than the other controlled blasting methods. • Is not very effective in non-homogeneous formations. 24 Controlled Blasting – Line drilling Free face Unloaded line drill holes 25 Cushion Blasting • Requires a single row of holes ( 2 to 3.5 in) in dia. • Permits a reduction in the No. of holes required by line-drilling • Unlike line-drilling holes, the cushion holes are loaded with light charges. • Holes are fully stemmed between charges, allowing no air gap, and are fired after the production shot has been excavated. • The stemming acts as a cushion to protect the finished wall from the shock waves. The larger the borehole, the greater the cushion. • Not suitable for underground - tough stemming requirements. • Drawbacks: (1) requires removal of excavated material before firing (costly due to production delay – no excavation for entire area at once). (2) Sometimes the production shot can break back to the cushion holes, creating redrilling problems and causing loading changes. 26 Smooth-wall Blasting • Similar to cushion blasting 27 Pre-splitting • Creates a plane of shear in solid rows along the desired excavation before the production blast. • All holes are loaded like cushion blasting • Reduces overbreak • Reduces the vibration 28
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