ABOUT BLASTING

BLASTING – It is technique in which , solid rock mass is brought in to small by using explosives.

Explosives- It is a solid or liquid substance or a mixture of chemical substance

which is converted instantaneously in to Large volume of gases at high temperature and pressure when subjected to combustion or detonation. chemical energy is converted in  to mechanical energy which is used to break the rock masses in to small pieces.

Detonation-  it is a process of giving sufficient violent shock to the explosives or it is a process of propagation of shock wave through an explosives.

Types of explosives

Low explosives & high Explosives

•Strength-Amount of energy by an explosive during blasting.

•Velocity of Detonation-Rate at which detonation wave passes through a column of Explosive . More the intimate contact between oxidizer and fuel , higher will be the VOD.

•Density- High density explosive required for blasting in hard rock in tunneling in hard ground. Low density explosive is required when lump coal is required and in soft rock like coal.

•Water Resistance-Explosive deteriorate under water and moisture penetration.

•Sensitivity- An explosive is required to be insensitive to normal handling , shock and friction.

•Fume characteristics-Explosives must produce minimum of harmful gases.

•Legal permission- In underground mine only permitted type explosive must be used.

Ingredient of explosives:  

vCombustible matter wood meal, fibre, sulpher , charcoal etc.

vOxidizing Agent- sodium nitrate, potassium nitrate, ammonium nitrate, etc.

vStabilizers such as magnesium and calcium carbonates .

vAn tisetting agent to prevent setting of salt.

vSensitizers like metallic powder like aluminum powder.

Low Explosive-

• Is blasted through the process of oxidation of the constituent particles by rapid combustion From particle to particle through mass of explosive.

•Effect of explosive is relatively Low.

•Fired by ignition or a flame.

•Produce heaving effect.

•Gun powder, propulsive compound for fire work.

HIGH EXPLOSIVE

vContain ingredient which itself is explosive when sensitized by proper mean.

vExplodes when a violent shock is applied to it with the help of a detonator

vOxidation takes place instantaneous with high velocity of reaction.

vProduce shattering effect.

vAN, ANFO, nitroglycirine, TNT, special gelatine, slurry explosive etc.

v GUN POWDER- Charcoal 15%, sulphur 10%, Potassium Nitrate  75%. Low explosive.

vNon of its constituents is explosive.

vHas heaving effect.

vHygroscopic, loose explosive power when damp.

vProduces flame of long duration.

vThe explosives is fired by safety fuse.

vNot used in underground coal mine

Nitroglycerine : Oily fluid, Sp. Gr. 1.6,feezing point 13 0 c, insoluble in water,

vVery sensetive to explosion by shock of any nature. Seansivity increases with freezing.

vSo to make it suitable for industrial use it must be absorbed with some inert material or gelatinised.

vIt is available in three consistencies-gelatenious, semi gelataneous  and powdery.

vHas shattering effect

vProduce fumes which produces fumes which  causes headache after long exposure.

vLow freezing agent   usually Di Nitro glycol which is itself is powerful explosive is mixed to

vLower the sensivity to detonation.

vLow freezing explosives are designated as Polar – polar viking,polar special gelatine etc

AMMONIUM NITRATE: White hygroscopic salt

vVery soluble in water, safe to handle

vIt canbe detonated by a booster of high explosive. It is difficult to detonate by itself and by a detonator. Non capsensetive

vTo use it sa a explosive it is mixed with diesel oil,N.G., or TNT.

vWhen detonated by extreme shock it decomposes as

v@NH4NO3=4H2O+O2+2N2 +346 Kcal/gm.

v Collodion cotton -  reaction between cellulose compound and nitric acid yeilds Colllodian cotton.

TNT- Reaction between nitric acid and benzene  or tolune compound yeilds Trinitrotolune T.N.T.

Booster- For effective detonation of some slluary, AN-FO mixture high detonating velocity booster is used . Indian explosives LTD. manufacture a booster with trade name PRIMEX. It is cast as cylindrical pallets with two longitudinal holes for threading on to down line of detonating fuse  for priming a detonating fuse is threaded through two holes and a knot is tied at the top.

This assemby is inserted into slurry explosive cartridge and its mouth is retied at the top. After lowering the primer cartridge other cartridge are lowered.

Primex pallets are also lowered down the hole , threaded through the detonating fuse at regular intervals throughout the slluary column.

The recommended interval are Above 150mm.dia.- one 500gnm pallets every 7-8 cartridge. In 150mm.dia.- one 250gm pallets every 5-6 cartridge. 83/100mm.dia.- one 100gnm pallets every 10-11 cartridge for Powerflow -3 and similar explosives.

PRIMEX a mixture of PETN and TNT. Water resistance , VOD 7000m/s. wt. strength 82, detonated by detonating fuse of detonator.

Booster manufactured by IDL chemical limited - PENTOLITE

Following can be used as primer or booster.

Gelatanious NG based explosives can be used .Example – O.C.G. or special gelatine,

Slurry explosive such as indoprime ,Aquadyne ,and superdyne also served as booster.

ANFO- Ammonium nitrate fule oil explosive – Ammonium nitate mixed with diesel oil .

Most balance oxygen balanced explosive is one with 5 to 6 % of diesel oil with wt.. Excess amount of diesel oil lower sensitivity to initiation of the  explosives.

Sp.gr. .8 to 1.wt.strength 75-80 , VOD 3500 m/s .

Non capsensetive and initiated by small quantity of O.C.G. or by a booster.

Not used in watery hole due to loe density of the explosive.

Burden- It is the  shortest distance to relief at the time the hole detonates. Relief is normally considered either a ledge face Or the internal face created by a row of holes that have been previously shot on an earlier delay.

Burden depends on hole diameter, type of rock, geological features and desired fragmentation level.

Lower burden is required for hard rock.

If burden is too small , rock is thrown a considerable distance from the face, air blast level is high, fragmentation is fine.

If burden is too big  severe back breake occurs behind the last row of the holes

Fly rocks

Ground vibration

Boulders formation

Bottom or toe problem

Burden=20 to 35 times the hole diameter. Max. 50 times the hole diameter.

Burden=within 40- 55% hole depth Burden B in ft.=0.67 De (Stv/Sgr)

De=Dia. Of explosive in inch

Stv= relative bulk strength

Sgr= specific gravity of the rock Blast hole diameter- Dia of blast hole depends upon bench height, type of Rock, fragmentation required , acceptable vibration level, fly rock, and location of blast site .

More the bench ht. larger the hole diameter.

 Hole diameter is generally kept H/40 TO H/60. H= depth of the hole in meter.

Two general conditions for initiation timing

Blast holes can be fired near simultaneously or  fired delayed from one another.

Simultaneous initiation in a row require larger blast hole spacing than firing on delay, also

Produce increased ground vibration ( many holes in a row fired simultaneously), formation of boulders

Will also take place.

Delay timing reduces ground vibration and produce fine fragmentation.

Hole to hole delay along a row  t = T * S , T= delay const.,S= spacing in ft.