Analysing Rock Blasting With Explosives Construction Essay

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Introduction

The main job of a quarry manager is to improve efficiency and hence profitability. This commences with the mass distribution optimization of a shot firing. The shot's mass distribution is a histogram showing the particle size analysis of the fragmented rock. The ideal particle size analysis is to achieve a minimum of very small and very big size blocks. In fact, the blocks which are very small (similar to dust diameter <5mm) have a low value due to their low uses in construction. And the very big blocks, (diameter > 1 meter) can not be processed straight by the crusher and they need to be reduced by a rock breaker. This is inefficient and hence increases operating costs

The design of the shot firing operation must be optimised to control fragmentation and hence particle size distribution. The author has experience of a hard rock quarry in Cameroun where more 15% of the shot rocks were very big blocks and they needed at full-time two rock-breakers to reduce block size suitable for mechanical crushing

Hypothesis

Oversize blocks may be obtained following blasting due to the geotechnical properties of the rock. those are believed to be caused by lack of knowledge of the influence of spacing natural discontinuities ( bedding, joints, faults) and their infill(void, clay, water, sand) in the rock blasting efficiency This problem can be minimised by re-designing to shot firing operation by altering shot-hole layout and charging

Aims

To understand tropical weathering and fracturing

To identify fracturing by blasting

Propose modified shot pattern to eliminate oversize

Abstract

Before commencing the report, a prerequisite explosive is indispensable for better understanding for a better understanding on chemistry and physical of the explosion phenomenon. After that, the methods used for the rock blasting will be studied in order to understand problems and causes leading to oversize blocks and corrective actions that could limit and reduce the oversize of the blocs. The last part is the approach of the oversize bloc problem by software.

Methods

Research have been achieved, the first part A. Prerequisite Explosive Courses. The final plan of the Honour Project is well advanced and the part B is being written. The last part of the honour project must be more difficult to achieve because Rock Blasting Software cost 2500 pound but I have contacted my Engineering school and it may possible to get software without paying.

B. Shot firing design

- General Points on the blasting agent use for mining extraction

- Products used in initiation system and delays

- Blasting pattern conception

- Nuisance and measure control linked to shot firing

C. Problems and causes leading to oversize blocks and corrective actions

Human causes: screen opening spacing, drilling control Explosive feed regulation

Geotechnical : Spacing of natural discontinuities, Infill of natural discontinuities, Ground/Rainwater, Rock strength: material and mass

Corrective actions: Drilling/explosive feed regulation adjustment, Intermediate tamping

D. Numerical Modelling software

Study on the particle size method with blasting rocks

A. Prerequisite Explosive Courses

I. Explosive History

Nowadays explosives are a major asset in Civil Engineering especially in rock blasting. Explosives permit the production large quantities of building material. Some data, 80.000 tons of explosives are used each year for the Civil Engineering and 67% for coal extraction and 14% for non-metal mine and quarry.

1) The beginning

According to Lacabane, L 1845. De la poudre a canon a son introduction en France, the great history of pyrotechnics commences around the VI century AD in China; a Doctor Alchemist named Sun Simiao sought the elixir of Immortality. During on of his experiments he mixed sulfides (PbS) and charcoal (CxH y) to saltpeter (KNO3) and he discovered an explosive mixture also known as the black powder. It was created by the merest chance, the first explosive and early fireworks. Rapidly between the VI and the X century, the black powder has been used in military applications mainly in Asia and Far East such as black powder grenade, incendiary arrows and flamethrower made of bamboo filled with gunpowder.

The use of black powder has been reported in Europe in the XIV century by Marco Polo during his long journey in Asia. 50 years later, guns appeared in Europe. During the following centuries the main use of the black powder is no longer for military applications but for Civil Engineering applications including mining extraction which is increasing. For example, the black powder permits to progress of 12meters/month (without explosives the task becomes extremely difficult fewer 2meter/month) in tunnels but it presents many drawbacks. The explosion produces a toxic black smoke (NO, CO) and it needs to be dispersed before the worker can return to the site. This loss of time incited research on new types of explosives, more powerful and especially smokeless during the second third of the nineteenth century.

2) XIX century, Industrial Revolution in Europe

In the middle of the XIX century, two major new products appeared TNT and Nitro-glycerine (dynamite). They are 6 to 10 times more powerful than the black powder. These explosives resulted in significantly increased the production in coal mines (the mainly source of energy in Europe) and of course, the military applications due to high destruction power. People make the confusion TNT and Dynamite. Theses explosives are completely different by first their composition and field applications

a) TNT

According to Chemical Business history of manufacturers, the trinitrotoluene was discovered in 1863 by a German chemist Joseph Wilbrand. It is made from the nitration of toluene (solvent for paints, derived from coal or oil). Its formula is C6H5CH3. This is an explosive of low sensitivity and moreover it is very toxic and carcinogenic. That is the reason why it is not used for Civil Engineering applications. Nevertheless, TNT is used in torpedoes and mortar bomb by German military for the 1st World War because of this low sensitivity. The projectile explodes after the shock with this objective (submarine, tank) and hence it causes more damages.

Although TNT will not be longer studied in the Honor Project it is essential to mention it because TNT is used as reference to describe the power of a explosive: 1kg of TNT corresponds to 4.2 MJ / kg. For bombs whose power is much greater than TNT is used multiple per kg i.e. kiloton (1,000 tons) and megaton (1,000,000 tons)

b) Nitro-glycerin

Nitro-glycerin, C3H5 (NO3)3 , was discovered by Ascanio Sobrero in 1847, an Italian Chemist. But unfortunately, he died in meantime. Nitro-glycerin is a composed very instable (the chemical/physical proprieties will be study in the Honour Project) above a temperature of 21 Celsius degree and it is very sensitive to shocks but it has a power equal 10 times to Black Powder. Despite its liability to detonate, the industrial manufacturing process was developed by Alfred Nobel in the 1860s. Many accidents continued to happen in the manipulation of the explosive causing several deaths and critical injuries. In 1864, Alfred Nobel's brother die in explosion in making nitro-glycerin in the Nobel Laboratory. In the following months the use of nitro-glycerin is forbidden in the all the Europe.

But nitro-glycerin is become indispensable in the coal mining extraction because its efficiency. Even this ban of use, artisanal production of nitro-glycerin occurs in mining, however, like any homemade explosives, accidents were frequents.

c) Dynamite

According to Alfred Nobel bibliography, in 1866s Alfred Nobel discovered a new process to stabilize nitro-glycerin. He added kieselgurh (diatomaceous fossil powder) in his composition and he molded the explosive in a form of sticks wrapped in paper; the first explosive, inexpensive to produce and stable has been created, the dynamite. The problem with the dynamite is with time or temperature above 32° C, sticks of dynamite ooze liquid nitroglycerin onto the exterior of the cartridge. This can accidentally detonate. That does can be dangerous and raises security problem on construction sites. Therefore it is not used anymore today. It has been removed by PETN, an explosive belonging to nitro-glycerin family.

3) XX century until Nowadays, Ammonium Nitrate Explosives NH4NO3

Ammonium Nitrate is before all a fertilizer used for agricultural production. This cost about 4 times less than PETN (60 cents / kg). It is mixed with diesel, fuel oil to be used as explosive. Nitrate Explosives are difficult to initiate; in fact, in mining extraction it must be initiated with dynamite (The chemical/physical proprieties will study in the Honour Project). Ammonium Nitrate is mainly used in rock blasting to reduce cost firing and decrease the quantity dynamite used. The preparation of Nitrate Explosives is very basic; the components can be bought in high quantity readily, the preparation does not need precautions because as the TNT, it has a very low sensitivity, thus it is safe to store in high quantity

II. Explosion principle

1). Combustion

Combustion is a chemical process which occurs between oxygen and a substance. It can become very rapid and highly exothermic; this is the reason why there is the presence of a flame during the combustion.

Combustion commences a low temperature; the first step is to increase the temperature of the substance until its ignition point. At the ignition temperature, the substance ignites it-self, that is generated the flame

In the chemistry of explosives, the combustion involves complex molecules CaHbNcOd which is decomposed in smaller and stabler molecules such as carbon monoxide or dioxide (CO, CO2), steam (H2O) and nitrogen gas N2). The combustion is a redox, a fuel oxidation by the oxidizing element. The combustion needs fuel (CaHb) and oxidizer elements (NcOd) and initiation energy to commence the reaction

According to"J.Akhava the chemistry of explosive, the decomposition of the product is determined by the Kistiakowsky - Wilson Rules

1. Carbon atoms are converted to carbon monoxide

2. If any oxygen remains then hydrogen is then oxidized to water

3. If any oxygen still remains then carbon monoxide is oxidized to carbon dioxide

4. All nitrogen is converted to nitrogen gas N2"

Example of the PETN decomposition

PETN is a derived from Nitro-glycerine and its formula is C5H8N4O12, following the Kistiakowsky - Wilson Rules its decomposition is

C5H8N4O12 → 5CO + 4H2O+ 7O + N2

There is enough oxygen to oxidize the carbon monoxide: 5CO + 7O → 5CO2 + 2O

To conclude C5H8N4O12→ 5CO2 + 4H2O+ 2O + N2; Explosives release oxygen (the quantity of oxygen is calculated by the oxygen balance cf. A.I.2.Chemistry of explosives) that means an explosive can explode without oxygen. That is the reason why the combustion phenomena occurs very rapidly with explosives, the combustion needs only some energy to commence the reaction

2) Chemistry of explosives

To explain the chemistry of the explosives, the author is going to describe and calculate the temperature/ pressure and oxygen balance from the explosion of 1 litre of nitro-glycerine.

C3H5 (NO3)3 → 3CO2 + 2.5H2O + 1.5N2 + 0.25O2

The density of the Nitro-glycerine is equal to 1, 6

The molar volume for gas is equal to 22, 4 L/mol

M (H) = 1 g/mol; M (C) = 12 g/mol; M (O) =16 g/mol; M (N) = 14g/ mol

M (C3H5 (NO3)3) = 12 x 3 + 5 x 1 + 3(14+ 16 x 3) = 227 g / mol

1 litre of Nitro-glycerine is equal to 1.600 g, hence 1.600/227 = 7, 04 moles

a) Volume of gas generated

Reaction is complete, hence x= 7.04

The explosion of 1 litre of nitro-glycerine generates 7, 25 x 7, 04 = 51 moles of gas

The molar volume is equal to 22, 4 mol/L. The volume of gas generated by an explosion of 1 litre of nitro-glycerine is 51 x 22, 4 = 1142 litres of gas

b) Pressure

At the beginning of the explosion, the volume of 1 litre does not change, but the temperature is about 2900-3175 Kelvin. The law of perfect gas permits to calculate the pressure before the rock blasting of 50 mole of burned gas (nitro-glycerine)

PV=nRT, P = nRT/V with P = 1.27 * 10^9 Pa or 12471 bars

V= 1litre or 0.001 m^3 T= 3000K

R = 8.314 J.K^-1.mol^-1 n = 51 moles

c) Oxygen balance

Explosives CaHbNcOd need oxygen for a complete oxidation of the fuel elements CaHb of the explosive but explosives release oxygen during the reaction due to its element oxidizers NcOd. The oxygen balance compares these two values. If oxygen balance is negative, that means the molecule does not have enough oxygen for a complete reaction (such as TNT).

The formula of Oxygen balance is: [d-(2a)-(b/2)]*1600 / M

For Nitro-glycerine C3H5N3O9 Oxygen balance is equal to: [9-2*3-5/2]*1600/227 = +3.5%

Some data from J.Akhavan.2005 The chemistry of Explosives 2nd Edition

3) Explosion

An explosion occurs when a confined explosive is suddenly ignited when subjected to a flame, spark, shock (cf. detonators). Explosive reactions burn more rapidly and violently than basic combustible.

Speed of Sound

There are two types of explosion which depend on the speed of propagation of the explosive. The speed of Sound varies the initial temperature: 343m/s is reached for a temperature of 20 Celsius degree. According to the thesis [Gas dynamic model of turbulent combustion in TNT explosions], the temperature reached by the combustion of PETN ranges from 2900-3175 Kelvin

Fourier formula of the speed of sound

Speed of sound ^2 = R * T * {1 + (gamma - 1) / (1 + (gamma-1) * [(theta/T)^2 * e^(theta/T) /(e^(theta/T) -1)^2]) }

R = gas constant (286 m^2/s^2/K° for air) T = absolute Temperature in Kelvin

Theta = 3056 K Gamma = 1.4 ratio of specific air

With an excel calculation; the speed of sound is obtained for different temperatures.

To conclude, the speed of sound of burned gas ranges from 1600m/s to 2000m/s during an explosion

b) Deflagration

A deflagration occurs when the flame front moves lower that the speed of sound of burned gas. The flame front propagates to some centimeter until a velocity of 1500meter /second is achieved. It is possible than a low explosive such as black powder will detonate like a high explosive if the explosion occurs in a confined space as inside a drill hole.

Detonation

This occurs when the flame front moves faster that the speed of sound of the burned gas, that is which generates a Shock wave. This happens with using a detonator or/and a high explosive in a confined area. The gases generated from the decomposition are blocked in the confined space, thus the stress and the burning surface increase. In detonating explosives, the front flame is so rapide that it exceeds the speed of sound in burning gas (1500m/s).Explosives can also detonated if they are initiated by a shockwave due to detonators in rock blasting with primary explosives (cf. detonator)

A High Explosive is an explosive which during an explosion exerts a pressure on the most resistant area. That's it called « Brisance » which comes from French language meaning « break »: it is the ability of an explosive to fragment in a confined space.

For example, the detonation of a small charge of high explosive on the wall will cause a perforation on the wall whereas with a basic explosive charge, the wall will not be perforated, the strong blast will propagated in the opposite direction to the wall.

High explosives have a detonation speed range from 1.5 to 10 km/second

High explosives are mainly used in mining extraction to fragment the rock. Some examples of high explosives; explosives derived from nitro-glycerin (PETN, RDX) are high explosives.

Deflagration and Detonation

The study of the difference between deflagration and combustion will be not studied in further details in the Honour Project. Theoretical knowledge is sufficient to understand the Rock Blasting.

As shows the scheme from the chemistry of Explosives 2nd Edition J.Akhavan, a detonation needs a delay of initiation; shock wave, high pressure given by the detonator

III. Classification of explosives

As described above, explosives have different properties for specifics uses: military or Civil Engineering and some explosives cannot be initiated alone e.g. Nitrate explosives and TNT. That is the reason why explosives need to be classified and differenced for a better understanding and use. Currently, explosives are classified following their sensitivity

Explosives are classified in three categories following their degree of sensitivity: primary, secondary and tertiary explosives. In Civil Engineering the three are used in a complementary way that is described below.

1) Primary Explosives

Primary explosives are highly sensitive to stimuli such as impact, electricity, heat or friction; they do not need a high amount of energy to be initiated. The limit between primary and secondary explosives is blurred; the rule of classification mentions than a primary explosive is more sensitive than PETN. In practical tests, a primary explosive can be initiated with an impact of a hammer. The PETN is considered as a secondary explosive but it can also be initiated by a hammer. Moreover there are some explosives that they can not be manipulated with handles without exploding such as Nitro-glycerin or nitrogen trioxide.

Primary explosives are mainly used in a small quantity (milligrams to be manipulated safety) in detonators to initiate secondary explosives. Primary explosives need a special place and safety rules for storage/transport. They are never with others explosives and have to transport alone. And the most of cases they are initiated at distance with an electrical signal

2) Secondary Explosives

Secondary explosives are less sensitive than primary explosive and hence, the need more energy to be initiated. They are safer to handle and store and that is the reason why they can used in high quantity (tons, in mining extraction). Secondary explosives are initiated by a small quantity of primary explosive which is in the detonator.

3) Tertiary Explosives

Tertiary explosives as known as blasting agents are really insensitive to shock. They cannot be initiated by a primary explosive; they need an intermediate explosive booster that brings the secondary explosives. They are mainly used in Mining extraction, construction and terrorism. Tertiary explosives are cheaper than secondary explosives with similar properties of powerful (Ammonium Nitrate).

VI. Manufacturing Process

An explosion is a spontaneous reaction that releases heat and a large quantity of gas.

As a thermodynamic point of view; explosives are substances which contain a high quantity of energy stocked in chemical bonds; this energy is released during the explosion. Hence, the powerful of an explosive is calculated following the strong double and triple bonds that can reach 1000 kJ/mol. That's the reason why in almost explosives there are nitro groups: -NO2,-ONO2,-NHNO2.

In fact, a good explosive is a molecule containing a carbon chain and nitro groups (cf. Combustion Principe). In manufacturing process of explosives, the main objective is to fix nitro groups to carbon chain. It must have several thousands of explosives; the author has decided to focus on the main one to show their manufacturing process: Nitro-glycerine and Ammonium Nitrate.

Nitro-glycerine, an easy but risked preparation

Nitro-glycerine molecule

The recipe of Nitro-glycerin can be found on website very easily.

Nitro-glycerin is made (or should be made) in laboratory by nitration of glycerin (glycerol) obtained by adding mixture of sulfuric and nitric acid. During the reaction, the temperature should never exceed more than 20 degrees under penalty explode

Glycerol (carbon chain) + acid nitrate (nitro groups) → Nitroglycerin + water

    C3H5 (OH) 3 + 3HNO3 → C3H5 (NO3) 3 + 3H2O

The temperature should not exceed 30 degrees during the reaction at risk of explosion. That's the reason why, There are stable and inert elements as nitro-glycol in the composition of dynamite to make it less sensible at shocks and temperature.

Nitrates Explosives : Slurries and gels

Ammonium Nitrate NH4NO3 is before all a fertilizer in farming but it is also used like explosives in mining extraction as a tertiary explosive. The Ammonium Nitrate is an oxidizer element; but it can be mix with fuels like diesel, fuel, oil, aluminium, to become a strong explosive.

Molecule of Nitrate

Fuel-Nitrate

94% Ammonium Nitrate - 6 % fuel oil. The fuel oil is used as the fuel. In some cases Fuel-Nitrate can contain more fuel oil.

Gels and slurries

35% Ammonium Nitrate, 5 - 40% Dynamite, aluminium 2-5% and water 5%: Gels and slurries differ by their consistence: liquid or pasty

c) Emulsions

70-80% Ammonium Nitrate- The fuel is a mix between mineral oil 4-10% and water ( 8-15%) with some emulsifier( <5% ). Some emulsions are not enough sensitive. They have to be sensitized with gas bubbles generated by a chemical agent.

Explosives can be delivering in bulk in 25 kg bag or in cartridge form. The packaging in cartridge for dynamite is compulsory

Cartridges are mainly used in case of water presence in borehole. Fuel-Nitrate, Gels and Emulsions can made on the site following some specific rules depending on the legislation of the country. In Europe, it needs a specific truck calls "Mobil Unit for explosives production".

B. Shot Firing Design

The scheme below (sectional view of the rock to extract) summarizes the second part of the Honor Project: Shot firing Design. In this section the determination and the use of primary, secondary and tertiary explosives and also electrical and non-electrical detonators will be studied. The shot firing design main objectives is to avoid to generate oversize blocs (diameter>800mm) during the rock blasting and reducing as much as possible the cost of the operation.

Drilling Holes

Rock Blasting

The grey volume is the rock which is going to be extracted and reducing in aggregates. There are two different ways to perform the work. The first one consists to snatch the rock along the front: for this, the rock has to be tender (limestone) and it suits for small exploitation (< 50.000 tons/ year), this process will be not studied in further details. The second method is the rock blasting: The rock is drilled at different spots by a drilling machine, then explosives are placed inside and permits once initiated by detonators to pulverize the rock in aggregates

Rock Blasting sizing

Determination of Explosives

Detonators

Delay Sequence and Rock Blasting

Initiation

Detonators are blasting agents; they are necessary for the implementation of shot firing. They initiate high explosives, causing a shockwave. The principle of a detonator is resumed in 3 steps: sending an electrical or mechanical energy, transfer of the initial energy into chemical energy, igniting the primary explosive. In rock blasting, 2 detonators are mainly used: electrical, non-electrical (there is also electronic detonator which has the use principle than the electrical detonator, but they are more accurate delay settling), they are complementary in rock blasting and have the same use principle.

Detonator scheme

A detonator is initiated by a mechanical (shockwave non-electronic) or an electronic (intensity from the exploder). The heat released initiates the pyrotechnic charges contained in the zinc and aluminum tube of the detonator. The primary charge, lead azide (Pb(N3)2) is very sensitive to the initiation and generates a shock. The lead azide is in too small quantity to initiates the explosives. Nevertheless, this charge is going to initiate the secondary charge (pentrite) which generates a sufficient impact to initiate explosives.

The detonation of a detonator occurs between 0 and 3ms after the initiation. The delay can be controlled by placing a retardant powder between the primary charge and the initiation as the scheme below shows. These detonators are called short and ordinary delay detonator.

A short delay detonator has a delay ranges from 25ms to 500ms (in 25ms increments) and the ordinary detonator ranges from 500ms to 6000ms (in 500ms increments). For electronic detonator, the delay is managed by the electronic system, that permits the choice of any delay and they are more accurate than electrical detonators.

Rock blasting design

To design a rock blasting scheme, 3 parameters are necessary:

Determination of basics parameters: the free face and the drilling diameter, the number of row

Establishment of condition for success: burden, initiation and delay sequence

Establishment of secondary parameters: dimension of the mesh, over depth and bourrage

Shooting parameters:

In order to realise a rock blasting, holes are drilled behind the fase. The distance between holes of a same row is called "the spacing". The distance between the front and the holes perpendicular to the front is called "the bench". Holes are placed according to the row. In rock blasting, there are, in the most of cases two rows in the rock blasting as shows the scheme below

Reference and bibliography

(Formby and Wharton 1996, Lynch, Brannon and Delfino 2002, Monteil-Rivera et al. Nyanhongo et al. 2009) (ANON. 1869, Ewing et al. 2001, Holmes 1927, Link 2002, Zeng, Chen and Liu 2007) (ANON. 1867, ANON. 1872, ANON. 1874, Mayer 1868, Venart 2004)

ANON. Biographie de Joseph Wilbrand. [online]. Chemical - Business History of Manufacturers. Available from http://www.kipnotes.com/chemicals.htm [ Accessed 22th of November 2010].

Lacabane,L 1845. De la poudre a canon a son introduction en France. Bibliotheque de l'ecole des chartres

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