© 2018 ijrar january 2019, volume 6, issue 1 (e … · 2019. 2. 4. · mining works. sometimes,...

© 2018 IJRAR January 2019, Volume 6, Issue 1 www.ijrar.org (E-ISSN 2348-1269, P- ISSN 2349-5138)

IJRAR19J1805 International Journal of Research and Analytical Reviews (IJRAR) www.ijrar.org 1113

A BRIEF OVERVIEW ON ROCK BLASTING FOR MINING

1Mithilesh Kumar Rajak, 2Dr. K. S. Siva Subramanian, 3M.J.A .Prince 1Assistant Professor, 2Professor, 3Assistant Professor

1Dept. of Mining Engineering 1AMET University, Chennai, India

Abstract

Fragmentation is one of the critical results of the blasting also treated as first quality demand in most types of

blasting which affects all the downstream processes of mine and mill production like loading, hauling, crushing, and

milling efficiency and has a great impact on the cost involved in these processes. In this study, the fragmentation

analysis of a chromite mine of Boula-Nuasahi Complex belt was conducted using WipFrag fragmentation analysis

software. Both single and merged image analysis were done and the merged image analysis was used to evaluate the

optimum fragmentation. The mean fragment size of the blasted rock was predicted from the analysis.

Keywords: Rock, Rock Blasting, Fragmentation, WipFrag, Cushion Blasting, Presplitting

1.0 Introduction:

Most of the economic mineral deposits occur in association with massive hard rocks1. These rock masses should

be fragmented to obtain the valuables and separate the materials for further processing. Breaking a rock mass

involves energy. This can be achieved by drilling and blasting. Hence drilling and blasting are considered to be

the first phase of the production cycle in most of the mining operations. Today technology has shown much

advancement in several activities of mining. Inspite of that, there is no alternative to blasting or drilling2. Quite a

lot of improvements have been shown by experts in controlled blasting operations which are more beneficial not

only on production point of view, but also on the safety point of view of mine workers. Let us see the details of

blasting methods adopted in mining operations.

Large scale mining in hard rock zones are difficult without the application of drilling and blasting

operations.

1.1 What is blasting?

Blasting is the process of breaking of bulk rock masses into loose forms, using explosive compounds. Here, the

primary role is played by the explosives. The explosives are the substances or devices used in blasting. The

explosives are used to produce a volume of rapidly expanding gas that exerts sudden pressure on its surroundings

and break the mass into pieces3. There are three common types of explosives used for blasting as chemical,

mechanical, and nuclear explosives. About 100 years ago, the Chinese invented explosives. The first chemical

explosive was gunpowder. Germans manufactured gunpowder in the early 1300s. A detonator is a device used to

trigger this explosive device. Detonators can be chemically, mechanically, or electrically initiated. Different

explosives require different amounts of energy to detonate. Detonation is a necessity for the explosive to get

triggered for blasting.

1.2 Purpose of blasting

Blasting is employed for breaking the massive rocks in quarries, open pits, surface and underground

mining works. Sometimes, blasting is also carried out for removing the overburden from the surface of the earth

for construction purposes. An optimum blast is associated with the most efficient use of blasting energy in the

rock-breaking process4. The mine blasting expenditure should also be very less. The process should not only

reduce the blasting cost but also reduce the consumption of explosives, and less wastage of explosive energy in

blasting. During any mining-related blasting, there should be less throw of materials, and there should not be

much blast-related vibration. All these should provide greater levels of safety and stability to the nearby

structures and also for all the people working around the mines.

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1.3 Blasting- an essential part of mining cycle

This diagram illustrates the overall organizational structure of a mine cycle. Once the ore body is

investigated through detailed surveys, the next steps include drilling and blasting, followed by loading, hauling,

primary crushing and other ore-dressing operations. Blasting helps in not only in

rock breaking, but also in primary crushing. So, Blasting is an essential aspect in the mining cycle.

Now, blasting has become a technology by itself. Many Mega engineering projects need some kind of blasting of

basement rocks. The blasting technology is the process of fracturing the material by the use of a calculated

amount of explosive so that a predetermined volume of material is broken down into pieces5. Everything can be

visualized by proper calculations and execution, with reference to time and space.

2.0 Steps involved in blasting

In surface mining operations, most of the rocks require blasting prior to their excavation.

The process of blasting includes the following stages:

1. Creation of blast holes and cleaning by using suitable tools.

2. The charge of explosives and place them at the breaking ends.

3. Filling of the remaining portion with clay and tamping.

4. Insertion of Fuse and keep it projecting 15-20 cm above the rock surface.

5. Firing from the free end of fuse by using a suitable detonator.




This diagram shows the drill hole charged with explosives at a depth. The burden face is shown as B.

The left-hand side is the bench over which the fragmented mass will fall. H is the depth of the borehole.

This diagram shows the schematic arrangement of blast holes.




2.1 Drilling and blasting

There are two basic operations that are necessary to achieve success in mining. One operation is drilling and the

other one is blasting. Drilling is mainly done for three reasons. The first reason is to

investigate the depth-wise distribution of minerals during prospecting. The second reason is to create

the blast-holes for rock blasting5. The third reason is to drill for dislodging the soft and friable

minerals like magnesite with shallow depth drills.




For surface explorations, in opencast mining operations, drillings rigs may be used.

For subsurface explorations, rock boring machines are used.




A shallow-hole driller may also be used for preparing blast holes in underground mines.

If we do not do these operations, the expenditure of mining would be more and the time taken for

recovering the economic deposit will also be more. In order to cut down the cost of the production and speed up

the processes, optimal fragmentation of the ore body is necessary. Fragmentation is a

necessity for mining and ore dressing also. Fragmentation helps rapid excavation of resources.

Through a properly designed blasting pattern and safe execution, in a mine, it is possible to achieve

the targeted productions. Rock blasting is an interesting aspect of study.

2.2 Concepts of blasting

Mine blast looks like a very simple but effective, thought provoking process. The physical

characteristics of the rocks are more important than the characteristics of the explosives used in blasting. The

success lies in proper design of the blasting process. The following are the important

points to be kept in mind:

When an explosive charge is detonated, chemical reaction is activated.

This chemical reaction will very rapidly change the explosive mass into a hot gas which generate

shockwaves. These are compressive waves, which also expand on all sides.

They act on the borehole wall and propagate through the explosive column.

If a series of holes are detonated simultaneously, the entire line of masses will be fragmented.

Due to this effect, blasting is always carried out in an array of interlinked blastholes.

Waves generate cracks and make the entire process very effective.

3.0 Good blast design and execution

A good blast design and proper execution of it , are very much essential for a successful mining operation.

Improper design or a poor design planned in blasting, can have a severely negative impact on the

economics of a mine.

Sometimes, it may complicate the situation and may need to excess work and expenditure.

Unexpected hazards may also crop out in the environment.

The use of excessive explosives at a mine site will result in full damage of the rock structures and

cause unwanted caving and large increases in support costs.

3.1 Factors of blast design

The procedures developed for a blast are aimed at the desired fragmentation and area of coverage.

Any kind of specific thermodynamic break should take into account of the following aspects:

1. Explosive and energy factors

2. Type and Quantity of explosives used

3. Diameter of blast hole

4. Orientation of the ore body

5. Dynamic rock/ore properties

There is a need to have effective free face for blasting. This diagram shows the effective free face. The location

of first blast hole, second blast hole and the third one also seen here




3.2 Selection of explosives

Usually four types of explosives are used in surface mining operations. They are slurries, dry mixes,

emulsions and the hybrid heavy ANFO(ammonium nitrate/fuel oil).

Selection of explosives depends on many factors. The primarily factors include the critical diameter,

hydrostatic pressure, temperature, minimum primer weight, density weight strength, bulk strength,

gap sensitivity, water resistance, loading procedures, coupling or decoupled properties, shelf life,

reliability for bulk operations and overall drilling and blasting economics.

3.3 Drilling pattern

Drilling is performed in order to blast the overburden, ore deposit, coal seams etc., so that the power

requirement for excavators to extract the materials becomes less.

This also reduces the wear and tear of the excavators, increases their life, reduces clearing time of

materials, and decreases operation cost. Drilling holes are usually made in a zig-zag pattern. They are done in the

form of rows and columns on the surface like grids. But inside their disposition may be dipping differently6.

The spacing between the rows and column should be of equal length. This is called as drilling

pattern. Selection of the drilling pattern varies with the type and size of the drill’s used, depth of

holes, kinds of rocks handled, quantity, rapidity of the explosive & amount of steaming. An array is

the line of drill holes made for blasting works.

4.0 Blasting pattern

The basic blast hole arrays may be single-rows, square-grid, or rectangular-grid and staggered arrays.

A typical surface blast design will have a set of blast holes charged with explosives.

In this illustration, we can see the layout. The rows are designated with sequential numbers.

The symbol B denotes the burden thickness and the symbol S denotes the spacing between blast

holes. If this sequence is executed, the array of line 1 will get triggered first, followed by the other

arrays, one after the other. The mass gets fragmented and will get fragmented towards the direction

of arrow mark shown in the diagram.




4.1 Types of blast pattern

Regular symmetrical or irregular arrays are used for blasting. To break any irregular areas at the edge of a

regular array, the blast holes may be distributed irregularly.

The standard blast patterns are of three major types. They are:

Square Grid pattern

Rectangular grid pattern

Staggered pattern

This illustration shows the square grid pattern

In staggered pattern the alignments are diagonal, as you can see in this diagram. First row blast holes

and third row blast holes are in alignments.

The drill holes may be vertical, inclined or horizontal. In opencast mines, both vertical and inclined

holes parallel to the bench face is practiced.

This illustration shows the vertical blast hole designed to remove the excessive burden on the left.




The row of the holes may be in single or multiple. Based on this, blasting pattern is also classified into

a) Single Row blasting pattern and

b) Multi-row blasting pattern

4.2 Single& Multi- row firing pattern

In single row blasting, the fragmentation will be low and the explosive consumption may be more than multi-row

blasting.

The multi-row blasting pattern is better and mostly preferred. Plan A Shows the multi-row blasting design. The

mass will move towards the direction of the arrow, that is vertically upwards.

Plan B shows another pattern.

It is called as box-cut design. In this, the area to be fragmented is not a square. Valley-like excavation

is aimed at, in this design. Fragmented mass will fall back within the trapezium.




Plan C shows a corner-cut design. It is also called as echelon design. The fragmented mass will fall

back on the cut bench itself.

This is one more corner-cut design- Plan D. The blast sequence is different from the previous plan-C.




Plan-E is another corner cut blast design. The cut bench will expand leftwards.

For any shallow-depth excavations, single-row design is preferred as shown in this plan F.

These blast designs are preferred for the following kinds of rock materials:

a) The alternate delay pattern (used for softer rocks),

b) Consecutive shot delay pattern (used for rocks with medium hardness),

c) Short delay firing with a cut (used for hard rocks).

4.3 bench blasting

Bench blasting is a common blast technique most often used for open pit mines. By definition, bench blasting is

blasting in a vertical or sub-vertical hole or a row of holes towards a free vertical surface7.

More than one row of holes can be blasted in the same round. A time delay in the detonation between the rows

creates new free surfaces for each row. There are three methods involved in this process as short-hole blasting,

long-hole bench blasting and ring drilling and blasting methods.

The short-hole blasting is usually limited to drilling rounds of 1.2 m to 5.0m length and hole

diameters of up to 43 mm.

Cut and fill and room and pillar underground mining methods commonly employ short-hole blasting.

The Longhole bench blasting is similar to bench blasting in open pits, using long holes drilled

downward either parallel to each other or in slight rings to cover the stope area. Initiation of the blast

is with a booster down the hole.




Ring drilling and blasting is done from a series of sub level drill drifts developed in the ore body.

The drill pattern is designed to cover off the extent of the ore in the stope. This type of blasting will cause the ore

to swell by 30%, and this must be allowed for when blasting otherwise the blast may freeze.

4.4 Delay pattern

In addition to these aspects, there is yet another factor which plays a dominant role. That is the blast

timing and triggering sequences. It is called as delay pattern. It is also called as firing sequences.

The delay patterns, and varying the hole array to fit natural excavation requirement, allow for an

efficient use of the explosive energy in the blast8. This diagram shows the delay in triggering with

reference to arrays

5.0 Methods of blasting

The major method adopted in mining is called as controlled blasting. Controlled blasting is a technique of

blasting for the purpose to reduce the amount of over break and to control the ground vibrations.

5.1 Controlled blasting

Following are the different types of controlled blasting techniques:

Pre-Splitting - this is an old but highly recognized technique with the purpose to form a fracture

plane beyond which the radial cracks from blasting cannot travel.

Other methods include

Trim (Cushion) Blasting,

Smooth blasting (contour or perimeter blasting) for underground mines and muffle blasting as a

solution to prevent fly-rock from damaging human habitants and structures.

Presplitting

Presplitting is a technique that involves loading a single row of holes that have been drilled along a

desired highwall crest or excavation line.




Here in this diagram, you can see the role presplitting. When the final limit of excavation is known,

there is a need to restrict the blasting zone. Presplitting helps to control the zone.

Presplitting is to create buffer holes and presplit holes in a closed network. When blasting is done,

since the zone is already weakened with series of holes, the blast has no role to play after this line.

Such methods reduce the crushing effect around the borehole and are shot before the main production

shot. The idea is to minimize or eliminate overbreak from the primary blast and to produce a smooth

rock wall. Presplitting will add a large drilling cost to an operation.

Borehole Diameters




Normally, the diameter of a borehole is limited by the capabilities of the drill used to create it. As a

rule, open-pit and coal strip mines using large drills will drill presplit holes that range from 9 to 12-¼

inches in diameter.

Quarry and construction presplits are much smaller, generally ranging from 2 to 4 inches in diameter.

Presplit holes may be drilled on an angle if the geology and drill allow for it.

Spacing

Presplit spacing will vary, depending upon rock characteristics, size of the operation, and bench

height. Trials should be conducted to determine the optimal borehole spacing.

Explosive Charge

Depending upon the rock characteristics and spacing used for the presplit, charge loads will vary.

Smooth Blasting

The term “smooth blasting” refers to lightly loaded holes that have been drilled along excavation

limits and are shot after the main excavation is removed. Typically, such holes are shot instantaneously or with

little delay, leaving a smooth wall with minimum overbreak. Smooth blasting is the most widely accepted

method for controlling overbreak in underground headings and stopes.

Line drilling

“Line drilling” provides a plane of weakness to which a primary blast may break. It may also protect a highwall

by reflecting some of the shock wave created by a blast. The distance from the back row to the line drill is

normally 50 to 75 percent of the production burden. Line drilling is normally limited to construction projects,

dimension stone quarries, and rock sculpting activities.

Cushion blasting

“Cushion blasting,” or trim blasting, is similar to smooth blasting. In this method, the holes are shot

after the main production shot9. Cushion blasting involves backfilling the entire borehole with crushed stone to

cushion the shock from the finished wall.

5.2 Major factors influencing blast efficiency

The major factors involved in blast efficiency are

• Burden

• Spacing

• Bench height

• Powder column

•Blast hole diameter and depth

• Powder factor

• Stemming.

5.3 CONCLUSION

Fragmentation control through effective blast design and its effect on productivity are the major

challenging tasks in mining. The easiest way to mine the valuable ore bodies is through the use of explosives and

well-planned blasting layouts which can produce good fragmentation profiles. The design of blasting patterns is

specific to not only the explosives that are being used, but also to the rock or ore type that is being fragmented.

In this lesson we have seen most of the important aspects related to rock blasting for mining.




Reference:

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10.1016/j.jrmge.2015.10.005

2. Johnson, Catherine E., "Fragmentation Analysis in the Dynamic Stress Wave Collision Regions in

Bench Blasting" (2014). Teses andDissertations--Mining Engineering. 16.

htps://uknowledge.uky.edu/mng_etds/16

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Retrieved from https://research.csiro.au/hardrockcutting/

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Southern African Institute of Mining and Metallurgy (2008), pp 113-114.

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Models- A Case Study”,4th UMaT Biennial International Mining and Mineral Conference, (2016),pp.

MP 1 – 11.

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8. Maerz, N. H., Palangio, T. C., Franklin, J. A., WipFrag image based granulometry system. In: Franklin,

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