ROCK FRAGMENTATION

Abhijit pal 2012JE1142

OBJECTIVES

At the end of this chapter, participants will be able to achieve:

Fragmentation principlesEvaluation of fragmentation Factors controlling fragment size

CONTENT

Fragmentation PrinciplesHow to Quantify the FragmentationEvaluation of Fragmentation in Tata Steel

West BokaroFactors Affecting Fragment Size

INTRODUCTION

The term ‘rock (or blast) fragmentation’ is an index that is used to estimate the effect of bench blasting in the mining industry.

it is generally understood that both the stress wave and the gas pressurization make significant contributions to rock fragmentation.

INTRODUCTION(contd.)

It is well known that the rock fragmentation in bench blasting is affected by blast condition such as specific charge, spacing and burden etc.

BLASTING MECHANICS

Upon detonation, explosives affect rock by various interrelated means.Which are-1. Detonation Shock Wave -an energy pulse is

generated and transmitted to the adjacent rock & the rock immediately surrounding the borehole is crushed to some extent.

2. Shock Wave Reflection -when the shock wave reaches a free face, the outward-bending compressive force releases, and the wave is reflected back into the rock as a tension wave.

BLASTING MECHANICS (contd.)

BLASTING MECHANICS (contd.)

3. Gas Pressure and Rock Movement – 1. Upon detonation the solid explosive is instantly

converted to superheated gas 2. Gas tries to occupy a space 10,000 to 20,000

times its original solid volume3. exerting a pressure that can exceed 1.5 million psi. 4. The fractured rock mass has a certain inertia which

the gas pressure must initially overcome to start rock movement

5. Once inertia is overcome, the rock moves outward away from the borehole at around 60 mph

Processes involved in primary blasting of rock in bench blasting.

1. Face survey - estimating overburden2. Drilling the shot holes - preparing charge hole3. Charging with explosive and stemming top 4. Detonating the explosive 5. Shot pile ready for loading

Secondary blasting

Blasting conducted to reduce the size of boulders resulting from a primary blast which can then be handled by the loading, hauling, and crushing system.

Secondary fragmentation can be accomplished by: 1. Drilling method - small diameter hole is drilled into the oversize fragment and a hydraulic rock breaker device inserted to split the rock .

Secondary blasting

2. Plaster blasting- An explosive charge may be packed loosely into a crack or depression in the oversize fragment then covered with a damp earth material and fired. plaster

boulder

3. Pop shooting - blast hole is usually drilled near a crack or depression in the rock, and is directed toward the centre of the mass and then charged is filled in it and detonated.

OPTIMUM FRAGMENTATION

Minimise oversize boulders (less secondary breaking)

Minimise ultra fines productionMaximise Lump productFragmentation enough to ensure efficient

digging and loadingMuck pile loose enough for fast cycle times

and full buckets

OPTIMUM MUCKPILE SHAPE

Depends mainly on-Pit geometryLoading machines

Blast hole Design

BENCH HEIGHT & HOLE DIAMETER

Large hole diameter and small bench Energy yield are difficult to controlBad blast effect charge hole

bench

BENCH HEIGHT & HOLE DIAMETER

In Small hole diameter and tall bench energy yield are reduced. Drill and blast cost is high Blast hole

bench

FRAGMENTATION RATIO

Blasting Crushing Ratio = 800mm

150mm

Ratio=infinite dimension 800 mm block

How delay interval can help to achieve better fragmentation in

different conditions

 The delay time between individual holes in a row:i)         The delay time between holes in a row should be between 1 ms and 5 ms per foot of burden, with 3 ms yielding good results in most instances.ii)       Where air blast is a problem or potential problem, the delay time between holes in a row should be at least 2 ms per foot of spacing.iii)      This will result in a blast progression along the face or along a row of holes that is approximately half the speed of sound (or less) and reduces the low frequency air blast generated by face area movement or by surface area mounding.iv)      Where possible, corner holes at the end of rows should be given extra delay time because of the greater degree of fixation of the rock in those locations requires more time for the rock blasted by previously fired adjacent holes to move away.

   Delay interval between rows:

i)         The delay interval between rows should be from two to three times longer than the delay interval between holes in a row.

ii)       The last row in the shot should often be delayed slightly more than preceding rows.

iii)      This serves to allow rock in previously fired rows time to move out and tends to reduce back-break in the rock behind the blast.

An additional hazard can exist where delay times (compared to burden and spacing) are excessively long, causing cutoffs of the initiation system or powder columns due to ground shift. Again, this needs to be analysed on a case by case basis and accounted for during blast design

Better fragmentation also involves minimizing ground vibrations, so

HOW TO QUANTIFY THE FRAGMENTATION

1 . A commonly used method today to quantify fragmentation is to use the mean fragment size, often designated by k50.

k50 is a figure which represents the screen size through which 50% of the loosened rock would pass if screened.

This implies that a low value represents a fine fragmentation and vice versa

HOW TO QUANTIFY THE FRAGMENTATION

Another way to quantify the fragmentation is by the oversize content.This could be expressed in percentage of the

broken material exceeding an acceptable stone size.

The oversize content is a very good complement to the k50 value as these two values together will provide a much better control of the fragmentation distribution than the k50 value alone.

Max Permitted Charge Calculation of the max permitted charge can be done according to two different sets of for-mulas,Method 1.

The formula can be rewritten as The constant k is determined by test blasting

Method 2.

or ,

The constants k and a must be determined, preferably by test blasting and regression analysis. where v is particle velocity and K represents the initial energy transferred from the explosive to the surrounding rockAnd R represents the distance.

CALCULATION OF FRAGMENTATION

Kuz-Ram ModelThe Kuz-Ram model is probably the most widely used approach for the prediction of rock fragmentation by blasting. The unique feature of this model is that the input data consists of the relevant blast design parameters.

Three key equations are the backbone of this model: Kuznetsov’s Equation: Rosin- Rammler equation: Uniformity index

CALCULATION OF FRAGMENTATION(contd.)

The n-value, which is dependent of drilling pattern, hole deviation, hole depth, charge length, etc., commonly varies between 0.8 and 1.5.

REPORT OF TATA STEEL WEST BOKAKO

FOR OVERBURDENAssuming Rock factor = 12 (Assuming rock is

sandstone)Mass of Explosive (Kg), Q = 200 Kg (Overburden)Weight strength relative to ANFO (RWS) = 152%; Where density of explosive(slurry) used =

1.31g/cc, Density of ANFO = 0.85g/cc RWS = Weight relative strength relative to

ANFO=152%

REPORT OF TATA STEEL WEST BOKAKO

Date of Blasting Powder factor (m3/Kg Explosive)

Mean Particle size of muck pile (cm)

DAY 1 2.12 47.18

DAY 2 2.23 49.63

DAY 3 2.0 44.51

DAY 4 1.7 37.83

DAY 5 1.6 35.60

DAY 6 1.9 42.28

DAY 7 2.18 48.51

DAY 8 2.0 44.51

DAY 9 2.0 44.51

DAY 10 2.1 46.73

REPORT OF TATA STEEL WEST BOKAKO

SIZE(cm)0

10

20

30

40

50

60

47.1849.63

44.51

37.8335.6

42.28

48.51

44.51 44.5146.73

FRAGMENT SIZE OF OVER BURDEN

DAY 1 DAY 2 DAY 3 DAY 4 DAY 5 DAY 6 DAY 7 DAY 8 DAY 9 DAY 10

REPORT OF TATA STEEL WEST BOKAKO

FOR COAL Assuming Rock factor = 3(for Coal)Mass of Explosive (Kg), Q = 90 Kg (Overburden)Weight strength relative to ANFO (RWS) = 152%  ; Where density of explosive used = 1.31g/cc, Density of ANFO = 0.85g/cc RWS = Weight relative strength relative to

ANFO=152%

REPORT OF TATA STEEL WEST BOKAKO

Date of Blasting

Powder factor (m3/Kg Explosive)

Mean Particle size of muck pile coal (cm)

DAY 1 4.5 16.02

DAY 2 4.3 15.45

DAY 3 4.3 15.45

DAY 4 4.3 15.45

DAY 5 4.1 14.87

DAY 6 4.4 15.90

DAY 7 4.6 16.08

DAY 8 4.3 15.45

DAY 9 4.3 15.45

DAY 10 4.3 15.45

REPORT OF TATA STEEL WEST BOKAKO

SIZE(cm)14.2

14.4

14.6

14.8

15

15.2

15.4

15.6

15.8

16

16.216.02

15.45 15.45 15.45

14.87

15.9

16.08

15.45 15.45 15.45

FRAGMENT SIZE OF COAL

DAY 1 DAY 2 DAY 3 DAY 4 DAY 5 DAY 6 DAY 7 DAY 8 DAY 9 DAY 10

WORK ON WIPFRAG SOFTWARE

We can get – 1) Mean particle size 2) Min & Max fragment size 3) standard deviation and most importantly- 4) percentage of fragment of particular size.

Step 1 –Open the Software

Step 2- open the file

Step 3- chose the image to be analyzed

Step-4 Select the proportionate scale

Step -5 select the option “Generate Net” for generating net

Step-6 Rectify the net using different tools

Step-7 select option “sieve” from tools

Step-8 Image analyzed and Data obtained

Step-9 Save the report

CONCLUSION

In this project so far we have read about different type of rock fragmentation (i.e. primary & secondary) and different stages involved in fragmentation of rock. This project also projects the necessity of different type of explosives & charge hole design for more power output.In project we have also learned about application of KUZRAM model and WIPFRAG software. This topic has void role in mining industry as it decides the total output and profit of ore.