Explosives today 4

Page 1

Explosives Today - Series 4, No 4

Explosives Today - Series 4, No 4

Series 4 I No 4

explosives.

Mean “Xm”

Fines typically result from how the rock breaks up - the rock properties but also from the drilling process. In the drilling operation, the rock is effectively ground out of the hole, thus volume of the material from these holes equates to a control of the fines. Also fines are generated during the detonation of the explosives column, the crush zone around the blasthole when it detonates, typically somewhere between 2 to 10 holes diameters depending on the rock properties and to a lesser extent the geology, figure 7.

Uniformity Accuracy

Explosives Today

(n)

-

Timing

The slope of the graph is affected by accurate timing – e.g. Electronics i.e. More of the material is generated in the same size range, more uniform, note this does not necessarily mean finer! The blasted material can be coarser and more uniform! (Figure 8.) Standard curve, blue, with a wide standard deviation range for the 40 to 80% passing sizes range from 250 to 600 mm. Accurate curve, red, with a narrow or tight standard deviation range for the 40 to 80% passing sizes range from 250 to 500 mm (Smaller range of material, more uniform) To illustrate what we mean by more uniform, figures 9 and 10 show a made up muckpile sample, where the black square in the photograph represents the mean size and the size distributions illustrate the more uniform material, i.e. more material around the mean size as the material becomes more uniform. In conclusion, to leverage fragmentation in our blasting, we must: • Understand the influence of the rock type, geology and rock properties • Use the appropriate explosive type oo To control the initial shape of “S” curve and distribution of the fragmentation sizes • Determine the explosive mass

Sample 3

40 30

n=1.50

20 n=1.00

10

Uniformity, “n”

Sample 1

0 0

2 20

5 50

10 100

20 200

40 400

Sieve size (mm)

80 800

n=0.75 160 1600

Figure 9 Distribution of fragmentation sizes around mean, more uniform fragmentation as uniformity index increases

Mean size equal, Xc, for the artificially created muckpiles Sample 1

n = 1.50

Leveraging Explosives and Initiating Systems in Blasting n = 0.75

Simon Tose, Group Consulting Mining Engeineer

Sample 3

All sizes below Xc will pass through this! Figure 10 Distribution of fragmentation sizes around mean, more uniform fragmentation as uniformity index increases

Whenever blasting is performed one of the first questions that arise is how the explosives can be best used.

oo Use the powder factor -- Manage the mean size of muckpile

In this edition of Explosives Today, we will look at this question in terms of managing the fragmentation of the blasted rock and we will ask:

• Stemming oo Hole diameter & Bench heights -- Influence proportion of oversize • Rock Breaking process oo Understand the drilling & crush zone -- Influence fines distribution • Electronic Initiation oo Control uniformity size of the muckpile

This document is a new addition to the Explosives Today series.

AEL Mining Services Limited (PTY) Ltd 1 Platinum Drive, Longmeadow Business Estate North Modderfontein, 1645 Tel: +27 11 606 0000 www.aelminingservices.com

“What are the various points at which we can consider leveraging the explosives and initiating systems to manage the size fractions and gain the desired results in the final muckpile?” The word “fragmentation” is however very loosely used and can mean

anything from “the limits of breaking” to “the percentage passing, above or below, a certain size.” The following definition will be used to discuss these blasting objectives:

“The economically significant size range of a definable volume of broken rock” This definition accommodates whatever is important to a particular operation and will vary with venue. The first item to establish when discussing fragmentation at a venue will be to ask:

“What are the important fragment sizes?” The economically significant fractions can usually be classified as: Oversize, Fines and Mid-range. • Oversize: oo Boulder size above which secondary breaking is necessary before further handling. -- e.g. In underground mines this can be as small as 300 mm, while in opencast mines it is seldom defined as greater than 1 000 mm (Typical range of values for oversize on the x-axis, figure 1.)


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