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.)
60.0 40.0
0.0 1
10
1000
10000
Figure 3 “Blast Envelope” - The “y” axis reflects the % Passing amount of rock at a given size, “x” axis
Fines
Mid Range
Figure 1 % Retained vs. Productivity rate (Red dash) vs. Fragment size for optimal haulage loading rates
Whilst we have no control over the Geology and Rock Properties found in our mining operations our first attempt to influence the fragmentation is however the need to fully understand and manage the Geology and Rock Properties. Are we dealing with a competent fine grained Granite, or a weathered, jointed limestone?
Blast envelope measured across different geological zones in Kimberlite
120.00
100.00
80.00
Rock Type
Granite
60.00
40.00
20.00
0.00
1
10
100
1000
10000
mm
Figure 4 Influenced by change in Geology and Rock Properties, same blast design across pit
Fine
100.0
Coarse
80.0
Figure 2 Understanding the influence of Geology and Rock Properties on the “Blast Envelope
Marble
60.0 More
Less
40.0 20.0 0.0 1
10
100 mm
Figure 5 Mass of explosives (Powder Factor) in effect controls the mean size
1000
What is the impact of the rock structure and how will this impact on the final result? (Figure 2.) If we apply an appropriate blast design then we will certainly break the rock. In order to determine if this meets the desired results, we will measure and generate a fragmentation distribution curve, often referred to as the “Blast envelope” or “S” curve, figure 3. If there are no major changes in our Geology and Rock Properties, then this curve should be fairly consistently repeated for each given blast design. In other words the main influences on the shape of the fragmentation “S” curve are the Geology and Rock Properties.
120.0
Limestone
Emulsion
100.0
Next we need to ask if this fragmentation is too coarse or fine in terms of our desired final muckpile and more importantly the downstream requirements such as Load and Haul, or the mineral processing systems. Often we just accept this “Blast envelope” and rather manage the fragmentation through a complex processing plant layout with more crushers.
Anfex
80.0 60.0 40.0 20.0 0.0 1
10
100
1000
10000
mm
Figure 6 Explosive Type impacts on the shape of the “Blast envelope”
Geology and Rock Properties
Oversize
Fragment Size
Andesite
120.0
10000
Figure 4 illustrates this as the “S” curve changes in response to the different geological domains across the pit, although the blast design remains constant.
120.0
Oversize – Top of blast, stemming, no explosives
100.0 80.0
% Passing
0
An illustration of the market value for these fractions and their impact on production rate is shown in the figure 1, which shows fragment size increasing from left to right.
It is very well established that loading and crushing operations are seriously hampered by large rocks,
100
mm
• Geology and Rock Properties • Mass of Explosives (Powder Factor) • Explosive Type • Control of Oversize & Fines (Stemming length and Hole diameter) • Uniformity - Timing Accuracy
Mass of explosives (Powder Factor)
“S” Curve representing fragmentation for “Blast envelope” for different explosives in similar rock type
% Passing
% Passing
80.0
20.0
• Poorly shot rock piles: oo Large oversize boulders oo Tightly keyed together oo Raise loading costs significantly oo Low productivity -- High wear and tear on the machines
The value of the fines is zero (or even negative if handling costs are incurred) below a certain range, while larger fragments can be sold at low prices, and mid-range product achieves premium pricing.
We will consider five key leverage points, in this document, to manage our desired production results and illustrate these using measured fragmentation results captured during blasting operations in a reasonably consistent rock body:
100.0
% Passing
• Well shot muckpile: oo Low cost loading oo High productivity oo Low wear and tear on equipment
even those of less than the defined oversize dimension.
120.0
% Passing
We can translate this into the fact that blasting is the first critical step in efficient loading, hauling, (the shaded portion found on figure 1.) and crushing. How well this is done significantly influences the downstream costs of the operation:
Explosives Today - Series 4, No 4
“S” Curve representing fragmentation for “Blast envelope” for emulsion explosive in given rock type
Haulage
• Fines: oo The particle size below which product can either not be sold, or which becomes difficult to handle due to flow, or other properties. -- e.g. It is common for a minimum size of 6 mm for coal or dolomite, but in gold ores this may be as small as 1 mm (Typical range of values for fines on x-axis, figure 1.)
Design fragmentation - Load & Haul?
% Retained
• Mid-range: oo Those sizes which have significant, but not terminal importance for handling, mineral recovery and the ability to achieve premium pricing
Explosives Today - Series 4, No 4
However this one is easy to tackle, as the old blasting hands know all too well, vary the distribution of explosive mass through the rock, i.e. change the powder factor and the “S” curve shifts either left or right, finer or coarser fragmentation results. Using our measurements and analysis in the given rock type, we observe that in effect by increasing or decreasing the explosives mass (powder factor) we are able to control the mean size of the broken rock, figure 5.
Explosives Type
60.0 40.0
Fines, Drilling, and Blast crush zone
20.0 0.0 1
10
100
1000
10000
mm
Figure 7 The control of Oversize & Fines determines the top and bottom shape of the “Blast envelope”.
120.0 100.0
% Passing
Explosives Today - Series 4, No 4
Control of Oversize & Fines (Stemming length and Hole diameter)
Digishot +
Electronic Detonator
80.0 60.0 40.0 20.0 0.0 1
10
100 mm
Figure 8 Uniformity, controlled through accurate timing – (Electronics)
One could argue that changing the explosive type could achieve a similar result in the control of fragmentation. However the explosive type in fact primarily impacts on the shape of the “Blast envelope”. In this illustration, the properties of the Anfex explosive encourage the development of breaking along the geological features inherent in this rock mass and impact on the shape of the “Blast envelope”. In this particular example, a measured change in the portion of oversize material, figure 6.
1000
10000
Oversize is primarily as a result of the geology and rock properties which influence how the rock tends to break up into blocks however it can also be affected significantly by the actual blast design. Typically we tend to see the control of the oversize in the front of the blast through the blast design burden and on the top of the blast in the stemming zone, i.e. where there are no