Explosives TodaySeries 4 I No 4

Leveraging Explosives and Initiating Systems in BlastingSimon Tose, Group Consulting Mining Engeineer

AEL Mining Services Limited (PTY) Ltd1 Platinum Drive, Longmeadow Business Estate NorthModderfontein, 1645Tel: +27 11 606 0000www.aelminingservices.com

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

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:

“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: o 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.)

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

Figure 10 Distribution of fragmentation sizes around mean, more uniform fragmentation as uniformity index increases

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explosives.

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 (n) - Timing AccuracyThe 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

o To control the initial shape of “S” curve and distribution of the fragmentation sizes

• Determine the explosive mass

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

• Stemming o Hole diameter & Bench heights

- Influence proportion of oversize

• Rock Breaking process o Understand the drilling & crush zone

- Influence fines distribution

• Electronic Initiation o Control uniformity size of the muckpile This document is a new addition to the

Explosives Today series.

Explosives Today - Series 4, No 4 Explosives Today - Series 4, No 4

• Mid-range: o Those sizes which have significant, but not terminal importance for handling, mineral recovery and the ability to achieve premium pricing

• Fines: o 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.)

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:

• Well shot muckpile: o Low cost loading o High productivity o Low wear and tear on equipment

• Poorly shot rock piles: o Large oversize boulders o Tightly keyed together o Raise loading costs significantly

o Low productivity - High wear and tear on the machines

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.

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.

It is very well established that loading and crushing operations are seriously hampered by large rocks,

Design fragmentation - Load & Haul?

Figure 1 % Retained vs. Productivity rate (Red dash) vs. Fragment size for optimal haulage loading rates

Figure 2 Understanding the influence of Geology and Rock Properties on the “Blast Envelope

Figure 3 “Blast Envelope” - The “y” axis reflects the % Passing amount of rock at a given size, “x” axis

Figure 8 Uniformity, controlled through accurate timing – (Electronics)Figure 5 Mass of explosives (Powder Factor) in effect controls the mean size

Figure 6 Explosive Type impacts on the shape of the “Blast envelope”

Figure 7 The control of Oversize & Fines determines the top and bottom shape of the “Blast envelope”.

“S” Curve representing fragmentation for “Blast envelope” for emulsion explosive in given rock type

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Blast envelope measured across different geological zones in Kimberlite

even those of less than the defined oversize dimension.

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:

• Geology and Rock Properties• Mass of Explosives (Powder

Factor)• Explosive Type• Control of Oversize & Fines

(Stemming length and Hole diameter)

• Uniformity - Timing Accuracy

Geology and Rock Properties

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?

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.

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.

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“S” Curve representing fragmentation for “Blast envelope” for different explosives in similar rock type

AnfexEmulsion

Figure 4 Influenced by change in Geology and Rock Properties, same blast design across pit

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Fines, Drilling, and Blast crush zone

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Mass of explosives (Powder Factor)

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.

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

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.

Control of Oversize & Fines (Stemming length and Hole diameter)

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

Explosives Today - Series 4, No 4 Explosives Today - Series 4, No 4 Explosives Today - Series 4, No 4

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