We have described how the processes involved in grinding developed in unison with new forms of energy and better materials: muscle power and stone, water power and iron, steam power and steel, and electricity and alloy steels. Explosive energy is the fifth form of energy, which is used to break rocks and to prepare them for further size reduction. The dangers and difficulties in blasting rock in mines have been great, and the priority of mining engineers has been to solve these problems rather than look downstream. It is only in the last 10 years that the development of precise drills, accurate explosives, and excellent sensors have given engineers the ability to understand and control explosive rock breakage, and they have used this knowledge to optimize blasting, crushing, and grinding to improve mineral extraction. Consequently, the objectives of blasts now include optimizing the feed to SAG mills, where the proportion of lump ore is important; minimizing the fines in products such as iron ores; enhancing the exposure of valuable minerals in deposits that are to be leached; and ensuring that the feed to primary crushers meets the required size. As knowledge increases, the link between blasting, crushing, and grinding will develop further.
The story of drilling and blasting, including the evolution of explosives from black powder to dynamite and ANFO, is told briefly in this chapter.
During the 18th and first part of the 19th century, the charging and firing of gunpowder shots was a somewhat risky procedure. A charge of gunpowder was poured into the shot hole. Sometimes a series of connected straws filled with powder or a train of powder laid in a paper twist were used to fill the needle hole (in the gunpowder). A piece of touch-paper that was supposed to burn for half a minute was lit and laid on the gunpowder trail, and the final performance of the shot-firer was to run as fast as possible (McAdam and Westwater 1958).
There are two reasons to break up large masses of rocks into fragments that can be readily transported away from the rock face:
1. To create space in the form of tunnels or surface excavations. For this purpose the broken fragments only have to be small enough to be transported from the rock mass for disposal.
2. To obtain broken rock for further processing. For this purpose the size distribution of the broken fragments is important because it affects downstream processing.
For thousands of years the sources of energy to break the rocks were fire, which cracked the surfaces, and human muscles, which wielded hammers and gads (see Chapter 3). Explosives were tested in the 17th century, and after initial problems blasting became the preferred technique to break rocks. Blasting was first used for military purposes in the mid-14th century, and there were good reasons that another 250 years elapsed before it was used for mining. Only a small amount of explosive was required to propel a missile, but moving masses of rock required very large amounts of explosives to be detonated. Because this was dangerous, miners were in no hurry to use blasting. But blasting was inevitable because the needs of mining and civil engineering left no alternative. The
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technology has now advanced to the stage at which the requirements of any civil or mining engineering application can be met accurately and safely.
The development of blasting technology has been difficult and dangerous:
■ Blasting required progress in two areas in which the problems encountered were probably as tough to solve as any in the industrial world: drilling holes in rock and making all the components that are required in safe explosives.
■ Explosives by their nature are dangerous, and innumerable tragedies occurred as progress was made in understanding and controlling blasting.
In this chapter it is possible only to briefly review the evolution of drilling and blasting, but this brevity does not reflect the importance of drilling and blasting in the spectrum of size-reduction processes. Indeed, it is the most complex and most difficult of all the breakage processes.