Print7.1.1: Why do We Blast?
Yes, we blast to break the rock, but we have specific goals beyond simply achieving fragmentation of the rock! The desired outcome of a blast is broken material:
- fragmented to a specified size distribution,
- thrown a certain distance, and
- distributed in a pile that is placed to facilitate loading.
The size distribution is important for several reasons, and these reasons vary with different commodities.
Generally, the blasted material must be handled in some fashion, e.g., to load it into a haul truck or to cast it onto a spoil pile. The size of the blasted material must be consistent with the capabilities of the equipment that will be used to move it.
Oversize rock, i.e., rock that is larger than can be handled, can’t be loaded or if it can be loaded, it is too large to fit into the crusher. This creates multiple problems. It introduces delays in production, it is time-consuming, costly, and sometimes dangerous to practice what is known as secondary breakage.
The production of fines, i.e., very small particles, in the blast can be desirable in certain cases where an ore is to be beneficiated. In other cases, excess production of fines is undesirable. In the crushed stone market, for example, fines are excluded from many products, and it is expensive to re-handle and dispose of an unsaleable product.
The throw distance and the pile placement are closely related.
Pile placement
Pile placement is the distribution of the blasted material. Is the pile of blasted rock 30’ wide, 10’ long, and 10’ deep, or is 30’ wide, 80’ long, and 15” deep, or somewhere in between? There will be an optimum depth for loading. If the pile is too deep, the loader will waste time digging to load the bucket. If the pile is too dispersed, time will be wasted maneuvering over a large area to load the material.
Throw distance
This is how far the blasted material is moved. There are examples in surface mining where the blast is used to move material to a previously mined strip, rather than using a dragline or shovel. This is known as cast blasting, and obviously the distance that the blasted material is moved is a critical performance parameter. Throw is also important to ensure that the blasted material can be loaded or dug. Without adequate throw, the fragmented material will sit back down, and be extremely difficult to access. This can be a problem when advancing a face in an underground mine and, accordingly, we design the blasting pattern to ensure that the fragmented rock is lifted and thrown away from the virgin rock.