The mining industry is a major consumer of explosives, and the ore winning process is heavily dependent on blasting. The purchase and use of explosives is a significant cost, but a potentially greater cost is the effect of the blast on the overall cost of the product. An improperly designed or executed blast can increase loading time and cost, create future ground control problems, i.e., delays and costs, increase crushing costs, cause excessive fines that may be unsalable, and so on. Paying close attention to the results of each blast, and taking appropriate corrective action, can yield immediate benefits. Of course, you have to know what to look for, and how to achieve a desired result. So, let’s start at the beginning, and answer two fundamental questions! Why do we blast, and how 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:
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 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.
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.
Very carefully! Actually, I am serious!!! There’s a difference between civil and military blasting. In the latter, their goal is usually to “blow things up.” In civil projects, such as mining, our goals are to use as little explosive as possible, as safely as possible, and to have no collateral damage, while achieving the design outcome for the blast. This is not easily accomplished and requires both good engineering and faithful implementation of the blast design by the drillers and blasters. The short answer to the question of “how do we blast” is as follows.
The placement of the holes and the timing sequence of when the holes are “fired” constitute the blasting pattern.
I said that one of the goals of blasting is to avoid collateral damage, which we do through proper design and execution of the blast. But, what do we mean by collateral damage? Here are the primary ones that we constantly have to assess.
This is when fractures from the blast propagate beyond the intended region. Imagine that you want to drill and blast a tunnel opening through a mountain, and you are designing for an opening that is 30’ wide and 15’ high. However, due to improper design or execution, cracks have propagated to 18’ high. Over time, it is likely that pieces of rock will begin to fall out of the tunnel roof, creating a safety hazard as well as delays in using the tunnel.
Flyrock is a large chunk of rock that is propelled well beyond the throw region for the blast. These chunks can weigh hundreds of pounds or even more than a ton, and travel distances of several hundred feet. Over the years, flyrock has caused numerous fatalities and millions of dollars of property and equipment damage. Proper design and execution are necessary to prevent flyrock.
Gas production is an intended action during a blast, but the production of excess quantities of toxic gases, notably CO and NOx, is to be avoided. Improper on-site formulation of the blasting agent and problems with loading are often responsible for this problem. It should be noted that this is a hazard in both surface and underground mines.
Like gas production, ground vibration is an intended consequence of blasting. However, excessive ground vibration can damage structures, and the level of vibration at the boundary of the mine’s property is regulated. Careful design of the blasting pattern, and especially the timing of the holes, is required to keep the ground vibration within prescribed limits.
This occurs when excess energy from the blast creates a shock wave in the air. Certain weather conditions will cause the air blast to be bounced back to the surface but at some distance from the blast. There is little danger of personal or structural damage from an air blast, but it can precipitate a barrage of angry complaints from people who live in proximity to a surface mine. Nothing good ever comes from irritating the locals! Air blast can be reduced through proper design the blasting pattern and the choice of detonators used to link the holes together.
These are not really “collateral damage” per se, but they are an unintended consequence. A misfire occurs when some of the blasting agent in a hole, or multiple holes, remain undetonated after the blast. This explosive could go off at a later time, such as during loading, and cause serious personal injury or death. Misfires can be avoided through careful attention to the execution, as well as the design, of the blast.
Throughout this discussion, I have emphasized the proper design and execution of the blast. By execution, I mean the drilling and loading of the holes. Poor drilling or loading procedures will compromise the best design, and similarly, proper drilling and loading cannot compensate for a poorly engineered blast round.
It starts with good engineering, and specifically:
Design of the blasting round is concerned with:
and ends with good execution by the drillers and the blasters. Specifically for the driller:
specifically for the blaster:
I want to conclude this introduction to explosives and blasting with a short video clip (4:04) showing multiple controlled explosions. There is no narration for the video, but the blasts are accompanied with a musical soundtrack.
Look for the following:
Direct link to video [1] if not showing below
I hope that you enjoyed this video. It is entertaining! It also represents good blasting practice, which you will come to better understand as you learn more about explosives and blasting practice.