Saving money from the start: a look at the effects of blasting on crushing and grinding efficiency and energy consumption

Pit & Quarry, Feb, 2004 by Lyall Workman, Jack Eloranta

In recent years there has been increasing attention paid to the effect of blasting on subsequent operations. In the past, the primary focus was on the ability of the excavation equipment to productively dig the blasted rock, and on the amount of oversize chunks produced. Now, more consideration is given to the effect of blasting on operations beyond loading, such as crushing and grinding.

There are two important aspects of blasting on fragmentation--one is seen and one is unseen. The first is the size distribution of blasted fragments. This is often assessed qualitatively, by inspection, as good or poor.

It can also be measured quantitatively by image analysis techniques. While these methods are not perfect, in terms of measuring fines, they provide much better results than previous techniques, are repeatable and not intrusive to production processes.

The size of fragments is the "seen" part of blasting results. It is important in crushing as it effects production and downtime. Overly coarse fragmentation will reduce primary crusher throughput. Coarse material will lead to more downtime for clearing crusher bridging and plugging.

Poor fragmentation will increase the load to secondary and tertiary crushing stages, if used, because there will be less undersize that can be split off to bypass these stages. This will affect productivity and energy consumption.

It is highly probable that the blasted size distribution introduced to the primary crusher will affect the feed size distributions throughout the crushing stages.

Second effect

The second effect of blasting, which is "unseen," is the crack generation that occurs within fragments. There is substantial evidence that such cracking occurs. The work by Nielsen and Kristiansen (Fragblast5, 1996) is an excellent example.

Fractures generated in the fragments may be macrofractures or microfractures. Microfractures develop around mineral grains and are seen through a microscope. Microfractures have the greatest chance of surviving the various stages of crushing and being present in grinding feed. The effect of internal fractures is to "soften" the fragments, making them easier to break. This has benefits to productivity, energy expenditure and wear of consumable items.

Therefore, in the process of optimizing blasting, it is important, but not enough, to know that the fragmentation distribution is adequate. Consideration must also be given to how blasting will precondition individual fragments by internal fracturing. While the first factor is now measurable directly, the second must be assessed through study of production, energy consumption and supply cost.

Two factors standout as being of essential importance in determining crushing and grinding effectiveness. One is productivity. There are certainly examples of processing plants where poor crushing and grinding production have controlled overall plant production.

The second is energy consumption. Large, hard rock mines expend enormous amounts of energy, with associated costs. A substantial portion of this energy is expended in crushing and grinding. Most particularly, energy consumption in grinding is large. The reason is that the change from feed size to product size, achieved in grinding, is typically much greater than in crushing.

There is significant evidence that blasting does affect crushing and grinding results, and that large savings in cost can accrue (Eloranta, 1995; Paley and Kojovic, 2001). It is reasonable to postulate that the size distribution of blasted fragments and the internal softening of individual fragments by blasting can affect crushing and grinding effectiveness, even though these processes are two to three unit processes downstream from drilling and blasting.

The role of microfractures is important, especially at the grinding stage. It is generally considered that fragments become harder at each stage of sizing, because the feed is smaller and there are fewer geologic and blast induced fractures present in the fragments. Since grinding feed is typically less than 3/4 in., it will only be the smallest macrofractures and the microfractures that survive to reduce the resistance to grinding.

The degree to which this happens is presently unclear. There is evidence that Bond work index is significantly reduced by heavier blasting (Nielsen and Kristiansen, 1996). There is, however, recent research that suggests that while significant softening is seen at the crushing stage, there is little change at the grinding level (Katsabanis et al, 2003).

The work by Katsabanis is currently confined to granodiorite, so the role of rock type is not considered. As cited above, there are also studies in operating plants that show important improvements to crushing and grinding production and cost associated with changes in blasting. For reasons made clear in this paper, it will be important to clarify the survivability and role of microfractures in future study.

A third factor of effectiveness in crushing and grinding is mineral liberation. Greater liberation means improved downstream recovery. A currently unanswered question is whether blasting that creates more microfractures around or through mineral grains will improve liberation and recovery.