The Effects of Blasting on Crushing and Grinding Efficiency and Energy Consumption Lyall Workman1 and Jack Eloranta2 Abstract
Blasting has an important impact on mining and milling well beyond the necessary ability to
dig and load the ore efficiently. There is an increasing body of blasting research indicating
significant impacts in crushing and grinding. These include increased production through higher
output and fewer delays for bridging and jamming by oversize. In addition, fragmentation better
suited to the crushing and grinding system is indicated to lead to reduce energy consumption by
these activities, an important result in today’s environment. An important component of
optimum fragmentation for this purpose appears to be micro-fracturing within individual
fragments. This differs from fragmentation criteria for loading which focus mostly on fragment
size. Therefore, one must analyze blasting broadly to obtain satisfactory results throughout the
This paper examines the role blasting plays in optimum crushing and grinding with the
emphasis on energy reduction. The role of different blasting energy input on fragmentation is
studied, and related to needs at the plant. The effect of different feed sizes on energy
consumption in crushing and grinding is studied. The role of micro-fracturing in this process is
In the current environment, a policy of using energy where it is least costly, and conserving it
where it is most expensive is essential. Both unit cost and efficiency of the various processes
must be considered. Based on the results of this study, methods for a rational allocation of
energy are discussed.
The need for more research in microfracturing and of how far downstream blasting
improvements affect the results of subsequent unit operations is pointed out.
1. Lyall Workman,
Calder & Workman, Inc,
2. Jack Eloranta, Eloranta & Associates, Inc.
In recent years there has been increasing attention paid to the effect of blasting on subsequent
operations. In the past, the primary focus was the ability of the excavation equipment to
productively dig the blasted rock, and the amount of oversize chunks produced. Now, more
consideration is given to the effect of blasting on operations beyond loading, such as crushing
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 very 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
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
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 very 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
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 very 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 inch, 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, 2 papers). 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. Energy Consumption in Crushing and Grinding
The energy input to size ore fragments is large. Overall reduction, performed in a series of
stages may be from an eighty percent feed size passing of 40 cm (15.8 inches) to a final product
size of 270 to 325 mesh (.053 to .045 mm). A lot of energy is expended to accomplish this, and
it is not particularly efficient, with much of the energy input being dissipated as heat. It has been
estimated that grinding efficiency may be as low as one percent (Hukki, 1975; Willis, 1988).
The third theory of comminution developed by Bond (1952) is still used today, although there
have been recent advances (King and Schneider, 1995). Using this theory, energy requirements
to reduce fragments from an 80% feed size to an 80% product size can be calculated.
The Bond equation of comminution is stated as follows:
W = 10W ? 1
? , where
W = work input, kwh/ton
Wi = work index for the specific rock type, kwh/ton
P = 80% passing size of the product
F = 80% passing size of the feed
One reason for using Bond’s third theory is that work index Wi has been measured and reported
for many rocks.
Using this relationship one can study the work input required for different feed sizes and work
indices in the stages of comminution. In the current study Wi is held constant throughout the
stages, although it may, in fact, vary. Provided consistency is maintained the trends in energy
consumption and cost will be correct.
As a base case, we assume that taconite ore is being blasted with a heavy ANFO (HANFO)
having absolute weight strength of 3.35 MJ/Kg (801 cal/gm). The 80% passing size of the
blasted ore has been measured and found to be 40 cm (15.75 inches). The ore passes through
primary and secondary crushing, and grinding. The final product is 80% passing 270 mesh.
Bond has published a Wi for taconite of 14.87 (1961). This value is used in these base case
Table 1 shows the feed and product size, the calculated total energy input, and the energy cost
for each unit operation. The explosive cost is based on the powder factor of 0.33 kg/tonne (0.65
lbs/ton) and an explosive cost of $0.264/kg ($0.12/lb). Electric energy cost is assumed to be
$0.07 per kwh. Table 1: Energy and cost calculations by unit operation Operation Feed size Product size Work input Energy cost
cm cm kwh/ton
By far the greatest work input is in grinding. Size is reduced by a factor of 360. In primary
crushing, it is reduced by a factor of four and in secondary crushing by about five times. Clearly,
changes in blasting that reduce grinding requirements will have the biggest impact for energy
The work input shown for blasting is calculated by the Bond equation. The actual energy input
is .33 kg/ton, or .31 kwh/ton. Thus, the efficiency compared to Bond is 77 percent. This is
likely due to the variable nature of rock and the transmission of the energy, and the possibility
that Wi is greater than 14.87 in the unblasted state. The cost for the explosives however is that
associated with the powder factor of .31 kwh/ton.
From an energy consumption viewpoint, it is clear that blasting that decreases the Bond work
index will produce large savings if that reduction carries through to grinding. Changing the Energy Consumption
The energy consumed can change in three ways. First, if the feed size to the primary crusher is
decreased, less energy will be required to crush the ore to the same product size. Second, a
decrease in Wi related to additional macrofracturing and microfracturing within individual
fragments. Third, an increased percentage of undersize that bypasses stages of crushing thereby
decreasing the percentage of total tons crushed.
Consider the hypothetical case where the powder factor in the example above is increased to 0.45
kg/ton (0.90 lb/ton), and that an associated reduction in P80 size to 30 cm in the blasted ore
occurs. The work input, assuming no change in work index, is .194 kwh/ton. For a mine
crushing 50 million tons per year, a reduction of 1.8 million kilowatt-hours, or about $125,000
per year is realized.
The second possibility is a decrease in Wi. There is evidence that work index bears a
relationship to powder factor. Nielsen and Kristiansen examined grinding results for core of
three rock types when the core sample was not subjected to blast action, and when samples were
subjected to blasting by one and by two pieces of detonating cord (1996). For taconite, a
substantial reduction in work index was calculated. The reduction was greater for the two
detonating cord case than for the one cord test.
We have used data about core dimensions and the detonating cord used, found in their paper, to
calculate a powder factor. A taconite density of 3.93 ton/m3 (6000 lbs/cyd) is assumed. This
derived powder factor is to be considered as a first order approximation only. The results are
displayed on the chart in figure 1.
This graph shows a marked decrease in Wi with increasing powder factor. It also illustrates that
the incremental decrease in work index for higher powder factor is less, and eventually there
would be no advantage to further increases in explosive energy applied. The trend suggests that
the level of explosive energy at which there is no further improvement in Wi may be quite high.
For the proposed increase in powder factor to 0.45 kg/ton, this chart would suggest Wi of about
9.5. However, to be somewhat conservative, we have chosen a work index of 10.4 for this case.
A Wi of 14.87 is still used for blasting the undisturbed ore.
Figure 1: Work Index as a function of Powder Factor, after Nielsen and Kristiansen
6Work Index, kwh/ton
1.8Powder Factor, Kg/ton
Once again two stages of crushing, and a grinding circuit are considered. The ore is sized to
80% passing 270 mesh. Table 2 provides the results of this analysis. The same HANFO is used
as the explosive, and the blasting cost is based on the powder factor, which is a higher work
input than calculated by the Bond Equation. Table 2: Energy and cost calculations by unit operation with a higher powder factor Operation Feed size Product size Work input Energy cost
cm cm kwh/ton
In the example, the required work input has decreased by 30%, and the total cost by 26%. The
cost in crushing and grinding has been reduced by 30%. No consideration is given here to
increased production, less wear, or increased undersize bypassing crusher stages.
The actual work input by the explosives is 0.42 kwh/ton, whereas the Bond equation yields a
requirement of 0.27 kwh/ton. The efficiency is 65%, and represents the variability of the field
environment and nature of energy transfer to the rock.
Assume a mine crushes 40 million tons per year. In the base case, the mine will expend $60.0
million per year for energy as calculated in table 1. When the powder factor is increased, the
requirement falls to $44.28 million. A savings of $15.72 million per year is realized, or $0.39
per ton. This is a substantial reduction in cost.
It is of interest to compare this to the savings reported by Paley and Kojovic, (2001) as a result of
a drill-to-mill implementation at the Red Dog Mine. They reported savings exceeding $30
million per year, with the potential for additional improvement. Their case considered more than
energy cost. What is important is that their results and the present analysis using assumed
parameters are of the same order of magnitude. This suggests that, at least in some ores,
improved internal fragmentation carries through the crushing and grinding circuits. However,
considerably more study is needed to determine if this is true. The magnitude of the potential
indicates that such study should be a priority. Effect of Decreasing Work Index
Clearly, reductions in work index have the potential to reduce costs for energy consumption. In
figure 2 the powder factor and reduction in crushing and grinding energy cost are presented.
Also plotted, on the second y-axis, is the increase in explosives costs associated with the
decreasing work index.
For each work index and associated powder factor the feed size to primary crushing is assumed
to be 30 cm. In reality this might change, but in the absence of specific data to support changes
to the feed size we have chosen to keep F80 to the crusher constant.
The chart relates to taconite. The powder factor trend is based on the values we derived from the
study by Nielsen and Kristiansen (1996), but is increased from the trend line in figure 1 for a
conservative estimate. These powder factors must be considered an estimate only. The
relationships must be confirmed by field study in operating mines.
A reduction in Wi from 10.4 to 5.0 is accompanied by a decrease in crushing and grinding cost of
$0.513 per ton. The associated increase in explosives cost for the HANFO at $0.264 per
kilogram is $0.219 per ton. The net decrease in energy cost is $0.294 per ton. For a mine
crushing 40 million tons of ore per year, the savings are $11.8 million per year. This is a very
worthwhile reduction in cost.
One observes in figure 2 that the decrease in sizing energy cost flattens as the work index
decreases and the increase in powder factor accelerates. However, the trend suggests that the
limits have not been reached, if the relationships hold true in field practice. This would be
consistent with findings by Eloranta (1995) and by Paley and Kojovic (2001).
Figure 2: Powder Factor And Sizing Energy Cost
0.10Explosives cost per ton, $Powder Factor, kg/ton and Crushing and Grinding Energy Cost per ton, $
5.00Work Index, kwh/ton
Estimated Powder Factor
Explosives cost Discussion
This paper is based on an examination of various research and implementation drill-to-mill
projects that have been reported. It is intended to inform drilling and blasting personnel of the
improvements that blasting can effect in the processing operations. The material presented
should make clear that blasting engineers need to work closely with process engineers to achieve
the best cost of operation.
The analyses presented are predicated on the assumption that internal fracturing that leads to
softening of individual fragments, and a reduced work index, carry through to the grinding stage.
By far the largest potential savings in energy input occur at this stage of sizing. There is
evidence, as cited, to support this view. It is dependent on the production of microfractures
within the fragments. There is, however, recent research that did not find reductions in work
index carrying through to grinding, at least in granodiorite, unless very high blasting energy
levels are employed.
The magnitude of potential savings make it imperative that these questions be resolved. Further
research, and work in the operating environment will make an important contribution to the drill-
to-mill understanding. Research should include various rock types and structural geologies, so
the role of geology on internal fracturing and rock softening can be understood.
This paper does not examine quantitatively other beneficial results of improved blasting.
However, these exist and they include:
1. Increased productivity in crushing and grinding.
2. More undersize that bypasses stages of crushing.
3. Reduced consumable wear in crushing, grinding, loading and hauling.
4. Increased shovel production and less energy expenditure in loading
5. Tertiary benefits such as the ability to use light weight truck boxes due to the less severe
service encountered. This will also decrease energy consumption.
These factors will further improve the cost picture. Even if all the energy savings in crushing
and grinding are not realized, significant cost savings are possible.
This study employs higher powder factor to achieve improved results downstream. We have
observed numerous cases where this approach is beneficial. However, the fragmentation
specialist must approach drill-to-mill optimization with an open mind. Depending on geology,
and the crushing and grinding equipment employed, increasing powder factor may not always be
This analysis does not consider drilling cost. However, depending on how increased powder
factor is achieved this cost may increase and reduce some of the projected savings. Blasting
costs such as labor, equipment and accessories are not included as these are considered to be
similar in all cases. The foregoing analyses indicate that drill-to-mill optimization opens up
many aspects of blast design and implementation. These include explosive selection, powder
factor, blasthole size, pattern dimensions and timing accuracy. Conclusions
The following conclusions are made
1. The greatest energy savings available are in grinding due to the large change in particle
2. Blasting related improvements in grinding will depend primarily on the degree of
microfracturing achieve, as it is these cracks that will survive earlier stages of crushing.
3. Substantial improvement in cost can be achieved.
4. The use of greater energy input in the blasting unit operation will often be less costly than
expending the energy downstream.
5. There remain unanswered questions about drill-to-mill optimization. The large cost
savings projected, and in some cases seen in actual practice make research in this field an
urgent priority for mining cost minimization. References
Bond, F. C. 1952, The Third Theory of Comminution, Mining Engineering, May pp 484-494.
Bond, F. C., 1961, Crushing and Grinding Calculations, Part II, British Chemical Engineering,
Eloranta, J., 1995, The Selection of Powder Factor in Large Diameter Blast Holes, Proc. Of 21st
Annual Conf. On Explosives and Blasting Research, Vol 1, Nashville, TN, pp 68-77
Hukki, R. T., 1975, The Principles of Comminution: an Analytical Summary, Engineering and
Mining Journal, Vol 176, pp 106-110
Katsabanis, P., Gregersen, S., Pelley,C., and Kelebek, S., 2003, Small Scale Study of Damage
Due to Blasting and Implications on Crushing and Grinding, Proc. Of 29th Annual Conf. On
Explosives and Blasting Research, Nashville, TN.
Katsabanis, P., Kunzel, G., Pelley,C., and Kelebek, S., 2003, Damage Development in Small
Blocks, Proc. Of 29th Annual Conf. On Explosives and Blasting Research, Nashville, TN.
Nielsen, K., and Kristiansen, J., 1996, Blasting-Crushing-Grinding: Optimisation of an
Integrated Comminution System. Proc. Of FRAGBLAST 5, Fragmentation by Blasting,
Montreal, Canada, August 25-29, pp 269-277.
Paley, N., and Kojovic, T., 2001, Adjusting Blasting to Increase SAG Mill Throughput at the
Red Dog Mine, Proc of 27th Annual Conf. On Explosives and Blasting Research, Orlando, FL.
Schneider, C. L., and King, R. P., 1995, A Comprehensive Simulation of an Industrial
Comminution Circuit Treating Taconite. XIX international Mineral Processing Congress, San
Francisco, CA, October
Willis, B. A., 1988, Enhancement of Mineral Liberation, Proceedings of XVI International
Minerals Processing Congress, pp 293-297
- Lyall Workman1 and Jack Eloranta2
- Lyall Workman, Calder & Workman, Inc,
- Jack Eloranta, Eloranta & Associates, Inc.
- Energy Consumption in Crushing and Grinding
- Table 1: Energy and cost calculations by unit operation
- Changing the Energy Consumption
- Table 2: Energy and cost calculations by unit operation with a higher powder factor
- Effect of Decreasing Work Index