Chapter 7: Drilling and Blasting For Downstream Benefit

Drilling and Blasting are essential components of most mining, quarrying and construction operations. These are the first unit operations of a process by which rock is removed from the ground. Consequently, the results of these activities impact every subsequent unit operation. For example, the unit operations of mining and quarrying are found in table 7.1.

Table 7.1 – Unit operations of mining and quarrying.

In construction work, the sequence usually only involves the first five operations through loading ore and waste. However, crushing may be involved in some cases.

Caution

In underground work the unit operations of mining and processing may differ in some respects to those at the surface, but it remains true that blasting is near the beginning and affects many subsequent processes.

It is clear then, that drilling and blasting can affect the performance of up to five major components of operation. Therefore, the importance of considering the needs of all unit operations is essential to developing a good drilling and blasting plan.

Traditionally, the effectiveness of a drilling and blasting program has been evaluated on pit goals like those listed in table 7.2.

Traditional Goals Used To Evaluate The Effectiveness Of a Drilling and Blasting Program in Open-Pit Operations

Table 7.2 – Goals often used to evaluate the effectiveness of a drilling and blasting program in open-pit operations.

These are all important goals. However, in recent years a broader range of goals has developed. Recognition that blasting can affect results further downstream leads to a desire to develop a more comprehensive set of goals as listed in table 7.3.

Comprehensive Goals Used To Evaluate The Effectiveness Of a Drilling and Blasting Program in Open-Pit Operations

Table 7.3 – Comprehensive goals used to evaluate the effectiveness of a drilling and blasting program in open-pit operations.

Electronic technology offers a significant and largely unfulfilled potential for drill and blast improvement. Managers now have the means to measure results in ways that were not available in earlier times. For example, emerging systems that measure fragmentation are relatively recent developments that, while not perfect, greatly enhance the ability to measure the results of blasting. Other equipment such as long-term laser survey systems, velocity of detonation monitors, computerized blast design, timing simulators, near field vibration monitoring, high speed video cameras, GPS, handheld dispatch systems, drill performance monitors, excavator performance monitors and spreadsheet software all contribute to the ability to quantify blasting results. Modern technology makes information available to more people in real time and enables a much faster information feedback loop.

The result is that drilling and blasting optimization is trending away from qualitative and subjective standards toward fact-based decision-making. This should not be interpreted to mean that the evaluation of experienced blasters is no longer important. That is far from the truth and when combined with information derived from technology will lead to the best optimization.

Caution

It is fundamentally important to understand that the cost of drilling and blasting is neither good nor bad. It is only in how these unit operations affect the final outcome that it becomes good or bad. Therefore, drilling and blasting, and indeed any and all operations must be evaluated on the impact it has on the end result.

Often, mine and quarry departments are evaluated primarily on factors within that department. So, for example, reducing cost within the department is considered good but no analysis of the effect of the cost change of the total cost of operation is made. If the result of the cost cutting is that costs increase in another unit operation those responsible for that downstream operation will be held accountable even though they may not be responsible for the increase.

Therefore, it is essential to determine the impact on the overall cost of mining, quarrying or construction of a change in drilling and blasting. Only then does one know if the change is "good" or "bad". Arbitrarily setting cost reduction goals for the annual or project budget is not a valid way to achieve an improved unit operation. For example, suppose the directive is made to reduce the cost of drilling and blasting by ten percent for the next year. The blasting department achieves this by reducing the powder factor and in fact reduces cost by twelve percent for which they are rightly congratulated since they met and exceeded the stated goal.

The same year it is found that the cost of production a unit of concentrated mineral has risen significantly. It is unclear why but a study finds that it is largely due to a loss of crusher throughput and a substantial increase in grinding energy cost. To a lesser extent the cost increase is tied to increased overtime in the pit and loss of excavation productivity. Using technology most if this is tied to a change in fragmentation distribution and muckpile swell. The result is that the blasting results are now deemed "bad" and the operation returns to previous designs.

Consideration is now given to the possibility that additional increases in powder factor might lead to more reductions in overall cost, so the powder factor is increased for the following year. The result is that overall cost of mining and processing show indeed due to increased excavation production, less oversize, and better throughput and energy consumption. Of course, the cost of drilling and blasting has risen, but is more than offset by the downstream decreases. Thus, the drilling and blasting results are presented "good" even though cost has risen within the department.

This example is realistic, but is not intended to imply that increasing powder factor is always a good thing. The opposite may be true in some cases. It is intended to say that arbitrary goal setting is not a valid approach. Blast optimization is a process and one that can pay large dividends. It is a process well suited to improvement techniques such as Six Sigma, Total Quality Management (TQM), Statistical Process Control (SPC) or other continuous improvement methods.

The first fundamental concept is that drilling and blasting results must be evaluated on the basis of their effect on the overall start to finish cost result and not in isolation.

The second fundamental concept is that drilling and blasting for downstream improvement is a multi-departmental effort. Its mining and quarrying this means the mine operations and processing departments must develop a close working relationship. In construction the same is true of the blasting contractor and the excavators, forming and concrete placement contractor. Without a cooperative effort, optimizing blasting for subsequent operations is difficult or impossible to achieve.

It is important to understand that blasting for downstream operations will not mean the same thing in all cases. Since ore or saleable material is handled differently from waste rock, optimization will differ in these cases.

Ore or saleable rock generally requires additional processing after loading and hauling and the full suite of blast optimization considerations come into play. In a best practices operation much emphasis should be placed on the effect of drilling and blasting on crushing and grinding, and on liberation. Much of the energy consumed in mining is used for grinding. Therefore, this is an excellent part of the operation to obtain savings. Drill to mill techniques can reduce grinding costs considerably. Often this is where the single largest savings are possible.

Quarries often have the further consideration that fines production should be minimized. This material is not saleable in most cases and storage space for fines is limited for most quarries. Therefore, establishing a blasting program for downstream benefit must include considerations of excess fines production. A specification for fines should be written and the blasting program must incorporate this. A comprehensive fines reduction study has been conducted in Europe over the past several years and quarry blast designers may wish to examine the results of this study.

Surface coal mining presents a different method of mining from base metal and iron ores mines and from construction blasting. Here most blasting is conducted in waste rock and is excavated with walking draglines or large mining shovels. Therefore, most blasting is conducted to condition the waste for digging. Waste rock does not have the size restriction imposed on ore by the size that the crusher can accept.

However, while these machines have very large buckets, the fragmentation distribution for high productivity digging is much smaller than that wall fill in the bucket. In addition a distribution of fragment sizes allows finer material to infill spaces between coarser pieces thereby increasing payload.

A blasting method often used in dragline mines is cast blasting. In this method the blast is purposely designed to displace overburden into the previously mined strip cut. In this manner, a percentage of the waste rock is moved to final location by blast action. The amount of overburden that must be moved by the dragline is reduced. Thus overall overburden production can be increased or a smaller dragline can be substituted to do the same job. Principles of cast blasting are discussed in chapter 34.

It is important that the designed "cast-to-final" percentage be achieved and maintained. Therefore, for these mines an important part of blast design for downstream operations is obtaining a high cast-over percentage.

Diggability And Excavator Performance

A primary goal of blasting is to prepare the rock for digging by the excavator selected for the job. Fragmentation goals should match the machine's bucket size and configuration. In construction operations bucket capacities are often relatively small. Blasting operations need to fragment rock to small sizes to flow readily into the bucket and fill it to high capacity.

In quarries, bucket capacity is often in the intermediate range. Material must be suitably prepared for equipment such as hydraulic shovels, backhoes and front end loaders.

Open pit mines tend to employ very large excavators such as electric mining shovels, large hydraulic shovels, and front end loaders. Preparation of the rock for this equipment is again different from the others in terms of size distribution.

Underground mines employ scooptrams and other loading equipment with characteristic bucket configurations and capacity. Blasting suitable for this kind of excavator must be designed.

Surface coal mines often employ draglines with very large buckets. Bucket capacities in the range of 61 cubic meters to 76 cubic meters (80 cubic yards to 100 cubic yards) are often used. These buckets can accept much larger fragments than those employed in construction and quarrying.

Caution

The true question is: what fragmentation distribution will yield the best productivity?

Optimum fragmentation for any type of excavator is not governed by pieces that will fit in the bucket. The size distribution should be considerably smaller than that to be optimum. Material must flow into the bucket easily and pack in a manner that maximizes payload. There should be a sufficient distribution of sizes so that finer fragments fill the voids between larger, blocky pieces maximizing the amount of material per bucket load. Figure 7.1 shows different packing results based on the nature of the fragmentation.

Figure 7.1 – Results of different gradations on fragment packing. (Courtesy: L. Workman)

It is found that large particle sizes and a narrow distribution, increases digging force and reduces payload. This was determined in a laboratory study of scoop tram productivity for example (Singh and Yalcin, 2002).

The three primary characteristics of blasted material that affect digging results are the (1) size distribution of the blasted rock. The material must be of a size to flow readily into the bucket and the distribution of sizes should allow for good packing. (2) loosening (swell) of the muckpile. The muckpile should displace and swell sufficiently to allow an excavator of given breakout force to enter the bank easily and pass the bucket readily through the bank and fill to capacity, and (3) profile of the blasted rock. The muckpile should leave a final profile that allows the equipment to dig efficiently. Figures 7.2a and 7.2b illustrates an example of good and poor muckpile profiles for shovels digging where there is a low productivity zone for shovel digging, whereas the lower diagram shows a more humped result. It is important to note that muckpiles should be kept at reasonable heights in relation to the equipment used. Any safety regulations in this regard should be complied with.

Figure 7.2a – Inefficient low-profile muckpile profile for a shovel. (After N. Paley)

Figure 7.2b – Efficient high-profile muckpile profile for a shovel. (After N. Paley)

Drilling and blasting affects the following five excavator performance factors listed in table 7.4.

Table 7.4 – Factors that affect excavator performance.

Clearly, blasting affects most of the productivity factors associated with excavators and in significant ways.

Figure 7.3 is shown a well-loaded dragline bucket. The bucket is well-filled and the material well fragmented.

In good digging material it is possible to fill the bucket in a length equal to twice the bucket length. Figure 7.3 illustrates a dragline bucket in good digging that is well-filled in two lengths. For example a dragline bucket suspended between two sets of wire ropes does not dig a tight bank very efficiently.

Figure 7.3 – Well-filled dragline bucket fully loaded in approximately two bucket in the path lengths. (Courtesy: Cloud Peak Energy)

Drilling and blasting affects truck maintenance cost and time in the five important ways listed in table 7.5.

Table 7.5 – Drilling and blasting results that affect excavator maintenance cost and time.

Therefore, good blasting is essential to high excavator availability and productivity.

Optimizing construction blasting involves many of the same principles found in other settings where blasting is employed. Breaking to grade and to excavation design limits becomes highly important in many cases because overbreak in any direction can be costly. Methods to avoid these problems are important and must be incorporated into best practices.

With the exception of very large projects, the bucket capacity and power of construction excavators are less than those of mining equipment. Therefore fragmentation needs and muckpile loosening must take available equipment into account to achieve high productivity. Higher powder factors may be required. The effect of the geology on fragmentation may need to be overcome.

Goals For Fragmentation

In the 1990s investigators started paying more attention to the effects blasting can have in crushing and grinding. It was recognized that that wise optimization uses energy sufficiently. Grinding is notoriously energy inefficient and many be as little as 0.1% efficient. There are two important effects of blasting on rock—seen and unseen.

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. Also these methods give results that can be fed back to the design process for continuing improvement. Progress is accelerating in this area. 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. Also these methods give results that can be fed back to the design process for continuing improvement. Progress is accelerating in this area.

Microcrack

A microcrack is a crack of small size when compared with a characteristic dimension of the structure of the material, often not visible to the naked eye. It is a crack-like opening in rock with an extension much smaller than the planar extensions and with a ratio of width to length (crack aspect ratio) of less than $10^{-2}$ and typically $10^{-3}$ to $10^{-5}$. The length of a microcrack typically is in the order of the grain size or in the order of a couple of millimeters (0.08 inches) or less. More often it is found on a microstructural scale. Microcracks can be subdivided into grain boundary crack (occur along grain boundaries), transgranular cracks (lying on the cleavage plane in a single grain) and inter-granular cracks extending along a grain boundary into the two adjacent grains (Simmons and Richter, 1976).

Fractures generated in the fragments may be macrofractures or microfractures. Microcracks develop around mineral grains, and can only be seen through a microscope. Microfractures have the greatest chance of surviving the various stages of crushing and being present in grinding feed.

Caution

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.

Figure 7.4 – Microcracks created within a rock fragment by blasting. (After Katsabanis, et al)

Therefore, in the process of optimizing blasting it is very important, but not enough, to know that the fragment size distribution is optimal. 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 and perhaps microstructurally.

Blasting For Crushing And Grinding

Optimized blasting can improve crushing and grinding in three principal ways: (1) increased throughputs, (2) reduced energy consumption and (3) reduced consumption of wear items.

It has been known that sending oversize to the crusher is costly in terms of downtime to remove blockages and in production. It is also understood that too much fine material can reduce crusher throughput.

It has also been understood that certain size distributions may lead to more productive grinding. However, this is not always well documented. The sizing considered optimum is not always correct.

Two factors standout as being of essential importance in determining crushing and grinding effectiveness: (1) energy consumption and (2) the potential of improved mineral liberation. There are certainly examples of processing plants where poor crushing and grinding production have controlled overall plant production.

The first factor of effectiveness 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.

Consider actual energy consumption in iron ore as reported by Eloranta, (Eloranta, 1997) in table 7.6.

Table 7.6 – Energy consumption costs in base year in U.S. dollars (Eloranta, 1997).

These results clearly show that most of the energy employed in size reduction is consumed in the grinding unit operation.

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 not altogether probable 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 four mass processes downstream from drilling and blasting.

The role of microcracks 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 19 millimeter (¾ inch), it will mostly be the microcracks that survive to reduce the resistance for grinding.

The degree to which microcracks are generated and resistance to grinding reduced has been unclear. There is evidence that the Bond work index is significantly reduced by heavier blasting (Nielsen and Kristiansen, 1996). Some research 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 was confined to granodiorites, so the role of rock type is not considered. However, there are studies from operating plants that show important improvements to crushing and grinding production and cost associated with changes in blasting.

More recent research (Katsabanis et al, 2008) has shown a decrease in the Bond work index with increase in powder factor for three granites. This decrease is less than that reported by Nielsen, et al. Moreover of his, from drop weight testing gave the same results.

A graph of work index as a function of powder factor was developed from the Katsabanis et al. data (Workman and Eloranta, 2009). This is presented as figure 7.5 and clearly shows the reduction in work index with increased energy input.

Figure 7.5 – Relationship of work index to powder factor for three types of granodiorites. (Workman and Eloranta, 2009)

When rocks experience a decrease in the Bond work index with an increase in powder factor, the energy expended in crushing and grinding will be reduced. It has been shown that reduction in work index can lead to substantial cost savings in energy cost and overall processing costs (Workman and Eloranta, 2003).

The work index is related to the difference between the product and feed sizes; not the absolute feed size. Finer fragmentation by blasting will mostly impact crusher throughput. A complex relationship exists between feed size and crusher performance. Oversize material may bridge, while fines can plug and halt production. Further variability arises from choke feeding versus allowing each load (See figure 7.6) to clear prior to dumping the next load.

Figure 7.6 – Choke feeding the crusher. (Courtesy: J. Eloranta)

Choke feeding tends to narrow the crushing zone higher where fragment-to-fragment crushing occurs (See figure 7.7). Choke feeding may cause any of three effects on crushing: (1) finer crusher product for the same crusher setting, (2) slower throughput due to the additional work being done, or (3) improvement of liner materials as crushing occurs across a greater vertical zone.

The most important fundamental is that both blasting and crushing are at least an order of magnitude more efficient (See table 7.7) in the production of new surface area than grinding. Small improvements in these two processes pay large dividends in grinding.

Figure 7.7 – Feeding the crusher truck-by-truck. (Courtesy: J. Eloranta)

Table 7.7 – Impact on efficiency of blasting and crushing on grinding.

Feed size into grinding mills is governed by crushing circuit product. Therefore, it is reasonable to assume that reduction in the Bond work index at the grinding circuit is predominantly related to internal softening of fragments.

The second factor of effectiveness in crushing and grinding is the potential of improved mineral liberation. Therefore, it is reasonable to assume that reduction in the Bond work index at the grinding circuit is predominantly related to internal softening of fragments. Greater liberation means improved downstream recovery. A currently unanswered question is whether blasting that creates more microcracks around or through mineral grains will improve liberation and recovery.

The foregoing discusses blasting for downstream operations where unit operations through crushing and grinding are required. However, not all blasting is performed in operations where this is necessary. The applications listed in table 7.8 have different downstream benefit goals.

Table 7.8 – Blasting applications that have more limited downstream benefit goals.

The important requirement is to take a global view of the effects of blasting on subsequent unit operations and not a narrow view encompassing only the immediately following operations like loading. One must determine what the priority downstream goals are and then design blasting to meet those goals. Priority goals are usually those that involve revenues and minimize costs. Therefore, those goals that impact cost and revenue the most should be identified and blasting designed to optimize them. In the process, some other goals may be left sub-optimized. The ultimate plan is the one that leads to better economics than other approaches and improves the financial position of the project or enterprise. This is the goal of blasting for downstream operations regardless of industry.

The foregoing discusses blasting for downstream goals listed in table 7.9 are more process oriented because the downstream processes are not usually needed.

Typical Surface Construction Blasting Downstream Benefit Goals

Table 7.9 – Typical surface construction blasting downstream benefit goals.

For example, avoiding overbreak at the perimeter may require a blast design not optimized for digging with the available equipment. However, overbreak control may be a priority because the cost of the job.

Aggregate quarry operations are usually only concerned with processes through multistage crushing and screening. Priority goals are listed in table 7.10.

Typical Quarrying Blasting Downstream Benefit Goals

Table 7.10 – Typical quarrying blasting downstream benefit goals.

From among these and other downstream goals the quarry operations must decide which have the greatest impact. Blasters must then design blasts that are optimized for these priorities and assist in best achieving a productive and profitable operation.

For example, if it is determined that introducing less tons of saleable products is the key goal then the crusher system feed size distribution that yields the desired result must be determined. Blasts that consistently lead to this distribution must be designed. Finally, fragmentation distribution of the crusher feed should be monitored to assure that goals are being met.

Prioritize Competing Goals

It is possible to optimize many variables, but if the focus is on the right ones the blasting program will make an important contribution to overall success of the operation.

In the case of coal stripping operations, key goals may be as listed in table 7.11.

Typical Coal Stripping Downstream Goals

Table 7.11 – Typical coal stripping downstream benefit goals.

Process Commonality

Every industry using blasting as part of its process uses the same criteria to develop its blasting program.

Regardless of industry, it is clear that every operation has the same ordered procedure to provide fragmentation and other blasting results that provide for the best downstream benefit. This procedure is provided in table 7.12.

Table 7.12 – Procedure to develop a blasting program that provides desired downstream benefits.

Throughput

Throughput refers to the rate at which ore and waste can be handled by a unit operation. It is most commonly applied to loading, crushing and grinding. However, the production of any unit operation can be thought of as throughput for that operation.

Construction operations may benefit from high productivity by being completed quickly. Profitability may then better and penalties due to late completion avoided.

Underground operations benefit from higher scooptrams, and other loading equipment production. In addition, crusher and grinding productivity also apply to many underground operations.

Similarly, strip mines may benefit from increased dragline or shovel production in overburden. Since the overburden allows the coal seams or inner benches adjacent to the coal is not further processed, excavator production at the waste is the key goal.

In iron ore mines cast blasting is often employed. The purposes are to excavate a portion of the overburden by explosive action directly and to optimize material fragmentation for productive digging. Some mines cannot make their annual coal production target if blasting does not displace a given percentage of the overburden to final location. Thus, cast blasting is directed at downstream result and is not a mine to mine option.

Throughput is an important consideration in all kinds of endeavor, not just when a crusher and grinding circuit is present.

When mineral demand and prices are high, a mine often benefits from producing some material product. There may be bottlenecks in the system that hinders its ability to produce more.

Blasting is a common symptom. Others include primary grinding and hauling on in the pit.

Crushing is a complex process that is affected by many variables. Blasting is one of these parameters. Blasting affects crusher throughput in two ways. One is the size of the fragments and the other is by cycles induced inside the fragments that reduce the resistance to breakage.

Table 7.13 – Mining and milling throughput improvements from an effective blasting program.

Energy Consumption

A fundamental driver to blasting for downstream benefit has been the attempt to allocate energy use in the best manner to decrease overall energy costs while achieving other improvements described. There is ample evidence that crushing and grinding use a lot of energy and that this energy consumption does change in response to changes in blasting operations. (Eloranta, 1999)

Energy Used in Blasting

The energy used in blasting must be evaluated against energy expended in other unit operations of mining and milling.

The energy used in blasting must be evaluated against energy expended in other unit operations of mining and milling as listed in table 7.14.

Table 7.14 – Energy consumption areas of mining and milling.

Energy expended in loading ore and waste increases as the fragmentation and swell of the muckpile decrease. The equipment experiences more difficulty penetrating the bank and passing the bucket through the broken rock. Bucket fill factors decrease, which means fewer tons per pass and a less efficient use of energy. Therefore, fragmentation suited to the loading equipment is essential to efficient energy consumption.

If there are no additional downstream operations, as may be the case in some construction projects or when mining overburden with a dragline, the decision of how to blast entails determining how much excavator energy use is reduced for a given increase in blasting energy. At some point, additional blasting will not further improve energy consumption and cost relationships. Thus, no further input to blasting energy can be justified on loading energy savings. Loader productivity may not yet be at maximum at this point however, so there are additional issues to weigh.

For example, in construction, a tight schedule for completion coupled with penalties for delay may override other factors and warrant additional energy input to blasting if this leads to higher production rates. Therefore, making the right decision requires consideration of all factors and the blaster must work cooperatively with others to define goals, measure results and determine the best course.

Mined ore is usually sent to crushing facilities for primary sizing. Crushing may consist of one or more stages of sizing to prepare the ore for grinding. Crushing expends energy and the user fragments can then be reduced in size in the energy expended.

Blasting can affect the crushability of fragments in two ways by either (1) affecting the size distribution of the blasted rock to be efficient for crusher operation, or (2) softening of the fragments through the generation of a web of microcracks within the individual pieces.

In the first instance, one must determine the size distribution that gives the least energy consumption per ton crushed. This typically entails the use of modern image analysis equipment, which is discussed in chapter 32. Size versus energy use can then be compared. Adjustments for water vs. moisture temperature, etc. need to be made when performing these analyses. Once an optimum distribution is determined, ongoing monitoring by size analysis is necessary to maintain optimum performance.

Softening of individual fragments is discussed earlier in the chapter. The ability to generate a web of minute cracks within particles reduces the energy required to break them down. The generation of microcracks will vary by rock type, but in general increased blasting energy does appear to increase the number of cracks produced. Experimentation at the property with energy consumption versus blasting powder factor is needed to determine the optimum operating point.

After crushing, ore processing usually requires multiple stages of grinding. Ball or rod mills or a combination of the two may be employed for this purpose. The feed to these mills is usually an inch or less. However, the size reduction ratio may be very large. Some iron ore operations for example grind to minus 270 or minus 325 mesh. Therefore, the energy consumed is very large.

The third theory of comminution was developed by Bond (1961). This theory relates the energy expended to the feed size, product size and a work index characteristic of the rock type. His equation is given as equation 7.1.

Equation 7.1 <!-- VERIFIED -->

$$W = 10 \times W_i \times \left( \frac{1}{\sqrt{P}} - \frac{1}{\sqrt{F}} \right)$$

Where:

• $W$ = work input (kilowatt hours/ton)
• $W_i$ = work index for the specific rock type (kilowatt hours/ton)
• $P$ = Passing size of rock at 80% (microns)
• $F$ = Feed size passing at 80% (microns)

One reason for the continued use of Bond's third theory is that work index ($W_i$) has been measured and reported for many rocks.

Equation 7.1 indicates both (1) as the difference between the feed size and the product size increases, the measured work input increases, and (2) the required work index for a given rock increases as the measured work input increases, including for the same size difference.

EXAMPLE 7.1

Calculate the work input for crushing required for fine taconite ore (Bond index = 14.87). The ore passes through primary and secondary crushing, and grinding. The blasted ore is 80% passing 40 centimeters (15.75 inches) (400,000 microns). Final crusher product is 80% passing 270 mesh. In primary crushing the rock is reduced from the 80% feed size of 40 centimeters to a product size 80% passing 10.2 centimeters (4.02 inches) (102,108 microns).

$$W = 10 \times 14.87 \times \left( \frac{1}{\sqrt{102108}} - \frac{1}{\sqrt{400000}} \right)$$

$$W = 148.7 \times (0.003129 - 0.001854)$$

$W \approx 2.21$ kWh/ton for primary crushing

The feed size into grinding is typically not large. However, the product size is usually quite small. In iron ore as an example, the feed size may be about 19 millimeters (0.75 inches) while the product size is 270 mesh. The large difference in sizes leads to high energy requirements to achieve the product size. This requirement is rather fixed by crushing characteristics and the particle size needed for liberation and the cannot be changed.

Therefore, reducing the energy required for grinding will result likely be done through the reduction of the work index. A fragment that has been softened through microcracking by the generation of a web of microcracks may have a lower $W_i$ than a fragment that has not been affected by blasting. Naibam (1990) reported softening of as much as a 65% decrease in work index.

Equipment must be properly instrumented to obtain quantitative data from which informed decisions can be made. Data required includes but are not limited to those listed in table 7.15.

Table 7.15 – Data required to obtain quantitative downstream blast performance information.

In the absence of quantitative data there is no accurate way to resolve anomalous results or to accurately determine when the optimum work index has been reached.

Control Of Fines By Blasting

The production of fines by blasting can be a problem for mining or quarrying operations. The problems experienced are listed in table 7.16.

Table 7.16 – Problems caused by fines for mining or quarrying operations.

The question arises as to how fines are produced in blasting. This does not appear to be entirely determined at this time; however, studies such as the large European "less fines" study (Moser, 2003) have led to better understanding. The amount of fines produced as a function of the factors listed in table 7.17.

Table 7.17 – Factors contributing to the production of fines by blasting.

The geologic structures and compressive strength characteristics of the rock are important to how the rock reacts under high-pressure gases and, therefore, the amount of fines produced. The importance of geology is discussed in chapter 8.

One concept of fines production in blasting is that, as the very high-pressure detonation gases cause expansion of the borehole volume, crushing occurs around the circumference of the borehole. Much of this crushed material is of fine size. Therefore, the greater the borehole expansion the more fines are generated.

The expansion around larger boreholes will be greater around smaller boreholes. However, the bigger boreholes employ larger burdens and spacing. Therefore, the total amount of fines produced may not be much greater except to the extent that volume expansion or blast pattern scale up is non-linear. In harder rock, the pattern expansion for larger boreholes works less well the toe pulling capability and the change in explosive weight associated with the change in diameter of the cylindrical borehole. This is less than a linear scale-up. In soft formations linear scale-up with borehole size is more possible.

Consequently, in some situations, it is possible that using a smaller diameter borehole may reduce fines production. For a given project, this must be determined by experimentation. The choice of borehole diameter must also be considered against production requirements, drill fleet size and overall economics.

The choice of explosive may also be important. Products with high velocity of detonation and associated high borehole pressure might produce more fines. Thus when fines are a problem, one more issue to consider explosive types with this in mind and do some experiments to determine the affect of different explosive choices on fines production.

The amount of explosive used in the blast will also affect fines production. This is often expressed as powder factor or ANFO equivalent powder factor. As an explosive concentration the basis of over-fragmentation will be met. Additional powder will likely generate more fines than desired. In this regard, the need to avoid fines and the desire to increase crushing and grinding throughput and reduce energy consumption may compete. Consequently, a decision will need to be made about what the overriding concern is and other considerations may then be left suboptimized.

The degree of confinement of the blast is important. Highly confined blasts such as buffer blasts and sinking cuts generally require higher powder factor and are very constrained. Under these circumstances, more fines can result. Confined blasting is a valid technique in some applications, but when possible, reducing the confinement may help reduce the quantity of fines produced.

Over-confinement may result if blast design reaches too much burden or poor explosive distribution. Such designs should be avoided because they can lead to overall poor fragmentation and fines.

Care should be taken in placing the front row of boreholes. Straight bench faces are helpful to accurate placement of front row boreholes. The use of technologies such as laser surveys or the more recent photogrammetric methods are helpful in optimizing front row borehole locations in relation to the free face.

Millisecond surface and down-the-hole delays are used to separate the detonation of boreholes in the blast not allow the muck to relieve and swell. When the delay timing is too short, the blast relieves poorly and the rock from hole's detonation in the next delay will jam into the previously detonated boreholes creating more impact and grinding of fragments. Thus, a higher percentage of fines may occur. Good timing design will not only add productive excavations of ore and waste, but it also may reduce the fines otherwise produced.

For best results, delay times should be accurate and reliable. This avoids situations where portions of a blast become over-confined or under-confined due to variable delay times or occasional failure of delays to detonate. Pyrotechnic delays have become increasingly reliable and accurate over the years, but continue to have some variability.

More recently, electronic delays have been introduce and are finding increasing acceptance. These devices typically have microsecond accuracy. Therefore, in situations where the need to limit fines is critical blasters should be aware of this technology.

Exacting Control Of Blasting For Optimum Results

Exacting control of the blasting process is essential to successful blasting for downstream benefit. The further practice deviates from design, the less effective the optimization will be. What blasting for downstream benefit tries to accomplish is a considerably more sophisticated blasting process than usually practiced in the past. One reason that this blasting process is becoming more accepted is that advancing technology has provided the blaster new highly accurate ways to implement, monitor and quantify blasts.

Therefore, blasting for downstream optimization will only be successful if the blasting is a carefully controlled process. When changes to Bond work index or other measures of softening are not large, poor quality control will produce inconclusive results.

If a large degree of softening is achieved, it will make loose the door to poor design and implementation. Therefore, production and economic benefit that could have been achieved will be lost. There are core practices in blast design and implementation that must be met in order to achieve optimized blasting. When introducing blasting for downstream optimization, even greater control is needed to consistently achieve goals. However, blasting for downstream benefit does not apply a whole new set of rules for blasting. The core principles, as discussed in this 18th Edition still apply as they do for high quality blasting in any context. However, tighter control is needed.

Modern technology has given us new ways to measure blast parameters and monitor results. Equipment such as size analysis equipment, laser profiling systems, new photogrammetric methods, field velocity of detonation monitors, near field vibration monitoring, GPS, on-board controllers on bulk explosives trucks, borehole deviation measuring equipment, and high speed video systems are all tools the blaster can employ to tightly control blast implementation and results. Chapter 32 discusses how these tools are used to evaluate blasting results. All of this technology helps make Six Sigma based process control possible.

What is certain is that without a control process that is exacting, in a realistic way, it is not likely that the benefits of drill-to-mill will be consistently achieved. Therefore, it is necessary to consider design and control of these blasting programs differently than has often been the case in the past. The goal is challenging but the evidence indicates that the reward is substantial.

In some cases the conditions under which blasting is conducted may mean that designs must be modified and less than optimum results may be achieved. This may be necessary, but one must recognize that the results achieved may not meet the usual goals of the overall operation. The result can be increased cost and decreased profitability. Therefore, it is important not to accept conditions as the "way it is." Everyone should work to correct the conditions causing suboptimal performance.

Some conditions cannot be corrected. For example in some situations of ethical blast vibration concerns it may not be possible to achieve the best fragmentation. Similarly final wall blasts next to a perplex line may have to be loaded in a manner that does not allow for the best diggability. Given that overbreak at the perimeter can be an enormously expensive good final wall control may be the goal that leads to the best economic results. Here the goal should be to protect the final pit or excavation wall and understand that digging may not be as productive as our normal goals require.

Interdepartmental Cooperation Is Essential

A primary goal of drilling for downstream benefit is to reduce the bottom line cost of producing a product whether that is an ore concentrate, crushed stone, cement, or a high quality civil excavation. This will not be achieved if these involved work in isolation.

Mines, quarries and construction projects are typically divided into a series of departmental responsibilities. The product of one department is the feedstock of the next group in line. However, the fundamental goal of everyone involved must be to impact the final cost of producing a unit of product. Therefore all groups have a role to play in the success of the venture and must be accountable for how well they contribute to that goal. If there is poor interdepartmental cooperation and communication it will be very difficult to achieve optimum results.

Caution

Everyone along the line organization should be accountable for the bottom line result, not just departmental results. When this does not occur individual departments may take actions that improve their results, while at the same time the overall cost of production is rising. This is counterproductive.

The Six Sigma method is a good way to involve interdepartmental team in the problem solving process. Whatever approach is taken senior management must foster a culture of cooperation throughout the operation. Once this is achieved blasting for downstream improvement will almost certainly achieve dramatic results.

References

Bond, F.C., 1961. Crushing and grinding calculations. British Chemical Engineering Part I 6 (6): pp. 378 –385 (Part II 6(8), pp. 543 –548).

Eloranta, Jack. 1997. The efficiency of blasting versus crushing and grinding. International Society of Explosives Engineers (ISEE) Research Proceedings of the 23rd Annual Conference on Explosives and Blasting Technique, February 2 – 5, La Vegas, NV. ISEE, Cleveland, OH.

Eloranta, 1995. Downstream costs and their relationship to blasting. Minefill 99. Surface Blasting Conference. Duluth, Minnesota, June 8

Katsabanis, P., A. Kunzel, C. Perley, and S. Sanchidrian. 2003. Damage development in small blasts. International Society of Explosives Engineers (ISEE) Proceedings of the 29th Annual Conference on Explosives and Blasting Technique, February 2 – 5. Nashville, TN. ISEE, Cleveland, OH.

Katsabanis, P., G. Gregoras, C. Perley, and J. Skinkine. 2004. Small scale study of damage due to blasting and implications on crushing and grinding. International Society of Explosives Engineers (ISEE) Proceedings of the 29th Annual Conference on Explosives and Blasting Technique, February 2 – 5. Nashville, TN. ISEE, Cleveland, OH.

Katsabanis, P. D., S. G. Kim, A. Panthania, and J. Sage. 2008. Effect of powder factor and timing on the impact of blasting on crushing for surface blocks. International Society of Explosives Engineers (ISEE) Proceedings of the 34th Annual Conference on Explosives and Blasting Technique, January 27 – 30, New Orleans, LA. ISEE, Cleveland, OH.

Moser, P.A., Gamasche A., de Morira, J P. A Hammer, E. 2003. Breakage energy in rock blasting. 2nd World Conference on Explosives and Blasting, R Holmberg ed., pp. 323 – 327. A. A. Balkema, Rotterdam, Netherlands.

Nabiom. M and F. 1990. Comminution, blasting and grinding of applications of breakage models. Biech Symposium on Blasting (FRAGBLAST). Proceeding of the de the Societe Francaise PYROTECHNIQUE, 3 – 27, Montre, Quebec, Canada. A. A. Balkema, Rotterdam, Netherlands.

Nielson, Kim and F. Kristiensen. 1996. Blasting and grinding in cemented rock. 5th intl symposium on rock fragmentation blasting. In Mohanty (Ed), Montr, Canada, pp 23.

Paley, Neil and F. Kojovic. 2001. Adjusting blasting to increase SAG mill throughput at the Red Dog Mine. International Society of Explosives Engineers (ISEE) Proceedings of the 27th Annual Conference on Explosives and Blasting Technique, January 28 – 31, Orlando, FL. ISEE, Cleveland, OH.

Simmons, G. and R. Richter. 1976. Microcracks in Rock. The Physics and Chemistry of Minerals and Rocks, pp. 105 -137. R. G. J. Stran (editor). Wiley, New York, NY.

Singh, Patel, R. Robert C. Vojtman, and Robert F. Cruvogie. Richard R.(2006). The Six Sigma Way. McGraw-Hill, New York, NY.

Navartnam, V. I 1983, We really need a revolution in comminution. Proceedings of XVI International Materials Processing Congress, pp 9/3-14.

Singh and Yalcin. (2002) Effects of muck size distribution on scooping operations. International Society of Explosives Engineers (ISEE) Proceedings of the 28th Annual Conference on Explosives and Blasting Technique, February 10 – 13, Las Vegas, NV. ISEE, Cleveland, OH.

Workman, Lyall and Jack Eloranta, 2003. The role of blasting in crushing and grinding efficiency and energy consumption. International Society of Explosives Engineers (ISEE) Proceedings of the 29th Annual Conference on Explosives and Blasting Technique, February 2 – 5, Nashville, TN. ISEE, Cleveland, OH.

Workman, Lyall and Jack Eloranta. 2009, Consideration on the effect of blasting on downstream performance. International Society of Explosives Engineers (ISEE) Proceedings of the 35th Annual Conference on Explosives and Blasting Technique, February 8 – 11, Denver, CO. ISEE, Cleveland, OH.