Efficient Blasting Raises Yields And Reduces Costs

By

Dr. Calvin J. Konya

PRECISION BLASTING SERVICES

Background

Blasting efficiency does not just reduce drilling and blasting costs but also drastically impacts other cost for an operation. In some operations the breaking or secondary blasting of large size boulders can increase cost by $20,000 per month or more. Blasting the proper size distribution of rock can drastically increase yields and decrease costs.

The production of fines in blasting also can have a major impact on costs. Some operations have no market for fines. Lime producers and some aggregate producers get a yield of only 50 %. What this means that as a result of blast design and mechanical crushing that only 50 % of the product can be used and the other 50 % is waste. This report will discuss costs as they relate to productions of fines which are considered waste to the operations mentioned above.

SIZE FROM MECHANICAL CRUSHING

The mechanical crushing of rock material produces a size distribution. Different crushers produce size distributions which depend on crusher type, opening size of crusher ( CSS for jaw crushers) and feed size. A typical crusher curve is shown in Figure 1. This figure shows the fraction of material that passes the different size screens.

Figure 1. Example of Crusher Curves for Jaw Crusher

If for example one sets the CSS (Closed Side Setting) for this crusher at 3.0 inches the size distribution on the crushed materials less than 0.375 inches in size is 12% of the material crushed. All material does not have to be crushed. Let us assume in this analysis that only material larger than the CSS of the crusher will be crushed. The smaller material falls through the crusher.

The size fractions of the crushed material is shown in Figure 2. It can be seen that crushing can produce a relatively small fraction of fines.

Figure 2. Size fractions from crushing

SIZE FROM BLASTING

For the example discussed below we will compare the size distribution which result from different blasting patterns. For this example all material less than 0.375 inches will be considered fines that are not usable and are considered waste. Figure 3 shows the size fractions and the fines which result from a blasting pattern which is drilled on a 10 foot (burden) by 18 foot (spacing) pattern.

Figure 4. shows the size fractions and the fines which result from a blasting pattern which is drilled on a 18 foot by 10 foot drill pattern. The powder factor for each blasting pattern is the same.

The following example shows the cost impact of improper blasting which results in a high percentage of unusable fines.

When the size fractions of the two blasting patterns in figure 5. and Figure 6. are compared it is obvious that the 18 X 10 foot pattern produce both a higher percentage of fines as well as more large material in spite of the fact that the powder factors are the same. The difference in size distribution is directly related to pattern design.

Figure 5. Size Fractions for a 10 Foot by 18 Foot Blasting Pattern

CALCULATION OF WASTE

The Example 1. calculation illustrates the effects of drilling blasting patterns that are 10 by 18 feet (10 feet burden and 18 feet spacing), 18 by 10 feet and a pattern which is drilled with one half the pattern drilled on 10 by 18 feet and the other half drilled on a 18 by 10 feet.

The example shows the sizes of the fractions in inches, the percent of material passing different screen sizes and the size fractions.

The example also shows the size distribution from crushing the rock which will not pass through the opening of the crusher without crushing. The CSS value is shown in the data and is three inches.

Figure 6. Size Fractions for a 18 Foot by 10 Foot Blasting Pattern

The example also shows the total fines (waste) generated by blasting and by crushing as well as the tons of waste produced for each pattern per million tons of rock produced. The 10 by 18 foot drill pattern produced 83,100 tons of waste per million tons of rock mined and processed. The 18 by 10 foot drill pattern produced 276,800 tons of waste per million tons of rock mined and processed. The same powder factor and same total drill footage was used for both patterns. The blasting efficiency accounts for a difference of 193,700 tons less waste per million tons of rock mined and crushed.

EXAMPLE 1.

NOTE: FOR ALL EXAMPLES THE POWDER FACTOR IN LB/TON IS CONSTANT. THE SIZE FRACTIONS BELOW 0.375 INCHES IS CONSIDERED WASTE.

Table 1. Size Distribution from Blasting Patterns

Table 2. Size Fractions from Blasting Patterns

Table 3. Size Distribution from Crushing

CRUSHER OPENING (CSS) AT 3 IN.

CALCULATION OF WASTE

To calculate the amount of fines (waste) produced one must determine the amount of fines produced from crushing. The amount of material larger than the CSS of the crusher is determined by evaluating the size distribution from blasting and determining the amount of material which is larger than the CSS of the crusher. Only the material larger than the CSS of the crusher is assumed to be in need of crushing. The material crushed will produce some percentage of fines as determined from the crusher curves (Table 3.) and for this example it is 12% of the material crushed. The fines produced from crushing is added to the fines produced from blasting (table 2.) to determine the total percentage of fines. The Cost of the finished product of unusable stone is increased by the cost of producing unusable waste. That cost increase is also shown in Table 4.

Table 4. Calculation of Waste

 

PATTERN		DRILLING & BLASTING COST INCREASE	WASTE PER MM TONS (tons)
10 X 18	(100% - 68.59% = 31.44%) x .12 = 3.76% FINES FROM CRUSHING 3.76% + 4.55% = 8.31%	1.09	83100
18 X 10	(100% - 56.75% = 43.25%) x .12 = 5.19% FINES FROM CRUSHING 5.19% + 22.49% = 27.68%	1.38	276800
50/50
SUM	(100% - 62.67% = 37.33%) x .12 = 4.48% FINES FROM CRUSHING	1.22	180000
	4.48% + 13.52% = 18%

There are also additional costs of excessive waste which are not shown in the above analysis. The additional cost are those for breaking boulders, Loading trucks, Haulage costs, Waste removal, Stripping, and reclamation. Some of these cost will be addressed in Example 2.

COST COMPARISON

To further understand the effect of blasting efficiency on variable cost we can compare the costs of two different cases in Example 2.. In the first case the waste from fines is 50% of the material blasted (50 % yield). In the second case we will evaluate the costs if only 20 % of the material blasted is waste (80 % yield). The variable costs of drilling and blasting is assumed to be $0.19 per ton. The cost of digging the broken rock, truck haulage to the crusher and the crushing cost is $1.00 per ton. The cost of moving and disposing of the fines (waste) is $0.10 per ton. For each million tons of usable rock produced there is a large savings in variable costs.

EXAMPLE 2.

General Information

• It costs $ 0.41 per cubic yard to drill and blast the rock.
• At 2.2 tons per yard the drilling and blasting cost per ton is $0.19 per ton.
• The cost of loading rock, haulage, crushing is $1.00 per ton.
• The cost of moving waste is $0.10 per ton

Example with 50 % yield

Annual production of finished product is 1,000,000 tons with a yield of 50%.

Cost to drill, blast, load, haul, and crush is $1.19 per ton (variable cost).

With a yield of 50% there is a need to produce 2,000,000 ton to get 1,000,000 tons of product.

Cost to produce 2,000,000 is 2,000,000 x $1.19 $ 2,380,000

Cost to move waste is 1,000,000 x $0.10 100,000

Total cost of production is $2,480,000

 Cost per ton of finished product is $2.48 per ton

Example with 80 % yield

Annual production of finished product is 1,000,000 tons with a yield of 80%

Cost to drill, blast, load, haul, and crush is $1.19 per ton (variable cost).

With a yield of 80% there is a need to produce 1,250,000 ton to get 1,000,000 tons of product

Cost to produce 1,250,000 is 1,250,000 x $1.19 $ 1,487,500

Cost to move waste is 250,000 x $0.10 25,000

Total cost of production is $1,512,500

Cost per ton of finished product is $1.51 per ton

DIRECT SAVINGS

The example shows that the cost to produce 1000,000 tons of usable stone is $2,480,000 at a 50% yield and $1,512,500 at a 80% yield. The difference is a savings of $967,500. This is a savings of $0.97 per ton of useable rock produced.

The example shows a direct saving of $967,500 per year. Less stripping of overburden is needed. There is less waste and reclamation needed. The high quality stone reserves are not wasted adding additional years of life to the quarry.

CONCLUSIONS

The cost of processing waste is far greater per ton than the drilling and blasting cost. In this actual example the cost savings in processing was 4.8 times the total drilling and blasting cost.

The example shows that mechanical crushing produces less waste per ton than poor blasting practices.

Less stripping of overburden is needed if blasting is efficient. There are less reclamation costs as well as a saving of reserves of high quality stone.