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Drilling blasting forum.

In open pit mines one of the most important factors that influences the productivity of the total operation is the performance of the rotary blasthole drilling. The efficiency of blasthole drilling is based on how fast the hole can be produced while the machine is actually operating. The speed of the drill is significantly influenced by the rate of penetration of the rock bits selected. The rate of penetration in making blastholes can influence productivity as well as reduce the overall cost of drilling by reducing the total drill operating time.

In order to illustrate the role of increased penetration rate on the productivity and drill operating costs, we will take the example of an open-pit copper mine producing at an average rate of 100K mt/d and operating 350 d/yr. A heavy-duty drill rig with a 12.25 in. rock bit is used to drill the holes. The bit life averages from 8,000 to 10K ft as the penetration rate increases from 75 to 100 ft/hr.

In order to evaluate the overall drilling efficiency and its variation caused by changes in penetration rate, the total feet drilled per year must be calculated. One method of determining drill footage required is by using the total annual amount of explosives consumed in blasting. This approach is considered an adequate generic to derive the total footage required, allowing for the fact that mines have different bench heights, depth, spacing, and pattern of holes, and other geologically influenced variations in their blasting practice.

Assuming a powder factor of 0.4 lb/mt, the annual explosive consumption is equal to the total material to be blasted multiplied by the powder factor, i.e. 14M lb.

The total annual footage of usable hole required is equal to the explosive usage divided by the loading density divided by the loading factor. Assuming the latter at typical values of 46 lb/ft and 0.7, the drill footage requirement works out at 435K ft/yr.

The total blasthole drilling time (hr/yr) and the total drilling operating costs can be calculated for different penetration rates.

Table 1 shows the influence of the penetration rate on the total drilling time.

It is assumed that the ownership and operating costs of the rotary drill rig are $425/hr (including purchase price, depreciation, overhead, operating costs, labor, and consumables).

The total operating cost in $/yr is equal to the total drilling hours multiplied by the operating cost, plus the total feet drilled per year divided by the bit life multiplied by the bit cost.

The bit cost for lower penetration rate bits is assumed to be $5,800/bit and for higher penetration rate bits $6,600/bit.

Applying these figures to the above total operating cost formula it can be demonstrated that the total costs decrease from $2.7M/yr to $2.1M/yr as the penetration rate increased from 75 to 100 ft/hr.

Figure 1 shows the total savings in drilling costs resulting from the reduction in total drilling time. It is clear that the higher savings noted at 100 ft/hr results from the shorter drilling time. Since bit costs are very low compared to the cost of operating the machine, using bits that increase the penetration rate can reduce the total operating cost significantly.
Table 1

Penetration    Drilling Time      Time Saved      Bits      Bit Cost
(ft/hr)               (hr)           (hr)        Needed        ($)

75                  5,800              -          41         5,800
80                  5,440            360          44         5,800
85                  5,120            680          47         5,800
90                  4,830            970          49         6,600
95                  4,580          1,220          52         6,600
100                 4,350          1,450          55         6,600
Table 2

Penetration      Time Saved    Additional Ore     Additional Revenue
(ft/hr)             (hr)            (mt)                 ($)

75                     -                 _                    -
80                   360           463,700            3,709,400
85                   680           930,600            7,444,600
90                   970         1,405,500           11,244,200
95                 1,220         1,866,000           14,927,000
100                1,450         2,334,500            8,676,000

In addition to the significant cost savings by using high-performance bits, increased productivity is another benefit.

The reduction in drilling time from 5,800 hr to 4,350 hr means that 1,450 drill hours are available to drill an additional 145K ft/yr with the same drill. By factoring in the figures for powder factor, blasting efficiency, etc. it can be shown that a potential increase in production from 0.5M to 2.5M mt/yr can be achieved by increasing the penetration rate from 75 to 100 ft/hr [ILLUSTRATION FOR FIGURE 2 OMITTED].

In the case of our hypothetical copper mine with a grade of 0.5% Cu, 80% recover, and a copper price of $1.00/lb, the influence of the increased penetration rate on potential additional revenues is shown in Table 2.


This article is based on a paper "Financial Impact of the Cost and Productivity Improvement of an Efficient Rotary Drilling Operation" by John W. Wilson and Y. Ding of the Dept. of Mining Engineering, University of Missouri-Rolla, presented at the seminar Rotary Blasthole Drilling in the '90s, South African Institute of Mining and Metallurgy, Johannesburg, 1994.
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Title Annotation:126th Annual Survey & Outlook Issue; rotary blasthole drilling in open-pit mines
Publication:E&MJ - Engineering & Mining Journal
Date:Mar 1, 1995
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