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Bored raises for development: raises could be used as vertical or near-vertical development openings.

In modern hard rock mining, development, maintenance and ventilation of main levels and sublevels account for a substantial part of the mining cost. Reduction of these costs has been sought by increasing the vertical spacing between levels using new technologies. Presently, a drill or production level every 40 m should be feasible. Further reduction of the costs could be achieved by implementing new development strategies. A promising option is to develop the orebody in the vertical rather than the horizontal direction. Potential advantages are minimising ore handling, maximum use of gravity and faster development. Because such a strategy has a major impact on mining operations, a financial assessment is presented here.

Mechanical excavation

Raise boring is a widely applied form of mechanical excavation. Although a raiseboring machine (RBM) uses the same roller cutter technology as tunnel boring machines (TBMs), vertical holes rather than horizontal tunnels are created[1]. When the thickness of the orebody exceeds the diameter of the raise, the RBM leaves a circular excavation while no waste is produced. To enable vertical development, two main levels are interconnected with raises. The diameter of each raise should be large enough for drilling equipment mounted on a platform to pass through. The platform is suspended on a cable and is hoisted through the raise. This has the advantage over an Alimak type platform in that no construction time for the Alimak rail is required.

Because drilling takes place in a circular raise, it is unavoidable to drill in a ring-like pattern, as illustrated in Figure 1. With this type of development, an accurate drill pattern is necessary to obtain uniform muck size after blasting. An option is to use a normal hydraulic drill hammer with an automatic drill steel handling system. In the following, we assume that it is possible to drill blastholes with a diameter of 30 to 40 mm for a length of up to 20 m. After a ring pattern is completed, the platform is hoisted to a new position and the next ring of blastholes is drilled.

To charge the blastholes, we suggest mounting a charging device on a specially designed platform for loading of explosives. Automatic loading of the blastholes should allow optimal distribution of explosives over the blastholes while minimising risks to the miners. In order to break the ore, it is necessary to blast towards a (free) face. The ore between the explosives and the free face is fragmented, increasing the occupied volume by roughly 40%. In order to have sufficient free face area and leave room for expansion, the initial blast should be adjacent to the lower main level. After the first blast, a new horizontal free face is formed. By blasting a series of horizontal slices in an upward sequence, a virtual stope is created. When maintaining this sequence on a larger scale, the ore is always available on the lower main level. This has significant advantage for mucking and drawing of the ore. It should be noted that, if an open stoping method is used, an enormous opening is created. In order to prevent wall collapse, it is advisable to blast in a pyramid sequence, as illustrated in Figure 2. It may also be necessary to leave some broken ore in the stope prior to total drawdown.

Total drawdown of the ore commences when the entire block has been blasted. While drawing ore on the lower main level, waste or tailings can be backfilled through the raise from the upper main level. In order to prevent dilution of the ore, mass flow of the ore is required. Whether this occurs will be determined by the size distribution and the shape of the drawpoints. For tentative modelling, the stopes with the drawpoints could be represented as a bin/hopper system.

Cheap development

The proposed strategy using bored raises as development openings is highly automated. By automating drilling and blasthole loading, miners are not exposed to the confined space and possible rock-fall. In principle, development could be performed by two miners working together: a miner monitoring operations and a miner clearing the cuttings at the bottom of the raise. Experience shows that mucking on a single level improves the utilisation of mucking equipment. With the proposed strategy, the extensive use of the lower main level could justify installing electrical mucking equipment. This has the advantage of reducing ventilation requirements.

Development using bored raises could be relatively fast as a medium-length raise of about 200 m is completed in about one month. However, this advantage may be lost during the subsequent drilling stage. In order to establish the overall attractiveness of the strategy, the proposed Raise Mining Method (RMM) was compared with a conventional Blasthole Stoping Method (BSM). For this purpose, an orebody was defined which consisted of a regularly shaped, steeply dipping (80%) block with the following dimensions: 180 m on strike, 240 m high and 8 m thick. The costs of developing this orebody with either method will be estimated. The cost of drifting in C$ can be approximated by the following equation[1,2]:

Cost per metre = 271 [A.sup.0.6]

(where A is the cross-sectional area of the drift)

This equation indicates that the average cost of a standard drift (2.44 x 2.44 m) equals C$790/m. With the conventional BSM, a ramp is required to provide access to the haul and sublevels. Construction of a ramp would be more expensive, typically in the order of C$1,400/m. The cost of development items for BSM are listed in Table 1.
Table 1 development for a blasthole stoping method

Development item                                Cost [CS]

4 x 4.5m ramp [at] -15%, (1618 m)              2,265,200
2, 4 x 3.5m mainlevels, (2 x 180 m)               474,948
3, 4 x 3.5m haullevels, (3 x 180 m)               728,064
4, 3 x 3m sublevels, (4 x 180 m)                  712,422
6, 3.5 x 3m loading crosscuts, (6 x 10 m)          66,834
Orepass, (180 m)                                  142,200
Total development cost                          4,389,668
Cost/tonne                                           4.70
Table 2 development for a raise mining method

Development item                                Cost [C$]

2, 4 x 3.5m mainlevels, (2 x 180 m)               474,948
6,12.75m raise, (6 x 240 m)                     1,717,254
6, 3.5 x 3m loading crosscuts, (6 x 10 m)          66,834
Total development cost                          2,259,036
Cost/tonne                                           2.42
Table 3 Cost/tonne for various items necessary to mine an orebody

Item                                        C$/t RMM     C$/t BSM

I. Diamond drilling                             0.41         0.41
II. Development                                 2.42         4.70
III. Stoping labour                             1.22         1.63
IV. Drawpoint mucking                           1.26         1.57
V. Drilling                                     1.67         1.33
VI. Blasting                                    0.92         0.77
VII. Ground support included in II              0            0
VIII. Timber, Aux. Ventilation, etc.            0.05         0.14
IX. Equipment operating & maintenance           1.07         1.07
Subtotal                                        9.02        11.62
Misc. cost [at] 10%                             0.90         1.16
Total                                           9.92        12.78




Table 2 lists the development items for the RMM. Given that the ore block has a density of 2.7 t/[m.sup.3], the total mass of ore equals 933,120 t. Consequently, the development costs for RMM and BSM are approximately C$2.42/t and C$4.7/t respectively.

The overall cost per tonne has been estimated using the cost items shown in Table 3. The values for [BSM.sup.3], were corrected for the 1997 price level by adding 20%. For RMM, the cost of drilling will be an estimated 25% higher because more smaller-sized blastholes have to be drilled. Similarly, the cost of blasting will be higher for RMM than BSM because more blastholes must be loaded (estimated extra cost: 20%). On the other hand, the cost of mucking for RMM is assumed to be 20% lower because mucking is only performed on a single level. Also stoping labour is less for RMM on account of the automated drilling (estimated savings: 25%). Finally, timber and auxiliary costs are lower for RMM in view of the smaller crew.

Table 3 shows that the costs per tonne for the proposed raise mining method (RMM) is roughly 25% less than for a blasthole stoping method (BSM).

Advantages/disadvantages of raise mining

The advantages of raise mining include:

* A lower cost of development and a lower overall cost per tonne are feasible

* Development is fast. Only some raises are required and no levels, ramp, slot raises or under/over cuts are required

* All the development is in ore, waste is only produced during the excavation of the main levels

* Raise mining is highly automated, minimising exposure of personnel to hazardous situations or unhealthy environment

* The non-entry stopes and raises reduce ventilation requirements. The established raises connect the upper and lower levels, creating a closed ventilation loop

* The loading and hauling system is set up on the lower level for the whole ore block. This possibly justifies the use of continuous mucking machines and/or electrically powered equipment, which would reduce ventilation requirements even further

* No pillars are left behind. The total extraction percentage is high.

However, there are some disadvantages associated with the raise mining method:

* Comparative lack of selectivity

* Inflexible path of drawing and mucking

* During drawing of ore, hang-up and dilution may occur as experienced in Vertical Crater Retreat stoping or shrinkage stoping. Consequently the ore has to be non-oxidising and non-packing

* New drill and explosive loading platforms have to be designed, possibly creating start-up problems and increasing costs

* Waste lenses are blasted with the ore, possibly making grade control difficult to achieve

* Requirements with respect to the orebody: a high vertical extent and a regular shape and thickness.

Acknowledgement

The authors wish to acknowledge the stimulating discussions and advice of Prof. C.W. Pelly, Queen's University at Kingston, and Ir. J.J. de Ruiter, Delft University of Technology.

References

1. Hartman, H.L., SME, Mining engineering handbook, AIME, Littleton, Colorado, 1992, pp 411, 1622-1623.

2. Mining Sourcebook, Canadian Mining Journal's, 1997, pp 41-47.

3. J.S. Redpath Limited, Estimating preproduction and operating costs of small underground deposits, CANMET, Ottawa, 1986, pp 2-8.
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Author:Bokum, E.R. ten; Glass, H.J.
Publication:Mining Magazine
Date:Sep 1, 1997
Words:1700
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