DILUTION CONTROL IN SOUTHERN AFRICAN MINES

Dilution Control in Southern African Mines R J Butcher1 ABSTRACT The economic viability of many Southern African massive mining operations is threatened by excessive dilution. In many operations currently in progress, some stopes are experiencing as much as 115 per cent waste ingress reporting to drawpoints. This paper examines the magnitude, causes and types of dilution and presents a set of general principles for the control of dilution in massive stoping operations.

INTRODUCTION All mining operations experience dilution at some time or another and the elimination of all waste ingress in most cases is impossible. However, experience has shown that dilution can be controlled to acceptable levels by the implementation of correct mining engineering principles (Butcher, 1997). This paper examines the magnitudes, causes and classification of dilution and proposes the reduction of excessive waste ingress through the application of the ‘define, design, draw’ principle. Based on Canadian experience (Pakalnis et al, 1995), dilution greater than 20 per cent is defined as excessive dilution.

MAGNITUDES AND CAUSES OF DILUTION A survey of massive mining operations in Southern African (Butcher, 1999a) has provided the information on dilution magnitudes and trends given in Table 1 and Figure 1.

1.

Senior Mining Engineer, SRK Consulting, 265 Oxford Road, Illovo, Johannesburg, South Africa.

TABLE 1 Dilution magnitudes associated with different mining methods. Mining method

Dilution %

Remarks

18 - 115

Unsupported

Sill and bench

5 - 48

Unsupported

Continuous undip retreat stoping

27 - 48

Unsupported

Sub level open stoping (SLOS)

Open benching

< 20

Unsupported

Fissure/vein mining

±40

Unsupported

Creeping cone

< 10

Unsupported/ Artificially supported

Cut and fill (CAF)

5 - 15

Artificially supported

Vertical Crater retreat (VCR)

10 - 38

Artificially supported

From the data in the table and figure the following can be concluded:

• From Figure 1 the average dilution is in the region of 40 per cent. If it is assumed that the average massive mine produces 500 000 tons per annum at a cost of $30/ton then the impact of this 40 per cent dilution rate would be about $6 million per annum. An approximate estimate of dilution costs per annum for three commodities is given in Figure 2. The cost per ton

FIG 1 - Dilution variation with time.

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information used in these estimates is based on real data obtained from international mining operations. This simple calculation illustrates the potential cost savings that can be made through the implementation of a dilution control strategy.

• Table 1 indicates that mining methods with unsupported stopes have higher dilution rates than artificially supported methods. This could be due to the lack of support afforded to incompetent stope rockwalls. It could be concluded that more orebodies should be extracted using mining methods involving artificially supported stopes (assuming that dilution control is the main design rationale).

• The data from Figure 1 (Butcher, 1999a) indicate that dilution levels vary with time. In this respect there could be two possible explanations: • variable stope rock wall competencies resulting in variable dilution levels; • poor mining practices due to: - poor blasting, resulting in the overbreak of the stope boundaries; - dilution due to a regular stope boundary profile being maintained when the orebody width fluctuates; and - poor mining discipline associated with a lack of draw control procedures. In summary, the main causes of excessive dilution are:

• incompetent ground conditions; • inappropriate mining methods; • poor mining practices.

THE ORIGIN AND CLASSIFICATION OF DILUTION In addition to the planned and unplanned types of dilution described by Dominy et al (1998), dilution can be classified according to its origin (Butcher, 1999a). Three types of dilution tend to affect massive stoping operations (see Figure 3).

Top dilution This can be defined as waste rock or ore which is of uneconomical value. This type of dilution normally occurs when crown pillars are wrecked or if sloughing of the back occurs during stoping.

Internal dilution This is the waste rock or low-grade ore that occurs within defined economic orebodies at the stope boundary (for example, shale floaters in a kimberlite orebody). This type of dilution can be thought of in a similar manner to the internal waste between reef bands. Internal dilution is the most difficult type of waste to control due to its close proximity to the ore. In certain cases, internal dilution can be as high as 40 per cent. This type of dilution is sometimes referred to as planned dilution (Scoble and Moss, 1994).

Side dilution

The first two points are important in understanding the origin of dilution for the purpose of formulating a prevention strategy. The last point focuses on the control and reduction of dilution.

This is the dilution that occurs due to the sloughing of the stope hangingwall and/or footwall in a steeply inclined orebody (or from the sidewalls in a massive deposit).

FIG 2 - Cost of dilution for three commodities based on an average mine production capacity of 500 000 tons per annum.

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FIG 3 - Classification of dilution.

DILUTION REDUCTION AND CONTROL STRATEGIES The magnitudes, causes and classes of dilution which affect Southern African massive stopes have been discussed above. The focus now changes to the prevention or control of the dilution using the define, design, draw principle. This principle enhances

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existing knowledge of dilution control for Southern African conditions. The method is currently being implemented at Rosh Pinah Mine, Namibia as part of the mine re-engineering process (Butcher, 1999b). There are two cornerstones to this principle:

• prevent dilution rather than control it, and • if dilution cannot be prevented, then control it.

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In essence. the define and design aspects act as the prevention component, and the draw aspect is the control portion of the principle. It may be considered that the control aspect is irrelevant since all mining methods should be designed to eliminate dilution. However, in many cases, the most dilution friendly mining methods do not fulfil the necessary tonnage requirements for operational viability. In these cases experience has shown that dilution can be reduced to tolerable levels by implementing a draw control strategy.

Dilution reduction using the define principle A major component in the prevention of stope dilution is good geological and geotechnical definition of the orebody and the surrounding country rock. The geological definition is important so that the amount of internal dilution can be determined and that the boundaries of stopes can correspond with the limits of the orebody. A common pitfall is to reduce or underestimate the amount of diamond drilling required for orebody definition. In the case of an irregular orebody, it may be necessary that drilling is conducted at intervals of less than 10 m. A study conducted in Canada (Puhakka, 1990) has shown that planned dilution decreased by ten per cent when the drilling definition interval was reduced from 25 m to 7.5 m. The geotechnical definition of the orebody and the country rock is important to:

• define weak country rock zones which could lead to dilution influx;

• define stable stope dimensions to prevent the failure of the rockmass surrounding the orebody; and

• determine the in situ rock mass strength, so that rib pillars can be correctly designed and stope spans limited, thus preventing dilution. Geotechnical definition can be accomplished with the use of a rock mass classification system incorporating the effects of blasting (Laubscher, 1990). It is important that geotechnical classification is not only carried out in the project planning stages, but also on an on-going basis during the mining stages, with the compilation of geotechnical plans being an essential part of the program. A useful exercise is the back analysis of dilution levels from old stopes and correlation with the classification values (Butcher, 1997). The purpose of this exercise is to identify high dilution geotechnical areas. Cavity monitoring systems can also aid this purpose (Gilbertson, 1995).

(1994) deals with the dilution associated with different types of mining methods. Taking cognizance of Elbrond’s observations and experience in Southern Africa (Butcher, 1997 and 1999a), the most suitable mining methods for dilution reduction are summarised in Table 2. The table indicates that cut and fill mining is the most dilution friendly mining method. However, this method tends to have the highest dollars per ton mining cost and the lowest production capacities. Figure 4 shows the potential cost-savings associated with changing the mining method from unsupported to artificially supported methods. With regard to the choice of mining method there is a dilemma in that a high tonnage/low mining cost method may be required due to the grade of the orebody, but in order to control dilution the preferred method may have the opposite characteristics. A compromise can be attained by using smaller open stope dimensions or a shrinkage method such as a creeping cone (Aplin, 1997). In these cases dilution can be reduced further by accurately setting out the stope boundary to the orebody contact. The non mining of the ore blocks with extremely complex and weak geologies can also assist. These measures will reduce the quantities of side and internal dilution (Butcher, 1997). The levels of side dilution can further be reduced by the implementation of good drilling and blasting practices (Dominy et al, 1998). One of the most common causes of dilution is poor drilling and blasting. The problem is essentially a design issue, although there are also control issues associated with it. Correction of the problem is difficult and time consuming. From experience with blasting projects on Zimbabwean mines in the late-1990s, a drilling and blasting improvement programme normally takes at least 24 months to show substantial results (Butcher, 1997). Although drilling and blasting aspects are beyond the scope of this paper, the following factors should be considered during the project design stage:

• the design of blasting rings/fans with blast hole lengths not exceeding 15 m, thus reducing potential blast hole deflection and stope rockwall damage;

• the design of stope fans/rings with small blast hole diameters rather than large diameters, hence reducing the charge per delay and the blasting damage;

• the determination of the correct powder factor; • the initiation of fans/rings using Nonel or Electrodet systems instead of detonating cord;

• the design of stope fans/rings using computer models to determine the effects of different charge lengths and timing systems;

Dilution reduction using the design principle The next stage in the dilution prevention strategy is the selection of the most dilution friendly mining method (taking cognizance of stable stope spans and pillar rockmass competencies). Elbrond

• the setting of realistic drilling and blasting targets, thus ensuring quality blasting;

TABLE 2 Dilution associated with different mining methods (modified after Brady and Brown, 1993). Orebody geometry

Rockmass competency

Dilution hazard

Massive/regular

Good

Little to none

All methods

Massive/regular

Poor/medium

Considerable

CAF/creeping cone, SLOS (with small stopes and post fill), VCR

Irregular changes from massive to narrow (eg vein) for example, over small strike distances

Good/medium

Internal and side dilution (due to stope boundary)

CAF, creeping cone, VCR

Irregular changes from massive to narrow, for example over short strike distances

Poor

Considerable side, internal and top dilution

CAF

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Mining method

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FIG 4 - Cost of dilution attributed to mining method.

• the provision for blast hole redrilling in the production planning stages, to avoid the charging of rings/fans with closed holes. This will eliminate overburdening and reduce rockwall damage.

Dilution control using the draw principle (draw control) The use of a draw control system in a stoping scenario differs from that which is used in block caving, the main focus being on grade control through the reduction of waste mining. The essential part of the draw management program is the establishment of drawpoint tonnage calls and acceptable dilution levels. In many mines these have not been determined and as a result it is difficult to ascertain the dilution level at which a drawpoint should be closed. The determination of drawpoint tonnage calls can be achieved with geological orebody modelling packages and production benchmarking exercises. The implementation of a dilution control program focuses on the prevention of excessive waste draw. In this, it is essential to have a draw control officer who regularly inspects the drawpoints, passes and tips for waste. The selection of the draw control officer is of the utmost importance and experience has shown that a very competent shiftboss is usually the most suitable person for this position. Such a person has sound knowledge of the production process. In addition, a draw control clerk is required to assist the draw control officer in compiling the relevant draw control statistics and preparing the monthly drawpoint calls. The setting of realistic monthly production tonnage calls is vital to prevent waste drawing. Experience has shown that if calls are set too high, underground production crews will draw waste to attain call. Unrealistic calls normally occur when over-optimistic forecasts of the production capability of particular mining methods are made, or when the mineral prices fall to such an extent that excessive production is required for mine viability.

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One of the main causes of excessive dilution is ignorance, and it is surprising that very few mines have dilution awareness campaigns which highlight the dangers of dilution on mine viability. An awareness program could be implemented at little cost and would involve posters at waiting places, lectures and regular reminders. A need which is sometimes overlooked is the requirement for dilution monitoring in mines which do not have excessive dilution. The main reasons for this are:

• to determine the correct level of dilution; • to ascertain whether dilution levels increase with the mining of different geotechnical areas. Even in mines which only suffer from a minor dilution problem, some form of draw monitoring is normally required to validate grades (Butcher, 1999a). Mines in this category normally overestimate the dilution level and lower stope grades accordingly. The correct mine dilution level can be controlled by a mine geologist conducting a dilution drawpoint audit on a quarterly basis. Figure 5 shows the potential cost-savings associated with dilution control.

CONCLUSIONS Excessive dilution can threaten the viability of most mining operations. In this respect it has been estimated that dilution could be costing some Southern African massive mining operations in the region of $6 million per year. However, with the correct definition of the orebody and the geotechnical environment, the most dilution friendly mining method can be selected. The implementation of a draw control system is fundamental to exercising effective control over the drawing of waste from stopes. These aspects can be summarised as the define, design, draw principle of dilution control. The application of these principles can result in major cost-savings at little cost to mining operations.

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FIG 5 - Potential cost saving associated with dilution control for mines with different capacity.

REFERENCES

BIBLIOGRAPHY

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