DILUTION CONTROL IN SOUTHERN AFRICAN MINES

Dilution Control in Southern African MinesR J Butcher1

ABSTRACTThe economic viability of many Southern African massive miningoperations is threatened by excessive dilution. In many operationscurrently in progress, some stopes are experiencing as much as 115 percent waste ingress reporting to drawpoints. This paper examines themagnitude, causes and types of dilution and presents a set of generalprinciples for the control of dilution in massive stoping operations.

INTRODUCTIONAll mining operations experience dilution at some time oranother and the elimination of all waste ingress in most cases isimpossible. However, experience has shown that dilution can becontrolled to acceptable levels by the implementation of correctmining engineering principles (Butcher, 1997). This paperexamines the magnitudes, causes and classification of dilutionand proposes the reduction of excessive waste ingress throughthe application of the define, design, draw principle. Based onCanadian experience (Pakalnis et al, 1995), dilution greater than20 per cent is defined as excessive dilution.

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

From the data in the table and figure the following can beconcluded: From Figure 1 the average dilution is in the region of 40 per

cent. If it is assumed that the average massive mine produces500 000 tons per annum at a cost of $30/ton then the impactof this 40 per cent dilution rate would be about $6 million perannum. An approximate estimate of dilution costs per annumfor three commodities is given in Figure 2. The cost per ton

MassMin 2000 Brisbane, Qld, 29 October - 2 November 2000 113

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

Mining method Dilution % RemarksSub level open stoping(SLOS)

18 - 115 Unsupported

Sill and bench 5 - 48 UnsupportedContinuous undip retreatstoping

27 - 48 Unsupported

Open benching < 20 UnsupportedFissure/vein mining 40 UnsupportedCreeping cone < 10 Unsupported/

Artificially supportedCut and fill (CAF) 5 - 15 Artificially supportedVertical Crater retreat (VCR) 10 - 38 Artificially supported

TABLE 1Dilution magnitudes associated with different mining methods.

FIG 1 - Dilution variation with time.

information used in these estimates is based on real dataobtained from international mining operations. This simplecalculation illustrates the potential cost savings that can bemade through the implementation of a dilution controlstrategy.

Table 1 indicates that mining methods with unsupportedstopes have higher dilution rates than artificially supportedmethods. This could be due to the lack of support afforded toincompetent stope rockwalls. It could be concluded that moreorebodies should be extracted using mining methodsinvolving artificially supported stopes (assuming that dilutioncontrol is the main design rationale).

The data from Figure 1 (Butcher, 1999a) indicate thatdilution levels vary with time. In this respect there could betwo 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 stopeboundaries;

- dilution due to a regular stope boundary profile beingmaintained when the orebody width fluctuates; and

- poor mining discipline associated with a lack of drawcontrol procedures.

In summary, the main causes of excessive dilution are: incompetent ground conditions; inappropriate mining methods; poor mining practices.

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

THE ORIGIN AND CLASSIFICATION OFDILUTION

In addition to the planned and unplanned types of dilutiondescribed by Dominy et al (1998), dilution can be classifiedaccording to its origin (Butcher, 1999a). Three types of dilutiontend to affect massive stoping operations (see Figure 3).

Top dilutionThis can be defined as waste rock or ore which is ofuneconomical value. This type of dilution normally occurs whencrown pillars are wrecked or if sloughing of the back occursduring stoping.

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

Side dilutionThis is the dilution that occurs due to the sloughing of the stopehangingwall and/or footwall in a steeply inclined orebody (orfrom the sidewalls in a massive deposit).

114 Brisbane, Qld, 29 October - 2 November 2000 MassMin 2000

R J BUTCHER

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

DILUTION REDUCTION ANDCONTROL STRATEGIES

The magnitudes, causes and classes of dilution which affectSouthern African massive stopes have been discussed above. Thefocus now changes to the prevention or control of the dilutionusing the define, design, draw principle. This principle enhances

existing knowledge of dilution control for Southern Africanconditions. The method is currently being implemented at RoshPinah 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.


DILUTION CONTROL IN SOUTHERN AFRICAN MINES

FIG 3 - Classification of dilution.

In essence. the define and design aspects act as the preventioncomponent, and the draw aspect is the control portion of theprinciple. It may be considered that the control aspect isirrelevant since all mining methods should be designed toeliminate dilution. However, in many cases, the most dilutionfriendly mining methods do not fulfil the necessary tonnagerequirements for operational viability. In these cases experiencehas shown that dilution can be reduced to tolerable levels byimplementing a draw control strategy.

Dilution reduction using the define principleA major component in the prevention of stope dilution is goodgeological and geotechnical definition of the orebody and thesurrounding country rock. The geological definition is importantso that the amount of internal dilution can be determined and thatthe boundaries of stopes can correspond with the limits of theorebody. A common pitfall is to reduce or underestimate theamount of diamond drilling required for orebody definition. Inthe case of an irregular orebody, it may be necessary that drillingis conducted at intervals of less than 10 m. A study conducted inCanada (Puhakka, 1990) has shown that planned dilutiondecreased by ten per cent when the drilling definition intervalwas reduced from 25 m to 7.5 m.

The geotechnical definition of the orebody and the countryrock 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, thuspreventing dilution.

Geotechnical definition can be accomplished with the use of arock mass classification system incorporating the effects ofblasting (Laubscher, 1990). It is important that geotechnicalclassification is not only carried out in the project planningstages, but also on an on-going basis during the mining stages,with the compilation of geotechnical plans being an essential partof the program.

A useful exercise is the back analysis of dilution levels fromold stopes and correlation with the classification values (Butcher,1997). The purpose of this exercise is to identify high dilutiongeotechnical areas. Cavity monitoring systems can also aid thispurpose (Gilbertson, 1995).

Dilution reduction using the design principleThe next stage in the dilution prevention strategy is the selectionof the most dilution friendly mining method (taking cognizanceof stable stope spans and pillar rockmass competencies). Elbrond

(1994) deals with the dilution associated with different types ofmining methods. Taking cognizance of Elbronds observationsand experience in Southern Africa (Butcher, 1997 and 1999a),the most suitable mining methods for dilution reduction aresummarised in Table 2.

The table indicates that cut and fill mining is the most dilutionfriendly mining method. However, this method tends to have thehighest dollars per ton mining cost and the lowest productioncapacities. Figure 4 shows the potential cost-savings associatedwith changing the mining method from unsupported toartificially supported methods.

With regard to the choice of mining method there is a dilemmain that a high tonnage/low mining cost method may be requireddue to the grade of the orebody, but in order to control dilutionthe preferred method may have the opposite characteristics. Acompromise can be attained by using smaller open stopedimensions or a shrinkage method such as a creeping cone(Aplin, 1997). In these cases dilution can be reduced further byaccurately setting out the stope boundary to the orebody contact.The non mining of the ore blocks with extremely complex andweak geologies can also assist. These measures will reduce thequantities of side and internal dilution (Butcher, 1997). Thelevels of side dilution can further be reduced by theimplementation of good drilling and blasting practices (Dominyet al, 1998).

One of the most common causes of dilution is poor drillingand blasting. The problem is essentially a design issue, althoughthere are also control issues associated with it. Correction of theproblem is difficult and time consuming. From experience withblasting projects on Zimbabwean mines in the late-1990s, adrilling and blasting improvement programme normally takes atleast 24 months to show substantial results (Butcher, 1997).

Although drilling and blasting aspects are beyond the scope ofthis paper, the following factors should be considered during theproject design stage: the design of blasting rings/fans with blast hole lengths not

exceeding 15 m, thus reducing potential blast hole deflectionand stope rockwall damage;

the design of stope fans/rings with small blast hole diametersrather than large diameters, hence reducing the charge perdelay 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 timingsystems;

the setting of realistic drilling and blasting targets, thusensuring quality blasting;


R J BUTCHER

Orebody geometry Rockmass competency Dilution hazard Mining methodMassive/regular Good Little to none All methodsMassive/regular Poor/medium Considerable CAF/creeping cone, SLOS

(with small stopes and post fill), VCRIrregular changes from massive to narrow (egvein) 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, forexample over short strike distances

Poor Considerable side, internal andtop dilution

CAF

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

the provision for blast hole redrilling in the productionplanning stages, to avoid the charging of rings/fans withclosed holes. This will eliminate overburdening and reducerockwall damage.

Dilution control using the draw principle (drawcontrol)The use of a draw control system in a stoping scenario differsfrom that which is used in block caving, the main focus being ongrade control through the reduction of waste mining. Theessential part of the draw management program is theestablishment of drawpoint tonnage calls and acceptable dilutionlevels. In many mines these have not been determined and as aresult it is difficult to ascertain the dilution level at which adrawpoint should be closed. The determination of drawpointtonnage calls can be achieved with geological orebody modellingpackages and production benchmarking exercises.

The implementation of a dilution control program focuses onthe prevention of excessive waste draw. In this, it is essential tohave a draw control officer who regularly inspects thedrawpoints, passes and tips for waste. The selection of the drawcontrol officer is of the utmost importance and experience hasshown that a very competent shiftboss is usually the mostsuitable person for this position. Such a person has soundknowledge of the production process. In addition, a draw controlclerk is required to assist the draw control officer in compilingthe relevant draw control statistics and preparing the monthlydrawpoint calls.

The setting of realistic monthly production tonnage calls isvital to prevent waste drawing. Experience has shown that if callsare set too high, underground production crews will draw wasteto attain call. Unrealistic calls normally occur whenover-optimistic forecasts of the production capability ofparticular mining methods are made, or when the mineral pricesfall to such an extent that excessive production is required formine viability.

One of the main causes of excessive dilution is ignorance, andit is surprising that very few mines have dilution awarenesscampaigns which highlight the dangers of dilution on mineviability. An awareness program could be implemented at littlecost and would involve posters at waiting places, lectures andregular reminders.

A need which is sometimes overlooked is the requirement fordilution monitoring in mines which do not have excessivedilution. 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 tovalidate grades (Butcher, 1999a). Mines in this categorynormally overestimate the dilution level and lower stope gradesaccordingly. The correct mine dilution level can be controlled bya mine geologist conducting a dilution drawpoint audit on aquarterly basis.

Figure 5 shows the potential cost-savings associated withdilution control.

CONCLUSIONSExcessive dilution can threaten the viability of most miningoperations. In this respect it has been estimated that dilutioncould be costing some Southern African massive miningoperations in the region of $6 million per year. However, with thecorrect definition of the orebody and the geotechnicalenvironment, the most dilution friendly mining method can beselected. The implementation of a draw control system isfundamental to exercising effective control over the drawing ofwaste from stopes. These aspects can be summarised as thedefine, design, draw principle of dilution control. The applicationof these principles can result in major cost-savings at little cost tomining operations.


DILUTION CONTROL IN SOUTHERN AFRICAN MINES

FIG 4 - Cost of dilution attributed to mining method.

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R J BUTCHER

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

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DILUTION CONTROL IN SOUTHERN AFRICAN MINES