dilution and losses

8/15/2019 Dilution and Losses

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McCarthy P L 2001. Mining Dilution and Losses in Proceedings Underground

Mining, in Mineral Resource and Ore Reserve Estimation – The AusIMM Guide to

Good Practice (Ed: A C Edwards), pp333-336 (The Australasian Institute of

Mining and Metallurgy: Melbourne). Reprinted with permission of The

Australasian Institute of Mining and Metallurgy.

Mining Dilution and Losses inUnderground Mining

By P L McCarthy 1

Abstract

Resource modelling and Ore Reserve estimation procedures are different for open pit and

underground mines. A statistical approach to the spatial location of orebody limits is unhelpful inunderground Ore Reserve estimation, while experience with practical mining outcomes and

economics is fundamental.

Resource modelling for underground ruining relies heavily on geological interpretation and

experience. When converting Mineral Resources to Ore Reserves, the chosen grade interpolation

technique, while important, has less significance than the raining, geotechnical and economic

considerations which determine mining dilution and recovery.

Mining dilution and recovery are difficult to measure and more difficult to predict. There is no

alternative to careful measurement coupled with experience-based adjustment. It is possible that a

`Competent Person' for the purpose of preparing a Mineral Resource estimate may not be `competent'

to estimate Ore Reserves for the same deposit.

Introduction

Conversion of a Mineral Resource estimate to an Ore Reserveestimate is a team effort involving, at minimum, a geologist,metallurgist and mining engineer. The factors to be consideredrelate to practical mining outcomes and economics, so theCompetent Person preparing the Ore Reserve estimate must bevery familiar with the proposed mining methods. The mainconsiderations are the amount of lower-grade or waste materialthat will become mixed with the ore (mining dilution), and the proportion of the resource that can be economically recovered(mining recovery).

Face-to-face involvement of the geologist who prepared the

resource estimate is essential. The assumptions and limitationsinherent in the resource model must be drawn out. The background of the resource geologist and his or her experiencewith underground mining estimates (as distinct from open-pit)should be understood.

For reasons set out in this paper, conversion of a MineralResource estimate to an Ore Reserve estimate is a challengingtask. The use of `text book' factors for dilution and recovery islikely to lead to errors.

1. MAusIMM, CPMin, Managing Director, Australian Mining Consultants

Pty Ltd, 19/114 William Street, Melbourne Vic 3000.

E-mail: [email protected]

Definitions

Dilution may be defined in several ways. To the metallurgistreceiving the ore for treatment, it is the percentage of thedelivered material which is waste.

Thus;

Dilution (%) = (mass of waste) x 100 / (mass of ore + mass of waste) (1)

The mining engineer often expresses dilution as a tonnageincrease.

Thus;

Dilution (%) = (mass of waste) x 100 / (mass of ore) (2)

Formulae (1) and (2) ignore the fact that `waste' may contain payable values, so that the economic impact of dilution is lesssevere. Dilution may also be expressed as a grade reduction.

Thus;

Dilution (%) = (resource grade - diluted grade) x 100 / (resource grade) (3)

All of the above measures of dilution are acceptable so long asthey are defined before use. An example of themisunderstandings that may otherwise arise is given by the

following example. Consider 100 t of ore of ten per cent gradediluted with 10 t of material of four per cent grade, to give 110 tat 9.127 per cent grade.

Equation (1) gives: 10 x 100 / 110 = 9.1% dilution.

Equation (2) gives: 10 x 100 / 100 = 10% dilution.

Equation (3) gives: (10 - 9.127) x 100 / 10 = 8.7% dilution.


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Mining Dilution and Losses in Underground Mining

AMC Reference Library – www.ausmin.com.au 2

Mining recovery may also be expressed in a variety of ways asfollows:

• What percentage of the total resource tonnage willultimately be mined'?

• How does the diluted tonnage delivered to the millcompare with the estimated resource tonnage?

•

What percentage of the total metal contained in theresource will be delivered to the mill?

• What percentage of the resource (tonnage orcontained metal) calculated at the resource cut-offgrade will be mined (or delivered for treatment) at thechosen mining cut-off grade?

• After elimination of those parts of the resourcedeemed inaccessible or otherwise uneconomic (forreasons of width, dip, deleterious elements, rockconditions, etc), what proportion of the remainderwill be recovered after leaving supporting pillars.

There may be other ways of defining mining recovery; it is

sufficient to state accurately what is meant by the term.

Suitability of the Resource Model

Resource models and the Ore Reserve estimation procedure aredifferent for open-pit and underground mints. To a scale of tensof metres, the location and spatial distribution of values may beunimportant in an open pit resource model. Provided the pinslocated to access the mineralised zone, any valuable materialcan be identified by grade control sampling and then markedout for mining. Thus, the emphasis in open-pit resourcemodelling is on the global accuracy of estimates of tonnes andgrade and internal variability at a scale that might affect pitoptimisation, so a statistical approach is often appropriate.

For underground mining the thickness, dip, continuity andspatial relationship of ore zones, the regularity of wall contacts,strength of ore and wall rocks are all critical inputs to the OreReserve estimate. These are drawn from the resource model, orfrom the geologist's knowledge of the deposit gained during thedata-gathering and modelling phase. Some parts of the resourcemay be impossible to mine; others may be located too far fromdevelopment to be economic; others may suffer severe dilution.

A sectional interpretation by an experienced geologist at one ormore possible cut-off grades is usually the first step in preparing the resource model. This sectional interpretation willinclude features inferred from the drill logs that would not begenerated by any grade interpolation software. The geologist's

experience tells him or her how variable the ore boundaries arein this type of deposit and what shapes the variations mighttake. When the sections are linked and wireframed, thenchecked and corrected in plan and back to section, the resultingthree-dimensional outlines can be used to validate the grade-interpolation process.

The above procedure is usually necessary regardless of whetheror not the limits of the mineralisation envelope have beeninterpreted (at a subeconomic cut-off) and wireframed as limitsto the grade block model. This is because the shape of themineralisation envelope may be quite different from the shapeof the economic material.

It may be acceptable to let the grade interpolation processdetermine limits of economic mineralisation in large deposits to

be mined by caving methods, where the edge inaccuracies of

the model become insignificant. For other cases, the geologistmust form a view about the spatial limits of ore at the chosencut-off grade, and must be prepared to model the ore boundaries realistically. To do this, the geologist needs tounderstand the style of mineralisation and to be able to inferirregularities, including structural dislocations such as faults, ata scale smaller than the drill spacing.

The mining engineer will design slopes which have geometriclimitations dictated by geotechnical factors, the economic

spacing and length of production blastholes, or the need tocombine blocks of `ore' and `waste' into mineable units. Whenthese shapes are overlaid on the resource model, the resourcegrade is diluted and some of the resource is lost. The resourcemodel will contain internal dilution according to the model block size (based on the assumed Selective Mining Unit) whichmay or may not accord with the engineer's proposed method.

In an underground mine, levels are planned on the basis ofrelatively coarse-spaced drilling. Stopes are designed and thenmined with limited flexibility for change. Pods of ore notidentified by drilling will be lost, even if they are expected

statistically to be present.

Proponents of geostatistics sometimes claim that a resourcemodel inherently contains an appropriate allowance for internaland edge dilution. This is an obvious fallacy; the dilutionestimate must derive from mining and geotechnicalconsiderations. A resource model which purports to includedilution provides an undefined starting point for the OreReserve estimator, who must somehow `remove' the diluentmaterial from the model before adding back mining dilution.This is an impractical task, so the only satisfactory approach isto refuse to accept such a model as a basis for an undergroundmine ore reserve.

The resource estimate for underground mining must include astatement of:

•

cut-off grade,

• minimum mining width,

• vertical limits (top and bottom RL), and

• lateral extent (plan limits).

The estimate should include a grade-tonnage curve. Thisenables the mining engineer to consider strategic slopingoptions (high tonnage bulk mining vs low tonnage selectivemining). The estimate should also quote resources at varyingcut-off grades and minimum widths to enable economic

optimisation.

Measurement of Dilu tion and Recovery

In many mines, ore from a number of sources is stockpiled and blended before treatment, making reconciliations difficult orimpossible. Assuming reconciliation is possible, the resultingcalculations of dilution and recovery may reflect on theaccuracy of the Mineral Resource and Ore Reserve estimatesrather than on actual mining performance. As in open pitmining there are several levels of reconciliation that may be ofinterest as performance measures:

• How does the material treated compare with the

Mineral Resource estimate'?• How does the material treated compare with the Ore

Reserve estimate'?


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•

How does the material treated compare with the slopedesign estimate, which was based on infill drilling,and perhaps a `call factor' grade adjustment`?

• What was actually drilled out, charged and fired (asdistinct from design)?

• What was delivered for treatment as measured bytruck factors, load cells, weightometers and grabsamples?

• What was really extracted from a stope as calculated

from slope surveys and back-calculation using allavailable data?

When overbreak occurs beyond the slope design line, it mayintroduce unexpected high-grade ore, low-grade, waste or a process contaminant such as graphite in a contact shear zone.Thus there is usually no direct correlation betweenmeasurements of slope overbreak and the variation of recoveredmetal from the treatment plant. Expressed globally in relation tothe resource estimate, 'dilution' is an experience-basedadjustment that takes account of a number of subjectivelyassessed variables.

Among the less obvious variables are:

• mixing of waste and spillage into ore in passes andonto stockpiles;

• blasthole damage to slope walls;

• turn-around time from grade control sampling tomark-up;

• selective mining by resuing or in-slope sorting; and

• loss of free gold in mining and transportation.

Recently surveying instruments have become available whichenable very accurate three-dimensional profiles of a slope voidto be determined. These are invaluable for mine planning,reconciliation and management, and their use can provide animproved understanding and control of dilution. There is agrowing database of these measurements.

Predicting Dilution

For ore reserve purposes dilution must be estimated from dataobtained from drilling and development, and from experience.

Key variables are:

• the mining method and size of equipment;

•

grade variability at the resource boundary;• ore width, dip, geometry and continuity;

• grade control method and proposed mining rate, and

• slope design criteria, including hydraulic radius, RQDand pillar dimensions

The availability of digital resource models has led some practitioners to calculate diluted grades based on an assumedaverage thickness of overbreak. For example, 0.5m on eachwall of a 3m wide stope represents a 33 per cent tonnageincrease. Grades from assays or composites within thisenvelope are used to interpolate a diluent grade. Caution isneeded, as the search ellipsoid used for òre' may have already

considered this material, or conversely the grade of thismaterial may be related to the sample grades lying outside thediluent boundaries in òre' or `waste'.

In practice, slope overbreak usually takes an arcuate shape,deepest at the mid-point of the slope and minimal al the pillar

sides. In large open stopes (∼20m spans), the `normal' arch may

be 3m deep at mid-span. If drill assays outside the stope (orore) limits are statistically analysed to calculate diluent grade,then this shape must be allowed for. The shape of overbreakmay be predicted using techniques such as the Radius Factor

(Dunne and Pakalnis, 1996).

Where there is a sharp geological cut-off between ore andwaste, simple geometric analysis, assuming dilution at zerograde, is often satisfactory. Where the boundary is gradational(a fiat grade-tonnage curve at the chosen cut-off) then somecredit should be given for values in the diluent.

Dilution is greatest in narrow ore zones with sharp contacts,and least in massive ore with gradational boundaries. Dilutionfrom backfill may be significant. If pillars are to be extractedagainst freestanding cemented fill in open slopes, then thestability and likely frequency of fill failure must be considered,even if rigorous control procedures are in place. In cut-and-fillmining, more dilution may be experienced if the slope miners

are paid on a piecework (tonnage) contract because they maydig deeper into the fill floor.

Dilution can be reduced over time as experience is gained andthe mining method is optimised. Decisions about the level ofslope-wall support using cable dowels are based on cost-benefitanalysis, and will affect dilution. As a general guide thefollowing suggestions are made for dilution expressed usingEquation 3 above:

•

dilution is not less than five per cent unless an errorwas made in the resource estimate;

• for selective methods (eg cut and fill), dilution istypically ten per cent;

• for open sloping dilution is typically 15 to 20 per cent

but can be more;

•

for caving methods dilution is 20 to 30 per cent; and

• for narrow vein mining, dilution of 50 to 100 per centis not uncommon.

Exceptions can be found to the above guidelines, and will bedependent on ore width, dip and stratigraphy. Improvements are possible with good mining practice. Contract mining ontonnage-based and metres-based schedules of rates may requiremore rigorous management to ensure control of dilution.

It may be useful to use dilution reconciliations for a similar

orebody and mining method as a check. This should be donecarefully with regard to the definition of dilution, the use ofhidden `call factors', and the methods of grade control andground support employed.

The proportion of a resource that can be recovered is typically70 to 90 per cent after removing ìnaccessible' or uneconomic blocks. The higher recoveries can be justified using moreselective mining methods in ore of higher unit value. With allmethods, some resource that would otherwise be classified asore will be left in pillars or abandoned due to premature groundfailure.

Pillar recovery may be justified as part of the on-going mining

process or as a retreating salvage operation at the end of minelife. For example, pillars were reduced on retreat in theCadjebut room and pillar operation, giving an improved


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recovery compared with the initial Ore Reserve assumptions. Ingeneral, resources remain 'open' along strike or at depth formost Ore Reserve estimates (i.e. there are Inferred Resources),so that any error in the recovery estimate is renderedinconsequential in time after further exploration and conversionto Ore Reserves.

In preparing a feasibility study it is critical to estimate miningrecovery accurately so that the tonnes of ore delivered to themill in the life-of-mine schedule relate to the expenditure ondevelopment and the amortisation of capital. As the miningrecovery is increased, less capital and operating costs areincurred in accessing each tonne of ore. Whether the increasedrecovery is desirable depends on how quickly thecorresponding sloping costs increase in achieving the higherrecovery.

Conclusions

The Ore Reserve estimate derives from a Mineral Resourceestimate. For an underground mine, particular limitations are

placed on the resource modelling technique. In particular,geostatistical models which purport to include dilution arelikely to lead to errors in estimation.

The conversion of a Mineral Resource estimate to an OreReserve estimate for an underground mine requiresconsideration of mining dilution and mining recovery. Thesetwo variables are the result of a multitude of factors that aredifficult to assess. Thus careful measurement, management, judgment, experience and a thorough understanding of the proposed mining method are required. The use of `textbook

factors' by inexperienced practitioners should he avoided. It is possible that a `Competent Person' for the purpose of preparinga Mineral Resource estimate may not he 'competent' to preparean ore reserve estimate for the same deposit.

There are several ways of expressing mining dilution andrecovery, all of which are valid. It is essential that these terms be defined wherever they are used.

References

Dunne, K and Pakalnis, R C, 1996. Dilution aspects of sublevelretreat .stope at Detour Lake Mine Rack Mechanics , (Eds:Aubertin, Hassani and Mitri) (Balkema).

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