Dilution and Losses in Underground Mining

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 in Underground 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 in underground 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

Definitions

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

Dilution may be defined in several ways. To the metallurgist receiving the ore for treatment, it is the percentage of the delivered 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 tonnage increase. Thus;

Face-to-face involvement of the geologist who prepared the resource estimate is essential. The assumptions and limitations inherent in the resource model must be drawn out. The background of the resource geologist and his or her experience with underground mining estimates (as distinct from open-pit) should be understood. For reasons set out in this paper, conversion of a Mineral Resource estimate to an Ore Reserve estimate is a challenging task. The use of `text book' factors for dilution and recovery is likely to lead to errors.

1. Managing Director, AMC Consultants 19/114 William Street, Melbourne E-mail: [email protected]

Pty Vic

Ltd, 3000.

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 less severe. 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 as they are defined before use. An example of the misunderstandings that may otherwise arise is given by the following example. Consider 100 t of ore of ten per cent grade diluted with 10 t of material of four per cent grade, to give 110 t at 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.

Mining Dilution and Losses in Underground Mining

Mining recovery may also be expressed in a variety of ways as follows: •

What percentage of the total resource tonnage will ultimately be mined'?

•

How does the diluted tonnage delivered to the mill compare with the estimated resource tonnage?

•

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

•

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

•

After elimination of those parts of the resource deemed inaccessible or otherwise uneconomic (for reasons of width, dip, deleterious elements, rock conditions, etc), what proportion of the remainder will 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 are different for open-pit and underground mints. To a scale of tens of metres, the location and spatial distribution of values may be unimportant in an open pit resource model. Provided the pins located to access the mineralised zone, any valuable material can be identified by grade control sampling and then marked out for mining. Thus, the emphasis in open-pit resource modelling is on the global accuracy of estimates of tonnes and grade and internal variability at a scale that might affect pit optimisation, so a statistical approach is often appropriate. For underground mining the thickness, dip, continuity and spatial relationship of ore zones, the regularity of wall contacts, strength of ore and wall rocks are all critical inputs to the Ore Reserve estimate. These are drawn from the resource model, or from the geologist's knowledge of the deposit gained during the data-gathering and modelling phase. Some parts of the resource may be impossible to mine; others may be located too far from development to be economic; others may suffer severe dilution. A sectional interpretation by an experienced geologist at one or more possible cut-off grades is usually the first step in preparing the resource model. This sectional interpretation will include features inferred from the drill logs that would not be generated by any grade interpolation software. The geologist's experience tells him or her how variable the ore boundaries are in this type of deposit and what shapes the variations might take. When the sections are linked and wireframed, then checked and corrected in plan and back to section, the resulting three-dimensional outlines can be used to validate the gradeinterpolation process. The above procedure is usually necessary regardless of whether or not the limits of the mineralisation envelope have been interpreted (at a subeconomic cut-off) and wireframed as limits to the grade block model. This is because the shape of the mineralisation envelope may be quite different from the shape of the economic material. It may be acceptable to let the grade interpolation process determine limits of economic mineralisation in large deposits to be mined by caving methods, where the edge inaccuracies of AMC Reference Library – www.amcconsultants.com.au

the model become insignificant. For other cases, the geologist must form a view about the spatial limits of ore at the chosen cut-off grade, and must be prepared to model the ore boundaries realistically. To do this, the geologist needs to understand the style of mineralisation and to be able to infer irregularities, including structural dislocations such as faults, at a scale smaller than the drill spacing. The mining engineer will design slopes which have geometric limitations dictated by geotechnical factors, the economic spacing and length of production blastholes, or the need to combine blocks of `ore' and `waste' into mineable units. When these shapes are overlaid on the resource model, the resource grade is diluted and some of the resource is lost. The resource model will contain internal dilution according to the model block size (based on the assumed Selective Mining Unit) which may or may not accord with the engineer's proposed method. In an underground mine, levels are planned on the basis of relatively coarse-spaced drilling. Stopes are designed and then mined with limited flexibility for change. Pods of ore not identified by drilling will be lost, even if they are expected statistically to be present. Proponents of geostatistics sometimes claim that a resource model inherently contains an appropriate allowance for internal and edge dilution. This is an obvious fallacy; the dilution estimate must derive from mining and geotechnical considerations. A resource model which purports to include dilution provides an undefined starting point for the Ore Reserve estimator, who must somehow `remove' the diluent material from the model before adding back mining dilution. This is an impractical task, so the only satisfactory approach is to refuse to accept such a model as a basis for an underground mine ore reserve. The resource estimate for underground mining must include a statement 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. This enables the mining engineer to consider strategic sloping options (high tonnage bulk mining vs low tonnage selective mining). The estimate should also quote resources at varying cut-off grades and minimum widths to enable economic optimisation.

Measurement of Dilution and Recovery In many mines, ore from a number of sources is stockpiled and blended before treatment, making reconciliations difficult or impossible. Assuming reconciliation is possible, the resulting calculations of dilution and recovery may reflect on the accuracy of the Mineral Resource and Ore Reserve estimates rather than on actual mining performance. As in open pit mining there are several levels of reconciliation that may be of interest 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'? 2

Mining Dilution and Losses in Underground Mining •

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

•

What was actually drilled out, charged and fired (as distinct from design)?

•

What was delivered for treatment as measured by truck factors, load cells, weightometers and grab samples?

•

What was really extracted from a stope as calculated from slope surveys and back-calculation using all available data?

When overbreak occurs beyond the slope design line, it may introduce 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 between measurements of slope overbreak and the variation of recovered metal from the treatment plant. Expressed globally in relation to the resource estimate, 'dilution' is an experience-based adjustment that takes account of a number of subjectively assessed variables. Among the less obvious variables are: •

mixing of waste and spillage into ore in passes and onto stockpiles;

•

blasthole damage to slope walls;

•

turn-around time from grade control sampling to mark-up;

•

selective mining by resuing or in-slope sorting; and

•

loss of free gold in mining and transportation.

Recently surveying instruments have become available which enable very accurate three-dimensional profiles of a slope void to be determined. These are invaluable for mine planning, reconciliation and management, and their use can provide an improved understanding and control of dilution. There is a growing database of these measurements.

Predicting Dilution For ore reserve purposes dilution must be estimated from data obtained 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, RQD and pillar dimensions

The availability of digital resource models has led some practitioners to calculate diluted grades based on an assumed average thickness of overbreak. For example, 0.5m on each wall of a 3m wide stope represents a 33 per cent tonnage increase. Grades from assays or composites within this envelope are used to interpolate a diluent grade. Caution is needed, as the search ellipsoid used for `ore' may have already considered this material, or conversely the grade of this material may be related to the sample grades lying outside the diluent boundaries in `ore' or `waste'. AMC Reference Library – www.amcconsultants.com.au

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 (or ore) limits are statistically analysed to calculate diluent grade, then this shape must be allowed for. The shape of overbreak may be predicted using techniques such as the Radius Factor (Dunne and Pakalnis, 1996). Where there is a sharp geological cut-off between ore and waste, simple geometric analysis, assuming dilution at zero grade, is often satisfactory. Where the boundary is gradational (a fiat grade-tonnage curve at the chosen cut-off) then some credit 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. Dilution from backfill may be significant. If pillars are to be extracted against freestanding cemented fill in open slopes, then the stability and likely frequency of fill failure must be considered, even if rigorous control procedures are in place. In cut-and-fill mining, more dilution may be experienced if the slope miners are paid on a piecework (tonnage) contract because they may dig deeper into the fill floor. Dilution can be reduced over time as experience is gained and the mining method is optimised. Decisions about the level of slope-wall support using cable dowels are based on cost-benefit analysis, and will affect dilution. As a general guide the following suggestions are made for dilution expressed using Equation 3 above: •

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

•

for selective methods (eg cut and fill), dilution is typically 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 cent is not uncommon.

Exceptions can be found to the above guidelines, and will be dependent on ore width, dip and stratigraphy. Improvements are possible with good mining practice. Contract mining on tonnage-based and metres-based schedules of rates may require more 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 done carefully with regard to the definition of dilution, the use of hidden `call factors', and the methods of grade control and ground support employed. The proportion of a resource that can be recovered is typically 70 to 90 per cent after removing `inaccessible' or uneconomic blocks. The higher recoveries can be justified using more selective mining methods in ore of higher unit value. With all methods, some resource that would otherwise be classified as ore will be left in pillars or abandoned due to premature ground failure. Pillar recovery may be justified as part of the on-going mining process or as a retreating salvage operation at the end of mine life. For example, pillars were reduced on retreat in the Cadjebut room and pillar operation, giving an improved 3

Mining Dilution and Losses in Underground Mining

recovery compared with the initial Ore Reserve assumptions. In general, resources remain 'open' along strike or at depth for most Ore Reserve estimates (i.e. there are Inferred Resources), so that any error in the recovery estimate is rendered inconsequential in time after further exploration and conversion to Ore Reserves. In preparing a feasibility study it is critical to estimate mining recovery accurately so that the tonnes of ore delivered to the mill in the life-of-mine schedule relate to the expenditure on development and the amortisation of capital. As the mining recovery is increased, less capital and operating costs are incurred in accessing each tonne of ore. Whether the increased recovery is desirable depends on how quickly the corresponding sloping costs increase in achieving the higher recovery.

Conclusions The Ore Reserve estimate derives from a Mineral Resource estimate. For an underground mine, particular limitations are placed on the resource modelling technique. In particular, geostatistical models which purport to include dilution are likely to lead to errors in estimation. The conversion of a Mineral Resource estimate to an Ore Reserve estimate for an underground mine requires consideration of mining dilution and mining recovery. These two variables are the result of a multitude of factors that are difficult 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 preparing a Mineral Resource estimate may not he 'competent' to prepare an ore reserve estimate for the same deposit. There are several ways of expressing mining dilution and recovery, 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 sublevel retreat .stope at Detour Lake Mine Rack Mechanics, (Eds: Aubertin, Hassani and Mitri) (Balkema).

AMC Reference Library – www.amcconsultants.com.au

4