CENTENNIAL SYMPOSIUM
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DEPARTMENT OF MINING ENGINEERING
The Mine
Call Factor
Isobel Clark
A. Introduction
In South African gold mines, the
major measure of the efficiency of the production process is the "Mine
Call Factor" (MCF). This factor
compares the gold estimated in situ
by the surveyors (or geologists) with the amount of gold finally produced by
the plant - with allowance for losses to tailings. The factor is generally
expressed as the ratio between the "gold called for" and the
gold accounted for, as a percentage.
In recent years, many mines
have been experiencing declining MCFs and numerous
investigations have been undertaken to try to
establish the causes on a mine by mine basis.
There. are many potential 'physical' causes for the loss of gold in a
producing mine, ranging from gold blasted into backfill bags up to gold theft
syndicates which seem to operate successfully on many mines. However, recent studies (de Jager, 1996) indicate that the popular mythology of gold
lost in cracks and washed into gullies cannot begin to explain the recent
plunges in the MCF in major mines.
Perhaps a centennial symposium
is the appropriate place to review the more traditional sources of apparent
gold loss - that is, in the initial valuation of the gold in place in the
stopes. In this paper
we investigate the difference between what is expected from stope valuation and
what is actually from a stope panel.
This exposition is based on a set of real
sample data from a
B. The Case Study
For the purposes of this study,
we have taken an area approximately 2000 metres square within a producing
mine. This area has
been mined continuously in recent years and is still in production. To avoid major complications, we have chosen
a reef which is lognormal but not too highly
skewed. This is not a very high grade area, but neither is it marginal at current
costs. The reef under study is also a
moderately thick reef, since extra complications arise in the sampling and
valuation of very narrow reefs. In
short, we have chosen an area which should be well
behaved as regards variations in value.
This area has been sampled on
the usual basis of face samples every five to six metres, taken at regular
intervals as the stope advances. As in
most traditional mines, the sample information follows a very rough 'grid' of
about five to six metres in the two-dimensional plane of the reef Sampling was
carried out by hand chipping channels across the reef. The individual sections of the reef are combined to provide a single average value across the
reef at the sampled location. This is usually expressed as an accumulation grade over the reef
times width of reef - so that the effective stoped grade can be directly
calculated.
It is usual to estimate the
value in the stope by averaging the accumulation values in the stope face. In some cases, more than one face is used to estimate the remainder of a stope. For this study, we have simplified the
situation as follows:
·
a face is
taken to be a 30 metre stretch of samples, usually 5 or 6 altogether;
·
a stope is
taken to be a 30 by 30 metre rectangular stope panel.
Obviously, the study as
described here can be carried out for other geometries
and stope sizes.
The estimated gold values as
produced by the survey office consist of the average accumulation for all
samples on the face, divided by the stoping width planned for that stope. We have emulated the 'true' stope panel
values by simply averaging all of the sample values within 30 by 30 metre
panels across the study area. This is
the closest we can expect to come to the actual resource figures. In this particular study area, we have just
under 2000 panels with sufficient sampling to serve our purposes.
C. Grade/tonnage curves
To compare the estimates
produced from the face sampling with the 'actual' values in the stopes we have
constructed 'grade/tonnage!' curves more commonly known on the gold mines as
'payability' graphs. In brief, we apply
a cutoff value or pay limit to the values and calculate the percentage of the area which is ted to be above this cutoff and the average of
the values over this 'payable' percentage.
This exercise was carried out on:
(a)
the
individual channel samples;
(b)
the 30 metre
face averages;
(c)
the 30x30 metre
stope panels.
Figure 1 shows the comparison
between the percentage payabilities for the three different 'support' sizes:
point, face average and area average. It
can clearly be seen that there is a significant
difference between the percentage payability in the three cases.
Of more concern, perhaps, are
the results shown in Figure 2. This graph shows the average value of the
payable proportion of the area. It is quite obvious from this graph that, the bigger the area
selected, the lower the achieved grade for a specific cutoff or pay limit. The two graphs taken together show that, for
a larger volume of ground, the average grade will always be lower than for a smaller
area. This is a direct consequence of
averaging over a volume or area. A
payable area on average may well contain unpay material which will be
mined. On the other hand, unpay stopes
may well contain payable material which will be left behind.
From Figure 1, it can also be seen that - for high cutoffs - the tonnage in
payable stopes is considerably less than that indicated by the face sampling.
D. Mine Call Factor
The Mine Call Factor (MCF) is
generally expressed as the "gold called for" versus the "gold
accounted for". However, this is expressed in different ways by different mines. In many cases, the ratio calculated is
between the average grade in grams of gold per tonne
of ore estimated versus grams per tonne of ore milled. That is, the MCF would be the grade found in
the stope divided by the grade measured on the face expressed as a percentage. If we perform this calculation on the lower
two lines in Figure 2, we obtain Figure 3 as an illustration for how the MCF
would change with rising pay limit in our case study area.
There seems to be little cause
for concern in this graph, since the MCF varies between 89 and 99 per cent
depending on the pay limit. That is, in
general, the stope value will be approximately 91% of that value predicted by
the face sampling. Most mines work with
this level of MCF without concern.
However, it is necessary to look also at Figure 1 to determine what
tonnage is being considered here. There is a crossover point at which the
tonnages in face and stope become equal.
At cutoffs below this point, there is more tonnage available in stope
panels than indicated by the faces. For
cutoffs above this point, the reverse is true with considerably less payable
tonnage available in stopes than might be expected. If we calculate the ratio between the tonnage
indicated by the face sampling and that indicated by the average value within
the stopes, we produce Figure 4. From this graph it is
clear that the ratio of grade called for to grade achieved is only really valid
at the point where the comparison between tonnages is 100%. This graph is analogous to the factor often
known as "surveyor's shortfall".
E. Production Controls
From the above analyses it can
be seen that many stopes which appear to be payable according to the face
sampling will go 'unpay' at some point.
There are two possible scenarios which can be considered
here:
1.
stop the
stope when the face sampling becomes unpayable;
2.
once the
decision to mine a stope has been made, the whole stope is mined even if the
face goes unpay.
For a particular production situation these two scenarios could be studied in
detail. For the purposes of this study,
we interpreted the two alternatives as follows:
1.
compare
grade obtained from payable stopes with that obtained from faces which were
payable at the same pay limit;
2.
assume that the
proportion of payable ground as indicated by the face samples is actually mined
and find the effective average grade for stopes totalling
that tonnage. This effectively means
that the pay limit being applied to the stopes is
lower.
These two scenarios were applied to the area under study and some rather
surprising results were obtained. Figure
5 shows the MCF for the scenario - stopping the stope when the face becomes
unpay - for increasing pay limits. It can be seen that the MCF stays very consistent at a small
but steady decline until the 'crossover' point on the grade/tonnage curve. Once this point has been
passed, the MCF declines drastically, eventually falling below 60% for
the range of cutoffs considered.
In contrast, Figure 6 shows the
effect on the MCF of rising pay limits if planned stopes are
mined regardless of the unpayability of subsequent faces. This graph shows a steady and consistent
decline in MCF with rising cutoff value.
However, the scale of this graph is somewhat different from that in
Figure 5. The decline in MCF for the same range of pay limits is only to 84%
not 55% as in scenario 1.
F. Conclusions
It has been
said that there are many possible physical causes for the apparent loss
of gold indicated by a falling Mine Call Factor. In this paper we
have not considered these factors although they can contribute significantly to
a low MCF. Neither have we considered
such problems as the accuracy of the sampling process underground and the
assaying process in the laboratories.
What we have attempted to show in this paper is that significant
declines in the Mine Call Factor may well be due to the valuation process
itself and to the management decisions which are based
on those valuations.
In previous years, when the pay
limit or cutoff values stayed at a consistent level - below the 'crossover
point' - Mine Call Factors remained reasonably consistent over long periods,
albeit with considerable fluctuations on a month to month basis. With rising costs in the mines and rising pay
limits, the actual decision process to mine or not to mine a stope becomes of
increasing importance in the maintenance of a reasonable Mine Call Factor and,
thus, of an efficient producing gold mine.
G. Reference
de Jager, E.J. (1996) "the
analysis of the Mine Call Factor in gold mining, with specific reference to
Western Holdings Mine", PhD thesis, University of the