ore dilution and ore recovery

Prof. Dr. Hassan Z. Harraz
Geology Department, Faculty of Science, Tanta University
hharraz2006@yahoo.com
Spring 2019
@Hassan Harraz 2019
Ore Dilution and Ore Recovery
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@Hassan Harraz 2019
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KEYWORDS
Dilution; Ore Dilution; Open Pit, Recovery in Mining;
Selectivity, Mining studies, Mine design; Mine evaluation

Outline of Topic
DILUTION
Planned and Unplanned Dilution
Internal and External Dilution
Primary and Secondary Dilution
Factors of Dilution
Mine Value Diminutions Due to Dilution
 ORE RECOVERY
 Room and Pillar Example
 Ore Dilution & Recovery in Mining
 Rate of Extraction
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Introduction
 As a mining project is developed from
conceptual to production phases, there
exist a variety of uncertainties and
difficulties that affect the operation’s
designs and economic value.
 A notable design parameter to be
taken into account is the factor of
dilution.
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Definition
Dilution refers to the waste material that is not separated from the ore
during the operation and is mined with ore. This waste material is mixed
with ore and sent to the processing plant (Jara, 2006; Sinclair, 2002).
Dilution is the result of mixing non-ore grade material with
ore-grade material during production,
generally leading to an increase in tonnage and a decrease in
mean grade relative to original expectations.
Dilution can be defined as the ratio of the tonnage of waste mined and
sent to the mill for processing over the combined the total tonnage of
ore and waste that are milled.
The following equation is the expression used for dilution:
For example if 10 tonnes of waste rocks (and/or below cut-off grade
mineralized rocks) are mined with 90 tonnes of ore and all (100
tonne) being sent to mill, dilution is calculated to be 10.0%. According
to this definition X percent of dilution in a mine suggests that X
percent of the feed is not economically profitable to be processed.
This X amount should not be sent to the crusher and proper actions
must be taken in the mine to separate them from the feed as much as
possible.
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Figure 1. An idealized view of the various sources of dilution at different stages
of mining and milling.
For each dilution source, the diagram indicates that some ore is potentially lost
and some waste is included with ore (after Elbrond, 1994)
A conceptual of
dilution during
various mining
operations
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Planned and Unplanned Dilution
 The following definitions are crucial in the estimations for planned and unplanned
percent dilution.
 Geological Reserves (Tg): The tonnage of ore above the cutoff grade
 Planned Waste (Wp): Rock with a lower mineralization content than the cutoff grade
within slope limits
 Mining Reserves (Tm): The ore tonnage within the planned stope limits
 Unplanned Waste (Wu): Rock with a lower mineralization content than the cutoff
grade, coming from beyond the planned stope limits
 Total Waste: Rock which includes mineralization below the cutoff grade
 Run of Mine Ore (Tt): The tonnage generally sent to the mill, sum of geological
reserves, planned waste, and unplanned waste
Figure 2 shows an example of the various types of waste, reserves, and dilution for
a simple stope.
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Figure 2: Visual representation of the planned and
unplanned dilution (DeSouza, 2010)
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Internal and External Dilution
 Referring to a Mining Block, dilution happens in two
different areas. Figure 3 shows a mining block and bench
depicting internal and external dilution. It is convenient to
consider dilution in two categories:
I) Internal Dilution,
II) External (Contact) Dilution
Figure 3: A mining block in an open pit visualizing internal and
external dilution (Ebrahimi, 2013)
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I) Internal Dilution
 Internal dilution occurs within a mining block in which pockets of waste are unable
to be separated and are mined with the block.
 Low grade material surrounded by high grade material.
 Sometimes within a mining block there are waste inclusions or low grade pockets of
ore that cannot be separated and are inevitably mined with the mining block.
 Degrees of internal dilution can vary within various types of deposits; specifically,
lithological and grade distributions significantly influence the degree of dilution.
 Internal dilution is difficult if not impossible to avoid. The amount of internal dilution
varies in different types of deposits. Lithology and grade distribution are important
factors in internal dilution.
 Furthermore, the following four main components govern internal dilution:
 Geology and Mineralogy: Typically fine-grained mineralization with local but
relatively small occurrences of mineralization.
 Data Density: Becomes a significant factor once the geology is understood.
 Estimation Method: Manual and automatic estimation methods tend to
overestimate grade and underestimate tonnes.
 Cutoff grade and grade control: When cutoff grade is applied to a deposit,
the engineer assumes that the grade contacts are definable at any given
grade.
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I) Internal Dilution…(Cont.)
 Internal dilution can be subdivided into:
i) Sharply defined geometric bodies:
Geometric internal dilution results from
the presence of well-defined waste bodies
within an ore zone, e.g. barren dykes
cutting an ore zone, 'Horses', etc.
ii) Inherent internal dilution results from
the decrease in selectivity that
accompanies an increase in the block size
(e.g., resulting from loss of equipment
digging selectivity) used as the basis for
discriminating ore from waste, even where
no entirely barren material is present.
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II) External Dilution
 External dilution:
 low grade material marginal to high grade material.
 refers to the waste outside of the orebody that is
mined within the mining block.
 External dilution varies based on geology, shape of
orebody, drilling and blasting techniques, scale of
operation and equipment size. This is the type of
dilution that can be controlled using proper equipment
and mining practices.
 It varies based on an assortment of parameters and
can be controlled using effective equipment and mining
practices.
 The following initiatives can be used to minimize
external dilution:
 Defining the contact surfaces of ore and waste
 Selection of the proper equipment to attain desired
selectivity
 Mining along the contact surfaces
 Modelling the effects of unavoidable dilution
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II) External Dilution….(Cont.)
External dilution is the result of sloughing of walls, difficulty of
sorting in open pits or the inadvertent or purposeful mining of
barren or low grade material at the margin of an ore zone. Such
dilution is generally significant where stope walls are physically
difficult to maintain because of rock properties, or where ore
widths are less than the minimum mining width.
External dilution can be of somewhat less significance in large
deposits with gradational boundaries in comparison with small
deposits because the diluting material:
can be a small proportion of the mined tonnage
contains some metal, possibly near the cut-off grade
In general, some uncertain proportion of waste must be taken
along with ore during the mining operation. This form of dilution
can be impossible to estimate with confidence in advance of
mining; experience is probably the best judge. Accepted
procedures for estimating dilution in underground mining
operations are summarized by Pakalnis et al (1995).
External dilution (e.g. wall slough, contact dilution, minimum
mining width dilution, overbreak) is not easily quantifiable in
totality and commonly is approximated based on experience. As
Stone (1986) suggests, the estimation of dilution is too commonly
done in retrospect and is .. "... designed to force a reserve
calculation to match the observed mill feed".
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 Highly formalized empirical methods of estimating
dilution in operating underground mines are
summarized by Pakalnis et al (1995) and involve
an extensive study of the physical characteristics
of a deposit and the development of a
deterministic model that relates these
characteristics to dilution.
 Such detailed modelling is difficult enough in
operating mines but is intractable for deposits
that are undergoing development or for which a
feasibility study is planned or in progress.
However, some components of external dilution
can be estimated geometrically (Figure 4); thus, it
is useful to consider various types of external
dilution as follows:
Vob is a volume of overbreak material
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Figure 4. Conceptual view of dilution associated with tabular deposits.
Note that the planned mining surface cannot be duplicated exactly during
production; there is always a tendency toward overbreak.
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a) Vein Widths Partly Less Than Minimum Mining Width
 In certain cases a significant component of external
dilution can be estimated with reasonable
confidence in advance of production, particularly at
the feasibility stage where some underground
access exists.
 Consider the case of a continuous vein that can be
mined to sharp boundaries except in those parts of
the vein where vein width is less than the
Minimum Mining Width (wm). (Figure 5).
Figure 5. Idealized plan of a vein, part of which (proportion pc) is less than the minimum mining
width (wm) leading to unavoidable dilution (darkest pattern).
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 An unbiased histogram of vein widths for a large part of the vein
provides an estimate of the proportion of the vein that has
widths less than the minimum mining width, i.e. (pc) (Figure 5).
The average vein thickness (wc), of this pc can be determined
from the unbiased histogram as a weighted average of the
appropriate class intervals (i.e. those class intervals less than
wm) and can be compared with the minimum mining width. The
difference between the two widths (wm - wc) is the average, ideal
(point) dilution perpendicular to the plane of the vein over that
portion of the vein that is less than minimum mining width, i.e.
So. pc.
 In this way the point estimate of dilution is modified to a volume
of diluting material in this equation :
Where:
Vd: volume of diluting material
Pc: proportion of the vein that has widths less than the
minimum mining width
Wm : Minimum Mining Width
wc : average vein thickness
So : the area in the plane of the vein, represented by the data.
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Consequently, the new volume, Vp, of mined (produced) material is
Where:
So is the known areal extension of the vein in the plane of the vein
vd is the volume of diluting material
Vo : is the original vein volume;
Vp : volume of mined (produced) material.
gp is weighted average grade
i) assuming that the vein material and the diluting country rock have the same bulk density
Weighted Average Grade (gp)
ii) assuming densities differ
In many practical situations, gw is approximately zero.
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 A comparable approach can be used where an unbiased histogram is approximated by a
normal or lognormal distribution. Where the histogram has the form of a normal distribution
the proportion of vein that is thinner than the minimum mining width can be estimated with
equation related to the normal distribution (see Sinclair and Blackwell, 2002)); and the
average thickness of that portion below minimum mining width (wc) can be determined as
well. Application of equations for a normal approximation requires that the minimum mining
width be expressed as a standard normal score (z-value); the equations apply directly if z is
positive but require some attention if z is negative. Comparable equations and procedures
exist for the case of a lognormal distribution (Sinclair and Blackwell, 2002).
 In general, it is virtually impossible to mine exactly to a planned mining limit or a contact,
however sharp; instead there is a tendancy for overbreak with the obvious result of
attendant dilution by some amount of wallrock. This effect is clearly a function of wallrock
properties and mining procedures (Pakalnis et al, 1995) and where rock is highly competent
and the mining process highly selective, can be relatively minor. In some cases limited
experience gained from exploratory and development work provides information that
permits reasonable estimation of the average overbreak that is likely. For example in the
case of the Silver Queen deposit the wallrock is mostly highly competent and overbreak will
be minimal, perhaps on the order of a foot or so (30 cm) of added thickness on average
throughout the vein.
This additional dilution amounts to
Total volume of mined material = (Vp + f.So)
where
Vob is a volume of overbreak material that is easily transformed to tonnage with an appropriate bulk density factor. The
total volume of mined material determined from the foregoing equation is thus increased to (Vp + f.So) and a
corresponding weighted average grade can be determined. Such a procedure is used at the Lupin gold mine (Bullis et
al, 1994)
Where
"Reserve Dilution is estimated by adding 1 m, at assay grade, on both sides of the ore in the Centre
zone. In the West zone, reserve dilution is estimated by the addition of 0.5 m, at assay grade, on both
sides of the ore outline". @Hassan Harraz 2019
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b) Contact Dilution
Stone (1986) describes an empirical, deterministic approach to the
estimation of a type of external dilution that he refers to as contact
dilution. This particular component of external dilution results from
intermittent protrusions of wall rock that penetrate beyond smooth,
interpreted mining margins into "ore". Stone (1986)'s estimation
procedure for this form of dilution is complex and difficult to implement
because the appropriate information is rarely available with which to
make an estimate to a reasonable level of confidence.
A simpler approach that meets his aim can be applied where
exploration has exposed a representative length of deposit-wallrock
contact such that it can be examined and characterized with
confidence. Given sufficient information that the detailed ore-waste
contact is well known in one or more parts of a deposit as a geological
map, it is a simple matter to estimate the area of waste protrusions into
ore on detailed sections and to extend this estimate to the third
dimension. Where such detail is known only at a few very local parts of
a deposit an alternative procedure is useful as described below.
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Figure 6. An approach to measuring 'contact' dilution as defined by Stone (1986). Vertical lines extending
from the irregular deposit margin to the smooth interpreted margin along the upper margin contact of the
hypothetical deposit have been superimposed on the original diagram and their lengths measured to
characterize the ore loss and dilution populations relative to the planned mining unit. An arbitrary scale
(32m between drill holes) has been assumed for the purposes of illustration.
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Figure 7. Histogram of measurements of thickness of diluting ground
(contact dilution) obtained for the hypothetical example of Figure 4. N = 67
equally-spaced measurements. Negative measurements are ore beyond the
proposed mining limit (i.e., ore lost to waste); positive values represent
wallrock dilution relative to the planned mining surface.
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 Consider Figure 6, from Stone (1986), for which a scale has been assumed for purposes of
illustration.
 Lines have been superimposed on Stone's original diagram parallel to the bordering
section traces and are sites used to measure distances from the interpreted
"Average Contact or Mining Margin" (dashed line) to the true ore-waste contact.
 Negative values are ore that is lost to wallrock;
 Positive values represent dilution by lobes of wallrock extending across the mining
margin into ore.
 A histogram of these distances is prepared and serves as a basis for estimating the
proportion of length between control points where waste penetrates across the mining
margin into ore. The average thickness of such waste extending into ore can be
determined as a weighted average using class frequencies as weights for the mid values
of corresponding class intervals.
 If the distribution of measurements is found to be approximately normally distributed then
equations tied to the assumption of normality (see Sinclair and Blackwell (2002) can be
used to determine the proportion of positive values and the average thickness of the
positive values. Of course, the length over which positive values occur can be measured
directly from the figure. The "thickness" data measured along the lines shown on Figure
4 are presented as a histogram in Figure 7.
 The positive values represent 44 percent of the measurements (this compares with 43%
of the actual measured length of the mining limit assumed in Figure 6) and have a
weighted mean value of 1.4 m. For the assumed scale, the proportion of positive values is
the proportion of the 32 m length between control points that is represented by waste
protruding into ore, and gives a length of 14.1 m. The product of this length and the
average thickness of protrusions is a 2-dimensional estimate of the quantity of "Contact"
dilution along one mining margin, in this case about 19.7 m2 or about 4.7% of the area
mined.
 A comparable estimate can be made along the opposite margin (not done here) to show
that the total contact dilution in this hypothetical example is about 5.1% of the area
outlined for mining. Several such estimates, averaged, provide a global estimate. The
method has the advantage that it uses point estimates and thus, can combine information
from another dimension, for example, where both a drift and a raise provide information
on the nature of the true ore-waste contact each can be used for independent estimates of
"contact" dilution or they can be combined to a single estimate.
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Primary and Secondary Dilution
 Estimating dilution prior to mining is a challenging
task, demanding the use of an engineer’s best
judgement to assess the feasibility and economic
value of mining a block, stope, or deposit.
 An important parameter to consider while doing so is
total dilution, a value which can be expressed in the
following equation:
 In general, primary dilution is found in narrow
deposits, as the thickness of the ore zone becomes
the main source for dilution. Figure 8 displays the
concept of primary dilution within a narrow orebody.
 Conversely, secondary dilution is the dilution that occurs
beyond the planned stope dimensions. Secondary dilution is
caused by a number of factors which include sloughing,
drilling and blasting, ground conditions, planar discontinuities,
mining method, equipment, and work practices. A visual
representation of secondary dilution is shown in Figure 9.
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Figure 8: Primary Dilution in a
narrow orebody (Crawford, 2004)
Figure 9: Secondary dilution in
an orebody (Crawford, 2004).
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Factors of Dilution
 Mine dilution occurs due to the mining method selected and from overbreak during
the mining process. There are multiple considerations in terms of dilution. Mining
methods such as block caving, sublevel stoping and room-and-pillar have mining
methods which are more predictable, where dilution can be modeled using empirically
generated equations.
 The major factors which have a direct effect on dilution are as follows:
 Mine Depth: Methods with greater selectivity exhibit lower dilution
 Rock Competency : More competent rock will be less susceptible to sloughing
and overbreaking
 Ore Type : Defines the selective and effective dilution parameters
 Ground Support: Support can be used to maintain ore and waste surfaces,
limiting the amount of dilution
 Self-supported openings are more selective and have lower dilutions than block caving
with typical dilution ranges from 5% to 15%. Additional factors which influence dilution
to a lesser extent are as follows:
 Rock Mechanics: Mechanical parameters and technical ability lead to increased
dilution to account for
 Ore Geometry : Layout of the ore in skewed orientation leads to increases in
unplanned dilution
 Hanging-wall Dip : The likelihood of wall slabbing and release of wedges will
depend on wall dip relative to the orientation of lamination and joints
 Geotechnical: Parameters are increased lead to an increase in dilution values
 Stope Span: Larger stope spans are less stable, increasing the risk of wall failure
and unplanned dilution
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Mine Value Diminutions Due to Dilution
1( One of the main consequences of dilution is the
reduction of mill feed grade. Lower feed grade
means less income. For marginal grade ore,
dilution may reduce the grades to a degree that
it becomes uneconomic to be processed, in
other words dilution may turn an ore block to
waste.
The feed grade after dilution can be calculated
using the equation
Where: The same unit must be used for all grades in this equation.
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 Based on the discussion above, it can be expected that due to higher milling cut-off
grade, caused by dilution, the overall reserve of the mine will decrease within a
given pit.
2) Due to dilution, energy and materials that are used in the processing plant to treat
the waste portion of the feed are wasted. As a result, the mill unit operating cost
increases directly by the amount of the dilution factor. For example in a project
whose processing cost has been estimated at $18/t,a 10% dilution means $1.80is
spent processing waste in the mill for every tonne of feed. For a 30,000 tonne a
day operation this amount of dilution means $54,000 is wasted every day.
3) Occupying part of the processing capacity by sending waste rocks to the plant
prolongs mine life. As a result it delays cashing the value of mineable resource on
time as planned. A longer mine life obviously lowers the net present value (NPV)
and internal rate of return (IRR). For example consider a mine that has the
potential to generate $20M/year net revenue by milling certain amount of ore for
up to 10 years. This is after $100M initial capital investment. Assuming a 10%
dilution the mine life increases to 11 years from 10 years. With a simple calculation
it is possible to compare mine economics for two cases of mining operation, with
dilution and without dilution. For this example there will be 21.0% decrease in NPV
(at 10% discount) and an 8.6% decrease in IRR after 10% dilution.
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Table 1 shows the effect of 10% dilution on ore grade for a gold mine for one tonne of
ore. Assaying showed that waste rocks have a small amount of gold. The gold grade in
waste rocks is 0.05 gram per tonnes. The milling cut-off grade for this operation is
calculated to be 0.30g/t. It means that a tonne of ore with 0.3g/t can be sent to the
mill and be expected to generate a small amount of profit. Ten percent dilution in this
mine will reduce the grade of ore from 0.3g/t to 0.28g/t which is below cut-off grade
and will not be able to return a profit. This block will be considered as waste after 10%
dilution.
Table 1: Effect of Dilution on Ore Grade in a Gold Mine
Original Ore
(g/t)
Dilution
(%)
Waste Au
(g/t)
Mill Feed
(g/t)
0.50 10% 0.05 0.46
0.40 10% 0.05 0.37
0.30 10% 0.05 0.28
0.20 10% 0.05 0.19
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FIG. 10: The -250M Plan View of the Deposit With Ore Greater
Than 0.49G/T (cut-off grade $1,300/OZ)
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ORE RECOVERY
 Ore recovery is based on the material within the model that is left behind to provide
structural support, thereby not being recovered.
 The generalized equation for recovery is given in the following equation:
 More specifically, ore recovery can be defined by the percentage of minable reserves
extracted in the mining process. The issue of balancing dilution and ore recovery is a
challenging one as profitability is to be optimized while not effecting the efficiency of
operation. It has been noted that instead of utilizing labour intensive mining methods
to high tonnage bulk mining has decreased the ability to control ore recovery.
 In a practical underground and open pit setting, the ore recovery is affected by three
main factors as follows:
 Ore wedges located at the top and bottom sills of the panel are unable to be
extracted as they provide access to mined-out areas in backfill and provide
support. This is comparable to room and pillar operations in which pillars are also
left for support.
 Ore left against backfill, as there exists a certain amount of ore adjacent to the
backfill that is unable to be extracted due to irregularities of the ore-backfill
contact surface
 Oxidization of ore stuck to the footwall is a direct consequence of the extraction
sequence. In this, ore blasted at the beginning of the extraction phase must stay
within the stope until the last slice of ore is extracted.

Room and Pillar Example
 Viewing a room and pillar operation from
a longitudinal cross section, see Figure
11, given the width of the pillar (wp) and
the width of the opening (wo) the
recovery is given by the following
equation:
Figure 11: Longitudinal view of room and
pillar recovery (DeSouza, 2010)
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 Viewing a room and pillar operation
from a plan view, see Figure 12,
given the width of the pillar (wp) and
the width of the opening (wo) the
recovery is given by the following
equation:
Figure 12: Plan view of room and pillar
recovery (DeSouza, 2010)
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 Depending on the mining method which is
being used, recovery can be estimated by
understanding the stope dimensions
relative to the mineralization. One must
also have an understanding of the
geotechnical environment to know if ore
will have to be left in place to provide
support. Once these characteristics are
known the recovery is easily calculated
using the generalized equation.
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Ore Dilution & Recovery in Mining
 Recovery and dilution usually are interrelated; with some methods
of stoping a high recovery involves contamination of the ore from
the walls or capping, and often clean ore can be obtained only by
leaving some ore in the mine.
 In open-stope mines the greatest loss of ore is that tied up in
pillars left for support of the back or hanging wall. The amount of
ore thus left varies considerably with the strength of ore and wall
rocks, thickness of the ore, its depth below surface, and the
requirements for permanent support to prevent movement and
subsidence of overlying strata and of the surface. Where eventual
subsidence is permissible, much of the pillar ore often can be
recovered on the retreat by robbing operations, the amount
recoverable depending to a considerable extent on roof
characteristics; sometimes almost complete extraction is possible.
Where it is imperative to prevent surface subsidence and resultant
damage to surface improvements or to prevent an influx of surface
or underground water, 20 to 40 percent of the ore in the deposit
may have to be left as pillars. Aside from ore left in pillars, high
extraction is usually possible in open stopes, since the stope
boundaries are exposed to view and all showings of ore can be
attacked and mined.
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 With shrinkage stoping, high extraction can be obtained if the lode is regular in
strike, dip, and thickness and the walls are firm. If the lode is very irregular, there are
numerous rolls in the footwall, or there are abrupt offsets due to strike or diagonal
faults or if tongues of ore make off into the walls, considerable ore may be lost. Ore in
the footwall in the form of bulges or branching tongues is likely to be overlooked
because covered by broken ore; however, if the walls are exceptionally firm much of
this often can be recovered during final drawing of the stope. When the stopes are
silled above the level, that is, on drift pillars (called boxhole pillars in Canada),
complete recovery of these pillars is frequently impossible. Likewise, some loss may be
incurred in recovery of floor or crown pillars. Loss in pillars may run up to 10 or 15
percent of the total ore in the deposit. As in open stoping, pillars may also be left at
various horizons in the stope for roof support, but the aim usually is to leave these in
low-grade sections as much as possible.
 Some dilution almost always occurs in shrinkage stoping, either because of
overbreaking the walls of the vein during mining or later sloughing, especially during
final drawing. Overbreaking is almost unavoidable when the vein is irregular in strike
and dip and especially so if there are numerous small offsets.
 From 85 to 95 percent of the ore usually can be recovered by shrinkage stoping with
dilution of 5 or 10 to 25 percent, except where some other method would be better
suited to the conditions. Sometimes dilution runs 30 percent or more, but in most such
instances cut-and-fill or some other method probably should be employed.
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 The use of cut-and-fill stoping under conditions to which it is
suited usually results in high recovery and little dilution, except in
very heavy ground, as at the Morning mine, although some ore may
be left in drift pillars where the stopes are silled above the level and
in floor pillars. As previously stated, the fact that irregularities in the
vein and tongues of ore can be mined and blind lodes prospected for
in the walls, and, when found, mined as a tributary stope, makes a
high recovery possible. Dilution is small, since the fill prevents
sloughing of the walls, and any wall rock that comes in or horses of
waste in the vein are left in the fill. Hand sorting is often employed
to remove coarse gangue material with resultant raising of the grade
of the ore trammed.
 A high recovery with little dilution usually can be obtained with
square-set stoping. This method finds its best application for
mining in heavy ground where the ore is high-grade and any loss or
dilution is to be avoided, regardless of cost. By square-set stoping
and close filling, high-grade sections of an ore body below or
adjacent to lower-grade ore often can be mined without disturbing
the lower-grade sections, which changes in economic conditions or
improved technology later may make it possible to mine profitably.
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 Recovery in block caving varies with conditions and is influenced by the amount of
dilution the ore can stand, since high recovery usually involves high dilution also, and the
amount of dilution permissible depends on metal prices. Thus, if the minimum grade of
ore that it is profitable to handle is 1 percent copper, drawing should stop when, owing to
dilution, the grade falls below this figure, regardless of whether or not all the ore has
been recovered. Some dilution always results from “piping” of broken capping through
the broken ore and from the margins, and the tonnage drawn from a block of ore usually
is greater than the estimated tonnage in the block, whereas the average grade of ore
usually is less than that estimated. If the margins of the deposit are very irregular in
outline, either the extraction or the grade of ore recovered will suffer, as it is impossible
to make the cave follow an irregular boundary closely. At Ray, Ariz., the dilution in
drawing 40,000,000 tons of ore was estimated as 10 percent. At Inspiration, 110 percent
of the estimated tonnage and more than 85 percent of the metallic contents are
recovered. At Miami, operations in 13 completed stopes or blocks resulted in the recovery
of 2.4 percent more copper than had been estimated previously, but 15.15 percent more
tonnage was drawn, the average grade being 88.93 percent of the calculated grade.
Careful regulation of drawing is essential to obtain minimum dilution and maximum
recovery.
 With sublevel caving the line of break may be controlled more closely around the
boundaries than with block caving, as the ore is partly excavated by slicing, which can be
confined strictly to the ore, and the blocks caved are relatively small both in vertical
height and in horizontal area. However, as the backs are caved under a gob consisting of
a mixture of broken capping, slice timbers, and flooring, which cannot always be
prevented from breaking through and cutting off undrawn ore, some ore is bound to be
lost; recovery usually is not as complete as with top slicing. Under the conditions on the
Gogebic iron range, Michigan, where much of the capping carries 40 percent or more
iron, a small amount of dilution does not seriously affect the grade of ore mined and the
dilution from this source may offset tonnage of ore lost; the net result is therefore likely
to be approximately 100 percent tonnage recovery, with a slight lowering of the total
contained iron below that of the ore body proper.
 Top slicing permits high recovery of ore with little dilution (provided a suitable mat is
maintained) under conditions where any method of mining from the bottom upward
would be difficult, hazardous, and costly. Small stubs of ore between the ends of slices or
between the end of a slice and the cave are sometimes lost, and 2 or 3 feet of ore may
have to be left around the margins of a deposit where a flat footwall comes up to the
overlying mat.
@Hassan Harraz 2019
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 Accurate figures on recovery and dilution are difficult to obtain unless
waste and ore faces are measured and sampled after each round and
the waste sorted out in the stopes is measured accurately. The finding
during stoping of additional ore in the form of bulges in the lode or
offshoots therefrom, inclusions of waste within the ore body, the
sorting of waste from the broken ore, and the admixture with the ore,
from sloughing, of waste and of capping or walls containing some ore
minerals may combine to obscure the true situation with regard to
recovery and dilution. Sometimes low-grade ore not included in the
original estimates is found in mining under conditions such that it is
profitable to extract it, and, on the other hand, low metal prices may
necessitate leaving considerable ore that would be mined under higher
prices.
 It thus appears that “dilution” and “recovery” as applied to stoping
results are not specific or rigid terms, and the data on recovery and
dilution in table 37 should be interpreted with this in mind. In circulars
dealing with cut-and-fill and square-set stoping, the authors have
usually stated or implied that complete recovery is obtained and no
figures are given; this accounts for the small amount of data on these
methods of stoping in the table.
@Hassan Harraz 2019
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@Hassan Harraz 2019
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Rate of Extraction
 Some stoping methods are inherently slow, whereas others permit rapid
extraction of ore from the stope and, as previously stated, permit flexibility
in rate of output. Thus,
 In cut-and-fill stoping the production rate cannot be forced very much
as ore-breaking must be stopped in sections being filled, although
production from an entire stope may be constant by breaking on one side
of the ore pass while filling on the other side.
 In square-set stoping production, rate is slow (1) because the ore is
broken in small blocks that usually can be drilled off in a short time,
whereas timbering and filling consume a large part of the working shift
and (2) because production cannot be forced in heavy ground without
inviting danger of collapsing the stope.
 In open stopes where there are large areas of stope faces, production
often can be forced by simply putting more machines to work.
 In sublevel stoping production can be speeded by simultaneous
operations on all the sublevels and often by increasing the number of
machines on each bench.
 Forced caving, as employed at Alaska-Juneau, Beatson, and Climax, and
block caving-all Nonselective Methods-permit a high production rate once
ore production begins, although considerable time is consumed in
preparatory work, especially for block caving. Best results are obtained with
block caving if the ore is drawn at a uniform rate; therefore, any forcing of
this rate is likely to cause the piping through of waste and incomplete
breaking up of the ore.
 The rate of output possible with any stoping method depends to a large
extent upon the size of the ore body, especially the width, and the width of
the stopes; a high rate of output is impossible in narrow stopes.
@Hassan Harraz 2019
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Effect on Recovery in Ore-Dressing Plant
 The stoping method may affect the percentage of ore minerals
recovered in the ore-dressing plant, especially where certain sulfide
ores are concentrated by flotation. Thus, a stoping method that
permits immediate removal of the ore after it is broken so as to
preclude its partial oxidation before it goes to the mill may be
desirable or necessary. Under these conditions, shrinkage stoping
and block caving, in which large masses of broken ore are
necessarily left in the mine for a considerable length of time, may be
unsuitable.
 In open-stope mines the greatest loss of ore is that tied up in pillars
left for support of the back or hanging wall. The amount of ore thus
left varies considerably with the strength of ore and wall rocks,
thickness of the ore, its depth below surface, and the requirements
for permanent support to prevent movement and subsidence of
overlying strata and of the surface. Where eventual subsidence is
permissible, much of the pillar ore often can be recovered on the
retreat by robbing operations, the amount recoverable depending to
a considerable extent on roof characteristics; sometimes almost
complete extraction is possible. Where it is imperative to prevent
surface subsidence and resultant damage to surface improvements
or to prevent an influx of surface or underground water, 20 to 40
percent of the ore in the deposit may have to be left as pillars. Aside
from ore left in pillars, high extraction is usually possible in open
stopes, since the stope boundaries are exposed to view and all
showings of ore can be attacked and mined.
@Hassan Harraz 2019
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References
Canadian Securities Administrators (2012). National Instrument 43-101 Standards of
Disclosure for Mineral Projects (NI 43-101).
Crawford, G. (2004). Dilution and Ore Recovery. [Online]. Available:
http://docslide.us/documents/mining-dilution.html#. [Accessed 7 February 2017]
Crawford, G.D. (2004). Pincock Allen and Holt, Pincock perspectives, Dilution and ore
Recovery, issue NO. 60
Darling, P. (2011). "SME Mining Engineering Handbook (3rd Edition) - 13.7.2.6 Layouts.
Society for Mining, Metallurgy, and Exploration (SME).," Society for Mining, Metallurgy,
and Exploration.
DeSouza, E. (2010). Underground Mining. Kingston, Ontario, Canada,
Ebrahimi, A. (2013). The Importance of Dilution Factor for Open Pit Mining Projects. [Online].
Available: http://www.srk.com/files/File/papers/dilution_factor_openpit_a_ebrahimi.pdf.
[Accessed 7 February 2017].
Jakubec, J. (2001)."Underground Mining Methods SME," 2001. [Online]. [Accessed 7 February
2017].
Pakalnis, R., Poulin, R.; Hadjieorgiou, J.(1995). Quantifying the Cost of Dilution in
Underground Mines, SME Annual Metallurgy and Exploration, Denver.
Parker, H. M. (2012). Reconciliation principles for the mining industry. Published by Maney on
behalf of the Institute of Materials, Minerals and Metallurgy and the AusIMM,
maneypublishing.com.
Pareja, L. (2000). "Deep Underground Hard-Rock Mining: Issues, Strategies, and
Alternatives," April 2000. [Online]. Available:
http://www.collectionscanada.gc.ca/obj/s4/f2/dsk1/tape4/PQDD_0011/NQ52844.pdf.
[Accessed 7 February 2017].
Scoble, M. J.; Moss, A. (1994). Dilution in underground bulk mining: implications for
production management; in Whateley, M. K. G., and P. K.
Sinclair, A.; Blackwell, G. (2002), Applied Mineral Inventory Estimation, Cambridge University
Press.
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