Geoscience Reference
In-Depth Information
7
Mining Dilution
Abstract
Dilution is a critical issue that affects many aspects in mining. It is generally due to the geo-
metric characteristics of the ore body, the mining operation, the characteristics of geologic
contacts, and the limitations of the mining equipment to recover material to the desired
boundaries or contacts. There are three types of dilution that need to be considered at the
time of mineral resource estimation. The dilution due to geologic contacts and the dilution
due to the mixing of material types within a block are best tackled by geologists and re-
source estimators at the time of modeling. Operational dilution is generally planned for by
the mining engineer at the time of developing a mine plan, but it also occurs unexpectedly,
and is called unplanned dilution.
7.1
Recoverable Versus In-Situ Resources
considered, different types of cutoffs are used. At the break-
even point, Revenue in Eq. 7.1 is zero, and the correspond-
ing economic cutoff grade is:
The objective of the resource model is to predict the ton-
nage and grade that the beneficiation plant will receive at
specified time intervals. This is true at all times in a min-
ing operation: at the initial evaluation of the project, as part
of pre-feasibility and feasibility studies, and in the context
of long-term and short-term resource models in operating
mines. The procedures for estimating and managing dilution
need to be updated regularly to capture all the new informa-
tion and experience collected as the deposit is being mined.
A model that attempts to satisfy this requirement is called a
“recoverable model” (David
1977
; Journel and Huijbregts
1978
; Rossi and Parker
1993
).
A recoverable resource model is an estimate of the tonnage
and grade of economic material above certain cutoffs, but
can also include other geo-metallurgical and geo-mechanical
characteristics that affect mill performance. Revenue is a
function of grades, product prices, metallurgical recoveries,
and operating costs such as mining, metallurgical, and gen-
eral and administration (G&A) costs:
Economic Cutof Grade
Mining
+
Metallurgical G &A Costs
Price * Recovery
+
(7.2)
=
Costs are usually expressed on a per unit basis, such as dol-
lars per ton. The units used in the calculation have to be con-
sistent, which often requires conversion factors.
Another important cutoff in an open pit mining operation
is the
marginal cutoff
, similar to the economic cutoff, except
that the mining cost is not considered. This is to fairly valu-
ate the rock when mining is progressing and the material has
to be mined. The only decision is where to send it, the mill, a
stockpile, or the waste dump. The mining cost must be spent
and is considered a sunk cost. The marginal cutoff is used for
example in grade control, as discussed in Chap. 13.
Cutoff calculations become complex if there are several
metals to consider, each with different metallurgical recover-
ies and costs. Also, there may be different mining costs as-
sociated with sending material to the mill, as opposed to the
waste dumps or stockpiles. In the case of stockpiles, re-han-
dling costs should also be considered. Finally, G&A costs are
a mixture of costs, not all of them directly related to the op-
eration. Mining companies have different policies for which
Revenue
=
Price * Recovery * Grade(s)
−
(Mining Cost
+
Metallurgical Costs
+
G & A Costs)
(7.1)
The grade for which revenue is nil is called the
break even
(or economic) cutoff grade
. Depending on which costs are
