ASSESSING
THE SUBAQUEOUS STABILITY OF OXIDIZED WASTE ROCK
Mine Environment Neutral Drainage at CANMET-MMSL |
MEND Report
2.36.3
April 1999
EXECUTIVE
SUMMARY
Waste
rock is typically stored in a subaerial environment, a setting that
may promote the oxidation of sulphide minerals and therefore be
conducive to the initiation of acid rock drainage (ARD) and commensurate
trace metal release. To mitigate this problem several strategies
are currently being employed and tested by the mining industry including
the subaqueous disposal of sulphide-rich waste rock. Subaqueous
disposal has a number of features that make it attractive as a long-term
storage option. However, the secondary mineral assemblages that
accumulate during subaerial exposure could have a profound influence
on the geochemical behaviour of the waste when submerged, such that
deleterious effects on water quality may result. In order to assess
adequately the environmental implications of placing oxidized waste
rock underwater, techniques must be developed to allow proponents
and government agencies to evaluate scientifically, and ultimately
predict, the potential water quality impacts of this waste rock
management strategy.
The ultimate
objective of this project was to design a laboratory test protocol
that could be used to quantify the chemical stability of waste rock
oxidation products in a range of subaqueous environments. To this
objective, the report first examines the mechanisms that control
the formation and stability of secondary minerals in waste rock
dumps, identifies the minerals that may be present in the dumps
and evaluates their subsequent stability in a subaqueous setting.
Second, the report examines available laboratory methods and their
applicability to assessing metal release from oxidized waste rock.
The extent
of oxidation is spatially variable in a subaerial waste rock dump;
hence the distribution of weathering products is heterogeneous.
The climate and physical characteristics of a waste rock dump indirectly
control the extent of these weathering products by regulating the
intensity of various physico-chemical conditions such as pH, temperature,
dump hydrology and mineral weathering. A general indication of the
mineralogical form taken by metal precipitates can be inferred from
the site-specific physico-chemical conditions and the elemental
constituents of the waste rock. An indication of which elements
will be retained as major or minor components of secondary minerals
in oxidized waste rock can be obtained by determining the elements
associated with various types of ore deposits and their relative
mobilities. However, iron and sulphate secondary minerals are common
in all oxidized sulphide-rich waste rock dumps and exert a major
control on the mobility of other less abundant elements.
The subaqueous
stability of a mineral is controlled by the thermodynamic and kinetic
properties of the mineral which are in turn influenced by variable
characteristics such as surface area, crystallinity, solution chemistry
and temperature. Four general mineral dissolution categories have
been examined including: water soluble; pH sensitive; reducible;
and oxidizable. Water soluble precipitates found in waste rock dumps
are typically hydrated sulphate minerals and many of these minerals
produce acidity upon their dissolution. Carbonate minerals and adsorbed
cations are extremely pH sensitive and are soluble in acidic solutions.
Mature oxyhydroxide and sulphate minerals are typically insoluble
in oxic waters but are susceptible to reductive dissolution. Organic
matter and sulphide minerals, which are susceptible to oxidation
have also been examined, although it is acknowledged that subaqueous
storage minimizes sulphide oxidation.
A number of
test methods were examined to evaluate the subaqueous stability
of oxidized waste rock including: partial extractions; sequential
extractions; shake flask; column; and tank tests. Sequential extraction
tests are best suited for use as an initial screening tool for the
evaluation of metal release from subaqueous waste rock. Shake flask,
column and tank tests have been used to determine the subaqueous
stability of oxidation products. However, they do not discriminate
between the geochemical processes or mineral phases responsible
for the release of metals to solution. Partial extraction tests
are limited to simulating one environmental condition and thus one
standard test cannot be applied to examine a range of receiving
environments. In contrast, sequential extraction can be used to
evaluate metal release for a range of extreme environmental conditions.
The application of sequential extraction methods to assess subaqueous
stability of oxidized waste rock is comparable to the use of acid-base
accounting (ABA) to assess the acid generating characteristics of
subaerially-exposed waste rock. Extractions are more powerful, however,
because they provide data on the element-phase associations which
are lacking in ABA testing.
A four step
sequential extraction scheme is proposed to examine metal partitioning
in oxidized waste rock by targeting the following four phases:
- F1: water
soluble (e.g., hydrated sulphates);
- F2: exchangeable/adsorbed/bound
to carbonates;
- F3: total
reducible (e.g., oxyhydroxides); and
- F4: total
oxidizable (e.g., sulphides and organic matter).
In order to
determine the elemental composition of the sample prior and subsequent
to the application of the proposed extraction scheme, two analytical
techniques have been considered. These are XRF and acid digestion
followed by ICP-MS analysis. The applicability of these techniques
should be assessed during the verification and standardization phase
for the proposed extraction procedure, with the optimum method being
chosen at that stage. Determination of whole rock elemental abundances
in the initial and residual fractions is critical for the interpretation
of results from the proposed extraction procedure.
It is of utmost
importance to note that sequential extractions are "operationally-defined",
therefore an extraction procedure that provides an accurate representation
of metal partitioning in one type of sample (i.e., soil)
may not be effective in another (i.e., waste rock). Thus,
prior to utilizing sequential extractions directly for assessing
metal release from oxidized waste rock, verification and validation
of the proposed procedure is required.
The proposed
sequential extraction method can be used as an effective tool to
assess metal-phase associations in waste rock when more direct methods
(e.g., SEM, XRD, etc.) become to expensive and time
consuming due to the fine grained/amorphous nature of many secondary
minerals. Although the extraction alone cannot predict quantitative
water quality impacts due to kinetic controls on mineral dissolution,
when combined with kinetic or in-situ testing it is an effective
tool to assess environmental risk associated with the subaqueous
disposal of oxidized waste rock.
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