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.
