Abstract:

Acid rock drainage (ARD) is a risk to water quality at mines that are associated with waste materials containing sulphide, such as tailings. The classification of mine materials as potentially acid generating (PAG) or non-potentially acid generating (non-PAG) is typically based on the ratio of effective neutralization potential to acid potential (NP/AP) in a sample as well as the rate at which the neutralization potential (NP) is consumed during neutralization of the acid produced by the oxidation of the sulphide minerals, representing the acid potential (AP). While a conservative NP/AP ratio (NPR) of 2 or 3 is typically recommended to account for some uncertainty in the neutralizing reactions (Price 2009), site‑specific values can be developed and justified based on appropriate evidence. The NPR for a mine material is not only important in the classification as PAG or non‑PAG, the value can also represent the ratio of the NP consumed relative to the AP reacted during sulphide mineral oxidation and accompanying neutralization reactions. The NP/AP consumption ratio is required to model NP consumption rates and to forecast the time to the onset of acid generation in PAG materials and as such, this ratio is important for mine material management. The methodology in this study implicitly determines the effectiveness of the NP at a case study mine site and applies this information to establish the non-PAG boundary in a sulphidic tailings and the approach can be applied at other sites. Lab-scale and field-scale static and kinetic tests results were assessed to evaluate a site‑specific NP/AP consumption ratio. The data were screened to ensure that the interpretation of the carbonate molar ratio (CMR) was represented in tests with the appropriate geochemical conditions. Data from the tests indicated that the average CMR from the applicable tests for sulphide tailings was 1.07, and a conservative upper‑bound value of 1.2, representing the highest average value identified in one of the kinetic tests, was selected to conservatively represent the NP/AP consumption ratio for site tailings. This representative value was proposed and accepted as a permit criterion to define non-PAG and the value was then applied in long-term oxidation modelling to forecast tailings oxidation risks of acidification and water quality effects through operations and post-closure.

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Ron Nicholson