Static Tests

A static test defines the balance between potentially acid-generating minerals (potential acidity) and acid-neutralizing minerals (neutralization potential) in a sample. In particular, acid-generating compounds include reactive sulphide minerals and acid-neutralizing compounds include carbonate minerals. A sample will theoretically generate net acidity at some point only if the potential acidity exceeds the neutralization potential (NP); otherwise the sample will not produce net acidity as long as the NP is not dissolved more quickly than the generation of acidity.

Despite the theoretical simplicity, static tests cannot be used to predict the quality of drainage emanating from waste materials at any future time. Acid-generation processes and therefore drainage quality are time dependent and functions of a large number of complex factors such as mineralogy, rock structure, and climate. For this reason, static tests should be treated as a qualitative predictive method; i.e., they can only indicate whether potential exists for the generation of net acidity at some unknown time.

Several types of static tests are available such as acid-base accounting and APP: sulphur ratio. However, all of these tests are simply variations on a basic procedure and all require variations of the same basic analyses for determining the balance between potential acidity (AP) and NP. Consequently, the basic, common procedure will be presented and the names of the variations will be deemphasized.

The initial step in defining the acid-generating/acid-neutralizing balance in a sample begins with a measurement of total sulphur in a sample, commonly performed with a Leco furnace/analyzer. The measurement of total sulphur allows the calculation of “maximum potential acidity,” which may overestimate the potential for acid generation if all sulphur in a sample is not acid generating. Therefore, additional analyses may be performed to refine the potential acidity. The analyses, which have not yet proven to be as reliable as total sulphur, are:

  • • Sulphur species, which defines short-term leachable sulphate and leachable sulphide using acid extractions
  • • Reactive sulphur, which defines short-term oxidizable sulphide using hydrogen peroxide

The unproven nature of these additional analyses makes them options in a static test.

Following the delineation of AP, the next parameter, NP, is defined. The measurement of NP provides a gross value for neutralization; however, this value may overestimate the capacity of the sample to neutralize the pH to an environmentally acceptable level above 6. An analysis of carbonate content will provide a more meaningful measure of NP from the perspective of pH neutralization. The carbonate analysis is recommended as an optional portion of static tests.

Paste pH is measured in a paste formed by water and the ground sample. The pH value will indicate the immediate reactivity of neutralizing minerals in the sample and will indicate whether significant acid generation occurred prior to the measurement.

Following these analyses, the potential for net acidity is calculated by subtracting AP from NP with a negative value indicating the potential for net acidity. Alternatively, a ratio of NP to AP can be used (APP: sulphur ratio), but the subtraction method (acid-base accounting) is adopted here.

The subtraction of maximum AP (based on total sulphur) from the gross NP yields the “net neutralization potential” (NETNP). Theoretically, a sample can be expected to generate net acidity at some point if the NETNP is <0. However, based on general experience, values of NETNP in the range of -20 and +20 tons of СаСОЗ/1,000 tons of sample (-2 to +2% CaCO,) may be considered to have the ability to generate net acidity. This range of uncertainty is attributed to the sources of error in:

  • • Obtaining the objective of defining true potential acidity and neutralization
  • • Converting total sulphur to acidity using a restricted conversion factor
  • • Analysis

The subtraction of AP (based on reactive sulphide) from carbonate content yields the “net neutralization potential from species” (NETNP(S)). This value will presumably reflect the actual NETNP due to the narrower range of uncertainty and, thus, provide more reliable predictions, although no database exists to confirm these conclusions. The primary sources of error are similar to those for the NETNP (above), except that estimating long-term reactive sulphide from a short-term test may result in some uncertainty.

In the event the samples from a geologic unit indicate the unit has or may have the potential for net acid generation, kinetic tests should be conducted.

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