Static tests identify the geologic units at a site that may have the potential to generate net acidity. Geochemical kinetic tests involve weathering (under laboratory-controlled or on-site conditions) samples of these units in order to confirm the potential to generate net acidity, determining the rates of acid generation, sulphide oxidation, neutralization, and metal depletion and test control/ treatment techniques. This information is critical because, for example, the rate of acid generation may be negligible or, in extremely rare cases, may be severe for only a short period of time so that long-term control or treatment techniques may not be necessary. Based on the results of kinetic tests, the optimization of treatment and control techniques to address the specific severity and duration of acid drainage from a geologic unit will minimize the overall costs of acid-generation abatement.
Whereas static tests provide some information on overall potential acid generation independent of time, kinetic tests explicitly define reaction rates through time under specific conditions. As a result, kinetic tests are significantly more expensive and continue for months or years. Laboratory kinetic tests conducted in the short term only provide semiquantitative information on drainage water quality because they do not reproduce site conditions. In order to provide quantitative data on water quality at the site, waste material test pads can be monitored for several years. Ultimately, true prediction of long-term drainage quality will only be possible through quantitative mathematical models that can reliably extrapolate results beyond the time of the tests.
The initial step in a kinetic test is the definition of material characteristics in addition to those measured in static tests, specifically surface area, mineralogy, and total metals. These characteristics are important to the interpretation of the results from kinetic tests as they can affect the acid- generation process or overall water quality.
The particle size of a material can affect acid production and acid-consumption results. Smaller grain-sized materials have a greater surface area per unit weight and a greater density of broken crystal bonds.
The mineralogy of a sample may also be directly related to reaction rates. Both the chemistry and crystal form of the minerals in a sample control the rates of acid generation and neutralization. For example, poorly crystalline minerals react faster than their crystalline counterparts, and some sulphide minerals oxidize faster than others. Additionally, the mineralogy of a sample may determine the metals that could be leached during acid generation and the extent to which pH may be neutralized by the sample.
Total metal analysis assists in the evaluation of the water quality from the tests. First, total metal analysis indicates any metals present in high levels that may warrant attention. Second, the leaching rates of a metal when compared to the total metal content will suggest when a metal may be depleted within the sample, resulting in negligible leach concentrations, even though it is difficult to extrapolate laboratory test concentrations to field leaching conditions.
Once the material characteristics have been determined, which include the sulphur and carbonate content determined during static tests, the overall programme objectives must be defined before the selection of a kinetic test. The programme objectives should be based on the mine plan and the proposed handling of acid-generating rock. Programme objectives could include one or more of the following:
- • Selection or confirmation of disposal options
- • Determination of the overall water quality impact
- • Determination of the effect of the flushing rates through a sample on water quality
- • Determination of the influence of bacteria on the acid-generation sample
Kinetic tests are selected for each acid-generating component based on the information required to meet programme objectives.
Both small-scale controlled tests (e.g., humidity cells) and large-scale on-site weathering trials have been used in assessing acid-generation reactions. The controlled tests have the advantage of simulating specific climatic and weathering conditions. On-site tests may be considered more representative than controlled tests because of the natural conditions under which the tests are conducted; however, because results vary as climatic conditions change, the interpretation and extrapolation of the test results are more complicated.
The data from kinetic tests are evaluated to define the rate and temporal variation of acid generation and water quality of a sample or a treatment/control technique. The results are assessed to determine if they are environmentally acceptable with respect to the proposed mine plan. For example, if the proposal is made to mix waste rock with limestone and the tests indicated that acidic drainage occurred in a kinetic test, then the results would not be environmentally acceptable.
If the results are not environmentally acceptable, then the mine plan and the programme objectives must be redefined. The mine plan must be redefined to ensure that the appropriate acid-generation control and treatment techniques are used. The programme objectives may have to be redefined to incorporate the changes in the mine plan and to test for additional appropriate acid-generation control and treatment techniques.
Additional tests would not be necessary if the existing data (through extrapolation) were sufficient to evaluate environmental acceptability concerning the new mine plan. If the existing data are not sufficient, then additional kinetic tests should be conducted to meet the new objectives.
When the results are environmentally acceptable, experimental results can be extrapolated to other conditions or into the future using mathematical models.