Reliability improvement for predicting acid-forming potential of rock samples using static tests

Abstract

In predicting the acid-forming potential of rock samples, a combination of acid-base accounting (ABA) and net acid generation (NAG) tests has been commonly used. While simple and economical, this method sometimes shows low reliability such as categorizing certain samples as uncertain (UC). ABA and NAG tests were modified to selectively recover valid minerals in nature and substituted for the original tests. ABA test overestimated acid-producing capacity (in the case of weathered samples) and acid-neutralizing capacity (in the case of plagioclase-including samples) compared to the modified ABA test. NAG test yielded lower NAG pH compared to modified NAG test for samples with high total C content and low total S content. By comparing the correlation coefficients between acid generation amounts by the two evaluation methods, it was confirmed that modified evaluation method (MEM) has a much higher reliability (R = 0.9582) than existing evaluation method (EEM) (R = 0.5873). It was also concluded that exploiting advantages of both EEM and MEM is recommended where EEM is initially applied for general classification and a supplemented static test of MEM is executed for the purpose of correcting the error of UC categorized samples.

Natural oxidation of coal tailings from Middelburg area (South Africa), and impact on acid-generation potential

“…As seen in Table V, S1, S2, S3, and S4 have positive NAPP and NAGpH values below 4.5 and are subsequently classified as PAF according to the criteria stipulated by Oh et al (2017) and Smart et al (2002. The relatively high NAPP values suggest that a large quantity of acid will be generated.…”

“…The effective ANC values obtained from the ABCC at a pH of 4 and MPA based on the pyrrhotite content (MPA min ) were used to recalculate the NAPP values of S1, S2, S3, and S4 (Table IX). The recalculated NAPP and NAGpH were used to re-evaluate the acidgeneration capabilities of S1, S2, S3, and S4 in order to reclassify the acid-generating capabilities according to the ABA and NAGpH criteria used by Oh et al (2017) and Smart et al (2002).…”

“…Most common evaluation methods are either kinetic or static, depending on the procedure of the experiment. A static test only predicts the acid-forming potential but does not provide an indication of the rate or duration of acid generation (Oh et al, 2017). Kinetic tests attempt to mimic the natural oxidation reactions and conditions that occur in the environment (Oh et al, 2017).…”

“…A static test only predicts the acid-forming potential but does not provide an indication of the rate or duration of acid generation (Oh et al, 2017). Kinetic tests attempt to mimic the natural oxidation reactions and conditions that occur in the environment (Oh et al, 2017). Kinetic test results are credible and provide information on parameters related to the rate of acid generation, sulphide reactivity, metal solubility.…”

“…However, kinetic tests such as the leach column tests are extremely time-consuming and expensive. Static tests are relatively inexpensive and conducted relatively quickly (Smart et al, 2002;Oh et al, 2017).…”