Geochemical effects of oxidation products and framboidal pyrite oxidation in acid mine drainage prediction techniques |
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| Authors: | P. A. Weber W. A. Stewart W. M. Skinner C. G. Weisener J. E. Thomas R. St. C. Smart |
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| Affiliation: | a Ian Wark Research Institute, University of South Australia, Mawson Lakes, Adelaide, SA 5095, Australia;b Environmental Geochemistry International Pty Ltd, 81A College Street, Balmain, NSW 2041, Australia |
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| Abstract: | Acid mine drainage predictive testwork associated with the Australian Mineral Industries Research Association (AMIRA) P387A Project: Prediction and Kinetic Control of Acid Mine Drainage (AMD) has critically examined static acid assessment and kinetic information from acid–base accounting techniques, including net acid production potential (NAPP), net acid generation (NAG) and column leach tests. This paper compares results on two waste rock samples that were obtained from the Kaltim Prima Coal mine (KPC) containing significant quantities of fine-grained framboidal pyrite. In agreement with other research, the authors' results indicated that framboidal pyrite is more reactive than euhedral forms due to the greater specific surface area of framboidal pyrite. This is evidenced by optical microscopy of reacted samples. Importantly, the results showed that NAPP testing is biased by the rapid acid generating oxidation of framboidal pyrite prior to, and during the acid neutralisation capacity (ANC) test. This can result in negative ANC values for samples containing significant framboidal pyrite (often “corrected” to zero kg H2SO4/t) when significant ANC is actually present in the sample. NAG testing using H2O2 indicated that samples containing a significant quantity of framboidal pyrite can result in the catalytic decomposition of the H2O2 prior to complete oxidation of the sulfide minerals present, requiring sequential addition of H2O2 for completion. A benefit of the NAG test, however, is that it assesses the net acid generation capacity of the sample without bias towards acid generation as is observed using NAPP methods. The kinetic NAG test also gives information on the reaction sequence of framboidal and euhedral pyrite. Periodic (kinetic) analysis of sub-samples from column leach tests indicated rapid oxidation of the framboidal pyrite compared to the euhedral pyrite, which was correlated with the greater framboidal pyrite surface area.Calculations to determine the sulfide/sulfate acidity derived from the oxidation of framboidal pyrite prior to; and during the ANC test have been developed to provide a better indication of the actual ANC (ANCActual) of the sample. Paste pH values of
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