Development of a CO2 accounting methodology for acid mine drainage active treatment plants: Evaluation of CO2 emissions from real plants in Japan

Abstract

Mining activities generate acid mine drainage (AMD), which requires treatment to prevent contamination and acidification of aquatic environments. Active treatment is widely applied because of its high efficiency and operational capacity; however, its substantial energy and chemical demands can lead to high CO₂ emissions. This study establishes detailed and simplified methods for estimating CO₂ emissions from AMD active treatment plants. Unlike existing approaches, the proposed method enables evaluation of carbon transformations among different carbon species throughout the treatment processes, allowing a more comprehensive carbon-flow assessment. CO₂ emissions from four full-scale AMD active-treatment plants in Japan were evaluated, and carbon flows during treatment were analyzed. The results showed that direct and indirect emissions varied among the plants due to differences in neutralizers. On average (except plant D), neutralizer-derived CO₂ accounted for 61% of total emissions, electricity-related emissions for 36%, and other sources for 3%, highlighting the dominant influence of neutralizer selection. Major contributors were the use of CaCO₃ as the neutralizer (through both its reaction with AMD and its calcination-derived Ca(OH)₂ supply) and electricity consumption. Across all plants, part of the inorganic carbon was converted into organic carbon, and plants using Ca(OH)₂ absorbed atmospheric CO₂ during treatment. The error between detailed and simplified estimates was small in CaCO₃-based systems and larger in Ca(OH)₂-based systems.

Keywords: Acid mine drainage; Active treatment; CO(2) emission; Carbon flow; Carbon neutrality.