New ARD / AMD Prevention Technology
In the battle to manage ARD/AMD, significant technological breakthroughs are few and far between. At the 9th Australian AMD Workshop in Burnie (November, 2017), Earth Systems announced a major advance in the prevention of ARD / AMD (acid and metalliferous drainage) from underground mines. The technology is applied in a two-stage process. Stage 1 involves engineering works that do not affect mine water discharge rates or water levels, but provide a primary control on air-entry into mine voids. Lowering air resupply into mine voids to levels that are below sulfide oxidation rates results in decreases in pollution production. Every cubic metre of oxygen consumed by sulfide oxidation within a mine void is replaced by a cubic metre of air, which only contains ~21 volume % oxygen. Hence if Stage 1 works are comprehensive, mine void oxygen reduction and nitrogen enrichment is inevitable.
Extensive monitoring from case study sites demonstrates that mine voids breathe in a process we refer to as barometric pumping. Voids “inhale” with the progress of external “high-pressure” weather systems, and “exhale” with atmospheric pressure drops associated with “low-pressure” fronts. While acid generation with a void can be fuelled during increasing atmospheric pressure scenarios, sulfide oxidation is unlikely to be related to the exhalation of nitrogen-rich gas from mine voids during decreasing external pressure scenarios. Consequently, effective Stage 1 works should be able to routinely halve acid generation processes.
To overcome the effects of barometric pumping during the passage of high pressure systems, Stage 2 works require the installation of inert gas injection systems. Such systems need to be configured to provide very small internal gas overpressures to counteract air entry due to external (climatic) pressure increases. In this way, inert gas can be used to prevent sulfide oxidation within mine voids.
Stage 1 inert atmosphere installations have been completed at two decommissioned metal mines in New South Wales (NSW), Australia, and monitoring is ongoing. These rehabilitation works were funded by the NSW State Government Legacy Mine Program. One site has been lowered by 50% and a 70% reduction has been recorded at the other site after less than 12 months. The case studies have provided unambiguous proof of concept, and the improvement at one site has far exceeded expectations.
High priority targets for application of this technology are legacy underground mines that are currently committed to treating in perpetuity. Close to 200 sites in this category have been identified worldwide. Dramatic reductions in treatment costs are the minimum expectation from this new remedial approach. Additional benefits include avoiding sludge production, lowering public safety risk and the ability to rapidly reinstate a site to a minable status.
For additional information on this method, please contact Dr Jeff Taylor at Earth Systems ([email protected] or +61 402 158 682).