It is a truth in geochemistry, as it is in life, that some things are better left buried – especially when it comes to sulfide-bearing minerals in rocks. The sulfides in these rocks, when exposed to air and water, usually by mining activity, break down to release acid that can mobilise trace elements such as arsenic, zinc, nickel, boron, copper and others from waste rock and tailings into ground and surface water. This can contaminate drinking water sources and have a harmful effect on plants and aquatic animals, even miles downstream from where the mining occurs.
An ancient problem
This process, called acid mine drainage (AMD) or acid rock drainage (ARD), is an age-old problem, with some Roman mining sites reportedly continuing to generate acid drainage some 2000 years after mining has ceased. Around the world, untreated AMD can affect thousands of kilometres of waterways and is currently the largest environmental problem facing the mining industry.
Researchers, particularly in the last few decades, have been making great strides in how to deal with the problem, but sometimes treating the problem can carry its own environmental costs. A team of researchers from the South Dakota School of Mines and Technology in the US and New Zealand’s University of Canterbury have been critically examining the various treatment options for the global coal industry by extensively reviewing (using a life cycle assessment technique) the active and passive treatment systems currently used at the Stockton coal mine in New Zealand, as well other treatment methods, to determine the environmental impacts of each treatment methodology per kilogram of acidity removed.
Different types of treatments
The study investigated five passive treatments and two active treatments. Active treatments generally include chemically dosing with lime, applied as calcium oxide (CaO) or as a slurry of hydrated calcium hydroxide (Ca(OH)2) to neutralise acidity, resulting in precipitation of metals, and a pumping system to move the AMD through. While active treatment is a proven and reliable AMD mitigation approach, their high energy and chemical costs result in high net environmental impacts.
Passive treatments generally are gravity fed and use biogeochemical processes within engineered biosystems. The researchers found that bioreactors using local waste – like mussel shells or wood byproducts (which can neutralise the acid and precipitate out trace elements) – and using gravity instead of pumping to move the contaminated water through the system produced the biggest improvements in environmental impact.
Passive treatments operate more sustainably
The results indicated that passive treatments generally had lower overall environmental impacts compared to active treatment technologies because they do not require continual pumping of chemical amendments, they incurred minimal transport distances of the recycled materials such as mussel shells or limestone and their use or the use of materials requiring a lesser degree of (pre)processing provided enhanced environmental benefits – all factors that allow passive systems to operate more sustainably.
The authors wrote in their published paper, “It is unlikely that large-scale mining operations would rely solely on passive treatment for AMD mitigation, as effective treatment systems rely on a number of site-specific factors such as required footprint, land availability, topography, AMD discharge (and chemical signature) and operational temperatures (impacting treatment efficiencies). However, one should consider [a] combination of both active and passive treatment systems to provide a balance between meeting operational AMD-treatment requirements and lowering environmental impacts compared to the sole consideration of active treatment. While this study focused specifically on conditions at the Stockton Coal Mine, several general recommendations could be inferred from our study results.
“Gravity-fed, passive treatments result in lower environmental impacts compared to active treatment systems, with the important caveat of increased process footprint requirement. Important design considerations for ‘sustainable’ AMD treatment should include utilizing materials with a reduced degree of processing, sourcing local materials, and minimizing pumping energy.”
Verifying assumptions about passive treatment
Environmental scientist and head of AMD research at CRL Energy Dave Trumm said he is impressed by the study. “Most AMD researchers have always assumed that the environmental impacts of passive treatment are less than for active treatment. In fact, that is partly why passive treatment came about so many decades ago. This study just goes through the exercise to actually document the environmental impacts of different treatment options. And yes, in our experience, we find similar results.”
Mr Trumm was part of a research team who, late in 2012, published a paper on removing highly toxic antimony from mine drainage through formation and precipitation as the mineral stibnite – the documented method is believed to be a world first.
This research was published in 2012 in the journal Resources, Conservation & Recycling.
In this activity, Site clean up, students investigate the clean up of contaminated sites around New Zealand.
Visit the Solid Energy website to learn more about the Stockton Opencast Mine and other coal mining operations in New Zealand.
Hengen, T.J., Squillace, M.K., O’Sullivan, A.D. and Stone, J.J. (2014). Life cycle assessment analysis of active and passive acid mine drainage treatment technologies. Resources, Conservation & Recycling. In press, published online ahead of print 12 February 2014. http://dx.doi.org/10.1016/j.resconrec.2014.01.003