Mineral acid is the liquid discharge that commonly flows from metal and coal mines, including abandoned mines, coal processing sites, tailings or ponds, and rock dumps. It often contains toxic metals such as copper, iron, zinc, and nickel, which, combined with a reduced pH level, have a hazardous effect on rivers and streams. In response to this undesirable situation, scientists working with stakeholders have been inventing ways to extract mineral acid in efforts to reduce the environmental impact.
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Why Does Mineral Acid Form?
Mining usually takes place underground below the water table and requires water to be pumped out of mines continuously to prevent flooding.
When mines are abandoned, water floods the mines again. Oxidization of the metal sulfides occurs within the surrounding rock and creates acidity. The discharge is commonly known as acid mine drainage (AMD).
Microorganisms and bacteria also act on metal ions to accelerate the process. AMD can discolor water and smother aquatic plant life and macro-invertebrates. It can kill aquatic and semi-aquatic species, such as fish and salamanders, which are often sensitive to changes in pH levels or die because of increased stress and disease from heavy metal exposure.
Economic Shift Towards Environmental Protection
Dealing with mining waste is notoriously expensive, so historically, disposing of mining waste in pool and dam systems known as tailings, has been largely left as the economic preference to reduce mining operation costs.
Tailings tend to cause massive environmental damage, including soil and water contamination due to their unstable nature. The creation of tailings has been prioritized over both environmental concerns and indigenous population protection, who depend on streams and rivers for what is often their only clean water source.
However, this situation has been slowly changing. Society has become more environmentally aware, and many governments have shifted towards creating stronger environmental legislation, including targets and international treaties for tackling pollution, with a strong focus on tackling climate change.
Net-Zero Ambitions Hindered by Demand for Increased Mining
The 2015 Paris Climate Agreement is a legally binding international treaty with a goal to limit warming to below 2 ˚C, preferably 1.5˚C, pre-industrial levels.
One of the ways to achieve this is for countries to become carbon net-zero, which requires both economic and social transformation. However, this transition increases demand for electrification of vehicles and renewable energy, both of which require more metals from mining. Mining requires burning fossil fuels to operate mining equipment. With more mining comes increased pollution and subsequently acid mine drainage.
It is therefore essential that solutions are sought for more sustainable ways of mining and dealing with waste, whilst also delivering economic benefits. The drive for carbon net-zero through electrification may become unsustainable if better solutions are not found.
Methods for Dealing with Acid Mine Drainage
The most common way for treating acid mine drainage is lime, a natural source of calcium carbonate. This is done by mixing the two together in a tank, which raises the pH so that toxic metals become insoluble and precipitate.
The resulting slurry is put in a clarifier to settle, and the overlying clean water is released. The remaining sludge is recycled in an acid mine drainage treatment tank or a settling pond.
This method is relatively economical, but is not particularly efficient, as reaction times can be long and produce a discharge with relatively higher trace metals. Therefore, they are more suited to small or less complex mining operations.
When calcium carbonate has been used in streams to neutralize acid, the neutralizing effects have been short-lived and can create harmful fluctuations in water chemistry.
Another method for neutralizing AMD systems is calcium silicate, made from steel slag, which reacts differently than lime, by removing free hydrogen ions in solution, thereby raising pH. In the presence of heavy metals, precipitation of metal hydroxides is accelerated. It is a cost-effective method and slightly more efficient than lime.
A third method is using carbonate chips such as limestone or other calcareous material, but the neutralizing effects have been disappointing due to the insoluble calcium sulfate layer covering the chips.
Ion exchange methods have been successful in South Africa, using a patented ion-exchange process that treats AMD economically. It works by removing toxic metals and sulfates from mine water through a cation exchange process.
One alternative solution that does not require plant equipment is the construction of wetlands over abandoned mine sites. Neutralized water is introduced, but the land sediments must remain submerged for it to work. This method has been used since the 1980s. It is relatively low-cost and creates an attractive landscape that develops its own ecosystem over time as flora and fauna colonize it.
Precipitation of metal sulfides using acidic solutions is something else researchers have developed, using biogenic sulfide, which relies on sulfate-reducing bacteria to oxidize organic matter and using sulfate instead of oxygen. Developing this method has been slow, but is considered cost-effective and innovative, as mine shafts can be used as long-term in-situ bio-reactors.
Alternatives to Neutralizing
Most methods used rely on neutralization to increase water pH and precipitate metals. Sludge is left over at the end and is often disposed of in tailing pool and dam systems, with the associated environmental impacts.
Therefore, a new method is being researched, which scientists working on gold mining wastewater have called liquid-liquid extraction (LLE).
Scientists used five different extractants: alamine, aliquat, cyanex, TEHA, and versatic acid, relying on the endothermic nature of reactions for acid recovery and then comparing efficiencies. TEHA (30% volume in 1-decanol solution) proved the most cost-effective.
This type of acid recycling could potentially address environmental concerns and improve economic savings due to lower requirements of the neutralizing agents for mining wastewater treatment, including subsequent sludge and its disposal.
LLE has development potential for other types of mining and could be made commercially available with investment and further research. This provides hope that sustainable solutions are being sought and researched.
LLE has technological potential that moves the necessity for increased metal mining a step further towards sustainability by addressing problems associated with acid mine drainage.
References and Further Reading
One step recycling of mineral acid from concentrated gold mining wastewater by high-temperature liquid-liquid extraction (04.22.2022) Elsevier Vol.286 in Science Direct online https://www.sciencedirect.com/science/article/abs/pii/S1383586622000077
The Paris Agreement (2015-16) in UNFCCC online https://unfccc.int/process-and-meetings/the-paris-agreement/the-paris-agreement
Report: How can research help the world hit net zero by 2050 ? (10.28.2022) in Elsevier online https://www.elsevier.com/connect/net-zero-report?dgcid=RN_AGCM_Sourced_400001413
Operational Lessons Learned During Bioreactor Demonstrations for Acid Rock Drainage Treatment. Bless.D, Park.B, Nordwick.S. (10.15.2008) in Springer Link online https://link.springer.com/article/10.1007/s10230-008-0052-6