Mining 101.1: Acid Mine Drainage

March 14th 2018 Liam Hardy for MiningIR.com;

Every stage of the mining process, from exploration to processing and marketing has the potential to introduce novel chemicals into natural environments. Once released, these chemicals can be transported throughout entire ecosystems, from source to ocean, by water, plants and animals…

The level and distribution of contamination surrounding any mine is often related to the resource sought and the region mining has taken place. Mining regulations on the use of water, release of waste products and processing of minerals are often poorly enforced in less developed regions such as Central Africa and across South America, where large corporations and small scale artisanal mining operations act without regard (or without knowledge) of their surroundings.

Stricter regulation is often simply ignored in more developed nations such as the USA where policy enforcers and corporate mining investors regularly exchange jobs with the enforcement agencies and fail to punish themselves (Milman and Rushe 2017) and in Australia, where mines are often abandoned in sensitive environments without any remediation works or investigation from authorities (Roche and Judd 2014).

This review aims to address one of the most common and widespread pollutants associated with mining. Sulphur, in the form of sulphates and sulphuric acid which drain from sulphide rich ores once they’re exposed to surface environments. This sulphuric run-off from mining is often referred to as acid mine drainage (AMD).

AMD was described as the largest environmental problem facing the U.S. mining industry. (USDA Forest Service 1993) (Ferguson and Erickson 1988) (Lapakko 1993). Some 7000km of waterways in the Western USA have been affected by AMD from coal mining alone and an estimated 16000km of streams are polluted by more than 20,000 mines on US forestry service land (USDA Forest Service 1993).

Figure 1: A global map of the world’s most toxic polluting processes. At least 6 of these are related to acid drainage. The USA was excluded from this map as data was not freely available. (Harris and Andrew 2011)

Sources of contamination

AMD is most commonly associated with the mining of coal and the sulphide rich ores of copper (chalcopyrite), zinc (sphalerite), Lead (Galena) and the associated gangue from large scale gold and iron mines (which can contain pyrite, pyrrhotite and arsenopyrite). This report will focus on the impacts of poorly managed sulphide ores.

Sulphide ores are found in a variety of environments, from 1cm vein hosted deposits to 40km wide sedimentary exhalative (SedEx) and volcanic massive sulphide (VMS) deposits. Ore minerals are very rarely found alone without associated gangue minerals. When found in veins, Copper is normally bound into chalcopyrite (CuFeS2) but is less abundant than iron in most sulphur rich hydrothermal solutions, so pyrite (FeS2), pyrrhotite (Fe7S8) and quartz also precipitate within the veins. Similarly, in SedEx & VMS ores, Cu is often not the most abundant mineral forming element, so it occurs alongside other sulphide minerals (see table 1).

While they are underground these minerals remain relatively stable in a closed system and pose little or no threat to their above ground environments. Common gangue minerals in this type of mining, such as quartz, micas and feldspars, are relatively stable even at surface conditions and have a limited chemical impact away from the mine itself, other than increased sedimentation in waterways.

Unlike most aluminosilicate and silicate minerals, sulphides readily oxidise when exposed to air and water, releasing their primary metal content, sulphate and H+ ions, all of which are highly reactive. The most common weathering reaction in AMD is that of pyrite show in equation 1. The weathering of chalcopyrite, sphalerite and other sulphide ores (table 1) is similar, but releases Cu/Zn sulphates.

2FeS2    +   7 O2   +   2H2O   →  2Fe2+   +   4SO42-   +   4 H+
(pyrite + oxygen + water → ferrous Iron + sulfate + acidity)

Equation 1: Simple weathering of pyrite (Lehigh University 2011)

According to the 1994 U.S. Environmental Protection Agency (EPA) report into AMD, a variety of common processes can contribute to AMD and its significance. Crushing the ore to remove it from the mine increases its surface area and thus its reactive potential. It is necessary to break up rocks in all mining types from open pit to underground.

Table 1: List of common ‘pyrite type’ sulphide minerals capable of contributing to AMD (Obreque-Contreras, et al. 2015)

Heap leaching is a common processing method at larger mining operations, where oxidising chemicals are sprayed onto the top of these stacked and crushed ore materials and the oxidised residue is collected at the base as shown in Figure 2. It is technically impossible to collect 100% of the oxidising chemicals and metalliferous residue when it is in an open and exposed system so, some of this always inevitable ends up entering the surrounding environment.

Figure 2: A combined heating and heap leaching plant from the MinTek facility in South Africa (MinTek 2011)

Some processing plants heat the ores to increases their entropy and the rate at which target sulphides oxidise. Only the highest grades of ore are financially viable to heat for this process, so lower grades are often stockpiled around the mine. Stockpiles of graded ore are often left to weather in the sun, wind and rain, often with little or no protection and themselves begin to oxidise and release sulphates and metals.

Once in aqueous form and away from the mine site, it is very difficult to control the spread of metals and sulphates as they freely enter rivers and groundwater systems, lowering the pH and increasing the salinity of their waters. Aqueous SO4 and H+ ions form sulphuric acid, which readily dissolves most carbonates, phosphates and weaker silicate minerals, releasing any metal content from them and adding it to the river fluids. AMD is not only a problem at source, but creates a chain of reactions along its reach, changing water and soil chemistry indefinitely.

Several rare examples exist of natural exposures of sulphur rich deposits which have not been exposed by mining, the original Rio Tinto copper deposit in Southern Spain was said to have been discovered in 3000bc by following the acidic red river to its highly metalliferous source. Similarly, the Kawah Ijen crater lake in East Java (Indonesia) is an actively forming terrestrial stockwork vein type deposit, circulating hot pH 0.3-1 fluids enriched in SO4, Cu, Fe, Zn and Pb, which leaks into surrounding water and soil systems.

Treatment of Acid Mine Drainage

The first laws were passed in 1968 in the USA for the control of mining waste in Pennsylvania (Lehigh University 2011). Companies responded quickly with simple chemical treatments to raise the pH of waste fluids and capture metals. The most common was filtering waste through a limestone buffer (equation 2), but proved to be costly created reservoirs of metal rich sludge which then needed to be further treated before disposal

H2SO4 + CaCO3 → CaSO4 + CO2 + H2O
(sulphuric acid + calcium carbonate → calcium sulphate + carbon dioxide + water)

Equation 2: Simplified reaction of aqueous sulphate when filtered through limestone (Lehigh University 2011)

In 1978 so called ‘passive treatment’ techniques were trialled, which involved the use of closed natural systems to treat waste, by passing it through specific metal absorbing plants and funghi rich soils which thrived in acidic environments and would act as a natural filter to the flow.

The use of extremophile sulphur reducing bacteria was also trialled. By using sulphur instead of oxygen for anaerobic energy production, these bacteria can either convert elemental or sulphate hosted sulphur to hydrogen sulphide (H2S). H2S is either released as a gas or can react back into pyrite/metal sulphides in the presence of Fe or suitable metals (Obreque-Contreras, et al. 2015). These pyrite group minerals are easier to store or manage than their aqueous phases as long as they are managed quickly after their capture.

Case Studies

The Thompson Creek deposit, Idaho, USA (EPA 1994)

Figure 3: (L) The Thompson Creek mine, Idaho, USA and (R) it’s waste storage lake from above. (EcoFlight 2017)

Thompson Creek has been mined for Molybdenite (Molybdenum sulphide, MoS2) since 1981. 130 million tons of waste rock was removed from above the deposit and an estimated 12.3 million m3 of waste rock was generated each year up until 1994. The relict volcanic complex of monzonite and argillised metasediments contained between 1.1wt% and 1.6wt% of sulphur in the form of pyrite and metal sulphides. Tailings were stored in two tailings stacks across some 150 acres above and below the mine site (shown in Figure 3) with a tailings pond for treatment below.

In 1994 and EPA investigation found that 180m high sand embankment that had been constructed to contain contamination at the site was potentially beginning to fail and that the waste rock’s AMD potential was far higher than originally claimed by the company. Monitoring from 1984 to 1991 showed that pH had decreased at 4 main locations, sulphate content had increased at al monitored locations, Fe content had increased at all locations. Although little of the contamination has escaped the mine, the only thing holding it back was now the sand embankment and the pH was much lower than the current treatment levels accounted for.

The EPA suggested the damn and run off was treated immediately with trisodium phosphate to trap metals and neutralise any fluids escaping the containment pond. The company also trialled removing the pyrite from crushed rock by passing it through the same flotation

 

facilities used to extract molybdenite and then re-burying the pyrite rather than leaving it to oxidise. It is expected that the contamination and tailings will likely require long-term management, long after the life of the mine and that failing to maintain the facility could endanger regional environments.

The mine and pond are situated at the top of the Bruno Creek which forms part of the Salmon River watershed, one of the last remaining migration routes for chinook salmon and a popular American camping, fishing, kayaking and climbing destination, it is still a point of conflict between the company and activists today with no long-term plan in place to control the waste at the end of the mine’s life (EcoFlight 2017).

The Lorraine Mine, Temiscamingue, Canada

The Lorraine mine was an active copper and nickel deposit covering around 24 acres of forest land until being abandoned in 1968. The site and was not investigated, remediated or managed until the early 2000s. Its tailings contain high amounts of pyrite, pyrothite, chalcopyrite and pentlantite which were all exposed to weathering for some 50 years creating pools of low pH fluids around the mine and seeping into the surrounding environment.

In 2011, three dolomite (CaMg(CO3)2) drains were built at the site by the regional government to treat the waste but this simple system was not effective in neutralising the waste, which was leaving the mine at pH 3 and containing some 2000Mg/l of Fe even after treatment. In 2012 a new low-cost system which used only local natural products and was installed in just 5 days. The system trialled the use of sulphate reducing bacteria in a three-step system as in Figure 4.

The system was designed to allow fluid to pass through at 0.8l/minute by compacting the substrates. This allowed adequate time for sulphates and Fe to react and be trapped by the filters. In just one year the system raised water pH from 5 to 6.5 and lowered metal content of the effluent by 2/3. (Genty, et al. 2012)

Figure 4: Installation and design of a three-step passive filtration system for the treatment of AMD. Red: Tailings and contaminated material, Green: Organic filter containing sawdust, manure, compost, calcite and sand, Brown: Wood ash filter to remove metals from solution (Genty, et al. 2012)

Conclusions

Acid mine drainage is a major issue facing any life which relies on a clean water supply, from plants, to fungi and bacteria and, humans. Although technology has now been trialled to deal with the effects of AMD, large businesses and governments in most of the world are not stepping up to their responsibilities.

There is currently no environmental department funded to investigate mining waste and water quality protection in the USA, the responsibility for which has been passed onto the department of economics, whose job it is to ensure the practices are profitable rather than safe.

Recent investigations have found countries such as Tanzania and Papua New Guinea have tighter (better regulated) mining and environmental authorities than Australia or the USA (McNamara 2009).

Now that technologies have been proven in local communities to be cost and performance effective for fixing at least the headline issues of AMD, their installation should be considered by all mining companies where appropriate. Without immediate action we face having more of our water ways worldwide, becoming like those in the unregulated regions of Dabaoshan in China (Figure 4).

Figure 5: A severely AMD contaminated lake near Dabaoshan mine in China (Reuters 2009)


The MiningIR – Mining 101 Series intends to simplify a wide range of mining topics from academic and professional sources to investors who want to gain a deeper knowledge their plays and the industry they’re contributing to.

We’ll be discussing a host of issues, from environmental reporting to basic geochemistry and mining terminology, from exploration and prospecting guides and even to worldwide stock exchange history and reviews.

Stay with us throughout 2018 and, feel free to comment or contact us with questions, or if there are any topics you’d like covered.

Until Next week,

Liam Hardy

Twitter: @BeefcakeHardy / @MiningIRMedia
Email: Liam [at] MiningIR.com

 


References

EcoFlight. Idaho thompson Creek Mine. 2017. http://ecoflight.org/issues/detail/Idaho—Thompson-Creek-Mine/.

EPA. ACID MINE DRAINAGE PREDICTION. Washington, DC, USA: Environmental Protection Agency, USA, 1994.

Ferguson, K D, and P M Erickson. Pre-Mine Prediction of Acid Mine Drainage. In: Dredged Material and Mine Tailings. Berlin: Springer-Verlag, 1988.

Genty, T, B Bussière, M Benzaazoua, G Zagury, and C M Neculita. “Acid mine drainage multi-step passive treatment system: the Lorraine case study.” Montreal, Canada: Goldschmidt Conference, the European Association of Geochemistry, 2012.

Harris, J, and M Andrew. The Top Ten of the Toxic Twenty, in The World?s Worst Toxic Pollution Problems Report. New York: Blacksmith Institute and Green Cross, 2011.

Lapakko, K. Mine Waste Drainage Quality Prediction: A Literature Review. St. Paul, MN, USA: Minnesota Department of Natural Resources, 1993.

Lehigh University. Basic AMD Chemistry. 2011. http://www.ei.lehigh.edu/envirosci/enviroissue/amd/links/science2.html.

McNamara, N. The environmental regulation of mining: An International Comparison. PhD thesis, University of Southern Queensland, 2009.

Milman, O, and D Rushe. New EPA head Scott Pruitt’s emails reveal close ties with fossil fuel interests. London: The Guardian, 2017.

MinTek. Heap Leaching and Heap Bioleaching. 2011. http://www.mintek.co.za/technical-divisions/biotechnology-bio/services-facilities/heap-leaching-and-heep-bioleaching/.

Mortel, P. Mining- Environmental Degradation & Sustainability: Newmont’s Mining Operations. http://pamelaenvironmentalscience.blogspot.nl, 2015.

Obreque-Contreras, J, D Pérez-Flores, P Gutiérrez, and P Chávez-Crooker. “Acid Mine Drainage in Chile: An Opportunity to Apply Bioremediation Technology.” Hydrology Current Research 6, 3, 2015.

Reuters. A contaminated lake near Dabaoshan mine in China. http://www.penki.lt/en/A-contaminated-lake-near-Dabaoshan-mine-in-China.media?id=213877, 2009.

Roche, C, and S Judd. Ground truths: Taking responsibility for Australia’s mining lagacies. Girrawheen WA, AU: Mineral Policy Institue, 2014.

USDA Forest Service. 1993. Acid Mine Drainage From Mines on the National Forests, A Management Challenge. p.12: Program Aid 1505, 1993.

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