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Acid Rock Drainage and Acid Mine Drainage: Processes, Effects and Practical Stud

Last Updated: 29th Apr 2019

By Chris Popham

Acid rock drainage is the process by which sulphide minerals decompose when exposed to oxygen to yield aqueous acidic products. As mining frequently exposes sulphides and workings underground require dewatering the result may become large scale and problematic.

Sulphide mineral deposits such as pyrite (FeS2) are naturally stable in the ground below the water table where conditions are anoxic. Should the water table be lowered due to: mining or excavation, the depletion of aquifers or through geological processes such as uplift or erosion as examples, then oxygen mediated sulphide decomposition can occur.

Pyrite has two chemically identical isoforms: the brassy, cubic form often termed Fool’s Gold, which is relatively stable, and the soft and gritty form known as marcasite which rapidly decomposes.

Where oxygenated water is the medium the decomposition of marcasite can be expressed in the following way:

2FeS2(s) + 7O2(g) + 2H2O(l) = 2Fe2+(aq) + 4SO42-(aq) + 4H+(aq)

The process may be accelerated by some micro-organisms that are able to take advantage of the energy released by the reaction.

The resulting acidification of the water by the dissolution of hydrogen ions leads to a positive feedback loop through further oxidation of the iron(II) ions:

4Fe2+(aq) + 2O2(g) + 4H+(aq) = 4Fe3+(aq) + 2H2O(l)

And then reduction of the iron(III) ions:

2FeS2(s) + 14Fe3+(aq) + 8H2O(l) = 15Fe2+(aq) + 2SO42-(aq) + 16H+(aq)

If sufficient acidification occurs, other mineral sulphides such as galena (lead), chalcopyrite (copper) and sphalerite (zinc) will also decompose along with ores of cadmium and nickel, all of which yield highly toxic metal ions.

Naturally however as geological process advance at very slow rates and water flow is usually unimpeded the resulting acidification is low level over a period of time and quickly dispersed. Consequently decomposition of many sulphides may not occur and if it does the products are often secondary minerals: cerrusite (lead), malachite (copper), smithsonite (zinc), rather than dissolved ions.

In mines however there may be a rapid exposure of large mineral reserves, water is invariably present and breathable air must be brought underground. Furthermore, the co-deposition of low value iron sulphide alongside other high value metal sulphides may result in stockpiling of mixed sulphide waste underground.

Nevertheless during the life of a mine active dewatering may result in only low levels of acidification and the outflow of water is typically managed through settling ponds so that metal ions are precipitated by treatment with lime to reduce the acidity or removed by biological filtering systems that are tolerant to the toxicity. The resulting pond slurry and/or biomass can then be safely managed and may have a residual value depending on the metal ions present.

As a case in point, in the 1920s a former mine worker by the name of John (aka Jack) Cloke obtained permission from several landowners in the Gunnislake area who had abandoned mines on their property to work discharges from the mines for copper. This was simply achieved by exploiting the difference in electropositivity of metals and placing scrap iron in the water onto which copper would accumulate.

When mines fall idle however, underground lakes and reservoirs may form in which the acidity becomes very high and into which copious quantities of metal ion slurry settles. Left unmanaged the reservoir dams may eventually fail resulting in a discharge of acidic, metal ion rich water and slurry into watercourses.

In the South-West of England perhaps the most notorious event in relatively recent times occurred in Cornwall at Wheal Jane in 1992 when an estimated 50 million litres of water and slurry discharged into the Carnon River over several days. In the 1960s and again in 1993 smaller events occurred at the Cwm Rheidol group of mines in Ceredigion with resulting pollution of the Afon Rheidol and in 2008 for a number of days the River Tamar downstream of Gunnislake on the Devon-Cornwall border was turned bright orange by discharge from an unidentified source.

The effect of such mass discharges into water courses is invariably catastrophic. In the first instance the water suffers severe oxygen depletion leading rapidly to the death of all aquatic animal life. The slurry will settle onto plant life choking and poisoning it, a process exacerbated if the acidity of the discharge is neutralised whereupon the metal ions will precipitate as hydroxides.

The settled slurry will retain the metal ions for an extended period much to the detriment of invertebrates which often burrow into river beds. The reduced invertebrate biomass not only constricts the food supply for vertebrates such as fish, but the passage of zinc, copper and cadmium metal ions through the food chain is detrimental to the gills of fish reducing their ability to survive in poorly oxygenated water. Additionally, lead causes bone and fin malformation in fish. Passing further along the food chain to birds and mammals the metal ions reduce fertility and result in birth defects or non-viable embryos.

Controlled management of discharges may be costly and in the case of abandoned mines in the UK that inevitably falls on the public purse. The settling ponds and on-going treatment and disposal of outflow from Wheal Jane in the 10 years after the 1992 event is thought to have cost upwards of £20m. At Cwm Rheidol the discharge of water is controlled, but the treatment involving beds of limestone chips proved poorly effective as the chips quickly became coated with insoluble calcium sulphate. Wetland ‘reed bed systems’ are effective but only after pre-treatment of the water and it is important that the resulting biomass is removed or remains submerged where decomposition results in an oxygen free environment. If not, the metal ions may be remobilized. An extensive wetland scheme is active at Force Crag Mine in Cumbria.

Case Study #1: Wheal Exmouth.
The dumps of this abandoned lead mine near Christow in Devon contain galena, pyrite and arsenopyrite. Not only are there dumps on the bank of the River Teign but between 2008 and 2012 a drainage scheme was put in place to dewater an adit and this produces a constant untreated discharge into the river.

Water in both the adit and dump discharges is mildly acidic (pH5.76). 800ml of adit discharge was filtered and the particulate material retained. Addition of a small amount of sodium carbonate to 200ml of the clarified water raised the pH to 9.56 and within a few minutes the water had taken on a brown tint compared to an untreated sample. Within 3 hours the carbonate treated sample had produced a fine, light brown deposit in the beaker whilst the untreated sample remained clear. Sodium not calcium carbonate was used to exclude the potential for insoluble calcium sulphate precipitation.

The reactions of iron(II) ions with carbonate are as follows:

Fe2+(aq) + CO32-(aq) = FeCO3(s) (a white precipitate)

Dissolved oxygen will further the reaction:

FeCO3(s) + O2(g) + 3H2O(l) = HCO3-(aq) + 2OH-(aq) + Fe(OH)3(s) (a brown precipitate)

Iron(III) ions will not react directly with dissolved carbonate but as follows:

CO32-(aq) + H2O(l) = HCO3-(aq) + OH-(aq)
Fe3+(aq) + 3OH-(aq) = Fe(OH)3(s)

As both iron(II) and iron(III) ions will ultimately produce a brown precipitate in the presence of dissolved oxygen a further sample of the filtered water was placed under vacuum in a stirred vessel until no further gas exsolved. The ‘degassed’ water was transferred to a smaller bottle containing a little sodium carbonate with the small headspace filled with pure nitrogen. After about an hour a white precipitate began to form, which settled as a thin layer on the bottom of the bottle overnight. The contents were then filtered and the paper gradually turned brown in the presence of oxygen indicting, as predicted by the preceding equations for the decomposition of the pyrite, that the water was rich in iron(II) ions.

Acidification of some of the particulates collected from the initial filtering with 0.01M hydrochloric acid (pH <2.0) produced no effect whereas similar treatment with 0.01M sulphuric or nitric acids slowly dissolved the solids as expected.

Case Study #2: Gawton Mine.
This mine is situated beside the River Tamar and is renowned for having a pool of blue water where run off from the dump is collected and where the bed of the outflow stream from the dumps is thickly populated with bright green pebbles.

The drainage is mildly acidic: pH5.52 and addition of sodium carbonate to filtered water produced a slight, off white precipitate. When the precipitate was dissolved in nitric acid and the product treated with ammonia a slight blue hexaqua-copper result was observed. However, when the coat was dissolved from pebbles with nitric acid a blue-green solution resulted which turned a vivid royal blue colour on addition of ammonia. The pebbles are lime rich country rock onto which 100 years after the mine closed copper carbonate still precipitates due to the change from acid to alkaline conditions.


Information display board at Cwm Rheidol Mine.

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