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What is AMD?

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When sulfidic material is exposed to atmospheric oxygen, sulfide minerals begin to oxidise and water subsequently transports reaction products including acidity, sulfate, iron and other metals into surface water and groundwater.  This water is referred to as acid and metalliferous drainage (AMD) or acid rock drainage (ARD).

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AMD/ARD can display one or more of the following chemical characteristics:

  • Low pH (typically < 4)
  • High soluble metal concentrations (eg. iron, aluminium, manganese, copper, lead, zinc, cadmium, arsenic)
  • Elevated total acidity (eg. 100 – 15,000 mg/L CaCO3 equivalent)
  • High sulfate salinity (sulfate typically 500-10,000 mg/L)
  • High salinity (1000 – 20,000 μS/cm)
  • Low dissolved oxygen concentrations (commonly < 6 mg/L)
  • Low turbidity or total suspended solids (TSS) (combined with one or more of the above).

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The generation of AMD/ML/ARD is the primary geochemical risk associated with most mine sites. In order to understand AMD and associated risks, it is important to consider the mechanism of AMD generation in some detail, as explained here.

AMD refers to the acidic, metalliferous and saline drainage that can be released from waste rock piles, tailings storage facilities, pit walls, underground workings and potentially other mine infrastructure such as ROM pads and road embankments. AMD is a common problem for mines worldwide and one of the most significant obstacles to pollution prevention and minimisation during operations and post-closure.

AMD commonly occurs when previously water-saturated sulfide-mineral-bearing rocks are excavated and stored in an unsaturated setting, as is typical in mining operations that store waste rock and tailings in unsaturated or partially unsaturated piles and impoundments.

When minerals with acid neutralising capacity (ANC) are present (eg. carbonates and some silicates) AMD can have near neutral to slightly alkaline pH.  In such cases the near neutral metal-rich water is referred to as neutral mine drainage (NMD), neutral and metalliferous drainage (NMD) or metal leaching (ML). While not acid these waters pose a similar risk to the environment in terms of metal pollution and salinity.

The key terms and processes involved in the generation, release and treatment of AMD / NMD are described in the following sections.

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Sulfide Oxidation

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AMD can be produced when reactive sulfide minerals such as pyrite (iron sulfide, FeS2) are disturbed as part of mine operations.  Many sulfide minerals, particularly pyrite but also chalcopyrite (copper sulfide, CuFeS2), pyrrhotite (iron sulfide, FeS) and some others, naturally undergo oxidation when exposed to atmospheric oxygen and moisture.  Oxidation of sulfides results in decomposition of the mineral to release sulfur in the form of sulfuric acid (H2SO4), and soluble metals such as iron, which contribute to ‘mineral acidity’.  The acid conditions and soluble iron generated during pyrite oxidation can attack and dissolve other minerals, resulting in elevated soluble concentrations of other metals such as aluminium, manganese, copper, lead, zinc, nickel, cobalt, cadmium, chromium, arsenic, antimony and mercury.

Acid and metal production associated with pyrite oxidation is shown in Reactions 1 to 4.  An initial oxidation reaction involves the oxidation of pyrite to produce ferrous ions (Fe2+), sulfate and acid, as shown in Reaction 1.

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    FeS2     +     7 O2     +           H2O      →      Fe2+     +     2 SO42–     +        2 H+                                                         [Reaction 1]
    Pyrite            oxygen                 water           ferrous iron       sulfate                  acid

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The ferrous ions (Fe2+) released by pyrite oxidation may be further oxidised to ferric ions (Fe3+) consuming some acid (Reaction 2).  Notice that this reaction does not involve pyrite.

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   Fe2+    +      14 O2      +      H+      →      Fe3+      +      12 H2O                                                                                       [Reaction 2]
ferrous ion      oxygen           acid           ferric iron                 water

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The ferric ions then react with water to form ferric hydroxide (Fe(OH)3), which precipitates out of solution, producing additional acid (Reaction 3).

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  Fe3+     +       3 H2O      →         Fe(OH)3      +        3 H+                                                                                                                                  [Reaction 3]
ferric iron         water              ferric hydroxide             acid
                                              (orange precipitate)

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As shown in Reaction 3, the precipitation of ferric hydroxide is a key acid producing stage.  Once sulfide minerals have oxidised and released Fe2+ ions, it is extremely difficult to prevent ferrous ions oxidising to ferric ions with concomitant iron hydroxide precipitation and further acid generation.

A summary reaction of the complete oxidation of pyrite (by oxygen) in mine waste materials may be expressed as follows (Reactions 1-3 combined):

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  FeS2     +     154 O2     +     72 H2O     →     2 SO42–     +     4 H+     +     Fe(OH)3                                                          [Reaction 4]
Pyrite             oxygen               water                 sulfate               acid          ferric hydroxide

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Furthermore, the presence of ferric ions (Fe3+) can accelerate the oxidation of pyrite, generating additional sulfate and acid, as shown in Reaction 5.

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  FeS2     +     14 Fe3+     +     8 H2O     →     15 Fe2+     +     2 SO42–     +     16 H+                                                        [Reaction 5]
 Pyrite          ferric iron              water             ferrous iron          sulfate                 acid

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Note that in Reaction 5, 16 moles of acid are produced per mole of pyrite oxidised, as compared with 4 moles of acid generated when pyrite is oxidised by molecular oxygen (Reaction 4).  Whether pyrite oxidation proceeds through Reaction 4 or 5 depends on the chemical conditions in solution at the pyrite surface.  Reaction 5 suggests that soluble ferric ions can play a significant role in promoting sulfide oxidising reactions that result in AMD.

Sulfide oxidation continues until all reactive sulfides have been converted to acid and metals.  Different sulfides oxidise at different rates.  It is not unusual for sulfide oxidation (and hence AMD issues) to persist for hundreds of years.  Following acid production soon after subaerial disposal, the amount of acid produced by sulfide oxidation per year tends to decrease over time as the bulk concentration of source sulfides decreases.

Some sulfide minerals, such as galena (PbS), sphalerite (ZnS), arsenopyrite (FeAsS) and stibnite (Sb2S3), are relatively geochemically stable (unreactive) and slow to oxidise.  However, these minerals can be dissolved by exposure to acid conditions and dissolved iron (Fe3+), resulting in the release of soluble metals, which contribute to acidity.

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Kinetics of Sulfide Oxidation

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Sulfide oxidation occurs at a rate that is determined by the intrinsic geochemical and physical properties of the sulfide minerals (eg. mineral type, mode of formation, geological history, crystal size), the grain size of the rock, temperature, moisture availability, oxygen availability and bacterial activity.

Sulfide oxidation is a first-order decay reaction that can be described in terms of a percentage of sulfide that oxidises each year, or as a sulfide half-life.  For example, if the sulfide oxidation rate is 50 wt% sulfide/year, half of the sulfide exposed to atmospheric oxygen would be oxidised (to form acid and soluble metal ions) in the first year (ie. a half-life of 1 year), and then half of the remaining sulfide (25% of the starting total) would be oxidised in the second year.  The amount of acid generated by this process decays over time accordingly.

The rate of oxidation can be determined through kinetic geochemical tests such as oxygen consumption cell tests and in certain cases column leach tests.

The kinetics of sulfide oxidation can therefore be used to estimate the duration or longevity of sulfide oxidation and acid generation (before neutralisation reactions) under freely oxidising conditions.

For rocks of the same geological characteristics (ie. from the same lithological unit) and grain size, the rate of sulfide oxidation is largely uniform, independent of absolute sulfide concentration.  This means that oxidation rates (in wt% sulfide/year) determined through kinetic geochemical test work can be applied to rocks of the same lithology for any sulfide content.  The sulfide oxidation rate is typically normalised to pyrite equivalent units for convenience.

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Acid, Acidity and Acidity Load

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Two distinct processes, both promoted by oxidation of sulfide minerals, are responsible for decreasing the pH of an aqueous solution:

  1. Acid (H+) is directly generated by the oxidation of sulfur (Reaction 1).
  2. Acid (H+) is generated by the precipitation of metal hydroxides (eg. Fe(OH)3, Mn(OH)4: Reaction 3) during oxidation / neutralisation / dilution reactions. 

While process 1 is controlled only by the availability of oxygen and water, process 2 depends on the solubility of the metal aqueous species, which in turn is controlled by the factors such as pH of the solution and oxidation state of the metal.  In other words, the generation of acid through process 1 is limited by the sulfide oxidation rate, while the generation of acid through process 2 is delayed until metals can precipitate from solution (thus the term “latent acidity” or “mineral acidity”).

The term “acid” quantifies only the actual amount of H+ present in solution and is generally expressed as pH.  The term “acidity”, on the other hand, accounts for both the actual H+ concentration of the aqueous solution and the potential for acid generation due to mineral or latent acidity (ie. H+ produced by process 2).

In general acidity increases as pH decreases, but there is not always a direct relationship between acidity and pH.  Based on earlier descriptions of metalliferous drainage, it is possible to have AMD with an elevated acidity but near neutral pH values.  It is therefore important to quantify the contributions of both hydrogen ion concentrations (acid) and mineral contributions (latent acidity) in order to determine the total acidity of a water sample.  Acidity is generally expressed as a mass of calcium carbonate (CaCO3) equivalent per unit volume (eg. mg/L CaCO3).

The measurement of acidity is equivalent to the amount of neutralising agent (such as calcium carbonate) that would need to be added to the affected water to raise the pH to 8.3.  Observations of pH alone, while a reasonable qualitative indicator of water quality, are insufficient to estimate total acidity.  For example, water with a pH of 3.0 can have an acidity of as low as 50 mg CaCO3/L and as high as 10,000 mg CaCO3/L or more.

Acidity is either measured in the field or laboratory by titration or estimates of acidity can be made from water chemistry data (pH and dissolved metal concentrations) using shareware such as ABATES.

Acidity load refers to the product of the total acidity (acid plus latent acidity) and flow rate (or volume) and is expressed as a mass of CaCO3 equivalent per unit time (or mass of CaCO3 for a given volume of water).

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          Acidity Load             =        10–3      x        Flow volume / year    x      Acidity                                                   [Equation 6]
(tonnes CaCO3 eq. / year)   (conversion factor)                     (ML/yr)                          (mg/L)

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Acidity loads can be used to establish the contributions of various acidity sources at a mine site and also provide an indication of the likely water treatment requirements.

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Secondary Acid Sulfate Minerals

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Acidity generated as a result of sulfide oxidation can react with silicate minerals to form secondary acid sulfate salts such as melanterite, jarosite and alunite (Reaction 7).

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3 FeS2     +     12 O2     +     212 H2O     +     KAl3Si3O10(OH)2     →
Pyrite               oxygen                water                    Muscovite
                                                  KFe3(OH)6(SO4)2        +         32 Al2Si2O5(OH)6         +         4 SO42-       +        8 H+     [Reaction 7]
                                                             Jarosite                                        clay                                     sulfate                    acid

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These are commonly observed as efflorescences (crusts) on the top of oxidising sulfide materials.  Melanterite is highly soluble in water, jarosite is sparingly soluble, and alunite is approximately ten times less soluble than jarosite.  Acidity stored in these minerals is released by dissolution in water (to produce AMD), and is not sensitive to oxygen availability (Reaction 8).  It is therefore important that any characterisation of partly oxidised / oxidised sulfidic materials establishes the presence or absence of any such secondary acid sulfate minerals.

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KFe3(OH)6(SO4)2     +     H2O     →     3 Fe2O3     +       2 K+      +     4 H2O     +     4 SO42-     +     6 H+                      [Reaction 8]
        Jarosite                      water               Hematite     potassium ions        water             sulfate               acid

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Depending on the rate of sulfide oxidation, jarosite/alunite formation as a result of sulfide oxidation can proceed faster than the rate of jarosite dissolution, resulting in an accumulation of jarosite in stockpiles of sulfidic materials.  Melanterite, if formed, is highly soluble and does not tend to accumulate in non-arid environments.

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Acid Neutralisation

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Certain carbonate minerals, primarily calcium- and magnesium-bearing carbonates such as calcite (CaCO3) and dolomite (CaMg(CO3)2), can neutralise the acidity produced by sulfide oxidation.  A rock’s neutralisation potential, as determined through test work, is referred to as its acid neutralisation capacity (ANC).  Iron- and manganese-bearing components of carbonates have no net contribution to ANC, as the metals oxidise and hydrolyse, thereby contributing to acidity.  However, depending on the laboratory procedures used, iron- and manganese-bearing components of carbonates may be included in the laboratory-measured ANC value.

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  CaCO3     +     2 H+     →       H2O       +       CO2       +       Ca2+                                                                                   [Reaction 9]
  Calcite              acid               water         carbon dioxide    calcium ions

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  CaMg(CO3)2      +      4 H+      →       2 H2O        +       2 CO2       +        Ca2+         +         Mg2+                               [Reaction 10]
    Dolomite                    acid                     water            carbon dioxide    calcium ions        magnesium ions

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  FeCO3     +     3 H+     +     14 O2     →     Fe3+     +     32 H2O     +     CO2                                                                  [Reaction 11]
 Siderite             acid               oxygen        ferric iron           water      carbon dioxide

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   Fe3+      +      3 H2O       →       Fe(OH)3      +      3 H+                                                                                                                             [Reaction 12]
 ferric iron          water               ferric hydroxide         acid

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In rocks containing carbonate ANC, acidity produced by sulfide oxidation is generally quickly neutralised by the acid neutralising minerals before it can be released as AMD.  This process continues until the carbonate minerals are exhausted or become less available for neutralisation due to surface coating or passivation.

Acid produced by sulfide oxidation can also react slowly with common silicate minerals, partially neutralising acidity and storing some acidity in precipitated secondary minerals such as jarosite or alunite.  Due to the slow rate of reaction, relatively long acidity contact times are required to induce silicate neutralisation, which can be achieved by ensuring slow water migration rates.  Silicate ANC is not included in the ANC value obtained by standard laboratory ANC measurements.

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Lag Period

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In sulfidic rocks containing ANC, drainage may initially have near-neutral pH due to the neutralisation of acidity as it is generated in the rock pile.  For materials containing an excess of sulfide with respect to available ANC, acid conditions will eventually develop as the ANC is exhausted and acid drainage with low pH will be produced.

The delay to the onset of acid conditions is called the lag period, and this delay can be hours to hundreds of years in length.  During the lag period, drainage will be of near-neutral pH, but may be metalliferous (ie neutral metalliferous drainage, NMD / Metal leaching, ML see below).

Depending on the balance of MPA and ANC, rocks can display three general lag-related behaviours:

  • No lag period with immediate onset of acid conditions (ANC = 0 , MPA > 0);
  • A discrete lag period followed by the onset of acid conditions (MPA >ANC);
  • No onset of acid conditions (ANC >> MPA).

The lag period can be predicted by calculation from the sulfide oxidation rate and the ANC.  Net acidity generation

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Neutral Metalliferous Drainage / Metal Leaching

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Occasionally, the acid drainage produced via Reactions 4 and 5 is completely neutralised by dissolution reactions with naturally occurring carbonate minerals such as calcite, dolomite, ankerite and magnesite.  This neutralisation process can result in the precipitation of metals such as aluminium (Al), copper (Cu) and lead (Pb) which have solubilities that are pH dependent.  Other metals, such as manganese (Mn), cadmium (Cd), zinc (Zn), arsenic (As) and antimony (Sb) are still relatively soluble at near neutral pH and so concentrations of these metals can remain elevated.  Sulfate concentrations are not always affected by these carbonate dissolution reactions and so can also remain elevated.  The resultant near-neutral, high sulfate salinity and variably metalliferous drainage is commonly referred to as neutral metalliferous drainage (NMD), neutral mine drainage (NMD) or metal leaching (ML).  While NMD still indicates the oxidation of sulfidic materials, it is less common due to the requirements for specific sulfide minerals (eg sphalerite, arsenopyrite) and a local excess of carbonate minerals.

In some environments the NMD may contain little or no soluble metals as a result of the reaction with available neutralising materials. In these environments the only indication of sulfide oxidation is high sulfate salinity or saline drainage (SD).  The concentration of sulfate within this saline drainage is dependent on the relative proportions of calcium and magnesium in the neutralising carbonate materials. If magnesium is the dominant component of the neutralising material, high salinity is more likely to be an issue, due to the high solubility of magnesium sulfate. Conversely, if calcium is the dominant component, then the formation of gypsum precipitates will contribute to lower salinity levels.

Saline drainage generated specifically as a result of sulfide oxidation is relatively rare, in comparison with acid and/or metalliferous drainage.  Nevertheless, sulfate salinity can be an important indicator of AMD issues at mine sites, and may require similar management strategies (that is, control of sulfide oxidation).

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