Introduction to Coal Mine Drainage
- Introduction to Coal Mine Drainage
- C2.1 Introduction
- C2.2. Pyrite in Sedimentary Strata
- C2.3 Carbonate Minerals in Coal-bearing Strata
- C2.4. How Geology Affects Coal Mine Drainage Quality
Coal deposits are found on every continent. Most of these deposits have been or are being mined. China is the world’s largest coal producer, while the United States has the world’s largest reserves. The other most important coal-producing countries are Australia, India, South Africa, and Russia. Yet the nature of the coal mine drainage (CMD) from the various coal deposits and, indeed, even within the same coal deposits, varies significantly. In part, this is because of climate but also generally the paleoenvironment of the coal deposit – that is, what the region was like when the organic-rich sediment that became the coal was first deposited.
This section discusses why the nature of CMD differs and how it is somewhat different from ARD at most other types of mines. The presentation starts at a very basic level, discussing the nature of sedimentary strata and how mining of these types of deposits is different from mining non-sedimentary (hard rock) ore bodies. Later, in the appropriate chapters, differences between coal and hard rock mining that affect pre-mining prediction of water quality, prevention/mitigation strategies, and water treatment are examined.
The concentrations of iron, manganese, and aluminium are generally very low in natural waters (<1 mg/L) because of chemical and biological processes that cause their precipitation in surface water environments. The same chemical and biological processes remove iron, manganese, and aluminium from contaminated CMD, but the metal loadings from some abandoned coal mine sites are so high that the deleterious effects of these elements persist. Thus, CMD can contain high concentrations of iron, manganese, and aluminium, as well as SO4, Ca, Mg, K, and Na. CMD generally contains much lower concentrations of other metals that can be so problematic at metal mines. However, some trace metals can be a cause for concern in certain coal mining areas (e.g., selenium). Drainage from coal refuse disposal sites (the reject material from coal washing or beneficiation plants) is more likely than drainage from coal mines themselves to contain elevated levels of Zn, Ni, and sometimes other potentially toxic metals.
C2.2. Pyrite in Sedimentary Strata
The principal difference between ARD and CMD is caused by the fact that coal is a sedimentary material. Coal develops from the settling of plant material and subsequent transformation under the pressure of subsequent sedimentary overburden. As most readers of this Guide already know, sedimentary rocks result from the weathering and erosion of other rocks. Streams and rivers transport the sediment to lakes or oceans, or deposit it on nearby floodplains, where it accumulates. On land, these sediments consist mainly of large boulders, cobbles, gravel, sand, and silt. Close to the shore, the deposited sediment is largely sand, silt, and clay. Further away from shore, in deeper water, clays, chemically precipitated calcium carbonate, and the remains of tiny plants and animals (e.g., coral) can accumulate. Over time, these sediment layers are buried by subsequent depositional material, are compacted, and eventually harden into sedimentary rock. Sand beds and beach deposits become sandstone, while finer grained material becomes siltstone or shale (which is cemented clay).
However, since this deposition and compaction is taking place over millions of years, the nature of the deposits at any one spot changes over time. Generally speaking, if the layers have not been disturbed by folding and faulting, the oldest layers are at the bottom. As sea levels rise and fall, near-shore deposits or fresh water sediments may be buried by marine sediments, or vice versa. Further complicating this ancient history are changes caused by regional climate changes, as areas became more or less arid, and the effects of mountain building (due to plate tectonics or volcanic eruptions) and erosion on the nature of the sediment and on the nearby weather patterns.
Strata can also change horizontally. For instance, a given stratum or formation can extend over hundreds of kilometres, but its nature may gradually change over these distances, from cemented gravel (conglomerate), to siltstone (or mudstone), and then to shale, reflecting the decreasing power of water to transport these different size materials away from their source. More rapid facies change is also possible, reflecting topographical features in the paleoenvironment. Also, because sedimentary rocks are formed from fragments of rocks, they are generally weaker than igneous or metamorphic rocks. Often, their strength is determined by how tightly the grains of sediment are stuck together.
In this discussion, an important sedimentary environment has been left out: swamps and wetlands. These represent the transition from terrestrial to lacustrine (lake) or marine deposits and, as one can observe around the world today, wetlands can be quite extensive. Wetlands can be marine, brackish, or fresh water environments and the nature of the sediment can change frequently due to seasonal changes or even large storm events. The sediment that accumulates in swamps is rich in organic matter (peat). When the organic matter is more than 50% by weight or 70% by volume, the compacted sediment is classified as coal; when there is less than 50% by weight or 70% by volume organic matter, the rock is classified as carbonaceous (organic-rich) shale.
Sedimentary strata were originally deposited horizontally, but in many cases, subsequent movement of the earth’s crust caused the strata to fold and tilt. As a consequence, many sedimentary strata, including coal seams, exist today at angles that are not horizontal. This feature is critically significant to how water flows in coal mines and where CMD might emerge.
In general, sedimentary strata tend to have a much simpler mineralogical composition than non-sedimentary mineral deposits. For example, of the common sulphide minerals listed in Table 2.1 of the Guide, only the first, FeS2, is commonly found in significant concentrations in sedimentary strata. Importantly, pyrite is commonly found in significant concentrations in organic (carbonaceous) deposits, such as coal and black shale. This is because the decaying organic matter in deltaic and marine wetlands and swamps can lead to the formation of hydrogen sulphide (H2S) and then the precipitation of iron monosulphides, which can be buried along with the plant material. Over time, these monosulphides transform into pyrite. The amount of pyrite that may form in sediment is limited by the amount of decomposable organic matter, dissolved sulphate, and reactive detrital iron minerals (Berner, 1984). The iron is a product of mineral weathering upland; iron concentrations are lower in marine sediments then they are in deltaic environments. Generally, sulphate concentrations have the opposite tendency, to the extent that they are often too low in fresh water wetlands to form much H2S. As a result, coals and shales that were formed in association with fresh water are generally low in pyrite. In marine sediments, the limiting component is typically the amount of organic matter, so that organic carbon often correlates with the percent sulphur (Goldhaber and Kaplan, 1982; Raiswell and Berner, 1986)
Several types of pyritic sulphur are found in coal and other sedimentary strata, based on the size and structure of the pyrite. Caruccio et al. (1988) provides an extensive review of the different forms, morphologies, and their relative reactivity. Although all of the sedimentary pyrite can oxidize, some forms oxidize faster than others. Figure 1 is a scanning electron microphotograph of one of these forms, framboidal pyrite, which owes its structure to the nature of the original bacterial formation of H2S and iron monosulphide. Framboidal pyrite is so small that it typically cannot be observed without magnification. The individual granules of framboidal pyrite are only tenths of a micron in size and so have a very large surface area – typically 2 - 4 m2/g – and so are highly reactive. In contrast, some pyrite in coal appears to have been introduced after the peat had been converted to coal, as is evidenced by pyrite coatings on the fracture surfaces, called cleats, in the coal seams. This pyrite reacts at a much slower rate.
In addition to pyrite, other forms of sulphur that are commonly found in sedimentary strata are organic sulphur and sulphate sulphur. Organic sulphur can be present because it was part of the original make-up of the plant material; alternatively, it can represent sulphate that formed complexes with the decaying organic matter, and so became combined with the structure of the coal. Generally, the organic sulphur component is not chemically reactive and has little or no effect on acid producing potential (Casagrande et al., 1989).
Sulphate sulphur is usually only found in relatively minor quantities in coal and other pyritic rocks, and is commonly the result of weathering and recent oxidation of sulphide sulphur. Some sulphate minerals, like melanterite, can dissolve and form acidity, while others, like gypsum, are not acid producing. Nordstrom (1982) provides a sequential summary of how these sulphate minerals can form Additional information on their characteristics can be found in Rose and Cravotta (1998).
C2.3 Carbonate Minerals in Coal-bearing Strata
As in hard rock deposits, the nature and quantity of the alkalinity that may be present in the form of mineral carbonates is critical in determining whether ARD will form. The principal carbonate minerals encountered in coal deposits are the minerals calcite (or its rock form, limestone) (CaCO3), dolomite (CaMg(CO3)2, and siderite (FeCO3). The presence or absence of calcareous carbonate minerals is extremely important in predicting CMD water quality. These minerals not only neutralize acidic water created by pyrite oxidation, there is also evidence that they actually inhibit pyrite oxidation by buffering the pH at a level where iron, that is released by pyrite oxidation, precipitates as ferric hydroxide rather than oxidizing additional pyrite. Brady et al. (1994) in studies in the southeastern US coal fields showed that the presence of 1-3% carbonate (on a mass-weighted basis) can determine whether a coal mine produces alkaline or acid water. Although pyrite is clearly critical to form acidic CMD, they found that pyrite concentration only correlated with CMD water quality when calcareous carbonates were largely absent. It is not known if this finding applies to coal deposits elsewhere in the world.
Of the carbonate minerals commonly found in sedimentary strata, the most significant is calcite, which is typically found as a cementing material in sedimentary rocks, a result of secondary emplacement (fracture filling), or as limestone, which is commonly found in marine and fresh water sedimentary strata. Dolomite generally forms after the limey sediment is buried and reflects the replacement of much of the calcium with magnesium. Dolomite is less soluble than calcite or limestone, though it will also neutralize acid and can thus inhibit pyrite oxidation. In some cases, the replacement is not significant enough for the rock to be called dolomite; in such instances, the term dolomitic limestone is used.
Siderite requires reducing environments to form and so it generally represents organic-rich fresh water or mildly brackish paleoenvironments that were too low in sulphate for the iron to precipitate as a sulphide. Siderite is less soluble than dolomite, and is not an effective acid neutralizer because the ferrous iron that is released when siderite dissolves eventually oxidizes and hydrolyzes. As a result, the alkalinity that forms initially when the siderite is dissolved is matched by the acidity that is generated by the eventual precipitation of the iron hydroxide. This becomes significant in prediction of ARD as discussed in Chapter 5. Other minerals listed in Table 2.2 of the Guide are generally not present in high enough concentrations to be relevant in coal mine settings.
C2.4. How Geology Affects Coal Mine Drainage Quality
Coal mining typically involves the disturbance of some of the sedimentary strata that are above, and to a limited extent, below the coal seam(s). At surface mines, all of the overburden strata are excavated and broken up, so all have the potential to affect the quality of the CMD. Moreover, most of the coal is generally removed, and so the effect of the overburden strata may be more important than the characteristics of the coal itself. At underground mines, some of the coal remains behind, often exposed to air and water, and so the amount and nature of the pyrite in the coal is much more important in predicting the quality of the subsequent drainage than the overburden strata, which is only partially fractured and disturbed. However, at either type of mine, the CMD is somewhat affected by all of the disturbed strata.
The nature of sedimentary strata affects the formation of CMD. Coal deposits that formed from fresh water organic-rich sediments, such as most coals found in the western United States, tend to be low in pyrite because there was typically not enough sulphate dissolved in such wetlands to generate H2S. In addition, freshwater limestones that may or may not be present in the stratigraphic neighbourhood can provide offsetting alkalinity. Therefore, CMD from fresh water coal deposits may be enriched in sulphate, but if so, it is likely from dissolution of evaporate minerals in the overlying strata rather than from pyrite oxidation.
Deltaic coal deposits are harder to generalize. If the wetland was upper delta, the sulphate concentrations in the sediment would be relatively low and the amount of pyrite present would therefore also be relatively low. However, lower deltaic wetlands tend to be influenced by tidal forces and therefore brackish. Such an environment is ideal for pyrite formation. In either case, unless the stratigraphy overlying the coal seam represents a substantially different paleoenvironment, there would be little chance for limestone formation, so even a relatively low concentration of sulphide could be problematic. Also, deltaic wetlands are very susceptible to changes in climate, storm events, etc. Consequently, the resultant coal deposit may reflect a highly variable paleoenvironment, with a high ash content and highly variable acid-generating potential.
Wetlands that form near oceans and bays tend to produce coal seams that are relatively high in pyrite. Yet, as discussed above, if marine limestone deposits overlie the coal seams, the alkalinity from the limestone may be enough to neutralize the acidity formed. If this is the case, the CMD will be near-neutral in pH, high in sulphate, and would likely contain some dissolved iron.
ARD from coal mines differs from ARD from metal mines in several ways. Although the pyrite oxidation reactions discussed in Section 2.4.4 in this Guide are the same, the other mineral sulphides typically present at metal ore mines are commonly absent, or if present, are minor components, and so CMD water quality tends to be simpler. In addition, since the rock is generally softer, explosives are used much less at coal mines than at most hard rock mines. Therefore, there is less dissolved ammonia present in CMD, which also simplifies water chemistry since metal-ammonia complexes can be difficult to remove in the water treatment process. As a result, the only cations of concern in CMD are typically iron, manganese, and aluminium, listed in the order in which they are most problematic. Iron is present because it is solubilised by the initial pyrite oxidation reactions. The other two metals are commonly dissolved by the acidic water from other sedimentary strata. Of the three, aluminium is the most toxic to aquatic life, but it readily precipitates at near-neutral pH. Iron is typically present in the highest concentrations. Accordingly, as discussed in Chapter 7, CMD water treatment technology is largely focused on iron removal.
Manganese, however, can be problematic in water treatment because removing it through conventional means (addition of alkalinity combined with aeration) requires a relatively high pH (typically above 10), which, if discharged into streams, could be much more toxic to aquatic life than the manganese itself. Therefore, except where the streams are already acidic and would benefit from the alkalinity, the water has to be re-acidified after the manganese is removed.