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  • 8/12/2019 Bandung Nbsp Perry Nbsp Paper

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    Estimating Mine Water Composition from

    Acid Base Accounting and Weathering Tests; Applications from U. S. Coal Mines

    Eric F. Perry(1)

    Mine water quality for coal mines in the United States is estimated using whole rockanalysis for Acid/Base Accounting (ABA), or with simulated weathering tests. ABAcompares the quantity of acidity that can be generated from pyrite oxidation to the

    amount of bases, mostly carbonates, that are available to neutralize acid. Studies of

    surface mine drainage and overburden rocks in the Appalachian region show that thequantity of acid neutralizers is the most important factor controlling mine water quality.

    Mines producing net alkaline drainage (alkalinity >acidity) contain more than 2 to 3 %

    neutralizers in overburden rocks, and had an excess of neutralization potential comparedto acid production potential. A ratio of about 2:1 or greater of neutralization potential to

    potential acidity also produces net alkaline mine drainage. Most mines can be classified

    as to potential to generate acid or alkaline waters. There is a small range of ABA

    properties where both acid and alkaline waters occur, and interpretation from ABA aloneis uncertain. These relationships are consistent across different coalbeds and overburden

    rocks. Similar ABA classifications have been proposed for base and precious metal

    mines. Concentrations of metals or sulfate cannot be determined directly from Acid BaseAccounting, however.

    Simulated weathering tests have the capacity to estimate mine water compositionincluding pH, and relative amounts of metals, sulfate, and trace elements. The relative

    rates of acidity and alkalinity production can also be estimated from weathering tests.

    Products of pyrite oxidation are soluble and are produced rapidly, while production ofalkalinity is limited by carbonate solubility. Weathering tests are especially useful where

    ABA results are inconclusive, or the rocks contain sulfide minerals other than pyrite.Different test protocols including columns, cells and soxhlet extractors are in use, so test

    results must be evaluated against the specific test procedure. Rock to water ratio,

    flushing frequency, pore gas composition and test length influence the results. A scaling

    factor relating the laboratory results to mine site conditions is usually required, andappears to be site specific.

    Examples of mine drainage prediction and actual mine water quality are given for bothAcid Base Accounting and simulated weathering tests. Both overburden test methods

    should be used in conjunction with other geologic, hydrologic and mine site data to

    estimate post-mining water quality.

    Eric Perry is a Hydrologist, U.S. Dept. of Interior, Office of Surface Mining, 3 ParkwayCenter, Pittsburgh, PA, 15220, USA. Email [email protected]

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    Introduction

    Mining of coal and minerals in the United States (U.S.) can sometimes produce acid

    drainage and elevated concentrations of metals, dissolved solids, and sulfate in surface

    and ground waters. To prevent water pollution by mining operations, testing ofoverburden and waste rock is conducted in advance of mining. The purpose of testing is

    to identify rocks with potential to generate acidic drainage, and determine which rockscan neutralize acidity and generate alkalinity. Some mines also test rock and soil to selectmaterials that can be used for reclamation and plant growth.

    Geochemical test methods are of two general types; static or whole rock analyses, andkinetic or simulated weathering tests. Static tests include Acid Base Accounting(ABA),

    X-ray diffraction for mineral identification, elemental analyses, exchangeable acidity,

    cation exchange capacity and others. Acid Base Accounting compares the quantity ofacidity that can be generated from pyrite oxidation to the amount of bases, mostly

    carbonates that are available to neutralize acid. It is the most common static test used for

    testing overburden and waste rock at U.S. coal mines.

    Kinetic or simulated weathering tests include various leaching protocols and batch

    extract tests. Kinetic tests attempt to simulate chemical weathering of rocks in contact

    with leach water. Mine water composition, including pH, metals, acidity, and alkalinityis estimated from the leachate chemistry. Column leaching tests are the most frequently

    used kinetic test method. Kinetic tests are especially useful where ABA results are

    inconclusive, or the rocks contain more than one sulfide mineral. They are often used toevaluate the acid drainage potential of waste rock and tailings from base and precious

    metal mines.

    The purpose of this paper is to review the use, assumptions and limitations of static and

    kinetic test methods for predicting mine drainage quality. Examples of mine drainageprediction and actual mine water quality are given for both Acid Base Accounting and

    simulated weathering tests. Both overburden test methods should be used in conjunction

    with other geologic, hydrologic and mine site data to estimate post-mining water quality.

    The U.S. has major coal deposits ranging from lignite to anthracite grade in several

    fields. Most coal mined is either subituminous or bituminous. Figure 1 shows the

    location of major U.S. coal deposits and the approximate percentage of mines in eachfield encountering acid forming materials. The most severe acid drainage associated with

    coal mining occurs in northern Appalachian and the Eastern Interior region. Rocks in

    these regions are Upper Pennsylvanian age, consisting of cyclothems of coal, shale,limestone and sandstone. The rocks generally contain moderate amounts of pyrite and

    carbonates. Annual precipitation is about 1000 to 1500 millimeters per year in these

    areas, and the climate is humid continental. The southern Appalachian region, which has

    less acid drainage, contains Lower Pennsylvanian age rocks, which generally contain lowconcentrations of pyrite and carbonates. The Powder River Basin is semi-arid and

    receives about 250 to 375 millimeters of precipitation per year. Coal bearing rocks are

    Cretaceous age, and generally contain low amounts of pyrite and moderate concentrationsof carbonates.

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    Figure 1. Extent of Acid Drainage From U.S. Coal Mines.

    Most active base and precious metal mining is concentrated in the western U.S. Aciddrainage from old metal mining occurs in Colorado, Montana, California and several

    other states. Geologic, geochemical, and hydrologic conditions largely determine the

    potential for acid drainage form coal and metal mines.

    Static Test Methods

    Origin of Acid Base AccountingAcid Base Accounting is the most frequently used static test for estimating acid drainage

    potential. ABA was developed at West Virginia University by soil scientists interested inreclamation (Skousen et al., 1990). The approach came from early attempts at classifying

    mine spoils for revegetation potential, based on acidity or alkalinity, and rock type. From

    these classifications, they could determine if plants could grow on the mine spoil, andwhether lime should be applied.

    In 1971, West Virginia University began to formally develop a system of balancing the

    acid and alkaline producing potential of rocks. This work included coal overburden rocks

    throughout the Appalachian and Interior coal basins. The importance of acid neutralizingminerals was recognized and quantified, and the term "neutralization potential" (NP) was

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    introduced. This work was published in a series of reports, including a manual of

    recommended field and laboratory procedures (Sobek et al.,1978).

    ABA, as originally developed, consists of measuring the acid generating and acidneutralizing potentials of a rock sample. These measurements of Maximum Potential

    Acidity (MPA) and Neutralization Potential (NP) are compared to obtain a Net

    Neutralization Potential (NNP), or net Acid-Base balance for the rock as follows:

    Net Neutralization Potential (NNP) = NP MPA (1)

    The measurements are usually reported in tons per thousand tons of overburden or parts

    per thousand(ppt). The units designation reflects the agricultural origins of ABA. One

    acre (0.40 hectatres) of plowed agricultural soil weighs about 1000 tons (907 kilograms).Liming requirements are usually expressed in tons per acre (kg/hectare). The units of

    measure for ABA are therefore comparable to lime requirement designations for

    agricultural lands.

    Maximum Potential Acidity(MPA)

    The acid generating potential, MPA, is calculated from a measurement of the total sulfurcontent of the rock by combustion in a sulfur furnace. It is assumed that sulfur is present

    in the form of pyrite (FeS2). For most coal overburden rocks, this is a good

    approximation, and potential acidity calculations are valid. If the rocks have undergonesignificant chemical weathering and contain some sulfate minerals such as gypsum

    (CaSO4* 2 H2O ) melanterite (FeSO4* 7 H2O) and others, total sulfur content may not

    accurately reflect potential acidity.Alkaline earth sulfate salts like gypsum are nonacid

    formers. Metal sulfate salts, however, are intermediate products of pyrite oxidation, andrepresent "stored acidity". These minerals can undergo dissolution and hydrolysis withacid generation. Sulfate sulfur cannot be ruled out

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