Technologies > Acid Drainage > Active Treatment
Conventional Lime and Limestone TreatmentObjective:
Limestone (CaCO3), hydrated lime (Ca(OH)2), and quicklime (CaO) are common
neutralizing reagents in conventional acidic drainage treatment. Chemical
neutralization of acidity occurs from the addition of the above chemicals,
followed by the precipitation of iron, aluminum, manganese, and other co-precipitates.
Description:
Limestone
Limestone (CaCO3) has been used extensively in active and passive treatment
systems to neutralize acidity and precipitate metals in acidic drainage. Limestone
is naturally occurring, extracted from open-pit and underground mines, and
generally consists of 50 to 90% calcium carbonate. Impurities in limestone
generally consist of silica, alumina, and iron oxide (Lewis and Boynton, 1995).
Acidic drainage treatment quality limestone must be selectively mined to maintain
a high calcium content (~90% CaCO3). Limestone with lower calcium content is
inadequate for the treatment of acidic drainage. Calcium carbonate is the standard
alkalinity measurement when calculating total acidity (refer to acid neutralization
calculation).
Quicklime
Quicklime (CaO) is produced from a process called calcination. During calcination,
limestone (CaCO3) is heated to over 1000oC in a rotary or vertical-shaft
kiln. Carbon dioxide gas is released from the limestone, leaving calcium
oxide, according to the following equation:
CaCO3 --------------> CaO + O2
Hydrated Lime
Hydrated (or slaked) lime (Ca(OH)2) is produced by hydrating quicklime with
water through the use of industrial hydrators. Hydrated lime, in powder form,
is the end product of this process. Hydrated lime is the most common liming
amendment used to treat large volumes of highly-acidic water.
Calcium in limestone is occasionally replaced by magnesium through a process called dolomitization. Dolomitization can result in significant magnesium content forming dolomitic limestone (CaCO3•MgCO3). Calcination of dolomitic limestone produces dolomitic lime (CaO•MgO) and dolomitic hydrate (Ca(OH)2•MgO). Dolomitic lime and dolomitic hydrate neutralize acid at slightly lower rates than quicklime and hydrated lime due to the lower solubility of magnesium hydroxide. Additionally, magnesium can precipitate out of solution at pH values of around 9.0, wasting the magnesium component (Lewis and Boynton, 1995). However, acidic treatment operations rarely raise pH above this 9.0 so magnesium precipitation is not a concern. Dolomite lime and hydrated usually have limited availability.
Neutralization of free acidity present in acidic drainage (sulfuric acid or H2SO4) by limestone and lime treatment occurs through the following reactions:
Limestone
CaCO3(aq) + H2SO4(aq) ---------> CaSO4(s) + H2CO3(aq)
3CaCO3(aq) + Fe2(SO4)3(aq) + 6 H2O ---------> 2CaSO4(s) + 2Fe(OH)3(s) + 3H22CO3(aq)
3CaCO3(aq) + Al2(SO4)3(aq)
+ 6H2O ---------> 3CaSO4(s) + 2 Al (OH)3(s) + 3 H2CO3(aq)
Quicklime and Hydrated Lime
Note that the first reaction involves the conversion of quicklime (CaO) to
hydrated lime (Ca(OH)2) with the addition of water.
CaO(s) +H2O --------> Ca(OH)2(aq)
Ca(OH)2(aq) + H2SO4(aq) -------->CaSO4 + 2H2O
Ca(OH)2(aq> + FeSO4(aq) --------> Fe(OH)2(s) + CaSO4(s)
3Ca(OH)2(aq) + Fe2(SO4)3(aq)
--------> 2Fe(OH)3(s) + 3CaSO4(s)
In the above reactions, addition of limestone, quicklime, and hydrated lime results in the neutralization of sulfuric acid and the formation of hydroxides complexes and gypsum salt (CaSO4•2H2O).
Hydrolysis reactions are the primary mechanisms of metal precipitation during active treatment of acidic drainage. During hydrolysis, oxidized dissolved metals are combined with water, resulting in the formation of metal oxides, hydroxides, and oxy-hydroxides and the release of proton acidity. Several metal cations can be removed by hydrolysis reactions, including Fe+3, Mn+4, and Al+3. A generic equation for a metal hydroxide forming hydrolysis reaction is:
Me+n + n(OH)- --------> Me(OH)n
Where Me is metal cation
Hydrolysis reactions for specific metals are shown in the following equations:
Fe+3 + 3H2O --------> Fe(OH)3 + 3H+
Fe+3 + 2H2O --------> FeOOH + 3H+
Al+3 + 3H2O --------> Al(OH)3 + 3H+
Mn+4 +2H2O --------> MnO2 + 4H+
In acidic drainage waters, the above metal oxide, hydroxide, and oxy-hydroxide complexes are generally hydrated. A generic equation for a hydrated metal hydroxide is:
Me(OH)n.nH2O
The majority of metal hydroxides have minimum solubility at pH values ranging
from 7 to 10. The solubility of most metals becomes limited around pH values
of 5.5 to 6.0 (Figure 1). Aluminum hydroxide, a white gelatinous precipitate,
forms begins to form at pH 4.5. Ferric hydroxide forms around 3.5, while
ferrous hydroxides do not form until a pH of 8.5. Iron hydroxide and oxy-hydroxide
precipitates are yellow and red in color. In addition to pH controls on dissolved
metal precipitation, dissolved oxygen content and oxidation-reduction potential
influence hydrolysis reactions.
Figure 1. Solubility of metal hydroxides as a function of pH. From MEND, 1994.
Metal oxides, hydroxides, and oxy-hydroxides, especially ferric iron and aluminum hydroxides can scavenge trace metals such as Cd, Cu, Ni, Pb, and Zn from the free water column in a process called co-precipitation (Parker and Robertson, 1999). During co-precipitation trace metal cations adsorb to negatively charged surface sites of metal hydroxide and oxy-hydroxide precipitates. Since co-precipitation usually occurs on metal hydroxides and oxy-hydroxides, co-precipitation primarily occurs under aerobic conditions.
In addition to the removal of dissolved metals through hydrolysis reactions and co-precipitation, metals may precipitate as metal carbonate complexes. Carbonates are present in acidic drainage from the dissolution of limestone, either added to acidic drainage waters during passive pre-treatment methods or during treatment. Metal carbonates generally require higher pH values to precipitate out of solution than metal hydroxides.
The accumulation of metal hydroxide, metal carbonate and gypsum precipitates results in sludge. The characteristics of treatment sludge vary depending on properties of the acid drainage and treatment process and neutralizing reagents used (Tremblay and Hogan, 2000).
Climate:
Conventional lime and limestone treatment of acidic drainage can be implemented
in all climates. However, some lime and limestone treatment operations that
use water or battery powered dosers may be sensitive to freezing conditions.
Treatment Process:
Conventional limestone and lime treatment of acidic drainage may occur along
a stream or in an acidic drainage treatment facility. Treatment facilities
are most often used in situations where large volumes of highly-acidic drainage
are being released into the environment. Conventional limestone and lime
treatment of acid drainage generally occurs through the following steps (Figure
2):
1) Equalization (holding tank)
2) Neutralization (mixing)
3) Aeration (metal oxidation)
4) Sedimentation/Clarification (particle settling)
5) Sludge Disposal
Figure 2. Simplified schematic of a conventional lime treatment process. From
Sengupta (1993)
Equalization occurs through the use of holding basins, ponds, or large sumps to collect acidic drainage in order to minimize flow variations which may complicate the treatment process. Flow in streams and ground water discharge from adits and mine tunnels vary seasonally. During spring, stream flow is increased due to runoff from snow melt and rainfall. Ground water discharge rates are also increased during the spring due to recharge from precipitation. Generally it is required that the volume of the equalization basin is sufficient to store 2 to 3 days flow of acid drainage (Sengupta, 1993). Equalization basins are not used at all lime treatment operations.
As acid drainage flows from the equalization basin or in an acid-drainage stream, the neutralization stage involved adding a neutralizing reagent to the acidic water. In lime and limestone treatment operations, dry finely-ground limestone or lime (calcium hydrate or calcium oxide) is stored in an on-site silo. Limestone and lime can be released into acidic waters in either dry, powdered form or as a slurry. Releasing dry powdered lime can result in incomplete mixing due to: 1) open channel velocity variations in a streams or 2) diffusion-limited transport in still surface water bodies such as treatment ponds. Incomplete mixing reduces the effectiveness of the neutralization process. If the treatment operation uses a lime slurry, mixing of water and lime occurs in a mixing tank. In the mixing tank, a specified amount of dry, powdered lime is added to water, creating a lime slurry referred to as “milk”. The milk is then dispensed at a controlled dosage rate into streams, wetlands, lakes, or ponds.
Dosers are typically used to dispense limestone, lime amendments, and other neutralizing reagents into streams (Figure 3). A doser is a fully-automated mechanical device used to release powdered or slurried neutralizing material into streams (Olem, 1991). Dosers work in conjunction with a storage silo. Dosers may be powered by direct electricity, batteries, or by stream flow. Doser selection for use in a liming operation is dependant on access to electricity, stream discharge, cost, and type of liming amendment.
Figure 3. Diagram of an electrically-powered doser. From Olem (1991) after
Ostensson (1983)
Electrical-powered dosers operate on 120/240 V AC, dispense a limestone slurry, and can be used to neutralize acid drainage in stream with up to 25 m3/s (880 cfs) (Fraser et al., 1985). Electrical-powered dosers can either mechanically mix the lime into a slurry in a mixing tank or use stream flow to aid in the mixing process. Mechanical mixing of lime is a power consuming process and increases treatment costs. Due to energy demands, electrical-powered dosers are sensitive to power failures. Additional maintenance of electrical dosers may be required if the slurry clogs the doser unit.
Battery-powered dosers use a 12V DC battery as a power source and require dry, powdered lime that is not mechanically mixed due to the energy demands that mixing requires. Battery-dosers are used for remote locations where electricity is not available and can be used to treat streamflows up to 8 m3/s (280 cfs) (Fraser et al., 1985). Battery powered dosers are sensitive to freezing.
Water-powered dosers are also an alternative to electrical-powered dosers in remote regions and can be used in steams that have up to 3 m3/s (100 cfs) of discharge (Fraser et al., 1985). An advantage of water-powered dosers is the mechanical rate of lime amendment is dependent on stream flow, accounting for stream flow variations. Electrical dosers also have the ability to release neutralizing amendments at varying rates, but water level is used to calculate steam discharge based on empirically determined stream level to discharge relationships. Water powered dosers are also sensitive to freezing. Refer to Fraser et al. (1985) for a more comprehensive review of doser types and their limitations.
Aeration is the next process once the lime amendment is added to the acid drainage waters. Aeration is the process of using oxygen or other oxidants such as chloride compounds, peroxides, and potassium permanganate to oxidize ferrous iron and other dissolved metals. Biological reactions can also facilitate the oxidation of ferrous iron. Once metals are oxidized, they can precipitate as oxides, hydroxides, and oxy-hydroxides. Aeration occurs in constructed aeration basins which are designed so that the detention time is sufficient to oxidize ferrous iron but insufficient for metal precipitates to settle of out solution. Mechanical aerators are usually used in the aeration process (Figure 4).
Figure 4. Floating mechanical aerator for a hydrated lime treatment system. In Phipps et al. (1996) after U.S. EPA (1983).
The rate of oxidation and consequent precipitation of metals depends on dissolved
oxygen content, oxidation-reduction potential, pH, and dissolved metal concentrations
in the solution. The equations for the oxidation of ferrous iron to ferric
iron using oxygen as the oxidant are:
The oxidizing capacity of an aeration system depends on the amount of iron in solution that needs to be oxidized. Maximum aeration capacity should not be greater than the saturation point of oxygen in water which is 8.25 mg/L at 25oC and atmospheric pressure (Langmuir, 1997). Note that oxygen solubility in water is temperature-dependant and decreases with decreasing temperature.
The theoretical oxygen demand for mine waters can be calculated using the
following equation (Sengupta, 1993):
Oxygen is usually obtained from atmospheric air which contains about 21% oxygen
by volume. Total air needed to supply the theoretical oxygen demand is calculated
by the following equation (Sengupta, 1993):
The oxygen transfer efficiency parameter is a function of how much air comes
in contact with the air-water interface and varies based on the type of aeration
system used.
After neutralization and oxidation of acid drainage, precipitated metals and
other suspended solids are allowed to settle out of solution by gravity into
sedimentation basins or clarifiers in acid-drainage treatment facilities. Flocculants
and coagulants are often added during the acid drainage treatment process to
enhance particle settling rates and efficiency of metal removal.
Settling basins are typically of earthen construction with lined bottoms to prevent seepage. Typically settling basins are designed to allow a residence time of 12 to 48 hours to allow sufficient particle settling. Clarifiers in treatment plants act as settling basins and have weir overflows where clarified water is released from the settling basin. In settling basins and clarifiers, metal precipitates settle on the bottom. The accumulation of metal precipitates on the floor of clarifiers and settling basins is called sludge. Settling basins and clarifiers must be designed to facilitate the sludge removal. The rate and volume of sludge accumulation is governed by neutralizing reagents and dissolved metal concentration in the acid drainage water.
High-density sludge (HDS) technology can be employed at most conventional lime treatment plants. High-density sludge (HDS) treatment is a process in which a fraction of the hydroxide sludge is recycled to act as a flocculant, promoting the formation of metal precipitate flocs. HDS reduces sludge volume and results in sludge with higher solids content than found in conventional acidic drainage treatment operations. Sludge from conventional lime treatment operations generally consists of 1 to 5 percent solids, while the high density sludge treatment process results in sludge that ranges from 10 to 40% solids (Tremblay and Hogan, 2000).
The removal of sludge is usually labor intensive and can be a major cost in acid drainage treatment operations. Sludge removal options include leaving the sludge at the bottom of the settling basin, pumping or hauling sludge from ponds to abandoned mine tunnels, open pits, tailings ponds, or designed repositories or dumping sludge into piles where the sludge is allowed to oxidize and dry. Dry, oxidized sludge can be placed on the ground surface which results in a relatively inert, crystalline sludge which can be incorporated into soil or rock. The decision of leaving sludge at the bottom of a sedimentation basin is dependant on the volume of the sedimentation basin, although most lime treatment operations are long-lived and sedimentation basins fill with sludge. When settling basins fill completely with sludge, the sludge is removed or the settling basin is abandoned and a new settling basin is constructed. In most acid drainage treatment operations, sludge is removed either constantly at a certain rate or at specific time intervals.
From the settling basin, treated water leaves the acid drainage treatment facility. This water has a near neutral or slightly alkaline pH and lower metal content then it had in its pre-treated state. Upper and lower limits on effluent pH have been established to protect aquatic organisms from highly acidic or alkaline conditions. The required pH values range from 6 (slightly acidic) to 9 (slightly alkaline) (Tremblay and Hogan, 2000). Based on dosage, pH of the effluent water may have to be lowered using sulfuric or carbonic acid before the effluent water is released.
Requirements and Limitations of Use:
Although limestone, quicklime, and hydrated lime have the potential to neutralize
acidity, selection of a liming amendment is dependant on the following criteria:
1) location (lake, pond, stream, soil, watershed)
2) acidity of water
3) volume of water body (lakes)
4) flow rate (streams)
5) dissolution rate
6) availability and cost
Limestone
Limestone is the most cost effective, safest, and easiest to handle of all
of the neutralizing compounds. Limestone is a carbonate compound which has
been shown to raise pH in acidic waters to around 7 or 8, limiting the risk
of over-liming. Acid neutralization with limestone produces heavy, low volume
sludge. However, drawbacks to using limestone for neutralization of acid
drainage is its slow dissolution rate (pKcalcite = 8.48) (in Langmuir, 1997
from Nordstrom et al., 1990) which is considerably slower than quicklime
or hydrated lime. This results in a gradual increase in pH which is not adequate
for stream or highly-acidic systems. To enhance limestone dissolution in
streams, limestone is finely ground to increase surface area of limestone
particles. In addition to slow dissolution rates, limestone particles can
become armored by iron, aluminum, and magnesium hydroxides and oxy-hydroxides
in stream systems, limiting the neutralization process. Due to armouring
and incomplete dissolution from impurities, limestone has a neutralizing
capacity of about 30% (Skousen et al., 1996).
The same properties of limestone that can be disadvantageous for rapid neutralization of acid drainage can be advantageous for other treatment technologies. Due to the slow dissolution rate of limestone, addition of limestone in conjunction with passive systems such as diversion wells, anoxic limestone drains, open limestone channels, wetlands and stream limestone addition has shown promising results as acid neutralization occurs through slow, time-dependent alkalinity releases. Passive systems may be used to pre-treat acidic drainage waters by adding alkalinity before conventional treatment.
For treating acid drainage, limestone should have a high calcium content (~90%), low magnesium content, and high specific surface area. Particle sizes of limestone used in acid-drainage treatment are dependent on the pH of the acid drainage and type of water body. Ground limestone passing a #325 mesh (Sengupta, 1993) is generally used in active treatment of acidic steams where rapid neutralization rates are desirable. The majority of passive systems such as anoxic limestone drains, limestone channels, and diversion wells use limestone pebbles and cobbles to slowly release alkalinity into the system.
Hydrated Lime and Quicklime
Hydrated lime and quicklime are best used in large flow, high acidity situations.
Hydrated lime is the most common lime amendment used for acid drainage. Selection
between these two lime amendments may depend on cost, availability, and extra
equipment associated with slaking quicklime. Refer to Lewis and Boynton (1995)
for further discussion on slaking of quicklime. Quicklime and hydrated lime
are extremely caustic and require additional care than limestone. These liming
reagents react very quickly and can raise pH in excess of 12. Extra caution
must be used when calculating dosage rates to avoid over liming. Quicklime
and hydrated lime have neutralizing efficiencies of 90% (Skousen et al.,
1996). The particle size of hydrated lime generally passes through a #200
mesh (Sengupta, 1993; Lewis and Boynton, 1995) while particle size of quicklime
is between a #8 and #100 mesh (Sengupta, 1993). Quicklime and hydrated lime
produce heavy, low volume sludge.
Sludge Stability
Sludge is composed of metal hydroxide precipitates and gypsum. A concern of
sludge management and disposal is the long-term stability of acid drainage
treatment sludge.
MEND (1997) conducted a survey of the physical properties and stability of treatment sludges from 11 lime treatment operations receiving acidic effluent from metal mines. The treatment operations incorporated three types of lime treatment: 1) batch treatment, 2) conventional limestone treatment, and 3) high-density sludge treatment. Samples of both “new” and “old” sludge were analyzed. Overall, they found that pH values of treatment sludged ranged from 8.2 to 10.8, with pH values decreasing with age. The decrease in pH was attributed to the consumption of alkalinity during the aging process. Solids content in the treatment sludges ranged from 2 to 32%. The solids content increased an average of 25% as the sludges aged. Upon acid leaching tests (acetic acid and synthetic acid rainfall), the older sludges exhibited higher stability which was attributed to the mineral formation. However, all sludges became unstable under constant acid leaching. Sludge stability was determined to be dependent on alkalinity present in the sludge. When alkalinity is exhausted, the sludge may become unstable. For this reason MEND (1997), did not recommend the storage of treatment sludges in tailings or waste rock where pyrite oxidation would result in acid formation. Providing that treatment sludge is stored in an area with minimal acid exposure, mineralogical data from this MEND study suggests that long-term aging (thousands of years) would transform the sludges into stable carbonate rock with a minor iron oxide component.
Calculating Neutralization Dosages for Acidic Drainage:
In order to neutralize acidic drainage, total acidity of the water body must
be calculated. First, acidity is determined from acid-base titrations of
water samples collected from an acid drainage affected water body (for a
review of acid-base chemistry see Langmuir, 1997). Then, acidity of the water
sample is multiplied by the volume or flow rate, depending on whether acid
neutralization is desired of a still surface water body or a stream. Two
methods of calculating amounts of chemical amendments required to neutralize
acidity are presented. The first method is volumetrically determines dosage
and is suitable for lakes, wetlands, and ponds. The second method uses stream
discharge measurements to determine dosage rates. In addition to quicklime,
limestone, and hydrated lime, alternative neutralizing reagents are included
for comparison.
The following calculations use the following assumptions:
1) Complete, rapid mixing
2) Chemical kinetics are ignored
3) The effects of particle settling velocities are negligible
4) Neutralization efficiencies are not taken into account
5) No dissolved organic matter
Useful Conversions
1 in = 2.54 cm
1 ft = 0.3048 m
1 ft3 = 28.32 L
1 gal = 3.79 L
1 cfs = 0.028 m3/s = 448.8 gpm
1 kg = 2.2 lbs
1 ton = 2000 lbs
Total alkalinity determination based on volume
Appropriate for lakes, wetlands, and ponds
Acidity Volume = Total Acidity (# moles CaCO3 equivalent acidity)
The amount of base required for acid neutralization is dependent on the molecular weight of the neutralizating compound (Table 1). The order of the amount of neutralizing reagents for the same acidity starting at the highest is soda ash, limestone, hydrated lime, quicklime, caustic soda, ammonia.
Table 1. Common neutralizing compounds and their molecular weight.
Total Acidity X Molecular Wt of Neutralizing Compound / 1000 (conversion
Factor) = Amount of Neutralizing Compound Needed.
(moles) (g/mol) (grams to kilograms) (kg)
Examples:
200,000 ft3 pond with 1000 mg/L acidity
Moles of CaCO3 equivalent acidity:
Neutralization Amounts:
Limestone
Hydrated Lime
Quicklime
Caustic Soda
NaOH can be in liquid form with 20% or 50% NaOH by volume. An additional step
is necessary to calculate the amount of caustic soda into gallons (Skousen,
1996):
Soda Ash
Ammonia
Total alkalinity determination based on stream discharge
Acidity Load = Moles of CaCO3 equivalent acidity per minute
Examples
Stream flowing at a rate of 20 cfs with 75 mg/L acidity
Limestone
Hydrated Lime
Quicklime
Caustic Soda
Soda Ash
Ammonia
Predicted Performance/Effectiveness:
Although costly, conventional lime treatment is the standard in acidic drainage
treatment, especially for high flow, high acidity situations. Lime (and limestone)
based metal precipitation technology has been proven and has been shown to
be effective in constantly providing acceptable effluent quality under normal
operating conditions (Tremblay and Hogan, 2000).
Conventional acidic drainage treatment can also incorporate other synergistic
technologies to either increase metal removal efficiencies under challenging
situations or
or decrease sludge volume. These technologies include oxidants, flocculants,
coagulants, and ion exchange to further enhance the ability to remove metals
from acid drainage waters. High-density sludge treatments aid in minimizing
the volume of metal-laden sludge produced at these lime treatment operations.
Table of pre and post treatment water chemistry
Summary conclusions based on case studies
Refer to MEND (1997) Report 3.42.2a
Synergistic Technologies:
Coagulants
Flocculants
Oxidants
High Density Sludge
Costs
Skouosen et al. (1996) presented the following table to compare the costs of
alternative technologies to treat acid mine drainage in West Virgina during
a 20 year operation period (Table 2). Alternative treatment chemicals are
included for comparison.
Table 2. Costs of alternative technologies to treat acid mine drainage in West Virginia, 20 year operation period. From Skousen et al., 1996. All money values are in 1990 U.S. dollars.
Case Studies:
Refer to MEND (1997) Project 3.42.2a
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