Overview of Acid Mine Drainage Treatment with Chemicals

Agricultural & Natural Resources Development Home Page | Land Reclamation Home Page

Jeff Skousen, Tiff Hilton, and Ben Faulkner

Contents

Acid Mine Drainage Formation
Chemical Treatment of Acid Mine Drainage
Table 1.Chemical Compounds used in AMD treatment.
Chemicals
Costs of Treating AMD
Other Aspects of AMD Treatment Technologies
Table 2. Costs in 1996 of six chemicals to treat acid mine drainage in West Virginia.
Table 3. Chemicals for acid neutralization, coagulation/flocculation, and oxidation.
Summary
Acknowledgments
References

Acid Mine Drainage Formation

Acid mine drainage (AMD) forms when sulfide minerals in rocks are exposed to oxidizing conditions in coal and metal mining, highway construction, and other large-scale excavations. There are many types of sulfide minerals, but iron sulfides common in coal regions, pyrite and marcasite (FeS2), are the predominant AMD producers. Upon exposure to water and oxygen, pyritic minerals oxidize to form acidic, iron and sulfate-rich drainage. The drainage quality emanating from underground mines or backfills of surface mines is dependent on the acid-producing (sulfide) and alkaline (carbonate) minerals contained in the disturbed rock. In general, sulfide-rich and carbonate-poor materials are expected to produce acidic drainage. In contrast, alkaline-rich materials, even with significant sulfide concentrations, often produce alkaline conditions in water.

Acidity in AMD is comprised of mineral acidity (iron, aluminum, manganese, and other metals depending on the specific geologic setting and metal sulfide) and hydrogen ion acidity. Approximately 20,000 km of streams and rivers in the United States are degraded by AMD. About 90% of the AMD reaching streams originates in abandoned surface and deep mines. Since no company or individual claims responsibility for reclaiming abandoned mine lands (AML), no treatment of the AMD occurs and continual contamination of surface and groundwater resources results. In a previous Green Lands article (Winter 1996 edition), Ziemkiewicz and Skousen reviewed strategies used to control AMD at its source. In this article, chemical treatment alternatives are discussed.

Chemical Treatment of Acid Mine Drainage

Since the passage of the Surface Mining Control and Reclamation Act (SMCRA) in 1977, coal mine operators have been required to meet environmental land reclamation performance standards established by federal and state regulatory programs. Operators must also meet water quality standards established in the Clean Water Act of 1972 (CWA), which regulates discharges into waters of the U.S. Control of AMD is a requirement imposed on operators by both SMCRA and CWA. In addition to the surface mining permit, each mining operation must be issued a National Pollutant Discharge Elimination System (NPDES) permit under CWA. Allowable pollutant discharge levels are usually determined by the U.S. Environmental Protection Agency's (EPA) technology-based standards, or the discharge levels may be based on the more stringent water quality-based standards where discharges are being released into streams with designated uses. If AMD problems develop during mining or after reclamation, a plan to treat the discharge must be developed. Treatment of AMD includes neutralization of acidity and precipitation of metal ions to meet the relevant effluent limits. In most cases, a variety of alternative treatment methods can be employed to meet the limits specified.

NPDES permits on surface mines usually require monitoring of pH, total suspended solids (TSS), and iron and manganese concentrations. Other parameters may be requested by the regulatory authority in a particular mining situation. However, in order for an operator to make a selection of an AMD treatment system, one must determine (in addition to the above parameters) the flow rate, the receiving stream's flow and quality, availability of electrical power, the distance from chemical addition to where the water enters a settling pond, and the settling pond's volume for water retention time. After evaluating these variables over a period of time, the operator can consider the economics of different chemicals and alternative AMD treatment systems. Most AMD chemical treatment systems consist of an inflow pipe or ditch, a storage tank or bin holding the treatment chemical, a means of controlling its application rate, a settling pond to capture precipitated metal oxyhydroxides, and a discharge point. The latter is the point at which NPDES compliance is monitored. The amount of chemical needed for neutralization can be calculated by multiplying the flow (gpm), the AMD's acidity (mg/l), and a factor of .0022. The product is the tons of acid that require neutralization per year (calcium carbonate equivalent). This value (tons of acid/yr) can then be multiplied by a conversion factor for each chemical to determine the amount of the chemical needed.

Overview of Chemicals Available to Treat AMD
Six primary chemicals have been used to treat AMD (Table 1). Each chemical has characteristics that make it more or less appropriate for a specific condition. The best choice among alternatives depends on both

technical and economic factors. The technical factors include acidity levels, flow, the types and concentrations of metals in the water, the rate and degree of chemical treatment needed, and the desired final water quality. The economic factors include prices of reagents, labor, machinery and equipment, the number of years that treatment will be needed, the interest rate, and risk factors.

Table 1. Chemical Compounds used in AMD treatment.
Common Name Chemical Name Formula Conversion Neutralization 1996 Cost3
      Factor1 Efficiency2 $ per ton or gallon
          Bulk <Bulk
Limestone Calcium carbonate CaCO3 1 30% $10 $15
Hydrated Lime Calcium hydroxide Ca(OH)2 0.74 90% $60 $100
Pebble Quicklime Calcium oxide CaO 0.56 90% $80 $240
Soda Ash Sodium carbonate Na2CO3 1.06 60% $200 $320
Caustic Soda (solid) Sodium hydroxide NaOH 0.8 100% $680 $880
20% Liquid Caustic Sodium hydroxide NaOH 784 100% $0.46 $0.60
50% Liquid Caustic Sodium hydroxide NaOH 256 100% $1.10 $1.25
Ammonia Anhydrous ammonia NH3 0.34 100% $300 $680

1 The conversion factor may be multiplied by the estimated tons acid/yr to get tons of chemical needed for neutralization per year. For liquid caustic, the conversion factor gives gallons needed for neutralization.

2 Neutralization Efficiency estimates the relative effectiveness of the chemical in neutralizing AMD acidity. For example, if 100 tons of acid/yr was the amount of acid to be neutralized, then it can be estimated that 82 tons of hydrated lime would be needed to neutralize the acidity in the water (100(0.74)/0.90).

3 Price of chemical depends on the quantity being delivered. Bulk means delivery of chemical in a large truck, whereas <Bulk means purchased in small quantities. Liquid caustic prices are for gallons. Others in tons.

Metal Precipitation and pH
Enough alkalinity must be added to raise water pH and supply hydroxides (OH-) so dissolved metals in the water will form insoluble metal hydroxides and settle out of the water. The pH required to precipitate most metals from water ranges from pH 6 to 9 (except ferric iron which precipitates at about pH 3.5). The types and amounts of metals in the water therefore heavily influence the selection of an AMD treatment system. Ferrous iron converts to a solid bluish-green ferrous hydroxide at pH >8.5. In the presence of oxygen, ferrous iron oxidizes to ferric iron, and ferric hydroxide forms a yellowish-orange solid (commonly called yellow boy), which precipitates at pH >3.5. In oxygen-poor AMD where iron is primarily in the ferrous form, enough alkalinity must be added to raise the solution pH to 8.5 before ferrous hydroxide precipitates. A more efficient way of treating high ferrous AMD is to first aerate the water (also outgassing CO2), causing the iron to convert from ferrous to ferric, and then adding a neutralizing chemical to raise the pH to 6 or 7 to form ferric hydroxide. Aeration after chemical addition is also beneficial. Aeration before and after treatment usually reduces the amount of neutralizing reagent necessary to precipitate iron from AMD. Aluminum (Al) hydroxide generally precipitates at pH > 5.0 but also enters solution again at a pH of 9.0. Manganese precipitation is variable due to its many oxidation states, but will generally precipitate at a pH of 9.0 to 9.5. Sometimes, however, a pH of 10.5 is necessary for complete removal of manganese. As this discussion demonstrates, the appropriate treatment chemical can depend on both the oxidation state and concentrations of metals in the AMD (U.S. Environmental Protection Agency 1983). Interactions among metals also influence the rate and degree to which metals precipitate. For example, iron precipitation will largely remove manganese from the water at pH 8 due to co-precipitation, but only if the iron concentration in the water is much greater than the manganese content (about 4 times more or greater). If the iron concentration in the AMD is less than four times the manganese content, manganese may not be removed by co-precipitation and a solution pH of >9 is necessary to remove the manganese. Because AMD contains multiple combinations of acidity and metals, each AMD is unique and its treatment by these chemicals varies widely from site to site. For example, the AMD from one site may be completely neutralized and contain no dissolved metals at a pH of 8.0, while another site may still have metal concentrations that do not meet effluent limits even after the pH has been raised to 10.

Chemicals

Limestone
Limestone has been used for decades to raise pH and precipitate metals in AMD. It has the lowest material cost and is the safest and easiest to handle of the AMD chemicals. Unfortunately, its successful application has been limited due to its low solubility and tendency to develop an external coating, or armor, of ferric hydroxide when added to AMD. In cases where pH is low and mineral acidity is also relatively low (low metal concentrations), finely-ground limestone may be dumped in streams directly or the limestone may be ground by water-powered rotating drums and metered into the stream. These applications have been tried recently in West Virginia in AMD-impacted streams with great success. Limestone has also been used to treat AMD in anaerobic (anoxic limestone drains) and aerobic environments (open limestone channels). These latter two techniques are especially useful in situations where specific discharge limits do not have to be met. They are both being installed on abandoned mine land reclamation projects and by operators wishing to reduce chemical treatment costs and improve compliance (Faulkner 1996).

Hydrated Lime
Hydrated lime is a commonly-used chemical for treating AMD. It is sold as a powder that tends to be hydrophobic, and extensive mechanical mixing is required to disperse it in water. Hydrated lime is particularly useful and cost effective in large flow, high acidity situations where a lime treatment plant with a mixer/aerator is constructed to help dispense and mix the chemical with the water (Skousen and Ziemkiewicz 1995). However, due to the kinetics of lime dissolution and its neutralization efficiency, increasing the lime rate above that required for acid neutralization increases the volume of unreacted lime that enters the metal floc settling pond. Hydrated lime can be purchased in 50-pound bags or in bulk. Bulk lime is preferred by mine operators due to cost and handling advantages. It can be delivered by barge, truck, or train to many sites and handled pneumatically. Proper storage of hydrated lime is important in order to maintain its flow characteristics and thus ensure efficient use. The appropriate silo volume depends on the daily lime requirement, but should be large enough to hold the amount of hydrate needed to last between scheduled deliveries with a safety margin to cover periodic unexpected delivery delays. The length of time that the system will be in operation is a critical factor in determining the annual cost of a lime treatment system due to the large initial capital expenditure that can be amortized over time. The topography of the site is also an important cost factor with design and structural costs increasing as the slope of the site increases.

Pebble Quicklime
CaO, has been recently used in conjunction with the Aquafix Pebble quicklime, Water Treatment System utilizing a water wheel concept (Jenkins and Skousen 1993). The amount of chemical applied is dictated by the movement of the water wheel, which causes a screw feeder to dispense the chemical. The hopper and feeder can be installed in less than an hour. This system was initially used for small and/or periodic flows of high acidity because calcium oxide is very reactive. Recently, however, water wheels have been attached to large bins or silos for high flow/high acidity situations. Preliminary tests show an average of 75% cost savings over caustic systems and about 20 to 40% savings over ammonia systems.

Soda Ash
Soda ash is generally used to treat AMD in remote areas with low flow and low amounts of acidity and metals, but its use is declining. Selection of soda ash for treating AMD is usually based on convenience rather than chemical cost. Soda ash comes as solid briquettes and is gravity fed into water by the use of hoppers mounted over a basket or barrel. The number of briquettes to be used each day is determined by the flow and quality of the water to be treated. One problem with the basket-hopper system is that the briquettes absorb moisture, causing them to expand and stick to the corners of the hopper. This hinders the briquettes from dropping into the AMD stream. For short-term treatment at isolated sites, some operators use a much simpler system employing a box or barrel with holes that allows water inflow and outflow. The operator fills the box or barrel with briquettes on a regular basis and places the box or barrel in the flowing water. This system offers less control of the amount of chemical used.

Caustic Soda
Caustic soda is often used in remote locations (e.g., where electricity is unavailable), and in low flow, high acidity situations. It is commonly the chemical of choice if manganese concentrations in the AMD are high. The system can be gravity fed by dripping liquid caustic directly into the AMD. Caustic is very soluble in water, disperses rapidly, and raises the pH of the water quickly. Caustic should be applied at the surface of ponded water because the chemical is more dense than water and sinks. The major drawbacks of using liquid caustic for AMD treatment are high cost and dangers in handling.

Tanks housing caustic soda can range in volume from 500 to 8,000 gallons. Large tanks are usually placed on a cement platform to limit the tendency for the tank to slip or twist as the ground swells and contracts with temperature changes. The discharge line is fixed at the bottom of the tank and transports the caustic solution to the seep, ditch, or pond. The rate of flow is controlled by a gate valve placed at the end of the discharge line.

Liquid caustic can freeze during winter months, but there are several options available to deal with the freezing problem. These include burying the caustic tank, installing a tank heater, switching from a 50 percent to a 20 percent caustic solution, using a freeze-proof solution containing some potassium hydroxide (KOH), and utilizing solid caustic. Burying a caustic tank is expensive because the operation must then comply with stringent EPA underground storage tank regulations. Heaters must be replaced often because of the corrosive effects of caustic. Of these options, the three most economical solutions are switching to the 20 percent caustic solution, adding some KOH, and switching to solid caustic. Switching from a 50 percent to a 20 percent caustic solution lowers the freezing point from 0oC to about -37oC. The addition of KOH (35% of the solution) also lowers the freezing point. Solid caustic, which may be delivered in 70-pound drums, beads, or flakes, has been used with good success. It is possible to regulate the rate at which solid caustic dissolves by metering the flow of water into the drum. Solid caustic can be used to make liquid caustic. A 20% solution of caustic requires 1.8 pounds of solid caustic to be dissolved in a gallon of water. Making a liquid solution from solid caustic is not cost effective when liquid caustic can be purchased, but the use of solid caustic for treating AMD is cost effective when compared to soda ash briquettes.

Ammonia
Ammonia, the common term for anhydrous ammonia, is a material that must be handled carefully (Hilton 1990). A gas at ambient temperatures, ammonia is compressed and stored as a liquid but returns to the gaseous state when released into water. In the gaseous state, ammonia is extremely soluble and reacts rapidly. It behaves as a strong base and can easily raise the pH of receiving water to 9.2. At pH 9.2, it buffers the solution to further pH increases, and therefore very high amounts of ammonia must be added to elevate the pH beyond 9.2. Injection of ammonia into AMD is one of the quickest ways to raise water pH. It should be injected into flowing water at the entrance of the pond to ensure good mixing because ammonia is lighter than water. The most promising aspect of using ammonia for AMD treatment is its cost, especially compared to caustic soda. A cost reduction figure of 50% to 70% can be realized when ammonia is substituted for caustic if the target pH for metal precipitation is <9.8 (Skousen et al. 1990). Major disadvantages of using ammonia include: 1) hazards associated with handling the chemical, 2) potential biological implications, and 3) the consequences of excessive application rates (Faulkner 1990). Specialized training and experience are important for the safe use of ammonia. Operators using ammonia are required to conduct additional analyses of discharge water where it is released into the stream and to monitor the biological conditions downstream. The extra analyses include temperature, total ammonia-N, and total acidity.

Operators must be careful to inject the appropriate amount of ammonia due to the potential consequences of excessive ammonia application. While ammonia can be effective for manganese removal in many cases, this requires careful monitoring and attention. Therefore, in situations where manganese is the ion of primary concern (low iron, high manganese water), a different chemical may be more appropriate. Low flow in the receiving stream may also require the substitution of another neutralizing chemical during dry conditions due to ammonia's toxic un-ioned state under these conditions (Faulkner 1990).

Costs of Treating AMD

Costs have been developed for five AMD treatment chemicals under four sets of flow (gpm) and acid concentration (mg/l) conditions (Table 2). These conditions are: (1) 50 gpm and 100 mg/l; (2) 1000 gpm and 100 mg/l; (3) 250 gpm and 500 mg/l; (4) 1000 gpm and 2500 mg/l. These conditions represent a sufficiently wide range for valid comparison of the treatment systems.

The costs for each technology were divided into two broad categories: installation cost and variable cost. Each of these can be broken down into several sub-categories. For example, installation cost includes materials, equipment, and labor. Materials consist of piping, extra material for the system foundation, and additional site preparation. Equipment includes conventional machinery and/or actual system hardware. Labor costs are based on man hours at a current union wage scale of $27 an hour. Variable cost includes reagent cost, annual labor, and maintenance. The amount of reagent was computed using acid neutralization formulas presented in Skousen and Ziemkiewicz (1995), but neutralization efficiencies were not included in the reagent calculation. Annual labor is estimated man-hours to run the system for one year multiplied by the current union scale of $27 an hour. Other variable costs include repair costs and electricity (Phipps et al. 1991).

The prices for the reagents, equipment, and labor were based on actual costs to mining operators in West Virginia in May 1996. All dollar values are in 1996 U.S. dollars. The net present value (NPV) is the value of the total treatment system plus annual operating and chemical expenses over the specified duration of treatment. A rate of 6% per year was used to devalue the dollar during future years of the treatment period. The annualized cost was obtained by converting the total system cost (NPV) to an equivalent annual cost so that each system could be compared equally on an annual basis. The parameters used in the analysis were entered in a spreadsheet and can be varied to conform to local conditions.

Use of soda ash has the highest labor requirements (10 hours per week) because the dispensers must be filled by hand and inspected frequently (Table 2). Caustic soda has the highest reagent cost per mole of acid- neutralizing capacity and soda ash has the second highest. But remember, soda ash is much more inefficient in treating water than caustic. Hydrated lime treatment systems have the highest installation costs of the four technologies because of the need to construct a lime treatment plant and install a pond aerator. However, the cost of hydrated lime is low. The combination of high installation costs and low reagent cost make hydrated lime systems particularly appropriate for long term treatment of high flow/high acid situations.

For a five-year treatment period, ammonia has the lowest annualized costs for the low flow/low acid situation (Table 2). Pebble quicklime was similar to ammonia in cost, and caustic was third. Soda ash is fourth because of its high labor and reagent costs, and hydrated lime is fourth because of its high installation costs. With the intermediate flow and acid cases, ammonia is the most cost effective, with pebble quicklime second. Hydrated lime and soda ash were next. Caustic soda is the most expensive alternative with these intermediate conditions. In the highest flow/acidity category, pebble quicklime and hydrated lime are clearly the least costly treatment systems, with an annualized cost of $260,000 less than ammonia, the next best alternative. The use of soda ash and caustic is prohibitively expensive at high flow and high acidity.

Other Aspects of AMD Treatment Technologies

Other Neutralizing Chemicals
While the primary AMD chemicals and applications have been discussed, particular circumstances may require a different chemical, a combination of chemicals, particular management patterns to implement the most cost effective method, or to meet more stringent effluent limits. Several operators have used potassium hydroxide, magnesium hydroxide, and magna lime with good results (Table 3). Potassium hydroxide is used because it is safer to use than caustic and reduces the potential for over treatment, but it is more expensive than caustic. Magnesium hydroxide and magna lime are dispensed in a manner similar to and behave like calcium hydroxide, but tend to be more expensive.

Table 2. Costs in 1996 of six chemicals to treat acid mine drainage in West Virginia. The analysis is based on a five-year operation period and includes chemical reagent costs, installation and maintenance of equipment, and annual operating costs. The "<Bulk" chemical prices in Table 1 were used to calculate the reagent costs for only the 50 gpm flow. The "Bulk" prices were used for higher flows. Neutralization efficiencies were not included in the reagent cost calculation.

Flow and Acidity Conditions

Flow (gpm)

50

1000

250

1000

Acidity (mg/l)

100

100

500

2500

CHEMICAL

Soda Ash 

reagent costs

repair costs

annual labor

installation costs

salvage value

Net present value

Annualized cost

$3,731

0

14,040

229

0

75,052

$17,817

$44,000

0

14,040

229

0

244,679

$58,086

$58,300

0

14,040

229

0

245,774

$58,346

$1,166,000

0

14,040

229

0

4,911,804

$1,166,046

Ammonia 

reagent costs

repair costs

tank rental

annual labor

electricity

installation costs

salvage value

Net present value

Annualized cost

$2,543

495

480

7,020

600

1,936

0

48,547

$11,525

$22,440

495

1,200

7,020

600

6,357

0

139,117

$33,026

$28,050

495

1,200

7,020

600

6,357

0

162,749

$38,636

$561,000

495

1,200

7,020

600

6,357

0

2,407,725

$571,586

Caustic Soda (20% Liquid) 

reagent costs

repair costs

annual labor

installation costs

salvage value

Net present value

Annualized cost

$5,174

0

7,020

283

0

51,601

$12,250

$79,341

0

7,020

5,478

0

368,398

$87,457

$99,176

0

7,020

5,478

0

451,950

$107,292

$1,983,520

0

7,020

5,478

0

8,389,433

$1,991,636

Pebble Quicklime

reagent costs

repair costs

annual labor

electricity

installation costs

salvage value

Net present value

Annualized cost

$1,478

500

6,500

0

16,000

0

49,192

$11,678

$9,856

2,500

11,200

0

80,000

5,000

162,412

$38,556

$12,320

2,500

11,200

0

80,000

5,000

172,790

$41,020

$246,400

10,000

11,200

0

120,000

20,000

1,127,220

$267,600

Hydrated Lime

reagent costs

repair costs

annual labor

electricity

installation costs

salvage value

Net present value

Annualized cost

$814

1,000

6,500

3,500

58,400

5,750

94,120

$22,344

$9,768

3,100

11,232

11,000

102,000

6,500

228,310

$54,200

$12,210

3,500

11,232

11,000

106,000

7,500

242,809

$57,642

$244,200

10,500

11,232

11,000

200,000

25,000

1,313,970

$311,932

Flocculants and Coagulants
Other chemicals used sparingly in AMD treatment include flocculants or coagulants, which increase particle settling efficiency (Table 3). These materials are usually limited to cases where unique metal compositions require a specialized treatment system, or where aeration and/or residence time in settling ponds are insufficient for complete metal precipitation. Coagulants reduce the net electrical repulsive forces at particle surfaces, thereby promoting consolidation of small particles into larger particles. Flocculation aggregates or combines particles by bridging the space between particles with chemicals. Bridging occurs when segments of a polymer chain absorb suspended particles creating larger particles (Skousen et al. 1993).

The most common coagulants/flocculants used in water treatment are aluminum sulfate (alum) and ferric sulfate. These materials are also called polyelectrolytes and produce highly charged ions when dissolved in water. Anionic polymers dissolve to form negatively-charged ions that are used to remove positively-charged solids. The reverse occurs with cationic flocculants. Polyampholytes are neutral, but when dissolved in water release both positively- and negatively-charged ions. Flocculants may be added to water as a liquid, or more commonly, placed in water as a gelatinous solid ("floc" logs).

Oxidants
Aeration is the process of introducing air into water. Oxidation occurs when oxygen in air combines with metals in the water. If the water is oxidized, metals generally will precipitate at lower pH values. For this reason, aeration of water can be a limiting factor in many water treatment systems. If aeration and oxidation were incorporated or improved in the treatment system, chemical treatment efficiency would increase and costs could be reduced. Oxidants (Table 3) are sometimes used to aid in the completion of the oxidation process to enhance metal hydroxide precipitation and reduce metal floc volume. The hypochlorite products, hydrogen peroxide, and potassium permanganate are used in AMD situations and have demonstrated very effective oxidation. Calcium peroxide has been shown to oxygenate AMD as well as neutralize acidity (Lilly and Ziemkiewicz 1992).

Residence Time in Ponds and Floc Generation
After chemical treatment, the treated water flows into sedimentation ponds so metals in the water can precipitate. Dissolved metals precipitate from AMD as a loose, open-structured mass of tiny grains called "floc". All chemicals currently used in AMD treatment cause the formation of metal hydroxide sludge or floc. Sufficient residence time of the water, which is dictated by pond size and depth, is important for adequate metal precipitation. Hilton (1993) found pond size to be too small on most AMD treatment sites to result in complete treatment of the water and precipitation of dissolved metals. The amount of metal floc generated by AMD neutralization depends on the quality and quantity of water being treated, which in turn determines how often the ponds must be cleaned. Knowing the chemical and AMD being treated will help determine the general floc properties and will provide an estimate of the stability of the various metal compounds in the floc.

Ackman (1982) investigated the chemical and physical characteristics of AMD floc and concluded that each floc varied depending on the nature of the AMD, the neutralization chemical, and the mechanical mixing or aeration device used during chemical treatment. He stated the most important physical property is the floc's settleability, which includes both the settling rate and final floc volume. Ackman found that calcium hydroxide and sodium carbonate produced a granular, dense floc versus a more gelatinous, loose floc generated by sodium hydroxide and ammonia. The chemical compositions of flocs were generally composed of hydrated ferrous or ferric oxyhydroxides, gypsum, hydrated aluminum oxides, calcium carbonate and bicarbonate, with trace amounts of silica, phosphate, manganese, copper, and zinc.

Payette et al. (1991) found that AMD neutralized by calcium hydroxide resulted in the formation of crystalline gypsum as well as various amorphic metal hydroxides. AMD floc was mostly amorphous at 1 hour after formation, while crystals were observed in the floc 24 hours after formation. In a series of experiments on floc generation and stability, Brown et al. (1994 a, b, c) found:

1. More floc was produced as the pH of the AMD solution was increased by chemical addition.

2. Using four AMD types, each was unique in its reaction to four neutralization chemicals.

3. The amount of floc produced as a function of the amount of chemical added (termed its "efficiency") remained about the same across all pH ranges for calcium hydroxide, sodium hydroxide, and sodium carbonate. Ammonia became less efficient at high pH.

4. Sodium carbonate was needed in the highest amount to raise water pH to 7.5 or greater.

Table 3. Chemicals for acid neutralization, coagulation/flocculation, and oxidation.
NAME CHEMICAL FORMULA COMMENTS
Acid Neutralization
Limestone CaCO3 Used in anoxic limestone drains and open limestone channels.
Hydrated Lime Ca(OH)2 Cost effective reagent, but requires mixing.
Pebble Quick Lime CaO Very reactive, needs metering equipment.
Soda Ash Briquettes Na2CO3 System for remote locations, but expensive.
Caustic Soda NaOH Very soluble, comes as a solid in drums, beads, or flakes, or as a 20% or 50% liquid. Cheaper in the liquid form.
Ammonia NH3 or NH4OH Very reactive and soluble; also purchased as aqua ammonia.
Potassium Hydroxide KOH Similar to caustic.
Magnesium Hydroxide Mg(OH)2 Similar to hydrated lime.
Magna Lime MgO Similar to pebble quicklime.
Calcium Peroxide CaO2 Used as a neutralizer and oxidant; either powder or briquettes.
Kiln Dust CaO, Ca(OH)2 Waste product of limestone industry. Active ingredient is CaO with various amounts of other constituents.
Fly Ash CaCO3, Ca(OH)2 Neutralization value varies with each product.
Coagulants/Flocculants
Alum (aluminum sulfate) Al2(SO4)3 Acidic material, forms Al(OH)3.
Copperas (ferrous sulfate) FeSO4 Acidic material, usually slower reacting than alum.
Ferric Sulfate Fe2(SO4)3 Ferric products react faster than ferrous.
Sodium Aluminate NaAlO2 Alkaline coagulant.
Anionic Flocculants   Negatively-charged surface.
Cationic Flocculants   Positively-charged surface.
Polyampholytes   Both positive and negative charges on surface based on pH.
Oxidants
Calcium Hypochlorite Ca(ClO)2 Strong oxidant.
Sodium Hypochlorite NaClO Also a strong oxidant.
Calcium Peroxide CaO2 Trapzene, an acid neutralizer.
Hydrogen Peroxide H2O2 Strong oxidant.
Potassium permanganate KMnO4 Very effective, commonly used.

5. Floc volumes were lowest with sodium carbonate and highest with calcium hydroxide after 1 week of settling.

6. Greater settling time caused floc consolidation.

7. Flocs were composed of metals in ratios similar to the metal ratios of the AMD from which it was generated.

8. Flocs were primarily amorphic (having no crystalline structure), except for sodium carbonate flocs.

9. Flocs collected from ponds on mined areas showed little similarity in composition to flocs generated with the same AMD and chemical in the laboratory. The field flocs had soil particles mixed with the chemical floc.

10. Aging of AMD flocs caused them to be more stable, thereby decreasing their likelihood of releasing metals (Watzlaf and Casson 1990). The greater stability of aged flocs remained even after re-introducing the flocs into acidic solutions. Aging in a dry environment resulted in better floc stability than flocs aged under water. Aging also caused floc consolidation.

Floc disposal options include: 1) leaving the floc submerged in a pond indefinitely, 2) pumping or hauling floc from ponds to abandoned deep mines or to pits dug on surface mines, and 3) dumping floc into refuse piles. Flocs pumped or dumped onto the surface of land or mixed with overburden during backfilling and allowed to age and dry is a good strategy for disposal. In its oxidized and dried condition, AMD flocs can become crystalline and become part of the soil. Injecting stable floc containing excess alkalinity into acidic deep mine pools has the potential to improve the quality of the discharge from those pools. Injection into abandoned mine works is cost-effective where field conditions allow its safe disposal.

Lovett and Ziemkiewicz (1991) estimated ammonia chemical costs for a site in West Virginia at $72,000 per year and floc handling costs of $486,000 per year. Based on a flow of 100 gpm for this site, Brown et al. (1994b) estimated that this site generated approximately 77,900 cubic yards of floc per year. Dividing $486,000 by 77,900 cubic yards of floc yields a cost of $6.25 per cubic yard for floc handling and disposal on this site. Several mine operators observed that floc handling and disposal may cost up to $15 per cubic yard. Due to their high water content and the sheer volume of material, floc handling costs frequently exceed chemical costs by several times.

Each AMD is unique and the chemical treatment of any particular AMD source is site specific. Each AMD source should be tested with various chemicals by titration tests to evaluate the most effective chemical for precipitation of the metals. The costs of each AMD treatment system based on neutralization (in terms of the reagent cost and capital investment and maintenance of the dispensing system) and floc volumes and disposal should be evaluated to determine the most cost effective system.

Summary

Acid mine drainage occurs when geologic materials containing metal sulfides are exposed to oxidizing conditions. Subsequent leaching of reaction products into surface waters pollute over 20,000 km of streams in the U.S. Chemicals used for treating AMD after formation are hydrated lime, pebble quicklime, caustic soda, soda ash briquettes, and ammonia. Each chemical has advantages for certain water conditions and treatment. Under low flow situations, pebble quicklime and ammonia are the most cost effective. Under high flow situations, hydrated lime and pebble quicklime are the most cost effective due to their low reagent cost compared to the other chemicals. Each chemical reacts differently with a specific AMD. Therefore, it is essential that each AMD source be treated and evaluated with each chemical to determine which is most environmentally sound, efficient and cost effective. Coagulants and flocculants may be used in water treatment where retention time in sedimentation ponds is insufficient for metal precipitation. Oxidants can be used to meet more stringent effluent limits and to make chemical treatment more efficient. Floc, the metal hydroxides collected in ponds after chemical treatment, are disposed of in abandoned deep mines, refuse piles, or left in collection ponds. Studies show that flocs are relatively stable materials and metals contained therein do not resolubilize after disposal, especially if aged and dried.

Acknowledgments

This paper is the second section in "Acid Mine Drainage Control and Treatment," a chapter in a new book entitled "Reclamation of Drastically Disturbed Lands," being prepared by the American Society for Agronomy and the American Society for Surface Mining and Reclamation. The anticipated release date for this book is 1997. The authors thank Paul Ziemkiewicz, Robert Darmody, John Sencindiver, Tim Phipps, Jerry Fletcher, Keith Garbutt, and two anonymous reviewers for helpful comments during the review process. Acid mine drainage research at West Virginia University is supported by grants from the National Mine Land Reclamation Center, the USDI Bureau of Mines, the West Virginia Division of Environmental Protection, and from funds appropriated by the Hatch Act.

References

Ackman, T. 1982. Sludge disposal from acid mine drainage treatment. U.S. Bureau of Mines, Report of Invest. 8672, Pittsburgh, PA.

Brown, H., J. Skousen, and J. Renton. 1994a. Floc generation by chemical neutralization of acid mine drainage. Green Lands 24(1): 33-51.

Brown, H., J. Skousen, and J. Renton. 1994b. Volume and composition of flocs from chemical neutralization of acid mine drainage. Green Lands 24(2): 30-35.

Brown, H., J. Skousen, and J. Renton. 1994c. Stability of flocs produced by chemical neutralization of acid mine drainage. Green Lands 24(3): 34-39.

Faulkner, B.B. 1996. Acid mine drainage treatment recommendations. West Virginia Mining and Reclamation Association, Charleston, WV.

Faulkner, B.B. 1990. Handbook for the use of ammonia in treating mine waters. West Virginia Mining and Reclamation Association. Charleston, WV.

Hilton, T. 1990. Handbook - Short Course for Taking A Responsible Environmental Approach towards Treating Acid Mine Drainage with Anhydrous Ammonia. West Virginia Mining and Reclamation Association. Charleston, WV.

Hilton, T. 1993. Technical information for fighting acid mine drainage. In: Proceedings, Fourteenth West Virginia Surface Mine Drainage Task Force Symposium. West Virginia University, Morgantown, WV.

Jenkins, M., and J. Skousen. 1993. Acid mine drainage treatment with the Aquafix System. In: Proceedings, Fourteenth Annual West Virginia Surface Mine Drainage Task Force Symposium. West Virginia University, Morgantown, WV.

Lilly, R., and P. Ziemkiewicz. 1992. Manganese removal at a lower pH with calcium peroxide: results of field trials. In: Proceedings, Thirteenth Annual West Virginia Surface Mine Drainage Task Force Symposium, West Virginia University, Morgantown, WV.

Lovett, R., and P. Ziemkiewicz. 1991. Calcium peroxide for treatment of acid mine drainage. p. 35-46. In: Proceedings, Second International Conference on the Abatement of Acidic Drainage. MEND, CANMET, Montreal, CAN.

Payette, C., W. Lam, C. Angle, and R. Mikula. 1991. Evaluation of improved lime neutralization processes. In: Proceedings, Second International Conference on the Abatement of Acidic Drainage. MEND, CANMET, Montreal, CAN.

Phipps, T., J. Fletcher, and J. Skousen. 1991. A methodology for evaluating the costs of alternative AMD treatment systems. In: Proceedings, Twelfth Annual West Virginia Surface Mine Drainage Task Force Symposium, West Virginia University, Morgantown, WV.

Skousen, J., R. Lilly, and T. Hilton. 1993. Special chemicals for treating acid mine drainage. Green Lands 23(3): 34-41.

Skousen, J., K. Politan, T. Hilton, and A. Meek. 1990. Acid mine drainage treatment systems: chemicals and costs. Green Lands 20(4): 31-37.

Skousen, J., and P. Ziemkiewicz. 1995. Acid Mine Drainage Control and Treatment. National Research Center for Coal and Energy, National Mine Land Reclamation Center, West Virginia University, Morgantown, WV. 243 pp.

U.S. Environmental Protection Agency. 1983. Neutralization of Acid Mine Drainage, Design Manual. USEPA-600/2-83-001, Cincinnati, OH.

Watzlaf, G., and L. Casson. 1990. Chemical stability of manganese and iron in mine drainage treatment sludge: effects of neutralization chemical, iron concentration, and sludge age. p. 3-9. In: Proceedings, 1990 Mining and Reclamation Conference, West Virginia University, Morgantown, WV.