DRAINAGE
Ireatment
MINE
oi t
—<4*.
I
p« ¦ ¦ n f ™
U.S. DEPARTMENT OF THE INTERIOR
FEDERAL WATER POLLUTION CONTROL ADMINISTRATION
CINCINNATI, OHIO
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MINE DRAINAGE TREATMENT
STATE OF THE ART AND RESEARCH NEEDS
By Ronald D. Hill
U. S. DEPARTMENT OF THE INTERIOR
FEDERAL WATER POLLUTION CONTROL ADMINISTRATION
MINE DRAINAGE CONTROL ACTIVITIES
CINCINNATI, OHIO 45226
December 1968
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TABLE OF CONTENTS
SECTION TITLE PAGE
1 Introduction ... 1
2 Chemistry of I line rainage 4
3 Neutralization 9
4 Iron Removal 39
5 Ion Exchange Treatment 51
6 Reverse Osmosis Treatment . 61
7 Distillation 70
8 Electrodialysis 78
9 Crystallization (Freezing) .......... 82
10 Biological Treatment 86
11 Summary ......... 90
References . . 95
- ii -
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1. INTRODUCTION
Stream pollution resulting from mining operations is a serious
(58)
problem in the United States, A U. S, Department of the Interior report
indicates that 4,800 miles of streams and 29,000 surface acres of impound-
ments and reservoirs are seriously affected by surface coal mining oper-
ations alone. Deep coal mining and other mining operations increase these
figures several fold. Inventories of stream pollution magnitude and source
(55)
have highlighted the seriousness of the problem. In Appalachia during
1966, more than 6,000 tons of acidity per day were discharged to streams
and over 10,000 miles of streams were polluted by all coal mining operations.
Figure 1,1 indicates the approximate relative distribution of the mine
drainage problem by source. About 40 percent of the mine drainage comes
from active operations and the remainder from abandoned surface, drift,
auger, etc., mines.
Methods of controlling mine drainage from surface mines are more
advanced than those for deep mines. By utilizing preventive measures,
such as good water control, mining techniques, and reclamation practices,
pollution from surface mines can be held to a minimum. Strong state laws
are being or have been enacted in most states to control pollution from
surface mining.
Deep mine discharges present an entirely different situation than
surface mines. Preventative control is not fully understood or developed.
The major control measures used to date are water diversion to prevent
water from entering the mine, flooding of the pyritic material to prevent
oxidation, and sealing to prevent air from entering the mine and subsequently
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25 % '
Abandoned
Surface Mines
f 35 %
Abandoned
Shaft and Drift
Mines
40%
Active Mines
Estimated Current Rate of Pollution 1
3.5 Million Tons per Year to all U.S. Streams
Figure I-1. Sources of Acid Pollution
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oxidizing the pyrite. While each of these methods has been successful to
some degree, documentation of the work performed and the results obtained
is sketchy and inconclusive. A broad spectrum of research is needed to
develop control methods for deep mine drainage.
At best preventative methods will not be 100 percent effective in
every situation. Thus, another approach will be necessary to control the
residual pollution from those measures. Further, there are many abandoned
mine situations where preventative measures will not be applicable. Active
mine operations also are usually not amenable to at-source control methods.
In each of these situations, treatment of mine drainage appears to be the
best method of pollution control.
This report reviews the current status of treatment methods for mine
drainage and outlines those areas needing further research and development.
- 3 -
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2. CHEMISTRY OF MINE DRAINAGE
The type of drainage produced by a particular mine is dependent
upon the product mined and the nature of the surrounding geologic formation.
In the case of coal mining, it is dependent upon the amount of sulfides
presentj the spatial distribution of these sulfides; the crystallinity of
the pyrite; the size of the individual sulfide particles; the presence of
bacteria associated with acid mine drainage; and the magnitude of the
fluctuation of the water level within the mine, if the workings are below
drainage. In addition, the presence or absence of calcium in the sulfide
aggregates seems to have some effect upon the rate of sulfide oxidation
( 8 )
and decomposition
The sulfide content and distribution are dependent upon the nature
of the paleo-environmental conditions which prevailed when the coals were
being deposited. In general, those coals which were deposited in
exclusively marine environments are more likely to produce acid waters
than are those which were deposited in essentially continental environ-
ments, The Pennsylvania bituminous coal fields show an increase in sulfide
content from east to west on the basal seams, reflecting the trend toward
more marine conditions toward the west. Further, these same seams show
a decrease in sulfur content upward in the stratigraphic column as pro-
(37)
gressively more continental conditions of deposition are encountered .
(17)
Emrich and Thompson made an analysis of the data on file with
the Pennsylvania Department of Health about the character of drainage
from underground bitu-ninous mines. They found that the younger coal seams
-------
(Sewickley-Pittsburgh) have more alkaline discharges and the older seams
(Clarion-Brockville) have predominantly acid discharges.
r • (93 )
Hinkle and Kochler ' who collected samples of mine waters, includ-
ing roof drips of deep mines in West Virginia, reported as follows:
"The analyses of mine waters - - - - have shown that some mine
drainage waters are decidedly acid and others are fairly alkaline. Roof
drip waters so far collected have all been found to be alkaline; however,
acid roof drips have been reported by others. The only significant diff-
erence between acid mine waters and alkaline drainage water and
roof drips - - - - was that the acid waters contained essentially sulfates,
whereas the roof drips and alkaline mine waters contained both sulfates
and bicarbonates, The compositions of the alkaline mine drainage waters
were dependent upon the composition of the roof drips. All three
waters contained compounds of essentially seven metals: iron, aluminum,
manganese, calcium, magnesium, sodium, and. possibly some potassium. Chlorides
were usually low in all waters. Silica (Si02) was relatively high in the
acid mine water, but low in the alkaline mine waters and roof drops."
While wide variations exist in the chemical characteristics of
mine drainage generally, acid mine drainage can be said to have a low
pH, net acidity (acidity greater than alkalinity), high iron (iron (II)
and/or iron (III), high sulfates, and significant amounts of aluminum,
manganese, calcium and magnesium. Alkaline mine drainage generally can
be said to have a pH near or greater than neutrality, net alkalinity,
high sulfate, significant calcium, magnesium, and manganese, and low
(12)
aluminum. Corbett and Growity reported that the 2n, Cd, Be, Cu, Ag,
Ni, Co, Pb, Cr, V, Ba, and Sr concentrations of mine drainage were less
-------
than 1 mg/1. Analyses of mine drainage samples collected in West Virginia
and Pennsylvania by Federal Water Pollution Control Administration personnel
revealed concentration of similar magnitude.
Although the exact mechanism is not fully understood, acid mine
drainage results from the oxidation of pyrite (FeS2) as illustrated in
equation (1):
2FeS2 + 2H20 + 702 ~ 2FeS04 + 2H2S04 U
(Pyrite) ^ (Ferrous sulfate) -t- (Sulfuric acid)
Subsequent oxidation of ferrous sulfate produces a ferric sulfate:
4FeS0^ + 202 + 2H2so4 ~ 2Fe2^S04b + 2H2° <2
(Ferrous sulfate) ~ (Ferric sulfate)
The reaction then may proceed to form a ferric hydroxide or basic ferric
sulfate:
Fe2(S04)3 + 6H20 2Fe(OH)3 + 3H2S04 (3
(Ferric sulfate) (Ferric hydroxide)
Fe2(S°4)3 + 2H20 ~ 2Fe(0H)(S04) + H2SO4 (4
(Ferric sulfate) ~ (Basic ferric sulfate)
From the above equations, it can be seen that dissolution of one mole of
iron pyrite ultimately leads to the release of four equivalents of acidity -
two equivalents from the oxidation of iron sulfate and two from the
oxidation of ferrous iron.
(52)
Pyrite oxidation also occurs due to ferric iron as illustrated
in equation (5:
14Fe3+ + FeS2 + 8H20 ~ 15Fe2+ + 2S042~ + 16H+ (5
(Ferric iron) + (Pyrite) ^ (Ferrous iron) (Sulfate)
- 6 -
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By either mechanism, an acid water is produced and the pH is lowered.
At low pH's many metallic ions in the mine for example, aluminum and
manganese become more soluble and enter into the mine discharge.
Mine drainage is a complex solution varying in quality from seam
to seam, mine to mine, and even within the same mine. The water quality
from mines low in pyrite may be alkaline and closely resemble ground
water. Often mines produce water high in ferrous iron and acidity, in-
dicating that reactions 1 and/or 5 are occurring. The discharge may
have high ferric iron and acidity concentration, indicating that reactions
2 and 3 are¦occurring. The discharge may also have been partially neutral-
ized within the mines thus reducing the acidity level.
Although there is no "typical" mine drainage, waters discharging
from mines can be divided into four general classes as shown in Table 2,1.
The wide variations in mine drainage characteristics indicate that
a number of treatment methods may be applicable. The best method for any
one site will depend on the quality of the mine discharge and the ultimate
use of the water. Treatment to meet stream water standards will be different
from that needed to meet domestic and industrial water use standards. The
following sections of this report review the current status of various
treatment methods.
- 7 _
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Class I
Acid Discharges
pH 2 - 4.5
Ac idity, Mg/l (CaC O3) 1,000 - 15,000
Ferrous Iron, Mg/l 500 - 10,000
Ferric Iron, 's/l 0
Aluminum, Mg/l 0 - 2,000
Sulfate, Mg/l 1,000 - 20,000
Source: In-house Studies, JWPCA
TABLE 2.1
MINE DRAINAGE CLASSES
Class 2
Partially Oxidized
and/or
Neutralized
Class 3
Oxidized and
Neutralized and/ or
Alkaline
Class 4
Neutralized
and
Not Oxidized
3.5 - 6.6
0 - 1,000
0 - 500
0 - 1,000
0-20
500 - 10,000
6.5 - 8.5
0
0
0
0
500 - 10,000
6.5 - 8.5
0
50 - 1,000
0
0
500 - 10,000
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3. NEUTRALIZATION
Introduction
The neutralization of acid mine drainage with an alkali has been
recognized as an acid mine drainage treatment method since the 1920's.
(7 )
In 1930 Carpenter and Davidson reported on four lime treatment plants
treating acid mine drainage. They conducted a number of studies them-
selves as did the U. S. Bureau of MinesInvestigators at that time
concluded that the process was not economic for practical use. In addition,
they pointed out the problem of handling and disposing of the sludge that
was formed.
In recent years, with the advent of stronger laws enacted to prevent
water pollution by acid mine drainage, interest in neutralization has been
renewed, a number of laboratory studies have been initiated, and pilot
plants and full-scale plants built.
Principle of Neutralization
The principle of the neutralization processes i3 as follows: an
alkali is mixed with acid mine waters to neutralize the acid and to precip-
itate the contaminating metal salts, which can then be separated by sedimen-
tation and/or filtration. The metal salts commonly found in acid mine drain-
age are separated because they are less soluble at neutral or higher pH's
than at lower pH's (Figure 3.1 and Figure 3.2).
The list of neutralizing agents suggested for acid mine drainage
is presented in Table 3.1. While most neutralization work to date has
utilized lime and limestone, other agents may be used successfully in some
situations.
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Figure 3- I
-200
-80
120 I
<-> 10-
80 q
-20
10 12
Solubility of Aluminum,
arid Manganese in
Acid Mine Drainage
at Various pH's
Sample Source :
Deep Mine, Elkins,
West Virginia
220
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Fe (OH),
Figure 3-2.
(after
Solubility of Iron
O'melia ft Stumm, 1967 (41 * )
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TABLE 3.1
Neutralizing Agents
Hydrated or calcined lime
Limestone
Magnesium hydroxide
Dolomite
Potassium permanganate
Sodium hydroxide
Potassium hydroxide
Ammonium hydroxide
Sodium sulfide
Tri-Sodium phosphate
Sodium cyanide
Sodium carbonate
Ammonia
Magnesium oxide
CaO
CaCO^
Mg(0H)2
Ca-MgCO^
KMnO^
NaOH
KOH
NH^OH
Na2S
Na3PO^
NaCN
Na2C0^
NH3
MgO
Basicity Factor (*)
1.78
1.00
1.72
1.006
1.25
0.915
0.94
2.94
2.482
Orams of 'urn carbonate equivalent per gram of agent.
12 -
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The choice of an alkaline agent should be based on the following
considerations:
Cost of Agent - The cheapest agent capable of fulfilling
the requirements should be used.
Availability of Agent - Availability is partially reflected
in cost. The availability of certain alkaline materials,
such as a by-product of another industry, may not be long-
termed.
Basicity Factor - The amount of alkali per unit weight of
material varies among different alkaline agents.
Reaction Time - The reaction rates of alkaline agents vary
over a considerable range and are important factors in the
size of mixing tanks, etc.
Sludge Characteristic - The settling rate and properties of
the sludge are important factors in the design of settling
tanks and lagoons, and in the disposal of the sludge.
Basicity Factor
Basicity factor is defined as the grams of calcium carbonate
equivalent per gram of alkaline agent. In Table 3»1 the basicity
factors for some of the commonly used alkaline agents are given. These
figures are slightly higher than those obtained in operation because
of the purity of the commercial product and other factors. The basicity
factor is a useful tool in comparing the cost of alkaline agents. Three
alkaline agents are compared on the following page.
- 13 -
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Basicity Price Cost
Agent Factor Dollars/Ton Dollars/Ton Basicity
Lime (CaO) 1.78 14 $ 7.86
Limestone (CaCO^) 1.0 5 5.00
Soda Ash (Na2C03) 0.94 31 32.98
In this comparison, lijne has the greater basicity factor, but is more
expensive per ton of basicity than limestone.
Reactivity
Reactivity is defined as the rate at which the neutralization process
(24)
occurs. Hoak, et al., reported the rate (in descending order) of the
following agents in relationship to each other to be: NaOU, CaO, Ca(0H)2,
Na2C03, and CaCO^. In most cases, an increase in temperature from room
temperature to 60° C increased the rate, as did aeration of the -waste.
Sludge Settleability
Sludge settleability is defined as the characteristic of the sludge
judged by the rate at which it settles and the final sludge volume. Hoak,
(24)
et al., rated the sludge settleability of the following agents (descend-
ing order): Na2C03, CaCO^, NaOH, CaO, and Ca(0H)2«
Caustic Soda
Caustic soda reacts rapidly, has a high settleability, reaction
products are highly soluble, thus reducing scaling problems, and cause no
increase in hardness. However, the cost of this agent is prohibitive
for treating acid mine drainage except in special cases.
Sodiu"i Sulfide
(6)
Care and Zawadzki evaluated the treatment of synthetic acid
m-'ne water with aqueous sodium sulfide solutions. The following equations
- 14 -
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illustrate the reaction:
Na2S + 2H20 —: 2NaOH + H2s(g) ^
(Sodium Sulfide) -r (Water)—~(Sodium Hydroxide) + (Hydrogen Sulfide)
The pH of a 0.0A- molar Na2S solution is 10 to 11.
4Na2S + Fe2 (504)3 + Fe S04 * ^^PP1^ * ^(PP^ r ^a2S0k ^
(Sodium Sulfide) + (Ferric Sulfate) + (Ferrous Sulfate) - ¦>
(Ferrous Sulfide) + (Ferric Sulfide) -r (Sodium sulfate)
Fe2S is a black precipitate and since the bulk of the precipitate
is ferrous sulfide, the precipitate is black. The precipitate was easy
to filter. At stoichiometric amounts of Na2S, no H2S odor was detected.
Upon addition of Na2S solution the pH initially increased to between
4 and 5. The pH later increased to between 6.8 and 7.1, after the iron
precipitated, and stabilized at this value.
Care and Zawadzki reported " essentially complete removal
of both ferrous and ferric iron, neutralization of the acidity, and in
addition, an innocuous sodium sulfate solution."
The major drawback was the cost of Na23 (1965 price was $142 per
ton of 100 percent Na2S). Therefore, the agent cost of treating a 500
ppm total iron discharge would be $0.15 per 1,000 gallons of water.
Zawadzki and Glenn^62^ conducted further tests on the use of sodium
sulfide to neutralize mine drainage and remove iron (II). The addition of
sodium sulfide at stoichiometric amounts to combine with the ferrous iron
caused a rapid increase in pH. The rise in pH was accompanied by rapid
reaction of the iron with sulfide. The reaction occurred beat at pH's
-------
of 4 and above. At pH's below k, there -was insufficient sodium to neutral-
ize the acidity.
Since, treatment with sodium sulfide was dependent on pH control,
(62)
Zawadzki and Glen conducted their later studies on a limestone-sulfide
system instead of a sodium sulfide system. The limestone-sulfide system
is reported on in the iron removal section of this report.
Limestone Treatment
In equations 8, 9, and 10, the reactions of limestone with acid mine
drainage are illustrated.
Limestone + Sulfuric AcidCalcium Sulfate + Water + Carbon Dioxide (8
CaC03 + H2SO4 » CaS04 + H20 + CO2
Limestone + Aluminum Sulfate + Water—'•Calcium Sulfate + Aluminum (9
Hydroxide -t- Carbon Dioxide
3CaC03 + A12(S0^)3 + 3H20 » 3CaS0^ + 2Al(0H)3 + 3C02
Limestone + Ferric Sulfate 4- Water * Calcium Sulfate + Ferric (10
Hydroxide f Carbon Dioxide
3CaC03 + Fe2(S04)3 + 3H20 » 3CaS04 + 2Fe(CH)3 + 3C02
Theoretically, one ton of limestone is required to neutralize one
ton of H2SO4.
The process will occur as long as there is an excess of limestone
available and the limestone surface is in an active state. Maintaining
the limestone in an active state is a major problem. 'When high acid con-
centrations are treated, calcium sulfate, because of its low solubility
(approximately 2,000 rig/l), will precipitate and coat the limestone.
Ferric hydroxide also coats the limestone, markedly reducing its efficiency,
- 16 -
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Limestone Treatment Studies
Braley^ ^ ^ reported on a study in which acid mine drainage having
both ferrous and ferric iron and 1,100 ppm acidity was passed through a
flume 50 feet long containing 5 tons of limestone (1 to 2 inches size).
The contact time was varied by changing the flow. The maximum neutralization
of 50 percent was obtained at the lowest flow rate (180 gph) and greatest
contact time (l hour). The amount of acid neutralization increased with
increased contact time. With continued use of the sane limestone, the amount
of neutralization decreased because the limestone became coated with gelat-
inous iron hydroxides, thus reducing the amount of reactive surfaces.
In a second series of tests by Braley, the above mentioned acid
mine drainage was batch-treated with crushed limestone in a tank provided
with an agitator. The agitator provided constant movement of the water
through the limestone, but did not agitate the limestone itself. With
new limestone, 5 hours were required to neutralize 135 gallons of acid
mine drainage, or 4.02 hours per pound of acid. By the 39th test, without
changing the limestone, 37.25 hours were required to treat the acid mine
drainage. The increase in time was due to the coating of the limestone
with iron.
Clifford and Snarley^ reported similar limestone coating in their
studies in which limestone was placed in cribs built in the bed of an acid
stream and in a flume through which the acid waters passed. The limestone
was coated not only with an iron deposit, but also with silt. Even when
operating in a new condition, the limestone increased the pH only slightly
because of the short contact time.
- 17 -
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(19)
In his studies, Glover confirraed that stationary beds of lime-
stone grit soon became blocked with reaction products. In view of the
large size of grit beds, he concluded that mechanical cleaning was not prac-
(63)
tical. Zurbuch overcame the coating of limestone by placing the lime-
stone in a drum turned by the streamflow. As the limestone revolved in
the drum, the abrasive action between the particles wore off the coating
and also helped the dissolution of the limestone. However, because of the
short contact time provided, the increase in pH was small.
(61)
Wheatland and Borne found that passing the waste upflow through
a limestone or calcined magnesite bed did not reduce the particle coating
problem.
Limestone can be successfully used to treat acid mine drainage only
if the coating on its surface can be prevented or removed. Abrasion between
the limestone particles or from an external force appears to be one method.
The revolving drum mentioned earlier, if driven by motors and given suffic-
ient contact time, might resolve the problem. A kiln or tumbler type
(39)
reactor was found by Mihok and Chambalain to supply the necessary
abrasion. Another approach would be the use of finely ground limestone,
such as that used for agricultural purposes.
(40)
The U. S. Bureau of Fines conducted a series of tests using a
3-foot-diameter by 24-foot long tube mill driven by a 15-horsepower motor.
The mill was charged with 3 x 1-inch limestone and optimum operation occurred
when the limestone discharged passed a 400 mesh screen. It was found that
the limestone generated having a size less than 400 mesh increased as the flow
rate decreased. For example, at 7.2 gallons per minute (gpm), the limestone
- 18 -
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generated was 8.4 pounds per minute, while at 14.6 gpm only 5.1 pounds per
minute was generated. These results indicated that instead of passing the
entire flow through the mill, a portion should be split off and passed
through the mill to make up the limestone slurry, then reunited with the
main stream. Data were not presented in the report of the cost of grind-
ing limestone by a mill as compared to buying ground limestone from the
quarry.
(19)
Glover in a series of tests using a tumbler reactor obtained
good results in treating the free acid and ferric and aluminum salts and
observed no apparent inactivation of the limestone. The rate of ferrous
iron oxidation in the tumbler was no higher than in a stationary bed of
(20)
limestone which was also tested. Glover, et al., have patented a
mechanical attrition device in the form of a rapidly rotating impeller
that operates in an upflow expanded limestone bed. This technique was
reported to be very effective in maintaining the activity of the limestone.
Another approach to preventing limestone coating is the use of powdered
limestone. Limestone in this state would have greater reactive surface area
(1«
available and possibly would be solubilized before a coating formed. Glover
found that the limestone powder reacted rapidly with free acid and ferric
(25
and aluminum salts, but not directly with the ferrous salts* Hoak, et'al.,
found pulverized limestone to be more effective than lump or crushed limestone
because the fine particles reacted more completely. This approach warrants
further consideration.
Limestone Reaction Rates
As mentioned earlierthe reaction rate of limestone is not as great
as that of other alkali agents. The ferrous to ferric iron reaction is the
limiting factor. The increase in pH and decrease in acidity usually occur
- 19 -
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in less than a minute, whereas the rate of ferrous oxidation is very slow
in a limestone bed. When oxygen is supplied to a bed of limestone ferrous
iron is oxidized. Under this condition, Gloverreported the oxidation
rate to be 6 grams iron per liter per 24 hours (gFe/l/24) hours. He observed
that the rate of ferrous oxidation was almost independent of the ferrous
concentration and concluded that since oxygen and limestone were present
in excess, the rate determining factor was probably the rate of stripping
of the carbon dioxide product. In an aerated flow through a limestone bed,
the oxidation rate ranged from 7 to 21 gFe/l/24 hours, suggesting that the
(13)
moving water carried away the carbon dioxide. Beul and Mihok ^' also
found carbon dioxide to be a limiting factor. In a limestone bed a terminal
pH of only 6.0 to 6.5 could be obtained. However, in a rotary drum, with
vigorous cascading agitation to introduce air, the CO2 was driven off
and a terminal pH of above 8 was reached. In these same studies, the
investigator found that the pH of the acid mine drainage in a rotary drum
with excess limestone could be increased to 6 or greater in less than a
minute; however, ferrous iron oxidation required a much longer time. The
final ferrous iron concentration after 20 minutes was dependent on the
initial concentration, since the oxidation rate was relatively constant
(ranging from 600 to 1,000 mg ferrous iron per liter per hour). The
oxidation rate was greater when the ferrous iron concentration exceeded
(95)
400 ppm than when the concentration was less than 200 ppm. Hoak, et al.,
reported that the reaction rate wa3 governed by the chemical characteristics,
fineness, and a specific reactivity peculiar to a particular limestone which
- 20 -
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cannot easily be evaluated by trial.
Mihok and Chambalain^ ^ conducted studies m a continuous limestone
neutralization pilot plant capable of handling 100 gpm. A rotary-kiln
type reactor was used followed by aeration. They found that ------
adequate treatment of acid waters containing less than 165 ppm ferrous iron
can be achieved at normal mine water temperatures (10°G) with a reaction
residence time of less than 5 minutes." The soluble ferrous iron concen-
tration was reduced at a rate of 30 to 35 ^g/l Per minute and the pH was
increased to between 6.5 and 7.5. The atmosphere within the reactor was
found to have abnormally high amounts of carbon dioxide (0.16 percent),
which may have surpassed higher terminal pH's. However, rapid pre-
cipitation of the ferrous hydroxide did not occur, even though an excess
of very fine limestone was present. Subsequent aeration of the limestone-
neutralized water reduced the residual ferrous iron at the rate of 1 to 4
mg/l per minute.
Calhoun^5 ^ reported on a limestone treatment plant used to treat
a mine discharge having the following characteristics: flow range 50 to
300 gpm, acidity 100 to 500 mg/l, and iron 20 to 120 mg/l (10 percent in
the ferrous state). A rotating drum, 30 feet long and 3 i*1 diameter,
containing 5,000 pounds of 3 inch by 1 inch limestone, and rotated at
15 rpin, was used as a reactor. The water flowed from the drum to a settl-
ing basin. The first year's operation is summarized as follows:
effluent pH 6.2-7.3
effluent iron 0.4 ~ 5.3 in;;/l
effluent alkalinity 4-26 mg/l
- 21 -
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total operating cost 5 cents per 1,000 gallons
limestone cost 1 cent per 1,000 gallons
labor cost 2 cents per 1,000 gallons
limestone usage efficiency 65 percent
Advantages stated for limestone were: more economical, less volume of
sludge and no danger of overtreatment.
(40)
Mihok, et al., conducted a series of field tests using a tube
mill to grind limestone which was subsequently fed to mine drainage. They
documented the importance of removing carbon dioxide from the process in
order to increase the rate of neutralization. Table 3.2 presents a cost
comparison for treating mine drainage with lime and limestone.
In summary, it can be concluded from the preceeding review that
acid mine drainage can be neutralized with limestone in less than 1 minute
of reaction time if the limestone is kept free of inhibiting coatings.
Ferrous iron must be treated either in a separate step or the reaction time
of the limestone treatment step must be extended. Fossible methods of
separate treatment of the ferrous iron are discussed in a later section.
Limestone treatment may not reduce the hardness or sulfate concentration.
In fact the hardness might be increased.
Line Treatment
Quicklime (CaO) and hydrated lime (Ca(0H)2) have been used in the
treatment of acid mine drainage. These limes may be either high-calcium
or high-magnesium (dolomitic). In equations 11 through 15, the reactions
of lime with acid mine drainage are illustrated.
Quicklime ¦+• 'iJater 1 •• Hydrated Lime (11
CaO i- H20 » Ca(0H)2
- 22 -
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TABLE 3.2
MATERIAL COST OF TREATING ONE MILLION GALLONS OF
MINE DRAINAGE PER DAY WITH
LIME AND LIMESTONE*
Lime Limestone
Material cost, dollars per ton
Material utilization, percent
Material required, tons
Cost per day, dollars
* Total Acidity - 2,000 mg/l as Ca CO3. From Mihok, et al.,
3
50
16.6
50
20
100
6.2
120
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Hydrated Lime -t- Sulfuric Acid = Calcium Sulfate + Water
Ca(0H)2 + H2S04 ^ CaSO^ + 2H20 (12
Hydrated Lime + Aluminum Sulfate = Calcium Sulfate + Aluminum Hydroxide
3Ca(0H)2 + A12(S04)3 > 3CaS0^ + 2Al(0H)3 (13
Hydrated Lime + Ferrous Sulfate = Ferrous Hydroxide + Calcium Sulfate
Ca(0H)2 -r FeSO^ ~ Fe(0H)2 + CaSO^ (14
Hydrated Lime + Ferric Sulfate = Ferric Hydroxide + Calcium Sulfate
3Ca(0H)2 + Fe2(S04)3 ~ 2Fe(0H)3 + 3CaS04 (15
Besides increasing the pH and decreasing the acidity, lime treat-
ment will remove many of the metallic salts. The aluminum and manganese
will precipitate if the proper pH level is reached (Figure 3.1). Calcium
sulfate will increase the hardness of the water until its maximum solubility
is reached (approximately 2,000 mg/l), then it will precipitate. Ferrous
hydroxide has a low solubility, which decreases as the pH increases. Ferric
hydroxide is even less soluble than ferrous hydroxide antl its solubility
decreases at higher pH's. The oxidation of ferrous iron results in a
decrease of the pH, which may result in an increase of the iron concentration
because of the higher solubility of iron at lower pH's.
Hydrated lime is most widely used for the treatment of mine drainage,
however, quicklime has been used in some cases. Quicklime is usually slak-
ed before use by the addition of water, thus forming lime hydrate. Hoak,
et al.,^"^ reported that lime hydrate reacted more effectively than the
hydrated lime. Quicklime is hazardous to work with and special hydrating
plants must be constructed to prevent explosions and burns during mixing of
quicklime and water. However, in some cases quicklime has a cost advantage.
- 24 -
-------
Both high-calcium hydrated lime and dolomitic lime have been used
to treat acid mine waters. The reaction rates of both types of lime are
very rapid. However, since calcium sulfate is insoluble at higher concen-
trations, dolomitic lime may hold some advantages in treating highly acid
U)
water. Hoak, et al., found that the pH could be increased to 7 or
greater in less than 2 minutes with both forms of hydrated lime. Similar
(1/')
results were reported by Dorr-Oliver x ^ .
The reaction rate of ferrous iron and lime is the limiting factor
in treating acid mine drainage. This reaction goes very slowly and requires
hours for completion even when the water is aerated. The process itself
is not fully understood; however, it is apparent that the reaction is con-
trolled by the pH of the system, the temperature, ion concentration, etc.
The oxidation rate is low at low pH's, as shown in Figure 3.3.
Lime neutralization of acid mine discharges dates back to the early
1920's (Clifford and onarely)^"^, A common practice at that time, still
in use today, was to apply the lime to the water and then pass the water
through a pond or lagoon, Lime is either fed as a lime-water mixture (slurry)
or as a dry chemical, Hydrated lime is reported to be difficult to handle
in the dry form since it has a tendency to arch or bridge in storage bins
and possesses poor flow properties.
( 3)
Braley, et al., conducted studies on treating acid mine drainage
with hydrated lime by batch treatment in an agitated tank. They reported
the time of neutralization to be short and the treatment complete, although
the gelatinous iron sludge settled very slowly. Approximately three-fourths
ton of hydrated lime was required to neutralize one ton of sulfuric acid.
-------
2 -
Stumm - Lee
I -
0 -
¦d loo fFeODl
dt
-I -
1
-2 -
o
o
-3 -
Singer- Stumm
Modification
Hgure 3-3. Iron (I) Oxidation Rate at pH 2-?
(after Singer and Stumm ))
-------
"Operation Yellowboy" was a program developed by the Pennsylvania
(? )
Department of Mines and Mineral Industries to demonstrate the treatment
of acid mine drainage by lime neutralization. The treatment consisted of
three steps in the following order: neutralization with lime in a flash
mixer, aeration for iron oxidation, and clarification by a thickener.
The sludge from the thickener was further treated by centrifuging. The
pilot plant was mounted on a semitrailer and taken to six different mine
drainage locations. Thus, the system was tested under a wide variety of
conditions.
Results of this study showed that the acid mine drainage could be
neutralized such that no acidity was present and the pH was greater than
6.5. Iron concentration could be reduced to less than 7 mg/1. However,
the calcium and hardness concentration increased and the sulfate concentra-
tion remained relatively constant.
Approximately 30 minutes of detention time was necessary in the
aeration tank, while the detention time in the thickener varied from site
to site and ranged from 7.35 to 13.3 hours. The amount of detention time could
not be related to the acidity or iron concentration of the raw water.
Thickener-sludge solids varied from 0.9 to 4.98 percent. A drum filter
was able to concentrate the sludge to between 12 and 26 percent solids.
Gypsum was successful as a filter aid.
(42)
As part of the "Operation Yellowboy" studies flocculating agents
were tested. Although some flocculents increased the settling rate, they
did not necessarily increase the solid concentration of the thickener
underflow. Flocculents were also found to be pH sensitive. Centrifuge
- 27 -
-------
and filtration tests demonstrated that within a few hours after the floe
was formed and settled it apparently went "stale",
(9)
In Figure 3.4 the cost data obtained from "Operation Yellowboy"
are reported. These data show that the unit cost increases as the concen-
tration of acidity and iron in the raw water increases and decreases as
the amount of water treated increases.
(13)
Deul and Xihok treated acid mine drainage in a reactor (rotating
drum) and found that neutralization took place very rapidly. The lime
treated acid mine drainage did not produce a sludge that settled as rapidly
or produced as dense a sludge as limestone treated acid mine drainage.
Up to 15 hours were required to reduce the sludge solids volume to 10 per-
cent.
(27)
Holland, et al., reported on the operation of a 230 gpm pilot
lime neutralization plant treating a mine discharge with high acidity and
ferrous iron concentrations. A lime slurry was fed to the water. A reaction
time of 30 seconds in a pipe was sufficient for neutralization to occur.
Enough lime was added to increase the pH to 10.5. According to the titra-
(27)
tion curves obtained by Holland, et al., a pH of this magnitude was
necessary to assure complete removal of iron. Neutralization 1vas followed
by aeration to oxidize the ferrous hydroxide to the insoluble ferric
hydroxide form. Following aeration, the water passed to a settling basin
having 1.5 days, retention time. Approximately 33 percent of the plant inflow
remained as sludge in the settling basin and was pumped to a sludge basin.
Here, the sludge was dewatereel further by settling and decanting of the clear
supernatant and by seepage and evaporation. In a three-week period the
-------
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Raw Water
Total Iron (ppm)_
IS)
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Flow Rate
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Total Acidity (ppm)
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sludge had a solids content of about 20 percent. The final sludge volume
in the sludge basin was three percent of the mine discharge pumped to the
plant. Table 3.3 presents an analysis of the residual sludge. Based on
(27)
the results obtained in this study, Holland, et al., estimated the cost
of treating mine drainage to range from 18.9 to 62.5 cents per 1,000 gallons.
Cost was dependent on plant capacity and the concentration of acidity and
iron. In Figure 3.5 these data are plotted.
Birch^ ^ ^ reported on laboratory bench tests conducted to determine
the most efficient and economical oxidation-neutralization methods for
different types of mine waters. He found that mine water that had aged
in a holding pond had a faster reaction time and the iron precipitated out
much more readily. The removal of carbon dioxide by loss to the atmosphere,
in the holding pond may have been the reason for the improvement. Further,
he found that sludge volume was about 10 percent of the original flow upon
immediate settling and, after several days, would compact to five percent.
When precipitated sludge was recycled to the influent water, the reaction
and settling times were reduced about 50 percent and a larger and heavier
floe was produced, Mihok, et al.,^^ conducted studies on the settling
and compaction of mine drainage treated with lime and limestone to a pH
of 6.9. As shown in Table 3.4 the sludge from the limestone treated water
settled faster and compacted more than the sludge from the lime treated
water.
Full scale lime acid mine drainage treatment plants are being placed
(60)
in operation in increasing numbers. Tybout reported 71 permits for the
construction of such plants in Pennsylvania during 1967. In 1966, Jones
- 30 -
-------
TABLE 3.3
ANALYSIS OF SLUDGE FROM MINE NEUTRALIZED
AERATED TREATED ACID MINE DRAINAGE*
Item Percent
Fe+-H- 19.2
A14++ 4.0
Ca-H- 12 < 0
Mg++ 0.2
CO3 3.9
so4 29.4
Insoluble 0.5
Water of Hydration 19.6
Specific Gravity 1,10
Total Solids 13.9
(27)
* From Holland, et al.,
31 -
-------
10
9
8
7
6
5
4
3
2
I
0
0
Curve Acidity, (ppm) Iron, (ppm)
A 600-700 322
B 1400 600-700
C 2800-4000 900-1200
_L
20 40 60 80
Cents per Thousand Gallons
100
Figure 3-5. Estimated Cost of Lime Neutralization
(Data from Holland, el al. )
-------
TABLE 3.4
HEIGHT OF SETTLED SOLIDS
OF LIME AND LIMESTONE-TREATED WATERS*
Days
Percent
of Total Volume^
Lime
Limestone
0
100.0
100.0
1
12.0
2.6
5
6.9
2.3
11
6.7
2.1
15
6.7
2.0
20
6.7
2.0
43
6.6
1.9
^-As percent of total volume (pH « 6.9, no detectable iron in supernatant).
* After Mihok, et al.,^4°^
- 33 -
-------
(10} (c>g)
and Laughlin ' y began operation of a plant in Washington County,
Pennsylvania. Mine drainage from a 10-inch borehole is placed in a
750,000-gallon raw-water surge lagoon (5 day storage). Considerable iron
oxidation is reported to occur in the lagoon which reduces the lime require-
ments. Approximately 150,000 gpd are withdrawn from the lagoon and mixed
with a slurry of hydrated lime. The treated water then flows through a small
tank containing a pH probe. The pH probe is connected to equipment that
controls the rate of lime feed and to a raw-water input valve that automat-
ically shuts the system off if the pH is too high or low. Subsequently, the
water flows to a 90,000-gallon settling lagoon where the sludge is removed.
Once a week the sludge is pumped from the lagoon and hauled away to a bore
hole, where it is discharged into an abandoned deep mine. Approximately
4,000 gallons per day of sludge are produced. Lime demand is two to five
pounds/1,000 gallons for a raw water supply having a pH of 3.5, acidity
200 to 560 mg/l and ferrous iron 100 mg/l. A chemical analysis of the
sludge resulted in the following: CaSO^ - 40 percent, HgS04 - 5 percent,
CaO - 3 percent, KgO - 1 percent, Fe203 - 15 percent, Mn2°3 - 4 percent,
Si02 - 20 percent, and AI2O3 - 12 percent. The sludge contained six
percent solids. Drying of the sludge increased the solids content to
sixteen percent. Tests indicated that recycling the sludge and mixing
it with the feed water did not increase the sludge solids content. Chemical
flocculents did not increase the solids content either.
A 7,000,000 gpd discharge at a Jones & Laughlin ' ^ mine in
Pennsylvania is treated with slaked quicklime and passed to an aeration
tank. Surface aerators are used. Overflow from the aerator is to two
- 34 -
-------
impoundment basins. The basins are used alternately to permit-the sludge
in the inactive basin to dry out for better compaction, oludge is not
removed from the basins and they were designed for .30 years of storage.
Only very limited work has been done on sludge handling and disposal.
(9)
In "Operation Yellowboy" studies were made on centrifuging and filter-
(22)
ing the sludge. Johns-nanville had some success with prscoat filters.
However, disposal of the thickened sludge is still a critical problem.
Lime treatment of acid wastes has been practiced for many years in
the plating industry; and this experience is applicable to acid mine drain-
age in many cases. The major problems in lime treatment are the oxidation
of the ferrous iron, the settling rate and density of the sludge, and the
disposal of the sludge. Table 3.5 presents design considerations for a
lime neutralization plant.
Limestone-Lime
(25)
Hoak, et al., found that neutralization could be accomplished
better when a combination of limestone and lime was used than when either
was used alone in treating pickle liquor. Pulverized limestone was used to
neutralize the acid and precipitate part of the iron and line to complete
the treatment. The split treatment was found to be more economical than
(13)
lime alone, Deul and MihokN •" reported that, when using limestone alone,
long reaction periods were needed to treat high ferrous iron waters, while
with lime alone the same waters were rapidly neutralized but larger amounts
of reagent were required to precipitate all the iron. When limestone and
lime were used together the time required to oxidize the ferrous iron was
reduced and the sludge settling rate and density of the settled sludge
were better than for lime treatment alone and similar to limestone treatment.
-------
TABLiS 3.5
LIKE NEUTRALIZATION DESIGN CONSIDERATIONS*
Reaction time (neutralization) 0,5 - 1.0 minutes
Sludge settling detention time 7.5 hours or more
Sludge volume 10-35 percent of plant influent
Sludge solids 1-5 percent
Sludge thickening in a basin (detention) 3 to 4 weeks
Sludge volume after thickening in basin 3 to 15 percent of plant influent
Sludge solids after thickening in basin 10 to 20 percent
* Ferrous iron to be removed in a separate operation.
- 36 -
-------
Thus, split treatment had some advantage over single treatment.
Research Needs
Although more information is available on the neutralization of
acid mine drainage than on any other treatment method, many areas need
additional study, ^ome neutralization processes need further laboratory
investigations; however, rcany are suitable for pilot plant evaluation
or large scale application.
Needed neutralization research areas are:
Analytical tools to evaluate acid mine drainage and determine the
amount of alkaline agent required, detention time to provide adequate
settling, and improved sludge characteristics.
Methods for utilizing low cost alkaline agents to accomplish higher
removal efficiencies e.g., limestone.
Methods of improving iron (II) oxidation and removal.
Methods for improving the settling and density characteristics of
sludges resulting from chemical treatment.
Methods for concentrating sludge.
Methods for disposing of the sludge, e.g., to deep mines and surface
mines for possible additional acid mine drainage control.
Byproduct recovery from sludges and/or sludge reuse.
Process analyses to determine how neutralization can fit in with
other processes for higher quality water at lower costs to meet
all water use requirements.
Optimization of neutralization process.
Cost analyses of the various neutralization processes.
- 37 -
-------
FWPCA Research Program
The Federal Water Pollution Control Administration has the follow-
ing research and development activities in the neutralization of mine
drainage area:
Grant to the Pennsylvania Department of Mines and Mineral Industries
and its subcontractor, Pennsylvania State University, for a project
titled "Construction of Mine Water Treatment Plant at Hollywood,
Pennsylvania," Grant Number WPRD-34.
Grant to the Pennsylvania Department of Mines and Mineral Industries
and its subcontractor, Bituminous Coal Research, Incorporated, for
a project titled "Optimization and Development of Improved Chemical
Techniques for the Treatment of Coal Mine Drainage." Grant Number
WTRD-63.
Grant to the Pennsylvania Department of Mines and Mineral Industries
and its subcontractor, Johns-Manville Products Corporation for a
project titled "Neutralization and Precoat Filtration of Concentrated
Sludges from I'ine Waters." Grant Number WPRD-150.
Grant to the Peabody Coal Company for a project titled "Lime/Lime-
stone Neutralization of Acid-Mine Drainage." Grant Number WPRD-272.
- 38 -
-------
k. IRON REMOVAL
Iron removal is an important aspect in the treatment of mine drain-
age. Iron must be removed to meet stream standards. Also, it is one of
the major problem areas in treatment. Ferrous iron (Fe II) presents a
number of problems to the neutralization of acid mine drainage; for
example, oxidation of ferrous iron produces acid (see equation 2-4) and
lowers the pH, thus increasing the amount of alkaline agent required. Fe
(II) removal is a limiting factor in the neutralization process. Ferric
iron (Fe III) presents problems in various other treatment processes;
for example, it fouls the membranes of an electrodialysis unit and coats
the exchange resin in an ion exchange process. Since iron removal is
often a separate step in the treatment of mine drainage, and because it
is a critical problem, it -warrants a separate section in this report.
Chemistry of Ferrous Iron Oxidation
(56)
Stumm and Lee reported that the rate of iron (II) oxidation
at pH values of 6 and above depends on the pH, temperature, concentra-
tion of dissolved oxygen, and certain catalysts. The rate of oxidation
was found to proceed 100 times faster at pH 7 than at pH 6. Catalysts
+2
such as Cu in trace quantities, and anions which form complexes with
Fe (III) increase the reaction rate significantly while small amounts
-2
of Fe (III), CI , and S0^ had no effect on the reaction rate.
The reaction of Fe (II) with oxygen generally leads to ferric
oxides or hydroxides, the stoichiometric relationship being:
Fe (II) + £ 02 + 2£ H20 ~ Fe (0H)3 (s) + 2H+ (16
- 39 -
-------
Stumm and Lee concluded that the rate of oxidation was first
order with respect bo Fe (II) and. independent of the Fe (ill) concen-
tration. x7urther, the rate was dependent on the partial pressure of
oxygen to the first power. The rate increased as the pll increased due
to the influence of the hydroxyl ions and as the temperature rose.
(51)
Singer and Ltum conducted ferrous iron oxidation studies
under conditions similar to acid mine drainage situations. One aspect
of their study was to determine the effect of various material found in
the nine environment on iron (II) oxidation. The Stumm-Lee rate law
was found to be not applicable at pH's less than 4«5. Between pH 4*5 and
3, a transition occurs; at pH 3 and below, the rate is relatively inde-
pendent of pH (Figure 3.3). The rate of iron (II) oxidation was found
to be greater in sulfate free solutions than in solutions containing
-a -5
10~J to 10 M of sulfate. On the premise that clay surfaces might
play a role in the oxidation of Fe (II), a study was undertaken to determine
the effect of silica (Si02), aluminum oxide (AlgO^i), bentonite and kaolinite
on the oxidation kinetics. It was postulated that those surfaces may
play a specific role due to their hydroxylated surface, Si-OH or A1-0H
groups, or a general,role resulting from surface absorption and localized
increased reactant concentrations. Aluminum, silica and bentonite was ¦
found to catalyze the Fe (II) oxidation, but only at very large surface
concentrations, which are not normally found in mine drainage, thus, these
materials did not appear to be of significance in explaining the rapid
Fe (II) oxidation rate observed in acid mine waters in the field. Kaolinite
clay, although it gradually neutralized the acid, showed no catalytic
- 40 -
-------
influence. Colloidal ferric hydroxide and soluble A1 (III) showed no
catalytic effect. Powdered charcoal increased the rate only slightly.
A series of tests was conducted to determine if the oxidation rate was
greater in a dark environment, similar to the inside of an abandoned
mine, or under normal light conditions. The oxidation proceeded at a
rate two to three times faster in the light. In summary, Singer and
Sturam were unable to explain on a chemical basis the high rate of iron
(II) oxidation observed under field conditions. They did not evaluate
the microbial aspects.
(31)
Kimv conducted studies of the effect of aeration upon the
oxidation of ferrous iron in acid mine water. She concluded that the
rate of oxidation increased until the solution is saturated with oxygen.
Aeration beyond oxygen saturation does not increase the rate of iron
(II) oxidation.
Removal of Ferrous Iron
(50)
Simpson and Rozelle , in a review of methods available for
removal of iron from solutions, listed the following:
Precipitation of iron by the addition of an alkaline agent
Electrolysis of iron (II)
Aeration-filtration or aeration-settling
Ultrasonic energy
Ozone -oxidation
Irradiation and photo-oxidation
Other methods could be added to this list, such as:
Chlorine oxidation
- 41 -
-------
Potassium permanganate treatment
Biochemical oxidation
Precipitation by Addition of an Alkaline Agent
The addition of an alkaline agent such as hydrated lime, sodium
carbonate or sodium hydroxide will result in the conversion of ferrous
sulfate to ferrous hydroxide and increase in pH. At the higher pH iron
(II) will oxidize rapidly to insoluble ferric hydroxide. Even where
alkaline mine discharges with high ferrous content are found, an alkaline
agent is sometimes added to increase the pH and thus increase the oxidation
(32)
rate. Kosowski and Henderson reported a situation in West Virginia
where the mine discharge had a pH of 6.5, an alkalinity of 252 mg/l,
and iron of 109 mg/l. The mine drainage was treated with lime to increase
the pH for rapid oxidation and to prevent a depression of the pH as a
result of the oxidation of the ferrous iron. Aeration and settling followed
(27)
lime addition. Holland, et al., applied lime to an acid discharge
high in ferrous iron to remove the iron. They found the pH had to be
raised to as high as 10 to obtain iron removal.
Electrolysis of Iron (II)
Direct current electrolysis of iron (II) sulfate solutions causes
the hydrogen ions to be discharged along with iron (II) at the cathode.
Discharge of hydrogen ions causes the pH in the area surrounding the
cathode to rise, resulting in the precipitation of iron (II) hydroxide.
The electrochemical reactions at the anode are not well defined, but it
appears iron (II) loses an electron to form iron (III) and that Fe (ill)
is hydrolyzed.The reactions of this process are:
- 42 -
-------
Cathode
H+ -r e~ = ^H2 (17
Fe+2 + 2e~ = Fe (18
Fe+2 + 2H20 = Fe(OH)2 + 21V (19
Anode
H20 = h °2 + 2H+ + 2e~ (20
Fe+2 = Fef3 + e- (21
Fe+3 + 3H20 = Fe(0H)3 + 3H"r (22
The electrolysis process has been studied in the laboratory but
has not been evaluated on mine drainage in the field.
Aeration
A common method of iron (II) removal is aeration to oxidise the
iron to the ferric form, followed by hydrolysis to ferric hydroxide, or
the formation of the hydroxide followed by aeration, as in equation 23.
The precipitated iron is then removed by settling or possibly filtration.
According to equation 23, one mole of oxygen reacts with four moles of
ferrous hydroxide to produce four moles of ferric hydroxide or one pound
of oxygen will oxidize 9.1 pounds of ferrous hydroxide.
4Fe 0H+ + 4 OH" + 6H20 + 02 » ¦ 4Fe(0H)3 + 4H20 (23
Ferrous Hydroxide + Water + Oxygen s Ferric Hydroxide + Water
As indicated earlier, the oxidation of iron (II) at low pH's is very slow,
and therefore, it is advantageous to raise the pH by the addition of an
alkaline agent.
(27)
Holland, et al,, reported studies conducted on the aeration
of acid mine drainage previously treated with lime. They listed the
- 43 -
-------
following three ideal conditions for aeration: maximization of the surface
areas between air bubble and solution for a given quaiitity of air (minimize
the bubble size), maximization of the amount of water which contacts the
air, and maximization of the amount of time the air is in contact with
the water. Three types of dispersion devices were evaluated for obtaining
small bubble size. A large number of small dispersion devices produced
a smaller bubble than a single large device because the velocity of air
discharge was decreased. The geometric placement of the dispersion devices
was also important.
(27)
Under field operation, Holland, et al., found that dispersion
type aerators required daily cleaning with acid to remove the iron deposit
which built up on the device. Surface aerators have also been used success-
, , (32), (10)
fully to oxidize the iron (II) in holding ponds
Once the iron is converted to the ferric hydroxide form, it is
usually removed by sedimentation in a settling tank or pond. The sludge
usually settles slowly, has a low solid content, and presents a disposal
problem.
Ultrasonic Energy
Ultrasonic energy has been used to oxidize iron (II) sulfate^1^.
Under the influence of ultrasound, hydrogen peroxide is generated which
may oxidize the iron (II) through the formation of free radicals, for
which oxygen is not needed, or the waves may catalyze the oxidation reaction.
The rate of oxidation is a function of the intensity of the waves. This
method is still in the research phase.
Radiation
Oxidation of iron (II) by alpha, beta, and gamma radiation is
- 44 -
-------
believed to occur by the same mechanism as ultrasonic oxidation. One
major problem with inoxidation is the short depth o? penetration of the
water by the inoxidating particles, Steinberg, et al.,^"^ studied the
removal of iron from artificial nine drainage with hirjh-energy radiation
and found that the ferrous iron could be removed in.a relatively short
time from a mine water that had been treated by limestone neutralization
60
to a pH of 5.7 and then exposed tc Co ' gamma radiation. Iron removal
was much slower in the non-irradiated water. Thus, radiation treatment
appeared to act as a catalyst. The rate of ferrous iron removal was
proportional to the square root of the intensity. An intensity of 4.3
megarads per hour obtained a rate of 45 ppra of ferrous iron removal per
minute.
The auth'ors suggest that a chain oxidation mechanism occurs which
includes a biradical chain termination step. The chain carrier may be
hydroxyl radicals. Sludge from the lijnestone-radiation process yields
a readily separated crystalline particle, while the limestone treatment
produced a flocculent precipitate which was difficult to handle,
(54)
Later studies by Steinberg, et al., with a sample of actual
acid mine drainage resulted in the following conclusions: (1) G-values#
ranging up to 285 were obtained at low intensities (130,000 rads per hour).
The G-values decreased with decreasing temperature and were found to be
three to five times less at 10° C, (2) At high intensities of 3.5 x 10^
¦HG-value is the radiation yield and is defined by the expression: G-value =
1,04 Q/I, where G-value is molecules (or atoms) of Fe-H- oxidized to Fe+t+
or removed from solution per 100 eV of ratiiation deposited, Q is rate of
Fe-H- removed (ppm/minute), and I is radiation intensity CO-60 gamma field
(megarad per hour)*
- 45 -
-------
rads per hour, a G-value of 12 was obtained, and the rate of Fe (II)
removal was 20 times higher than the control, and temperature had relatively
little effect. (3) Increasing the pH and aeration increased the radiation
yield and rate of Fe (II) removal and (4) the preliminary cost analysis
indicated that if iron (II) could be removed at a rate of 10 ppm per minute
with limestone treatment alone, then it would be difficult for the radia-
/
tion treatment to compete on an economic basis solely.
Ozone
Ozone is a powerful oxidizing agent which is produced by passing
an electrical discharge through oxygen, The reaction of ozone with iron (II)
is:
2 Fe^2 -i- 03 t 2H"r » 2 Fe"'"3 -t- 02 + H20 (24
Simpson and Rozelle^0^ reported that the rate of oxidation of iron (II)
was independent of the concentration of iron (II) and temperature and
dependent on the rate of flow of ozone through the solution. The oxidation
of an acid mine drainage sample is illustrated in Figure 4.1. Terminal
pH was approximately 2.8, The miHiequivalent of iron (II) oxidized
per nilliequivalent of ozone was about 0,85 in acid mine drainage samples
(Figure 4.1)• Ozonation of iron (II) appears promising from laboratory
studies, but has not been attempted on a pilot scale.
Oxidizing Arent
Oxidizi'ir agents such as chlorine, iodine, and potassium perman-
ganate have been used in the water treatment field for the oxidation of
iron (II). The oxidation process is usually followed by filtration to
remove the precipitated iron. These agents have had very little evaluation
- 46 -
-------
27
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MEQ Ozone
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for treating mine drainage. Birch reported that studies were con-
ducted on potassium permanganate and calcium hypochlorite. Potassium
permanganate was found to react slowly in oxidizing the iron and was
expensive. Calcium hypochlorite was found to do an excellent job of
converting ferrous to ferric iron.
Biochemical Oxidation
(19)
Glover has developed a biochemical oxidation process for ferrous
iron removal. An autotrophic bacteria that derives its energy from the
promotion of an inorganic reaction and its cellular carbon from carbon
dioxide is used. These bacteria promote the oxidation of ferrous iron.
An activated sludge type process is used. During startup, the acid mine
drainage is seeded with an ochreous deposit from a mine and fed to an
aeration chamber. From this chamber, the waste goes to a settling tank.
The sludge is collected, recirculated, and mixed with the incoming acid
mine drainage. Once the process is started, the seed is no longer necessary.
The rate of oxidation was much greater in the activated sludge system
than in a system of aeration alone. In general, the rate of oxidation
was first order to the ferrous iron concentration. The quantity of act-
ivated sludge produced in a pilot plant remained adequate and a small
surplus occurred only at temperatures in excess of 20° G. Chemical analyses
of the influent and effluent showed that the ferrous iron was reduced from
levels of 100 to 300 mg/l to less than 5 mg/l> and that a corresponding
increase occurred in the ferric iron. The activated sludge process is
followed by limestone neutralization to remove the ferric iron.
- 48 -
-------
Sulfide Treatment
(62)
Zawadaki and Glenn conducted studies on the removal of iron
with sulfide. As indicated in equations 25 thru 27, both ferric and
ferrous iron can be removed in this manner.
3II2S -r Fe2(S0^)3 * Pe2S3(ppt) + 3^30^ (25
Hydrogen Sulfide + Ferric Sulfate 1111 ~' Ferric Sulfide Sulfuric Acid
H23 -i- FeSC^ 11 ^ FeS(ppt) + H2S0^ (26
Hydrogen Sulfide + Ferrous Sulfate * Ferrous Sulfide + Sulfuric Acid
4Na2S + Fe2(S0^)^ + Fe 30^—"- ~ Fe2S^ (ppt) -r FeS(ppt) -r 4Na2S0^ (27
Sodium Sulfide Ferric Sulfate -t- Ferrous Sulfate ——* Ferric Sulfate -r
Ferrous Sulfate +
Sodium Sulfate
The pH must be above 5 for the hydrogen sulfide to be effective.
One disadvantage of the use of hydrogen sulfide is the formation of sulfuric
acid. Thus, additional alkaline must be used to counteract this acid.
Research Needs
A number of the ferrous iron oxidation schemes considered have
shown promise; however, the majority have not been tested on a pilot
plant scale. Only aeration and precipitation with an alkaline agent
have received any degree of testing. Each of these methods needs to be
studied under field conditions to determine the system's efficiency,
operating problems, factors affecting operation, best conditions for use,
and its economics. The use of catalysis to increase the rate of iron
removal needs to be studied further. Microbiological removal of iron
requires study. The possibility of developing a magnetic sludge needs
evaluation.
- 49 -
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FWPCA Research Program
The Federal Water Pollution Control Administration has the following
research and development activities in the removal of iron from mine drainage:
Grant to Bituminous Coal Research, Incorporated for a project
titled, "Sulfide Treatment of Acid Mine Drainage," Grant Number
WPRD-271.
Grant to Continental Oil Company for a project titled, "Micro-
biological Removal of Iron from Mine Drainage Waters," Grant
Number WPRD-36.
Contract to Harvard University for a project titled, "Oxidation
of Iron in Acid Mine Waters," Contract Number PH 36-66-107.
Grant to Pennsylvania Department of Mines and Mineral Industries
and its subcontractor, Pennsylvania State University, titled
"Construction of Mine Water Treatment Plant at Hollywood,
Pennsylvania," Grant Number WPRD-34.
Grant to Pennsylvania Department of Mines and Mineral Industries
and its subcontractor, Bituminous Coal Research, Incorporated,
titled, "Optimization and Development of Improved Chemical
Techniques for the Treatment of Coal Mine Drainage," Grant
Number WPRD-63.
Grant to Pennsylvania Department of Mines and Mineral Industries
and it's subcontractor, Applied Science Laboratories, Incorporated,
titled, "Feasibility of the Purification of Acid Mine Water by
a Partial Freezing Process," Grant Number WPRD-265.
Contract to Syracuse University Research Institute titled,
"Biological Treatment of Acid Mine Water," Contract Number WP-01460.
- 50 -
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5. ION EXCHANGE TREATMENT
Ion exchange can be used to remove any number of undesirable con-
stituents in mine drainage. In combination with neutralization, lime
softening, and other processes, a highly refined water can be obtained.
Ion exchange appears to be well suited to the removal of calcium and sul-
fate, which are not removed by neutralization processes. While a number
of ion exchange schemes have been proposed for treating mine drainage,
none have been tested on actual mine wastes.
Principles of Ion Exchange
Ion exchange has been defined as a reversible exchange of ions be-
tween two phases, usually a solid and liquid in which there is no substantial
change in the structure of the solid. The solid is the ion exchange mater-
ial or resin and is based upon a matrix composed of addition copolymers.
This structure gives a maximum resistance to oxidation, reduction, mech-
anical wear and breakage, and is insoluble in common solvents.
The nature of the ionizable groups attached to the hydrocarbon
network determines the chemical behavior of an ion exchange resin. In
general, there are four major types of ion exchange resin; namely, strongly
and weakly acidic (cation) resins and strongly and weakly basic (anion)
resins. Of these resins, the weakly basic anion exchange resins appear
best suited for mine drainage treatment because of their acid absorbing
properties. The weakly basic resins are often used in the removal of
free acids from solution and have ion exchange activity only below pH 7.
However, schemes for using combinations of all the resins have been
developed.
- 51 -
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Although a variety of weakly basic exchanges are available, the
more common of these contain mixtures of secondary and tertiary amine
(1
groups. The following characteristics have been reported for these resins
Chemistry analogous to that of ammonia.
Free-base form of resin absorbs strong acid (HC1, HgSO^, HNO^)
to form hydrochloride, bisulfate, nitrate, etc.
Acid form of weak-base resins liberates free acid when in
contact with water. The stronger the acid, the greater the
tendency toward hydrolysis.
Acid forms can be interchanged; e.g., sulfate will replace
chloride.
Complete regeneration to free-base form is achieved with
nearly stoichiometric quantities of alkali.
Regeneration to free-base form can be carried out with
ammonia or soda ash as well as with caustic soda.
Capacity to strong acid increases with the valence of the
anion.
The weak-base resins can only form very unstable salts with weak
acids; therefore, they cannot effectively remove carbon dioxide from water,
though they do remove a limited amount. Illustrations of weak-base resin
usage are shown in equations 32 and 37.
Strong-base resins have quaternary ammonium groups in their structure'
They will remove salts by exchanging hydroxyl ions for other anions as
in equations 28 and 29.
RN(CIl3)3 + OH" + NaCl RN(CH3)3 t- Cl~ + NaOH (28
- 52 -
-------
(RMIlboH -r H2G0. (m:b2JC4 1- 2H20 (29
Unless removed before ion exchange treatment, iron could cause a trouble-
some problem because of the exchange would result in ferric and/or ferrous
hydroxide which would precipitate and coat the exchange resin. Removal
of these precipitates would be difficult.
leak-acid resins usually contain carboxyl groups in their structure.
They cannot split neutral salts and are operable in the pH range above 7.
These resins do not appear to be applicable to mine drainage.
Strong-acid resins are commonly composed of sulfonated styrene
and divinylbenzene. In the hydrogen form they are used to remove cations
from solution as in equation .30. Sulfuric acid in excess of the stoichio-
metric amount is required for good regeneration. Calcium and other metal
ions can be removed as in equation 31. Sodium chloride is usually used
for regeneration in this case.
Ii£0JH+ ->¦- WaCl IfSO'rJa"'" -i- HC1 (30
3 (RSO3H) t Ketal-i-H- ^ (RSO3 )3 metal + 3H"r (31
These resins could be useful in removing the troublesome cations
found in mine drainage.
Ion Exchange Treatment of Mine Drainage
A number of schemes for treating acid mine drainage by ion exchange
have been suggested or seem feasible. These are reviewed on the follow-
ing pages.
The weak-base anion resin can be used in the bicarbonate form by
Utilizing carbon dioxide during a carbonation step to produce RHCO3.
Once the resin is in the bicarbonate form, mine drainage is passed through
- 53 -
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it and the following conversion occurs:
IIx(S04) + 2RHC03 » r2S0U + f^(HC03 )x (32
R =s Ion Exchange Resin, II = Metal Ions
For example, the equation for ferrous sulfate would be
Fe(SO^) + 2RHCO3 » R230^ + Fe(HC03) (33
The resin once exhausted is regenerated with CO2 and ammonium
hydroxide (NH^OH). Ammonium hydroxide is passed through the column first,
then water saturated with CO2; this results in the following conversion:
R2S0^ + 2NH4OH 2R0H + (NH4)2S0^ (34
ROH + H2C03 ** RHCO3 + H20 (35
The waste is therefore, ammonium sulfate.
/) o ^
Pollio and Kunin conducted studies on this process. In their
scheme the ion exchange process was followed with aeration to expel C02,
and to precipitate iron, aluminum, and manganese as the insoluble hydrous
oxides. Some calcium and magnesium also coprecipitated or occluded with
the hydrous oxides as CaCO^ and Kg(0H)2. After aeration a bicarbonate water
with a pH of 8.0 to 8.2 containing calcium and magnesium hardness remains.
Ca( 11003)2 -r Ca(0H)2 2 CaCX^ + 2H20 (36
(j±3 )
Pollio and Kunin operated the ion exchanger as an upflow unit
to prevent precipitation problems. Their study was on a small laboratory
scale using a synthetic acid mine drainage water. The process worked . .
successfully under this condition. Cost of the ion exchange process was
estimated as 44 cents per 1,000 gallons for a 100,000 gpd plant and 38
oents per 1,000 gallons for a 1 mgd plant. The cost of ion exchange would
increase as the anion content increased. They suggest that for economic
- 54 -
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treatment the ion exchange process should be limited to mine waters with
an anion content below 4,000 ppm expressed as CaCGj,
Disposal of the waste regenerant (primarily ammonium sulfate) from
the ion exchange unit might present a problem unless the regenerant can be
used as a fertilizer or put to some other use. A process could be develops
where lime was added to the waste regenerant and IIII^ recovered and reused.
Another method would be to use the weak-base anion resin in the
chloride cycle. lline drainage as it passes through the resin is converted
as follows:
Mx(SG^) -r 2EC1 K IlxCl -r (37
The resin once exhausted is regenerated with MaCl as follows.
R2S04 -i- 2NaCl 2RC1 -i- Ha2S0^ (32
The effluent water from this process would contain high concen-
trations of ferrous chloride, v/ith aeration the iron could be removed as
ferric hydroxide. The chloride that remained would be somewhat difficult
to remove.
The waste regenerant, •which would be primarily sodium sulfate,
would create some disposal problems.
The chloride cycle would be cheaper to operate than the bicarbonate
cycle because of the lower cost of the regenerating agent.
Other ion exchange processes may have some merit. One process
suggested^) for sulfuric acid pickle liquor calls for the use of a cation
exchange resin. The resin bed would remove the iron and produce sulfuric
acid. During the regeneration step, the iron is removed from the resin
as iron nitrate. Upon heating to 350° F, iron oxide is formed. The iron
-------
oxide and sulfuric acid are then marketable by-products. However, the
concentration of iron and acid in acid mine drainage may not be sufficient
to be economically recovered.
Use of ion exchange process to treat acid mine drainage and also
to produce fertilizer can be visualized. The mine drainage would first
be neutralized with NH^, then settled. The sludge would contain the iron
and aluminum while the effluent would be diluted (NH^)2S0^, The ammonium
sulfate would be passed through a cation resin bed to remove the ammonia
and then through an anion resin bed to remove the sulfate. The cation
bed would be regenerated with HHO3 and the anion bed with NH^OH. The
waste regenerates would be NH^NO^ and (NH^^SO^ which could be used for
fertilizer.
(6)
Care and Zawadzki conducted studies using a strong anion resin
in the sulfate form to treat acid mine drainage. The chloride form of
the resin is converted to the sulfate form with sulfuric acid. When acid
mine drainage is passed through the resin, the following conversion occurs!
R2SO^ + H2S0^ ^ RHSO^ + RHSO^ (39
By this reaction divalent sulfate ions on the exchange sites are
converted to monovalent bisulfate ions. This conversion frees one of
the two exchange sites initially required to hold the divalent sulfate
ion. The newly freed site is now capable of accepting an additional anion',
(16)
The bed was regenerated with water as shown in equation 40
RHSO4 + RHSO4 -T H2O »¦ R2SO4 T H2SO4 1- H20 (40
(6)
Care and Zawadzki regenerated the bed by downflow. They found
that for every unit of acid mine drainage treated, three units of water
were required to regenerate the resin. Iron fouling occurred when excess
acid was not present to prevent the precipitation of ferric hydroxide.
- 56 -
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Under conditions of 3:1 ratio of regenerant to feed, and flow rate of
4 gpm per square foot of bed cross section, a maximum of 90 percent of
the iron was separated into a fraction containing only 20 percent of the
acid. By this method the acid could be separated from the salt fraction.
However, the acid fraction would have greater volume and be more dilute
in sulfuric acid than the original feed solution because of the high ratio
of regenerant to feed. Cost of treating 1 mgd was estimated as 30 cents
per 1,000 gallons.
(49)
Schroeder and Marchello describe the Nalco process in which
three beds are used, that is, a weakly acidic bed, strongly acidic bed,
and strongly basic bed. The raw water is filtered, then passed through
the beds in the order listed above. Sulfuric acid is used to regenerate
the cation bed and lime is used for alkali rinse regeneration of the acid-
exhausted anion bed. The concentrated waste brine is acidic and under-
ground injection is suggested as a disposal method. Schroeder and
Marchello suggested the following possible difficulties: calcium, iron
and manganese may greatly reduce resin lifej calcium sulfate precipitating
in the columns may plug themj and resin capacity may decrease markedly
with continued use. The estimated costs for the Nalco ion exchange
system are:
Plant Size, Capital Cost, Dollars, Per
MGD Dollars 1.000 Gallons
0.1 254,000 $2.53
1.0 1,450,000 1.04
10 10,180,000 0.71
100 78,250,000 0.61
- 57 -
-------
Another possibility vrould be a strong acid resin followed by a
strong base. The first bed would remove the metal ions as shown in
equation 29.
A barium-loaded strong-acid cation exchanger could be used to
remove the sulfate and carbonates. The insoluble barium sulfate and
carbonate formed would then be separated by precipitate. A method for
(18)
recovering and reusing the barium has been developed . Studies to
date using barium resin have been made with saline water where sodium is
the predominant cation. Sodium levels are low in mine drainage and it
would be necessary to determine if the process would operate with calcium
and/or iron as the main cation.
Summary
A number of different ion exchange schemes have been suggested
for treating acid mine drainage. Only, two of these have been actually
tested on mine waters, and in both cases a synthetic mine water was used.
The various proposed schemes are summarized in Table 5.1. However, this
list may not be all inclusive, and better and more economical methods
might be developed.
Table 5.1 gives the estijnated cost of ion exchange processes. Costs
per 1,000 gallons vary from $0.30 to $2.53. The costs given by Schroeder
and Marchello are probably close to the true costs, as they were quite
inclusive, including such items as brine disposal.
Research Needs
Many problems still need to be investigated before the most
economical and functional system can be developed. More than one system
- 58 -
-------
TABLE 5.1
ION EXCHANGE SCHEMES
Estimated Cost
Ion Exchange Dollars/l.QOQ Gallons Reference
Weak-base anion-bicarbonate 0.38^, 0.44a 43
cycle
Weak-base anion-chloride
cycle
Cation-nitrate cycle 28
Cation-anion-preceded by
treatment _____
Strong anion resin 0.30^ 6
_ S Vj
Weak acid-strong acid-
Strong base beds (Nalco 0.71 , 0.16 49
Process)
Strong acid-strong base
Barium strong acid ____
a - 100,000 gpd plant
b - 1,000,000 plant
c - 10,000,000 gpd plant
d - 100,000,000 gpd plant
- 59 -
-------
may serve to treat mine drainage and each will depend on such factors as
the characteristics of the mine drainage, ultimate use of the treated
water, and ultimate use or disposal of the waste regenerant. There is
a definite need for ion exchange processes to be tested under actual field
conditions, A partial list of areas needing investigation follows.
The advantages, disadvantages, economics, and application of
each ion exchange process.
The upper limits of acid, iron (ferric and ferrous), sulfate,
and other ions that can be treated economically.
The effect of calcium, magnesium and iron, on the life and
activity of the resin.
The effect of bacteria, especially iron bacteria, found in mine
drainage on the resin.
The effect of suspended solids, sediment and algae, found in
mine drainage on the resin.
Best regenerant for resin efficients, and long life.
Possible marketable use of the waste regenerant.
Disposal of the regenerant,
FWPCA Research Program
At the current time, the Federal Water Pollution Control Adminis-
tration is conducting no in-house or supporting any research On the treat-
ment of mine drainage by ion exchange. The Commonwealth of Pennsylvania
has announced that a full scale plant utilizing the process reported by
(43 )
Pollio and Kunin will soon be constructed in Pennsylvania.
- 60 -
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6. REVERSE OSMOSIS TitLATMiSNT
Osmosis is defined as the transport of a fluid across a boundary
which separates two solutions of differing solvent activity. The direc-
tion of solvent flow under an osmotic gradient is always from the dilute
into the concentrated solution. Reverse osmosis is defined as the flow
of solvent across a boundary from the more concentrated to the less con-
centrated solution as a result of application of pressure to the concen-
trated solution in excess of the osmotic pressure difference between the
two solutions. In essence, the reverse osmosis, process produces two
discharges, one of concentrated solute, and one of "pure" water. The
reverse osmosis process will renove acidity, sulfates, calcium, magnesium,
iron, and other ions, from acid mine drainage and concentrate them as
a brine. The product water has a low concentration of these constituents
but the pH is raised only slightly. However, because of the removal of
acidic components, only small amounts of alkalinity are required to in-
crease the pH.
The reverse osmosis treatment process is in its early stage of
development. The Office of Saline tfater, U. S. Department of the Interior,
has supported a variety of research on water desalination by reverse osmosis.
One small study of mine drainage treatment has been made and another is
in progress. A few pilot plants have been built for treating brackish
water, but no full-scale plants. Reverse osmosis appears to be a promis-
ing process. It has the attractive feature of effecting the separation
without change in state and therefore, there is a potential for utilizing
- 61 -
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energies close to the theoretical thermodynamic minimum.
Principles of Reverse Osmosis
In the reverse osmosis process, a membrane which is highly permeable
to water and relatively impermeable to solutes, is used to separate product
water from a feed solution. Water normally tends to flow through an osmotic
membrane from the dilute to the concentrate solution. In the reverse
osmosis process, the normal osmotic tendency is reversed by increasing
the activity of the water in the brine by the application of pressure in
excess of the osmotic pressure. The rate of flow through the membrane
is proportional to the difference between applied pressure and the osmotic
pressure.
The salt flow through a membrane is relatively independent of pressure
and depends primarily on the gradient of the salt concentration, therefore,
increasing the applied pressure produces not only in more water per unit
time, but also water of higher quality.
The rate of water flow through a membrane is called the water flux
and is defined by the following equation:
The rejection properties of a membrane are called the salt flux
and are defined by equation 42,
J = A(AP - Atf)
(41
js = b(Ac)
(42
Where J = product water flux, gpm/ft2 of membrane
2
A = membrane constant, gpm/ft - p»i
Ap - the differential pressure across the membrane, psi
ait= the osmotic pressure, psi
Js = salt flux, g/cm sec
- 62 -
-------
B = salt flux coefficient, cm/sec
A C = difference in salt concentration between the brine and product,
g/cm3
As noted previously and as seen in the equations, the water flux
is hydraulic-pressure dependent and the salt is pressure independent;
thus, the product water quality improves as the applied pressure is in-
creased.
The critical feature of the reverse osmosis process is the membrane.
The membrane is required to discriminate between solute and solvent and
to permit the passage of the solvent at a rate 'sufficiently rapid to render
the process economically feasible at reasonable hydraulic pressure gradient.
To date, the cellulose acetate membrane is the only proven one, but others
are in the development stage.
Several different configurations of membranes have been developed.
The "flat plate" membrane configuration is a membrane held flat by a frame.
A "spiral-wound" membrane is essentially a "flat plate" wound into a
cyclinder. A space is provided between the roll of membrane to provide
movement of the brine and product water. The "tubular" membrane is in
the form of a long tube made of fiberglass or some other material. Coat-
ing the inside of the tube is the membrane. Feed water and brine pass
through the center of the tube and the product water passes through the
tube to be collected on the outerside. A recently developed configuration
is a hollow fiber. This membrane is still in the early stages of develop-
ment .
- 63 -
-------
Some terms used in reverse osmosis studies are not common to other
treatment areas and are therefore, defined as follows:
Water recovery is the ratio of product water to feed water
expressed as a percent.
Salt rejection is defined as the ratio of the specific conductance
of the product water to the feed water expressed as a percent.
It has been observed^-^ that after a period of time, the salt
rejection of cellulose acetate reverse osmosis membranes decreases. This
increase in salt flux can be ascribed to the hydrolysis of the cellulose
acetate. Further, it has been shown that the hydrolysis is strongly de-
pendent on pH and temperature. The hydrolysis rate is a minimum at a
pH of 4.8 and a temperature of 23 degrees centigrade, and increases rapidly
at higher and lower pH values. Mine waters directly from the mine usually
have a temperature of 8 to 12 degrees centigrade, a level that would result
in low hydrolysis. The pH of mine drainage varies widely from as low as
2.0 to above 7. However, the most troublesome waters have a pH of 2.5
to 5.5. Therefore, hydrolysis for some acid mine drainages may be near
the minimum.
(38)
Merten, et al., reported that the performance of cellulose
acetate membranes may have deteriorated as a result of rust (iron) deposits
on the membrane. Acid mine drainage with its high concentration of iron
( 3/l )
may cause rapid deterioration of membrane performance. Larson found
that a reverse osmosis unit could remove 98 percent of the sodium and
^hloride ion and 99 percent of the calcium sulfate and bicarbonate.
- 64 -
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Reverse Osmosis Treatment of Mine Drainage
Three studies have been conducted on the treatment of mine drainage.
Two were field tests and the third a cost analysis.
(i-7)
Riedinger and Schultz conducted tests using reverse osmosis
to treat acid mine drainage at two mine sites near Kittanning, Pennsylvania.
Their purpose was to explore the effectiveness of reverse osmosis in con-
centrating and reclaiming drainage containing acid and iron from coal
mines. The tests were conducted on a 24-hour a day basis for about 10
days each. The modules used were of cellulose acetate membrane material
in a spiral-wound configuration. Two types of membranes were used: a
high-selectivity and a low selectivity, a high-selectivity membrane has
greater salt rejection and less water recovery than a lower-selectivity
membrane. Operating pressures of 400 and 600 psi were used. The mine
drainage was pretreated with coarse screen and a five-micron cartridge
filter.
Tests indicated that the osmotic pressure of the acid mine drainage
at a conductivity of 1,040 micromhos/cm was 19 psi and was proportional
to the conductivity^4"^.
The results of this test show that a high quality water can be pro-
duced from feeds with pH of 3.0 and lower, and containing 100 ppm or more
of dissolved iron. Recovery ratios (product/feed) were in excess of 90
percent. The brine was highly concentrated, in fact, calcium sulfate,
and magnesium sulfate were concentrated beyond saturation. Ferrous iron
did not appear to cause problems, but ferric iron deposited on the mem-
brane and reduced the water flux. The high-selectivity membrane produce
- 65 -
-------
the best results. The product waters were low in dissolved solids but
had a pH of 5 or less.
( n I )
Larson reported that sulfate scale does not adhere as readily
to membrane surfaces and tends to collect as large particles on brine
channel spacers, whereas, carbonate scale tends to form at the membrane
surface and adheres tenaciously. Sulfate scale causes a large increase
in brine side pressure drop and carbonate scale a substantial reduction
in product water flow. Calcium sulfate could be flushed from the module
by increasing brine flow. Sodium tripolyphate was used to inhibit precip-
itation.
Cellulose acetate membranes in the spiral-wound configuration are
being tested at the Federal Water Pollution Control Administration's Kine
Drainage Treatment Laboratory, Norton, West Virginia. Results of one
run are presented in Table 6.1. Salt rejection was high with all para-
meters, except acidity, reduced to less than one mg/1. Some difficulties
with module failures occurred during these tests. No problems were encounter
with iron precipitating on the membrane or with cleaning the modules follow-
ing shutdown.
(49)
Schroeder and Marchello estimated the costs of treating mine
drainage by reverse osmosis and disposing of the brine by deep well
disposal as follows:
Plant Capacity, Capital Cost, Water Cost,
HOD Dollars Dollars/1.000 Gallons
0.1 248,000 2.57
1.0 1,407,000 1.09
10.0 10,120,000 0.77
100.0 81,250,000 0.68
- 66 -
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TABLE 6.1
REVERSE OSMOSIS TEST RESULTS - NORTON, WEST VIRGINIA
Feed
'.later
Product
VJater
Brine
Water
Percent
Reduction
pH
2.9
4.5
2.4
Acidity, Mg/l
504
4
2,050
99.2
Total Iron, Mg/l Fe
93
0.1
391
99.9
Iron (II), Mg/l Fe
3.8
0.1
5.1
97.4
Hardness, Mg/l CaCO^
298
0
1,252
100
Calcium, Mg/l Ca CO^
196
0
350
100
Sulfate, Mg/l SO^
790
0.5
2,940
99.9
Specific Conductance
1,510
13
4,395
99.1
Aluminum, Mg/l A1
24
0.1
100
99.8
Salt Rejection - 99 Percent
Water Recovery - 76,5 Percent
Pressure - 560 psi
- 67 -
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3xperiments in reverse osmosis indicate the process has great
possibilities for concentrating the dissolved solids in mine drainage and
producing a high quality water, low in sulfate, iron calcium and acidity.
The feed water requires prefiltration for removal of suspended solids
and the disposal of the brine waste is a major problem.
Research Heeds
Research needs for the development of reverse osmosis for the
treatment of mine drainage are as follows:
Development of membranes best suitable for mine drainage
treatment.
Determine the effect of acid mine drainage on deterioration
of membranes.
Evaluation of membrane plugging problem due to iron, calcium,
sulfate, and organisms.
Determination of membrane life under field conditions.
Development of methods of cleaning membranes.
Development of methods for reuse of brine material.
Development of methods of disposing of and treating brine
waste.
Determine the cost of treating mine drainage by reverse
osmosis.
Fi'JPCA Research Program
The Federal Water Pollution Control Administration is conducting
or sponsoring the following research on the treatment of mine drainage
by reverse osmosis:
Cooperative research project between FWPCA and Office of Saline
- 68 -
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Water at Norton, West Virginia, "evaluation of a 10,000 gpd
Reverse Osmosis Unit in the Treatment of Acid Mine Drainage."
Research project by FWPCA at the Mine Drainage Treatment Labor-
atory, Norton, West Virginia, on the treatment and disposal
of brine from the reverse osmosis process.
Grant to the Pennsylvania Department of Mines and Mineral
Industries and its subcontractor, Haven Industries, for a
project titled, "Abatement of Acid Mine Drainage Pollution by
Reverse Osmosis."
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7. DISTILLATION
Distillation has proved the best method developed to date for the
desalination of sea -water. Many full-scale plants with capacities exceed-
ing one mgd have been built throughout the world. Extensive knowledge is
available on the construction of such plants. Two major advantages of
distillation are: the process is relatively insensitive to the degree
of contamination of the feed, and a water with 50 ppm of dissolved solids
can be produced. The major problems of adopting distillation processes
to mine drainage treatment are scaling and corrosion. Calcium sulfate
which causes major scaling in all evaporators, is usually present in a
high concentration in mine drainage and the acid nature of mine drainage
makes it highly corrosive.
Frinciples of Distillation Processes
Distillation processes can be grouped into three main categories;
(a) multistage flash evaporation, (b) long tube evaporation, and (c)
vapor compression.
The multistage flash distillation process is based on the fact that
water boils at progressively lower temperatures as it is subjected to
progressively lower pressures. The feed water is heated (200° F) and
then introduced into a chamber where the pressure is sufficiently low to
cause some "flash" immediately into vapor, leaving the salt and other
impurities behind. The vapor rises in the chamber and comes in contact
with condenser tubes. As the vapor condenses against the cool tubes,
droplets of water fall into a separator and are carried away as the
- 70 -
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product water. The brine produced in the .first flash chamber, now lower
in temperature, flows to a second chamber where the pressure is lower
than in the previous chamber. The same process occurs again. Finally,
the brine passes to a heat exchanger where the incoming water is heated
and the brine condensed.
This method is most widely used in the largest of the desalination
plants. It is an efficient system insofar as heat energy is concerned,
since as much as 90 percent of the heat energy can be recovered.
The long-tube vertical distillation or full drop system is con-
structed of a series of bundles of long tubes. .The feed water is heated
to 250° F and is introduced at the top of the first series of long-tube
bundles within a large cylindrical chamber. The first chamber or evapor-
ator is a steam chest, and receives steam from an outside source. As
the water pours down the sides of the tubes in a thin film, some of it
vaporizes. Part of the vapor condenses and is drawn off as product water
and the remainder is piped to the second chamber and acts as the "steam
source," Unvaporized feed water is introduced at the top of the second
chamber and the process is repeated, Aa many as 12 chambers may be used.
The pressure in each chamber is progressively reduced to permit vapor-
ization to occur at lower temperatures. The brine is collected after
the final chamber. Scale control at high temperatures (250° F) has been
a major problem, A one million gallon per day plant of this type is
in use.
Vapor compression distillation depends on the fact that when the
saturated vapor rising from a boiling liquid is compressed, it increases
- 71 -
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in temperature and in fact becomes slightly superheated. Since the vapor
is at a higher pressure, its condensing temperature is correspondingly-
increased. Because its condensing temperature is now higher, this com-
pressed vapor can transfer its heat of condensation to the boiling liquid.
Water is evaporated at the cost of whatever power is required to compress
the vapor. In single-stage vapor compression distillation, feed water
is pumped upward through a bundle of tubes into a large spherical chamber.
As the water travels upward, it is heated by steam condensing on the
outside of the tubes. This transfer of heat causes some of the water in
the tube to vaporize. When the mixture of vapor and brine leaves the tubes
and enters the chamber, part of the brine is returned to the bottom and
recirculated and the remainder is discharged. The vapor is compressed and
conducted to the steam chest surrounding the tubes. Here it condenses,
as noted above, and is withdrawn as product water. Two stage units of
slightly different design are also used in saline water distillation
processes.
The primary difference between this process and other distillation
processes is in the method in which heat is added to the system. In
other processes heat is added to the feed water, while in this process
heat is added to the vapor by compressing it. Most of the energy is consumed
by the motor which drives the compressor. Heat is added directly only
during startup.
As in all distillation processes, vapor compression suffers from
scaling and corrosion. The Office of Saline Jater has one demonstration
plant of this type with a capacity of one mgd. It has been plagued with
- 72 -
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scaling and hydraulic difficulties.
Distillation Treatment of ;!ine Water
Westinghouse Electric Corporation, under a contract v.dth the otate
of Pennsylvania, studied the application of distillation processes for
(35)
the treatment of acid mine drainage. Lemezis reported that flash
distillation was the most promising process because the plants are simple
in construction, are easily designed to handle large volumes of flow, and
do not involve boiling or a heat transfer surface. All contamination
in acid mine drainage can be reduced and the product water would have
approximately 50 ppm dissolved solids. The major problems were described
as: "(a) The waste (brine) must be disposed of in such a way that it xvill
not return to the streams or to the water table, (b) The mine water is
acid from hydrolysis of its metallic sulfates and is, therefore, very
corrosive."
(57)
'./estinghouse Electric Corporation studied the two aspects
mentioned above for the State of Pennsylvania, They concluded that the
best and cheapest way to handle the waste brine was to cast it into solid
block. They failed to outline the method of casting the block and gave
no costs for this process. Cost of hauling the brine to sea from Penn-
sylvania was estimated as 12-16 cents per 1,000 gallons of brine.
Laboratory tests by V/estinghouse indicated, that if the temperature
was kept near 200° F, no scaling occurs. At higher temperatures, a scale
consisting mainly of calcium sulfate formed.
Corrosion tests were made to determine the best material for con-
structing the system. Titanium was found to be the most corrosion resis-
tant material for use in acid mine drainage flash evaporators. The
- 73 -
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presence of free acid in acid mine drainage increased the corrosion rate
on all materials.
It was concluded in the 'Jestinghouse report that it was feasible
to treat acid nine drainage by flash distillation and that a high quality
water could be produced. Cost estimates for a five mgd plant were devel-
oped. Two types of plants were discussed. A single purpose plant just
for treating acid nine drainage and a dual purpose plant for treating
mine drainage and producing electrical power. The following cost figures
were given:
Single Purpose Dual Purpose
Capital Cost (5 mgd plant) £7,535,034 $9,570,444
Treatment Cost (cents/1,000 gallons) SI.31-95.36 33.3-51.3
(49)
'¦¦chroeder and I'archello ' analyzed the cost of treating mine
drainage by distillation methods. Since the cost of treatment by distillation
is relatively independent of the feed water concentration, the coot figures
developer, should be applicable to most acid mine drainage situations.-
As noted earlier, because of the highly corrosive nature of acid mine
drainage, special high cost material would be required to construct the
reactors, etc. ochroeder and I'archello suggested as an alternate to high
cost building materials that the acid mine drainage be neutralized first.
They supported their contention with the following estimated costs for
a one ngd multistage flash plant.
- 74 -
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Capital Cost,
Dollars
Water Cost,
Dollars/1.000 Gallons
Neutralized Raw Water
Conventional Materials $2,154,459 1.13
Unneutralized Raw Water
Stainless Steel Construction $3,712,540 1.50
Unneutralized Raw Water
Titanium Construction $3,257,720 1.36
Capital cost estimates, which included raw water intake, neutralization
and settling facilities, process facilities and auxiliaries, fresh water
storage, and brine disposal by underground injection, for the three dis-
tillation processes were as follows:
CAPITAL COSTS (DOLLARS)
Plant Capacity, Multistage
MGD Flash Long-Tube Vapor Compression
0.1 477,000 486,900 445,500
1.0 2,152,000 2,513,000 2,431,000
10 11,570,000 14,650,000 15,953,000
100 68,440,000 91,564,000 115,200,000
Schroeder and Marchello noted that since a distillation plant
produced water containing 50 ppm solids, (a higher quality water than usually
required), costs could be reduced by mixing the product water with some
neutralized raw water to produce a water with 500 ppm solids. Water costs
for the 50 and 500 ppm solids content given on the following page, show
that multistage flash produces the cheapest water, verifying Westinghouse's
findings.
- 75 -
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Water Costs (Dollars/1,000 Gallons)
Production Multistage
MGD Flash Long-Tube Vapor Compression
50 ppn 500 ppm 50 ppm 500 ppm 50 ppm 500 ppm
0.1
3.05
2,36
3.12
2.41
3.24
2.50
1.0
1.13
0.87
1.28
0.99
1.49
1.15
10
0.64
0.49
0.77
0.59
1.05
0.81
100
0.47
0.36
0.58
0.45
0.88
0.68
Summary
The distillation process can treat acid mine drainage and produce
a very high quality water. The major problem areas are in scale control,
corrosion, and brine disposal. Westinghouse Electric Corporation has
developed some answers to the scale control and corrosion problems, and
the neutralization of the acid mine drainage before distillation, as suggested
by Schroeder and Karchello, -will also reduce corrosion problems. No sound
solution to the brine disposal problem has been developed. At this time,
multistage flash distillation appears to be the most economical process.
Suggested water costs are as follows:
- 76 -
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Process
Water Cost,
Dollars/l.000 Gallons
Reference
Flash - Dual Purpose
Flash - Dual Purpose
Flash - Single Purpose
0.81 - 0.95a
0.33 - 0.51a
0.40a
Pennsylvania
Westinghouse
Westinghouse
Long Tube
Vapor Compression
Flash
0.47b, 0.64°, 1.13d, 3.05e
0.58b, 0.77°, 1.28d, 3.12®
0.88°, 1.05°, 1.49d, 3.24®
Schroeder & Marchello
n
it
Note: a - 5 MGD
b - 0.1 MGD
c - 1 MGD
d - 10 MGD
e - 100 MGD
Research Needs
The distillation processes are highly developed to a stage where
full-scale demonstration plants are in order. The major area that requires
further study is brine disposal and/or utilization. The Commonwealth of
Pennsylvania has been evaluating various sites for the construction of
a full scale plant.
FWPCA Research Program
The Federal Water Pollution Control Administration currently haa
no in-house research nor is supporting any research on the treatment of
mine drainage by distillation.
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8. KLKCTR0DIALY3IS
Electrodialysis for the treatment of brackish water has been
developed to the point where a 650,000-gpd demonstration plant is in
operation. Electrodialysis appears to be best adapted to desalination
of waters of lower salt concentrations than sea water because the electric
current required in this process varies directly with the amount of
dissolved salt, rather than with the amount of water. For this reason,
the treatment of acid mine drainage by this process may be feasible.
Principles of Electrodialysis
Electrodialysis, like reverse osmosis, utilizes membranes; however,
electricity is used as the driving force in electrodialysis. The conver-
sion assembly is essentially an electrolytic cell which contains two differ-
ent types of ion selective membranes. One membrane will pass only neg-
atively charged ions (anions), while the other will pass only positively
charged ions (cations). Pairs of membranes are placed in an alternate
pattern in a cell between two electrically charged plates. As feed water
passes through the cell, the negative anions (such as sulfate) are attracted
to the positive plate (anode), while the positive cations (such as iron,
aluminum, etc.) are attracted to the negative plate (cathode). The anion-
permeable membrane allows passage of the negative ions and the cation-
permeable membrane allows passage of the positive ions, thus yielding
fresh water between the membranes.
The amount of electric current required in the unit depends on
amount of salt to be removed. Therefore, the cost of the energy consumed
- 78 -
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in the process depends on the concentration of salt in the feed water.
Electrical power requirements might be lowered by operating the system
at elevated temperatures, since high temperatures result in low electrical
resistance of the electrolyte.
Membrane stacks are known to increase resistance to flow with
time. The rate of resistance increases rapidly at the beginning of a
run, then tapers off at a much lesser rate. This increase was found to
result from deposition of suspended solids in feed waters on membrane
surfaces, and not from any changes in the membrane properties'®^. Brunna' ^
reported the fouling of membranes by both turbidity and microorganisms.
Therefore, acid mine drainage will probably require some type of pretreat-
ment to remove suspended solids and control microorganisms.
Studies by the Office of Saline Water have indicated that the
optimum pH needed to mitigate the stack resistance increase is between
5.5 and 5.1. Ilany acid mine drainages have pH's in this range or lower,
and thus should be in a favorable range in many cases.
Waters of high hardness concentration cause scale problems as
well as high treatment costs. The hardness of mine drainage waters is
often high, a factor to be considered. The Office of Saline Water is
conducting studies on pretreatment methods for high hardness waters. "
Iron and manganese form deposits on membranes and thus increase
membrane-stack resistance and power consumption. The recommended iron
concentration for feed waters to an electrodialysis unit is less than
one mg/l. Almost all acid mine drainage will have much higher concen-
trations than this, necessitating pretreatment to remove iron,
- 79 -
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Electrodialysis Treatment of Mine Drainage
A small bench-scale electrodialysis unit was tested by F'.iPCA at
its Mine Drainage Treatment Laboratory, Norton, Vest Virginia, in cooperation
(45)
with the Office of Saline Water . When used on water receiving no
pretreatment, the cathode cell quickly became fouled with iron. The
current reversal technique showed promise of preventing flocculation in
the membrane system. In those cases where the mine drainage was pretreated
by lime neutralization for iron removel, the unit operated satisfactorily.
C 1 g\
The 1966 Saline Water Conversion Report states that a cooperative
study between the Office of Saline Water and the Bureau of Mines was
initiated on the use of the electrodialysis process to treat acid mine
drainage. The purpose of this two-phase investigation was to determine
methods of pretreatment of acid mine drainage to remove insoluble iron
and other suspended solids, and to use these results in the construction
and. testing of a 10,000-gpd field unit. Results of this work have not
been published,
(49)
Schroeder and Marchello conducted a study on the estimated
costs of treating acid mine drainage by electrodialysis. Their findings
were as follows:
'¦'/ater Cost,
Plant oize (MOD) Dollars/l.OQO Gallons
0.1 $2.52
1.0 1.01
10 0.68
100 0.5s
- 30 -
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Research Needs
From the studies conducted to date, there is no reason to believe
that the electrodialysis process could not be used for the treatment of
acid mine drainage, providing pretreatment was included. The high hard-
ness, iron, and low temperature of acid mine drainage will result in mem-
brane problems. However, when pretreatment to remove these contaminants
is included in the process, removal of the remaining dissolved solids
should present few problems.
The electrodialysis process is highly developed, with a number of
full-scale demonstration plants in operation. However, only limited
laboratory and field studies have been made to test the procedure on
actual acid mine drainage. Research and development effort should be
placed on pretreatment, sludge disposal and/or utilization, and optimization
of the process.
F.tTPCA Research Program
The Federal Water Pollution Control Administration is not conducting
or funding work in this area at this time.
- 61
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9. CRYSTALLIZATION (Freezing)
Introchict ion
Separation of salts from water by crystallization has been under
study for a number of years by the Office of Saline water. Several pilot
plants are in operation. Crystallization processes have distinct energy
advantages over many other methods of demineralization, since the separ-
ation of pure water ffrom a salt solution requires the removal of only 144
btu per pound of water.
Principles of Crystallization Treatment
When mineralized water freezes, fresh-water ice crystals form, and
the salts remain in solution in the unfrozen water. If the crystals are
separated from the brine, washed, and melted, fresh water stripped of
other substances is produced. The freezing (heat of fusion) of water re-
quires only 144 btu per pound of water, or less than one-sixth of the heat
of vaporization. Low temperatures also minimize scale and corrosion
problems.
To further explain the crystallization process, wheri a feed water
is cooled to below 32° F, ice begins to form. As more ice is formed, the
salt concentration of the'remaining solution increases. If the temperature
continues to decline, a point is reached where the minerals in the solution,
begin to precipitate. This is called the eutetic point. For an NaCl solution)!
it is reached at a temperature of -6° F and a concentration of 23.3 percent.
Let us now consider an acid mine drainage water which contains in-
organic ions such as Ca, Mg, Na, 30^, and Fe. When water is removed from
such a solution, the solubility of one ion is usually exceeded first, and
- 82 -
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it will precipitate with the ice. Soon a point is reached where the sol-
ubility of a second salt is exceeded. Further removal of water as ice
precipitates 3alts and ice simultaneously until the solubility of a third
salt is reached, and so forth. Calcium sulfate, which is commonly found in
acid mine drainage at high concentrations, should be one of the first to
precipitate.
Two methods of crystallization appear equally promising, i.e., the
freezing method and the gas hydration method. In the freezing process,
direct refrigeration is performed without transferring heat through a
metal barrier. A portion of the precooled feed water is evaporated under
reduced pressure, thus reducing the temperature, or a refrigerant such as
n-butane is vaporized in direct contact with the feed water. The refrig-
erant is then separated from the water by a simple stripping process.
In the gas hydration process, a chemical reaction separates fresh
water from a feed water by the use of hydrate-forming materials, notably
propane. The hydrate-forming gas is used in its liquid phase. IThen
introduced into the feed water, it combines with the water in complex
crystals that reject ionic (salt) constitutents. These crystals are sep-
arated from, and worked free of, the mother liquor, and then melted into
two immiscible liquids, i.e., the liquid hydrating agent, which is recycled,
and fresh water.
In the freezing process, separating the ice crystals from the brine
and precipitated solids is a major problem. However, techniques have been
developed to make the separation effective.
Crystallization Treatment of Mine Drainage
The crystallization processes appear applicable to the treat-
— 83 —
-------
ment of acid mine drainage, although these techniques have not, "been tested.
on actual acid mine drainage. The Office of Saline Water has sponsored
a number of research projects on these processes for the treatment of
brackish water and a few demonstration plants are in operation.
(49)
Schroeder and ilarchello estimated the coat of treating mine
drainage by vacuum freezing, secondary refrigerants (n-butane); and hydrate
process as follows:
Water Cost. Dollars/1 ..000 Gallons
Production,
'-'gd Direct Freezing IJ-Butane Hydrate
0.1 3.10 3.13 3.23
1.0 1.32 1.34 1.38
10 0.85 0.85 0.39
100 0.68 0.67 0.71
Cost vrould increase as the concentration of pollutants in the acid
mine drainage increased.
P.esearch Meeds
The immediate research needs in the area of crystallization is a
feasibility study to determine if the more troublesome ions found in mine
drainage, such as iron (II), iron (III), sulfate, calcium, aluminum,
magnesium and manganese can be removed efficiently and economically. An
important aspect of the crystallization process would be disposal and/or
utilization of the brine. Research is needed in this area.
- 84 -
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F'vfPCA Research Program
The Federal Water Pollution Control Administration is sponsoring
the following research on the treatment of mine drainage by freezing:
Grant to the Pennsylvania Department of Kines and Mineral
Industries and its subcontractor, Applied Science Laboratories,
Incorporated, for a project intitled "Feasibility of the
Purification of Acid J line Water by a Partial Freezing Process."
- 85 -
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10. BIOLOGICAL TREATMENT
Introduction
Biological treatment of mine drainage has been proposed as a method
of controlling pollution from mines. As noted in Chapter 4 on iron re-
(19)
moval, Glover ' has developed a biochemical oxidation process for ferrous
iron removal. An autotrophic bacteria that derives its energy from the
promotion of an inorganic reaction and its cellular carbon from carbon
(21
dioxide is used to promote the oxidation of ferrous iron. Hanna, et &1.}
suggested the use of sulfate-reducing bacteria to dispose of acid in acid
mine drainage. Wood dust, sewage or sewage sludge could be employed as
an inexpensive source of organic nutrients for sulfate-reducing bacteria.
Various biological schemes are described in the succeeding text.
Sulfate Reduction
(59)
Tuttle reported of finding the reduction of sulfate occurring
naturally in a stream in Ohio. He observed a stream polluted with acid
mine drainage flowing through a pile of saw dust. The stream below the
wood dust pile contained less sulfate, and iron, and had a higher pH
than above the pile. A strictly anaerobic gram negative mesophilic
spore-forming sulfate-reducer of the genus Desulfotoma clum was isolated
from the wood dust. Laboratory studies by Tuttle demonstrated that in
wood dust-acid mine water cultures the sulfate could be reduced to sulfide
with a concomitant removal of hydrogen ion and precipitation of iron and
sulfide as FeS. Wood dust provided the oxidizable organic substrate for
the sulfate-reducing bacteria and created the anaerobic conditions necesss^
for the growth of the microorganisms. Other organic material such as
sewage sludge could be used in place of the wood dust.
- 86 -
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The reactions occurring in the process are shown in equations
43 and 44.
Fe-H- *h SO^ = -rSlf i" Ce h Feo -r 4^20 (43
2Fe+-H- + 3S0^ = +2.4H' r 21+e h- Fe^-Jj ~r 4H2'--1 (44
Sulfate in the presence of hydrogen would be reduced to hydrogen
sulfide, which would consequently react with ferrous and ferric ions to
form insoluble ferrous and ferric sulfide* After the iron is depleted
additional hydrogen would combine with the relon;~oc! oxygen to form water
or with the excess sulfide to produce hydrogen sulfide. For these reactions
to take place, a source of electrons must be present, which could be provided
by organic compounds. Under anaerobic conditions the organic matter would
be transformed to methane and carbon dioxide. It is anticipated that
the above process would raise the pH of the water to at least 6.
A sulfate reducing process would include an anerobic reaction
chamber to which the mine drainage and organic matter were fed. After
sulfate reduction the water would pass to a settling tank where the in-
soluble matter such as ferric sulfide, activated bacteria, and residual
organic matter would be removed.
Through further treatment the ferrous sulfide precipitate might be
converted to a useable byproduct. For example the iron component could
be converted to iron oxide and the sulfide to elemental sulfur.
(44)
Various studies have been conducted that show that bacteria
can reduce sulfate and many advances have been made in our understanding
of the sulfate-reducing bacteria. The bacteria ltesulfovibrio desulfuricans
has been associated with sulfate reduction. However, the problem of
- 87 -
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putting these bacteria to work in a waste treatment process has been only
superficially evaluated. Both laboratory and pilot plant studies are
needed before full scale demonstration plants can be constructed.
Ferrous Iron Oxidation
In Chapter 4, the Glover process for the biological treatment of
(36)
ferrous iron was discussed. Lundgren has conducted extensive studies
on the physiology of iron oxidizing bacteria. The bacteria Thiobacillus
thiooxidans. Thiobacillus ferrooxidans. and Ferrobacillus ferrooxidens.
have been associated with acid mine drainage. These bacteria have also
been reported to play a role in sulfur oxidation. Except for the Glover
process, only limited work has been conducted to develop full scale
treatment plant for the biological removal of iron (II).
Research Needs
Biological treatment of mine drainage for sulfate reduction is a
different process than that for ferrous iron oxidation. Thus the research
needs have been listed separately.
Sulfate Reduction
Effect of pH on kinetics of reactions.
Optimum ratio of organic material to mine drainage.
Corrosion problems as a result of the *acid water and hydrogen
sulfide production.
Prevention of air pollution by hydrogen sulfide production.
Feasibility of sulfur and iron oxide recovery.
Optimization of a sulfide reduction process.
Evaluation of organic material sources.
- 88 -
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Effect of such variables as sulfate concentration, iron (II)
and iron (III) concentration, temperature, oxidation potential
and organic solids.
Ferrous Iron Oxidation
Kinetics of the reactions and those factors that affect it, such
as pH, iron and sulfate concentration and temperatures.
Development of laboratory and pilot systems for removal of iron.
Nutrient requirements of the iron oxidizing bacteria.
Optimization of the iron oxidation process.
FWPCA Research Program
The Federal Water Pollution Control Administration is sponsoring
the following research on biological treatment of mine drainage:
Grant to Syracuse University for a project intitled, "Inorganic
Sulfur Oxidation by Iron-Oxidizing Bacteria," Grant Number
vJP-01367
Grant to Syracuse University for a project intitled, "Biological
Treatment of Acid Mine Water," Grant Number V/P-01460
Grant to Continental Oil Company for a project intitled, "Micro-
biological Removal of Iron from Mine Drainage Waters," Grant
Number WPRD-63.
-------
11. SUMMARY
Mine drainage is a critical pollution problem in the Appalachian
Region as well as other smaller areas of the United States. Over 10,000
miles of streams have been polluted by mine discharges in the United
States, Although the ultimate control of mine drainage pollution lies
in preventing its formation at the source, no proven method has been
developed for underground and active nines. In the interim, treatment
appears important in mine drainage control methodology, particularly in
active mine situations, mine drainage not amenable to at-source control,
and the control of residual pollutants from at-source control measures.
The type of treatment applied to mine drainage depends primarily
on the quality of the water to be treated and the quality desired in the
finished water. Table 10.1 outlines the treatment methods most promising
for mine drainage. Neutralization, usually coupled with aeration, is the
most commonly used method and removes acidity and iron but does not reduce
the hardness or sulfate content. The remaining methods are essentially
demineralization processes leading to pure water. Some of these processes
require pre and post-treatment. The high quality of the product water
would permit its use as industrial and municipal supplies.
Table 10.2 presents estimated efficiencies of the various treatment
methods for removal of contaminants found in mine drainage. Table 10.3
compares estimated costs for the various acid mine drainage processes
described in this report.
- 90 -
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TAELii 10.1
r 1 LilA 11 liiil'J J. 1' • )XjO , ' : i i'.ilijj L'iii'. j imO.
Process
Comments
Benefits
Problem Areas
Neutralization
Reverse Osmosis
Electrodialysis
Crystallization
Ion Exchange
Distillation
Process usually Includes
aeration and so;ii-ientation.
Lime and limestone used
as alkaline agents.
Three basic types: of
modules, i.e., spiral
wound, plate, and tube.
Has energy cost superior
to distillation at low
TDS.
Freezing process
Various schemes have been
proposed. Each scheme
has its own operating
characteristics and
removes different ions.
TDS of acid mine drainage
often too low for economic
removal.
riemove? acidity,
iron, aluminum,
and manganese.
Increases pH.
water less cor-
rosii \ e.
Demineralization
Demineralization
Demineralization
Demineralizat ion
Possible reduction
of acidity.
Demineralization
iJoes not remove
hardness, sul-
fate. kludge
a major problem.
Requires pre-
treatiiient for
removal of sedi-
ment and control
of organisms.
CaSO^ precipitation.
Brine disposal.
Resulting water has
low pH and is corro-
sive and requires
post-treatment for
stabilization.
Pretreatment to
remove iron, man-
ganese, sediment,
and microorganisms
required. Brine
disposal. CaCO^
precipitation.
Brine disposal.
Ice separation.
Determination of best
ion exchange scheme.
Brine disposal,
regeneration, iron
fouling, precipitates.
Brine disposal.
Corrosion problems.
- 91 -
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TABLE 10.2
EFFICIENCY OF IIINE DRAINAGE TREATMENT
Efficiency (Percent)
Acidity
Iron
Aluminum
Sulfate
Manganese
Hardness
i/tat er
Recovery
Neutralization
98-100
90-100
95-100
0-5
95-100
0*
67-90
Reverse Osmosis
95-99
95-99
95-99
95-99
95-99
98-100
70-90
Slectrodialysis
90-95
0#*
95-99
85-95
0-i:-::-
85-95
70-90
Crystallization
90-99
90-99
90-99
90-99 •
90-99
90-99
25-50
Ion Exchange
80-99
70-99
70-99
80-99
70-99
90-99
60-95
distillation
95-99
98-100
98-100
98-100
98-100
98-100
60-80
^ May increase
Cannot operate at high iron and manganese concentrations
- 92 -
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TABLE 10.3
ESTIMATED COSTS OF TREAT DIG MINE DRAIIIAGE
Process
Dollar Cost per 1,000 gallons
Controlling Factors
Neutralization
Reverse Osmosis
Electrodialysis
G'rystallizat ion
Ion Exchange
Distillation
0.05 - 1.10
0.68 - 2.57
0.58 - 2.52
0.67 - 3.23
0.30 - 2.53
0.33 - 3.24
Water quality, size of plant
Size of plant
Size of plant, amount of
pretreatment, dissolved
solids concentrate
Freezing process, size of
plant
Size of plant, ion exchange
scheme, dissolved solids
concentration
Size of plant, type of
distillation unit
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Research Needs
The demineralization processes have received considerable research
support from the Office of Saline Water which has led to their refinement
and illustrated their potential for use in mine drainage treatment. These
processes need now to be modified to meet the criteria of mine drainage
and to be demonstrated. Cost data are also needed.
Ion exchange probably requires the greatest development to ascertain
the best schemes. Neutralization requires greater refinement to demonstrate
cost and operational parameters. Ferrous iron removal needs to be optimized.
All of the processes need to be optimized to obtain maximum water
recovery. Brine and/or sludge disposal is a major weak point of all the
suggested processes. Thus the quantity of the waste materials should be
held to a minimum. A major research program is needed in the area of the
disposal and utilization of brine and sludges. The development or reuse
of a useful byproduct would be an ideal solution. Mine drainage contains
sulfur, iron and aluminum, which have commercial value. Of these materials,
sulfur has the most promising price structure. Sludge and brine disposal
techniques that lead to at-source control of mine drainage appear to be
promising. Disposal to strip pits or to deep mines to coat and/or bury
pyritic material is a possibility.
This report has examined those treatment processes which are most
amenable to mine drainage. One purpose in reviewing the state of the art
is to stimulate action in the development of other treatment methods.
The Federal Water Pollution Control Administration through its
contract and grant program is prepared to help support research, development,
and demonstration of new promising methods.
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In its assigned function as the Nation's princiiaal natural resource
agency, the United States Department of the Interior bears a special
obligation to ensure that our expendable resources are conserved,
that renewable resources are managed to produce optimum yields, and
that all resources contribute their full measure to the progress,
prosperity, and security of America - now and in the future.
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