WATER POLLUTION CONTROL RESEARCH SERIES • 14010 FUI 10/71
Foam Separation of
Acid Mine Drainage
U.S. ENVIRONMENTAL PROTECTION AGENCY
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BATER POLLUTION CONTROL RESEARCH SERIES
the Water Pollution Control Research Series describes the
results and progress in the control and abatement of pollution
in our Nation*s waters. They provide a central source of
information on the research, development, and demonstration
activities in the Environmental Protection Agency, through
inhouse research and grants and contracts with Federal, State,
and local agencies, research institutions, and industrial
organizations.
Inquiries pertaining to Water Pollution Control Research Reports
should be directed to the Head, Publications Branch (Water),
Research Information Division, R&M, Environmental Protection
Agency, Washington, D.C. 20460.
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Foam Separation of Acid Mine Drainage
Horizons Incorporated
23800 Mercantile Road
Cleveland, Ohio 44122
for the
Environmental Protection Agency
Project No. 14010 FUI
Contract No. 14-12-876
October 1971
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EPA Review Notice
This report has been reviewed by the Environmental
Protection Agency and approved for publication. Approval
does not signify that the contents necessarily reflect
the views and policies of the Environmental Protection
Agency, nor does mention of trade names or commercial
products constitute endorsement or recommendations for
use.
ii
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ABSTRACT
Laboratory studies of continuous flow foam separation were
conducted to determine the optimum operating conditions for
maximum extraction of dissolved metal cations (Fe, Ca, Mg,
Mn and Al) from acid mine drainage. Foaming experiments
were condircted in a 6 in.-diameter glass column capable of
liquid flow rates of 3-12 gal. per hour. The approach to
foam separation taken was the production of persistent
foams which allowed protracted foam drainage to reduce
liquid carry-over in the foam. The effects of pH, chelate
addition, surfactant type and concentration, air sparging
rate, metal concentration and foam drainage were investi-
gated in relation to metal extraction.
_y
The average extraction rate obtained was 1.9x10 moles total
metal per cm column cross-section area per minute which is
approximately 4.0xlO~ equivalents per cm per minute.
Operation in simple and countercurrent foaming modes produced
similar extraction rates for acid mine drainage.
The low extraction capacity of foam fractionation renders the
process economically unfeasible for the treatment of acid
mine drainage. The principal chemical cost is for surfactant
followed by air; either alone makes the process noncompetitive,
Surfactant regeneration from collapsed foam by the addition of
base was investigated as a means for surfactant reuse and
cost reduction.
This report was submitted in fulfillment of Project Number
14010 FUI, Contract 14-12-876, under the sponsorship of the
Water Quality Office, Environmental Protection Agency-
iii
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CONTENTS
Section Page
I Conclusions 1
II Recommendations 3
III Introduction 5
IV Foam Separation Apparatus 11
V Analytical Methods 19
VI SAMD Experiments-Results and Discussion 21
VII AMD Experiments-Results and Discussion 41
VIII Surfactant Regeneration 43
IX Economic Evaluation and Summary 47
X Acknowledgements 51
XI References 53
XII Notations and Glossary 57
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FIGURES
Number Page
1 Modes of Continuous Foam Separation 8
2 Separation Column 12
3 Equilibrium Column 13
4 Column Nomenclature 14
5 Sparging Head 16
6 Distribution Ratios 30
7 Volume Reduction versus Sparging Rate 33
8 Foam Density versus Foam Residence Time 34
9 Dynamic Surface Excess versus Pool
Concentration 36
10 Total Metal Fraction Removed and Total
Metal Extraction Rate versus Total
Metal Feed Rate 38
11 Process Flow Scheme for Foam Separation
of Metals and Surfactant Regeneration 48
VI
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TABLES
No. Page
1 Independent Control Variables in Continuous
Foam Separation Processes 9
2 Dynamic Surface Excess Values for Selected
Surfactants in SAMD 24
3 Effect of pH on Metal Separation from SAMD 26
4 Dilute SAMD 29
5 Comparison of Simple and Stripper Mode
Experiments 31
6 Summary of Results for SAMD Experiments and
Comparison with AMD Experiments 39
7 Grassy Run Acid Mine Discharge 41
8 Recovery of NaDS from Collapsed Foam by
Addition of NaOH 44
vii
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SECTION I
CONCLUSIONS
1. Continuous foam separation of mixtures of acid mine
drainage (AMD) and anionic surfactants achieved the ex-
traction of dissolved metal cations at a practical rate
of 1.9xlO~7 moles total metal per cm column cross-sec-
tion area per minute which is approximately 4.0xlO~7
equivalents per cm2 column cross-section area per minute.
These rates represent 16 percent of the estimated theo-
retical maximum extraction capacity of foam separation.
2. Operation of foam fractionation in a persistent foam
regime in order to minimize foam density requires exce's-
sive surfactant consumption which results in increased
chemical costs.
3. The low extraction capacity of foam fractionation
renders the process economically unfeasible for the treat-
ment of AMD. The principal chemical cost is for surfactant
followed by sparging air; either alone makes the process
noncompetitive.
4. Bubble production by sparging air through porous
materials, although convenient and effective, is not en-
tirely satisfactory for control of optimum bubble diameter
and a narrow bubble distribution. Bubbles tend to be
larger than optimum.
5. Synthetically prepared AMD proved to be an accurate
substitute for AMD in foaming tests.
6. Increasing the pH above the 2.2 - 2.3 of synthetic AMD
tended to reduce metal separation, particularly for iron.
7. The use of EDTA with anionic and cationic surfactants
did not improve metal separation over anionic surfactants alone.
8. Foam column operation in simple and countercurrent mode pro-
duced comparable metal removals.
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SECTION II
RECOMMENDATIONS
The economic unfeasibility of foaming AMD in a persistent
foam regime precludes further study of that system. The
high chemical costs for foaming AMD in a persistent foam
regime could be reduced by foaming in a transient foam
regime which requires less surfactant. The low extraction
capacity of foam fractionation, which is governed by
stoichiometric adsorptive interaction of-metal cation and
surfactant anions, could be increased by operation in a
non-stoichiometric system, such as in froth flotation, which
may require less surfactant and sparging air per mass of
metal removed from AMD. Further research on foam separa-
tion treatment of AMD should be directed to non-stoichio-
metric, transient systems.
The use of waste surfactant and air should be investigated.
The combination of waste surfactant in municipal sewage,
the tendency for autopurification of AMD and sewage mix-
tures, and the high pressure method of foam separation
(bubble formation by degasification of air supersaturated
liquid) appears to have favorable technical and economic
feasibility and should be investigated.
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SECTION III
INTRODUCTION
Environmental degradation caused by the acidic, iron rich
effluent from coal producing regions has required assiduous
efforts to find cost-effective'-abatement and treatment
methods. The treatment method for acid mine drainage (AMD)
under investigation in this study is classically known as
foam fractionation and often called foam separation.
Separation is based on selective adsorption of dissolved
surface active material (surfactant) at the liquid-gas
interface of rising bubbles. Surface inactive material can
be separated by coadsorption with surfactant. Separation of
adsorbed and coadsorbed material from liquid is accomplished
by the production and collection of foam.
The purpose of this investigation is to determine the tech-
nical and economic effectiveness of continuous-flow foam
separation as a treatment method for AMD. Towards this goal,
a laboratory scale, pilot plant apparatus capable of 3-12 gal.
per hour was constructed and operated to define and
evaluate the variables which control separation and, thus,
the practical application of the method to AMD on a field
scale. For expediency, the majority of work was conducted
using a synthetically prepared acid mine drainage (SAMD)
followed by confirming experiments using freshly sampled AMD.
Numerous small scale, laboratory investigations of foam
separation, generally of a theoretical nature utilizing
simplified solutions, have empirically developed the basic
theory on which this practical investigation of a complex
solution (AMD) is based (1-22). Even so, the complexity
of a foam separation system has not permitted a single uni-
fying theory and thus an empirical approach is necessitated.
Applied investigations of foam separation for treatment of
various polluting liquid wastes, a few on a pilot plant scale,
have appeared (20, 23-27). Several reviews of foam separa-
tion processes are available (28-30).
This research program is divided into two main efforts; the
construction phase in which the foaming apparatus was
assembled and tested, and the experimental phase in which
SAMD and AMD were treated. Once the apparatus was opera-
tional, the operating variables which control separation
efficiency were determined within the practical limits of a
potential pollution control process which in large part are
restricted by hard economics.
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Foam separation is based on the phenomenon of surface
activity which results from the ability of certain solutes
(surfactants) to reduce the surface free energy of their
solutions, and therefore the total free energy of the
system, by accumulating at an interface. Surface activity
as it relates to foam separation process is described using
the concept of Gibbs surface excess.
~ fl
- d / = RT£ I; d In a; (D
~ M L L
where / = interfacial surface tension
R = gas constant
T = absolute temperature
n = number of solute components
I; = surface excess of the i component
a = chemical activity of the i component
In practice foam separation consists of passing bubbles
through a solution of surface active solute(s) with the aim
to adsorb the solute(s) onto the gas-liquid interfaces and
to remove these surfaces intact as foam, thus effecting a
separation. Further, by coadsorption of non-surface active
solute with surface active solutes, the former can be
separated from solution with the latter. This is the case
in the treatment of AMD.
An understanding of the experimental approach used here can
be gained by considering three factors: (1) the particular
goal of foam separation as applied to acid mine drainage,
(2) the separation characteristics of the various operational
modes of foam separation processes, and (3) practical con-
sideration of the final application of this research.
Foam separation can operate in four basic modes, with each
mode offering potential separation characteristics appli-
cable to different goals (Figure 1). These goals can be
described as:
1. Decontamination, in which the goal is separation
of constituents from the feed to produce a puri-
fied bottom flow.
2. Recovery, in which the goal is separation and
recovery in the foam of constituents in the feed.
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3. Separation, in which the goal is sepp.ration of
multiple constituents, one from another, by
fractionation in the foam.
Obviously, the prime goal for the treatment of acid mine
drainage is decontamination and, thus, the operational
mode of foam separation is determined by this goal.
In comparison to simple mode operation which can be con-
sidered single theoretical plate operation, the stripper,
enricher and combined modes can be considered multiple
plate operations. Theoretically, the stripper mode should
enhance the decontamination of the feed by providing ad-
sorption equilibrium between bubble film and interstitial
liquid of feed concentration instead of pool concentration
as in the simple mode. This is accomplished by counter-
current flow of feed and foam, as depicted in Figure 1.
The purpose of recycling (external reflux) a portion of the
collapsed foam back into the rising foam, as in the enricher
and combined modes, is to further enhance the bubble film-
interstitial liquid equilibrium by providing an even richer
interstitial liquid than in the stripper mode. The effect
of external reflux should be a higher concentration in the
net overhead product, Lc-r (Figure 1) and thus a more
efficient operation for recovery and concentration of feed
constituents. Thus, the stripper mode should enhance de-
contamination of the feed, the enricher mode should enhance
the recovery in the foam and the combined mode should pro-
vide the benefits of both the stripper and enricher modes.
Since the prime goal in the treatment of acid mine drainage
is decontamination, the stripper mode should be given prime
consideration. However, concentration of the overhead pro-
duct is still desirable, not for the purpose of recovering
the contaminants in acid drainage but rather for reducing
the liquid volume carried over in the foam. A foam of low
liquid volume is produced by drainage of the interstitial
liquid out of the foam. Provision for conditions which pro-
mote foam drainage is thus required for efficient overall
operation designed for the decontamination of mine drainage.
In a practical sense, the use of complex modes is often re-
stricted by operational difficulties. For example, the
distribution of liquid feed or reflux into a rising foam
without causing channeling or bubble rupture is a real
problem. Also, the effect of micelles introduced in con-
centrated reflux streams can adversely affect the bubble
film-interstitial liquid equilibrium and cause a reduction
of concentration in the overhead flow.
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t
Foam
Pool
Jb
tG
Simple Mode
c
t
Foam
Pool
T75"
Stripper Mode
t
Foam
Pool
•+L
c-r
TG
Enricher Mode
f
Foam
Pool
•*• L
c-r
TG
Combined Mode
FIGURE 1
Modes of Continuous Foam Separation
Lf Liquid Feed Flow
L, Liquid Bottom Flow
G Gas Sparging Flow
c ™~
Total Liquid Overhead Flow
Net Liquid Overhead Flow
L Liquid Recycle Flow
8
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Certain independent operating variables are known to sig-
nificantly affect the separation achieved in a foam column
(7) (Table 1). Not every variable is continuously variable,
as bubble diameter is restricted by the flow dynamics of
the system (1) and available bubble producing equipment.
TABLE 1
Independent Control Variables, in
Continuous Foam Separation Processes
Geometric Variables Solution Variables Operation Variables
1. Bubble diameter
2. Column diameter
3. Column length
1. Ratio of the
concentration of
surfactant to
contaminant.
2. Chemical
character of
the feed.
3. Specific
chemical and
physical inter-
ferences .
1. Ratio of the
flow rate of gas to
liquid feed.
2. Liquid feed rate
as a function of
column dimensions.
3. Foam drainage
time.
4. Modification of
column to allow
operation in various
modes (Figure 1).
For this program, the solution variables are of secondary
importance since a standard SAMD is used. The exceptions are
the amount and type of surfactant required and pH effects.
The main variables considered here are bubble diameter;
column length, particularly as it effects liquid and foam
residence times in the column; surfactant concentration or
better stated for the continuous flow system the mass flow
rate; gas flow rates particularly in relation to bubble
diameter, solute throughput, foam density and foam stability;
contaminate (metal ion) mass flow rate and operation in
simple and stripper modes.
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SECTION IV
FOAM SEPARATION APPARATUS
Foam Column Description - Foam separation processes are
not in wide use. With the exception of particle flotation
methods used in the mining industry, which are not com-
pletely analogous to foam separation of dissolved consti-
tuents in mine drainage, the state-of-the-art in foam
separation knowledge and equipment is still in its infancy.
As a result, not only must the physical-chemical and pro-
cess engineering aspects of applying foam separation to
mine drainage be studied, but, also, equipment must be
designed and fabricated. The development of equipment is
generally costly and time consuming, thus every effort was
made to make use of commercially available equipment and/or
components in order to speed fabrication.
The foam column (Figure 2) was constructed of 6 in.-ID pyrex
pipe using teflon lined, stainless steel couplings at the
joints. The couplings are constructed with a nearly smooth
inside surface which can be significant to foam stability
in the drainage sections. The foam originates at the
sparging head in the lower part of the vertical section
and flows up and then to the right in the horizontal drainage
section. The foam passes from the drainage section down into
the foam breaker (not visible) behind the control panel.
Structural support for the column and auxiliary equipment is
provided by an open pipe structure of dimension 15 ft. L. x
9 ft. H. x 3 ft. D. The horizontal drainage section is pro-
vided with valved ports which allow foam drainage to be re-
moved from the column, collected in a plastic pipe which
hangs below the glass pipe and either removed from the system
or reintroduced at a desired point. Also, shown are liquid
storage tanks, air and liquid control equipment, liquid
pumps and timing devices.
The column was constructed of multiple short lengths of
pipe to provide rapid manipulation of such parameters as
pool depth and drainage section length which in turn control
bubble residence time in the liquid and foam drainage time,
respectively. Referring to Figures 2 and 3 the column has
two basic configurations, the separation column and the
equilibrium column. The separation column is of general
design, adaptable to operation in any of the four modes
(Figure 1), and was used to optimize the geometric and
operating variables (Table 1) once the soluble variables
were defined. The equilibrium column was designed to study
the solution variables which in this case are determinations
of the dynamic surface excess (an equilibrium measurement
11
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Figure 2.
tion.
Separation Column. The column is shown in opera-
from which the term equilibrium column was derived) for the
metallic constituents of mine drainage as a function of
both various types and concentrations of surfactants in the
feed. The subject of dynamic surface excess will be dis-
cussed later.
Filtered, humidified laboratory air was used for sparging.
Air pressure control is provided by adjustable spring dia-
phragm-type regulators and gauges. Air flow was controlled
with neec'le valves and rotameters. The various types of
sparging heads tested are discussed later.
SAMD, AMD and surfactant solutions were held in poly-
ethylene tanks. The solutions were pumped into the foaming
column by positive displacement diaphragm metering pumps.
The liquid flow rates are controlled by adjustment of the
stroke rate and stroke length of the pumps, by needle valves
and rotameters.
12
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FIGURE 3. Equilibrium Column. The column is shown in
operation. The absence of an extended drainage section
is the only significant structural difference from the
separation column. The foam breaker is shewn.
Column Operation - Column operation was begun by pumping
feed solution(Surfactant and SAMD or AMD) into the vertical
foaming section, the pool volume of which is controlled by
height adjustment of the overflow discharge tube (Figure 4).
The surfactant and SAMD solutions can be mixed in line or
in batch prior to pumping; the later method was found more
reproducible and used almost exclusively. When the pre-
determined static pool volume was reached, sparging was
begun and the column was allowed to operate until a steady
state was reached. Ostensive indications of steady state
operation were stability in the height of the dynamic flooding
13
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a
o
•H
-P
O
0
K!
a
•H
o
EH
si
o
•H
-P
Horizontal Drainage Section
Counter-
current
Length
Dynamic
Flooding
Level
Bottom
Exit
Foam
Foam
Pool
Drainage Collector
Stripper Mode
Feed Entry
Foam Exit
Feed Entry
t
Sparging
Air Entry
FIGURE 4
Column Nomenclature
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level and steady rate of foam drainage return. As the foam
proceeds up the vertical section, drainage returns counter-
current to foam flow. In the horizontal section drainage
is perpendicular to foam flow and is removed through valved
ports in the bottom of the section (Figure 2). Drainage
is collected in the drainage return pipe (Figure 2) and
returned to the vertical section by gravity or pumping.
Usually from one to two hours were required to reach
steady state; separation results are based on samples
taken at steady state.
Sparging Heads - Production of a stable and persistent
foam is a fundamental requirement for achieving efficient
separation of metal ions from acid mine drainage. Since
the goals of separation are to decontaminate the feed and
to collect the contaminating metal ions in as small a
foam volume as possible, a foam of sufficient stability to
allow protracted drainage is desirable. As a foam drains
its persistence decreases due to thinning of bubble films.
Thus in practice it is often difficult to separate foam
drainage and bubble coalescense. Since the foam becomes
progressively more susceptible to rupture as drainage pro-
ceeds, the drainage section of the column must reduce to a
minimum the chances of physical shock. A horizontal drainage
section provides a minimum countercurrent flow of interstitial
liquid through the foam by allowing gravitational drainage
perpendicular to foam flow instead of parallel through the
length of the column. By reducing countercurrent drainage
flow, the chances of bubble rupture are reduced especially
at low foam densities. Drainage sections must be designed
to provide minimum constriction or expansion of the drainage
foam, and as smooth an inside surface as practical.
A foam is produced from a number of individual bubbles; how-
ever the structure of a foam is best visualized as a network
of interconnecting films surrounding gas spaces. The pres-
sure difference across a bubble film is
is the surface tension and r is the radius of curva-
ture. Thus it is evident that the pressure difference in-
creases as the radius decreases. This explains the tendency
for larger bubbles to grow at the expense of smaller ones in
a foam of varying bubble sizes. A foam with a wide distribu-
tion of bubble sizes tends to be self destructive. As the
bubbles rupture, surface adsorbed material is released into
the interstitial liquid and the efficiency of separation is
reduced. Thus it is necessary to produce a foam with as
narrow a bubble size distribution as possible.
One practical method to produce a foam for continuous foam
separation is by sparging air through an prifice(s) into a
15
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liquid and have the foam form above the liquid. Since bubble
size and size distribution are important to foam separation,
considerable time was spent designing and testing sparging
heads.
An explicit relationship for the bubble size obtained by
sparging as a function of pore size, solution density and
surface tension has not been determined for multi-pore
devices. Thus, it is necessary to conduct experiments to
find the sparging device most suited to the conditions.
Seven different sparging heads were constructed and tested.
Two heads were made from glass Buechner style filtering
funnels (Corning No. 36060) with 125 mm and 90 mm diameter
fritted discs of coarse porosity. Porosity varied over
the discs which resulted in channeling and thus streams of
large bubbles. The small diameter disc produced less
channeling, but still the bubble size distribution was too
wide for a stable foam.
Three and four glass gas dispersing tubes (Corning No. 39533)
with fritted cylinders of extra coarse porosity were mounted
in specially constructed manifolds and adapted to the 6 in.-
dia. column for use as sparging heads (Figure 5). The tube
stems were shortened from 250 to 57 mm. These heads showed
very little channeling and produced a stable foam.
FIGURE 5. Sparging Head. The head is made from four pyrex gas
dispersing tubes with extra coarse fritted cylinders. The
diameter of the fritted cylinder is 2 mm. The support struc-
ture and a column adapter coupling is made of PVC and poly-
ethylene plastic.
16
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A special holder was fabricated from PVC plastic to hold
2 in.-dia. discs of scintered stainless steel (Pall Corp.
Type 316L). Six porosity grades ranging from 5 to 165 u
mean pore size were tested and found to be less uniform
and to channel more than the fritted glass discs in the
Buchner funnels. The foam produced was very unstable.
A sparging head was fabricated from stainless steel to hold
four cup-shaped spinnerettes which contained precision
drilled holes on their upper faces. The spinnerettes were
designed for previous unrelated research but were tested to
determine if holes of one known size and spacing would pro-
duce a uniform, narrow bubble size distribution. The four
spinnerettes had 60 holes (15 each) of 125 u-dia. The
bubbles formed were very uniform and the foam was stable.
However, the sparging air capacity was too small for use
in the 6 in.-dia. column. Quotes for fabrication of a
larger sparging head of sufficient air capacity were approxi-
mately 1.5 dollars per hole. Since at least 2000 holes of
80 u-dia. would be necessary, the cost of 3000 dollars was
considered excessive at the present stage of research.
The same sparging head used to hold the spinnerettes was
also used to hold discs of collimated hole structure
(Brunswick Corp.). One disc contained 5,500 holes of
125 u-dia. and the other contained 335,000 holes of 12.5 u-dia;
both discs were tested as currently available without special
fabrication. Both discs produced streams of bubbles widely
varying in size, not because of channeling but due to bubble
coalescence in the swarm. The hole spacing (length between
hole centers) is less than three hole radii apart; this
allows for bubble coalescence and the resulting wide bubble
size distribution and unstable foam. Collimated hole struc-
ture can be made in a wide variety of hole sizes and spacings
and may be a less expensive alternative for drilled spin-
nerettes.
The results of the test indicate that the sparging head
constructed of four dispersing tubes (Figure 5) is adequate
for producing a reasonably stable foam; it was used in all
subsequent tests. Its low cost is an attractive feature.
Feed Distributors - Operation of the foam column in the
countercurrent mode requires that descending liquid feed be
introduced into ascending foam without causing significant
bubble rupture or liquid channeling. The feed stream must,
therefore, impinge the foam stream with minimum momentum.
Uniform distribution of the feed stream across the column
diameter is desired to provide liquid-foam contact over
a minimum downflow length. To meet these requirements,
special liquid distributors were constructed and tested.
17
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Previous work with countercurrent foam separation has lead
to liquid distributors of various designs. Spaced, packed
columns have been used to distribute feed streams (9).
Distributors constructed of single tubes, multiple tubes
and screen mesh in various configurations have been used
(3) (21). In general, the multiple tube distributors are
reported to have operated most effectively.
Three multiple tube distributors were designed, constructed,
and tested with the 6 in.-ID column. Based_on the expected
liquid feed rates (approximately 500 ml-min 1), the number
of distributor tubes of various sizes necessary to provide
the required pressure drop, was calculated such that the
feed liquid would impinge the foam at near ambient pressure.
This would provide minimum momentum transfer to the foam
and, thus, reduce foam collapse. The number of tubes should
be sufficient to provide effective feed distribution over
the column cross-section, but not so numerous that their
cumulative surface area exposed to the foam would be large
enough to disrupt foam flow. Based on these considerations,
the three distributors had 8, 12 and 21 tubes each of
0.062 in.-ID, 0.045 in.-ID and 0.030 in.-ID tubing,
respectively.
The distributors are constructed entirely of polyethylene
tubing and silicon rubber potting material. The smaller
distributing tubes are potted directly into 0.25 in.-ID
tubing which connects directly to the feed stream tubing.
The silicon rubber provided an economical and durable
method for distributor construction.
While all three distributors operated satisfactorily, the
8-tube distributor caused some channeling, and the 21-tube
distributor appeared to disrupt the foam flow more than the
others. The 12-tube distributor provided adequate distri-
bution with minimal channeling and foam collapse. The
12-tube distributor was used in all subsequent countercurrent
experiments.
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SECTION V
ANALYTICAL METHODS
Total Fe, Ca, Mg, Al and Mn - These metals in AMD are deter-
mined by atomic absorption spectrophotometry using standard
comparator methods (31). The samples are aspirated directly
into the flame with dilution necessary only to bring the
samples into the concentratiob range of the particular
resonance line. A high solids burner head (Perkin-Elmer
three slot) is used and no interferences from the surfactant
present has resulted.
Surfactant - Most of the surfactants used to date have been
anionic and possess a sulfonate or sulfate hydrophylic
group. Such surfactants form strongly colored complexes
with the triphenylmethane dyes which thus provide the basis
for a spectrophotometric method of surfactant determination
(32). In this case, the dye crystal violet is complexed with
the surfactant, the colored complex is extracted by benzene
and the absorbance of the benzene layer determined at 615 mu.
Ferrous Iron - Since the Fe II/Fe III ratio present in mine
water can theoretically have an effect on the foam separa-
tion of iron, determination of this variable was considered
important. Fe(II) is determined spectrophotometrically (33),
and Fe(III) is determined by difference from total iron de-
termined by atomic absorption. Fe(II) in aqueous solution
is complexed by 4,7-diphenyl-l-10-phenanthroline at pH 4,
the colored complex is extracted by n-hexyl alcohol and the
absorbance of the alcohol layer measured at 533 mu.
pH - The surfactants under test are weak acids and, thus,
their solubility and speciation in solution is a function
of pH. pH is monitored by a glass electrode - calomel
reference electrode pair and a pH meter.
Bubble Diameter Measurement - As will be seen in Section VI,
determination of the average bubble diameter in the foam is
necessary for calculation of surface excess. Bubble dia-
meters can be determined by direct or indirect methods. An
example of an indirect method is the empirical relationship
of optical density to bubble diameter. A direct measurement
would be bubble photographs. It is known that photographs
of a foam taken through a glass surface can yield useful
measurements of bubble diameters (34). The bubble diameters
at the periphery of the foam column are not significantly
different from those in the interior. Thus a method of ob-
taining photographs of the bubbles was developed. A 35 mm
19
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single-lens reflex camera with a bellows unit and 55 mm lens
was used to obtain 1-2 x photographs of the moving foam. A
strob-flash provided the light.
Bubble diameters are measured directly from the negative
with an optical comparator which magnifies the bubbles and
allows visual comparison of the bubble to a graduated
reticle. The measured diameters, d. are used to calculate
a mean diameter,
3 -
/^
i i
where n. is the_number of bubbles of .various diameters d..
Calculation of d was by computer.
20
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SECTION VI
SAMD EXPERIMENTS - RESULTS AND DISCUSSION
Surfactant Screening - A great number of surfactants are
commercially available from an equally great number of
companies. Surfactants are commonly sold under trade
instead of generic names because their chemical composi-
tions are considered proprietary. In addition, test in-
formation on the chemical and physical properties of sur-
factants is often fragmentary and/or inconsistent or
noncomparable from one manufacturer to another. Therefore,
the screening of surfactants for those suitable to foam
separation under the conditions encountered in acid mine
drainage presents a formidable task.
Since the metal ions in acid drainage are not surface
active, a surfactant must be added to allow coadsorption
of metal. Coads-orption of metal may arise from charge
interactions between adsorbed surfactant and a diffuse
double layer of metal counterions, or from bonded inter-
actions between surfactant and metallic species. Both
methods for obtaining oeadsorption have been considered.
Since the major metallic constituents of acid mine drainage,
i.e., Fe, Al, Ca, Mg and Mn, occur as cations at low pH, an
anionic surfactant would be required for coadsorption by
the charge interaction mechanism. Thus, of the three charge
types, i.e., nonionic, cationic, and anionic, anionic sur-
factant has received greatest attention.
Metal-ligand complexes of high bond strength and metal speci-
ficity are formed by a class of multidentate ligands known
as chelates. Ethylenediamine tetraacetic acid (EDTA) and
diethylene triamine pentaacetic acid (DTPA) (and their
associated salts) are well known, examples of ligands which
form water soluble, multidentate complexes (chelates) with
a variety of metal ions. These chelates are characterized
by exceptional complex stability and metal specificity; how-
ever they have minimal surface activity and are not directly
useful in foam separation. The chelating ability of EDTA
and the surfactant properties of the dodecyl-benzyl func-
tional group are combined in dodecylbenzyl-diethylene triamine
tetraacetic acid (DBDTTA) to form a water soluble, chelating
surfactant. Another chelating surfactant is dodecylimino
diacetic acid (DIDAA).
To screen the large number of available surfactants by
laboratory experiments would be very costly in time. Thus,
21
-------
an extensive literature search was considered the most prac-
tical method for picking the few best surfactants for labora-
tory testing. Research and commercial data was consulted;
in addition, surfactant manufacturers and suppliers were
consulted and asked to consider surfactant requirements and
recommend suitable materials. By these methods surfactants
were chosen for testing.
Anionic type surfactants were chosen which produce stable
foams which have sufficient solubility at_low pH and in
hard water to resist precipitate and/or micelle formation
and which are low cost and readily available. Unfortunately,
chelating surfactants are not commercially available and can
be obtained only in research quantities at considerable cost.
Thus, the testing ©f chelating surfactants has not been further
considered.
Surfactant Tests - The potential for effective foam separa-
tion can be expressed by the surface excess concentration
gamma P (equation 1, page 7) for surfactant and coadsor-
bants, in general the greater the P values, the greater the
potential.
The equilibrium surface excess of a solute is usually deter-
mined by measuring surface tension as a function of solute
concentration. Solute concentration is kept low so that
the activity coefficient can be assumed equal to one and,
therefore, the chemical activity is assumed the same as
concentration. Rvalues measured in this manner are not
representative of dynamic foam separation systems (35) and,
thus, have little value. Instead, useful equilibrium values
can be determined for surfactant and coadsorbed metals
under dynamic conditions in a foam column designed for
that purpose. The equilibrium column (Figure 3) was de-
signed to allow measurement of P values. The column
operates in the continuous, simple mode.
By mass balance and surface equilibrium considerations
around the foam, the system can be described by
22
-------
6GP
where [x] is the concentration of surfactant or coadsorbed
component
LC is the liquid volume rate of foam
G is the air volume sparging rate
d is the average bubble diameter
P
x ls the surface excess in equilibrium with [x],
and the subscripts c and b refer to the foam exit and pool
exit flows, respectively.
Rearrangement of equation (2) gives an expression for I
P ,r i r , ,5 LC
I = (l-xl -f-vl ^ — (1}
•v ^!-Jr«LjAJv1'crl v«J /
X CD oLr
Thus, by measuring [x], Wfcj ^» Lc and G' values of '
can be calculated for x equal to surfactant, Fe, Ca, A'l,
Mn and Mg. The basic requirement is that sufficient bubble
liquid contact time be provided to assure attainment of sur-
face adsorptive and coadsorptive equilibrium and that foam
coalescence is minimal. Such equilibria appear to have
rapid kinetics (36), and approximately one foot of pool
depth gives sufficient bubble contact time (29).
Five surfactants, determined in the initial screening to
have properties suitable for foam separation at low pH,
were selected for testing. The tests were carried out in
the equilibrium column to determine (values for surfactant
and metal constituents. Each of the surfactants were tested
at four feed concentrations ranging from 100-400 ppm in
standard SAMD.
The surfactants tested were sodium dodecyl sulfate, Alkanol
189-S (DuPont), Alipal EO-526 (GAF), Alipal CD-128 (GAF)
and Aerosol AY (Cyanamid). Alkanol 189-S is a sodium alkyl
sulfonate with an average carbon chain length of 15. Alipal
EO-526 is a sodium salt of a sulfated alkylphenoxy poly(ethyl
eneoxy) ethanol with an undisclosed chain length. Alipal
23
-------
CD-128 is a sulfate-ester type surfactant based on a linear
chain (hydrophobe) of undisclosed length. Aerosol AY is
sodium diamyl sulfosuccinate.
Upon addition of Alkanol 189-S to SAMD, a coarse reddish
precipitate formed. Alkanol189-S is normally a high
foaming, acid stable surfactant, but mixed with SAMD it
produced only a transient froth. A sample of the precipi-
tate was dried at 100°C, producing a viscous substance
obviously containing a large amount of surfactant. The
surfactant is incorporated in the precipitate and the
precipitate is not solely the result of oxidation of Fe(II)
and hydrolysis of Fe(III) due to pH increase.
Aerosol AY would not produce a stable foam in SAMD over a
concentration range of 100-500 ppm surfactant.
In the tests using sodium dodecyl sulfate, Alipal EO-526,
and Alipal CD-128, stable foams were produced and the data
necessary for \ calculations were obtained.
For purposes of surfactant comparison, individual | values
for surfactant and SAMD metals were averaged for each sur-
factant and surfactant concentration; the results are in
Table 2.
TABLE 2
Dynamic Surface Excess Values for
Selected Surfactants in SAMD
Equilibrium Column, G = 10 liters-min
Lf = 700 ml-min 1, [S]f = 100-400 ppm
_i
r r r
1 s ' Fe ' Ca
Surfactant, S (Units: x
Sodium dodecyl
sulfate, NaDS
(M.W. = 288)
Alipal EO-526
(M.W. = 586)
Alipal CD-128
(M.W. = 352)
Aerosol AY
(M.W. = 360)
Alkanol-189S
(M.W. = 354)
26.1 5.22 5.11
27.7 3.22 1.92
48 2.5 1.5
r r r r /r
1 Mn ' Mg ' tm ' tm/ ' s
1011 moles -cm 2)
0.131 0.768 11.2 0.430
0.1 0.166 5.40 0.195
0.14 0.99 5.09 0.106
No foam produced over surfactant concentration
range 100-500 ppm
Precipitate formed;
produced
only transient foam
24
-------
The economics of foam separation are greatly dependent on
the magnitudes of the adsorptive J""1 for surfactant and the
coadsorptive \ for the metals to be separated. Increased
adsorption of surfactant to bubble walls increases the
capacity to separate coadsorbed metals and thus promotes
the most efficient use of surfactant. Because foam
separation is inherently of low exchange capacity compared
to ion exchange or solvent extraction (1), the economics
of foam separation will depend on maximizing exchange
capacity (i.e., maximizing coadsorptive) per consumption
of materials and power (i.e., surfactant and sparging air).
Generally a stoichiometric relationship is believed to
exist for foam separation of dissolved constituents, but
the exact relationship has not been clearly defined. In
foams produced from pure aqueous solutions of ionic sur-
factants, the surface concentrations of adsorbed surfac-
tant and coadsorbed metal will be equal (1), e.g., for
NaDS in deionized water PN + = f^q- and tne ratio
•Na+/ IDS" = 1" Considering metal coadsorption to arise
from charge interactions with adsorbed surfactant, and
considering the requirement of electroneutrality for the
system, a one-to-one ratio of surfactant and metal in the
foam would be expected on an equivalent basis. Ratios less
than one indicate coadsorptive competition from metal im-
purities.
At a pH of 2.2, the hydrogen ion concentration in SAMD is
6.3 x 10~3M compared to about 7 x 10~3M total added metal.
Sodium in SAMD derived from surfactant and tap water
(approximately 13 ppm in tap water) is at least equal molar
with surfactant (sodium analyses were not conducted). Iron
in SAMD is initially greater than 98 percent Fe(II); the
extent of oxidation to Fe(III) during sparging and, thus
the exact equivalence of iron in the foam system is unknown.
For these reasons, equivalence ratios of coadsorbed metal
to adsorbed surfactant are not determined. Molar ratios
are used to provide information on the relative effective-
ness of a particular surfactant for foam separation in
standard SAMD.
An optimum value for adsorptive \ , the value to which ex-
perimental values for surfactants can be compared, can be
estimated at least as to order of magnitude (1). A (value
of about 3 x 10~10 moles-cm" would be expected under
equilibrium conditions for the surfactant sodium dodecyl
sulfate (M.W. = 288). Comparing the average \s values in
Table 2 with this theoretical value, it is apparent that
adsorptive equilibrium was reached for NaDS, EO-526 and
CD-128. Considering the individual surface excess values
for each metal | , and then the sum of the individual values
1 m'
25
-------
P. ?it is clear that NaDS with a molar ratio of total
coadsorbed metal to adsorbed surfactant P /p equal
to 0.430 is the most efficient surfactant Tor metal
separation.
Effect of pH - Experiments were conducted at pH 2.4, 2.7
and 2.9 to determine the effect of hydrogen ion concen-
tration on foam separation of SAMD metals with NaDS. pH
was adjusted from 2.2 withNaOH, the upper limit being
restricted by iron precipitation at about pH 3.0. The re-
sults in Table 3 indicate a slight decrease in P and a
more significant decrease in P, with increasing pH. The
decrease in R and ft
coadsorption 01 iron.
, .
results mostly from decreased
TABLE 3
Effect of pH on Metal Separation from SAMD
Equilibrium Column, G = 10 liters-min"1
Lf = 700 ml-min"1, [S]f = 100 ppm
PH
r.
mt
_2
(Units: x 101 moles-cm" )
r /r
1 mt/' s
NaDS and SAMD
2.2
2.4
2.7
2.9
EO-526 and SAMD
without iron
2.8
3.5
4.4
2.61
2.40
2.22
2.34
1.56
1.52
1.41
1.12
1.08
0.761
0.542
0.303
0.212
0.202
0.430
0.450
0.343
0.232
0.194
0.139
0.143
Three additional experiments were conducted over a wider pH
range at 2.8, 3.5 and 4.4 with EO-526 and SAMD without iron.
The results in Table 3 also indicate a slight decrease in
and a more significant decrease in R . The relative
~
decrease of
.
with pH is greater with iron present.
26
-------
Effect of the Chelating Agent EDTA - A number of experiments
were conducted to determine the effect of the chelating
agent EDTA on the coadsorption of metals in SAID with both
anionic and cationic surfactants. EDTA alone will not pro-
duce a foam in SAMD, but in combination with a surfactant
it is possible that its ability to form strong, selective,
ionic metal complexes coupled with its tendency to associate
with and stabilize foams could enhance selective foam
separation of metals. The stability of EDTA complexes with
SAMD metals is: Al=» Fe(II) =- Mn=~ Ca=- Mg. In general, ex-
cept for the alkaline earths, the metal chelates are stable
to a pH of near 1.0. Decreasing pH favors formation of
less highly ionized, less strongly chelated complexes. The
effect of increasing pH is to stabilize the completely
ionized chelatent species, but at high pH this advantage
is balanced and eventually nullified by metal hydroxide
formation.
Three experiments using NaDS and SAMD at pH 2.3 were con-
ducted with EDTA to NaDS molar ratios of 0.59, 1.02 and 1.62;
[NaDS]f was 4.38 x 10~ M. P for the three experiments was
10 M ; for \tm it was 3.51 x 10"11 which gives a
ratio of 0.193. Comparing these values to those in
TalSle 2 for similar conditions except without EDTA, it is
apparent that adsorption of surfactant and coadsorption of
metal is reduced by EDTA.
Six additional experiments were conducted to test higher con-
centrations of EDTA with both anionic NaDS and cationic
Hyamine-1622 surfactants. Hyamine-1622 (Rohm and Haas) is
di-isobutyl phenoxy ethoxy ethyl dimethyl benzyl ammonium
chloride, monohydrate. These experiments were conducted
under conditions similar to those above except with an EDTA
to total SAMD metal ratio of 1 and pH at 2.9 - 3.1, 6.0 -
6.2 and 7.8 - 7.9. In three experiments [NaDS]f was
6.89 x 10~4M; in the other three [Hyamine-1622]f was
7.22 x 10~4M.
At pH 7.8 - 7.9 Hyamine-1622 and SAMD produced a dark red-
brown turbid solution containing a grey-black precipitate;
at pH 6.0 - 6.2 the solution was yellow-green and turbid;
at pH 2.9 - 3.1 the solution was green and clear. In all
three pH ranges foam production and stability was marginal;
and no measurable amounts of metal were removed.
With NaDS and EDTA, metal separation at all three pH_ranges
was lower than without EDTA as indicated by r^/Ps ratios
of less than 0.190. The results of these tests indicate
no enhancement of metal coadsorption with EDTA compared to
surfactant alone. Hyamine-1622 partially precipitated with
27
-------
SAMD and EDTA, thus results from its tests were questionable.
The possibility of enhanced separation and metal specificity
with chelating surfactants remains to^be examined.
Effect of Surfactant Concentration - Results of testing
several surfactants in SAMD at various pH values with and
without EDTA indicate that NaDS without EDTA at the 2.2 -
2.3 pH of standard SAMD are the most efficient conditions
tested. Therefore, NaDS was used in all subsequent ex-
periments .
The I values for a surfactant will increase with surfactant
concentration until complete saturation of the air/solution
interface is reached. Further increase of surfactant con-
centration will not increase [""" . As surfactant concentration
increases the critical micelle concentration (CMC) is
reached, above which additional surfactant will form micelles
which are bulk rather than surface active. Above the CMC,
the " for surfactant will remain constant while the | ,
for coadsorbed metal will decrease due to coadsorption com-
petition between surfactant adsorbed at bubble interfaces
and surfactant present as micelles. The maximum surfactant
concentration for efficient coadsorptive separation will be
determined by the CMC and/or the concentration at which
surface saturation is maximum (theoretically these two
concentrations are the same). The lower limit of surfactant
concentration is usually that necessary to produce a stable
foam. Thus, an optimum surfactant concentration or concen-
tration range will exist at which maximum coadsorptive
separation of metal will be achieved.
SAMD was foamed in a series of experiments with NaDS over a
surfactant feed concentration range of 180-300 ppm. The
column was operated in the simple continuous mode with an
extended horizontal drainage section. A second series of
experiments were conducted under similar conditions but
with a more dilute mine drainage DSAMD, the composition of
which is given in Table 4. For reasons of analytical
sensitivity, the metal concentrations are not in the same
relative proportions as in SAMD. The results are shown
in Figure 6.
The distribution coefficients for both SAMD and DSAMD,
which differ by a factor of approximately 200 in total
molar metal concentration, indicate the optimum [NaDS],
is in the range 4.7 to 6.1 x 10 M.
28
-------
TABLE 4
Dilute SAMD
Constituent Metal Concentration, M x 106
FeS04-7H20
CaSO4-2H20
MgSO4-7H20
MnSO4-H20
A12(S04)3-18H20
2.24
11.5
1.42
0.574
20.4
Effect of Countercurrent (Stripping Mode) Column Operation -
Two series of experiments were conducted with SAMD under
conditions similar to those reported in Figure 6 except
that the column was operated in the stripping instead of
the simple mode. Two countercurrent lengths were tested
to determine the effect on separation of increased contact
time between counterflowing feed and foam. [NaDS]f was
set such that [NaDS], would be in the optimum range
(Figure 6).
For comparison, results of the stripping experiments are
shown in Table 5 with those from an analogous experiment
conducted in the simple mode. Extension of the countercur-
rent length from 15 to 29 inches produced an absolute in-
crease in both surfactant adsorption and metal coadsorption;
however, the relative metal coadsorption per unit surfac-
tant adsorbed remained the same. Comparison of \\.j/\s
and VR values for stripping and simple operation suggests
the stripping mode to be less efficient at metal separation.
Greater liquid carry-over with countercurrent operation
would be expected considering the higher adsorbed and inter-
stitial liquid surfactant concentrations compared to simple
operation. Extension of the horizontal drainage section
would increase VR to about 20 without significant reduction
in separation due to foam collapse.
29
-------
ID'1-,
s
u
10
-3.
o SAMD VR = 11-20 >
• Dilute SAMD VR = 15-23
.-"r
,AT
4.5
5.0
7.0
10
-3
-10
o
10
10'
7.5
6.0
[NaDS] b, ¥ (xlO 4)
FIGURE 6 DISTRIBUTION RATIOS
Simple Mode, 18 in. Vertical Section, 72 in. Horizontal Section,
G = 8 liter • min '1, Lf = 500 ml- min -1 . Each data point
represents the average of 4 to 5 measurements.
30
-------
TABLE 5
Comparison of Simple and Stripper Mode Experiments
O A TUTT-k — « *3 XTn T"\O /~* O 1-34-.n..»»__-!u ^~ T rr f\/\ T ! ~1
(SAMD and NaDS, G = 8 liter-min
, Lf = 500 ml-min )
Variable
Column Mode
Vert. Sect. , in.
Horz. Sect. , in.
C.C. Length, in.
VR
FRT, min.
, , moles • cm
tm'
1 , moles -cm
s '
^n/Q
Stripper,.
24 in.
72 in.
15 in.
13.6
6.02 min.
9.06 x 10""11
2.30 x 10"1
0.394
Stripper
36 in.
72 in.
29 in.
14.2
5.94 min.
10.3 x 10'11
2.70 x 10'1
0.381
Simple
18 in.
72 in.
-
19.0
5.99 min.
11.1 x 10'11
2.22 x 10"1
0.500
' Because adsorptive phenomena are in equilibrium with feed,
liquid in the stripping mode rather than pool liquid as in
the simple mode, the proper form of equation 3 for
calculation in the stripper mode is (29) :
r =
3 L
x
f' 6.59 G
In the simple mode, adsorptive phenomena are in equilibrium
with pool liquid; in the stripping mode equilibrium is with
feed liquid. Since surfactant optimization tests (Figure 6)
were conducted in the simple mode, and since [NaDS]f [NaDSJb,
the surfactant concentration in the stripping tests was
above optimum for maximum coadsorption of metal. A reduction
of [NaDSK from 8.7 - 8.9 x 10~4M to 4.7 x 6.1 x 10 M would
increase fL/P to about °'5 and thus increase separation
by about 3 to 21 percent. The small relative magnitude of
this increase with respect to metal separation from SAMD
precluded allocation of time for further tests.
31
-------
Volume Reduction - Production of dry foams is required to
reduce interstitial liquid carry-over in the foam and, thus,
to reduce the waste volume for a foam column operating to
decontaminate the feed. Volume reduction factors (Lf/Lc)
were determined as a function of sparging rate G and sur-
factant concentration (Figure 7). Volume reduction factors
equivalent to approximately 1.7 to 3.8 percent liquid carry-
Over are easily achieved at low gas rates. Further increase
in volume reduction factors may be possible through increase
of foam residence time FRT which is accomplished by reducing
sparging rate and/or by lengthening the drainage section.
The practical factors which operate to limit volume reduction
in a 6 in.-dia. column are the sparging heads at high gas
rates and foam collapse at low gas rates, i.e., long FRT.
At high gas rates, bubble diameters increase and their size
distribution widens. Thus surface throughput per volume of
gas and foam stability declines markedly. At low gas rates
a maximum FRT is reached above which further drainage to
reduce interstitial liquid results in foam collapse and re-
duction in separation efficiency. With protracted drainage
the foam becomes dry and its suceptibility to rupture in-
creases until mechanical shock from flow in contact with the
column wall produces complete collapse.
The inverse log relationship of foam density and drainage
time (1) and the practical limits for foam drainage in the
6 in. column are shown in Figure 8. For [NaDS]f = 200 ppm
the foam density decreases with drainage for FRT values
approximately three and greater; this section of the curve
follows the expected exponential relationship. At low FRT
values, which result experimentally from high sparging
rates, the average bubble diameter is increased and the
diameter size distribution is widened. The result is more
rapid drainage and a significant reduction in foam stability.
Also, at high gas rates turbulence in the pool causes voids
to form in the foam at the foam-liquid interface. These
voids progressively enlarge and destroy plug flow in the
drainage section. The low foam densities at low FRT values
(Figure 8) are caused by extensive foam collapse which, of
coarse, destroys separation efficiency. The destructive
effect of high sparging rates can be seen in Figure 7 where
the curve of decreasing VR with increasing gas rate changes
shape above 10 liters-min
These data indicate the operational restrictions for the 6 in,
dia. column. A minimum surfactant concentration is required
for foam production; under present conditions the minimum
can be expressed as 100 ppm-= [NaDS] f*= 200 ppm. Gas rates of
approximately 10 liters-min"1 or less are required with the
EC fritted glass sparging heads. With these restriction
foam densities in the range of 2 - 5 x 10 3 ml liquid per ml
32
-------
70
60
50
40
30
20
10-
9-
8
7-
6-
5-
4-
D
[NaDS]
f
o 100 ppm
D 200 ppm
a 300 ppm
A 400 ppm
10 15
G, liter • min -1
20
25
30
FIGURE 7
Volume Reduction versus Sparging Rate
(Simple Mode, 18 in. Vertical Section, 36 in. and 72 in. Horizontal Sections,
Lf = 500 ml • min"1)
33
-------
10-
9
8-
/-^
no 7-
i—i
- 6
6
o 5
g 4-
3-
•H
S 2-
0
p
Cj
[KADS]f
100 ppm
200 ppm
300 ppm
400 ppm
8
10
FRT, min
FIGURE 8
Foam Density versus Foam Residence Time
34
-------
foam can be easily obtained; lower values could result from
further development of drainage sections. However, since
drainage rate and extraction rate are opposite functions
of bubble diameter, a compromise must be made between
separation efficienty and foam density. For this study
the controlling factor is the narrow range of sparging
rates over which a stable foam can be produced, the
sparging head determining the upper limit and a minimum
gas flow to produce a stable -foam determining the lower
limit.
Effect of Metal Concentration on Coadsorptive Separation -
In the presence of dissolved electrolytes, the surface
tension and critical micelle concentration of ionic sur-
factants are reduced. Ions of charge opposite to the
surface active species tend to reduce repulsion between
surfactant ions and allow closer packing of surfactant
in the surface film. Thus, the presence of cations in
solutions of anionic surfactants, e.g., SAMD and NaDS,
should tend to increase j values. Experimental data
showing this effect is plotted in Figure 9; [~* values tend
to be greater in SAMD than in DSAMD. S
As shown in Figure 6, the decline in |, /[M], with in-
creasing [NaDS], indicates that the CMC is reached at [NaDS],
greater than approximately 6x10 M which is equivalent to
[NaDS]f greater than 7.5xlO~4M under the operating condition
shown. The reduction in CMC by dissolved salts is evident by
comparing these values to 7x10 M, the CMC for NaDS in pure
aqueous solution (4) .
Literature data for surface excess of NaDS in pure aqueous
solution as measured by the foaming method in included in
Figure 9 (4) (37) . This data suggests that l in SAMD
"1 "2
would be larger than SxlO" moles -cm" and possibly
larger than 4X10"1 moles -cm . Our data (Figures 6^ and
9) show that practical PNaDS values equal to 2-3x10"
moles • cm"2 are the equilibrium values for conditions in
the 6 in. column. As indicated above (Figure 6), the
foam column is most efficiently operated with a surfactant
concentration below the CMC and with the concentration
optimized for metal coadsorption under existing conditions.
Coadsorptive P values for total metals show the characteris
tic equilibrium relationship with increasing metal concen-
tration in the pool (Figure 9) . The total metal \ values
increase by a factor of approximately 10 while total metal
concentration in the pool increases by a factor of 700; sur
factant P values increase by a factor of 3 or less over
the same increase in metal concentration.
35
-------
x = NaDS in Dist. H?0 (37)
x = NaDS in SAMD
9 3'
x = NaDS in H20 (4)
Gi
S
o
•
01
0)
rH
O
E
1-
x = Mt SAMD
-H_g_JL
7 8 9
[X]b, M (xlO3)
FIGURE 9
Dynamic Surface Excess versus Pool Concentration
(For x = NaDS in SAMD: • is Dilute SAMD, • is SAMD]
-------
Optimum Operating Conditions for the 6 in.-Dia. Column -
The metal separation rate increases with higher metal feed
rates (Figure 10) as would be expected from P. data
"tin
(Figure 9); the separation rate increases by a factor less
than 5 while the metal feed rate increases by a factor of
about 200. Optimum surfactant feed concentrations were
found to have a narrow range over the wide range of total
metal concentrations in SAMD and dilute SAMD (Figure 6).
Thus, the range of surfactant feed rates (approximately
3.5 - 4,5x10 4 moles-min ) was considerably narrower
than the metal feed rates.
Date in Figure 10, which includes all simple and stripping
experiments with standard and dilute SAMD, shows that the
metal separation rates do not significantly differ as a'
function of operating mode and surfactant type (CD-128 and
EO-526 experiment are included). The fraction of total
metal removed depends strongly on the metal feed rate.
For standard SAMD, an average metal separation rate is
about 3.5xlO~5 moles-min" which is equivalent to 1.93x10"
moles-cm 2 of column_cross section area per minute or
approximately 4.0x10 equivalent cm 2-min . The corres-
ponding experimental rate for surfactant is 4.8x10"
equivalent cm" -min" . The difference between total metal
and surfactant capacities represents unaccounted metal
coadsorption such as from sodium derived from surfactant
and metal impurities.
Based on theoretical calculations (1) and empirical measure-
ments (4) the maximum exchange capacity for a foam separa- _
tion system would be approximately 3x10" equivalent cm" -min
based on an optimum bubble diameter of 0.08 cm, a maximum
gas rate of 9.4 cm3-min~1 per cm of column cross section
and a packing density of 40 sq-8 per NaDS molecule on the
bubble surface. Thi| theoretical maximum sparging rate is
equivalent to 1.7x10 cm -rain"1 for the 6 in.-dia. column
and is 2.1 times larger than the practical rate of 8x10
cm3-min which was restricted by foam instability at high
rates using the EC fritted glass sparging heads. The
average bubble diameter obtained in this study at gas rates
of 8xl03 cm -min was 0.13 cm and would reduce the surface
area throughput by a factor of 1.6.
The 6 in.-dia. column operates at a practical capacity of
16 percent of theoretical. Improvement in sparging head
design, possibly the use of precision drilled spinnerettes,
or change in the method of sparging, such as bubble
37
-------
OJ
00
1.0
.9
.8-
.7
.6
.4
.3
.2
•H
10-5
L [M,]f, moles-min
10
-i
-3
10
9
8
7
6
5
4
3
2
10
-2
c
•H
e
en
0)
rH
O
e
o
FIGURE 10
Total Metal Fraction Removed and Total Metal Extraction Rate Versus Total Metal Feed Rate.
-------
formation by degasification of supersaturated solutions,
may allow closer approach to optimum gas rates and bubble
diameters. Allowing these improvements, the capacity
would increase to 54 percent of theoretical.
Derived from the experimental results with SAMD, the opti-
mum conditions for practical operation of the 6 in.-dia.
column demonstrate the effectiveness of continuous foam
separation for treating acid mine drainage (Table 6).
Average values for VR, [NaDS],, [M,]f, SR and SR, de-
termined within the practical operating values for G and
L- in the 6 in.-dia. column were used to calculate the
average material balances for all SAMD experiments.
Approximately 6 percent metal removal with 5 percent
liquid carryover indicates that 1 percent metal is separa-
ted as coadsorbed metal. Twenty-six percent of the added
surfactant is removed in the foam, 23 percent is removed
as adsorbed material and 74 percent remains in the bottoms
stream.
TABLE 6
Summary of Results for SAMD Experiments
and Comparison with AMD Experiments
SAMD and NaDS
AMD and NaDS
(3 runs)
Column Mode
G, liter -min
L.., ml-min
VR
[NaDS],, _i
moles -liter
moles -liter"1
SR , moles -min"
s '
SRtm» moles"min~
[NaDS]f, _i
moles- liter
Simple and
Stripping
8
500
20
G.OxlO"4
7.0xlO~3
8.8xlO"5
3.5xlO"5
7.7xlO~4
Simple
4
250
27(11)
4
5.4x10
7.9xlO~3
4.3xlO~5
1.7xlO~5
7.5xlO~4
Simple
8
500
13
4.8xlO~4
7.5xlO~3
7.3xlO~5
4.3xlO~5
7.1xlO~4
Stripping
8
500
23
5.3xlO~4
7.9xlO~3
2.8xlO~5
1.8xlO~5
7.4xlO~4
39
-------
TABLE 6 (continued)
SAMD and NaDS
AMD and NaDS
(3 runs)
[NaDS]c,
moles • liter"
[Mt]b>
moles - liter
[Mt]c>
moles • liter
„ , . -i
Rf _ , moles -mm
R, , moles -min"
R , moles -min"
cs '
Rftm' m°les'min
R, . , moles -min"
R , , moles -min
Material Balance
Metal, % diff.
H20, % diff.
NaDS, % diff.
SRtm/SRs
SRs/Rfs.102
Rbs/Rfs-1()2
Rcs/Rfs-lo2
R , /R». -102
c tm i tm
SR, /R_,
tm f tm
4.1xlO~3
6.9xlO~3
8.3xlO~3
3.8xlO~4
2.8xlO~4
l.OxlO"4
3.5xlO~3
3.3xlO~3
2.0xlO~4
-
-
-
0.40
23%
74%
26%
6.0%
1.0%
5.2xlO~3
7.7xlO~3
9.6xlO~3
1.9xlO~4
1.2xlO~4
5.2xlO~5
2.0xlO~3
1.7x10"
8.9xlO~5
7
5
9
0.40
23%
63%
27%
4.5%
0.85%
2.4x10 3
7.4xlO~3
8.6xlO~3
3.5xlO~4
2.2xlO~4
9.0xlO~5
3.7xlO~3
3.4xlO~3
3.2xlO~4
1
0.4
13
0.59
21%
63%
26%
8.6%
1.2%
1.8xlO~3
7.8xlO~3
8.6xlO~3
3.7xlO~4
2.7xlO~4
4.0xlO~5
3.9xlO~3
4.0xlO~3
1.9xlO~4
8
6
16
0.62
7.7%
73%
11%
4.8%
0.45%
40
-------
SECTION VII
AMD EXPERIMENTS-RESULTS AND DISCUSSION
To verify the results with SAMD, AMD was foamed with NaDS
in the 6 in.-dia. column operating in both simple and
countercurrent modes. The AMD originated from Grassy Run
(Sample Site GT6-1) near Elkins, W. Va. and was trucked to
Cleveland; the tests began approximately 24 hours after
sampling. Analyses of the water was conducted approximately
30 hour's after sampling (Table 7) .
TABLE 7
Grassy Run Acid Mine Discharge (GT6-1)
Total Fe 112 ppm
Ca 114 ppm
Mg 32 ppm
Mn 3.8 ppm
Al 34 ppm
pH 2.75
Two simple mode experiments at different liquid feed and air
sparging rates were conducted on the second and third days
after delivery of the Grassy Run AMD. The results of these
experiments compare favorably with those using SAMD (Table 6).
Total metal removal was 8.6 and 4.5 percent with 1.2 and 0.85
percent separated as coadsorbed metal. Approximately 63
percent of the added surfactant remained in the bottoms stream.
The error in material balances was typical of SAMD experiments.
Separation efficiencies were essentially the same at the two
different feed and sparging rates.
An additional experiment in the countercurrent mode was
conducted on the fourth day after delivery of the acid
drainage. The results (Table 6) are consistent with pre-
vious tests except for the abnormally low measured surfactant
concentration and surface throughput in the foam. The
measured surface throughput of coadsorbed metal also appears
to be low. The surfactant balance between feed and bottom
41
-------
flows indicates that surfactant analysis of the foamate
was incorrect due most probably to surfactant loss onto
sampling containers at the high VR of 23. As with SAMD,
no advantage for operation in the stripping mode compared
to the simple mode was found with AMD. SAMD appears to
be a valid laboratory substitute for low pH, high Fe(II)
AMD, at least for foam separation studies.
42
-------
SECTION VIII
SURFACTANT REGENERATION
Surfactant represents the principle expense in foam separa-
tion of AMD and thus its recovery and reuse in a cyclic
pattern would be required for the process to be economically
competitive. As a preliminary investigation of surfactant
recovery, the addition of base to collapsed foam was tested
to determine if surfactant and hydrolyzable metals (Fe,
Al and Mn) could be separated by pH increase.
The hydrous oxide of Fe(III) begins to precipitate in weak
acidic solution of pH about 4; precipitation is generally
complete at or before pH 7. Hydrous aluminum oxide begins
to precipitate at pH 3 or higher and is completely precipi-
tated at or before pH 7; at about pH 9 resolution is evident.
Thus for initial tests, sufficient base was used to bring
the pH up to approximately 7.
Hydrous ferric and aluminum oxides tend to coprecipitate
cations at higher pH, which would be desirable in this case.
However, excess base would also be a cost factor. Copre-
cipitation of cationic species can be promoted by rapid
addition of base which promotes precipitation in local ex-
cess of hydroxide.
Experiments were conducted with the collapsed foam collected
from SAMD and AMD experiments. Standardized NaOH was added
to filtered and unfiltered foamate samples. The samples
(three replicates) were continuously stirred and pH was
continuously recorded. Once the precipitate was formed it
was allowed to settle briefly before supernatant aliquots
were taken for surfactant and metals analysis. Some ali-
quots were taken up to 3 days after precipitate formation to
determine if aging had any effect on supernatant composition.
Supernatant samples were either contrifuged or filtered
(0.45 u membrane filter) to remove solids before analysis.
Some foamate samples were filtered to determine the amount
and nature of solid material.
Tests results for SAMD and AMD, although different in detail,
generally agree (Table 8). From 41 to 62 weight percent
of the collapsed foam is solid surfactant. Fe, Al and Mn
are precipitated to a greater extent than Ca and Mg.
43
-------
TABLE 8
Recovery of NaDS from Collapsed Foam
by Addition of NaOH
NaDS Fe
Ca Mg Mn
(Units; ppm)
Al
pH
Foam from SAMP
Collapsed foam
Collapsed foam,
filtrate
42 Whatman
Supernatant
solution
(1)
Percent removed
filtration,
42 Whatman
Percent removed
filtration plus
precipitation
Foam from AMD
Collapsed foam
Collapsed foam,
filtrate
42 Whatman
Collapsed foam,
filtrate
0.45 membrane
487 209 85 28 8.5 27 2.3
286 206 70 25 8.2 25
88 0.75 53 8.3 0.03 0.33
41 1.4 6.2 9.0 3.5 10
82 99 38 70 >• 99 =» 98
2.3
8.0-8.5
510 136 123 35 3.8 46
194 99 99 35 3.8 43
194 102 99 35 3.8 44
2.7
2.7
2.7
(1)
Supernatant solution from addition of 0.150N NaOH to
filtered collapsed foam (42 Whatman)
44
-------
TABLE 8 (continued)
NaDS Fe Ca Mg Mn
(Units; ppm)
Al
pH
(2)
Supernatant
solution,
centrifuged
replicate 1
2
3
Supernatant
solution,
filtered (0.45)
replicate 1
2
3
Percent removed
filtration
42 Whatman
Percent removed
filtration
0.45 membrane
Percent removed
precipitation
265 0.10
270 0.20
265 0.15
265 0.12
265 <.l
265 -=.1
62 27
62 25
48 =-99
72
82
78
75
77
77
20
20
37
25
27
27
26
27
28
0
0
21
0.92 «= 1 7.2
1.2 -= 1 6.9
1.2 «= 1 7.0
0.85 •= 1 7.2
1.3 •= 1 6.9
1.2 «cl 7.0
0 6
0 4
71 >98
(2)
Supernatant solution from addition of 0.147N NaOH to
unfiltered collapsed foam
The solution resulting from base treatment of foamate con-
tains on the average 265 ppm NaDS, less than 2 ppm total
Fe, Mn and Al, 77 ppm Ca and 27 ppm Mg. This represents
52 percent recovery of surfactant present in the foamate
with the removal of =» 99 percent iron, =-98 percent Al and
71 percent Mn.
No noticeable difference in separation occurred between rapid
(14 min.) or slow (90 min.) base titratlon of foamate.
45
-------
Supernatant pH values tended to drop slightly during the
first two hours after which they remained constant. For
example, with the AMD foamate samples (Table 8) the pH
of the first replicate dropped from 7.2 to 6.3, the
second replicate from 6.9 to 6.2 and the third replicate
from 7.0 to 6.3 all within two hours; 24 hours later the
pH had not changed.
Calculations based on the foamate from the AMD experiments
indicate an average of 1.5xlO~ moles NaOH are required to
precipitate (at pH 7) the metals from a liter of foamate.
Titration curves indicate that possibly 9.4x10 moles
NaOH may be sufficient at pH 4-5.
46
-------
SECTION IX
ECONOMIC EVALUATION AND SUMMARY
The effectivness of foam separation for the extraction of
metals from AMD is low (Figure 10). Approximately 6 per-
cent of the metals are removed per pass under the practical
operating limits of the 6 in.-dia. column; of this 6 percent
approximately 1 percent is removed as coadsorbed metal, the
remainder as interstitial liquid in concentration equal to
that in the pool. Minimum surfactant concentration is de-
termined by that required to produce a foam of sufficient
persistence to remain intact during protracted drainage
which is necessary to reduce foam density and liquid carry-
over in the tops. The inherent low foamability of AMD re-
quires a surfactant concentration of about 200 ppm, the
majority of which remains in the bottoms.
With surfactant concentration equal to 265 ppm in the super-
natant liquid after the surfactant regeneration step, re-
usable surfactant amounts to 6 percent of-that in the feed,
dissolved in liquid equal to approximately 5 percent of the
feed stream. Loss of surfactant, 74 mole percent to the
bottoms and 20 mole percent to the sludge, represents a
major technical and economic obstacle to efficient operation.
Chemical and air process costs can be estimated based on
the flow scheme presented in Figure 11. A ratio of 1.90
moles surfactant used per mole total metal removed leads to
a surfactant cost of 235 dollars per Ib-mole metal removed
assuming 0.43 dollars per Ib surfactant and no surfactant
recovery and reuse. This is equivalent to 0.78 dollars per
1000 gal. per foam separation stage which removes approxi-
mately 6 mole percent total AMD metals in the ratio 1 mole
Fe:0.61 mole Ca:0.32 mole Mg:0.05 mole Mn:0.27 mole Al.
To reduce the Fe concentration to 10 ppm, assuming an initial
Fe concentration of 200 ppm as in SAMD, would require 19
process stages at a cost of 0.20 dollars per stage per 1000
gal. for each of the first 18 stages plus 0.78 dollars for
the last stage giving total cost of 4.50 dollars per 1000
gal. for surfactant. This calculation is based on no sur-
factant recovery from the overheads but with make-up sur-
factant added to each successive stage. This calculation
assumes no significant reduction in metal extraction rate
at reduced metal concentration and thus is only an esti-
mate (Figure 10).
At a sparging air-to-liquid feed volumetric ratio of 16,
the cost of air would range between 0.04 and 0.12 dollars
per 1000 gal. per stage depending on power costs, or be-
tween 0.76 and 2.28 dollars for 19 stages.
47
-------
so
Bottoms
Rbg=2.8xlO
Rbtm=3'3xl(
Lb=0.95 Lf
A
3
Tops
R =1.0x10
cs
R , =2.0x10
ctm
L =0.05 Lf
O J-
~4
~4
Foam
Separa-
tion
-4
Feed
Rf =3.8x10
Rftnr3-5xl°
Lf=500 ml-min
-4
-i
Gas
G=8 liter-min
Surfactant
Recycle
Surfactant
Regeneration
Base (NaOH)
R =1.5xlO~2-L
u c
moles-min"1
Rrtm=7-7xl°"
L =L
r c
Sludge
R =7.7x10
ws
R . =1.2x10
wtm
~5
~4
FIGURE 11
Process Flow Scheme for Foam Separation
of Metals and Surfactant Regeneration
-------
It is evident from these cost figures that the inherent low
extraction capacity of foam fractionation disallows economic
feasibility for treatment of waters as concentrated as AMD.
Air costs for foaming are comparatively low since the process
operates near ambient pressure with little loss. Even so
the air costs alone would be prohibitive.
Six mole percent of the total surfactant (23 mole percent
of surfactant in the overheads) can be recovered for reuse
at a cost of 0.06 dollars per 1000 gal. per stage based on
a NaOH cost of 0.25 dollars per Ib. This degree of re-
covery has no economic advantage.
Reduction in surfactant costs may be effected by operation
with less surfactant and recovery of a greater percentage
of surfactant. This research program investigated the
extraction of dissolved metals from AMD by foam separation
under conditions which allowed production of persistent
foams and protracted foam drainage. Operating with persis-
tent foams requires surfactant concentrations and sparging
rates large enough to support a stable foam without sur-
passing the critical micelle concentration or allowing air
velocities high enough to cause foam rupture. At lower sur-
factant concentrations and sparging rates a transient foam
regime is generally produced in which foam drainage and
collapse are rapid but controllable. Extraction in a transient
foam regime must be lower than for a persistent foam regime
due to collapse of surface film and loss of adsorbed material
to interstitial liquid followed by drainage back to the pool.
Reduction in extraction efficiency could be offset by reduc-
tion in surfactant requirements. An optimum sparging rate
appears to exist for maximum extraction in a transient foam
regime (15); the exact rate being a complex function of
chemical and physical parameters must be determined em-
pirically. The maximum occurs at a sparging rate such that
the foam drainage rate is significantly greater than the
rate of foam collapse (surface loss). This would tend to
minimize liquid carryover while maintaining a reasonable
metal extraction rate. Such a transient foam system would
be characterized by low foam residence times.
A method for surfactant recovery which appears feasible is
solvent extraction of surfactant from aqueous solution
followed by distillation or spray drying to concentrate
surfactant. Such a system has been mentioned (36) but no
information on its effectiveness was reported. Surfactant
loss to the bottoms flow could be reduced by transient
foaming or bubble fractionation, but again air costs would
be prohibitive for the treatment of concentrated AMD.
49
-------
Economic feasibility of foam separation for AMD could be
improved with the use of waste air and surfactant. A major
source of waste surfactant is sewage, which suggests the
waste-plus-waste approach for the synergistic treatment of
AMD with alkaline municipal sewage. Studies of the auto-
purification of AMD and treated sewage effluent have
achieved reductions of 80-95 percent iron, 94-98 percent
phosphate, 60-76 percent surfactant, 3-63 percent COD and
70-84 percent acidity in the combined wastes (23) (38).
The process consisted of aeration and settling unit
operations.
The feasibility of foam separation for sewage treatment
has been investigated with the general conclusion that
the foamability of sewage is too low since the change from
alkyl benzene sulfonate (ABS) to linear alkylate sulfonate
(LAS) detergents. Recent research into foam separation of
primary and secondary sewage effluents utilizing a combina-
tion of foam fractionation and froth flotation has achieved
reductions of 70 percent COD, 90 percent phosphate and 40-80
percent suspended solids (27). Air bubbles for particulate
flotation and adsorptive extraction were produced by spon-
taneous degasification of air supersaturated sewage upon
pressure release. The best results were achieved with the
addition of 300-400 ppm ferric chloride or alum as coagulants;
ferrous sulfate was less successful as a coagulant.
These two independent lines of research suggest that if mix-
tures of AMD and sewage were foamed by degasification, a
process which produces fine bubbles capable of maintaining
a transient foam in sewage, a significant reduction in COD,
phosphate, AMD metals such as Fe and Al, suspended solids,
and surfactant could result in addition to neutralization
of the wastes. Waste constituents in this type of system
are removed non-stoichiometrically with surfactant which
tends to reduce treatment costs compared to foam separation
of only dissolved, non-surface active constituents of AMD.
Preliminary costs for high pressure foaming of sewage along
appear promising (27).
50
-------
SECTION X
ACKNOWLEDGEMENTS
The research program was conducted under the direction of
Dr. Peter J. Hason. Construction and operation of the foaming
apparatus and analytical work were performed by Messrs. Paul H. Hartman
and Jerome H. Jacobs. The report was prepared by Dr. Hanson.
A significant objective of this project was to investigate practical
means of abating mine drainage pollution. Such research projects,
intended to assist in the prevention of pollution of water by
industry, are required by Section 6 b of the Water Pollution Control
Act, as amended. This project of EPA was conducted under the
direction of the Pollution Control Analysis Section, Ernst P. Hall,
Chief, Dr. James M. Shackelford, Project Manager, and Ronald D. Hill,
Project Officer.
51
-------
SECTION XI
REFERENCES
1. Wace, P. F. , P. J. Alder and D. L. Banfield, 1968, Foam
Separation Process Design, In:Unusual Methods of
Separation, Chem. Eng. Progress Symposium Series
(A. I. Ch. E.) 65:19-28.
2. Weinstock, J. J., et al., 1963, Fission Product Separa-
tion by Foam Extraction, (Radiation Applications Inc.,
Long Island City, N. Y., NYO-10038) 123 p.
3. Schonfeld, E. and A. H. Kibbey, 1967, Improving Strontium
Removal from Solution by Controlled Reflux Foam Separa-
tion, Nucl. Appl. 3:353-359.
4. Kishimoto, H., 1963, The Foam-Separation of Surface-Active
Substances, Part I, Fundamental Treatments, Kolloid-
Zeitschrift and Zeitschrift fur Polymer, 192:66-101.
5. Newson, I. H., 1966, Foam Separation:The Principles
Governing Surfactant Transfer in a Continuous Foam Column,
J. Appl. Chem. 16:43-49.
6. Rubin, E., C. R. LaMantia and E. L. Gadan, Jr., 1967,
Properties of Dynamic Foam Columns, Chem. Eng. Sci.
22:1117-1125.
7. Grieves, R. B., 1968, Studies on the Foam Separation Process,
British Chem. Eng. 13:77-82.
8. Rubin, A. J. , J. D. Johnson and J. C. Lamb III, 1966,
Comparison of Variables in Ion and Precipitate Flotation,
Ind. Eng. Chem. Process Design Develop. 5:368-375.
9. Bauer, D. J. , 1962, Foam Concentration of Scandium, (U. S.
Dept. Interior, Bureau Mines, Wash., D. C., RI-5942) 15 p.
10. Rubin, A. J. and W. L. Lapp, 1969, Foam Separation of
Lead (II) with Sodium Lauryl Sulfate, Anal. Chem. 41:1133-1135,
11. Rubin, A. J., 1968, Removal of Trace Metals by Foam Separa-
tion Processes, J. Am. Water Works Assoc. 60:832-846.
12. Wace, P. F. and D. L. Banfield, 1966, Foam Separation,
Chem. and Process Eng. 47:70-76 and 90.
13. Fineman, M. N., et al., 1952, Foaming of Non-Ionic Surface
Active Agents, J. Phys. Chem. 56:963-966.
53
-------
14. Saturnine, F. and R. Lemlich, 1965, Predicting the Per-
mance of Foam Fractionation Columns, A. I. Ch. E.-I.
Chem. E. Symposium Series 9:75-78.
15. Robertson, G. H. and T. Vermeulen, 1969, Foam Fractiona-
tion of Rare-Earth Elements (California Univ.,
Berkeley, Lawrence Radiation Lab., UCRL-19525) 138 p.
16. Grieves, R. B., 1966, Foam Separation for the Treatment of
Low-Quality Waters (Illinois Institute of Tech., Chi-
cago, AD 478452L) 24 p.
17. Wallace, G. T. and D. F. Wilson, 1969, Foam Separation
as a Tool in Chemical Oceanography (Naval Research Lab.,
Wash. D. C., NRL-6958) 17 p.
18. Weinstock, J. J., 1965, The Removal of Metallic Ions by
Foaming Agents and Suspensions:Laboratory and Engineering
Studies, Annual Report, July 1, 1963-June 30, 1964
(Radiation Applications Inc., Long Island City, N. Y. ,
RAI-350) 41 p.
19. Davis, W. , Jr., £t a_l. , 1965, Laboratory Demonstration
of the Two-Step Process for Decontaminating Low-Radio-
activity-Level Process Waste Water by Scavenging-Pre-
cipitation and Foam Separation (Oak Ridge National Lab.,
Tenn., ORNL-3811) 32 p.
20. Bikerman, J. J. , et, a_l. , 1970, Treatment of Acid Mine
Drainage, (Horizons Inc., Cleveland, Ohio, Water Pollution
Control Research Series 14010 DEE, FWQA) 88 p.
21. Hass, P. A., 1965, Engineering Development of a Foam
Column for Counter-Current Surface-Liquid Extraction of
Surface-Active Solutes (Oak Ridge National Lab., Tenn.,
ORNL-3527) 245 p.
22. Banfield, D. L. and I. H. Newson, 1967, Improvements in
or Relating to Foam Deionization Apparatus, British
Pat. No. 1,062,346.
23. Streeter, R. C. and D. C. McLean, 1966, A Study of the
Interactions and Foam Fractionation of Sewage Effluent-
Acid Mine Drainage Mixtures (Pennsylvania State Univer-
sity, Water Resources Research Publication No. 6-66) 51 p.
24. Rubin, E. , et a.1. , 1963, Contaminant Removal from Sewage
Plant Effluents by Foaming, (Radiation Applications, Inc.,
Long Island City, New York, USPHS Publ. 999-WP-5) 56 p.
25. Brunner, C. A. and D. G. Stephan, 1965, Foam Fractionation,
Ind. Eng. Chem. 57:40-48.
54
-------
26. King. L. J. , ^t al. , 1968, Pilot Plant Studies of the
Decontamination of Low-Level Process Waste by a
Scavenging-Precipitation Foam Separation Process (Oak
Ridge National Lab., Tenn., ORNL-3803) 57 p.
27. Miller, J. P. K., et al., 1970, Investigation of a
High-Pressure FoanflVastewater Treatment Process (Garrett
Research and Development Inc., LaVerne, California,
EPA, Water Pollution Control Research Series 17020) 39 p.
28. Schoen, H. M. , 1966, Foam Separation as a Purification
and Preparative Tool, Annals New York Acad. Sci.
137:148-161.
29. Lamlich, R., 1968, Principles of Foam Fractionation,
In:Progress in Separation and Purification, E. S. Perry
(ed), Interscience, New York, p. 1-56.
30. Lemlich, R., 1968, Adsorptive Bubble Separation Methods,
Ind. Eng. Chem. 60:16-29.
31. Perkin-Elmer Corp., 1968, Analytical Methods for Atomic
Absorption Spectrophotometry, Norwalk, Conn., looseleaf.
32. Hedrick, C. E. and B. A. Berger, 1966, Extraction of Anions
Using Triphenylmethane Dyes, Anal. Chem. 38:791-793.
33. Lee, G. F. and W. Stumm, 1960, Determination of Ferrous
Iron in the Presence of Ferric Iron with Bathophenantb.ro-
line, Jour. AWWA 52:1567-1574.
34. Chang, R. C., H. M. Schoen and C. S. Grove, Jr., 1956,
Bubble Size and Bubble Size Distribution, Ind. Eng.
Chem. 48:2035-2039.
35. Banfield, D. L., et al., 1966, Surface Excess in Solutions
of Surface ActiveTgents, I:A Comparison of Static and
Dynamic Results (United Kingdom Atomic Energy Authority,
AERE-R-5124) 30 p.
36. Grieves, R. B. , et al. , 1969, Optimization of the Ion
Flotation of Dichromate, Jour. Sanit. Eng. Div. Am. Soc.
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of the Autopurification of Sewage Effluent-Acid Mine
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Publication No. 55) 37 p.
55
-------
SECTION XII
NOTATIONS AND GLOSSARY
AMD acid mine drainage
SAMD synthetic acid mine drainage
CMC critical micelle concentration
d average bubble diameter
FRT foam residence time, min
L volumetric liquid rate, ml-min~
G volumetric air sparging rate, liter-min
M metals in mine drainage, Fe, Ca, Mg, Mn and Al
R material mass rate, moles-min
S surfactant
SR surface material mass rate, moles-min~
VR volume reduction (Lf/L )
-L O
[ ] concentration
\ dynamic surface excess, moles-cm
lx/[X] distribution ratio, cm
Subscripts
b refers to liquid pool or bottom flow
c refers to overhead, foamate or tops flow
f refers to feed flow
r refers to recycle of regenerated surfactant
s refers to surfactant
t refers to total, e.g., [Mt]f refers to total
metal concentration in the feed flow
-------
Subscripts
tm refers to total AMD or SAMD metals, e.g.,
R.P. refers to feed rate of total metal
± tm
u refers to base in surfactant regeneration
w refers to sludge in surfactant regeneration
x refers to any variable denoted in text
Adsorption - The concentration of a surface active component
of a solution at a phase boundary. In the case of foam
separation, the adsorption of surfactant at the gas-liqiiid
interface.
Chelation - An equilibrium reaction between a metal ion and
a complexing agent, characterized by formation of multiple
bonds between the metal and a molecule of complexing agent
and resulting in the formation of a ring structure incorpora-
ting the metal ion (see p. 21 and 27).
Coadsorption - The concentration of a surface inactive com-
ponent of a solution at a phase boundary as a result of its
association with a surface active component of the solution.
In the case of foam separation, the coadsorption of metal
ions with adsorbed surfactant at the gas-liquid interface.
Critical Micelle Concentration - The surfactant concentra-
tion at which significant aggregation of surfactant molecules
into micelles begins.
Dynamic Surface Excess, I - Defined by equation (2), p. 23.
The surface excess measured in a flowing foam column (see
p. 22-23).
Exchange Capacity - The theoretical maximum time-rate of
material removal per unit cross-sectional area of foam
column. (See p. 37-39 for discussion and derivation.)
Extraction Rate - The time-rate of adsorbed or coadsorbed
material being removed from solution by the foam. (See
p. 37-40 and H.gure 10. Extraction rate and separation rate
used synonymously.)
Foam Coalescence - The rupture of bubble walls resulting in
foam collapse and a change from a two phase ;liquid-gas
system to a single liquid phase.
58
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Foam Density - The ratio of the gas-free volume of liquid
contained in a foam, to the volume of the original foam.
Micelles. - Aggregates of about a hundred monomer surfactant
units of roughly spherical shape which form as surfactant
concentration is increased. The change from monomeric to
polymeric units marks the boundary between true and col-
loidal solutions.
Surface Excess, |) - Defined by Gibbs model, equation (1),
p. 6. The surf acre excess of solvent is zero. Thus, a
solute £ with a positive surface excess means that the
liquid containing a unit area of surface contains ["/ more
of solute than the volume of bulk liquid which con-
tains the same amount of solvent.
Surfactant - An acronym for "surface active agent". Often
characterized by a linear molecular structure which is com-
posed of solvent compatible radicals at one end and solvent
incompatible radicals at the other end, these solutes in-
variably reduce the surface energy of their solvents by
collecting at an interface. (See p. 6, 21, and 22 for
further discussion of surfactants as they apply to this
study.
Synthetic Acid Mine Drainage, SAMP -
CaS04-2H20
MnS04-H20
A12(S04)3-18H20
FeS04-7H20
MgS04-7H2O
0.344 g
0.024 g
0.186 g
0.997 g
0.246 g )
Dissolved in one
liter of 0.01 N H2S04
59
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Accession Number
Subject Field & Group
SELECTED WATER RESOURCES ABSTRACTS
INPUT TRANSACTION FORM
Organization
HORIZONS INCORPORATED, Cleveland, Ohio
Title
FOAM SEPARATION OE ACID MINE DRAINAGE
1 Q
Authors)
Hanson, Peter J.
16
Project Designation
EPA Project 14010 FUI, Contract 14-12-876
21
Note
22
Citation
Water Pollution Control Research Series, 14010 FUI 10/71
Environmental Protection Agency, Washington, D. C.
23
Descriptors (Starred First)
Acid Mine Drainage, Foam Separation, Mine Drainage Treatment,
* * *
Surfactants, Iron, Economics, Metal Cations, Surfactant
Regeneration
25
Identifiers (Starred First)
27
-4t>s(rac(Laboratory studies of continuous flow foam separation in a persis-
foam regime were conducted to determine the optimum operating conditions
for maximum extraction of dissolved metal cations (Fe, Ca, Mg, Mn and Al)
from acid mine drainage. Foaming experiments were conducted in a 6 in.-dia-
meter glass column capable of liquid flow rates of 3-12 gal. per hour. The
effects of pH, chelate addition, surfactant type and concentration, air
sparging rate, metal concentration and foam drainage were investigated in
relation to metal extraction.
The average extraction rate obtained was 1.9xlO~ moles total metal per
cm column cross-section area per minute which is approximately 4.0xlO~7
equivalents per cm2 per minute. Operation in simple and countercurrent
foaming modes produced similar extraction rates for acid mine drainage.
The low extraction capacity of foam fractionation renders the process
economically unfeasible for the treatment of acid mine drainage. The prin-
cipal chemical cost is for surfactant followed by air.
Surfactant regeneration from foam by addition of base was investigated
for surfactant reuse and cost reduction. (Hanson-Horizons)
Abstractor
Peter J. Hanson
Horizons Incorporated, Cleveland, Ohio 44122
WR:I02 (REV. JULY 1969)
WRSI C
SEND TO: WATER RESOURCES SCIENTIFIC INFORMATION CENTER
U.S. DEPARTMENT OF THE INTERIOR
WASHINGTON, D. C. 20240
» CPO: 1969-359-339
For sale by the Superintendent of Documents, U.S. Government Printing Office
Washington, B.C., 20402 - Price 65 cents
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