WATER POLLUTION CONTROL RESEARCH SERIES + 12010 EQF 03/71
An Electromembrane Process
for Regenerating Acid
from Spent Pickle Liquor
ENVIRONMENTAL, PROTECTION AGENCY • WATER QUALITY OFFICE
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WATER POLLUTION CONTROL RESEARCH SERIES
The Water Pollution Control Research Series describes the
results and progress in the control and abatement of pollu-
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Reports should be directed to the Head, Project Reports
System, Office of Research and Development, Water Quality
Office, Environmental Protection Agency, Washington, D.C. 20242.
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AN ELECTROMEMBRANE PROCESS FOR REGENERATING
ACID FROM SPENT PICKLE LIQUOR
Submitted to
ENVIRONMENTAL PROTECTION AGENCY
WATER QUALITY OFFICE
through the
ALABAMA WATER IMPROVEMENT COMMISSION
Southern Research Institute
Birmingham, Alabama
Project Number 12010 EQF
March 1971
For sale by the Superintendent of Documents, U.S. Government Printing Office, Washington, D.C. 20102 - Price $1
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WQO Review Notice
This report has been reviewed by the Environmental
Protection Agency Water Quality Office and approved for
publication. Approval does not signify that the contents
necessarily reflect the views and policies of the Environ-
mental Protection Agency Water Quality Office, nor does
mention of trade names or commercial products constitute
endorsement or recommendation for use.
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ABSTRACT
Studies of an electromembrane process for regenerating
acid from spent sulfuric acid pickle liquor have indicated
that the process is technically feasible. The studies
have shown that the iron ions in spent pickle liquor can
be removed and replaced by hydrogen ions to regenerate
in electromembrane cells.
A method of removing iron from spent liquor that involves
the formation of insoluble iron hydroxides is preferable
to plating iron metal onto cathodes.
Estimated treatment costs were $0.045 ± 0.002 per gallon,
whereas the combined costs of purchasing acid and dispos-
ing of spent liquor by existing methods were in the range
of $0.015 to $0.06 per gallon of spent liquor.
A determination of the long-term performance of the ion-
exchange membranes when treating actual pickle liquors
that contain organic pickling aids is needed.
This report was submitted in fulfillment of Project No.
12010 EQF under the partial sponsorship of the Water
Quality Office.
Key Words: Industrial wastes, spent pickle liquor,
pickling of steel/ acid regeneration, ferrous
sulfate, membrane processes, treatment costs,
ion-exchange membranes, electrolytic cells.
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CONTENTS
Section Page
I Conclusions 1
II Recommendations 3
III Introduction 5
IV Summary 7
V Experimental 11
A. Preliminary Studies 11
1. Membrane properties and selection
of membranes 11
2. Studies of methods of conversion
of iron to a solid form for
removal 21
3. Studies of voltage, current,
temperature relationships 28
4. Discussion 29
B. Bench-Scale Experiments 29
VI Continuous Regeneration Process 55
VII Estimates of Costs 63
Acknowledgments 71
Appendix 73
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FIGURES
PAGE
1 DIAGRAM OF CELL AND AUXILIARY EQUIPMENT
USED TO DETERMINE TRANSFERENCE NUMBERS
OF H+ AND Fe++ IONS 13
2 RELATIONSHIP BETWEEN CELL VOLTAGE AND
CURRENT DENSITY AT 55°C AND 65°C 30
3 DIAGRAM OF FLOWS IN BATCH-RECYCLE
REGENERATION PROCESS 33
4 DIAGRAM OF THREE-COMPARTMENT CELL USED
IN STUDIES IN BATCH-RECYCLE REGENERATION
PROCESS 36
5 HYDRAULIC FLOWS IN EXPERIMENTAL
REGENERATION UNIT 37
6 VARIATIONS OF AVERAGE COULOMB EFFICIENCY
AND AVERAGE CURRENT DENSITY WITH CELL
VOLTAGE 41
7 VARIATION OF COULOMB EFFICIENCY AND CURRENT
DENSITY WITH TIME 43
8 VARIATIONS IN CURRENT DENSITY WITH TIME 46
9 EFFECTS OF CHANGES IN OPERATING CONDITIONS
ON CURRENT DENSITY 49
10 FLOW DIAGRAM OF CONTINUOUS REGENERATION
PROCESS 56
11 VARIATION OF THE TRANSPORT NUMBER OF H+ IONS
THROUGH MA 3148 ANION-EXCHANGE MEMBRANE WITH
THE CONCENTRATION OF HaSO., IN THE ANOLYTE
SOLUTION 60
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TABLES
PAGE
I Transport Properties of Ion-Exchange
Membranes Considered for Use in the
Process 16
II Stability of Ion-Exchange Membranes in
1 N HzSOi* Solutions at Elevated Temper-
atures 19
III Removal of Iron from Solutions of Ferrous
Sulfate by Electrolysis and Precipitation
of Insoluble Iron Hydroxides 26
IV Coulomb Efficiencies and Average Current
Densities with Different Cell Voltages 40
V Performance of Experimental Cell with
Synthetic Solutions and with Actual Spent
Pickle Liquor 52
VI Capital Costs for Electromembrane Acid-
Regeneration Processes for Treating Spent
Pickle Liquor 65
VII Total Costs for Electromembrane Acid-
Regeneration Processes for Treating Spent
Pickle Liquor 66
VIII Total Costs for an Electromembrane Acid-
Regeneration Process 67
IX Operating Costs for an Electromembrane
Acid-Regeneration Process 68
X Estimated Capital Costs for Acid-
Regeneration Process 78
XI Operating Costs for Electromembrane
Acid-Regeneration Process 79
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AN ELECTROMEMBRANE PROCESS FOR REGENERATING
ACID FROM SPENT PICKLE LIQUOR
I. CONCLUSIONS
The main conclusions from this study are:
1. Either a batch-recycle or a continuous electro-
membrane acid-regeneration process appears to
be technically feasible on the basis of the
relatively short-term experiments performed so
far. One of the major questions remaining is
the long-term behavior of the membranes used
in the process. (The longest term of service
of any set of membranes, so far, is 24 hr.)
(See Section VI.)
2. The cost estimates indicate the operating costs
for the electromembrane acid-regeneration process
should be competitive with all but the lowest
cost processes for disposing of spent pickle
liquor. Many plants cannot use the lowest cost
existing processes (such as deep-well injection,
or direct discharge to sewers after neutralization)
Moreover, many of the existing processes (such as
neutralization with limestone plus lagooning)
produce residues that are not entirely satis-
factory for discharge to receiving streams. The
only material to be disposed of from the electro-
membrane process will be hydrous iron oxides (22
to 26% moisture) that are no more harmful to the
environment than naturally occurring iron ores.
These solids can be disposed of by low-cost
earth-fill methods. (See Section 7.)
Other conclusions are:
3. Service lifetimes of MC 3142 and MA 3148 membranes*
could be as long as 2 yr (based on the data of
Kramer1 and Forgacs2). (pp. 17 through 20.)
*Ionac Chemical Company, Birmingham, New Jersey,
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4. The method of removing iron from spent pickle
liquor that involves the formation of insoluble
iron hydroxides is more practical for use than
the method that involves plating of iron onto
cathodes. (pp. 21 through 24.)
5. The resistance of unit cells decreases with in-
creasing temperature, and it will be important
to operate the process at as high a temperature
as possible to minimize energy costs. (pp. 28
through 30.)
6. The probable maximum temperature of operation
is 70°C, since there is evidence that the glass-
transition temperatures of MC 3142 and MA 3148
membranes are only slightly higher than 70°C
and ion-exchange membranes lose their desirable
properties when the glass-transition temperature
is exceeded. (pp. 17 through 20.)
7. It should be possible to achieve coulomb
efficiencies of at least 50% when the process
is operated on a large scale. Coulomb effi-
ciencies from 50% to 70% were achieved in the
laboratory, (p. 52.)
8. The specific permselectivity of H+ and Fe+2 ions
through MC 3142 cation-exchange membranes is
1.7 ± 0.4. The specific permselectivity is a
convenient parameter for use in designing con-
tinuous processes. (pp. 57 through 60.)
9. For full-scale acid-regeneration processes, it
should be possible to achieve average current
densities of 48 to 55 mA/cm2 with cell voltages
of 5 V and temperatures of 65° to 70°C. (pp. 30
and 52.)
10. The insoluble iron hydroxides formed in the
electromembrane process are easily separated
from the NazSCU solution. They settled to 50%
of the total volume in 30 min and to 25% of the
volume in 60 min. (p. 44.)
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II. RECOMMENDATIONS
On the basis of the foregoing conclusions it is recom-
mended that research on the electromembrane process for
regenerating usable pickling acid from spent pickle
liquor be continued to study the long-term behavior of
the cell and membranes on a pilot-plant scale. For the
most effective studies, the pilot plant should be
located at one of the industrial sponsors plants and
should operate on the actual pickle liquor produced at
the plant. Simultaneously, studies in the bench-scale
cell would be performed to study the effects on cell
performance of particular components in actual pickle
liquors from other sources including the liquors from
other industrial sponsors.
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III. INTRODUCTION
This report covers the research performed to study an
electromembrane process for treating spent sulfuric acid
pickle liquor to produce usable acid. The objectives of
the research were to determine the technical feasibility
of an electromembrane process for regenerating usable
sulfuric acid from spent pickle liquor, to develop
engineering data for the design of larger units, and to
prepare cost estimates based on the experimental findings
to indicate the economic feasibility of the process.
The rationale of the research program was based on the
following considerations:
In the pickling of steel products with HaSCU, iron oxides
are dissolved from the surfaces of the products and I^SO.,
is converted to FeSCU. The pickling solution is normally
used until its content of FeSCH is increased and its con-
tent of HaSOi, is decreased to levels at which efficient
dissolution of additional iron oxides from the surfaces
of the products is no longer possible. The pickling
solution is then considered to be "spent". The pickling
rates achievable with 10% to 15% sulfuric acid solutions
(the usual range of concentrations in fresh pickling acid)
become slower as iron is dissolved and become unacceptably
slow when the ferrous sulfate concentrations reach the
range of 15% to 20% with corresponding remaining acid
concentrations of 2 to 5%.
If usable pickling acid is to be regenerated from spent
pickling liquor, iron ions must be removed from the liquor
and replaced by hydrogen ions. The iron ions can be re-
placed by hydrogen ions by an electromembrane process.
Removal of the dissolved iron can then be facilitated
by conversion to a solid form that can be readily
separated from solutions. Two promising ways for
accomplishing this in an electromembrane cell were
studied. One way was to plate the iron on the cathode
in a loosely adherent form; the other was to convert the
dissolved iron to iron hydroxides that could be separated
from solution by filtration or settling. Therefore, an
electromembrane process seemed to offer considerable
promise for treating spent pickle liquor to regenerate
usable pickling acid and to remove the iron in a solid
form that might be used or sold, or, at least, present
no serious disposal problems.
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IV. SUMMARY
The studies described in this report indicate that
either a batch-recycle or a continuous method of acid-
regeneration with electromembrane cells is technically
feasible and that the operating costs should be competitive
with all but the lowest cost existing methods of disposal
of spent pickle liquor. The most serious remaining
question about the operation of the electromembrane pro-
cess is the long-term performance of the cell when treat-
ing actual pickle liquors that contain organic pickling
aids. Only one actual pickle liquor was studied in this
program. Further research to study the behavior of
large cells during long periods of operation with actual
pickle liquors is needed.
The results of studies of ten commercially available
membranes indicated MC 3142 cation-exchange and MA 3148
anion-exchange membranes,* both products of the lonac
Chemical Company, have the best physical and electro-
chemical properties of the membranes studied for the
proposed use. These studies suggest that these mem-
branes might have useful service lifetimes of 2 years,
or more, under the expected conditions of service and
that 70°C would probably be the maximum permissible
operating temperature because the transport properties
of both the MC 3142 and MA 3148 are seriously degraded
above that temperature.
*We have used the terms cation- and anion-exchange mem-
branes throughout this report in preference to other
nomenclature found in the literature (e_.g_. cationic
membranes, cation-selective membranes), since a
standard reference book, "Ion Exchange" by F. Helferrich
(McGraw-Hill), uses cation-exchange in preference to
other terms.
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Studies of plating iron from iron sulfate solutions were
made under a variety of operating conditions (current
densities from 25 to 100 mA/cm2, temperatures from 50°C
to 80°C, and with the pH of the solution at 2.0 and 3.6).
The results of these studies indicate that it will be
difficult to deposit the iron on cathode materials
(seven different materials were tried) in a loosely
adherent form so that it can be easily and cheaply
removed. The results of other studies indicate that
the iron from spent pickle liquor can be converted to
readily separable insoluble iron hydroxides in an
electromembrane cell and that H+ ions can be generated
at the anode of the same cell simultaneously to replace
the iron ions that were removed from the pickle liquor.
These results indicate that the preferred method of
removing iron from spent pickle liquor is the method
that involves the formation of iron hydroxides.
Data from further studies of the electromembrane process
in which hydrous iron oxides are formed and removed by
filtration or settling show that coulomb efficiencies
for acid production in the anolyte compartment of three-
compartment unit cells can reasonably be expected to be
50%, or higher (coulomb efficiencies from 50% to 70% were
obtained in the experiments).
Either a batch-recycle method, which is described on
pages to , or a continuous method, which is de-
scribed on pages to , appear to be technically
feasible. For either method, three-compartment cells
formed by an anode, an anion-exchange membrane, a
cation-exchange membrane, and a cathode would be used.
For the batch-recycle method, the spent pickle liquor
would be circulated from a storage tank through the
center compartment (bounded by the two membranes) from
which iron ions would transfer to the catholyte compart-
ment and sulfate ions to the anolyte compartment. A
solution of NazSOi* (about 7% by weight) would be circu-
lated through the catholyte compartment, where Na+ ions
would be discharged at the cathode to form sodium metal
which would react with water to form NaOH. The OH" ions
would react with the iron ions transferred through the
cation-exchange membrane to form insoluble iron hydroxides,
which would be filtered from the NaaSOi, solution before
recirculation. A solution from which the iron had been
removed during a previous time cycle would be circulated
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through the anolyte compartment where the sulfate ions
entering through the anion-exchange membrane would
combine with H+ ions formed at the anode to regenerate
HaSOii. After the iron had been removed from the pickl-
ing solution during the circulation through the center
compartments of the cells and the HaSCH had been
regenerated during the circulation through the anolyte
compartments, the pickling solution would be ready for
reuse as pickling acid.
The continuous process would operate in a similar manner
in that pickle liquor from the pickling operation would
be circulated through the center compartment and iron
ions would be transferred through the cation-exchange
membrane into the catholyte solution where insoluble
iron hydroxides would be formed. However, in the con-
tinuous process the pickle liquor would flow directly to
the anolyte compartment as soon as it left the center
compartment. In the anolyte compartment, E2SO^ would be
formed in an amount equal to the iron ions removed through
the cation-exchange membrane. The pickling acid would
then be returned to the pickling operation.
The continuous method is slightly simpler than the batch-
recycle method, but both methods might find use depending
on whether the pickling operation is a continuous or
batch process.
Of the cell voltages studied (4, 4.5, 5, and 6 volts),
voltages of 4.5 or 5 volts appeared to offer the best
combination of reasonably rapid regeneration rates and
reasonably high coulomb efficiencies, and estimates of
processing costs were prepared with those two assumed
cell voltages. In the cell used for these studies the
spacings between membranes (or between a membrane and
an electrode) were 0.125-in. and the temperatures were
55° and 60°C.
The cost estimates indicated that the probable operating
costs for the acid regeneration process would be equivalent
to about $0.043 to $0.047/gal. of spent pickle liquor for
intermediate and large pickling operations (2500 and 6700
gpd). These costs should be competitive with all but the
cheapest existing processes for disposing of spent pickle
liquor, since the total costs of purchasing acid and dis-
posing of spent liquor by existing processes are from
$0.015 to $0.060/gal. This range is based on costs of
$0.01 to $0.02/gal. for purchasing acid plus $0.005 to
$0.040/gal. for disposal as shown below.
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Method Cost, $/gal.
Deep-well injection 0.005 to 0.01
Neutralization and lagooning 0.01 to 0.02
Neutralization and land-fill 0.02 to 0.03
Hauling 0.02 to 0.04
The cheaper processes for disposal (e_. g_. deep-well
disposal or neutralization and lagooning) cannot be used
at all locations, nor under all circumstances. With the
electromembrane process, the only material to be disposed
of is solid iron hydroxide, which is no more damaging to
the environment than iron ore. Moreover, it may be
possible to sell the solid iron hydroxide for credits
to offset the cost of disposal.
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V. EXPERIMENTAL
Preliminary studies were made to determine the important
properties of available ion-exchange membranes, to select
the best membranes for use in subsequent experiments, and
to determine whether plating of iron onto cathodes or a
method that involved the formation of insoluble iron
hydroxides was the better method for converting the iron
removed from the spent pickle liquor by electrolysis to
a solid form that could be separated easily. Experiments
were then made in bench-scale apparatus to determine the
relationships between the main controllable operating
variables (voltage and temperature) of the selected
process and the main performance factors (current density
and coulomb efficiency), to check the stabilities of the
materials used in the electromembrane cell, and, finally,
to determine whether there are any materials in actual
pickle liquor that would cause the electromembrane cell
to perform differently than with the solutions of
FeSCH-HaSOit with which the previous experiments were
performed.
In addition to the above experiments that were concerned
with the performance of the cell itself, experiments were
performed to determine the settling rates of the iron
hydroxides produced in the bench-scale cell to provide
data with which the costs of solids separations equip-
ment for use in the full-scale process could be estimated,
A. Preliminary Studies
1. Membrane properties and selection of membranes
The more important properties of the ion-exchange mem-
branes that were available were determined so that the
best membranes for use in subsequent studies could be
selected. The most important membrane properties for
the proposed use in an electromembrane process for
regeneration of sulfuric acid were (a) the transport
numbers of H+ and Fe"1"2 ions through cation-exchange
membranes, (b) the transport numbers of H+ and S0£2
ions through anion-exchange membranes, (c) the resist-
ances of the membranes when equilibrated in solutions
containing FeSCU and H2SOi», and (d) the physical and
electrochemical stabilities of the membranes when in
contact with solutions of FeSO4 and HaSOi* at the temper-
atures anticipated for use (50°C to 70°C).
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Experimental determinations were made of the transport
numbers and resistances of the membranes under selected
conditions, but for information about the stabilities
of the membranes under conditions of anticipated use,
we depended on the previously developed data of Kramer.1
a. Determination of transport numbers of membranes
The cell used to determine transport numbers consisted
of two identical, half-cells made of Micarta plastic
laminate from Westinghouse Electric Corporation. Each
half-cell contained a 3x3-in. graphite electrode. When
the membrane to be tested was clamped between the two
half-cells, the membrane separated two 3x3x0.125-in.
solution compartments formed by cavities adjacent to
the electrodes. Each solution compartment had a
solution entrance manifold at the bottom and an exit
manifold at the top. A schematic diagram of the cell
and its auxiliary equipment is shown in Figure 1.
In a typical determination of the transference numbers
of H+ and Fe+* ions through cation-exchange membranes, or
of H+ and SOi;* ions through anion-exchange membranes, the
solution loop through the anode side of the cell was
charged with 3 liters of anolyte solution, which con-
tained about 10% of FeSCH and 5% of HzSCK. The solution
loop through the cathode side of the cell was charged
with 3 liters of catholyte solution, which contained
about 5% of FeSCH and 4% of H2SCU. The solutions were
circulated through the respective compartments and
voltage was applied to the electrodes for a timed
period of about 2 hours, during which time the current
was maintained at 100 mA/cm2 and the temperature was
maintained at 60°C. Samples of the anolyte and
catholyte solutions were taken at the start and end
of the timed period and analyzed for Fe+2, Fe+3, and
H+. From the change in the compositions of the two
solutions and from the number of coulombs passed
through the cell, the transference numbers of H+ and
Fe+z ions (as eq/F; in the membrane being tested or of
H+ and SOi;2 ions, were calculated.
1. R. M. Kramer, "Electrodialytic Concentration of
Sulfuric Acid", Ph.D. Dissertation, Chemical
Engineering Department, Polytechnic Institute of
Brooklyn (1963).
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MEMBRANE
CATHOLYTE
SOLUTION
©
PUMP
CATHODE
ANODE
1
ANOLYTE
SOLUTION
PUMP
Figure 1. Diagram of Cell and Auxiliary Equipment Used to
Determine Transference Numbers of H+ and Fe++ Ions
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A few difficulties were encountered during the first
several experiments. In the anolyte solution, Fe+2
ions were oxidized to Fe+3 ions in a non-reproducible
manner, probably because of the oxygen gas evolved at
the anode of the cell. This oxidation reaction produced
H+ ions also, so that the values of transference numbers
calculated from changes in the composition of the anolyte
solution were unreliable and not reproducible. However,
the formation of H+ and Pe+3 ions in the anolyte solution
did not affect the compositions of the catholyte solutions
and good material balances of the Fe+2, H+, and SO£2 ions
were obtained from the chemical analyses of the initial
and final samples of the catholyte solutions.
The following analytical methods were found to be
reliable for use in our studies. Difficulties were
encountered with the first methods used for analysis
of the solutions for E2SOk because of the presence of
the hydrolyzable Fe+2 and Fe+3 ions, but the methods
described below proved to be satisfactory.
• For Fe+2 ions - The potassium permanganate method
described by Joseph Rosin, "Reagent Chemicals and
Standards", Van Nostrand, N.Y., 1967, p. 206.
• For Fe+3 ions - A method involving the reduction
of all Fe"1"3 to Fe+2 followed by a potassium
permanganate titration that is described by
Koltoff and Belcher, "Volumetric Analysis",
Vol. 3, Interscience, N.Y., 1957, p. 83.
* Fo.r H+ ions - Several methods for the determination
of H+ ions in the presence of the hydrolyzable Fe+2
and Fe+3 ions were studied. The method of Moskowitz,
Dasher, and Jamison, Anal. Chem. 32, 1362 (1960)
was found to give acceptable accuracy and to be
quick and simple enough for use as a routine
method for obtaining values for calculation of
transference numbers. In this method, the
hydrolyzable ions are complexed by the addition
of a measured amount of NHi»F so they do not
interfere with the subsequent titration with
standardized NaOH solution.
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The basis for the determination of transference numbers
is illustrated below with cation-exchange membranes.
The transference numbers for anion-exchange membranes
were calculated by similar methods. During electrolysis
in the cell shown in Figure 1, the following changes in
composition would be expected in the catholyte solution.
(The difficulties with oxidation in the anolyte solution
were mentioned previously.)
• 1 eq/F of H+ ions would be converted to hydrogen
gas at the cathode and thus disappear from the
catholyte solution. With the pH conditions in
the catholyte solution, only Ir and no Fe+2 or
Fe+3 ions/ would be discharged at the cathode.
• tH+ eq/F of H+ ions, tFe+2 eq/F of Fe+2 ions, and
tFe+3 eq/F of Fe+3 ions would be transferred from
the anolyte solution through the cation-exchange
membrane into the catholyte solution.
The net change in the H+ ion content of the catholyte
solution would be (tH+-l) eq/F. The net change in the
Fe+2 ion content would be tFe+2 eq/F, and the net
change in the Fe+3 ion content would be tFe+s eq/F.
Thus, knowing the values for the initial volumes and
the Fe+2, Fe*3, and H+ ion contents of the catholyte
solution initially and after electrolysis along with
the value for the total number of coulombs passed
through the cell would permit calculation of tfl+,
tFe+2 f and tFe+3•
The transference numbers determined by this method for
the ten membranes considered for use in this program are
given in Table I.
b. Determination of membrane resistances
The cell used for the determinations of areal resist-
ances of the ion-exchange membranes consisted of two
identical half-cells. Each half-cell was a 3x3-in.
piece of 1-in. Micarta plastic laminate (from Westing-
house Electric Corporation) provided with a 0.500-in.
diameter cavity centered on one of the 3x3-in. surfaces.
The cavity was 3/16-in. deep. The bottom of the cavity
was covered with a disk (0.500-in. diameter) of platinum
to serve as an electrode. A platinum wire, to serve
as an electrical connection, was spot-welded to the disk
and led through a small hole in the Micarta laminate.
This small hole was subsequently filled with Silastic
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Table I. Transport Properties of Ion-Exchange Membranes
Considered for Use in the Process
Res i s tances,b ohm-cm 2
Membrane
Transference number*
designation Supplier
For H"1"
For Fe"1"2
For SO.,"2
In 1
.!>%
and 10%
H2S(X,
FeSCK
In 10% H2SO«,
and 5% FeSCK
Cation-exchange
l
H
ON
1
MC 3142
MC 3470
CSV
CL-2.5T
CMV-2
AZL183
lonac Chemical Co.
lonac Chemical Co.
Tokuyama Soda Co . , Ltd .
ToJcuyama Soda Co . , Ltd .
Asahi Glass Co., Ltd.
Ionics, Inc.
0.5
0.8
0.8
0.5
0.8
0.8
0
0
0
0
0
0
.5
.2
.2
.5
.2
.2
0.
o-.
0.
0.
0.
0.
0
0
0
0
0
0
7
35
17
4
7
to
to
to
to
5
to
9
49c
40C
8
8
0.5 to 1.0
d
1.7
d
d
0.6
Anion-exchange
MA 3148
AV-4T
AST- 2
BZL183
lonac Chemical Co.
Tokuyama Soda Co . , Ltd .
Asahi Glass Co., Ltd.
Ionics, Inc.
0.4
0.4
0.6
0.4
0
0
0
0
.0
.0
.0
.0
0.
0.
0.
0.
6
6
4
6
11
6
to
4
12
to
18
12
1 to 2
d
d
1.2
a. Measured at 60±1°C with an anolyte solution containing about 10% of FeSOi* and 5% of
and a catholyte solution containing about 5% of FeSO* and 4% of H2SCK .
b. Measured at 60±1°C. Varied with current density.
c. Resistance increased with time.
d. Not measured.
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RTV cement to seal it. Each half-cell was also provided
with one tube at the bottom and one at the top to intro-
duce and withdraw solutions.
In operation, two half-cells were clamped together with
the membrane to be tested between them so that each
cylindrical cavity had an electrode at one end and a
surface of the membrane at the other. The inlet tubes
to the half-cells were connected with tubing to a pump,
and thence to a reservoir that contained a solution
containing 10% of FeSCK and 1.5% of H2SCU. The outlet
tubes from the half-cells were returned to the reservoir
so that the solution would circulate through each half
of the cell when the pump was on. The entire system was
maintained at 60±1°C.
For measurements, the solution was circulated through
the cell, a d-c voltage to the electrodes was increased
stepwise, and the electrode voltage and the current were
recorded for each value of electrode voltage. When a
sufficiently high current density was reached (ca 300 to
400 ma/cm2), the voltage was decreased stepwise and the
electrode voltage and current were again recorded at
each value of electrode voltage.
The cell was then disassembled and the test membrane was
removed. The two half-cells were reassembled,.but this
time without a membrane, and the electrode voltage re-
quired for each current density recorded without the
membrane in the cell was determined. The areal resist-
ance (in ohm-cm2) of the test membrane was calculated
from the difference between the cell voltages with and
without the membrane, the current density, and the area
of membrane exposed to the current (1.226 cm2).
The entire procedure was then repeated in an identical
manner except that the solution contained about 5% of
FeSCK and 10% of H2SOi» (instead of 10% of FeSCH and 1.5%
of H2SOO .
The values for the resistances of the membranes considered
for use in this program are given in Table I.
c. Stability of membranes in HaSQ^
Two of the membranes for which data are given in Table I
(the CMV-2 and the AST-2 membranes made by the Asahi Glass
Co., Ltd.) showed some evidence of physical degradation
during the relatively brief exposures to the
-17-
-------
solutions (at 60°C) that were used in the above procedures.
The estimated cumulative time of exposure was 6 hours.
There was no evidence of degradation of the other mem-
branes .
Kramer has presented data on the physical and electro-
chemical stabilities of a number of ion-exchange membranes
after exposure to sulfuric acid solutions similar to
those proposed for use in the electromembrane regeneration
process.l Kramer measured the transport numbers and
resistances characteristic of various ion-exchange mem-
branes before and after various periods of exposure of
the membranes to H2SCK solutions at 50°C and at 80°C.
The differences in the measured properties were considered
to indicate the degree of degradation of the electro-
chemical (i_.e_. transport) properties of the membranes.
He used a concentration potential method for determining
transport numbers in which the membranes separated 1 N
and 0.5 N solutions of KCl, and resistances were measured
with the membranes equilibrated in 0.5 N KCl. He expected
that measurements in KCl solutions would indicate changes
in the characteristics of the membranes better than
measurements in HaSOi* solutions.
Along with a number of other membranes, Kramer evaluated
the MC 3142 and MA 3148 membranes.* His data for these
membranes are given in Table II with his data for the
membranes he selected as the most stable of those that he
evaluated, C-60 and A-60. The C-60 and A-60 membranes,
which were manufactured by American Machine and Foundry
Company, are no longer available.
*From the lonac Chemical Company.
-18-
-------
Table II. Stability of Ion-Exchange Membranes in 1 N H2SOH
Solutions at Elevated Temperatures
After storage
Membrane
designation
MC 3142C
MA 3148C
C-60d
A-60d
0
0
0
0
0
months
ta
.94
.90
.92
.94
RD
8
9
4
8
at 50°C for
4 months
ta
0.85
0.87
0.86
0.93
RD
6
7
2
8
9 months
ta
0.86
0.88
0.86
0.93
Rb
6
9
3
6
After storage
0 months
ta
0.98
0.90
0.92
0.93
RD
8
16
2
7
at 80°C for
4 months
ta
0.73
0.80
0.83
0.91
RD
6
12
2
4
8 months
ta
0.76
0.79
0.87
0.91
RD
5
" 11
2
4
a. Transport number of the counterion measured by a concentration-
potential method when the membrane separated 1.0 N and 0.5 N KC1.
b. Resistance in ohm-cm2 when the membrane was equilibrated in 0.5 N
KC1.
c. From lonac Chemical Company, Birmingham, New Jersey.
d. From American Machine and Foundry Company, Stamford, Connecticut.
-------
The data in Table II indicate that, at 50°C the MC 3142
cation-exchange membrane was essentially as stable as the
C-60 cation-exchange membrane. However, the decrease in
transport number of the MC 3142 membrane during 8 months
of storage in H2SCU solution at 80°C was significantly
larger than the decrease in the transport number of the
C-60 membrane.
Similarly, the MA 3148 membrane was almost as stable as
the A-60 membrane when stored for 9 months in 1 N HaSOt*
solution at 50°C/ but was significantly less stable than
the A-60 membrane when stored at 80°C.
Kramer pointed out that the glass-transition temperatures
of some of the membranes he tested were less than 80°C
and he attributed the decreases in transport numbers of
such membranes when stored at 80°C to changes in the
structure of the polymers that occurred when the glass-
transition temperature was exceeded.
In extended tests, Kramer also showed that the transport
properties of the C-60 and A-60 membranes declined only
slightly during 2 years storage in 1 N HzSCH solution at
80°C, which suggests those membranes might have usable
lifetimes of 2 years.
Judging from these indications of the stability of the
MC 3142 and MA 3148 membranes when stored at 50°C, these
membranes should have acceptably long lifetimes (perhaps
2 years) if used in the proposed electromembrane process
at 50°C.
Forgacs2 used MC 3142 and MA 3148 membranes to desalt
NaCl solutions by electrodialysis at temperatures of
70°C and 90°C. He found that the MC 3142 and MA 3148
membranes retained their transport properties in long-
term experiments when the operating temperature was 70°C,
but the transport properties deteriorated in experiments
at 90°C.
The results of both Kramer and Forgacs suggest that the
glass-transition temperature of the MC 3142 and MA 3148
membranes is slightly above 70°C and that these membranes
should have acceptable lifetimes when used in HaSOi*
solutions at 70°C or less. As stated before, there is a
suggestion in Kramer's data that the usable lifetimes
might be 2 years, or longer.
2. C. Forgacs, Desalination 3_, 129 (1967)
-20-
-------
Considering all of these data on the transport properties
and the stabilities, we selected the lonac MC 3142 and
MA 3148 membranes as the main ones for our further studies.
2. Studies of methods of conversion of iron to a solid
form for removal
At the outset of this program two methods for converting
the iron removed from the spent pickle liquor to a readily
handled solid form were considered: plating the iron out
onto cathodes and precipitation as iron hydroxides.
Plating the iron out appeared to be the simpler. However,
to be successful it would be necessary to form loosely
adhering deposits that could be removed easily from the
cathode and also necessary to prevent the growth of
dendrites on the electrodes that could puncture the ion-
exchange membranes. In the method involving conversion
to insoluble hydroxides, the major problems appeared to
be (a) the development of an easy method of partially
oxidizing the ferrous iron to ferric iron so that easily
filterable iron hydroxides (presumed to be ferroso-
ferric hydroxides) could be formed, since either ferrous
or ferric hydroxides by themselves form as gelatinous
precipitates that are hard to filter, and (b) the
development of an electromembrane cell in which insoluble
hydroxides would not form within the membranes and in-
crease the electrical resistance of the cell.
We studied both of these methods.
a. Plating
(1) Comparison of various cathode materials
The plating experiments were performed by immersing a
small rectangular piece (typically 12 x 10 cm) of the
cathode material to be studied in a solution of FeSOi*
(typically 15% of FeSO^) along with a graphite anode.
The cathode and anode were held 3 cm apart by a Micarta
spacer. Voltage was applied to the cells and adjusted
to give the desired value of current density in the
range of 50 to 100 mA/cm2, while the FeSCK solution was
maintained at the desired temperature (50°C or 60°C).
Experiments were made with FeSCU solutions with a pH of
3.6 and a pH of 2.0.
-21-
-------
Electrodeposition on the cathode was carried out for
2 hours and then the cathode was removed from the bath,
rinsed/ and the deposit of iron was examined to note the
approximate length, number, and location of any dendrites
formed, and the tenacity with which the deposited iron
was attached to the cathode material. It was planned
that more quantitative means of determining the adherence
of coatings would be used for the cathode materials that
showed some promise in the initial visual screening tests,
but none of the materials studied showed enough promise
to warrant further study.
The cathode materials studied were: low-carbon steel,
Armco iron with extremely low carbon content, 316 stain-
less steel, 430 stainless steel, aluminum, chrome-plated
steel with a matte finish and with a mirror finish, and
titanium.
In these initial studies, titanium was the only cathode
material of those studied to which the deposited iron
adhered only loosely. Tightly adherent deposits with
dendrites formed on all of the other cathode materials.
(2) Studies with titanium cathodes
In further studies with titanium cathodes, the temperature
of the FeSOi* solution was varied from 50°C to 80°C, and pH
of the solution was either 2.0 or 3.6, and the current
density was varied from 25 to 100 ma/cm2 in attempts to
find conditions that would result in deposits of iron
that were completely free of dendrites and that could be
easily and completely removed from the titanium (e_.g_.
by rapping the cathodes or by simple scraping procedures).
No operating conditions were found that resulted in de-
posits that could be completely removed by simple, low-
cost procedures. Moreover, dendrites or other protuber-
ances were formed in all of the experiments. These
experiments were stopped when the results of experiments
(described later) showed that iron would be precipitated
from solutions simulating spent pickle liquor in the form
of readily filterable iron hydroxides.
b. Precipitation as hydroxides
(1) Experiments in two-compartment cell
Seven experiments were performed in a two-compartment
cell to explore various conditions for the removal of
iron from solutions containing sulfates of iron by
electrolysis and precipitation of insoluble iron
hydroxides. The information obtained in these experi-
ments was mainly qualitative in nature so the experiments
will be only briefly discussed.
-22-
-------
The main components of the cell used in these experi-
ments were an anode, a cathode, and an lonac MC 3142
cation-exchange membrane. The cation-exchange membrane
was clamped between two identical half-cells to form an
anolyte solution compartment and a catholyte-solution
compartment. Each compartment was provided with means
of introducing and withdrawing solutions from the
compartment and contained a graphite electrode.
A ferrous sulfate solution (ferric sulfate in one experi-
ment) was circulated through the anolyte-solution com-
partment; the concentration of the solution was varied
from experiment to experiment over the range of 1% to
10% of ferrous sulfate. A solution of Na2SOi» was
circulated through the catholyte solution compartment
in some experiments; a solution of (NKU^SOi* was circu-
lated through this compartment in other experiments.
The catholyte solutions were 1.0 N, except in Experiment
5723-59-1, in which a 0.1 N solution was used. The
current density was 100 mA/cm2 and the temperature was
60°C in all of the experiments. Voltage was applied to
the cell for periods varying from 2 hours to almost
7 hours in the several experiments. Samples of the
anolyte were taken at intervals during each experiment
and analyzed for total iron and ferrous iron contents.
Changes in composition with time were calculated from
the analyses. The catholyte solutions were filtered
at the end of each experiment and the solid was dried
and weighed.
In the anolyte-solution compartment H+ ions and 62 gas
were formed at the anode and Pe+2 ions were oxidized to
Fe+3 ions, either by gaseous 02 or by direct electro-
chemical action. In the catholyte-solution compartment
Na+ ions (except when (NHOzSOi* was used) were discharged
at the cathode to form Na which reacted with water to
form NaOH. Insoluble iron hydroxides were formed by the
reaction of OH~ ions with Fe+2 and Fe+3 ions transferred
into the catholyte-solution compartment through the
cation-exchange membrane.
-23-
-------
The main things learned in these experiments were:
• Easily filtered iron hydroxides were formed in
all experiments except the one in which the
anolyte solution contained only ferric sulfate.
The use of only ferric sulfate in the anolyte
solution resulted in reddish gelatinous pre-
cipitates that were difficult to filter.
• The cell voltages needed to produce a given
current density (100 mA/cm2) depended on the
concentration of FeSCK and of H2SOi, in the
anolyte.
• The use of (NHit)2SOif in the catholyte solutions
resulted in slightly lower cell voltages than
the use of NazSOi*. This behavior is in accord
with the higher conductance of (NHOzSOt* solutions.
(2) Studies in three-compartment cell at 60°C
After the preliminary experiments in a two-compartment
cell were completed, center frames (one 1/8-in. thick
and one 1/16-in. thick) were fabricated to fit between
the two end frames and convert the two-compartment cell
into a three-compartment cell. Additional studies of
methods of reducing the iron content of spent pickle
liquor by electrolysis and formation of insoluble iron
hydroxides were performed in this three-compartment
cell, to determine relationships between cell voltage
and current densities, as well as to study the formation
of precipitates of iron hydroxide. The cell was assembled
with an anion-exchange membrane clamped between the anode
end frame and the center frame, and a cation-exchange
membrane clamped between the cathode end frame and the
center frame. Each compartment had provisions for
introducing and withdrawing solution.
We planned initially to use partially oxidized ferrous
sulfate solutions from previous studies as feeds to the
middle compartment of the three-compartment cell. How-
ever, in the initial shakedown experiment with the cell,
a fresh FeSCK solution containing no FeafSOiJa was used
as feed to the middle compartment, and it was found that
a black, easily filterable precipitate of iron hydroxide
was formed. Apparently, the FeSOit in the feed solution
-24-
-------
was oxidized sufficiently during the recirculation and
electrolytic processes to form a filterable precipitate,
instead of the slimy greenish precipitate usually result-
ing from the formation of ferrous hydroxide. Since a
filterable precipitate was obtained, the remaining
experiments were made with ferrous sulfate solutions
as feeds to the middle compartment.
In all of these experiments a 1 N solution of either
NaaSOif or KzSCK was circulated through the catholyte
compartment. During electrolysis, the alkali metal
ions were discharged at the cathode to form alkali
metal, which reacted immediately with water to form
an alkali metal hydroxide. The hydroxide ions combined
with the iron ions entering the catholyte compartment
through the cation-exchange membrane to form insoluble
precipitates.
Solutions of FeSOi* ranging between 2% and 7% in
concentration were circulated through the center
compartment.
A solution of HzSOit was circulated through the anolyte
compartment. During electrolysis, sulfate ions entered
the anolyte compartment through the anion-exchange mem-
brane and hydrogen ions were formed at the anode as a
result of the electrolytic decomposition of water. The
result of this transfer and decomposition was an increase
in the concentration of the circulating acid with time.
A summary of the experiments that were performed is
given in Table III. All of these experiments were con-
ducted at 60 °C, for a period of 2 hours, and with a
current density of 100 mA/cm2 . Although the MC 3142
and MA 3148 membranes were indicated by previous experi-
ments to be the most probable candidates for eventual
use, we included CL2.5-T cation-exchange membranes
(which also appeared to be good) in some of these
experiments to confirm our choice of the MC 3142
cat ion- exchange membranes.
Experiments 5723-85, -87, and -89 indicated that the
voltage needed to maintain a current density of 100
mA/cm in the experimental cell is decreased when the
H2SOit content of the anolyte solution is increased.
However, the change in voltage (1.1 volt) between
Experiments 5723-87 and 5723-89, with only a small
change in H2SO4 concentration (11% to 12%), appeared
anomalously large.
-25-
-------
Table III. Removal of Iron from Solutions of Ferrous Sulfate by Electrolysis and
Precipitation of Insoluble Iron Hydroxides
(Current density-100 ma/cm2; all experiments for 2 hours at 60eC)
Initial Approximate
Composition composition composition
Experiment Membranes of catholyte of FeSO* of anolyte Applied
1
NJ
O"\
1
5723-
85
87
89
Changed to
membrane .
91
95
used3
CL2.5-T
MA 3148
CL2.5-T
MA 3148
CL2.5-T
MA 3148
solution
8.7% K2SO.,b
8.7% K2SOi,b
8.7% K2S01(b
solution
4% FeSCU
4% FeSCK
4% FeSOi,
different cation-exchange membrane; used
MC 3142
MA 3148
MC 3142
MC 3148
Cell modified to thinner
installed
105
107
109
new MC 3142
MC 3142
MA 3148
MC 3142
MA 3148
MC 3142
MA 3148
8.7% K2SO,b
8.7% K2SO,,b
4% FeSOi,
2% FeSO*
center compartment (1/16-in
solution
7% HzSOi,
11% H2SO,,
12% H2SO«,
voltage
7.3
7.0
5.9
same anion-exchange
12% H2SOi.
9% HjiSO,.
. instead of
8.1
8.4
1/8-in.};
and MA 3148 membranes.
6.7% NazSOi.c
6.7% Na2SO.,c
6.7% Na2SO,,c
7% FeSCK
4% FeSO.,
4.5% FeSCH
5% H2SOi,
12% H2SO^
7.5% H2SO.t
7.3
6.3
6.8
a. MC 3142 and MA 3148 designate cation- and anion-exchange membranes,
respectively, obtained from lonac Chemical Co. CL2.5-T designates a
cation-exchange membrane obtained from Tokuyama Soda Co., Ltd.
b. 8.7% K2SOi, is 1 N in K2SOi».
c. 6.7% Na2SOi, is 1 N in Na2SCK.
-------
Prior to Experiment 5723-91 the three-compartment cell
was re-assembled with an MC 3142 cation-exchange mem-
brane to replace the CL2.5-T membrane used in the three
prior experiments. Experiment 5723-91 was then performed
under the same conditions as Experiment 5723-89. There
was a large increase in cell voltage (5.9 volts with the
CL2.5-T membrane, 8.1 volts with the MC 3142 membrane)
required to obtain the same current density, which
suggested the MC 3142 membrane had higher resistance
than the CL2.5-T membrane.
Experiment 5723-95 was intended primarily to determine
the effect of a lower concentration of FeSOi* solution
on cell voltage. Because of an error in preparing the
anolyte solution the concentration of acid in the anolyte
solution was not the same as in Experiment 5723-91.
Therefore, it is not certain whether the increased
voltage in Experiment 5723-95 (8.4 volts compared with
8.1 volts in Experiment 5723-91) was a result of the
decreased FeSCH content, the decreased HaSOt* content,
or both.
After Experiment 5723-95, the cell was reassembled with
a thinner center frame (1/16 in. instead of the previous
1/8-in. frame) and with new pieces of MC 3142 and MA 3148
membranes.
The conditions for Experiments 5723-105, -107, and -109
were chosen to explore a possible batch-recycle method of
operation of the eventual process that is described only
briefly here but in detail in Section III-C. During a
given time period, say 6 hours, in such a batch-recycle
process:
• The pickling line would be run with acid that had
been regenerated previously.
• The spent pickle liquor from the previous time
period would be circulated through the center
compartments of cells to reduce the iron and
sulfate contents.
• The solution from the previous time period that
had been reduced in iron and sulfate contents
would be circulated through the anolyte compart-
ments to regenerate pickling acid.
-27-
-------
The concentrations of the iron sulfate solutions and the
H2SOi» solutions in Experiments 5723-105 and -107 were
chosen to be representative of the concentrations near
the end of the time period for the batch-recycle pro-
cess (Experiment 5723-107) , and near the mid-point of
the time period (Experiment 5723-105). The catholyte
solution was 1 N Na2SOi», since Na2SOif (which is lower
in cost than K2SOit) would probably be used in commercial
practice.
The solution concentrations in Experiment 5723-109
represent the concentrations to be expected at a point
in the time period of the batch-recycle process between
the mid-point and the end.
The general indications from this series of experiments
were that as the HzSOi, concentration in the anolyte
solution increases, the voltage needed to maintain a
constant current density will decrease. Conversely,
with a constant voltage applied to the cell the current
density will increase as time passes and the H2SOif
concentration in the anolyte solution increases.
3. Studies of voltage/ current, temperature relation-
ships
In addition to the above experiments at 60°C, experiments
were performed at 55°C and at 65°C to determine the
relationships between voltage, current, and temperature.
The solutions used in the three compartments of the
cell were:
Catholyte - 6.7% Na2SO^ (1 N Na2SOif)
Center - 4.0% FeSO^
Anolyte - 12.0% H2SOit
The membranes used were MC 3142 and MA 3148 membranes from
lonac Chemical Co. The cell spacings (i_.e_. spacings be-
tween membranes or between membranes and electrodes) were:
Catholyte - 0.125 in.
Center - 0.0625 in.
Anolyte - 0.125 in.
_"i Q
f. O —
-------
The cell voltages at various current densities are shown
in Figure 2 for temperatures of 55°C and 65°C. The points
for each temperature lie on straight lines that intercept
the zero axis at 2.4 and 2.5 volts, respectively.
These values correspond with the values of 2.4 and 2.5
volts reported in the literature3'1* for the decomposition
potentials of such cells. The slopes of the voltage-
current curves indicate the effective resistances of
the cell were 50 ohm cm2 at 55°C and 45 ohm cm2 at 65°C.
4. Discussion
In these experiments to explore the methods of removing
iron from iron sulfate solutions by electrolysis and
formation of insoluble iron hydroxides, the most
important information found was:
• The precipitation method of removing iron seemed
better for eventual use than the plating method—
primarily because the insoluble hydroxides formed
can be removed easily by settling or filtration,
whereas the solids formed in the plating method
cannot be easily or cheaply removed.
• FeSOi» solutions circulating through the center
compartments of three-compartment cells are
apparently oxidized sufficiently to allow
formation of easily filterable precipitates
in the cathode compartment.
• The experimental apparatus used in these experi-
ments needed changes (described later in Section
III-C) to improve the accuracy of some of the
measurements.
B. Bench-Scale Experiments
The cell and equipment used in the bench-scale studies
were designed to study an electromembrane process in
3. D. J. Lewis and F. L. Tye, J. Appl. Chem. 9_, 279
(1959).
4. C. L. Man tell and L. G. Grenni, J. Water Pollutiojn
Control Fed. 3_4_, 951 (1962).
-29-
-------
8.0
7.0
6.0
5.0
LU
tu
o
3.0
2.0
1.0
CATHOLYTE - 6.7% Na2SOi,
ANOLYTE - 2% H2SOi,
FeSOi. SOLUTION - «*% FeSOi,
I
1
25 50 75 100
CURRENT DENSITY, ma/cm2
125
Figure 2. Relationship between Cell Voltage and
Current Density at 55°C and 65°C
-30-
-------
which iron is removed from spent pickle liquor by elec-
trolysis and precipitated as insoluble iron hydroxides,
and acid is regenerated at the anode of cells.
Before describing the equipment and methods used in
the experimental program, we will describe a newly
conceived electromembrane cell and the way in which it
can be used to remove iron from spent pickle liquor and
to regenerate acid so that the reader can more easily
understand the reasons for the experimental techniques
used.
1. Batch-recycle process
a. Prior electromembrane acid-regeneration process
Our considerations of methods of reducing the iron con-
tent of spent pickle liquor by a method involving the
formation of insoluble iron hydroxides stemmed from our
studies of U. S. Patent 3,394,068 (assigned to lonac
Chemical Co.), and a private communication from Mr.
Allyn Heit (one of the co-inventors) that described
the studies made in support of the patent.
U. S, Patent 3,394,068 points out that the iron content
of spent pickle liquor can be reduced by adding OH" ions
to form insoluble iron hydroxides. If it were not for
the fact that both ferrous hydroxide and ferric hydroxide
precipitate as gelatinous materials that are difficult
and costly to filter, the reduction of the iron content
by this method would be easy and relatively cheap. How-
ever an easily filterable ferroso-ferric hydroxide can
be formed if the ferrous ions in the spent liquor are
partially oxidized to ferric ions before the formation
of hydroxides.
The process described in U. S. Patent 3,394,068 makes
use of an electrolytic cell comprised of an anode, a
cathode, and two anion-exchange membranes. The two
anion-exchange membranes divide the electrolytic cell
into an anolyte compartment, a center compartment and
a catholyte compartment. A sulfuric acid solution is
circulated through the anolyte compartment, the spent
pickle liquor (containing iron mainly as FeSOO is
circulated through the center compartment, and a sodium
hydroxide solution is circulated through the anolyte
compartment. When electrical current flows through the
cell: (a) OH~ ions are transferred through one of the
-31-
-------
anion-exchange membranes to the center compartment, Na+
ions are discharged at the cathode, and the sodium reacts
with water to re-form NaOH; (b) S01T2 ions are transferred
from the center compartment to the anolyte compartment;
and (c) H+ ions are formed in the anolyte compartment.
Thus, OH~ ions are transferred into the pickle liquor,
and SO"2 ions are transferred out of the pickle liquor
and insoluble iron hydroxides are formed.
In the process described in U. S. Patent 3,394,068 the
coulomb efficiencies achieved for the regeneration of
acid were no higher than 30%. Our preliminary experi-
ments indicated higher coulomb efficiencies for re-
generation of acid (50 to 70%) could be achieved with
a new electromembrane process that differed from that
described in U. S Patent 3,394,068.
b. New electromembrane regeneration process
(1) In the new electromembrane process, the three com-
partments of the electrolytic cells were formed by an
anode, a cathode, one anion-exchange membrane, and one
cation-exchange membrane (instead of an anode, a cathode,
and two anion-exchange membranes). The operation is
carried out as a batch-recycle process as described
below.
A schematic flow diagram of the batch-recycle regeneration
process is shown in Figure 3. Only a single three-
compartment cell is shown for convenience in depicting
the flows of solutions in the regeneration circuit. In
practice, many such three-compartment cells would be
assembled into a stack of such cells with appropriate
provisions for manifolding the solutions by an arrange-
ment similar to that in a filter press. Stacks such as
this, with multiple cells in series, have been used
industrially to generate hydrogen and oxygen gases. In
the batch-recycle regeneration process, the following
operations would be carried out during each of the
three steps of a cycle.
• In Step 1, the pickling line would be run with
acid that had been regenerated previously. This
stream is shown in Figure 3 to be circulating
from Tank I to a steam-heated heat exchanger
(to avoid the dilution resulting from heating
with an open steam line), and then to the
-32-
-------
IRON HYDROXIDE
HEATER
PICKLING
LINE
Figure 3. Diagram of Flows in Batch-Recycle Regeneration Process
-------
pickling line and back to Tank I. In this operation,
the acid content of the pickling liquor decreases
and the iron content increases.
In Step 2 , spent pickle liquor from a previous
pickling operation, which is high in iron content
and low in acid content, would be circulated from
Tank II through a filter to remove particles, such
as flakes of mill scale, and then through the
center compartments of repeating cells in the
regeneration stack and back to Tank II. The iron
content of the spent pickle liquor would be re-
duced in this step.
In Step 3 , the solution in which the iron content
was reduced in a previous step would be circulated
from Tank III through the anolyte compartments of
the cells. In this step, this solution would be
enriched in H+ ions, as a result of the reactions
at the electrodes, and in S0"£2 ions, as a result
of transfer of SO~2 ions through the anion-
exchange membrane. At the end of the step the
solution in Tank III would be ready for reuse
for pickling; a small addition of make-up acid
might be necessary.
• During Steps 2 and 3, a solution of NaaSCK would
be circulated from Tank IV through the catholyte
compartments of the cells and then through a
filter and back to Tank IV. In the catholyte
compartments, Na+ ions would be discharged to
form Na°, which would immediately combine with
water to form NaOH. The OH~ ions would react
with Fe+2 and Fe+3 ions transferred through the
cation-'-exchange membranes to form insoluble iron
hydroxides. The iron hydroxides would be
filtered and the filtrate, which would be an
NazSOi, solution of the original composition,
would be returned to Tank IV.
At the end of each step of a cycle, Tanks I, II, and III
would be switched to new circulation lines for the next
step of a cycle.
-34-
-------
(2) Experimental apparatus
The three-compartment cell used in the experimental
studies of the batch-recycle regeneration process is
shown schematically in Figure 4. The frame of the
cell consisted of two end sections and one center
section made of reinforced phenolic resin. The
dimensions of the end sections were 1x5x5 in. and each
end section had a 0.625x3x3-in. cavity. One cavity
was fitted with a 0.5x3x3-in. graphite electrode; the
other was fitted with an antimonial lead electrode.
The spaces through which the anolyte and catholyte
solutions were circulated were each 0.125x3x3 in.
These spaces were provided with mesh spacers (Vexar
69-PDS-49 from Du Pont) to support the ion-exchange
membranes. Each end-section was also provided with
means of introducing and removing solutions and with
an electrical connection to the electrode. Graphite
was used for the cathode and an antimony-lead alloy
was used for the anode.
The center section was 0.125x5x5 in. with a 3x3 in.
open space in the center. The center frame was also
provided with connections for introducing and removing
solutions.
The cell was assembled with an anion-exchange membrane
at the anode side of the center section and a cation-
exchange membrane at the cathode side to form three
solution compartments.
In addition to the cell described above, the auxiliary
apparatus included a source of d-c voltage, a reservoir
and pump for each of the three circulating solutions,
and instruments for indicating—and in some cases,
recording—voltages, and current flows. The hydraulic
flows are shown in Figure 5.
The solution reservoirs were 30 mm in diameter. Solution
reservoirs used in preliminary experiments were 3-liter
aspirator bottles, but these preliminary experiments
showed that small volumes of solution (about 300 ml)
were desirable so that large changes in composition
could be effected within a reasonable time. Therefore,
the reservoirs had to be much smaller in diameter than
the 3-liter bottles to improve the accuracy of reading
the volumes of the circulating solutions. Initially,
electrical heating tapes were wrapped around the
reservoirs to provide a means of maintaining the temper-
atures at 60°C. However, it was found in shake-down
-35-
-------
CATHODE
Na2SCU 1
|
ANODE
4-
r^
ANOLYTE
IRON SULFATE
SOLUTION
LEGEND:
C - CATION-EXCHANGE MEMBRANE
A - AN ION-EXCHANGE MEMBRANE
Figure 4. Diagram of Three-Compartment Cell used in Studies
of Batch-Recycle Regeneration Process
-36-
-------
I
CATHODE
PUMP
II
ANODE
III
PUMP
LEGEND:
c
A
I
II
III
PUMP
CATION-EXCHANGE MEMBRANE
ANION-EXCHANGE MEMBRANE
RESERVOIR FOR CATHOLYTE SOLUTION
RESERVOIR FOR ANOLYTE SOLUTION
RESERVOIR FOR IRON-SULFATE SOLUTION
Figure 5. Hydraulic Flows in Experimental Regeneration Unit
-37-
-------
experiments that these tapes were not needed. When the
circulating systems were filled initially with solutions
at 60°C, the energy input from the pumps and the IR
losses in the cells maintained the temperature at
60°±2°C.
The pumps were Eastern Industries, Model D-ll centrif-
ugal pumps. Flow was controlled by a pinch clamp on
the rubber tubing at the exit of each pump. The solution
flow rates were determined by measuring the amounts of
solutions being returned to the reservoirs during a timed
interval. With these circulating systems, the estimated
uncertainty in the measurements of volume was ±5 ml, or
±2% of the total volume.
The d-c power source was an Eico Model 1060 d-c rectifier.
The cell voltage was indicated by a General Electric Type
DP-9 voltmeter (1% accuracy) and recorded on a Leeds and
Northrup, Speedomax W recorder. The current was indicated
by a Weston, Model 931, ammeter (1% accuracy) and recorded
on the recorder.
(3) Experiments with FezSCK-HaSCU solutions
In a typical experiment in the apparatus described above,
the composition of the solution circulated through each
of the three compartments was chosen to be representative
of the composition expected if the process were used in
a plant.
The composition of the solution circulated through the
center compartment simulated a typical spent pickle
liquor—about 10% of FeSOi, and 3% of H2SOj» initially.
The composition of the solution circulated through the
anolyte compartment simulated a typical solution in which
the contents of iron sulfate and acid had been reduced in
a previous cycle of operation—about 2% of FeSOi, and 0.25%
of HaSOi,. The composition of the NazSOi, solution circu-
lated through the catholyte compartment was 1 N.
Three hundred milliliters of each of the three circulating
solutions was preheated to 60°C and placed in the appro-
priate reservoir of the acid-regeneration unit. The
solutions were circulated for 5 to 10 min to remove
entrapped air, and then the level of the solution in
each reservoir was marked and the temperature of the
solution was measured. The normal flow rates of the
-38-
-------
circulating solutions were about 30 ml/sec. Voltage was
applied to the cell and adjusted to the desired value
(either 4, 4.5, 5, or 6 V). Voltages and currents were
recorded continuously and also read on the meters at
1-hr intervals. Five-milliliter samples of the iron sul-
fate solution and the anolyte solution were taken at 1-hr
intervals and analyzed later for contents of Fe+2, Fe+3,
and H+ ions. The levels in the reservoirs were measured
each hour.
With these data, the number of equivalents of each ion
transferred into or out of each compartment during each
time period was calculated and compared with the number
of faradays passed through the cell as calculated from
the recorded electrical current.
Experiments were performed in the bench-scale equipment
to study the batch-recycle acid-regeneration process
previously described. The relationships between the
controllable variables (applied cell voltage and temper-
ature) and performance factors (coulomb efficiencies
and current densities) that are important to the operat-
ing costs of the batch-recycle process were studied.
These experiments also afforded information about the
behavior of materials of construction under operating
conditions.
The solutions circulated through the three compartments
of the cell were prepared in the laboratory.
Experiments were performed with applied cell voltages of
4, 4.5, 5, and 6 V. The results of these experiments are
summarized in Table IVf and are presented graphically in
Figure 6. In Figure 6, the current densities and the
coulomb efficiencies for the production of HaSCK in the
anolyte chamber of the cell are plotted vs cell voltage.
The average current density increased with increasing
cell voltage, from about 22 mA/cm2 with 4 V to about 50
mA/cm2 with 6 V. The current density curve in Figure 6
was not extended to the 6-V value because of uncertainty
as to where it should be drawn. A duplicate experiment
was not performed with an applied voltage of 6 V because
the lower values of coulomb efficiencies obtained with
higher voltages made it seem highly improbable that the
process would ever be run on a plant scale with cell
voltages as high as 6 V. The data on current densities
for cell voltages of 4, 4.5, and 5 V describe a reasonably
-39-
-------
Table IV. Coulomb Efficiencies and Average Current Densities
with Different Cell Voltages
Coulomb efficiencies, % Average
Experiment Cell Center
5723- voltage compartment
i
*».
o
1
119-2
120-2
120-3
122-1
119-1
119-3
120-1
119-4
4
4
4.5
4.5
5 to 6C
5
5
6
71
71
67
50
40
56
50
44
Anolyte ,
compartment
70
67
53
61
50
60
49
52
current density
ma/cm2
24
21
33
36
47
45
39
50
Calculated from the difference between the total equivalents
of iron ions at the start and end of the experiment in the
solution circulated through the center compartment.
Calculated from the difference between the total equivalents
of H+ ions at the start and end of the experiment in the
solution circulated through the anolyte compartment.
Voltage control was not as good in this experiment as in
the others.
-------
2 80
LU
s:
«o c* 70
-.0.
o o
2 u 60
UJ
o t-
M V
u. _i 50
u.0 p
CO
x z <*o
o <-•
o o
u- JO
111 O
o.
o
« 10
g
60
50
to
z 30
Ul
at.
c&
o 20
UJ
ss 10
VARIATION OF AVERAGE COULOMB EFFICIENCY
WITH APPLIED VOLTAGE
_L
I
I
2 3 «* 5 6
CELL VOLTAGE, VOLTS
VARIATION OF AVERAGE CURRENT DENSITY WITH APPLIED VOLTAGE
Figure 6. Variations of Average Coulomb Efficiency and
Average Current Density with Cell Voltage
-41-
-------
good straight line/ as expected. However, if the
straight line is extrapolated to zero current density,
the indicated decomposition potential at the electrodes
is about 2.7 V, instead of the 2.4 to 2.5 V found in
preliminary experiments. This may indicate the line
in the figure should be drawn with a slightly lower slope,
but on the basis of the data, it is difficult to justify
any lower slope than is shown.
The average coulomb efficiencies (i_. e_., the averages for
the 6-hr periods of the experiments) decreased with
increasing cell voltage (and the attendant increasing
current density). Almost certainly, one of the reasons
for the decrease in coulomb efficiency at the higher
voltages is that the acid concentration in the anolyte
solution increased to higher values during the 6 hours of
each experiment when high current densities (i^.e^ , high
voltages) were used. With increasing acid concentrations
in the anolyte solution, the anion-exchange membrane
separating the anolyte and center solutions becomes more
permeable to H+ ions and more of the H+ ions formed at
the anode are transferred out of the anolyte solution
through the anion-exchange membrane.
In addition to the analyses of the experimental data
that resulted in the average current densities and
coulomb efficiencies reported in Table IV, the current
densities and coulomb efficiencies were calculated for
each hour of the 6-hr period of operation. Typical
variations in current density and coulomb efficiency with
time are shown in Figure 7. The variation of current
desnity was taken from data printed by the recorder, but
the coulomb efficiencies had to be calculated for 1-hr
periods and are shown as average efficiencies for each
hour.
The current densities usually increased slightly during
the course of the 6-hr runs, as indicated in Figure 7.
This increase in current density is believed to be a
result of the increasing acid contents of the anolyte
solutions. However, in two of the experiments the
current density decreased appreciably during the last
hour of operation (from 48 to 35 mA/cm2 in Experiment
5723-120-1, and from 50 to 44 mA/cm2 in Experiment
5723-119-3). It was thought that these decreases in
current density might be caused by deposition of some
-42-
-------
70
UJ
f-
>-
o 60
o z
Z 1-1
ILI
u. i
U. I
at D
m O
o n.
_j
=> o
of
o
u_
50
JO
CONDITIONS: «».5 VOLTS, 6o°c
_L
123^5
TIME, HOURS
TYPICAL VARIATION OF COULOMB,EFFICIENCY WITH TIME
140
530
20
ILI
o
z
m
of.
at
u
o
10
CONDITIONS:
-------
of the particles of iron hydroxide in the circulating
catholyte solution upon the cation-exchange membrane
or the cathode, which would increase the resistance of
the cell. The cation-exchange membrane and the electrode
were inspected after the cell was disassembled and black
deposits were found on both. From the inspection it was
difficult to tell whether the deposits were formed during
the experiment or after the run was over. (The catholyte
slurries were not filtered during the experiments and
the solids contents increased throughout the runs. The
slurries were filtered or allowed to settle after the
runs were over.) Two additional experiments, described
later, were performed in an attempt to determine whether
deposition of iron hydroxide in the catholyte chamber
caused increases of cell resistance.
The settling times of the slurries of precipitates formed
in several of the experiments were determined. In these
determinations, measured volumes of well-stirred slurries
were placed in graduated cylinders and the positions of
the line of demarcation between the clear liquid above
the settling slurry and the slurry itself were recorded
at timed intervals. The precipitates settled to 50% of
the total volume in 30 minutes and to 25% of the volume
in 60 minutes.
The coulomb efficiencies for acid production usually
remained relatively high during the first 4 to 5 hr
of each 6-hr run, but decreased somewhat during the
last 1 or 2 hours of the run, as shown in Figure 7.
When the concentration of acid in the anolyte solution
exceeded about 45 g/1, the coulomb efficiencies began
to decrease and in most of the experiments the acid
concentration reached about 45 g/1 in 4 to 5 hours.
As stated previously, in some of the experiments there
were indications that particles of iron hydroxide in the
circulating catholyte solution could have deposited on
the cation exchange membrane facing the cathode of the
three-compartment cell and caused an increase in the
resistance of the cell.
The following experiments were performed to determine
whether deposition of particles on the cation-exchange
membrane or the cathode (or both) during operation
contributed to the increases in cell resistance noted
in some of the runs. A first acid-regeneration experi-
ment (Experiment 5723-121-1) was performed in the usual
-44-
-------
manner except that the rate of circulating the catholyte
solution was decreased from the normal rate of about
30 ml/sec to about 10 ml/sec. A decreased rate of
circulation should result in a lower rate of mixing
within the cathode compartment, and, therefore, should
increase the tendency of particles to deposit on sur-
faces in the cathode compartment. This first experi-
ment was followed by an experiment {Experiment 5723-122-1)
under identical conditions except that the circulation
rate was increased to the usual 30 ml/sec.
The conditions used for these two experiments were: cell
voltage = 4.5 V, temperature = 60°C, initial composition
of anolyte solution = 3 g/1 of H2SOit and 21 g/1 of FeSCU,
initial composition of solution circulated through center
compartment = 25 g/1 of H2S04 and 93 g/1 of FeSOi,,
composition of catholyte solution = 1 N NazSCu, the circu-
lation rates of anolyte and center solution were about
30 ml/sec, and the circulation rate of the catholyte
solution was about 10 ml/sec in one experiment and
30 ml/sec in the other.
A graph showing current density as a function of operat-
ing time for the two experiments is given in Figure 8.
In the experiment performed with a low flow rate of circu-
lating catholyte solution (Experiment 5723-121-1) the
current density stayed at about 24 mA/cm2 for slightly
more than the first hour. It then decreased to about
one-third of that value over the next 2 hours and remained
at 8 to 9 mA/cm2 during the final 3 hours of the experiment,
In the experiment performed with a higher flow rate of
catholyte solution, the current density was initially
about 33 mA/cm2 and increased slightly over the 6 hours of
the experiment. On the basis of these two experiments
it appeared that the flow rate of the catholyte solution
will be important to the operation of the cell. The
lower flow rate used in these two experiments apparently
did not "scrub" deposited solids off of the surfaces in
the catholyte compartments as thoroughly as the higher
flow rate did.
In a full-scale acid-regeneration process, solids are
not expected to build-up to as high a concentration in
the circulating catholyte solution as they did in these
experiments because in a full-scale process the solutions
would be filtered continuously, whereas the solution was
not filtered (or settled) until the end of the experiments.
-45-
-------
50
s
o
30
en
Z
ID
Q
UJ
20
10
EXPERIMENT
5723-122-1
EXPERIMENT
5723-121-1
12345
TIME, HOURS
Figure 8. Variations in Current Density with Time
-------
Nevertheless, the problem of solids build-up was con-
sidered to be serious and was studied further in the
first experiments with actual pickle liquor.
(4) Experiments with actual pickle liquor
The results of the previous experiments performed with
solutions of FeSOi* and HaSCU prepared in the laboratory
afforded valuable information about the operation of the
electromembrane cell in the batch-recycle regeneration
process. However, it was desirable to determine whether
the performance of the cell when treating actual spent
pickle liquor would be different from that observed when
the cell was used to treat the synthetic solutions. The
organic pickling aids are one type of material that is
known to be present in most actual spent pickle liquors
that might be detrimental. Some organic material, such
as humic acids, are known to foul ion-exchange membranes
and cause their resistances to increase. If the resist-
ances of membranes increase, the resulting energy costs
for treating spent pickle liquor could be high.
A sample of spent liquor that contained an organic
pickling aid (Activol made by the Henry Miller Company)
was obtained from one of the industrial sponsors. The
actual pickle liquor chosen was selected because its
composition appeared to be typical of that of four of
the six industrial sponsors. The composition of the
sample used in the experiments, as determined by
analyses, was:
eg/1 g/1 wt %
H2SO^ 0.61 30.0 2.8
FeSOi, 1.55 117.7 11.3
Fe2(S04)3 0.089 4.6 4.4
The specific gravity of the pickle liquor was 1.153.
In addition, an unknown amount of a proprietary organic
pickling aid was known to be present. We did not have a
method for analysis for the proprietary product, nor did
we find out what materials were in it. However, the
concentrations of the product normally used in pickling
operations is about 1 g/1 (0.1 wt %).
The three-compartment cell that was used in the experi-
ments with synthetic solutions was used for the experi-
ments with actual pickle liquor. The actual spent
-47-
-------
pickle liquor (300 ml) was circulated through the center
compartment of the cell, the same amount of a prepared
solution that contained about 20 g/1 of FeSOi* and about
5 g/1 of HaSOi* was circulated through the anolyte compart-
ment, and the same amount of 1 N NazSCN solution was
circulated through the catholyte compartment.
The first experiment with actual spent pickle liquor
(5723-124-1, Figure 9) was performed to study further
the possibility that precipitates of iron hydroxide
would be deposited on surfaces in the catholyte compart-
ment and cause an increase in the resistance of the cell.
This experiment was performed at 60°C with a cell voltage
of 5 volts and the low flow rate of catholyte solution
used previously (about 10 ml/sec). The membranes used in
this experiment had been used in the two prior 6-hr
experiments (5723-121-1 and 5723-122-1) with synthetic
solutions to determine whether increases in cell resist-
ances were a result of deposition of iron hydroxides on
membrane or electrode surfaces in the catholyte compart-
ment. After the prior experiments the membranes were
stored 12 days; they were initially immersed in dis-
tilled water, but, because of evaporation, some areas
of the membranes became dry.
During the course of Experiment 5723-124-1, several
measures were tried in an attempt to gain more infor-
mation about the effects of deposits of iron hydroxides
on membrane and electrode surfaces on the resistance of
the cell. From results of the previous experiments it
seemed probable that deposits were formed on either
the electrode or the membrane, or both. The deposits
may have resulted from electrophoretic transport of
the particles to the membrane or the electrode, or they
may have resulted simply as a result of particles imping-
ing against the surfaces as the slurry circulated through
the compartment. If the particles were being transported
electrophoretically and being held against the membrane
or electrode as a result of the electric field, halting
the electric current temporarily should permit some of
the adherent particles to be stripped off of the sur-
faces by the action of the circulating solution, which
should result in a reduction of cell resistance.
Similarly, reversal of the electric current should be
even more effective in reducing cell resistance.
Accordingly, we tried both halting and reversing the
-48-
-------
50
30
to
I
Q
OJ
a:
u
20
10
0123^567
TIME, HOURS
Figure 9. Effects of Changes in Operating Conditions on Current Density
-------
electric current. We also tried replacing the catholyte
solution, at least partially/ with fresh solution after
the solids content of the catholyte solution had in-
creased to form a fairly concentrated slurry to see
whether this would result in lower cell resistance.
The results of these experiments are shown in Figure 9
as changes in current density (with a constant cell
voltage) resulting from the changes in operating
conditions.
During the first hour of operation the current density
increased, probably as a result of the increasing acid
content in the anolyte solution. After 2 hr and 8 inin
of operation the power was turned off for 2 min. When
the power was turned back on, the current density was
almost as high as it was after 1 hr of operation, but
the current density dropped rapidly and the decrease
in current density then continued at about the same
rate as before the interruption of current. After a
total of 3 hr and 8 min the power was turned off for
4 min and most of the catholyte solution (that in the
reservoir) was replaced by fresh 1 N NaaSOi*. When
the power was turned on, the current increased from
the previous 22 to 34 mA/cm2. This increase was not
as great as that at the previous interruption, but
the current remained high for a longer period (almost
30 min). The replacement of catholyte solution was
repeated after 4 hr and 20 min, and again after 5 hr
and 40 min. After 6 hr and 45 min, the electrical
current was reversed for 3 min and then returned to
the original direction. The reversed current was
65 mA/cm2. When the current was returned to the
original direction, the current density was initially
52 mA/cm2, but declined to 30 mA/cm2 over a period of
about 18 min. Over the last 24 min of the experiment
the current density decreased to 27.5 mA/cm2.
All of the above changes in current density as a result
of changes in operating conditions indicated that the
decreases in cell resistance that had been observed
previously were partly, and perhaps largely, the re-
sult of deposition of iron hydroxides in the catholyte
compartment—on the membrane, on the electrode, or on
both. The results also suggested that replacement of
the catholyte solution with clean 1 N NaaSOi, solution
periodically in future experiments would come closer
to simulating the operation to be expected in a plant
operation, in which the catholyte solution would be
continuously filtered so that the solids content would
not build up.
-50-
-------
The coulomb efficiency for acid production during Experi-
ment 5723-124-1 was 65% compared with efficiencies of
60% and 49% for Experiments 5723-119-3 and -120-1 (see
Table V) in which the cell voltage was also 5 volts but
in which Fe2SOit-H2SOit solutions were used instead of
actual spent liquor. However, the coulomb efficiency
for iron removal was only 25% in 5723-124-1 compared
with efficiencies of 56 and 50% for the two experiments
made with synthetic solutions. Because of these changes
in coulomb efficiencies along with our knowledge that
the membranes had partially dried during storage, it was
thought that the transport properties of the membranes
used for Experiment 5723-124-1 might have been altered
by the partial drying or by accumulations of deposited
iron hydroxide. Accordingly, we installed a new set of
MC 3142 and MC 3148 membranes in the cell for two more
experiments.
These two experiments were performed with the actual
spent pickle liquor circulating through the center
compartment of the three-compartment cell and with the
same operating conditions as before except that the flow
rates of the circulating solutions were approximately
30 ml/sec. In these experiments (Experiments 5723-125-1
and -125-2, Table V) the catholyte solution was replaced
each hour with fresh 1 N Na2SOi, solution in an attempt
to simulate a commercial operation in which the catholyte
solution would be continuously filtered. In replacing
the catholyte solution, the slurry of iron hydroxide
was drained from the reservoir and an equal volume of
1 N NaaSOit was added. Thus, the portion of the slurry
that was in the tubing and the cell was not replaced;
nevertheless, most of the solids were removed by the
replacement procedure.
The average current density of 32 mA/cm2 in the second
experiment (5723-125-2) was lower than that in Experi-
ment 5723-125-1 (41 mA/cm2). At the end of Experiment
5723-125-1, the solutions were left in the cell overnight
to keep the membranes moist. Just before Experiment
5723-125-2, the solutions were drained from the cell,
the compartments were rinsed briefly with water, and
filled with new solutions. There was some possibility
that the overnight storage of the membranes in the
presence of the iron hydroxide slurry in the catholyte
compartment could have caused the lower current density.
-51-
-------
Table V. Performance of Experimental Cell with Synthetic Solutions
and with Actual Spent Pickle Liquor
Experiment
5723-
With Fe2SO»-
119-2
120-2
122-1
120-3
119-3
120-1
1
Ui 119-4
NJ
' With actual
125-1
125-2
127-1
127-2
n f* a 1 <^n 1 a
Cell
voltag
•H2SO,
4
4
4.5
4.5
5
5
6
spent
5
5
5
5
4-ari a«j
Average
current
Final H2SO,,
Average area! concentration
density, resistance o
re ma/cm ohm-cm
solutions
24
21
36
33
45
39
50
pickle liquor
41
32
47
62
p = Efcotal 7
62
72
56
60
56
64
70
60
78
53
41
rp
decomp .
f cell, in anolyte
2 g/1
32
33
50
39
59
44
57
66
48
60
77
Etotal - 2.5
Wt. , % £
3.1
3.2
4.9
3.8
5.7
4.3
5.6
6.4
4.7
5.8
7.5
where E. _ ,
Average coulomb
efficiencies, %
For acid For iron
iroduction removal0 Comments
70
67
61
53
60
49
52
61
53
66
46
, is the
71
71
50
67
56
50
44
60
56
72
32
New membranes
Membranes used
New membranes
Membranes used
New membranes
Mew membranes
New membranes
New membranes
Membranes used
New membranes
Membranes used
total voltage, E, is
6 hours previously
12 hours previously
6 hours previously
5 hours previously
the
decomposition potential (2.5 volts), and i = current density, amp/cm2.
Calculated from the difference between the total equivalents of H"1" ions at th
-------
Experiments 5723-127-1 and 5723-127-2 were performed
with operating conditions identical to those in Experi-
ment 5723-125-1 and -2 except that the flowrates of the
solutions were increased to approximately 50 ml/sec.
Experiment 5723-127-2 was performed as soon after
Experiment 5723-127-1 as the three solutions could be
drained and replaced.
In contrast to the two prior experiments, the average
current density in Experiment 5723-127-2 was higher (62
ma/cm2) than that in Experiment 5723-127-1 (47 mA/cm2).
Both current densities were higher than those observed
in Experiments 5723-125-1 and -2, which suggests the
more rapid circulation rates of the solutions in Experi-
ments 5723-127-1 and -2 aided in reducing the resistance
of the cell.
If the higher current density (62 mA/cm2 at 60°C) could
be maintained, the costs estimated later (Section VII)
might be considerably reduced, since those estimates
were based on average current densities of 48 to 54
ma/cm2.
In Experiment 5723-127-2, there was a large decrease in
the coulomb efficiency below that achieved in Experiment
5723-127-1. In contrast, the difference in coulomb
efficiencies in Experiments 5723-125-1 and -2 was no
greater than the difference in Experiments 5723-119-3
and 5723-120-1, which were performed with FeSCH-HaSOi,
solutions and with new membranes for each experiment.
After Experiment 5723-127-2 the cell was dismantled and
the components were inspected. It was found that the
membranes were in good physical condition, but the
catholyte compartment contained heavy deposits of pre-
cipitate. It is not known whether the presence of this
precipitate affected the coulomb efficiencies. A filter
in the circulation loop for the catholyte solution would
certainly have been desirable, but it would have caused
pressure imbalances between the three compartments and
attendant bowing or bending of the membranes without
modifications of the apparatus. Limitations on time
and money did not permit making those modifications
and performing additional experiments.
-53-
-------
The results of the experiments with actual pickle liquor
indicate that the electromembrane cell was as satisfactory
for treating actual spent pickle liquor as for treating
the FeSCK-HaSCK solutions. However, some uncertainties
remain concerning the effects of organic materials in
actual spent pickle liquor in long-term operations. The
trial with only one actual pickle liquor (and one type
of organic pickling aid) does not provide an adequate
basis for making a final conclusion regarding the suit-
ability of the process for general plant use. In addition
the performance of the cell should be evaluated over long
periods of time before a final conclusion is made. We
also believe a modified laboratory-scale apparatus with
a filter in the catholyte-solution loop should be con-
structed and experiments with other actual pickle liquors
should be made to obtain a broader base of data on which
to base a decision on the technical feasibility of the
process for plant operations.
-54-
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VI. CONTINUOUS REGENERATION PROCESS
Some of the data from the batch-recycle experiments per-
formed in the bench-scale equipment indicated it might
be feasible to operate a continuous process for regenerat-
ing acid from spent pickle liquor. This process was not
investigated experimentally because of limitations of
time and money.
A flow diagram for the continuous acid-regeneration pro-
cess is shown in Figure 10.
The pickling acid would contain enough free acid to
assure adequate pickling, for example, 40 g/1., and a
sufficiently low amount of FeSOi, (e_.g_. , 120 g/1.) so as
not to impair the pickling action. In the continuous
process, the liquor from the pickling line is circulated
to the electromembrane cells and back to the pickling
line. In the pickling line, the concentration of iron
ions in the solution is increased; in the cells the iron
ions added in the pickling line are removed and replaced
by hydrogen ions to regenerate pickling acid. In the
cells, the pickling solution flows first through the
center compartments, where iron ions (along with some
hydrogen ions) are transferred through the cation-
exchange membranes into the catholyte compartments where
iron ions combine with hydroxide ions to form insoluble
iron hydroxides, which are filtered off. The pickling
solution leaves the center compartments and enters the
anolyte compartments where H+ ions are formed to combine
with SO^2 ions transferred through the anion-exchange
membranes and regenerate HzSO.,. The regenerated acid
is sent back to the pickling line.
This continuous process is simpler and would require
less equipment than the batch-recycle process.
Consideration of the electrolytic transfers that occur
within the cells shows that:
• The coulomb efficiency for iron removal will be
determined by the value of the transport number
for iron ions in the cation-exchange membrane,
-55-
-------
Ul
FILTER
TANK
I
CELLS
r r
i
i
ACID
TANK
PICKLING
LINE
FILTER
C - CATION-EXCHANGE MEMBRANE
A - ANION-EXCHANGE MEMBRANE
Figure 10. Flow Diagram of Continuous Regeneration Process
-------
• The change in the hydrogen-ion concentration of
the solution in the center compartments will be
determined by the difference between t§, which is
the equivalents of H+ ions per faraday entering
the center compartment through the anion-exchange
membrane, and t§, which is the equivalents of
H+ ions per faraday leaving the center compartment
through the cation-exchange membrane.
• The coulomb efficiency for enrichment of hydrogen
ions in the anolyte compartment will be deter-
mined by the difference between the 1 equivalent
per faraday of H+ ions formed at the anode and the
t§ equivalents of H+ ions leaving through the
anion-exchange membrane.
As a part of the determination of the feasibility of such
a process, the specific permselectivity of cation-exchange
membranes for H+ ions relative to Fe+ ions was calculated
from the data obtained from the bench-scale experiments.
The specific permselectivity of an ion-exchange membrane
for one ion relative to another is defined as the ratio
of the ionic velocities of the two ions in the membrane.
Thus, the specific permselectivity of H+ ions relative
to Fe+2 ions can be calculated from the following equation:
+ H+ Fe+2
m11 _ i _ j.
TFe+2 ~ H+ '
H+
where: Tpe+2 = specific permselective of H+
ions relative to Fe+2 ions
H+
t = transport number of H ions
t = transport number of Fe+2 ions
H+
C =, concentration of H+ ions in the
mixed solution, eq/1.
pe+2
C = concentration of Fe+2 ions in
the mixed solution, eq/1.
-57-
-------
The transport numbers of H4" and Fe 2 ions through the
cation-exchange membranes were calculated from the data
obtained in the batch-recycle experiments. The concen-
trations of H+ and Fe"1"2 ions in the mixed solutions of
H2SOi, and FeSOi* that were circulated through the center
compartment of the cell were also calculated. The
specific permselectivities that were calculated from
the transport numbers and concentrations ranged from
values of 1.2 to 2.1, and the average value was 1.7.
Because this appeared to be a fairly wide range we
attempted to find relationships between the specific
permselectivities and the concentrations of IT" ions, the
concentrations of Fe+2 ions, or the ratio of H+ to Fe"1"2
in the solutions of H2S(H and FeSCK. However, there
appeared to be little relationship between the above
concentrations or ratios and the specific permselectiv-
ities. It appeared that the spread in the values of
specific permselectivity was a result of slight in-
accuracies inherent in the experiments used to determine
the transport numbers. Since tH+ = 1 - tFe+ the
specific permselectivities calculated from Equation 1
would be sensitive+to sligh£ variations in the calcu-
lated values of tH or tFe"*" that might result from
slight inaccuracies in experimental measurements.
The specific permeability of H+ to Fe+2 ions in
MC 3142 membranes was taken to be 1.7 ± 0.4 and this
value of specific permeability for H+ to Fe+2 ions was
used in the calculations of cost estimates for the
continuous acid-regeneration process.
Data obtained in the bench-scale equipment while
performing the batch-recycle experiments were also
used to calculate the transport numbers of H+ ions
(tl[+) through the MA 3148 anion-exchange membrane
used in the cell for use in the determination of the
feasibility of the continuous process of acid regen-
eration.
The fact that the coulomb efficiency for acid production
decreased as the concentration of acid in the anolyte
solution increased indicated that the transport of H+
ions through the MA 3148 membrane increased and the
transport of SO"2 ions decreased as the concentration
of Ha SO i* in the circulating anolyte solution increased
during each 6-hr experiment. Therefore, we calculated
values of t§ from data for the changes in the concen-
tration of H2SOit in the anolyte solution during 1-hr
-58-
-------
periods of operation during each experiment, since the
concentration of I^SOit increased with time during each
experiment. In this way, we obtained an idea of the
variation of t§+ with acid concentration.
Equation 2* was used for these calculations.
H+ = AF - Ae (2)
^a AF ^'
TT+ ,
where: t§ = transport number of H"1" through the
anion-exchange membrane
AF = faradays passed through the cell
during a given time period
Ae = the change in the total equivalents
of HeSOi, in the anolyte solution
during the given time period
The variation of the calculated values of t§+ with the
concentration of HaSO4 in the anolyte solution is shown
in Figure 11. As indicated by the two lines, there were
two groups of data points—one for cell voltages of 4 V
and the other for cell voltages of 4.5 to 5 V. The
reason for the separation of the two groups of points is
not known; perhaps the difference is a result of some
difference in concentration gradients in the boundary
layers adjacent to the membrane surfaces.
In the calculations made as a part of the estimations of
costs for the continuous process of acid regeneration
(given later in Section VII), we used values of tl+ taken
from the top line in Figure 10 because the assumption of
high values of t§ resulted in the highest and, therefore,
most conservative estimations of operating costs.
*Equation 2 stems from the fact that the increase in the
total equivalents of H2SOi, in the anolyte solution equals
the equivalents of SO^2 transferred through the anion-
exchange membrane, and the equivalents of S0£2 trans-
ported divided by the faradays passed through the cell is
t|<"2, and t§+ = 1 - *
-59-
-------
0.7
33
i
(Ti
O
I
0.6
to
Z UJ
o z
I- <
+ CO
xs °-5
u. s:
o
co
CC J-
UJ ^
CO fv
o <
z 2: 0.4
H- Z
o: >-•
o
Q.
to
Z
0.3
.5 AND 5 VOLTS
VOLTS
0
0.1 0.2
1.2 1.3
0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 1.1
CONCENTRATION OF HzSO^ IN ANOLYTE, eq/1
Figure 11. Variation of the Transport Number of H+ Ions through MA 3148
Anion-Exchange Membrane with the Concentration of H2SOi, in the Anolyte Solution
-------
We concluded that a continuous process for acid regen-
eration was feasible and for some pickling operations a
continuous acid-regeneration process might be prefer-
able to the batch-recycle type of processing.
-61-
-------
VII. ESTIMATES OF COSTS
The capital and operating costs of the acid-regeneration
process were estimated for pickling operations in which
2750 Ib/day, 485 Ib/day, and 44 Ib/day of iron are
dissolved in the pickling acid. The details of one of
the cost estimates are given in the appendix to illustrate
the methods used and the assumptions made.
These sizes of operation correspond approximately to the
pickling operations of three of the industrial sponsors.
The largest assumed scale of operation (2750 Ib Fe/day)
corresponds to a pickling operation that requires dis-
posal of about 6700 gal. of spent liquor each day. The
other two sizes (485 Ib and 44 Ib Fe/day) correspond to
operations which require disposal of about 2400 and
200 gal./day of spent liquor, respectively. (The lack
of correspondence between the ratios of pounds of iron
dissolved to gallons of spent liquor for the three
assumed sizes of plants illustrates the variations in
pickling practice that result in wide variations in the
concentrations of EzSOt, and FeSOi* in the spent liquors.)
The assumptions pertaining to materials of construction
for the cost estimates were based on the behavior of the
materials used in the laboratory units during 80 hr of
operation at 60°C. Some of the information about
materials of construction is:
• The reinforced phenolic plastic from which the
end frames and center frames of the cell were
made (Micarta, grade 223, from Westinghouse
Electric Corp.) showed no apparent degradation.
• Neither the graphite cathode nor the antimontial-
lead anode (6% Sb, from National Lead Company)
showed any signs of degradation.
• The Hastalloy pumps that were used gave satis-
factory service except that the bearing packing
on the pump that was used for the catholyte
solution had to be replaced once. It is
believed that this need for replacement was
due largely to the solid particles in the
catholyte solution, which would not be present
in a large-scale process because that solution
would be continuously filtered.
-63-
-------
• The room-temperature-vulcanizing silicone rubber
(Silastic 731 RTV, Dow Corning Corp.) used to
form gasketing surfaces on the end frames and
center frames gave satisfactory service.
It appears that the materials used for the components
of the cell and in the pump should give satisfactory
service in large units. However, longer runs in a
pilot-plant cell will be needed to provide more
meaningful data on service lifetimes of materials.
The capital costs and operating costs for the 2750 Ib
Fe/day operation are compared in Tables VI and VII for
three variations of processing: batch-recycle with
4.5 V/unit cell, batch-recycle with 5 V/unit cell, and
continuous processing with 5 V/unit cell.
Table VI shows that the capital needed for the batch-
recycle process will be slightly less when 5 V/unit
cell is used than when 4 V/unit cell is used because
of the lower cost of the cells. Table VII shows that
the operating cost also will be lower with 5 V/unit
cell. The data in these tables also show that both
the capital needed and the operating costs will be less
for the continuous process than for the batch-recycle
process.
The capital costs for a plant to remove 485 Ib of dis-
solved iron per day and regenerate the acid by the
continuous process were estimated to be $66,900. The
operating costs with 5 V/unit cell are shown in Table
VIII.
The capital costs for a plant to remove 44 Ib of dis-
solved iron per day and regenerate the acid by the
continuous process were estimated to be $12,200. The
operating costs with 5 V/unit cell to remove 44 Ib
Fe/day are shown in Table IX.
The daily operating costs for removal of 485 Ib Fe/day
were not as much below those for removal of 2740 Ib
Fe/day as might be expected. The main reason is that
the estimated cost of operating and maintenance labor
did not decrease at all.
-64-
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Table VI. Capital Costs for Electromembrane Acid-Regeneration Processes
for Treating Spent Pickle Liquor
(Size—2750 Pounds of Iron Dissolved per Day)
Capital costs,
Item
Cells
Rectifiers
Tanks
Pumps
Filters
Heat exchanger
Voltmeter
Ammeter
Timer and solenoid valves
(or other instruments)
Cost of installation3
Installed cost of principle items
of equipment
Cost of piping and wiringb
Sub-total I
Other costs of construction0
Total cost
Batch-recycle
with 4.5 volts/cell
45,500
33,200
6,360
1,160
7,200
1,900
300
300
1,450
$97,370
4,800
>ms
$102,170
61,300
$163,470
81,700
Batch-recycle
with 5 volts/cell
38,800
37,000
6,360
1,160
7,200
1,900
300
300
1,450
$94,470
4,800
$99,270
59,500
$158,770
79,000
Continuous process
with 5 volts/cell
38,800
37,000
3,180
720
7,200
1,900
300
300
1,450
$90,850
3,900
$94,750
56,600
$151,350
75,650
$245,170
$237,770
$217,000
a. It is assumed the equipment can be housed in an existing building and that no additional
foundations, supports, vents, or chutes will be needed (see appendix).
b. 60% of principal items of equipment.
c. 50% of sub-total 1. Includes costs of providing utility services (electrical, steam, etc.),
site preparation, interest on funds for construction, contractor's profit, and contingencies,
-------
Table VII. Total Costs for Electromembrane Acid-Regeneration Processes
for Treating Spent Pickle Liquor
(Size—2750 Pounds of Iron Dissolved per Day)
Conditions
In batch-recycle process:
Composition of spent pickle liquor:
HzSCU - 25 g/1 (0.51N) 2.3 wt.
FeSO, - 74 g/1 (0.97Ni 6.7 wt.
Composition of regenerated acid:
H2SO» - 60 g/1 (1.22SN)5.7 wt.
FeSO* - 20 g/1 (0.26N7 1.9 wt.
Time of cycle - 6 hr
Temperature - 70*C
Coulomb efficiency - 50%
In continuous process:
Average composition of pickle liquors
H2SO, - 40 g/1 (0.815N) 3.6 wt. I
FeSO» - 120 g/1 (1.600N)10.5 wt. %
Temperature - TO'C
Coulomb efficiency - 50%a
Item
Make-up acid
Energy
Operating labor
Maintenance labor
Supervision
Payroll extras
Operating supplies
Membrane replacement
Electrode replacement
General overhead
Taxes and insurance
Interest on working capital
Total direct costs
Amortization:
at 8% - 15 yr
at 10% - 15 yr
Total costs
Batch-recycle
with 4.5 volts/cell
3.50
94.58
24.00
8.00
Total costs, $/day
3.20
7.06
8.20
28.80
.78
.80
3.
4.
8.20
6.67
201.92
Batch-recycle
with 5 volts/cell
3.50
104.88
24.00
8.00
3.20
7.06
7.24
21.80
3.
Continuous process
with 5 volts/cell
3.50
104.88
24.00
8.00
.22
,80
,24
3.20
7.06
7.24
21.80
3.22
.80
.24
6.67
198.61
4.
7.
6.67
198.61
95.00
92.00
84.50
108.00
104.00
$296.92 S309.92 $290.61 $302.61 $283.11
98.50
$297.11
Total costs on the basis of $/equivalent gallon of spent liquorb
$0.0443 $0.0462 $0.0435 $0.0451 $0.0420
$0.0442
a. Calculations show the coulomb efficiency will be 50% when the pickling acid contains
0.815 eq/1 of H2SO» and 1.60 eq/1 of FeSO* and the specific permselectivity is 1.7.
b. Estimated daily operating costs divided by the gallons of spent liquor being disposed
of in the existing process.
-66-
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Table VIII. Total Costs for an Electromembrane Acid-Regeneration Process
(Continuous process assumed with 5 volts/cell)
(Size—485 Pounds of Iron Dissolved per Day)
Conditions
Average composition of pickling solution:3
H2SO., - 40 g/1 (0.815N) 3.6 wt %
FeSOi, - 120 g/1 (1.600N) 10.5 wt %
Spacings between membranes tor membranes and electrodes) - 0.4 cm
Temperature - 70°C
Coulomb efficiency - 50%
Cost,
Item S/day
Make-up acid 0.80
Energy 21.60
Operating labor 24.00
Maintenance labor 8.00
Supervision 3•20
Payroll extras 7.06
Operating supplies 2.23
Membrane replacement 3.90
Electrode replacement 0.57
General overhead 4.80
Taxes and insurance 2.23
Total direct costs 78.39
Amortization:
at 8% - 15 yr 26.00
at 10% - 15 yr 30.40
Total cost $104.39 $108.79
Operating costs on the basis of $/equivalent gallon of spent liquor
With amortization:
at 8% - 15 yr $0.0435/gal. of spent pickle liquor
at 10% - 15 yr $0.0455/gal. of spent pickle liquor
a. Calculations show the coulomb efficiency will be 50% when the
pickling acid contains 0.815 eq/1 of HjSOi, and 1.60 eq/1 of
FeSOi, and the specific permselectivity is 1.7.
-67-
-------
Table IX. Operating Costs for an
Electromembrane Acid-Regeneration Process
(Continuous process assumed with 5 volts/cell)
{Size - 44 pounds of iron dissolved/day)
Conditions
Average composition of pickling solution:3
H2SOi, - 40 g/1 (0.815N) 3.6 wt %
FeSCU - 120 g/1 (1.600N) 10.5 wt %
Spacings between membranes (or membranes and electrodes) - 0.4 cm
Temperature - 70°C
Coulomb efficiency - 50%a
Cost,
Item $/day
Make-up acid 0.14
Energy 2.94
Operating labor 24.00
Maintenance labor 8.00
Supervi s ion 3.20
Payroll extras 7.04
Operating supplies 0.27
Membrane replacement 0.42
Electrode replacement 0.06
General overhead 0.53
Taxes and insurance 0.27
Total direct costs 46.87
Amortization:
at 8% - 15 yr 7.48
at 10% - 15 yr 8.70
$54.35 $55.57
a. Calculations show the coulomb efficiency will be 50% when the
pickling acid contains 0.815 eq/1 of H2SOi, and 1.60 eq/1 of
FeSOi, and the specific permselectivity is 1.7.
-68-
-------
For the small plant for removal of 44 Ib Fe/day, the
estimated capital costs are much higher per unit of
throughput ($96/daily gal. of spent liquor) than for
the larger plants $28 and $33/daily gal.). This was
expected because the smaller processing equipment often
costs nearly as much as larger sizes. The costs for
operating labor, maintenance labor, supervision and
payroll extras ($32.24 total) are by far the major
costs for this scale of operation. It may be possible
to assign an existing operator (with some other major
task) to make the routine checks of instruments and
thus reduce the costs indicated in Table IX appreciably,
The estimated operating costs should be considered in
relation to the increasing problems of disposal and to
the present costs of purchasing acid and disposing of
spent pickle liquor, which range from about $0.015/gal.
to $0.06/gal., as indicated below.
Cost of
Disposal disposal, Cost of acid, Total cost,
method $/gal. $/gal. $/gal.
Deep-well
injection 0.005 to 0.01 0.01 to 0.02 0.015 to 0.03
Neutralizing
and
lagooning 0.01 to 0.02 0.01 to 0.02 0.02 to 0.04
Neutralizing
and
land-fill 0.02 to 0.03 0.01 to 0.02 0.03 to 0.05
Hauling 0.02 to 0.04 0.01 to 0.02 0.03 to 0.06
The cheaper methods of disposal (deep-well injection and
neutralizing and lagooning) cannot be used at all local-
ities without additional costs for transporting the
spent liquor.
The estimated costs for the large and intermediate sizes
of treatment plants are in the range of costs for the
hauling and neutralization and land-fill. Therefore,
the estimated costs suggest that the electromembrane
process will be competitive in cost with the present
-69-
-------
operations. In addition, if the electromembrane process
is used, it will be necessary to dispose only of the
solid hydroxides of iron. Disposal of these insoluble
solids can be achieved by earth-fill methods with no
problems of water or air pollution. Thus, the proposed
electromembrane process should eliminate the pressures
to find methods of disposal that will not result in
pollution problems.
-70-
-------
ACKNOWLEDGMENTS
This work was performed under the direction of Robert
E. Lacey, Head, Membrane Processes Section, Southern
Research Institute with the assistance of Mr. John J.
Holmes, Assistant Chemical Engineer, Mr. J. B. Powell,
Assistant Chemist, Mr. Don B. Hooks and Mr. Samuel
Edward, Chemical Technicians. The research was supported
in part by the following companies:
Amax Lead and Zinc, Inc.
Armco Steel Corporation
Continental-Emsco Company
Pittsburgh Tube Company
Stockham Valve and Fittings, Inc.
Vulcan Rivet and Bolt Corporation
This report was submitted in fulfillment of Project No.
12010 EQF under the partial sponsorship of the Water
Quality Office.
-71-
-------
APPENDIX
Cost Estimates
Estimates of capital and operating costs for the acid-
regeneration process have been prepared for three sizes
of pickling operation, for two variations in type of
processing (batch and continuous), and for two assumed
cell voltages. The details of one of the cost estimates
are given below to illustrate the estimating methods
used.
For this estimate, we assumed the batch-recycle method
of operation described in Section III would be used.
We assumed that the unit to treat spent pickle liquor
would be required to remove approximately 2750 Ib Fe/day
from the liquor, which is roughly the amount of iron that
one of the industrial sponsors now accumulates in the
pickling bath each day. (This sponsor now must dispose
of about 6700 gal./day of spent pickle liquor.) Other
assumptions were as follows:
- The unit will be operated 24 hr/day and 300 days/yr,
- Unit cells of the type shown in Figure 5 will be
used with spacings of 0.4 cm between membranes
and electrodes, or between membranes for all
three compartments. Many three-compartment unit
cells will be assembled into a filter-press type
of stack like those used for production of hydrogen
and oxygen to conserve floor space.
- Bipolar electrodes will be used in the filter-press
cells. They will be low carbon steel on the sur-
faces used as cathodes and antimonial lead on the
surfaces used as anodes.
- The operating temperatures will be 70°C.
- The cell voltage will be 4.5 V/unit cell.
- The coulomb efficiency will be 50%. For cell
voltages of 4.5 V, coulomb efficiencies from 50%
to 56% were achieved in the experiments.
- The efficiency of rectification will be 94%.
-73-
-------
- The cost of energy will be $0.008/kwh. This is
higher than the national average of $0.007/kwh.
- The cost of membranes will be $3.00/ft2.
- The percentage of the membranes utilized will
be 85%.
- Average membrane lifetime will be 2 yr.
- Average electrode lifetime will be 6 yr.
- The cost of bipolar electrodes will be $2.65/ft2
(based on cost of $1.98/ft2 for lead, $0.42/ft2
for steel, and $0.25/ft2 for fabrication). The
electrodes consist of a 1/16-in. thickness of
antimonial lead rolled onto 16-gage steel.
- Rectifiers will cost $70 per kw of capacity.
- The average current density will be 48 mA/cm2,
or 44.5 A/ft2 (based on our experimental data).
- Six hours per day of an operator's time will be
needed.
- Two hours per day of a maintenance man's time
will be needed.
- Supervision will be 10% of direct labor.
- The operator and maintenance man will receive
$4.00/hr.
- Payroll extras will be 20% of direct labor costs.
- Cost of operating supplies will be 1% of capital
investment.
- General administrative overhead will be 15% of
direct labor cost.
- Capital investment will be depreciated over the
15 years estimated as the lifetime of the plant.
The capital investment includes the cost of all
components, except the membranes and electrodes
which are treated as expendable items and costs
for their replacement are included in addition
to the amortization of capital costs.
-74-
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- The interest to be paid for the money needed for
the capital investment was calculated for two
rates—8% and 10%. Since this investment will
almost certainly not be made until 1972, it is
difficult to foresee what an appropriate rate
of return would be at that time. Therefore,
the cost of amortization is given on two bases.
- Taxes and insurance will be 1% of the total
investment.
The costs of standard items of equipment, such as tanks,
pumps, and filters, were estimated by updating information
given by H. E. Mills.5
The general method of estimating capital costs was that
described by C. Miller.6 In this method of estimation,
the installed cost of the principal items of equipment
is adjusted by certain factors to obtain an estimate of
the total capital cost.
The total area of unit cells that will be needed to
remove 2750 Ib Fe/day (i.e_., 0.516 g eq/sec) with a
coulomb efficiency of 50~% was estimated as follows:
0.516 g eq/sec x 96,500 coul/g eq x (1/0.5)=99,500 coul/sec
99,500 A v 44.5 A/ft2 = 2240 ft2
A design of unit cells was selected in which the membranes
and electrodes will be 39.4 in. square. With a total area
of 10.8 ft2/cell and an effective area of 9.4 ft2, the
number of unit cells needed will be 2240 •=• 9.4 = 238. A
unit cell consists of a cathode, an anode, an anion-
exchange membrane, a cation-exchange membrane and the
5. H. E. Mills, "Costs of Process Equipment", in Modern
Cost-Engineering Techniques, H. Popper, Editor, McGraw-
Hill Book Company, New York (1970).
6. C. Miller, "New Cost Factors Give Quick, Accurate
Estimates", in Modern Cost-Engineering Techniques,
H Popper, Editor, McGraw-Hill Book Company, New
York (1970).
-75-
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three mesh-like spacers needed to maintain the proper
spacing between membranes and electrodes. Each unit
cell will require three cell frames and spacers, one
anion-exchange membrane, one cation-exchange membrane,
and one bipolar electrode. The cost of the 238 unit
cells complete with end frames and clamping bolts is
as follows:
Cell frames—238 x 3 x $25/frame = $17,900
Ion-exchange membranes—
238 x 2 x $3/ft2 x 10.8 ft2 = 15,500
Electrodes—238 x $2.65/ft2 x 10.8 ft2 = 6,800
End frames and bolts 1,250
Assembly cost and contingency (10% of above items) 4,150
$45,500
The cost of the rectifiers was estimated as follows:
99,500 A x 4.5 V = 447,000 V A d-c or 447,000 f 0.94 =
475,000 W or 475 kw.
Cost of rectifiers = 475 x $70/kw = $33,250.
Four 4000-gal. Fiberglas-reinforced tanks will be needed.
In Owens-Corning Corporation Catalog, Type 104MC2 tanks
(4000 gal.) sell for $1590 each. The freight cost will
just about offset the 10% discount that was quoted. The
cost of four tanks will be 4 x $1590 = $6360.
Four pumps will be needed. Eastern Manufacturing
Company's Model D-ll pumps with Hastalloy impellers and
cases should be satisfactory. These pumps sell for $290
each, so the cost will be 4 x $290 = $1160.
One sand filter will be needed to filter particulate
matter from the spent pickle liquor. The cost was
estimated to be $1200.
The continuous pressure filters needed for the filtration
of the precipitated iron hydroxides were estimated to
cost $6000, if made of 316 stainless steel. Two quick-
opening, vertical-leaf pressure filters (each with 130
square feet of filtering area will be needed. One
filter will be on-stream while the cake is being dumped
from the other.
-76-
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The heat exchanger needed in the pickling line to eliminate
the need for heating with direct steam is estimated to cost
$1900.
The instruments needed (a voltmeter, ammeter, and timer
with solenoid valves) are estimated to cost $2050.
Table X summarizes the costs of the principal items of
equipment and the other costs comprising the estimated
total costs.
The operating costs were then estimated.
Cost,
$/day
The cost of energy was estimated to be
d-c energy = 475 kwh/hr x 24 hr/day x $0.008/kwh 91.50
a-c energy (for pumps, and filter) 16 kwh/hr x 24
x $0.008/kwh 3.08
Total 94.58
The cost of operating labor = 6 hr x $4.00/hr 24.00
The cost of maintenance labot = 2 hr x $4.00/hr 8.00
The cost of supervision, 10% of direct labor,
10% x $32.00 3.20
The cost of payroll extras, 20% of total labor,
20% x $35.20 7.06
The cost of operating supplies
1% of $245,170 for 300 days (1 yr) 8.20
The cost of membrane replacement
$15,500 for 600 days (2 yr) 25.80
The cost of electrode replacement
$6,800 for 1800 days (6 yr) 3.78
The cost of general overhead, 15% of direct
labor, 15% of $32.00 4.80
The cost of taxes and insurance, 1% of capital
cost, 1% of $245,170 for 300 days (1 yr) 8.20
The cost of interest on working capital
10% of $20,000 for 300 days (1 yr) 6.67
The operating costs are summarized in Table XI.
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Table X. Estimated Capital Costs for
Acid-Regeneration Process
(For pickling operation that
dissolves 2750 Ib Fe/day)
Item
Cells
Rectifiers
Tanks
Pumps
Filters
Heat exchanger
Voltmeter
Ammeter
Timer and solenoid valves
Installation13 ($10/hr x 480)
Installed cost of principal items of
equipment (PIE)
Piping and wiring (60% of PIE)
Sub-total
Other construction costsc
(50% of sub-total
Total cost
Cost, $
45,500
33,200
6,360
1,160
7,200
1,900
300
300
1,450
97,370
4,800
102,170
61,300
163,470
81,700
245,170
Estimated
hours to
install3
120
80
120
60
50
20
5
5
2£
480
Amortization: at 8% for 15 yr
at 10% for 15 yr
0.117 ($245,170) » $28,600
0.132 ($245,170) - $32,400
a. Estimated installation times from reference 6.
b. It is assumed that no additional building, foundations,
or vents will be needed.
c. Includes the costs of providing utility services, site
preparation, interest on construction funds, and contin-
gency .
-78-
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Table XI. Operating Costs for Electromembrane
Acid-Regeneration Process
Item Cost,$/day
Acid (250 Ib/day make-up at $0.014/lb) 3.50
Energy 94.58
Operating labor 24.00
Maintenance labor 8.00
Supervi s ion 3.20
Payroll extras 7.06
Operating supplies 8.20
Membrane rep1acement 28.80
Electrode replacement 3.78
General overhead 4.80
Taxes and insurance 8.20
Interest on working capital 6.67
201.92
Amortization: at 8% = $28,600/300 95.00
at 10% = $32,400/300 108.00
Daily operating cost: amortized at 8% 296.92
amortized at 10% 309.92
-79-
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BIBLIOGRAPHICi
Southern Research Institute, An Electromerabrane Process
for Regenerating Acid from Spent Pickle Liquor, Final Re-
port WQO Grant No. 12010 EQF, March, 1971.
ABSTRACT
Studies of an electromembrane process for regenerating acid
from spent sulfuric acid pickle liquor have indicated that
the process is technically feasible. The studies have shown
that the iron ions in spent pickle liquor can be removed and
replaced by hydrogen ions to regenerate HjSO, in electro-
membrane cells.
A method of removing iron from spent liquor that involves
the formation of insoluble iron hydroxides is preferrable to
plating iron metal onto cathodes.
Estimated treatment costs were $0.045 1 0.002 per gallon,
whereas the combined costs of purchasing acid and disposing
of spent liquor by existing methods were in the range of
$0.015 to $0.06 per gallon of spent liquor.
A determination of the long-term performance of the ion-
exchange membranes when treating actual pickle liquors that
contain organic pickling aids is needed.
This report was submitted in fulfillment of Project 12010
EQF under the partial sponsorship of the Water Quality Office.
ACCESSION NO.
KEY WORDS:
Industrial wastes
Spent Pickle Liquor
Pickling of Steel
Acid Regeneration
Ferrous Sulfate
Membrane Processes
Treatment Costs
Ion-Exchange Membranes
Electrolytic Cells
BIBLIOGRAPHIC:
Southern Research Institute, An Electromerabrane Process
for Regenerating Acid from Spent Pickle Liquor, Final Re-
port WQO Grant No. 12010 EQF, March, 1971.
ABSTRACT
Studies of an electronembrane process for regenerating acid
from spent sulfuric acid pickle liquor have indicated that
the process is technically feasible. The studies have shown
that the iron ions in spent pickle liquor can be removed and
replaced by hydrogen ions to regenerate HjSO* in electro-
membrane cells.
A method of removing iron from spent liquor that involves
the formation of insoluble iron hydroxides is preferrable to
plating iron metal onto cathodes.
Estimated treatment costs were $0.045 i 0.002 per gallon,
whereas the combined costs of purchasing acid and disposing
of spent liquor by existing methods were in the range of
$0.015 to $0.06 per gallon of spent liquor.
A determination of the long-term performance of the ion-
exchange membranes when treating actual pickle liquors that
contain organic pickling aids is needed.
This report was submitted in fulfillment of Project 12010
EQF under the partial sponsorship of the Water Quality Office.
ACCESSION NO.
KEY WORDS:
Industrial Hastes
Spent Pickle Liquor
Pickling of steel
Acid Regeneration
Ferrous Sulfate
Membrane Processes
Treatment Costs
Ion-Exchange Membranes
Electrolytic Cells
BIBLIOGRAPHIC:
Southern Research Institute, An Electromembrane Process
for Regenerating Acid from Spent Pickle Liquor, Final Re-
port WQO Grant No. 12010 EQF, March, 1971.
ABSTRACT
Studies of an electromembrane process for regenerating acid
from spent sulfuric acid pickle liquor have Indicated that
the process is technically feasible. The studies have shown
that the iron ions in spent pickle liquor can be removed and
replaced by hydrogen ions to regenerate Hi so* in electro-
membrane cells.
A method of removing iron from spent liquor that involves
the formation of insoluble iron hydroxides is preferrable to
plating iron metal onto cathodes.
Estimated treatment costs were SO.045 2 0.002 per gallon,
whereas the combined costs of purchasing acid and disposing
of spent liquor by existing methods were in the range of
$0.015 to $0.06 per gallon of spent liquor.
A determination of the long-term performance of the ion'
exchange membranes when treating actual pickle liquors that
contain organic pickling aids is needed.
This report was submitted in fulfillment of Project 12010
EQF under the partial sponsorship of the Water Quality Office.
ACCESSION NO.
KEY WORDS:
Industrial Hastes
Spent Pickle Liquor
Pickling of steel
Acid Regeneration
Ferrous Sulfate
Membrane Processes
Treatment Costs
Ion-Exchange Membranes
Electrolytic Cells
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1
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SELECTED WATER RESOURCES ABSTRACTS
INPUT TRANSACTION FORM
Organization
Southern Research Institute, Birmingham, Alabama
Title
AN ELECTROMEMBRANE PROCESS FOR REGENERATING ACID
FROM SPENT PICKLE LIQUOR
10
Authors)
Robert E. Lacey
16
Project Designation
Project No. 12010 EQF
21
Note
22
citation Southern Research Institute, An Electromembrane Process for
Regenerating Acid from Spent Pickle Liquor, Final Report WQO Grant
No. 12010 EQF, March, 1971.
23
Descriptors (Starred First)
*Industrial wastes, *Treatment, *Spent pickle liquor treatment,
*Membrane process
25
Identifiers (Starred First)
*Pickling of steel, *Sulfuric acid, *Electrolysis process
27
Abstract
Studies of an electromembrane process for regenerating acid from
spent sulfuric acid pickle liquor have indicated that the process is
technically feasible. The studies have shown that the iron ions in spent
pickle liquor can be removed and replaced by hydrogen ions to regenerate
H2SOi» in electromembrane cells.
A method of removing iron from spent liquor that involves the formation of
insoluble iron hydroxides is preferrable to plating iron metal onto cathodes.
Estimated treatment costs were $0.045 ± 0.002 per gallon, whereas the
combined costs of purchasing acid and disposing of spent liquor by existing
methods were in the range of $0.015 to $0.06 per gallon of spent liquor.
A determination of the long-term performance of the ion-exchange mem-
branes when treating actual pickle liquors that contain organic pickling
aids is needed.
This report was submitted in fulfillment of Project 12010 EQF under the
partial sponsorship of the Water Quality Office.
Abstractor
Robert
E.
Lacey
Institution
Southern
Research
Tnst-it-nt-
e, Bi
rmi ng
ham.
A
labama
WR:I02 IREV JULY !»»»)
WRSIC
SEND TO: WATER RESOURCES SCIENTIFIC
US DEPARTMENT OF THE INTERIOR
WASHINGTON. D C 2O24O
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