EPA-600/2-77-194
September 1977
Environmental Protection Technology Series
REGENERATION OF CHROMATED ALUMINUM
DEOXIDIZERS - Improved Diaphragm
Fabrication and Performance
Industrial Environmental Research Laboratory
Office of Research and Development
U.S, Environmental Protection Agency
Cincinnati, Ohio 45268
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RESEARCH REPORTING SERIES
Research reports of the Office of Research and Development, U.S. Environmental
Protection Agency, have been grouped into nine series. These nine broad cate-
gories were established to facilitate further development and application of en-
vironmental technology. Elimination of traditional grouping was consciously
planned to foster technology transfer and a maximum interface in related fields.
The nine series are:
1. Environmental Health Effects Research
2. Environmental Protection Technology
3. Ecological Research
4. Environmental Monitoring
5. Socioeconomic Environmental Studies
6. Scientific and Technical Assessment Reports (STAR)
7. Interagency Energy-Environment Research and Development
8. "Special" Reports
9. Miscellaneous Reports
This report has been assigned to the ENVIRONMENTAL PROTECTION TECH-
NOLOGY series. This series describes research performed to develop and dem-
onstrate instrumentation, equipment, and methodology to repair or prevent en-
vironmental degradation from point and non-point sources of pollution. This work
provides the new or improved technology required for the control and treatment
of pollution sources to meet environmental quality standards.
This document is available to the public through the National Technical Informa-
tion Service, Springfield, Virginia 22161.
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EPA-600/2-77-197
September 1977
REGENERATION OF CHROMATED ALUMINUM DEOXIDIZERS
Improved Diaphragm Fabrication and Performance
By
Harry C. Hicks, Donald F. Sekits
Manufacturing Research and Development
Boeing Commercial Airplane Company
Seattle, Washington 98124
Grant No. S803064
Project Officer
Hugh B. Durham
Industrial Pollution Control Division
Industrial Environmental Research Laboratory
Cincinnati, Ohio 45268
INDUSTRIAL ENVIRONMENTAL RESEARCH LABORATORY
OFFICE OF RESEARCH AND DEVELOPMENT
U.S. ENVIRONMENTAL PROTECTION AGENCY
CINCINNATI. OHIO 45268
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DISCLAIMER
This report has been reviewed by the Industrial Environmental Research Laboratory
Cincinnati, U.S. Environmental Protection Agency, and approved for publication. Approval
does not signify that the contents necessarily reflect the views and policies of the U.S.
Environmental Protection Agency, nor does mention of trade names or commercial products
constitute endorsement or recommendation for use.
11
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FOREWORD
When energy and material resources are extracted, processed, converted and used, the
related pollutional impacts on our environment and even on our health often require that
new and increasingly more efficient control methods be used. The Industrial Environmental
Research Laboratory, Cincinnati (IERL-CI) assists in developing and demonstrating new
and improved methodologies that will meet these needs both efficiently and economically.
This report describes a regeneration process which maintains continuous operation of a
chemical processing solution for aluminum. Continuous regeneration minimizes chemical
additions, stabilizes solution effectivity, and eliminates periodic dumping of toxic
concentrated solutions. The regeneration process can be utilized by those members of the
metal finishing industry who are involved in surface finishing of aluminum products.
Additional information regarding this project can be obtained from the Industrial Pollution
Control Division, Industrial Environmental Research Laboratory, U.S. Environmental
Protection Agency, Cincinnati, Ohio 45268.
David G. Stephan
Director
Industrial Environmental Research Laboratory
Cincinnati
ui
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ABSTRACT
"Regeneration of Chromated Aluminum Deoxidizers-Phase I," report number EPA-660/
2-73-023, U.S. Government Printing Office, dated December 1973, provides the develop-
ment information for the regeneration concept from theory through pilot scaleup testing.
A laminated ion-selective diaphragm was developed during phase I as a necessary part of the
electrolytic process. Phase II was initiated to improve the diaphragm fabrication techniques
and performance.
Fabrication operations were optimized and improved diaphragms were tested in newly
designed test equipment. Pilot scaleup tests were conducted to verify performance of the
newer diaphragms. Fabrication improvements were made in terms of less material, lower
laminating pressures, and shorter hydraulic press time. These improvements resulted in a
lower cost diaphragm "with increased electrical current-carrying capacity and no sacrifice in
function. An alternate, newly marketed diaphragm material by E. I. DuPont called Nafion
36-3089 was also tested. In comparison to the laminated diaphragms, the service life of
Nafion appears to be longer with comparable regenerating performance but with higher
installation cost and some sacrifice in ruggedness.
The pilot scaleup electrolytic regeneration equipment was installed in a shop production
tank and successfully operated for 4 months. The pilot operation maintained the hexavalent
chromium content under widely varying workloads and at an operational cost savings.
The objectives of the program were met and it is concluded that the concept of regenerating
chromated aluminum deoxidizers is a feasible and practical method for significantly reducing
the quantity of discarded toxic chromium compounds and conserving chromium resources.
This report was submitted in fulfillment of Phase II, grant number S803064-01, between
the United States Environmental Protection Agency and the Boeing Commercial Airplane
Company. This report covers the period November 1, 1973 to October 31, 1975.
IV
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CONTENTS
Page
Foreword iii
Abstract iv
Figures ' vi
Tables vi
List of Abbreviations and Symbols vii
Acknowledgements viii
1. Introduction 1
2. Conclusions 4
3. Recommendations 5
4. Technical Discussion 6
Optimize Press Fabrication 6
Alternate Polyester Material 10
Electrical Evaluation 10
Calendering Evaluation 13
New Product Evaluation 15
Pilot Verification 15
Presentations 24
Economic Analysis 24
Bibliography 27
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FIGURES
1
->
3
4
5
6
7
8
9
10
11
1
2
3
4
5
6
Diaphragm permeability test apparatus
Diaphragm permeability test apparatus
Diaphragm permeability and electrical test apparatus
Potentiostat test setup for electrical evaluation of diaphragms
Electrical test data for current-carrying capacities of diaphragms ....
Three-section cathode chamber for 220-liter pilot tank
Pilot tank for testing regeneration diaphragms
8700-liter deoxidizer tank with regeneration equipment installed
DC power supply and recording instrumentation used with 8700-liter
deoxidizer
Regeneration cathode chamber in 8700-liter deoxidizer tank
Regeneration performance in 8700-liter deoxidizer
TABLES
Diaphragm Permeability and Resistance Test Data
Polyester Material Test Data "
Calendered Diaphragm Evaluation
New Product Membranes Test Data
Material Resistance Measurements in Pilot Tank
Economics of Electrolytic Regeneration Installation
7
8
9
12
14
16
18
20
21
22
23
7
11
13
15
17
25
vi
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LIST OF ABBREVIATIONS AND SYMBOLS
Cr(III), Cr , Cr Trivalent oxidation state of chromium
Cr(VI), Cr ° Hexavalent oxidation state of chromium (also expressed as
E,V
Potential, volts
Total potential measured between anode and cathode
(g)
Reversible electrode potential
Gas
I or A
Na
Nc
Pg
R
Dichromate
sulfuric
ElsE2 — E
Current, amperes
Current efficiency
Current density, amperes per square foot
Anode polarization, volts
Cathode polarization, volts
Membrane permeability
i Resistance, ohms
Membrane resistance
Solution resistance
An aluminum deoxidizer whose makeup chemistry consists of only the
dichromate (Cr^Oa"^) ion plus sulfuric acid
A series of data points 1 through "x*
Vll
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ACKNOWLEDGMENTS
The support of Equipment Engineering, Quality Control, Operations, and Manufacturing
Research and Development of the Boeing Commercial Airplane Company is gratefully
acknowledged.
John M. Watkins, Jr., Plastics Department, E. I. duPont de Nemours and Company, provided
valuable technical assistance in the evaluation of membranes.
vm
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SECTION I
INTRODUCTION
BACKGROUND
Chromium-containing chemical compounds have long been recognized as a major contrib-
utor to water pollution problems because of the high toxicity of chromium's ionic forms
and the prevalence of chromium in a wide variety of industrial processes. Approximately
10% of the total U.S. chromium consumption is in the chemical industries (in excess of
45 400 metric tons annually). Nearly one-third of this is used for paint pigments and hence
can be considered as nonpolluting, as are the metallurgical and refractory uses.
Metal surface treatments and corrosion control measures use a large quantity of chromium,
estimated at 27 200 metric tons annually. In this metal-finishing industry, chromated
processing solutions are used extensively to treat aluminum surfaces prior to operations such
as anodizing, conversion coatings, painting, welding, and adhesive bonding. A specific process
commonly referred to as deoxidizing of aluminum (part of a cleaning cycle) is of special
interest. Chromated aluminum deoxidizing solutions have a relatively high concentration of
chromium in the hexavalent state, and this chromium is used up in three ways: (1) a minute
amount remains on the surface of the aluminum as a complex chemical conversion coating;
(2) a somewhat larger amount is lost by drag-out into rinse waters; and (3) high concentra-
tions are lost when the processing solution is discarded for various nonfunctional reasons.
It is predicted that for many technical and economic reasons, chromated aluminum deoxi-
dizers will continue to be used.
The loss described in item 3 is the problem to which the efforts of this project were directed.
A concept for a regeneration process (by electrolytic oxidation of trivalent chromium and
dissolved-metals removal by crystallization) was devised to extend the useful life of the
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chromated deoxidizers. The advantages of the regeneration process described in this
report are:
• Environmental
• Major reduction in the quantity of chromium-containing effluent
• Conservation of chromium resources
• Industrial
• Reduction in the requirements of treatment plants
• Increased production through reduced downtime
• Lower processing costs
• .Reduction of chemical additions
• Reduction of quality control costs
• Elimination of dumping and recharging costs
• Increased process reliability
Preliminary research work has proved that it is feasible to regenerate spent deoxidizer
solutions rather than discard them. Thus it is possible, by applying chemical engineering
technology, to maintain the acceptable performance of chromium-containing solutions
indefinitely. By making this technology available to all metal finishers, a significant con-
tribution to antipollution efforts can be made.
SCOPE
The engineering techniques developed in phase I of this project involve regeneration of
active chromium compounds by electrolysis plus removal of undesirable dissolved metals
by crystallization and separation. A deoxidizer consisting of sodium dichromate and sul-
furic acid was selected for this investigation for two reasons: (1) the initial composition is
known exactly; and (2) the workload and performance of this solution within the facility
of the project contractor are critical and precisely controlled.
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Phase II of the project was undertaken to develop better fabrication techniques for the
membrane used in the electrolytic section of regeneration, and to test and evaluate the
improved membrane for lower cost and improved performance.
PROJECT OBJECTIVES-PHASE II
The objectives of phase II were to investigate methods of diaphragm fabrication that would
reduce cost and flowtime without impairment of diaphragm function.
Other objectives were to evaluate recent commercially developed alternate membrane
materials for use in this regeneration process, and subject improved and new materials to
pilot testing in order to verify equal or improved performance.
An economic analysis of the improved diaphragms was also planned and presentations of
the details of the project were scheduled for two EPA-sponsored national conferences.
TECHNICAL APPROACH
Efforts during the previous phase of this grant produced a diaphragm that effectively
impeded migration of the dichromate ion from the anode section to the cathode chamber.
At the same time, the diaphragm allowed electrical current to pass to complete the circuit
for reoxidization of trivalent chromium at the anode surface. The material used for the
diaphragm was 100% polyester fabric by DuPont, designated Reemay 2024. To produce a
laminated structure with very low permeability, 60 layers of this material were subjected to
elevated temperature and pressure. The process for fabrication was lengthy, required very
high pressures (140 kg/cm^), and occupied a hydraulic press for up to 8 hr for one 15- by
20-cm diaphragm.
Since there are other configurations of 100% polyester fabrics commercially available, the
technical approach was to isolate the variables of diaphragm fabrication, establish the
individual effects of these variables on fabrication performance, and establish the least
costly method of fabrication consistent with suitable diaphragm performance. The accept-
ance criteria were permeability of the diaphragm to dichromate ion and electrical conduc-
tivity per unit area. Both woven and nonwoven polyester fabrics were scheduled for
investigation for equivalent or better performance. Recently, other proprietary materials
have become available. Some of these materials were obtained and tested for chemical
resistance, permeability, and electrical conductivity. Finally, the most promising diaphragms
were selected and pilot scaleup operations were performed to establish production suita-
bility. The final test was a 3-month operation in a 220-liter tank, followed by a 4-month-test
period in a shop production tank of 8700 liters.
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SECTION 11
CONCLUSIONS
Regeneration of chromated aluminum deoxidizers is feasible, practical, and economical,
and results in a cost saving to the operator. The useful life of this type of deoxidizer can be
extended appreciably, thereby minimizing disposal of concentrated chromium salts. From
phase I of this project it has been shown that regeneration system effectiveness is more
efficient in the conventional dichromate-sulfuric deoxidizer than is some proprietary solu-
tions. From phase II, improved diaphragms for the electrolytic section of regeneration have
been tested and proved satisfactory.
Suitable diaphragms for production use can be made using either 30 layers of DuPont
Reemay 2024 or 10 layers of Reemay 2470, when properly laminated. Permeability of the
diaphragm is satisfactory with either material when a laminating pressure of 70 kg/cm2 is
used for 30 min at 150°C followed by a rapid cooling time of 10 min to 38°C. A condi-
tioning presoak of 3 days at 70°C in the deoxidizer is necessary to achieve long-term
performance. Another DuPont product, Nafion, is a readymade perfluorosulfonic acid
diaphragm material that has been tested and shows equivalent regeneration performance.
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SECTION III
RECOMMENDATIONS
During phase I of this project, electrolytic regeneration of chromated aluminum deoxidizers
was demonstrated to be feasible and practical for production use. It was also shown that
dissolved metals which build up in the deoxidizer solution can be effectively removed by
crystallization and separation. The process of regeneration was shown to be a cost-saving
procedure. In phase II, improved diaphragms were developed, pilot and production
tested, and proved to be satisfactory. Therefore, it is recommended that the process for
regeneration of dichromate-sulfuric deoxidizers as developed in this project be encouraged
for use by industrial metal finishers.
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SECTION IV
TECHNICAL DISCUSSION
Phase II consisted of eight tasks as detailed in the following subsections.
TASK I-OPTIMIZE PRESS FABRICATION
Phase I performance for a Reemay 2024 diaphragm made with 60 plies laminated in a
hydraulic press at 150 kg/cm2 and held at 150°C for 4 hr was the standard for continuing
work. Using these conditions, 30- and 60-ply diaphragms were fabricated and permeability
and electrical resistances were measured. At the start, a test apparatus (Figure 1) was fabricated
for determining permeability. Using this apparatus, it was found that more than 24 hr were
required to establish permeability for each test diaphragm.
In order to shorten the laboratory test time, a greater hydraulic head was added to the per-
meability test apparatus. Figure 2 illustrates this first revision. In this setup, appreciable
column height differences could be maintained. This difference in hydrostatic head did not
appreciably change the test time for permeability. Therefore, electrodes were installed in
each chamber to add electro-osmotic pressure. This step reduced individual diaphragm test
time to 4 hr. The final test apparatus configuration is illustrated in Figure 3.
Diaphragms were made using Reemay 2024 (15, 30, and 60 plies) and varying pressure, time
under pressure, and pressing temperature. Pressures used were 35, 70, 105, 120, and 140
kg/cm2. Test times under pressure were selected at 14, 1, 2, 3, and 4 hr. Pressing tempera-
tures were 93°C, 149°C, and 177°C. The resultant diaphragms were tested for permeability
and electrical resistance (task III). The data are summarized in (Table 1).
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TABLE 1. DIAPHRAGM PREMEABILITY AND RESISTANCE TEST DATA
Plies
Fabrication p'ressure. Press time.
kg/cm"
hr
Press temp, Pg, Rm,
°C (mg/cm2-hr)x 10'4 ohms/cm2
60
60
30
15
35,70,105,120,140 4
70 1/4,1,2,3,4
35,70,105,120,140 1,4
70 1
149
149
149, 177
149
0 to 0.06
0.75 to 8. 7
0.4 to 55.7
10.4
0.018 to 0
0.028 to 0.029
0.027 to 0
(Pg too high)
AIR AGITATORS
]
'//////////>
CLEAR
WATER
N
-v_
>X//////////////////////
DIAPHRAGM
UNDER TEST-
Figure 1. Diaphragm permeability test apparatus
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Figure 2. Diaphragm permeability test apparatus
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vO
Figure 3. Diaphragm permeability and electrical test apparatus
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At this stage it was also determined that a minimum soak condition of 72 hr at 70°C was
necessary to stabilize the functional performance of the diaphragms. From these data,
minimum diaphragm fabrication conditions are established as:
Laminating layers 30
Pressure 70 kg/cm2
Time at pressure 1 hr
Press temperature 149°C
These conditions are considerable improvements over the phase I fabrication requirements
of 60 layers, 140 kg/cm2, and 4 hr at pressure. In addition, it was found that a rapid cooling
of the press platens was beneficial to diaphragm reproducibility and visual appearance, as
well as reducing the overall fabrication time. A cooling rate of 10 min from 149°C to 38°€
was established for all diaphragms.
TASK II-ALTERNATE POLYESTER MATERIAL
The minimum diaphragm fabrication conditions were used to evaluate Reemay 2470, 2421,
and 2408, which are crimped, nonwoven fabrics, and Reemay 2011 and 2033, which are
straight-fibered fabrics similar to 2024. Six woven-polyester fabrics were also used to fabri-
cate diaphragms under the same operating conditions. Reemay 2470 was found to be
approximately equivalent to 2024 in performance, and was therefore selected for further
testing. None of the woven fabrics was found to be suitable; all required much higher
pressures (and thus a higher cost) to achieve adequate adhesion between layers, one material
having melted at the normal press temperature. Table 2 provides a summary of the test data.
TASK III-ELECTRICAL EVALUATION
The same U-tube test apparatus that was used for permeability determinations was used for
the first part of the electrical evaluation. Under stabilized conditions, amperage readings
were taken at predetermined voltage settings. The resistances of the individual diaphragms
were isolated and calculated algebraically: ,
E2-El EX-EX.1
10
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TABLE 2. POL YESTER MA TERIAL TEST DA TA
D
U
P
0
N
T
A
M
E
T
E
K
Material
2470
2421
2408
2024
2011
2033
HG6E2D
X4M7
H4E6
ZOM2
H2EO
XBHV4K2
Pg, Rm,
{mg/cm • hr) x 10 ohm/cm2
0 to 0.65 0.023 to 0.040
1.1 0.025
0
0 to 1.5 0.018toO:029
00
21.8
0 0.59
2.2 0.03
ic 7
1 J. / ~"~ •""
Remarks
OK
OK
Low amperage
Resistance too high
OK
Permeability too high
Permeability too high
Resistance too high
High permeability
High permeability
Delaminated
Delaminated
Melted in press
Table 2 includes the diaphragm electrical resistances as determined in the equation. It should
be mentioned that in many cases involving alternate materials, poor diaphragm performance
such as no passage of current (infinite resistance), diaphragm delamination, or unacceptable
permeability negated the need for electrical evaluation.
From examination of diaphragm breakdowns in phase I, it was surmised that electrical over-
loading contributed to early failures; therefore, the second part of task III consisted of
determining the current-carrying capacities of the best candidate materials. The procedure
involved subjecting a small, precise circle (1.27-cm diameter) of the test diaphragm to a
programmed potential from 0 to 20 V over a period of 30 minutes and recording the resultant
amperage. The laboratory potentiostat with its programming controller and sensing probe
accessories was used (figure 4). When each diaphragm was tested in this manner, in all cases a
break occurred in the current buildup prior to reaching maximum voltage. The recorded
curves were reproducible and nearly the same for each material. The current-recorded area
beyond the breakdown point is of added interest, and could be interpreted as being an
11
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N>
Figure 4. Potentiostat test setup for electrical evaluation of diaphragms
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indication of a diffusion controlled, electrical conductivity process in a restricted opening.
Posttest examination revealed a physical deterioration in each diaphragm. Figure 5 provides
the recorder tracings for three diaphragm materials. Since several of the tasks in this phase
were run concurrently, this figure also illustrates the current-voltage breakdown curve for
Nafion, one of the materials tested in task V. From this information, a maximum of 16
amp/dm^ was established for the durability test in task VI.
TASK IV-CALENDERING EVALUATION
Since production of the diaphragms at the present state of the art includes the use of a
heated-platen hydraulic press, it is logical, in an improvement program, to inquire into
alternate lower cost methods of achieving the necessary laminating pressures and tempera-
tures. Calendering, a procedure used for hot-roll pressing of fabrics and paper, was investi-
gated as a substitute for diaphragm fabrication. Stacks of 30 layers of Reemay 2024 were
passed through heated rolls under various pressures and temperatures. Rolling speed was
established at 175 rpm for a 10.2-cm-diameter roll by preliminary test runs. The fabric was
passed through the rolls by themselves and also when contained between two layers of
aluminum sheet stack. The thickness of the aluminum was also varied to some extent. A
final diaphragm thickness of 0.215 to 0.254 cm was the objective.
In most cases, visual examination of the resultant diaphragm was sufficient cause for
rejection. The diaphragms frequently were warped, smeared, and/or delaminated in the
as-produced condition. A few appeared to have some of the necessary requirements of a
diaphragm and subsequently were tested for permeability. None of the diaphragms passed
acceptance testing; consequently, the calendering process was abandoned.
Table 3 summarizes the efforts.
TABLES. CALENDERED DIAPHRAGM EVALUATION
(30 Layers Reemay 2024, Roller Speed - 175 rpm)
Preheat,
°C
171
None to 171
204
204
Backing
plates
Yes
Yes
No
Yes
Roller
heat
None
71
60
60
Roller gap,
cm
0.03 to 1.7
0.67- to 1.13
0.51 to 1.4
1.25
P
Diaphragm , , 2 Rm, 0 .
..u- i (mg/cm • , , o Remarks
thickness, cm nr\ x ^-4 ohm/cm*
0.27 to 0.78
0.21 to 0.60
0.47 to 0.60
0.23
46.6
1.9
10.8
6.5
Not tested
0.02
Not tested
Not tested
Warped and
Warped and
Too thick
Warped
too thick
too thick
13
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20
MINUTES
10 -
30
20
MINUTES
10
0 246 0246
CONTROL VOLTAGE AMPERES
Straight Fiber Polyester
02460246
CONTROL VOLTAGE AMPERES
Crimped Fiber Polyester
30
20
MINUTES
10
0 24 60246
CONTROL VOLTAGE AMPERES
Dupont Nafion 36-3089
Figure 5. Electrical test data for current-carrying capacities of diaphragms
14
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TASK V-NEW PRODUCT EVALUATION
A search of recent market developments in membranes revealed two potential candidates as
alternates for the ,diaphragm developed during this grant. Past investigations of available
diaphragms produced no material that would withstand the strong oxidizing environment of
a hot deoxidizer. However, recently there has become available from several sources a Teflon
semipermeable membrane and also a proprietary organic film called Nafion, marketed by
DuPont. The Teflon semipermeable membrane was acquired from Southwestern Analytical
Chemicals, Incorporated. The DuPont Company provided a quantity of two types of Nafion
membrane.
The Teflon membrane provided low permeability, but no appreciable electrical current
could be forced through. The material was rejected as unsatisfactory for this reason. Both
types of Nafion membrane material provided encouraging results. Consequently, Nafion
36-3089 was selected for pilot scaleup evaluation.
Table 4 provides U-tube test results from the new product evaluation effort.
TABLE 4. NEW PRODUCT MEMBRANES TEST DATA
Material
name
Teflon
Nafion
Nafion
Material
number
236
XR-480
36-3089
Volts
10.0
7.2
7.0
Watts
0
31.7
34.3
Pg.
mg/cm.hr x 10"4
0.44
0.33
0.29
Rm,
ohm/cm^
OG
0.020
0.017
TASK VI-PILOT VERIFICATION
Three diaphragm materials were selected for pilot scaleup durability testing. These were
Reemay 2024, Reemay 2470, and Nafion 36-3089. A three-section cathode chamber was
fabricated as illustrated in figure 6. Because of the improvements in diaphragm fabrication
techniques resulting from work in phase II, it was possible to produce larger sized dia-
phragms with the available press capability. After each larger sized diaphragm was fabri-
cated, permeability was measured in at least three different areas. It was found that
permeability varied widely between the areas, and permeability was roughly proportional
to thickness. Therefore, the press platens were examined for flatness. Such a wide variation
15
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Figure 6. Three-section cathode chamber for 220-liter pilot tank
16
-------
in platen gap was found that the platens were removed from the press and reground.
Additional diaphragms were made and it was determined that acceptable permeability
could be obtained when diaphragms were fabricated in areas where the platen gap did not
vary greater than 0.25 mm.
Acceptable large diaphragms, each with an exposed area of 2.34 dm , were installed in the
three-section cathode chamber and the chamber was placed in a 220-liter pilot tank cqn-
/•^
taining a dichromate-sulfuric deoxidizer at 63° C. An electrical load of 13 A/dmz was
imposed on each diaphragm. Periodic analyses for chromium migration into the cathode
chamber were made. The complete pilot installation is shown in figure 7.
The 220-liter pilot tank was operated continuously for 2 months without shutdown except
for periodic examination of components. The collected data verify that dichromate migra-
tion into the cathode chamber was effectively prevented, that electrical conductivity was
high, and that the improved Reemay diaphragms, as well as the Nafion material, resisted the
corrosive environment. Resistance measurements were made in the pilot tank and the results
are shown in table 5.
TABLE 5. MA TERIAL RESISTANCE MEASUREMENTS IN PILOT TANK
.. . . Electrical resistance.
Material . . 9
ohm/cnrr
Reemay 2024 0.0207
Reemay 2470 0.0215
Nafion 0.0186
After an additional month of pilot operations, results were sufficiently encouraging to
warrant a longer term of exposure. A shop production tank containing 8700 liters of the
dichromate-sulfuric deoxidizer was selected for further exposure tests. This solution was
new in April 1974. At the time of installation of the regeneration equipment, dissolved
aluminum content had risen to 13 800 ppm. Since this aluminum content is well below the
concentration at which crystallization becomes feasible, no metals separation equipment
was installed.
Using the data from phase I of this project, it was calculated that 111 amperes of direct
current (16 A/dm^) would be adequate to maintain a constant hexavalent chromium level
and offset the reduction of chromium caused by the average workload. An existing constant-
17
-------
Figure 7. Pilot tank for testing regeneration diaphragms
18
-------
voltage-controlled dc power supply was used. From this basic starting point, it was deter-
mined that the pilot tank cathode chamber could be used with the existing diaphragm
^
sizes. A maximum cathode area of 1160 cm^ was used. A sheet lead (Pb) anode area of
^
5750 cmz was selected. The anode was suspended at one end of the tank and the cathode
chamber was placed at the other end, approximately 6.7 meters away. Instrumentation not
normal to a production installation was added for monitoring during this test period. A
two-channel recorder monitored voltage and amperage at the dc power supply. A digital
ampere-hour meter registered energy consumption for data reduction. Figures 8, 9, and 10
illustrate the shop installation.
Since it had been observed previously that water was lost from the cathode chamber during
elevated temperature operations, a metering pump with a 221-liter water reservoir was also
installed to maintain liquid level at the cathode.
The sulfate ion was driven through the diaphragm as well as the hydroxyl ion by the electro-
osmotic pressure. Therefore, daily additions of sulfuric acid were made as required by
chemical analyses.
After 2 months of operation, the data verified that all operating parameters had been stable
and predictable for this installation. The water was replaced with a 10% sulfuric acid solu-
tion, which was metered into the cathode chamber. This improvement eliminated the daily
manual additions of acid.
A comparison of analytical records for the deoxidizer solution illustrates the advantage of
the electrolytic regeneration process. Prior to installation, this 8700-liter solution consumed
an average of 68 kg of sodium dichromate per month. During the test run of 4 months, no
additions of sodium dichromate were made. The analysis for hexavalent chromium remained
constant. The amount of sulfuric acid consumed was stoichiometrically equivalent to the
amount of aluminum metal dissolved. The aluminum content rose from 13 800 ppm in late
February to 16700 ppm at the end of May. Figure 11 summarizes the regeneration
performance.
The diaphragms were examined at the time the sulfuric acid metering was initiated. It was
found that both the Reemay 2024 and 2470 were deteriorated to the extent that replace-
ment was required. Two new Reemay 2024 diaphragms were installed. The Nafion dia-
phragm was left in place. Thus, the Reemay material endured 6 months of regeneration at
13 to 16 A/dm^. This compares to approximately twice the current-carrying capacity of
the original diaphragms for the same service life.
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Figure 8. 8700-liter deoxidizer tank with regeneration equipment installed
-------
Figure 9. D. C. power supply and recording instrumentation used with 8700-liter deoxidizer
-------
Figure 10. Regeneration cathode chamber in 8700-liter deoxidizer tank
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REGENERATION
44.9
SODIUM 37-5
DICHROMATE
(9/l) 30.0
22.5
30M
DISSOLVED
ALUMINUM 20M
(ppm)
10M
JAN
MAY
Figure 11. Regeneration performance in 8700-liter deoxidizer
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Summarizing, two diaphragm materials-100% polyester resin-laminated fabric and a per-
fluorosulfonic acid membrane-have been identified as providing suitable electrical conduct-
ance and ionic separation for successful reoxidization of trivalent chromium.
• The polyester diaphragm utilizes low-cost material, is physically strong and rigid,
and requires only simple clamping in order to seal it effectively in a cathode
chamber. Its disadvantage is that it requires an elevated temperature and high
pressure to achieve satisfactory lamination.
• The perfluorosulfonic acid diaphragm is ready to install as received and may have
a longer service life. Its disadvantage is that it is slightly more costly, is very
flexible, and requires more critical cathode chamber design for support and
sealing.
TASK VII-PRESENTATIONS
Details of the regeneration project were summarized in presentations at the National Confer-
ence on Management and Disposal of Residues from the Treatment of Industrial Wastewaters
sponsored by Information Transfer Inc., 6110 Executive Blvd, Rockville, Md 20852 in
association with The Office of Research and Development, U.S. Environmental Protection
Agency; American Institute of Chemical Engineers, and Manufacturing Chemists Association.
A second presentation was made at the Second National Conference on Complete WateReuse
sponsored by American Institute of Chemical Engineers and Environmental Protection
Agency Technology Transfer.
TASK VIII-ECONOMIC ANALYSIS
Previously, a 15- by 20-cm, 60-ply, Reemay 2024 diaphragm was estimated to cost $30.
This amounts to $10/dm2. This charge includes labor, material, and equipment amortiza-
tion. As a result of improving the diaphragm press time, material and labor have been reduced
so that the polyester diaphragm cost is now $3.16/dm2. Nafion diaphragm cost is nearly the
same at about $3.25/dm2
Cost data have been updated from the phase I report to reflect the economics of an elec-
trolytic regeneration hi a 8700-liter deoxidizer tank (see table 6).
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TABLE 6. ECONOMICS OF ELECTROL YTIC REGENEFtATION INSTALLATION
Cost item Baseline period, 2nd quarter 1975
Equipment amortization 10 yr
Electrical cost $1.52/109 Joules
Water cost (purchase) $3.53/m
Na2Cr2O7 • 2H20 (tech grade) $31.00 per cwt
Deoxidizer disposal 3.170/2
150 A, 15 Vdc power supply $650
Regeneration accessories $1057
(cathode chamber, electronics, diaphragms,
metering pump)
Regeneration installation $655
Effect on Waste Volume and Characteristics
Implementation of this regeneration process will reduce the sporadicity of plant waste
effluent normally due to periodic dumping of concentrated solutions. As a result of regen-
eration, it is reasonable to assume that the chromated deoxidizer need never be changed, or
at the most—in the case of some proprietary formulations—changed only every 4 or more
years. Thus, the total amount of plant effluent containing chromium compounds can be
greatly reduced, and the effluent is expected to be more uniform in concentration and
character per unit of time.
Effect of Reuse
The added benefits of regeneration are immediately obvious:
• Capital investment in waste treatment facilities can be lower because it is no
longer necessary to size the facility to handle the periodic heavy overloads.
• This same waste facility can be more easily automated because of the lack of
annual surges of high concentrates.
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During the period of testing and production simulation in this regeneration program, no
detrimental effects caused by continual reuse of the chromium compounds could be
detected on the deoxidized surfaces. Thus, continual regeneration of chromated aluminum
deoxidizers will contribute to preservation of chromium resources.
Effectiveness of Treatment
The electrolytic section of regeneration is a wholly self-contained system since the reoxida-
tion of the chromium occurs within or adjacent to the aluminum processing tank. When
dissolved aluminum concentration reaches the point at which separation procedures should
be implemented, the waste product (salts of various metals) is sufficiently dry to be used as
a chrome-bearing, solid-fill material. Under the most effective conditions encountered in
this program, chromium metal constituted about 1% of the dried salts weight In very large
installations, metals recovery operations may be considered.
Capital Costs
Capital costs are based on regeneration equipment sized to meet the workload throughput
of a 8700-liter deoxidizer tank that has been monitored for several years.
Capital equipment for electrolytic regeneration in this tank consists of one dc power supply
at a cost of $650, a cathode chamber plus electrodes at $686, and a metering system at
$371. The total capital cost is $1707.
Maintenance
Maintenance costs for the types of equipment described in this report are generally low com-
pared with those for other chemical processing facilities. The diaphragm cost as fabricated in
this report has been reduced, while labor rates have risen since the previous report. Yearly
maintenance cost for the 8700-liter regeneration equipment is now estimated at $214.
Operating Costs, Regeneration Versus Nonregeneration
8700-Liter Deoxidizer-During regeneration, electrical cost for the rectifier and metering
pump is $7.12 per month, equipment amortization is $19.68 per month, and maintenance
is $17.83 per month. The total regeneration operating cost is $44.63 per month. This figure
is contrasted to nonregenerated deoxidizer costs of $46.50 per month for addition of chem-
icals, $39.20 per month for prorated dumping charge, and $31.00 per month for prorated
new solution chemicals. The total nonregeneration cost is $116.70 per month.
26
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SECTION V
BIBLIOGRAPHY
r
1. Hempel, C. A.,Rare Metals Handbook, Second Edition, Reinhold Publishing Company.
2. Continuous Regeneration of Aluminum Deoxidizer, Manufacturing Development
Report 6-92013, The Boeing Company.
3. Continuous Regeneration of Aluminum Deoxidizer, Service Request/Program Authori-
zation 692-038, The Boeing Company.
4. Gross and Hickling,/. Chem Soc., 235, 1937.
5. Hickling and Richards, ibid, 256, 1940.
6. Glasstone, An Introduction to Electrochemistry, Van Nostrand, pp 107-128, 1956.
7. Potter, Electrochemistry, Cleaver-Hume, London, pp 31-100, 1961.
8. Metals Handbook, Eighth Edition, American Society for Metals.
9. Bulletin 172 CL-FL, the Leon J. Barrett Company, Worcester, Massachusetts.
10. Bulletins, the Duriron Company, Inc., Dayton, Ohio.
11. Solubilities of Inorganic and Metal Organic Compounds, Fourth Edition, American
Chemical Society.
12. Bulletin S-4, E. I. DuPont de Nemours & Co., Inc., Wilmington, Delaware.
13. Purchas, Derek B., Industrial Filtration of Liquids, Second Edition, Leonard Hill
Books.
27
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14. Bulletin A13 5A, Ametek, Inc., East Moline, Illinois.
15. Drawing 9665-2900, Ametek, Inc., East Moline, Illinois.
16. Bulletins, Troy Mills, Inc., New York, New York.
17. Form 19, Chicopee Mills, Inc., Milltown, New Jersey.
18. Bulletin JPB 3/21/68, Diamond Shamrock Corp., Cleveland, Ohio.
19. Seegmiller, R. and Lamb, V. A., Re-Oxidation of Trivalent Chromium in Chromic Acid
Plating Baths, National Bureau of Standards, Washington, D. C.
20. Telex 920407, Dennison Manufacturing Company, Renton, Washington.
21. Hicks,. H. and Jarmuth, R., Regeneration of Chromated Aluminum Deoxidizers-
Phase I, report no. EPA-660/2-73-023, December 1973, Superintendent of Documents,
U.S. Government Printing Office.
22. Hicks, H. and Sekits, D., Regeneration of Chromated Aluminum Deoxidizers, Boeing
document D6-22251-16, The Boeing Company.
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TECHNICAL REPORT DATA
(Please read Instructions on the reverse before completing)
1. REPORT NO.
EPA-600/2-77-194
3. RECIPIENT'S ACCESSION NO.
4. TITLE AND SUBTITLE
REGENERATION OF CHROMATED ALUMINUM DEOXIDIZERS-
Improved Diaphragm Fabrication and Performance
5. REPORT DATE
September 1977 issuing date
6. PERFORMING ORGANIZATION CODE
7. AUTHOR(S)
Hicks, H. C. and Sekits, D. F.
8. PERFORMING ORGANIZATION REPORT NO.
D6-22251-16
9. PERFORMING ORGANIZATION NAME AND ADDRESS
The Boeing Commercial Airplane Company
P.O. Box 3707
Seattle, Washington 98124
10. PROGRAM ELEMENT NO.
1BB610
11. CONTRACT/GRANT NO.
S803064
12. SPONSORING AGENCY NAME AND ADDRESS
Industrial Environmental Research Laboratory - Cin., OH
Office of Research and Development
U.S. Environmental Protection Agency
Cincinnati, Ohio 45268
13. TYPE OF REPORT AND PERIOD COVERED
Final Nov. 73-Oct 75
14. SPONSORING AGENCY CODE
EPA/600/12
15. SUPPLEMENTARY NOTES
This report is supplementary to the Phase I Report, EPA Report number EPA 660/2-73-023, December 1973, "Regeneration
of Chromated Aluminum Deoxidizers" available.from U.S. Government Printing Office, Wa. D.C. 20402-Price $1.95.
16. ABSTRACT
In the metal finishing industry highly concentrated hexavalent chromium solutions are used extensively to deoxidize
aluminum surfaces prior to anodizing, conversion coatings, prepaint preparation, welding and adhesive bonding. A
regeneration process was conceived and tested to reduce the frequency of discarding the spent chromated deoxidizers.
The engineering techniques developed in this project involve reoxidation of trivalent chromium to the hexavelent state
by electrolysis thru a diaphragm plus removal of undesirable dissolved metals by crystallization and separation. Results
of the accomplished work establish that regeneration of chromated aluminum deoxidizers is feasible, practical and economical.
In the second phase of this project diaphragm fabrication techniques were refined to produce an improved diaphragm in
terms of cost and performance. In addition, electrolytic regeneration equipment installed in a large production tank for
six months exceeded technical and economic objectives.
17.
KEY WORDS AND DOCUMENT ANALYSIS
DESCRIPTORS
b.lDENTIFIERS/OPEN ENDED TERMS
cos AT I Field/Group
Regeneration, Water Pollution, Centrifuge
Crystallization, Drum Filters, Chemical
cleaning, Metal cleaning
Chromate recycle, Toxic
metal control, Electro-
lytic reoxidation, pH
titrimetry, Permeable
diaphragm, Aluminum de-
oxidizer reuse, Chromate
deoxidizer
13B
18. DISTRIBUTION STATEMENT
Release Unlimited
19. SECURITY CLASS (ThisReport)
Unclassified
21. NO. OF PAGES
37
20. SECURITY CLASS (Thispage)
Unclassified
22. PRICE
EPA Form 2220-1 (9-73)
•ft U.S. GOVERNMENT PRINTING OFFICE: 1977-757-O56/6546
29
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