United States
Environmental Protection
Agency
Industrial Environmental Research
Laboratory
Cincinnati OH 45268
Research and Development
EPA-600/S2-84-077 Apr. 1984
Project Summary
Evaluation of Process
Systems for Effective
Management of Aluminum
Finishing Wastewaters and
Sludges
F. M. Saunders, E. S. K. Chian, C. B. Harmon, K. L Kratz, J. M. Medero,
M. E. Pisani, R. R. Ramirez, and M. Sezgin
Innovative processes for use in treat-
ment of wastewaters and sludges pro-
duced in anodizing, etching and painting
extruded aluminum were investigated.
Due to the low quantities of wastewater
aluminum from painting processes,
emphasis was placed on those process-
es amenable to treatment of anodizing
and etching wastewaters.
Segregated neutralization of spent
caustic etch and spent anodize acids at
temperatures of 60 to 90°C and pH
values of 5.5 to 10 were examined.
Major improvements in thickening and
dewatering properties were achieved
with increasing values of neutralization
pH while neutralization temperature
had minimal impact. Solids contents of
dewatered sludge samples ranged from
33 to 54 percent while those of con-
ventional aluminum-finishing sludges
ranged from 9 to 17 percent, indicating
the potential of segregated neutraliza-
tion for major reductions in the mass of
wet dewatered-sludge solids for dispos-
al.
Recovery of spent caustic etch by
precipitation of aluminum with calcium
(i.e., lime) addition was studied. Stoi-
chiometric precipitation of aluminum at
temperatures of 25 to 60°C was
achieved at calcium-aluminate (Ca/AI)
ratios of 4 to 5.5 (mass basis). Calcium-
aluminate sludges produced had excel-
lent dewatering properties with de-
watered-sludge solids contents of 45 to
53 percent. Recovery of spent etch for
reuse would therefore be accompanied
by a significant reduction in the mass of
wet dewatered sludge solids for dispos-
al.
Recovery of aluminum-finishing sludg-
es using sulfuric-acid extraction to
produce liquid alum (i.e., Alz(SO4)3-14
H2O) was examined with numerous
types of sludges. Sludges produced by
conventional neutralization, segregated
neutralization, and those from proprie-
tary etch-recovery processes were suc-
cessfully extracted to produce com-
mercial-strength (i.e., 8-8.3 percent as
AI2O3) liquid alum. Sludge solids con-
tent was a critical variable with a
minimum value of 20 percent required
for production of a commercial-strength
product.
Results of the research can be imme-
diately implemented at many aluminum-
finishing plants where sludge disposal
restrictions and costs are increasing.
Segregated neutralization and recovery
of spent caustic etch can be used to
increase the net solids content of de-
watered sludge solids and thereby re-
duce the volume of dewatered-sludge
available for disposal. Reclamation of
dewatered-sludge solids using acid ex-
traction for production of liquid alum
has potential for virtual elimination of
the need for sludge disposal while
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producing a net income from this
sludge-reclamation process.
This research was co-sponsored by
the Aluminum Extruders Council and
the Industrial Environmental Research
Laboratory. U.S. Environmental Pro-
tection Agency, Cincinnati, OH.
This Project Summary was developed
by EPA's Industrial Environmental Re-
search Laboratory, Cincinnati. OH. to
announce key findings of the research
project that is fully documented in a
separate report of the same title (see
Project Report ordering information at
back).
Introduction
Treatment of wastewaters from clean-
ing, milling, etching, anodizing and paint-
ing extruded aluminum results in the
production of large quantities of residual
suspended solids for disposal. These
solids are composed primarily of alumi-
num hydroxides and are contained in
suspensions that are highly gelatinous
and difficult to thicken and dewater. The
mass of wet dewatered sludge solids
produced by an aluminum extrusion/
anodizing plant, for example, may ap-
proach the mass rate of production of
finished aluminum products. A major
waste disposal problem for aluminum
finishing plants is therefore disposal of
large quantities of highly gelatinous
sludge solids.
Characterization of wastewaters and
sludges produced by aluminum-finishing
plants has been examined earlier by the
authors, as presented in a companion
report. Wastewater suspensions from
numerous eastern U.S. plants were exam-
ined with respect to conventional waste-
water parameters, as well as priority-pol-
lutant metals. Sludge thickening, de-
watering, gravity-drainage and condition-
ing properties were examined in detail to
establish optimal means of treating con-
ventional aluminum-finishing waste-
waters.
Thorough examination of properties of
conventional wastewaters and sludges
from aluminum-finishing plants resulted
in the identification of numerous inno-
vative treatment options which merited
further consideration in the present study.
Using wastewaters, sludges and spent
finishing solutions and suspensions from
aluminum-finishing plants, the following
innovative treatment options were exam-
ined: (1) segregated neutralization of
spent acids and bases, (2) reclamation of
spent caustic etch by lime addition, and
(3) reclamation of sludges as liquid alum.
In addition, an extensive industrial-waste
survey was conducted at an aluminum-
finishing plant to establish sources and
quantities of wastewater and waste
aluminum. The overall impact of alumi-
num-finishing and waste-treatment prac-
tices on waste disposal economics and
reclamation potential were presented.
Procedures
Laboratory-scale studies were conduct-
ed using wastes obtained from aluminum-
finishing plants with production capaci-
ties ranging from 15 to 25 ton/d of
finished architectural aluminum. Typical
aluminum-finishing steps used by the
participating plants included alkaline clean-
er, caustic etch, acidic desmut, bright dip,
conventional sulfuric-acid anodize, inte-
gral-color, sulf uric-acid anodize, dye, and
seal. Waste treatment practices included
neutralization, polymer conditioning, and
gravity sedimentation followed by waste-
water discharge to a sewer or stream.
Gravity-thickened sludges were dewa-
tered in lagoons and with pressure or
vacuum filtration followed by land dis-
posal.
Results and Discussion
The project was focused on four major
topics which are presented below.
Industrial Waste Survey
A 24-hour survey was conducted at
plant A-1 to determine the quantities of
wastewater and waste aluminum dis-
charged from individual finishing and
rinsing tanks included in the anodize line
and a parallel, paint line. During the
survey, equal quantities of similar alumi-
num alloys were finished on each line,
i.e., 18-19 tons of extruded aluminum
with a total surface area of 7-8x103 m2.
For the anodize line, 93 percent of the
wastewater discharged was attributable
to wastewater from rinse tanks following
the various finishing steps in the line, as
shown in Table 1. The majority (65
percent) of waste aluminum was con-
tained in spent finishing solutions which
accounted for only 6 percent of the total |
wastewater flow. "
The paint line was a fully-automated
system which had only one combined
wastewater flow from the alkaline-rinse,
chrome-conversion, and acidulating-
rinse portions of the unit. The total
wastewater flow from the paint line was
43 rnVd and was only 8 percent of that
from the anodize line. Waste aluminum
was similarly low at 1.7 kg/d. Paint-line
wastes, therefore, accounted for an ex-
tremely small portion of the total flow of
wastewater and waste aluminum.
An examination of dragin rates from
finishing tanks in the anodize line was
made using mass flows of aluminum, as
well as those for chromium and cadmium,
in conjunction with wastewater flow
data. Dragin rates ranged from 0.053 to
3.08 m3/d (14 to 814 gal./d) with the
higher rates attributable to the viscous
solutions used in caustic etch, desmut,
anodize and acid cleaner. Overall waste-
water and waste aluminum discharge
rates for the plant are presented in Table
2. The quantity of wastewater and waste
aluminum from the paint line were mar-
ginal compared with those for an anodize
line. Waste aluminum, which is centra I to
the sludge disposal problem, was 2.95
percent (mass basis) of the aluminum fin- I
ished on the anodize line but was only
0.01 percent (mass basis) of that finished
on the paint line. In addition, wastewater
flow was considerably higher from the
anodize line. Therefore, major sludge
disposal problems originate with the
intensive surface treatments, such as
caustic etch and sulfuric-acid anodize,
which are required in anodizing extruded
aluminum.
Segregated Neutralization
Major sources of waste aluminum in
anodizing wastewaters were shown to be
spent caustic etch and spent anodize
acid. Neutralization of these concentrated
wastes, apart from dilute rinsewaters and
hence referred to as segregatedneutra/i-
Table 1. Summary of Wastewater Flow and Waste Aluminum from Anodize Line at Plant A -1
During 24-h Survey
Waste Source
Rinsewater
Spent Etch
Spent Anodize Acid
Other
Flow
m3/d
495
6
26
7
Waste Aluminum
kg/d
195
335
36
--
Total
534
566
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Table 2. Wastewater and Waste Aluminum Flow Normalized to Finished-Metal Production
Unit Waste Discharge Rates
Source
Surface Area
Basis
Mass
Basis
Flow
Anodize Line
Paint Line
Waste Aluminum
Anodize Line
Paint Line
(m^/IOOO m3) x 24.5 gal./1000 ft3.
(m3/tonx0.12 = gal./lb.
73.4m3/WOO m3
4.2m3/1000m3
77.7 kg/1000m2
0.2 kg/1000 m2
27.8 rtf/ton
2.5 m3/ton
29.5 kg/ton
0.1 kg/ton
zation, was examined to determine the
impact on sludge treatment, handling and
disposal. Samples of spent caustic and
spent anodize acid were collected from
plant A-3 and used immediately for
neutralization experiments in a complete-
ly-mixed flow reactor with a hydraulic
retention time of approximately 10 min-
utes. Thickening and dewatering proper-
ties of suspensions which were neutral-
ized at temperatures ranging from 60 to
90°C and pH values from 5.5 to 10 and
stored for periods of 0 to 24 h were
investigated. Neutralization temperature
and storage for up to 24 h had minimal
impacts on thickening and dewatering
properties.
Neutralization pH had a dramatic im-
pact on sludge thicken ing and dewatering
properties. Sludge thickening properties,
as measured by interfacial settling veloc-
ity, were improved for unconditioned
suspensions as pH was increased. Simi-
larly, specific resistance measurements
indicated dewatering properties improved
with increasing values of neutralization
pH. In addition to improved dewatering
rates, significant reductions in the wet
mass of dewatered sludge solids were
achieved. Cake solids concentrations
ranged from 33 to 54 percent (mass basis)
following standard laboratory dewatering
tests and, as indicated in Figure 1,
improved with increasing values of neu-
tralization pH. When compared with cake
solids concentrations of 9 to 17 percent
obtained in a similar manner with con-
ventional aluminum-finishing sludges,
the values presented in Figure 1 indicate
the potential of segregated neutralization
for significant reductions in the quantity
of sludges produced for disposal. As an
example, it is assumed that 65 percent of
waste aluminum at a plant is treated by
segregated neutralization to achieve a
dewatered sludge with a solids content of
40 percent, and the remainder of the
55
I 50
I
| 45
I
i«
35
30
= 10.0
pH = 8.5 (Run #5)
0 50 100 150 200 250 300
Suspended Solids Concentration, G/L
Figure 1. Solids content of laboratory-
dewatered sludge following
segregated neutralization
at 80°C.
waste aluminum is dewatered to a solids
content of 15 percent. In comparison to a
plant producing one combined dewatered
sludge by conventional neutralization
with a solids content of 15 percent, use of
segregated neutralization to treat a small
portion of the total wastewater flow
would result in a 40 percent reduction in
the wet mass of sludge to be disposed.
Improved thickening and dewatering
properties following segregated neutrali-
zation were attributed to the formation of
crystalline aluminum hydroxides as op-
posed to amorphous precipitates. De-
creased sludge compressibility, as well as
improved thickening and dewatering prop-
erties, were indicative of the apparent
formation of hydroxides such as gibbsite,
boehmite, pseudo-boehmite and nord-
strandite.
Caustic Etch Recovery
Spent caustic etch is a major source of
waste aluminum with aluminum con-
centrations as high as 70 g/L. Removal of
aluminum from this concentrated source
of waste aluminum has potential for
recovery of spent etch for reuse in
finishing extruded aluminum, as well as
reduction in handling of waste sludge
solids. Addition of calcium to spent caustic
etch was examined to establish the
extent to which aluminum was precipi-
tated in the form of calcium aluminates;
the ease with which the precipitate could
be removed from suspension; and, to a
lesser extent, the potential for recovery of
the remaining caustic etch solution. All
experimental studies were conducted in
batch reactors maintained at constant
temperature. Lime (Ca(OH)2) was added
to fresh samples of spent caustic etch,
mixed and examined with respect to
removal of aluminum and calcium, as
well as dewatering properties of precip-
itated solids.
Removal of aluminum from spent etch
was a function of reaction time, temper-
ature, and calcium addition. For a reaction
time of 6h, the Ca/AI ratio for stoichio-
metric removal of aluminum ranged from
2.7 to 3.7 on a molar basis (4.0 to 5.5 on a
mass basis) at temperatures of 60°C and
25°C, respectively. Production of alumi-
num-free etch from spent etch was
therefore feasible and could be controlled
by the level of lime addition used.
Since aluminum concentrations in
spent etch frequently range from 20 to 70
g/L, precipitation of aluminum by calcium
addition produced a suspension of suffi-
cient concentration (e.g., suspended
solids of 80 to 300 g/L) to warrant direct
filtration to remove precipitated alumi-
nates. Specific resistance data, collected
at temperatures of 25 to 60°C, indicate
good dewatering properties for the sus-
pensions produced. In addition, solids
concentrations for dewatered cakes
ranged from 45 to 53 percent. Therefore.
precipitation of aluminum from spent
caustic etch with lime produced a sus-
pension which was easily dewatered to
high solids contents. Controlled lime
addition would, furthermore, allow for
regulation of aluminum concentrations in
reclaimed etch, depending on aluminum-
finishing requirements.
Sludge Reclamation as Liquid
Alum
Extensive examination of the metal
composition of aluminum-finishing sludg-
es indicated that aluminum was the
major metal present. Based on inert
suspended solids (ISS), aluminum content
ranged from 0.27 to 0.6 kg Al/kg ISS,
3
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averaged 0.35 kg Al/kg ISS, and was
typical of that for aluminum hydroxide,
i.e., 0.346 kg Al/kg AI(OH)3. Other metals
commonly contained in sludge solids
included arsenic, cadmium, chromium,
copper, lead, nickel and zinc. These latter
metals were however, only minor constit-
uents with concentrations ranging from
10 to 2000 mg/kg. Therefore aluminum-
finishing sludges were established to be
excellent sources of aluminum, presum-
ably as a collection of numerous alumi-
num hydroxides, which contained low
levels of other metals.but contained high
levels of moisture, e.g., 83 to 91 percent
moisture. These characteristics indicated
the potential for use of aluminum-finish-
ing sludges in production of aluminum
sulfate, i.e., "alum."
A laboratory-scale investigation of the
production of alum from aluminum-finish-
ing sludges was conducted with a heated
batch reactor. Various types of sludges
were examined including: (1) sludge from
two conventional anodizing plants (sam-
ples A-2 and A-3); (2) sludge produced by
segregated neutralization of spent caustic
etch and anodize acid (sample SN); and (3)
sludge solids from two proprietary etch-
recovery systems (samples ER-1 and ER-
2). All sludge samples were collected
from field or laboratory dewatering sys-
tems and examined without pretreatment,
except sample A-3-2 which was air dried
to a solids content similar to that of
sample A-2. Sludge sample A-3-1 was
typical of the majority of the aluminum-
finishing sludges examined during a
previous study, while sample A-2 was a
mixture of a suspension produced by
segregated neutralization and a suspen-
sion produced by conventional neutral-
ization of dilute rinse-waters.
Characteristics of the sludges examined
are presented in Table 3. The solids
content of the sludges varied consider-
ably while aluminum content (based on
dry inert solids) was consistent, between
32 and 38 percent, and vyas, in general,
typical of aluminum hydroxide precipi-
tates. Based on aluminum content of
each sludge sample, sulf uric acid require-
ments were calculated according to the
following equation:
2AI(OH)3 + 3H2SCu -
6H2O
and were based on stoichiometric extrac-
tion of sludge-aluminum and the goal to
produce a commercial-strength product.
Commercial grade alum has an aluminum
content of 8.0 to 8.3 percent as AI2C>3.
Solubilization of aluminum in sludge
samples A-2, A-3-1, A-3-2, and SN was
rapid and was complete within 60 min-
utes. Aluminum solubilization for samples
ER-1 and ER-2 was complete after 120
minutes, when conducted at an elevated
acid strength during extraction. The qual-
ity of the alum products produced from
sludge samples is indicated in Table 4.
"Predicted" values of AI2C>3 content in
Table 4 were target values used to
establish sulf uric acid requirements. Data
for conventional sludges from two alumi-
Table 4.
Composition of Alum Produced
by Acid Extraction of Aluminum-
Finishing Sludges
Sample
percent
Predicted
Measured
A-2
A-3-1
A-3-2
SN
ER-1
EFt-2
8.3
5.3
8.3
8.3
8.3
8.3
8.9
5.9
8.4
8.5
8.7
8.5
Table 3.
Characteristics of Aluminum-Finishing Sludges Used in Alum Production Studies
Sample
A-2
A-3-1
A-3-2**
SN
ER-1
ER-2
Solids Content
percent
21.3
13.5
21.1
32,9
95.1
90.4
Aluminum Content
percent"
35
34
34
32
37
38
"Percent as dry inert so/ids.
"Air dried to increase solids content.
num-finishing plants (A-2; A-3) indicated
the potential for production of commercial-
grade alum from these sludges. Extraction
of samples A-2 and A-3-2 produced
commercial-strength alum while that for
A-3-1 did not. The solids content of
sample A-3-1 was the lowest of sludges
examined and the moisture contained in
the dewatered sludge resulted in the
production of a diluted product. However,
when air-dried to a solids content of 21.1
percent, the sludge (A-3-2) was effective-
ly used to produce a commercial-strength
product. Using Equation 1, the theoretical
minimum value for sludge solids content
required to produce a commercial-
strength alum (i.e., 8.0 percent AlaOa)
was estimated to be 20 percent. Data for
samples A-2 and A.-3 experimentally
confirmed this theoretical value.
In addition to aluminum content, free-
acid and free-aluminum concentrations
and concentrations of iron, calcium,
potassium, magnesium and priority pollut-
ant metals were investigated. All param-
eters were within acceptable limits for
product quality and no restrictions on the
commercial use of alum produced from
aluminum-finishing sludge was antici-
pated.
Process economics were investigated
for an aluminum extrusion/anodizing
plant finishing approximately 25 ton/d of
extruded aluminum. Acapital investment
of $80,000 was estimated for a plant
producing a dewatered sludge with a
solids content of 20 percent or greater.
The estimated payback period for the
capital investment was 14 to 21 months,
exclusive of any economic benefits real-
ized as a result of elimination of the need
to dispose of sludge solids and using a
price for the sale of alum equal to 60
percent of the current market value. The
production of liquid alum from aluminum-
finishing sludges therefore has the poten-
tial to profitably reclaim waste aluminum
and eliminate a major sludge disposal
problem.
Conclusions
The results of the research on innova-
tive treatment processes indicate that
they have excellent potential for achieving
major reductions in wet mass of sludges
available for disposal; recovery of spent
caustic etching solutions; and economical
reclamation of waste aluminum as a
marketable product.
The results of an initial industrial plant
survey provided the justification for pur-
suit of the three innovative processes
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investigated. From the survey it was con-
cluded that:
1. Themajorityof waste metal from an
anodize line was aluminum re-
moved from alloy surfaces during
etching and anodizing with finish-
ing-solution additives providing
minor metal loadings. Rinsewaters
contributed the bulk of the waste-
water discharged while spent caus-
tic etch, spent anodize acids and
dragout from etching tanks were
the sources of more than 90 percent
of waste aluminum.
2. Waste-metal quantities in paint
line wastes were significantly lower
than those in anodize lines and
were equally attributable to alumi-
num removed from alloy surfaces
and chromium discharged from
finishing solutions.
Segregated neutralization of concen-
trated finishing solutions was investi-
gated as a means of reducing the volume
of sludge solids produced. It was con-
cluded that:
1. Segregated neutralization of con-
centrated spent etch and acid solu-
tions could be achieved at temper-
atures of 60 to 90°C in 9 to 10
minutes.
2. Thickening properties of sludges
produced by segregated neutraliza-
tion were improved significantly by
alkaline neutralization over neutral
or acidic neutralization.
3. Batch flux analysis indicated that
thickened sludge concentrations of
4 to 5 percent solids could be
routinely achieved in sedimentation
basins conventionally used in the
industry as compared to conventional-
neutralization sludge concentra-
tions of 1 to 2 percent.
4. Dewatering properties of sludges
produced by segregated neutraliza-
tion were improved by use of alka-
line pH values. Cake solids concen-
trations from 35 to 54 percent
solids were achieved at alkaline pH
values indicating a major reduction
in final sludge volume.
5. Evaluation of implementation of
segregated neutralization to treat
spent finishing solutions at plant A-
3 indicated that predicted reduc-
tions in wet sludge mass ranged
from 73 to 80 percent, resulting in a
major reduction in sludge disposal
costs.
Recovery of spent caustic etch by
orecipitation of aluminum with lime was
westigated. It was concluded that:
1. Removal of aluminum from caustic
etch solutions was achieved by
precipitation of calcium aluminate
using lime addition at reaction
temperatures of 25 to 60°C.
2. Reaction kinetics were affected by
reaction temperature, reaction time
and the Ca/AI ratio.
3. Sludge solids produced at Ca/AI
mass ratios of 1.5 to 4.5 and
temperatures of 25 to 60°C had
excellent dewatering character-
istics.
4. Analysis of the impact of imple-
mentation of etch recovery with
lime addition at a full-scale anodiz-
ing plant indicated a 24-percent
reduction in wet sludge mass was
achieved as well as a potential
chemical saving of $500/day
through recovery of spent etching
solution.
F. M. Saunders, E. S. K. Chian, G. B. Harmon, K. L Kratz, J. M. Medero, M. E.
Pisani, R. R. Ramirez, and M. Sezgin are with School of Civil Engineering,
Georgia Institute of Technology, Atlanta, GA 30332.
Alfred B. Craig, Jr. is the EPA Project Officer (see below).
The complete report, entitled "Evaluation of Process Systems for Effective
Management of Aluminum Finishing Wastewaters and Sludges, "(Order No. PB
84-170 661; Cost: $16.00. subject to change) will be available only from:
National Technical Information Service
5285 Port Royal Road
Springfield, VA 22161
Telephone: 703-487-4650
The EPA Project Officer can be contacted at:
Industrial Environmental Research Laboratory
U.S. Environmental Protection Agency
Cincinnati, OH 45268
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United States
Environmental Protection
Agency
Center for Environmental Research
Information
Cincinnati OH 45268
Official Business
Penalty for Private Use $300
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U.S. GOVERNMENT PRINTING OFFICE: 1964-759-102/934
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