United States
Environmental Protection
Agency
Industrial Environmental
Research Laboratory
Research Triangle Park NC 27711
Research and Development
EPA-600/S2-84-064 May 1984
f/EPA Project Summary
Laboratory Feasibility Studies for
the Fluidized-Bed Combustion of
Spent Pot I'm ing from Aluminum
Reduction
B.C. Kim, E.J. Mezey, D.R. Hopper, A. Wensky, R. Heffelfinger, and S.E.
Rogers
This study was undertaken with the
objective of providing a preliminary
assessment of the technical feasibility
and environmental acceptability of a
fluidized-bed combustion (FBC) process
for the disposal of spent potlining waste
from the aluminum reduction process. In
accomplishing the above, technical
efforts were directed to two main areas
of laboratory studies: (1) differential
thermal analysis experiments to establish
the operating temperature range to
prevent agglomeration in a FBC process,
and (2) fixed-bed combustion experi-
ments to determine cyanide destruction
and gaseous emissions expected from a
FBC process.
The results from the differential
thermal analysis and the fixed-bed
combustion experiments indicated that
FBC should be limited to temperatures
below 760°C due to agglomeration of
potlining waste that occurs above that
temperature. Analysis of the combus-
tion residues showed that 99.9 to
99.99 percent of cyanide was removed
from the potlining and that substantial
burning of the carbon in the potlining
could be achieved at temperatures as
low as 760°C. The results on cyanide
removal and carbon combustion are
very encouraging and indicate that
potential application of a FBC process
to the disposal of spent potlining
wastes is technically feasible.
A small fraction (0.01-2 percent) of
CM and a substantial fraction (15-35
percent) of F were found in the gaseous
effluent from the combustion of potlin-
ing. The gaseous emissions, however,
can be adequately controlled and
should not pose an insurmountable
technical barrier in the FBC process. No
significant emissions of SO2 or Na2O
were found during combustion of
potlining waste.
Results of the laboratory studies
indicate that further investigation of the
FBC process, through a pilot-plant
scale test program (particularly efforts
to prove the operability of a fluidized
bed over a range of combustion tempera-
• tures to accomplish effective destruc-
tion of cyanide in the spent potlining
waste), would provide valuable infor-
mation in this technological area. "
This Project Summary was developed
by EPA's Industrial Environmental Re-
search Laboratory, Research Triangle
Park, NC. to announce key findings of
the research project that is fully docu-
mented in a separate report of the same
title (see Project Report ordering
information at back).
Introduction
Approximately 230,000 tons of spent
potlining is generated each year in the
production of aluminum metal in the U.S.
This discarded carbon potlining material
contains significant quantities of alumi-
num, and sodium metals, aluminum
oxides, fluorides of sodium and aluminum.
carbides and nitrides of aluminum, and
quantities of cyanide which have pene-
trated the potliners over their 3 to 5 year
useful life.
-------
Because of the significant energy,
chemical, and environmental implications
of the generation and storage of spent
potlining, efforts by the aluminum
industry have been directed toward
developing environmentally sound re-
source recovery alternatives. In December
1981, The Environmental Committee of
the Aluminum Association, Inc. sponsored
a workshop on the storage, disposal, and
recovery of spent potlining.
Although in July 1980, spent potlining
was listed by EPA as a hazardous waste
under RCRA, its listing is currently
suspended because of relevant court
challenges. However, EPA's interest and
responsibility in working with concerned
groups in addressing the potlining dispo-
sal problem remains active.
The scope of this program was to
conduct laboratory studies to evaluate
the thermal destruction of cyanide in the
potlining waste and potential gaseous
emissions from FBC of spent potlining.
The objective was to assess the technical
feasibility and environmental acceptabil-
ity of the FBC process and provide a basis
for recommending and evaluating test
plans for follow-up pilot-scale FBC of
spent potlining.
Conclusions
The study results have identified
agglomeration as a potential problem for
a FBC process due to softening or melting
of the potlining from the aluminum
reduction process during the combustion
process. Although the combustion residue
(ash) from the potlining could be main-
tained free flowing without agglomeration
at temperatures above 900°C using a
kaolin clay as an additive, the potlining
itself was observed to agglomerate at
temperatures as low as 760°C even with
the clay additive. Whether the turbulent
mixing obtained in the FBC process could
overcome the agglomeration problem
appears to be the key technical issue that
must be resolved to prove the FBC process
for application in spent potlining disposal.
The results from fixed-bed combustion
experiments have also indicated that
99.9 to 99.99 percent of the cyanide in
the potlining can be removed at 760°C.
Small quantities of HCN were found in
the gaseous combustion products from
fixed-bed combustion tests performed on
potlining samples. It is expected that the
HCN generated during combustion of
potlining waste can be destroyed by
providing sufficient gas-phase residence
time at combustion temperature in a FBC.
No detectable emission of SOa was found
in the flue gas from combustion of
potlining.
A «ignificafrt evolution of HF has been
.found from •combustion of potljning.
Therefore, the flue gas from potlining
waste combustion will need to be treated
to control HF emission and possibly
reclaim the fluoride value.
Results from fixed-bed combustion
experiments have indicated that the
carbon in potlining waste can be ignited
and burned efficiently at temperatures as
low as 760°C. The reactivity of the carbon
in spent potlining waste appears to be
sufficiently high to obtain a high combus-
tion efficiency (>90%) and possibly
sustain combustion without an auxiliary
fuel in a FBC.
No significant volatilization of Na20
from possible reactions between potlining
and CaO or water vapor was detected at
760°C. Therefore, generation of Na20
from combustion of potlining and asso-
ciated slagging and fouling problems are
not expected in the FBC process.
Recommendations
Based on the laboratory feasibility
studies, a FBC test program should be of
value. A major technical issue that could
be addressed in such a program would be
demonstration of the operation of a
fluidized bed over a practical range of
temperature to effectively destroy cyanide
without bed agglomeration. The target
temperature range would be at least 50°C
(preferably 100°C) to allow for normal
temperature fluctuations and process
upsets that can be anticipated in a full-
scale FBC unit. The clinkers formed by
molten potliner waste are extremely
hard, and formation of such clinkers in a
fluidized bed by temperature excursions
would cause a major shutdown and
possibly extensive damage to the refrac-
tory in a FBC. If bed agglomeration causes
a severe problem in fluidized bed opera-
tion, other combustion technologies (e.g.,
a rotary kiln incinerator) could be con-
sidered as an alternative.
Other technical issues that could be
addressed in a fluidized-bed test program
are:
1. Multiple sampling of the feed and
ash during the test program, due to
the inhomogeneous nature of pot-
line
waste.
2. The use of kaolin clay as an additive
to increase combustion temperature
to the suggested 600 to 800°C
combustion temperature range.
3. Flue gas analysis for HCN and HF
emissions to further establish the
need for downstream treatment of
the flue gas. (The level of HF in the
flue gas has an important bearing on
possible recovery ,of fluoride «alue
and waste heat.)
4. Confirmation of cyanide destruction
by FBC by leaching tests on .the
combustion residue.
Results
Laboratory studies were conducted in
two parts: (1) differential thermal analyses
(DTAs) to establish the operating tempera-
ture for the FBC process dictated by ash
agglomeration, and (2) fixed-bed combus-
tion experiments to determine cyanide
destruction and gaseous emissions at the
operating temperature established in the
experiments of differential thermal
analysis.
Potlining Samples
Potlining samples were supplied by
Aluminum Company of America (Alcoa)
and Reynolds MetaJs Company Chemical
analyses of the potlining samples are
shown in Table 1. Both samples exhibited
a strong odor of ammonia, probably
produced by hydrolysis reaction between
moisture in ambient air and aluminum
nitride present The samples contained
some coarse (about 1/4-m or 0.635 cm)
particles and appeared nonhomogeneous.
Therefore, the samples were dumped
from the shipping containers, mixed,
coned, and split into smaller aliquots by
sequential quartering and coning or by
using a sample splitter. The head samples
prepared in this manner were the 1/32
cut of the Alcoa sample and the 1/128
cut of the Reynolds sample. The head
samples were pulverized and stored in
sealed containers for use in the DTA
studies.
Table 1. Composition of Potlining Samples"
Composition. Percent
c
Al
Si
Fe
Na
F
CN
Al Carbide
Ash
Alcoa
28.6
142
4.76
3.23
139
18.2
009
046
68.7
Reynolds
42.7
47
0.11
0.27
20.0
17.6
0135
571
^Analyses provided by Alcoa and Reynolds,
except ash analyses performed by Battelle
Differential Thermal Analyses
DTAs were carried out on two potlining
samples and the ash samples prepared
from the potlining samples. Ash samples
were prepared by burning off the combus-
tible matter from the potlining samples in
a muffle furnace in ambient air at 750°C
-------
to constant residue weight. DT&s were
.also carried out <®n mixtures of the
potlining ash samples with a reagent-
grade fcaikewm 3
TiO2
CaO
MgO
Na2O
Loss on Ignition at 1000°C
469
38.2
0.35
1 42
0.43
0.58
004
13.9
Results from the ash agglomeration
tests are presented in Table 3 and
summarized below:
• Alcoa potlining ash has higher
agglomeration temperature than
Reynolds potlining ash.
• Adding CaO has little or no adverse
effect on raising ash agglomeration
temperature.
• Adding clay significantly increases
the ash agglomeration temperature
to above 900°C.
Fixed-Bed Combustion Studies
Fixed-bed combustion experiments
were performed to determine cyanide
destruction and emissions of cyanide and
fluwr*de in ithve gaseous combustion
products. The reactor was a quartz tube,
2 in. IDt>y44 in. long(6.08 by 111.76 cm),
heated by a3 that is formed by hydrolysis
of AIN in potlining. For this hypothesis,
agglomeration could be avoided in a
fluidized bed, since dehydration of
AI(OH)3 is commercially practiced in
fluidized-bed calciners. However, the
probability of the AI(OH>3 sintering
process' being the sole cause of agglom-
eration is remote in view of the complex
salts included in the potlining composi-
tions. Therefore, agglomeration of potlin-
ing poses a potential problem m a FBC
process. Results of the fixed-bed combus-
tion tests on bed agglomeration are, in
summary:
1. The Alcoa potlining sample showed
greater tendency for agglomeration
than the Reynolds potlining sample.
(Compare Tests 5 and .9, or Tests 7
and 8.) This behavior of the potlining
samples is opposite from that
observed with the ash samples in
the DTA tests which showed higher
agglomeration temperatures for the
Alcoa sample compared with the
Reynolds sample.
2. Reducing the sample size decreased
the apparent extent of agglomeration
of potlining. (Compare Tests 4 and
7.) However, even a smaller quantity
of test samples showed agglomera-
tion at 760°C (Tests 8 and 9).
Potlining
Test
1
2
3
4
5
6
7
8
9
10
;;"
Type
Alcoa
Alcoa
Reynolds
Reynolds
Reynolds
Alcoa
Reynolds
Alcoa
Alcoa
Reynolds
Reynolds
Sample Wt.. g
75
75
75
75
12
12
12
12
12
12
12
Wt.,g
7.5
7.5
7.5
37.5
6
4.1
6
7.2
7.2
6+6(CaO)
6
% of
Ash"
15
15
17
87
87
50
87
87
87
87+87(CaOj
87
Temperature,
°C
911
762
760
762
760
875
760
760
76O
760
760
Residence
Time, mm
30
120
120
30
120
30
30
30
120
120
120
Results
Very severe agglomeration:
Quartz reactor cracked.
apparently caused by
fused potlining
Very severe agglomeration
Very severe agglomeration
Very severe agglomeration
Slight agglomeration
Very severe agglomeration
Slight agglomeration
Severe agglomeration
Severe agglomeration
Slight agglomeration
Slight agglomeration
"Based on 68.7 percent ash in the Alcoa potlining and 57.1 percent ash in the Reynolds potlining.
''Combustion air was humidified to approximately 2 percent water vapor concentration.
-------
Due to the severe agglomeration of the
bed that resulted in poor gas/solid
contact, the ash residue and impinger
train samples that were selected for
analysis were chosen from those tests
performed during the latter part of the
combustion test program in which bed
agglomeration was substantially reduced
by decreasing the combustion tempera-
ture
and the amount of potlining sample used.
Material balance results describing the
fate of cyanide, fluorine, carbon, and
sodium during the fixed-bed combustion
of potlining were calculated from these
sample analysis results.
Tests 5, 7, 8, and 9 were performed to
determine cyanide destruction, emissions
of cyanide and fluoride in flue gas, and
carbon combustion efficiency. Results
are:
1. Cyanide was removed from the
potlining in the range of 99.9 to
99.99 percent with both Alcoa and
Reynolds potlining samples at 760°C
and 30 to 120 minutes residence
time. Small quantities of cyanide in
the range of 0.01 to 2 percent of feed
were found in the flue gas. It is
expected that a FBC should provide
sufficient gas-phase residence time
in the bed and in the freeboard to
destroy the cyanide evolved as HCN
from the potlining.
2. Approximately 20 to 30 percent loss
of fluoride from potlining was
observed under the conditions
employed in the combustion tests.
Fluoride collection from the flue gas
was measured to be in the range of
0.2 to 8.7 percent. The fluoride
unaccounted for was probably lost
by reaction with the quartz reactor
and other glass surfaces that con-
tacted the flue gas. Nevertheless,
the results strongly suggest that a
significant fraction of fluoride is
generated as HF from potlining
during the combustion process.
Therefore, flue gas should be treated
to control HF emission and possibly
recover the fluoride value.
3. Results on carbon balance show
approximately 70 to 90 plus percent
carbon combustion efficiencies in
the combustion tests. The total
carbon found in the ash and the flue
gas was found to be approximately
25 to 35 percent above the carbon
introduced with the potlining, indi-
cating a reasonable closure on the
carbon balance. Combustion effi-
ciency in a fluidized bed is expected
to be higher due to improved mixing
and gas/solid contact in a fluidized
bed. Therefore, combustion of pot-
lining in a fluidized bed at tempera-
tures as low as 760°C appears to be
feasible.
4. No detectable emission of SOz was
found in the flue gas from combus-
tion of potlining.
Tests 10 and 11 were performed to
determine volatilization of Na20 by
reactions of a potlining sample with CaO
and with H20 vapor. The tests were
carried out with the Reynolds potlining
sample that has a higher Na content than
the Alcoa potlining sample. Test 10 was
carried out with CaO addition and dry
combustion air. Test 11 was carried out
with the combustion air humidified to
approximately 2 percent water vapor.
Results on sodium balance show essen-
tially no volatilization of Na2O due either
to CaO or HaO vapor under the conditions
employed in the combustion tests.
B. Kim, E. Mezey, D. Hopper, A. Wensky. Ft. Heffelfinger, and S. Rogers are with
Battelle-Columbus Laboratories, Columbus, OH 43201.
David C. Sanchez is the EPA Project Officer (see below).
The complete report, entitled "Laboratory Feasibility Studies for the Fluidized-Bed
Combustion of Spent Potlining from Aluminum Reduction," (Order No. PB
84-168 764; Cost: $10.00. 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
Research Triangle Park, NC 27711
•fr U S GOVERNMENT PRINTING OFFICE, 1984 — 759-015/7685
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