Characterization
of Carbide Lime
to jdentify Sulfite
Oxidation Inhibitors
Interagency
Energy/Environment
R&D Program Report
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EPA-600/7-78-176
September 1978
Characterization of Carbide
Lime to Identify Sulfite
Oxidation Inhibitors
by
L J. Holcombe and K. W. Luke
Radian Corporation
P. O. Box 9948
Austin, Texas 78766
Contract No. 68-02-2608
Task No. 21
Program Element No. EHE624A
EPA Project Officer: Julian W. Jones
Industrial Environmental Research Laboratory
Office of Energy, Minerals, and Industry
Research Triangle Park, NC 27711
Prepared for
U.S. ENVIRONMENTAL PROTECTION AGENCY
Office of Research and Development
Washington, DC 20460
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TABLE OF CONTENTS
Page
1.0 INTRODUCTION 1
2.0 SUMMARY AND CONCLUSIONS 2
3.0 RECOMMENDATIONS 6
4.0 BACKGROUND 8
4.1 Carbide Lime Characteristics 8
4.2 Sulfite Oxidation 11
4.3 Inhibition of Sulfite Oxidation 11
4.4 Sulfur Oxide Chemistry 14
5.0 CARBIDE LIME CHARACTERIZATION 17
5.1 Spark Source Mass Spectrometry 19
5.2 Gas Chromatography-Mass Spectrometry... 23
5. 3 Sulfur Analysis 24
5.4 X-Ray Powder Diffraction 28
5.5 Infrared Spectrophotometry 29
6.0 ASSESSMENT OF POSSIBLE OXIDATION INHIBI-
TORS 35
6.1 Inhibition of Sulfite Oxidation by
Carbide Lime ^~*
6.2 Possible Oxidation Inhibitors 40
6.3 Thiosulfate as the Sulfite Oxidation
Inhibitor
6.4 Oxidation Inhibiting Reactions 44
7.0 EVALUATION OF POTENTIAL INTERFERENCE-FREE
ANALYTICAL METHODS FOR SULFITE, SULFATE AND
THIOSULFATE 47
iii
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TABLE OF CONTENTS (continued)
7 .1 Ion Chromatography 47
7 . 2 lo dome trie Analysis 49
7.3 Potentiometric 56
7.4 Acidimetric Analysis of Sulfate 57
8.0 ANALYTICAL METHODS FOR SULFITE, SULFATE, AND
THIOSULFATE 58
8 .1 Sulf ite Analysis 58
8. 2 Sulfate Analysis 61
8.3 Thiosulfite Analysis 62
8.4 Mixtures of Two or More Sulfur Species.. 63
REFERENCES 65
APPENDIX - STANDARD OPERATING CONDITIONS FOR THE
DETERMINATION OF SULFITE, SULFATE AND THIOSULFATE
CONCENTRATIONS IN CARBIDE LIME LIQUORS USING THE
ION CHROMATOGRAPH 67
IV
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LIST OF FIGURES
Figure Page
5-1 Infrared Spectrum of Reagent Grade Sodium
Thiosulfate, Na2S203«5H20 30
5-2 Infrared Spectrum of Freshly Dried Carbide
Lime Solids 31
5-3 Infrared Spectrum of Freshly Dried Carbide
Lime Solids 32
5-4 Infrared Spectrum of Dried Solids from the
Standard Carbide Lime Additive 33
6-1 Oxidation of Sulfite to Sulfate in Commercial
Lime and Carbide Lime Slurries 37
6-2 Oxidation of Sulfite to Sulfate in Commercial
Lime and Carbide Lime Slurries 39
6-3 Oxidation of Sulfite to Sulfate in Slurries of
Commercial Lime with Thiosulfate, Commercial
Lime and Carbide Lime 45
v
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LIST OF TABLES
Table Page
4-1 Carbide Lime Composition 10
4-2 Inhibitors of Sulfite Oxidation 13
5-1 Spark Source Mass Spectrometer Results on
Solids from Carbide Lime Slurry 20
5-2 Spark Source Mass Spectrometer Results on
Filtrate from Carbide Lime Slurry 22
5-3 Concentration of Polynuclear Aromatic Hydro-
carbons and Related Species in Carbide Lime
Solids 25
5-4 Concentration of Major Organics in Carbide
Lime Liquors as Detected on the Gas Chroma-
tograph-Mass Spectrometer 24
5-5 Analysis of Sulfur Species in Carbide Lime
Solids Taken from Waste Heap Adjacent to
Paddy' s Run 26
5-6 Sulfur Analysis on Filtrate from Carbide Lime
Additive and on Carbide Lime Solids Extracted
With Water 28
5-7 X-Ray Diffraction Analysis of Carbide Lime
Solids 28
VI
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LIST OF TABLES (continued)
Table Page
6-1 Concentration of the Interferent vs_ Thio-
sulfate and Reduced Sulfur in Carbide Lime
Liquor 43
7-1 Evaluation of the Back-Titration in the lodo-
metric Determination of Sulfite in Carbide
Lime Liquors 52
7-2 Comparison of Sulfite Value Obtained by
Fomaldehyde-Iodometric Titration with Ion
Chromatography 55
A-l Chromatographic Conditions 69
vxi
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1.0 INTRODUCTION
Recent testing under EPA sponsorship at Louisville
Gas and Electric1s (LG&E) Paddy's Run flue gas desulfurization
(FGD) unit has shown a significant difference between carbide
lime, a byproduct of acetylene manufacture, and commercial lime.
The most significant difference was the extremely low rate of
oxidation of sulfite to sulfate when carbide lime was used. The
low oxidation rate results in a greatly reduced tendency for
calcium sulfate (gypsum) scaling. Also, a significant inter-
ference with Radian1s liquid sulfite analytical technique (io-
dometric titration in a pH 6 buffer) was apparent when carbide
lime was being used.
The main component of carbide lime is hydrated calcium
oxide, Ca(OH)z, but it also contains numerous impurities. Car-
bide lime is made under generally reducing conditions and thus
many of the contained impurities are in reduced oxidation states.
This study was undertaken to characterize carbide lime,
identify the possible sulfite oxidation inhibitors and evaluate
methods for sulfite and sulfate analysis in carbide lime liquors.
The characterization of carbide lime involved various instru-
mental and wet chemical techniques to identify the major and
minor species in carbide lime. These species were then evalua-
ted as possible oxidation inhibitors. Candidates for sulfite
oxidation inhibitors were tested in the laboratory. Finally,
interference-free methods for sulfite and sulfate analysis
were evaluated.
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2.0 SUMMARY AND CONCLUSIONS
Carbide lime was characterized using several analytical
techniques. A survey analysis for elemental composition was
done by spark source mass spectrometry. The organic compounds
in carbide lime were determined on a gas chromatograph-mass
spectrometer. And the sulfur species present were analyzed by
both wet chemical and instrumental methods. The physical
characteristics of carbide lime were determined by X-ray powder
diffraction and infrared spectrophotometry.
Laboratory tests were conducted to compare the rate
of sulfite-to-sulfate oxidation in the presence of:
carbide lime,
commercial lime and
oxidation inhibitors found in carbide
lime.
Several methods were tested for analyzing sulfite and
sulfate in carbide lime liquors. Radian normally uses an iodo-
metric titration in a pH 6 buffer to determine sulfite concentra-
tions. However, in carbide lime liquors a species other than
sulfite reacts to consume the iodine, resulting in a high apparent
sulfite concentration. There is no interference with normal
sulfate analysis in carbide lime, but the sulfite present must
be prevented from oxidizing to sulfate.
From the results of this work, it can be concluded
that sulfur species in reduced oxidation states in carbide lime
cause both the inhibited sulfite oxidation and the interference
with the sulfite analysis.
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The characterization studies showed that most of the
sulfur in carbide lime was present in a reduced oxidation state.
One reduced sulfur species, thiosulfate, was identified in the
solid fraction of the carbide lime slurry through X-ray powder
diffraction and infrared spectrophotometry. It was also present
in the liquor of the slurry, as determined by ion chromatography.
Thiosulfate inhibits the oxidation of sulfite to sulfate in
carbide lime liquors. Thiosulfate also interferes with Radian's
iodometric sulfite determination in carbide lime, but the pH
and time dependence of this interference suggests that other
species are also involved.
The organic analysis of carbide lime revealed several
polynuclear aromatic hydrocarbons in ppb concentrations in carbide
lime. These include:
naphthalene,
fluorine,
anthracene/phenanthrene,
carbazole,
pyrene, and
benzo(a)pyrene.
Some of these compounds have been shown to be harmful to man and
his environment. Further work is needed to determine if these
compounds are leachable from carbide lime and therefore a
potential threat to the environment.
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Laboratory tests have confirmed that, in comparison
to commercial lime slurries,
carbide lime inhibits the oxidation of
sulfite to sulfate and
thiosulfate in a commercial lime slurry
inhibits sulfite oxidation.
The exact mechanism of sulfite oxidation inhibition is unknown
but the thiosulfate in carbide lime probably reacts with sulfite
to form an intermediate species other than sulfate. One pos-
sible reaction, from Schroeter (Ref. 4), involves the trithio-
nate ion (S30s2~).
S2032~ + SOa2" + 20H*(radical) + 2H+ = S30s2" +2H20 (2-1)
The reaction to a third, stable sulfur species (in this case,
trithionate) prevents the formation of sulfate.
Antioxidants have been suggested for use in scrubbers
to inhibit sulfite oxidation and thus reduce gypsum scaling pot-
ential. The amount of thiosulfate required for scale-free
scrubber operation is unknown. However, to bring the thio-
sulfate level of commercial lime up to that found in carbide
lime would cost $1.50 per ton of lime (using sodium thio-
sulfate pentahydrate at $12 per 100 Ibs). Since commercial lime
currently costs over $40 per ton (in bulk) this would represent
an additional cost of less than 5 per cent. However, it is not
certain that the price of thiosulfate would remain this low if
a major increase in demand occurred.
The thiosulfate and reduced sulfur species in carbide
lime also interfere with Radian's iodometric analytical tech-
nique. The interfering reaction is:
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time dependent. More iodine is consumed
with time after the carbide lime liquor
has been added.
pH dependent. At low pH's the inter-
ference appears to decrease. However,
this may be due to the increased air
oxidation of iodide at low pH's thus
creating a counter effect to the
interference.
The bulk of the analytical interference can be attributed to the
thiosulfate and reduced sulfur species in carbide lime. But the
time dependence and pH dependence of the reaction suggests that
other factors are involved.
The ion chromatograph was found to be the best analy-
tical tool for determining sulfite, sulfate and thiosulfate in
carbide lime liquors. The operating conditions of the ion chroma-
tograph for analyzing each species are given in the appendix.
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3.0 RECOMMENDATIONS
Further work concerning carbide lime should be concen-
trated in three areas, outlined below in order of commercial
importance:
the feasibility of adding sulfite oxida-
tion inhibitors to FGD systems,
further development of methods for
analyzing sulfite, sulfate and thio-
sulfate in carbide lime liquors, and
evaluation of the potential hazards
of polynuclear aromatic hydrocarbons
found in carbide lime on man and his
environment.
If the oxidation of sulfite to sulfate could be in-
hibited in a FGD system, then the scrubber solution would
remain subsaturated with respect to gypsum and scale would not
form. Sulfite oxidation could be controlled by adding inhi-
bitors, such as those found in carbide lime, to a wet scrubber.
Therefore, it would be important to study the mechanism of sul-
fite oxidation in the presence of inhibitors in order to deter-
mine the following:
the amounts of inhibitor to add to the
scrubber to prevent scale formation,
the economic feasibility of adding the
inhibitor to FGD systems and
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the effect of the inhibitor on the
scrubber slurry. Considered here will
be the effect of the inhibitor on the
particle size of the scrubber solids,
on the pH of the solution, and on the
overall scrubber chemistry.
Once the mechanism of inhibition has been identified, thio-
sulfate, or the antecedent to thiosulfate in carbide lime, should
be tested as an inhibitor in a bench scale scrubber.
The development of field analytical methods for sulfur
species would enhance the understanding of carbide lime FGD
chemistry. A wet chemical technique for sulfite should be
perfected in case an ion chromatograph is not available. Also,
a method should be found to preserve sulfite in carbide lime
liquors. This would prevent the sulfite from oxidizing to
sulfate in going from the sampling site to the lab. A possible
preservant for both wet chemical and ion chromatographic methods
is formaldehyde.
Finally, several polynuclear aromatic hydrocarbons were
found in carbide lime solids in ppb concentrations (see Table 5-2)
These compounds may be hazardous to the environment. Therefore,
the following studies are recommended:
the leachability of these compounds in
water,
their behavior in the FGD units and
the best method to dispose of them if
they are found to be hazardous.
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4.0 BACKGROUND
In this section results of a literature search conducted
under this study are presented. The search covered the following
areas:
the chemical make-up of carbide lime, the
conditions under which it is made and its
differences from commercial lime,
sulfite oxidation, including the various
mechanisms of sulfite oxidation, known
inhibitors of sulfite to sulfate oxi-
dation and possible indirect inhibi-
tors of sulfite oxidation,
various aspects of sulfur chemistry,
including the many oxidation states
sulfur may have in aqueous solution,
their reactions with sulfite, and ways
in which these sulfur species may
interfere in the sulfite analysis, and
analytical methods for determining
sulfite, sulfate and thiosulfate.
(These methods will be presented in
Section 8.0 along with descriptions
of analytical methods used in this
work).
4.1 Carbide Lime Characteristics
Carbide lime is a byproduct of the manufacture of
acetylene. Calcium carbide and water are reacted together to
8
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produce acetylene and carbide lime:
CaC2 + 2H20 = Ca(OH)2 + C2H2 (4-1)
The reaction is exothermic.
The calcium carbide (CaC2) in Reaction 4-1 is made
from the reaction of calcium oxide and carbon in an electric
arc furnace:
CaO + 3C = CaC2 + CO (4-2)
Commercially, calcium carbide is most often produced by burning
coke and limestone (Ref. 1). All of the nonvolatile impurities
in the coke and limestone are retained in the calcium carbide.
The contained impurities undergo some chemical changes due to the
high temperatures and reducing conditions involved.
Carbide lime is mainly composed of calcium hydroxide
and calcium carbonate. Examples of carbide lime compositions
are given in Table 4-1. Along with the inorganic compounds
noted, carbide lime may also contain organic compounds (Ref. 1).
Carbide lime can be used in place of commercial lime
in FGD units because of its high calcium hydroxide content. The
important differences between the two limes are mainly due to the
higher impurities in carbide lime. Some chemical and physical
differences in the two limes are given below (Ref. 1).
There is much less magnesium and
phosphorous in carbide lime than in
commercial lime.
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TABLE 4-1. CARBIDE LIME COMPOSITION
Compound
Ca(OH)2
CaC03
CaS03
CaSO>»
Si02
AlzOs
Fe203
CO
CaS
CNS
Weight 7o
Source:
Chemico/Mitsui
(Ref. 2)
84.3
6.9
1.8
1.0
1.7
0.7
1.1
N.R.
N.R.
N.R.
Source :
Miller, S.A.
(Ref. 1)
96.30
N.R.
N.R.
0.34
1.41
1.33
0.12
0.14
0.08-0.12
0.01
N.R. : Not Reported
10
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The amount of silicon, aluminum,
carbon and iron is greater in carbide
lime than in commercial lime.
The particle size of carbide lime
is less than that of commercial lime.
4.2 Sulfite Oxidation
The oxidation of sulfite (S032~) to sulfate (SO,,2")
in aqueous solution takes place through a complex chain reaction
(Ref. 3 and 4). With no catalysts or inhibitors present,
the oxidation is first order with respect to sulfite.
Fuller (Ref.3) postulated that in the reaction of
sulfite with oxygen, unstable intermediates are formed. Through
the reduction of oxygen the following reactive radicals could
be formed:
H02- H202- OH- (4-3)
Also, the oxidation of sulfite could produce the following radicals
S206 = SOs^ (4-4)
These radicals then react to give the overall oxidation reaction:
S03" + %02 = SO*2" (4-5)
4.3 Inhibition of Sulfite Oxidation
The most often proposed mechanism for inhibiting sulfite
oxidation is through the breaking of the chain reaction going
11
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from sulfite to sulfate. The inhibitor species terminates the
free radical chain reaction by complexing with or deactivating
one of the intermediates. Schroeter (Ref. 4) states that the
rate equation of the inhibition reaction is bimolecular with a
first order dependence on both the radical and the inhibitor.
There has been much work done on the inhibition of
sulfite oxidation (Ref. 3, 4, 5 and 6). Many organic compounds
and many nitrogen compounds may be inhibitors (Ref. 4). Also,
inorganic compounds such as arsenite, antimonite, phosphite and
cyanide may terminate the chain reaction in sulfite to sulfate
oxidation. A list of known inhibitors is presented in Table
4-2.
Another mechanism for inhibiting sulfite oxidation
could be through the complexing of sulfite. Schroeter (Ref. 4)
notes that pyridine possibly retards sulfite oxidation by
forming the N-pyridinium sulfonic acid complex with sulfite.
Not only is oxidation reduced but the end product contains sig-
nificant amounts of dithionate in addition to sulfate. Formalde-
hyde also forms a stable complex with bisulfite, which greatly
retards oxidation:
HSOa" -I- CH20 = HOCHaSOa" (4-6)
Sulfite oxidation is also inhibited if another species
competes with the sulfite for oxygen. Aqueous compounds which
might inhibit air oxidation of sulfite in this way are other
reduced sulfur species, organic matter, and reduced inorganic
compounds.
12
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TABLE 4-2. INHIBITORS OF SULFITE OXIDATION*
methanol
ethanol
propanol
normal-, secondary-, iso- and tertiary-butyl alcohol
alkyl alcohol
ethylene glycol
glycerin
mannitol
benzyl alcohol
acetaldehyde
benzaldehyde
acetone
ethyl acetate
potassium tartrate
sodium succinate
phenol
ortho-, meta-, para-cresol
aniline
benzene
* May inhibit oxidation of sulfite when present in amounts as
low as 0.00005 mole/liter.
Source: Schroeter, L. C. (Ref. 4).
13
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4.4 Sulfur Oxide Chemistry
Through the course of this work it became apparent
that there were significant concentrations of sulfur compounds
in reduced oxidation states in carbide lime. These compounds
are important because they may,
inhibit sulfite to sulfate oxidation
or
interfere with the iodometric sulfite
de terminat ion.
Therefore a brief review of pertinent sulfur chemistry is
presented.
The chemistry of aqueous sulfur oxides is complex
to say the least. In order to appreciate the number of reactions
possible, it will help to describe some of the possible species
(Ref 7):
Sulfuric acid (H2SOO is the highest
oxidation state of sulfur at +6. The
sulfate ion combines with calcium in
scrubber liquors to form gypsum. If
gypsum scale build-up is to be avoided,
then sulfate formation must be inhibited.
Thiosulfuric acid (H2S203) contains
sulfur atoms of oxidation states +6
and -2.
14
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Sulfurous acid (HaSOs) has a sulfur
oxidation state of +4. This is the
form sulfur dioxide gas takes when
it is dissolved in water.
Disulfurous acid (HaSaOs) has sulfurs
of +3 and +5 and has the chemistry of
the normal sulfites.
Dithionic acid (H2S206) with a +5
sulfur is stable in aqueous solution.
It is an oxidation product of bisulfite.
Polythionic acid (H2Sn06, where n is 3 to
as much as 80) is also a stable species
in water. Thiosulfate reacts with
iodine below a pH of approximately 9
to form tetrathionate (Si»0s2~).
Many sulfur compounds may react with sulfite ion. Some
of these reactions lead to a retarding of sulfite oxidation.
A mechanism which slows sulfite oxidation is the change
of sulfite to another species less prone toward air oxidation.
If free sulfur is present in an alkaline sulfite solution,
thiosulfiCbe is produced (Ref.7):
Jb.
S032" + S = S2032" (4-7)
Thiosulfate is less prone toward air oxidation than is sulfite.
Thiosulfate can react with sulfite to create trithionate (Ref,4):
SaOs2" + S032~ + 20H-(radical) + 2H+ = S3062" + 2H20 (4-8)
15
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Tetrathionate (SitOe2") has also been shown to react with sulfite
(Ref 8) to form trithionate:
S«,062" + S032" = SaOs2" + S2032" (4-9)
However, researchers disagree as to the stability of trithionate
in solution.
Other sulfur compounds may react to create interme-
diates which subsequently react with sulfite. One example is:
S2032" = 8203" + e and 2S203~ = S^062~ (4-10)
The tetrathionate may then combine with sulfite to produce tri-
thionate.
The above-mentioned mechanisms for the inhibition of
sulfite oxidation all involve reactions with sulfur oxides in
reduced oxidation states (below the oxidation state of sulfate).
These same sulfur oxides, because of their reduced state, can
react with iodine and interfere with the sulfite analysis
(see Section 7.2). Sulfur oxides in reduced oxidation states are
abundant in carbide lime and are capable of inhibiting sulfite
oxidation and reacting with iodine.
16
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5.0 CARBIDE LIME CHARACTERIZATION
Carbide lime collected at Paddy's Run was analyzed
to identify possible oxidation inhibitors and analytical
interferents. The concentrations of trace elements in carbide
lime were determined and compared to the magnitude of the inter-
ference to help identify possible interfering species. This
section reports the results of the various analyses of carbide
lime and suggests possible oxidation inhibitors and analytical
interferences.
There is no way to directly measure the concentration
of the sulfite oxidation inhibitor without prior identification
of the reponsible compound or compounds. However, if the inter-
fering species and the inhibitor are the same, then the known
concentration of the interfering species can be used as the
inhibitor concentration. Preliminary work at LG&E strongly sug-
gested that the interference with the sulfite iodometric deter-
mination was caused by a sulfur species, based on analyses and
sulfur balances performed on carbide lime slurry liquors. The
background work presented in this report (Section 4.4) also shows
that reduced sulfur oxides can inhibit sulfite oxidation. In
fact, any species in a metastable reduced oxidation state could
both react with iodine and compete with sulfite for oxidation.
Therefore, in this section, elements present in amounts compar-
able to the known analytical interferent concentration may be
considered as possible oxidation inhibitors.
Later in this report, (see Section 6.3), it is shown
that the concentration of the interfering species in the carbide
lime slurry* liquor is on the order of 2 mmoles/liter, assuming the
*The lime slurry as it is added to the scrubber at Paddy's Run.
17
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equivalency of the interfering species is the same as sulfite.
If the interfering species does not have the same equivalency as
sulfite then the molarity may be in error within an order of
magnitude. Knowing the weight per cent solids in the carbide
lime slurry, and assuming the dissolved species in the liquor
originally came from the solids, the concentration of the inter-
ference in the solids is at least 10~2 mmole/gm:
2 mmole of interferent 1000 gm of liquor /-i -i»/ -i j > \
1000 gm of liquor124 gm of solid° '
_ 10~2 mmole of interferent (5-1)
gm solid
In this section, results of chemical analyses of
both the solids and the liquors from carbide lime are reported.
These analyses were conducted in light of the concept that
elements present in amounts comparable to the concentration of the
analytical interference were to be considered as potential oxi-
dation inhibitors.
The solids used in the carbide lime analysis came
from two sources:
Solids filtered from the carbide lime
additive slurry. The slurry contained
approximately 11% by weight carbide
lime solids slurried in water from the
acetylene manufacturing plant.
Solids taken from the storage piles ad-
jacent to Paddy's Run station. These
solids were taken from a settling pond
in the acetylene plant and stored in
piles. They were 48% solids by weight.
18
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All analyses are based on the weight of the respective solids,
dried at 45°C.
The liquor used in the analyses was filtered from the
carbide lime additive. In certain analyses the solids from the
storage pile were leached using deionized water. The liquor
from these analyses will be referred to as carbide lime extract.
5.1 Spark Source Mass Spectrometry
The spark source mass spectrometer (SSMS) is a survey
analytical instrument for determining as many as eighty elements
in trace amounts. A spark source ionizes the elements in the
sample. The ions are then focused into a beam electrostatically.
The ions are magnetically segregated and detected according to
their mass to charge ratio. The results are accurate within a
factor of plus or minus two. Any element with a concentration
greater than about 1000 ppm is designated as a major component
(MC). Table 5-1 contains the SSMS data on the carbide lime
solids from the slurry additive.
As can be seen from Table 5-1, only certain elements
are of great enough concentration to cause the interference
observed. At least 10"2 mmole/gm of the element is necessary
before it can be considered as the interfering species. The
following elements are of large enough concentration:
barium, strontium, iron, manganese,
titantium, calcium, potassium, chlorine,
sulfur, phosphorous, aluminum, silicon,
magnesium, and sodium.
19
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TABLE 5-1. SPARK SOURCE MASS SPECTROMETER RESULTS ON SOLIDS
FROM CARBIDE LIME SLURRY. SLURRY IS THE SAME AS
THAT ADDED TO SCRUBBER AT PADDY'S RUN
CONCENTRATION IN PPM WEIGHT
ELEMENT CONC.
Urani urn 4
Thorium 2
Bismuth
Lead 4
Thallium
Mercury NR
Gold
Platinum
Irldlum
Osmium
Rhenl urn
Tungsten
Tantalum
Hafnium
Lutetlum
Ytterbi urn
Thulium
Erbium fO.l
Hoi mi urn 0. 1
Dysprosium 0.3
ELEMENT
Terbium
Gadolinium
Europl um
Samarium
Neodymlum
CONC.
<0.1
0.6
0.2
3
2
Praseodymium 1
Cerium
Lanthanum
Barium
Cesium
Iodine
Tellurium
Antimony
Tin
Indium
Cadmium
Silver
Palladium
Rhodium
13
5
120
0.1
<0.5
0.5
0.4
STD
<0.2
<0.1
ELEMENT
Ruthenium
Molybdenum
Niobium
Zirconium
Yttrium
Strontium
Rubidium
Bromine
Selenium
Arsenic
Germanium
Gallium
Zinc
Copper
Nickel
Cobalt
Iron
Manganese
Chromium
CONC.
11
0.3
6
4
100
0.4
<4
1
<1
0.5
5
32
13
5
0.9
MC
490
22
ELEMENT
Vanadium
Titanium
Scandium
Calcium
Potassium
Chlorine
Sulfur
Phosphorus
Silicon
Aluminum
Magnesium
Sodium
Fluorine
Oxygen
N1 trogen
Carbon
Boron
Beryllium
Lithium
Hydrogen
CONC
12
MC
1
MC
>600
270
MC
330
MC
HC
MC
MC
=43
NR
NR
NR
5
0.3
27
NR
NR - Not detected by SSMS
All element! not reported <0.1 ppm weight
Sample was thermally ashed S350°C for
prior to analysis.
MC - Major Component
one hour in a laooratory furnace in a quartz crucible
20
-------
Several of these elements:
aluminum, silicon, magnesium, sodium,
potassium, titanium, barium, strontium,
and calcium,
should be considered as nonreactive ions or insoluble silicates.
Nitrogen and carbon are not detected in the SSMS data. However,
they both are present in compounds which are known to inhibit
sulfite oxidation (Ref 4), and therefore will be included as
possible inhibitors. This makes a total of eight elements in
the carbide lime solids which can be suspected as being the
interfering/inhibitor species.
Table 5-2 lists the results of the SSMS analysis of
carbide lime liquors. These liquors were filtered from the
carbide lime additive.
The concentration of the analytical interferent at
Paddy's Run FGD unit is on the order of 2 millimoles per liter
of scrubber liquor. Only a few elements in the liquor are pre-
sent in this large of a concentration:
calcium,
potassium,
chlorine, and
sulfur.
In addition, carbon and nitrogen are not detected by the SSMS
and they may be interfering species. If the possible inter-
21
-------
TABLE 5-2. SPARK SOURCE MASS SPECTROMETER RESULTS ON FILTRATE
FROM CARBIDE LIME SLURRY. SLURRY IS THE SAME AS
THAT ADDED TO SCRUBBER AT PADDY'S RUN.
CONCENTRATION IN ug/ml
ELEMENT CONC.
Uranium 0.01
Thorlun
Bismuth
Lead 0.007
Thallium
Mercury NR
Gold
Platinum
Irldlum
Osmium
Rhenium
Tungsten
Tantalum
Hafnium
Luteti urn
Ytterbium
Thulium
Erbi urn
Holmlum
Dysprosium
aEMENT CONC.
Terbium
Gadolinium
Europium
Samarium
Neodymlum
Praseodymi urn
Cerium <0.001
Lanthanum <0.008
Barium 6
Cesium 0.02
Iodine 0.08
Tellurium
Antimony fO.006
Tin 0.001
Indium STD
Cadmium <0.003
Silver
Palladium
Rhodium
ElEMENT
Ruthenium
Molybdenum
Niobium
Zirconium
Yttrium
Strontium
Rubidium
Bromine
Selenium
Arsenic
Germanium
Gallium
Zinc
Copper
Nickel
Cobalt
Iron
Manganese
Chromium
CONC
0.02
<0.001
0.003
<0.003
13
0.4
1
<0.02
0.2
<0.002
0.01
0.03
0.02
INT
10.001
0.06
0.005
0.05
aEMENT
Vanadium
Titanium
Scandium
Calcium
Potassium
Chlorine
Sulfur
Phosphorus
Silicon
Aluminum
Magnesium
Sodium
Fluorine
Oxygen
Nitrogen
Carbon
Boron
Beryllium
Lithium
Hydrogen
CONC
0.006
0.005
<0.001
MC
MC
MC
>3
0.5
2
0.4
0.4
MC
=6
NR
NR
NR
0.08
>1
NR
NR - Not detected by SSMS
Alt element! not reported <0.001 ug/ml
Sample was thermally ashed 3. 35Q°C for one hour in a laboratory furnace in a quartz crucible
prior to analysis. INT-Interference
MC - Major Component
22
-------
faring/inhibitor species found in the liquor are cross-checked
with those possible in the solids, only four remain as candi-
dates for the interfering/inhibitor species:
chlorine,
sulfur,
carbon and
nitrogen.
5.2 Gas Chromatography-Mass Spectrometry
The organic fraction of the carbide lime solids was
analyzed on a gas chromatograph-mass spectrometer (GC-MS). The
ability of many organics to inhibit sulfite oxidation is men-
tioned in Section 4.
The solids were filtered from the standard carbide
lime additive. The organic fraction was then extracted into
ethyl ether for GC-MS analysis. Radian's gas chromatograph has
a six foot column with a 2.0 mm inner diameter. The packing is
1% SP 2250 on 100/120 mesh Supelcoport. The mass spectrometer
is a Hewlett Packard 5980 A with glass jet separator and 70 eV
power source.
The only organic materials found in the solids were
several polynuclear aromatic hydrocarbons and some aliphatic
hydrocarbons. The aliphatic hydrocarbons are long chain al-
kanes and would not affect FGD scrubber chemistry. The poly-
23
-------
nuclear aromatics are listed in Table 5-3. Although these
compounds are not important within the confines of this report,
several are known cancer-causing agents. Therefore, it is re-
commended that further work be conducted to determine the con-
centration of these compounds in leachates of carbide lime.
The results of the GC-MS analysis on the carbide lime
liquors are in Table 5-4. The aliphatic hydrocarbons found are
not listed; these alkanes have carbon chains ranging around
20 carbon atoms long with concentrations in the liquor no greater
than 150 ppb. They are not important in this study.
TABLE 5-4. CONCENTRATION OF MAJOR ORGANICS IN CARBIDE LIME
LIQUORS AS DETECTED ON THE GAS CHROMATOGRAPH-MASS
SPECTROMETER
Compound
Napthalene
Anthracene/ Phenanthrene
Concentration, ppb
8
2
(pg/kg)
In summary, no organic species were found on the GC-MS
in either the solid or liquid portion of the carbide lime addi-
tive which could cause the analytical interference or the oxida-
tion inhibition observed.
5.3 Sulfur Analysis
This section reports analyses of carbide lime for
several different sulfur compounds. Sulfur was one of the four
elements found by SSMS which is in high enough concentrations to
be suspected as the interfering/inhibitor species.
24
-------
TABLE 5-3. CONCENTRATION OF POLYNUCLEAR AROMATIC HYDROCARBONS AND RELATED SPECIES IN CARBIDE LIME
SOLIDS
ro
Compound
Naphthalene
2-Methyl naphthalene
1-Methyl naphthalene
Biphenyl
Cz-Alkylnaphthalene
Ca-Alkylnaphthalene
C2-Alkylnaphthalene
Acenaphthylene
Acenaphthene
Fluorene
Methyl fluorene
Anthracene/Phenanthrene
Acridine
Methylanthracene
Carbazole
Acephenanthrene
Fluoranthene
Pyrene
Methylpyrene
Dimethylpyrene
Chrysene
Benzofluoranthene
Benzo(a)pyrene
Indeno ( c , d ) py rene
Benzo (g,h, i)perylene
Methylbiphenyl
Methylanthracene
Concentration, ppb (yg/kg)
ABC
470.
94.
64.
36.
44.
10.
6.
84.
42.
108.
49.
150
17.
22.
120.
2.
59
37.
16.
3.
30.
11.
12.
1.
2.
450.
58.
46.
26.
35.
10.
79.
22.
74.
52.
190.
24.
134.
99.
54.
55.
38.
19.
52.
26-
600.
73.
78.
46.
58.
14.
99.
75.
144.
104.
480.
76.
170.
240.
120.
1 f\
140.
87.
35.
87.
59.
Aliquots A, B and C arc all oolid samples filtered from the standard carbide line additive. They
were not dried before the extraction.
-------
5.3.1 Sulfide
The results of the sulfide (S2~) analyses on both the
storage pile solids and the carbide lime additive solids showed
no sulfide. An Orion Ag2S electrode specific for sulfide ion
was used for the determinations. This electrode is capable
of detecting sulfide as low as 10"17 molar. There are no
known interferences with the technique (Ref. 9).
5.3.2 Total Sulfur, Sulfate Sulfur and Pyritic Sulfur
Total sulfur, sulfate and pyritic sulfide were analyzed
on one solid sample according to the standard method for "Forms
of Sulfur in Coal" (Ref. 10).
The results are in Table 5-5. The total sulfur in
carbide lime solids was 0.31 % by weight. There was 0.15%
sulfur (or about half of the total sulfur) as sulfate and no
pyritic sulfur (FeSz). Therefore, a little over half of the
total sulfur present in carbide lime is in some other form than
sulfide or sulfate. This means that over half of the sulfur
present in the solids is in a reduced oxidation state. This
amount of reduced sulfur could create the interference with the
sulfite analytical method or cause the inhibition of sulfite
oxidation observed at Paddy's Run FGD unit.
TABLE 5-5. ANALYSIS OF SULFUR SPECIES IN CARBIDE LIME SOLIDS
TAKEN FROM WASTE HEAP ADJACENT TO PADDY'S RUN
Dry Weight 7, as Sulfur (S)
Total Sulfur Sulfate Sulfur Pyritic Sulfur Other
0.31 0.15 0 0.16
26
-------
5.3.3 Sulfur Species in Carbide Lime Liquors
The purpose of these analyses was to determine the
amount and form of the sulfur that would leach out of the solid
carbide lime. This is important if a sulfur species is the ana-
lytical interferent or oxidation inhibitor in carbide lime
slurries.
Two sets of sample analyses were done. One sample
came from the liquor of the carbide lime additive and one
from an aqueous extraction of the solids from the storage piles.
A comparison of the two shows to what extent the interfering/
inhibitor species is concentrated in the acetylene process
water. Acetylene process water is used to slurry the solids
for the standard caribde lime additive, while the aqueous extrac-
tion was carried out using deionized water under a nitrogen
atmosphere.
Table 5-6 lists the sulfur species in the liquor of
the carbide lime additive and in the aqueous extract of carbide
lime solids. Both of the samples were analyzed by ion chroma-
tography. The very low concentrations of thiosulfate in carbide
lime liquors prohibit the use of conventional wet chemical
techniques. Radian developed a method for the analysis of
thiosulfate on the ion chromatograph near the end of this study.
Therefore, some of the work in this report was done before the
thiosulfate procedure was developed and thiosulfate values are
not available.
27
-------
TABLE 5-6. SULFUR ANALYSIS ON FILTRATE FROM CARBIDE LIME ADDI-
TIVE AND ON CARBIDE LIME SOLIDS EXTRACTED WITH WATER
Millimoles/Liter as Sulfur (S)
Sample SO*2' S0s2~ SzOa2' S Total Other (by difference)
Filtrate 0.017 0 1.52 2.14 0.60
Extract 0.0225 0 N. R. 0.128 0.106
N. R. - Not reported. The technique for thiosulfate was not
developed at this time.
5.4 X-Ray Powder Diffraction
The carbide lime solids from the storage piles were
analyzed by X-ray powder diffraction using copper radiation.
Table 5-7 lists the major and minor species found. Also, some
trace species are included but these are present in amounts
that approach the lower detection limits of the X-ray diffracto-
meter.
TABLE 5-7. X-RAY DIFFRACTION ANALYSIS OF CARBIDE LIME SOLIDS
Compound Name Chemical Formula CRYSTAL SYSTEM
Major Species
Portlandite Ca(OH)2 Hexagonal
Moderate Species
Calcite CaCOs Hexagonal
Minor Species
a-Quart2 SiOz Hexagonal
Probable Trace Species
Ettringite |Ca6[Al(OH)6]2'24HZ0}
J(SOO3-l%H2Of
Possible Trace Species
(continued)
28
-------
Lime CaO Cubic
Gypsum CaSOit-ZHzO Monoclinic
Arconite K2SO<» Orthorhombic
K 2 SO ij Hexagonal
Aphthiotolite (Na, K)2SO^ Hexagonal
Glauber's Salt NaaSO*-10H20 Monoclinic
K2Ca2(SOOs
Hydroxyl Ellestadite Cai 0 (SiOO 3 (SOO 3 (OH) 2 Hexagonal
CaS203'6H20
As expected, the major component is portlandite,
Ca(OH)2. Calcite (CaCOs) and quartz (Si02) are present in
smaller amounts. The remaining solids are probably sulfate
salts with the possibility that some thiosulfate salts are
present.
5.5 Infrared Spectrophotometry
A Perkin-Elmer Model 283 infrared (IR) spectrophoto-
meter was used to study the carbide lime solids. Thiosulfate ex-
hibits characteristic bands at approximately 1000 and 1130
wave numbers (cm~l). Figure 5-1 is the IR spectrum of reagent
grade sodium thiosulfate, Na2S2Oa-5H20.
In Figure 5-2 and 5-3 are the IR spectra of freshly
dried carbide lime solids. They both have bands near 1000 and
1130 cm"1 similar to thiosulfate. A carbonate band is present
at about 1470 cm"1.
Figure 5-4 is an IR spectrum of carbide lime solids
which have been dried and stored for almost a year. Note the
absence of the large bands characteristic of thiosulfate. It
appears that thiosulfate is present in the fresh solids but has
29
-------
CO
O
1.0
0.8
0.6
9
g
g 0.4
0.2
3.0
4.0
r~
Mlcrometera
5.0 6.0
10
46 20
3500 3000 2500 2000 1800 1600 1400 1200 1000 800 600
Havenumber (cm ')
Figure 5-1. Infrared Spectrum of Reagent Grade Sodium Thlosulface, NazSzOj
*Thiosulfate bands.
02-2617-
-------
0.8
0.6
0.2
3.0
4.0
5.0
Micrometers
6.0
3500 3000 2500 2000 1800 1600 1400 1200 1000 800
Wavenumber (cm ')
Figure 5-2. Infrared Spectrum of Freshly Dried Carbide Lime Solids.
600
400 200
*Thiosulfate bands.
02-2611
-------
CO
N>
3.0
4.0 5.0
Micrometers
6.0
8.0
10
16 20 25
0.8
0.6
0.2
3500 3000
2500 2000 1800 1600 1400 1200 1000 800
Wavenumber (cm"1)
Figure 5-3. Infrared Scan of Freshly Dried Carbide Lime Solids.
600 400
*Thiosulfate bands.
02-2816
-------
UJ
to
3.0
Micrometers
6.0
16 20 25
3500 3000 2500 2000 1800 1600 1400 1200 1000
Wavenumber (cm"1)
800
600 400
Figure 5-4. Infrared Spectrum of Dried Solids from the Standard Carbide Lime Additive. The IR Spectrum was
Taken One Year After the Solids Were Dried.
*Thiosulfate bands.
02-2618-
-------
almost disappeared in the stored solids. It is unknown why
the thiosulfate is gone in the dried solid. Most likely the
thiosulfate has oxidized to sulfate.
34
-------
6.0 ASSESSMENT OF POSSIBLE OXIDATION INHIBITORS
One main goal of this study was to assess carbide lime
as an inhibitor of sulfite oxidation. At LG&E's Paddy's Run
station, no scaling was observed in the FGD unit when carbide
lime was used as the scrubbing agent. Scaling was prevented
because carbide lime inhibited the oxidation of sulfite to sul-
fate. Scaling did occur when commercial lime was used.
This section considers how carbide lime inhibits
sulfite oxidation in an FGD unit. First, it will be shown that
carbide lime does inhibit the oxidation of sulfite to sulfate.
And second, the source and nature of the oxidation inhibitor
will be discussed.
6.1 Inhibition of Sulfite Oxidation by Carbide Lime
6.1.1 LG&E Studies
No gypsum scale formed when carbide lime was used in
the SOz scrubber at Paddy's Run station. When commercial lime
was used, gypsum scaling in the scrubber occurred relatively
early in the testing. The reason for the reduced scaling with
carbide lime is the lowered sulfite oxidation. This is best
represented numerically by the oxidation fraction, which is
defined as:
Oxidation Fraction = (sulfate)
(sulfate + suTtTte)
where "sulfate" and "sulfite" are the fraction of calcium sulfate
and calcium sulfite, respectively, found in the scrubber solids.
35
-------
The oxidation fraction at Paddy's Run S02 scrubber was around
0.1 when carbide lime was used. This fraction increased to
about 0.15 or more when commercial lime was used under the same
operating conditions. This shows the reduction in sulfate for-
mation when carbide lime is used.
6.1.2 Laboratory Studies
A test cell containing a carbide lime slurry was shown
to inhibit sulfite oxidation in comparison to a similar test
cell containing a commercial lime slurry. The carbide lime
solids were taken from the storage piles adjacent to Paddy's
Run station; the commercial lime was Austin white lime.
The lime was dissolved in boiled deionized water by
the addition of 6 M HC1 until a stable pH of 5 was reached. Then
a weighed amount of sodium sulfite was added to the solution.
The pH and major species concentrations were selected to avoid
precipitation of CaS03'%H20. Compressed air was bubbled through
the solution at a constant flow rate to oxidize the sulfite.
Aliquots of the solution were taken every ten minutes and run
on the ion chromatograph to measure the amount of sulfate being
produced. Sulfite concentration was also monitored.
The results of these tests may be seen graphically in
Figure 6-1. When using commercial lime, most of the sulfite
was oxidized within ten minutes. With carbide lime, sulfate
was still being produced for up to two hours. The inhibition
of sulfite oxidation by carbide lime was significant.
36
-------
CO
-vl
3
0)
u
o
2
1.0
.9
.8
.7.7
.6
.5
.4
.3
.2
.1 -
El Commercial Lime Slurry
O Carbide Lime Slurry
0 10 20 30 40 50 60 70 80 90 100 110 120
Time (min)
Figure 6-1. Oxidation of Sulfite to Sulfate in Commercial Lime and Carbide Lime
Slurries
02-2699-1
-------
The total Na2S03 added initally to both lime slurries
made a 2 millimolar (mmole/J,) solution. The carbide lime test
solution initially had approximately 0.3 mmole/£ of sulfate and
the commercial lime 0.1 mmole/2. . After complete sulfite oxida-
tion, the total sulfate present in the carbide lime and commer-
cial lime test cells should have been 2.3 mmole/£ and 2.1
mmole/X,, respectively. However, neither lime solution attained
this sulfate concentration after oxidation. The carbide lime
solution leveled off at about 1.4 mmole/2. sulfate, leaving 0.9
mmole/X, sulfate unaccounted for as either sulfite or sulfate.
And the commercial lime solution finished at 1.7 mmole/S, sulfate
leaving 0.4 mmole/Z sulfate unaccounted for.
This loss of sulfite to a species other than sulfate
can be seen graphically in Figure 6-2. Here, the same data as
in Figure 6-1 is plotted as a function of sulfate formation:
[S0^2"]/[S032"] initially added^
If all the sulfite originally added were oxidized to sulfate,
then the curves would level off at 1.0. However, both solutions,
especially the carbide lime solution, seemed to have lost much
of the sulfite before it oxidized to sulfate. Some explana-
tions for this are:
loss of sulfite as SOa to the air during
the experiment. Since the experiments
were run at a pH of 5, it is conceivable
that some loss occurred. This could
account for the loss in both the
commercial lime and the carbide lime
solution, or
38
-------
to
vo
TS
3
a
CO
M
d
>M
- Commercial Lime Slurry
© - Carbide Lime Slurry
10
20
50 60 70 80
Time (minutes)
100 110
120
Figure 6-2. Oxidation of Sulfite to Sulfate in Commercial Lime and Carbide Lime
Slurries.
-------
conversion of sulfite to another species
besides sulfate. This is what probably
occurred in the carbide lime test to
account for the large amount of missing
sulfate.
6-2 Possible Oxidation Inhibitors
From the analysis of carbide lime (Section 5.1)
four elements were designated as possible sulfite oxidation
inhibitors:
carbon,
nitrogen,
sulfur and
chlorine.
Of the four possible elements, a sulfur species is
the most likely oxidation inhibitor. This is based on two facts
Much of the sulfur present in the
carbide lime is in a reduced oxidation
state (see Table 5-5 and 5-6);
a reduced species could compete with
sulfite for oxidation, thus limiting
the amount of sulfite that is oxidized.
Many sulfur species are known to
react with sulfite (see Section 4.3).
These reactions could inhibit sulfite
40
-------
oxidation by changing sulfite to a
species other than sulfate.
In this section the magnitude of the analytical inter-
ference will be compared to the concentrations of sulfur species.
Finally, a reduced sulfur species (thiosulfate) will be assessed
as a sulfite oxidation inhibitor.
6.2.1 Magnitude of Inhibitor
The magnitude of the analytical interference at
LG&E's Paddy's Run scrubber was calculated by subtracting the
true sulfite number from the value obtained by iodine titration
at pH 6. The true sulfite number was taken as that obtained at
a pH of 1. However, the number obtained at a pH of 1 may be
low due to the increased air oxidation of iodine at low pH's
(see Section 8). Unfortunately, these were the only sulfite
values obtained at Paddy's Run.
The magnitude of the sulfite analytical interference
was also determined in the laboratory at Radian. These values
were obtained by comparing the pH 6 iodine titration with the
"true" sulfite as analyzed on the ion chromatograph (1C). The
scrubber liquor was generated on a bench scale SOz scrubber.
S02 gas was bubbled through carbide lime additive in a. controlled
concentration and flow rate. Parameters such as pH, gas flow
and solids content were monitored to simulate scrubber operating
conditions at LG&E's Paddy's Run FGD unit.
Finally, the carbide lime additive liquor was analyzed
on the 1C for thiosulfate, sulfate and total sulfur. The total
sulfur was determined by oxidizing all of the dissolved sulfur
species to sulfate and then analyzing for sulfate on the 1C.
The amount of sulfate (SCK2~) originally present in the liquor
41
-------
was subtracted from the total sulfur to obtain the "Total Non-
Sulfate Sulfur". These values and those calculated for the
interferent are compared in Table 6-1.
As can be seen in Table 6-1, the magnitude of the sum
of thiosulfate and other reduced sulfur is the same as that of
the analytical interference. This amount of reduced sulfur
species would also produce the sulfite inhibition observed.
6.3 Thiosulfate as the Sulfite Oxidation Inhibitor
In this section, it will be shown that thiosulfate was
verified as an inhibitor of sulfite oxidation. The exact mech-
anism by which thiosulfate prevents sulfite oxidation is probably
through the formation of an intermediate sulfur species.
The experimental procedure for verifying thiosulfate
as an oxidation inhibitor was the same as that presented in
Section 6.1.2. But in this experiment the test cell contained
commercial lime and sodium thiosulfate (NaaSaOa). The lime and
thiosulfate constituted a simulated carbide lime slurry.
Enough sodium thiosulfate was added to make a 0.82 mM SaOa2"
solution. This concentration represents an extreme case in
a carbide lime slurry, where all of the reduced sulfur in the
solids is dissolved as thiosulfate. Next, sodium sulfite
(NaaSOs) was added to make a 2 mM SOa" solution. This is again,
the same sulfite concentration used in the previous experiment.
Compressed air was bubbled through the solution at the same
flow rate and aliquots were analyzed on the ion chromatograph
for sulfate and sulfite.
42
-------
TABLE 6-1. CONCENTRATION OF THE INTERFERENT VS THIOSULFATE AND REDUCED SULFUR IN
CARBIDE LIME LIQUOR
Sulfite Analytical Interfered Standard Carbide Lime Liquor 2
mmole/liter as SOs2" mmole/liter as Sulfur
Total Non-Total *
At From Simulated Sulfate Sulfur Available Non-
LG&E's Paddy's Run Scrubber Runs Thiosulfate (Total Sulfur - Sulfate Sulfur
Sulfate)
2-5
3.7 1.5
5.6
3.5
8.1
2.1 4.7
If all the nonsulfate sulfur available in the carbide lime solids were in solution,
the concentration in standard carbide lime liquor would be 4.7 mmoles/liter, This
is calculated from the sulfur analysis in carbide lime solids (Table 5-5) and
the 11 weight % solids in standard carbide lime liquor.
lodometric Titration at pH 6.
Analyzed by Ion Chromatography.
-------
Figure 6-3 compares the results of this experiment
with the two previous sulfite oxidation studies. It can be
seen that thiosulfate significantly inhibits sulfite oxidation.
In this test about half of the sulfite added initially remained
unoxidized after 115 minutes. The inhibition of sulfite oxidation
was much greater in this test than in the test using carbide lime
slurry. Part of the reason for the greater inhibition is the
large amount of thiosulfate used. The molar concentration of
thiosulfate used was equivalent to the total amount of reduced
sulfur found in carbide lime slurry, which seems to indicate
that the other reduced sulfur species in the carbide lime do not
inhibit sulfite oxidation.
Once again, the sum of the sulfite and sulfate in
solution did not approach the total sodium sulfite originally
added. Approximately 2 mmole/J, sulfite was added at time zero and
at the end of 115 minutes there was 0.36 nnnole/A sulfate and 0.86
mmole/i sulfite for a total of 1.22 mmole/Z. This leaves about
0.8 mmole/A of the original sulfite in a form different than
sulfite or sulfate. It is conceivable that some sulfite was
lost to the atmosphere but not this large an amount. The thio-
sulfate apparently reacts with the sulfite to form another sulfur
species, thus preventing the formation of sulfate ion.
6.4 Oxidation Inhibiting Reactions
Several reactions may take place between thiosulfate
and sulfite to inhibit sulfite oxidation. Schroeter (Ref 4)
suggests that thiosulfate and sulfite in solution produce
trithionate:
S2032" + S032" + 20H-(radical) + 2H+ = S3062" + 2H20 (6-1)
44
-------
in
B
4-1
O
ra
a
o
1.0
.9 .
3
!
i,
&
i
o
en
l^"
.0
.7
.6
.5
.4
.3
.2 .
.1
0
O Carbide Lime Slurry
0 Commercial Lime Slurry
A Commercial Lime Slurry with Thiosulfate
10
20
30
50
90
100 110 120
60 70 80
Time (minutes)
Figure 6-3. Oxidation of Sulfite to Sulfate in Slurries of Commercial Lime
with Thiosulfate, Commercial Lime and Carbide Lime.
02-2898-1
-------
Or, if thiosulfate were oxidized to tetrathionate
the following reaction could occur (Ref . 8) .
" + S032" = S3062" + S2032~ (6-2)
Reactions such as these would effectively inhibit sulfite oxi-
dation by making sulfite unavailable for reaction. An important
aspect of these reactions would be the stability of the product
species (such as trithionate) toward oxidation.
Another possible mechanism of sulfite oxidation inhi-
bition would be the oxidation of thiosulfate. Thiosulfate
is a reduced sulfur species and can be oxidized to sulfate.
Thiosulfate would therefore compete with sulfite for oxidation,
thus lowering the amount of sulfite which is oxidized. The
other reduced sulfur species present in the liquors could also
compete for oxidation.
From the oxidation studies in Section 6.1.2 and 6.3,
it appears that the mechanism of inhibition in thiosulfate solu-
tions is most similar to reactions such as 6-1 and 6-2. These
reactions would account for both the inhibited sulfite oxidation
and for the loss of sulfite to some species other than sulfate.
A. competing oxidation reaction would not account for the loss
of sulfite.
46
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7.0 EVALUATION OF POTENTIAL INTERFERENCE-FREE ANALYTICAL
METHODS FOR SULFITE. SULFATE AND THIOSULFATE
Knowing the concentration of sulfur species in FGD
liquors is important in understanding the overall chemistry of
the system. In carbide lime liquors the sulfur chemistry is
complicated by the presence of thiosulfate and other sulfur
species in reduced oxidation states. These other species
cause interferences with normal sulfite analysis. Therefore,
several methods for analyzing sulfite, sulfate and thiosulfate
in carbide lime liquors were tested. The analytical method
chosen for each species must be reproducible, reliable at low
concentrations (millimolar) and free from interferences.
Two different sample sets were used to test the methods
One set came from the filtrate of the carbide lime additive
slurry. The other set was simulated scrubber liquor, generated
on a bench scale SOa scrubber. After each test run the liquors
were analyzed for sulfur species using various trial methods.
7.1 Ion Chromatography
Ion Chromatography was found to be the most reliable
method for the analysis of sulfite, sulfate and thiosulfate in
carbide lime liquors. Because of this, the ion chromatograph
(1C) was used as the referee for other methods that were tested.
Section 8 contains a brief description of the 1C.
The advantage the 1C offers over other analytical
techniques is that the concentration of the ion of interest is
47
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determined directly. Concentration is not measured through
reaction as, for example, in the iodine titration where other
species can react to give false results.
The main disadvantage with the 1C concerns logistics.
First, the instrument is bulky and it may not always be possible
to have one on sampling trips. Second, with carbide lime liquors,
the sulfite may oxidize to sulfate in transporting it from the
sampling site to the 1C. The latter problem could be solved,
however, if the sample was preserved.
To preserve a carbide lime FGD sample, the sulfite
must be prevented from oxidizing to sulfate. Several techniques
are available for preventing sulfite oxidation in carbide lime
liquors:
The sample is added to a 60% isopropanol
solution. This worked well in preventing
sulfite oxidation in synthetic solutions.
However, when carbide lime liquors genera-
ted from the simulated scrubber were pre-
served in 6070 isopropanol some precipita-
tion occurred. Some species that were
water soluble were not soluble in iso-
propanol solutions.
Another sulfite oxidation inhibitor
is a 570 glycerol solution. In this case
no precipitation occurred when scrubber
liquors were preserved in the glycerol
solution.
48
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Finally, a promising preservant which was
not used in this study is a formalde-
hyde solution. Only enough formaldehyde
need be present to complex all of the
sulfite. The complex formed would
resist air oxidation. Furthermore,
the complex might prevent reaction
of sulfite with interfering species
in solution. Once the sample is in-
jected into the 1C, the complex breaks
down on the ion exchange column and
sulfite is eluted off. Further work
is needed to test this on carbide
lime liquors.
The instrument operating conditions for determining
sulfite, sulfate and thiosulfate on the 1C are in the Appendix.
7.2 lodometric Analysis
The existence of the interfering/inhibitor species
was first discovered while doing iodometric sulfite analyses of
carbide lime liquors at Paddy's Run FGD unit. Both the inter-
fering species and sulfite act as reducing agents. Therefore,
an iodometric titration at pH 6 to determine the sulfite
concentration resulted in a measured concentration that was too
large; the iodine was consumed by both the sulfite and the
interferent. Also, the reaction of the interfering species
with the iodine was time dependent, that is the reaction of the
interferent with iodine was relatively slow so that the longer the
time before back-titration, the larger the measured concentration
of "sulfite".
49
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7-2.1 lodometric Titration as a Function of pH
When the iodine Citration was carried out at a pH of
1 there seemed to be less interference. The rate of the inter-
fering reaction seemed to decrease from pH 6 down to pH 1. How-
ever, at low pH's a competing reaction is the oxidation of iodide
to iodine by molecular Oz :
61" + 02 + 4H+ = 2I3~ + 2H20 (7-1)
where Is is a complex formed between Iz and I~ to solubilize
the Ii . Reaction (7-1) produces iodine and the interfering
reaction consumes iodine. Therefore, the two occurring together
would result in an apparent decrease in the interference.
The following experiment was performed to test the
reliability of the icxlometric titration at low pH's in carbide
lime liquors. The carbide lime additive liquor was titrated iodo-
metrically at a pH of 1. According to ion chromatography,
there should be no sulfite present in these liquors. The
results of each of four runs showed that iodine was actually
produced in the carbide lime liquors. This means that the
air oxidation of I~ to la was catalyzed by the low pH liquors.
Therefore, if sulfite were present in the low pH liquors, several
reactions could take place during an iodine titration:
the I~ could be air oxidized to I2,
the I2 could be reduced to I~ by sulfite,
the interfering species also reduces
12, and
50
-------
any number of side reactions involving
sulfite and the interferent could
take place.
7.2.2 Errors With the Back-Titration
The iodometric titration involves two steps (see
Section 8) :
the reaction of the sample with an
excess of iodine and
the titration of the unreacted iodine
with a thiosulfate standard.
The interference with the analysis could occur in either step.
A back- titration using standard arsenious oxide instead of thio-
sulfate was done to determine if the interference is in the
back- titration .
Arsenious oxide quantitatively reduces iodine between
pH 7 and 9:
HAs02 + la" + 2H20 = HsAsO* -I- 31" + 2H+ (7-2)
Samples of both the standard carbide lime additive and the
simulated scrubber liquor were added to an excess of standard
iodine and back- titrated. Thiosulfate and arsenious oxide were
used as back-titrants for comparison.
Different results were obtained with each back-titrant,
as seen in Table 7-1. Both back-titrations show the affect of
the interfering species. The "apparent" sulfite determined by
51
-------
both titrations is greater than the true sulfite as determined
by ion chromatography. However, the extent of the interference
is different for the two titrations. More work would be
needed to explain the difference between the two back-titrations
TABLE 7-1. EVALUATION OF THE BACK-TITRATION IN THE IODOMETRIC
DETERMINATION OF SULFITE IN CARBIDE LIME LIQUORS
Sample
Standard Carbide Lime
Liquor
Standard Carbide Lime
Liquor
Simulated Scrubber
Liquor
Simulated Scrubber
Liquor
Apparent Sulfite
Back-Titrant (mmole/A)
Sodium Thiosulfate
Arsenious Oxide
Sodium Thiosulfate
Arsenious Oxide
0.20
0.63
3.28
5.55
True sulfite as determined on the ion chromatograph:
Simulated Scrubber Liquor - 0.72 mM
Standard Carbide Lime Liquor - 0 mM
7.2.3 Catalytic Effects
If the interfering species in the iodometric analysis
of sulfite acts as a catalyst then it would not be consumed by
the iodine titration. This would mean that only small amounts
of interfering species are needed to cause the errors found.
Experimental results show that the interferences with
the iodometric determinations are not catalytic.. A 50.0 ml
aliquot of standard carbide lime liquor was added to an excess
of 0.100 N iodine in a pH 6 buffer. The unreacted iodine was
back-titrated with 0.100 N NazS203 to a clear end point. The
52
-------
liquor aliquot consumed 13.5 micromoles of iodine. Next, an add-
tional 5.0 mis of iodine was added to the solution. If the
reaction to consume iodine was catalytic then more iodine should
react. However, no iodine reacted. Therefore, the interferent
in the carbide lime liquor is consumed by reaction with iodine.
7.2.4 Isolating the Sulfite from the Interfering Species
by Purging off Sulfur Dioxide
One method for determining the sulfite concentration
in carbide lime liquors is to physically separate the sulfite
and the interferent. The sulfite may then be analyzed iodo-
metrically with no interference. This may be done by adding the
sulfite sample to dilute hydrochloric acid. Sulfur dioxide gas
is formed,
S032" + 2H+ = H2S03(aq) = S02(g) + H20 (7-3)
and is purged off with an inert gas. The purged S02 is then
trapped in a 0.1 M NaHCOa solution. The interferent should
remain in the acid impinger and sulfite is determined in the
NaHCOa trap.
Scrubber liquors from three simulated scrubber runs
were analyzed for sulfite by this purge and trap method. The
results are inconsistant. In some instances, it appears that
the method may be suitable for sulfite analysis of carbide lime
scrubber liquor. However, overall it appeared that the inter-
ferent might also be purged and trapped along with the sulfite.
Futher work will be needed to exactly assess the value of this
method, although it appears to be of little value for carbide
lime liquors.
53
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7.2.5 Isolation of Sulfite from the Interfering Species by
Complexing Sulfite with Formaldehyde
When complexed with formaldehyde, sulfite is inert to
iodine oxidation (see Section 4). A concentrated formalde-
hyde solution was required to prevent the reaction of sulfite
with iodine. Concentrated formaldehyde reagent is 37% formalde-
hyde by weight. The pH of the formaldehyde reagent is approxi-
mately 6.
In this method the sulfite is isolated from the
interfering species by complexing with formaldehyde. The sample
is added to the 377« formaldehyde solution and titrated iodo-
metrically. This titration yields the equivalents of the
interfering species. A second titration is carried out on the
sample in a pH 6 buffer without adding formaldehyde. This titra-
tion yields the equivalents of the sulfite plus interfering
species. The equivalents of interfering species is then sub-
tracted from the equivalents of sulfite plus interfering species.
This gives the net sulfite concentration.
The results of the formaldehyde-iodometric titration
are compared to the sulfite obtained by ion chromatography in
Table 7-2. The test samples were generated on a bench scale SOa
scrubber using standard carbide lime additive. The sulfite con-
centrations determined by the formaldehyde-iodine method are
within a factor of two of the true sulfite measured by ion chroma-
tography. This error is probably due to the slow reaction of
the interfering species with iodine.
This is the most reliable wet chemical technique for
sulfite analysis in carbide lime liquors. Further work is needed
to prevent the time dependent reaction of the interfering species
with iodine.
54
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Cn
ui
TABLE 7-2. COMPARISON OF SULFITE VALUE OBTAINED BY FORMALDEHYDE-IODOMETRIC TITRATION
WITH ION CHROMATOGRAPHY
Formaldehyde-Iodometric
Titration
Ion Chromatograph (mM) (mM)
Sample S0sz~ S0i»2~ Total Sulfur S032~
Simulated Scrubber
Liquor, Run #2 1.0 6.0 10.4 1.9
Simulated Scrubber
Liquor, Run #3 0.72 3.7 4.4 1.8
-------
The formaldehyde-iodine method could also be used to
determine thiosulfate in carbide lime liquors. One iodine titra-
tion of the sample is done with formaldehyde present to give the
thiosulfate concentration. The next titration without formalde-
hyde will give the concentration of thiosulfate plus sulfite.
The difference of the two titrations will yield both sulfite
and thiosulfate concentrations. However, the time dependence
of the iodine titration in carbide lime liquors would again create
errors.
7.3 Potentiometric
This section describes a method of analyzing sulfite
potentiometrically. The method involves measuring the current
peaks observed when linear voltage sweeps are applied to a
stationary platinum wire electrode. The electrodes are immersed
in an unstirred pH 6 buffer solution containing the sulfite
sample. An anodic potential sweep is used to determine sulfite.
This method is relatively free from interference since
each current peak is obtained over a limited potential. In
carbide lime liquors generated from the simulated scrubber both
the sulfite and an unknown peak were observed. However, the
instrument proved too insensitive for the measurement of both
peaks. The peak for sulfite overlapped the unknown peak.
The technique was also too insensitive for identi-
fying the interferent. Several candidates for the interfering
species were spiked into carbide lime liquors and then run on
the potentiostat. It was impossible to judge if their oxida-
tion potentials matched that of the unknown peak.
56
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7.4 Acidimetrie Analysis of Sulfate
Radian has used an acid-base titration to determine
sulfate in scrubber liquors. First the sample is purged of
sulfite by acidifying with HC1 and driving off the SOa formed
with an inert gas. The sample is then passed through a cation
exchange column in the hydrogen form. In the column the cations
in the sample are exchanged for protons (H ), forming HzSCH.
Next, the sample is evaporated to dryness at 75°C leaving only
nonvolatile acids. In scrubber liquors only sulfuric acid re-
mains with possibly small amounts of phosphoric acid. Sulfuric
acid is then determined by titrating with 0.05 N sodium hy-
droxide. Any phosphates present must be analyzed separately.
This method was used successfully on carbide lime
scrubber liquors at LG&E's Paddy's Run station. In order to
prevent oxidation of sulfite to sulfate the sample could be
acidified immediately upon collection, thus driving the sulfite
off as SOz gas.
57
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8.0 ANALYTICAL METHODS FOR SULFITE. SULFATE. AND THIO-
SULFATE
It is important to know the sulfite and sulfate con-
centrations in FGD liquors . The degree to which sulfite oxidizes
to sulfate influences when scaling occurs . Thiosulf ate is
important because of the significant amounts found in carbide
lime and its ability to retard sulfite oxidation.
8.1 Sulfite Analysis
8.1.1 lodometric
The standard method for sulfite analysis is an iodine
titration. The iodine titrant is made by solubilizing iodine
with potassium iodide, KI, to form the tri-iodide ion, I3~.
Normally, the sulfite sample is added to an excess of iodine:
H2S03 + Is" + H20 = SO*2" + 31" + 4H+ (8-1)
and then the excess iodine is back-titrated with thiosulfate,
Is" + 2S2032~ - 31" + S^Os2" (8-2)
The amount of iodine which originally was consumed by the sulfite
is found by difference and is used to calculate the sulfite con-
centration in the sample (Ref 9) .
The iodine titration is usually carried out between
a pH of 1 and 8. Very low pH's promote the oxidation of iodide
by the air. And in mildly alkaline solutions triiodide may dis-
proportionate :
la" + OH" = 21" + HOI. (8-3)
58
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It is, therefore, best to avoid either very acidic or alkaline
solutions during an iodine titration. Radian uses a pH 6 buffer
for iodine titrations.
The iodine titration may be changed to fit different
sample conditions. It may be done as a direct titration with the
sample being the titrant. This would minimize side reactions
which take place between an excess of iodine and the sample.
Secondly, in the back-titration arsenious oxide (HA.s02) may be
used instead of thiosulfate if there are harmful side reactions
with thiosulfate.
The major problem with sulfite determination by
iodine titration is the numerous species which may interfere.
Other sulfur species, such as thiosulfate, sulfide, and poly-
sulfides may also react with iodine. Many organic compounds are
oxidized by iodine. Also, metal cations can act as catalysts
for the reduction of iodine.
8.1.2 Ion Chromatography
Ion chromatography is a relatively new technique for
measuring ions in solution. Ion exchange columns are used to
separate the ions of interest which are then detected on a
conductivity meter. The concentrations of the ions are de-
termined by comparison with standard solutions. This instru-
ment has been used by Radian to detect sulfite and sulfate in
FGD liquors. Stevens and Turkelson (Ref. 11) used the ion
chromatograph to measure chloride, phosphate, sulfite and
sulfate in boiler blowdown waters. They were able to achieve
complete separation of all the anions mentioned.
59
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Care must be taken in making the sulfite standards
because sulfite is easily oxidized to sulfate. One method is
to make the standard by dissolving sodium sulfite in an oxi-
dation inhibitor. Some suitable inhibiting solutions are:
60% isopropanol,
5% glycerol or
formaldehyde (the amount has not yet
been determined).
These solutions will prevent the sulfite standard from oxidizing
A second method for making a sulfite standard is to dissolve
the sodium sulfite in deionized water. The sulfate which sub-
sequently forms from sulfite oxidation can be measured on
the 1C. This sulfate value is then subtracted from the sodium
sulfite initially added to obtain the concentration of sulfite
in the standard.
8.1.3 Gravimetric Analysis
Sulfite can be oxidized to sulfate using any number
of oxidizing agents. The sulfate formed is then precipitated
as the barium salt and determined gravimetrically.
8.1.4 Other Oxidizing Titrations
Blasius et al. (Ref. 7) mentions several other
titrants of sulfite. They are all oxidizing agents. Among
them are bromine, permanganate, and hydrogen peroxide.
60
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8.1.5 Compleximetric
The titration of a sulfite solution with mercuric
chloride forms the complex [Hg(803)2]2". The complexation reaction
causes a potential difference in a platinum wire coated with
mercury (Ref.7).
8.2 Sulfate Analysis
8.2.1 Ion Chromatography
The use of the ion chromatograph for sulfate analysis
is well documented (Ref, 11). There is no problem in making
a stable standard.
8.2.2 Gravimetric
Sulfate is precipitated with BaClz in an acidic
medium. The solution is digested by heating for several
minutes to minimize coprecipitation of other barium salts. The
sulfate is determined gravimetrically as BaSCK (Ref, 7).
8.2.3 Color imetrie
Bertolacini and Barney (K,ef, 12)have-used barium
chloranilate to detect sulfate. The barium chloranilate is added
to a sulfate solution causing the precipitation of barium sulfate.
The highly colored acid-chloranilate ion is then detected colori-
metrically. The reaction is carried out at a pH of 4 and the
absorption is measured at 530 my.
61
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8.2.4 Compleximetric
The compleximetric determination of sulfate involves
titrating the solution with either Pb2+ or Ba2+ to form an in-
soluble salt. The sample is added to an excess of lead or barium
and the remaining cation is back-titrated with EDTA. The end
point is detected using Eriochrome-black T (Ref. 7).
8.3 Thiosulfite Analysis
8.3.1 Ion Chromatography
There is no method in the literature for analyzing
thiosulfate by ion chromatography. However, thiosulfate does
have a definite retention time on the ion chromatograph and the
use of this instrument for thiosulfate determination could
greatly eliminate the interferences found in wet chemical
methods. Radian is currently optimizing a method for thio-
sulfate analysis by ion chromatography.
8.3.2 Gravimetric
Thiosulfate can be converted to sulfate using a suitable
oxidizing agent such as H2 02, Br2, or Naz 02. The sulfate pro-
duced is then determined gravimetrically using BaCla as the pre-
cipitant (Ref.7).
8.3.3 - lodometric
Iodine may be used in a direct titration of thiosulfate.
Thiosulfate reacts with iodine between pH's of 4.5 and 9.5 to
form tetrathionate (SivOs2"). At more alkaline pH's the reaction
goes to sulfate non-stoichiometrically. Therefore, it is necessary
to carry out the reaction below a pH of 9.5 (Ref. 7).
62
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8.3.4 Compleximetric
The compleximetric analysis of thiosulfate involves
titrating a sample with HgClz. The complex HgSaOa is formed and
the end point is determined potentiometrically or by the dead
stop method (Ref, 7).
8.4 Mixtures of Two or More Sulfur Species
8.4.1 Sulfate-Sulfite Analysis
Sulfate and sulfite may be determined in the same
solution by ion chromatography. Under the right operating
conditions sufficient separation between the two species is
obtainable. Also, sulfite may be analyzed iodometrically in the
presence of sulfate with no interference. Sulfate may be deter-
mined gravimetrically or on the ion chromatograph.
Care must be taken to prevent sulfite from oxidizing
to sulfate. Radian has developed a technique where sulfite is
separated from sulfate and then both can be analyzed separately.
The sample is added to 0.4 M HC1 and purged with an inert gas.
The sulfite decomposes to form SOa gas which is then expelled
and trapped in an impinger containing a bicarbonate solution.
The sulfite trapped in the bicarbonate is measured iodometrically.
The sulfate in the original solution is analyzed on the ion
chromatograph or determined gravimetrically.
8.4.2 Sulfite-Thiosulfate Analysis
The most often cited method for sulfite and thiosulfate
analysis is the formaldehyde method (Ref. 7 and 8-) . Formalde-
hyde and sulfite in solution form the stable complex hydroxy-
methanesulfinate (HOCHzSOa')- This complex is inert to iodine
63
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oxidation. Formaldehyde can be added to a solution containing
both sulfite and thiosulfite. The thiosulfate can then be
determined iodometrically without sulfite interference. A
second aliquot can be titrated with iodine without the addition
of formaldehyde. The value of the second titration minus that
of the formaldehyde titration gives the sulfite concentration by
difference.
8.4.3 Analysis of Several Sulfur Oxides Together
The ion chromatograph could be a powerful analytical
tool for sulfur species analysis in complex solutions. Radian
is working on techniques for the simultaneous analysis of several
sulfur oxides. Now there are few reliable wet chemical techniques
for complicated sulfur solutions, especially when the concentra-
tions are millimolar or less.
Several methods of analysis for solutions containing
two or more of sulfate, sulfite, thiosulfate, sulfide and poly-
thionate are listed by Blasius, E. et al. (Ref. 7) . Most of these
are variations on the wet chemical methods already mentioned
in this report.
One final method specific for thiosulfate in the pre-
sence of other sulfur oxides merits attention. Danehy and Zubrit-
sky (Ref. 8) use formaldehyde and NaaSOs to determine thiosulfate
iodometrically in the presence of dithionite and bisulfite. This
method-is of interest because it is specific for thiosulfate.
The method requires at least millimolar concentrations of thio-
sulfate to be reliable.
64
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REFERENCES
1. Miller, S. A., Acetylene, Its Properties, Manufacture and
Uses. Vol. L. NY, Academic, 1965.
2. Environmental Protection Agency, Flue Gas Desulfurization
Symposium. 1973. Proceedings, EPA 650/2-73-038. Research
Triangle Park, N. C., 1973.
3. Fuller, E. C. and R. H. Crist, "The Rate of Oxidation of
Sulfite Ions by Oxygen:, JACS 63. 1644 (1941).
4. Schroeter, L. C. , Sulfur Dioxide. Elmsford, N.Y. Permagon,
1966.
5. Altwicker, E. R., "Sulfur Dioxide Absorption, Oxidation,
and Oxidation-Inhibition", DECHEMA-Monogr. 80 (1939-1669),
343-64 (1976).
6. Kawamoto, Kensuke, et al. , "Antioxidant for the Aqueous
Solutions of Sulfite and/or Bisulfite of Sodium or
Potassium and Process for Preventing the Oxidation of
Said Aqueous Solution", U. S. Patent 3,88,969 (June 1975).
7. Nickless, G., ed., Inorganic Sulfur Chemistry. N.Y., Elsevier,
1968.
8. Danehy, James P. and Charles William Zubritsky. Ill,
"lodometric Method for the Deterination of Dithionite, Bi-
sulfite, and Thiosulfate in the Presence of Each Other and
Its Use in Following the Decomposition of Aqueous Solutions
of Sodium Dithionite", Anal. Chem. 46(3). 391 (1974).
65
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REFERENCES (continued)
9. Peters, Dennis G., John M. Hayes, and Gary M. Hieftje,
Chemical Separations and Measurements. Philadelphia,
W. B. Saunders Co., 1974.
10. American Society for Testing and Materials, 1977 Annual
Book of ASTM Standards. Part 31. Water. Philadelphia, PA,
1977.
11. Stevens, Rimoty S. and Virgil T. Turkelson, "Deter-
minations of Anions in Boiler Blow-Down Water with
Ion Chromatography," Anal. Chem. 49 (8), 1176 (1977).
12. Bertolacini, R. J. and J. E. Barney, "Colorimetric Determ-
ination of Sulfate with Barium Chloranilate," Anal. Chem.
29, 281-83 (1957).
66
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APPENDIX
STANDARD OPERATING CONDITIONS FOR THE DETERMINATION OF SULFITE,
SULFATE AND THIOSULFATE CONCENTRATIONS IN CARBIDE LIME LIQUORS
USING THE ION CHROMATOGRAPH
67
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APPENDIX
Radian has applied ion chroma to graphy to the analysis
of carbide lime flue gas desulfurization solutions. Ion chroma -
tography has been used to measure sulfite and sulfate. And
recently, a method has been developed for the analysis of thio-
sulfate. The operating conditions for each ion are presented.
The Dionex Model 14 Ion Chromatograph uses ion ex-
change columns to separate individual ions , a patented suppressor
column and a conductivity cell for the detection of the separated
ions. The instrument can be used for the determination of either
cations or anions.
Sulfite (S032") and Sulfate (SCU2")
Both sulfite and sulfate are determined in one in-
jection on the ion chromatograph . Care must be taken to pre-
vent sulfite from oxidizing to sulfate in the sample prior to
injecting into the ion chromatograph. This was done by storing
the sample in either 60% isopropanol solution or 5% glycerol
solution. The chromatographic conditions are shown in the
table.
Thiosulfate
The technique for determining thio sulfate on the ion
chromatograph is relatively new. It was used for one sample in
this report and the results are accurate. More work is needed to
optimize the method for all conditions. Perhaps sulfite, sulfate
and thiosulfate can all be determined in one injection.
68
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TABLE A-l. CHROMATOGRAPHIC CONDITIONS
SOs and
S203
2-
Eluent
0.003 M Na2C03
0.0024 M NaHCOs
0.009 M Na2C03
0.0072 M NaHCOs
Pump Pressure
Analytical Column
400 psi
Dionex Corp. Anion
Exchanger
500 mm
150 psi
Dionex Corp.
Anion Exchanger
150 mm
Detector Sensitivity
Injector Volume
30 umhos
100
30 ymhos
100 yL
Standard Solutions
Sulfate standards were made from diluted
sulfuric acid.
Sulfite standards were made by dissolving
reagent grade Na2S03 in a 60% isopropanol
solution. Any sulfate formed was measured
on the ion chromatograph and subtracted
from the original sulfite.
Thiosulfate was made by dissolving reagent
grade N2S203 in water. Thiosulfate is
stable toward air oxidation.
69
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TECHNICAL REPORT DATA
(Please read Instructions on the reverse before completing)
, REPORT NO.
2.
EPA-600/7 -78-176
«. TITLE AND SUBTITLE
Characterization of Carbide Lime to Identify Sulfite
Oxidation Inhibitors
3. RECIPIENT'S ACCESSION NO.
5. REPORT DATE
September 1978
6. PERFORMING ORGANIZATION CODE
. AUTHOR(S)
B. PERFORMING ORGANIZATION REPORT NO.
L.J. Holcombe and K.W. Luke
3. PERFORMING ORGANIZATION NAME AND ADDRESS
Radian Corporation
P.O. Box 9948
Austin, Texas 78766
10. PROGRAM ELEMENT NO.
EHE624A
11. CONTRACT/GRANT NO.
68-02-2608, Task 21
:. SPONSORING AGENCY NAME AND ADDRESS
EPA, Office of Research and Development
Industrial Environmental Research Laboratory
Research Triangle Park, NC 27711
13. TYPE OF REPORT AND PERIOD COVERED
Task Final; 9/77-7/78
14. SPONSORING AGENCY CODE
EPA/600/13
i6. SUPPLEMENTARY NOTES T£RL-RTP project officer is Julian W. Jones, Mail Drop 61, 919/
541-2489.
16. ABSTRACT
The report gives results of a study of carbide limea by-product of acety-
lene manufacture, primarily calcium hydroxideused in a flue gas desulfurization
(FGD) system at Louisville Gas and Electric (LGE). The study was undertaken to:
identify sulfite ion oxidation inhibitors in carbide lime, and develop an analytical
method for sulfite that avoids the interferences observed in analyzing scrubber liquors
from LGE's FGD system. Thiosulfate was identified as the oxidation inhibitor in
carbide lime; it was also identified (along with other reduced sulfur species) as a
source of interference in the iodine titration method used at LGE for sulfite analysis.
Bench-scale tests verified the presence of thiosulfate as a major inhibition to sul-
fite oxidation in simulated scrubber liquors. This means that the low oxidation rate
(e.g., that reported at LGE with carbide lime) results in a greatly reduced tendency
to calcium sulfate (gypsum) scaling, therefore a greatly improved FGD system reli-
ability. The amount of thiosulfate required for scale-free scrubber operation is
unknown. However, to bring the thiosulfate level of commercial lime up to that found
in carbide lime would cost jh. 50 per ton of lime (using sodium thiosulfate pentahydrate
at ^12 per 100 pounds). The ion chromatograph was found to be the best analytical tool
for determining sulfite concentrations in carbide lime liquors.
17.
KEY WORDS AND DOCUMENT ANALYSIS
DESCRIPTORS
b.lOENTIFIERS/OPEN ENDED TERMS
c. COSATi Field/Group
Pollution
Calcium Hydroxides
Properties
Sulfites
Oxidation
Thiosulfates
Flue Gases
Desulfurization
Iodine
Volumetric Analysis
Calcium Sulfates
Gypsum
Scale
Pollution Control
Stationary Sources
Carbide Lime
13B
07B
14B
07C
2 IB
07A,07D
08G
J1E-
18. DISTRIBUTION STATEMENT
Unlimited
19. SECURITY CLASS (ThisReport/
Unclassified
21. NO. OF PAGES
76
20. SECURITY CLASS (This page/
Unclassified
22. PRICE
EPA Form 2220-1 (9-73)
70
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