Znd. Eng. Chem. Res. 1994,33, 1145-1149
1145
MATERIALS AND INTERFACES Characterization of the Product of the Thiosulfate Process for Desulfurization of Flue Gases Ahmed M. Gadalla' and Anurag Guptat Chemical Engineering Department, Texas A&M University, College Station, Texas 77843
X-ray diffraction, infrared spectroscopy, thermal analysis, scanning electron microscopy,and particle size analysis were used to characterize the product of the thiosulfate process. It was found to consist of a mixture of a solid solution of calcium sulfate in calcium sulfite hemihydrate, calcium sulfate dihydrate, and calcium carbonate. Reactions taking place on heating the byproduct were identified in oxidizing and reducing atmospheres. Pure calcium sulfite hemihydrate as well as solid solutions of Cas04 in CaSOV(1/2)H20with different sulfate concentrations were synthesized in the laboratory. Thermal analyses of pure sdid solutions proved that the higher the sulfate content, the higher is the dehydration temperature. Accordingly, taking S042. in solution increases the stability of the byproduct. Above the solubility limit, thermal analysis detected the formation of gypsum which precipitated as a second phase. Introduction
With stricter regulations by the EnvironmentalProtection Agency to get cleaner air, more industries are adding units for flue gas desulfurization (FGD) by bringing the sulfur dioxide in the flue gas in contact with lime or limestone slurry. To get clean air, most operational engineers in these experimental units do not concentrate their efforts on controlling the operating conditions to get a byprodud with reproducible properties. Slight changes in the amount, purity, or particle size of lime and additions including compressed air drastically affect the process and the quality of the byproduct. The calcium sulfite formed is oxidized by excess oxygen and precipitates as gypsum (CaS04.PH20).To avoid scaling inside the scrubber,pipes, and pumps and to decrease the operational and maintenance costs, more plants are retarding the oxidation by additives such as sodium thiosulfate. The product consists mainly of calcium sulfite hemihydrate and calcium carbonate [I]. It was found that, even when the byproduct was free from CaS0~2H20,sulfate could still be detected in the byproduct. The formation of a solid solution of calcium sulfate in calcium sulfite was suggested by Jones and others [23 to be responsible for sulfate purge which leads to unsaturated operation. Jones et al. 121confirmed the formation of this solid solution using infrared spectroscopy. Although the reduced byproduct does not form hard scale, the sludge poses dewatering and disposal problems. It increases the water consumption of the scrubber and requires larger disposal area. The material exhibits a thixotropic behavior and has a high viscosity at low shear rates t31. It is to be noted that the phase diagram of the system CaSOa-CaS04-HzO was not published. Also the available diagram by Slack 151 on the state of hydration of calcium sulfate at different temperatures and pH value is not complete.
* Author to whom correspondence should be addressed.
+ Present
address: PRC Environmental Management, Inc.,
350 North St. Paul St., Suite 2600, Dallas, TX 75201.
In the present investigation, FGD byproduct obtained from Texas Utilities (TU), Martin Lake power plant, was characterized using various experimental techniques. Its behavior was compared to those of pure calcium sulfate dihydrate, calcium sulfite hemihydrate, and solid solutions of sulfate in sulfite. Such a study is important in understanding the system CaSOa-CaS04-HaO and is technically essential to recommend additional units and/ or steps to get a byproduct which can be utilized as aggregate for road construction or as a raw material for producing wall board, bricks, etc., as will be explained in following papers. Experimental Procedure 1. Characterization of the Byproduct. Characterization of the byproduct is essential to study the effect of the desired additive on the oxidation kinetics in the slurry form or after drying. This step will be reported later by Gadalla et al. FGD byproduct was obtained from TU Electric-Martin Lake power plant. The material was dried to a constant weight at 60 "C to remove the free water present. This temperature was selected to avoid any dehydration of the constituents of the FGD byproduct. The dry material was stored in a powder form in plastic containers. The material was stored in a powder form in plastic containers. The material was characterized using the following techniques. (a) Thermal Analysis. A Netzsch STA-409 simultaneous thermal analyzer was used to determine qualitatively and quantitatively, the composition of the sample. The analyzer recorded the change in weight (TG) and the heat effects associated with the exothermic and endothermic changes (DTA) taking place in the sample as a function of temperature, as well as their first derivative. Calcined kaolin fired at high temperature to a stable structure was used as the reference material which was heated side by side with the sample in alumina crucibles under the required atmospheres,using the selected heating and cooling rates. (b) Infrared Spectroscopy. Infrared spectroscopy was used for the qualitative analysis of the FGD byproduct
0888-5885/94/2633-ll45$04.50/00 1994 American Chemical Society
1146 Ind. Eng. Chem. Res., Vol. 33, No. 5, 1994
and was compared with that for pure calcium sulfate dihydrate and calcium sulfite hemihydrate. (c) X-ray Diffraction. A Rigaku X-ray diffraction machine was used to perform X-ray analysis on the FGD byproduct and its intermediate products obtained by thermal treatment. The d-spacings were compared with JCPDS powder diffraction files to identify the initial mineralogical constituents of the byproduct as well as the phases developed on heating in different atmospheres. The use of the quantitative analysis is not very accurate, and titrations as well as thermal analysis were used for this purpose. The broadening of the peaks indicates the finer crystals or particles. Particle size was determined as follows. (d) Scanning Electron Microscopy and Particle Size Analysis. A JEOL 6400 scanning electron microscope was used to study the morphology of the particles present in FGD byproduct and pure calcium sulfite hemihydrate. The particle sizedistribution of the samples was measured using a Horiba CAPA-'700 particle size analyzer. The centrifugal sedimentation measurement method wasused to accelerate the measurementof particle size distribution as smaller particles do not settle rapidly under natural gravitational force. The particle size distribution was based on the equivalent volume assuming that the particles are spherical in shape. The samples were dispersed in SEDISPERSE A-14 particle dispersion liquid supplied by Micromeritics Instrument Corporation. This liquid is a stable, balanced formulation of highly purified, nonpolar, saturated aliphatic hydrocarbons. Ultrasonic stirring was used prior to particle size analysis to break the agglomerates and uniformly disperse the particles. Since the density of the particles is required to determine the size distribution, it was measured by a helium pycnometer. 2. Preparation of Calcium Sulfite Hemihydrate. Pure calcium sulfite hemihydrate (CaS0~(1/2)H20)was prepared in the laboratory b passing sulfur dioxide gas through an aqueous solution of calcium hydroxide. Twenty grams of calcium oxide was mixed in 1000 mL of distilled water at room temperature (25 "C). The slurry was stirred for 15min and then it was filtered using vacuum filtration to obtain a clear solution. SO2 gas was then passed through this solution at a flow rate of 10 cm3/min. Oxidation of calcium sulfite to calcium sulfate takes place at pH values lower than 5.0. In order to inhibit the oxidation of sulfite to sulfate and the formation of a solid solution of calcium sulfite and calcium sulfate, the pH of the solution was monitored and when the pH of the solution dropped to 6.0, the slurry was filtered quickly using vacuum filtration and the produced precipitate (consisting of pure CaSOV(1/2)H20)wasrinsedwithmethanoland acetone toremove excess water. 3. Preparation of Solid Solution (CaSO& (CaSOs)l,.(l/2)HzO with Different Compositions. Solid solutionswith different compositions of sulfate were prepared by reading a known amount of pure calcium sulfate hemihydrate with different amounts of sulfuric acid as shown in Table 1. After 15 min of reaction time, the solutions were filtered and rinsed with methanol and acetone. Thermal analysis was used to study the behavior of different members in the solid solution series and to establish the limit of solubility, above which the second phase of gypsum is detected.
Table 1. Preparation of Solid Solution (CaSO,).(CaSOs)l-,.( 1/2)Ht0 with Different Compositions w t of vol of value phases sample no. CaSOdl/fl)H10 (a) 1N H8O1(mL) of a present" 1 0.5 o s 2 0.5 0.4 0.05 ss 3 0.5 0.8 0.10 ss 4 0.5 1.2 0.15 ss 5 0.5 1.6 O . N b SS+ G 6 0.5 2.0 0.18'' SS + G 7 0.5 2.4 0.18* SS + G ~~
a s
~
~~
= pure CaSO8.(1/2)H20; SS = solid solution; G = gypsum.
b Gypsum was separated as a secondary phase indicating a saturated
solid solution.
from Texas Utilities-Martin Lake power plant consists of a mixture of calcium sulfate dihydrate (CaS04e2H20), calcium sulfite hemihydrate (CaSOdl/2)HzO),and calcium carbonate (CaCO3). The undetected shift in 2Bvalues could not confirm that a solid solution of calcium sulfate in calcium sulfite is formed. As stated above, the X-ray diffraction patterns were not used for a quantitative analysis or to determine the average particle size (see below). The infrared spectral pattern for the byproduct is shown in Figure 1. This pattern was compared to those obtained for pure CaS0~(1/2)H20, CaS0~2Hz0, and CaC03. The spectral peaks in the range 94&1000 cm-l are attributed to the presence of S032-ions. The peak at 1104 cm-l in the infrared spectra is due to Sod2-ions. The spectral peaks in the 3300-3600- and 1600-1700-cm-1 ranges are as a result of water of hydration of calcium sulfate dihydrate and calcium sulfite hemihydrate. The spectral peaks in the 1400-1500-~m-~ range are due to Cos2-ion. The peaks in 600-700-~m-~ range are also due to sulfate and sulfite ions but are not as distinctive as other peaks. It is evident that the byproductconsists of hydrated sulfite and sulfate as well as CaCOs. When SO2 was passed through a slurry of FGD byproduct, peaks corresponding to excess CaC03 were eliminated. The peak at 1213cm-1 which was not observed in spectra for pure CaS04-2Hz0may be attributed to the substitution of sulfite ion by sulfate ion forming a solid solution. To confirm the presence of a solid solution of calcium sulfite and calcium sulfate in the FGD byproduct and to determine the limit of solubility, solid solutions were prepared by adding different amounts of H2SO4 to CaS03. (1/2)H20 to form (CaS04),(CaSOs)1,.(1/2)HzO, where a is the molar fraction of sulfate present in the solid solution. Figures 2 and 3 show the thermal analysis curves for the FGD byproduct in N2 and air, respectively. Nitrogen with a flow rate of 10 cm3/min and stagnant air with a heating rate of 5 "C/min were used, respectively. The curves were compared with those obtained for pure componentsunder the same conditions, and the weight loss as well as the X-ray diffraction for the intermediate products were used to establish the following reactions: In N2, the first weight loss occurs at about 110 "C and is due to the dehydration of calcium sulfate dihydrate (gypsum)present as a separate phase. The decomposition takes place in two steps and two endothermic peaks are observed on the DTA curve.
Results and Discussion 1. Byproduct Characterization. The X-ray diffraction pattern indicated that the FGD byproduct obtained
The two endothermic peaks are not clear in Figure 2 due to a relatively small quantity of gypsum. However,
Ind. Eng. Chem. Res., Vol. 33, NO.5, 1994 1147
t
4ooo
I
I
3500
3000
I
I
I
1500
loo0
I
2500 2000 Wavenumben
0
Figure 1. Infrared pattern for FGD byproduct. 100,
7) crso,
,
.
,
'
1bO
,
,
.
I
,
,
300
'
4bO
,
I
,
,
.
,
.
,
.
'
800
'
,
.
,
,
,
--->c.0 + sq (9)
8) O-WO4 -> U.Ck%
A
*
9) CIS04
-> c.0 + SI% (B)
3) ( C ~ S O ~ ) , ( C ~ S ~ ~ ) I . ~ -+ .~HZ (CaSOi)a(CaSO~l~.a O + fHlO(g) 4) 4caso3
--->
3 C 9 0 4 + Cas
I\
I 70!
--->
5) 4c.SD)
3CaS01
t
--->CaO +
CIS 7)
.LL '
200
'
560 ' 600 7b0 EWERATURE. Rp. C '
Wd
J-
900
'
1000
I100
Figure 3. Thermal analysis curve for FGD byproduct sample in air atmosphere.
decomposition of calcium carbonate. CaCO,(s) -CaO(s)
Assumingthat the solid solutionis saturated with sulfate and the limit of solubility is 18% (as will be explained), the loss in weight indicates the sulfite content in the byproduct to be nearly 62.3%. Iodometric titration, on the other hand, yields a result of about 62 % sulfite, which is in good agreement with the thermal analysis results. An exothermicpeak occurred on the DTA curve at about 650 "C and was not accompanied by a weight change on the TG curve. This is due to the disproportionation reaction of calcium sulfite [4,51 accordingto the reaction 4CaSO,(s) -.3CaS04(s)+ CaS(s)
(4)
The third weight loss takes place at round 700 "C. Carbon dioxide is liberated due to the endothermic
+ CO,(g)
(5)
The composition of the various cornponenta present in the samplecan be calculated from these three weight losses, indicatingthat the product consista of 5.3% gypsum, 77.7% solid solution, and 16.8% CaC03. When heating is continued above 700 OC, another weight loss startsaround 900 OC, which is due to the decomposition of calcium sulfide and the remaining calcium sulfite according to the reactions CaS(s)
-
CaS,(s)
-
CaS03(s)
+ (1- r)S(g)
CaO(s) + SO,(g)
(6) (7)
An endothermic peak is observed on the DTA curve around 1190 O C . This is due to the phase transformation of orthorhombic B-CaSOr to hexagonal ar-CaSOd. Above 1300 OC, decompositionof calcium sulfate takes place and a series of complicated reactions takes place which are indicated by various peaks on the DTA curve. When FGD byproduct is heated in air (Figure 31, two exothermic peaks are observed above 550 "C on the DTA curve. These peaks correspond to the disproportionation
1148
Ind. Eng. Chem. Res., Vol. 33, No. 5, 1994
..
.. .~-
Figure 4. SEM photograph of FGD byproduct gypsite.
Di2rnC,C,
Figure 6. SEM photograph of CaSO3.(1/2)H20.
,","I
Figure 5. Particle size distribution for FGD byproduct gypaite.
Figure 7. Particle size distribution for pure calcium sulfite hemihydrate prepared in the laboratory.
calcium sulfate in the solid state. The oxidation causes a detected increase in weight. CaS(s) + 20,(g)
-
CaSO,(s)
I
(8)
Figure 4 shows the photomicrograph of the FGD byproduct obtained using scanning electron microscopy -mb,l"".. (SEM). The material consists of thin platelike particles with different sizes. The density of the FGD byproduct was measured using a helium pycnometer and was found , , to be 2.53 g/cm3. For particle size distribution measurements, the particles were assumed to be spherical and u5 340 w E4 $s ?sa 151 370 T D F e A T U L0.c c having a uniform density. The particle size distribution Figures. Firstd~vativefordehydrationofsolidBolutionaofcalcium was based on the volume equivalent. The particle size sulfite with different compositions. ranges from 0 to 50 pm with a median particle size of 8.2 pm. Figure 5 shows the particle size distribution for the very small, rod-shaped particles which tend to form FGD byproduct. The particle size distribution appears agglomerates (Figure 6). The particle size ranges from 0 to have a bimodal or trimodal distribution. This can be to 18pm with a median diameter of 8.8 pm. Particle size due to the fact that the byproduct consists of different distribution for CaS03.(1/2)H20is shown in Figure 7. The components, the sulfite, the gypsum, and the calcium density of the sample was found to be 2.48 g/cm3. carbonate. Calcium sulfite solid solutions with different sulfate 2. Characterization of Synthetic Sulfite Phase concentrations were prepared by reacting a fixed amount with Different Compositions. As mentioned earlier, thes~lfitephasecontainssomeCaSO~inthesolidsolution. of CaS03.(1/2)H20 with differentamountsof sulfuricacid as shown in Table 1. Figure 8 shows the first derivative However, pure calcium sulfite hemihydrate was prepared of the TG curves (TG') for different samples in the in the laboratory. The pure material was studied as the temperature range 335-385 "C. It was observed that as end member in the solid solution series, and its X-ray the amount of Cas04 in the solid solution (a) increases up pattern, infraredspectraandthermalanalysescurveswere to0.15, thecurvesshift toward the right. Incompositions used in identifying . . the constituents of the industrial corresponding to values of a of 0.20 and higher, gypsum byproduct. precipitated as a secondary phase and on thermal analysis The X-rav diffraction oattern was identical to that curves the two characteristic endothermic peaks were reported for"calcium sulfi& hemihydrate. It consists of
1,.
,Jj
Ind. Eng. Chem. Res., Vol. 33, No. 5, 1994 1149
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't 1
96
: 5
I
95 94
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t sa
- - -U ---
'
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1
On using S02, finer Cas03 grains are formed due to the reaction with residual CaC03. This process also caused the disappearance of gypsum which dissolves and is taken in solid solution. The sulfite formed by this treatment has finer grains and accordingly starts dehydration at a slightly lower temperature. Conclusions The FGD byproduct obtained from Texas Utilities was analyzed using X-ray diffractionand infrared spectroscopy and was found to consist of a mixture of a solid solution of calcium sulfate in calcium sulfite hemihydrate, calcium sulfate dihydrate, and unreacted calcium carbonate. Thermal analysis was used to determine the composition. The material consists of 5.3% gypsum, 77.7% solid solution, and 16.8% calcium carbonate. With the use of scanning electron microscopy, the material was found to consist of platelike particles which account for its thixotropic property. The size of the particles was found to be in the range 0-50 pm with a median particle diameter of 8.2 pm. Pure calcium sulfite hemihydrate, CaSOr(1/2)Hz0,was prepared in the laboratory to study and characterize the solid solution. Calciumsulfite never precipitates in a pure form inside the scrubber because of the presenceof oxygen in the flue gas and low pH in the scrubber which causes oxidation of sulfite to sulfate. Calcium sulfate is precipitated along with calcium sulfite in the form of a solid solution till a certain limit after which it precipitates as gypsum. Solid solutions with different sulfate contents were prepared by reacting a fixed amount of CaSOy(l/ 2)HzO with different amounts of sulfuric acid. The limit of solubility of sulfate in sulfite was established to be 18% (molar basis). As the sulfate content in the solid soIution increased, the dehydration temperature of the solid solution also increased.
1
91 90 0
100
50
150
200
2W
300
350
400
450
500
TEWFXATURE. WO. C
Figure 9. TG curves for (a) CaSO~(1/2)H20, (b) industrial byproduct, and (c) sample prepared by passing SO2 through a slurry of industrial byproduct.
t
.'
4
\
t
330
1
340
310
360
370
380 390 400 TEMPERATWE. DSp. C
410
420
430
440
450
Figure 10. First derivative of TG curves for (a) CaSO3.(1/2)H20, (b) industrial byproduct, and (c) sample prepared by passing SO2 through a slurry of industrial byproduct.
detected. Accordingly it was concluded that the limit of solubility is reached when a = 0.18 f 0.02. Within the range of this solid solubility,the higher the sulfate content (i.e., the higher the amount of HzS04 that was reacted with a fixed amount of CaSOy(1/2)H20), the higher was the dehydration temperature. In other words, increasing the concentration of Cas04 in the solid solution increases its stability toward dehydration. 3. Effect of Treating a Slurry of the Byproduct by SOz. Figures 9 and 10 show the TG and TG' curves respectively for (a) pure CaS03.(1/2)H:!O, (b) the industrial byproduct sample, and (c) the sample prepared by passing SO:! through a slurry of industrial byproduct. The dehydration of pure CaSOy(1/2)H:!O takes place in the temperature range 340-380 OC, whereas the dehydration of the solid solution present in the FGD byproduct takes place from 360 to 430 "C. The differencecan be attributed to the presence of Cas04 in the solid solution, thus increasingits stability. It is to be noted that the industrial material dehydrated at a temperature slightly higher than the synthetic saturated solid solution (Figures 8 and 10). This can be attributed to the fact that the average grain size of the industrial material is larger than that of the synthetic material.
Acknowledgment The authors thank Texas Utilities, Electric Power Research Institute, and Center for Energy and Mineral Resources, Texas A&M University, for supporting this work. Literature Cited (1) Chang, J. C. 5.;Bma, T. G. Environ. B o g . 1986,5 (4), 225233. (2) Jones, B. F.; Lowell, P. S.;Meserole, F. B. "Experimentaland Theoretical Studiesof Solid SolutionFormation in Lime & Limestone SO2 Scrubbers"; EFA-600/2-76-273a, Vol. 1, Final Report; U.S. Environmental Protection Agency: Washington, DC, 1976. (3) Goodwin, R. W. J. Air Pollut. Control Assoc. 1978, 28 (l), 35-39. (4) Karleson, H. T.;Bengtason,S.;Bjerle, I.; Klingspor,J.; Nibson, L. I.; Stromberg, A. M. Processing and Utilization of High Sulfur Coakr;Eteevier: Amsterdam, 1985. (5) Slack, A. V.; Hollinden, G. A. Sulfur Dioxide Removal from Waste Gases; Noyes Data Corporation: NJ, 1975.
Received for review July 6 , 1993 Accepted December 21, 1993. Abstract published in Aduance ACS Abstracts, February 15, 1994.
