Environ. Sci. Technol. 1992, 26, 1976-1981
(51) Solomon, P. A.; Moyers, J. L. Atmos. Enuiron. 1986,20, 207-213. (52) Graf, J.; Snow, R. H.; Draftz, R. G. Aerosol Sampling and Analysis-Phoenix, Arizona; EPA-600/3-77-015;U.S.En-
vironmental Protection Agency, Research Triangle Park, NC, 1977. Received for review February 3, 1992. Revised manuscript re-
ceiued May 1 , 1992. Accepted June 12, 1992. X-ray diffractometer was purchased with National Science Foundation Grant DMR-8406823. This work was supported in part by NSF Grant ATM-9007796. Microscopy was done at the Facility for HighResolution Electron Microscopy in the Center for Solid State Science at Arizona State University, established with support from the National Science Foundation (Grant DMR-86-11609) and also ASU.
Mechanism and Kinetics of Autoxidation of Calcium Sulfite Slurries Wanda Pasluk-Bronikowska, Tadeusz Bronikowski, and Marek Ulejczyk Institute of Physical Chemistry, Polish Academy of Sclences, Kasprzaka 44/52,
rn The kinetics of Co-catalyzed autoxidation of calcium sulfite was studied to deepen knowledge of the mechanism of this chain reaction. Laboratory experiments were performed under heterogeneous conditions using two different reactors: a stirred tank with a plane gas-liquid (slurry) interface and an impinger. The reaction course was followed by monitoring the conductivity of the reacting solution and by quenching with iodine solution, respectively. Mechanistic judgments were derived from the influence of sulfite and catalyst concentrations, and the solid CaS0, load on the kinetics of oxygen absorption. Appropriate reaction orders and rate constants were determined. Phenomena related to the solubility product law that imposed autoxidation rate limitations were analyzed. A soluble sulfate was shown to be a dual-action additive markedly accelerating the autoxidation due to increased sulfite and catalyst solubilities. The results are useful for designers of air pollution control processes.
Introduction In recent years the attention of investigators working in the area of air pollution control has concentrated on catalytic oxidation of calcium sulfite (CaSOJ with molecular oxygen (1-5). The interest in this reaction is, on the one hand, because of its commercial applicability in flue gas desulfurization systems and, on the other hand, because of the still obscure chemistry and kinetics of the radical chain transformations involved. The overall reaction stoichiometry corresponds to the equation
catalyst
+ f/202
1876 Envlron.
Scl. Technol., Vol. 26, No.
10, 1992
Warsaw, Poland
thors formulated a simple mathematical model for the overall gas-liquid-solid process. Karlsson et al. ( 4 ) intended to outline the mechanism of calcium sulfite slurry oxidation when the reagent was obtained by spray dryer absorption of sulfur dioxide. The work by Pasiuk-Bronikowska and Ziajka (5) was an attempt to arrange the information on bisulfite autoxidation in order to explain the different kinetics observed under varying reaction conditions. A summary of the literature, including this work, is given in Table I. In this paper we describe our studies on the autoxidation of calcium sulfite slurries in the presence of a cobaltous catalyst, until now not used in aqueous systems containing calcium.
Reaction Model It is usually accepted that the autoxidation of sulfite sulfur catalyzed by transition metal ions is a chain reaction (6-8).We can distinguish two main sequences of reaction steps, both involving free radicals. One is connected with oxygen-imposedtransformations of sulfite-derived species: Transformations of SIv initiation
+ -
(MeL)(Z+l-")+ + S032propagation I SO3"-
propagation I1
ki
O2
SO5'- + S032final product formation
kP2
HS04-, SO?This reaction takes place equally in the absence and in the presence of calcium. In the latter case, however, its kinetics is strongly influenced by phenomena related to the solubility product law. The aim of this work is to study more thoroughly the impact of calcium on the behavior of reaction 1to determine how to overcome some of the rate limitations. The reaction considered takes place in a gas-liquid-solid system where one of substrates is gaseous (oxygen),another one is solid (calcium sulfite), and the reaction environment is liquid (aqueous solution). Also, the final product (calcium sulfate) may crystallize from the solution. Because of the complexity of this system, a variety of approaches is needed to determine experimentally the kinetics of sulfite autoxidation. The goal of Jimell(2) was to determine certain kinetic parameters (mainly rate constants) using clear solutions of calcium bisulfite [Ca(HS03)2]. Weisnicht et al. (3) studied the liquid-phase reaction between bisulfite and oxygen and the solid to liquid mass transfer. These auHS03-, SO3'-
01-224
+
(MeL)(Z-n)+ SO,'-
k*l
(2)
SO5'-
(3)
S052- + SO3*-
(4)
ki
S052- + SO:-
2S042(5) Although reactions 2-5 are expressed using sulfite ions and radicals, we are aware that the question of whether sulfite or bisulfite species (or both) are active in the transformations has not yet been finally answered. L"- in eq 2 represents a ligand. We assume that the ligand plays a distinct role in the autoxidation kinetics. The other important sequence of reaction steps comprises transformations of transition metal species: Metal Cation Reduction-Oxidation initiation (MeL)(z+l-n)+ + SO32-
k
A (MeL)("-")++ SO3'-
(2)
initiator regeneration
+
2(MeL)("-")+ O2 -t- 2H+
0013-936X/92/0926-1976$03.00/0
22(MeL)(Z+1-n)+ +H 0 2
2
(6) 0 1992 American Chemical Society
Table 1. Survey of Experimental Conditions technique
reactor tubular flow calorimeter tubular reactor or semibatch reactor with bubbled gas sparged stirred tank absorber stirred tank with plane gas-liquid interface conductivity cell or impinger
catalyst
recording the temperature increase standard iodometric titration
MnS04 metal impurities or MnS04
iodine-thiosulfate titration FeS04 or MnS04 or mixture volumetric measurements of O2 MnSO, absorption electrical conductivity measurements or CoS04 iodometric titration
Step 6 is written as an overall reaction which is developed into a set of elementary steps elsewhere (9). The two sequencies are coupled through the common step, i.e., through initiation. It is characteristic for the autoxidation of SIv that different kinetics may be observed, depending on the reagents' (including metal ions) concentration ratios. At an excess of oxygen with respect to sulfite and metal, it is reasonable to suppose that the overall autoxidation rate becomes controlled by the propagation I1 step (see eq 4): r, = kp2[S05*-][S032-]
temp,OC
pH
ref
25 50
3.8-4.7 2 4.6 3
50 26
4-6 4 3.7-6.5 5
19-25
7.3
this work
Cas03
SUSPENSION
(7)
By applying the steady-state assumption to peroxysulfate radicals, Barron and OHern (10) derived the following rate expression for the autoxidation catalyzed by CuS04: r, = (ki/kt)1/2kp2[C~2+] [S1v]3/2
(8) Here, we consider this expression in a more generalized form: r, = (ki/kt) 1 4 z p 2[(MeL)(Z+l-n)+] l I 2 [SI"] 3/2 (9) It results from eq 9 that the rate of autoxidation is of the order with respect to the catalyst only when [(MeL)(Z+l-n)+] = a[cat.] (10) where a denotes a constant and [cat.] is the initial concentration of the catalyst, e.g., CoS04. Solubility of Calcium Sulfite The concentration of Srvin slurry solutions containing calcium is governed by the following equilibrium only as long as the concentration of calcium sulfate (fiial product) is below its precipitation level:
== KSI
CaS03,solid
Ca2++ S032-
KsI = [Ca2+][S032-]
(11)
where KsI = (3.1 f 1.5) X lo-' mo12/dm6at 25 "C as based on the activities (11). The corresponding solubility of calcium sulfite is (4.5 f 1.0) X mol/dm3. With the concentration of calcium increasing due to the formation of more soluble calcium sulfate, the concentration of SN diminishes to achieve the minimum value fixed by the additional equilibrium: KSA
CaS04,solid KSA
Ca2++ S042-
= [Ca2+][S042-]
(12)
where KSA= 104.63mo12/dm6at 25 "C (12). This minimum value is approximately half the amount soluble in pure water (11). Experimental Section Apparatus and Procedure. To study the kinetics of sulfite autoxidation in the presence of calcium precipitates (calcium sulfite and calcium sulfate), the reaction was performed in gas-liquid-solid reactors of the following
Figure 1. Schematic of experimental apparatus: (A) stirred-tank with a plane gas-liquid interface (1. air-thermostated tank; 2. magnetic stirrer; 3. conductivity sensor); (B)impinger (1. air pump: 2. flowmeter; 3. humidifier; 4. oxygen absorber: 5. thermostating unit).
types: a stirred-tank (conductivity cell) with the plane gas-liquid (slurry) interface and an impinger with a gaseous stream blown through the liquid phase (slurry). Both reactors (Figure 1) were made of glass and were operated semibatchwise at ambient temperatures and at atmospheric pressure. The conductivity cell had a total volume of 0.25 dm3and an inside diameter of 0.71 dm. It was supplied with a magnetic stirrer rotated at 1175 f 50 rpm. The reactor was charged with 0.15 dm3of liquid (slurry). The progress of calcium sulfite autoxidation was followed continuously using a conductivity probe to measure the concentration of sulfate anions in the reacting solution. The concentration of oxygen dissolved in the bulk of the solution was determined using an oxygen probe. The impinger (cylindrical part) was 1.5 dm high with a 0.45-dm inside diameter. This reactor was charged with the same volume of liquid (slurry) as that introduced into the stirred tank. Air was introduced through a nozzle (diameter 0.025 dm) installed downward. The distance between the spherical bottom and the nozzle was 0.088 dm. The amount of calcium sulfite consumed during autoxidation was determined at the end of an experiment by the standard iodine-thiosulfate titration of the slurry contained in the impinger. The reaction studied was initiated by introducing a known volume of the standard Co catalyst solution to the reactor filled with the aqueous slurry of calcium sulfite (known amounts). Both the catalyst solution and the slurry solution were presaturated with air. Runs in the conductivity cell were carried out at varied duration times, whereas those in the impinger were stopped after 4 min Environ. Sci. Technol., Vol. 26, No. 10, 1992
1977
,I I 1 c
3
g
3/2
,diffu;;n;I
reg;r.e
.............................
,~,,;~',~~~~ >
.-,-E
, n
12-8
_ ^ - I
1 s 5
I!
,-->
C~SO,..mo~z'dn13
1
c-'1 o
- ~
1 o -1 c~ -
S w , rnol/drn
~
Figure 2. SIv concentration dependence of the Co-catalyzed autoxidation rate for CaSO, slurry (black points): [cat.] = 1.67 X lo-' mol/dm3, Co* = 2.5 X lo4 mol/dm3, pH = 7.3,t = 23.0 f 0.1 OC, CaSO, load 0.06 mol/dm3 (conductivity cell). For the Na2S03clear mol/dm3, [O,]= 6 X solution (dashed line) (7): [cat.] = 1 X 10-5-1 X lo-, mol/dm3, pH = 7.0,f = 30 OC.
to keep the consumption of calcium sulfite precipitate negligible. Materials. Redistilled water was used in all experiments. The calcium sulfite precipitate was obtained by treating Ca(N03)2predissolved in water with an aqueous solution of Na2S03and separating the resulting precipitate by filtration, washing, and drying, at 1110 "C. The operations were conducted under a nitrogen blanket. The final product contained 93.64% (by weight) CaS03J/2H20, 6.34% (by weight) H 2 0 (humidity), and calcium sulfate at the trace level. Before introduction to the reactor, a sample of the precipitate was ground in the agate mortar to obtain particles (spheres) of 1100 pm in size. All other chemicals used were reagent grade (POCH, Gliwice, Poland). Results and Discussion Undisturbed Reaction Kinetics. The validity of eq 8 for the CoS04-catalyzed autoxidation of SIv has been proved by Bengtsson and Bjerle (1)in experiments with aqueous solutions of sodium sulfite (clear solutions). Their results showing the 3/2 order of the autoxidation rate with respect to the concentration of SIv are given in Figure 2 (dashed line). These data were obtained from a series of experiments made in a batch reactor under homogeneous conditions. They are compared with the rate data determined by us for the calcium sulfite slurry in a semibatch reactor under heterogeneous conditions (black points). In this work, the CaSO, slurry is regarded as quasi-homogeneous as suspended particles are fine and mixing is adequate. We calculated the data from oxygen absorption measurements using the following equation for the rate of absorption accompanied by the zero-order reaction with respect to the absorbing gas (compare eq 9): ~ , =a a
(
m
(13)
where R, denotes the rate of chemical absorption (mol/dm2 s), a the gas-liquid interface area per unit volume of liquid (l/dm), Do the diffusivity of oxygen (dm2/s), CO* the interface value of the oxygen concentration equal to the gas-liquid equilibrium value (mol/dm3),and r, the reaction 1078 Envlron. Scl. Technol., Vol. 26, No. 10, 1992
Flgure 3. CoSO, concentration de endence of the autoxldatlon rate for CaSO, slurry (black polnts): [S ] = 6.4 X mol/dm3, Coo = 2.5 X mol/dm3, pH = 7.3,t = 20 f 1 OC, CaSO, load 0.06 mol/dm3 (impinger). For Na2S03clear solution (dashed line) ( 7): [SIv] = 1 X lo-' mol/dm3, Co' = 1.4X mol/dm3, pH = 7.5, t = 50 OC.
R
rate (mol/dm3s) as given by eq 9. Equation 13 is valid under the fast-reaction regime conditions (13)which were fulfilled in the conductivity cell described above. By combining eq 13 with eq 9 and by rearranging, one obtains the expression (R,a)2/Do = a2Co*(ki/k,)1~2kp2cr[cat.]1/2[S1V]3/2 (14) which can be simplified to give (R,a)2/Do = c ~ n s t ~ [ S ' ~ ] ~ / ~ (15) when a, Co*, and [cat.] may be assumed constant. The actual concentration of SIv in the slurry solution was calculated from eq 11, taking [Ca2+]= [S042-] (16) and activity coefficients equal to 1. The concentration of sulfate anions was that measured using a conductivity probe. In this instance, the values of [Sot-] were always lower than those at equilibrium with calcium sulfate solids. It is evident from Figure 2 that the intrinsic reaction order with respect to the sulfite concentration is the same for sodium and calcium sulfites. Similarly, the reaction orders with respect to the cobalt catalyst concentration were the same in both cases and equal to 1/2, as shown in Figure 3. The experiments were carried out in the impinger under conditions of the kinetic regime for the slow reaction (see ref 13). In this regime the reaction studied behaved as if performed in a homogeneous system (negligible mass-transfer limitations). The results of Bengtsson and Bjerle were obtained from experiments in the reactor with a plane gas-liquid interface. The reactor was operated under heterogeneous conditions in the fast-reaction regime. It was possible to determine the reaction order with respect to the catalyst from the rate of oxygen absorption measured by these authors for varied concentrations of the catalyst at constant a, CO*,DO,and [S'V] : (R,a)2 = c ~ n s t ~ [ c a t . ] ~ / ~ (17) where constz = a200(ki/kt)1/2kp2cr[S1V]3/2Co*(18)
To ascertain the regions of oxygen absorption in the reactors, a series of additional experiments was performed. The bulk concentration of oxygen was measured during a typical run in the conductivity cell. Its value was found
I
.c-5
1
' 003
004 006 0CE CaS03 load, rnol/dm3
032
.-
0
A
A
+
impinger I
0 10
I irnpinger 11
I 2.0
A
0O Y 000 1 0-'
3-8
10-5
I
n-I
Figure 4. Experimental evidence for the rate of CaSO, autoxidation limited by the liquid-side diffusion of oxygen: C o o = 2.5 X mol/dm3, t = 20 f 1 'C, CaSO, load 0.02mol/dm3 (A), [cat.] = 1.33 X lo-' mol/dm3 (B) (impinger).
to be equal approximately to zero, indicating that the reaction studied was sufficiently fast in comparison with the rate of oxygen diffusion. The value of kLa, where k~ is the physical absorption coefficient (dm/s), was determined for the impinger using varying cobalt catalyst concentrations and varying calcium sulfite loads. Data collected in these experiments are given in Figure 4A for two similar impingers and in Figure 4B for one of these impingers, respectively. It is clear that the region where no influence of the catalyst concentration and, simultaneously, of the calcium sulfite load on the rate of oxygen is observed must be the liquid-side diffusional one (see ref 13). In such a case the kLa product can be calculated from the equation k L U = (R,a)/Co* (19) The following values are obtained from Figure 4: (kLa)impingerI = 0.114 s - ~
-
0.142 s-l (Figure 4A) or 0.120 s-l (Figure 4B) The importance of the gas-side resistance has been rejected on the basis of preliminary calculations. To prevent the occurrence of rate limitations due to solid sulfite dissolution, a sufficiently large weighed portion of ground CaS03 precipitate was introduced into a reactor. Thus, the rate of solid sulfite dissolution necessary to maintain the concentration of dissolved sulfite nearly at the solubility level was ascertained. The influence of calcium sulfite dissolution is shown in Figure 5 for one of the highest rates for the intrinsic reaction attained in this work. The figure indicates that in this case no dependence of the autoxidation rate on the dissolution of CaS03 is observed at loads exceeding about 8 X mol/dm3. Evidently, for lower rates of the intrinsic reaction the above condition was met at correspondingly lower solid loads, provided the granulation of solid particles was identical. Moreover, we arranged absorption experiments to keep the fraction of solid CaS03 consumed relatively low. Numerical values of the composite rate constant (being totally the second-order rate constant)
k, =
(ki/kJ1&,2a
I
I
1
1
I
304
006
008
010
012
1C-4
C ~ S O , rnol/drn3
(kLa)impingerII
1
002
(20)
are given in Table 11,as found by us for CaS03 slurries and by Bengtsson and Bjerle (1) for Na2S03solutions. To calculate the value of k, at homogeneous conditions we used eqs 9 and 10. The concentration of S" was assumed equal to the average solubility of calcium sulfite, being the
CaS03 load, mol/dm3
Flgure 5. Influence of CaSO, solids' dissolution on the rate of oxygen absorption in implnger 11: [cat.] = 1.33 X 10"' mol/dm3, Co' = 2.1 x 1 0 - ~ mol/dm3, p~ = 7.3,t = 23.0 f 0.1 'C, [K2S04] = 0.2 mol/dm3.
Table 11. Numerical Values of k, for Co-Catalyzed Reaction temp, O
system CaS03,8-water homogeneous heterogeneous Na2S03-water homogeneous heterogeneous
pH
k, dm3/mol.s
ref
20 f 1 23
7.3 7.3
1.3 X lo4 3.3 X IO4
this work this work
20 30 23 50
7.3 7.3 7.5 7.5
5.1 x 1.1 x 1.0 x 7.0 x
1
C
103" 104 103" 103
1
Calculated by use of the activation energy E = 57 kJ/mol determined by Bengtason and Bjerle (I).
arithmetic mean for the solubility in pure water (5.6 X mol/dm3 by the conductivity method) and in the solution mol/dm3 by saturated with calcium sulfate (1.44 X the iodometric method). From the results one can draw the general conclusion that, in principle, the autoxidation of SIv catalyzed by C0S04 obeys the same kinetic law, irrespective of whether sodium or calcium cations neutralize sulfite anions in the reacting solution. This conclusion, however, is true only at appropriate concentrations of the catalyst. Behavior of the Catalyst. Keeping the above statement in mind, it seems reasonable to raise the problem of homogeneous catalyst persistency in solutions containing suspensions of reactive solids. Results obtained in the conductivity cell for a series of prolonged runs at very low concentrations of C0S04are given in Figure 6. The figure shows the rate of SIv oxidation continuously decreasing with time. The value determined in the absence of the catalyst introduced purposely corresponds well to the rate of oxygen absorption in the diffusional regime (see, for example, ref 19), where (kLa)ceu= 2.50 x 10-4 11s This "decreasing-rate phenomenonn is explicable in terms of the adsorption behavior of trace metals observed on inorganic sediments (14). The percentage adsorption depends not only upon time of contact but also upon the ratio of metal concentration to available surface area of suspended solids. With increasing concentration of C0S04 at a constant surface area (the same load of CaS03 used), the effect of metal ion adsorption diminishes. Figure 7 shows short experiments where the influence of metal ion adsorption on the concentration of the catalyst in the reacting solution may be disregarded. The curvature of the plot at C0S04 1.66 X mol/dm3 may be easily Environ. Sci. Technol., Vol. 26, No. 10, 1992
1979
C OGCG J
2033
130'2
3cco
1
I c: [: :;
time. s
Flgure 6. Time dependence of CaSO, concentration for the autoxidation of CaSO, slurry at lower concentrations of CoSO, (cobalt adsorption important): (W) 1.0 X (0) 6.6 X lo-', and (A)0 mol/dm3 [[S'"] = 5.6 X lo-, mol/dma, Co* = 2.5 X IO-, mol/dm3, pH = 7.3,t = 24.0 f 0.1 O C , CaSO, load 0.06 mol/dm3 (conducthri cell)].
Table 111. Conditions for Appearance of Co Precipitates [COSO41,
mol/dm3
(2-10) 26.65
X X
lo4
[Srv],mol/dm3
temp, "C
pH
Na2S03,0.25-1.0
room
CaSOn,5.6 X
23.5 f 0.5 7.3
7.5-9.0
ref
15 this work
explained as resulting from CaS03 solubility changes due to the accumulation of dissolved reaction product, CaS04. When calcium sulfite is oxidized at concentrations of Co catalyst of 16.65 X lo* mol/dm3, brown solids appear at the gas-liquid (slurry) interface. Thence, the rate of CaS03 autoxidation ceases to be sensitive to varying the catalyst concentration (Figure 7). Precipitation of Co catalyst was also observed by others in experiments with sodium sulfite solutions, however, at much higher Co concentrations (Table III). When much stronger sodium sulfite solutions were used, no Co precipitate formation was observed even at catalyst concentrations as high as 1 X mol/dm3. One can summarize these observations as follows: (1) Precipitation of Co catalyst occurs only at a deficiency of sulfite with respect to the catalyst concentration. (2) The precipitate appears only in the presence of oxygen in the liquid phase. [In our experiments the formation of brown solids was confined to the region penetrated by oxygen, i.e., to the film of liquid adjacent to the gas-liquid interface. According to the theory of gas absorption with chemical reaction (13),the bulk of the liquid contains no oxygen under the conditions of the fast-reaction regime.] As at precipitation of the catalyst its concentration in the reacting solution is governed by the solubility product law, eq 10 becomes invalid. Mechanistic Aspects. In light of the results described above, the necessity of completing the basic chain mechanism with additional reactions is evident: Initiator Precursor Transformations complexation Me(z+l)++ S032- + (MeSo3)(Z+1-2)+ (21) hydrolysis Me(a+l)++ (z
-
+ 1)H20
Me(OH)(,+1)
KHD
Me(OH)(,+l,
+ ( z + 1)H+ (224
Me(OH)(z+l),solid
(22b)
Further we suppose that reaction 21 is followed by step 1980 Environ. Sci. Technol., Vol. 26,No. 10, 1992
Figure 7. Time dependence of CaSO, concentration for the autoxidation of CaS0, slurry at higher concentratlons of CoSO, (cobalt adsorption negligible): (0)1.33 X (A)1.66 X lo-', (A)6.65 X IO-', (0) 1.33 X and (0)2.00 X mol/dm3 [[SI"] = 5.6 X lo4 mol/dm3, Co* = 2.5 X lo4 mol/dm3, pH = 7.3,t = 23.5 f 0.5 O C , CaSO, load 0.06 mol/dm3 (conductivity cell)].
2. It means that the SIv autoxidation chain is initiated by a labile complex of the type (MeL)(Z+l-n)+, where L"- = SO:- (or HSOc) as assumed for simplicity. Actually, it is not clear how many sulfite ligands surround the cation Me(Z+U+. For verification of this point we added nitroso-R salt to the reacting solution to bind selectively Co3+in a stable complex (16). Nitroso-R salt caused strong inhibition, simultaneously preventing formation of the brown solid. The existence and fate of transient ironLS" complexes has been shown by Kraft and van Eldik (17,18). The combination of manganese'" with S" ions as the rate-determining reaction has been suggested by Siskos et al. (19). Remarkable similarities may be expected in complex formation reactions between various transition metal cations and SIv anions by analogy to the greatly similar behavior of the cations with respect to hydroxy anions (20). Therefore, cobaltll'-S1v transient complexes are equally probable. To avoid the situation where the initiator precursor is lost forming insoluble hydroxides (eqs 22), reaction 21 should be favored. This can be achieved by raising the concentration of SIv. It is possible to increase the solubility of S" in the slurry solution by addition of a solubility enhancer. Let us consider the possibility for the w e when the concentration of SIv anions is governed by the relationship
[so:-]
=
(KSI/KSA)
[so42-l
(23)
Equation 23 is obtained by combining eq 11and eq 12. It shows that the higher concentration of anionic SIv at equilibrium with calcium sulfite and sulfate precipitates may be forced by addition of a sulfate, the solubility of which exceeds that of calcium sulfate. In this work, experiments were performed in the impinger with potassium sulfate at a concentration 0.2 mol/dm3 (Figure 8). Results of the experiments show distinct enhancement of the autoxidation rate. Moreover, sensitivity of the rate to the catalyst concentration has been restored (no precipitation of Co compounds is observed at these conditions) and the 1/2 order with respect to the catalyst regained. Data in Figure 8 concern the rate of autoxidation in the fast-reaction regime of oxygen absorption. Therefore the order seen in Figure 8 is lIq, as resulting from eq 13. The effect of ionic strength (I)on the rate of SIv autoxidation was also considered. Results obtained at calcium
I
1:
I
1 1
C -'
13 5
C -L
c o s o , m01/*rn3
Flgure 8. Rate of @SO3 slurry autoxidation in the presence of K2S04 (0.2mol/dm3)(black points) compared with that in the absence of the mol/dm3, Co' = 2.1 X additive (dashed line): IS'"] = 1.44 X lo-' and 2.5 X lo-' mol/dm3, respectively, pH = 7.3, t = 20 f 1 OC, CaSO, load 0.07 mol/dm3 (impinger).
1
1 0-'
4
0 00
0 05
0 10
P2, mol
0 15
0 20
0 25
/dm2'3
Flgure 9. Plot of SI" autoxldation rate against ionic strength [cat.] = 6.65 X lo-' mol/dm3, Co+ = 2.5 X lo-' mol/dm3, pH = 7.3, t = 23 f 0.1 OC, @SO3 load 0.06 md/dm3 (conductlvtty cell, fast-reaction regime).
sulfate concentrations changing continuously up to the supersaturation value at which precipitation of the sulfate occurred are given in Figure 9. They indicate the net effect-the influence of the ionic strength being partly compensated by that of the decreasing solubility of calcium sulfite. Having estimated the latter, one comes to the conclusion that the effect of the ionic strength still may be assumed a secondary factor.
Conclusions It has been shown that the Co-catalyzed oxidation of CaS03J/zHz0suspended in aqueous solution may be analyzed in terms of a chain reaction, as in the case of more soluble sulfites. The mass-transfer effects that must be taken into account are concerned primarily with gas absorption and solid dissolution, both processes supplying autoxidation substrates. The presence of calcium precipitates causes peculiarities in the reaction kinetics by maintaining the concentration of SN at a very low level. This concentration is often not sufficient for complexing higher valency metal cations to prevent their elimination due to hydrolysis combined with the precipitation of hydrolysis products.
Values of the rate constant for the Co-catalyzed autoxidation of calcium sulfite are given, together with conditions for performing the reaction in practice. The rate-limiting effect of the scarcity of dissolved sulfite for the initiator is hampered by addition of soluble sulfates at concentrations exceeding the solubility of calcium sulfate. Thus, the catalytic activity of Co catalyst, being the moat widely used transition metal catalyst in autoxidation of hydrocarbons, can be fully made use of in the case of autoxidation of inorganic compounds. Particularly, the results demonstrate the possibility of improving the flue gas desulfurization systems in which SOz is absorbed in aqueous slurries of lime or limestone. The application of C0S04with a solubility enhancer under slightly alkaline conditions leads to reasonably high rates of calcium sulfite (SOpabsorption product) oxidation at much lower concentrations of a catalyst than in the case of the MnS04 usually used under acidic conditions. Registry No. CaSOs, 10257-55-3;C0S04, 10124-43-3; SOz, 7446-09-5.
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Environ. Scl. Technol., Vol. 26, No. 10, 1992 1981
