Kinetics of the Reaction of Calcium Sulfite and Calcium Carbonate with Sulfur Dioxide and Oxygen in the Presence of Calcium Chloride G. Van Houte, L. Rodrique,t M. Genet, and B. Delmon" Universite Catholique de Louvain, Groupe de Physico-Chimie Minerale et de Catalyse, Place Croix du Sud 1,
81348 Louvain-la-Neuve, Belgium
w The kinetics of the reactions of CaC03 or CaSO3-impregnated by 2% of CaCl2-with SO2 and 0 2 are compared with those of the pure samples. The rate of reaction of Cas03 with 0 2 is substantially increased in the presence of the additive, and the reaction is completely unblocked above 460-470 "C, in contrast with the case of pure Cas03 where the reaction became completely blocked even at high temperature (650 OC). The activation energy is not modified by the presence of the additive. The rate of reaction of CaC03 with SO2 0.502 is also substantially increased in the presence of CaC12. When impregnated CaC03 is contacted with a stoichiometric mixture of the gases, the rate limitation comes from the oxidation of CaS03, formed by the reaction of CaC03 with SO,; on the contrary, with pure carbonate, the reaction with SO2 constituted the rate-limiting step. The role played by the additive on the reaction scheme is discussed.
Using doped CaS03, it is possible to isolate reaction 2 for a kinetic study. When SO2 and 0 2 are contacted together with doped CaC03 both reactions 1 and 2 take place. It will thus be possible to give an overall picture of the sequence of reactions with activated samples (identified by i). Because of the suppression of reaction 3, the sequence of reaction simplifies as follows: 1
CaCOs(i)
+0.502
+
Introduction The present article completes a series of publications dealing with the kinetics of the various reactions taking place during the fixation of SO2 on CaC03, either pure or activated. We have shown that some additives increase considerably the rate of fixation of SO2 on lime or limestone and allow the entire utilization of the solid, namely, the quantitative transformation to Cas04 ( I ) . The practical interest of using an additive to promote the reactivity of lime or limestone has been demonstrated by bench-scale experiments with a fluidized bed of absorbents ( 2 , 3 ) . Three previous articles have dealt with kinetics of the various reactions taking place with pure CaC03 or Cas03 (4-6). Another article has examined the kinetics of the reaction, with SO2 alone, of CaC03 and of Cas03 impregnated with CaClz, which is one of the best promotors (7). The present article concerns the reaction of SO2 and 0 2 with the modified carbonate and of 0 2 with the modified sulfite. In the presence of 0 2 , the reaction of SO2 with CaC03 is complex (8):
CaCO, + SO,
1
+0.50,fl CaSO, + CO,f 0.5 so,
CaSO,
CaSO, + 0.5s f
In the previous publication (7) dealing with doped CaC03 and CaS03, we have shown that reaction 3 is extremely slow in the presence of CaC12 and can be completely neglected. We have thus to study a simplified consecutive system of only two reactions, namely, reactions 1and 2. Section de Physico-Chimie Minerale du Musee Royal de 1'Afrique Centrale, Place Croix du Sud 1, B1348 Louvain-la-Neuve, Belgium.
2
+ SO2 -+CaSOs(i) + C02 t +Cas04
Reagents and Experimental Methods Three different powdered samples were used for this study: (1)CaC03 I = CaC03 p.a. Merck 2064. Diaheter of the particles: 2-15 pm. BET N2 specific surface area: 0.5 m2 g-l. (2) CaCO3I1 = CaCO3 p.a. Merck 2064, second delivery. Diameter of the particles: 1-8 pm. BET N2 specific surface area: 0.95 m2 g-l. (3) Anhydrous Cas03 = obtained, as described below, from C a S 0 ~ 2 H 2 0p.a. Riedel-de Haen A.G. Diameter of the particles (dehydrated sample): 0.5-2 pm. BET N2 surface area (dehydrated sample): 7.6 m2 g-l. Calcium chloride (2.0 mol %) is added to the solid absorbent by impregnation and drying a t 100 OC. Preliminary experiments made with different quantities of CaC12 have shown that 2.0 mol % is sufficient for obtaining the optimum effect. It has been verified that the quantity of impregnation water added to the absorbent, i.e., 5,10, or 100 mL of water for 100 g of absorbent, is without influence on the reactivity of the latter. Impregnation using methanol instead of water also gives identical results. Anhydrous CaSOs(i) was prepared by dehydration of modified C a S 0 ~ 2 H 2 0by heating under vacuum at 200 "C for 1h. For each experiment, we have used 3.6 g of CaCOs(i) or 5.6 g of CaS03.2H20(i); no significant difference of specific surface area has been noted between modified and unmodified carbonate and anhydrous sulfite. Measurement of the Reaction Rates. The reactions are conducted in a fixed-bed reactor, in a closed apparatus, allowing a circulation of the gases (pure S02, pure 0 2 , or mixtures). A detailed description of the apparatus has been given in two publications (5, 6). The apparatus used for this study does not permit a direct measurement of the progress of reaction 1 (CaC03 + SO2 Cas03 COz), since this reaction does not bring about any modification of the pressure in the apparatus; indeed, one molecule of C02 is evolved each time CaC03 absorbs one molecule of SO2. Reaction 2 (Cas03 f 0.502 CaS04) brings about a pressure drop in the apparatus and allows the measurement of the quantity of CaC03 converted into CaS04. Without additional experiment, our measurement thus reflects the progress of reaction 2 proceeding consecutively to a progress of reaction 1 which is unknown (except that it necessarily exceeds that of reaction 2). We shall see, however, that one can evaluate the progress of reaction 1at a given time by contacting the partially reacted sample with oxygen and measuring the absorption of this gas: Cas03 0.502 CaS04. In this case, we have verified, by heating the sample up to 560 "C and reintroducing 0 2 , that the oxidation of
0013-936X/81/0915-0327$01.25/0 @ 1981 American Chemical Society
-
+
-
+
-
Volume 15, Number 3, March 1981 327
Cas03 by 02 was complete. As will he shown, no blocking of reaction 2 is observed at this temperature. Examination of t h e Samples. The pulverulent products have been analyzed by X-ray diffraction. Some samples have been submitted to microscopic examination, either directly with a scanning electron microscope or by the intermediate of replicas with a conventional transmission electron microscope. For scanning microscopy, the powdered samples have been coated with vacuum-evaporated gold-palladium and examined with a JEOL JSM-U3 instrument operated a t a potential of 15-20 kV. Ultimate resolution should he -200 A. For the examination of materials by transmission electron microscopy, we have perfected a method which permits the preparation of hull replicas on pulverulent samples by simultaneous codeposition of platinum and carbon (Pt/C) with an arrangement described by Bradley ( 9 ) . The observation of such a replica can he made with the full resolving power (10 A) of the AEI EMGG electron microscope operated at a potential of 100 kV. The sharpness with which details of the specimen surface can he seen is limited only by the quality of the deposit; in principle, the used technique allows the visualization of surface details, the dimensions of which are of the order of 25-30 A.
of the lower reactivity of this sample); in this latter case, the surface area diminishes. The hydrated sulfite (Figure 3a) is made of tabular, often elongated, particles with variable forms and of nearmicron sizes; the face of the platelets presents a relatively smooth appearance. Dehydration seems to provoke a partial breaking of the sulfite platelets (Figure 3h) which, moreover, show a surface with uneven relief. The sample prepared from CaC12 impregnated sulfite, after 87%conversion into CaS04 a t 473 "C, is constituted of grains with irregular and ill-defined outlines and with a greatly altered surface (Figure 3c); this feature is probably associated-at least partially-with the
Reaction 2. A first set of experiments is conducted with the sulfite sample, either pure or impregnated with CaC12. After being heated under vacuum to the desired temperature, the sample is contacted with 02 a t nearly constant pressure, namely, starting at 760 torr (1atm) and recharging the apparatus as soon as the pressure of 0 2 drops to 720 torr (0.95 atm). Figure 1 shows different curves obtained for 490 and 585 "C in the case of pure CaSOs (dashed lines) and in the range of 395-485 "C for impregnated CaS03 (solid lines). An activation energy of 32 kcal mol-" has been calculated from the initial rates of the reaction. The calculated value, in the case of pure CaSOs. between 490 and 560 "C, WBS -35 kcal mol-' in initial kinetics (5). Figure 2 gives the evolution of the BET specific surface area of the partially reacted CaSOdi) sample vs. the degree of conversion a into CaS04at a temperature of 470 "C. From a = 0 to a = 0.75, the surface area increases from 7.6 to 9.5 m2 g-1. The dashed line in Figure 2 shows the evolution of BET surface area of pure sulfite a t 580 "C (a temperature chosen higher than in the case of the impregnated sample, because
t (rnin.) +re 1. Renction of CaS03. eHher pue (dashed lines) or impregnated with 2 MI % CaCI2 (solid lines),with 02.
328 Environmental Science 8 Technology
I
4
Results
I
I
0
a2
I
0.4
1
-
I 0.8
0.6
U Figure 2. Evolution 01 the BET N2 specific surface area of &SO3 vs. degree of transformation a. Comparison 01 pure (dashed line) or inpregnated with CaCi, (solid line) CaS03.
I-^
.
"
FWre 3. Transmission deckon miaogapb of replicas wepared from (a) hydrated sulfite. (b) dehydrated sulfite. (c) impregnated CaS03 reacted wim O2 until an 87% CaSO. conv6fsIon at 473 OC. and (d) p r e CaS03 reacted with O2 until a 30% CaSO, conversion at 573 OC.
increase of volume due to the transformation of Cas03 (molar volume: 41.9 cm3) into CaS04 (molar volume: 45.7 cm3). I t should be observed that a similar appearance is observed with pure sulfite having undergone 30%transformation into Cas04 (Figure 3d). Reactions 1 plus 2. Unless otherwise stated, the proportion of the gaseous reactants (i.e., SO2 and 0 2 ) has been held a t a stoichiometric ratio of 2:l. The solid lines in Figures 4 and 5 represent the degree of transformation ( a )vs. reaction time at various temperatures for, respectively, CaC03 I(i) and CaC03 II(i) in the presence of SO2 0.502. The total pressure was kept between 1and 0.95 atm by periodically evacuating and recharging with a fresh mixture of the gases as soon as the pressure dropped by 5%. The finer-grained sample (CaC03 II(i)) reacts more rapidly and to a higher degree of transformation than CaC03 I(i). The former transforms completely and quickly into sulfate above 480 "C; for the latter, the temperature necessary to attain the same result is -525 OC. At the lower temperatures, the curves present a slightly sigmoidal shape. For the sake of comparison, the same curves obtained in the case of pure carbonates are also presented (dashed lines) in Figures 4 and 5. The following experiments were made with the most reactive sample, CaC03 II(i). The activation energy has been calculated from the maximum rates (that is, corresponding
+
to the rate at the inflection point of the sigmoidal curves) observed in the presence of the stoichiometric mixture of SO2 and 0 2 . Because of the sigmoidal shape of the curves, it is clear that the phenomenon we measure is complex. Therefore, the value of 16 kcal mol-l which can be calculated from the rates of reaction observed between 380 and 520 "C does not represent an activation energy of a given reaction, but rather the overall thermal increment of the succession of reactions 1plus 2. A less precise measurement of the thermal increment in the case of CaC03 I(i) gives similar values. In the case of the pure samples, an activation energy of 50 kcal mol-l had been calculated in initial kinetics ( 4 ) . Figure 6a indicates the dependence of the initial reaction rate (first 3 min) on the total pressure of an SO2-02 stoichiometric mixture at 525 "C. The overall order with respect to the gaseous reactants is 1. Figure 6, b and c, corresponds to experiments where the partial pressure of SO2 or 0 2 was held constant, with the pressure of the other reactant being changed. The initial order of reaction with respect to 0 2 was 1 (Figure 6b); the initial order of reaction with respect to SO2 was zero, except at low pressures (Figure 6c). The overall order of 1 observed in Figure 6a can thus be interpreted as the sum of a zero order with respect to SO2 and an order of 1with respect to 0 2 . This suggests that, for nearly stoichiometric conditions, reaction 2 limits the rate. Figure 7 gives the evolution of the BET surface area of the solid vs. the degree of conversion a at 495 "C (solid line). The surface area substantially increases during the reaction: after C
0.75
0.50 C?Z O+? 0 0 2 0 4 0 6 p,,atm I ; P,,atm
1 0
6
(sOz+f&),atm
0
0.25
-0
' 1
2
3
I
,
I
,
4 0 0 5
2
1
P
12
'lP
Figure 6. Influence of the pressure of the gaseous reactants on the initial rate of reaction: (a)stoichiometric mixture (Sop 0.50,); (b) influence of the pressure of 0 2 , the pressure of SOn being maintained constant (psOz= 0.2 atm); (c) influence of the pressure of SOP,the pressure of O2 being maintained constant (po, = 0.3 atm) (p is defined as the ratio 2p02/pso2;for the stoichiometric mixture of the gases, p = 1).
+
n
25
50
t , min.
75
Figure 4. Reaction of CaC03 I, either pure (dashed lines) or impregnated with 2 mol % CaC12 (solid lines), with SO2 0.5O2.
+
0
CaSO,+ C02
II
m" 050
0.25
I
0.8 0
0
0
25
50
75
t, min. Figure 5. Reaction of CaCO3 11, either pure (dashed lines) or impregnated with 2 mol % CaC12 (solid lines), with SO2 i- 0.502.
Q2 5
I 0.50
1 ! Q75 a
Figure 7. Evolution of the BET N2 specific surface area of CaC03 II vs. the degree of transformation (a).CaC03 was either pure (dashed lines) or impregnated with 2 % CaCI2 (solid lines).
Volume 15, Number 3, March 1981 329
2.5% (a= 0.025) transformation of CaC03 II(i) into CaS04, the surface becomes more than twice as large as that of the initial reactant. The dashed line in Figure 7 shows the evolution of the surface of a pure CaC03 sample a t 550 "C ( 4 ) . The starting calcium carbonate consists of well-developed rhombohedra with essentially smooth surfaces (Figure 8a). After a transformation into Cas04 of only 2.5% (Figure ab), a t a temperature of 495 "C, a deep alteration of the surfaces can be noted. This effect can be attributed to the swelling of the reaction products (the molar volumes of CaCOs and CaSOl are respectively 36.9 and 45.7 cm3) and to the evolution of COz, which presumably tears off the solid product layer. A similar aspect is observed for the 15% CaS04 sample where the original outlines of the rhombohedral particles are still apparent (Figure &); the replicas of that powder reveal small areas of grains which still present a smooth appearance (see arrows in Figure 8d), but such a texture has completely disappeared after complete conversion in CaS04 (Figure 8, e and 0. Furthermore, product growth ultimately deforms the particles beyond recognition of their original rhombohedral features. For the sake of comparison, a micrograph of the 23% Cas04 product obtained from pure CaC03 a t 570 "C, and corresponding to the maximum attainable conversion, is also presented (Figure 8g). In that case, the surface has a relatively less uneven relief. X-ray Analysis of the Samples. Some samples were examined by X-ray diffraction (Cu K a ) . The results are presented in Table I.
I ' I
.
! I
1
i
Discussion Reaction 2. The comparison of reaction rates in Figure 1 for reaction 2 shows that both the initial rate and the maximum absorption capacity of the solid are greatly increased by the addition of CaC12. At -490 "C, the only temperature where direct comparison is possible because of the large difference of reactivity, the initial rate is 7-8 times greater with the modified reactant. Moreover, the transformation of the impregnated sample gives 100%sulfation even helow 500 "C; such a degree of conversion cannot be obtained for the pure sample a t 600 "C (5). During the transformation, the surface area strongly diminishes in the absence of additive (Figure 2, dashed line), probably owing to a partial sintering of the sulfite platelets (Figure 3d), whereas it increases with the impregnated sulfite (Figure 2, solid line); in the latter case, the general aspect of reacted grains is considerably altered (Figure 3 4 . In our study of the reaction of pure Cas03 with 0 2 (5),we had concluded that the reaction became more and more blocked by the narrowing of the pores of the reactant and the formation of an impervious CaSO4 layer preventing oxygen from reacting with
I
1
' I
Table 1. -Pi.
1. 2.
3. 4. 5.
PO((UU.
CaSOs.2H2qi) after dehydration at 500 OC (vacuum) CaSOs(i) after partial reaction (a= 0.4) with O2 at 460 "C CaSOs(i)after complete reaction with O2 at 485 OC CaC03 Mi) CaCOs ll(i) after partial reaction (U = 0.15) with SO2 + 0.502
CaSO3
CaC03 ll(i) after complete reaction with SO2 0.502 at 495 'C
CaSO.
+
CaSO3 CaSO. CaSO.
+
Transmission electron micrographs (sample II)of replicas prepared from (a) pure calcium carbonate. (d) impregnated CaCO, reacted with SO2 0.502 until a 15 % CaSO, conversion at 495 OC, (1) impregnated CaC03 reacted with SO2 0.502 until a complete CaSO. conversion at 495 OC, and (9)pure CaC03reacted with SO2 + 0.502 until a 23% CaSO, conversion at 570 OC. Scanning electron micrographs of impregnated CaC03 II reacted with SO2 0.502 until (b) 2.5 % CaSO, conversion at 495 "C, (c) 15% CaSOI conversion at 495 'C, and (e) 100% CaSO, conversion at 495 OC. Flgure 8.
CaCOS CaC03
at 495 'C 6.
Obu.".d
330 Environmental Science 8 Technology
+ CaSO3 + CaSO.
+
+
+
the underlying unreacted CaS03. It seems that the promoting effect of the additive is essentially to bring about the formation of a more favorable, less compact texture, with Cas04 forming a much less impervious barrier. It may be imagined that the presence of the additive makes easier the recrystallization of the Cas04 product into a more open polycrystalline structure (Figure 9). This could be the result of a decrease of the rate of nucleation of new Cas04 crystallite, or of a more active crystal growth, or of the adoption, by the growing crystals, of a habit different from the habit that they have when pure. The difference of 3 kcal mol-l between thermal increments observed in initial kinetics for impregnated and pure samples does not seem significant. The value of 32-35 kcal mol-l probably corresponds to the real activation energy of the initial chemical reaction, Le., the uniform attack of the surface of the particles of Cas03 by 0 2 . Reactions 1 plus 2. General Kinetic Characteristics. The maximum rate observed in the case of impregnated CaC03 is at least 10-15 times greater than in the case of pure absorbents (compare dashed and solid lines in Figures 4 and 5 ) . In some cases (at the lower temperatures), the rate of reaction is increased by a factor of 30. The maximum absorption capacity of the absorbents is also greatly increased in the presence of CaCl2 additive (from 3 to. 7 times). A complete transformation of impregnated calcium carbonate can be obtained at a moderately high temperature (500 “C for CaC03 11). Figure 6 suggests that the rate limitation comes from the reaction involving 0 2 , namely, reaction 2. On the contrary, with pure samples, reaction 1was found to be rate limiting ( 4 ) . The easiest way to explain this modification is to admit that the speed of reaction 1 (absorption of SO2 by CaC03 and formation of CaS03) is so greatly increased in the presence of the additive that reaction 2 (absorption of 0 2 by Cas03 and formation of CaS04) becomes rate limiting. One can prove directly that reaction 2 cannot keep up the pace with reaction 1.An impregnated carbonate sample has been treated with a stoichiometric mixture of the gases (SO2 + 0.502; total pressure: 1atm) for 3 min at 500 “C. A pressure drop corresponding to a conversion of CaC03 into Cas04 of 8%was observed. The apparatus was then rapidly evacuated and pure oxygen introduced (0.33 atm). An important decrease of the pressure was noted, corresponding to a subsequent sulfation of the sample, until a transformation of 24% was attained. This confirms that reaction 2 becomes ratelimiting in impregnated samples; in the present case, reaction 1 was -3 times as rapid as reaction 2. Reaction 1being easy, it is not surprising that the specific surface area of the solid increases during reaction (Figure 7 ) . Reaction 1evolves a gas, namely, COZ, and, if it proceeds easily and rapidly, this effect may bring about an efficient tearing off of the layers of the reactant and, hence, the formation of an open porosity and some dispersion of the reaction product. We encounter more difficulties in interpreting the data on activation energy or thermal increment. The activation energy, in initial kinetics, of the reaction of pure CaC03 with SO2 + 0.502 amounted to 50 kcal mol-l(4). The thermal increment of the rate of reaction a t the inflection point of modified CaC03 is 16 kcal mol-l. It is not surprising that the values are different. The value of 50 kcal mol-l corresponds to reaction 1 (which is rate determining in pure samples) on CaC03, presumably covered with CaS04. The much lower value found with modified CaC03 is the complex result of reaction 1, proceeding a t a relatively high rate, and of the somewhat slower reaction 2. A simple hypothesis would be to assign an activation energy value of 16 kcal mol-l to reaction 2. On theoretical grounds, this is not justified, as reaction 1is more
(I=
0.1
cr.o.2
1
I
a=0
a-0.1
(IE0.2
E r r
coco,
CUSO,
COSO,
L
b
Figure 9. Schematic representation of the evolution of the reaction starting from pure (a)or modified (b)carbonate.
rapid only by a relatively modest factor (-3 a t 500 OC). This is no more justified in view of the measured activation energy for reactiod 2: 32-35 kcal mol-’. We have no explanation for this discrepancy. The problem of consecutive reactions, in heterogeneous kinetics, is much more complicated than in homogeneous kinetics. This is due, in general, to what can be called “coupling effects” (IO). Coupling effects, in the present case, could correspond to modifications, by reaction 1, of the texture of the produced CaS03, a modification which would obviously alter the kinetics of reaction 2. For the sake of illustration of the possible effects of the coupling effect, let us imagine, for example, that there is a decrease of the disperseness of Cas03 produced by reaction 1 according to a negative thermal incremefit of -16-19 kcal mol-l. The measured rate of reaction would then correspond to the difference between the true activation energy of reaction 2 (32-35 kcal mol-l) and this value, thus explaining an observed thermal increment of 16 kcal mol-l. g u t this explanation is far from being the only possible one. Although this kind of explanation, and especially the argument indicated in the previous paragraph, look quite plausible, we have no sufficient evidence for supporting them. For this reason, we shall continue the discussion in two directions. We shall first examine the data concerning texture, which obviously constitute a clue to the understanding of the mechanism of the involved reaction. Thereafter, we shall, in a more general perspective, present sotne speculative considerations on the role that the promoter may exert upon the reaction network. Textural Changes. Two facts deserve discussion, when considering the textural data reported in this paper. The first fact is the increase of surface area observed when CaC03 is contacted with SO2 o.502 (Figure 7 ) , and the associated changes in texture (Figure 8).Let us mention especially, in this respect, micrographs a and b of Figure 8, which represent respectively the nonreacted CaCOs(i) sample and the same sample partly converted into Cas04 (2.5%).This last micrograph thus corresponds to the very beginning of the formation
+
Volume 15, Number 3, March 1981 331
of sulfate (reaction 2). It already reveals a very important modification in the sample texture. In view of the fact that, in the presence of SO2 0.502, CaCOa(i) first reacts with SOz to give Cas03 (step 1)and that the rate of this step is relatively high as compared to reaction 2, Figure 8b actually represents a solid which contains a small quantity of Cas04 (2.5%),and a substantially larger quantity of Cas03 (37.5%). The formation of this intermediary product is clearly the cause of most of the characteristic swollen texture observed in Figure 8b. We have thus several direct proofs that reaction 1produces a highly dispersed CaS03. With respect to our former discussion, this certainly has much bearing on the kinetics of the “coupled” reactions 1 plus 2. The second fact concerns the comparison of treated and pure CaC03 samples. Figure 7 indicates that the tendency to get an increased surface area as the reaction progresses is rapidly counteracted by a strong decrease for a! > 0.25 in pure samples. The comparison of parts d and g of Figure 8 shows striking differences, although the modified (Figure 8d) and pure (Figure 8g) samples have been reacted to comparable extents (respectively, 15 and 23%). The modified sample keeps much open porosity (also clearly seen on the scanning micrographs, Figure 8c), whereas the pure sample exhibits scales, which seem to tightly cover the surface. Figure 9 is an attempt to represent schematically the two situations. The micrographs thus confirm and somewhat refine our interpretations. In pure CaC03 samples, a covering layer of CaS04 forms very soon. The activation energy of 50 kcal mol-l should thus correspond to reaction 1with CaC03 covered with a compact Cas04 layer. In the modified samples, reaction 1 with the continuously renewed surface of CaC03 forms a finely dispersed and porous Cas03 which ultimately transforms to a porous Cas04 layer. Possible Mechanism of Action of CaCla. As proposed, this last part of the discussion will deal with speculations and a few additional data concerning the possible mechanisms by which CaCl2 modifies the reaction network when CaC03 reacts with SO2 0 2 . We must examine the various causes which, in principle, might explain the observed phenomenon. These causes might be a chemical reaction of CaC12 with SO2 or with 0 2 , a chemical reaction of CaC12with CaC03 or with CaS03, or a physical action, e.g., on the nucleation rate, growth rate, or growth habitus of Cas03 or CaS04, or formation of a eutectic. We have made some attempts for determining whether CaClz was able to react with the gas phase. No modification of the pressure was observed at 500 “C. CaC12, after exposure to the gases, remained also unchanged.,We can therefore conclude that there is no interaction between pure CaCl2 and the gases used in our experiments. This is in agreement with the results of Derlyukova ( I I ) , who did not observe any chemical reaction between SO,, 0 2 , and CaC12 (20-850 “C), unless an oxidation catalyst was added. If a chemical reaction occurs between CaC12 and CaC03, e.g., during the absorption of SO2 and 0 2 , one should observe, for example, the emission of gaseous Cl2 and the disappearance of C1- in the solid phase. The titration for C1 ion in the impregnated CaC03 sample, before and after transformation into CaS04, shows that the initial quantity of the chloride ion remains constant in the solid phase. Another possibility would be that the evolution of CO2 from CaC03 could be facilitated in the presence of CaC12. We have checked that C02 is not evolved more rapidly in the presence of the additive.
+
+
332 Environmental Science & Technology
One could imagine that CaCl2 forms a eutectic phase with CaC03, CaS03, or CaS04. No such eutectic is mentioned in phase-diagram handbooks (12) for temperatures under 700 “C, although we have observed that CaCl2 is active down to temperatures lower than 400 “C. As the above-mentioned hypotheses do not seem to hold, it is probably reasonable to ascribe the effect to modification in the kinetics of nucleation of the new phases and/or of growth of the various faces of the Cas03 and Cas04 crystallites. This is a phenomenon which is currently observed in the presence of impurities, even when in minute quantities. As the specific surface area and the porosity of the surface undergo the most dramatic changes when CaClz is present, this looks to be a reasonable explanation.
Conclusion Although the specific mechanism of the action by CaC12 is not clearly understood, we can tentatively propose the following picture to explain why the overall reaction rate is increased and not blocked in the presence of this additive. Reaction 1 becomes sufficiently fast compared with reaction 2 (reaction 3 on the other hand becoming negligible) that it breaks and efficiently disperses the sulfite layer, which will eventually react further with 0 2 along reaction 2. Thus, easy access to unreacted CaC03 is maintained throughout the reaction. The oxidation of Cas03 is slower, but in stoichiometric mixtures it proceeds with a rate sufficient to allow a complete reaction to CaS04. This is achieved thanks to the fine dispersion of CaS03, when produced by modified CaC03. It seems that this dispersion is sufficient for the diminution of reactivity, due to the formation of covering Cas04 ( 5 ) ,not to prevent complete transformation. It would also appear that, in the delicate balance of the two consecutive reactions, it is essential that reaction 2 be slower than reaction 1.Presumably, the too easy oxidation of Cas03 in the untreated samples causes the broken layer of Cas03 coming from reaction 1to sinter too rapidly and brings about a blocking of the reaction, by the formation of a too compact layer of CaS04. Literature Cited (1) Van Houte, G.; Delmon, B. Chem.-1ng.-Tech. 1976,48,863. (2) Van Houte, G.; Maon, J. Cl.; Dumont, Ph.; Delmon, B. Actual. Chim. 1977,9,25.
(3) Van Houte, G.; Maon, J. Cl.; Dumont, Ph.; Delmon, B. J. Air Pollut. Control Assoc. 1978,28,1030. (4) Van Houte, G.; Delmon, B. J. Chem. SOC.,Faraday Trans. 1 1979, 75,1593. (5) Van Houte, G.; Delmon, B. Bull. SOC.Chim. Fr. 1978,11-12, 1-413. (6) Van Houte, G.; Delmon, B. Bull. Soc. Chim. Belg. 1978,87, 241.
( 7 ) Van Houte, G.; Delmon, B. Bull. SOC.Chim. Belg. 1979,88, 205.
(8) Tarradellas, J.; Bonnetain, L. Bull. SOC.Chim. Fr. 1973, 6,
1903. (9) Bradley, D. E. Br. J . Appl. Phys. 1959,10,198. (10) Boldyrev, V. V.; Bulens, M.; Delmon, B. “The Control of the Reactivity of Solids”; Elsevier: Amsterdam, 1979. (11) Derlyukova, L. E. Zh. Neorg. Khim. 1973,18,2341. (12) Levin, E. M.; Robbins, C. R.; McMurdie, H. F. “Phase Diagrams for Ceramists”, 1969 Supplement; The American Ceramics Society: Columbus, OH, 1969. Received for review June 10,1980.Accepted October 14,1980.
