Ind. Eng. Chem. Res. 1990,29, 2118-2123
2118
Inoue, K.; Kawano, Y.; Nakashio, F.; Sakai, W. Extraction Equilibrium of Hydrochloric Acid by Long-chain Alkylamine Solution. Kagaku Kogaku 1974,38, 41-46. Inoue, K.; Tomita, S.; Maruuchi, T. Extraction Kinetics of Nickel with a Hydroxyoxime Extractaat. J . Chem. Eng. Jpn. 1985,18, 445-449. Inoue, K.; Baba, Y.; Yoshizuka, K.; Oka, T. The Solvent Extraction of Palladium(I1) from Aqueous Chloride Media with 7-Tridecanone Oxime. Bull. Chem. SOC.J p n . 1988, 61, 803-807. Ma, E.; Freiser, H. Mechanistic Studies on the Extraction of PallaOxime (LIX65N). dium(I1) with 2-Hydroxy-5-nonylbenzophenone Extr. Ion Exch. 1983, 1, 485-496.
Ma, E.; Freiser, H. Solvent Extraction Equilibria and Kinetics in the Palladium(I1)-Hydrochloric Acid-7-( l-Vinyl-3,3,5,5-tetramethylhexyl)-8-quinolinol System. Inorg. Chem. 1984,23, 3344-3347. Rund, J. V. Kinetics of the Reactions of Tetrahalo Complexes of Palladium(I1) and Platinum(I1) with 1,lO-Phenanthroline. Inorg. Chem. 1971, 13, 738-740. Szymanowski, J. The Hydrophilic Lipophilic Balance of Hydroxyoximes and the Mechanism of Copper Extraction. Polyhedron 1985, 4 , 269.
Received for review December 15, 1989 Accepted May 22, 1990
Sintering and Sulfation of Calcium Silicate-Calcium Aluminate R. H. Borgwardt Air and Energy Engineering Research Laboratory, U.S. Environmental Protection Agency, Research Triangle Park, North Carolina 2771 1
G. T. Rochelle* Department of Chemical Engineering, The University of Texas at Austin, Austin, Texas 78712
The reactivity of a solid at high temperature can be greatly retarded by physical changes occurring in its pore structure due to sintering. The nature of that effect was studied with calcium silicatecalcium aluminate reacting with SO2 between 665 and 800 "C, where sintering of this material progresses rapidly. The isothermal kinetics of the sintering and sulfation processes were measured independently as a function of the specific surface area of the solid. The rates of both processes were increased by the presence of water vapor in the gas as well as by higher temperatures. A combined sinter/sulfation model, based on the parameters evaluated independently, is in qualitative agreement with sulfation rates observed when both processes occur simultaneously.
Introduction An earlier study that evaluated the sulfation kinetics of CaO (Borgwardt and Bruce, 1986) showed the rate to increase rapidly with both the temperature and the specific surface area of the solid. At high temperatures, the surface area begins to decline because of sintering, a process caused by coalescence of the CaO micrograins that comprise the porous particles. The rate of surface reduction of CaO was shown (Borgwardt, 1989a) to follow the German-Munir (1976) sintering model given by
where So is the nascent surface area of the CaO existing immediately after thermal decomposition of the precursor compound (e.g., Ca(OH),], S is the specific surface area at time t , y is a mechanism-dependent constant, and k is a function of temperature. In many practical applications, H 2 0 or C 0 2 will be present during sintering, both of which catalyze the surface area reduction of CaO (Anderson et al., 1965; Beruto et al., 1984). Equation 1 can be modified to include the catalytic effects of HzO and C02 on the singering of CaO by empirical correlations of y and k in terms of partial pressure and temperature (Borgwardt, 1989b). The sulfation behavior of small CaO particles at high temperatures is explained by the combined sintering and sulfation kinetics when the surface area effects are accounted for (Borgwardt, 1989b). Prior investigations of the sulfation of calcium silicate (Rochelle et al., 1987) have shown it to be an effective sorbent for flue gas desulfurization at 50-100 "C, especially at high relative humidity. It, might also be used in con-
junction with bag filters at 300-500 "C (Chu and Downs, 1989) and in fluidized beds at 800-950 OC (Yang and Shen, 1979). The study reported here is aimed at an evaluation of the sintering of calcium silicate-calcium aluminate (CSA) as it affects the reactivity of that material with SOz when both processes occur simultaneously. The approach follows the method previously used for evaluating these effects on the CaO reaction. That method assumes three important steps are involved, each of which can be evaluated by independent measurement: (i) thermal decomposition of the solid to produce a nascent surface of maximum specific area, (ii) reduction of the surface area, by sintering, at a rate dependent on temperature and the gas-phase concentrations of HzO and COP,and (iii) reaction of the solid with SO2 at a rate dependent on its specific surface area and temperature.
Experimental Section The CSA consisted of the product formed by the reaction of calcium hydroxide with the silica and alumina in fly ash when 1 part Ca(OH), is slurried with 3 parts fly ash (Clinch River) for 12 h at 90 "C as described by Jozewicz and Chang (1989). Scanning electron micrographs suggest that the hydrated CSA is deposited as a porous layer on the surface of the excess fly ash. Analysis of the dry product showed 13.1 wt 70calcium, a BET surface area of 16.0 m2/g, a pore volume of 0.103 cm3/g, and a true density of 2.62 g/cm3. Additional work with related CSA reagents suggests that silicate, rather than aluminate, is responsible for the surface area and reactivity to SOp (Peterson and Rochelle, 1990). X-ray diffraction of CSA was inconclusive but demonstrated the absence of Ca(OH), and other crystalline phases, suggesting that CSA is mostly amorphous. Se-
0888-5885/90/2629-2118$02.50/0 0 1990 American Chemical Society
Ind. Eng. Chem. Res., Vol. 29, No. 10, 1990 2119 lective dissolution of CSA in 0.1 M HC1 indicated that the reaction product had about 0.5 mol of &/mol of Ca (Peterson and Rochelle, 1988). The reactivity of the identical sample was studied by White (1989) a t 50-100 "C. No characterization of CSA was performed after calculation or after its reaction with SO2. Calcination and Sintering Procedure for Surface Area Measurement. Two differential reactors (Borgwardt et al., 1986) were used, one of them dedicated to calcination and the other to sulfation, which was usually carried out at another temperature. A 10-g reactor sample tube, made of quartz and containing a quartz wool plug, was weighed and the CSA sample (normally 250 mg) placed on the wool as a thin layer. The tube was reweighed and placed in the ground-glassjoint of the reactor exhaust tube and the assembly inserted in the calcination reactor, normally a t 665 "C, with nitrogen flowing through the reactor a t 17 L/min. With the exhaust tube sealed into the reactor, calcination continued for 2 min, after which the nitrogen was turned off, the sample assembly removed, and the sample tube placed under flowing nitrogen to cool for 30 min. The Calcine was transferred to a BET flask and the calcination process repeated to obtain a composite sample of 700 mg. The surface area was measured by nitrogen adsorption using the multipoint BET method at 77 K. To determine the porosity, nitrogen adsorption was continued to a relative pressure of 0.99 to obtain the total volume of pores up to 0.19-Nm diameter according to the Kelvin equation. Comparisons were made with 25-mg samples that were dispersed in the quartz wool prior to calcination and sintering to ensure that the resulting surface areas were not affected by sample size. The small samples required 14 runs for each BET analysis followed by HC1 extraction and EDTA titration (to establish CSA content). The comparisons, made at 665 and 800 "C, verified that the use of 250-mg samples gave valid results and that procedure was used for all subsequent sintering measurements. Nitrogen of required purity was obtained from a liquid N2 tank. The N2 gas from this tank was fed to the sintering reactor in one of four modes: (i) as pure N,, (ii) as a mixture with C02gas, (iii) as a mixture with water vapor, (iv) as a mixture with both C02 and water vapor. The nitrogen was humidified by passing it through two water flasks in series suspended in a constant-temperature bath. The bath temperature was set to deliver 7 vol 70 H 2 0 in the reactor feed at 6 L/min N2 flow rate, as measured by gravimetric absorption. C 0 2 was added after humidification to a concentration of 12 vol %. These concentrations were selected as representative of the CO, and H 2 0 content of combustion gases from a coal-fired furnace. Sulfation Measurements. A sample of 82 mg of CSA, dispersed in quartz wool, was calcined and sintered in the calcination reactor and transferred to the sulfation reactor a t the required temperature. The calcination time and temperature were selected according to the conditions established in the above experiments that provide a calcine of known surface area. Those conditions were also chosen to minimize sintering during sulfation. A 3-min period of temperature equilibration was provided in the sulfation reactor during which pure N2was fed at 6 L/min. SO, feed commenced at t = 0 at a point between the humidifier and reactor inlet. The source of SO2 gas was a cylinder of the pure liquid. It was blended with Nz and air to yield a reactor feed containing 4600 ppm SO2 and 5% O2 and continued until time = t , when all feeds were terminated, and the sample was immediately withdrawn from the reactor and cooled under flowing N2. The reactor was
DEHYDRATION WT LOSS AT INDICATED TEMP,
25r
o
t 100l
0
1.0
2.3
I
4,2 6.0
I
I
7.1 7.6 8.0 a5 I
I
I
I
0
200 300 LOO 500 600 700 800 903 CALCINATION TEMPERATURE, OC
Figure 1. Specific surface area and pore volume of calcium silicate-calcium aluminate as a function of calcination temperature. Samples calcined 8 min in dry nitrogen.
flushed with N2 prior to inserting the next sample for exposure at a different value oft. A conversion versus time curve was thus established from a series of exposures at a given temperature and a fixed CSA surface area. In all sulfation tests, the reactor feed contained 5% O2 added as air prior to humidification. Analytical Section. Because sulfation was measured over a range of temperatures at which both CaSO, and CaS03can be formed, several analyses were necessary on the exposed solids. These included weight gain, calcium content, sulfate content, and sulfite content. Calcium, determined by atomic absorption spectrometry and EDTA titration, established the amount of calcine recovered. Total sulfur, determined as sulfate by ion chromatography (IC) after oxidation with H202,established the conversion as the ratio of S/Ca. Sulfite was measured separately by dissolving exposed reactor samples in standard iodine solution followed by back-titration with standard arsenite. The weight gain measurements provided a check on the other data. They were corrected for entrainment losses of 0.2-1.5 mg, depending on time and temperature, which were measured in separate calibration tests.
Results Evaluation of So. The loss of water from the raw sorbent by thermal decomposition in flowing nitrogen increased the surface area and porosity as indicated in Figure 1. At a calcination temperature of 665 "C, the surface area reached a maximum of 20 m2/g, which is taken as the value of So. The weight loss at 665 "C was 7.4%, corresponding to 1.2 mol of H20/mol of Ca. Although dehydration continued to increase with temperature to 1000 "C, both surface area and porosity declined sharply above 700 "C, revealing the onset of sintering. Sintering was complete at 850 "C. Sintering Kinetics. The rate of surface reduction was measured as a function of temperature in atmosphere containing 12% C02and/or 7% H20. In all cases, nascent CSA with an initial surface area (So)of 20 m2/g was prepared by calcination in pure N2 at 665 "C for 2 min. This calcine was transferred to the second reactor for sintering,
2120 Ind. Eng. Chem. Res., Vol. 29, No. 10, 1990 TEMPERATURE *C
oo7
BOO
750
700 665
i
0 6
&
1
101
l
I
$ 1 a m
Symbol
Sinter temp., O c
i
oooogzo
Curve parameters
7
'
'
'
k
'
'
IO 0
'
'
'
1
110
L X I O ' , K-'
i
T
Figure 4. Correlation of sintering parameter k from data of Figures 2 and 3.
0
4
8
12
16
20
24
28
32
36
SINTER TIME, minutes Figure 2. Surface reduction of calcium silicate-calcium aluminate by sintering at low temperatures. Closed symbols = 7% H,O in sinter atmosphere; open symbols = 7% H20 + 12% COz, balance N2
" CT
tu
k
7=
w
U
a n.
0
sz
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Symbol
Sinter Temp, "C
Curve parameters
IL
exp(0.01I T - 9.50)
n -
1
20 e
I
750
800
I
SINTER TEMPERATURE, 'C Figure 5. Correlation of sintering parameter y from data of Figures 2 and 3.
750 10.0
I
I
700
650
0 028
I
1
0
I
2
I
4
I
I
8
I
10
I
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12 SINTER TIME, minutes 6
14
16
I 18
Figure 3. Surface reduction of calcium silicate-calcium aluminate by sintering a t high temperatures. Closed symbols = 7% HzO in sinter atmosphere; open symbols = 7% H20 + 12% COz, balance NP.
usually at a different temperature. A series of calcination and sintering runs at various temperatures and sintering atmospheres resulted in the data shown in Figures 2 and 3. It is evident that the rate of surface reduction was dominated by H20,with no apparent effect when C02was added to the sinter atmosphere. The curves shown in Figures 2 and 3 are eq 1,fitted to yield the minimum total error. The sinter parameters, k and y, required for the best fit are also shown. The same empirical procedure was used to correlate these parameters with temperature as in prior studies of catalyzed CaO sintering. The results, shown in Figures 4 and 5, yield a
lot
i
/o
i;' I
oL
5
15
IO TiUE
t
20
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-?der
Figure 6. Sulfation of calcined calcium silicate-calcium aluminate at 665 OC with 4600 ppm SOz + 5% O2 Balance of sulfation atmosphere is N2 for lower curve; N, + 7% H 2 0 for upper curve. S = 20 m2/g.
satisfactory representation of the CSA sintering parameters a t constant H20 partial pressure with k = exp(23.6 - 29270/T) (2) y = exp(O.0112' - 9.496) (3) Equations 1-3 reproduce the isothermal sintering data with an average error of 0.5 m2/g. Sulfation of Calcines without Sintering. Figure 6 shows sulfation as a function of time for CSA that was
Ind. Eng. Chem. Res., Vol. 29, No. 10, 1990 2121 TEMPERATURE " C
1
1
t u 10 11 12 13 14
oooo19
'L 0
5
IO
15
20
-
TIME minutes
Figure 7. Sulfation of calcined calcium silicate-calcium aluminate with 4600 ppm SOz + 5% Oz, balance Nz. Curves are eq 4 with indicated k, values for best fit.
precalcined at 665 "C to its maximum surface area of 20 m2/g and then exposed at the same temperature to 4600 ppm SOz and 5% Oz in nitrogen. The lower curve was obtained without water vapor in the feed gas; the tests shown in the upper curve contained 7% H20. At this temperature, the effect of water vapor on sintering is negligible. Two significant conclusions are drawn from Figure 6. First, both curves follow the characteristic response of product layer diffusion, given for spherical particles (Levenspiel, 1962) by 1 - 3(1 - x ) 2 /+3 2(1 = kdt (4)
x)
The curves are quite similar to those found for the sulfation of CaO (Borgwardt and Bruce, 1986) where alternative X versus t responses predicted by other assumed mechanisms were shown to yield correlations inferior to eq 4. The second conclusion is that the presence of water vapor increases the sulfation rate when S is constant. A t 665 "C, this enhancement of product layer diffusion is indicated by the ratio of kd values to be 0.00432/0.00287 = 1.5. To establish the independent effect of temperature on sulfation, additional measurements were made without water vapor using calcined CSA and a series of sulfation temperatures at which the preestablished surface area would not be rapidly changed by sintering. The results are shown in Figure 7. Equation 4 was fitted to the data at each temperature to obtain the best estimate of kd. The principal cause of deviation at high temperatures is attributed to sintering effects that become significant at the longest exposure times, and those data were excluded from the curve-fitting procedure. At the low temperatures, deviations were caused by a competing reaction to form sulfite during the initial stage of reaction, which will be discussed below. In Figure 8, the kd values are plotted as a function of temperature. The k d values are normalized to 20 m2/g surface area by the factor 2OZ/S2,which assumes product-layer diffusion control. Two lines are required to fit the data of Figure 8. The inflection is interpreted to indicate a change in mechanism from diffusion through a product of CaSO, at the higher
&IO: T
K-'
Figure 8. Correlation of sulfation rate data, normalized to 20 m2/g surface area. Open symbols = CaSO, product; closed symbols = CaS04 + CaS03 product.
temperatures to diffusion through a mixture of CaS03and CaSO, a t the lower temperatures. Chemical analyses of the reaction product showed only sulfate present above 665 "C. The product formed at 665 "C contained 4 mol % sulfite, and a t 500 "C, it contained 14 mol %. From the slope of the high-temperature line, the apparent activation energy is estimated to be about 35 kcal/mol, which compares closely with the value of 36.6 kcal/mol obtained for CaO sulfation (Borgwardt and Bruce, 1986) under product-layer diffusion control. From Figure 8, the following correlations are obtained:
hd =
S2 400
kd
exp(-17680/T
S2 400
+ 13.08)
= - exp(-11400/T
above 665 "C
+ 6.38)
(5)
below 665 "C (6)
The inflection indicated by eqs 5 and 6 at 665 "C, which signifies a reduced activation energy at the lower temperatures, is qualitatively consistent with the CaO/S02 reaction noted by Tremple (1989), which showed an activation energy of only 10 kcal/mol when pure CaS03 is formed below 700 "C. Simultaneous Sintering and Sulfation. A final series of experiments was performed in which uncalcined CSA samples were exposed to SO2and water vapor at varying temperatures. In this case, the hydrate decomposed to form nascent CSA followed by simultaneous sintering and sulfation. The objective was to compare the sulfation responses with those expected from eqs 4-6 using values of S calculated from the sinter model. The comparison is made in Figure 9. The curves shown in Figure 9 were calculated at a given temperature by using eqs 1-3 to determine S at each time increment. The value of kd was obtained from eq 5 or eq 6 for the argument S , and the conversion X was calculated from eq 4 as reformulated by Ruthven (1984)
The calculations require an estimate of the sulfation enhancement factor, which was shown in Figure 6 to be 1.5
2122 Ind. Eng. Chem. Res., Vol. 29, No. 10, 1990
and Squires (1972) showed that C02greatly accelerates the reaction of H2S with CaC03, and other work has shown that C 0 2 catalyzes the sintering of CaC03. It should also be noted here that water vapor has been shown to increase the sulfation rate of CuO a t 400 "C (Hedges et al., 1985). Possible relationships between these effects and the mechanisms by which they occur should be a fruitful area of future research.
Nomenclature
TIME m W e i
Figure 9. Comparison of sinter/sulfation model with measured sulfation rates of uncalcined calcium silicate-calcium aluminate. balance N,. Reactor feed = 7% water vapor, 4600 ppm SOz, 5% 02,
a t 665 "C. This value was assumed to apply to all temperatures involving sulfite formation. The enhancement factor at higher temperatures was estimated to be 5.0 by fitting the model to the 775 "C data, and that value was assumed characteristic of the sulfate product. The correlation was not improved by assuming either factor to be temperature dependent within the range of temperatures studied.
Discussion The above calculations assume that sintering proceeds at a rate unaffected by the growth of the product layer. This was tested by comparisons with calculations based on the alternative assumption: that sintering ceases when sulfation reaches some given value of conversion. In no case was the agreement as good as in Figure 8, even though both the enhancement factor and the sinter cutoff value were treated as freely adjustable parameters. The observed effect of water vapor on CSA sintering is similar to that found in the prior study of CaO (Borgwardt, 198913). Contrary to CaO, however, CSA sintering was not affected by C02. The reason appears to be related to the difference in reactivity of C02 with the two solids. Experiments in which they were exposed to 12% C 0 2 at temperatures where CaO could be completely recarbonated gave no detectable reaction with CSA-nor was reaction evident at any other temperature. An affinity for reaction at low temperatures appears to be a requirement for the catalysis of sintering, as might be expected, even though sintering occurs above the equilibrium temperature at which a stable product can be formed. Enhancement of the sulfation of reagent-grade monocalcium silicate by water vapor was reported by Yang and Shen (1979). Although the mechanism by which it occurs is unknown, a similar effect is found for CSA. In view of the well-established effect of water vapor on sintering-an effect that requires increased mobility of the ionic constituents of the solid-it is not unreasonable that a similar effect on ionic mobility through the product layer might occur. The results suggest that the two effects are probably related. It is known that salts containing aliovalent cations (valence +1 or +3 in a calcium oxide or calcium sulfate lattice) can accelerate both sintering and sulfation and that both effects can be explained by the contribution of such ions to the defect structure that mediate solid-state diffusion. A more subtle process is apparently required to explain the similar effects of gas-phase constituents. Ruth
k = sintering rate parameter, defined by eqs 1 and 2 for CSA exposed to 7 % water vapor, min-* kd = rate constant for product layer diffusion, defined by eq 4, min-' S = specific surface area of solid, m2/g So= initial specific surface area following dehydration of the solid, m2/g t = exposure time of solids in the reactor, min X = conversion, mole ratio S/Ca in reacting solid at time t y = sintering parameter defined by eqs 1 and 3 for CSAc exposed to 7 % water vapor, min-' Registry No. SOz, 7446-09-5; calcium silicate, 1344-95-2; calcium aluminate, 11104-48-6.
Literature Cited Anderson, P. J.; Horlock, R. F.; Avery, R. G. Some Effects of Water Vapor During the Preparation and Calcination of Oxide Powders. Proc. Br. Ceram. SOC. 1965,3, 33-42. Beruto, D.; Barco, L.; Searcy, A. W. C02-Catalyzed Surface Area and Porosity Changes in High-Surface-Area CaO Aggregates. J . Am. 1984,67, 512-515. Ceram. SOC. Borgwardt, R. H. Sintering of Nascent CaO. Chem. Eng. Sci. 1989a, 44, 53-60. Borgwardt, R. H. Calcium Oxide Sintering in Atmospheres Containing Water and Carbon Dioxide. Ind. Eng. Chem. Res. 1989b, 28,493-500. Borgwardt, R. H.; Bruce, K. R. Effect of Specific Surface Area on the Reactivity of CaO with SOz. AIChE J. 1986,32, 239-246. Borgwardt, R. H.; Roache, N. F.; Bruce, K. R. Method for Variation of Grain Size in Studies of Gas-Solid Reactions Involving CaO. Ind. Eng. Chem. Fundam. 1986,25, 165-169. Chu, P.; Downs, A. R. Sorbent and Ammonia Injection a t Economizer Temperatures Upstream of a High-Temperature Baghouse. Presented at AIChE National Meeting, Houston, TX; April 11, 1989. German, R. M.; Munir, Z. A. Surface Area Reduction During Isothermal Sintering. J . Am. Ceram. SOC. 1976,59, 379-383. Hedges, S. W.; Diffenbach, R. A.; Fauth, D. J. Properties of CuO Sorbents Related to SOz Reactivity. Presented a t AIChE National Meeting, Chicago, IL, Nov 1985. Jozewicz, W.; Chang, J. C. S. Evaluation of FGD Dry Injection Sorbents and Additives, Volume 1: Development of High Reactivity Sorbents. Final report by Acurex Corp. for the US.Environmental Protection Agency, EPA-600/7-89-006a (NTISNo. P B 89-208920), May 1989. Levenspiel, 0. Noncatalytic Fluid-Solid Reactions. Chemical Reaction Engineering; Wiley: New York, 1962; pp 346-348. Peterson, J. R.; Rochelle, G. T. Production of Lime/Fly Ash and Absorbents for Flue Gas Desulfurization. Presented at the First Combined FGD and Dry SO2Control Symposium, St. Louis, MO, 1988; Paper 9A-1. Peterson, J. R.; Rochelle, G. T. Lime/Fly Ash Materials for Flue Gas Desulfurization: Effects of Aluminum and Recycle Materials. Presented at the 1990 SOz Control Symposium, New Orleans, LA, May 8-11, 1990. Rochelle, G. T.; Chu, P.; Peterson, J. R. SOz and NO, Removal from FLue Gas by Reaction with Solids Prepared by Slurrying Fly Ash and Ca(OH),. Final report, U.S.Department of Energy, Grant NO. DE-FG22-85PC81006, 1987. Ruth, L. A,; Squires, A. M. Desulfurization of Fuels with Half-Calcined Dolomite: First Kinetic Data. Enuiron. Sci. Technol. 1972, 12, 1009-1014. Ruthven, D. M. Macropore Diffusion Control. Principles of Adsorption and Adsorption Processes: Wiley: New York, 1984; pp 181-182.
2123
Znd. Eng. Chem. Res. 1990,29, 2123-2128 Trempel, D. P.; Rochelle, G. T. Reaction of SO2with Ca(OH)z and CaO a t 100 to 800 “C. Presented a t AIChE National Meeting, Philadelphia, PA, Aug 1989. White, W. G. Differential Reaction of SOz in Flue Gas with LimeBased Sorbents at 66 OC for 10 to 7200 Seconds. M.S. Thesis, The University of Texas a t Austin, 1989.
Yang, R. T.; Shen, M. S. Calcium Silicate: A New Class of Highly Regenerative Sorbents for Hot Gas Desulfurization. AZChE J. 1979,25,811-819.
Received for review December 27, 1989 Accepted J u n e 11, 1990
Penetration Wetting of Asbestos Insulation Products Stephen K. Brown* CSIRO Division of Building, Construction and Engineering, P.O.Box 56, Highett, Victoria 3190, Australia
Factors controlling the penetration wetting of several “model“ asbestos insulation products have been determined by experiment in order to optimize the wetting of building insulation products where in asbestos abatement activities. In general, penetration height = constant(y/17)-’/2t~1/2, y and 17 are the surface tension and viscosity of the liquid and t is time of penetration, similar to previous equations for capillary rise. Water exhibited slower penetration than expected from its 7/17value. Addition of surfactant t o the water caused no change in penetration wetting in most cases, reduction in several cases (as predicted from the above equation), and an increase with a few surfactants and the insulation most difficult to wet. Increasing the 7/17value of water by heating to 60 ‘C provided the most consistent method to (moderately) improve penetration wetting.
Introduction Asbestos insulation is a generic term for a diverse group of insulation products widely installed in buildings, ships, and industrial equipment until the late 1970s. They are based on different types of asbestos (e.g., chrysotile, amosite, or crocidolite) and binders (cement, plaster, or calcium silicate) and exhibit different physical and mechanical properties appropriate to end application. Much interest is now being given to assessment and abatement of health hazards from these products. It has been estimated that in the US.30 million tons of asbestos has been used in construction since 1900 and that it would cost $51 000 million to abate (remove or seal) friable asbestos products in the 733 000 public and commerical buildings where they are installed, although it is expected that abatement will be necessary only in a small proportion of cases (Krizan, 1988). Removal of asbestos insulation products from buildings, ships, and industrial equipment is a hazardous operation due to the high asbestos dust concentrations that may be generated. Stringent removal codes are enforced in most countries to ensure that dust emissions are contained and that removal contractors and building occupants are protected from asbestos exposure. These codes usually specify that insulation products be prewet during removal since this is known to markedly suppress dust emission, c.f., dry removal (Sawyer, 1977; Cross, 1976). Dust suppression is considered most effective when wetting is thorough since dry patches of insulation are not encountered. It has been reported in one case that the addition of a nonionic surfactant led to more rapid and thorough wetting and as a consequence improved dust suppression above that found with water alone (Sawyer, 1977). Use of this surfactant has now become part of the US.Environmental Protection Agency’s “amended water” methodology for asbestos stripping (Sawyer et al., 1985) and other surfactants and stripping aids have become commercially available.
