Ind. Eng. Chem. Res. 2000, 39, 335-341
335
Fixation of Lead Chloride on Kaolinite and Bentonite at Temperatures between 550 and 950 °C Jie Wang and Takayuki Takarada* Biological and Chemical Engineering Department, Gunma University, 1-5-1 Tenjin-cho, Kiryu, Gunma 376-8515, Japan
Vaporization of lead chloride (PbCl2) on sorbents was carried out at linearly rising temperatures in flowing nitrogen with the use of a thermogravimetric apparatus. A modeling method has been proposed to depict the vaporization rate of lead chloride and to calculate the fraction of lead chloride fixed on sorbents during heat-up. The results revealed that dehydrated kaolinite (metakaolinite) had a moderate ability to fix lead chloride while fresh kaolinite showed a significantly increased fixation ability. The lead component formed by reaction with kaolinite or dehydrated kaolinite could hardly volatilize, even at a temperature up to 1200 °C. Bentonite also showed the moderate effectiveness for capturing lead chloride. However, at least part of the components or reaction products adsorbed on bentonite appeared to be thermally unstable and could volatilize gradually with a prolonged heating time at elevated temperatures. It was seen that during dehydroxylation of kaolinite, part of the chloride was released in the form of hydrochloric acid. Chemically bound water in kaolinite appeared to dramatically promote its reaction with lead chloride. Introduction
Table 1. Chemical Composition of Kaolinite and Bentonite Samples
Legislation has widely been made for the control of emissions of toxic metals in waste incineration plants. The U.S. Clean Air Act 1990 amendment also identified 11 potentially hazardous trace elements commonly found in coal, which have potential hazards to human health related to their emissions in coal-fired power stations. Lead is one of many volatile metals which may be emitted from flue gas as tiny particulates to cause air pollution in waste incineration and coal combustion. In waste incineration, lead chloride, a highly volatile and toxic form, is commonly formed because many municipal and industrial wastes contain both lead and chloride in it. Even if particulates are effectively collected as a solid residue, lead compounds in the residue may have potential adverse impacts on the aquatic and land environments when the residue is landfilled, depending on the form and stability of lead compounds in it. It is generally assumed,1 in coal combustion, that a fraction of mineral matter in coal serves a host at capturing various potentially toxic trace elements, although the form of lead vapor is unclear. Previous research efforts showed that lead was concentrated in submicron fly ash.2 Further, Querol et al.3 fractionated the fly ash by density and magnetic separation and found that lead was enriched in aluminosilicates; aluminosilicates may have a strong affinity to lead compounds by the chemical interaction and thus exhibit a preferential lead capture over other ash species. However, very little has been known concerning the chemical reaction of lead compounds and minerals in coal. A few previous studies4-6 have been involved in using aluminosilicate-type materials as the scavengers of leadcontaining particulates generated in the incineration of wastes. Ho et al.4,5 investigated the in situ lead retention * To whom correspondence should be addressed. E-mail:
[email protected].
chemical composition (wt %) sample
SiO2 Al2O3 Fe2O3 CaO MgO K2O Na2O TiO2 ig. loss
kaolinite 44.2 bentonite 65.4 a
38.6 14.6
0.6 2.5
Ignition loss at 600 °C
0.01 0.00 1.5 2.0 b
0.1 0.7
1.7 2.7
14.5a 10.5b
Ignition loss at 1100 °C.
by injecting sorbents into a fluidized-bed combustor. Experimentally, a piece of wood was permeated by a solution of lead chloride and the mixture was employed as a substitute of waste. Their results indicated that lead vapor could be captured effectively by sorbents such as sand, alumina, and bentonite. Uberoi et al.6 screened several sorbents for the removal of lead chloride from hot flue gas. Kaolinite and bauxite were found to be the most effective sorbents. Silica and emathlite also had a marked adsorbing effect. In the paper, we have examined the ability of some mineral sorbents for the capture of lead chloride. The minerals used include kaolinite, bentonite, and quartz, all of which are generally the major minerals present in coal.7 It should be noted that bentonite is not ordinarily identified in coal but it constitutes the main composition of montmorillonite, a mineral phase commonly present in coal. A straightforward experimental method combined with a simple modeling method have been proposed in the present study. No attempt has been made to simulate the complex combustion conditions. The study aims at obtaining some underlying information about the reaction between lead chloride and the minerals. Experimental Section Materials. The samples of kaolinite and bentonite were provided by Technical Laboratory of Okutama Kogyo Co., Ltd. The compositions of as-received samples of kaolinite and bentonite are listed in Table 1. Com-
10.1021/ie9905097 CCC: $19.00 © 2000 American Chemical Society Published on Web 01/08/2000
336
Ind. Eng. Chem. Res., Vol. 39, No. 2, 2000
pared with kaolinite, bentonite had a high SiO2/Al2O3 ratio. Throughout this work bentonite was heat-treated at 600 °C for 1 h and then used as a sorbent. During heat treatment the weight loss was 6.5%. Kaolinite was dehydrated in a similar manner and then used as a sorbent in part of the experiments. The interior water of kaolinite was wholly removed upon this heat treatment. Quartz sand, lead chloride (99% PbCl2), and hematite, of reagent grade, were purchased from Wako Chemicals Co. in Japan. The particle size of the sorbent samples was measured by using a laser micron sizer, LMS-30. Particle diameters ranged between 1 and 100 µm, between 1 and 40 µm, and between 0.8 and 8 µm, respectively, for quartz, bentonite, and kaolinite, with the specific outer surface areas estimated from the particle size distribution of 0.21, 1.00, and 1.61 m2/g in the sequence. Thermogravimetric Analysis. The main part of the experiment in this study was the heat treatment of the mixtures of lead chloride and the sorbent on a thermogravimetric apparatus, TGD-9600 (Sinko Riko Co.). The sorbent was mixed with a proportion of lead chloride in agate mortar in such a way as to result in the uniform dispersion of lead chloride on sorbents but not to cause a salient alteration in the sorbent particle size. Each heat treatment used about 100 mg of the mixture which was held in a platinum cylindrical holder (diameter, 8 mm; height, 10 mm). The thermocouple tip contacted a spot inside the sample. Prior to pressing the power button to on, the reaction atmosphere with a well-sealed system was evacuated and then purged by a nitrogen gas (purity, 99.98%). In this work, each sample was treated at a heating rate of 1.667 °C/s unless stated otherwise, kept isothermal for a predetermined time, and then cooled with the power off. It took about 4 min for the temperature to drop from 950 to 500 °C. The experiment was already conducted to examine the effect of the heating rate (from 0.83 to 5 °C/s) on the vaporization of lead chloride and the fixation of lead chloride on sorbents (not shown here). Weight loss of the sample and its differential form versus heating time or temperature were recorded by the TG apparatus. From these data, the vaporization rate of lead chloride was obtained by considering the dehydration of the sorbent. The vaporization rate depends not only on the chemical interaction between lead chloride and the sorbent but also on the physical factors in operation which affect the mass transfer. We put forward a vaporization model to calculate the lead capture fraction during the course of heat treatment, which may be difficult to directly determine when some lead chloride remains in the mixture (see below). XRF and XRD Analyses. Upon the completion of heat treatment, the lead and chloride contents of mixtures were determined by energy-dispersive X-ray fluorescence spectroscopy (XRF) with the use of a Rayny EDX-700 apparatus. A powdery glassy sample of the lead and chloride contents was used as a standard sample. The energy-dispersive XRF method for the analysis of trace elements in both coal ash and whole coal was documented in the literature.8 To enhance the sensitivity of chloride measurement, the sample was exposed in the vacuum; however, the spectra of chloride was still weak. Parallel measurements showed that the variations of chloride concentration detected were up to 10%. The spectra of lead were very strong within the range of lead concentrations used.
Room-temperature X-ray diffraction (XRD) and hightemperature X-ray diffraction (HT-XRD) were used to identify the products formed from the reaction between the sorbent and lead chloride. Analyses were implemented on a M03XHF22 diffractometer using Cu Ka radiation at 40 kV and 30 mA with the scanning speed of 1°/min. In the case of the HT-XRD analysis, the sample was mounted on a corundum plate and heated in flowing nitrogen. None of the peaks were reflected from plate materials. Theoretical Calculation Vaporization of Lead Chloride. The melting point of lead chloride is 501 °C and its saturated vapor pressure is given by4
ln P* (mmHg) )
-8819.8 + 15.911 T (°C)
(1)
Simplified hypothesis is made that lead chloride evaporates via two-stepwise mass transfer upon heat treatment. First, it diffuses from the surface of particles to the void space within a packed sample bed. Lead chloride pressure at the surface of particles, P*, could be inferred from eq 1 at the temperature measured. Suppose that the lead chloride concentration or pressure (P) in the void space of the sample bed (called bulk intraparticle pressure below) is dimensionally uniform. Second, lead chloride vapor passes through the interfacial area of the packed sample in the holder, into the outer stream of gas. In the first step, the rate of the lead chloride transfer, N1 (g s-1 g-1 of the initial mixture), can be described as
(P* - P)M RT
N1 ) (X0 - X)kg1a
(2)
where X0 (wt %) is the initial lead chloride concentration in the mixture, X (wt %) is the lead chloride loss varying with the heating time; kg1 (g s-1 cm-2) is the local masstransfer coefficient which is affected by the packing state of the sample in the holder, a (cm2 g-1) is the evaporating surface area of lead chloride particles based on the “per gram of sample” which depends on their dispersion on the mixture, M is the molecular weight of lead chloride, T (K) is the absolute temperature, and R (cm3 mmHg mol-1 K-1) is the gas constant. The lead chloride concentration in the outer gas stream was so small that it can be regarded approximately as zero. In the second step, the rate of lead chloride transfer, N2 (g s-1 g-1 of the initial mixture), can thus be expressed by
N2 )
kg2SPM W0RT
(3)
where kg2 (g s-1 cm-2) is the local mass-transfer coefficient, S (cm2) is the interfacial area of the sample contacting the outer stream of gas, and is the packed voidage. Equation 3 can be represented in another form,
N2 )
kg2PM hFRT
(4)
where h is the height of the sample layer in the holder and F is the density of the mixture. If the weight loss of the sample is only due to the vaporization of lead
Ind. Eng. Chem. Res., Vol. 39, No. 2, 2000 337
chloride, N2 can be regarded as equal to the differential weight loss, dX/dt, which is experimentally measurable. The intraparticle concentration of lead chloride in a sample bed varies with heating time. Thus, the rate of mass accumulation in it, ∆N, can be given by
(RTP )
∆N ) MF d
(5)
As vaporization occurs significantly, the values of ∆N are small as compared with those of N2. ∆N is thus neglected for simplicity of calculation. In the absence of reaction and adsorption on the sorbent, the following mass conservation equation is established:
( )/( ) [ P* RT
dX hF 1 + ) dt kg2M (X0 - X)kg1aM
]
(6)
Because X and dX/dt as functions of heating time or temperature are measurable, the values of kg1 and kg2a can thus be derived from eq 6 by using the least-squares linear regression method. Integration of eq 6 between Ta and T corresponding to Xa and X gives the following form,
( ) ( )
P* ∫TT φRT a
dT )
Fh (X - Xa) + kg2M
(
) (
Figure 1. Weight loss of a mixture of hematite and lead chloride (6.6% PbCl2) measured by TG analysis and that simulated by the vaporization model. Vapor pressure within sample bed was calculated, compared with the saturated vapor pressure. Heating rate, 1.667 °C/s.
)
X0 - X a 1 ln (7) kg1aM X0 - X
where φ is the heating rate. The value of weight loss (X) can be numerically obtained from eq 7 to compare with the experimental one. Reaction Fraction. In the presence of reaction between lead chloride and sorbent, the mass balance ignorant of the rate of mass accumulation ∆N leads to the following equation,
(P* - P)M kg2PM dY ) + (8) RT hFRT dt
(X0 - X - Y)kg1a
where Y is the fraction of lead chloride fixed or adsorbed on the sorbent, which can be calculated according to the integrated equation:
{ [(
Y ) X0 - X - (X0 - Xa) exp
)
Fhkg1a (X - Xa) kg2
kg1aM
P* dT ∫TTφRT a
]}
(9)
where X and Xa are determined by TG analysis. The mass-transfer parameters shown in the right side of eq 9 can be deduced from eq 6 in the absence of reaction between lead chloride and the sorbent. Results and Discussion Vaporization Behavior. To obtain the unknown values of kg1a and kg2 from eq 6, it is necessary to find a powdery material which can barely react with lead chloride. In the study, we used such a mixture of lead chloride and hematite. No reaction was observed between these two phases in the temperature range of 700-950 °C. It appears impossible for any reaction to take place at lower temperatures. Figure 1 shows the weight loss with heating temperature for the mixture of lead chloride and hematite at
Figure 2. Least-squares linear regression plot for deducing the mass-transfer parameters according to eq 6 described in the text.
a heating rate of 1.667 °C/s, which was measured by TG analysis (solid curve). A slight weight loss (0.2%) occurred at 500 °C; this may be due to the evolution of a small amount of readily volatile impurities contained in them. The strongest change in weight loss occurred at 775 °C. Above 850 °C, the weight loss leveled off, with a weight loss quite close to the initial content of lead chloride in the mixture. Weight loss of this mixture was essentially caused by lead chloride vaporization. It is thus known that no lead chloride remained on the hematite at the end. Figure 2 shows the least-squares linear regression plot in terms of eq 6 for the part of data only ranging in the middle region of vaporization shown in Figure 1 (solid curve). At the initial or end stage, the differential weight loss (dX/dt) measured had such large variations that it may be of no significance; omitting eq 5 would also have relatively large errors. As shown in Figure 2, the regression coefficient obtained was good. The values of Fh/kg1M and 1/kg2aM were 6.945 × 10-6 and 2.278 × 10-5 s g-1, respectively. The magnitude of two parameters could stand for the resistance of mass transfer through either of the steps. It is reasonable that the mass transfer was less resisted on the second step by virtue of the convection of outer flowing gas. The weight loss over the heat-up calculated according to eq 7 is shown in Figure 1, the curve being very consistent with the experimental one. The theoretical figures slightly deviate from the experimental ones at the initial and final vaporization stages; this may be due to the omission of the item in eq 5. Figure 1 also
338
Ind. Eng. Chem. Res., Vol. 39, No. 2, 2000
Figure 3. Weight loss of mixtures of lead chloride and sorbents measured by TG analysis (had subtracted the water loss) and the fixation or sorption fraction of lead chloride calculated by modeling. Heating rate, 1.667 °C/s. (a) PbCl2/dehydrated kaolinite mixture (PbCl2/DK); (b) PbCl2/kaolinite (PbCl2/K); (c) PbCl2/quartz; (d) PbCl2/bentonite (PbCl2/bent.).
illustrates the saturated vapor pressures of lead chloride and the calculated bulk intraparticle pressure with temperature. The bulk intraparticle pressure was much lower than the saturated pressure and decreased to zero at high temperatures. Fixation Behavior. In calculation of the fraction of lead chloride fixed or adsorbed on the sorbents according to eq 9, the parameters kg1 and kg2a were taken as the same as those obtained by heat-treating the mixture of lead chloride and hematite. Caution was taken in the experiment to keep the least number of variations in preparing the mixture and packing sample and to control the same flowing rate of gas and heating ramp. Suppose that h and F are invariable during the heat treatment. The value of F used in calculation was 5.0 g cm-3 for hematite, 2.6 g cm-3 for kaolinite, and 2.2 g cm-3 for quartz and bentonite. Figure 3a-d shows the TG results of weight loss (had subtracted water loss of a pure sorbent) and the calculated fixation or sorption fraction of lead chloride on different sorbents varying with temperature. The curve (total) is the sum of other two curves, which represents the theoretical lead chloride release history if any sorption does not occur. Figure 3a shows the result obtained for the mixture of lead chloride and dehydrated kaolinite (referred to as PbCl2/DK mixture below). At temperatures above 500 °C, the weight loss could only be attributed to the vaporization of lead chloride because kaolinite had thoroughly been dehydrated. Above 850 °C, the weight loss almost reached a plateau, with about 35% of lead chloride fixed on dehydrated kaolinite. This indicates the formation of a thermally stable compound between lead chloride and dehydrated kaolinite. In an accessory experiment (not shown), the mixture was heat-treated until the final temperature was increased as high as 1300 °C. No weight loss was observed below 1220 °C and weight loss slightly occurred above 1220 °C. Vaporization of lead chloride for the mixture of lead chloride and kaolinite (referred to as PbCl2/K below) is shown in Figure 3b. Interestingly, weight loss signifi-
cantly occurred in a lower-temperature region and even was larger than the total weight loss; calculated fixation fractions of lead chloride were minor values. It should be pointed out that kaolinite incorporates hydroxylated water. Under the experimental conditions, dehydroxylation of a pure kaolinite markedly occurred in the temperature range from 500 to 700 °C; the strongest change in weight loss was located at 600 °C. The weight loss plotted in the figure reflects subtraction of the water loss caused by the kaolinite part of the mixture from the total weight loss. However, the reaction may take place between lead chloride and steam water as described in a general form
(x + y)PbCl2 + yH2O f x(PbCl2)‚y(PbO) + 2yHCl (a) where the molecular weight of released hydrochloric acid is larger than that of water. Such a reaction could account for a larger weight loss than that caused simply by vaporization of lead chloride. Despite rapid volatilization during the dehydroxylation of kaolinite, weight loss became slight at temperatures above 700 °C. It is estimated that upon the completion of heating, about two-thirds of the lead chloride was captured by kaolinite, much larger than the amount captured by pre-dehydrated kaolinite. If any products such as x(PbCl2)‚y(PbO) are formed at the initial stage, the lead vaporization could subsequently decrease because of the reduced saturated vapor pressures of these compounds compared with that of lead chloride. The melting points of PbO‚PbCl2, 2PbO‚PbCl2, 4PbO‚PbCl2, and PbO were 524, 693, 711, and 835 °C, respectively.9 Compared with kaolinite and dehydrated kaolinite, quartz had a weak ability to capture lead chloride, as shown in Figure 3c. The calculated results indicate that there likely exists some interaction between quartz and lead chloride which retarded the vaporization of lead chloride on quartz, although we have not known if this
Ind. Eng. Chem. Res., Vol. 39, No. 2, 2000 339
Figure 4. The content of lead retained in sorbents after various heat treatments by XRF analysis.
interaction is a physisorption or chemical reaction process. The adsorbed part or reaction compound appeared to be reversible or unstable and this may account for its evaporation with decreasing intraparticle vapor pressure and with increasing temperature. At temperatures greater than 850 °C, the weight loss roughly approached the initial content of lead chloride in the mixture. Uberoi et al.6 reported that silica (silica gel) was a good lead sorbent at 700 °C and little of the lead captured was soluble in water, unlike lead chloride which could be dissolved. We note that they used a mixed flowing gas containing 2% H2O. The difference between their results and ours may be due not only to the type of silica but also to the presence of water. This may be why the capture results contrasted sharply between kaolinite and dehydrated kaolinite, although Uberoi et al. had not mentioned how the existence of water steam impacted the sorption reaction. Bentonite appeared to have a stronger capability for capturing lead chloride at lower temperatures than dehydrated kaolinite, as shown in Figure 3d. Like quartz, bentonite showed the gradual reduction in fixation fraction at high temperatures. However, the reduction rate was relatively low. Upon the completion of heat treatment at 950 °C, a significant fraction of lead chloride was still retained in bentonite, contrary to that of quartz. This indicates that the interaction between lead chloride and bentonite may form both stable and unstable components, and only part of the latter could be reversible or evaporate in the later heating stage. Because bentonite has a high content of silica and a low content of alumina, it is reasonably doubted whether the weakly bound components are due to the possible interaction between the silica part and lead chloride as in the case of quartz. According to the study by Ho et al.4, the lead capture efficiency of sand in a fluidizedbed combustor indeed decreased with increasing temperature above 700 °C. Results of the analysis of lead content in typically heat-treated mixtures obtained by energy-dispersive XRF method is illustrated in Figure 4. Kaolinite retained a significantly higher lead content than dehydrated kaolinite. Bentonite retained a higher lead content at 700 °C than dehydrated kaolinite; however, at 950 °C, the remaining lead content of bentonite was close to that of hydrated kaolinite. A little bit of the lead was detected by XRF for the mixture of silica and lead chloride, which was subjected to the heat treatments
Figure 5. High-temperature XRD patterns of a PbCl2/DK (10.5% PbCl2) at different temperatures.
under the conditions denoted in the figure. All these results were in good agreement with those obtained by TG analysis combined with the theoretical calculations. Reaction between Lead Chloride and Dehydrated Kaolinite. Reaction to turn lead chloride into a less volatile reaction product should be a dominant process for the capture of lead chloride. In this study, we have examined the reaction of lead chloride with dehydrated kaolinite and fresh kaolinite by X-ray diffraction analysis. Limited studies in connection with this reaction were reported in the literature. We also examined some of the reacted samples for mixtures of bentonite and lead chloride but we were not able to reach some reliable information on this aspect, so the reaction of lead chloride with bentonite is not intended to be discussed here. Figure 5 shows the changes in high-temperature X-ray diffraction patterns of a PbCl2/DK mixture with heating temperature. The temperature of the sample was raised at 20 °C/min to the predetermined temperature and then held there isothermally for 20 min before the X-ray analysis started. X-ray diffraction analysis was isothermally conducted at a scanning rate of 1 °C/ min in flowing nitrogen. At room temperature, the XRD pattern demonstrated metakaolinite and initial lead chloride in the sample. A broad peak ranging approximately between 18° and 32° (2θ) was indicative of amorphous metakaolinite. The pattern of lead chloride was consistent with that reported by Nitsch et al.10 except for a strong peak at a d spacing of 2.16 Å (2θ, 41.8°) which was not recorded in the literature. At 400 °C, the XRD pattern still showed the same crystalline phase of lead chloride with slightly increased distance spacings as compared with those at room temperature, indicating the thermal expansion of lead chloride at a high temperature. However, the strong peak at 2.16 Å almost disappeared. It is not clear whether this peak is due to an impurity in the lead chloride sample or lead chloride itself. At 550 °C, a broad peak approximately between 25° and 38° (2θ) appeared as the peaks of lead chloride thoroughly disappeared. The peak partly overlapped that of metakaolinite but was distinct from the
340
Ind. Eng. Chem. Res., Vol. 39, No. 2, 2000
Figure 6. Change in lead content and Pb/Cl atomic ratio after heat treatment for PbCl2/DK and PbCl2/K mixtures. Heating rate, 1.667 °C/s; no holding at final temperature.
latter. At 700 °C, it became more protruding. The peak is most likely due to a glassy product formed. At 900 °C, the distinction between the two broad peaks became vague, probably because lead chloride may penetrate into the whole metakaolinite. Besides the formation of a glassy product, some very weak peaks occurred, probably representing the formation of a minor crystalline phase. Note that the broad peak of the glassy product failed to be seen by room-temperature XRD analysis, even though the PbCl2/DK mixture was treated at any similar heating temperatures and times. Reaction between Lead Chloride and Kaolinite. The main weight loss of the PbCl2/K mixture occurred in the temperature region between 500 and 700 °C where dehydroxylation of kaolinite took place. In the above description, we have explained the possible reaction in the presence of water steam. Figure 6 shows the changes in the lead content and Pb/Cl atomic ratio for the PbCl2/DK and PbCl2/K mixtures. The Pb/Cl atomic ratio in both of the initial samples was 0.5, corresponding to the stoichiometry of molecular lead chloride. In the case of the PbCl2/DK mixture, the Pb/Cl ratio kept around 0.5 with the reduction of lead content. This suggests that the vaporization of lead chloride on metakaolinite was virtually congruent. In the case of the PbCl2/K mixture, however, the Pb/Cl atomic ratio increased up to about 1 with the rapid decrease in lead content at lower temperatures. This means that half of the chloride in lead chloride was preferentially released from molecular lead chloride while the interior water was lost from kaolinite. The release of hydrochloride acid gas was markedly observed with a silver nitrate solution for the PbCl2/K mixture whereas it was hardly observed for the PbCl2/DK mixture. To confirm the formation of any lead oxychlorides or lead oxides, we had examined the samples heated at 550 °C for varying times (not shown). The results showed the changes in the peak intensity of lead chloride with heating time but there were no distinct peaks possibly accounting for any lead oxychlorides or lead oxides known in the Power Diffraction File compiled by the International Center for Diffraction Data (1997). Figure 7 shows the XRD patterns of the mixtures of PbCl2/DK and PbCl2/K with the same mixing ratio which were subjected to heat treatment at 550 °C for 20 min. A significant amount of PbCl2 remained for the PbCl2/DK mixture. However, the crystal phase of lead chloride was different from the initial one. It has been known that lead chloride has a phase transformation
Figure 7. XRD patterns of a PbCl2/DK mixture (a) and a PbCl2/K mixture (b) after the same heat treatment, both of which initially contained 20% PbCl2. Heating rate, 1.666 °C/s; final temperature, 550 °C; holding time, 20 min.
at about 483 °C and the high-temperature phase is not reversed upon quenching.10 In contrast to the PbCl2/ DK mixture, the PbCl2/K mixture exhibited no traces of lead chloride remaining, with a small but distinct peak at 2θ of 25.3°. This indicates that the reaction of PbCl2 with kaolinite proceeded much faster than that with metakaolinite. According to the above results, the reaction between lead chloride and kaolinite may take place mainly as the following equation:
2PbCl2 + Al2Si2O5(OH)4 f (PbCl2‚PbO)(Al2Si2O7) (glassy) + 2HCl + H2O (b) The reaction takes place rapidly so that the products x(PbCl2)‚y(PbO) possibly formed by reaction (a) could hardly be observed. The small peak at 25.3° shown in Figure 7 was also observed in most of the heat-treated PbCl2/K and PbCl2/DK mixtures; this peak was not identified but it may indicate the simultaneous formation of a minor crystalline phase. Uberoi et al.6 observed that the reaction between lead chloride and kaolinite in a reacting gas containing 2% H2O formed hexagonal and monoclinic PbAl2Si2O8. None of the peaks in our treated samples could possibly trace these two species. It is estimated according to eq 4 that the rate of lead chloride vaporization at 550 °C is less than 7.6 × 10-4 g s-1 per gram of mixture; this means that it must take at least 138 min for a mixture containing 6.6% lead chloride to evaporate entirely. It is certain that reaction occurred not only between lead chloride vapor and the sorbent but also between molten lead chloride and the sorbent. In combustion, once lead vapor condenses on the surface of fly ash, the reaction between deposited lead and ash would be an important process in the lead retention. It is conjectured from this study that lead vapor may readily be enriched in kaolinitic materials rather than in hematite or quartz particles. A fairly fast heating rate was used in the experiment to make it possible for lead chloride to be kept volatilizing at higher temperatures (but a little lower than the boiling temperature of lead chloride). We consider that this may make the reaction relatively close to that occurring between condensing lead vapor and sorbents in combustion.
Ind. Eng. Chem. Res., Vol. 39, No. 2, 2000 341 Table 2. Change of d Spacing in the 001 Reflection of Kaolinite Lattice after Heat Treatment without or with Lead Chloride sample
heat treatmenta
pure kaolinite 6.7% PbCl2/K
510 °C, 3 min 600 °C, 0 min 510 °C, 3 min 600 °C, 0 min
water content (%)
d001 (Å)b
14.5 9.7 6.8
7.196 7.155 7.138 7.155 7.138
a Final temperature and holding time denoted in column; heating rate, 1.667 °C/s; naturally cooling.
Kaolinite has a layer structure with a basal d spacing of around 7.1 Å. The hydroxyl group is bound by the gibbsite layer at which, as is well-known, ion exchange usually takes place when kaolinite is used in ion adsorption. However, the layer structure is disrupted when kaolinite is transformed into metakaolinite. The question here is whether the addition of lead chloride may cause the alteration in the disruption process of kaolinite if any lead chloride vapor or molten lead chloride penetrates into the basal layer. This has been investigated because it may provide another reason of how the kaolinite structure possibly affects its reaction with lead chloride. Table 2 shows the changes of d100 with the water loss of kaolinite without the addition of lead chloride and with the addition of lead chloride. However, no difference in the alteration of d100 was observed between the two cases, indicating that the dehydroxylation of kaolinite was not affected by the presence of lead chloride, even with reaction (b) occurring simultaneously. Conclusions 1. The vaporization rate of lead chloride on various sorbents at linearly rising temperatures was measured by thermogravimetric analysis. The lead chloride fraction fixed on the sorbent with the heating temperature was calculated according to a vaporization model proposed in the work. The calculated result revealed that the component fixed on both kaolinite and dehydrated kaolinite were thermally stable and little volatilized at 950 °C whereas part of the component adsorbed on bentonite tended to volatilize at elevated temperatures. 2. TG and XRF analyses showed that the capture ability of the sorbents studied are in the order kaolinite > dehydrated kaolinite ∼ bentonite > quartz. Quartz was weak in lead capture under the present conditions. The enhanced capture efficiency of kaolinite in com-
parison with dehydrated kaolinite may be mainly caused by the dehydroxylation of kaolinite. The reaction of lead chloride with water steam led to the partial release of hydrochloric acid and promoted the lead fixation reaction on kaolinite. 3. The reaction product formed between dehydrated kaolinite and lead chloride was mainly a glassy product. High-temperature X-ray diffraction analysis showed a broad peak at a heating temperature between 550 and 700 °C as a result of the glassy product being formed. None of the crystalline phases were definitely identified as a reaction product, although some small peaks were observed in most heat-treated samples of PbCl2/DK and PbCl2/K mixtures. Acknowledgment We wish to express appreciation to Mr. H. Nishiguchi at the Technical Laboratory of Okutama Kogyo Co., Ltd. for his providing kaolinite and bentonite samples and their composition data. Literature Cited (1) Davidson, R. M.; Clarke, L. B. Trace Elements in Coal. IEA Coal Res. 1996, IEAPRR/21, 1-60. (2) Linak, W. P.; Wendt, J. O. L. Trace Metal Transformation Mechanism during Coal Combustion. Fuel Process. Technol. 1994, 39, 173. (3) Querol, X.; Fernandez-Turiel, J. l.; Lopez-Solar, A. Trace Elements in Coal and Their Behavior during Combustion in a Large Power Station. Fuel 1995, 74, 331. (4) Ho, T. C.; Chen, C.; Hopper, J. R.; Oberacher, D. A. Metal Capture During Fluidized Bed Incineration of Wastes Contamination with Lead Chloride. Combust. Sci. Technol. 1992, 85, 101. (5) Ho, T. C.; Chen, J. M.; Shukla, S.; Hopper, J. R. Metal Capture during Fluidized Bed Incineration of Solid Waste. AIChE Symp. Ser. 276 1990, 86, 51. (6) Uberoi, M.; Shadman, F. Sorbents for Removal of Lead Compounds from Hot Flue Gas. AIChE J. 1990, 36, 307. (7) Wang, J.; Tomita, A. Removal of Mineral Matter from Some Australian Coals by Ca(OH)2/HCl Leaching, Fuel 1998, 77, 1747. (8) Evan, J. R.; Sellers, G. A.; Johnson, G. R.; Vivit, D. V.; Kent, J. Analysis of Eight Argonne Premium Coal Samples by X-ray Fluorescence Spectrometry. Energy Fuels 1990, 4, 440. (9) Podsiadlo, H. Phase Equiliberia in the Binary System PbOPbCl2. J. Thermal Anal. 1991, 37, 626. (10) Nitsch, K.; Kniizek, K.; Rodova M. X-ray Diffraction Study of Lead Chloride. Solid State Commun. 1994, 91, 611.
Received for review July 12, 1999 Revised manuscript received November 15, 1999 Accepted November 18, 1999 IE9905097
