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Pyrolysis study of poly(vinyl chloride)–metal oxide mixtures: Quantitative product analysis and the chlorine fixing ability of metal oxides
J. Anal. Appl. Pyrolysis 77 (2006) 159–168 www.elsevier.com/locate/jaap
Pyrolysis study of poly(vinyl chloride)–metal oxide mixtures: Quantitative product analysis and the chlorine fixing ability of metal oxides Y. Masuda a, T. Uda a,1, O. Terakado b,*, M. Hirasawa b a
Institute of Multidisciplinary Research for Advanced Materials, Tohoku University, Katahira 2-1-1, Aoba-ku, 980-8577 Sendai, Japan b Department of Materials Science and Engineering, Graduate School of Engineering, Nagoya University, Furo-cho, Chikusa-ku, 464-8603 Nagoya, Japan Received 5 September 2005; accepted 7 March 2006 Available online 18 April 2006
Abstract Thermal decomposition of mixtures of poly(vinyl chloride) ( PVC) and a variety of metal oxides including rare earth oxides has been studied under helium atmosphere at 400 and 800 8C. Volatile pyrolysis products have been quantitatively analysed. The formation of benzene at 800 8C is suppressed by the addition of oxides except aluminium oxide, especially in the case of zinc oxide, reducing 80% compared with the case of pure PVC. The emission of hydrogen chloride changes significantly with the oxides used. This feature is strongly concerned with the chlorine fixing ability of oxides. Together with the analysis of pyrolysis residues, the behaviour of fixation of chlorine from PVC by oxides has been compared. The trivalent rare earth oxides (Ln2O3) show great ability to fix considerable amount (more than 95% in the case of lanthanum oxide) of initial chlorine in PVC in the form of oxychlorides (LnOCl). The water-insoluble property of LaOCl is advantageous for the use of La2O3, which is abundant in the sludge discharged from magnet production industry, as chlorine fixing agent. # 2006 Elsevier B.V. All rights reserved. Keywords: PVC; Metal oxides; Chlorine fixing; Plastic recycling
1. Introduction Recycling of waste plastics is of great importance from the environmental point of view. Among various treatment processes, pyrolysis method is a powerful tool which can be applied to the feedstock recycling of synthetic polymers. An example is the usage of waste plastics in coke ovens for steel making process [1]. Poly(vinyl chloride) (PVC) is a widely used cheap, commodity polymer, showing good mechanical properties as well as great chemical resistance. However, incineration of waste PVC results in the emission of harmful hydrogen chloride (HCl), and organochlorine compounds, such as chlorobenzene, polychlorinated dibenzo-p-dioxins (PCDDs) and polychlorinated dibenzofurans (PCDFs) [2,3]. It is, therefore, very important to understand the thermal decom-
* Corresponding author. Tel.: +81 52 789 5308; fax: +81 52 789 5308. E-mail address: [email protected] (O. Terakado). 1 Present address: Division of Materials Science and Engineering, Graduate School of Engineering, Kyoto University, Yoshida-hommachi, Sakyo-ku, 6068501 Kyoto, Japan. 0165-2370/$ – see front matter # 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jaap.2006.03.001
position behaviour of PVC, and a number of studies have so far been carried out, see e.g. [4–6]. During the course of our investigations on the pyrolysis of mixtures of polymer and metal oxides, we have examined the possibility for the simultaneous recycling process of large amount of waste plastics and metallurgical dusts, consisting mainly of metal oxides [7–9]. The discharge of the electric arc furnace (EAF) dust reaches up to 500,000 tonnes per year in Japan [10]. Our previous studies [7,8] showed that zinc could be separated from ZnFe2O4, main component of EAF dust, in the form of zinc chloride gas by thermal decomposition of the mixture of PVC and ZnFe2O4, while iron remains in the pyrolysis residue as metallic iron. This result suggests a potential application of steel making dust as sorbent of HCl from PVC. Furthermore, the Zn-eliminated dust may be recycled for steel making process. Zinc is known to corrode furnaces so that it must be thoroughly removed for recycling of dusts. As for the plastic industry, various additives are used for the enhancement of the functionality of polymers or flame retardancy of plastics. The influences of metal oxide additives on the thermal decomposition or stabilisation of PVC have been also extensively investigated in many works [11–16]. They
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focus mainly on the kinetics of decomposition and the emission of volatile compounds like benzene by applying experimental methods such as TG and pyrolysis-gas chromatography. Taking into account the influence of volatile compounds to the environment, it is necessary not only to investigate qualitative analysis but also to acquire quantitative information on the influence of additives. In the present study, we have performed quantitative analysis of the pyrolysis products (gas, liquid, and pyrolysis residue) of mixtures of PVC and various metal oxides, including ZnO, Fe2O3, rare earth oxides, CaO, PbO, and Al2O3. It should be noted that the latter three compounds are often abundant in used plastics. As for the rare earth oxides, quite large amount of sludge containing these oxides is discharged from magnet production industry. In comparison with the reported studies on this subject, such as the influence of additives on the flame retardancy of PVC [17], higher concentration of metal oxides has been examined in the present study for the purpose of the investigation of the recycling process of the wastes containing metal oxides as mentioned above. From this point of view, we will also discuss the chlorine fixing of metal oxides through pyrolysis of the PVC–metal oxide mixtures. 2. Experimental Table 1 summarises the physical properties of oxide used in the present study. The measurement of particle size was performed with a laser micron sizer equipment, LMS-30 (Seishin Enterprise Co. Ltd.) for the suspension of oxide powder in water. The BET surface area was determined with a TriStar 3000 equipment (Micrometrics). The reagent grade powders of PVC (Wako Pure Chemicals; average molecular weight = 6.9 104 and average particle size = 160 mm) and metal oxides were mixed mechanically and pressed to a disc of 8 mm diameter (thickness 2–3 mm). The amount of PVC was kept constant (250 mg). In the present study, the composition of [PVC]:[oxide] = 2:1 in molar ratio was investigated, where one mole of PVC means that of the structural unit of polymer, C2H3Cl. The experimental set-up of pyrolysis is schematically shown in Fig. 1. All pyrolysis runs were conducted under helium (99.9999%) gas flow with the flow rate of 100 ml/min. After the temperature of high temperature zone in the furnace was Table 1 Physical properties of metal oxide Oxide
Supplier
ZnO CaO Fe2O3 PbO La2O3 Nd2O3 CeO2 Al2O3
Kanto Chemicals Kanto Chemicals Kanto Chemicals Kanto Chemicals Wako Pure Chemicals Chimie Rhodia Wako Pure Chemicals Kojundo Chem. Lab. Co. Ltd.
Mean particle size (mm) 1.9 a
9.9 8.4 15 9.0 10.6 3.6
BET surface area (m2/g) 2.8 4.9 5.9 0.2 1.0 1.2 4.1 2.9
a It is not possible to obtain the data for CaO because of the formation of hydroxide in water.
Fig. 1. Schematic experimental set-up for pyrolysis.
stabilised at 400 or 800 8C, sample, which had been placed in an alumina reaction boot, was introduced quickly from the cold part to the reaction zone. In the present work, we carried out pyrolysis for 30 min at 400 8C and 10 min at 800 8C, respectively. The quartz wool inside the reaction tube prevents the contamination of soot into the subsequent traps. Acetone traps collect organic liquid products. The following water trap is that for HCl emitted from the sample. Other gaseous products were first collected at the last trap cooled with liquid nitrogen, and then introduced to a gasbag. This procedure avoids the contamination of water into the gasbag. Some of organic substances were adhered as soot on the reaction tube and quartz glass wool. They were rinsed with tetrahydrofuran (THF) after each experimental run. We regard the dissolved compounds as tar and not-dissolved ones as wax in the present study. Liquid and gaseous products and tar solutions were analysed by using a Hewlett-Packard Gas Chromatograph/Mass Spectrometer (GC/MS) analyser 6890/5973. As for the GC/MS column we used HP-5MS column for liquid products (except for the quantitative analysis of benzene and toluene) and HP-PLOTQ column for gaseous products, benzene, and toluene. Quantitative analysis of the gaseous and liquid products was performed by standard calibration method. The typical experimental errors of the amount of gaseous and liquid products were 15 and 10%, respectively. The quantity of emitted HCl was analysed through the Cl anion content by a liquid chromatography apparatus (LC10AD VP, Shimadzu). Since the arrangement of traps in Fig. 1 might result in the collection of HCl also at the acetone traps, we carried out separate pyrolysis runs without acetone traps and analysed the water trap. If water-soluble metal chlorides formed during the pyrolysis would be collected at the water trap, they could affect the result of ion chromatography. The water trap was, therefore, analysed with an ICP spectrometer (Optima 3300, Perkin-Elmer) to examine the existence of metal species in the trap. The result showed that less than 0.1% of all Cl was due to metal chlorides, which attributed the amount of Cl anion solely to the emission of HCl. Thus, the HCl content was obtained directly from the Cl concentration in the trap. The error of the concentration was within 10% in most cases, but in the case of the PVC–La2O3 system, it was 30% because the emission of HCl in this case was very small, as to be seen in Section 3 of the present report. Pyrolysis residues in the reaction boot were characterised with X-ray diffractometer RINT 2200 (Rigaku). The carbon
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contents in the residues were determined at Kawasaki Steel Techno-research Co. Ltd. by combustion method under oxygen flow equipped with IR spectrometer. The hydrogen content was determined in the following way. After a pyrolysis run the residue was heated to 700 8C under oxygen atmosphere for 15 min. The resulting H2O was collected with a cold trap (100 8C, at which CO and CO2 gases were not trapped), which was then introduced to a tube filled with calcium chloride. The amount of water was determined from the weight change of the tube. The carbon content of soot was determined by the similar combustion method. In this case, CO and CO2 gases were collected and analysed with GC/MS. Separate run after THF dissolution of the reactor allows the determination of carbon content of wax. That of tar was, thus, evaluated by subtraction of carbon content in wax from that in soot. The errors of those of wax and tar were not estimated because the complete collection of tar and wax components on the wall of reaction tube as well as in the gas flow system was not possible. 3. Results 3.1. Product analysis of organic compounds and gases Some typical results of GC/MS analysis of the liquid, gaseous, and tar products for the pyrolysis at 800 8C are shown in Figs. 2–5, respectively. In all cases, we have not found any difference in chemical species of products by addition of metal oxides. As shown in Figs. 2 and 3, various aromatic hydrocarbons of C6–C14 including benzene and toluene have been detected as liquid products. The figure also demonstrates the suppression of the formation of liquid products by addition of metal oxides, i.e. ZnO in this case. The main gaseous products are CO, CO2, and hydrocarbons with C1–C3 (Fig. 4),
Fig. 2. Representative GC/MS results of liquid products by pyrolysis of PVC and PVC–ZnO at 800 8C (column: HP-5MS).
Fig. 3. Chromatogram for benzene and toluene by pyrolysis of PVC and PVC– ZnO at 800 8C (column: HP-PLOTQ).
and various aromatics with 3–6 benzene rings (depicted in Fig. 5) have been observed as tar. In this case, again, any changes in chemical species of the product have not been observed, so that it is rather important to carry out quantitative analysis to find out the influence of metal oxides in detail. In the present study twelve light organic compounds, indicated in Figs. 2 and 3, and gaseous products have been quantitatively analysed with GC/MS by standard calibration method. The results of the products at 800 8C are summarized in Fig. 6. Each amount is normalised by that of reactant PVC in the initial sample, i.e. (mass of product)/(mass of PVC in the initial mixture). In all cases benzene and toluene are major liquid compounds. Obviously, the amounts of liquid products are reduced by addition of metal oxides, especially significantly in the case of Fe2O3 and ZnO, where the amount of the liquid products has been less than 30% of that in the case of pure PVC. In the pyrolysis of PVC–Al2O3 mixture, the formation of benzene and toluene is enhanced, as reported also in literature [15]. Concerning the organochlorine compounds, the formation of chlorobenzene, one of the precursors of PCDD/Fs, was analysed in the present study. The yield is listed in Table 2. Any significant correlation has been hardly found between the amount of liquid products and organochlorine compounds from the present results. The addition of Fe2O3 and CeO2 enhances the formation of chlorobenzene, whereas significant suppression is observed for zinc oxide. The influence of iron oxide is indicative, since the Friedel–Crafts halogenation of benzene is promoted by Lewis acids, such as iron chloride. This feature is to be further considered in Section 4. As for the gaseous products, considerable amounts of CO and CO2 were emitted by the reaction of PVC–Fe2O3 mixture,
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Table 2 Yield of chlorobenzene, normalized by the weight of initial PVC Wchlorobenzene (mg)/WPVC (g) PVC PVC–ZnO PVC–CaO PVC–Fe2O3 PVC–PbO PVC–La2O3 PVC–Nd2O3 PVC–CeO2 PVC–Al2O3
0.15 0.03 0.07 0.24 0.07 0.06 0.15 0.25 0.09
as presented in Fig. 6 (upper panel). This is due to the reduction process of iron oxide to metal iron, which has been confirmed also by our previous XRD and simultaneous TG–MS measurements [7,8]. In contrary to the case of Fe2O3, the addition of alumina leads to repression of gaseous compounds. The results of pyrolysis at 400 8C for several PVC–metal oxide systems are shown in Fig. 7, together with those at 800 8C. In most cases, less gaseous and liquid products are formed at 400 8C in comparison with the pyrolysis at higher temperature. The fraction of benzene to liquid products becomes higher, and the abundance of Cn with n > 8 is very small at low temperature. The latter tendency was also reported in literature on the vacuum pyrolysis of pure PVC, for which the enhancement of polymer chain scission and the secondary reaction were responsible with increasing temperature [18].
Fig. 4. Gas chromatograms of gaseous products for the pyrolysis of PVC and PVC–PbO at 800 8C.
Fig. 5. Gas chromatograms of tar products for the pyrolysis of PVC at 800 8C.
Other important result of the pyrolysis at 400 8C is that the formation of chlorobenzene has not been detected. 3.2. Mass balance of carbon The mass balance of carbon for pyrolysis products at 800 8C is summarised in Fig. 8. Most of miscellaneous compounds are
Fig. 6. Product yields of thermal decomposition of PVC with existence of metal oxides at 800 8C: gaseous products (upper panel) and liquid products (lower panel).
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Fig. 9. Mass balance of carbon of pyrolysis products at 400 8C, together with the result at 800 8C for comparison. The symbol ‘*’ indicates that the carbonaceous compounds in residue are hydrocarbons. See the text for details.
Fig. 7. Comparison of gaseous and liquid product yields at 400 and 800 8C.
presumably not-collected tar and wax adhered on the wall of reaction tube as well as in the gas flow system. Thus, quantitative discussion on the tar and wax is less accurate. On the other hand, the following features are clearly observed, some of which have been also reported in literature [11–16]. Formation of char is enhanced by the addition of ZnO, while calcium oxide causes the reduction of carbon in residue. As
Fig. 8. Mass balance of carbon for the pyrolysis products at 800 8C.
described in the previous section, the amount of gaseous products is considerably high for Fe2O3. On the other hand, the addition of Al2O3 suppresses more than 70% of gas in comparison with the case of pure PVC. In contrary to this, the mixture of PVC–PbO and PVC–rare earth oxides looks to give similar results with pure PVC. The carbon mass balance at 400 8C is presented in Fig. 9, together with that of 800 8C for comparison. By addition of oxides the proportion of carbons in residue, which exist as hydrocarbons (see next section for detail), increases, and the yields of gaseous and liquid products do not change significantly. An exception is the case of ZnO where the considerable decrease of liquid products has been observed, as is also indicated in Fig. 7. As for the influence of pyrolysis temperature, gas formation is promoted at high temperature. We do not find a systematic difference in the amount of tar at different pyrolysis temperature. 3.3. Pyrolysis residues In Fig. 10, XRD patterns of solid residues of pyrolysis at 800 8C are presented. For a pyrolysis residue of PVC without additives, the XRD pattern shows a broad peak around 2u 268, indicating the partial formation of layered structure of graphite [19,20]. The addition of oxides results in the existence of metal compounds in the residues. In the case of alumina and ZnO, XRD peaks solely of oxides are found. It should be noted that thermal decomposition of PVC–ZnO mixture proceeds with the formation of ZnCl2 and the consequent quasi binary ZnCl2– ZnO fluid, then evaporation of chloride due to its high vapour pressure with increasing temperature [7,8]. The XRD patterns of the Fe2O3 and PbO systems contain those of metal Fe and Pb, respectively, indicating the reduction of each oxide, although, in the latter system, the influence of lead chloride and the subsequent PbCl2–PbO quasi binary mixture, similar to the zinc system, should also be considered for the reaction process in the mixture. In the case of PVC–CaO mixture not only calcium oxide but also CaClOH are present in the residue. This is presumably concerned with the chlorination and hydration process of oxide, as will be described in Section 4. As for the
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rare earth oxides, most of lanthanum oxide is converted to oxychloride. Less oxychloride is formed for Nd2O3, and lesser for CeO2, that can be expected from the intensity ratio of the peaks attributed to respective oxychloride and oxide in the XRD patterns of these oxide–PVC systems. The pyrolysis residue at lower temperature (400 8C) exhibits slightly different XRD pattern, as depicted in Fig. 11. The broad peak for pure PVC is shifted to the lower angle compared with that at 800 8C. We have examined the hydrogen contents of pyrolysis residues of PVC–ZnO and PVC–Al2O3. In both cases ca. 2 wt.% of hydrogen at 800 8C and 30–50 wt.% at 400 8C were observed. Thus, the carbonaceous compounds in pyrolysis residues at 400 8C are mainly hydrocarbons. For the PVC–ZnO system, the formation of Zn5(OH)8Cl2H2O is observed, which is owing to the formation of ZnCl2 and H2O [7,8]. The absorption of water after the experimental run, e.g. during the XRD measurement, in ambient atmosphere can also contribute to the complex form of residue compounds because of the hygroscopic property of zinc chloride. As will be mentioned in the next section, the reduction or chlorination of oxides is not completed at 400 8C. This is also the case for the La2O3 system, though its chlorination proceeds to higher degree than the zinc system. 3.4. HCl emission and chlorination of oxide Fig. 10. XRD patterns of the residues of the PVC–metal oxide mixtures (800 8C). The peak of graphite at 2u = 268 is depicted as reference. Since XRD data of CeOCl are not available in literature, that of LaOCl is shown in the XRD data of PVC–CeO2. A good agreement indicates the similarity of structure for both oxychlorides.
In addition to the liquid organochlorine compounds, oxychlorides and metal chlorides as chlorine containing pyrolysis products, thermal decomposition of PVC causes the emission of HCl that is collected at water trap in the present pyrolysis set-up. The analytical results of the amount of trapped Cl anion are summarised in Table 3. The HCl emission is, in general, reduced by the addition of oxide and is especially low in the case of La2O3 at 800 8C. Taking into account the result of the XRD measurement and the order of the amount of organochlorine products, i.e. <1 mg/1 g PVC, we regard that almost all chlorines from PVC are converted to lanthanum oxychloride. The addition of PbO results in the conversion of only 30% of initial chlorine into HCl at 800 8C. The remainder is mainly responsible for the formation of lead chloride. This result is consistent with a TG study for a 2PVC–1PbO mixture, where the mass balance calculation of lead showed the formation of Table 3 The ratio of trapped Cl at water trap and the initial chlorine at 800 8C, together with the result at 400 8C for the five systems (in percent)
Fig. 11. XRD patterns of the residues of the PVC–metal oxide mixtures (400 8C).
PVC PVC–ZnO PVC–Fe2O3 PVC–CaO PVC–PbO PVC–Al2O3 PVC–La2O3 PVC–Nd2O3 PVC–CeO2
400 8C
800 8C
100 49
92 36 69 51 30 90 3.1 25 78
31 89 24
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quasi binary PbCl2–PbO mixture with the composition of X(PbO) 0.3 in molar fraction (liquidus temperature = 490 8C): in other word ca. 70% of chlorine from the initial sample was converted to lead chloride [21]. As for the other metal oxides, ca. 70–80% of Cl is converted to HCl for Fe2O3 and CeO2 at 800 8C, indicating that the chlorination of metals or metal oxides is not a major reaction for these systems. On the other hand, the pyrolysis behaviour of the PVC–Al2O3 system is very similar to that of pure PVC with regard to HCl emission. The chlorination of alumina has not been observed under the present experimental conditions. The results for the Fe2O3, CeO2, and Al2O3 systems are consistent with the slow kinetics of the reaction between these oxides and HCl gas, compared with other oxides like ZnO and PbO [22]. Also at lower temperature (400 8C) the addition of metal oxide reduces the emission of HCl in comparison with the case of pure PVC. For the PVC–ZnO and PVC–La2O3 systems, the results indicate that the formation of chloride or oxychloride undergoes slower than that at high temperature, while the chlorination of PbO seems to complete at <400 8C, as indicated by almost the same HCl emission at different temperature for the latter system.
the other hand, ZnCl2 has high vapour pressure, so that the kinetics of chloride evaporation accompanied by char formation should be considered. With respect to this point, it is noteworthy to mention the work by Ballistreri et al. [13] on the direct pyrolysis of PVC–metal oxide mixture in mass spectrometer, in which the simultaneous emission of ZnCl2 and benzene was observed. They anticipated that the presence of zinc chloride was essential for the suppression of liquid products. Furthermore, our previous TG–MS study showed that the ZnCl2 evaporation started above 400 8C [8]. Taking into account the existence of ZnCl2 species in pyrolysis residue as well as high abundance of residue materials for the PVC–ZnO system at 400 8C (Fig. 9), we deduce the following scenario. The precursor species of char (presumably cross-linked hydrocarbons) are formed simultaneously with the formation of zinc chloride below 400 8C. The ZnCl2 starts to vaporise above 400 8C, and consequently the carbonisation of residue proceeds. However, the chlorination of ZnO does not complete at 400 8C, as seen in the data of HCl emission at this temperature (Table 3). Further studies on the kinetics of chlorination of ZnO and the evaporation of ZnCl2 are required in order to verify the scenario suggested above in more detail.
4. Discussion
4.1.3. Fe2O3 The substantial suppression of liquid products by Fe2O3 indicates that the oxide can also play a role of Lewis acid, as described in the present section. The formation of catalytic amount of iron(III) chloride can enhance the char formation, although no trace of iron chloride was so far found in the XRD measurement of the pyrolysis residue and ICP measurement in water trap. In the work by Ballistreri et al. [13], the existence of another mechanism for the suppression of liquid products was speculated. Nevertheless, we cannot explicitly exclude the possibility of the formation of catalytic amount of iron chloride species, since the slight dissipation of iron from the residue has been observed under the present experimental condition [7]. This may be also supported by the subtle decrease of HCl emission for the PVC–Fe2O3 in comparison with the case of pure PVC (see Table 3). Moreover, the formation of intermediate species, such as Cl–Fe, to suppress the liquid products may be considered, although further studies are required to clarify the peculiarity of the influence of iron oxide.
4.1. Influence of oxides on the decomposition products of PVC 4.1.1. General remarks The mechanism of PVC pyrolysis has so far been studied very extensively. Polyene structure due to dehydrochlorination causes either cyclisation, leading to the formation of aromatics like benzene, or char formation owing to the bridging of the polyene structures. Since benzene is the major liquid product, the formation of benzene is a proper measure of the cyclisation of dehydrochlorinated PVC. The formation of benzene is known to involve the detachment of precursor cyclohexadiene intermediates from the polymer chain [23]. On the other hand, it is known that Lewis acid, such as iron(III) chloride, enhances the bridging of polyene chains through cationic polymerisation, leading to the char formation [24,25]. In the present case, the influence of metal oxides on the decomposition process of PVC as well as the transformation of oxides, such as reduction and chlorination, are of special interest. Because of the presence of miscellaneous compounds with a few exceptions, i.e. PVC– ZnO and -Fe2O3 at 800 8C, quantitative discussion on the whole pyrolysis products is not possible. On the other hand, since the miscellaneous compounds are considered mainly as uncollected tar and wax, these can be categorised into a group of polycyclic aromatic hydrocarbons and relevant compounds, designated hereafter as PAHs group. 4.1.2. ZnO The addition of ZnO causes the enhancement of char formation and the reduction of liquid products, as seen in Fig. 8. A possible explanation of this result is that it owes to the formation of ZnCl2, which is known as strong Lewis acid. On
4.1.4. Al2O3 Aluminium oxide is neither chlorinated nor reduced through the pyrolysis under the present experimental conditions. The catalytic effect of alumina directs to the promotion of detachment of precursor cyclohexadiene intermediates, leading to the formation of benzene, as suggested by Blazso´ and Jakab [15]. Another interesting observation is the suppression of gaseous products. The detailed mechanism is, however, still unclear for the relationship between the enhancement of benzene and the repression of gaseous products formation. 4.1.5. CaO and other oxides The addition of calcium oxide leads to the reduction of residue. This observation indicates that the bridging of polyene
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chains is hindered to some extent. The relation with the enhanced formation of PAHs group is interesting, although the detailed mechanism is an open question because of the lack of precise quantitative information. Other metal oxides do not significantly affect the amount of liquid and gaseous products, although reduction or chlorination of oxide occurs during the pyrolysis. This can be ascribed to the less catalytic activity of these oxides for the detachment of preformed cyclohexadienes and for the char formation, or to the less affinity between oxide and polymer to affect the decomposition of PVC. On the one hand, experimental supports such as study on the interfacial properties between PVC and single crystal metal oxide are desired to get insight into the catalytic role of metal oxides onto the thermal decomposition of PVC. On the other hand, it is intriguing to understand the influence of oxides onto the pyrolysis products systematically in terms of their chemical properties such as acidity. It should be noted here that no correlation is, so far, found between the amount of specific product and physical surface characteristics of the oxides (see Table 1). The study is now under way to get further insight into the role of metal oxides. 4.2. Fixation of Cl by metal oxides 4.2.1. Comparison of oxides As apparent from the results of XRD and the HCl emission, the fixing of chlorine by metal compounds changes significantly among oxides used. We now summarise the chlorine fixing of metal oxide through the pyrolysis. This information will be useful for the environment-friendly treatment of waste PVC and metallurgical dusts. As mentioned in the previous sections, alumina does not possess chlorination ability under the present experimental conditions, because the chlorination and the formation of oxychloride have not been observed. In the case of iron oxide, the reduction process is dominant, and hence the chlorine from PVC is emitted chiefly as HCl. On the other hand, it is of interest that the amount of chlorobenzene increased considerably by the addition of Fe2O3 (Table 2). Though the reaction mechanism is not clear yet, a reasonable account is based on the scheme of Friedel–Crafts chlorination of benzene through Lewis acid like FeCl3. It is possible that a part of initial Cl can contribute to form the iron chloride or Cl–Fe intermediate species at the interface between molten PVC and Fe2O3, which can play a role to generate the chlorobenzene according to the Friedel–Crafts scheme. This is further to be clarified in the future work. Nearly half of chlorine is fixed as the complex form of calcium compounds, such as CaClOH. Our previous TG–MS study demonstrated a possibility of the dehydration of Ca(OH)2 occurs at 630–720 8C [26]. Complex compounds, consisting of CaO, CaCl2, and Ca(OH)2, would be formed during pyrolysis. Zinc oxide and lead oxide are chlorinated, over 50 and ca. 70% of Cl being converted to volatile chlorides, respectively. As for the rare earth oxides, lanthanum oxide can convert up to 95% of initial chlorine in PVC to lanthanum oxychloride,
LaOCl. Taking the water-insoluble property of LaOCl into account, we consider that the use of La2O3 is beneficial to fix the chlorine in the form of solid oxychloride and to avoid the flowage through soil into the environment in comparison with the conventional Cl fixation agent, such as CaO. It should also be noted here that the oxychloride can be converted back to the corresponding oxide and HCl by conventional hydrolysis method at high temperature. Both products are used in various processes: the industrial production of rare earth metals is based on the electrolysis of the oxide, whereas hydrogen chloride can be utilised as chlorination agent. The present observation serves good basis for the utilisation of chlorine in processing of rare earths as well as wastes containing the oxides in the view of chlorine circulation [27,28]. 4.2.2. Interpretation with optical basicity of oxides The obtained results indicate that no straightforward correlation is found between the degree of chlorination or reduction and their physical surface characteristics of oxides examined. Thus, it is of interest to summarise the chlorine fixing ability of oxides in terms of their chemical properties systematically. For this purpose, we examine the concept of optical basicity, defined by Duffy and Ingram [29], which is widely applied in the fields of glass science and metallurgy in order to discuss the acidity or basicity of oxides. Fig. 12 shows the amount of HCl at 800 8C as a function of optical basicity of the corresponding oxide. The basicity value has been estimated by using a relation after Lebouteiller and Courtine [30] with appropriate parameters [31]. We see fairly well correlation between the basicity of oxide and the HCl emission. The emission of HCl decreases with increasing the basicity of oxide. However, large deviation is found for the trivalent rare earth oxide systems.
Fig. 12. The normalised amount of HCl from the pyrolysis of PVC–MO mixture at 800 8C, plotted as a function of optical basicity of the corresponding oxide. The basicity data are taken from [30]. That of Nd2O3 is evaluated from the empirical equation given in [30] with the proper parameter in [31]. For comparison, the optical basicity of FeO is also indicated in the figure with the same HCl data of Fe2O3. The line is the guide to the eye.
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5. Summary
5.6. Rare earth oxides
In the present study, we have investigated the influence of variety of metal oxides onto thermal degradation of PVC under inert atmosphere. As a general tendency, the addition of metal oxides reduces the amount of liquid products, while the formation of gaseous products is not influenced by the oxide except Fe2O3 and Al2O3. The characteristic features of the influence of each oxide addition can be summarised as follows.
The formation of chlorobenzene is enhanced by the addition of CeO2, though the reason is still unclear yet. The peculiar feature is the change in the chlorine fixing ability by the oxide examined. It changes with the order of La2O3 > Nd2O3 CeO2. The addition of La2O3 converts Cl from PVC to oxychloride with the efficiency of more than 95% at 800 8C and 75% at 400 8C. This is quite efficient compared with the conventional HCl sorbent such as CaO. The resulting LaOCl is not dissolved into water, dissimilar to the CaCl2 which can flow out through soil into the environment. Considering the sufficient suppression of the formation of chlorobenzene by the addition of La2O3, we regard the present result indicates a potential application for the fixation of chlorine in incineration process. Further investigations under the atmosphere with the presence of oxygen are required for the practical application.
5.1. ZnO This oxide significantly suppresses the formation of liquid products and enhances the char formation. This is presumably due to the formation of ZnCl2. The reduction of chlorobenzene formation indicates a potential application of the oxide for the inhibition of organochlorine compounds. 5.2. Fe2O3 The formation of gaseous products, CO and CO2, is enhanced by the addition of iron oxide due to its reduction to metallic Fe under the present pyrolysis condition. The oxide promotes the formation of chlorobenzene. One possible explanation for this observation is the formation of the catalytic amount of iron chloride or intermediate Cl–Fe species by the pyrolysis, although further study should be carried out to get insight into the mechanism. 5.3. Al2O3 The abundance of benzene becomes higher and the formation of gaseous products is suppressed by the addition of this oxide (ca. 20% enhancement for the former and 70% reduction for the latter, respectively, in comparison with the case of pure PVC at 800 8C). These tendencies are not observed in other oxides systems. Neither chlorination nor reduction of oxide occurs under the present experimental conditions. 5.4. PbO The influence of this oxide for the liquid and gaseous pyrolysis products is not remarkable. Circa 70% of the initial Cl is converted to volatile PbCl2. 5.5. CaO The formation of carbonaceous residues is suppressed, and presumably the abundance of a group of polycyclic aromatic hydrocarbons and related compounds (PAHs group) becomes high. Approximately half of Cl is consumed for the formation of chloride, giving rise to the complex hydration compounds which are due to the hydration process during the pyrolysis as well as absorption of water from the ambient atmosphere after pyrolysis run.
References [1] A. Okuwaki, Polym. Deg. Stab. 85 (2004) 981. [2] J.C. Wagner, A.E.S. Green, Chemosphere 26 (1993) 2039. [3] T. Katami, A. Yasuhara, T. Okuda, T. Shibamoto, Environ. Sci. Technol. 36 (2002) 1320. [4] D. Braun, Thermal decomposition of poly(vinyl chloride), in: N. Grassie (Ed.), Developments in Polymer Degradation, vol. 3, Applied Science Publishers, London, 1981. [5] K.S. Minsker, S.V. Kolesov, G.E. Zaikov, Degradation and Stabilization of Vinyl Chloride Based Polymers, Pergamon Press, Oxford, 1988. [6] R. Knu¨mann, H. Bockhorn, Comb. Sci. Tech. 101 (1994) 285. [7] B. Zhang, X.Y. Yan, K. Shibata, M. Tada, M. Hirasawa, High Temp. Mater. Processes 18 (1999) 197. [8] B. Zhang, X.Y. Yan, K. Shibata, T. Uda, M. Tada, M. Hirasawa, Mater. Trans. JIM 41 (2000) 1342. [9] O. Terakado, M. Hirasawa, J. Anal. Appl. Pyrol. 73 (2005) 248. [10] H. Sasamoto, Kinzoku 74 (2004) 1155. [11] T. Iida, K. Goto, J. Polym. Sci. Polym. Chem. Ed. 15 (1977) 2427. [12] A. Ballistreri, S. Foti, P. Maravigna, G. Montaudo, E. Scamporrino, J. Polym. Sci. Polym. Chem. Ed. 18 (1980) 3101. [13] A. Ballistreri, G. Montaudo, C. Puglisi, E. Scamporrino, D. Vitalini, J. Polym. Sci. Polym. Chem. Ed. 19 (1981) 1397. [14] M.C. Gupta, S.G. Viswanath, Ind. Eng. Chem. Res. 37 (1998) 2707. [15] M. Blazso´, E. Jakab, J. Anal. Appl. Pyrol. 49 (1999) 125. [16] G. Sivalingam, R. Karthik, G. Madras, Ind. Eng. Chem. Res. 42 (2003) 3647. [17] S.V. Levchik, E.D. Weil, Polym. Adv. Technol. 16 (2005) 707. [18] R. Miranda, H. Pakdel, C. Roy, H. Darmstadt, C. Vasile, Polym. Deg. Stab. 66 (1999) 107. [19] M. Inagaki, Y. Okada, H. Miura, H. Konno, Carbon 37 (1999) 329. [20] M. Inagaki, K. Fujita, Y. Takeuchi, K. Oshida, H. Iwata, H. Konno, Carbon 39 (2001) 921. [21] See B. Zhang, Ph.D. Thesis, Tohoku University, 2000, for details. In brief, the formation of quasi binary fluid, xPbOyPbCl2 is considered and its decomposition can be described as: xPbOyPbCl2 ! xPb(l) + xCO(g) + yPbCl2(g).The weight reduction in TG curve above 650 8C is assumed to correspond to the evaporation of xCO and yPbCl2. Since the value x + y corresponds to the total amount of the initial lead, it is possible to estimate the apparent composition of the quasi binary fluid. [22] S. Kasaoka, Y. Sakata, M. Shirata, J. Chem. Soc. Jpn. Chem. Ind. Chem. 11 (1977) 1728. [23] R.P. Lattimer, W.J. Kroenke, J. Appl. Polym. Sci. 25 (1980) 101; R.P. Lattimer, W.J. Kroenke, J. Appl. Polym. Sci. 27 (1982) 1355. [24] P. Carty, E. Metcalfe, S. White, Polymer 33 (1992) 2704.
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[25] J. Mu¨ller, G. Dongmann, J. Anal. Appl. Pyrolysis 45 (1998) 59. [26] See B. Zhang, Ph.D. Thesis, Tohoku University, 2000, for details. The gradual increase of mass number of m/z = 1 and 2 was observed at this temperature range, that can be ascribed to the dehydration. [27] T. Uda, K.T. Jacob, M. Hirasawa, Science 289 (2000) 2326.
[28] [29] [30] [31]
T. Uda, Mater. Trans. 43 (2002) 55. J.A. Duffy, M.D. Ingram, J. Am. Chem. Soc. 93 (1971) 6448. A. Lebouteiller, P. Courtine, J. Solid State Chem. 137 (1998) 94. J. Portier, G. Campet, J. Etourneau, B. Tanguy, J. Alloys Compd. 209 (1994) 285.
Pyrolysis study of poly(vinyl chloride)–metal oxide mixtures: Quantitative product analysis and the chlorine fixing ability of metal oxides Y. Masuda a, T. Uda a,1, O. Terakado b,*, M. Hirasawa b a
Institute of Multidisciplinary Research for Advanced Materials, Tohoku University, Katahira 2-1-1, Aoba-ku, 980-8577 Sendai, Japan b Department of Materials Science and Engineering, Graduate School of Engineering, Nagoya University, Furo-cho, Chikusa-ku, 464-8603 Nagoya, Japan Received 5 September 2005; accepted 7 March 2006 Available online 18 April 2006
Abstract Thermal decomposition of mixtures of poly(vinyl chloride) ( PVC) and a variety of metal oxides including rare earth oxides has been studied under helium atmosphere at 400 and 800 8C. Volatile pyrolysis products have been quantitatively analysed. The formation of benzene at 800 8C is suppressed by the addition of oxides except aluminium oxide, especially in the case of zinc oxide, reducing 80% compared with the case of pure PVC. The emission of hydrogen chloride changes significantly with the oxides used. This feature is strongly concerned with the chlorine fixing ability of oxides. Together with the analysis of pyrolysis residues, the behaviour of fixation of chlorine from PVC by oxides has been compared. The trivalent rare earth oxides (Ln2O3) show great ability to fix considerable amount (more than 95% in the case of lanthanum oxide) of initial chlorine in PVC in the form of oxychlorides (LnOCl). The water-insoluble property of LaOCl is advantageous for the use of La2O3, which is abundant in the sludge discharged from magnet production industry, as chlorine fixing agent. # 2006 Elsevier B.V. All rights reserved. Keywords: PVC; Metal oxides; Chlorine fixing; Plastic recycling
1. Introduction Recycling of waste plastics is of great importance from the environmental point of view. Among various treatment processes, pyrolysis method is a powerful tool which can be applied to the feedstock recycling of synthetic polymers. An example is the usage of waste plastics in coke ovens for steel making process [1]. Poly(vinyl chloride) (PVC) is a widely used cheap, commodity polymer, showing good mechanical properties as well as great chemical resistance. However, incineration of waste PVC results in the emission of harmful hydrogen chloride (HCl), and organochlorine compounds, such as chlorobenzene, polychlorinated dibenzo-p-dioxins (PCDDs) and polychlorinated dibenzofurans (PCDFs) [2,3]. It is, therefore, very important to understand the thermal decom-
* Corresponding author. Tel.: +81 52 789 5308; fax: +81 52 789 5308. E-mail address: [email protected] (O. Terakado). 1 Present address: Division of Materials Science and Engineering, Graduate School of Engineering, Kyoto University, Yoshida-hommachi, Sakyo-ku, 6068501 Kyoto, Japan. 0165-2370/$ – see front matter # 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jaap.2006.03.001
position behaviour of PVC, and a number of studies have so far been carried out, see e.g. [4–6]. During the course of our investigations on the pyrolysis of mixtures of polymer and metal oxides, we have examined the possibility for the simultaneous recycling process of large amount of waste plastics and metallurgical dusts, consisting mainly of metal oxides [7–9]. The discharge of the electric arc furnace (EAF) dust reaches up to 500,000 tonnes per year in Japan [10]. Our previous studies [7,8] showed that zinc could be separated from ZnFe2O4, main component of EAF dust, in the form of zinc chloride gas by thermal decomposition of the mixture of PVC and ZnFe2O4, while iron remains in the pyrolysis residue as metallic iron. This result suggests a potential application of steel making dust as sorbent of HCl from PVC. Furthermore, the Zn-eliminated dust may be recycled for steel making process. Zinc is known to corrode furnaces so that it must be thoroughly removed for recycling of dusts. As for the plastic industry, various additives are used for the enhancement of the functionality of polymers or flame retardancy of plastics. The influences of metal oxide additives on the thermal decomposition or stabilisation of PVC have been also extensively investigated in many works [11–16]. They
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focus mainly on the kinetics of decomposition and the emission of volatile compounds like benzene by applying experimental methods such as TG and pyrolysis-gas chromatography. Taking into account the influence of volatile compounds to the environment, it is necessary not only to investigate qualitative analysis but also to acquire quantitative information on the influence of additives. In the present study, we have performed quantitative analysis of the pyrolysis products (gas, liquid, and pyrolysis residue) of mixtures of PVC and various metal oxides, including ZnO, Fe2O3, rare earth oxides, CaO, PbO, and Al2O3. It should be noted that the latter three compounds are often abundant in used plastics. As for the rare earth oxides, quite large amount of sludge containing these oxides is discharged from magnet production industry. In comparison with the reported studies on this subject, such as the influence of additives on the flame retardancy of PVC [17], higher concentration of metal oxides has been examined in the present study for the purpose of the investigation of the recycling process of the wastes containing metal oxides as mentioned above. From this point of view, we will also discuss the chlorine fixing of metal oxides through pyrolysis of the PVC–metal oxide mixtures. 2. Experimental Table 1 summarises the physical properties of oxide used in the present study. The measurement of particle size was performed with a laser micron sizer equipment, LMS-30 (Seishin Enterprise Co. Ltd.) for the suspension of oxide powder in water. The BET surface area was determined with a TriStar 3000 equipment (Micrometrics). The reagent grade powders of PVC (Wako Pure Chemicals; average molecular weight = 6.9 104 and average particle size = 160 mm) and metal oxides were mixed mechanically and pressed to a disc of 8 mm diameter (thickness 2–3 mm). The amount of PVC was kept constant (250 mg). In the present study, the composition of [PVC]:[oxide] = 2:1 in molar ratio was investigated, where one mole of PVC means that of the structural unit of polymer, C2H3Cl. The experimental set-up of pyrolysis is schematically shown in Fig. 1. All pyrolysis runs were conducted under helium (99.9999%) gas flow with the flow rate of 100 ml/min. After the temperature of high temperature zone in the furnace was Table 1 Physical properties of metal oxide Oxide
Supplier
ZnO CaO Fe2O3 PbO La2O3 Nd2O3 CeO2 Al2O3
Kanto Chemicals Kanto Chemicals Kanto Chemicals Kanto Chemicals Wako Pure Chemicals Chimie Rhodia Wako Pure Chemicals Kojundo Chem. Lab. Co. Ltd.
Mean particle size (mm) 1.9 a
9.9 8.4 15 9.0 10.6 3.6
BET surface area (m2/g) 2.8 4.9 5.9 0.2 1.0 1.2 4.1 2.9
a It is not possible to obtain the data for CaO because of the formation of hydroxide in water.
Fig. 1. Schematic experimental set-up for pyrolysis.
stabilised at 400 or 800 8C, sample, which had been placed in an alumina reaction boot, was introduced quickly from the cold part to the reaction zone. In the present work, we carried out pyrolysis for 30 min at 400 8C and 10 min at 800 8C, respectively. The quartz wool inside the reaction tube prevents the contamination of soot into the subsequent traps. Acetone traps collect organic liquid products. The following water trap is that for HCl emitted from the sample. Other gaseous products were first collected at the last trap cooled with liquid nitrogen, and then introduced to a gasbag. This procedure avoids the contamination of water into the gasbag. Some of organic substances were adhered as soot on the reaction tube and quartz glass wool. They were rinsed with tetrahydrofuran (THF) after each experimental run. We regard the dissolved compounds as tar and not-dissolved ones as wax in the present study. Liquid and gaseous products and tar solutions were analysed by using a Hewlett-Packard Gas Chromatograph/Mass Spectrometer (GC/MS) analyser 6890/5973. As for the GC/MS column we used HP-5MS column for liquid products (except for the quantitative analysis of benzene and toluene) and HP-PLOTQ column for gaseous products, benzene, and toluene. Quantitative analysis of the gaseous and liquid products was performed by standard calibration method. The typical experimental errors of the amount of gaseous and liquid products were 15 and 10%, respectively. The quantity of emitted HCl was analysed through the Cl anion content by a liquid chromatography apparatus (LC10AD VP, Shimadzu). Since the arrangement of traps in Fig. 1 might result in the collection of HCl also at the acetone traps, we carried out separate pyrolysis runs without acetone traps and analysed the water trap. If water-soluble metal chlorides formed during the pyrolysis would be collected at the water trap, they could affect the result of ion chromatography. The water trap was, therefore, analysed with an ICP spectrometer (Optima 3300, Perkin-Elmer) to examine the existence of metal species in the trap. The result showed that less than 0.1% of all Cl was due to metal chlorides, which attributed the amount of Cl anion solely to the emission of HCl. Thus, the HCl content was obtained directly from the Cl concentration in the trap. The error of the concentration was within 10% in most cases, but in the case of the PVC–La2O3 system, it was 30% because the emission of HCl in this case was very small, as to be seen in Section 3 of the present report. Pyrolysis residues in the reaction boot were characterised with X-ray diffractometer RINT 2200 (Rigaku). The carbon
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contents in the residues were determined at Kawasaki Steel Techno-research Co. Ltd. by combustion method under oxygen flow equipped with IR spectrometer. The hydrogen content was determined in the following way. After a pyrolysis run the residue was heated to 700 8C under oxygen atmosphere for 15 min. The resulting H2O was collected with a cold trap (100 8C, at which CO and CO2 gases were not trapped), which was then introduced to a tube filled with calcium chloride. The amount of water was determined from the weight change of the tube. The carbon content of soot was determined by the similar combustion method. In this case, CO and CO2 gases were collected and analysed with GC/MS. Separate run after THF dissolution of the reactor allows the determination of carbon content of wax. That of tar was, thus, evaluated by subtraction of carbon content in wax from that in soot. The errors of those of wax and tar were not estimated because the complete collection of tar and wax components on the wall of reaction tube as well as in the gas flow system was not possible. 3. Results 3.1. Product analysis of organic compounds and gases Some typical results of GC/MS analysis of the liquid, gaseous, and tar products for the pyrolysis at 800 8C are shown in Figs. 2–5, respectively. In all cases, we have not found any difference in chemical species of products by addition of metal oxides. As shown in Figs. 2 and 3, various aromatic hydrocarbons of C6–C14 including benzene and toluene have been detected as liquid products. The figure also demonstrates the suppression of the formation of liquid products by addition of metal oxides, i.e. ZnO in this case. The main gaseous products are CO, CO2, and hydrocarbons with C1–C3 (Fig. 4),
Fig. 2. Representative GC/MS results of liquid products by pyrolysis of PVC and PVC–ZnO at 800 8C (column: HP-5MS).
Fig. 3. Chromatogram for benzene and toluene by pyrolysis of PVC and PVC– ZnO at 800 8C (column: HP-PLOTQ).
and various aromatics with 3–6 benzene rings (depicted in Fig. 5) have been observed as tar. In this case, again, any changes in chemical species of the product have not been observed, so that it is rather important to carry out quantitative analysis to find out the influence of metal oxides in detail. In the present study twelve light organic compounds, indicated in Figs. 2 and 3, and gaseous products have been quantitatively analysed with GC/MS by standard calibration method. The results of the products at 800 8C are summarized in Fig. 6. Each amount is normalised by that of reactant PVC in the initial sample, i.e. (mass of product)/(mass of PVC in the initial mixture). In all cases benzene and toluene are major liquid compounds. Obviously, the amounts of liquid products are reduced by addition of metal oxides, especially significantly in the case of Fe2O3 and ZnO, where the amount of the liquid products has been less than 30% of that in the case of pure PVC. In the pyrolysis of PVC–Al2O3 mixture, the formation of benzene and toluene is enhanced, as reported also in literature [15]. Concerning the organochlorine compounds, the formation of chlorobenzene, one of the precursors of PCDD/Fs, was analysed in the present study. The yield is listed in Table 2. Any significant correlation has been hardly found between the amount of liquid products and organochlorine compounds from the present results. The addition of Fe2O3 and CeO2 enhances the formation of chlorobenzene, whereas significant suppression is observed for zinc oxide. The influence of iron oxide is indicative, since the Friedel–Crafts halogenation of benzene is promoted by Lewis acids, such as iron chloride. This feature is to be further considered in Section 4. As for the gaseous products, considerable amounts of CO and CO2 were emitted by the reaction of PVC–Fe2O3 mixture,
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Table 2 Yield of chlorobenzene, normalized by the weight of initial PVC Wchlorobenzene (mg)/WPVC (g) PVC PVC–ZnO PVC–CaO PVC–Fe2O3 PVC–PbO PVC–La2O3 PVC–Nd2O3 PVC–CeO2 PVC–Al2O3
0.15 0.03 0.07 0.24 0.07 0.06 0.15 0.25 0.09
as presented in Fig. 6 (upper panel). This is due to the reduction process of iron oxide to metal iron, which has been confirmed also by our previous XRD and simultaneous TG–MS measurements [7,8]. In contrary to the case of Fe2O3, the addition of alumina leads to repression of gaseous compounds. The results of pyrolysis at 400 8C for several PVC–metal oxide systems are shown in Fig. 7, together with those at 800 8C. In most cases, less gaseous and liquid products are formed at 400 8C in comparison with the pyrolysis at higher temperature. The fraction of benzene to liquid products becomes higher, and the abundance of Cn with n > 8 is very small at low temperature. The latter tendency was also reported in literature on the vacuum pyrolysis of pure PVC, for which the enhancement of polymer chain scission and the secondary reaction were responsible with increasing temperature [18].
Fig. 4. Gas chromatograms of gaseous products for the pyrolysis of PVC and PVC–PbO at 800 8C.
Fig. 5. Gas chromatograms of tar products for the pyrolysis of PVC at 800 8C.
Other important result of the pyrolysis at 400 8C is that the formation of chlorobenzene has not been detected. 3.2. Mass balance of carbon The mass balance of carbon for pyrolysis products at 800 8C is summarised in Fig. 8. Most of miscellaneous compounds are
Fig. 6. Product yields of thermal decomposition of PVC with existence of metal oxides at 800 8C: gaseous products (upper panel) and liquid products (lower panel).
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Fig. 9. Mass balance of carbon of pyrolysis products at 400 8C, together with the result at 800 8C for comparison. The symbol ‘*’ indicates that the carbonaceous compounds in residue are hydrocarbons. See the text for details.
Fig. 7. Comparison of gaseous and liquid product yields at 400 and 800 8C.
presumably not-collected tar and wax adhered on the wall of reaction tube as well as in the gas flow system. Thus, quantitative discussion on the tar and wax is less accurate. On the other hand, the following features are clearly observed, some of which have been also reported in literature [11–16]. Formation of char is enhanced by the addition of ZnO, while calcium oxide causes the reduction of carbon in residue. As
Fig. 8. Mass balance of carbon for the pyrolysis products at 800 8C.
described in the previous section, the amount of gaseous products is considerably high for Fe2O3. On the other hand, the addition of Al2O3 suppresses more than 70% of gas in comparison with the case of pure PVC. In contrary to this, the mixture of PVC–PbO and PVC–rare earth oxides looks to give similar results with pure PVC. The carbon mass balance at 400 8C is presented in Fig. 9, together with that of 800 8C for comparison. By addition of oxides the proportion of carbons in residue, which exist as hydrocarbons (see next section for detail), increases, and the yields of gaseous and liquid products do not change significantly. An exception is the case of ZnO where the considerable decrease of liquid products has been observed, as is also indicated in Fig. 7. As for the influence of pyrolysis temperature, gas formation is promoted at high temperature. We do not find a systematic difference in the amount of tar at different pyrolysis temperature. 3.3. Pyrolysis residues In Fig. 10, XRD patterns of solid residues of pyrolysis at 800 8C are presented. For a pyrolysis residue of PVC without additives, the XRD pattern shows a broad peak around 2u 268, indicating the partial formation of layered structure of graphite [19,20]. The addition of oxides results in the existence of metal compounds in the residues. In the case of alumina and ZnO, XRD peaks solely of oxides are found. It should be noted that thermal decomposition of PVC–ZnO mixture proceeds with the formation of ZnCl2 and the consequent quasi binary ZnCl2– ZnO fluid, then evaporation of chloride due to its high vapour pressure with increasing temperature [7,8]. The XRD patterns of the Fe2O3 and PbO systems contain those of metal Fe and Pb, respectively, indicating the reduction of each oxide, although, in the latter system, the influence of lead chloride and the subsequent PbCl2–PbO quasi binary mixture, similar to the zinc system, should also be considered for the reaction process in the mixture. In the case of PVC–CaO mixture not only calcium oxide but also CaClOH are present in the residue. This is presumably concerned with the chlorination and hydration process of oxide, as will be described in Section 4. As for the
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rare earth oxides, most of lanthanum oxide is converted to oxychloride. Less oxychloride is formed for Nd2O3, and lesser for CeO2, that can be expected from the intensity ratio of the peaks attributed to respective oxychloride and oxide in the XRD patterns of these oxide–PVC systems. The pyrolysis residue at lower temperature (400 8C) exhibits slightly different XRD pattern, as depicted in Fig. 11. The broad peak for pure PVC is shifted to the lower angle compared with that at 800 8C. We have examined the hydrogen contents of pyrolysis residues of PVC–ZnO and PVC–Al2O3. In both cases ca. 2 wt.% of hydrogen at 800 8C and 30–50 wt.% at 400 8C were observed. Thus, the carbonaceous compounds in pyrolysis residues at 400 8C are mainly hydrocarbons. For the PVC–ZnO system, the formation of Zn5(OH)8Cl2H2O is observed, which is owing to the formation of ZnCl2 and H2O [7,8]. The absorption of water after the experimental run, e.g. during the XRD measurement, in ambient atmosphere can also contribute to the complex form of residue compounds because of the hygroscopic property of zinc chloride. As will be mentioned in the next section, the reduction or chlorination of oxides is not completed at 400 8C. This is also the case for the La2O3 system, though its chlorination proceeds to higher degree than the zinc system. 3.4. HCl emission and chlorination of oxide Fig. 10. XRD patterns of the residues of the PVC–metal oxide mixtures (800 8C). The peak of graphite at 2u = 268 is depicted as reference. Since XRD data of CeOCl are not available in literature, that of LaOCl is shown in the XRD data of PVC–CeO2. A good agreement indicates the similarity of structure for both oxychlorides.
In addition to the liquid organochlorine compounds, oxychlorides and metal chlorides as chlorine containing pyrolysis products, thermal decomposition of PVC causes the emission of HCl that is collected at water trap in the present pyrolysis set-up. The analytical results of the amount of trapped Cl anion are summarised in Table 3. The HCl emission is, in general, reduced by the addition of oxide and is especially low in the case of La2O3 at 800 8C. Taking into account the result of the XRD measurement and the order of the amount of organochlorine products, i.e. <1 mg/1 g PVC, we regard that almost all chlorines from PVC are converted to lanthanum oxychloride. The addition of PbO results in the conversion of only 30% of initial chlorine into HCl at 800 8C. The remainder is mainly responsible for the formation of lead chloride. This result is consistent with a TG study for a 2PVC–1PbO mixture, where the mass balance calculation of lead showed the formation of Table 3 The ratio of trapped Cl at water trap and the initial chlorine at 800 8C, together with the result at 400 8C for the five systems (in percent)
Fig. 11. XRD patterns of the residues of the PVC–metal oxide mixtures (400 8C).
PVC PVC–ZnO PVC–Fe2O3 PVC–CaO PVC–PbO PVC–Al2O3 PVC–La2O3 PVC–Nd2O3 PVC–CeO2
400 8C
800 8C
100 49
92 36 69 51 30 90 3.1 25 78
31 89 24
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quasi binary PbCl2–PbO mixture with the composition of X(PbO) 0.3 in molar fraction (liquidus temperature = 490 8C): in other word ca. 70% of chlorine from the initial sample was converted to lead chloride [21]. As for the other metal oxides, ca. 70–80% of Cl is converted to HCl for Fe2O3 and CeO2 at 800 8C, indicating that the chlorination of metals or metal oxides is not a major reaction for these systems. On the other hand, the pyrolysis behaviour of the PVC–Al2O3 system is very similar to that of pure PVC with regard to HCl emission. The chlorination of alumina has not been observed under the present experimental conditions. The results for the Fe2O3, CeO2, and Al2O3 systems are consistent with the slow kinetics of the reaction between these oxides and HCl gas, compared with other oxides like ZnO and PbO [22]. Also at lower temperature (400 8C) the addition of metal oxide reduces the emission of HCl in comparison with the case of pure PVC. For the PVC–ZnO and PVC–La2O3 systems, the results indicate that the formation of chloride or oxychloride undergoes slower than that at high temperature, while the chlorination of PbO seems to complete at <400 8C, as indicated by almost the same HCl emission at different temperature for the latter system.
the other hand, ZnCl2 has high vapour pressure, so that the kinetics of chloride evaporation accompanied by char formation should be considered. With respect to this point, it is noteworthy to mention the work by Ballistreri et al. [13] on the direct pyrolysis of PVC–metal oxide mixture in mass spectrometer, in which the simultaneous emission of ZnCl2 and benzene was observed. They anticipated that the presence of zinc chloride was essential for the suppression of liquid products. Furthermore, our previous TG–MS study showed that the ZnCl2 evaporation started above 400 8C [8]. Taking into account the existence of ZnCl2 species in pyrolysis residue as well as high abundance of residue materials for the PVC–ZnO system at 400 8C (Fig. 9), we deduce the following scenario. The precursor species of char (presumably cross-linked hydrocarbons) are formed simultaneously with the formation of zinc chloride below 400 8C. The ZnCl2 starts to vaporise above 400 8C, and consequently the carbonisation of residue proceeds. However, the chlorination of ZnO does not complete at 400 8C, as seen in the data of HCl emission at this temperature (Table 3). Further studies on the kinetics of chlorination of ZnO and the evaporation of ZnCl2 are required in order to verify the scenario suggested above in more detail.
4. Discussion
4.1.3. Fe2O3 The substantial suppression of liquid products by Fe2O3 indicates that the oxide can also play a role of Lewis acid, as described in the present section. The formation of catalytic amount of iron(III) chloride can enhance the char formation, although no trace of iron chloride was so far found in the XRD measurement of the pyrolysis residue and ICP measurement in water trap. In the work by Ballistreri et al. [13], the existence of another mechanism for the suppression of liquid products was speculated. Nevertheless, we cannot explicitly exclude the possibility of the formation of catalytic amount of iron chloride species, since the slight dissipation of iron from the residue has been observed under the present experimental condition [7]. This may be also supported by the subtle decrease of HCl emission for the PVC–Fe2O3 in comparison with the case of pure PVC (see Table 3). Moreover, the formation of intermediate species, such as Cl–Fe, to suppress the liquid products may be considered, although further studies are required to clarify the peculiarity of the influence of iron oxide.
4.1. Influence of oxides on the decomposition products of PVC 4.1.1. General remarks The mechanism of PVC pyrolysis has so far been studied very extensively. Polyene structure due to dehydrochlorination causes either cyclisation, leading to the formation of aromatics like benzene, or char formation owing to the bridging of the polyene structures. Since benzene is the major liquid product, the formation of benzene is a proper measure of the cyclisation of dehydrochlorinated PVC. The formation of benzene is known to involve the detachment of precursor cyclohexadiene intermediates from the polymer chain [23]. On the other hand, it is known that Lewis acid, such as iron(III) chloride, enhances the bridging of polyene chains through cationic polymerisation, leading to the char formation [24,25]. In the present case, the influence of metal oxides on the decomposition process of PVC as well as the transformation of oxides, such as reduction and chlorination, are of special interest. Because of the presence of miscellaneous compounds with a few exceptions, i.e. PVC– ZnO and -Fe2O3 at 800 8C, quantitative discussion on the whole pyrolysis products is not possible. On the other hand, since the miscellaneous compounds are considered mainly as uncollected tar and wax, these can be categorised into a group of polycyclic aromatic hydrocarbons and relevant compounds, designated hereafter as PAHs group. 4.1.2. ZnO The addition of ZnO causes the enhancement of char formation and the reduction of liquid products, as seen in Fig. 8. A possible explanation of this result is that it owes to the formation of ZnCl2, which is known as strong Lewis acid. On
4.1.4. Al2O3 Aluminium oxide is neither chlorinated nor reduced through the pyrolysis under the present experimental conditions. The catalytic effect of alumina directs to the promotion of detachment of precursor cyclohexadiene intermediates, leading to the formation of benzene, as suggested by Blazso´ and Jakab [15]. Another interesting observation is the suppression of gaseous products. The detailed mechanism is, however, still unclear for the relationship between the enhancement of benzene and the repression of gaseous products formation. 4.1.5. CaO and other oxides The addition of calcium oxide leads to the reduction of residue. This observation indicates that the bridging of polyene
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chains is hindered to some extent. The relation with the enhanced formation of PAHs group is interesting, although the detailed mechanism is an open question because of the lack of precise quantitative information. Other metal oxides do not significantly affect the amount of liquid and gaseous products, although reduction or chlorination of oxide occurs during the pyrolysis. This can be ascribed to the less catalytic activity of these oxides for the detachment of preformed cyclohexadienes and for the char formation, or to the less affinity between oxide and polymer to affect the decomposition of PVC. On the one hand, experimental supports such as study on the interfacial properties between PVC and single crystal metal oxide are desired to get insight into the catalytic role of metal oxides onto the thermal decomposition of PVC. On the other hand, it is intriguing to understand the influence of oxides onto the pyrolysis products systematically in terms of their chemical properties such as acidity. It should be noted here that no correlation is, so far, found between the amount of specific product and physical surface characteristics of the oxides (see Table 1). The study is now under way to get further insight into the role of metal oxides. 4.2. Fixation of Cl by metal oxides 4.2.1. Comparison of oxides As apparent from the results of XRD and the HCl emission, the fixing of chlorine by metal compounds changes significantly among oxides used. We now summarise the chlorine fixing of metal oxide through the pyrolysis. This information will be useful for the environment-friendly treatment of waste PVC and metallurgical dusts. As mentioned in the previous sections, alumina does not possess chlorination ability under the present experimental conditions, because the chlorination and the formation of oxychloride have not been observed. In the case of iron oxide, the reduction process is dominant, and hence the chlorine from PVC is emitted chiefly as HCl. On the other hand, it is of interest that the amount of chlorobenzene increased considerably by the addition of Fe2O3 (Table 2). Though the reaction mechanism is not clear yet, a reasonable account is based on the scheme of Friedel–Crafts chlorination of benzene through Lewis acid like FeCl3. It is possible that a part of initial Cl can contribute to form the iron chloride or Cl–Fe intermediate species at the interface between molten PVC and Fe2O3, which can play a role to generate the chlorobenzene according to the Friedel–Crafts scheme. This is further to be clarified in the future work. Nearly half of chlorine is fixed as the complex form of calcium compounds, such as CaClOH. Our previous TG–MS study demonstrated a possibility of the dehydration of Ca(OH)2 occurs at 630–720 8C [26]. Complex compounds, consisting of CaO, CaCl2, and Ca(OH)2, would be formed during pyrolysis. Zinc oxide and lead oxide are chlorinated, over 50 and ca. 70% of Cl being converted to volatile chlorides, respectively. As for the rare earth oxides, lanthanum oxide can convert up to 95% of initial chlorine in PVC to lanthanum oxychloride,
LaOCl. Taking the water-insoluble property of LaOCl into account, we consider that the use of La2O3 is beneficial to fix the chlorine in the form of solid oxychloride and to avoid the flowage through soil into the environment in comparison with the conventional Cl fixation agent, such as CaO. It should also be noted here that the oxychloride can be converted back to the corresponding oxide and HCl by conventional hydrolysis method at high temperature. Both products are used in various processes: the industrial production of rare earth metals is based on the electrolysis of the oxide, whereas hydrogen chloride can be utilised as chlorination agent. The present observation serves good basis for the utilisation of chlorine in processing of rare earths as well as wastes containing the oxides in the view of chlorine circulation [27,28]. 4.2.2. Interpretation with optical basicity of oxides The obtained results indicate that no straightforward correlation is found between the degree of chlorination or reduction and their physical surface characteristics of oxides examined. Thus, it is of interest to summarise the chlorine fixing ability of oxides in terms of their chemical properties systematically. For this purpose, we examine the concept of optical basicity, defined by Duffy and Ingram [29], which is widely applied in the fields of glass science and metallurgy in order to discuss the acidity or basicity of oxides. Fig. 12 shows the amount of HCl at 800 8C as a function of optical basicity of the corresponding oxide. The basicity value has been estimated by using a relation after Lebouteiller and Courtine [30] with appropriate parameters [31]. We see fairly well correlation between the basicity of oxide and the HCl emission. The emission of HCl decreases with increasing the basicity of oxide. However, large deviation is found for the trivalent rare earth oxide systems.
Fig. 12. The normalised amount of HCl from the pyrolysis of PVC–MO mixture at 800 8C, plotted as a function of optical basicity of the corresponding oxide. The basicity data are taken from [30]. That of Nd2O3 is evaluated from the empirical equation given in [30] with the proper parameter in [31]. For comparison, the optical basicity of FeO is also indicated in the figure with the same HCl data of Fe2O3. The line is the guide to the eye.
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5. Summary
5.6. Rare earth oxides
In the present study, we have investigated the influence of variety of metal oxides onto thermal degradation of PVC under inert atmosphere. As a general tendency, the addition of metal oxides reduces the amount of liquid products, while the formation of gaseous products is not influenced by the oxide except Fe2O3 and Al2O3. The characteristic features of the influence of each oxide addition can be summarised as follows.
The formation of chlorobenzene is enhanced by the addition of CeO2, though the reason is still unclear yet. The peculiar feature is the change in the chlorine fixing ability by the oxide examined. It changes with the order of La2O3 > Nd2O3 CeO2. The addition of La2O3 converts Cl from PVC to oxychloride with the efficiency of more than 95% at 800 8C and 75% at 400 8C. This is quite efficient compared with the conventional HCl sorbent such as CaO. The resulting LaOCl is not dissolved into water, dissimilar to the CaCl2 which can flow out through soil into the environment. Considering the sufficient suppression of the formation of chlorobenzene by the addition of La2O3, we regard the present result indicates a potential application for the fixation of chlorine in incineration process. Further investigations under the atmosphere with the presence of oxygen are required for the practical application.
5.1. ZnO This oxide significantly suppresses the formation of liquid products and enhances the char formation. This is presumably due to the formation of ZnCl2. The reduction of chlorobenzene formation indicates a potential application of the oxide for the inhibition of organochlorine compounds. 5.2. Fe2O3 The formation of gaseous products, CO and CO2, is enhanced by the addition of iron oxide due to its reduction to metallic Fe under the present pyrolysis condition. The oxide promotes the formation of chlorobenzene. One possible explanation for this observation is the formation of the catalytic amount of iron chloride or intermediate Cl–Fe species by the pyrolysis, although further study should be carried out to get insight into the mechanism. 5.3. Al2O3 The abundance of benzene becomes higher and the formation of gaseous products is suppressed by the addition of this oxide (ca. 20% enhancement for the former and 70% reduction for the latter, respectively, in comparison with the case of pure PVC at 800 8C). These tendencies are not observed in other oxides systems. Neither chlorination nor reduction of oxide occurs under the present experimental conditions. 5.4. PbO The influence of this oxide for the liquid and gaseous pyrolysis products is not remarkable. Circa 70% of the initial Cl is converted to volatile PbCl2. 5.5. CaO The formation of carbonaceous residues is suppressed, and presumably the abundance of a group of polycyclic aromatic hydrocarbons and related compounds (PAHs group) becomes high. Approximately half of Cl is consumed for the formation of chloride, giving rise to the complex hydration compounds which are due to the hydration process during the pyrolysis as well as absorption of water from the ambient atmosphere after pyrolysis run.
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