B. Guías, Protocolos y Educación

Y. Masuda, T. Uda, O. Terakado¹* and M. Hirasawa¹

Institute of Multidisciplinary Research for Advanced Materials, Tohoku University, Sendai, Japan

1Current Address: Department of Materials Science and Engineering, Graduate School of Engineering, Nagoya University, Nagoya, Japan, [email protected]

Abstract: Thermal decomposition of poly(vinyl chloride), PVC, in the presence of various metal oxide, MO, has been studied under inert atmosphere. The pyrolysis products have been quantitatively analysed. In general, the addition of oxide leads to the reduction of the amount of liquid products such as benzene: the addition of ZnO and Fe₂O₃ is espe-cially effective, reducing more than 70 % compared to the case of the pure PVC. With re-spect to the environmental are-spect of the emission of hydrogen chloride from the pyrolysis of PVC, the chlorination ability of metal oxide has been discussed and compared among the oxides. It was found that the trivalent rare earth oxide showed remarkable ability to fi x Cl from PVC in the form of oxychloride.

1. Introduction

Pyrolysis treatment of waste plastics is an attractive method, in which the volume can be drastically reduced and the feedstock recycling of synthetic polymers are expected.

Thermal decomposition of PVC, one of the commodity polymers, causes the emission of hydrogen chloride, that is crucial for the corrosion of pyrolysis pipe line. In the present study, we have carried out quantitative product analysis of pyrolysis of the variety of mix-ture of PVC and metal oxide. This information is especially important for the feedstock recycling of PVC as well as simultaneous recycling process of metallurgical dusts, con-taining mainly metal oxides, and waste polymers, that we have proposed in our previous study [1].

2. Experimental

The details of the all experimental procedures are described elesewhere [2]. The sample was prepared by the physical mixing of PVC powder (Wako pure chemicals Co., Ltd.:

particle diameter ~ 160 μm) and reagent grade of metal oxide (particle size: 1-10 μm).

The composition of [PVC]:[MO]=2:1 was mainly investigated in this study. Pyrolysis was carried out at various temperauters in alumina reaction boot placed in quartz tube. A helium gas fl ow of 100 ml/min was passed through the quartz tube. After the pyrolysis compartment acetone trap, water trap and gas bag were placed in a line to collect light

organic compounds, HCl and gaseous products, respectively. Typical reaction time was 10 min at 800 ^oC.

Pyrolysis products have been carefully characterised by the following method. Light or-ganic compounds and gaseous products were analysed with GC/MS (Hewlett-Packard, 6890/5973). After a pyrolysis run quartz tube contains tar and wax adhered on the wall.

The tube was rinsed with THF. The dissolved compounds are designated as tar, while not-dissolved ones as wax. The amount of tar plus wax as well as wax was determined in separate runs by combustion method under oxygen atmosphere. The emission of HCl was examined by using ion chromatography (Shimadzu, LC-10AD VP) for the water trap in the pyrolysis line. It was confi rmed that the acetone trap was not contaminated by HCl.

3. Results and Discussion

3.1. Quantitative analysis of pyrolysis products of PVC-MO mixtures

Fig. 1 shows the gaseous and liquid pyrolysis products of various PVC-MO mixtures at 800 ^oC. The signifi cant feature of the gaseous products is the enhanced formation of CO and CO₂ in the case of PVC-Fe₂O₃. This is related to the reduction of Fe₂O₃ to metallic iron, as has been also confi rmed by our previous TG-MS and XRD study [1]. The major liquid products are benzene and toluene, and further ten major products have been quan-titatively analysed in the present study. Obviously, the addition of metal oxide leads to the reduction of these products, especially in the case of the PVC-ZnO and Fe₂O₃ system with more than 70 % of suppression in comparison to the pure PVC. On the other hand, the addition of Al₂O₃ gives rise to the enhancement of benzene formation as is also reported in literature [3].

The carbon mass balance of the pyrolysis residues is depicted in Fig. 2. The addition of iron oxide leads to the considerable enhancement of gaseous products, as described above, while Al₂O₃ affects to suppress these compounds with reduction of ca. 70 % compared to pure PVC. As for the formation of char, zinc oxide is especially effective to increase the ratio of carbon in residue, whilst the calcium oxide leads to the suppression. As well known in literature, thermal decomposition of pure PVC takes place with the following three stages with increasing temperature: Stage 1 (250-400^oC) dehydrochlorination and the consequent formation of polyene structure; Stage 2 (400-570 ^oC) decomposition of the polyenes in two ways, i.e. either cyclisation to form benzene and related products or bridging each other, giving rise to the formation of char; Stage 3 (>570 ^oC) carbonisation as well as further decomposition. The formation of benzene involves the detachment of a precursor cyclohexadiene ring from the polyene chains [4]. On the other hand, it is known that the existence of Lewis acid, such as FeCl₃, catalyses to cross-linking of polyene chains, giving rise to the char formation [5].

Figure 1: Yields of pyrolysis products of 2PVC-1MO mixture at 800 ^oC: gaseous (upper panel) and liquid products (lower panel). Major six gaseous and twelve liquid products, listed in the fi gure,

have been quantitatively analysed.

From these considerations and the present experimental results, the followings can be deduced about the infl uence of metal oxide additives. (1) The infl uence of Al₂O₃ directs to the promotion of detachment of precursor cyclohexadiene intermediates, giving rise to the formation of benzene. The suffi cient reduction of gaseous products is also an interesting observation, although the detailed mechanism is unclear yet. (2) Taking into account the suppression of liquid products, mainly benzene, the addition of ZnO and Fe₂O₃ is consid-ered to enhance the cross-linking of polyene chains. In the case of the latter oxide, howev-er, the char is consumed for the reduction of the oxide to metal iron. As mentioned above, a possible explanation for the catalytic effect of the char formation is owing to Lewis acid, such as zinc chloride, as has been found for the former system in our previous study [1].

On the other hand, the formation of iron chloride has not been so far confi rmed in the

0 10 20 30 40 50 60

XRD analysis of pyrolysis residue in the present work and in the literature concerning the direct pyrolysis analysis in the mass spectrometer [6]. This feature as well as the kinetics of the evaporation of volatile ZnCl₂ accompanied by the char formation should be further investigated in the future in order to clarify the infl uence of ZnO.

0 10 20 30 40 50 60 70 80 90 100

misc.

Wax Tar Carbon in residue Liquid Gas

PVC / Al₂O₃

W_C / W_{C PVC0} X 100 PVC / CeO₂

PVC / Nd₂O₃ PVC / La₂O₃ PVC / PbO PVC / Fe₂O₃ PVC / ZnO

PVC / CaO PVC

Figure 2: Mass balance of carbon of the pyrolysis products of 2PVC-1MO at 800 ^oC.

3.2. Chlorine fi xing ability of metal oxides

One of the important topics of the pyrolysis study of PVC-MO mixtures is to examine the chlorination of oxides with PVC at high temperature. This feature is strongly related to the suppression of the emission of harmful HCl gas and PCDD/Fs by additives (metal oxides in this case) in the incineration or pyrolysis process. In the present systems, the chlorine containing pyrolysis products are (a) HCl, (b) metal chloride or oxychloride and (c) organochlorine compounds. As for the type (c), we have carried out the quantitative analysis of chlorobenzene, one of the major type (c) compounds. The yield is summarised in Table 1, together with the data of the amount of chlorine anion (Cl^-) captured at water trap, normalised by the initial Cl in the PVC-MO mixture. Obviously, compounds of type (c) is formed with the order of less than 1 mg / 1 g PVC for all PVC-MO systems. Thus, major chlorine containing products are type (a) and/or (b). On the other hand, it is note-worthy here to discuss the difference in the emission of chlorobenzene by the addition of oxide in the view of the emission of organochlorine compounds. Among the oxides ex-amined, zinc oxide is effective to suppress the formation, while the addition of Fe₂O₃ and CeO₂ enhances the emission of chlorobenzene. The latter observation is indicative, since the Friedel-Crafts halogenisation is catalysed by the existence of Lewis acid. Although

the formation of FeCl₃ has not been observed, as mentioned above, a possibility cannot be explicitly excluded for the generation of catalytic amount of Lewis acid, as described above. This is further to be investigated in the future work.

Table 1: Change in yield of chlorobenzene in liquid products and chlorine anion at water trap by the addition of metal oxides for the pyrolysis of 2PVC-1MO mixture at 800 ^oC.

Oxide None ZnO CaO Fe₂O₃ PbO La₂O₃ Nd₂O₃ CeO₂ Al₂O₃ W_CB(mg)/W_PVC(g) 0.15 0.03 0.07 0.24 0.07 0.06 0.15 0.25 0.09

Trapped Cl (wt.%) 92 36 51 69 30 3.1 25 78 90

W_CB: mass of chlorobenzene, W_PVC: mass of PVC

As for the formation of hydrogen chloride gas, the results listed in Table 1 directly refl ect the amount of emitted HCl, since chemical analysis showed the negligible amount of metal cation found in the trap, which attributes neither metal chloride nor oxychloride to the origin of Cl^- in the trap. As shown in the table, the emission of HCl is, in general, suppressed by the addition of metal oxide, especially effectively in the case of La₂O₃. A XRD analysis of the pyrolysis residue indicates the formation of lanthanum oxychloride (LaOCl). We can, therefore, conclude that ca. 95 % of the initial chlorine is fi xed as pyrolysis residue in the form of LaOCl. It is also an interesting issue to consider the dif-ference in HCl emission among the rare earth oxides examined in the present work. The tetravalent cerium oxide shows far less chlorine fi xing ability than trivalent oxides. This is presumably explained by the fact that the latter oxides do not involve the reduction process to form oxychlorides, while tetravalent CeO₂ requires the reduction process from Ce(IV) to Ce(III). As for the difference in the chlorination between La₂O₃ and Nd₂O₃, a possible explanation is based on a thermodynamical consideration, whereby LaOCl can exist in wider range of partial pressure of chlorine and oxygen than neodymium oxychlo-ride. Further studies on the kinetics of the chlorination reaction are required to get insight into the mechanisms of chlorine fi xing by rare earth oxides.

As for the effect of other oxides, the emission of HCl is also suppressed by the addition of ZnO and PbO, though this is in conjunction to the formation of volatile metal chloride, as found also in our previous study [1]. The addition of iron oxide does not signifi cantly affect the suppression of HCl formation because the oxide is not mainly converted to chloride nor oxychloride but converted to metallic iron owing to the reduction process under the present experimental conditions. In the case of CaO, which is commonly used as chlorine absorbent in many applications, nearly half of chlorine is fi xed as the complex form of calcium compounds as found in XRD measurement. The detailed identifi cation of the compounds is not possible because of the hydration of a possible product CaCl₂

after experiment. Taking into account the water-insoluble property of LaOCl, the use of La₂O₃ as chlorine capture agent is considered to be advantageous in comparison to the conventional absorbents, e.g. calcium oxide, since the incineration products of the latter, such as CaCl₂, can easily fl ow out through soil into the environment. It should be here also noted that sludge discharged from magnet production industry contains rich amount of rare earth oxides and that the oxychloride can be further converted to the oxide by the conventional hydrolysis process at high temperature.

References

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JIM 41:1342 (2000).

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Sci. Polym. Chem. Ed. 19:1397 (1981).