Chemosphere 117 (2014) 353–359
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Thermochemical reaction mechanism of lead oxide with poly(vinyl chloride) in waste thermal treatment Si-Jia Wang a,b, Hua Zhang a,⇑, Li-Ming Shao b,c, Shu-Meng Liu b, Pin-Jing He b,c,⇑ a
State Key Laboratory of Pollution Control and Resources Reuse, Tongji University, 1239 Siping Road, Shanghai 200092, PR China Institute of Waste Treatment and Reclamation, Tongji University, 1239 Siping Road, Shanghai 200092, PR China c Centre for the Technology Research and Training on Household Waste in Small Towns & Rural Area, Ministry of Housing and Urban-Rural Development of PR China, 1239 Siping Road, Shanghai 200092, PR China b
h i g h l i g h t s The thermochemical reaction mechanism of PVC with PbO was investigated. HCl decomposed from PVC reacted with PbO via an exothermal gas–solid reaction. Chlorination effect of PVC on Pb was apt to lower-temperature and rapid. The product PbCl2 melted, volatilized and transferred into flue gas at >501 °C.
a r t i c l e
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Article history: Received 10 April 2014 Received in revised form 16 July 2014 Accepted 17 July 2014
Handling Editor: O. Hao Keywords: Lead Poly(vinyl chloride) Thermal treatment Reaction mechanism Volatilization
a b s t r a c t Poly(vinyl chloride) (PVC) as a widely used plastic that can promote the volatilization of heavy metals during the thermal treatment of solid waste, thus leading to environmental problems of heavy metal contamination. In this study, thermogravimetric analysis (TGA) coupled with differential scanning calorimeter, TGA coupled with Fourier transform infrared spectroscopy and lab-scale tube furnace experiments were carried out with standard PVC and PbO to explicate the thermochemical reaction mechanism of PVC with semi-volatile lead. The results showed that PVC lost weight from 225 to 230 °C under both air and nitrogen with an endothermic peak, and HCl and benzene release were also detected. When PbO was present, HCl that decomposed from PVC instantly reacted with PbO via an exothermal gas–solid reaction. The product was solid-state PbCl2 at 501 °C, PbCl2 melted, volatilized and transferred into flue gas or condensed into fly ash. Almost all PbCl2 volatilized above 900 °C, while PbO just started to volatilize slowly at this temperature. Therefore, the chlorination effect of PVC on lead was apt to lower-temperature and rapid. Without oxygen, Pb2O was generated due to the deoxidizing by carbon, with oxygen, the amount of residual Pb in the bottom ash was significantly decreased. Ó 2014 Elsevier Ltd. All rights reserved.
1. Introduction Incineration, with the advantages of volume reduction, sanitation, and energy recovery, has been an important treatment alternatives for municipal solid waste (MSW) or combustible hazardous waste. Though waste can be greatly reduced during thermal treatment, heavy metals contained in the waste will be concentrated in
⇑ Corresponding authors. Tel./fax: +86 21 6598 1383 (H. Zhang). Address: Institute of Waste Treatment and Reclamation, Tongji University, 1239 Siping Road, Shanghai 200092, PR China. Tel./fax: +86 21 6598 6104 (P.-J. He). E-mail addresses: [email protected] (H. Zhang), [email protected]. cn (P.-J. He). http://dx.doi.org/10.1016/j.chemosphere.2014.07.076 0045-6535/Ó 2014 Elsevier Ltd. All rights reserved.
the byproducts of incineration (i.e., flue gas, air pollution control residues and bottom ash) via physicochemical processes (Zhang et al., 2008). The occurrence of heavy metals in flue gas and fly ash (FA) can lead to environmental contamination, which is problematic in secondary pollution control for waste incineration (Belevi and Moench, 2000). Lead as an important semi-volatile heavy metal with environmental toxicity and it is universal in combustible wastes, hence its migration and transformation in thermochemical condition have attracted great concerns. Chlorination reactions are key for distribution of heavy metals into gas or solid phases. Previous studies have demonstrated that the incineration temperature, content of chlorine atom and species of chloride impact the distribution of lead in incineration
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byproducts. Increasing the temperature (650 to 900 °C) can promote the movement of lead into FA and fuel gas in the presence of chlorine, and the increasing content of chlorine atom and chloride (poly(vinyl chloride) (PVC), C2Cl4 and NaCl) can lead to more volatilization of lead (finally condensing on FA or being emitted with flue gas), among which, more lead distribution in flue gas was observed when C2Cl4 was present (Wang et al., 1997; Chiang et al., 2007). As a result of this behavior, the lead content in the FA from medical waste (which has more chlorinated compounds) incineration was significantly higher than that from MSW incineration (Zhao et al., 2009). Yu et al. (2012) found that adding HCl to nitrogen, synthetic gas (5% O2 and 95% N2) and air also increased the volatilization ratio of lead. In the light of this information, chlorinated compounds can be used to remove lead in MSW incineration FA (Jakob et al., 1996; Rio et al., 2007; Nowak et al., 2012), sewage sludge ash (SSA) (Vogel and Adam, 2011; Vogel et al., 2012) and other residues (Lee and Song, 2007) by means of thermal treatment. It has been reported that the volatilization ratio of lead increased by 10–15% during thermal treatment of MSW incineration FA and dredged sediment (treated with phosphoric acid) by adding PVC (5% by weight) (Rio et al., 2007), and the ratio increased by 90% when the temperature changed from 600 °C to 1000 °C when PVC was added into electric arc furnace (EAF) (Lee and Song, 2007). Others (Vogel and Adam, 2011; Vogel et al., 2012) found that the increased temperature, PVC content in the ash and HCl concentration in the inlet gas could all enhance the removal ratios of lead in SSA. CaCl2 and MgCl2 can also be used as Cl-donors to remove lead in SSA (Adam et al., 2009). The increase of lead volatilization and mobility by chlorine during thermal treatment mainly results from the formation of more volatile heavy metal chlorides. Nowak et al. (2012) proposed that chloride can react with heavy metal oxides by direct chlorination, or through indirect reactions with HCl or Cl2, which are reaction products of chloride with H2O or O2. Jakob et al. (1996) and Yoo et al. (2005) proposed that the chlorination reaction of lead with NaCl occurs when a mineral matrix (SiO2 or Al2O3) is present, and this resulted in the formation of PbCl2 in the evaporation products of FA and EAF. Although many researches have reported the promotion effect of chlorine on lead volatilization and migration into the gas phase, the reaction mechanism and variation patterns of chlorine with lead have not been clearly revealed. Therefore, the objective of this study is to investigate the mechanism of PVC reacting with lead compounds, including how and when they react, and how the temperature, and atmosphere influence both their reaction and lead volatilization.
equipped with a differential scanning calorimeter (DSC) as well as with a TGA instrument (SDTA851e, Mettler-Toledo, USA) equipped with a Fourier transform infrared spectroscopy (FTIR) (Netxus 670, Nicolet, USA). In the previous experiments, it was observed that the appropriate heating rate was 10 °C min1, at which hysteresis caused by heat diffusion did not occur, therefore, 10 °C min1 was adopted in the TGA experiments. 10 mg of PVC or PVC–PbO samples were heated in air or under nitrogen at a flow rate of 100 mL min1, with the temperature increasing from 50 °C to 350 °C, 450 °C, 600 °C or 900 °C, and then the temperature was maintained for 60 min. The FTIR spectral region was set as 4000– 400 cm1, with a scanning velocity of 0.6329 cm s1 and a resolution of 8 cm1. In order to reduce the gas condensation along the transfer line, the temperature in the gas cell and transfer line was set to 180 °C (Zhu et al., 2008). 2.3. Tube furnace experiments Fig. 1 shows the tube furnace system, equipped with a quartz tube that was 95 cm in length and 3.5 cm in diameter. According to the modified US EPA Method 5 (Chiang et al., 2007), the absorption system was equipped with six impingers, where the first one was empty, the second to the fifth ones were filled with 100 mL solutions (5% HNO3 and 20% H2O2, v/v), and the last one was filled with silica gel. The connection between the quartz tube and impingers was maintained above 120 °C with a heating belt to prevent flue gas condensing. The tube was washed thoroughly with 100 mL of the above-mentioned solution after the tube was cooled down to room temperature. Each sample (2.00 g ± 0.05 g) was filled in a corundum boat and heated. Based on the TGA results, the heating programs in the tube furnace experiments were set as follows: increase the temperature from 50 °C to 350 °C, 450 °C, 600 °C or 900 °C with a heating rate of 10 °C min1, and then maintain the temperature for 60 min (for 350 and 450 °C) or 30 min (for 600 and 900 °C). The residues in the corundum boat and condensates on the surface of the glass ring at the end of the quartz tube were bottom ash (BA) and FA, respectively. The chemical species in the BA and FA samples were tested by X-ray powder diffraction (XRD) (D8 Advance X diffractometer with Cu Ka radiation, Bruker, Germany) operated at 40 mA, 40 kV, with a step size of 0.02° and a step time of 0.1 s. Triplicate samples were taken from BA and respectively measured by ICP-OES after digestion using HCl, HNO3, HClO4 and HF. 3. Results
2. Materials and methods 3.1. Thermal transformation of PVC 2.1. Materials According to the thermochemical handbook of pure substances (Cheng et al., 2003), most lead-containing compounds decompose or transform to PbO during thermal treatment (>400 °C), therefore PbO was chosen to represent lead compounds. The thermal experiments were carried out with standard PVC powder (molecular weight = 48000, particle size = 75–150 lm) purchased from Sigma–Aldrich. PbO powder (analytical reagent grade) was purchased from Sinopharm Chemical Reagent. PbO was dried at 105 °C and then mixed with PVC at a molar ratio of Cl/Pb = 3 (referred to as PVC–PbO hereafter) to ensure the excessiveness of chlorine atom. 2.2. Thermogravimetric analysis experiments Thermogravimetric analysis (TGA) conducted on pure PVC and PVC–PbO using a TGA (Q600 SDT, TA Instrument, USA) instrument
According to the derivative thermogravimetry (DTG)–DSC (Fig. 2a) and TGA curve (Fig. 3a) of PVC from 50 °C to 900 °C under air flow, the thermal decomposition of PVC can be divided into two stages. In the first stage (230 to 350 °C), the weight loss of PVC reached 64% (if all the chlorine atoms formed HCl and was released, the weight loss should be 58%), and there was an endothermic peak at around 277 °C on the DSC curve. According to the FTIR result (Table 1 and Fig. 4a), the gaseous products were HCl that decomposed from PVC and benzene generated by carbochain polycondensation, which was approximately in agreement with the investigations of Levchik and Weil (2005) and Zhu et al. (2008) on PVC decomposition. Moreover, a small amount of naphthalene, methylbenzene and other ethyl benzenes other than HCl and benzene was detected by Matuschek et al. (2000). As shown in Fig. 4a, HCl started to be generated at 229 °C (Tg) and reached a maximum value at 285 °C (Tmax), then its generation decreased as the temperature continued to increase. In the second stage
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Fig. 1. Schematic diagram of the tube furnace system. r Gas cylinders. s Flow meters. t Thermocouple. u Tube furnace. v Quartz tube. w Heating belt. 10 Aspirator Pump. x Thermocontroller. y Corundum crucible. z Impingers. 11 Ice-water bath. 12 Glass ring.
2.0 1
(a)
DTG DTG DSC DSC
0
1.5
PVC PVC PVC PVC
Air Nitrogen Air Nitrogen
0.8
2
15 10
4
DTG PVC-PbO Air
(b)
DTG PVC-PbO Nitrogen DSC PVC-PbO Air DSC PVC-PbO Nitrogen
0
2
0.6
200
300
5
400
1.0
0
-2 200
300
0
400
0.4 -2
-5 0.5 -10
0.2
-4
-15
0.0 0
200
400
600
800
-20 1000
Heat flow (W/g)
DTG (%/°C)
-1
-6
0.0 0
200
400
600
800
1000
Temperature (°C)
Temperature (°C)
Fig. 2. DTG–DSC curves of (a) PVC and (b) PVC–PbO under air and nitrogen flows.
100
(a)
PVC-PbO Tf =900°C PVC-PbO Tf =600 °C
PVC-PbO Tf =900 °C PVC-PbO Tf =600 °C PVC-PbO Tf =450 °C PVC-PbO Tf =350 °C PVC Tf =600 °C PVC Tf =450 °C PVC Tf =350 °C
PVC Tf = 450°C PVC Tf = 350 °C
80
Weight (%)
(b)
PVC-PbO Tf =450 °C PVC-PbO Tf =350 °C PVC Tf = 600 °C
60
40
20
0 0
20
40
60
80
100
120
Time (min)
0
20
40
60
80
100
120
Time (min)
Fig. 3. Variations of weight versus time of PVC–PbO and PVC under (a) air and (b) nitrogen flows (Tf is the final temperature of TGA and ‘‘X’’ indicate the final temperature achieved).
(350 to 554 °C), there were intensive exothermic peaks on the DSC curve caused by combustion of the sample, with a weight loss of 36% due to the emission of CO2 (Table 1). Under nitrogen flow, PVC experienced an endothermic peak at around 289 °C on the DSC curve (Fig. 2a). HCl and benzene release were also detected by FTIR (Table 1). The temperature range (225 to 355 °C) and weight loss (62%) under nitrogen flow during
the first stage were not significantly different from those under air flow. While the weight loss (26%) during the second stage under nitrogen was less than that under air and there were no endothermic or exothermic peaks. Carbon was generated under nitrogen flow, which accounted for about 12% of the PVC sample in the end. The gaseous products of PVC decomposition under nitrogen flow were heptane and cyclohexane. Linear organic compounds,
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Table 1 Temperatures and gaseous products corresponding to the DTG and DSC peaks. Sample
Curve
Peak 1
Peak 2
Peak 3
Peak 4
Peak 5
PVC under air
DTG DSC
276 °C (HCl/Benzene) 277 °Ca (HCl/Benzene)
435 °C (CO2) 444 °Cb (CO2)
523 °C (CO2) 525 °Cb (CO2)
542 °C (CO2) 541 °Cb (CO2)
552 °C (CO2) 551 °Cb (CO2)
PVC under nitrogen
DTG
285 °C (HCl/Benzene)
328 °C (HCl/Benzene)
454 °C (Heptane/ Cyclohexane) 583 °C (CO2)
677 °C (CO2)
PVC–PbO under air
DSC
289 °C HCl/Benzene
DTG
298 °C (HCl/Benzene/ H2O) 314 °Cb (HCl/Benzene/ H2O)
431 °C (CO2/H2O)
481 °C (CO2)
478 °Cb (CO2)
577 °Cb (CO2)
300 °C (HCl/Benzene/ H2O) 317 °Cb (HCl/Benzene/ H2O)
329 °C (HCl/Benzene/H2O)
460 °C (Benzene/H2O)
DSC PVC–PbO under nitrogen
DTG DSC
a b
a
667 °C (CO2)
a
501 °C (HCl/H2O/CO2/Heptane/ Cyclohexane)
Endothermic peak. Exothermic peak.
Fig. 4. 3D-FTIR patterns of gaseous products from PVC–PbO and PVC under air and nitrogen flows.
like alkane and alkene, or cyclic carbon organic compounds were found in this stage by Ma et al. (2002) and Yannick et al. (2007), which was in agreement with this study. 3.2. Thermal transformation of PVC–PbO As shown in Figs. 2b and 3, the TGA curve of PVC–PbO can also be divided into two stages. In the first stage with either air (260 to 340 °C) or nitrogen flow (265 to 353 °C), the gaseous products (HCl and benzene) were detected by FTIR (Fig. 4b and e and Table 1), similar to PVC. There were no significant differences in the temperature range and mass loss between the samples exposed to air and nitrogen flows, and both DSC curves exhibited exothermic peaks. The initial temperatures for weight loss of PVC–PbO under air and nitrogen were about 30 °C and 40 °C higher than those of PVC, and the Tg in the PVC–PbO system also increased
by 48 °C under air flow and 36 °C under nitrogen flow compared with the PVC runs. In this stage, the weight loss ratios of PVC in the PbO–PVC mixture were significantly reduced, and furthermore, the endothermic peak (corresponding to dechlorination) on the DSC curve changed to an exothermic peak (corresponding to the PbCl2 formation reaction, discussed later) with the presence of PbO, indicating that PbO reacted with PVC or HCl which decomposed from PVC. The integral absorption intensity of gas decomposed a solid sample due to heating is linearly related to the volume of gaseous sample (Marsanich et al., 2002), as indicated in Eq. (1) (Chen et al., 2009). According to the calculation, the integral absorption intensities of HCl from PVC–PbO under air and nitrogen flows were only 34% and 28%, respectively, compared to those from PVC. If there was no reaction between PVC and PbO, the integral absorption intensities of HCl from PVC–PbO under air and nitrogen flows
S.-J. Wang et al. / Chemosphere 117 (2014) 353–359
should both be 46% of those from PVC. This suggests that chlorine reacted with lead and was trapped in the residues, and under nitrogen flow, HCl emission was significantly reduced and was slower (Fig. 4c and f) than that under air flow for PVC–PbO.
It1 ;t2 ¼
Z
t2
t1
Z
m2
½Aðm; tÞdmdt
ð1Þ
m1
where A(m, t) was the absorbance at wavenumber m at time t; m1 and m2 were the wavenumbers at time t1 and t2; t1 and t2 were the time range of HCl evolution. In the second stage, two intensive exothermic peaks on the DSC curve were observed under air flow, all corresponding to CO2 emission (Table 1 and Fig. 4b). There was only one small endothermic peak at around 500 °C under nitrogen flow, which may be caused by the melting of the product. Owning to pyrolysis, the gaseous products from the sample under nitrogen flow contained hydrocarbons, such as heptane or cyclohexane. The weight loss in the second stage finished at 695 to 713 °C, and the mass ratio of the residue under nitrogen (25%) was significantly higher than that (9%) under air. Even the PVC residue under nitrogen was considered and deducted in proportion, the residue of PVC–PbO under nitrogen was still about 10% higher than that under air, which suggested that oxygen promoted the chlorination and volatilization of lead. In order to investigate the physicochemical changes of the samples and their solid products at characteristic temperatures, the TGA curves (Fig. 3) of PVC and PVC–PbO from 50 °C to 350 °C, 450 °C, 600 °C or 900 °C were compared. In air (Fig. 3a), PVC–PbO experienced the first stage of weight loss and then remained constant after heating to 350 °C. PbCl2 was detected in BA (Fig. 5a), and PbO was almost undetectable by XRD, indicating that below 350 °C (314 to 317 °C based on the exothermal peak), PbO had begun to react with chlorine and formed PbCl2. When the final temperature increased from 350 °C to 450 °C, PVC–PbO began to lose weight again with a slow rate similar to PVC, and PbCl2 was still detected in BA. No FA was observed for the 350 °C or 450 °C trials since the melting point of PbCl2 is 501 °C. When the temperature further increased to 600 °C, the weight of PVC–PbO or PVC greatly decreased and tended to ultimately be stable, but PVC–PbO needed a longer time to reach a stable point than PVC. FA occurred and
357
PbCl2 was found in both FA and BA. The volatilization of PbCl2 prolonged the time for weight change. When heated to 900 °C, the weight loss was mainly due to the volatilization of PbCl2 and less BA was left in the end. The changing pattern of weight loss (Fig. 3b) and products (Fig. 5b) for PVC–PbO under nitrogen were similar to that under air flow at temperatures below 600 °C, but a little PbO was detected in BA by XRD. Furthermore, besides PbO, Pb2O was found in BA at 450 °C and 900 °C. This suggests that PbO reacted with C and produced Pb2O and CO2, which reduced the weight loss and increased the proportion of lead in BA. The distribution ratios of lead in BA at different final temperatures were calculated. The result indicated that 97%, 98%, 20% and 3% of lead remained in BA at 350 °C, 450 °C, 600 °C and 900 °C, respectively, under air flow, while 97%, 100%, 28% and 16% of lead remained in BA at 350 °C, 450 °C, 600 °C and 900 °C, respectively, under nitrogen. There was more lead in BA under nitrogen atmosphere than under air, which was consistent with the TGA results (Fig. 3), and this indicated that the air atmosphere assisted the volatilization of PbCl2. 4. Discussion Based on the results of TGA–DSC and XRD, it can be concluded that PbO reacted regardless of the atmosphere and the product was PbCl2. Though the reaction happened during the process of dechlorination of PVC to form HCl, there were still two possible reactions: (1) a solid–solid reaction, namely PVC reacted with PbO directly; (2) a gas–solid reaction, namely HCl decomposed from PVC reacted with PbO. In the case for ZnO (Zhang et al., 2000; Takashi et al., 2012), the reaction of PVC and ZnO at a molar ratio of 4/3 was investigated. Based on the lower initial temperature (200 °C) for the weight loss of PVC–ZnO than that of PVC (230 °C), it was concluded that PVC reacted with ZnO directly. In this study, the initial temperatures and Tg for PVC–PbO under air or nitrogen flow were about 30 to 40 °C higher than that of PVC. If PbO reacted with PVC via a solid–solid reaction, excess PVC in the PVC–PbO system (with a molar ratio of 3) must be decomposed to form HCl from 230 °C. According to the result of FTIR, however, HCl was detected at around 280 °C, which contradicted what was
Fig. 5. XRD patterns of FA and BA under (a) air and (b) nitrogen flows from PVC–PbO.
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found in the solid–solid reaction. Supposing that the reaction was 2HCl(g) + PbO = PbCl2 + H2O(g), the theoretical weight loss of PVC–PbO during the first stage (DW1(PVC–PbO)theory) can be calculated according to Eqs. (2) and (4) and the theoretical weight loss during the second stage (DW2(PVC–PbO)theory) can be calculated according to Eqs. (3) and (5) based on the weight loss of the PVC system and amounts of PVC, PbO in the PVC–PbO system. Under air flow:
DW 1ðPVCÞ M ðPVCÞ 100 ðPbOÞ M SðPbOÞ 2MWðHClÞ MWðH2 OÞ MWðPbOÞ ¼ 18%
DW 1ðPVCPbOÞtheory ¼
ð2Þ SðPbOÞ ¼ RðPVCPbOÞ ¼ 9%
DW 2ðPVCÞ M ðPVCÞ 100 ðPbOÞ M SðPbOÞ MWðPbCl2 Þ þ MWðPbOÞ ¼ 73%
DW 2ðPVCPbOÞtheory ¼
ð3Þ
Under nitrogen flow:
DW 1ðPVCÞ M ðPVCÞ 100 ðPbOÞ M SðPbOÞ 2MWðHClÞ MWðH2 OÞ MWðPbOÞ ¼ 19%
DW 1ðPVCPbOÞtheory ¼
ð4Þ
SðPbOÞ ¼ RðPVCPbOÞ SðCÞ RðPVCPbOÞ ¼ 24% SðCÞ ¼ RðPVCÞ MðPVCÞ ¼ 5%
DW 2ðPVCÞ M ðPVCÞ 100 ðPbOÞ M SðPbOÞ MWðPbCl2 Þ þ MWðPbOÞ ¼ 57%
DW 2ðPVCPbOÞtheory ¼
ð5Þ
where DW1(PVC) and DW2(PVC) were the weight loss ratios (%) of the PVC system during the first stage and the second stage; M(PVC) and M(PbO) were the mass fraction (%) of PVC and PbO in the PVC–PbO system, respectively; R(PbO–PVC) was the mass fraction (%) of residue in PVC–PbO system at the end of the run; R(PVC) was the mass fraction (%) of residue for the PVC system under nitrogen flow; MW(PbO), MW(PbCl2), MW(HCl), and MW(H2O) were the molecular weights of PbO, PbCl2, HCl and H2O, respectively; S(C) and S(PbO) were the carbon and PbO that remained in the PVC–PbO system. Comparing the theoretical arithmetic results with the experimental values, there were deviations of 7.7% and 0.7% for DW1(PVC) and DW2(PVC), respectively, under air and 2.1% and 2.0% for DW1(PVC) and DW2(PVC), respectively, under nitrogen, which were acceptable values. Thus, the assumption for the reaction was confirmed by the experimental results and calculations. According to the dechlorination mechanism of PVC (Amer and Shapiro, 1980), the first step was the random generation of a single carbon–carbon double bond in the cis configuration; the second step was 1,4-elimination of HCl via a six-centered transition state yielding a polyene; the third step was HCl catalyzed isomerization of the polyene formed by HCl elimination to regenerate the initial structure. For the PVC–PbO system in this study, HCl was
Fig. 6. Thermochemical reaction process of PVC with PbO.
S.-J. Wang et al. / Chemosphere 117 (2014) 353–359
decomposed in the first stage, and reacted with PbO to generate PbCl2. Based on FTIR of PVC (Fig. 4a and d), HCl produced from the decomposition of PVC increased as the temperature rose firstly and then decreased. In the beginning, PbO was excessive and all HCl reacted with PbO, thus the initial temperature of the first stage and Tg in the PVC–PbO system also increased. With the increase of HCl formation, the HCl amount became excessive, so some of it escaped into gas, which was represented as weight loss on the TGA curve. Fig. 6 is the diagrammatic sketch of the reaction between PVC and PbO. The temperature can influence the volatilization of the product. When the temperature reached the melting point of PbCl2, PbCl2 transferred from the solid phase to the liquid phase, and then volatized gradually as the saturated vapor pressure increased. When the temperature was high enough, PbCl2 volatized completely and the volatilization rate increased. Oxygen can promote the reaction and volatilization of the product. It can be inferred that the interior temperature was higher because of the oxidation, and a higher temperature can promote the reaction of HCl and PbO as well as volatilization of PbCl2. In addition, BA under nitrogen contained Pb2O, which was deoxidized by residual reducible carbon. Residue carbon was produced at higher temperatures (>400 °C) when HCl had already released or reacted with PbO completely, thus residue carbon had no effect on the chlorination reaction. The chlorination reaction mechanism of PbO with PVC indicated that, when lead recovery is targeted in waste thermal treatment, transformation of PbO to PbCl2 can be realized at a relatively low temperature (
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Chemosphere journal homepage: www.elsevier.com/locate/chemosphere
Thermochemical reaction mechanism of lead oxide with poly(vinyl chloride) in waste thermal treatment Si-Jia Wang a,b, Hua Zhang a,⇑, Li-Ming Shao b,c, Shu-Meng Liu b, Pin-Jing He b,c,⇑ a
State Key Laboratory of Pollution Control and Resources Reuse, Tongji University, 1239 Siping Road, Shanghai 200092, PR China Institute of Waste Treatment and Reclamation, Tongji University, 1239 Siping Road, Shanghai 200092, PR China c Centre for the Technology Research and Training on Household Waste in Small Towns & Rural Area, Ministry of Housing and Urban-Rural Development of PR China, 1239 Siping Road, Shanghai 200092, PR China b
h i g h l i g h t s The thermochemical reaction mechanism of PVC with PbO was investigated. HCl decomposed from PVC reacted with PbO via an exothermal gas–solid reaction. Chlorination effect of PVC on Pb was apt to lower-temperature and rapid. The product PbCl2 melted, volatilized and transferred into flue gas at >501 °C.
a r t i c l e
i n f o
Article history: Received 10 April 2014 Received in revised form 16 July 2014 Accepted 17 July 2014
Handling Editor: O. Hao Keywords: Lead Poly(vinyl chloride) Thermal treatment Reaction mechanism Volatilization
a b s t r a c t Poly(vinyl chloride) (PVC) as a widely used plastic that can promote the volatilization of heavy metals during the thermal treatment of solid waste, thus leading to environmental problems of heavy metal contamination. In this study, thermogravimetric analysis (TGA) coupled with differential scanning calorimeter, TGA coupled with Fourier transform infrared spectroscopy and lab-scale tube furnace experiments were carried out with standard PVC and PbO to explicate the thermochemical reaction mechanism of PVC with semi-volatile lead. The results showed that PVC lost weight from 225 to 230 °C under both air and nitrogen with an endothermic peak, and HCl and benzene release were also detected. When PbO was present, HCl that decomposed from PVC instantly reacted with PbO via an exothermal gas–solid reaction. The product was solid-state PbCl2 at 501 °C, PbCl2 melted, volatilized and transferred into flue gas or condensed into fly ash. Almost all PbCl2 volatilized above 900 °C, while PbO just started to volatilize slowly at this temperature. Therefore, the chlorination effect of PVC on lead was apt to lower-temperature and rapid. Without oxygen, Pb2O was generated due to the deoxidizing by carbon, with oxygen, the amount of residual Pb in the bottom ash was significantly decreased. Ó 2014 Elsevier Ltd. All rights reserved.
1. Introduction Incineration, with the advantages of volume reduction, sanitation, and energy recovery, has been an important treatment alternatives for municipal solid waste (MSW) or combustible hazardous waste. Though waste can be greatly reduced during thermal treatment, heavy metals contained in the waste will be concentrated in
⇑ Corresponding authors. Tel./fax: +86 21 6598 1383 (H. Zhang). Address: Institute of Waste Treatment and Reclamation, Tongji University, 1239 Siping Road, Shanghai 200092, PR China. Tel./fax: +86 21 6598 6104 (P.-J. He). E-mail addresses: [email protected] (H. Zhang), [email protected]. cn (P.-J. He). http://dx.doi.org/10.1016/j.chemosphere.2014.07.076 0045-6535/Ó 2014 Elsevier Ltd. All rights reserved.
the byproducts of incineration (i.e., flue gas, air pollution control residues and bottom ash) via physicochemical processes (Zhang et al., 2008). The occurrence of heavy metals in flue gas and fly ash (FA) can lead to environmental contamination, which is problematic in secondary pollution control for waste incineration (Belevi and Moench, 2000). Lead as an important semi-volatile heavy metal with environmental toxicity and it is universal in combustible wastes, hence its migration and transformation in thermochemical condition have attracted great concerns. Chlorination reactions are key for distribution of heavy metals into gas or solid phases. Previous studies have demonstrated that the incineration temperature, content of chlorine atom and species of chloride impact the distribution of lead in incineration
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byproducts. Increasing the temperature (650 to 900 °C) can promote the movement of lead into FA and fuel gas in the presence of chlorine, and the increasing content of chlorine atom and chloride (poly(vinyl chloride) (PVC), C2Cl4 and NaCl) can lead to more volatilization of lead (finally condensing on FA or being emitted with flue gas), among which, more lead distribution in flue gas was observed when C2Cl4 was present (Wang et al., 1997; Chiang et al., 2007). As a result of this behavior, the lead content in the FA from medical waste (which has more chlorinated compounds) incineration was significantly higher than that from MSW incineration (Zhao et al., 2009). Yu et al. (2012) found that adding HCl to nitrogen, synthetic gas (5% O2 and 95% N2) and air also increased the volatilization ratio of lead. In the light of this information, chlorinated compounds can be used to remove lead in MSW incineration FA (Jakob et al., 1996; Rio et al., 2007; Nowak et al., 2012), sewage sludge ash (SSA) (Vogel and Adam, 2011; Vogel et al., 2012) and other residues (Lee and Song, 2007) by means of thermal treatment. It has been reported that the volatilization ratio of lead increased by 10–15% during thermal treatment of MSW incineration FA and dredged sediment (treated with phosphoric acid) by adding PVC (5% by weight) (Rio et al., 2007), and the ratio increased by 90% when the temperature changed from 600 °C to 1000 °C when PVC was added into electric arc furnace (EAF) (Lee and Song, 2007). Others (Vogel and Adam, 2011; Vogel et al., 2012) found that the increased temperature, PVC content in the ash and HCl concentration in the inlet gas could all enhance the removal ratios of lead in SSA. CaCl2 and MgCl2 can also be used as Cl-donors to remove lead in SSA (Adam et al., 2009). The increase of lead volatilization and mobility by chlorine during thermal treatment mainly results from the formation of more volatile heavy metal chlorides. Nowak et al. (2012) proposed that chloride can react with heavy metal oxides by direct chlorination, or through indirect reactions with HCl or Cl2, which are reaction products of chloride with H2O or O2. Jakob et al. (1996) and Yoo et al. (2005) proposed that the chlorination reaction of lead with NaCl occurs when a mineral matrix (SiO2 or Al2O3) is present, and this resulted in the formation of PbCl2 in the evaporation products of FA and EAF. Although many researches have reported the promotion effect of chlorine on lead volatilization and migration into the gas phase, the reaction mechanism and variation patterns of chlorine with lead have not been clearly revealed. Therefore, the objective of this study is to investigate the mechanism of PVC reacting with lead compounds, including how and when they react, and how the temperature, and atmosphere influence both their reaction and lead volatilization.
equipped with a differential scanning calorimeter (DSC) as well as with a TGA instrument (SDTA851e, Mettler-Toledo, USA) equipped with a Fourier transform infrared spectroscopy (FTIR) (Netxus 670, Nicolet, USA). In the previous experiments, it was observed that the appropriate heating rate was 10 °C min1, at which hysteresis caused by heat diffusion did not occur, therefore, 10 °C min1 was adopted in the TGA experiments. 10 mg of PVC or PVC–PbO samples were heated in air or under nitrogen at a flow rate of 100 mL min1, with the temperature increasing from 50 °C to 350 °C, 450 °C, 600 °C or 900 °C, and then the temperature was maintained for 60 min. The FTIR spectral region was set as 4000– 400 cm1, with a scanning velocity of 0.6329 cm s1 and a resolution of 8 cm1. In order to reduce the gas condensation along the transfer line, the temperature in the gas cell and transfer line was set to 180 °C (Zhu et al., 2008). 2.3. Tube furnace experiments Fig. 1 shows the tube furnace system, equipped with a quartz tube that was 95 cm in length and 3.5 cm in diameter. According to the modified US EPA Method 5 (Chiang et al., 2007), the absorption system was equipped with six impingers, where the first one was empty, the second to the fifth ones were filled with 100 mL solutions (5% HNO3 and 20% H2O2, v/v), and the last one was filled with silica gel. The connection between the quartz tube and impingers was maintained above 120 °C with a heating belt to prevent flue gas condensing. The tube was washed thoroughly with 100 mL of the above-mentioned solution after the tube was cooled down to room temperature. Each sample (2.00 g ± 0.05 g) was filled in a corundum boat and heated. Based on the TGA results, the heating programs in the tube furnace experiments were set as follows: increase the temperature from 50 °C to 350 °C, 450 °C, 600 °C or 900 °C with a heating rate of 10 °C min1, and then maintain the temperature for 60 min (for 350 and 450 °C) or 30 min (for 600 and 900 °C). The residues in the corundum boat and condensates on the surface of the glass ring at the end of the quartz tube were bottom ash (BA) and FA, respectively. The chemical species in the BA and FA samples were tested by X-ray powder diffraction (XRD) (D8 Advance X diffractometer with Cu Ka radiation, Bruker, Germany) operated at 40 mA, 40 kV, with a step size of 0.02° and a step time of 0.1 s. Triplicate samples were taken from BA and respectively measured by ICP-OES after digestion using HCl, HNO3, HClO4 and HF. 3. Results
2. Materials and methods 3.1. Thermal transformation of PVC 2.1. Materials According to the thermochemical handbook of pure substances (Cheng et al., 2003), most lead-containing compounds decompose or transform to PbO during thermal treatment (>400 °C), therefore PbO was chosen to represent lead compounds. The thermal experiments were carried out with standard PVC powder (molecular weight = 48000, particle size = 75–150 lm) purchased from Sigma–Aldrich. PbO powder (analytical reagent grade) was purchased from Sinopharm Chemical Reagent. PbO was dried at 105 °C and then mixed with PVC at a molar ratio of Cl/Pb = 3 (referred to as PVC–PbO hereafter) to ensure the excessiveness of chlorine atom. 2.2. Thermogravimetric analysis experiments Thermogravimetric analysis (TGA) conducted on pure PVC and PVC–PbO using a TGA (Q600 SDT, TA Instrument, USA) instrument
According to the derivative thermogravimetry (DTG)–DSC (Fig. 2a) and TGA curve (Fig. 3a) of PVC from 50 °C to 900 °C under air flow, the thermal decomposition of PVC can be divided into two stages. In the first stage (230 to 350 °C), the weight loss of PVC reached 64% (if all the chlorine atoms formed HCl and was released, the weight loss should be 58%), and there was an endothermic peak at around 277 °C on the DSC curve. According to the FTIR result (Table 1 and Fig. 4a), the gaseous products were HCl that decomposed from PVC and benzene generated by carbochain polycondensation, which was approximately in agreement with the investigations of Levchik and Weil (2005) and Zhu et al. (2008) on PVC decomposition. Moreover, a small amount of naphthalene, methylbenzene and other ethyl benzenes other than HCl and benzene was detected by Matuschek et al. (2000). As shown in Fig. 4a, HCl started to be generated at 229 °C (Tg) and reached a maximum value at 285 °C (Tmax), then its generation decreased as the temperature continued to increase. In the second stage
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Fig. 1. Schematic diagram of the tube furnace system. r Gas cylinders. s Flow meters. t Thermocouple. u Tube furnace. v Quartz tube. w Heating belt. 10 Aspirator Pump. x Thermocontroller. y Corundum crucible. z Impingers. 11 Ice-water bath. 12 Glass ring.
2.0 1
(a)
DTG DTG DSC DSC
0
1.5
PVC PVC PVC PVC
Air Nitrogen Air Nitrogen
0.8
2
15 10
4
DTG PVC-PbO Air
(b)
DTG PVC-PbO Nitrogen DSC PVC-PbO Air DSC PVC-PbO Nitrogen
0
2
0.6
200
300
5
400
1.0
0
-2 200
300
0
400
0.4 -2
-5 0.5 -10
0.2
-4
-15
0.0 0
200
400
600
800
-20 1000
Heat flow (W/g)
DTG (%/°C)
-1
-6
0.0 0
200
400
600
800
1000
Temperature (°C)
Temperature (°C)
Fig. 2. DTG–DSC curves of (a) PVC and (b) PVC–PbO under air and nitrogen flows.
100
(a)
PVC-PbO Tf =900°C PVC-PbO Tf =600 °C
PVC-PbO Tf =900 °C PVC-PbO Tf =600 °C PVC-PbO Tf =450 °C PVC-PbO Tf =350 °C PVC Tf =600 °C PVC Tf =450 °C PVC Tf =350 °C
PVC Tf = 450°C PVC Tf = 350 °C
80
Weight (%)
(b)
PVC-PbO Tf =450 °C PVC-PbO Tf =350 °C PVC Tf = 600 °C
60
40
20
0 0
20
40
60
80
100
120
Time (min)
0
20
40
60
80
100
120
Time (min)
Fig. 3. Variations of weight versus time of PVC–PbO and PVC under (a) air and (b) nitrogen flows (Tf is the final temperature of TGA and ‘‘X’’ indicate the final temperature achieved).
(350 to 554 °C), there were intensive exothermic peaks on the DSC curve caused by combustion of the sample, with a weight loss of 36% due to the emission of CO2 (Table 1). Under nitrogen flow, PVC experienced an endothermic peak at around 289 °C on the DSC curve (Fig. 2a). HCl and benzene release were also detected by FTIR (Table 1). The temperature range (225 to 355 °C) and weight loss (62%) under nitrogen flow during
the first stage were not significantly different from those under air flow. While the weight loss (26%) during the second stage under nitrogen was less than that under air and there were no endothermic or exothermic peaks. Carbon was generated under nitrogen flow, which accounted for about 12% of the PVC sample in the end. The gaseous products of PVC decomposition under nitrogen flow were heptane and cyclohexane. Linear organic compounds,
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Table 1 Temperatures and gaseous products corresponding to the DTG and DSC peaks. Sample
Curve
Peak 1
Peak 2
Peak 3
Peak 4
Peak 5
PVC under air
DTG DSC
276 °C (HCl/Benzene) 277 °Ca (HCl/Benzene)
435 °C (CO2) 444 °Cb (CO2)
523 °C (CO2) 525 °Cb (CO2)
542 °C (CO2) 541 °Cb (CO2)
552 °C (CO2) 551 °Cb (CO2)
PVC under nitrogen
DTG
285 °C (HCl/Benzene)
328 °C (HCl/Benzene)
454 °C (Heptane/ Cyclohexane) 583 °C (CO2)
677 °C (CO2)
PVC–PbO under air
DSC
289 °C HCl/Benzene
DTG
298 °C (HCl/Benzene/ H2O) 314 °Cb (HCl/Benzene/ H2O)
431 °C (CO2/H2O)
481 °C (CO2)
478 °Cb (CO2)
577 °Cb (CO2)
300 °C (HCl/Benzene/ H2O) 317 °Cb (HCl/Benzene/ H2O)
329 °C (HCl/Benzene/H2O)
460 °C (Benzene/H2O)
DSC PVC–PbO under nitrogen
DTG DSC
a b
a
667 °C (CO2)
a
501 °C (HCl/H2O/CO2/Heptane/ Cyclohexane)
Endothermic peak. Exothermic peak.
Fig. 4. 3D-FTIR patterns of gaseous products from PVC–PbO and PVC under air and nitrogen flows.
like alkane and alkene, or cyclic carbon organic compounds were found in this stage by Ma et al. (2002) and Yannick et al. (2007), which was in agreement with this study. 3.2. Thermal transformation of PVC–PbO As shown in Figs. 2b and 3, the TGA curve of PVC–PbO can also be divided into two stages. In the first stage with either air (260 to 340 °C) or nitrogen flow (265 to 353 °C), the gaseous products (HCl and benzene) were detected by FTIR (Fig. 4b and e and Table 1), similar to PVC. There were no significant differences in the temperature range and mass loss between the samples exposed to air and nitrogen flows, and both DSC curves exhibited exothermic peaks. The initial temperatures for weight loss of PVC–PbO under air and nitrogen were about 30 °C and 40 °C higher than those of PVC, and the Tg in the PVC–PbO system also increased
by 48 °C under air flow and 36 °C under nitrogen flow compared with the PVC runs. In this stage, the weight loss ratios of PVC in the PbO–PVC mixture were significantly reduced, and furthermore, the endothermic peak (corresponding to dechlorination) on the DSC curve changed to an exothermic peak (corresponding to the PbCl2 formation reaction, discussed later) with the presence of PbO, indicating that PbO reacted with PVC or HCl which decomposed from PVC. The integral absorption intensity of gas decomposed a solid sample due to heating is linearly related to the volume of gaseous sample (Marsanich et al., 2002), as indicated in Eq. (1) (Chen et al., 2009). According to the calculation, the integral absorption intensities of HCl from PVC–PbO under air and nitrogen flows were only 34% and 28%, respectively, compared to those from PVC. If there was no reaction between PVC and PbO, the integral absorption intensities of HCl from PVC–PbO under air and nitrogen flows
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should both be 46% of those from PVC. This suggests that chlorine reacted with lead and was trapped in the residues, and under nitrogen flow, HCl emission was significantly reduced and was slower (Fig. 4c and f) than that under air flow for PVC–PbO.
It1 ;t2 ¼
Z
t2
t1
Z
m2
½Aðm; tÞdmdt
ð1Þ
m1
where A(m, t) was the absorbance at wavenumber m at time t; m1 and m2 were the wavenumbers at time t1 and t2; t1 and t2 were the time range of HCl evolution. In the second stage, two intensive exothermic peaks on the DSC curve were observed under air flow, all corresponding to CO2 emission (Table 1 and Fig. 4b). There was only one small endothermic peak at around 500 °C under nitrogen flow, which may be caused by the melting of the product. Owning to pyrolysis, the gaseous products from the sample under nitrogen flow contained hydrocarbons, such as heptane or cyclohexane. The weight loss in the second stage finished at 695 to 713 °C, and the mass ratio of the residue under nitrogen (25%) was significantly higher than that (9%) under air. Even the PVC residue under nitrogen was considered and deducted in proportion, the residue of PVC–PbO under nitrogen was still about 10% higher than that under air, which suggested that oxygen promoted the chlorination and volatilization of lead. In order to investigate the physicochemical changes of the samples and their solid products at characteristic temperatures, the TGA curves (Fig. 3) of PVC and PVC–PbO from 50 °C to 350 °C, 450 °C, 600 °C or 900 °C were compared. In air (Fig. 3a), PVC–PbO experienced the first stage of weight loss and then remained constant after heating to 350 °C. PbCl2 was detected in BA (Fig. 5a), and PbO was almost undetectable by XRD, indicating that below 350 °C (314 to 317 °C based on the exothermal peak), PbO had begun to react with chlorine and formed PbCl2. When the final temperature increased from 350 °C to 450 °C, PVC–PbO began to lose weight again with a slow rate similar to PVC, and PbCl2 was still detected in BA. No FA was observed for the 350 °C or 450 °C trials since the melting point of PbCl2 is 501 °C. When the temperature further increased to 600 °C, the weight of PVC–PbO or PVC greatly decreased and tended to ultimately be stable, but PVC–PbO needed a longer time to reach a stable point than PVC. FA occurred and
357
PbCl2 was found in both FA and BA. The volatilization of PbCl2 prolonged the time for weight change. When heated to 900 °C, the weight loss was mainly due to the volatilization of PbCl2 and less BA was left in the end. The changing pattern of weight loss (Fig. 3b) and products (Fig. 5b) for PVC–PbO under nitrogen were similar to that under air flow at temperatures below 600 °C, but a little PbO was detected in BA by XRD. Furthermore, besides PbO, Pb2O was found in BA at 450 °C and 900 °C. This suggests that PbO reacted with C and produced Pb2O and CO2, which reduced the weight loss and increased the proportion of lead in BA. The distribution ratios of lead in BA at different final temperatures were calculated. The result indicated that 97%, 98%, 20% and 3% of lead remained in BA at 350 °C, 450 °C, 600 °C and 900 °C, respectively, under air flow, while 97%, 100%, 28% and 16% of lead remained in BA at 350 °C, 450 °C, 600 °C and 900 °C, respectively, under nitrogen. There was more lead in BA under nitrogen atmosphere than under air, which was consistent with the TGA results (Fig. 3), and this indicated that the air atmosphere assisted the volatilization of PbCl2. 4. Discussion Based on the results of TGA–DSC and XRD, it can be concluded that PbO reacted regardless of the atmosphere and the product was PbCl2. Though the reaction happened during the process of dechlorination of PVC to form HCl, there were still two possible reactions: (1) a solid–solid reaction, namely PVC reacted with PbO directly; (2) a gas–solid reaction, namely HCl decomposed from PVC reacted with PbO. In the case for ZnO (Zhang et al., 2000; Takashi et al., 2012), the reaction of PVC and ZnO at a molar ratio of 4/3 was investigated. Based on the lower initial temperature (200 °C) for the weight loss of PVC–ZnO than that of PVC (230 °C), it was concluded that PVC reacted with ZnO directly. In this study, the initial temperatures and Tg for PVC–PbO under air or nitrogen flow were about 30 to 40 °C higher than that of PVC. If PbO reacted with PVC via a solid–solid reaction, excess PVC in the PVC–PbO system (with a molar ratio of 3) must be decomposed to form HCl from 230 °C. According to the result of FTIR, however, HCl was detected at around 280 °C, which contradicted what was
Fig. 5. XRD patterns of FA and BA under (a) air and (b) nitrogen flows from PVC–PbO.
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found in the solid–solid reaction. Supposing that the reaction was 2HCl(g) + PbO = PbCl2 + H2O(g), the theoretical weight loss of PVC–PbO during the first stage (DW1(PVC–PbO)theory) can be calculated according to Eqs. (2) and (4) and the theoretical weight loss during the second stage (DW2(PVC–PbO)theory) can be calculated according to Eqs. (3) and (5) based on the weight loss of the PVC system and amounts of PVC, PbO in the PVC–PbO system. Under air flow:
DW 1ðPVCÞ M ðPVCÞ 100 ðPbOÞ M SðPbOÞ 2MWðHClÞ MWðH2 OÞ MWðPbOÞ ¼ 18%
DW 1ðPVCPbOÞtheory ¼
ð2Þ SðPbOÞ ¼ RðPVCPbOÞ ¼ 9%
DW 2ðPVCÞ M ðPVCÞ 100 ðPbOÞ M SðPbOÞ MWðPbCl2 Þ þ MWðPbOÞ ¼ 73%
DW 2ðPVCPbOÞtheory ¼
ð3Þ
Under nitrogen flow:
DW 1ðPVCÞ M ðPVCÞ 100 ðPbOÞ M SðPbOÞ 2MWðHClÞ MWðH2 OÞ MWðPbOÞ ¼ 19%
DW 1ðPVCPbOÞtheory ¼
ð4Þ
SðPbOÞ ¼ RðPVCPbOÞ SðCÞ RðPVCPbOÞ ¼ 24% SðCÞ ¼ RðPVCÞ MðPVCÞ ¼ 5%
DW 2ðPVCÞ M ðPVCÞ 100 ðPbOÞ M SðPbOÞ MWðPbCl2 Þ þ MWðPbOÞ ¼ 57%
DW 2ðPVCPbOÞtheory ¼
ð5Þ
where DW1(PVC) and DW2(PVC) were the weight loss ratios (%) of the PVC system during the first stage and the second stage; M(PVC) and M(PbO) were the mass fraction (%) of PVC and PbO in the PVC–PbO system, respectively; R(PbO–PVC) was the mass fraction (%) of residue in PVC–PbO system at the end of the run; R(PVC) was the mass fraction (%) of residue for the PVC system under nitrogen flow; MW(PbO), MW(PbCl2), MW(HCl), and MW(H2O) were the molecular weights of PbO, PbCl2, HCl and H2O, respectively; S(C) and S(PbO) were the carbon and PbO that remained in the PVC–PbO system. Comparing the theoretical arithmetic results with the experimental values, there were deviations of 7.7% and 0.7% for DW1(PVC) and DW2(PVC), respectively, under air and 2.1% and 2.0% for DW1(PVC) and DW2(PVC), respectively, under nitrogen, which were acceptable values. Thus, the assumption for the reaction was confirmed by the experimental results and calculations. According to the dechlorination mechanism of PVC (Amer and Shapiro, 1980), the first step was the random generation of a single carbon–carbon double bond in the cis configuration; the second step was 1,4-elimination of HCl via a six-centered transition state yielding a polyene; the third step was HCl catalyzed isomerization of the polyene formed by HCl elimination to regenerate the initial structure. For the PVC–PbO system in this study, HCl was
Fig. 6. Thermochemical reaction process of PVC with PbO.
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decomposed in the first stage, and reacted with PbO to generate PbCl2. Based on FTIR of PVC (Fig. 4a and d), HCl produced from the decomposition of PVC increased as the temperature rose firstly and then decreased. In the beginning, PbO was excessive and all HCl reacted with PbO, thus the initial temperature of the first stage and Tg in the PVC–PbO system also increased. With the increase of HCl formation, the HCl amount became excessive, so some of it escaped into gas, which was represented as weight loss on the TGA curve. Fig. 6 is the diagrammatic sketch of the reaction between PVC and PbO. The temperature can influence the volatilization of the product. When the temperature reached the melting point of PbCl2, PbCl2 transferred from the solid phase to the liquid phase, and then volatized gradually as the saturated vapor pressure increased. When the temperature was high enough, PbCl2 volatized completely and the volatilization rate increased. Oxygen can promote the reaction and volatilization of the product. It can be inferred that the interior temperature was higher because of the oxidation, and a higher temperature can promote the reaction of HCl and PbO as well as volatilization of PbCl2. In addition, BA under nitrogen contained Pb2O, which was deoxidized by residual reducible carbon. Residue carbon was produced at higher temperatures (>400 °C) when HCl had already released or reacted with PbO completely, thus residue carbon had no effect on the chlorination reaction. The chlorination reaction mechanism of PbO with PVC indicated that, when lead recovery is targeted in waste thermal treatment, transformation of PbO to PbCl2 can be realized at a relatively low temperature (
