Capture of hydrogen chloride by copyrolysis of polyvinyl chloride waste
materials with steelmaking dust
Jakub Korpas1, Václav Slovák2, Kamil Wichterle1
1
VSB – Technical University of Ostrava, Institute of Environmental Technology, 70833 Ostrava Poruba, Czech
Republic
2
Faculty of Science, University of Ostrava, 701 03 Ostrava
least recommended way, as dangerous pollutants
may be released by a slow stepwise degradation.
Abstract
There are experiments with application of the waste
Experimental study of pyrolysis of PVC based
PVC particles instead of gravel as a filler in
materials in presence of various amount of
concrete (Najjar et al.,2013; Siddique et al., 2008).
steelmaking dust was performed. The dust from
The material recycling is a commonly employed
steel manufacture employing zinc plated scrap
with well-defined PVC material, as the one rejected
contains considerable amount of zinc oxide and its
directly during the polymer processing. After the
utilization in metallurgy is quite complicated.
melting and granulation, this material may be
However the dust can react with the hydrogen
processed like a virgin polymer (La Mantia, 1996;
chloride released from the heated PVC in the
Braun, 2002; Sadat-Shojai and Bakhshandeh,
temperature range 200-400°C. Material balance of
2011). Problems arise from a stepwise
the
pyrolysis
process
was
studied
by
dehydrochlorination by heat and light, resulting in
thermogravimetry, and the data obtained were
the formation of conjugated bonds, oxidation and
checked by comparison with the results of larger
branching (Starnes, 2002; Jian et al.,, 1990, 1991;
laboratory oven experiments. In excess of PVC, the
Osawa and Aiba, 1982). Therefore, the re-melting
amount of captured HCl stoichiometrially
is problematic, and postconsumer recycling is
approximately corresponds to the content of ZnO;
practically infeasible.
additional HCl is probably captured to FeCl2. In
The aim of chemical recycling is to destroy the
excess of the dust, the captured amount of HCl
polymer chains with the yield of simple organic
approximates 100 %. The suggested copyrolysis
compounds, suitable as a feed for organic
seems to be a promising method to prevent
syntheses. Unlike the material recycling, it can be
formation of dangerous chlorinated organic
employed with more complex polymer feed.
compounds during the thermal treatment of the
However, the polymer chain breaking processes,
waste PVC. Furthermore, the obtained ZnCl2 is a
like pyrolysis, hydrogenation, or gasification
valuable material and the zinc depleted dust may be
require complex reactors and are also energetically
reused in metallurgy instead of the dumping.
demanding.
Keywords: Waste PVC; Steelwork dust;
Combustion in proper furnace seems to be the
Thermogravimetry; Pyrolysis; Zinc chloride
simplest way which does not require any polymer
feed separation. (La Mantia, 1996; Braun, 2002;
1. Introduction
Sadat-Shojai
and
Bakhshandeh,
2011).
Poly-(vinyl chloride), PVC, is one of the most
Nevertheless, the released hydrogen chloride and,
popular synthetic polymers. With respect to its
in the presence of oxygen highly corrosive
specific mechanical and electric properties, taking
elementary chlorine, are precursors of a number of
in account its environmental stability and resistance
extremely toxic polychlorinated substances, e.g. the
toward many chemicals, it is preferred in a great
ones based on dibenzo-p-dioxines, dibenzofuranes,
number of technologies. More than a half of the
or polycyclic aromatic hydrocarbons (Everaert and
PVC production is targeted to building applications
Baeyens, 2002, Wang et al., 2003).
as window frames, sewage pipes, flooring. PVC is
The pyrolysis is a more promising way of PVC
also used in packaging applications, in rainproof
recycling. However, it should be carefully
fabric, and it is superior material for electric cable
controlled. The PVC decomposition takes place in
insulation. Lifetime of PVC products is estimated
two steps. In the range 200-360°C, hydrogen
to be 30-100 years and considering that the PVC
chloride is released with a minor amount of
mass production started in fiftieth of the 20th
hydrocarbon gas. Later, in the range 360-500°C the
century, amount of the PVC waste rapidly increases
polymer chains are cracked releasing hydrocarbon
(Mleziva, 2000; Vinyl Progress Report, 2010).
gases and tar. There are many papers devoted to the
There are four essential ways of the PVC waste
process under different conditions (Bhaskar et al.,
treatment, i.e. dumping, material recycling,
2006; Czégény et al., 2012; Slapak et al., 2000;
chemical recycling and combustion. Dumping is the
McNeill et al., 1995; Marongiu et al., 2003;
1
Matuschek et al., 2000; Miranda et al., 1999).
Miranda et al.(1999) have examined production of
hydrocarbons during the pyrolysis in vacuum
equipment. Matuschek et al. (2000) have observed
more degradation steps in the air atmosphere. The
stage of hydrogen chloride release period has been
observed in the same temperature range by all
investigators. However, it is necessary to separate
hydrogen chloride as soon as possible and at lower
temperatures, to prevent formation of chlorinated
organic compounds. All remaining organic
compounds can be used as a fuel or as a reducing
agents (Zevenhoven and Saed, 2000).
Hydrogen chloride in the presence of inorganic
basic compounds is fixed to chloride salts and this
is a common way of the chlorine separation.
Usually basic oxides, hydroxides or carbonates are
used for this purpose in laboratories. Borgianni et
al.(2002) have suggested Na2CO3 to be more
efficient than CaO and Ca(OH)2. Jacksland et al.
(2000) obtained reliable results with CaCO3. Zhu et
al. (2008) have compared application of CaO,
Ca(OH)2 a CaCO3. Effects of various other oxides,
as Fe2O3, ZnO, La2O3, Nd2O3 Co3O4, TiO2, partly
acting as catalysts, were also studied during the
degradation of PVC (Blazso and Jakab, 1999;
Terakado et al., 2009; Sivalingam and Madras,
2004; Iida and Goto, 1977; Masuda et al., 2006).
However, the best basic compounds are
comparatively expensive and the chloride products
are valueless, making this approach impractical for
large scale application. Another option is to employ
the waste metallurgy dust; major components of
them are iron oxides, zinc oxide, zinc ferrite, and
lime. The dust is usually captured by a wet scrubber
in a form of a sludge. Content of iron and zinc in
steelmaking dust depends on the ladle feed, and on
the technology. Fe content may be within the range
20-73% and when zinc plated scrape is used, Zn
content may be as high as 35%, and CaO content
within 3-20%. Separation of particular metals is a
complicated process, and when the zinc ferrite is
present in the form of franklinite (Doronin and
Svyazhin, 2011) most of the sludge is simply
dumped.
Pyrolytic coprocessing of the dust with PVC
produces different chlorides, zinc chloride being a
valuable product. Lee and Song (2007) pelletized
the dust from electric arc furnaces with PVC
powder and released practically all zinc from it at
800-1000°C in a laboratory oven. To recover zinc,
reaction of the steelmaking dust with gaseous
hydrogen chloride at high temperature was studied
(Wichterle et.al.,2010; Cieslar, 2009). At the
temperatures beyond 300°C, hydrogen chloride did
not react with Fe(3) and ZnCl2 was formed
preferably (Cieslar, 2009). However, with respect
to the high corrosiveness of HCl, the process has
not been found to be industrially feasible.
The aim of our research was to investigate the
utility of the metallurgy dust for hydrogen chloride
capture. The main goal was to obtain chlorine free
pyrolytic gases from PVC, with a secondary goal to
reduce zinc content in the dust and extract it as zinc
chloride.
2. Experimental
2.1. Materials
Four samples of PVC were studied experimentally.
One was the virgin powdered polymer Neralit, three
others were taken from typical PVC waste materials
shredded to 1 mm particles. In the following text
the abbreviations according the Tab.1 are used:
The PVC samples were mixed with the metallurgy
dust, obtained by drying an arbitrary sample of
Table 1
PVC samples used.
Abbreviation
V
WP
CR
CI
Origin of the PVC samples
Virgin polymer Neralit
Sewadge piping
C onstruction rod
Electric cable insulation
State
Powder
Hard
Hard
softened
centrifuged sludge collected from the emissions of
tandem ovens of the steelwork Arcelor Mittal Steel
Ostrava. Elementary composition of the dust, as
determined by AAS, is presented in Chyba!
Nenalezen zdroj odkazů.
1 g of the dust contains such amount of oxides that
all hydrogen chloride formed from 0.285 g of pure
PVC Present metal oxides in HCl environment can
react to produce ZnCl2, CaCl2, CdCl2 and FeCl2
while Fe(III) prefers the oxide form in the
temperature range 200-400°C, as obvious from
Table 2
Steelmaking-dust composition and chlorine required for the dissolution at 400°C.
Fe(II) Fe(III)
Zn
Pb
Ca
Cd
O
a)
53.49
14.08 0.90
0.86
0.06
b)
2.00
51.49
26.57
mol metal/100 g
0.036 0.922
0.215 0.004 0.021 0.001
mol Cl/100 g c) 0.072 0.431 0.009 0.043 0.001
a) - by elementary analysis
b) - Fe speciation estimated by RTG analysis; oxygen calculated
c) - in the chlorides stable at 400°C
Total
%
2
95.96
0.555
i.e. 10 K/min within the range 20-400°C, which
corresponds to complete release of HCl. After the
cooling, the solid residuum was crushed and
leached by water and chloride content in the extract
was analyzed by the argentometric titration, using
potassium chromate as an indicator.
Other experiments were performed at 400°C with
PVC samples under the nitrogen stream to
determine the released HCl captured in a bubble
trap with NaOH solution, again using the
argentometric titration.
Gibbs free energy data plotted in Fig. 1. Reduction
Fe(III) Fe(II) does not probably take place
during the low-temperature PVC pyrolysis.
3.
Fig. 1. Gibbs free energy for the reactions of metal oxides
with HCl.
Results and discussion
3.1. Thermal behavior of PVC containing
samples
Thermal decomposition of the PVC samples is
presented by thermogravimetry curves in Fig.2
together with the MS results for HCl. The data are
consistent with a two-stage decomposition; one step
occurs within the interval 200-370°C, where
practically all HCl is released (Slapak et al., 2000;
McNeill et al., 1995; Marongiu et al., 2003;
Matuschek et al., 2000; Miranda et al., 1999).
Observed difference in the behavior of pure and
technical PVC is likely due to differences in various
fillers, softeners and other additives.
2.2. Procedure
Two kinds of experiments were completed. First,
the mixture of PVC with various ratios of the dust
were treated by thermogravimetric (TG) analysis to
check the pyrolysis process. Then, laboratory scale
experiments have been done to check the efficiency
of chlorine capture during the process.
2.3. Thermoanalytical experiments
Setsys Evolution (Setaram) thermoanalytical
apparatus was used for all experiments. The device
was interconnected with mass spectrometry (MS)
detector (QMG 700, Pfeifer) through Supersonic
system (Setaram) for analysis of released gases.
TG measurements were used for preliminary testing
of thermal behavior of pure samples and for
pyrolysis of mixtures of PVC containing samples
and the sludge. The mixtures were prepared by
direct weighing of the components to TG crucible
and homogenization (intensive shaking of the
crucible covered with a lid).
All thermoanalytical experiments were carried out
in opened alumina crucibles with heating rate 10
K/min in the range 20 – 500 °C. The furnace was
purged with argon (200 ml/min, 25 min) before the
experiments to ensure inert atmosphere, the
measurements were performed under argon flow of
20 ml/min.
HCl in evolved gases was detected as MS signal
with m/z = 36 (in multi-ion detection mode of
measurement).
2.4. Laboratory scale experiments
The batch was prepared for any experiment by
mixing 4 grams of particular PVC sample with the
dust in mass ratios 1:1 to 1:4; for PVC-V the ratio
1:6 was used as well. The mixture was transferred
to the corundum crucible and heated in an electric
furnace. The rate of heating was programmed to the
slope equivalent with the previous TG experiments;
Fig. 2. Mass changes and HCl evolution during heating of
PVC-containing samples
Average mass change during pyrolysis of pure
PVC-V sample in temperature interval 200 – 370°C
was found to be 62.0±1.2 % (95% confidence
interval, 8 measurements). The value is higher than
stoichiometric content of HCl in (C2H3Cl)n (58.4
%), the difference (3.6 %) can be attributed to slow
cracking of the structure and release of volatile
hydrocarbons (process immediately following HCl
elimination at higher temperatures). The loss of
mass for all pure samples under the same conditions
is shown in Chyba! Nenalezen zdroj odkazů..
3.2. TG analysis of mixtures with sludge
3
Table 3
Composition of the tested mixtures and the mass change in the
temperature interval 200 - 370°C as determined by TG
experiments.
Sample
Mass of sample
mg
Mass of dust
mg
40.50
46.10
43.17
41.75
41.02
40.05
43.80
67.30
80.13
92.62
113.80
159.40
26.40
30.85
27.54
33.76
26.49
61.77
89.36
131.08
42.55
35.50
40.30
29.95
40.19
70.65
129.07
132.41
39.34
43.08
42.92
29.57
45.36
78.81
118.58
128.74
V
WP
CR
CI
Mass ratio
dust:sample
0
1.1
1.5
1.9
2.2
2.8
4.0
0
1.0
2.0
3.2
3.9
0
0.9
2.0
3.2
4.4
0
1.2
1.8
2.8
4.4
∆ where m(HClE) and m(CmHn) are the masses of the
released HCl and hydrocarbons, respectively. In
contrast, in the presence of an excess of the dust the
total mass change ∆m of the feed during the thermal
treatment in given temperature range should take
into consideration also the mass of HCl captured
into the dust m(HClC), and corresponding mass of
water m(H2O) formed by the reaction
Mass change
∆m ,%
62.0
38.1
34.9
32.2
29.0
27.8
26.5
49.3
33.0
22.4
27.0
23.1
55.8
32.2
30.4
29.8
24.2
60.6
43.0
37.3
37.9
36.3
2HCl O 2Cl H O
and released immediately as a vapor. By using
stoichiometry, we can calculate
18.02
2
2 ∗ 36.46
The mass change in presence of the dust is
∆ By combining of the above equations, the hydrogen
chloride mass captured,
m(HClC),
can be
determined on the basis of two decomposition
experiments as
1.328∆ ∆
Thermal decomposition curves for PVC-V mixed in
various mass ratios with the metallurgy dust are
presented in Fig. 3. Percentage of the remaining
mass is related to the input mass of PVC samples.
The observed decrease of the mass loss is likely due
to the capture of HCl in the dust. Significant
decrease of the temperature at which the
decomposition starts is probably due to the catalytic
action of the metallic oxides contained in the dust,
as mentioned by Terakado et al.(2009) and
Sivalingam and Madras (2004).
Main quantitative results of thermogravimetry
analysis for all samples are also presented in
.
Fig. 4. Proximate analysis of the pyrolysis products of
studied materials in absence of the dust
3.3. Laboratory oven experiments
During the laboratory oven tests, the released
hydrogen chloride was absorbed in a trap, and its
mass was determined by titration. Accordingly, the
amount of released hydrocarbons from the samples
in the absence of the dust was calculated, as
presented in Tab. 4.
Supposing that all chlorine contained in the sample
had been released in the form of HCl, content of
pure PVC in these materials was estimated, as
shown also in Tab. 4. For 100% virgin PVC, the
error of such estimation is less than 3%.
Primary results of the measurements in the presence
of the dust are given in Tab. 5.
Fig. 3. TG curves of pyrolysis of PVC – sludge mixtures in
comparison with pure PVC-V.
It should be noted that the mass change ∆m0 for
PVC alone consists of two components, e.g.:
4
Table 4
Balance of PVC decomposition samples characteristics 200 – 370°C.
V
WP
CR
CI
Mass change
(TG)
%
62.0
49.3
55.8
60.6
Relased HCl
(LO)
%
55.1
48.0
46.6
28.9
Relased
hydro-carbons
%
6.9
1.3
9.3
31,7
PVC estimate
PVC actual
%
97.1
84.6
82.0
51.0
%
100
-
3.4. Products of PVC materials pyrolysis
According to the experiments, practically all
chlorine has been released from PVC. Proximate
composition of the products is shown in Fig. 4. The
amount of volatile organic compounds is
considerably higher for the CI sample, likely due to
the presence of softeners in the cable insulations.
4. Conclusion
Metallurgy dust from the steel processing that
employs zinc plated scrap contains some amount of
zinc oxide. Here we demonstrate its use for the
abatement of hydrogen chloride from pyrolyzed
polymeric PVC based technical materials. The
produced zinc chloride can be further used as a
suitable raw material for zinc recycling.
The laboratory pyrolysis employed in our study
enabled determination of the content of pure PVC
in
the
complex
polymeric
materials.
Thermogravimetry turned out to be a simple
method for investigation of the polymers
decomposition both in the absence and in the
presence of solid oxide materials added to facilitate
fast transformation of the released hydrogen
chloride to the inorganic form. Evidently, the dust
was capable to capture more HCl, than corresponds
to the amount of Zn present. We suppose that under
these conditions additional HCl reacts with Fe(II)
oxides. In excess of the dust, more than 90 % of
chlorine was captured. We estimate that the
residence time of HCl inside the fixed bed of the
dust was about 1 s. We anticipate that in more
efficient reactors (e.g. with countercurrent moving
bed) the residence time will be longer and the
conversion will be even higher.
Presented results indicate that by synergy of two
waste materials – waste PVC and metallurgy dust –
Table 5
Composition of laboratory oven experiment mixture and
mass of captured HCl.
Mass of the PVC based material was 4 g and different
amount of the dust was added.
Sample
V
WP
CR
CI
Mass ratio
dust : sample
1
2
3
4
6
1
2
3
4
1
2
3
4
1
2
3
4
Moles Cl
in 100 g
of the sample
1.510
1.316
1.276
0.794
Mass of HCl captured, g
1.24
1.67
1.83
1.91
1.91
0.98
1.26
1.51
1.63
1.19
1.37
1.56
1.65
1.15
1.15
1.17
1.17
3.5. Efficiency of HCl capture in the
metallurgy dust
By stoichiometry, 1 g of the dust at 200-400°C is
capable to trap HCl contained in 0,35 g PVC in
form of ZnCl2 and other chlorides.
When all obtained data were plotted as the
efficiency of HCl capture versus the mass ratio of
the dust to the pure PVC for all measured systems,
we have obtained the graph shown in Fig.5.
Interestingly, our data indicate that at lower ratios
of the dust additional HCl is captured as FeCl2,
while at higher ratios practically all HCl is
captured.
Fig. 5. Captured portion of HCl as a function of the mass ratio of the
dust to the recalculated mass of pure PVC in the samples.
TG - estimated by thermogravimety experiments
LO – estimated by the laboratory oven pyrolysis
5
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oxides/chlorides on the thermal degradation of poly
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Slapak, M. J. P., Van Kasteren, J. M. N., Drinkenburg, A.
A. H., 2000. Determination of the pyrolytic degradation
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Starnes Jr, W. H., 2002. Structural and mechanistic
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chloride). Progr. Polym. Sci. 27, 2133-2170.
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two useful product may be obtained: zinc chloride
and chlorine free organic residuum, which can be
safely combusted.
Acknowledgements
The work has been performed as part of the project
financially supported by the Ministry of Education,
Youth and Sports of the Czech Republic in the
“National Feasibility Program I”, project LO1208
“Theoretical Aspects of Energetic Treatment of
Waste and Environment Protection against
Negative Impacts”. Other part has been financed by
Structural Funds of European Union and from the
means of state budget of the Czech Republic within
the frame of the project Institute of Environmental
Technologies, reg. no. CZ.1.05/2.1.00/03.0100
supported by Research and Development for
Innovations Operational Program.
Abbreviations
LO Laboratory oven
PVC Poly-(vinyl chloride). (Detailed specification
of PVC materials by Table 1).
TG Thermogravimetry
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