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Chlorination-Vaporization Reaction of Pb-Oxychloride in Pyrometallurgical Recycling Process of Dust and Ash.

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Dev. Chem. Eng. Mineral Process. 14(3/4), pp. 385396, 2006.
Chlorination-Vaporization Reaction of
Pb-Oxychloride in Pyrometallurgical
Recycling Process of Dust and Ash
Nan Wang’*, Ai-jun Jin2, Min Chen’
College of Materials and Metallurgy, Northeastern University,
Shenyang 110004, P R. China
.’ Nanjing Iron & Steel Group Co. Ltd, Nanjing 2 I0035 P R. China
A molten salt layer composed of PbO-PbCI2 is formed when the vaporization rate of
PbCl2 is slower than its formation rate. Kinetic analysis has been performed of the
chlorination-vaporization reaction of Pb-oxychloride in the pyrometallurgical
recycling process of dust and ash. Simulation calculationsfor the bulk dflusion have
been carried out, and also by applying the quasi-steady principle the profiles of
relative chemical potentials, compositions, and mass frux across the molten salt as
well as the intermediate compound layers of Pb-oxychloride, have been obtained. For
the chlorination-vaporizationreaction, the eflects of temperature and CI-potential in
the gas phase (log( pic, pH,*)) have been evaluated.
Introduction
Due to increasing amounts of EAF dust, attention to the social responsibility of
industry, and economic issues has caused a focus on the recycling process and
recovery of valuable resources from the EAF dust. Pyrometallurgical processes, such
as Wealtz kiln or MF process, are currently used to isolate Zn from EAF dust through
the chlorination-vaporizationreaction. One problem is that the impurities included in
EAF dust, e.g. Pb and CI, polluted the ZnO formed during vaporization and
condensation processes. Many “insoluble chlorides” are included in the crude ZnO
which cannot be removed by a simple washlleaching process, and will reduce the
value of crude ZnO. Moreover, in the melting processing of primary bottom ash and
fly ash from the incineration of municipal solid waste, heavy metal chlorides are
thought to be generated and possibly evaporate through the reaction with other
chlorides (chlorine sources), so that the environmental pollution problems caused by
elution of heavy metals will occur. Therefore, study of the chlorination-vaporization
reaction of heavy metals in pyrometallurgical recycling processing of dust and ash is
significant not only for the optimization of current industrial processes, but also for
* Author for correspondence (wangnan@mail.neu.edu.cn).
385
Nan Wang, Ai-jun Jin and Min Chen
the development of novel recycling processes for various industrial and municipal
solid wastes. In our previous work [11, the driving force for chlorination-vaporization
reaction of heavy metals has been elucidated, and a determination of the optimized
thermodynamic parameters for PbO-PbC12 system employing the CALPHAD
approach has also been obtained. In the present study, a kinetic analysis of the
chlorination-vaporization reaction of Pb-oxychloride has been made using the
quasi-stationary principle for the case of a molten salt layer composed of PbO-PbCI2
formed when the vaporization rate of PbC12 is slower than its formation rate. The bulk
diffusion of the corresponding components across the molten salt and intermediate
compound layers has also been analyzed by applying the phenomenological mass
transport equations. Finally, simulation calculations of relative chemical potentials,
molar fractions, and the normalized mass flux of the related components have been
performed based on the established rate equations.
Reaction Mechanisms
In the pyrometallurgical processing of EAF dust, vaporization of heavy metal
chlorides formed by the chlorination reaction takes place, in addition to the simple
vaporization of chlorides charged in the raw materials. Considering PbO, for example,
its chlorination-vaporizationreaction can be described as the reactions (1-a) and (1-b)
below. There is gaseous water (H20) in the industrial processing system, therefore
CI-potential is controlled in practice by reaction (2), and 1og(p&,/pHlo) is employed
to express the CI-potential in the gas phase.
As the temperature increases, the chlorination reaction occurs between PbO and
the chloridizing gas agent, such as gaseous HC1, and then, the formed PbClz vaporizes
from reaction site to gas phase. In our previous work [2], thermodynamic analysis
identified that the relative rate of the chlorination reaction versus the vaporization
reaction varies significantly due to the effects of temperature and C1-potential in the
atmosphere. On the other hand, from the kinetics view point, depending on the
relative ratio of PbClz formation rate to its vaporization rate, then the
chlorination-vaporizationreaction can be considered as two cases:
( I ) Case I: Vaporization rate of PbCI2 is faster than its rate of formation, and the
kinetic model of shrinking unreacted-core without product layer can be
employed to describe this case [3].
(2) Case 11: Vaporization rate of PbC12 is slower than its rate of formation, which
means a portion of the formed PbC12 remains at the reaction site. A molten salt
layer then forms and covers the outside of the unreacted solid PbO (below the
melting point of PbO), this is illustrated schematically in Figure 1.
386
Chlorination-Vaporization Reaction of Pb-Qxychloride in Recycling Dust and Ash
Gas Film
pqc12
. .. .. .. .. .. .. .. .
......
(Pbo-Pbcl2)
Figure I. Formation of molten salt layer composed of PbO-PbC12.
n
H
a"
Gas
Phase
0.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
M i z e d Distmce from inter fa^^ (.d6)
Figure 2.
Variation of PbO-PbC12molten salt layer:
Since no valid kinetic data are available, the calculated results are used to make a
thermodynamics analysis. Figure 2 shows the activity profile of PbCI2 across the
molten salt layer of PbO-PbC12 as a hnction of normalized distance from molten
salt/gas-phase interface, at the given temperature and C1-potential.
38 7
Nan Wang, Aijun Jin and Min Chen
At higher temperatures, PbCI2 in molten salt will evaporate to the gas phase due to
the increase of vapor pressure, and then the interface will move toward the interior of
the molten salt. Eventually either no molten salt layer is left outside the solid PbO, or
the molten salt layer of PbO-PbCI2is always present. Once the PbO-PbC12 molten salt
layer (Case 11) is formed, two kinds of reaction model can be used to describe the
corresponding chlorination-vaporization reaction at the temperature below the melting
point of PbO. The phase diagram for PbO-PbC12 system [4] is employed to provide a
detailed discussion. If the temperature range is relatively high, e.g. above the eutectic
point e l , then as the chlorination-vaporization reaction proceeds, the existing phases
are molten salt and solid PbO. Therefore, a molten salt layer composed of PbO-PbCIz
can be formed outside the unreacted PbO core, and the so-called mono-layer reaction
model can be applied to analyze the reaction process.
'Oo0
5
900
800
Y
2
v
700
c)
(D
Y
L
600
F
500
400
300
0
P bC I,
0.2
0.4
0.6
0.8
XP bO
1 .o
PbO
Figure 3. Phase diagramfor PbO-PbCiI system.
Alternatively, if the temperature is lower than the eutectic point e l , such as given
by temperature Tz, then the possible included phases are molten salt, Pb3O2CIz([ 1:2]
compound), Pb5O4CI2([I :4] compound), and solid PbO, i.e. a multi-layer reaction
model as shown in Figure 3 can be used to analyze the reaction process. In practice,
the existing phases which can be formed during the chlorination-vaporization reaction
of Pb-oxychloride are determined by both the thermodynamic and kinetic conditions
of the corresponding reactions simultaneously. The elementary steps included in the
above Case I1 can be summarized as follows: chlorination-vaporization reaction of
Pb-oxychloride at the interface (gas-phase/molten salt interface), mass transport
across the gas phase, the molten salt layer, and the intermediate compound layers
respectively, as illustrated in Figure 4.
388
Chlorination-Vaporization Reaction of Pb-Oxychloride in Recycling Dust and Ash
. . ..
..o
Figure 4. Schematic model and elementary steps.
Mass Transport across the Molten Salt and the Intermediate
Compound Layers
In order to discuss the mass transport phenomena across the molten salt and the
intermediate compound layers, kinetic parameters such as dieusivities of the
corresponding components in the molten salt and intermediate compounds are
required. However, the chemical potential profiles of the related components across
molten salt and intermediate compound layers were estimated only for steady-state
conditions. According to the phenomenological theory, the mass fluxes of each ionic
species in PbO-PbCl, molten salt are assumed to be linear functions of the
electrochemical potential gradients, and can be expressed as:
...(4)
where represents qualities per equivalent; li,j are the transport coefficients for the
corresponding dependent set of equations; ci and iti are the concentration and
velocity of species I; w is the velocity of the reference system; qi is the
electrochemical potential qi = pi + z i F p ; subscripts I , 2, 3 represent 02-,
CI',and Pb2+,
respectively. In order to define the coordination system, choosing Pb" as the
reference component, w = v 3 , Equation (4) can be transformed into the independent
set of flux equations:
389
Nan Wang, Ai-jun Jin and Min Chen
...(5 )
113131
L,, =I,, --
where
.L
9
I2
-1
113132
-
12
are the coefficients of the independent
4 3
I33
L,, = I 2 , -!&;L
22
-1
-
123132
22--
4 3
[3 3
-
-
transport equation. According to electroneutrality conditions, 3 j+
l 3 j=
2o , local
electrochemical equilibrium can be expressed by Equations (6-a) and (6-b) following,
-
-
-
-
and V q1- V q 2 = V q,, - V q~~~ , the chemical potential gradients of neutral
components, 13 and 23 (PbO and PbCI2),can be used to represent the mass fluxes of
ionic species as given by Equation (7).
...(7)
Equation (7) also represents the mass fluxes of the neutral molecules 13 and 23
-
-
-
-
-
_
-
-
(PbO and PbCI2) since 3j13=3jl+3j3=3jl
and3jz3=3j2+3
j 3 = 3 j 2 .Therefore, the
overall rate equation was established as given by:
Furthermore, the effects of operating conditions, such as temperature and
C1-potential in the atmosphere (log(p&, I pHz0)), on the chlorination-vaporization
reaction of Pb-oxychlorides required evaluation. The variations of the relative
chemical potential of PbO and PbC12 across the molten salt layer for the mono-layer
model, and across the intermediate compound layers for the multi-layer model, were
calculated by means of quasi-steady principle, assuming that all the difision
coefficients are composition-independent over these diffision barrier layers. In
addition, the composition profiles of PbO and PbClz across the molten salt layer were
assumed to be a function of the normalized distance away from the molten
390
Chlorination-VaporizationReaction of Pb-Oxychloride in Recycling Dust and Ash
salt/gas-phase interface, and can be determined by using the obtained optimized
thermodynamic parameters of the PbO-PbC12 quasi-binary system [ 11. However, by
applying Fick's second law in Equation (8), the normalized mass fluxes of PbO,
jpbo
/ L ~ ,as~ a, hnction of log(P&, / pHlo), were obtained at temperatures of 983 K,
1023 K and 1123 K.
Results and Discussion
(i) Variation of relative chemical potentials and molar fraction profiles of PbO
and PbC12
Profiles of the relative chemical potentials of PbO and PbCI2 across the molten salt
layer were calculated for the higher temperature range based on the mono-layer
reaction modeland are shown in Figure 5(a). The corresponding calculated results for
different values of log(P&, pHlo) are shown in Figure 5(b).
The composition profiles of PbO and PbCI2 (as mole fractions) across the molten
salt layer for different values of log(P,& / p H l o ) and at three temperatures were also
calculated, and the results are shown in Figures 6(a) and 6(b), respectively.
-20
'P
-60il
I
PbCI.
I
g -80.1
I
i
p -100 J
I
d's -I20
-140
-
-.""
0.0
0.2
0.4
0.6
0.8
.""
0.0
1.0
Figure 5.
Variations of ApPbOand
0.2
0.4
0.6
0.8
1.0
normalized distance from interface (x/6)
normalized distance from interface (x/6)
~
p
~ with
~ normalized
~ , ,
distancefrom
the
interface as a function of (a) ]0g(pic, pHlo and (b) temperature.
391
Nan Wang, Ai-jun Jin and Min Chen
-T-
0.2
00
0.4
lWK
0.6
1.0
0.8
normaliaed distance fmm interface (d6)
Figure 6. Molar fraction variations ofPb0 and PbC12 with normalized distance
from the interface at different values ox (a) log( picl I pHI01 and (b) temperature.
I
0.0
0.5
1.0
1.5
2.0
2.5
3.0
normalized distance from interface (x/8)
(a)
-024
0.0
0.5
1.0
1.5
2.0
2.5
3.0
normalized distance from interface ( d 6 )
(4
~ ~ ,the,molten salt and intermediate
Figure 7. Variations of Ap PbO and ~ p ~ across
compounds layers showing: (a) Cl-potential dependence; and (b) temperature
dependence.
392
Chlorination- VaporizationReaction of Pb-Oxychloride in Recycling Dust and Ash
12001
I
I
I
I
I
-4
-2
0
2
4
log @,,,'lp,p)
Figure 8. Normalized mass fluxes of PbO as a function of bg(P&, i pH,*) at
different temperatures.
Based on the multi-layer reaction model for the lower temperature range, the
variations of relative chemical potentials of PbO and PbCI2 across molten salt,
intermediate compound layers, [ 1 :2] and [ 1 :4], were also calculated for different
iog(p&, / pHz0) and temperatures, as shown in Figure 7(a) and 7(b), respectively.
For simplification in the calculation due to a lack of available data, the thicknesses of
the [1:2] and [1:4] compound layers were considered to be the same as that of the
molten salt layer. From these calculated results, higher CI-potential in the gas phase
and lower temperatures are favorable to promote the chlorination-vaporization
reaction of Pb-oxychlorides. Figure 8 shows the relationship between the normalized
mass flux of PbO and IO~(P&, /pHz0) for different temperatures. An increase of
temperature and CI-potential in the atmosphere increases the mass flux across the
molten salt layer.
(ii) Discussion on ionic structure of PbO-PbCI, system
In order to describe the transport properties across the molten salt and the
intermediate compounds of Pb-oxychloride, information on the ionic structure of the
PbO-PbC12 system is needed. We used the stability function to evaluate the effect of
the degree of stability of PbO-PbC12 liquid solution on the formation of intermediate
compounds. For a binary system, the defining equation of the stability function, \v , is
expressed below by Equation (9), and its strong positive value at stoichiometric
393
Nan Wang, Ai-jun Jin and Min Chen
composition can be anticipated due to the presence of intermediate solid-state phases
at lower temperature [5]. The calculated results of stability function ( w ) for the
PbO-PbC12system at 823 K, 923 K, 1023 K, and 1123 K are shown in Figure 9.
It can be seen that peak positive values of w occur, which implies the possibility
of complex ionic species existing in the PbO-PbClz melting salt. In this study, the
ideal associated solution model is employed in order to make a further analysis [6].
For PbO-PbC12 liquid solution, the complex ionic species: [PbO4I6‘, [Pb03CI]”,
[PbO2ClZl4-,[PbOCl3l3-,[Pb2O4CI3]’-,and [PbCI4]’- are assumed to exist, and these
correspond to the hypothetical molecules of PbO, Pb706C12 ([1:6]), Pb302C12 ([1:2]),
Pb5O2CI6 ([3:2]), PbllO8Cl6 ([3:8]) and PbCI2. The mole fractions of these
hypothetical molecules at equilibrium were calculated at 823 K, 923 K, 1023 K and
1 123 K as a function of mole fraction of PbO (X,,,) as shown in Figure 10. These
calculated results are consistent with the presence of the stable intermediate
compounds, Pb20CI2 ([ 1:I]), Pb302C12([ 1:2]), and Pb5O4CI2([ 1:4]), at lower
temperatures. Considering the Stokes-Einstein diffusion model, the diffusion
coefficient in liquid is inversely proportional to its viscosity, which is mainly
dependent on the nature of bonding between the ions, i.e. the ionic structure. For the
PbO-PbC12 quasi-binary system, difhsion of the oxygen bonded to the complex ionic
species is difficult compared to its simple discrete ion, and the related oxygen
diffusion in molten salt can be considered to be the rate-controlling step.
”
0.0
0.2
0.6
0.4
0.8
1.0
XPb,
Figure 9. Stability function of PbO-PbCl2 system.
3 94
Chlorination-VaporizationReaction of Pb-Oxychloride in Recycling Dust and Ash
c
CN
s
e
*%
6
L
*%
Figure 10. Calculated molefractions of hypothetical molecules at the equilibrium
state as a function of x,,,for the PbO-PbClz system,from 823 K to 1123 K
Conclusions
A kinetic analysis on the chlorination-vaporization reaction of Pb-oxychloride in the
pyrometallurgical recycling process of dust and ash has been performed. The
following conclusions can be stated:
(1) Higher CI-potential in the gas phase and lower temperatures are favorable to
promote the chlorination-vaporization reaction of Pb-oxychlorides.
(2) A molten salt layer composed of PbO-PbC12 is assumed to form when the
vaporization rate of PbClz is slower than its rate of formation, caused by the
effects of temperature and CI-potential in the gas phase ( log(&, / pHz0) ).
(3) An increase in temperature and CI-potential in the gas phase increases the mass
flux across the molten salt layer.
(4) The related oxygen diffusion in molten salt can be considered to be the
rate-controlling step.
395
Nan Wang, Ai-jun fin and Min Chen
Nomenclature
i
w, v,
c,
rl
P
4,j
Mass flux (mo1.i')
Velocity of reference system and species i (m.s-')
Molar concentration of species i (m01.m'~)
Electrochemical potential of i species (J.moP')
Chemical potential (kJ.mor')
Onsager coefficient for independent set (mol*.J-'.m-'.s-')
Charge number of i and k species
F
Faraday constant (Jvolt-I )
v
Electrical potential (V)
6
Thickness of molten salt and intermediate compound layer (m)
R
Gas constant (J.moP')
Temperature (K)
T
t
Time (s)
X
Distance from molten salt/gas phase interface (m)
Gibbs fiee energy of mixing (Jmor')
Gmix
Molar fraction of PbO
xPbO
xPbC12 Molar fraction of PbCI2
zi,zk
References
1. Yamaguchi, S., Iwasawa, K., Wang, N., et al. 2003. Thermodynamic analysis on the chlorination
reaction in dust recycling processing. Proc. TMS-Yarawa Inlernafional Symposium, San Diego,
California, USA, 3, 173-184.
2. Yamaguchi, S., Iwasawa, K., and Maeda, M. 2002. Chlorination in pyrometallurgical processing of
solid wastes. Proc. Recycling and Waste Treatment in Mineral and Metal Processing, Lulea, Sweden,
457-460.
3. Otake, T., Tone, S., Komasawa, I., et al. 1984. Removal of heavy metals through
chlorination-vaporization reaction. Japan Kagaku Kogaku Runbunshu, 11,68-73.
4. Podsiadlo, H. 1991. Phase equilibrium in the binary system PbO-PbC12.J. Thermal Anal., 37,613-626.
5 . Lupis, C.H.P. 1983. Chemical Thermodynamics of Materials, Elsevier Science Publishing Co., Inc.,
New York, USA; pp. 86-89.
6. Guggenheim, E.A. 1952. Mixtures. Oxford University Press, Oxford, UK.
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