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Transcript
Slovak Society of Chemical Engineering
Institute of Chemical and Environmental Engineering
Slovak University of Technology in Bratislava
PROCEEDINGS
36
th
International Conference of Slovak Society of Chemical Engineering
Hotel Hutník
Tatranské Matliare, Slovakia
May 25 – 29, 2009
Editor: J. Markoš
ISBN 978-80-227-3072-3
36th International Conference of SSCHE
May 25–29, 2009, Tatranské Matliare, Slovakia
Po-Tu-4, 091p.pdf
Effect of convection for gaseous hydrochloride reactions with a fixed bed
Jan Cieslar, Kamil Wichterle
Department of Chemistry, VSB-Technical University of Ostrava, 17.listopadu 15, 70833
Ostrava Poruba, Czech Republic, phone 420596994304. E-mail [email protected]
Key words: fixed bed, metallurgy dust, zinc recycling, leaching, evaporation, convection
Reaction of gaseous hydrochloride with a fixed bed of metallurgy dust, consisting mostly
from zinc and iron oxides and zinc ferrite, is investigated. Quite different rate of reaction and
yield of zinc chloride has been obtained in a tubular reactor with upflow and downflow.
Apparently it is an effect of convection in gaseous mixture of hydrogen chloride, and
produced zinc chloride and water steam. Downflow arrangement is recommended when the
product is efficiently removed.
Introduction
In steel production the recycling plays an important role. At present the steelmaking
feedstock consists of about 40% of virgin pig iron consisting mostly of Fe, C, Si, P Mn and S.
and 60% of scrap, containing number of different metals. Some of them, Ni, Cr, V make steel
better etc., however metals like Zn, Cd, Pb are unwelcome. In modern steelmaking equipment
using oxygen atmosphere in presence of lime, C is removed as CO gas, Si, P and S form slag.
Small percentage of Fe is oxidized as well, and leaves the batch together with oxidized heavy
metals. It is collected as a dust. For the steel production, 6 millions ton/year in the Czech,
100 000 t of the dust /year is produced. It cannot be recycled as a iron feedstock because it
contains not only 50% of Fe but also 5-15% Zn, mostly in form of zinc ferrite (franklinite)
ZnFe2O4, and further minor compounds (about 1%) of Pb, Cd etc. The steelmaking dust
should be therefore dumped as a dangerous waste.
There is number of methods how to separate the metals from fine metallurgy dust. The
process have to be very cheap, because price of the zinc available in 1 metric tone of dust is
no more 100€, price of iron recyclable is order less and so, the main driving force for the
process development is in a charge 200€/t for the dust dumping.
One option is treatment of the fine metallurgy dust by gaseous hydrogen chloride at
higher temperatures. Above 300°C, franklinite decomposes according the scheme
ZnFe2O4 + 2HCl → ZnCl2 + Fe2O3 + H2O
ZnO + 2HCl → ZnCl2 + H2O
and zinc becomes soluble form.
Appearance of soluble iron by
FeO + 2HCl → FeCl2 + H2O
is suppressed by metathesis reaction
FeCl2 + ZnO → ZnCl2 + FeO
when oxidic or ferritic form of zinc is still present.
Reaction
Fe2O3 + 6HCl → 2FeCl3 + 3H2O
producing volatile Fe(3) chloride does not take place above 200°C. This is apparent from the
values of Gibbs free energy for quoted reactions (Fig.1).
091–1
36th International Conference of SSCHE
May 25–29, 2009, Tatranské Matliare, Slovakia
60
Po-Tu-4, 091p.pdf
2-
-
2 HCl + O --> H2O + 2 Cl
40
20
∆G / kJ/mol
0
0
100
200
300
400
500
600
700
800
900
-20
1000
T °C
-40
-60
FeO->FeCl2
Fe3O4->FeCl2+FeCl3
Fe2O3->FeCl3
ZnO->ZnCl2
ZnFe2O4->ZnCl2+FeCl3
ZnFe2O4->ZnCl2+Fe2O3
Fe3O4->FeCl2+Fe2O3
-80
-100
-120
Fig.1. Gibbs energy for the reactions of HCl with selected oxides and ferrites
Aim of present research was to study the reaction of gaseous hydrogen chloride with
actual metallurgy dust (collected from exhaust gas line leaving tandem ovens of steelmaking
plant Mittal Steel Ostrava Kunčice).
Experimental
Sample TP600 of the dust under investigation was analyzed by atomic absorption
spectrometry and its composition is presented in Table 1.
Tab.1. Elementary composition of the dust TP600
Elementary composition (mass %):
Fe
Zn
Pb
Cd
48.7
14.5
1.8
0.02
Forms of the solid phase was analyzed by X-ray diffraction and the results are presented in
Tab.2.
Tab.2. Crystalline form of TP600 components
tp600
Hematit
3.22
Magnetit
35.6
Wuestit
3.22
Franklinit
36.6
Grafit
4.37
Zinkit
5.56
amorf
11.4
091–2
36th International Conference of SSCHE
May 25–29, 2009, Tatranské Matliare, Slovakia
Po-Tu-4, 091p.pdf
Fixed bed of fine metallurgy dust in quartz tube was placed vertically in an electric oven and
heated to selected temperature in the range 100-900°C. Gaseous hydrogen chloride was
prepared by the reaction
2NH4Cl + H2SO4 → (NH4)2SO4 + 2HCl
and temporarily stored in a soft PVC balloon. Gas was injected to the tubular reactor by the
calibrated peristaltic pump. The superficial velocity was typically 0.7 mm/s. Hot gaseous
products condensate partly just in the quartz tube at the outlet from the heated section (See
Fig.2 and 3.) of the oven, other part was solved in water trap.
Fig. 2. Condensed crystals of ZnCl2 in the quartz tube and in the trap
Fig.3 Detail of ZnCl2 crystals inside the tube
Excess of hydrogen
chloride was trapped in NaOH solution and analyzed quantitatively by titration.
091–3
36th International Conference of SSCHE
May 25–29, 2009, Tatranské Matliare, Slovakia
Po-Tu-4, 091p.pdf
Results
Three main quantities were studied:
- Conversion XZn of zinc is percentage of zinc converse to the soluble form.
- Conversion XFe of iron is percentage of iron converse to thesoluble form.
- Conversion XHCl is percentage of hydrogen chloride neutralized in the dust bed.
In a first set of experiments, HCl gas was introduced to the fixed bed from bottom as usual in
common fixed bed processes. However, there appeared some condensed crystals of the
product also at the entrance of the tube to the heated section. Apparently, there was an
important effect of free convection, as the ZnCl2 vapor density was significantly higher than
the density of other present gases, HCl and H2O. Therefore, the reverse orientation of gas
flow was preferred. Increase of the reaction conversion with downward gas flow was evident,
as can be seen from zinc conversion in Fig. 4..
100
90
80
X(Zn) / %)
70
60
50
40
30
20
HCl upw ard flow
10
HCl dow nw ard flow
0
500
550
600
650
700
750
800
850
900
T / °C
Fig. 4.Effect of flow orientation on the zinc conversion,
reaction time 40 min
This paper is an attempt to explain such a spectacular behavior.
Discussion
Exothermic reaction takes place
ZnO(s) + 2HCl(g) → ZnCl2(l) + H2O(g) ∆H = -104 kJ/mol
which is followed by local adiabatic heating and partial evaporation of ZnCl2 to the stream of
H2O vapor according to the tension of ZnCl2 and starting temperature. Vapor pressure of
ZnCl2 in relevant range of temperatures is

15500 
0
PZnCl
2 [kPa ] = exp
 20.0 − T [K ]  .


When some zinc chloride is evaporated, following enthalpy change should be taken into
consideration
ZnO(s) + 2HCl(g) → ZnCl2(g) + H2O(g) ∆H = +25 kJ/mol
Proportional amount of both products in gaseous form is obtained at temperatures higher than
685°C when the vapor pressure of ZnCl2 is not less than 500 kPa. To reach this temperature
adiabatically, the injected reactants temperature should be at least 760°C.
091–4
36th International Conference of SSCHE
May 25–29, 2009, Tatranské Matliare, Slovakia
Po-Tu-4, 091p.pdf
We can suppose that the local adiabatic heating or cooling is more significant than conduction
heat transfer keeping the bed isothermal. The adiabatic temperature is close to the boiling
temperature of ZnCl2 for large extent of reactant temperatures as seen in Fig.5.
Adiabatic temperature / °C
700
680
660
640
X=20%
X=40%
620
X=60%
600
X=80%
580
X=100%
560
200
300
400
500
600
700
800
Inlet temperature / °C
Fig.5. Adiabatic temperature as a function of inlet temperature for various conversions
When the inlet temperature is less than 760°C, part of ZnCl2 remains in liquid phase
and covers the dust surface. Particles of the dust (size distribution is presented in Fig.6) are
covered by layer up 5µm of liquid zinc chloride.
25
percentage %
20
15
10
5
0
00,32- 0,063- 0,10,032 0,063 0,1
0,2
0,2- 0,315- 0,40,315 0,4
0,5
0,50,63
0,63- > 0,9
0,9
particle diameter / mm
Fig. 6 Particle size distribution for the dust TP600
This thin layer retards the mass transfer – diffusion of HCl to the oxide components of the
particles and countercurrent diffusion of H2O. Even in such a thin liquid layer, downward
flow occurs and the zone of liquid zinc is shifted downwards which cleans surface in upper
part of the particles in the reaction zone of the bed. By diffusion of heat accompanied by
condensation and evaporation of ZnCl2 and by natural convection of heavier vapor products,
can be explained the advantage of downstream orientation of gas flow through the solid
particle bed.
Extended results with the downstream flow are presented in Fig. 7. Apparently, low
temperatures when ZnCl2 stays in a condensed form retard the process and yield of soluble
zinc is about 50% in wide range 100-700°C. At lowest temperatures is also formed volatile
FeCl3, which complicates separation of pure zinc. For an efficient separation of zinc,
temperatures above 700°C should be preferred.
091–5
36th International Conference of SSCHE
May 25–29, 2009, Tatranské Matliare, Slovakia
Po-Tu-4, 091p.pdf
Conversion for sample TP600
90
80
X (%)
70
60
50
40
30
X(HCl)
20
X(Zn)
10
0
100
X(Fe)
200
300
400
500
600
700
800
900
T (°C)
Fig. 7.Effect of temperature on the yield of soluble zinc and iron as a function of temperature
As concerns the theoretical mechanism of studied reaction in the fixed bed, we can estimate
quantitative effects of heat conduction, evaporation, reaction heat, and both forced and natural
convection. The process is so complex, that it was impossible to make a complete analysis in
a frame of our project. However, the explanation of the process behavior presented here is
based on the observed reality and it has been confirmed by quantitative estimate of particular
effects.
Acknowledgments
Support of Grant Agency of the Czech Republic through the grant GAČR 104/06/1606 was
highly appreciated.
091–6