Slovak Society of Chemical Engineering Institute of Chemical and Environmental Engineering Slovak University of Technology in Bratislava PROCEEDINGS

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1 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, 9 Editor: J. Markoš ISBN

2 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, 7833 Ostrava Poruba, Czech Republic, phone kamil.wichterle@vsb.cz 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 4% of virgin pig iron consisting mostly of Fe, C, Si, P Mn and S. and 6% 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, 1 t of the dust /year is produced. It cannot be recycled as a iron feedstock because it contains not only 5% of Fe but also 5-15% Zn, mostly in form of zinc ferrite (franklinite) ZnFe 2 O 4, 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 1, price of iron recyclable is order less and so, the main driving force for the process development is in a charge /t for the dust dumping. One option is treatment of the fine metallurgy dust by gaseous hydrogen chloride at higher temperatures. Above 3 C, franklinite decomposes according the scheme ZnFe 2 O 4 + 2HCl ZnCl 2 + Fe 2 O 3 + H 2 O ZnO + 2HCl ZnCl 2 + H 2 O and zinc becomes soluble form. Appearance of soluble iron by FeO + 2HCl FeCl 2 + H 2 O is suppressed by metathesis reaction FeCl2 + ZnO ZnCl 2 + FeO when oxidic or ferritic form of zinc is still present. Reaction Fe 2 O 3 + 6HCl 2FeCl 3 + 3H 2 O producing volatile Fe(3) chloride does not take place above C. This is apparent from the values of Gibbs free energy for quoted reactions (Fig.1). 91 1

3 6 2 HCl + O 2- --> H 2 O + 2 Cl - 4 G / kj/mol FeO->FeCl2 Fe3O4->FeCl2+FeCl3 Fe2O3->FeCl3 ZnO->ZnCl2 ZnFe2O4->ZnCl2+FeCl3 ZnFe2O4->ZnCl2+Fe2O3 Fe3O4->FeCl2+Fe2O3 Fig.1. Gibbs energy for the reactions of HCl with selected oxides and ferrites T C 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 TP6 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 TP6 Elementary composition (mass %): Fe Zn Pb Cd Forms of the solid phase was analyzed by X-ray diffraction and the results are presented in Tab.2. Tab.2. Crystalline form of TP6 components tp6 Hematit 3.22 Magnetit 35.6 Wuestit 3.22 Franklinit 36.6 Grafit 4.37 Zinkit 5.56 amorf

4 Fixed bed of fine metallurgy dust in quartz tube was placed vertically in an electric oven and heated to selected temperature in the range 1-9 C. Gaseous hydrogen chloride was prepared by the reaction 2NH 4 Cl + H 2 SO 4 (NH 4 ) 2 SO 4 + 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.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 ZnCl 2 in the quartz tube and in the trap Fig.3 Detail of ZnCl 2 crystals inside the tube Excess of hydrogen chloride was trapped in NaOH solution and analyzed quantitatively by titration. 91 3

5 Results Three main quantities were studied: - Conversion X Zn of zinc is percentage of zinc converse to the soluble form. - Conversion X Fe of iron is percentage of iron converse to thesoluble form. - Conversion X HCl 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 ZnCl 2 vapor density was significantly higher than the density of other present gases, HCl and H 2 O. 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 X(Zn) / %) HCl upw ard flow HCl dow nw ard flow T / C Fig. 4.Effect of flow orientation on the zinc conversion, reaction time 4 min This paper is an attempt to explain such a spectacular behavior. Discussion Exothermic reaction takes place ZnO(s) + 2HCl(g) ZnCl 2 (l) + H 2 O(g) H = -14 kj/mol which is followed by local adiabatic heating and partial evaporation of ZnCl 2 to the stream of H 2 O vapor according to the tension of ZnCl 2 and starting temperature. Vapor pressure of ZnCl 2 in relevant range of temperatures is 155 [ ] [ ] P ZnCl 2 kpa = exp.. T K When some zinc chloride is evaporated, following enthalpy change should be taken into consideration ZnO(s) + 2HCl(g) ZnCl 2 (g) + H 2 O(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 ZnCl 2 is not less than 5 kpa. To reach this temperature adiabatically, the injected reactants temperature should be at least 76 C. 91 4

6 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 ZnCl 2 for large extent of reactant temperatures as seen in Fig.5. 7 Adiabatic temperature / C X=% X=4% X=6% X=8% X=1% Inlet temperature / C Fig.5. Adiabatic temperature as a function of inlet temperature for various conversions When the inlet temperature is less than 76 C, part of ZnCl 2 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 % ,32,32-,63,63-,1,1-,2,2-,315,315-,4,4-,5 particle diameter / mm,5-,63 Fig. 6 Particle size distribution for the dust TP6,63-,9 >,9 This thin layer retards the mass transfer diffusion of HCl to the oxide components of the particles and countercurrent diffusion of H 2 O. 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 ZnCl 2 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 ZnCl 2 stays in a condensed form retard the process and yield of soluble zinc is about 5% in wide range 1-7 C. At lowest temperatures is also formed volatile FeCl 3, which complicates separation of pure zinc. For an efficient separation of zinc, temperatures above 7 C should be preferred. 91 5

7 9 Conversion for sample TP6 X (%) X(HCl) X(Zn) X(Fe) 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 14/6/166 was highly appreciated. 91 6