PROCESSING OF ZINC-CONTAINING METALLURGIC SLUDGES FOR THEIR FURTHER UTILIZATION. Moscow

Transcription

1 PROCESSING OF ZINC-CONTAINING METALLURGIC SLUDGES FOR THEIR FURTHER UTILIZATION David Lempert 1, Inessa Gudkova 1, Valery Dyubanov 2, George Manelis 1 1 Institute of problems of chemical physics, Rus.Ac.Sci., Chernogolovka, 2 Baikova Institute of Metallurgy and Material Science, Rus.Ac.Sci., Moscow lempert@icp.ac.ru The method of filtration combustion may be effectively used for man-made wastes beneficiation with zinc and then for zinc extraction together with the applying Fe-containing residue. Regularities of filtration combustion, mass transfer and volatile Zn-containing products extraction from metallurgic sludges with high iron content have been studied. The burning conditions and the level of Zn extraction have been studied at wide interval of control parameters. Up to recent years in Russia millions tons of metallurgic sludges with high content of Fe (up to 55%) and Zn (from 3 to 20%) are accrued. These products can not be processing further with metallurgic technologies. Thanks to high Zn content they can not be returned into blast furnace processing Zn is accumulated in the work space of the blast furnace and it impedes normal eliquation in the blast furnace. On the other hand the use of these sludges in the recent industry of Zn extraction is not effective economically if Zn content is lower than 15%. That is why the development of the technology for Zn and Fe separation may become very relevant. In principle, there are no difficulties to separate Zn from Fe, these metals are rather different in chemical and physical properties, the main problem is to create a cost-effective technology. For example, one may simply ignite a source mixture at temperature considerably higher than 1000 C in nitrogen flow and Zn-containing compounds that are rather more volatile that analogous derivatives of Fe, would be carried out together with the nitrogen flow. But using such a technology one wastes a huge amount of heat energy, so it will not be cost-effective. It was proposed to use the process of filtration combustion that differs from other combustion processes it goes with an extremely high heat efficiency because in countercurrent flows of solid porous fuel and 80

2 gaseous oxidizer the maximal temperature in combustion wave is higher in a few times than so called adiabatic temperatures (that may be reached at ordinary furnace regime [1-4]) while the temperature of gaseous products leaving the reactor is almost close to the ambient temperature. There is another particularity of filtration combustion process besides an abrupt separation of the reaction space onto different temperatures zones there is also an abrupt separation accordingly reductive-oxidative properties of the reactive area the zone from the point of maximal temperature to the side from where the air moves has oxidizing properties while the other zone (from the point of maximal temperature to the side where volatile combustion products move, e.g. CO and H 2 if water was presented in initial mixture,) has reductive properties. Thus simultaneously a few factors favor the extraсton of Zn from its mixtures with Fe: Making high temperature (even in a rather narrow zone); The presence of the reductive zone, where Zn-containing products may be reduced into free metal, which is rather volatile and finally High energetic efficiency of the process. The investigations of possibilities to extract metals in filtration combustion regime have been starting in Institute of problems of chemical physics latterly [5-8]. The task may be resolved by two paths To try to extract Zn completely from the source mixture obtaining Fe-containing concentrate with considerably lower Zn content and separately to obtain products enriched with Zn but practically with no iron or Processing filtration combustion not to very end, that is by stopping the combustion at some moment so one obtains the mixture impoverished with Zn in the bottom zone (if the gas-oxidizer moves bottom-up), while atop one obtains the mixture enriched with Zn. This investigation is considering the second path wishing to obtain Zn-containing sources with Zn concentration enough (higher than 15%) for being further processing by traditional methods. In this investigation we have studied samples of metallurgic sludges of different nature, and, consequently, of different element content (Table 1). 81

3 Table 1 Iron and zinc content (mass %) in products under studying # Product Fe Zn 1 Mixture of sludges from Ltd «Severstal» Electrosteelmaking sludges from Ltd «Severstal» 3 Electrosteelmaking sludges from Ltd «Serp & molot» 4 sludges from Ltd «Severstal» Experimental investigations on Zn extraction from metallurgic sludges have been accomplished in model vertical quartz reactors (diameter mm). Temperature in reaction zone was controlled with chromel-alumel or Pt/Pt-Rh thermocouples. Oxidizer (air) moved bottom-up with the velocity between m 3 /(m 2 s) (further we write m/s). All samples of sludges from Table 1 are powders, that is why before filtration combustion processing they were granulated with bentonite clay (the sludges was mixed with the clay in ratio 90:10 and then we made granules of dimension mm). Also some samples have been tableted by pressing, that is with no clay. As a fuel we used activated coal AG-3 (size mm) with ash content 10% (experimental data). Obtained products analysis has been done by energy dispersion X-ray fluorescence spectrometry (device «x-art M»). We have measured the ratio Zn/Fe in samples before burning and after burning at different points of the reactor zone. As Fe-containing products are not volatile in these temperatures (lower than 1400 C), the ratio Zn/Fe lets to resolve if there is a mass transfer of Zn with the burning front moving. Different parameters have been varied, mainly, coal content, air flow velocity, the presence or absence of heat-resisting enclosure. The following results were obtained for different samples: For sludges with 1% Zn the ratio Zn/Fe almost did not change, that it was an exiguous Zn content fall, but this fall was of the order of the experimental error. For sludges with 3.85 % Zn after burning the ratio Zn/Fe in the sample gotten from the caked part fell almost in two times. It means that 82

4 a considerable part of Zn left (by diffusing) the source granules with evaporative Zn-containing products. Thus, at air flow being 0.14 m/s and the front combustion temperature being 830 C the ratio Zn/Fe in caked granules in the burnt area is equal 0.07 while it is equal 0.13 for the initial granules. Sludges with 16.9 % Zn showed that the air flow increase from 0.14 to 0.18 and 0.21 m/s at the same content of the charge increases the maximal temperature in the combustion front (from 1005 to1040, then to 1210 C). At temperatures higher than 1200 C we observed white ZnO needles. Electronic microscope showed (increasing *200) that these crystals have length about 1000 mcm and thickness about mcm. The ratio Zn/Fe in the initial charge (granules 50%+coal 50%) was equal 0.53, in caked granules from the burnt area 0.35, in non-caked granules 0.47, in products taken on the reactor walls The obtained data allow to conclude that the combustion temperature increase was rather high (higher than 1200 C) and therefore the ratio Zn/Fe in the burnt zone falls on about more than 35%. Anyway the level of Zn extraction is rather low. Even if maximal combustion temperature reached 1800 C (at air flow 0.4 m/s) the ratio Zn/Fe in the burnt zone fell from 0.53 to 0.39 that is about 33% of Zn has left the burnt zone. So, controlling the filtration combustion process (e.g. changing air flow) one can increase Zn fraction in the caudal fractions of the charge with simultaneous fall of Zn content in the burnt part. The level of the enriching caudal fractions with Zn may be controlled with the combustion temperature. Moreover one has to set so called reaction leading heat structure regime, when the zone ahead the combustion front is rather heated and where reducing gases (CO, H 2 ) are present in considerable concentration. The study of sludges with 12.05% Zn (Table 1, #4) has been carried more elaborately. The initial sample was a fine powder with particles size a bit lower than 10 mcm. As one can not process the filtration combustion filling the reactor with powder (gaseous oxidizer will not be able to overcome such a resistance) one has to make granules. We made granules of this sample with no clay, only by pressing tablets of size 5.0 mm diameter and height about 5-6 mm. For stopping the filtration process abruptly, but not to let Zn to leave the reaction zone the bottom part of the reactor was filled with the mixture 83

5 of tableted sludges with coal, and the upper part with the same tablets, but with no coal. So, as soon the combustion front achieves the boarder between these two parts the combustion blows out. The upper tablets have to collect Zn which leaves the combustion zone. After burning solid products have been analysed and compared by the Zn/Fe ratio in different zones inside the burnt charge and in the tablets in the upper part. The processing of this kind of sludges have been carried varying the mixture content, as well as varying the air flow rate. At the air flow rate being fixed as 4 litre/min at different coal content (from 10 up to 50%) the combustion wave reconstructs gradually from a so called reaction leading heat structure to a reaction trailing one. This phenomenum is widely known the higher is the released heat the wider is the reducing zone. The maximal temperature is achieved at rather low coal content (10% or so) namely at this ratio so called superadiabatic regime undergoes [2-4]. After the bottom part has been burnt we analysed the upper part of the enriched tablets and residues of the burnt mixture with chemical analysis and X-ray phase one. The results are presented in Table 2. Analysing the obtained data we may notice that the level of the enrichment with Zn increases if the coal fraction increases (Fig.1). Table 2 Data of chemical analysis of the tablets enriched with Zn in result of filtration combustion at different coal content. Air flow is 4 litre/min. In the bottom line the data of metals content in the initial sludges samples are represented Coal content in mixture, % Zn, % (chem. analysis) Zn, % X-ray phase Fe, % (chem. analysis) Fe, % X-ray phase T max, C ,8 15,2-33, ,5 16,9-32, in the initial sludges samples

6 Zinc fraction in enriched wastes,% Chemical analysis X-ray analysis Mass fraction of coal in the charge,% Fig. 1. Dependence of the mass fraction of zinc in enriched wastes on coal content The best results have been shown if coal content was 50%. Hereby the sludges is enriched from initial 12 to 18-19% Zn, that means that 25-30% Zn has been extracted. Then the air flow rate has been varied from 2 to 9 litre/min (Table 3, Fig.2). Table 3 Zn content in upper (caudal) part at filtration combustion of systems with 50% coal at different air flows v, litre/min V lin., m/s Zn, % Fe,% T, o C heat-resisting enclosure presence Zn and Fe content in initial samples

7 In this serie of experiments we used reactors with different sizes (diameter from 20 to 30 mm). In Fig. 2 two values charterizing the air flow are represented linear air flow (vlin (m/s) and the flow rate (v, litre/min). Dependence of Zn content in enriched sludges on vlin and v is represented in Fig.2. Zn, mass.% V, l/min Zn, mass% ,00 0,05 0,10 0,15 0,20 0,25 0,30 V lin, m/c a b Fig. 2. Dependence of the Zn mass fraction in enriched wastes on the air flow (v) (a), and on the linear flow velocity (v lin ) (b) at the same charge (coal:sludges = 1:1) The highest level in Zn enrichment is achieved at middle values of v lin (4-6 litre/min) or v ( m/s). At high flow rates the adventage of higher temperature decrerases a bit - the burning wave velocity is higher, tablets remain in a heated state during less time, it influences negatively the process of ZnO reducing, as well as the diffusion of zinc vapour through tablets. Low air flow rates in case of considerable heat loss level do not allow to reach high temperatures, that does not increase the Zn yield any more. After these experiments the necessity to set a heat-resister enclosure became clear because without such a device a part of coal (near walls) had not burnt and high temperatures were achieved only in the center of the tube reactor. Thus, a heat-resister enclosure was set. It was a quartz tube with wall thickness 4 mm, the space between wall of this tube and the reactor was fulfilled with basalt fibre. The heat insulation of reactor walls allowed obtaining more uniform warm-up of all charge volume in the same cross-section. Anyway the presence of the heat-resisting enclosure did not improved considerably the Zn yield (Table ## 3, 4, 4*; 5, 5*; 7, 7*). 86

8 Thus, during filtration combustion zinc partially leaves the reaction zone and enriches the caudal part. It is necessary to create the combustion regime with so called reaction trailing structure wave. It was possible to get product with 20% Zn basing on an initial source - sludge with 12% Zn. So, the obtained product may be further processed in standard technology for zinc refining because that technologies require Zn content at least 15%. It is possible that the reason of not enough full Zn extraction is the fact that the filtration combustion process though having many advantages over simple furnace process has some drawbacks when being used for metal extraction. One of them is the following: the combustion wave and consequently all high-temperature zones move with relatively high velocity regarding the charge, so at a definite cross-section of the reactor the temperature increases rather quickly, then it falls rather quickly too. So some granules of the charge (we may use only granules with rather considerable size, elsewise the air can not come through the tube with the reaction charge) may be heated not enough (the temperature inside the granule would remain considerably colder than on the surface); second, the reducing gas (CO) will not have enough time to come though the solid phase and to reduce oxides; third, the diffusion of gaseous Zn from solid granules may be cumbered too because of time missing. That is why we hope that considerable increase of reactor size, e.g. up to industrial size, will allow to decrease remarkable this negative side of filtration combustion technology heat losses through reactor walls will decrease essentially, the deepness of high-temperature zone will be intensely expanded, and each granule will remain at higher temperature during considerably longer time. Totally, the investigation under consideration allows to guess that the filtration combustion may become one of industrial methods for some metals extraction from non-traditional sources, particularly for obtaining Zn-containing concentrate for further Zn extraction using known methods (distillation and hydrometallurgy). Acknowledgement The investigation was partially sponsored by the federal planned program Scientific and Scientific-pedagogical staff of innovative Russia for Contract , and by the 87

9 program of the department of chemistry and material science of Rus.Ac.Sci, program 26 Combustion & Explosion. References. 1. Voice, E.W., and Wild, R., Iron Coal Trade Review Importance of Heat Transfer and Combustion in Sintering. V P Aldushin A.P., Rumanov I. E., Matkowsky B. J. // Maximal energy accumulation in a superadiabatic filtration combustion wave //. Combustion and Flame. V P Manelis, G.B.; Polianchik, E.V. & Fursov V.P. (2000). Energetic technology of burning basing on the phenomenon of superadiabatic heating, Chemistry for Sustainable Development., Vol. 8., N 4, pp (Rus), 4. Salganskii, E.A.; Kislov, V.M.; Glazov, S.V.; Zheludev, A. F. & Manelis, G. B. (2008). Filtration Combustion of a Carbon Inert Material System in the Regime with Superadiabatic Heating, Combustion, Explosion, and Shock Waves, Vol. 44, N. 3, pp ,. 5. Rozenberg A.S., Grigoryan L.A., Gudkova I.Yu., Lempert D.B., Manelis G.B.// Mass transfer of Zn-containing products at filtration combustion in contreflow. Mass transfer at high Zn content. Chemical Physics (Rus) v. 28. N 9. P Rozenberg A.S., Grigoryan L.A., Gudkova I.Yu., Lempert D.B., Manelis G.B.// Mass transfer of Zn-containing products at filtration combustion in contreflow. Mass transfer at small Zn content. Chemical Physics (Rus) v. 28. N 10. P Rozenberg A.S., Grigoryan L.A., Gudkova I.Yu., Zemlyakova L.A., Lempert D.B., Medvedev A.S., Manelis G.B.// Molybdenum trioxide extraction from used industrial catalysts at filtration combustion in superadiabatic regime// Chemical Physics (Rus) v. 25.N5.P Rozenberg A.S., Lempert D.B., Medvedev A.S., Grigoryan L.A., Gudkova I.Yu., Zemlyakova L.A., Manelis G.B.// Evaporation of Mo-containing products from molybdenite concentrates at filtration combustion in superadiabatic regime // Chemical Physics (Rus) v. 25. N2. P