RF Thermal Plasma Synthesis of Ferrite Nanopowders from Metallurgical Wastes

RF thermal plasma synthesis of zinc-ferrite nanopowders has been studied from metallurgical wastes of high iron oxide and zinc oxide and/or hydroxide content. XRD measurements of the products revealed that an extensive spinel formation took place on RF plasma treatment. However, instead of expectable ZnFe2O4, formation of zinc ferrous ferrites (Zn0,7Fe2,3O4 and Zn0,4Fe2,6O4) was concluded from the XRD investigations. Addition of some nickel salt to the waste mixture resulted in the formation of Ni-Zn ferrites of different composition. Measurements of magnetic properties revealed that the saturation magnetization of ferrite powders produced by thermal plasma treatment was much higher than that of ferrites synthesized by the conventional ceramic route. Introduction Radiofrequency (RF) thermal plasmas offer unique advantages for the synthesis of special ceramic powders due to their high temperatures and energy densities. In addition, a high temperature gradient exists between the hot plasma flame and the surrounding gas phase. The resulting rapid quenching rate is favourable for producing fine particles with unstable structures (e.g. the inverse spinel structure) in thermodynamic terms. In our previous paper synthesis of zinc ferrites from pure oxides and from co-precipitated hydroxides was described [1]. It was concluded, that nanosized (30-60 nm) spinel phases of various composition can be produced in RF plasma reactor in spite of the very short residence time in the hot zone, although, conversion of precursors was not complete. In this paper, results on the thermal plasma synthesis of zinc ferrites from metallurgical wastes are presented. Experimental Two wastes of different origin and composition, a precipitated, dried sludge from the hot galvanizing process (HPS) and a converter flue dust from steelmaking (CFD) were mixed in a proper amount for setting the optimum Fe/Zn ratio in the final product. The starting mixtures were treated in a RF thermal plasma reactor (3-5 MHz, max. 35 kW plate power, TEKNA PL-35 torch). In each run products were collected from the reactor wall (R) and from the reactor bottom (RB). For bulk chemical compositions the samples were digested in diluted HNO3 solution using a CEM microwave digestion system. Dissolved samples were analysed by ICP-AES technique (Labtest PSX7521). Crystalline phases were identified by X-ray diffraction (XRD) using a Philips Xpert diffractometer and CuKα radiation (λ=0.15418 nm). The lattice parameters (a) of the crystalline phases were determined from the positions of the diffraction peaks. The spinel composition was estimated by assuming the change of lattice parameter due to Zn incorporation to be proportional the Zn concentration. Key Engineering Materials Vols. 264-268 (2004) pp. 2359-2362 online at http://www.scientific.net © 2004 Trans Tech Publications, Switzerland Licensed to Gubicza ([email protected]) Eötvös University Hungary All rights reserved. No part of the contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of the publisher: Trans Tech Publications Ltd, Switzerland, www.ttp.net. (ID: 157.181.173.76-23/09/04,10:39:40) Results and Discussion Chemical compositions of the industrial samples are shown in Table 1. Iron and zinc have the highest concentration in both samples. Chlorine content of HPS (6.99%) is also worth mentioning. In addition to data in Table 1 both CFD and HPS samples contained other components, such as Al, Cr, Ni, Mn, Cu, Cd, P and C in a total amount of less than 1%. Table 1. Bulk chemical composition of CFD and HPS samples Concentration [wt%] a Sample Fe Zn Pb Ca Mg Si Cl CFD 65.0 4.22 0.98 1.96 0.15 1.02 0 HPS 17.5 39.5 0.23 2.76 0.29 2.22 6.99 a concentrations refer to the dried (105°C) samples The main crystalline phase of CFD sample was magnetite (FeFe2O4). Traces of hematite (Fe2O3) were also detected. Main diffraction peaks of the HPS sample could be assigned as simonkolleite (Zn5(OH)8Cl2⋅H2O). Iron compounds was most probably present in the original HPS sample as amorphous hydroxides. During plasma treatment it was expected that these compositions release HCl and H2O vapours. A considerable part of the plasma energy may be lost on the dissociation of particular gases. For this reason, prior to the plasma treatment sample HPS was preheated at 300°C to eliminate H2O vapour. For the synthesis of zinc ferrite of stoichiometric composition (ZnFe2O4) CFD and HPS samples were mixed in an appropriate ratio. From the chemical analysis the Fe/Zn molar ratio of the mixture was 2.18. Conditions of the plasma treatments can be seen in Table 2. In order to improve heat transport between the hot gases and the particles O2 was mixed to the sheath gas. Although during conventional ferrite production the starting oxide mixture is heated for several hours above 1000°C, in the RF plasma a residence time of some milliseconds was high enough to produce fine ferrite powders from the waste mixture. Table 2. Conditions of the ferrite synthesis Flow rate of gases (l/min) Power Feed rate Espec Waste powder Plasma Sheath Auxiliary Carrier kW g/min kWh/g CFD+HPS 11 (Ar) 20 (Ar) 20 (O2) 2 22.3 2.4 0.16 CFD+HPS+Ni salt 10 (Ar) 20 (Ar) 20 (O2) 3 24.5 1.9 0.22 Both powders collected from the reactor wall (R) and from the reactor bottom (RB) showed similar diffraction patterns (Fig. 1.). It indicated presence of spinel as dominant phase. From the lattice parameter (a) for the compositions of sample R and sample RB Zn0.7Fe2.3O4 and Zn0.4Fe2.6O4 were calculated, respectively. Consequently, some of the Zn content of the starting mixture could not build into the spinel structure and left the system in the form of very fine ZnO powder through the exhaust. Chemical analysis of the products supported results of XRD (Fig. 2): Fe/Zn ratio from the ICP and XRD data were very close to each other. NiZn ferrites could also be synthesized in RF plasma reactor from the CFD and HPS powders by admixing Ni3H4CO7·4H2O to the waste mixture. Molar ratio of the metals was set for Ni/Zn/Fe=0.4/0.6/2. For comparison, a NiO/ZnO/Fe2O3 mixture, containing Ni/Zn/Fe with the same ratio, and the waste/Ni-carbonate mixture were treated in a resistively heated furnace at 900°C for 6h as well. Euro Ceramics VIII 2360