Kinetics of zinc ferrite formation in the rate deceleration period
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I.
INTRODUCTION
ZINC ferrite formation is of interest to both the electronics and the metallurgical industries. In the electronics industry, it has been found that both the magnetic and the electrical properties of ferrites can be substantially improved if some zinc oxide is present.[1,2,3] For example, the addition of zinc ferrite (ZnFe2O4) to magnetite (Fe3O4), manganese ferrite (MnFe2O4), and nickel ferrite (NiFe2O4) results in the formation of ferrites which have substantially higher magnetic moments than the pure ferrites.[1–4] It is noteworthy that in its pure form, zinc ferrite has a magnetic moment of zero.[1,2,3] In the metallurgical industry, zinc ferrite is found in the calcine from the roasting of zinc ores and also in the dust which is generated when galvanized scrap is melted in the electric arc furnace (EAF). The zinc, in the zinc ferrite, is difficult to recover, since it is insoluble in both dilute sulfuric acid and caustic solutions. Ferrite formation is a relatively simple solid-state reaction and is often selected as a model system for homogeneous rate processes.[5] Some previous research has been performed on the formation of zinc ferrite. As early as 1931, Guillissen and Van Rysselberghe investigated the effects of temperature and time on zinc ferrite formation.[6] A fine mixture of zinc oxide and iron oxide was heated at successively higher temperatures for various time increments. Then, the unreacted zinc oxide in the mixture was extracted with an ammoniacal solution. They found that above 873 K, the reaction proceeded rapidly. It began at 853 K and was completed at 1343 K. Their results also showed that for the first 3 hours the reaction followed a linear relationship in accordance with Tammann’s law:[7] C 5 A log t 1 B
[1]
where C is the fraction of ferrite formed at time t for a
DAN K. XIA, Doctoral Candidate, and CHRISTOPHER A. PICKLES, Doctor and Professor, are with the Materials and Metallurgical Engineering Department, Queen’s University, Kingston, ON, Canada K7L 3N6. Manuscript submitted July 8, 1996. METALLURGICAL AND MATERIALS TRANSACTIONS B
given temperature and A and B are constants. A similar study by Kedesdy and Katz employed X-ray diffraction (XRD) to measure the extent of reaction between the constituent oxides.[8] The mixed oxides were pressed at about 1758 kg cm22 and heated for 5 hours at various temperatures in air. Then, the compacts were quenched, ground, and examined by XRD. Their results showed that only a small amount of reaction took place below 873 K and the reaction was complete at about 1373 K. Heating to higher temperatures merely increased the crystallinity of the reaction product. In their study of the reaction, Furuichi et al.[9] modified Jander’s equation [1 2 (1 2 X)1/3]2 5 kt
[2]
by incorporating the condition that the initial surface reaction was much faster than the later diffusion-controlled process. Their modified Jander’s equation is as follows: {1 2 [1 2 (X 2 Xs/1 2 Xs)]1/3}2 5 kt/(1 2 Xs)2/3
[3]
where X is the total fraction reacted, Xs is t
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