Ind. Eng. Chem. Res. 1992,31,82&833
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Kinetic Study of Producing Barium Hexaferrite from 7-Fe203and Ba(OH)2by the Hydrothermal Method Maw-Ling Wang,*tt Zong-Whie Shih,?and Cheng-Hsiung Lint Departments of Chemical Engineering and Materials Science and Engineering, National Tsing Hua University, Hsinchu, Taiwan 30043, ROC
The kinetics of the reaction of y-FezOa(s) and Ba(OH)z in an alkaline solution by the hydrothermal method was investigated in the present study. The reaction was carried out in a batch closed reador which can withstand high temperature and high pressure. The effects of the concentrations of y-FezO3, Ba(OH)z, and NaOH in the feed and temperature on the reaction rate were studied. A kinetic model, based on homogeneous-phase reaction, was built to describe the dissolution of yFe203(s)and the reaction of the dissolved 7-Fe20, (or Fe(OH)4-(aq)) and Ba(OH)2 in sequence to produce the desired magnetic product, Ba0-6Fe203. It is found that the reaction rate is highly dependent on the concentration of Fe(OH),-(aq), Ba(OHI2, and the hydroxide ion. The obtained reaction rate is expressed as functions of Fe(OH)4- and Ba2+ concentrations, Le., R = 0.2[Fe(0H4-)l2[Ba2+]/ (1 + 1.12[Fe( OH),-] 1. Introduction In recent years, techniques for developing new ceramic materials have been studied by many scientists (Ulrich, 1990). A lot of research activities have been focused on the development of new synthetic methods to produce ultrafiie oxide powders with a particle size in the nanometer range. Reducing the particle size and increasing the chemical and morphological homogeneity of the powders have led to some specific problems in synthesis and processing. A review of the literature reveals that ceramic materiale can be produced through the solid state sintering (Bye and Hooward, 1971; Gumaste et al., 1988),chemical coprecipitation (Sapieszko and Matijevic, 1980; Veale, 1972), melting (Hibst, 1982; Vallet et al., 1988), and hydrothermal (Kiyama, 1976; Hadj Farhat and Jonbert, 1986) methods. Among these methods, the hydrothermal method will probably be the most attractive one. The greatest advantage of using hydrothermal synthesis is that it yields an oxide suspension of very fine crystalline metal oxides, which in some cases can even be used for ceramic processes without a calcination step. In addition, one can obtain a perfect crystal structure of the product with defiite chemical composition using the hydrothermal method. The product can be purified easily, simply by only washing the product with water. A variety of different oxides have been synthesized by hydrothermaltechniques (Hattori et al., 1989; Kriechbaum and Kleinschmit, 1989; Kutty et al., 1990, Li et al., 1989). Past efforts (Kiyama, 1976; Lin et al., 1990) have concentrated on searching for the optimum operating conditions for obtaining the ultrafine powders of complex oxides with high purity, narrow particle size distribution, phase homogeneity, controlled particle morphology, and a high degree of crystallinity in the hydrothermal method. However, studies of the reaction mechanism and kinetics of producing the ultrafine powder of complex oxides were rare in the past. In general, the hydrothermal method involves heating the aqueous suspension in a closed vessel above the boiling point at high pressure. Wang, Shih,and Lin (1991) carried out experiments to produce barium hexaferrite by reacting y-FezOS(s) and Ba(OH), in an alkaline solution by the hydrothermal method in order to study the reaction mechanism. They identified that the
* To whom all correspondence should be addressed. t Department
of Chemical Engineering.
* Department of Materials Science and Engineering.
reaction took place in a homogeneous phase, rather than in a solid-liquid two-phase heterogeneous solution. They also found that a pure form of the desired magnetic product, Ba0-6Fez03,can be obtained by adjusting the mole ratio of Ba(OH), and y-FezO,(s) appropriately in the feed. In the design of scaleup of a reactor, an understanding of the reaction mechanism and the formulation of the kinetic model are very important in light of the reaction engineering. The kinetics of the reaction of -y-FezO&) and Ba(OH), in an alkaline solution by the hydrothermal method was investigated in the present study. Those factors, such as the concentrations of reactanta and sodium hydroxide and temperature, which affect the reaction rate, were studied. On the basis of the experimental data, a kinetic model concerning the dissolution of y-FezO&) and the reaction of the dissolved yFezO3 and Ba(OHIzin the homogeneous phase was developed. The reaction rate constant was obtained. Experimental Section Materials. All chemicals used in this study were G. R. grade except for yFezO3, a product of Bayer Co., which was commercial grade (Bayferrox 420). Procedures. The batch reactor was a high-pressure autoclave, made of seamless stainless steel pipe which could withstand a vessel pressure of 2000 psi. One end of the stainless steel pipe was closed. The other open end of the stainless steel pipe was fabricated with a flange which was used to connect the top plate cover using eight screws. There were four ports, which served the purposes of inserting the thermocouples, sensing pressure, taking samples and connecting the cooling unit on the top plate cover. The stirrer within the reactor for agitating the slurry mixture was a sealless device driven by a magnetic inductor. Two seta of propeller blades were attached to the shaft of the stirrer for agitating the slurry mixture. The reactor was heated by a thermal mantle by electric power, and the temperature of the slurry mixture was controlled to within f0.5 O C by a proportional integral derivative (PID)controller. To start an experimental run,a measured quantity of y-Fez03(s),Ba(OH),, NaOH, and water was introduced into the reactor and the slurry mixture was heated to a desired temperature. At a chosen time, the reaction was stopped by turning off the power supply of the heater and the agitator, cooling the solution quickly, and releasing the pressure in the vessel slowly. The BUS-
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pension was withdrawn from the reactor, neutralized by a diluted HCl solution, and then filtered and washed thoroughly with water. The product was examined using a Rigaku X-ray diffractometer with Cu Ka. The constituents of the product were determined by comparing the relative intensity of diffraction peaks with those of pure phases. The composition of the mixture was determined by direct comparison of the integrated intensity of the correspondence peaks (Cullity, 1981) with a correlation factor k,, described by Wang, Shih, and Lin (1991).
Kinetic Model On the basis of experimental observation, the reaction of y-Fe203and barium hydroxide in an alkaline solution (sodium hydroxide) at high temperature and high pressure can be identified as taking place in the homogeneous phase. For the main reaction, y-Fe203(s)(or HFe508(s)) was first dissolved in sodium hydroxide solution to form Fe(OH),-(aq). Then, Fe(OH),-(aq) reacted with Ba(OH)2 in the liquid phase to produce the desired product, Ba0.6Fe2O3.A precipitation step was required to get a crystal form of Ba0*6Fe203. In addition to the main reaction, y-Fe203(s)(or HFe508(s))also converted to a-Fe203(s)at the early stage of reaction. When the concentration of Ba2+ was low, where the condition is unfavorable for the main reaction, Fe(OH),-(aq) also converted to a-FepO3. Therefore, the reaction path based on the mechanism can be described as follows: (a) dissolution of y-Fe203(s)in the alkaline solution or in pure water 2HFe508(s)+ 100H- + 14H20& lOFe(OH),-(aq) (1) 2HFe508(s)+ 14H20+ lOFe(OH),(aq) (2) where K is the equilibrium solubility constant; (b) main reaction for producing Ba0.6Fe203 12Fe(OH),-(aq) + Ba2+ Ba0.6Fe203(s)+ 100H- + 19H20 (3) and (c) side reaction for producing a-Fe203(s) 2Fe(OH)3(aq) a-Fe203(s)+ 3H2O (4)
-
-
2Fe(OH),a-Fe203(s)+ 20H- + 3H20 (5) As indicated, the production of a-Fe203(s)from y-Fe203(s) shown by eq 4 only takes place at the early stage of the reaction. Nevertheless, the reaction shown in eq 5 takes place after the termination of the main reaction shown in eq 3 or at the conditions unfavorable for the main reaction. Equation 3 involves the reaction of Fe(OH),-(aq) and Ba2+to produce BaO*6Fe203(aq)in the aqueous phase and the precipitation of BaO*6Fe203(s) as crystals. From the reaction rate obtained from experiments, it is known that eq 3 is not the fundamental equation. Therefore, it is reasonable to assume that eq 3 involves a series of reactions, i.e., xFe(OH),-(aq) [Fe(OH),],Y-
kl
[Fe(OH),]."-(aq);
0
