2888
Ind. Eng. Chem. Res. 1993,32, 2888-2891
Preparation and Characterization of Fine Magnetite Particles from Iron(111) Carboxylate Dissolved in Organic Solvent Yasuhiro Konishi,' Takeshi Kawamura, and Satoru Asai Department of Chemical Engineering, University of Osaka Prefecture, 1-1, Gakuen-cho, Sakai, Osaka 593, Japan
The preparation of fine, crystalline magnetite from nonaqueous iron(II1) carboxylate solution at elevated temperatures was investigated. On treatment with water, the iron carboxylate solution precipitated pure hematite (a-Fe203)at 200 "C and a mixture of hematite and magnetite (FesOr) a t 245 "C. In the absence of water, a heat treatment of the organic solution at 245 "Cand 0.45 MPa for 60 min resulted in the formation of magnetite alone. The resulting magnetite was found to be crystalline particles with size ca. 0.1 pm and be free from contamination by the organic starting material.
Introduction Magnetite (Fe304) is an important magnetic material with spinel structure. There are several wet methods for the preparation of magnetite. Various studies have indicated that fine magnetite particles can be prepared by the following methods: oxidation of aqueous Fe(OH)2 suspensions with air at 50-80 "C (Kiyama, 1974), aging aqueous Fe(OH)2 gels at 90 "C for several hours in the presence of nitrate ion (Sugimoto and Matijevic, 19801, heating aqueous mixed solution of FeCl2 and FeCl3 a t 100 "C for 3-7 h in the presence of urea (Matauda et al., 19871, and oxidation of aqueous FeCl2 solutions at 100 "C with alkaline KN03 in the presence of phosphite (Couling and Mann, 1985). These conventional methods gave fine particles of magnetite with mean diameters ranging between 0.02 and 2.2 pm. However, these oxide powders were produced in aqueous media containing foreign anions, which tend to be a major source of contamination of the oxide products. This paper considers the preparation of fine magnetite particles from nonaqueous iron(II1) carboxylate solution at elevated temperatures. The chemical synthesis of solid particles in organic phase has an advantage that the resultant precipitates are not affected by foreign anion contamination, because the organic solution having a low dielectric constant is free from ionic species. Another advantage of this synthetic route includes the use of metal carboxylate as a starting material, which can be readily prepared by solvent extraction of metal ions from aqueous solutions with carboxylic acids. Above all, tertiary carboxylic acids, which are widely used as a solvent extractant, are suitable for preparing the starting material because of their low cost and thermal stability. Precipitation of oxides and hydroxides from metal carboxylate solutions has been investigated, as discussed in a review (Doyle, 1992). On a heat treatment with pure water in sealed tubes a t 200 "C for 3 h, nonaqueous carboxylate solutions of Fe(III), Cu(II),Mn(IV), and Ni(II1) are hydrolyzed and precipitate crystalline powders of a-Fe203, CuO + Cu20, y-Mn203, and Ni(0H)Z; moreover, binary mixed-metal carboxylate solutions precipitate magnetic spinel ferrites MFe204 (M: Ni, Co, Zn, Mn) (Doyle and Monhemius, 1985). Although the ability to precipitate various oxides including complex metal oxides directly from the inexpensive organic solu-
* To whom all correspondence should be addressed.
tions is very attractive, the resulting precipitates have been poorly characterized. Moreover, no reports have appeared concerning the preparation of magnetite from iron carboxylate solution. The purpose of this paper is to find a suitable condition for the preparation of fine, crystalline magnetite particles from nonaqueous iron(II1) carboxylate solution at a temperature up to ca. 250 "C and to characterize the resulting magnetite in details.
Experimental Section Materials. The carboxylic acid used in this work was Versatic 10, a synthetic tertiary monocarboxylic acid produced by Shell Chemical Co. The materials contains 98.5% CIOacid, and the acid value was 318. The Versatic 10 was diluted to the desired concentration level using Exxsol D80, aliphatic hydrocarbons produced by Exxon Chemical Co. These organic materials were used without further purification. Carboxylate solution of Fe(II1) was prepared by exchange extraction with Ca(I1) in the Versatic solution. Calcium loading was first carried out by dissolving CaO powder in the Versatic acid solution, and then the calciumloaded organic solution was shaken with aqueous FeC13 solution for several minutes in a separating funnel. After the exchange extraction, the iron-loaded organic solution was washed with dilute HC1 to remove residual calcium and washed with deionized water to remove residual anions. The organic solutions were then filtered through glass fiber paper and passed through phase separating paper to remove physicallyentrained water. To determine the organic-phase iron concentration, aliquota of the organic solution were mixed with 6 kmol m-3 HC1 to strip the iron, and the strip solutions were analyzed by ion chromatography. The initial concentration of Fe(II1) in the organic solution was 0.30 kmol/m3, and the initial concentration of free Versatic 10 was 0.86 kmol/m3. Apparatus and Procedure. A stainlesssteel autoclave lined with glass was used to synthesize iron oxides. The autoclave was 6.0-cm i.d. and 16-cm high, and the liquid stirrer was a turbine agitator of 4.5-cm diameter. The stirrer was placed 3.0 cm above the bottom of the autoclave and was driven at 500 rpm. The experimental temperature varied from 140 "C to 245 "C. The duration of synthesis time was 60 min. A 200-cm3 volume of the Fe(II1)-loaded carboxylate solution was charged into the autoclave with or without
Q888-5885/93/2632-2888~Q4.oo/o 0 1993 American Chemical Society
Ind. Eng. Chem. Rea., Vol. 32, No. 11,1993 2889 pure water. The stirring of the solution was started at room temperature, and nitrogen gas was bubbled into the solution for 30 min. After that, the solution was heated and maintained at the experimental temperature for 60 min. The time taken to raise the experimental temperature was from 5 to 20 min. The total pressure changed markedly with the temperature and the amount of water added. When 50 em3of water was added to the autoclave, the total pressure at the operating temperature was approximately equal to the saturated steam pressure because of lower volatility of the organic solution, Le., 0.4 MPa a t 140 O C and 1.6 MPa at 200 "C. When no water was present, or even when 2 cm3 of water was added, the total pressure in the autoclave was equal to the saturated vapor pressure of the organic solution, Le., 0.45 MPa a t 245 "C. After the continued heating for a period of 60 min, the autoclave was cooled to room temperature. Precipitated material in the autoclave was filtered,washed with deionized water and acetone, dried at room temperature, and stored in a desiccator. The precipitates were characterized by X-ray diffraction (XRD) analysis, infrared (IR) absorption spectrophotometry, and therm* gravimetry and differential thermal analysis (TG-DTA). The particle size and morphology were observed by using transmission electron microscopy (TEM), and the magnetic properties were evaluated with a vibrating sample magnetometer (VSM).
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Results and Discussion Conditions for Formation of Magnetite. A suitable condition for the preparationofmagnetitealonewas found by controlling such factors as temperature, pressure, and the amount of water. The iron carboxylate solution gave precipitation except at temperatures up to 200 OC in the absence of water. Figure 1 shows the XRD patterns of powders precipitated at different temperatures and pressures. It is evident that only hematite (a-Fe,Oa) is precipitated when the carboxylate solution is mixed with 50cm3ofwaterat 140°C(0.4MPa)and2000C(1.6MPa). This is consistent with the result previously reported by DoyleGarner and Monhemius (1985). When the iron carboxylate solution is contacted with 2 cm3 of water a t 245 O C (0.45 MPa), the precipitate has sharp X-ray diffractionpeaksofmagnetitealongwiththoseofhematite. The presence of magnetite was also evidenced by the color change of the priciptate powders: a dark reddish precip itate was obtained at 200 O C (1.6MPa), whereas the oxide prepared a t 245 O C (0.45 MPa) consisted of blackish powders. Thus, althoughan increaseintemperaturerather than total pressure is favorable for the preparation of magnetite, a side reaction occurs with the formation of hematite because of the presence of water. In order to suppress the formation of unwanted hematite, the iron carboxylate solution was heated and maintained a t 245 OC in the absence of water. Comparing Figure l a with lb, it is demonstrated that the absence or presence of water affects the nature of the resulting oxides. Evidently, the ironcarboxylate solutiondidnot precipitate hematite when nowaterispresent,resultinginthe formationofmagnetite alone. Thusitcan beconcluded thatthemagnetite powder of high crystallinity is synthesized when the iron(II1) carboxylate solution is heated a t 245 "C for 60 min in the absence of water. Under this condition the total yield of magnetite powder on the iron carboxylate in the organic solution was 51%. CharacterizationoftheMagnetiteParticles. Figure 2 shows TEM of the magnetite particles prepared at 245 O C in the absence of water. The unaggregated magnetite
2 8 (degrees) Figtm 1. X-raydiffiacfionpattemsofprscipitatss~mnoaaqu~ earborylste solution of Fe(l11) at different conditione: (a) 245 O C and 0.45 Mps in the abnenee of water: (b) 245 'C and 0.45 MPa in the pnasncs of water; (e) 200 O C and 1.6 MPa in the presence of water: (d) 140 OC and 0.4 MPa in the presence of water.
Figure 2. TEM of magnetite particles obtained at 245 O C and 0.46 Mpa for 60 min in the a h n e e of water.
particles are of hexagonal morphology with a well-defined habit, and the particle size is 0.10-0.15 wm. Figure3givestheIRspeetrumofthemagnetiteparticleg. There is no distinguishing absorption band a t 3000 em-' and 1700-1400 cm-', which is assigned to the Versatic 10 diluted in aliphatic hydrocarbons (Stefanakis and Monhemius, 1985). This demonstrates that the magnetite mrticles are free from contamination by the organic starting material.
TheTG-DTAdataforthemaenetiteareshowninFimve 4. The magnetite exhibits an exothermic peak centired
2890 Ind. Eng. Chem. Res., Vol. 32, No. 11, 1993
resented as:
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Fe,(OH),R4 + H,O = Fee03+ 4HR W
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W a v e n umb e r s [cm-') Figure 3. IR absorption spectrum of magnetite prepared at 245 OC and 0.45 MPa in the absence of water.
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Figure 4. TG-DTA curves of magnetite prepared at 245 OC and 0.45 MPa in the absence of water.
at 250 "C on the DTA curve and a gain in weight on the TG curve. The appearance of the gain in weight between 200 and 300 "C is assigned to the oxidation of magnetite to hematite, and the observed gain in weight follows an exact stoichiomtric relationship to the oxidation reaction of magnetite: 4Fe304 + 0 2 6Fe203. It should be noted that a weight loss at around 300 O C on TG curve, which is a result of combustion of hydrocarbons, is not detected. In the TG-DTA data, there is not evidence that the magnetite particles are contaminated by the organic starting material, demonstrating the formation of monolithic magnetite particles. This result of the thermal analysis is consistent with that of the IR analysis. A VSM was used to evaluate the magnetic properties of the magnetite particles at room temperature. The saturation magnetization and the coercive force were determined as 77.4 e m d g and 160Oe, respectively. These results demonstrate that magnetite particles with spinel structure were synthesized from the iron carboxylate solution after heating at 245 "C for 60 min. Reaction Mechanism. It is evident from the experimental results that the reaction temperature and the presence or absence of water affect the nature of the iron oxides precipitated from the iron(II1) carboxylate solution: pure hematite was formed at 140-200 "C in the presence of water, whereas pure magnetite was prepared at 245 "C in the absence of water. The nature of the precipitated oxides is presumably closely related to the structure of iron carboxylate complex within the organic solution. The predominant species in freshly prepared Fe(II1)-loaded carboxylate solutions is thought to be the dihydroxo-bridgeddimeric complex,Fe2(0H)zR4,and then slowly transform to the hydroxo-centered trinuclear complex, Fe3(0H)4RvHR (Doyle, 1988),where HR represents the carboxylic acid. In view of the structure of the iron(II1) carboxylate complexes, hematite appears to be formed by hydrolyzing the dimeric complex with water. The overall formation reaction of hematite can be rep-
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(1)
where overbars denote species present in the organicphase. Thus, the addition of water to the organic carboxylate solution yields hematite a t 140-200 "C. The heat treatment of the iron carboxylate solution without water a t 245 "C leads to the synthesis of magnetite alone. The most likely reason for the formation of magnetite is that at elevated temperatures around 245 "C, the structure of iron carboxylate is changed from the dimeric complex into the trimeric complex. When the iron carboxylate trimers serve as a precursor of magnetite, water is unlikely to be required for the formation of magnetite as judged by the structure of the partially hydrolyzed iron species. In forming magnetite with spinel structure (Fe(I1)-Fe(111)oxide) from the iron(II1) carboxylate solution, it is necessary to reduce some of the Fe(II1) within the organic solution into Fe(I1). Similar behavior was observed when Cu(I1)-loaded carboxylate solution was hydrolyzed with water at 200 "C and then precipitated CuO CueO. It has been reported that during the reduction of Cu(I1) to Cu(I), the carboxylic acid serves as a reducing agent and is partially decomposed into carbon dioxide (DoyIe-Garner and Monhemius, 1985). From the previous result it is inferred that the formation of magnetite takes place with reduction of Fe(II1) and simultaneous oxidation of the carboxylic acid. Such the consumption of the carboxylic acid must be considered as an inevitable consequence of the production of the valuable magnetic material.
+
Conclusions The nature of the iron oxides precipitated from the Fe(111)-loadedVersatic solution was found to be affected by the presence or absence of water in the autoclave. In the presence of water, the heat treatment a t 245 "C (0.45 m a ) resulted in a mixture of hematite and magnetite, indicating that a side rection takes place with a hydrolysis of the iron carboxylate solution and subsequent precipitation of hematite. In the absence of water, the heat treatment of the iron carboxylate solution at 245 "C (0.45 MPa) led to the formation of magnetite alone. The resultingmagnetite was crystalline particles with size ca. 0.1 pm and was free from contamination by the organic starting material. Acknowledgment We wish to thank Mr. Hiroshi Doi, Japan New Metals Co., L a . , Osaka, Japan, Mr. Takashi Goto and Mr. Michiya Ohashi, Santoku Metal Industry Co., LM., Kobe, Japan, and Mr. Sadao Murasawa and Mr. Shinsuke Takumi, Ishihara Sangyo Kaisya, Ltd., Kusatau, Japan, for their assistance in characterizing the precipitated powders. Literature Cited Couling, S. B.; Mann, S. The Influence of Inorganic Phosphate on the Crystallization of Magnetite (FerO,) from Aqueous Solution. J. Chem. SOC., Chem. Commun. 1985, 1713-1715. Doyle, F. M. The Physical Chemistry of the PrecipitationStripping Process for Removing Iron(II1) from Carboxylate Solutions with Dilute Sulphuric Acid. Hydrometallurgy 1988,20,65-85. Doyle, F. M. Integrating Solvent Extraction with the Proceeeing of Advanced Ceramic Materials. Hydrometallurgy 1992,29, 527545. Doyle-Garner,F. M.; Monhemius,A. J. HydrolyticStripping of Single and Mixed Metal-VersaticSolutions. Metall. Tram.B 1985,16B, 671-677.
Ind. Eng. Chem. Res., Vol. 32, No. 11,1993 2891 Kiyama, M. Conditions for the Formation of FesO4 by the Air Oxidation of Fe(OH)2 Suspensions. Bull. Chem. SOC. Jpn. 1974, 47,1646-1650. Matauda, K.; Sumida, M.; Fujita, K.; Mitauzawa, S. The Control of the Particles of Magnetite. Bull. Chem. Soc. Jpn. 1987,60,44414442. Stefanakis, M.I.; Monhemius, A. J. Determination of Organic Phase Complexes Formed on Extraction of Iron(II1) from Aluminium Nitrate Solutions with Versatic 10. Hydrometallurgy 1985,15, 113-139.
Sugimoto, T.; Matijevic, E. Formation of Uniform Spherical Magnetite Particles by Crystallization from Ferrous Hyroxide Gels.J. Colloid Interface Sei. 1980, 74,227-243. Received for review April 23, 1993 Accepted August 9,1993.
* Abstract published in Advance ACS Abstracts, October 1, 1993.
