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ADVANTAGES OF THE FLUIDIZED BED
TECHNIQUE FOR THE PREPARATION OF HARD
FERRITE POWDERS
L. Giarda, A. Cattalani, A. Franzosi
To cite this version:
JOURNAL D E PHYSIQUE Colloque C l , suppl&rnent au no 4, Tome 38, Avril 1977, page Cl-325
ADVANTAGES
OF THE FLUIDIZED BED TECHNIQUE
FOR THE PREPARATION OF HARD FERRITE POWDERS
L. GIARDA, A. CATTALANI and A. FRANZOSI
Montedison S. p. A., Istituto Ricerche cc G. Donegani )) Novara, Italy
Rbumh. - On dkrit la preparation de poudres de ferrite dur par reaction a I'etat solide dans un lit fluidise. On rapporte les propriktes magnktiques et ceramiques de ces poudres, qui ont et6 preparks L partir d'oxydes de fer pigmentaires, soit regenerts soit naturels. Les resultats demontrent que la technique du lit fluidise peut etre appliquke
L
la preparation d'aimants frittes et plastiques de hautes performances.Abstract. - Preparation of hexaferrite powders by means of a solid state reaction in a fluid bed furnace is described. Magnetic and ceramic properties of these compounds, which were prepared starting from pigmentary, regenerated and natural iron oxides are reported. Results show that the fluidized bed technique is suitable for high performance plastically bonded and ceramic magnets.
1. Introduction.
-
In the production process of both ceramic and plastically bonded permanent magnets, the preparation of hexaferrite powders (firing) is of essential importance for the performance of the final products [l]. Their magnetic propertiesare infact determined by the chemical and geometrical Pythagoras Reactor
properties of the powders as well as by the chemical composition, purity, particle size, size distribution, etc.
In recent years besides conventional methods, a ubular Muffle
number of wet and dry methods have been proposed for the production of hexaferrite powders [2, 3, 4, 51.
Some of these methods are well suited to prepare fine Electric Resistance
and homogeneous powders at a laboratory scale, but not for an industrial production.
Of these methods which could be used as alterna-
tive to the rotary kiln furnace, the fluidized bed reactor S Grid Distributor
represents a promising substitute, as it allows the continuous production of fine and homogeneous powder at temperatures below those used in conven- tional furnaces [ 6 ] .
We report here some results of experiments carried Air
out with the fluidized bed reactor a t a laboratory scale for the preparation of hexaferrites starting from
different iron oxides, which show the advantages of N2
this technique for the preparation of higher perfor- FIG. 1. - Laboratory fluid bed reactor. mance hexaferrite powders.
2. Experimental procedures.
-
The discontinuous ~~~~~i~~~~~ were carried out with the fo~lowing laboratory fluid bed reactor we used is schematically raw materials :shown in figure 1.
It consists of a vertical Pythagoras tube = 38 mm, - Pigmentary iron oxides derived from flanged at the two extremities and heated by an electri- FeSO,. 7H,O
cal oven. The gas distributor is a Pt grid. aFeOOH (99.5
X).
S = 0.38%,
needleC1 -326 L. GIARDA, A. CATTALANI A N D A. FRANZOSI shaped, SBET = 13.1 mZlg (S,,, = specific sur-
face area) ;
aFe,O, (99.5
X),
obtained by the calcina- tion of the previous goethite at ca 800 OC,S = 0.3
%,
S,,, = 3.5 m2/g ;Fe304 (99.2
X)),
S = 0.40X ,
SBET = 3.5 m2/g. - Regenerated Ruthner iron oxideaFe203 (98.0
X),
ca 1.0 Fe304, ca 1.0 NaCI, S,,, = 3.0 m2/g.-- Mineral natural iron oxide
aFe,03 (ca 98.0
X),
ca 1.5 SiO,. This oxide, before being reacted with SrCO, was wet-milled for 3 hours in a vibratory mill,SBET = 4.6 m2/g.
Each of these oxides were wet mixed with technical grade SrCO, (S,,, = 6 . 5 m2/g) in a Fe203/ SrCO, = 5.7 molar ratio. The mixture was then dried and micropellettizzed (granulate size range between 0.15-0.49 mm). Solid state reactions were all performed between 1 000 and 1 200 O C with a soaking time of 2 hours a t the maximum temperature in air flow.
3. Experimental results. - The most relevant magnetic and ceramic parameters of the strontium hexaferrite powders are reported in figures 2 and 3. Magnetic measurements of the saturation magnetic moment per mass unit, a,, and the intrinsic coercive force, H,,, were performed using a vibrating sample
magnetometer (P. A. R.) in a magnetic field of 18 kOe,
55 ' I d
l000 l050 1KX) l150 1200
Temperature (OC) .---a From ct FeOOH Plgmentary lron Oxide
0-- 0 From oc Fe2 O3 Pigmentary iron Oxide
&---A From Fe3 0, Pigmentary lron Oxide
A---A From cx Fe2 O3 Regenerated lron Ox~de X---4 From ot Fe2 O3 Natural Iron Oxide FIG. 2.
-
Magnetic parameters of Sr0.5.7 Fe203 powdersstarting from various oxides.
on pressed powder disks of 5 mm diameter. Surface area measurements, S,,,, were carried out with a Per- kin Elmer-Shell sorptometer, and the apparent densi- ties, d,, were taken with a mercury pycnometer, in vacuum, on loose powders after firing. From figure 2 which reports the magnetic values vs temperature, we observe that all samples are fully reacted at l 050 O C because the U, values correspond to the theoretical
value. Independently of a, measurements we carried out X-ray powder diffraction which confirmed the magnetic measurements results. Hexaferrites from uFe,03 are nearly reacted even at 1 000 OC.
When starting with very fine and acicular a-FeOOH [7], it has been found that reaction with SrCO, takes place at a higher temperature (750 vs 600-650 OC) which depends apparently on the change of shape [8] of the reacting iron oxide powder. Conse- quently, the reaction products exhibited very high
HCi values, compared to those obtained by reacting
pigmentary uFe20,. Between 1 100 and 1 150 OC we observed a considerable reduction of HCi values,
especially with pigmentary uFe,03 and aFeOOH, due to the particle size growth.
Figure 3 shows the increase of the densities and the decrease of the specific surface area measurements as a
O ,
1000 1050 1100 1150 1200
Temperature (OC)
e-• From W FeOOH Pigmentary lron Oxide
0-0 From cx Fe2 O3 Plgmentary lron Oxide
A--& From Fe3 O4 Pigmentary lron Oxide
A-.-A From cx Fe2 O3 Regenerated lron Oxide X----+ From Fe2 O3 Natural lron Oxide FIG. 3. - Ceramic parameters of Sr0.5.7 Fe203 powders
starting from various oxides.
THE FLUIDIZED BED TECHNIQUE FOR THE PREPARATION O F HARD FERRITE POWDERS Cl-327 the hexaferrite powders from aFe,O, are reported in
figure 4 and show the shape, size and agglomeration of the particles as a function of the firing temperature. Up to l l00 OC we observe small single magnetic
FIG. 4. - T. E. M. micrographs vs. firing temperature start-
ing from aFezO3.
monodomain particles, having an average size of the order of magnitude of the magnetic monodomain. At 1 150 OC and higher temperatures the particles are agglomerated and grow to sizes bigger than the monodomain particle size. Starting with the other oxides we have obtained almost the same results. Hexaferrites obtained at l lOOOC are well shaped, with an average particle size of d , , = 0.4 pm which strongly adhere to each other due to magnet forces. These powders are not suitable to be magnetically oriented during the pressing process and, moreover, the pressed samples have rather low density (< 50
X ) .
Figure 5 shows S. E. M. micrographs of samples obtained from pigmentary uFeOOH, fired at 1 150 OC and 1 200 OC, respectively. It is worth noting here the homogeneous and regular hexagonal platelet-like shape of the particles. Due to these properties, samples fired at 1 150 OC were shown to be suited for oriented plastically bonded and anisotropic ceramic magnets. Crystal size is well known [9, 101 to be dependent not only on the firing temperature but also on the Fe,O,/ SrCO, ratio. At 1 125 OC, with pigmentary ctFe,O,, we have infact found that by changing the molar ratio from 5.9 to 5.5 the particle size increases remar-FIG. 5. - S. E. M. micrographs vs. firing temperature statirng from pigmentary aFeOOH.
kably, while from 5.5 to 5.1 it remains almost the same.
To obtain powders which are fully reacted, the reactor soaking time can be shortened when the firing temperature rises. A soaking time of 30 minutes at 1 125 OC is enough to have strontium hexaferrite powders fully reacted.
Cl-328 L. GLARDA, A. CATTALANI A N D A. FRANZOSI Experiments, performed by heating the reagent
mixture (SrC03
+
5.7 Fe203) at 5, 10, 15 O C per minute give nearly the same crystal sizes. The mixing procedure of the reagents was shown not to influence too much the oxides reactivity [12]. Reagent powders, wet and dry mixed, have reacted at the same rate at 1 125 OC.On pigmentary a F e 2 0 3 the hexaferrite powder preparation has been carried out also at a pilot plant size, with a continuous fluidized bed reactor having a n internal diameter of 156 mm. Results obtained with this reactor have confirmed laboratory results as far as size and magnetic properties are concerned. With these industrial reactors, to reach the established tem- peratures, a subsidiary gas burner is needed. In this case some precautions must be taken to have an uni- form heat distribution in order to avoid local inter- particle sintering which may cause the sticking of the bed.
I) Magnetic powders can thoroughly reacted a t temperatures lower than those necessary with conven- tional methods. As a result we have submicronic particles (i. e. single magnetic domain), weakly adher- ing to each other. Size particle can be controlled by varying the temperature. Lower firing temperatures avoid the volatilization of earth alkali oxide [l 31.
11) The good thermal control of the continuous stirred reagent particles, together with the small micropellet sizes, give homogeneous powders, fully reacted even with slightly reactive reagents. The narrow particle size distribution gives high H,; values
with better properties of the final products [14].
111) Magnetic powders obtained in a fluid bed need only a dispersion for plastically bounded magnets and a mild milling for anisotropic ceramic magnet. The short milling time reduces the contamination, as a result we have a more accurate control of the Fe/Sr(Ba) ratio.
As a conclusion, we believe that the fluidized bed 4. Conclusion. - The main advantages of the technique is very promising and particularly suitable
fluidized bed technique are : for high performance materials.
References
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Material, Amsterdam (1 974) 53. (1971) 324.
[2] DE LAU, J. G. M., Am. Ceram. Soc. BUN. 49 (1970) 572. [g] W ~ L K O P P , M., znt. J. Magnetism 3 (1972) 179. [3] HANEDA, K., MIYAKAWA Ch., KOJIMA, N., Bull. Res. Inst.
Scien. Measur. 22 (1973) 67. [l01 WULLKOPF, M., Znt. J. Magnetism 5 (1973) 147.
[4] SHIRK, B. T., BUESSEM, W. R., J. Amer. Ceram. Soc. 53 [l11 BYE, G . C., HOWARD, C. R., J. Appl. Chem. Biotechnol. 21
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