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[136] have studied the kinetics of the crystallization of barium hexaferrite in detail.
The reaction rate of producing barium hexaferrite was highly dependent on the con-
centrations of Fe
Þ 4 (aq.), Ba(OH) 2 , and NaOH in the aqueous solution. These
authors have found that the reaction should be carried out at a temperature greater
than 280 C to obtain the main desired product, BaO
ð
OH
6Fe 2 O 3 . A kinetic model,
based on the homogenous phase reaction has been built to describe the dissolution
of
Þ 4
γ
-FeOOH (s) and the precipitation of the dissolved
γ
-FeOOH (s) {or Fe
ð
OH
(aq.)} and Ba(OH) 2 in sequence to produce the desired product, BaO
6Fe 2 O 3 .The
reaction rate at 280 C is expressed as a function of Fe
Þ 4
(aq.) and Ba 2 1
ð
OH
concentration, i.e.,
Þ 4 aq
2
Ba 2 1
0
:
19
½
Fe
ð
OH
½
Rp
ð
9
:
12
Þ
5
Þ 4 ð
1
8
:
6
½
Fe
ð
OH
aq
Þ
1
Figure 9.30 shows the effect of an alkali concentration on the extent of conver-
sion of the BaO
6Fe 2 O 3 : The reaction rate increases with the increase of the con-
centration of NaOH solution. Higher concentration of NaOH also results in a
higher yield of the production. The maximum yield of BaO
6Fe 2 O 3 achieved is
81% for [OH] 2 5
2 N. In case of the nitrate source for the nutrient, the particle
size decreased with an increase in alkali molar ratio defined as
OH 2
NO 3 :
½
Particles of about 0.1
m in size were produced at molar ratios above six. As the
concentration of NaOH increases, the mean size of the particles decreases. These
experimental evidences may support the concept that during hydrothermal treat-
ment at a constant temperature (e.g., 300 C), an increase in the crystallinity of pre-
cipitated particles mainly proceeds instead of particle growth driven by Ostwald
ripening.
The synthesis of manganese ferrites is carried out in the same way as that of
barium hexaferrites. Wolski et al. [44,133] have studied the hydrothermal synthesis
and treatment of manganese ferrites in detail. The main problem associated with the
manganese ferrites is the tendency of manganese to form an enormous number of
individuals with various valencies, different O/Mn ratios, and with the vacancies
μ
Figure 9.30 Effect of alkali
concentration on the conversion of
the BaO 6Fe 2 O 3 [136] .
1.0
[OH - ]=0.28 N
k 6 =0.19
k 7 =8.6
[OH - ]=1.00 N
[OH - ]=1.50 N
[OH - ]=2.00 N
0.8
0.6
0.4
0
200
400
600
800
1000
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