FORMATION OF THE MORPHOLOGY OF CONTINUOUS-PRECIPITATION ALUMINUM HYDROXIDES 175
the temperature ranges of thermal decomposition.
The presence of aluminum nitrate is confirmed by IR
The derivatogram of a hydroxide stabilized for
90 min shows the following changes (Table 3, sample
no. 2). The endothermic peak associated with the de-
hydration of gibbsite is entirely absent in the DTA
curves because of its phase transformation into pseu-
doboehmite [8], in agreement with the results of an
X-ray phase analysis of the samples (Table 1, sample
no. 2). It is of interest that the curves of thermal de-
composition show, on the background of the disappear-
ance of a broad peak at 290 C, a low-intensity peak
at 275 C, with a loss of mass equal to 3%, which is
due to thermal decomposition of basic aluminum salts.
spectra (not given here), which contain an absorption
1
band at 1385 cm , associated with NO ions [13].
3
In addition, the X-ray diffraction pattern of a sam-
ple subjected to thermal treatment at 270 C shows
a certain decrease in the intensity of pseudoboehmite
peaks and the appearance of weak lines with d/l 1.827
and 1.409 . It is known [6] that pseudoboehmite,
which is characterized as a hydroxide with a low crys-
tallinity because of the presence in its structure of an
excess amount of water, is converted to -Al O at
2
3
2
80 330 C. This gives reason to believe that further
The increase in the fraction of Pb in this sample
1
removal of moisture at 305 C is due to thermal de-
composition of Pb containing 2.08 mol H O per mole
by approximately 40 wt % leads to a substantial loss
of mass by the monohydroxide in the course of ther-
mal decomposition, which is manifested in the DTA
curves as an endothermic peak at 460 C (Table 3,
sample no. 2). The observed shift of this peak by
30 C to higher temperatures is probably due to an in-
2
2
of Al O . It was shown in [6, 14] that low tem-
2
3
peratures of the phase transition pseudoboehmite
Al O are characteristic of fine and poorly crystal-
-
2
3
lized hydroxides. The result obtained suggests that
the phase transition temperature is determined largely
by the crystallinity of pseudoboehmite, as well as by
the size of crystallites. According to the results of
calculations, the content of -Al O ( ) formed in
crease in the crystallinity of the structure of Pb , which
1
is indicated (according to XPA, by a decrease in the
lattice constants (Table 1, sample no. 2). The high-
temperature endothermic peak at 870 C, associated
2
3
2
gradual dehydroxylation of Pb in the given stage of
with the phase transition -Al O3
x-Al O , is not
2
2
2 3
thermal treatment is 1 wt %. Because of the pseudo-
morphism [1, 15], the dimensions of coherent domains
observed in the DTA curves because of the absence
of gibbsite in the precipitate.
in Pb are comparable with those of its precursor
2
The derivatogram of a hydroxide that passed the
stage in which the pH value is brought to 8.9 by
adding a solution of sodium aluminate (Table 3, sam-
ple no. 3) shows an endothermic effect in the range
240 300 C, with a peak at 290 C and a pre-effect
at 265 C. This is due to dehydroxylation of gibbsite
formed in accordance with the scheme
(
[
950.0
200] [002]).
along the crystallographic planes [020]
The absence of lines associated with gibbsite in
the X-ray diffraction pattern of a sample calcined at
30 C indicates that it completely thermally decom-
poses into -Al O , whose content in the precipitate is
3
2
3
4
0 wt %. According to calculations, a certain increase
+
in the intensity of peaks associated with -Al O3
NaAlO2 + 2H O = Na + Al(OH)4,
2
2
2
results from the completion of dehydration of Pb2.
The presence of broad bands associated with pseudo-
boehmite in the X-ray diffraction patterns indicates
Al(OH)4
Al(OH)3 + OH ,
in the first case, and to thermal decomposition of
aluminum nitrate, in the second. The total loss of
mass, associated with this effect, is 4 wt %.
that the precipitate contains Pb ( 57 wt %).
1
The stage of dehydration of Pb is observed in the
1
DTA curves at 350 550 C, with a peak at 430 C. It is
known [6, 16] that thermal decomposition of a pseudo-
boehmite with a high crystallinity occurs in this tem-
perature range. According to [17], the temperature of
dehydration of pseudoboehmite can be used to evalu-
ate the dispersity of particles of the starting hydrox-
ide. The dimensions of the coherent domains of Pb1,
which undergoes dehydration at around 430 C, do not
exceed 100 (Table 1), in agreement with published
data [15]. The presence of a high-temperature peak at
The derivatogram of a hydroxide that passed the
stage of stabilization for 4 h shows, in the range 240
3
00 C, only the effect of thermal decomposition of
basic aluminum salts with a loss of mass equal to 3%
Table 3, sample no. 4). No changes are observed in
(
other stages of dehydration of the given sample.
On the whole, the phase composition of the hy-
droxides does not change substantially in the subse-
quent stages of the technological process (Table 3),
and, therefore, the pattern of thermal decomposition of
sample nos. 5 8 is the same. It can be stated, on the
8
70 C indicates that there occurs a phase transforma-
tion -Al O3 x-Al O , with loss of 5% of mass.
2
2
3
RUSSIAN JOURNAL OF APPLIED CHEMISTRY Vol. 78 No. 2 2005