stirring intensively and continuously. The obtained precipitate was filtered and washed
with distilled water until no Cl
-
ions were detected in wash water.
Zirconium hydroxide was dehydrated by azeotropic distillation. Azeotropic
distillation was done as follows: 90 g of n-butanol were added to 100 g of zirconium
hydroxide and azeotropic mixture was distilled off while stirring intensively and
continuously. The obtained xerogels were roasted at 600-650° C for 1 hour.
The composition of intermediate compounds formed during the process of
coprecipitation and products of thermal decomposition of precipitates was determined
by
methods
of
chemical,
E(lectronic)P(paramagnetic)R(esonance).
N(uclear)M(agnetic)R(esonance) and radiographic analyses.
The calculations of equilibrium distribution of ions and hydroxocomplexes in
system Zr(IV)-Me(III)-H
2
O and solubility of hydroxides depending on pH of medium
were made in accordance with procedures presented in publications [2,3]. Hydrolysis
constants for zirconium and scandium were taken according to authors' data [4,5].
The Chemistry of Aqueous Solutions of Zirconium and Scandium Salts. In
aqueous medium Zr(IV) hydrolyzes forming hydroxo complexes with the general
formula Zr
m
(OH)
h
(4m-h)+
according to the following equation:
(where m is the number of zirconium atoms in the complex, h is the number of
hydroxo groups)
The authors [2] have shown the possibility of existence (Zr(OH)
3
+
)
n
, (Zr
2
(OH)
7
+
)
n
and (Zr(OH)
4
)
n
, hydroxo complexes in Zr(IV) solutions where n is the degree of
polymerization. As the solution gets less acidic Zr
4
(OH)
15
+
is likely to be formed.
Hydroxo complex Zr
4
(OH)
15
+
undergoes hydrolysis producing neutral tetrameric
complex Zr(OH)
4
)
4
gets polymerized to form a solid phase.
Zr
4
(OH)
15
+
+H
2
O → (Zr(OH)
4
)
4
(Zr(OH)
4
)
4
particles are primary base units of hydroxide zirconia solid phase.
Further transformations of tetrameric complex are connected with olation and
oxolation reactions.
Olation: ≡Zr−OH + HO−Zr≡ → ≡Zr−HO−Zr≡ + OH‾
Oxolation: ≡Zr−HO−Zr≡ → ≡Zr−O−Zr≡ + H
+
The formed olation bonds are very strong and it is practically impossible to break
them.
Thus, in general the formation of zirconium oxide solid phase can be described
by the following equation:
Zr
m
(OH)
h
(4m-h)+
→ (Zr(OH)
4
)
4
→ (ZrO(OH)
2
)
n
→ a-ZrO
2
Coprecipitation process can occur by chemical interaction, absorption or
occlusion.
()
+
+
−
+
+
+
O
hH
OH
Zr
O
hH
тZr
h
m
h
m
3
)
4
(
2
4
2
- 1401 -