Thermodynamic Analysis of Coprecipitation Process Zr(IV)-Sc(III).
Coprecipitation process is a heterogeneous process so the composition of the solid
phase in the Zr(IV) – Ме(III) – ОН
-
system is significantly dependent upon medium
pH and upon chemical properties of coprecipitating cations.
Formation of complex hydroxides such as Zr
x
Me
y
(OH)
z
and their precipitation as
a solid phase is the most important stage in the process of coprecipitation.
It is commonly thought [6] that adding alkali solution to salts mixture is bound to
cause a stepwise hydroxide precipitation i.e. first metal with lower precipitation pH is
precipitated and then that with higher one.
To analyze the processes occurring in the process of scandium and zirconium
hydroxide coprecipitation, Fig. 1 shows relation between logarithmic solubility of
zirconium hydroxides and scandium and pH value.
Figure 1. Relation between zirconium hydroxides (1) and scandium (2)
and pH value. Dots show experimental data of scandium content
in the solution above the precipitate
It was established (Fig. 1) that zirconium precipitates at pH levels ranging from
0.4 to 4.4 and scandium precipitation occurs at pH levels ranging from 4.5 to 6.
To determine the pH value of the beginning and ending of scandium hydroxide
precipitation in the presence of zirconium we performed a series of precipitation
experiments at various pH values.
Compositional analysis of obtained precipitates and measurement of the metals
residual concentration in the solution showed that precipitation of scandium begins at
pH value as low as 1 i.e. as soon as zirconium hydroxide solid phase appears in the
solution.
It can thereby be concluded that precipitation pH range for scandium is the same
as that for zirconium. This fact is well accounted for possibility of scandium and
pH
0
2
4
6
8
10
lgS
-8
-6
-4
-2
0
1
2
pH0
Zr
pHk
Zr
pH0
Sc
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