Environmental Engineering Reference
In-Depth Information
Figure 6.29
Superimposed phase stability diagram of Ni-O-S on the stability region
of Na
2
SO
4
in Na-O-S diagram at 1173 K. The broken lines represent isoactivity lines for
NiSO
4
and NaNiO
2
at activities of 10
2
and 10
4
[50].
diagram. The obvious choice would be to superimpose the Ni-O-S diagram (Fig.
6.28) on the stability region of Na
2
SO
4
in Na-S-O diagram (Fig. 6.27). Such
superimposition is illustrated in Fig. 6.29. However, while doing so, one should
not forget to consider the formation of some other new phases (like Na
2
NiO
2
and
NaNiO
2
) and the mutual solubilities of the different phases, e.g., NiO in Na
2
SO
4
or NiSO
4
in Na
2
SO
4
. Solid NiSO
4
is stable only at high SO
3
activities but gets
dissolved in molten Na
2
SO
4
at activities lower than unity. In a similar way,
NaNiO
2
also gets dissolved in Na
2
SO
4
, whereas Na
2
NiO
2
is thermodynamically
unstable in the temperature range of 1173-1273 K. The mutual solubilities of
NiO and the nickel sulfides are insignificant.
Gupta and Rapp [57] have measured the solubility of NiO in fused Na
2
SO
4
at 1200 K and 1 atm O
2
as a function of
a
Na
2
O
(or
p
SO
3
), and their results are
presented in Fig. 6.30. This figure demonstrates that the solubility passes through
a minimum at log
a
Na
2
O
10.3, which prompted the authors to conclude that
at higher values of Na
2
O activity, NiO dissolves as nickelate (NiO
2
), whereas
at lower values it dissolves as Ni
2
ions. These two types of dissolution processes
are termed as basic and acidic fluxing, respectively, which will be discussed in