Environmental Engineering Reference
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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
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