Solubilities of CO₂ and Redox Equilibria of Sb and As in NaO_0.5–SbO_m and NaO_0.5–AsO_m Melts

抄録

Solubilities of CO₂ and redox ratios of Sb and As (n_Sb³⁺⁄n_Sb⁵⁺, n_As³⁺⁄n_As⁵⁺, n: the number of moles) in NaO_0.5–SbO_m and NaO_0.5–AsO_m melts were studied. The equilibrium measurements were conducted over the composition range of 1>N>0.7 {=n_NaO0.5⁄(n_NaO0.5+n_MOm), M:Sb or As} under the partial pressures of CO₂ and O₂ of 0.01∼0.1 MPa and 0.005∼1000 Pa, respectively, at the temperatures of 1423 and 1523 K.
Antimony exists in NaO_0.5–CO₂–SbO_m melts not only in pentavalent (SbO₄³⁻) but also in trivalent (SbO₂⁻) state, their relative proportions depending on the experimental conditions. The CO₂ solubility in the melts is, therefore, determined by the following reactions:
3CO₃²⁻+2SbO_2.5=2SbO₄³⁻+3CO₂
CO₃²⁻=CO₂+O²⁻
and
SbO₂⁻+O²⁻+1/2O₂=SbO₄³⁻.
Arsenic exists predominantly in the form of pentavalent anion (AsO₄³⁻) in NaO_0.5–CO₂–AsO_m melts. As the pentavalent arsenic is more stable compared with antimony, the redox reaction of arsenic has almost no effect on the CO₂ solubility in the melts. Thus CO₂ solubility is determined only by the following two reactions:
3CO₃²⁻+2AsO_2.5=2AsO₄³⁻+3CO₂
and
CO₃²⁻=CO₂+O²⁻.
However, with increasing content of arsenic oxide in the melts, the evaporation reaction of trivalent arsenic oxide (As₄O₆),
AsO₄³⁻=1/4As₄O₆(g)+1/2O₂+3/2O²⁻,
becomes significant.