The value of the activity coefficient of bismuth oxide γBiO1.5 in converter slags must be known
because it is needed as an input to computational thermodynamics models of continuous copper converting. Such models can be used to predict the behaviour of minor elements, such as bismuth, during converting.
Several studies have investigated the activity coefficient of bismuth oxide, γBiO1.5, in various
slag systems. Their findings are summarised in Table 2.5.4.
Fukatsu et al. (1976) investigated the distribution of bismuth in liquid lead and PbO-SiO2
slags at 900 °C by using the EMF method and calculated the activity BiO1.5 to be 1.6~2.2. The
temperature used in this work is much lower than that used during copper converting, so their value of the activity coefficient can be expected to be higher than that at 1300 oC in lead
silicate slags. The relevance of measurements in lead silicate slags to the behaviour of BiO1.5
in copper converting slags is also doubtful.
Table 2.5.4 Summary of literature data on the value of γBiO1.5 in slags.
Fukatsu et al. (1976) Takeda et al. (1983) Jimbo et al. (1984) Eerola et al. (1984)
Slag type PbO-SiO2
iron silicate and calcium ferrite slags silica- saturated iron silicate slag calcium ferrite slag Temperature 900ºC 1250ºC 1200 – 1250ºC 1250ºC Oxygen potential (atm.) 10 -10 – 10-11 10-6 – 10-11 10-7 – 10-11 10-5 – 10-8 Activity coefficient of BiO1.5 1.6 ~ 2.2 calcium ferrite: 0.3 at 10-9 atm. 1.5 at 10-6 atm. iron silicate: 0.6 3.3 at 1200oC 1.6 at 1250ºC 0.8
Takeda et al. (1983) equilibrated calcium ferrite slag and copper metal containing bismuth as a minor element at 1250 ºC under oxygen partial pressures from 10-9 to 10-6 atm. They calculated the activity coefficient of BiO1.5 from their experimental distribution ratio data.
Although the data for iron silicate slag is very scattered, as seen on Figure 2.5.18, it does suggest that γBiO1.5 is constant at about 0.6.
There are no errors bars on the figure so it is not possible to estimate the likely error in the activity coefficient value. However, for calcium ferrite slag the value of γBiO1.5 increased from
about 0.3 at 10-9 atm to 1.5 at 10-6 atm. Such a strong dependence of activity coefficient on oxygen partial pressure is unlikely and suggests a systematic error in the distribution data for calcium ferrite slag.
Figure 2.5.18 Activity coefficients of BiO1.5 at 1250 oC in iron silicate slag (solid
line) and calcium ferrite slag (dashed line) (Takeda et al., 1983)
Takeda et al. (1983) also presented calculated activity coefficient data as a function of the bismuth content of the copper alloy used, as set out in Figure 2.5.19. Not surprisingly, they found that the activity coefficient was not a function of the bismuth content of the alloy over a large range.
Figure 2.5.19 Activity coefficient of BiO1.5 as a function of bismuth content of the copper alloy (Takeda et al., 1983)
The bismuth slag/metal distribution ratio is small, so the BiO1.5 concentration in the slag is
very small over the range of bismuth contents in copper used in their work. This can be seen on Figure 2.5.20 where the maximum mole fraction of BiO1.5 in the slag is only 10-4. This
partial pressure, which verifies that the activity of BiO1.5 obeys Henry’s Law over this
composition range.
Figure 2.5.20 Activity coefficient of BiO1.5 as a function of the mole fraction of BiO1.5
in slag at an oxygen partial pressure of 10-8 atm (Takeda et al., 1983)
Jimbo et al. (1984) investigated the solubility of bismuth in silica-saturated iron silicate slag at 1200 ºC and 1250 ºC, and oxygen partial pressures between 10-7 atm and 10-11 atm. Bismuth was assumed to exist in the slag as two oxides, BiO and BiO1.5. By using regression
analysis, the authors calculated the activity coefficient of BiO1.5 at infinite dilution at both
temperatures. The values were 3.3 and 1.6 at 1200 and 1250 ºC respectively. The difference in the two temperatures is small, so the activity coefficient values would not be expected to differ as much as reported. They considered that the value of γBiO1.5 calculated at 1250 ºC i.e.
1.6, was the more accurate of the two values. This value is much higher than that reported by Takeda et al. (1983), and may be a consequence of the assumption that bismuth was present as both BiO and BiO1.5. This assumption is not supported by the results of Takeda et al.
(1983).
Eerola et al. (1984) investigated the distribution of bismuth in calcium ferrite slags under copper refining conditions at 1250 ºC and calculated the activity coefficient of BiO1.5.
Contrary to the data reported by Takeda et al. (1983) for the same oxygen partial pressure range, they found that the activity coefficient of BiO1.5 was not a function of the oxygen
partial pressure, as seen on Figure 2.5.21.
These results are believed to be more reasonable than those of Takeda et al. (1983), as there is no reason for a strong dependency of the activity coefficient on oxygen partial pressure at
such low concentrations of BiO1.5 in slag. They reported the value of γBiO1.5 to be 0.8 ± 0.1,
which is within the range of values reported by Takeda et al. (1983).
Figure 2.5.21 Activity coefficients of minor components in calcium ferrite slags at 1250 oC (Eerola et al., 1984)
It can be concluded that the values of the activity coefficients of BiO1.5 in both iron silicate
and calcium ferrite slags at 1250 oC are very similar and are most likely to be within the range of 0.6 to 0.8.