Dissolution of Basic and Acidic Minor Elements in FCS Slag

0.35 and that for iron silicate slag is approximately 0.45. Thus at similar converting conditions, FCS slag not only has a lower copper content in slag but also the total dissolution loss of copper is lower than both iron silicate and calcium ferrite slag. Such values once again support Yazawa and co-author’s (1999) prediction on the dissolution loss of copper in ferrous calcium silicate slag.

2.9.3 Dissolution of Basic and Acidic Minor Elements in

FCS Slag

The predicted distribution behaviour of acidic and basic oxides in FCS slag has been discussed in detail in Section 2.7.4. However there exists limited experimental data on minor element distribution in ferrous calcium silicate slag to support the predictions. Although, the distribution of PbO in the ternary FeOx-SiO2-CaO slag system has been investigated by

Takeda and Yazawa (1989) at 1300oC and the activity coefficients of PbO in the ternary system are reproduced in Figure 2.9.13. In Figure 2.9.13, at the composition of iron silicate slag γPbO = 0.3 and for calcium ferrite slag γPbO is between 3 and 4. For the composition of

ferrous calcium silicate slag, γPbO is between 1 and 2. This reconfirms that whilst the

interactions between PbO and SiO2 in the FCS slag are not as strong as those between PbO

and SiO2 in iron silicate slag, the removal of PbO in FCS slag is nonetheless more efficient

than in calcium ferrite slag. The line trends of the γPbO isobars in Figure 2.9.13 are similar to

Figure 2.9.13: Activity coefficient of PbO (solid lines) in slag (Takeda and Yazawa, 1989)

In agreement with Takeda and Yazawa (1989) are the results on lead distribution by Vartiainen et al. (2003). Figure 2.9.14 illustrates the distribution coefficient of lead between slag and blister copper for iron silicate, calcium ferrite and FCS slag as a function of sulphur content in blister copper (i.e. as a function of oxygen partial pressure). Vartiainen et al. (2003) indicate that the lower the sulphur content in blister, the higher the oxygen partial pressure and the target sulphur content in blister copper set by the operators in the direct-to-blister flash smelting pilot plant testing is 0.1wt%. In general the distribution coefficient of lead between slag and blister is lower in the FCS slag than in iron silicate slag. The distribution coefficient of lead between calcium ferrite slag and blister copper is also given in Figure 2.9.14. In comparison with calcium ferrite slag, slag Cu

Pb

L / _{for FCS slag is higher at the same}

oxygen partial pressure (i.e. the sulphur content), indicating a greater ability to remove lead from blister copper. It should be kept in mind that a comparison between the results from Takeda and Yazawa (1989) and Vartiainen et al. (2003) work cannot be made, as the experimental conditions and the ternary slag composition used by the two sets of investigators are different. Takeda and Yazawa’s (1989) results are based on the oxygen partial pressure of 10-6 atm. and the oxygen partial pressure used by Vartiainen and co-authors (2003) for the pilot plant tests was highly oxidising, oxygen partial pressure of 10-4.74 atm. The FCS slag composition applied by Vartiainen et al. (2003) is situated in the ‘neck’ of the liquid region, as defined by Yazawa et al. (1999). The data presented by Takeda and Yazawa (1989) in Figure 2.9.13 is for various compositions within the FeOx-CaO-SiO2 system. This work was

compiled much earlier (1989) than when FCS slag was proposed for copper converting (1999) and hence gives an approximate value of γPbO in FCS slag. Nonetheless, even though the

numerical data by Takeda and Yazawa (1989) and Vartiainen et al. (2003) cannot be compared, the general behaviour of the basic oxide in the three slags is in agreement with one another as well as with the thermodynamic predictions.

Figure 2.9.14: Distribution coefficient of Pb as a function of %S in blister copper.

(Vartiainen, Kojo and Rojas, 2003)

Although a ternary diagram is not available for the AO-BO-MO system when MO is an acidic oxide, it is expected that the removal of acidic oxides such as arsenic and antimony with application of FCS slag will be similar to calcium ferrite slag but more superior to iron silicate slag. The activity coefficient data for the acidic arsenic oxide, as determined by Yazawa et al. (1999) is demonstrated in the ternary FeOx-SiO2-CaO slag in Figure 2.9.15, in

Figure 2.9.15: Activity coefficient of AsO1.5 (solid lines) in slag (Yazawa, Takeda and

Nakazawa, 1999)

As expected, the trends of γPbO and γAsO1.5 are opposite, such that γPbO increases from

FeOx-SiO2 binary to FeOx-CaO binary slags and for γAsO1.5 the opposite effect is observed and

γAsO1.5 increases from FeOx-CaO slag to FeOx-SiO2 slag. The γAsO1.5 in iron silicate slag is

approximately 0.7-1 and for calcium ferrite slag γAsO1.5 is between 0.04 and 0.1, depending on

the slag composition. In the area of ferrous calcium silicate slag, γAsO1.5 is estimated to be 0.1.

Hence, as predicted, the removability of arsenic to FCS slag is similar to calcium ferrite slag but better than iron silicate slag.

Vartiainen et al. (2003) calculated the distribution coefficients of arsenic between slag and blister copper and the results are in agreement with Yazawa et al. (1999). The results forLslagAs Cu

/

are illustrated in Figure 2.9.16 for both FCS slag and iron silicate slag. Observations of Figure 2.9.16 indicate that the arsenic distribution coefficient between ferrous calcium silicate slag and blister copper is much higher than that of iron silicate slag. In ferrous calcium silicate slag the arsenic distribution coefficient increases with an increasing CaO/SiO2

ratio in the slag. At a given Fe/SiO2 ratio, when the CaO/SiO2 ratio of the slag increases the

distribution coefficient of arsenic, slag Cu As

FeOx SiO2 CaO A’ A FCS

Figure 2.9.16: Distribution coefficient of As, slag Cu As

L / , in FeOx-SiO2-CaO system at constant