Effect of FeO and CaO on dephosphorization:
Figure 8.1 shows the variation of dephosphorization index  as a function of wt%FeO for CaO-FeO- slag at different basicities. The dephosphorization ratio increases with
| Figure 8.1: |
Dephosphorization index as a function of weight percent FeO content of slag for different basicities. |
increase in FeO content of slag and becomes maximum in between 15-16% FeO at all basicities. Further increase in FeO beyond 15-16%, dephosphorization decreases. The above behaviour can be observed at all basicities of slag.
The above behaviour is due to the dual role of FeO. FeO is the source of oxygen for oxidation of P according to the following reaction
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(8) |
For a given basicity of slag, as FeO content of slag increases oxidizing power of slag increases and phosphorus oxidation according to reaction 8 will be favoured because CaO of slag decreases the activity of  by forming a stable compound. Beyond the optimum value of FeO in slag FeO replaces CaO and may either combine with CaO or with . FeO is a weak base compared with CaO as a result of which the dephosphorization ratio decreases with addition of FeO beyond an optimum value.
The maximum dephosphorization ratio increases with the increase in the basicity of slag as can be seen in the figure 7.1. Higher basicity requires higher amount of CaO dissolved in slag. Any undissolved CaO will not be effective for dephosphorization. Optimum value of FeO is more or less independent of the basicity of slag. Thus control of FeO in slag is important for efficient dephhosphorization.
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