To remove P from steel, lime( CaO) and oxygen

( FeO) is needed to produce the product P2O5.CaO to trap in the slag . However if the temp go over 1640 degC the reaction is reversed and rephosphorisation takes place. So high activity of FeO favours deP.

2P + CaO+ 5FeO = P2O5.CaO + 5 Fe

Tanmoy Mandal Why is the addition compound(formation of CaO.P2O5) required?Does it increase/decrease viscosity(why?)What effect will the increase or decrease in viscosity have?

Rajorshi Koyal , you can get elaborated explanation while studying the ellingham diagram.

In an oxygen rich top blowing steelmaking process, dephosphorisation occurs at the interface between slag and metal. The direct removal of P by oxygen into slag doesnt happen as P2O5 is unstable at steelmaking temperature and it will reduce immediately after its formation.

2 [P] +5 [O] = (P2O5) ∆ G° = −740375 + 535.365T J/mol

For example, at temperature > 1382 K, the delG becomes positive and P2O5 decomposes to P and O. How can we make the delG negative at high temperature?

Del G = del G0 + RT ln (aP2O5/aP^2*aO^5)

Decreasing the activity of P2O5 or increasing activity of P and O will help to move the reaction in forward direction. Therefore common practice is to add basic oxide such as CaO (strong basic oxide) to reduce the activity of P2O5 and makes the P2O5 phase stable in the slag phase. As mentioned in previous comment, the dephosphorisation reaction in steelmaking process proceeds via the following reaction:

[P] + 5[O] + 3(O2-) = (PO43-)

As you can see from the above reaction, P in the melt requires a oxidising environment([O]), which is generally provided by FeO in slag ( FeO = Fe+ [O]) and also it requires a basic slag (O2-) supplied by the addition of basic oxide ( CaO= Ca + O2-) to reduce the P2O5 activity in slag.

So to answer your question, a high FeO slag supplies [O] and favours the oxidation of P to P2O5. However, a basic slag is absolutely necessary to make P2O5 stable in the slag phase. Therefore beyond certain percentage, generation of more FeO does not help the dephosphorisation process . Also you have to keep in mind that oxidation of P is an exothermic reaction and a reduced T always assists the P removal process.

Mr. Rout explained this so thoroughly.

FeO provides oxidizing atmosphere in slag and this is basic in nature also.

2P +5FeO = P2O5+ 5Fe, In this reaction FeO is providing Oxygen to P. However CaO does not provide Oxygen to Phosphorus because it does not dissociate and CaO is just basic is nature.

Hence in total reaction,

2P +5FeO +3CaO= 3CaO.P2O5+ 5Fe

Hence FeO has two roles, first is to supply Oxygen and second is to provide basicity(although less than CaO). Hence if FeO starts to replace CaO, dephosphorization would decreased. As wt%FeO increases dephophorization index increases but after achieving maximum value of Lp, on increasing wt%FeO, Lp starts to reduce. This behaviour is true basicity B=2, 3 and 4. Where B= %CaO/%SiO2. Initial effect is due to oxidizing effect of FeO and later effect occurs due to the fact that FeO starts to replace the CaO since basic nature of slag is reduced. The optimum FeO is around 15-20 wt% . Most importantly, stirring is needed for proper dissolution of CaO and FeO.

High activity of FeO will help in the lime dissolution. FeO preferentially attack grain boundaries of lime crystallites which breaks down the solid lime structure. Thus lime dissolution rate increases. FeO also decreases the viscosity of slag. Thus in increasing basicity of slag in BOF, FeO plays a major role. Rest already explained above.

Role of FeO cannot be understood solely in case of dephosphurization.

FeO supplies oxygen for formation of P2O5. However, if the FeO content reduces (25%). In addition to this CaO (basicity) is essential to bind the the P2O5 in compound state so that the activity of P2O5 will be reduced for reversion reaction or make P2O5 stable int he slag itself.

Direct removal of Phosphorus with Oxygen into the BOF slag will not happen because of thermodynamic re-strictions. Phosphorus Pent-Oxide P2O5 is not stable at steelmaking temperatures and will be reduced immediately after formation during hot metal refining. Therefore its activity must be reduced by offering of liquid CaO. Since the pure lime has a very high melting point of > 2.800 °C a flux must be used to liquefy the lime. This flux is FetO in today’s state-of-the-art operations. Of course SiO2 also supports the solution of lime but lowers the activity.

When looking into the phase diagram CaO–FeO–P2O5 it becomes evident, that equilibrium will be achieved at an sufficient CaO/P2O5-ratio > 3. Based on the P-O reaction equation it can be estimated that De-Phosphorization (De-P) is encouraged by:

- High oxidizing conditions in the BOF (aFeO ↑)

- High lime activity in the slag (aCaO↑) and

- Low process temperature

Since the De-P reaction is not a direct reaction with the blow Oxygen, but with iron oxide in the slag, it is favorable to work at elevated (%FetO) content. The P partition ratio is initially enhanced with increasing (%FetO) in the range of 15% to 20%, but this effect is reversed at higher content over 20%.

FeO concentration along with basicity is the most significant variable characteristic of chemical composition of BOF slag. Increase in FeO concentration initially increases the phosphorus partition ratio upto a certain limit (~15-20% FeO)

Sorry, I just saw this post today. In BOF: Content of P2O5 in liquid slag is small: only around 1%. P2O5 gets captured in liquid slag. It is important to form and retain di-calcium silicate to retain phosphorus in slag, else it can revert, as usually happens. Solubility of P2O5 in dicalcium silicate is around 5% (nearly 5 times of that in liquid part of the slag). The end slag (at taping time) usually contains around 15% dicalcium silicate if basicity is more than 2.5. FeO only provides oxygen to P to form P2O5. Capture of P2O5 is more efficient in dicalcium silicate. This is proven now by several researchers, including my own work. Researchers discuss equilibrium distribution of phosphorus in slag and metal: these studies provide only a guide line to explain effect of basicity, temperature, and FeO content of slag. In actual practice phosphorus control is quite difficult. For the same end point conditions of temperature, basicity, and FeO content one can end up with widely different phosphorus distribution ratios at tap. So the success of Phos control is in the techniques of managing the blow and the additions sequence during the blow. At no pint of approach towards equilibrium only towards the end of blow. So if we keep retain the slag and metal in BOF (or in laboratory experiments) for a very long time then Phos distribution can be predicted. But it is not possible to hold metal and slag for long times in vessel for practical reasons. So the secret of phos control in BOF is in control of blow.