Experiments have been made to study the effect of Na
2O(7-13wt%) and BaO(4wt%) additions on the phosphorus distribution between liquid iron and CaO-MgO-Fe
tO-SiO
2-P
2O
5(≤1wt%) slags at 1550°C. The effect of Na
2O and BaO on the logarithm of the equilibrium quotient, k
p=(%P
2O
5)/([%P]
2(%Fe
tO)
5), was found to be 1.2 and 0.9 times as large as that of CaO on weight percent basis, respectively. The values for log k
p can be expressed as log k
p=0.145[(%CaO)+0.3(%MgO)-0.5(%P
2O
5) +1.2(%Na
2O)+0.9(%BaO)]+22 810/T-20.506
The effect of Na
2O or BaO on the activity coefficients of P
2O
5 was also studied by the Turkdogan and Pearson's treatment. The following expression for the log γP
2O
5 is obtained for the Na
2O- or BaO-containing slaps. log γP
2O
5=-1.02(23N
CaO+17N
MgO+8N
FetO-26N
P2O5 +33N
Na2O+42N
BaO)-22990/T+9.490
It was found from the plot of activity coefficients of Fe
tO vs. mole fraction of SiO
2 that activity coefficients of Fe
tO decrease with the addition of Na
2O, while it does not change with the addition of BaO. The dephosphorization equilibrium was discussed on the basis of the theoretical optical basicity proposed by Duffy and Ingram. The following equation was obtained as a function of optical basicity. log N
PO3-4/([P][O]
5/2)=22.22∧
th+30890/T-28.752 where, N
PO3-4: the anionic fraction of PO
3-4 ion [ ]: weight percent in metal phase.
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