A Thermodynamic Model for Predicting Phosphorus Partition between CaO-based Slags and Hot Metal during Hot Metal Dephosphorization Pretreatment Process Based on the Ion and Molecule Coexistence Theory

Abstract

A thermodynamic model for predicting phosphorus partition L P between a CaO-based slags and hot metal during hot metal dephosphorization pretreatment process has been developed based on the ion and molecule coexistence theory (IMCT), i.e., the IMCT–L P model. The reaction abilities of structural units or ion couples in the CaO-based slags have been represented by the calculated mass action concentrations N i through the developed IMCT–N i model based on the IMCT. The developed IMCT–L P model has been verified to be valid through comparing with the measured L P as well as the predicted L P by two reported L P models from the literature. Besides the total phosphorus partition L P between the CaO-based slag and hot metal, the respective phosphorus partitions L P,i of nine dephosphorization products as P2O5, 3FeO·P2O5, 4FeO·P2O5, 2CaO·P2O5, 3CaO·P2O5, 4CaO·P2O5, 2MgO·P2O5, 3MgO·P2O5, and 3MnO·P2O5 can also be accurately predicted by the developed IMCT–L P model. The formed 3CaO·P2O5 accounts for 99.20 pct of dephosphorization products comparing with the generated 4CaO·P2O5 for 0.08 pct. The comprehensive effect of CaO+Fe t O, which can be described by the mass percentage ratio (pct Fe t O)/(pct CaO) or the mass action concentration ratio $$ {{N_{{{\text{Fe}}_{t} {\text{O}}}}} \mathord{\left/{\vphantom {{N_{{{\text{Fe}}_{t} {\text{O}}}}} {N_{\text{CaO}}}}} \right. \kern-0pt} {N_{\text{CaO}}}} $$ as well as the mass percentage product (pct Fe t O) × (pct CaO) or the mass action concentration product $$ N_{{{\text{Fe}}_{t} {\text{O}}}}^{5} \times N_{\text{CaO}}^{3} $$ , controls dephosphorization ability of the CaO-based slags. A linear relationship of L P against (pct Fe t O)/(pct CaO) can be correlated compared with a parabolic relationship of L P against $$ {{N_{{{\text{Fe}}_{t} {\text{O}}}}} \mathord{\left/{\vphantom {{N_{{{\text{Fe}}_{t} {\text{O}}}}} {N_{\text{CaO}}}}} \right. \kern-0pt} {N_{\text{CaO}}}} $$ , while the linear relationship of L P against (pct Fe t O) × (pct CaO) or $$ N_{{{\text{Fe}}_{t} {\text{O}}}}^{5} \times N_{\text{CaO}}^{3} $$ can be established. Thus, the mass percentage product (pct Fe t O) × (pct CaO) and the mass action concentration product $$ N_{{{\text{Fe}}_{t} {\text{O}}}}^{5} \times N_{\text{CaO}}^{3} $$ are recommended to represent the comprehensive effect of CaO+Fe t O on dephosphorization ability of the CaO-based slags. Furthermore, a parabolic relationship of L P against binary basicity or complex basicity CB2 and CB3 can be established at binary basicity in 1.8 or at complex basicity CB2 and CB3 in 2.0 corresponding to the maximum of dephosphorization ability of the CaO-based slags. However, the linear relationship between L P and optical basicity can only be correlated with the mathematically regressed $$ \varLambda_{\text{FeO}} $$ = 1.0 and $$ \varLambda_{{{\text{Fe}}_{ 2} {\text{O}}_{ 3}}} $$ =0.75. A great gradient of oxygen potential or oxygen activity a %,O between the dynamically formed metal film beneath slag–metal interface and hot metal bath is the main driving forces of hot metal dephosphorization by the CaO-based slags. The formed metal film with high oxygen content and low carbon content can dynamically be exchanged or replaced by hot metal elements from bath during hot metal dephosphorization process until dephosphorization products mainly as 3CaO·P2O5 are saturated in the CaO-based slags.