Critical Assessment of P 2 O 5 Activity Coefficients in CaO-based Slags during Dephosphorization Process of Iron-based M
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IT is a widely accepted viewpoint that dephosphorization products from iron-based melts by oxidizing slags are phosphate, which are mainly composed of in the case of the mass monomer phosphate PO3 4 percentage of phosphorus in slags (pct P) less than 1.0,[1–4] which corresponds to the mass percentage of P2O5 in slags (pct P2O5) smaller than 2.29 as summarized by the present authors elsewhere.[5] However, no values of activity aPO3 of monomer phosphate PO3 4 in slags have 4 been measured or determined by common experimental techniques up to now. Thus, the phosphorous content in
XUE-MIN YANG and DONG-PING DUAN, Research Professors, are with the Key Laboratory of Green Process and Engineering, Institute of Process Engineering, Chinese Academy of Sciences, Beijing 100190, P.R. China. Contact e-mail: [email protected] JIN-YAN LI, Ph. D. Candidate and Joint-Training Student, is with the School of Metallurgical and Ecological Engineering, University of Science and Technology Beijing, Beijing 100083, P.R. China, and also with the Key Laboratory of Green Process and Engineering, Institute of Process Engineering, Chinese Academy of Sciences. GUO-MING CHAI, formerly Ph.D. Candidate with the Key Laboratory of Green Process and Engineering, Institute of Process Engineering, Chinese Academy of Sciences, is now Engineer with the No. 2 Steelmaking Steel Plant, Shanxi Taigang Stainless Steel Corporation Limited, Taiyuan 030003, P.R. China. JIAN ZHANG, Professor Emeritus, is with the School of Metallurgical and Ecological Engineering, University of Science and Technology Beijing. Manuscript submitted October 2, 2015. METALLURGICAL AND MATERIALS TRANSACTIONS B
metallurgical slags is usually represented by the mass percentage of P2O5. The CaO-based slags, which CaO dominates the main basic oxides, have widely been applied to be dephosphorization fluxes in commercial practice. Principally, the dephosphorization reaction of iron-based melts by CaO-based slags can be described by two coupling steps[5]: first, phosphorous in iron-based melts is oxidized by iron oxides FetO to form P2O5 in slags through 2½Pþ5ðFet OÞ¼ðP2 O5 Þþ5t½Fe
½1
second, the extracted P2O5 is stabilized by CaO to form phosphate as 3CaOÆP2O5 via ðP2 O5 Þ þ 3ðCaOÞ ¼ ð3CaOP2 O5 Þ
½2
Thus, the dephosphorization reaction in CaO-based slags can be represented in molecular form from Eqs. [1] and [2] as 2½P þ 5ðFet OÞ þ 3ðCaOÞ ¼ ð3CaOP2 O5 Þ þ 5t½Fe Dr GH m; 3CaOP2 O5 ¼ 832; 302 þ 318:67T
ðJ/molÞ ½3a
KH 3CaOP2 O5 ¼
a3CaOP2 O5 a5t R;Fe a2%;P a5Fet O a3CaO
¼
a3CaOP2 O5 a5t R;Fe ½pct P2 f 2%;P a5Fet O a3CaO
ðÞ ½3b
The dephosphorization reaction in Eq. [3a] can also be expressed in ionic form as[5–8] 5 3 ½P þ ½O þ O2 ¼ PO3 4 2 2 aPO3 4 KPO3 ¼ ðÞ 5=2 3=2 4 a%;P a%;O aO2
½4
Activity aP2 O5 of P2O5 is an important and indispensable parameter on investigation of the dephosphorization reaction in Eq. [1] from the viewpoint of dephosphorization thermodynamics. Turkdogan and Pearson[9,10] proposed a famous expression of activity c
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