Thermodynamic models for predicting phosphorus distribution ratio P and phosphate capacity C PO 32 4 of CaO-FeO-Fe 2 O 3 -Al 2 O 3 -P 2 O 5 slags during secondary refining process of molten steel, according to the ion and molecule coexistence theory (IMCT), i.e. IMCT2 P and IMCT2C PO 32 4 4 against the aforementioned parameters can be observed for the slags in the higher temperature range from 1918 to 1927 K. The influences of the aforementioned parameters on dephosphorisation ability are significantly different from those on dephosphorisation potential of the slags.

A thermodynamic model for calculating sulphur distribution ratio between CaO-SiO 2 -MgO-Al 2 O 3 ironmaking slags and carbon saturated hot metal has been developed by using a thermodynamic model for calculating mass action concentrations of structural units or ion couples of ironmaking slags based on the ion and molecule coexistence theory.The calculated mass action concentrations of structural units or ion couples in CaO-SiO 2 -MgO-Al 2 O 3 ironmaking slags equilibrated with carbon saturated hot metal at 1 773 K can be applied to represent reaction ability, like classic concept of activity. The calculated total sulphur distribution ratio shows an acceptable agreement with the tested sulphur distribution ratio between CaO-SiO 2 -MgO-Al 2 O 3 ironmaking slags and carbon saturated hot metal from desulphurization experiments at 1 773 K. Meanwhile, the developed thermodynamic model for calculating sulphur distribution ratio between CaO-SiO 2 -MgO-Al 2 O 3 ironmaking slags and carbon saturated hot metal can quantitatively determine the respective contribution of free CaO and MgO in CaO-SiO 2 -MgO-Al 2 O 3 slags. A very significant difference of desulphurization ability between free CaO and MgO has been found with free CaO accounting for 97 % desulphurization potential comparing with free MgO as about 3 % in CaO-SiO 2 -MgO-Al 2 O 3 slags equilibrated with carbon saturated hot metal at 1 773 K.

A sulphide capacity prediction model of CaO-SiO 2 -MgO-Al 2 O 3 ironmaking slags has been developed based on the ion and molecule coexistence theory (IMCT) and verified by two groups of sulphide capacity data of CaO-SiO 2 -MgO-Al 2 O 3 ironmaking slags by different researchers. A hot metal pretreatment slags of CaO-SiO 2 -MgO-Al 2 O 3 with high binary basicity is also applied to verify the feasibility of the developed IMCT model. The predicted sulphide capacity of CaO-SiO 2 -MgO-Al 2 O 3 ironmaking slags at 1 773 K as well as high alumina CaO-SiO 2 -MgO-Al 2 O 3 ironmaking slags in a temperature range of 1 773-1 873 K by the developed IMCT model has higher accuracy than the measured as well as the predicted by other sulphide capacity prediction models. The calculated equilibrium mole numbers, mass action concentrations of structural units or ion couples and optical basicity are recommended to represent slag composition for correlating with sulphide capacity of the slags compared with mass percentage of components or binary slag basicity. The developed IMCT model can calculate not only the total sulphide capacity of the slags but also the respective sulphide capacity of free CaO and MgO in the slags. Largely increasing Al 2 O 3 content from 15 to 25 % and decreasing CaO content from 40 to 34 %, MgO content from 9 to 4 % can improve contribution of free CaO from 97 to 99 % while decreasing contribution of free MgO from 3 to about 1 % to the total sulphide capacity of CaO-SiO 2 -MgO-Al 2 O 3 ironmaking slags.KEY WORDS: sulphide capacity; CaO-SiO 2 -MgO-Al 2 O 3 ironmaking slags; sulphur distribution ratio; sulphide capacity model; the ion and molecule coexistence theory; blast furnace ironmaking; desulphurization potential; mass action concentration; structural units; ion couples.plied to some limited slags without widespread acceptance from viewpoint of metallurgical physicochemistry. It is necessary and interesting to develop a sulphide capacity prediction model according to intrinsic relation of sulphide capacity and sulphur distribution ratio from new viewpoint.A sulphide capacity prediction model of CaO-SiO 2 -MgO-Al 2 O 3 ironmaking slags has been developed according to the ion and molecule coexistence theory [22][23][24][25][26] (2) However, the desulphurization reaction between slag and metal can be traditionally written by ion exchange reaction as Therefore, the relationship between L S and C S 2Ϫ of a slag can be obtained from Eq. (6) as 14,19) .... (7) Obviously, the uneasily measured C S 2Ϫ of slags equilibrated with gas can be calculated from the easily measured L S of slags equilibrated with metal by Eq. (7). Sulphide Capacity Model Based on IMCTAccording to the reported L S prediction model 22) between CaO-SiO 2 -MgO-Al 2 O 3 slags and hot metal based on IMCT, [22][23][24][25][26] . .

A thermodynamic model for calculating the phosphorus distribution ratio between top-bottom combined blown converter steelmaking slags and molten steel has been developed by coupling with a developed thermodynamic model for calculating mass action concentrations of structural units in the slags, i.e., CaO-SiO 2 -MgO-FeO-Fe 2 O 3 -MnO-Al 2 O 3 -P 2 O 5 slags, based on the ion and molecule coexistence theory (IMCT). Not only the total phosphorus distribution ratio but also the respective phosphorus distribution ratio among four basic oxides as components, i.e., CaO, MgO, FeO, and MnO, in the slags and molten steel can be predicted theoretically by the developed IMCT phosphorus distribution ratio prediction model after knowing the oxygen activity of molten steel at the slag-metal interface or the Fe t O activity in the slags and the related mass action concentrations of structural units or ion couples in the slags. The calculated mass action concentrations of structural units or ion couples in the slags equilibrated or reacted with molten steel show that the calculated equilibrium mole numbers or mass action concentrations of structural units or ion couples, rather than the mass percentage of components, can present the reaction ability of the components in the slags. The predicted total phosphorus distribution ratio by the developed IMCT model shows a reliable agreement with the measured phosphorus distribution ratio by using the calculated mass action concentrations of iron oxides as presentation of slag oxidation ability. Meanwhile, the developed thermodynamic model for calculating the phosphorus distribution ratio can determine quantitatively the respective dephosphorization contribution ratio of Fe t O, CaO + Fe t O, MgO + Fe t O, and MnO + Fe t O in the slags. A significant difference of dephosphorization ability among Fe t O, CaO + Fe t O, MgO + Fe t O, and MnO + Fe t O has been found as approximately 0.0 pct, 99.996 pct, 0.0 pct, and 0.0 pct during a combined blown converter steelmaking process, respectively. There is a great gradient of oxygen activity of molten steel at the slag-metal interface and in a metal bath when carbon content in a metal bath is larger than 0.036 pct. The phosphorus in molten steel beneath the slag-metal interface can be extracted effectively by the comprehensive effect of CaO and Fe t O in slags to form 3CaOAEP 2 O 5 and 4CaOAEP 2 O 5 until the carbon content is less than 0.036 pct during a top-bottom combined blown steelmaking process.

slags at the steelmaking endpoint during an 80-ton top-bottom combined blown converter steelmaking process has been developed based on the ion and molecule coexistence theory (IMCT). The phosphate capacity has a close relationship with the phosphate capacity index, whereas the logarithm of phosphate capacity is 12.724 greater than that of phosphate capacity index at 1873 K (1600°C). The developed phosphate capacity prediction model can be also used to predict the phosphate capacity index with reliable accuracy compared with the measured and the predicted phosphate capacity index of the slags by other models in literatures. The results from the IMCT phosphate capacity prediction model show that the comprehensive effects of iron oxides and basic components control the dephosphorization reaction with an optimal ratio of (pct FeO)/(pct Fe 2 O 3 ) as 0.62. The determined contribution ratio of Fe t O, CaO + Fe t O, MgO + Fe t O, and MnO + Fe t O to the phosphate capacity or phosphate capacity index of the slags is approximately 0.0 pct, 99.996 pct, 0.0 pct, and 0.0 pct, respectively. The generated 2CaOAEP 2 O 5 , 3CaOAEP 2 O 5 , and 4CaOAEP 2 O 5 as products of dephosphorization reactions accounts for 0.016 pct, 96.01 pct, and 3.97 pct of the phosphate capacity or phosphate capacity index of the slags, respectively.