Estimation of Various Properties of CaO–“FeO”–SiO₂ System at 1,673 K by Mass Triangle Model

Density of CaO–“FeO”–SiO₂ system

The density of slag is of great importance because it directly affects the volume of slag. Therefore, knowing the density of the slag can give instruction for the many processes, such as electroslag remelting, molten-salt electrolysis, etc. There were many researchers [26, 27, 2829] having reported reliable density data of this system.

Lee [26] studied the density and structure of CaO–“FeO”–SiO₂ in the temperature range of 1,573 K~1,873 K by maximum bubble pressure method. The starting materials included wustite prepared by melting and reducing ferric oxide in iron crucibles. Meanwhile, argon gas was used as the bubbling gas. Based on his experimental data, densities of CaO–“FeO”–SiO₂ at 1,673 K were calculated by the model described above. The results are shown in Figure 2.

Figure 2:

Densities (g/cm³) of CaO–“FeO”–SiO₂ system at 1,673 K.

As discussed by Lee, when the mole fraction of SiO₂ increased, the average silicate anion size would increase as well. As shown in the figure, with increasing content of SiO₂, the density value got decreased. That was identical with the atmosphere presented by Lee. Meanwhile, Lee also pointed out that the calcium silicate melts were less polymerized than the corresponding iron silicates. Again, the calculation gave the same trend, that is, the increasing content of “FeO” elevated the density, while the effect of CaO content to the density is not very obvious.

In order to measure the accuracy of this new model, two experimental data were taken to compare the calculation results, marked by a circle and a cross. The deviation is defined by eq. (3). Table 1 showed the calculation error rates of these two data were –0.346 % and 1.003 %, respectively.

It is demonstrated that the new model successfully predicted the trend of density change with different composition. When the composition is fixed, the model can also calculate the property using the boundary point with a lower deviation.

Viscosity

Viscosity is one of the most important properties in both ironmaking and steelmaking, in that slag reacted with both metal and gas. If the viscosity is too high, the slag cannot absorb detrimental element and gas in unable to get through the slag smoothly. However, if the viscosity is too low, foaming is likely to happen, and it could cause serious difficulty and harm. Therefore, viscosity is also a basic factor that is taken to consideration in process modeling. Previous papers [30, 31, 32, 33, 34, 35] made a great contribution to understanding the viscosity of CaO–“FeO”–SiO₂ system.

Ji [30] reported his experimental study on viscosity of CaO–“FeO”–SiO₂ system in temperature range of 1,423 K–1,753 K. In his experiment, iron crucible was employed to both mix the slag and measure the viscosity. And the oxygen partial pressure was kept between 2.5×10^–10 and 1×10^–6 atm. Based on the experimental data, the viscosity contour lines were obtained, shown in Figure 3. Generally, the viscosity increased as the SiO₂ content increased. This was because the SiO₂ always acted as a network former, which would increase viscosity of slag. On the contrary, increasing content of “FeO” and CaO will decrease the viscosity, due to their function of a network modifier. Ji also indicated that replacement of SiO₂ by “FeO” caused a dramatically decrease in viscosity. The experimental data reported by Gudenau [35] was slightly higher than the calculation, partly due to the slag containing ferric oxide.

Figure 3:

Viscosities(Pa × s) of CaO–“FeO”–SiO₂ system at 1,673 K.

Surface tension

Information on the surface tension of slags is related to the interface behavior of slag phase and has combined effect with other properties. In sintering process, the surface tension plays an important role in pelleting of raw materials. In ironmaking and steelmaking, the decrease of ratio of surface tension to viscosity is necessary to obtain a stable foaming slag. In refining and continuous casting, this property affects the absorption capacities to inclusions and erosion to the refractory and mold channels. Valuable data can be found in previous work [36, 37, 38, 39, 40, 41, 42, 43, 44, 45, 46]. Lee et al. [43] studied the surface tension of CaO–“FeO”–SiO₂ system in different temperatures and based on his experimental data, iso-surface tension lines were calculated, presented in Figure 4.

Figure 4:

Surface tensions (mN/m) of CaO–“FeO”–SiO₂ system at 1,673 K.

As can be seen, the calculation results satisfactorily predicted trend of surface tension with altered composition. The increasing content of CaO severely elevated the surface tension of this system while the increasing content of SiO₂ slightly lowered the surface tension. However, the effect of “FeO” content was not as notable as other oxides. The point marked by a dot was calculated to have an error rate of 1.04 %, as presented in Table 1. Therefore, the new model can also estimate the surface tension of CaO–“FeO”–SiO₂ system successfully.

Sulfide capacity

Sulfur removal in ironmaking and steelmaking processes is now of even more importance than in the past because of the increasing demand for extremely clean steel. Sulfide capacity is regarded as the capacity of a slag to hold sulfur, which is of great importance to the industry application. As for the system concerned, Nzotta et al. [47] have studied the sulfide capacity of CaO–“FeO”–SiO₂ system at 1,673 K and 1,773 K by gas–slag equilibrium. FeO used in his studied was examined by X-ray Diffraction for confirmation. Thus, these data obtained by experiment work were used as “boundary data”, and then the calculation performed. The sulfide contours are exhibited in Figure 5.

Figure 5:

Sulfide capacities in logarithm of CaO–“FeO”–SiO₂ system at 1,673 K.

As seen in the figure, the increase of SiO₂ content considerably hindered the desulfurization capacity. That was because SiO₂ was an acid oxide which could not offer much O^2– so it had a lower affinity to sulfur. Generally, increasing content of “FeO” and CaO favored desulfurization. However, the effect of “FeO” and CaO could not be distinguished easily. Nzotta also mentioned the marginal difference observed in C_S values between “FeO”–SiO₂ slag and CaO–SiO₂ slag. Comparison between the reference data and the calculated data was shown in Table 1. An error rate of –7.3 % was obtained when evaluating sulfide capacity of the point marked by a round dot. Similarly, the new model worked well with the sulfide capacities of CaO– “FeO” –SiO₂ both in contour lines derivation and calculation for fixed composition.