A)
Experiments
Listed in Table 5.1.4 are the experimental conditions at which the slag/brick experiments were conducted. The experiments were initially conducted at 1300oC and oxygen partial pressure of 10-6 atm for 8 hours, to determine the nature of slag attack caused by ferrous calcium silicate slag on magnesia-chrome refractories at converting conditions. These tests were conducted using two FCS slag compositions as mentioned previously. For comparison, the tests were repeated using calcium ferrite slag under the same conditions of
temperature, oxygen partial pressure and time. The experiments were repeated at 32 hours for both FCS and calcium ferrite slags, keeping all other conditions constant, in order to observe the effects of time on the rate and extent of slag attack. Finally, changes to the physico- chemical properties of FCS slag were induced by increasing the experimental temperature to 1400oC, keeping all other conditions constant, including an oxygen partial pressure of 10-6 atm., contact time of 8 hours and slag composition.
Table 5.1.4: The experimental conditions of slag/brick experiments
Experiment Temperature (oC) Oxygen Partial Pressure (atm) Experiment Time (hours) Slag Composition SB-4 1300 10-6 8 MS-4 SB-5 1300 10-6 8 MS-5 SB-12 1300 10-6 8 MS-CF SB-14 1300 10-6 32 MS-5 SB-15 1300 10-6 32 MS-CF SB-1400 1400 10-6 8 MS-5
If refractory attack by FCS slag is a mass transfer controlled process then a higher temperature should accelerate the attack greatly as both slag viscosity and the rate of solid- state diffusion between species in the slag and the refractory are a function of temperature according to the Arrhenius relationship (Equation 5.1.1 and Equation 5.1.2).
The Arrhenius Relationship between the rate of solid state diffusion and temperature is given by Equation 5.1.1. ) / exp( Q RT D D= o − Equation 5.1.1
where T is temperature, D is the rate of diffusion, Do is a constant, Q is the activation energy
and R is the universal gas constant.
Equation 5.1.1 states that as temperature increases the rate of solid state diffusion also increases. The increase in rate of diffusion is controlled by the activation energy, which is the energy required for atoms to migrate from one lattice site to another. If the value of the activation energy is small then the rate increase will also be small. The activation energies for
solid state diffusion of some oxides in iron silicate, calcium ferrite and FCS slag and the magnesia-chrome refractories were found in literature at various temperatures and are given in Table 5.1.5.
Table 5.1.5: Activation energies for solid state diffusion of some oxides in iron silicate,
calcium ferrite and FCS slag and the magnesia-chrome refractories (Kofstad, 1966) at various temperatures
Diffusion Activation Energy (Q)
kJ/mol Coefficient (Do) Temperature Range (oC) Cr3+ in Cr2O3 255.7 0.137 1050-1550 Mg2+ in MgO 330.6 0.250 1425-1625 Fe2+ in MgO 174.1 0.000089 1000-1850
In Table 5.1.5, the activation energies for solid state diffusion are quite high, indicating that solid state diffusion rates are very temperature sensitive. However the data in Table 5.1.5 for Cr2O3 and MgO is self-diffusion data, i.e. the rate of solid state diffusion of
the ions within the oxide in its own lattice. This is the only data available on solid state diffusion and thus will be used as an indication of the temperature sensitivity of solid state diffusion in the current process. The diffusion of Fe2+ in MgO was the only data available on solid state diffusion of the ions from one lattice site to another. Kofstad also reviewed the diffusion of Co2+ and Ni2+ in MgO in the temperature range of 1000-1850oC and found that the activation energy of the diffusing ion decreased with increasing ionic radius of the ion. The ionic radius of Fe2+ is 78 pm, Co2+ is 74 pm and Ni2+ is 69 pm. The data for the self diffusion of Mg2+ and the diffusion of Fe2+ in MgO in Table 5.1.5 is in accord with the relationship between activation energy and the ionic radius of the diffusing ion. The ionic radius of Mg2+ is 72 pm. Table 5.1.5 shows that the diffusion of Fe2+ in MgO is both much slower (i.e. activation energy is lower) and less temperature sensitive than the self-diffusion of Mg2+. Based on the same relationship, it can assumed that given the ionic radius of Fe3+ (64.5 pm) is higher than Cr3+ (63 pm), the activation energy for the diffusion of Fe3+ in Cr2O3 will
be lower than the self diffusion of Cr3+ in Cr2O3 given in Table 5.1.5. With interdiffusion, the
diffusing ions move in opposite directions at an equimolar rate and the rate of diffusion of the slowest moving ion sets the rate of the interdiffusion process. Therefore Fe3+ and Fe2+ determine the rate of interdiffusion between Fe3+ and Cr3+ in chromite spinel and between Fe2+ and Mg2+ in periclase.
According to Equation 5.1.2, the viscosity of liquids decreases exponentially with increase in temperature. The activation energy for the viscosity of FCS slag is not available in the literature; however it may be expected that this value will be between the activation energies for viscous flow for iron silicate and calcium ferrite slags at the same temperature increase, although closer to the value for iron silicate slag as FCS slag contains a significant amount of silica and thus its structure will most probably be closer to that of iron silicate. This data for the silicate and ferrite slag at a temperature increase from 1300oC to 1400oC is given in Table 5.1.6, from various sources.
) / exp(E RT AA A = η Equation 5.1.2
where T is temperature, η is viscosity, AA is a coefficient, EA is the activation energy and R is
the universal gas constant.
Table 5.1.6: Activation energies for viscous flow for iron silicate and calcium ferrite slags when temperature increases from 1300oC to 1400oC.
Activation Energy (EA)
kJ/mol
Coefficient (AA)
Iron Silicate Slag
Toguri et al. 85.0 0.14
Shiraishi et al. (1978) 67.6 0.36
Kauira et al. (1977) 77.5 0.15
Calcium Ferrite Slag
Sumita et al. (1980) 120.9 0.04
Saito et al. (2003) 128.9 0.01
Wright et al. (2001) 101.8 0.03
In general when comparing the activation energies for solid state diffusion to viscous flow, the activation energies for viscous flow are smaller, although the activation energies for calcium ferrite slag of the same order of magnitude to those of self-diffusion in the solid state. It is assumed that FCS slag is more like iron silicate slag so the activation energies for iron silicate slag are more likely to be appropriate to FCS slag. Thus viscous flow is less affected by temperature rise than solid state diffusion.
If refractory attack is chemically controlled and depends therefore on the activity of FeO and Fe3O4 in the slag and the intrinsic rate of the chemical reactions, then refractory
attack should not increase significantly with increase in temperature. The effects of temperature on the activity of FeO and Fe3O4 in FCS slag are unreported however it is
assumed that the temperature effects on the aFeO and aFe3O4 in FCS slag will be similar to that
in calcium ferrite and iron silicate slags. Michal et al. (1952), Schuhmann et al. (1951) and Sehnalek et al. (1972), have studied the effects of temperature on the activity of FeO in iron silicate slag and found that it did not change significantly with an increase in temperature, for constant oxygen partial pressure and slag composition. For example, Michal et al. (1952) found that with temperature increased from 1250 to 1350oC, aFeO in a silica saturated iron
silicate slag at an oxygen partial pressure of 10-8 atm was 0.36 and 0.37, respectively. Similarly Sehnalek et al. (1972) measured the activity of Fe3O4 in the silica saturated iron
silicate slag at 1200 and 1300oC and found aFe3O4 to be 0.005 at 1200oC and 0.004 at 1300oC.
The effects of temperature on the activity of FeO and Fe3O4 in calcium ferrite slag have been
studied by Henao et al. (2006), Kongoli et al. (2003) and Takeda et al. (1980). All agree that at a given slag composition and oxygen partial pressure, there is no appreciable change in the aFeO and aFe3O4 in calcium ferrite slag with an increase in temperature of 100
o
C. Takeda et al. (1980) found that as temperature increased from 1200 to 1300oC, at a fixed slag composition and oxygen partial pressure, aFeO is 0.34 at 1200oC and 0.35 at 1300oC. Similarly, Takeda et
al. (1980) found aFe3O4 to be 0.2 at both 1200oC and 1300oC.