Technical Paper


Depending on the oxidation state of iron, the phases spinel, corundum, and CA6


replace Mg2+


can form iron containing solid solutions with Fe2+ and Al3+


depends on the atmosphere and temperature. At 1 atm O2 and/or Fe3+ 20% iron oxide can be incorporated into the corundum lattice at 1490°C [15] to


respectively. The solubility of iron oxide in corundum pressure around .


Melting temperatures of these solid solutions decrease by increasing iron oxide content.


Another dominant reaction is the spinel formation by reaction of iron oxide (Fe2+


) with corundum, so that FeAl2 O4 melts at 1780°C [15] . O4 spinel (hercynite) becomes the


dominant phase instead of corundum with increasing iron oxide content. FeAl2


Thermodynamic calculations were made using the FactSage 6.4 version with FToxid and FACTPS database at McGill University Canada (Figure 3-9).


The phase diagram of the Al2O3 O19 -Fe2 O3


is calculated in Figure 3. There is CA2 (Ca[Al,Fe]12


-CaO ternary system in air at 1700°C solution (Ca[Al,Fe]4


O7 ) and corundum solution ([Al,Fe]2 O3 ), CA6 solution ) in this system. As can be


shown, the addition of iron oxide can expand the liquid area at 1700°C. CA6 solid solution and corundum solid solution co-exist in the small area close


the corundum angle. Based on the above analysis it can be concluded that, at 1700°C, iron oxide has a severe corrosive effect on the lime containing refractory system.


CaO - Al2 O3 - Fe2 O3 O3


p(O2) = 0.21 atm, 1700o Fe2


- O2 C, 1 atm


is plotted in Figure 4. The weight percentage of Fe2 Fe2


and refractory, is presented as the X-axis. In this system, the phases are corundum solid solution (corundum ss), (Mg,Fe)(Al,Fe)2


O3 O4 (spinel ss), CA6 O3 solid solution (CA6 ss), and CaO. 2MgO. 8Al2 O3


solid solution . By increasing


the content of iron oxide, the stability range of the above mentioned phases decreases and the liquidus temperature falls. At a fixed amount of Fe2


, the amount of solid phases decreases by increasing the temperature, due to liquid phase formation.


The reaction of purging plug refractory and iron rich slag occurs at the interface of the hot face of the material or in the slit area. Here, the ratio of slag to slag+refractory can be assumed as 50 %, shown by the red


line in Figure 4. The co-existing phases are spinel solid solution and CA6 solid solution at temperatures below 1575°C. Above this temperature in addition to these two phases, liquid phase also appears. At 1602°C, melting of the CA6


solid solution is complete. The spinel solid solution, the last remaining solid phase, continues to melt with increasing temperature and is completely molten at 1737°C.


The equilibrium phase distribution at 1600°C is calculated in Figure 5. Solid phases include corundum, CM2


A8 phase CM2 A8 transforms to CA6 , CA6 ss and spinel ss at Fe2 disappears at around 50 wt% Fe2 O3 O3


than 2 wt%. Corundum reacts with iron oxide and the magnesium aluminate spinel forming spinel ss, and disappears at 6 wt% of Fe2


O3 . CA6 Liquid phase appears at 37.5 wt% Fe2 solid solution.


until it approaches the composition of magnetite (Fe3 O3


. At 50 wt% Fe2 O3 O4 ss , which is in line with the above analysis.


The spinel ss phase is the dominating phase over the entire composition range of Fe2


). , the phase


composition is 28 wt% of liquid and 69 wt% of spinel solid solution, and only 3 wt% CA6


Figure 6 shows the Al2O3 L iquid (L ) L + CorS .S . L + Ca(Al,Fe)12O19 + CorS .S . L + Ca(Al,Fe)12O19 Ca(Al,Fe)12O19 + CorS .S . L + CaOS .S . CaO 90 80 70 60 L + Ca(Al,Fe)4O7 50 40 weight percent


Figure 3: Isothermal sections through the system Al2 1700°C


O3 -MgO O3 -Fe2 O3 30 20 10 Al2 O3 -CaO in air at


3.2 The effect of iron oxide on the systems Al2 CaO and Al2


O3 -MgO-


The specific composition of recipe C5S26 was taken as an example for thermodynamic analysis. The bulk chemistry of the castable is 92.8 wt% Al2


O3 ,


5.7 wt% MgO, and 1.5 wt% CaO. During slag attack on refractories, coarse aggregates are relatively inert when compared to the fine matrix material, due to much lower porosity and fewer chemical compounds. The lime- containing cement is only present in the matrix. Therefore, all aggregates above 0.5 mm were excluded for the thermodynamic calculations. The matrix material includes tabular alumina fines, alumina magnesia spinel, reactive alumina and calcium aluminate cement. The chemical composition for the matrix material is 87.1 wt% Al2


O3


Transforming into mole composition, the components are 74.1 mol% Al2 20.6 mol% MgO and 5.3 mol% CaO.


O3


, 9.5 wt% MgO and 3.4 wt% CaO. O3


,


Assuming that a pure iron oxide is formed during oxygen lancing, a phase calculation for the effect of iron oxide to the Al2


-MgO-CaO ternary system 18 1800 L + Spinel 1700 1600 1500 1400 0 Spinel + CorS.S. + CaMg2Al16O27


1602oC 1575oC


1737oC Fe2 O3 -MgO-CaO-Fe2 O3 phase calculation at 1700°C.


The stability range of spinel ss with regard to melting has decreased significantly. Liquid phase has already appeared at 6 wt% Fe2


O3 . At 50 wt%


, the phase composition is 75.5 wt% of liquid, and 22.5 wt% of spinel ss, much lower than the 69 wt% spinel ss existing at 1600°C .


To evaluate the CaO effect on the phase relationships of the above Al2 MgO-CaO-Fe2


O3 O3 - quaternary system, the CaO component is completely


eliminated as shown in Figure 7. The percentage of MgO (from spinel raw material) remains the same. The specific composition of recipe A4S26 was taken as an example for thermodynamic analysis. The bulk chemistry of the castable consists of 94.3 wt% Al2 includes 79 mol% Al2


O3 O3 2000 1900 Liquid (L) and 5.7 wt% MgO. The matrix material and 21 mol% MgO. In this case, it becomes a ternary


ss, and spinel ss. The metastable O3


www.ireng.org O3 to the combination of


contents greater


L + Spinel + Ca(Al,Fe)12O19


Spinel + Ca(Al,Fe)12O19 Spinel + Ca(Al,Fe)12O19 + CorS.S.+ CaMg2Al16O27


+ CorS.S. 20


wt% Fe2 O3 / ((Al2 40 O3 )0.741 Figure 4: Calculated phase diagram of the Fe2 60


(MgO)0.206 O3


-Al2


(CaO)0.053 O3


Spinel + Ca(Al,Fe)12O19 80 + Fe2 O3 ) -MgO-CaO in air


simulating the chemical reaction between C5S26 refractory (matrix) and iron oxide


ENGINEER THE REFRACTORIES July 2018 Issue 100


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