Phase evaluation of refractories on slag line in used tundishes
In case of acidic slag, the percentage of tundish where the skull was locked into the safety lining increased with the amount of slag present in the tundish. Thus, the volume of slag carried over slag from ladle to tundish has a direct effect on refractory wear in the tundish. When the composition of the slag in the tundish was compared, it was observed that the percentage of tundishes where the skull was locked into the safety lining depends on the slag basicity and the amount of Al2O3 in the slag in the tundish. The percentage of tundish where the skull was locked into the safety lining in acidic slag decreased with the amount of Al2O3 in the slag present in the tundish, but when in contact with basic slag, it was not dependent on alumina content.
When the acidic slag with a low amount of alumina came into contact with the refractory, precipitation of new phases was observed between MgO grains, which were the product of the reaction between slag and refractory. Since the slag was not saturated with MgO and silicate phases, it would be possible to dissolve 2MgO.SiO2, MgO.SiO2 and MgO, leading to the corrosion of the refractory. The melting point of Enstatite (MS) is 1557 °C which is a liquid at the casting temperature (1550 °C-1650 °C). A new reaction product phase, Monticellite, CaO.MgO.SiO2 (CMS) (with minor Al2O3, MnO and FexO) was found and it was deduced that the silicate phases from the magnesia-olivine mix refractory dissolved into the penetrating slag by forming a low melting 3CaO.MgO.(SiO2)2 (1577°C), Ca.MgO.SiO2 (1488 °C) and probably Gehlenite (1593 °C). No remnant phases of M2S and MS were present which indicates that they completely dissolved in the penetrating slag. As indicated by the CaO-MgO-SiO2 phase diagram, CaO penetrating to these areas would react with M2S to form low melting Ca.MgO.SiO2 and/ or 3CaO.MgO.(SiO2)2, depending on the CaO/SiO2 ratio. This assertion is supported by the formation of CMS. The phase diagram suggests that M2S is most likely to get dissolved into the penetrating slag by reacting with CaO from the slag to form low melting CMS. Ca (and other minor) ions from the slag could still penetrate into these areas by the process of diffusion. These results indicate that although liquid slag may not always fully penetrate into a solid refractory as a single liquid phase, some of its ions may penetrate more deeply forming liquid phases by reacting with the refractory components. Such behavior would be expected to decrease hot strength and corrosion/erosion resistance of the refractory.
Acidic slags containing high alumina.
When the acidic slag with a high amount of alumina comes in contact with the lining but the deskulling problem associated with tundish is not serious. Al2O3 often plays an ambiguous role: in an acidic slag it will act as a base affecting basicity in a CaO-SiO-Al2O3- MgO slag as follows:
Acidic slag =(CaO+MgO+Al2O3) / (SiO2)
In this case, Al2O3 will act as a fluxing component and will enhance slag fluidity and thus the penetration potential into the refractory material. At the same time, it can also reduce the MgO solubility in the slag by decreasing periclase dissolution. More importantly, the slag Al2O3 content may influence the corrosion mechanisms of magnesia-based refractories by direct or indirect penetration and dissolution of refractory in slag.
Zhang and Lee studied the corrosion of magnesia and predicted the critical condition of MgAl2O4 (MA) spinel formation through the CaO-SiO2-MgO-Al2O3 phase diagrams.
Herzog examined the effect of Al2O3 content (ranging from 20 to 40 wt%) on the corrosion rate of magnesia refractories by a rotary slag test, indicating that in basic slags, it decreases with increasing Al2O3 content because of the indirect dissolution. Clearly, the slag Al2O3 content has a great influence on the corrosion mechanisms of magnesia-based refractories. According to references and phase diagrams, the formation of the MA spinel layer would be expected for an alumina content higher than 30 wt%. with a dense layer between the refractory and slag after the reaction.
Dissolution of MgO in CaO-MgO- Al2O3-SiO2 (or CaO-Al2O3-SiO2) slag can be directly or indirectly dependent on whether an MA spinel layer forms at the MgO/slag interface. When the content of Al2O3 is higher than 30 wt% and C/S is less than 1.1, thermodynamically there is a strong tendency towards the formation of the MA phase such that this solid phase is spinel produced by the reaction between MgO and slag. The formation of spinel on the surfaces of MgO grains is described by the following reaction equation: MgO(s) + (Al2O3) = MgAl2O4(s)
where (Al2O3) means Al2O3 in slag. This means that slag reacted with MgO during the penetration process. The role of the spinel layer is its resistance to further slag penetration. The formation of a dense layer of spinel prevented the coating material from being penetrated by slag during its service. Since the melting point of MgAl2O4 spinel is 213 °C which is well above the casting temperature (1550 °C-1650 °C ), and being stable in high alumina slag, its formation reduces further corrosive wear of the lining.
In basic slags.
When the basic slag comes in contact with the lining, slag gets penetrated into the working lining via open pores and also MgO dissolves directly into the slag.
The existence of low melt phases of Mayenite, Akermanite, and calcium magnesium aluminium oxide was observed. Mayenite had come from carried over slag from the ladle because of poor operating procedures or poor sensitivity of slag detector.
Although some papers have reported that dusting would occur in the region in direct contact with the original slag in basic refractories, after corrosion in slag with high CaO/SiO2, due to the precipitation of 2CaO.SiO2 (C2S) which undergoes a polymorphic transformation involving a large volume change, no dusting and C2S phase were observed. The open pores provide easy pathways for the slag to initially enter into the working lining. Capillaries, such as open pores and micro-cracks, are the main channels of initial slag penetration into the refractory. The slag, as a single liquid phase, is driven into the refractory by the capillaries and the penetration L can be described by the following equations:
L=(rcos θ/2)1/2(tγ η) (for horizontal penetration)
L=2γcos θ/(rρg) (for vertical steady-state penetration)
(Where
L=penetration depth [m];
h=viscosity of slag [Pa*s=kg/m*s];
γ=slag surface tension [N/m];
t=contact time [s],
θ=contact angle between refractory and slag [°];
r=pore radius [m];
ρ=slag density [kg/m3];
g=gravitational constant [ms ])
After this initial penetration, the penetrated slag will either dissolve the surrounding refractory components, making new “open channels” and leading to further penetration, or the slag ions penetrate separately and more deeply via diffusion processes. The dissolution of refractory components in the slag not only makes new “open channels” (e.g. by connecting some closed pores) but also changes the local slag composition, and thus the slag viscosity and surface tension.
The formation of new “open channels” will accelerate the penetration, whereas the slag viscosity and surface tension change will have different effects on the penetration. If the slag viscosity increases or surface tension decreases due to the dissolution of refractory components, it will inhibit the penetration. However, if the dissolution of refractory components in the slag decreases the slag viscosity or increases the slag surface tension, then the slag penetration will be accelerated. The slag no longer penetrates the refractory as a single liquid phase, but its ions penetrate separately via the diffusion rate of each slag ion. The basic slag was not saturated with silicate phases and MgO, so after initial slag penetration into the working lining, the basic slag would dissolve M2S, MS, and MgO. These dissolutions result in further slag penetration. That slag reacted with silicate phases during the dissolution process to form Akermanite (2CaO.MgO.2SiO2) and calcium magnesium aluminium oxide. Dissolution at refractory/ slag interfaces is governed by chemical reaction (or solution) at the interface, or transport (or diffusion) of reacting species through the liquid boundary layer. In this case, the dissolution process may be directly controlled by a reaction. Dissolution of the refractory components, making new open channels (e.g. by connecting some closed pores) leads to further penetration. In this case, the slag as a liquid phase is driven into the refractory by the capillaries.
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