Technical Paper


was in this case only in the range of 100 – 200 µm. At one small area of the interface, a slag infiltration of 600 µm was observed, most likely due to presence of a pore.


SEM investigation of the slag-refractory interface of reference sample MgO-C-REF is displayed in figure 5a. After oxidation of graphite and the carbon binder components the slag easily attacks the MgO coarse grains and matrix particles. Due to their bigger diameter and lower specific surface area, the coarse grains remained longer, whereas the fine matrix particles dissolved in the penetrated slag already at an early state of the corrosion process. Also the coarse grains are infiltrated by slag, which will result in their stepwise decomposition. Figure 5b presents an interspace between coarse grains filled with slag located approx. 200 µm beyond the slag line. The slag is obviously composed of three different compositions, which were analysed by EDX.


An overview of the composition at all EDX spots is given in table 3. The main dark grey area (spot 1) contains about 45 wt.% Al2


O3 CaO and 5 wt.% MgO. According to the Al2 O3 O3 , 20 wt.% SiO2 -CaO-MgO-SiO2 , 30 wt.% quaternary


system with 5 wt.% MgO, the detected composition at spot 1 lies in the field of stability of spinel and nearby melilite phase. The grey area at spot 2 consists of about 38 wt.% Al2


, 15 wt.% SiO2 , 7 wt.% MgO, 3 wt.% of


sulphur containing phase, and shows a depletion of CaO. This composition lies on the liquidus line of anorthite and CaO·Al2


O3 . In comparison to spot


2, the light grey spot 3 shows a low silica content and high amounts of TiO2 and sulphur. Compared to initial synthetic slag composition (table 3), an


increase of alumina and silica as well as depletion of CaO can be detected in the slag between the MgO grains. There is no added Al2


O3 in sample MgO-C-REF, which would explain the increase in Al2 portion


source available O3


by dissolution. The discrepancy of composition of detected EDX-spots in comparison to synthetic slag implies an interaction of MgO-C-REF and its impurities especially with CaO of the slag. However, according to MgO- CaO binary system, in the range of 1600°C MgO will not be dissolved by CaO but forms a solid solution. This was also proven by higher CaO content nearby slag-refractory interface compared to CaO content detected in slag far away from the MgO-C-REF material. The interaction with CaO results in a eutectic slag with lower melting point and lower viscosity, which will easily penetrate theMgO-C-REF material. Al2 minor role on the corrosion, and TiO2


O3


and sulphur seem to play only a does not take part at the corrosion


reaction, but these constituents enrich in the slag between the MgO grains. Furthermore, spinel, melilite, or anorthite are able to crystalize in the slag during cooling.


Figure 6: Slag-refractory interface of sample MgO-C-MA at 30-times magnification (a) and grain filled with slag at 200-time magnification with marks of EDX spot analysis


Figure 7 displays two slag-refractory interfaces of sample MgO-C-CMA. There are two different zones detectable: (a) an infiltration zone with coarse and fine particles surrounded by slag, and (b) the initial refractory material. In contrast to sample MgO-C-MA, the transition from slag to infiltration zone is very sharp, which accompanies the findings of digital microscope investigations (figure 4). Hence, only small infiltration and dissolution inside the refractory matrix takes place and less of the solved material can potentially transported away from the refractory matrix into the slag as the reaction takes mainly place at the brick surface only.


www.ireng.org dark grey area (spot 4) consists of about 40 wt.% Al2O3 O3 . , 20 wt.% SiO2 , 30


wt.% CaO, 5 wt.% MgO and some sulphur. Like EDX spot 2, this composition lies on the liquidus line of anorthite and CaO·Al2


Composition at spot 5 is rich in CaO (50 wt.%) and SiO2 has a very low amount of Al2 TiO2


O3 O3 (35 wt.%), and (5 wt.%) and MgO (2 wt.-%). In addition,


and sulphur is present next to rankinite. Light grey spot 6 correlates to a Ca- and S-rich phase. Compared to initial synthetic slag, CaO content is nearly constant, there is an increase in SiO2 of Al2


content, and a depletion will additionally react with Al2


. Hence, in contrast to sample MgO-C-REF, sample MgO-C-MA O3


O3 . This will intensify the basic character of


the residual slag, which results in higher melting point, higher viscosity, and reduced slag penetration. EDX investigations of the slag far away from MgO-C-MA material verify these findings, as lower Al2


content


but higher CaO content were detected compared to slag far away from sample MgO-C-REF. The slag inside sample MgO-C-MA has the potential of crystallisation of rankinite as well as anorthite and CA. In addition, there is a crystallisation of most likely CaS.


Figure


5:


Slag-refractory


interface of sample MgO-C-REF at


30-times


magnification (a) and space between grains filled with slag at 1000-times magnification with marks of EDX spot analysis


Investigation of sample MgO-C-MA revealed a slag-refractory interface composed of a mixture of coarse grains and fine particles surrounded by slag (figure 6a). In contrast to sample MgO-C-REF the fine particles will not be washed out by slag so fast during the corrosion test. As it was already observable at sample MgO-C-REF, inside the coarse grains of MgO-C-MA slag is detectable with three different compositions (figure 6b). The main


18


Figure 7: Slag-refractory interface of sample MgO-C-CMA at 30-times magnification with marks of EDX spot analysis


The infiltration zone (spot 8) is composed of 48 wt.% Al2 O3 , 10 wt.% SiO2 ,


37 wt.% CaO, and 4 wt.% MgO, which lies in the stability field of spinel. Like observable at sample MgO-C-MA, sample MgO-C-CMA with CMA-grains comprising fine spinel particles and some CA and CA2 react with Al2


, will additionally O3 , which will result in higher melting point, higher viscosity,


and reduced slag penetration. EDX investigations of the slag far away from sample MgO-C-CMA revealed nearly similar composition like those of sample MgO-C-MA. The residual penetrating slag inside one of the bigger


ENGINEER THE REFRACTORIES November 2018 Issue


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