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Technical Paper


wear in the joints (figure 8). Two of the three brick layers that contained the CMA-addition showed a uniform slag coating while it seems that the slag coating on the upper CMA-containing brick layer broke off, probably during cooling of the ladle.


3-3: Results from a 120t ladle


A MgO-C brick with addition of 5% CMA (0-3 mm) as replacement for fused magnesia (97.5% MgO, C/S>2, 0-3 mm) was produced and a full ladle metal zone was lined with this new brick material that contained 10% graphite, 2.5% antioxidants, and bonded with 3% resin. The plant produced the steel grades SPHC11, SPHC21, SS400-2, SS400Cr-1, and 65Mn. It had a refining ratio of 95% with ladle furnace (LF) and 5% with degasser (RH). Refining took 35-40min for LF and 40min for RH, with the average steel holding time of 100min. The converter was tapped around 1620-1650°C. After 25 cycles the lining was in good condition and had only locally some rough surfaces. No slag coating was observed at this point. After 50 cycles the brick surface in the metal zone was flat and smooth, and had now visibly a slag coating. After 74 cycles (new slag line bricks) the metal zone had a residual thickness of 170mm against the original of 200mm and the slag coating on the bricks surface could still be observed. After 114 cycles the thickness was 140mm. Reaching 138 cycles the ladle went offline. Slag coating was observed on the hot face of each brick (figure 9b). When replacing the bricks it was also observed that they were somewhat sticking together. In comparison, the lining with MgO-C bricks without CMA addition lasts typically 132 heats in average and shows significantly higher corrosion with strong wear in the joints and no slag coating (figure 9a).


The slag coating on the CMA-containing bricks has been further investigated. As can be seen in figure 10 the slag coating is strongly connected to the brick surface. Behind that about 1 mm thick slag layer a small zone (1-3 mm) of de-carburized material was found and behind that the material looked macroscopically almost unaffected with clear presence of graphite. The protective slag layer itself consists mainly of light-grey calcium-silicate (close to C2S composition), bright-white calcium-ferritic phase (close to C2F) and as major crystalline phase (dark-grey) iron oxide-enriched MgO grains (Figure 11, Table 5). The coating itself appears to be quite dense and obviously protects the carbon in the brick from oxidation. Since it was observed that the bricks were sticking together it is assumed that they create in-situ a small amount of high viscous liquid phase resulting in a joint sealing effect that minimizes the corrosion in the joints and might at the same time reduce thermomechanical stresses.


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Figure 10: Sample of CMA containing MgO-C brick after 138 heats in a 120t steel ladle (metal zone)


Phase


Dark grey Light grey


Bright white


MgO Al2 85.5


- 0.8 O3 -


3.3 6.5


SiO2 -


33.2 3.3


CaO -


61.9 41.5


TiO2 -


- 4.9 Table 5: Chemical composition of main phases in the slag coating 3-4: Results from a 70t ladle


Building on the positive results described above, the effectiveness of the coating against oxidation of carbon was further studied in the metal zone of a 70t steel ladle. While the CMA-free reference material contains 2.5% anti-oxidants, it has been reduced to 1.5% (balanced by addition of 1% fused MgO powder) when 5% CMA-aggregates were added as replacement for 5% fused magnesia of similar grain sizes. Despite the reduction of anti- oxidants, the CMA-containing composition lasted for a greater number of cycles (98, with LF 88) than the standard (93, with LF 85). The lining without CMA shows already after 47 heats during an intermediate repair significant wear especially in the brick joints (figure 12a). For the CMA-containing lining


the intermediate repair happened after


55 heats and still showed a much smoother and homogenous surface without strong joints defects (figure 12b). A protective coating is clearly visible on the brick lining. Thus, CMA addition was found to be more cost effective due to direct cost reduction through expensive anti-oxidants minimization and indirectly for the performance enhancement.


3-5: Other industrial trials


Multiple other industrial trials are running to further explore the effect of CMA in different application areas,


refractory


both, carbon-bonded and carbon-free bricks


and monolithics. First results


Figure 9: 120 t steel ladle linings after operation: a) MgO-C brick lining without CMA after 132 cycles; b) MgO-C brick lining with CMA aggregate after 138 cycles


20 ENGINEER THE REFRACTORIES


indicate for example that CMA could have also its benefit in alumina-carbon products for flow control applications, but also in monolithic ladle repair mixes. Amongst others, also investigations are running that explore the effect of CMA additions to Magnesia-spinel bricks for OPC kilns.


November 2018 Issue Fe2 O3


15.5 1.6


43.1


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