THE
REFRACTORIES ENGINEER
2.2 – CMA applied as aggregates (MagArmour) in monolithics
Wöhrmeyer et al. [18] investigated the effect of CMA-aggregate addition to alumina-spinel and alumina-magnesia castables. Formulations with up to 38% CMA as replacement for sintered alumina aggregates have been studied. The expected lightening effect (Fig. 11) was confirmed when partly replacing dense alumina aggregates by the porous CMA- aggregates that exhibit a porosity of approx. 30%. Interestingly, despite the higher porosity, the strength wasn’t impacted (Fig. 12). Corrosion resistance measured in a static cup test was equivalent to the reference A-MA castable (Fig. 13). With the increase of CMA-agg an increase in positive permanent linear change was observed (Fig 14), most likely linked to the higher total CaO-content in the castable and consequently a higher amount of CA6 formation.
Another monolithic application is the use of CMA-aggregates in dry- gunning repair mixes for steel ladles where it not only achieves sufficient penetration and corrosion resistance but also helps to make a good bonding between the used substrate refractory and the repair material. Service life improved significantly with the introduction of CMA-agg. Fig. 15 shows the ladle wall in operation. A postmortem sample (Fig. 16) of the dry-gunned layer taken from that ladle shows only little penetration of slag. The material contains 45% CMA-agg as replacement for dense sintered alumina aggregates.
Technical Paper
2.3 – CMA (MagArmour) applied as addition to carbon bonded bricks
Gehre and Aneziris [31] were the first who published in 2016 about the effect of CMA-addition to Magnesia-Carbon bricks. Although no clear differences were seen in thermomechanical properties with standard laboratory tests further investigations [32] revealed a significant difference of the materials in contact with ladle slag (Fig. 18). While the spinel-free MgO-C material was strongly attacked by slag, addition of MA-spinel resulted in an improvement. However, when adding CMA almost no slag attack was visible but slag sticked very well on the brick surface. Pagliosa et al. [20] were then the first who reported that in steel ladle trials with MgO-Al2O3-C bricks the formation of a slag coating was observed when CMA was added to the brick. Trials were conducted with bricks that were installed in the metal zone of a 205t steel ladle that were used to produce both Si- and Al-killed steel. For both types of steel this slag coating had been observed.
Fig. 15: Steel ladle metal zone with dry-gunning repair material based on CMA-aggregates
Fig. 18: MgO-C bricks after slag corrosion test [30]
The formation of a protective slag coating associated with an improved service life in steel ladles metal zone has then also been confirmed when MagArmour was added in small quantities to MgO-C [33], Al2O3-MgO-C, and MgO-CaO-C bricks for steel ladles ranging from 70 to 250 t. While this effect was systematically observed in the ladle metal zone for CMA- additions in the range of 3-8%, for bricks in the slag zone the content had to be adjusted to a lower content to achieve a positive effect. The effect in the slag zone was mainly visible in form of less vertical cracking (Fig. 19).
Hot face in contact with steel & slag Fig. 16: Post mortem sample of dry gunning material containing CMA-aggregates
Also as addition to dry-vibratables MagArmour CMA-aggregates have shown excellent performance. Fig. 17 shows a 25t foundry ladle with CMA-containing Alumina-Magnesia dry-vibratable material.
Especially in the metal zone, not only the formation of a protective coating has been observed but also a significant better resistance of the joints against wear-out (Fig. 20). Furthermore the slag coating protects the brick better against excessive carbon burnout. It allows to reduce the amount of metallic antioxidants as shown in Fig. 20 where the amount of antioxidants had been reduced from 2.5% (left, without MagAromour) to 1.5% when MagArmour was added (right). As a result both, lower brick costs and lower refractory consumption per ton of steel were achieved. Hypotheses for the formation mechanism of the protective layer were elaborated by Gao et al [34]. It results from a complex reaction mechanism as described in which on one hand slag can penetrate the porous CMA-grains and enrich the CMA with SiO2. At the same time, the micro-spinel can react with iron oxide which in consequence increases the slag viscosity.
2.4 – CMA-addition to functional products for continuous steel casting
Applied as an addition to functional products for steel production like spinel-carbon monoblock-stoppers as shown in Fig. 21 and MgO-C
Fig. 17: Foundry ladle with CMA-containing A-M dry-vibratable refractory July 2019 Issue 21
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