THE
REFRACTORIES ENGINEER
Technical Paper
Fig. 5: Castable matrix with conventional CAC + MA versus matrix with CMA-binder [17]
Fig. 6: Castable matrix with CMA-binder after firing at 1550°C
Alumina-Magnesia castables bonded with CMA72 in steel ladle bottoms and walls. In many cases new service life records were achieved. In the few cases where no improvement was achievable due to operational conditions, e.g. strong oxygen lance utilization, still often a better cost per ton of steel has been recorded. This is due to the more economical way to form micro-spinel via CMA than via pure sinter-spinel due to its lower formation temperature inside the C-M-A system.
Fig. 7: CMA-aggregate MagArmour material has been commercialized under the trade name MagArmour.
Like the CMA72-binder, this CMA-aggregate has high refractoriness (>1700°C) which makes it suitable for high demanding applications, both in bricks and monolithics. But unlike traditional dense high alumina-, spinel- or magnesia-aggregates the CMA-aggregate has a higher porosity and lower density. In consequence castables and bricks with a high content of CMA-agg can achieve lower densities, which means less material is needed for a given lining design [18, 19] and potentially a lower thermal conductivity. Beside this economical and sustainability aspect it can also mean that steel ladle capacity can be increased in cases where crane capacity is the limiting factor.
Added in small portions to carbon-bonded bricks for steel ladles the CMA-agg has a corrosion reducing effect as it forms on the brick surface a protective layer [20]. This reduces the burnout of carbon and offers a potential to reduce costs by reduction of the expensive anti-oxidants addition.
In the following chapters the main refractory application fields of CMA- binder and CMA-aggregates will be discussed in more details.
2 – CALCIUM MAGNESIUM ALUMINATE APPLICATIONS 2.1 – CMA applied as binder (CMA 72) for monolithics Steel ladle castables and precast shapes
Applied in a high alumina castable (A, A-MA, A-M) and in contact with slag from steel ladles, the fine and homogeneous network of micro-spinel particles blocks the slag from deep penetration into the castable matrix by uptake of Fe2O3 and increasing slag viscosity at the interface between slag and refractory material. That results in a better wear resistance as shown in dynamic corrosion tests (Fig. 8).
First industrial tests were done successfully with precast shapes like well blocks and injection lances for steel ladles, followed by gas purging plugs. Cölle et al. [21] reported about CMA72-containing precast products for ladle bottoms (impact pad). With the positive results of these trials, both from operational & safety point of view and with respect to the ultimate performance, the next step was to install Alumina-Spinel and also
CMA72 CAC + MA Fig. 8: Slag rotary kiln test with A-MA castables bonded with CMA 72 vs CAC+MA
Also the impact of CMA72-dosage on castable performance was investigated. It was found that at equal total CaO-content in the castable, the one with CMA develops higher strength than the reference material based on 70% alumina cement mixed with spinel powder [22]. The higher strength is positive for example for the abrasion resistance of CMA-bonded material. On the other hand it might impact the thermal shock resistance. To optimize for thermal shock resistance the CMA- content in the castable can be slightly lowered to reach the same strength as the reference material. This has no negative effect on the penetration and corrosion resistance since the negative effect of lower content of micro-spinel is offset by the lower total lime content and lower risk of crack formation. A high-temperature cycling test has been conducted that aims at simulating real thermal cycling conditions in steel ladles [23]. Here it could be demonstrated that a CMA-bonded Alumina-Magnesia castable showed less damage during thermal cycling than a CMA- bonded Alumina-Spinel castable. While the A-M castable contained 0.5% added silica fume to control the permanent expansion, the A-MA castable was silica-free to prevent shrinkage during firing. Braulio et al. [24] investigated the effect of the positive permanent linear change of
July 2019 Issue 19
www.irengineers.co.uk www.ireng.org
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