THE


REFRACTORIES ENGINEER


A Review of Calcium Magnesium Aluminate Applications in Refractory Bricks &


Monolithics C. Wöhrmeyer, J. Gao, G. Bhattacharya, C. Parr, P. Edwards. Imerys Aluminates, Paris, France


ABSTRACT


This paper reviews the application of Calcium Magnesium Aluminate in refractory monolithics and bricks. Launched in 2011 as a new refractory binder (CMA72) for monolithics, its success is based on the engineered powder particles containing both, micro-spinel and calcium aluminate phases inside each particle. This CMA-binder has since then proven its benefit in a large range of monolithic and precast applications, e.g. steel ladle bottoms, walls, impact pads, well blocks, purging plugs, injection lances and EAF-delta pieces. The main benefits are superior penetration and corrosion resistance. More recently CMA has also proven its benefit as refractory aggregate, commercialized with the trade name MagArmour. This paper reviews the effects that can be achieved by an addition to bricks for steel ladles e.g. MgO-C, Al2O3-MgO-C, MgO-Al2O3-C, MgO- CaO-C, Al2O3-MgAl2O4, and functional refractory products like mono- bloc stopper, ladle shrouds or submerged entry nozzles (SEN) for the continuous steel casting process. In Magnesia-Spinel bricks for rotary kilns for Ordinary Portland Cement (OPC) production, an addition of CMA can improve the thermo-elastic/plastic properties and the formation of the accretion on the brick surface that protect the brick against strong wear. CMA-aggregates applied in Alumina-Spinel based monolithics, one of the benefits is a lower material requirement for a given dimension of a ladle wear lining or permanent lining. Also in dry-vibratables for foundry applications as well as in A-MA dry-gunning mixes for ladle repairs CMA- aggregates are successfully in use.


Fig. 1: CaO-MgO-Al2O3 phase diagram at 1600°C and 1725°C


1 – INTRODUCTION & PRODUCTS Phase diagram


Rankin and Merwin [1] were one of the first researchers who published in 1916 about the ternary system CaO-Al2O3-MgO (C=CaO, A=Al2O3, M= MgO). In their investigation they concentrated on phases with low melting temperature close to the field C3A-C12A7. Many other publications followed and in 1995 Hallstadt [2] made a new thermodynamic assessment of the whole CaO-MgO-Al2O3 system while Göbbels et al. [3] and Iyi et al. [4] looked specifically at the Al-rich part of this particular system. The thermodynamic simulation software FactSage®


suggests that


in the system CaO-MgO-Al2O3 at 1400°C and chemical composition within the blue marked triangle in Fig 1, the three solid phases MA, CA, and CA2 are in thermodynamic equilibrium.


Sintering trials with alumina, lime, and magnesia powder confirm that only these three phases occur. When moving to very high temperatures, e.g. 1725°C then compositions that fall into the blue marked field (Fig. 1) result in an equilibrium between solid MA-spinel, evtl. some solid CA2 and a CaO-Al2O3-rich liquid. Using the PCE method (pyrometric cone equivalent) for determining the refractoriness of a MA-CA-CA2 assemblage that had a composition of 70% Al2O3, 20% MgO, and 10% CaO resulted in a refractoriness (pyrometric cone fall temperature) of 1750-1770°C, about 150°C higher than a calcium aluminate cement with 70% Al2O3 and 30% CaO [5].


On the contrary, by moving in the Al2O3-MgO-CaO system into a more lime rich area and adding small amounts of silica, iron- and titanium


oxide, a very low melting temperature can be achieved. A composition with 40.5% Al2O3, 33.5% CaO, 12.8% MgO, 4.7% SiO2, 2.3% Fe2O3, 2.1% TiO2 has for example a melting temperature <1345°C [6].


CMA-flux for ladle slags


A composition with a low melting temperature has its advantage as fluxing agent for ladle slags [7, 8] as shown in Fig. 2. Also the effect on corrosion of Magnesia-Carbon bricks in contact with CMA-fluxed slag was investigated and it was found that it attacked the refractory material less than a slag that uses a mix of calcium aluminates and magnesia as fluxing agent.


Technical Paper


Fig. 2: Rapid melting and steel covering with CMA-flux Optimet® CMA-binder for refractory castables


De Aza et al. [9, 10] investigated more specifically the field of crystallization of spinel in the subsystem MgAl2O4-CaAl4O7-CaO-MgO


RG at 1600°C [7]


July 2019 Issue


17


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