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ladle castables to very low levels it has become a challenge to flocculate them rapidly when they need to be installed by means of shotcreting. Typically only very high pH-aqueous solutions are able to stiffen those castables within a very short period of time. A new development has resulted in a solution where a very efficient deflocculant and the flocculant (gelling agent) build components of a system that are well adapted to each other. The result of this development is a gelling agent that no longer requires a very high pH which improves the health and safety at job sites significantly.
BINDER DEVELOPMENTS
Binder with increased stability against aging in a castable dry-mix
Low cement (LCC) or deflocculated castable technology has been widely adopted in the past decades driven by the improved thermomechanical properties and corrosion resistance that these castables display in service. These improvements have been made possible by the increased sophistication of refractory raw materials and formulations, especially through the introduction of filler and additive systems into the formulations while cement content could be reduced. However, a negative side effect is frequently observed with LCC’s as they tend to be more sensitive to aging during dry-mix storage. This can result in unpredictable castable
Technical Paper
setting times. Humidity can either reach the dry-mix by diffusion through the packaging (“external water”) or can be packed with the raw materials into the bag (“internal water”) as some raw materials like silica fume can contain a significant amount of humidity. To provide a better stability of the hydraulic calcium aluminate inside a castable dry-mix, a binder with a protective surface, Secar®
712, has been developed [5] . It makes castable
setting and early strength development more predictable as shown in Figure 8 and 9.
Figure 10: Samples after rotary slag corrosion test
Binder that increases performance of steel ladle castables
Important parameters for the performance of ladle castables
are and spinel distribution inside their magnesium
aluminate (MA) spinel content, spinel grain size,
the Figure 8: Working time as function of dry-mix
castable to build-in an efficient protection mechanism against slag penetration, structural spalling, and corrosion. Even better performance can be achieved by creating very fine spinel in-situ through the reaction of magnesia powder with alumina during the heat-up of the ladle. However while taking advantage of the in-situ formation of very fine spinel one needs to put special measures in place to minimize risk related to hydration of magnesia which could potentially negatively influence the casting properties. Magnesia hydration during the dry-out can eventually create cracks. And finally, expansion during spinel formation on the hot side of the castable needs to be well controlled to prevent high stresses in the ladle wall.
A new calcium magnesium aluminate binder, CMA 72, has been developed that consists mainly of microcrystalline MA-spinel phases and hydraulic It allows
calcium aluminate phases. employing homogeneously a
high amount of very small spinel particles into the castable with spinel particle sizes similar to those that are formed in-situ in alumina-magnesia castables while avoiding
Figure 9: Early strength development as function of storage time dry-mix storage time January 2018 Issue ENGINEER THE REFRACTORIES 17
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