Technical Paper Robust deflocculant for steel ladle castables


Typically, high purity synthetic raw materials are used to formulate castables that have to resist very high temperatures and direct steel and slag contact for example in steel ladles. These high purity raw materials have a relatively lower impact on calcium aluminate hydration than the medium purity raw materials that are typically used in silica fume containing LCC. Nevertheless one aspect still strongly impacts the setting characteristics of a ladle castable and that’s the ambient temperature. Calcium aluminate hydration is a temperature dependent reaction, and it is not only the reaction kinetics that change but also the reaction paths. Low ambient temperature favors the precipitation of meta-stabile hydrates as CAH10


and C2 AH8 ambient temperature turns the reaction path towards the stable C3


while high AH6


. The


cumulated effect is that at low ambient temperature the setting takes much longer and the strength development is slower than at high temperature. Furthermore the generally retarding effect of polymer-based deflocculants amplifies the difference between low and high ambient temperature. Figure 5 shows an example for a castable deflocculated with a PCE (Peramin® AL200) that has been designed for high purity systems. This PCE has a different polymer structure than the PCE that has been used in the previous section for the silica fume containing castables.


A new active compound, REFPAC® 500 (RP500), has been developed that


combines a deflocculating organic polymer with a mineral component that triggers calcium aluminate hydration at low temperature without accelerating the hydration at medium and high ambient temperature. Figure 5 shows the time to reach an ultrasound speed of 4000 m/s which roughly corresponds to the time at which massive calcium aluminate precipitation has occurred (here also called setting time) and which typically creates enough strength to demold and handle the castable.


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surface. However, they cannot change the de-hydration temperature of the calcium aluminate hydrates which occurs mainly in the range between 150 and 350°C, in some cases even up to 550°C. As higher the de-hydration temperature (change from water bonded in solid phases into steam) as higher the risk of high pressure build up even in the presence of pore channels created by PP-fibers as the steam needs to be transported to the surface first to reduce pressure. When the transport distance to the surface is very long the steam pressure release remains difficult.


A new Active Compound, REFPAC® MIPORE 20 (MP20), has been


designed to create 3 effects: • Castable deflocculation for most types of castable systems


• Higher gas permeability already at lower temperatures than achievable with PP-fiber


• Modifying the hydration path of calcium aluminate binders and creates calcium aluminate hydrate gel which releases its water at lower temperature than the crystalline hydrates C2AH8 and C3AH6


Figure 6 and 7 show thermogravimetric measurements of an MCC (AS10) and an LCC (AM6) with temperature measurements in the center of a 10x10x10 cm test cube. When employing the MP20, water release occurs at significantly lower temperature which reduces the risk of high pressure build-up. The MCC with MP20 shows a similar TG-profile as a silica sol/gel bonded No-Cement-Castable (AS0) known for its ease of dry-out.


Figure 6: Macro-TGA of MCC and NCC


Figure 5: Time to reach an Ultrasound speed of 4000 m/s


While the efficiency of both deflocculation systems are very little impacted by the ambient temperature it is obvious that the hardening of the castable with the new active compound RP500 is much less temperature dependent. This is a key advantage for the organization of work in precast shops as well as castable installation in steel ladles as the time frame for each step in the process becomes more predictable despite unpredictable ambient temperatures.


Permeability enhancing and deflocculating active compound that facilitates castable dry-out


Dense castables are difficult to dry and explosive steam spalling can occur when the steam pressure in the pores exceeds the strength of the castable. Polymer fibers are typically added to castables to create small pore channels through which steam can more easily be transported to the


16


Figure 7: Macro-TG of LCC (300 K/h)


Shotcrete gelling agent for ladle castables While extremely efficient deflocculants have brought water demand of


ENGINEER THE REFRACTORIES January 2018 Issue


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