The manufacturing of lightweight refractories by direct freeze foaming technique

Refractories are a key component for cost effective and energetic sustainable processes. Depending on the operation temperature of industrial furnace thermal characteristics of porous refractory linings have to be adjusted. There are a lot of possible porosity adjusting methods, e.g. placeholder, replica and gas injection/development approaches. Now, the freeze-foaming as direct foaming technique shall be introduced as an environmentally friendly way to refractories. In this presented work an aqueous ceramic mullite suspension is foamed within minutes just by the reduction of the ambient pressure in a freeze drying device. The foam structure suddenly freezes when the suspension temperature, related to the vacuum pressure, reaches the liquid-solid equilibrium line (p, T-diagram o f water). The porous structure is then dried by sublimating the frozen water. The resulting bricklike lightweight refractories exhibit a high amount of open porosity and dense struts. Just 5-10 mass-% organic additives, required for a stable foaming, minimize the effect of crack formation during the sintering step and provide an environmentally friendly processing route to the final product. The pore morphology is being determined by X-ray computed tomographic images and mercury porosimetry. Measurements of the thermal conductivity, compressive strength and creep have been carried out to evaluate the freeze-foaming process as a promising approach for manufacturing refractories.