The manufacturing of refractories with larger dimensions by direct freeze-foaming technique
Herstellung größerformatiger Feuerfestmaterialien über die Gefrier-Direktschäumung
Refractories are a key component for cost effective and energetic sustainable processes. For instance, most important for the economic operation of industrial furnaces is the degree of thermal insulation. Refractories with a low bulk density and therefore low thermal conductivity are highly preferable. Thus, depending on the operation temperature thermal characteristics have to be tuned. There are a lot of possible porosity adjusting methods, e.g. the burnout of sawdust or petrol coke, gas injection approaches, inflation processes or the use of volatile additives and fiber structures. Now, the freeze-foaming as direct foaming technique shall be introduced as an environmentally friendly and cost effective way to refractories. In the present work an aqueous ceramic mullite suspension is f oamed within a few minutes by mere pressure reduction in a freeze drying device. During decreasing pressure the vapour pressure acts as the driving force for foaming. The foam structure is suddenly frozen when the suspension temperature, related to the vacuum pressure, reaches the liquid-solid equilibrium line. The porous structure is then dried thoroughly by sublimating the frozen water. The resulting large-sized foams (length x width x height, 235 x 114 x 70 mm in the green state) exhibit open porosity and dense struts. The open porosity provides an equal thermal distribution and the dense struts improve the mechanical properties. Just 5-10 wt.-% organic additives, required for stabile foaming, minimize the effect of crack formation during the sintering step and provide an environmentall y friendly and effective processing route to the final product. The pore morphology is being determined by X-ray computed tomographic images and mercury porosimetry. The measurement of the thermal conductivity has been carried out to evaluate a first benefit of the freeze-foaming process as a promising approach for manufacturing refractories.