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After having passed the casting and the drying process steps, semi-finished sanitary ware can still show locally different degrees of humidity. Locally increased residual moisture in the ceramic material will inevitably cause cracking during calcination or even before. Most of these spots containing an increased degree of moisture are located in the bulk of the material. Due to not only being absorbed on the surface of many materials but mainly in their core, microwave drying is a most feasible means for selective volumetric removal of humidity. Apart from the development of a reliable procedure for detection of invisible cracking in the core of the ceramic material, the work in DRY-CONTROL project aimed at engineering a microwave system for focused and tailored heating of these spots in the ceramics material containing an increased degree of moisture. Having undergone this microwave drying process, the semi-finished sanitary ware had to show a homogeneous and low degree of humidity of approximately 0.3% which is most advantageous for the subsequent calcination process. Muegge developed a compact and lightweight iso-launcher system based on a coaxial isolator with optimized distribution of the electrical field strength. This iso-launcher system even allowed for dedicated drying of spots in the ceramic material which are difficult to access. For exact positioning of the iso-launcher system towards the surface of the semi-finished sanitary ware, a flexible waveguide system based on contactless and axially symmetric power transmission lines was developed for microwave power values up to 10 kW. In a different approach, a drying chamber with fixed microwave antennas was designed and constructed. The semi-finished sanitary product inside the drying chamber was moved by a robotic positioning system accurately towards the fixed microwave antennas. Dosing of the microwave power was due to inline measurement of the surface temperature of the material to be dried by means of an infrared sensor. Due to increased microwave absorption by the material showing the highest degree of humidity compared with the surrounding material being in a less humid state, the microwave drying of the semi-finished sanitary product proved to produce the homogeneous distribution of the low degree of humidity of approximately 0.3% to be achieved. For microwave shielding, Muegge designed several microwave traps for the different access and feedthrough sites of the drying chamber. The most feasible microwave traps with regard to being resistant towards contamination by ceramic residues were based on three λ /4 traps in combination with an absorber strap. Final tests showed the resource efficiency of the microwave drying process: use of 1.6 kWh of electrical power for homogeneous drving will avoid wastage of 200 kWh of eneray for production of sanitary ware in case of cracking.
Entwicklung einer ressourceneffizienten Trocknungstechnologie für keramische Produkte (Dry-Control), TP4: Mikrowellensystemtechnik. Schlussbericht. Berichtszeitraum: 01.07.2009 - 30.06.2013