Crucibles made of recycled ceramic powders

The production of ceramics typically results in large quantities of ceramic waste from breakage, worn kiln furniture and crucibles. This waste contains valuable materials, such as SiC and Al₂O₃. The KMU-innovativ project ”SiAluPor“ (033RK070), funded by the Federal Ministry of Education and Research, investigated how far such waste products can be collected, sorted, processed and recycled into new crucibles.

The recovery and processing of the raw materials SiC and Al2O₃ is of great interest because approx. 7–8 MWh of electrical energy is required to produce one metric ton of new SiC – in a process generating approx. 6.6 t of CO₂.
The project partner, Spitzer Rohstoffhandelsgesellschaft mbH, has succeeded in identifying sources of ceramic waste products with high proportions of SiC and Al₂O₃. The collected waste products from various sources can be sorted and processed in a way that enables obtaining reproducible powder batches from recycled SiC and Al₂O₃. Because of their narrow grain size distribution, they are suitable for processing using common ceramic forming methods, such as pressure die casting (Fig. 1).

In addition to removing metallic impurities typically introduced in the grinding process, the team developed recipes for pressure die casting masses based on the recycled powder fractions in close cooperation with the project partner Rösler CeramInno. These casting masses are characterized by the highest achievable proportion of recycled material, with the coarsest possible grain size of 1–2 mm and stable ceramic bonding. Suitable compositions and processing parameters were developed for both clay-bonded SiC and clay-bonded Al₂O₃. These materials were used to produce firing crucibles as demonstration parts, which can be used, for example, for calcining ceramic powders.

In endurance tests (Figs. 2 and 3), it was shown that the recycled SiC crucibles did not show any signs of degradation at temperatures around 1180 °C during the test period. It was only at higher temperatures that a reaction with the fired goods occurred, which led to cracking in some of the crucibles. The Al₂O₃-based crucibles showed similar results.

The project was able to successfully demonstrate that reproducibly adjustable powder fractions can be prepared from SiC and Al₂O₃ waste products, which, after suitable cleaning, only show slightly increased levels of iron from grinding. These recycled powders are very well suited for the production of oxide-bonded SiC and Al₂O₃ crucibles thanks to their very good processing properties.