This type of ceramic, which can contain more than 99% alumina, builds in porosity to parts that make them ideal for supporting catalysts or as filters. After preparation, usually by spray drying, the material is shaped then fired at temperatures in excess of 1500°C. Shrinkage is carefully controlled and the part exhibits the same characteristics as a densified ceramic: high mechanical strength and hardness with resistance to wear and chemical corrosion and excellent thermal and electrical resistance properties. But, careful control of the alumina particle size distribution enables porosity to be created. The pore size and volume maximizing the available surface area for a given volume. Also often important for these ceramics is consistent control of the alumina chemistry as impurities can interfere with catalyst reactions and also need to be minimized for food and medical applications. Examples of porous ceramic applications:
- Bed filtration
- Foam filters for liquid metal
- Ultrafiltation membranes for the food and medicine industry
- Catalyst support
- Catalyst exhaust gas filters and diesel particulate trap filters for the automotive industry
- Catalyst carriers e.g. petrochemicals and environmental industries
Ground calcined alumina
These aluminas are ground in continuous ball-mills or air-jet mills to median particle sizes (d50) 3, 4, 5 or 6 µm.
Hard calcined alumina
These aluminas have low specific surface area (BET < 1m²/g) and are almost entirely alpha alumina. Standard soda content is 3000-4000 ppm and intermediate soda content is 2000-3000 ppm. Hard calcined aluminas are produced in rotary kilns.
Low soda alumina
This alumina has very low soda content: between 300 and 500ppm. Carefully controlling calcination using the proprietary Reynolds process also allows a wide range of primary crystal size to be offered.