Dehydration

Dehydration takes place in silicate ceramics when the shards are heated to higher temperatures - between 500°C and 700°C, for example. During this process, the water of crystallisation contained in the clay raw materials is removed from the shards (dehydration). For example, kaolinite is transformed into metakaolin by dehydration.

If the shards are heated up too quickly, the water vapour released inside the shards cannot escape quickly enough. This leads to an increase in pressure, which can destroy the cullet. In the same temperature range at approx. 573°C, the transformation of the quartz raw materials contained in the cullet also takes place. In the process, the quartz transforms into its high-temperature modification and expands by approx. 0.8% (quartz jump). The increase in volume generates mechanical stresses that can also lead to damage to the component.

The optimisation of the process parameters during dewatering is done by in-situ measurements of the weight changes and, if necessary, the strains and subsequent FE simulation. The verification of the optimised process parameters can then be carried out on large components with the help of the ThermoOptic Measuring System TOM_gas.

Similar optimisation strategies as for dewatering are also used if the raw materials contain carbonates. These decompose in the temperature range around 800°C to 900°C, whereby the gases released can also lead to excess pressure inside the cullet.