Indirect calciners are very similar to indirect heat dryers: the main difference is that indirect calciners operate at very high temperature and sometimes can work in a special or inert gaseous atmosphere.
Indirect calciner purpose is not limited to water evaporation, but depending on the desired process, chemical reaction (ie thermal dissociation, oxidation or reduction) can occur inside the calciner so that final product can differ from inlet product.
The heat transfer is mainly by radiation, while a small rate is given by conduction.
Indirect calciner consists of a rotating cylinder made with special alloys selected to resist to high temperature and eventually also to corrosion. Internally, depending on the processed product, some flights of different shape can be welded to cylinder wall to facilitate product movement and agitation inside the calciner.
The calciner is enclosed in a refractory lined steel furnace along its active length.
The wall of the calciner is indirectly heated by a series of tangentially-fired burners, located along one side of the furnace casing, so arranged as to give the required heat demand.
Slope of cylinder can be adjusted as well as its rotation speed in order to control retention time of product inside the calciner.
Products that can be processed by indirect calciners:
- Silica gel
- Powdered products
- Hazardous products