Residence time investigation of a multiple hearth kiln using mineral tracers

Kaolin is used in many applications (e.g. paint, plastic and rubber) due to its unique properties, including fine particle size, brightness and chemical inertness. Properties can be improved by calcination, heating the kaolin to high temperatures inducing physico-chemical transformations. If calcination time is excessive, kaolin becomes harder and abrasive, due to mullite formation. This is detrimental to product quality; causing processing problems and machinery damage. If calcination time is too short, the kaolin is not fully calcined, becoming highly reactive, an issue for pharmaceutical applications. By controlling the exposure time to high temperature, such negative effects are minimised. Multiple hearth calciners are often used for this transformation, but their operating conditions and configuration make it difficult to determine kaolin residence time. To improve product consistency, trials were performed to measure the residence time distribution of an industrial furnace used to calcine kaolin. Titania, TiO2, and talc, Mg3Si4O10(OH)2, were used to individually dope the kaolin, Al2Si2O5(OH)4, and the concentration of these mineral tracers was detected using X-ray fluorescence. Talc was the preferred mineral tracer, with a mean residence time of 42 min under standard operating conditions, with a throughput of 5.3 tonnes h−1 of 0 wt.% moisture feed.