Appts. has at least two chambers, the particles to be treated being fed to the first chamber and subsequently being transported through one or more succeeding chambers to a discharge, and each chamber having a bottom plate with openings through which a stream of gas is blown to maintain the particle bed in that chamber in a fluidised state and to subject it to a required treatment, and also having a gas outlet at the top. Additionally the appts. has (a) a connecting passage at the discharge side of one chamber which can be closed by a valve connects more or less smoothly to the bottom of the chamber so that particles discharge takes place exclusively as a result of the difference in level between adjacent chambers, (b) the stop valves in these passages are connected to operating devices so that at any given instant at least one valve is closed, in order to prevent the continuous discharge of particles while the particles are retained in the chamber or chambers succeeding the first chamber for a predetermined residence time since the inlet and outlet valves of the (these) chamber(s) is kept completely closed during this time.
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