Abstract: A method of operating a multiple hearth furnace with a plurality of vertically aligned hearth floors includes feeding a first material onto the uppermost hearth floor and moving it over the hearth floor before it falls through a drop hole onto the next lower hearth floor. The first material is processed in this way from hearth floor to hearth floor to the lowermost hearth floor. A second material is fed onto one of the hearth floors to be mixed into the first material. This second material is moved separately from the first material in a separate annular zone of the hearth floor onto which it is fed before it is mixed into the first material, providing an efficient thermal preconditioning of the second material prior to mixing it with the first material without requiring any supplemental equipment.

Abstract: A method of operating multiple hearth furnace (10) with a plurality of vertically aligned hearth floors (361, 362, 363, 364, 365), comprises in particular following steps. A first material is fed onto the uppermost hearth floor and moved over this uppermost hearth floor (361) before it falls through a drop hole (40) onto the next lower hearth floor. This first material is processed in this way from hearth floor to hearth floor down to the lowermost hearth floor (366). A second material is fed onto one of the hearth floors to be mixed into the first material. This second material (64) is moved separately from the first material in a separate annular zone (68) of the hearth floor onto which it is fed before it is mixed into the first material. The method allows to provide an efficient thermal preconditioning of the second material prior to mixing it into the first material without requiring any supplementary equipment therefore.