PURPOSE: To enable the efficient mixing and agitation of Glauber's salt dust and black liquor by disposing a grinding device in the supply path for Glauber's salt connecting the hopper of a recovery boiler and a mixing tank and grinding the Glauber's salt dust.

CONSTITUTION: A grinding device 16 consisting of a pair of rolls 17a, 17b is provided in a pipeline 15 for supplying a lumped material (e.g.; Glauber's salt dust) to a mixing tank 12 and after the lumped material is ground by the device 16, the ground material is supplied to the tank 12, in a device for mixing and agitating the lump material and a prescribed soln. (e.g.; black liquor) in the tank 12. If such device is used, the lump material and the soln. are satisfactorily mixed, and problems such as clogging of the screen plate 13 in the tank 12 or the like are eliminated. If the rolls of the device 16 are constituted of a flexible material such as rigid rubber or the like, the sealing between the recovery boiler side and the mixing tank is effectively accomplished. It is possible to supply the lump material at a desired rate by controlling the rotating speed of the rolls.