PURPOSE: To suppress the scattering of dust and decrease the amount of dust attached to a heat-conduction pipe by controlling the properties of black liquor to be burnt, especially the viscosity and further the solid-content of the black liquor.

CONSTITUTION: Ash (dust) attached to a heat-conduction pipe and falling from the lower part of an evaporation water pipe 4 and an economizer 5 is collected in a dust hopper 6 and classified by a classifier 9. The dust having particle diameter of about >3mm is crushed with a crusher 10 into fine dust having particle diameter of about ≤3mm, stored in a dust tank 11 and supplied from a dust-feeder 12 to a mixing tank 7 in such a manner as to keep the viscosity or the solid concentration of the black liquor in the mixing tank 7 at respective specific level.