PURPOSE: To save energy during the calcination of a CaCO3 slurry formed in a caustification vessel and to improve the rate of caustification of chemicals recovered, by feeding powder consisting of MgCO3 and iron oxide to the burning section of a soda recovery boiler when waste liquor from a pulp manufacturing stage is burned in the boiler.

CONSTITUTION: Concd. alkali waste liquor from a pulp manufacturing stage is blown into the bottom burning section of a recovery boiler 10 as a mist from burners 12, and it is burned at a high temp. with primary heated air and secondary heated air fed from pipes 13, 14 to produce smelted matter. Powder contg. essentialy MgCO3 and iron oxide is spouted into the bottom burning section from feeding inlets 15, and part of MgO. Fe2O3 formed by calcination is captured with a dust collector 22 connected to the flue section of the boiler. It is introduced into a soda dissolving vessel 16 under the boiler through a pipe 23 to recover sodium, Mg and iron components. At the same time, the remaining MgO.Fe2O3 is allowed to flow in the vessel 16 together with the smelted matter flowing downward along the wall of the boiler, and the smelted matter is dissolved in water fed from a pipe 17 beforehand to recover sodium, Mg and iron components in accordance with a reaction represented by the reaction formula.