PURPOSE: To improve the quality of tar remarkably, and to eliminate process of discharging, ammonic water together with tar from the system, by spraying a colloidal mixture of ammonia water, tar, and powdered coal into hot gas before spraying ammonia water, thereby coagulating the colloid.

CONSTITUTION: A colloidal mixture consisting of ammonia water, tar and powdered coal, produced in a decanter 7 is sprayed into a bent pipe 3 by a pump 15 through a line 16 and a nozzle 17. The amount of spraying is 0.01W0.3 m3 per 1 ton of coal. The sprayed mixture is brought into contact with hot gas of a temperature of 600W800°C and evaporated or partly decomposed, and is coagulated. The hot gas is cooled to the final state by spraying 3W10 m3 per 1 ton of coal of cooling ammonia water through a nozzle 4. When the above colloidal mixture is diluted with ammonia water which forms the upper layer of the decanter 7, the blockage of the nozzle 17 is prevented, and the spraying can be continued for a long period.