PURPOSE: To salvage the hard and large grain crystal slag to be utilized as a substitute material for a ballast and the like by a method wherein a slag cooling tank composed of a fluidized bed is connected to a slag tap port for the removal of a molten ash, then the molten ash is cooled gradually.

CONSTITUTION: A slag tap type cyclone burning furnace is composed of a cyclone burner 2, a furnace main body 1, a slag separating chamber 17 and a slag cooling tank 18 connected to the furnace main body 1 at just under thereof. The cyclone burner 2 forms a revolving fire flame, the furnace main body 1 catches an ash content by forming a molten slag film on a lining wall. The slag cooling tank 18 is installed at the lower part of the slag separating chamber 17. The slag cooling tank 18 keeps a grain group under floating and suspending condition by blowing a fluid upward from lower side of a grain group, and cools the molten ash. The tank is so-called a fluidized bed type cooling tank, introduces an exhaust gas at a temperature of 200W300°C, fluidizes a sand 24. Thus, the melted slag is cooled gradually by the fluidized sand and the fluid for fluidizing of the sand, accordingly, a crystal slag is formed.