Getting the landfill space for the municipal solid waste incinerator ash is the very important issue. In order to reduce the volume of the municipal solid waste incinerator ash, ash melting furnaces were introduced in Japan late in 1990's. The molten ash is rapidly cooled by being put into water, resulting in shrunken vitrified slag. Such furnaces, however, require high temperatures such as 1300-1500°C, consuming high energy. Molten ash quickly erodes refractory of the furnace. These lead to a high cost of the maintenance. This study developed a new method, in which a mixture of incinerated ash and molten slag was heated under small pressures, resulting in a decrease in its volume. The molten slag itself softened between 800°C and 900°C. The incinerator ash had a void of about 65%. It was predicted that 2 times mass of molten slag can be pushed into the void of the incinerator ash. When a mixture of the incinerator ash and the molten slag with the ratio of 1 : 2 was heated at 900°C under a pressure of 0.52 MPa, it was shrunken to the ideal volume. At a lower temperature, 800°C, it could be shrunken to almost the ideal volume by an increased pressure of 2.3 MPa.
