Abstract: A tank having a hinged top and sealing means is provided with a series of heaters mounted on the inside of the front, rear and side walls and also the bottom of the tank. A rack containing fiberboards placed in upright, spaced position is set on the bottom heaters and a solution of solvent and asphalt or the like is pumped into the tank, then removed. The top of the tank is sealed and a heating medium is pumped through the heaters, while a vacuum pump is started to produce a vacuum inside the tank and suction exerted for evaporating solvent. One or two condensers, between the tank and the vacuum pump, condense the solvent vapors for collection and reuse. With one condenser, an excess condensate tank is connected to the lower condensate section of the condenser. A valved pipe connecting with the pipe between a lower condensate section of the condenser and the vacuum pump is utilized to bleed air into the tank after treatment and may be used to control the initial vacuum applied.