Abstract: A solution containing atactic polypropylene by-produced during the preparation of a propylene-containing polymer is passed through a tubular, elongated heating zone having a pressure difference of at least 3 kg/cm.sup.2 between its inlet and outlet so that the calculated flow velocity at the outlet is at least 40 m/sec, whereby sticky fine particles of molten atactic polypropylene substantially free of the solvent are formed in suspension in the vaporized solvent. Then, the sticky fine particles are separated from the vaporized solvent.

Abstract: A polymer slurry composed of solid polymer particles and a diluent such as an inert hydrocarbon and or an inert halogenated hydrocrbon, kept at a sufficient super-atmospheric pressure to maintain the diluent in the liquid state, is continuously introduced into a heating tube divided into a first and a second heating zone, the diameter of the second heating zone being larger than that of the first heating zone. The flow rate of the slurry passing through the first heating zone is controlled by regulating the amount of heat supplied to the first heating zone. Substantially all of the vaporized diluent is then introduced into a separation zone and separated from the solid polymer particles.