Verfahren und Anlage zum Gefriertrocknen

1,251,526. Freezing liquids; drying frozen materials. WESTERN ELECTRIC CO. Inc. 4 Dec., 1968 [6 Dec., 1967], No. 57508/68. Headings F4G and F4H. In a method of producing a particulate material, a solution of the material is introduced into the lowermost region of a body of refrigerant liquid so that the solution is frozen and rises towards the surface of the liquid. As shown, solution from a pressurized bath 2 passes through line 6 to an atomizer 5 which injects droplets of solution into a body of refrigerant contained in a chamber 1, and the frozen droplets collect on the surface of the refrigerant beneath a partially immersed apertured screen 18. Chamber 1 has a jacket 1 which may be supplied with a heat exchange gas, e.g. nitrogen, through line 15, and atomizer 5 has a heater 10, the heater and the admission of said gas being controlled so as to prevent the solution from coalescing in or immediately above the atomizer. After completion of freezing, a further screen 17 is rotated by 180 degrees to define a spherical enclosure with screen 18, the refrigerator is drained through line 8 and a drying gas, e.g. air or nitrogen, is introduced at atmospheric pressure through line 8 while the enclosure 17, 18 is rotated about axis 19. The drying gas temperature is initially - 40‹ C. and is increased to - 5‹ C. Residual moisture is removed from the frozen material by successively reducing the pressure in the chamber, e.g. to 1 mm. Hg, while maintaining the initial temperature, e.g. -5‹C., raising the temperature by 5‹ C., introducing a dry gas to restore atmospheric pressure and repeating the cycle from the new initial temperature of 0‹ C. In a modification (Fig. 3, not shown) freezing is carried out continuously in chamber 1 which is connected to a plurality of freeze drying chambers, the screens 17, 18 being provided within the latter chambers. The process is suitable for use in the preparation of ferrites and other oxidic materials.