ENTRAINMENT OF NON-VOLATILE SOLIDS IN SUBLIMATION AT REDUCED PRESSURE

A fundamental analysis of some of the factors affecting entrainment in sublimation is proposed, and the predictions are verified experimentally by investigation of the separation of particulate silicon carbide from ammonium chloride. It was found that for the impurity particle size distribution studied increased system pressure is effective in reducing entrainment at a given mass rate of sublimation. The separation is also improved at a given pressure by sublimation at lower rate. The sublimation rate was found to be a function of temperature and pressure. The temperature necessary to initiate sublimation is approximately the temperature at which the vapor pressure of ammonium chloride is equal to the system pressure. Above this initiation temperature the sublimation rate increases linearly with temperature. Thorium tetrachloride of 200 ppm oxygen content is prepared by double sublimation at controlled pressure. In the second sublimation the vapor is passed through turnings of thorium metal to remove volatile metal chloride impurities. The purity of the thorrum tetrachloride sublimed by this procedure was found to be much superior to that sublimed at low pressure under a dynamic vacuum. (auth)