GB664376A - Improvements in or relating to the continuous crystallisation of solids from their solutions - Google Patents

Abstract

In the crystallization of solids from solutions wherein the solution is passed continuously into the top of a vertically-disposed cylindrical vessel, provided with a closely-fitting stirrer and a jacket through which cooling liquid is circulated in counter-current with the flow of solution, while a suspension of crystals in cooled solution is continuously discharged from the bottom of the vessel; the average temperature-difference between the solution and the cooling liquid and the speed of the stirrer are adjusted so that more seed crystals are formed per unit of time than developed crystals are discharged, the excess of seed crystals being dissolved in the warmer parts of the solution as a result of the stirring. The process enables a coarse crystalline product to be obtained with high rate of output. Several crystallizing vessels may be connected in series. In an example, 500 kg. per hour of hot anthrocene oil is passed into the top of a cylindical crystallizer, 2 metres high and 1 metre in diameter, provided with a helicoidal stirrer and scraper making 20 revolutions per minute, and with a jacket corresponding to a cooling-surface of 4.1 m2. Water is passed into the bottom of the jacket at 17 DEG and leaves the top at 34 DEG C. The average temperature of the oil in the crystallizer is 45 DEG C. The suspension of anthracene crystals in oil discharged at the bottom of the crystallizer is passed to the top of a second and similar crystallizer. Cooling water enters the jacket of the second crystallizer at 13 DEG and leaves at 17 DEG C. The suspension of crystals discharged from the bottom of the second crystallizer is passed to a storage vessel provided with a stirrer, and is then conducted to a centrifuge. The product from the centrifuge was coarse-grained (greater than 0.25 mm.) and contained 20 per cent of pure anthracene. A second example describes the crystallization of calcium nitrate tetrahydrate from a nitric acid solution of raw phosphate. The method may also be applied to the crystallization of naphthalene from crude naphthalene oil, to the recrystallization of crude anthracene from centrifuged naphthalene oil, and to the separation of ammonium chloride and of potassium ferrocyanide from their aqueous solutions. Specifications 260,133, 260,691, 261,085, [all in Class 32], and 559,992 are referred to.