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Zone Refining is the method of choice for ultra-purification of organic chemicals.
Zone Refining is the method of choice for ultra-purification of organic chemicals.
In Zone Refining, a short molten zone travels slowly through an elongated sample [MiniZone Refiner Diagram] of a crystalline material. A molten zone traversing a solid sample has two liquid-solid interfaces; a melting interface and a freezing interface.
At the melting interface the composition sample of a crystalline material. A molten zone traversing a solid sample has two liquid-solid interfaces; a melting interface and a freezing interface. At the melting interface the composition does not change but at the freezing interface, the impurities favor the liquid phase because the crystal structure tends to exclude molecules that do not readily fit into the crystal lattice. Passing the molten zone(s) through the sample many times concentrates the impurities at one end leaving the opposite end highly purified
The photo shows a sample during the refinement process in the MiniZone refiner. The sample is contained in one (or two) disposable, thinwalled Teflon® sample tubes. These tubes are filled with the sample and clamped on each end in the tube transport mechanism. (The metal blocks at the top and bottom of the photo.) The transport mechanism slowly moves the tubes downward, passing them through the four heating and four cooling zones of the temperature regulated metal blocks. After the tubes move downward the distance of one hot zone and one cold zone (two centimeters), the transport mechanism is reversed and the tubes are quickly moved upward to the original position. The net result of this motion is that the molten and crystalline zones migrate slowly and continuously upward with respect to the tubing.
With a typical zone rate of 2 cm/hr and with four active zones, each portion of the sample is subjected to one purifying phase transitions per hour. Usually within 18 hours of automatic operation (18 recrystallizations), the sample is purified. After the sample tube is cooled, it is removed from the MiniZone. The refined sample is easily recovered by cutting the tube into sections. The major portion of the impurities are concentrated at one end of the tube.