Process and Apparatus for the Crystallization of a Melt

The invention concerns a process and an apparatus for the crystallization of melts, wherein at least one wall coolable to a temperature below the crystallization point of the melt is provided, the wall being in contact with the melt so that crystallization takes place on it.

Processes and apparatus are known (e.g., see Swiss Patent 501,421) wherein crystals are formed by the extraction of heat from the melt of a chemical product. In the known processes and apparatuses, tubular crystallizers are used, wherein a melt and a coolant run down along internal and external walls thereof, respectively, in the form of trickling films, so that the desired crystallization takes place. The crystallization is induced for the purpose of solidifying a melt or purifying a chemical product.

The purifying process is based upon the fact that crystals are purer than the initial melt. Thus, by carrying out the above-described crystallizing process, impurities can be pressured against the grain boundaries upon the formation of crystals, so that the impurities remain in the residual melt. The known processes and configurations operate discontinuously. Initially, the melt is caused to crystallize on the cooled walls of the crystallizers. Afterwards, the crystalline layer is melted and runs down the walls.

It is an object of the present invention to develop a process and an apparatus of the above-mentioned type so that it is no longer necessary to work discontinuously, but rather so that a continuous recovery of the crystallized product is possible.

To attain this object, initially in a process of the afore-mentioned type two walls moving in opposite directions are provided one of which walls is supplied with the melt, while the other wall is a cooled wall brought into contact with a surface of the melt. That surface of the melt crystallizes on that other wall and is continuously drawn off therewith to be further processed.

Preferably, the cooled wall carrying the melt is at a temperature in the vicinity of the crystallization point, whereby the melt surface in contact with the cooled wall crystallizes very rapidly.

An apparatus according to the invention wherein the walls are formed by two endless belts defining a gap between two opposing flights thereof. The flights travel in different directions. The melt is applied to one flight and travels thereon into the gap. The other flight is cooled and is in contact with an opposing surface of the melt to crystallize that surface. The crystallized layer leaves the gap on the cooled belt and is removed therefrom.

Preferably, the cooled belt constitutes an upper one of the belts, and the lower belt is heated. A scraper or the like removes residual melt from the lower belt. The residual melt can be remelted and re-fed to the gap. The belts can be heated and cooled by sprayers which spray hot or cold liquid thereagainst.

The objects and advantages of the invention will become apparent from the following detailed description of a preferred embodiment thereof in connection with the accompanying drawings in which like numerals designate like elements, and in which:

The Sole FIGURE shows schematically in side elevation an apparatus for carrying out the process, according to the invention.

The apparatus comprises a double belt installation, including a lower belt 1 and an upper belt 2, which are guided around reversing rolls 3, 3a and 4, 4a. The axles 5 of the reversing rolls 3, 3a, 4, 4a are horizontal. The belts 1, 2 may comprise steel bands. The lower belt 1 is somewhat longer than the upper belt 2 and its upper flight 1a projects upstream past the reversing roll 4 of the upper belt 2. Disposed above that upstream portion of the flight 1a is a feeder device 6 for feeding a melt 7 produced by the melting of a material capable of crystallization. The feeder device 6 may comprise a feeder pipe with a slit in its bottom side extending transversely to the running direction 8 of the lower belt 1. However, a known strip caster or means to apply the melt in the form of drops, may also be used.

The upper belt 2 revolves in a direction 9 (i.e., clockwise in the FIGURE) which is the same as that of the lower belt. The flight 1a of the lower band and the lower flight 2a of the upper band form a parallel gap therebetween, and those flights travel in opposite directions.

Within areas surrounded by the belts 1, 2, respectively, are disposed spraying devices comprising a plurality of spray nozzles 10 and 11. The spraying devices are located within a cover 12 and 13 in a manner such that the upper flight 1a of the lower belt 1 is sprayed from below, and the lower flight 2a of the upper belt 2 is sprayed from above. A liquid is supplied to the lower spray nozzles 10 through conduit system 14, and a liquid is supplied to the spray nozzles 1 by means of a conduit system 15. Those conduit systems 14, 15 are connected in a manner (not shown in detail) with a fluid circulation system laterally relative to the endless belts.

The liquid supplied to the conduit system 14 is maintained at a temperature in the vicinity of, but preferably slightly higher than, the crystallization temperature of the melt 7. The conduit system 15, on the other hand, is supplied with a cooling liquid whose temperature is substantially below the crystallization temperature of the melt 7.

The spraying liquids thus impact against the sides of the respective belts 2 and 1 facing away from the gap 16 in which the melt 7 is located in order to add heat to, or remove heat from, the melt 7 in the gap. That is, the liquid sprayed against the upper strand 1a of the lower belt, maintains at a predetermined temperature the surface of the melt contacting the flight 1a, while the surface of the melt 7 in contact with the lower flight 2a of the upper belt 2 is strongly cooled within the gap 16 by means of the liquid sprayed from nozzles 11. Thus, on the lower flight 2a of the upper belt beginning in an area upstream of the reversing roll 4a, a crystallization layer 17 will be formed from the melt 7, which layer is moved out of the gap 16 in the traveling direction 9 of the upper belt 2a, i.e., to the left in the FIGURE, opposite the direction of feeding of the melt. The crystallization layer 17, which continues to adhere to the upper belt 2 outside of the gap, is removed from the belt 2 by means of a take-off deice 18, for example a stripping blade and is placed on a conveyor 19 capable of moving the crystalline layer. The crystalline layer 17 crumbles upon being taken off the belt and is transported laterally out of the range of the revolving belts 1 and 2 for further processing.

The reversing roll 3a of the lower belt 1 is combined with a stripping deice 20, for example in the form of a spatula or the like, whereby the residual melt 7' is removed from the terminal area of the lower belt 1 and passed into a collector device 21. From this collector deice 21 the melt enters a recycle line 23 and is transported to the feeder device 6 through a heat exchanger 22 which remelts and heats the melt to the initial feeding temperature. It would also be possible to equip the collector deice 21 itself with a heat exchanger in order to recycle the residual melt. The apparatus described with reference to the drawing renders possible the continuous production of crystals for further processing.

Although the present invention has been described in connection with a preferred embodiment thereof, it will be appreciated by those skilled in the art that additions, modifications, substitutions, and deletions not specifically described may be made without departing from the spirit and scope of the invention as defined in the appended claims.