GB911837A - Improvements in or relating to the dehydration of aluminium fluoride hydrates - Google Patents

Abstract

Aluminium fluoride hydrates are dehydrated by heating within a very short time, i.e. at the most 5 seconds, to the temperature required for the desired degree of dehydration. Preferably the rapid heating is effected by introducing the crude material, the granular size preferably not exceeding 0,5 mm. at the required dehydration temperature into an upwardly flowing current of gas. A fluidized bed may be used which consists of anhydrous aluminium fluoride of a granular size not exceeding 0,5 mm. and which is maintained by blowing in air from below; the hydrate is continuously introduced and is discharged at some other point by natural overflow of the fluidized bed. The fluidized bed may be heated directly by heating the current of gas or air, or preferably indirectly by externally heating the container for the fluidized bed. Instead of using fluidized bed calcination, the material to be calcined may be sluiced into a current of air or gas circulating in a tube system, which is preferably heated externally, wherein it is dehydrated and separated in the form of an anhydrous product by means of a cyclone. Calcination may also be carried out in rotating horizontal containers which are adapted to provide the required rapid heating of the hydrate. When the water-content of an aluminium fluoride hydrate is high, it may be advantageous to effect the dehydration in stages. In Example 1, aluminium fluoride trihydrate at a temperature of about 20 DEG C. is continuously introduced for 1 hour into an externally heated fluidized bed into which air is supplied from below; the temperature is 590 DEG C. The product, which is withdrawn by means of an overflow, contains 0,2% water, and 67,5% fluorine corresponding to an aluminium fluoride content of 99,6%. The product is dehydrated in one second and the average period that the product remains in the bed is 20 to 30 minutes. In Example 2, the water of crystallization is removed in stages from the trihydrate. Two fluidized beds are used, one is kept at 200 DEG to 220 DEG C. and the other at 580 DEG to 600 DEG C., the two beds being connected by an overflow. The product overflowing from stage 1 to stage 2 contained 10,4% water, whilst the end product contained 0,3% water and 99% aluminium fluoride. In each stage the product is dehydrated within one second and left in the bed for 20 minutes.