GB408212A - The manufacture of polymorphous substances in a distinct crystal form - Google Patents

Abstract

The manufacture of a polymorphous substance in a distinct crystal form is effected by mixing therewith a different substance having the same crystal structure as that desired for the polymorphous substance and heating the mixture to the necessary temperature. Alternatively, a compound which on heating decomposes to give a substance of the required crystal structure may be used for mixing with the polymorphous substance. Admixture may be effected before or during calcination; or in a two-stage heating process at the end of the first stage. In one example, dried titanic acid hydrate, consisting of anastase crystals, is converted wholly to rutile by heating at 900 DEG C. for one hour in admixture with stannic oxide or a tin salt such as the chloride which is transformed into stannic oxide by heat; in a second example, using manganous sulphate or other manganese compound giving manganous oxide on heating, the conversion is effected at 800 DEG C. In a third example, aluminium oxide or a compound such as ammonium alum yielding it on heating is transformed into corundum, say at 1150 DEG C. by heating with vanadium sesquioxide, ferric oxide or hydroxide, or iron alum. In a fourth example, precipitated zinc sulphide is heated with cobalt sulphate or another compound giving rise to cobalt sulphide and the wurtzite formed above 600 DEG C. is stable between 700 and 1000 DEG C. The cobalt sulphide may be replaced by iron or nickel sulphide.