Abstract: Hydrargillite is converted into boehmite by preparing a suspension (A) of hydrargillite in water with a proportion from 150 g/l to 700 g/l of dry material expressed as Al.sub.2 O.sub.3, subjecting it to heat treatment under pressure (D) at a temperature of from 200.degree. C. to 270.degree. C., the speed of the rise in temperature of said suspension being at least 1.degree. C./minute, and causing it to pass a period of time of from 1 minute to 60 minutes in a holding zone (D) at a temperature in the range of from 200.degree. C. to 270.degree. C.The boehmite produced has a granulometry which is at most identical to that of the initial hydrargillite, and has a much lower content of alkaline material.

Abstract: A process for cleaning the walls of heat exchangers or reactors which are covered with essentially titaniferous incrustations but which can also contain silico-aluminous incrustations formed during the attack of ores and causing a considerable reduction in the heat exchange capacity, the said process being intended to restore the fundamental characteristics of the said walls, which is characterized in that the incrustations are removed by means of an aqueous treatment liquor containing from 3 to 30% by weight of hexafluosilicic acid and at most 10% by weight of hydrofluoric acid. The aqueous treatment liquor can contain a corrosion inhibitor and the treatment can be carried out at a temperature between 20.degree. C. and 80.degree. C.