with the sodium hydroxide solution to achieve this objective it is necessary to have as high a solids content in the solution as possible so as to create as large as possible a surface suitable for the reaction based on the reactor volume and by providing suitable apparatus to ensure that all the reaction components are thoroughly mixed to achieve this objective a circulating suspension reactor 1 is used for the process according to the invention consisting of a jet loop mixer 11 as the suspending apparatus surmounted by a gravity thickener 12 with a conically shaped or tapered lower part as clarifier components of the jet loop mixer include a cylindrical pressure tight jet loop mixer container 11 an insertion tube 13 arranged or suspended coaxially in the jet loop mixer 11 at least one upwardly directed power jet nozzle 16 arranged in the base of the reactor 1 and preferably at least one tangential nozzle 17 arranged near the base and directed parallel thereto and tangentially of the reactor wall a recirculation conduit 14 leading through a recirculating pump 15 connects the head of the gravity thickener 12 i e a zone in which the solution is adequately clarified to the above mentioned nozzles 16 and 17 the reactor as a whole is in the form of a pressure vessel and consists of nickel lined steel or of high nickel content steel all pipes which come into contact with the hot strongly alkaline solutions also consist of high nickel content steels the power jet nozzle 16 generates an intensive flow of liquid from.