from about 5 to about 30 rpm preferably at least a portion of the acid filtrate is fed to the mixing tank it is also preferred that an acid aqueous medium being a portion of the acid filtrate or acid aqueous medium from the mixing tank or a mixture thereof is transferred to the reaction zone of the chlorine dioxide generator suitably the process is operated to obtain an acidity of the aqueous medium transferred to the reaction zone of the chlorine dioxide reactor exceeding 4 8 n preferably exceeding about 5 5 n most preferably exceeding about 6 5 n generally it is hard to obtain an acidity exceeding about 7 n suitably the at least partially neutralized alkali metal sulfate is washed on the filter with an aqueous medium preferably water so to obtain alkali metal sulfate containing less then about 2 by weight of h 2 so 4 preferably less then about 1 5 by weight of h 2 so 4 most preferably less then about 1 by weight of h 2 so 4 the acid filtrate preferably is a mixture of filtered medium from the mixing tank and spent washing water it is preferred to operate the process to achieve an acidity in the acid filtrate exceeding about 4 8 n preferably exceeding about 5 5 n most preferably exceeding about 6 5 n generally it is hard to obtain an acidity above about 7 n suitably an aqueous medium is supplied to the mixing tank preferably in addition to the acid filtrate the aqueous medium may for example be water or an aqueous solution of methanol sodium chlorate.