heating type quartz tube while passing therethrough a mixed nitrogen gas of 94 n 2 and 6 o 2 or a nitrogen gas of 100 n 2 and it was calcined for 15 minutes after the calcining the dish was placed in a desiccator and cooled thereafter the amount of the residual magnesium sulfate contained in the cooled calcined material was measured the conversion was determined according to the following equation the results of the calcining tests are shown in fig1 and 2 the calcining atmospheres used in fig1 and 2 were respectively the mixed nitrogen gas of 94 n 2 and 6 o 2 and the nitrogen gas of 100 n 2 in these graphs curves 1 2 and 3 indicate the results obtained by using respectively petroleum coke boiler soot and lignite coke as reducing agents the properties of the reducing agents used above are shown below ______________________________________reducing particle composition agent size u03bc carbon ash others______________________________________petroleum less than 94 5 2 1 3 4 coke 500lignite less than 94 5 2 0 3 5 coke 500boiler less than 72 0 3 0 25 0 soot 100______________________________________ as can be seen from the graphs the use of lignite coke and soot as reducing agents according to this invention increases markedly the conversion of magnesium sulfate into magnesium oxide in the calcination thereof thus fig1 reports.