When birchwood (Betula platyphylla var. japonica) was cooked H2SO3+MgSO4, the amount of the chemicals, cooking temperatures and cooking times were examined in order to obtain the high yield chemical pulp comparable to kraft pulp in the strength, and following results were given.
1) In case of constant liquor of 28% H2SO3 and 24% MgSO4 (based on o.d., wood) cooking times required for pulping were 1.2 and 3hours at the cooking temperatures of 170, 160 and 150°C, respectively. pulps of good quality were obtained in the range of 150 to 160°C.
2) The chemicals of such amount as 25 to 30% H2SO3 and 24 to 35% MgSO4 based on o.d.wood needed to obtain chemical pulps, in the cooking of 150°C-3hr. as well as 160°C-2hr.
3) The former cooking condition showed better pulp properties than the latter in the same amount of the chemicals.
4) The cooking of 150°C-3hr. using these amounts of chemicals gave the pulps of similar yield and strength.
5) It was shown that the pulp cooked at 150°C for 3hours using the liquors of 25 to 30% H2SO3 and 24 to 35% MgSO4 is 10% higher in the yield than convetional kraft pulp and stands comparison with it in the strength.