Several cooking conditions were compared with regard to pulp yield and pulp qualities when Magnesium-base high yield sulphite pulping was carried out in laboratory scale, using mixed Yezo-spruce and Saghalien-fir chips. It was found that Magnesium-base high yield sulphite pulps at the yield of 70% produced with the ilquor of ph 2-4 and C.A. 1.7% had better brightness and strength properties than Calcium-base high yield sulphite pulp at the yield of 65%.
In mill operations, side relief liquor is usually gathered into an accumulator and re-used for the next cooking, after mixed with fresh liquor. In that case, the stability of the liquor is very important.
Experiments were carried out to study about that stability. Cooking liquors were prepared by mixing several types of spent liquors with fresh liquor and kept in autoclaves at 70°C and 130°C respectively. After 12-16 hours of heating, the compositions of those liquors at 70°C were not so much changed, but those at 130°C rather deteriorated. Especially when the spent liquor which had beenn obtained from the cooking of larch was used, the concentrations of both thiosulphate ion and sulphate ion increased considerably.
The above mixed spruce and fir chips were cooked with Magnesium-base liquors at various thio sulphate ion concentrations, and the liquor compositions were analysed at various yield levels. When the cookings were stopped in the high yield region such as 65-70%, there were no troubles, even if thiosulphate ion was initially added in as much as 6
g/l, and burnt cooking occured only below the yield of 53%.
As a result, the high yield sulphite pulping of Yezo-spruce and Saghalien-fir has no trouble with respect to burnt cooking, but some cautions have to be taken if larch wood is mixed in raw materials.
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