JAPAN TAPPI JOURNAL

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In order to clarify the mechanism of the formation of coloring structure during sulfite cooking, solid lignosulfonic acid being of light cream color, was treated under various conditions of acid sulfite cooking.
1. The change of color of solid lignosulfonic acid during sulfite cooking was dependent on the concentration of combined SO₂ in cooking liquor, and in case of less than 0.1% of combined SO₂, the change of color was very remarkable.
2. The effect of cooking time on color of solid lignosulfonic acid was very little in the case of combined SO₂ higher than 1%. The color of solid lignosulfonic acid was mainly due to the cooking time in the case of combined SO₂ less than 0.1%.
3. Moleculare weight distribution of lignosulfonic acid by gel filtration shows the formation of lower and higher moleculare weight fractions compared with the original solid lignosulfonic acid.
4. The changes of some functionalgroups of lignosulfonic acid during sulfite cooking were studied by the use of sulfurous acid alone which was similar to the composition of cooking liquor at the final stage. The results show that methoxyl group decreased, and non-conjugated phenolic hydroxyl and α-carbonyl groups increased, and catechol group plus o-quinone structure decreased.
5. The contribution of quinone structure to color of lignosulfonic acid, which was estimated by reduction with sulfurous acid, decreased during sulfite cooking in spite of the strong deepening of color by the cooking.
6. From the results described above, it can be concluded that the o-quinone structure which was formed by demethylation and oxidation during cooking, condensed and gave very deep color. The α-carbonyl group which increased during cooking, also takes parts in the formation of another coloring structure.

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This report summarizes the results of both laboratory scale and pilot scale tests for rapid vapor-phase alkaline cooking and high sulfidity chemical recovery processes. The pilot plant tests include rapid vapor-phase alkaline cooking of various wood species by a 0.5 ton per day scale continuous pilot digester and operation of a 0.5 ton per day scale (as Na₂CO₃·H₂O crystal) pilot crystallizer which is one of the most important parts of the newly developed high sulfidity chemical recovery system.
The works were performed in cooperation of Jujo Paper Mfg. Co., Ltd. and Mitsubishi Heavy Industries Ltd.

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