A new cooking process with sodium sulfite, formaldehyde, and a quinone compound (1, 4-dihydro-9, 10-dihydroxyanthracene sodium salt: DDA) was devised in order to produce soft wood semichemical and chemical pulps which are superior to neutral sulfite and kraft pulps, respectively. In this studies, this process was evaluated in terms of the optimum compositions of cooking liqnor, delignification selectivity, and pulp qualities.The results were as follows:(1) The addition of formaldehyde to a sodium sulfite-DDA solution accelerates delignification of yezo spruce. The optimum molar ratio of formaldehyde to sodium sulfite for delignification is in the range of 0.15-0.70.(2) Delignification selectivity in sodium sulfite-formaldehyde-DDA cooking is better than that in neutral (weakly alkaline) sulfite-DDA cooking. Lignin-free yields by the former cooking are 2-5% higher than those by the latter at a given delignification.(3) Delignification rate of yezo spruce in this cooking is comparable to that in kraft-DDA cooking, when the dose of DDA is increased to 0.5% on o.d. wood. Lignin-free yields by the former are 9-10% higher than those by the latter at a given delignification.(4) Yezo spruce/saghalien fir pulp produced by this cooking has higher tensile and burst indices, and higher brightness than kraft-DDA pulp.

The delignification rate constants were calculated as observed (pseudo) first-order reactions, and the effects of [OH-] and [SO32-] on the rate constants were evaluated in the Salkaline sulfite cooking with and without DDA (1, 4-dihydro-9, 10-dihydroxy-anthracene sodium salt). The delignification rates were also compared with those for kraft and kraft-DDA cookings.In the alkaline sulfite-DDA cooking of spruce and beech wood meals under the same conditions, the delignification rate constants for beech were not necessarily higher than those for spruce wood meals. This is quite different from the general observation for the soda, soda-DDA, kraft, kraft-DDA cookings, in which beech is higher than spruce. Especially in the case of beech, the delignification was not accelerated when the sodium sulfite concentration was increased from 0.4 M to 0.8 M. The addition of DDA could increase the effect of [32-] on the delignification in the alkaline sulfite cooking while it was not the case for [SH-] in the kraft cooking. Total alkali charge was higher in the alkaline sulfite-DDA cooking than those in the soda-DDA, kraft, kraft-DDA cookings to have a given rate constant.