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Abstract

This paper deals with the various properties related to the combustion and chemical recovery from the waste liquors of five different methods of magnesium-base semichemical pulp production, and the reuse of the recovered chemicals and relief liquors. As shown in Table 1, the heating value of the dried solids from the waste liquors was found to be closely related to the amounts of organic matter present, as well as COD values. While the weight loss during heating of the waste liquors was also connected with the amount of organic matter, the difference within the pulps of the five methods is small, as shown in Fig. 1. Except for the slurry method, recovery yields of magnesium from four of the methods are fairly high, as shown in Table 3. This is because the original cooking liquor of the slurry method comprises magnesium hydroxide slurry at room temperature; and its waste liquor, which passed at an elevated temperature (160°C), contains the precipitate of magnesium monosulfite (MgSO3·6HO) partly dispersed over the surface of the pulp. As shown in Fig. 2, the order of magnitude of the recovery yields of sulphur from the five condensed liquors corresponds directly to the amounts of free acid presest and therefore, to that of the pH values of each of the original cooking liquors. Evaporation of the free acids from four of the waste liquors results in a positive increment of the pH values of those condensates ; but this is not the case in the two-component method, as shown in Fig. 3. During the condensation of the latter liquor, [HSO4-] product instead of [HSO3-], as in the other cases, causes the low pH values. The furnace temperature necessary for complete recovery of sulphur from the high-yield sulfite and the two-component liquor is 900°C, but 800°C is adequate for that from the other three liquors, as indicated in Fig. 4. No problem in the reuse of the recovered chemicals from any of the five waste liquors, except for the relief liquor reuse in the vapor-phase magnefite method, could be presumed. The results obtained here, and summarized in Table 6, suggest that the vapor-phase magnefite and the (liquid-phase) magnefite methods seem to be more prif itable than the others. The former, with the above noted demerit and its longer cooking time, allow the conclusion that the (liquid-phase) magnefite may be the best one within the range of the experiments studied.