The two stage soda oxygen pulping of Japanese red pine and beech woods was studied to obtain unbleached pulps of newsprint level or higher brightness.
In the first stage, wood chips were cooked with NaOH to a relatively high residual lignin content (cf. Table 2). The partially-delignified chips were then defiberized in a beater. In the second stage, the obtained first stage pulp was further delignified by cooking with NaOH and O
2 at pressure above 7 kg/cm
2 using an autoclave of 2 litter volume (cf. Table 3).
The pulps prepared by the soda oxygen method were yellowish white in color and their yield, % based on wood, was somewhat less than that of the comparable sulfate pulps (cf. Table 3 and 4). When the first stage pulps of lignin content of 15% for pine and 10% for beech were further delignified by the second stage cooking, unbleached pulp of brightness 62-66 and 75-82 was obtained in yield of 42.4-43.1% and 47.7-49.3% based on wood, respectively for pine and beech (cf. No. 6, 7, 16, and 17 in Table 3). The second stage delignifications with MgCO
3 were compared with those without additive of MgCO
3. The results showed that MgCO
3 had only a small effect on yield, brightness, and color of pulp.
The pressure of O
2 increased with the elevation of cooking temperature, and reached at a maximum pressure just before the time when the cooking temperature reached up to the maximum, and then decreased with the consumption of O
2 by the pulping reaction (cf. Fig. 1).
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