Two-stage chemical treatment of mechanical wood pulp with sodium sulfite

A method of improving the properties of mechanical refiner wood pulp, comprising the steps of:

• applying an aqueous solution of sodium sulfite, in the range of about 1%-10% sodium sulfite based on bone dry weight of wood, to wood particles, selected from the group consisting of wood chips, shredded wood chips, and shavings, the solution having a pH in the range of about 4.5-11;
• heating the sodium sulfite treated wood particles to a temperature in the range of about 100.degree.-160.degree. C. and maintaining the particles in the temperature range for a period of time in the range of about 20 seconds to 10 minutes;
• refining the heated sodium sulfite treated wood particles into mechanical wood pulp;
• separating the pulp into a long fiber fraction and an accept fraction, the long fiber fraction containing a higher proportion of shives and long fiber material, said long fiber fraction representing about 10%-65% by weight of the whole refiner pulp;
• applying an aqueous solution of sodium sulfite in the range of about 4%-50% sodium sulfite based on bone dry weight of wood, to the long fiber fraction, the solution having a pH in the range of about 4.5-11;
• cooking the sodium sulfite treated long fiber fraction at a temperature in the range of about 100.degree.-160.degree. C. for a period of time in the range of about 2-120 minutes;
• refining the cooked sodium sulfite treated long fiber fraction to provide a refined long fiber fraction that provides handsheets prepared according to TAPPI standard T205 om-81 and, when tested according to TAPPI standard T220 os-71, having a freeness in the range of about 100-300 ml, sheet density in the range of about 0.4-0.55 g/cm.sup.3, burst index in the range of about 3.2-4.6 g/cm.sup.3, breaking length in the range of about 6500-7800 m and tear index in the range of about 8-14 mN.m.sup.2 /g;
• recombining at least part of the refined long fiber fraction with at least part of the accept fraction;
• and
• the combined weight of the accept fraction and the refined long fiber fraction, being at least about 90% based on bone dry weight of wood particles.