Process for high consistency oxygen delignification of alkaline treated pulp followed by ozone delignification

A process for delignifying and bleaching lignocellulosic material which comprises:

• partially delignifying a lignocellulosic material to form an unbleached brownstock pulp;
• oxygen delignifying said pulp by:
• reducing the consistency of the pulp to a low consistency less than about 5% by weight;
• uniformly mixing the low consistency pulp with a quantity of alkaline material in an aqueous alkaline solution for a predetermined time and at a predetermined temperature correlated to the quantity of alkaline material without interruption to expose substantially all the pulp to the alkaline solution to substantially complete a substantially uniform distribution of the alkaline material throughout the pulp;
• increasing the consistency of the pulp after completion of the mixing step to at least about 20% by weight by removing liquid from the pulp to form high consistency brownstock while retaining at least about 1.4 percent by weight based on oven dry pulp of alkaline material on the increased consistency pulp, said pulp fibers containing the alkaline material being directly passed from the combining step to the consistency increasing step;
• recycling substantially all of the liquid removed from the pulp in the consistency increasing step to the low consistency pulp mixing step;
• and
• subjecting the increased consistency alkaline material containing pulp to high consistency oxygen delignification to obtain enhanced delignification of the pulp without a corresponding decrease in pulp viscosity compared to pulp which is not treated with alkaline material at low consistencies, the combination of the partial and oxygen delignifying steps providing a partially delignified pulp having a first K No. of about 9 or less and a first viscosity of greater than about 13 cps;
• directing the partially delignified pulp to ozone delignification without any intervening delignification or bleaching steps;
• and
• further delignifying said partially delignified pulp with an effective amount of ozone for a sufficient time to obtain a substantially delignified pulp having a second K No. of about 5 or less, a second viscosity of greater than about 10 cps and a GE brightness of at least about 50% prior to any additional bleaching steps.