Environmentally improved process for bleaching lignocellulosic materials with oxygen, ozone and chlorine dioxide

A process for delignifying and bleaching a lignocellulosic material which comprises the following steps in the order stated without additional intervening delignification or bleaching steps:

• partially delignifying a lignocellulosic material by pulping to form a pulp, washing said pulp, and delignifying said pulp with oxygen to form a partially delignified pulp having a first K No. of about 10 or less and a first viscosity of greater than about 13 cps;
• washing the partially delignified pulp;
• further delignifying said partially delignified pulp at a consistency of at least about 20% by increasing the consistency of the oxygen delignified pulp to at least about 28%, adjusting the pH of the pulp to below 4, forming discrete pulp particles of a size having a sufficiently small diameter and a sufficiently low density to facilitate substantially complete penetration of a majority of the particles by ozone gas, and intimately contacting and turbulently mixing the pulp particles with an ozone containing gaseous mixture having an amount of ozone sufficient to remove a substantial portion, but not all, of the lignin remaining in the pulp in a dynamic reaction zone for a sufficient time to allow access of the ozone gas to substantially all surfaces of the pulp particles for reaction therewith while the particles advance through substantially all of the reaction zone to obtain a substantially uniformly delignified pulp having a second K No. of about 5 or less, a viscosity of greater than about 10 cps and a GE brightness of at least about 50%;
• washing the substantially delignified pulp;
• combining the substantially delignified pulp with an effective amount of alkaline material in an aqueous alkaline solution for a predetermined time and at a predetermined temperature correlated to the quantity of alkaline material to solubilize a substantial portion of any lignin which remains in the pulp;
• extracting a portion of the aqueous alkaline solution so as to remove substantially all of the solubilized lignin therefrom and form a substantially lignin-free pulp;
• washing the substantially lignin-free pulp;
• bleaching the substantially lignin-free pulp with chlorine dioxide to raise the GE brightness thereof to at least about 70%;
• and
• washing the bleached pulp;
• wherein at least a portion of the wash water effluent from one or more of the partially delignified pulp washing step, the substantially delignified pulp washing step, the substantially lignin free pulp washing step or the bleached pulp washing step is recycled countercurrently to a previous pulp washing step, and further wherein the effluent from washing the bleached pulp has an amount of total organic chlorides of no more than about 2 pounds per ton prior to any treatment steps for organic chloride removal.