Process for the delignification of unbleached chemical pulp

Claim 1

A process for the delignification of unbleached Kraft pulp comprising the steps of: washing the unbleached Kraft pulp; diluting the washed pulp by the addition of part of the effluents extracted from the pressing step following the oxidation step to provide a quantity of dissolved materials of between 0.5% and 5% by weight with respect to the pulp; treating the diluted unbleached Kraft pulp with a peroxide in an alkaline medium without using bleaching agents containing chlorine and without the addition of inorganic buffers or cellulose preservatives, to oxidize and fragment lignin molecules in the pulp, with a percentage ratio of peroxide with respect to the pulp per unit of Kappa index of said pulp between 0.01 and 0.1, with a pH between 10 and 12, at a temperature between 70° and 95° C., during a period of time between 20 minutes and 3 hours, while keeping during the oxidation step an autogenous overpressure slightly greater than atmospheric in the column of pulp by carrying out the reaction in a closed reactor, the overpressure in the column of pulp being limited to a value no greater than 2 Kg/cm 2 ; diluting the oxidized pulp to a concentration of between 1 and 5% by weight; pressing the diluted oxidized pulp mechanically to a consistency of 30% or more; recycling a first portion of the effluents extracted from the pressing step to the dilution step prior to the treating step; and recycling a second portion of the effluents extracted from the pressing step downstream of the treating step for ensuring the dilution of the oxidized pulp before the pressing step.