to mean the reaction process usually know under the name of cooking which under the conditions of the invention leads in addition to paper pulp to easily separable products which is not the case in most conventional processes according to another embodiment the fractionation reaction is performed at a temperature below or equal to the reflux temperature of the mixture the liquid solid mass ratio will preferably be between 4 and 11 separation of the paper pulp from the organic phase at the end of cooking is preferably performed by pressing another preferred embodiment specifies that the pulp separated in this way is washed with a mixture of formic acid and acetic acid or with pure acetic acid the pulp from which most of the lignin residues and sugars have been removed is then washed with hot water another preferred embodiment proceeds with cooking in at least two stages in order to improve delignification and therefore the quality of the pulps the first stage is performed in the presence of the formic acid acetic acid mixture the second stage is performed after having separated the pulp produced in the first stage in the presence of anhydrous acetic acid pulp washings are carried out with acetic acid one preferred embodiment specifies the control of ph during washing in an organic acid medium so that the paper pulp is at an ideal ph for bleaching with ozone in 1 or 2 sequences at a dryness of the pulp of the order of 40 to 60 another preferred embodiment specifies the separation of formic acid and acetic acid by evaporation under vacuum the separation of entrained water the recycling of formic and acetic acids in the required proportions and the recovery of excess acetic acid and water another.