Cooking liquor containing low b.pt. solvents and quinone reducing catalysts - contains alkali sulphite(s) and sulphide(s) in aqueous solution

A process for the preparation of pulp by cooking material containing lignin in an alkaline aqueous cooking liquid which contains one or more low b.pt. solvents and one or more reducing catalysts - comprises the use of a liquid which contains aqueous solutions of alkali sulphites and sulphides. A similar process comprising the following steps is also claimed: (i) the lignin-containing cellulosic material is pre-hydrolysed with acid, steam and/or water in at least one step prior to cooking ; (ii) the material is impregnated with an alkaline liquor containing alkalis, sulphites and/or sulphides and optionally reducing catalysts ; (iii) the low b.pt. solvent is added before cooking, optionally along with the catalysts ; (iv) the pre-cooked material is cooked in a liquor containing at least one sulphide, at least one sulphite, at least one low b.pt. organic solvent (present in a concentration of ca. 2.5-60 %) and quinone reducing catalysts ; (v) the pulp is separated from the black liquor ; (vi) the solvent is recovered from the black liquor or directly from the cooking step equipment, and it may also be recovered from the associated kraft pulp line ; (vii) alkali present in the excess liquor and filtrate from the kraft pulp bleaching step is transferred to an evaporation and recovery step ; (viii) black liquor is evaporated, optionally as a mixture with the excess liquor from the bleaching step, to form a concentrated black liquor ; (ix) the concentrated black liquor is burnt under reducing conditions to form a solid residue ; (x) the residue is dissolved to form green liquor containing alkali carbonate and alkali sulphide ; (xi) the green liquor is divided into two parts ; (xii) the sulphur components are separated from at least one these parts using a sulphur separation method ; and (xiii) the sulphur-free solution is divided into two parts, one of which is treated to convert it into alkali sulphite for use in the (pre-)cooking step, whilst the other part is converted into sulphur-free hydroxide for use in the bleaching step. Preferably the solvent is methanol and the catalysts are chosen from quinone derivatives such as anthra(hydro)quinone. Sodium, potassium and/or ammonium sulphites and sulphides are used, to which the corresponding alkali hydroxides and/or (hydrogen) carbonates are added. A method for producing pulp from material containing lignocellulose by cooking in an alkaline, aqueous cooking liquid which contains one or more organic solvents having