Pretreatment of chips before cooking

The strength properties of chemical cellulose pulp (particularly kraft pulp) are improved by substituting a cold impregnation soak for conventional impregnation procedures. After steaming, wood chips are soaked in an alkaline liquid at a temperature of about 80-110° C. (preferably 80-100° C., or 90-105° C.) for between one-half-72 hours (typically about 2-4 hours) at a pressure of about 0-15 bar (preferably about 1-5 bar), to dissolve at least about 8% of the wood (preferably about 10-20%) and at least about 15% of the lignin. The alkaline liquid used preferably contains sulfide (e.g. black liquor, green liquor, white liquor, or mixtures thereof), but almost any alkaline liquid having an alkali concentration of about 1.0 mole of NaOH/liter or less (typically about 0.75 m/l or less) is suitable. The wood chips are then raised to a cooking temperature of about 145-180° C. and cooked to produce the cellulose chemical pulp. There may be an intermediate step, between soaking and raising the cooking temperature, of heating the wood chips to a temperature of about 110-150° C. (preferably about 120-140° C.) for about 10-90 minutes (preferably about 10-30 minutes). Alternatively or additionally the majority of the dissolved lignin (and/or other solids) may be removed before raising the wood chips to cooking temperature. The system for treating the wood chips need only include a low pressure soaking vessel (with an associated pump or other transfer device) in addition to conventional equipment.