IMPROVED CELLULOSIC PULP PROCESS

1,217,833. Bleaching pulp. ELECTRIC REDUCTION CO. OF CANADA Ltd. 16 Jan., 1968 [16 Jan., 1967], No. 2240/67. Heading DIP. Pulp is produced by digesting a cellulosic material with an aqueous white liquor to form an unbleached pulp and separating black liquor from the unbleached pulp, washing the unbleached pulp, subjecting the washed unbleached pulp to at least one bleaching stage and/or neutralisation or alkaline extraction stage, washing the pulp after at least one of these stages, passing the black liquor to a chemical recovery system which comprises evaporating the liquor to leave a solid residue, burning the residue, dissolving the burnt residue in water to form green liquor and causticising the green liquor with lime to form fresh white liquor, at least a part of the liquid effluent from the wash after at least one of the stages being used to wash the unbleached pulp and at least part of the liquid effluent from the washing of the unbleached pulp being combined with the black liquor and passed to the chemical recovery system. At least part of the effluent from the washing of the unbleached pulp may be used for dissolving the smelt resulting from the burning of the black liquor residues in order to form fresh white liquor for digesting further quantities of pulp prior to evaporation in the black liquor recovery system. The pulp is preferably subjected to a plurality of bleaching and/or alkaline extraction stages and a plurality of washings, at least part of the effluent from at least one of the washings being employed as at least part of the wash water in at least one other washing, preferably a preceding washing in the sequence so that a completely countercurrent washing process is possible in which at least part of the liquid effluent from each washing is used as wash water in the next preceding washing. The wood or other cellulosic material may be digested with an aqueous solution containing sodium hydroxide and/or sodium sulphide or, in the sulphite processes, an aqueous solution of sodium sulphite and/or sodium bisulphite or of sodium bisulphite and sulphurous acid. The pulp may be subjected to a chlorination using chlorine alone or in admixture with chlorine dioxide, caustic extraction, chlorine dioxide, caustic extraction, chlorine dioxide bleaching system or to a chlorination, caustic extraction, hypochlorite bleaching system.