GB1106841A - Process for the production of bleached pulp - Google Patents

Abstract

1,106,841. Bleaching pulp; treating cellulosic pulp. ELECTRIC REDUCTION CO. OF CANADA Ltd. 24 Jan, 1966 [25 Jan., 1965], No: 3130/65. Heading DIP and D2W. Bleached pulp is produced by heating in a digestion stage a fibrous cellulosic material in an aqueous solution containing sodium hydroxide or sodium sulphide or both, separating the fibrous pulp from the solution, sending the solution to a recovery stage which includes evaporating the solution, burning the concentrated solution to form a smelt, dissolving the smelt in water and treating the solution so obtained to form a liquor for re-use in the digestion stage, and partially bleaching the pulp with an aqueous solution of chlorine or chlorine dioxide or both, extracting the partially bleached pulp in a caustic extraction step with a hot aqueous alkali solution and further bleaching it in at least one more bleaching step, e.g. with hypochlorite or chlorine dioxide, the effluent from the caustic extraction step being used at least partially as the agent for quenching and dissolution of the smelt from the furnacing of the solution at the recovery stage. In the drawing wood chips are digested in digestor 1 in a white liquor comprising an aqueous solution of sodium hydroxide and sodium sulphide and the pulp mixture is passed to separator 2 in which the pulp is separated from the black liquor which is then concentrated in evaporator 3, sodium sulphate is added to the concentrated black liquor and the liquor burnt in furnace 4, the smelt so obtained containing sodium carbonate and sodium sulphide being passed to smelt dissolver 5 where it is quenched with water and the green liquor so formed passed to clarifier 6 from which the dregs obtained are passed to dregs washer 7 and the clarified green liquor passed to causticiser 8 in which it is mixed with lime and the resulting liquor passed to white liquor clarifier 9 where the precipitated calcium carbonate is removed as a mud and passed to mud washer 10 and the white liquor passed via line 21 to digester 1. The pulp is washed with water in washer 11, bleached in the bleacher 12 with an aqueous solution containing Cl 2 , ClO 2 or both, washed in washer 13 where it is passed as a mat over a wire mesh on the periphery of rotating drum 14, mixed with steam and recycled white liquor which enters via line 20 from white liquor clarifier 9 in repulper 19, subjected to caustic extraction in tank 22, washed in washer 24 and passed via repulper 38 and washer 40 to further bleaching steps with chlorine dioxide. The effluent sucked from the washer 24 through line 27 passes to seal tank 28 from which it is pumped by pump 28a, a portion being recycled via lines 29 and 30 to the surface of the wash mat on drum 14 of washer 13, a portion passing through lines 31, 32 and 33 to the dregs washer 7 and through lines 31, 34 and 35 to mud washer 10 and a further portion if desired passing through lines 31, 34 and 36 to mud thickener 37. The water passing through line 33 to dregs washer 7 is subsequently passed through line 45 to mud washer 10 and the wash water passing through line 36 to mud thickener 37 is subsequently passed to mud washer 10 through line 46, the wash water leaving the mud washer 10 passing through line 47 to smelt dissolver 5 where it is used to quench and dissolve the smelt from the furnace 4. The thickened mud from 37 is passed to lime kiln 49 where it is calcined to regenerate lime which is returned through line 50 to causticiser 8 for causticisation of the green liquor.