GB1035456A - Production of pulp - Google Patents

Abstract

1, 035, 456. Treating wood pulp. DOMTAR Ltd. Oct. 21, 1963 [Oct. 20, 1962], No. 41407/63. Heading D2W. [Also in Division B1] Material such as wood chips or other cellulosic raw materials containing a vaporizable liquid are impregnated with an impregnant in a vaporizable liquid, such as sulphidic alkali liquor, by continuously introducing said materials into said liquid to effect impregnation, continuously removing from the system impregnated material together with vaporizable liquid, continuously evaporating an amount of the liquid left in the system equal to the difference between the total amount of liquid entering the system and the amount leaving the system and removing the so-formed vapour, to prevent accumulation of liquid in the system. As shown in Fig. 1, wood chips are passed from storage bin 10 to presteamer 11 and thence to impregnator 13, cooking tube 15, washer 19 and further washer 22 from which pulp product is obtained. The impregnating liquor is supplied through line 28 to a circulating supply to the impregnator after being preheated by black liquor formed in the process, in heat exchanger 42. The circulating impregnating liquor is heated by high pressure steam in heat exchanger 26 to a temperature such that flashing takes place in the impregnator, and released steam is vented through line 30 under the control of valve 30<SP>1</SP>, so that a constant level L is maintained in the impregnator. By this means optimum impregnation is achieved, irrespective of the amount of water in the chips. The impregnated chips then pass to the cooker 15 to which high pressure steam is admitted through line 16. Black liquor is withdrawn through line 32 and the cooked material passes to counter-current washer 19 which removes further black liquor through line 41 which is combined with that from the cooker and passed through heat exchanger 42 to flasher 43, if desired to oxidation tower 54, and then to concentrators 45, 44 arranged in multiple effect before final discharge through flash tank 58 as concentrated black liquor. High pressure steam in line 30 derived from the impregnation stage is employed to perform this concentration and the process renders hot water available for causticising, and low pressure steam. In the embodiment illustrated in Fig. 2 (not shown), flashing of the impregnating liquor is arranged to take place in a tank externally of the impregnation vessel. Fig. 3 (not shown) shows an alternative washing system in which pulp from cooker 15 passes through a pool of washing liquor and thence to a tank from which it is withdrawn for final washing and dewatering. Washing liquors are recycled.