Patent GB-419543-A (GB)
Title: Improvements in or relating to methods of bleaching fibrous cellulose material
IPC: D21C9/10
Filed: 1933-06-12
Abstract: <PICT:0419543/IV/1> <PICT:0419543/IV/2> Fibrous cellulose extracted from woody materials by a chemical cooking process are bleached by subjecting the materials in a substantially but not completely dry state to the action of a gaseous bleaching agent such as chlorine, in which the materials originally in a water suspension have air and a large part of the water removed therefrom, and are allowed to expand naturally following the removal of the water, so as to cause the remaining water to line the walls of the fibres and thus permit the gaseous bleaching agent to enter into and between the fibres and come into contact with the inner and outer walls of the fibres.  Air may be removed from the interstices and cells of the material by vacuum before introducing the bleaching agent.  The fibrous material has a water content of about 2 to 85 per cent when it comes in contact with the bleaching agent.  After the bleaching treatment, the cellulose material may advantageously be treated in a similar manner with gaseous sulphur dioxide, ozone, oxygen, carbon monoxide, oxides of nitrogen, or oxides of chlorine.  The material may be impregnated, before carrying out the bleaching process, by means of an alkaline substance, such as sodium, potassium, calcium, or magnesium oxide, hydroxide, peroxide, carbonate, or bicarbonate, ammonia, or ammonium salts.  The apparatus described comprises a preferably conical vessel 10 which contains the substantially dry pump, and is mounted by means of trunnions 16 and 17 in bearings 18, so that it may be inverted for discharging its contents.  The vessel is provided with an air-tight lid 12, and connections at 26 for a vacuum pump 23, and at 11 for a tank 29 containing the compressed bleaching agent.  Alternatively, the bleaching agent may be supplied from a tank 291 connected to the hollow trunnion 16, from which it is led by way of pipe 281 and the connection 11 to nozzle 27.  In operation, the pulp, for example, in the condition in which it leaves a wet machine or screw press, or after impregnation with alkaline materials, is introduced into the vessel and the lid 12 closed.  The vacuum pump 23 is first operated to bring the pressure down to about 15 inches of mercury, and the chlorine is introduced until the pressure is atmospheric or slightly above.  Finally, the excess chlorine is removed by means of the vacuum pump, the lid opened and the vessel inverted to discharge the pulp.  In another form of the apparatus (not shown), the pulp in the form of a relatively thick sheet is passed firstly through a vessel divided by means of partitions and pairs of rollers into three air-tight compartments, in the first of which the pulp sheet is subjected to reduced pressure, in the second to the action of chlorine or other gaseous bleaching agent, and in the third to reduced pressure to remove the bleaching agent.  After washing, the pulp may be subjected to further treatment, by passage through a similar apparatus which is supplied, e.g. with sulphur dioxide.  A modified apparatus for the continuous treatment of pulp in sheet form (Fig. 3) comprises a rotating filter drum 83, provided with a hood 84 adapted to supply an atmosphere of chlorine, and suction orifices 85 and 86 within the drum.  As the pulp passes between the hood 84 and suction area 85, air will be withdrawn from the pulp and replaced by chlorine. The residual chlorine is removed during passage over the suction area 86.