Patent GB-736491-A (GB)
Title: Process and apparatus for treating vegetable fibrous materials with gaseous chlorine or chlorine dioxide
IPC: D21C9/12
Filed: 1950-05-11
Abstract: In the treatment of vegetable fibre pulp with chlorine or chlorine dioxide in two stages, the pulp in the first stage has a fibre concentration of 10-40 per cent and is treated with gaseous chlorine or chlorine dioxide for 10-45 minutes while in descent under the action of gravity.  Thereafter the pulp, without washing, passes directly by gravity to an apparatus wherein it is diluted and agitated with fresh water, or with water which has been used for washing the finally treated pulp, so that its fibre concentration is reduced to below 10 per cent, treatment with chlorine or chlorine dioxide in aqueous solution taking place in this apparatus for a period up to about one hour, with or without the addition of further gas.  The apparatus for carrying out the treatment may comprise means for initially consolidating the moisture-containing fibre so that the pulp has a 10-40 per cent fibre concentration, and an opening machine for opening-out or fluffing the consolidated fibres before their introduction into one or more upright towers in which the first stage of the treatment is effected.  Each tower may have a smooth chlorine-resisting lining, a circular cross-section of about 21-36 in. diameter and a height of 10-40 feet, and is mounted on and communicates with a closed chest so that the pulp is able to pass by gravity into the chest at a rate determined by the adjustment of an outlet valve in the lower part of the chest. Each tower is provided with means for introducing chlorine or chlorine dioxide, such as a perforated pipe extending some distance down the axis of the tower and/or inlets evenly spaced around the walls of the tower.  Each tower may also be provided with glazed windows to enable the colour changes of the pulp descending the tower to be observed.  Each chest is provided with means for introducing water into it at a controlled rate and with an agitator for mixing the water with the pulp, and may also be provided with means for the introduction of chlorine or chlorine dioxide.  The consistency of the diluted pulp in each chest may be automatically maintained at a predetermined value by means responsive to the power required to drive the agitator, the arrangement being such that increase of power consumption increases the rate of flow of water, and vice versa.  When chlorine is employed in the process, the amount used in the first stage may be such as to render the acidity of the water in the pulp equivalent to 10-40 grams of hydrogen chloride per litre, while the acidity of the pulp suspension in the second stage may be maintained within the same range by using for the dilution of the pulp acid washwater recirculated from the chlorinated pulp wash filter which is used in a subsequent stage of the process. In an example, straw which has been digested with aqueous caustic soda solution is fed in the form of fluffed pulp of 27 per cent concentration into a chlorination tower, down which it passes in about 45 minutes and absorbs 6.2 per cent of chlorine.  In the chest below the tower the pulp is diluted to 2.5 per cent concentration and is treated for a further 25 minutes during which it absorbs a further 0.5 per cent of chlorine, this chlorine having been entrained in the pulp during its passage to the chest. Specifications 415,224, [Group VIII], and 489,302 are referred to.