Verfahren zur Herstellung bleichfähiger Halbzellulose aus Nadel- oder Laubholz

摘要

Half-stuff is produced from wood by first subjecting the wood chips or pieces to vacuum and then treating with a weakly alkaline sodium sulphite solution at 100-140 DEG C. under a pressure of at least 6 atmospheres under such conditions that the evolution of steam within the capillaries of the wood is avoided, after which the material is defibrinated. The subjection to vacuum, which facilitates penetration of the sulphite solution, is preferably conducted at a vacuum of 710-750 mm. of mercury and lasts at least half an hour. It is effected in a boiler and the vacuum is maintained whilst the treating liquid is introduced, thereafter the liquid is heated by indirect means and pressure applied by a pump. The pump is of an oscillatory type, e.g. a piston pump, so that a pulsating pressure is communicated to the treating liquid, which facilitates its penetration into the wood. The treating liquid is a solution of sulphite containing 100-200 gms. of sodium sulphite per litre as is made weakly alkaline, e.g. with a pH value of 7.5-8.5. At the end of the treatment, the liquid is drawn off and the wood may be washed. If the wood is in the form of sticks it is allowed to lie in a moist condition for at least 24 hours and is then ground; if the wood is in the form of chips, penetration of the treating liquid is better and the treated chips are ground straight away. Grinding is effected at about 70 DEG C. in a refiner. the material then being graded and dehydrated. It may be bleached if desired with hypochlorite, peroxides, or other known bleaching agents. A suitable apparatus comprises a boiler 1 having pressure gauges 8, 81, thermometer 9, indirect steam heating pipe 11 and safety valve 10. 15 is a water supply pipe. The wood pieces are introduced through manhole 2, which is then sealed. Air is extracted through valve 31 by pump 4. At the end of half an hour's subjection to vacuum, valves 611 and 20 are opened to draw in treating liquor from tank 5. Heat is applied by steam pipe 11 until a temperature of 115-140 DEG C. is reached and a pulsating pressure by triple-piston pump 7 until a pressure of e.g. 10 atmospheres is reached, the pressure oscillating between 9.9 and 10.1 atmospheres. After 2-5 hours, the pump is stopped, treating liquor drained back to tank 5 for re-use and the wood washed with water from pipe 15. After grading, e.g. on rotary sieves or vibratory screens, and dehydrating, the yield is 80-90 per cent.