GB480252A - Improvements in or relating to processes and apparatus for the extraction of cellulosic fibres from wood and other highly lignified material - Google Patents

Abstract

A process for the extraction of cellulosic fibres from wood, bamboo and other highly liquefied material by an alkali treatment followed by a chlorination treatment comprises carrying out the alkali reaction under pressure in a closed vessel at a temperature between 100 DEG C. and 150 DEG C. and a pressure of 20-50 lb. per sq. inch and carrying out the chlorine reaction in an open vessel at a temperature not exceeding 65 DEG C. The alkali employed is caustic soda of 2,4 per cent, 4,6 parts of solution being employed to 1 part of wood. The alkali should be employed in quantities only just sufficient to react with the non-cellulose constituents of the wood. The chlorine used is moist and is diluted with air (e.g. ,5-1 volumes of air to 1 of chlorine) which facilitates temperature control and minimizes the action of hydrochloric acid formed in the chlorination. The ratio of chlorine to air and the rate of passage through the material of the gas is controlled. After the chlorine treatment, the material is treated with alkali of ,1 to 10 per cent strength at temperatures up to 100 DEG C. The wood chips are fed from hoppers 10 to a mixing device 11, Fig. 4, serving two alkali towers 8 and comprising a double screen conveyer 15 in a V-shaped trough 14. A deflector 16 controls the amounts fed to each tower. Caustic soda solution is supplied at 23 and is intimately mixed with the wood in the desired proportion before it enters the towers 8. A constant level device maintains the level of solution in the trough below the blades of the conveyer. The alkali impregnated wood is compressed by a ram 74 in a cylinder 72 leading into an opening in the conical cover plate 71 of <PICT:0480252/IV/1> <PICT:0480252/IV/2> the oval reaction tower 8, the compressed material forming a seal at the top of the tower. Steam is introduced by pipes 24 into the towers to generate the desired temperature and pressure. The material descends the tower by gravity, becoming compacted as it descends and is removed by a rotatable valve at the bottom and delivered to a screw conveyer 32 which feeds it through screening devices 33, 34 to separate the alkali from the treated wood, washing water being fed to the material in the second portion of the conveyer and run off into tank 37. The washed and partly treated fibres are treated in a screw press 39, expressed spent liquor passing to tank 40, converted into a floccular state in an opener 43 and then blown up conduits 45 against deflecting plates 46 at the top of chlorine towers 47. Chlorine with the required degree of moisture is prepared electrolytically and after being mixed with the desired amount of air is introduced by pipes 49, 51 about 1/3 of the way down the tower and reacts with the descending material which is removed at a controlled rate from the bottom of the tower by an extractor 55 comprising two parallel oppositely rotating shafts each carrying a set of interlacing star-shaped members. The material is delivered to a tank 60 with stirrers 61 through which cold water is circulated, and passes thence along trough 62 to a rotating conical sieve 63 where hydrochloric acid is separated to screw presses 65 and a mixing vat 66 containing caustic soda of less than 2 per cent strength. The fibres pass thence to the usual washing and sorting apparatus where knots are removed. British Specification 415,224, [Group VIII], and U.S.A. Specification 1,932,904 are referred to.