GB685368A - Method and plant for the production of cellulose - Google Patents

Abstract

<PICT:0685368/IV (a)/1> Cellulose pulp is obtained from vegetable material such as straw by subjecting it to the action of a single solution containing several different reagents, including (1) acid radicals capable of reacting with the calcium and magnesium tetragalactouronates, present as binding substances in the raw vegetable material, so as to combine with and fix the alkaline earth metals, and (2) hydrolysing alkaline radicals, the material being moved through the solution in a direction opposite to the direction of flow of the solution. The process may be carried out in apparatus comprising a cylindrical body, an axial rotary shaft on which is fixed a perforated helicoidal partition, orifices for the admission of the material to be treated, for the water and for the treating lye, which are so disposed on the cylindrical body that the material moves in counter-current to the increasingly active lye and to the washing water; and means for recovering the heat from the black lye in order to heat the treating lye and the material undergoing treatment. As the de-calcifying acid there may be used hydrofluoric acid or oxalic acid. Salts of these acids may also be used. The treatment may be carried out at raised temperature, e.g. 90-180 DEG C. The material may be submitted to crushing or stirring during the treatment. As shown, vegetable material such as straw is fed from hopper b to the treating chamber 8 and is conveyed from inlet 11 to the outlet zone 15 by means of a screw. Grooves 23 prevent rotation of the vegetable material. An apertured section 10 of chamber 8 is provided for removal of spent lye. A heating medium such as hot black lye, or steam or other gas therefrom, obtained from outlet a is passed through jacket 12. The solution containing the required chemical agents is introduced through ring distributer 13, and washing-water enters through distributer 14. The conveyer-screw comprises a helical partition 17 perforated with flared holes 18 and fixed to shaft 16. A disc 20 is provided with recesses for removal of the cellulose. The charge may be subjected as it progresses along the chamber 8, successively, to heating by the spent lye or vapour therefrom, to the action of the hydrofluoric or oxalic acid or salt thereof in the treating solution, to the solvent action of the solution at a stage where the alkali has been exhausted but where organic sodium salts are present, to the action of alkali carbonates, to the action of alkali hydroxide in the fresh solution containing all the required chemicals, and then to washing with water. The treating solution introduced at 13 may comprise 10-20 per cent of caustic soda or sodium carbonate, sulphite, or sulphide, and about 0.25-0.5 per cent of sodium oxalate or other decalcifying agent. Mixtures of caustic soda and sodium carbonate together with sodium oxalate may be used. The Specification as open to inspection under Sect. 91 comprises also methods in which the vegetable material is treated with the chemical reagents while remaining stationary. An apparatus comprising a series of towers containing vegetable material at various stages of the treatment and means for passing the liquid continuously through all the towers in succession is described. In another form of apparatus, the vegetable material is forced by a screw-conveyer into the bottom of a vertical tower which is provided with a rotary stirrer. The material gradually moves up the tower, meeting a descending current of treatment liquor. The fresh liquor is introduced through a ring distributer at a level some distance from the top, and water for washing is introduced at the top of the tower. The treated cellulosic material passes through an outlet near the top. The tower is provided with a heating jacket. This subject-matter does not appear in the Specification as accepted.