SU934912A3 - Process for producing bleached sulphate cellulose - Google Patents

Abstract

Bleached cellulose fibre pulp are prepd. by first cooking a cellulose fibre material, pref. wood, then sepg. the pulp from the spent cooking liquor (I). (I) is then evapd., and burnt in an oven to give a melt which is dissolved up to give a green liquor (II). One portion of (II) is treated with caustic and recycled to the cooking stage; one portion is processed to make NaCO3 and a third is used to generate H2S. This is oxidised (partly) to H2SO4, (part of) which is reacted with MClO3 and MCl (when M is alkali metal, esp. Na) to give ClO2 and M2SO4. The M2SO4 is used as feed for the oven. Pref. the CO2 used for NaHCO3 prodn. is provided by combustion gases from the oven, and the ClO2, esp. as aq. soln., is used to bleach the pulp. By including a S recovery/recycle system max. utilisation of chemicals is achieved with minimisation of chemical losses and pollution.

Description

Gases with the formation of sodium bisulfite, sodium carbonate, sodium bicarbonate and hydrogen sulphide, hydrogen sulphide formed during the treatment of the second part of green liquor with flue gases, is oxidized to sulfuric acid and sent to the chlorine dioxide preparation stage, which is carried out by the interaction of sulfuric acid with sodium chloride and sodium chlorate to produce chlorine dioxide and sodium sulfate; the chlorine dioxide obtained is then sent to bleaching, and sodium sulphate is used as a raw material when burning spent cooking liquor in a furnace.

The fibrous cellulosic material is wood.

The drawing shows the scheme of the proposed method.

Fibrous cellulosic material, such as wood chips, is fed via line 1 to digester 2, where chips are treated with white sulphate liquor fed through line 3. The resulting pulp and spent liquor are sent through line k to washer 5. Washed pulp through line 6 is fed to bleaching plant 7, where the pulp is subjected to bleaching fe several stages, at least in one of them with chlorine dioxide.

The spent liquor is sent through line 8 to the evaporator 9 and then followed by SRI 10 to furnace 11. The melt obtained in the furnace 11 is fed to the melt solvent 12 to produce green liquor, which is divided into two parts, and the flue gases from the furnace 11 are fed through precipitator 13 enters tower 1. One part is green. the liquor is treated with soda ash and causticized at unit 15 to produce white liquor, which enters digester 2 through line 1b. The second part of the green liquor is removed from solvent 12 of the melt through line 17 and transferred to the reactor 18 for precarbonization. Carbon dioxide gas is fed into this reactor via Ikct from bicarbonate tower 14. The pre-carbonized substance enters through line 19 to hydrogen sulphide distilling tower 20, where additional bicarbonate solution is introduced or slurry through line 21, and the depleted solution is returned via line 22 to bicarbonate tower ik. Almost pure gaseous hydrogen sulphide through line 23 enters oxidizer 2C, where sulfuric acid is obtained, which is fed through line 25 as raw material to generator 2b of chlorine dioxide. Chlorine dioxide generator 26 uses sulfuric acid, sodium chlorate and sodium chloride to produce chlorine dioxide, which is sent along line 27 to the bleaching plant 7, and the resulting sodium sulfate directs the lines 28 to the furnace 11.

The flue gases supplied to the bicarbonate tower 1 through the precipitator 13 contain about 15 wt. Soo The incoming carbon dioxide reacts with sodium sulfide and sodium carbonate to form sodium bicarbonate. The bicarbonate tower operates in the following mode: temperature 50-80 C, pressure from 2.1 atm to atmospheric. The main products of the bicarbonate tower are sodium bicarbonate and chlorides supplied to the chlorine dioxide generator 26. Sodium bicarbonate in the presence of a small amount of carbon dioxide is fed via line 17 to the pre-carbonation reactor of green liquor to convert the hydrogen sulphide into sodium sulfite. The treated solution from the preliminary carbonization reactor 18 is fed, via line 19, to the hydrogen sulphide distillation tower 20, where hydrogen sulphide is distilled off in a concentrated and almost pure form.

Bicarbonization and distillation can be carried out in any type of equipment commonly used for gas absorption or distillation.

Claims (1)

The table shows the comparative data on the consumption of chemicals with the known and proposed methods. The proposed method allows using chemical compounds formed at various stages of the process with maximum efficiency at minimum losses and excludes environmental pollution. The invention of the method for producing bleached sulphate pulp, including boiling fibrous cellulosic material, separating spent cooking liquor from fibrous material, bleaching it with chlorine dioxide, evaporating and burning spent cooking liquor in a furnace to produce a flue gas, dissolving the melt to form green liquor, separating green liquor into two parts, causticization of one part of green liquor, followed by its feeding to cooking, processing of the second part of green liquor by flue gases with the formation of sodium bisulfite, sodium carbonate, sodium bicarbonate and hydrogen sulphide, characterized in that, in order to expand the technological capabilities of the process by using waste cooking liquor processing products, hydrogen sulphide formed during the treatment of the second part of green liquor with flue gases, oxidized to sulfuric acid and sent to a chlorine dioxide preparation stage, which is carried out by reacting sulfuric acid with sodium chloride and sodium chlorate to produce two chlorine oxide and sodium sulfate; the chlorine dioxide produced is then sent to bleaching, and sodium sulfate is used as a raw material when burning spent cooking liquor in an oven. Sources of information taken into account in the examination 1. US patent number ,, ,, Cl. D 21 11/00, 1973 prototype).