METHOD OF PRODUCING DISSOLVING PULP

Abstract

A process for producing dissolving pulp from comminuted wood-based fibrous material. The process includes the following consecutive stages: cooking comminuted fibrous material with alkaline cooking liquor in a kraft cooking process to produce pulp; treating the cooked pulp in caustic extraction at a temperature of 70-110 C. and in an effective alkali concentration of 60-120 g/l for at least 5 minutes, and washing and oxygen delignifying the caustic extracted pulp.

Claims

1. A method for dissolving pulp from comminuted hardwood-based fibrous material, the method comprises: treating comminuted fibrous material under acidic conditions in which a P factor is in a range of 5 to 250; cooking the treated comminuted fibrous material with an alkaline cooking liquor at a temperature in a range of 120 C. to 175 C. in a kraft cooking process to produce pulp; treating the pulp in caustic extraction at a temperature is in a range of 70 C. to 110 C. for at least 5 minutes to produce caustic extracted pulp, wherein the caustic extraction includes a liquid phase in which the pulp is in suspension and an alkali concentration of the liquid phase is in a range of 60g/l to 120 g/l, washing the caustic extracted pulp, and oxygen delignifying the caustic extracted pulp after the washing.

2. The method according to claim 1, further comprising introducing white liquor with an effective alkali concentration of over 90 g/l to the pulp before the treatment of the pulp by the caustic extraction.

3. The method according to claim 1, further comprising separating filtrates from the caustic extracted pulp.

4. The method according to claim 1, further comprising extracting a first filtrate from the pulp after caustic extraction, and the first filtrate is delivered for use as pulp wash liquor flowing counter-currently to a pulp flow.

5. The method according to claim 4, further comprising separating a second filtrate from the pulp and delivering the second filtrate to the cooking step as at least a portion of the cooking liquor.

6. The method according to claim 1, further comprising treating the comminuted hard-wood based fibrous material in acid hydrolysis before the cooking step.

7. The method according to claim 1, wherein the temperature of the temperature of the caustic extraction step is in a range of 80 C. to 100 C.

8. The method according to claim 1, wherein during the caustic extraction step, the effective alkali concentration is in a range of 70 g/l to 110 g/l.

9. The method according to claim 3, wherein the pulp is treated in a fractionating wash to form the filtrates.

10. The method according to claim 4, wherein the first filtrate is delivered to a digester wash.

11. A The method according to claim 1, further comprising processing of the oxygen delignified caustic extracted pulp in an acid stage.

12. The method according to claim 1, wherein the cooking step is performed in at least one of: a continuous single hydraulic vessel, a two vessel hydraulic system, and a vapor phase digester vessel.

13. The method according to claim 1, the cooking step is performed as a batch digester process.

14. A method comprising: a) treating comminuted fibrous material under acidic conditions in which a P factor is in a range of 5 to 250; b) cooking the treated comminuted fibrous material with alkaline cooking liquor at a cooking temperature in a range of 120 C. to 175 C. to produce pulp, c) feeding alkaline wash liquor to the pulp to cool and/or wash the pulp; d) feeding white liquor to the pulp to form a mixture of the white liquor and the pulp, after step c; e) treating the mixture of the white liquor and the pulp at a temperature of 70 C. to 110 C. for 5 to 120 minutes; f) removing a first filtrate from the mixture and delivering the first filtrate as a pulp wash liquor flowing counter-currently to a pulp flow; and g) separating a second filtrate from the mixture, and using the second filtrate as at least a portion of the alkaline cooking liquor; and h) delivering the pulp to further processing after step g).

15. A method comprising: treating comminuted hardwood based fibrous material under acidic conditions in which a P factor is in a range of 5 to 250; producing pulp by a cooking the treated comminuted fibrous material with an alkaline cooking liquor at a temperature to a range of 120 C. to 175 C. in a kraft cooking process; adding the pulp to a caustic extraction vessel in which the pulp is suspended in a liquid, controlling a temperature of the liquid with the suspended pulp is in a range of 70 C. to 110 C. for at least 5 minutes to produce caustic pulp, wherein an alkali concentration of the liquid with the suspended pulp is in a range of 60g/l to 120 g/l, washing the caustic pulp after discharge from the caustic extraction vessel, and oxygen delignifying the washed caustic pulp.

16. The method of claim 15, wherein the treatment of the comminuted hardwood based fibrous material is performed in a hydrolysis reactor, the cooking of the treated comminuted fibrous material is performed in a digester cooking vessel, and the caustic extraction vessel is a blow tank.

Description

EXAMPLE 1

[0058] A method according to the invention was analyzed in a laboratory. The raw material was hardwood chips with a xylan concentration of 12.1%. When the chips were cooked in a normal alkali profile, the cooking yield was 53.3% at kappa number 17.1 and the xylan concentration in the pulp was 14.5%, meaning that 62% of the original xylan in the chips remained.

[0059] When the chips were cooked in a higher alkali concentration according to the method, the cooking yield was 50.4% at kappa number 14.5 and the xylan concentration in the pulp was 12.3%, meaning that 50% of the original xylan in the chips remained. When this pulp was caustic extracted at a temperature of 50 C., it produced pulp with a kappa number of 8.7 and a xylan concentration of 5.0%. Thus, only 16% of the original xylan in the chips remained. When the temperature of the corresponding caustic extraction was 90 C., the pulp's kappa number was 8.8 and its xylan concentration was 5.9%, and 20% of the original xylan in the chips remained. The laboratory tests show that both pulps can be used as dissolving pulp, especially after appropriate further processing and/or pre-treatments, and that caustic extraction can also be carried out quite successfully in the normal brown stock wash temperature range of 70-100 C., and that high alkali profile cooking creates a better than normal starting point for successful caustic extraction.

EXAMPLE 2

[0060] A method according to the invention was analyzed in a laboratory. The raw material was hardwood chips with a xylan concentration of 15.5%. When a prehydrolysis stage with 200 P factors was first carried out for the chips, along with a cooking stage in a high alkali concentration, the cooking yield was 44.2% at kappa number 10.2 and the xylan concentration in the pulp was 5.5%. Thus, 16% of the original xylan in the chips remained. When this pulp was caustic extracted at a temperature of 90 C. and an alkali concentration of about 80 g/l, it produced pulp with a kappa number of 6.9 and a xylan concentration of 2.6%. The total yield after prehydrolysis, cooking and caustic extraction was 42.3%. Thus, only 7% of the original xylan remained. When the same raw material was used in the laboratory to produce dissolving pulp using conventional prehydrolysis cooking with 500 P factors, the yield was 39.4%, the kappa was 6.6 and the xylan concentration in the pulp was 2.5%. These laboratory tests show that, with caustic extraction, good-quality dissolving pulp can be produced with a significantly higher yield than when using the conventional prehydrolysis process.

[0061] Advantages of the new solution:

[0062] The method connects caustic extraction more simply and economically than previously to a cooking process in the same line because the cook's alkali profile avoids excess xylan precipitation in the chips. When caustic extraction is done before the oxygen stage, transformation of residual sulfide to hydrogen sulfide in the subsequent acidic stages does not take place. With caustic extraction in accordance with the method, the prehydrolysis stage can be lightened considerably, which significantly improves the pulp yield.