Process for pretreating particulate lignocellulosic material to remove heavy metals

• US 4,599,138 A
• Filed: 10/30/1981
• Issued: 07/08/1986
• Est. Priority Date: 05/02/1977
• Status: Expired due to Fees

• Current Assignee: MO OCH Domsjo AB A Limited Company of Sweden
• Original Assignee: MO OCH Domsjo AB
• Inventors: Lindahl, Jonas A. I.
• Primary Examiner(s): ALVO, MARC S
• Application Number:
  US06/316,461
• Time in Patent Office: 1,712 Days
• Field of Search: 162/18, 162/56, 162/23, 162/24, 162/25, 162/26, 162/19, 162/65, 162/78, 162/90, 162/20, 162/37, 162/40, 162/41, 162/261, 162/28, 162/84, 162/86
• US Class Current: 162/19
• CPC Class Codes: D21B 1/021 by chemical means
• CPC Class Codes: D21C 1/00 Pretreatment of the finely-divided materials before digesting of waste paper D21C5/02

Abstract

A process is provided for pretreating particulate lignocellulosic material to remove heavy metals and resin without any delignification or defibration, which comprises washing particulate lignocellulosic material; compressing the washed material to a solids content of at least 40% to remove absorbed and excess liquid; impregnating the compressed material with an alkaline aqueous solution comprising alkali and at least one of a heavy metal ion complexing agent and a heavy metal ion reducing agent; heating the impregnated material at a temperature within the range from abut 50° to 100° C. for up to approximately 0.75 hour; compressing the pretreated material to a solids content of at least 40%; and separating undiluted liquor squeezed out during the compression, while maintaining conditions during the pretreating such that the pH of the squeezed-out liquor is within the range from about 4 to about 9.5; thereby separating heavy metal ions, resins and alkali-extracted substances in solution in the expressed liquor, but not any lignin.

Claims

1. A process for pretreating particulate lignocellulosic material to remove heavy metals and resin without any delignification or defibration, which comprises washing particulate lignocellulosic material;

compressing the washed material to a solids content of at least 40% to remove undiluted, absorbed liquid;

impregnating the compressed material with an alkaline aqueous solution comprising alkali and at least one member selected from the group consisting of a heavy metal ion complexing agent and a heavy metal ion reducing agent;

heating the impregnating material at a temperature within the range from about 50°

to 100°

C. for from about 0.1 to about 0.75 hour;

compressing the pretreated material to a solids content of at least 40%; and

separating undiluted liquor squeezed out during the compression;

while maintaining conditions during the pretreating such that the pH of the squeezed-out liquor is within the range from about 4 to about 9.5;

thereby separating heavy metal ions, resins and alkali-extracted substances in solution in the expressed liquor but no lignin.

2. A process according to claim 1, in which following the pretreatment the particulate lignocellulosic material is delignified and pulped by a chemical pulping process.

3. A process according to claim 2 in which the lignocellulosic material after compression to a solids content of at least 40% is chemically pulped using a chemical pulping process and an aqueous pulping liquor selected from the group consisting of acidic sulfite liquor, bisulfite liquor, sulfite liquor, alkaline sodium hydroxide liquor, alkaline sodium carbonate liquor, alkaline sodium bicarbonate liquor, white liquor, and sodium hydroxide/oxygen gas.

4. A process according to claim 1, in which following the pretreatment the particulate lignocellulosic material is defibrated and refined by a mechanical defibrating process.

5. A process according to claim 1, in which following the pretreatment the particulate lignocellulosic material is delignified, pulped and defibrated by a combined chemical and mechanical delignifying, pulping and defibrating process.

6. A process according to claim 1, in which the amount of alkali in the pretreating solution is so controlled that the pH of the expressed and separated pretreating liquor at the conclusion of the pretreatment is within the range from about 5 to about 7.5.

7. A process according to claim 1, in which the amount of complexing agent in the pretreating solution is within the range from 0.05% to about 0.80%, based on the dry weight of the particulate lignocellulosic material.

8. A process according to claim 1, in which the heavy metal ion reducing agent is selected from the group consisting of sulfur dioxide, sulfurous acid and sulfite salts, and the amount of reducing agent is within the range from about 0.1% to about 3.0%, based on the dry weight of the particulate lignocellulosic material.

9. A process according to claim 1 in which after compression to a solids content of at least about 40% the lignocellulosic material is pulped by heating in a pressure vessel in the presence of at least one of steam and compressed air at a temperature within the range from about 90°

to about 180°

C. for from about 1 to about 15 minutes, and is then subjected to mechanical defibration.

10. A process according to claim 1 in which the particulate lignocellulosic material after compression to a solids content of at least 40% is defibrated directly.

11. A process according to claim 1 in which after compression to a solids content of at least about 40% the lignocellulosic material is digested and thereby partially chemically pulped by heating in a pressure vessel in the presence of at least one of steam and compressed air at a temperature within the range from about 90°

to about 180°

C. for from about 1 to about 15 minutes, and is then subjected to mechanical defibration in a disc refiner, in the presence of a bleaching agent supplied to the disc refiner so that it is mixed with the lignocellulosic material in the vicinity of the circumference of the grinding disc, and at a distance of at least one third of the disc radius from its center.

12. A process according to claim 11, in which the bleaching agent is a lignin preserving bleaching agent.

13. A process according to claim 12, in which the bleaching agent is a mixture of a peroxide bleaching agent, sodium hydroxide, sodium silicate and magnesium sulfate.

14. A process according to claim 13, in which the sodium silicate is applied separately at the circumference of the grinding discs or at a distance of at most 200 mm from their circumference, and the remaining bleaching agent is applied at the center of the grinding discs, or at a distance from the center corresponding to at most one quarter of the grinding disc radius.

15. A process according to claim 1 in which the heavy metal ion reducing agent is SO₂, and all of the SO₂ supplied via the alkaline aqueous solution is consumed so that the expressed liquor contains substantially no free SO₂.

16. A process according to claim 1, in which the alkaline aqueous solution comprises alkali.

17. A process according to claim 1, in which the alkaline aqueous solution comprises alkali and a heavy metal ion complexing agent.

18. A process according to claim 1, in which the alkaline aqueous solution comprises alkali and a heavy metal ion reducing agent.

19. A process according to claim 1, in which the alkaline aqueous solution comprises alkali, a heavy metal ion complexing agent, and a heavy metal ion reducing agent.