Ozone bleaching of low consistency pulp

• US 20030006017A1
• Filed: 07/24/2002
• Published: 01/09/2003
• Est. Priority Date: 05/08/1998
• Status: Active Grant

• Current Assignee: Air Liquide America LP (Mitsubishi Chemical Group Corp.)
• Original Assignee: Air Liquide America LP (Mitsubishi Chemical Group Corp.)
• Inventors: Fisher, Steven A., Hornsey, Derek, Corbeil, Sebastien, Sundaram, V.S. Meenakshi
• Granted Patent: US 6,579,412 B2
• Time in Patent Office: Days
• US Class Current: 162/65
• CPC Class Codes: D21C 9/144 with ClO2/Cl2 and other bleaching agents in a multistage process
• CPC Class Codes: D21C 9/153 with ozone

Abstract

Provided is a process for bleaching pulp with ozone. The process involves preparing a slurry of cellulosic pulp having a consistency in fibers of from 1-5 weight %. Such a low consistency slurry is then mixed with ozone under high shear conditions. The ozone is then maintained in contact with the cellulosic fibers to effect bleaching of the fibers. The present process offers the advantages of bleaching using a low consistency slurry, with the added advantages of employing ozone.

Claims

1. A process for bleaching pulp with ozone, which comprises:

preparing a slurry of cellulosic pulp having a consistency in fibers of from 1-5 weight %;

adding ozone to the cellulosic pulp with high shear mixing; and

maintaining the ozone in contact with the cellulosic fibers for a time sufficient to bleach the fibers.

2. The process for bleaching pulp with ozone of claim 1, wherein the bleached fibers are passed onto a chlorine dioxide bleaching stage.

3. The process for bleaching pulp of claim 1, wherein the cellulosic pulp used to prepare the slurry is obtained from a chlorine dioxide bleaching stage.

4. The process for bleaching pulp of claim 1, wherein the ozone/cellulosic pulp is passed into a pressurized retention tube where the ozone reacts with the lignin in the cellulosic pulp.

5. The process for bleaching pulp of claim 4, wherein the ozone/cellulosic pulp from the retention tube leaves the retention tube through a pressure control valve and is discharged into a separate vessel, where the gas is separated and then passed into an ozone destruct unit before venting to the atmosphere, and the pulp slurry is pumped to a subsequent bleaching stage.

6. The process for bleaching pulp of claim 1, wherein the ozone used in the process is generated on-site from oxygen in a pressurized ozone generator.

7. The process for bleaching pulp with ozone of claim 1, wherein the ozone charge added to the pulp is in the range of from about 2-10 kg/ton of pulp.

8. The process for bleaching pulp of claim 7, wherein the ozone charge added to the pulp is in the range of from about 5-6 kg/ton of pulp.

9. The process for bleaching pulp of claim 6, in which the ozone generator produces ozone from oxygen at a concentration in the range of 10-20%.

10. The process for bleaching pulp of claim 9, wherein the ozone generator operates at a pressure in the range of from about 20-60 psig.

11. The process for bleaching pulp of claim 1, wherein the pulp slurry consistency is in the range of from 3-4 weight %.

12. The process for bleaching pulp of claim 1, wherein the ozone is mixed with the cellulosic fibers under high shear conditions for a period of time ranging from 0.1 to 10 seconds.

13. The process for bleaching pulp of claim 4, wherein the residence time in the retention tube ranges from 1 to 10 minutes.

14. The process for bleaching pulp of claim 1, wherein the temperature of the pulp slurry entering the mixing with ozone is in the range of from 20 to 80°

C.

15. The process for bleaching pulp of claim 5, wherein the subsequent bleaching stage involves chlorine dioxide as the bleaching agent.

17. The reactor of claim 16, wherein the high shear mixer produces high shear by high rotational speeds across a narrow gap through which the pulp slurry flows.

18. The reactor of claim 16, wherein the pulp slurry into which the ozone is dispersed in of a consistency of from 3 to 4 wt %.

19. The reactor of claim 16, wherein an ozone generator is used to generate the ozone dispersed into the pulp slurry in the high shear mixer, with the ozone generator being connected with the high shear mixer.

20. The reactor of claim 16, wherein an ozone compressor is connected to the high shear mixer such that the ozone delivered to the high shear mixer has been first compressed.

21. The reactor of claim 16, wherein a gas meter is present in a conduit to a high shear mixer in order to regulate the flow of gas mixture to the mixer.

16. A reactor for bleaching pulp at low consistency with ozone, comprising a high shear mixer wherein ozone is dispersed into a pulp slurry having a consistency in the range of from 1 to 5 wt %, and a retention tube connected to the mixer which operates at a pressure of from 20 to 60 psig, and wherein the ozone bleaches the pulp in the pulp slurry.