METHODS FOR OXYGEN DELIGNIFICATION AND OZONE BLEACHING OF PULP

Abstract

A method for treating pulp through oxygen delignification and ozone bleaching is disclosed. The method uses a modified oxygen recovery unit which separates a mixture of oxygen and ozone thereby generating a stream of ozone and a stream of oxygen; feeding the stream of oxygen to an oxygen delignification unit; feeding a pulp to the oxygen delignification unit; feeding the stream of ozone to an ozone bleaching unit; feeding the delignified pulp to the ozone bleaching unit; and recovering the delignified and bleached pulp.

Claims

1. A method for delignifying and bleaching pulp comprising the steps: a) Feeding a mixture of oxygen and ozone to an oxygen recovery unit thereby generating a stream of ozone and air and a stream of oxygen; b) Compressing and feeding the stream of oxygen to an oxygen delignification unit; c) Feeding a pulp mixture which comprises a pulp slurry and a liquor to the oxygen delignification unit; d) Feeding steam to the pulp mixture thereby raising its temperature; e) Mixing the pulp mixture of pulp, liquor, steam and oxygen thereby enabling the oxygen to react with lignin and become water soluble and mixed with the liquor; f) Feeding the pulp mixture to at least one washing step thereby removing dissolved lignin from the pulp mixture; g) Feeding the stream of air and ozone to an ozone bleaching unit; h) Feeding the delignified, washed pulp mixture of step f) to the ozone bleaching unit; and i) Recovering the delignified and bleached pulp.

2. The method as claimed in claim 1 wherein the pressure in step b) ranges from 5 to 12 Barg.

3. The method as claimed in claim 1 wherein during step c), the temperature of the pulp mixture increases from about 90 to 120 C.

4. The method as claimed in claim 1 wherein the mixing is up to 100 minutes in length.

5. The method as claimed in claim 1 wherein the at least one washing step is two or more washing steps.

6. The method as claimed in claim 1 wherein the pulp is selected from the group consisting of medium and high consistency pulp.

7. The method as claimed in claim 1 wherein the liquor is selected from the group consisting of white liquor, partially oxidized white liquor and completely oxidized white liquor.

8. The method as claimed in claim 1 wherein the concentration of oxygen in the stream of oxygen is greater than 80%.

9. The method as claimed in claim 1 wherein the oxygen delignification unit is a reactor.

10. The method as claimed in claim 9 wherein the oxygen is present in the pulp between 0.011 to 0.026 tons of oxygen per one ton of pulp.

11. The method as claimed in claim 1 wherein the mixture of oxygen and ozone is produced by an ozone generation unit.

12. The method as claimed in claim 10 wherein the mixture of oxygen and ozone is about 10% ozone and 90% oxygen.

13. The method as claimed in claim 1 wherein the oxygen recovery unit is a pressure swing adsorption system.

14. The method as claimed in claim 13 wherein the pressure swing adsorption system contains an adsorbent.

15. The method as claimed in claim 14 wherein the adsorbent is selected from the group consisting of silica gel and high silica zeolite.

16. The method as claimed in claim 1 wherein the ozone bleaching unit is a reactor.

17. The method as claimed in claim 6 wherein the pulp is about 10% consistency.

18. The method as claimed in claim 1 further comprising adding acid to the pulp before the pulp enters the ozone bleaching unit.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0043] The FIGURE is a schematic of an oxygen delignification and ozone bleaching process according to the invention.

DETAILED DESCRIPTION OF THE INVENTION

[0044] Turning to the figure, a schematic of an oxygen delignification and ozone bleaching process in communication with an oxygen recovery unit is described.

[0045] The pulp which is typically a medium or high consistency pulp is fed through line 1 to an oxygen delignification unit A. The pulp will optimally be about 10% pulp slurry. This pulp feed will typically be at a temperature of about 90 to 120 C. and a pressure of 5 to 12 barg. The pulp solution is typically a caustic solution aided by the presence of magnesium sulfate. A blow tank will usually be present to assist in pressure reduction and separation of the pulp slurry and dissolved gases.

[0046] The oxygen delignification unit A will receive oxygen through line 4 from the modified oxygen recovery unit C which will separate a mixture of ozone and oxygen. Typically the oxygen is gaseous oxygen and is present between 0.011 to 0.026 tons of oxygen per one ton of pulp. The modified oxygen recovery unit C will typically have an external line 2A which will typically utilize clean dry air to assist in desorbing the beds present in the oxygen recovery unit C.

[0047] The modified oxygen recovery unit C will receive through line 2 a mixture of ozone and oxygen from an ozone generator E. The mixture of ozone and oxygen is typically about 10% ozone and 90% oxygen.

[0048] The reactor will thoroughly mix the oxygen and the pulp and the resulting delignified pulp will be fed through line 3 to a series of washers step B where dissolved solids and soluble lignin is removed through line 5. The pulp in line 3 will have the same consistency as when fed through line 1 but be nearer to atmospheric pressure. The Kappa reduction is also greater than 35%. The dissolved solids and soluble lignins removed through line 5 form a weak black liquor. The delignified pulp will then be fed through line 7 to an ozone bleaching unit D. The consistency of the pulp slurry will remain the same at about 10% and the pressure remains at near atmospheric after unit B. Acid is typically added to this stream prior to it reaching the ozone bleaching unit D and this stream is raised in pressure through a pump to a range of 5 to 12 barg.

[0049] The ozone bleaching unit D will also be a reactor which will thoroughly mix the ozone and delignified pulp. The ozone will be derived from the modified oxygen recovery unit C and will be fed through line 6 to the ozone bleaching unit. The mixture is approximately 10 wt % ozone with the remaining 90% being clean dry air. The reactor will ensure that the ozone and the delignified pulp are thoroughly mixed to provide the desired level of bleaching. The final bleached pulp may then be recovered through line 9 and sent for further processing in the pulp mill.

[0050] While this invention has been described with respect to particular embodiments thereof, it is apparent that numerous other forms and modifications of the invention will be obvious to those skilled in the art. The appended claims in this invention generally should be construed to cover all such obvious forms and modifications which are within the true spirit and scope of the invention.