Source: http://www.google.com/patents/US5554259?dq=7,003,515
Timestamp: 2014-09-15 09:40:06
Document Index: 63905669

Matched Legal Cases: ['art 1', 'art 2', 'art 1', 'art 2', 'art 2', 'art 2', 'art 2', 'art 2', 'art 2', 'art 1', 'art 2', 'art 1', 'art 2', 'art 1', 'art 1']

Patent US5554259 - Pulp bleaching - Google PatentsSearch Images Maps Play YouTube News Gmail Drive More »Sign in<nobr>Advanced Patent Search</nobr>PatentsA method for reducing or eliminating the formation of salt scale upon process equipment caused by precipitation of salts during the bleaching of pulp. The pulp is subjected to a bleaching sequence which includes a plurality of pulp treatment steps, wherein at least one pulp treatment step is conducted...http://www.google.com/patents/US5554259?utm_source=gb-gplus-sharePatent US5554259 - Pulp bleachingAdvanced Patent SearchPublication numberUS5554259 APublication typeGrantApplication numberUS 08/130,619Publication dateSep 10, 1996Filing dateOct 1, 1993Priority dateOct 1, 1993Fee statusLapsedAlso published asCA2173215A1, EP0721525A1, EP0721525A4, US5693184, WO1995009944A1Publication number08130619, 130619, US 5554259 A, US 5554259A, US-A-5554259, US5554259 A, US5554259AInventorsThomas P. Gandek, James C. Joseph, Michael A. Pikulin, George Pangalos, Philip E. BowdenOriginal AssigneeUnion Camp Patent Holdings, Inc.Export CitationBiBTeX, EndNote, RefManPatent Citations (100), Non-Patent Citations (213), Referenced by (1), Classifications (19), Legal Events (5) External Links: USPTO, USPTO Assignment, EspacenetPulp bleachingUS 5554259 AAbstract A method for reducing or eliminating the formation of salt scale upon process equipment caused by precipitation of salts during the bleaching of pulp. The pulp is subjected to a bleaching sequence which includes a plurality of pulp treatment steps, wherein at least one pulp treatment step is conducted under alkaline conditions and at least one pulp treatment step is conducted under acidic conditions. A filtrate stream which contains dissolved salts therein is thus generated, and at least a portion of the filtrate stream is combined with an alkaline stream and pulp to cause the salts to associate with the pulp, thus removing the salts from the filtrate stream to reduce or eliminate the formation of salt scale upon process equipment during the bleaching of the pulp.
What is claimed is: 1. A method for reducing or eliminating the formation of salt scale upon process equipment caused by precipitation of salts during the bleaching of pulp which comprises:subjecting the pulp to a bleaching sequence which includes a plurality of pulp treatment steps, wherein at least one pulp treatment step is conducted under alkaline conditions and at least one pulp treatment step is conducted with ozone under acidic conditions; generating an acidic filtrate stream which contains salts therein by washing or pressing the pulp after the acidic ozone treatment step; combining at least a portion of the filtrate stream with a caustic material and pulp to cause the salts to associate with the pulp, thus removing the salts from the filtrate stream to reduce or eliminate the formation of salt scale upon process equipment during the bleaching of the pulp; and directing the pulp and associated salts to at least one subsequent bleaching or brightening step. 2. The method of claim 1 wherein the caustic material is an alkaline stream.
3. The method of claim 1 wherein the caustic material is added to the filtrate stream portion prior to combining the filtrate stream with pulp.
4. The method of claim 1 wherein the pH of the acidic filtrate stream portion is raised to at least 6 by the addition of the caustic material before the filtrate stream portion is combined with the pulp.
5. The method of claim 1 wherein the acidic filtrate stream portion is initially mixed with the pulp and the pH of the mixture is then raised to at least 6 by the addition of the caustic material to promote salt association with the pulp.
6. The method of claim 1 wherein the acidic filtrate stream portion is neutralized by raising the pH to above about 7 with the addition of caustic material prior to countercurrently recycling the neutralized filtrate for mixing with the pulp.
7. The method of claim 1 wherein the concentration of salt cations or anions in the acidic filtrate stream is reduced by cocurrently recycling the filtrate stream portion to an alkaline portion of the process containing the caustic material prior to mixing with the pulp.
8. The method of claim 7 wherein the acidic filtrate stream portion is recycled to an alkaline effluent stream containing the caustic material prior to combining with the pulp.
9. The method of claim 7 wherein the acidic filtrate stream portion is recycled to an alkaline pulp stream containing the caustic material and the pulp to promote salt association with the pulp.
10. The method of claim 1 wherein the bleaching sequence is in a closed bleach plant where substantially all wash water effluents or filtrates are countercurrently recycled.
11. The method of claim 1 wherein a diluent stream comprising water is added to the acidic filtrate stream which contains the salts before mixing the filtrate stream with the alkaline material and pulp.
12. The method of claim 11 wherein the diluent stream is a low ion concentration stream.
13. The method of claim 12 wherein the diluent stream is fresh water, stripped condensate or ozone stage filtrate.
14. A method for reducing or eliminating the formation of salt scale upon process equipment caused by precipitation of salts during the bleaching of pulp which comprises:subjecting the pulp to a bleaching sequence which includes a plurality of pulp treatment steps, wherein at least one pulp treatment step is conducted under alkaline conditions and at least one subsequent pulp treatment step is conducted with ozone under acidic conditions; generating an acidic filtrate stream which contains salts therein by washing or pressing the pulp after the acidic ozone treatment step; recycling at least a portion of the acidic filtrate stream for combining with a caustic material and pulp to cause the salts to associate with the pulp, thus removing the salts from the filtrate stream to reduce or eliminate the formation of salt scale upon process equipment during the bleaching of the pulp; and directing the pulp and associated salt to at least one subsequent bleaching or brightening step. 15. The method of claim 14 wherein the caustic material is an alkaline stream.
16. The method of claim 14 wherein the pH of the acidic filtrate stream portion is increased by mixing with the caustic material to form a mixture prior to recycling the mixture to the pulp for mixing therewith.
17. The method of claim 16 wherein the pH of the acidic filtrate stream portion is increased to at least 6.
18. The method of claim 16 wherein the pH of the acidic filtrate stream portion is increased to at least about 7 prior to recycling the mixture to the pulp.
19. The method of claim 16 wherein the pH of the acidic filtrate stream portion is increased by adding the caustic material to a washing unit which washes the acidic pulp to generate a higher pH filtrate stream before the stream portion is combined with the pulp.
20. The method of claim 19 wherein the pH of the acidic filtrate stream is increased by adding the caustic material to the water used to wash the acidic pulp in the washing unit.
21. The method of claim 14 wherein the pH of the acidic filtrate stream portion is increased by the addition of caustic material after recycling the stream and mixing the pH increased stream with the pulp.
22. The method of claim 21 wherein the acidic filtrate stream portion is recycled to an alkaline effluent stream containing the caustic material prior to combining the mixture with pulp.
23. The method of claim 21 which further comprises recycling the acidic filtrate stream portion to an alkaline pulp stream to promote salt association with the pulp.
24. The method of claim 14 wherein the pulp is acidified prior to the ozone treatment to a pH of between 1 and 4.
25. The method of claim 14 wherein the acidic filtrate portion is recycled to wash the pulp prior to the ozone treatment prior the being combined with the caustic material and the pulp.
26. The method of claim 14 wherein the alkaline pulp treatment step comprises an oxygen treatment.
27. The method of claim 26 wherein the acidic filtrate portion is recycled to wash the pulp after the oxygen treatment for mixing with the pulp and alkaline material of that treatment.
28. The method of claim 14 wherein the salts comprises calcium or barium cations and the pH is increased to at least about 7.
29. The method of claim 14 wherein the pulp which is combined with the alkaline material and acidic filtrate stream is pulp which has been subjected to at least one treatment step of the bleaching sequence.
30. A method for reducing or eliminating the formation of salt scale upon process equipment caused by precipitation of salts during the bleaching of pulp which comprises:subjecting the pulp to a bleaching sequence which includes a plurality of pulp treatment steps, wherein at least one pulp treatment step is conducted under alkaline conditions and includes an alkaline oxygen delignification stage, at least one pulp treatment step is conducted under acidic conditions and includes an acidic ozone delignification stage, and a final pulp treatment step includes a brightening stage; washing or pressing the pulp after the acidic ozone delignification stage, thus generating a filtrate stream which contains salts therein; combining at least a portion of the filtrate stream with a caustic material and pulp which has been subjected to at least one treatment step of the bleaching sequence to cause the salts to associate with the pulp, thus removing the salts from the filtrate stream to reduce or eliminate the formation of salt scale upon process equipment during the bleaching of the pulp; and subsequently treating the pulp with associated salts in a brightening stage. 31. The method of claim 30 wherein the caustic material is an alkaline process stream.
32. The method of claim 30 wherein the acidic filtrate stream portion is generated by washing the pulp after the ozone delignification stage and the pH of the acidic filtrate stream portion is increased by adding caustic material to the pulp after the ozone delignification stage.
33. The method of claim 32 wherein a portion of the caustic material is added to the acidic ozone delignified pulp before the pulp is washed to increase the pH of the resulting filtrate stream.
34. The method of claim 30 wherein the brightening stage is a chlorine dioxide or peroxide stage.
35. The method of claim 34 wherein the pulp with associated salts is added to an alkaline extraction prior to the brightening stage.
36. The method of claim 35 wherein the acidic filtrate stream portion is recycled cocurrently for combining with the caustic and pulp in an alkaline stream downstream of the ozone delignification stage.
37. The method of claim 36 wherein the acidic filtrate stream portion is added to an alkaline effluent stream containing the caustic material to form a mixture prior to combining the mixture with the pulp.
38. The method of claim 36 wherein the caustic material and pulp is an alkaline pulp stream.
39. The method of claim 30 wherein the oxygen and ozone delignification stages are each conducted on high consistency pulp.
40. The method of claim 39 wherein the pH of the acidic filtrate stream portion is increased to at least 6 by the addition of the caustic material prior to combining with pulp.
41. The method of claim 40 wherein the salts comprise calcium or barium cations and the pH of the acidic filtrate stream portion is increased to at least about 7.
42. The method of claim 30 wherein the pulp with associated salts is washed after the brightening stage and the resultant wash water is recycled countercurrently to wash pulp exiting a bleaching stage.
43. A method for reducing or eliminating the formation of salt scale upon process equipment caused by precipitation of salts during the bleaching of pulp in a continuous pulp processing stream which comprises:subjecting the pulp stream to a bleaching sequence which includes a plurality of pulp treatment steps, wherein at least one pulp treatment step is conducted under alkaline conditions and at least one pulp treatment step is conducted with ozone under acidic conditions; generating a filtrate stream which contains salts therein by washing or pressing the pulp after the acidic ozone treatment step; combining at least a portion of the filtrate stream with a caustic material and the pulp stream to cause the salts to associate with the pulp, thus removing the salts from the filtrate stream to reduce or eliminate the formation of salt scale upon process equipment during the bleaching of the pulp; and directing the pulp and associated salts to at least one subsequent bleaching or brightening step. Description
TECHNICAL FIELD The present invention relates to a method for preventing or minimizing the formation of calcium or other insoluble salt precipitates in equipment used for washing and processing pulp during a bleaching sequence where a countercurrent wash water effluent recycle strategy is utilized. Salt scale precipitation is reduced by control of the pH and salt concentration of various process streams and selective association with the pulp fiber.
BACKGROUND OF THE INVENTION In any bleaching pulp process, filtrate management is an important factor in the overall economy or cost of operation of the process. The water which is used in the process requires both access to a suitable source and treatment of the effluent prior to discharge.
U.S. Pat. No. 5,164,043 discloses an environmentally improved chlorine-free process for bleaching pulp with oxygen ("O"), ozone ("Z") and then chlorine dioxide ("D") or a peroxide ("P"). A modified oxygen ("Om ") stage followed by a modified ozone ("Zm ") stage is a preferred delignification sequence. The Om stage is conducted by reducing the consistency of the pulp to less than about 5%, substantially uniformly combining the pulp with alkaline material, increasing the consistency of the pulp to obtain the desired amount of alkaline material uniformly distributed thereon, and subjecting the pulp to high consistency oxygen delignification. The Zm stage is conducted by acidifying the pulp, adding a chelating agent, increasing the consistency of the pulp to greater than 20%, and turbulently mixing the pulp at the high consistency with ozone in a dynamic reaction chamber.
Instead of Om Zm ED, one may use the Om EP process of the invention to obtain additional substantial advantages over the prior art in that no chlorinated compounds whatsoever are produced. This enables all of the effluent to be recycled without experiencing the problems of chloride build-up in the process wash water streams.
It has now been found that the effluent from the washer downstream of the ozone reactor becomes acidic, primarily due to the relatively low pH conditions of the pulp in the ozone reactor. During typical continuous operation of the Zm stage, the washer effluent achieves a pH of about 3 to 4 due to the countercurrent flow of alkaline E-stage filtrate. When this washer effluent is recycled to the washer upstream of the ozone stage, the pH of the wash water in that washer drops, calcium, barium and other metals desorb from the pulp and salts of divalent cations such as calcium and barium, and in particular, calcium and barium oxalates, precipitate from the wash water. It has been found that this precipitation generally occurs in the washer, although it can occur in downstream process lines and equipment, such as in the acidification step or the ozone reactor, where it causes operability problems. The extent of scaling can be sufficiently large to cause plugging or blocking of the equipment and require shutdown of the process to remove the precipitated salts. To resolve this problem, it is necessary to reduce the concentration of the divalent cation in this part of the process, or to not recycle the stream that contains it.
Accordingly, what is needed is a method for preventing or controlling precipitation of salts from the wash water effluents or filtrates which are recycled in order to avoid forming substantial amounts of salt scale in process equipment, but without purging or discharging the effluents or filtrates which contain such salts. The present invention provides a simple, yet effective, method for resolving this problem.
SUMMARY OF THE INVENTION The present invention relates to a method for reducing or eliminating the formation of salt scale upon process equipment caused by precipitation of salts during the bleaching of pulp which comprises: subjecting the pulp to a bleaching sequence which includes a plurality of pulp treatment steps, wherein at least one pulp treatment step is conducted under alkaline conditions and at least one pulp treatment step is conducted under acidic conditions; generating a filtrate stream which contains dissolved salts therein; and combining at least a portion of the filtrate stream with a caustic material and pulp to cause the salts to associate with the pulp, thus removing the salts from the filtrate stream to reduce or eliminate the formation of salt scale upon process equipment during the bleaching of the pulp. For convenience, the caustic material may be an alkaline process stream.
BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic flow diagram of the wash water treatment processes of the present invention.
DETAILED DESCRIPTION OF THE INVENTION U.S. Pat. No. 5,164,043 discloses the preferred Om Zm Eo D process which is utilized in the present invention. Accordingly, the content of the '043 patent is expressly incorporated herein by reference thereto. FIG. 2 of the '043 patent schematically illustrates the entire bleaching process. For convenience in understanding the present invention and for comparing the present process to that of the '043 patent, like numerals will be used to refer to the equipment and process streams which are the same in each process.
According to one embodiment of the present invention, an alkaline solution is added to the washer effluent 86 from the washer 84 which is downstream of the ozone reactor 58 before or as such effluent 86 is countercurrently recycled in order to avoid precipitating salts in the equipment to which that effluent is recycled.
This dilution tank 68 receives the acidic pulp and effluent 72 is added to act as an ozone seal and also reduce the consistency of the pulp to facilitate movement of the pulp 74 to subsequent bleaching treatments. Caustic material 225 may instead be added with dilution water 72 or can be added separately, as shown. 0f course, adding caustic material 205 to effluent 86 prior to the takeoff of stream 72 avoids the need for the separate addition at line 225.
According to another embodiment of the present invention, a portion 51 of the acidification filtrate 50 is cocurrently recycled to the Eo stage pulp or to a point further downstream thereof in order to avoid the precipitation of salts in the equipment to which that filtrate was previously recycled. Filtrate portion 51 is generated by the difference in consistency of the pulp exiting wash press 34 and that exiting displacement press 48 and chemical and water addition, and must be discharged or directed to another point in the process to maintain water balance in the acidification loop. In a closed bleach plant, this stream must be directed to another point in the process. Filtrate portion 51 previously was recycled to blow tank 32 for mixing with the pulp 30 which exits the oxygen reactor 26. It was found, however, that this procedure was disadvantageous in that precipitation of such salts would occur in the wash press 34 where the pulp 30, acidic filtrate portion 51 and acidic effluent 86 were combined.
In order to prevent the formation of scale from the precipitation of such salts in displacement press 48, the salt concentration of the filtrate 50 must be decreased. This is easily accomplished by the cocurrent recycle of filtrate portion 51 to a downstream location where alkaline pulp is present. The mixing of the acidic filtrate portion 51 with alkaline pulp causes the salts to become associated with the pulp fiber for removal from the system by being physically carried forward with the fiber. This causes those salts to be removed from the acidification loop, with the concentration of such salts being reduced in filtrate 50. The precipitation problem in displacement press 48 is then eliminated or substantially reduced since the concentration of such salts in the process fluid is reduced below the precipitation point. Furthermore, when the effluents used for washing the ozone bleached pulp are neutralized by the addition of alkaline material as explained above, the introduction of salts into the acidification loop is reduced, with the concentration of such salts being further reduced. Conducting both a cocurrent recycle of filtrate portion 51 in addition with a neutralization of the ozone bleached pulp portion of the system is thus advantageous for optimum elimination of salt scale precipitation.
To help control the concentration of such salts in acid filtrate 50, it is useful to introduce fresh water or filtrate 72 into the filtrate loop. Then, a relatively larger quantity of filtrate portion 51 is removed for cocurrent recycle as noted above. Typically, only about 1 to 10 percent of filtrate 50 is removed as filtrate portion 51. When fresh water or filtrate 72 is added, the quantity of filtrate portion 51 increases to about 10 to 20 percent. For the situation where the consistency of the pulp exiting wash press 34 is about the same as that exiting displacement press 48, adding fresh water or filtrate 72 helps generate a filtrate portion 51 that can be removed to control the concentration of salts in the acidification loop.
EXAMPLES The following examples provide illustrations of the preferred modes of carrying out the processes of the present invention without limiting its scope.
Example 1 A pulp bleaching sequence incorporating the use of ozone has been implemented on a 1000 ADTPD commercial scale. The bleaching sequence is an Om Zm Eo D sequence which incorporates full countercurrent flow of effluents from the Eo stage back through brownstock washing and ultimately to the liquor recovery system. As described above and in U.S. Pat. No. 5,164,043, the Om and Eo stages are operated under alkaline conditions (pH 10-12), and the Zm stage is operated under acidic conditions (pH 2-3).
Example 2 The pulp bleaching sequence of Example 1 uses an acidification step for reducing the pH of the pulp prior to the Zm stage. Since the consistency of the pulp entering the Zm reactor is higher than that which exits the post-oxygen washer, a portion of the acidification filtrate is countercurrently recycled. Since this stream was recycled to a point upstream of the acidification step, calcium and barium salts were not removed and continued to build up in this step. It was observed that substantial scaling in the form of calcium and barium sulfates also occurs in the acidification step wash press, particularly inside the drain holes where the wash water would be removed. The extent of scaling required cleaning of the equipment on a regular basis to maintain an operable process.
While the preceding examples have specifically illustrated the operability of the present process to prevent the formation of calcium and barium salt scale formation, the same principles are believed to apply to other compounds which are in alkaline filtrates and which can precipitate when contacting such filtrates. Thus, scale which results from precipitates of such other compounds can be prevented by following the principles and disclosure presented herein.
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