BACKGROUND OF THE INVENTION There is continuous research in the pulp and paper industry aimed at improving efficiencies in the various aspects of pulp and paper processes. The bleaching process is one aspect which has received ongoing attention.

The present invention is concerned with improvements in the process for improving brightness in mechanical and ultra-high yield pulps, such as groundwood pulp (GW), thermomechanical pulp (TMP), chemithermomechanical pulp (CTMP), and alkaline peroxide mechanical pulp (APMP). The processes of interest accomplish the bleaching utilizing hydrogen peroxide in an alkaline environment.

The efficiency of such processes depends on various factors, a very important one of which is the full utilization of the hydrogen peroxide. The objective is to obtain the dual efficiency of the improved brightness and reduced chemical usage that comes from increased utilization.

It is known that chemicals present in the pulp slurry will result in decomposition of a part of the hydrogen peroxide. This is particularly the case in the presence of transition metal ions. The hydrogen peroxide decomposition obviously reduces potential bleaching power, but also affects brightness by causing the formation of new chromophores. Manganese is known to be the most harmful transition metal species in this regard.

It is common to utilize stabilizers and sequestering agents to reduce the peroxide decomposition.

Notwithstanding the use of these methods, the peroxide

decomposition has been an ongoing problem.

Against this background the present invention provides a process in which the peroxide decomposition is reduced and brightness enhanced relative to known processes.

PRIOR ART Typical state of the art processes are described in the following references: 1. Presley, J. R. and Hill, R. T., Pulp Bleaching : Principles and Practice, Edited by C. W. Dence and D. W. Reeve, Page 480. This is the so-called cascade system for preparation of bleach liquor, in which magnesium sulfate and sodium silicate are added to water and intimately mixed, followed by the addition of caustic soda and finally by the addition of hydrogen peroxide. The resulting liquor is subsequently mixed with pulp.

2. Presley, J. R. and Hill, R. T., Pulp Bleaching : Principles and Practice, Edited by C. W. Dence and D. W. Reeve, Page 481. This is the so-called in-line system, where similar mixing and addition occurs but without the cascade arrangement.

3. Ni, Y. et al., Proceedings, PAPTAC Annual Meeting, Montreal, 1999, Page B183. This process provides a sequential addition of chemicals beginning with the addition of hydrogen peroxide to a pulp slurry and the subsequent and simultaneous addition of caustic soda and silicate stabilizer.

The following patents deal with bleaching processes for mechanical or high yield pulp, but do not address the process of the present invention: Canadian Patents 686,115; 820,190; 1,294,655; 1,310,797; 2,041,588; 2,070,556.

United States Patents 2,872,280; 3,023,140; 4,029,543; 4,731,161; 4,812,206; 4,915,785; 4,938,842; 5,118,389.

BRIEF SUMMARY OF THE INVENTION It has now been determined that improvement in the bleaching process can be obtained if at least one stabilizer is added to the pulp slurry prior to the addition of the hydrogen peroxide.

Thus, the invention provides a process for peroxide bleaching of mechanical or high yield pulp, the process comprising adding to a pulp slurry at least one stabilizer for stabilizing transition metal ions in said slurry ; subsequently adding hydrogen peroxide to said slurry at a preselected point; adding an alkali source to said slurry simultaneously with or subsequent to adding said at least one stabilizer; and subjecting said slurry to preselected conditions to complete said bleaching process.

In a further embodiment, the process further comprises the pretreatment steps of adding a cheating agent to said slurry and subsequently removing cheated transition metal ions from said slurry.

In a further embodiment, the process further comprises the pretreatment steps of adding a cheating agent and a reducing agent to said slurry and subsequently removing cheated transition metal ions from said slurry In a further embodiment there is provided a process for peroxide bleaching of mechanical or high yield pulp, the process comprising adding to a pulp slurry sodium hydrosulfite and DTPA; subsequently removing from the slurry cheated transition metal ions; adding to the slurry a stabilizer comprising sodium silicate, magnesium sulfate and DTPA; subsequently adding hydrogen peroxide to said slurry at a preselected point in said process; adding caustic soda to said slurry simultaneously with or subsequent to adding said at least one stabilizer ; and subjecting said slurry to preselected conditions to complete said bleaching process.

BRIEF DESCRIPTION OF THE DRAWING The figure illustrates in schematic form a process according to the invention. While the invention will be described in conjunction with the

illustrated embodiments, it will be understood that it is not intended to limit the invention to such embodiments. On the contrary, it is intended to cover all alternatives, modifications and equivalents as may be included within the spirit and scope of the invention as defined by the appended claims.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS In the following description, similar features in the drawings have been given similar reference numerals.

It is useful in understanding the invention to consider the problem that arises in the peroxide bleaching process as a result of the naturally occurring transition metal ions in the wood. The particular transition metals present and their concentrations vary geographically and seasonally, but the problem caused by the transition metals will always be present.

As a fundamental part of process efficiency, there is a desire to maximize peroxide utilization in the bleaching process. At the same time, it is desired to minimize the production of chemicals in the process which adversely affect brightness by producing chromophores.

The invention therefore concerns the realization that if the transition metal ions can be stabilized by the addition of stabilizers to a pulp slurry prior to addition of hydrogen peroxide, the decomposition of peroxide will be greatly reduced and the overall efficiency of the process will be raised.

As will be discussed below, one preferred embodiment of the invention involves a pre-treatment process to remove some transition metal ions.

This comprises an enhanced chelation process preceding the bleaching process itself.

Turning now to the drawing, the flow sheet can be divided as between the pre-treatment section 10 and the bleach section 12. The process of , the present invention is not dependent on the presence of a pre-treatment section 10 and can be said to begin at the tank 14. From tank 14 the unbleached pulp slurry is pumped via pump 16 through T-mixer 18 to tank 20. The stabilizers and bleach chemicals are added between tanks 14 and 20. It is convenient to add

stabilizers via lines 22 and 24 at pump 16 to facilitate mixing at pump 16. Caustic can also be added via line 30 at pump 16 or farther downstream, via, for example, line 31 at mixer 18. Hydrogen peroxide and water are subsequently added through lines 26 and 28 to T-mixer 18.

One or more stabilizers are preferably chosen from sodium silicate, magnesium sulfate, or DTPA or EDTA or their salts. In a preferred embodiment of the process the stabilizer comprises silicate or its solution, and in a most preferred embodiment comprises a combination of silicate, DTPA and manganese sulfate.

The alkali source can be chosen from those used in conventional such processes, but is preferably NaOH.

It is highly preferable that the inventive process described above be preceded by a pre-treatment process, as is common in the art, for the removal of a significant proportion of the transition metal ions which are present in varying concentrations in the pulp.

It is further preferred that the pre-treatment process comprise a reducing agent assisted chelation process. One such process is described in Ni, Y. et. al., Pulp & Paper Canada, 100 (10), 51-55 (1999).

Therefore, with reference to the drawing, the pre-treatment process comprises adding at least one of a cheating agent at line 32 and preferably a reducing agent at line 34. In the preferred case both the cheating agent and the reducing agent are used in the pretreatment process.

While known suitable cheating and reducing agents may be used in the pre-treatment process, it is preferred that the cheating agent be DTPA and that the reducing agent be sodium hydrosulfite.

An advantage of these two additives is that conditions for their use are similar, so that they can be added in a single step.

Subsequent to the addition of the additives, preferably downstream of a pump 36, the pulp slurry is dewatered and washed at press and washers 38 to remove the cheated transition metal ions. The slurry is then transferred into tank 14 and hence into the bleach process.

With reference to the bleach process, the following are preferred parameters: Hydrogen peroxide charge 1-10% Caustic soda charge 0.5-5% Sodium silicate charge 1-5% Magnesium sulfate charge 0-0.2% DTPA charge 0.05-0.5% The following examples illustrate the invention.

Example 1.

Equivalent to 10 grams o. d. mill cheated TMP pulp from a mill in Eastern Canada (60 ppm Mn, 53% ISO initial brightness) was treated in a polyethylene bag with 2% Na2SiO3, reagent grade, and 0.05% MgSO4 for 1 minute; and then with addition of 3% H202for about 5 minutes. A thorough mixing was provided after each reagent addition. Subsequently 1.5% NaOH was added to the pulp in the polyethylene bag. The bag, along with its contents, was placed in a temperature bath at 60°C. The bleaching conditions were 120 min, 12% pulp consistency.

After the completion of the required reaction time, a portion of the filtrate was then taken to determine the residual H202 and ending pH. The remaining pulp slurry was transferred from the bag to a beaker, further diluted, and neutralized with sulfuric acid to pH 5. The neutralized pulp slurry was subsequently filtered and washed thoroughly with deionized water. A handsheet was then made following TAPPI test method T272, air-dried and determined for brightness.

The residual hydrogen peroxide was 0.65% on pulp, and the brightness of the resulting pulp was 69.6% ISO.

The process described above is designated as the PM process.

Example 2.

The following is provided to illustrate that the residual hydrogen peroxide is much less and the brightness of the resulting pulp is lower if the same TMP pulp was subjected to a conventional peroxide stage (P) under otherwise the same conditions.

The conventional peroxide stage (designated as P process in the subsequent discussion) was performed as follows : Equivalent to 0.05% MgSO4, 2.0% Na2SiO3, 1. 5% NaOH, and 3.0% H202 were first added in that order to a beaker containing distilled water to form a mixture. This mixture was then added to a polyethylene bag which contains equivalent to 10 grams o. d. of the same TMP pulp as in Example 1. The contents were mixed thoroughly. Subsequently, the polyethylene bag, along with its contents, was placed in a temperature bath at 60°C to start bleaching. The bleaching conditions were the same as those in Example 1. After the completion of the required reaction time, samples were collected for residual H202, ending pH and brightness, in accordance with the procedures in Example 1.

The residual hydrogen peroxide was 0.29% on pulp and the pulp brightness was 68.7% ISO. These results are compared with those in Example 1 of 0.65% and 69.6% ISO, respectively, supporting that the peroxide bleaching performance is improved by the process outlined in Example 1.

Example 3.

In this example, it will be shown that the PM process, described in Example 1, can be varied and the improvement in bleaching performance over the P process can be maintained, even enhanced. In this case, DTPA was added as part of the stabilizers, along with sodium silicate and magnesium sulfate. These stabilizers were mixed with the pulp slurry for 5 minutes. Subsequently, the required amount of caustic soda was added. The same TMP pulp as that in Example 1 was used. The procedures for the PM and P process were the same as those in Example 1, except that 0.1 % DTPA solution was added to the pulp slurry in a polyethylene bag, along with 2% Na2SiO3 and 0.05% MgSO4.

The residual hydrogen peroxide was 0.87% on pulp, and the brightness of the resulting pulp is 70.8% ISO. Evidently, in comparison with Example 2, the PM process given in this example leads to much improved bleaching results.

Example 4.

In this example, it will be shown that the PM process, described in Example 1, can be further varied, and the improvement in bleaching performance

over the P process can be maintained. In this case, the stabilizers, namely sodium silicate, magnesium sulfate, DTPA and other chemicals, needed for peroxide bleaching, namely, caustic soda and hydrogen peroxide, are added to the pulp slurry in various orders. Three more orders were conducted, namely : A: 1S'Sodium silicate and magnesium sulfate 2"d Caustic soda 3rd DTPA 4th H202 B: 1St Sodium silicate and magnesium sulfate 2"d Caustic soda 3rdH202 4th DTPA C: 1 St DTPA 2nd Sodium silicate and magnesium sulfate 3rd Caustic soda Always a thorough mixing is provided before the next chemical is charged to the pulp. The key here, however, is that at least one of stabilizers, such as sodium silicate and DTPA, is charged to the pulp with sufficient mixing before the addition of hydrogen peroxide.

The same TMP pulp as that in Example 1 was used. The chemical charges and other procedures were the same as those in Example 1. The results are listed in Table 1.

Table 1. Sequence A Sequence B Sequence C Brightness (% ISO) 70.7 70.7 70.8 ResidualH202 (% on pulp) 1. 01 0. 89 0. 95 In comparison with the P process, Example 2, one can conclude that the bleaching performances of Sequences A, B, and C are much better.

Example 5.

In this example it will be shown that the benefit of the PM process, in

comparison with the P process, can still be achieved when applied to a TMP pulp, which was cheated in the Qy process, the so-called sodium hydrosulfite assisted chelation process.

Equivalent to 20 grams o. d. TMP pulp (brightness 54.3% ISO, 149 ppm Mn) was treated in a polyethylene bag with 0.2% DTPA (as active DTPA on pulp) and 0.1% sodium hydrosulfite (on pulp) under the conditions of 3% pulp consistency, 60°C and 10 minutes. After the completion of the required time the pulp slurry was filtered and pressed to about 30% pulp consistency. The cheated pulp has a residual manganese content of about 34 ppm, which is then ready for the bleaching experiments.

One half of the Qy treated pulp was then treated in a polyethylene bag with 3% sodium silicate, 0.05% MgSO4 for about 10 minutes. Subsequently, 2% H202 and 1.6% caustic soda were added to the pulp slurry in that order. A thorough mixing was provided before the addition of each chemical. The other bleaching conditions and procedures were the same as those in Example 1.

The residual hydrogen peroxide was 0.25% (on pulp) and the brightness of the resulting pulp was 69.0% ISO.

The other half of the Qy treated pulp was subjected to a conventional peroxide stage following the same procedure as those in Example 2 except that the chemical charges were the same as those in the previous paragraph, i. e. 3% Na2Si03, 0. 05% MgSO4, 1. 6% NaOH and 2% H202. The residual H202 was 0.05% (on pulp) and the brightness of the resulting pulp was 65.8% ISO.

Example 6.

It will be shown that PM process, described in Example 1, can be further varied and the improvement in bleaching performance over the P process can be maintained. In this case, instead of reagent grade, a silicate solution (37.56% solid, 8.90% Na2O, 28.66% SiO2, specific gravity @ 20°C, 1.394, viscosity @ 20°C, 177 cP), which is commercially available was used. The stabilizers, namely the silicate solution, and/or MgSO4, were mixed with the required NaOH.

The above mixture was then added to the pulp.

A TMP pulp with initial brightness of 57.8% ISO and 48 ppm Mn content was used. Equivalent to 10 grams o. d. of above pulp at a consistency of

about 15%, was added to a polyethylene bag. A mixture prepared with 4% of the above specified silicate solution (on o. d. pulp), 0. 1% MgSO4 (on o. d. pulp) and 2.8% NaOH (on o. d. pulp), was then admitted to the same polyethylene bag and thoroughly mixed with the pulp fibers (the mixing time was about 1 min).

Subsequently, 3.5% H202 was added to the bag and mixed thoroughly with its contents. The bag, along with its content, was placed in a temperature bath at 70°C. The bleaching conditions were: 120 min, 12% pulp consistency. After the completion of the required reaction, samples were collected for residual H202, ending pH and brightness, in accordance with the procedures in Example 1. The residual hydrogen peroxide was 1.28% and the pulp brightness was 72.5% ISO.

The same mill chelated TMP pulp, initial brightness of 57.8% ISO and 48 ppm Mn content was subjected to a conventional peroxide stage (P) with the same chemical charges as above (0.1 % MgS04,4% silicate solution, 2.8% NaOH, 3.5% H202, all based on o. d. pulp) under the same conditions (70°C, 12% pulp consistency). The above chemicals were added to a beaker containing distilled water to form a mixture. The mixture was then added to a polyethylene bag which contains equivalent to 10 grams o. d. pulp. The subsequent procedures were the same as those in Example 2.

The residual hydrogen peroxide was 0.28% and the pulp brightness was 70.7%. The results are compared with above of 1.28% and the pulp brightness of 72.5% ISO respectively, supporting that the peroxide bleaching performance is improved by the PM process.

Thus, it is apparent that there has been provided in accordance with the invention a peroxide bleaching of wood pulp that fully satisfies the objects, aims and advantages set forth above. While the invention has been described in conjunction with specific embodiments thereof, it is evident that many alternatives, modifications and variations will be apparent to those skilled in the art in light of the foregoing description. Accordingly, it is intended to embrace all such alternatives, modifications and variations as fall within the spirit and broad scope of the invention.