Pulp bleaching process including final stage treatment step with salt of peroxymonosulfuric acid

Description

BACKGROUND OF THE INVENTION

1. Field of the Invention

[0001] The present invention relates to the field of bleaching and brightening of pulp obtained from lignocellulosic fiber, and in particular to a final treatment stage in which the pulp is treated with one or more salts of peroxymonosulfuric acid.

2. Description of the Related Art

[0002] Cellulosic fiber used in the production of pulp and paper products is derived from wood materials and non-wood materials, such as kenaf, bagasse, and reed. These wood and non-wood materials are typically subjected to mechanical treatment, chemical treatment or a combination of mechanical and chemical treatments to provide cellulosic fiber. The cellulosic fiber thus obtained is typically dark in color and contains mostly cellulose and lignin (and thus the term lignocellulosic fiber) and some extraneous materials. The lignocellulosic fiber obtained mainly from mechanical treatment is treated with hydrogen peroxide or hydrosulfite in one or two stages to produce semi-bleached pulp (80% ISO brightness) suitable for various grades of paper and paper products. Brightness improvement of these lignocellulosic fibers is achieved primarily by modifying the light absorbing chromophoric groups of lignin and cellulose without much degradation and dissolution. Thus the bleached pulp retains most of the lignin and is known as lignin retaining bleaching.

[0003] Cellulosic fibers having very high brightness of 88-90% ISO and good brightness stability are obtained from bleaching pulp obtained from chemical pulping. Chemical pulping conducted under severe conditions reduces the lignin content to 2-3% on wood (based on total lignin content in the feedstock). Bleaching of chemical pulp is conducted in multiple stages using one or more bleaching chemicals, including for example, chlorine (C), chlorine dioxide (D), sodium or calcium hypochlorite (H), oxygen (O), ozone (Z), hydrogen peroxide (P), and one or more chemicals for caustic extraction (E), such as sodium hydroxide. Typical multi-stage bleaching sequences include CEHD, CEHH, (C/D)EDED, (C/D)EopDED, DEopDED, DEopD, DEopDP, and other sequences that are known in the art. In recent times it has also been known to include an oxygen delignification stage between the pulping process and the multi-stage bleaching treatment to reduce the lignin content by another 40-50% in the pulp. In all these multi-stage bleaching sequences the first two stages, for example, CE or DEop, or (C/D)Eop and the like are generally considered as pre-delignification or pre-bleaching stages. In these two-stages degradation and dissolution of lignin occurs. The subsequent stages of bleaching are referred to as brightening stages where brightness improvement is the primary function of the chemicals.

[0004] After completion of the bleaching in the multi-stage sequences, the bleached pulp is stored in a suitable storage vessel or chest typically found in pulping plants, such as a high density storage tower or chest. These storage chests can store pulp for varying periods of time, typically from 5 minutes to several hours (24-72 hours). The pulp typically moves through the vessel or chest in a continuous or discontinuous flow before being discharged for final processing on a papermachine into articles such as printing and writing paper or other paper products.

[0005] Oftentimes, the desired final brightness of the pulp is not achieved from the multi-stage bleaching treatment due to one or more factors such as, process variations, increased production rate, customer demand for higher brightness, and/or brightness stability. A simple solution for increasing the final brightness of completely processed pulp by up to 4% ISO involves a post-treatment, in which the fully bleached or bleached pulp having low brightness (semi-bleached pulp) is further treated with small amounts of hydrogen peroxide and caustic at an initial pH of 11.0 - 11.3 in the high density storage chest. This hydrogen peroxide treatment, which is typically conducted at about 45°C to 65°C requires long retention times of about 6-8 hours to utilize fully the peroxide and enhance the brightness by, for example, 2% ISO to in some cases up to 4% ISO.

[0006] In the high density storage chest since a constant level of pulp is not possible to maintain and no washing of pulp is done following the treatment, practically, several problems arise from the unused hydrogen peroxide (residual) and caustic left in the pulp when it leaves the high density storage chest. Following are the drawbacks of this treatment.

• Variable reaction time that could-vary from 5 min to as long as 72 hours, resulting in an uncontrolled pH profile, and either incomplete chemical consumption or early consumption of peroxide.
• Unconsumed peroxide or caustic or both may remain as residual chemical with the pulp. Depending upon the available reaction time, the peroxide can be used up and the pulp is left with caustic only. This results in what is known as "Caustic yellowing" or "Alkali darkening". This will revert the brightness gain achieved from peroxide treatment. On the other hand, if the caustic is used up and unconsumed peroxide is left, this will interfere with the chemicals added in the following wet end operation of papermaking. Under such circumstances the residual peroxide has to be destroyed by reaction with reducing agents like sodium thiosulfate, sodium bisulfite or sulfur dioxide in water. When peroxide and caustic remain as unconsumed then additionally the caustic should be neutralized to reach a pH of around 6-8. This is the required pulp pH for papermaking.

[0007] In order to circumvent the aforementioned problems associated with the peroxide treatment of pulp in the high density storage chest, application of Caro's acid or peroxymonosulfuric acid was discovered as described herein.

[0008] Numerous publications and patents discuss the use of peroxymonosulfuric acid (or Caro's acid) for delignification of unbleached chemical pulp in the pre-bleaching stage. These disclosures generally involve pre-bleaching with peroxymonosulfuric acid in acidic solutions followed by an alkaline extraction with sodium hydroxide solution. For example, U.S. Patent No. 5,004,523 to Springer et al. discloses a process in which lignocellulosic material is delignified with peroxymonosulfuric acid in a strongly acidic solution having a pH less than 2. U.S. Patents Nos. 5,091,054 and 5,246,543, both to Meier et al., relate to delignification processes in which lignocellulosic materials are treated with peroxymonosulfuric acid to arrive at a final pH of 3-5. The Meier patents disclose the beneficial effect of Caro's acid treatment that is realized by employing a subsequent oxidative extraction stage.

[0009] Ozone is also known in the field of pulp bleaching and is primarily used in the prebleaching stage as a delignifying agent. Treating chemical pulp with a mixture of Caro's acid and ozone as an effective means for removing residual lignin in pulp is discussed in U.S. Patent No. 5,411,635 to Francis et al. However, the Francis patent discloses that the delignification stage is preferably conducted at the low pH range of 3-4, and precedes an alkaline extraction or oxidative extraction stage conducted in the presence of oxygen and/or hydrogen peroxide. Ozone is a powerful oxidant and readily attacks lignin and cellulose under acidic conditions. However, ozone is such a non-selective bleaching chemical that it attacks cellulose much more than the lignin, thus producing pulp having low strength properties.

[0010] Liebergott (Pulp & Paper Canada, Vol. 97, No. 3, p. T105) discusses the effect of peroxy chemicals such as peracetic acid, mixed peracids and dimethyl dioxirane on brightness improvement for semi-bleached (81.5 %ISO brightness from DEopD sequence) and fully bleached (89.2 %ISO brightness from DEopDnD) pulps. Also, the Liebergott publication mentions that due to anticipated decomposition of Caro's acid occurring in a solution having an alkaline pH, treatment with Caro's acid was not investigated for semi-bleached or fully bleached pulps at a pH above 6.0.

[0011] The present invention is characterized by the unexpected discovery that the brightness of a prebleached lignocellulosic pulp can be improved by up to about 4 %ISO by performing a final bleaching treatment in which an oxidizer consisting essentially of at least one peroxymonosulfuric acid salt is introduced to the pulp while avoiding a final pH of the pulp in an undesirable range.

[0012] Accordingly, we have found a process for treating a prebleached pulp in a final bleaching stage with the bleaching agent, peroxymonosulfuric acid or its salts, to produce a more fully bleached pulp that is more compatible for subsequently producing paper products. For instance, the final pH of the final bleaching stage is more compatible with the papermaking process - little or no pH adjustment is necessary prior to papermaking. Also, there is little or no risk of alkali darkening from early consumption of bleaching agent. Furthermore, the process is more amenable to highly variable retention times.

[0013] The present invention thus provides a significant advancement over the above-mentioned final stage hydrogen peroxide/caustic treatment in which the final pH of the pulp is in the range of 10 or more and has potential deleterious effects.

SUMMARY OF THE INVENTION

[0014] In accordance with the principles of the present invention, a process is provided for performing a final stage of bleaching, in for example a high density storage tower or chest, said bleaching comprising treating a prebleached lignocellulosic pulp having a brightness with an oxidising agent consisting essentially of at least one peroxymonosulfuric acid salt to increase the brightness of the prebleached lignocellulosic pulp and provide a more fully bleached pulp, wherein the more fully bleached pulp is not subjected to further brightening or delignification operations subsequent to said final bleaching stage, and wherein said treatment is conducted at an initial pH at commencement of said final bleaching stage in a range of from about 7 to about 12 and a final pH at the end of said final bleaching stage in a range of from about 3 to about 9. Preferably, the initial pH is in a range of from about 8 to about 11, and the final pH is in a range of from about 6 to about 8.

[0015] The prebleached pulp exiting the preceding stage from a conventional bleaching process may be in the range of 4-8, in which case the initial pH at the commencement of the final bleaching stage according to the present invention may be attained by the addition of alkali, for example a caustic solution.

[0016] The reaction of peroxymonosulfuric acid or its salts with prebleached lignocellulosic pulp in a high density storage chest to produce a more fully bleached pulp proceeds to completion in say 1-2 hours, and the final pH of reaction falls in the range of preferably 6-8. Thus the more fully bleached pulp produced according to the process of the present invention need not be subjected to any chemical treatment or washing before being discharged to the wet end of a paper machine. However, optionally the pulp can be washed prior to such discharge.

[0017] The bleaching process according to the present invention can be conducted with about 0.01% on pulp to about 3.0% on pulp as active oxygen (based on oven dried weight of the pulp) of an oxidizing agent consisting essentially of at least one peroxymonosulfuric acid or its salts. Preferably, the charge of oxidizing agent is in the range of from about 0.05% on pulp to about 1.5% on pulp as active oxygen.

[0018] As referred to herein, an oxidizing agent consisting essentially of at least one peroxymonosulfuric acid salt preferably means an oxidizing agent consisting exclusively of peroxymonosulfuric acid salt, although the term permits for the presence of insubstantial amounts of other oxidizing agents, including, for example, those from previous bleaching operations, so long as the residual oxidizing agents do not materially affect the bleaching operation of the peroxymonosulfuric acid salt or the pH of the pulp during the final bleaching stage.

[0019] The present invention can be practiced in connection with the preparation of more fully bleached pulp from virgin pulp, from recycled furnish or from a combination of virgin pulp and recycled furnish.

[0020] The principles of the present invention outlined above are applicable to all types of bleached pulps, but have particular applicability to the production of high brightness chemical pulp for the production of various writing, printing, coated and uncoated free sheets.

[0021] These and other objects, features and advantages of the present invention will become apparent from the following detailed description which explains, by way of examples, the principles of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

[0022] Lignocellulosic materials such as untreated wood chips and annual plants such as wheat straw, kenaf, reed and the like can be used in accordance with the present invention, although it is understood that the scope of the invention is not limited to these exemplary materials. Especially suitable are materials that have been produced in a mechanical pulping process, a chemical pulping process, or a combination of both mechanical and chemical pulping processes, such as groundwood (GW), thermomechanical, chemithemomechanical, NSSC, sulfite, soda, kraft, organosol, and the like, and any combinations thereof. It is this kind of material in an aqueous suspension, hereafter referred to as pulp, which is treated in accordance with the present invention with at least one salt of peroxymonosulfuric acid in the final stage of a bleaching process. It should be noted that where no prebleaching occurs prior to a final peroxymonosulfuric acid treatment according to the process of the present invention, that is the peroxymonosulfuric acid treatment is the only bleaching step, then such treatment is considered to be within the scope of the present invention. This may be applicable to the mechanical type pulps or sulfite pulp. Kraft pulp and/or recycled pulp are preferred materials for use in the present invention for the production of the bleached articles.

[0023] The peroxymonosulfuric acid salt used in the treating step can be obtained by dissolving commercial grades of its salts such as Caroat® supplied by Degussa AG or Oxone® supplied by Dupont® in water and/or an alkaline material, and thereafter treating the pulp with the mixture. Alternatively or in addition thereto, peroxymonosulfuric acid, also known as Caro's acid can be produced on-site by mixing concentrated sulfuric acid and hydrogen peroxide prior to mixing with caustic solution and/or pulp. Also, in practicing the treating step the alkaline material can be added to the pulp prior to or at the same time that the peroxymonosulfuric acid is added to the pulp. Suitable alkaline materials include, sodium hydroxide, potassium hydroxide, calcium hydroxide, sodium carbonate, magnesium oxide, and the like.

[0024] The peroxymonosulfuric acid salt final treatment stage is carried out, preferably, with about 0.01% on pulp to about 3.0% on pulp (based on oven dry weight of the pulp) as active oxygen of Caro's acid or one or more of its salts, although it is understood that the present invention is not so limited. Less than about 0.01% on pulp may be insufficient to effect desirable brightness improvements, whereas more than about 3.0% on pulp is unnecessary to obtain satisfactory results and becomes quite uneconomical. More preferably, the chemical charge is in a range of from about 0.05% on pulp to about 1.5% on pulp as active oxygen of Caro's acid or one or more of its salts.

[0025] Additives which serve to facilitate the peroxymonosulfuric acid salt final treatment stage may optionally be used. Among the additives that can be used are hydrogen peroxide stabilizing agents, such as sodium silicate; chelating agents such as Na5DTPA, Na4EDTA, DTMP, and the like; or, cellulose protecting agents, such as magnesium sulfate and the like; and any combination thereof. These additives can be applied to the pulp prior treating the pulp with the mixture of peroxymonosulfuric acid or salt and alkali, or simultaneously therewith, or added to the peroxymonosulfuric acid or its salt prior to the addition to pulp.

[0026] The consistency of the pulp in the peroxymonosulfuric acid salt final treatment stage is not critical and can be, for example, in a range of from about 1% to about 45%, and preferably in a range of from about 5% to about 15%.

[0027] The pH at the commencement of the peroxymonosulfuric acid salt final treatment stage is preferably in a range of from about 7 to about 12, and more preferably in a range of from about 8 to about 11. Over the course of the reaction, the pH drops due to the liberation of sulfuric acid from the peroxymonosulfuric acid. Generally, the pH drops to a final pH in a range of from about 3 to about 9, and preferably in a range of from about 6 to about 8.

[0028] By way of example and without limitation, the peroxymonosulfuric acid salt final treatment stage can be carried out at temperatures in a range of from about 10°C to about 100°C. Preferred temperatures for the peroxymonosulfuric acid final treatment stage are in a range of from about 40°C to about 70°C.

[0029] Depending upon the temperature, pH and chemical charge selected, the retention time or reaction time of the peroxymonosulfuric acid salt treatment can be, for example and without limitation, in a range from about 10 minutes to about 60 hours, and preferably in a range from about one hour to about 6 or 8 hours.

[0030] Pressure conditions selected for the peroxymonosulfuric acid salt treatment stage can vary for this process, as is conventional in pulp operations. Suitable pressures can range from, but are not restricted to, atmospheric to 0.5 MPa.

[0031] The above process conditions are merely illustrative and are not considered limiting.

[0032] The peroxymonosulfuric acid salt final treatment is the final bleaching stage prior to any other post-bleaching operations, which may generally include refining of pulp, treatment with additives and chemicals to make it suitable for pulp or paper sheets production. It is noted that minor adjustments to the pH of the pulp can optionally be effected prior to these post-bleaching operations by including a washing and filtering step.

[0033] In carrying out the process according to the present invention, conventional equipment well known in the pulp industry can be used. By way of example, the final treatment stage of the present invention can be carried out by injecting or mixing the chemicals with pulp in a mixer or at the suction side of the pump discharging the pulp in to a suitable storage tower or chest. A high density storage chest or tower, which is typically found in pulping plants, is well suited for the final bleaching treatment (prior to any post-bleaching operations).

[0034] The following examples serve to illustrate the present invention without limiting it in any way.

EXAMPLES

Example 1

[0035] Southern pine kraft pulp bleached to a brightness of 86.7 %ISO by DEopD sequence was treated with 0.05% Caro's acid (AO) in accordance with the process of the present invention as follows.

[0036] A 45-60 gram sample of the Southern pine kraft pulp received from the mill was pressed down (filtered) to a consistency of 24% and fluffed. The sample was then treated with 0.49 wt% (based on the total weight of the pulp sample) of sodium hydroxide. The sample was then treated with a mixture of 0.05 wt% Caro's acid, 0.49 wt% sodium hydroxide, and 0.1 wt% Na5DTPA (based on the total weight of the pulp sample) and diluted with water to bring the consistency of the pulp sample to 10%. The pulp sample was bleached in a plastic bag immersed in a water bath having a temperature of 45°C. Following the reaction time, the pH of the pulp was measured and then the pulp sample was diluted to 0.2% consistency, before being pressed down to about 30% consistency without further washing. The brightness of the bleached pulp was measured with an Elrepho-Data Color 2000 meter using 3 gram handsheet.

[0037] The reverted brightness was estimated by subjecting the handsheet to 105°C ± 3°C in a conventional oven for 60 minutes, and thereafter measuring the brightness with the Elrepho-Data Color 2000 meter.

[0038] For comparative purposes, a substantially identical procedure was conducted with hydrogen peroxide applied in an equivalent basis, which is 0.1 wt%. The experimental conditions and results of Example 1 are reported below in TABLE I:

TABLE I

	Caro's Acid (0.05 wt%)			Hydrogen Peroxide (0.1 wt%)
Temperature	65°C			65°C
Magnesium Sulfate	---			0.1 wt%
Na5DTPA	0.1 wt%			---
Reaction time (hr)	2	4	6	2	4	6
Brightness (%ISO)	88.5	88.7	88.6	88.1	88.5	88.5 2
Reverted Brightness (%ISO)	86.6	87.0	87.3	86.7	87.3	87.2
Brightness Loss (%ISO)	1.9	1.7	1.3	1.4	1.2	1.3
NaOH (wt%)	0.98	0.98	0.98	0.22	0.22	0.22
Initial pH	10.0	10.0	10.0	11.3	11.3	11.3
Final pH	6.5	6.2	6.1	10.3	10.2	10.0
Residual (wt%)	0.02	0.02	0.02	0.06	0.04	0.02
Viscosity (mPa·s)	31.7	--	30.5	33.3	--	30.3

[0039] The Caro's acid treatment at 65°C with 0.05% Caro's acid over 6 hours resulted in a 1.9 %ISO increase in brightness and a 0.6 %ISO increase in reverted brightness of the pulp, which had an initial brightness of 86.7 %ISO prior to being subjected to the inventive operation. The brightness and reverted brightness increases realized by the Caro's acid treatment were slightly better than that realized with the hydrogen peroxide treatment (which were 1.8 %ISO and 0.5 %ISO, respectively).

[0040] However, the final pH levels of the pulp sample after 2, 4, and 6 hours of treatment with 0.05% Caro's acid were 6.5, 6.2, and 6.1, respectively. These pH levels are significant less than the pH levels of the pulp sample treated with 0.1% hydrogen peroxide, which ranged from 10.0 to 10.3. Thus, unlike the comparative samples treated with hydrogen peroxide, the inventive samples advantageously did not have to be neutralized prior to post-bleaching operations.

Example 2

[0041] The procedures set forth above in connection with Example 1 were repeated, with the exception that 0.1 wt% Caro's acid and 0.2 wt% peroxide were employed, and the temperature of the bath was 45°C. The experimental conditions and results of Example 2 are reported below in TABLE II:

TABLE II

	Caro's Acid (0.1 wt%)			Hydrogen Peroxide (0.2 wt%)
Temperature	45°C			45°C
Magnesium Sulfate	---			0.1 wt%
Na5DTPA	0.1 wt%			---
Reaction time (hr)	2	4	6	2	4	6
Brightness (%ISO)	88.5	89.5	89.4	88.3	88.7	88.9
Reverted Brightness (%ISO)	86.6	87.4	87.6	86.0	86.4	86.6
Brightness Loss (%ISO)	1.9	2.1	1.8	2.3	2.3	2.3
NaOH (wt%)	2.0	2.0	2.0	0.28	0.28	0.28
Initial pH	10.0	10.0	10.0	11.3	11.3	11.3
Final pH	6.1	6.0	6.0	11.0	10.8	10.8
Residual (wt%)	--	--	--	0.16	0.15	0.14
Viscosity (mPa·s)	32.4	--	31.9	32.8	--	27.2

[0042] The Caro's acid treatment at 45°C with 0.1% Caro's acid over 6 hours resulted in a 2.7 %ISO increase in brightness and a 0.9 %ISO increase in reverted brightness of the pulp, which had an initial brightness of 86.7 %ISO prior to being subjected to the inventive operation. The brightness and reverted brightness increases realized by the Caro's acid treatment were better than that realized with the hydrogen peroxide treatment (which were 2.2 %ISO and -0.1 %ISO, respectively).

[0043] However, the final pH levels of the pulp sample after 2, 4, and 6 hours of treatment with 0.1% Caro's acid were 6.1, 6.0, and 6.0, respectively. These pH levels are significant less than the pH levels of the pulp sample treated with 0.2% hydrogen peroxide, which ranged from 10.8 to 11.0. Thus, unlike the comparative samples treated with hydrogen peroxide, the inventive samples advantageously did not have to be neutralized prior to post-bleaching operations.

[0044] The foregoing detailed description of the preferred embodiments of the invention has been provided for the purposes of description and elaboration. It is not intended to be exhaustive or to limit the invention to the precise embodiments disclosed. Obviously, many modifications and variations will be apparent to practitioners skilled in this art. The embodiments were chosen and described in order to best explain the principles of the invention and its practical application, thereby enabling others skilled in the art to understand the invention for various embodiments and with various modifications as are suited to the particular use contemplated. It is intended that the scope of the invention be defined by the following claims and their equivalents.

Claims

1. A process for bleaching lignocellulosic pulp comprising a final bleaching stage, said final bleaching stage comprising treating prebleached lignocellulosic pulp having a brightness with an oxidizing agent consisting essentially of at least one peroxymonosulfuric acid salt to increase the brightness of the prebleached lignocellulosic pulp and provide a more fully bleached pulp, wherein the more fully bleached pulp is not subjected to further brightening or delignification operations during the remainder of said process subsequent to said final bleaching stage, and wherein said treatment is conducted at an initial pH at commencement of said final bleaching stage in a range of from about 7 to about 12 and a final pH at the end of said final bleaching stage in a range of from about 3 to about 9.

2. A process according to claim 1, wherein the initial pH is in a range of from about 8 to about 11, or the final pH is in a range of from about 6 to about 8.

3. A process according to claim 1, wherein said treatment is conducted with about 0.01% on pulp to about 3.0% on pulp as active oxygen (based on oven dried weight of the pulp) of peroxymonosulfuric acid or one or more salts of peroxymonosulfuric acid.

4. A process according to claim 1, wherein the processed pulp is prepared by the Kraft process.

5. A process according to claim 1, wherein said final bleaching stage is conducted in a high density storage tower or chest.