EP2606175B1

EP2606175B1 - Method for removing hexenuronic acids

Info

Publication number: EP2606175B1
Application number: EP11770836.2A
Authority: EP
Inventors: Pedro Fardim; Malin Ekroos
Original assignee: Abo Akademi University
Current assignee: Abo Akademi University
Priority date: 2010-08-18
Filing date: 2011-08-18
Publication date: 2016-04-20
Anticipated expiration: 2031-08-18
Also published as: EP2606175A1; BR112013003758B1; FI20105862A0; WO2012022840A1; BR112013003758A2

Description

FIELD OF THE INVENTION

The present invention relates to a method for removing hexenuronic acids from pulp according to preambles of the enclosed claims.

BACKGROUND OF THE INVENTION

Wood comprises several different components: cellulose; hemicelluloses, such as xylan; lignin and extractives. During chemical pulping in a kraft, i.e. sulphate, pulp mill the xylan chain forms side groups called hexenuronic acids (HexAs) which are unsaturated sugars. The amount of HexAs varies from pulp to pulp, because different wood species contain different amounts of xylan, which can be transformed into HexAs during cooking process. Also cooking parameters contribute to different amounts of HexAs.
The process of kraft pulping comprises alkaline cooking and bleaching, and it begins with wood handling where wood is debarked and made into chips. The chips are screened so fine material and oversized chips are eliminated. The chips are then fed to a digester where they first are treated with steam and then with cooking liquid, while the temperature is raised to the desired cooking temperature. When desired rate of delignification is achieved, cooking is interrupted and the content in the digester is moved to a blow tank and onwards to a screener. After the pulp is screened it is washed several times and pumped to the following delignification stage, i.e. bleaching. The cooking chemicals are recovered in the chemical recovery plant.
The main target for chemical pulping process is delignification in order to liberate the fibres without harming them. Alkaline delignification occurring during cooking is alkaline hydrolyses of phenol ether bonds that make lignin soluble. Phenols are weak acids that dissociate in alkali environment (pH> 10). The lignin will be partly demethylated by nucleophilic attack of sulfide ions on methoxyl groups in lignin.
Bleaching of the obtained pulp comprises typically a number of discrete steps or stages. In the oxygen delignification, which may occur either as pre-bleaching or bleaching step, more lignin is dissolved and washed away. This is also the case in the different following bleaching stages; peroxide bleaching, ozone bleaching and chlorine dioxide bleaching. Finally the pulp is moved to the drying machine where it is dried, cut and packed for further transportation to paper mills.
Oxygen delignification occurring in pre-bleaching or bleaching step may comprise only one stage, but usually the process is carried out in a two-stage system with or without washing between the stages. In typical one stage oxygen delignification system the cooked pulp is washed in the filtrate from the post-oxygen washer before it is charged with NaOH or oxidized white liquor. The pulp is preheated in a low-pressured steam mixer before it is transferred by a medium consistency pump to the high-shear, medium-consistency mixer. Oxygen is added to the mixer and the oxygen delignification process begins.
The first stage after oxygen delignification may be a delignification stage using chlorine dioxide to dissolve lignin. The typical following alkaline extraction stage (EOP) stage is an alkaline extraction stage enhanced with the oxidizing agents: oxygen and peroxide.
Alkaline oxygen and peroxide bleaching stages do not affect the HexA content in pulp. Chlorine dioxide and ozone on the other hand have a great impact on the HexA content and will react with the HexA groups in the pulp. HexAs are consumed by chlorine in the chlorine dioxide stage by forming unchlorinated and chlorinated dicarboxylic acids. The HexAs thus consume bleaching chemicals and also increase brightness reversion of fully bleached pulps.
Moreover, the HexAs also bind heavy metal ions and increase the problems with non-process elements (NPEs) which will lead to an increase in deposits in the bleaching stages. This is why it is in interest to remove these components from the pulp before the bleaching stages. In that case a lower chemical batch can be used in each delignification or bleaching stage and higher brightness stability can be achieved.
The kappa number, that is a measure of lignin content in pulp, is also affected by HexAs. HexAs consume potassium permanganate that is one of the reactants used in the kappa number analysis. Permanganate reacts with carbon-carbon double bonds in the lignin structure but HexAs also contribute to the consumption because of its carbon-carbon double bond.
The hot acid stage (A-stage, at pH 3, temperatures of 50-90 °C and retention time of 1-3 hours), that is disclosed in US 6,776,876 and the hot chlorine dioxide bleaching (at temperatures 60-90 °C) disclosed in WO 2008/044988 are two methods to eliminate HexAs that are used today. Both these methods leave residual HexAs in the pulp, increase the retention time in the bleaching lines, increase the costs of effluent treatment, reduce the amount of charged groups on the fibre surface and reduce the fibre strength properties.
WO 92/13993 discloses bleaching of lignocellulosic material with activated oxygen, where chemical pulp contains reactants able to generate dioxirane within the pulp.

OBJECT AND SUMMARY OF THE INVENTION

An object of the present invention is to minimise or even totally eliminate the problems and/or disadvantages existing in the prior art.
An object of the invention is to provide a method for eliminating hexenuronic acids (HexA) more efficiently from the lignocellulosic pulp after chemical cooking.
Another object is to lower the production and capital costs for the chemical, such as kraft, pulp mills while at the same time providing a pulp that is at least as useable as when manufactured in a traditional manner.
A yet further object of the invention is to provide a method enabling the development of new pulp products with enhanced optical and mechanical properties.
These objects are attained with the invention having the characteristics presented below in the characterising parts of the independent claims.
A typical method for removing hexenuronic acids from pulp comprises

obtaining pulp by chemical pulping,
treating the obtained pulp by using a further delignification process comprising an oxygen treatment stage, and
carrying out the oxygen treatment stage in the presence of at least one perbenzoic acid.

Typical pulp according to the present invention is obtainable by the method of this invention.
Some preferred embodiments are described in the dependent claims.

DETAILED DESCRIPTION OF THE INVENTION

Now it has been surprisingly found out that carrying out the oxygen treatment stage in the presence of at least one perbenzoic acid the amount of hexenuronic acids is significantly decreased. In the present invention, perbenzoic acid (PBA) or its derivatives are added directly into the oxygen treatment stage, typically together with chemicals that are traditionally used in this stage. It is thus surprisingly found that these compounds could effectively remove HexAs from the pulp. The amount of HexAs removed can be for example up to 60 %, even 100 % from the amount originally present after the cooking stage.
Thus, according to the invention, the method for removing hexenuronic acids from a cellulosic pulp during a pulping process comprises an oxygen treatment stage, which is carried out in the presence at least one perbenzoic acid. The oxygen treatment stage of a pulping process refers in this application to a stage which can also be called oxygen bleaching stage or oxygen delignification stage. This stage is thus different from the chlorine dioxide treatment stage and from the alkaline extraction stage. In the oxygen treatment stage oxygen is added to the pulp under alkaline conditions. The oxygen treatment stage may also be a pre-bleaching stage, occurring after the cooking of the pulp and before actual bleaching stage, or the oxygen treatment stage may be part of the bleaching sequence.
Some advantages of the present invention are a time gain in the overall process, since there is no need for a separate stage for removing HexAs, therefore no need for a supplemental retention time. The present method can also be implemented in existing installations without the need for any specialised equipment. Most importantly, the method allows the manufacture of pulps having improved characteristics compared to prior art methods.
Indeed, the results obtained and discussed in more detail below show that the use of at least one perbenzoic acid reduces the hexenuronic acid content in the pulp and the obtained pulp shows a higher resistance towards cellulose degradation. Further, a water retention value is improved and a higher brightness is achieved. Also higher kappa number reduction is achieved. It is also to be noted that perbenzoic acids are not known to be harmful for the environment. Compared to pulps of similar origin where the amount of hexenuronic acids has been reduced by using conventional prior art methods, the pulp produced by the present invention shows an increase of at least 25 %, typically 30 % in tensile stiffness index, an increase of at least 25 %, typically 30 % in tensile strength index, and/or an increase of at least 20 %, typically 25 %. The water retention value of the pulp produced according to the present invention is decreased typically at least 6 %, more typically 10 %, compared to pulp of similar origin produced by using conventional prior art methods. An improved brightness value may be achieved for the pulp according to the present invention after oxygen bleaching in comparison with similar pulp bleached by using conventional oxygen bleaching. Typically an increase of ISO brightness of at least 10 points may be achieved when the amount of perbenzoic acid used is about 150 kg/ton of pulp.
The pulp treated according to the present invention may comprise fibres originating either from hardwood or softwood, and it can be obtained in any suitable manner. The present invention is also suitable for treating pulps obtained by pulping or fiberising of non-wood material, such as bamboo, sugar cane bagasse, hemp, wheat or rice straw.
According to an embodiment of the invention, the at least one perbenzoic acid is selected from the group consisting of perbenzoic acids, salts of perbenzoic acids, derivatives or precursors of perbenzoic acids and mixtures thereof. For the time being, perbenzoic acid as such, at least in the solid form, is quite unstable. Therefore, its derivatives are preferred, and it is believed that the nature of derivative group does not have any or only minor influence on the activity of the compound and thus on the present invention. Salts of perbenzoic acid, both inorganic and organic, may be used in the present invention. For example, sodium or potassium salt of perbenzoic acid may be used.
According to one embodiment of the invention the at least one perbenzoic acid is perbenzoic acid, sodium salt of perbenzoic acid, metachloroperoxybenzoic acid, 4-tert-butylperbenzoic acid, 4-methylperbenzoic acid or 4-methoxyperbenzoic acid.
Some useful perbenzoic acids are 4-cyanoperbenzoic acid, 3-tert-butylperbenzoic acid, 2-tert-butylperbenzoic acid, 4-nitroperbenzoic acid, 4-fluoroperbenzoic acid, 3-chloroperbenzoic acid, 2,4-dichloroperbenzoic acid, 4-chloroperbenzoic acid, 2-methylperbenzoic acid, 3-methylperbenzoic acid, 3,4,5-trimethoxyperbenzoic acid, monoperphthalic acid and 1-pernaphthoic acid. Particularly preferred are 4-tert-butylperbenzoic acid, 3-chloroperbenzoic acid, 4-methylperbenzoic acid and 4-methoxyperbenzoic acid.
Some useful perbenzoic acid precursors are benzoic acid, phthalic anhydride, substituted and unsubstituted benzoyl oxybenzene sulfonates, N-benzoyl succinimide, tetrabenzoyl ethylene diamine, N-acylated lactam, tetraacetyl ethylene diamine, lactose octaacetate and 4-trimethyl ammonium methyl derivative of benzoyl oxybenzene sulfonate.
The amount of perbenzoic acid used and/or present in the oxygen treatment stage can be for example 1-500 kg/ton of cellulosic pulp, according to one embodiment 1-300 kg/ton of cellulosic pulp. The amount depends on the type of pulp (for example on the origin of the cellulose), as different pulps contain different amounts of HexAs. The amount is given as calculated to pure active perbenzoic acid. Naturally, if derivatives, salts or precursors of perbenzoic acid are used, the added amounts should be calculated and converted as pure perbenzoic acid.
According to an embodiment of the invention, the at least one perbenzoic acid, salt of perbenzoic acid, derivative or precursor of perbenzoic acid or a mixture thereof is added to the oxygen treatment stage in a form selected from the group consisting of powder, solution, slurry or suspension. Preferably, the perbenzoic acid is added in the form of powder or slurry, which had been found to be the optimal form in order to achieve a rapid and effective mixing of the components in the oxygen treatment stage.
The at least one perbenzoic acid, salt of perbenzoic acid, derivative or precursor of perbenzoic acid or a mixture thereof can be added before or during the oxygen treatment stage. In case the oxygen treatment stage is carried out in a system comprising two or more discrete steps with or without washing between the steps, the at least one perbenzoic acid, salt of perbenzoic acid, derivative or precursor of perbenzoic acid or a mixture thereof may be added during the first or the following step, or to all of the steps of the oxygen treatment stage. The addition may thus be done in the first, second and/or the following treatment vessels, tanks or reactors of the oxygen treatment stage. According to one embodiment of the invention the oxygen treatment stage may be a pre-bleaching stage or it is a treatment stage incorporated into the bleaching sequence. According to another embodiment of the invention the at least one perbenzoic acid, salt of perbenzoic acid, derivative or precursor of perbenzoic acid or a mixture thereof may be added to the pulp during the alkaline extraction stage when oxygen is added to the pulp.
According to an embodiment of the invention, the general conditions of the oxygen treatment stage can be as follows: the alkali dosage is 10-30 kg/ton of pulp, the temperature 80-120 °C, the retention time is 20-120 minutes, the dosage of magnesium sulphate 1-4 kg/ton of pulp and the oxygen pressure is 50-100 psi.
The present invention yet further relates to the use of a perbenzoic acid for removing hexenuronic acids from a cellulosic pulp. This use preferably occurs during an oxygen treatment stage of a pre-bleaching or bleaching process.
The embodiments listed above in connection to the method apply mutatis mutandis to the pulp and the use as described above.
The invention is described in more detail in the following, non-limiting experimental part.

EXPERIMENTAL PART

In the following examples, two different pulps were bleached in a full sequence, as described below. Kappa number, viscosity, hexenuronic acid content, amount of total anionic groups, water retention value and brightness were determined from these pulps.

Materials

The pulp used for these examples was a eucalyptus pulp from Fibria and-Stora Enso pulp mill Veracel in Brazil. The pulp was collected from the last washing stage before oxygen delignification and properly washed at the pulp mill in Brazil before shipping it.
The perbenzoic acid derivative used was metachloroperoxybenzoic acid (mCPBA), in the form of powder. Also sodium hydroxide (NaOH) and magnesium sulfate (MgSO₄) were used as traditionally in the oxygen treatment stage.
The amounts of the components used in each bleaching are presented in Table 1, and given as kg/ton of pulp. Table 1 The amounts of the components used in each bleaching.

mCPBA (kg/ton) NaOH (kg/ton) MgSO₄ (kg/ton)

- 20 3

5.0 20 3

10.0 20 3

15.0 20 3

25.0 20 3

75.0 20 3

100.0 20 3

150.0 20 3

Oxygen delignification

The initial pulp in the oxygen delignification stage was an unbleached kraft pulp. Oxygen pressure, heat, alkali, magnesium sulfate (MgSO₄) arid different charges of mCPBA (presented in table 5 above) was applied in the oxygen delignification stage. After oxygen bleaching the pulp was washed with water.

The parameters presented in Table 2 below were used in the first set of trials in the oxygen delignification stage. All trials were performed under the same conditions using the same equipment and chemical charges, only the mCPBA charge was changed for each experiment.

Table 2 Parameters used in the oxygen delignification stage.

Parameter	Value	Unit
Pulp (dry)	200	gram
Consistency
	10	%
Reactor time
	60	minutes
Temperature
	100	°C
Oxygen pressure
	6	bar
Mixing sequence
	10 every 120	seconds
Alkali
	20	kg/ton pulp
MgSO₄	3	kg/ton pulp

200 grams of dry pulp was weighted and preheated in a microwave oven. A special lid was used for the pulp to prevent loss of water during the heating. A Quantum mixer was filled with distilled water and preheated so that the temperature was as close to the target temperature as possible. The water was removed and the mixer dried before adding the preheated pulp and some preheated water.
The pulp was then mixed with the Quantum set to manual in order to adjust the temperature to the desired 100 °C. When this temperature was reached, the chemicals were added to the reactor. Some more preheated water was also added, in order to dilute the pulp consistency to about 10 %. The reactor was closed carefully and the oxygen valve was opened. The temperature was set once again in order to have it as close to 100 °C as possible. Also the pressure was adjusted to be precisely 6 bars during the delignification sequence. The Quantum mixer was set to automatic and the settings were adjusted to 10 seconds of mixing every second minute for a reaction time of 60 minutes.
When the reaction time was over, the heating and the reactor were shut off. The pressure inside the reactor was carefully decreased by opening the pressure valve very slowly (lasted several minutes). The reactor was opened carefully and the pulp was removed. Some pulp was lost due to expansion of the pulp during the reaction time.
The pulp was placed in a container and distilled water was added. The mixture was then filtered in a Buchner-funnel. Water was added until the filtrate was clear. The pulp was then centrifuged in order to increase the dry content, placed in a plastic bag and stored in a refrigerator.

HexA removal with benzoic acid

One trial was done with the use of only benzoic acid in order to confirm that the reagent needs the oxidative effect to remove the HexAs. This trial was done by adding 100 kg/ton benzoic acid (BA) to the oxygen delignification stage at the exact same way as the mCPBA was added. The same process parameters and chemical charges were also used as in the former trials described above.

HexA removal with mCPBA in solution

To investigate if the mCPBA had a bigger effect on the HexAs if it was added as a solution instead of a powder, two trials were done with mCPBA in solution. The solution was prepared by dissolving mCPBA in tert-butyl alcohol. The tert-butyl alcohol was mixed with water in a ratio of 1:1 and mCPBA was mixed in to this solution so it represented approximately 12 % of the total amount of the solution. In order to be able to compare the results with the earlier trials two charges used earlier were selected; 25.0 kg/ton and 75.0 kg/ton. In Table 3 below the mixtures of mCPBA in solution are presented. Table 3 The used mixtures of mCPBA.

12% mCPBA 44% tert-butyl alcohol 44% distilled water

25 kg/ton 5 18.3 18.3

75 kg/ton 15 55 55
All the process parameters and chemical charges presented in Tables 1 and 2 were also used in these experiments. Kappa number, viscosity, brightness, water retention value and HexA content were analysed for both these experiments.

Bleaching

Three of the oxygen delignified pulps prepared above were subjected to two conventional bleaching steps and one alkali extraction step.

The following bleaching steps were selected; D₀ (initial chlorine dioxide step), EOP (oxygen and peroxide reinforced alkali extraction step) and D (chlorine dioxide step). Pulps selected for these bleaching sequence trials were the reference pulp (without any additional mCPBA), the pulp with a mCPBA charge of 75 kg/ton and the pulp with a mCPBA charge of 150 kg/ton. The parameters used in the different bleaching steps are presented in Table 4 below.

Table 4 The parameters used in the different bleaching steps.

	Do	EOP	D
Consistency (%)	4.5	8.0	8.0
Temperature (°C)	70	75	75
pH	4.5	10.5	5.5
Time (min)	15	70	200
ClO₂ (kg/ton)	6	-	6
H₂O₂ (kg/ton)	-	8	-
Pressure (bar)	-	2.8	-
MgSO₄ (kg/ton)	-	2	-

D_o-stage

Three plastic bags, one for each pulp, were prepared and about 150 g of dry pulp were used for the experiment. The amount of active chlorine was calculated from the chlorine dioxide solution. This was done by mixing distilled water, 1 M potassium iodide solution, 2M sulfuric acid and the chlorine dioxide solution and titrating it with 0.2M sodium thiosulfate solution. The amount of active chlorine was then calculated by the following equation: $1.42 \times d = active chlorine ({Cl}_{2}) (g / l)$
where d = sodium thiosulfate consumption (ml)
Since the process parameter for ClO₂ was not expressed as active chlorine but as chlorine dioxide, it was necessary to recalculate the active chlorine to chlorine dioxide. This was done by multiplying the desired amount of ClO₂ with 2.63 in order to transform the process parameter into active chlorine. This could then easily be translated into the desired amount of ClO₂.

One plastic bag at a time was prepared by weighing the right amount of pulp, adding the right amount of ClO₂ and water. The pH was also adjusted with 1 M H₂SO₄. The amounts used are presented in Table 5 below. The plastic bag was then placed in a water bath with the temperature adjusted to 70 °C, and left there for 15 minutes.

Table 5 The amounts used in D₀ stage.

Do	Unit	Ref.	75 kg mCPBA/ton	150 kg mCPBA/ton
m _pulp dry	g	150.00	138.77	147.75
ClO₂	kg/ton	6	6	6
ClO₂	ml	278.28	257.45	274.11
Consistency	%	4.50	4.50	4.50
Total	g	3333.33	3083.78	3283.33
Water	ml	2392.51	1804.20	2263.77

When the reaction time was over, the plastic bag was removed from the water bath and opened. The pulp was immediately put in a Buchner-funnel equipped with a wire. The chlorine residues were measured from the filtrate and used to calculate the chlorine consumption. Warm water was added to clean the pulp and in a last step, cold water was added in order to cool down the pulp. The pulp was transferred to a centrifuge bag and centrifuged in order to increase the dry content of the pulp.
The residue chlorine was determined after the bleaching operation. This was done by mixing 10 ml 1 M potassium iodide solution, 5 ml 2M sulfuric acid, 50 ml of the filtrate from the bleaching operation and a few drops of starch solution. The mixture was titrated with a 0.01 M sodium thiosulfate solution, and the consumption was noted as a (ml). The residue chlorine was calculated from the equation below: $Residual chlorine (ClO) (_{2}), (kg / ton) = \frac{0.0071 \times a (Y - R)}{R}$

where a = sodium thiosulfate consumption (ml)
Y = total amount in the step (g)
R = amount of pulp in the step (g)

Kappa number, viscosity, brightness and HexA content were analyzed for each pulp.

EOP-stage

The oxygen and peroxide reinforced alkali extraction stage was carried out in a quantum mixer. First the concentration of the hydrogen peroxide, H₂O₂, was determined. 10 ml of the H₂O₂-solution, 10 ml 1M potassium iodide solution, 5 ml 2M sulfuric acid and about 5 drops 15 % ammonium molybdate solution was mixed and titrated with 0.2M sodium thiosulfate solution. The sodium thiosulfate consumption was noted as e and the H₂O₂-concentration was calculated according to the equation below. $H_{2} O_{2} (g / l) = 0.34 \times e$

The amounts of H₂O₂-solution, water and pulp used in this stage are presented in Table 6 below.

Table 6 The amounts of used in EOP-stage.

EOP	Unit	Reference	75 kg mCPBA/ton	150 kg mCPBA/ton
m _pulp dry	G	100.00	80.00	80.00
H₂O₂	kg/t	8.00	8.00	8.00
H₂O₂	ml	95.24	76.19	76.19
Consistency	%	8.00	8.00	8.00
Total	G	1250.00	1000.00	1000.00
Water	ml	828.98	654.07	667.35

The quantum mixer was filled with water and preheated until the temperature of the water was the desired 75 °C. The weighted pulp was preheated in a microwave and the added water in a Teflon-covered pot.
The preheated pulp, water and the right H₂O₂ amount was mixed in a big glass beaker. The pH was adjusted by adding a few drops of NaOH. The quantum mixer was emptied and water residues were removed. The mixture was transferred from the glass beaker to the quantum reactor and the lid was carefully closed. The pulp inside the reactor was mixed with the quantum set on manually to the desired temperature before oxygen was applied. The pressure was now adjusted to 2.8 bar. The quantum was set to automatic and the reaction time was 70 minutes.
When the reaction time was over, the applied oxygen pressure was taken off and the pressure inside the quantum was slowly released. The reactor was opened and the pulp was taken out and put into a Buchner-funnel. The pulp was washed with enough water until the filtrate was clear. The pulp was transferred to a centrifuge bag and centrifuged in order to increase the dry content. The amount of pulp after the stage was weighted in order to determine the yield.
Kappa number, viscosity, HexA content and brightness were determined for each pulp.

D-stage

The D-stage was carried out exactly the same way as the D₀-stage, except that the reaction time was 200 minutes instead of 15 minutes. In Table 7 below the different amounts of pulp, ClO₂ and water for each pulp are presented.

Table 7 The amounts used in D-stage.

D	Unit	Reference	75 kg mCPBA/ton	150 kg mCPBA/ton
m _pulp dry	g	56.94	37.15	34.09
Active Cl	kg/ton	15.78	15.78	15.78
ClO₂	ml	105.58	68.89	63.44
Consistency	%	8.00	8.00	8.00
Total	g	711.75	464.38	427.63
Water	ml	412.10	267.61	250.50

When 200 minutes had past the reaction was interrupted and the pulp was transferred to a Buchner-funnel equipped with a wire. The residue chlorine was determined from the filtrate. The pulp was then washed with hot water until the filtrate was clear. The pulp was washed one more time with cold water in order to cool it down. The pulp was then transferred to a centrifuge bag and centrifuged to increase the dry content.
The pulp was analyzed for kappa number, viscosity, HexA content and brightness.

Results

The pulps were analysed as follows. Kappa number was analysed according to the standard SCAN -C 1:00, in force on July 2010, viscosity according to the standard SCAN-CM 15:99, in force on July 2010, water retention value according to the standard SCAN-C 62:00, in force on July 2010 and brightness according to the standard SCAN-CM 11:9, in force on July 2010. Other analyses performed on the pulps were HexA content analysis and total anionic group analysis.

Determination of hexenuronic acid content

The pulp was freeze dried in order to measure the HexA content. A hydrolysis solution consisting of 22 mmol/l of mercuric chloride (0.6 %) (HgCl₂) and sodium acetate (CH₃COONa·3H₂O) was prepared for the HexA content analysis.
Three parallel tests were done for each pulp. 0.05 g of freeze dried pulp was weighted into each test tube and 10 ml hydrolysis solution was added. The test tubes were well mixed in order to remove all clumps in the sample. The mixing was done by hand shaking. The test tubes were put in a water bath with a temperature between 60 and 70 °C for 30 minutes. After the reaction time they were cooled down to room temperature very quickly. The UV absorption was measured at 260 and 290 nm for each sample and the hydrolysis solution was used as blank. The HexA content was then calculated from the following formula (Chai X.-S., Zhu J.Y., Li J., A simple and rapid method to determine hexenuronic acid groups in chemical pulps, Journal of Pulp and Paper Science, 27, 165-170 (2001)): $C_{HexA} = 0.287 \times \frac{(A_{260} - 1.2 \times A_{290}) \times V}{w}$

where A₂₆₀ = Absorbance at 260 nm
A₂₉₀ = Absorbance at 290 nm
V = Volume of the hydrolysis solution in ml
w = Weighted amount of pulp

Determination of anionic groups

The anionic groups in the pulp were determined by methylene blue sorption. A 60 mM barbital buffer mother solution was prepared by dissolving pure 5-5 diethyl barbituric acid in deionized water. Sodium hydroxide (NaOH) was added to promote the dissolution of the 5-5 diethyl barbituric acid. A 0.4 mM methylene blue solution was prepared by dissolving methylene blue powder in deionized water with the addition of 10 ml barbital buffer mother solution.
A calibration curve was made by diluting the 0.4 mM methylene blue solution at a ratio of 25:250 by using a 0.6 mM buffer barbital solution as a solvent.
About 50 mg of oven dry pulp was measured and transferred to a 100 ml mixing flask. Different volumes of the methylene blue solution were added to the flasks and the reaction time was 15 minutes under continuous stirring at 500 rpm. The mixtures were then filtered in a sintered glass filter. Each sample was diluted 25 times, including the blank, with the 0.6 mM barbital buffer solution. The solutions were then analyzed in a UV-visual spectroscopy at 664 nm. The solution without pulp was used as a blank.

Oxygen delignification with mCPBA

Seven different charges of mCPBA were tested in the first experiments to eliminate HexAs from the pulp. In the first experiments mCPBA was added as powder. A reference pulp was also made by adding only NaOH and MgSO₄ to the oxygen delignification stage. The effect on kappa number, viscosity, water retention value, brightness, HexA-content and anionic groups will be discussed below.

Kappa number

The kappa number dropped as expected several units for the reference pulp after the oxygen delignification stage. The drop was mainly caused by lignin removal since oxygen delignification does not degrade HexAs.
In the first oxygen delignification experiments with low amounts of mCPBA added, no further decrease in kappa number could be noticed. When the amount of mCPBA was increased to 75 kg/ton a decrease could be registered and an even larger decrease could be noticed when 100 and 150 kg/ton were added. This was the first proof that mCPBA addition in the oxygen delignification stage was working since HexAs also contribute to the kappa number. The results from the first trials are presented in Figure 1. In this and the following Figures, a stands for unbleached pulp and b for oxygen-bleached pulp. The references A-G refer to pulps treated with different amounts of mCPBA, as follows:

A: 5.0 kg of mCPBA for 1 ton of pulp
B: 10.0 kg of mCPBA for 1 ton of pulp
C: 15.0 kg of mCPBA for 1 ton of pulp
D: 25.0 kg of mCPBA for 1 ton of pulp
E: 75.0 kg of mCPBA for 1 ton of pulp
F: 100.0 kg of mCPBA for 1 ton of pulp
G: 150.0 kg of mCPBA for 1 ton of pulp

Viscosity

A slight drop in viscosity could be noticed after the oxygen delignification of the reference pulp, which was expected since cellulose degradation always occurs in the oxygen delignification stage. No drop in viscosity could be registered for the smaller amounts of mCPBA, but when the larger dosages (75, 100 and 150 kg/ton mCPBA) were added a drop could be recognized. The viscosity results are presented in Figure 2, in ml/g.

Water retention value

The water retention value, or WRV, tends to increase in conventional oxygen delignification. This is not good since then the paper web tends to hold more water and this results in difficulties with dewatering of the paper web in the paper making process and also dewatering in the fiber line. In Figure 3 the results from the first set of experiments are presented, and it is clearly proved that by adding mCPBA the WRV decreases and the fibers will release bonded water more easily.

Anionic groups

Total anionic groups were determined for each pulp by methylene blue adsorption followed by UV-measurements. The WRV is also affected by anionic groups; the more anionic groups the higher WRV. In Figure 3, a drop in WRV can be recognized.

Brightness

The brightness of the pulps was measured as mentioned above and the results are shown in Figure 4, as percentage. It can be seen that the brightness slightly increases with smaller amounts of mCPBA and that a more notable increase can be achieved with higher loads of mCPBA.

HexA

The HexA-content was measured as explained above and the results can be seen from Figure 5 as µmol of HexA per g of the material. The effect of smaller amounts of mCPBA on the amount of HexAs is minimal, but the effect improves exponentially at higher loads.

Oxygen delignification with BA

The results from the trial where only benzoic acid was used show that the amount of HexA was clearly lower (28 µmol/g) when perbenzoic acid was used than when benzoic acid (53 µmol/g) was used. However, the partial removal of HexAs when benzoic acid was used indicated that perbenzoic acid precursors can also be used.
The viscosity of the pulp treated with benzoic acid was about 1300 ml/g, whereas for a pulp treated with the same amount of perbenzoic acid the viscosity about 930 ml/g. The water retention value of the pulp treated with benzoic acid was 1.52 compared to 1.39 for the pulp treated with mCPBA.

Oxygen delignification with mCPBA in solution

Figures 6-10 show the effect of the form of the mCPBA added to the process, the powder form being on the left and the solution form on the right. The first group (on the left) shows the effect at a load of 25.0 kg/ton and the second group on the right the effect of a load of 75.0 kg/ton.
Figure 6 shows the effect on Kappa number, Figure 7 the effect on viscosity, Figure 8 the effect on water retention value, Figure 9 the effect on brightness and Figure 10 the effect on the amount of HexAs.

Results after the complete bleaching sequence

Figures 11-14 show the results of Kappa number (Figure 11), viscosity (Figure 12), brightness (Figure 13) and the amount of HexAs (Figure 14) for the samples that were subjected to the complete bleaching sequence. In these Figures, in each group the reference sample is on the left, the sample treated with 75.0 kg of mCPBA/ton of pulp in the middle and the sample treated with 150.0 kg of mCPBA/ton of pulp on the right.
It can be seen that the effect on each of these characteristics remains identical to that after the oxygen treatment stage, thus showing that the effect on HexAs and consequently on these properties is indeed due to the use of perbenzoic acids in the oxygen treatment stage.
Even if the invention was described with reference to what at present seems to be the most practical and preferred embodiments, it is appreciated that the invention shall not be limited to the embodiments described above, but the invention is intended to cover also different modifications and equivalent technical solutions within the scope of the enclosed claims.

Claims

A method for removing hexenuronic acids from pulp, method comprising
- obtaining pulp by chemical pulping,

- treating the obtained pulp by using a further delignification process comprising an oxygen treatment stage which is an oxygen bleaching stage, oxygen delignification stage or oxygen pre-bleaching stage, where oxygen is added to the pulp under alkaline conditions,
carrying out the oxygen treatment stage in the presence of at least one perbenzoic acid, wherein the amount of perbenzoic acid used is 1-500 kg/ton of pulp.
A method according to claim 1, characterised in that the at least one perbenzoic acid is selected from the group consisting of perbenzoic acids; salts of perbenzoic acids; precursors of perbenzoic acids selected from benzoic acid, phthalic anhydride, substituted and unsubstituted benzoyl oxybenzene sulfonates, N-benzoyl succinimide, tetrabenzoyl ethylene diamine, N-acylated lactam, tetraacetyl ethylene diamine, lactose octaacetate and 4-trimethyl ammonium methyl derivative of benzoyl oxybenzene sulfonate; and mixtures thereof.
A method according to claim 1 or 2, characterised in that the at least one perbenzoic acid is perbenzoic acid, sodium salt of perbenzoic acid, metachloroperoxybenzoic acid, 4-tert-butylperbenzoic acid, 4-methylperbenzoic acid or 4-methoxyperbenzoic acid.
A method according to any of the preceding claims, characterised in that the amount of perbenzoic acid used is 1-300 kg/ton of pulp.
A method according to any of the preceding claims, characterised in that the at least one perbenzoic acid, salt of perbenzoic acid, derivative or precursor of perbenzoic acid or a mixture thereof is added to the pulp in a form selected from the group consisting of powder, solution, slurry and suspension.
A method according to any of the preceding claims, characterised in that the at least one perbenzoic acid, salt of perbenzoic acid, derivative or precursor of perbenzoic acid or a mixture thereof is added before the oxygen treatment stage.
A method according to any of the preceding claims 1-5, characterised in that the at least one perbenzoic acid, salt of perbenzoic acid, derivative or precursor of perbenzoic acid or a mixture thereof is added during the oxygen treatment stage.
A method according to any of the preceding claims, characterised in that at said oxygen treatment stage the alkali dosage is 10-30 kg/ton of pulp, the temperature is 80-120 °C, the retention time is 20-120 minutes, the dosage of magnesium sulfate is 1-4 kg/ton of pulp and the oxygen pressure is 50-100 psi.
A method according to any of the preceding claims, characterised in that the oxygen treatment stage is a pre-bleaching stage or it is a treatment stage incorporated into the bleaching sequence.