JPH0229791B2 - - Google Patents

Description

[Detailed description of the invention]

(Industrial Field of Application) The present invention relates to a method for bleaching mechanical pulp. (Prior Art) Mechanical pulp is produced by physically grinding wood into fibers containing cellulose and lignin, rather than chemically dissolving the lignin in wood and leaving only a fibrous cellulose residue. Ru. Since all of the wood can be converted into pulp suitable for manufacturing paper products, the mechanical pulping method has the advantages of a high yield of pulp based on the wood used as raw material and a substantially small amount of waste material. Both are considered preferable. Mechanical pulp can be produced by grinding logs with a "stone" wheel (groundwood pulp) or by grinding wood with cold metal refiner blades or hot metal refiner blades (refiner pulp or thermomechanical pulp). Refiner mechanical pulp and thermomechanical pulping materials are usually of a lower grade than groundwood pulping logs. Most of the wood for refiner and thermomechanical pulping comes from sawn plank offcuts. This material contains more bark (and mud) than molted logs. Even though the bleachability of the pulp is not affected by this impurity, the brightness of both unbleached and bleached pulps is. Two main methods are used for bleaching mechanical pulp: reduction and oxidation. These two methods can be used alone or in combination with a two-stage bleaching process: a first stage oxidative bleaching and a second stage reduction. Reductive bleaches are bisulfites, dithionites and borohydride compounds. The primary oxidizing bleach is hydrogen peroxide. Nithionite bleaching is the most commonly used reductive bleaching method. Sodium dithionite (Na ₂ S ₂ O ₄ ) is the preferred form. Zinc dithionite is also used to some extent. It is more stable than the sodium form and can be produced in factories, but has the disadvantage that it is toxic to fish. Nithionite bleaching is usually performed at low concentrations, with maximum bleaching results of approximately 4%
Obtained in concentration. Maximum brightness is usually achieved at pH 5-6. Nithionite can be produced by reducing sulfite with sodium borohydride (“borole”).
commercially available as a liquid). All percentages herein are by weight. Concentrations are percent based on oven dry weight of pulp in aqueous slurry. Unless otherwise specified, additives are expressed in percent by weight based on the weight of the oven-dried pulp. Although sodium borohydride is a strong reducing agent,
Its bleaching effect is commonly used in dithionite bleaching
In the acidic region of PH4 to 7, it is surprisingly small compared to dithionite. Boron hydride compounds form an alkaline solution, but this alkalinity exceeds the self-decomposition limit of borohydride compounds during bleaching.
not enough to keep it. Good results can be obtained using higher alkalinity and higher concentrations. The presence of a sequestering agent (DTPA) and sodium silicate also appears to improve bleaching properties. Sodium borohydride is usually too expensive for use in pulp bleaching, but is often used in the in-situ production of sodium dithionite. Oxidative bleaching using peroxides has certain advantages over dithionite reductive bleaching. The brightness achieved with oxidative bleaching is significantly greater than with dithionite bleaching, and the corrosion problems in peroxides are less than in dithionite bleached pulp. Although many oxidizing agents have been tested for mechanical pulp bleaching, peroxides, primarily hydrogen peroxide, are the only ones for mechanical pulp bleaching. A simple mixture of hydrogen peroxide and alkali does not give the best bleaching results. One or more chemicals are usually added to improve results. The most common chemical used is sodium silicate. Other sequestering agents include magnesium sulfate and DTPA and phosphates. Sodium silicate has a buffering effect in the pH range where peroxide is most active as a bleaching agent. Another effect of sodium silicate is to render metal ions inert. Magnesium sulfate has a similar inert effect and is particularly useful when water hardness is low. Sodium silicate and magnesium are thought to act cooperatively, but the mechanism is not clear. Calcium has similar potency to magnesium. Rather, large amounts of sodium silicate are used in peroxide bleaching of mechanical pulp. Using a 40% sodium silicate solution, approximately 5% by weight based on the pulp weight
(2.0% sodium silicate based on pulp weight). Magnesium sulfate (Epsom salts) is used in small amounts, generally 0.05 to 0.2% based on pulp. The concentration in bleaching depends on the dewatering equipment and the pulp and bleach mixing equipment. A concentration of 10 to 20% is used;
The average is about 15%. Retention time is usually 1.5 to 3
It's time. At the bottom of the column the pulp is diluted with water to a concentration of 3-4%. Since the pulp is alkaline, the slurry pH must be lowered to prevent color reversion. The pH is usually lowered to about 5.5, which is the same as unbleached pulp. If sulfur dioxide or bisulfite is used to lower the pH, residual peroxide will also be consumed. Chemical pulps are usually bleached in multiple stages, and multi-stage methods are also used for mechanical pulps if high brightness values are required. Oxidative bleaching with peroxide followed by reductive bleaching with dithionite is an industrially used method for multi-step bleaching of mechanical pulp. The residual peroxide needs to be destroyed before the dithionite is added because the peroxide bleaching will be finished before it is all consumed. Adjustment of pulp PH must also be done before addition of dithionite. Sulfur dioxide and sulfite solutions are suitable for both purposes. Reductive/oxidative bleaching methods (using a dithionite first stage and a peroxide second stage) are believed to be inferior to bleaching methods using peroxide in the first stage. This is because certain color-forming groups reduced by the dithionite are then oxidized by the peroxide, offsetting some of the benefits of reductive bleaching. A two-step oxidative/reductive bleaching process using dithionite or borohydride compounds followed by peroxide gives high whiteness values. Boron hydride compounds and peroxides can be used together in one-stage bleaching. This is surprising since the two chemicals do not react with each other under normal bleaching conditions. This effect is 2
It is not as good as what can be obtained with chemical two-step bleaching methods, but it is better than peroxide alone. Whiteness values of 87-88% have been obtained by three-step bleaching using borohydride-peroxide-nithionite. (Description of the invention) The present invention provides (a) PH10 of mechanical pulp with a concentration of 4% or more.
(b) optionally washing the slurry pulp from the first stage to substantially remove residual reducing bleach; (c) Adjust the concentration of the washed pulp slurry to 10% or more, and (d) Add to the pulp slurry from step (c) a hydrogen peroxide concentration of 1% or more based on the oven dry weight of the pulp in the slurry. Sufficient amount of PH10 to 11 to
A method for bleaching mechanical pulp in an aqueous slurry, comprising the steps of: adding an alkaline hydrogen peroxide bleaching solution to the pulp slurry and maintaining the temperature of the pulp slurry at an elevated temperature to increase the whiteness of the pulp in the slurry. . Sequestering agents can be used to remove polymerized metals from the pulp. Generally ethylenediaminetetraacetate (EDTA), pentansodium diethylenetriaminepentaacetate (DTPA),
Salts such as nitrilotriacetate (NTA), sodium tripolyphosphate, and sodium citrate are used industrially. In the case of refiner pulp or thermomechanical pulp, a bleaching solution is fed into the refiner together with the chips as a feed liquid. For groundwood pulp, it is recommended to add bleach to the pulp slurry. Sodium borohydride is most commonly used to produce sodium dithionite by reacting sulfur dioxide or sulfite with it. However, it can be used at pH 10-11 and concentrations above 4% without forming dithionite. Peroxygen bleaching method has a concentration of 40 to 20%.
It requires a temperature of 60°C and sufficient bleaching can be achieved in 1.5 to 3 hours. Usually about 1 based on pulp weight
3% to 3% sodium silicate and 0.05 to 0.2% magnesium sulfate are added to the bleach solution. Sequestering agents such as DTPA or 2-hydroxyethyl-ethylenediamine triacetate (HEDTA) can be used to pre-treat the pulp before the peroxygen bleaching step. The peroxide content of the bleaching solution must be 1% or more of H ₂ O ₂ , preferably 3 to 10%. 35-70 _{% H2O2}
Any source of hydrogen peroxide can be used, such as liquid, sodium peroxide, sodium peroxide carbonate, etc. (Examples and Comparative Examples) The present invention will be explained in more detail using the following Examples and Comparative Examples. Pulp is generally 0.25% DTPA at 4% concentration
Preprocessed using. An amount equivalent to 50 g of oven-dried (OD) pulp was weighed out and adjusted to the desired concentration using 60°C (140°C) deionized water. Sodium dithionite bleaching method The pulp was made into a 4% consistency and placed in a closed container. Immediately before adding the required amount of sodium dithionite, the pH was adjusted to 6 with an aqueous sulfur dioxide solution. The sodium dithionite solution was poured into the container below the liquid level using a pipette to avoid contact between the sodium dithionite solution and air as much as possible. The bottle was immediately closed and shaken by hand to mix thoroughly. The treated pulp was kept at 60°C for 1 hour.
The pulp was then filtered and washed with water to give the appropriate consistency for the next operation. Sodium borohydride bleaching method Sodium borohydride bleaching method was performed in the same manner as described above. However, the PH is on the base side (10−11),
The pulp was adjusted to a 4% concentration. Bleaching took less time. Hydrogen peroxide bleaching After washing the pulp from the pre-reduction step, bleaching using hydrogen peroxide was carried out in a conventional manner. sample is concentration
Bleached at 12%. Heat the pulp sample in a microwave oven to the desired temperature and add a bleaching solution (sodium silicate, caustic soda,
(made by adding magnesium sulfate and hydrogen peroxide) was added to the pretreated pulp and mixed well.
The pulp mixture was placed in a peaker, covered with a glass plate, and heated in a constant temperature bath. Samples were taken out periodically to check their brightness. Method for manufacturing handsheets for measuring whiteness A 6 g sample of OD pulp was taken for each measurement. The sample was diluted with deionized water to a concentration of 0.75%.
The pH of this slurry was adjusted to 4.5 with an aqueous sulfur dioxide solution. 6.25 according to appendix of TAPPI standard T218 05−69 after removing fiber lumps using walling high speed mixer
Test sheets were produced in an inch TAPPI sheet mold. The sheets were allowed to dry overnight and the whiteness was measured on a Datacolor 2000 reflectometer. The unit is
Expressed as ELREPHO or ISO whiteness percentage. Comparative Example 1 A two-stage bleaching process was conducted using hydrogen peroxide in the first stage and sodium dithionite in the second stage.
The pulp was a 50/50 mixture of hardwood/softwood TMP. The test conditions and results are shown in the table. Comparative Example 2 The same batch of pulp used in Comparative Example 1 was bleached in the reverse order using first sodium dithionite and then hydrogen peroxide. The results are shown in the table. A single hydrogen peroxide bleach method was tested following the same other conditions as in Example 1. The whiteness was 85%. Also, 7% hydrogen peroxide was used in all cases. Example 1 and Comparative Examples 3-4 Sodium borohydride was used as a reducing bleach for Nanpo thermomechanical soft wood pulp. In Experiment 1 (Comparative Example 3), there was an oxidation stage first and then a reduction stage. In Experiment 2 (Comparative Example 2), the reduction stage was first followed by the oxidation stage. Experiment 3 (Comparative Example 4) was a single reduction using sodium borohydride as a reducing agent. The results are shown in the table. The two-step bleaching method using a reducing agent as the first step is different from the conventional method. On the contrary, it was surprising that it was superior to the ordinary oxidation/reduction method.The only document on the reduction/oxidation bleaching method using ground wood pulp is Burton R.
There is a pulverized wood bleaching method using peroxide and dithionite acid (TAPPI, Vol. 41 (3), pp. 161A-165A) by W. In this literature 0.4% _H2O2 as _peroxide
(2% Na ₂ O ₂ ) is used.

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Claims (1)

[Claims] 1 (a) PH10 to 12 of mechanical pulp with a concentration of 4% or more
(b) contacting the aqueous slurry of with a reducing bleach consisting of sodium borohydride in a first step;
optionally washing the slurry pulp from the first stage to substantially remove any residual reducing bleach; (c) adjusting the washed pulp slurry concentration to 10% or more; and (d) from step (c). A sufficient amount of alkaline hydrogen peroxide bleaching solution with a pH of 10 to 11 is added to the pulp slurry to make the hydrogen peroxide concentration 1% or more based on the oven-dried weight of the pulp in the slurry, and the temperature of the pulp slurry is increased. 1. A method for bleaching mechanical pulp in an aqueous slurry, comprising the steps of: increasing the whiteness of the pulp in the slurry by keeping it under water; 2 Peroxygen bleaching solution contains 1 to 3% sodium silicate
and 0.05 to 0.2% magnesium sulfate.