JPS5927436B2 - Peroxide bleaching method for wood pulp - Google Patents

Description

TECHNICAL FIELD OF THE INVENTION The present invention relates to a method for bleaching wood pulp with peroxide.

(Prior art) In the conventional method of bleaching wood pulp with peroxide, known peroxides include hydrogen peroxide, sodium peroxide, and peracetic acid, but hydrogen peroxide is the most common. used in

As is well known, hydrogen peroxide bleaching must be carried out under alkaline conditions (preferably pH 9.0 to 11.0), so a large amount of caustic soda (NaOH) or sodium silicate (Na2SiO3) must be added as an alkaline agent. No. Sodium silicate is generally used as a buffer to keep the pH of the bleach bath alkaline, and as a chemical to seal the heavy metal layer in the bleach bath so that peroxides can work effectively. Regarding caustic soda, Na2Si is usually used as a bleaching aid to keep the pH of the bleaching bath alkaline.
03/NaOH ratio (as NaOH) 1/4 to 1
It is added in large amounts such as /1.

In addition, a small amount of a chelating agent such as magnesium sulfate or EDTA (ethylenediaminetetraacetate) may be added as a stabilizer for hydrogen peroxide. In the conventional peroxide bleaching method, sodium silicate and caustic soda were added to a pulp slurry with a pulp concentration of about 10%, peroxide was further added, and the slurry was maintained at a straining temperature of 60 to 70°C for 2 to 4 hours. After that, the pH is lowered to about 5.0 to 6.0 by adding sulfite water or sulfuric acid to decompose the remaining peroxide and obtain bleached pulp.

According to these conventional methods, since sodium silicate is soluble, it is not used as an adjuvant for peroxide bleaching after use, and most of it is discarded by flowing into wastewater.
This method is not only uneconomical, but also has the problem of increasing the amount of inorganic sludge in subsequent wastewater treatment, especially coagulation and sedimentation treatment.

Furthermore, in conventional hydrogen peroxide bleaching, as mentioned above, caustic soda, which is a strong alkali, is added three to four times as much as sodium silicate (as NaOH), and it is treated at high temperatures of 60 to 70 degrees Celsius for a long time. , the wastewater has a high pollution load such as COD (chemical oxygen demand) and BOD (biological oxygen demand), and furthermore, the fine fibers in the pulp are eluted by the influence of alkali, resulting in pulp failure. The problem was that the transparency was significantly reduced. In order to solve the problems of conventional peroxide bleaching, the applicant of this patent has
A method of fixing silicic acid content to pulp by adding a water-soluble compound of divalent or higher metal to pulp slurry after hydrogen peroxide bleaching (using caustic soda and sodium silicate as bleaching aids) 143704) or, if caustic soda and sodium silicate are used, the BOD of the wastewater, . Not only does it have a high COD load, but it is also impossible to concentrate and burn the wastewater due to the scale that accumulates in the silica. It has already been proposed that a method using a chelating agent (see Japanese Patent Application No. 53-115164) is effective.

Furthermore, the applicant of this patent has developed a peroxide-bleached pulp characterized by adding and mixing sodium silicate and a magnesium compound as bleaching aids to unbleached pulp, performing peroxide bleaching, and then adding an aluminum compound. He also proposed a manufacturing method (see Japanese Patent Application No. 151481/1983). (
Purpose of the Invention) In order to further solve the problems of conventional peroxide bleaching, the present inventors developed a new bleaching aid, effectively utilized the used bleaching aid, and improved pulp quality (opacity, etc.). As a result of repeated research aimed at this purpose, we have arrived at the present invention.

(Summary of Structure and Effects of the Invention) The present invention provides a method for bleaching wood pulp with peroxide, in which sodium silicate, soda carbonate, and calcium hydroxide and/or calcium oxide are added to unbleached pulp as bleaching aids. A method for peroxide bleaching wood pulp, which is characterized by mixing and peroxide bleaching of wood pulp, and a method of peroxide bleaching wood pulp, which is characterized by mixing and peroxide bleaching the auxiliary agent, and then adding an aluminum compound. This is a peroxide bleaching method.

The present invention will be explained in detail below.

In the method of bleaching wood pulp with peroxide, the present invention forms caustic soda in the system by adding soda carbonate and calcium hydroxide and/or calcium oxide in addition to sodium silicate as a bleaching aid. In other words, the in situ method is adopted.

That is, the present invention has a major feature in that it does not use the conventionally used caustic soda at all. In the present invention, since caustic soda, which is a strong alkali, is not used, the pH of the bleaching bath does not become very high, and the pH is 9.0, which is suitable for hydrogen peroxide bleaching. It is easy to maintain 0 to 11.0. As a result, it has become possible to extremely reduce the pollution load of wastewater, which was extremely large in the conventional method using caustic soda. Further, the method of the present invention includes adding an aluminum compound such as aluminum sulfate to the bleaching system after the in-situ bleaching process, and fixing the bleaching aid in the pulp as a white precipitate, thereby effectively using it as a filler. It is also characterized by Although the chemical structure and other details of the generated precipitate are unknown, X-ray diffraction results show that it is a compound of calcium, aluminum, and silica, CaAl2S.
iO3 was fixed. The bleached pulp obtained by the method of the present invention has greatly improved opacity compared to that obtained by the conventional method. Furthermore, when an aluminum compound such as aluminum sulfate is added, the produced white filler has the effect of improving not only the opacity but also the whiteness. An example of an embodiment of the method of the present invention will be described with reference to the accompanying drawings (FIG. 1).

The unbleached pulp is introduced into a dehydrator 2 via an unbleached pulp transfer pipe 1.

The waste water from the dehydrator passes through the drain pipe 3 and is either circulated to the previous process or subjected to appropriate treatment and then disposed of. The unbleached pulp from the dehydrator is transferred to a high concentration mixer 5 using dilution water supplied by a dilution water inlet pipe 4 to obtain a bulk concentration suitable for peroxide bleaching, i.e. 5 to 20 weight percent. diluted in The calcium oxide and/or calcium hydroxide stored in the calcium oxide and/or calcium hydroxide storage tank 6 is added as a solid or slurry to the high concentration mixer 5, and then added to the soda carbonate storage tank 7.
The carbonated soda stored in is also added. Furthermore, the sodium silicate stored in the sodium silicate storage tank 8 is also added to the high concentration mixer and stirred well. The peroxide in the peroxide storage tank 9 is then fed into a high concentration mixer, thoroughly mixed, and then fed into the bleaching tower 10. Bleaching of the pulp is completed by keeping it at a temperature of 50 to 70° C. for 2 to 6 hours in a bleaching tower. Sulfite water is supplied to this bleached pulp in a conventional manner to decompose unreacted peroxide, and the pulp is transferred to the next step.

Alternatively, the pulp after bleaching is introduced into the reaction tank 11, and the aluminum compound from the aluminum compound storage tank 12 is added thereto, followed by thorough stirring and reaction. The pulp concentration at the time of reaction is pulp,
It is sufficient that the calcium compound and the aluminum compound are sufficiently stirred and brought into contact with each other, and around 5.0% is good. The reaction temperature and time may vary depending on the strength of stirring, but about 10 minutes at 30 to 40°C is sufficient. After the reaction is completed, the pulp is led to the next step. (calcium oxide and/or
(or calcium hydroxide) In the method of the present invention, the calcium hydroxide that reacts with soda carbonate to produce caustic soda in situ is a calcium hydroxide slurry;
Alternatively, it may be added to the pulp slurry as a powder of calcium hydroxide.

The calcium hydroxide to be added may be a powder rich in calcium carbonate in the center and calcium oxide in the surface layer.
The addition rate of calcium oxide and/or calcium hydroxide to the pulp may be within the range of about 1 to 10%, and the preferred range is about 1 to 5%.

The addition rate of peroxide per pulp is about 1 to 10%, preferably about 2 to 6%, in terms of hydrogen peroxide. (
Soda carbonate) Soda carbonate can be used in any form, but if it is a sulfite pulp mill in connection with the chemical recovery process in the pulp manufacturing process, for example, the Tampera method (Japanese Patent Publication No. 46-10057) is used as the chemical recovery process. If the factory adopts the method (see US Pat. No. 3,508,863), sodium carbonate monohydrate extracted from the crystal separation step of the recovery step may be used.

In addition, if the pulp mill has a kraft pulping process, it is possible to use soda carbonate, which is obtained by evaporating and concentrating an aqueous solution of smelt (green liquor) obtained by concentrating and burning black liquor, and crystallizing it. Good too. The addition rate of soda carbonate per pulp may be about 1 to 10%, but the preferred range is about 2 to 6%.

(Aluminum Compound) Aluminum sulfate, aluminum chloride and aluminum nitrate are preferred as the aluminum compound used in the method of the present invention.

The amount of aluminum compound added is based on calcium oxide and/or
It also varies depending on the amount of calcium hydroxide, soda carbonate, and sodium silicate, but generally speaking, the amount of aluminum oxide is about 0.5 to 2 times that of calcium hydroxide Ca(0H)2, preferably about 2 times as much as aluminum oxide. It is necessary to add 0.9 to 1.5 times as much. The pH of the pulp slurry after treatment is preferably 7.0 or less, preferably around 5.0. (Wood Pulp) The wood pulps to be bleached by the method of the present invention include SP (sulfide pulp), GP (ground pulp), CGP (chemical ground pulp), and RGP (refined ground pulp). ), TM
Examples include P (thermomechanical pulp) and waste paper pulp, but they are not necessarily limited to those listed in +, and any type of wood pulp may be used.

(Effects of the present invention) The present invention does not use caustic soda, which is a strong alkali, but instead adds soda carbonate as an alkali, and then adds calcium hydroxide (and/or calcium oxide). In other words, caustic soda is generated within the system by an in situ method.

Further, after bleaching is completed, an aluminum compound is added to fix the bleaching aid to the pulp or paper as a white filler. This results in various advantages, such as: (1) Chemical costs can be reduced because soda carbonate and calcium hydroxide and/or calcium oxide are used, which are cheaper than caustic soda.

(4) Calcium hydroxide and/or added to soda carbonate
Alternatively, by adjusting the amount of calcium oxide, it is possible to freely adjust the amount of caustic soda generated, and since sodium carbonate also has a PH buffering power, depending on the conditions, the amount of sodium silicate added can also be reduced. .

(Almost the entire amount of calcium hydroxide and/or calcium oxide and sodium silicate used as IiO bleaching aids can be fixed as a white filler in the method of the present invention.

●) The opacity of the paper is improved by fixing the white filler.

Also calcium hydroxide and/or calcium oxide,
Depending on the amount of aluminum sulfate etc. added, it is possible to greatly improve the whiteness. () Since caustic soda, which is a strong alkali, is not used, the CODl and BOD loads of wastewater can be significantly reduced. As a result, the bleaching yield also improves. An example of COD load reduction is shown in FIG. 10.

In Figure 10, the upper curve is JP-A-53-14370.
The COD according to the invention described in Publication No. 4 is shown, and the lower curve shows the COD according to the invention. As mentioned above, the present invention greatly contributes to the rationalization of the pulp and paper industry.

Next, the present invention will be explained in more detail by showing examples of the present invention, but the present invention is not limited in any way by these examples.

Comparative Example 1 After diluting RGP (unbleached whiteness 51.8%) to a pulp concentration of 10%, bleaching chemicals were added to the bone-dried pulp at 3% sodium silicate, 3.0% caustic soda, and 3.0% caustic soda, respectively. After adding 6% hydrogen peroxide and stirring well, the mixture was kept at 70° C. for 4 hours for bleaching.

The pH at the beginning of bleaching is 11.6, and the pH at the end is 9.6.
It was hot. After diluting the bleached pulp to a pulp concentration of 5% and adjusting the pH to 5.5 with sulfite water, a handsheet was prepared using a conventional method, and the whiteness, opacity, and sheet ash content were measured. Also, pulp slurry treated under the same conditions.
/166 types▲It was filtered through paper and dried at 105°C to determine the bleaching yield. The COD value of the sap liquid was measured by the potassium permanganate method. The results were: whiteness 71.0%, opacity 90%, sheet ash 0.7%, bleaching yield 93.0%.
, the COD load was 40k9/TOn. Example 1 After diluting absolutely dry 30f7 RGP (unbleached brightness 51.8%) to a pulp concentration of 10%, bleaching chemicals were added to each absolutely dry bulb, such as 3.0% sodium silicate and 0.0 sodium carbonate. %〜
After changing the molar ratio of Na2CO3/Ca(0H)2 such as 8.0% and calcium hydroxide 0.0% to 6.0%, adding 6.0% hydrogen peroxide and stirring well. , 70
℃ for 4 hours for bleaching.

PH at the beginning of bleaching is 10.5-11.5, P after bleaching is finished
H ranged from 8.9 to 9.9. The pulp after bleaching was diluted to a pulp concentration of 5.0%, aluminum sulfate was added at 1% (as Al2O3) based on the pulp, and the mixture was thoroughly mixed.
The pH after addition was 4.5 to 5.0. Then, a handmade sheet was prepared by a conventional method, and the whiteness, opacity, and sheet ash content were measured.

In addition, in the same manner as Comparative Example 1, the bleaching yield and COD load were also measured.
The results are shown in FIGS. 2 to 6. Comparative Example 2 Using exactly the same method as Comparative Example 1, the alkali addition rate was changed to 2.5.
%, 3.0%, 3.5%, 4.0%, and 4.5%, the whiteness, opacity, and sheet ash content were measured.

When the alkali addition rate is 2.5%, 3.0%, and 3.5%, the caustic soda addition rate is 1.8%, 2.1%, and 2.5%.
The addition rate of sodium silicate is 2.4%, 2.9%,
It is 3.4%, and the hydrogen peroxide addition rate at this time is 4%. Also, the addition rate of alkali is 3.5%, 4.0%, 4,5
% addition rate of caustic soda is 2.5%, 2.8%,
3.2%, and the addition rate of sodium silicate is 3.4%, 3
．． 8% and 4.3%, and the hydrogen peroxide addition rate at this time was 6%.

Here, the addition rate of alkali means the sum of the amount of NaOH produced from sodium silicate and the amount of NaOH added, and the addition rate is based on bone dry pulp. The results are shown in solid lines in FIGS. 7 to 9.

Figures 7-a, 8-a and 9-a show the case where the hydrogen peroxide addition rate is 4%, and Figures 7-1b, 8-b and 9-b show the case of hydrogen peroxide addition. The case where the hydrogenation rate is 6% is shown. Example 2 Using the same method as in Example 1, the alkali addition rate was changed to 2.5.
%, 3.0%, 3.5%, 4.0%, and 4.5%, the whiteness, opacity, and sheet ash content were measured.

When the alkali addition rate was 2.5%, 3.0%, and 3.5%, the content of caustic soda generated by the causticizing reaction between soda carbonate and calcium hydroxide was 1.8% and 2.1%. %, 2
．． 5%, and the addition rate of sodium silicate is 2.4%, 2
．． 9% and 3.4%, and the hydrogen peroxide addition rate at this time was 4%. Also, the addition rate of alkali is 3.5% and 4.0%.
, 4.5%, the content of caustic soda generated by the causticizing reaction is 2.5%, 2.8%, and 3.2%, and the addition rate of sodium silicate is 3.4%, 3.8% and 4.3%, and the hydrogen peroxide addition rate at this time was 6%. Here, the addition rate of alkali is the amount of NaOH generated from sodium silicate and the amount of NaOH generated by the causticizing reaction.
It means the sum with OH, and the addition rate is based on bone-dry pulp. The results are shown in dashed lines in FIGS. 7-9. In these graphs, the hydrogen peroxide addition rate is as explained in Comparative Example 2.

[Brief explanation of drawings]

FIG. 1 is a flow sheet of a preferred embodiment of the method of the present invention, and the reference numerals in the figure indicate the following, respectively. 1... Unbleached pulp transfer pipe, 2... Dehydrator, 3... Drain pipe, 4... Dilution water introduction pipe, 5... ... High concentration mixer, 6 ... Calcium oxide and/or calcium hydroxide storage tank, 7
... Sodium carbonate storage tank, 8 ... Sodium silicate storage tank, 9 ... Peroxide storage tank, 10 ...
... Bleaching tower, 11 ... Reaction tank, 12 ...
・Aluminum compound storage tank Figures 2 to 6 are graphs showing the test results of handmade sheets produced using the pulp obtained in Example 1, and Figure 2 shows the whiteness,
Figure 3 shows the opacity, Figure 4 shows the sheet ash content, Figure 5 shows the bleaching yield, and Figure 6 shows the COD load.

Claims (1)

[Claims] 1. A method for bleaching wood pulp with peroxide, in which sodium silicate, soda carbonate, and calcium hydroxide and/or calcium oxide are added to unbleached pulp as bleaching aids without adding caustic soda. A method for bleaching wood pulp with peroxide, which comprises adding, mixing, and bleaching. 2. A method for bleaching wood pulp with peroxide, in which peroxide is added to unbleached wood pulp as a bleaching agent, and sodium silicate, soda carbonate, and calcium hydroxide and/or as bleaching aids are added to unbleached wood pulp without adding caustic soda. A method for bleaching wood pulp with peroxide, which comprises adding and mixing calcium oxide for bleaching, and then adding an aluminum compound.