JPH0413476B2 - - Google Patents

Description

[Detailed description of the invention]

[Industrial Field of Application] The present invention relates to a pulp bleaching method, and particularly to a bleaching method using peroxide. [Prior art] In recent years, wood pulps such as groundwood pulp (GP), refined groundwood pulp (RGP), mechanical pulp (MP) such as thermomechanical pulp (TMP), chemical ground pulp (CGP), semi-chemical pulp (SCP), etc. Most pulps, including so-called high-yield pulps, chemical pulps (CP) such as soda pulp (AP) and kraft pulp (KP), as well as deinked pulp (DIP) made from waste paper, are bleached with peroxides such as hydrogen peroxide. Adoption of the law is increasing. Due to the recent demand for high whiteness of paper and environmental problems such as increased wastewater load, this method has been replaced by conventional reductive bleaching method using sodium hydrosulfite and chlorine bleaching agent such as chlorine dioxide by peroxide bleaching method. This appears to be due to switching to bleach. Here, the circumstances and problems of peroxide bleaching will be explained using mechanical pulp as an example. In the past, most mechanical pulp was used for papermaking with medium to low brightness, but now the demand for mechanical pulp is increasing due to its use as an alternative to high brightness kraft pulp for newspapers and medium-quality paper. increases,
In addition, there is a growing demand for materials with high whiteness. Therefore, the search for an efficient bleaching method for mechanical pulp is one of the important issues in the paper and pulp industry. The first bleaching method that greatly improves the whiteness of mechanical pulp is peroxide bleaching, typified by hydrogen peroxide. In addition to greatly improving whiteness, hydrogen peroxide bleaching produces less verignin than other bleaching agents.
It has the characteristics of good bleaching yield and good opacity. However, hydrogen peroxide has the property that in the presence of metal ions, the metal ions act as a catalyst and react and decompose as shown in the following formula. H ₂ O ₂ +M ^* →H ₂ O + 1/2O ₂ M ^* is a metal ion Metal ions in the mechanical pulp manufacturing process mainly flow in from the outside through wood chips, and are contained in large amounts in the circulating white water within the process. . Therefore,
During bleaching, some hydrogen peroxide is decomposed and loses its bleaching ability. Conventionally, organic chelating agents (hereinafter abbreviated as chelating agents) have been used to control the decrease in bleaching ability due to hydrogen peroxide decomposition.
and sodium silicate (Na _{2 Si} O ₃ ) have been used.
The reason for this is that chelating agents have the effect of chelating and inactivating metal ions, and sodium silicate has a buffering effect that maintains the pH at which hydrogen peroxide is most effective. This is because it has an activating effect. Conventionally, the chelating agent and sodium silicate were added to the bleaching mixer in the process shown in the drawings at almost the same time as other bleaching agents. However, in conventional bleach mixers, the chelating agent and hydrogen peroxide are added at the same time, resulting in competitive reactions between the metal ions and the chelating agent, and between the metal ions and hydrogen peroxide, which prevents the chelating agent from fully demonstrating its ability. Not yet. Since the chelating agent is added to a high-concentration pulp dispersion with a pulp concentration of 10 to 30% by weight, contact with metal ions is incomplete. Sodium silicate, which has the ability to deactivate metal ions, is underutilized. There were other problems. These problems in peroxide bleaching of mechanical pulp are common problems in peroxide bleaching of other pulps, and there has been an urgent need to solve these problems. [Problems to be Solved by the Invention] The present invention solves the above-mentioned problems, and in peroxide bleaching of pulp, the chelating agent and sodium silicate added to inactivate metal ions are
The object of the present invention is to provide a method for bleaching pulp that works effectively to reduce ineffective decomposition due to decomposition of peroxide, and which can yield pulp with high whiteness. [Means for Solving the Problems] As a result of various studies regarding the above-mentioned problems, the present inventors changed the addition location of the chelating agent in peroxide bleaching of pulp from medium and high concentration pulp to low concentration pulp. In addition, they discovered that by adding sodium silicate in portions, the chelating agent and sodium silicate work effectively, significantly reducing the ineffective decomposition of peroxide, and improving the whiteness of the pulp, and completed the present invention. It came to this. That is, in the bleached pulp manufacturing process, which is performed in the order of pulping process, dust removal and selection process, dehydration process, and bleaching process, the present invention adds an organic chelating agent in the dust removal and selection process, and adds sodium silicate to the bleaching process after the dehydration process and after the bleaching process. This is a method for bleaching pulp, which is characterized in that it is added at two separate points in the process. In addition, in the bleached pulp manufacturing process described above, the white water generated in the dehydration process of unbleached pulp is circulated and used in the dust removal and cleaning process, and an organic chelating agent is added to the unbleached circulating white water, and then a sodium silicate The scope of the invention also includes a pulp bleaching method characterized in that the pulp is added in two parts: after the dehydration step and in the bleaching step. The drawing shows an outline of the flow sheet for the dust removal/selection process, dewatering process, and bleaching process in the general mechanical pulp manufacturing process, and this flow sheet is almost the same for other chemical pulps and waste paper DIP. In mechanical pulping, chips are disintegrated with a refiner to produce refined ground pulp (RGP).
The low-concentration pulp dispersion liquid is processed through the dust removal and selection process using a screen and cleaner, and after dust removal and selection, it is dehydrated by a dehydrator in the dehydration process to achieve a high concentration, and then the next bleaching mixer is used to bleach the liquid. A chelating agent and sodium silicate were added together with the bleaching agent, and after bleaching in a bleaching tower, the bleached pulp (bleached pulp) was sent to a preparation process. In the present invention, the chelating agent and the sodium silicate are added in separate steps in the dust removal/selection step, dehydration step, and bleaching step of the pulp manufacturing process. Specifically, this is a bleaching method in which a chelating agent is added to the dust removal/selection process, and sodium silicate is added separately after dehydration in the dehydration process and to the bleaching mixer. This is a bleaching method in which it is added separately after the dehydration process and to the bleach mixer. When sodium silicate is added after dehydration in the dehydration step, it is preferably added at the outlet of the dehydrator, but it may be added as far as possible before entering the bleach mixer. The pulp bleaching method of the present invention can be applied not only to the exemplified mechanical pulp, but also to high-yield pulp such as CGP and SCP.
It can be applied to peroxide bleaching of chemical pulp such as KP and AP, and DIP of waste paper. As the peroxide bleaching agent, hydrogen peroxide, sodium peroxide, percarbonate, perborate, peracetic acid, ozone, etc. can be used. The amount of peroxide to be added may be a known amount; for example, hydrogen peroxide may be added in an amount of 0.5 to 5% based on bone dry pulp.
(calculated as 100% H ₂ O ₂ ). There are no particular limitations on the chelating agent, and known diethylenetriaminepentaacetate (DTPA),
Organic chelating agents such as ethylenediamine tetraacetate (EDTA) and hydroxyethylenediamine tetraacetate (HEDTA) are used. The addition rate of the chelating agent may be 0.05% by weight or more (as a 40% stock solution, the same applies hereinafter) to the bone dry pulp, but preferably 0.05 to 1.00% by weight, more preferably 0.20 to 0.50% by weight. This is because if it is less than 0.05% by weight, there is no effect of sequestering metal ions, and if it exceeds 1.00% by weight, no noticeable proportional effect can be obtained as the amount added increases. A commercially available commercially available sodium silicate product may be used, and the amount added may be the same as that used for normal peroxide bleaching. For example, in the case of hydrogen peroxide bleaching, 1 part hydrogen peroxide to 0.4 to 1 part sodium silicate. The ratio of Sodium silicate is added in portions to the unbleached pulp after dehydration and to the bleaching mixer, but there is no particular restriction on the dividing ratio. Preferably, the amount is 1/3 to 2/3 after dehydration, and 2/3 to 1/3 in the bleach mixer. [Function] By adding the chelating agent to the dust removal/selection process where the pulp concentration is relatively low before adding bleach or to unbleached circulating white water containing a large amount of metal ions, good contact between the metal ions and the chelating agent is achieved. Since the reaction between the metal ions and the chelating agent has priority over the reaction between the metal ions and the peroxide, decomposition of the peroxide is suppressed. Chelating agents are effectively used for these reasons. In addition, by adding sodium silicate in portions and adding a portion before adding peroxide bleach (after dehydrating the unbleached pulp), the ability of sodium silicate to inactivate metal ions can be utilized, and the ability of sodium silicate to inactivate metal ions can be utilized. The sodium silicate added to the bleach mixer also maintains a buffering effect that maintains the pH during bleaching. It is believed that for the above-mentioned reasons, the ineffective decomposition of peroxide was suppressed (residual peroxide ratio increased), and the whiteness was improved. On the other hand, adding the entire amount of sodium silicate to the unbleached pulp after dehydration is not preferred because the unbleached whiteness decreases due to alkali burning. In addition, adding sodium silicate to the dust removal/selection process or circulating white water system makes it difficult to maintain the pH during bleaching (because sodium silicate comes out of the pulp during the dehydration process).Decrease in unbleached whiteness due to alkali burning. This is not preferable because problems such as scaling may occur. Therefore, in the present invention, it is preferable that sodium silicate is added in parts after the unbleached pulp is dehydrated and after the bleaching mixer is added. [Example] Examples of the present invention will be shown below. In addition, all % in an example shows weight %. Example 1 Unbleached mechanical pulp RGP made from softwood (25% wood from Hokkaido, 75% wood from outside) was used, and the pulp concentration was 1.5%.
A pulp suspension (corresponding to the pulp in the dust removal and selection process) was made. 0.20% of DTPA (Versenex 80 manufactured by Dow Chemical Company) was added to this pulp suspension, and the temperature was 80°C.
The mixture was stirred at ℃ for 3 minutes. Add this to Buchnarot (40
A pulp suspension with a pulp concentration of 17% (hereinafter referred to as high-concentration pulp, which corresponds to the pulp at the outlet of the unbleached pulp dehydrator) was prepared by suction filtration using a mesh wire (equipped with a mesh wire). To this high concentration pulp, 1.5% of sodium silicate was added for the first time (corresponding to the outlet of the unbleached pulp dehydrator) and mixed. After that, 4.3% caustic soda, 1.5% sodium silicate (corresponding to a bleach mixer) for the second time, and 5.5% hydrogen peroxide were added to make the pulp concentration 15%.
After mixing, bleaching was carried out at a temperature of 80°C for 120 minutes. After bleaching, the pulp concentration was adjusted to 1.0% and the pH was adjusted to 5.0 with sulfuric acid. Next, this pulp slurry was made into paper with a basis weight of 100 g/m ² by a conventional method using a Tappi standard method test hand paper machine. PH before and after bleaching, residual hydrogen peroxide (H ₂ O ₂
) and whiteness are shown in the table. Test method PH Measured according to JIS Z8802 Residual H ₂ O ₂ Kingset method (iodate reduction titration method)
Measured by Whiteness Measurement according to JIS P8123 Example 2 The same procedure as in Example 1 was carried out except that 0.50% of DTPA was added. The results are shown in the table. Example 3 In Example 1, the first sodium silicate was added to 0.75
Example 1 was carried out in exactly the same manner as in Example 1, except that 2.25% of sodium silicate was added for the second time. The results are shown in the table. Example 4 In Example 1, the first sodium silicate was 2.25
Example 1 was carried out in exactly the same manner as in Example 1 except that 0.75% of sodium silicate was added for the second time. The results are shown in the table. Example 5 The same procedure as in Example 1 was carried out except that 0.10% of DTPA was added. The results were tabulated. Example 6 The same procedure as in Example 1 was carried out except that 1.10% of DTPA was added. The results are shown in the table. Example 7 In Example 1, instead of using pulp with a concentration of 1.5%, which corresponds to the pulp in the dust removal and screening process, DTPA was added to unbleached circulating white water prepared as follows, and the mixture was heated at a temperature of 80°C for 10 minutes. After stirring for seconds, the concentration is 8% again.
The same procedure as in Example 1 was carried out except that the mixture was returned to the pulp and added to the pulp before dehydration. The results are shown in the table. Preparation of unbleached circulating white water The pulp suspension with a concentration of 1.5% produced in Example 1 was suction filtered using a Buchner funnel (equipped with 40 mesh wires) until the pulp concentration reached 8%, and the filtrate at this time was is unbleached white water. Conventional Example 1 The process was carried out in exactly the same manner as in Example 1, except that DTPA was not added in Example 1, sodium silicate was not added in portions, and 3.0% was added at once in the second time (corresponding to a bleach mixer). Ivy. The results are shown in the table. Conventional Example 2 In Example 1, instead of adding 0.20% DTPA to the pulp suspension with a concentration of 1.5% corresponding to the pulp in the dust removal and sorting process, it was added to the high concentration pulp after dehydration of the unbleached pulp, At the same time, the same procedure as in Example 1 was carried out except that 3.0% of sodium silicate was added at one time. The results are shown in the table. Comparative Example 1 The same procedure as in Example 1 was carried out except that DTPA was not added. The results are shown in the table. Comparative Example 2 The same procedure as in Example 1 was carried out except that 0.02% of DTPA was added. Comparative Example 3 In Example 1, the entire amount of sodium silicate was
The procedure was carried out in exactly the same manner as in Example 1, except that 3.0% was added. The results are shown in the table. Comparative Example 4 The same procedure as in Example 1 was carried out except that the first addition of Na _{2 Si} O ₃ in Example 1 was not performed, and 3.0% of NaSi _{O 3} was added in the second time. The results are shown in the table. Comparative Example 5 Instead of adding sodium silicate for the first time in Example 1 to a pulp suspension with a concentration of 17% corresponding to the pulp after dehydration of the unbleached pulp, the unbleached white water obtained in Example 6 was added. After stirring for 10 seconds at a temperature of 80°C, the suspension was returned to a pulp suspension with a concentration of 8% and added to the pulp to which DTPA had been added before dehydration.
It was carried out in exactly the same manner as in Example 1. The results are shown in the table.

[Table] From the results shown in the table, it can be seen that in Examples 1 to 6, in which the chelating agent of the present invention was added to the dust removal/selection process, and sodium silicate was added dividedly to the outlet of the unbleached pulp dehydrator and the bleaching mixer, only sodium silicate was added. (Conventional Example 1), when adding the chelating agent and sodium silicate at once using a bleach mixer (Conventional Example 2), and when adding the sodium silicate in portions without adding the chelating agent (Comparative Example) 1) The whiteness of the pulp obtained by bleaching is higher than either method (Comparative Example 4) in which a chelating agent is added in the dust removal/selection process and sodium silicate is added all at once in the subsequent bleaching mixer. The residual H ₂ O ₂ ratio was also high in Examples 1 to 5, which proves that there is little ineffective decomposition of hydrogen peroxide. Furthermore, the results of Example 7 show that even when a chelating agent is added to unbleached circulating white water, the same results as when added to the dust removal/selection process can be obtained. Comparative Example 2 shows that if the amount of chelating agent added is too small, the effect of improving whiteness cannot be obtained, and Comparative Example 3 shows that even if a chelating agent is added, sodium silicate is not added at once to the unbleached pulp at the dehydrator outlet. This shows that if it is added, the effect of improving whiteness cannot be expected. Comparative Example 5 shows that when the first addition of sodium silicate is made to unbleached circulating white water, the whiteness decreases rapidly. [Effects of the Invention] As detailed above, the pulp bleaching method of the present invention includes adding a chelating agent to the dust removal/selection step of the pulp manufacturing process or unbleached circulating white water during peroxide bleaching, and By adding soda separately to the unbleached pulp after dehydration and to the bleaching mixer, the chelating agent and sodium silicate each act effectively on metal ions, reducing ineffective decomposition caused by peroxide decomposition, and producing high whiteness pulp. is obtained.

[Brief explanation of drawings]

The drawing is a schematic diagram of a flow sheet showing a part of a general mechanical pulp manufacturing process to which the method of the present invention is applied.

Claims (1)

[Scope of Claims] 1. In the manufacturing process of bleached pulp, which is processed in the order of pulping process, dust removal and selection process, dehydration process, and bleaching process, an organic chelating agent is added in the dust removal and selection process, and sodium silicate is added at the end of the dehydration process. A pulp bleaching method characterized in that the pulp is added in two parts: after the bleaching process and after the bleaching process. 2. In the bleached pulp manufacturing process, which is performed in the order of pulping process, dust removal and selection process, dehydration process, and bleaching process,
The white water generated in the dehydration process of unbleached pulp is circulated to the dust removal and cleaning process, an organic chelating agent is added to the unbleached circulating white water, and the sodium silicate is divided into two parts: after the dehydration process and in the bleaching process. A pulp bleaching method characterized by adding the following: