JPH02251692A - Bleaching method for wood pulp - Google Patents

Abstract

PURPOSE:To obtain pulp of high whiteness with reduced invalid decomposition in peroxide treatment by adding an organic chelating agent and sodium silicate at different stages, respectively and adding the sodium silicate by portions, when pulp is bleached with a peroxide. CONSTITUTION:In the production process for bleached pulp where pulp is bleached with a peroxide, an organic chelating agent and sodium silicate are added at stages different from each other, respectively and the sodium silicate is added by portions. It is preferred that the organic chelating agent is added at the dusting and cleaning process, while the sodium silicate is added after dehydration at the dehydration stage and to the bleaching mixer. Further, the organic chelating agent is preferably added to the unbleached circulation water and the sodium silicate is preferably added, after dehydration at the dehydration stage, and to the bleaching mixer, separately.

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Application Field] The present invention relates to a method for bleaching pulp, and more particularly to a method for bleaching pulp using peroxide.

[Prior Art] In recent years, wood pulps such as ground ice pulp (GP), refined ground wood pulp (RGP), and thermomechanical pulp (T
Mechanical pulp (MP) such as MP), so-called high-yield pulp such as chemical ground pulp (CGP), semi-chemical pulp (SCP), chemical pulp (CP) such as soda pulp (AP), kraft pulp (KP), etc. Bleaching methods using peroxides such as hydrogen peroxide are increasingly being used for most types of pulp, such as deinked paper pulp (DIP). This is due to the recent demand for higher whiteness of paper and environmental problems such as increased wastewater load.
This appears to be due to the reduction bleaching method using sulfide and the switch from chlorine bleach such as chlorine dioxide to peroxide bleach.

Here, we will explain the circumstances and problems of peroxide bleaching using mechanical pulp as an example.

In the past, most mechanical pulp was used for papermaking with medium and low brightness, but now, with the use of mechanical valves as an alternative to kraft valves,
Demand for mechanical pulp is increasing, and pulp with high whiteness is required.

Therefore, searching for an efficient bleaching method for mechanical pulp is one of the important issues in the paper and valve industry.

Bleaching methods that greatly improve the whiteness of mechanical pulp include:
The first method is peroxide bleaching, typified by hydrogen peroxide.

In addition to greatly improving whiteness, hydrogen peroxide bleaching has the characteristics of good bleaching yield and good opacity because it releases less verigenin compared to other bleaching agents.

However, in the presence of metal ions, hydrogen peroxide
Metal ions act as catalysts and have the property of reacting and decomposing as shown in the formula below.

H2O2+M"→H20+17202 M1 is metal ion Metal ions in the mechanical valve manufacturing process flow in from the outside mainly from wood chips, and are contained in large quantities in the circulating white water within the process. Therefore, during bleaching, some of the Hydrogen oxide is decomposed and loses its bleaching ability.

Conventionally, organic chelating agents (hereinafter abbreviated as chelating agents) and sodium silicate (Na2S, 03) have been used to control the decrease in bleaching ability due to hydrogen peroxide decomposition. 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, and like chelating agents, it also inactivates metal ions. 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, when adding to a conventional bleach mixer, the chelating agent and hydrogen peroxide are added almost simultaneously, 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.

(2) Since the chelating agent is added to a high-concentration bulb dispersion liquid with a bulb concentration of 10 to 30% by weight, contact with metal ions is incomplete.

■ Sodium silicate, which has the ability to inactivate metal ions, is not utilized.

There were problems such as.

These problems in peroxide bleaching of mechanical pulp are also common problems in peroxide bleaching of other valves.
This necessitated a solution.

[Problems to be Solved by the Invention] The present invention solves the above-mentioned problems, and solves the above-mentioned problems by making it possible for the chelating agent and sodium silicate, which are added to inactivate metal ions, to be effective in peroxide bleaching of bulbs. The present invention aims to provide a bulb bleaching method that reduces ineffective decomposition due to peroxide decomposition and provides a bulb with high whiteness.

[Means for Solving Problem 1] As a result of various studies regarding the above-mentioned problems, the present inventors have changed the location of adding the chelating agent during peroxide bleaching of bulbs from medium concentration bulbs to low concentration bulbs. It was discovered that by modifying the change and 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 bulb, and completed the present invention. I ended up doing it.

That is, the present invention is characterized in that the organic chelating agent and sodium silicate are added in separate steps, and the sodium silicate is added in portions in the manufacturing process of bleached bulbs in which peroxide is added to bleach the bulb. This is a method of bleaching bulbs.

2. The method for bleaching a valve according to claim 1, wherein the organic chelating agent is added in the dust removal/selection step, and the sodium silicate is added in portions to the bleaching mixer and after the dehydration step.

2. The valve bleaching method according to claim 1, further comprising adding an organic chelating agent to the unbleached circulating white water, and adding sodium silicate in portions to the bleaching mixer and after the dehydration step.

The drawing shows an outline of the flow sheet for the deburring/selection process, dehydration process, and bleaching process in the general mechanical valve manufacturing process.
But it's almost the same.

In the case of mechanical pulp, the chips are disintegrated in a RIF 7 iner to form a refiner ground pulp (RGP), which is a low-concentration pulp dispersion that is processed through the dust removal and selection process using a screen and cleaner, and then dehydrated in the dehydration process. It is dehydrated to a high concentration in a machine, and in the next bleaching mixer, a chelating agent and sodium silicate are traditionally added along with bleach.
After being bleached in the bleaching tower, the bleached pulp (bleached pulp) was sent to the preparation process.

In the present invention, the chelating agent and the silicate powder 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 adding sodium silicate after dehydration in the dehydration step, it is preferable to add it to the outlet of the dehydrator, but it may be added as far as possible before entering the white mixer.

The pulp bleaching method of the present invention is applied to high-yield pulp such as Mochimon, CGP, SOP, KP, AP, etc.
It can be applied to peroxide bleaching of chemical pulp such as 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 can be added to absolutely dry pulp;
It is added in an amount of about 0.5 to 5% (H20□ 100% conversion).

The chelating agent is not particularly limited, and known organic chelating agents such as diethylenetriamine pentaacetate (DTPA), ethylenediamine tetraacetate (EDTA), and human oxyethylenediamine tetraacetate (HEDTA) can be used.

The addition rate of the chelating agent is O, OS weight% based on the bone dry pulp.
(As a 40% stock solution, the same applies hereinafter) or more may be used, but preferably 0.05 to 1.00% by weight, more preferably 0.05% by weight.
It is 20 to 0.50% by weight. This is because if the amount of O,OS is less than 1.00% by weight, there is no effect of sequestering metal ions, and if it exceeds 1.00% by weight, no significant proportional effect can be obtained with respect to an increase in the amount added.

Sodium silicate is a commercially available product, 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 parts sodium silicate A ratio of ~1 is sufficient.

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 173-2/3 after dehydration, 2/3-1 in bleach mixer
/3.

[Function] By adding the chelating agent to the dust removal/selection process where the pulp concentration is relatively low before adding bleach or to the unbleached circulating white water containing a large amount of metal ions, (1) Good contact between the metal ions and the chelating agent is achieved.

■ Since the reaction between metal ions and the chelating agent takes precedence over the reaction between metal ions and peroxide, the decomposition of peroxide is suppressed.

Chelating agents are effectively used for these reasons.

In addition, by adding sodium silicate in portions and adding a portion to 1 t1 of peroxide bleach (after dehydration of unbleached pulp), the ability of sodium silicate to inactivate metal ions is utilized, and the ability of sodium silicate to inactivate metal ions is utilized. The sodium silicate added to the bleaching 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 will cause: ■ Difficulty maintaining pH during bleaching (because sodium silicate is removed from the pulp in the 12-water process) ■ Decrease in unbleached whiteness due to alkali burning ■ It is not preferable because it causes scaling problems in white water systems.

Therefore, in the present invention, it is preferable that sodium silicate be added in parts after dehydration of the unbleached bulb and in the bleaching mixer.

[Example] Examples of the present invention will be shown below. In addition, all % in an example shows weight %.

Example 1 An unbleached mechanical valve RGP manufactured from needle R14 (25% domestic material, 75% foreign material) was used, and the valve concentration was 1.5% (
Bulb suspension of bone-dry pulp (hereinafter omitted) (
(equivalent to pulp in the dust removal and cleaning process) was produced.

0.20% DTPA (Versenex 80, manufactured by Dow Chemical Company) was added to this valve suspension, and the mixture was stirred at a temperature of 80° C. for 3 minutes. This was suction filtered using a Buchner funnel (equipped with 40 mesh wire), and the valve concentration was 1.
A 7% valve suspension (hereinafter referred to as high consistency pulp, corresponding to the pulp at the outlet of an unbleached pulp dehydrator) was prepared.

1.5% of sodium silicate was added to this high concentration valve for the first time (corresponding to the outlet of the unbleached pulp dehydrator) and mixed. Then 4.3% caustic soda, 1.5% sodium silicate for the second time (corresponding to a bleach mixer) and 5.5% hydrogen peroxide.
After mixing, bleaching was carried out at a temperature of 80'C for 120 minutes.

After bleaching, the bulb 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 using a Tappt standard method test hand paper machine in a conventional manner to a paper size of 1100 tsubo and 1/7 rt.

The table shows the pH before and after bleaching, the percentage of residual hydrogen peroxide after bleaching (as T1□02), and the degree of whiteness.

Test method pHJIS Z8802 Standard UTill constant residue H2
Whiteness measured by O2 Kingsett method (iodic acid reduction titration method) 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%,
The same procedure as in Example 1 was carried out 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%,
The same procedure as in Example 1 was carried out except that 0.15% 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% DTPA was added. The results were tabulated.

Example 6 The same procedure as Example 1 was carried out except that 1.10% 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/selection process, DT and PA were added to unbleached circulating white water prepared as follows, and the mixture was heated to a temperature of 8. (+
After stirring at "c" for 10 seconds, return to pulp with a concentration of 8%,
The same procedure as in Example 1 was carried out except that it was added to the pulp before dehydration. The results are shown in the table. Preparation of unbleached circulating white water The valve suspension with a concentration of 1.5% prepared in Example 1 was poured into a Buchner funnel (40 mesh wires were fitted).
A suction line is passed through the bulb until the concentration reaches 8%, and the filtrate at this time is used as unbleached white water.

Conventional Example 1 The procedure was exactly the same 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). I went. The results are shown in the table.

Conventional Example 2 Instead of adding 20% of DTPAo to the pulp suspension with a concentration of 1.5% corresponding to the pulp in the dust removal and sorting process in Example 1, it was added to the high concentration pulp after dehydration of the unbleached pulp. However, the same procedure as in Example 1 was carried out except that at the same time, 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 The same procedure as in Example 1 was conducted except that 3.0% of the total amount of sodium silicate was added in the first step. The results are shown in the table.

Comparative Example 4 Comparative Example 4 was carried out in exactly the same manner as in Example 1 except that the first addition of Na2S 103 in Example 1 was not performed and 3.0% of Na5103 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 having a concentration of 17% corresponding to the pulp after dehydration of the unbleached pulp, the unbleached white water obtained in Example 6 was used. After stirring for 10 seconds at a temperature of 80°C, the pulp suspension was returned to a pulp suspension with a concentration of 8%, and the same procedure as in Example 1 was performed except that the suspension was added to the pulp to which DTPA had been added before dehydration. .

The results are shown in Table 6.

From the results shown in the table, 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 in portions to the outlet of the unbleached valve dehydrator and the bleach mixer, were different from those in which only sodium silicate was added. (Conventional Example 1), when the chelating agent and sodium silicate were added at once with a bleach mixer (Conventional Example 2), and when the sodium silicate was added in portions without adding the chelating agent (Comparative Example 1), Compared to either method (Comparative Example 4), where a chelating agent is added in the dust removal/selection process and sodium silicate is added at once in the subsequent bleaching mixer,
The whiteness of the bulb obtained by bleaching is improved, and residual H2O2
The 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 the chelating agent added is too small, the effect of improving whiteness cannot be obtained, and Comparative Example 3 shows that even if the chelating agent is added, the sodium silicate is not added at once at the dehydrator outlet of the unbleached valve. 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 bulb bleaching method of the present invention includes adding a chelating agent to the dust removal/selection step of the bulb manufacturing process or unbleached circulating white water during peroxide bleaching, and By adding soda separately to the unbleached bulb 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 bulbs. is obtained.

[Brief explanation of drawings]

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

Claims (3)

[Claims] (1) In the bleached pulp production process in which peroxide is added to the pulp and bleached, the organic chelating agent and the sodium silicate are added in separate steps, and the sodium silicate is added in portions. How to bleach pulp. (2) The pulp bleaching method according to claim 1, characterized in that the organic chelating agent is added in the dust removal/selection step, and the sodium silicate is added in portions to the dehydration step and to the bleaching mixer. (3) The method for bleaching pulp according to claim 1, characterized in that the organic chelating agent is added to the unbleached circulating white water, and the sodium silicate is added in portions to the bleaching mixer and after the dehydration in the dehydration step.