JP2546224B2 - Method for removing organochlorine compounds in effluent - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for removing organic chlorine compounds mixed in the wastewater from a multi-stage bleaching process of hardwood kraft pulp.

[0002]

2. Description of the Related Art The current methods for producing chemical pulp for papermaking include:
There are a Kraft method using caustic soda and sodium sulfide as cooking chemicals, and a sulfite method mainly using acidic sulfite.

Of these, kraft pulp is more difficult to bleach and has a lower yield than sulfite pulp, but on the other hand, it has the advantage of being strong. Further, the craft method has a wider range of tree species of raw material wood that can be used as compared with the sulfite method. Furthermore, the process of drainage and recovery of cooking chemicals is incorporated into the manufacturing system, and by burning the organic matter in the drainage, mainly lignin, the energy of the entire system can be fully self-supplied. That is, it has an economic advantage not found in the sulfite method.

Under such circumstances, the production of chemical pulp for papermaking is currently dominated by the Kraft method. The residual lignin content of the unbleached kraft pulp and the color of the pulp differ depending on the tree, but usually 2 to 5% of lignin remains and the color is brown. In order to remove a slight amount of residual lignin from unbleached kraft pulp, a multi-stage bleaching process using chlorine (C), alkali extraction (E), hypochlorite (H), chlorine dioxide (D), etc. Process and
It is usual to use white pulp having a whiteness of 85 to 90%.

Therefore, the effluent discharged from this bleaching process is
It contains an organic chlorine compound produced by the reaction of lignin or a resin component with chlorine. Since the organic chlorine compound mainly has a carbon-chlorine covalent bond, it is biologically inactive, and the amount released to the natural world should be minimized from the viewpoints of its toxicity and accumulation. Therefore, efforts to reduce the emission of organochlorine compounds have been continued for a long time, usually after reacting the unbleached pulp with oxygen to further reduce the lignin content by 40-50% and then CE-
A method of performing multi-stage bleaching of HD has been adopted. But,
At present, chlorine is still used as a bleaching agent in terms of cost and quality.

For this reason, a method of treating the effluent containing an organic chlorine compound becomes a problem, but generally, a method of treating by a coagulation sedimentation method to reduce the amount of the organic chlorine compound as much as possible is adopted. The flocculation-precipitation method is a method in which a sulfuric acid band and, if necessary, a polymer flocculant are added to the effluent to separate and remove the generated precipitate, which is used to remove suspended solids and high-molecular weight lignin. It is said to be effective. In fact, when looking at the bleaching effluent of softwood kraft pulp, there are more lignin parts with higher molecular weight than the bleaching effluent of hardwood kraft pulp, so even if the treatment with a conventional sulfuric acid band-polymer flocculant is carried out About 70% removed. However, when the hardwood kraft pulp bleaching effluent is treated under the same conditions, the organic chlorine compound removal rate is less than 30%. Thus, the bleaching effluent of bleached hardwood kraft pulp has a low organic chlorine compound removal efficiency by the usual coagulation-sedimentation treatment.

[0007]

[Problems to be Solved by the Invention] Looking at the production volume of kraft pulp by type in Japan, hardwood kraft pulp is 8
It accounts for 0% or more, and it is certain that its production will increase as the demand for paper increases. Nevertheless, in view of the fact that the removal rate of organic chlorine compounds is low in the conventional coagulation-sedimentation effluent treatment of bleaching bleached from hardwood kraft pulp, the cause was investigated and the following findings were obtained. .

First, the low molecular weight organochlorine compounds mainly originate from the chlorine stage bleaching step (C). Second, the low molecular weight organochlorine compounds from this chlorine stage bleaching process are labile to alkalis. Third, the flocculation-precipitation method has low removal efficiency for low molecular weight compounds.

In light of the above findings, it is clear that the conventional method of mixing the chlorine stage bleaching effluent with another effluent and then coagulating and precipitating at pH 5-8 is not suitable.

Accordingly, it is an object of the present invention to provide a novel method for efficiently removing organic chlorine compounds from the bleaching effluent of hardwood kraft pulp.

[0011]

The present invention firstly separates the effluent of the chlorine stage bleaching step from the effluent of each step in the multi-stage bleaching process of hardwood kraft pulp, and then separates the effluent with an alkali metal or By adding hydroxide or oxide of alkaline earth metal and exposing it to high alkaline conditions of pH 10 or higher, the removal of carbon-bonded chlorine unstable to alkali containing carbon-bonded chlorine derived from low molecular weight compounds, And further, in order to remove alkali stable polymer compounds,
It is characterized in that the drainage is purified by performing coagulation and precipitation by adding a conventional aluminum compound.

The effluent to be treated in the present invention is a bleaching effluent of hardwood kraft pulp. In other words, chlorine (C),
Alkali extraction (E), hypochlorite (H), chlorine dioxide (D), hydrogen peroxide (P) were added to C-E-H-D and C-E-
HP-D, C-E-D-E-H, or C-E-H-
D-E-D is an effluent discharged from a multi-stage bleaching process that is appropriately combined and includes a chlorine stage bleaching process (C). When processing, separate the wastewater from the chlorine stage bleaching process from other wastewater so that it can be handled separately.

In the present invention, the hydroxide or oxide of an alkali metal or alkaline earth metal added to bring the chlorine stage bleaching effluent into a highly alkaline condition is sodium hydroxide, sodium oxide, potassium hydroxide. , Potassium oxide, magnesium oxide, calcium hydroxide, calcium oxide, barium hydroxide can be exemplified,
Calcium hydroxide is particularly preferable. Alkaline condition is p
Although it is set to H10 or more, the higher the pH, the more effective the removal of the organic chlorine compound. If the pH is less than 10, the effect of removing low molecular weight organochlorine compounds that are unstable to alkali is insufficient.

As the aluminum compound added in the next step, aluminum sulfate (sulfuric acid band) and aluminum chloride can be mentioned, but the sulfuric acid band is usually used. By adding this aluminum compound, an alkali-soluble or high-molecular weight organochlorine compound can be coagulated and precipitated. In this case, a polymer flocculant may be used together.

Since the organic chlorine compounds in the effluent treated by the present invention are decomposed or aggregated and precipitated, the impact on the natural world can be minimized by discharging only the supernatant of the bleaching effluent. The chlorine stage bleaching effluent after efficient removal of organic chlorine compounds may be mixed with other E, H, D and P stage effluents and treated again, or may be discharged alone. good.

When the processing order is reversed from that of the present invention,
That is, to the hardwood kraft pulp chlorine stage bleaching effluent, first add the sulfuric acid band, then add potassium hydroxide,
Even when the pH is raised to 10 or higher, a temporary effect on the removal of AOX can be expected. However, since the sulfuric acid band has a buffering function in the vicinity of pH 4 to 5, a large amount of alkali is consumed in order to raise the pH to 5 or higher after the addition of the sulfuric acid band, and when the treated liquid is discharged as waste water. However, since the pH is too high, it is necessary to add an acid to lower the pH, and as a result, the amounts of both the alkali and the acid used are extremely increased, which is not practical on an industrial scale. In addition, when treated in this order, a large amount of electrolyte is generated in the treated wastewater,
This electrolyte may adversely affect the natural ecosystem.

[0017]

In the present invention, among the organochlorine compounds in the bleaching effluent of hardwood kraft pulp, the low molecular weight organochlorine compounds, which were difficult to remove by the conventional method, are mainly present in the chlorine stage bleaching effluent, and It is based on the fact that it is unstable. That is, in the treatment method of the present invention, in the first step, only the chlorine stage bleaching effluent is separated and brought to a highly alkaline condition of pH 10 or more. By doing so, it is possible to cleave the alkali-unstable carbon-chlorine bond contained in the low-molecular-weight organic chlorine compound, which cannot be removed by the usual coagulation-precipitation method. In this case, the elimination of chlorine occurs mainly in the chlorine atom bonded to the carbon atom adjacent to the carbonyl group, and becomes chlorine ion after the elimination.

Further, in the present invention, in the second step, by adding an aluminum compound to the drainage liquid, an alkali-stable organochlorine compound having a particularly high molecular weight can be coagulated and removed to be removed. According to the present invention, the removal of the organochlorine compound is carried out by successively performing these two treatments, that is, the carbon-chlorine bond of the low-molecular-weight organochlorine compound is cleaved, and the high-molecular-weight organochlorine compound is coagulated and precipitated. The removal rate of organochlorine compounds can be significantly improved as compared with the coagulation-precipitation method of. CO, which is indispensable for drainage treatment
The removal of D can be satisfied at the same time.

[0019]

EXAMPLES The present invention will be specifically described with reference to the following examples. The amount of organic chlorine is shown as the amount of AOX.

[Example 1] C-of hardwood kraft pulp
Of the effluents from the E-HD multi-stage bleaching process, the chlorine stage (C) effluent was separated so as not to mix with other effluents. This chlorine-stage drainage has a pH of 2.0 and an AOX amount of 46.
It was 5 mg / l. Next, various bases shown in Table 1 were gently stirred while adding to the chlorine stage drainage, the pH was raised to 10 or more, and the amount of residual AOX in the supernatant was measured. Furthermore, after adding a sulfuric acid band to promote coagulation and precipitation, the amount of residual AOX in the supernatant was measured again. The results are summarized in Table 1. It can be seen that the AOX removal rate is significantly improved by adding the aluminum compound as the secondary treatment in addition to the alkali treatment. The alkaline treatment is most effective when using calcium hydroxide.

[0021]

[Table 1]

[Comparative Example 1] A sulfuric acid band was added to the chlorine bleaching effluent of the hardwood kraft pulp treated in Example 1 at various concentrations, and calcium hydroxide was used to add p.
It was adjusted to H6.5. Since coagulation and precipitation occurred in the drainage,
The amount of residual AOX in the supernatant was measured. It should be noted that pH 6.5 is a pH value suitable for discharging as treated wastewater. As a result, as shown in Table 2, AOX cannot be effectively removed only by adding the aluminum compound.

[0023]

[Table 2]

[0024]

As described above, the method for treating a bleaching effluent of hardwood kraft pulp of the present invention has the following effects.

(1) Chlorine-stage bleaching effluent in the bleaching effluent is determined to contain a low-molecular-weight organic chlorine compound which is unstable to alkali, and the chlorine-stage bleaching effluent is intensively treated with alkali. The amount of AOX can be efficiently reduced. (2) By performing the coagulation-precipitation treatment with an aluminum compound subsequent to the alkali treatment, it is possible to efficiently remove not only a high molecular weight organic chlorine compound but also COD. (3) Calcium hydroxide and caustic soda used in the paper manufacturing process can be used, and in a factory having a coagulating sedimentation facility, the existing facility can be used. (4) According to the present invention, it is possible to stably and efficiently obtain high-quality treated water at low cost.

Claims (2)

(57) [Claims] 1. The effluent of the chlorine stage bleaching process is separated from the effluent of each process in the multi-stage bleaching process of hardwood kraft pulp, and the alkali metal or alkaline earth metal hydroxide or oxide is added to this effluent. After adjusting the pH to 10 or more, an aluminum compound is further added to cause coagulation / precipitation, and the waste liquid is purified to remove the organic chlorine compound in the waste liquid. 2. The method for removing an organochlorine compound in a drainage according to claim 1, wherein calcium hydroxide is used when the pH is adjusted to 10 or more.