JP2002045887A - Scale preventing method for kraft pulp cleaning and bleaching process step - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a scale preventing method for a kraft pulp cleaning and bleaching process step in a paper pulp production factory which can efficiently prevent the adhesion of the scale produced in the cleaning and bleaching process step of a kraft pulp manufacturing process step of a paper pulp mill without using a conventional scale preventing agent. SOLUTION: This scale preventing method for the kraft pulp cleaning and breaching process step is characterized in that seed crystals of scale materials are added to the process water of the cleaning and breaching process step of the kraft pulp manufacturing process step.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a method for preventing scale in a kraft pulp washing and bleaching process. More specifically, the present invention relates to a kraft pulp washing method capable of effectively preventing the adhesion of scale generated in the washing and bleaching process of the kraft pulp manufacturing process of a paper pulp mill without using a conventional scale inhibitor. The present invention relates to a method for preventing scale in a bleaching process.

[0002]

2. Description of the Related Art Kraft pulp is obtained by mixing chips obtained by crushing wood or the like with caustic soda and sodium sulfate, heating and digesting the mixture. The cooked pulp is washed in the washing process and lignin solubilized in the cooking process,
Resin, organic acids and residual cooking liquor are removed. further,
In order to increase the whiteness of the pulp, multi-stage bleaching is performed in the bleaching step by combining a chlorine agent such as chlorine gas, an alkaline agent such as caustic soda, and a bleaching agent such as hypochlorous acid and chlorine dioxide. The washing and bleaching process of kraft pulp varies depending on the paper company, but basically, the oxygen delignification tower, chlorine gas stage, chlorine dioxide stage, caustic soda stage, oxygen gas stage, sodium hypochlorite stage, peroxide It is performed by combining processes such as hydrogen stage and water washing. Water used in various processes of pulp and paper mills is often recycled for saving water, and the calcium content is high as calcium ions or ash. The main sources of calcium are calcium contained in wood raw materials,
And calcium in the filler. Since the kraft pulp obtained by cooking wood chips is highly alkaline, when washed with water containing calcium ions in the washing step, calcium carbonate becomes supersaturated, and carbon dioxide is screened with screens, strainers, pipes, etc. Calcium scales. Further, in the bleaching step, since caustic soda is added in the caustic soda stage, a high alkali condition is established, and here also calcium carbonate becomes supersaturated, and scale is generated in the piping. If scale adheres to the device or the inner wall of the pipe, not only will the flow rate decrease, resulting in a decrease in productivity, but also a great deal of trouble, such as malfunction of measuring instruments such as flow meters or blockage of the pipe. There is. When calcium carbonate scale adheres to an apparatus or a pipe, it is usually removed by acid washing with hydrochloric acid or sulfamic acid, or by physically spraying with high-pressure water. This descaling operation not only requires a great deal of labor and time, but also has a problem in terms of operation safety, especially when an acid is used. In order to solve such a problem, a conventional scale inhibitor such as a polymer having a carboxyl group or a phosphonic acid-based drug may be used. However, since the target is a highly supersaturated solution of calcium carbonate, the scale preventing effect is not sufficient, and the oxidizing agent such as sodium hypochlorite and hydrogen peroxide coexists as a bleaching agent. May be decomposed and the original function may not be exhibited, and there is a strong demand for improvement measures.

[0003]

SUMMARY OF THE INVENTION It is an object of the present invention to provide an effective method for preventing the adhesion of scale generated in the washing and bleaching process of the kraft pulp manufacturing process of a paper pulp mill without using a conventional scale inhibitor. The purpose of the present invention is to provide a method for preventing scale in the kraft pulp washing and bleaching step that can be carried out.

[0004]

Means for Solving the Problems The inventors of the present invention have conducted intensive studies to solve the above-mentioned problems, and as a result, in the washing and bleaching process of the kraft pulp manufacturing process, the seed crystals of the scale substance were removed from the process water in the process. By adding to the process water, the scale substance contained in the supersaturated state in the process water precipitates on the surface of the seed crystal, the concentration in the water decreases, and it has been found that scale adhesion can be effectively prevented. Thus, the present invention has been completed. That is, the present invention provides (1) a method for preventing scale in the kraft pulp washing and bleaching step, wherein a seed crystal of a scale substance is added to the step water in the washing and bleaching step in the kraft pulp manufacturing step. Is what you do. Furthermore, as a preferred embodiment of the present invention, (2) the first substance, in which the scale substance is calcium carbonate,
Item 4. The method for preventing scale in the kraft pulp washing and bleaching step according to Item 1, (3) the method for preventing scale in the kraft pulp washing and bleaching step according to Item 1, wherein the average particle diameter of the seed crystal is 50 μm or less, and (4) Scale substance. The concentration of seed crystals of
2. The method for preventing scale in the kraft pulp washing and bleaching step according to item 1, wherein a seed crystal of the scale substance is added to the process water so as to give 1,000 mg / L.

[0005]

BEST MODE FOR CARRYING OUT THE INVENTION In the method for preventing scale in the kraft pulp washing and bleaching step of the present invention, a seed crystal of a scale substance is added to the step water of the step. The scale to which the method of the present invention is applied is not particularly limited, but the main component of the scale that adheres in the washing and bleaching step of the kraft pulp production step is often calcium carbonate, which is particularly effective for preventing the adhesion of calcium carbonate scale. Also, when calcium carbonate seeds are added to prevent calcium carbonate scale,
Since the calcium ion concentration in the system decreases, the generation of calcium oxalate scale and calcium sodium carbonate scale can also be prevented. The seed crystal of the scale substance used in the method of the present invention, when added to the process water,
It is a crystal that acts as a nucleus to start crystallization of the scale substance and increase the deposition rate of the scale substance. When the scale material is known, the same or similar crystal as the scale material can be used as a seed crystal. Examples of the crystals similar to the scale substance include crystals of a mixture containing 30% by weight or more of the scale substance or crystals of a compound having a similar structure or crystal form to the scale substance. The effectiveness of a crystal having a structure or crystal form similar to that of the scale substance as a seed crystal is as follows. Can be confirmed by adding crystals of the above and observing the state in which crystals of the scale substance precipitate. When the scale substance is unknown, the circulating water to be applied is concentrated, and the precipitated crystals are finely pulverized and can be used as the same seed crystal as the scale substance.

For example, when the scale substance to be prevented is calcium carbonate, it is preferable to use a calcium carbonate crystal as a seed crystal of the scale substance. There are no particular restrictions on the calcium carbonate crystals used, and any of the calcium carbonate polymorphs calcite, aragonite, and vaterite can be used. If the scale substance to be prevented is a magnesium silicate-based substance,
Magnesium orthosilicate (forsterite), magnesium metasilicate, magnesium tetrasilicate (talc), magnesium trisilicate,
Chrysotile, sepiolite, magnesium calcium orthosilicate, magnesium calcium metasilicate and the like can be used. Further, according to various scale substances to be prevented, for example, calcium phosphate, calcium sulfate, aluminum silicate, calcium silicate, or the like can be used as a seed crystal. If the scale substance to be prevented is silica, for example, a solid such as silica gel can be pulverized and used as a seed crystal. The seed crystal of the scale substance used in the method of the present invention has an average particle diameter of 50 μm.
m, preferably 20 μm or less, more preferably 5 μm or less. Since the activity increases as the surface area of the seed crystal increases, the smaller the average particle diameter of the seed crystal, the more effectively the scale can be prevented. In the method of the present invention, the seed crystal of the scale substance is preferably added immediately before a portion where scale is likely to be generated or where it is desired to prevent adhesion of scale. For example, it can be added to the cooked kraft pulp slurry, can be added in advance to the washing liquid, or can be added to the inlet of the caustic soda stage of the bleaching step.

In the method of the present invention, the method of adding the seed crystal of the scale substance is not particularly limited, and the addition method can be appropriately selected in consideration of workability. For example, the seed crystal can be added as it is in a powder state, or can be added as a slurry state in which the seed crystal is dispersed in a liquid. In the method of the present invention, the amount of the seed material of the slurry substance is not particularly limited, but the concentration in the process water is 1 to 1,000 m.
g / L, more preferably 2 to 100 mg / L. Concentration of seed crystal in process water is 1mg / L
If it is less than the above, the scale prevention effect may be insufficient. When the concentration of the seed crystal in the process water exceeds 1,000 mg / L, the amount of the seed crystal used increases, and the ash content in the pulp increases, which may affect the quality of the pulp. In the method of the present invention, if necessary, a scale inhibitor other than the seed crystal of the scale substance can be used in combination. Other scale inhibitors include, for example, polyacrylic acid, polymaleic acid, copolymers of acrylic acid and 2-acrylamido-2-methylpropanesulfonic acid, and acrylic acid and 2-hydroxy-3-allyloxypropanesulfonic acid. Organic polymers such as copolymers, and phosphorus-based compounds such as nitrilotrimethylenephosphonic acid, hydroxyethylidene diphosphonic acid, phosphonobutanetricarboxylic acid, and sodium hexametaphosphate can be given. According to the scale prevention method of the present invention, a seed crystal of the scale component is added to a highly supersaturated solution of the scale component, which is process water of the washing and bleaching process of the kraft pulp manufacturing process of the pulp and paper mill, thereby adding the seed crystal of the scale component. Crystals become nuclei, and scale components in water precipitate. In this case, since the rate at which the scale is deposited using the added seed crystal as a nucleus is higher than the rate at which the scale adheres to the apparatus wall, the degree of supersaturation of water is reduced, and scale adhesion can be effectively prevented.

[0008]

EXAMPLES The present invention will be described in more detail with reference to the following Examples, which should not be construed as limiting the present invention. In Examples and Comparative Examples,
In a bleaching process of a kraft pulp manufacturing process of a paper pulp mill, caustic soda and oxygen gas are injected into a pulp slurry that has passed through a C / D (chlorine gas / chlorine dioxide) stage.
A process up to a / O (caustic soda / oxygen gas) stage was assumed. Scale generation occurs under high temperature and high alkali conditions immediately after the addition of the caustic soda. Assuming that the washing outlet water at the C / D stage and the pulp slurry at the C / D stage outlet have the same water quality, the test was conducted under conditions where caustic soda was added to the C / D stage washing outlet water of the actual device until the pH reached 10. Was done. The liquid temperature was 60 ° C. The water quality of the C / D stage washing outlet water used for the test was conductivity 5,800 μS / cm, pH 2.6, calcium ion 325 mg / L, M alkalinity 2.0 mg / L or less, and sulfate ion 72.0 mg / L. , Silica 12.8mg / L,
233 mg / L of suspended substance, 16.6 mg of magnesium ion
/ L. Example 1 100 mL of C / D stage washing outlet water was put into a 300 mL beaker, immersed in a constant temperature water bath at 60 ° C., and stirred with a stirrer. When the liquid temperature reaches 60 ° C., the average particle size is 1.1 μm.
Calcite powder is added to a concentration of 2 mg / L,
Subsequently, a 1 mol / L aqueous sodium hydroxide solution was added dropwise to adjust the pH.
Was adjusted to 10. Ten minutes after the pH reached 10, the calcium ion concentration was 20 mg / L. Example 2 5 mg of calcite powder having an average particle size of 1.1 μm was added.
The same operation as in Example 1 was performed except that / L was used. The calcium ion concentration 10 minutes after the pH reaches 10 is
It was 2.2 mg / L. Example 3 The amount of calcite powder having an average particle diameter of 1.1 μm was adjusted to 20 m.
The same operation as in Example 1 was performed except that g / L was used. pH
10 minutes after the concentration reached 10, the calcium ion concentration was 0.4 mg / L. Example 4 Instead of calcite powder having an average particle diameter of 1.1 μm, a 40 w / v% dispersion slurry of aragonite powder having an average particle diameter of 2.4 μm was added so that the concentration of the aragonite powder became 50 mg / L. Performed the same operation as in Example 1. Ten minutes after the pH reached 10, the calcium ion concentration was 0.2 mg / L. Comparative Example 1 Instead of calcite powder having an average particle size of 1.1 μm, 1
The same operation as in Example 1 was performed, except that -hydroxyethylidene-1,1-diphosphonic acid was added so as to have a concentration of 20 mg / L. Ten minutes after the pH reached 10, the calcium ion concentration was 280 mg / L. Comparative Example 2 Instead of calcite powder having an average particle diameter of 1.1 μm, sodium polyacrylate having a molecular weight of 5,000 was used at a concentration of 20 mg.
/ L, the same operation as in Example 1 was performed. Ten minutes after the pH reached 10, the calcium ion concentration was 235 mg / L. Comparative Example 3 100 mL of C / D stage washing outlet water was put into a 300 mL beaker, immersed in a constant temperature water bath at 60 ° C., and stirred with a stirrer. When the liquid temperature reached 60 ° C., the pH was adjusted to 10 by dropwise addition of a 1 mol / L aqueous sodium hydroxide solution. pH 10
10 minutes after the calcium ion concentration reached, the calcium ion concentration was 15
It was 0 mg / L. Table 1 shows the results of Examples 1 to 4 and Comparative Examples 1 to 3.

[0009]

[Table 1]

As shown in Table 1, in Examples 1 to 4 in which calcite powder or aragonite powder was added as a seed crystal of calcium carbonate, the calcium ion concentration in water was reduced to 0.2 to 20 mg / L. It is presumed that calcium carbonate precipitated on the surface of the seed crystal particles. The calcium ion concentration in water of Comparative Example 3, which was adjusted to pH 10 without adding a drug, was 150 mg / L, and remained high in supersaturation. However, the calcium ion concentration of Examples 1 to 4 was reduced to 1/10 to 1/100 of that of Comparative Example 3, and it is considered that the adhesion of scale was sufficiently prevented. In Comparative Example 1 in which 1-hydroxyethylidene-1,1-diphosphone as a conventional scale inhibitor was added, and in Comparative Example 2 in which sodium polyacrylate was added, the calcium ion concentration in water was higher than that in Comparative Example 3. Is high, and calcium ions are kept in water in a solubilized state, but lower than the calcium ion concentration of the C / D stage washing outlet water, and the amount of calcium ions corresponding to the difference may be scaled. is there.

[0011]

According to the scale prevention method of the present invention, the scale substance precipitates on the surface of the seed crystal added to the process water, and the concentration of the scale component in the process water decreases. The water quality becomes rugged and scale adhesion can be effectively prevented.

Claims (1)

[Claims] 1. A method for preventing scale in the kraft pulp washing and bleaching step, wherein a seed crystal of a scale substance is added to water in the washing and bleaching step in the kraft pulp manufacturing step.