JP2005314824A - Method for cleaning lime mud filter - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method for cleaning a lime mud filter by which the clogging of the lime mud filter is cleaned in a short time, while not reducing the productivity and operability so much, and while reducing the loss of energy at a firing kiln, and the cost required for dehydration of the lime mud. <P>SOLUTION: The filter cloth of the lime mud filter clogged at a caustification process in the production of a kraft pulp is cleaned by an aqueous alkali solution of EDTA-4Na. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to a method for cleaning a lime mud filter for removing scale adhered to the lime mud filter by cleaning in a causticizing step of kraft pulp production.

  In the production of kraft pulp, first, in a cooking process, wood chips are mixed with a strong alkaline white liquor composed of sodium sulfide and sodium hydroxide and cooked in a digester at high temperature and high pressure. By this cooking step, lignin and the like contained in the wood chips are oxidatively decomposed and eluted into the white liquor, and a kraft pulp slurry is obtained.

  The kraft pulp slurry thus obtained is then washed in a washing step and separated into recovered black liquor and pulp. Further, the recovered black liquor is concentrated by an evaporator to obtain a concentrated black liquor having a solid content of about 70% by mass. This concentrated black liquor contains not only sodium sulfide and sodium hydroxide contained in the white liquor but also a large amount of organic substances such as lignin derived from wood. For this reason, the organic matter in the concentrated black liquor is burned in the boiler in the next combustion step, and a smelt composed of a mixture of sodium sulfide and sodium carbonate is obtained.

  The smelt thus obtained is dissolved in water to become a green liquor in the next causticizing step, and further, quick lime is added to the green liquor to proceed with the decontamination reaction, and the supernatant is left as a white liquor. Regenerated and a precipitate is obtained as lime mud. This lime mud is baked in a lime kiln and reused as quick lime.

When calcining lime mud with a lime kiln and returning it to quick lime, the less water is contained in the lime mud, the less energy is required for the calcining. For this reason, lime mud is dehydrated with a lime mud filter until the water content becomes about 15 to 30% by mass (see, for example, Patent Document 1).
Japanese translation of PCT publication No. 2003-505612 (paragraph number 0004)

  The lime mud filter usually includes a cylindrical drum having a large number of holes and a filter cloth that wraps around the drum surface, and the lime mud is filtered by decompressing the inside of the drum with a decompression pump. It is filtered with a cloth and dehydrated. Examples of such a lime mud filter include an Oliver filter, a Young filter, a precoat filter, and the like.

  When dewatering lime mud with a lime mud filter, in order to eliminate clogging due to scale adherence to the filter cloth of the lime mud filter, jet cleaning is usually performed once a day for about 10 to 20 minutes. Yes. However, this method cannot sufficiently eliminate clogging due to the scale of the filter cloth, and it is inevitable that the dewatering rate of the filter cloth gradually decreases.

  For this reason, a method of washing the filter cloth with mineral acid or the like and chemically dissolving the scale is also performed. However, in this method, the lime mud decomposes with mineral acid during the cleaning of the filter cloth and a large amount of carbon dioxide gas is generated, which obstructs the removal work, or the lime mud is decomposed by the mineral acid remaining after washing, Carbon dioxide gas was generated from the lime mud filter and its peripheral devices, causing problems such as hindering the process.

  After all, at present, the most frequently used method is to replace the filter cloth of the lime mud filter with a new filter cloth once a month. However, in this method, while replacing the clogged filter cloth with a new filter cloth, the supply of lime mud to the calcining kiln is stopped, resulting in a decrease in productivity and workability. It was. In addition, while the supply of lime mud to the firing kiln is stopped, the firing kiln is idled at a high temperature of 600 to 800 ° C., the operation of the firing kiln is stopped, and the operation of the lime mud filter is started. In response to this, the operation of the firing kiln must be resumed, resulting in a large energy loss. For this reason, the cost required for dehydration of lime mud has been increased.

  The present invention has been made in view of the above-described conventional situation, and can eliminate clogging of the lime mud filter in a short period of time, without significantly reducing productivity and workability, and the energy of the firing kiln. It is an object to be solved to provide a filter cleaning method with low loss and low cost for dewatering lime mud.

  The inventor has conducted intensive research to solve the above problems, and found that the above problems can be solved by washing the filter cloth clogged by the adhesion of scales with an alkaline aqueous solution of aminocarboxylic acid, thereby completing the present invention. It came.

  That is, the filter cleaning method of the present invention is the lime mud filter cleaning method for removing the adhesion scale of the lime mud filter in the causticizing process of kraft pulp production, wherein the lime mud filter is washed with an alkaline aqueous solution of aminocarboxylic acid. It is characterized by that.

  As a result of analyzing the filter cloth of the clogged lime mud filter, the inventor has found that a considerable amount of barium sulfate is contained in addition to calcium carbonate which is the main component of lime mud. This barium sulfate is considered to have adhered to the filter cloth of the lime mud filter for the following reasons.

  That is, the bark wood contains a slight amount of inorganic barium salt, and the barium salt moves into the strong alkaline black liquor during the wood chip cooking process. Then, sodium sulfate is added to the recovered black liquor and burned in the black liquor combustion boiler, thereby accumulating as barium sulfate in the smelt. Furthermore, it is considered that the smelt is contained in the lime as the smelt dissolves and causticizes, and this barium sulfate together with the calcium carbonate adheres to or clogs the filter cloth of the lime mud filter.

  In the filter cleaning method of the present invention, the clogged lime mud filter is cleaned with an alkaline aqueous solution of aminocarboxylic acid. Since the alkaline aqueous solution of aminocarboxylic acid has an action of chelating calcium ions and barium ions, calcium carbonate and barium sulfate adhering to the lime mud filter that cause clogging can be dissolved in a short time. For this reason, clogging of the filter cloth of the lime mud filter can be eliminated in a short time, the empty operation time and stop time of the kiln are shortened, energy loss is reduced, and the cost required for dehydration of lime mud. Will be cheaper.

  Although the temperature of the alkaline aqueous solution of aminocarboxylic acid should just be determined suitably according to the condition of the clogging of a lime mud filter, it is normally set to 20-95 degreeC. If the temperature of the alkaline aqueous solution of aminocarboxylic acid is 95 ° C. or higher, the alkaline environment is handled at a high temperature, and the working environment may be deteriorated. On the other hand, when the temperature of the alkaline aqueous solution of aminocarboxylic acid is 20 ° C. or lower, the reaction rate of chelation of barium ions and calcium ions becomes slow, and it is necessary to lengthen the washing time. Particularly preferred is a range of 40 ° C. or more and less than 80 ° C.

  In the present invention, aminocarboxylic acid refers to a compound having both an amino group and a carboxylic acid group in one molecule. In addition, an alkaline aqueous solution of aminocarboxylic acid is an aqueous solution obtained by dissolving aminocarboxylic acid in an aqueous solution of an alkali metal hydroxide (eg, sodium hydroxide, potassium hydroxide). Japanese-type and partially-neutralized sodium salts, potassium salts, ammonium salts, and composite salts thereof are also dissolved in water or an aqueous alkali metal hydroxide solution.

  Among aminocarboxylic acids, ethylenediaminetetraacetic acid (EDTA), diethylenetriaminepentaacetic acid (DTPA), nitrilotriacetic acid (NTA), hydroxyethyliminodiacetic acid (HIDA), hydroxyethylethylenediaminetetraacetic acid (HEDA), triethylenetetraminehexaacetic acid (TTHA), aspartic acid diacetic acid (ASDA), glutamic acid diacetic acid (GLDA), and methylglycine diacetic acid (MGDA) are preferable because they are relatively inexpensive and easily available. Among these, ethylenediaminetetraacetic acid is particularly preferable because it is particularly inexpensive and easily available.

  Although the aminocarboxylic acid concentration in the alkaline aqueous solution of aminocarboxylic acid can be used without any particular limitation, it is usually used as 1 to 50% by mass. According to the test results of the inventors, for example, in the case of an alkaline aqueous solution using ethylenediaminetetraacetic acid, if it is less than 1% by mass, it may take a long time to eliminate clogging of the lime mud filter. On the other hand, if it exceeds 50% by mass, the viscosity from the aqueous solution becomes high and it becomes difficult to handle, or crystals may precipitate at low temperatures, which may be undesirable.

  Examples 1 to 10 embodying the present invention will be described below in comparison with Comparative Examples 1 to 8.

(Examples 1-9)
In Examples 1 to 9, alkaline aqueous solutions of various aminocarboxylates shown in Table 1 below with respect to the filter cloth adhesion of the lime mud filter used in the causticizing process in a kraft pulp manufacturing factory that is actually in operation The dissolution test was conducted. Sodium hydroxide was used as the alkali (hereinafter, “alkali” refers to sodium hydroxide). That is, first, the deposits on the used filter cloth of the lime mud filter are scraped with a spatula and ground with a mortar to obtain a uniform powder. Then, 0.1 g of the powder was put into a 200 mL Erlenmeyer flask, and 100 g of an alkaline aqueous solution of aminocarboxylate was added and gently shaken, and then allowed to stand in a thermostatic bath heated to 70 ° C. for 8 minutes. Then, no. It filtered with 6 filter papers, and the barium density | concentration in a filtrate was measured by the atomic absorption method. The composition of the deposits was barium sulfate: 15.3% by mass, calcium carbonate: 84.0% by mass, and others: 0.7% by mass.

(Comparative Examples 1-6)
In Comparative Examples 1 to 6, dissolution tests using various aqueous solutions or water shown in Table 1 were performed on the same deposits on the used filter cloth of the lime mud filter in Examples 1 to 9.

<Result>
In Examples 1 to 9, it was confirmed that the adhered matter on the used filter cloth was dissolved and disappeared visually in each case. Further, as shown in Table 1, in Examples 1 to 9 to which an aqueous alkaline solution of aminocarboxylic acid was added, 40% of barium contained in the filter cloth deposit was obtained despite the short elution time of 8 minutes. % Or more could be dissolved. On the other hand, in Comparative Examples 1 to 6, dissolved barium was 1.5% or less and could not be almost eluted.

  From the above results, if the filter cloth of the lime mud filter is washed with an alkaline aqueous solution of aminocarboxylic acid, it can dissolve the calcium carbonate and barium sulfate that cause the filter cloth to be clogged, thereby eliminating the clogging. I understood that I could do it.

(Example 10)
In Example 10, a 10 wt% alkaline aqueous solution (pH: 11) of EDTA · 4Na was washed with respect to the filter cloth adhering to the lime mud filter used in the causticizing process in a kraft pulp manufacturing factory actually in operation. Used as.

  That is, as shown in FIG. 1, this lime mud filter has a slurry bat 1 having a semicircular cross section for storing lime mud slurry sent from the causticizing reaction tanks 100 a, 100 b, 100 c, and a slurry bat 1. And a drum 2 provided to immerse the lower part.

  The inside of the drum 2 is separated by a large number of partition plates (not shown), and the pressure regulating valve is operated with rotation in each separated portion. A large number of holes are formed in the peripheral surface, and are wrapped by a plastic filter cloth 3. The inside of the drum 2 can be decompressed by a decompression pump (not shown). Above the drum 2, a stainless scraper 4 is installed with the cutting edge close to the drum 2. The rear end of the scraper 4 is fixed by a blade pressing plate 4a so that the angle formed by the blade surface of the scraper 4 and the surface of the drum 2 is always a constant angle and a constant distance is maintained.

  A spray nozzle 5 is installed facing the drum 2 at a position slightly advanced from the position of the scraper 4 in the direction opposite to the rotation direction of the drum 2. The spray nozzle 5 can be sprayed with 5% by mass alkaline water of fresh water or EDTA · 4Na by a compressor (not shown). The spray nozzle 5 is appropriately selected from various nozzles such as a uniform fan type nozzle, a wide angle fan type nozzle, a single fan type nozzle, an empty cone nozzle, a full cone type nozzle, a full cone type nozzle, and a straight nozzle.

  The dehydrated lime mud scraped off by the scraper 4 is conveyed to the calcining kiln 7 by the belt conveyor 6 and calcined at 600 to 800 ° C. to be quick lime.

  Using the lime mud filter configured as described above, the lime mud is recovered for 10 days, the clogging of the filter cloth proceeds, and the lime mud filter immediately before the replacement time is described below. The filter cloth was washed.

(Recovery of lime mud)
First, lime mud is deposited on the outer surface side of the filter cloth 3 by rotating the drum 2 with a motor (not shown) while reducing the pressure inside the drum 2 with a vacuum pump. Fresh water is sprayed from the spray nozzle 5 to the lime mud thus deposited on the outer surface of the filter cloth 3 to form a cake. Further, the scraper 4 scrapes off a portion having a certain thickness or more. The lime mud thus scraped off is transported to the firing kiln 7 by the belt conveyor 6. On the other hand, the lime mud that has not been shaved by the scraper 4 forms a precoat layer on the filter cloth 3 and is dipped in the slurry vat 1 together with the drum 2 to serve as a filter medium.

(Washing the filter cloth)
After the lime mud is collected for 10 days, the blade pressing plate 4a is adjusted to bring the scraper 4 into contact with the surface of the filter cloth 3, and all the lime mud deposited on the filter cloth 3 is peeled off. Then, a 5 mass% alkaline aqueous solution of EDTA · 4Na is sprayed from the spray nozzle 5 at a rate of 100 L / min for 8 minutes. Thereafter, the blade pressing plate 4a is adjusted to return the setting position of the scraper 4 to the position for collecting lime mud again. Thus, the collection of lime mud and the washing of the filter cloth were repeated alternately.

(Comparative Examples 7 and 8)
In Comparative Example 7, the conventional city water was sprayed from the spray nozzle 5 in washing the filter cloth. Further, in Comparative Example 8, washing was performed using an alkaline aqueous solution of 1-hydroxyethylidene-1,1-diphosphonic acid (HEDP) · 4Na “Diquest 2016” (trade name, manufactured by Nihon Monsanto Co., Ltd.).

<Result>
In Example 10 in which an alkaline aqueous solution of EDTA · 4Na was sprayed from the spray nozzle 5, the drainage decreased to the extent that the initial replacement was necessary, and the filter cloth was drained to the extent that there was no difference in normal operation after 3 days. Improved. And as shown in Table 2, the continuous operation of the lime mud filter for 90 days or more was further completed. On the other hand, in Comparative Example 8 in which the alkaline aqueous solution of HEDP was sprayed from the spray nozzle 5, the operating days of the lime mud filter could be extended only for 2 days. Further, in Comparative Example 7 in which mere water was sprayed from the spray nozzle 5, the operating days of the lime mud filter could not be extended at all.

  By carrying out the filter cleaning method of the present invention, clogging of the filter cloth of the lime mud filter for dewatering lime mud in kraft pulp production can be eliminated and the filter can be used again.

6 is a schematic diagram of a lime mud filter used in Example 10. FIG.

Explanation of symbols

  3. Filter cloth

Claims (4)

In the lime mud filter cleaning method for removing the adhesion scale of the lime mud filter in the causticizing process of kraft pulp production,
The lime mud filter washing | cleaning method characterized by wash | cleaning the said lime mud filter with the alkaline aqueous solution of aminocarboxylic acid.   The temperature of the alkaline aqueous solution of aminocarboxylic acid is 40 degreeC or more and less than 80 degreeC, The lime mud filter washing | cleaning method of Claim 1 characterized by the above-mentioned.   Aminocarboxylic acids are ethylenediaminetetraacetic acid, diethylenetriaminepentaacetic acid, nitrilotriacetic acid, hydroxyethyliminodiacetic acid, hydroxyethylethylenediaminetetraacetic acid, triethylenetetraminehexaacetic acid, aspartic acid diacetic acid, glutamic acid diacetic acid and methylglycine diacetic acid. The lime mud filter cleaning method according to claim 1 or 2, wherein the method is at least one.   The lime mud filter cleaning method according to any one of claims 1 to 3, wherein the aminocarboxylic acid is ethylenediaminetetraacetic acid.

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