JPS6252634B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention is directed to a method of depositing liquid discharged from a digester (hereinafter referred to as black liquor) on the inner wall of the black liquor side of an evaporative concentrator (hereinafter referred to as a can) in the chemical ground method or semi-chemical pulp manufacturing process. The present invention relates to a novel method for removing scale deposited from the inner wall of a can by chemically dissolving the scale deposited on the inner wall of the can. Pulp production using the Chemiground method or semi-chemical method has been carried out for quite some time, and the black liquor produced in the manufacturing process is once evaporated and concentrated and then collected or disposed of. Insoluble substances gradually accumulate on the inner walls of the can and eventually become attached as scale, reducing the heat transfer efficiency. Therefore, the scale adhering to the evaporative concentration can is occasionally removed. This adhered scale is composed of calcium, oxalic acid, and other complex inorganic and organic components, and cannot be removed with washing water or commonly used detergents.Also, it cannot be removed with strong acids such as hydrochloric acid, nitric acid, sulfuric acid, etc. Even with strong alkaline chemicals such as caustic soda and caustic potash, these methods cannot be applied because they cannot be efficiently removed and the base material of the can is easily damaged by corrosion. A method of peeling off scale by applying external mechanical force to it is used. A relatively efficient way to remove the scale by applying external mechanical force is to apply a force of 200 to 350
There is a method of removing scale by spraying high-pressure water from a nozzle at a high speed of around 10 tons/hour and targeting the scale, but this method requires high-pressure generators, pressure-resistant equipment, human power, and a large amount of energy. In addition, it is difficult to easily remove the scale that adheres to the curved parts and narrow corners of the can, and furthermore, it is difficult to remove the scale that adheres to the can's curved parts and narrow corners. In some cases, it is unavoidable to unnecessarily aim the high-speed water jets mentioned above, which causes a non-negligible amount of wear on the can base material, and the work itself involves aiming the jet water inside the can. It has many disadvantages, such as requiring troublesome work such as disassembling the device, removing and attaching the lid of the can. Apart from the above-mentioned method of removing scale using external mechanical force, in some cases, a hot water solution containing 20 to 25% caustic soda and 10 to 15% soda carbonate is poured into a can with scale. There is also a two-step dissolution removal method in which the alkaline aqueous solution in the can is replaced with an aqueous solution of sulfamic acid and brought into contact with the scale, but in this method, the alkali aqueous solution is brought into contact with the scale. If contact is not carried out for a long period of time in the first stage of contact, there will be almost no effect of contact dissolution and removal in the second stage, making it an extremely inefficient method and requiring multi-stage processing operations to effectively remove scale. Not the method. The present inventors intended to establish a simple and efficient method for removing scale adhering to the inner wall of a black liquor evaporation concentration container, and as a result of detailed research on the effects of various chemical solutions on the scale, an aqueous solution of aluminum salt was found. discovered the surprising new fact that the above-mentioned scale can be dissolved extremely quickly, and completed the present invention. It is an object of the present invention to provide a method for dissolving and removing scale that adheres to the inner wall of a black liquor evaporator concentrator in a simple, efficient and work-safe manner. The object of the present invention is to provide a method for removing the scale that does not cause corrosion, damage, wear, etc. The scale removal method of the present invention includes formic acid, acetic acid, glycolic acid, oxalic acid, lactic acid, malonic acid, malic acid, tartaric acid,
It is characterized by bringing into contact an aqueous solution in which aluminum ions coexist with an anion of one or more organic acids selected from the group consisting of gluconic acid and citric acid. Usually, the material of the inner wall of the black liquor evaporation concentration container is SS-
41, STB-35, SUS-304, etc., mild steel, stainless steel, etc., and the scale that adheres to the inner wall is mainly composed of calcium and oxalic acid, which together account for about 60 to 90%. It contains small amounts of water, inorganic and organic components as other components, and forms a hard tissue structure that adheres to the inner wall of the can. When the thickness of this scale becomes approximately several millimeters, the transmission efficiency decreases significantly, and removal of the scale is required. As mentioned above, this scale is
Even if it comes into contact with strong acid solutions such as sulfuric acid, nitric acid, phosphoric acid, or sulfamic acid, or with strong alkaline solutions such as caustic soda or caustic potash, it is difficult to dissolve and remove it economically and safely in an efficient manner. It was hot. In particular, the above acids tend to corrode the inner wall of the can, and phosphoric acid and sulfuric acid are undesirable because they form precipitates. The aqueous solution used in the present invention contains formic acid,
It is characterized by the coexistence of anions of acetic acid, glycolic acid, oxalic acid, lactic acid, malonic acid, malic acid, tartaric acid, gluconic acid, and citric acid and aluminum ions, and as long as the object of the present invention is achieved. It may contain any components. The aqueous solution used in the present invention is easily prepared by supplying in water a substance that generates anions of the organic acid and a substance that generates aluminum ions. Generally speaking, it can be obtained by dissolving in water a soluble compound containing the anion of the organic acid and a soluble compound containing aluminum ion, and specific examples include aluminum formate, aluminum acetate, A method of dissolving aluminum glycolate, aluminum oxalate, aluminum lactate, aluminum malonate, aluminum malate, aluminum tartrate, aluminum gluconate, aluminum citrate or a mixture thereof; formic acid, acetic acid, glycolic acid, oxalic acid, lactic acid in water. , malonic acid, malic acid, tartaric acid, gluconic acid, citric acid, or a mixture thereof and aluminum hydroxide. However, even if the purpose of the present invention is not newly prepared, as long as the purpose of the present invention is achieved, an aqueous solution which is a by-product of a chemical factory and contains the anion of the above-mentioned organic acid, aluminum ion, etc. can be used in an appropriate combination. . The aqueous solution used in the present invention preferably does not contain ions such as sulfate groups and phosphate groups that generate poorly soluble salts of calcium, and generally the solution after the scale has been dissolved is discarded or recycled. It is desirable that the liquid does not contain precipitable substances or substances that cause pollution. The aqueous solution used in the present invention is as described above,
It is characterized by the coexistence of aluminum ions and the anions of organic acids such as formic acid, acetic acid, glycolic acid, oxalic acid, lactic acid, malonic acid, malic acid, tartaric acid, gluconic acid, and citric acid, which dissolves scale. In this case, since the anion of the organic acid mentioned above is involved in the scale dissolution action at a ratio of 1 equivalent of anion of the organic acid to 1 equivalent of aluminum ion, the anion of the organic acid is contained in a ratio of approximately 1 equivalent to 1 equivalent of aluminum ion. Preferably. However, as long as the aqueous solution contains aluminum ions and anions of the organic acid in amounts necessary to dissolve and remove scale, the equivalent ratio of aluminum ions and anions of the organic acid coexisting in the aqueous solution must be limited to 1. There is no problem even if one of the ions is in excess. In particular, when the scale contains acid-soluble carbonates and sulfites such as calcium carbonate and calcium sulfite in addition to calcium oxalate, the amount of acid necessary to dissolve these salts must be added to the above aqueous solution in advance. It is preferable to include it in Since the aqueous solution containing scale components after dissolving the scale is subjected to appropriate treatment, it is preferable to use an aqueous solution with a composition that is designed in advance to minimize energy consumption. The anion of the organic acid and the aluminum ion in the aqueous solution used in the present invention participate in the dissolution of scale in a consumptive manner at an equivalent ratio of 1, so when there is a large amount of attached scale, contact with it The aqueous solution must contain sufficient amounts of the above-mentioned anions and aluminum ions to dissolve all of the attached scale. Usually, the chemical composition of the scale to be removed is not constant, but it is preferable that the above aqueous solution is brought into contact with the scale at a ratio of 50 to 1000 parts by weight per 1 part by weight of the scale. The preferred amount of aqueous solution to be contacted with the deposited scale depends on the aluminum ion content and the concentration of the anions mentioned above. If the concentration of the aqueous solution is too high, dissolved components are likely to precipitate, and if the concentration is too low, the time for contacting the aqueous solution with the scale becomes undesirable.
A generally preferred concentration range is 0.1 to 40% by weight, particularly 1.0 to 15% by weight, as the total amount of aluminum ions and anions of the organic acid. The preferred concentration and amount of the aqueous solution used for removing scale adhesion also depends on the conditions under which it is brought into contact with the scale. The method of the present invention is characterized in that the aqueous solution is brought into contact with the scale to be removed, whereby the scale is dissolved into the liquid. In order to quickly dissolve scale, it is best to supply an aqueous solution into a can and bring the stored liquid into contact with it under stirring or circulating flow. The temperature of the aqueous solution during contact is usually 20~
The temperature is preferably about 90°C, particularly about 50 to 70°C. Below 20°C, the dissolution rate of scale is low, and above 90°C, it tends to be difficult to work, which is not preferable. The aqueous solution used in the present invention may optionally contain a surfactant, an acid, a corrosion inhibitor, etc. in addition to the specific components described above, as long as the purpose of the present invention is achieved. Furthermore, the method of the present invention can also be carried out in combination with a conventional method of peeling off scale using external mechanical force, a method of contacting scale with an alkaline aqueous solution and an acidic aqueous solution in two stages, and the like. The method of the present invention is used to dissolve and remove scale that adheres to the inner wall of a black liquor evaporation concentration can during the chemical ground method or semi-chemical pulp manufacturing process, and the main components of the scale are calcium and oxalic acid. It is believed that these components are extremely well dissolved by the aqueous solution used in the present invention, so that the scale-forming tissue structures are easily dissolved. It is useful not only for scales that adhere, but also for removing scales that have adhered to equipment such as cans in manufacturing processes in other fields, as long as the chemical composition of the main components of the scale and the scale deposition process are similar. It is. The present invention will be described in more detail below with reference to Examples and Reference Examples, but the technical scope of the present invention is not limited thereto. Example 1 A brown scale with a thickness of about 1 mm attached to the inner wall of a black liquor evaporation concentration can in a semi-chemical pulp manufacturing factory was peeled off and collected, cut into a size of approximately 5 mm x 10 mm, and 2.5 g of it was collected as shown in Table 1. 10% of the solute components shown in
It was poured into 100g of aqueous solution. Then, after stirring at 60°C for 4 hours, the dissolution rate was determined by measuring the amount of insoluble matter, and the results shown in Table 1 were obtained.
Regarding insoluble matter, when the aqueous solution was renewed and the dissolution test was performed again under the same conditions, No. 1, 6, 7, 9, 10
The scale of No. 2, 3, 4, 5, and 8 is dissolved.
Almost dissolved in the second time.

[Table] Next, test pieces of mild steel and stainless steel were subjected to a corrosion test using a 10% aqueous solution of the solute components shown in Table 2. Test piece A: SS-41 (JIS G3101, #320 polished finish) Dimensions are 1 x 12 x 75 mm Degrease and clean with acetone Test piece B: SUS-304 (JIS G4305, #320 polished finish) Dimensions are 1 ×12×75mm Items that can be degreased and cleaned with acetone.The test method is as follows. Experiment No. 1 listed in Table 2 in a 100ml glass container
Pour 90 g of an aqueous solution of ~10, then add 0.5
% of a commercially available corrosion inhibitor is added and dissolved therein, and the above test piece A or B is preliminarily washed with acetone.
The entire surface of each sample was immersed separately and left at 60°C for 6 hours. Thereafter, the test piece was taken out and washed with running water. Test piece B was washed with acetone and dried, and test piece A was washed with running water and then immersed in a 10% diammonium citrate aqueous solution at 70°C for 1 minute. Then, the corrosion rate was calculated by washing again with running water, washing with acetone, drying, and weighing.

[Table] As shown in the above table, it was confirmed that all of the aqueous solutions used in the present invention had extremely low corrosiveness to steel materials, and could be sufficiently put to practical use.

Claims (1)

[Claims] 1. Formic acid, acetic acid, glycolic acid, oxalic acid, lactic acid, malonic acid, malic acid, tartaric acid, glucone, The method for removing scale, which comprises bringing into contact an aqueous solution in which aluminum ions coexist with one or more organic acid anions selected from the group consisting of acids and citric acid.