JP2003027263A - How to remove silicate scale - Google Patents

Abstract

(57) [Abstract] [Problem] To reduce silicate scale in various industrial water systems,
A water-soluble carboxylic acid having a pKa value of less than 4 for a first proton is provided, which provides a cleaning method that efficiently removes chemically and does not generate highly irritating toxic gases such as hydrochloric acid and hydrofluoric acid. 1 selected from acids and sulfamic acids
A step of contacting with an aqueous solution containing a species or more and having a pH of 4 or less,
Thereafter, the method is characterized by comprising a step of contacting with an aqueous solution containing fluoride which dissolves in water to generate fluoride ions.

Description

Description: TECHNICAL FIELD The present invention relates to a silicate scale,
In particular, the present invention relates to a chemical cleaning method for removing silicate scale attached to a metal surface or the like that comes into contact with water. [0002] In various industrial water systems such as a cooling water system, a boiler water system, a paper pulp manufacturing process water, a steel mill process water, and a wet dust collector, dissolved components in the water are present on the surface of the device that comes into contact with water. Precipitation results in a so-called "scale", which hinders the operation of the apparatus. [0003] The most common scale generated on the surface of the device in contact with water is calcium carbonate scale caused by calcium ions and carbonate ions contained in the water.
Natural water contains silica that reacts with metal ions present in the water, such as calcium, magnesium, iron, aluminum, and zinc, under alkaline conditions to produce silicates that form silicate scales. . For example, in an open circulation cooling water system of a factory,
Although industrial water is received and used as cooling water for the process, silica and metal ions in the water are concentrated by evaporation of water in the cooling tower, and this forms silicate scale, especially magnesium silicate, A silicate scale consisting of zinc silicate is generated. Scale has adverse effects such as worsening the heat transfer effect and obstructing the flow of water,
In addition, if scale adheres to metal surfaces such as stainless steel, titanium, copper alloys, and nickel alloys, chloride ions will concentrate at the lower part of the scale, causing stress corrosion cracking. Therefore its countermeasures are important. [0005] In a boiler, ion-exchanged water or softened water is often used as water, but a small amount of silica or metal ions still generates silicate scale such as magnesium silicate or calcium silicate in an evaporating tube or the like. Not only does the scale of the boiler evaporator tube hinder heat transfer, but in severe cases, overheating can lead to a boiler breakage accident. In a paper mill, aluminum ions eluted from alkali disintegration of waste paper made using aluminum sulfate combine with silicate ions in process water to form aluminum silicate scale, which reduces productivity. This is a direct problem. [0007] Since calcium carbonate and calcium phosphate are dissolved in inorganic acids such as nitric acid, hydrochloric acid and sulfuric acid, the scale of calcium carbonate and calcium phosphate can be removed by chemical cleaning using a cleaning method using these inorganic acids. The scale mainly composed of silica can be removed to some extent by using a fluoride such as hydrofluoric acid. On the other hand, silicate is found in various places in industrial water systems, and it is common to form silicate scale on the surface of an apparatus which comes into contact with water. It is difficult to dissolve in acid and cannot be completely removed even by a method using hydrofluoric acid or fluoride. In addition, a method of adding an aminopolycarboxylic acid, β-glycerophosphate glycerin, or the like to an aqueous alkali metal solution and washing with a solution having a pH of 9 or more [Japanese Patent Publication No. 49-14468] has been proposed. There was no effective chemical cleaning method available, relying exclusively on mechanical removal. SUMMARY OF THE INVENTION An object of the present invention is to efficiently remove silicate scale in various industrial water systems and to remove highly irritating toxic gases such as hydrochloric acid and hydrofluoric acid. An object of the present invention is to provide a cleaning method that does not occur. [0010] In order to achieve the object, the present inventors diligently studied the dissolution of silicate scale and completed a method of removing silicate scale. A step of contacting with a water-soluble carboxylic acid having a pKa value of one proton of less than 4 and an aqueous solution having a pH of 4 or less containing at least one selected from sulfamic acid,
The method is characterized by including a step of contacting with an aqueous solution containing fluoride which dissolves in water to generate fluoride ions. DETAILED DESCRIPTION OF THE INVENTION The object of the present invention is a silicate scale formed from silica and a polyvalent metal, and specifically, magnesium silicate, calcium silicate, iron silicate, aluminum silicate, silicate This scale includes zinc, manganese silicate, strontium silicate, copper silicate, nickel silicate, and the like, and includes both crystalline and amorphous forms. The present invention relates to a method for removing silicate scale, wherein the surface to which the scale is attached is a water-soluble carboxylic acid having a pKa of the first proton of less than 4 (hereinafter referred to as "water-soluble carboxylic acid"), sulfamic acid. 1 to be selected from
An aqueous solution containing a species or more and having a pH of 4 or less (hereinafter, referred to as a “first cleaning liquid”), and then an aqueous solution containing a fluoride that dissolves in water to generate fluoride ions (hereinafter, referred to as a “second cleaning liquid”) "). The water-soluble carboxylic acids in the first washing solution are water-soluble carboxylic compounds having at least one carboxyl group in the molecule and having a pKa value of the first proton of less than 4. The pKa value is the logarithm of the reciprocal of the dissociation constant Ka of the first proton in the water-soluble carboxylic acids, and is expressed by the following equation. pKa = log [1 / Ka] Specific examples of the water-soluble carboxylic acids and the pKa value of the first proton thereof are formic acid (pKa = 3.55) and malonic acid (pKa = 2. 6
5), oxalic acid (pKa = 1.04), maleic acid (p
Ka = 1.75), fumaric acid (pKa = 2.85), malic acid (pKa = 3.24), citric acid (pKa = 2.
87), glycolic acid (pKa = 3.63), lactic acid (pKa
Ka = 3.66) and tartaric acid (pKa = 2.82). In addition, many water-soluble carboxylic acids have pK
The a value is known, and is shown in, for example, Chemical Handbook (Basic Edition II), 3rd revised edition [edited by The Chemical Society of Japan, p. 339, published in 1984 (Maruzen)]. The pKa value of the water-soluble carboxylic acids used in the present invention is less than 4, but if the pKa value is 4 or more, the scale removing effect is not sufficient. [0015] The first cleaning solution contains one or more compounds selected from water-soluble carboxylic acids and sulfamic acid.
This is an aqueous solution in which H is reduced to 4 or less. The optimum concentration of the water-soluble carboxylic acid or sulfamic acid in the aqueous solution varies depending on the type of the compound, but is usually 1 to 20% by weight. The pH is adjusted so as to be 4 or less, but if the pH is higher than 4, the effect of removing the silicate scale is not sufficient. The cleaning with the first cleaning liquid is achieved by bringing the first cleaning liquid into contact with the surface to which the scale is attached. The contact time varies depending on the properties of the scale, the degree of adhesion, or the type and concentration of the cleaning solution, but is usually 30 minutes to 48 hours. Washing temperature is usually 20-100 ° C
It is. When the temperature is lower than 20 ° C., the scale dissolution rate is low, and when the temperature is higher than 100 ° C., the boiling point of the cleaning solution is reached, and neither is practical. At this time, the cleaning liquid may be in a stationary state, but it is preferable to circulate or agitate the cleaning liquid. The pH may be adjusted by adding ammonia, amines, alkali hydroxide and the like to the first cleaning liquid, but the present invention is not limited to these pH adjusting methods. Next, it is brought into contact with a second cleaning liquid. The transition from cleaning with the first cleaning liquid to cleaning with the second cleaning liquid is as follows:
Usually, it is basically to discharge the first cleaning liquid and then add the second cleaning liquid. However, fluoride is added while leaving all or one part of the first cleaning liquid, and used as the second cleaning liquid.
It can also be washed. The fluoride in the second cleaning solution is dissolved in water to generate fluoride ions. For example, ammonium fluoride, sodium hydrogen fluoride, potassium hydrogen fluoride, amine hydrogen fluoride, amine fluoride amine Such as hydrogen fluoride compounds, ammonium fluoride, sodium fluoride, potassium fluoride, and amine fluoride.
However, hydrofluoric acid is not preferred in the present invention because highly toxic hydrogen fluoride is easily vaporized. The optimum concentration of the fluoride varies depending on the kind of the compound, but is usually 1 to 20% by weight in the washing solution. If the concentration of fluoride is less than 1% by weight, the scale dissolution rate is slow, and if the concentration of fluoride exceeds 20% by weight, the increase in the scale dissolution rate beyond the increase in fluoride concentration cannot be expected. Impractical. The pH of the second cleaning solution is usually 2 to 8. The cleaning with the second cleaning liquid is achieved by bringing the second cleaning liquid into contact with the surface to which the scale adheres. The contact time varies depending on the properties of the scale, the degree of adhesion, or the type and concentration of the cleaning solution, but is usually from 15 minutes to
12 hours. The washing temperature is usually from 10 to 50C. If the temperature is lower than 10 ° C., the scale dissolution rate is low, and if the temperature is higher than 50 ° C., no further increase in the scale dissolution rate can be expected. At this time, the cleaning liquid may be in a stationary state, but it is preferable to circulate or agitate the cleaning liquid. The method of performing the cleaning with the first cleaning liquid and the second cleaning liquid in reverse order, or mixing the first cleaning liquid and the second cleaning liquid from the beginning and cleaning them at once, does not provide the scale removing effect intended by the present invention. I can't expect enough. The silicate scale removal according to the method of the present invention comprises:
First, it is considered that the metal component is dissolved by the water-soluble carboxylic acids and sulfamic acid in the first cleaning solution by being brought into contact with the first cleaning solution, and the remaining silica component is dissolved by fluoride ions from the second cleaning solution. Can be On the other hand, when first contacting with an aqueous solution containing fluoride, fluoride ions selectively dissolve the silica component in the metal silicate salt, but the metal component remains as an oxide, so that the water-soluble carboxylic acid It is thought that it cannot be sufficiently dissolved with sulphamic acid or the like. Also, when water-soluble carboxylic acids or sulfamic acid are used together with fluoride, the reaction rate of fluoride to the scale is faster than that of water-soluble carboxylic acids or sulfamic acid. Since the silica component is selectively dissolved and the metal component remains as an oxide, it is considered that the scale cannot be sufficiently dissolved for the same reason as described above. Although the mechanism of these actions is not always clear, water-soluble carboxylic acids and sulfamic acid can easily act on metal ions because metal ions and silicate ions are connected by ionic bonds in metal silicates. This is presumed to be because water and water-soluble carboxylic acids and sulfamic acid cannot act on the metal atom because the metal atom and the oxygen atom are firmly linked by a covalent bond. The cleaning of the present invention is intended for silicate scales. However, scales such as calcium carbonate and calcium phosphate can be removed by cleaning with a first cleaning liquid, and silica scales can be removed with a second cleaning liquid. The method of the present invention can effectively act on scales in which calcium carbonate, calcium phosphate, silica, and the like are mixed in addition to silicate. In carrying out the method of the present invention, an acid corrosion inhibitor may be added to each cleaning solution, particularly the first cleaning solution, to suppress metal corrosion during cleaning. Such acid corrosion inhibitors include, for example, those commercially available as "IBIT" (trade name) from Asahi Chemical Co., Ltd. and "Kirehibit" (trade name) from Kirest Chemical Co., Ltd. As the acid corrosion inhibitor, those commercially available for organic acids, sulfamic acids and fluorides can be used, but acid corrosion inhibitors for hydrochloric acid and sulfuric acid may be diverted. When a copper alloy is present in the system to be cleaned, thioureas, ammonia,
Copper ion sequestering agents such as amines can be used in combination. The material of the apparatus to which the cleaning method of the present invention is applied is not particularly limited, and includes metals, resins, ceramics, and the like. Examples of the metal material include stainless steel, titanium, nickel alloy, carbon steel, and copper alloy. EXAMPLES The present invention will be specifically described below, but the present invention is not limited to these examples. (Example 1) A stainless steel tube (material: SUS304, outer diameter: 19 mm, wall thickness: 2 mm) with scale attached was extracted from a heat exchanger of an open circulation type cooling water system operating in a factory, and 5 cm in diameter. And cut into lengths to give test tubes for evaluating the cleaning effect. A scale having a thickness of 3 mm was adhered in the tube. The composition of the attached scale is SiO ₂ : 46.4%, MgO: 12.3%, Ca
O: 14.7%, Al ₂ O ₃ : 7.3% (indicating weight%), the adhesion scale was magnesium silicate,
It was a mixture of calcium silicate and aluminum silicate.
The test tube for evaluating the cleaning effect and the circulating pump are connected by a vinyl hose, and the cleaning solution separately placed in a plastic container is used for 0.1 ml.
Circulation was performed at a flow rate of 1 m / s. In an experiment in which washing was performed in two stages, the first washing solution and the subsequent washing solution were all replaced and replaced. After circulating and washing for a predetermined time, the thickness of the residual scale was measured with a microscope. The results are shown in Table 1. [Table 1] From these results, it was confirmed that the silicate scale can be removed by performing the cleaning solution of the present invention in the correct order. (Example 2) A stainless steel tube (material: SUS316, outer diameter: 19 mm, scale attached) from a heat exchanger of an open circulation type cooling water system different from Example 1
(Thickness: 2 mm) was extracted and cut into a length of 5 cm to obtain a test tube for evaluating the cleaning effect. A scale with a thickness of 2.5 mm was adhered in the tube. The composition of the adhered scale is SiO ₂ : 27.7%, MgO: 15.9.
%, CaO: 18.5%, CO ₂ : 14.4% (each represents% by weight), and the scale was a mixture of magnesium silicate and calcium carbonate. According to the same method as in Example 1, first, a 5% aqueous solution of malic acid (pH 2.0) was used as a first cleaning solution, and washed at 50 ° C. for 12 hours. Washed for 3 hours at ° C. The residual scale thickness after the completion of the washing was 0.1 mm or less. It has been found that the method of the present invention enables washing and removal of scales in which magnesium silicate and calcium carbonate are mixed. According to the present invention, it is possible to efficiently remove silicate scale without using dangerous chemicals such as hydrochloric acid and hydrofluoric acid which are highly irritating and may generate toxic gas. it can. This makes it possible to efficiently remove scale and contributes to stable operation and safe operation of related devices.

──────────────────────────────────────────────────続 き Continued on the front page (51) Int.Cl. ⁷ Identification symbol FI theme coat ゛ (Reference) C11D 17/08 B08B 9/02 Z F term (Reference) 3B116 AA13 BB03 BB82 BB87 3B201 AA13 BB03 BB82 BB87 BB92 BB96 4H003 BA12 DA09 DC02 EA05 EA06 EB08 FA01 FA21 4K053 PA01 PA02 PA03 PA06 PA07 PA09 PA18 QA01 RA14 RA21 RA22 RA23 RA25 RA45 RA46 RA47 RA52 RA63 SA06 SA08 TA16 TA17 XA24 YA03

Claims (1)

Claims: 1. A water-soluble carboxylic acid having a pKa value of a first proton of less than 4 and 1 selected from sulfamic acid.
A method for removing silicate scale, comprising a step of contacting with an aqueous solution containing at least seeds and having a pH of 4 or less, and thereafter, a step of contacting with an aqueous solution containing fluoride which dissolves in water to generate fluoride ions.