Filtration membrane regeneration method and silica microparticle dispersion liquid

Abstract

This filtration membrane regeneration method comprises a chemical liquid cleaning step in which a filtration membrane is cleaned using a chemical liquid containing 0.1 wt% to 14 wt% of fluoride ions.

Description

Method for regenerating filtration membrane and silica microparticle dispersion

[0001] The present invention relates to a method for regenerating a filtration membrane and a silica fine particle dispersion concentrated using the regenerated filtration membrane.

[0002] Membrane separation methods using filtration membranes are being used in a wide range of fields due to their energy-saving properties. For example, they are used in water purification processes to produce industrial water or tap water from river water or groundwater, and in seawater desalination processes. In recent years, they have also been used in the separation of nanoparticles. 。 Ultrafiltration membranes are used for concentration and other purposes.

[0003] However, when various raw waters are filtered through a membrane, the turbidity, organic matter, inorganic matter, and other substances contained in the raw water that are to be removed accumulate on the membrane surface, causing clogging. This is particularly noticeable when separating and concentrating nanoparticles. This increases the membrane's filtration resistance, eventually making it impossible to continue filtration. Therefore, to maintain membrane filtration performance, membrane cleaning is necessary. Membrane cleaning methods include backflow cleaning, in which membrane-filtered water or pure water is reversed from the secondary side (filtrate side) of the membrane to the primary side (raw water side); air cleaning, in which gas is supplied to the primary side of the membrane to remove membrane fouling; and chemical cleaning, in which a chemical solution is left in contact with the membrane for a certain period of time to clean it. Effective cleaning of these methods is extremely important for stable membrane filtration operation.

[0004] Membrane clogging depends on the quality of the raw water, but the most common problem is clogging caused by organic matter. Backwashing with water containing an oxidizing agent such as sodium hypochlorite is effective in eliminating clogging caused by organic matter, and water containing an oxidizing agent is generally used as the cleaning water for backwashing. On the other hand, clogging caused by inorganic matter is removed by dissolving it in acid.

[0005] Japanese Patent Application Laid-Open No. 2001-070763 describes a method for effectively removing both organic and inorganic substances by backwashing with an acid in addition to backwashing with an oxidizing agent. However, this method does not use a component that is effective against silica, which is insoluble in common acids, and has problems such as being insufficiently effective when treating water containing silica fine particles or against clogging caused by soluble silica contained in raw water.

[0006] The present invention can effectively dissolve and remove clogging in the membrane, extend the life of the filtration membrane, and reduce processing costs.

[0007] [Concept 1] The method for regenerating a filtration membrane according to the present invention may include a chemical solution cleaning step of cleaning the filtration membrane with a chemical solution containing 0.1 wt % to 14 wt % of fluoride ions.

[0008] [Concept 2] In the method for regenerating a filtration membrane according to Concept 1, the chemical solution may be a hydrogen fluoride aqueous solution, a potassium fluoride aqueous solution, or an ammonium fluoride aqueous solution containing 10 wt % or less of fluoride ions.

[0009] [Concept 3] In the method for regenerating a filtration membrane according to Concept 1 or 2, the chemical solution may be a hydrogen fluoride aqueous solution containing 1 wt % or more of fluoride ions.

[0010] [Concept 4] The method for regenerating a filtration membrane according to any one of Concepts 1 to 3 may further include a rinse solution cleaning step performed after the chemical solution cleaning step, wherein a fluoride ion concentration contained in the rinse solution at the end of the rinse solution cleaning step may be 10 ppmw or less.

[0011] [Concept 5] In the method for regenerating a filtration membrane according to Concept 4, a fluoride ion concentration contained in the rinse liquid at the end of the rinse liquid cleaning step may be 2 ppmw or less.

[0012] [Concept 6] In the method for regenerating a filtration membrane according to any one of Concepts 1 to 5, the filtration membrane may be a hollow fiber membrane, and the chemical solution containing fluoride ions may be circulated so as to flow inside and outside the hollow fiber membrane.

[0013] [Concept 7] In the method for regenerating a filtration membrane according to Concept 6, the hollow fiber membrane may be made of polyacrylonitrile or polysulfone.

[0014] [Concept 8] In the method for regenerating a filtration membrane according to Concept 6 or 7, the molecular weight cutoff of the hollow fiber membrane may be 10,000 to 50,000.

[0015] [Concept 9] The silica fine particle dispersion according to the present invention may be obtained by ultrafiltration concentration using a hollow fiber membrane regenerated by the method for regenerating a filtration membrane according to any one of claims 6 to 8.

[0016] According to the present invention, even when filtering water containing silica microparticle dispersions or soluble silica, clogging of the membrane can be dissolved and removed by washing with a chemical solution containing fluoride ions, thereby providing a method for regenerating a filtration membrane that can extend the life of the filtration membrane and reduce processing costs, and a concentrated silica microparticle dispersion obtained using the regenerated filtration membrane.

[0017] 1 is a diagram showing a cross-flow filtration system that can be employed in an embodiment of the present invention; 2 is a diagram showing a dead-end filtration system that can be employed in an embodiment of the present invention;

[0018] The method for regenerating a filtration membrane according to this embodiment includes a chemical cleaning process in which the filtration membrane is cleaned using a chemical solution containing fluoride ions. This chemical solution may contain 0.1 wt% to 14 wt% hydrogen fluoride, or may be a 0.1 wt% to 14 wt% hydrogen fluoride aqueous solution, a potassium fluoride aqueous solution, or an ammonium fluoride aqueous solution. If the fluoride ion concentration is too high, the membrane will deteriorate and the process will be uneconomical. On the other hand, if the fluoride ion concentration is too low, sufficient effects will not be obtained in terms of regenerating the filtration membrane. From this perspective, it is more preferable to set the upper limit of fluoride ions to 10 wt% and the lower limit to 1 wt%.

[0019] As a guideline for regeneration, it is preferable that the regeneration rate, expressed by the following formula, is 1.6 or more: Regeneration rate = filtration rate after regeneration (L min -1 ・m 2) / filtration rate before regeneration (L・min -1 ・m 2 ) However, the filtration rate of the target filtration membrane is assumed to have decreased to less than half of that of the initial (new) state.

[0020] In order to further increase the regeneration efficiency while preventing membrane degradation, it is preferable that the lower limit of the fluoride ion concentration contained in the chemical solution is 1 wt% and the upper limit is 10 wt%. Furthermore, from the viewpoint of increasing the regeneration efficiency, it is more preferable that the lower limit of the fluoride ion concentration is 4 wt%. From the viewpoint of prioritizing prevention of membrane degradation, it is more preferable that the upper limit of the fluoride ion concentration is 8 wt%, even more preferably 7 wt%, and even more preferably 6 wt%. It has been confirmed that the regeneration efficiency is higher when a hydrogen fluoride aqueous solution is used than when a potassium fluoride aqueous solution is used.

[0021] After the chemical cleaning step, a rinse cleaning step may be performed in which the filtration membrane is cleaned with a rinse liquid such as pure water. At the end of this rinse cleaning step, the fluoride ion concentration contained in the rinse liquid may be 10 ppmw or less. The fluoride ion concentration contained in the rinse liquid is preferably 5 ppmw or less, and more preferably 2 ppmw or less.

[0022] The regeneration method may be a method of washing the hollow fiber membrane. In this case, a chemical solution containing fluoride ions, such as a hydrogen fluoride aqueous solution or a potassium fluoride aqueous solution, may be circulated so as to flow inside and outside the hollow fiber. In this case, a cross-flow filtration method as shown in Figure 1 or a dead-end filtration method as shown in Figure 2 may be used.

[0023] The cross-flow filtration method shown in FIG. 1 and the dead-end filtration method shown in FIG. 2 use a tank 10 that stores a chemical solution containing fluoride ions, such as a hydrogen fluoride aqueous solution or a potassium fluoride aqueous solution, a circulation pump 20 that provides a driving force for circulating the chemical solution containing fluoride ions in the tank 10, and filtration membranes 30, 80 made of hollow fiber membranes or the like.

[0024] In the crossflow filtration system shown in Figure 1, a membrane primary side inlet 40 and a membrane primary side outlet 50, and a membrane secondary side inlet 60 and a membrane secondary side outlet 70 are provided. In the dead-end filtration system shown in Figure 2, a membrane primary side 90 and a membrane secondary side 100 are provided. In Figures 1 and 2, the solid arrows indicate cleaning from the primary side (raw water side) to the secondary side (filtrate side). On the other hand, the dotted arrows indicate backflow cleaning, in which water flows back from the secondary side (filtrate side) to the primary side (raw water side).

[0025] The hollow fiber membrane may be made of polyacrylonitrile (PAN) or polysulfone (PS).

[0026] The molecular weight cut-off of the hollow fiber membrane may be 10,000 to 50,000.

[0027] In this embodiment, there is also provided a silica fine particle dispersion obtained by ultrafiltration concentration using a regenerated hollow fiber membrane.

[0028] The membrane was regenerated using an ultrafiltration (hollow fiber) membrane having the membrane material and molecular weight cutoff shown in Table 1. The filtration rate was measured using pure water at a temperature of 25°C and a pressure (transmembrane pressure difference) of 0.15 MPa.

[0029] Pure water, which was a rinse liquid provided to the membrane, was passed through the hollow fiber membrane in the order of outside → inside → outside → inside, 20 L each, totaling 80 L, by cross-flow method.

[0030] At the end of the rinse cleaning step, the concentration of fluoride ions in the rinse solution was determined by sampling the solution immediately before the end of the rinse and quantifying the concentration by ion chromatography.

[0031] The initial filtration rate was measured when the membrane was new, and then the membrane was used to concentrate a silica dispersion with an average particle size of 26 nm as measured by DLS (dynamic light scattering), after which the filtration rate before regeneration was measured. The membrane was regenerated under the conditions shown in Table 1 below, and the filtration rate after regeneration, the fluoride ion concentration in the liquid at the end of the rinse solution washing step, and the visual condition of the regenerated membrane were confirmed. The results are shown in Table 1. Good visual conditions are indicated by "◯", poor visual conditions by "Δ", and very poor visual conditions by "×".

[0032]

[0033] In each example, it was confirmed that the filtration rate recovered and the membrane was regenerated for both polyacrylonitrile (PAN) and polysulfone (PS) materials. This was also confirmed for molecular weight cutoffs of 10,000 to 50,000.

[0034] On the other hand, as shown in Comparative Examples 1 and 2, it was confirmed that if the fluoride concentration in the chemical solution used for regeneration was too low, sufficient effect could not be obtained. On the other hand, as shown in Comparative Examples 3 and 4, if the fluoride concentration was too high, deterioration of the membrane was observed. Furthermore, if the concentration was high, not only did the amount of fluoride required increase, but the amount of pure water used for rinsing also increased, which was not economical.

Claims

1. A method for regenerating a filtration membrane, comprising a chemical washing step of washing the filtration membrane with a chemical solution containing 0.1 wt % to 14 wt % fluoride ions.

2. The method for regenerating a filtration membrane as described in claim 1, wherein the chemical solution is a hydrogen fluoride aqueous solution, a potassium fluoride aqueous solution or an ammonium fluoride aqueous solution containing 10 wt % or less of fluoride ions.

3. The method for regenerating a filtration membrane according to claim 1 or 2, wherein the chemical solution is an aqueous hydrogen fluoride solution containing 1 wt % or more of fluoride ions.

4. A method for regenerating a filtration membrane as described in claim 1 or 2, further comprising a rinse liquid cleaning process carried out after the chemical cleaning process, wherein the fluoride ion concentration contained in the rinse liquid at the end of the rinse liquid cleaning process is 10 ppmw or less.

5. A method for regenerating a filtration membrane as described in claim 4, wherein the fluoride ion concentration contained in the rinsing liquid at the end of the rinsing liquid cleaning process is 2 ppmw or less.

6. A method for regenerating a filtration membrane as described in claim 1 or 2, wherein the filtration membrane is a hollow fiber membrane, and a chemical solution containing fluoride ions is circulated so as to flow inside and outside the hollow fiber membrane.

7. The method for regenerating a filtration membrane according to claim 6, wherein the material of the hollow fiber membrane is polyacrylonitrile or polysulfone.

8. The method for regenerating a filtration membrane according to claim 6, wherein the molecular weight cut-off of the hollow fiber membrane is 10,000 to 50,000.

9. A silica fine particle dispersion obtained by ultrafiltration concentration using a hollow fiber membrane regenerated by the method for regenerating a filtration membrane according to claim 6.