CN110023250B - Treatment system and treatment method for treating water containing silicon dioxide - Google Patents

Treatment system and treatment method for treating water containing silicon dioxide Download PDF

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CN110023250B
CN110023250B CN201780074970.8A CN201780074970A CN110023250B CN 110023250 B CN110023250 B CN 110023250B CN 201780074970 A CN201780074970 A CN 201780074970A CN 110023250 B CN110023250 B CN 110023250B
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magnesium
silica
containing water
reaction
liquid
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CN110023250A (en
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福水圭一郎
中野徹
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Organo Corp
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    • CCHEMISTRY; METALLURGY
    • C02TREATMENT OF WATER, WASTE WATER, SEWAGE, OR SLUDGE
    • C02FTREATMENT OF WATER, WASTE WATER, SEWAGE, OR SLUDGE
    • C02F1/00Treatment of water, waste water, or sewage
    • C02F1/52Treatment of water, waste water, or sewage by flocculation or precipitation of suspended impurities
    • CCHEMISTRY; METALLURGY
    • C02TREATMENT OF WATER, WASTE WATER, SEWAGE, OR SLUDGE
    • C02FTREATMENT OF WATER, WASTE WATER, SEWAGE, OR SLUDGE
    • C02F1/00Treatment of water, waste water, or sewage
    • C02F1/52Treatment of water, waste water, or sewage by flocculation or precipitation of suspended impurities
    • C02F1/54Treatment of water, waste water, or sewage by flocculation or precipitation of suspended impurities using organic material
    • C02F1/56Macromolecular compounds
    • CCHEMISTRY; METALLURGY
    • C02TREATMENT OF WATER, WASTE WATER, SEWAGE, OR SLUDGE
    • C02FTREATMENT OF WATER, WASTE WATER, SEWAGE, OR SLUDGE
    • C02F1/00Treatment of water, waste water, or sewage
    • C02F1/58Treatment of water, waste water, or sewage by removing specified dissolved compounds
    • C02F1/60Silicon compounds

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  • Hydrology & Water Resources (AREA)
  • Engineering & Computer Science (AREA)
  • Environmental & Geological Engineering (AREA)
  • Water Supply & Treatment (AREA)
  • Chemical & Material Sciences (AREA)
  • Organic Chemistry (AREA)
  • Removal Of Specific Substances (AREA)
  • Separation Of Suspended Particles By Flocculating Agents (AREA)

Abstract

The present application provides a silica-containing water treatment system and method that can effectively treat silica-containing water. The silica-containing water treatment system 1 includes: a magnesium dissolution tank 10 in which a magnesium salt and an acid are mixed with each other and reacted under a condition of a pH value of 7 or less to produce a magnesium-containing liquid; and a magnesium reaction tank 12 in which a mixed liquid obtained by mixing the silica-containing water and the magnesium-containing liquid is reacted at a pH of 10 to 12.

Description

Treatment system and treatment method for treating water containing silicon dioxide
Technical Field
The present invention relates to a silica-containing water treatment system and a method for treating silica contained in silica-containing water.
Background
The recovery and reuse of the silica-containing water causes a problem of scale formation in a pipeline, a reverse osmosis membrane (RO) device at a subsequent stage, or the like, which makes it difficult to increase the recovery rate of the silica-containing water and to perform a stable operation in some cases. Therefore, there is a need to reduce the amount of silica in silica-containing water.
Research methods for reducing the amount of silica in silica-containing water include methods using magnesium salts.
For example, patent document 1 discloses a method for reducing the amount of silica, which comprises adding a magnesium salt to silica-containing water, and adding Mg (OH) to the resultant2Adsorbing silica, flocculating the silica by adding an iron salt, and then performing solid-liquid separation in which a higher silica removal rate is achieved by heating silica-containing water to 50 ℃ or higher. However, the method of patent document 1 requires a heat source for heating, and thus causes a problem of an increase in processing cost.
Non-patent document 1 describes a method for reducing the amount of silica, which comprises: adding a mixture to silica-containing water, wherein the mixture is obtained by adding sulfuric acid to a magnesium slurry containing MgO or the like; separating the solid component, wherein the recovery of silica is improved compared to the case where no sulfuric acid is added to the magnesium slurry. However, in the method of non-patent document 1, about 20mg/L of silica remains in the treated water. When the risk of silica scale is considered, in the case where a reverse osmosis membrane apparatus is provided at a later stage, it is difficult to increase the recovery rate of the reverse osmosis membrane apparatus.
Reference list
Patent document
Patent document 1: JP 2014-168742A
Non-patent document
Non-patent document 1: isabel Latour, Ruben Miranda, Angeles Blanco, "silica removal with sparingly soluble magnesium Compounds A first part," isolation and purification techniques, 138(2014), pp 210-218
Disclosure of Invention
An object of the present invention is to provide a silica-containing water treatment system and method capable of efficiently treating silica-containing water.
According to the present invention, there is provided a silica-containing water treatment system comprising: a magnesium dissolution unit in which a magnesium salt and an acid are mixed with each other and reacted at a pH of 7 or less to produce a magnesium-containing liquid; and a magnesium reaction unit in which a mixed liquid obtained by mixing silica-containing water and a magnesium-containing liquid is reacted at a pH of 10 to 12.
The silica-containing water treatment system preferably comprises: a magnesium dissolution tank as a magnesium dissolution unit in which a magnesium salt and an acid are mixed with each other and reacted under a condition of a pH value of 7 or less to produce a magnesium-containing liquid; and a magnesium reaction tank as a magnesium reaction unit, wherein a mixed liquid obtained by mixing silica-containing water and a magnesium-containing liquid is reacted at a pH of 10 to 12.
The silica-containing water treatment system preferably comprises: a magnesium dissolution tank as a magnesium dissolution unit in which a magnesium salt and an acid are mixed in silica-containing water and reacted under a condition of a pH value of 7 or less to prepare a mixed liquid containing a mixture of a magnesium-containing liquid and silica-containing water; and a magnesium reaction tank as a magnesium reaction unit, wherein the mixed liquid is reacted at a pH of 10 to 12.
The silica-containing water treatment system preferably includes a removal unit for separating and removing insoluble matters resulting from the reaction performed by the magnesium reaction unit at a stage subsequent to the magnesium reaction unit.
In the silica-containing water treatment system, the removal unit preferably includes: a flocculation unit that flocculates insoluble matter by using a flocculant; a solid-liquid separation unit for performing solid-liquid separation on the flocculated matter at a stage subsequent to the flocculation unit.
In the silica-containing water treatment system, the flocculant is preferably at least one of an iron-based inorganic flocculant and a cationic polymer flocculant.
Further, according to the present invention, there is provided a silica-containing water treatment method comprising: a magnesium dissolution process in which magnesium salts and an acid are mixed with each other and reacted at a pH of 7 or less to produce a magnesium-containing liquid; and a magnesium reaction process in which a mixed liquid obtained by mixing a silica-containing water and a magnesium-containing liquid is reacted at a pH of 10 to 12.
In the magnesium dissolution process of the silica-containing water treatment method, the magnesium salt and the acid are preferably mixed with each other and reacted under a condition of pH less than or equal to 7 to prepare a magnesium-containing liquid, and in the magnesium reaction process, a mixed liquid obtained by mixing the silica-containing water and the magnesium-containing liquid with each other is preferably reacted under a condition of pH 10 to 12.
In the magnesium dissolution process of the silica-containing water treatment method, the magnesium salt and the acid are preferably mixed in the silica-containing water and reacted at a pH of 7 or less to prepare a mixed liquid containing a mixture of the magnesium-containing liquid and the silica-containing water, and in the magnesium reaction process, the mixed liquid is preferably reacted at a pH of 10 to 12.
The silica-containing water treatment method preferably further comprises: a removing process of separating and removing insoluble matters obtained from the reaction performed during the magnesium reaction process at a stage after the magnesium reaction process.
In the silica-containing water treatment method, the removal process preferably includes: a flocculation process of flocculating insoluble matters using a flocculant, and a solid-liquid separation process of subjecting the flocculated matters to solid-liquid separation at a post-flocculation stage.
In the silica-containing water treatment method, the flocculant is preferably at least one of an iron-based inorganic flocculant and a cationic polymer flocculant.
According to the present invention, silica-containing water can be efficiently treated.
Drawings
Fig. 1 is a schematic configuration diagram for explaining an example of a silica-containing water treatment system according to an embodiment of the present invention.
Fig. 2 is a schematic configuration diagram for explaining another example of a silica-containing water treatment system according to an embodiment of the present invention.
Fig. 3 is a schematic configuration diagram for explaining still another example of a silica-containing water treatment system according to an embodiment of the present invention.
FIG. 4 is a graph for explaining the silica concentration (mg/L) of the flocculation and sedimentation treated water in the examples and the silica removal rate (%) with respect to the pH value of the magnesium dissolution tank.
FIG. 5 is a graph for explaining the removal rate (%) of silica with respect to the amount (mg/L) of magnesium salt added in examples and comparative examples.
Detailed Description
Embodiments of the present invention will be described below. The present embodiment is an example of implementing the present invention, and the present invention is not limited to the present embodiment.
In the silica-containing water treatment method and the silica-containing water treatment system according to the embodiment of the invention, a magnesium salt and an acid are mixed with each other and reacted under a condition of pH value of 7 or less to prepare a magnesium-containing liquid (magnesium dissolving process), and a mixed liquid in which silica-containing water and the obtained magnesium-containing liquid are mixed with each other is reacted under a condition of pH value of 10 to 12 (magnesium reacting process). After the magnesium reaction process, insoluble matters obtained by the reaction performed during the magnesium reaction process are separated and removed as necessary (removal process).
An example of a treatment system including a magnesium dissolution tank for producing a magnesium-containing liquid and a magnesium reaction tank for reacting a mixed liquid in which silica-containing water and the resultant magnesium-containing liquid are mixed with each other will be described below. However, the processing system is not limited to the following configuration. As examples of treatment systems employing the silica-containing water treatment method according to the present embodiment, system example 1 (fig. 1) in which a magnesium dissolution tank is separated from a flow of treated water obtained from silica-containing water via a magnesium reaction tank, and system example 2 (fig. 2) in which treated water is obtained from silica-containing water via a magnesium dissolution tank and a magnesium reaction tank arranged in series, and configurations thereof will be described below.
[ example 1 of System ]
The silica-containing water treatment system 1 shown in fig. 1 comprises a magnesium dissolution tank 10 and a magnesium reaction tank 12, wherein magnesium salts and an acid are mixed with each other in the magnesium dissolution tank 10 and reacted at a pH of 7 or less to produce a magnesium-containing liquid, and the magnesium reaction tank 12 is used for reacting a mixed liquid obtained by mixing silica-containing water and the magnesium-containing liquid with each other at a pH of 10 to 12. The silica-containing water treatment system 1 is an example of a system in which a magnesium dissolution tank 10 is separated from a flow of treated water obtained from silica-containing water via a magnesium reaction tank 12.
A silica-containing water pipe 18 is connected to a silica-containing water inlet of the magnesium reaction tank 12, and a treated water pipe 20 is connected to a treated water outlet thereof. The outlet of the magnesium dissolution tank 10 and the magnesium-containing liquid inlet of the magnesium reaction tank 12 are connected together by a magnesium-containing liquid pipe 26. Stirring devices 14 and 16 including stirring blades are installed as stirring units in the magnesium dissolution tank 10 and the magnesium reaction tank 12, respectively. An acid feed pipe 22 and a magnesium salt feed pipe 24 are connected to the magnesium dissolution tank 10. A pH adjustor feeding pipe 28 is connected to the magnesium reaction tank 12.
The operation of the silica-containing water treatment method and the silica-containing water treatment system 1 according to the present embodiment will be described below.
An acid is fed into the magnesium dissolution tank 10 through an acid feed pipe 22, and for example, a water slurry of magnesium salt is fed thereto through a magnesium salt feed pipe 24. They are stirred by the stirring device 14, and the magnesium salt and the acid are mixed and reacted at a pH of 7 or less to prepare a magnesium-containing liquid (magnesium dissolving process). The solid magnesium salt may be added as it is to the magnesium dissolution tank 10 to be mixed with the acid.
On the other hand, silica-containing water containing silica is supplied to the magnesium reaction tank 12 through the silica-containing water pipe 18. In the magnesium reaction tank 12, a magnesium-containing liquid containing magnesium salts and acids obtained in the magnesium dissolving process is added to the silica-containing water through the magnesium-containing liquid pipe 26, and stirred and mixed by the stirring device 16. In addition, in the magnesium reaction tank 12, a pH adjuster is added to the magnesium-containing liquid through a pH adjuster feeding tube 28 and the reaction is carried out under the condition that the pH value is 10 to 12, and the silica is not dissolved (magnesium reaction process). The reaction liquid is discharged as treated water through the treated water pipe 20. Alternatively, after the magnesium reaction process, the liquid after the reaction may be subjected to a process in which insoluble substances obtained by the reaction performed during the magnesium reaction process are separated and removed by a removal unit (not shown) as necessary (removal process).
In the silica-containing water treatment system 1, the magnesium dissolution tank 10, the stirring device 14 and the like are used as a magnesium dissolution unit in which a magnesium salt and an acid are mixed with each other and reacted under a condition of pH 7 or less to produce a magnesium-containing liquid, and the magnesium reaction tank 12, the stirring device 16 and the like are used as a magnesium reaction unit for reacting a mixed liquid in which silica-containing water and the magnesium-containing liquid are mixed with each other under a condition of pH 10 to 12.
The present inventors have found that effective treatment of silica-containing water can be achieved by reacting an acid and a magnesium salt at a pH of 7 or less to form a magnesium-containing liquid, and reacting a mixed liquid obtained by mixing the resulting magnesium-containing liquid and silica-containing water with each other at a pH of 10 to 12. The present inventors have also found that the removal rate of silica (removal process) can be significantly improved by separating and removing insoluble silica at a stage after the magnesium reaction tank 12.
Therefore, when the treated water is recovered and reused by a reverse osmosis membrane apparatus or the like at a stage subsequent to the silica-containing water treatment system 1, treated water in which a large amount of silica is removed is obtained. Therefore, the formation of silica scale can be suppressed, and the recovery rate can be improved.
[ example 2 of System ]
A silica-containing water treatment system 3 shown in fig. 2 includes a magnesium dissolution tank 30 and a magnesium reaction tank 32, wherein a magnesium salt and an acid are mixed with each other in silica-containing water in the magnesium dissolution tank 30 and reacted under a condition of pH value of 7 or less to prepare a mixed liquid in which a magnesium-containing liquid and silica-containing water are mixed with each other; the magnesium reaction tank 32 is used for reacting the mixed liquid under the condition of pH value of 10-12. The silica-containing water treatment system 3 is an example of a system that obtains treated water from silica-containing water via a magnesium dissolution tank 30 and a magnesium reaction tank 32 that are arranged in series.
A silica-containing water pipe 38 is connected to the silica-containing water inlet of the magnesium dissolving tank 30. The outlet of the magnesium dissolution tank 30 and the inlet of the magnesium reaction tank 32 are connected together by a magnesium-containing liquid pipe 40. A pipe 42 of treated water is connected to an outlet of the magnesium reaction tank 32. Stirring devices 34 and 36 including stirring blades are installed as stirring units in the magnesium dissolution tank 30 and the magnesium reaction tank 32, respectively. An acid feed pipe 44 and a magnesium salt feed pipe 46 are connected to the magnesium dissolution tank 30. A pH adjustor feed pipe 48 is connected to the magnesium reaction tank 32.
The operation of the silica-containing water treatment method and the silica-containing water treatment system 3 according to the present embodiment will be described below.
The silica-containing water containing silica is supplied to the magnesium dissolution tank 30 through the silica-containing water pipe 38. In the magnesium dissolution tank 30, an acid is added to the silica-containing water through an acid addition pipe 44, and a magnesium salt water slurry is added thereto through a magnesium salt addition pipe 46, for example. By the stirring of the stirring device 34, the magnesium salt, the acid and the silica-containing water are mixed with each other and reacted at a pH of 7 or less to prepare a mixed liquid in which the magnesium-containing liquid and the silica-containing water are mixed with each other (magnesium dissolving process). The solid magnesium salt may be added as it is to the magnesium dissolution tank 10 to be mixed with the acid.
The magnesium-containing liquid containing magnesium salts therein obtained in the magnesium dissolving process and the mixed liquid obtained by mixing the acid and the silica-containing water with each other are supplied to the magnesium reaction tank 32 through the magnesium-containing liquid pipe 40 and are stirred by the stirring device 36. In addition, in the magnesium reaction tank 32, a pH adjusting agent is added to the mixed liquid through a pH adjusting agent feeding tube 48, and the reaction is carried out under a condition that the pH value is 10 to 12, and the silica is not dissolved (magnesium reaction process). The reaction liquid is discharged as treated water through the treated water pipe 42. Alternatively, after the magnesium reaction process, the liquid after the reaction may be subjected to a process in which insoluble substances obtained by the reaction performed during the magnesium reaction process are separated and removed as necessary by a removal unit (not shown) (removal process).
In the silica-containing water treatment system 3, the magnesium dissolution tank 30, the stirring device 34 and the like are used as a magnesium dissolution unit in which a magnesium salt and an acid are mixed with each other and reacted under a condition of pH 7 or less to produce a magnesium-containing liquid, and the magnesium reaction tank 32, the stirring device 36 and the like are used as a magnesium reaction unit for reacting a mixed liquid in which silica-containing water and the magnesium-containing liquid are mixed with each other under a condition of pH 10 to 12.
In the case of using the silica-containing water treatment system 1 shown in FIG. 1, since the magnesium salt and the acid are directly mixed with each other in the magnesium dissolution tank 10, the time for dissolving the magnesium salt can be shortened, and the volume of the magnesium dissolution tank can be reduced. In addition, the magnesium dissolving process may be set as a batch process. Furthermore, even if the silica concentration in the silica-containing water to be treated is changed, the influence thereof on the treatment can be reduced.
The silica-containing water treatment system 3 shown in fig. 2 is an effective system in the case where the silica-containing water is acidic. Further, since the silica and the dissolved magnesium in the silica-containing water flow into the magnesium reaction tank 32 in a state of coexisting, the reactivity of silica and magnesium can be high, and a higher silica removal rate than the treatment system 1 can be obtained.
The concentration of silica in the silica-containing water to be treated is, for example, 10mg/L to 500mg/L.
The silica-containing water to be treated is not particularly limited as long as it is a silica-containing water, and examples thereof include silica-containing water produced by a semiconductor manufacturing plant and silica-containing water produced by a power plant.
The magnesium salt is not particularly limited as long as it is a magnesium salt or a hydrate thereof such as magnesium oxide (MgO), magnesium hydroxide (Mg (OH)2) Or magnesium chloride (MgCl)2·6H2O), but from the viewpoint of chemical cost and the like, magnesium hydroxide (Mg (OH) is preferable2). As the magnesium salt, CaMg (CO) of the formula3)2Magnesium-containing minerals such as dolomite are shown. In addition, from the viewpoint of handling and the like, the magnesium salt is preferably used as a slurry of a solvent such as water.
The acid is not particularly limited, and inorganic acids such as hydrochloric acid and sulfuric acid can be used. As the acid, organic acids such as oxalic acid and citric acid may be added. However, since magnesium depending on raw water may undergo a chelating reaction with an organic acid, the removal rate of silica may be reduced. Therefore, caution is required.
The amount of the magnesium salt added is preferably in the range of 0.1 to 10 times, more preferably 0.5 to 5 times, the weight concentration of silica in the silica-containing water in terms of magnesium concentration. When the amount of the magnesium salt added is less than 0.1 times the weight concentration of silica in the silica-containing water, the silica insolubilization reaction may be insufficient in some cases. When the magnesium salt is added in an amount of more than 10 times the weight concentration of silica in the silica-containing water, the amount of sludge generation may become excessive in some cases
The pH value during magnesium dissolution may be less than or equal to 7, and is preferably in the range of 4 to 7, more preferably in the range of 4 to 6. When the pH during magnesium dissolution is higher than 7, the dissolution of the magnesium salt may be insufficient. When the pH is less than 4, since the silica removal rate is hardly increased, the acid injection cost may be wasted in some cases. However, when the pH is lowered, the reaction time of the magnesium dissolution process may be shortened.
The temperature during the dissolution of magnesium is not particularly limited as long as magnesium can be dissolved, and for example, it may be in the range of 1 ℃ or more and less than 50 ℃, and more preferably in the range of 10 ℃ or more and less than 50 ℃. When the temperature during the dissolution of magnesium is lower than 1 c, the dissolution of magnesium salts may be insufficient in some cases. When the temperature during the magnesium dissolution is 50 ℃ or more, the treatment cost may be increased in some cases.
The reaction time during the dissolution of magnesium is not particularly limited as long as magnesium can be dissolved, and for example, it may be in the range of 1 minute to 60 minutes, and more preferably in the range of 5 minutes to 30 minutes. When the reaction time during the dissolution of magnesium is shorter than 1 minute, the dissolution of magnesium salt may be insufficient in some cases. When the reaction time during the magnesium dissolution is longer than 60 minutes, the reaction tank may become excessive in some cases. As described above, the reaction time can be shortened by lowering the pH during the dissolution of magnesium.
As the pH adjuster, an alkali such as sodium hydroxide or calcium hydroxide may be used, and an inorganic acid such as hydrochloric acid or sulfuric acid may be used as needed.
The pH during the magnesium reaction may be in the range of 10 to 12, and preferably in the range of 10.5 to 11.5, more preferably in the range of 11 to 11.5. When the pH during the magnesium reaction is below 10 or above 12, the silica removal rate decreases.
The temperature during the magnesium reaction is not particularly limited as long as the silica can undergo an insolubilization reaction at the temperature, and for example, it may be in the range of 1 ℃ or more and less than 50 ℃, and more preferably in the range of 10 ℃ or more and less than 50 ℃. When the temperature during the reaction of magnesium is lower than 1 c, the insolubilization reaction of silica may be insufficient in some cases. When the temperature during the magnesium reaction is 50 ℃ or more, the treatment cost may be increased in some cases.
The reaction time during the magnesium reaction is not particularly limited as long as the insolubilization reaction of silica can proceed. For example, the reaction time is 1 minute to 60 minutes, more preferably 5 minutes to 30 minutes. When the reaction time during the reaction of magnesium is shorter than 1 minute, the insolubilization reaction of silica may be insufficient in some cases. When the reaction time during the magnesium reaction is longer than 60 minutes, the reaction tank may become excessive in some cases.
The method for separating and removing silica may be, for example, a silica reduction method such as: the flocculation sedimentation method, the pressure flotation method, the sand filtration method, the membrane filtration method (for example, membrane filtration using a microfiltration membrane (MF membrane), an ultrafiltration membrane (UF membrane) and the like) are preferable from the viewpoint of the amount of sludge produced and the like.
At a stage subsequent to the silica-containing water treatment system 1 or 3, insoluble silica may be separated and removed, and then reverse osmosis membrane (RO membrane) treatment, decarburization treatment, ion exchange treatment, distillation treatment, and the like may be further performed. At a stage after the silica-containing water treatment system 1 or 3, silica can be separated and removed at a high removal rate. Thus, even if the resulting treated water is subjected to these treatments at a later stage, the risk of scaling is reduced.
[ example 3 of System ]
Fig. 3 shows a schematic configuration of an example of a silica-containing water treatment system in which insoluble silica is separated and removed by flocculation and sedimentation at a stage after a magnesium reaction tank.
The silica-containing water treatment system 5 shown in FIG. 3 is an example of a configuration in the case where the flocculation sedimentation treatment is performed at a stage subsequent to the magnesium reaction tank 32 of the silica-containing water treatment system 3 having the structure shown in FIG. 2. The silica-containing water treatment system 5 shown in fig. 3 includes a magnesium dissolution tank 30, a magnesium reaction tank 32, and a flocculation tank 50, a flocculation tank 52, and a precipitation tank 54 as removal units, wherein in the magnesium dissolution tank 30, magnesium salts and an acid are mixed with each other in silica-containing water, and a reaction is performed under a condition that a pH is less than or equal to 7 to prepare a mixed liquid in which a magnesium-containing liquid and silica-containing water are mixed with each other; the magnesium reaction tank 32 is used to react the mixed liquid at a pH of 10 to 12.
A silica-containing water pipe 38 is connected to the silica-containing water inlet of the magnesium dissolution tank 30. The outlet of the magnesium dissolution tank 30 and the inlet of the magnesium reaction tank 32 are connected together by a magnesium-containing liquid pipe 40. The outlet of the magnesium reaction tank 32 and the inlet of the flocculation tank 50 are connected together by a reaction liquid pipe 60. The outlet of the flocculation tank 50 and the inlet of the floc forming tank 52 are connected together by a flocculation liquid pipe 62. The outlet of the floc-forming tank 52 and the inlet of the settling tank 54 are connected together by a floc-forming liquid pipe 64. A treated water pipe 66 is connected to the treated water outlet of the settling tank 54. The sludge pipe 68 is connected to the sludge outlet of the precipitation tank 54. Agitating devices 34, 36, 56 and 58 including agitating blades are installed as agitating units in the magnesium dissolution tank 30, the magnesium reaction tank 32, the flocculation tank 50, and the flocculation-forming tank 52, respectively. An acid feed pipe 44 and a magnesium salt feed pipe 46 are connected to the magnesium dissolution tank 30. A pH adjustor feed pipe 48 is connected to the magnesium reaction tank 32. An inorganic flocculant feed pipe 70 is connected to the flocculation tank 50. A polymer flocculant feed pipe 72 is connected to the floc forming tank 52.
The silica-containing water containing silica is supplied to the magnesium dissolution tank 30 through the silica-containing water pipe 38. In the magnesium dissolution tank 30, an acid is added to the silica-containing water through an acid addition pipe 44, and a water slurry of magnesium salt is added thereto through a magnesium salt addition pipe 46, for example. By the stirring of the stirring device 34, the magnesium salt, the acid and the silica-containing water are mixed with each other and reacted at a pH of 7 or less to prepare a mixed liquid in which the magnesium-containing liquid and the silica-containing water are mixed with each other (magnesium dissolving process). The solid magnesium salt may be added as it is to the magnesium dissolution tank 10 to be mixed with the acid.
The magnesium-containing liquid containing magnesium salts therein obtained in the magnesium dissolving process and the mixed liquid in which the acid and the silica-containing water are mixed with each other are supplied to the magnesium reaction tank 32 through the magnesium-containing liquid pipe 40 and stirred by the stirring device 36. In addition, in the magnesium reaction tank 32, a pH adjusting agent is added to the mixed liquid through a pH adjusting agent feeding pipe 48, and the reaction is carried out under a pH value of 10 to 12, and silica is not dissolved (magnesium reaction process). The reaction liquid is supplied to the flocculation tank 50 through the reaction liquid pipe 60.
In the flocculation tank 50, an inorganic flocculant is added to the reaction liquid through an inorganic flocculant feed pipe 70, and insoluble matters are flocculated (flocculation process). The flocculated liquid is supplied to the floc forming tank 52 through a flocculated liquid pipe 62.
In floc-forming tank 52, a polymer flocculant is added to the flocculated liquid through polymer flocculant feed pipe 72 to form flocs (floc-forming process). The floe-forming liquid is supplied to the settling tank 54 through a floe-forming liquid pipe 64.
In the precipitation tank 54, aggregates formed by flocs are solid-liquid separated (solid-liquid separation process). The treated water is discharged through a treated water pipe 66. On the other hand, the sludge is discharged through the sludge pipe 68.
In the stage subsequent to the silica-containing water treatment system 5, reverse osmosis membrane (RO membrane) treatment, decarburization treatment, ion exchange treatment, distillation treatment, and the like may be further performed. The treated water obtained in the silica-containing water treatment system 5 has silica removed at a high removal rate. Thus, even if these treatments are performed at a later stage, the risk of fouling is reduced.
Examples of the inorganic flocculant used in the flocculation process include iron-based inorganic flocculants such as ferric chloride, and aluminum-based inorganic flocculants such as polyaluminum chloride (PAC), and the iron-based inorganic flocculants are preferable from the aspects of chemical cost and pH range of flocculation.
The amount of the inorganic flocculant to be added is preferably 0.1 to 10 times, more preferably 1 to 5 times, the amount of the magnesium salt to be added in terms of a weight ratio. When the inorganic flocculant is added in an amount of less than 0.1 times by weight the amount of the magnesium salt added, flocculation may be insufficient in some cases. When the amount of the inorganic flocculant added is more than 10 times the amount of the magnesium salt added in terms of weight ratio, the sludge production amount may become excessive in some cases.
The pH value during flocculation is for example in the range of 3 to 11. When the pH during flocculation is below 3 or above 11, poor flocculation may occur in some cases. In addition, when the pH during flocculation is below 9, the silica may dissolve from the flocs. Therefore, it is desirable to perform the flocculation process in the pH range of 9 to 11.
The temperature during flocculation is for example in the range of 1 ℃ to 80 ℃. When the temperature during flocculation is lower than 1 ℃ or higher than 80 ℃, poor flocculation may occur in some cases.
Examples of the polymer flocculant used in the floc formation process include cationic polymer flocculants such as polyacrylamide-based or polyacrylate-based flocculants, anionic polymer flocculants, and nonionic polymer flocculants, and preferably cationic polymer flocculants from the viewpoint of flocculability.
Examples of commercially available polymer flocculants include cationic polymer flocculants such as ORFLOCK OX-304 (manufactured by Organo Corporation).
The amount of the polymer flocculant to be added is preferably in the range of 0.1 to 10mg/L, more preferably in the range of 1 to 5mg/L, relative to the amount of raw water. When the amount of the polymer flocculant added is less than 0.1mg/L relative to the amount of raw water, floc formation is not improved in some cases. When the amount of the polymer flocculant added is more than 10mg/L, the polymer flocculant existing by dissolving in the treated water may remain in some cases.
The pH during floe formation is for example in the range of 3 to 11. When the pH during floc formation is below 3 or above 11, undesirable flocculation may occur in some cases. In addition, when the pH during flocculation is below 9, the silica may dissolve from the flocs. Therefore, it is desirable to perform the floc formation process at a pH in the range of 9 to 11.
The temperature during floc formation is for example in the range of 1 ℃ to 80 ℃. When the temperature during floc formation is below 1 ℃ or above 80 ℃, poor flocculation may occur in some cases.
In the above flocculation treatment, an inorganic flocculant and a polymer flocculant are used for the flocculation process and the floc formation process. At least one of an inorganic flocculant, a polymer flocculant, and the like may be used, and preferably at least one of an iron-based inorganic flocculant and a cationic polymer flocculant is used. When the magnesium salt reacts to flocculate the insoluble silica, the use of at least one of an iron-based inorganic flocculant and a cationic polymer flocculant can improve flocculation performance and solid-liquid separation performance.
Examples of the solid-liquid separation include a pressure flotation treatment, a membrane filtration treatment, and the like in addition to the sedimentation separation, and the sedimentation separation is preferable from the viewpoint of separability and the like.
Examples of the invention
The present invention will be described more specifically below with reference to examples and comparative examples, but the present invention is not limited to the following examples.
< example 1 and comparative example 1>
A continuous water flow experiment was performed on the silica-containing water by using the treatment system 5 shown in fig. 3 while changing the pH values in the magnesium dissolution tank to 4, 5, 6, 7 (example 1), 8, 9, 10 (comparative example 1), and the effect of removing silica was examined. The experimental conditions are shown in table 1.
[ Table 1]
Table 1: conditions of the experiment
Figure BDA0002083294870000121
Figure BDA0002083294870000131
The results of the experiment are shown in FIG. 4. The silica concentration was measured by molybdenum yellow absorption spectrophotometry using a spectrophotometer (U-3900, manufactured by Hitachi High-Tech Science Corporation).
As shown in fig. 4, it was found that the silica concentration of the flocculation sedimentation-treated water was successfully reduced when the pH value in the magnesium dissolution tank was less than or equal to 7.
< comparative example 2>
When the amount of magnesium salt added (250mg/L, 500mg/L, 750mg/L, 1000mg/L and 1500mg/L) is changed, the reaction is carried out under the conditions described in non-patent document 1 (magnesium salt: Mg (OH))2+ sulfuric acid, dissolution pH: 9.5, reaction pH: 11) the results of the experiment using the method of the present invention are shown in fig. 5. The results of example 1 in which the pH in the magnesium dissolution tank was set to 7 (addition amount of magnesium salts: 230mg/L, 460mg/L and 920mg/L) are also shown in FIG. 5.
As shown in fig. 5, when compared with the same addition amount of the magnesium salt, the result is that the silica removal rate of the method of example 1 is higher than that of the method of comparative example 2 described in non-patent document 1.
The above results indicate that treated water with high silica removal can be obtained. Therefore, even if reverse osmosis membrane treatment is performed in a subsequent stage and the treated water is recovered and reused, the risk of silica scale and the like can be reduced.
As shown above, the silica-containing water can be effectively treated by the method of example 1.
List of reference numerals:
1,3,5: silica-containing water treatment system
10, 30: magnesium dissolving tank
12, 32: magnesium reaction tank
14, 16, 34, 36, 56, 58: stirring device
18, 38: water pipe containing silicon dioxide
20, 42, 66: treated water pipe
22, 44: acid feed tube
24, 46: magnesium salt feeding tube
26, 40: magnesium-containing liquid tube
28, 48: PH regulator filling tube
50: flocculation tank
52: flocculation forming tank
54: settling tank
60: reaction liquid tube
62: flocculation liquid pipe
64: floc-forming liquid tube
68: sludge pipe
70: inorganic flocculant charging tube
72: polymer flocculant feed tube

Claims (8)

1. A silica-containing water treatment system comprising:
a magnesium dissolution tank in which a magnesium salt and an acid are mixed in silica-containing water and reacted at a pH of 4 to 7 to prepare a mixed liquid including a mixture of a magnesium-containing liquid and the silica-containing water; and
a magnesium reaction tank in which a reaction is performed by subjecting the mixed liquid to a pH of 10 to 12.
2. The silica-containing water treatment system of claim 1 further comprising:
A removal unit for separating and removing insoluble substances obtained from the reaction in the magnesium reaction tank at a stage after the magnesium reaction tank.
3. The silica-containing water treatment system according to claim 2,
the removing unit includes:
a flocculation unit for flocculating the insoluble matter by using a flocculant; and
a solid-liquid separation unit for performing solid-liquid separation of the flocculated material at a stage subsequent to the flocculation unit.
4. The silica-containing water treatment system according to claim 3,
the flocculant is at least one of an iron-based inorganic flocculant and a cationic polymer flocculant.
5. A method of treating silica-containing water comprising:
a magnesium dissolution process in which a magnesium salt and an acid are mixed in silica-containing water and reacted at a pH of 4 to 7 to prepare a mixed liquid comprising a mixture of a magnesium-containing liquid and the silica-containing water; and
a magnesium reaction process in which the reaction is carried out by reacting the mixed liquid at a pH of 10 to 12.
6. The silica-containing water treatment method as claimed in claim 5, further comprising:
a removal process for separating and removing insoluble materials obtained by the reaction performed by the magnesium reaction process at a stage after the magnesium reaction process.
7. The silica-containing water treatment method according to claim 6,
the removing process comprises the following steps:
a flocculation process for flocculating the insoluble matter by using a flocculant; and
a solid-liquid separation process for performing solid-liquid separation on the flocculated matter at a stage subsequent to the flocculation process.
8. The silica-containing water treatment method according to claim 7,
the flocculant is at least one of an iron-based inorganic flocculant and a cationic polymer flocculant.
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