JPH0315512B2 - - Google Patents

Description

[Detailed description of the invention]

TECHNICAL FIELD OF THE INVENTION This invention relates to a method for treating water containing fluoride ions and sulfate ions, and in particular to a treatment method for removing fluoride ions and sulfate ions without forming scale. [Prior art] Water containing fluoride ions and sulfate ions is reacted with a calcium compound to form a precipitate,
There is a treatment method that involves solid-liquid separation. This method precipitates fluoride ions and sulfate ions in the form of calcium fluoride and calcium sulfate, but it is said that scale mainly composed of calcium sulfate is generated in the reaction tank, solid-liquid separation tank, or pipes. There was a problem. In order to improve this problem, a method has been proposed in which the precipitate separated by solid-liquid separation is returned to the reaction system as a seed crystal (Japanese Patent Application No. 58-45058).
issue). In this method, new precipitates are precipitated using the returned precipitates as nuclei, which prevents scaling. However, if the concentration of sulfate ions in the raw water is low or the amount of calcium compounds added is small, the degree of supersaturation will be low. However, there was a problem in that unless the reaction time was prolonged, there would be no seed crystal effect and scale formation could not be prevented. [Purpose of the Invention] This invention is intended to solve the problems of the conventional method as described above. After mixing a calcium compound with a solid-liquid separated precipitate, the supersaturation level is reduced by reacting it with raw water. It is an object of the present invention to provide a method for treating water containing fluoride and sulfate ions, which can remove fluoride ions and sulfate ions without forming scale even when the fluoride and sulfate ions are low. [Structure of the Invention] The present invention provides a method for reacting raw water containing fluoride ions and sulfate ions with a calcium compound to generate a precipitate and separating the solid-liquid,
This is a method for treating water containing fluoride and sulfate ions, which is characterized by mixing a calcium compound with a precipitate that has been separated into solid and liquid, and then reacting the mixture with raw water. Fluoride ion and sulfate ion-containing water to be treated in this invention includes wastewater discharged from phosphoric acid production processes, phosphate fertilizer production processes, cryolite recovery processes, flue gas desulfurization processes, flue gas denitrification processes, etc. For example, This invention will be explained below with reference to the drawings. The drawings are system diagrams showing preferred embodiments of the invention,
1 is a dissolution tank, 2 is a first reaction tank, 3 is a solid-liquid separation tank,
4 is a second reaction tank, and 5 is a solid-liquid separation tank. The treatment method is to first introduce raw water into the dissolution tank 1 from the raw water pipe 6, introduce the precipitate from the solid-liquid separation tank 5 through the return pipe 7, and add acid from the chemical injection pipe 8 as necessary.
The pH is adjusted to 4 or less, preferably 2 to 4, and the returned precipitate is dissolved. Since this precipitate contains magnesium hydroxide and calcium carbonate as described below, magnesium ions and calcium ions are eluted by dissolution. The reason for adjusting the pH to below 4 is to completely dissolve the precipitates and to prevent the formation of gypsum scale in the dissolution tank. If the raw water is acidic and the pH drops to below 4 after dissolving the precipitates, there is no need to add acid, but in other cases it is necessary to add acid. Although the acid is not particularly limited, sulfuric acid is not preferred because it consumes calcium ions, nitric acid is not preferred because it becomes a nitrogen source, and hydrochloric acid is most suitable. Outflow water from the dissolution tank 1 flows into the first reaction tank 2. On the other hand, the solid-liquid separation tank 3 returned from the return pipe 9
The precipitate of
~60 minutes). The calcium compound added from the drug injection tube 11 is a calcium compound that is reacted with raw water, and includes, for example, calcium chloride, calcium carbonate, calcium hydroxide, and the like. The amount of calcium compound added is sufficient to be added to raw water and reacted with fluoride ions and sulfate ions.
It is about 1 to 3 times the Ca equivalent as CaF ₂ , and preferably about 2 times the equivalent. However, if a large amount of aluminum ions are contained, such as in flue gas desulfurization or (and) desulfurization wastewater, the amount of calcium ions added can be reduced, and the amount required will vary depending on the salt concentration of the wastewater, etc. , the amount of calcium ions added can be easily confirmed experimentally. When a calcium compound is mixed and reacted with the returned precipitate in the aging tank 10, the calcium sulfate in the precipitate is aged and the particle size increases, making it suitable for seed crystals. 2, mixed with the raw water flowing out from dissolution tank 1, and the pH reached 5.
When the pH is adjusted to ~8.5, preferably 6 to 7, calcium sulfate precipitates on the surface of the precipitate even when the supersaturation degree of the reaction system is low, thereby eliminating the supersaturation state and preventing scaling. In this case, aluminum ions may be present in addition to calcium ions. Adjustment of PH is carried out by injecting a PH adjusting agent from the drug injection pipe 12 if necessary. As the pH adjuster, sodium hydroxide, sodium carbonate, calcium hydroxide, etc. can be used as the alkali, and among these, calcium hydroxide is preferable because it can also be used as a calcium ion source. As the acid, hydrochloric acid, sulfuric acid, nitric acid, etc. can be used, and among these, hydrochloric acid is preferred. By adding such a PH adjuster to adjust the PH to the above range,
A precipitate forms. The precipitates are mainly _CaSO4 , which is the reaction of calcium ions with sulfate ions, and _CaF2 , which is the reaction with fluoride ions.If aluminum ions are present, Al(OH) ₃ is included in the form of fluoride. It is assumed that this is the case, but the details are unknown. The PH range in this process is _CaF2 and
A range in which the solubility of Al(OH) ₃ precipitates is low and the formation of magnesium hydroxide precipitates to be returned in the process described later is small, that is, a pH range of 5 to 8.5 is preferable, and a pH of 6 to 7 is particularly preferable. The reaction liquid in the first reaction tank 2 undergoes solid-liquid separation in the solid-liquid separation tank 3, and a part of the separated precipitate is returned to the aging tank 10 from the return pipe 9, and the remainder is transferred to the sludge drain pipe 13.
is discharged from the system. On the other hand, the supernatant liquid is allowed to flow out into the second reaction tank 4. In the second reaction tank 4, the pH is further adjusted to 9.5 or higher in the presence of magnesium ions and carbonate ions (including bicarbonate ions) to form a precipitate.
Further reduces residual fluoride ions in water.
Note that in this step, residual calcium ions in the water are also removed at the same time. In the second reaction tank 4, magnesium salt is added from the chemical injection pipe 14, carbonate is added from the chemical injection pipe 15, and PH adjuster is added from the chemical injection pipe 16 as necessary. Magnesium chloride or the like can be used as the magnesium salt, but it may not be added if it already exists in the reaction solution. As the carbonate, sodium carbonate, sodium bicarbonate, etc. can be used, but carbon dioxide gas may also be blown into it. Further, the pH adjuster is added when the predetermined pH is still not reached even after adding the magnesium salt and carbonate, and the same one as in the first step can be used. When the amount of magnesium ions present in the reaction solution is at least 20 times the weight of fluoride ions, the amount of residual fluoride ions can be reduced to 1 mg/min or less. Further, the amount of carbonate ions is typically about 1/2 equivalent or more relative to calcium ions. Said PH
By adjusting the temperature, precipitates of magnesium hydroxide and calcium carbonate are formed, and fluoride in the liquid is also trapped and precipitated. In this case, since calcium carbonate and magnesium hydroxide are precipitated in a mixed state, the resulting floc is dense and heavy, resulting in a higher fluoride removal rate than in the case of magnesium hydroxide alone. Separability is also improved, and the product can be returned in a highly concentrated state. The reaction liquid in the second reaction tank 4 undergoes solid-liquid separation in the solid-liquid separation tank 5, and the supernatant water is discharged as treated water to the outside of the system from the treated water pipe 17, and is subjected to treatments such as neutralization as necessary. Further, the precipitate is returned to the dissolution tank 1 through the return pipe 7. In particular, in flue gas desulfurization wastewater, metal hydroxides such as manganese and iron are included in the precipitates discharged from the solid-liquid separation tank 5, so when returning the precipitates, the first reaction tank 2 It is preferable to carry out aeration to precipitate these substances. The returned precipitate is dissolved in the dissolution tank 1, and the released fluoride ions are subjected to the above-mentioned treatment together with the fluoride ions in the raw water. In this case, the eluted calcium ions react with fluoride ions, so the amount of calcium compound added from the chemical injection tube 11 is to make up for the shortage of calcium ions and aluminum ions in the raw water and calcium ions eluted from the precipitates. Just do it. Further, the eluted magnesium ions flow directly into the second reaction tank 4 and are recycled. Therefore, the amount of magnesium ions added in the second reaction tank 4 only needs to be the amount corresponding to the magnesium precipitate discharged from the solid-liquid separation tank 3;
When adjusting the pH to 7 or less, magnesium hardly precipitates, so it is only necessary to add magnesium ions at the beginning. In addition, if the raw water contains magnesium ions, by adjusting the amount of PH adjuster added in the second reaction tank 4 to an appropriate amount, the magnesium necessary for removing fluoride ions can be circulated within the system. The surplus will be discharged into the treated water. In addition, if you want to reduce the amount of magnesium ions in the treated water,
It is sufficient to add a PH adjuster corresponding to the amount in the reaction tank 4 and take out the excess magnesium precipitate in the solid-liquid separation tank 5. In the above treatment, water containing fluoride ions and sulfate ions is reacted with calcium ions to separate precipitates without causing scale damage, and then magnesium hydroxide and calcium carbonate precipitates are generated and returned. Therefore, fluoride ions can be efficiently and highly removed, and the fluoride ion concentration in the resulting treated water is low. In addition, since unreacted calcium ions are returned as precipitates, the concentration of calcium ions in the treated water can be reduced, and calcium ions can be used effectively, and the amount of calcium salt added from the chemical injection pipe 11 can be reduced. can. When the treated water is further treated with a synthetic adsorbent such as a weakly basic one, since it has been previously subjected to dehardening treatment, precipitation of calcium, etc. in the resin layer is prevented, and the resin can be used effectively. Furthermore, the precipitates that are subject to sludge treatment are only the precipitates from the solid-liquid separation tank 3, which reduces the amount of sludge that must be treated, and the treatment is also easy because the amount of magnesium hydroxide, which is difficult to dewater, is small. become. Furthermore, since the pH in the dissolution tank 1 is adjusted to 4 or less, no scale is generated and the treatment effect is improved. In the above description, the calcium compound is added only to the aging tank 10 from the chemical injection pipe 11, but it may also be added to the first reaction tank 2. Furthermore, when an alkaline compound such as calcium hydroxide is used as the calcium compound added to the aging tank 10, the addition of the PH adjuster from the chemical injection pipe 12 in the first reaction tank 2 may be omitted. Furthermore, depending on the required water quality, the dissolution tank 1, the second reaction tank 4, and the solid-liquid separation tank 5 can be omitted. [Effects of the Invention] According to the present invention, the calcium compound is mixed with the solid-liquid separated precipitate and then reacted with raw water, so that the precipitate can be aged and activated as crystal seeds. As a result, even if the raw water quality fluctuates and the supersaturation degree of the reaction system decreases, calcium sulfate can be precipitated on the surface of the precipitate to prevent scaling and increase the removal rate of fluoride ions. Can be done. [Example of the invention] F660mg/, SO ₄ 5000mg/, Ca644mg/,
Add 3200 mg of Ca(OH) ₂ to flue gas desulfurization wastewater with a pH of 1.6, react for 30 minutes, leave to stand for 10 minutes, solid-liquid separation, add Ca(OH) ₂ to the precipitate, and stir for 20 minutes. After mixing, it was added to the waste water to adjust the pH to 6.4, and was stirred for 30 minutes to react. The amount of precipitate added per raw water was 200 ml. Then, 1 mg/anionic polymer flocculant was added to the above reaction solution to perform solid-liquid separation, and the separated water was filtered and the quality of the filtrate obtained was analyzed, and the filtrate was left to stand for 3 days. The state of scale precipitation was observed (No. 1). As a comparative example, when no precipitate was added (No.
2) and precipitates were added, but Ca(OH) ₂ was added directly to the raw water without mixing with the precipitates, and
A similar test was conducted when Ca(OH) ₂ was also added directly to raw water (No. 3). The results are shown in Table 1.

[Table] From the results in Table 1, it can be seen that in the case of the present invention (No. 1), there was no scale precipitation and the removal rate of fluoride ions was superior compared to the comparative example. In addition, in method No. 3, in which the precipitate is added as it is, if the precipitate is used for a reaction time of 3 hours or more, the result is No. 1.
The degree of scale precipitation is the same as that of F=60
mg/.

[Brief explanation of drawings]

The drawing is a system diagram showing one embodiment of the present invention, in which 1 is a dissolution tank, 2 is a first reaction tank, 3 and 5 are solid-liquid separation tanks, 4 is a second reaction tank, and 10 is an aging tank.

Claims (1)

[Scope of Claims] 1. In a method of reacting raw water containing fluoride ions and sulfate ions with a calcium compound to form a precipitate and separating the solid-liquid, the calcium compound is mixed with the precipitate separated from the solid-liquid. After that,
A method for treating water containing fluoride and sulfate ions, which comprises reacting with raw water. 2. The method for treating water containing fluoride and sulfate ions according to claim 1, wherein the calcium compound is mixed with the precipitate in the aging tank. 3. The method for treating water containing fluoride and sulfate ions according to claim 1 or 2, wherein the calcium compound and the precipitate are added to the raw water together with a PH regulator after mixing.