JPH0126755B2 - - Google Patents

Description

[Detailed description of the invention]

This invention relates to a method for treating wastewater containing fluoride ions and sulfate ions, particularly for removing fluoride ions and sulfate ions as precipitates without causing scale damage. This invention relates to a method for treating denitrified wastewater. As a method for treating water containing fluoride ions, there is a known treatment method in which calcium ions are added in an amount equivalent to about twice the amount of fluoride ions to form a precipitate and then removed. The fluoride ion concentration of the treated water was high, and it could not be said to be a perfect treatment method. Moreover, when sulfate ions coexist in addition to fluoride ions in the raw water, the calcium compounds added to precipitate the fluoride ions react with the sulfate ions in the raw water, and some of them become gypsum. Although it precipitates, the remainder becomes scale and deposits on the reaction tank, precipitation tank, or pipe wall. When gypsum scale precipitates, it becomes impossible to operate, so operations are sometimes interrupted and the scale is removed manually, but gypsum scale cannot be removed sufficiently even with acid, and is considered an extremely troublesome problem. was. The present invention aims to improve upon such conventional methods and provide a method for efficiently removing fluoride ions from raw water without causing scale damage. That is, this invention includes a step of adding a calcium compound to flue gas desulfurization or denitrification wastewater containing fluoride ions and sulfate ions to obtain a suspension, and adding the suspension of the above step to treated water and calcium sulfate and calcium fluoride. The present invention relates to a water treatment method comprising a step of separating the precipitate and a precipitate as a main component, further comprising a step of returning the precipitate to the step of adding the calcium compound. be. The present invention will be explained below with reference to embodiments of the drawings.
The drawings are based on the patent application filed in 1983 by the inventors.
It is a system diagram showing a preferred embodiment of this invention applied to the invention of No. 104241, in which 1 is a dissolution tank, 2 is a first reaction tank, 3 is a precipitation tank, 4 is a second reaction tank, and 5 is a final precipitation tank. be. First, raw water is introduced into the dissolution tank 1 from the raw water pipe 6,
Further, the precipitate from the final precipitation tank 5 is introduced from the return pipe 7, and if necessary, acid is added from the chemical injection pipe 9 to adjust the pH to 4 or less, thereby dissolving the returned precipitate. 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 precipitate 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 precipitate, 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. Next, in the process of the present invention, the outflow water from the dissolution tank 1 and the precipitate from the settling tank 3, which is returned via the return pipe 8, are introduced into the first reaction tank 2, and the water is heated to pH 5 in the presence of calcium ions. ~8.5, preferably 6-7, and further generate a precipitate. In this case, aluminum ions may be present in addition to calcium ions. Calcium ions that originally exist in the raw water and those eluted from the returned precipitate are involved in the reaction, but if there is a shortage, calcium salts are added from the chemical injection tube 10.
The calcium salt added from the drug injection tube 10 is as follows:
Examples include calcium chloride, calcium carbonate, calcium hydroxide, and the like. The required amount of calcium ions is about 1 to 3 times the equivalent of Ca as CaF ₂ , and preferably about 2 times the equivalent of CaF 2 . However, since flue gas desulfurization and/or denitrification wastewater contains a large amount of aluminum ions, it is possible to reduce the amount of calcium ions added.In addition, the amount required varies depending on the salt concentration of the wastewater, etc.
The amount of calcium ions added can be easily confirmed experimentally. Adjustment of PH is performed by injecting a PH adjusting agent from the drug injection pipe 11 if necessary. As the pH adjuster, sodium hydroxide, sodium carbonate, calcium hydroxide, etc. can be used, and among these, calcium hydroxide is preferred because it can also be used as a calcium ion source. A precipitate is generated by adding such a pH adjuster to adjust the pH to the above range. The precipitate is mainly composed of _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 step is the range in which the solubility of the precipitated products of CaF ₂ and Al(OH) ₃ is low and the precipitation of magnesium hydroxide to be returned in the process described later is small, that is, the PH range.
5 to 8.5, particularly preferably PH6 to 7. The reaction liquid in the first reaction tank 2 undergoes solid-liquid separation in the precipitation tank 3, and a part of the precipitate is passed through the return pipe 8.
The sludge is returned to the first reaction tank 2 where the calcium compound is added, and the remainder is discharged outside the system via the sludge pipe 12. On the other hand, the supernatant liquid is allowed to flow out into the second reaction tank 4. Part of the precipitate discharged from the settling tank is returned to the first reaction tank because calcium sulfate, which is produced at the same time as calcium fluoride, does not partially precipitate, but instead is deposited on the reaction tank or pipe walls. This is to prevent it from precipitating as scale. In other words, when a calcium compound is added to raw water in the presence of precipitate, CaSO ₄ in the precipitate becomes a crystal nucleus, causing gypsum to crystallize in the reaction tank, causing gypsum to scale. The purpose is to form a precipitate with good sedimentation properties. Since calcium fluoride is also included in the precipitate, it becomes the core of calcium fluoride crystals, reducing the amount of fine particles of calcium fluoride that have extremely poor sedimentation properties, and reducing the amount of calcium fluoride that is formed between gypsum and calcium fluoride. A coprecipitation phenomenon was also observed, which greatly improved the fluoride ion removal effect. Furthermore, the use of precipitates has the advantage that undissolved calcium in the precipitates can be effectively used as a fixing agent for sulfate ions and fluoride ions. It should be noted that if by-product gypsum or the like is used instead of the precipitate, it will not be effective against ions other than sulfate ions, which is not preferable. The treated water, from which sulfate ions and fluoride ions have been removed as much as possible, is then sent to 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 generate a precipitate.
Further reduces residual fluoride ions in water.
Note that this step also removes residual calcium ions in the water and also serves as a dehardening treatment step. In the second reaction tank 4, a chemical injection pipe 13 is used as necessary.
Magnesium salt is added from the tank, carbonate is added from the chemical injection pipe 14, and PH adjuster is added from the chemical injection pipe 15. 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. Carbonates include sodium carbonate,
Sodium bicarbonate or the like can be used, but carbon dioxide gas may also be blown in. 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. The amount of magnesium ions present in the reaction solution is 20% by weight relative to fluoride ions.
If the amount is more than doubled, the amount of residual fluoride ions will be 1mg/
It can be less than l. Further, the amount of carbonate ions is about 1/2 equivalent or more relative to calcium ions. By adjusting the pH to the above value, precipitates of Mg(OH) ₂ 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 is subjected to solid-liquid separation in the final settling tank 5, and the supernatant water is discharged as treated water to the outside of the system from the treated water pipe 16, 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, the precipitate discharged from the final settling tank 5 also contains metal hydroxides such as manganese and iron, so when returning the precipitate, it is necessary to ensure that these are removed in the calcium compound addition step. It is preferable to aerate to remove. 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 in the calcium compound addition step is such that the calcium ions and aluminum ions in the raw water and the calcium ions eluted from the precipitate replenish the insufficient water. Just that is enough. Further, the eluted magnesium ions flow directly to the second reaction tank and are recycled and used.
Therefore, the amount of magnesium ions added in the second reaction tank only needs to be the amount corresponding to the magnesium precipitate discharged from the precipitation tank 3;
When adjusting the pH to 7 or lower, magnesium ions hardly precipitate, 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 to an appropriate amount, the magnesium necessary for removing fluoride ions will be circulated within the system, resulting in excess will be discharged into the treated water. Moreover, if it is desired 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 second reaction tank and remove the excess magnesium precipitate in the final precipitation tank. As described above, according to a preferred embodiment of the present invention, flue gas desulfurization or denitrification wastewater containing fluoride ions and sulfate ions is reacted with calcium ions and precipitated and separated without causing scale damage, and then magnesium hydroxide is Since precipitates of calcium carbonate and calcium carbonate are generated and returned, fluoride ions can be efficiently and highly removed, and the fluoride ion concentration in the resulting treated water is low. Furthermore, 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, making it possible to reduce the amount of calcium salt added in the first reaction tank. . 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. In addition, since the calcium ions added in the second reaction tank are recycled and used, only the amount lost needs to be replenished, and the alkaline agent used to precipitate the magnesium ions is also returned to the first reaction tank in the form of hydroxide. It is returned and used effectively as a neutralizing agent for acidic raw water. Furthermore, the only precipitate that is subject to sludge treatment is the precipitate from sedimentation layer 3, which reduces the amount of sludge that must be treated, and also simplifies treatment because the amount of magnesium hydroxide, which is difficult to dewater, is small. . In addition, since the pH in the dissolution tank is adjusted to 4 or less, no scale is generated and the processing effect is improved. Next, Examples and Comparative Examples will be described. Example F508mg/l, SO ₄ 4400mg/l, Ca653mg/l,
As shown in the table, Ca(OH) ₂ and precipitate were added to flue gas desulfurization wastewater with a pH of 1.6, stirred for 45 minutes to react, and then 1 ppm of anionic polymer flocculant was added and precipitated. . The quality of the supernatant water obtained by filtering the supernatant water was analyzed, and the degree of scale precipitation was observed after the supernatant water was allowed to stand for 3 days. The results are shown in the table. The precipitate added was the precipitate obtained after the suspension in case No. 2 was allowed to stand for 30 minutes.

[Table] As is clear from this table, by returning the precipitate, the supersaturation degree of calcium sulfate is broken,
Almost no scale precipitates even if the treated water is left for 3 days. It is also clear that returning the precipitate is more effective than adding the gypsum itself.

[Brief explanation of drawings]

The drawing is a system diagram showing a preferred embodiment of the present invention, in which 1 is a dissolution tank, 2 is a first reaction tank, 3 is a precipitation tank, 4 is a second reaction tank, 5 is a final precipitation tank, 7, 8
is the sediment return pipe.

Claims (1)

[Claims] 1. A step of adding a calcium compound to flue gas desulfurization or denitrification wastewater containing fluoride ions and sulfate ions to obtain a suspension, and using the suspension of the above step as treated water, which mainly contains calcium sulfate and calcium fluoride. A method for treating water comprising a step of separating the precipitate into a precipitate, further comprising a step of returning the precipitate to the step of adding the calcium compound.