JPS6055197B2 - Wastewater treatment method - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for treating wastewater containing mercury and fluoride ions.

For example, in garbage incineration plants, combustion gas is washed with an alkaline absorption liquid, but as a result, smoke washing wastewater is generated that contains organic substances, fluoride ions, and large amounts of various heavy metals, especially mercury.

Conventionally, when treating such wastewater, components to be removed, particularly mercury and fluoride ions, have been treated using separate methods. In other words, mercury has been treated using a combination of an alkaline coagulation-precipitation method, a sulfide coagulation-precipitation method in which sulfide is added to produce mercury sulfide, a poorly soluble compound, and activated carbon, ion exchange resins, and chelate resins. However, with such treatment methods, the removal rate was not always satisfactory. On the other hand, for fluoride ions, there are a precipitation method in which calcium salts are added to produce calcium fluoride, and an aluminum salt is added to form aluminum hydroxide flocs, and the flocs adsorb fluoride ions. was being processed by. Therefore, if these known methods are used to treat wastewater containing mercury and fluoride ions, multiple coagulation-sedimentation steps will be required, the entire process will be long, and the equipment and plant installation required for the treatment will be expensive. The disadvantages are that the area increases and that a considerable level of operating skill is required to maintain optimal operating conditions in each process.

This invention was developed to overcome the shortcomings of these conventional methods, and by simultaneously removing fluoride ions and mercury using a specific method, it is possible to greatly shorten the processing steps and processing time. This paper proposes a method that can significantly improve treated water quality through synergistic effects.

This invention involves adding a water-soluble salt of an aluminum compound, a water-soluble salt of a ferric compound, and a heavy metal fixative to wastewater containing mercury and fluoride ions, and then adjusting the pH to around neutrality. This is a wastewater treatment method characterized by subsequent solid-liquid separation.

The wastewater containing mercury and fluoride ions that is the subject of this invention includes, in addition to the above-mentioned wastewater from garbage incineration plants, wastewater from the manufacturing process of fluorescent lamps and cathode ray tubes, and wastewater from testing laboratories and research institutes. Examples of the aluminum compound to be added to such wastewater include known aluminum compound flocculants such as aluminum sulfate and polyaluminum chloride.

Further, as the ferric compound, ferric chloride and ferric sulfate can be used. Furthermore, in recent years, flocculants containing an aluminum compound and a ferric compound have been commercially available, and using such a flocculant is preferable because the aluminum compound and the ferric salt can be added at the same time. As the heavy metal fixing agent, those containing S2- ions such as sodium sulfide and sodium hydrogen sulfide, or commercially available selective heavy metal fixing agents can be used.

Selective heavy metal fixatives usually have chelating ability and form chelates with nitrogen atoms and sulfur atoms, such as thiourea type and dithiocarbamate type, and those that form chelates with nitrogen atoms and sulfur atoms, such as dithioate type and thiol group type. There are those that form a chelate with one sulfur atom and those that form a chelate with a sulfur atom and an oxygen atom, such as the thiol acid type, but those that are liquid at room temperature and soluble in water are preferred. The amounts of aluminum compounds, ferric compounds, and heavy metal fixatives to be added vary depending on the mercury and fluoride ion concentrations contained in the wastewater to be treated, as well as the salt concentration. You can decide.

For example, mercury has a number M9/e1 fluoride ion 407
Y! In order to treat wastewater containing about 9/e to below the emission standard values for mercury and fluoride ions, aluminum compounds must have a concentration of 250 to 300 as A]203.
About m9/e, 50 as FeCl in ferric compounds
-500 m9/', and for heavy metal fixatives, the addition of S is about 5 to 50 m9/e. In addition, if the pH of the water to be treated exceeds 5.5 after adding the aluminum compound and ferric compound, increase the amount of ferric salt added or add acid to increase the pH to 5.5. If the following adjustments are made, as will be clear from the examples below,
This is preferable because the processing effect is further improved. Note that the pH of the water to be treated after adding the aluminum compound, ferric compound, and heavy metal fixing agent may be adjusted to 5.5 or less, but since the heavy metal fixing agent is generally an alkaline substance, If the heavy metal fixing agent is in a small amount, it may be added at the same time or in any order so that the pH becomes 5.5 or less. However, if the heavy metal fixing agent is in a large amount, the aluminum compound and the second (PHf) by adding iron compounds
It is preferable to add the heavy metal fixative after adjusting the temperature to 5 or less. After adding these chemicals to wastewater, P
Adjust H to around neutrality, preferably 6 to 8.

A known acid or alkaline solution can be used to adjust the pH. If this pH adjustment exceeds 8 or falls below 6,
As time goes by, the coagulation-sedimentation effect tends to worsen, and even after solid-liquid separation, mercury and fluoride ions remain in the supernatant water, and the aluminum flocs that have precipitated once again start to dissolve. , undesirable.
After adding a pH adjuster to the wastewater to adjust the pH to a predetermined value, if stirring is continued, a precipitate will form.

At this time, it is preferable to add a polymer flocculant to speed up the formation of the precipitate and to increase its particle size. Any commercially available polymer flocculant can be used, but polyacrylamide or its hydrolyzate is particularly preferred.

The treated water containing sediment is then separated into supernatant water and sediment by solid-liquid separation.

As the solid-liquid separation means, any means such as a settling tank can be used. Furthermore, a type in which a coagulation tank and a sedimentation tank are integrated, such as a coagulation-sedimentation tank, can also be used. The supernatant water separated in this way contains almost no mercury or fluoride ions, and can be discharged as treated water as is. However, since it usually contains fine suspended matter, it is discharged after filtration. It is preferable to do so.

On the other hand, the separated precipitate is further concentrated and dehydrated using a dehydrator before being discharged.

The dehydrator is not particularly limited, and any known dehydrator such as a vacuum dehydrator, pressure dehydrator, press dehydrator, centrifugal dehydrator, etc. can be used.

In this invention, wastewater containing fluoride ions and mercury is
By adding an aluminum compound, a ferric compound, and a heavy metal fixative to the same tank, the pH of the water to be treated drops extremely rapidly, and some aluminum hydroxide and
Iron compounds create hydroxide flocs, into which fluoride ions and mercury in water are effectively adsorbed.

Further, some fluoride ions combine with aluminum to form aluminum fluoride and form a precipitate. Although there is some mercury remaining in water without being adsorbed, ionization becomes more likely to occur due to the rapid decrease in pH, and as a result, it effectively reacts with the heavy metal fixative. Furthermore, as the pH of the liquid decreases, the alkalinity components, mainly bicarbonate ions, that were present in the wastewater volatilize, resulting in the effect of increasing the reactivity of mercury and heavy metal fixatives. can get. As detailed above, this invention not only synergistically removes fluoride ions and mercury by adding an aluminum compound, a ferric compound, and a heavy metal fixative in the same tank;
Instead of multiple reaction tanks that were conventionally required, only one reaction tank is required, which simplifies the equipment and reduces the site area.

It also provides many benefits, such as easier driving. Example Waste incineration plant wastewater (water quality: PH8.6, Hf2.8m9/
', F36.4m9/F. NaCl3%, Na2SO4
O. 4%, Fel3.5m9/E. ．． ZnO. 85m9
/E. ．． PbO. 85m9/e), the sulfate band was added to Al
2O3 and ferric chloride as FeCl3 were added in the amounts shown in Table 1, and the pH at that time was measured.

In addition, case No. 1 in Table 1 below. The pH of 4 is the value obtained by further adding sulfuric acid. Thereafter, 5 m9/' of sodium sulfide as a heavy metal fixing agent was added as S, and the mixture was stirred. Next, sodium hydroxide was added to adjust the pH to 7.8, and after stirring for another minute, 2 m9/' of polymer flocculant was added.
Table 1 shows the water quality of the permeate water, which was filtered from the supernatant water after coagulation and precipitation. Table 1 shows that the method of this invention can effectively remove mercury and fluoride ions. In particular, aluminum compounds are
250 m9/e or more as O3, ferric chloride as FeCl
If 50m9/e or more is added as 3, this method alone can meet the emission standards (mercury 0.0057n9/11 fluoride 15m9 without further treatment with chelate resin).
/') can be satisfied. . Furthermore, even if the amount of aluminum compound or ferric chloride added is less than the above-mentioned amount, if the pH after addition is 5.5 or less, a much better effect can be obtained compared to the conventional example. It will be done.
In addition, the above case No. 2, AL. Case NO. except that 10 m9/e of M-648 (product of Nippon Soda Co., Ltd., aliphatic polydiocarbamic acid chelating agent) was added. As a result of treatment using the same procedure as in 2, the quality of the treated water was 0.003 mercury.
0m9/e1 fluoride ion 10.2m9/e,
It showed almost the same effect.

Comparative Example (1) To the same wastewater as in Example, 300 m9/e of ferric chloride was added as FeCl3 (PH 6.8), then 5 mg/e of sodium sulfide was added as S, and the pH was adjusted to 7.0 with sodium hydroxide. 9 and stirred once.

Thereafter, 2 WLg/e of the same polymer flocculant as in Example was added to flocculate and precipitate. When a part of the supernatant water was filtered and the water quality was analyzed, the amount of mercury was 0.0090 mg/e and the amount of fluoride ions was 35.0 m9/e. In addition, when the amount of ferric chloride added was 550 m9/e, mercury was 0.0
065 m9/' and fluoride ion was 32.5 m9/e. (2) In the above (1), Al sulfate is used instead of ferric chloride. Addition of 250 m9/e as O3 (P
H6. O) The mercury in the filtrated supernatant water is 0.0
15m9/e1 fluoride ion was 10.0m9/f. Also, when 550 m9/f of sulfuric acid band was added, mercury 0.0058 mg/e1 fluoride ion was 5.
It was 2m9/'. (3) Next, in (1) above, the supernatant water obtained by adding 300 m9/e of ferric chloride (mercury 0.0090 mg/e, fluoride ion 35.0
m9/e) with sulfuric acid band as Al2O3 and 1500m
9/e was added, the pH was adjusted to 7.0 with sodium hydroxide, and after stirring again for about 10 minutes, the same amount of the above-mentioned polymer flocculant was added to coagulate and precipitate.

When the supernatant water was filtered and water quality analyzed, fluoride ions were found to be IO. Om decreased to 9/e, but mercury was 0
．． There was no change at 0,090 m9/', and mercury exceeded the emission standard value. As is clear from the above comparative examples (1) and (2), case NO. Even when the same amount of chloride sulfate and ferric chloride were used together with a heavy metal fixative, the mercury could not be lowered below the emission standard value.

Also, this case NO. Even when the total amount of sulfuric acid band and ferric chloride was increased to 550Tn9/e and a heavy metal fixative was used together, the mercury emission standard value could not be satisfied. Furthermore, in Comparative Example (3), even if a large amount of sulfuric acid was added later, the mercury still exceeded the emission standard value.

Claims (1)

[Claims] 1. After adding a water-soluble salt of an aluminum compound, a water-soluble salt of a ferric compound, and a heavy metal fixative to wastewater containing mercury and fluoride ions, the pH is adjusted to around neutrality. A method for treating wastewater, the method comprising: solid-liquid separation; 2. After adding water-soluble salts of aluminum compounds and water-soluble salts of ferric compounds to wastewater containing mercury and fluoride ions, the pH was once adjusted to 5.5 or less, and then a heavy metal fixative was added. A wastewater treatment method characterized by adjusting the pH to around neutrality again.