JPS5854634B2 - Treatment method for wastewater containing heavy metals - Google Patents

Description

[Detailed Description of the Invention] The present invention relates to a method for treating wastewater containing heavy metals.
This invention relates to a method for treating wastewater containing heavy metals, in which sulfate and slaked lime are added to wastewater containing dissolved heavy metal ions to produce sludge, and after drying, water is added to solidify the produced solids.

Conventionally, various means have been proposed and put into practical use as methods for treating wastewater containing heavy metal ions such as zinc, chromium, copper, cadmium, and others.

Among these, wastewater treatment by adding coagulants has become widely adopted.

Furthermore, recently, a wastewater treatment method has been proposed in which heavy metal ions in wastewater are made into ferrites, insolubilized, and removed.

In this method, ferrous salt is added to heavy metal-containing wastewater, and then caustic soda is added to adjust it to near neutrality. A precipitate with poor settling properties (ferrous hydroxide) is produced, and heavy metal ions are removed by this iron hydroxide. When the ferrous hydroxide is co-precipitated and air is blown into it, the ferrous hydroxide is oxidized and becomes ferric hydroxide, which will settle if left to stand, so the supernatant liquid after separation will contain no heavy metals. There is no such thing.

However, in all of these wastewater treatment methods, emphasis was placed only on the quality of the treated water, and little attention was paid to the handling of the produced sludge containing heavy metals. The treatment of generated sludge containing large amounts of these harmful substances has become a problem.

In other words, if such sludge is piled up in the open or buried in the ground, there is a strong possibility that environmental re-contamination will occur due to scattering or runoff due to wind and rain.

Therefore, at present, when using the conventional wastewater treatment method described above, a solidifying agent is added to the generated sludge to solidify it to prevent environmental recontamination.

However, in solidification treatment using such a solidification agent, (1) the volume of the treated product increases, and (2) it takes several days to obtain sufficient strength.

(3) The ferriticization method uses caustic soda as a neutralizing agent, which has drawbacks such as increased wastewater treatment costs.

In addition, the ferriticization method has the disadvantage that a large amount of salts flows out into the treated water because the S)-IJium salts produced are water-soluble.

The purpose of the present invention is to remove heavy metal ions contained in wastewater to obtain clean treated water, and at the same time, to convert sludge containing harmful substances into self-hardening solids that are easy to handle and safely dispose of. An object of the present invention is to provide a method for treating wastewater containing heavy metals.

That is, in the present invention, 0.02 mol/j? of sulfate is added to heavy metal-containing wastewater as a sulfate group. After adding the above and further adding slaked lime to carry out a neutralization reaction to precipitate the heavy metal ions in the wastewater as a co-precipitation product of hydroxide,
This method for treating wastewater containing heavy metals is characterized in that the obtained sludge is dried at 110 to 163° C., and then water is added thereto and kneaded to promote solidification of the produced solids.

Heavy metal-containing wastewater to be treated in the method of the present invention includes wastewater from various factories, schools, and public facilities, and the heavy metal ions contained in this include zinc, chromium, copper, cadmium,
Examples include ions such as mercury, tin, lead, arsenic, and cobalt.

Note that the number of heavy metal ions contained in the wastewater is not limited to one type, but may be two or more types, or they may be present in the form of a complex.

Moreover, the content of heavy metal ions does not pose a particular problem.

In the present invention, after sulfate is added to such heavy metal-containing wastewater, slaked lime is added to cause a neutralization reaction, and solids containing harmful substances (heavy metals, other suspended substances, etc.) are combined with gypsum (CaSO4).・2H20) is obtained.

Since the gypsum produced here has low solubility in water, most of it precipitates.

In other words, in the coagulation-sedimentation method, sulfate (
(e.g. aluminum sulfate, ferrous sulfate, zinc sulfate, etc.)
When is added as a sulfate group at a concentration of 0.02 mo I /1 or more, heavy metals in wastewater will coagulate without losing ions, and some of the flocculants will be aluminum,
Hydroxides (gel-like precipitated solids) of iron, zinc, etc. are formed, and these come to contain the above-mentioned aggregates (turbid substances).

Then, when slaked lime is added in such a way that the sulfate solution is neutralized, i.e. contains an equal amount of hydroxyl groups to the remaining sulfate groups, the remaining sulfates and slaked lime react with each other.
Hydrogypsum is produced.

On the other hand, in the ferritization method, ferrous sulfate is added as a sulfate radical at a concentration of 0.02 mol/1 or more, and when air is blown into this, the ferrous hydroxide is oxidized and becomes water. It becomes a ferric oxide salt, and the heavy metal ions in the wastewater are co-precipitated more quickly by the ferric hydroxide.

When slaked lime is then added, the heavy metal ions become ferritic and insolubilized, and at the same time, the remaining sulfate and slaked lime react to produce dihydrate gypsum.

In addition, pH is an important factor in determining the quality of flocculation in the coagulation-sedimentation method, and is also important in the ferriticization method, and the operation is preferably carried out in the pH range of 6 to 8, preferably near neutrality. desirable.

The amount of flocculant (ferrous sulfate in the ferriticization method) added is as follows:
0.02 mo f as sulfate root, 41? If the amount is less than this, it will not be possible to obtain a solid material with sufficient strength.

When the series of reactions described above is completed, the sludge containing harmful substances is separated as a precipitate.

The supernatant liquid after separation does not contain any heavy metals.

The separated sludge is then dehydrated and heated to 110-163°C.
Gypsum (CaS) in the sludge is dried by
04・2H20) is calcined gypsum (Ca S 04 ・% H
20), and it becomes a solid material with self-hardening and quick-hardening properties.

When dried at temperatures below 110°C or above 163°C, it is not possible to obtain a solid that achieves the intended purpose.

Next, when water is added to this and kneaded, it becomes a strong solidified body within a few minutes.

The drawing shows an example of the implementation of the method of the present invention in the form of a flow sheet, in which 1 is a neutralization reactor, 2 is a solid-liquid separator, and 3 is a flow sheet.
4 indicates a drying device, and 4 indicates a kneading device.

Heavy metal ion-containing wastewater is introduced into the neutralization reactor 1, and sulfate (ferrous sulfate in the ferritization method) and slaked lime are added thereto.

The order of addition of sulfate and slaked lime requires that sulfate be added first.

In the neutralization reactor 1, taking the case where iron sulfate is used as an example, the following reaction occurs, and dihydrate gypsum and hydroxide
Iron FeSO4+Ca(OH)2→Fe(OH)2+CaS
O4.2I-(20) are formed and these precipitate out in such a way that the gypsum contains ferrous hydroxide, which has poor settling properties.

In addition, heavy metal ions aggregate without losing ions and are also included in gypsum.

In the ferritization method, air is introduced to form Fe(OH)2
is converted into F e (OH) s, and the sedimentation property is improved.

Note that the above reaction is carried out near neutrality.

These are then transferred to the solid-liquid separator 2, and the supernatant liquid is discharged as treated water.

After dehydration, the sludge containing harmful substances is transferred to the drying device 3 and dried, and the dihydrate gypsum is converted into hemihydrate gypsum (CaSO4.3A
H20).

Next, this self-hardening, fast-hardening solid material is kneaded with water in a kneading device 4, and taken out as a solidified material.

Although the above example is suitable for batch operation, the addition of the flocculant and the neutralization reaction may be carried out in separate tanks for continuous operation.

The solidified material extracted in this way is obtained by adding water to the solid material and hardening in about a few minutes, so it does not require a long curing period as in the conventional method.

Furthermore, there is no risk of re-contamination of the environment due to re-elution of harmful substances, scattering by wind and rain, or spillage.

In particular, if the method of the present invention is applied to the treatment of wastewater containing heavy metal ions using the ferriticization method, it is possible not only to prevent large amounts of salts from flowing into the treated water, but also to dump the recovered material without reusing it. When disposing of it, the cost is lower than neutralization with caustic soda.

The hardened material can also be effectively used as building materials.

Next, examples will be shown.

Example l Zn2+, Cr", Cu2+, Cd2+, H
Add sulfuric acid to the wastewater containing 20.0 pl of each
The sludge obtained by adding iron, neutralizing with slaked lime, and air oxidation was dehydrated and dried at 150°C. The solid material obtained was subjected to a re-elution test (
The above sludge 209 was added to 20 oml of pH-adjusted water (PH6, 0), shaken for 6 hours, and then filtered using filter paper 5C.

The results are shown in Table 1, and it was extremely stable.

Example 2 The sludge obtained by adding ferrous sulfate to the same wastewater used in Example 1 and neutralizing it with slaked lime was dried at 150°C after dehydration, and then water was added and kneaded to obtain a sludge. The results of measuring the crushing strength of the assimilated product after 30 minutes are shown in Table 2.

As can be seen from Table 2, the sulfate group is 0.02 mo 17
If it is 73 or higher, a strength suitable for disposal can be obtained in a short period of time, so there is almost no need for a curing period until hardening.

[Brief explanation of drawings]

The drawing is a flow sheet showing an example of implementing the method of the present invention. 1... Neutralization reaction device, 2... Solid-liquid separation device, 3... Drying device, 4... Kneading device.

Claims (1)

[Claims] 1 0.02m of sulfate as a sulfate group in wastewater containing heavy metals
ol/J or more and further add slaked lime to perform a neutralization reaction to precipitate the heavy metal ions in the wastewater as a coprecipitated product of hydroxide. 1. A method for treating wastewater containing heavy metals, which comprises drying the wastewater at a temperature of 0.degree.