JPH03224691A - Treatment method for chromium-containing wastewater - Google Patents

Abstract

PURPOSE:To perform reduction and neutralization in the same tank by adjusting the pH of waste water contg. Cr<6+> to >=9 and adding ferrous ions so that the oxidation reduction potential is regulated to <=-200mv. CONSTITUTION:When ferrous ions are added to waste water contg. Cr<6+> to reduce the Cr<6+> to Cr<3+>, the pH of the waste water is adjusted to >=9 and the ferrous ions are added so that the oxidation-reduction potential is regulated to <=-200mV.

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Application Field] The present invention relates to a method for treating chromium-containing wastewater, and particularly to an improvement in a method for reducing hexavalent chromium-containing wastewater by adding ferrous ions thereto.

[Prior Art] As a method for removing hexavalent chromium from wastewater containing hexavalent chromium, a method has been used in which hexavalent chromium is reduced to trivalent chromium and then precipitated and removed as an insoluble compound. There are a method of using ferrous salt and a method of using sulfite as a reducing agent for hexavalent chromium. Among these methods, the method using ferrous salts has the disadvantage that it generates a relatively large amount of sludge, but has the advantage that ferrous salts are inexpensive and can be reduced in any pH range from acidic to alkaline. This is an industrially advantageous method. Specifically, in Japanese Patent Publication No. 82-30838, wastewater containing hexavalent chromium is treated with an oxidation-reduction potential (r
It is abbreviated as ORPJ. ) A method is disclosed in which a ferrous salt is added to provide a voltage of about -50 to -150 mV.

[Problems to be Solved by the Invention] However, in the past, reduction treatment using ferrous salts in an alkaline range has not been performed, but reduction has been carried out in an acidic to neutral range. For this reason, in the past, after reduction treatment, the pH was adjusted and precipitation treatment (neutralized IA It is necessary to do so. Therefore, the two steps are reduction treatment and neutralization treatment.
This method has drawbacks such as requiring stage processing and complicating the equipment. The reason for this is that it was thought that chemical injection control using an ORP meter would not be possible unless the chemical was acidic to neutral.

That is, the concept of chemical injection control using an ORP meter is whether to keep the atmosphere of the solution oxidizing or reducing. For example, in the oxidative decomposition of cyanide, the potential of chlorine, which is an oxidizing agent, is detected to control the injection of sodium hypochlorite. In the sulfite reduction method of hexavalent chromium, the potential of sulfite (Cr” (500 to 7
00mV)-bNaH5C1+ +Cr”(200~3
00mV)) to control chemical injection.

On the other hand, when ferrous ions are used, the difference between the potential before reduction and the potential after reduction is small (Cr'10 vs. Cr"+
It was thought that ORP control would be difficult to apply unless the material was neutral or acidic. Therefore, after reduction,
It is necessary to adjust the pH to alkaline and perform precipitation treatment.

By the way, the solubility of ferrous hydroxide is determined from Chemistry Handbook Basic Edition II.
(1981: Maruzen), KSp = 3X10-8, so 45 ppm at pH 8, 4.5 pp at pH 8.5
m, 0.45 ppm at pH 9. When ferrous ions are used as a reducing agent, since the ferrous ions are precipitated as hydroxides, sedimentation separation has conventionally been carried out at a pH of 9 to 10, and this value coincides with the theoretical pH.

In addition, in Japanese Patent Publication No. 62-30838, pH 4.5-8
．． 5, but as mentioned above, precipitation of ferrous ions requires a pH of 9 or higher, and pH 8.5 is necessary for precipitation of ferrous ions.
However, it is not possible to perform sufficient precipitation treatment, and there is a problem that ferrous ions and other coexisting heavy metal ions remain in the treated water. Further, at pH 7 or lower, it is difficult to precipitate reduced chromium ions. By the way, Tokuko Sho 6
No. 2-30838 lists an example treated at pH 11 as a comparative example, but when the amount of ferrous salt added is 0RP-
At 100 mV, the amount necessary for reduction cannot be secured, so good results have not been obtained.

The present invention solves the above conventional problems, and in the alkaline region,
An object of the present invention is to provide a method for treating chromium-containing wastewater that can easily and efficiently reduce hexavalent chromium-containing wastewater and obtain high-quality treated water.

[Means for Solving the Problems] The method for treating wastewater containing chromium of the present invention adds ferrous ions to wastewater containing hexavalent chromium to remove hexavalent chromium.
In the method of reducing treatment to valent chromium, the pH of the wastewater is
is adjusted to 9 or more, and ferrous ions are added so that the redox potential becomes 1200 mV or less.

That is, the present inventors believe that if it is possible to control chemical injection at pH 9 to 10, which is necessary for the precipitation of ferrous ions mentioned above, reduction treatment and precipitation can be performed. ! ! (neutralization) treatment can be performed at the same time, simplifying the treatment equipment, and as a result of intensive studies on chemical dosing control of ferrous ions using an ORP meter in such an alkaline region, we found that The present invention was completed based on the discovery that a much more acute ORP inflection point can be obtained in alkaline conditions and that chemical injection of ferrous ions can be controlled using an ORP meter.

The present invention will be described in detail below with reference to the drawings.

FIG. 1 is a system diagram showing one implementation method of the chromium-containing wastewater treatment method of the present invention.

In the method of the present invention, raw water is first mixed with the stirrer 11, p.
The mixture is introduced into a reduction neutralization tank 1 equipped with an H meter 12 and an ORP meter 13, and a ferrous salt, a pH adjuster, a flocculant (polymer), etc. are added as necessary, and the pH is adjusted to 9 or more, preferably 9 to 10. ,
ORP -200mV or less, preferably -200 to -35
Adjust the voltage to 0 mV and react.

That is, near the equivalence point, ORP changes sharply, and this ORP
P is -200mV or less, preferably -200 to -350
Add ferrous salt to give mV. Furthermore, this OR
Since the change in P reaches equilibrium in 20 to 30 minutes after addition of the ivy-iron salt, at least 20 minutes are required for the reaction.

In the reduction-neutralization tank 1, since reduction is carried out in an alkaline region with a pH of 9 or more, trivalent chromium, ferrous ions, ferric ions, and other coexisting heavy metal ions precipitate as hydroxides, so they must be separately neutralized. The reduced treated water is then introduced into the precipitation tank 2 without any operation, that is, without passing through a neutralization tank, and precipitates of various metal ions are removed to obtain treated water.

Note that the method of the present invention can be carried out in either continuous processing or batch processing, and the amount of ferrous salt, p)l regulator, and flocculant to be added is determined based on the detected values of the pH meter and ORP meter. It can be carried out automatically using a controlling device.

The type of ferrous salt used in the method of the present invention is not particularly limited; for example, ferrous sulfate is the most common, but in addition,
Ferrous chloride, ferrous ammonium sulfate, ferrous nitrate, ferrous hydroxide, etc. can be used. In addition to these pure solutions, general wastewater containing these ferrous salts, such as pickling wastewater from the iron industry, iron ore wastewater from the steel industry, etc. can also be used.

As the pH adjuster, an alkali such as caustic soda, slaked lime, or soda ash can be used, and as the flocculant, various organic polymers can be used.

[Effect] As a result of a detailed study of the ORP characteristics of ferrous salts, the present inventors found that, contrary to conventional understanding, an ORP inflection point that is much more sensitive in alkaline conditions than in acidic conditions was obtained, making it easier to control chemical dosing. found that it is possible. That is, conventionally, in alkaline conditions, O
RP control was thought to be difficult because the meaning of the ORP value during reduction by ferrous ions was not well understood, and the ORP value normally reaches equilibrium in a response time of about 5 minutes; In the case of alkalinity, the response time is 20 to 30 minutes, so it is thought that the presence of an inflection point was overlooked.

The detection ORP in the present invention indicates the potential of dissolved oxygen. That is, in neutral to alkaline conditions, ferrous ions and dissolved oxygen easily react, so the injection of ferrous ions is controlled by measuring ORP, which indicates the potential of dissolved oxygen.

Cr"+3 Fe"-+ Cr"+3 Fe"
-...■2Fe"+O+H20-+ 2Fe3++20H-...■ In the reduction with ferrous ions, the reaction (2) above takes precedence at the beginning of the reduction reaction, but after the reduction of hexavalent chromium, the reaction (2) above releases dissolved oxygen. is consumed and becomes anoxic, resulting in a rapid decrease in ORP.In acidic conditions where ferrous ions and oxygen are less likely to react, the ORP value shows a slow change.

[Example] The present invention will be explained in more detail with reference to Experimental Examples, Working Examples, and Comparative Examples below.

Experimental Example 1 A solution containing ferrous sulfate (100pp (converted to 100pp+)) was adjusted to pH 10 with caustic soda, and the ORP response time was measured. As a result, it becomes constant at about -350 mV in about 20 to 30 minutes, and the time required for the ORP response of ferrous hydroxide is 2
It was found that it took 0 to 30 minutes.

Similarly, when the ORP response time was measured when the pH was set to 1.5, it became constant at 550 mV in about 5 minutes.

From this result, it was confirmed that the response time of ferrous ions was slow on the alkaline side.

Experimental example 2 pH: 1.5, Cr": 360 ppm, Fe": 15
Using 0 ppm wastewater, adjust the pH to various concentrations, add ferrous salt at various concentrations, measure the ORP value 20 minutes after addition, obtain the OR2 curve, and find the relationship between pH and OR2 curve. Examined. The results are shown in Figure 2.

As is clear from FIG. 2, the inflection point of the OR2 curve becomes clear from pH 5. This corresponds to the empirical value of pH 4 to 5, at which the reaction between ferrous ions and oxygen tends to occur. It is clear that the higher the pH, the sharper the inflection point of the ORP curve.

Experimental Example 3 pH: 1.5, Cr": 360ppm, Fe": 15
Using 0 ppm Metsuki wastewater as raw water, reduction treatment was performed by adding ferrous salt at various concentrations at pH 10 and pH 1 and 5, respectively, and the relationship between ORP and Crs÷ was investigated. In addition,
ORP and Cr6+ are all values 20 minutes after addition of ferrous salt. The results are shown in Figure 3 (pH+10) and Figure 4 (pH:
1.5).

The following is clear from FIGS. 3 and 4.

That is, even in the case of pH 1.5 (FIG. 4), an inflection point with a small OPR was obtained, and hexavalent chromium was reduced after the inflection point.

On the other hand, in the case (pH 1077) (Figure 3), the change in OR, P is sharp, and as soon as the reduction of hexavalent chromium is completed, the DRP
is almost constant at -400 mV.

Examples 1 and 2, Comparative Example 1 pH: 2.3, Cr": 50ppm, Zn": 10p
pm, Ni”: Synthetic wastewater containing 10 ppm was subjected to reduction treatment at the pH shown in Table 1 by adding ferrous salt to achieve the ORP setting value shown in Table 1, and the treated liquid was left as is. The filtrate was analyzed after filtration with No. 5A filter paper.The reaction time was 30 minutes, and the reaction rate was 10 j2/h.
Continuous reduction was carried out at r. The results are shown in Table 1.

From Table 1, it is clear that according to the present invention, ferrous ions and other coexisting heavy metals can be treated to a high degree in addition to hexavalent chromium.

[Effects of the Invention] As detailed above, according to the method for treating chromium-containing wastewater of the present invention, in order to control the chemical injection of ferrous salt on the alkaline side with a pH of 9 or more, the reduction operation and the neutralization operation are performed. can be carried out in the same tank. Therefore, there is no need to separately install a reduction tank and a neutralization tank, and the processing equipment can be simplified, the installation area can be reduced, and the number of installed instruments can be reduced. On the alkali side, by adjusting the amount of ferrous salt added such that the ORP is less than -200 mV, efficient reduction can be achieved! It is possible to perform A filling.

Therefore, according to the method for treating chromium-containing wastewater of the present invention,
By controlling the chemical injection of ferrous salt using an ORP meter on the alkali side, it is possible to easily and efficiently treat wastewater containing hexavalent chromium.

[Brief explanation of drawings]

Fig. 1 is a system diagram showing one implementation method of the chromium-containing wastewater treatment method of the present invention, Fig. 2 is a graph showing the results of Experimental Example 2, and Figs. 3 and 4 show the results of Experimental Example 3. It is a graph. ...reduced neutralized species, 2...sedimentation tank.

Claims (1)

[Claims] (1) In a method of reducing hexavalent chromium to trivalent chromium by adding ferrous ions to wastewater containing hexavalent chromium, the pH of the wastewater is adjusted to 9 or higher, and the redox potential is increased. −
A method for treating wastewater containing hexavalent chromium, characterized by adding ferrous ions so that the voltage becomes 200 mV or less.