JPH0824910B2 - Metal-containing waste liquid treatment device - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial field of application] The present invention relates to an apparatus for treating a metal-containing waste liquid, and in particular, it can suitably treat a hexavalent chromium-containing waste liquid by utilizing a pickling waste liquid.

[Conventional technology]

For example, when manufacturing cold-rolled or hot-rolled steel sheets by surface treatment to produce electroplated steel sheets, hydrochloric acid or sulfuric acid is used as a pretreatment to remove the rust and oxide film on the original plate surface and activate the surface. It is common practice to subject the original plate to pickling and, after electroplating, to chromate for the purpose of corrosion resistance. Therefore, a waste liquid containing a large amount of Fe ²⁺ and Fe ³⁺ is generated from the pickling line. Also,
Waste liquid containing Cr ⁶⁺ is generated from the chromate treatment line.

The pickling waste liquid and the chromium-containing waste liquid generated during such a metal surface treatment process are obliged to be properly treated in accordance with the laws and regulations for preventing environmental pollution. Conventionally, various waste liquid treatment methods and devices have been proposed. Has been done. For example, as a treatment device for waste liquid containing hexavalent chromium, a device as shown in Fig. 4 has been published (a magazine issued by the Japan Equipment Designers Association,
"Urban design" vol12, No.4, P180-186).

In this case, the wastewater containing hexavalent chromium stored in the chromium-containing wastewater tank 1 is transferred to the reducing tank 2 and sodium sulfite (Na
₂ SO ₃ ) and sulfuric acid (H ₂ SO ₄ ) are added, and hexavalent chromium is reduced to trivalent chromium by the following reaction.

2H ₂ CrO ₄ + 3Na ₂ SO ₃ + 3H ₂ SO ₄ → Cr ₂ (SO ₄ ) ₃ + 3Na ₂ SO ₄ +5
H ₂ O (1) On the other hand, the waste water containing ferrous iron Fe ²⁺ stored in the pickling waste water tank 3 is transferred to the reaction tank 4 and contains the above-mentioned reduced trivalent chromium. The wastewater is also transferred and an alkaline agent such as slaked lime is added to precipitate chromium ions and iron ions by the following reactions.

2Cr ³⁺ + 3Ca (OH) ₂ → 3Cr (OH) ₂ ↓ (2) Fe ²⁺ + Ca (OH) ₂ → Fe (OH) ₂ ↓ + Ca ²⁺ ...... (3) Wastewater slurry containing the resulting precipitate Is sent to the thickener 6 through the flocculation tank 5, and the supernatant liquid separated into solid and liquid is discharged as it is. The concentrated precipitate is dehydrated by a dehydrator 8 through a sludge storage tank 7.

[Problems to be Solved by the Invention]

The ferrous salt in the pickling wastewater has a reducing function. For example, in the case of iron sulfate FeSO ₄ , hexavalent chromium can be reduced as in the following formula.

2H ₂ CrO ₄ + 6FeSO ₄ + 6H ₂ SO ₄ → Cr ₂ (SO ₄ ) ₃ + 3Fe ₂ (SO ₄ )
₃ + 3H ₂ O (4) However, in the conventional wastewater containing hexavalent chromium, the ferrous salt in the pickling wastewater is neutralized as it is with the alkaline agent, and is represented by the above formula (3). However, there is a problem in that the reducing function is not effectively utilized because it is simply separated as a precipitate by the reaction described above.

Therefore, it is an object of the present invention to provide a metal-containing waste liquid treatment apparatus that can effectively utilize the reducing function of the ferrous salt in the pickling waste liquid to render the hexavalent chromium-containing waste liquid harmless.

[Means for solving the problem]

The present invention that achieves the above-mentioned object includes a chromium-containing waste liquid reduction tank, an addition system of a divalent iron ion-containing waste liquid switchably connected to the reduction tank, and a chromium reducing agent such as sulfite or bisulfite. A reducing agent adding means having an adding system, and a switching timing of the reducing agent adding means is controlled according to the amount of waste liquid stored in the adding system of the iron ion-containing waste liquid, and the divalent value is determined according to a measured value of the participating reduction potential. Control means for controlling the addition amount of the iron ion-containing waste liquid and the chromium reducing agent, and a separating means for adding an alkali to the reduced chromium-containing liquid to precipitate and remove it as chromium hydroxide.

[Action]

If the storage amount of the iron ion-containing waste liquid (pickling waste liquid) is not less than a predetermined value, 2 of the reducing agent adding means are instructed by the control means
The amount of waste liquid containing high-valent iron ions operates preferentially. As a result, Fe ²⁺ having a reducing function is added to the hexavalent chromium-containing waste liquid in the chromium-containing waste liquid reduction tank.
The reaction of the formula occurs, and chromium is reduced from hexavalent to trivalent (Cr ⁶⁺ → Cr ³⁺ ).

Thus, when an aqueous solution containing chromic acid is reduced with an aqueous solution of ferrous salt, it is well known that the oxidation reduction potential (oxidation reduction po
tential, hereinafter abbreviated as ORP) changes rapidly. Utilizing this, the amount of the Fe ²⁺ containing waste liquid added can be adjusted to the OR in the reduction tank.
It can be controlled based on P measurements.

On the other hand, if the pickling waste liquid does not reach the predetermined storage amount, the addition system of the chromium reducing agent such as sulfite or bisulfite in the reducing agent addition means operates according to the instruction of the control means. In this case, for example, the reaction of the above formula (1) occurs in the reducing tank, and chromium is reduced from hexavalent to trivalent. OR in this case
Although the P value is different from the case of adding ferrous iron, the phenomenon of rapid change at the equivalence point is the same, so the addition amount of the chromium reducing agent is based on the ORP measurement value in the reducing tank as above. Can be controlled.

In the precipitation separating means, an alkaline agent such as slaked lime (Ca (OH) ₂ ) is added to the trivalent chromium-containing liquid in which the hexavalent chromium has been reduced. As a result, the above reactions (2) and (3) occur, and the trivalent chromium ion and the divalent ferrous iron ion respectively become hydroxide and precipitate. The wastewater slurry containing this precipitate is then subjected to solid-liquid separation treatment in the same manner as conventional.

〔Example〕

Embodiments of the present invention will be described below with reference to the drawings. The same or corresponding parts as in the conventional case are designated by the same reference numerals.

This metal-containing waste liquid treatment apparatus has a reduction tank 2 for chromium-containing waste water, which is connected via a pump P ₁ to a chromium-containing waste water tank 1 for storing waste water discharged from a chromate treatment line, as well as a conventional chromium-containing waste water tank. It has a reducing agent addition means 10 for introducing a reducing agent into the waste liquid tank 2. The reducing agent adding means 10 is a chromium reducing agent (hereinafter referred to as iron-based reducing agent) addition system 11 composed of a divalent iron ion-containing waste liquid switchably connected to the reduction tank 2 of the chromium-containing waste water, and sulfurous acid. It has an addition system 12 of a chromium reducing agent such as a salt (for example, Na ₂ SO ₃ ) or bisulfite (hereinafter referred to as a sulfite reducing agent).

The former 11 is a pickling wastewater tank 3 that stores the wastewater discharged from the pickling line as an iron-based reducing agent, a liquid level gauge 13 that detects the liquid level in the tank, and the wastewater in the pickling wastewater tank 3 is chromium. A pump P ₂ for sending to the reduction tank 2 of the contained waste water is provided. The latter 12 is provided with a chemical storage tank 14 for storing a sulfite reducing agent and a pump P ₂ for sending the sulfite reducing agent to the reducing tank 2 of the chromium-containing wastewater.

The pumps P ₂ and P ₃ do not operate at the same time, and the operation is controlled by the setter 15 as a control means. That is, the signal from the liquid level gauge 13 and the signal from the ORP measuring instrument A are input to the setter 15, and the start and stop signals of the pumps P ₂ and P ₃ are output, as will be described later. Pickling waste water tank 3
The starting and stopping of the pump are automatically controlled in accordance with the storage amount of the iron-based reducing agent and the measured value of the ORP in the reducing tank 2 of the chromium-containing waste water in.

Further, as the separation means 20 for precipitating and removing the chromium-containing waste liquid reduced in the reduction tank 2 of the chromium-containing waste water as chromium hydroxide by the addition of alkali, as in the conventional case.
It is equipped with a reaction tank 4, a flocculation tank 5, a thickener 6, a sludge storage tank 7, and a dehydrator 8.

In the figure, S is, for example, a rotary blade type agitator, P is a pump, and B is a pH meter.

 Next, the operation will be described.

The waste water from the chromate treatment line is temporarily stored in the chromium-containing waste water tank 1 and then transferred to the reduction tank 2 by the pump P ₁ . In the reduction tank 2, first the pH suitable for chromium reduction
Sulfuric acid is added based on the measured value of the pH meter B so that it falls within the range of (normally 2 to 3). Next, a reducing agent for hexavalent chromium is added.

This reducing agent addition is performed by selectively using one of the iron-based reducing agent addition system 11 and the sulfite-based reducing agent addition system 12 of the reducing agent addition means 10.

The reason is that, as described below, the ORP, which is an index when adding the reducing agent, is significantly different between the iron-based reducing agent and the sulfite-based reducing agent, and therefore both reducing agents are controlled by the same ORP. This is because things cannot be done. Furthermore, the iron content contained in the pickling wastewater is not only Fe ²⁺ but also partly forming Fe ^{3 + .That} is, both the iron-based reducing agent and the sulfite-based reducing agent are assumed. When is mixed and added at that time, SO ₃ ^{2− in} sodium sulfite forms a complex with Fe ³⁺ . As a result, the mixed liquid exhibits the electric potentials of Fe _x (SO ₃ ) _y and SO ₃ ²⁻ , and ORP control becomes difficult.

The pickling wastewater and its selection are made according to the storage amount of the pickling wastewater stored in the pickling wastewater tank 3.
If the liquid level detected in step 3 is equal to or higher than the pump injectable level value set in the setter 15, the setter 15 outputs a start signal for the pump P ₂ to reduce the pickling waste liquid in the pickling waste water tank 3. Tank 2
Added to.

Fig. 2 shows the ionic Cr of hexavalent chromium when the pickling wastewater containing iron sulfate FeSO ₄ as the first iron salt was added to the chromium-containing wastewater containing about 50 ppm of hexavalent chromium.
⁶ shows changes in ⁶⁺ concentration and changes in ORP at that time. FeSO ₄
ORP of the liquid shows about 680mv up to the concentration of 600mg /
During that time, the Cr ⁶⁺ concentration decreases only slightly. But Fe
When the SO ₄ concentration exceeds 600 mg /, the Cr ⁶⁺ concentration sharply decreases to 900 mg / zero. In response to this change, the ORP curve sharply changes to about 480 mv. This inflection point is detected by the ORP measuring meter A and sent to the setter 15, and the stop of the pump P ₂ is commanded.

On the other hand, if the liquid level detected by the liquid level gauge 13 is lower than the pump injectable level value set in the setter 15 (that is, when the pickling waste water is insufficiently stored), the setter 15 changes the pump P ₃ A start signal is output, and the sulfite reducing agent in the chemical storage tank 14 is added to the reducing tank 2.

Fig. 3 shows the change in the concentration of hexavalent chromium ions Cr ⁶⁺ when a sodium Na ₂ SO ₃ aqueous solution of phosphite was added to the chromium-containing wastewater containing approximately 50 ppm of hexavalent chromium. Shows the transition of ORP. Simultaneous addition of Na ₂ SO ₃ and Cr
^{The 6+} concentration decreases almost linearly and becomes zero at 400 mg / addition. During that time, the ORP of the liquid shows a gradual decrease from about 460 mv, but when the Na ₂ SO ₃ addition amount exceeds 300 mg /, it rapidly decreases and the addition amount decreases to about 200 mv at 400 mg /. The inflection point of this ORP curve is detected by the ORP measuring meter A and sent to the setter 15, and the stop of the pump P ₃ is commanded.

In this way, slaked lime is added in the reaction tank 4 to the chromium-containing wastewater in which hexavalent chromium is reduced to trivalent chromium. As a result, trivalent chromium ions become chromium hydroxide insoluble in water. Further, the trivalent iron ions remaining in the pickling waste water form ferric hydroxide Fe (OH) ₃ which is insoluble in water. Therefore, the wastewater containing these insoluble precipitates is collected in the coagulation tank 5
Then, the solid-liquid separation process is performed in the same manner as the conventional method. That is, in the flocculation tank 5, a flocculant and a flocculation aid are added to flocculate the insoluble product. The waste water containing the flocs is treated with a thickener 6 and the supernatant liquid is re-neutralized and then discharged or used as cooling water for a cascade cooler. The sludge collected at the bottom of the thickener 6 is pumped and stored in the sludge storage tank 7, and finally pumped by the dehydrator 8 to be dehydrated, used for landfill, or recycled.

It should be noted that, although the above-mentioned embodiment has been described with respect to the treatment of the chromate treatment waste liquid and the pickling treatment waste liquid in the steel mill, it is needless to say that the present invention is not necessarily limited to such an application.

〔The invention's effect〕

As described above, according to the present invention, in the treatment device for the hexavalent chromium-containing waste liquid, as the reducing agent adding means,
The addition system of the divalent iron ion-containing waste liquid, which was conventionally only the object to be treated, and the addition system of the original chromium reducing agent, such as sulfite or bisulfite, are switchably provided to provide the divalent iron. The ion-containing waste liquid is preferentially used effectively as a reducing agent. Therefore, the consumption of the original chromium reducing agent such as sulfite or bisulfite can be significantly reduced, and the effect of reducing the waste liquid treatment cost can be obtained.

[Brief description of drawings]

FIG. 1 is a treatment system diagram of an embodiment of the present invention, FIGS. 2 and 3 show treatment data of chromium-containing wastewater by the treatment apparatus shown in FIG. 1, and FIG. 2 shows the amount of iron sulfate added. And graph showing the relationship between the concentration change of hexavalent chromium ion and ORP,
FIG. 3 is a graph showing the relationship between the added amount of sodium sulfite, the concentration change of hexavalent chromium ions, and ORP. FIG. 4 is a system diagram of a conventional metal-containing waste liquid treatment device. In the figure, 1 is a chromium-containing wastewater tank, 2 is a chromium-containing wastewater reduction tank, 3 is a pickling wastewater tank, 10 is a reducing agent addition means, 11 is an iron ion-containing wastewater addition system (an iron-based reducing agent addition system) ), 12 is a system for adding a chromium reducing agent such as sulfite (a system for adding a sulfite reducing agent), 13 is a level gauge, 15 is a control means, and 20 is a separation means.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Kajii Kaoru, 3-4-7 Nishishinjuku, Shinjuku-ku, Tokyo Kurita Industry Co., Ltd. (72) Satoru Nagai 3-4-7 Nishishinjuku, Shinjuku-ku, Tokyo Kurita Industry Co., Ltd.

Claims (1)

[Claims] 1. A reduction having a chromium-containing waste liquid reduction tank, a divalent iron ion-containing waste liquid addition system and a chromium reducing agent addition system such as sulfite or bisulfite, which are switchably connected to the reduction tank. Agent addition means, while controlling the switching timing of the reducing agent addition means according to the waste liquid storage amount of the addition system of the iron ion-containing waste liquid,
Control means for controlling the addition amount of the divalent iron ion-containing waste liquid and the chromium reducing agent according to the measured value of the redox potential, and alkali is added to the reduced chromium-containing liquid to precipitate and remove as chromium hydroxide. And a separation means for removing the metal-containing waste liquid.