JPH0571318B2 - - Google Patents

Description

[Detailed description of the invention]

[Industrial Application Field] This invention relates to a method for treating chromium-containing wastewater, which reduces the chromium-containing wastewater and separates and removes chromium as hydroxide. [Prior art] As a method for treating chromium-containing wastewater, after reducing hexavalent chromium to trivalent chromium by adding sulfite,
A method is known in which trivalent chromium is separated and removed as a hydroxide by precipitation separation or the like (for example, published by Maruzen Co., Ltd., Waste Water Handbook). In this method, in order to completely reduce hexavalent chromium, the ORP setting value is set low, sulfite is injected excessively for reduction, and the generated trivalent chromium is removed as hydroxide by adding alkali. ing. [Problems to be Solved by the Invention] However, in such conventional treatment methods, it is necessary to add a large amount of sulfite to completely reduce hexavalent chromium.
There was a problem in that even though valent chromium was almost completely reduced to trivalent chromium, trivalent or hexavalent chromium was sometimes detected in the discharged water. [Means for Solving the Problems] This invention is intended to solve the above problems, and it is possible to almost completely remove trivalent and hexavalent chromium without using a large amount of sulfite. The purpose is to provide a method for treating chromium-containing wastewater. This invention involves adding sulfite to chromium-containing wastewater to reduce hexavalent chromium to trivalent chromium, and then separating and removing trivalent chromium as hydroxide. After reducing chromium to trivalent chromium, 5 to 20 mg of ferrous salt is added as iron in the presence of sulfite to suppress the production of hexavalent chromium and separate and remove trivalent chromium. This is a method for treating chromium-containing wastewater. When we investigated the cause of trivalent chromium being detected in the water discharged from the settling tank, we found that it was found to be in large excess (e.g. 500mg/
In the presence of sulfite (above), the reaction from trivalent chromium to hexavalent chromium does not occur, but the sulfite ion and trivalent chromium form a complex according to the following formula, which prevents the precipitation of chromium hydroxide. It was estimated that Cr ³⁺ +xSO ₃ ^2- → Cr(SO ₃ ) _x ^-2x+3 …[] On the other hand, the reason why hexavalent chromium is detected in effluent water even though hexavalent chromium is reduced to trivalent chromium When examined, the excess sulfite was found to be 10~
It is presumed that this is because when 300 mg of chromium is present, sulfite is oxidized by dissolved oxygen using heavy metals as a catalyst to generate oxides, and the remaining trivalent chromium reacts with this to be oxidized to hexavalent chromium. The reaction in which trivalent chromium is oxidized to hexavalent chromium is presumed to be as follows. First, sulfite reacts with dissolved oxygen using a heavy metal as a catalyst, and oxides of sulfite ions are generated according to the following equation. SO ^- +O ₂ +OH ^- → SxOy...[] Next, the oxide of sulfite ions reacts with trivalent chromium to produce hexavalent chromium according to the following formula. Cr ³⁺ +SxOy→ Cr ⁶⁺ +SO ₄ ^2-... [] It was concluded through experiments that the following four conditions are necessary for the production of hexavalent chromium. Excess sulfite Dissolved oxygen Coexisting heavy metals such as Cu, Ni, and Mn Hydroxide ion (OH ^- ) Of the four conditions above, dissolved oxygen is constantly supplied from the air, heavy metals are already present in the waste, and Hydroxyl ions are necessary to precipitate trivalent chromium, and it is difficult to reduce them to zero. Regarding excess sulfite, it is difficult to reduce excess sulfite to zero because the addition of sulfite to reduce hexavalent chromium is controlled by ORP. Therefore, in the present invention, after adding excess sulfite to reduce hexavalent chromium to trivalent chromium, 5 to 20 mg of ferrous salt is added as iron in the presence of sulfite to generate hexavalent chromium. Prevents trivalent chromium from occurring and separates trivalent chromium as hydroxide. 6 of the ferrous salt at this time
The inhibitory effect on valent chromium production is presumed to be due to the following mechanism. (1) Ferrous ions act as catalyst poisons against coexisting heavy metals. (2) In alkaline conditions, ferrous ions are oxidized by dissolved oxygen and become ferric ions, but when sulfite ions coexist, ferrous ions form a complex and become stable against air oxidation. In addition, the produced hexavalent chromium and ferrous ions react, resulting in 6
It can prevent the formation of valent chromium. The reaction is expressed as follows. Fe ²⁺ +xSO ₃ ^2- → Fe (SO ₃ ) ^-2x+2 … [] Cr ⁶⁺ +3Fe (SO ₃ ) ^-2x+2 → Cr ³⁺ +3Fe (SO ₃ ) ^-2x+3 … [] 1st The amount of iron salt to be added is theoretically calculated from the formulas [ ] and [ ], but generally 5 to 20 mg of iron is sufficient. Hereinafter, the present invention will be explained based on the drawings. 1st
The figure is a system diagram showing one embodiment of the present invention. In the figure, 1 is a reduction tank, 2 is an addition line, 3 is a neutralization flocculation tank, and 4 is a settling tank. The treatment method is to introduce chromium-containing wastewater into reduction tank 1, add sulfite and sulfuric acid there, and adjust the pH to 2.
~2.5, ORP250~300mV, reaction time 10~30 minutes, perform the reduction process according to the following formula to reduce hexavalent chromium to 3
Reduces to valent chromium. At this time, sulfite is added so that the excess sulfite is 300mg/or less.
It is desirable to have a slight excess. 2H ₂ Cr ₂ O ₇ +6NaHSO ₃ +3H ₂ SO ₄ → 2Cr ₂ (SO ₄ ) ₃ +3Na ₂ SO ₄ +8H ₂ O … [] The reaction solution that has been reduced in reduction tank 1 is taken out to addition line 2 and neutralized. On the way to the flocculation tank 3, ferrous salt is injected into the reaction liquid through the addition line 2. Ferrous salt may be added to the neutralization flocculation tank 3, but in alkaline conditions, ferrous ions undergo air oxidation before forming a complex with sulfite ions, so it is necessary to add a large amount. Ferrous salt is mixed with excess sulfite by line injection, and a sulfite complex is produced according to the above formulas [] and []. After adding the ferrous salt, the reaction solution is introduced into the neutralization flocculation tank 3, and alkalis such as sodium hydroxide and slaked lime are added, and if necessary, an organic polymer flocculant is added, and the pH is adjusted to 7 to 10 using the following formula. Neutralization and coagulation are performed to generate trivalent chromium hydroxide. Here, the presence of a sulfite complex of ferrous ions prevents the formation of hexavalent chromium. Cr ₂ (SO ₄ ) ₃ +6NaOH→ 2Cr(OH) ₃ ↓+3Na ₂ SO ₄ … [] The liquid that underwent the neutralization and flocculation reaction is introduced into the precipitation tank 4 for solid-liquid separation, and trivalent chromium hydroxide is is separated and removed as sludge, and the separated liquid is discharged as treated water. As mentioned above, when sulfite is excessive, the cohesiveness of hydroxide deteriorates, and when excess sulfite is kept low, hexavalent chromium is more likely to be generated. However, if a ferrous salt is added, trivalent chromium hydroxide with good sedimentation properties can be obtained, and the formation of hexavalent chromium can be prevented. Therefore, 3 of the treated water
As the amount of valent and hexavalent chromium decreases,
The amount of sulfite added for chromium reduction can also be reduced. Hereinafter, the present invention will be explained in more detail based on experimental examples and examples. Experimental Example 1 NaHSO ₃ was added to wastewater containing 50 mg of hexavalent chromium, and the mixture was reacted for 20 minutes while maintaining the pH at 2.2 to 2.4. Next, after reacting for 20 minutes with NaOH to adjust the pH to 7.5 to 11, the mixture was filtered through filter paper No. 5A to measure total chromium and hexavalent chromium. The equivalent amount of NaHSO ₃ is 150mg/
However, in order to complete the reduction of hexavalent chromium, it was added in excess. Figure 2 shows the measurement results for total chromium. Hexavalent chromium was 0.02 mg/or less in all cases. As is clear from FIG. 2, cohesiveness worsened and dissolved chromium increased in proportion to the excess amount of NaHSO ₃ . And NaHSO ₃ 1200mg/Cr(OH)
No precipitation of ₃ occurred at all. From the results below, it can be seen that when the amount of excess sulfite is less than 300 mg, the flocculation of total chromium is excellent, but when it is more than that, for example, when it is more than 500 mg, the flocculation is inhibited. Experimental example 2 Cu as a coexisting heavy metal and NaHSO ₃ as a reducing agent were added to a solution containing 50 mg of trivalent chromium.
While stirring with nitrogen gas, the pH was adjusted to 10 with NaOH, and the mixture was placed in an Erlenmeyer flask that was open or sealed with nitrogen gas, and the change in hexavalent chromium production over time was measured while stirring with a magnetic stirrer. The results are shown in Table 1.

[Table] As shown in Table 1, it can be seen that hexavalent chromium is produced when the conditions of Cu, NaHSO ₃ and air are aligned (Nos. 4 to 6). Furthermore, in the case of 300 mg/NaHSO ₃ (No. 7), the production of hexavalent chromium is delayed. However, as shown in FIG. 2, as the amount of excess sulfite increases, the aggregation of trivalent chromium becomes worse. NaHSO ₃ 10
mg/, hexavalent chromium is also produced, but
In the absence of NaHSO ₃ (No. 1 and No. 3),
Alternatively, if NaHSO ₃ is present but Cu is not present (No. 2), hexavalent chromium is not generated. Also
Similar results were obtained when transition metals such as Ni, Co, Mn, and V were used instead of Cu. According to experimental results, hexavalent chromium is not produced at a pH of 7 or lower, but trivalent chromium cannot be precipitated as hydroxide at a pH of 7 or lower. From the results below, it can be seen that the formation of hexavalent chromium can be prevented if excessive sulfite is not present. Example A simulating solution of wastewater after reduction of hexavalent chromium was prepared by coexisting sulfite and transition metals such as Cu and Mn in a solution containing trivalent chromium and adjusting the pH to 2.5. then
Fe ²⁺ , Fe ³⁺ , Zn ²⁺ or Pb ²⁺ was added as an inhibitor of hexavalent chromium production, and the pH was adjusted to 10 with NaOH. After stirring with a stirrer for 30 minutes, filtration was performed using filter paper No. 5A, and hexavalent chromium was measured. The results are shown in Table 2.

〔Effect of the invention〕

As described above, according to the present invention, after adding excess sulfite to reduce hexavalent chromium to trivalent chromium, ferrous salt is converted to iron in the presence of sulfite, and then
20mg/added to suppress the production of hexavalent chromium
Since valent chromium is separated and removed, it is possible to improve the separation of trivalent chromium and reduce the total amount of chromium in the treated water, while also preventing the formation of hexavalent chromium and almost completely removing hexavalent chromium. In addition, there is no need to add a large excess of sulfite in the chromium reduction process, and the amount of ferrous salt added is small, resulting in a small amount of sludge.

[Brief explanation of the drawing]

FIG. 1 is a system diagram showing an embodiment of the present invention, and FIG. 2 is a graph showing the results of Example 1. 1... Reduction tank, 2... Addition line, 3... Neutralization flocculation tank, 4... Sedimentation tank.

Claims (1)

[Claims] 1. In a method of adding sulfite to chromium-containing wastewater to reduce hexavalent chromium to trivalent chromium, and then separating and removing trivalent chromium as hydroxide, an excess of sulfite is added. After reducing hexavalent chromium to trivalent chromium, add 5 to 20 mg of ferrous salt as iron in the presence of sulfite to suppress the production of hexavalent chromium and separate and remove trivalent chromium. A method for treating chromium-containing wastewater, characterized by: 2 The amount of sulfite added is 300mg/excess sulfite.
The method according to claim 1, wherein the amount is: