JPH0379079B2 - - Google Patents

Description

[Detailed description of the invention]

TECHNICAL FIELD The present invention relates to a method for treating wastewater containing hexavalent chromium. This method is particularly useful when the wastewater contains radioactive substances as well as hexavalent chromium.

[Conventional technology]

Hexavalent chromium is a harmful substance that causes respiratory disorders and skin disorders, and wastewater containing such hexavalent chromium compounds is mainly discharged from the chromium plating process and the chromate treatment process associated with galvanizing. At nuclear power plants, potassium chromate is also included in rust preventive agents for equipment, especially pressure suppression pool water containers and piping. This wastewater is
Furthermore, they often contain radioactive materials. Ion exchange resins have long been used to treat wastewater containing hexavalent chromium. The method is based on the fact that hexavalent chromium exists in the form of chromic acid or its salts in wastewater, so chromic acid ions are
CrO ₄ ^2- is adsorbed on an anion exchange resin, eluted with caustic soda after adsorption, and purified with a cation exchange resin to recover chromic acid. Since chromic acid and its salts themselves are inexpensive,
This method aims to reuse hazardous waste in a closed system rather than recovering valuables. If you do not want to reuse the eluent described above,
Another method is to treat hexavalent chromium with a reducing agent such as NaHSO ₃ to convert it to harmless trivalent chromium, then add an alkali to neutralize it and precipitate the trivalent chromium in the form of hydroxide. This precipitate is further solidified with cement and then disposed of, making it harmless, but the drawback of this method is that it increases the volume of waste. As is well known, chromic acid has a strong oxidizing effect, so no matter which method is used, the ion exchange resin is likely to be oxidized and deteriorated, resulting in a short service life. Wastewater from nuclear facilities is
As mentioned above, in addition to hexavalent chromium, radioactive substances are also often included, so there is also the problem of how to dispose of them. Therefore, unless the content of hexavalent chromium is high, the ion exchange resin that has been treated with wastewater is not reused, but a method is used in which it is directly solidified.
However, since the volume of waste is large, we must try to reduce the volume.

[Problem to be solved by the invention]

A general object of the present invention is to provide a method for treating hexavalent chromium-containing wastewater using an ion exchange resin, resulting in a small amount of final waste. A special object of the present invention is to provide a safe method for treating wastewater containing radioactive substances as well as hexavalent chromium using an ion exchange resin, which is safe and produces a small amount of final waste. be.

[Means to solve the problem]

The method for treating wastewater containing hexavalent chromium of the present invention is to remove wastewater containing hexavalent chromium by bringing it into contact with an ion exchange resin and adsorbing hexavalent chromium onto it, and to suppress oxygen in the used ion exchange resin. This process consists of reducing hexavalent chromium to trivalent chromium to render it harmless by incinerating it in an atmosphere, and reducing the volume of the ion exchange resin. Incineration in an oxygen-suppressed atmosphere ranges from incomplete combustion, which is performed by supplying less than the amount of oxygen required for combustion of the ion exchange resin, to heating the ion exchange resin under conditions where there is virtually no oxygen supply. It also includes thermal decomposition. In many cases, partial combustion and pyrolysis will occur one after the other or in parallel. Although combustion is carried out in the presence of a limited amount of oxygen, it is preferable that oxidation proceed as homogeneously as possible with respect to the ion exchange resin to be incinerated. It is recommended to add about 10 to 40% by weight. LPG
may be introduced. In actual operation, the total air ratio m defined by the following formula is 1.5 or less m = A / (A ₀ + A ₁ ) [In the formula, A is the actual amount of air introduced into the incinerator, A ₀ represents the amount of air theoretically required for combustion of the auxiliary fuel, and A ₁ represents the amount of air theoretically required for combustion of the ion exchange resin. ] It is preferable to form an oxygen-suppressed atmosphere by adjusting the amount of air introduced into the incinerator. Any incinerator can be selected and used, such as a fixed grate furnace, inclined grate furnace, cyclone furnace, or rotary kiln. The incineration ash produced by incineration is preferably subjected to solidification treatment. For this purpose, any one of asphalt solidification, plastic solidification (using a thermosetting resin), and vitrification is suitable. It can also be done by cement solidification.

[Effect]

Incinerators loaded with ion exchange resin (and auxiliary fuel, if used) emit O ₂ , CO,
Finely adjust the air volume while checking the concentration of CO ₂ , etc., and keep the O ₂ concentration in the furnace at an appropriately low level.
Create an oxygen-suppressed atmosphere. As a result, incomplete combustion of the ion exchange resin occurs, the inside of the furnace becomes a reducing atmosphere, and Cr(→) is changed. Now, the ion exchange resin including the anion exchange group is
Expressed in R ²⁺ , the adsorption reaction of chromate ions is expressed as follows, and the reduction reaction during incineration is as follows: R ²⁺ −(OH ⁻ ) ₂ +CrO ₄ ²⁻ → R ²⁺ −CrO ₄ ²⁻ +2OH ⁻ It can be considered as follows. R ²⁺ −CrO ₄ ²⁻ +xO ₂ → yCr ₂ O ₃ +zCO ₂ +aH ₂ O +bNO _x As mentioned above, this reaction proceeds under the condition of a total air ratio of 1.5 or less. In the case of normal substantially complete combustion, the total air ratio is approximately 2 or more, and the above reaction cannot be expected. The produced Cr ₂ O ₃ is an extremely stable substance that is hardly soluble in water, and can be stably solidified.

【Example】

By dissolving K ₂ CrO ₄ in pure water, the concentration of CrO ₄ ^2-
A 500 ppm aqueous solution was prepared. Anion exchange resin “Amberlite IRA−”
400B"0.9 was packed into a column with a diameter of 3 cm and a length of 130 cm, the above K ₂ CrO ₄ solution was passed through the column at a flow rate of 5/h, and the CrO ₄ ^2- concentration at the column outlet was measured. At a feed volume of 200, the CrO ₄ ^2- concentration at the column outlet is
When the concentration reached 0.5 ppm, the liquid supply was stopped and the ion exchange resin was taken out. 100g of used ion-exchange resin is placed in a large combustion boat and placed in a horizontal cylindrical electric furnace to heat it, while air is introduced from one end and the amount is adjusted to create an oxygen-suppressed atmosphere. formation and incomplete combustion. Three combustion experiments were conducted, each with a total air ratio of 1.2, 0.8, and 0.5. The combustion residue was analyzed with the results shown in the table below.

【table】

【table】

【Effect of the invention】

According to the treatment method of the present invention, wastewater containing harmful hexavalent chromium is treated with an ion exchange resin to remove the hexavalent chromium and rendered harmless for reuse or discharge. The resin can be safely incinerated to reduce its volume, and hexavalent chromium can be reduced to trivalent chromium to render it harmless. Since this reduction is not carried out by adding a reducing agent, the volume reduction effect can be obtained as is. The generated incineration ash can be safely solidified and disposed of by known solidification means.

Claims (1)

[Claims] 1. Removing wastewater containing hexavalent chromium by contacting it with an ion exchange resin so that the hexavalent chromium is adsorbed thereon, and incinerating the used ion exchange resin in an oxygen-suppressed atmosphere. A method for treating wastewater containing hexavalent chromium, which comprises reducing hexavalent chromium to trivalent chromium to render it harmless, and reducing the volume of ion exchange resin. 2. Form an oxygen-suppressed atmosphere so that the total air ratio m defined by the following formula is 1.5 or less: m = A / (A ₀ + A ₁ ) [In the formula, A is the actual air introduced into the incinerator] A ₀ represents the amount of air theoretically required for combustion of the auxiliary fuel, and A ₁ represents the amount of air theoretically required for combustion of the ion exchange resin. ] The treatment method according to claim 1, wherein the treatment method is carried out by adjusting the amount of air introduced into the incinerator. 3. The treatment method according to claim 1, wherein the wastewater containing hexavalent chromium also contains radioactive substances, and the radioactive substances are also removed by adsorption with an ion exchange resin. 4. The treatment method according to claim 1, comprising the step of solidifying the incineration ash produced by the incineration by a means selected from asphalt solidification, plastic solidification, vitrification, and cement solidification.