JP2902235B2 - How to recover iodine - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for economically and safely recovering iodine from iodine or iodine compounds contained in waste.

[0002]

2. Description of the Related Art Iodine is used as a raw material for daily life, as an X-ray contrast agent, a pharmaceutical, a bactericidal and antifungal agent, and as an industrial product.
It is used as a catalyst stabilizer, a raw material for photography, and as a feed additive and a herbicide for agricultural use, and is a valuable resource worldwide.
Recovering expensive iodine from waste containing iodine or iodine compounds is very advantageous from the viewpoints of economical, environmental protection and resource saving. As described above, iodine is used for raw materials, intermediates, catalysts, etc. of various products. For example,
In the synthesis of organic compounds, iodine is used alone or as a compound, but not all iodine used is involved in the synthesis. Some may remain unreacted or become other compounds. Further, if a solvent is used in a purification step for increasing the purity of a synthetic substance, not all of the synthetic substance may be dissolved in the solvent, and not all of the synthetic substance may be recovered. Therefore, the waste discharged from the manufacturing process contains iodine in the form of a simple substance or various inorganic and organic compounds, and the shape thereof is various, such as waste liquid, waste oil, and sludge. Conventionally, various proposals have been made regarding iodine recovery. In particular, the following proposals are mainly made for iodine recovery relating to the combustion method. For example, Japanese Patent Publication No. 54-13234 discloses that
A method is described in which waste containing iodine or an iodine compound is continuously introduced into a combustion furnace to generate free iodine, and the iodine gas is absorbed by an aqueous solution of sodium thiosulfate or sodium sulfite. Further, Japanese Patent Application Laid-Open No. 1-108102 discloses a combustion furnace for recovering iodine used for recovering iodine from waste containing an organic compound, iodine and / or an iodine compound, and an alkali metal compound. .

[0003]

The above-mentioned conventional method is a very useful method in that iodine is recovered from iodine-containing waste. However, there are disadvantages in that chemicals are used more than necessary and the operation is complicated.

[0004]

In order to advantageously solve the above-mentioned problems, in the method for recovering iodine of the present invention, an alkali metal compound and a solvent are mixed with a waste containing iodine or an iodine compound. The mixture is introduced into a combustion furnace having a combustion device and heat-treated, and the iodine compound contained in the heat-treated gas is absorbed by an alkaline aqueous solution.

Next, the present invention will be described in detail. The waste containing iodine or an iodine compound includes radiographic agents such as iohexol and iopamidol, 5-acetamido-2,4,6-triiodo-N, N'-bis (2,3-dihydroxypropyl) -isophthalamide The properties of each of the intermediates such as X-ray contrast agents and organic solvents containing sodium iodide are highly viscous liquids, solids or liquids.

Although there are various alkali metal compounds used in the present invention, sodium hydroxide is preferred from the viewpoints of easy handling of the resulting compound and economy. The amount of sodium hydroxide added is such that the molar ratio of alkali metal to the amount of iodine in the waste is 1: 1.5 or less, preferably 1: 0.6 to 1.2.
Good range.

[0007] The solvent is added to make the shape of the waste liquid or slurry in order to pass through the spray nozzle of the combustion furnace, and the added substance and the amount vary depending on the type and shape of the waste. In general, use of water is preferable, but depending on the waste, an organic solvent such as methanol, ethanol, or acetone may be used. In the present invention, a sufficient hydrogen compound is required to convert iodine to hydrogen iodide, but water or natural gas is preferred because of easy handling.

[0008] The heat treatment in the present invention means that a chemical reaction such as the following formulas (1), (2) and (3) occurs in a furnace.
The temperature is 800-1400 ° C., preferably 950-110
0 ° C and the amount of air was adjusted so that the oxygen concentration of the exhaust gas was 4-12.
%, Preferably 6 to 10%. Excess combustibles that do not participate in the reaction are burned by excess air after the reaction and provided for maintaining the temperature in the furnace. CH ₄ + 2H ₂ O → CO ₂ + 4H ₂ (1) I ₂ + H ₂ → 2HI (2) HI + NaOH → NaI + H ₂ O (3)

By properly adjusting the temperature and the amount of combustion air according to the type and nature of the waste, there is almost no free iodine in the gas discharged from the combustion furnace. Therefore, there is an advantage that the iodine compound, preferably hydrogen iodide and sodium iodide can be recovered only by cooling the exhaust gas.

[0010] To absorb in an alkaline aqueous solution means that when hydrogen iodide and carbon dioxide gas in the exhaust gas are absorbed, the pH of the recovered liquid becomes acidic and the metal material is corroded. In order to prevent this, the pH of the recovered solution is 7.0 to 10.0, preferably 8.0 to 9.5.
Adjust so that it is within the range. Although there are various chemicals for adjusting the alkalinity, it is preferable to use the sodium hydroxide used above because using a variety of chemicals complicates the equipment and operation.

FIG. 1 shows an example of steps for carrying out the method of the present invention.
It will be described based on. A waste 6 containing iodine or an iodine compound is adjusted to a constant concentration with sodium hydroxide 7 and water 8 in a waste concentration adjusting tank 5. Next, this adjusted liquid is supplied to the combustion furnace 1
And heat-treated. The temperature in the combustion furnace 1 is previously maintained at 950 to 1100 ° C. Next, after the water 8 is sprayed for a certain period of time, the adjusting liquid is introduced from the waste dissolution adjusting tank 5 to an appropriate position in the combustion furnace and sprayed by the spray nozzle 12.
The liquid in the waste dissolution adjusting tank 5 may be a slurry containing solid matter as long as it passes through the spray nozzle 12. The heat treatment gas is introduced again into the cooling and dissolving tank 2 by injecting air from 13 again and introduced into the cooling and dissolving tank 2 to cool the gas temperature to 90 ° C. or lower, and dissolve sodium iodide and sodium carbonate contained in the gas. The gas is cooled again to 50 ° C. or lower in the cooling tower 3. The cooled gas absorbs hydrogen iodide in the demister 4, collects dust and mist, and removes the exhaust gas 14.
To be discharged. There is no free iodine gas in the exhaust gas 14, and the iodine content in the exhaust gas is 1 mg / m ³ _N or less, depending on the iodine concentration in the absorbing solution and the amount of scattered mist. (The symbol m ³ _N is the gas volume converted to the standard state of 0 ° C. and 760 mmHg. Hereinafter, this symbol is used for the gas volume.) Sodium hydroxide 7 and water 8 are supplied from the demister 4 so that the iodine concentration and pH of the iodine recovery liquid 15 become constant, and the cooling tower 3 and the cooling and dissolving tank 2
And an iodine recovery liquid 15 is obtained. The recovered liquid is air cooling tower 1
It is cooled in 6, 17 and used for circulation. Iodine recovery liquid 15
Is an alkaline solution containing sodium iodide and sodium carbonate. To recover purified iodine from this recovered solution,
After acidification with sulfuric acid or the like, chlorine is blown in to precipitate free iodine, and then purified by a known method of melting under pressure. This method has advantages in that a reducing agent for free iodine (such as sodium thiosulfate) or a large amount of a collecting agent (sodium hydroxide) is not required, and that it can be treated continuously.

Iodine is an extremely corrosive element, and most metallic materials are corroded. Therefore, it is generally very difficult to select a material for the iodine recovery apparatus. However, in the present invention, as described above, the proportion of corrosive gas in the exhaust gas is small, and the processes after the combustion furnace 1 are under conditions in which the reducing and oxidizing corrosion effects are extremely suppressed. In addition, the temperature in the process is as low as 90 ° C. or less, and the solution is weakly alkaline, so that it is safe. Therefore, materials such as stainless steel and vinyl chloride can be selected, the apparatus can be easily designed, and there are many advantages that iodine can be recovered economically and safely.

[0013]

Next, the method of the present invention will be described in detail with reference to examples. Example 1 1 kg of sodium hydroxide and 40 kg of water are added to 10 kg of a highly viscous liquid waste composed of 74% by weight of iohexol, 13% by weight of isopropyl alcohol, and 13% by weight of water, and mixed to prepare a 51 kg adjusted liquid. The iodine content is 3433 g
It is. The molar ratio of iodine to alkali metal is 1: 0.93. After spraying water for several minutes into a combustion furnace in which the temperature is kept constant by burning 2 m ³ _{N /} hour of natural gas and 20 m ³ _{N /} hour of air, the above-mentioned adjusted liquid is sprayed at 6 kg / hour and heat-treated. The temperature in the combustion furnace is adjusted to the air amount and the natural gas amount so as to maintain the temperature at 1050 ° C. after spraying the adjusting liquid. The heat-treated waste is replenished with air at 20 m ³ _{N /} h from the middle of the combustion furnace. The heat-treated gas passes through the cooling / dissolving tank 2, the cooling tower 3, and the demister 4, and is discharged into the atmosphere through cooling, melting, and absorbing operations. Gas temperature of cooling / dissolution tank outlet is 85 ℃
Hereinafter, the cooling tower outlet gas temperature was 42 ° C. or less. The demister was replenished with 50 liters of water and an appropriate amount of sodium hydroxide per hour to obtain 186 liters of a recovered liquid having a pH of 8.5. The recovered iodine amount was 3423 g, and the recovery rate was 99.6%. Also, no undecomposed waste was detected in the recovered liquid. Free iodine is not detected in the exhaust gas released into the atmosphere, and iodine ions in the absorbing solution scattered as mist are 0.
It was measured at a concentration of 9 mg / m ³ _N. The amount of iodine that escaped to the atmosphere was 0.7 g, and the iodine loss rate was 0.02%. The oxygen concentration in the exhaust gas was 10%. The solid content of sodium iodide on the combustion furnace wall was 0.37% in terms of iodine.

Sulfuric acid is added to 186 liters of the recovered
After that, the mixture was treated and refined by a known method of adding chlorine and pressurizing and melting to obtain 3354 g of purified iodine crystal. The purity of the purified iodine was 99.85%. The recovery rate of purified iodine from iodine-containing waste was 97.6%.

Example 2 Composition of 5-acetamido-2,4,6-triiodo-
28% by weight of N, N'-bis (2,3-dihydroxypropyl) -isophthalamide, 8% by weight of sodium chloride,
1% by weight of sodium acetate and 63% by weight of water 10
0.45 kg of sodium hydroxide and 10 kg of water are added to and mixed with Kg to prepare a preparation of 20.45 kg. The iodine content is 1513
g. The molar ratio of iodine to alkali metal is 1: 0.97. Heat treatment was carried out in the same manner as in Example 1 to obtain 85 liters of a recovered solution having a pH of 8.9. The amount of iodine recovered was 1505 g and the recovery rate was 99.5%. Also, no undecomposed waste was detected in the recovered liquid. No free iodine was detected in the exhaust gas discharged into the atmosphere, and iodine ions in the absorbing solution scattered as mist were measured at a concentration of 0.8 mg / m ³ _N. The amount of iodine that escaped to the atmosphere was 0.6 g, and the iodine loss rate was 0.04%. The oxygen concentration in the exhaust gas was 9%. The solid sodium iodide adhered to the combustion furnace wall was 0.44 as iodine.
%Met.

Sulfuric acid is added to 85 liters of the recovered solution to adjust the pH to 2
After that, treatment and purification were carried out by a known method of adding chlorine and adding pressure to melt, thereby obtaining 1474 g of purified iodine crystals. The purity of the purified iodine was 99.78%. The recovery rate of purified iodine from the iodine-containing waste was 97.4%.

Example 3 The composition was 12% by weight of sodium iodide, 5% by weight of dimethylacetamide, 6% by weight of formaldehyde, and 2% by weight of methanol.
A heat treatment is performed on 20 kg of a liquid containing 75% by weight of water and 75% by weight of water. The iodine content is 2032 g. The molar ratio of iodine to alkali metal is 1: 1. Heat treatment was performed in the same manner as in Example 1 to obtain 120 liters of a recovery liquid having a pH of 9. The recovered iodine amount was 2002 g and the recovery rate was 98.5%. or,
No undecomposed waste was detected in the recovered liquid. No free iodine was detected in the exhaust gas released into the atmosphere, and 0.9 mg / m ³ _N of iodine ions in the absorbing solution scattered as mist was detected.
It was measured at a concentration of 1.0g of iodine escaping into the atmosphere
And the iodine loss rate was 0.05%. The oxygen concentration in the exhaust gas was 7%. The solid content of sodium iodide on the combustion furnace wall was 1.44% in terms of iodine.

Sulfuric acid is added to 120 liters of the collected solution to adjust the pH.
Then, the mixture was treated and refined by a known method of adding chlorine and adding pressure to obtain 1960 g of purified iodine crystals. The purity of the purified iodine was 99.81%. The recovery rate of purified iodine from the iodine-containing waste was 96.5%.

Comparative Example The following test was performed as a comparative example. Sulfuric acid was added to 5 kg of the same waste liquid as in Example 3 to adjust the pH to 2, then chlorine was added, and the mixture was treated and purified by a known method, but the released iodine was easily dissolved in the solvent, and the black specific gravity was large. It was not possible to obtain purified iodine crystals just by becoming liquid. The recovery of purified iodine from the iodine-containing waste was 0%.

[0020]

According to the present invention, a waste containing iodine or an iodine compound is mixed with an alkali metal compound and a solvent, and the mixture is introduced into a combustion furnace having a combustion device and heat-treated. Since the iodine compound contained therein is absorbed by the alkaline aqueous solution, purified iodine can be economically recovered in high yield from various wastes containing iodine and iodine compounds.

[Brief description of the drawings]

FIG. 1 is a flow sheet showing an example of a process for implementing the method of the present invention.

[Explanation of symbols]

 Reference Signs List 1 combustion furnace 2 cooling and melting tank 3 cooling tower 4 demister 5 iodine-containing waste dissolution adjusting tank 6 iodine-containing waste 7 sodium hydroxide 8 water 9 natural gas 10 air 11 combustion device 12 spray nozzle 13 secondary air 14 exhaust gas 15 Iodine recovery liquid 16 Air cooling tower 17 Air cooling tower

──────────────────────────────────────────────────の Continued on the front page (51) Int.Cl. ⁶ Identification code FI F23G 7/04 601 F23G 7/04 601L

Claims (1)

(57) [Claims] 1. An alkali metal compound and a solvent are mixed with a waste containing iodine or an iodine compound.
A method for recovering iodine, comprising introducing into a combustion furnace having a combustion device, performing heat treatment, and absorbing an iodine compound contained in the heat treatment gas into an alkaline aqueous solution.