JPH0218288B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION (Industrial Application Field) The present invention relates to a method for recovering mercury by solvent extraction. In particular, it concerns methods for recovering mercury contained in wastewater and solid waste.

(Prior art) Mercury is contained in wastewater discharged from mercury-method electrolytic soda plants, chemical plants that use mercury catalysts, and manufacturing plants for pharmaceuticals and disinfectants containing mercury, which causes a great deal of environmental pollution. It is a well-known fact that this has happened. For this reason, various measures have been taken up to now to separate and remove mercury ions contained in wastewater, particularly mercuric ions that have an adverse effect on the human body.

On the other hand, in recent years, environmental pollution caused by mercury contained in solid materials such as used mercury batteries and silver oxide batteries has attracted serious social concern, and complete separation and recovery of mercury from these solid wastes has become a major concern. desired.

Conventionally, wastewater containing mercury has been treated using sedimentation methods, adsorption methods, ion exchange methods, etc. Among these methods, the sedimentation method has the drawbacks of consuming a large amount of chemical solution and being difficult to operate continuously. In addition, the adsorption method using activated carbon or chelate resin, and the ion exchange method have a disadvantage in that when continuous operation is performed, a large amount of adsorbent or ion exchange resin is required in relation to the throughput.

In contrast to these techniques, in recent years, from the viewpoint of energy saving, techniques for selectively and efficiently separating trace metal ions using solvent extraction methods and liquid membrane methods have attracted attention. As the component which acts as an extractant in the case of solvent extraction or a carrier in the case of the liquid membrane method, various high molecular weight amines, neutral phosphorus compound extractants such as trioctylphosphine oxide and tributyl phosphate, etc. can be considered. However, these components cannot necessarily be said to be suitable extractants in terms of selectivity to mercury.

Recently, a method for separating and removing mercury using a liquid membrane using dicyclohexyl-24-crown-8 as a carrier has been developed (Japanese Unexamined Patent Application Publication No. 153786-1986-Japanese Patent No. 1257206-). However, this method has problems in practical use because the carrier dicyclohexyl-24-crown-8 is very expensive, and furthermore, this publication makes no mention of selectivity to mercury.

According to Mr. Bierson's concept of “hard acids, soft acids and hard bases, soft bases”, Reference: RG Pearson; Journal of American Chemical Society (RG
Pearson; J. Amer. Chem. SoC., 3533 (1963)), mercury is classified as a typical soft acid. Therefore, when selectively extracting mercury, it is more efficient to use a sulfur compound extractant that acts as a soft base than to use the aforementioned high molecular weight amine or neutral phosphorus compound extractant that acts as a hard base. be.

Dialkyl sulfides, petroleum sulfides contained in crude oil, and sulfoxides obtained by oxidizing these are known as sulfur compound extractants, and research on their extraction characteristics is being conducted in the Soviet Union and other countries. It is being said.

(Problems to be Solved by the Invention) The aforementioned dialkyl sulfides and petroleum sulfides are excellent extractants with high selectivity for mercury, but their extraction ability is still insufficient from a practical point of view. It's not something I can be satisfied with.

The object of the present invention is to develop an extractant with a much better extraction ability than the above-mentioned sulfides and to find advantageous operating conditions therefor. On the other hand, the excellent extraction performance of the extractant means that back extraction is sometimes very difficult. It is also an object of the present invention to find an effective back extraction solution to overcome this point.

(Means for solving the problems) In order to solve these problems, the present invention has the following features:
A method for recovering mercury using 1,2-bis(alkylthio)ethanes as an extractant is provided. As mentioned above, dialkyl sulfides and petroleum sulfides are excellent extractants with high selectivity for mercury, but 1,2-bis(alkylthio)ethanes are highly selective for mercury. It was found that the extraction ability was far superior to that of

_The 1,2 _- bis(alkylthio)ethanes used in _the present _invention have _the general formula _: (represents a chain or branched alkyl group). These 1,2-bis(alkylthio)ethanes may be a single type or a mixture of two or more types. A mixture may be used from the viewpoint of availability and price in the market.

The number of carbon atoms in R ₁ and R ₂ in the formula is preferably 6 to 8. When the number of carbon atoms is too low, problems arise such that the complex formed by the reaction between the extractant and mercury is not soluble in organic solvents and forms a precipitate, or the extractant becomes easily soluble in water, resulting in loss of the extractant. . Also,
If the number of carbon atoms is too large, the extractant becomes viscous and the aqueous phase/
A problem arises in that phase separation of the organic phase becomes difficult. Therefore, a compound having a carbon number within the above range is preferred because it does not have such problems, can be obtained at the lowest cost as an industrial reagent, and has excellent extraction performance.

The 1,2-bis(alkylthio)ethanes used in the present invention can be prepared, for example, by the following reaction with one or two corresponding alkanethiols, 1,2-dibromoethane, and potassium hydroxide, which can be obtained at low cost as industrial chemicals. According to the formula R ₁ SH+R ₂ SH+CH ₂ BrCH ₂ Br+2KOH →R ₁ −S−CH ₂ CH ₂ −S−R ₂ +2KBr +2H ₂ O, it can be easily synthesized in methanol in high yield. As the alkanethiol, linear or branched chain compounds are used, and in addition to single products,
The isomer mixture can be used as is. Although the obtained compound can be easily purified by distillation under reduced pressure, the mixture may be used as it is as an extractant.

Prior to solvent extraction, an aqueous solution such as hydrochloric acid, chloride, nitric acid or nitrate can be used as the aqueous phase medium in which the mercury is dissolved. In terms of solubility, an aqueous solution of nitric acid or nitrate is preferred.

In Figure 1, using hydrochloric acid as the aqueous phase medium, approximately
In extraction from hydrochloric acid containing mercuric ions at a concentration of 0.005 mol/ ^dm3 , 1,2-bis(n-hexylthio)ethane, one of the extractants used in the present invention, was used as an extraction solvent at 30°C. Comparison of extraction performance between the case of using dioctyl sulfide at 25℃ (marked with ○) and the case of using dioctyl sulfide, a type of sulfide known as an efficient extractant for mercury, at 25℃ (solid line). show. The vertical axis shows the extraction percentage (%), which represents the rate at which mercury initially present in the aqueous phase migrated into the organic phase after extraction. The horizontal axis indicates the aqueous phase hydrochloric acid concentration (mol/dm ³ ). In both cases, 1,2-dichloroethane was used as the diluent, and its concentration was
It is 0.01 mol/ ^dm3 . The solid line is Mikhailov,
Reference: Izvestiya Sibirskogo Otdeleniya Akademii Nauk S.S.S.R.
Nauk SSSR, Seriya Khimicheskikh.Nauk,
(6), 29 (1975)) are the calculation results obtained by using the extraction equilibrium constant of mercury obtained through research with the above extraction system and corresponding to the above extraction conditions.

Comparing the two from the figure, it is clear that 1,2-bis(n-hexylthio)ethane used in the present invention has better extraction performance than sulfides.

The extractant used in the present invention can be used alone or in combination with a diluent, a modifier, etc.

Diluents include aliphatic hydrocarbons such as kerosene and heptane, aromatic hydrocarbons such as toluene and xylene, and polar organic compounds such as halogen-containing organic compounds such as 1,2-dichloroethane and chloroform. However, when an aliphatic hydrocarbon such as kerosene or an aromatic hydrocarbon such as toluene is used as a diluent, if the mercury concentration in the organic phase increases to a certain extent, precipitates that are insoluble in the organic phase tend to occur. Therefore, it is desirable to use a highly polar organic solvent as a diluent or to add a modifier such as a higher alcohol to kerosene or the like.

Furthermore, as is clear from Figure 1, when hydrochloric acid or an aqueous chloride solution is used as an aqueous phase medium for dissolving mercury, mercury forms a stable complex with chloride ions in the aqueous phase, reducing the extraction efficiency. The use of such media, especially those containing chloride ions in concentrations greater than 0.2 mol/dm ³ , should be avoided. In this case, it is preferable to use nitric acid or an aqueous nitrate solution that hardly forms a complex with mercury. Of course, when removing mercury from wastewater that already has dissolved mercury, the wastewater can be used as is, as long as it contains only a small amount of ions that form stable complexes with mercury, such as chloride ions. extraction.

An aqueous solution containing ions that form stable complex ions with mercury is used as a back-extraction solution for extracting and concentrating mercury back into the aqueous phase from the extraction solvent from which mercury has been extracted. Such an aqueous solution may be an ammonia aqueous solution or hydrochloric acid, but the former produces precipitates, and the latter is inconvenient in actual operations because it requires the use of a very highly concentrated acid. On the other hand, an aqueous solution of a thiocyanate such as ammonium thiocyanate can completely back-extract mercury even at a concentration of about 1 mol/dm ³ .

The present invention will be specifically explained below using Examples.

(Example) Example 1 Take 0.03 dm ³ of nitric acid aqueous solution of each concentration shown below containing 0.005 mol/dm ³ of mercuric ion as the aqueous phase, and add 1,2-bis(n) with a concentration of 0.05 mol/dm ³ −
When the mixture was shaken with 0.03 dm ³ of a solution of hexylthio)ethane in 1,2-dichloroethane, the mercury in the aqueous phase was extracted into the organic phase in the following proportions.

Nitric acid concentration [mol/dm ³ ] Mercury extraction percentage [%] 0.01 96.1 0.1 96.0 1 96.7 When the mercury extracted into the organic phase was shaken with an aqueous ammonium thiocyanate solution of about 1 mol/dm ³ , it became 100% aqueous phase. Back-extracted.

Example 2 A nitric acid aqueous solution containing 0.005 mol/dm ³ of mercuric ion at each concentration shown below was used as the aqueous phase at 0.02 dm ³
1,2-bis(n) at a concentration of 0.05 mol/ ^dm3 .
- Kerosene solution in hexylthio)ethane 0.02dm ³
When an organic solution containing 0.005 dm ³ of octyl alcohol was mixed by shaking, mercury in the aqueous phase was extracted into the organic phase at the following ratio.

Nitric acid concentration [mol/dm ³ ] Mercury extraction percentage [%] 0.01 92.3 0.10 97.8 1.0 99.3 When octyl alcohol was not added to the organic phase, precipitation was observed in the organic phase; No formation of precipitate was observed when it was added at a ratio of .

Example 3 Take 0.03 dm ³ of an aqueous nitric acid solution containing 0.005 mol/dm ³ of mercuric ion at each concentration shown below as the aqueous phase ^, and add 1,2-bis(tertiary octylthio ) When ethane was shaken and mixed with 0.03 dm ³ of 1,2-dichloroethane solution, mercury in the aqueous phase was extracted into the organic phase in the following proportions.

Nitric acid concentration [mol/dm ³ ] Mercury extraction percentage [%] 0.01 94.2 0.1 95.9 1 94.9 Example 4 Equimolar amounts of tertiary octanethiol, tertiary hexanethiol, and 1,2
- 1.45 g of 1,2-bis(alkylthio)ethane having alkyl chains with different carbon numbers synthesized from dipromethane was dissolved in 1,2-dichloroethane to make 0.1 dm ³ and 0.03 dm ³ take,
When the mixture was shaken and mixed with 0.03 dm ³ of an aqueous nitric acid solution containing 0.005 mol/dm ³ of mercuric ion at each concentration shown below, the mercury in the aqueous phase was extracted into the organic phase at the following ratio.

Nitric acid concentration [mol/dm ³ ] Mercury extraction percentage [%] 0.01 94.5 0.1 94.4 1 93.9 (Effect of the invention) According to the present invention, as shown in the examples, mercury contained in the aqueous phase is almost completely removed. can be recovered. Furthermore, back extraction can be easily performed. Further, according to the present invention, as shown in FIG. 1, it is possible to provide a recovery method that has a higher extraction ability than dialkyl sulfides, which have conventionally been excellent as extractants for mercury.

[Brief explanation of drawings]

FIG. 1 is a graph comparing the mercury extraction performance according to the conventional method and the mercury extraction performance according to the present invention. The solid line indicates the conventional method using dioctyl sulfide, and the circle indicates the method of the present invention using 1,2-bis(n-hexylthio)ethane.

Claims (1)

[Claims] 1. The extractant has the following formula R ₁ -S-CH ₂ CH ₂ -S-R ₂ (R ₁ and R ₂ in the formula are linear or branched chains having the same or different number of carbon atoms) A method for recovering mercury by solvent extraction, characterized by using one or more types of 1,2-bis(alkylthio)ethanes represented by (indicating an alkyl group). 2. The recovery method according to claim 1, wherein R ₁ and R ₂ have 6 to 8 carbon atoms. 3 As an extractant, the following formula R ₁ -S-CH ₂ CH ₂ -S-R ₂ (R ₁ and R ₂ in the formula represent a straight chain or branched alkyl group having the same or different number of carbon atoms) is used. Mercury by solvent extraction characterized by using one or more of the following 1,2-bis(alkylthio)ethanes, solvent extracting mercury, and further back-extracting the extracted mercury into an aqueous solution of thiocyanate. collection method.