EP0021383A1 - Process for the recovery of benzene and chlorine from hexachlorocyclohexanes - Google Patents

Abstract

An improved process for recovering benzene and chlorine from hexachlorocyclohexanes by reacting the waste isomers from the production of lindane with zinc and electrolysis of the zinc chloride obtained, characterized in that the reaction is effected in the cathode compartment of an electrolytic cell divided by a diaphragm, the zinc being produced in situ at the cathode.

Description

The invention relates to a new process for the recovery of benzene and chlorine from waste products which are obtained in the production of the insecticidal Y-hexachlorocyclohexane ("lindane"). These waste products are isomeric hexachlorocyclohexanes. They are formed in considerable quantities in addition to the desired (isomer. The recovery of the starting materials benzene and chlorine is therefore of great importance. It is known that the 1,2,3,4,5,6-hexachlorocyclohexane isomers (HCH isomers ) can be reacted with zinc dust in an aqueous slurry to give benzene and zinc chloride (Chem. Abstr. 46 (1952), 3962 e; 48 (1954), 2096 ab) , since two solid phases have to react with each other and the resulting third phase (zinc chloride) has to be dissolved in the water in order to avoid blocking the surfaces. Because of the complicated heterogeneous course of the reaction, the implementation of larger amounts is very expensive Methanol) for dissolving the organic substrate is also known (Chem. Abstr. 47 (1953), 5906 b; 46 (1952), 3962 e); however, very large amounts of zinc are required and the workup is sword.

It is also known to recover zinc from zinc chloride by electrolysis.

Surprisingly, it has now been found that the processes for obtaining benzene from hexachlorocyclohexanes and chlorine from the zinc chloride obtained can advantageously be combined with one another under suitable conditions.

The new process is characterized in that the reaction between the hexachlorocyclohexane isomers and zinc takes place in the cathode compartment of an electrolysis cell divided by a diaphragm, the zinc being generated electrolytically in situ at the cathode.

In the cathode compartment there is a heated mixture of aqueous zinc chloride solution with 30-60, preferably about 50 percent by weight zinc chloride, and the solution of hexachlorocyclohexane isomers in an organic solvent which is essentially insoluble in water under the reaction conditions.

The reaction temperature in the process according to the invention is expediently chosen so that only little solid zinc is present during the reaction. It is therefore generally carried out at temperatures above 95 ° C.

• Reaktion I:
  
• Reaktion II:
  

The combined reactions can be specified as follows:

• Reaction I:
  
• Reaction II:
  

The organic solvents are e.g. Chlorobenzene, 1,2,4-trichlorobenzene, benzene, xylene, if necessary also in a mixture.

It is favorable to add a small amount of an emulsifier that is as stable as possible under the reaction conditions.

The anolyte preferably consists of an aqueous zinc chloride solution, similar to the aqueous phase of the catholyte. Carbon (e.g. as "charcoal" or graphite) has proven to be favorable as the cathode material.

The anode consists of a material whose suitability for anodic chlorine deposition is known. The chlorine is anodically deposited in parallel with the reactions in the cathode compartment.

An advantage of the method according to the invention follows from the fact that the Cl concentration in the anolyte can be kept relatively high during the entire electrolysis period. This suppresses unwanted anodic side reactions and corrosion of the anode remains minimal even if it is made of carbon.

The diaphragm has the task th, the contact of the organic phase of the Kathol ^y and prevent the zinc to the anode and the resultant of their chlorine gas. It is not necessary to separate the aqueous phase of the catholyte from the anolyte. Porous inorganic materials that do not conduct electricity are inexpensive diaphragm materials (eg porous ceramics, stable asbestos in the weakly acidic area, glass fiber fleece). Anion exchange membranes with sufficient stability under the reaction conditions can also be used.

The properties of the emulsifier are not critical. However, it is advantageous to use a sufficiently stable, neutral-reacting, nonionic emulsifier in order to avoid its migration in the electric field with minimal decomposition. In order to also make the transition into the To keep anode space small, its water solubility should also be as low as possible. In addition, it is very cheap if it is made up of only C, H and 0 atoms. This is because when the aqueous electrolyte is circulated, decomposition and anodic degradation of the emulsifier cannot be completely avoided. Therefore, the electrolyte would become contaminated with inorganic ions over time and the anode corrosion would be increased if the emulsifier contained other atoms than the ones mentioned above (eg S or N).

B-HCH is decomposed much more slowly than the other HCH isomers. Therefore, it remains largely undecomposed if the decomposition of technical HCH isomer mixture according to the invention (approx. 10% by weight β-HCH content) is terminated before passage of the theoretical amount of electricity. If largely pure β-HCH is decomposed, a somewhat larger amount of zinc occurs as an intermediate than in the decomposition of the other isomers.

The process according to the invention can be carried out batchwise or continuously in vat or plate frame cells. The shape of the electrodes and that of the diaphragms depend on the design requirements of the electrolysis cells.

The implementation of the method is explained in more detail in the following examples.

Examples A) vat electrol cell

The reaction according to the invention can be carried out in the tub electrolysis cell described below. The cell housing is a glass pot that can be heated by an external oil bath and has a flat, flared rim at the top, on which a round glass lid lies tightly. It is equipped with several openings with standard cuts, into which the individual parts explained below are inserted. The asymmetrical arrangement of diaphragms, electrodes and stirrer causes an intensive mixing of the catholyte.

Figure 1 shows the lid of the electrolytic cell in plan view. The openings (1) serve to insert the diaphragm holders, (2) to insert the cathode holder.

Opening (3) receives the reflux condenser with gas discharge pipe (4) the stirrer with a stirrer closure and finally (5) a thermometer. In addition, the cathode compartment can be filled or emptied through (5). (6) is the inner trace of the glass pot.

FIG. 2 shows a vertical section through the electrolysis cell along the line AB in FIG. 1. The glass pot (7) forms the cell housing with the cover (8). The porous clay cups (9) are used as a diaphragm. These are cemented onto the diaphragm holder (10). The upper part of the diaphragm holder is provided with a cut (11) into which a corresponding cut core of the anode holder (12) fits. The anode holder is provided with a gas discharge pipe (13), anolyte storage container (14) and a glass pipe (15) into which the anode rod (16) made of carbon is sealed. The The cathode holder (17) consists of a glass tube into which the cathode rod (18) is tightly glued. The cathode holder (17) is ground with the lid. (8) connected. The marks (19) and (20) indicate the fill level in the cathode and anode space.

B) Implementation

unter Verwendung von Chlorbenzol als inertem organischem Lösungsmittel und einer kleinen Menge eines nichtionischen, in Wasser schwer löslichen, nur C, H und 0 enthaltenden Emulgators (Kathodenstromdichte: 10 A/dm²; elektrochemisch wirksame Fläche: 0,785 _dm ²)_. 1) Decomposition of a technical mixture of waste isomers from lindane production (technical a-HCH) of the following composition:

using chlorobenzene as an inert organic solvent and a small amount of a nonionic, sparingly water-soluble, only C, H and 0 containing emulsifier (cathode current density: 10 A / dm ² ; electrochemically effective area: 0.785 _dm ² ) _.

140 g (-0.47 mol) techn. a-HCH of the above specification and 1 drop of emulsifier of the carboxylic ester type are dissolved in 200 ml of chlorobenzene at 90 ° C. The portion that remains undissolved at this temperature (ß-HCH and inorganic impurities) is suctioned off at this temperature via a steam-heated double-wall glass frit filter and washed with 100 ml of 90 ° C hot chlorobenzene (6.0 g of undissolved residue). The filtrate (approx. 300 ml) is added to the cathode compartment of the electrolytic cell after it has passed was charged with 2.3 l of 50% by weight aqueous zinc chloride solution at 90 ° C. and the two anode compartments with such amounts of the same solution that they were filled to the extent indicated in FIG. 2 (approx. 140 ml solution per anode compartment). In the cathode compartment, after the organic solution has been added, enough hot aqueous solution is refilled so that the fill level indicated in FIG. 2 is reached. From now on, the two-phase catholyte is stirred well and further heated until a slight reflux can be observed (approx. 100 ° C internal temperature). The electrolysis current is then switched on and regulated to 8.0 A (corresponding to 10 A / dm ² cathode current density). A voltage of 4.7-4.8 V was set between the electrodes, which remained practically constant during the entire electrolysis period. During the electrolysis, the stirrer speed is adjusted so that the phases are mixed well, but little foam is formed.

The chlorine gas generated at the anodes during electrolysis is passed through a flask cooled to 0 - 5 ° C. In it entrained zinc chloride solution is deposited; in addition, a large part of the water vapor contained in the hot chlorine gas condenses in it. The gas treated in this way is then condensed in a tared cold trap cooled with dry ice / methanol (crude yield: 69 g).

During the subsequent evaporation, some water remains (about 1 g).

The chlorine yield can be determined more precisely by absorbing the resulting chlorine glass in excess aqueous potassium iodide solution (1 kg of potassium iodide in 1.5 l of water) instead of condensing. Then a small aliquot is titrated for iodine and converted the result (pure yield: 67.5 g of chlorine).

The amount of anolyte decreases during the electrolysis because some solution passes into the cathode compartment and the escaping chlorine gas entrains some liquid. This amount is replaced during the electrolysis in order to keep the filling level constant (approx. 30 ml per anode compartment).

During the electrolysis, metallic zinc forms on the cathode, which reacts immediately with the HCH dissolved in the organic phase of the catholyte to the extent that only a small amount of metal is present in the cathode compartment. In addition, some hydrogen is generated, which is collected and measured (approx. 700 ml).

After 54.0 Ah (2.0 F) of electrical current has passed, the electrolysis is stopped. After the current has been switched off, the mixture is boiled under gentle reflux with stirring until no more solid particles can be seen; this is the case after a few minutes. Now allow to cool to 80 ° C, replace the reflux condenser with a descending and then distill a mixture of organic phase and water until the top temperature is constant at 98 ° C, so all the benzene has passed over. This is the case when about 180 ml of organic phase are present in the distillate. The two phases of the distillate are separated. The benzene contained in the organic is determined by GC (22.5) and largely isolated by fractional distillation over a column (19 g).

After the organic phase and water from the cathode compartment have just been distilled off, the residue is allowed to cool to 70 ° C. and transferred to a separating funnel to separate the two phases. The two anode compartments are also emptied. The two anolytes are combined and filtered. The filtrate, the aqueous phase of the catholyte and that of the above distillate are combined. The clear aqueous solution thus obtained can be used for the next batch; volatile organic impurities (e.g. methanol from technical HCI) are removed by distillation and any losses are compensated for.

The organic phase separated in the separating funnel is evaporated to dryness in vacuo. What remains is undecomposed HCH (39.0 g). Instead, this organic phase can be used in the following batch (taking into account its HCH content).

2) Decomposition of the techn. a-HCH using a mixture of 90% by weight 1,2,4-trichlorobenzene and 10% by weight benzene as an inert organic solvent and a small amount of a nonionic, water-insoluble emulsifier (cathode current density 10 A / dm ² ).

The procedure is as in Example 1, but instead of chlorobenzene, the same volume of a mixture of 90% by weight 98% 1,2,4-trichlorobenzene and 10% by weight benzene (368 g 98% 1, 2,4-trichlorobenzene and 40 g benzene).

The removal of the benzene is easier here than when using chlorobenzene as an inert solvent. The distillation from the cathode compartment after the electrolysis has ended can be stopped after the transition from 80-90 ml of organic phase. Summary of results:

3) Decomposition of the techn. a-HCH using a mixture of 90% by weight distilled technical trichlorobenzene isomer mixture, which contains about 75% 1,2,4-isomer, and 10% by weight benzene as an inert organic solvent and a small amount of a nonionic, in water poorly soluble emulsifier. The procedure is as in Example 2, but instead of the 98% 1,2,4-trichlorobenzene, the distilled trichlorobenzene isomer mixture is used with a content of about 75% 1,2,4-isomer. The results are the same as those of Example 2.

4) Decomposition of the techn. a-HCH under the conditions of Example 2 with the modification that the electrolysis current is regulated to 12 A (corresponding to 15 A / dm ² cathode current density). The procedure is as in Example 2, with the exception that the electrolysis current is adjusted to 12 A (corresponding to 15 A / dm ² cathode current density). At this current, a voltage of 5.9 - 6.0 V is established between the electrodes. The other results are the same as those of Example 2.

5) Decomposition of largely pure ß-HCH using chlorobenzene as an inert organic solvent and a nonionic, water-insoluble emulsifier (cathode current density: approx. 8.8 _{A / dm} ² ) _. The procedure is as in Example 1, but with the following changes: 89 g (0.3 mol) ß-HCH (instead of 140 g <0.47 mol) HCH isomer mixture)

The hot, cloudy mixture of β-HCH and chlorobenzene is added to the cathode compartment of the electrolytic cell, which has previously been charged with the hot 50% aqueous zinc chloride solution, without prior filtration.

10 drops of emulsifier are added. The electrolysis current is adjusted to 7 A. After a flow of 35 A h (1.3 F), the electrolysis is stopped. The catholyte is then refluxed for 2 hours with thorough stirring.

Summary of results:

Claims (4)

1. A process for the production of benzene and chlorine from hexachlorocyclohexanes by reacting the waste isomers of lindane production with zinc and electrolysis of the resulting zinc chloride, characterized in that the reaction takes place in the cathode compartment of an electrolysis cell divided by a diaphragm, the zinc being generated in situ at the cathode becomes. 2. The method according to claim 1, characterized in that the zinc chloride content in catholytes is between 30 and 60, preferably about 50 percent by weight. 3. The method according to claim 1 or 2, characterized in that the reaction temperature is above about 95 ° C. 4. The method according to claim 1, 2 or 3, characterized in that the hexachlorocyclohexane isomers are used as a solution in a stable under the reaction conditions, water-insoluble organic solvent.

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