BE1011880A4 - Method of treatment of brine. - Google Patents

Abstract

Process for purifying an aqueous solution of organic compounds, according to which: (a) the pH of the solution is brought to at least 13; (b) the solution is heated to at least 100 degrees C for at least 1 hour; (c) at least one chlorinated oxidant is gradually added to the solution.

Description

       

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   Brine cleaning process
The present invention relates to a process for purifying aqueous solutions, and in particular brines, into organic compounds.



   The aqueous solutions collected as effluents from organic synthesis processes are often loaded with troublesome organic compounds, the elimination of which is desirable in many cases, and in particular in the case of brines which it is intended to recover by electrolysis in installations provided with ion-exchange membranes, for example in order to produce aqueous solutions of sodium hydroxide.

   Indeed, several organic compounds seriously affect the performance of said membranes,
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 especially the cation trimethy) su! fbnium ((CHS)
There are certainly known methods for removing organic compounds, for example chlorination methods, but these known methods do not make it possible to sufficiently reduce the content of organic compounds (as measured for example by the total content of organic carbon, commonly known as "TOC") aqueous solutions. In addition, these known methods are ineffective with respect to certain particular organic compounds, for example trimethylsulfonium salts.



   It has already been proposed to subject wastewater containing unwanted organic substances to a thermochemical treatment by oxidation, said substances thus being oxidized to carbon dioxide and water. Such a process, described for example in document DE 26 40 603, is generally known as wet oxidation
The wet oxidation process is comparable to combustion in the liquid phase. To achieve satisfactory decomposition of organic compounds and sufficient conversion rates,

   a determined oxygen concentration as well as high temperatures and pressures are necessary. This treatment process is therefore very expensive because of its high energy consumption and the expensive apparatuses used. The temperatures and pressures used can certainly be reduced by the addition of heavy metal catalysts, but this leads to an additional purification step to remove these catalysts
The present invention therefore aims to provide a process for purifying

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 simple and effective aqueous solutions of organic compounds, both in terms of the variety of organic compounds removed and in terms of the final contents of the organic compound solutions
To this end,

   the present invention relates to a process for purifying an aqueous solution of organic compounds, according to which (a) the pH of the solution is brought to at least 13; (b) the solution is heated to at least! 00 C for at least 1 hour; (c) at least one chlorinated oxidant is gradually added to the solution
This process can take place continuously or discontinuously (batch) We prefer to carry it out discontinuously
This process is advantageously applied to a brine. The term "brine" is intended to denote, in the usual manner, an aqueous solution of sodium chloride (NaCl), in which one or more other constituents may possibly be present in smaller amounts.

   The process gives good results when applied to brines with a sodium chloride content of at least 100 g / t. Their maximum sodium chloride content is that which corresponds to saturation; in practice, their NaCI content generally does not exceed 300 g /!.



   The organic compounds which the process according to the invention aims to eliminate are in particular the compounds of the carboxylic acid (formic, acetic acids, etc.), alcohols (methanol, pentanol, etc.), aldehydes (acetaldehyde, etc.) types. , amines (alamine ,.), and in particular the sulfur derivatives (dimethylsulfide, dimethylsulfoxide, trimethylsulfonium chloride.) The process gives particularly remarkable results when the solution to be purified contains at least one sulfonium salt.

   The term “sulfonium salt” is intended to denote any compound of formu) e (R} RRS) X, in which RI, R2 and R3 are identical or different and denote alkyl, aryl, aralkyl or alkaryl radicals, substituted or not, and X denotes a valence anion n Anion X is generally a halogen, usually chlorine (n = 1). The invention applies especially to the case where the sulfonium salt is trimethylsulfonium chloride.

   In this case, good results have been obtained with trimethylsulfonium chloride contents of the order of 50 to 280 mg / l, without however these values being limiting in nature.
In the process according to the invention, the solution to be purified can naturally have a pH value equal to or greater than 13 This is however generally not the case, and it is then necessary to bring the pH to at least 13

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 by adding a strong base, or by any equivalent means (for example by electrochemical means) A water-soluble base is preferably used, for example chosen from hydroxides and salts of alkali metals (sodium carbonate, etc.). alkali metal hydroxides are preferred, especially sodium hydroxide (NaOH).

   The amount of strong base to be used will depend on the solution to be purified, its initial pH and the desired pH.



  Advantageously, the strong base content of the solution at the end of step (a) is at least 10 mmol per kg of solution, preferably at least 300 mmollkg.



   During the heating step (b), the temperature of the aqueous solution is advantageously brought to at least 120 ° C., without however exceeding its boiling point. Very good results have been obtained above 130 ° C., for example at about 140OC. Preferably, the duration of step (b) is 2 to 6 hours. This step advantageously takes place under autogenous pressure, in particular in order to avoid boiling of the aqueous solution.



   If the aqueous solution contains a sulfonium salt, steps (a) and (b) have the effect of decomposing this salt into alcohol on the one hand and into sulfide on the other hand For example, in the case where the solution contains trimethylsulfonium chloride, step (b) generally leads to the formation of methanol and
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 dimethyl sulfide ((CHS).



   According to an advantageous variant, after step (b), hydrochloric acid (HCl) is added to the solution. This makes it possible to reduce the amount of chlorinated oxidant used during step (c) In this variant, the amount of hydrochloric acid used is advantageously such that the pH of the solution is less than 5 or even more than 9.



   The oxidation step (c) makes it possible to decompose most of the organic compounds still present in the aqueous solution (for example methanol or dimethylsyulfide, or also an organic acid such as for example formic acid)
The chlorinated oxidant is gradually added to the aqueous solution, so as to avoid the formation of chlorates.

   To this end, the flow rate of chlorinated oxidant is preferably adjusted so that the total amount of oxidizing agent is introduced in 5 to 30 minutes
Advantageously, the chlorinated oxidant used in step (c) is chosen from chlorine gas (C12) and sodium hypochlorite (NaCIO) If a chlorinated oxidant gas is used, it can be used in any conventional manner , for example by bubbling through the aqueous solution, if necessary after dilution with

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 using an inert gas such as nitrogen
At the end of step (c), the solution can be both acidic and basic. However, preferably, during step (c), a quantity of chlorinated oxidant is added to the solution sufficient for the pH to become less than 2, and preferably less than 1.

   Obtaining a very low pH at the end of the reaction is advantageous in that it leads to further purification, even in cases where the subsequent use of the purified aqueous solution requires a higher pH, for example close to 7 (neutral solution); for this purpose, a conventional technique can be used, for example a reaction with a base
According to an advantageous variant, in step (a), the pH of the solution is brought to at least 13 by adding an alkaline sodium derivative, and the ratio between the number of chlorine atoms of the chlorinated oxidant added during step (c) and the number of sodium atoms added during step (a) is 0.5 to 1.5.



   Steps (a) and (b) on the one hand, and (c) on the other hand, take place in the same reactor or in separate reactors The reactor (s) used must be made up or internally coated with a material having sufficient mechanical and chemical resistance, for example titanium.



   Step (c) may optionally be followed by one or more conventional purification steps, such as filtration, additional purification using activated carbon or by ozonization, etc.



  Example
To one kg of brine containing 270 g / l of sodium chloride, 60 mg / l of trimethylsulfonium chloride (TMSC) (i.e. 19 mg C / 1) and 383 mg / l of formic acid ( HCOOH) (i.e. around 100 mg C / 1), overall
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 a TOC of 232 mg C / 1, 300 mmol (12 g) of NaOH was added, then it was heated to 13 OOC, in a stirred autoclave, under autogenous pressure (approximately 2.7 bars after 3 hours) After 3 hours of reaction, the concentration of TMSC was reduced to 37% of its initial value.

   Chlorine was then added, in large excess relative to the amount of organic compounds (total amount of chlorine: 7.06 g) After 15 minutes, the TOC was reduced to about 30 ppm (30 mg of organic carbon per liter ), the concentration of TMSC being reduced to a value below the threshold of quantification by capillary electrophoresis (<5 mg / 1) The formic acid was practically completely consumed

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It was found that a progressive addition of chlorine (in about 25 minutes) made it possible to obtain a lower final chlorate content than in the case of rapid addition The final pH was 13.4 (pH normalized at room temperature )


    

Claims (10)

 CLAIMS 1-Process for purifying an aqueous solution of organic compounds, according to which (a) the pH of the solution is brought to at least 13;  EMI6.1  (b) the solution is heated to at least 100 ° C for at least 1 hour; (c) at least one chlorinated oxidant is gradually added to the solution 2 - Process according to claim 1, in which the aqueous solution is a brine 3 - Method according to one of the preceding claims, wherein the solution to be purified contains at least one sulfonium salt.  4-Method according to one of the preceding claims, wherein the pH of the solution is raised to at least 13 by adding a strong base, and the content of strong base of the solution at the end of step ( a) is at least 10 mmol per kg of solution.  5-Process according to one of the preceding claims, in which the pH of the solution is brought to at least 13 by adding sodium hydroxide.  6-Method according to one of the preceding claims, wherein the duration of step (b) is 2 to 6 hours 7 - Method according to one of the preceding claims, in which, after step (b), hydrochloric acid (HCl) is added to the solution.    8 - Method according to one of the preceding claims, wherein the chlorinated oxidant used in step (c) is chosen from chlorine gas (C12) and sodium hypochlorite (NaCIO) 9 - Method according to one of the preceding claims, wherein, during step (c), is added to the solution a sufficient amount of chlorinated oxidant so that the pH becomes less than 2 10 - Method according to one of the preceding claims, wherein, in step (a), the pH of the solution is brought to at least 13 by adding an alkaline sodium derivative, and the ratio between the number of chlorine atoms of the oxidant  <Desc / Clms Page number 7>  chlorine added during step (c) and the number of sodium atoms added during step (a) is 0.5 to 1.5.