CN113382968A

CN113382968A - Process for producing salts from aqueous waste streams of organic peroxide production

Info

Publication number: CN113382968A
Application number: CN202080011294.1A
Authority: CN
Inventors: M·斯迪恩斯马; M·C·塔默; C·德克
Original assignee: Norion Chemicals International Ltd
Current assignee: Norion Chemicals International Ltd; Nouryon Chemicals International BV
Priority date: 2019-01-31
Filing date: 2020-01-28
Publication date: 2021-09-10

Abstract

A process for the production of salts comprising NaCl and/or KCl from an aqueous waste stream from one or more organic peroxide production processes, the process comprising the steps of: (a) ensuring that the pH of the aqueous waste liquid is in the range of 1-5, (b) separating the waste liquid into a liquid organic layer and an aqueous layer, (c) removing the organic layer, (d) raising the pH of the aqueous layer to a value in the range of 6-14, and (e) crystallizing a salt from the aqueous layer having a pH in the range of 6-14.

Description

Process for producing salts from aqueous waste streams of organic peroxide production

The present invention relates to a method for producing salt (more specifically, a salt containing NaCl and/or KCl) from chemical process waste (effluent), more specifically, organic peroxide production waste.

The production of various organic peroxides (e.g. peroxydiacyl, peroxyester and peroxydicarbonates) involves the reaction of an acid chloride or chloroformate with an organic hydroperoxide or H₂O₂Reaction under alkaline conditions. Depending on the specific process, alkaline conditions are obtained by addition of NaOH, KOH or a combination thereof.

As a by-product, a large amount of salt (KCl and/or NaCl) is formed. May also existOther salts, e.g. Na₂SO₄. High salt concentrations generally do not allow for the direct disposal of saline waste liquids to biological wastewater treatment plants.

It is more attractive from an environmental and economic point of view than diluting the spent liquor to isolate the salt in a form that makes it usable in other processes, such as textile production, leather tanning or chlor-alkali processes.

The salt can be recovered by conventional evaporative crystallization. However, direct treatment of waste streams in such systems can lead to safety and operational problems. First, a dangerously concentrated organic peroxide residue phase may form in the crystallizer. Furthermore, the resulting salts may contain peroxide residues and/or high levels of organic impurities, which prevent the reuse of the salts or may cause health and safety problems in the reuse of the salts. Furthermore, the settling of benzoic acid residues present in process effluents using benzoyl chloride as a reactant may lead to fouling and cleaning problems. Finally, higher carboxylic acids (≧ 8 carbon atoms) or their salts present in the waste stream of the process using higher acyl chlorides as reactants may end up in the aqueous layer and may cause bubbling, thereby reducing the capacity of the crystallizer.

In order for the salt to be suitable for reuse, it should contain no more than 1000ppm, preferably no more than 500ppm, even more preferably no more than 300ppm, most preferably no more than 200ppm of organic impurities, based on the weight of dry salt. The content of this organic impurity is defined as the non-purgable organic compound (NPOC) content, which can be determined as described in the examples below.

It has now been found that in order to alleviate these problems and to obtain salts suitable for reuse, the waste liquor is first acidified to a pH in the range of 1-5, followed by separation of an organic liquid layer, and then the pH of the waste liquor is raised again to a value in the range of 6-14.

Acidification brings the benzoic acid and higher carboxylic acids (typically present in process streams when producing organic peroxides) into the organic layer, which separates them from the aqueous layer before crystallization. The subsequent high pH is used to prevent corrosion of the stainless steel crystallization equipment and/or prevent precipitation of organic acids.

Accordingly, the present invention relates to a process for the production of a salt comprising NaCl and/or KCl from an aqueous waste stream of one or more organic peroxide production processes, said process comprising the steps of:

a) ensuring that the pH value of the aqueous waste liquid is within the range of 1-5,

b) the waste liquid is separated into a liquid organic layer and a water layer,

c) the liquid organic layer is removed and the organic layer,

d) the pH of the aqueous layer is raised to a value between 6 and 14,

e) the salt is crystallized from an aqueous layer having a pH in the range of 6-14.

It is noted that CN108423908 discloses a method for treating waste streams of organic peroxide processes, comprising the steps of: the waste liquid is acidified, 4-methylbenzoic acid is separated in solid form by precipitation and the pH of the remaining stream is increased to between 5 and 7, followed by a precipitation step to separate NaCl. The purpose of the process disclosed in CN108423908 is mainly to separate and recover unreacted 4-methylbenzoic acid, rather than mainly to recover salts of high purity. It has surprisingly been found that when the separation in step b) is a liquid-liquid separation separating an organic liquid phase and an aqueous liquid phase, the purity of the separated salt is significantly improved, the salt comprising a significantly lower amount of (non-purgeable) organics compared to the process disclosed in CN 108423908. Alternatively, in the process of CN108423908, if it is desired to obtain low salts with a lower organic content, further purification steps are required, which makes the process unattractive and results in less separation of such salts, since some yield is lost in any purification step.

Aqueous waste liquid

By reacting acid chlorides or chloroformic acid salts with organic hydroperoxides or H₂O₂The reaction under alkaline conditions produces a peroxydiacyl, peroxyester, peroxycarbonate, or peroxydicarbonate, thereby producing an aqueous waste stream.

Examples of peroxy esters are tert-butyl peroxybenzoate, tert-amyl peroxybenzoate, cumyl peroxybenzoate, 1,3, 3-tetramethylbutyl peroxybenzoate, tert-butyl peroxyisobutyrate, tert-amyl peroxyisobutyrate, 1,3, 3-tetramethylbutyl peroxyisobutyrate, cumyl peroxyneodecanoate, 1,3, 3-tetramethylbutyl peroxyneodecanoate, tert-amyl peroxyneodecanoate, tert-butyl peroxyneodecanoate, 1,3, 3-tetramethylbutyl peroxypivalate, tert-amyl peroxypivalate, tert-butyl peroxypivalate, cumyl peroxypivalate, 1,3, 3-tetramethylbutyl peroxy-2-ethylhexanoate, tert-butyl peroxy2-ethylhexanoate, tert-butyl peroxy-2-ethylhexanoate, Cumyl peroxy-2-ethylhexanoate, tert-amyl peroxyacetate, tert-butyl peroxyacetate, cumyl peroxyacetate, tert-butyl peroxy-3, 5, 5-trimethylhexanoate, tert-amyl peroxy-3, 5, 5-trimethylhexanoate and cumyl peroxy-3, 5, 5-trimethylhexanoate.

Preferred peroxyesters are tert-amyl peroxypivalate, tert-butyl peroxypivalate, tert-amyl peroxy-2-ethylhexanoate, tert-butyl peroxy-3, 5, 5-trimethylhexanoate, cumyl peroxyneodecanoate and tert-butyl peroxyneodecanoate.

Examples of peroxycarbonates are tert-butyl peroxy-2-ethylhexyl carbonate, tert-amyl peroxy-2-ethylhexyl carbonate, cumyl peroxy-2-ethylhexyl carbonate, tert-butyl peroxy-2-ethylhexyl carbonate, 1,3, 3-tetramethylbutyl peroxy-2-ethylhexyl carbonate, tert-butyl peroxyisopropyl carbonate, tert-amyl peroxyisopropyl carbonate, cumyl peroxyisopropyl carbonate, tert-butyl peroxyisopropyl carbonate and 1,1,3, 3-tetramethylbutyl peroxyisopropyl carbonate.

The preferred peroxycarbonate is t-butyl peroxy-2-ethylhexyl carbonate.

Examples of diacyl peroxides are diisobutyryl peroxide, di-n-butyryl peroxide, diisovaleryl peroxide, di-n-valeryl peroxide, di-2-methylbutyryl peroxide, dihexanyl peroxide, dioctanoyl peroxide, dibenzoyl peroxide, acetyl isobutyryl peroxide, cyclohexanoyl peroxide, acetyl benzoyl peroxide, lauroyl peroxide, hexanoyl peroxide, propionyl isobutyryl peroxide, propionyl benzoyl peroxide, di (p-methylbenzoyl) peroxide, di (o-methylbenzoyl) peroxide, dilauroyl peroxide, di (3, 5, 5-trimethylhexanoyl) peroxide and didecanoyl peroxide.

Preferred diacyl peroxides are diisobutyryl peroxide, acetoacetisobutyryl peroxide, benzoyl peroxide, bis (p-methylbenzoyl) peroxide, bis (o-methylbenzoyl) peroxide, dilauroyl peroxide, and bis (3, 5, 5-trimethylhexanoyl) peroxide.

Examples of peroxydicarbonates are 3-methoxybutyl peroxydicarbonate, di-2-ethylhexyl peroxydicarbonate, di-sec-butyl peroxydicarbonate, diisopropyl peroxydicarbonate, diacetyl peroxydicarbonate, di- (4-tert-butylcyclohexyl) peroxydicarbonate, dimyristyl peroxydicarbonate and di-2-propylheptyl peroxydicarbonate.

A preferred peroxydicarbonate is di- (2-ethylhexyl) peroxydicarbonate.

Aqueous waste streams from one single peroxide production process may be used in the process of the invention, but mixtures of aqueous waste streams from two or more peroxide production processes may also be used. The advantage of using such a mixture is that the (combined) liquid organic phase formed and removed in step b) can be used as an extraction solvent for extracting water-soluble organic impurities from the aqueous layer of the mixture.

The aqueous waste liquid to be used in the process of the present invention typically contains more than 3 wt.%, more preferably more than 7 wt.%, even more preferably more than 12 wt.% of salts comprising NaCl and/or KCl.

The concentration of organic species in the waste stream is typically in the range of from 0.1 to 10 wt%, more preferably 0.3 to 7 wt%, most preferably 0.5 to 4 wt%. A part of these organics consists of organic (hydro) peroxide residues. These residues are generally present in the waste liquid in a concentration of 0.01 to 4 wt.%, preferably 0.05 to 3 wt.%, most preferably 0.1 to 2.5 wt.%.

Other organic species that may be present in these waste streams are peroxide decomposition products (e.g., acetone, methanol, methyl ethyl ketone, ethanol, t-butanol, or tert-amyl alcohol) and solvents (e.g., dimethyl phthalate, isododecane, odorless mineral oil, ethyl acetate, or toluene).

The pH of the waste liquid to be used in the process is preferably at least 8, even more preferably at least 10, most preferably at least 11.

Step a)

Step a) requires ensuring that the pH of the aqueous waste stream is in the range of 1 to 5, preferably in the range of 2 to 4. This means that if the pH is already within this range, no measures need to be taken.

However, since the waste liquid comes from a process carried out under alkaline conditions, it is generally necessary to lower the pH by adding an acid.

While many acids can be used, it is preferred to use HCl for spent liquors containing chloride anions. If the waste stream also contains sulfate anions, H may suitably be used₂SO₄Or NaHSO₄。

Step b)

The acidification of step a) usually results in the formation of an organic phase. Before or during the acidification step a), an organic solvent may optionally be added. If no or hardly any liquid organic phase is formed, it is necessary to add an organic solvent before or during the acidification step a) in order to extract the organic matter from the waste liquid, thereby reducing contamination and improving the phase separation and safety characteristics of the organic layer. In embodiments where little liquid organic phase is formed such that good liquid-liquid separation cannot be performed, an organic solvent is added.

The organic solvent is preferably a polar solvent having only limited solubility in water. Examples of suitable solvents are lower phthalates, C_6-18Carboxylic acid, C_6-18Alcohols, alcohols having more than 5 carbon atoms, ethers having more than 5 carbon atoms, alkanes having more than 5 carbon atoms, aromatics having more than 6 carbons, and mixtures of such solvents. Specific examples are 2-ethylhexanol, dimethyl phthalate, oleic acid, diesel oil, isomeric C₁₂Mixtures (e.g., isododecane), ester mixtures containing dimethyl or diethyl adipate, dioctyl adipate, dibutyl sebacate, dibutyl maleate, ethyl benzoate, toluene, xylene, and mixtures thereof.

The liquid organic layer will contain most of the organic components present in the waste stream, including organic peroxide residues, benzoic acid, alcohols and higher carboxylic acids.

Step c)

Removing the liquid organic layer formed in step b) from the aqueous layer. This can be achieved in a number of ways. For example, this can be done by gravity settling and then decanting the upper layer. An oil skimmer, i.e., a device comprising a rotating belt or slowly moving scraper which dips into the organic layer and removes it, may also be used. The liquid organic layer may also be removed by liquid-liquid separators, by applying centrifugal force, by plate separators, by flotation or by extraction.

The removed liquid organic layer can be transferred to a biological wastewater treatment unit as such or dispersed in (alkaline) water. The organic layer may also be disposed of as an organic liquid waste or used as a fuel after washing and/or neutralization. If an organic solvent is added during step b, the organic layer in a preferred embodiment is optionally purified, for example by washing with an aqueous alkaline solution, for reuse as extraction solvent in step b).

If desired, volatile organic compounds, such as acetone, methanol, acetic acid, formic acid, isobutyric acid, n-butyric acid, pivalic acid, tert-amyl hydroperoxide and tert-butyl hydroperoxide, can be removed by stripping. This can be done after the acidification step a) and before or after the removal of the liquid organic layer.

The stripping is generally carried out at a temperature in the range from 90 to 120 ℃ and a pressure in the range from 0.1 to 0.2 MPa.

The resulting vapor stream may be condensed and sent to a biological wastewater treatment unit.

Step d)

After separation of the liquid organic layer, the pH of the aqueous layer is increased to a value in the range of 6-14, preferably 7-13, more preferably 8-13, even more preferably 9-13 and most preferably 11-13, to prevent corrosion of the crystallization equipment used in step e) and/or to prevent precipitation of organic acids.

The pH is preferably raised by addition of KOH or NaOH.

Step e)

The salt may be crystallized in various ways. One of these is evaporative crystallization. The crystallization temperature and pressure depend on the boiling temperature of the salt solution and the configuration of the crystallizer. The temperature is generally between 50 and 150 ℃; the pressure is between 50 mbar and 4 bar.

The crystallizer may be of any conventional type, such as a forced circulation crystallizer driven by Mechanical Vapor Recompression (MVR) or a steam driven single or multiple effect crystallizer, optionally in combination with Thermal Vapor Recompression (TVR); or simply a spray dryer.

For KCl crystallization, cooling crystallization is also possible.

For NaCl crystallization, a 2-effect or 3-effect steam-driven crystallizer or MVR is preferred.

Crystallizing to obtain salt slurry. The slurry can be washed, if required for quality reasons, using for example (push) centrifuges, elutriation tubes, washing columns or washing vessels, etc. The washing may be performed with clean water or brine, e.g. brine from a previous washing cycle. The salt is collected from the slurry as "wet salt" by gravity settling, centrifugation, filtration or any other suitable solid-liquid separation technique. The filtrate may be discarded or (partly) recycled to step a) or step e).

In one embodiment, the wet salt, after separation from the adhering liquid, may also be washed on a filter or another suitable device.

The resulting "wet" salt (washed or unwashed) is dried if necessary. Drying may be carried out in any conventional dryer, such as a fluid bed dryer or a belt dryer.

The water content of the salt obtained is preferably less than 10% by weight, more preferably less than 5% by weight, and after drying preferably less than 0.5% by weight. It preferably contains less than 1000ppm, more preferably less than 500ppm, even more preferably no more than 300ppm, and most preferably no more than 200ppm of non-purgeable organic compounds (NPOC), based on dry salt weight, to make it suitable for reuse.

If desired, an anti-caking agent may be added to the salt. Examples of anti-caking agents are sodium ferricyanide (sodium xanthate, YPS) or silicon dioxide. The amount of the anti-caking agent added is generally 5 to 100 ppm. The resulting salt can be discarded as waste, but is preferably recycled. It can be reused in various applications, such as textile production, leather tanning, fertilizer or chlor-alkali processes.

Examples

In all examples, NPOC was measured using a shimadzu TOC (total organic carbon) analyzer. First, the salt is dissolved in water, acidified by addition of HCl, and then acidified with N₂And (5) purging. The sample was then burned at 680 c in a tube in the presence of a Pt catalyst. Determination of CO formed Using a non-dispersive Infrared (NDIR) Detector₂And the amount of carbon relative to an external standard (potassium hydrogen phthalate) was calculated.

Example 1

To the pilot scale reactor was added 30l of a mixture of several aqueous waste streams from the organic peroxide production process. The mixture had a pH of 10.5 and an approximate composition of:

NaCl	15wt％
		sodium benzoate	1wt％
2-Ethyl hexanoic acid sodium salt	2wt％
		Tert-butyl hydroperoxide	2wt％
Water (W)	Balance of

The mixture was acidified to pH 2.5 by addition of 30 wt% HCl solution followed by addition of 0.5 wt% dimethyl phthalate (based on the total weight of the mixture). A clear organic layer was obtained which could be easily separated from the aqueous phase.

To the resulting aqueous phase was added a 30 wt% NaOH solution until the pH was 10.5. The mixture was then transferred to a pilot scale batch crystallizer and heated to about 110 ℃ at a pressure of 1 bar. During the crystallization, water vapor is removed overhead. After a period of time, NaCl crystals began to form. The resulting salt slurry was transferred to a centrifuge. During centrifugation, the salt was washed with a total amount of wash water of 200ml/kg salt. The resulting NaCl had a water content of 4 wt% and a non-purgeable organic carbon (NPOC) content of 233 ppm.

Example 2

To the pilot scale reactor was added 30l of a mixture of several aqueous waste streams from the organic peroxide production process. The mixture had a pH of about 11 and an approximate composition of:

KCl	15wt％
		new sodium decanoate	2wt％
Cumyl hydroperoxide	1.5wt％
		Water (W)	Balance of

The mixture was acidified to pH 2.5 by addition of 30 wt% HCl solution. A clear liquid organic layer was formed on top of the aqueous phase. The organic layer was separated from the aqueous phase using a mini-skimmer (ex-Abanaki).

To the resulting aqueous phase was added a 30 wt% KOH solution to a pH of 10.5. The mixture was then transferred to a pilot scale batch crystallizer and heated to about 50 ℃ at a pressure of 60 mbar. During the crystallization, water vapor is removed overhead. After a period of time, KCl crystals begin to form. The resulting salt slurry was transferred to a centrifuge. During centrifugation, the salt was washed with a total amount of wash water of 300ml/kg salt. The KCl obtained had a water content of 4 wt% and an NPOC content of 215 ppm. The KCl was transferred to a pilot scale fluid bed dryer. The water content of the resulting dried salt was 0.2 wt%.

Example 3

To the pilot scale reactor was added 30l of a mixture of several aqueous waste streams from the organic peroxide production process. The mixture had the following approximate composition:

NaCl	15wt％
		sodium benzoate	0.3wt％
2-Ethyl hexanoic acid sodium salt	2wt％
		Tert-butyl hydroperoxide	2wt％
Water (W)	Balance of

The mixture was acidified to pH 2.5 by addition of 30 wt% HCl solution and the organic layer obtained in example 2 was added. A clear organic layer was obtained which could be easily separated from the aqueous phase.

To the resulting aqueous phase was added a 30 wt% NaOH solution until the pH was 10.5. The mixture was then transferred to a pilot scale batch crystallizer and heated to about 110 ℃ at a pressure of 1 bar. During the crystallization, water vapor is removed overhead. After a period of time, NaCl crystals began to form. The resulting salt slurry was transferred to a centrifuge. During centrifugation, the salt was washed with a total amount of washing water of 400ml/kg salt. The NaCl content obtained was 4% by weight and the NPOC content 177 ppm.

Example 4

NaCl	15wt％
		KCl	3wt％
Na₂SO₄	2wt％
		mixing organic matter	4wt％
Water (W)	Balance of

The mixture was acidified to pH 2.5 by addition of 30 wt% HCl solution. A clear liquid organic layer was formed on top of the aqueous phase. The organic layer was separated from the aqueous phase.

To the resulting aqueous phase was added a 30 wt% NaOH solution until the pH was 10.5. The mixture was then transferred to a pilot scale batch crystallizer and heated to about 110 ℃ at a pressure of 1 bar. During the crystallization, water vapor is removed overhead. After a period of time, salt crystals begin to form. The resulting salt slurry was transferred to a centrifuge. The resulting salt mixture had a water content of 6 wt% and an NPOC content of 760 ppm.

Comparative example 5

30l of the waste stream mixture used in example 1 were transferred to a pilot scale batch crystalliser and heated to about 110 ℃ at a pressure of 1 bar. After a period of time, NaCl crystals began to form. At approximately the same time, a precipitate of sodium benzoate begins to form. Crystallization was attempted to continue until a reasonable slurry density was formed, but sodium benzoate and other precipitates plugged the equipment. Since the salt/organic mixture did not dewater well, the slurry could not be centrifuged to a water content < 10%.

This experiment illustrates that the acidification step performed in example 1 is necessary for proper separation of the salt.

Comparative example 6

30l of the waste stream mixture used in example 2 were transferred to a pilot scale batch crystalliser and heated to about 50 ℃ at a pressure of 60 mbar. During the crystallization, water vapor is removed overhead. After a period of time, KCl crystals begin to form. The resulting salt slurry was transferred to a centrifuge. In the centrifuge, foaming was observed, which resulted in slow and incomplete centrifugation. An attempt was made to wash the salt with a total amount of wash water of 300ml/kg salt. The salt was transferred to a pilot scale fluid bed dryer. The NPOC content of the resulting dried salt was 2600 ppm.

This experiment illustrates that the acidification step performed in example 2 is necessary to obtain salts with sufficiently low organic content.

Example 7

Benzoyl peroxide is prepared from benzoyl chloride, H2O 2-30%, NaOH-25% and surfactant. The reaction mixture was filtered to separate the product and the aqueous layer. The aqueous layer had the following composition:

NaCl	7wt％
		sodium benzoate	0,6wt％
Sodium peroxybenzoate	0.03wt％
		Water (W)	Balance of

To 100.4g of the aqueous layer was added 8.04g of dimethyl phthalate (DMP), measured with a Knick pH meter and a Mettler Toledo Inlab pH electrode, the pH of the aqueous layer was > 6. The combined mixture was acidified to pH 2.3 by addition of 0.84g of 18 wt% HCl solution at 20-25 ℃ with stirring. After further stirring for 5 minutes or more, the layers were separated. The lower DMP layer was separated from the aqueous layer. The resulting aqueous phase, which had a benzoic acid content of 0.07%, was neutralized with a 30 wt% NaOH solution to a pH > 7. The mixture was then transferred to a batch crystallizer and heated to about 110 ℃ at a pressure of 1 bar. During the crystallization, water vapor is removed overhead. After a period of time, salt crystals begin to form. The resulting salt slurry was transferred to a centrifuge. The resulting salt had a water content of 6 wt% and an NPOC content of 620 ppm. The DMP layer was washed with 3% NaOH solution until pH >7 and reused in the extraction of benzoic acid.

Comparative example 8

The aqueous layer of 77.3g of the benzoyl peroxide process of example 7 was acidified to pH 2.3 by the addition of 0.64g of 18 wt% HCl solution at 20-25 ℃ with stirring. A white precipitate formed in the aqueous phase. After stirring for more than 5 minutes, the solid was filtered. The resulting clear aqueous phase, having a benzoic acid content of 0.17%, was neutralized with 30 wt% NaOH solution to a pH > 7. The mixture was then transferred to a batch crystallizer and heated to about 110 ℃ at a pressure of 1 bar. During the crystallization, water vapor is removed overhead. After a period of time, salt crystals begin to form. The resulting salt slurry was transferred to a centrifuge. The resulting salt had a water content of 6 wt% and an NPOC content of 1490 ppm.

This experiment illustrates that the addition of solvent in the acidification step and the subsequent liquid-liquid separation performed in example 7 are necessary to obtain salts with sufficiently low organic content.

Claims

1. A process for the production of salts comprising NaCl and/or KCl from an aqueous waste stream from one or more organic peroxide production processes, the process comprising the steps of:

a) ensuring that the pH of the aqueous waste liquor is in the range of 1-5,

b) separating the waste liquid into a liquid organic layer and an aqueous layer,

c) the organic layer is removed and the organic layer is removed,

d) raising the pH of the aqueous layer to a value in the range of 6-14,

e) crystallizing the salt from the aqueous layer having a pH in the range of 6-14.

2. The method of claim 1, wherein step a) involves acidifying the spent liquor to a pH in the range of 1-4.

3. The method of claim 2, wherein the acidifying is performed using HCl.

4. The process according to any of the preceding claims, wherein prior to step d), the aqueous layer is subjected to stripping to remove volatile organic compounds.

5. The process of claim 4, wherein the volatile organic compound comprises isobutyric acid, n-butyric acid, pivalic acid, tertbentyl hydroperoxide and/or tert-butyl hydroperoxide.

6. The process according to any of the preceding claims, wherein an organic solvent is added before or during step b).

7. The process according to claim 6, wherein the organic layer collected in step c) is recycled and reused as organic solvent, optionally after washing with aqueous alkaline solution.

8. The method according to any of the preceding claims, wherein step c) involves the use of an oil skimmer.

9. The process according to any one of the preceding claims, wherein step e) involves evaporating the aqueous phase, thereby crystallizing the salt to form a salt slurry.

10. The process according to any of the preceding claims, wherein the salt obtained in step e) is dried to a water content of less than 0.5 wt%.