PROCESS FOR THE RECOVERY OF EPSILON-CAPROLACTAM FROM POLYAMIDE 6 COMPRISING FISHING NETS

Abstract

The present invention provides a process and a plant for recovering purified-caprolactam from polyamide 6 comprising fishing nets, wherein the plant comprises a depolymerization section [B], a recovery section [C], and a purification section [D]. The present invention also provides purified -caprolactam that has a particularly low product carbon footprint and is obtained via depolymerization of polyamide 6 from fishing nets.

Claims

1. A process for recovering purified -caprolactam from material derived from polyamide 6 comprising fishing nets in a plant, wherein the plant comprises a depolymerization section [B], a recovery section [C], and a purification section [D], and wherein the process comprises the steps of: a) charging material derived from polyamide 6 comprising fishing nets to the depolymerization section [B]; b) depolymerizing the material derived from polyamide 6 comprising fishing nets in the depolymerization section [B] at a temperature ranging from 180 C. to 400 C. so that an -caprolactam comprising stream is obtained; c) discharging the -caprolactam comprising stream from the depolymerization section [B] and recovering crude -caprolactam from said stream in the recovery section [C]; and d) purifying said crude -caprolactam in the purification section [D] to obtain purified -caprolactam wherein the purification comprises the steps of (i) extracting the crude -caprolactam with an organic solvent, whereby an organic phase is obtained, and wherein the organic phase comprises the organic solvent, -caprolactam and impurities; (ii) switching the solvent by replacing the organic solvent at least partially with water, whereby an aqueous phase comprising water, -caprolactam and impurities with lower- or higher-boiling points than -caprolactam is obtained and wherein the solvent switch step (ii) is selected from a process based on back-extraction with water, and a process based on solvent swap distillation, in which the organic solvent is distilled off and water is charged; and (iii) obtaining purified -caprolactam by distillative removal of impurities with lower- or higher-boiling points than -caprolactam from said aqueous phase; and at least one additional step comprising: after stepd)(i), the organic phase obtained in stepd)(i) is washed with water or with an aqueous alkaline solution, and wherein prior to the distillative removal in stepd)(iii), an alkali metal hydroxide, is added to the aqueous phase.

2. The process according to claim 1, wherein the depolymerization in step b) is performed in the presence of water, whereby the -caprolactam comprising stream is a vapor stream comprising -caprolactam and water in a weight to weight ratio of 1:2 to 1:15; and wherein in the extraction in stepd)(i) both an aqueous phase and an organic phase are obtained.

3. The process according to claim 1, wherein prior to the distillative removal in stepd)(iii), the purification in step d) also comprises the step of oxidizing with an oxidant in aqueous solution at a temperature ranging from 20 C. to 85 C., wherein the oxidant is selected from the group consisting of potassium permanganate, sodium permanganate and hydrogen peroxide and combinations thereof.

4. The process according to claim 2, wherein the water present in step b) is in the form of steam, which is charged to the depolymerization section [B] in step b) as superheated steam having a temperature ranging from 220 C. to 575 C.

5. The process according to claim 1, wherein the solvent switching in stepd)(ii) is a process based on back-extraction with water.

6. The process according to claim 1, wherein the solvent switching in stepd)(ii) is a process based on solvent swap distillation.

7. The process according to claim 3, wherein the organic solvent in stepd)(i) is selected from the group consisting of cyclohexane, benzene, toluene, methylene chloride, chloroform, trichloroethane, 4 methyl-2-pentanol, 1-octanol, 2-ethylhexanol and mixtures thereof.

8. The process according to claim 3, wherein the oxidizing in step d) is performed in an aqueous solution comprising water and -caprolactam in a weight to weight ratio of 5:1 to 1:5.

9. The process according to claim 1, wherein the depolymerization in step b) is carried out in the absence or presence of a catalyst, wherein the catalyst is selected from an acid and a base catalyst, the acid catalyst being selected from the group consisting of orthophosphoric acid, boric acid, sulfuric acid, organic acid, organic sulfonic acid, salts of the aforementioned acids, Al.sub.2O.sub.3 and SiO.sub.2, and combinations thereof, and the base catalyst being selected from the group consisting of alkali hydroxide, alkali salt, alkaline earth hydroxide and alkaline earth salts, organic bases and solid bases, and combinations thereof.

10. The process according to claim 1, wherein the depolymerization in step b) is carried out in the absence of a catalyst or in the presence of orthophosphoric acid.

11. The process according to claim 1, wherein prior to step a) the material derived from polyamide 6 comprising fishing nets is obtained as the product of a pre-treatment in a pre-treatment section [A], wherein the pre-treatment is selected from at least one of: in particular a cleaning in cleaning section [], a and/or mechanical size reduction in a mechanical size reduction section [] and/or increasing the bulk density in a densification section [].

12-15. (canceled)

16. The process of claim 1, wherein prior to the distillative removal in stepd)(iii), the alkali metal hydroxide is NaOH.

17. The process of claim 3, wherein the oxidant is potassium permanganate.

18. The process of claim 4, wherein the superheated steam has a temperature ranging from 275 C. to 500 C.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0172] In the following, the invention is described with reference to the Figures, which depict certain embodiments of the invention. The invention, however, is as defined in the claims and as generally described herein. It should not be limited to the embodiments shown for illustrative purposes in the Figures below.

[0173] FIG. 1 is a schematic of the process of the invention comprising processing steps performed in an optional pre-treatment section [A], a depolymerization section [B], a recovery section [C] and a purification section [D].

[0174] FIG. 2 illustrates two embodiments of the pre-treatment section [A], in which the polyamide 6 comprising fishing nets are cleaned in a cleaning section [] by removal of foreign materials and by washing with a washing solvent and fragmented in a mechanical size reduction section [B] to obtain cleaned and fragmented pieces of polyamide 6 comprising fishing nets.

[0175] FIG. 2A depicts an embodiment of the pre-treatment section [A], in which polyamide 6 comprising fishing nets are first cleaned in a cleaning section [] by removal of foreign materials and by washing with a washing solvent and then fragmented in a mechanical size reduction section [B] to obtain cleaned and fragmented pieces of polyamide 6 comprising fishing nets.

[0176] FIG. 2B depicts an embodiment of the pre-treatment section [A], in which polyamide 6 comprising fishing nets are first fragmented in a mechanical size reduction section [] and then cleaned in a cleaning section [] by removal of foreign materials and by washing with a solvent to obtain cleaned and fragmented pieces of polyamide 6 comprising fishing nets.

[0177] FIG. 3 illustrates two embodiments of the purification section [D], in which crude -caprolactam is purified to obtain high purity -caprolactam.

[0178] FIG. 3A depicts an embodiment of the purification section [D] of the process of the invention, that comprises an extraction section [], optionally a washing section [], a back-extraction section [], optionally an oxidation section [], and a distillation section [A].

[0179] FIG. 3B depicts an embodiment of the purification section [D] of the process of the invention, that comprises an extraction section [], optionally a washing section [], a solvent swap distillation section [], optionally an oxidation section [], and a distillation section [].

DETAILED DESCRIPTION OF THE DRAWINGS

[0180] The process of the invention is schematically illustrated in FIG. 1. The process is carried out in the following plant sections:

[0181] Optionally, the polyamide 6 comprising fishing nets [1] are cleaned by removal of foreign materials and by washing with a washing solvent [2] in pre-treatment section [A], whereby contaminated washing solvent [3] is obtained. Next, polyamide 6 comprising fishing nets are fragmented by mechanical size reduction. The cleaned and fragmented polyamide 6 comprising fishing nets [6] are discharged from the pre-treatment section [A]. Optionally, the polyamide 6 comprising fishing nets [1] are additionally cleaned by removal of foreign materials in the pre-treatment section [A]. Removal of foreign materials can be done prior and/or after fragmentation of the polyamide 6 comprising fishing nets. Optionally, the optionally cleaned and fragmented nylon 6 comprising fishing nets are densified before being depolymerized to -caprolactam in the depolymerization section [B] (not shown in FIG. 1).

[0182] The optionally cleaned and fragmented polyamide 6 comprising fishing nets [6] are depolymerized to -caprolactam in the depolymerization section [B]. An -caprolactam comprising stream [7] is discharged from the depolymerization section [B]. In addition, residual material [8] is discharged. Optionally, superheated steam [9] and catalyst are charged to the depolymerization section [B].

[0183] Crude -caprolactam is recovered from the -caprolactam comprising stream [7] that is discharged from the depolymerization section [B] in recovery section [C]. In addition, an aqueous phase is discharged from the recovery section [C] in case water or superheated steam [9] was charged to depolymerization section [B].

[0184] Crude -caprolactam that is discharged from recovery section [C] is purified to yield high purity -caprolactam in purification section [D]. Water and impurities are also discharged from the purification section [D].

[0185] FIG. 2A depicts an embodiment of the pre-treatment section [A] (area enclosed by dashed line), in which polyamide 6 comprising fishing nets [1] are first cleaned in a cleaning section [a] by removal of foreign materials and by washing with a washing solvent [2] whereby foreign materials, contaminated washing solvent [3] and cleaned polyamide 6 comprising fishing nets [4] are obtained. Subsequently, the cleaned polyamide 6 comprising fishing nets [4] are fragmented in a mechanical size reduction section [] to obtain cleaned and fragmented pieces of polyamide 6 comprising fishing nets [6]. The cleaned and fragmented pieces are then discharged. Optionally, the cleaned and fragmented nylon 6 comprising fishing nets are densified before being depolymerized to -caprolactam in the depolymerization section [B] (not shown in FIG. 2A).

[0186] FIG. 2B depicts an embodiment of the pre-treatment section [A] (area enclosed by dashed line), in which polyamide 6 comprising fishing nets [1] are first fragmented in a mechanical size reduction section [] to obtain fragmented pieces of polyamide 6 comprising fishing nets [5]. Subsequently, the fragmented pieces of polyamide 6 comprising fishing nets [5] are cleaned in a cleaning section [] by removal of foreign materials and by washing with a washing solvent [2] to obtain foreign materials, contaminated washing solvent [3], and cleaned and fragmented pieces of polyamide 6 comprising fishing nets [6] that are discharged. Optionally, the cleaned and fragmented nylon 6 comprising fishing nets are densified before being depolymerized to -caprolactam in the depolymerization section [B] (not shown in FIG. 2B).

[0187] FIG. 3A depicts an embodiment of the purification section [D] (area enclosed by dashed line), that comprises the following sections:

[0188] In the extraction section [], crude -caprolactam [11] is extracted with an organic solvent [13] to obtain an aqueous phase comprising water and impurities [14] and an organic phase comprising the organic solvent, -caprolactam and impurities [15]. Both phases are discharged from the extraction section [].

[0189] In the optional washing section [], the organic phase comprising the organic solvent, -caprolactam and impurities [15] is washed with water or an aqueous alkaline solution [16] to obtain an aqueous residue-comprising phase [17] and a washed organic phase that comprises organic solvent, -caprolactam and impurities [18]. Both phases are discharged from the washing section []. In the back-extraction section [E], the optionally washed organic phase that comprises the organic solvent, -caprolactam and impurities [18] is back-extracted with water [19] to obtain an organic solvent phase that comprises impurities [20] and an aqueous phase comprising water, -caprolactam and impurities with lower- or higher-boiling points than .Math.-caprolactam [22]. Both phases are discharged from the back-extraction []. Optionally, residual organic solvent is removed by stripping and/or distillation from the aqueous phase comprising water, -caprolactam and impurities with lower- or higher-boiling points than -caprolactam [22] (not shown in FIG. 3A).

[0190] In the optional oxidation section [], the aqueous phase comprising water, -caprolactam and impurities with lower- or higher-boiling points than -caprolactam [22], from which optional residual organic solvent has been removed by stripping and/or distillation, is oxidized with an oxidant [23] to obtain an oxidized -caprolactam-water phase comprising water, -caprolactam, and impurities [24]. This phase is discharged from the oxidation section []. Optionally, the oxidized -caprolactam-water phase comprising the water, -caprolactam and impurities [24] is filtered to remove solid manganese (IV) oxide particles before being discharged from the oxidation section [] (not shown in FIG. 3A).

[0191] In distillation section [], the optionally oxidized-caprolactam-water phase comprising water, -caprolactam and impurities [24] from which optionally solid manganese (IV) oxide particles have been removed by filtration, is distilled to obtain water and impurities [25] (i.e., mainly water, low-boiling organic impurities, and high-boiling organic impurities) and high purity -caprolactam [26]. All of the distillation products are discharged from the distillation section []. Optionally, prior to the distillation in distillation section [], an alkali metal hydroxide is dosed to the optionally oxidized -caprolactam-water phase comprising water, -caprolactam and impurities [24] (not shown in FIG. 3A).

[0192] FIG. 3B depicts an embodiment of the purification section [D] (area enclosed by dashed line) that comprises the following sections:

[0193] In extraction section [], crude -caprolactam [11] is extracted with an organic solvent [13] to obtain an aqueous phase comprising water and impurities [14] and an organic phase comprising organic solvent, -caprolactam and impurities [15]. Both phases are discharged from the extraction section [].

[0194] In the optional washing section [], the organic phase comprising the organic solvent, -caprolactam and impurities [15] is washed with water or an aqueous alkaline solution [16] to obtain an aqueous residue-comprising phase [17] and a washed organic phase that comprises organic solvent, -caprolactam and impurities [18]. Both phases are discharged from the washing section [].

[0195] In the solvent swap distillation section [], the optionally washed organic phase comprising the organic solvent, -caprolactam and impurities [18] is solvent swap distilled with addition of water [19] to obtain organic solvent [21] and an aqueous phase comprising water, -caprolactam and impurities with lower- or higher-boiling points than -caprolactam [22]. Both distillation products are discharged from the solvent swap distillation section [].

[0196] In an optional oxidation section [], the aqueous phase comprising water, -caprolactam and impurities with lower- or higher-boiling points than -caprolactam [22], from which optionally residual organic solvent has been removed by stripping and/or distillation, is oxidized with an oxidant [23] to obtain an oxidized -caprolactam-water phase comprising water, -caprolactam and impurities [24]. This phase is discharged from the oxidation section []. Optionally, the oxidized -caprolactam-water phase comprising water, -caprolactam and impurities [24] is filtered to remove solid manganese (IV) oxide particles before being discharged from the oxidation section [] (not shown in FIG. 3B).

[0197] In distillation section [], the optionally oxidized -caprolactam-water phase comprising water, -caprolactam and impurities [24] from which optionally solid manganese (IV) oxide particles have been removed by filtration, is distilled to obtain water and impurities [25] (i.e., mainly water, low-boiling organic impurities, and high-boiling organic impurities) and high purity -caprolactam [26]. All of the distillation products are discharged from the distillation section []. Optionally, prior to the distillation section [], an alkali metal hydroxide is dosed to the optionally oxidized -caprolactam-water phase comprising water, -caprolactam and impurities [24] that is charged to the distillation section [] (not shown in FIG. 3B).

EXAMPLES

[0198] The following examples serve to explain the invention in more detail, in particular with regard to certain forms of the invention. The examples, however, are not intended to limit the present disclosure.

[0199] -caprolactam, that can be used for all major polyamide 6 polymerization applications, without dilution with purer qualities of -caprolactam, fulfils all of the following specifications: [0200] PAN: max. 5 [0201] E290: max. 0.05 [0202] VB: max. 0.5 mmol/kg [0203] Alkalinity: max. 0.1 mmol/kg.
The parameters and measurement methods are defined as follows: [0204] PAN: ISO DIS 8660 Caprolactam for industrial use-Determination of permanganate index of caprolactam-Spectrometric method, revision of first edition ISO 8660; 1988, [0205] E290: ISO 7059-Caprolactam for industrial use-determination of absorbance at a wavelength of 290 nm, 1982, [0206] Volatile bases (VB) ISO 8661-Caprolactam for industrial use-Determination of volatile bases content-Titrimetric method after distillation, 1988.

[0207] Alkalinity of -caprolactam product: the alkalinity is determined by titration at a temperature of 25 C. using a Tashiro indicator in a 1:2 ratio of 0.1 wt./V.sub.Ethanol % Methylene blue:0.1 wt./V.sub.Ethanol % Methyl red, which is grey at its end point. A flask containing water and indicator is first titrated to grey, then X grams of an aqueous -caprolactam solution containing Y wt. % -caprolactam (as determined by refractive index) is added and the solution is titrated back to grey using a 0.01 N H.sub.2SO.sub.4 solution. Alkalinity is then given by:

[00001]$\mathrm{Alkalinity}\left(\mathrm{mmol}/\mathrm{kg}-\mathrm{caprolactam}\right)=v*t*1000/\left(X*Y\right)$

[0208] Where: [0209] v=volume of H.sub.2SO.sub.4 solution added (ml) [0210] t=normality of H.sub.2SO.sub.4 solution (=0.01 N) [0211] X=weight of sample (g) [0212] Y=concentration -caprolactam (wt. %)

[0213] The polyamide 6 comprising pellets that were used in the EXAMPLES and the COMPARATIVE EXPERIMENTS were made from discarded fishing nets. The pre-treatment included removal of foreign materials, washing, grinding, melting and conversion into chips/pellets. The pellets were obtained from a fishing net recycling company in China.

[0214] The pellets were rod-shaped and had an average diameter of about 3 mm and an average length of about 4 mm and the weight of most of the pellets was between 20 and 30 mg.

[0215] Thermal Gravimetric Analysis (TGA), combined with qualitative information from Differential Scanning calorimetry (DSC), revealed that the polyamide 6 content of the pellets was >98 wt. % (on dry basis).

Example 1

Depolymerization of Polyamide 6 and Recovery of -Caprolactam

[0216] 48 grams of polyamide 6 comprising chips/pellets and 14 grams of 20 wt. % phosphoric acid were charged to a Premex high pressure autoclave. First, the reactor content was heated under nitrogen and subsequently superheated steam was injected continuously at a rate of 4 grams per minute during the 120-minute reaction. The temperature and the pressure in the reactor were maintained at 260 C. and 0.11 MPa, respectively. During the reaction, a vapor stream was continuously discharged from the reactor and was cooled down to ca. 20 C., whereby an -caprolactam and water comprising condensate was obtained.

[0217] The condensate that was composed of circa 44 grams of -caprolactam, most of the remainder being water, was concentrated by evaporation in a rotavap (rotary evaporator) that was operated under vacuum (9.5 kPa; water bath temperature was ca. 65 C.) to an -caprolactam concentration of 49.7 wt. %.

[0218] (This mixture, crude -caprolactam, is the mixture to be purified.)

[0219] The specifications of the crude -caprolactam were: [0220] PAN: 16 [0221] E290: 2.33

[0222] This EXAMPLE shows that crude -caprolactam can be obtained by depolymerization of polyamide 6 that originates from discarded polyamide 6 comprising fishing nets. Due to the very poor quality, this crude -caprolactam cannot be used as-such for all major polyamide 6 polymerization applications.

Comparative Experiment 1

Depolymerization of Polyamide 6, Recovery of -Caprolactam and Distillative Purification

[0223] The procedure of EXAMPLE 1 was followed. Subsequently, 75 mmol of aqueous sodium hydroxide per kg -caprolactam was added. Then, water and impurities with lower boiling points than -caprolactam were removed as top products by distillation under reduced pressure in a batch-wise operated distillation set-up, whereby the pressure was stepwise reduced. -caprolactam was distilled at 300 Pa, while the impurities with higher boiling points compared to -caprolactam remained as bottom product in the distillation set-up. The specifications of the distilled -caprolactam were: [0224] PAN: 5 [0225] E290: 0.09 [0226] VB: 0.34 mmol/kg [0227] Alkalinity: 0.25 mmol/kg.

[0228] This COMPARATIVE EXPERIMENT shows that the quality of -caprolactam that is obtained from depolymerization of polyamide 6 that originates from discarded polyamide 6 comprising fishing nets and was purified by distillation is very poor as it does not meet most of the required specifications for major polymerization applications.

Comparative Experiment 2

Depolymerization of Polyamide 6, Recovery of -Caprolactam and Purification by Oxidation

[0229] The procedure of EXAMPLE 1 was followed. Subsequently, the crude -caprolactam was treated with 0.2 wt. % KMnO.sub.4 with regard to -caprolactam at 50 C. for 2 hours. The solids formed were then removed from the oxidized reaction product by means of a filtration. The specifications of the purified -caprolactam were: [0230] PAN: 30 [0231] E290: 3.62

[0232] This COMPARATIVE EXPERIMENT shows that the quality of -caprolactam that is obtained from depolymerization of polyamide 6 that originates from discarded polyamide 6 comprising fishing nets and was purified by oxidation is very poor and cannot be used as-such for all major polyamide 6 polymerization applications.

Comparative Experiment 3

Depolymerization of Polyamide 6, Recovery of -Caprolactam and Purification by Oxidation and Distillation

[0233] The aqueous -caprolactam solution which was purified by oxidation that was obtained in COMPARATIVE EXPERIMENT 2 was then, after addition of 75 mmol of aqueous sodium hydroxide per kg -caprolactam, distilled according to the procedure described in COMPARATIVE EXPERIMENT 1. The specifications of the obtained purified -caprolactam were: [0234] PAN: 3 [0235] E290: 0.09 [0236] VB: 0.84 mmol/kg [0237] Alkalinity: 0.35 mmol/kg.

[0238] This COMPARATIVE EXPERIMENT shows that the quality of -caprolactam that is obtained from depolymerization of polyamide 6 that originates from discarded polyamide 6 comprising fishing nets and was purified by oxidation and distillation is poor and cannot be used as-such for all major polyamide 6 polymerization applications.

Comparative Experiment 4

Depolymerization of Polyamide 6, Recovery of -Caprolactam and Purification by Oxidation, Carbon Treatment and Distillation

[0239] The procedure of EXAMPLE 1 was followed. Subsequently, the crude -caprolactam was treated with 0.2 wt. % KMnO.sub.4 with regard to -caprolactam at 50 C. for 2 hours. Then, the resulting oxidized solution was treated with 0.4 wt. % powdered activated carbon at 50 C. for 0.5 hours. Afterwards, the solids formed and the activated carbon particles were removed from the aqueous -caprolactam solution by means of a filtration. This activated carbon treated aqueous -caprolactam solution was then, after addition of 75 mmol of aqueous sodium hydroxide per kg -caprolactam, distilled according to the procedure described in COMPARATIVE EXPERIMENT 1. The specifications of the obtained purified -caprolactam were: [0240] PAN: 3 [0241] E290: 0.08 [0242] VB: 0.43 mmol/kg [0243] Alkalinity: 0.29 mmol/kg.

[0244] This COMPARATIVE EXPERIMENT shows that the quality of -caprolactam that is obtained from depolymerization of polyamide 6 that originates from discarded polyamide 6 comprising fishing nets and was purified by oxidation, carbon treatment and distillation is poor and cannot be used as-such for all major polyamide 6 polymerization applications.

Example 2

Depolymerization of Polyamide 6, Recovery of -Caprolactam and Purification by Extraction, Back-Extraction, Oxidation and Distillation.

[0245] The procedure of EXAMPLE 1 was followed. The condensate that was composed of 44.5 grams of -caprolactam, most of the remainder being water, was concentrated by evaporation in a rotavap that was operated under vacuum (9.5 kPa; water bath temperature was ca. 65 C.) to an -caprolactam concentration of 50.2 wt. %. (This mixture, crude -caprolactam, is the mixture to be purified.)

[0246] The crude -caprolactam was ten times batch-wise extracted with a solvent mixture of 4-methyl-2-pentanol (50 wt. %)/cyclohexane (50 wt. %) at a temperature of ca. 25 C. Total amount of extraction solvent used was 8.05 grams of 4 methyl-2-pentanol/cyclohexane per gram of crude -caprolactam. The combined organic extracts were batch-wise washed with 7 grams of an aqueous 2 wt. % NaOH solution. The resulting washed organic extracts were concentrated by distillation under vacuum conditions to an -caprolactam concentration of about 40 wt. % and then fresh cyclohexane was added. The -caprolactam concentration of the resulting mixture was about 27 wt. % and the weight ratio of the solvent mixture 4-methyl-2-pentanol/cyclohexane was 50 wt. %: 50 wt. %. This mixture was 7 times batch-wise extracted with water at a temperature of ca. 25 C. Total amount of water used was 5.75 grams of water/gram of recovered -caprolactam.

[0247] The then obtained aqueous -caprolactam solution was concentrated by evaporation in a rotavap that was operated under vacuum (9.5 kPa; water bath temperature was ca. 65 C.) to an -caprolactam concentration of 45.9 wt. %. The resulting mixture was treated with 0.04 wt. % KMnO.sub.4 with regard to -caprolactam at 50 C. for 2 hours. The solids formed were then removed from the oxidized reaction product by means of a filtration. The -caprolactam in the obtained aqueous solution was after addition of 75 mmol of aqueous sodium hydroxide per kg -caprolactam further purified by distillation as described in COMPARATIVE EXPERIMENT 1. The specifications of the obtained purified -caprolactam were: [0248] PAN: 1.5 [0249] E290: 0.032 [0250] VB: 0.082 mmol/kg [0251] Alkalinity: 0.1 mmol/kg.

[0252] From this EXPERIMENT, it can be concluded that purified -caprolactam that meets all the required specifications for major polymerization applications can be obtained from depolymerization of polyamide 6 that originates from discarded polyamide 6 comprising fishing nets and was purified by extraction, back-extraction, oxidation and distillation.

Example 3

Depolymerization of Polyamide 6, Recovery of -Caprolactam and Purification by Extraction, Back-Extraction and Distillation.

[0253] The procedure of EXAMPLE 1 was followed.

[0254] The obtained aqueous -caprolactam solution was concentrated by evaporation in a rotavap that was operated under vacuum (9.5 kPa; water bath temperature was ca. 65 C.) to an -caprolactam concentration of 50.7 wt. %. (This mixture, crude -caprolactam, is the mixture to be purified.)

[0255] The crude -caprolactam was 11 times batch-wise extracted with toluene at a temperature of ca. 25 C. Total amount of extraction solvent used was 12.5 grams of toluene per gram of crude -caprolactam. The combined organic extracts were concentrated by distillation under vacuum conditions to an -caprolactam concentration of 30 wt. %. Subsequently, the resulting concentrated organic extracts were 4 times batch-wise extracted with water at a temperature of ca. 25 C. Total amount of water used was 1.79 grams of water/gram of combined organic extracts. The resulting combined aqueous -caprolactam solution was concentrated by evaporation in a rotavap that was operated under vacuum (9.5 kPa; water bath temperature was ca. 65 C.) to an -caprolactam concentration of 52.2 wt. %.

[0256] The -caprolactam in the obtained aqueous solution was after addition of 75 mmol of aqueous sodium hydroxide per kg -caprolactam further purified by distillation as described in COMPARATIVE EXPERIMENT 1. The specifications of the obtained purified -caprolactam were: [0257] PAN: 4 [0258] E290: 0.04 [0259] VB: 0.2 mmol/kg [0260] Alkalinity: 0.1 mmol/kg.

[0261] From this EXPERIMENT, it can be concluded that purified -caprolactam that meets all the required specifications for major polymerization applications can be obtained from depolymerization of polyamide 6 that originates from discarded polyamide 6 comprising fishing nets and was purified by extraction, back-extraction and distillation.

Example 4

Depolymerization of Polyamide 6, Recovery of -Caprolactam and Purification by Extraction, Back-Extraction and Distillation.

[0262] The procedure of EXAMPLE 1 was followed twofold. Both obtained aqueous -caprolactam solutions were concentrated by evaporation in a rotavap that was operated under vacuum (9.5 kPa; water bath temperature was ca. 65 C.) to -caprolactam concentrations of 70.2 wt. % and 67.6 wt. %, respectively. Then, both concentrated aqueous -caprolactam solutions were added together. One third of the resulting mixture, crude -caprolactam, is used for further purification.

[0263] The crude -caprolactam was diluted to 65.0 wt. % and 5 times batch-wise extracted with benzene at a temperature of ca. 25 C. The total amount of extraction solvent used was 10.7 grams of benzene per gram of crude -caprolactam. The combined organic extracts were batch-wise washed with 3.1 grams of an aqueous 2 wt. % NaOH solution. The resulting washed organic extracts were concentrated by distillation under vacuum conditions to an -caprolactam concentration of about 17 wt. %. Subsequently, the resulting concentrated organic extracts were 4 times batch-wise extracted with water at a temperature of ca. 25 C. The total amount of water used was 1.19 grams of water/gram of concentrated combined organic extracts. The resulting combined aqueous -caprolactam solution was concentrated by evaporation in a rotavap that was operated under vacuum (9.5 kPa; water bath temperature was ca. 65 C.) to an -caprolactam concentration of 56.3 wt. %.

[0264] The -caprolactam in the obtained aqueous solution was after addition of 75 mmol of aqueous sodium hydroxide per kg -caprolactam further purified by distillation as described in COMPARATIVE EXPERIMENT 1. The specifications of the obtained purified -caprolactam were: [0265] PAN: 3 [0266] E290: 0.01 [0267] VB: <0.02 mmol/kg [0268] Alkalinity: 0.1 mmol/kg.

[0269] From this EXAMPLE, it can be concluded that purified -caprolactam that meets all the required specifications for major polymerization applications can be obtained from depolymerization of polyamide 6 that originates from discarded polyamide 6 comprising fishing nets and was purified by extraction, back-extraction and distillation.

Example 5

Calculation of Carbon Footprint of Purified -Caprolactam

[0270] A continuous process according to the invention for the production of purified -caprolactam from polyamide 6 comprising fishing nets in a plant was simulated. The process included: [0271] Mechanical removal of foreign materials from polyamide 6 comprising fishing nets; [0272] Cutting polyamide 6 comprising fishing nets in small pieces; [0273] Washing of the pieces of polyamide 6 comprising fishing nets with water; [0274] Separation of washed pieces of polyamide 6 comprising fishing nets and aqueous extract by centrifugation; [0275] Drying of washed pieces of polyamide 6 comprising fishing nets; [0276] Melting and pelletization of washed pieces of polyamide 6 comprising fishing nets; [0277] Depolymerization of polyamide 6 under influence of H.sub.3PO.sub.4 and superheated steam; [0278] Recovery of crude-caprolactam (75 wt. % -caprolactam) by partial condensation of vapors discharged from depolymerization reactor; [0279] Counter-current extraction of concentrated crude -caprolactam with toluene; [0280] Washing of organic extract with diluted caustic solution; [0281] Counter-current back-extraction of washed organic extract with water; [0282] Evaporative concentration of aqueous extract; [0283] Oxidation of concentrated aqueous extract with KMnO.sub.4; [0284] Addition of caustic; and [0285] Recovery of pure -caprolactam by vacuum distillation.

[0286] The carbon footprint of purified -caprolactam was calculated based on the consumption figures of raw materials, and utilities of the above described process are based on data originating from ecoinvent version 3.7.1.

[0287] The outcome revealed that the product carbon footprint of purified -caprolactam obtained from polyamide 6 comprising fishing nets is less than 2.0 tons CO.sub.2 eq./ton of -caprolactam (location Europe).

[0288] While the invention has been described and with reference to specific embodiments thereof, it will be apparent to those of ordinary skill in the art that various changes and modifications, including (semi-) continuous operations and upscaling to commercial scale, can be made therein without departing from the spirit and scope thereof.