Lactam may be prepared by subjecting a cyclic ketoxime to the Beckmann rearrangement in the presence of sulphuric acid or oleum, resulting in a Beckmann rearrangement mixture. The Beckmann rearrangement mixture may be neutralized with ammonia or ammonium base. The neutralized Beckmann rearrangement mixture typically comprises an ammonium sulfate solution phase and an aqueous lactam phase. The ammonium sulfate concentration in the ammonium sulfate solution phase is generally between 25 an 50 wt. %. The lactam concentration in the aqueous lactam phase is generally between 60 and 80 wt. %. Typically, organic impurities are present in both phases. In the processing of the neutralized Beckmann rearrangement mixture, the organic impurities are often discarded in one or more waste streams, which often also include ammonium sulfate. See for examples GB-A-1353448.

In CN-C-1023790 a process is described for treating an ammonium sulfate solution phase of a neutralized Beckmann rearrangement mixture. Ammonium sulfate crystals are recovered from the ammonium sulfate solution phase, and organic impurities

originating from the ammonium sulfate solution phase are oxidized in a wet air oxidation process. Ammonium sulfate crystals are recovered from the ammonium sulfate solution phase using a crystallizer which is operated by evaporation. Mother liquor which contains organic impurities, is removed from the crystallizer and treated in a wet air oxidation process. In the wet air oxidation process the mother liquor is contacted with an oxygen-containing gas in a wet air oxidation reactor, designated as oxygenolysis column, resulting in gaseous oxidized products and purified ammonium sulfate solution. The purified ammonium sulfate solution, which has a decreased amount of organic impurities, is recycled to the crystallizer. In the process of CN-C-1023790 no treatment is provided for waste containing organic impurities originating from the aqueous lactam phase.

It is the aim of the invention to provide a process for treating a mixture, in particular a neutralized Beckmann rearrangement mixture, comprising i) an ammonium sulfate solution phase containing first organic impurities and ii) an aqueous lactam phase containing second organic impurities, in which process the amount of waste containing organic impurities is reduced in an efficient manner.

According to the invention the process comprises forming an aqueous liquid containing the first and second organic impurities, and subjecting the aqueous liquid to a wet air oxidation process to purify the aqueous liquid.

Liquid containing organic impurities originating from the ammonium sulfate solution phase WO01/12549

and from the aqueous lactam phase and ammonium sulfate can be purified and recycled to a crystallizer. A significant reduction of the amount of waste which contains ammonium sulfate and organic impurities as well as an increased recovery of ammonium sulfate as crystals can be effected according to the invention.

Ammonium sulfate crystals with good quality can be obtained.

There are many possibilities to recover the aqueous liquid from the mixture, in particular from the neutralized Beckmann rearrangement mixture.

In a preferred embodiment (embodiment A) the process comprises the step of: Aa) contacting the mixture with an organic solvent, resulting in a lactam-containing organic liquid and an ammonium sulfate solution.

As a result of step Aa) lactam is extracted, and an ammonium sulfate solution is obtained containing the first and second organic impurities. The ammonium sulfate solution obtained may be subjected to a wet air oxidation process. Purified solution may then be fed to a crystallizer, and mother liquor withdrawn from the crystallizer may be recycled to the wet air oxidation process. More preferably, the process comprises the steps of: Ab) processing the ammonium sulfate solution through a crystallizer to form ammonium sulfate crystals and a mother liquor Ac) combining the mother liquor with aqueous dilution liquid, resulting in an oxidation mixture Ad) subjecting the oxidation mixture to a wet air oxidation process, resulting in purified ammonium WO 01/12549

sulfate solution.

This has the advantage that the organic impurities are concentrated before they are fed to the oxidation reactor, so that a smaller oxidation reactor may be used. Step Ac) effects that an oxidation mixture is obtained having a lower ammonium sulfate concentration than the mother liquor, this being advantageous since it minimizes and preferably prevents the occurrence of crystallization in the purified ammonium sulfate solution, while or after leaving the oxidation reactor.

Most preferably, the process comprises the step of: Ae) recycling the purified ammonium sulfate solution to the crystallizer.

In another preferred embodiment (B) the process comprises the steps of: Ba) separating the ammonium sulfate solution phase and the aqueous lactam phase, resulting in an ammonium sulfate solution and aqueous lactam Bb) contacting the aqueous lactam with an organic solvent, resulting in a lactam-containing organic liquid and an aqueous effluent Bc) combining the ammonium sulfate solution with the aqueous effluent.

As a result of step Bb) lactam is extracted, and an aqueous effluent is obtained containing organic impurities (second organic impurities). As a result of step Bc) a combined ammonium sulfate solution and aqueous effluent is obtained, containing the first and second organic impurities. Step Bc) may be carried out by feeding the WO 01/12549

ammonium sulfate solution and the aqueous effluent to the same crystallizer or the same wet air oxidation reactor. Preferably, small amounts of lactam which may have remained in the ammonium sulfate solution and/or the aqueous effluent are extracted from the ammonium sulfate solution and/or aqueous effluent using an organic solvent. The combined ammonium sulfate solution and aqueous effluent may be subjected to a wet air oxidation process. Purified solution may then be fed to a crystallizer, and mother liquor withdrawn from the crystallizer may be recycled to the wet air oxidation process. More preferebly, the process comprises the steps of: Bd) processing the combined ammonium sulfate solution and aqueous effluent through a crystallizer to form ammonium sulfate crystals and a mother liquor Be) combining the mother liquor and aqueous dilution liquid, resulting in an oxidation mixture Bf) subjecting the oxidation mixture to a wet air oxidation process, resulting in purified ammonium sulfate solution.

This has the advantage that the organic impurities are concentrated, before they are fed to the oxidation reactor, so that a smaller oxidation reactor may be used. Step Be) effects that an oxidation mixture is obtained having a lower ammonium sulfate concentration than the mother liquor, this being advantageous since it minimizes and preferably prevents the occurrence of crystallization in the purified ammonium sulfate solution, while or after leaving the oxidation reactor.

Most preferably, the process comprises the step of: Bg) recycling the purified ammonium sulfate solution to the crystallizer.

In another preferred embodiment (C) the process comprises the steps of: Ca) separating the ammonium sulfate solution phase and the aqueous lactam phase, resulting in an ammonium sulfate solution and an aqueous lactam Cb) contacting the aqueous lactam with an organic solvent, resulting in a lactam-containing organic liquid and an aqueous effluent Cc) processing the ammonium sulfate solution through a crystallizer to obtain ammonium sulfate crystals and a mother liquor Cd) combining the mother liquor and the aqueous effluent, resulting in an oxidation mixture Ce) subjecting the oxidation mixture to a wet air oxidation process, resulting in purified ammonium sulfate solution.

As a result of step Cb) lactam is extracted, and an aqueous effluent is obtained containing organic impurities (second organic impurities). As a result of step Cd) an oxidation mixture is obtained containing the first and second impurities. Embodiment (C) has the advantage that the flow rate of the mother liquor removed from the crystallizer can be kept smaller, without raising the concentration of organic impurities in the crystallizer. A small flow rate of mother liquor removed from the crystallizer has the advantage that a small oxidation reactor can be used. Combining the WO01/12549

mother liquor with aqueous effluent having a lower ammonium sulfate content than the mother liquor has also the advantage that the ammonium sulfate concentration of the oxidation mixture is decreased, so that no, or at least less aqueous dilution liquid is needed from external sources. Decreasing the amount of aqueous dilution liquid which does not originate from the effluent has also the advantage that less water is evaporated in the crystallizer and/or oxidation reactor, which saves energy. Combining the mother liquor with aqueous effluent, which contains organic impurities, has the advantage that the COD content of the oxidation mixture is higher than in the case that the mother liquor is diluted with aqueous dilution liquid not containing organic impurities. Preferably, the COD content of the oxidation mixture is increased with respect to the COD content of the mother liquor.

An increased COD content of the oxidation mixture has the advantage that less energy is needed to operate the oxidation reactor. If the COD content of the oxidation mixture is sufficiently high, it is even possible that the oxidation reactor can be operated without the use of energy from external sources, or that a surplus of energy is produced. As used herein, the values for the COD (chemical oxygen demand) content, which is a measure for the concentration organic impurities, refers to values as determined according to ASTM D 1252-95 (dichromate method).

Most preferably, the process comprises the step of: Cf) recycling the purified ammonium sulfate solution to the crystallizer.

WO 01/12549

In another preferred embodiment (D), the process comprises the steps of: Da) separating the ammonium sulfate solution phase and the aqueous lactam phase, resulting in an ammonium sulfate solution and an aqueous lactam Db) contacting the aqueous lactam with an organic solvent, resulting in a lactam-containing organic liquid and an aqueous effluent Dc) processing the ammonium sulfate solution through a crystallizer to obtain ammonium sulfate crystals and mother liquor Dd) combining the mother liquor with the mixture De) subjecting the aqueous effluent to a wet air oxidation process, resulting in purified effluent.

As a result of step Dd), organic impurities present in the mother liquor and originating from the ammonium sulfate solution phase (first organic impurities) enter the aqueous lactam phase. As a consequence aqueous lactam and aqueous effluent are obtained containing the first and second impurities.

Embodiment (D) has the advantage that a small reactor may be used, since only the effluent needs to be treated in the oxidation reactor.

More preferably, the process comprises the step of: Df) recycling the purified effluent to the crystallizer.

The wet air oxidation process is preferably carried out by contacting the aqueous liquid containing the first and second impurities, also referred to as WO 01/12549

oxidation mixture (embodiments A, B, C) or aqueous effluent (embodiment D), with an oxygen-containing gas at elevated temperature and elevated temperature in an oxidation reactor. The aqueous liquid may include two or more combined liquids, such as for instance mother liquor and aqueous dilution liquid in the case of embodiments A and B or mother liquor and aqueous effluent in the case of embodiment C. Said liquids may be fed to the oxidation reactor as separate flows, in which case they are combined in the oxidation reactor.

Preferably, they are combined, before they are fed to the oxidation reactor. It is known to the skilled person how a wet air oxidation process may be carried out, see for instance CN-C-1023790. Usually, the aqueous liquid is pressurized, after which it is combined with pressurized oxygen-containing gas. The amount of oxygen-containing gas used usually depends on the COD content of the feed of the oxidation reactor and can be determined by the skilled person. Usually, the combined oxidation mixture and oxygen-containing gas is subsequently heated in a heat exchanger, after which it is fed into the oxidation reactor. The oxidation reactor, which is preferably a bubble column, is usually operated at a pressure between 5 and 200 bar, preferably between 20 and 170 bar. The oxidation reactor is usually operated at a temperature between 125 and 320 °C, preferably between 175 and 320 °C, more preferably between 200 and 300 °C. Increasing the temperature generally results in an increased conversion of organic impurities into gaseous oxidized products. In general a higher pressure is used if the oxidation reactor is operated at a higher temperature.

WO 01/12549

The oxygen containing gas may be pure molecular oxygen.

However, the oxygen containing gas is preferably a mixture of oxygen with nitrogen, such as for instance air. The concentration of molecular oxygen in the oxygen containing gas is usually 5 to 50 vol. %, preferably 10 to 40 vol. %. In the oxidation reactor organic impurities are oxidized, resulting in gaseous oxidized products, typically CO2, and a purified aqueous liquid, also referred to as purified ammonium sulfate solution or purified effluent. The oxidation reaction may be carried out with or without catalyst. Carrying out the oxidation without catalyst is preferred. This results in a better quality of the ammonium sulfate crystals, in case the purified ammonium sulfate solution is recycled to a crystallizer.

In a preferred embodiment, the average COD content of the feed of the oxidation reactor is at least 20 g/kg, more preferably at least 45 g/kg. The feed of the oxidation reactor is understood to be all liquids which are fed to the oxidation reactor, and which are to be contacted with the oxygen containing gas. An increased COD content of the feed of the oxidation reactor has the advantage that less energy from external sources is needed to operate the oxidation reactor. If the COD content of the feed of the oxidation reactor is sufficiently high, the oxidation reactor may even be operated without the use of energy from external sources. If the COD content of the feed of the oxidation reactor further increases a surplus of energy is produced. Preferably, the COD content of the feed of the oxidation reactor is lower than 300 g/kg.

WO 01/12549

As a result of the occurrence of evaporation of water during the oxidation process or during the separation of the purified ammonium sulfate solution and gaseous oxidized products, crystallization in the purified aqueous liquid may occur if the ammonium sulfate concentration in the feed of the oxidation reactor is not sufficiently low. The occurrence of crystallization in the purified aqueous liquid may give rise to clogging problems. Preferably, the ammonium sulfate concentration of the feed of the oxidation reactor is sufficiently low that crystallization in the purified aqueous liquid, is prevented. Preferably, the ammonium sulfate concentration in the feed of the oxidation reactor is lower than 40 wt. %, preferably lower than than 35 wt. %, and more preferably lower than 30 wt. %. Decreasing the ammonium sulfate concentration in the feed of the oxidation reactor has the advantage that more water can be allowed to evaporate, without the occurrence of crystallization in the purified aqueous liquid.

The gaseous oxidized products and purified aqueous liquid may be separated by any known means.

Following separation of the gaseous oxidized products from the purified aqueous liquid, the purified aqueous liquid may be recycled to a crystallizer. The purified aqueous liquid may be directly recycled to the crystallizer, but recycling may also be done indirectly, for instance by combining the purified aqueous liquid with the mixture, in particular the neutralized Beckmann rearrangement mixture or by feeding the purified aqueous liquid to a step in which neutralization of a Beckmann rearrangement mixture is WO 01/12549

effected. The purified aqueous liquid and the gaseous oxidized products, may, with advantage, be used to heat the feed of the oxidation reactor or other steps in the production process of lactam before they are separated.

Preferably, separation of the gaseous oxidized products and the purified aqueous liquid is performed, before the gaseous oxidized products and/or steam are used to heat the feed of the oxidation reactor or other steps in the production process of lactam.

Preferably, the process according to the invention involves obtaining ammonium sulfate crystals from the mixture by crystallization. This may be done by any means known to the skilled person. An ammonium sulfate solution may for instance be obtained by extracting the lactam from the mixture with an organic solvent. The organic solvent is preferably benzene or toluene. The extraction is preferably carried out at a temperature of between 20 and 60 °C. An ammonium sulfate solution may also be obtained by separating the ammonium sulfate solution phase and the aqueous lactam phase, preferably by phase separation. The ammonium sulfate solution thus obtained is preferably contacted with an organic solvent, such as for instance benzene or toluene, to recover small amounts of lactam which may have remained in the ammonium sulfate solution.

Ammonium sulfate crystals and mother liquor may be obtained from the ammonium sulfate solution in a crystallizer. Examples of suitable crystallizers are described in"Perry's Chemical Engineers Handbook", by Don W. Green and James 0. Maloney, 7th edition, McGraw Hill, 1997, chapter 18, pages 44-55. Crystallizers which are operated by evaporation, optionally in WO 01/12549

combination with cooling, are particularly suitable for the process according to the invention. Usually, the crystallizer is operated at a temperature between 20 and 180 °C, and a pressure between 20 mbar and 8 bar.

Preferably the crystallizer is operated at a temperature between 40 and 130 °C, and a pressure between 50 mbar and 2 bar. It is to be understood that it is also possible to use more than one crystallizer.

In that case each of these crystallizers may be operated at a different pressure and/or temperature.

The ammonium sulfate concentration in the ammonium sulfate solution fed to the crystallizer is generally between 25 and 50 wt. %. The COD content of the ammonium sulfate solution fed to the crystallizer is generally between 0.1 and 20 g/kg in particular between 0.2 and 15 g/kg, and more in particular between 0.5 and 10 g/kg.

Mother liquor may be withdrawn from the crystallizer by any means known to the skilled person.

A slurry comprising ammonium sulfate crystals and mother liquor may for instance be withdrawn from the crystallizer, and the mother liquor may be separated from the slurry, for instance by centrifuging.

Preferably mother liquor which contains substantially no ammonium sulfate crystals is withdrawn from the crystallizer. The ammonium sulfate concentration of the mother liquor withdrawn from the crystallizer is usually between 35 and 60 wt. %, preferably between 38 and 55 wt. % and more preferably between 40 and 50 wt. %.

The concentration organic impurities in the mother liquor withdrawn from the crystallizer usually corresponds to a COD content between 8 and 120 g/kg, WO 01/12549

preferably between 12 and 80 g/kg, and more preferably between 15 and 60 g/kg. Keeping the concentration of organic impurities in the mother liquor in the crystallizer low improves the quality of the ammonium sulfate crystals, i. e. they will have less color, they will contain fewer impurities, and the production of large crystals is facilitated. The lower limit for the COD content is not critical, and is mainly determined by process economics. The flow rate of the mother liquor withdrawn from the crystallizer is usually between 0.5 and 30 parts by volume per unit of time, preferably between 1 and 20 parts per unit of time for a flow rate of 100 parts by volume per unit of time for the ammonium sulfate solution which is fed to the crystallizer. If the flow rate of the mother liquor withdrawn from the crystallizer decreases, the COD content in the mother liquor generally increases, if the other circumstances are equal. If the feeding of the ammonium sulfate solution and/or the removal of mother liquor would occur discontinuously or batchwise, it is to be understood that the above mentioned flow rates are the average amounts fed or withdrawn per unit of time. If more than one crystallizer is used, it is advantageous to withdraw mother liquor from one crystallizer, feed said mother liquor to a subsequent crystallizer, in which the COD content may increase further, and withdraw the mother liquor with an increased COD content. This has the advantage that mother liquor with a relatively high COD content may be fed to the oxidation reactor, while it is avoided that the COD content of the mother liquor is at this high level in all crystallizers.

WO01/12549

Preferably, lactam is recovered from the mixture, which preferably involves extracting the lactam with an organic solvent to form a lactam- containing organic liquid. This may be achieved by contacting the mixture or neutralized Beckmann rearrangement mixture with an organic solvent. It is also possible to obtain an aqueous lactam by separating the ammonium sulfate solution phase and the aqueous lactam phase. The aqueous lactam obtained may be contacted with an organic solvent, resulting in a lactam-containing organic liquid and an aqueous effluent. The organic solvent is preferably benzene or toluene. The lactam concentration in the lactam- containing organic liquid is preferably less than 30 wt. %. The extraction is preferably carried out at a temperature of between 20 and 60 °C.

The aqueous effluent usually contains between 0.1 and 10 wt. % ammonium sulfate dissolved in water, in particular between 0.2 and 8 wt. % ammonium sulfate dissolved in water, and more in particular between 0.5 and 6 wt. % ammonium sulfate dissolved in water. The COD content of the aqueous effluent is usually between 5 and 150 g/kg, in particular between 15 and 100 g/kg, and more in particular between 20 and 90g/kg.

Lactam may be recovered from the lactam-containing organic liquid by any method known to the skilled person, for instance by distillation.

The process according to the invention is in particular suitable if the lactam contains 5 to 12 carbon atoms in the ring, and more in particular if the lactam is e-caprolactam.

The invention is now illustrated with the aid of the following examples, without, however, being limited thereto.

Brief description of the drawings Figure 1 shows a flow chart of an embodiment (A) of the process according to the invention.

Figure 2 shows a flow chart of another embodiment (B) of the process according to the invention.

Figure 3 shows a flow chart of another embodiment (C) of the process according to the invention.

Figure 4 shows a flow chart of another embodiment (D) of the process according to the invention.

All COD contents mentioned in the examples are determined in accordance with the dichromate method according to ASTM D 1252-95.

Example I This example illustrates how the process may be carried out according to embodiment A. The example is described with reference to figure 1. The numbers in the text and figure refer to flows.

Composition and flow rates of various flows are indicated in table 1.

Neutralized Beckmann rearrangement mixture 1 and benzene 4 are fed to extraction apparatus EO in which extraction caprolactam is effected. Caprolactam- containing organic liquid 5, comprising caprolactam WO 01/12549

dissolved in benzene, and an ammonium sulfate solution 2 are withdrawn from extraction apparatus E0.

Caprolactam is separated from the carpolactam- containing organic liquid using distillation. The ammonium sulfate solution 2 is fed to crystallizer C0.

In crystallizer CO water is evaporated, and water vapour 7 and ammonium sulfate crystals 8 are withdrawn.

Mother liquor 10 is withdrawn from crystallizer CO, and mixed with water 11 (aqueous dilution liquid), resulting in an oxidation mixture. Oxidation mixture 12 and air 13 are mixed, heated, pressurized, and fed to oxidation reactor W0. Oxidation reactor WO is operated at a pressure of 65 bar and at a temperature of 250 °C.

In oxidation reactor WO oxidation of organic impurities is effected, resulting in gaseous oxidized products and purified ammonium sulfate solution. Upon separation of the purified ammonium solution and the gaseous oxidized products, a gaseous phase is obtained comprising steam and oxidized products (not shown). The gaseous phase is used to heat the feed of the oxidation reactor (not shown). Purified ammonium sulfate solution 14 is recycled to crystallizer C0.

In this example all streams which contain ammonium sulfate and organic impurities are recycled into the process. The COD content of the mother liquor in the crystallizer is kept at a low value, so that ammonium sulfate crystals with good quality are obtained. Crystallization in the purified ammonium sulfate solution does not occur.

Example II This example illustrates how the process may be carried out according to embodiment B. The example is described with reference to figure 2. The numbers in the text and in figure refer to flows.

Composition and flow rates of various flows are indicated in table 2.

Neutralized Beckmann rearrangement mixture 101 is fed to separation apparatus Sl in which separation of the ammonium sulfate solution phase and aqueous crude caprolactam phase is effected by phase separation. Ammonium sulfate solution 102 and aqueous crude caprolactam 103 are withdrawn from separation apparatus S1. Aqueous crude caprolactam 103 and benzene 104 are fed to extraction apparatus E1, in which extraction of caprolactam is effected. Caprolactam- containing organic liquid 105, comprising caprolactam dissolved in benzene, and aqueous effluent 106 are withdrawn from extraction apparatus E1. Aqueous effluent 106 and ammonium sulfate solution 102 are mixed and fed to crystallizer Cl. In crystallizer Cl water is evaporated, and water vapour 107 and ammonium sulfate crystals 108 are withdrawn. Mother liquor 110 is withdrawn from crystallizer Cl, and mixed with water 111 (aqueous dilution liquid), resulting in an oxidation mixture. Oxidation mixture 112 and air 113 are mixed, heated, pressurized, and fed to oxidation reactor W1. Oxidation reactor Wl is operated at a pressure of 65 bar and at a temperature of 250 °C. In oxidation reactor W1 oxidation of organic impurities is effected, resulting in gaseous oxidized products and purified ammonium sulfate solution. Upon separation of

the purified ammonium solution and the gaseous oxidized products, a gaseous phase is obtained comprising steam and oxidized products (not shown). The gaseous phase is used to heat the feed of the oxidation reactor (not shown). Purified ammonium sulfate solution 114 is recycled to crystallizer Cl.

In this example all streams which contain ammonium sulfate and organic impurities are recycled into the process. The COD content of the mother liquor in the crystallizer is kept at a low value, so that ammonium sulfate crystals with good quality are obtained. Crystallization in the purified ammonium sulfate solution does not occur.

Example III This example illustrates how the process may be carried out according to embodiment C. The example is described with reference to figure 3. The numbers in the text and figure refer to flows.

Neutralized Beckmann rearrangement mixture 201 is fed to separation apparatus S2 in which separation of the ammonium sulfate solution phase and aqueous crude caprolactam phase is effected by phase separation. Aqueous crude caprolactam 203 and benzene 204 are fed to extraction apparatus E2, in which extraction of the caprolactam is effected. Caprolactam- containing organic liquid 205, comprising caprolactam dissolved in benzene, and aqueous effluent 206 are withdrawn from extraction apparatus E2. Ammonium sulfate solution 202 is fed to crystallizer C2. In crystallizer C2 water is evaporated, and water vapour 207 and ammonium sulfate crystals 208 are withdrawn.

Mother liquor 210 is withdrawn from crystallizer 207 and mixed with aqueous effluent 206, resulting in an oxidation mixture. Oxidation mixture 212 and air 213 are mixed, heated, pressurized, and fed to oxidation reactor W2. Oxidation reactor W2 is operated at a pressure of 65 bar and at a temperature of 250 °C. In oxidation reactor W2 oxidation of organic impurities is effected, resulting in gaseous oxidized products and purified ammonium sulfate solution. Upon separation of the purified ammonium sulfate solution and the oxidized products, a gaseous phase is obtained comprising steam and oxidized products (not shown). The gaseous phase is used to heat the feed of the oxidation reactor (not shown). Purified ammonium sulfate solution 204 is recycled to crystallizer C2.

In this example all streams which contain ammonium sulfate and organic impurities are recycled into the process. The COD content of the mother liquor in the crystallizer is kept at a low value, so that ammonium sulfate crystals with good quality are obtained. Crystallization in the purified ammonium sulfate solution does not occur. This example further shows that flow rates of mother liquor 210 and oxidation mixture 212 are considerably smaller than in examples I and II, so that a smaller oxidation reactor may be used. Since the mother liquor to be fed to the oxidation reactor is mixed with the aqueous effluent, no aqueous dilution liquid from external sources is needed to decrease the ammonium sulfate concentration of the feed of the oxidation reactor, and no water from external sources needs to be evaporated. Due to the COD content of the aqueous effluent 206, the COD content of

the oxidation mixture 212 is considerably higher than in examples I and II, which results in an energy- efficient operation of the wet air oxidation reactor.

Example IV This example illustrates how the process may be carried out according to embodiment D. The example is described with reference to figure 4. The numbers in the text and in figure 4 refer to flows.

Composition and flow rates of various flows are indicated in table 4.

Neutralized Beckmann rearrangement mixture 301 and mother liquor 310 are combined and fed to separation apparatus S3 in which separation of the ammonium sulfate solution phase and the aqueous crude caprolactam phase is effected by phase separation.

Ammonium sulfate solution 302 and aqueous crude caprolactam 303 are withdrawn from separation apparatus S3. Aqueous crude caprolactam 303 and benzene 304 are fed to extraction apparatus E3, in which extraction of caprolactam is effected. Caprolactam-containing organic liquid 305, comprising caprolactam dissolved in benzene, and aqueous effluent 306 are withdrawn from extraction apparatus E3. Caprolactam is separated from the caprolactam-containing organic liquid using distillation. Aqueous effluent 306 and air 313 are mixed, heated, pressurized, and fed to oxidation reactor W3. Oxidation reactor W3 is operated at a pressure of 65 bar and at a temperature of 250 °C. In oxidation reactor W3 oxidation of organic impurities is effected, resulting in gaseous oxidized products and purified effluent. Upon separation of the purified

ammonium sulfate solution and the gaseous oxidized products, a gaseous phase is obtained comprising steam and oxidized products (not shown). The gaseous phase is used to heat the feed of the oxidation reactor (not shown). Purified effluent 314 and ammonium sulfate solution 302 are fed to crystallizer C3. In crystallizer C3 water is evaporated, and water vapour 307 and ammonium sulfate crystals 308 are withdrawn.

Mother liquor 310 is withdrawn from crystallizer C3 and recycled to separation apparatus S3.

In this example all streams which contain ammonium sulfate and organic impurities are recycled into the process. The COD content of the mother liquor in the crystallizer is kept at a low value, so that ammonium sulfate crystals with good quality are obtained. Crystallization in the purified effluent does not occur. The flow rate of the aqueous effluent is small, so that a small oxidation reactor can be used.

No aqueous dilution liquid from external sources is needed to decrease the ammonium sulfate concentration of the feed of the oxidation reactor, and no water from external sources needs to be evaporated. Due to the high COD content of the aqueous effluent the oxidation reactor is operated very efficiently.

Table 1. Composition of flows in example I Stream Water Amm. COD Capro-Flow rate No. (wt. %) Sulf. (g/kg) lactam (Weight parts/unit (wt. %) (wt. %) of time) 2 61 39 7 27 47.5501050 11 100 3.2 14 66 33. 5 7 6. 7 2830Oxid.70 Mixt. (12) Table 2. Composition of flows in example II Stream Water Amm. COD Capro-Flow rate No. (wt. %) Sulf. (g/kg) lactam (Weight parts/unit (wt. %) (wt. %) of time) 103 30 1 69 7. 5 102 58 42 3 27 4702.210692.5 110 50 47. 5 50 6 111100 114 66 33. 5 7 8. 5 Oxid. 70 28 30 10 Mixt. (112)

Table 3. Composition of flows in example III Stream Water Amm. COD Capro-Flow rate No. (wt. %) Sulf. (g/kg) lactam (Weight parts/unit (wt. %) (wt. %) of time) 203 30 1 69 7.5 202 58 42 3 27 210 50 47. 5 50 2.8 206 92. 5 4 70 2.2 214 58 42 12 3.5 Oxid. 69 28 59 5.0 Mixt. (212) Table 4. Composition of flows in example IV Stream Water Amm. COD Capro-Flow rate No. (wt. %) Sulf. (g/kg) lactam (Weight parts/unit (wt. %) (wt. %) of time) 303 30 1 69 7.5 302 58 42 3 27 310 50 47. 5 50 3.5 314 92 6. 5 20 1.4 Aq. 92 3 100 3.0 efl. (306)