Process for the purification of caprolactam from a solution of crude caprolactam without organic solvent extraction

Abstract

The present invention relates to a process for purifying caprolactam from solutions of cmde caprolactam by a direct treatment with one or more alkaline compounds of polyvalent metals, preferably bivalent and trivalent, without resorting to organic solvent extraction as used in the usual purification process. A further subject of the present invention is a facility devoid of a unit for organic solvent extraction and designed to carry out the caprolactam purification process described herein.

Claims

1. A process for the purification of caprolactam from a solution of crude caprolactam, characterized in that said process comprises the following steps: (i) treating the solution of crude caprolactam by means of direct contact with one or more alkaline compounds of polyvalent metals, at a temperature between 50 C. and 110 C., for a duration from 1 hour to 5 hours, under stirring, so as to thus obtain a suspension, wherein said one or more alkaline compounds of polyvalent metals are selected from the group formed of: bivalent alkaline earth metal hydroxides, bivalent alkaline earth metal oxides, amphoteric polyvalent metal hydroxides having alkaline behavior when placed in contact with acids, amphoteric polyvalent metal oxides having alkaline behavior when placed in contact with acids and mixtures thereof; (ii) cooling the suspension of step (i) and holding it at a temperature between 20 C. and 35 C., for a duration between 1 hour and 6 hours, under stirring; (iii) separating the solids from the suspension of step (ii) to obtain an aqueous solution of crude caprolactam.

2. The process according to claim 1, wherein said one or more alkaline compounds of polyvalent metals are present overall in an amount between 0.5% and 5% by weight relative to the total weight of the suspension.

3. (canceled)

4. The process according to claim 1, wherein said polyvalent metal is an alkaline earth metal selected from the group formed of calcium and magnesium.

5. The process according to claim 1, wherein the alkaline compound is aluminum hydroxide.

6. The process according to claim 1, wherein said one or more alkaline compounds of polyvalent metals is/are present overall in an amount between 2.0% and 3.5% by weight relative to the total weight of the suspension and in the form of an aqueous dispersion comprising 30-40% by weight of said alkaline compounds relative to the total weight of the aqueous dispersion.

7. (canceled)

8. The process according to claim 1, wherein the temperature in step (i) is between 80 C. and 95 C.

9. The process according to claim 1, wherein the duration of step (i) is between 2 hours and 3 hours.

10. The process according to claim 1, wherein the temperature in step (ii) is between 25 C. and 30 C.

11. The process according to claim 1, wherein the duration of step (ii) is between 2 hours and 30 minutes and 3 hours and 30 minutes.

12. The process according to claim 1, wherein the separation of the solids in step (iii) is performed in two steps, whereby in the first step the separation of most of the suspended solids is performed by centrifugation, and in the second step the filtration of the residual fine solids is performed by means of pressurized equipment equipped with filter cloths, whereby a layer of diatomaceous earth is applied to the filter cloths.

13. The process according to claim 1, wherein the solution of crude caprolactam is obtained by a process of caprolactam synthesis by way of Beckmann rearrangement of cyclohexanone oxime, or by a process for the recovery of caprolactam.

14. The process according to claim 1, further comprising, at the end of step (iii), the following steps: (iv) dehydrating the aqueous solution of caprolactam obtained in step (iii), at a temperature below 100 C. and at a pressure between 35 mbar and 45 mbar, thus obtaining an anhydrous crude caprolactam; (v) collecting and maintaining the anhydrous crude caprolactam at 110-120 C. under inert atmosphere of nitrogen for 1.5-2.5 hours in a stirred reactor to complete the transformation of the residual organic salts; (vi) evaporating the crude anhydrous caprolactam obtained in step (v), thus separating the salified polyvalent alkaline compounds; (vii) rectifying the caprolactam obtained in step (vi) by means of a vacuum fractional distillation column equipped with structuring packages, separating and purging the light byproducts; (viii) rectifying the caprolactam obtained in step (vii) by means of a vacuum fractional distillation column equipped with structured packages, separating the purified caprolactam from the heavy byproducts; (ix) squeezing the product remaining at the bottom of the column at the end of step (viii) and rectifying the caprolactam thus obtained by means of a vacuum fractional distillation column, equipped with structured packages, thus recovering a further amount of residual caprolactam.

15. The process according to claim 14, further comprising a squeezing of caprolactam from the residual materials obtained from step (vi) of evaporation, and rectifying the liquid component thus obtained by way of a vacuum fractional distillation column, equipped with structured packages, thus recovering a further amount of residual caprolactam.

16. The process according to claims 14 and 15 wherein the dehydrated anhydrous caprolactam from step (vi) is mixed with traces of NaOH in the amount from 0.05-0.5 by weight relative to anhydrous caprolactam, and said mixture is then submitted to the rectification/distillation steps.

17. (canceled)

18. A facility for purification of caprolactam by means of the process according to one or more of the preceding claims, characterized in that it comprises: (a) a first reactor made of stainless steel and equipped with an agitator and outer jacket, for performing step (i) of the process; (b) a second reactor made of stainless steel and equipped with an agitator and outer jacket, for performing step (ii) of the process; (c) a separation station comprising centrifuge and one or more pressurized filters equipped with filter cloths, for performing step (iii) of the process; (d) a column for dehydrating the filtered caprolactam solution obtained in step (iii), removing the water therefrom; (e) an evaporation station for removing the salts of the alkaline compounds by evaporating the caprolactam; (f) a distillation station, comprising a column for vacuum fractional distillation, equipped with structured packages, for separating the caprolactam from the light byproducts and a column for vacuum fractional distillation, equipped with structured packages, for separating the caprolactam from the heavy byproducts.

19. (canceled)

Description

EXAMPLES

[0096] In the tables presented in the examples, the following abbreviations correspond to the meanings below:

[0097] CPL=caprolactam;

[0098] GC=gas chromatography;

[0099] CPL refract=concentration of caprolactam in aqueous solution, determined by refractometry;

[0100] H.sub.2O KF=content of H.sub.2O in the solution, determined using the Karl-Fischer analysis method; In the tables, the sum of CPL refract and H.sub.2O KF is close to, but not exactly equal to 100%, given the intrinsic error of these analyses, which are nevertheless useful in providing an indication of the progress of the process and can be performed very quickly.

[0101] The consumption of permanganate (KMnO.sub.4) is an indicator of the amount of oxidizable substances that are present in the solution of crude caprolactam; it is not a specific quality parameter, but an indicator of the amount of present impurities sensitive to the oxidizing agents. The gas-chromatography analysis was performed by dissolving the sample in methanol using a capillary column with stationary phase of the crossbond-PEG type (or also other type like Stabilwax Cap column) with predetermined temperature scanning: in addition to verifying the purity (in terms of area) of the peak corresponding to caprolactam, this analysis is used to verify which impurities remain following the treatment. Although not all the byproducts can be identified or quantified directly, this method makes it possible to perform a valid comparison of the effects of the treatment.

Example 1

Treatment with Ca(OH).SUB.2

[0102] The following were introduced into a pilot reactor made of AISI 316 steel with a volumetric capacity of at least 100 liters, provided with an agitator and outer jacket:

[0103] solution of crude caprolactam with 72-73% by weight caprolactam from a HAS Raschig process: 45.00 kg;

[0104] anhydrous solid Ca(OH).sub.2: 1.15 kg.

[0105] The term HAS Raschig process refers to the process in which the hydroxylamine sulfate (HAS) used for the preparation of the cyclohexanone oxime was obtained with an elevated amount of sulfuric acid, which leads to the final co-production of approximately 4.5 kg of ammonium sulfate per 1 kg of caprolactam.

[0106] The concentration of the hydroxide is therefore equivalent to 2.5% by weight. The suspension, under continuous stirring, was brought to 50 C. by means of water at 95 C. fed into the outer jacket of the reactor and maintained under these conditions continuously for 2 hours. The flow of heating water was then shut off, and cooling with water at 25-30 C. was performed, keeping the stirring constant. The suspension was held at this temperature for 2 hours and 30 minutes, then it was discharged into a tank and subjected to filtration over a filter cloth. After the filtration, a dark yellow, but clear solution was obtained, with no solids in suspension.

[0107] The analytical data of the initial, untreated solution and of the treated solution are as follows:

TABLE-US-00002 Consumption Acidity or of KMnO.sub.4 GC purity CPL H.sub.2O KF alkalinity [mg/kg CPL [% GC n.sup.o Sample refract [%] [%] pH [meq/kg] solut.] Ashes [%] area] peaks Initial 72.1 28.0 5.9 9.4 1185 0.0 99.7 9 + 1 (CPL) solution (acidity) Treated 73.4 26.0 11.4 82.34 1185 0.260 99.8 5 + 1 (CPL) solution (alkalinity)

[0108] In this case the HPLC analysis, performed in a column of the Zorbax Eclipse XDB-C18 type, revealed that no oligomers of caprolactam could be found in the treated solution, neither cyclic nor linear oligomers, demonstrating that the treatment according to the invention creates conditions that inhibit the pre-polymerization forms of caprolactam.

[0109] The analytical data reveal that the treatment fundamentally modifies the initial solution with the following primary effects:

[0110] 1) the acidic solution becomes alkaline, which is an indispensable condition for the subsequent phases of distillation insofar as the highly acidic caprolactam never reaches the typical parameters of the commercial specifications;

[0111] 2) the number of byproducts evidenced by way of gas chromatography is significantly reduced, which signifies that they have been transformed, salified and separated during the filtration phase.

[0112] The inspection of the effects of the treatment is indispensable in order to track and monitor the process, whereas the quality of the final caprolactam is verified by the distillation and rectification tests described further below.

Example 2

Treatment with Ca(OH).SUB.2

[0113] The same amounts of caprolactam solution and of Ca(OH).sub.2 (45.00 kg of solution of crude caprolactam with 72-73% by weight of caprolactam from a HAS Raschig process and 1.15 kg of anhydrous solid Ca(OH).sub.2) were introduced into the same reactor from Example 1 under application of the same conditions. The concentration of the hydroxide was thus equivalent to 2.5% by weight. The temperature of the treatment in the hot phase was maintained at 85 C.

[0114] The analytical data of the initial, untreated solution and of the treated solution are as follows:

TABLE-US-00003 Consumption Acidity or of KMnO.sub.4 GC purity CPL H.sub.2O KF alkalinity [mg/kg CPL [% GC n.sup.o Sample refract [%] [%] pH [meq/kg] solut.] Ashes [%] area] peaks Initial 72.1 28.0 5.9 9.4 1185 0.0 99.7 9 + 1 solution (acidity) (CPL) Treated 80.8 19.0 11.34 60.84 1580 0.263 99.9 3 + 1 solution (evap.sup.(*.sup.)) (alkalinity) (CPL) .sup.(*.sup.)the rise in the refractometry concentration of caprolactam and the decrease in the content of H.sub.2O revealed by the analyses indicate that there was a loss of some water by evaporation, which in no way influences the treatment of the byproducts.

[0115] In this case the lower final alkalinity, with the same amount of Ca(OH).sub.2 used, can indicate a greater reactivity of the mixture, which is consistent with the fact that the gas chromatography demonstrates a further reduction of the number of byproducts. The behavior and conditions most suitable for the proposed treatment and the results obtained at this stage are dependent in each case on the effective composition of the starting solution of crude caprolactam, but remain within the ranges described in the text.

Example 3

Treatment with Ca(OH).SUB.2

[0116] The following were introduced into the same reactor from Example 1 and under application of the same conditions:

[0117] solution of crude caprolactam with 73-74% by weight of caprolactam from a HAS from NO reduction process: 45.00 kg;

[0118] anhydrous solid Ca(OH).sub.2: 1.15 kg-1.65 kg.

[0119] The HAS from NO reduction process means a process of preparing caprolactam via hydroxylamine sulfate (HAS) obtained by reduction of nitrogen oxide (NO) with hydrogen, which makes it possible to reduce the final co-production of ammonium sulfate to 2.2 kg per 1 kg of caprolactam.

[0120] The concentration of the hydroxide is thus equivalent to 2.5% and 3.5% by weight, respectively. The operating temperature was held at 85 C.

[0121] The analytical data of the initial, untreated solution and of the solutions after treatment are as follows:

TABLE-US-00004 Consumption Acidity or of KMnO.sub.4 GC purity CPL H.sub.2O KF alkalinity [mg/kg CPL [% GC n.sup.o Sample refract [%] [%] pH [meq/kg] solut.] Ashes [%] area] peaks Initial 74.41 25.3 5.73 27.67 1285 0.0 99.69 8 + 1 solution (acidity) (CPL) Solution 82.18 17.60 11.90 96.1 1216 0.381 99.85 6 + 1 treated (evap) (alkalinity) (CPL) with 2.5% Ca(OH).sub.2 Solution 82.65 17.40 11.85 90.4 1200 0.43 99.91 5 + 1 treated (evap) (alkalinity) (CPL) with 3.5% Ca(OH).sub.2

[0122] It is noted that the solution of crude caprolactam obtained by means of a process different from the traditional Raschig process (but equally widespread however, or more widespread) also presents characteristics similar to those obtained in the examples above.

Example 4

Treatment with Ca(OH).SUB.2

[0123] The following were introduced into the same reactor from Example 1 under application of the same conditions:

[0124] solution of crude caprolactam with 82-84% by weight of caprolactam from a recovery process: 45.00 kg;

[0125] anhydrous solid Ca(OH).sub.2: 1.15 kg.

[0126] The crude caprolactam used in this example originates from a recovery process, and therefore in itself is contaminated to a greater extent by impurities. It is noted that the percentage by weight of caprolactam, which is greater compared to the examples above, is dependent on the use of a smaller amount of water in the recovery process compared to the synthesis process.

[0127] The concentration of the hydroxide is thus equivalent to 2.5% by weight. The operating temperature was held at 80 C. for 2 hours, then the temperature was reduced to 25-30 C. for 2 hours and 30 minutes.

[0128] The analytical data of the initial, untreated solution and of the solution after treatment are as follows:

TABLE-US-00005 Consumption Acidity or of KMnO.sub.4 GC purity CPL H.sub.2O KF alkalinity [mg/kg CPL [% GC n.sup.o Sample refract [%] [%] pH [meq/kg] solut.] Ashes [%] area] peaks Initial 84.1 15.10 4.20 74.6 n.a. 0.33 88.9 24 + 1 solution (acidity) (CPL) Treated 84.8 15.40 10.40 56.1 n.a. 0.59 96.8 14 + 1 solution (alkalinity) (CPL)

[0129] It is noted that the solution of crude caprolactam obtained by means of a recovery process, and therefore having a greater concentration of byproducts, after the treatment according to the present invention also presents characteristics rather similar to those obtained in the examples above.

Example 5

Treatment with Ca(OH).SUB.2

[0130] Proceeding from a single batch of crude caprolactam originating from a recovery process, and therefore particularly rich in byproducts and having an elevated initial acidity value, the following were introduced into the same reactor from Example 1 under application of the same conditions:

[0131] solution of crude caprolactam with 70-75% by weight of caprolactam from a recovery process: 45.00 kg;

[0132] anhydrous solid Ca(OH).sub.2: 1.15 kg.

[0133] The concentration of the hydroxide is thus equivalent to 2.5% by weight. Two different processes were performed: one at 80 C. and one at 95 C. for 2 hours and 30 minutes. The cooling at 25-30 C. was kept the same for both processes.

[0134] The analytical data of the initial, untreated solution and of solution after treatment are as follows:

TABLE-US-00006 Consumption CPL Acidity or of KMnO.sub.4 GC purity refract H.sub.2O KF alkalinity [mg/kg CPL [% GC n.sup.o Sample [%] [%] pH [meq/kg] solut.] Ashes [%] area] peaks Initial 70.6 29.79 4.50 141.7 n.a. 0.14 86.20 38 + 1 solution (acidity) (CPL) Solution 71.26 30.3 11.1 181 n.a. 1.25 97.67 26 + 1 treated at (alkalinity) (CPL) 80 C. Solution 71.12 30.75 11.2 202 n.a. 1.24 96.10 21 + 1 treated at (alkalinity) (CPL) 95 C.

[0135] In this case as well it is noted that the solution of crude caprolactam obtained by means of a recovery process and particularly rich in byproducts, after the treatment according to the present invention also presents characteristics rather similar to those obtained in the examples above. It was also observed that some byproducts are completely eliminated, and therefore the number of byproducts reduces, whereas the amount of other byproducts is significantly reduced.

Example 6

Treatment with Ca(OH).SUB.2

[0136] The following were introduced into a pilot reactor made of AISI 316 steel, having a volumetric capacity of 3000 liters and provided with an agitator and outer jacket:

[0137] solution of crude caprolactam with 75-80% by weight of caprolactam from a recovery process: 2500.00 kg;

[0138] anhydrous solid Ca(OH).sub.2 (amounts used in successive tests): 37.5 kg-50.0 kg-62.0 kg-75.0 kg-87.5 kg.

[0139] The concentration of the hydroxide in the solution of crude caprolactam is thus, in the various experimental tests, equivalent to: 1.5%; 2.0%; 2.5%; 3.0%; 3.5% by weight, respectively.

[0140] The suspension, under continuous stirring, was brought to a temperature of 85 C. by means of low-pressure steam fed into the outer jacket of the reactor and maintained under these conditions for 2-3 hours. The flow of heating water was then shut off, and cooling with water at 25-30 C. was performed, keeping the stirring constant. The suspension was held at this temperature for 3 hours, then it was subjected to filtration over a filter press with filter cloths made of low-permeability polypropylene. With each filtration, the preparation of a coating of diatomaceous earth was continued by means of the conventional procedure known for operations of this kind. After the filtration, a dark yellow, but clear solution was obtained, with no solids in suspension.

[0141] The analytical data of the initial, untreated solution and of the treated solutions are as follows:

TABLE-US-00007 Consumption CPL Acidity or of KMnO.sub.4 GC purity refract H.sub.2O KF alkalinity [mg/kg CPL [% GC n.sup.o Sample [%] [%] pH [meq/kg] solut.] Ashes [%] area] peaks Initial 76.0 21.20 4.0 215.8 3350 n.a. 82.8 solution (acidity) Solution 75.1 25.9 9.9 142.0 3318 n.a. 93.1 treated with (alkalinity) 1.5% Ca(OH).sub.2 Initial 76.3 21.9 3.8 232.8 3334 n.a. 86.7 solution (acidity) Solution 77.9 23.6 10.4 230.6 3397 n.a. 96.66 treated with (alkalinity) 2% Ca(OH).sub.2 Initial 83.5 15.7 4.6 197.1 2844 n.a. 87.7 solution (acidity) Solution 83.5 16.5 10.9 117.6 2812 n.a. 98.0 treated with (alkalinity) 2.5% Ca(OH).sub.2 Initial 79.2 19.4 4.5 217.6 2686 n.a. 81.32 solution (acidity) Solution 80.8 20.1 11.6 208.0 2148 n.a. 97.13 treated with (alkalinity) 3% Ca(OH).sub.2 Initial 79.5 20.4 5.1 180.0 2907 n.a. 96.32 solution (acidity) Solution 78.9 20.6 10.7 66.2 3033 n.a. 98.24 treated with (alkalinity) 3.5% Ca(OH).sub.2

[0142] It is noted from the data presented in the table that, also with use of solutions of crude caprolactam with great variability and relatively low degree of purity, as shown in particular by the indicator acidity, the treatment according to the present invention makes it possible to obtain a neutralizing effect and a significant improvement of the purity measured by way of gas chromatography.

[0143] The solid discharged from the filtration during the course of these tests had, on average, the following composition range:

[0144] Total dry material: 70-80%

[0145] Humidity: 30-20%

[0146] On the basis of the total humid mass, the ashes accounted for approximately 30-35%, of which 35-45% was formed by organic substances. Caprolactam was revealed to be present in very modest levels (equal to a few percentage points), which signifies a considerable removal of the organic impurities, even if these were present in elevated amounts in the starting solution of crude caprolactam.

Example 7

Treatment with Ca(OH).SUB.2

[0147] The following were introduced into the same reactor for Example 1 under application of the same process parameters:

[0148] solution of crude caprolactam with 72-73% of caprolactam from a HAS Raschig process: 45.00 kg;

[0149] 30% by weight anhydrous solid Ca(OH).sub.2 dispersion in demineralized water maintained under stirring: 3.85 kg (1.15 kg dry).

[0150] In this case the Ca(OH).sub.2 was mixed with the solution of crude caprolactam in the form of a 30% by weight slurry pre-dispersed in demineralized water, keeping the stirring constant.

[0151] The concentration of the hydroxide is thus equivalent to 2.5% by weight of the solution of crude caprolactam. The temperature of the hot stage was 85 C.

[0152] The analytical data of the initial, untreated solution and of the treated solution are as follows:

TABLE-US-00008 Consumption Acidity or of KMnO.sub.4 GC purity CPL H.sub.2O KF alkalinity [mg/kg CPL [% GC n.sup.o Sample refract [%] [%] pH [meq/kg] solut.] Ashes [%] area] peaks Initial 72.1 28.0 5.9 9.4 1185 0.0 99.7 9 + 1 (CPL) solution (acidity) Treated 69.9 29.8 11.2 58.7 1450 0.21 99.9 3 + 1 (CPL) solution (alkalinity)

[0153] It is noted that the pre-dispersion of Ca(OH).sub.2 in demineralized water leads to the same results obtained with the direct addition of solid Ca(OH).sub.2 in the solution of crude caprolactam.

Example 8

Treatment with Mg(OH).SUB.2

[0154] The following were introduced into the same reactor for Example 1 under application of the same process parameters:

[0155] solution of crude caprolactam with 72-73% of caprolactam from a recovery process: 45.00 kg;

[0156] anhydrous solid Mg(OH).sub.2: 1.15 kg.

[0157] The concentration of the hydroxide is thus equivalent to 2.5% by weight. The temperature of the hot stage was maintained at 85 C.

[0158] The analytical data of the initial, untreated solution and of the treated solution are as follows:

TABLE-US-00009 Consumption Acidity or of KMnO.sub.4 GC purity CPL H.sub.2O KF alkalinity [mg/kg CPL [% GC n.sup.o Sample refract [%] [%] pH [meq/kg] solut.] Ashes [%] area] peaks Initial 82.0 17.4 4.4 133.40 n.a. 0.12 95.60 22 + 1 solution (acidity) (CPL) Treated 83.1 17.1 6.7 49.7 n.a. 0.29 97.34 11 + 1 solution (evap) (alkalinity) (CPL)

[0159] Magnesium hydroxide has effects similar to those of calcium hydroxide.

Example 9

Treatment with CaO

[0160] The following were introduced into the same reactor for Example 1 under application of the same process parameters:

[0161] solution of crude caprolactam with 72-73% by weight of caprolactam from a Raschig process: 45.00 kg;

[0162] anhydrous solid CaO: 0.90 kg.

[0163] Calcium oxide, in contact with water, transforms almost immediately into hydroxide, with production of heat, according to the following reaction:

CaO+H.sub.2.fwdarw.Ca(OH).sub.2+1136 kJ/kg of CaO

[0164] By using calcium oxide (or magnesium oxide) in aqueous pre-dispersion, or by subsequent contact of the oxide with the water of the solution of crude caprolactam, the effective compound that reacts is therefore still the hydroxide.

[0165] The operating temperature was held at 85 C. The concentration of the oxide is equivalent to 1.95% by weight, but for the above-cited reaction it is equivalent to the concentration of 2.5% by weight of calcium hydroxide.

[0166] The analytical data of the initial, untreated solution and of the treated solution are as follows:

TABLE-US-00010 Consumption Acidity or of KMnO.sub.4 GC purity CPL H.sub.2O KF alkalinity [mg/kg CPL [% GC n.sup.o Sample refract [%] [%] pH [meq/kg] solut.] Ashes [%] area] peaks Initial 72.1 28.0 5.9 9.4 1185 0.0 99.7 9 + 1 solution (acidity) (CPL) Solution 78.3 21.8 11.2 73.4 1073 0.31 99.9 4 + 1 treated at (evap) (alkalinity) (CPL) 85 C.

[0167] It is noted that, also with the use of calcium oxide, effects equivalent to those obtained with the use of calcium hydroxide are obtained, with the same starting material and conditions.

Example 10

Treatment with Al(OH).SUB.3

[0168] Aluminum hydroxide (Al(OH).sub.3) is obtained by hydrating aluminum oxide. Aluminum is not part of the family of alkaline earth metals and is trivalent, whereas the alkaline earth metals used in this process are bivalent. However, aluminum demonstrates behaviors highly similar to the alkaline earth metals, and aluminum hydroxide is widely available at very modest cost.

[0169] The following were introduced into the same reactor for Example 1 under application of the same conditions:

[0170] solution of crude caprolactam with 73-75% by weight of caprolactam from a recovery process: 45.00 kg;

[0171] anhydrous solid Al(OH).sub.3: 0.45 kg-1.35 kg.

[0172] The concentration of the hydroxide is equivalent to 1.0% and to 3.0% by weight, respectively. The temperature of the hot stage was held at 75 C., whereas it was held at 25-35 C. during the course of the subsequent cooling. For aluminum hydroxide as well, the contact times for both stages of treatment were maintained at the levels indicated in the examples above.

[0173] The analytical data of the initial, untreated solution and of the treated solution are as follows:

TABLE-US-00011 Consumption Acidity or of KMnO.sub.4 GC purity CPL H.sub.2O KF alkalinity [mg/kg CPL [% GC n.sup.o Sample refract [%] [%] pH [meq/kg] solut.] Ashes [%] area] peaks Initial 72.41 27.6 5.01 38.14 n.a. n.a. 97.93 18 + 1 solution (acidity) (CPL) Solution 81.9 18.8 10.2 24.4 n.a. 0.19 98.48 14 + 1 treated (evap) (alkalinity) (CPL) with 1.0% of Al(OH).sub.3 Solution 80.7 19.1 11.4 36.4 n.a. 0.45 99.29 11 + 1 treated (evap) (alkalinity) (CPL) with 3.0% of Al(OH).sub.3

[0174] It is noted that, also with the use of aluminum hydroxide, both a reduction of the number of byproducts and a reduction of the amount of some byproducts compared to the initial level are observed.

Example 11

Treatment with Ca(OH).SUB.2 .and Final Purification of Caprolactam

[0175] A solution of crude caprolactam was treated with Ca(OH).sub.2 in the same reactor for Example 2 and under application of the same conditions. The solution of crude caprolactam used was derived from a traditional process, in which a reaction is performed between cyclohexanone and hydroxylamine sulfate in order to produce the cyclohexanone oxime, followed by a Beckmann rearrangement and separation of the ammonium sulfate. The treatment was performed using a temperature of 85 C. in the hot stage and two concentrations of Ca(OH).sub.2: 3.0% by weight in the first case and 3.5% by weight in the second case. The treated solution, after filtration, had the following characteristics:

TABLE-US-00012 Consumption Acidity or of KMnO.sub.4 GC purity CPL H.sub.2O KF alkalinity [mg/kg CPL [% GC n.sup.o Sample refract [%] [%] pH [meq/kg] solut.] Ashes [%] area] peaks Solution 73.0 26.80 10.8 83.4 1580 0.41 98.48 14 + 1 treated (alkalinity) (CPL) with 3.0% of Ca(OH).sub.2 Solution 72.9 26.75 11.9 95.6 1200 0.45 99.29 11 + 1 treated (alkalinity) (CPL) with 3.5% of Ca(OH).sub.2

[0176] The two samples of treated solution were then subjected to a final purification according to the present invention: firstly, a dehydration was performed in order to remove the water, followed by evaporation of the caprolactam in order to separate the salts of the alkaline compounds of polyvalent metals and by rectification of the caprolactam in order to separate the light byproducts and heavy byproducts.

[0177] The final purification of the two solution samples was performed in a batch-type facility equipped with two columns: the first was used for complete dehydration of the samples, and the second (equipped with structured packages of the Sulzer BX type) was used to perform, in succession, the phases of evaporation of caprolactam in order to separate the salts of the alkaline compounds of polyvalent metals and of rectification in order to separate the light byproducts and the heavy byproducts. In particular, the characteristics of the final purification process performed for these samples were as follows:

[0178] evaporation of water under moderate vacuum and at temperatures below 100 C. by means of the smaller column; final water evaporation parameters: pressure equal to 120 mbar; temperature between 48 C. and 50 C.;

[0179] evaporation and rectification (by means of the larger column equipped with Sulzer structured packages) of the fractions of caprolactam obtained under high vacuum in multiple steps; the temperature at the top of the column was between 122 C. and 127 C. and the pressure was equal to approximately 6 mbar.

[0180] The facility was fed with approximately 45-55 kg of solution; the removal of water and the fractionation of the caprolactam required the same time of two days each.

[0181] The results of the main quality parameters of the central fraction, equivalent to those of the final yield of rectified caprolactam following treatment with 3.0% and 3.5% of Ca(OH).sub.2 as described above are presented below, compared with the standard specification of commercial caprolactam.

TABLE-US-00013 Volatile Permanganate bases Permanganate number [dry] Absorbance Acidity Alkalinity Sample [meq/kg] index (IP) [PN] at 290 nm APHA pH [meq/kg] [meq/kg] Standard <0.5 <5 >15000 <0.05 <5 n.a. <0.05 <0.1 Solution 0.35 3.6 15512 0.043 2.96 6.9 0.01 treated with 3.0% Ca(OH).sub.2 Solution 0.45 2.94 15138.8 0.04 1.93 7.5 0.05 treated with 3.5% Ca(OH).sub.2

[0182] The analytical methods used to verify the quality parameters are those standardized and commonly used for caprolactam.

Example 12

Treatment with Ca(OH).SUB.2 .and Final Purification of Caprolactam Using Traces of Sodium Hydroxide in Rectifications Steps

[0183] A solution of crude caprolactam was treated with Ca(OH).sub.2 in the same reactor for Example 2 and under application of the same conditions. The solution of crude caprolactam used was derived from a traditional process through Beckmann rearrangement after separation of the ammonium sulfate. The treatment was performed using a temperature of 85 C. in the hot stage and the concentration of Ca(OH).sub.2: 2.5% by weight. The treated solution, after filtration, had the following characteristics:

TABLE-US-00014 Consumption Acidity or of KMnO.sub.4 GC purity CPL H.sub.2O KF alkalinity [mg/kg CPL [% GC n.sup.o Sample refract [%] [%] pH [meq/kg] solut.] Ashes [%] area] peaks Solution 70.33 31.0 11.43 133.4 1025 0.21 99.68 7 + 1 treated (alkalinity) (CPL) with 2.5% of Ca(OH).sub.2

[0184] The sample of treated solution were then subjected to a final purification according to the present invention: firstly, a dehydration was performed in order to remove the water, followed by evaporation of the caprolactam in order to separate the remaining soluble organic salts of the alkaline compounds of polyvalent metals and then the collected raw evaporated caprolactam was divided in two parts: [0185] 1-First part was sent to final rectification as it was; [0186] 2-Second part of raw evaporated caprolactam was first melted (REMARK: FOR THIS TRIAL THE PRODUCT WAS RECOVERED AND REMELTED, WHILE IN THE CONTINOUS PROCESS THE CAPROLACTAM IS ALWAYS MANTAINED IN MOLTEN SATE) and mixed with 0.2% of NaOH and was left in contact for 2 hours at 90 C. under nitrogen and then sent to final rectification. In both cases the final purification was performed in a batch-type facility equipped with two columns: the first was used for complete dehydration of the samples, and the second (equipped with structured packages of the Sulzer BX type) was used to perform, in succession, the phase of evaporation of caprolactam in order to separate the salts of the alkaline compounds of polyvalent metals. The following conditions were used in the first step:

[0187] evaporation of water under moderate vacuum and at temperatures below 100 C. by means of the smaller column; final water evaporation parameters: pressure equal to 120 mbar; temperature between 48 C. and 50 C.;

[0188] evaporation of the raw caprolactam under high vacuum with the temperature at the top of the column between 122 C. and 127 C. and pressure equal to approximately 6 mbar.

[0189] The raw evaporated caprolactam obtained from this first step was used to perform two final rectifications in order to separate the light byproducts and the heavy byproducts without and with some Sodium Hydroxide contact.

[0190] The usual parameters conditions for caprolactam distillation were adopted:

[0191] rectification (by means of the larger column equipped with Sulzer structured packages) of the fractions of caprolactam at the temperature on the top of the column maintained between 122 C. and 127 C. and the pressure equal to approximately 6 mbar.

[0192] The parameters for distillation were the same for both cases.

[0193] The results of the main quality parameters of the central fraction, equivalent to those of the final yield of rectified caprolactam, without and with presence of the NaOH are presented below compared with the standard specification of commercial caprolactam.

TABLE-US-00015 Volatile Permanganate bases Permanganate number [dry] Absorbance Acidity Alkalinity Sample [meq/kg] index (IP) [PN] at 290 nm APHA pH [meq/kg] [meq/kg] Standard <0.5 <5 >15000 <0.05 <5 n.a. <0.05 <0.1 Solution 0.45 5.5 5300 0.055 1.51 6.9 0.01 treated with 2.5% Ca(OH).sub.2 No NaOH Solution 0.27 1.3 19100 0.034 1.93 7.4 0.045 treated with 2.5% Ca(OH).sub.2 With NaOH

[0194] The final rectification using some trace of NaOH in anhydrous condition in contact with the caprolactam remain one additional method to improve the final quality of the product also in combination with the polyvalent metals alkaline treatment.

[0195] The analytical methods used to verify the quality parameters are those standardized and commonly used for caprolactam.