GB2083068A - A method for the separation and purification of anthracene from an anthracene oil - Google Patents

Abstract

A method for the separation and purification of anthracene from an anthracene oil obtained from coal tar by crystallization and solvent extraction, by crystallization of anthracene in the presence of methyl alcohol as a diluent and by optional subsequent solvent extraction of crystallized and separated anthracene with N-methylpyrrolidone.

Description

SPECIFICATION A method for the separation and purification of anthracene from an anthracene oil The present invention relates to a method for the separation and purification of anthracene from an anthracene oil obtained from coal tar by crystallization and solvent extraction.

According to the known industrial processes, a crude anthracene containing 2045% of pure anthracene is separated by a gradual cooling of the anthracene oil. Cooling down to 20--50"C usually takes about 24-36 hours, both in batch and in continuous apparatuses. The crude anthracene obtained this way is a solid of yellow-green to brown and even black colour, sometimes of a greasy consistency, resulting in difficulties of separation of the precipitate from the oil. The yield of the crude anthracene depends on the cooling temperature: at lower temperatures higher yields and lower concentrations of anthracene (and higher concentrations of the oil) in the precipitate are obtained.

The known methods make it possible to manufacture crude anthracene containing slightly more than 40% of pure anthracene; however the yield is low and on a commercial scale such methods are inefficient. To enrich further the crude anthracene and, especially, to remove the oily components there is usually carried out a single stage or multi-stage crystallization from known solvents such as pyridinium bases, solvent naphta, light oil from coke oven tar, benzene and acetone.

According to the known methods the proportion of anthracene separated is, however, rather low and only occasionally reaches 60%.

The known methods yield as a by-product a coal carbolineum containing a substantial proportion of crystallizable components, which adversely affect its quality and impose limitations upon its application.

To obtain coal carbolineum of a required quality and, especially, of a low crystallization temperature, it should be recrystallized and the crystals should once more be separated by decantation or centrifugation (H. G. Franck, G. Collin "Steinkohlenteer", Springer Verlag, Berlin 1 968, pp. 56-60).

The known methods for purification of anthracene comprise either distillation separation of the anthracene fraction and its subsequent purification by crystallization, or separation of phenanthrene and carbazole (which accompany anthracene in crude products) by extraction-crystallization, or else by purging of these impurities by chemical methods such as acid treatment or alkali fusion.

Since the chemical methods for anthracene purification give rather low yields and lead to formation of burdensome waste products and at the same time involve complex technoloyical problems, methods of this kind are not used any more on a commercial scale.

German Patent Specification No. 966 864 describes an example of a distillation method wherein crude anthracene is fractionated using a column of 20 plates, at a reflux of 10:1, at a pressure of 200-250 torr. Freezing temperatures of anthracene and carbazole fractions are 1 70-2000C and 185--2000C, respectively. The anthracene fraction obtained this way contains about 65% of anthracene, and the remainder is mainly phenanthrene which, because of its higher solubility than that of anthracene, can be removed by subsequent crystallization from pyridinium bases. As a result, 95% pure anthracene at a yield of 6065% is obtained.

The method described in this specification has a number of disadvantageous features, resulting mainly from high freezing temperatures of the distillate, which tend to impair condensers and pipelines, making the distillation inefficient.

In consequence, purification of anthracene by vacuum distillation should be avoided and therefore distillation requires high temperatures, making it energy consuming and economically inefficient.

It is possible to avoid some of the difficulties mentioned above by applying distillation in a stream of a superheated vapour of a neutral solvent. For instance, USSR Patent Specification No. 531 789 describes a method for distillation purification of anthracene by use of solvents having boiling temperatures lower than 1 800C, such as mixtures of xylenes or trimethyl benzenes (solvent naphtha).

The solvents are introduced to a distillation column in a proportion of 0.5:1 to crude anthracene.

The condensed liquid anthracene and the solvent are directed to a crystallizer and the resulting suspension of crystalline material is centrifuged. The purity of anthracene obtained this way is, however, not higher than 90%, which limits possible applications of the method. At the same time the capacity of the distillation unit is substantially reduced, increasing correspondingly costs of investment.

Thus the disadvantageous features of distillation methods determine that their use on a commercial scale is rather limited.

Crystallization and/or extraction methods of purification of anthracene are more widely used in practise.

USSR Patent Specification No. 386 889 describes crystallization of anthracene from acetone containing 1 0-20% of water, addition of which increases both selectivity of separation and yield of the process. According to this specification, the solvent and crude anthracene are mixed in a tank in a proportion of 4:1 to 8:1, and directed to an autoclave, where the mixture is heated at a constant volume and at a pressure of 4-9 atm until all solids are dissolved. Then the solution is transferred to crystallizers and centrifuged. To obtain anthracene of a purity higher than 95% the process should be repeated at least twice.The yield calculated for 100% anthracene is 5878%. The method is, however, disadvantageous in that it requires the use of high-pressure apparatuses, high temperature and large quantities of an explosive solvent.

The use of other solvents allows the crystallization temperature to be reduced, but at the same time the purity of the final product is lowered. Karanashev and Zhurovski (Khimiya i Industriya 45, 151-153 (1973)) have reported that by use as a solvent of benzene, a benzene-toluene-xylene fraction from coke benzol or solvent naphta it is possible to obtain anthracene of 8085% purity, if extraction of impurities is carried out at boiling temperature of the solvent. A similar quality of the final product can be acheived by use of low-boiling pyridinium bases as a solvent (e.g. J. Shuba "Processing of Coal Tar" WNT, Warsaw, or USSR Patent Specification No. 476 844).However, this degree of a purity is not sufficient for subsequent processing of anthracene, especially for catalytic oxygenation to anthraquinone.

The present invention provides a method for the separation and purification of anthracene from an anthracene oil obtained from coal tar by crystallization, wherein the anthracene oil is crystallized in the presence of methyl alcohol as a diluent and crude anthracene is subsequently separated.

By means of the present invention it is possible to separate crude anthracene from an anthracene oil and to purify the crude anthracene to a purity of at least 95 weight per cent and at the same time to increase the output of anthracene from an anthracene oil, to reduce the processing time and to improve the quality of the final product.

The crystallization of the anthracene oil in the presence of methyl alcohol and subsequent extraction of crude anthracene with N-methylpyrrolidone yields anthracene of purity of at least 95 weight per cent, which is useful for further processing.

Anthracene oil at a temperature of 30--900C, especially at 60-800C, is preferably treated with methyl alcohol in an amount of 5-20 weight per cent with respect to the initial anthracene oil and cooled down in one or two stages to a temperature in the range of 20--600C. Crystals of the crude anthracene are separated by filtration, centrifugation or in any other known way. Cooling down of the anthracene oil-methyl alcohol mixture is carried out using a mixer, a crystallizer or any other known apparatus for 1 to 5 hours, more especially for 1 to 2 hours.

By means of the method of the present invention one-stage crystallization yields crude anthracene containing 3550% of pure anthracene, while two-stage crystallization yields at the first stage crude anthracene of 4050% purity and at the second stage crude anthracene of 1 5-25% purity.

To remove completely the remaining oily components from the final product the separated crystals of crude anthracene can be additionally rinsed with a small quantity of methyl alcohol (20--50 weight per cent of separated crystals). Methyl alcohol used for rinsing can be re-circulated directly to the process, whereas methyl alcohol from the mother liquor is regenerated and also re-circulated.

It has been further found that the anthracene output from anthracene oils I and Il can be substantially increased, if to the anthracene oil I at a temperature of 40-1 200C, especially at 80 to 1 200 C, is added at least 10 weight per cent of low-grade crude anthracene containing 1020% of pure anthracene and the mixed oil is crystallized in the presence of methyl alcohol as a diluent.

This finding is quite unexpected since addition of a low-grade anthracene to anthracene oil crystallized without diluent has no effect on the crystallization yield, giving only products of higher purity, if the crystallization temperature is higher than 600C. It was found also that a similar effect results from using low-grade anthracene separated from the anthracene oil il, where its content is 14%, making separation of enriched anthracene (containing more than 40% of anthracene) by usually applied simple methods rather difficult.

However, a low-grade anthracene, containing at least 10% of the pure component, is mixed with the anthracene oil I in a proportion not exceeding 50 weight per cent and the mixture is crystallized at a temperature 40-700C in the presence of a diluent, especially methyl alcohol, used in a proportion of 525% to the anthracene oil I.

As the low-grade crude anthracene is used crude anthracene separated by crystallization from the anthracene oil II (cryzene oil) containing 1-4 weight per cent of anthracene or by crystallization of mother liquor obtained as a by-product from the method for separation of anthracene according to the present invention, or else crude anthracene separated-by crystallization from mixtures of anthracene oil II with mother liquors obtained as by-product from the method according to the invention.

Separation of crude low-grade anthracene from all of the mentioned raw materials is achieved by crystallization at a temperature 20--50aC, resulting in obtaining coal carbolineum or oils for carbon black of low anthracene content, respectively.

The separated crystals of crude anthracene are yellow-green and well-shaped and contain no oily components. The crystals are mixed for 1 to 3 hours at a temperature of 20-600C with Nmethylpyrrolidone in an amount of 50-200 weight per cent with respect to crude anthracene, and the solids are separated by centrifugation, filtration or in any other known way.

The separated crystals of anthracene are rinsed with a small quantity of a volatile, polar solvent, such as water, methyl alcohol, ethyl alcohol, acetone or ethyl acetate, to remove N-methylpyrrolidone from the surface, analyzed and processed further to obtain anthraquinone.

The N-methylpyrrolidone extract is treated at 20--500C with water in an amount of 20-100 weight per cent with respect to the extract, the fine-crystalline precipitate containing mainly phenanthrene and carbazole is separated, water is distilled off from the filtrate under vacuum, and regenerated N-methylpyrrolidone is re-circulated to the process without additional purification.

The extraction with N-methylpyrrolidone can also be carried out in two stages: the first stage at 10---300C with N-methylpyrrolidone in an amount of 50---150 weight per cent with respect to crude anthracene and the second stage at 40-600C with N-methylpyrrolidone in an amount of 100-- 200 weight percent with respect to enriched anthracene as an intermediate product.

The yield of pure anthracene is 70-90% of the theoretical yield and it usually contains about 97% of anthracene, 3% of phenanthrene, traces of carbazole and is useful for subsequent processing to anthraquinone.

It has also been found that if at the first stage of extraction an N-methylpyrrolidone-carbazole complex is used as a solvent, the selectivity of extraction can be increased by reducing the solubility of anthracene in the extract.

Crude anthracene containing 2050% of pure anthracene is mixed for 1 to 3 hours at 1 0-300C, especially at 250C, with a solution of a carbazole complex in N-methylpyrrolidone, which is re-circulated from the second stage of extraction of anthracene. The amount of pure extraction solvent is 50 to 1 50 weight per cent, especially 100 weight per cent of crude anthracene. Under these conditions, Nmethylpyrrolidone dissolves phenanthrene remaining in the anthracene precipitate and forms a liquid intermolecular complex with carbazole. The purified anthracene forms a precipitate suspended in the extract. The precipitated anthracene is separated from the liquid extract by filtration or centrifugation, rinsed with water or methyl alcohol to remove remaining N-methylpyrrolidone and dried.

The N-methylpyrrolidone extract from the first stage of extraction is treated with water in an amount of 20 to 100%, especially 60 weight per cent, with respect to the extract. Fine-crystalline precipitate containing mainly phenanthrene and carbazole is separated and the filtrate is dehydrated by distillation. The extract from the second stage of extraction is completely recycled to the first stage of extraction of crude anthracene.

The method according to the invention gives a product containing more than 95 weight per cent of anthracene in a yield exceeding 90%.

The method according to the invention is technologically more advantageous than the known processes for separation of crude anthracene from an anthracene oil. Addition of methyl alcohol results in decreasing the density and viscosity of the initial oil, permitting reduction of the time of crystallization and obtaining of better shaped crystals, as well as complete removing of mother liquor from the crystals separated. Addition of low-grade anthracene containing more than 10 weight per cent of anthracene to the anthracene oil I results not only in increasing the concentration of anthracene in the oil (which is a raw material for separation of enriched anthracene), but also in an advantageous shifting of the liquidsolid equilibrium, especially in systems containing diluents such as methyl alcohol.Increase in concentration and introducing of crystal nuclei in the form of the precipitate added to the oil result in a substantial increase of the crystallization rate, followed by further reducing of the processing time, increasing the anthracene output and volume capacity of the crystallizer.

Other polar solvents, e.g. ethyl alcohol, can also be used in the method according to the invention.

Crude anthracene separated according to the invention can be used for further processing and purification. Addition of methyl alcohol decreases the solubility of anthracene in the initial oil, resulting in an increase of anthracene output to more than 70%.

Two-stage processing of the oil allows anthracene to separate completely and in consequence to reduce the crystallization temperature, i.e. to improve the quality of carbolineum, which is a by-product of the process.

Increasing the crystallization rate results in an increase of the volume capacity of the apparatuses, making possible reduction of their volume, as well as their cost.

Regeneration of methyl alcohol diluent, because of the substantial difference in the boiling points of alcohol and anthracene, can be carried out using distillation methods and involves no technological difficulties.

Extraction of crude anthracene with N-methylpyrrolidone is carried out at a low temperature, i.e. at conditions of low solubility of anthracene in N-methylpyrrolidone and therefore the extraction yield is high, even in the case of low-grade crude anthracene. The solvent is non-toxic, non-corrosive and nonvolatile at the extraction temperature, and therefore its loss is low. Regeneration of the solvent is simple thanks to its miscibility with water which makes possible precipitation of the dissolved components of crude anthracene by addition of water.

Carbazole, which is extremely hard to remove by use of the known methods, may be completely removed as an intermolecular complex with N-methylpyrrolidone using the method according to the present invention.

Two-stage extraction with recycling of N-methylpyrrolidone from the second stage of extraction to the first stage of extraction of crude anthracene results in reducing the solubility of anthracene in the extract increasing the anthracene yield to more than 90%, a high purification degree of anthracene with lower amounts of N-methylpyrrolidone, and a high concentration of the extract and consequently higher output of the precipitate from the volume unit of the extract.

Since the solubility of phenanthrene in N-methylpyrrolidone is high, it is possible to achieve high purity of the final product.

Anthracene obtained according to the invention contains 96-98 weight percent of anthracene.

2-3 weight per cent of phenanthrene and traces of carbazole, which makes it very useful for further processing to anthraquinone.

The invention will be further described with reference to the following illustrative Examples.

EXAMPLE I To an anthracene oil (1000 g) containing 6.7 weight per cent of anthracene and 25.8% of phenanthrene were added 200 g of methyl alcohol and the resulting mixture was heated under reflux with stirring to 820C, until solids in the system were completely dissolved. The solution was cooled down to 550C and kept at this temperature for 2 hours, under slow stirring. The dark-yellow, crystalline precipitate was filtered by suction through a heated funnel and 146 g of crude anthracene, containing 43.0 weight per cent of anthracene, 1 5.7 weight per cent of carbazole and 1 5.5 weight per cent of phenanthrene, as determined by chromatography, were obtained. The anthracene output from the anthracene oil was 81%.

In a comparative experiment carried out under the same conditions, but without addition of methyl alcohol, 36 g of a greasy, dark brown, hard to filtrate product, containing 25 weight per cent of anthracene, 21.1 weight per cent of carbazole and 43 weight per cent of phenanthrene was obtained.

Crude anthracene (100 g) was mixed with 100 g of N-methylpyrrolidone and stirred at 500C for 2 hours. The precipitate of purified anthracene was filtered off at 500C, rinsed with methyl alcohol, dried and analyzed. The N-methylpyrrolidone filtrate was treated with water to precipitate remaining soluble components of crude anthracene. The process yielded 29 g of pure anthracene containing 96.1 weight per cent of anthracene and 66 g of a phenanthrene fraction containing 23.2% of anthracene.

EXAMPLES II TO XII In a flask fitted with a reflux condenser, 100 p.b.w. (parts by weight) of an anthracene oil of melting temperature 820C and boiling range for 50% of the oil of 330-3450C were heated to 900C and cooled down to 700 C. At this temperature 10 p.b.w. of methyl alcohol were quickly introduced at the bottom of the flask and the mixture was slowly cooled down to 500 C. After 1.5 hours stirring precipitated crystals of crude anthracene were filtered off by suction and rinsed with 10 p.b.w. of methyl alcohol yielding 15 p.b.w. of crude anthracene containing 44 weight per cent of anthracene, 1 0 weight per cent of carbazole and 26 weight per cent of phenanthrene. Methyl alcohol was distilled off from the filtrate.

Crystallization of the same oil with methyl alcohol diluent at various temperatures gave crude anthracene of composition and with yields as tabulated below:

Final Yield of Composition of crude anthracene, Methyl crystallization crude wt. % Example alcohol temperature anthracene No. p.b.w. C wt. % Anthracene Carbazole Phenanthrene 111 5 45 18.8 38.2 11 34 IV 5 50 17.8 40.1 10 33 V 10 35 19.4 38.6 10.9 34 VI 10 45 19.1 40.0 10.2 31 VII 10 50 18.3 41.5 9.2 29 VIII lox* 35 17.6 44.2 10.2 26 IX 10x* 45 15.3 49.1 9.8 25 X 20 35 18.1 44.0 10 31 XI 20 45 17.3 45.1 9.2 30 XII 20 50 15.8 48.0 9.0 26 *Concentrations of anthracene, carbazole and phenanthrene were determined by a polarographic method.

EXAMPLE Xlil An anthracene oil (100 pbw) obtained from continuous distillation of coal tar was cooled to 80--900C, and treated in a metallic crystallizer with 20 bpw of methyl alcohol introduced carefully at the bottom. The mixture was gradually cooled down to 550C, kept at this temperature for 1 hour and filtered using a heated funnel. The filtered off green crystals of crude anthracene I were rinsed with a small volume of methyl alcohol (about 20 g), dried, analyzed and purified further. Mother liquors from crude anthracene I were cooled down to room temperature (about 200 C), stirred for 2 hours and filtered to give crystals of crude anthracene II. The filtrate was distilled to regenerate methyl alcohol used (48 g). The residual coal carbolineum met requirements of corresponding Polish standards.The material balance of the process is tabulated below:

Composition, wt. %* Quantity Product p.b.w. Anthracene Carbazole Phenanthrene Anthracene oil 100 6.32 2.2 25.8 Crude anthracene 1 9.8 44.3 8.9 28 Crude anthracene 11 30 6.6 4.4 57 Coal carbolineum ] 60.2 0 0 10.2 *Determined by a chromatographic method.

EXAMPLES XIV to XVI II Crude commercial anthracene (100 g) containing 38.0 weight per cent of anthracene, 14.8 weight per cent oF carbazole and 30 weight per cent of phenanthrene was stirred for 2 hours at 25"C with 100 g of N-methylpyrrolidone. A precipitate was filtered off and stirred with 55 9 of Nmethylpyrrolidone at 450C for 1 hour. The resulting precipitate of purified anthracene was filtered through a funnel heated to 450C, rinsed with 100 ml of distilled water and dried, yielding 36.5 g of pure anthracene containing 96.5% of anthracene.

The N-methylpyrrolidone filtrate from a cold filtration was treated with 100 ml of distilled water, yielding 44.5 g of phenanthrene fraction containing 4.5 weight per cent of anthracene, 11.2 weight per cent of carbazole and 56.2 weight per cent of phenanthrene. The N-methylpyrrolidone filtrate from extraction at 450C was treated with water used for rinsing of purified anthracene. The precipitate was filtered off to yield 1 5.5 g of anthracene fraction, containing 32 weight per cent of anthracene, 20 weight per cent carbazole and 35.1 weight per cent of phenanthrene. This anthracene fraction, obtained as a by-product, was recycled to the next portion of crude anthracene to be purified.

Using the same raw material and treating it in a similar way, the results tabulated below were achieved:

Extraction temp., "C Quantity Purified anthracene of or crude Stage Stage anthracene, Quantity Purity Output Example I II g. g wt. % % XV 20 50 100 32.2 95.7 81 XVI 25 50 100 30.8 96.1 78 XVIII i 30 40 100 1 30.0 96.0 76 EXAMPLE XIX Anthracene oil II (chrysene oil, 250 g) containing 3.1 weight per cent of anthracene was heated with stirring to 800C, then cooled down to room temperature and stirred for an additional 2 hours.

Precipitated crystals were filtered off by suction to obtain 43 g of precipitate of low-grade anthracene containing 1 6.5 weight per cent of anthracene and 207 g of filtrate, which was non-crystallizable at room temperature.

The low grade anthracene was added to 250 g of the anthracene oil I, containing 9.8% of anthracene and to which was added previously a precipitate (40 g) containing 1 4.5 weight per cent of anthracene obtained from the second stage of crystallization of mother liquor after separation of crude 40% anthracene To the anthracene oil enriched this way were added 40 g of methyl alcohol, the mixture was heated under reflux with stirring to 800C and the resulting solution was cooled down to 450C and stirred for 1 hour. Crystals precipitated were filtered off by suction to obtain 65.5 g of crude anthracene containing 42 weight per cent of pure component and 300 g of mother liquor.The mother liquor was cooled down to 25"C and filtered to give 40 g of precipitate containing 14.5% of anthracene, which was recycled to the anthracene oil I and 260 g of a carbolineum-methyl alcohol mixture.

The crude anthracene separated this way (50 g) was mixed with 50 g of liquid Nmethylpyrrolidone-carbazole complex, i.e. with N-methylpyrrolidone extract from the second stage of extraction, and the mixture was stirred at 250C for 2 hours. The precipitate was filtered off and underwent the second stage of extraction by mixing with 35 g of N-methylpyrrolidone at 450 for 1 hour.

The residue after second stage of extraction was filtered off by suction through a funnel heated to 400C and rinsed with 30 ml of distilled water at 800C to remove N-methylpyrrolidone. Purified anthracene (19.0 g) containing 97.1 weight per cent of anthracene, 2.4 weight per cent of phenanthrene and 0.3 weight per cent of carbazole was obtained.

The filtrate obtained at the first stage of extraction was treated with 30 ml of water used for rinsing of purified anthracene to obtain 30 g of phenanthrene-carbazole fraction containing 35 weight per cent of anthracene, 59 weight per cent phenanthrene and 23.2 weight per cent of carbazole and 62.5 g of aqueous solution of N-methylpyrrolidone, which was subsequently regenerated. The Nmethylpyrrolidone extract from the second stage of extraction was recycled to the first stage of extraction of crude anthracene.

EXAMPLE XX To 100 p.b.w. of the anthracene oil II, i.e. chrysene oil, were added 5 p.b.w. of methyl alcohol and the mixture was heated with stirring up to 820 C. Then the mixture was cooled down to room temperature and a grey-yellow precipitate of low-grade anthracene (10.5 p.b.w.) containing 20.8 weight per cent of anthracene was filtered off. The precipitate was added to 100 p.b.w. of the anthracene oil I containing 7.8 weight per cent of anthracene to obtain enriched anthracene oil containing 9.05 weight per cent of anthracene. To the resulting oil were added 10 b.p.w. of methyl alcohol and the mixture was heated under reflux, with stirring, to 800 C. In order to dissolve solids the solution was cooled down to 400C and was maintained at this temperature for 1 hour.The resulting precipitate was centrifuged to give 23 b.p.w. of crude anthracene containing, according to a chromatographic analysis, 41.5 weight per cent of anthracene, 15 weight per cent of carbazole and 32 weight per cent of phenanthrene and 87.5 p.b.w. of a coal carbolineum-methyl alcohol mixture.

The filtrate from separation of low-grade anthracene was mixed with the filtrate from separation of enriched anthracene and underwent distillation to regenerate methyl alcohol. The resulting residue was a raw material for manufacturing of coal black.

Crude anthracene was mixed with N-methylpyrrolidone extract from the second stage of extraction and, by treating it in the same way as described in Example XIX, 8.5 p.b.w. of anthracene of 97.6% purity were obtained.

EXAMPLE XXI Low-grade anthracene (18 p.b.w.) containing 1 5.6 weight per cent of anthracene, 47 weight per cent of phenanthrene and 20 weight per cent of carbazole and 100 p.b.w. of the anthracene oil I containing 7.62 weight per cent of anthracene, 1 6.6 weight per cent of phenanthrene and 1.52 weight per cent of carbazole were introduced into a mixer and heated to 900C until solid matter was dissolved.

Then, under the surface of the solution at 800C, 15 p.b.w. of methyl alcohol were introduced. the mixture was thoroughly mixed and cooled down to 500C and left to crystallize for 1 hour. The resulting suspension of crude anthracene crystals in mother liquor was centrifuged to give 1 7 p.b.w. of solids, containing 43 weight per cent of anthracene, 25.5 weight per cent of phenanthrene and 1 2.5 weight per cent of carbazole and 11 5 p.b.w. of mother liquor, containing 2.6 weight per cent anthracene.The mother liquor was cooled down to 200C in order to crystallize 1 8 p.b.w. of low-grade anthracene, containing 1 5.6 weight per cent of anthracene, 47 weight per cent of phenanthrene and 2 weight per cent of carbazole and to obtain 97 p.b.w. of a filtrate containing 0.3 weight per cent anthracene. Lowgrade anthracene precipitate was recycled to the anthracene oil 1, and from the filtrate methyl alcohol was distilled off to give 82 p.b.w. of carbolineum. Crude anthracene was processed in the same way as described in Example XIX yielding 7.4 weight per cent anthracene of a purity of 96.4 weight per cent.

EXAMPLE XXII To 100 g of N-methylpyrrolidone were added 7.5 g of 94% carbazole and the mixture was stirred at 50-550C for 1 5 minutes. Then to the resulting brown solution were added 100 g of crude anthracene of the composition given in Example IV and the mixture was stirred for 3 hours at 45--500C. Non-dissolved fine-crystalline precipitate was filtered off and rinsed with methyl alcohol to obtain 36 g of enriched anthracene of purity of 91.4%. The product was purified by extraction with pure N-methylpyrrolidone at 400C and the filtrate was treated with water from rinsing of the precipitate to obtain a mixture of phenanthrene and carbazole. The final product was 97.4 weight per cent pure.

Claims (13)

1. A method for the separation and purification of anthracene from an anthracene oil obtained from coal tar by crystallization, wherein the anthracene oil is crystallized in the presence of methyl alcohol as a diluent and crude anthracene is subsequently separated.

2. A method as claimed in Claim 1. wherein crude anthracene is extracted with Nmethylpyrrolidone.

3. A method as claimed in Claim 2, wherein the extraction is carried out at 1 0-600C and wherein the proportion of N-methylpyrrolidone to crude anthracene to be purified is 50-200 weight per cent.

4. A method as claimed in Claim 2, wherein the extraction is carried out in two stages, and wherein in the first stage the temperature is 1 0-300C and the proportion of N-methylpyrroiidone to crude anthracene is 50-1 50 weight per cent, while in the second stage the temperature is 40-600C and the proportion of N-methylpyrrolidone to enriched anthracene as a by-product is 100--200 weight per cent.

5. A method as claimed in Claim 2, wherein the extraction is carried out in two stages, and wherein in the first stage the extraction solvent is a liquid complex of N-methylpyrrolidone with carbazole.

6. A method as claimed in Claim 5, wherein the carbazole-N-methylpyrrolidone complex is Nmethylpyrrolidone extract from the second stage of the extraction.

7. A method as claimed in any of Claims 1 to 6, wherein the anthracene oil is treated with 5-20 weight per cent of methyl alcohol, and wherein the crystallization is carried out at 20-900C for 1-5 hours.

8. A method as claimed in Claim 7, wherein the crystallization is carried out at 40-700C for 1-2 hours.

9. A method as claimed in any of Claims 1 to 6, wherein the anthracene oil is treated with 5-20 weight per cent of methyl alcohol and wherein the crystallization is carried out in twp stages at 40-700C and at 20--500C, respectively.

10. A method as claimed in any of Claims 1 to 6, wherein low-grade anthracene containing at least 10% of pure anthracene is mixed with the anthracene oil and wherein the resulting mixture is crystallized at 40-700C in the presence of methyl alcohol as a diluent.

11. A method as claimed in Claim 10, wherein the low-grade anthracene containing at least 10% of pure anthracene is crude anthracene separated by crystallization from the anthracene oil, or crude anthracene separated by crystallization from mother liquor being a by-product of the method of separation of enriched anthracene, or crude anthracene separated by crystallization from a mixture of the said anthracene oil from which crude anthracene is separated by crystallization with mother liquor being a by-product of the method.

12. A method as claimed in any of Claims 1 to 11, wherein separated crystals of anthracene are rinsed with methyl alcohol.

13. A method according to Claim 1 for the separation and purification of anthracene, substantially as herein described in any of the foregoing Examples.