EP0000493B1 - Process for the preparation of 1-amino-8-hydroxy-naphthalene-3,6-disulphonic acid (h acid). - Google Patents

Description

The present invention relates to a process for the preparation of 1-amino-8-naphthol-3,6-disulfonic acid (H-acid) as the monoalkali salt from 1-naphthylamine-3,6,8-trisulfonic acid by alkaline pressure hydrolysis.

1-Amino-8-naphthol-3,6-disulfonic acid, which is often also referred to as H-acid, is an important intermediate for the production of dyes (see Ullmanns Enzyklopadie der Technischen Chemie, 3rd edition, 12th volume, p . 621).

From FIAT Final Report No. 1016, pp. 32 to 39 it is known that H-acid can be produced as follows: naphthalene is mixed with sulfuric acid monohydrate (= 100% H ₂ S0 ₄ ) and 65% oleum while maintaining a certain temperature program and stepwise addition of sulfuric acid monohydrate and oleum to a naphthalene-trisulfonic acid isomer mixture, which is nitrated with mixed acid. After dilution with water, expulsion of the nitrous gases and separation of the sulfuric acid as calcium sulfate, the isomer mixture of the nitronaphthalene-trisulfonic acids is reduced with iron and then dissolved iron salts are precipitated with magnesium oxide and separated off. By adding rock salt and hydrochloric acid, the acidic calcium sodium salt of T-acid (1-naphthylamine-3,6,8-trisulfonic acid) is precipitated, which is filtered off and washed several times. This salt is added to the washing water and soda is added. Then the chalk that has precipitated is pressed and the salt solution is concentrated. The concentrated trisodium salt solution of the T-acid is reacted with 50% sodium hydroxide solution under pressure. Then sulfuric acid is added first, then water, and finally the H acid is obtained as the monosodium salt by filtration, washing and drying.

In this process, considerable amounts of by-products are formed in the alkaline hydrolysis of T-acid, e.g. B. the 1-amino-6-naphthol-3,8-disulfonic acid, a known under the name W-acid isomer of H-acid, and the 1,8-dihydroxy-naphthalene-3,6-disulfonic acid, one under the Chromotropic acid is a known by-product of H-acid. The yield of H acid in the process described above is generally only 70 to 72%, based on the T acid used.

A process has now been found for the preparation of monoalkali metal salts of 1-amino-8-naphthol-3,6-disulfonic acid by reacting 1-naphthylamine-3,6,8-trisulfonic acid and / or its salts and / or naphthylamine. trisulfonic acid - mixtures of isomers and / or their salts with alkali metal hydroxide solution at elevated pressure and elevated temperature and separation of 1-amino-8-naphthol-3,6-disulfonic acid monoalkali metal salts by acidification, which is characterized in that the reaction in the presence of Alcohols and / or alcoholates.

• 75 bis 85 Gew.-% 1-Naphthylamin-3,6,8-trisulfonsäure
• 5 bis 15 Gew.-% 1-Naphthylamin-3,5,7-trisulfonsäure
• 1 bis 10 Gew.-% 1-Naphthylamin-4,6,8-trisulfonsäure
• 0,5 bis 2 Gew.-% 1-Naphthylamin-2,5,7-trisulfonsäure
• 0,1 bis 2 Gew.-% 2-Naphthylamin-3,5,7-trisulfonsäure
• 0,1 bis 2 Gew.-% 2-Naphthylamin-4,6,8-trisulfonsäure
• 0,1 bis 2 Gew.-% 2-Naphthylamin-3,6,8-trisulfonsäure

In the process according to the invention, 1-naphthylamine-3,6,8-trisulfonic acid (T-acid) can be used in pure form and / or in the form of naphthylamine-trisulfonic acid isomer mixtures. The naphthylamine-trisulfonic acid isomer mixtures generally contain more than 65% by weight of 1-naphthylamine-3,6,8-trisulfonic acid, based on the total amount of diazotizable substance. If naphthylamine-trisulfonic acid isomer mixtures are used, preference is given to using those containing 70 to 90% by weight of 1-naphthylamine-3,6,8-trisulfonic acid. A naphthylamine-trisulfonic acid isomer mixture to be used with particular preference contains

• 75 to 85% by weight of 1-naphthylamine-3,6,8-trisulfonic acid
• 5 to 15% by weight of 1-naphthylamine-3,5,7-trisulfonic acid
• 1 to 10% by weight of 1-naphthylamine-4,6,8-trisulfonic acid
• 0.5 to 2% by weight of 1-naphthylamine-2,5,7-trisulfonic acid
• 0.1 to 2% by weight of 2-naphthylamine-3,5,7-trisulfonic acid
• 0.1 to 2% by weight of 2-naphthylamine-4,6,8-trisulfonic acid
• 0.1 to 2% by weight of 2-naphthylamine-3,6,8-trisulfonic acid

Naphthylamine - trisulfonic acid - isomer mixtures can contain other products in addition to the naphthylamine trisulfonic acids. Such products can be, in particular, by-products, decomposition products or unreacted intermediates from the production stages for naphthylamine-trisulfonic acid, for example naphthalene-di-, tri- and tetra-sulfonic acids, nitronaphthalene-mono-, di- and trisulfonic acids, naphthylamine-mono - and disulfonic acids, e.g. B. 1-naphthylamine-3,6- and -5,7-disulfonic acid, also dinaphthylsulfone sulfonic acids and their amino and nitro derivatives, and oxidation products of naphthalene and / or naphthalene sulfonic acids, which are formed in the sulfonation and / or nitration can.

The 1-naphthylamine-3,6,8-trisulfonic acid or the naphthylamine-trisulfonic acid isomer mixtures can be used in free form, in the form of neutral salts or in the form of acid salts. Mixtures containing free acids and salts can also be used. If all or part of the 1-naphthylamine-3,6,8-trisulfonic acid or the naphthylamine-trisulfonic acid isomer mixtures are present as salts, the alkali and alkaline earth metal salts, in particular the sodium and potassium salts, are preferred.

Suitable 1-naphthylamine-3,6,8-trisulfonic acid or salts of this acid for use in the process according to the invention can be obtained by trisulfonating naphthalene, nitriding the resulting mixture, then reducing the nitro-naphthalene-trisulfonic acid mixture present, the acidic Calcium-sodium salt of the T-acid precipitates, this salt is mixed with soda in solution, the precipitated chalk is pressed out and the salt solution is concentrated. These reactions can be carried out according to the FIAT Final Report procedure described at the beginning or in any other way.

A naphthylamine-trisulfonic acid isomer mixture suitable for use in the process according to the invention can be obtained in a similar manner if you stop the reaction sequence after the reduction of the nitro-naphthalenetrisulfonic acid mixture has ended.

The 1-naphthylamine-3,6,8-trisulfonic acid and / or its salts and / or the naphthylamine-trisulfonic acid isomer mixture and / or its salts can be, for example, in solid form or as an aqueous solution with a content of, for example, 20 to 50% by weight. -%, preferably 30 to 40 wt .-%, calculated as the free acid with the molecular weight 383, are used.

Alkali hydroxide solutions for the process according to the invention are, in particular, aqueous potassium or sodium hydroxide solution. The use of potassium hydroxide leads to better yields compared to sodium hydroxide solution, but sodium hydroxide solution is generally cheaper. For example, 2.5 to 12 moles of alkali metal hydroxide can be used per mole of diazotizable substance (calculated with a molecular weight of 383 = T acid). The use of 6 to 9 moles of alkali metal hydroxide per mole of diazotizable substance is particularly preferred. The concentration of alkali hydroxide in the reaction mixture can be, for example, 10 to 50% by weight (based on the sum of alkali hydroxide plus water plus alcohol). This concentration is preferably 25 to 35%.

An essential aspect of the process according to the invention is that it is carried out in the presence of alcohols and / or alcoholates. For this purpose, alcohols in pure form, in a mixture with water or in the form of alcoholates, for example in the form of alkali metal alcoholates, can be added to the reaction mixture. Suitable alcohols are those alcoholic compounds which are miscible with water under the reaction conditions and which undergo no or only minor undesirable side reactions with strong alkali. In this case, the formation of alcoholate is not an undesirable side reaction. Aliphatic alcohols with, for example, 1 to 6 carbon atoms are preferred. For example, primary, secondary and tertiary monohydric and polyhydric alcohols can be used, the hydroxyl groups of which can also be wholly or partially etherified. As monohydric alcohols come e.g. B. in question: methanol, ethanol, n-propanol, 2-propanol, n-butanol, iso-butanol, tert-butanol. As polyhydric alcohols such. B. in question: ethylene glycol, propanediols, butanediols, glycerol, butanetriols, monoglyme, diglyme. Mixtures of alcohols can of course also be used. Methanol is particularly preferably used.

The amount of alcohol or alcoholate to be used can be chosen, for example, so that 10 to 80% by weight, preferably 25 to 60% by weight, of alcohol or alcoholate, based on the sum of water plus alcohol, is present.

The process according to the invention can be carried out, for example, at temperatures from 150 to 250 ° C., preferably at 180 to 220 ° C., in a closed vessel. The pressure which arises is generally completely sufficient to carry out the process according to the invention in a satisfactory manner. Of course, the method according to the invention can also be carried out with pressures other than those which set themselves in closed vessels. For example, pressures in the range from 5 to 100 bar are possible for the process according to the invention.

The reaction time essentially depends on the reaction temperature and the alkali hydroxide concentration. It is shorter at relatively high reaction temperatures and at relatively high alkali hydroxide concentrations and longer at relatively low reaction temperatures and relatively low alkali hydroxide concentrations and is generally from 10 minutes to 10 hours. For example, good results are obtained at a reaction temperature of approx. 200 ° C. and an alkali hydroxide concentration of 30% by weight with a reaction time of 45 to 60 minutes.

The substances to be used in the process according to the invention are most advantageously introduced into the reaction vessel at a temperature such that the desired reaction temperature is present after the heat of mixture and optionally the heat of neutralization has been released. The substances to be entered can also be brought together at lower temperatures and heated to the desired reaction temperature in the reaction vessel.

After the reaction has ended and before the H acid has been separated off as the monoalkali metal salt, it is advantageous to cool the reaction mixture and / or to dilute it with water. One can, for example, cool to temperatures in the range from 20 to 150 ° C., preferably to temperatures in the range from 80 to 120 ° C. The amount of water to be added depends on the reaction conditions, eg. B. the type of alkali metal hydroxide, its amount and concentration, and according to the amount of alcohol that may still be present. It is advantageous to choose the amount of water so that the alkali sulfite formed in the reaction is dissolved or remains dissolved.

The H-acid can be separated off as the monoalkali salt by acidifying the reaction mixture with mineral acids.

Sulfuric acid is preferably used for this. Sufficient mineral acid is added to form the sparingly soluble monoalkali salt of H-acid. By appropriate choice of the concentration of the mineral acid and / or by adding water before and / or during the addition of the mineral acid, it is expedient to ensure that the inorganic salt which forms, for. As sodium sulfate or potassium sulfate, does not fail. Good results can be obtained, for example, if a pH in the range from 0 to 4, preferably 0.5 to 2.5, is set for the separation of the H-acid as the monoalkali salt and by dilution with water and / or by appropriate choice of the concentration the mineral acid, based on the weight of the mixture present in the pressure hydrolysis, which brings in 0.1 to 5 times, preferably 0.5 to 2 times, the amount of water. The monoalkali salt of the H-acid can be separated off in a customary manner, for example by filtration. It is advantageous, before the monoalkali salt of the H acid to be separated off, to cool the temperature, for example by Evaporative cooling, set to less than 80 ° C and the separation at a temperature of less than 80 ° C. The separation is preferably carried out at a temperature in the range from 20 to 60 ° C.

To remove sulfur dioxide completely, it is advantageous, after the precipitation conditions have been set and before the monoalkali salt of the H-acid has been separated off, to reflux the acidified and diluted mixture for some time, for example from 0.5 to 2 hours, or to adhere it in vacuo or the sulfur dioxide with an inert gas, e.g. B. nitrogen to blow out.

The monoalkali salt of H-acid present after the separation is usually washed with water and dried, for example in vacuo.

After the reaction, the alcohol can be separated off at various points during the working up of the reaction mixture. It is possible to separate the alcohol from an alkaline, neutral or acidic solution before or after the H-acid has been separated off as the monoalkali salt. The alcohol is preferably separated from alkaline or neutral solution and by distillation. It is particularly preferred to distill off the alcohol directly from the reaction mixture, optionally after cooling and / or dilution with water, via a column. If low-boiling alcohols, for example methanol, are used, it may be sufficient to start the distillation without external heat input by depressurization. If the alcohol used separates from the reaction mixture at temperatures lower than the reaction temperature by forming its own phase, it is possible to separate the alcohol by a simple phase separation after cooling the reaction mixture.

The separated alcohol is preferably reused in the process according to the invention. It is then only necessary to add any alcohol portions lost during the alkaline pressure hydrolysis and / or during the workup.

The process according to the invention provides the advantage over the known processes for the preparation of 1-amino-8-naphthol-3,6-disulfonic acid (H-acid) as the monoalkali salt that higher yields can be achieved and the formation of by-products, in particular the formation of 1-Amino-6-naphthol-3,8-disulfonic acid (W acid) and the formation of 1,8-dihydroxynaphthalene-3,6-disulfonic acid (chromotropic acid) is significantly reduced. The reduced content of the W-acid, which is sparingly soluble in acidic solution, also means that the monoalkali salt of the H-acid can be isolated in a particularly pure form without intensive washing which is associated with losses in yield.

Examples example 1

450 g of 1-naphthylamine-3,6,8-trisulfonic acid - trisodium salt (content 15.3 g of nitrite / 100 g, 85.1% by weight of 1-naphthylamine-3, 6,8-trisulfonic acid with a molecular weight of 383; a total of 69 g of nitrite), 325 g of water and 310 g of methanol and heated to 200 ° C. In a 1.3 l steel autoclave, 480 g of 70% sodium hydroxide solution (8.4 mol NaOH) are heated to 220 ° C. and pressed with nitrogen into the 2.7 l autoclave, so that based on the sum of water + methanol , a 30 wt .-% sodium hydroxide solution is formed. A temperature of 220 ° C is set. The reaction mixture is kept at 220 ° C. for 15 minutes, cooled as quickly as possible, the methanol is distilled off over a column and the melt is run into a hot mixture of 600 g of sulfuric acid and 2 l of water. The resulting suspension is refluxed for one hour to remove sulfur dioxide, then cooled to 40 to 45 ° C and kept at 40 to 45 ° C for another hour. The precipitated product is filtered at 40 ° C, washed with a total of 500 g of water and vacuum dried at 80 ° C. The yield is 82% of theory. The H-acid quality was determined by high pressure liquid chromatography as follows:

Examples 2a to 2n

• 1. Molverhältnis NaOH zu T-Säure-Trinatriumsalz
• 2. NaOH-Konzentration bezogen auf Wasser + Methanol
• 3. Gewichtsverhältnis Wasser zu Methanol
• 4. Temperatur
• 5. Reaktionszeit ergeben die in Tabelle 1 zusammengefaßten Ergebnisse.

Several reactions carried out as in Example 1, varying the reaction parameters

• 1. Molar ratio of NaOH to T-acid trisodium salt
• 2. NaOH concentration based on water + methanol
• 3. Weight ratio of water to methanol
• 4. Temperature
• 5. Response time gives the results summarized in Table 1.

Example 3

513 g of T-acid tripotassium salt (content 13.4 g nitrite / 100 g, 74.7% by weight T-acid with a molecular weight of 383; a total of 69 g nitrite), 325 g water and 315 g methanol are in I-nickel autoclaves heated to 210 ° C. 480 g of 70% strength by weight potassium hydroxide solution (6.0 mol of KOH) at 210 ^° C. are pressed in using nitrogen as in Example 1. A temperature of 220 ° C. is established, and a 30% by weight KOH solution results, based on water + methanol. The reaction mixture is kept at 220 ° C. for 17 minutes and cooled to 150 ° C. in about 2 minutes using a cooling coil. After further cooling, the methanol is distilled off and the reaction solution is run into a hot mixture of 450 g of sulfuric acid and 5.5 kg of water. The reaction mixture is stirred for 2 hours at 100 to 110 ° C. to remove sulfur dioxide, cooled to room temperature and stirred for 1 hour at room temperature. The product is filtered, washed with a total of 500 g of water and dried at 80 ° C. in vacuo. The yield is 88% of theory. The H-acid quality was determined by high pressure liquid chromatography as follows:

Examples 4a to 4m

• 1. Molverhältnis KOH zu T-Säure-Trikaliumsalz
• 2. KOH-Konzentration bezogen auf Wasser + Methanol
• 3. Gewichtsverhältnis Wasser zu Methanol
• 4. Temperatur
• 5. Reaktionszeit ergaben die in Tabelle 2 zusammengefaßten Ergebnisse.

Several reactions carried out as in Example 3, varying the reaction parameters

• 1. Molar ratio of KOH to T-acid tripotassium salt
• 2. KOH concentration based on water + methanol
• 3. Weight ratio of water to methanol
• 4. Temperature
• 5. Response time gave the results summarized in Table 2.

Example 5 (according to the FIAT Final Report, cited above)

A reaction carried out as in Example 1 with 1-naphthylamine-3,6,8-trisulfonic acid trisodium salt, but in which the methanol is replaced by the same amount by weight of water, gives a yield of 73% of theory. The contents of the isolated product were determined by high pressure liquid chromatography as follows:

These levels are calculated on free acids. In fact, these are in the form of the salts given in Example 1.

Example 6

4.5 g of T-acid trisodium salt (content 15.3 g of nitrite / 100 g, 85.1% by weight of T-acid with a molecular weight of 383; a total of 0.69 g of nitrite) are mixed in a mixture of 30 g of sodium hydroxide, 42 g of water and 28 g of the specified alcohol are suspended and the suspension is kept at 215 ° C. for 20 minutes with shaking. The composition of the reaction mixtures was determined by means of high-pressure liquid chromatography. The alcohols used and the contents of the various acids in the reaction mixture are shown in Table 3 below.

Example 7

Several reactions carried out as in Example 6 using methanol as alcohol and varying the water / methanol weight ratio give the following composition of the reaction mixtures (see Table 4):

Example 8

(Prozentgehalte jeweils bezogen auf diazotierbare Substanz)
das zusätzlich

• 0,3 Gew.-% 1-Naphthalamin-3,6-disulfonsäure-dinatriumsalz,
• 1,3 Gew.-% Naphthalin-1,3,6-trisulfonsäure-trinatriumsalz,
• 0,6 Gew.-% 1-Nitronaphthalin-3,6,8-trisulfonsäure- trinatriumsalz,

580 g of a naphthalene trisulfonic acid mixture in the form of the trisodium salts (content 11.9 g total nitrite / 100 g, 52.8% by weight T-acid with a molecular weight of 383; a total of 69 g nitrite) are placed in a 2.7 l nickel autoclave , 0.80 mol T acid) of the following composition:

(Percentages based in each case on diazotizable substance)
that in addition

• 0.3% by weight of 1-naphthalamine-3,6-disulfonic acid disodium salt,
• 1.3% by weight of naphthalene-1,3,6-trisulfonic acid trisodium salt,
• 0.6% by weight of 1-nitronaphthalene-3,6,8-trisulfonic acid trisodium salt,

Contains 4.6% by weight of water, and quantitatively undetermined quantities of amino and nitro derivatives of dinaphthylsulfonic sulfonic acid and of oxidation products of naphthalene and naphthalene-trisulfonic acids, as well as 275 g of water and 280 g of methanol and heated to 190.degree . In a 1.3 l steel autoclave, 430 g of 70% strength by weight sodium hydroxide solution (7.5 mol NaOH) are heated to 185 ° C. and pressed into the 2.7 l autoclave with nitrogen, causing, based on water + Methanol, a 30 wt .-% sodium hydroxide solution. A temperature of 200 ° C is reached. The reaction mixture is kept at 200 ° C. for 45 minutes, cooled, diluted with 500 g of water and the methanol is distilled off. The hot reaction solution is pH-controlled, acidified at pH 1 to 1.5 with about 1000 g of 50% strength by weight sulfuric acid, refluxed for 1 hour to remove sulfur dioxide completely, cooled to 40 ° C. under evaporative cooling and 2 hours at 40 ° C kept. The product is filtered at 40 ° C, washed with a total of 500 g of water and dried at 80 ° C in a vacuum.

The yield is 62%, based on the T-acid isomer mixture, or 78% based on T-acid. The H-acid quality was determined by high pressure liquid chromatography as follows:

The stated contents relate to free acids. The salts mentioned in Example 1 are in fact present. Reaction products from the isomeric naphthylamine-trisulfonic acids are not contained in the isolated product.

Example 9

A reaction carried out as in Example 8, but in which the methanol is distilled off from the reaction mixture previously neutralized with sulfuric acid, gives a yield of 63%, based on the T-acid / isomer mixture, or 79%, based on T-acid.

Examples 10a to 10i

• 1. Molverhältnis NaOH zu T-Säure-Isomerengemisch-Trinatriumsalz,
• 2. NaOH-Konzentration bezogen auf Waser + Methanol,
• 3. Temperatur
• 4. Reaktionszeit ergeben die in Tabelle 5 zusammengefaßten Ergebnisse.

Several reactions carried out as in Example 8 with a weight ratio of water to methanol of 3: 2 in the reaction mixture and with variation of the reaction parameters

• 1. molar ratio of NaOH to T acid / isomer mixture / trisodium salt,
• 2. NaOH concentration based on Waser + methanol,
• 3. Temperature
• 4. Response time gives the results summarized in Table 5.

Example 11

(Prozentgehalte jeweils bezogen auf diazotierbare Substanz) das zusätzlich 0,2 Gew.-% 1-Naphthylamin-3,6-disulfensäure-dinatriumsalz, 1,1 Gew.-% Naphthalin-1,3,6-trisulfonsäure-trinatriumsalz, 5,4 Gew.-% Wasser,
und quantitativ nicht bestimmbare Mengen an Amino-und Nitroderivaten der Dinaphthylsulfon-sulfosäure und an Oxidationsprodukten des Naphthalins und der Naphthalin-trisulfonsäuren enthält, sowie 2,8 kg Wasser und 2,8 kg Methanol werden in einem 20 I-Nickel-Autoklaven auf 180° C aufgeheizt. Unter Stickstoff- überlagerung werden im Verlaufe von ca. 5 Minuten 4,3 kg 70 gew.-%ige Natronlauge (75 Mol NaOH) von 180° C eingepumpt, wobei sich eine Temperatur von 200° C einstellt. Die Reaktionsmischung wird 55 Minuten bei 200° C gehalten und in einem 25 I-Edelstahl-Kessel entspannt, in dem 10 1 kaltes Wasser vorgelegt sind. Das Methanol wird abdestilliert und die verdünnte H-Säure-Isomerengemisch-Lösung im Verlaufe von 40 bis 60 Minuten mit Stickstoff in ein Glas-Fällungsgefäß gedrückt, in dem 4 kg Wasser (bzw. Waschwasser aus der Vorpartie) vorgelegt sind. Durch gleichzeitigen Zulauf von ca. 5,2 kg Schwefelsäuremonohydrat (= 100%ige Schwefelsäure) wird der pH-Wert bei 1 bis 1,5 gehalten und die Reaktionsmischung zum Sieden gebracht. Die heiße, saure H-Säure-Suspension wird durch Anlegen von Vakuum vom restlichen Schwefeldioxid befreit, im Verlaufe von einer Stunde auf 40° C abgekühlt, eine Stunde bei 40° C gehalten und filtriert. Das Produkt wird mit insgesamt 5,4 kg Wasser gewaschen und im Vakuum bei 80° C getrocknet. Die Ausbeute beträgt 65%, bezogen auf T-Säure-Isomerengemisch, bzw. 78%, bezogen auf T-Säure. Die H-Säure-Qualität wurde durch Hochdruck-Flüssigkeits-Chromatographie wie folgt ermittelt:

5.9 kg of a naphthylamine-trisulfonic acid mixture in the form of the trisodium salts (content 11.7 g of total nit / 100 g, 53.9% by weight of T-acid with a molecular weight of 383, a total of 0.69 kg of nitrite, 8.3 Mol T acid) of the following composition:

(Percentages based in each case on diazotizable substance), the additional 0.2% by weight of disodium salt of 1-naphthylamine-3,6-disulfenic acid, 1.1% by weight of trisodium salt of naphthalene-1,3,6-trisulfonic acid, 5. 4% by weight of water,
and quantitatively undeterminable amounts of amino and nitroderivatives of dinaphthylsulfonic sulfonic acid and of oxidation products of naphthalene and naphthalene trisulfonic acids, as well as 2.8 kg of water and 2.8 kg of methanol are heated to 180 ° in a 20 l nickel autoclave C. heated up. 4.3 kg of 70% strength by weight sodium hydroxide solution (75 mol of NaOH) at 180 ° C. are pumped in over a period of about 5 minutes with a blanket of nitrogen, a temperature of 200 ° C. being established. The reaction mixture is held at 200 ° C. for 55 minutes and expanded in a 25 liter stainless steel kettle in which 10 liters of cold water are placed. The methanol is distilled off and the dilute H-acid isomer mixture solution is pressed over a period of 40 to 60 minutes with nitrogen into a glass precipitation vessel in which 4 kg of water (or washing water from the previous batch) are placed. By simultaneously adding about 5.2 kg of sulfuric acid monohydrate (= 100% sulfuric acid), the pH is kept at 1 to 1.5 and the reaction mixture is brought to a boil. The hot, acidic H-acid suspension is freed of residual sulfur dioxide by applying a vacuum, cooled to 40 ° C. in the course of one hour, kept at 40 ° C. for one hour and filtered. The product is washed with a total of 5.4 kg of water and dried in vacuo at 80 ° C. The yield is 65%, based on the T-acid isomer mixture, or 78%, based on T-acid. The H-acid quality was determined by high pressure liquid chromatography as follows:

The contents of the specified organic acids are calculated on free acids. In fact, the salts given in Example 1 are present.

Claims (9)

1. A process for the preparation of mono-alkali metal salts of 1-amino-8-naphthol-3,6-disulphonic acid by reacting 1-naphthylamine-3,6,8-trisulphonic acid and/or salts thereof and/or naphthylamine-trisul- phonic acid isomer mixtures and/or salts thereof with an alkali metal hydroxide solution at elevated pressure and elevated temperature and separating out the mono-alkali metal salts of 1-amino-8-naphthol-3,6- disulphonic acid by acidification, characterised in that the reaction is conducted in the presence of alcohols and/or alcoholates.

2. A process according to claim 1, characterised in that the reaction is conducted in the presence of aliphatic alcohols having from 1 to 6 carbon atoms.

3. A process according to claims 1 and 2, characterised in that the reaction is conducted in the presence of methanol.

4. A process according to claims 1 to 3, characterised in that from 10 to 80% by weight of alcohol and/or alcoholate, based on the total weight of water and alcohol is present during the reaction.

5. A process according to claims 1 to 4, characterised in that the reaction is conducted at temperatures of from 150 to 250° C.

6. A process according to claims 1 to 5, characterised in that the reaction is conducted in a closed vessel under the pressure which forms therein.

7. A process according to claims 1 to 5, characterised in that the reaction is conducted under a pressure of from 5 to 100 bars.

8. A process according to claims 1 to 7, characterised in that after the reaction the alcohol is separated from the alkaline or neutral solution by distillation.

9. A process according to claim 8, characterised in that the alcohol which is separated off is reused.