DE2516173A1 - METHOD FOR PURIFYING SULFURIC ACID CONTAMINATED WITH ORGANIC SULPHONIC ACIDS - Google Patents

Description

PATENT ANWALTSBÜRO TlEDTKE - BüHLING "KlNNE

TEL. (089) 53 96 53-56 TELEX: 524845 tipat CABLE ADDRESS: Germaniapalent Müncheft C 1 C 1 7 ^ l

8000 Munich 2

Bavariaring 4

P.O. Box 20 2403, April 14, 1975

case P 26927 - B 6575

IMPERIAL CHEMICAL INDUSTRIES LIMITED London, United Kingdom

Process for the purification of sulfuric acid contaminated with organic sulfonic acids

The invention relates to the purification of sulfuric acid residues.

Many organic chemistry reactions involve hydrolysis of organic sulfonic acids. Such hydrolysis Sulphonic acids generally proceed only to a certain extent, since sulphonic acid hydrolysis to sulfur acid, so that when its concentration reaches a certain level, the reverse reaction, i.e. the SuIfurierung, prevailed. The result of such hydrolysis

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sulfuric acid formed, which shall be referred to herein as "crude acid", is generally dark in color and it generally contains unhydrolyzed sulfonic acids along with water and other reaction products like tars.

The release or removal of the crude acid causes difficulties because it is indeed with lime or calcium carbonate Can be neutralized in the form of chalk or limestone, but the calcium salts have organic sulfonic acids often a considerable solubility in water, so that there is a possibility that they can be found in the groundwater or the water supply can be achieved if the neutralized acid is discarded or dumped away.

The aim of the invention is therefore to eliminate such difficulties, for which a method is provided after which the raw acid is not neutralized and discarded as usual, but with economically viable means is processed. The acid purified and concentrated in this process can then be used for further reactions be used.

According to the process according to the invention, the crude acid is first reduced to a sulfuric acid content of at least

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90% by weight concentrated or brought and then with Treated nitric acid at an elevated temperature.

More particularly, the invention comprises a method for purifying organic sulfonic acids contaminated Sulfuric acid, which is characterized in that the contaminated sulfuric acid is reduced to a sulfuric acid content of at least 90 wt.96 brings or concentrated and then the concentrated contaminated acid Temperatures between 270 and 335 ° C treated with nitric acid and the vaporized or brought into the vapor state Impurities distilled off.

This process according to the invention is surprising, since organic sulfonic acids are very stable - especially with respect to an oxidation - are and yet with nitric acid with the formation of vaporizable and / or colorless organic Connections can be implemented.

It is assumed that the organic impurities are hydrolyzed, nitrated and / or in the process according to the invention are oxidized to form volatile or colorless organic materials.

In the process according to the invention, the organic impurities in the sulfuric acid are treated with salt

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pitric acid implemented. This nitric acid can be concentrated nitric acid, fuming nitric acid, nitric acid constant boiling point or nitric acid vapor.

The temperature at which the nitric acid is reacted with the concentrated crude acid is preferably in the range from 300 to 315 ⁰ C, since the cleaning proceeds only slowly at lower temperatures, while larger amounts of nitric acid are required at higher temperatures, since a Decomposition occurs before it can react with the organic contaminants,

The temperature used is preferably in the range of 310 to 315 ° C, since then acid may be formed with a sulfuric acid content of about 96 wt.%, And this is generally suitable for use in further chemical reactions, such as sulfurations or sulfonations.

It has been found that it is necessary for the removal of the organic impurities by treating the sulfuric acid with nitric acid at a temperature above 270 ⁰ C, that the sulfuric acid to be treated has a sulfuric acid content of more than 90 wt.%. Because the crude acid generally has a lower sulfuric acid content

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has, therefore, it is first necessary to bring the crude acid to a sulfuric acid content of more than 90 wt.%, before it is treated with the nitric acid at more than 270 ⁰ C.

The crude acid preferably contains less than 20% by weight organic sulfonic acids. When the sulfonic acid content is above this value, the crude acid should be subjected to further hydrolysis to be more organic Hydrolyze sulfonic acid. The crude acid preferably contains at least 70% by weight sulfuric acid.

If the crude acid contains only a small amount of unhydrolyzed sulfonic acids and other organic impurities (less than 3% by weight), concentration can be effected simply by distilling off water from the crude acid by heating. This heating can simply be formed by that which is necessary to bring the crude acid to the temperature at which it is reacted with the nitric acid. Since 90 Cew.% Sulfuric acid containing at atmospheric pressure a boiling point of about 255 ⁰ C has (the actual boiling point is, however, be altered by the presence of the organic sulfonic acids), is by heating the crude acid at a temperature in the range 250-260 ⁰ C.

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and distilling off the water a sufficient increase in concentration causes. If the crude acid contains larger amounts of non-hydrolyzed sulfonic acids, can Effective concentration by adding concentrated sulfuric acid with a sulfuric acid content over 90% by weight (For example, such as is formed in the method according to the invention). the added amount of concentrated sulfuric acid should be such that the resulting mixture, if necessary, after distilling off of water by heating it to the temperature at which the reaction with nitric acid takes place Has sulfuric acid content of at least 90% by weight.

A modified and preferred method for concentrating the crude acid containing such larger amounts of unhydrolyzed sulfonic acids consists in adding some nitric acid to the crude acid at a lower temperature, for example at room temperature, ie about 20 ^° C. to 250 ^° C. and heating the mixture to for example, 250 to 26O ⁰ C to distill off water and any volatile materials by reaction of the nitric acid formed to produce a concentrated sulfuric acid having a sulfuric acid content of more than 90 wt.%. The amount of nitric acid required for this concentration level increases with the content of sulfonic acid and other organic impurities in the crude acid.

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This concentration stage, which works with nitric acid, can also be used to advantage when the crude acid contains only a small amount of non-hydrolyzed sulfonic acids. Namely, it has been found that the total amount of nitric acid required for purification is reduced when a nitric acid concentration process is used. It has been found that up to 90% and preferably at least 10% _1, in particular 25 to 50 wt.% Can be used (based on the total weight of the Salpexersäure applied) at this concentration step. The total amount of nitric acid required depends on the amount of organic sulfonic acids and other organic impurities in the crude acid. Under the prevailing conditions in the method as little nitric acid is consumed by reaction with sulfuric acid, generally at least 1 Gew.96 should nitric acid (calculated as HNO,) on the weight of the crude acid to be used based, at least or about 1.5 wt.% concentrated nitric acid (constant boiling acid; 69% ENT). If fuming nitric acid (95 % ENT) is used instead of the constant boiling acid, a smaller amount is of course required. The total amount of nitric acid used is preferably at least 3 % and should be less than 20 % _t, preferably less than 10% by weight (calculated as HNO,) based on the

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Weight of the crude acid because the yield of sulfuric acid because of the reaction between nitric acid and Sulfuric acid is inappropriately reduced when larger amounts of nitric acid are used.

The nitric acid is preferably added slowly to the concentrated crude acid. This enables the required The amount of nitric acid kept to a minimum and the risk of violent reactions kept as low as possible that can occur when the concentrated crude acid contains impurities such as phenols and aromatic Contains hydrocarbons. In terms of precautions to be taken, it is advisable to remove the contaminated sulfuric acid to be analyzed before carrying out the method according to the invention.

The nitric acid, which can be in the form of constant boiling acid (with about 69 % HNO,) or fuming nitric acid (with about 95 % HNCU), can be in liquid or vapor form or as a mixture with sulfuric acid - a mixture with 30 to 40 % By weight of constant-boiling nitric acid and correspondingly 70 to 60% by weight of concentrated (98-6%) sulfuric acid is particularly suitable - be added. It has been found that the amount of nitric acid required is kept to a minimum

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is kept when the nitric acid is in vapor form or when such a mixture with sulfuric acid is added. Furthermore, the required amount of nitric acid becomes so small as far as possible, if the nitric acid - regardless of whether alone or in a mixture with sulfuric acid - in the concentrated crude acid containing reaction vessel is fed below the level of the vessel contents. if If nitric acid is present during the concentration step, it can advantageously be mixed with the crude acid prior to heating be mixed by the latter.

In one embodiment of the invention, the The pretreatment and the cleaning stage are carried out consecutively without any interruption, and are carried out continuously or intermittent addition of nitric acid to the crude acid when the temperature rises above room temperature 270 ° C is increased or rises.

The reaction mixture is preferably supplied with gas, e.g. Air or an inert gas such as nitrogen or argon, cleaned or blown through to remove volatile To support reaction products. It is also preferred that the cleaning gas bubbled through the reaction liquid or is pressed.

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The vaporous 'product!' of the procedure can be carried out by a Water or alkali absorption towers can be sent for interception or reaction with acidic gases and the other gases can be burned or condensed.

The process according to the invention can be carried out batchwise or continuously. In a continuous process, the crude acid is ^{fed intermittently or continuously to a first vessel kept at a temperature of 220 to 280 °} C., in particular 250 to 260 ^° C., in which a concentration takes place continuously and the concentrated crude acid is then continuously or intermittently in a second vessel kept at a temperature in the range from 270 to 335 ^° C. is transferred. Nitric acid is slowly introduced into the second vessel (and if a concentration process using nitric acid is envisaged, it can also be slowly introduced into the first vessel or mixed with the crude acid before it is introduced into the first vessel) and water and volatile products distill off . The purified acid is then continuously or intermittently removed from the second vessel.

A simplified scheme of the apparatus is shown in the attached drawing. The apparatus includes a heated and jacketed first reaction vessel 1 from

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an acid-resistant material such as glass or quartz or "silica" with an inlet 2 through which crude acid can be fed with a controlled throughput. A mixture of nitric acid and sulfuric acid can be fed in through a second inlet 3 with a controlled throughput below the surface of the contents of the vessel. Alternatively, inlet 3 can be omitted and nitric acid added to the crude acid prior to feeding through inlet 2. An effluent column 4 is provided through which water distilling off the acid can be removed along with any other volatile materials present or formed by reaction of the nitric acid with the crude acid. This effluent column 4 leads to suitable cleaners (not shown). A gas supply or distribution 5 for air or nitrogen is also provided to aid in the removal of the volatilized water and other volatile material. A stirrer 6 is also located in the vessel and concentrated crude acid formed in vessel 1 can pass through an outlet into the second heated and jacketed reaction vessel 8 made of acid-resistant material. This vessel 8 is also provided with a gas inlet 9 _S, a controlled feed line 10 for a nitric acid / sulfuric acid mixture, a stirrer 11 and an outlet 12 through which the purified acid enters a suitable receptacle 13

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transgresses. Gases outgoing from the vessel 8 reach the vessel 1 through the conduit 7 and from there to the effluent column 4. Valves 14 are provided to control the flow rates of the acids. These valves can be used for continuous or intermittent feeding or transfer of the acids.

The inventive method is particularly useful when the organic impurities are essentially only formed by organic sulfonic acids along with a small amount of tar-like materials.

An example in which the invention is useful relates to the preparation of bis (4-chlorophenyl) sulfone. As described in US Pat. No. 3,855,312, this is formed by reacting 4-chlorobenzenesulfonic acid with chlorobenzene. As described in the applicant's GB patent application 16 448/74, the bis (4-chlorophenyl) sulfone can be separated from the reaction product by crystallization, leaving behind mother liquors with some bis (4-chlorophenyl) sulfone together with chlorobenzene , Chlorobenzenesulfonic acid and isomers of bis (4-chlorophenyl) sulfone. These mother liquors can be hydrolyzed by treatment with water in the presence of sulfuric acid at elevated temperatures of, for example, 220 to 250 ^° C., the sulfones

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and SuIfonsäure be hydrolyzed to chlorobenzene, the (and can be reused) distilled off, leaving behind rcher sulfuric acid with some non-hydrolysed SuIfonsäure together with tars or lubricating ^{n M.} This crude acid can be purified and reused by the process according to the invention, for example for the production of chlorobenzenesulfonic acid from chlorobenzene, which in turn can be used for the production of bis (4-chlorophenyl) sulfone.

The invention is explained below with the aid of examples.

example 1

A black viscous residual liquid was analyzed and a sulfuric acid content of 80 to 85 wt g) the liquid or press-in into a 250 ml glass flask equipped with a condenser for distillation, a dropping funnel and inlet tube given for nitrogen constant boiling nitric acid (5 g; density:.. 1.41 g / cnr, at 20 ⁰ C) was added to the mixture at room temperature (25 ^° C.) and the mixture was ^{heated to 310 °} C. It formed

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- IU -

Nitrogen oxides and some liquid distillate was collected. Another amount (5 g) of concentrated nitric acid

was added dropwise over 10 minutes at 310 ⁰ C-

given; the liquid in the flask lightened with each drop added, and nitrogen oxides again became developed. After adding the nitric acid, the liquid in the flask was very pale. Residual nitrogen oxides were through Air bubbled through for 15 minutes. The liquid was allowed to cool to 25 ° C, at which Temperature it was clear as water. The liquid contained about 96% by weight sulfuric acid, a trace of nitric acid and traces of organic residues. The liquid was for suI-furierung immediately suitable for aromatics.

Example 2

The procedure of Example 1 was repeated, but changing the sequence of the addition of constant boiling nitric acid using a black viscous crude sulfuric acid with 86 wt.% sulfuric acid and organic sulfonic acids (4 wt.%) .. The rest was largely water. Details of these experiments are given in the table below.

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Addition of nitric acid (parts 310 ⁰ C ■ 4th rawer Duration at Color of the result i black, impenetrable per 100 parts by weight 310 ⁰ C animal sulfur sighted Sulfuric acid) at (Minutes) acid black, impenetrable room 0 those sighted temperature temperature water-clear, clear 1.2 0 a) 8th 30th 8 (26O ⁰ C) b) 0 25th 0 c) 4th 5

The product from experiment c) contained only trace amounts of organic material. A comparison of tests b) and c) shows that 1.2 parts of nitric acid were insufficient to achieve cleaning even if 8 parts were used in the concentration step during the application of 4 parts in each stage gave the desired result.

Example 3

The procedure of Example 1 was followed with different samples of black crude sulfuric acid organic sulfonic acids and tars as impurities repeatedly using different amounts of nitric acid and reaction conditions. The results are given in the table below.

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No. H ₂ SC composition (Weight 96) Pretreatment 100 Type of cleaning Type of duration cn the raw Acid) used used at CD sulfuric acid at room temperature Saltpetre added Saltpetre 310 ⁰ C ) - j organic rature added acid Saltpetre acid (Min) Look (% By weight): sulfonic acids nitric acid amount of acid the right quantity (parts per (Parts per resulting attempt Parts by weight raw 100 wt. acid ; ² , 4 share raw Acid) 1 j 70 % 310 ⁰ C 70 96 5 . '2.4 2 2 2 86 95 % 95 % 5 clear, water cn 3.3 bright O
^t £ L 2 1.3 OO 3 86 3.3 » 95 % 5 clear wai "s him cn bright 4th no 95 96 5 95 % 5 CD 86 clear but OO 5 2 95 96 2.4 95 % 10 dark co 86 clear, water cn 6th 3 70 96 1.25 70% 25th bright 86 clear, water 7th 4th C. 70 96 6th 70% 10 bright 86 clear, water 8th 4th 70 % ^, 5 70 96 10 bright 9 84 3.3 95% 95% 5 clear, water 3 5.5 bright 84 3 (+4 at 220-240 ° 1.25 clear, dunk
Red 83 clear, water
bright 5 5 7th

A comparison of experiment 3 with experiments 4 and 5 shows that by adding some of the nitric acid at a low temperature, smaller amounts of nitric acid to be used are possible. It is believed that the acid would have become clear as water when trying 3 when more acid had been added at 310 ⁰ C. In the same way it is assumed that the acid in experiment 8 would have become water-white at 310 ° C. if more nitric acid had been added. A comparison of experiments 7 and 8 thus indicates that the total amount of nitric acid required can be reduced by using more nitric acid in the concentration stage (ie heating from room temperature).

Example 4

The apparatus used was a 250 ml flask with a thermometer, inlet for dry nitrogen, dropping funnel with fine adjustment and a splash guard or distillation attachment with air cooler with attached template.

Crude sulfuric acid (200.1 g with 80 % sulfuric acid, 13.3 % organic sulfonic acids, 0.05 % other organic impurities and water as the remainder) was added to the flask and nitric acid (69%) was added to the dropping funnel.

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The flask was heated up and while the temperature rose to 120 ^° C., nitric acid (3 ml.) Was added dropwise. The addition of nitric acid was then interrupted and the temperature increased further. The mixture foamed quite violently when it reaches a temperature of 220 ⁰ C, but was "less" if the temperature exceeds 235 ° C. The temperature was raised to 305 ⁰ C and the mixture was allowed to cool to about 240 ⁰ C. It was then held at about 240-250 ° C. while another 5 ml of nitric acid was slowly added from the dropping funnel over a period of 30 minutes. The crude acid turned dark red. The temperature was then raised to 310 C - the acid started at about 280 ⁰ C to boiling (refluxing) - and the mixture was then allowed to cool to 245 ⁰ C. Another 2 ml of nitric acid was added dropwise while the mixture was kept at about 240 ^° C. The resulting orange-yellow mixture was then heated to 31 ° C. and a further 0.5 ml of nitric acid was added. The mixture was then lemon yellow and was allowed to cool to about 70 ^0C. The cold acid was pale yellow in color.

The acid was then again heated to 31o ⁰ C and a further 2 ml of nitric acid was added dropwise. The mixture was then allowed to cool to about 60 ⁰ C. The cold acid was clear and water-white.

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The total amount of nitric acid used was 12.5 ml, i.e. 9.2% by weight (based on the weight of the crude Acid) of 69% acid corresponding to 6.3 wt. # Calculated as

Example 5

The apparatus of Example 4 was modified by connecting the dropping funnel and nitrogen inlet in such a way that that the nitrogen and the drops from the dropping funnel are fed below the surface of the contents of the 250 ml flask could become.

Crude acid (201.1 g - a further sample of the crude acid as used in Example 4) was added to the flask and 23O ⁰ C erhitzt.Salpetersäure (1.5 ml) was then added and the mixture was then 3l4 ° C heated. The temperature was then maintained at 310-312 ° C while nitric acid (6.5 ml) was added dropwise over a period of 42 minutes. (After adding a total of 2.5 ml of nitric acid, the mixture was very dark red-brown; it then became lighter: it was red after adding a total of 6.3 ml of nitric acid and yellow after adding a total of 7.0 ml of acid). The mixture was then allowed to cool to about 70 ° C. The cold acid was clear and water-white. The total amount of nitric acid used was 8 ml, ie 6.8% by weight (based on the weight of the

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crude acid) corresponding to about 4.7 wt.? o calculated as

Example 6

Example 5 (#ig 69) using a mixture of 36 wt.% Nitric acid and 64 wt.% Sulfuric acid (lg 98%) and a crude acid (96 g _t 2C7) of similar composition (79.7% H ₂ SO ^; 13.8% sulfonic acids; O, C1 % other organic impurities) as that used in Example 5 was repeated. 7, the sulfuric acid / nitric acid mixture were ml added at room temperature (22 ⁰ C) and the piston to 308 ⁰ C is heated, after which the sulfuric acid / Salp ± ersäuremischung (15 ml) was dropwise added over a period of 30 minutes, the Temperature was maintained at 307 to 311 ° C. The result! - .: ·: lime-yellow acid was cooled, however it was not water-white.

The mixture was then reheated to 310 ⁰ C and it was then further 0.5 ml of the sulfuric acid / Salpcfcersäure mixture added dropwise. On cooling to about 80 ^° C., the resulting acid was very pale yellow in color.

The mixture was then ^{heated again to 310 °} C., and a further 0.5 ml of the sulfuric acid / nitric acid mixture were added dropwise. The resulting acid was then to about

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Chilled 90 ° C, at which temperature it was clear and colorless.

The total amount of nitric acid required was 13.89 g of 69 % acid, ie 4.6 % (based on the weight of the real acid) calculated as HNCU.

Example 7

The procedure of Example 6 was followed using a black colored crude acid (211.45 g) of the the following composition repeated:

H ₂ SO ^: about 82.5% by weight;

organic sulfonic acids: about 2.5 wt. $ 6; other organic impurities: 0.1% by weight ; Remainder: water

69% nitric acid (14.8 g) was added to the crude acid before it was placed in the flask and the sulfuric acid / nitric acid mixture was not added until the temperature reached 310 ^° C. While maintaining the temperature at 310 to 312 ° C, 8.5 ml of the sulfuric acid / nitric acid mixture was added dropwise over a period of 15 minutes. After cooling, the resulting acid was clear and water-white. The total amount of nitric acid used was 9.5% by weight of 69% nitric acid (based on the weight of the crude

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Acid) corresponding to 6.5% by weight, calculated as HNO ,.

Example 8

As a comparison, an attempt was made to purify another sample (154.81 g) of the crude acid used in Example 7 using oleum instead of nitric acid. 5 ml of oleum (with 20% by weight free sulfur trioxide) was added dropwise to the crude acid at room temperature (21 ° C) and the mixture was then heated to 220 ° C. Another 5 ml gleum was added dropwise and the mixture was then brought to 310 ⁰ C. While the temperature

was maintained at 305-31 ⁰ ^ C, more oleum was added dropwise. Even after adding a total of 99 ml of oleum (189.36 g, ie about 25% by weight of free sulfur trioxide based on the weight of the crude acid), no change in the color of the crude acid was observed.

Example 9

The procedure of Example 7 was followed with the addition varying amounts of nitric acid to the crude acid repeatedly before adding to the flask. In all cases the acid was clear and practically colorless after the cleaning process. The results are in the table below summarized.

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added at the beginning
Amount of 69% nitric acid (% by weight of
raw acid)

To obtain a 'yield of decolorless product; colored acid (%; required total '■ based on the quantity 69% weight of the crude acid; pitric acid (% by weight of the '+ of the

raw acid)

put sulfur / nitric acid mixture 'contained sulfuric acid)

8.5 7.8 7.8 9.5

82.6 77.6 76.8 75.7

Example 10

The crude acid as used in Examples 7-9 was processed by a continuous process using a system with two vessels as described in connection with the attached drawing is cleaned.

Vessels 1 and 8 each contained approximately 750 ml of acid from a previous run of the procedure. The vessel 1 contained about 4 % by weight of nitric acid and was

O,

kept at temperatures in the range of 220 + 5 C while

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the vessel 8 was kept at temperatures in the range of 310 ± 5 ° C. A mixture of the crude acid and 4% by weight (based on the weight of the crude acid) nitric acid was fed into vessel 1 at a rate of 40 ml / h (654 g crude acid per hour) and a mixture of nitric acid (69 % Ig) and sulfuric acid (98 % ± g) with 0.4 g / ml nitric acid (calculated as ENT) were fed through the inlet 10 into the vessel 8 with a throughput of about IGO ml / hour. The valve 14 controlling the overflow or drainage from the vessel 8 was actuated intermittently, whereby it was only opened every 5 or 10 minutes in order to let the decolorized acid into the receiving vessel 13. The procedure was carried out for about 6 hours and then interrupted. The procedure was then repeated in vessel 1 using different temperatures. The results obtained are summarized in the table below.

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temperature 1 in the Throughput of the 1 100 Total indicated Yield of ent vessel ³ C) Saltpeter / Schwe 90 turned saltpeter colored acid (be ( ^c fic acid AO acid amount (in moved to the Ge mixture 40 ENT, based on weight of the raw (ml / hour) the weight of the Acid and the in raw acid) the fed Saltpeter / sulfur acid contained 5 ⁰ C sulfuric acid 220 i 5 ⁰ C 8.6 % 92 % 250+. 5 ° C 8.1 % 85 % 280 + 5 ⁰ C 5.0 % 81 % 310 + 5.2 % 77 %

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Claims (7)

Claims (lj process for the purification of sulfuric acid contaminated with organic sulfonic acids, characterized in that the contaminated sulfuric acid is concentrated to a sulfuric acid content of at least 90% by weight and then "treated with nitric acid at temperatures between 270 and 335 ° C and evaporated impurities are distilled off . 2. The method of claim l, wherein the contaminated acid comprises less than 3 wt.% Organic sulphonic acids, characterized in that the concentration is effected by distilling off the water while the contaminated acid heated to temperatures in the range 270-335 ⁰ C. where it is treated with nitric acid .. 3. The method according to claim 1 or 2, characterized in that IC up to 90% by weight of the total amount of nitric acid used to the contaminated acid at temperatures
below 270 ⁰ C are added. 4. The method according to any one of claims 1 to 3, characterized characterized that the total amount of nitric acid used (calculated as HNO,) 1 to 20 wt.% (based on the Weight of the contaminated acid). 509845/089 5 5. The method according to any one of claims 1 to 4, characterized by a continuous implementation by feeding contaminated acid into a first, ^{heated to a temperature in the range of 220 to 280 0} C vessel and distilling off water from this and transferring the more concentrated acid from this first vessel in a second vessel, which is kept at a higher temperature than the first vessel in the range of 270 to 335 ⁰ C, with feeding of nitric acid into the second vessel and distilling off water and volatile products from this second vessel, from which then purified sulfuric acid is removed. 6. The method according to claim 5, characterized in that the nitric acid in the second vessel below the Level of contaminated acid is introduced. 7. The method according to claim 5 or 6, characterized in that nitric acid with the contaminated acid is mixed before it is fed into the first vessel, 509845/0895 it Blank page