GB1561604A

GB1561604A - Process for purifying and concentrating sulphuric acid

Info

Publication number: GB1561604A
Application number: GB1551777A
Authority: GB
Original assignee: Hoechst AG
Current assignee: Hoechst AG
Priority date: 1976-04-15
Filing date: 1977-04-14
Publication date: 1980-02-27
Also published as: JPS52126693A; DE2616768B2; DE2616768C3; FR2348149A1; DE2616768A1

Description

(54) PROCESS FOR PURIFYING AND CONCENTRATING SULPHURIC ACID (71) We, HOECHST AKTIENGESELLSCHAFT, a body corporate organised according to the laws of the Federal Republic of Germany, of 6230 Frankfurt/Main 80, Postfach 80 03 20, Federal Republic of Germany, do hereby delcare the invention for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: The present invention relates to the reprocessing of aqueous waste sulphuric acid having organic impurities.

Dilute sulphuric acid is frequently concentrated according to the process of H. Pauling described in German Patent Specification No. 299 774(1915. At the same time impurities of an organic nature that may be present are removed to a large extent. In this process, the aqueous sulphuric acid having a concentration of, for example, 70% is passed through a distillation column mounted on a cast iron boiler. This is heated from the outside by oil or gas, and contains boiling concentrated sulphuric acid, for example, having a concentration of from 96 to 97%. The water contained in the waste acid fed in evaporates on contact with the hot contents of the boiler and escapes through the distillation column in the form of steam at a temperature of about 1300C. As a result, the waste sulphuric acid fed into the boiler is pre-heated.The purified and concentrated sulphuric acid is removed from the boiler in an amount corresponding to the amount of new waste sulphuric acid entering from above.

The boiler and boiler lid are generally made of lamellar grey cast iron having a pearlitic structure. The distillation column is generally manufactured from cast silicon and is usually in the form of a bubble-plate column. In practical operation, the process is normally carried out with waste sulphuric acid containing from 20 to 30% of water, up to 2% of organic impurities and inorganic constituents, for example, metal salts or ammonium sulphate.

Detailed descriptions of the Pauling process may be found in the following literature references: P. Parrish, Transactions Inst. Chem. Engineers, Vol. 19 (1941), pages 1 to 24; in B.I.O. S. Final Report No. 243; B. Waeser, Schwefelsäurefabrikation (Sulphuric Acid Production), Braunschweig 1961, page 422; F. Rumford, "Chemical Engineering Materials", London 1954, pages 54 to 75; FIAT Fin Rep. 1187, Film F-18; and E. Maahn, Brit.

Corrosion J. 1 (1966), page 350. In addition, the Pauling process is also discussed in British Patent Specification No. 1 265 526 and U.S. Patent Specification No. 3 792 987.

If the waste sulphuric acid contains more than 0.1% of organic impurites, for example 1%, these constituents may be oxidised by the hot sulphuric acid during the process, according to the following equation: [C] + 2H2SO49 CO2 + 2SO2 + 2H2O This reaction is, however, detrimental to the process because the yield of sulphuric acid is reduced and the large amounts of sulphur dioxide formed can often be removed only with difficulty.

Furthermore, the reducing action of the organic impurities leads to a lowering of the redox potential of the contents of the boiler, whereby the passivation of the cast iron boiler is rendered more difficult or is prevented and corrosion of the boiler is intensified.

Sometimes, the organic impurities or the carbon resulting from their decomposition are oxidised by the boiling concentrated sulphuric acid at about 320"C only so slowly that with normal, average dwell times, the reaction remain incomplete, and only yellow to brown-coloured sulphuric acid can be withdrawn from the boiler. Thus, for example, some polycyclic compounds or sulphonated benzothiazoles as impurities of sulphuric acid are only partially oxidatively degraded even by boiling the concentrated sulphuric acid for about 20 hours.

To eliminate these disadvantages, in accordance with German Patent Specifications Nos.

679 850, 751 909 and 1 079 606, British Patent Specification No. 441 452 and U.S. Patent Specification No. 2 390 316, nitric acid is added repeatedly to the organically contaminated sulphuric acid; under the conditions of the Pauling process the nitric acid reacts preferentially with the organic constituents, these being oxidised and the nitric acid being reduced either to nitrogen or to nitrous gases, according to the process used.

Moreover, the addition of nitric acid has the advantage that ammonium sulphate present in the waste acid or formed as a result of the breakdown of organic nitrogen compounds is decomposed.

According to German Patent Specification No. 699 770, nitric acid can also be fed, as an oxidising agent, into the boiler of the Pauling plant serving as the distillation vessel and filled with boiling concentrated sulphuric acid. It has, however, been shown that this requires relatively large amounts of nitric acid and/or long dwell times.

German Patent Specification No. 1 079 606 describes a process in which nitric acid or nitrosylsulphuric acid is mixed with the waste acid. These acids are intended then to bring about the decomposition of the organic impurities at from 110 C to 1200C and at about 1700C respectively. The process is so designed that it does not result in nitration of the organic impurities. Nitrated organic impurities are in many cases more resistant to oxidative attack than are non-nitrated organic impurities. Besides this, their presence represents a certain risk in operation. At the relatively low temperatures of the process decomposition of organic impurities succeeds only in a few cases.

The present invention provides a process for the purification and concentration of sulphuric acid, wherein aqueous sulphuric acid having a concentration not exceeding 90% by weight and containing organic impurities is fed together with nitrosylsulphuric acid into a distillation column connected to a vessel containing boiling concentrated sulphuric acid to which an oxidising agent is added.

The process of the invention is primarily intended to be carried out in Pauling apparatus.

In the process, the waste sulphuric acid is mixed with nitrosylsulphuric acid and is then passed to the distillation column of the Pauling plant, whilst an oxidising agent is fed into the boiler of the Pauling plant, i.e. the distillation vessel, which contains boiling concentrated sulphuric acid.

The oxidising agent may be fed periodically or continuously into the boiler of the Pauling plant.

The amount of nitrosylsulphuric acid to be added may vary within wide limits.

Advantageously, from 1 to 43, preferably from 1 to 5, parts by weight of nitrosylsulphuric acid per part by weight of carbon in the waste sulphuric acid may be used. Nitric acid is especially suitable for use as the oxidising agent. The amount of nitric acid used is preferably from 0.001 to 0.1 parts by weight, advantageously from 0.02 to 0.06 parts by weight, per part by weight of waste acid running into the boiler. Besides nitric acid, other oxidising agents, for example, perchloric acid, Caro's acid or hydrogen peroxide, may be used.

The nitrosylsulphuric acid required for the process of the invention is advantageously used in the form of a solution in concentrated sulphuric acid. Such solutions, having concentrations of up to 50%, can be prepared, for example, in accordance with German Patent Specification No. 1 111 158. For laboratory experiments, nitrosylsulphuric acid can be prepared in a simple manner by dissolving sodium nitrite in concentrated sulphuric acid while cooling.

Surprisingly, the process of the invention requires considerably smaller amounts of additives conducive to the decomposition of the organic impurities in the waste sulphuric acid (i.e. oxidising agent and nitrosylsulphuric acid) than are required in previously proposed processes. Moreover, the process of the invention permits substantially shorter average dwell times of the boiling sulphuric acid in the distillation boiler.

It will be understood in the art that the concentration of the boiling sulphuric acid must be high enough to avoid rapid attack of the container vessel, for example concentrations of 94% or more must be used where the vessels are made of grey cast iron as this type of vessel, which is generally used in industry, is attacked rapidly if the concentration falls below 94% by weight. In other cases this concentration may be as low as 90% by weight.

Generally, however, a colourless sulphuric acid having a concentration of from 96 to 97% by weight and a carbon content of at most 0.05% may be obtained from the dilute waste acid used. The dilute waste sulphuric acid itself may be used in a concentration as low as 65% by weight when fed into the distillation column of a Pauling unit. The amounts of nitrosylsulphuric acid and oxidising agent giving a minimum residual amount of nitrogen-oxygen compounds in the resulting sulphuric acid can readily be determined in simple experiments.

Compared with the prior art process mentioned above which operates merely with added nitric acid, the process according to the invention leads in many cases to lower residual contents of nitrogen-oxygen compounds; the amount of coloured nitrous gases which have to be eliminated is also considerably reduced. The partial replacement of nitric acid by nitrosylsulphuric acid also reduces the possibility that organic nitro compounds will be formed.

The amounts of nitrosylsulphuric acid to be added and of oxidising agent are, in general, independent of one another. However, in order to purify a particular sulphuric acid as economically as possible, it is sensible to try to use a minimum of auxiliaries. A weight ratio of nitrosylsulphuric acid to oxidising agent in the range of from 10 to 0.1, especially in the range from 5 to 0.5, is especially advantageous. In general, the amount of oxidising agent required to achieve a particular decomposition can be ascertained by means of a simple experiment. Upon the simultaneous supply of waste sulphuric acid through the distillation column of the Pauling plant and supply of oxidising agent into the distilling vessel, the progression of an oxidation can easily be recognised by the fact that the waste sulphuric acid become lighter in colour.The coloured impurities of the sulphuric acid are, as it were, titrated with the added oxidising agent. This procedure allows the dwell time necessary for decolouration to be reduced, by using an excess of oxidising agent.

In various industrial processes, for example, the process of the invention, the process of German Patent Specification No. 751 909 as carried out in a Pauling plant, and the concentration of contaminated waste sulphuric acid from a concentration of about 20% by weight to a concentration of about 70% by weight in the presence of nitric acid, nitrous gases can be evolved. These can easily be absorbed in concentrated sulphuric acid, for example by means of a trickling tower or a packed column, and the nitrosylsulphuric acid thereby formed can be used with particular advantage in the process of the invention.

It is known that nitrosylsulphuric acid is decomposed by water to give nitrous gases and sulphuric acid. Since, however, in the process of the invention the mixture of aqueous waste sulphuric acid and nitrosylsulphuric acid is subjected on the upper tray of the distillation column to an intense steam of hot water vapour (at about 130"C), it would be expected that the nitrosylsulphuric acid added would be quickly destroyed. The favourable effect of the nitrosylsulphuric acid mixed with the waste sulphuric acid is therefore unexpected.

The process according to the invention is especially suitable for reprocessing waste acids which accumulate in the manufacture of dyestuffs or dyestuff intermediates.

The carbon content of the waste acids used is not critical. Preferably, acid having a carbon content of from 0.2 to 2%, especially from 0.4 to 1.0%, is used. Higher contents are also possible but require larger amounts of oxidising agent.

The following Examples illustrate the invention.

Examples 1.Experimentation.

The Pauling apparatus used for the experiments consisted of a fractionating column, 30 cm in length, filled with glass spirals and mounted on an electrically heated round-bottomed flask of 1 litre capacity. Almost the same results were obtained with a 40 cm long column mounted on a flask of 2 litres capacity. The experiments generally consisted of three steps.

In the first step a small portion of the sulphuric acid to be purified was first heated in the flask and concentrated (exceptions: experiments 7 and 8). The water contained in the acid distilled over the column and condensed by means of a descending Liebig condenser.

In the second step, the bulk of the sulphuric acid to be purified was added uniformly over a period of about 1.5 to 3.5 hours via the fractionating column (where appropriate, after the addition of nitrosylsulphuric acid) to the small amount of boiling concentrated crude sulphuric acid already present in the flask from the first step. At the same time an oxidising agent was passed through an immersion tube into the flask (exceptions : experiments 1, 3, 6 and 7), while the contents of the flask were kept boiling.

In the third step, the Liebig condenser was replaced by a reflux condenser and boiling was carried out under reflux until the sulphuric acid started to become colourless, (exception : experiment 7). If the dwell time was to be shortened, boiling was carried out with the addition of further amounts of oxidising agent.

The average dwell time in the flask was calculated according to the formula T = l/2 x (duration of the second step) + (duration of the third step).

Experiments 4 to 6 and 8 to 11 were carried out again with the addition of a few drops of Bayer 7800 anti-foaming agent. Experiments 5, 8, 9, 11 and 12 correspond to the process of the invention.

2. Experimental conditions Experiment Acid No. NOHSO4 NOHSO4 HNO3 Oxidising agent No. in fraction- in distill- in fraction- type amount in ating column ation flask ating column. distillation # # # flask 1 - - 8.09 - 1 - - - HNO3 10.3 3 1 - - 10.35 - 4 1 - - - HNO3 8.84 0 5 1 2.03# - - HNO3 4.36 6 1 - 50.74 # - 7 2 - 1.79 ( 8 2 1.78 # - - - HNO3 0.39 9 3 2.88 # - - HNO3 3.90 10 1 - - - HC104 5.04 11 3 2.88 0 - - HCl04 4.16 12 4 3.10 - - HNO3 3.21 # 3.Experimental results Experiment Sum of average dwell acid obtained additives # time required color Carbon content 1 12.8 1,275 min colourless 0.045% 2 16.4 360 " 0.02 3 16.4 1,225 " 0.045 4 14 420 " 0.045 5 8.5 232 " 0.065 6 40.2 816 " 0.165 7 1.4 68 black- 0.57 brown R 8 2 91 colourless v 0.014 9 8.5 91 " 0.021 10 5 1,262 " 0.06 11 6.3 95 " 0.011 12 7.6 94 a 0.013 4. Explanations of the table 1. Parts by weight per part by weight of carbon in the waste sulfuric acid; added to the waste sulfuric acid; addition via fractionating column; 2. addition to the distillation flask; parts by weight per part by weight of carbon in the waste sulfuric acid.

3. Oxidizing agent in distillation flask added together with the waste sulfuric acid.

4. In the form of an acid containing 57% by weight of HNO3 and 43% by weight of concentrated H2SO4.

5. In the form of concentrated H2SO4 containing 5.52% of NHSO4.

6. In the form of an acid containing 42% by weight of concentrated H2SO4 and 58% by weight of NOHSO4.

7. In the form of an acid containing about 50% by weight of concentrated H2SO4 and about 50% by weight of HNO3.

8. In the form of an acid containing 42% by weight of concentrated H2SO4, 17% by weight of H2O and 41% by weight of HClO4.

9. In the form of an acid containing 50% by weight of concentrated H2SO4, 15% by weight of H2O and 35% by weight of HClO4.

10. The term "additives" includes both nitrosylsulfuric acid and oxidizing agents (NO3 and HClO4). The sum of the additives is given in mol/100 g of the waste sulfuric acid.

11. Experiment broken off after a total duration of 115 minutes.

5. Acid used No. 1 68.5% H2SO4; 0.68% C; 0.08% N according to Kjeldahl; 0.12% ash. (Mixture of waste acids from the manufacture of dyestuffs and dyestuff intermediates).

2 70% H2SO4 (remainder water) reacted with 1% by weight of aniline. (In the distilling vessel concentrated H2SO4 was introduced with 1% of aniline before the start of the run.) 3 69.9% H2SO4; 0.48% C; 0.09% N according to Kjeldahl;0.24% ash; (Mixture of waste acids from the manufacture of dyestuffs and dyestuff intermediates).

4 70% H2SO4 (remainder water) reacted with 1% starch.

WHAT WE CLAIM IS: 1. A process for the purification and concentration of sulphuric acid, wherein aqueous sulphuric acid having a concentration not exceeding 90% by weight and containing organic impurities is fed together with nitrosylsulphuric acid into a distillation column connected to a vessel containing boiling concentrated sulphuric acid to which an oxidising agent is added.

2. A process as claimed in claim 1, which is carried out in a Pauling apparatus.

3. A process as claimed in claim 1 or claim 2, wherein the concentration of the sulphuric acid in the vessel is at least 90%.

4. A process as claimed in claim 3, wherein the concentration of the sulphuric acid in the vessel is from 96 to 97% by weight.

5. A process as claimed in any one of claims 1 to 4, wherein the concentration of the sulphuric acid fed into the distillation column is at least 65% by weight.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. 3. Experimental results Experiment Sum of average dwell acid obtained additives # time required color Carbon content 1 12.8 1,275 min colourless 0.045% 2 16.4 360 " 0.02 3 16.4 1,225 " 0.045 4 14 420 " 0.045 5 8.5 232 " 0.065 6 40.2 816 " 0.165 7 1.4 68 black- 0.57 brown R 8 2 91 colourless v 0.014 9 8.5 91 " 0.021 10 5 1,262 " 0.06 11 6.3 95 " 0.011 12 7.6 94 a 0.013 4. Explanations of the table 1.Parts by weight per part by weight of carbon in the waste sulfuric acid; added to the waste sulfuric acid; addition via fractionating column; 2. addition to the distillation flask; parts by weight per part by weight of carbon in the waste sulfuric acid. 3. Oxidizing agent in distillation flask added together with the waste sulfuric acid. 4. In the form of an acid containing 57% by weight of HNO3 and 43% by weight of concentrated H2SO4. 5. In the form of concentrated H2SO4 containing 5.52% of NHSO4. 6. In the form of an acid containing 42% by weight of concentrated H2SO4 and 58% by weight of NOHSO4. 7. In the form of an acid containing about 50% by weight of concentrated H2SO4 and about 50% by weight of HNO3. 8. In the form of an acid containing 42% by weight of concentrated H2SO4, 17% by weight of H2O and 41% by weight of HClO4. 9. In the form of an acid containing 50% by weight of concentrated H2SO4, 15% by weight of H2O and 35% by weight of HClO4. 10. The term "additives" includes both nitrosylsulfuric acid and oxidizing agents (NO3 and HClO4). The sum of the additives is given in mol/100 g of the waste sulfuric acid. 11. Experiment broken off after a total duration of 115 minutes. 5. Acid used No. 1 68.5% H2SO4; 0.68% C; 0.08% N according to Kjeldahl; 0.12% ash. (Mixture of waste acids from the manufacture of dyestuffs and dyestuff intermediates). 2 70% H2SO4 (remainder water) reacted with 1% by weight of aniline. (In the distilling vessel concentrated H2SO4 was introduced with 1% of aniline before the start of the run.) 3 69.9% H2SO4; 0.48% C; 0.09% N according to Kjeldahl;0.24% ash; (Mixture of waste acids from the manufacture of dyestuffs and dyestuff intermediates). 4 70% H2SO4 (remainder water) reacted with 1% starch. WHAT WE CLAIM IS:

1. A process for the purification and concentration of sulphuric acid, wherein aqueous sulphuric acid having a concentration not exceeding 90% by weight and containing organic impurities is fed together with nitrosylsulphuric acid into a distillation column connected to a vessel containing boiling concentrated sulphuric acid to which an oxidising agent is added.

6. A process as claimed in any one of claims 1 to 5, wherein from 1 to 43 parts by weight

of nitrosylsulphuric acid per part by weight of carbon in the sulphuric acid fed into the distillation column are used.

7. A process as claimed in claim 6, wherein from 1 to 5 parts by weight of nitrosylsuphuric acid per part by weight of carbon in the sulphuric acid fed into the distillation column are used.

8. A process as claimed in any one of claims 1 to 7, wherein the oxidising agent comprises nitric acid.

9. A process as claimed in claim 8, wherein from 0.001 to 0.1 parts by weight of nitric acid per part by weight of sulphuric acid fed into the distillation column are used.

10. A process as claimed in claim 9, wherein from 0.02 to 0.06 parts by weight of nitric acid per part by weight of sulphuric acid fed into the distillation column are used.

11. A process as claimed in any one of claims 1 to 7, wherein the oxidising agent comprises perchloric acid, Caro's acid or hydrogen peroxide.

12. A process as claimed in any one of claims 1 to 11, wherein the weight ratio of nitrosylsulphuric acid to oxidising agent is within the range of from 0.1 to 10.

13. A process as claimed in claim 12, wherein the weight ratio of nitrosylsulphuric acid to oxidising agent is within the range of from 0.5 to 5.

14. A process as claimed in any one of claims 1 to 13, wherein the carbon content of the sulphuric acid fed into the distillation column is within the range of from 0.2 to 2.0% by weight.

15. A process as claimed in claim 14, wherein the carbon content of the sulphuric acid fed into the distillation column is within the range of from 0.4 to 1.0% by weight.

16. A process for the purification and concentration of sulphuric acid, carried out substantially as described in any one of Experiments 5, 8, 9, 11 and 12 of the Examples herein.

17. Sulphuric acid whenever prepared by a process as claimed in any one of claims 1 to 16.