HK1125621A1 - Device for purification of waste sulfuric acid - Google Patents

Abstract

The invention relates to a device for purifying waste sulphuric acid; wherein, the addition of oxidant and high-temperature treatment are adopted for destroying organic impurities, so as to purify acid. The device comprises a reactor (R), a circulation pump (P), a heat interchanger (W) which is specially used for adding required energy into the acid and a pipeline which is connected with the heatinterchanger (W); wherein, as the size of the reactor (R) is designed in this way, the retention time is long enough for oxidizing the organic impurities, moreover, high-temperature heat medium oil is used for heating the heat interchanger (W). The device not only can be used for purifying waste sulphuric acid, but also can be used for purifying waste acid and for concentrating waste acid at the same time. The purified waste acid can be preferentially led back to the process generating the waste acid or used as an alternative scheme to be put into market.

Description

Equipment for purifying waste sulfuric acid

Technical Field

The invention relates to a plant for purifying spent sulfuric acid, in which organic impurities in the sulfuric acid are destroyed by treatment at high temperatures and the addition of oxidizing agents and the acid is thereby purified.

Background

DE 2404613B 2 has disclosed a method for purifying a used acid stream, which is used for the nitration of aromatic compounds, which used acid comprises, in addition to organic impurities, approximately 60 to 85% sulfuric acid, by removing the acid stream for removing the main share of volatile organic compounds and subsequently contacting the acid stream containing organic impurities, which is obtained in the process, of at least 50ppm of non-volatile, consisting of nitrated cresols and other nitrated phenolic compounds, with an oxidizing agent, characterized in that the removed acid stream is contacted with an oxidizing agent consisting of ozone, hydrogen peroxide, chlorates, peroxodisulfates or mixtures of these compounds in such a way that, per equivalent of non-volatile organic compounds, at least 1 stoichiometric equivalent of the oxidizing agent is contacted in a share of approximately one-third of the equivalents or less during a period of 1 minute to approximately 60 minutes from the time of approximately one-third of the equivalents of the oxidizing agent per equivalent of non-volatile organic compounds The addition is carried out at a temperature of from 130 to 230 ℃ and the purified acid stream is withdrawn.

EP 0052548B 1 discloses a process for purifying residual sulfuric acid from ethanol production by hydration of the corresponding olefin, in which process one continuously and simultaneously feeds sulfuric acid previously concentrated to at least 70% and charged with HNO at a concentration of at least 60% into a reactor₃Wherein the nitric acid is charged at a temperature of at least 150 ℃, characterized in that the nitric acid is added in an amount of 0.5 to 1 nitric acid molecule per carbon atom, wherein carbon is dissolved as an organic substance in the sulfuric acid to be purified, and the liquid thus obtained is continuously poured into a second reactor, which is maintained in a temperature range above 150 ℃ and to which no nitric acid is added.

EP 0117986 a2 and EP 0117986B 1 disclose a circulation process for concentrating and purifying sulfuric acid containing organic impurities, in which process the sulfuric acid to be treated is continuously fed into the evaporation process with the input of heat and reaches a higher concentration with continuous separation of the vapor mixture, characterized in that the sulfuric acid to be treated is fed into a part of the acid leaving the evaporation process in a concentrated manner with a spacing between the separation points of the vapor mixture and the gas leaving the liquid phase, which corresponds to the necessary heating path, forming a reaction circuit, and is fed to the separation points together with this part of the acid, wherein an oxidizing agent is added to the acid to be treated in the cocurrent before it is fed into the liquid phase of the acid in the evaporation process. The materials specifically used for the heater are not described in detail herein.

DE 19807632A 1 discloses a device for concentrating sulfuric acid to 95 to 98% H₂SO₄The apparatus comprises at least one natural circulation evaporation system which is used for purifying sulfuric acid at a temperature of 270 to 340 DEG CThe system consists of a two-component steam drum, a tube bundle heat exchanger, a circulation line and a distillation column, characterized in that the lower part of the steam drum, the heat exchanger and the circulation line, which are the parts of the plant conducting hot sulfuric acid in the liquid state at temperatures up to 270 to 340 ℃, are made of a silicon-containing, austenitic-ferritic iron alloy with the respective specified composition and the upper part of the steam drum and the distillation column are made of enamelled steel.

From bartholome.e. [ u.a.hrsg ]]：UllmannsderTechnischen Chemie，4.neubearbeitete und erweiterte Auflage，Weinheim[u.a.]: verlag Chemie, 1982, Band A21, page 157-159 discloses the stability of Si-Fe castings with a Si content of 14-18% to concentrated sulfuric acid, but also mentions a lower stability of the material to thermal and mechanical shocks.

The use of cast ferrosilicon, i.e. an iron alloy with a silicon content of 14 to 18%, for a heat exchanger for hot sulfuric acid has already been proposed in DE 3050562 a1, DE 3015957 a1, DE 972412B 1 and GB 1175055 a. However, it is pointed out in DE 3320527C 2 that materials with 14 to 18% silicon are not used as heat exchangers due to the hardness and brittleness of the material. The stability of the silicon-containing alloys with a silicon content of between 4 and maximally 9% silicon with respect to sulfuric acid and the use as heat exchangers is described in EP 0615950 a1, EP 0378998 a1, DE 19719394C 1, DE 4213325 a1 and DE 3320527C 2. However, it is stated in said document that materials with 14 to 18% silicon do not function as heat exchangers due to the hardness and brittleness of the material.

The sensitivity of materials with a high silicon content to thermal shock is also described in US 2002/0009382a1 and it is pointed out that such materials are not available in a workable form.

It is also stated in US 1861568 that materials with 8-20% silicon are not available in a workable form. The fragility of the material is also mentioned here. Here, processing can only be carried out by addition of antimony.

DE 3015957 a1 describes the possibility of using silicon-containing cast steel as a heat exchanger, but does not describe the silicon content and composition.

The possibility of using silicon-containing cast steel as a heat exchanger is described in GB 429267. But here only the case of using as a condenser and a cooler at a temperature of 140 to 150 c at maximum is described. The use as a heater at higher temperatures is not described here. The silicon content and composition are also not specified here.

In GB 1175055, the possibility of using silicon-containing cast steel as a heat exchanger for heating sulfuric acid is described, but the silicon content and the composition are not described.

The possibility of using silicon-containing cast steel as a heat exchanger is also described in US 1861568. However, here again only the use as condenser and cooler for SO is described₃And H₂And O steam. The silicon content and composition are likewise not specified here.

It is therefore known to remove organic impurities from large amounts of organic-contaminated sulfuric acid by adding oxidizing agents and/or by treatment at high temperatures. However, it is not possible according to the prior art to use a heat exchanger made of a material having a silicon content of silicon of 14 to 18% for heating acid, since this material cannot be used as a heater for heating sulfuric acid at high temperatures due to its sensitivity to thermal and mechanical shocks and poor processability.

Disclosure of Invention

The object of the present invention is to provide a cost-effective plant for purifying waste sulfuric acid, which is capable of treating different waste acids at high temperatures with correspondingly different oxidizing agents.

The inventive device comprises a reactor, a circulation pump, a special heat exchanger for adding the required energy to the acid, and a line connecting the heat exchanger, wherein the reactor is dimensioned such that the residence time is sufficient for destroying the organic impurities, and wherein the heat exchanger is heated with high-temperature heat transfer oil.

To select suitable, corrosion-resistant materials, the materials named for their stability to hot sulfuric acid, such as enamelled steel, steel lined with polytetrafluoroethylene or polyfluorinated acid copolymers, special ceramic materials, silicon-containing cast steels and silicon-containing special steel alloys, were first checked for their workability.

As expected, enamelled steel and steel lined with teflon proved to work very well for the reactor and the pipe. A problem arises in the selection of the materials used for the heat exchanger and in the selection of the heating medium to be used, since for most materials, limits are reached here with regard to mechanical and special resistance to temperature changes. When steam is used as heating medium, one encounters the problem that one has to work in a pressure range of > 20 to 40bar for achieving a reasonable temperature difference between the heating medium and the acid, due to the desired temperatures of > 190 ℃ to 260 ℃ in the acid. Furthermore, slow and uniform heating with steam is very difficult to avoid local temperature differences. During disconnection, a low pressure arises on the steam side due to the condensation of the residual steam, which low pressure places additional higher demands on the seal between the acid side and the heating medium. The use of heating medium steam in the device according to the invention is therefore dispensed with, and high-temperature heat carrier oil is selected as the heating medium. This provides the advantage that the heat exchanger can be operated with a constant, moderate pressure of about 6 to 10 bar. The heat exchanger can also be heated uniformly in this case, since the heat exchanger surface is covered uniformly by the oil and the oil temperature rises uniformly and slowly during heating. All natural and synthetically produced heat transfer oils which are stable at the required temperatures of from 200 to 350 ℃ can be used as heat transfer media. Other heat carrying liquids having similar properties are also contemplated.

Despite these advantageous conditions, enameled heat exchangers also always exhibit the disadvantage that cracks occur in the enamel when the installation is frequently started and stopped, which cracks lead to leaks. Enamel steel has therefore been abandoned as a material for heaters. This does not occur in enameled pipes and installations, since the enamel layer is much thicker than on enameled heaters on the one hand, and the temperature changes are not so extreme on the other hand. Tantalum cannot be used as a material for heaters because for 96 weight percent H₂SO₄The following sulfuric acid concentrations show corrosion on the acid side from about 210 ℃ and for > 96 weight percent H₂SO₄The corrosion occurred on the acid side from about 190 c onwards. The material with extremely corrosion resistant properties with respect to sulphuric acid is more particularly a cast steel with a very high silicon content. For components which are not subjected to intense thermal and mechanical loads, such as column inserts, the cast steel has been used for decades in sulfuric acid concentration. Such cast steels with a high silicon content of 14-18% silicon have in the past exhibited a very low mechanical stability like glass, i.e. local stresses occur in the event of rapid changes in temperature, and this leads to stress fractures. Stress fractures also occur during thermal expansion. Great difficulties also arise in the production of homogeneous, pore-free, gas-tight shaped parts from silicon-containing cast steel, since the material shrinks on cooling. Silicon-containing cast steel with 14 to 18% silicon has therefore not hitherto been used as a material for a heat carrier, which is then used for heating sulfuric acid, and has not hitherto been considered for this purpose. Owing to the special casting technique and the corresponding finishing, at the same time, corresponding shaped parts, in particular tubes made of silicon-containing cast steel, can be produced with stable quality. In this case, the mold for the pipe is installed so that the pipe is vertically cast. The melt is introduced from belowInto the casting mould so that the melt is distributed uniformly in the casting mould and without any inclusions of gas bubbles. After casting, the tube is allowed to cool uniformly in the mold for 12 to 36 hours, typically about 24 hours. By this measure a tube with a very uniform molecular structure is obtained.

Tests were now carried out with such specially manufactured tubes, to what extent heaters for sulphuric acid could be made with this material. The tube is heated from the outside in an oil bath containing high-temperature heat transfer oil. Through which sulfuric acid having different concentrations at 60 weight percent H was pumped₂SO₄To 98 weight percent H₂SO₄Within the range of (1). On the one hand, the thermal expansion of the pipe is measured and, on the other hand, it is examined to what extent corrosion or severe corrosion can occur through the flow rate of the acid. The temperature change stability was also checked by rapid heating and cooling of the oil sump.

It has surprisingly been found that silicon-containing cast steels on pipes manufactured as described above show a much higher stability against temperature changes than silicon-containing cast steels according to previous experience with similar composition. The thermal expansion of the material is also in a range that can be tolerated by appropriate design and force compensation, so that no mechanical stresses are exerted on the tube, which would lead to stress fractures. It was also found, surprisingly, that even with high flow velocities of the acid in the pipe of between 1 and 5m/s, no corrosion and high material corrosion occurred. For the device according to the invention, therefore, silicon-containing cast steel with a silicon content of 14 to 18% silicon is used as the material for the acid-wetted parts of the heat exchanger. Several possible ingredients are illustrated below by way of example:

material 1

Silicon 15 to 17 weight percent

Carbon 0.4 to 0.7 weight percent

Manganese 0.3 to 0.5 weight percent

Phosphorus 0.05 weight percent

Sulfur 0.009 to 0.05 weight percent

Molybdenum-weight percent

Chromium (wt.%)

Weight percent of iron remainder

Material 2

Silicon 14.5 to 15.5 weight percent

Carbon 0.4 to 0.7 weight percent

Manganese 0.5 weight percent

Phosphorus 0.05 weight percent

0.01 percent by weight of sulfur

Molybdenum 3.0 weight percent

Chromium (wt.%)

Weight percent of iron remainder

Material 3

Silicon 14.5 to 15 weight percent

Carbon 0.4 to 0.7 weight percent

Manganese 0.5 weight percent

Phosphorus 0.05 weight percent

0.01 percent by weight of sulfur

Molybdenum-weight percent

Chromium 5 weight percent

Weight percent of iron remainder

All three materials have proven to be very suitable not only in laboratory tests but also in industrial applications and are used according to the invention. Depending on the impurities in the respective waste acid, the various materials exhibit specific advantages, so that they can be selected on the basis of existing working experience or with the aid of laboratory tests. For industrial applications, tubes made of silicon-containing cast steel are inserted into tubes made of steel, such as RSt 37.2, and high-temperature heat transfer oil is then conducted through the tubes. The sealing between the inner tube made of silicon-containing cast steel and the outer tube made of steel is then carried out by means of corresponding seals which are stable against high-temperature heat carrier oil and at operating temperatures on the oil side of up to 350 ℃.

Due to the viscosity of the sulfuric acid, in particular in the higher concentration range between 75 and 98 percent by weight, in the apparatus according to the invention the sulfuric acid is pumped through a heat exchanger. As a material for the pump, the silicon-containing cast steel can be used as well. Other corrosion resistant materials such as fluoropolymer plastics may also be used as the pump material depending on the desired processing temperature.

Drawings

FIG. 1 is an apparatus for purifying spent sulfuric acid according to the present invention;

FIG. 2 is an apparatus for purifying spent sulfuric acid according to the present invention;

FIG. 3 is an apparatus for purifying spent sulfuric acid according to the present invention.

Detailed Description

Fig. 1 shows a plant according to the invention for purifying spent sulfuric acid. The plant comprises a reactor R made of corrosion-resistant material, such as enamelled steel or steel lined with fluoropolymer, a circulation pump P made of silicon-containing cast steel or other corrosion-resistant material, such as fluoropolymer plastic, a heat exchanger W made of an inner pipe i made of silicon-containing cast steel and an outer pipe a made of steel, and pipes connecting said heat exchangers, also made of corrosion-resistant material, such as enamelled steel or steel lined with fluoropolymer.

The volume of the reactor R is chosen such that the acid remains in the apparatus for a sufficient time for destroying organic impurities. The residence time required is determined on the basis of working experience accumulated in the treatment of similar waste acids or with the aid of laboratory tests. The circulation power of the pump P is determined according to the amount of waste sulfuric acid to be treated and is likewise generated from working experience accumulated when treating similar waste acids or by means of laboratory tests. The size of the heat exchanger W is calculated from the energy required for the treatment of the acid. Depending on the installation of the heat exchanger W, a plurality of heat exchangers W can be operated preferably in series or in parallel. The heat exchanger W can be installed both horizontally and vertically, since according to the invention both acid and high-temperature heat transfer oil are sucked in, and thus a throughflow of the medium is achieved independently of the installation. In terms of process flow, it is expedient to guide the acid in countercurrent to the high-temperature heat carrier oil, i.e. the acid flows into the heat exchanger W at the point 4 and leaves it at the point 5, while the high-temperature heat carrier oil flows into the heat exchanger at the point 6 and leaves it at the point 7. The high-temperature heat carrier oil flows only in the outer casing a and wets the outer surface of the inner tube i uniformly, while the acid flows past the inside of the inner tube i, absorbs energy from the high-temperature heat carrier oil and is heated in the process. The heated sulfuric acid then flows into the reactor R. In principle, the acid can also be heated with the high-temperature heat transfer oil in cocurrent flow; however, the countercurrent mode is more interesting from a process flow point of view, since here a higher final temperature of the acid can be achieved. The purified acid is recycled back to the circulation pump P. The purified acid can optionally be extracted from the reactor or from the discharge line of the reactor at position 3 or at position 2. The gases produced in the destruction of the organic impurities are discharged from the reactor at 13. The spent sulfuric acid to be treated is preferably added to the recycled, hot and purified sulfuric acid at point 1. However, as an alternative, it is also possible to add at the point 11 on the pressure side of the circulation pump P. Alternatively, the oxidant may be metered at positions 8, 9, 10 and 12. Whether the metering of the oxidizing agent is necessary for the purification, at which point the oxidizing agent is metered, which oxidizing agent is used and how much oxidizing agent is required, depends on the type of waste acid and is determined by means of working experience accumulated when treating similar waste acids or by means of laboratory tests. The device according to the invention is then provided with a corresponding metering point or, if necessary, a plurality of metering points. If different waste sulfuric acids are to be treated by the apparatus, corresponding metering points can be provided on the apparatus according to the invention. These metering points are then used only if necessary on the basis of acid.

The apparatus according to the invention can be adapted to the purification to be carried outThe concentration of the waste acid and the temperature required for purification are operated at working pressures of between 10mbar and 10 bar. For many waste acids, it has proven particularly effective to operate approximately at the boiling temperature or even at slightly superheated conditions of the acid relative to the boiling temperature, at the corresponding operating pressure. In use with different concentrations of H at 60 weight percent₂SO₄To 98 weight percent H₂SO₄In the range between and having an acid temperature of > 190 ℃ to 260 ℃, the best purification results are achieved.

The device according to the invention can be used optionally in a charging mode (ansatzweise) or alternatively in continuous operation.

During operation in the charging mode, the waste acid is introduced into the apparatus according to the invention. In this case, it is already possible to add an oxidizing agent. The acid is then circulated and heated to the operating temperature by heating with heat exchanger W. At the points 9, 10 and 12, it is already possible to add the oxidizing agent to the acid during the heating process and also for the entire working duration. For better mixing of the acid with the oxidizing agent, a mixer, preferably a static mixer, can additionally be installed in the line immediately after the addition point. A possible mixer position is shown as M in fig. 2. If the purification of the spent acid is finished, the purified acid is extracted and re-injected accordingly. Optionally, the purified acid is extracted while it is hot or after it has cooled. The next batch of charge is then loaded into the apparatus according to the invention.

Operation in the charging mode is advantageous if longer residence times and larger amounts of oxidizing agent are required for the purification of the spent acid. It is of further interest to continue the operation if the desired purification effect is already obtained after one cycle.

In continuous operation, the spent acid is continuously added to the circulating, hot, purified acid. The advantage of adding the waste acid to the recirculated, hot, purified sulfuric acid is, on the one hand, that the waste acid is brought to an approximate operating temperature at once, as a result of which many organic impurities have already been destroyed, and, on the other hand, that the concentration of organic impurities is reduced in the mixing accordingly, depending on the ratio between the fed waste acid and the recirculated, purified acid, as a result of which, in particular, a reliable dissipation of energy in the form of heating of the sulfuric acid is ensured in the strongly exothermic decomposition reaction of the various organic compounds. The inventive device can thus also be used for waste acid from the production of explosive materials, which can be reliably purified using the inventive device. The ratio of the waste acid fed to the purified acid recycled is generally, in the case of continuous operation of the plant according to the invention: the ratio of spent acid to recycled acid is between 1: 1 and 1: 400. However, this can also be adapted accordingly for the particular purification task.

The oxidizing agent can be added to the acid continuously at positions 8, 9, 10 and 12 even during continuous operation. The mixing effect can also be improved here by additionally installing a mixer, which is shown in fig. 2 as an example as M, in the line immediately after the oxidant metering point. For example, an oxidizing agent (via point 8 or 12) can be added to the possibly still cold waste sulfuric acid fed in (via points 1 and/or 11) in such a way that it is optimally mixed with the waste acid in the cold state before the mixture is introduced into the plant according to the invention and then into the hot, recirculated, purified sulfuric acid. This procedure has the advantage that the organic impurities already react with the oxidizing agent in the cold waste acid and continue to react smoothly after metering by means of vigorous heating. Tests have shown that the corresponding operation of the plant according to the invention makes it possible to reduce the requirement for the oxidizing agent for purification on several acids, if the waste acid is already mixed with the oxidizing agent in the cold state. It is also advantageous to preheat the spent acid before or after addition of the oxidizing agent before it is introduced into the apparatus according to the invention, since the energy which has to be introduced into the system via the heat exchanger W is thereby reduced and the recirculated acid is additionally not cooled down as greatly after the addition of the spent acid. Possible locations for the respective preheaters are shown in fig. 3 as W2 and W3. Which mode of operation brings most advantages and must be ascertained by trial or learned from working experience.

Alternatively, it is of course also possible to feed the waste acid into the plant according to the invention at different points (1, 9, 10, 12) with or without prior metering of the oxidizing agent. It is also advantageous to pretreat a part of the waste acid or all of the waste acid before it is fed to the plant according to the invention. Such a pretreatment can, for example, comprise a desorption (striping), concentration or extraction (extraction), whereby it is already possible for several applications to reduce the proportion of organic impurities and thus to simplify the purification task for the device according to the invention.

Additional energy can also be added to the acid in the apparatus according to the invention by means of the heat exchanger W, and the acid is then discharged from the apparatus according to the invention in the form of water vapor together with the offgas produced during the decomposition of the organic compounds and with the organic compounds volatilized by the water vapor and the decomposition products at the point 13. Whereby steam-volatile compounds are removed from the recycled acid in a purification step and the acid is additionally concentrated.

The apparatus according to the invention can be used not only for purifying waste sulfuric acid, but also for purifying and simultaneously concentrating waste acid.

In principle, the device according to the invention can also be used only for concentrating sulfuric acid without performing a purification task, since the device has corrosion-resistant properties under the respective operating conditions. The purified spent acid may preferably be directed back to the process for producing the spent acid or sold as an alternative. The purified acid may also be subsequently concentrated.

As the oxidizing agent, oxidizing agents disclosed from the literature such as nitric acid, hydrogen peroxide, ozone and the like can be used for purification. If the concentration of sulfuric acid is not to be reduced unnecessarily by adding an oxidizing agent, a correspondingly highly concentrated oxidizing agent solution must be used.

The functional principle of the device according to the invention is explained in the following in different examples.

Example 1:

2-Butanol (SBA) and Methyl Ethyl Ketone (MEK) were mixed in a spent acid having about 60 weight percent H₂SO₄The concentration of sulfuric acid is continuously treated with the apparatus according to the invention. The cleaner material having a weight percentage of 77% H was stored in the apparatus₂SO₄And the sulfuric acid is heated to a boiling temperature of about 197 c at an operating pressure of about 1300 mbar. The spent sulfuric acid is transported at position 1. Nitric acid is added to the cooled spent sulfuric acid at about 20 c through point 8 in a ratio of nitric acid to spent acid of 0.01: 1. The mixture is then indirectly heated to 130 ℃ by means of a heat exchanger before being metered into the circulating sulfuric acid. The ratio of the recycled sulfuric acid to the spent acid was 50: 1. The energy supplied to the apparatus according to the invention via the heat exchanger W is regulated by the temperature of the acid circulated, so that this temperature is constantly maintained at 197 ℃. The exhaust gases and the water vapour produced are discharged from the apparatus at 13. The purified acid was at position 2 at a constant 77 weight percent H₂SO₄Leaving the plant. The content of organic impurities in the acid may be from about 4000mg of O in the spent acid₂/kg CSB down to about 2000mg O in the product acid₂/kg CSB. The acid can be used again in the production process after subsequent concentration.

Example 2:

alkyl sulfonic acid mixed in waste acidAnd has about 60 weight percent H₂SO₄Sulfuric acid concentration of (2), byThe waste acid is treated continuously by the open equipment. The purer material having a weight percentage of 80H is stored in the apparatus₂SO₄And the sulfuric acid is heated to a boiling temperature of about 210 ℃ at an operating pressure of about 1000 mbar. The waste sulfuric acid is fed in at position 1. Hydrogen peroxide is added to the cold spent sulfuric acid at about 20 c via point 8 in a 0.03: 1 ratio of hydrogen peroxide to spent acid. The ratio of recycled purified sulfuric acid to spent acid was 100: 1. The energy supplied to the apparatus according to the invention via the heat exchanger W is regulated by the temperature of the acid circulated, so that this temperature is constantly maintained at 207 ℃. The exhaust gases and the water vapour produced are discharged from the apparatus at 13. The purified acid was at position 2 at a constant 80 weight percent H₂SO₄Leaving the plant. This acid is then continuously introduced into the second apparatus according to the invention and treated again with the apparatus. In the second apparatus, a cleaner material having 85 weight percent H was stored₂SO₄And the sulfuric acid is heated to a boiling temperature of about 230 ℃ at an operating pressure of about 1000 mbar. The pre-purified sulfuric acid from the first plant is fed in at point 1. Hydrogen peroxide is added to the hot spent sulfuric acid at about 207 ℃ via point 8 in a 0.03: 1 ratio of hydrogen peroxide to spent acid. The ratio of recycled purified sulfuric acid to spent acid was 100: 1. The energy supplied to the apparatus according to the invention via the heat exchanger W is regulated by the temperature of the acid circulated, so that this temperature is constantly maintained at 230 ℃. The exhaust gas and the generated water vapour are discharged from the apparatus at 13. The purified acid was at position 2 at a constant 85 weight percent H₂SO₄Leaving the plant. The content of organic impurities in the acid may be from about 9000mg of O in the spent acid₂The COD dropped to < 100mg O in the product acid/kg₂/kg COD。

Example 3:

first of all by desorption and concentration of the product from DNT productionAnd (4) pretreating waste acid. The sulfuric acid thus produced had about 85 weight percent H₂SO₄And is continuously treated with the apparatus according to the invention. The cleaner material having 96 weight percent H was stored in the apparatus₂SO₄And heating the sulfuric acid to a boiling temperature of about 230 ℃ at an operating pressure of about 90 mbar. Nitric acid is added to the recycled sulfuric acid via position 9. The energy supplied to the apparatus according to the invention via the heat exchanger W is regulated by the temperature of the acid circulated, so that this temperature is constantly maintained at 230 ℃. The exhaust gases and the water vapour produced are discharged from the apparatus at 13. Purified acid at position 2 at a constant 96 weight percent H₂SO₄Leaving the plant. The content of said organic impurities in the product acid can be kept constant at < 200ppm TOC. The acid can be reused in the production process.

Claims (19)

1. An apparatus for purifying spent sulfuric acid, in which organic impurities in the sulfuric acid are destroyed by treatment at high temperature and addition of an oxidizing agent, comprising a reactor, a circulation pump, a heat exchanger and a pipe connecting the heat exchanger, wherein the reactor is dimensioned such that the residence time is sufficient for the destruction of organic impurities,

a) the reactor is made of enamelled steel or steel lined with polytetrafluoroethylene;

b) the pipe is made of enamelled steel or steel lined with polytetrafluoroethylene;

c) high-temperature heat-carrying oil is used as a heating medium;

d) as a material for the acid-wetted parts of the heat exchanger, cast steel containing silicon with a silicon content of 14 to 18% silicon is used;

e) the heat exchanger comprises an inner tube made of silicon-containing cast steel and an outer tube made of steel;

f) the inner tube is cast from a silicon-containing cast steel, wherein

i. Vertically mounting a casting mold;

charging the melt from below into a casting mould;

and cooling the tube in the mold for 12 to 36 hours after casting;

g) the pump is made of a corrosion resistant material;

h) the apparatus is provided with a dosing arrangement for dosing the oxidizing agent into the acid.

2. The apparatus as claimed in claim 1, wherein said silicon-containing cast steel for heat exchangers has the following composition:

silicon 15 to 17 weight percent

Carbon 0.4 to 0.7 weight percent

Manganese 0.3 to 0.5 weight percent

Phosphorus 0.05 weight percent

Sulfur 0.009 to 0.05 weight percent

Molybdenum-weight percent

Chromium (wt.%)

Iron remaining weight percent.

3. The apparatus as claimed in claim 1, wherein said silicon-containing cast steel for heat exchangers has the following composition:

silicon 14.5 to 15.5 weight percent

Carbon 0.4 to 0.7 weight percent

Manganese 0.5 weight percent

Phosphorus 0.05 weight percent

0.01 percent by weight of sulfur

Molybdenum 3.0 weight percent

Chromium (wt.%)

Iron remaining weight percent.

4. The apparatus as claimed in claim 1, wherein said silicon-containing cast steel for heat exchangers has the following composition:

silicon 14.5 to 15 weight percent

Carbon 0.4 to 0.7 weight percent

Manganese 0.5 weight percent

Phosphorus 0.05 weight percent

0.01 percent by weight of sulfur

Molybdenum-weight percent

Chromium 5 weight percent

Iron remaining weight percent.

5. The device according to claim 1, characterized in that the sealing on the heat exchanger between the inner tube made of silicon-containing cast steel and the outer tube made of steel is performed by means of a seal which is stable to high-temperature heat transfer oil and stable at operating temperatures of the oil side up to 350 ℃.

6. The apparatus of claim 1 wherein said heat exchanger is mounted vertically.

7. The apparatus of claim 1 wherein said heat exchanger is mounted horizontally.

8. The apparatus of claim 1, wherein a plurality of heat exchangers are connected in parallel.

9. The apparatus of claim 1 wherein a plurality of heat exchangers are connected in series.

10. The apparatus of claim 1 wherein a plurality of heat exchangers are connected in parallel and in series.

11. The apparatus as claimed in claim 1, wherein a flow velocity of said acid in said inner tube of between 1 and 5m/s is achieved.

12. The apparatus as claimed in claim 1, wherein the apparatus is operated at an operating pressure of between 10mbar and 10bar on the acid side.

13. The device according to claim 1, characterized in that the heat transfer oil is used at a pressure of 6 to 10 bar.

14. The apparatus as claimed in claim 1, characterized in that the heat carrier oil is pumped.

15. The apparatus as claimed in claim 1, characterized in that the heat carrier oil in the heat exchanger is conducted in countercurrent to the acid.

16. The apparatus as claimed in claim 1, characterized in that the heat carrier oil in the heat exchanger is conducted co-currently with respect to the acid.

17. The apparatus as claimed in claim 1, wherein a mixer for mixing the oxidizing agent and the acid is provided in the apparatus.

18. The device according to claim 1, characterized in that the device has a metering point (1, 9, 10, 12).

19. The apparatus of claim 1, wherein said corrosion resistant material is silicon-containing cast steel or a fluoropolymer plastic.