WO2013132294A1

WO2013132294A1 - Process for treatment of waste water from nitro-aromatic production

Info

Publication number: WO2013132294A1
Application number: PCT/IB2012/051127
Authority: WO
Inventors: Martin EKSTRAND; Jean-Pierre Landert; Raphaël SIGGEN
Original assignee: Biazzi S.A.
Priority date: 2012-03-09
Filing date: 2012-03-09
Publication date: 2013-09-12

Abstract

The present invention relates to a process for working up aqueous waste water generated in the alkaline washing step of a nitro-aromatic manufacturing process alone or in combination with waste water produced during the corresponding aromatic amine purification step. The waste water worked up according to the present invention is suitable for biological treatment before final disposal. This process comprises: a) A chemical oxidation treatment to degrade aromatic by-products using hydrogen peroxide and iron as catalyst under acidic conditions. b) Recycle of the iron catalyst under alkaline conditions without any specific regeneration treatment.

Description

PROCESS FOR TREATMENT OF WASTE WATER FROM NITRO-AROMATIC

PRODUCTION

Field of the invention

The present invention relates to a process for working up waste waters from nitro- aromatic and/or aromatic amine production, and in particular the waste water generated in the alkaline washing step of a nitro-aromatic manufacturing process. Background of the invention

Nitro-aromatic compounds are widely used in the chemical industry for their suitability as intermediates for bulk chemical production. For example, dinitrotoluene produced by nitration of toluene is reduced to toluenediamine which is further processed into isocyanates to finally obtain polyurethanes, a major component in the car industry and furniture's. In the nitro-aromatic production process, the nitration step is followed by a purification of the product which involves the use of acidic and alkaline washing steps. The aqueous waste water generated during this process contains in addition to limited quantities of the nitro-aromatic produced also nitration by-products, such as for example nitrocresols and picric acid. These substances are hardly biodegradable and therefore a pre-treatment is necessary to remove or degrade these pollutants before the waste water is released to a classical biological waste water treatment plant. Nowadays, with increasing plant capacities the amount of waste waters generated from nitro-aromatic production increases constantly. An efficient and cheap process to handle large amounts of polluted water is therefore needed.

Advanced oxidation processes (AOPs) are based on the degradation of organic compounds dissolved in an aqueous media. These processes involve the generation of organic radicals produced either by photolysis of organic substrate or by reaction with hydroxyl radicals. Chemical oxidation by Fenton reaction using hydrogen peroxide is a well-known AOP for degradation of pollutants. The process involves iron and hydrogen peroxide to generate hydroxyl radicals as described in the following equations:

(1) Fe²⁺ + H₂0₂ -> Fe³⁺ + ΌΗ + OH^"

( 2 ) Fe³⁺ + H₂0₂ -> Fe²⁺ + OOH^'+ H⁺ The generated OH radicals are capable of oxidizing a wide range of organics in wastewater such as phenols, nitrophenols, nitrocresols, etc.

The main advantages of the Fenton process are the simplicity of the needed equipment,-no pressurized equipment necessary for example-, the moderate temperature applied and the cheap and environmentally safe features of the reactants used.

The main drawback is the high amount of iron sludge generated during the process that is directed to the final biological waste water treatment plant. With increasing industrial capacities, the amount of iron sludge generated becomes a major issue and has to be removed and correctly disposed, as it contains dangerous pollutants adsorbed on its surface, before releasing the treated water into the waste water treatment plant.

Patent U S 4,604,214 describes a method for removal of nitrocresols from dinitrotoluene waste streams using Fenton's reagent. The patent discloses the use of hydrogen peroxide and ferrous iron under defined conditions to oxidize nitrocresol material to nitric acid, carbon dioxide and carboxylic acids. There is no mention about possibilities of recycling the iron sludge or the problems brought by the disposal of the iron sludge. Furthermore, the patent describes also the relatively low degradability by Fenton's reagent of the dinitrotoluene present in the wastewater compared to the dinitrocresols and trinitrocresols. Thus, a high content of dinitrotoluene in the waste water to be treated results in excessive use of reactants.

Patent HU 226079 discloses an extraction process for recovery of dinitrotoluene and organic by-products from the wastewater generated in the toluene nitration process. The process comprises the extraction, using mononitrotoluene, of alkaline waste water generated during the DNT washing step, phase separation and the recycle of the mononitrotoluene into the nitration step. The aqueous phase is further subjected to a stripping to remove residual mononitrotoluene before the extracted wastewater is further treated by Fenton oxidation. The main advantage of this treatment is to increase the product yield combined with a reduction of the chemicals required for oxidative degradation.

Kavitha and Palanivelu describe in Chemosphere 55 (2004), 1235-1243, a treatment of phenolic waste water based on an oxidative Fenton process, the treatment including reuse of the iron by dissolving the precipitated ferric hydroxide in sulfuric acid followed by reduction with a hydroxylamine solution to recover the ferrous ion. These authors observe that when the iron sludge is recycled without reducing to ferrous ion, the degradation process of the phenolic waste water takes only slowly place after a time interval of 2 hours attributed to the lower reactivity of ferric ion for initiating Fenton reactions and they recommend to use solar-Fenton or UV-Fenton processes instead of a mere Fenton process.

Cao et al. report in Journal of Hazardous Materials 172, 1446-1449, a treatment of wastewater originating from a fine chemical facility, having a pH of 6.2-6.8 and a COD of 1 100-1300 mg.dm³, including a method for regeneration and reuse of iron catalyst for Fenton-like reactions: the ferric hydroxide sludge is dewatered, dried in an owen and further baked at 350-400°C for 20-30 minutes in a furnace; the residual solid is dissolved in sulfuric acid to form again the catalyst for Fenton and Fenton-like reactions. The process appears to be energy and reagent consuming. Cao et al mention also electrolysis as an alternative regeneration method with the disadvantage being not effective, as only reducing the ferric ion to ferrous, without removing the organics adsorbed on the newly formed ferric hydroxide floes. Patent US 5,538,636 describes a process for chemically oxidizing highly concentrated waste waters using Fenton's reagent and teaches the necessity for a treatment of the iron (III) sludge by reducing it electrochemically to iron (II) before recycling into the process. The patent discloses also the problem of organic contaminants adsorbed on the iron sludge resulting in recycling problems and difficulties concerning its treatment and elimination.

Therefore, there is still a need, for treatment of waste water from nitro-aromatic production for an AOP including a both effective and cost effective recovery of the catalyst.

Summary of the invention

Thus, the invention proposes a process for treating a waste water from nitro- aromatic and/or aromatic amine production by oxidative degradation of aromatic by-products involving a Fenton-like reaction said process comprising following steps: a) reacting said wastewater with hydrogen peroxide and ferric ion in an aqueous phase under acidic conditions, b) elevating the pH of said aqueous phase to an alkaline condition and separating said aqueous phase into a supernatant and a sludge ,

c) recycling of the sludge into step (a). Without being bound by theory, since the ferric ion consumed in the Fenton like reaction is recovered as ferric hydroxide sludge in step (b), the latter being recycled directly to the acidic reaction medium of step (a) whereby the ferric ion is formed again, this ferric ion is considered to act overall as a catalyst. In particular, the said nitro-aromatic produced can be dinitrotoluene and the aromatic amine can be toluenediamine.

The said waste water may comprise alkaline waste water generated by the washing steps in a dinitrotoluene process.

The said waste water may be formed by combination of alkaline waste water from the washing step in the dinitrotoluene process and a neutralized condensate from a sulfuric acid concentration plant. The said waste water may be further combined with an aqueous distillate from the toluenediamine purification step.

The process according to the present invention is preferably continuous. The ferric iron is in particular separated in step (b) under alkaline conditions, preferably by decantation of said sludge.

The concentration of ferric iron in step (a) is in the range of 200-600 ppm, preferably 350-450 ppm, and more preferably 375-400ppm.

The concentration of ferric iron in step (a) can be adjusted by adding a ferrous salt. The concentration of ferric iron in step (a) can be adjusted by adding a ferric salt. The temperature in step (a) can be between 40-100°C, in particular between 85- 95°C.

The pH in step (a) can be adjusted in a range between 3 and 6, in particular at about pH 4.

The hydrogen peroxide concentration can be between 5-20 kg per m^ of waste water, in particular between 8-15 kg per m^ of waste water. The process can be continuous and the residence time in step (a) is 60-90 minutes for a reduction of 80% of the initial Chemical Oxygen Demand (COD).

In the process, step (a) is carried out in a series of successive stirred reaction vessels, wherein reagents are introduced and mixed under stirring in the first reaction vessel and further processed in subsequent vessel(s), wherein step (b) is initiated in a last reaction vessel by addition of alkaline hydroxide under stirring and continued in a settler.

In particular, step (a) can be carried out in two successive stirred reaction vessels, wherein reagents are introduced and mixed under stirring in the first reaction vessel and further processed under less strong stirring in the second vessel, wherein step (b) is initiated in a third reaction vessel by addition of alkaline hydroxide under stirring and continued in a settler. After the process according to the invention as set forth above, the supernatant leaving step (b) is further processed in a biologic treatment plant.

According to a preferred embodiment, as a pre-treatment step, the aqueous waste waters are previously contacted with an organic solvent to extract nitro-aromatics, followed by separation of two phases, an aqueous phase and an organic phase.

In particular the solvent for extraction is nitrotoluene or toluene and the recovered organic phase is recycled into the nitration process, whereas the aqueous phase is subjected to a stripping to remove residual solvent which is recycled to the extraction stage.

Detailed description of a preferred embodiment.

Further features and advantages of the present invention will appear to those skilled in the art from the description of a preferred embodiment in form of a continuous process, in relation to the drawing, wherein figure 1 shows a schematic set-up for continuous Fenton-like reaction with iron recycle, and from several examples of treatments at laboratory scale demonstrating advantageous features of such a process. The set up shown in figure 1 illustrates a process for working up the aqueous waste water generated in the alkaline washing step of a nitro-aromatic manufacturing process.

The aqueous waste is degraded by a chemical oxidation using hydrogen peroxide in the presence of iron as catalyst (Fenton reaction). The reaction is carried out at moderate temperature (between 40°C and 100°C, preferably between 85-95°C) and under acidic conditions (between pH 3 and pH 6, preferably at pH 4). The amount of ferric iron is in the range of 200-600 ppm, preferably between 375-400 ppm. The hydrogen peroxide is dosed according to the organic content present in the waste water followed by a post-reaction of 60-90 minutes. The pH is then increased, which results in the precipitation of the iron catalyst as ferric hydroxide. The ferric hydroxide sludge is then recycled into the process. The treated waste water shows an 80% reduction of the Chemical Oxygen Demand and a 60% removal of the Total Organic Carbon (TOC).

The process is preferably carried out at industrial scale continuously in stirred tanks using a static settler or the like and a continuous recycle flow of the iron sludge. The iron particles lost during the settling being compensated by a constant flow of fresh iron catalyst solution.

As shown in Fig .1 , into a first oxidation reactor 1 , a heated vessel under vigorous stirring is continuously added:

(a) a waste water stream with a composition as set forth in example 1 below

(b) sulfuric acid 50%,

(c) hydrogen peroxide 35%,

(d) a ferrous sulfate 1 % aqueous solution,

(e) a recycle stream of iron sludge from settler 4;

The mixture is continuously overflowing from the first vessel 1 into a post-reactor composed of a second heated vessel 2 under moderate stirring. Into a third stirred vessel 3 is continuously added (f) sodium hydroxide 50% to maintain pH 8. The outlet of the third vessel 3 is directed to a static settler 4. From the top of the settler 4 the supernatant aqueous stream W is sent to a biologic treatment plant (not shown in the drawing). From the bottom of the settler 4 is continuously pumped an iron sludge S stream (e) at a flow rate corresponding to 10% of the exiting flow of treated waste water. This sludge stream (e) is fed to the first reaction vessel 1 . It is to be noted that the amount of iron introduced into vessel 1 by stream (d), compensating losses of iron vehiculated in the supernatant stream W, is between about 1 -2% by weight of the iron recycled into vessel 1 by stream (e). Surprisingly, it was found that despite contrary information present in the literature, the iron sludge resulting from the Fenton reaction can be recycled without any further treatment i.e. reduction to ferrous ion by chemical or electrochemical process. It is believed that the alleged lower activity of the ferric ion is compensated by a higher concentration as well as an increased temperature.

The direct recycling of the iron sludge resulting from the chemical oxidation of organic by products present in the waste water avoids both large amounts of the iron sludge to be released into the final biological waste water treatment plant and costly working up of said sludge.

Examples :

Example 1a (Fenton reaction)

Alkaline waste water from dinitrotoluene plant and a neutralized condensate from the sulfuric acid concentration plant (SAC) are combined to obtain waste water with the following composition:

300 ml of the waste water is charged in a 500 ml glass bottle with a magnetic rod stirrer placed in a bain-marie on an electrical heating plate. The temperature is set at 85°C and the pH is reduced from 7.8 to 4.8 using 1 .27g H₂SO4 97%. Ferrous sulfate heptahydrate is added (0.65g FeSO4 ^*7 H2O, corresponding to 415 ppm Fe). The stirring is increased to position 10 (max.) and a 35% hydrogen peroxide solution is dosed at a rate of 1 .4 g/min during 5 minutes (13.2g corresponding to 15.4 kg H2O2 /nv of waste water or expressed as molar ratio H2O2 :Fe = 58). After the dosing, a post-reaction is carried out during 85 minutes at reduced stirring (pos. 3) at 85°C. The pH is then increased from 2.7 to 8.6 using 2.65g sodium hydroxide 50% to precipitate the iron as ferric hydroxide. The stirring is interrupted and the suspension is left for settling during 2 hours. After settling, the supernatant (90% of the volume) is recovered while the remaining liquid (10% of the volume) mainly Fe(OH)3, is recycled. The supernatant is analyzed as per following composition:

Compound Concentration % Removal

Nitrocresols < 1 ppm 99.9

Dinitrotoluene < 1 ppm 99.9

COD 709 mg 02 /I 88.0

Fe³⁺ 7 mg/l Example 2a (Fenton-like reaction)

The iron sludge from example 1 (approx 34g) was mixed with 300 ml of wate r identical to the one used in example 1 a and the resulting mixture was analyzed as per follows: COD 5500 mg 02 /I. The pH was adjusted to 4.2 using 1 .56g H2SO4 97% and 13.2g hydrogen peroxide were dosed at 1 .4 g/min during 5 minutes. The same work-up as example 1 a was then performed to recover 345g of treated water with the following composition:

Examples 3a-5a (Fenton-like recycles)

3 recycles were carried out following the procedure of example 2a and treating the same quality of waste water as example 1 a. Ferrous sulfate heptahydrate was added periodically to compensate the ferrous losses during the settling step and to maintain 350-400 ppm Fe concentration. The COD reduction was constantly superior to 80%.

The treated water was collected, mixed and subjected to a Zahn-Wellens test for inherent biodegradability and showed degradation superior to 85% after 13 days.

Example 1 b (Fenton reaction on extracted waste water) Combined alkaline waste water from dinitrotoluene plant and a neutralized condensate from the sulfuric acid concentration plant (SAC) are extracted with toluene under alkaline conditions and stripped to obtain extracted waste water with the following composition:

300 ml of the extracted waste water is charged in a 500 ml glass bottle with a magnetic rod stirrer placed in a bain-marie on an electrical heating plate. The temperature is set at 85°C and the pH is reduced from 8.5 to 4.8 using 0.7g H2SO4 97%. Ferrous sulfate heptahydrate is added (0.65g FeSO4 ^*7 H2O, corresponding to 420 ppm Fe). The stirring is increased to position 10 (max.) and a 35% hydrogen peroxide solution is dosed at a rate of 1 .4 g/min during 5 minutes (7.07g corresponding to 8.2 kg H2O2 /m³ of waste water, or expressed as molar ratio H2O2 :Fe = 31 ). After the dosing, a post-reaction is carried out during 85 minutes at reduced stirring (pos. 3) at 85°C. The pH is then increased from 3.2 to 8.5 using 1 .1 g sodium hydroxide 50% to precipitate the iron as ferric hydroxide. The stirring is interrupted and the suspension is left for settling during 2 hours. After settling, the supernatant (90% of the volume) is recovered while the remaining liquid (10% volume, mainly Fe(OH)3) is recycled The supernatant is analyzed as per following composition:

Example 2b (Fenton-like reaction)

The iron sludge from example 1 b (approx 36g) was mixed with 300 ml of extracted water identical to the one used in example 1 b and the resulting mixture was analyzed as per follows: COD 1910 mg 02 /I, TOC 741 mgC/l. The pH was adjusted to 4.0 using 0.86g H2SO4 97% and 7.07g hydrogen peroxide were dosed at 1 .4 g/min during 5 minutes. The same work-up as example 1 b was then performed to recover 307g of treated water with the following composition:

Examples 3b-26b (Fenton-like recycles)

24 recycles were carried out following the procedure of example 2b and treating the same quality of extracted waste water as example 1 b. Ferrous sulfate heptahydrate was added periodically to compensate the ferrous losses during the settling step and to maintain 350-400 ppm Fe concentration. The COD reduction was constantly superior to 80%.

The treated water was collected, mixed and subjected to a Zahn-Wellens test for inherent biodegradability and showed degradation superior to 85% after 10 days.

Example 1c (Fenton reaction with toluenediamine distillate)

Alkaline waste water from dinitrotoluene plant and a neutralized condensate from the sulfuric acid concentration plant (SAC) are extracted with toluene and stripped. The resulting mixture is combined with an aqueous distillate from the toluenediamine purification step (weight ratio 75:40) to obtain combined waste water with the following composition:

300 ml of the combined waste water from dinitrotoluene and toluenediamine plants is charged in a 500 ml glass bottle with a magnetic rod stirrer placed in a bain- marie on an electrical heating plate. The temperature is set at 85°C and the pH is reduced from 8.5 to 4.9 using 0.5g H2 SO4 97%. Ferrous sulfate heptahydrate is added (0.65g FeSO4 ^*7 H2O, corresponding to 420 ppm Fe). The stirring is increased to position 10 (max.) and a 35% hydrogen peroxide solution is dosed at a rate of 1 .8 g/min during 5 minutes (8.93g corresponding to 10.4 kg H2 O2 m^ of waste water, or expressed as molar ratio H 2 O 2 :Fe = 39). After the dosing, a post reaction is carried out during 85 minutes at reduced stirring (pos. 3) at 85°C. The pH is then increased from 3.1 to 8.5 using 0.9g sodium hydroxide 50% to precipitate the iron as ferric hydroxide. The stirring is interrupted and the suspension is left for settling during 2 hours. After settling, the supernatant (90% of the volume) is recovered while the remaining liquid (10% of the volume), mainly Fe(OH)3, is recycled. The supernatant is analyzed as per following composition:

Example 2c (Fenton-like reaction with toluenediamine distillate)

The iron sludge from example 1 c (approx 31 .6g) was mixed with 300 ml of combined water identical to the one used in example 1 c and the resulting mixture was analyzed as per follows: COD 1647 mg 02 /I, TOC 632 mg C/l. The pH was adjusted to 4.0 using 0.6g H2 SO4 97% and 8.84g hydrogen peroxide were dosed at 1 .8 g/min during 5 minutes. The same work-up as example 1 c was then performed to recover 31 1 g of treated water with the following composition:

Examples 3c-17c (Fenton-like recycles)

15 recycles were carried out following the procedure of example 2c and treating the same quality of combined waste water as example 1 c. Ferrous sulfate heptahydrate was added periodically to compensate the ferrous losses during the settling step and to maintain 350-400 ppm Fe concentration. The COD reduction was constantly superior to 80%.

The treated water was collected, mixed and subjected to a Zahn-Wellens test for inherent biodegradability and showed degradation superior to 90% after 13 days.

In summary, advantages of the process include:

- reduction of the consumption of Fe reagent by about 98%;

- an ability to degrade industrial waste water by chemical oxidation without generating large amounts of iron sludge that have to be separated and treated as toxic waste.

- a simple, safe and efficient process to treat large amounts of industrial waste water. The obtained waste stream can be released to a biological waste water treatment plant for final treatment.

References U.S. Patent documents:

- US 4'604'214, 08/1986, Carr

- US 5'538'636, 07/1996, Gnann

- HU 226079, 07/2006, Kovacs Other publications:

G. Cao, M. Sheng, W. Niu, Y. Fei, D. Li, Regeneration and reuse of iron catalyst for Fenton- like reactions, Journal of Hazardous Materials 172, 1446-1449;

V. Kavitha, K. Palanivelu, The role of ferrous iron in Fenton and photo-Fenton processes for the degradation of phenol, Chennosphere 55, 1235-1243.

Claims

1 . A process for treating a waste water from nitro-aromatic and/or aromatic amine production by oxidative degradation of aromatic by-products involving a Fenton-like reaction, characterized in that said process comprises following steps:

a) reacting said wastewater with hydrogen peroxide and ferric ion in an aqueous phase under acidic conditions,

b) elevating the pH of said aqueous phase to an alkaline condition and separating said aqueous phase into a supernatant (W) and a sludge (S) ,

c) recycling of the sludge (S) into step (a).

2. The process of claim 1 , wherein the said nitro-aromatic produced is dinitrotoluene and the aromatic amine is toluenediamine.

3. The process of claim 1 or 2, wherein the said waste water comprises alkaline waste water generated by the washing steps in a dinitrotoluene process.

4. The process of claim 2 or 3, wherein the said waste water is formed by combination of alkaline waste water from the washing step in the dinitrotoluene process and a neutralized condensate from a sulfuric acid concentration plant (SAC).

5. The process of claim 4, wherein the said waste water is further combined with an aqueous distillate from the toluenediamine purification step.

6. The process of anyone of the preceding claims, wherein said process is continuous.

7. The process of anyone of claims 1 to 6, wherein the ferric ion is separated in step (b) under alkaline conditions, in particular by decantation of said sludge.

8. The process of anyone of claims 1 to 7, wherein the concentration of ferric ion in step (a) is in the range of 200-600 ppm, in particular 350-450 ppm.

9. The process of claim 8 wherein the concentration of ferric ion in step (a) is adjusted by adding a ferrous salt.

10. The process of claim 8, wherein the concentration of ferric ion in step (a) is adjusted by adding a ferric salt.

1 1 . The process of anyone of claims 1 to 10, wherein the temperature range in step (a) is between 40-100°C, in particular between 85-95°C.

12. The process of anyone of claims 1 to 11 wherein the pH in step (a) is adjusted in a range between 3 and 6.

13. The process of anyone of claims 1 to 12, wherein the hydrogen peroxide concentration added in step (a) is between 5-20 kg per m^ of waste water, in particular between 8-15 kg per m^ of waste water.

14. The process of anyone of claims 6 to 13 wherein the process is continuous and the residence time in step (a) is 60-90 minutes for a reduction of 80% of the initial Chemical Oxygen Demand.

15. The process of anyone of claims 6 to 14, wherein step (a) is carried out in a series of successive stirred reaction vessels, wherein reagents are introduced and mixed under stirring in the first reaction vessel and further processed in subsequent vessel(s), wherein step (b) is initiated in a last reaction vessel by addition of alkaline hydroxide under stirring and continued in a settler.

16. The process of anyone of claims 1 to 15, wherein the supernatant (W ) leaving step (b) is further processed in a biologic treatment plant.

17. The process of anyone of claims 1 to 16, wherein as a pre-treatment step, the aqueous waste water is previously contacted with an organic solvent to extract nitro-aromatics, followed by separation of two phases, an aqueous phase and an organic phase.

18. The process of claim 14, wherein the solvent for extraction is nitrotoluene or toluene and the recovered organic phase is recycled into the nitration process, whereas the aqueous phase is subjected to a stripping to remove residual solvent which is recycled to the extraction stage.