U.S. patent application number 13/111337 was filed with the patent office on 2011-11-24 for process and apparatus for separating nitroaromatics from wastewater.
This patent application is currently assigned to Verein fuer Kernve. und Analyt. Rosse. e.V. (VKTA). Invention is credited to Holger Allardt, Hans-Juergen Friedrich, Rudiger FRITZ, Stefanie Haase, Reiner Reetz, Michael Zoellinger.
Application Number | 20110284391 13/111337 |
Document ID | / |
Family ID | 44971566 |
Filed Date | 2011-11-24 |
United States Patent
Application |
20110284391 |
Kind Code |
A1 |
FRITZ; Rudiger ; et
al. |
November 24, 2011 |
PROCESS AND APPARATUS FOR SEPARATING NITROAROMATICS FROM
WASTEWATER
Abstract
The invention relates to a process for the electrochemical
treatment of aromatic nitro compounds, which comprises the steps:
introducing an aqueous composition comprising at least one aromatic
nitro compound into the anode space of an electrolysis cell and
carrying out an electrolysis at an anodic current density in the
range from 0.1 to 10 kA/m.sup.2 and a cell potential in the range
from 4 to 15 V.
Inventors: |
FRITZ; Rudiger; (Bernsdorf,
DE) ; Haase; Stefanie; (Bretnig-Hauswalde, DE)
; Allardt; Holger; (Schwarzheide, DE) ;
Zoellinger; Michael; (Eislingen, DE) ; Reetz;
Reiner; (Schwarzheide, DE) ; Friedrich;
Hans-Juergen; (Stolpen, DE) |
Assignee: |
Verein fuer Kernve. und Analyt.
Rosse. e.V. (VKTA)
Dresden
DE
BASF SE
Ludwigshafen
DE
|
Family ID: |
44971566 |
Appl. No.: |
13/111337 |
Filed: |
May 19, 2011 |
Related U.S. Patent Documents
|
|
|
|
|
|
Application
Number |
Filing Date |
Patent Number |
|
|
61346914 |
May 21, 2010 |
|
|
|
Current U.S.
Class: |
205/703 ;
204/252 |
Current CPC
Class: |
C02F 1/4672 20130101;
C02F 2103/36 20130101; C02F 2001/46157 20130101; C02F 2101/166
20130101; C25B 9/19 20210101; C25B 11/075 20210101; C02F 1/725
20130101; C02F 2201/4617 20130101; C02F 2201/46135 20130101; C25B
11/057 20210101; C02F 2001/46147 20130101; C02F 2201/4614 20130101;
C02F 2209/02 20130101; C02F 2201/4618 20130101; C02F 2201/46115
20130101; C02F 2101/322 20130101; C02F 2101/38 20130101; C25B 13/08
20130101; C02F 2101/345 20130101 |
Class at
Publication: |
205/703 ;
204/252 |
International
Class: |
C25B 3/00 20060101
C25B003/00; C25B 9/00 20060101 C25B009/00 |
Claims
1. A process for the electrochemical treatment of aromatic nitro
compounds, which comprises the steps: a) introducing an aqueous
composition comprising at least one aromatic nitro compound into
the anode space of an electrolysis cell, where the electrolysis
cell has at least one anode space and at least one cathode space
which are separated from one another by a separator, and the
electrolysis cell has at least one anode which comprises at least
one anode segment comprising platinum or an anode segment
consisting of a support material and a coating, where the support
material comprises at least one metal selected from the group
consisting of niobium (Nb), tantalum (Ta), titanium (Ti) and
hafnium (Hf), and the coating consists of boron-doped diamond; b)
carrying out an electrolysis at an anodic power density in the
range from 0.1 to 10 kA/m.sup.2 and a cell potential in the range
from 4 to 15 V.
2. The process according to claim 1, wherein the aqueous
composition is alkaline process wastewater from the nitration of
aromatic compounds.
3. The process according to claim 1, wherein the aromatic nitro
compounds are at least one compound selected from the group
consisting of nitrobenzene (NB), dinitrobenzene (DNB),
trinitrobenzene (TNB), mononitrotoluene (MNT), dinitrotoluene
(DNT), trinitrotoluene (TNT), nitrochlorobenzene (NCB),
mononitroxylenes, dinitroxylenes, trinitroxylenes, mononitrocresol,
dinitrocresol, trinitrocresol, mononitrophenol, dinitrophenol,
trinitrophenol, mononitrobenzoic acid, dinitrobenzoic acid,
trinitrobenzoic acid, mononitroxylenols, dinitroxylenols and
trinitroxylenols, with all isomeric forms of the compounds
mentioned being encompassed.
4. The process according to claim 1, wherein the aqueous
composition comprises at least one aromatic nitro compound in
suspended form.
5. The process according to claim 1, wherein the aqueous
composition comprising at least one aromatic nitro compound
additionally comprises a redox mediator.
6. The process according to claim 1, wherein the temperature of the
aqueous composition in the anode space in the electrolysis is in
the range from 30 to 90.degree. C.
7. The process according to claim 1, wherein the electrolysis cell
has an anode which comprises at least one anode segment comprising
platinum and the electrolysis is carried out at an anodic current
density in the range from 2.5 kA/m.sup.2 to 10 kA/m.sup.2 and a
cell potential in the range from 4 to 15 V.
8. The process according to claim 1, wherein the electrolysis cell
has an anode which comprises at least one anode segment consisting
of platinum, wherein the anode segment is in the form of foils,
sheet or woven wire mesh and is fastened to a base frame comprising
at least one metal selected from among niobium (Nb), tantalum (Ta),
titanium (Ti) and hafnium (Hf), and the anode segment covers the
surface of the base frame to an extent of not more than 30%.
9. The process according to claim 1, wherein the electrolysis cell
has an anode comprising at least one anode segment comprising a
support material and a coating, where the support material
comprises at least one metal selected from the group consisting of
niobium (Nb), tantalum (Ta), titanium (Ti) and hafnium (Hf); the
coating comprises boron-doped diamond; the anode segment is
fastened to a base frame comprising at least one metal selected
from among niobium (Nb), tantalum (Ta), titanium (Ti) and hafnium
(Hf) and the electrolysis is carried out an anodic current density
in the range from 0.1 kA/m.sup.2 to 2 kA/m.sup.2 and a cell
potential in the range from 4 to 15 V and the coating of
boron-doped diamond on the support material has a layer thickness
in the range from 5 to 50 .mu.m.
10. An apparatus for the electrochemical treatment of aromatic
nitro compounds, which comprises at least one electrolysis cell,
wherein the electrolysis cell has at least one anode space and at
least one cathode space which are separated from one another by a
separator, and the electrolysis cell has at least one anode which
comprises at least one anode segment comprising platinum or an
anode segment comprising a support material and a coating, where
the support material comprises at least one metal selected from the
group consisting of niobium (Nb), tantalum (Ta), titanium (Ti) and
hafnium (Hf) and the coating comprises boron-doped diamond.
11. The apparatus according to claim 10, wherein the separator is
selected from among unspecific separators based on inorganic or
organic porous materials, cation-exchange membranes based on
polyethylene composite polymers and/or polyvinyl chloride composite
polymers and/or polyvinylidene fluoride (PVDF) and/or
polytetrafluoroethene (PTFE) and anion-exchange membranes.
12. The apparatus according to claim 10, wherein the electrolysis
cell has a spacer between the separator and the anode, where the
spacing between anode and cathode is in the range from 2 to 12
mm.
13. The apparatus according to claim 10, wherein the spacer between
anode and cathode is a multilayer woven mesh composed of
nonconductive polymer, with the various layers having a gradated
aperture.
Description
[0001] The present invention relates to a process and an apparatus
for the electrochemical treatment of aromatic nitro compounds, in
particular an electrolytic process for treating alkaline process
wastewater, e.g. from processes for the nitration of aromatic
compounds to form mononitroaromatics, dinitroaromatics and
trinitroaromatics. The aromatic nitro compounds comprised in the
process wastewater and any nitrite comprised in the wastewater are
reacted or destroyed by anodic oxidation or by means of anodically
generated, oxidizing compounds. The process makes, in particular,
complete oxidation of the aromatic nitro compounds to carbon
dioxide and nitrate possible. The process can also be operated on a
large scale and industrially.
[0002] Aromatic nitro compounds are usually prepared by nitration
of the corresponding aromatic compounds (e.g. benzene, toluene,
xylene, chlorobenzene) by means of a mixture of concentrated nitric
acid and concentrated sulfuric acid, also referred to as nitrating
acid. This forms an organic phase comprising the crude product of
the nitration and an aqueous phase comprising essentially sulfuric
acid, water of reaction and water introduced by the nitrating acid.
The nitric acid used is generally largely consumed in the
nitration.
[0003] After separation of the two phases, the aqueous, sulfuric
acid-comprising phase is, depending on the technology of the
nitration process, mixed again, either directly or after having
been concentrated, with fresh nitric acid and used for the
nitration. However, it is usually necessary to discharge at least
part of the sulfuric acid, either continuously or discontinuously,
from the overall process in order to avoid an increase in the
concentration of impurities, in particular metallic salts (see, for
example, DE 101 43 800).
[0004] The crude product from the nitration reaction of aromatic
compounds (e.g. benzene, toluene, xylene, chlorobenzene) to form
the corresponding nitroaromatics usually comprises the desired
nitroaromatics (e.g. nitrobenzene (NB), dinitrobenzene (DNB),
mononitrotoluene and dinitrotoluene (MNT and DNT),
nitrochlorobenzene (NCB), nitroxylene) together with small amounts
of aromatic nitro compounds which additionally have one or more
hydroxyl and/or carboxyl groups.
[0005] These compounds are undesirable by-products. As undesirable
by-products, it is possible for, for example, mononitrophenols,
dinitrophenols and trinitrophenols (hereinafter also summarized as
nitrophenols), mononitrocresols, dinitrocresols and trinitrocresols
(hereinafter also summarized as nitrocresols), mononitroxylenols,
dinitroxylenols and trinitroxylenols (hereinafter also summarized
as nitroxylenols) and mononitrobenzoic and dinitrobenzoic acids
(hereinafter summarized as nitrobenzoic acids) to be formed.
[0006] The crude product from the nitration has to be freed of the
undesirable by-products before further use. The by-products are
usually, after the aqueous phase (comprising sulfuric acid as
nitrating acid) has been separated off, separated off by multistage
washing of the organic phase with acidic, alkaline and neutral
washing liquids, with washing generally being carried out in the
order indicated. The alkaline washing is usually carried out using
aqueous sodium hydroxide solution, aqueous sodium carbonate
solution and/or aqueous ammonia solution.
[0007] The alkaline process wastewater formed comprises, inter
alia, nitrophenols, nitrocresols, nitroxylenols and nitrobenzoic
acids in the form of their water-soluble salts of the base used.
They are usually present in a concentration of from 0.2 to 2.5% by
weight, based on the alkaline process wastewater. The alkaline
process wastewater also comprises neutral nitro molecules formed in
the nitration, in particular reaction products. Neutral nitro
molecules can usually be comprised in the alkaline process
wastewater in an amount of greater than 1000 ppm. The alkaline
process wastewater frequently also comprises from 500 to 5000 ppm
of nitrates, from 500 to 5000 ppm of nitrite and more than 100 ppm
of sulfate. These ions originate predominantly from the nitration.
The constituents present give a typical chemical oxygen demand of
from 1 to 20 g/l.
[0008] The nitrophenols, nitrocresols, nitroxylenols, nitrobenzoic
acids and especially their salts have an intensive color and may be
highly toxic to the environment. In addition, the nitrophenols, and
especially their salts, are explosive in relatively high
concentrations or in neat form and have to be removed from the
wastewater before the latter is released. They are disposed of in
such a way that they do not pose a risk to the environment. The
alkaline process wastewater from the nitration of aromatics
additionally comprises neutral nitro molecules formed in the
nitration, in particular reaction produces. Since the aromatic
nitro compounds can also have biocidal or bactericidal properties
and thus make biological purification of the wastewater impossible,
purification or work-up of the wastewater comprising aromatic nitro
compounds is necessary.
[0009] This is also useful in order to be able to send the
wastewater to a conventional wastewater treatment comprising a
microbiological purification stage.
[0010] Numerous methods have been described in the literature for
removing nitrophenols, nitrocresols, nitroxylenols, nitrobenzoic
acids and the neutral nitroaromatics from process wastewater, for
example from processes based on extraction, adsorption, oxidation
or thermolysis.
[0011] The Encyclopedia of Chemical Technology, Kirk-Othmer, Fourth
Edition 1996, Vol. 17, p. 138, describes an extraction process for
separating off nitrobenzene, in which the nitrobenzene dissolved in
the wastewater at the respective temperature is removed by
extraction with benzene. Benzene which has dissolved in the water
is removed by stripping before the final treatment of the
wastewater.
[0012] EP-A 005 203 describes a thermal process for treating
wastewater comprising hydroxy-nitroaromatics. Here, the wastewater
comprising the hydroxynitroaromatics in the form of their
water-soluble salts is heated to temperatures in the range from 150
to 500.degree. C. under pressure with exclusion of air and
oxygen.
[0013] The dissolved nitroaromatics and hydroxynitroaromatics can
also be removed in an acid medium by extraction with an organic
solvent (see Ullmanns Enzyklopadie der technischen Chemie, 4th
edition, 1974, volume 17, page 386).
[0014] Furthermore, various methods for separating off organic
nitro compounds, especially nitroaromatics from water and
wastewater, which comprise a chemical or electrochemical reduction
or oxidation step and optionally a further physical separation
process (for example an extraction), are known.
[0015] DE-A 197 48 229 describes the electrochemical, cathodic
reduction of nitrophenols and of nitroresorcinol to the
corresponding amines at carbon cathodes. EP-A 808 920 describes the
electrochemical reduction of nitrobenzene and nitrophenol at
cathodes composed of various materials. In general, the known
processes for the cathodic reduction of nitroaromatic compounds
have the disadvantage that the nitroaromatics are not removed
completely, or further process steps are required for separating
off any environmentally damaging reaction products formed.
[0016] In addition, the above-described cathodic reduction
processes often suffer from severe foaming in the cathode space,
and the processes are therefore technically difficult to carry out,
especially on a relatively large scale. A series of oxidative
processes, e.g. nonelectrochemical, electrochemical or combined
processes, have been described for the treatment of nitration
wastewater or for the removal of aromatic nitro compounds.
[0017] U.S. Pat. No. 6,953,869 describes the removal of
trinitrocresols and picric acid from nitrating acid by oxidation by
means of concentrated nitric acid at temperatures of 70.degree. C.
U.S. Pat. No. 4,604,214 describes an oxidation process using
Fenton's reagent for the oxidation of trinitrocresols and picric
acid, in which an excess of oxidant is necessary and, in addition,
complete removal of the aromatic nitro compounds is not possible.
CN-A 1 600 697 describes the oxidation of p-nitrophenol with
combined use of UV light, Fenton's reagent (H.sub.2O.sub.2/Fe(II))
and/or anodic oxidation at PbO.sub.2 anodes.
[0018] The publication J. Hazardous Materials, Vol. 161, No. 2-3,
2009, pp. 1017-1023, describes a process for the electrochemical
removal of dinitrotoluene and trinitrotoluene from the waste acid
from the nitration of aromatics, in which the aromatic nitro
compounds are oxidized by means of hydrogen peroxide which has
previously been cathodically generated in situ from oxygen
dispersed in the waste acid. The electrolytic process described
here operated with introduction of oxygen and without separation of
anode space and cathode space.
[0019] The publication "Proceedings of the 1992 Incineration
Conference", 1992, pp. 167-174, reports the indirect anodic
oxidation of dinitrotoluene and trinitrotoluene in nitric acid
solution by means of anodically generated Ag.sup.2+ ions. The
process is carried out in a parallel plate reactor having platinum
anodes and a Nafion cation-exchange membrane. The electrochemical
process described is very sensitive to sulfate or halide ions and
additionally involves the production of toxic carbon monoxide. In
addition, complicated process steps are necessary for separating
the silver ions from the liquid output.
[0020] CN-A 1 850 643 describes the removal of aniline and
nitrobenzene from wastewater by means of an electrochemical
process. Here, the aromatic nitro compounds are removed by
oxidation in the anode space, with, in particular, an electrolysis
apparatus comprising an anode comprising a titanium base material
coated with ruthenium oxide, iridium oxide or lead oxide and also a
cation-exchange membrane being used.
[0021] In general, the above-described, in part electrochemical
oxidation processes have the disadvantage that additional
complicated separation steps and an often complicated technology in
terms of apparatus, e.g. introduction of gas, are necessary. In
addition, a large excess of oxidant often has to be employed. It is
usually not possible to achieve sufficient or complete removal of
the environmentally damaging aromatic nitro compounds by the
oxidation processes known from the prior art. In addition, the
known processes and the apparatuses for carrying out the processes
have an unsatisfactory period of uninterrupted operation.
[0022] The processes and apparatuses known from the prior art for
removing aromatic nitro compounds from wastewater and process water
also cannot ensure that simultaneous oxidation of a plurality of
different aromatic nitro compounds is possible and that formation
of possibly toxic degradation products does not occur.
[0023] It is therefore an object of the present invention to
provide a process which can be carried out industrially using
simple means and a suitable apparatus for carrying out an
electrochemical oxidation of aromatic nitro compounds in aqueous
compositions, and in particular a process for treating alkaline
process wastewater from the nitration of aromatic compounds. In
this process, complete or virtually complete oxidative degradation
of the aromatic nitro compounds to form toxicologically acceptable
compounds should be ensured.
[0024] The treated process water obtained should comprise no toxic,
environmentally toxic and/or explosive substances and it should be
able to be passed to a conventional wastewater treatment, including
a microbiological purification stage. The wastewater should satisfy
all present EU limits as a result of the process of the invention
and/or as a result of a subsequent conventional wastewater
treatment. Furthermore, the process should have stable operation
and the apparatus should give a high process running time/operating
time without interruption of the electrolysis cell. The process
should be inexpensive and able to be realized in a simple fashion
technically, also on a relatively large scale.
[0025] It has surprisingly been found that the process described
below and the electrolysis apparatus described enable various
nitroaromatic compounds which may be present in various forms (e.g.
dissolved, emulsified and suspended) in an aqueous composition to
be completely decomposed in an anodic oxidation using specific
anodes at high anodic current density.
[0026] A further advantage of the electrochemical process of the
invention is that the durability of the electrode materials, in
particular the anode material, and of the separator material is
maintained over a long period of time. Thus, long process running
times of up to 2 years can be realized by means of the process
described in the present invention and by means of the apparatus
according to the invention. In particular, it has been able to be
shown that stable operation of the process of the invention and of
the apparatus of the invention for at least 750 hours is
possible.
[0027] In addition, the foaming of the electrolyte composition and
in particular of the anolyte caused by formation of gases, which
usually represents a problem in the processes described in the
prior art, is reduced or prevented completely in the process of the
invention.
[0028] DE-A 10 2004 026 447 describes an electrolysis cell having a
three-part structure for the removal of sulfate ions from water,
with, inter alia, an anode coated with boron-doped diamond being
used.
[0029] The present invention provides a process for the
electrochemical treatment, in particular the electrochemical
oxidation, of aromatic nitro compounds, which comprises the steps:
[0030] a) introducing an aqueous composition comprising at least
one aromatic nitro compound (hereinafter also referred to as
anolyte) into the anode space of an electrolysis cell, [0031] where
the electrolysis cell has at least one anode space and at least one
cathode space which are separated from one another by a (at least
one) separator, [0032] and the electrolysis cell has at least one
anode which comprises at least one anode segment comprising (or
consisting of) platinum or an anode segment consisting of a support
material and a coating, where the support material comprises at
least one metal selected from the group consisting of niobium (Nb),
tantalum (Ta), titanium (Ti) and hafnium (Hf), but preferably
comprises or consists of niobium (Nb) and the coating consists of
boron-doped diamond; [0033] b) carrying out an electrolysis at an
anodic power density in the range from 0.1 to 10 kA/m.sup.2 and
preferably a cell potential in the range from 4 to 15 V, preferably
in the range from 5 to 9 V.
[0034] In general, the inventive process described can be carried
out in the entire pH range, i.e. both using acidic anolytes and
using basic anolytes.
[0035] In a preferred embodiment, the invention relates to a
process for the electrochemical treatment, in particular
electrochemical oxidation, of aromatic nitro compounds as described
above, with the aqueous composition (anolyte) being alkaline
process wastewater from the nitration of aromatic compounds.
Furthermore, the aqueous composition preferably has a pH in the
range from 4 to 14, in particular in the range from 4 to 12, in
particular from 4 to 10.
[0036] For the purposes of the present invention, aromatic nitro
compounds are organic compounds which have a 6- to 14-membered
aromatic ring, in particular a ring selected from among phenyl,
naphthyl, anthracene and phenanthrene, to which at least one nitro
group (--NO.sub.2) is directly bound, with the aromatic ring also
being able to have further substituents, in particular substituents
selected from among C.sub.1-C.sub.6-alkyl, C.sub.2-C.sub.6-alkenyl,
C.sub.2-C.sub.6-alkynyl, phenyl, benzyl, halo, --OH, --COOH and
--COOR.sup.1, where R.sup.1.dbd.C.sub.1-C.sub.6-alkyl,
C.sub.2-C.sub.6-alkenyl, C.sub.2-C.sub.6-alkynyl, phenyl or benzyl.
The aromatic nitro compound preferably has a 6-membered aromatic
ring.
[0037] In particular, the present invention provides a process for
the electrochemical treatment, in particular the electrochemical
oxidation, of aromatic nitro compounds of the general formula
(I)
##STR00001## [0038] where: [0039] n is an integer from 1 to 6;
[0040] m=0 or m is an integer from 1 to 5; [0041] R is selected
from among C.sub.1-C.sub.6-alkyl, C.sub.2-C.sub.6-alkenyl,
C.sub.2-C.sub.6-alkynyl, phenyl, benzyl, --F, --Cl, --Br, --OH,
--COOH or --COOR.sup.1, where R.sup.1.dbd.C.sub.1-C.sub.6-alkyl,
C.sub.2-C.sub.6-alkenyl, C.sub.2-C.sub.6-alkynyl, phenyl or
benzyl.
[0042] In particular, the present invention provides a process for
the electrochemical treatment of one or more of the above-described
aromatic nitro compounds.
[0043] A preferred embodiment of the invention is a process for the
electrochemical treatment of aromatic nitro compounds, wherein the
aromatic nitro compounds are at least one compound selected from
the group consisting of nitrobenzene (NB), dinitrobenzene (DNB),
trinitrobenzene (TNB), mononitrotoluene (MNT), dinitrotoluene
(DNT), trinitrotoluene (TNT), nitrochlorobenzene (NCB),
mononitroxylenes, dinitroxylenes, trinitroxylenes, mononitrocresol,
dinitrocresol, trinitrocresol, mononitrophenol, dinitrophenol,
trinitrophenol, mononitrobenzoic acid, dinitrobenzoic acid,
trinitrobenzoic acid, mononitroxylenols, dinitroxylenols and
trinitroxylenols, with all isomeric forms of the compounds
mentioned being encompassed.
[0044] Aromatic nitro compounds which do not comprise a hydroxyl
group or carboxyl group in the molecule will also, for the purposes
of the invention, be referred to as "neutral nitro molecules" or
"neutral nitro aromatics. Nitrophenols, nitrocresols, nitroxylenols
and nitrobenzoic acids will hereinafter also be summarized as
hydroxynitroaromatics. In a preferred embodiment, the aromatic
nitro component is a mixture of at least two of the abovementioned
compounds.
[0045] The aromatic nitro compound(s) can be present in dissolved,
emulsified or suspended form in the aqueous composition. In
particular, the invention provides a process as described above for
the electrochemical treatment, in particular the electrochemical
oxidation, of aromatic nitro compounds, where the aromatic nitro
compound(s) is/are present in at least two of the abovementioned
forms.
[0046] Preference is also given to a process as described above,
wherein the aqueous composition comprises (optionally in addition
to at least one dissolved nitro compound) at least one aromatic
nitro compound, preferably in suspended form.
[0047] In a preferred embodiment of the invention, the aqueous
composition (anolyte) comprises at least one aromatic nitro
compound in an amount in the range from 0.1 to 5% by weight,
preferably in the range from 0.5 to 2.5% by weight (based on the
total aqueous composition).
[0048] In a preferred embodiment of the invention, the aqueous
composition comprises not only the aromatic nitro compound but also
further additives, in particular in a concentration of from 0.001
to 30 g/l, preferably in a concentration of from 0.01 to 10 g/l.
The aqueous composition can also comprise inorganic nitrites in
addition to the aromatic nitro compound.
[0049] As further additives, water-soluble salts can be added to
the aqueous composition comprising at least one aromatic nitro
compound in order to increase the conductivity. These salts are
selected, in particular, from among water-soluble inorganic salts,
in particular salts comprising nitrate, sulfate and/or carbonate,
in particular alkali metal salts comprising nitrate, sulfate and/or
carbonate. The abovementioned salts for increasing the conductivity
can be comprised, in particular, in a concentration in the range
from 0.1 to 30 g/l, preferably in the range from 0.1 to 10 g/l,
particularly preferably in the range from 1 to 10 g/l (based on the
aqueous composition) in the aqueous composition.
[0050] For the purposes of the present invention, a water-soluble
salt is generally a salt having a solubility in water of greater
than or equal to 1 mol/l.
[0051] The present invention further provides a process as
described above, wherein the aqueous composition (anolyte)
comprising at least one aromatic nitro compound additionally
comprises a (at least one) redox mediator (e.g. an alkali metal
sulfate).
[0052] The redox mediator can be comprised, in particular, in a
concentration in the range from 0.001 to 0.2 mol/l, in particular
in the range from 0.01 to 0.05 mol/l (based on the aqueous
composition), in the aqueous composition.
[0053] The redox mediator can be, in particular, at least one
compound selected from among inorganic salts comprising sulfate
ions, in particular an alkali metal or alkaline earth metal
sulfate; and inorganic salts of cerium (Ce) or praseodymium (Pr),
in particular nitrates, sulfates and/or hydrogenphosphate salts of
cerium or praseodymium.
[0054] In one embodiment of the invention, the aqueous composition
(anolyte) comprises from 0.1 to 10 g/l, preferably from 1 to 5 g/l,
of cerium and/or praseodymium ions.
[0055] As catholyte, particular preference is given to using a
solution selected from the group consisting of lithium hydroxide
solution, sodium hydroxide solution, potassium hydroxide solution
and ammonium sulfate solution.
[0056] The concentration of the catholyte solution is preferably
from 0.1 to 5 mol/l, particularly preferably from 0.5 to 2 mol/l.
In particular, it is possible to use an ammonium sulfate solution
having a concentration in the range from 0.5 to 2.5 mol/l as
catholyte.
[0057] The process of the invention can, in particular, be carried
out so that the temperature of the aqueous composition in the anode
space during the electrolysis is in the range from 30 to 90.degree.
C., particularly preferably in the range from 40 to 70.degree.
C.
[0058] The duration of the electrolysis can preferably be in the
range from 0.3 to 10 hours, in particular from 1 to 10 hours,
preferably in the range from 1 to 5 hours. The process described
can be operated in the continuous mode, in a mode with partial
recycling of the process stream, e.g. with a recycling ratio in the
range from 80 to 98%, preferably in the range from 90 to 95%, or
batchwise.
[0059] The present invention provides a process as described above,
in which the electrolysis cell has at least one anode space and at
least one cathode space which are separated from one another by a
separator and the separator is selected from among [0060]
unspecific separators based on inorganic or organic porous
materials (e.g. separators made of sintered silica); [0061]
cation-exchange membranes based on polyethylene composite polymers
and/or polyvinyl chloride composite polymers and/or polyvinylidene
fluoride (PVDF) and/or polytetrafluoroethene (PTFE), in particular
comprising sulfonate groups (e.g. MC-3470, manufacturer Ionac)
[0062] strongly basic anion-exchange membranes, in particular on
the basis of composite polymers, e.g. the abovementioned polymers,
comprising tertiary amino groups (e.g. MC-3450 from the
manufacturer Ionac).
[0063] The anode of the electrolysis cell used in the process of
the invention preferably comprises a base frame to which at least
one anode segment is fastened (e.g. screwed), with the base frame
comprising a metal which has a high overvoltage for the formation
of oxygen and is electrochemically passive under the given
conditions.
[0064] In particular, the base frame can comprise one or more
"valve metals". For the present purposes, a valve metal is a metal
which when connected as anode becomes coated with a semiconducting
(insulating) oxide layer which does not become conductive even at a
high overvoltage and thus blocks the electrolysis. As valve metals,
mention may be made of, inter alia, niobium, tantalum, titanium,
hafnium, zirconium and tungsten.
[0065] The base frame preferably comprises at least one metal
selected from among titanium (Ti), niobium (Nb), tantalum (Ta) and
hafnium (Hf). It is also possible to use alloys of the metals
mentioned.
[0066] A preferred embodiment of the invention provides a process
as described above, wherein the electrolysis cell has an anode
which comprises at least one anode segment comprising a support
material and a coating, [0067] where the support material comprises
at least one valve metal, preferably at least one metal selected
from the group consisting of niobium (Nb), tantalum (Ta), titanium
(Ti) and hafnium (Hf), particularly preferably niobium (Nb), and
the coating comprises boron-doped diamond, [0068] and the
electrolysis is carried out at an anodic current density in the
range from 0.1 kA/m.sup.2 to 2 kA/m.sup.2, preferably in the range
from 0.1 to 1.25 kA/m.sup.2, preferably in the range from 0.75 to
1.25 kA/m.sup.2, and a cell potential in the range from 4 to 15 V,
preferably in the range from 5 to 9 V.
[0069] In a preferred embodiment, the above-described anode segment
comprising a support material and a coating has a coating
comprising boron-doped diamond in an amount of from 90 to 100%
(based on the electrochemically active anode area or based on the
electrochemically active area of the anode segment/segments).
Preference is given to using anode segments which have a
corresponding coating on two sides, preferably on all sides. In
particular, the boron-doped diamond has a dopant content of from
0.01 to 3%, in particular from 0.1 to 0.5%.
[0070] In a further embodiment, the above-described anode segment
(comprising a support material and a coating) has a coating of
boron-doped diamond in a layer thickness in the range from 5 to 50
.mu.m, in particular from 10 to 40 .mu.m. The support material
comprising a metal selected from among niobium (Nb), tantalum (Ta),
titanium (Ti) and hafnium (Hf), in particular comprising niobium
(Nb), preferably has a thickness of from 1 to 4 mm, in particular
from 2 to 3 mm.
[0071] An embodiment of the invention provides a process for the
electrochemical treatment of aromatic nitro compounds as described
above, wherein the electrolysis cell has an anode comprising at
least one anode segment comprising a support material and a
coating, where the support material comprises at least one metal
selected from the group consisting of niobium (Nb), tantalum (Ta),
titanium (Ti) and hafnium (Hf); where the coating comprises
boron-doped diamond, the anode segment is fastened to a base frame
comprising at least one metal selected from among niobium (Nb),
tantalum (Ta), titanium (Ti) and hafnium (Hf) and the electrolysis
is carried out at an anodic current density in the range from 0.1
kA/m.sup.2 to 2 kA/m.sup.2 and a cell potential in the range from 4
to 15 V and the coating comprising boron-doped diamond on the
support material has a layer thickness in the range from 5 to 50
.mu.m.
[0072] In the above-described embodiment of an anode comprising at
least one anode segment comprising a support material and a
coating, the base frame is preferably almost completely or
completely covered by the anode segment or the anode segments. In
particular, a coverage of the base frame by the anode segment or
the anode segments can, in this embodiment, also be greater than
100%. In this embodiment, the base frame can, in particular, not be
manufactured with a solid area and can, for instance, comprise a
grid of metal strips which are electrically conductive and
mechanically stably joined to one another (e.g. screwed, welded).
In particular, the actual anode segments can then be fastened to
these metal strips. The base frame grid structure preferably has on
its rear side contact pins or contact tabs for the supply of
electric power and can, in particular, be fixed in the anode shell
by means of the power leads.
[0073] A further preferred embodiment of the invention provides a
process as described above, wherein the electrolysis cell has an
anode which comprises at least one anode segment comprising or
consisting of platinum.
[0074] In particular, the invention provides a process as described
above, wherein the electrolysis cell has an anode which comprises
at least one anode segment consisting of platinum, [0075] and the
electrolysis is carried out at an anodic current density in the
range from 0.1 to 10 kA/m.sup.2, preferably in the range from 2.5
to 10 kA/m.sup.2, preferably in the range from 4 to 10 kA/m.sup.2,
particularly preferably in the range from 2 to 6 kA/m.sup.2, and a
cell potential in the range from 4 to 15 V, preferably in the range
from 5 to 9V.
[0076] The electrolysis is preferably carried out at a platinum
segment electrode as described above at an anodic current density
above 2.5 kA/m.sup.2, in particular above 4 kA/m.sup.2.
[0077] The electrolysis cell can preferably have an anode which
comprises at least one anode segment comprising or consisting of
platinum, [0078] wherein the anode segment or the anode segments
is/are in the form of foils, sheet or woven wire mesh and is/are
fastened to a base frame comprising at least one metal selected
from among niobium (Nb), tantalum (Ta), titanium (Ti) and hafnium
(Hf), [0079] and the anode segment or anode segments cover(s) the
surface of the base frame to an extent of not more than 30%,
preferably not more than 10%.
[0080] In particular, coverage of the base frame by the
above-described electrochemical active anode segments in the range
from 1 to 30%, preferably in the range from 1 to 10%, is
advantageous.
[0081] In particular, at least one anode segment comprising finely
polished pure platinum is used in the preferred embodiment. Such
platinum electrodes are known to those skilled in the art and are
commercially available. A smooth, finely polished platinum
electrode can typically be obtained by polishing using a silica gel
polishing composition having a particle size of about 0.05
.mu.m.
[0082] As coproduct of the electrolysis, hydrogen, in particular,
is obtained in a high purity. This hydrogen can be used in other
processes, e.g. in the further processing of the aromatic nitro
compounds by catalytic reaction (reduction).
[0083] Furthermore, the present invention provides an apparatus for
the electrochemical treatment, in particular the electrochemical
oxidation, of aromatic nitro compounds, which comprises at least
one electrolysis cell [0084] wherein the electrolysis cell has at
least one anode space and at least one cathode space which are
separated from one another by a (at least one) separator [0085] and
the electrolysis cell has at least one anode which comprises at
least one anode segment comprising (or consisting of) platinum or
an anode segment comprising a support material and a coating, where
the support material comprises (or consists of) at least one metal
selected from the group consisting of niobium (Nb), tantalum (Ta),
titanium (Ti) and hafnium (Hf) and the coating comprises
boron-doped diamond.
[0086] As separator, it is possible to use both specific and
unspecific separators. For this purpose, it is possible to use, for
example, cation-exchange membranes, organic porous materials,
inorganic porous materials and/or strongly basic anion-exchange
membranes. In a preferred embodiment, the separator is selected
from among: [0087] unspecific separators based on inorganic or
organic porous materials (e.g. separators made of sintered silica);
[0088] cation-exchange membranes based on polyethylene composite
polymers and/or polyvinyl chloride composite polymers and/or
polyvinylidene fluoride (PVDF) and/or polytetrafluoroethene (PTFE),
in particular comprising sulfonate groups (e.g. MC-3470,
manufacturer Ionac) [0089] strongly basic anion-exchange membranes,
in particular on the basis of composite polymers, e.g. the
abovementioned polymers, comprising tertiary amino groups (e.g.
MC-3450 from the manufacturer Ionac).
[0090] The invention also provides a process in which the
electrolysis cell has an anode comprising at least one anode
segment comprising a support material and a coating, where the
support material comprises at least one metal selected from the
group consisting of niobium, tantalum, titanium and hafnium; the
coating comprises boron-doped diamond; the anode segment is
fastened to a base frame comprising at least one metal selected
from among niobium (Nb), tantalum (Ta), titanium (Ti) and hafnium
(Hf) and the electrolysis is carried out an anodic current density
in the range from 0.1 kA/m.sup.2 to 2 kA/m.sup.2 and a cell
potential in the range 4-15 V and the coating of boron-doped
diamond on the support material has a layer thickness in the range
from 5 to 50 .mu.m.
[0091] The invention also provides an apparatus for the
electrochemical treatment of aromatic nitro compounds, which
comprises at least one electrolysis cell, wherein the electrolysis
cell has at least one anode space and at least one cathode space
which are separated from one another by a separator.
[0092] Here, the electrolysis cell has at least one anode which
comprises at least one anode segment comprising platinum or an
anode segment comprising a support material and a coating, where
the support material comprises at least one metal selected from the
group consisting of niobium (Nb), tantalum (Ta), titanium (Ti) and
hafnium (Hf) and the coating comprises boron-doped diamond.
[0093] The invention also provides an apparatus in which the
separator is selected from among unspecific separators based on
inorganic or organic porous materials, cation-exchange membranes
based on polyethylene composite polymers and/or polyvinyl chloride
composite polymers and/or polyvinylidene fluoride (PVDF) and/or
polytetrafluoroethene (PTFE) and anion-exchange membranes.
[0094] In particular, it has been found to be advantageous to
provide a spacer between the anode and the separator, which
separator ensures maintenance of a sufficient spacing between anode
and separator in order to ensure protection of the separator from
the strong oxidants formed in the anode reaction (e.g. hydroxyl
radicals, perhydroxyl radicals, peroxodisulfate anions, Caro's acid
and ozone). In particular, the spacing between anode and separator
can be in the range from 2 to 12 mm, preferably from 4 to 8 mm.
This enables damage to the separator (the membrane) by the strong
oxidants to be avoided.
[0095] One embodiment of the invention is an apparatus as described
above, wherein the electrolysis cell has a spacer between the
separator and the anode, where the spacing between anode and
cathode is in the range from 2 to 12 mm, preferably in the range
from 4 to 8 mm.
[0096] In particular, the spacer between anode and cathode can be a
multilayer woven mesh, preferably a three-layer, three-dimensional
woven mesh (gauze), composed of nonconductive polymer, with the
various layers having a different, in particular gradated, aperture
(proportion of open area). The spacer used is preferably composed
of an oxidation-resistant, electrically nonconductive polymer,
where the oxidation-resistant, electrically nonconductive polymer
is selected from among polyesters and polyalkylene (e.g.
polyethylene, polypropylene).
[0097] In a particular embodiment of the invention, the spacer has
three different layers having a different aperture (proportion of
open area), with the layers of the spacer adjacent to the anode or
to the separator having an aperture in the range from 50 to 90%,
preferably from 60 to 75%, and the middle layer of the spacer
having an aperture of from 30 to 60%, preferably an aperture of
from 40 to 60%. This gradation of the aperture of the spacer can,
in particular, aid establishment of turbulent flow close to the
anode.
[0098] In particular, the apparatus of the invention comprises a
cell body formed by at least one cathode space and at least one
anode space in the numbers required in each case, provided with
connections for introduction and discharge of the anolyte and
catholyte and also sealing systems, pressure frames and power
supply leads for the electrodes, with selection and arrangement
being known to those skilled in the art. Preference is given to a
two-chamber electrolysis cell or a three-chamber electrolysis
cell.
[0099] The electrolyte preferably flows upward, i.e. from the
bottom toward the top, through the electrolysis cell.
[0100] The cathode preferably comprises a sufficiently
corrosion-resistant metal or carbon (in the form of graphite or
glassy carbon). The cathode is preferably made of stainless steel
sheet. In particular, the cathode material is resistant to
embrittlement by hydrogen.
[0101] Furthermore, the present invention provides for the use of
an apparatus as described above for treating wastewater or process
water, in particular from the nitration of aromatic compounds, in
particular alkaline process water.
[0102] The invention is illustrated by the following examples.
EXAMPLE 1
[0103] An aqueous process solution from a chemical production
process, which was characterized as follows: [0104] 1.5 g/l of
NaNO.sub.2 [0105] 20 g/l of Na.sub.2CO.sub.3 [0106] 10 mg/l of
mononitrotoluene isomer mixture with a proportion emulsified [0107]
1 g/l of dinitrotoluene isomer mixture with a proportion suspended
[0108] 0.5 g/l of trinitrocresol isomer mixture with a proportion
suspended was used.
[0109] 100 ml of the aqueous process solution described were
introduced into the anode space of an electrolysis cell, while a
sodium hydroxide solution having a concentration of 1 mol/l was
present in the cathode space. The electrode spaces were separated
by means of a diaphragm composed of sintered silica having a pore
opening of from 30 to 50 .mu.m. A smooth platinum sheet having a
size of 1.5 cm.sup.2 and a thickness of 0.25 mm was used as anode.
After application of a DC voltage, the solution was electrolyzed at
an anodic current density of 0.6 A/cm.sup.2 and a temperature of
70.degree. C.
[0110] The electrolysis process was carried out batchwise. After an
electrolysis time of four hours, the previously dark brown liquid
was clear and decolorized.
[0111] The chemical oxygen demand of the solution was reduced from
6500 mg/l (original process solution) to 150 mg/l, corresponding to
a conversion of 98%. Nitrite was no longer detectable in the
solution after the end of the electrolysis. The chemical oxygen
demand was determined in accordance with DIN 38409/H41.
EXAMPLE 2
[0112] A process solution as described in example 1 was introduced
into an electrolysis cell in which a 4 cm.sup.2 anode composed of
boron-doped diamond on niobium sheet having a thickness of 1.5 mm
was used as anode. The electrolysis was carried out at a
temperature of 60.degree. C. and an anodic current density of 0.125
A/cm.sup.2. The experimental setup otherwise corresponded to the
setup as described in example 1. The electrolysis process was
carried out batchwise. After an electrolysis time of two hours, the
solution in the anode space was completely clear and colorless. The
chemical oxygen demand had been reduced from an initial 6500 mg/l
to below 50 mg/l, corresponding to a conversion of more than 99%.
Nitrite was no longer detectable in the solution from the anode
space.
EXAMPLE 3
[0113] An aqueous process solution comprising: [0114] 1.0 g/l of
sodium nitrite [0115] and also mononitrotoluene, dinitrotoluene,
nitrocresols, and having a chemical oxygen demand (COD) of 5100
mg/l was used.
[0116] An electrolysis cell having an anode comprising a
titanium/auxiliary frame with a boron-doped diamond electrode on a
niobium support fastened thereto was used. The anode area was 75
cm.sup.2.
[0117] Furthermore, the electrolysis cell comprised a spacer in the
form of a woven polymer mesh having 3 layers of different aperture
in the anode space. The layers of the spacer adjacent to the anode
and to the separator had an aperture in the range from 60 to 75%
and the middle layer of the spacer had an aperture of from 40 to
60%. The spacing between anode and separator was 6.5 mm.
[0118] An MC/3470 cation-exchange membrane (manufacturer Ionac,
USA) and a cathode made of CrNiTi stainless steel were used. The
cathode area was 100 cm.sup.2. A 1M sodium hydroxide solution was
used as catholyte. 2.5 l of the abovementioned process solution
were electrolyzed batchwise at a current of 10 A and a temperature
of 70.degree. C.
[0119] The electrolysis process was carried out batchwise and with
partial recycling in the range from 80 to 95%, with the results
described below being similar for the modes of operation. After an
electrolysis time of four hours, a chemical oxygen demand of 600
mg/l was measured on a sample from the anode space, corresponding
to a conversion of 88%.
[0120] During the electrolysis, 150 ml of the liquid went over from
the anode space into the cathode space. The chemical oxygen demand
(COD) of the catholyte was 40 mg/l after the end of the
electrolysis (0.0 mg/l before the beginning of the experiment).
EXAMPLE 4
[0121] An aqueous process solution as described in example 1 was
used; the electrolysis cell as described in example 3 was
employed.
[0122] Sodium sulfate was additionally added to the liquid in the
anode space in such an amount that a concentration of 5 g/l based
on the total amount of liquid was obtained. The electrolysis
process was carried out batchwise with partial recycling in the
range from 80 to 95%; the results described below for the mode of
operation were similar. The electrolysis was carried out at a
current of 10 A and a temperature of 60.degree. C. After an
electrolysis time of four hours, the chemical oxidation demand of
the liquid in the anode space was found to be 360 mg/l,
corresponding to a conversion of 94%.
EXAMPLE 5
[0123] The electrolysis was carried out as described in example 4,
with the difference that praseodymium nitrate instead of sodium
sulfate was added to the liquid in the anode space such that a
concentration of praseodymium ions in the liquid in the anode space
of 0.005 mol/l was obtained. The electrolysis process was carried
out batchwise with partial recycling in the range from 80 to 95%;
the results described below for the mode of operation were similar.
After an electrolysis time of 3.5 hours, the chemical oxygen demand
had been reduced to 270 mg/l, corresponding to a conversion of
96%.
EXAMPLE 6
[0124] An aqueous process solution comprising nitrotoluene and
nitrocresols and having a chemical oxygen demand of 7200 mg/l was
used. The electrolysis cell comprised anode, cathode and spacer as
described in examples 4 and 5. An MA/3450 anion-exchange membrane
(manufacturer Ionac, USA) served to separate the electrode spaces.
A 2 M ammonium sulfate solution was used as catholyte. The
electrolysis process was carried out batchwise with partial
recycling in the range from 80 to 95%; the results described below
for the mode of operation were similar. After an electrolysis time
of 1 hour at a current of 10 A, the value for the chemical oxygen
demand (COD) of the liquid flowing out of the anode space was 150
mg/l. An increase in volume of the liquid in the cathode circuit
was not observed. The value for the chemical oxygen demand of the
catholyte over the total time of the electrolysis experiment was 1
mg/l.
EXAMPLE 7
[0125] An aqueous process solution comprising aromatic nitro
compounds and having a chemical oxygen demand of 6000 mg/l was
used. A 2 M ammonium sulfate solution was used as catholyte. An
electrolysis cell having an anode area of 600 cm.sup.2 was used. A
boron-doped diamond anode was used as anode. The electrolysis
process was carried out continuously at a current density of 1
kA/m.sup.2, with an anolyte volume flow of 1.5 l/h and at
70.degree. C. A conversion based on the COD of the anolyte of 89%
was able to be achieved.
* * * * *