U.S. patent application number 10/499770 was filed with the patent office on 2005-06-09 for method and apparatus for electro-catalytical hydrogenation of vat dyes and sulphide dyes.
Invention is credited to Dossenbach, Otmar, Marte, Walter, Meyer, Ulrich, Roessler, Albert, Rys, Paul.
Application Number | 20050121336 10/499770 |
Document ID | / |
Family ID | 25738483 |
Filed Date | 2005-06-09 |
United States Patent
Application |
20050121336 |
Kind Code |
A1 |
Marte, Walter ; et
al. |
June 9, 2005 |
Method and apparatus for electro-catalytical hydrogenation of vat
dyes and sulphide dyes
Abstract
The invention relates to a method for electro-catalytical
hydrogenation of vat dyes and sulphide dyes in aqueous solutions,
and to an apparatus for carrying out said method. The inventive
method is suitable for batch operation and continuous operation. It
works entirely without any reducing agents and provides a
substantially salt-free dye concentrations of up to 200 g/l. The
dyed threads are characterized by good friction qualities and by a
high weaving yield. As far as waste water is concerned, no toxic
load occurs and the salt loads are much lower, thereby enabling the
waste water to be recycled in a substantially less complex manner
in comparison with other conventional dye systems. The invention
also relates to an apparatus for carrying out said method.
Inventors: |
Marte, Walter; (Ulisbach,
CH) ; Dossenbach, Otmar; (Frauenfeld, CH) ;
Roessler, Albert; (Zurich, CH) ; Meyer, Ulrich;
(Zurich, CH) ; Rys, Paul; (Zurich, CH) |
Correspondence
Address: |
MERCHANT & GOULD PC
P.O. BOX 2903
MINNEAPOLIS
MN
55402-0903
US
|
Family ID: |
25738483 |
Appl. No.: |
10/499770 |
Filed: |
January 19, 2005 |
PCT Filed: |
December 20, 2002 |
PCT NO: |
PCT/CH02/00718 |
Current U.S.
Class: |
205/687 ;
204/263 |
Current CPC
Class: |
D06P 1/221 20130101;
D06P 5/2016 20130101; D06P 1/22 20130101; D06P 1/30 20130101 |
Class at
Publication: |
205/687 ;
204/263 |
International
Class: |
C25B 001/00; C25D
017/00 |
Foreign Application Data
Date |
Code |
Application Number |
Dec 20, 2001 |
CH |
232201 |
Mar 13, 2002 |
CH |
80102 |
Claims
1. Method for electrocatalytic hydrogenation of vat and sulfide
dyes in aqueous solutions wherein at a cathode (8) of an
electrochemical reaction cell (7) adsorbed hydrogen (H.sub.ad) is
formed due to a cell voltage being applied, that a dye (A) is
present at the same cathode (8) as adsorbed dye (A.sub.ad), where
it is brought by the electrochemically generated, adsorbed hydrogen
(H.sub.ad) into an adsorbed, reduced form (P.sub.ad) of the dye,
that the adsorbed, reduced form (P.sub.ad) transitions through the
desorption by the cathode (8) into a reduced form (P), or rather
into a species (P), which forms the leuco form of the dye (A),
where the formed species (P) is used for the dye process.
2. Method according to claim 1, wherein four reaction equations
(I-IV) form the basis: H.sub.2O+e.sup.-.fwdarw.H.sub.ad+OH.sup.-
(I)A.fwdarw.A.sub.ad (11)A.sub.ad+2H.sub.ad.fwdarw.P.sub.ad
(III)P.sub.ad.fwdarw.P (IV) where A is a dye, A.sub.ad is an
adsorbed dye, H.sub.ad is the electrochemically generated, adsorbed
hydrogen, P.sub.ad is the adsorbed leuco form of the dye and P is
the leuco form of the dye.
3. Method according to claim 1 wherein the electrochemically
generated, adsorbed hydrogen (H.sub.ad) is formed through the
application of a cell voltage of 1-5 V, preferably 2-3 V.
4. Method according to claim 1, wherein it is carried out as a
batch or continuous process.
5. Method according to claim 1, wherein used as dye are indigoids
or an anthraquinoids, as well as sulfide dyes and other vat
dyes.
6. Method according to claim 1, wherein the concentration of the
dye (A) is to up to 200 g/l, preferably 80 -120 g/l.
7. Method according to claim 1, wherein the electrochemical
reaction cell (7) is operated with a pulsing cell voltage, or to be
precise, current.
8. Method according to claim 1, wherein the temperature amounts to
20-100.degree. C., preferably 25-60.degree. C.
9. Method according to claim 1, wherein the pH value amounts to
9-14, preferably 12-13.
10. Method according to claim 1, wherein the surfactant
concentration amounts to 0.1-10 g/l, preferably 1-5 g/l.
11. Method according to claim 1, wherein used as additives are
ketones, alcohols, preferably methanol and iso-propanol, acetals,
glycols and glycol ethers, pyridines, lactams, acids, naphthaline
sulfonic acid derivatives, and acid amides.
12. Method according to claim 11, wherein the amount of additive
referenced to the introduced amount of dye amounts to 0.1-90%,
preferably 1-30%.
13. Method according to claim 1, wherein at the beginning of the
process a pre-reduction of a part of the dye (A) to the species (P)
takes place through a one time addition of reducing agent, whereby
the reaction time is shortened.
14. Method according to claim 13, wherein used as reducing agents
are sub-stochiometric quantities of dithionite and its derivatives,
thiourea dioxide, glucose, .alpha.-hydroxyketones,
.alpha.-hydroxyaldehydes, triose-reducton or reductin acids.
15. Apparatus for the implementation of the method according to
claim 1, wherein provided for the suspended dye (A) present in a
catholyte tank (1) is a first circuit with a first circulation
stream (V1) that prevents the sedimentation of the dye suspension,
that a part of the first circulation stream (V1) in a second
circulation stream (V2) forms a second circuit through an
electrochemical reaction cell (7) past the cathode (8), which
second circuit leads back into the catholyte tank (1), the
thermostatically controlled electrochemical reaction cell (7) being
fitted with a cathode (8) and an anode (8'), that a third
circulation stream (V3) coming out from an anolyte tank (31)
through the electrochemical reaction cell (7) past an anode (8')
forms a third circuit that leads back into the anolyte tank (31),
that located in the electrochemical reaction cell (7) is a membrane
(9) that separates the cathode (8) and the anode (8') and that a
cell voltage is provided for the electrochemical reaction cell
(7).
16. Apparatus according to claim 15, wherein provision is made for
batch operation of the apparatus.
17. Apparatus according to claim 15, wherein a first volume stream
(V4) with a supply tank (11) for the dye suspension is connected to
the catholyte tank (1) by way of the first circulation stream (V1),
that the dye suspension from the catholyte tank (1) is connected to
the storage tank (21) by way of a second volume stream (V5), the
first volume stream (V4) and the second volume stream (V5) being
essentially the same size, and that provision is made for
continuous operation of the apparatus.
18. Apparatus according to claim 15, wherein the second circuit
consists of a second pump (P2), a steel pipe spiral (3), a second
inlet pipe (6) and a second line system (17, 17', 17", 17'"), an
ultrasound vibrator (5) being arranged under the steel pipe spiral
(3) and serving the dispersion of the dye.
19. Apparatus according to claim 17, wherein the catholyte tank (1)
and the storage tank (21) are tightly sealed and are free of
oxygen.
20. Apparatus according to claim 15, wherein the pressure drop
across the electrochemical reaction cell (7) remains essentially
constant over time.
21. Apparatus according to claim 15, wherein the second circulation
stream (V2) remains essentially steady over time.
22. Apparatus according to claim 15, wherein the electrochemical
reaction cell (7) is designed for a pressure of 1-10 bar,
preferably 1-6 bar.
23. Apparatus according to claim 15, wherein t in the scope of
continuous dye hydrogenation a second volume stream (V5) from the
catholyte tank (1) directly into a storage tank (21), or to be
precise, into a dye bath is provided for discharge and that this
takes place in a `just-in-time` dosing corresponding to the portion
of dye consumed by the dyed goods.
24. Apparatus according to claim 15, wherein t cathode (8) is
electrically conductive, has a large surface area and is
catalytically active for the electrocatalytic hydrogenation.
25. Apparatus according to claim 15, wherein the cathode (8)
consists of a substrate and a coating located thereon.
26. Apparatus according to claims 25, wherein the carrier consists
of a fine-meshed netting, of expanded metal or of smooth sheet
metal, on the particles of which the coating is located as an
electrically conductive, porous film, the latter being permanently
fixed thereon.
27. Apparatus according to claim 25, wherein the substrate consists
of a fixed bed or a fluidized bed, on the particles of which is
located the coating as an electrically conductive, porous film, the
latter being permanently fixed thereon.
28. Apparatus according to claim 15, wherein the cathode (8)
consists of graphite granules.
29. Apparatus according to claim 15, wherein the cathode (8) is
designed to be rotating.
Description
[0001] The present invention relates to a method for
electrocatalytic hydrogenation of vat dyes and sulfide dyes in
aqueous solutions according to patent claim 1 and an apparatus for
carrying out said method according to patent claim 15.
[0002] The application of vat and sulfide dyes to textile materials
takes place in the reduced form, since only this form is water
soluble and possesses a high substrate affinity. Through the
oxidation carried out after the dyeing, the dye is again converted
from its leuco form into the water-insoluble pigment structure.
[0003] The use of vat and sulfur dyes for printing and coloring of
textile fibers has until now been associated with the application
of over-stoichiometric reduction-agent amounts (relative to the dye
amount to be reduced). The reduction of the vat dyes conventionally
takes place in alkaline (pH>9), aqueous solutions with sodium
dithionite (hydrosulfite) or reduction agents derived therefrom
(e.g. RONGALIT C, BASF) in conjunction with wetting agents and
complexing agents. Other reduction agents such as thiourea dioxide
or endiolates have become little accepted due to cost
considerations.
[0004] The reduction agents suitable for reduction of vat dyes have
a redox potential, under the conditions necessary for the vatting
of the dyes, of -400 mV to -1000 mV. Both the application of
hydrosulfite and of thiourea dioxide lead to a high sulfite or
sulfate loading of the effluent. These salt loads are on the one
hand toxic, and on the other hand are corrosive and lead to the
destruction of the concrete conduits. A further problem of the
sulfate load in the effluent arising from the sulfite is the
hydrogen sulfide formation in the sewer system pipes, caused by
anaerobic organisms.
[0005] Likewise, newer methods could only partially solve the
problems mentioned. Here, the reduction using ultrasound reactors
in conjunction with the conventional reduction agents is worthy of
mention. This method offers the advantage that the reduction-agent
consumption is lowered to stoichiometric proportions and that the
hydrosulfite can be replaced with endiols. A known electrochemical
method uses hydrosulfite, from which additional, dye-reducing
reaction products arise, leading to a lowering of the hydrosulfite
use amount necessary for the vatting of the dye (E. H. Daruwalla,
Textile Asia, 165-169, September 1975). In addition, known from WO
90/15182 is a method in which an electrochemical vatting is carried
out with the aid of a mediator. The mediators are a matter of
reversible redox systems such as iron (II/III) complexes that
reduce the dye and are constantly regenerated at the cathode. Based
on the high use amounts and the ecological seriousness of such
mediators, there exists as before an acute environmental problem
that can only be solved through additional investments in an
adequate wastewater technology or through a recycling process. A
further disadvantage of this method is the perpetual additional
mediator feeds necessary for maintenance of the redox cycle in the
continuous dyeing technology. The additional dosing of the mediator
system results from liquor discharge proportional to the fabric- or
yarn-flow.
[0006] The mentioned problems led to a new solution approach that
in essence permitted a reduction-agent free vatting of the dye.
Here, it is a matter of an electrochemical reduction that,
proceeding from different start mechanisms, requires no additional
reduction agent during the continuous operational mode of the
reactor (WO 00/31334).
[0007] The mentioned electrochemical methods (WO 90/15182 and WO
00/31334) have a common disadvantage in the limited specific
reactor power, for the increase of which very large electrode
surfaces must be provided.
[0008] Known from DE 4310122-A1 as well as the patent specification
U.S. Pat. No. 1,247,927 of A. Brochet (1917) is a method in which
leucoindigo produced through catalytic hydrogenation is used for
the dyeing. While in this method the consumption of reduction agent
can be significantly reduced, the necessary dithionite leads once
again to a very poor eco-efficiency. Moreover, resulting from the
necessary gas-form hydrogen, in addition to a high risk of
explosion and fire, is a large equipment expense for pressure
vessels and compressors. A hydrogenation directly in the dyeing
operation is thus scarcely possible.
[0009] A current approach is described in WO 94/23114, in which a
leucoindigo produced through catalytic hydrogenation is used for
dyeing of cellulose-containing textiles material and the portion of
the leucoindigo in the dye liquor oxidized through air contact
during the dyeing is electrochemically reduced with the application
of a mediator system. The dyeing, after the absorption of the
leucoindigo into the textile material, takes place in a
conventional manner. Thus, this method is likewise afflicted with
the above-mentioned disadvantages of the mediator technique.
[0010] In addition, known from WO 01/46497 is a method for
electrochemical reduction of vat dyes, which method is based on the
principle of the so-called precoat-layer-cell method described in
EP 0808920-B1. Here, the dye, in the presence of a base, is brought
into contact with a cathode comprising a porous,
electrically-conducting carrier formed as a filter and an
electrically-conducting, cathodically polarized film formed on the
carrier in situ through deposition, and is electrochemically
reduced through application of a voltage. The catalytically active
electrode is stabilized through the loss of pressure at the film
formed through deposition.
[0011] The use, as in the prior art, of solubilizing agents
necessary for a quick vatting with high conversion factors and, in
particular, the application of ultrasound for generation of an
essentially homogeneous and fine-grained distribution of the
pigments would indeed lead to very large pressure losses and to
clogging of the electrode formed as a filter.
[0012] The electrocatalytic hydrogenation of nickel or similar
large-surface, conductive, catalytically active materials with low
hydrogen overvoltage represents a method long known and was
successfully used in the case of numerous organic compounds.
Platinum, nickel, palladium, and rhodium were used for the
hydrogenation of acetophenone (S. J. C. Cleghorn, D. Pletcher,
Electrochim. Acta 1993, 38, 425-430), palladium in the case of
alkenes (K. Junghaus, Chem. Ber. 1974, 107, 3191-3198) and
palladium as well as nickel for hydrogenation of nitrobenzene (S.
J. C. Cleghorn, D. Pletcher, Electrochim. Acta 1993, 38,
2683-2689). Nickel surfaces were very often used for reasons of low
costs and the relatively simple possibility of forming extremely
large surfaces (Raney nickel). This electrode type was successfully
applied in the electrocatalytic hydrogenation of unsaturated
hydrocarbons such as polycyclic compounds (D. Robin, et. al., Can.
J. Chem. 1990, 68, 1218-1227), phenols (A. Martel, et. al., Can. J.
Chem. 1997, 75, 1862-1867), ketones (P. Dabo, et. al.,
Electrochimica Acta 1997, 42, 1457-1459), nitro compounds (U.S.
Pat. No. 4,584,069), nitrites (U.S. Pat. No. 5,266,731 and WO
93/02230), imines, unsaturated fatty acids (WO 91/19774), and
carbohydrates such as glucose (K. Park, et. al., J. Electrochem.
Soc. 1985, 132, 1850-1855).
[0013] In this, the cathode was used in different configurations.
Conductive metal (plate or grid form) such as, for example, nickel
or V2A steel can be covered with a likewise metallic, porous film,
e.g. nickel black (A. Bryan, et. al., Electrochimica Acta, 1997,
42, 2101-2107), in which particles of Raney nickel-aluminum alloy
(U.S. Pat. No. 4,302,322) or Raney copper-aluminum alloy (U.S. Pat.
No. 4,584,069) can be embedded. In the case of application of
not-yet-active Raney catalyst, the activation must be carried out
through an appropriate pretreatment. In addition, it is possible to
use polytetrafluorethylene (PTFE) as a binder for the catalyst
particles (e.g. noble metal) on a metallic substrate.
[0014] The object of the present invention is to provide a
completely reduction-agent free vatting method for producing fully
reduced dye solutions, while avoiding the above-mentioned
disadvantages of known reduction methods. A further object of the
invention consists in specifying an apparatus for carrying out this
method.
[0015] The object is achieved through a method according to patent
claim 1 and an apparatus according to patent claim 15.
[0016] The method and the associated apparatus are described in the
following. In the drawing:
[0017] FIG. 1: shows a schematic representation of an apparatus for
continuous electrocatalytic vatting
[0018] Within the scope of the present invention, understood by the
term vat dyes are, in addition to indigoid dyes (of which indigo
itself is preferred), also anthraquinoids as well as sulfide dyes
and other vat dyes. They are referred to in the following as dye A,
this generally being present as dye pigment.
[0019] The method is based essentially on the electrocatalytic
hydrogenation of dye A to form reduced dye species P, for brevity
called species P, which represents the leuco form of dye A
(reaction equations I-IV). Involved in this are, on the one hand,
the formation of adsorbed hydrogen at the cathode (I) and, on the
other hand, the hydrogenation process known from catalytic
hydrogenation (II-IV):
H.sub.2O+e.sup.-.fwdarw.H.sub.ad+OH.sup.- (I)
A.fwdarw.A.sub.ad (II)
A.sub.ad+2H.sub.ad.fwdarw.P.sub.ad (III)
P.sub.ad.fwdarw.P (IV)
[0020] In order to achieve this hydrogenation, a voltage suitable
to hydrogen formation is applied to the electrodes present. Through
the proper selection of voltage ratios, the formation of molecular
hydrogen occurring as a side reaction can be minimized and the
hydrogenation process can be optimized. Here, in contrast to the
method described in WO 00/31334, it is not necessary to carry out a
start reaction, as the vatting can take place directly with the dye
A as the starting material.
[0021] The dye A, according to its individual characteristics, can
have 2 to n (n=maximum 6) reducible keto functions, whose enol
forms formed in the reaction can be present in more or less
deprotonated form according to their pKs values.
[0022] The radical species appearing during the hydrogenation under
certain conditions can be hydrogenated in an analogous manner.
However, in contrast to the method described in WO 00/31334, they
are not necessary for maintaining the reaction, since the dye A is
itself electrocatalytically hydrogenated directly a
[0023] The electrocatalytic hydrogenation is clearly
distinguishable from so-called electrochemical hydrogenation.
Namely, electrochemical hydrogenation relates to a process that
takes place at an electrode with low or no catalytic hydrogenation
activity, a small surface, and a large hydrogen overvoltage, with
electrons being transferred directly onto the substrate. The
electrocatalytic hydrogenation, on the other hand, is carried out
using a conductive, catalytically active electrode with a large
surface and a small hydrogen overvoltage, which simultaneously acts
as the electrode for the electrochemical generation of adsorbed
hydrogen atoms and as hydrogenation catalyst for the reduction of
the dye. In the ideal case, the arising hydrogen is not formed in
molecular form, and thus is not desorbed by the cathode and
readsorbed at the catalyst, but rather the reaction proceeds
simultaneously with the generation of the adsorbed hydrogen atoms
at the same cathode surface.
[0024] The method according to the invention is also
distinguishable from a process in which electrochemically produced,
gaseous hydrogen is used for catalytic hydrogenation of organic
substances. This catalytic hydrogenation, in contrast to the
present invention, requires two separate reaction steps--first the
electrochemical hydrogen production, then the purely chemical,
catalytic hydrogenation--in spatially separated reactors.
[0025] The dye hydrogenation takes place in an oxygen-free,
electrochemical reaction cell. Different cell connections enable
both continuous and batch operation of the electrolysis
apparatus.
[0026] Dye A, in an aqueous suspension containing various
additives, is placed on the cathode side into an electrolysis
vessel or into a catholyte tank. The alkaline pH value required for
dye hydrogenation lies in the range of pH 9 to 14, preferably
12-13, and is adjusted using alkali hydroxide, in particular
caustic soda solutions. The acidic or alkaline anolyte spatially
separated by a separator (e.g. membrane or diaphragm) consists
preferably of an aqueous solution of sulfuric acid or alkali
hydroxide.
[0027] As additions or additives, the following dye-affined
solubilizing or dispersing agents are used:
[0028] alcohols, as for example methanol, ethanol, isopropanol,
with methanol and isopropanol being especially preferable;
[0029] acetals, as for example glycol ether, propylene glycol,
ethylene glycol monomethyl, ethyl, or -butyl ether, diethylene
glycol monomethyl or -ethyl ether;
[0030] pyridines, as for example pyridine and .alpha.-, .beta.-, or
.gamma.-picoline;
[0031] lactams, as for example pyrrolidone, N-methylpyrrolidone,
and 1,5-dimethylpyrrolidone;
[0032] acids and acid amides, as for example benzene sulfonic
acids;
[0033] naphthaline sulfonic acid derivatives, as for example
Setamol WS (naphthaline sulfonate condensed with formaldehyde);
[0034] N,N-dimethylformamide and acetamide.
[0035] These additives are applied in amounts of approximately 0.1
to 90%, preferably 1 to 30%, relative to the dye mass used. For
promotion of the solubilization or dispersion through the described
additives, the use of ultrasound as a dispersion aid have proved
effected. In this case, during or before the hydrogenation of the
dye, the suspension is acted on with ultrasound energy.
[0036] According to the invention, ionic or non-ionic surfactants
as well as protic and aprotic solvents (as previously described)
are also used as additives, which have both a dye affinity and an
electrode affinity and do not themselves act in a reducing manner.
Typical representatives of these substances are alcohol
propoxylates, as for example Lavotan SFJ, alcohol sulfates, as for
example Sandopan WT, Subitol MLF, and alkyl sulfonates such as
Levapon ML. The application quantities of these additives lie in
the range of 0.1 to 10 g/l, and preferable concentrations lie
between 1 and 5 g/l.
[0037] In general, also used are auxiliary substances for adjusting
the conductivity of the electrolyte solutions. Used in this context
as auxiliary substances are salts of metal cations such as sodium,
potassium, or tetraalkylammonium ions, as for example
tetramethylammonium and anions such as halide ions, sulfates or
sulfonates, as for example toluolsulfonate. The content of these
lies between approximately 0.1 and 10% by weight, preferably 1-5%
by weight.
[0038] As cathode material, in principle any catalytically active
material that is constantly in the alkaline region (pH 9 to 14),
electrically conductive, large surfaced, and has a low hydrogen
overvoltage can be used. Examples are metals such as Raney copper,
Raney cobalt, Raney molybdenum, platinum black, ruthenium black,
and palladium black, or corresponding active Raney alloys (e.g.
Raney nickel-molybdenum and nickel-molybdenum), with Raney nickel
being preferably applied. These are applied in conventional manner
to different electrically conductive substrate materials and
configured as an electrode. Examples of suitable substrates are
metals such as nickel, V2A steel, or carbon, which are used in the
form of porous, perforated materials such as mesh, expanded sheet
metal, grids, and smooth sheet metal.
[0039] In place of the conventional planar or structured
electrodes, a heap of conductive particles can also serve as the
substrate. In this case, the current is fed via a contact
electrode. This particle heap is located in a current channel and
is flowed through by the electrolytes, whereby the carrying away or
evacuation of particles is avoided. In this configuration also, the
catalytically active film is permanently fixed on the substrate
particles. The particle heap is, for example, flowed through
upwardly from the bottom. If here the rate of oncoming flow exceeds
the so-called loosening speed, then a fluidized bed is present,
while at lower speeds the electrode operates as a fixed bed.
[0040] Unexpectedly, with a cathode consisting of graphite
granulate material alone, a similarly advantageous electrochemical
reduction behavior has occurred.
[0041] The selection of the anode material is not critical, but is
dependent on the solvent of the anolytes. Possible materials are,
for example, graphite, iron, nickel, platinum, titanium coated with
platinum, and titanium coated with ruthenium oxide.
[0042] The voltage applied to the electrodes is a function of the
hydrogen overvoltage of the respective electrode material and
depends further on the reaction medium. Normally, cell voltages
between 1 and 5 V, preferably between 2 and 3 V, are applied. The
current density amounts to 50-10,000 A/m.sup.2, preferably
100-2,000 A/m.sup.2. In addition to the use of a constant current,
it is also possible to use pulsing currents.
[0043] The process is carried out in conventional manner at
atmospheric pressure and temperatures between 20 and 100.degree.
C., preferably between 25 and 60.degree. C.
[0044] The electrocatalytic hydrogenation can be carried out both
in a batch reactor and in a continuous reactor, the structure of
which is substantially simpler and cheaper compared to normal
hydrogenation reactors.
[0045] However, the electrochemical reaction cell can also be
formed as a pressure vessel and operated with pressures of 1-10
bar, preferably 1-6 bar. The pressure drop over the electrochemical
reaction cell remains essentially constant when measured over time.
There occurs no clogging, thus eliminating the necessity of
backflushing by means of flow reversal.
[0046] With the method according to the invention, unexpected
advantages in the field of the dyeing of textile materials with vat
dyes and sulfur dyes, especial indigo, are attained.
[0047] The great advantage of this reaction conducting lies in the
minimal addition of chemicals to be activated. In an oxygen-free
reaction cell, only the vat dye to be colored, the alkali necessary
for the pH adjustment, an appropriate electrical voltage for
maintaining the reaction, and possible small amounts of additives
are necessary. Through the elimination of the addition of the
reduction agent, there remains to be activated only the amount of
caustic soda that is necessary for the conversion of the
hydrogenated dye A into its water-soluble enolate form. Compared to
the current practice, this corresponds to a 60-70% reduction of the
consumption of caustic soda and a corresponding salt unloading of
the effluent, whereby a direct, partial recycling becomes possible
for the first time.
[0048] The described vatting technique also permits a renewed
reaction start after longer stand-still times, without requiring
any addition of reduction agents. The controlling of the formation
of hydrogen on the catalyst surface by the flowing current or the
applied voltage enables an avoidance of an over-reduction of the
dye, as very often occurs in the case of hydrosulfite and thiourea
dioxide as reduction agents. Due to the largely salt-free
condition, dye concentrations up to 200 g/l, but preferably 80-120
g/l, can be achieved in the primary vat.
[0049] The high dye solubility is of special importance, since
through concentrated primary vat dye liquors, color over runs in
the color bands can be prevented. This reduction technique further
leads to a largely salt-free coloring, whereby a stronger
reproducibility and better fabric and/or yarn quality can be
automatically assured. It is in this way that the warp threads that
are dyed with these solutions distinguish themselves through good
friction qualities as well as a high weaving yield factor. Other
advantages are the high degree of stability of the reduced primary
vat dye liquor in the acid-free electrolysis tank, the strong dye
solubility of the vatted species, the continuous dye reduction and
therewith the "Just in Time" production of the dye solution.
[0050] The electrocatalytic hydrogenation in accordance with the
invention is suitable for dye starters as well as for dye liquors.
The enormous economic advantages lie in the reduction of the use of
chemicals (reduction agents and caustic soda), the production of a
better quality product and lower waste water costs due to the now
available biocompatability of the remaining substances contained in
the waste water. On the waste water side, no toxic loading occurs,
wherewith recycling of the waste water becomes possible with
considerably less expense, as compared to conventional dyeing
systems.
[0051] Furthermore, resulting through the method in accordance with
the invention, compared to known catalytic hydrogenation, are the
following advantages: (1) the kinetic barrier of splitting the
hydrogen molecule is completely circumvented; (2) transport
suppression of the lesser soluble hydrogen is likewise
circumvented; (3) hydrogen and the catalyst material are employed
essentially more efficiently, wherewith a lesser loading of the
reactor with active catalyst is needed; (4) only small amounts of
gaseous hydrogen are released and the danger of explosion and fire
is minimized; (5) formation of hydrogen on the catalyst surface can
be regulated and controlled much more easily by the amount of
current flowing or the applied voltage, which leads to improved
product selectivity and possibly avoids so-called over reduction of
the dye; (6) the operating temperature is low; (7) no pressure
vessels or compressors are needed for transport of the gaseous
hydrogen.
[0052] It is possible, but not necessary, right at the beginning of
the process to execute a pre-reduction by a one-time addition of
reduction agent, in order to shorten the reaction time. Resulting
by this means is the combining of the method in accordance with the
invention and a known method, in which a shorter reaction time
conflicts above all with a need for a reduction agent having
appropriate pollutants for the waste disposal. Used as reduction
agents are the following compounds:
[0053] dithonite and its derivatives, e.g. such as formalde
hydesulfoxylate (e.g. RONGALIT C, BASF),
[0054] thiourea dioxide,
[0055] glucose,
[0056] hydroxyketone, e.g. such as monohydroxyacetone,
dihydroxyacetone,
[0057] hydroxyaldehyde, e.g. such as glycolaldehyde,
triose-reducton (2,3-dyhydroxyacrylaldehyde or
[0058] reductin acid (cyclopendentiol, -one).
[0059] FIG. 1 shows in a schematic representation an apparatus for
continuous, electrocatalytic dye hydrogenation.
[0060] A catholyte tank 1 with cover 1', tightly closed with
gaskets 2, is a component of a first circuit with first lines 13,
13' and 13", a first pump P1, and a supply pipe 4 that leads back
via the cover 1' into the catholyte tank 1. The dye suspension,
with the alkali and the selected additives, located in the
catholyte tank 1 is driven through the circuit in a circulation
stream V1 by means of the pump P1, in order to prevent
sedimentation in the catholyte tank. After pump P1, branched off is
a second circuit with a second, time-measured, essentially constant
flow volume V2, consisting of second lines 17, 17' and 17", a
second pump P2, a steel tube coil 3, an electrochemical reaction
cell 7 and a second feed pipe 6 that likewise leads back into the
catholyte tank 1 via the cover 1'. The steel tube coil 3 is located
on an ultrasonic vibrator 5. The energy fed in over the ultra-sonic
vibrator 5 amounts to 100-1000 watts and serves for dye dispersion,
whereby the steel tube coil 3 with the ultrasonic vibrator 5 act as
a dispersion aid.
[0061] Further provided is a third circuit for the anode,
consisting of an anolyte tank 31 with cover 31', tightly sealed by
gaskets 32, with third lines 18, 18' and 18", with a third pump P3
and a third feed tube 19 that leads back into the anolyte tank 31
via the cover 31'.
[0062] Located in the electrochemical reaction cell 7, separated by
a membrane 9, is an electrode pair consisting of a cathode 8 and an
anode 8', to which is applied an electric cell voltage of about 2
to 3 V.
[0063] As a rule, it is a matter here of a normal direct-current
voltage; however, pulsing direct-current voltages are also
used.
[0064] As soon as a current flows through the electrodes 8, 8', the
process according to reaction equations I-IV begins to run, i.e.
the dye A is hydrogenated electrocatalytically as described.
[0065] This condition is maintained until the entire amount of dye
produced is completely reduced, the apparatus described up to here
sufficing for batch hydrogenation.
[0066] With the following described equipment supplements, the
apparatus is expanded to a continuous hydrogenation apparatus.
[0067] Static reaction conditions set in when a first flow volume
V4 of the suspended dye A is fed in, and a second flow volume V5 of
hydrogenated dye is carried off.
[0068] Additionally, from a supply tank 11 with cover 11' a dye
suspension equal to the one originally supplied is introduced, by
means of a fourth pump P4 in a first flow volume V4, via fourth
lines 14, 14' into the first line 13 and therewith supplied to the
circulating stream V1.
[0069] At the same time, taken from the catholyte tank 1 is a
second flow volume V5 corresponding to the first flow volume V4,
and is dosed by means of a fifth pump P5 via fifth lines 15, 15'
and a fourth supply tube 16, into an acid-free supply tank 21 that
is tightly closed with the cover 21' and gaskets 22.
[0070] The reduction-agent-free, electrocatalytically dye
hydrogenation carried out in this way corresponds to the principles
of performing continuous reaction in an ideally mixed agitator
vessel.
[0071] The described apparatus is suitable for laboratory operation
and can be operated with different sizes of electrochemical
reaction cells. In particular, the arrangement presented and the
method of construction are suitable for a "scale up" process, up to
electrochemical cells and catholyte tanks on an industrial scale,
the upward sizes of which are hardly measurable. Thus catholyte
tanks with volumes from 8-500 liters are normal.
[0072] The present invention will be explained in detail through
the following examples 1-10, without laying claim to having fully
described the technical potential in accord with the invention.
[0073] Example 1 describes an electrocatalytic hydrogenation of
indigo in a batch reactor, as well as the construction and
activation of electrodes for electrocatalytic hydrogenation.
[0074] Construction of the Electrodes:
[0075] A netting made of stainless steel (square mesh, 250 .mu.m
mesh width) having outside dimensions of 4.times.10 cm is first
cleaned in aqueous lye (NaOH 30 g/l) at 50.degree. C., and
afterwards, during a 15-minute electroplating step, is coated with
a layer of nickel. The nickel bath, at 50.degree. C., displays the
following composition: 300 g/l NiSO.sub.4.6H.sub.2O; 45 g/l
NiCl.sub.2.6H.sub.2O; 30 g/l H.sub.3BO.sub.3. A nickel sheet is
used as anode. Current density amounts to about 1 A/dm.sup.2. Next
follows a second electroplating treatment, during which time
processing is done in a suspension of Raney nickel-aluminum alloy
(10 g/l). This nickel plating is carried out at 50.degree. C. and a
current density of 5 A/dm.sup.2.
[0076] Activation of the Electrodes
[0077] In order to optimize the electrocatalytic characteristics,
the electrode must be activated at 70.degree. C. for approximately
10 hours in 20% concentration of caustic soda. Following that is
washing process with deionized water.
[0078] Electrocatalytic Hydrogenation:
[0079] The activated electrodes 8, 8' are built into an
electrochemical batch reactor 7 (H-cell) in which the anode and
cathode spaces are separated by a membrane (diaphragm) 9 (Nafion
324, DuPont).
[0080] O.1 g of indigo are dispersed in 95 ml of water and 5 ml of
methanol, which at the same time contains 4.0 g of caustic soda and
2 g of Setamol WS as a dispersion agent, and poured on the cathode
side into the electrolysis vessel 7, thermostatically maintained at
50.degree. C. After about 2 hours of degassing of the reaction
mixture with nitrogen (99%), applied is a cathode potential of
-1200 mV vs. Ag/AgCl in 3 M KCL solution. Serving as anolyte is a
mixture of 95 ml water and 5 ml methanol that contains 4.0 g of
caustic soda. The working current amounts to about 0.3 A. These
conditions are maintained for 10 hours in order to completely
hydrogenate dye A.
[0081] Produced with 20 ml of this primary vat dye mixture is a dye
solution whose dye concentration amounts to 0.1 g/l. Dyeing ensues,
under exclusion of oxygen, with 10 g of cotton fabric, at a
temperature of 30.degree. C. during 10 minutes. After ending the
dyeing time, the sample is oxidized in air, rinsed and finally
washed at 50.degree. C.
[0082] The thusly produced sample displays a brilliant blue tone,
the color depth is identical to that of a sample produced based on
the conventional dyeing methods with sodium hydrosulfite.
[0083] Example 2 describes an electrocatalytic hydrogenation in a
flowthrough reactor, of filter press manner of construction, in
batch operation.
[0084] The reactor 7 (Electro MP-cell, Electrocell AB, Sweden)
consists of two anodes 8' (nickel sheet) that are located on both
sides of the centrally placed cathode 8. This latter also consists
of nickel sheet to which are spot welded, on both sides, several
layers of Raney nickel electrodes known from example 1, with
outside geometrical measurements of 10.times.10 cm. The entire
geometric cathode surface amounts to 1 m.sup.2. Catholyte and
anolyte flow through the respective electrode spaces, vertically
from bottom to top, with a flow volume of 0.6 l/min. Used as
diaphragms is a commercially-available Nafion membrane 9 (Nafion
324, DuPont).
[0085] Dispersed in the catholyte tank 1 are 20 g of indigo in 2
liters of water, which at the same time contain 80 g of caustic
soda and 4 g of Setanol WS (BASF) as a dispersion agent. Placed on
the anode side are 2 liters of water containing 80 g of caustic
soda. Hydrogenation of the dye suspension is obtained at 30.degree.
C. in the reactor following appropriate degassing with nitrogen
(99%) by simple application of a cathode potential of -1200 mV vs.
Ag/AgCl in 3 M KCl solution. The working current amounts to about 3
A. These conditions are maintained for 45 minutes, in order to
completely hydrogenate the dye A.
[0086] The colorings produced with this solution correspond in all
criteria (color depth and fastness) to those obtained from
conventionally produced vat dye liquors.
[0087] Example 3 describes a continuous electrocatalytic
hydrogenation in a flowthrough reactor of a filter press manner of
construction. In analogous manner of example 2, batch hydrogenation
is carried out in a first step.
[0088] Dispersed in the catholyte tank are 35 g of C.I. Vat Green 1
in 2 liters of water, which at the same time contains 80 g of
caustic soda and 4 g of Setamol WS (BASF) as a dispersion agent.
Placed on the anode side are 2 liters of water containing 80 g of
caustic soda. Hydrogenation of the dye suspension is obtained at
30.degree. C. in the reactor after appropriate degassing with
nitrogen (99%) by simple application of a cathode potential of 1200
mV vs. Ag/AgCl in 3 M KCL solution. The working current amounts to
about 3 A. These conditions are maintained during 45 minutes in
order to completely hydrogenate dye A.
[0089] Next, conveyed from the supply tank 11 is a 1.75% dye
suspension into circulation stream VI by means of the fourth pump
P4, with a first flow volume V4 of 10 m/min. The indigo suspension
in the supply tank 11 has the same composition as described at the
beginning. In parallel fashion, from the catholyte tank a second
flow volume V5 of 10 ml/min, corresponding to the color inflow,
i.e. flow volume V4, is taken and dosed into the acid-free storage
tank 21 by means of the fifth pump P5.
[0090] The operating condition is maintained during another 24
hours, in order thereby to demonstrate continuous electrocatalytic
hydrogenation. The reduction rates analyzed within this time showed
values >95%. The colorings produced with this solution
correspond in all criteria (color depth and fastness) to those
obtained from conventionally produced vat dye liquors.
[0091] Example 4 describes a continuous electrocatalytic
hydrogenation, on an industrial scale, in a flowthrough reactor of
a filter press manner of construction.
[0092] The reactor 7 consists of ten parallel-connected reaction
cells (Electro Prod-Cell, Electro-cell AB, Sweden), of a filter
press manner of construction, each containing two anodes 8' (nickel
sheet) that are located on the two sides of the centrally placed
cathode 8. This latter likewise consists of a nickel plate on which
are spot-welded, on both sides, several layers of the Raney nickel
electrodes described in example 1, with the external geometric
measurements of 60.times.60 cm. The entire outer geometric cathode
surface of the reactor amounts to 120 m.sup.2. The catholyte flows
through the respective reaction cells vertically bottom to top with
a flow volume V1 of 20 l/min. Used as diaphragms between the
individual cells is a commercially-available Nafion membrane
(Nafion 324, DuPont). In a manner analogous to example 2 and 3,
batch hydrogenation is carried out in a first step.
[0093] Dispersed in the catholyte tank 1 are 20 kg of indigo in a
mixture of 190 liters of water and 10 liters of methanol, which at
the same time contains 8 kg of caustic soda and 800 g of Setamol WS
(BASF) as a dispersion agent. Placed on the anode side are 200
liters of water containing 8 kg of caustic soda. Hydrogenation of
the dye suspension is obtained at 60.degree. C. in the reactor
after appropriate degassing with nitrogen by simple application of
a cathode potential of -1200 mV vs. Ag/AgCl in 3 M KCl solution.
These conditions are maintained during 24 hours in order to
completely hydrogenate dye A.
[0094] Next, conveyed from the supply tank 11 into the circulation
stream V1 by means of pump P4, with a flow volume of V4 of 2.5
l/min, is a 100 g/l indigo suspension. The indigo suspension in the
supply tank 11 has the same composition as was described at the
beginning. In parallel fashion, taken from the catholyte tank 1 is
a flow volume V5 of 2.5 l/min corresponding to the dye inflow V4,
and dosed into the acid-free storage tank 21 by means of the pump
P5. The indigo primary vat dye liquor that is available in the
storage tank is used with a flow volume of 1.75l/min in the dye bed
for coloring a warp thread. The warp thread, weighing 250 g/Lm, is
continuously dyed at a speed of 35 m/min during 8 hours. Based on
the general condition of the warp dyeing machine and the supplied
primary vat dye flow volume of 1.75 l/min, there results a 2%
coloration (referenced to the weight of the warp thread).
[0095] This operating condition is maintained during another 24
hours of operation, in order to demonstrate the continuous
electrocatalytic hydrogenation. The reduction rates analyzed within
this time show values >95%. The colorings produced with this
solution correspond in all criteria (color depth and fastness) to
those obtained from conventionally produced vat dye liquors. The
thusly dyed warp threads are characterized, as a result of the low
salt load, by good friction qualities as well as by a high weaving
yield factor.
[0096] Example 5 describes an electrocatalytic hydrogenation in the
fixed bed reactor in batch operation.
[0097] The reactor 7 consists of a cathode 8 that is structured as
a bed electrode. Serving as electrode material are 50 g nickel
spheres (balls) 1 mm in diameter that were coated beforehand by
electroplating with a layer of platinum black. Underneath is a
platinum netting as a contact electrode. The spheres are located in
a glass flow channel (cross section 7 cm.sup.2) between two sieves
(mesh width 0.5 mm). Located in the anode room spatially separated
by a membrane 9 (Nafion 324, DuPont) is the anode 8' (DeNora DSA)
with an electrode area of 20 cm.sup.2. Serving as anolyte is 2%
sulfuric acid which is not circulated.
[0098] Dispersed in the catholyte tank are 0.1 g of indigo in 49 ml
of water and 1 ml of isopropanol, which at the same time contains
10 g of caustic soda. Hydrogenation of the dye suspension is
obtained at 50.degree. C. in the reactor after appropriate
degassing with nitrogen (99%) by simple application of a cathode
potential of -1000 mV vs. Ag/AgCl in 3M KCl solution. The working
current amounts to 0.4 A. The catholyte flows through the reactor
at 30 l/h vertically from bottom to top. These conditions are
maintained during 6 hours, in order to completely hydrogenate dye
A.
[0099] Example 6 describes an electrocatalytic hydrogenation in the
fluidized bed reactor in batch operation.
[0100] The reactor 7 consists of a cathode 8 that is structured as
a bed electrode. Serving as electrode material are 50 g nickel
spheres (balls) 1 mm in diameter that were coated beforehand by
electroplating with a layer of platinum black. Underneath is a
platinum netting as a contact electrode. The spheres are located in
a glass flow channel (cross section 7 cm.sup.2) between two sieves
(mesh width 0.5 mm). However, above the bed there is enough
distance to not impede expansion of the eddy layer. Located in the
spacious anode room separated by a membrane 9 (Nafion 324, DuPont)
is the anode 8' (DeNora DSA) with an electrode surface of 20
cm.sup.2. Serving as anolyte is 2% sulfuric acid which is not
stirred up.
[0101] Dispersed in the catholyte tank are 0.1 g of indigo in 49 ml
of water and 1 ml of isopropanol, which at the same time contains
10 g of caustic soda. Hydrogenation of the dye suspension is
obtained at 50.degree. C. in the reactor after appropriate
degassing with nitrogen (99%) by simple application of a cathode
potential of -1000 mV vs. Ag/AgCl in 3M KCl solution. The working
current amounts to 0.6 A. The catholyte flows through the reactor
at 110 l/h vertically from bottom to top. These conditions are
maintained during 5 hours, in order to completely hydrogenate dye
A.
[0102] Example 7 describes an electrocatalytic hydrogenation of a
rotating bed electrode in batch operation.
[0103] The reactor 7 consists of a cathode 8 that is structured as
a bed electrode. Serving as electrode material are 250 g nickel
spheres 2 mm in diameter that were coated, as described above in
Example 1, by electroplating with a layer of nickel, in which were
embedded Raney nickel particles. The spheres are located in a
circularly constructed, rotating fixed-bed basket (mesh width 1
mm). The electrolyte is suctioned axially by the self- pumping
action of the reactor and flows through the fixed bed radially
outwardly. Supplying of current to the inside of the electrode is
done through sliding contacts. The fixed-bed basket displays an
external diameter of 3.5 cm, an internal diameter of 2.5 cm and a
height of 4 cm. It is driven at 1800 rpm. Located in the spacious
anode room separated by a membrane 9 (Nafion 324, DuPont) is the
anode 8' (DeNora DSA) with an electrode area of 20 cm.sup.2.
Serving as an anolyte is 1.5% sulfuric acid.
[0104] In the catholyte tank are dispersed 1 g indigo in a solution
of 490 ml of water, 5 ml methanol and 5 ml of ethanol, the solution
at the same time containing 10 gm of caustic soda. The
hydrogenation of the dye suspension is accomplished at 55.degree.
C. in the reactor after appropriate degassing with nitrogen (99%)
through the simple application of a cathode potential of -1000 mv
vs. Ag/AgCl in 3 M KCl solution. The working current amounts to 1.2
A. The catholyte flows through the reactor with a flow volume of 40
l/h. These conditions are maintained for 12 h, in order to
hydrogenate completely the dye A.
[0105] Example 8 likewise describes an electrolytic hydrogenation
in the fixed bed reactor in batch operation. The applied
graphite-type electrode material is activated through the
introduction of platinum.
[0106] Production of the Electrodes:
[0107] Serving as electrode material are 40 g of graphite granules
(material 00514, enViro-Cell Umwelttechnik GmbH, Oberursel,
Germany) of 2-4 mm diameter. This is overlaid galvanically with a
layer of platinum. Used as an electrolyte for this is 0.1 molar
sulfuric acid 2% hexachloroplatinate solution (H.sub.2PtCl.sub.6).
A platinum sheet serves as anode; the granulate is contacted by
means of platinum wire and cathodically polarized for 15 Minutes (1
A current flow).
[0108] Electrolytic Hydrogenation:
[0109] The reactor 7 consists of a cathode 8, which is constructed
as a bed electrode. Serving as electrode material are 40 g of the
modified graphite granules. Serving as contact electrode is a
centrally arranged platinum wire. The balls are located on a
perforated glass plate in a flow channel of glass (cross-section of
7 cm.sup.2). In the anode space, separated spatially by a membrane
9 (Nafion 324, DuPont), is located anode 8' (DeNora DSA; electrode
area 20 cm.sup.2). Serving as anolyte is caustic soda with a
concentration of 40 g/l.
[0110] Dispersed in the catholyte tank 1 are 2 g of indigo in 2000
ml of water, the latter containing at the same time 80 g of caustic
soda. The hydrogenation of the dye suspension is achieved at
50.degree. C. in the reactor after appropriate degassing with
nitrogen (99%) through the simple application of a cathode
potential of -1100 mV vs. Ag/AgCl in 3 M KCl solution. The working
current is 5.5 mA. The catholyte flows vertically through the
reactor from below to above at 1.23 l/h . These conditions are
maintained for 2.5 h in order to completely hydrogenate the dye
A.
[0111] Example 9 describes an electrocatalytic hydrogenation in the
flow-through reactor constructed in the manner of a filter press.
In contrast to examples 2, 3 and 4, used as a electrode here is
paladium on aluminum oxide, built into a float-like glass-carbon
structure.
[0112] Production of the Electrode:
[0113] The plate shaped, commercially available RVC material (100
ppi, reticulated vitreous carbon, ERG Materials and Aerospace
Corporation, Oakland, USA) with outer geometric dimensions of
10.times.10.times.0.5 cm is contacted by a copper wire and wetted
for 1 hour with 1 l phosphate buffer solution (1 M potassium
dihydrogen phosphate (KH.sub.2PO.sub.4) and 1 M caustic soda NaOH),
adjusted to a pH of 7). Added afterwards is 7 g of
Pd/Al.sub.2O.sub.3 (5 w/o Pd) catalyst and the suspension is
stirred moderately (450 rpm) for approximately 2 h at 50.degree. C.
During this time the fluidized carbon as cathode is polarized with
a constant current of 20 mA. With the passage of time the clouding
of the suspension disappears through the incorporation of the metal
particles into the network of the carbon.
[0114] Electrolytic Hydrogenation:
[0115] The reactor 7 (Electro MP-Cell, Electrocell AB, Sweden)
consists of two anodes 8' (nickel sheet), which are located on
either side of the centrally placed cathode 8. The latter consists
likewise of a nickel sheet on which at both sides is placed a piece
of RVC material with the outer geometric dimensions of 10.times.10
cm. Catholyte and anolyte flow through the respective electrode
spaces in each case vertically from bottom to top with a volume
flow of 1.2 l/m. Used as a diaphragm is a commercially available
Nafion membrane 9 (Nafion 324, DuPont). Dispersed in the catholyte
tank are 20 g indigo in 2 l water, which at the same time contains
as a dispersing agents 80 gm NaOH and 4 g Setamol WS (BASF).
Provided at the anode side are 2 liters of water that contain 80 g
of caustic soda. The hydrogenation of the dye suspension is
achieved at 50.degree. C. in the reactor after appropriate
degassing with nitrogen (99%) through simple application of a
cathode potential of -1100 mV vs. Ag/Ag/Cl in 3 M KCl solution.
These conditions are maintained for 60 minutes in order to
completely hydrogenate the dye.
[0116] Example 10 describes a further vatting in the fixed bed
reactor in batch operation. Reactor 7 consists of a cathode 8,
which is designed as a bed electrode. Serving as electrode material
is 40 g of graphite granules (material 00514, enViro-cell
Umwelttechnik GmbH, Oberursel, Germany) of 2-4 mm diameter. Serving
as contact electrode is a centrally arranged platinum wire. The
spheres are located in a flow channel of glass (diameter 7
cm.sup.2) on a perforated glass plate. In the anode space separated
spatially by a membrane 9 (Nafion 324, DuPont) is located anode 8'
(DeNora DSA; electrode area 20 cm.sup.2). Serving as anolyte is
caustic soda at a concentration of 40 g/l.
[0117] In the catholyte tank 1 0.4 g indigo is dispersed in 2000 l
of water that at the same time contains 80 g of caustic soda. The
hydrogenation of the dye suspension is achieved at 50.degree. C. in
the reactor after appropriate degassing with nitrogen (99%) through
simple application of a cathode potential of -1100 mV vs. Ag/Ag/Cl
in 3 M KCl solution. The working current is 7 mA. The catholyte
flows vertically through the reactor from below to above at 1.23
l/h. These conditions are maintained for 5 hours in order to
completely hydrogenate the dye A.
[0118] The following aspects thus arise as fundamental to the
invention:
[0119] No environmentally relevant problem materials are
introduced.
[0120] The method functions without any addition of reducing
agents.
[0121] Beyond dye, caustic soda and, at most, small quantities of
additives, no other chemicals active in the redox process are
introduced.
[0122] The addition of caustic soda serves merely the adjustment of
the ph value, whereby there results a 60-70% diminution of caustic
soda referenced to known methods.
[0123] Regarding waste water, there arises no toxic loading and
greatly reduced salt load, whereby a recycling of the waste water
becomes possible with substantially reduced expense compared to
conventional dye systems.
[0124] The recovery of materials relevant to cost and the
environmental considerations is obviated (e.g., mediator
system).
[0125] No pressure vessels or compressor are necessary for
transport of the gaseous hydrogen, whereby the fire and explosion
danger is minimized.
[0126] Through a combination of ultrasound and electrocatalytic dye
hydrogenation, substantially higher reaction times with, at the
same time, a minimization of additives can be achieved.
[0127] Side reactions as, for example, dye precipitation, mud
formation and corrosion as with the use of mediators cannot
arise.
[0128] For a rapid vatting with high exchange rate the necessary
use of solubilizing materials and especially the introduction of
ultrasound do not lead to pressure loss through clogging of the
electrodes, whereby a backflushing through flow reversal is
obviated.
* * * * *