Patent Publication Number: US-2005121336-A1

Title: Method and apparatus for electro-catalytical hydrogenation of vat dyes and sulphide dyes

Description:
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 .  
      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.  
      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&gt;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.  
      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.  
      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.  
      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).  
      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.  
      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.  
      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.  
      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.  
      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.  
      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).  
      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.  
      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.  
      The object is achieved through a method according to patent claim  1  and an apparatus according to patent claim  15 . 
    
    
      The method and the associated apparatus are described in the following. In the drawing:  
       FIG. 1 : shows a schematic representation of an apparatus for continuous electrocatalytic vatting 
    
    
      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.  
      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 2 O +e   − →H ad +OH −   (I)
 
A→A ad   (II)
 
 A   ad +2H ad →P ad   (III)
 
P ad →P  (IV) 
 
      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.  
      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.  
      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  
      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.  
      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.  
      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.  
      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.  
      As additions or additives, the following dye-affined solubilizing or dispersing agents are used: 
          alcohols, as for example methanol, ethanol, isopropanol, with methanol and isopropanol being especially preferable;     acetals, as for example glycol ether, propylene glycol, ethylene glycol monomethyl, ethyl, or -butyl ether, diethylene glycol monomethyl or -ethyl ether;     pyridines, as for example pyridine and α-, β-, or γ-picoline;     lactams, as for example pyrrolidone, N-methylpyrrolidone, and 1,5-dimethylpyrrolidone;     acids and acid amides, as for example benzene sulfonic acids;     naphthaline sulfonic acid derivatives, as for example Setamol WS (naphthaline sulfonate condensed with formaldehyde);     N,N-dimethylformamide and acetamide.        

      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.  
      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.  
      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.  
      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.  
      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.  
      Unexpectedly, with a cathode consisting of graphite granulate material alone, a similarly advantageous electrochemical reduction behavior has occurred.  
      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.  
      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 2 , preferably 100-2,000 A/m 2 . In addition to the use of a constant current, it is also possible to use pulsing currents.  
      The process is carried out in conventional manner at atmospheric pressure and temperatures between 20 and 100° C., preferably between 25 and 60° C.  
      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.  
      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.  
      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.  
      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.  
      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.  
      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.  
      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.  
      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.  
      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: 
          dithonite and its derivatives, e.g. such as formalde hydesulfoxylate (e.g. RONGALIT C, BASF),     thiourea dioxide,     glucose,     hydroxyketone, e.g. such as monohydroxyacetone, dihydroxyacetone,     hydroxyaldehyde, e.g. such as glycolaldehyde, triose-reducton (2,3-dyhydroxyacrylaldehyde or     reductin acid (cyclopendentiol, -one).        

       FIG. 1  shows in a schematic representation an apparatus for continuous, electrocatalytic dye hydrogenation.  
      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 P 1 , 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 V 1  by means of the pump P 1 , in order to prevent sedimentation in the catholyte tank. After pump P 1 , branched off is a second circuit with a second, time-measured, essentially constant flow volume V 2 , consisting of second lines  17 ,  17 ′ and  17 ″, a second pump P 2 , 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.  
      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 P 3  and a third feed tube  19  that leads back into the anolyte tank  31  via the cover  31 ′.  
      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.  
      As a rule, it is a matter here of a normal direct-current voltage; however, pulsing direct-current voltages are also used.  
      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.  
      This condition is maintained until the entire amount of dye produced is completely reduced, the apparatus described up to here sufficing for batch hydrogenation.  
      With the following described equipment supplements, the apparatus is expanded to a continuous hydrogenation apparatus.  
      Static reaction conditions set in when a first flow volume V 4  of the suspended dye A is fed in, and a second flow volume V 5  of hydrogenated dye is carried off.  
      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 P 4  in a first flow volume V 4 , via fourth lines  14 ,  14 ′ into the first line  13  and therewith supplied to the circulating stream V 1 .  
      At the same time, taken from the catholyte tank  1  is a second flow volume V 5  corresponding to the first flow volume V 4 , and is dosed by means of a fifth pump P 5  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 .  
      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.  
      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.  
      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.  
      Example 1 describes an electrocatalytic hydrogenation of indigo in a batch reactor, as well as the construction and activation of electrodes for electrocatalytic hydrogenation.  
      Construction of the Electrodes:  
      A netting made of stainless steel (square mesh, 250 μm mesh width) having outside dimensions of 4×10 cm is first cleaned in aqueous lye (NaOH 30 g/l) at 50° C., and afterwards, during a 15-minute electroplating step, is coated with a layer of nickel. The nickel bath, at 50° C., displays the following composition: 300 g/l NiSO 4 .6H 2 O; 45 g/l NiCl 2 .6H 2 O; 30 g/l H 3 BO 3 . A nickel sheet is used as anode. Current density amounts to about 1 A/dm 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° C. and a current density of 5 A/dm 2 .  
      Activation of the Electrodes  
      In order to optimize the electrocatalytic characteristics, the electrode must be activated at 70° C. for approximately 10 hours in 20% concentration of caustic soda. Following that is washing process with deionized water.  
      Electrocatalytic Hydrogenation:  
      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).  
      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° 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.  
      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° C. during 10 minutes. After ending the dyeing time, the sample is oxidized in air, rinsed and finally washed at 50° C.  
      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.  
      Example 2 describes an electrocatalytic hydrogenation in a flowthrough reactor, of filter press manner of construction, in batch operation.  
      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 10×10 cm. The entire geometric cathode surface amounts to 1 m 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).  
      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° 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.  
      The colorings produced with this solution correspond in all criteria (color depth and fastness) to those obtained from conventionally produced vat dye liquors.  
      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.  
      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° 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.  
      Next, conveyed from the supply tank  11  is a 1.75% dye suspension into circulation stream VI by means of the fourth pump P 4 , with a first flow volume V 4  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 V 5  of 10 ml/min, corresponding to the color inflow, i.e. flow volume V 4 , is taken and dosed into the acid-free storage tank  21  by means of the fifth pump P 5 .  
      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 &gt;95%. The colorings produced with this solution correspond in all criteria (color depth and fastness) to those obtained from conventionally produced vat dye liquors.  
      Example 4 describes a continuous electrocatalytic hydrogenation, on an industrial scale, in a flowthrough reactor of a filter press manner of construction.  
      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 60×60 cm. The entire outer geometric cathode surface of the reactor amounts to 120 m 2 . The catholyte flows through the respective reaction cells vertically bottom to top with a flow volume V 1  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.  
      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° 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.  
      Next, conveyed from the supply tank  11  into the circulation stream V 1  by means of pump P 4 , with a flow volume of V 4  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 V 5  of 2.5 l/min corresponding to the dye inflow V 4 , and dosed into the acid-free storage tank  21  by means of the pump P 5 . 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).  
      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 &gt;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.  
      Example 5 describes an electrocatalytic hydrogenation in the fixed bed reactor in batch operation.  
      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 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 2 . Serving as anolyte is 2% sulfuric acid which is not circulated.  
      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° 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.  
      Example 6 describes an electrocatalytic hydrogenation in the fluidized bed reactor in batch operation.  
      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 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 2 . Serving as anolyte is 2% sulfuric acid which is not stirred up.  
      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° 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.  
      Example 7 describes an electrocatalytic hydrogenation of a rotating bed electrode in batch operation.  
      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 2 . Serving as an anolyte is 1.5% sulfuric acid.  
      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° 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.  
      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.  
      Production of the Electrodes:  
      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 2 PtCl 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).  
      Electrolytic Hydrogenation:  
      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 2 ). In the anode space, separated spatially by a membrane  9  (Nafion 324, DuPont), is located anode  8 ′ (DeNora DSA; electrode area 20 cm 2 ). Serving as anolyte is caustic soda with a concentration of 40 g/l.  
      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° 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.  
      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.  
      Production of the Electrode:  
      The plate shaped, commercially available RVC material (100 ppi, reticulated vitreous carbon, ERG Materials and Aerospace Corporation, Oakland, USA) with outer geometric dimensions of 10×10×0.5 cm is contacted by a copper wire and wetted for 1 hour with 1 l phosphate buffer solution (1 M potassium dihydrogen phosphate (KH 2 PO 4 ) and 1 M caustic soda NaOH), adjusted to a pH of 7). Added afterwards is 7 g of Pd/Al 2 O 3  (5 w/o Pd) catalyst and the suspension is stirred moderately (450 rpm) for approximately 2 h at 50° 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.  
      Electrolytic Hydrogenation:  
      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 10×10 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° 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.  
      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 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 2 ). Serving as anolyte is caustic soda at a concentration of 40 g/l.  
      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° 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.  
      The following aspects thus arise as fundamental to the invention: 
          No environmentally relevant problem materials are introduced.     The method functions without any addition of reducing agents.     Beyond dye, caustic soda and, at most, small quantities of additives, no other chemicals active in the redox process are introduced.     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.     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.     The recovery of materials relevant to cost and the environmental considerations is obviated (e.g., mediator system).     No pressure vessels or compressor are necessary for transport of the gaseous hydrogen, whereby the fire and explosion danger is minimized.     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.     Side reactions as, for example, dye precipitation, mud formation and corrosion as with the use of mediators cannot arise.     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.