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.

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 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 ad ), where it is brought by the electrochemically generated, adsorbed hydrogen (H ad ) into an adsorbed, reduced form (P ad ) of the dye, that the adsorbed, reduced form (P 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 2 O+ e   − →H ad +OH −   (I) A→A ad   (11)   A   ad +2H ad →P ad   (III) P ad →P  (IV)  where A is a dye, A ad  is an adsorbed dye, H ad  is the electrochemically generated, adsorbed hydrogen, P 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 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° C., preferably 25-60° 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, α-hydroxyketones, α-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 (V 1 ) that prevents the sedimentation of the dye suspension, that a part of the first circulation stream (V 1 ) in a second circulation stream (V 2 ) 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 (V 3 ) 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 (V 4 ) with a supply tank ( 11 ) for the dye suspension is connected to the catholyte tank ( 1 ) by way of the first circulation stream (V 1 ), that the dye suspension from the catholyte tank ( 1 ) is connected to the storage tank ( 21 ) by way of a second volume stream (V 5 ), the first volume stream (V 4 ) and the second volume stream (V 5 ) 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 (P 2 ), 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 (V 2 ) 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 (V 5 ) 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.