Leuco dyes (e.g. indigo white) as dispersing aids

Abstract

An electrolytic dye reduction process comprises preparing a cathode electrolyte by dispersing the dye in an electrolyte containing a leuco dye. In a preferred process, a leuco dye such as indigo is the only dispersing aid.

Description

Technical Field

[0001] The present invention belongs to dyeing with sulfur dyes or vat dyes such as indigo, and more particularly to a process for electrolytic dye reduction. Background Art

[0002] Dyeing with sulfur dyes or vat dyes has a long history. Although electrochemical dyeing is still the most commonly used method, its environmental impact is significant due to the chemicals used. Therefore, many attempts have been made to establish an electrolytic reduction method for producing leuco dyes, particularly indigo. An electrolytic reactor suitable for electrolytic leuco dye production is described in WO2007 / 147283, for example. The problem of using electrolytic reduction is the insolubility of dyes in aqueous electrolytes such as caustic soda solution (which is preferred, for example, in indigo reduction). In order to electrolyze well, dyes must be finely and evenly dispersed in the electrolyte. This is achieved, for example, by adding anionic dispersants such as anionic dispersants based on methylated naphthalenesulfonic acid-formaldehyde condensates. Because these reagents remain in the electrolyte or worse remain on the dyed fabric, so that at least a part of them can leave the production plant and pollute the environment, so it is highly desirable to have a reduction method without chemical additives. Summary of the invention

[0003] It is therefore a general object of the present invention to provide an electrolytic dye reduction process which operates without the addition of any chemical auxiliaries such as reducing agents or dispersants or dispersing agents, respectively.

[0004] Now, in order to carry out these and other objects of the invention, which will become clearer as the specification proceeds, the electrolytic dye reduction process is characterized in that it includes preparing a cathode electrolyte by dispersing the dye in an electrolyte containing a leuco dye.

[0005] In the following description, some terms are used, and their meanings are further defined as follows:

[0006] In combinations such as a diluted suspension, a suspension containing a leuco dye, the term "suspension" also means the cathode electrolyte, provided that the suspension is disclosed as being fed to an electrolytic cell. The final reduced product is also referred to as a leuco dye solution.

[0007] The term "alkaline electrolyte" does not necessarily mean that it has an alkaline pH, ie pH>7, but rather that it is a freshly supplied (ie dye-free) electrolyte solution. However, it is generally preferred that the alkaline electrolyte has an alkaline pH.

[0008] As used herein, the term "tube" encompasses all hollow cylindrical devices, ie, includes not only rigid tubes but also flexible tubes.

[0009] In the context of the present invention it has surprisingly been found that leuco dyes such as indigo white have a suspending effect on dyes such as indigo.For environmental and health reasons it is desirable to perform dyeing without using any additional chemicals such as dispersants.

[0010] In a preferred embodiment of the invention, the leuco dye is a reduced dye, ie no mixing of the dyes occurs, although such mixing is not excluded for specific effects.

[0011] In a more preferred embodiment, the dye is indigo, and the leuco dye is indigo white.

[0012] In general, particularly for indigo, it has been found that solutions of leuco dye in alkaline electrolytes (leuco dye concentration being only 5% weight / volume (w / v)) can stably suspend up to 20% w / v of the dye, e.g. 5-10% w / v leuco dye for 10-20% w / v dye.

[0013] If no other dispersants than leuco dyes should be used, which is more preferred, the problem is to first start up the electrolytic reactor if no suitable leuco dye solution is available (e.g. from another plant). This problem has been solved by a start-up method comprising:

[0014] (i) dispersing a starting amount of dye in an electrolyte, and electrolytically reducing it to obtain a diluted leuco dye solution to prepare a diluted leuco dye suspension,

[0015] (ii) adding further dye to the diluted leuco dye solution of step (i), and electrolytically reducing it to obtain a more concentrated or concentrated leuco dye solution,

[0016] (iii) Optionally repeating step (ii) until the desired concentration of leuco dye is obtained.

[0017] In more detail, the method comprises the steps of:

[0018] (i) preparing in a main container or in a first and main container a dilute suspension of the dye in an alkaline electrolyte by circulating and heating the suspension, optionally modifying the suspension by circulating it through one or more ultrasound devices,

[0019] (ii) electrolytically treating the diluted suspension of step (i) in at least one electrolytic cell to obtain a diluted leuco dye solution,

[0020] (iii) optionally transferring the diluted leuco dye solution of step (ii) to a first or main vessel where it is supplied with additional dye by circulation and heating to obtain a suspension containing the leuco dye,

[0021] (iv) then circulating the suspension containing the leuco dye of step (iii) through at least one electrolytic cell to electrochemically convert the dye into a leuco dye, or to electrochemically convert the suspension containing the leuco dye into a solution containing the leuco dye, respectively, optionally

[0022] (v) Repeating steps (iii) and (iv) one or more times using the solution obtained in step (iv) and additional dye to form a leuco dye solution in an alkaline electrolyte.

[0023] Steps (iii) to (v) are optional, ie they are only carried out when the concentration of the leuco dye obtained in step (ii) is not sufficiently concentrated to provide a suitable dispersion effect.

[0024] For indigo as a preferred example, the preferred concentration of the dye suspension in step (i) is 100-200 g / L of an alkaline electrolyte, such as sodium hydroxide at a concentration of 2-10% w / v, preferably 4% w / v. The indigo and indigo white concentrations of the indigo suspension in the first step (iii) are 150-250 g / L, in the second step (iii) are 250-350 g / L, and in the third step (iii) are 300-380 g / L.

[0025] Once the desired leuco dye concentration in the electrolyte is available, an electrolytic dye reduction process is performed, i.e. the concentrated leuco dye solution is diluted with electrolyte and replenished with dye, optionally and preferably circulated, preferably passed through at least one ultrasonic device to improve the homogeneity of the suspension, and then sent to an electrolytic cell for electrolytic reduction, optionally repeating this step until the desired amount of leuco dye solution with the desired leuco dye concentration is obtained.

[0026] Because the solution containing leuco dye used as dispersion medium at the beginning of leuco dye production usually derives from the previous production cycle, it must be diluted to obtain new batch.Therefore, the amount of suspendable dye is limited.So, dyestuff can be added in portions, for example at first 150kg, then it is carried out above-mentioned treatment to obtain suitable cathode electrolyte, then reduction, then in main container or in the first container with other dyestuff and optionally with other electrolyte, until the leuco dye solution of the desired concentration of the desired amount is obtained.

[0027] In more detail, the method for producing a concentrated leuco dye solution comprises:

[0028] (i) adding a first portion of the dye to be reacted to an alkaline electrolyte solution containing a leuco dye in a first or main vessel and circulating, optionally passing through one or more ultrasound devices and preferably heating to form a first cathode electrolyte,

[0029] (ii) passing the first cathode electrolyte of step (i), preferably via a filter for removing oversized particles and optionally a heating device, to an electrolytic cell, and

[0030] (iii) starting the electrolytic cell by gradually increasing the voltage to the conversion voltage and maximum conversion power,

[0031] (iv) adding a further portion of the dye to be reacted, thereby increasing the power, preferably to maximum power and continuing the conversion,

[0032] (v) optionally repeating step (iv) until the desired concentration of the leuco dye is reached,

[0033] (vi) converting the dye into a leuco dye until the power drops to a threshold switching power due to the dye conversion,

[0034] (vii) Removal of the leuco dye solution.

[0035] The further dye may be added by adding the solid dye to the main container, or preferably by feeding a portion of a solution or suspension containing the leuco dye from the main container or the cathode electrolyte circulation loop into a first container where it may be diluted with and usually with an alkaline electrolyte and replenished with the solid dye and circulated to form a suitably uniform suspension which is then supplied to the main container.

[0036] Typically, particularly for indigo, it has been found that leuco dye solutions in alkaline electrolytes (leuco dye concentration being only 5% w / v) can stably suspend up to 20% w / v of the dye, e.g. 5-10% w / v leuco dye for 10-20% w / v dye.

[0037] To produce a concentrated indigo solution, for example a solution containing 300 kg of indigo in 1000-1500 L of alkaline electrolyte, the following procedure has proven to be very suitable:

[0038] Initially, a portion of the indigo solution is left in the main container or pumped into the first container, replenished with alkaline electrolyte and indigo. It has proven to be suitable to keep about 200 L of indigo solution in the container, which is then replenished with 300-600 L of alkaline electrolyte. Due to additional indigo solution in the tubes and electrolytic cells, etc., the indigo concentration has to increase once it mixes with the contents of the tubes, cells, etc. Therefore, although a 5% indigo concentration is sufficient to stabilize suspensions with up to 20% indigo, higher indigo concentrations are usually used. The volume information given below refers to the volume in the first container and the main container.

[0039] The first portion of indigo to be reacted to indigo is added, for example 150 kg, to 500 L of an alkaline electrolyte solution containing indigo (preferably 15%-20% indigo concentration, 2%-10% NaOH concentration, in particular about 4%) in a first container or, if there is no first container, a main container, and circulated, optionally passed through one or more ultrasound devices, and heated to form a first indigo suspension. If there is a first container, the suspension is circulated through the first container and the ultrasound device for about 30 min. Once pumped into the main container, it is circulated again for about 20 min through a second ultrasound device and at least when ready to be supplied to the cathode through a particle filter and a heat exchanger.

[0040] Once the suspension is homogeneous (indigo particle size below 50 μm) and has the desired temperature of about 50° C.-65° C., preferably 60° C., the first indigo suspension is fed to at least one electrolytic cell (in industrial applications, usually several electrolytic cells in more than one stack and all electrolytic cells of all parallel working stacks), and then the electrolytic cells (in a preferred embodiment all stacks) are started by increasing the voltage stepwise to the conversion voltage and maximum conversion power, for example from 7 V to 11 V stepwise at 0.5 V / about every 2 min. All electrolytic cells of one stack and preferably all stacks are fed and started simultaneously. The preparation of step (i) takes about 1-2 hours and the start-up of the electrochemical process takes about 15 min.

[0041] At a maximum conversion voltage of 11 volts, the conversion power is about 170 A for an indigo suspension containing 150 kg indigo in 500-800 L indigo catholyte. Once the maximum current is reached, additional indigo, for example 50 kg, is prepared in the indigo solution in the first vessel and supplied to the main vessel, which again generates an enhanced current and continues the reduction. The indigo solution used to suspend the indigo is typically diluted with electrolyte to an indigo concentration of 5-20% weight / volume (w / v), more preferably 10-20%.

[0042] Once all the indigo has been added, for example three times 50 kg indigo in about 150-400 L to a total of 300 kg indigo in 1000-1500 L electrolyte, the power decreases with decreasing indigo concentration. Since no further indigo should be supplied, the voltage also decreases slowly, depending on the measured power or on the indigo concentration, respectively. In general, all the dye can be reduced to a leuco dye by this method.

[0043] Since the quality of the dye suspension also depends on the particle size, it is preferred to circulate the suspension, preferably through at least one ultrasound device, preferably until about 95% weight / weight (w / w) of the particles are smaller than 50 μm, more preferably at least 97% w / w, most preferably about 99% w / w, i.e. the particles are not individualized, but the percentage is determined by weighing the residue in the corresponding particle filter.

[0044] Dye suspension or cathode electrolyte preparation can be directly carried out in main container, or first in first container, then supplied in main container and carry out.In the case of first container, circulation can be carried out in first container and / or in main container.In case suspension reaches desired quality, i.e. when this suspension is ready to be used as cathode electrolyte, this suspension is sent to electrolyzer, specifically to the cathode chamber comprising particle carbon cathode such as graphite cathode.

[0045] Since it is usually not possible to remove all oversized particles by circulation, even in the presence of ultrasound equipment, it is preferred that the cathode electrolyte supply pipe comprises a particle filter upstream of the cathode chamber inlet. In another preferred embodiment, this cathode electrolyte supply pipe or the dye suspension circulation pipe of the main container comprises a heating device such as a heat exchanger.

[0046] It has been found that, due to the osmotic pressure difference between the cathode and anode chambers, impurities (which are present, for example, in the dye, such as aniline in the case of indigo) migrate through the semipermeable membrane, where they form an insoluble film on the anode, thereby affecting its performance and life. It has now been found that if the anolyte circulation pipe is equipped with an adsorption filter for adsorbing such impurities, the performance and life of the anode can be significantly improved. Since in the process of the invention, no additional dispersant or other chemical auxiliaries are required, there is no risk of their depletion due to the osmotic pressure difference (which is maintained throughout the process because the molecules are removed on the anode side). Therefore, there is no risk of suspension destabilization caused by the removal of the dispersant in the early reduction stage.

[0047] Another advantage is that the electrolytic cell can be easily regenerated.

[0048] This can be done by cleaning the electrode compartment, in particular the particle electrode, with a strong acid (pK<1), circulating an acidic cleaning solution through the electrode bed, followed by washing the bed with water, optionally and preferably after washing the acid treated bed with an alkaline solution, preferably caustic soda, to more effectively remove acid and electrode contaminants.

[0049] This cleaning step is particularly suitable for particle carbon-based electrodes, preferably cathodes, because it removes metals from the particle surface. Such metals may be present in the particles themselves or brought to the particles during the electrolysis process. These metals, usually mainly iron and / or nickel and / or copper in metallic or ionic or complex form, can have a catalytic effect on H2 generation that competes with the desired reduction and must therefore be avoided. However, the cleaning step has also been shown to be very suitable for anode cleaning.

[0050] The acid wash is carried out for a suitable time, for example 10-60 min, followed by washing the bed with an alkali such as caustic soda (to remove acid and electrode contamination), followed by washing with water, or less preferably directly with water. Although the cleaning can be carried out simultaneously for both electrodes (i.e. cathode and anode), in the case of several stacks of electrolytic cells, it is preferred to carry out the cleaning / regeneration of cathodes and anodes in a subsequent manner, i.e. first the cathodes of all stacks, one stack at a time in a preferred embodiment, and subsequently, during the preparation of the suspension, all anodes of preferably one stack are cleaned. Since the anodes require less regeneration than the cathodes, it has proven to be advantageous to clean them stack by stack after cleaning all the cathodes, i.e. once a week for the cathodes of each stack and once for all 7 weeks for all anodes of one stack.

[0051] The advantage of cleaning the electrodes during the preparation of the suspension is that the entire stack remains in the production of the leuco dye, and since the preparation of a fresh suspension in a batch procedure takes at least 1 hour, while a longer cycle through an ultrasound device, i.e. up to 2 hours, improves the suspension quality, careful cleaning / regeneration of the electrodes and preparation of the suspension require almost the same amount of time. Therefore, carrying out both steps simultaneously does not extend the required time in any way or only minimally.

[0052] Cleaning the cathodes of at least only one stack at a time has several advantages, namely

[0053] - the water required for the preparation of the suspension can be supplied via the cathode of the stack to be regenerated, thereby avoiding the loss of leuco dye,

[0054] - the water required for the suspension preparation is sufficient to remove all the leuco dye from one stack, but may be less effective in the case of several stacks,

[0055] - Less cleaning solution is required because the cleaning solution is recirculated.

[0056] The concentration of a suitable acidic solution is 10-100 g / L, more preferably 40-60 g / L, most preferably about 50 g / L, or 0.25-30 M, preferably 1-2 M, more preferably 1.3-1.4 M (referring to protons) in deionized water. If an alkaline solution, especially a solution of a base with pKa>10, such as caustic soda, is used after the acid solution cleaning, the concentration is typically 10-100 g / L, more preferably 20-60 g / L, most preferably 40 g / L, or 0.1-2.5 M, preferably 0.5-1.5 M (referring to hydroxide) in deionized water.

[0057] For the washing step, the strong acid is preferably selected from HCl, H2SO4, HNO3, and mixtures thereof.

[0058] The cleaning solution may be circulated through a filter to retain small particles of metal sources or worn electrode material, and through a carbon or other adsorbent filter to adsorb dissolved contaminants.

[0059] An electrolytic reactor suitable for carrying out the method described in any one of the preceding claims, comprising a main container and a first container for preparing a dye suspension or a cathode electrolyte, respectively, an anolyte container and at least one electrolytic cell, the electrolytic cell comprising a cathode chamber and an anode chamber separated by a partition. The main container is connected to the cathode chamber via a cathode electrolyte circulation loop or a cathode electrolyte circulation pipe for circulating the cathode electrolyte, and the anolyte container is connected to the anode chamber via an anode electrolyte circulation loop or an anode electrolyte circulation pipe for circulating the anode electrolyte. In addition, the main container and the first container are connected to each other via a colorless dye return pipe and a first suspension supply pipe, and the main container comprises a main suspension circulation loop, and / or the first container comprises a first suspension circulation loop. The main suspension circulation loop and / or the first suspension circulation loop are preferably equipped with an ultrasonic device.

[0060] For industrial applications, the electrolysis reactor comprises at least 4 electrolytic cells, wherein the electrolytic cells are provided in the form of at least two stacks, each stack having at least two electrolytic cells, the stacks and the electrolytic cells of a stack being arranged in parallel and allowing at least one stack to be isolated for cleaning / regeneration while the remaining colorless dye solution circulates through the remaining one or more stacks, and / or at least one stack to be shut down for maintenance while the remaining stacks remain in production and preferably can be operated so that they provide full production capacity.

[0061] In a more preferred embodiment of the electrolytic cell, the cathode chamber comprises a plurality of freely suspended electrically conductive, in particular non-spherical particles in contact with a conductor, the conductor forming at least a side wall opposite to the partition, the cathode chamber having an upper grid and a lower grid. The width of the mesh and the arrangement of the grids are selected so that the liquid cathode electrolyte can pass through them, but the particles are prevented from passing through them so that they remain in the cathode chamber. The anode chamber has a wall opposite to the partition formed by an electrode (anode), and each chamber has an inlet and an outlet for the liquid cathode electrolyte or the liquid anode electrolyte, respectively, at the top or bottom of each chamber, wherein the flow of the liquid cathode electrolyte and / or the liquid anode electrolyte can be regulated.

[0062] The preferred number of stacking body is at least 4, more preferably 6.In the situation of 6 stacking bodies, each stacking body can be operated separately, namely separates one for cleaning / regeneration, and the leuco dye solution circulates through the remaining 5 stacking bodies.Or at least 1 and preferably 1 stacking body can be closed for maintenance (more than only cleaning / regeneration), and the remaining stacking bodies (normally 5) are at work.Preferred 6 stacking bodies, because in the reactor of continuous production, for example, in the reactor of producing a batch every day, it is enough to regenerate the cathode once a week usually, and the regeneration frequency that anode needs is lower, for example, all regenerate once in seven weeks.For the reactor with 6 stacking bodies, this means that after six days, cleaned the cathode of whole stacking bodies, cleaned the anode of a stacking body on the 7th day.Each stacking body preferably comprises 1-10 electrolyzer, preferred 4-6 electrolyzer, particularly 5 electrolyzers. For an electrochemical reactor suitable for the production of 1000 kg of leuco dye, in particular of, for example, 30% strength indigo white solution within 24 h, an electrochemical reactor having 6 stacks of 5 electrolytic cells each (at least 5 stacks in operation) has proven suitable, in particular electrolytic cells with a cathode chamber having a carbon cathode with the following dimensions and particles:

[0063] -0.3m 2 Separator area / cell;

[0064] - The dimensions of the cathode chamber containing granular graphite carbon are 0.4 m high, 0.7 m wide and 0.04 m thick;

[0065] -Carbon particle size is 1mm-0.3mm;

[0066] - The dimensions of the stainless steel cathode current collector and anode are 0.6m high and 0.9m large.

[0067] More information about suitable electrodes can be found in WO 2007 / 147283 A2, the disclosure of which is incorporated herein by reference in its entirety. Such information relates to the determination of, for example, the sphericity and flow properties of the microparticles.

[0068] One problem with the combination of particle electrodes and suspensions is clogging, which also results in reduced transport of electrolyte through the particle electrodes and / or results in increased pressure.

[0069] In order to avoid any clogging of the particle electrode, the electrolyte circuit direction (i.e. the electrolyte flow direction) is preferably reversed regularly, for example, for 2-5 minutes, for example, for about 3 minutes. Since the electrolysis should not be affected by the reversal of the flow direction, it is important that the particle bed of the electrode is filled equally in both directions. This is achieved by ensuring that the flow and the particle bed are such that the bed is always pressed tightly against the upper or lower grid, keeping the particles in the electrode chamber.

[0070] Depending on the type of electrode, the electrolyte orientation of the particle cathode and optionally the anode may be reversed.

[0071] The cathode electrolyte supply pipe is preferably equipped with a particle filter for removing oversized particles, in particular particles >50 to 150 μm, and / or a heating device (heat exchanger) for heating the indigo to, for example, about 50°C-65°C, preferably 60°C. Optionally, the cathode electrolyte supply pipe can also be equipped with an ultrasound device. In a more preferred embodiment, the main container also contains a main suspension circulation loop, which is equipped with a main suspension circulation pump and preferably also has an ultrasound device. This main suspension circulation loop can be a separate loop, or provided as a bypass of the cathode electrolyte supply pipe, that is, leaving the main container at the cathode electrolyte outlet and returning via the reduced cathode electrolyte inlet, but not via the cathode chamber, and optionally also passing only through a heating device and usually not through a filter. The currently preferred ultrasound device is a cylindrical ultrasound device. The main suspension can be circulated through the circulation pipe for a sufficient time to produce a uniform dye suspension (cathode electrolyte) having a desired temperature and only a few oversized particles. The time required to obtain a good suspension is about 1 hour, but better results are achieved with a suspension obtained after about 2 hours.

[0072] Since the object of the present invention is to avoid chemicals such as dispersants, it is more preferred to prepare the suspension in a first container, since in the first container the suspension can be circulated to improve the suspension, in particular the particle size, without interrupting the circuit through the electrolytic cell, as would be the case if the circulation were carried out in the main container.

[0073] This first container comprises a first suspension circulation loop, which is equipped with a first suspension circulation pump and preferably an ultrasonic device, more preferably a cylindrical ultrasonic device. The dye suspension in the first container is also prepared by circulating it through the circulation loop for a sufficient time to produce a suspension of the desired uniformity (i.e., the dye particle size is close to or below 50 μm). In the case of having an ultrasonic device in the circulation loop of the first container, the ultrasonic device in the circulation loop of the main container can be eliminated or the circulation loop of the main container can be eliminated completely, but it is preferred that they exist.

[0074] Because the presence of oxygen must be avoided, the cathode part of the reactor is operated under an inert atmosphere, usually nitrogen, which can be supplied and removed at any point in the cathode electrolyte loop, provided that the inert atmosphere is maintained throughout the loop. The preferred location for nitrogen supply and removal (usually together with the hydrogen produced during the electrolysis process) is above the main vessel. BRIEF DESCRIPTION OF THE DRAWINGS

[0075] The present invention will be better understood when considering the following detailed description, and objects other than those mentioned above will become apparent. This specification refers to the accompanying drawings, in which:

[0076] Figure 1 The main components of the entire electrochemical reactor are shown, and the electrolytic cell is shown in an enlarged view.

[0077] Figure 2a Six stacks are shown schematically, each with associated supply and withdrawal lines, and

[0078] Figure 2b A stack is shown in more detail, having five electrolytic cells.

[0079] Figure 3 The preparation of a dye suspension in a first container and the supply to a main container are schematically shown.

[0080] Figure 4 Components of an electrochemical reactor are shown in more detail for preparing a cathode electrolyte in the presence of a first container.

[0081] Figure 5 The preparation of a dye suspension is shown in the presence of a first container and using an in-house produced leuco dye as a dispersant.

[0082] Figure 6 The reactor and the method for continuous cleaning of the anolyte with an adsorption filter are shown.

[0083] Figure 7A reactor and method for regenerating electrodes by means of cathode and anode cleaning are shown. For clarity, the cathode electrolyte and anode electrolyte circulation loops are omitted. It is not necessary to circulate the anode electrolyte during the cathode cleaning process, but it is preferred to circulate the cathode electrolyte during the anode cleaning process. DETAILED DESCRIPTION

[0084] The invention will now be further described based on the accompanying drawings for a preferred dye reduction apparatus, in particular an indigo reduction apparatus.

[0085] Figure 1 The core of the preferred electrolytic reactor for the present invention is shown, having only one electrolytic cell 4 for reasons of clarity, and having an inlet for the dye in suspended form, i.e. transferred from the first container 2. Figure 2a The assembly of six stacks is schematically shown in Figure 2b A stack with five electrolytic cells is shown in more detail in .

[0086] The main container 1 has a main suspension circulation loop 14, which includes a main suspension circulation pump P03 and preferably an ultrasonic device 141 (see Figure 4 ), used to circulate the suspension, thereby improving its homogeneity.

[0087] After the suspension has circulated for a certain time (depending on the quality of the dye, i.e. its particle size and particle distribution), the main suspension circulation loop 14 is closed and the valve to the cathode electrolyte outlet 12 is opened. The cathode electrolyte is then circulated through the cathode electrolyte circulation loop 15 by pumping it with the cathode electrolyte pump P01, through another optional ultrasound device 154, a particle filter 152 (for removing oversized particles if still present) and a cathode electrolyte heating device (heat exchanger) 153 via the cathode electrolyte supply pipe 151, and enters the cathode chamber 41 of the electrolytic cell 4 separated from the anode chamber 42 by a partition 43 (preferably a semipermeable membrane) via the cathode electrolyte inlet 411. After passing the cathode, the cathode electrolyte returns to the main container 1 via the reduced cathode electrolyte outlet 412a, the reduced cathode electrolyte return pipe 412b and the reduced cathode electrolyte inlet 13. As shown in the figure numerals 413a and 413b, in a preferred embodiment, the cathode electrolyte direction can be reversed.

[0088] The anolyte is supplied to the anolyte container 3 via the anolyte supply pipe 31 or once reduction has begun via the anolyte inlet 35, and to the anode chamber 42 via the anolyte outlet 32, the anolyte pump P02, the anolyte heating device 331 and the anolyte inlet 421. After leaving the anode chamber 42 via the anolyte outlet 422a, the anolyte is recycled to the anolyte container 3 via the anolyte return pipe 422b and the anolyte inlet 35.

[0089] In a preferred embodiment, an adsorption filter 332 is provided in the anolyte loop. Such a filter 332 can be placed anywhere, but it is preferably placed just before the anolyte heating device 331, because here the temperature is lowest and thus adsorption is best.

[0090] All vessels additionally have supplies for nitrogen, caustic soda and optionally further supplies as well as a degasser and a solution removal line for removing leuco dye or, in the case of anode cleaning, anolyte.

[0091] Figure 2 schematically shows six stacks of five electrolytic cells 4 each, all connected in parallel. Due to the independent anolyte and catholyte supply to each stack, any of them can be independently isolated for regeneration or shut down for maintenance.

[0092] Figure 3 and 4 The preparation of a dye suspension in a first container 2 is shown. Figure 4 As shown, this preparation includes circulating the solid dye from the solid dye container 21a via the solid dye inlet 21b to the caustic soda solution of the desired concentration in the first container 2. The first container may have a first suspension circulation loop 22, optionally equipped with an ultrasonic device 221. Once the dye suspension has reached the desired uniformity, it is supplied to the main container 1 via the suspension outlet 23 and the dye suspension inlet 11b, driven by a pump P05. Once the suspension is in the main container 1, it can be further circulated or used directly as a cathode electrolyte as desired. Once the reduction process begins, the main container 1 and all pipes and equipment in contact with the colorless dye are purged with nitrogen (or other inert gas), for example introduced into the main container 1, to prevent oxidation of the reduced dye. After the reduction process has been as described above, the main container 1 and all pipes and equipment in contact with the colorless dye are purged with nitrogen (or other inert gas), for example introduced into the main container 1, to prevent oxidation of the reduced dye. Figure 1 After the cycle shown, the catholyte thus formed is circulated through the cathode chamber driven by pump P01 until all the dye has been reduced to a leuco dye.

[0093] As indicated above, most dyes cannot be suspended in desirably high amounts without the addition of large amounts of dispersants. Figure 5 The dye reduction process of the present invention is schematically shown in , which uses little and preferably no dispersant other than the leuco dye.

[0094] To start the dye reduction, in a first preparation step, a diluted suspension is prepared as described above and electrolytic reduction is carried out in the electrolytic cell 4. Once the dye has been reduced to a leuco dye, it can be replenished with additional dye in the main container 1, or some of the produced leuco dye can be transferred from the main container 1 to the first container 2 via the leuco dye outlet 16, the leuco dye return pipe 24 equipped with a leuco dye return pump P04 and a leuco dye inlet 241, to be replenished with additional dye. The suspension containing the leuco dye can then be first processed in the first suspension circulation loop 22 to improve its homogeneity and then supplied to the main container 1 and finally to the electrolytic cell, as described above.

[0095] Similarly, once leuco dye production has begun, a portion of the concentrated leuco dye is removed and a portion of the leuco dye solution remains in the reactor. This remaining leuco dye solution can then remain in the main container 1, or preferably as Figure 5 As shown, the leuco dye is supplied to the first container 2 via the leuco dye return pipe 24. In the main container 1 or preferably the first container 2, the leuco dye solution is diluted with further electrolyte, such as caustic soda. Since the leuco dye acts as a dispersant, a more concentrated dye suspension can be prepared directly. However, it has proven to be advantageous to add the leuco dye in several portions, such as Figure 5 In the case of indigo, a concentration of 5-20% has been found to be suitable for stabilizing suspensions containing up to 20% indigo. Once the indigo has been reduced to the desired concentration of indigo, the indigo is removed from the reactor via the concentrated leuco dye outlet 44.

[0096] As mentioned above and as Figure 1 and 6 As shown, an adsorption filter 332 may be provided in the anode electrolyte circulation loop 33. If the anode electrolyte circulation loop 33 comprises a heating device 331, the adsorption filter 332 is preferably provided near but upstream of the heating device 331, and the heating device 331 is preferably provided upstream of the anode chamber. Such an adsorption filter is used to remove small molecules that may be present and are typically present in the dye and have been found to migrate across the separator 43, such as a semipermeable membrane, and directly or after a reaction such as polymer deposition on the anode, thereby affecting its activity.

[0097] Suitable adsorption filter materials are, for example, activated carbon and molecular sieves such as zeolites. For a good anolyte flow through the adsorption filter, a particle size of about 1 to 3 mm, in particular about 2 mm, is preferred.

[0098] Despite all the improvements described within the scope of the invention, which allow a cleaner process and thus longer time intervals between maintenance activities, cleaning is still sometimes necessary, since the quality of the electrolytic cells decreases with continued use. In order to avoid shutting down the entire reactor, several stacks of electrolytic cells are provided, preferably in such a number that each time one of them is not required for the full reactor capacity, it can be isolated for maintenance without affecting the subsequent dyeing process that may be associated.

[0099] However, it has also been found that the time intervals between actual shutdowns for maintenance can be substantially extended by using the cleaning / regeneration method described further below.

[0100] Such method is carried out, at least one and preferably one stack is separated, and the leuco dye solution is circulated through the remaining stacks simultaneously. This stack separated for cleaning can be cleaned or regenerated respectively, such as Figure 7 shown.

[0101] Although in particular the particle cathode must be regenerated, the anode quality may also suffer due to use. Therefore, it is preferred to clean the anode compartment as well. Although the cleaning / regeneration of cathodes and anodes can be carried out simultaneously with the same scheme, it is preferred to first clean the cathodes of all stacks, for example one by one, and then clean all anodes of a stack. This allows the actual desired cleaning / regeneration characteristics such as cleaning solution and application time to be adopted.

[0102] For the reactor with 6 stacks each with 5 electrolyzers, it has been found that the regeneration step is roughly the same as the suspension preparation time. Because dye reduction is carried out in batches, it is more advantageous to carry out the cleaning / regeneration step when preparing new dye suspension. This allows whole stacks to be maintained in the reduction process until the maintenance that needs more time-consuming. Good regeneration program comprises whole negative electrodes of a stack cleaned / regenerated together at intervals of one day, i.e. one stack every day, whole stacks subsequently. On the 7th day, after all negative electrodes have been cleaned, clean all the anodes of a stack, i.e. clean the cathode of each stack at intervals of one week, clean all the anodes of a stack at intervals of 7 weeks. Although these orders can change, considering productivity, the required amount of leuco dye loss and cleaning solution, this program has been confirmed to be optimal.

[0103] For this cleaning or regeneration step, the electrochemical reactor also has a device for supplying a cleaning solution to the stack of electrolyzers or electrolyzers 4 itself. These devices include at least one cleaning medium supply pipe 61, for supplying a cleaning / regeneration solution to cathode or anode, and at least one cleaning medium on the side of the electrolyzer opposite to the inlet removes pipe 62, for removing acidic cleaning solutions and water and preferably alkaline solutions. These cleaning medium supply pipes 61 can be the bypass (i.e., using the same inlet and outlet) of cathode electrolyte supply pipe 151 and anode electrolyte supply pipe 31, or an independent pipe with its own inlet and outlet adjacent to the inlet and outlet of cathode electrolyte circulation loop 15 or anode electrolyte circulation loop 33. Typically, one or more cleaning medium supply pipes 61 are connected to acid container 63a to supply acid, and are optionally connected to alkali container 63b to supply alkali, and are connected to the water line for supplying deionized water. One or more cleaning media remove pipe 62 and are directly supplied to a wastewater treatment plant (WWTP), or are supplied to a wastewater container for storing wastewater. In a preferred embodiment, the cleaning solutions are circulated for a certain time, i.e. until their contamination reaches an undesirable level. In the case of circulation, the cleaning medium removal pipe 62 is connected to the containers 63a, 63b via a cleaning medium circulation device 64. In this case, it is preferred to have a cleaning medium circulation device equipped with one or more particle filters and / or adsorption filters, preferably just downstream of the electrolytic cell.

[0104] Cleaning or regenerating the particle electrode or two electrodes in the electrochemical reactor of the present invention, respectively, comprises cleaning the first and / or second chambers 41, 42, in particular the bed or the particle electrode, respectively, with a strong acid (pK<1) by circulating the acidic solution through the chambers 41, 42, in particular the electrode bed, and then cleaning the chambers 41, 42, in particular the bed, with water, which is optionally and preferably carried out after cleaning the acid-treated chambers 41, 42 with an alkaline solution, preferably caustic soda, to more effectively remove acid and electrode contaminants.

[0105] While there have been shown and described presently preferred embodiments of the invention, it is to be distinctly understood that the invention is not limited thereto but may be otherwise variously embodied and practiced within the scope of the appended claims.

[0106] List of reference numerals:

[0107] 1 main container

[0108] 11a Solid dye inlet

[0109] 11b Dye suspension inlet

[0110] 12 Catholyte outlet

[0111] 13 Reduced cathode electrolyte inlet

[0112] 14 Main suspension circulation loop

[0113] P03 Main suspension circulation pump

[0114] 141 Ultrasonic equipment in the main suspension circulation loop

[0115] 15 Catholyte circulation pipe or cathode electrolyte circulation loop

[0116] 151 Catholyte supply pipe

[0117] P01 Catholyte supply pump

[0118] 152 Particle filter in cathode electrolyte supply pipe

[0119] 153 Cathode electrolyte heating device (heat exchanger)

[0120] 154 Ultrasonic device in the cathode electrolyte supply pipe

[0121] 16 Leuco dye export

[0122] 2 First container

[0123] 21a Solid dye container

[0124] 21b Solid dye inlet

[0125] 22 First suspension circulation loop

[0126] 221 Ultrasonic device in the first suspension circulation loop

[0127] 23 First suspension outlet

[0128] 231 First suspension supply pipe

[0129] P05 First suspension supply pump

[0130] 24 Leuco dye return pipe

[0131] 241 Leuco Dye Inlet

[0132] 25. First suspension mixer

[0133] P04 Leuco dye return pump

[0134] 3 Anolyte container

[0135] 31 Anode electrolyte supply pipe

[0136] 32 Anolyte outlet

[0137] 33 Anode electrolyte circulation pipe or anode electrolyte circulation loop

[0138] 331 Anode electrolyte heating device such as heat exchanger

[0139] 332 Adsorption filter

[0140] P02 Anolyte Pump

[0141] 35 Anolyte inlet

[0142] 4 Electrolytic Cell

[0143] 41 First chamber or cathode chamber

[0144] 411 Catholyte inlet

[0145] 412a Reduced cathode electrolyte outlet

[0146] 412b Reduced cathode electrolyte return pipe

[0147] 413a Bypass for changing the direction of cathode electrolyte

[0148] 413b Bypass for changing the direction of cathode electrolyte

[0149] 42 Second chamber or anode chamber

[0150] 421 Anode electrolyte inlet

[0151] 422a Anode electrolyte outlet

[0152] 422b Anode electrolyte return pipe

[0153] 43. separator, semipermeable membrane

[0154] 44 Concentrated leuco dye export (in batches)

[0155] 5. Stack of electrolytic cells

[0156] 61 Cleaning medium supply pipe

[0157] 62 Cleaning medium removal tube

[0158] 63a Acid container

[0159] 63b Alkali container

[0160] 64 Cleaning medium circulation device

Claims

1. An electrolytic dye reduction method, comprising preparing a cathode electrolyte in a main container (1) and / or a first container (2) by dispersing a dye in an electrolyte containing a leuco dye, the cathode electrolyte having a leuco dye concentration of 5-20% w / v and a dye concentration of 10-20% w / v, wherein the preparation comprises adding the dye in solid form to the cathode electrolyte containing the leuco dye, wherein the leuco dye is the only dispersant for the dye, wherein the dye is indigo, and the leuco dye is indigo white.

2. The electrolytic dye reduction method according to claim 1, wherein the concentrated leuco dye solution is diluted with an electrolyte and supplemented with a dye to form a suspension and then sent to an electrolytic cell (4) for electrolytic reduction.

3. The electrolytic dye reduction method according to claim 2, wherein the suspension is circulated through an ultrasonic device (141, 154, 221) to improve the homogeneity of the suspension before being sent to the electrolytic cell (4) for electrolytic reduction.

4. The electrolytic dye reduction process according to claim 1, wherein the dye suspension or the cathode electrolyte is at least partially prepared in a first container (2) and then supplied to the main container (1).

5. The electrolytic dye reduction method according to claim 1, wherein preparing the suspension in the first container (2) and / or in the main container (1) comprises circulating the suspension in the first container (2) and / or in the main container (1) via an ultrasonic device (141, 154, 221).

6. The electrolytic dye reduction method according to claim 5, wherein preparing the suspension in the first container (2) and / or in the main container (1) further comprises circulating the suspension in the first container (2) and / or in the main container (1) via a heating device (153).

7. The electrolytic dye reduction method according to claim 2, wherein the cathode electrolyte is supplied to the electrolytic cell (4) via a particle filter (152) and / or a heating device (153).

8. The electrolytic dye reduction process of claim 2, wherein the cathode electrolyte is circulated through the electrolytic cell (4) until all the dye is reduced.

9. The electrolytic dye reduction method according to claim 1, comprising: (i) adding a first portion of the dye to be reacted to an alkaline electrolyte solution containing a leuco dye in a first container (2) or main container (1) and circulating to form a first cathode electrolyte, (ii) passing the first cathode electrolyte of step (i) into at least one electrolytic cell (4), and (iii) starting the at least one electrolytic cell (4) by gradually increasing the voltage to a conversion voltage and a maximum conversion power, (iv) adding a further portion of the dye to be reacted, thereby increasing the power and continuing the conversion, (vi) converting the dye into a leuco dye until the power drops to a threshold switching power due to the dye conversion, (vii) Removal of the leuco dye solution.

10. The electrolytic dye reduction method according to claim 9, further comprising a step (v) between steps (iv) and (vi): (v) Repeat step (iv) until the desired concentration of leuco dye is reached.

11. The electrolytic dye reduction method according to claim 9, wherein the alkaline electrolyte solution containing the leuco dye is obtained by a preparation method comprising the following steps: (i) dispersing a starting amount of dye in an electrolyte and reducing it to obtain a diluted leuco dye solution to prepare a diluted leuco dye suspension, (ii) adding further dye to the diluted leuco dye solution of step (i) and reducing it to obtain a more concentrated or concentrated leuco dye solution.

12. The electrolytic dye reduction method according to claim 11, wherein the preparation method further comprises step (iii): (iii) Repeating step (ii) until a leuco dye having a desired concentration is obtained.

13. The electrolytic dye reduction method according to claim 9, wherein the alkaline electrolyte solution containing the leuco dye is prepared by a preparation method comprising the following steps: (i) in the main vessel (1) or in the first vessel (2) and the main vessel (1), a dilute suspension of the dye in an alkaline electrolyte is prepared by circulating and heating the suspension, (ii) electrochemically treating the diluted suspension of step (i) in an electrolytic cell (4) to obtain a diluted leuco dye solution, (iii) sending the diluted leuco dye solution of step (ii) to the first container (2) or the main container (1) where it is supplied with additional dye by circulation and heating to obtain a suspension containing leuco dye, (iv) the suspension containing the leuco dye of step (iii) is then circulated through an electrolytic cell to electrolytically convert the dye into the leuco dye, or to electrolytically convert the suspension containing the leuco dye into a solution containing the leuco dye, respectively.

14. The electrolytic dye reduction method according to claim 13, wherein the preparation method further comprises step (v): (v) Repeating steps (iii) and (iv) one or more times with the solution of step (iv) and additional dye to form a leuco dye solution in an alkaline electrolyte.

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