This invention relates to an apparatus and a system for reduction of dye. This invention has particular, but not exclusive, application in the field of dyeing involving electrochemical reduction.

BACKGROUND

Sulphur dyes are one of the most commonly used dyes in the dyeing industry. They are inexpensive, generally have good wash-fastness and are easy to apply. Out of all the sulphur dyes perhaps 50% of production is of the sulphur black color as black is the most popular fabric color. The Sulphur black is insoluble in water and cannot be carried by water into fibers until made water-soluble. It has to be treated with a reducing agent and an alkali at a temperature of around 80 degrees Celsius where the dye breaks into small particles which then becomes water soluble and hence can be absorbed by the fabric. Heating and adding a substance like common salt facilitates the absorption. After this the fabric is removed from the dye solution and then taken for oxidation. This oxidation can be done in air or by using oxidizing agents, such as hydrogen peroxide or sodium bromate, in a mildly acidic solution. Oxidation renders the dye insoluble in water again which facilitates fixing the dye permanently to the fabric. The above chemical processes are carried out in dyeing baths.

The popular reducing agent used above is sodium dithionite. However, the disposal of dyeing baths containing the unspent reducing agent and the by-products of the reduction and oxidation processes (sulphites, sulphates, thiosulphate and sulphide) above is toxic and can pollute water bodies in the proximity, resulting in environmental pollution. In addition, as a result of the considerable quantity of reducing agent required to stabilize the oxidation-sensitive dyeing baths, the waste water may contain excess dithionite which affects aerobic processes in waste-water treatment. The problems described above have been mitigated by employing electrochemical reduction processes which does away with using the reducing agent described above, resulting in non-polluting byproducts from the dyeing process.

The electrochemical bath used for reducing sulphur dye comprises a membrane dividing the oxidizing and reducing portions of the bath, the oxidizing and reducing portions comprising an anode and cathode respectively. The membrane used in the electrochemical bath is a cation exchange membrane, which allows cations to pass through the same enabling the electrochemical process to take place. Apart from allowing cations to pass through them to carry out the reduction process, the cation exchange membranes also allow anions such as hydroxide ions to leak through the membrane which is undesirable. Another disadvantage of using a cation exchange membrane is a reduction in the efficiency of the electrochem ical processT

SUMMARY

In accordance with an aspect of the invention, there is disclosed an apparatus for reduction of dye. The apparatus comprises an electrochemical tank arranged for containing an electrolyte. The electrochemical tank is further arranged to contain a cathode and an anode adapted for electrochemical reaction through the electrolyte. The apparatus further comprises a membrane that divides the electrochemical tank into an oxidation portion and a reduction portion. The oxidation portion comprises the anode and the reduction portion comprises the cathode. The membrane comprises a carboxylate layer. The apparatus works such that the dye contained in the reduction portion is reduced chemically when electrically driven by the anode and the cathode.

In accordance with another aspect of the invention, there is disclosed a system for reduction of dye. The system comprises an electrochemical tank for containing an electrolyte. The electrochemical tank is further arranged to contain a cathode and an anode adapted for electrochemical reaction through the electrolyte. The system further comprises a plurality of membranes disposed between the cathode and the anode. Each of the plurality of membranes comprises a carboxylate layer. The system also comprises a plurality of bipolar plates, wherein each of the plurality of bipolar plates is disposed between a pair of the plurality of membranes. The system works such that the dye contained in the electrolyte is chemically reduced when electrically driven by the anode and the cathode.

BRIEF DESCRIPTION OF DRAWINGS

FIG 1 shows an exemplary arrangement of an electrochemical cell used for the reduction of sulphur dye

FIG 2 shows an exemplary arrangement of the anode and the cathode with respect to the membrane

FIG 3 shows another exemplary arrangement of the anode and the cathode with respect to the membrane

FIG 4 shows an exemplary arrangement of a system used for the reduction of sulphur dye DETAILED DESCRIPTION

Sulphur dyes are the most commonly used dyes for dyeing cotton fabrics. As described earlier in the background section, as sulphur dyes are insoluble in water, they have to be reduced to break the dye into smaller particles. The smaller particles of the dye are soluble in water. This enables the dye to be absorbed by the fabric. The dye along with the fabric is then oxidised to fix the dye to the fabric permanently. The dye intermediate comprising smaller particles of the dye is referred to as a leuco compound.

As described earlier, the electrochemical reduction process employs electrons to reduce the dye compound rather than chemical compounds. The electrochemical reduction and dyeing process is carried out in an electrochemical cell. An electrochemical cell is a device that is capable of either deriving electrical energy from chemical reactions, or facilitating chemical reactions through the introduction of electrical energy. For this invention, the second application of the electrochemical cell is utilized, i.e. facilitating chemical reactions through the introduction of electrical energy.

FIG 1 shows an exemplary arrangement of an apparatus for reduction of dye. As illustrated in FIG 1 , the apparatus 100 comprises an electrochemical tank 102. The electrochemical tank 102 can also be referred to as an electrochemical cell, an electrochemical bath or an electrochemical apparatus. The dye that is being reduced is a sulphur dye. Alternatively, other dyes can also be reduced. The electrochemical tank 102 is arranged for containing an electrolyte, the electrolyte being further described below. The electrochemical tank 102 further comprises a cathode 105 and an anode 1 10. The cathode 105 and the anode 110 are connected to an electrical battery 1 15, the battery being held externally to the electrochemical tank 102. The cathode 105 is connected to a negative terminal 1 15a of the battery 115 and the anode 1 0 is connected to a positive terminal 115b of the battery 115. As is understood by a person skilled in the art, the cathode 105 repels electrons from the surface and the anode 1 10 attracts electrons towards the surface. Reduction reactions take place at the cathode and oxidation reactions take place at the anode, which will be described further below. The anode 110 and the cathode 105 are adapted for electrochemical reaction through the electrolyte which will be further described below.

Between the cathode 105 and the anode 110, there is a placed a membrane 120 which functions like a salt bridge. Generally, a salt bridge in an electrochemical cell or tank separates the cell into two portions, one portion having the cathode and the other having the anode. In other words, the salt bridge separates the electrochemical cell into the reduction portion and the oxidation portion and allows only the flow of ions to maintain a balance in charge between the oxidation and the reduction portions of the electrochemical cell 100. The salt bridge does not allow the mixing of fluids in the oxidation portion and the reduction portion of the electrochemical cell. The voltage level of the electrical battery 1 15 is 1 V to 6V. Any other suitable voltage levels can also be used. A first portion of the electrochemical cell 100 comprising the cathode 105 is called a reduction portion 100a and a second portion of the electrochemical cell 100 comprising the anode 1 10 is called an oxidation portion 100b. In other words, the membrane 120 divides the electrochemical tank 100 into the reduction portion 100a and the oxidation portion 100b. The reduction portion 100a and the oxidation portion 100b comprise electrolytes in which the cathode 105 and the anode 1 10 are suspended or placed. In electrochemistry, an electrolyte is a conducting medium in which the flow of current (in the form of electron transportation) is accompanied by the movement of matter in the form of ions. Electrolytes are mostly liquids, but they can also be solids. Liquid electrolytes can be solutions of acids, bases and salts. In the electrochemical cell 100, the electrolyte used is Sodium Hydroxide (NaOH) in both the reduction portion 100a and the oxidation portion 100b. The electrolyte is mixed with water (H ₂ 0) in both the portions 100a and 100b. In the reduction portion 100a, apart from the electrolyte the sulphur dye to be reduced is also introduced. The membrane 120 comprises a carboxylate layer, which is further described below. The dye in the reduction portion 100a is chemically reduced when electrically driven by the anode 1 10 and the cathode 105.

The chemical formula of the sulphur dye is C ₆ H ₄ N ₂ 0 ₅ .

Described below are the reactions taking place at both the anode 1 10 and the cathode 105. The reactions at the anode are as follows:

NaOH _■ . > Na ^* + OH ^"

The sodium hydroxide molecules break down into sodium and hydroxide ions. The sodium ions are positively charged and the hydroxide ions are negatively charged. Further, the hydroxide ions breakdown into oxygen, water and electrons, which is illustrated in the reaction below:

40 — > 0 ₂ +.2H ₂ 0 + 4e ^'

The resulting oxygen gas bubbles up from the oxidation portion 100b. The electrons generated are attracted towards the anode 110. As shown in the above reactions, the sodium is oxidised in the oxidation portion 100b.

The sodium ions, being positively charged are repelled by the anode 1 10 and pass through the membrane 120, which will be further described below.

The reactions at the cathode are as follows:

2H ₂ 0 + 2e ^{' ■} > H ₂ + 20H The water molecules in the electrolyte react with the electrons from the cathode to produce hydrogen gas and hydroxide molecules. The hydrogen gas bubbles up from the reduction portion 100a. The hydroxide ions in the reduction portion 100a react with the positive sodium ions to form sodium hydroxide.

The free electrons in the reduction portion 100a react with the sulphur dye particles to reduce the particles. The reduced sulphur dye particles are now soluble in aqueous solution. The solution with the reduced sulphur dye particles are then routed or piped to a chamber (not shown) where the fabric to be dyed is placed. The reduced sulphur dye attaches to the fabric and imparts the colour to the fabric.

The membrane 120 used here also comprises a cation exchange layer which allows the movement of positive ions or cations through the same, apart from the carboxylate layer. The cation exchange layer is a sulphonate layer. The sulphonate layer that is used as a cation exchange layer can be a Nafion 324 membrane from DuPont. The properties of the Nafion 324 membrane are as follows:

(a) Thickness: 0.007 in

(b) Perfluorosulfonic acid cation exchange membrane with strong polytetrafluoroethylene fiber reinforcement

(c) The cell voltage is 3.74V

(d) The current efficiency is 97%

(e) NaOH percentage 2%

Other membranes from the Nafion family such as the Nafion 424 can also be used as the cation exchange layer or the sulphonate layer. The Nafion family of membranes are sulfonated tetrafluoroethylene based fluoropolymer-copolymers. In order to avoid confusion, the term "membrane" used in conjunction with Nafion is distinct from the membrane 120. Further, the term "membrane" used in conjunction with Nafion refers to the cation exchange layer or the sulphonate layer. Nafion "membrane" is a term that is commonly used in the relevant art and the relevant industry and that is the reason for its usage in the current description. The carboxylate layer is attached or coupled to the sulphonate layer so that the membrane 120 becomes a dual or double layered membrane. The side of the membrane 120 on the side of the anode 110 comprises the sulphonate layer and the side of the membrane 120 on the side of the cathode 105 comprises the carboxylate layer. The advantage of using the carboxylate layer attached to the sulphonate layer of the cation exchange membrane is to reduce leakage of hydroxide ions through the membrane when used in a dyeing electrochemical cell. This improves the efficiency of the electrochemical process as reduced leakage of hydroxide ions translates to better chemical separation of the oxidation portion 100b and the reduction portion 100a. Better chemical separation of the oxidation portion 100b and the reduction portion 100a is desired. In other words, this can improve the stability of the electrochemical process as well. The other advantage is to reduce protonation of the membrane. Protonation reduces conductivity. Hence, by reducing protonation, the conductivity of the ions is maintained thereby running the process of sulphur dye reduction for longer periods of time.

The sulphonate layer used is not limited to Nafion membranes as described above. Membranes similar to Nafion membranes and with similar properties can be used as well.

As illustrated in FIG 2, the membrane 120 has a first face 120a and a second face 120b, the first face being opposite the second face. To elaborate, the first face 120a is disposed on the side of the anode 110 and the second face 120b is disposed on the side of the cathode 105.

The cathode and the anode used are porous electrodes. Porous electrodes are also referred to as foam electrodes, mesh electrodes or 3-D electrodes because of their structure. Porous electrodes, because of their porosity have a surface area higher than non-porous electrodes, available for electron exchange and current collection during the process of oxidation and reduction in an electrochemical cell. The higher surface area enables speeding up the process of reduction of the sulphur dye in the reduction portion 100a and the oxidation of sodium in the oxidation portion 100b, To explain further, the electrolyte used in the electrochemical cell 100 can actually pass through the recesses and cavities in the porous electrode, thus utilizing the higher surface area of the electrodes. Alternatively, the anode 110 can be in the form of a plate or a sheet as well. In other words, the anode can also be plate or sheet electrode. The characteristics of the plate electrode and a porous electrode are compared below. Since the plate electrode has a substantially planar surface which- is non- permeable, the surface area available for the electrochemical reaction is a function of the surface area. In other words, the surface area available for the electrochemical reaction is approximately twice the surface area of the electrode plate, counting both the sides of the plate. In. the case of a porous or foam electrode, and as explained earlier, the surface area available for the electrochemical reaction also includes the surface area exposed to the pores. Hence, this substantially increases the surface area available for the electrochemical reaction. The thickness of the porous electrodes can range from 0.1 mm to 20 mm. For the purposes of an illustration, for a plate electrode and a porous electrode of the same dimension, the surface area of the porous electrode available for electrochemical reaction is far higher than the surface area available in a plate electrode. As the pores or holes in the porous electrode increase, the surface area available for electrochemical reaction also increases.

In the invention as described herein, if the anode 110 is a plate electrode and the cathode 105 is a foam electrode, the cathode 105 has a substantially higher surface area available for reduction when compared to the anode 110, the reason for which has been explained above. In this case, according to the principles of electrochemistry, the surface area of the cathode 105 used for reduction is equal to the surface area of the anode 110. Hence, the cathode 105 is not fully utilized for reduction. Whereas, if both the anode 110 and the cathode 105 are porous or foam electrodes, the surface area of both the anode 110 and the cathode 105 are approximately equal and therefore the anode 110 and the cathode 105 can be fully utilized for the electrochemical reduction, which increases the speed of the reaction.

In the invention as described herein, the anode 110 can be a plate electrode as well.

Moreover, the electrodes can also have a coating of a catalyst on the surfaces. The catalyst is coated on the surface of the electrode with the help of a polymer or an ionomer solution to enable the catalyst to remain coated on the electrode. To prepare the solution, the catalyst is mixed with the solution and then applied on the electrode. Alternately, the catalyst can also be coated on the surface of the electrode without the help of the polymer or ionomer.

The particle size of the catalyst is in the range of 1nm to 50nm.

The catalyst can be platinum black, ruthenium black, nickel, gold, palladium, platinum, silver, aluminium, antimony, cadmium, copper, indium, iridium, iron, kovar, lead, osmium, rhodium, ruthenium, tin, cobalt, stainless steel or manganese. If the electrodes are coated with the catalyst as mentioned above, then it is called a supported electrode or specifically, a supported cathode and a supported anode. If the electrodes are not coated with the catalyst as mentioned above, then they are called a non-supported cathode and non-supported anode. Supported anodes and supported cathpdes are preferred as the electrochemical reduction process is speeded up in the presence of a catalyst, which is understood by a person skilled in the art. By using a catalyst, the usage of hydrogen for reduction is enhanced or increased, thereby leading to increased speed of the reduction process. Increased speed of the reduction process leads to improved efficiency of the reduction process. In addition, even after the anode 110 and the cathode 105 are disconnected from the battery 115, the reduction continues for a period of time before dying down. This also contributes to the increase in efficiency of the overall reduction process.

The catalysts described above are different in the way they speed up the reduction process. Among the catalysts mentioned above, all of them speed up the electrochemical reduction process. The most effective among the above indicated catalysts is platinum black and the least effective among the above indicated catalysts is stainless steel. The difference in the effectiveness mentioned above translates into the speed of the reduction process. The catalyst to be used is selected based on the speed of the electrochemical reduction that is required.

FIG 2 shows an exemplary arrangement of the anode and the cathode with respect to the membrane. As illustrated in FIG 2, the cathode 105 and the anode 110 are each separated from the membrane 120 by a distance ranging from 0 mm to 20 mm. As the separation between the membrane 120 and anode 110 and the cathode ^~ 105 increases, the voltage drop-across the electrodes also increases, which results in a proportionately higher increase in current consumption from the battery 115. This is as per Ohm's law. As the current consumption increases, then the power consumed from the battery also increases reducing the efficiency of the process. Hence, the efficiency of the reduction process is inversely proportional to the separation between the membrane 120 and the electrodes (anode 110 and the cathode 105).

A separation of 0 mm to 20 mm is optimum when it comes to maintaining the efficiency of the process. Beyond 20 mm, the efficiency is reduced further as explained earlier.

FIG 3 shows another exemplary arrangement of the anode 110 and the cathode 105 with respect to the membrane 120. The anode 110 and the cathode 105 are placed adjacent the membrane 120. A surface 10a of the anode 1 0 which is adjacent the membrane 20 is coated with an ionomer as the same polymer as the membrane 120. In this context, the polymer refers to sulphonate in the membrane 120. An ionomer comprises repeated units of both electrically neutral units and a fraction of ionized units of a polymer. In this example, the ionomer comprises repeated units of the polymer of the membrane. This is to have better contact. The sulphonate is mixed with alcohol or water and then coated on to the surface of the anode 110 as a layer of ionomer 110b. The advantage of the above arrangement is to have better contact between the anode 110 and the membrane 120. To elaborate, the anode 110 and the membrane 120 are sandwiched together with the ionomer coating between the anode 110 and the membrane 120. The coating of ionomer between the anode 110 and the membrane 120 helps improve the contact between the anode 110 and the membrane 120.

Similarly, a surface 105a of the cathode 105 which is adjacent the membrane 120 is coated with an ionomer as the same polymer as the membrane 120 to form a layer of ionomer 105b. The advantage of using the ionomer is to have a better contact between the cathode 105 and the membrane 120. In the example of. FIG 3, there is zero or very minimal gap or separation between the membrane 120 and each of the anode 110 and the cathode 105. As mentioned earlier, the efficiency of the reduction process is inversely proportional to the separation between the membrane 120 and the electrodes. Hence as the separation between the membrane 120 and the electrodes is less, the efficiency of the reduction process is increased due to lesser consumption of power from the battery 115. In the example of FIG 3, both the anode 110 and the cathode 105 are porous electrodes, so that electrolyte can pass through the pores and the holes in the porous electrode or mesh electrode.

FIG 4 shows an exemplary arrangement of a system used for the reduction of sulphur dye. In the example of FIG 4, a system 400 for reduction of dye comprises an electrochemical tank 402 for containing electrolyte, which will be described further below. The electrochemical tank 402 further comprises an anode 404 and a cathode 406 adapted for electrochemical reaction through the electrolyte. As illustrated in FIG 4, the anode 404 and the cathode 406 are arranged at each of the electrochemical cell 402. The system 400 further comprises a plurality of membranes 408 disposed between the anode 404 and the cathode 406. Each membrane of the plurality of membranes 408 comprises a carboxyiate layer. The properties and the structure of each membrane of the plurality of membranes 408 is the same as the membrane 120 of the apparatus 100 as described above. In other words, the description of the membrane 120 of the apparatus 100 and the advantages associated therewith provided above is applicable to each membrane of the plurality of membranes 408. The system 400 further comprises a plurality of bipolar plates 410. Each of the plurality of bipolar plates 410 is disposed between a pair of membranes of the plurality of membranes 408. Bipolar plates are readily understood by a person of ordinary skill in the art. Bipolar plates are conductive plates used in an electrochemical cell or environment that acts as an anode for one cell and as a cathode for the next cell. The structure of the bipolar plate is further described below. Bipolar plates can be made of carbon, metal or conductive composite polymer.

As illustrated in FIG 4, each bipolar plate of the plurality of bipolar plates 410 comprises a first side 410a and a second side 410b. The first side 410a is arranged to operate as a cathode and the second side 410b is arranged to operate as an anode. This is explained below in conjunction with FIG 4. As illustrated in FIG 4, a bipolar plate 414 is disposed between membranes 412 and 416. As described earlier, the bipolar plate 414 comprises the first side 410a and the second side 410b. The membrane 412 is disposed between the anode 404 and the first side 410a of the bipolar plate 414 which operates as the cathode. This constitutes the first electrochemical cell for reduction of the dye. The membrane 416 is disposed between the bipolar plates 414 and 418. Specifically, the second side 410bjof the bipolar plate 414 operates as the anode for the membrane 416 and the first side 410a of the bipolar plate 418 operates as the cathode for the membrane 416. This constitutes the second electrochemical cell for reduction of the dye. This explanation can be applied to the next. set of bipolar plates and membrane. The anode 404 and the cathode 406 are connected to a positive terminal and a negative terminal of a power supply (not shown). The electrolyte surrounding the cathodes contains the dye to be reduced. Hence, when the anode 404 and the cathode 406 are connected to the power supply, the electrochemical reduction of the dye takes place, which is as described in the apparatus 100. After reduction of the dye, the reduced dye is drained from the cathode regions by suitable conduits (not shown) which is understood by a person skilled in the art.

The number of bipolar plates and membranes are not limited to that illustrated in FIG 4. Any number of membranes or bipolar plates can be used. The arrangement of the system 400 can be compared to that of connecting a plurality of electrochemical cells in series and specifically the apparatus 100 for reduction of dye as explained earlier. The advantage provided by the system 400 is increased yield of reduced dye per unit time. The example of FIG 4 is used to reduced sulphur dye, but is not limited to this alone. Other types of dyes can also be reduced with the example of FIG 4.

The resulting reduced dye indicated above is able to dye products at room temperature. The electrolyte and the dye to be reduced are generally pumped into the reduction portion to be reduced. With a higher flow rate of the electrolyte and the dye to be reduced going into the reduction portion of the electrolytic Ttank, the yield of the ^~ reduced dye is higher: It is to be understood that the foregoing description is intended to be purely illustrative of the principles of the disclosed techniques, rather than exhaustive thereof; and that changes and variations will be apparent to those skilled in the art, and that the present invention is not intended to be limited other than as expressly set forth in the following claims.