GB2592306A

GB2592306A - A method and system for dyeing material

Info

Publication number: GB2592306A
Application number: GB2100842.0A
Authority: GB
Inventors: Mark Brook Jonathan; Nicholas Gledhill Timothy
Original assignee: Huddersfield Dyeing Co
Current assignee: Huddersfield Dyeing Co
Priority date: 2019-08-20
Filing date: 2019-08-20
Publication date: 2021-08-25
Anticipated expiration: 2039-08-20
Also published as: GB202100842D0; GB2592306B

Abstract

Material is dyed in a first dye liquor within a vat. The used first dye liquor is removed from the vat. The removed used first dye liquor is passed through an aerator. The aerated used first dye liquor is mixed with water. This is preferably used to form a second dye liquor. The dyed material is preferably fibrous. The preferred process involves extracting heat from some of the first dye liquor using heat exchangers and using this preheat the water and vat. Boilers are preferably used to further heat water. The preferred system uses venturi aerators.

Description

A Method and System for Dyeing Material

Field of the Invention

The present invention relates to a method and a system for dyeing materials. In particular the present invention relates to a method and system for dyeing materials such as fibres in a dye liquor.

Background the Invention

Materials, such as fibres, are commonly dyed using a solution of water and dye known as a dye liquor or dyebath. Dye liquors comprise water and dye and typically comprise additional dyeing auxiliaries such as levelling agents or pH modifiers; dye liquors are typically heated as materials are dyed therein.

During the dyeing of material using a dye liquor, dye is taken up from the dye liquor into the material, the water is typically de-oxygenated, and the pH of the dye liquor is typically changed. Additionally, during the dyeing of wool fibres, sulphates typically build up in the water of the dye liquor as a by-product of the dyeing process and the is water is hardened.

After material is dyed in the dye liquor, the used dye liquor is then typically released as wastewater or effluent, such as into a watercourse. This release of the used dye liquor may have negative impacts on the environment and in some jurisdictions may necessitate the payment of specific fees or taxes.

After a first amount of material has been dyed using a dye liquor, dyeing additional material typically necessitates using a new batch of dye liquor. Producing such a new batch of dye liquor typically necessitates obtaining a new volume of water from a water source and then heating the water and adding dye and dyeing auxiliaries. Obtaining and heating new batches of water increases costs and energy consumption and may have further negative impacts on the environment from which the water is sourced.

Within the fibre dying industry it is important for many reasons to ensure consistency of take up of dye by material being dyed such that the colouring of multiple batches of material are as near to identical as possible. This is because several batches may be used to produce a single item, such as a roll of carpet, or a set of items, such as a range of fabrics, where colour consistency is important. Due to the different nature of natural fibrous materials, such as wool and cashmere, it is often difficult to maintain consistency of colour between batches.

Systems for dying fibrous materials, such as wool, are sometimes referred to as loose fibre dying systems. Other types of dying techniques include yarn dying, fabric dying and wool top dying. Each technique and related dying system require slightly different equipment and skills in order to carry out efficient and consistent dying of natural and synthetic fibres.

Another factor that has made consistent colouring difficult to achieve, in particular in loose fibre dying systems, is that the water carrying dye is often repeatedly heated, usually to boiling point. This heating of the water has been found to lead to hardening of the water which can affect the take up of dye.

Additionally, during maintenance or idle periods, the dyeing system cools, increasing the risk of anaerobic reactions commencing in the still warm water and bacteria can have degrading effects upon take up of dye as they can cause the water to become acidic having negative effects on the dyeing process.

Additionally, in many judications, operators of dyeing plants and related processing equipment are taxed by way of higher fuel costs and there is increasing pressure on processors to reduce their so-called carbon footprint and the amount of energy consumed, as well as to reduce the amount of liquid effluent or wastewater that has zo to be returned to water courses.

An aim of the present invention is to provide an improved method and system for dyeing materials.

Statement of the Invention

According to a first aspect of the present invention, there is provided a method of dyeing material, the method comprising the steps of: dyeing material in a first dye liquor in a vat; removing used first dye liquor from the vat; passing removed used first dye liquor through an aerator; and mixing the aerated used first dye liquor with water.

Removing used dye liquor from a vat in which material is dyed and passing it through an aerator may advantageously improve its quality before it is mixed with water, either for subsequent reuse in additional dye liquors, or during or before its release as effluent or wastewater.

In preferred embodiments, the method comprises the additional step of forming a second dye liquor comprising the mixed water and aerated used first dye liquor. The method may comprise the additional step of dyeing additional material in second dye liquor comprising the mixed water and aerated used first dye liquor.

Such a second dye liquor may be prepared by adding dye and optionally additional dyeing auxiliaries (such as one or more levelling agents or pH modifying agents) to the mixture of the water and aerated used first dye liquor.

The first dye liquor preferably comprises a solution of dye and water and may additionally comprise one or more dyeing auxiliaries (such as one or more levelling agents or pH modifying agents).

When the material is dyed in the first dye liquor, dye comprised by the first dye liquor will be taken up by the material, reducing the concentration of dye within the first dye liquor. The material is preferably dyed in the first dye liquor until the majority of, or substantially all of the dye of the first dye liquor is taken up by the material. After this step is complete the first dye liquor may be described as spent, exhausted, or nearly fully exhausted.

Additionally, during the first step of the method, the water comprised by the first dye liquor may be hydrolysed, be de-oxygenated, be hardened, increase in pH, and/or increase in concentration of sulphates.

The material dyed in the first dye liquor may be or may comprise fibres. These fibres may be or may comprise vegetable fibres such as cotton, animal fibres such as wool, and/or synthetic fibres such as nylon or Viscose (RIM). The fibres may be loose fibres and the material may be dyed in the first dye liquor using loose stock dyeing techniques. Loose stock dyeing is the dyeing of loose fibres before they are woven or otherwise bonded to form a fabric. Loose stock dyeing is a common method for dyeing wool.

The vat in which the material is dyed in the first dye liquor is preferably a dye pan or dyeing machine, such as a conical-pan loose stock dyeing machine or a radial flow loose stock dyeing machine.

In preferred embodiments the first dye liquor is heated. The first dye liquor may be heated within the vat in which the material is dyed in the first dye liquor, for example by a heating means within, comprised by or in thermal communication with the vat. Alternatively, or additionally, the first dye liquor may be heated before it is introduced to the vat and/or water may be heated before dye or concentrated liquor is added thereto to form the first dye liquor, for example, by a boiler.

In some embodiments, the method comprises the additional step of removing the material which has been immersed in the first dye liquor from the vat after it has been dyed therein. The material may be removed from the vat before, during and/or after first dye liquor being removed from the vat.

In the step of removing used first dye liquor from the vat, some, the majority, substantially all, or all of the used first dye liquor within the vat may be removed therefrom. In some embodiments, the used first dye liquor may be removed from the vat through one or more outlets. One or more valves and/or pumps may be operated to control the removal of the used first dye liquor from the vat.

Some of, the majority of, substantially all of or all of the used first dye liquor removed from the vat may subsequently be passed through an aerator. In some embodiments, a first volume of the removed used first dye liquor may be passed through an aerator and a second volume of the removed used first dye liquor may be delivered elsewhere, for example to a heat exchanger and/or to be released as wastewater or effluent.

The used first dye liquor which is passed through the aerator may be delivered to the aerator from the vat via a conduit, for example an aerator may be installed on a conduit between the vat and a holding or mixing tank to which the used first dye liquor is delivered from the vat in which the material is dyed.

In some embodiments, the removed used first dye liquor may be passed through the aerator after removal of the used first dye liquor from the vat is complete; in other embodiments used first dye liquor may be passed through an aerator simultaneously to the removal of used first dye liquor from the vat.

Aerating the used first dye liquor may advantageously reoxygenate the dye liquor 30 and/or the water comprised thereby. Reoxygenating the water may advantageously improve the properties of a mixture of water and used first dye liquor for subsequent reuse (for example as pad of a second dye liquor) or may reduce the negative impact on the environment when the used first dye liquor is mixed with environmental water during its release as effluent or wastewater.

Some of, the majority of, substantially all of, or all of the used first dye liquor passed 5 through the aerator may subsequently be mixed with water.

In some embodiments, some, the majority, substantially all, or all of the aerated used first dye liquor may be delivered to a tank from the aerator where it may be mixed with water. Alternatively, or additionally, some, the majority, substantially all, or all of the aerated used first dye liquor may be mixed with water when it is released as effluent or wastewater.

Mixing aerated removed used first dye liquor with water in a tank may advantageously decrease the concentration of remaining dye and/or dyeing auxiliaries within the dye liquor and other impurities such as sulphates introduced by the dyeing process. This may allow the mixture of water and aerated used first dye liquor to be reused, for example to produce a second dye liquor.

The water with which the aerated used first dye liquor is mixed is preferably preheated (in such embodiments, the first dye liquor is preferably heated before and/or as the material is dyed therein). The mixture of water and aerated first dye liquor may therefore advantageously be at a higher temperature than their surroundings, for example for use as a second heated dye liquor.

In preferred embodiments, a first volume of the used first dye liquor removed from the vat is passed through an aerator and a second volume of the removed used first dye liquor is passed through a heat exchanger to transfer heat from the second volume of used first dye liquor to the water some or all of which may be mixed with the aerated first volume of used first dye liquor and/or may be used to heat a vat in which material is dyed.

In such embodiments, thermal energy from the second volume of the used first dye liquor is transferred to water, some or all of which may then be mixed with the first volume of the used first dye liquor -for example, to form a second dye liquor -and/or may be used to heat a vat in which material is dyed.

A mixture of and the first volume of the used dye liquor and water to which heat is transferred from the second volume (such as a second dye liquor) will therefore comprise and retain the water and heat of the first volume of the used first dye liquor as well as heat from the second volume of the used first dye liquor. Therefore, a large amount of heat from the first dye liquor may be retained along with a portion of the water while retaining less of the impurities or remaining non-exhausted dye or dyeing auxiliaries.

Alternatively, or additionally, water to which heat is transferred from the second volume of the used dye liquor may be used to heat a vat in which material is dyed, such as in a second dye liquor formed from recycled used dye liquor (or a first volume thereof), thereby transferring the reclaimed heat to a subsequent dyeing process.

The first and second volumes of used first dye liquor may be substantially all, or all of the used first dye liquor removed from the vat.

The water which is mixed with the aerated first volume of used first dye liquor may be 15 passed through the heat exchanger as it is transferred from a water source to a tank or other location where it is mixed with the aerated used first dye liquor.

In some such embodiments, all or some of the second volume of removed used first dye liquor is passed through a first heat exchanger to transfer heat from that used first dye liquor to a first volume of water, which may then be mixed with the aerated first volume of used first dye liquor, and all or some of the second volume of removed used first dye liquor is passed through a second heat exchanger to transfer heat from that used first dye liquor to a second volume of water, which may be used to heat a vat in which material is dyed (for example, using a dye liquor comprising a mixture of water and aerated used dye liquor).

In some such embodiments, all of the second volume of the used first dye liquor is passed through both the first heat exchanger and the second heat exchanger. The second volume of the used first dye liquor may be passed through the second heat exchanger and then the first heat exchanger or may be passed through the first heat exchanger and then the second heat exchanger.

In some such embodiments, the first volume and/or the second volume of water may be heated after passing through the heat exchanger and before being mixed with the aerated first volume of used first dye liquor, or being used to heat a vat in which material is dyed (for example using a heat well, an economiser, and/or a boiler).

A second volume of water which is heated within the second heat exchanger and which is used to heat a vat in which material is dyed may be heated (in the heat exchanger, or subsequently as described above) to produce steam, In particularly preferred embodiments, this second volume of water is passed through the second heat exchanger to receive heat from the second volume of removed used first dye liquor, and may then be subsequently heated (for example, to around 115°C using a heat well and an economiser) before entering a steam boiler, where steam is produced from it. The steam is preferably then used to heat one or more vats in which material is dyed.

The first volume of water passed through the first heat exchanger may be mixed with the aerated first volume of used first dye liquor (for example, in a tank) and the second volume of the water passed through the second heat exchanger may be used to heat one or more vats for dyeing material.

In preferred embodiments, the second volume of the used first dye liquor is released as wastewater or effluent (for example, into a natural watercourse) after being passed through the one or more heat exchangers for transferring heat to water for mixing with the aerated first volume of the used first dye liquor and/or for heating vats for dyeing zo material.

In some embodiments, the second volume of the used first dye liquor will advantageously be cooled by passing it through the heat exchanger or heat exchangers, which will reduce any negative impact it has on the environment when it is released as wastewater or effluent.

In some embodiments, the second volume the used first dye liquor may be passed through and/or stored within a tank between being removed from the vat and being passed through a heat exchanger and/or between being passed through the first and second heat exchangers. The tank may be wastewater or effluent tank.

In some embodiments, the second volume of the used first dye liquor is passed 30 through an aerator. The second volume of the used first dye liquor may be passed through an aerator before passing through the one or more heat exchangers, before passing to the tank, and/or after being passed through the one or more heat exchangers and before being released as wastewater or effluent. This may advantageously reduce any negative impact it has on the environment when it is released as wastewater or effluent.

In preferred embodiments, the second volume of the used first dye liquor is passed through one or more filters. The filters preferably being for removing fibres from the used dye liquor. The second volume of the used first dye liquor is preferably passed through one or more filters before passing through the one or more heat exchangers, as material such as fibres within the used dye liquor may coat the interior of the one or more heat exchangers thereby reducing their efficiency. Alternatively, or additionally, the second volume of the used first dye liquor may be passed through one or more filters before passing to the tank, and/or after being passed through the one or more heat exchangers and before being released as wastewater or effluent. This may advantageously The used first dye liquor or first volume thereof which is passed through an aerator and which is to be mixed with water may be transferred to a tank after passing through the aerator and may be mixed with water within the tank. The tank is preferably a tank for mixed water and used first dye liquor (and/or a second dye liquor formed therefrom) to be transferred to the vat and/or to other vats for dyeing materials from. For example, one or more conduits for delivering mixed water and used first dye liquor (and/or a second dye liquor formed therefrom) may extend from the tank to one or more vats for dyeing material.

In some embodiments the removed used first dye liquor or first volume thereof, which is passed through an aerator before being mixed with water, may be passed through an aerator and one or more filters before being mixed with water. The removed used first dye liquor or first volume thereof may be passed through a filter before being passed through the aerator or may be passed through a filter after passing through the aerator and before being mixed with water. In preferred embodiments the filter is for removing fibres from the removed used first dye liquor.

The aerator through which the used first dye liquor or first volume thereof is passed may be a venturi aerator such as a Venturi tube. Venturi aerators comprise a conduit which narrows from a wider section to a narrow section, as a consequence of the Venturi effect, the pressure of a fluid passing through the conduit drops as the conduit narrows. This drop in pressure is used to suck air or another gas into the fluid to aerate it.

As described above, the method may comprise the additional step of forming a second dye liquor comprising the mixture of water and the aerated used first dye liquor or an aerated first volume thereof.

The second dye liquor may be formed by adding dye to the mixed water and aerated first dye liquor. Dyeing auxiliaries, such as levelling agents and/or pH modifiers, may also be added to the mixed water and aerated first dye liquor.

The second dye liquor is preferably formed in a vat for dyeing material. Such a vat may be the vat in which material was dyed in the first dye liquor or may be another vat. The mixture of aerated used first dye liquor and water may be transferred to the vat from the tank.

In alternative embodiments, the second dye liquor may be formed in the tank in which the aerated used first dye liquor is mixed with the water, or may be formed intermediate the tank in which the aerated used first dye liquor is mixed with the water and the vat for dyeing material with the second dye liquor, for example in a vat to which the mixture is transferred from the tank and from which second dye liquor is transferred to the vat in which additional material is to be dyed in the second dye liquor.

In preferred embodiments, the second dye liquor is for dyeing material to a darker colour than the first dye liquor or comprises a darker dye than the first dye liquor. This may advantageously prevent any remaining non-exhausted dye from the first dye liquor contaminating the colour of material dyed using the second dye liquor.

According to a second aspect of the invention there is provided a system for dyeing material, the system comprising: a dyeing vat for dyeing material in dye liquor therein; a mixing tank; an aerator; a first transfer means for transferring used dye liquor from the dyeing vat to the mixing tank via the aerator; and a water delivery means for delivering water to the mixing tank.

The system may be used to perform the method of the first aspect of the invention, by dyeing material in a first dye liquor within the dyeing vat, using the first transfer means to pass the used first dye liquor through the aerator to the mixing tank, and using the water delivery means to deliver water into the mixing tank to mix with the aerated used first dye liquor.

In preferred embodiments, the system comprises a filter and the first transfer means is for transferring used dye liquor from the dyeing vat to the mixing tank via the aerator and the filter. The first transfer means may pass the used dye liquor through the filter and then the aerator before delivering it to the mixing tank.

In some embodiments the system comprises a plurality of dyeing vats. Such a system may comprise a plurality of first transfer means, each for transferring used dye liquor from one of the plurality of dyeing vats to the mixing tank via the aerator or via one of a plurality of aerators comprised by the system.

The one or more dyeing vats may be dye pans or dyeing machines, such as conical-pan loose stock dyeing machines or radial flow loose stock dyeing machines.

In some embodiments the system comprises one or more transfer means for transferring liquid (such as mixed water and aerated used dye liquor, or a second dye liquor formed therefrom) from the mixing tank to the dyeing vat or one of the plurality of dyeing vats. In other embodiments the system may comprise one or more transfer means for transferring liquid (such as mixed water and aerated used dye liquor) from the mixing tank to a vat for forming a dye liquor from mixed water and aerated used dye liquor (for example, by adding dye and optionally dyeing auxiliaries thereto) and may comprise one or more transfer means for transferring dye liquor from said vat to the dyeing vat or to one of the plurality of dyeing vats.

In some embodiments the system further comprises a means for transferring used dye liquor from the dyeing vat (or a plurality of such means for transferring used dye liquor form the plurality of dyeing vats) to a wastewater or effluent holding tank. In some embodiments this means may deliver the used dye liquor to the wastewater or effluent holding tank via an aerator.

In preferred embodiments the system comprises a means for transferring used dye liquor from the dyeing vat (or from each of the plurality of dyeing vats) or from the wastewater or effluent holding tank to or via a heat exchanger arranged to transfer heat between used first dye liquor transferred thereto and water being delivered therethrough by the water delivery means for delivering water to the mixing tank.

Alternatively, or additionally the system may comprise a means for transferring used dye liquor from the dyeing vat (or from each of the plurality of dyeing vats) or from the wastewater or effluent holding tank to or via a heat exchanger arranged to transfer heat between used first dye liquor transferred thereto and water being delivered therethrough by a means for delivering water to a means for heating the water (such as a steam boiler) for heating the one or more of the dyeing vats.

The system may comprise a transfer means for transferring used dye liquor from one or both of the heat exchangers described above to a wastewater or effluent outlet or drain, such as for releasing wastewater or effluent into a watercourse. Such a transfer means may transfer the used dye liquor to the outlet or drain via an aerator.

Alternatively, or additionally, the system may comprise a transfer means for transferring used dye liquor from one or both of the heat exchangers described above back to the wastewater or effluent holding tank.

In some embodiments, the system may comprise a boiler, heat well and/or economiser 15 for heating water for heating the one or more dyeing vats.

One, any, some or all of the transfer means described above may be or may comprise conduits, pipes, valves and/or pumps.

The system may further comprise any optional feature described above with reference to the method of the first aspect of the invention.

According to a third aspect of the invention there is provided a dye liquor comprising mixed water and an aerated used other dye liquor. The dye liquor may be produced by a method according to the first aspect of the invention and/or using a system according to the third aspect of the invention.

According to a fourth aspect of the invention there is provided material dyed using the method of the first aspect of the invention, using the system of the second aspect of the invention, and/or using the dye liquor of the third aspect of the invention.

Preferred embodiments of the invention will now be described with reference to the figures.

Brief Description of the Figures

Figure 1 is a diagram of a system according to the second aspect of the invention for performing a method according to the first aspect of the invention.

Detailed Description of the Figures

Figure 1 is a block diagram of a system 100 according to the second aspect of the invention for performing a method according to the first aspect of the invention.

The system comprises a plurality of dye pans 30, a water mixing and holding tank 20, an effluent holding tank 40, a plurality of filters 60, a plurality of aerators 65, 70, two heat exchangers 50, 55, a heat well 75, an economiser 80 and a boiler 85. The system 100 is arranged to receive water from a water source 10 and to release effluent to a drain 90.

The system 100 receives water from a water source 10 such as a canal or other watercourse. Water is collected to from the water source 10 and transferred to a water holding and mixing tank 20 via a first route 12 and to the boiler 85 via a second route 14.

The first route 12 passes collected water through a first heat exchanger 50 within which thermal energy is transferred from used dye liquor to the water so as to heat the water before it is delivered to the water holding and mixing tank 20.

The second route 14 passes collected water through a second heat exchanger 55, the heat well 75, the economiser 80 and then to the boiler 85. The water may be boiled within the boiler 85 to produce steam for heating the plurality of dye pans 30.

The water passed along the second route 14 to the boiler 85 is heated by a greater number of steps and components of the system than the water passed along the first route 12 to the holding and mixing tank 20. The water delivered to the boiler 85 by the second route 14 is therefore hotter than the water delivered to the holding and mixing tank 20 by the first route 12.

Within the water holding and mixing tank 20 the water delivered by the first route is mixed with filtered and aerated reclaimed used dye liquor delivered from the dye pans 30 via a first dye liquor recycling route 32.

The mixture of water and of filtered and aerated reclaimed used dye liquor is delivered to the plurality of dye pans 30 via route 22. Additional dye and dyeing auxiliaries (such as levelling agents and pH modifiers) are then added to the mixture to form a new dye liquor within the pan 30. The new dye liquor may use darker dye than the dye liquor which was reclaimed and mixed into the water.

The dye pans 30 are heated by steam produced by the boiler 85 from the hot water delivered by the second water route 14. Material, such as woollen fibre is then dyed within the dye pans 30. During the dyeing process, dye is taken up from the dye liquor into the material, such that the dye liquor is substantially exhausted. After the dyeing of the material is complete the material is removed form the dye pans and the used dye liquor is removed from the dye pans via first and second dye liquor recycling routes 32 and 34.

The first dye liquor recycling route 32 transfers a first volume of the used dye liquor from the dye pans 30 to the holding and mixing tank 20 where it may be mixed with water as described above to repeat the process. The dye liquor passed along the first dye liquor recycling route 32 is passed through one of a filter 60 and an aerator 65.

The second dye liquor recycling route 34 transfers a second volume of the used dye liquor from the dye pans 30 to the effluent holding tank 40. The dye liquor passed along the second dye liquor recycling route 34 is passed through an aerator 70.

Used dye liquor from within the effluent holding tank 40 is passed along a route 42 from the effluent holding tank 40, through the second heat exchanger 55, and back to the effluent holding tank 40, so as to transfer heat from the used dye liquor to water passing along the second water route 14.

Used dye liquor from within the effluent holding tank 40 is also passed along a route 44 from the effluent holding tank 40, through the first heat exchanger 50, to an outlet or drain 90 where it is released as wastewater or effluent, for example into a watercourse. This transfers heat from the used dye liquor to water passing along the first water route 12 and cools the used dye liquor before it released as effluent. Optionally, this route 44 also passes used dye liquor through an additional aerator (not shown) before it is released as effluent or wastewater.

In an example of a method of operating the system 100, the water collected from the source 10 is at approximately 10°C to 17°C and the water passed along the first water route 12 is delivered to the holding and mixing tank 20 at approximately 40°C after being heated by the first heat exchanger 50.

The water passed along the second route 14, is heated to approximately 52°C by the second heat exchanger 55, to approximately 80°C by the heat well, to approximately 115°C by the economiser 80 and is boiled to form steam by the boiler 85 before being used to heat the dye pans 30.

The used dye liquor reclaimed from the dye pans via the first and second routes 32, 34 is at approximately 60°C. The mixture of water and used dye liquor produced in the mixing and holding tank is at approximately 50°C. The used first dye liquor released to the drain 90 after passing through the first heat exchanger (and optionally the second heat exchanger beforehand) is at approximately 25°C.

The invention has been described by way of example only and it will be appreciated that variation may be made to the described embodiments without departing from the scope of the invention as defined by the claims.

Claims

Claims 1. A system for producing a diluted dye liquor mixture from a used dye liquor when dyeing a material, the system comprising: a dyeing vat for dyeing the material in a dye liquor; a mixing tank; an aerator; a first transfer means for transferring used dye liquor from the dyeing vat to the mixing tank via the aerator; a water delivery means for delivering water to the mixing tank; and a pump which removes the used dye liquor from the vat, characterised in that a first volume of the used first dye liquor is passed through an aerator and a second volume of the used first dye liquor is passed through a heat exchanger to transfer heat from the second volume of the used dye liquor to the water, the water is mixed with the aerated first volume of the used dye liquor to produce a diluted dye liquor mixture.
2. A system according to claim 1 wherein the aerator, through which the used first dye liquor, or a first volume thereof, is passed is a venturi aerator.
3. A system according to claim 1 or 2 includes a filter through which the diluted dye liquor mixture, or a first volume thereof, is passed.
4. Material dyed using the diluted dye liquor mixture of any of claims 1 to 3.
5. Material according to claim 4 wherein the material is a fibrous material.