WO1993015258A1

WO1993015258A1 - Process of removing dyes

Info

Publication number: WO1993015258A1
Application number: PCT/GB1993/000135
Authority: WO
Inventors: Francis Michael John Mccollam
Original assignee: W.L. Gore Associates (Uk) Limited
Priority date: 1992-01-24
Filing date: 1993-01-22
Publication date: 1993-08-05
Also published as: GB9201496D0; AU3362093A; GB9301275D0; JPH07503264A; GB2263476B; GB2263476A; EP0623181A1; CA2128229A1

Abstract

Residual reactive dye remaining in a salt-containing dye bath after use is removed by adsorption onto active carbon particles, and subsequent removal of the carbon by filtration. The presence of salt enhances the adsorption of dye onto the active carbon. Filtration may be carried out using a hollow fibre crossflow filter containing expanded porous polytetrafluoroethylene hollow fibres, or by back-pulse filtration.

Description

PROCESS OF REMOVING DYES

FIELD OF THE INVENTION

The present invention relates to a process of removing dye from a dyeing solution comprising a dye and a salt, so that the cleaned salt solution may be reused or dumped; and to an apparatus for carrying out the process.

PRIOR ART

According to conventional dyeing technology, cloth is treated with a solution of a reactive dye. In order to encourage the dye to leave the solution and enter the cloth fibres, it is normal to include a substantial concentration of a salt (such as sodium sulphate or sodium chloride) in the dyeing solution typically in amounts of 20 to 100 g/L depending on the dye and depth of shade required. The presence of the salt effectively reduces the solubility of the dye in the solution and promotes adsorption of the dye onto the cloth by disturbing the equilibrium between dissolved dye and dye on the cloth. Typically, 80% of the dye in the dyeing solution is transferred onto the cloth. Of this 80%, typically 70% is reacted with the cloth, whilst the remaining 10% of dye is unreacted. Currently, the used dyeing solution comprising residual dye and salt is then dumped. However, the disposal of the used dyeing solution in an environmentally acceptable way poses significant problems. In addition to salt, the dye bath may include conventional additives such as sequesterants, lubricants, and alkalis.

Subsequently, the dyed cloth containing reacted and unreacted dye is washed with clean water a number of times. Typically, the first rinse uses cold water, followed by a second rinse at 60°C for 30 to 60 minutes; followed by a third rinse at 95°C; followed by two further rinses at 60°C. The exact sequence of washing steps employed will vary depending on the nature and concentration of the dye used, higher concentrations generally requiring more washing steps.

A typical dye bath capable of dyeing 250 kg of cloth will hold about 3,500 litres of liquid. Thus, that same volume of used dye and salt solution requires to be safely disposed of. Whilst in a five stage washing process, five times that volume of dilute dye solution also requires to be safely disposed of. This poses a significant problem.

It is an object of the present invention to mitigate these problems, and in particular to provide a process for the removal of dye from dyeing solution containing dye and salt.

SUMMARY OF THE INVENTION

The present invention provides a process of removing dye from a dyeing solution comprising dye and a salt, which comprises: - contacting the dyeing solution with particles of active carbon such that the dye becomes adsorbed onto the particles; and

- removing the particles having adsorbed dye thereon from the dyeing solution, so as to generate a cleaned salt solution of reduced dye concentration.

The invention also extends to a corresponding apparatus.

Active carbon is known to be useful for decolourising aqueous liquids. However, the invention is based on the surprising discovery that the presence of salt in the dyeing solution improves the adsorption of dye onto the active carbon particles. The absorption of dye is improved at salt concentrations as low as 1 g/L, though higher concentrations of 3, 5 and 10 g/L show particular enhancement. Typically reactive dye baths contain 30-90 g/L of salt (usually NaCl or Na2Sθ4) . On the other hand, the presence of salt has been found to adversely affect the adsorption of dye onto other solid substrates. It is also found that dye removal is improved at high pH, due for example, to the presence of NaOH or Na2Cθ3«

The process is particularly applicable to reactive dyes, i.e. those dyes which react with the material being dyed.

The active carbon is generally the crude charcoal material without any added substances. Suitable grades are available under the trademark Celite. The weight of active carbon used is generally from 0.1 to 20 times (preferably 1 to 20 times, particularly 2 to 10 times) the amount of dye in the solution. The particle size of the active carbon particles is usually in the range 0.1 to 200 microns (preferably 1 to 40 microns) . It is found that the salt in the dyeing solution is not absorbed to any significant extent by the active carbon.

The carbon particles having adsorbed dye thereon are then removed from the dyeing solution. This may be achieved by having the active carbon particles formed as a fixed bed through which the dyeing solution is passed. However, in a preferred embodiment, the carbon particles are freely dispersed within the dyeing solution and are then removed therefrom, for example by filtration, sedimentation or centrifugation. The type of filter used depends on the particle size of the active carbon and thus a variety of filtration processes will be suitable. However, in order to provide a continuous process, a cross-flow filtration technique or a back pulse filtration technique is advantageously employed.

In a cross-flow filter, the dyeing solution having dispersed active carbon particles therein is passed through the lumen of a bundle of fibres having porous walls (e.g. porous expanded polytetrafluoroethylene) . The cleaned salt solution (of reduced dye concentration) passes through the fibre walls, whilst the carbon particles are retained in a concentrated stream of active carbon particles. The active carbon particles having adsorbed dye thereon may then be filtered from the concentrated solution to leave a substantially solid residue which may then be safely disposed of in conventional manner e.g. by incineration or as landfill. The cleaned salt solution may then be reused.

Back pulse filtration typically involves passing the dyeing solution and dispersed carbon particles through a candle filter, the solution being passed from outside to inside the filter. Candle filters are known in the art and comprise a hollow sleeve of filter fabric which is closed at the lower end and open at the upper end (the outlet) , and is supported on a frame. The open end opens into a plenum chamber for receiving the filtrate. A particularly useful filter fabric comprises porous PTFE membrane laminated to a fiberglass or polyester support material (available under the Gore-tex trade mark) . Carbon is filtered out and deposits on the outside of the candle filter. Periodically, the flow of liquid is reversed in a back-flow pulse or pulses to dislodge the accumulated filter cake, which settles on the bottom of the filter vessel. The solid filter cake can then be removed, e.g. by a screw conveyor.

Depending on the residual concentration of dye in the cleaned salt solution, this solution may be used in some or all of the subsequent washing stages to remove unreacted dye from the cloth. For example, the clean salt solution may be used to rinse the dyed cloth. The used salt rinse solution is then cleaned by addition of active carbon to adsorb dye, followed by removal of the active carbon particles, and recycling of the cleaned rinse salt solution to the rinsing bath. In this way, rinsing and removal of dye may be carried out in a substantially continuous manner, leading to substantial reductions in the amount of rinse solution employed and speeding up the rinse operation.

Final rinsing before the cloth is dried may be carried out using pure salt-free water. This may also be achieved using carbon or other solid adsorbent in an analogous clean water rinse cycle, wherein the adsorbent is added and then removed from the water prior to recycling the cleaned water.

The process allows a reduction in time and heating energy to be achieved, together with savings in water and salt used. The adsorption of dye onto active carbon substantially alleviates the problems of disposal of large quantities of liquid effluent.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Embodiments of the present invention will now be described by way of example only with reference to the drawings wherein;

Figure 1 is a schematic flow diagram of a process according to the invention.

Figure 2 shows the variation in dye removal as a function of salt concentration; and

Figure 3 shows the cross flow filtration apparatus referred to in Example 7.

Figure 1 shows a dye bath B containing a roll of cloth 2 to be dyed. The bath contains a solution of a reactive dye and a salt. Dyeing is typically carried out at 95°C. After dyeing is completed, the used dye/salt solution is removed from the tank along line 4 and is collected in tank T. In the tank, active carbon particles C are introduced into the used dye/salt solution and the mixture is agitated until the dye becomes adsorbed onto the active carbon particles. The liquid is then pumped from the tank T by circulating pump P along line 6 to a cross-flow filter F. The cross-flow filter F comprises a bundle of hollow fibres formed of a porous material, such as polytetrafluoroethylene (PTFE) , polypropylene or polysulphone. The pore size is usually in the range 0.1 to 2 microns. The stream of dye/salt solution containing carbon particles having adsorbed dye thereon passes through the lumen of the fibres. A stream of cleaned salt solution having a reduced (or preferably zero) dye concentration passes through the walls of the fibre and exits from the cross-flow filter along line 8 and is returned for use in the dye bath B (or is stored for future dyeing operations) . The returned cleaned salt solution is used in bath B for further rinsing operations on the dyed cloth.

The concentrated stream containing carbon particles having dye thereon exits from the filter F along line 10 and is returned to the tank T. A portion of the recycle is diverted from line 10 along line 12 to D where the used active carbon having adsorbed dye is removed and disposed of. Alternatively, at the end of a dyeing operation the stream may be concentrated further by repeated passage through the filter before being discarded.

If desired, the cloth may then be subjected to an analogous rinse cycle using clean salt-free water, optionally using carbon or a different solid adsorbent active in salt free rinse water. The salt solution is replaced by clean water and the carbon replaced with fresh adsorbent, if desired. Alternatively, a second dedicated clean water rinse circuit (including a further tank T, pump P and filter F) might be provided in addition to the salt solution circuit shown.

The presence of salt in the dye solution improves the adsorption of dye onto the active carbon, as will be demonstrated hereafter in the following examples.

EXAMPLE 1 (comparison)

A solution of reactive dye, type Procion yellow HE4R was made up in water at 80°C to a strength of 1 g of dye per litre of water.

150 ml of this solution (0.15 g dye) was measured into a beaker and 2 g of activated carbon type Celite Z850 (NW) was added. This has a particle size wherein approximately 65% by weight is less than 45 microns, the balance being larger than 40 microns. This mixture was mechanically stirred for five minutes and then filtered through a number 5 filter paper onto a Buchner funnel. By colour comparison with samples of known dye concentration of Procion yellow HE4R the concentration of dye in the filtrate was judged to be about 0.5 g/litre.

EXAMPLE 2

The procedure of Example 1 was repeated except that the dye solution additionally included 5 g per litre of sodium chloride (analytical grade) so as to resemble a conventional salt/containing dye solution. 2 g of active carbon was added as before and the mixture stirred for five minutes before being filtered. The filtrate contained no dye as measured by visual assessment.

It can therefore be concluded that the presence of salt in the solution of reactive dye enhances the ability of the particulate active carbon to adsorb dye from the dye solution. This has application particularly in the dyeing of cotton and cotton-containing cloth. EXAMPLE 3

Further tests were conducted using the same type of activated carbon adsorbent and salt as in Examples 1 and 2. The same experimental procedure was also followed, but varying combinations of dye, dye concentration, salt concentration, pH and dye: adsorbent weight ratio were tested. Zero salt concentrations are included for comparison.

The colour of each filtrate from each combination was assessed visually as in Examples 1 and 2. Test details and resulting filtrate colour are shown in Table 1.

Remazol Blue R (Spec) 1.0 o 0.3 0.125 Remazol Blue R (Spec) 1.0 40 0 1.0 80 0

1.0 80 2.0 0.062

(a) pH was increased to 12.4 using sodium hydroxide

(b) pH was decreased to 2.0 using acetic acid.

EXAMPLE 4 (varying salt concentration)

Further tests were carried out at varying salt concentrations using the procedure of Example 1. The dye wa Procion Navy HER 150.

The results are given in Tables 2 and 3 and are graphed in Figure 2 which shows the variation in residual dye concentration as a function of salt concentration at dye: carbon absorbent ratios of 0.3 and 0.15.

TABLE 2

Dye Salt Dye:Adsorbent pH Filtrate D

Cone. (g/L) Cone. (g/L) Weight Ratio

1 0.3 0.93

3 0.74

5 0.84

10 0.63

20 0.45

40 0.39

Table 3

Dye Salt Dye: dsorbent pH Filtrate D

Cone. (g/L) Cone. (g/L) Weight Ratio fcr/L,

1 0. 15 0.40

3 0.21

5 0. 13

10 0.08

20 0.03

40 0.02 EXAMPLE 5 (varying pH)

Further tests were carried out at varying pH (by addition of NaOH or Na2Cθ3) in respect of two dyes.

The results for Procion Navy HER 150 are given in Table 4 and are given in Table 5 for Remazol Red RB. Improved dye removal at high pH is demonstrated.

EXAMPLE 6 (effluent treatment)

The following effluent was taken from a dye bath after dyeing:

Dyelube NF 1.75L

Drimagen ER 1.75L

NaCl 45g/L

Soda Ash 15g/L

Procion Yellow HE4R 28g

" Red HEGXL 1.092kg " Red HEXL 0.72kg

Total liquid volume = 1750L Cloth weight = 207.8kg

(a) In order to simulate low salt levels, 270 ml of water was added to 30 ml of the above effluent. Active carbon powder Celite Z850 (0.016g) was added to this 300ml solution and stirred for 5 ins.

The 300ml was then filtered as in previous Examples.

(b) This procedure was repeated, but salt was added to the 300ml of solution at 45g/L to approximately restore the original salt concentration.

The results showed that filtrate from the salt addition test was about one half the colour concentration of that with no additional salt. Thus the presence of high levels of salt (49.5g/L) in the effluent in case (b) doubled the level of dye removal as compared to case (a) where only low levels (4.5g/L) of salt were present. EXAMPLE 7 (crossflow filtration)

The following test was carried out to demonstrate reactive dye removal from a dye bath effluent using carbon adsorption following by crossflow filtration.

A sample of dye bath contents was removed from a dye bath at the end of dyeing.

The dye bath contents was as follows

8 litres of the dye bath liquor sample was placed in the tank of the pilot plant shown in Figure 3.

Valve Vrj; was closed before the 8 litres of liquor was poured into the tank. 37g of activated carbon powder type Celite Z 850 (Celite NW) was added to the tank. The tank contents were stirred for 5 minutes using a low shear hand held stirrer.

Valve Vrp was now opened with valves Vp, V_j and V₀ closed.

The pump P was turned on and Vj and V₀ opened to give Pj - 2 bar, P_σ = 1.5 bar, Pp = 1.5 bar. Valve Vp was now opened and filtrate collected (P_p = 0) . The filtrate flowrate was measured at 0.5 litres/minute.

The temperature was 20°C approx. The tube bundle comprised ten membrane tubes each of 6mm outside diameter, 4mm inside diameter and 0.5 metres long, giving a filtration area of 0.062 m².

The tubes were of Gore-Tex (trademark) expanded porous PTFE tubular membranes. The isopropanol bubble point of the tubular membranes used was 3psi. (see EP 0106494) .

The filtrate was colourless showing that substantially all the dye had been removed. The filtrate flux was 484 (litres/m² hour) .

The dye concentration was 1.05g/L at the start of dyeing. Based on 70% exhaustion removal of dye during dyeing, approximately 30% of dye would be left in the liquor with the other 70% being retained on the cotton. Hence at the end of dyeing around 0.315 g/L of dye is left in solution. Thus, the dye:adsorbent ratio is about 0.068.

Claims

1. A process of removing dye from a dyeing solution comprising dye and a salt, which comprises

- contacting the dyeing solution with particles of active carbon such that the dye becomes adsorbed onto the particles; and

2. A process according to claim 1 wherein the concentration of salt in the dyeing solution is at least 1 g/litre.

3. A process according to claim 1 wherein the salt concentration in the dyeing solution is at least 3 g/L.

4. A process according to claim 1 wherein the salt concentration in the dyeing solution is at least 5 g/L.

5. A process according to claim 1 wherein the salt concentration in the dyeing solution is at least 10 g/L.

6. A process according to any preceding claim wherein the dyeing solution has an alkaline pH.

7. A process according to any preceding claim wherein the salt is selected from sodium chloride and sodium sulphate.

8. A process according to any preceding claim wherein the weight of active carbon used is from 1 to 20 times the weight of dye in the solution.

9. A process according to claim 8 wherein the weight of active carbon used is from 2 to 10 times the weight of dye.

10. A process according to any preceding claim wherein the particle size of the active carbon is 1 to 40 microns.

11. A process according to any preceding claim wherein the particles are removed from the dyeing solution by filtration.

12. A process according to claim 11 wherein filtration is carried out by cross-flow filtration wherein the dyeing solution and carbon particles are passed through the lumen of a bundle of porous hollow fibres and the cleaned salt solution passes through the fibre walls.

13. A process according to claim 12 wherein the porous hollow fibres are formed of expanded polytetrafluoroethylene.

14. A process according to claim 11 wherein filtration is carried out in a back-pulse liquid filtration apparatus comprising a liquid filtration membrane.

15. A process according to claim 14 wherein the liquid filtration membrane comprises a porous polytetrafluoroethylene membrane laminated to a support fabric.

16. The use of active carbon particles for the removal of dye from a salt-containing dyeing solution.