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«EUROPEAN COMMISSION Integrated Pollution Prevention and Control (IPPC) Reference Document on Best Available Techniques for the Textiles Industry July 2003 ...»

-- [ Page 53 ] --

Plants in Spain, Belgium (UCO-Sportswear), Italy, Portugal, China, Turkey, India, Pakistan and Korea are operating with the Econtrol® process.

4.6.14 pH-controlled dyeing techniques Description Fibres such as wool, polyamide and silk contain weak acid and weak base groups (e.g.

carboxylic and amino functions). Just like the parent amino acids from which all proteins are derived, these fibres show zwitterionic characteristics at pH values close to the isoelectric point (i.e. the pH at which the fibre contains equal numbers of protonated basic and ionised acidic groups).

At a pH below the iso-electric point, the carboxylate anions are progressively neutralised by the

adsorption of protons and the fibre acquires a net positive charge (see equation 1):

(1) H3N+—(fibre)—COO- + H+ = H3N+—(fibre)—COOH

–  –  –

Conversely, as the pH rises above the isoelectric point, the fibre becomes negatively charged as a result of the dissociation of the carboxylic acid groups (equation 2) and deprotonation of the

amino groups by adsorption of hydroxide ions or other anions as shown below in equation 3:

(2) H3N+—(fibre)—COOH + OH- = H3N+—(fibre)—COO- + H2O (3) H3N+—(fibre)—COO- + OH- = H2N—(fibre)—COO- + H2O Based on these reactions, fibres with zwitterionic characteristics can be dyed by imposing a pH profile at iso-temperature, instead of a temperature profile at iso-pH.

The dyeing process is started in alkaline conditions, above the isoelectric point. At this pH, the carboxylic groups become dissociated and the anionic charged groups repulse anionic dyes.

This makes it possible to control the adsorption of the dye on the fibre by gradually decreasing the pH.

At a low enough pH when the number of cationic charges on the fibre increases, the dye becomes attracted to the fibre via coulombic interactions, which provides additional bonding forces that cannot be broken by thermal agitation.

At iso-pH, part of the carboxylic groups are neutralised and at higher temperatures, the dye can move rapidly and with minimal energy through the fibre.

The main difference between temperature- and pH-controlled dyeing is that in the temperaturecontrolled dyeing the process is controlled by the dye bath exhaustion and thermal migration of the dye, whereas with a pH-controlled profile the dyeing process is controlled by the adsorption of the dye onto the ionic fibre.

The pH profile can be controlled during the dyeing process either by dosing a strong acid or base or by creating a buffer system during the dyeing process (mixture of a weak acid and their conjugates base or vice versa). Two methods are normally used to generate a buffer system. One method is to dose a weak acid (e.g. acetic acid) starting from an alkaline bath containing a strong base (or a strong acid starting from a weak base); another method consists in using acidor base-donors for pH-sliding (ammonium sulphate and hydrolysable organic esters are examples of acid donors).

Main achieved environmental benefits One of the advantages of iso-thermal dyeing is that the use of special organic levelling agents or retarders (typically added to the dye bath to allow even dyeing) can be avoided.

Time and energy use with pH-controlled dyeing is lower than with the temperature-controlled process. Energy is saved because the dye bath (and the machine) do not need to be heated from room temperature up to the migration temperature (above the optimum dyeing temperature).

Time is saved because the heating and cooling phases are shorter and no extra time is required for the migration process.

Moreover, this technique offers new opportunities for recycling and recovery of spent dye baths.

With a pH-controlled system, the hot spent bath can be recycled as such for the next batch, instead of being cooled down before re-use. This is not possible in a temperature-controlled dyeing system because in that case the dyeing cycle cannot be started at the so called “treatment temperature”, but must be started at a lower temperature (e.g. 50°C) in order to prevent uneven dyeing.

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Operational data As stated earlier, pH steering during batch dyeing can be performed by fitting the machine with dosing systems for acids and alkalis. This is the best and most effective method because it minimises the amount of chemicals consumed to shift the pH. However, precise control of the pH profile with this method is difficult as the pH must be measured continuously and the bath must be fully homogenised. This technique is therefore limited to machines where the goods and liquor are well mixed, such as jets and modern carpet winches. Moreover, if a mineral acid (e.g. sulphuric acid) and an alkali are used, the salt content of the dye bath may increase above acceptable levels when recycling water.

Instead of using pH-measuring instrumentation another technique is the generation of a pHbuffer during the dyeing process. In this case, there is no need to measure the pH in a fully contained system. In fact, pH-chemistry and dynamic mass-balancing can predict pH and more importantly, can create a consistent repeatable pH profile [171, GuT, 2001]. For these reasons this technique, although more expensive (higher consumption of chemicals) and more polluting (higher organic load in the effluent), tends to be preferred by companies in the sector.





The use of decarbonated water is the best way to ensure optimal pH control, especially when weak acid donors are used (when process water is not decarbonated the acid will be consumed in the formation of CO2 rather than for shifting the pH of the bath).

Cross-media effects The application of the proposed technique doesn’t give rise to significant cross-media effects.

However, the thermal splitting of ammonium sulphate releases ammonia to the atmosphere.

Applicability The pH-controlled process is applicable to fibres with zwitterionic behaviour such as wool, polyamide, silk, etc. The technique is commonly applied in uni-dyeing processes, whereas it presents some limitations when blends of fibres are dyed to obtain differential shades (differential dyeing). Here, if the two (or more) fibre-types do not have compatible pHexhaustion/adsorption profiles, dyeing at iso-pH may be preferable.

The pH-controlled dyeing process is less common for fibres with only basic or only acidic functional groups. Neverthelss it is also advantageous for dyeing acrylic fibres with basic dyes and in principle it can be used for all types of fibres with “neutral pH-dyeable” reactive dyes.

The referenced technique is generally considered the most valuable technique in batchwise and continuous carpet dyeing and may set an example for other textile products [59, L. Bettens, 2000].

Economics The bath does not need to be warmed up and cooled down according to a preset temperature profile. The resulting saving in processing time is therefore one major economic advantage of this technique.

Additional benefits in terms of time and energy savings can be achieved when the hot spent dye bath is recycled because the dye liquor can then be re-used for the next dyeing cycle without the need to cool it down and warm it up again.

Investment costs, although fairly acceptable, are associated with fitting the dyeing machine with dosing and pH-control units.

No investment cost is required when the pH control takes place via buffer systems or acid/alkali donors.

–  –  –

Driving forces for implementation Time and energy savings are the main driving forces for the implementation of this technique.

Moreover, the technique overcomes the limited potential for dye bath recycling often found with temperature-controlled dyeing processes.

Reference plants The technique has been applied by many dyehouses (especially in the carpet sector) since the early seventies.

Reference literature [171, GuT, 2001], [59, L. Bettens, 2000] 4.6.15 Low-chrome and ultra-low-chrome afterchroming methods for wool Description Chrome dyeing of wool is still an extremely important process to obtain deep full shades at an economical price and with excellent fastness properties.

In 1995, the world market for wool dyestuffs was about 24000 t, with a higher percentage in Asia, especially China and Japan, than in Europe. Chrome dyes represent about 30 % of the global market. They are used for dark shades specifically, 50 – 60 % for black shades, 25 - 30 % for navy and the remaining 10 – 25 % for specific colours, such as brown, bordeaux or green [179, UBA, 2001].

The afterchrome method (see also Sections 2.7.4 and 9.6) is now the most widely adopted technique for the application of chrome dyes, and chromium (as sodium or potassium dichromate) is the metal used almost universally as mordant. In the application of chrome dyes, inefficient chroming methods can lead to the discharge of chromium in spent dye liquors (see also Section 2.7.8.1 “oxidising agents”). In order to minimise the amount of residual chromium in the final effluent much attention has recently been given to the low-chrome (stoichiometric) and ultra-low (substoichiometric) chrome dyeing techniques, where only the minimum amount of dichromate required to form the dye complex in the fibre is dosed.

During the last 10 - 15 years, so-called low-chrome dyeing technology has been increasingly used. The method consists in stoichiometric dosage of chrome (up to a maximum of 1.5 % o.w.f.) together with careful pH control (3.5 - 3.8) and optional addition of a reducing agent, which assists in the conversion of CrVI to CrIII and promotes its exhaustion onto the fibre [191, VITO, 2001].

Every major chrome dye manufacturer has published data relating to chrome additions and dyeing techniques which are widely adopted (e.g. Bayer, Ciba-Geigy, Sandoz).

By the use of low-chrome techniques it is possible to reduce residual CrIII in the spent chroming bath from about 200 mg/l (typical of conventional process) to about 5 mg/l in practical mill conditions. Residual CrVI is almost eliminated. In the laboratory, lower residual CrIII concentrations (about 1 mg/l) can be achieved but, although such results are reported in the literature, they are not regularly achievable in practice [191, VITO, 2001].

Ultra-low chroming techniques are applied to achieve even lower residual chromium levels or in particular cases (e.g. wool that has to be dyed in deep shades) when low-chroming techniques cannot guarantee residual chromium levels below 5 mg/l in the spent chroming bath. Chrome is dosed substoichiometrically, based on the dye uptake of the fibre.

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With ultra-low chroming techniques additional measures are applied, compared to the lowchrome process, in order to ensure maximum exhaustion of the dye bath. If the dye bath exhaustion is incomplete before chroming, the residual dye in the liquor will be chromed and remain in the liquor, adding to the discharged chromium. By ensuring maximum dye exhaustion, contamination from this source can be reduced and this will also give maximum fastness performance. Dye bath exhaustion can be improved by ensuring that the dye bath pH is sufficiently low or, as Bayer have shown, by allowing the dye bath to cool to 90 - 80 °C at the end of the dyeing stage. Optimum results will be obtained by draining the dye liquor and setting a fresh bath for chroming [191, VITO, 2001].

Main achieved environmental benefits Methods using exactly calculated quantities of dichromate and special process conditions result in minimisation of chromium in the waste water.

An emission factor of 50 mg chromium per kg of wool treated is achieved, which corresponds to a chromium concentration of 5 mg/l in the spent chroming bath when a 1:10 liquor ratio is used [191, VITO, 2001].

Operational data In order to ensure accurate dosing and minimum handling of hazardous chemicals by the operator, the application of the low-chroming/(ultra-low chroming) techniques requires the use of automated dosing and dispensing system for dichromate, dyes and pH-control. The required amount of dichromate is fed directly to the dyeing machine through pipework (no manual transfer, no human contact, no losses). The system is fitted with control devices for the volumetric control of the delivered quantities, which switch the entire system into emergency mode if normal operating parameters are breached [161, Comm., 2001].

In addition, special safety precautions are recommended for the storage of dichromate. The containers for the solution of sodium dichromate must be stored within isolated bunded areas in order to contain potential spillage and avoid interaction with other chemicals (in case of spillage).

For maximum chroming efficiency, it is essential to eliminate from the chroming bath any chemicals that will inhibit the chromium/dye interaction. Two main classes of chemicals can have this effect. The first class includes all chemicals that can form soluble complexes with chromium, thereby holding the metal in solution in the bath and adding to the effluent load.

Examples of such products are sequestering agents and polycarboxylic acids, such as citric acid.

The second class of compounds are those that inhibit the exhaustion of the dichromate anion;

the most common example is the sulphate anion. The use of sodium sulphate and sulphuric acid should therefore be avoided, except in the manner indicated in the specific Bayer method [191, VITO, 2001].

Note that, even without added reducing agent, reducing species released from the wool into the dye bath will convert CrVI almost quantitatively to CrIII. An exception is represented, for example, by wool that has been submitted to oxidising shrink-resist treatments, because in this case wool molecules will already be oxidised and the reduction potential will be lower.

Cross-media effects Taking the conventional method as a reference, there are no cross-media effects to be mentioned.

It has to be taken into account that even when: 1) special application methods are employed, which involve reduction of Cr(VI) to Cr(III); 2) the chrome is encouraged to complex with the

–  –  –

carboxyl groups within the fibre; 3) a further effluent dilution from rinsing is incorporated, it is still a huge challenge to reduce the chromium level in the total chrome dyeing effluent (spent dye bath + rinsing water) from over 300 mg/l to just 1 mg/l. It is for this reason that the future of afterchrome dyestuffs has been questioned [188, VITO, 2001].

If a fresh bath is set for chroming, as required with the ultra-low chroming technique, the additional water consumption has to be taken into account [280, Germany, 2002].

Applicability Low-chroming methods are cheap and easy to apply and are already widely used.

The optimised levels of dichromate additions give complete and level chroming of the dye, under specified chroming conditions. This minimises oxidation and cross-linking of the fibre, and therefore also reduces fibre damage.

However, it has to be taken into account that too low amounts of dichromate may adversely affect the required reproducibilities of shades [280, Germany, 2002].

Economics It is commonly accepted that in the long term, the introduction of automated dosing/dispensing systems brings about savings in chemicals thanks to improved dosing accuracy, but no quantitative data were made available to this respect [161, Comm., 2001].

The addition of reducing agents increases costs because of the longer dyeing cycles and the resulting reduced productivity [161, Comm., 2001]. The same is valid for the setting of a fresh bath for the chroming step, as required by the ultra-low chroming techniques [280, Germany, 2002].

Driving force for implementation

Pressure and safety requirements set by the legislation are probably the main driving forces for the application of this technique. It should be noted, however, that many initiatives discourage the use of chrome mordant dyes (OSPAR, GuT, EU-Ecolabel, etc.). Chromium-free dyeing is therefore becoming more and more attractive for companies that are not obliged to use chromium dyes.

Reference plants Many plants in Europe.

Reference literature [51, OSPAR, 1994] P091, [161, Comm., 2001], [188, VITO, 2001], [179, UBA, 2001], [191, VITO, 2001].



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