«EUROPEAN COMMISSION Integrated Pollution Prevention and Control (IPPC) Reference Document on Best Available Techniques for the Textiles Industry July 2003 ...»
Driving force for implementation The introduction of legislation restricting colour in the discharged effluent has been the main driving force for the development of high fixation dyes. Another important drive is the reduction of total processing costs achievable thanks to high dye fixation [179, UBA, 2001].
Reference plants Many plants in Europe.
Reference literature [179, UBA, 2001], [190, VITO, 2001], [180, Spain, 2001].
4.6.11 Exhaust dyeing with low-salt reactive dyes Description Traditionally, exhaust dyeing of cellulosic fibres with reactive dyestuffs required high amounts of salt to improve exhaustion (usually 50 - 60 g/l, but also up to 100 g/l for dark shades - see also Sections 2.7.3 and 22.214.171.124 – “Salt”). Several dye manufacturers have developed innovative dye ranges and application processes that only need about two-thirds of this quantity. Examples
· Cibacron LS (Ciba) · Levafix OS (Dystar) · Procion XL+ (Dystar) · Sumifix HF (Sumitomo).
Most of these dyes are polyfunctional dyes and offer very high level of fixation, thus bringing the added benefit of a reduced amount of unfixed dye in the effluent.
Because of the reduced amount of salt needed for their exhaustion, low-salt dyes are more soluble and can be kept in solution at a higher concentration than necessary for low liquor ratio dyeing machines. This offers further possibilities for reducing the overall salt requirement, as illustrated in the following table.
Table 4.21: Quantities of salt required for dyeing 1000 kg of fabric to a medium depth of shade Main achieved environmental benefits Salt consumption for exhaust dyeing of cellulose fibres is reduced by about one-third of the quantity needed for conventional reactive dyestuffs, with positive effects on effluent salinity and smooth running of waste water treatment units [179, UBA, 2001].
Low-salt reactive dyes are high affinity dyes, which makes them less easy to wash off than lowor medium-affinity types. However, dyes with low affinity in the hydrolysed (unfixed) form are increasingly available, making post-rinsing operations much easier.
Operational data The lower the salt concentration, the more sensitive the system becomes to any change in parameters that influence exhaustion. To give the dyer the high flexibility needed, dye manufacturers have developed trichromatic combination dyes with high mutual compatibility (by matching the affinity and reactivity of each dye and minimising interactions among components). Products with very similar application properties are now available, which makes them little affected by (or virtually insensitive to) changes in dyeing conditions. Right-first-time production is claimed (for example, with Cibacron LS) even when batch sizes and liquor ratio vary widely, as for example when dyeing blends such as polyester/cotton [190, VITO, 2001].
Individual manufacturers provide comprehensive technical information for their low salt dye ranges, including detailed salt recommendations according to depth of shade, type of substrate, equipment in use, etc.
The most impressive feature of advanced reactive dyes is the mutual compatibility of the dyes included in each dye range (matching the affinity and reactivity of each dye and minimising interactions among components). Thanks to sophisticated molecular engineering techniques it has been possible to design reactive dyes which have the optimum profile required to maximise the right-first-time production. They exhaust at very similar rates. Curves plotted for each dye colour by temperature and time can be laid over one another with practically no variation. This is important in order to obtain high reproducibility, low dependency on dyeing conditions (e.g.
liquor ratio, dyeing temperature, salt concentration) and therefore “ right-first-time” dyeing.
Cross-media effects None believed likely.
Applicability Low-salt reactive dyestuffs are applicable to both existing and new dyeing equipment, but offer particular advantage in the most modern low liquor ratio dyeing machines where additional advantages of reduced energy and water consumption can be exploited [179, UBA, 2001].
Low-salt reactive dyes are significantly more expensive than conventional reactive dyes (mainly because of the sophisticated molecular engineering techniques applied in their manufacture).
However, depending on the special circumstances of each dyehouse, the application of low-salt dyes can be of economic benefit.
Driving force for implementation
Low-salt reactive dyes were introduced first in areas having arid climate conditions and negative water balance (e.g. North Carolina in the US and Tirupur, Tamil Nadu in India). They have also been successful in regions where dyehouses discharge directly to fresh water and there is a need to minimise salination effects.
Moreover, it should not be forgotten that corrosion caused by the presence of salt is the main cause of failure in water recycling.
Reference plants Many plants in Europe.
Reference literature [179, UBA, 2001], [190, VITO, 2001], [180, Spain, 2001], [61, L. Bettens, 1999].
4.6.12 Omitting the use of detergents in afterwashing of cotton dyed with reactive dyes Description Both international literature and practical experience in textile mills show that detergents do not improve removal of hydrolysed reactive dyestuffs from the fabric.
On the contrary, high temperatures do have an affect on rinsing effectiveness. Tests carried out with rinsing at 90 – 95 °C have shown that rinsing is more effective and faster at high temperatures. About 30 % more unfixed hydrolysed reactive dyestuff is rinsed out after 10 minutes of rinsing at 95 °C than at 75 °C.
Many dyehouses already carry out hot rinsing and omit the use of detergents in rinsing after reactive dyeing. The product quality is not negatively affected. On the contrary, most often the fastness of the goods are better after the hot rinsing than after the traditional rinsing with detergents, complexing agents and neutralisation in the first rinse.
Energy should be recovered when using large volumes of hot process water. Energy reclamation can be done either by heat exchange between the hot outgoing process water and the cold incoming clean water or by reclamation of the hot water and re-use of both energy and water.
Main achieved environmental benefits The main benefit is the reduction in consumption of detergents and pollution load discharged to the waste water. Obviously, the potential for reduction will vary according to the existing dyeing procedure at the company.
The experience of two dyehouses (one mainly dyeing knitted fabrics and the other dyeing garments) shows that the average potential load reduction can be in the order of 1 kg detergent, 1 kg complexing agent and 1 kg acetic acid per 100 kg of textile.
Additional advantages are the savings achievable in the amount of chemicals consumed to destroy reactive dyes by free radical treatment processes. In the Fenton reaction for example since the OH* radicals react very fast not only with the dyestuffs but also with many detergents, a large amount of expensive H2O2 can be saved by omitting the use of detergents.
The high degree of fixation and the excellent wash-off properties typical of some new low-salt, polyfunctional reactive dyes (see Sections 4.6.10 and 4.6.11) are important factors that help obtain sufficient wash fastness with hot rinsing without the need for detergents.
It has been reported that difficulties might arise with accidental stops of the machinery. In such conditions the high temperature of the rinsing water could cause irreversible cleavage of the bond between the reactive groups of the dye and the hydroxyl groups of cotton or viscose [297, Germany, 2002].
Cross-media effects Substituting cold rinsing with hot rinsing leads to higher energy consumption, unless thermal energy from the rinsing effluent is recovered.
Applicability A Danish textile mill has during the last 5 years totally omitted the use of detergents in the rinsing process after reactive dyeing. The company treats knitted and woven goods made of cotton or cotton/PES and dyes them in all colours and shades. The application of this technique may involve a change in the type of dyes employed. The referenced company works with bifunctional reactive dyes as Cibachron C or Bezaktiv S. Soft water is used.
Another textile mill dyeing garments of knitted and woven fabric has not used detergents during the last 5 – 6 years, apart from a few exceptions (i.e. red, dark red or bordeaux colours).
Economics The only change in operating procedures is to omit the addition of detergents. Savings will depend on the number of reactive dyeings carried out at the company.
Driving force for implementation High costs for chemicals and waste water treatment.
Reference plants Many plants in Europe. In particular, a few examples of plants applying this technique in Denmark are: Kemotextil A/S, Sunesens Textilforædling ApS, Martensen A/S.
Reference literature [78, Danish EPA, 1999], [7, UBA, 1994], “Environmentally friendly method in reactive dyeing of cotton”. Water Science and Technology Vol. 33, No.6, pp.17-27, 1996” “Reclamation and re-use of process water from reactive dyeing of cotton”. Desalination 106 (1996) 195-20” 4.6.13 Alternative process for continuous (and semicontinuous) dyeing of cellulosic fabric with reactive dyes Description The technique referenced is a continuous dyeing process for cellulose fibres that uses selected reactive dyestuffs. Unlike the conventional pad/continuous dyeing systems, it requires no additional substances such as urea, sodium silicate and salt, or long dwell time to fix the dyes.
The recipe includes: x g/l dye 1, y g/l dye 2, z g/l dye 3, 1 - 2 g/l wetting agent and alkali. The other auxiliaries normally used in a conventional process are replaced here by operating with controlled steam content during drying.
The dye liquor is applied to the textile using a padder (cotton is squeezed to about 70 % pick-up and viscose to about 80 %) and, after a short passage through air, the fabric is fed directly to the dryer (hot-flue), where it remains for 2 minutes.
In the conventional process, urea is used as solvent for the dye in dry heat. Urea melts at 115 °C and binds water above 100 °C, thus allowing penetration of the dyestuff in the fabric during fixation in the steamer. With the referenced process, this is not needed because the conditions in the dryer are set (120 °C and 25 vol.- % steam content) so that the fabric remains at a specific temperature of 68 °C as long as it is damp.
Since highly reactive dyes are used, only a low fabric temperature (68°C), a weak alkali and a short time (2 minutes) are needed for fixation.
Main achieved environmental benefits Significant reduction in chemicals consumption is possible as shown in Figure 4.18.
Figure 4.18: Basic chemicals consumption per 10 million metres with pad-dry-pad-steam, padbatch, pad-dry-thermofix and the reference technique [180, Spain, 2001] No urea, salt (chloride/sulphate), or sodium silicate is consumed and the alkalinity is often lower (less NaOH, due to substitution with Na2CO3 depending on the selected dyes).
One company operating a three-shift system for dyeing continuously by the pad-dry-thermofix process or the pad-dry-pad-steam process, at a rate of 40 m/min, would consume approximately 423 t/yr of urea or 540 t/yr of NaCl. On the other hand, a company operating the referenced technique on a three-shift basis would consume only 22 t/yr of sodium bicarbonate, which ends up in the effluent. In conclusion, the waste water from washing contains only 4 – 5 % of the chemical load produced by other dyeing processes carried out in accordance with the latest technology [190, VITO, 2001].
The elimination of urea, in particular, results in a lower amount of nitrogen-containing compounds in the waste water and avoids the presence of urea break-down products in the exhaust air, typically found in pad-thermofix processes.
The absence of salt is advantageous not only because it results in a lower salt load in the final effluent, but also because without salt the unfixed dye is easier to wash off (less water and energy consumption in post-rinsing operations). In addition, dyes that have low substantivity in the hydrolysed form are now employed, which show very good washing-off properties.
In addition, energy consumption is minimised through control of the exhaust air.
Operational data The temperature and humidity profile during the fixation process is illustrated in the figure below.
Figure 4.19: Fabric temperature and humidity during the dyeing process using the referenced technique [180, Spain, 2001] A dampening unit is used during start-up of the machine in order to ensure that ambient conditions in the chamber of the dryer are set at 25 vol.
Sometimes, the amount of water given off from very lightweight fabrics is not enough to keep the chamber at 25 vol.-%. In this case, the steam injector is used to spray-in the required amount of steam.
It has to be stressed that maximum performance is obtained only with the right choice of fabric pretreatment and well-engineered selected dye formulations.
Cross-media effects None believed likely.
Applicability The process itself is simple and ideal for both small and large batches. It is an economically viable option for dyehouses that are reinvesting.
In addition to high versatility and applicability to a wide variety of fabrics, a number of benefits are achieved in terms of fabric quality compared with other dyeing techniques. These include
for example [180, Spain, 2001]:
· soft handle due to mild fixing conditions · migration minimised by rapid fixation and humidity control (especially important on pile fabrics, where rub fastness is improved due to less dye migration to the tips) · improved penetration of difficult fabrics (compared to pad-thermofix) due to presence of humidity at high fabric temperature · improved coverage of dead cotton compared to pad-batch or exhaust dyeing · dyeing PES/viscose and PES/cotton blends in a single bath with excellent results.
Economics No IR pre-dyer is needed, unless heavy fabric is being dyed. Nevertheless, the initial investment cost for new hot-flue is around 0.75 million euros, excluding the cost of an automatic dye kitchen [190, VITO, 2001]. This investment cost, however, is compensated by huge savings in chemicals/auxiliaries, energy, flexibility, higher productivity and environmental improvement (less emissions to air or waste water pollution to treat).
The lower chemicals/auxiliaries costs arise from the avoidance of sodium silicate, sodium chloride and urea in the dyeing recipe. In many cases, the dye consumption is also reduced compared with the other processes such as pad-batch. This is illustrated in the example reported in the table below.
Table 4.22: Comparison between a conventional pad-batch process and the referenced technique applied to mercerised 100 % cotton twill, 300 g/m, 75 % pick-up The increased productivity obtained by the elimination of long batching times, produces significant savings over the traditional pad-batch process.
Despite the much lower machinery cost for pad-batch equipment, this alternative process has shown to be more cost effective in terms of total processing costs. Moreover, the demand for rapid response by the industry can be met more easily. Not having to wait until the next day to view shades means a much improved service and a faster delivery to the customer.
Driving force for implementation Minimised consumption, sustainable clean technology, market share.
Reference plants The referenced technique is available commercially under the name of Econtrol®, which is a registered trademark of DyStar.