«EUROPEAN COMMISSION Integrated Pollution Prevention and Control (IPPC) Reference Document on Best Available Techniques for the Textiles Industry July 2003 ...»
Textiles Industry 315 Chapter 4 Separate dispensing of chemicals and colourants avoids unnecessary wastage of raw materials that cannot be considered for further re-use, if already pre-mixed. The preparation of the dye liquor batch based on on-line measurement of the pick-up allows a reduction of the residual dye liquor in the feeding tanks from up to 150 litres down to 5 – 15 litres.
Furthermore, modern dyeing ranges are operated with minimised flow of rinsing water, which allows additional 25 % water savings.
Operational data Routine maintenance is essential in order to ensure the precision and proper performance of the equipment. In particular, the precision of the dosage system (e.g. pumps) and pick-up measurement should be checked at regular intervals. In measuring the pick-up, the specific weight of the fabric has to be determined on the fabric before dyeing and not on the raw material before pretreatment.
Cross-media effects None believed likely.
Applicability The described techniques are recommended both for new and existing continuous and semicontinuous dyeing ranges. However, it is often more convenient to wait and install completely new machinery, where all the recommendations have already been taken into account, rather than upgrading parts of the existing machine [59, L. Bettens, 2000].
Dyeing in the nip is not suitable for light fabrics (below 220 g/m) or fabric with good wettability. With brushed or sheared textiles, the pick-up time may be too short and reproducibility is adversely affected [179, UBA, 2001].
Special attention has to be paid to knit and elastic fabrics.
Economics Investment cost for an automated dosage system and a volume-minimised trough (e.g. U-shaft) is about 85000 euros (taking as a reference a width of 1800 mm). On the other hand, annual savings of 85000 euros are already achieved in the case of finishing mills performing 15 batches per day (assuming savings of 50 litres per batch at a cost of the dye liquor of 0.5 euros/l). This means a short payback time. [179, UBA, 2001] Furthermore, the additional benefits achievable thanks to the reduced volume of waste water to be treated have to be taken into account.
Driving force for implementation The restrictions set by environmental legislation (e.g. colour in the effluent) have certainly encouraged mills to apply measures aimed at reducing the emission of concentrated effluent.
However, they are not to be regarded as the only driving force. The avoided wastage of expensive raw materials (colourants and auxiliaries) together with the increased reproducibility (right-first-time) and productivity achieved thanks to improved process control play an important role in the implementation of this technique.
Reference plants In Europe and countries outside Europe, there are about 40 plants successfully operating the described technique. These plants are equipped with the on-line system for individual dosage of
dyestuffs/auxiliaries combined with pick-up measurement devices for preparation of the next comparable batch. The rapid batch dosing system for liquid dyes is successfully operating in at least one finishing mill in Europe.
The rapid batch dosing system is supplied by [179, UBA, 2001]:
· E. Kusters Maschinenfabrik GmbH & Co. KG, division textile, D-47805 Krefeld · Kleinewefers Textilmaschinen GmbH, D-47803 Krefeld · Seybert & Rahier GmbH & Co. Betriebs-KG, 34376 Immenhausen Reference literature [51, OSPAR, 1994] (P082), [179, UBA, 2001], [59, L. Bettens, 2000].
4.6.8 Enzymatic after-soaping in reactive dyeing Description Dyeing and printing with reactive dyes entails a number of soaping and rinsing steps to remove from the substrate the amount of unreacted and hydrolysed dye. The removal of all unfixed dyestuff from the fibre is essential for obtaining optimum wet fastness, while contributing significantly to energy, water and chemicals consumption of the overall dyeing process.
The suggested technique consists in adopting an enzymatic treatment to remove the non-fixed dyestuff not only from the fibre, but also from the exhausted dye bath. Enzymatic decolourisation of reactive dyestuffs has been proved with Levafix, Remazol, Cibacron, Procion and Synozol types [179, UBA, 2001].
The application of the enzymatic compounds usually takes place in the fourth or fifth rinsing step (see table below).
Table 4.20: Comparison of the sequence steps between a conventional and an enzymatic aftersoaping treatment (exhaust dyeing) Main achieved environmental benefits As Table 4.
20 shows, one of the hot rinsing steps can be avoided when using enzymatic aftertreatment. Savings in water, energy and detergent consumption are the main advantages achievable with this technique [179, UBA, 2001].
The enzymatic treatment is carried out as follows (batch process) [179, UBA, 2001]:
· filling with fresh water (50°C) · addition of a buffer for adjusting the pH · control of pH (addition of acetic acid, if necessary)
· addition of the enzymatic compound (0.25 g/l) · running: 10 min · draining Cross-media effects None believed likely.
Applicability The technique is already applicable to exhaust dyeing with reactive dyestuffs. Application to continuous dyeing and printing is currently under development.
Most of the reactive dyestuffs can be enzymatically decolourised. However, a precautionary test on laboratory scale is recommended.
Economics Savings in water and energy consumption and reduced process time are among the economic benefits achievable.
No information has been made available about the cost of the enzymatic compounds.
Driving force for implementation Cost-saving potential and improved quality (higher fastness) of the final product [179, UBA, 2001].
Reference plants Enzymatic after-treatment is applied in Germany in several finishing mills and world-wide [179, UBA, 2001].
Reference literature [179, UBA, 2001].
4.6.9 Silicate-free fixation method for cold pad batch dyeing Description Sodium silicate is often used in cold pad-batch dyeing, mainly to increase the pad liquor stability and to avoid selvage carbonisation. On the other hand, sodium silicate gives rise to a number of problems such as the formation of silicate deposits on the textile surface and on the equipment, increased salt in the effluent, etc.
Silicate-free highly concentrated aqueous alkali solutions have been developed and are now available on the market. They are ready-made products (carefully adjusted mixture of alkali in aqueous solution), which can be easily applied with modern dosing systems. They are particularly suitable for the cold-pad-batch process.
Main achieved environmental benefits
The following advantages are achievable:
· no residues of alkali in the preparation tank because, unlike with sodium silicate, the alkali can be added as ready-made solution and does not need to be prepared · no formation of difficult-to-wash-off deposits on the substrate and on the equipment · no need for additional auxiliaries in the padding liquor to avoid the formation of deposits · lower electrolyte content in the effluent · possibility of using membrane techniques in waste water treatment (no crystallisation in filters, pipes and valves and no membrane blocking, which is the case with sodium silicate).
Operational data Membrane pumps such as the sera-pumps with 4:1 ratio (alkali solution to dyestuffs solution) are suitable for the application of the product [179, UBA, 2001].
Figure 4.16 shows a typical dosing curve.
Using a curve instead of the conventional alkali addition steps brings additional advantages in terms of increased reproducibility [179, UBA, 2001].
Figure 4.16: Dosing curve for ready-made alkali solution [179, UBA, 2001] Cross-media effects None believed likely.
Applicability The technique is applicable to both existing and new installations. However, for existing installations additional measures for process optimisation and control may be needed in order to guarantee constant conditions [179, UBA, 2001].
Economics The ready-made alkali solutions are more expensive than the conventional fixation methods.
The ready-made alkali solutions are designed for modern minimum-volume troughs, where pad liquor exchange time is very short and where extremely long pad liquor stability is not needed.
Nevertheless, the absence of sodium silicate affects the stability of the pad liquor. More efficient
process control (e.g. temperature control of the dyeing liquor) is therefore required and the corresponding investment costs have to be taken into account.
The following economic benefits have to be considered [179, UBA, 2001]:
· investment in advanced dosing systems is lower because only two dosing units are needed (1 for the dyestuff solution and 1 for the ready-made alkali solution), while with the conventional fixation methods based on silicate, three dosing units are required (1 for the dyestuffs solution, 1 for the silicate and 1 for the alkali). Investment cost for 1 dosing unit is around 12000 euros · no need to change the rubbers of the padder at short time intervals because of silicate deposit formation. The cost for removing the rubber of a padder is estimated at around 7000 - 10000 euros · the lower electrolyte content of the liquor reduces the substantivity of the hydrolysed dyestuffs, which is therefore easier to wash off. This results in lower energy and water consumption in the washing-off step of the process · higher productivity of the padders and washing ranges · better reproducibility thanks to monitored process conditions.
In conclusion, reduction of total process costs is achieved, compared to the conventional fixation methods.
Driving force for implementation
Among the main driving forces it is worth highlighting [179, UBA, 2001]:
· better reproducibility · reduction of total process costs · easy handling of the product (possibility of having the alkali in liquid form and dosable in the required concentration without crystallisation problems) · no deposits and better washing-off behaviour · possibility of using membrane techniques for waste water treatment.
Reference plants In Europe there are many finishing mills applying ready-made alkali solutions. Some example plants are Miroglio in Italy, T.I.L., F-Lyon in France, Riedel & Tietz Textil GmbH, D-09212 Limbach-Oberfrohna in Germany and Fussenegger, A-Dornbirn in Austria.
Reference literature [179, UBA, 2001]
Description One of the main objectives of research & development studies is the development of reactive dyes with the greatest possible degree of fixation (see also Sections 9.8 and 188.8.131.52 – “Dyes”).
Bifunctional (polyfunctional) reactive dyes, containing two similar or dissimilar reactive systems, offer very high levels of fixation in exhaust dyeing. Because of their two reactive groups, bifunctional reactive dyes have increased probability of chemical reaction with cellulose fibres compared to monofunctional dyes with one reactive group. If one of the reactive groups hydrolyses during the dyeing process, the other one can still chemically react with the hydroxyl groups of cellulose. Moreover, combining two reactive systems in the same dye delivers the advantages of the two individual groups (e.g. high degree of fixation with high fastness levels
and wash-off). However, polyfunctional dyes are not necessarily better. Only the right combination of reactive groups makes them superior to conventional monoreactive dyes.
Examples of these high-fixation dye ranges are [179, UBA, 2001]:
· Cibacron FN (exhaust warm) (Ciba) · Cibacron H (exhaust hot) (Ciba) · Drimarene HF (Clariant) · Levafix CA (Dystar) · Procion H-EXL/ XL+ (Dystar) · Sumifix HF (Sumitomo).
Figure 4.17 illustrates two examples.
The first has two monofluoro triazine reactive groups bridging the chromophore; the second (reactive black 5) has two vinyl sulphone reactive groups.
There are also reactive dyestuffs with two different reactive groups.
Figure 4.17: Two examples of polyfunctional dyestuffs [179, UBA, 2001] Main achieved environmental benefits The fixation of a reactive dye with cellulose can be expressed either as a percentage of the total dye applied (fixation rate, sometimes also called “absolute fixation”) or as a percentage of the dye exhausted (exhaustion rate, sometimes also called fixation efficiency).
In the case of monofunctional dyes, the fixation rate is approximately 60 % (with an exhaustion rate of about 70 %) so that 40 % of the dye applied is lost in the effluent. In the case of bifunctional reactive dyes, 80 % fixation rate and over 90 % exhaustion rate is achieved. The immediate consequence is a significant reduction of unused dyestuff ending in the waste water stream (reduced colour and organic load).
This is particularly advantageous when advanced oxidation techniques are applied to treat the dye in the effluent (see Section 4.10.7). It has to be noted, however, that a lower amount of dye in the effluent does not necessarily produce a reduction in visual colour. The tinctorial strength of new reactive dyes has significantly improved. This means that deeper and more intense shades are possible with less dye than necessary with other dyes or dye classes. Less dye is therefore left in the effluent, but visual colour may be still high [190, VITO, 2001].
With new dyes (and processes) there is also potential for water, energy and chemicals savings.
For example, the recently introduced Levafix CA dyes (Dystar) reach more than 90 % fixation with moderate salt quantities.
Post-rinsing to obtain the required level of wet-fastness can be performed quickly and with relatively little energy and water. This is partly a consequence of the high fixation yield of the new bifunctional dyes (only a small amount of unfixed dye needs to be washed off). More important, however, is the fact that some new reactive dyes have molecules especially designed to show reduced affinity when they are in the hydrolysed form, which means that they have excellent wash-off properties.
A recent innovation from Dystar (Procion XL+ dyes) allows much shorter processing time on certain substrates by combining the pretreatment and dyeing steps using polyfunctional dyes that fix at 90 °C. Savings of up to 40 % in water and energy consumption and more than 30 % of salt are claimed.
To facilitate selection and application, dye manufacturers introduced small dye ranges each comprising highly compatible dyes with virtually identical behaviour in the dye bath. Each of these compact ranges is geared to specific application segments. Also dyeing compatibility matrixes are provided. 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 High fixation reactive dyes can be applied in all types of dyeing machines, but offer particular advantage on the most modern low liquor ratio dyeing machines fitted with multi-task controllers where additional advantages in terms of reduced energy and water consumption can be exploited [179, UBA, 2001]. In particular, new reactive dyes with very good solubility can be applied at ultra-low liquor ratios [190, VITO, 2001].
Economics Compared to conventional reactive dyes, polyfunctional reactive dyestuffs are more expensive per kilogram, but the higher fixation efficiency, the savings on salt usage and the reduced water and energy consumption lead to a significant reduction of total cost.