«EUROPEAN COMMISSION Integrated Pollution Prevention and Control (IPPC) Reference Document on Best Available Techniques for the Textiles Industry July 2003 ...»
Prices vary from country to country. UBA reports, for the example plant described above, savings of 3.20 euros/m3 (0.6 euros/m3 for fresh water, including treatment, and 2.60 euros/m3 for disposal fee). ENco indicates UK costs in the region of 1.09 euros/m3 for town water and average effluent disposal cost of about 1.62 euros/m3 (calculated according to the standard Modgen strength formula) [32, ENco, 2001].
In the example plant reported by UBA, investment costs for the tanks, piping and control devices amounted to 0.8 million euros [179, UBA, 2001].
Reference plants Many plants in Europe.
Reference literature [204, L. Bettens, 2000], [179, UBA, 2001], [11, US EPA, 1995], [51, OSPAR, 1994] (P087, P088), [32, ENco, 2001].
4.7 Printing 4.7.1 Urea substitution and/or reduction in reactive printing Description Description Urea content in reactive printing paste can be up to 150 g/kg paste. Urea is also used in printing
pastes for vat dyes, but in much lower concentration (about 25 g/kg paste). Urea is applied to:
· increase the solubility of dyestuffs with low water solubility · increase the formation of condensate, which is necessary for allowing migration of the dyestuff from the paste to the textile fibre · form condensates with a higher boiling point (115°C) which means that requirements for constant operating conditions are lower (non-constant operating conditions can negatively affect reproducibility if urea is not used).
Urea can be substituted by controlled addition of moisture (10 wt-% for cotton fabric, 20 wt-% for viscose fabric and 15 wt-% for cotton blends). Moisture can be applied either as foam or by spraying a defined quantity of water mist.
Main achieved environmental benefits In the textile finishing industry the printing section is the main source of urea and its decomposition products (NH3/NH4+). During waste water treatment, the nitrification of the excess ammonia involves high energy consumption. Discharge of urea, ammonia and nitrate
contributes to eutrophication and aquatic toxicity. Minimisation/elimination of urea at source significantly reduces these adverse effects.
In reactive printing the urea content in the printing paste can be reduced from 150 g/kg paste to zero by application of moisture. In case of phthalocyanine complex reactive dyes, however, reduction is possible only to 40 g/kg paste because of the bad migration properties typical of these large-molecule dyestuffs [179, UBA, 2001].
Operational data By avoiding the use of urea in reactive printing pastes, the ammonia concentration in the mixed effluent of a finishing mill with a significant printing section, decreased from about 90 - 120 mg NH4+-N/l to about 20 mg NH4+-N/l [179, UBA, 2001].
Without using the referenced technique, the amount of urea consumed can be reduced to about 50 g/kg of printing paste for silk and 80 g/kg for viscose, and still maintain the quality standards.
Cross-media effects The application of moisture needs energy, but this consumption is significantly lower than the energy required for the production of urea [179, UBA, 2001].
Applicability The foam and the spraying systems are applicable to both new and existing plants performing reactive printing [179, UBA, 2001].
For silk and viscose articles, however, it is not possible to completely avoid the use of urea with the spraying system. The technique is not reliable enough to ensure a uniform dosage of the low moisture add-on required for these fibres and, especially for fine articles, cases have been reported where spraying systems did not meet the quality standards.
On the contrary, the foaming system has proven successful for several years for viscose in complete elimination of urea [179, UBA, 2001]. This technique should in principle be technically viable also for silk, although it has not yet been proven. Silk is known to be less problematic as a fibre than viscose, but it is typically processed in smaller runs.
The investment costs for spraying equipment including on-line moisture measurement are about 30000 euros, but the foam system is significantly more expensive. Approximately 200000 euros are required for a foaming machine, which relates to a production capacity of up to about 80000 linear metres per day. The foaming technique has been operated under economically viable conditions in plants of capacity of about 30000, 50000 and 140000 linear metres per day.
Driving force for implementation Stringent limits applied by local authorities for NH4+-N in waste water, due to eutrophication and aquatic toxicity in surface water, have promoted the introduction of this technique.
The application of this technique is also very attractive for mills discharging to a municipal waste water treatment plant. Because of the high energy consumption for biological nitrification, many municipalities now charge indirect dischargers for nitrogen emissions.
Reference plants Many plants in Europe. Example plants where the technique is currently applied are: Ulmia, DRavensburg-Weissenau; KBC, D-Loerrach; Textilveredlung Wehr, D-Wehr [179, UBA, 2001] Reference literature [179, UBA, 2001].
4.7.2 Reactive two-step printing Description As already described in Section 4.7.1, in conventional one-step printing with reactive dyes, urea is used to increase solubility of the dyestuffs and to improve dyestuff migration from the printing paste to the textile fibre.
Minimisation or even substitution of urea is possible in the one-step method by controlled addition of moisture.
Another option is the two-step printing method, involving the following steps:
· padding of the printing paste · intermediate drying · padding with alkaline solution of fixating agents (especially water-glass) · fixation by means of overheated steam · washing steps (to remove thickeners and improve fastness properties).
The process is carried out without urea.
Main achieved environmental benefits Significant reduction of ammonia content in waste water and reduced off-gas problems are the main ecological improvements. In addition the life-time of printing pastes is lengthened, which increases the opportunities to recycle the residual printing pastes [179, UBA, 2001].
Operational data A typical recipe for the printing paste is given below [179, UBA, 2001].
Cross-media effects None believed likely.
Applicability Two-step reactive printing can be carried out on cotton and viscose substrates. Application of the fixation liquor needs the combination of an impregnation device with a steamer. Overheated steam is necessary. Reactive dyes based on monochlorotriazine and vinylsulphone types can be used [179, UBA, 2001].
It should be noted that this technique is more complex and slower than the one-phase process.
Economics No information was made available.
Driving force for implementation Environmental and quality aspects are the reasons for implementing a two-step printing method.
Reference plants Many plants in Europe and world-wide.
Reference literature [179, UBA, 2001] with reference to:
“DyStar, 2000” DyStar, D-Frankfurt/Main Product information (2000)
Description The final stages in pigment printing consist in drying and fixing the printed fabric with hot air.
In both stages, emissions of volatile organic compounds to air may be significant (see also Sections 2.8.3 and 8.7.2).
White spirit used for emulsion thickeners was a major source of volatile organic compounds in the past. Water-in-oil (full emulsion) thickeners seem no longer to be applied in Europe and half-emulsion printing pastes (oil in water) are only occasionally used. Hydrocarbons (predominantly aliphatic) are still found in exhaust air, however, mainly arising from mineral oil contained in synthetic thickeners. Their emission potential can be up to 10 g Org.-C/kg textile.
New generation thickeners contain minimal amounts of volatile organic solvents. They are based on polyacrylic acid or polyethylene glycol compounds instead of mineral oils. Products have also been developed that do not contain any volatile solvent. They are supplied in the form of non-dusting granules or powders.
Another potential source of air emissions in pigment printing is fixing agents. The cross-linking agents contained in these auxiliaries are largely based on methylol compounds (melamin compounds or urea-fomaldehyde pre-condensates) that give rise to formaldehyde and alcohols (mainly methanol) in the exhaust air. New low-formaldehyde products are now available.
Furthermore, optimised printing pastes are APEO-free (pigment printing auxiliaries may be found in the waste water as a result of cleaning operations, etc.) and have a reduced ammonia content. Ammonia is used as additive in binders and is also a source of air pollution.
Main achieved environmental benefits Table 4.31 shows for three typical thickeners the emission levels that may arise from drying and fixing.
Table 4.31: Volatile organic carbon emissions in pigment printing Recipe I is an already optimised thickener, but still containing hydrocarbons.
In the optimised recipe II the mineral oil is replaced by polyethylene glycol, while recipe III uses a powder thickener. With recipe III formaldehyde emissions can be kept below 0.4 g CH2O/kg textile (assuming 20 m³ air/kg textile). As a result, the carry-over of volatile substances from printing to finishing processes can also be reduced to 0.4 g Org.-C/kg.
Using optimised printing pastes, ammonia emission can also be reduced below 0.6 g NH3/kg textile (assuming an air/textile ratio of 20 m³/kg).
Operational data There has been no evidence of variations in operating conditions, product quality or process control [180, Spain, 2001].
Cross-media effects None believed likely.
Applicability The technique is applicable in new and existing installations [179, UBA, 2001], [180, Spain, 2001].
Thickeners based on powders may generate dust or block the stencils [179, UBA, 2001].
Economics The higher costs of the printing pastes are offset by the reduced air treatment costs (lower emissions of organic volatile compounds) [180, Spain, 2001], [179, UBA, 2001].
Driving force for implementation Environmental legislation (regarding, in particular, emission limits set for volatile organic carbon, formaldehyde and ammonia) is the main motivation for the use of optimised printing pastes [179, UBA, 2001].
Reference plants Many plants in Europe and world-wide.
Reference literature [180, Spain, 2001], [51, OSPAR, 1994] (P096, P106), [61, L. Bettens, 1999], [179, UBA, 2001] 4.7.4 Volume minimisation of printing paste supply systems in rotary screen printing machines Description A typical printing paste supply system for textile rotary screen printing machines is illustrated in Figure 2.22. The volume depends on the diameter of the pipes and squeegee as well as on the pump design and the length of the pipes.
Printing paste supply systems, of which there may be up to 20 per printing machine, have to be cleaned at each change of colour or pattern and a considerable amount of printing paste finds its way into the waste water. For fashion patterns, a typical number of different colours per design is 7 to 10. Typical system volumes in conventional and optimised printing paste supply systems for textiles are shown in Table 4.32. Note that the figures given in this section are not applicable to carpet rotary screen printing (the screens in carpet rotary screen printing are much bigger – 5 - 6 m width – and the the volumes of printing paste to be delivered are much higher than in most textile printing applications).
Table 4.32: Volume of conventional and optimised printing paste supply systems in rotary screen printing machines for textiles In addition to this volume, residual printing paste in the rotary screens has to be taken into account, which may be about 1 - 2 kg.
Thus, in conventional systems the loss per supply system is up to 8 kg.
If this quantity is compared with the amount of paste applied on the printed fabric (see table below), it becomes apparent that the volume of the supply system is highly significant and may even exceed the amount of paste printed on the fabric (for pattern/batch lengths of about 120 m). In Europe in recent years, average printing batch lengths have significantly decreased to only 400 - 800 m.
Table 4.33: Amount of printing paste required for printing various lengths of fabric at different degrees of coverage Minimising the system volume therefore has major effects in reducing printing paste emissions to waste water.
Diameters of pipes and squeegees have been reduced to 20 - 25 mm, leading to substantial reductions.
Along with minimum-volume supply systems, paste residues can be minimised by improving paste recovery from the supply system itself. New pumps can be operated in both directions.
Thus, at the end of each run, the printing paste can be partly pumped back into the drum. The problem of air being drawn in via the holes in the squeegee can be solved by applying the technique described in Section 4.7.5.
Additional measures are:
· using squeegees with an even paste distribution over the whole width (minimisation of residual printing pastes in the screens) · manual stopping of printing paste supply shortly before finishing a run in order to minimise the residual printing pastes in the rotary screens.
For small run lengths in particular, the following measures may also be considered:
· not using the supply system, but instead injecting small quantities of printing paste (1 - 3 kg) directly into the squeegee manually or by manual insertion of small troughs (with a cross-section of 3x3 cm or 5x5 cm) · minimisation of pipes by supplying the printing pastes through funnels positioned directly above the pumps.
Main achieved environmental benefits System volumes and consequent losses can be reduced significantly (according to industrial experience, a reduction of up to one third is possible), depending on the age of the equipment [179, UBA, 2001].
Operational data Measures such as the manual injection or insertion of printing paste and the manual stopping of the printing paste supply system shortly before the end of the run need trained and motivated staff. Meanwhile some companies have put in practice the manual injection of printing paste, but others report that even for small lots (up to 120 m), the technique is difficult to apply.
Reproducibility may be affected because the quantity of printing paste to be injected varies according to the pattern, which is difficult to manage for the staff. Moreover, a constant level of printing paste within the screen cannot be maintained, which may affect constancy of printing quality.
Cross-media effects None believed likely.
Applicability The described measures are applicable to both existing and new installations.
Economics The installation of 12 sets of volume-minimised pipes and squeegees requires investment cost of about EUR 25000.
Driving force for implementation The need to minimise production costs by minimising printing paste losses and problems with waste water disposal have been the main driving forces.
Reference plants Many plants in Europe and world-wide have been retrofitted with minimised printing paste supply systems that operate successfully.
Reference literature [179, UBA, 2001], [51, OSPAR, 1994] (P100).