«EUROPEAN COMMISSION Integrated Pollution Prevention and Control (IPPC) Reference Document on Best Available Techniques for the Textiles Industry July 2003 ...»
4.7.5 Recovery of printing paste from supply system in rotary screen printing machines Description This technique allows the recovery of the printing paste remaining in the supply system in rotary screen printing machines at the end of each run. Before filling the system, a ball is inserted in the squeegee and then transported by the incoming paste to its end. After finishing a print run, the ball is pressed back by controlled air pressure, pumping the printing paste in the supply system back into the drum for re-use. Systems available for re-using residual printing paste are described in Section 4.7.6.
The technique is illustrated in the figure below, showing the ball during the phase in which the pump is transporting the paste back to the drum.
Figure 4.27: Recovery of printing paste from the paste supply system by back-pumping an inserted ball [179, UBA, 2001] with reference to “Stork, 2001” Main achieved environmental benefits Printing paste losses are reduced drastically.
In textiles, for instance, at a printing width of 162 cm, the loss is reduced from 4.3 kg (in the case of a non-optimised printing paste supply system) to 0.6 kg.
Rotary screen printing machines have up to 20 supply systems, although in practice, for fashion patterns, 7 - 10 different printing pastes are common. Therefore, the 3.7 kg of printing paste saved per supply system have to be multiplied by 7 - 10. Water pollution can thus be minimised considerably.
To achieve maximum benefit from this measure, modern printing machines with minimumvolume feed systems should be used (see Section 4.7.4).
Operational data The described technique is successfully applied in textile finishing mills, especially in combination with recycling of the recovered printing paste.
Cross-media effects None believed likely.
Applicability The technique is applicable to new installations (new rotary screen printing machines). Certain existing machines can be retrofitted. There is only one supplier for this technique (Stork Brabant B.V., NL-5830 AA Boxmeer). All their machines newer than the so-called RD-III (that is RDIV and all subsequent models) can be retrofitted with the described system.
As mentioned earlier the technique is applied in textile finishing mills (for flat fabrics). In principle this system can also work for carpets, but it is not applied for various reasons.
Probably the main reason is related to the type of thickeners most commonly used in carpet printing pastes. These are based on guar-gum, which is quite inexpensive, but has a limited shelf-life and therefore cannot be stored for long time before re-use (it is biodegradable and the growth of bacteria and other organisms such as yeasts, rapidly destroy the viscosity).
Economics Investment for retrofitting this recovery system to a rotary screen printing machine with 12 new squeegees and pipes (for a printing width of 185 cm) is about EUR 42000. The next table shows the savings achievable in the reference mill.
Table 4.34: Calculation of savings achievable in a typical textile mill by installing the referenced printing paste recovery system (the number of changes as well as the number of printing pastes per design may be higher in industrial practice) The example does not include the investment cost of new pumps, so a certain range of viscosity needs to be maintained.
When a wide viscosity range is required, the pumps have to be replaced.
The total investment cost is then reported to range between about 90000 and 112000 euros.
A payback time of about two years can therefore be considered realistic, but only when the whole amount of recovered printing paste is re-used. In practice this does not happen, especially with mills that use several different types of printing pastes. In such mills, due to logistical problems (limited storage and handling capacities), re-use rates of only 50 - 75 % are reported, which significantly extends the payback period [179, UBA, 2001].
Driving force for implementation Severe waste water problems and the need to reduce printing paste losses for economic and environmental reasons have been the main driving forces.
Reference plants World-wide, but especially in Europe, there are many rotary screen printing machines in operation, equipped with the described technique.
Reference literature [179, UBA, 2001], [180, Spain, 2001].
4.7.6 Recycling of residual printing pastes Description Printing pastes are highly concentrated preparations that consist of dyestuff, thickener and various other auxiliaries according to the different types of pastes (e.g. pigment, reactive, vat and disperse printing pastes). Printing paste residues can reach levels as high as 40 – 60 % in non-optimised equipment. A large portion of these residues is still discharged to the effluent, thus causing considerable waste water problems (see Section 2.8.3).
For a long time, printing pastes were prepared manually, which meant that residues were reused, but the extent and manner of that re-use were highly dependent on the staff responsible and on the production schedule. Today, computer-assisted systems offer more opportunities for recycling printing pastes. In most companies printing pastes are prepared with computerised 366 Textiles Industry
Another option is to empty all drums with residual printing paste and sort it according to its chemical characteristics (that is, type of dyestuff and thickener). The drum is cleaned with a scraper to minimise the printing paste loss, then is washed and re-used for the preparation of new printing pastes.
Main achieved environmental benefits The quantity of residual printing pastes to be disposed of is significantly reduced both as waste water and solid waste. The amount of residual printing paste is reduced by at least 50 % and in many cases by about 75 % [179, UBA, 2001], [192, Danish EPA, 2001].
Operational data Textile finishing mills that have applied these techniques report that such systems need some time for implementation in daily routine, but after managing starting problems, they work satisfactorily. The recycling systems described here are most efficient in companies having only one or two kinds of printing pastes (e.g. pigment and reactive printing pastes). In companies using various different types of printing pastes, the management of a large number of different mixtures may be difficult (logistical problems, limited capacity of the storage area, etc.). In these cases, the recycling rate may be in the range of 50 – 75 % only [179, UBA, 2001].
Operating with modern printing machines with a built-in system for mechanical removal of printing paste from pipes and hoses of the feeding system will increase the amount of printing paste that can be collected and re-used (see Section 4.7.5).
Cross-media effects None believed likely.
Applicability Systems for recycling of printing pastes are applicable in the textile industry both in existing and new installations. However, for totally computerised systems, the printing section must have a minimum size of 3 rotary screen and/or flat screen printing machines [179, UBA, 2001].
Trained and motivated personnel are needed for manual operations, such as scraping buckets, aimed at increasing paste recovery rate. Printers claim that specialised personnel are not willing to scrape buckets, especially given the number of buckets a day that need to be scraped for a typical 12 position printer that has to be changed every 600 - 800 m on average.
This technique is not applicable in the carpet industry for the same reasons explained in Section 4.7.5.
Economics Investment costs are about 0.5 - 1 million euros, depending on size and number of different printing pastes to be recycled. Payback periods are reported to vary between two and five years depending on individual circumstances [179, UBA, 2001].
Driving force for implementation Economic considerations and problems regarding residual printing paste disposal have been the main driving forces [179, UBA, 2001].
Reference plants Many plants in Europe and world-wide.
Important suppliers are:
· Stork Brabant B.V., NL-5830 AA Boxmeers · GSE Klieverik Dispensing, NL-6971 GV Brummen · I.A.S. Industrial Automation Systems S.R.L., I-22077 Oligate Comasco Reference literature [179, UBA, 2001], [192, Danish EPA, 2001].
4.7.7 Reduction of water consumption in cleaning operations Description The equipment used around the printing machine (e.g. screens, buckets and the print paste feed system) needs careful cleaning before it can be used for new colours. In connection with
cleaning operations there are several ways of reducing water consumption:
A. start/stop control of cleaning of the printing belt B. mechanical removal of printing paste C. re-use of the cleanest part of the rinsing water from the cleaning of the squeegees, screens and buckets D. re-use of the rinsing water from the cleaning of the printing belt.
A. Start/stop control of cleaning of the printing belt.
In many cases water dosage for the cleaning of the printing belt continues when the fabric and (therefore) the printing belt are stopped for whatever reason. Start/stop control of the water dosage can be automatically connected to the start/stop control of the printing belt.
B. Mechanical removal of printing paste.
Large amounts of water for cleaning of squeegees, screens and buckets are used within the print-house. Improved printing paste removal before flushing this equipment reduces the amount of water needed for flushing. Physical devices for removal of dye from buckets are available (e.g. scrapers). Modern printing machines have a built-in system for mechanical removal of residual printing paste from pipes and hoses.
C. Re-use of the cleanest part of the rinsing water from the cleaning of the squeegees, screens and buckets.
Typically, the first half of the effluent from the washing equipment is heavily loaded with printing paste and will have to be discharged as waste water. The water used for this first stage does not, however, need to be of a high quality, which means that recycled water can be used. In the second half of the washing process, clean water must be used, but the effluent can be collected for re-use, potentially as first-rinse water in the next cycle.
D. Re-use of the rinsing water from the cleaning of the printing belt.
The rinsing water from the cleaning of the printing belt is only slightly coloured and contains small amounts of fibres (depending on the fabric) and very small amounts of glue. The rinsing water can be mechanically filtered, collected in an overflow vessel and re-used for the same purpose if minor amounts of fresh water are added to the recycling system.
Main achieved environmental benefits
The environmental benefit is a reduction in water consumption. In a Danish pigment print-house the implementation of all the above options has reduced the annual consumption of water by approximately 25000 m3 (55 % reduction). For option A alone the savings are estimated at approximately 2 m3 for every hour the printing machine is stopped and the water dosage for cleaning of the printing belt is still running for whatever reason. For option C 50 % of the water can be re-used. For option D approximately 70 % of the water can be recycled [192, Danish EPA, 2001].
Operational data No information was made available.
Cross-media effects None believed likely.
Applicability Options A – D can be implemented in all types of textile companies involving a printing section: new or existing, large or small. Space availability is a minor factor: the company only needs space for collection tanks (option C and D). However, older printing machines can probably not be retrofitted with a device for mechanical removal of the residual printing paste from pipes and hoses in the printing machine as suggested in option B [192, Danish EPA, 2001] (see Section 4.7.5).
In the Danish case mentioned above, the total capital costs (tanks, mechanical filters, pumps and pipes) for all options are estimated at approximately 100000 DKK (13500 euros). Options C and D together amount to approximately 95000 DKK (12825 euros). Option A costs approximately 5000 DKK (675 euros) and the cost of option B is negligible. Changes in operating costs (before and after implementation) for all options are negligible. The costs for fresh water and waste water discharge are 9 and 18 DKK per m3 respectively (1.2 and 2.43 euros/ m3). Hence the total annual savings are approximately 675000 DKK (90000 euros). For the case study considered here, the payback time for all options together is estimated at about two months [192, Danish EPA, 2001].
Driving force for implementation The high cost of waste water discharge and, in some countries, of fresh water are the main driving forces for implementing the measures described.
Reference plants Many plants in Europe. One example plant is mentioned by [192, Danish EPA, 2001]: Danish Colour Design Textile Print A/S
Reference literature [192, Danish EPA, 2001] 4.7.8 Digital jet printing of carpet and bulky fabric Description The principles of jet printing and the latest development in this technique are described in Section 2.8.2.
Jet printing is today a full digital technique (right from the design stage to industrial production). The designer makes the sample on the computer and from this digital information the sample can be exactly reproduced on an industrial scale without need for corrective adaptations/measures.
Main achieved environmental benefits
In digital jet printing, selected trichromatic dyes are dosed on-line, based on computed requirements. The colour is applied on demand. This avoids dye losses or printing paste residues at the end of the process, which represents a revolutionary change from conventional printing methods (flat-screen and rotary screen).
Another big advantage of this technique is that the amount of thickeners can be reduced depending on the type of substrate because the dye is shot directly deep into the face of the fabric.
Further environmental advantages are [171, GuT, 2001]:
· reduced water consumption (only 20 % compared with conventional printing) · less trial-and-error sampling (one of the most important sources of pollution, often forgotten in evaluations) · the printing process can be carried out in the final stage of the manufacturing chain (reduction of material losses, particularly of coloured fabric).
Operational data No information made available.
Cross-media effects Compared to analogue printing techniques, no negative cross-media effect needs to be mentioned. However it is important to point out that the substrate must still undergo fixation and afterwashing. Therefore water emissions cannot be completely avoided.
Applicability Digital jet printing can be applied to carpet and bulky fabrics. It is also applicable for patterning runners, mats and tiles, in which case the operation is done very conveniently at the end of the process on the ready-made product (which doesn’t need to be washed afterwards).
The investment cost is still significant, which means that this technique is applied only in large companies.
There is still room for developments in [171, GuT, 2001]:
· increasing productivity · wider application (uneven pile density and un-level pile-height, blends, unfixed structure) · reducing investment cost · expanding the dye-range to cover more, if not all, shades and depths.