«EUROPEAN COMMISSION Integrated Pollution Prevention and Control (IPPC) Reference Document on Best Available Techniques for the Textiles Industry July 2003 ...»
Applicability This machine can be used both for knit and woven fabric and for nearly all types of fibres.
Fabrics containing elastane fibres, which are always difficult to dye, due to dimensional stability, can be dyed successfully using the airflow system. Limitations to the use of this machine are found with wool and wool blends with a percentage of wool higher than 50 % because of felting problems. This technique cannot be recommended for dyeing linen fabric because the linen lint causes scaling of the machine. The technique has been approved for silk, but it is rarely applied.
A process has been developed to minimise the oxidation of vat and sulphur dyes by the oxygen in the injected air.
Economics The application of this technique means investment in new equipment. Existing machines cannot be retrofitted. Investment cost for this type of machine is around one third higher than conventional jets, but due to high savings the payback period is relatively short. [179, UBA, 2001].
Driving force for implementation High productivity and repeatability still remain the main driving forces, followed by savings in water, chemicals and energy consumption.
Reference plants Airflow dyeing machines are in operation in many textile finishing mills world-wide. Bath-less airflows such as the one described in this section, where the textile is moved by air only, are produced by THEN GmbH, D-74523 Schwäbish Hall. ATYC SA Terrassa Barcelona produces an ULLR airflow (AIRTINT EVO H.T. machine) where the fabric is driven by air and water in separate jets. Other producers of machines using air and water for moving the fabric, with reduced and variable bath (e.g. Thies GmbH, 48653 Coesfeld; MCS, I-24059 Urgnano – Bergamo; Scholl AG, CH-5745 Safenwil).
Reference literature [179, UBA, 2001], [176, VITO, 2001].
22.214.171.124 Soft-flow dyeing machines with no contact between the bath and the fabric Description This model of jet uses water to keep the fabric in circulation. The concept that distinguishes this equipment from conventional jets operating with a hydraulic system is that the fabric rope is kept in circulation during the entire processing cycle (from loading to unloading) without stopping either the liquor or the fabric circulation for normal drain and fill steps.
The principle behind this technique is that fresh water enters the vessel via a heat exchanger and arrives at a special interchange zone whilst at the same time the contaminated liquor is channelled to the drain without coming into contact with the fabric or the new bath in the machine.
Rinsing is carried out in continuous mode, as in the airflow machine described earlier. Rinsing efficiency is increased thanks to the application of a special countercurrent system.
Main achieved environmental benefits The features of this machine lead to significant savings in processing time (17 – 40 %), water (about 50 %) and steam consumption (11 – 37 %), compared with other soft-flow machines of the same category. Performance data are reported under “Operational data”.
The efficient separation of the different streams offers further advantages such as optimum heat recovery and the possibility of re-use or dedicated treatment.
Operational data Table 4.29 shows the results of a comparison that has been made by running the same dyeing procedure on a conventional machine, on a “new generation” machine (typified by having charge tanks, pumped drain & fill options and continuous rinsing systems) and on the referenced soft-flow machine described above.
Notes (1) Including rinsing (2) Including loading/ unloading Table 4.29: Comparison of the performance for cotton dyeing with reactive dyestuffs in a conventional machine, a "new generation machine" (typified by having charge tanks, pumped drain & fill options and continuous rinsing systems) and the referenced soft-flow machine Cross-media effects None believed likely.
Applicability Typical applicability of soft-flow machines.
Economics The application of this technique means investment in new equipment. Existing machines cannot be retrofitted. No data regarding investment costs for this type of equipment were made available.
Driving force for implementation Increased productivity.
Reference plants The machine described in this section is produced by Sclavos (VENUSTM with AquachronTM process).
Reference literature [176, VITO, 2001]
126.96.36.199 Single-rope flow dyeing machines Description The configuration of this jet machine is reported in Figure 4.26. The way in which it handles the fabric and the dyeing cycle is very different from conventional rope dyeing machines. Firstly, there is only one fabric rope which passes through all flow groups and compartments, returning to the first compartment after the lap is complete.
The single rope approach ensures both optimum uniformity of the system and repeatability of the results.
High uniformity is achieved because the fabric passes continuously through all the different nozzles and different troughs in each lap. Whilst in multi-rope machines different conditions are generated in each compartment for various reasons (e.g. different speed of the ropes due to different nozzle flows, etc.), the single rope approach ensures homogenous operating conditions in the system as whole. This also means that the bath reaches equilibrium more rapidly when the operating conditions change (e.g. alkali/dyestuff injection, temperature increase/decrease). An immediate consequence is that chemical injection can be much faster and temperature gradients can be significantly increased without damage to the fibre.
This single-rope technique introduces a new concept in securing high repeatability: the use of the number of laps, rather than hold time for controlling the process. Except for time for fixation, which remains a time dependent parameter, addition of dyes and chemicals into the machine, temperature increase/decrease, etc. are done over a number of laps, instead of by a pre-set time. The counting of laps is very easy and ensures that from dye lot to dye lot the fabric always undergoes an equivalent process. Another advantage of the application of this lapcounting approach is that the cycle time is automatically adjusted to the speed of the rope and the load of the machine (the shorter the length of the rope, the lower the machine cycle time).
A lot of the latest time-saving devices are also incorporated, such as power filling and draining, a full volume heated tank, advanced rinsing programmes, etc.
The machine can maintain a constant liquor ratio (typically 1:6) whilst being loaded at a level as low as 60 % of its nominal capacity.
Figure 4.26: Representation of the single-rope dyeing machine
Main achieved environmental benefits Very short cycles and other features described above result in significant water and energy savings (up to 35 %) compared to conventional multi-rope machines.
High repeatability and reliability of the final results bring about additional environmental benefits. "Right-first-time" production is one of the most effective pollution prevention measures because it avoids additional consumption and waste of chemicals and resources for corrective measures such as rework, re-dyes, stripping, shade adjustments, etc. A reduction of reworks from 5 % to 2 % has been observed in companies where this technique is applied [177, Comm., 2001].
Last but not least, the use of a single-rope machine reduces the amount of sewing and cutting at the end of each dyeing cycle. On average 1 – 1.5 metres of fabric are wasted for each join. A typical three-rope machine performing 3 dyeing cycles per day for 300 days/year would waste 2700 metres of fabric per year more than the same process carried out in a single-rope machine.
For a medium-size finishing mill this is equivalent to about 3000 – 4500 kg/year of wasted fabric [177, Comm., 2001].
The table below shows the results of the same dyeing process in a conventional machine (L.R.
1:10 – 1:12), a "new generation machine" (typified by L.R. of 1:8 and fitted with latest timesaving devices) and the single-rope machine described above (L.R. 1:6). The data are derived from measurements taken at production sites.
Notes (1) Including rinsing (2) Including loading/ unloading Table 4.30: Comparison of the performance for cotton dyeing with reactive dyestuffs in a conventional machine (L.R. 1:10 – 1:12), a "new generation machine" (L.R. 1:8 and equipped with latest time-saving devices) and the single-rope machine described above (L.R. 1:6) Cross-media effects None believed likely.
Applicability The single-rope machine is used successfully for processing both knit and woven fabric of nearly all types of fibres. Unless the horizontal model of this machine is used, limitations are observed when dyeing wool, silk and blends of these two fibres.
Economics Investment cost for this type of machine is 20 – 30 % higher compared to new conventional type machines, but thanks to savings and the higher productivity the payback period can be less than 10 months.
Driving force for implementation High productivity, repeatability and versatility are the strong points of this machine.
Minimisation of energy consumption is a welcome bonus.
Reference plants Many are in operation in textile finishing industries world-wide.
The single-rope machine described in this section is produced by MCS Urgnano (BG) Italy.
Reference literature [177, Comm., 2001], [116, MCS, 2001], [176, VITO, 2001].
4.6.22 Water re-use/recycling in batch dyeing processes Description Opportunities to minimise water consumption in dyeing processes may be found in dye bath reconstitution and re-use or re-use of the rinse water for the next dyeing.
Dye bath re-use is the process by which exhausted hot dye baths are analysed for residual colourant and auxiliary concentration, replenished and re-used to dye further batches. Two procedures are possible. With the first method the dye bath is pumped to a holding tank (or to a second identical machine), while the product is rinsed in the same machine in which it was dyed. The dye bath is then returned to the machine for the subsequent batch of material. With the second option, the product is removed from the exhausted dye bath and placed in another machine for rinsing. In this case no holding tank is required, but the material needs additional handling. Dye bath analysis can be performed using spectrophotometer and/or may be determined by production experience based on exhaustion level, volatilisation and dye liquor drag-out [11, US EPA, 1995].
Since the spent dye bath is usually hot, it is of course convenient to save time and energy by dye bath re-use. However, to assure level dyeing it is normally necessary to start the dyeing process at 50 °C. Therefore, the hot spent bath is cooled down and then warmed up again. In some cases this can be avoided. New technologies have been developed which allow dyeing to start at process temperatures. Instead of piloting the temperature one can control the chemical potential of the dye (which is what happens, for example, by adding the sodium hydroxide to reactive dyes). These techniques are suitable for wool dyeing with acid dyes, acrylic dyeing (this would exclude the addition of levelling agents) and for cotton in the case of sulphur dyeing or reactive exhaustion dyeing processes [204, L. Bettens, 2000].
The second technique proposed here is similar, but this time the spent rinse bath is re-used to form the next dye bath.
Main environmental benefits Reduction in water and chemicals consumption. Energy saving (re-use of the hot dye bath) is also possible, in some cases (see above) when dye adsorption is controlled by pH and the bath becomes nearly completely exhausted without cooling down at the end of dyeing.
Operational data Operational data are reported by UBA for a plant dyeing PES and wool loose fibre. Wool is dyed with afterchrome or with metal-complex dyes, whereas PES fibre is dyed with disperse dyes. Both dyes are characterised by high exhaustion rates, which allows re-use of the spent dyeing bath for the next batches. All dyeing machines with capacity ranging from 50 to 100 kg (L.R. 1:8) have been fitted with holding tanks, temperature and pH control devices and automated dosage systems for formic acid. Most of the holding tanks are constantly used for the same type of dyes and shades (e.g. afterchrome bath for dark shades, etc.). As a result of the improvements, the company has achieved a decrease in specific water consumption from 60 to 25 l/kg [179, UBA, 2001].
Another operational experience is reported by ENco for a mill dyeing wool loose fibre. The company operates conical pan type machines and loads the fibre carriers with dry fibre. Mean specific water consumption figures for the conventional dyeing and rinse cycle are 9.5 l/kg and
7.8 l/kg, respectively (1.7 l/kg is retained by the fibre load between dyeing and rinsing). Overall water consumption for a conventional cycle would be 17.3 l/kg.
When re-using the rinse liquor for the next dyeing it is necessary to add on average 1.7 l/kg of fresh water to the dyeing to make up for the water lost when the wet fibre from the previous dyeing is removed. Experience indicates that on average only four cycles of the same shade can be sustained with re-use. Overall water consumption for this four-batch dyeing system is reduced by approximately 33 % when compared with the conventional cycle [32, ENco, 2001].
Cross-media effects None believed likely.
Applicability For re-using water in dyeing processes (both techniques) holding tanks are normally needed to store the spent baths. Some models of modern batch dyeing machines (e.g. jiggers, jets and winches) have built-in holding tanks, thus allowing for uninterrupted automatic separation of concentrates from rinsing water.
When using top-loading dyeing machines (typically used for loose fibre and in some cases for yarn) the rinse bath can be retained in the machine at the end of the process and re-used for dyeing the next lot of material without need for holding tanks.
The re-use of dye baths and rinse waters involves some fundamental differences from the use of fresh bath. The easiest systems to manage are dye classes which have high affinity (exhaustion) and which undergo mimimum changes during the dyeing process. Examples are acid dyes for nylon and wool, basic dyes for acrylic, direct dyes for cotton and disperse dyes for synthetic fibres. The easiest situation is to re-use a dye bath to repeat the same shade with the same dyes and equipment and the same fibre. Some production planning to progress from pale to deep shades is required (which may somewhat limit the flexibility of batch dyeing operations).
The number of re-use cycles is limited by build-up of impurities from several sources. One source is represented by the impurities present on the textile material, which include natural impurities in cotton and wool, knitting oils, fibre preparation agents, etc. Impurities can also
accumulate from components of dye formulations, auxiliaries (e.g. levelling agents), electrolytes, salt build-up from addition of acids and bases for pH control, etc.
In conclusion, the limitations are less severe where machinery is available for internal separation of contaminated spent bath and rinsing water, where trichromatic dye systems are used, dye adsorption is controlled by pH (saving the hot dye bath) and where the bath becomes nearly completely exhausted without cooling down at the end of dyeing [204, L. Bettens, 2000].
Direct savings are related to both process water purchase price and effluent disposal costs.