«EUROPEAN COMMISSION Integrated Pollution Prevention and Control (IPPC) Reference Document on Best Available Techniques for the Textiles Industry July 2003 ...»
These problems were recently tackled by machine manufacturers and dyestuff suppliers. Recent technological developments have decreased specific water consumptions in batch processing to levels more typical of continuous operations. A constant liquor ratio across variable load sizes is now a standard feature of modern equipment for batch processes. Efficient washing techniques have also been especially developed for batch operations (see Section 4.9.1). Furthermore, various functions typical of continuous processing have been transferred to batch machines,
such as (see Section 4.6.19):
· in-process separation of the bath from the substrate · internal separation of process-liquor from the washing liquor · mechanical liquor extraction to reduce carry-over and improve washing efficiency · internal countercurrent flow in the batch washing process.
· by combining processes Combining and scheduling processes reduces the number of chemical dumps. This is often feasible for pretreatment operations (e.g. scouring/desizing, scouring/desizing/bleaching – see for example Section 4.5.3). Combining pretreatment into the colouration stage is also possible in some cases.
Batch processes do not easily allow for water recycling. When trying to re-use waste water in batch operations, storage facilities for re-usable waste water must be provided. Other problems associated with re-use of waste water from batch bleaching and scouring are the non-continuous character of the waste stream and the higher liquor ratios.
Continuous countercurrent flow of textiles and water is now also possible in batch processing.
Machines are now available with built-in facilities for waste stream segregation and capture. For example, the wash water from a previous load can be recovered and fully used in the bleach bath for the current load, which can then be used to scour the next load. In this way, each bath is used three times.
Some examples of water recycling and re-use are reported in this chapter (see Sections 4.6.22 and 4.7.7).
The internal separation of process-liquor from the washing liquor applied to some modern batch dyeing machines (see above) is essential to allow easier bath segregation and re-use, in cases where the characteristics of the liquor make it feasible.
Main achieved environmental benefits Significant savings in water and energy consumption are possible (energy is used to a great extent to heat up the process baths).
Economics In existing mills, investment in new equipment and/or structural modifications (e.g. for the segregation of streams) is likely to be necessary.
Reference plants See cross-referenced techniques in other sections of this document.
Reference literature [179, UBA, 2001], [204, L. Bettens, 2000], [208, ENco, 2001], [11, US EPA, 1995].
4.1.5 Insulation of High Temperature (HT) machines Description Insulation of pipes, valves, tanks and machines is a general principle of good housekeeping practice that should be applied at the general level in all processes.
In this section an example is given of the energy savings achievable by heat insulation of HT dyeing machines.
Main achieved environmental benefits More rational use of energy.
It is reported that insulation can save up to 9 % of the total energy requirement on wet processing machines [146, Energy Efficiency Office UK, 1997]. An integrated approach to energy conservation is, however, preferable to ad hoc measures.
Operational data The nature of the process means that insulation material may be exposed to water, chemicals and physical shock. Any insulation should therefore be covered or coated with a hard-wearing, chemical/water resistant outer layer.
Cross-media effects None believed likely.
Applicability General applicability.
Economics A calculation of the payback for heat-insulation of HT dyeing units is given in the table below [179, UBA, 2001].
Source: [179, UBA, 2001] Table 4.2: Payback periods for heat insulation of dyeing units Driving force for implementation Savings in energy costs.
Reference plants Many plants.
Reference literature [179, UBA, 2001], [146, Energy Efficiency Office UK, 1997]
4.2 Quality management of incoming fibre 4.2.1 Man-made fibre preparation agents with improved environmental performance Description Man-made fibres cannot be produced and processed without auxiliaries. As a consequence of pretreatment operations (e.g. washing and heat-setting) these auxiliaries find their way into the waste water and exhaust air of finishing mills.
Among the auxiliaries used, coning oils and other preparation agents applied to the fibre after it has been manufactured have been identified as the major causes of the pollution in the downstream processes. This is due to the quality of the formulations employed and to the high loads applied (see also Section 184.108.40.206).
Conventional preparation agents are mainly based on mineral oils, with their well known disadvantages of high add-on, low temperature stability (they smoke during high-temperature treatments), poor biodegradability, presence of polyaromatic hydrocarbons and generation of difficult-to-sediment sludge in biological waste water treatment plants (see also Section 8.2).
Alternatives to the mineral oil-containing preparation systems are preparation agents based on [179, UBA, 2001]:
· polyether/polyester or polyether/polycarbonates · special polyolesters · special steric hindered fatty acid esters.
Main achieved environmental benefits Alternative preparation systems are less volatile and have higher thermal stability. Moreover, they can be applied in lower amounts on the fibre. As a result, reduced odour nuisance in the workplace and reduced emission levels of organic volatile compounds in the exhaust air are achieved.
The table below compares the performance of the alternative products with the conventional systems in heat-setting conditions for grey materials.
Table 4.3: Emission factors and corresponding organic-C concentration in the off-gas The optimised products indicated in the table above are easier to wash out (lower consumption of water, energy and chemicals) and are in general characterised by a higher level of biodegradability compared to mineral oil-based preparation agents.
Polyesterpolyetherpolycarbonates compounds, in particular, show extremely good biodegradability in comparison with mineral oils. Sterically hindered fatty acid esters, on the other hand, only represent an improvement with respect to classic fatty acid esters for air emissions from thermal treatment (thermofixation). They are in fact less volatile, but they are more difficult to biodegrade due to the increased branching of the chain.
Yarn producer Some machine components have to be made up of high-grade steel due to potential corrosion problems. With polyeter/polyethercarbonate-based products compatibility problems with conventional hydrophobic preparation systems means that thorough equipment cleaning is needed following use.
Fabric producer Because of compatibility problems the equipment has to be cleaned carefully (especially in the case of polyester-/polyethercarbonate-based auxiliaries).
Finishing mill Processes in pretreatment have to be adjusted to the new preparation systems. In some cases (e.g. with polyester-/polyethercarbonate-based auxiliaries) washing steps in pretreatment can be simplified or even omitted.
Cross-media effects Since new products are less volatile, off-gas emissions are reduced, but a higher amount remains on the fabric after heat-setting and eventually ends up in the waste water.
However, because of the lower quantities applied and the better biodegradability of the new products, the replacement will always bring benefits [179, UBA, 2001].
Applicability Low-emission preparation agents are applicable on PES, PA 6.6, PA 6, CV and their blends with PES or CV. However, the applicability depends on the type of fibre and the particular enduse of the final product. As a result, specific trials should be carried out [179, UBA, 2001].
Commission finishers often receive no information from the supplier concerning the quality of preparation agents used. Conventional preparation agents are cheaper and spinning mills look mainly at the economic aspects and at the performance of a given substance in the spinning process. The environmental problems produced in the downstream processes (at the finishing mill) are not seen as a priority by spinning mills.
Economics The following economic aspects affecting all members of the textile processing chain have to be taken into consideration [179, UBA, 2001].
Yarn producer Low-emission auxiliaries are high-price products. This can be compensated by a lower add-on.
Finishing mill Saving of exhaust-air cleaning equipment, simplified waste water treatment and prevention of oil-contaminated wastes will reduce investment, maintenance, and disposal costs.
Additional cost savings can be achieved with those preparation agents that allow total or partial omission of the washing step. Increased operational reliability is also to be expected.
Driving force for implementation Minimising off-gas loads caused by preparation agents (compliance with national regulations) and water saving in washing are the main reasons for the use of low-emission preparation agents.
Reference plants Some fibre/yarn and fabric producers in Europe are using low-emission preparation agents.
Examples of fibre/ yarn producers are listed below [179, UBA, 2001].
Inquitex S. A.
Via Augusta 158, 5a planta E-08006 Barcelona
Nurel S. A.
P. delle Gracia 53 E-08007 Barcelona Nylstar GmbH Postfach 2209 D-24531 Neumünster Nylstar CD Italy Via Friuli 55 I-20031 Cesano Maderno (MI) Textilwerke Deggendorf GmbH Postfach 1909 D-94459 Deggendorf Trevira GmbH & Co KG D-60528 Frankfurt am Main Unifi Textured Yarns Europe LTD.
Co. Donegal Letterkenny, Ireland Reference literature [77, EURATEX, 2000], [179, UBA, 2001].
4.2.2 Mineral oils substitution in wool spinning lubricants Description Spinning lubricants are usually removed during pretreatment to ensure uniform penetration of the dye and finishing agents and to avoid reaction and precipitation with them. Since in the case of wool the processes that take place first in the finishing mill are wet treatments (washing/scouring), the presence of lubricants affects primarily water rather than air emissions.
In carded wool and wool blend yarns, where a higher load of lubricants is applied (compared to worsted wool) spinning oils (together with detergents used in the scouring process) may contribute up to 80 % of the oxygen-demanding load in dyehouse waste water [32, ENco, 2001].
Mineral oil-based lubricants were once used universally in the wool sector. These substances may not be fully degraded in biological sewage treatment works.
Moreover, the formulations of conventional spinning lubricants may contain variable amounts of even more hazardous substances such as polyaromatic hydrocarbons and APEO or other “hard surfactants” as emulsifiers (see also Section 8.2).
Mineral oils have now largely been replaced with formulations based on glycols and this trend continues. Biodegradable substitutes are readily available [32, ENco, 2001]. APEO compounds can now also be substituted by less problematic surfactants.
Main achieved environmental benefits Elimination of mineral oil from scouring and dyeing waste water and the effluents received by sewage treatment works.
Using APEO-free spinning lubricant formulations helps to reduce the amount of potentially toxic endocrine disrupters in the receiving water.
Operational data In the carpet sector, the use of mineral oil-based lubricants is reported to have been in decline for some years, indicating that carpet yarn of commercial quality can be produced without the use of this material [32, ENco, 2001]. It is believed that the same is valid for the textiles sector.
Cross-media effects Compared to mineral oil-based lubricants, increased foaming in the waste water treatment may be observed.
There is some evidence that mineral oil-based lubricants are more amenable to on-site treatment than are the more water-soluble glycol-based products. Where there is on-site pretreatment, the choice of lubricant may be critical and mineral oil-based products may be the more viable option. Further research may be required [32, ENco, 2001].
Spinning lubricants are applied during fibre blending. Undertakings which include the manufacturing processes leading up to wet processing, such as sales yarn spinning, and vertically-integrated companies have the means to control the use of these materials “in house”.
Commission dyers receive yarn on which the lubricant is already present. In these cases it would be necessary to work with clients to eliminate these materials from the supply chain [32, ENco, 2001].
Economics The consequences of substituting one spinning lubricant for another are difficult to predict as the yarn yield (the quantity of yarn obtained from a given mass of raw fibre) is notoriously difficult to measure accurately and very small changes in yield markedly affect the economics of yarn production. The lubricant type and level of application can have a significant impact on yield [32, ENco, 2001].
Driving force for implementation Environmental legislation.
Reference plants Many plants in Europe.
Reference literature [32, ENco, 2001] 4.2.3 Mineral oils substitution in knitted fabric manufacturing Description The production of knitted fabric requires an efficient lubrication of the needles and mechanical elements of the knitting machine. The quantity of lubricants used depends on the technology of the machine and its speed.
The yarn driven by the needles during the manufacturing of the fabric carries part of the lubricant. As a result, the final knitted fabric can contain about 4 – 8 % w/w of lubricant oils that then needs to be removed during pretreatment.
Conventional knitting oils (mineral oil-based formulations) can only be removed through emulsification using detergents, emulsifiers and antiredeposition agents. The process is carried out under alkaline conditions and at temperatures between 80 and 100 ºC. Water consumption is approximately 10 l/kg of fabric, and the time required for the process is about 30 - 60 min.
The proposed technique suggests using hydrosoluble oils instead of conventional lubricants.
With knitted fabrics made of cotton or cotton blends with synthetic fibres these hydrosoluble oils can be easily washed out with water at 40 ºC. This makes it possible to scour and bleach the fabric in one single step, thus saving time, water and energy.
Knitted fabrics made of synthetic fibres (e.g. polyester or polyamide) are often thermofixed before being washed. If conventional oils are present on the fabric, an intense emission of fumes is generated and the remaining oil becomes more difficult to remove from the fabric in the subsequent washings.
Also in this case it is often possible to use water-soluble oils instead of conventional lubricants and to carry out the washing step before thermofixation. Washing takes place in a continuous high-efficiency washing unit (e.g. TVE-Escalé type). After this step the fabric is sent to the stenter and then dyed, washed and finished. In this way emissions of fumes from the stenter are minimised.
Main achieved environmental benefits
Unlike conventional mineral oil-based lubricants, hydrosoluble oils can be easily washed out from the fabric. This helps reduce water, energy and chemicals consumption along with processing time. Moreover, these oils are reported to be biodegradable according to OECD test 301C [295, Spain, 2002], which makes the resulting effluent suitable for treatment in a biological waste water treatment plant.