«EUROPEAN COMMISSION Integrated Pollution Prevention and Control (IPPC) Reference Document on Best Available Techniques for the Textiles Industry July 2003 ...»
126.96.36.199 Hydro-extraction by squeezing The fabric is squeezed by means of a padding machine through two or three rollers covered with rubber. This process cannot be applied to delicate fabric.
188.8.131.52 Hydro-extraction by suction The fabric is transported flat over a "suction drum" which is linked to a pump. The external air is sucked through the fabric and thereby removes the excess water. The resulting residual humidity is still about 90 %.
184.108.40.206 Centrifugal hydro-extractor The design of this machine is similar to the one described earlier for loose fibre and yarn hydroextraction. With heavy fabric, an horizontal axis machine may be used.
This is the most efficient method for mechanical water removal, but it cannot be applied to delicate fabrics prone to form permanent creases.
220.127.116.11 Stenter This machine is used for full drying of the fabric. The fabric is conveyed through the machine in open width. A hot current of air is blown across the fabric thereby producing evaporation of the water.
The fabric is sustained and moved by two parallels endless chains. The fabric is hooked undulating and not taut to allow its shrinking during drying.
Most common stenter designs are horizontal and multi-layer, but many new designs exist. In the horizontal stenter machine, the fabric enters wet from one side and exits dried from the other. In the multi-layer type it enters and exists from the same side. While in the first one the fabric moves horizontally without direction changes, in the second it is deviated many times, which makes this equipment unsuitable for delicate fabrics. On the other hand horizontal stenter frames occupy more space and are less efficient (in terms of energy consumption) 18.104.22.168 Hot-flue dryer This machine is composed of a large metallic box in which many rolls deviate the fabric (in full width) so that it runs a long distance (about 250 m) inside the machine. The internal air is heated by means of heat exchangers and ventilated.
22.214.171.124 Contact dryer (heated cylinder) In this type of machinery the fabric is dried by direct contact with a hot surface. The fabric is longitudinally stretched on the surface of a set of metallic cylinders. The cylinders are heated internally by means of steam or direct flame.
126.96.36.199 Conveyor fabric dryer The fabric is transported within two blankets through a set of drying modules. Inside each module the fabric is dried by means of a hot air flow.
This equipment is normally used for combined finishing operations on knitted and woven fabrics when, along with drying, a shrinking effect is also required in order to give the fabric a soft hand and good dimensional stability.
188.8.131.52 Airo dryer This machine can be used for washing, softening and drying operations on woven and knitted fabrics in rope form.
During the drying phase the fabric in rope form is re-circulated in the machine by means of a highly turbulent air flow. Water is thus partly mechanically extracted and partly evaporated.
Thanks to the particular design of this machine it is possible to carry out in the same machine wet treatments such as washing. In this case the bottom of the machine is filled up with water and the required chemicals and the fabric is continuously soaked and squeezed.
The capacity of this machine is determined by the number of channels (from 2 to 4).
2.14 Textiles industry categories Up to this point, this chapter has described the fundamental unit processes in the textiles industry that are within the scope of this document. The information given has been structured by fibre type, which has made it possible to explain those issues that are determined by the physico-chemical behaviour of the fibre. From a practical point of view, however, a subdivision of the textile industry activities into sub-sectors based on the type of the fibre offers little practical aid.
In practice, there are established patterns of activity, with finishing mills tending to concentrate on particular kinds of make-up or end-product (e.g. yarn, woven fabric, carpet, etc.), because this is defined by the specialist machinery used. This degree of specialisation does not apply to the same extent to the fibre. Although in the past the predominance of natural fibres made possible the identification of separate sectors based on the fibre (mainly cotton and wool), nowadays the proliferation of man-made fibres means that finishers almost always process a wide variety of fibres, even if one type is dominant within a particular mill (e.g. wool, cotton, etc.).
As an aid to the application of this BREF, therefore, the rest of this chapter gives practical information on the main categories of mills that are actually found in this sector (integrated mills should be seen as a combination of these main categories).
The typical mill categories listed below also prepare the ground for the presentation of the emission and consumption levels in Chapter 3.
· Wool scouring mills
The carpet industry is kept in a separate group from the other finishing mills. This is slightly inconsistent with the categorisation system adopted (based on the processing operations), in which a category of mills finishing yarn consisting mainly of wool is already identified in the list under the heading "Mills finishing yarn". However, the peculiarity of carpet as an end
product involves such specific requirements that companies tend to specialise in the processing of products that are suitable only for the carpet sector.
2.14.1 Wool Scouring Mills Wool processed in Europe is mostly imported. Most of the wool produced in Europe is in fact coarse wool which is suitable for the manufacturing of carpets, but not for use in apparel.
Merino wool (fine wool) is therefore imported mainly from Australia, New Zealand, South Africa, Argentina, Uruguay and Brazil.
The organisation of the scouring sector reflects the two main systems used to process wool: the woollen and the worsted system. Scourers tend to specialise in one or the other. Woollen system scourers normally only scour the wool (sometimes they may blend it before dispatching it to the customer). Worsted system scourers usually scour, card and comb the wool thus producing a sliver of parallel fibres which is called top. Because of this difference, worsted system scourers are usually referred to as combers.
Within Europe, significant quantities of wool are also obtained from skins of slaughtered animals by a process called fellmongering in which the skins are treated chemically or biochemically to loosen the wool roots so that the wool can readily be separated from skins.
The scouring process is usually the only wet process carried out in scouring mills and it has already been described in detail in Section 184.108.40.206.
Most of the scourers have an on-site waste water treatment plant to treat their effluent. The majority discharge the treated effluent to sewer, but there are several scourers who discharge directly to surface waters. Those in the latter category have to treat their effluent to higher standards. Of the scourers who discharge to sewer, some treat only the heavily contaminated effluent from the scouring section and discharge the rinse water flowdown without treatment;
others mix the two effluent streams before treatment.
Broadly speaking, there are four main types of effluent treatment process used by scourers:
· evaporation (sometimes combined with incineration with full closure of the water cycle);
· membrane filtration;
· anaerobic/aerobic biological treatment · spreading to land or lagooning (after grease separation, in extensive wool producing areas).
Some scourers use combinations of the above processes. Neither the heavily contaminated effluent from the scouring section nor the mixed scouring and rinsing effluents can be treated directly by aerobic digestion, because their COD values are too high. It is normal to subject these effluent streams to anaerobic biological treatment or coagulation/flocculation before aerobic biological treatment (coagulation/ flocculation before aerobic treatment may result in huge amounts of sludge).
All of the effluent treatment processes employed by scourers produce a sludge or a concentrate which has to be disposed of safely. Sludge disposal routes used include landfill, composting, incineration, pyrolysis/gasification and brick manufacture.
2.14.2 Mills finishing yarn and/or floc A common feature of floc and yarn finishing is that all process steps are normally carried out in
the same equipment. The basic process sequence is:
· pretreatment (scouring/bleaching) · dyeing · finishing (mainly softening by addition of softening agents in the last rinsing bath, but also flame retardant or mothproofing treatments for carpet wool fibre).
120 Textiles Industry
Chapter 2Pretreatment can be carried out as a separate step, especially in the case of natural fibres, or together with dyeing by adding additional specific auxiliaries in the dye bath. The second option is common when the amount of impurities on the fibre is not significant and they are easy to remove, or when auxiliaries (e.g. preparation agents, spinning lubricants) are specially chosen not to interfere with the dyeing process.
Bleaching is normally not applied for synthetic fibres. With natural fibres, bleaching is commonly omitted for dark shades, whereas for light shades it is often combined with scouring.
After washing, the material is dyed in the same machine and then submitted to final washing and rinsing.
For dyeing, it is common practice, in the case of floc and tops material, to achieve the desired final shade by thorough mixing of individual dyeings. With yarn, on the other hand, the required shade has to be achieved with only one dyeing since, unlike floc and tops, the shade cannot be corrected by compensation. For this reason, a higher standard of accuracy is required in the development of the dye recipe in the laboratory.
As explained in other parts of this document, the dyes and auxiliaries applied vary with the fibres processed.
Mercerising treatment may be desired for cotton. If so, the material is processed in hank form.
Mercerisation is carried out in a separate machine and is normally the first treatment applied.
Anti-felt treatment is another optional operation; it is applied only on wool and mainly on tops.
2.14.3 Mills finishing knitted fabric Mills finishing knitted fabric consisting mainly of CO The typical process sequence for finishing knitted fabric consisting mainly of cotton is shown in Figure 2.42 (only the wet processes are reported). The dotted lines indicate processes that are not obligatory or are not common practice. Acidic demineralisation, for example, is applied only in a few mills. Mercerisation is also indicated with a dotted line because this additional treatment is only required for certain articles.
Figure 2.42: Typical process sequence for the finishing of knitted fabric mainly consisting of cotton [179, UBA, 2001] Scouring is generally a batch operation, but large installations often do it in continuous mode.
Hydrogen peroxide is the most commonly applied bleaching agent in cotton mills today.
Cotton knitted fabric can be dyed with different classes of dyestuffs such as reactive, direct, sulphur and vat dyestuffs. Reactive dyestuffs are the most commonly used. Direct dyestuffs may be used for lighter shades and sulphur dyestuffs for dark shades. Vat dyestuffs may be used for very high light fastness requirements.
In printing, two further subclasses can be identified:
· mills finishing cotton knitted fabric without a printing section and · and mills finishing cotton knitted fabric with a printing section.
Pigment printing is widely applied for knitted fabric and does not need the after-washing step required when printing with reactive, disperse and vat dyes (also quite common techniques in this sector).
Mills finishing knitted fabric consisting mainly of synthetic fibres The typical process sequence for finishing knitted fabric mainly consisting of man-made fibres is shown in Figure 2.43 (only the wet processes are indicated). Optional operations are indicated with dotted lines.
Figure 2.43: Typical process sequence for the finishing of knitted fabric consisting mainly of manmade fibres [179, UBA, 2001] Before dyeing, the fabric is normally washed out in order to remove preparation agents and impurities.
Heat-setting is not always needed, but when carried out this operation can take place either before washing (on the raw fabric) or after the washing step.
Depending on the required degree of white, bleaching may be needed.
Mills finishing knitted fabric consisting mainly of WO The process sequence that is reported in Figure 2.45 is also applicable to this category of finishing mills.
2.14.4 Mills finishing woven fabric Mills finishing woven fabric consisting mainly of CO and/or CV The typical process sequence for the finishing of woven fabric consisting mainly of cotton is shown in Figure 2.44. Optional operations are indicated with dotted lines.
Figure 2.44: Typical process sequence for the finishing of woven fabric mainly consisting of cotton [179, UBA, 2001] Woven fabric consisting mainly of cotton and cotton blends is finished on semi-continuous/ continuous lines or in discontinuous mode mainly depending on the size of the lot.
Pretreatment operations such as desizing, scouring and bleaching are very often combined in one single step in continuous lines. Pretreatment of viscose usually requires alkali treatment and washing only, provided that the sizing agents are water-soluble, which is normally the case.
In addition to the processes mentioned in Figure 2.44, further treatments may be exceptionally applied, such as pretreatment with liquid ammonia (carried out at a very few sites in Europe only).
For printing, two further subclasses can be identified:
· mills finishing cotton woven fabric without a printing section · and mills finishing cotton woven fabric with a printing section.
Mills finishing woven fabric consisting mainly of WO The typical process sequence for the finishing of woven fabric consisting mainly of wool (woollen and worsted wool) is shown in Figure 2.45.
Figure 2.45: Typical process sequence for the finishing of woven fabric mainly consisting of wool [31, Italy, 2000] Both water washing and solvent washing (dry cleaning) are common in the wool sector.
Washing in an aqueous medium is carried out either in rope (in batch) or in open-width (mostly in continuous, but also in batch). Heavy fabrics (woollen wool) are preferably treated in rope form, whereas washing in open width is preferred for fine worsted fabric.
Carbonising and fulling are optional treatments in the basic process sequence. Carbonising is applied only on woollen wool fabric, which is also the most common application of fulling treatments.
Crabbing can be carried out before or after dyeing, depending on the desired effect. Crabbing on raw fabric is done in order to set the dimensions of the fabric, so that they will not change during use or during the subsequent processes.
Mills finishing woven fabric mainly consisting of synthetic fibres The process sequence for the finishing of woven fabric consisting mainly of man-made fibres is similar to the one illustrated in Figure 2.43 for knitted fabric. However, here the washing/desizing step is more important because all sizing agents need to be removed.
Synthetic sizing agents are normally used, which are easily removed with water, often in continuous washing machines.
In fabric with a certain percentage of elastane, silicones are also present. The complete removal of these substances can be very difficult. In some cases, tetrachloroethylene is applied; totally closed systems are mainly used for this purpose today, which severely limit losses of solvent.