«EUROPEAN COMMISSION Integrated Pollution Prevention and Control (IPPC) Reference Document on Best Available Techniques for the Textiles Industry July 2003 ...»
Peracetic acid is produced from acetic acid and hydrogen peroxide. It can be purchased as ready-made product or produced in-situ. Its optimal bleaching action is reached only in a very narrow pH range between 7 and 8. Below pH 7 the degree of whiteness decreases sharply and above pH 9 depolymerisation of the fibre with consequent damage of the fibre occurs.
Peracetic acid is sometimes applied for synthetic fibres (e.g. polyamide) where hydrogen peroxide cannot be used.
18.104.22.168 Environmental issues The main environmental issues associated with cotton pretreatment arise from emissions to water.
The characteristics of the emissions vary according to a number of factors: the make-up, the sequence adopted, the fact that some treatments are often combined in a single step, etc.
The most relevant issues are highlighted below for the various operations involved in pretreatment.
Desizing and scouring In a typical mill processing cotton or cotton-blend woven fabric, desizing represents the main emission source in the overall process. As already stated in Section 22.214.171.124, the washing water from desizing may contain up to 70 % of the total COD load in the final effluent, especially in the case of native sizing agents. Neither enzymatic nor oxidative desizing allows size recovery, while for some synthetic sizing agents size recovery is technically feasible, but difficult to apply for commission companies (see also Section 4.5.1).
Textiles Industry 47 Chapter 2 Compared to woven cotton fabric, the removal of impurities from yarn and knitted fabric is less significant in terms of pollution load. Nevertheless, consideration must be given to the COD load associated with the removal of knitting oils (for knitted fabric), spinning oils and preparation agents (when cotton is in blend with synthetic fibres) and the biodegradability of these compounds. Substances such as synthetic ester oils are readily emulsified or soluble in water and easily biodegraded. More problems are posed by the presence of substances that are difficult to emulsify and poorly biodegradable such as silicone oils, which are found in elastane blends with cotton or polyamide.
Mercerising is responsible for a large amount of strong alkali that is discharged in waste water and needs to be neutralised. The corresponding salt is formed after neutralisation. In this respect, the cold process involves higher emission loads than the hot one. In order to allow the required retention time and make possible the cooling of the bath in continuous mode, a portion of the bath needs to be taken out and cooled down in continuous mode. This means that higher volumes of bath are necessary in cold mercerising, which result also in higher emissions if the caustic soda is not recovered. Mercerising baths are usually recovered and re-used. When this is not possible, they are used as alkali in other preparation treatments (caustic soda recovery is discussed in Section 4.5.7).
Bleaching with hydrogen peroxide
The decomposition of hydrogen peroxide, which takes place during the bleaching reaction, forms only water and oxygen. However, environmental concerns associated with the use of stabilisers have to be taken into account (see Section 8.5). These products may contain complexing agents such as EDTA and DTPA with poor bio-eliminability, which may pass undegraded through the waste water treatment systems. Their ability to form very stable complexes with metal makes the problem even more serious because they can mobilise heavy metals present in the effluent and release them in the receiving water (see also Section 4.3.4 and 4.5.6 for alternative techniques).
It has been reported ([77, EURATEX, 2000]) that, depending on concentration, pH, time and temperature, effluents containing inorganic chlorides (e.g. NaCl from dyeing and printing, MgCl2 from finishing) can be oxidised to their chlorites/hypochlorites when they are mixed with hydrogen peroxide bleach waste water. In this way AOX may be found in waste water even if bleaching is carried out without chlorine-based bleaching agents. However, experimental measurements have shown that this does not play an important role, if any [7, UBA, 1994].
Bleaching with sodium hypochlorite
For ecological reasons the use of sodium hypochlorite is now limited in Europe to just a few particular cases, connected with knitted fabric and, in some cases, bleaching of yarn when a high degree of whiteness is required. Bleaching with sodium hypochlorite leads, in fact, to secondary reactions that form organic halogen compounds, commonly quantified as AOX.
Trichloromethane (which is suspected of being carcinogenic) accounts for the bulk of the compounds formed, but other chlorinating subsidiary reactions might occur, besides haloform reaction (see also Section 4.5.5).
When bleaching with hypochlorite, air emissions of chlorine may also occur (although only under strong acid conditions [281, Belgium, 2002]).
According to the Institute of Applied Environmental Research (Stockholm University), in 1991 – 1992 dioxins were found in sludge from a textile plant that bleached with sodium hypochlorite. Further investigations showed that pentachlorophenol was present in the hypochlorite [316, Sweden, 2001].
Bleaching with sodium chlorite Sodium chlorite bleaching may also be responsible for AOX generation in the waste water.
However, compared to sodium hypochlorite, the amount of AOX formed during chlorite bleaching is much lower (on average only 10 % to a maximum of 20 % of the amount of AOX generated by hypochlorite) and it is not correlated with sodium chlorite concentration or with temperature [7, UBA, 1994]. Recent investigations have shown that the formation of AOX is not originated by the sodium chlorite itself, but rather by the chlorine or hypochlorite that are present as impurities or are used as activating agent. Recent technologies (using hydrogen peroxide as the reducing agent of sodium chlorate) are now available to produce ClO2 without generation of AOX (see also Section 4.5.5).
As already mentioned earlier, handling and storage of sodium chlorite need particular attention because of toxicity and corrosion risks. Mixtures of sodium chlorite with combustible substances or reducing agents can constitute an explosion hazard, especially when they are subject to heat, friction or impact. Sodium chlorite forms chlorine dioxide when put in contact with acids. Moreover, sodium chlorite decomposition produces oxygen, which supports combustion.
2.6.2 Wool preparation before colouring
Typical preparation wet treatments applied on wool fibre before dyeing are:
· carbonising · washing (scouring) · fulling · bleaching.
Other possible treatments are those that go under the definition of anti-felt and stabilisation treatments. Although they are often carried out before dyeing, they are not obligatory preparation steps. For this reason they have been described under finishing operations (Section 126.96.36.199).
188.8.131.52 Principal manufacturing processes Carbonising Sometimes scoured wool contains vegetable impurities that cannot be completely removed through mechanical operations. Sulphuric acid is the chemical substance used for destroying these vegetable particles and the process is called carbonising.
Carbonising can be carried out on floc/loose fibre or on fabric (this operation is not applied in the carpet sector).
Loose fibre carbonising is performed only on fibres that are later used to produce fine fabric for garments (worsted fabrics) and usually takes place at the scouring mill.
In typical equipment for carbonisation of loose fibre, the still damp scoured wool is soaked in a solution containing 6 – 9 % of mineral acid (generally sulphuric acid). Excess acid and water are removed by pressing or by centrifugation until on average 5 – 7.5 % of sulphuric acid and 50 – 65 % of water remain. The fibres are then dried at 65 – 90 ºC to concentrate the acid and baked at 105 – 130 ºC (carbonising).
As soon as the wool is dried, it is fed into a machine, which consists of two counter-rotating rolls. These rolls crush the carbonised particles into very small fragments, which are then easily removed. The addition of small quantities of detergent to the sulphuric acid improves the yield
and reduces the attack on the wool. In order to prevent the fibre from being gradually degraded, the pH is finally set to 6 by neutralisation with sodium acetate or ammonia.
Sulphuric acid can be replaced by gaseous HCl or by aluminium chloride. The latter releases HCl when it is heated: this method is useful for the carbonisation of wool/synthetic fibres mixtures such as wool/PES, that are too sensitive to sulphuric acid.
After carbonising, the fibre can be carded and then spun before being dyed or it can be dyed directly in floc form.
Fabric carbonising is typical of woollen fabrics. The operation can be carried out by either the traditional or the more modern "Carbosol" system.
The conventional procedure is substantially similar to that used for loose fibre. The fabric can be previously soaked in a water or solvent bath and squeezed, but this step is optional. Then it is impregnated in a concentrated sulphuric acid solution (acidification) and squeezed (hydroextraction) before passing through the carbonising chamber. The carbonised particles are then removed by mechanical action and subsequent washing. In modern carbonising plants all these steps are carried out in continuous mode.
The "Carbosol" process, licensed by SPEROTTO RIMAR, uses an organic solvent instead of water. The equipment consists of three units. In the first one the fabric is impregnated and scoured with perchloroethylene, in the second the material is soaked in the sulphuric acid solution and in the third, carbonising and solvent evaporation take place. At this stage the perchloroethylene is recovered by distillation in a closed loop.
The “Carbosol” system is reported to have several technical advantages over the traditional process. The level of acidity of the fabric after carbonising is much lower and the risk of damage to the wool fibre is reduced. Thanks to full recovery of the organic solvent, the process can also be considered more efficient from the environmental point of view.
Figure 2.13: Representation of a conventional carbonising installation [71, Bozzetto, 1997]
Figure 2.14: "Carbosol" system [71, Bozzetto, 1997] Scouring Both yarn and fabric contain, besides accidental impurities, a certain amount of spinning oils and in some cases also sizing agents such as CMC and PVA.
All these substances are usually removed before dyeing in order to make the fibre more hydrophilic and allow the penetration of the fibre by dyestuffs. However, this operation is not always necessary. In some cases, if the preparation agents are applied in low amounts and they do not interfere with the dyeing process, a separate scouring/ washing step can be omitted.
As pointed out in Section 2.4.1, the percentage of spinning oils on woollen wool is quite relevant and it is always above 5 %, while on worsted wool it never reaches 2 %.
Typical substances that have to be removed by scouring can be classified as:
· soluble in water · insoluble in water, but emulsifiable thanks to the action of surfactants · insoluble in water and non-emulsifiable (or difficult to emulsify) with surfactants. These substances can be removed only by using organic solvents (in general, halogenated solvents like perchloroethylene).
As a result the material can be washed (scoured):
· with water or · with solvent (dry cleaning) Water washing is carried out in neutral or weakly alkaline conditions (by sodium carbonate or bicarbonate) in the presence of detergents. Commonly used detergents are mixtures of anionic and non-ionic surfactants such as alkyl sulphates, fatty alcohols and alkylphenol ethoxylates. In wool carpet yarn production the scouring process can include simultaneous chemical setting of yarn twist with reductive agents (sodium metabisulphite) and/or application of insect-resist agents (see also Section 184.108.40.206).
Water scouring is normally a batch operation which is carried out in the equipment in which the textile material will be subsequently dyed. This means that an autoclave is the commonly used equipment for yarn, while jets and overflows are the machines typically applied for fabric. In this respect, the carpet sector is an exception. Wool yarn for carpet is scoured on continuous or semi-continuous basis in tape scouring machines (hanks) or in package-to-package scouring machines (package yarn), where the yarn is passed through a series of interconnected bowls (see Section 220.127.116.11.2).
Textiles Industry 51 Chapter 2 Dry cleaning is less common and is applied when the fabric is heavily soiled and stained with oils from the weaving or knitting process. The most widely used solvent is perchloroethylene. In some cases water and surfactants are added to the solvent to provide a softening effect.
Solvent washing can be carried out either in discontinuous mode in a tumbler (generally for knitted fabric) or in continuous mode in open-width (for woven and knitted fabric). Impurities are carried away by the solvent, which is continuously purified and recycled in a closed loop (see also Section 2.12.2).
Fulling This treatment takes advantage of the felting tendency typical of wool fibre when it is submitted to friction under hot humid conditions and is a typical pretreatment for woollen fabric.
It is usually carried out after carbonising, but in some cases (e.g. heavy woollen fabrics) it can be done directly on raw fabric. The material is kept in circulation in a bath containing fulling auxiliaries. Both acids (pH 4.5) and alkali (pH 8) speed up the fulling process. Fulling auxiliaries available on the market, however, also produce excellent results under neutral conditions. As a result, fulling either in acid or alkaline conditions is becoming less common.
After fulling, the fabric is washed.
Machines specially designed for this process are still in use. However, today they have been largely replaced by multifunctional machines, where both fulling and washing can be carried out by simply adjusting the set-up of the equipment.
Bleaching Wool is bleached with hydrogen peroxide (sodium hypochlorite may discolour and damage the wool). An additional reductive bleaching is, however, indispensable for achieving high levels of whiteness (full bleach). A typical reductive bleaching agent is sodium dithionite (hydrosulfite), which is often used in combination with optical brightners to enhance its effect.
In the oxidative bleaching step, hydrogen peroxide is applied in the presence of alkali and stabilisers, which slow down the decomposition of the hydrogen peroxide (see also Section 8.5).
When wool has been previously submitted to an anti-felt treatment it is preferable to use hydrogen peroxide in weakly acid conditions.
18.104.22.168 Environmental issues Wool pretreatment gives rise mainly to water emissions, although there are also specific operations (e.g. carbonising with the “Carbosol” system and dry cleaning) where halogenated (mainly perchloroethylene) solvents are employed. The use of halogenated organic solvents solvents can produce not only emissions to air, but also contamination of soil and groundwater if their handling and storage is not done using the necessary precautions. Preventive and end-ofpipe measures include closed-loop equipment and in-loop destruction of the pollutants by means of advanced oxidation processes (e.g. Fenton reaction). See Sections 4.4.4, 4.9.3 and 4.10.7 for more detailed information. The process also involves the generation of exhausted active carbon from the solvent recovery system. This solid waste has to be handled separately from other waste material and disposed of as hazardous waste or sent to specialised companies for regeneration.