Wool preparation before colouring

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.

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 (hydro-extraction) 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 2.14.5.1).

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 2.14.5.1.2).

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.

2.6.2.2 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-of-pipe 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.

Due to the predominantly batch nature of wool pretreatment operations for all types of make-ups, the resulting emissions will be discontinuous and with concentration levels largely influenced by the liquor ratios used. An exception is represented by carpet yarn which can be scoured/ bleached and mothproofed on tape or "package to package" scouring machines (see Section 2.14.5.1.2) giving rise to continuous flows.

The pollutants that can be found in the waste water, originate in part from the impurities that are already present on the fibre when it enters the process sequence and in part from the chemicals and auxiliaries used in the process.

Pollution originating from impurities present on the raw material

Residues of pesticides used to prevent the sheep becoming infested with external parasites can still be found on scoured wool in amounts which depend on the efficiency of the scouring process. These are mainly organophosphates (OPs) and synthetic pyrethroid (SPs) insecticides and insect growth regulators (IGRs), but detectable residues of organochlorine pesticides (OCs) can be observed. They partition between the fibre and the water according to their stronger or weaker lipophylic character and, as a consequence, traces of these compounds are released in the waste water. More information about ectoparasiticides can be found in Sections 2.1.1.9 and 2.3.1.2. The partition factors of the different classes of pesticides are discussed in more detail for the carpet sector in Sections 3.4.1.1 and 3.4.1.2.

Note that because of their steam volatility some pesticides (OPs) end up in the air emissions from open machines. This must be taken into account in input/output balances.

Spinning lubricants (see 8.2.3), knitting oils (see 8.2.5) and other preparation agents also represent an important issue in wool pretreatment. These substances are removed during the scouring process, contributing to the COD load and aquatic toxicity in the final effluent. The main concerns are about:

• poorly refined mineral oils (content of aromatic hydrocarbons)

• APEO (non-biodegradable and giving rise to toxic metabolites)

• silicones (non-biodegradable and difficult to remove without scouring assistants)

• biocides (toxic to aquatic life).

The dry spinning route in the carpet sector, described in Section 2.14.5.1, represents one exception because in this case spinning lubricants do not reach the water effluent.

Pollution originating from chemicals and auxiliaries used in the process

Considerable amounts of surfactants are used in pretreatment as detergents, wetting agents, etc.

Surfactants with good biodegradability with acceptable performance are now available (see Section 4.3.3). Nevertheless, the use of alkylphenol ethoxylates is still common in some companies due to their low cost. Alkylphenol ethoxylates (APEOs) and in particular nonylphenol ethoxylates (NPEs) are under pressure due to the reported negative effects of their metabolites on the reproduction system of aquatic species. The environmental issues arising from surfactants in common use are discussed in Section 8.1.

Other pollutants of concern that may be found in water effluent from pretreatment activities are:

• reducing agents from bleaching treatments and chemical setting of carpet wool yarn (sodium metabisulphite): they contribute to oxygen demand in the waste water

• poorly bio-eliminable complexing agents (e.g. EDTA, DTPA, phosphonates) from hydrogen peroxide stabilisers, etc.

• AOX from sodium hypochlorite bleaching

• insect-resist agents in wool carpet yarn production.

More details regarding environmental issues associated with the above-mentioned substances are given elsewhere in this document, in particular in Section 2.6.1.2 and Section 8.5.