«EUROPEAN COMMISSION Integrated Pollution Prevention and Control (IPPC) Reference Document on Best Available Techniques for the Textiles Industry July 2003 ...»
Following primary spinning, the applied treatments vary, depending on the final product and the
processed fibre. Two simplified process sequences can be identified for this stage:
1. process for the manufacturing of continuous filament in flat or texturised form
2. process for the manufacturing of staple fibres.
Figure 2.2: Simplified process sequences for manufacturing of continuous filaments (flat and texturised) and staple fibres As shown in the diagrams above, preparation agents can be applied at various stages during the manufacture of the chemical fibres.
Application of preparation agents in primary spinning (fibre manufacturing) is usually followed by further additions during secondary spinning, when the yarn is manufactured, including operations such as coning, twisting, warping, etc.
Textiles Industry 23 Chapter 2 The preparation agents need to be removed to ensure uniform penetration of dyes and finishing agents and to avoid reaction or precipitation with them. Due to their high organic content and their often-low bioeliminability, these substances are responsible for waste water pollution and air emission in the subsequent fibre pretreatment. Particularly relevant in this respect is the production of continuous filaments destined for the production of knitted fabric and the manufacturing of elastomeric fibres, because in this case the amount of preparation agents applied is higher.
The amount of preparation agents applied varies according to the fibre (e.g. PES, PA, etc.) and make-up (flat or texturised filament, staple fibre). Information about the chemistry of the preparation agents and the amount applied on the fibre is given in Section 8.2.
2.3 Fibre preparation: natural fibres 2.3.1 Wool Wool is usually opened and de-dusted before it is fed to the scour. This is a mechanical process designed to shake out dirt from the wool and to open the fleeces in order to improve the efficiency of the scour in removing contaminants. The process also roughly blends the wool and produces a layer of fibres suitable for presentation to the scour. The opening and de-dusting processes vary considerably in severity depending on the characteristics of the particular wool being processed. The process produces a solid waste comprising dirt, sand, fibre fragments and vegetable matter.
The object of subsequent raw wool scouring processes is to remove contaminants from the wool fibre and to make it suitable for further processing.
Almost all of the scouring plants are based upon aqueous washing. Solvent scouring is much less widely practised. There are world-wide only about five companies that degrease with organic solvents [18, VITO, 1998].
22.214.171.124 Cleaning and washing with water A conventional wool scouring set is shown in Figure 2.3. The process is carried out by passing the wool through a series of 4 – 8 wash bowls, each followed by a mangle or squeeze press which removes excess scouring liquor from the wool and returns it to the bowl. Clean water is added at the last bowl and passes via a counter-flow system from bowl to bowl with final disharge from the first bowl in a controlled manner to drain.
Figure 2.3: Conventional wool scouring arrangement [8, Danish EPA, 1997] In the scouring bowls, suint is removed from the wool by dissolution, grease by emulsification and dirt by suspension.
For merino wools, the first bowl may be charged with water only and, in that case, its purpose is the removal of water-soluble suint before the wool enters the scouring process proper (this bowl is usually called “de-suint”).
In order to achieve grease emulsification, the scouring bowls are charged with detergent and often with sodium carbonate, or other alkali, which acts as a detergent builder. Concentrations of detergent and builder are usually highest in the first scour bowl and they decrease in subsequent bowls.
Detergents used by scourers are mainly synthetic non-ionic surfactants, namely alcohol ethoxylates and alkylphenol ethoxylates. Some scourers also report the use of "solvent-assisted detergents" for the removal of marking fluids from fleeces.
Finally, the wool is rinsed by passing it through bowls containing water only.
In coarse wool scouring plants the final bowl of the scouring train is sometimes used for chemical treatments. In this case, it is isolated from the countercurrent liquor flow system and may also be isolated from the drain if the chemical treatment uses ecotoxic chemicals. The most commonly used treatment is bleaching, in which hydrogen peroxide and formic or acetic acid are added to the bowl. Other treatments sometimes applied include mothproofing, using a synthetic pyrethroid insecticide and acetic or formic acid, and sterilisation (of goat hairs) using formaldehyde.
Wool grease has a melting point around 40 ºC. Since removal of solid grease from wool by detergents is slow and difficult, 40 ºC is the lowest temperature at which aqueous scouring liquors are effective for removing grease. In addition, non-ionic detergents lose efficiency rather rapidly below 60 ºC, which means that scour and rinse bowls are typically operated at 55 - 70 ºC.
After leaving the final squeeze roller the wool will contain 40 to 60 % moisture. It is therefore dried by convection in a hot-air drier. The drier is usually heated either by closed steam pipes or by direct gas firing. The heat supply to the drier may be controlled by a signal from a device which senses the humidity of the drier atmosphere or the moisture content of the wool at the output end, thus saving energy and avoiding overdrying the wool.
The mechanical design of wool scours and the arrangements for circulating the scour and rinse liquors vary widely. Since these matters have a direct influence on energy and water usage, as well as the partial removal of contaminants from the effluent, it is important to illustrate them in more detail.
New generation scouring plants like the one illustrated in Figure 2.4 have an integrated system for grease and dirt recovery.
Figure 2.4: Schematic diagram showing a scour line, integrated waste handling process and on-site effluent treatment plant [187, INTERLAINE, 1999] The dirt tends to settle at the bottom of the bowl and modern scour bowls usually have hoppershaped bottoms from which the sludge is removed by gravity through a valve.
Opening of the valve may be under the control of a timer or may respond to a signal from a turbidity meter which senses the thickness of the dirt suspension in the hopper bottom. The discharge from the scour bowl hopper bottoms is led to a heavy solids settling tank where it is gravity-settled and the settled liquor partly recycled to scour bowl 1 and partly discharged. Flocculant may be added to the heavy solids settling tank to assist the separation of dirt, or a decanter centrifuge or hydrocyclone may be used in preference to gravity settling for dirt removal.
For grease recovery, modern scour bowls have a side tank in which the grease-rich liquors removed from the wool by the squeeze press are collected. From here, part of the flow may be pumped to the previous bowl or, in the case of bowl 1, to a primary grease centrifuge. The centrifuge separates the liquor into three phases. The top phase, referred to as the cream, is rich in grease and passes to secondary and possibly tertiary centrifuges for further dewatering, finally producing anhydrous grease; the bottom phase is rich in dirt and goes to the heavy solids settling tank; the middle phase is impoverished in both grease and dirt compared with the input and this is split, part being recycled to scour bowl 1 and part being discharged.
In a commonly used variation of the above recycling arrangements, the dirt and grease removal and recycling loops may be combined. In this case, scouring liquor may be removed from the bottoms of the bowls only, or from top and bottom, and passed first through the dirt removal equipment, then through the primary grease centrifuge.
Some scourers recycle rinse water (see Figure 2.4). The flowdown from the first rinse bowl can be treated to make it suitable for addition to the feed to the final rinse bowl. Normally, this is accomplished by removing dirt in a hydrocyclone and processing the water through a membrane filtration plant to remove other impurities.
It is normally necessary to purge dirty liquors which collect at the bottoms of the rinse bowls, but this is not always the case.
Purging of rinse bowls will depend upon the efficiency of the bowls. Some modern scours have rinse bowls discharge controlled by solid detectors, but generally rinse bowls merely have a timed discharge of bottom liquor which operates automatically whatever the state of the liquor [208, ENco, 2001].
The dirt removal and grease recovery loops described above serve several purposes. They save water, by recycling effluent to the scour, and they act as a process-integrated partial effluent treatment. The recovered wool grease can be sold, although the market for this by-product has been variable in most recent years. Finally, since the discharges from the loops are the only points at which heavily contaminated scour liquors are discharged, valves and meters at these points can be used to control the rate of water usage in the scouring section.
For more information about the performance of the dirt removal and grease recovery loops, see Sections 3.2.1, 4.4.1 and 4.4.2.
126.96.36.199 Environmental issues associated with wool scouring (with water) This section discusses the environmental issues associated with the overall scouring process including the treatment of the process effluent.
The main environmental issues associated with the wool scouring process arise from emissions to water, but solid waste and the air emissions also need to be taken into account.
Potential for pollution of water The removal of contaminants present on the raw fibre leads to the discharge of an effluent in
which the main polluting contributors are:
· highly concentrated organic material in suspension and in solution, along with dirt in suspension · micro-pollutants resulting from the veterinary medicines applied to protect sheep from external parasites.
There are also detergents in the discharged water, which contribute to the increase of the chemical oxygen demand of the effluent. The detergent is recycled via the grease recovery/dirt removal loop. As a result, low efficiency in this recovery system is associated with higher amounts of detergent in the effluent. Compared to the chemical oxygen demand attributable to wax, dirt, etc., the detergents can be considered minor contributors to water pollution, but this is not the case when harmful surfactants such as alkylphenol ethoxylates are used as detergents (for more detail on environmental issues regarding detergents, see Section 8.1).
As to the organic matter coming from wax and dirt, according to “Stewart, 1988” the COD of
effluent and of greasy wool can be calculated using the following equation:
COD(mg/kg) = (8267 x suint(%)) + (30980 x oxidised grease(%)) + (29326 x top grease(%)) + (6454 x dirt(%)) + 1536.
Since the coefficients for top grease and oxidised grease2 in this equation are similar and since approximately equal quantities of top grease and oxidised grease are present in many wools, it is
possible to combine the two grease terms in the above equation as follows:
COD (mg/kg) = (8267 x suint(%)) + (30153 x total grease(%)) + (6454 x dirt(%)) + 1536
It is then possible to calculate the COD content of "typical" merino and crossbred wools:
Top grease is unoxidised grease which is readily separated from scour liquors by centrifuging; oxidised grease is less hydrophobic and is less readily separated.
COD = (8.267 x 8) + (30.153 x 13) + (6.454 x 15) + 1.536 = 556 g/kg greasy wool
- Crossbred wool: suint = 8 %; grease = 5 %; dirt = 15 % COD = (8.267 x 8) + (30.153 x 5) + (6.454 x 15) + 1.536 = 315 g/kg greasy wool.
These high levels of oxygen-depleting substances must be removed from the effluent before it can be discharged to the environment without potential for harmful effects.
Organohalogen, organophosphorus compounds and biocides are among the priority substances listed for emission control in the IPPC Directive.
Worldwide, the most common ectoparasiticides used for treating sheep are diazinon (OP), propetamphos (OP), cypermethrin (SP) and cyromazine (fly-specific IGR), for control of blowfly. Insect growth regulators such as dicyclanil, diflubenzuron and triflumuron are registered only in Australia and New Zealand. Organochlorine pesticides (in particular, hexachlorocyclohexane) are still found on wool coming from the former Soviet Union, the Middle East and some South American countries [187, INTERLAINE, 1999] (see also Section 188.8.131.52).
Concerning the fate of ectoparasiticides when they enter the scouring process, a distinction has to be made between lipophilic and hydrophylic compounds such as cyromazine. The lipophilic compounds – OCs, OPs and SPs – associate strongly with the wool grease and are removed with it during scouring (although a fraction – up to 4 % – is retained by the fibre and will be released in the subsequent finishing wet processes). This behaviour applies also to diflubenzuron (IGR).
Recent studies have shown that triflumuron (IGR) associates partially with the dirt and partially with the grease. As a result, compared to other lipophilic compounds, a higher proportion of this pesticide is likely to be retained on-site (on the wool fibre and on the recovered wool grease and dirt) and not be discharged in the aqueous effluent [103, G. Savage, 1998]. On the contrary, IGRs such as cyromazine and dicyclanil are appreciably water-soluble (11 g/l at 20 ºC, for cyromazine), which means that they are not removed in wool grease recovery systems.
In the waste water treatment systems an additional fraction of the pesticide residues is removed.
Physico-chemical separation techniques remove the biocide residue at approximately the same rate as the grease and the dirt with which they are associated. On the other hand, evaporation systems remove OCs and SPs in significant quantities, but up to 30 % of the OPs may appear in the condensate because they are steam volatile. The water-soluble compounds, such as the IGR cyromazine are probably not removed from the effluent stream except by evaporating treatments [187, INTERLAINE, 1999].
Despite these treatments, the removal of pesticides is often incomplete and there is potential for pesticides to enter the aquatic environment when the effluent is discharged. The environmental concentrations of ectoparasiticides in the receiving water depend greatly on local circumstances, in particular, the amount of scouring activity concentrated in a given catchment and the dilution available between scouring discharges and the river which receives the treated effluent.
In areas of Europe with a high concentration of scouring activity, there is a risk of high concentration levels of pesticides in the receiving water. In this case, it is preferable to define discharge limits on the basis of risk assessment models. In UK for example, statutory environmental quality standards (EQS) for the OCs and non-statutory standards for the OPs and cypermethrin have been defined. Discharge limits are set up for processing mills by comparing the given EQS targets with predicted environmental concentrations based on tonnage of wool processed and typical effluent treatment systems.
28 Textiles Industry