«EUROPEAN COMMISSION Integrated Pollution Prevention and Control (IPPC) Reference Document on Best Available Techniques for the Textiles Industry July 2003 ...»
Applicability The measures described here are of mixed applicability. Many are readily applicable in existing plant but some are not, or would involve such changes that the plant would, in effect, become a new plant. For many of the measures, applicability is discussed in the descriptions above [187, INTERLAINE, 1999].
The most important means for wool scourers to reduce energy consumption is by reducing water consumption and effluent volume. The economics of doing this by installation of countercurrent scouring and integrated dirt removal/grease recovery loops have already been discussed (Section 4.4.1). Information on the costs of other measures is not available at the time of writing [187, INTERLAINE, 1999].
Driving force for implementing these techniques
Economic considerations are the major driving force from the industry’s point of view. From a governmental viewpoint, the main driving force is reduction of emissions to air, in order to achieve commitments made in international agreements [187, INTERLAINE, 1999].
Reference plants All the measures discussed here are in operation in mills throughout the world, although there may be no one mill which operates all. An exception is the use of co-generation (combined heat and power). No wool scouring mill is known to be operating such a system, though systems operate in other industries. It is likely that a scouring mill would generate an excess of power (electrical energy) and in most member states the excess could be sold and fed into the grid. For the wool scouring sector, co-generation must be regarded as an emerging technique [187, INTERLAINE, 1999].
Reference literature [187, INTERLAINE, 1999] 4.4.4 Wool scouring with organic solvent Description The Wooltech wool cleaning system involves the use of a non-aqueous solvent (trichloroethylene) and does not use any water in the washing process. The process has already been described in Section 220.127.116.11.
Main achieved environmental benefits The described technique avoids the use of water in the actual wool cleaning process. The only source of water emission is moisture introduced with the wool, steam used in vacuum ejectors and moisture recovered from air drawn into the equipment. This water is treated in two steps, comprising a solvent air stripping unit and a residual solvent destruction unit. Here the residual traces of solvent are destroyed, using a free-radical process based on the Fenton reaction (iron and hydrogen peroxide).
Since pesticides partition strongly to the solvent and leave with the grease, the clean wool is reported to be pesticide free. This has positive implications for the downstream processes where wool is finished.
Another positive effect of this technique is reduced energy consumption, due to the low latent heat of evaporation of an organic solvent compared to water.
Operational data A nominal consumption of 10 kg/h of solvent is reported for the production of 500 kg/h of clean wool fibre. Part of this solvent ends up in the water stream and is destroyed. The remaining portion is partly emitted in the exhaust air (0.01 kg/h) and partly accounted for as uncaptured losses (5 kg/h).
It is reported ([201, Wooltech, 2001]) that uncaptured losses can generally be very low, but this is directly related to how the plant operators undertake maintenance and how the plant is managed. The Wooltech process has prepared a Code of Conduct for operators with strict maintenance, quality control and management practices to address all the environmental, health and safety issues [201, Wooltech, 2001].
The referenced technique uses trichloroethylene as solvent. Trichlorethylene is a non biodegradable and persistent substance. Unaccounted losses of this solvent arising from spills, residues on the fibre, etc., if not adequately treated on site to destroy the solvent, may lead to
diffuse emissions resulting in serious problems of soil and groundwater pollution. This has been taken into account in the latest design of the described technique.
Applicability The technique is reported to be applicable to any kind of wool. Typically plants with 250 kg/h or 500 kg/h of clean wool (852 kg/h of greasy wool) capacity are used, but smaller plants can be considered [201, Wooltech, 2001] Economics According to information submitted by the supplier ([317, Comm., 2002]), the capital investment required for a solvent scouring line (and ancillary plant) with a capacity of 500 kg/h clean wool is in the order of A$ 5000000 (corresponding to about 2.8 million euros).
An estimate of the operating costs can be derived from the consumption levels reported in Table 3.8.
Driving forces for implementation Scarcity of water is probably the main driving force for the implementation of this technique.
Reference plants The Wooltech system is applied in one plant in Trieste-Italy.
Reference literature [201, Wooltech, 2001]
4.5 Pretreatment 4.5.1 Recovery of sizing agents by ultrafiltration Description Sizing agents are applied to warp yarn in order to protect it during the weaving process and have to be removed during textile pretreatment, thus giving rise to 40 – 70 % of the total COD load of woven fabric finishing mills.
Water-soluble synthetic sizing agents such as polyvinyl alcohol, polyacrylates and carboxymethyl cellulose can be recovered from washing liquor by ultrafiltration. Recently, it has been confirmed that modified starches such as carboxymethyl starch can also be recycled.
The principle of recovery by ultrafiltration is shown in Figure 4.9. After sizing and weaving, sizing agents are removed during textile pretreatment by hot washing with water in a continuous washing machine (in order to minimise water consumption, the washing process may need to be optimised). Sizing agents concentration in the washing liquor is about 20 - 30 g/l. In the ultrafiltration plant, they are concentrated to 150 - 350 g/l. The concentrate is recovered and can be re-used for sizing, whereas the permeate can be recycled as water in the washing machine.
Note that the concentrate is kept at high temperature (80 - 85°C) and does not need to be reheated [179, UBA, 2001].
Figure 4.9: Recovery of sizing agents by ultrafiltration [179, UBA, 2001] Figure 4.
10 shows the mass balance of sizing agents and water for the process with and without recovery in a representative case study. It can be noticed that, even with recovery, some losses of sizing agent still occur at various steps of the process, especially during weaving.
Furthermore, a certain amount of sizing agent still remains on the desized fabric and a fraction ends up in the permeate. In conclusion, the percentage of sizing agents which can be recovered is 80 – 85 %.
Figure 4.10: Representative example of mass balance for sizing agents and water with and without recovery [179, UBA, 2001] Main achieved environmental benefits COD load of waste water from finishers of woven fabric is reduced by 40 – 70 %.
Sizing agents are recovered by 80 – 85 %. In addition, sizing agents in waste water do not need to be treated.
Thus energy consumption for treatment is reduced significantly as well as quantity of sludge to be disposed of [179, UBA, 2001].
Ultrafiltration is very efficient in reducing high organic load from textile mills. However, it has to be remembered that the polymers used for recoverable sizing agents are also widely applied in products such as household detergents, which are found in great quantities in other effluents.
[61, L. Bettens, 1999].
Operational data In order to minimise scaling and fouling, fibres have to be removed before ultrafiltration. The same applied to fine particles, such as singeing dust. A pre-filtration step is carried out for this purpose.
When desizing coloured woven fabric (dyed warp yarn), the desizing liquor becomes slightly coloured. Dyestuff particles are more difficult to remove and the liquor needs to be submitted to microfiltration (which is more complex, but still feasible) [179, UBA, 2001].
The operation/management of ultrafiltration units for recovery of sizing agents requires qualified staff and accurate maintenance.
Cross-media effects Ultrafiltration needs energy, but the amount consumed is much less than the energy required to produce new sizing agents (if they are not recovered) and to treat them in a waste water treatment plant [179, UBA, 2001].
Applicability As explained earlier, this technique is suitable only for specific types of sizing agents. These are water-soluble synthetic sizing agents such as PVA, polyacrylates and carboxymethyl cellulose.
Recently, it has been confirmed that some modified starches such as carboxymethyl starch can also be recycled.
Re-use in the weaving plant is not always without problems. Stock and the recovered size need to be kept under sterile conditions when stored and mixed with virgin size. In the past, failure of protection against bacterial growth (biological degradation of concentrates and contamination of the ultrafiltration equipment) resulted in the shutdown of a recycling plant in Belgium [61, L.
Bettens, 1999]. Nowadays, recovered sizing agents are kept at temperatures above 75 °C. It is reported that under these conditions there are no problems of microbial attack and therefore no addition of biocides is needed to maintain sterile conditions [280, Germany, 2002].
Limitations in the applicability of this technique may arise from cases where the auxiliaries applied to the yarn are not only sizing agents, but also waxes, antistatic agents, etc. These compounds remain in the concentrate after UF. The concentrate can be re-used for sizing, but limitations can be found when re-using the same concentrate for different kind of yarns (with different applications and end-uses) which may need specific additives [281, Belgium, 2002].
To date, the weavers’ acceptance of recovered sizes is still limited. Weavers are concerned about the quality of the recovered size. Furthermore, certain effects such as minting can only be carried out with non-desized fabric. For these reasons, re-use of the concentrate is typically applied in integrated companies with a uniform production.
A further issue to consider is the transport distances. Long-distance shipments cancel out any ecological advantages because the liquor needs to be transported in adequate conditions in insulated tankers [179, UBA, 2001]. Although, there are mills where recovery is carried out in spite of a considerable distance between the weaving and finishing departments (up to 300 km in one company in the USA), sizing agents are usually recovered in integrated mills having a weaving and a finishing section at the same site.
When weaving and finishing (desizing) take place in completely different places, a more practicable option would probably be to remove and recover the sizing agents directly in the weaving mill, which would therefore produce desized fabric. However, while the quantity of processed fabric must be higher than 1000 t/yr to make the process cost-effective in an integrated mill, the minimum amount in a weaving mill producing desized fabric is much higher (about 5000 - 8000 t fabric/yr) because, in addition to the ultrafiltration plant, a washing machine and a dryer have to be installed [179, UBA, 2001]. Additionally the textile finishers’ acceptance of already de-sized fabric is still limited and certain effects such as minting can only be carried out with non-desized fabric.
Economics A cost/benefit assessment should take into account not only the costs of ultrafiltration, but also the recipe and overall process and treatment costs, especially when considering that changing over from starch and starch derivatives to synthetic sizing agents also has implications for weaving efficiency. Synthetic sizing agents are more expensive than starch-based sizing agents, but they are applied in lower amounts and the weaving efficiency may be higher.
Table 4.15: Typical example of annual savings achievable when introducing recovery of sizing agents [179, UBA, 2001] In the example given in the table, there will be additional savings because of the higher weaving efficiency and the reduced cost of pretreatment (time saving and significantly reduced consumption of chemicals for degradation and removal of sizes compared to starch-based products) and waste water treatment.
The payback time of an ultrafiltration plant may then be less than one year [179, UBA, 2001], which suggests that in most cases companies primarily invest in this technique not because of the environment, but because of the economical benefit.
The investment costs for the ultrafiltration plant referenced above illustrated above are the
following [179, UBA, 2001]:
Driving force for implementation Waste water problems and cost reductions have been the most important driving forces to implement recovery of sizing agents [179, UBA, 2001].
Reference plants The first plant for recovery of polyvinyl alcohol went into operation in 1975 in the USA.
Meanwhile there are two plants that have been in operation in Germany for many years and various plants are now in operation in Brazil, Taiwan and USA. There are not many suppliers of ultrafiltration plants [179, UBA, 2001].
Reference literature [61, L.
Bettens, 1999], [179, UBA, 2001] with reference to:
“Klaus Stöhr, ATA Journal, Oct/Nov, 2001, 50-52” “Heinz Leitner, Melliand Textilberichte 10 (1994) E, 205-209” “Technical information BASF, T/T 372e, July 2000” “Size UCF-4, Techn. Info BASF (2000)” 4.5.2 Application of the oxidative route for efficient, universal size removal Description Many woven fabrics contain a variety of different sizing agents, depending on the origin and quality of the substrate. Most textile finishers deal with many different types of fabrics, and therefore sizing agents, so they are interested in fast, consistent and reliable removal of nonfibrous material (be it the impurities and fibre-adjacent material or any preparation agent) independent of the origin of the fabric.
Enzyme desizing removes starches but has little effect in removing other sizes. Under specific conditions (above pH 13), H2O2 generates free radicals which efficiently and uniformly degrade all sizes and remove them from the fabric. This process provides a clean, absorbent and uniform base for subsequent dyeing and printing, no matter which size or or fabric type is involved [189, D. Levy, 1998].
Recent studies ([203, VITO, 2001]) show that above pH 13 the oxide radical anion O*- is the predominant form. This species is highly reactive, but it will attack non-fibrous material (sizing agents, etc.) rather than cellulose, for various reasons. First because it is negatively charged like the cellulose polymer in strongly alkaline medium (coulombic repulsion effect) and secondly because, unlike the OH* it does not react by opening the aromatic rings.
It is recommended to first remove the catalyst that is not evenly distributed over the fabric (e.g.
iron particles, copper, etc.). One possible process sequence would therefore be: removal of metals (modern pretreatment lines are equipped with metal detectors), oxidative desizing (peroxide and alkali), scouring (alkali), demineralisation (acid reductive or, better still, alkaline reductive/extractive), bleaching (peroxide and alkali), rinsing and drying.
Main achieved environmental benefits The proposed technique allows significant environmental benefits: water & energy consumption along with improved treatability of the effluent.
The oxidative route is a very attractive option where peroxide bleaching is carried out. Taking advantage of hydrogen peroxide also being used as an active substance for bleaching, it is advantageous to combine alkaline bleaching with scouring and regulate the countercurrent flow of alkali and peroxide through the different pretreatment steps, so as to save water, energy and chemicals.