«EUROPEAN COMMISSION Integrated Pollution Prevention and Control (IPPC) Reference Document on Best Available Techniques for the Textiles Industry July 2003 ...»
188.8.131.52.1 Process A: Application of agent from the final bowl of the yarn scour – uses conventional, existing equipment and eliminates emissions at the dyeing stage Description Mothproofer is applied from the final bowl of a conventional yarn scour as a substitute for conventional dye bath application in loose fibre colouration.
Conventional sized scour bowls are used, typically 1200 –2000 litres operating volume.
Mothproofer concentrate is metered to the bowl at a rate proportional to yarn throughput. The treatment liquor is acidified to promote adsorption of the active substance, metering of acid is in proportion to throughput. Bowl liquor is discharged when judged to be too contaminated for further use.
Main achieved environmental benefits
A reduction in active substance emissions when compared to the baseline process. Emissions reduction depends on the quantity of yarn that can be treated before the liquor becomes too contaminated to use. Typical emissions figures equivalent to 92mg active/kg yarn have been reported from an installation of this type (Allanach and Madden, Proceedings of the 9th International Wool Textile Research Conference Vol. 1, 182 -190 (1995).
This process was used extensively in the UK until the introduction of surface water Environmental Quality Standards in the early 1990’s. Strict trade effluent consents, limiting the discharge of mothproofers followed, and operators adopted alternative processing routes, invariably either Process B or Process C described below. In light of this experience the above technique can no longer be considered to represent BAT for application in conjunction with yarn scouring.
Applicability The technique can be used with any existing yarn scouring machine. A minimum of four scouring bowls is normally required to carry out the scouring, rinsing and mothproofing operation successfully.
Reference literature [32, ENco, 2001] 184.108.40.206.2 Process B: Mothproofer application using dedicated low volume application equipment, incorporating liquor clean-up and re-cycling Description Specialised application equipment has been developed to meet many of the shortcomings encountered in Process A above. Minimal operating volume both limits liquor contamination and permits re-cycling of the liquor with intermediate cleaning to remove contaminants.
Electronic measurement and process control are used to eliminate fugitive emissions and control mothproofer application levels and acidification conditions. Machines of this design are variously referred to as mini-bowl, low volume bowl or EnviroProof installations.
An installation of the type described above is capable of operation in a number of modes:
· liquor storage between yarn batches – discharge to drain when liquor is contaminated · liquor storage between batches with adsorptive renovation of liquor to eliminate discharge.
Further developments of the low volume application system have taken place in which residual liquor from the hydroextractor is treated with alkali at high temperature. This procedure is applicable to all insect resist agent formulations based on synthetic pyrethroids, which undergo rapid hydrolysis at elevated temperatures. The degradation products from this reaction are several orders of magnitude less toxic to invertebrates than the parent molecule (Hill, I. R.
Pesticide Science. 1989, 27,429-465) and can thus be safely discharged to drain with other waste water streams.
The above technique has also been adapted to treat the spent bowl liquor in cases where heavy contamination cannot be managed with the adsorptive filter system.
A further variant of the system deals with the hydroextractor liquor by segregation from other effluents, followed by addition to a dye bath. Dark shades are usually chosen to accommodate any slight shade change due to contaminants in the hydroliquor and only 10 - 20 % of the dye bath volume is made up using hydroliquor. Active ingredient present in the hydroliquor behaves in a similar manner to that present in the original formulation and is adsorbed by the fibre under the hot acidic dyeing conditions. Overall residues from this process are of a similar magnitude to those encountered when IR agent is applied from the dye bath.
Main achieved environmental benefits Dedicated application machinery eliminates emissions from the dyeing process and fugitive emissions from the conventional scour bowl application.
Allanach and Madden (Proceedings of the 9th International Wool Textile Research Conference Vol. 1, 182 -190 (1995) quantified the permethrin release from an installation processing respectively 22 and 92 tonnes of carpet yarn. Both operating modes mentioned above were assessed (1. discharge to drain when liquor is contaminated and 2. adsorptive renovation of liquor to eliminate discharge).
Textiles Industry 387 Chapter 4 Permethrin released from the installation amounted to 0.97mg/kg in the case of simple multiple batch processing without clean up and 0.23mg/kg for the system running with full clean up and recycling of the process liquor. In both cases a significant proportion of the emissions arose when the treated yarn was hydroextracted prior to evaporative drying.
Cross-media effects None believed likely Applicability Installations of this type may be retrofitted to any continuous yarn scouring machine. Versions of the technology have been adapted for both hank and continuous end to end yarn processing machines.
Economics Both commercial and self-build versions of the low volume application system described above are known. A commercial installation (EnviroProof) retrofitted to an existing scour line normally includes all the control features associated with chemical dosing and liquor management, together with refurbishment of the final squeeze press of the existing scour.
Installation costs are in the region of EUR 185000. The chemical dosing system fitted to these installations is capable of closely regulating application levels in proportion to yarn throughput.
As a result, the nominal treatment level can normally be reduced without compromising the insect resistance of the treated yarn. The magnitude of these savings can vary from site to site, but a 50 % reduction is not uncommon, equivalent to a saving of approximately EUR 7.0/tonne of treated yarn.
Equipment for the control and in-plant treatment of hydroextractor residues and highly contaminated liquors using the alkali hydrolysis technique is often assembled on a self build basis to suit local site conditions and space. Depending on complexity, costs are estimated to be upwards of EUR 7000. Treatment costs, excluding energy, are in the region of EUR 1.4/m3
A significant number of plant in the UK use variations of the low volume application techniques described above and their associated spent liquor management systems for the insect resist treatment of yarn in both hank and single end form. Further installations operating similar systems are known to be in operation in Europe and New Zealand.
Reference literature [32, ENco, 2001] 220.127.116.11.3 Process C: Insect resist application to the carpet pile using foam technology Description Figure 4.30 shows that this candidate technique does not form part of the yarn manufacturing process. In this case IR agent is applied directly to the pile of the carpet during the back-coating or latexing operation. Specialised equipment has been developed in which a high-density foam incorporating the IR agent is generated on site and applied to the carpet using a specially profiled application head. This is a continuous process and can be carried out simultaneously with backing/ latexing. The use of foam minimises moisture addition. The foam is generated within a contained area on demand and any rinse water is recycled and used to prepare the next batch of foam. There are no emissions to the mill waste water stream. For the treatment to be 388 Textiles Industry
Chapter 4fully effective the active ingredient must penetrate to the base of the carpet pile structure. The density of the pile structure, the foam density and foam blow ratio, controls penetration.
Technical aspects of the technology are described by Allanach and Greenwood (Proceedings of the 9th International wool Textile Research Conference Vol.3,325-332, 1995) Main achieved environmental benefits When operated with a rinse liquor re-use system the process is effluent-free and eliminates the emission of mothproofer active substance to the aquatic environment.
Cross-media effects None believed likely.
Applicability The technique relies on application of the mothproofing finish directly to the pile of the carpet and is, therefore, applicable only on sites where the final carpet manufacturing processes are carried out. Commission dyers and sales yarn spinners are unable to make use of this technique.
Practical experience indicates that penetration of the foam to the base of the pile may be difficult to achieve with some densely tufted and woven pile structures. The application of a foam to the surface of some pile structures is also known to cause unacceptable changes in the surface appearance of the finished carpet and these effects have to determined on a case by case basis. In comparison to yarn application techniques a somewhat higher overall application rate is required to ensure that the base of the pile is adequately protected against insect damage. The foam adds moisture to the carpet which must be removed by evaporation, additional drying capacity may be required over and above that required to cure the latex or backcoat.
Dedicated equipment designed specifically to achieve the level application of foam at a low moisture add-on is required. At the time of writing only one commercially available system has been fully evaluated for insect resist treatment. The equipment can be retrofitted to an existing carpet finishing line if space allows, at a cost of approximately EUR 160000. Depending on the construction of the carpet, additional drying capacity may need to be installed, or alternatively the finishing line may need to be run at a slower speed. Due to the higher required application rate, chemical costs are approximately twice those incurred when a mothproofing finish is applied during yarn wet processing.
Reference plants The technique is employed by a number of UK and European carpet manufacturers.
Reference literature [32, ENco, 2001]
Figure 4.30: Process map for mothproofing stock dyed – yarn scoured production [32, ENco, 2001] 4.
8.4.4 Specific process modifications to minimise the emission of insect resist agent active substance during the treatment of yarn dyed production Figure 4.31 illustrates the baseline process for the mothproofing of carpet yarn produced via the yarn dyeing route. In this process mothproofing is carried out simultaneously with colouration, the required quantity of agent being added to the dye vessel at the commencement of dyeing.
Active substance is adsorbed by the fibre as the temperature of the dye liquor increases. This adsorptive process reaches equilibrium at the boil, with more than 98 % of the active substance partitioning towards the fibre. The spent dye liquor contains residual active substance at a concentration that depends on dyeing conditions. Acidic dyeing conditions (pH 4) yield the lowest residues. Dyeing carried out under more neutral conditions, for example, in the case of dyeing with pre-metalised dyestuffs generally result in higher residues in liquors.
The dyeing may be followed by a rinsing cycle, which may also contain residual active substance. In the conventional process both these spent liquors are discharged to the mill’s waste water attenuation tank. The liquor expelled during hydrextraction of the dyed yarn may also contain active substance at trace levels.
Emissions from the baseline process depend on a large number of interrelated variables as each dyeing is virtually unique. For practical purposes the pH of the dye liquor can be considered to
have the single most important effect and emission values from a large number of dyeings
generally fall in the following range (Enco Environmental Network, unpublished):
dyeing at a pH 4 0.1 – 0.4mg/kg yarn dyeing at a pH 4 2.0 – 7.0mg/kg yarn Inspection of Figure 4.31 indicates that there are four alternative process modifications (numbered 1 – 4) which are capable of minimising the emission of mothproofer when compared with the baseline process. Two of these alternatives, continuous low volume application (Process 2) and foam application (Process 4) have been described above in the section on minimising emissions from the stock dye – yarn scour yarn production route. The two candidate techniques which are suitable only for the yarn dye production route (Process 1 and Process 3) are described below.
18.104.22.168.1 Process 1. Aftertreatment under acidic conditions Description In this process mothproofing is carried out from a separate bath to colouration as an aftertreatment, following rinsing. The aftertreatment bath is acidified to provide optimum conditions for mothproofer uptake.
Main achieved environmental benefits Treatment is carried out in the absence of any retarding chemicals and at a pH optimal for mothproofer adsorption. Residual active substance loads are typically between 0.1 and
Cross-media effects None believed likely.
Applicability In many dyehouses wool and wool blend dyeing is carried out using a variety of dyeing conditions, dependent on the particular dyestuffs selected to meet end-product requirements.
This process is applicable to the mothproofing of yarn that would be dyed under conditions which are not optimised for mothfproofer uptake. In practice the technique is used to mothproof yarn when the dyeing conditions specify a liquor pH greater than approximately 5.0.
The process requires the use of an additional after-treatment bath, equal in volume to the original dyeing. Depending on machine design and yarn load water consumption will lie between 15 and 25 m3/tonne. Additional energy is required to raise the aftertreatment bath to dyeing temperature, together with additional time to complete the lengthened process.
Reference plants This technique is used in UK dyehouses to minimise emissions from a dyeing that would otherwise yield unacceptably high residues.
Reference literature [32, ENco, 2001]
22.214.171.124.2 Process 2. Low volume semi-continuous application Description The basic technique and equipment are described in the section on mothproofing stock dyed – yarn scoured production. Identical equipment can be employed to treat yarn that is dyed in hank form rather than as loose stock. In this case a stand alone machine may be required consisting of only the low volume application bowl and squeeze press or alternatively the feed to an existing machine installed on the end of a conventional scouring line can be modified to accept hanks between the scour and the low volume application bowl.
In this application contamination of the treatment liquor with residual colour from the previously dyed yarn can be a significant problem and the yarn must be dyed with dyes of good wet fastness.
Main Achieved Environmental Benefit See previous section (Process B).
Applicability The technique is applicable where yarn dyeing is carried out with dyes of good wet fastness.
Economics See previous section (Process B).
Reference plants One dyehouse is known to mothproof yarn dyed production by this technique, but no detailed information is available.
Reference literature [32, ENco, 2001]