«EUROPEAN COMMISSION Integrated Pollution Prevention and Control (IPPC) Reference Document on Best Available Techniques for the Textiles Industry July 2003 ...»
4.6.16 Chromium-free dyeing of wool Description Concerns associated with the use of sodium (or potassium) dichromate as mordant in wool dyeing with chrome dyes are already discussed in Section 188.8.131.52 (“oxidising agents”). Lowchrome dyeing techniques (see Section 4.6.15) allow a considerable improvement in the efficiency of this process, but they cannot avoid the presence of free chromium in the water effluent and in the sludge.
Furthermore, many initiatives discourage the use of chrome mordant dyes (e.g. OSPAR, GuT, EU-Ecolabel).
Until recently the use of chrome dyes was considered unavoidable for certain types of wool articles, in particular for dark shades, due to the excellent wet fastness of these dyes. Quite recently, new reactive dyestuffs have been put on the market that can provide levels of fastness comparable with those achievable with chrome dyes, even for dark shades.
These new colourants are bifunctional reactive dyestuffs generally containing bromoacrylamide or vinylsulphone reactive groups. The structure of a typical bifunctional reactive dyestuff of the bromo-acrylamide type is shown in Figure 4.20. The dye range is based on a trichromatic system, where Yellow CE (or Golden Yellow CE), Red CE and Blue CE can be used as the basis for the coloured shade area, and Navy CE and Black CE as the basis for highly fast navies and blacks.
Table 4.24: Composition and ecological information of two auxiliaries to be applied with “Lanasol Dyes” It has to be noted that the formulated product also contains a variable amount of auxiliaries (e.
anti-dusting agents) that are completely discharged with the exhausted liquor.
Thanks to the high fixation rate now achieved, the released dye only accounts for a minor amount of the total COD from dyeing, whereas the real contribution comes from the other constituents of the dye formulation and from the auxiliaries used in the process (e.g. levelling agents).
The general features of these new reactive dyes, in comparison with chrome dyes, are reported in the following table.
Dyeing process two-step dyeing process (dyeing and one-step dyeing process (but for dark chroming) shades an after-treatment is required) Source: [179, UBA, 2001], [191, VITO, 2001]
(1) non fixed dye: metal complex dyes: 3 – 7 % (“Entec” and “Ciba”); 1:2 metal complex dyes: 2 – 5 % (“Ciba”); chrome dyes: 1 – 2 % (“Entec”); reactive dyes: 7 – 20 % (“Entec”), 5 – 15 % (“Ciba”) Table 4.25: Comparative analysis of the features of chrome dyes and reactive dyes for wool dyeing Main achieved environmental benefits By changing over to reactive dyestuffs, the handling of hexavalent chromium, which requires special safety precautions, due to its chronic toxicity and carcinogenic effects, can be avoided.
As concerns waste water, the presence of chromium, not only in the chelated form, but more importantly as free metal, is avoided. In this respect, it has to be taken into account that dyehouses that accept dyeing without chrome dyes may still use the metal complex dyes.
Nevertheless, in metal-complex dyes the metal is present in the chelated form, which brings about less risk than the same amount of chromium released from afterchroming (see Section 184.108.40.206 – “Heavy metals emissions”).
Dyeing cycles are reported to be longer when dyeing with reactive dyes, due to rinsing and clearing (approximately one hour more than a standard cycle of 2h30 with chrome dyes) [163, Comm., 2001]. However, this technique is improving very rapidly and industrial experience shows that in most cases it is possible to dye at 105 °C and carry out the afterclearing process in the dye bath, without lowering the wet fastness (see the dyeing curve in Figure 4.21) [280, Germany, 2002]. As a result, water and energy consumption can be reduced.
Figure 4.21: Dyeing curve for the application of reactive dyestuffs for wool exhaust dyeing Cross-media effects Attention should be paid to colour, AOX and organic load from non-biodegradable levelling agents.
Reactive dyestuffs produce more highly coloured effluents than chrome dyes. This is attributable to the higher colour strength typical of these colourants. In quantitative terms, however, for the same level of colour the amount of dyestuff discharged may be comparable or even less than traditional dyes [61, L. Bettens, 1999]. EU research shows that effective and economic viable routes (Enhanced Thermal Fenton ETF & Enhanced Photo Fenton reaction EPF) are available for destruction of the residual dyestuff [191, VITO, 2001].
For an objective assessment of the organic load produced by the dyeing process, the composition of the dye and the COD-values of the other constituents of the dye formulation, along with the auxiliaries used in the process, should be considered. The dyestuff itself, thanks Textiles Industry 337 Chapter 4 to its relatively high fixation rate, makes only a minor contribution to the organic load in the final effluent. On the other hand, the organic load produced by the levelling agents added to the dye liquor is significant in the overall balance. These are fatty amine ethoxylates, which are hardly biodegradable and only 60 – 70 % bioeliminable. Since these compounds have affinity for the wool, it is estimated that 50 % of the amount applied will remain on the fibre, while the remaining 50 % will end up in the waste water or in the sludge (that is, transferred to other media). Nevertheless, at a minimum application level of 1 % o.w.f. (10 g/kg fibre), 1.5 g/kg of CD from the levelling agent would still be released into the waste water (10g/kg x 0.5 x 0.3 x 1025 mg COD/g), unless free-radical destruction methods are applied.
Dyeing with pH-controlled profile (starting in acid conditions when reduced reaction occurs and shifting to alkaline pH as soon as the boiling temperature is reached) would allow optimum dye exhaustion with lower environmental impact (no need for levelling agent). A pH-buffer can be used as alkali spender, instead of neutralising the acid bath with alkali (which would give uneven results). However, these compounds also need to be assessed for their hazardousness and the possible formation of harmful products.
The fact that reactive dyestuffs may contain organically bound halogens is regarded as an environmental issue, especially in countries where AOX is a parameter regulated by the environmental legislation. However, in the case of reactive dyes, the AOX found in the waste water are not the result of a haloform reaction and therefore they represent a much lower level of risk for the environment. Furthermore AOX originated from reactive dyes are not persistent in the environment because of hydrolysis (see also Section 220.127.116.11, “AOX”).
Note also that most navy and black dyestuffs (shade range in which chrome dyes are most used) do not contain any AOX at all [280, Germany, 2002].
Finally water and energy consumption: when dyeing with reactive dyes two rinsing steps at about 80 °C are normally carried out after dyeing, in order to remove the unfixed dye. This leads to higher consumption of water (approximately 30 % more [163, Comm., 2001]) and energy.
However, as already mentioned earlier, recent industrial experience shows that in most cases the afterclearing process can be carried out directly in the exhausted dye bath, thus saving water and energy (specific water consumption figures of about 25 l/kg are reported) [280, Germany, 2002].
Applicability Reactive dyestuffs described in this section are suitable for wool and polyamide in all make-ups and can be applied in all types of dyeing machines.
Fastness properties can be very good and even comparable/equivalent to those obtained with chrome dyes. However, the importance of reactive dyes in substitution of chrome dyes is
increasing only slowly for a number of reasons:
· not all operators agree that wool articles treated with the two different classes of dyestuffs both meet the final quality standards, especially for fastness levels. Some finishers still consider that chrome dyes are the only dyes that can guarantee the level of fastness required for overdyeing · it is not possible to match the same shade (metamerism) and thus a slightly different product is obtained by substitution · it is difficult, especially for commission dyehouses, to change over to reactive dyestuffs because customers often expressly require the use of a specific class of dyestuffs · operators find it difficult to adapt to new techniques because this requires radical changes to a well-established procedure · dyeing with reactive dyes is claimed to be more expensive than with chrome dyes.
Economics UBA states that costs are comparable with the chroming method when taking into account the overall process costs [179, UBA, 2001].
According to CRAB- Italy, on the other hand, dyeing with reactive dyes is more expensive than with chrome dyes [163, Comm., 2001]. The economic aspects involved with changing over from chrome to reactive dyes are summarised in the following table.
Table 4.26: Assessment of the economic aspects involved when changing over from chrome to reactive dyes Driving force for implementation Economic reasons are not considered to be the driving force.
Pressure and safety requirements set by legislation play a more important role in the ongoing process of substitution for chrome dyes.
As mentioned at the beginning, the use of chrome dyes is also discouraged by various initiatives at European level (e.g GuT label for carpets, Eco-label for textile products, etc.). GuT members, for example, have agreed to no longer use chrome dyes for carpet wool and the European Ecolabel Criteria are taking the same direction for textiles.
GuT and EU labelling do not exclude metal complex dyes. OSPAR recognises the need for chrome dyes, but strict limits are recommended in order to minimise the amount of chrome discharged.
Reference plants Reactive dyes have been on the market for about 15 years and are now successfully applied in many finishing mills in Europe and world-wide.
Reference literature [179, UBA, 2001], [163, Comm., 2001], [61, L. Bettens, 1999], [59, L. Bettens, 2000], [51, OSPAR, 1994].
4.6.17 Emission reduction in dyeing wool with metal-complex dyestuffs Description Dyeing of loose wool fibre and combed tops is still often carried out using afterchrome or metal-complex dyestuffs. Afterchrome dyestuffs can be substituted by metal-free reactive dyestuffs in many cases. However, when substitution is not possible, another alternative is using metal-complex dyes under optimised conditions (especially pH control).
In the case of 1:2 metal complex dyestuffs, the dyeing process can be improved by:
· using a special auxiliary (mixture of different fatty alcohol ethoxylates with high affinity for the fibre and the dyestuff) · replacing acetic acid by formic acid.
The optimised process is the well-known "Lanaset TOP process", launched by a dyestuffs and textile auxiliaries supplier in 1992.
The control of pH and the application of a mixture of different fatty alcohol ethoxylates shorten the dyeing time drastically compared to the conventional process. In addition exhaustion rate is almost 100 %, which makes dyeing in a standing bath easier [179, UBA, 2001].
In addition to environmental advantages, the process enables reproducible dyeings with very high fastness properties.
Main achieved environmental benefits Because of the higher exhaustion and fixation rate, the amount of dye in the exhausted liquor is reduced, which directly correlates with lower chromium content of the effluent. Residual chromium levels down to 0.1 mg/l have been reached in the exhausted dye bath in a laboratory test for a marine shade (i.e. a dark shade). Such low values were confirmed as achievable.
However, concerning daily practice in companies, higher values of 1 mg/l are considered more realistic. Emission factors of 10 - 20 mg/kg of treated wool can therefore be achieved, which correspond to 1 - 2 mg/l of chromium in the spent dye bath when a 1:10 liquor ratio is used [320, Comm., 2002].
Such low concentration levels open the way for dyeing in a standing bath without the negative effects/limitations arising from build-up of chromium.
The substitution of acetic acid (which has a specific COD of 1067 mg/g) by formic acid (which has a specific COD of only 235 mg/g and is a stronger acid than acetic acid) contributes to lowering the COD load in the effluent.
An additional benefit is achieved due to the reduction of the dyeing cycle time. When applying this technique, boiling time can be shortened to one-third that of the conventional process, which saves energy as well as time.
Operational data Figure 4.22 shows the dyeing curve for the conventional process and the optimised process (Lanaset TOP process).
Figure 4.22: Dyeing of loose wool fibre and combed tops: comparison between the dyeing curve for the conventional process (whole curve) and the optimised process (Lanaset TOP process) (dotted part of the curve) [179, UBA, 2001] Cross-media effects None believed likely.
Applicability The technique is applicable in new and existing installations. It is mainly applied for dyeing loose wool fibre and combed tops, which still represent about half of the wool fibre processed annually.
Economics Savings are achieved due to shorter process time and less rinsing water.
Driving force for implementation The requirements set by environmental legislation to reduce the chromium content in waste water and the desire to increase productivity have been the main driving forces for the implementation of this technique.
Reference plants The process has been successfully put into practice in many dyehouses worldwide.
Reference literature [179, UBA, 2001] 4.6.18 Use of liposomes as auxiliaries in wool dyeing Description The use of liposomes as auxiliary products in wool dyeing with acid dyestuffs allows good dye
bath exhaustion at 80 ºC and in 40 min. The advantages are:
· lower superficial damage of the wool fibre (due to the lower operating temperature the hand-feel of the fabric is softer) · energy saving · no electrolyte use · lower COD load in the waste water.
With wool/polyester mixtures, in order to allow the diffusion of disperse dye into the polyester fibre, it is necessary to operate at higher temperatures (100 ºC), and to add a low concentration of carriers. Liposomes have the effect of increasing the diffusion of disperse dyestuffs into the wool fibre (see Section 2.7.7 - “Polyester-wool blends”). It is therefore important to carry out selection essays of the suitable disperse dyestuffs in order to avoid negative effects on the fastness properties of the dyed product.
Main achieved environmental benefits
The main environmental benefits associated with the use of liposomes include:
· energy saving · lower COD load in the waste water · lower conductivity of the waste water.
Operational data Wool dyeing with acid dyes in the presence of liposomes is carried out at 80 ºC for 40 min., in a
bath containing [308, Spain, 2002]:
· liposome 0.1 - 0.2 % o.f.w.
· formic acid · acid dyestuff.
Cross-media effects None believed likely.
Applicability Liposomes-based auxiliaries have general applicability in wool dyeing mills [308, Spain, 2002].
Economics Energy savings and better quality of the fabric compensate for the cost of the liposomes [308, Spain, 2002].
Driving force for implementation The improved product quality is the main driving force for the implementation of this technique.
Reference plants Two plants in the Barcelona region are reported to have implemented this technique [308, Spain, 2002].