«EUROPEAN COMMISSION Integrated Pollution Prevention and Control (IPPC) Reference Document on Best Available Techniques for the Textiles Industry July 2003 ...»
Acrylic-cellulose blends PAC-cellulose blends are used for household textiles (drapery and table linen) and imitation fur ("peluche", in which the pile consists of PAC fibres and the back is made of cotton). The percentage of PAC in the mixtures varies between 30 and 80 %.
PAC can be dyed with cationic or disperse dyes, while direct, vat or reactive dyes can be used for the cellulose component.
The following methods are the most commonly used for dyeing this blend:
· continuous dyeing with cationic and direct dyes according to the pad-steam process (to avoid precipitation of cationic and anionic dyes present in the pad liquor at relatively high concentration, combination of anionic and non-ionic surfactants are added to the solution) · batch dyeing (usually according to the one-bath, two-steps method) with cationic and vat dyes or with cationic and reactive dyes.
Acrylic-wool blends Among synthetic fibres, PAC fibres are the most suitable for obtaining blends with wool that keep a wool-like character. This makes this blend widely used, especially for knitwear and household textiles. The blending ratio of PAC to wool varies from 20:80 to 80:20.
Metal-complex, acid and reactive dyes are the dyestuffs typically used for the wool part, while PAC is dyed with cationic dyes.
Cationic dyes stain wool fibre. As a matter of fact cationic dyes attach first to wool and then migrate to PAC fibre at higher temperature. Even if well-reserving dyes are selected, dyeing must be conducted for a sufficiently long time (from 60 to 90 minutes) in order to obtain good wool reserve [186, Ullmann's, 2000].
PAC-wool blends can be dyed using the following exhaustion methods:
· one-bath one-step · one-bath two-step · two-bath.
The first one allows shorter dyeing times and lower consumption of water. However, it is not always applicable because the simultaneous presence in the dye bath of anionic and cationic compounds can produce the precipitation of the formed adducts on the fibre. Precipitation can be prevented using dispersing agents and selecting adequate dyes.
When dyeing with the one-bath, two-step method the use of reserve agents is not necessary. In fact, wool absorbs the cationic dye and slowly releases it, acting as a retarding agent (exerting a retardant effect on PAC).
Table 2.11: Overview of the typical emissions generated in dyeing processes As the table shows, most of the emissions are emissions to water.
Due to the low vapour pressure of the substances in the dye bath, emissions to air are generally not significant and can be regarded more as problems related to the workplace atmosphere (fugitive emissions from dosing/dispensing chemicals and dyeing processes in “open” machines). A few exceptions are the thermosol process, pigment dyeing and those dyeing processes where carriers are employed.
In pigment dyeing the substrate is not washed after pigment application and therefore the pollutants are quantitatively released to air during drying. Emissions from carriers are to air and water.
In the first part of the following discussion the environmental issues related to the substances employed will be described, while in the second part the environmental issues related to the process will be mentioned.
188.8.131.52 Environmental issues related to the substances employed
Water-polluting substances in the above-mentioned emissions may originate from:
· the dyes themselves (e.g. toxicity, metals, colour) · auxiliaries contained in the dye formulation · basic chemicals (e.g. alkali, salts, reducing and oxidising agents) and auxiliaries used in dyeing processes · contaminants present on the fibre when it enters the process sequence (residues of pesticides on wool are encountered in loose fibre and yarn dyeing and the same occurs with spin finishes present on synthetic fibres).
Dyes Spent dye baths (discontinuous dyeing), residual dye liquors and water from washing operations always contain a percentage of unfixed dye. The rates of fixation vary considerably among the different classes of dyes and may be especially low for reactive dyes (in the case of cotton) and for sulphur dyes. Moreover, large variations are found even within a given class of colourants.
This is particularly significant in the case of reactive dyes. Fixing rates above 60 % cannot be achieved, for example, in the case of copper (sometimes nickel) phthalocyanine reactive dyes especially used for turquoise-green and some marine shades. In contrast, the so-called double anchor reactive dyes can achieve extremely high rates of fixation (see Sections 4.6.10 and 4.6.11).
The degree of fixation of an individual dye varies according to type of fibre, shade and dyeing parameters. Therefore fixation rate values can be given only as approximations. However, they are useful to give an idea of the amount of unfixed dyes that can be found in waste water.
Information from different authors is given in the table below.
Source: [11, US EPA, 1995], [77, EURATEX, 2000] and [293, Spain, 2002] EPA: US Environmental Protection Agency OECD: Organisation for Economic Co-operation and Development ATV: Abwasser Technishe Vereinigung (Waste Water Technical Association) Notes: (1) Now Dystar (including BASF) (2) New reactive dyestuffs with higher fixation rates are now available (see Section 4.6.10 and 4.6.11) Table 2.12: Percentage of non-fixed dye that may be discharged in the effluent for the principal classes of dyes
As stated earlier, as a consequence of uncomplete fixation a percentage of the dyestuff used in the process ends up in the waste water.
Dyestuffs are not biodegradable in oxidative conditions, although some of them may degrade under other conditions (e.g. azo dyes may cleave under anoxic and anaerobic conditions). Less water-soluble dyestuffs molecules (typically, disperse, vat, sulphur, some direct dyestuffs and pigments) can be largely bio-eliminated from waste water by coagulation/ precipitation or absorption/ adsorption to the activated sludge. The quantity of activated sludge in the waste water treatment plant and the quantity of dyestuff to be eliminated are key factors in determining the efficiency of removal of a dyestuff from the effluent.
Another factor to take into consideration is the colour strength of the colourant. For example, with reactive dyestuffs a lower amount of colourant is needed to achieve a given shade compared to other classes of dyes (e.g. direct, vat and sulphur dyes). As a result a lower amount of dyestuff will need to be removed from the waste water.
Dyestuffs that are poorly bio-eliminable (unless they are submitted to destructive treatment techniques) will pass through a biological waste water treatment plant and will ultimately end up in the discharged effluent. The first noticeable effect in the receiving water is the colour.
High doses of colour not only cause aesthetic impact, but can also interrupt photosynthesis, thus affecting aquatic life. Other effects are related to organic content of the colourant (normally expressed as COD and BOD, but could be better expressed as organic carbon, using TOC, DOC as parameters), its aquatic toxicity and the presence in the molecule of metals or halogens that can give rise to AOX emissions.
These issues are discussed in more detail for each class of dyestuff in Section 9. Only some general key issues are considered in this section.
AOX emissions Vat, disperse and reactive dyes are more likely to contain halogens in their molecule.
The content of organically bound halogen can be up to 12 % on weight for some vat dyes. Vat dyes, however, usually show a very high degree of fixation. In addition, they are insoluble in water and the amount that reaches the effluent can be eliminated with high efficiency in the waste water treatment plant through absorption on the activated sludge.
Reactive dyes, on the contrary, may have low fixation degrees (the lowest level of fixation is observed with phthlocyanine in batch dyeing) and their removal from waste water is difficult because of the low biodegradability and/or low level of absorption of the dye onto activated sludge during treatment. The halogen in MCT (monochlorotriazines) reactive groups is converted into the harmless chloride during the dyeing process. In calculating the waste water burden it is therefore assumed that the MCT reactive groups react completely by fixation or hydrolysis so that they do not contribute to AOX emissions. However, many commonly used polyhalogenated reactive dyes, such as DCT (dichlorotriazine), DFCP (difluorochloropyrimidine) and TCP (trichloropyrimidine) contain organically bound halogen even after fixation and hydrolysis. Bound halogen is also found in discharges of dye-concentrate (pad, kitchen) and non-exhausted dye baths that may still contain unreacted dyestuff.
For the other classes of colourants the AOX issue is not relevant because, with few exceptions, halogen content is usually below 0.1 %.
PARCOM 97/1 recommends strict limits for AOX. Even stricter limits are set by the EUEcolabel and German legislation. Extensive investigation of AOX in textile effluents was performed, but AOX as an indicator remains a matter of discussion.
Dyestuffs containing organically bound halogens (except fluorine) are measured as AOX. The only way to limit AOX from dyeing is by dye selection, by more efficient use of dyes or by treating the resulting effluent by decolouration. Effluent decolouration can be achieved using destructive techniques, such as the free radical oxidation or non-destructive techniques (e.g.
However, it should be noted that AOX from dyes do not have the same effect as the AOX derived from chlorine reactions (haloform reaction, in particular) arising from textile processes such as bleaching, wool shrink-resist treatments, etc.
Dyestuffs are not biodegradable compounds and the halogens in their molecule should not give rise the haloform reaction (main cause of hazardous AOX).
In this respect it is interesting to consider that PARCOM 97/1 does not set a general discharge limit value for AOX, but rather allows discrimination between hazardous and non-hazardous AOX [50, OSPAR, 1997].
Heavy metals emissions Metals can be present in dyes for two reasons. First, metals are used as catalysts during the manufacture of some dyes and can be present as impurities. Second, in some dyes the metal is chelated with the dye molecule, forming an integral structural element.
Dye manufacturers are now putting more effort into reducing the amount of metals present as impurities. This can be done by selection of starting products, removal of heavy metal and substitution of the solvent where the reaction takes place.
ETAD has established limits in the content of heavy metal in dyestuffs. The values have been set to ensure that emission levels from a 2 % dyeing and a total dilution of the dye of 1:2500, will meet the known waste water requirements [64, BASF, 1994].
Examples of dyes containing bound metals are copper and nickel in phthalocyanine groups, copper in blue copper-azo-complex reactive dyes and chromium in metal-complex dyes used for wool silk and polyamide. The total amount of metallised dye used is decreasing, but there remain domains (certain shades such as greens, certain levels of fastness to light) where phthalocyanine dyes, for example, cannot be easily substituted.
The presence of the metal in these metallised dyes can be regarded as a less relevant problem compared to the presence of free metal impurities. Provided that high exhaustion and fixation levels are achieved and that measures are taken to minimise losses from handling, weighing, drum cleaning, etc., only a little unconsumed dye should end up in the waste water. Moreover, since the metal is an integral part of the dye molecule, which is itself non-biodegradable, there is very little potential for it to become bio-available.
It is also important to take into account that treatment methods such as filtration and adsorption on activated sludge, which remove the dye from the waste water, also reduce nearly proportionally the amount of bound metal in the final effluent. Conversely, other methods such as advanced oxidation, may free the metal.
Dyestuffs showing aquatic toxicity and/ or allergenic effects are highlighted in Section 9. Here it is also important to mention that about 60 % to 70 % of the dyes used nowadays are azo dyes [77, EURATEX, 2000]. Under reductive conditions, these dyes may produce amines and some of them are carcinogenic. A list of carcinogenic amines that can be formed by cleavage of certain azo dyes is shown in the Table 2.13.
Table 2.13: List of carcinogenic amines The use of azo-dyes that may cleave to one of the 22 potentially carcinogenic aromatic amines listed above is banned according to the 19th amendment of Directive 76/769/EWG on dangerous substances.
However, more than 100 dyes with the potential to form carcinogenic amines are still available on the market [77, EURATEX, 2000].
Auxiliaries contained in dye formulations Depending on the dye class and the application method employed (e.g. batch or continuous dyeing, printing) different additives are present in the dye formulations. Since these substances are not absorbed/ fixed by the fibres, they are completely discharged in the waste water. Typical additives are listed in the table below.
Source [18, VITO, 1998]
(1) Statistical elimination test (Zahn-Wellens Test) Blank cells mean that data are not available Table 2.14: Ecological properties of dye formulations additives While these additives are not toxic to aquatic life, they are in general poorly biodegradable and not readily bioeliminable. This applies in particular to the dispersants present in the formulations of vat, disperse and sulphur dyes. These dyes are water-insoluble and need these special auxiliaries in order to be applied to the textile in the form of aqueous dispersions. These dispersants consist mainly of naphthalene sulphonate-formaldehyde condensation products and lignin sulphonates, but sulphomethylation products derived from the condensation of phenols with formaldahyde and sodium sulphite can also be found. Other not readily eliminable additives are acrylate and CMC-based thickeners and anti-foam agents.
The difference between liquid and powder formulations should also be mentioned. Dyes supplied in liquid form contain only one third of the amount of dispersing agent normally contained in powder dyes (see Table 2.15). The reason for this difference stems from the manufacturing process of powder dyes: the very small particles generated during grinding must be protected during the subsequent drying process and this is possible only by adding high proportions of dispersing agents.
Note that liquid formulations include liquid dispersions and true solutions (solutions without solubilising aids), whereas powder dyes can be supplied as dusting, free-flowing, non-dusting powders or granulates.
Basic chemicals and auxiliaries used in the dyeing process Regarding the environmental concerns associated with the chemicals and auxiliaries used in dyeing processes it is worth mentioning the following key issues.
Sulphur-containing reducing agents Waste water from sulphur dyeing contains sulphides used in the process as reducing agents. In some cases the sulphide is already contained in the dye formulation and in some other cases it is added to the dye bath before dyeing. In the end, however, the excess of sulphide ends up in the waste water. Sulphides are toxic to aquatic organisms and contribute to increasing COD load. In addition, sulphide anions are converted into hydrogen sulphide under acidic conditions, thereby giving rise to problems of odour and corrosivity.