«EUROPEAN COMMISSION Integrated Pollution Prevention and Control (IPPC) Reference Document on Best Available Techniques for the Textiles Industry July 2003 ...»
Applicability The technique is suitable for cellulose and cellulose/polyester blends. The applicability, however, is restricted to pastel to pale shades (up to approximately 5 g/l of dyestuff at 50 % liquor pick-up).
Economics Significant benefits are achieved compared to the conventional pad-steam method, thanks to savings in energy, time, water and chemicals.
Driving force for implementation Economic benefit is the main motivation for the implementation of this technique.
Reference plants Many plants in Germany and world-wide [179, UBA, 2001].
Reference literature [179, UBA, 2001] with reference to “BASF Aktiengesellschaft, Technical Information TI/T 7063 d, 1998 and 7043 d, 1998”.
4.6.5 Aftertreatment in PES dyeing Description A major problem in the dyeing of PES fibres and PES blends using disperse dyestuffs is wash fastness. In order to meet washing fastness requirements, an after-treatment step is carried out,
which removes the non-fixed disperse dyes from the fibre. Reductive after-clearing is normally preferred over simple washing with surfactants because disperse dyestuff molecules absorbed on the surface are broken down into smaller, often colourless and more readily water-soluble fragments (see 184.108.40.206). The precondition is that no dyestuffs susceptible to reduction have been used for dyeing.
In the conventional process, after dyeing the polyester at 130 °C, the dye bath (acidic) needs to be cooled down to 70 °C before draining, in order to bring the fibre below its glass transition temperature. The reductive aftertreatment is carried out in a new bath using hydrosulphite and a dispersing agent in alkaline conditions (the temperature is raised again to 80 °C during the process). Afterwards the bath is drained and one or two more rinsing steps are needed in order remove the remaining alkali and reducing agent. The pH of the textile before entering the steamer needs to be between 4 and 7 in order to avoid yellowing. Rinsing is therefore carried out in acidic conditions.
Beside the environmental concerns involved with the use of hydrosulphite as reducing agent (see 220.127.116.11), this process entails three bath changes (including temperature raising/cooling cycles) and two changes in the pH of the treatment baths: from acidic pH of the dyeing liquor to the high alkalinity of the aftertreatment bath and back again to acidic levels in the rinsing baths.
The double change produces higher consumption of water, energy and chemicals, greater demands on time and increased levels of salt in the effluent.
Two different approaches are proposed:
· Approach A) consists of using a reducing agent based on a special short-chain sulphinic acid derivative that can be added directly in the exhausted acidic dye bath. This reducing agent is liquid and can therefore be metered automatically. Moreover, it has very low toxicity and is readily biodegradable [179, UBA, 2001], [181, VITO, 2001].
· Approach B) consists of using disperse dyes that can be cleared in alkaline medium by hydrolitic solubilisation instead of reduction. These are azo disperse dyes containing phthalimide groups [182, VITO, 2001].
Main achieved environmental benefits Approach A) First of all, because this reducing agent can be applied in the acidic pH range, significant water and energy savings can be achieved. Compared to the conventional process, up to 40 % of water can be saved.
Moreover, these aliphatic short-chain sulphinic acid derivatives are readily biodegradable (the product is non volatile and water-soluble, with more than 70 % DOC reduction within 28 days, under the OECD test 302B). The sulphur content of the product is approximately 14 %, compared to 34 % of sodium dithionite and the amount of by-products (sulphites and sulphates) can be reduced by half relative to the conventional process (see table below).
It is also worth noticing that unlike sodium dithionite, the product is non-corrosive, non-irritant, non-flammable and does not have an unpleasant odour. As a result, workplace safety compared to dithionite is improved and odour nuisances minimised.
Table 4.19: Sulphur and sulphite concentration and load in the mixed effluent from typical processes using sodium dithionite or sulphinic acid based reducing agents Approach B) With alkali-dischargeable dyes the use of hydrosulphite or other reducing agents can be avoided, which means a lower oxygen demand in the final effluent.
There is the possibility of dyeing PES/cotton blends using a one-bath two-step dyeing method, as alkali-clearable dyes can be applied in the same bath with cotton reactive dyes. This brings about additional environmental benefits in terms of water and energy consumption.
Operational data Approach A) In most cases concentrated hydrosulphite can be replaced by the same quantity of the referenced product. In a typical process 1.0 – 1.5 ml/l of reducing agent (for medium shades) and 1.5 –
2.5 ml/l (for dark shades) are added to the exhausted dye bath. The process is carried out for 10 – 20 min at 70 – 80 °C. Hot and cold rinsing follow [179, UBA, 2001].
In order to derive the maximum environmental and economic benefit from the proposed technique, it is of primary importance that only the strict amount of reducing agent needed to reduce the dyestuff is consumed. Consumption of the reducing agent by the oxygen in the machine should therefore be avoided as much as possible. One effective technique to ensure this is to use nitrogen to remove oxygen from the liquor and the air in the machine [182, VITO, 2001].
For some types of polyesters that show a higher percentage of oligomer migrating to the surface during the dyeing process, it is advantageous to carry out the aftertreatment in a fresh bath.
The example below gives a comparison between two equivalent recipes, one with alkaliclearable dyes and the other with standard disperse dyes:
With alkali-clearable disperse dyes there is no need for levelling agents, dispersing agents or detergent. Moreover the amount of dye used is reduced. The resulting environmental benefits are evident.
Cross-media effects Approach A) When the product is used in the dye bath, the unfixed dispersed dye particles are destroyed by reduction so that the effluent is largely free of colour. On the other hand, the byproducts of the reductive reaction may be more hazardous than the original dyestuff (e.g.
aromatic amines originating from reduction of azo dyes). The effluent therefore needs to be treated before being discharged (for polyester oligomers and aromatic amines).
Approach B) None believed likely.
Applicability Approach A) The technique can be used in all types of dyeing machines, not only for polyester fibres, but also for PAC, CA and their blends. The only limitation on applicability is with blends with elastane fibres.
Approach B) Alkali-clearable dyes are currently applied for both for PES and PES/cotton blends, with greater environmental and economic advantages being achieved with PES/cotton blends.
Economics Approach A) Significant savings can be achieved as a result of higher productivity, reduced consumption of energy, water and chemicals and the lower burden in waste water.
Approach B) The use of alkali-clearable dyes implies higher recipe costs compared to conventional disperse dyestuffs (about twice as much: the total cost of a recipe with alkaliclearable dye is about EUR 0.5/kg, whereas with standard disperse dyes this is in the order of EUR 0.2/kg). However, Approach B is expected to bring savings in time (higher productivity) and reduce water, energy and chemicals costs, (particularly when these dyes are applied for PES/cotton bleands).
Driving force for implementation Cost savings (higher productivity) and improvement of the environmental performance (especially with regard to sulphite content in the waste water) are the main reason for application of both these techniques.
Reference plants Approach A) The proposed technique is applied in at least five finishing mills in Germany and world-wide as well [179, UBA, 2001].
Approach B) Many plants in Europe.
Reference literature [179, UBA, 2001] with reference to “BASF Aktiengesellschaft, Technical information TI/T 7043 d, 1998”, [180, Spain, 2001], [182, VITO, 2001], [181, VITO, 2001], [59, L. Bettens, 2000].
4.6.6 Dyeing with sulphur dyes Description Sulphur dyestuffs are of great importance world-wide in dyeing cotton in medium to dark shades (especially black) with a high fastness to light and washing. Sulphur dyes are insoluble in water and they need to be converted to the water-soluble “leuco-form” at some stage during the dyeing process (see also Section 9.9).
Conventional sulphur dyes are available in powder form. Before dyeing, they have to be reduced with sodium sulphide in alkaline conditions. Other typical sulphur dyes are the “prereduced”/“ready-for-use” dyes. They are supplied in liquid form and already contain the reducing agent in their formulation (the sulphide content may be higher than 5 % [179, UBA, 2001]).
Excess of sulphide (from the dyestuff and reducing agent) is responsible for aquatic toxicity and odour nuisances (workplace atmosphere) (see also Section 18.104.22.168 – “Sulphur-containing reducing agents”).
The ecological profile of sulphur dyeing has decisively improved thanks to the introduction of new sulphur dyes and alternative reducing agents.
The classic powder and liquid sulphur dyes can be successfully replaced by [179, UBA, 2001]:
· pre-reduced dyestuffs (liquid formulations with sulphide content 1 %) (with reference to “DyStar, 2001”) · non-pre-reduced sulphide-free dyestuffs (water-soluble in the oxidised form) (with reference to “DyStar, 2001”) · non-pre-reduced sulphide-free stabilised dispersed dyestuffs (in powder or liquid form) (with reference to “DyStar, 2001”) · non-pre-reduced sulphide-free dyestuffs (stable suspension) (with reference to “Clariant, 2001”).
Unlike the old sulphur dyes with low reduction potential, all these types of dyestuffs can be used without any sodium sulphide (in the pre-reduced liquid formulations a low amount of sodium sulphide is still present in the formulation).
The following binary systems are in use (“DyStar, 2001”):
· combination of dithionite and glucose · combination of hydroxyacetone and glucose (seldom) · combination of formamidine sulphinic acid and glucose (seldom).
Glucose is added to sodium dithionite to prevent over-reduction. Looking at the first list of bullet points, the addition of glucose can be omitted when using stabilised sulphide-free dyestuff formulations (3rd bullet point). With the non-pre-reduced sulphur dyestuffs mentioned at the last bullet point, the reduction step can be carried out with glucose alone (“Clariant 2001”).
In the past, an additional concern associated with sulphur dyeing was raised by the use of sodium dichromate as oxidising agent (applied to reconvert the dye to the original oxidised
insoluble form, after adsorption into the fibre). Sodium dichromate has now been fully replaced by hydrogen peroxide, bromate, iodate and chlorite.
Hydrogen peroxide is the preferred oxidising agent. Bromate, iodate and chlorite are detected as AOX. Nevertheless, they are not organohalogen compounds and they are not likely to give rise to hazardous organohalogen products (only certain chlorite products that contain Cl2 or use chlorine as activator are likely to give rise to hazardous AOX).
Main achieved environmental benefits Main environmental benefit resulting from the application of sulphide-low or sulphide-free sulphur dyes in combination with sulphide-free reducing agents is that sulphide content in waste water is minimised.
In order to derive the maximum environmental and economic benefit from the proposed technique, it is of primary importance that only the strict amount of reducing agent needed to reduce the dyestuff should be consumed. Consumption of the reducing agent by the oxygen in the machine should therefore be avoided as much as possible. One effective technique to ensure this is to use nitrogen to remove oxygen from the liquor and the air in the machine [182, VITO, 2001].
A typical recipe for cotton dyeing on a jet machine (liquor ratio 1:6 to 1:8; dyeing for 45 min at 95 °C) is given below [179, UBA, 2001] (with reference to “DyStar, 2001”):
· non-pre-reduced sulphur dye: 10 % · wetting agent: 1 g/l · caustic soda solution (38 Bé): 15 - 20 ml/l · soda ash: 8 - 10 g/l · salt: 20 g/l · glucose: 10 - 12 g/l
· sodium dithionite: 8 - 10 g/l or hydroxyacetone: 4 - 5 g/l or formamidine sulphinic acid:
4 - 5 g/l.
Cross-media effects When using sodium dithionite as reducing agent, the sulphite content in waste water has to be taken into account (see Section 22.214.171.124).
Applicability The dyestuffs and reducing agents described in this section can be used in existing and new dyeing machines (exhaust dyeing as well as continuous techniques). Possible differences of shade compared to common sulphur dyeing should be taken into consideration [179, UBA, 2001].
Economics Stabilised non-pre-reduced sulphide-free dyestuffs are more expensive than sulphur dyes.
Detailed information is not available [179, UBA, 2001].
Driving force for implementation Worker health & safety, odour nuisance and waste water problems related to the presence of sulphides.
Reference plants Many plants in Europe and world-wide.
Reference literature [179, UBA, 2001], [51, OSPAR, 1994] P071, [183, VITO, 2001].
4.6.7 Minimisation of dye liquor losses in pad dyeing techniques Description Main emission sources in pad dyeing processes arise from the discharge of the residual dyeing liquor in the pad, pumps and pipes at the end of each lot when a new colour is started (see Section 126.96.36.199.4 for further details about emission & consumption levels).
Reduction of these losses can be achieved by carrying out the impregnation step in a nip or by minimising the capacity of the dip trough (e.g. flex-shaft, U-shaft).
Figure 4.15: Representation of a U-shaft (A) and nip (B) dye liquor application systems
Further reduction of losses can be achieved by means of:
· systems for controlled dosage of the input raw materials. The dyestuff solution and auxiliaries are dosed, based on the specific recipe and are dispensed as separate streams, being mixed only immediately before being fed to the pad · dosage of the padding liquor based on measurement of the pick-up. The amount of dyeing liquor consumed is measured by reference to the quantity of processed fabric by measuring the length of the fabric times its specific weight. The resulting values are automatically processed and used for the preparation of the next comparable batch in order to minimise residues of unused dyeing liquor. This system, however, cannot avoid the presence of residual dye liquor in the feeding tank. The rapid batch dyeing technique represents a further improvement in this respect. In this case, rather than prepared in one single step (for the whole batch) before starting the dyeing batch, the dyestuff solution is prepared just in time, in several steps, based on on-line measurement of the pick-up.
Main achieved environmental benefits
Conventional pad-bath troughs have a capacity ranging from 30 litres to up to 100 litres.
Changing over to U-shaft troughs (12 litres capacity) will enable a reduction of the residues of unused liquor from 60 % to nearly 90 % per batch, compared to the conventional system.
Correspondingly, in the case of nip dyeing (5 litres), up to 95 % reduction will be achieved.