«EUROPEAN COMMISSION Integrated Pollution Prevention and Control (IPPC) Reference Document on Best Available Techniques for the Textiles Industry July 2003 ...»
Sodium hydrosulphite (also called sodium dithionite) is another sulphur-containing reducing agent, which is commonly used not only in sulphur and vat dyeing processes, but also as reductive after-cleaning agent in PES dyeing. Sodium hydrosulphite is less critical than sodium sulphide. However, during the dyeing process sodium dithionite is converted into sulphite (toxic to fish and bacteria) and in some cases this is further oxidised into sulphate.
In the waste water treatment plant sulphite is normally oxidised into sulphate, but this can still cause problems. Sulphate, in fact, may cause corrosion of concrete pipes or may be reduced under anaerobic conditions into hydrogen sulphide.
Hydroxyacetone, although it produces an increase in COD load, is recommended to lower the sulphur content in waste water, but it cannot replace hydrosulphite in all applications. New organic reducing agents with improved reducing effects have been developed (see Section 4.6.5 and Section 4.6.6 for further details).
Consumption of the reducing agent by the oxygen present in the machine (partially-flooded dyeing machines) needs also to be taken into account. Instead of applying only the amount of reducing agent required for the reduction of the dyestuff, a significant extra amount of reducing agent often needs to be added to compensate for the amount consumed by the oxygen contained in the machine. This obviously increases oxygen demand of the effluent.
Dichromate should no longer be used in Europe as an oxidising agent when dyeing with vat and sulphur dyes, but it is still widely used for the fixation of chrome dyes in wool dyeing.
Chromium III exhibits low acute toxicity, while chromium VI is acutely toxic and has been shown to be carcinogenic towards animals. During the dyeing processes with chrome dyes, Cr VI is reduced to Cr III if the process is under control. Nevertheless, emissions of Cr VI may still occur due to inappropriate handling of dichromate during dye preparation (care must be taken as dichromate is carcinogenic and may cause health problems for workers handling it). Emissions of trivalent chromium in the waste water can be minimised (see Section 4.6.15), but cannot be avoided, unless alternative dyestuffs are applied (see Section 4.6.16).
The use of bromate, iodate and chlorite as oxidising agents in vat and sulphur dyeing processes and the use of hypochlorite as stripping agent for decolouring faulty goods or for cleaning dyeing machines (e.g. before subsequent lighter-coloured dyeing) may produce AOX emissions.
However, only hypochlorite and elemental-chlorine-containing compounds (e.g. certain chlorite products that contain Cl2 or use chlorine as activator for formation of chlorine dioxide gas) are likely to give rise to hazardous AOX.
Salt Salts of various types are used in dyeing processes for different purposes (e.g. to promote level dyeing or increase dye exhaustion). In particular, large amounts of salt are used in cotton batch dyeing processes with reactive dyes. The amount of salt employed is quite significant compared to other classes of dyestuffs, for example direct dyes (Table 2.16) and efforts have been made by dye manufacturers to solve this problem (see Section 4.6.11).
Table 2.16: Amount of salt employed in cotton batch dyeing processes with reactive and direct dyes In addition to the use of salt as a raw material, neutralisation of commonly employed acids and alkalis produces salts as a by-products.
Salts are not removed in conventional waste water treatment systems and they are therefore ultimately discharged in the receiving water. Although the mammalian and aquatic toxicity of the commonly employed salts are very low, in arid or semi-arid regions their large-scale use can produce concentrations above the toxic limit and increase the salinity of the groundwater.
Countries have set emission limits at 2000 ppm or below. River quality standards must also be taken into account.
Carriers The use of these auxiliaries, which were widely employed in the past, has now been reduced due to ecological and health problems. They are still an issue in dyeing of polyester in blend with wool.
Carriers may already be added to the dyes by manufacturers. In this case textile finishers will have little knowledge of the loads discharged ([4, Tebodin, 1991] and [61, L. Bettens, 1999]).
Carriers (see Section 8.6.7) include a wide group of organic compounds, many of them steam volatile, poorly biodegradable and toxic to humans and aquatic life. However, as the active substances usually have high affinity for the fibre (hydrophobic types), 75 – 90 % are absorbed by the textile and only the emulsifiers and the hydrophilic-type carriers such as phenols and benzoates derivatives are found in the waste water. The carriers that remain on the fibre after dyeing and washing, are partially volatilised during drying and fixing operations and can give rise to air emissions. Traces can still be found on the finished product, thus representing a potential problem for the consumer.
Alternative options are described in Sections 4.6.1 and 4.6.2.
Other auxiliaries of environmental interest Other substances that may be encountered in the dyeing auxiliaries and that may give rise to
water pollution are:
· fatty amine ethoxylates (levelling agent) · alkylphenol ethoxylates (levelling agent) · quaternary ammonium compounds (retarders for cationic dyes) · polyvinylpyrrolidone (levelling agent for vat, sulphur and direct dyes) · cyanamide-ammonia salt condensation products (auxiliaries for fastness improvement) · acrylic acid-maleic acid copolymers (dispersing agent) · ethylenediamine tetraacetate (EDTA) complexing agents · diethylenetriaminepentaacetate (DTPA) · ethylenediaminetetra(methylenephosphonic acid) (EDTMP) · diethylenetriaminepenta(methylenphosphonic acid) (DTPMP) These are water-soluble hard-to-biodegrade compounds which can pass untransformed or only partially degraded, through waste water treatment systems.
In addition, some of them are toxic (e.g. quaternary amines) or can give rise to metabolites which may affect reproduction in the aquatic environment (APEO).
188.8.131.52 Environmental issues related to the process Both water and energy consumption in dyeing processes are a function of the dyeing technique, operating practices and the machinery employed.
Batch dyeing processes generally require higher water and energy consumption levels than continuous processes. This is due to a number of different factors.
The higher liquor ratios involved in batch dyeing represent one of these factors. As previously mentioned in Section 2.7.2, higher liquor ratios mean not only higher water and energy uses, but also a higher consumption of those chemicals and auxiliaries that are dosed based on the volume of the bath.
Consistently with the quality of the different types of substrates, all equipment manufacturers now can offer machines with reduced liquor ratios. Terms like “low liquor ratio”and “ultra-low liquor ratio” are now commonly used to define the performance/ features of modern machines.
For dyeing fabric in rope form nominal reference values for “low liquor ratio machines” are in the range of 1:5 - 1:8 for cotton and correspondingly 1:3 - 1:4 for PES. The liquor ratio can be higher for other types of substrates/fibres.
The term “Ultra-low liquor ratio” is used to define machines that can be operated at liquor ratios as low as the minimum volume required to completely wet out the substrate and avoid cavitations of the pumps. This term applies only to machines for dyeing fabric in rope form.
It is important to show the difference between the nominal and the real liquor ratio. As already stated in Section 2.7.2, the nominal liquor ratio is the liquor ratio at which a machine can be operated when it is loaded at its maximum/ optimal capacity. It is often the case that the machine is underloaded compared to its optimal capacity. This often occurs in commission companies where a high production flexibility is required to serve variable lot sizes according to customer’s demands. Modern machines can still be operated at approximately constant liquor ratio whilst being loaded at a level as low as 60 % of their nominal capacity (or even 30 % of their nominal capacity with yarn dyeing machines – see Section 4.6.19). In this way the same benefits achievable with low liquor ratios can be kept even with reduced loading. It is obvious however, that when a machine is loaded far below its optimal capacity (e.g. below 60 % of its nominal capacity for fabric dyeing machines) the real liquor ratio will differ greatly from the nominal liquor ratio. This will result not only in lower environmental performances (higher water, energy and chemicals consumptions), but also in higher operating costs.
In conclusion, the use of low liquor ratio machinery, or selection of the most adequate machine for the size of the lot to be processed, is fundamental to the resultant environmental performance of the process.
Having said that, high energy and water consumption in batch dyeing is not only the result of high liquor ratios.
Another factor to take into consideration is the discontinuous nature of the batch dyeing operating mode, especially with regard to operations such as cooling, heating, washing and rinsing.
Furthermore, shade matching can be responsible for higher water and energy consumption, especially when dyeing is carried out without the benefit of laboratory instruments. In a manual regime the bulk of the dyestuff is normally applied in the first phase to obtain a shade which is close to that required in the final product. This is followed by a number of matching operations, during which small quantities of dye are applied to achieve the final shade. Shades which are difficult to match may require repeated shade additions with cooling and reheating between each addition [32, ENco, 2001].
Increased energy and water consumption may also be caused by inappropriate handling techniques and/or poorly performing process control systems. For example, in some cases displacement spillage may occur during immersion of the fibre in the machine, while the potential for overfilling and spillage exists where the machines are only equipped with manual control valves, which fail to control the liquor level and temperature correctly (see also Section 4.1.4).
Continuous and semi-continuous dyeing processes consume less water, but this also means a higher dyestuff concentration in the dye liquor. In discontinuous dyeing the dye concentration varies from 0.1 to 1 g/l, while in continuous processes this value is in the range of 10 to 100 g/l.
The residual padding liquor in the pads, pumps and pipes must be discarded when a new colour is started. The discharge of this concentrated effluent can result in a higher pollution load compared with discontinuous dyeing, especially when small lots of material are processed.
However, modern continuous dyeing ranges have steadily improved in recent years. The use of small pipes and pumps and small pad-bath troughs help to reduce the amount of concentrated liquor to be discharged. In addition, it is possible to minimise the discard of leftovers, by using automated dosing systems, which meter the dye solution ingredients and deliver the exact amount needed (see also Sections 4.1.3 and 4.6.7 for more detailed information about recent improvements).
In both continuous and batch dyeing processes, final washing and rinsing operations are waterintensive steps that need to be taken into consideration. Washing and rinsing operations actually consume greater quantities of water than dyeing itself (see Sections 4.9.1 and 4.9.2 for water and energy conservation techniques in batch and continuous processing and Sections 4.1.4 and 4.6.19 for equipment optimisation in batch processing).
2.8 Printing2.8.1 Printing processes
Printing, like dyeing, is a process for applying colour to a substrate. However, instead of colouring the whole substrate (cloth, carpet or yarn) as in dyeing, print colour is applied only to defined areas to obtain the desired pattern. This involves different techniques and different machinery with respect to dyeing, but the physical and chemical processes that take place between the dye and the fibre are analogous to dyeing.
A typical printing process involves the following steps:
86 Textiles Industry
Chapter 2· colour paste preparation: when printing textiles, the dye or pigment is not in an aqueous liquor, instead, it is usually finely dispersed in a printing paste, in high concentration · printing: the dye or pigment paste is applied to the substrate using different techniques, which are discussed below · fixation: immediately after printing, the fabric is dried and then the prints are fixed mainly with steam or hot air (for pigments). Note that intermediate drying is not carried out when printing carpets (too much energy would be needed for removing the highly viscous liquor) · after-treatment: this final operation consists in washing and drying the fabric (it is not necessary when printing with pigments or with other particular techniques such as transfer printing).
When describing the different printing techniques, a distinction should be made between printing with pigments, which have no affinity for the fibre, and printing with dyes (reactive, vat, disperse, etc.).
184.108.40.206 Printing with pigments Pigment printing has gained much importance today and for some fibres (e.g. cellulose fibres) is by far the most commonly applied technique. Pigments can be used on almost all types of textile substrates and, thanks to increased performance of modern auxiliaries, it is now possible to obtain high-quality printing using this technique.
Pigment printing pastes contain a thickening agent, a binder and, if necessary, other auxiliaries such as fixing agents, plasticizers, defoamers, etc.
White spirit-based emulsions, used in the past as thickening systems, are used only occasionally today (mainly half-emulsion thickeners). More information regarding the characteristics of the auxiliaries used can be found in Section 8.7.2 After applying the printing paste, the fabric is dried and then the pigment is normally fixed with hot air (depending on the type of binder in the formulation, fixation can also be achieved by storage at 20 °C for a few days). The advantage of pigment printing is that the process can be done without subsequent washing (which, in turn, is needed for most of the other printing techniques).
220.127.116.11 Printing with dyes Printing paste preparation The process traditionally starts with the preparation of the paste. Compared to pigment printing, the composition of the pastes is more complex and variable, being determined not by the dye used, but by the printing technique, the substrate, the application and the fixation methods applied.
Apart from the dye, printing pastes contain a thickening agent (see also Section 8.7.1) and
various other auxiliaries, which can be classified according to their function as follows:
· oxidising agents (e.g. m-nitrobenzenesulphonate, sodium chlorate, hydrogen peroxide) · reducing agents (e.g. sodium dithionite, formaldehyde sulphoxylates, thiourea dioxide, tin(II) chloride) · discharging agents for discharge printing (e.g. anthraquinone) · substances with a hydrotropic effect, like urea · dye solubilisers, which are polar organic solvents like glycerine, ethylen glycol, butyl glycol, thiodiglycol, etc.
· resists for reactive resist printing (e.g. sulphonated alkanes) · defoamers, (e.g. silicon compounds, organic and inorganic esters, aliphatic esters, etc.).