«EUROPEAN COMMISSION Integrated Pollution Prevention and Control (IPPC) Reference Document on Best Available Techniques for the Textiles Industry July 2003 ...»
Because of the action of free radicals generated by activation of hydrogen peroxide,the size polymers are already highly degraded. The process produces shorter and less branched molecules, glucose, more carboxylated molecules such as oxalate, acetate and formate, which are easier to wash out with a reduced amount of water in efficient washing machines.
The pre-oxidation of size polymer is also advantageous at waste water treatment level (improved treatability). With enzymatic desizing, starches are not completely degraded (the long molecules are not completely broken down after desizing). This means higher organic load to be degraded in the biological plant and it is often the cause of problems such as the production of bulky difficult-to-settle sludge.
Operational data It is well known that in an oxidative alkaline medium (with hydrogen peroxide) there is potential risk of fibre damage during bleaching if OH* formation is not controlled. Size and the cellulose have similar molecular structure and therefore the attack of the cellulose polymer from non-selective OH* is possible. To achieve good results and avoid damage to the fibre when removing starch-like size, it is essential to add hydrogen peroxide at pH 13. These operating conditions minimise OH* radicals, which are responsible for cellulose damage.
An example of desizing-bleaching padding recipe for PVA/starch blends is:
· detergent (0.3 %) · sequestrant (0.1 %) · sodium hydroxide (0.7 - 2.0 %) · hydrogen peroxide (0.2 - 0.4 %) · salt (0.04 %) · emulsifiers as needed.
Cross-media effects None believed likely.
Applicability The technique is particularly suitable for commission finishers (independently of their size), who need to be highly flexible because their goods do not all come from the same source (and consequently they do not have goods treated with the same type of sizing agents). In the interests of high productivity, these companies need to operate with a universally applicable technique to enable a right-first-time approach.
There is no need for sophisticated control devices as these are already be available for control of oxidative bleaching. Equipment is no different from modern preparation lines.
Economics The steps and liquors are combined so that the resource consumption is optimised at overall minimal cost.
Driving force for implementation With the increased usage of hydrogen peroxide as a replacement for hypochlorite in bleaching, the cost of hydrogen peroxide will continue to drop relative to other oxidants. Selective use of hydrogen peroxide (minimising non-selective reaction pathways) will be important for reducing overall costs, including raw material, energy and environmental clean-up.
Reference literature [203, VITO, 2001] with reference to:
“Ref. 1995, Catalytic oxidations with oxygen: An Industrial Perspective, Jerry Ebner and Dennis Riley “Ref. 1998, Peroxide desizing: a new approach in efficient, universal size removal, David Levy” “Ref. 1995, Environmentally friendly bleaching of natural fibres by advanced techniques, Ludwich Bettens (SYNBLEACH EV5V – CT 94- 0553) - Presentation given at the European
Workshop on Technologies for Environmental Potection, 31 January to 3 February 1995, Bilbao, Spain – Report 7” 4.5.3 One-step desizing, scouring and bleaching of cotton fabric Description For cotton woven fabric and its blends with synthetic fibres, a three-stage pretreatment process
has been the standard procedure for many years, comprising:
· desizing · scouring · bleaching.
New auxiliaries’ formulations and automatic dosing and steamers allow the so-called “Flash Steam” procedure which telescopes desizing, alkaline cracking (scouring) and pad-steam peroxide bleaching into a single step [180, Spain, 2001].
Main achieved environmental benefits Combining three operations in one allows significant reductions in water and energy consumption.
Operational data Within the space of 2 - 4 minutes (with tight strand guidance throughout) loom-state goods are brought to a white suitable for dyeing. This is a big advantage, especially when processing fabrics that are prone to creasing [180, Spain, 2001].
The chemistry is simple and completely automated with full potential for optimum use.
One of the possible recipes consists of:
· 15 - 30 ml/kg phosphorus-free mixture of bleaching agents, dispersant, wetting agent and detergent · 30 - 50 g/kg NaOH 100 % · 45 - 90 ml/kg H2O2 35 %
The sequence of the “Flash Steam peroxide bleach” is:
1. application of the bleaching solution
2. steam 2 - 4 min (saturated steam)
3. hot wash off.
Cross-media effects None believed likely.
Applicability Companies with new machinery suitable for this process can apply this technique [180, Spain, 2001]. No more detailed information was made available.
Economics No information was made available.
Driving force for implementation Increase in productivity.
Reference plants Several plants in Europe.
Reference literature [180, Spain, 2001] with reference to “International dyer, October (2000), p.10” 4.5.4 Enzymatic scouring Description Enzymatic desizing using amylases is an established process that has been in use for many years. More recently, pectinases have shown promise in replacing the traditional alkaline scouring treatment. Some auxiliaries suppliers have introduced an enzymatic process to remove hydrophobic and other non-cellulosic components from cotton. The new process operates at mild pH conditions over a broad temperature range and can be applied using equipment such as jet machines.
It is claimed that, due to a better bleachability of enzyme-scoured textiles, bleaching can be carried out with reduced amounts of bleaching chemicals and auxiliaries. Enzymes actually make the substrate more hydrophilic (which could explain better bleachability), but they are not able to destroy wax and seeds, which are therefore removed in the subsequent bleaching process.
Main achieved environmental benefits Sodium hydroxide used in conventional scouring treatment is no longer necessary. Furthermore, the following advantages are reported over the traditional procedure (see next table).
Source: [179, UBA, 2001] Table 4.16: Environmental benefits achieved with an enzymatic scouring process Operational data A typical process for a pad-batch process combining scouring and desizing in one single step is
as follows [179, UBA, 2001]:
Cross-media effects The environmental benefits remain unclear as enzymes contribute to the organic load and their action is based on hydrolysis rather than oxidation. The organic load not removed with enzymatic scouring may appear in the later wet processing steps. A more global balance would probably reveal no significant improvement.
Applicability The enzymatic scouring process can be applied to cellulosic fibres and their blends (for both woven and knitted goods) in continuous and discontinuous processes.
When enzymatic desizing is applied, it can be combined with enzymatic scouring.
The process can be applied using jet, overflow, winch, pad-batch, pad-steam and pad-roll equipment.
Economics Price performance is claimed to be economical when considering the total process costs.
Driving force for implementation Quality aspects (good reproducibility, reduced fibre damage, good dimensional stability, soft handle, increased colour yield, etc.), technical aspects (e.g. no corrosion of metal parts) as well as ecological and economical aspects are reported as reasons for the implementation of the enzymatic scouring technique [179, UBA, 2001].
Reference plants Many plants in Europe [179, UBA, 2001].
Reference literature [179, UBA, 2001] with reference to:
“kahle, 2000” Kahle, V.
Bioscouring ein neues, modernes Bio Tech-Konzept Product information, Bayer AG, D-Leverkusen (2000)
4.5.5 Substitution for sodium hypochlorite and chlorine-containing compounds in bleaching operations Description The application of hypochlorite gives rise to subsidiary reactions leading to the formation of a number of chlorinated hydrocarbons such as the carcinogenic trichloromethane (which is also the most frequently formed as it is the end of the reaction chain). Most of these by-products can be detected as adsorbable organic halogens by means of the sum parameter AOX. Similar contributions to the formation of hazardous AOX come from chlorine or chlorine-releasing compounds and strong chlorinated acids (e.g. trichloroacetic acid). Halogenated solvents are a different category of problematic AOX (see also Section 184.108.40.206).
Sodium hypochlorite was for a long time one of the most widely used bleaching agents in the textile finishing industry. Although it has been largely replaced in Germany and many other European countries, it is still in use not only as a bleaching agent, but also for cleaning dyeing machines or as a stripping agent for recovery of faulty dyed goods.
In certain conditions, sodium chlorite may also give rise to the formation of AOX, although to a lesser extent than hypochlorite. However, recent investigations have shown that the cause is not sodium chlorite itself, but the chlorine or hypochlorite present as impurities (from nonstoichiometric production) or used as activating agent. Recent technologies (using hydrogen peroxide as the reducing agent of sodium chlorate) are now available to produce ClO2 without generation of AOX [18, VITO, 1998], [59, L. Bettens, 2000].
Hydrogen peroxide is now the preferred bleaching agent for cotton and cotton blends as a substitute for sodium hypochlorite.
When a single-stage process using only hydrogen peroxide cannot achieve the high degree of whiteness required, a two-stage process with hydrogen peroxide (first step) and sodium hypochlorite (second step) can be applied, in order to reduce AOX emissions. In this way the impurities on the fibre – which act as precursors in the haloform reaction – are removed, thus producing a reduction in AOX in the effluent. Nevertheless, a two-stage bleaching process using only hydrogen peroxide is today possible, thus completely eliminating the use of hypochlorite (cold bleaching at room temperature followed by a hot bleaching step).
There is also increasing support for peroxide bleach under strong alkaline conditions, which achieves a high degree of whiteness after careful removal of catalysts by a reduction/extraction technique. The additional advantage claimed is the possible combination of scouring and bleaching. The reduction/extraction followed by a strong oxidative combined bleaching/scouring step (high alkali and high active oxygen concentration) is applicable for bleaching highly contaminated textiles in all make-ups and on all types of machines (discontinuous and continuous). This method takes the oxidative route and uses the active oxygen.
Main achieved environmental benefits The presence of hazardous AOX such as trichloromethane and chloroacetic acid in the effluent is avoided.
Operational data Particular attention needs to be paid to the combination or sequence of pretreatment operations and to the mixing of streams containing hypochlorite or chlorine. For example, the application of the two-step bleaching method where hypochlorite as well as peroxide is used, is potentially hazardous if the hypochlorite bleaching is performed when large quantities of organohalogen precursors are still present on the substrate. The risk would thus be reduced if hypochlorite Textiles Industry 293 Chapter 4 bleach came as a last step after an alkaline peroxide bleach that removes the precursors from the fibre. However, no data were made available that show the importance of reversing the sequence of the two steps from hypochlorite ® peroxide into peroxide ® hypochlorite. It is actually more important to avoid mixing hypochlorite bleach waste water with certain other streams and mixed effluents, in particular from desizing and washing, even when the right sequence of pretreatment and bleaching is adopted. The formation of organohalogens is highly possible in combined process streams.
For chlorite bleach, handling and storage of sodium chlorite needs particular attention because of toxicity and corrosion risks. Machinery and equipment need to be inspected frequently because of the high stress to which they are subjected (see also Section 220.127.116.11).
Complexing agents (e.g. EDTA, DTPA, phosphonates) are normally applied as hydrogen peroxide stabilisers. The main concerns associated with the use of these substances arise from their ability to form stable complexes with metals (remobilisation of heavy metals), their N- and P- content and their often low biodegradability/bioeliminability. The addition of strong sequestering agents, however, can be avoided by fine control of the pH conditions during the bleaching process (see Section 4.5.6) and with the assistance of silicates, magnesium, acrylates or biologically degradable carboxylates, slowing down the uncontrolled decomposition of hydrogen peroxide (see Section 4.3.4).
Optical brighteners are often applied when peroxide bleaching is not sufficient to achieve the required level of whiteness. The resulting COD load and smoke during fixation in the stenter have to be taken into account. Moreover, optical whiteners are potentially irritating and thus not always acceptable for white goods coming into close contact with the skin (e.g. underwear, bedsheets).
Applicability Substitution for hypochlorite as bleaching agent is applicable to both new and existing installations.
Hydrogen peroxide is a valid substitute for bleaching yarn and woven fabric made of most cellulosic and wool fibres and most of their blends. Today a full hydrogen peroxide bleaching process is also applicable to cotton & cotton-blend knitted fabric and a high degree of whiteness (75 BERBER Whiteness Index) can be obtained (with a strong alkaline scour/bleach after removal of the catalyst).
Exceptions are flax and other bast fibres that cannot be bleached using peroxide alone. Unlike chlorine dioxide, the anionic bleaching agent is not strong enough to remove all coloured material and does not preferentially access the hydrophobic region of the fibre. A two-step hydrogen peroxide-chlorine dioxide bleaching is an option for flax.
It is claimed that a sequence where precursors of halogenation are removed with a peroxide bleach followed by a hypochlorite bleach (or a peroxide pre-bleach followed by a combined hydrogen peroxide/hypochlorite bleach) is still necessary for high whiteness and for fabrics that are fragile and would suffer from depolymerisation.
Sodium chlorite is an excellent bleaching agent for flax, linen and some synthetic fibres.
Economics In general, bleaching with hydrogen peroxide is no more expensive than with hypochlorite because of market saturation.
The two-stage bleaching process with hydrogen peroxide proposed for knitted fabric is reported to be from two to six times more expensive than the conventional process using hydrogen peroxide and hypochlorite [179, UBA, 2001].
If using chlorine dioxide as bleaching agent, investment may be needed (in existing installations) for equipment resistant to the highly corrosive conditions in which this bleaching agent is used.
As far as the production of elemental chlorine-free chlorine dioxide is concerned, this process is fully investigated and described in another BREF (pulp & paper industry).
Driving force for implementation Market demands for chlorine-free bleached textiles and the requirements set by legislation (regarding waste water discharge) are the main driving forces for the implementation of this technique.
Reference plants Many plants in Europe and worldwide largely use substitutes for sodium hypochlorite as bleaching agent.
Reference literature [179, UBA, 2001], [51, OSPAR, 1994] (P059, P063), [203, VITO, 2001].