«EUROPEAN COMMISSION Integrated Pollution Prevention and Control (IPPC) Reference Document on Best Available Techniques for the Textiles Industry July 2003 ...»
4.5.6 Minimising consumption of complexing agents in hydrogen peroxide bleaching Description When bleaching with hydrogen peroxide, oxygen species of differing reactivity may be present in water (O2**, H2O2/HOO-, H2O/OH-, HOO*/O2*-, OH*/O*-, O3/O3*-). The kinetics of formation and disappearance depend on concentration of oxygen, energy for activation, reduction potential, pH, catalyst and other reagents. These processes are very complex and can only be explained with dynamic simulation models. It is widely accepted that the OH* radical is responsible for attacking the cellulose fibre and leading to its damage (depolymerisation) and that the formation of the OH* radical is mainly due to the reaction of H2O2/HOO- with transition metals such as iron, manganese and copper. The prevention of “catalytic” damage of the fibre as a consequence of the uncontrolled formation of OH* radical is usually achieved by using complex formers that inactivate the catalyst (stabilisers). See also Section 8.5.
Complexing agents (see Figure 4.6) that are typically applied in finishing mills are based on polyphosphates (e.g. tripolyphosphate), phosphonates (e.g. 1-hydroxyethane 1,1-diphosphonic acid) and amino carboxylic acids (e.g. EDTA, DTPA and NTA). The main concerns associated with the use of these substances arise from their N- and P- content, their often-low biodegradability/bioeliminability and their ability to form stable complexes with metals, which may lead to the remobilisation of heavy metals (see also Section 8.5).
The use of high quantities of sequestering agents can be avoided by removing the responsible catalysts from the water used in the process and from the textile substrate and by scavanging away the OH*.
Softening of fresh water is largely applied by textile mills to remove the iron and the hardening alkaline-earth cations from the process water (magnesium hydrate has a stabilising effect and techniques that remove transition metals and calcium are therefore preferred).
Iron carried with the raw fibre can be present as fibre impurity, rust or coarse iron particles on the surface of the fabric. The latter can be detected and removed by a dry process using magnetic detectors/ magnets (modern continuous lines are equipped with magnetic detectors).
This treatment is convenient when the process starts with an oxidative scouring/desizing step, because otherwise a huge amount of chemicals would be required to dissolve these coarse iron particles in a wet process. On the other hand, the previous removal of coarse iron particles is not necessary when an alkaline scouring treatment is carried out as a first step before bleaching.
Magnetic sensors cannot detect non ferromagnetic particles and magnets cannot remove the iron that is inside the fibre (fibre impurities and rust in heavily contaminated goods). This iron fraction has to be solubilised and removed from the substrate by acid demineralisation or reductive/extractive treatment before bleaching. In the case of acid demineralisation, Fe(III) oxide, iron metal and many other forms of iron (some organic complexes) are solubilised in strongly acid conditions (by hydrochloric acid at pH 3). This means that the metal parts of the equipment must withstand these conditions. The advantage of the reductive treatment is that there is no need to use strong corrosive acids. Moreover, with the new non-hazardous reductive agents (see Section 4.6.5), it is possible to avoid a drastic change of pH.
As mentioned above, OH* radicals can be scavanged away in order to minimise fibre damage without the need for complexing agents.
In-depth research into the reactivity of hydrogen peroxide (SYNBLEACH EV5V-CT94-0553 EC funded research project) has shown that the control of the process is fundamental to prevent uncontrolled decomposition of hydrogen peroxide and to allow optimum use of hydrogen peroxide.
Figure 4.11 shows that under optimal conditions (pH approximately 11.
2, homogeneously distributed catalyst and controlled peroxide concentration) the hydroxyl radical OH* is scavenged away by hydrogen peroxide, forming the true bleaching agent, the dioxide radical ion (maximum formation of dioxide radical anion O2*- in accordance with the peak). Under these conditions hydrogen peroxide itself acts as a scavenger and the reaction product is the active bleaching agent itself (which allows optimal use of hydrogen peroxide). The addition of formic acid (formate ion) as scavenging agent is also useful to further control the formation of the OH* radical, generating more O2*- and even repairing damage to the fibre.
Figure 4.11: Production of the peroxide radical ion by scavenging hydroxyl radicals (OH*) using hydrogen peroxide [203, VITO, 2001] Main achieved environmental benefits With the proposed technique it is possible to bleach cellulose in full and even to high whiteness,
without damage to the fibre with:
· no use of hazardous sequestering agents · minimal consumption of peroxide (50 % compared with uncontrolled conditions) · (pre-)oxidation of the removed substances.
As mentioned above, as an alternative to acid demineralisation, pre-cleaning of heavily soiled fabric (rust) is possible in more alkaline conditions using non-hazardous reducing agents, without any need for a drastic change in pH. The reduction/extraction is effective for all types of substrates and qualities of fabrics (highly contaminated, uneven distribution of iron-rust). This step is easy to integrate with discontinuous and continuous pocesses following the oxidative route under mildly or strongly alkaline bleach conditions [203, VITO, 2001].
Cross-media effects None to be expected.
Applicability The measures described in this section may be generally applicable to existing and new plants.
However, fully automated equipment is necessary for the application of hydrogen peroxide under controlled process conditions. Dosing of the bleaching agent, controlled by a dynamic simulation model, is still limited [203, VITO, 2001].
Reduction of peroxide consumption by more than 50 % is possible. There is no increase, but rather a decrease in organic load, along with better treatability of the effluent. The chemistry needed is not expensive and is reliable, provided that there is a good knowledge of the complex control parameters [203, VITO, 2001].
Reference plants The technique described in this section is provided directly by some auxiliaries suppliers. With the help of dynamic simulation models they are able to prepare a recipe that is suitable for the specific substrate, equipment used, etc. under defined process conditions.
Reference literature [203, VITO, 2001] with reference to:
“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”.
4.5.7 Recovery of alkali from mercerising Description During the mercerisation process, cotton yarn or fabric (mainly woven fabric but also knit fabric) is treated in a solution of concentrated caustic soda (270 - 300 g NaOH/l, or also 170 - 350 g NaOH/kg textile substrate) for about 40 - 50 seconds. The textile substrate is then rinsed in order to remove caustic soda. This rinsing water is called weak lye (40 - 50 g NaOH/l) and can be concentrated by evaporation for recycling. The principle is shown in the figure below.
Figure 4.12: Representation of the caustic soda recovery process by evaporation followed by lye purification [179, UBA, 2001] After removal of lint, fluff and other particles (using self-cleaning rotary filters or pressure micro-filtration), the weak lye is first concentrated, for instance in a three-stage evaporation process.
In many cases, purification of the lye is required after evaporation. The purification technique depends on the degree of lye contamination and can be simple sedimentation or oxidation/flotation with injection of hydrogen peroxide.
Main achieved environmental benefits The alkaline load of waste water is reduced drastically and acid required for waste water neutralisation is minimised.
Operational data The concentration of weak lye is usually 5 - 8 °Bè (30 - 55 g NaOH/l) and is increased to 25 - 40 °Bè (225 - 485 g NaOH/l), depending on the mercerising process applied. When mercerisation is carried out on the greige dry textile substrate (raw mercerisation) it is possible to achieve a concentration of caustic soda not higher than 25 - 28 °Bè, whereas a concentration of 40 °Bè can be obtained in non-raw mercerisation. In raw mercerisation, the concentration of impurities is significantly higher, as is viscosity, which makes it difficult to reach higher concentrations (circulation in evaporators is disturbed) [179, UBA, 2001].
The higher the number of stages for evaporation, the more often the heat is re-used, the lower the steam consumption and, therefore, the running cost. Investment, however, obviously increases with the number of stages [179, UBA, 2001].
Cross-media effects Evaporation requires approximately 0.3 kg steam /kg water evaporated in a 4-stage evaporation plant. This corresponds to 1.0 kg steam/kg of recovered NaOH at 28 °Bé or 1.85 kg steam / kg of recovered NaOH at 40 °Bé.
Applicability The technique is applicable to both existing and new installations.
Due to the action of active oxygen generated by the decomposition of hydrogen peroxide it is possible to recover and decontaminate coloured alkali for re-use (hydrogen peroxide is already used in the water stream when applying the oxidative route – see Section 4.5.2).
Economics Investment costs mainly depend on plant size and purification technique and typically vary from 200000 to 800000 euros. Payback time depends on plant size and operating time per day.
Usually, if mercerisation is practised full-time, the payback period is less than one year. In companies where non-recovered caustic soda lye has to be neutralised with acid, payback time is less than six months. Thus, from the economic point of view, caustic soda recovery may be very attractive [179, UBA, 2001].
Driving force for implementation High alkali content of waste water and economic aspects of caustic soda losses are the main driving forces [179, UBA, 2001].
Reference plants The first caustic soda recovery plant went into operation more than one hundred years ago.
Today, there are more than 300 plants in operation worldwide, especially for recovery of caustic soda from woven fabric mercerisation and yarn mercerisation and some from knit fabric mercerisation (the latter process is not applied very often).
Main suppliers in Europe are:
· KASAG Export AG, CH-9259 Kaltenbach, Switzerland · Körting Hannover AG, D-30453 Hannover, Germany Reference literature [179, UBA, 2001], [5, OSPAR, 1994] P040.
4.5.8 Optimisation of cotton warp yarn pretreatment Description In the production of white, non-dyed cotton sheets (e.g. sheets to be used under bed sheets and table-cloths) cotton warp yarn is bleached before weaving (for the production of this type of article the fabric does not need to be desized after the weaving process).
The conventional process consists of five steps, including wetting/scouring, alkaline peroxide bleaching and three subsequent rinsing steps. The last rinsing water is re-used for making the first bath.
This process can be further improved by combining wetting, scouring and bleaching in one step and performing rinsing in two steps, re-using the second rinsing bath for making the bleaching/scouring bath (as above).
In addition, the energy consumption of the process has been reduced by heat recovery. The heat from the scouring/bleaching bath (110°C) is recovered (by means of a heat exchanger) and used for heating the fresh water for the first rinsing. The bath is therefore cooled to about 80°C, while the fresh water reaches a temperature of 60 – 70 °C.
This cooled scouring/bleaching bath is collected in a tank together with the warm rinsing water from the first rinsing step. This waste water still has a valuable energy content. Therefore, before being drained, this stream is used to heat the water from the second rinsing step (which is then used for making the bleaching/scouring bath as explained above).
Main achieved environmental benefits Water consumption and waste water discharge before and after optimisation can be seen from the following table: 50 % reduction of water consumption is achieved.
Operational data The operating conditions of the optimised process are illustrated in Table 4.18, which also contains the calculation of COD-input and -output.
Table 4.18: Optimisation of warp yarn scouring/bleaching: recipe and operating conditions for the optimised process Cross-media effects None believed likely.
Applicability The optimisation of the process is possible for both existing and new installations. For the recovery of heat, space for additional tanks is required, which may be a limiting factor in some cases. The quality of the cotton yarn has to be considered (as regards content of iron, seeds etc.) in order to make sure that the process can be applied.
The considerable savings of time, water, chemicals and energy make the process highly economic. The optimised process does not require new equipment for pretreatment, but tanks, heat exchangers, pipes and control devices for energy recovery from waste water are required.
Driving force for implementation Environmental motivation has been the main driving force for the development of the process, but the economic benefit also justifies the investment of effort.
Reference plants Two textile finishing plants in Germany are using the described optimised process successfully.
Reference literature [179, UBA, 2001] with reference to:
“van Delden, 2001” van Deleden, S.
Prozessoptimiwerung durch Wasserkreislaufführung und Abwasservermeidung am Beispiel einer Kettbaumbleiche Proceedings of BEW-Seminar "Vermeidung, Verminderung und Behandlung von Abwässern der Textilindustrie" on 06.03.2001 (2001)
4.6 Dyeing 4.6.1 Exhaust dyeing of polyester and polyester blends with carrier-free dyeing techniques or with use of environmentally optimised carriers Description Due to the high glass transition point of polyethyleneterephthalate, which is in the range of 80 - 100 °C, the diffusion rate of disperse dyestuff molecules into the standard PES fibres (PET based) at normal dyeing temperatures is very low. As a result, dyeing conditions typically used for other types of substrates are not applicable. Exhaust dyeing of single polyester and polyester blends can be carried out either in autoclaves at high temperature (HT-dyeing at 130 °C, which is usually applied for pure PES and wool-free PES blends) or at normal dyeing temperatures (95 °C – 100 °C, which is applied for PES/wool blends) with the help of so-called carriers (see also Sections 18.104.22.168, 22.214.171.124, 2.7.7 and 8.6.7).
Carriers are absorbed to a great extent onto the PES fibre. They improve fibre swelling and encourage colourant migration. In dyeing and rinsing a significant amount of carriers is emitted to waste water. The fraction that remains on the fibre may be emitted to air during subsequent drying, heatsetting and ironing.
Active substances used in carrier formulations include: