Cost reduction in dyeing of cotton with reactive dyes
COST REDUCTION IN DYEING OF COTTON WITHREACTIVE DYESR.B.ChavanDepartment of Textile TechnologyIndian Institute of TechnologyHauz-Khas, New Delhi 110016E-mail: email@example.comIntroductionAmong the various fibres and dye classes, cotton and reactive dye system is the mostpopular. Reactive dye molecules are characterised by a chromophore, to impart colour,attached to a reactive group capable of reacting with cellulose, and thereby binding thechromophore to the cellulose covalently.The total world-wide production of cotton is estimated at 21 x 106 te p.a. (metric tonnesper annum). The corresponding amount of reactive dye used for cotton coloration isestimated to be 120,000 te p.a. Of this about two thirds is dyed by exhaust technology.Salient statistics, which follow from these figures, include: 4 x 106 te p.a. of cotton is exhaust dyed with reactive dyes; 4 x 108 te p.a. of fresh water is used in the overall process, and all of this is ultimately discarded in a contaminated state; 2.8 x 106 te p.a. of salt is used in the process; and all of this is ultimately discarded in the aqueous effluent; 8 x 104 te p.a. of reactive dye is applied, with an average fixation yield of 70%, thus: 2.4 x 104 te p.a. of dye is discarded in the aqueous effluent.The reactive dye business, due to its maturity, is very competitive and price sensitive. Asolder dye technologies have moved out of patent, these have been taken up bymanufacturers in the developing world, driving down manpower costs and total
production costs. Therefore, a large proportion of cotton processing has also moved to thecheaper and less environmentally conscious economies of the developing world.In the present paper a modified process for rinsing after dyeing has been suggested forcost reduction and environment benefits.Reactive dyeing processThe process for reactive dyeing of cotton can be divided into three steps: the pre-treatment, the dyeing and the rinsing after dyeing. Traditionally, the consumption ofenergy, chemicals and water in rinsing is crucial; approximately half of the total energyconsumption and of the total water consumption are attributed to the rinsing processafter dyeing. Therefore there is much scope for cost reduction in rinsing operation.Water consumption in reactive dyeing.During the pre-treatment, the cotton fabric is scoured and bleached and washed. Aftersome rinses, the dyestuff is poured into the dye bath and a diffusion of the dyestuffmolecules between the cellulose fibres takes place. After some time, salt is added toobtain adsorption of the dyestuff to the cellulose fibre. After this, adjusting temperature(50-80°C) and pH (10,5-11,5) completes the reaction between the dyestuff and thecellulose. Some of the dyestuff will be hydrolysed during this dyeing process, and theadsorbed hydrolysate must be removed in the succeeding rinsing after dyeing.The rinsing traditionally consists of several baths, as in table 1.Table 1 Typical water consumption in reactive dyeing (100 kgbatch) Step Process Water Litres 1 Dyeing 700 2 Overflow rinse 7300 3 Warm rinse 700
4 Neutralisation 700 5 Overflow rinse 7300 6 Hot soaping 700 7 Warm rinse 700 8 Overflow rinse 4300 9 Hot soaping 700 10 Warm rinse 700 11 Overflow rinse 4300 12 Neutralisation & softening 700 Total water 20800The large water consumption in the rinsing after dyeing is primarily caused by the largenumber of baths but also by the common use of overflow rinses. Before the temperatureis raised in the rinse, the dyestuff producers recommend neutralisation to pH around 8,when dyestuffs with vinyl sulphone reactive groups are used. This neutralisation has,however, in some dye-houses, become usual practice for all sorts of reactive dyestuffs.After neutralisation, the rinsing consists of a number of soaping sequences: hot soaping,warm rinse and overflow rinse. In the hot soaping steps 6 and 9 in table1 soapingadditives are used, in the form of surface active agents (detergents), complexing agentsand dispersing agents. The reasons for the use of these auxiliary agents are protectionagainst hardness in the water and/or the cotton, and keep the unfixed dye in dispersedform.The process is completed with neutralisation to pH around 7 and treatment withsoftening agents, necessary for the subsequent sewing process.
Cost saving rinsing operationNew water saving, chemical free, high temperature and high speed rinsing steps areshown in Table 2Table 2 Modified rinsing processStep Process water (Litre)1 Dyeing 7002 Hot rinse 7003 Hot rinse 7004 Hot rinse 7005 Neutralisation & Softening 700 Total 350050 industrial scale trials with the new recipe documented that a chemical free, hightemperature rinse, using a reduced number of rinses, and thus saving water, chemicalsand process time can be implemented in the dye house with no adverse effect on productquality. When implementing the water saving, chemical free, high temperature and highspeed rinse after reactive dyeing of cotton in batch, the following cost reduction andcleaner production options should be considered:A) Change from overflow rinsing to stepwise rinsing.Rinsing by overflow, i.e. pouring clean cold water directly into the process water in themachine while excess water is drained out of the machine, is used both for rinsing and forcooling purposes. Overflow is quick but causes unnecessary water consumption.Table 3 Stepwise rinsing as substitute for overflow rinse.A Fill the machine according to liquor ratioB 10 minutes rinsing
C Discharge rinsing waterD 5 minutes drainingB) Omit the use of detergents in the rinsing after reactive dyeing of cotton.Surplus and un-fixed reactive dyestuffs are highly water-soluble, in spite of this ,detergents are often used during rinsing after dyeing. In international literature, it hasbeen documented that detergents do not improve removal of hydrolysed reactivedyestuffs from the fabric. More than 50 full-scale dyeings carried out at variousdye-houses without the use of detergents. All have successfully proven thatdetergents can be omitted without negative impact on product quality.C) Omit the use of complexing agents in the rinsingIf soft water with a quality of below 5h dH is used, complexing agents can be omittedwithout any negative effects on dyed fabric. However, if hardness builders e.g. calciumand magnesium are present in the dyeing processes and in the rinsing after dyeing, theymight have a negative effect on the dyeing result, e.g. change in shade or problems withreproducibility. For that reason, soft water is recommended as standard procedure in thedyeing processes. However, water softening in the dyeing machine by using complexingagents, forming bonds with the hardness-builders, are both economically andenvironmentally a bad solution. Water softening can profitably be done in a separateplant by the ion-exchange technique or the membrane filtration technique.D) Use only neutralisation after dyeing when using Vinyl sulphone(VS)reactive dyestuffs.Neutralisation in the first rinse after dyeing can be restricted to the vinyl sulphone (VS)reactive groups. Some VS dyestuffs have poor alkaline washing fastness (low bondstability) and thus sensitive to high pH and high temperature simultaneously.Nevertheless, it is not uncommon that all recipes for reactive dyeing in a dye-house
include neutralisation in the first rinse after dyeing, whether VS reactive dyestuffs areused or not.The dyeing can be successfully carried out without the use of neutralisation in the firstrinse after dyeing. This in spite of the fact that more than half of the dyeings were carriedout with dyestuffs based on VS-groups. As it is not possible to put forward generalguidelines on when to neutralise dyestuffs based on VS-groups, it is recommendedalways to neutralise these. There is no reason to neutralise in this step when all otherreactive dyestuffs are used, e.g. based on monochlorotrazine (MCT), dichlorotriazine(DCT), trichloropyrimidine (TCP) or difluorochloropyrimidine (DFCP).In general, it is recommended to select dyestuffs with a superior alkaline washingfastness when selecting VS-dyestuffs.E) Chemical-free high speed rinsing after reactive dyeing of cotton.Tests have shown that rinsing is more effective and faster at elevated temperatures – e.g.around 30% more unfixed hydrolysed reactive dyestuff is rinsed out after 10 minutes at95°C than at 75°C.Tests using hot 90-95°C rinsing after reactive dyeing of cotton have proved that thetechnique has no negative effects on the dyeing results. Most often the fastness of thegoods were better after the hot rinsing than after the traditional rinsing with overflow,detergents, complexing agents and neutralisation in the first rinse. Furthermore, whenusing 90-95°C rinsing water, a few stepwise rinses (table 2) can reduce the rinsing timeby around 50% compared to a standard recipe (table 1).Main achieved cost and environmental benefitsThe benefits of the new rinsing operation are shown in Figure 1
Fig. 1 Benefits of step wise rinsing operation 1. Reduction in water consumption and wastewater generation. 2. Cost saving in chemical consumption and reduction in pollution load of waste water 3. Time and energy saving.2.1.4 Applicability1. The new process can be implemented in all types of textile companies involved in reactive dyeing of cotton.2. The new process can only be implemented if the company do have availability of soft groundwater or is operating with a soft-water system (which is normally the case).3. It is recommended always to neutralise in the first rinse when dyestuffs based on VS-groups are used. There is no reason to neutralise in this step when all other reactive dyestuffs based on monochlorotrazine (MCT), dichlorotriazine (DCT), trichloropyrimidine (TCP) or difluorochloropyrimidine (DFCP).
Economics1. The economic feasibility is obvious. 50-70% reduction in the consumption of water for rinsing. Total savings will depend on the number of reactive dyeings at the company.2. Omit the use of detergents, complexing agents and acetic acid. Savings will depend on the number of reactive dyeings at the company.Driving force for implementation1. High costs for water and wastewater discharge and/or low availability for water of appropriate quality.2. High costs for chemicals and wastewater load.3. A desire for reduced operation time per lot and increased capacity per machine.