Introduction

Among the various fibres and dye classes, cotton and reactive dye system is the most popular. Reactive dye molecules are characterised by a chromophore, to impart colour, attached to a reactive group capable of reacting with cellulose, and thereby binding the chromophore to the cellulose covalently.

The total world-wide production of cotton is estimated at 21 x 106 te p.a. (metric tonnes per annum). The corresponding amount of reactive dye used for cotton coloration is estimated 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. As older dye technologies have moved out of patent, these have been taken up by manufacturers in the developing world, driving down manpower costs and total production costs. Therefore, a large proportion of cotton processing has also moved to the cheaper and less environmentally conscious economies of the developing world.

In the present paper a modified process for rinsing after dyeing has been suggested for cost reduction and environment benefits.


Reactive dyeing process

The 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 of energy, chemicals and water in rinsing is crucial; approximately half of the total energy consumption and of the total water consumption are attributed to the rinsing process after 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. After some rinses, the dyestuff is poured into the dye bath and a diffusion of the dyestuff molecules between the cellulose fibres takes place. After some time, salt is added to obtain adsorption of the dyestuff to the cellulose fibre. After this, adjusting temperature (50-80C) and pH (10,5-11,5) completes the reaction between the dyestuff and the cellulose. Some of the dyestuff will be hydrolysed during this dyeing process, and the adsorbed hydrolysate must be removed in the succeeding rinsing after dyeing.

The rinsing traditionally consists of several baths, as in Table 1.



The large water consumption in the rinsing after dyeing is primarily caused by the large number of baths but also by the common use of overflow rinses. Before the temperature is 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 soaping additives are used, in the form of surface active agents (detergents), complexing agents and dispersing agents. The reasons for the use of these auxiliary agents are protection against hardness in the water and/or the cotton, and keep the unfixed dye in dispersed form.

The process is completed with neutralisation to pH around 7 and treatment with softening agents, necessary for the subsequent sewing process.


Cost saving rinsing operation

New water saving, chemical free, high temperature and high speed rinsing steps are shown in Table 2



50 industrial scale trials with the new recipe documented that a chemical free, high temperature rinse, using a reduced number of rinses, and thus saving water, chemicals and process time can be implemented in the dye house with no adverse effect on product quality. When implementing the water saving, chemical free, high temperature and high speed rinse after reactive dyeing of cotton in batch, the following cost reduction and cleaner 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 the machine while excess water is drained out of the machine, is used both for rinsing and for cooling purposes. Overflow is quick but causes unnecessary water consumption.



B) 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 has been documented that detergents do not improve removal of hydrolysed reactive dyestuffs from the fabric. More than 50 full-scale dyeings carried out at various dye-houses without the use of detergents. All have successfully proven that detergents can be omitted without negative impact on product quality.


C) Omit the use of complexing agents in the rinsing

If soft water with a quality of below 5 dH is used, complexing agents can be omitted without any negative effects on dyed fabric. However, if hardness builders e.g. calcium and magnesium are present in the dyeing processes and in the rinsing after dyeing, they might have a negative effect on the dyeing result, e.g. change in shade or problems with reproducibility. For that reason, soft water is recommended as standard procedure in the dyeing processes. However, water softening in the dyeing machine by using complexing agents, forming bonds with the hardness-builders, are both economically and environmentally a bad solution. Water softening can profitably be done in a separate plant 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 bond stability) 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 are used or not.

The dyeing can be successfully carried out without the use of neutralisation in the first rinse after dyeing. This in spite of the fact that more than half of the dyeings were carried out with dyestuffs based on VS-groups. As it is not possible to put forward general guidelines on when to neutralise dyestuffs based on VS-groups, it is recommended always to neutralise these. There is no reason to neutralise in this step when all other reactive 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 washing fastness 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 at 95C than at 75C.

Tests using hot 90-95C rinsing after reactive dyeing of cotton have proved that the technique has no negative effects on the dyeing results. Most often the fastness of the goods were better after the hot rinsing than after the traditional rinsing with overflow, detergents, complexing agents and neutralisation in the first rinse. Furthermore, when using 90-95C rinsing water, a few stepwise rinses (table 2) can reduce the rinsing time by around 50% compared to a standard recipe (table 1).

Main achieved cost and environmental benefits

The benefits of the new rinsing operation are shown in Figure 1



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 Applicability

1. 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).


Economics

1. 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 implementation

1. 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.