Cleaner Technology Transfer to the Polish Textile ... - Miljøstyrelsen

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The strategy for the water reclamation research has been to introduce reclamation
and re-use closely integrated in the dyeing process. This implies to work
upstream, where water characteristics are still process specific, and not downstream,
where substreams have been mixed and water characteristics represent
an overall average. This strategy is believed to be optimal as long as large scale
advantages and flexibility are not lost by the tight process integration. For the
water types in reactive dyeing of cotton the strategy has been found very suitable,
and our experience shows that it will result in the environmentally and
economically optimal solution.
Furthermore, and not least important, the strategy has been to seek for re-use
not only of water but also of the energy and chemical content in the water.
Recipes
In dyeing processes the recipe is the fundamental specification of the process.
Water consumption, chemical consumption, temperature, salinity, pH, etc. are
all specified by the recipe. Process water characteristics, therefore, are given
by the recipe and the quality of the cotton.
A comprehensive investigation of recipes in reactive dyeing of cotton form the
background for the research of optimisations and water re-use. The essential
process water characteristics have been found to be: dye-stuff content, salinity,
temperature, COD (deriving from additives such as acetic acid, detergents
and complexing agents), pH, and to some extent suspended solids incl. cotton
fibres. Essential points to the applicability of water reclamation techniques
are:
Dye-stuff content Reactive dyes are typically azo-based chromophores combined with different
types of reactive groups (Ciba-Geigy, 1987). The relative large chromophores
place reactive dyes in the molarweight area of 700-1000 g/mol. The reactive
groups can be based on e.g dichlortriazin or vinyl sulphonic acid and will be
negatively charged. Beside the reactive groups, other negatively charged
groups are found, too, typically sulfonic acid groups. The reactive groups react
with hydroxyl groups in the cellulose fibre during the dyeing reaction. However,
a relatively large part of the reactive groups will react with water, leaving
then the dye-stuff at an unreactive stage. Due to the properties of the chromophore,
the dye-stuff is quite adsorptive to the cellulose, and the hydrolysed
dye-stuff requires large amounts of water, preferably at high temperature, in
the preceeding rinses to be washed out properly.
Salinity Salinity in the dye-bath typically varies in the area of 40.000-80.000 mg/l. In
the preceeding rinses, salinity will decrease typically by a factor 3 for the
batchwise rinse and much more by the very water consuming »over flow«
rinse.
Temperature At pH above 10 and temperatures above 50-60°C certain types of reactive dyestuffs
are sensitive to breaking the established covalent bond to the cellulose.
Therefore, the dye-bath (having pH >10) and the first batch rinse to follow
need a temperature not higher than 50-60°C. After this first rinse a temperature
of 90-95°C is preferable to the rinsing. For economical reasons some of
the rinses are often kept colder.