Page 155 - Basic Principles of Textile Coloration

144 WATER TREATMENT

salts with dyes, or form complexes that are invariably duller and even different in
shade. If the amounts are excessive, the water may be unusable without treatment.
On adequate aeration of the water at pH values around 7, iron precipitates as
insoluble Fe(OH)3 and can be removed. There is always concern about the
possibility of iron from corrosion inside the water pipes in a textile plant.

Addition of a sequestering agent to the water avoids many problems from
relatively low concentrations of undesirable metal ions. Sequestering agents react
with the metal ions to form very stable complex ions. Examples of such chemicals
used in textile processing include EDTA, and related aminocarboxylic acids, as
well as polyphosphates such as sodium tetrametaphosphate Na4P4O12. EDTA is a
most effective sequestering agent, particularly in neutral or weakly alkaline
solution. It forms such stable complexes with metal ions that it often removes the
metal ion from a metal–complex dyestuff molecule, to give the EDTA–metal
complex and uncomplexed dye.

Polyphosphates can bind alkaline earth metal ions and thus decrease the effects
of water hardness. The product Calgon, sodium hexametaphosphate Na6P6O18, is
widely used for this purpose. Calcium ions replace sodium in the
hexametaphosphate, forming a stable complex. The free calcium ion
concentration in the water is then so low that calcium soaps do not precipitate.
Polyphosphates are frequently present in domestic washing powders for the same
reason.

8.4 BOILER WATER

Boilers for steam generation have varying operating pressures and capacities. The
use of a high pressure boiler allows a greater generation capacity but to minimise
some of the problems discussed below requires a much higher water quality than a
low pressure system.

Temporary hardness in boiler feed water gives an accumulation of chalk scum in
the boiler and scale on the walls and heating tubes. Deposition on the latter
greatly reduces the rate of heat transfer and the boiler becomes increasingly less
efficient. At high temperatures and pressures, both calcium and magnesium
carbonates and magnesium hydroxide are much less soluble than under ambient
conditions and contribute to scaling. Simple phosphates such as Na2HPO4, added
to boiler feed water, will precipitate insoluble calcium and magnesium phosphate
in a form that does not form a crust on the boiler walls and pipes. Polyphosphate