Hardness is defined as the presence of soluble calcium and magnesium salts in the water. If these are present in the form of bicarbonates, the hardness is temporary. Heating hard water containing bicarbonates eliminates dissolved carbon dioxide and the causes precipitation of calcium carbonate. Magnesium carbonate is slightly soluble in water but heating will cause its hydrolysis into the much less soluble magnesium hydroxide . Simply boiling and filtering the water therefore eliminates temporary hardness. In regions where water has high temporary hardness, and is used directly without treatment, it is not uncommon to see hot water rinsing and washing baths with a generous crust of chalk (CaCO3) on the inner surfaces. This type of precipitation inside a boiler is also undesirable because the scale reduces the efficiency of heat transfer.
MgCO3(aq) + H2O = Mg(OH)2(s) + CO2(g)
The use of hard water in a textile dyeing or finishing mill can have some serious consequences. These include:
(1) precipitation of soaps;
(2) redeposition of dirt and insoluble soaps on the fabric being washed – this can cause yellowing and lead to unlevel dyeing and a poor handle;
(3) precipitation of some dyes as calcium or magnesium salts;
(4) scale formation on equipment and in boilers and pipelines;
(5) reduction of the activity of the enzymes used in desizing;
6) decreased solubility of sizing agents;
(7) coagulation of some types of print pastes;
(8) incompatibility with chemicals in finishing recipes.
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