WATER QUALITY FOR THE DYEHOUSE

During the Industrial Revolution, the textile industry invariably grew in regions having an abundant supply of soft water such as in West Yorkshire. Such water is relatively free from calcium and magnesium salts. It does not precipitate soap or other chemicals during washing, or form a scale on the boiler walls when generating steam. Water for a textile plant may come from various sources. These include surface water from rivers and lakes, and subterranean water from wells. The water may be obtained directly from the source or from the local municipality. Natural and pretreated water may contain a variety of chemical species that can influence textile wet processing in general, and dyeing in particular. The various salts present in water depend on the geological formations through which the water has flowed. These salts are mainly the carbonates (CO32–), hydrogencarbonates (HCO3–, more commonly named bicarbonates), sulphates(SO42–) and chlorides (Cl–) of calcium (Ca2+), magnesium (Mg2+) and sodium(Na+). Although calcium and magnesium carbonates in limestone are relatively insoluble, the action of dissolved carbon dioxide in rain water can leach them out in the form of the more soluble bicarbonates (Scheme 8.1). Hard water contains calcium and magnesium ions and gives an immediate precipitate with soap. Soap is a water-soluble sodium salt of a high molecular weight carboxylic acid, such as stearic acid. Its calcium and magnesium salts are much less soluble and give a gummy precipitate with a soap solution in hard water (Scheme 8.2). When this happens, the cleaning efficiency of the soap decreases dramatically. The dirt in suspension, as well as the precipitated calcium and magnesium soaps, can deposit back onto the material being washed. This gives a dingy fabric with a poor handle that will be difficult to dye in a level shade. CaCO3(s) + CO2(aq) + H2O(l) = Ca(HCO3)2(aq) Scheme 8.1 2CH3(CH2)16CO2Na+ (aq) + Ca2+ (aq) = (CH3(CH2)16CO2-)2 Ca2+ (s) + 2Na(aq) Scheme 8.2 Besides dissolved salts of natural origin, water may contain a variety of other salts from human or industrial activity. These include nitrates (NO3–), phosphates(HPO42– and H2PO4–) and various metal ions. The ions of certain transition metals, such as those of iron, copper and manganese, can precipitate dyes, or form complexes with them, dulling or even changing their shade. In addition, many of these ions catalyse the decomposition of hydrogen peroxide in bleaching baths. It is not uncommon for a particle of iron rust on the surface of a cotton fabric to generate such rapid peroxide decomposition that the cellulose is totally oxidised and a hole results. These transition metal ions may be present in the natural or municipal water, or, in the case of iron, come from corrosion within pipelines. Both dissolved and colloidal silica in boiler feed water can lead to the formation of a hard resistant scale on the boiler’s internal surfaces, considerably decreasing heat transfer efficiency. Organic compounds from the decomposition of vegetable matter in the water, or from sources of pollution, can be very varied. They can have such high concentrations that the water is coloured. Table 8.1 illustrates the quality of water usually considered acceptable for textile processing and steam generation. Water entering a textile dyeing or finishing plant usually passes through a grill to eliminate floating debris. Small particles in suspension must also be removed.