Hard Water Softening Methods For Dyeing and Finishing Water

Water containing impurities such as calcium and magnesium ions is called hard water. The purpose of hard water softening is to reduce the content of calcium and magnesium ions in the water according to the need, so that the hard water is softened into soft water, and this treatment process is called water softening.

At present, the water softening is mainly carried out by chemical methods, often using water softeners and ion exchange method.

A: water softening agent method

The method is to use chemicals as water softeners, and the water of calcium, magnesium ions, or generate insoluble precipitation to remove from the water, or the formation of stable soluble complexes, to reduce the hardness of water.

1.Precipitation method

Usually use lime and soda ash, so that the calcium ions in the water to form CaCO3 precipitation, magnesium ions to form Mg (OH) 2 precipitation and remove from the water, thereby reducing the hardness of water, known as lime - soda ash method, more economical and practical.

Ca(HCO3)2+Ca(OH)22CaCO3↓+2H2O

Mg(HCO3)2+Ca(OH)2MgCO3+CaCO3↓+2H2O

MgCO3+Ca(OH)2Mg(OH)2↓+CaCO3↓

CaCl2+Na2CO3CaCO3↓+2NaCl

MgCl2+Ca(OH)2Mg(OH)2↓+CaCl2

CaSO4+Na2CO3CaCO3↓+Na2SO4

MgSO4+Ca(OH)2Mg(OH)2↓+CaSO4

In the removal of calcium, magnesium ions at the same time, water iron, manganese salts, etc. can also be converted into insoluble hydroxide precipitation and removed.

Hard water only add soda ash (Na2CO3), after boiling can also reduce the hardness of water, but because of magnesium carbonate in the water still has a certain solubility, so the degree of softening is not high.

Trisodium phosphate is also commonly used as a water softener, with a good water softening effect.

3Ca(HCO3)2+2Na3PO4Ca3(PO4)2↓+6NaHCO3

3CaSO4+2Na3PO4Ca3(PO4)2↓+3Na2SO4

Soda ash or trisodium phosphate treatment of soft water, often containing high alkalinity, can only be used directly for alkaline conditions in the practice of bleaching, dyeing and finishing process.

Industrial use of lime - soda ash method for soft water treatment, is the amount of water and the need for chemicals in the reactor mix, after the treatment of soft water, precipitates from the bottom of the reactor discharge, and soda ash and trisodium phosphate is often in the practice of bleaching, printing and dyeing working fluid added in the right amount, to play the role of water softener.

2. Complexation method

Inorganic complexing agent such as sodium hexametaphosphate, is often used in dyeing and finishing process complexing type water softener, it can form a stable water-soluble complex with calcium and magnesium ions in water, and will remove calcium and magnesium salt impurities, to soften the effect.

Na4[Na2(PO3)6]+Ca2+Na4[Ca(PO3)6]+2Na+

Na4[Na2(PO3)6]+Mg2+Na4[Mg(PO3)6]+2Na+

As sodium hexametaphosphate does not produce insoluble precipitates in the water softening process, it is more suitable for use as a water softener directly in the dyeing and finishing working solution, but the price is relatively high.

Organic type complexing agents such as sodium ethylenediaminetetraacetate and sodium ammonia triacetate and some compound organic chelating dispersant, organic complexing agent, is a better complexing water softener, a small amount of use both good water softening effect, the generation of water-soluble complexes and stable, in industry has been widely used.

KEY WORDS:Hard Water Softening,Dyeing and Finishing Water,WATER QUALITY,water softening agent method,Precipitation method, Complexation method

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