Hard water softening method for dyeing and finishing water

Date:Sep 17, 2020

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 plasma in the water according to the needs and to soften hard water into soft water. This treatment process is called water softening.


At present, the softening of water is mainly carried out by chemical methods, and the water softener method and ion exchange method are frequently used.


1. Water softener method

This method uses chemicals as water softeners to interact with calcium and magnesium ions in the water, or generate insoluble precipitates to remove them from the water, or form stable soluble complexes to reduce the hardness of water.


(1). Precipitation method

Lime and soda ash are usually used to make calcium ions in the water form CaCO3 precipitation and magnesium ions form Mg(OH)2 precipitation to be removed from the water, thereby reducing the hardness of the water, called the lime-soda ash method, which is 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


While removing calcium and magnesium ions, iron and manganese salts in the water can also be converted into insoluble hydroxides and removed by precipitation.

Only adding soda ash (Na2CO3) to hard water can reduce the hardness of the water by boiling, but because magnesium carbonate still has a certain solubility in water, the degree of softening is not high.


Trisodium phosphate is also a commonly used water softener, which has a good softening effect.

3Ca(HCO3)2 2Na3PO4Ca3(PO4)2↓ 6NaHCO3

3CaSO4 2Na3PO4Ca3(PO4)2↓ 3Na2SO4


Soft water treated with soda ash or trisodium phosphate often contains high alkalinity and can only be used directly for scouring, bleaching, dyeing and finishing under alkaline conditions.


When the lime-soda ash method is used in the industry for soft water treatment, the water is mixed with the required amount of chemicals in the reactor. After the treatment, the soft water is released, and the sediment is discharged from the bottom of the reactor. Soda ash and trisodium phosphate are often practiced. Add an appropriate amount to the bleaching, printing and dyeing working fluid to act as a water softener.


(2). The network is legal

Non-type complexing agents such as sodium hexametaphosphate are complex water softeners often used in dyeing and finishing. It can form stable water-soluble complexes with calcium and magnesium ions in water, and remove calcium and magnesium salt impurities. Remove and soften:

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

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


Since sodium hexametaphosphate does not produce insoluble precipitates in the water softening process, it is more suitable to be used directly as a water softener in the dyeing and finishing working fluid, but the price is relatively high.


Organic complexing agents such as sodium ethylenediaminetetraacetate and sodium nitrilotriacetate, as well as some complex organic chelating dispersants, organic complexing agents, are more excellent complexing type water softeners, use a small amount of existing good softener The result is that the water-soluble complex formed is stable and has been widely used in industry.



2. Ion exchange method

Use ion exchange materials with certain chemical reaction activity to adsorb calcium and magnesium ions in hard water into solid materials, thereby reducing the hardness of water and achieving the purpose of soft water. Commonly used ion exchange materials include zeolite, sulfonated coal, and ion exchange resin. Because ion exchange resin has the outstanding advantages of good mechanical strength, excellent chemical stability, high exchange efficiency, and long service life, it has become a large number in the industry. The main treatment method for the production of softened water.

There are two types of ion exchange resins: cation exchange resin and anion exchange resin.


(2). Cation exchange resin

Cation exchange resins can exchange various cations in water, such as Ca2, Mg2, etc. At present, domestic 732 type polystyrene strong acid cation exchange resins are widely used. Most of them are sodium type when they leave the factory. This polymer material contains sulfonic acid groups. The reactive functional groups can absorb most of the Ca2, Mg2, etc. in the water, thereby softening the water. The principle of action is as follows: first, the relatively stable sodium resin is transformed into a hydrogen resin with higher activity with acid:




Then the hard water is slowly passed through the hydrogen resin layer to produce ion exchange and soften the water:

2R—SO3H Ca2 (R—SO3)2Ca 2H


(2). Anion exchange resin

Anion exchange resins can exchange various anions in water, such as Cl-, SO42-, etc. At present, domestic 717 type polystyrene strong basic anion exchange resins are widely used. Most of them are chlorine type when they leave the factory. This polymer material contains The quaternary ammonium reactive functional group can absorb Cl- and SO42- in the water, and the water softened by the cation exchange resin can be changed from acidic to neutral, and the soft water can be further purified. The principle of action is as follows:


First of all, the more stable chlorine-based alkali is transformed into a more reactive hydroxide resin:

R—CH2N(CH3)3Cl OH-R—CH2N(CH3)3OH Cl-

Then slowly contact with water or hard water that has been softened by the cation exchange resin to produce ion exchange, making the soft water neutral or further purifying the water quality:

2R—CH2N(CH3)3OH SO42- 2H [R—CH2N(CH3)3] 2SO4 2H2O


(3) Resin regeneration treatment

Generally, after ion exchange resin is used for a period of time, the adsorption capacity of the resin is saturated and the chemical reaction activity is lost. It needs to undergo necessary regeneration treatment to make it have adsorption reaction activity again.


The inactive cation exchange resin can be regenerated with HCl solution:

(R—SO3)2Ca 2H 2R—SO3H Ca2


The inactive anion exchange resin can be regenerated with NaOH solution:

[R—CH2N(CH3)3]2SO4 2OH-2R—CH2N(CH3)3OH SO42-


The regenerated cation and anion exchange resin can be put into the soft water treatment process again. It can be seen from the above that ion exchange resins can be used repeatedly, regenerated, and recycled for a long period of time without frequent replacement.


Electrodialysis is a more advanced ion-exchange softening method, but the equipment investment is relatively high, maintenance is complicated, and the power consumption is large. It is rarely used in general dyeing factories.


In addition, if surface water such as rivers, rivers, and lakes are directly used as the water source, the removal of suspended solids in the water must be completed in advance before the water is softened. Sand layer filtration, coagulant adsorption, activated carbon layer filtration and other treatment methods are often used to make After the water source is colorless, tasteless, clear and transparent, it then enters the hard water softening process. Using groundwater or tap water as a water source can generally enter the hard water softening stage directly. Refer to related reference books for commonly used measurement methods of water quality.