Water Hardness & Its Potential Problems in Wet Processing

Water Hardness is a measure of the quantity of divalent ions (salts with two positive charges) such as calcium, magnesium present in water i.e. bi-carbonates, sulphates, chlorides.
The water that contains these kinds of salts is called hard water.
 
Classification :

1. Temporary hardness
2. Permanent hardness
   Temporary hardness: this hardness is caused due to the presence of bi-carbonates of calcium and magnesium. Since this hardness can be removed easily by boiling the water, is called temporary hardness. When boiled, carbonates decomposes and liberates carbon-dioxide and forms an insoluble precipitate.
   Permanent hardness: The presence of chlorides or sulphates of calcium and magnesium cause permanent hardness. This hardness can not be removed easily by boiling. Ions causing permanent hardness of water can be removed using a water softener, or ion exchange column.

Methods of expressing hardness of water:
1.Parts Per Million(PPM): The number of grains of calcium carbonates which is present in one million grains of water is called PPM.
2.Grains per Gallon(GPG): The number of grains of calcium carbonates present in 70,000 grains of water.
PPM= GPG/0.07
Scales of Hardness:

Hardness unit conversion:

 
Water hardness parameter/classification of water according to hardness:

Problem with Hard Water: hard water may cause a number of problems during textile processing . since every state of textile processing is interrelated with each other any problem caused by hard water in one stage might affect the sub sequent processes.
 
In boiler: When hard water is boiled in boiler, temporary hardness causing element is dissociated and CaCO₃ and Mg(OH)₂  are formed . When they combine, create a stone like very hard insoluble material that is stored at the inner side of vessel or inner surface of tube. This material creates a layer progressively that is called ‘Scale’.

Ca(HCO₃)₂ → CaCO₃ + CO₂ ↑+ H₂O
Mg(HCO₃)₂→ MgCO₃ + CO₂↑ +H₂O
MgCO₃ + H₂O → Mg(OH)₂ + CO₂↑
[CaCO₃ + Mg(OH)₂] → Scale
This scale resist heat transfer and affects the free flow of water in tube. As a result more heat is required  ,therefore, more fuel will be needed. When scale is present in the boiler equal transformation of heat can not be achieved in the tubes so that over heating in a particular portion of the tube may cause burst in the tube.
Corrosion can be a serious problem in boiler, if hard water is used in it. Dissolved O₂ in the presence of CO₂ is the common cause of corrosion. Fe present in hard water reacts with CO₂ to form Fe CO₃, which is the main process of corrosion. This Fe CO₃ is hydrolyzed & produce Fe(OH)₂, this agent / component damage the boiler.
Fe + H₂O+ CO2 → Fe CO₃ + H₂O
FeCO₃ + H₂O     → Fe(OH)₂ + CO₂
Potential problem caused by hard water in textile wet processing:
Desizing : Hard water de-active enzymes used in desizing process & insolubilize size materials such as starch, PVA etc.
Scouring:  Soap is used in scouring process. Soap is the Na & K salt of higher fatty acid (C₁₇H₃₅COONa). The Ca & Mg salt of hard water react with soap and produce organic salts they are insoluble. As a result, an amount of soap is wasted that are of no use.
CaSO₄ + 2 RCOONa → (RCOO)₂Ca + Na₂SO₄
MgSO₄ + 2 RCOONa → (RCOO)₂Ca + Na₂SO₄
 Bleaching : Decomposes bleach bath.
H₂O₂→ H₂O + [O]
Mercerizing : It forms insoluble metal acid, reduce absorbency and luster.
Dyeing : Ca²⁺ and Mg²⁺ ions of hard water reacting with dyestuff precipitates dyes. As a result a lot of dyestuffs may be wasted also uneven shade may appear.
Printing :  It break the emulsion, change it’s thickness and efficiency and it is also harmful for thickener. It causes difficulty achieving required viscosity using thickener. Viscosity of printing paste since the prime concern of printing, hard water harms printing accuracy and production efficiency.
Finishing: Hard water interfering with catalysts, cause resins and other additives to become non reactive, break emulsion and deactivates soap.