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 of Water Hardness

Water hardness is classified in the following ways –

1. Temporary hardness
2. Permanent hardness

Among these types, permanent hardness creates the most troubles. Let’s look at them in datail.

 

Temporary Hardness of Water

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. These kind of hardness creates the maximum hassles in boilers and other heating equipments.

Permanent Hardness of Water

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.
Now, this is the type that proves to be the biggest challenge for dyehouses accross the globe. We’ll talk about the problems that arise due to the permanent hardness. But let’s look at the method in which these properties are expressed first.

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.
The units can converted to each other by using the following equation –

PPM= GPG/0.07

Scales of Water Hardness

Description	Definition
German hardness(odh)	10mg CaO in 1L of water
French hardness(ofh)	10mg CaCO3  in 1L of Water
English hardness(oeh)	10mg CaCO3  in 0.7L of Water
American hardness(oah)	1mg CaCO3  in 1L of water

Hardness Unit Conversion

Unit	PPM	GPG	odh	ofh	oeh
PPM	1.00	17.12	17.85	10	14.25
GPG	0.05842	1.00	1.043	0.5842	0.8327
odH	0.5603	0.9591	1.00	0.5603	0.7986
ofh	0.1	1.712	1.785	1.00	1.425
oeh	0.07016	1.201	1.252	0.7016	1.00

Water Hardness Parameters / Classification of Water Hardness

Classification	Hardness in PPM	Hardness in odh
Soft	>60	0-10.7
Moderately hard	60-120	10.8-21.4
Hard	120-180	21.5-32.1
Very hard	<180	≥10.57

Problems with Hard Water in Wet Processing

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 subsequent processes.

Scale Formation 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

Deactivation of Enzymes in Desizing

Hard water de-active enzymes used in desizing process & insolubilize size materials such as starch, PVA etc.

Wastage of Soap in 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 reacts with soap and produce organic salts, which 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

Oxidative Damage in Bleaching

The minerals present in the water promotes the decomposition of belaching agents, such as hydogend peroxide. This unstabilty leads to oxidatiove damage of cellulosic substrates- known as pin-hole formation.

H₂O₂→ H₂O + [O]

Reduced Luster in Mercerizing

It forms insoluble metal acid, reduce absorbency and luster.

Anionic Dye Precipitation in 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.

Reduced Viscosity of the Paste in 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.

Resin Deactivation in Finishing

Hard water interfering with catalysts, cause resins and other additives to become non reactive, break emulsion and deactivates soap.

REFERENCE

Classification of water on the basis of total hardness Durfor and Becker