Hardness of any substance in equivalents of CaCO3

can be calculated as

Hardness=[Mass of hardness producing substance in mg/L] X [Eqv. wt of CaCO3]________________________________________________________

[Eqv. Wt of hardness producing substance]

[Mass of sub. in mg/L]x[Mol. Wt of CaCO3]___________________________________

Mol. Wt of Subs.Or=

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For e.g. If a sample of hard water contains 10 mg/L of MgSO4, then its CaCO3 equivalent is given by

Mass of MgSO4 (mg/L)X mol. Wt of CaCO3__________________________________

Mol. wt of MgSO4

= 10x100/120 = 1000/120 = 8.33 mg/L (ppm)

CaCO3 equivalents of MgSO4 (Hardness)

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=

DIFFERENT UNITS OF HARDNESS

1. PPM: May be defined as the no. of parts (by wt) ofCaCO3 equivalent hardness +nt per million parts (bywt) of water

2. mg/L: It is the no of mg of CaCO3 equivalent hardnesspresent per liter of water1mg/L=1mg of CaCO3 eq. hardness per liter of waterAs mass of 1 L water = 1 Kg = 1000g = 1000x1000mg = 106 mg;So, 1mg/L = 1mg of CaCO3 eq. hardness per 106 mg ofwater=1ppm 1mg/L = 1ppm

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One litre of water has a mass of almost exactly one kilogram when measured at its maximal density, which occurs at about 4 °C.

3. Degree Clarke (°Cl): defined as the no. of parts ofCaCO3 equivalent hardness present per 70,000parts of water. It is denoted by °Cl

1°Cl= 1 part of CaCO3 equivalent hardness present per70,000 parts of water

4. Degree french (°Fr): defined as the no. of parts ofCaCO3 equivalent hardness in 105 parts of water.Denoted by °Fr.

1°Fr= 1 part of CaCO3 equivalent hardness in 105 partsof water

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Relationship between different units of hardness

1ppm=1part per 106 parts of water,

1°Fr = 1part per 105 parts of water

°Cl = 1part per 70,000 parts of water;

So, 106 ppm = 105 °Fr = 70,000 °ClOr 1ppm = 0.1 °Fr = 0.07 °Cl

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SPECIFICATION OF WATER FOR DIFFERENT USE

• Boilers• Paper Industry• Textile Industry• Sugar Industry• Beverage Industry• Laundries• Cooling Systems• Pharmaceutical Industries• Cooking

Boilers:• Water used in boilers should be free

from hardness because hard watercauses sludge and scale formationwhich prevent efficient heat transfer

• Dissolved solids may cause causticembrittlement (material of boilerbecomes brittle due to accumulationof caustic substances)

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Paper Industry:• Should be free from silica (produces

cracks in paper) and hardness (increases ash content of paper)

• Fe & Mn can affect brightness & color of the paper

• Water should be free from alkalinity as alkaline water consume more alum and increase cost of production.

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Textile Industry:

• Should be free from hardness as hardwater precipitates basic dyes anddecreases the solubility of acidic dyes

• Presence of Fe, Mn salts causes unevendyeing on fabrics

• Should be free from turbidity, colour andorganic matter

Sugar Industry:

• Should be free from sulphates, carbonatesand nitrates otherwise crystallization andrefining of sugar becomes difficult and sugarobtained is deliquescent (absrob moisturefrom air)

Laundries:

• Hard water increases consumption of soaps

Cooking:

• Hard water increases fuel consumption andcooking time

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SCALE & SLUDGE FORMATION

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In BoilersWater is continuously converted into steam, resulting increasein conc. of dissolved salts due to evap. of water.

When the conc. reaches to the saturation point, the dissolvedsalts start precipitating out.

Sludge: If the pptes are soft, loose and slimly, these are calledsludges.

Scales: If the pptes are hard and adhering on the inner walls ofboiler, these are called scales.

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SLUDGE

Soft, loose and non-sticky precipitate formed within the boiler

It can be easily removed by a wire brush

It is formed at the colder portion of a boiler due to substanceswhich have greater solubility in hot water than in cold water(e.g. MgSO4 , MgCl2, CaCl2)

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Disadvantages of sludge

1. Sludge is a poor conductor of heat, leads to increase in fuelconsumption

2. Decreases efficiency of boiler3. Leads to blocking of pipes

(as sludge settles in the regions of poor water circulation)

Prevention of sludge formation

by using soft water by removing a portion of water having conc. salts and

replacing it by fresh water (blow down operation)

SCALEHard deposits firmly adhering to the inner walls of boiler(very difficult to remove)

Cause of formation:1. At high temp, the bicarbonates +nt in water are converted into insoluble

carbonates (scales)

Ca(HCO3)2 → CaCO3 ↓ + H2O + CO2scale

2. Magnesium salts get hydrolysed at high temperature formimg scales

MgCl2 + 2H2O → Mg(OH)2 ↓ + 2 HClScale

3. Solubility of CaSO4 decreases with temperature, so CaSO4 is ppted as scale athigh temperature

Some Scale Constituents: CaSO4, CaCO3, Mg(OH)2, Fe2O3, SiO2, Na2SO4

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Disadvantages of scales:1. Scales are poor conductors of heat, so leads to the

wastage of fuel2. Distortion of boiler material

Prevention: Internal treatment of boiler feed water

Removal of Scales: • By wire brush or by blow down operation• Sudden cooling• By chemical dissolution-

CaCO3 by 5-10% HCl, CaSO4 by EDTA

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