water technology (unit i)

8/8/2019 Water Technology (Unit i)

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Hardness of water

Hardness in water is that characteristic , which prevents the lathering of soap

Due to presence of salts of Ca, Mg, and other heavy metals

Sample of hard water , when treated with soap ( Na or K salt of higher fatty acid,

Oleic , palmitic or stearic) does not produce lather but on the other hand

forms a white scum or precipitate . This is due to insoluble soaps of Ca and Mg .

Water which does not produce lather with soap solution but forms white curd

called Hard water

Water which lathers easily on shaking with soap solution is called soft water


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Hardness of water

Water which does not produce lather with soap solution but forms white curd

called Hard water

Water which lathers easily on shaking with soap solution is called soft water

2 17H35 a a l2+ ( 17H35 )2 a +2 a l

2 17H35 a g 4+ ( 17H35 )2 g +2 a l

odium tearate

(soap)

Hardness

In soluble

In solubleodium tearate


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Temporary or carbonate Hardness

Is caused by dissolved bicarbonates of Ca, and Mg and other heavy

metals and carbonate of ion.

This hardness can be destroyed by mere boiling of water, when

bicarbonates decomposed yielding carbonates or hydroxides which are

deposited as crust at the bottom of vessel

( 3)2

t

3 + 2 + 2

( 3)2

t( )2 + 2 2

In soluble

In soluble


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Permanent or non carbonate Hardness

Is caused by dissolved chlorides and sulphates of Ca, and Mg , iron

and other heavy metals ion.

This hardness can not be destroyed by boiling of water


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Concentration of hardness as well as non hardness constituting

ions are usually expressed in terms of equivalent amount of CaCO3

This mode permits multiplication and division of concentration

and this choice is due to the Molecular weight of CaCO3: = 100;Equivalent weight = 50

And CaCO3 is most insoluble in water and can be precipitated in

Water treatment


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Units of Hardness

1. Parts per million (ppm)1 part of CaCO3 equivalent hardness per 10

6parts of water

2. Milligrams per liter (mg/L): Number of mg of CaCO3 equivalenthardness present per liter of water1mg /L = 1 mg of CaCO3 equivalent hardness of 1L of water

but 1L water weights = 1Kg = 1000 g = 1000X1000mg = 106 mg

Hence 1mg/L = 1 ppm

3. Clarkes degree (oCl): number of grains (1/7000 lb) of CaCO3equivalent hardness per gallon (10 lb) of water

Or parts of CaCO3 equivalent hardness per 70000 parts of water


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4. Degree French (oFr): parts of CaCO3 equivalent hardness per

105parts of water

1 oFr = 1 parts of CaCO3 equivalent hardness per 105parts of

water

5. Milli equivalent per liter(meq/L): Number of milli equivalents

of hardness present per liter

1 meq/L = 1meq of CaCO3per L of water

= 10-3

X 50 g of CaCO3 eq per liter= 50 mg /L of CaCO3 eq = 50 ppm


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Hardness of water

The equivalents of CaCO = [Mass of hardness producing substance X chemical equivalent of CaCO ]

Chemical equivalent of Hardness producing substance

The equivalents of CaCO = [Mass of hardness producing substance X 0]

Chemical equivalent of Hardness producing substance

Ca(HCO ) : Molar Mass: 6 chemical equivalent = 8

Mg(HCO ) : MM = 6 C.E. =

CaSO

: MM = 6 C.E. = 68

MgSO : MM = 0 C.E. = 60

CaCl : MM = C.E. = .

MgCl : MM = 9 C.E. = .

MgCO : MM = 8 C.E. =

CaCO : MM = 00 C.E. = 0


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Disadvantages of Hard Water

In domestic :

Washings :

Wastage of lot of soap, white sticky precipitate adheres on the

fabric giving spots and streaks. Irons salts could cause for

staining of cloth

Bathing : White mass deposits on the bath tub and body, cleaning

quality soap is depressed

Cooking :boiling point of water is elevated due to the presence

of ions. More fuel and time is required. Pulses, beans and peas

do not cook properly with hard water. Salty material depositson cooking utensil. Un-pleasent taste of tea or coffee if prepared

using hard water

Drinking : Bad effect on our digestive system.

Possibility of forming calcium oxalate crystals in urinary tract is

increased


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In industrial use :

Textile industry : Wastage of lot of soap, white sticky precipitate

adheres on the fabric giving spots and streaks. These fabrics when

dyed do not provide exact shade of color

Irons salts could cause for staining of cloth.

Sugar industry : Water containing sulphate, nitrite, alkali carbonate

refining causes difficulties in the crystalization of sugar

Dyeing industry : Ca, Mg, and Fe salts in hard water may react with

costly dyes forming undesirable precipitates. It could give impure

shades of color and spots on the fabric


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Pharmaceutical industry: Undesirable products will be obtained if

hard water is used in drug manufacturing industry

Paper industry : Ca, Mg, salts in hard water tend to react with

chemicals and other material employed to provide smooth and

shining finish to paper. Iron salt could give undesired color to thepaper.

Laundry: Waste of soap; Iron salt could give undesired color to the

cloth

Concrete making: Water containing chlorine and sulphates couldaffect

Hydration of cement which eventually affects the strength of

concrete


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In steam generation in boilers:To generate stream , boilers are extensively used. If

hard water used in boilers, many troubles arises due to

the salt content

i) Scale and sludge formation

ii) Corrosion

iii) Priming and foaming

iv) Caustic embrittlement


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In Boiler, water evaporates continuously and salt concentration

Increases progressively.

When salt concentration reaches its saturation point, they

precipitate out of water.

If precipitate is loose and slimy, it is called Sludge

If the precipitated matter forms hard , adhearing crust/coating on the inner

walls of boiler, it called Scale

Sludge and Scale


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Scale and sludge formation

Sludge: It can be easily scrapped off with wire brush

It formed at colder part of the boiler

It collects in the area where flow rate is slow or bends

Scale: These are formed by the salts which has greater solubility

in hot water than cold water

MgCO3,MgCl2, CaCl2,MgSO4

Disadvantages_ Sludge

Sludges are poor conductors; wastage of heat that generated

IfSludges formed with scale, former gets trapped in the latter

and both get deposited as scales

Excessive sludge formation disturbs the working boiler . It settle

In the region of poor water circulation such as pipe connection,

Plug opening, gauge-glass connection: choking of the pipes


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Scale and sludge formation

Disadvantages_ Scale

1. Wastage of fuel

Scales are low thermal conductivity: rate of heat transfer fromboiler to water is low: To get excess heat, fuel consumption is high

2. Lowering the boiler safety

Over heating results boiler material softener and weaker and it

distorts boiler tube

3. Decrease in effeciency

Scales sometimes deposits on valves and condenser of the

boiler. It chokes them partially

4. Danger of explosionWhen thick scale crack due to uneven expansion, water

comes out and touches over heated iron plates. This results

high stream supply subsequently generates high pressure

which may even cause explosion


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Sludge and Scale

Ca(HCO ) CaCO H O CO

Formation of Scales is due to

1. Decomposition of CaHCO3

ScaleCaCO3 scale is soft. This type scale forms in low pressure

Boilers.

However, in high pressure boilers, CaCO3 is soluble

CaCO H O Ca(OH) CO

soluble


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Sludge and Scale


2. Deposition of CaSO4

Solubility of CaSO4in water decreases with rise of

Temperature.

At 15oC, 3200 ppm,

At 55oC, 55 ppm

At 320oC 27 ppm

Hence, CaSO4

gets precipitated as hard Scale at heated

Portions of the Boiler

This is main cause of scales in high pressure Boilers

CaSO4 scale is quite adherent and difficult to remove

soluble


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Sludge and Scale


3. Hydrolysis of Mg salts

Dissolved Mg salts undergo hydrolysis at high temperature

In side the boiler forming MgOH2precipitate as soft Scale

MgCl2 + 2 2 Mg( )2 + 2 Cl

Scale


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Sludge and Scale


4. Presence of silica (SiO2)

Even small amount of silica in water deposits as Calcium

silicate or Magnesium silicate

Ca(SiO3) and Mg(SiO3)

These stick very firmly and very difficult to remove

Source of silica in water: sand filter


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Removal of Scale

1. With the help of scraper or piece of wood or wire brush

If they are loosely adhering

2. Thermal shocks ( heating and sudden cooling with cold water)

if they are brittle

3. Dissolving scales by using chemicals

if they are adherent and hard

CaCO3 scales: 5-10% HCl

CaSO4 Scales: EDTA (which forms soluble complexes)

4. By frequent blow down operationif they are loosely adhering


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Prevention of Scales Formation

1. External treatment : Using softening water

2. Internal treatment (sequestration):

Dissolved salt ions are complexed with chemicals to give

soluble salts

a. Precipitate the scale forming impurities n the form of sludge

which can be removed by blow-down operation

b. Convert to dissolved compounds which stays in water but do not cause

any harm

Blow down operation:

Partial removal of hard water through top at the bottom of the boiler

Addition of fresh softened water to make up after blow-down


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1. Colloidal conditioning: In low pressure boiler scale can be

Avoided by using kerosene, tannin, agar-agar gel etc.Scale gets coated to give non sticky and loose deposits and thiscan be removed by blow-down operation

2. Phosphate conditioning: In high pressure boiler, scale can

be avoided by adding sodium phosphate

3 CaCl2 + 2 a3P 4 Ca3(P 4)2 + aCl

Soft sludgeHard scale

Soft, non-adherent sludge of calcium and magnesium

phosphates can be removed by blow-down operation

NaH2PO4 (acidic); Na2HPO4 (weakly alkaline);

Na3PO4 (alkaline)

Choice of salt depends upon the alkalinity of boiler-feed water


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3. Carbonate conditioning: In low pressure boilers, Na2CO3

addition avoids scale formation

CaSO4 is converted to CaCO3

CaSO Na CO CaCO Na SO

scale Loose sludgeCan be removed by blow down


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To be continued in next class

4. Calgon conditioning

5. Treatment with sodium Aluminate

6. Electric conditioning

7. Radioactive conditioning

8. Complexometric method

water technology (unit i)

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