Internal Treatment Methods

Can be achieved by adding certain chemicals to water in the boiler

• Colloidal conditioning• Carbonate conditioning• Phosphate conditioning• Calgon conditioning

1

External Treatment of water

• Lime Soda Process, • Zeolite Process• Ion Exchange Process

ZEOLITE PROCESS

Zeolites• Naturally occuring hydrated, microporous, sodium aluminosilicate

minerals

• General formula Na2O. Al2O3 . xSiO2· yH2O

Zeolites are Capable of exchanging their Na+ ions with hardness producing ions like Ca2+, Mg2+ present in water

2

• Natural zeolites are non-porous, e.g. Natrolite, Na2O.Al2O3 . 3SiO2· 2H2O

• Synthetic zeolites are porous and have a gelstructure.

• Can be prepared by heating together sod. Carbonate(Na2CO3 ), alumina (Al2O3) and silica (SiO2)

• Synthetic zeolites have higher ion exchange capacitythan Natural Zeolites

3

Microporousmolecular structure of a zeolite

Structure of Zeolite

Zeolite crystal consists of SiO4 tetrahedrajoined to each other and each Oxygen isbeing shared with an adjacent one.

Has an open structure with cavities inwhich Na+ ions are loosely held and canbe exchanged with hardness producingions of water during softening.

4

Process of water treatment

Hard water is passed at a specific rate through a bed ofzeolite in a cylindrical unit.

As hard water percolates through zeolite, the hardnessproducing ions i.e. Ca2+, Mg2+ are exchanged with Na+ ions ofzeolite

A zeolite with general formula Na2O.Al2O3.xSiO2·yH2O isconveniently represented as Na2Z where Z representsaluminosilicate complex

5

Removal of Ca2+, Mg2+ ions from hard water

Regeneration of Exhausted Zeolite

6

7

1. Hardness of water is removed almost completely and softened water of about 10 ppm is obtained

2. The process requires much less time for completion and operation is quite clean

3. The equipments used is compact and can be installed in a limited space

4. No impurities are precipitated so there is no danger of sludge formation in the treated water at later stage

5. The process automatically adjusts itself for variation in hardness of incoming water

6. Maintenance and operation is easy and requires less skills.

Advantages of Zeolite Process

8

Disadvantages of Zeolite Process

1. Water containing impurities of mineral acids can destroy theZeolite Bed. So mineral acids must be neutralized beforefeeding water into Zeolite bed

2. Turbid water blocks the pores of zeolite bed and decreasesthe efficiency- Turbidity & suspended matter must beremoved before feeding

3. Water should not be hot- (Dissolves Zeolites)

4. Cations Ca2+, Mg2+ are removed but sulphates, bicarbonatesand chlorides are not removed.

9

In industry mainly three external methods are employedfor softening of water, they are.

1. Lime-Soda process2. Zeolite process3. Ion-Exchange process

WATER SOFTENING“Process of removal of hardness causing salts from water”

It is very important and essential process since hard wateris unsuitable for domestic and industrial use.

10

LIME-SODA PROCESSIn this method,• The soluble calcium and magnesium salts in water are

chemically converted into insoluble compounds (ppt),by adding calculated amount of Lime (Ca(OH)2) andSoda (Na2CO3)

• Precipitates of CaCO3 and Mg(OH)2

these precipitates are filtered off.

• Lime soda process has two types• Cold Lime-soda process• Hot Lime-soda process

11

• In this method, calculated quantity of lime and sodaare mixed with water at room temp.

• At room temp. the precipitates formed are finelydivided, so they do not settle down easily andcannot be filtered easily.

• Consequently, it is essential to add small amounts ofcoagulants like alum, aluminium sulphate, sodiumaluminate (NaAlO2), etc.

COLD LIME SODA PROCESS

12

• Coagulants form gelatinous precipitates of aluminumhydroxide and entraps fine precipitates.

• Use of sodium aluminate, NaAlO2 as coagulant alsohelps the removal of silica as well as oil, if present inwater

• Cold L-S process provides water,containing a residual hardness of 50 to 60 ppm

NaAlO2 + 2H2O → NaOH + Al(OH)3

Al2(SO4)3 + 3Ca(HCO3)2→ 2Al(OH)3 + 3CaSO4+ 6CO213

cellulosic elements that are extracted from trees, straw, bamboo, cotton seed, hemp, sugarcane and other sources.14

Raw water and calculated quantities of chemicals (Lime+ soda + Coagulants) are fed from the top into theinner vertical circular chamber, fitted with a verticalrotating system carrying a number of paddles.

As the raw water and chemicals flow down, there is avigorous stirring and continuous mixing, wherebysoftening of water takes place.

The softened water comes into the outer co-axialchamber, where it rises upwards.

METHOD OF TREATMENT

15

The heavy sludge or precipitates settles down in theouter chamber by the time the softened waterreaches up.

The softened water then passes through a filteringmedia this is usually made of wood fibres to ensurecomplete removal of sludge.

Filtered soft water finally flows out continuouslythrough the outlet at the top.

Sludge settling at the bottom of the outer chamber isdrawn of occasionally.

METHOD OF TREATMENT

16

In Hot lime-soda process hard water is treated withsoda and lime at 94-100°CHot lime-soda plant essentially consists of three parts

→ A ‘Reaction tank’ in which raw water,chemicals and steam are thoroughly mixed.

→ A ‘Conical sedimentation vessel’ in whichsludge settles down, and

→ A ‘sand filter’ which ensures complete removalof sludge from the softened water.

HOT LIME SODA PROCESS

17

• The reaction tank has three separate inlets, one each for rawwater, lime-soda and steam

• Superheated steam is used to increase the temperature.

• Due to high temp., reactions of water softening proceed fasterand sludge is collected in conical sedimentation tank.

• The softened water is passed through sand filter which ensurescomplete removal of sludge from softened water.

• Due to high temperature, the dissolved gases are driven out ofwater.

• The resulting softened water has a residual hardness of 15-30ppm.

METHOD OF TREATMENT

18

METHOD OF TREATMENT

19

1. Very economical

2. When combined with sedimentation, lesseramounts of coagulants shall be needed.

3. The quantity of minerals in the water is reduced.

4. To certain extent, iron and manganese are alsoremoved

ADVANTAGES OF LIME-SODA PROCESS

20

1. For efficient and economical softening, carefuloperation and skilled supervision is required.

2. Disposal of large amounts of sludge or insolubleprecipitates poses a problem.

3. This can remove hardness only upto 15 ppm,which is not good for boilers.

DISADVANTAGES OF LIME-SODA PROCESS

21