2. Contents
❖Defination of Softening Process
❖Types
❖Lime-Soda Process
❖Zeolite or permutit process
❖Ion exchange or demineralization process
❖Difference between Lime-Soda, Zeolite and Ion-exchange Process
2
3. Softening of Hardwater
3
❑The process of removing hardness producing salts from water is known as
softening of water.
❑There are two mode of softening the hard water.
i. External Treatment
ii. Internal Treatment
❑The five important chemical methods used for softening of water internally
are:
1. Colloidal Conditioning / Treatment
2. Phosphate Conditioning / Treatment
3. Carbonate Conditioning / Treatment
4. Calgon Conditioning / Treatment
5. Sodium Aluminates Conditioning / Treatment
4. Softening of Hardwater(Internally)
4
❖Internal treatment of boiler water is carried out by adding proper
chemicals to precipitate the scale forming impurities in the form of
sludge and to convert the scale forming chemicals into compounds which
will stay in dissolved form in water.
(a) COLLODIAL CONDITIONING:-
❖The addition of organic substances such as Kerosene, tannin, Gel etc.,
to the surface in low pressure boilers may prevent the scale formation.
❖These substances gets coated over the scale forming precipitates and
gives a loose and non-sticky precipitates which can be removed by using
blow-down operation.
5. Softening of Hardwater(Internally)
5
(b) PHOSPHATE CONDITIONING:-
❖The addition of sodium phosphate in hard water reacts with the
hardness causing agents and gives calcium and magnesium phosphates
which are soft and non-adhere and can be removed easily by blow-down
operation.
❖In this way, scale formation is removed in high-pressure boilers.
3CaCl2 + 2 Na3PO4 → Ca3(PO4)2 + 6NaCl
6. Softening of Hardwater(Internally)
6
(c) CARBONATE CONDITIONING:-
❖In low-pressure boilers, scale-formation can be avoided by adding
sodium carbonate to boiler water, when CaSO4 is converted into calcium
carbonate in equilibrium.
CaSO4 + Na2CO3 → CaCO3 + Na2SO4
❖Consequently, deposition of CaSO4 as scale doesn’t take place and
calcium is precipitated as loose sludge of CaCO3 which can be removed
by blow-down operation.
7. Softening of Hardwater(Internally)
7
(d) CALGON CONDITIONING:-
❖It involves in adding calgon to the boiler water. It prevents the scale
and sludge formation by forming soluble complex compound with
CaSO4.
Calgon = Sodium hexa Meta phosphate = Na2[(Na4(PO3)6]
8. Softening of Hardwater(Internally)
8
(e) SODIUM ALUMINATE CONDITIONING:-
❖Sodium aluminate gets hydrolyzed yielding NaOH and a gelatinous
precipitate of aluminum hydroxide.
NaAlO2 + 2H2O → NaOH + Al (OH)3
❖The sodium hydroxide, so-formed, precipitates some of the magnesium
ions as Mg (OH)2, i.e.,
MgCl2 + 2NaOH → Mg (OH)2 + 2NaCl
❖The precipitate of Mg(OH)2 plus aluminum hydroxide,
produced inside the boiler, entraps finely suspended and
colloidal impurities, including oil drops and silica. The loose
precipitate can be removed by pre-determined blow-down
operation.
9. Softening of Hardwater(Externally)
9
❑The three important industrial methods used for softening of water
externally are:
1. Lime – Soda process
2. Zeolite or permutit process
3. Ion exchange or demineralization process
10. 10
Lime-Soda Process
❖In this process, the soluble calcium and magnesium salts are
chemically converted into insoluble compounds by adding calculated
amounts of lime [Ca(OH)2] and soda [Na2CO3].
❖Calcium carbonate [CaCO3] and magnesium hydroxide [Mg(OH)2]
so precipitated are filtered off. By this method both temporary as
well as permanent hardness are removed.
12. 12
Lime-Soda Process
❖Hence to remove equivalent amount of Ca and Mg hardness, the amount
of lime requirement is in the ratio 1:2.
❖Again for the removal of permanent hardness, the reactions are:
CaSO4 + Na2CO3→ CaCO3 + Na2SO4
MgSO4 + Na2CO3 → MgCO3 + Na2SO4
13. 13
Lime-Soda Process
❖In this method, soluble calcium and magnesium salts in water are
chemically converted into insoluble compounds like calcium
carbonate CaCO3 and magnesium hydroxide Mg(OH)2 by adding
calculated amount of lime Ca(OH)2 and soda Na2CO3 to it. So
precipitates are filtered off.
❖Generally there are two types of lime-soda Process.
(a) Cold-Lime Soda Process
(b) Hot-Lime Soda Process
15. 15
Cold Lime-Soda Process
❖In this method, calculated quantity of chemicals (lime and soda) are
mixed with water at room temperature.
❖At room temperature, the precipitates formed are finely divided, so
they do not settle down easily and cannot be filtered easily.
❖Consequently, it is essential to add small amounts of coagulants (Like
Alum, aluminium sulphate, sodium aluminates, etc.), Which hydrolyse
to gelatinous precipitate of aluminium hydroxide, and entraps the fine
precipitates.
16. 16
Cold Lime-Soda Process
❖Use of sodium aluminates as coagulant, also helps the removal of
silica as well as oil, if present in water. 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 2 Al(OH)3 + 3 CaSO4 + 6CO2
17. 17
Cold Lime-Soda Process
❖Raw water and calculated quantities of chemicals (Lime +soda +
coagulant) are fed from the top into the inner vertical circular chambers,
fitted with a vertical rotating shaft carrying a number of paddles.
❖As the raw water and chemicals flow down, there is a vigorous stirring
and continuous mixing, whereby softening of water takes place.
18. 18
Cold Lime-Soda Process
❖The softened water then passes through a filtering media (usually
made of wood fibers) to ensure complete removal of sludge.
❖Filtered soft water finally flows out continuously through the outlet at
the top sludge settling at the bottom of the outer chamber is drawn off
occasionally.
20. 20
Hot Lime-Soda Process
❖The reaction proceeds faster.
❖The softening capacity of hot process is increased to many fold.
❖The precipitate and sludge formed settle down rapidly and hence, no
coagulants are needed.
❖Much of the dissolved gases such as CO2 and air driven out of the
water.
❖Viscosity of softened water is lower, so filtration of water becomes
much easier. this in-turn increases the filtering capacity of filters.
21. 21
Hot Lime-Soda Process
❖Hot lime-soda process produces water of comparatively lower residual
hardness of 15 to 30 ppm.
❖Hot lime-soda plant consists essentially of three parts
❑A ‘Reaction tank’ in which raw water, chemicals and steam are
thoroughly mixed.
❑A ‘Conical sedimentation vessel’ in which sludge settles down.
❑A ‘sand filter’ which ensures completes removal of sludge from the
softened water.
22. 22
Advantages of Lime-Soda Process
❖It is very economical.
❖If this process is combined with sedimentation, then lesser amount of
coagulants shall be needed.
❖The process increases the pH value of the treated-water; thereby
corrosion of the distribution pipes are reduced.
❖Besides the removal of hardness, the quantity of minerals in the water
is reduced.
❖Due to alkaline nature of treated-water, amount of pathogenic bacteria
in water is considerably reduced.
23. 23
Disadvantages of Lime-Soda Process
❖For efficient and economical softening, careful operation and
skilled supervision are required.
❖Disposal of large amounts of sludge or insoluble precipitates
poses a problem. However, the sludge may be disposed off in
raising low-lying areas of the city.
24. 24
Zeolite Process
❖Zeolites are also known as Permutits (means ion-exchange resins)
❖Zeolites are classified in to 2 types:
1. Natural Zeolites are non-porous, eg;- Natrolite-
Na2O.Al2O3.4SiO2.2H2O.
2. Synthetic Zeolites posses gel structure.
❖Synthetic Zeolites posses higher exchange capacity than natural
Zeolites.
25. 25
Zeolite Process
❖For Softening of water by Zeolite process, hard water is entered at a
specified rate through a bed of Zeolite; kept in a cylinder.
❖The Hardness causing ions (Ca+2, Mg+2 etc.) are retained by the
Zeolite as CaZe and MgZe; while the outgoing water contains sodium
salts.
❖Reactions taking place during the softening process are:
Na2Ze +Ca(HCO3)2 CaZe +2NaHCO3
Na2Ze +Mg(HCO3)2 MgZe +2NaHCO3
27. 27
Zeolite Process
❖Reactions taking place during the softening process are:
Na2Ze +CaCl2 (or CaSO4) CaZe +2NaCl ( or Na2SO4 )
Na2Ze +MgCl2 (or MgSO4) MgZe +2NaCl ( or Na2SO4 )
28. 28
Zeolite Process
Regeneration
❑After some time, the Zeolite is completely converted into calcium
and magnesium Zeolites and it ceases to soften water, i.e., it gets
exhausted.
❑At this stage, the supply of hard water is stopped and the exhausted
Zeolite is reclaimed by treating the bed with a concentrated Brine
solution (10% NaCl).
CaZe + 2 NaCl Na2Ze + CaCl2
MgZe + 2 NaCl Na2Ze + MgCl2
31. 31
Ion-Exchange Process
❖Ion exchange process also known as demineralization or de-ionization
process.
❖Ion-exchange resins are insoluble. Crossed linked long chain organic
polymers with a micro porous structure, and the “functional groups”
attached to the chains are responsible for the ion-exchanging properties.
❖Basically resins with acidic functional groups are capable of exchanging
H+ ions with other cations, where as resins with basic functional groups are
capable of exchanging OH- ions with other anions.
32. 32
Ion-Exchange Process
❖Ion exchange resins are classified in to two categories.
(a) Cation exchange resin:- Resins containing acidic functional
groups (-COOH, -SO3H). This is represented as RH+ .
(b) Anion exchange resin:- Resins containing basic functional
groups(-NH2OH). This is represented as (ROH).
33. 33
Ion-Exchange Process
❖In ion-exchange process, hard water is allowed to pass through cation
exchange resins, which remove Ca+2 and Mg+2 ions and exchange
equivalent amount of H+ ions.
2RH+ + Ca+2 R2Ca+2 + 2H+
2RH+ + Mg+2 R2Mg+2 + 2H+
(RH+ = cation exchange resin)
34. 34
Ion-Exchange Process
❖In ion-exchange process, hard water is allowed to pass through anion
exchange resins, which remove bicarbonates, chlorides and
sulphates from water and exchange equivalent amount of OH-
ions.
R’OH + Cl+2 R’ Cl- + OH-
2R’OH- + SO4
-2 R’2SO4
-2 - + 2OH-
2R’OH + CO3
-2 R’2 CO3
-2 + 2OH-
35. 35
Ion-Exchange Process
❖H+ and OH- ions, thus released in water from the respective cation and
anion exchange columns, get combined to produce water molecules.
H+ + OH- H2O
❖The water coming out from the exchanger is ion free i.e., free from
anions and cations. Thus water of zero hardness is obtained.
37. 37
Ion-Exchange Process
Regeneration
❖When cation exchanger looses it’s capacity of producing H+ ions and
anion exchanger losses it’s capacity of producing OH- ions, then they are
said to be exhausted.
❖The exhausted cation exchanger is regenerated by passing it through
dilute sulphuric acid (H2SO4).
R2Ca+2 + 2H+ 2RH+ + Ca+2
❖The exhausted anion exchanger is regenerated by passing a dilute
solution of NaOH.
R2SO4
-2 + 2OH- 2R’OH- + SO4
-2
38. 38
Ion-Exchange Process
Advantages
❖The process can be used to soften highly acidic or alkaline
water.
❖It produces water of very low hardness (2 ppm).
❖So it is very good for using treating water in high-pressure
boilers.
39. 39
Ion-Exchange Process
Disadvantages
❖The equipment is costly and more expensive chemicals are
needed.
❖If water contains turbidity, the output of the process is reduced,
the turbidity must be below 10 ppm; otherwise it has to be
removed by coagulation and filtration.
40. 40
Lime-Soda Process Zeolite Process Ion-Exchange Process
❑Water treatment
plant occupies more
area or place.
❑Water treatment
plant occupies less
area.
❑Water treatment plant
occupies less area.
❑Water after
treatment has lesser
dissolved solids.
❑Water after
treatment has much
more dissolved solids.
❑Water after treatment
free from dissolved solids.
❑This method of water
treatment plants is not
much expensive and
material used
is cheap.
❑This method of
water treatment plants
is more expensive and
material used for
softening is expensive.
❑This method of water
treatment plants is more
expensive and material
used for softening is
expensive.
Differences
41. 41
Lime-Soda Process Zeolite Process Ion-Exchange Process
❑Operation expenses
are higher.
❑Operation expenses
are low.
❑Operation expenses are
higher.
❑The cost incurred on
softening of water is
high.
❑The cost incurred on
softening of water is
low.
❑The cost incurred on
softening of water is high
❑It cannot operate in
under pressure.
❑It can even operate
in under pressure.
❑It can even operate in
under pressure.
Differences
42. 42
Lime-Soda Process Zeolite Process Ion-Exchange Process
❑It can be used for
treating acidic water
also.
❑This method of
softening the water is
not used for treating
❑acidic water.
❑It can be used for
treating acidic and
alkaline water also.
❑There is a problem
of settling, coagulation
and removal of sludge.
❑There is no problem
of settling, coagulation
and removal of sludge.
❑There is problem of
turbidity.
❑In order to meet the
changing hardness of
incoming water,
frequent control and
adjustment of reagents
is needed.
❑Control test
comprises only in
checking the hardness
of treated-water.
❑Control test comprises
only in checking the
hardness of treated
water.
Differences
43. 43
Lime-Soda Process Zeolite Process Ion-Exchange Process
❑Residual hardness is
low about 15 to 50
ppm.
❑Residual hardness is
low about 10 to 15
ppm.
❑Residual hardness is
low about 0 to 2 ppm.
❑It is not good for
boilers.
❑It is not good for
boilers.
❑It is very good for
treating water for use in
high pressure boilers.
❑Skilled persons
required.
❑It required less skill
for maintenance as
well as operation.
❑It required less skill for
maintenance as well as
operation.
Differences
