engineering chemistry

2. Introduction:-
Name:- G.ASHWITHA
Roll no:- 22D21A6612
Branch:- computer science and engineering(ALML)

3. TOPIC:
EXTERNAL
TREARTMENT/
SOFTENING OF WATER

4. EXTERNAL TREATMENT:-
• the treatment given to water for the removal of hardness causing
salts before it is taken into the boiler is called external treatment.

5. EXTERNAL TREATMENT:
Lime-soda process
Zeolite process
Ion-exchange

6. LIME-STONE PROCESS:-
In this process, Calcium and Magnesium ions are precipitated by the addition of lime
(Ca(0H)2) and (Na2CO3).
standard water-softening process.
 Carried out either hot or cold.
Use lime (Ca(OH)2) and soda ash (Na2CO3) to reduce the hardness of the treated water by
precipitating the dissolved calcium and magnesium salts as insoluble calcium carbonate and
magnesium hydroxide respectively.
Also used in the preparation of caustic soda (NaOH),by mixing slaked lime (Ca(OH)2)with
soda and filtering off the precipitated calcium carbonate (CaCO3).

7. As slacked lime is added to a water, it will react with any carbon dioxide present as follows:
Ca(OH)2 + CO2 CaCO3 + H2O(Eq.1)
The lime will react with carbonate hardness as follows:
Ca(OH)2 + Ca(HCO3)2 2CaCO3 + 2H2O(Eq.2)
Ca(OH)2 + Mg(HCO3)2 MgCO3 + CaCO3 + 2H2O(Eq.3)
The product magnesium carbonate is soluble. To remove it, more lime is added:
Ca(OH)2 + MgCO3 CaCO3 + Mg(OH)2 (Eq.4)

8. Also, magnesium non-carbonate hardness, such as magnesium sulphate, is removed:
Ca(OH)2 + MgSO4 CaSO4 + Mg(OH)2 (Eq.5)
Lime addition removes only magnesium hardness and calcium carbonate hardness. In equation 5
magnesium is precipitated, however , an equivalent amount of calcium is added . The water now contain the
original calcium non-carbonate hardness produced in equation 5. soda ash is added to remove
calcium non-carbonate hardness:
Na2CO3 + CaSO4 Na2SO4 + CaCO3 (Eq.6)
To precipitate CaCO3 requires a pH of about 9.5; and to precipitate Mg(OH)2 requires a pH of about 10.8,
therefore, an excess lime of about 1.25m eq/L is required to raise the pH.

10. LIMITATIONS:-
1. Lime soda softening cannot produce a water at completely free of hardness because of minute solubility of CaCO3
and Mg(OH)2.
2. Thus the minimum calcium hardness can be achieved is about 30 mg/L as CaCO3, and the magnesium hardness
is about 10 mg/L as CaCO3.
ADVANTAGES:-
1. It increase pH value , there by corrosion is reduced.
2. Due to alkaline nature of water the pathogenic bacteria in water is reduced.
DISADVANTAGES:-
1. Disposal of large amounts of sludge is a problem.
2. The hardness is 50-60 ppm by cold process & about 15-30 ppm by hot process.

11. ZEOLITES PROCESS:-
Naturally occurring hydrated , microporous ,sodium aluminosilicate minerals.
General formula Na2O. AI2O3.xSiO2.YH2O
Zeolites are Capable of exchanging their Na+ ions with hardness producing ions like
Ca2+,Mg2+ present in water.
 Natural zeolites are non-porous, e.g. Natrolite, Na2O.AI2O3.3SiO2.2H2O
 Synthetic zeolites are porous and have a gel structure.
Can be prepared by heating together sodium Carbonate (Na2CO3),alumina(Al2O3) and
silica(SiO2).

12. Zeolite crystal consists of SiO4 tetrahedra joined to each other and each Oxygen is being
shared with an adjacent one.
As hard water percolates through zeolites, the hardness producing ions i.e. Ca2+,Mg2+ are
exchanged ions of zeolite.

13. Removal of Ca2+,Mg2+ ions from hard water
Na2Z + Ca(HCO3)2 CaZ + 2NaHCO3
(Zeolite) (Hard water)
Na2Z + MgSO4 MgZ + Na2SO4
Na2Z + CaCl2 CaZ + 2NaCl
Regeneration of Exhausted Zeolite
CaZ + 2NaCl Na2Z + CaCl2
(Exhausted Zeolite) (Regenerated Zeolite)
MgZ + 2NaCl Na2Z + MgCl2

15. LIMITATIONS:-
1. Raw water must not contain any coloured ions like Mn2+ and Fe2+,because they form manganese zeolite and
ferrous zeolite which cannot be regenerated.
2. Minerals acids if present in water will destroy the zeolite bed permanently. Water must be neutralized with soda
before it is fed into the zeolite bed.
ADVANTAGES:-
1. The removes hardness almost completely and the treated water contains hardness upto in 10ppm.
2. The process adjust itself for variation of hardness of incoming water.
DISADVANTAGES:-
1. The treated water contains more sodium salts than in lime- soda process.
2. These NaHCO3 salts decompose and liberate CO2 which causes corrosion.

16. ION EXCHANGE PROCESS:-
This process also known as Demineralization or Deionization process.
The process by which a mixture of similar charged ions can be separated by using an ion
exchange resin.
Mostly similar charged ions like cations or anions can be separated by this technique.
An Ion-exchange resin is a cross linked organic polymer network having some ionisable group.
It may be two types depending upon the nature of the ionisable group.
1. Cation exchange resin
2. Anion exchange resin

17. 1. CATION EXCHANGE RESIN:-
These resins exchange the cationic portion of minerals by their hydrogen
atom,they are known as cation exchangers.
2. ANION EXCHANGE RESIN:-
These resins exchange the anionic portion of the minerals and they are known as
anion exchanger.

19. PROCESS:-
In this process first we passed hard water through cation exchange column,which removes all the cations(like ca+2,mg+2
etc.) From it and equivalent amount of H+ ions are released from this column to water.
Thus,
2RH+ + CA+2 R2CA+2 + 2H+
2RH+ + MG+2 R2MG+2 +2H+
After this process,hard water is passed through anion exchange column,which removes all the anions (like SO4
-2,Cl- etc.)
from it, and equivalent amount of OH- ions are released from this column to water.
R`OH-2 + Cl-2 R`Cl- + OH-
2R`OH- + SO2- R`2 SO2-
4 + 2OH-
2R`OH- +CO2-
3 R`2CO2-
3 + 2OH-
H+ and OH- ions get combined to produce water molecule H+ + OH- + H2O

20. LIMITATIONS:-
1. Long-term costs are high when it comes to operating ion exchange equipment.
2. Waste is highly concentrated and requires careful disposal.
ADVANTAGES:-
1. Highly acidic or alkaline water samples can be purified by this process.
2. The hardness possessed by the deionised water is 2 ppm.
DISADVANTAGES:-
1. The ion exchanging resin are expensive,hence the cost of purification is high.
2. Raw water should contain turbidity below 10ppm.