Water Hardness &
Precipitation Softening
Precipitation softening processes are used to
reduce raw water hardness, alkalinity, silica, and other constituents.
This helps prepare water for direct use as
cooling tower makeup or as a first-stage treatment followed by ion exchange for
boiler makeup or process use.
The water is treated with lime or a
combination of lime and soda ash (carbonate ion). These chemicals react with
the hardness and natural alkalinity in the water to form insoluble compounds.
The compounds precipitate and are removed
from the water by sedimentation and, usually, filtration.
Waters with moderate to high hardness and
alkalinity concentrations (150-500 ppm as CaCO3) are often treated
in this fashion.
In almost every raw water supply,
hardness is present as calcium and magnesium bicarbonate, often referred to as
carbonate hardness or temporary hardness.
These compounds result from the action of
acidic, carbon dioxide laden rain water on naturally occurring minerals in the
earth, such as limestone. For example:
CO2
|
+
|
H2O
|
=
|
H2CO3
|
|||||
carbon dioxide
|
water
|
carbonic acid
|
|||||||
H2CO3
|
+
|
CaCO3 ¯
|
=
|
Ca(HCO3)2
|
|||||
carbonic acid
|
calcium carbonate
|
calcium bicarbonate
|
|||||||
Hardness may also be present as a sulfate or
chloride salt, referred to as non-carbonate or permanent hardness.
These salts are caused by mineral acids
present in rain water or the solution of naturally occurring acidic minerals.
The significance of "carbonate" or
"temporary" hardness as contrasted to "non-carbonate" or
"permanent" hardness is that the former may be reduced in
concentration simply by heating.
In effect, heating reverses the solution
reaction:
Ca(HCO3)2
|
+
|
Heat
|
=
|
CaCO3 ¯
|
+
|
H2O
|
+
|
CO2
|
calcium bicarbonate
|
calcium carbonate
|
water
|
carbon dioxide
|
Reduction of non-carbonate hardness, by
contrast, requires chemical addition.
A combination of lime and soda ash, along
with coagulant and flocculant chemicals, is added to raw water to promote a
precipitation reaction. This allows softening to take place.
Precipitation softening accomplished at
ambient temperatures is referred to as cold lime softening.
When hydrated lime, Ca(OH)2, is
added to the water being treated, the following reactions occur:
CO2
|
+
|
Ca(OH)2
|
=
|
CaCO3 ¯
|
+
|
H2O
|
|||||||||||||||
carbon dioxide
|
calcium hydroxide
|
calcium carbonate
|
water
|
||||||||||||||||||
Ca(HCO3)2
|
+
|
Ca(OH)2
|
=
|
2CaCO3 ¯
|
+
|
2H2O
|
|||||||||||||||
calcium bicarbonate
|
calcium hydroxide
|
calcium carbonate
|
water
|
||||||||||||||||||
Mg(HCO3)2
|
+
|
2Ca(OH)2
|
=
|
Mg(OH)2 ¯
|
+
|
2CaCO3 ¯
|
+
|
2H2O
|
|||||||||||||
magnesium bicarbonate
|
calcium hydroxide
|
magnesium hydroxide
|
calcium carbonate
|
water
|
|||||||||||||||||
If the proper chemical control is maintained
on lime feed, the calcium hardness may be reduced to 35-50 ppm.
Magnesium reduction is a function of the
amount of hydroxyl (OH-) alkalinity excess maintained.
Non-carbonate or permanent calcium hardness,
if present, is not affected by treatment with lime alone.
If non-carbonate magnesium hardness is
present in an amount greater than 70 ppm and an excess hydroxyl alkalinity of
about 5 ppm is maintained, the magnesium will be reduced to about 70 ppm, but
the calcium will increase in proportion to the magnesium reduction.
For example, in cold lime treatment of a
water containing 110 ppm of calcium, 95 ppm of magnesium, and at least 110 ppm
of alkalinity (all expressed as calcium carbonate), calcium could theoretically
be reduced to 35 ppm and the magnesium to about 70 ppm.
However, an additional 25 ppm of calcium
would be expected in the treated water due to the following reactions:
MgSO4
|
+
|
Ca(OH)2
|
=
|
Mg(OH)2 ¯
|
+
|
CaSO4
|
magnesium sulfate
|
calcium hydroxide
|
magnesium hydroxide
|
calcium sulfate
|
MgCl2
|
+
|
Ca(OH)2
|
=
|
Mg(OH)2 ¯
|
+
|
CaCl2
|
magnesium chloride
|
calcium
|
magnesium hydroxide
|
calcium chloride
|
To improve magnesium reduction, which also
improves silica reduction in cold process softening, sodium aluminate may be
used.
The sodium aluminate provides hydroxyl ion
(OH-) needed for improved magnesium reduction, without increasing
calcium hardness in the treated water.
In addition, the hydrolysis of sodium
aluminate results in the formation of aluminum hydroxide, which aids in floc
formation, sludge blanket conditioning, and silica reduction. The reactions are
as follows:
Na2Al2O4
|
+
|
4H2O
|
=
|
2Al(OH)3 ¯
|
+
|
2NaOH
|
||||||||||||
sodium aluminate
|
water
|
aluminum hydroxide
|
sodium hydroxide
|
|||||||||||||||
Mg
|
[
|
SO4 |
]
|
+
|
2NaOH
|
=
|
Mg(OH)2¯
|
+
|
[
|
Na2SO4 |
]
|
|||||||
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