Transcript Slide 1

LIME SOFTENING
& STABILIZATION
Murder Parties
Ralph Rottingrape
Marilyn Merlot
George W. Taliban
Queen Illicibeth;
Gridlock, the
Merchant of Tennis
Pen Ta Gon
Advisor to
the Emperor
Classifying Water Hardness
Description of water
Hardness expressed as mg/L of CaCO3
Soft
less than 50
Moderately soft
50–75
Slightly hard
75–150
Hard
150–300
Very hard
greater than 300
What is “lime”?
• Limestone, mainly
calcium carbonate,
CaCO3
• Quicklime, calcium
oxide, CaO
• Slaked lime,
calcium hydroxide,
Ca(OH)2
Limestone mining,
preparation
and use
Limestone Mining, cont’d
Limestone Mining, cont’d
Limestone Handling and Shipping
Lime Conversions
• Limestone can be converted to quick lime:
CaCO3
 CaO + CO2 (1800F)
• Quick lime is slaked with water:
CaO + H2O
 Ca(OH)2
• Slaked lime slowly reacts with CO2 in air:
…Ca(OH)2 + CO2  CaCO3 + H2O
• Dissolved lime (hardness) can precipitate:
Ca(HCO3)2
 CaCO3 + H2O + CO2
Lime Kilns
Calcination of Limestone
CaCO3 
CaO + CO2
Hydration: slaking with water
Water reacts with the quicklime to form hydrated lime:
CaO + H2O

Ca(OH)2
Water Softening
The
removalofofdivalent
divalentmetal
metal
ions
The removal
ions
Precipitation/settling or
Precipitation/settling
orIXIXresins
resins
Ca(HCO3)2 + Ca(OH)2
 2CaCO3¤
+ 2H2O
Mg(HCO3)2 + 2Ca(OH)2  Mg(OH)2¤ + 2CaCO3¤
Note that the CaCO3 precipitated
is chemically identical to limestone
Typical lime softening process…
Ca(OH)2
Lime



Lime sludge
…presented as a complete
drinking water treatment process
LIME
SLAKING
…and
DOSING
Precipitation occurs in a settling tank
LAGOON
DEWATERING
Why are we concerned about corrosion?
Stability Relationship
• The basic Relationship in Stabilization
pH
s

 p Ca
2
  p  Alk .  pK i Ca C 0   pK sp Ca C 0  
3
3
Has been simplified by Powell, Bacon and Lill to
pHs
=
p(Ca2+) + p(Alk) + C
Powell, Bacon and Lill Diagram
Langelier index
mg/L
Modified Caldwell Lawrence Diagram