LIME SOFTENING LIME SOFTENING & STABILIZATION& STABILIZATION
Murder Parties
Ralph Rottingrape
Marilyn MerlotGeorge W. Taliban
Queen Illicibeth; Gridlock, the Merchant of Tennis
Pen Ta GonAdvisor 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”?What is “lime”?
• Limestone, mainly calcium carbonate,
CaCO3
• Quicklime, calcium oxide, CaO
• Slaked lime, calcium hydroxide,
Ca(OH)2
Limestone mining, Limestone mining, preparation preparation and useand 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 SofteningWater Softening
The removal of divalent metal ions Precipitation/settling or IX 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
The removal of divalent metal ionsPrecipitation/settling or IX resins
Typical lime softening process…
Lime sludge
Ca(OH)2
Lime
…presented as a complete drinking water treatment process
LIMESLAKING
…andDOSING
Precipitation occurs in a settling tank
LAGOONDEWATERING
Why are we concerned about corrosion?
Stability Relationship
• The basic Relationship in Stabilization
pHs p Ca2 p Alk. pKi CaC03 pKsp CaC03 Has been simplified by Powell, Bacon and Lill to
pHs = p(Ca2+) + p(Alk) + C
Powell, Bacon and Lill Diagram
mg/L
Langelier index
Modified Caldwell Lawrence Diagram