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