Water Hardness: Temporary and Permanent

Soap

• Soap is a sodium or potassium salt of long chain fatty acids like palmitic, oleic, or stearic acids
• They form a colloidal medium in water that degreases fibers, coagulates dirt, and dislodges particles by jerking clothes
• Calcium (Ca2+) and magnesium (Mg2+) salts react with soap, rendering it unavailable for washing

Types of Hardness

• Hardness in water is due to the presence of Ca and Mg salts, classified into two types, depending on the chemical behavior

Temporary Hardness

• Caused by thermally unstable bicarbonates of Ca and Mg, also known as 'Carbonate hardness'
• Decomposition on heating:
  • Ca(HCO3)2 decomposes into CaCO3, CO2, and H2O when heated
  • Mg(HCO3)2 decomposes into Mg(OH)2 and CO2 when heated
• Lime treatment eliminates temporary hardness like this
  • Ca(HCO3)2 reacts with Ca(OH)2 to form CaCO3 and H2O
  • Mg(HCO3)2 reacts with Ca(OH)2 to form Mg(OH)2, CaCO3, and H2O

Permanent Hardness

• Exhibits thermal stability unlike temporary hardness, but can be eliminated by lime and soda
• Reactions involved:
  • CaCl2 reacts with Na2CO3 to form CaCO3 and NaCl
  • MgCl2 reacts with Na2CO3 to form MgCO3 and NaCl
  • MgCO3 is soluble in water and contributes to hardness, but lime removes it, as it reacts with Ca(OH)2 to form Mg(OH)2 and CaCO3
• Notes on hardness removal:
  • Temporary Ca-hardness is removed using one equivalent of lime
  • Temporary Mg-hardness is removed using two equivalents of lime
  • Permanent Ca-hardness is removed using one equivalent of soda
  • Permanent Mg-hardness is removed by one equivalent of lime and soda

Scale of Hardness

• Hardness is expressed in 'CaCO3 scale,' where salts are converted into equivalents of CaCO3
• Formula for calculating hardness in CaCO3 scale:
  • (Mass of hardness producing substance × 50) / Equivalent mass of hardness producing substance
• This also equals "No. of equivalents of hardness producing substance × 50"
• Example: Hardness caused by 38 mg/l of MgCl2 equals 40 mg/l of CaCO3
• Whenever hardness is reported, the value should be followed by 'of CaCO3'

Units of Hardness

• Commonly used units to express hardness:
  • Parts per million (ppm): International unit
    • 1 ppm of hardness equals 1 part of CaCO3 equivalent in 10^6 parts of water
    • 1 mg/l = 1 ppm (since 1 ml of water weighs ≈ 1 g)
  • Degree Clarke (°Cl): British unit
    • 1 °Cl of hardness equals 1 part of CaCO3 equivalent in 70,000 parts of water
  • Decree French (°Fr): French unit
    • 1 °Fr of hardness equals 1 part of CaCO3 equivalent in 10^5 parts of water, equating to 10mg/l
• Relationships between hardness units:
  • 1 ppm ≈ 0.1 °Fr ≈ 0.07 °Cl
  • 1 °Cl ≈ 14.3 ppm ≈ 1.43 °Fr
  • 1 °Fr ≈ 10 ppm ≈ 10 mg/l ≈ 0.7 °Cl
• Equivalence in grams for various compounds related to hardness:
  • 100 g of CaCO3 ≡ 162 g of Ca(HCO3)2 ≡ 146 g of Mg(HCO3)2
  • 136 g of CaSO4
  • 111 g of CaCl2
  • 148 g of Mg(NO3)2 ≡ 44 g of CO2
  • 2 g of H+

Degree of Hardness

• Approximate representation of hardness:
  • 0-50 ppm is soft water.
  • 50-100 ppm is moderately soft water.
  • 100-150 ppm is moderately hard water.
  • 150-250 ppm is hard water.

  • 250 ppm is very hard water.

• Note: Hardness removal is preferred for moderately soft water or harder, but not for soft water (already soft) or very hard water (high sodium salt concentration).

Softening of Hard water

• It is the process of removing hardness from water, with methods like boiling, lime/Clarke process, lime-soda process, and ion-exchange
• Boiling or lime use shows minimal softening; lime-soda or ion-exchange are effective
• Combined methods yield better results

Lime-Soda (L-S) Process

• Both lime and soda are used as softening agents
• Lime-soda process variations:
  • Cold lime-soda process: uses both lime and soda agents at room temperature
  • Hot lime-soda process: uses only soda at higher temperatures

Cold Lime-Soda Process

• Operated at room temperature, employs two methods of operation:
  • Intermittent Softening
  • Continuous Softening
• Both lime and soda are used; generally, 10% excess chemicals are used for better softening
• Solubility limits of CaCO3 (~17ppm) and Mg(OH)2 (~15ppm) restrict residual hardness to 32ppm
• Temporary hardness reactions
  • Ca(HCO3)2 + Ca(OH)2 => 2CaCO3↓ + 2H2O
  • Mg(HCO3)2 + Ca(OH)2 => MgCO3 + CaCO3↓ + 2H2O
  • MgCO3 + Ca(OH)2 => Mg(OH)2↓ + CaCO3↓
• Permanent hardness reactions
  • CaCl2 + Na2CO3 => CaCO3↓ + 2NaCl
  • MgCl2 + Na2CO3 => MgCO3 + 2NaCl
  • MgCO3 + Ca(OH)2 => Mg(OH)2↓ + CaCO3↓

Reactions and Requirements of Lime and Soda for Softening

• Provides a table summarizing chemical reactions and equivalent requirements of lime and soda for softening different constituents of hardness

Lime Requirement

• Lime requirement = 74/100 [Temp. Ca + 2 x Temp. Mg + Perm (Mg + Fe + Al) + CO2 + H (free acid) + HCO3- - NaAlO2 in equiv. of CaCO3]

Soda requirement

• Soda requirement = 106/100 [perm (Ca + Mg + Al + Fe) + H+ (free acid) - HCO3- in equiv. CaCO3]

Intermittent Softener

• Consists of multiple large tanks for continuous water supply
• A small amount of alum is also mixed with lime and soda for effective separation of sludge within 2 hours
• The efficiency of the process depends upon mixing chemicals with water
• A horizontal stirrer operated by an externally placed motor is used to do this
• The bottom having a good slope effects easy separation and removal of sludge
• Residual hardness = 50 – 60 ppm (under optimum pH 8.5)
• Duration:
  • In stepwise process it takes 15 mins. mixing, ~ 2 hours settling and rest sludge removal, supply and recharging total = 3 – 4 hours

Continuous Softener

• Usually smaller with a cylindrical tank
• Has a coaxial cylinder with a vertical stirrer
• The coaxial cylinder is fixed to the outer one by a filter bed consisting of synthetic fibrous material
• Water and chemicals are added from the top to the inner cylinder
• Precipitates separate and move to the bottom, while water moves upward through a filter bed
• Clear soft water is released through a valve.

Hot Lime-Soda Process

• Operates with water at 80-100°C with only soda
• Water and soda under spray and superheated steam (300°C)
• Reaction accelerates CaCO3 and Mg(OH)2 production from salts

Advantages of Hot Lime-Soda Process

• Faster reaction rate (10-15 mins) compared to cold process (2 hrs)
• Increased softening efficiency; residual hardness is 17-34 ppm versus 50-60 ppm in the cold process
• Coarse and heavy sludge separates easily

Hot Lime-Soda Process, Disadvantages

• The process may contaminate water with dissolved silica from alkaline medium filtration
• The alkaline water causes turbidity, which produces scale in boiler
• Heating increases production cost
• Requires careful operation and skilled supervision
• Produces a large amount of sludge
• Limited to a small quantity of water
• Residual hardness is unsuitable for boiler use, supplemented by ion-exchange for high-pressure boilers

Ion-Exchange Process

• The process replaces hardness-causing ions with non-hardness-producing ions, featuring a reaction between liquid and solid phases
• Two types of ion-exchange: cation exchange and double exchange (demineralization/deionization)
• Cation exchange uses a mobile ion with higher electropositivity to displace cations into the solid phase
• Occurring ion-exchangers called zeolites have a formula, Na2O • Al2O3 • xSiO2 • yH2O (x = 2-10, y = 2-6). Synthetic zeolites are also prepared

Cation Exchange Process

• The process can be written Na₂Ze (sodium zeolite where 'Ze' stands for O·Al2O3 ·xSiO2·yH2O)
• Reactions:
  • Ca(HCO3)2 + Na2Ze -> CaZe + 2NaHCO3
  • Mg(HCO3)2 + Na2Ze -> MgZe + 2NaHCO3
  • CaCl2 + Na2Ze -> CaZe + 2NaCl
  • MgCl2 + Na2Ze - MgZe + 2NaCl
  • CaSO4 + Na2Ze -> CaZe + Na2SO4
  • MgSO4 + Na2Ze -> MgZe + Na2SO4
• Due to sodium's electropositivity, Fe2+, Mn2+, and Al3+ ions are also removed
• Zeolites are large molecules forming a solid phase where cations pass and sodium ions are released
• Softening:
  • Hard water is added from the top and passes down through the zeolite bed. Hard water softens and passes out through the outlet at the bottom.
• Regeneration:
  • Zeolite is exhausted and needs regeneration after complete exhaust for further softening.
  • Treatment with brine solution (10% NaCl solution) reverses the equilibrium reaction.
• Washing:
  • During regeneration Ca and Mg chlorides form which are likely to stick to the system and reduce the strength of the exchangers.

Advantages of Ion-Exchange Process

• The residual hardness is 8-10 ppm
• The equipment is compact and occupies less space
• There is no precipitate and no sludge disposal problem
• The system adjusts automatically and it has less softening time
• Iron and manganese ions are also removed

Disadvantages of Ion-Exchange Process

• Turbidity can choke the filter bed.
• Substances like color, turbidity, acidity, alkalinity or Fe/Mn salts damages the zeolite.
• Zeolite dissolves in hot water.
• The softened water is only suitable for laundry but not for boilers

Double Exchange Process (Demineralisation or Deionisation)

• Used for water purification by eliminating all ions
• Two exchangers are used in series-cation exchanger, followed by anion exchanger
• Cation exchanges are high molecular weight, cross-linked polymer containing anionic and active mobile cation groups, while anion exchange resins contain amine or quanternary ammonium groups as bulky, inmobile cation
• Functional groups of synthetic ion-exchangers: strongly acidic/basic and weakly acidic/basic
• Acceptable hardness for boilers depends on operating stem pressure. Acceptable hardness decreases with increasing pressure

Organic Polymers

• Organic Polymer treated with concentrated sulfuric acid produces a sulfonic acid of the polymer.
• Organic Polymer + H2SO4 → Sulfonated organic polymer + H2O

Operational Procedure and key information

• High exchange capacity is exhibited by both hydrogen and sodium cycles
• Highly basic/weakly basic regenerations are achieved using caustic soda/ammonium hydroxide

Anion Exchanger

• A weakly basic ion-exchanger exchanges strong acids (Cl-, SO42-, NO3-) but not weak anions
• Strongly basic anion exchanger used for quick and efficient reaction There are issues with high pressure boilers employing the other exchanges
• Advantages: It caters to the is essential for high pressure boilers
• Disadvantage:
  • Costly equipments are there
• Since super saturation with CaCO3 and Mg(OH2) leads to after precipitation, this is prevented by treating with excess lime and subsequent filtration by pressure filter prior to the ionexchange process.