JNTU B.Tech I ENGINEERING CHEMISTRY Previous Question Paper (May - 2019 - R16)

Permanent hardness in water is caused by the presence of dissolved calcium and magnesium salts that do not precipitate upon boiling. These salts remain in the water even after prolonged heating, unlike temporary hardness which is caused by bicarbonates.

The primary chemical substances responsible for permanent hardness include:

• Calcium Chloride (CaCl₂)
• Magnesium Chloride (MgCl₂)
• Calcium Sulphate (CaSO₄)
• Magnesium Sulphate (MgSO₄)

These compounds are highly soluble in water and contribute to the formation of scale in industrial boilers and pipes, necessitating chemical treatment methods such as ion exchange or lime-soda processes for removal.

Caustic embrittlement is a form of intercrystalline corrosion caused by the presence of sodium hydroxide (NaOH) in boiler water, which leads to the formation of brittle cracks in steel, particularly at stressed areas like joints, rivets, and bends. It can be prevented through the following methods:

• Addition of Sodium Phosphate: Sodium phosphate acts as a chemical conditioning agent. It reacts with sodium hydroxide to form sodium phosphate, which is non-corrosive and does not cause embrittlement. The reaction is:
  3Na₂SO₄ + 2Na₃PO₄ → 2Na₃PO₄ + 3Na₂SO₄ (neutralization)
• Addition of Tannin or Lignin: These organic compounds are added to the boiler water to form a protective film over the metal surface, preventing the concentration of caustic soda at the stressed regions.
• Addition of Sodium Sulfate: Sodium sulfate acts as an inhibitor by filling the cracks and crevices, thereby preventing the penetration of caustic soda. The recommended ratio of sodium sulfate to sodium hydroxide is maintained based on the boiler pressure:
  1:1 for pressures up to 10 atm, 2:1 for 10-20 atm, and 3:1 for pressures above 20 atm.
• Maintaining Proper pH: Ensuring that the boiler water remains slightly alkaline but not excessively so, by strictly controlling the softening process and blow-down operations to prevent the accumulation of sodium hydroxide.