Explain internal treatment methods for removal of scales.

Internal treatment (Sequestration):

 In this method some chemicals reagents are added directly to the boiler water for removing dangerous scale forming salts.

 The chemicals convert the scale forming impurities into sludge or more dissolved compounds.

 This method is generally followed by blow down operation.

Important internal conditioning/treatment methods:

Carbonate conditioning:

Reagent added is sodium carbonate.

In low pressure boilers scale formation can be avoided by adding sodium carbonate to boiler water

 CaSO4 +Na2CO3 -->CaCO3↓+Na2SO4  Sludge

Deposition of CaSO4 as scale doesn’t takes place and calcium is precipitated as loose sludge of CaCO3 which can be removed by blow down operation.

Phosphate conditioning:

Reagent added is sodium phosphate.

In high pressure boilers scale formation can be avoided by adding sodium phosphate. It reacts with hardness of water forms non adherent, easily 

removable soft sludge of Ca / Mg phosphates.

 3CaCl2 +2Na3PO4--> Ca3 (PO4)2 ↓+ 6NaCl

 Sludge

We can use different phosphates based on PH

of water

 Na3PO4 (alkaline in nature) is used when alkalinity is low.

 Na2HPO4 (weakly alkaline) is used when alkalinity is sufficient.

 NaH2PO4 (acidic) is used when alkalinity is high

Calgon conditioning:

Reagent added is Calgon.

Calgon = sodium hexa meta phosphate

The process involves the addition of calgon to boiler water. 

It prevents scale and sludge formation by forming highly soluble calcium hexa meta phosphate complex compound with CaSO4

 

1.{Na2 [Na4(PO3)6] --->2Na+ + [Na4(PO3)6]2-}

2.{2CaSO4 +[Na4(PO3)6]2-  --> [Ca2(PO3)6]2- +2Na2SO4}

Highly Soluble 

complex ion