Question

You are considering ferrous sulfate (FeSO4) as a coagulant because it is cheap (JD240/ton)
compared to ferric chloride (JD520/ton). But you will need to use chlorine (Cl2) to oxidize ferrous
ion to ferric.
a. The chlorine is reduced to chloride in oxidizing the iron. Balance the overall reaction and
determine how much chlorine is needed (kg Cl2/kg Fe).
b. If chlorine costs JD285/ton, which coagulant strategy would you select?

Answer 1

Molecular weight of

• Ferrous sulphate = 151 gm per mole
• Chlorine = 71 gm per mole

a) Here we have the balanced reaction of ferrous sulphate with chlorine as,

From the above reaction it is clear that 3 moles of chlorine are required to oxidise 6 moles of ferrous sulphate.

We can also say that 213 gm (3×71= 213 gm) of chlorine is required to oxidise 906 gm (6×151= 906 gm) of ferrous sulphate.

Thus (213/906 = 0.235 Kg of Chlorine is required per Kg of ferrous sulphate.

b.) Now, As chlorine is required to oxidise ferrous sulphate into ferric chloride, overall cost per ton of ferrous sulphate will be

And as cost of using ferric chloride directly is JD 520 per ton,we will use the first strategy in which we will reduce the ferrous sulphate to ferric chloride by using chlorine.