Objective 03: Cold-Lime Soda Softening

Question 1

How do chemical methods treat water?

Answer 1

Chemicals are used to react with the dissolved solids to:
1) change them into highly soluble dissolved solids that do not easily form scale

2) precipitate them out as insoluble and separate from the water

Question 2

How does Cold Lime-Soda Softening work?

Answer 2

1. Raw water (room temperature), softening chemicals, and a coagulant are all mixed together in a central mixing zone by motor driven agitators.
2. Insoluble compounds are produced when the softening chemicals react with the scale forming materials in the water
3. The insoluble compounds, with the aid of coagulant, form a sludge which can be discharged from the bottom of the softener
4. Part of the sludge is kept in suspension by the action of the agitators and the flow of water, forming a sludge blanket through which the water passes as it flows upwards towards the outlet so that it can aid in removing impurities not already precipitated from the water

Question 3

What are the 2 chemicals used in Cold Lime-Soda Softening?

Answer 3

1. Calcium Hydroxide

2. Sodium Carbonate

Question 4

Describe the compounds that Calcium Hydroxide reacts with in Lime-Soda Softening

Answer 4

1. Calcium Bicarbonate -> Calcium Carbonate + Water
2. Magnesium Bicarbonate -> Magnesium Hydroxide + Calcium Carbonate + Water
3. Magnesium Sulphate -> Magnesium Hydroxide + Calcium Sulphate
4. Magnesium Chloride -> Magnesium Hydroxide + Calcium Chloride

Question 5

Why is Calcium Carbonate and Magnesium Hydroxide produced?

Answer 5

They form soft sludge that can usually be removed by blowdown.

Question 6

Describe the compounds that Sodium Carbonate reacts with in Cold Lime-Soda Softening

Answer 6

1. Calcium Sulphate (produced from Magnesium Sulphate) -> Calcium Carbonate + Sodium Sulphate
2. Calcium Chloride (produced from Magnesium Chloride) -> Calcium Carbonate + Sodium Chloride

Question 7

What are the 2 objectives of Cold Lime-Soda Softening?

Answer 7

1. Complete precipitation of all calcium and magnesium salts
2. If the salts do enter the boiler, it should be in the form of calcium carbonate and magnesium hydroxide which are easier to remove