Lime removes hardness from water by reacting with the calcium and magnesium hardness minerals and the natural alkalinity present, forming insoluble compounds that precipitate out of the water. These precipitates are then removed through sedimentation and filtration.
Here's a more detailed breakdown:
The Chemistry Behind Lime Softening
Hardness in water is primarily caused by dissolved calcium (Ca2+) and magnesium (Mg2+) ions. Lime softening involves adding lime (calcium hydroxide, Ca(OH)2) to the water, which causes a series of chemical reactions.
Reactions with Calcium Hardness
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Calcium bicarbonate hardness (temporary hardness): Lime reacts with calcium bicarbonate (Ca(HCO3)2) to form calcium carbonate (CaCO3), which is insoluble and precipitates:
Ca(HCO3)2 + Ca(OH)2 → 2CaCO3(s) + 2H2O
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Calcium non-carbonate hardness (permanent hardness): If soda ash (Na2CO3) is also used, it reacts with calcium non-carbonate hardness (like calcium sulfate, CaSO4), producing calcium carbonate that precipitates and sodium sulfate (Na2SO4) remains in solution:
CaSO4 + Na2CO3 → CaCO3(s) + Na2SO4
Reactions with Magnesium Hardness
Magnesium removal is pH-dependent. Lime reacts with magnesium bicarbonate (Mg(HCO3)2) to form magnesium hydroxide (Mg(OH)2) and calcium carbonate (CaCO3). Magnesium hydroxide is more soluble than calcium carbonate, so a higher pH is required to precipitate it effectively.
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First Stage: Lime reacts with magnesium bicarbonate similar to calcium:
Mg(HCO3)2 + Ca(OH)2 → MgCO3 + CaCO3(s) + 2H2O
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Second Stage (High pH): Further addition of lime raises the pH, converting magnesium carbonate (MgCO3) to magnesium hydroxide (Mg(OH)2) which precipitates.
MgCO3 + Ca(OH)2 → Mg(OH)2(s) + CaCO3(s)
Table Summarizing Reactions:
| Hardness Type | Reagent(s) | Product(s) Precipitated | Other Products |
|---|---|---|---|
| Calcium Bicarbonate | Lime (Ca(OH)2) | Calcium Carbonate (CaCO3) | Water (H2O) |
| Calcium Non-Carbonate | Soda Ash (Na2CO3) | Calcium Carbonate (CaCO3) | Sodium Sulfate (Na2SO4) |
| Magnesium Bicarbonate | Lime (Ca(OH)2) | Calcium Carbonate (CaCO3) | Magnesium Carbonate (MgCO3), Water (H2O) |
| Magnesium Carbonate (High pH) | Lime (Ca(OH)2) | Magnesium Hydroxide (Mg(OH)2), Calcium Carbonate (CaCO3) | None |
The Process: Sedimentation and Filtration
After the chemical reactions occur, the insoluble calcium carbonate and magnesium hydroxide form a precipitate (a solid that separates from the solution). This precipitate is then removed from the water through two main processes:
- Sedimentation: The water is allowed to sit quietly in large basins, allowing the precipitate to settle to the bottom due to gravity.
- Filtration: The water is then passed through filters (usually sand or other granular media) to remove any remaining fine particles of precipitate.
In summary:
Lime softening is an effective method for reducing water hardness by converting dissolved calcium and magnesium ions into insoluble forms that can be easily removed through sedimentation and filtration. It relies on chemical reactions between lime and the hardness-causing minerals, aided by pH adjustment and sometimes the addition of soda ash.
