Calcium, in the form of lime (calcium hydroxide), plays a crucial role in certain large-scale water softening processes by facilitating the removal of calcium hardness already present in the water.
Water hardness is primarily caused by dissolved minerals like calcium and magnesium. To make water "soft," these minerals must be removed or neutralized. While ion exchange (like household softeners using sodium) is common, chemical precipitation methods are used on a larger scale.
The Role of Lime in Water Softening
One common chemical softening method is the Lime-Soda Ash Process. In this process, calcium (as part of lime) is not added to soften water directly, but rather used as a reagent to remove the calcium and magnesium ions causing the hardness.
Here's how it works based on the reference:
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Step 1: Adding Lime
Just enough lime (calcium hydroxide, Ca(OH)₂) is added to the hard water. The reference states this is done "to precipitate the calcium as carbonate and the magnesium as hydroxide". This means the added lime reacts with the calcium bicarbonate and magnesium bicarbonate present in the hard water (the primary sources of temporary hardness).- Calcium bicarbonate + Calcium hydroxide → Calcium carbonate (solid) + Water
- Magnesium bicarbonate + Calcium hydroxide → Magnesium carbonate + Water (followed by magnesium carbonate reaction with lime to form magnesium hydroxide solid)
- Magnesium sulfate/chloride + Calcium hydroxide → Magnesium hydroxide (solid) + Calcium sulfate/chloride
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Step 2: Precipitating Hardness
These reactions convert the dissolved calcium and magnesium ions into solid, insoluble forms: calcium carbonate (CaCO₃) and magnesium hydroxide (Mg(OH)₂). Because these are solids, they no longer contribute to water hardness and can be removed from the water, typically through settling (sedimentation) and filtration. -
Step 3: Removing Remaining Calcium
After the lime addition and removal of precipitates, sodium carbonate (soda ash, Na₂CO₃) is added. The reference notes this is done "to remove the remaining calcium salts". This step targets non-carbonate calcium hardness (like calcium sulfate or calcium chloride) that wasn't removed by the lime alone.- Calcium sulfate + Sodium carbonate → Calcium carbonate (solid) + Sodium sulfate
This second precipitation step ensures that most of the calcium is removed as solid calcium carbonate.
Summary Table
| Reagent Added | Primary Function | Ions Targeted | Product Formed | Removal Method |
|---|---|---|---|---|
| Lime (Ca(OH)₂) | Precipitate temporary hardness (Ca & Mg bicarbonates) and Mg | Dissolved Ca²⁺, Mg²⁺ | CaCO₃(s), Mg(OH)₂(s) | Settling, Filtration |
| Sodium Carbonate (Na₂CO₃) | Precipitate remaining non-carbonate Ca hardness | Remaining dissolved Ca²⁺ (from non-carbonate salts) | CaCO₃(s) | Settling, Filtration |
In essence, calcium (as part of the added lime) is used as a reactant to chemically transform dissolved calcium and magnesium ions into solid precipitates that can be physically removed from the water, thereby reducing hardness.
