Sodium silicate (Na2SiO3) functions as a selective chemical agent designed to facilitate the physical separation of impurities from molten salt. By reacting specifically with magnesium chloride (MgCl2) and calcium chloride (CaCl2), it forces these contaminants to change phases from liquid to solid while leaving the desired sodium chloride (NaCl) in a liquid state.
High-temperature processing utilizes sodium silicate to chemically convert soluble impurities into solid silicate minerals. This induced phase difference ensures that contaminants can be physically separated from the purified liquid sodium chloride.
The Mechanism of Phase Transition
Targeting Specific Impurities
In raw molten salt mixtures, impurities like magnesium chloride (MgCl2) and calcium chloride (CaCl2) typically exist in the same liquid phase as sodium chloride.
Sodium silicate is introduced specifically to disrupt this homogeneity. It acts as a reactant that targets these specific chloride impurities rather than the primary sodium chloride salt.
The Chemical Conversion
The addition of Na2SiO3 triggers a chemical reaction within the molten bath.
This reaction transforms the chemical structure of the impurities. They cease to exist as simple chlorides and are converted into complex silicate minerals.
Formation of High-Melting-Point Solids
The primary outcome of this reaction is the creation of new solid compounds: Na4Ca4Si6O18 and Na2Mg2Si2O7.
Unlike the original chlorides, these silicate minerals possess high melting points. Consequently, they precipitate out of the solution as solids, even at temperatures where the sodium chloride remains molten.
Achieving Efficient Separation
Creating a Phase Differential
The core value of this process is the creation of a distinct phase difference.
Before the additive is introduced, the mixture is a single liquid phase. After the reaction, the mixture becomes a suspension of solid silicate particles within liquid sodium chloride.
Facilitating Physical Removal
Because the impurities are now solid and the product (NaCl) is liquid, mechanical separation becomes straightforward.
The liquid NaCl can be drained or filtered away from the solid mineral waste, resulting in a purified sodium chloride product.
Understanding the Constraints
Reactant Specificity
The success of this process relies entirely on the chemical affinity between sodium silicate and the specific impurities (Mg and Ca).
If the molten salt contains impurities that do not react with silicates to form high-melting-point solids, this phase transition method will not be effective for those specific contaminants.
Thermal Management
The process is strictly bound by temperature controls.
The furnace temperature must be maintained in a specific window: high enough to keep the sodium chloride liquid, but strictly below the melting points of the newly formed silicate minerals (Na4Ca4Si6O18 and Na2Mg2Si2O7). If the temperature rises too high, the impurities will re-melt, nullifying the separation effort.
Making the Right Choice for Your Process
If you are designing or optimizing a molten salt purification system, consider how this additive aligns with your specific contamination profile.
- If your primary focus is removing Calcium and Magnesium: This method is highly effective as it chemically locks these specific elements into insoluble solid minerals.
- If your primary focus is recovering pure Sodium Chloride: This approach allows you to maintain the NaCl in a liquid state for easy extraction while leaving waste products behind as solids.
By leveraging phase transition additives, you convert a complex chemical separation problem into a simpler physical separation task.
Summary Table:
| Feature | Description |
|---|---|
| Additive Function | Selective chemical conversion of impurities (MgCl2, CaCl2) |
| Mechanism | Formation of high-melting-point silicate minerals (Na4Ca4Si6O18, Na2Mg2Si2O7) |
| Phase Change | Liquid impurities transition into solid precipitates |
| Separation Method | Physical filtration/draining of liquid NaCl from solid waste |
| Critical Control | Precise furnace temperature management below silicate melting points |
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References
- Effect of physical properties of liquid phase by MD simulation on NaCl separation behavior during the phase transition of molten salt chloride slag. DOI: 10.1038/s41598-025-05909-x
This article is also based on technical information from Kintek Furnace Knowledge Base .
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