When calcium carbonate (CaCO₃) reacts with sulfur dioxide (SO₂), the process primarily leads to the formation of hemihydrate calcium sulfite, which then undergoes oxidation to produce gypsum.
The reaction between calcium carbonate, commonly known as limestone, and sulfur dioxide is a significant chemical process, particularly in industrial contexts aimed at mitigating air pollution. This reaction occurs in a two-stage mechanism, as described in scientific literature (Guri et al. 1982).
Stage 1: Formation of Hemihydrate Calcium Sulfite
In the initial step of the reaction, calcium carbonate reacts with sulfur dioxide to form hemihydrate calcium sulfite. This intermediate compound is represented by the chemical formula CaSO₃·½H₂O.
- Reactants:
- Calcium Carbonate (CaCO₃): A naturally occurring compound found in limestone, marble, and chalk.
- Sulfur Dioxide (SO₂): A major air pollutant, often released from burning fossil fuels.
- Intermediate Product:
- Hemihydrate Calcium Sulfite (CaSO₃·½H₂O): The primary product formed in the first phase of the reaction.
Stage 2: Oxidation to Gypsum
Following the formation of hemihydrate calcium sulfite, a crucial subsequent step occurs: oxidation. In this phase, the sulfur within the calcium sulfite, which is in a +4 oxidation state (S(IV)), undergoes oxidation to a higher oxidation state, specifically +6 (S(VI)). This transformation results in the formation of gypsum, which is hydrated calcium sulfate (CaSO₄·2H₂O).
This oxidation step is critical for the final product's stability and utility.
Summary of the Reaction Process
The entire reaction sequence can be summarized as follows:
| Component | Chemical Formula | Role in Reaction |
|---|---|---|
| Calcium Carbonate | CaCO₃ | Initial reactant, acts as a sorbent for SO₂. |
| Sulfur Dioxide | SO₂ | Primary gaseous reactant, often an atmospheric pollutant. |
| Calcium Sulfite | CaSO₃·½H₂O (Hemihydrate) | Intermediate product, formed in the first stage. |
| Gypsum | CaSO₄·2H₂O | Final stable product, formed after oxidation of calcium sulfite. |
Practical Applications and Significance
This chemical reaction is fundamental to flue gas desulfurization (FGD) systems, widely employed in power plants and industrial facilities to remove sulfur dioxide from exhaust gases. By reacting SO₂ with calcium carbonate (limestone slurry), industries can effectively reduce harmful sulfur emissions, preventing environmental issues such as acid rain and improving air quality. The resulting gypsum is a valuable byproduct used in construction materials like plasterboard.
