1. Introduction
Calcium carbide slag is an industrial by-product mainly composed of calcium hydroxide (Ca(OH)₂). It can be used as an alternative alkaline reagent in wet flue gas desulfurization (FGD) systems to remove sulfur dioxide (SO₂) from industrial exhaust gases.
Compared with conventional limestone-based desulfurization, calcium carbide slag may offer advantages in chemical reactivity and industrial waste utilization. However, its application requires proper material preparation and process control.
2. Desulfurization Principle
The main active component of calcium carbide slag is calcium hydroxide, which reacts with sulfur dioxide in flue gas.
The main chemical reaction is:
Ca(OH)₂ + SO₂ → CaSO₃ + H₂O
The resulting calcium sulfite can be further oxidized into calcium sulfate in the presence of oxygen:
2CaSO₃ + O₂ → 2CaSO₄
Under suitable operating conditions, calcium sulfate can form gypsum, which may be separated from the slurry for further treatment or qualified reuse.
The actual desulfurization efficiency depends on the alkalinity of the raw material, gas-liquid contact, slurry circulation, pH, and oxidation conditions.
3. Step-by-Step Operating Process
Step 1: Raw Material Inspection
Before use, calcium carbide slag should be analyzed to determine its chemical composition and suitability for desulfurization.
Important parameters include:
- Calcium hydroxide content and available alkalinity
- Moisture content and particle size
- Impurities and inert solids
- Residual calcium carbide content, where relevant
Consistent raw material quality helps improve process stability and reduce operating costs.
Step 2: Slurry Preparation
The calcium carbide slag is prepared as an aqueous slurry in a suitable mixing tank.
The slurry concentration should be determined according to laboratory testing, material characteristics, and the design of the desulfurization system.
Effective mixing is necessary to maintain a uniform slurry and prevent excessive sedimentation. Any pretreatment or screening should be based on the characteristics of the raw material.
Safety note: Calcium carbide slag must not be assumed to be free of residual calcium carbide. Residual CaC₂ reacts with water and releases flammable acetylene gas. Material safety and composition must be verified before preparation.
Step 3: Flue Gas Absorption
The prepared slurry is pumped into the desulfurization absorber, where it contacts the incoming flue gas.
Sulfur dioxide dissolves into the liquid phase and reacts with the alkaline components of the slurry.
Operators should monitor:
- Inlet and outlet SO₂ concentrations
- Slurry pH
- Flue gas flow rate and temperature
- Slurry circulation rate
- Absorber pressure drop
Operating conditions should be adjusted according to the approved process design and measured performance.
Step 4: Oxidation and By-Product Separation
In systems designed to produce gypsum, oxidation air may be introduced to convert calcium sulfite into calcium sulfate.
The resulting slurry is then processed using suitable solid-liquid separation equipment, such as a thickener or filter.
The quality and usability of the recovered solids depend on the oxidation process, impurity levels, and applicable product or environmental standards.
Step 5: Process Monitoring and Optimization
Continuous monitoring is essential for stable desulfurization performance.
Regular inspections should cover slurry pH, reagent consumption, SO₂ removal efficiency, circulation pumps, spray nozzles, pipelines, and solid-liquid separation equipment.
Process adjustments should be based on operating data rather than fixed assumptions about reagent dosage.
4. Conclusion
Calcium carbide slag can be an effective alkaline reagent for wet flue gas desulfurization when its composition is suitable and the process is properly designed.
Successful application requires raw material testing, controlled slurry preparation, efficient gas-liquid contact, appropriate oxidation, and reliable by-product treatment.
For industrial projects, laboratory testing and engineering evaluation should be completed before determining specific operating parameters.