The mainstream process for calcining hydromagnesite (magnesium hydroxide) in a rotary kiln is the production of light-burned magnesia. The core process consists of: raw material pretreatment → sealed feeding → staged calcination in a rotary kiln → rapid cooling → grinding and classification → finished product packaging, supported by waste heat recovery and environmental protection measures to ensure compliance with indicators and product reactivity.
I. Full Process Flow
1. Raw Material Pretreatment (Impurity control, particle size uniformity, moisture reduction)
- Coarse crushing → fine crushing → screening: Raw ore is coarsely crushed by a jaw crusher to 30–50 mm, then shaped by a hammer crusher to 10–30 mm; vibratory screening ensures uniform particle size.
- Impurity removal and purification: Remove red inclusions and waste rock, wash to remove clay; for high-silicon/high-iron materials, magnetic separation is added to remove iron and improve purity. Alternatively, wet milling and carbonation can be used for impurity removal.
2. Sealed Feeding Section (Air leakage prevention, stable metering)
- Belt metering and conveying → sealed screw feeder → kiln head sealed bin, achieving slight positive pressure feeding to prevent cold air from entering the kiln and affecting the temperature profile.
3. Calcination Section in Rotary Kiln (Core, five-zone temperature control)
| Zone | Temperature Range | Core Function | Key Control Points |
| Preheating zone | 200–400°C | Remove free water, expel adsorbed gases | Heating rate ≤10°C/min, prevent cracking |
| Pre-decomposition zone | 600–700°C | Begin decomposition of Mg(OH)₂ | Release H₂O, accumulate energy for main decomposition |
| Main decomposition zone | 700–780°C | Main reaction: Mg(OH)₂ → MgO + H₂O↑ | Precise temperature control ±10°C, ensure complete decomposition |
| Homogenization & activity stabilization zone | 780–850°C | Grain growth, stabilize reactivity | Residence time ≥15 min, prevent overburning |
| Pre-cooling zone | 500–600°C | Cool down, protect reactivity | Rapid cooling, inhibit grain growth |
- Total residence time: 60–90 min (matched with particle size).
- Heating method: Direct heating with natural gas/gas.
4. Rapid Cooling Section (Activity preservation)
- Kiln discharge → single-tube cooler, rapid cooling to below 100°C to avoid loss of activity and pulverization.
- Waste heat recovery: Hot cooling air is returned to the calcination zone for preheating, saving 15%–20% energy.
5. Post-Treatment and Packaging (Quality improvement)
- Grinding and classification: Ball mill/vertical mill to 325 mesh – 5 μm (depending on customer requirements), with air classifier controlling particle size D50.
- Inspection and packaging: Test for MgO content, reactivity, whiteness; package in moisture-proof lined bags to prevent moisture absorption.
II. Key Process Parameters and Control Points
- Feed particle size: Powder: 100–325 mesh, lump: 10–30 mm.
- Decomposition temperature: 700–780°C (core range for light burning).
- Atmosphere: Slightly oxidizing, O₂ ≥ 5%, to avoid iron contamination due to reduction.
- Kiln speed and slope: Speed 1–3 r/min, slope 2%–4%, matching material residence time.
III. Environmental Protection and Compliance Points
- Dust control: Bag filters at kiln head/kidney, emission ≤ 30 mg/m³.
- Desulfurization and denitrification: For gas firing, equipped with desulfurization tower + SCR to meet SO₂/NOₓ standards; natural gas firing does not require these.
- Noise: Equipment enclosed in soundproof covers, plant boundary ≤ 60 dB(A).
IV. Common Issues and Countermeasures
| Problem | Cause | Countermeasure |
| Ring formation / Build-up | Non-uniform particle size, high moisture content, excessive temperature | Uniform particle size, dry material, control temperature ≤850°C, clean kiln |
| Insufficient reactivity | Low temperature, insufficient residence time | Increase temperature to 750–780°C, extend residence time |
| Overburning / Pulverization | Excessive temperature, slow cooling | Control temperature, rapid cooling |
