Precautions for powder calcination in high-temperature rotary furnace

When calcining powder in a high-temperature rotary furnace, strict control is required from multiple aspects such as equipment operation, process control, safety protection, and post maintenance to ensure production safety, stable product quality, and extended equipment life. The following are specific precautions:

1. Equipment operation specifications
Pre startup inspection
Mechanical components: Check the lubrication condition of the rotating mechanism of the furnace body (such as gears and bearings) to ensure that there is no jamming or abnormal noise; Verify the tension of the drive belt to avoid slipping.
Electrical system: Confirm that heating elements (such as resistance wires and silicon carbide rods) are not broken or aged, and check the calibration status of temperature control instruments and sensors (thermocouples/infrared thermometers).
Gas pipeline: Check the sealing of the intake valve, exhaust valve, and vacuum pump to prevent gas leakage; For flammable gases such as hydrogen, it is necessary to install flame arresters and regularly detect leaks.
Furnace body sealing: Check whether the sealing rings of the furnace door, observation window, feeding port and other parts are intact to avoid vacuum leakage or air infiltration.
Monitoring during operation
Temperature control: Real time monitoring of temperature in each section to avoid local overheating (exceeding the design temperature by more than 50 ℃ may cause equipment damage); For temperature sensitive materials (such as lithium battery positive electrode materials), an over temperature alarm should be set and linked to shutdown.
Rotation speed adjustment: Adjust the rotation speed according to the powder particle size. For fine powder, reduce the rotation speed to reduce dust, and for coarse powder, increase the rotation speed appropriately to enhance the mixing effect.
Gas flow rate: For processes that require atmosphere protection (such as silicon nitride calcination), it is necessary to stabilize the gas flow rate (such as N ₂ flow rate fluctuations should be<5%) to prevent oxidation or insufficient reaction.
Vacuum degree monitoring: During vacuum calcination, ensure that the vacuum pump continues to operate and the vacuum degree remains stable at the target value (such as 10 ⁻ Pa level) to avoid product oxidation caused by leakage midway.
Shutdown operation
Cooling control: Adopt segmented cooling (such as cooling by 50-100 ℃ per hour) to prevent cracking of the furnace body or abnormal material phase transition caused by thermal stress.
Atmosphere replacement: Before shutdown, inert gas (such as N ₂) should be used to replace reactive gas (such as H ₂) in the furnace to avoid residual gas mixing with air and causing explosions.
Equipment cleaning: After the furnace body cools down to below 80 ℃, clean the residual powder to prevent clumping or corrosion of the furnace lining.

2. Process parameter control
Temperature uniformity optimization
Preheating stage: Low temperature section (<300 ℃) slowly heats up (such as heating up 20-30 ℃ per hour) to eliminate internal stress in the powder and reduce the risk of cracking.
Insulation time: Set the insulation time according to the material characteristics (such as 2-4 hours for alumina crystal transformation) to ensure complete reaction.
Cooling rate: For metal powders (such as titanium powder), the cooling rate should be controlled (e.g.<50 ℃/min) to prevent oxidation; For ceramic materials such as silicon nitride, rapid cooling can suppress grain growth and improve strength.
Atmosphere purity management
Gas purity: Use high-purity gas (such as N ₂ ≥ 99.999% Ar≥99.999%）， Avoid impurities (such as O ₂, H ₂ O) affecting product quality.
Gas humidity: For water sensitive materials (such as lithium metal powder), a drying device (such as a molecular sieve adsorption tower) should be equipped to control the gas dew point below -60 ℃.
Reaction gas ratio: For reactions such as nitriding and carbonization, it is necessary to accurately control the gas ratio (such as N ₂/H ₂=4:1) to avoid side reactions.
Powder pretreatment
Particle size control: Ensure uniform powder particle size (such as D50=5-10 μ m) through screening or air flow classification to avoid uneven calcination of coarse and fine particles.
Moisture content: For powders that are prone to agglomeration (such as aluminum hydroxide), they need to be pre dried to a moisture content to prevent expansion and cracking during calcination.
Additive usage: For difficult to sinter materials (such as zirconia), a small amount of sintering aid (such as Y ₂ O3) can be added to lower the sintering temperature and increase the density.

3. Safety protection measures
Fire and explosion prevention
Explosion proof design: The furnace body is equipped with an explosion-proof port (such as a bursting disc), which automatically releases pressure when the pressure exceeds 0.02 MPa; For hydrogen atmosphere furnaces, it is necessary to set a hydrogen concentration alarm (threshold<1% VOL) and link exhaust.
Static elimination: Install static grounding devices at the feed and exhaust ports to prevent powder friction from generating static sparks.
Fire protection configuration: Dry powder fire extinguishers or carbon dioxide fire extinguishing systems are equipped around the furnace body, and the use of water-based fire extinguishers is prohibited (which may cause explosions to high-temperature equipment).
Personnel protection
Personal equipment: Operators are required to wear heat-resistant gloves (temperature resistance ≥ 300 ℃), gas masks (for toxic gases such as SO ₂), and goggles to prevent burns or inhalation of dust.
Operation training: Regularly conduct safety drills, including emergency shutdown, gas leak handling, fire extinguishing, and other processes.
Health monitoring: For positions that are exposed to dust or toxic gases for a long time, regular occupational health examinations (such as lung function tests) are required.
Emergency Response Plan
Overtemperature treatment: If the temperature is out of control, immediately cut off the heating power and start forced ventilation to prevent equipment damage or fire.
Vacuum leakage: When the vacuum degree drops, quickly close the intake valve and start the backup vacuum pump to avoid product oxidation.
Gas leakage: In case of hydrogen leakage, immediately turn off the gas source and evacuate personnel. Dilute with nitrogen to a safe concentration (<1% VOL) before handling.

4. Equipment maintenance and upkeep
routine maintenance
Lubrication and maintenance: Add high-temperature lubricating grease (such as molybdenum disulfide based grease) to the rotating bearings every week to ensure flexible rotation.
Seal replacement: Check the furnace door seal ring every 3 months and replace it promptly if aging or cracks are found to prevent vacuum leakage.
Cleaning the furnace: After each batch of production, use compressed air to clean the residual powder in the furnace to avoid clumping and affecting heat transfer.
Regular maintenance
Heating element detection: Every six months, use an infrared thermometer to check the temperature distribution of the resistance wire or silicon carbide rod, and replace locally overheated or broken components.
Transmission system calibration: Conduct annual wear testing on gears and belts, adjust tension or replace worn parts.
Vacuum system maintenance: Clean the oil circuit of the vacuum pump every quarter, replace deteriorated lubricating oil, and ensure stable performance of the vacuum pump.
Spare Parts Management
Key spare parts reserve: Reserve commonly used vulnerable parts (such as sealing rings, thermocouples, resistance wires) to ensure replacement within 4 hours in case of failure.
Spare parts life tracking: Establish a spare parts usage file, record replacement time and reasons, optimize inventory management, and predict equipment life.