How to Control Moisture Content in Gypsum Powder Production

 How to Control Moisture Content in Gypsum Powder Production?

Controlling moisture content is a fundamental aspect of gypsum powder production. Excessive moisture in the final stucco product can lead to numerous issues, including clogging in conveyors and silos, premature setting during storage, and weakened final products like gypsum boards. Effective moisture management is required throughout the production line.

1. Control at the Raw Material Stage:
The process begins with the raw gypsum rock. Quarried rock often contains significant surface and inherent moisture. While some moisture is unavoidable, it should be managed.

• Stockpile Management: Storing crushed rock in covered or well-drained areas can reduce absorption of rain and environmental humidity.

• Pre-drying: In regions with consistently high humidity, a pre-drying step before the raw material enters the grinding mill may be considered. This reduces the energy load on the mill and the subsequent calciner.

2. Moisture Removal during Grinding:
In many modern systems, the grinding mill also functions as a preliminary dryer. This is typically achieved by introducing hot air into the mill chamber.

• Airflow and Temperature: The volume and temperature of this hot air are controlled to evaporate the majority of the free moisture from the raw gypsum as it is being ground. The moist air is then exhausted through a baghouse or other dust collection system. Proper adjustment of this air stream is critical; insufficient heat will leave the powder damp, while excessive heat can start premature and inefficient calcination within the mill.

3. The Primary Role of Calcination:
The calcination stage is the core of chemical moisture removal. Here, the chemically combined water of crystallization (CaSO₄·2H₂O) is driven off to form hemihydrate (CaSO₄·½H₂O).

• Precise Thermal Control: The calciner must provide a consistent and accurately controlled temperature, typically between 150°C and 180°C for most systems. The system must supply enough thermal energy to complete the calcination reaction without causing localized overheating that creates dead-burned anhydrite.

• Efficient Vapor Removal: The calcination process generates a large amount of steam. The equipment design must facilitate the efficient and complete removal of this water vapor from the powder bed. In kettles, this is done through vents. In flash calciners, the powder is suspended in the hot gas stream, and the vapor is carried away with the exhaust gases. Any restriction or inefficiency in vapor removal can lead to moisture being trapped and re-absorbed by the powder.

4. Post-Calcination Cooling and Handling:
After calcination, the stucco is very dry and hygroscopic, meaning it readily absorbs moisture from the surrounding air.

• Cooling with Dry Air: The hot stucco must be cooled before storage to prevent caking. However, this cooling must be done using air with low humidity. Special coolers that use dehumidified or ambient air (conditioned if necessary) are employed. Introducing cool, moist air at this stage would directly contaminate the product.

• Closed Conveying and Storage: The transfer of the finished stucco from the cooler to storage silos should use closed conveying systems, such as screw conveyors or bucket elevators, to minimize contact with ambient air. Storage silos should be equipped with airtight seals and breather filters to prevent humid air from being drawn in during filling or emptying cycles.

5. Monitoring and Process Adjustment:
Continuous control relies on continuous monitoring.

• Online Moisture Analyzers: Installing online moisture gauges at key points, such as after the mill and after the calciner, provides real-time data for operators to make adjustments.

• Laboratory Testing: Regular grab samples should be tested in a lab using a loss-on-ignition (LOI) method or a portable moisture meter to verify online readings and ensure the final product meets the specified moisture content, which is typically well below 1%.

In conclusion, controlling moisture is not a single-step operation but a holistic process strategy. It involves managing the raw material, utilizing the grinding and calcining stages effectively for dehydration, and protecting the hygroscopic final product from the environment. The machinery design from Hengshui Decheng Machinery & Equipment Co., Ltd. incorporates features aimed at facilitating this essential control throughout the entire production system.