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Gypsum powder, also known as stucco or calcined gypsum, is the primary raw material for gypsum board. Its manufacturing process is a controlled thermal operation that transforms raw gypsum rock into a usable powder. This process is crucial as it determines the setting time and final strength of the gypsum products.
The process begins with the quarrying of natural gypsum rock. Large pieces of raw gypsum are first crushed by primary crushers into smaller, manageable fragments. These fragments are then further reduced in size by secondary crushers, often hammer mills or impact crushers, to produce a gravel-sized material.
The next stage is grinding. The crushed gypsum is fed into a grinding mill, typically a ball mill or a Raymond mill, where it is pulverized into a fine powder. This raw gypsum powder, with a specific particle size distribution, is now ready for the calcination process. In some processes, grinding may occur after calcination.
Calcination is the core of the process. It involves heating the raw gypsum powder to a specific temperature to drive off a portion of its chemically combined water. The most common type of calciner is a kettle calciner, a large, stationary vessel with internal heating and mixing mechanisms. The powder is heated to between 150 and 180 degrees Celsius. During this heating process, the dihydrate calcium sulfate (CaSO4·2H2O) loses three-quarters of its water content and is converted into hemihydrate calcium sulfate (CaSO4·½H2O), also known as beta-hemihydrate plaster.
The control of temperature and residence time in the calciner is critical. Under-calcination leaves unreacted dihydrate, which can cause inconsistent setting times. Over-calcination can form insoluble anhydrite, which lacks binding properties and weakens the final product. The calcined gypsum, now called stucco, is discharged from the calciner.
After calcination, the stucco may go through a conditioning or aging process. It is stored in silos for a period, allowing it to stabilize and for any residual uncalcined or over-calcined particles to equilibrate. This aging step helps achieve more consistent setting characteristics. Additives may be introduced during or after grinding and calcination to modify the properties of the final powder, such as its set time, fluidity, or strength.
Finally, the finished stucco powder is stored in large silos, ready to be transported to the gypsum board production line or bagged for other uses. The entire process from crushing to storage is a continuous, closely monitored industrial operation designed to produce a consistent and reliable material for downstream manufacturing.
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