To make a pellet sample, the process involves several key steps:
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Grinding the Sample: The sample must be ground to a fine particle size, ideally less than 50µm. This is crucial for ensuring that the sample compresses and binds together effectively when pressed, and to minimize sample heterogeneities that can affect analytical results. Grinding can typically be done using a ring and puck pulverizing mill.
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Mixing with a Binder: The ground sample is then mixed with a binder or grinding aid in a vessel. The binder helps in forming a cohesive pellet and can vary depending on the specific requirements of the sample and the desired pellet size.
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Loading the Die: The mixture is loaded into a die, ensuring it is filled to the brim and leveled. A small amount of pellet pressing lubricant is added to the top of the sample to prevent sticking.
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Pressing the Sample: The loaded die is placed into a press machine, and the lid is closed. The press is programmed to apply a specific pressure, typically between 15 to 35 tons, and a pressing cycle is initiated. The hydraulic pressure compresses the sample into a small, flat disc.
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Removing and Inspecting the Pellet: After the pressing cycle, the pellet is removed from the die. It should be smooth, uniform in shape, and thickness. If the pellet meets the required specifications, the process is complete. If not, adjustments may be needed in the grinding, binder, or pressing parameters.
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Repeat for Additional Samples: If multiple samples need to be processed, the steps are repeated for each sample.
Additional Considerations:
- Contamination Prevention: To avoid contamination from the die surface, it is important to clean the die before each use and consider using a film between the powder and the die.
- Handling Small Sample Quantities: For samples that are too small for direct pelletization, methods such as double pelletization or embedding can be used. These involve either covering a previously pressed powder with the new sample and pressing again, or placing the new sample in the center of a formed pellet and reapplying pressure.
- Use of Polypropylene Discs: For small quantities of dried grass samples or powders, the sample can be pressed onto polypropylene discs with adhesive surfaces, using cylinder-type dies.
This detailed process ensures that the pellet samples are prepared accurately and consistently, which is critical for reliable analytical results in applications such as X-ray fluorescence (XRF) analysis.
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