Editorial Feature

Pelletizing - Pellet Sample Preparation

Pelletizing is an important sample preparation technique, which involves the formation of a solid 'pellet' for analysis via methods that require flat, round samples.

The pellets are formed via the agglomeration of fine, amorphous powders in the presence of moisture in a piece of apparatus known as a pelletizer. A solid or liquid binder can also be added before and during pelletizing if needed for process considerations or increasing the hardness of the product.

As more raw powder is continuously fed into the pelletizer, spherical pellets are discharged from the edge of a rotating drum, whilst relatively smaller pellets are retained in the bottom. The size of the pellet is controlled by adjusting the speed and angle of the pelletizer, the amount of liquid added at any given time, and the placement of water-sprays and feed. Furthermore, the moisture, retention time and availability of dry powder can also be controlled.

Pellet Preparation

X-ray fluorescence (XRF) analysis, optical emissions spectroscopy and infrared spectroscopy are the commonly used analytical methods that require pellet samples.

When samples are prepared for XRF analysis, the size of the particles to be analyzed should be maintained within the saturation depth of the x-rays. A preliminary size reduction is carried out for the sample materials of large feed sizes. Following this, a small part of the sample is subjected to fine grinding. Dry and bulk samples can be introduced into the rotating dividers through vibratory feeders for grinding. Sample splitters are used for heavily flowing materials. Finally, the part sample thus obtained is subjected to pulverization.

Vibratory disc mill are the most widely used milling tools for reducing the size of the hard and brittle samples used in XRF analysis. The required analytical fineness is obtained within very short grinding times.

Another common method is the potassium bromide (KBr) pellet method, in which the sample is well mixed with fine alkali halide powder and finely pulverized. The pulverized sample is then fed into a pellet-forming die when applies a vacuum force on the sample to form transparent pellets. In addition to KBr, cesium iodide may also be used to form pellets.

The below video is an instructive look at how this potassium bromide (KBr) pellet method can be used in a laboratory setting.  

Thermal Processing Of Pellets

After the pellets have been created, they are then subjected to thermal processing to impart appropriate characteristics and high resistance metallurgic properties to the pellets. The thermal processing of pellets involves the following stages:

  • Drying
  • Daily pay burn
  • After-burn
  • Cooling

The duration and temperature at which each stage is performed has a strong influence on the final product quality.

Companies Involved in Pelletizing

The list of companies below are involved in producing pelletizing equipment or producing pelletized products:

SPEX SamplePrep’s laboratory pellet presses are suitable for preparing sample discs for pelletizing. Its product range includes an air-actuated press and manual and automatic hydraulic presses. These presses are designed to fit on the benchtop, and require very less space than standard floor model presses.

Glatt offers customized pelletizing systems with simple, logical operator guidance. Glatt spheronizers/pelletizers are used for producing homogeneous pellets with throughputs up to 10,000 kg/h.

Eirich has developed intensive mixers that perform mixing and granulating/pelletizing in a single machine. These machines are mainly used for producing glass, metal, building materials and ceramics.

Gala is a leading manufacturer of underwater pelletizing systems suitable for pellet rates up to 15,000 kg/h. Gala pelletizing systems are used for applications that include high viscosity to very low viscosity materials and high additive content.

In recent years, Thermo Fisher Scientific have increasingly helped to bring the lab to the field for mining applications. This can be seen from the below video, which demonstrates their in-situ pelletizing method.

Further Applications Of Pelletizing

Outside of sample preparation, some of the other major applications of pelletizing include the following:

  • Manufacturing of plastics - Raw and recycled plastics that are being reprocessed are converted into pellets as this form is easy to use and carry. The manufacturer introduces the plastic pellets into a plastic molding equipment, which melts them. The liquid mixture is then shot into molds, extruded or processed to produce a finished plastic product. These plastic pellets, known as nurdles, are available in different range of colors.
  • Fuel - Fuels can also be pelletized. Scrap wood and other paper products are converted into pellets so that they can be burned in stoves without wasting excess materials.
  • Metals - Certain metals are converted into pellets during production to create pieces of uniform size used during the refining process.
  • Fertilizer - Organic fertilizer products can be obtained in pellet form for those who prefer a dry and tidy way to deliver nutrients to their gardens

References

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