Why does briquette density matter?

Shredded cotton banknotes are briquetted for various reasons, but the most important reason is volume reduction. In addition to this, the dust can also be compressed and added to the briquettes. However, there is often a lot of confusion about the density of the briquettes and how to interpret the various figures that are associated with it. This confusion arises from the distinction between shreds destroyed online and those destroyed offline, as well as the various shapes of the briquettes themselves and the way briquettes are collected. In this blog we explain more about the impact of density and compression in the banknote destruction process.

What is density?

Density is a physical property of matter that describes the amount of mass present in a given volume. In other words, it is a measure of how much “stuff” is packed into a specific amount of space. The formula for density is: Density = Mass / Volume.

Density is measured in units of mass per unit volume, such as g/cm³ or kg/m³, and can vary based on factors like compression and material composition.

Density is used to describe how heavy something feels or how much space it takes up. For example, gold feels heavier than aluminium, even if they have the same physical dimensions.

See below the density per type of shreds.

The interesting part is the density of the bulk weight in the collection unit. The shape of the briquette then suddenly really matters. Cylindrical shaped briquettes fill gaps in a big-bag, container or other collection method which makes it possible to get most of the briquettes into the collection unit. We explain this further below but first let us also explain the compression. 

The explanation about compression?

Compression is often used to create more stuff in the same amount of space, such as in the production of briquettes made from shredded banknotes. Because of this compression it's possible to pack more mass into a smaller volume. This will increase their density and making them more efficient to transport and store.

The more technical explanation of the compression process is that force is applied to a material in a perpendicular direction to its surface, causing it to deform or change shape. The amount of force applied determines the degree of compression, with greater force resulting in more significant changes to the material. The optimum of this force (and thus energy consumption) and compression rate benefits are crucial to when it comes to the total environmental impact of the briquetting effect.

The difference between offline and online destruction

Offline destruction involves bulk destruction of banknotes  This method is used to handle large volumes of banknotes efficiently. Online destruction involves immediately shredding banknotes after they have been processed in the sorting process of the Bank. They are destroyed one by one through high speed sorters or via Kusters' Online Granulating Systems (OGS).

For both methods the shredded pieces are transported by air and collected, compressed and can be stored as briquettes.

In this blog, we explain the difference between offline and online destruction methods.

The working principle of banknote briquetting

After the banknotes have been shred, the collected shreds are fed to the screw feeder that has a highly accurate dosing of shreds to the press, which helps the press to compress the shreds into cylindrical briquettes with a diameter of approx. 66 or 90 mm. The hydraulic cylinders of the press are compressing the shreds into high-density briquettes based on using only pressure without additives. The density of these briquettes is approx. 800-1000 kg/m³ and the briquettes will remain in compressed form after discharge.

In the flowchart below you see the working principle of the banknote destruction and shred handling. 

Why is briquetting being used?

The destruction of the unfit banknotes is often done with advanced disintegration systems where the banknotes are reduced to shreds of maximum 6x6 mm in a random confetti shape. Cotton based banknotes become a bit fluffy when they are shredded, which will increase their volume. Hydraulic pressure is used to effectively reduce the volume of these shredded banknotes, transforming them into compact briquettes without the need for any binding additives.

Cylinder versus rectangle shaped briquettes

In particular, cylinder-shaped briquettes can be more beneficial than rectangle-shaped ones. For example, if the briquettes are formed into small, dense cubes, they will likely have a higher overall density than if they are formed into larger, more loosely packed shapes. This is because smaller cubes are able to fit more tightly together, reducing the overall volume of the briquette and increasing its density.

Reduction of costs for transportation and storage

Cylinder-shaped briquettes are typically more efficient in terms of space utilization, as they can be packed more tightly together. This means that for a given amount of shredded currency, a cylinder-shaped briquette will take up less space than a rectangle-shaped one when collected randomly in a recepticle.

In addition to the space-saving benefits of cylinder-shaped briquettes, they can also be easier to handle and transport. This is because their cylindrical shape allows them to roll more easily, making them easier to load and unload from vehicles or storage areas.

If the briquettes are formed into larger, more irregular shapes, they may have a lower overall density, as there may be more empty space between the briquettes. This can make the briquettes more difficult to handle and transport, as they may take up more space than denser briquettes and have a large environmental impact accordingly.

When the briquettes end up in a collection unit the density can decrease because of the presence of air. This is because there occur small spaces or gaps between the briquettes. These gaps are filled up with air, lighter than briquettes and thus reducing the overall density of collection device such as a big-bag or container. But for example when the container or bag is moved or jostled, the briquettes can shift and settle, creating even less gaps and allowing less air to fill the space. This can cause that the bulk density of these briquettes increases further, making it more efficient to transport or store. 

Conclusion

First of all, it needs to be understood that the density can be measured at different stages in the briquetting process, from loose shreds, to a single briquette, ending in bulk collection. It is important to realize and be clear on which density figure is used at what point. If a single briquette density is used as a measurement unit, but final collection will take place in bulk, one could find themselves having an additional >20% of transport volume than expected. This coud cause additional unexpected transport costs and additional unexpected impact on the environment.

Overall, the kind of briquette shape can have a significant impact on the practicality and cost-effectiveness. By choosing a cylinder shape, it may be possible to reduce transportation and storage costs while also making the briquettes easier to handle and use. High volume reduction of the briquette isn’t enough, you also need high density in the collection unit to get the most of the briquettes into a collection. And that is easier with a cylinder-shaped briquette.

Also it's important to note that the actual difference in density between briquettes made from online and offline destroyed banknotes can vary depending on the specific processes and equipment used by the bank or organization responsible for destroying the currency. To determine the exact reasons for the density difference in a particular case, you would need to examine the specific procedures and equipment used for banknote destruction.

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