Silo cleaning and emptying of a damaged silo system for plastic granulate

Silo cleaning and emptying of a damaged silo system for plastic granulate

Plastic granulate in silo systems is a bulk material that rarely causes problems in raw material storage. There are no abrasive influences that could damage the container walls. Hygroscopic properties rarely cause silo blockages.

Biological accidents due to mold ingress or contamination are just as rare as serious silo blockages. And yet these silo systems very rarely have to deal with so-called accidents. These situations are often so serious for the plant operator that they are difficult to understand. But what exactly happens? Plastic granulate (small plastic beads) is required in industry for various applications. In injection molding machines, the granulate is used to produce molded or cast parts.

This granulate is stored temporarily in large silo systems and stockpiled for further use. Certain chemical and physical processes can cause the granulate to fuse together in the silo system, compressing it under pressure. When high heat is generated in the silo system, the granulate particles fuse together to form a coherent block. This block is largely resistant to external mechanical influences and can take on enormous shapes.

In our project, the plastic block has a weight of approx. 35 tons. The granulate is almost completely fused together and is now present in the silo system as a large lump.

Fig. 1 – Plastic pellets melted in the silo system

It is not possible to understand exactly how this chemical and physical process takes place in detail. What is certain, however, is that this process could have been prevented in advance and is due to the fact that the bulk material storage was at a standstill for a longer period of time and the granulate in the silo system had sufficient time to approach pressure compaction. If it were only a matter of simple pressure compaction, this could be quickly eliminated by mechanical means.

But the granulate has completely melted together under its own dynamic thermal influences to form a huge block. In our more than 10 years of company history, we have already encountered this case 3 times. The plastics involved were pe, pp, abs.

The initial situation was always identical and led to complete melting of the stored bulk material in every situation. In some cases, we observed considerable temperature developments, which led to enormous amounts of condensation and fueled the conversion process; in another case, in a similar situation, we even found soot spots on the inner wall of the silo, which indicate enormous heat development.

We do not know whether the thermal process can ultimately lead to a material fire; the flash point of the plastic granulate in question speaks against it. However, until the process has been investigated and analyzed in more detail, we cannot give the all-clear.

The fused block is mechanically insensitive and virtually unbreakable. We subjected the blocks of material to a whole series of experiments: strong vibrations, sledgehammers, maximum pressure technology (2200 bar), cryogenic exposure to liquid nitrogen, compressed-air hammering technology, chain saws to separate the blocks. Nothing really helped, no method led to a satisfactory result.

Hammer-proof silo blockade

It is astonishing that every plastic granulate for which this process is known to us has different melting points. And yet the end result is always the same. Unfortunately, we are only ever on site when the melting process is almost complete; we would like to find out whether the process could be prevented by external influences.

Fig. 2 – Melting pattern of the plastic storage in the silo

Fig. 3 – Recovered fused ABS plastic block

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