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The light at the end of the silo

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World Cement,

John Hodson, Cardox, describes the company’s silo clean out strategy for Hanson’s cement plant in Purfleet, UK.

For several years, the Hanson Cement Plant at Purfleet persevered with a gradually reduced throughput in its GGBS (ground granulated blast-furnace slag), caused by moisture ingress which had resulted in an ever decreasing ‘rat hole’ that was becoming smaller and smaller, until it became totally blocked with approximately 80% dead stock product. Hanson then called in Cardox International and requested a total silo clean out.

The silo in question is a steel construction measuring 30 m in height and 10 m in diameter, with the bottom 15 m forming a cone section. At the base of the cone, a discharge valve releases material onto the conveyor belt. An inspection of the silo revealed that the cement slag material was bridged approximately 2 m above the discharge valve and over the years, a thick hardened crust had formed, above which the material had been subjected to moisture and weight that had transformed the product into a solid dense mass.

From a manhole at the top of the silo, it was determined that the top of the cement slag material was approximately 5 m from the roof of the silo, so the total depth of product was around 23 m thick and amounted to over 2000 t.

The silo clean out strategy devised by Cardox engineers involved three phases:

  • Phase 1: Clearing the lower area of the cone by installing four small fixtures (Cardox sockets and plugs) through which specifically designed silo tubes could be inserted into the hardened material, locked in position, and activated.
  • Phase 2: Once the lower area of the cone had been cleared, the second phase of the operation could begin. This would involve installing ‘angled’ sockets on the cone so that Cardox tubes could be positioned vertically upwards to pull down the material above. The objective of this phase was to create a huge void or ‘rat hole’ through the centre of the material so that Phase 3 could be deployed.
  • Phase 3: The last phase of the operation involved a Cardox hydraulic SiloWhip system, used to clear the remaining material from the walls of the silo.

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