Blue Nodules Degradation

Polymetallic nodules degrade as they are transported from the seafloor to the sea surface through the vertical transport system (VTS). Twelve pumps, a jumper hose and several kilometers of vertical pipe will reduce the average size of the nodules and produce fine nodule dust. The nodules are dewatered at the sea surface onboard of the processing vessel, however the dust particles (<63μm) are practically impossible to dewater. An estimation of the nodule degradation rate and nodule dust production is required to help design dewatering equipment.
Without having access to a full scale VTS, the nodule degradation was measured using a very fundamental approach. Several degradation mechanisms can be observed in the VTS. Abrasive wear takes place as a nodule slides against the wall of the vertical pipe, fragmentation wear due to nodules being smashed by the pump and attrition is possible due to nodules bumping into each other on their way to the sea surface. These degradation mechanisms were studied separately, each with its own specific experiment, which allows  to compare and to judge which process is dominant.
Dropping nodules on a steel plate, placing them in a constantly tilting pipe for days and sieving the degraded nodules provided us with the following insights:
• The dominant process that produces fine nodule dust is the abrasive wear of the nodules against the pipe wall
• Fragmentation wear in the pump will quickly reduce the diameter of the nodules but produces a limited amount of fine particles
• A conservative estimate of the particle size distribution  and nodule dust production.

Impact degradation of a manganese nodule impacting at 6 m/s

The concept tilting pipe test setup for measuring abrasive wear of manganese nodules

The realized abrasive wear tester for measuring abrasive wear of manganese nodules

A single manganese nodule degraded into fractions of various sizes.