Deep-Sea Mining

Researchers: Professor Andy Woods & Dr Nicola Mingotti

The Problem

The increasing demand for certain metals crucial to the energy transition is driving interest in deep-sea mining. These mining processes produce large quantities of waste sediment which is disposed of into the ocean.

Work Needed

We need to better understand the environmental risks to marine life posed by deep-sea mining.

Our Work

We have been exploring the fundamental fluid mechanics of how this waste sediment moves in the ocean. Investigating the behaviour of sediment plumes has allowed us to demonstrate how these mining operations could have widespread impacts.

Background

The energy transition is increasing the demand for materials like copper, cobalt, and magnesium, which are essential for battery and electric vehicle production. The scale of this demand has driven renewed interest in deep-sea mining, especially in the Pacific Ocean, where there are large deposits of polymetallic nodules on the sea floor. In this process, nodules are collected by specialised sea floor vehicles and transported to the surface for processing.

Large quantities of waste are generated after the sifting and processing of collected material, which is disposed of back into the ocean. This disposal poses environmental risks as fine particles can remain suspended in the water for extended periods, and there may be some dissolution of material back into the water column.

Our Work

Our work investigates some of the fundamental fluid mechanics of sediment transport through the water column. Laboratory experiments explore how sediment plumes disperse in water when a suspension of particles is discharged from a pipe at depth, as relevant for deep-sea mining operations, focusing on how different densities and particle loads affect plume behaviour and sediment spread.

Exemplar Results – Types of Plume Behaviour:

(1) Dense particle-laden plume with dense fluid: In this scenario, the plume contains particles suspended in denser, saline fluid. As the plume descends, its downward speed reduces below the particles’ settling speed, causing the particles to separate from the plume and settle out independently

(2) Dense particle-laden plume with buoyant fluid: Here, the particles are in a dense plume, but the fluid itself is less dense. As the plume descends, the buoyant fluid gradually rises out of the plume, creating a secondary upward plume, while the particles continue to settle downward.

(3) Buoyant particle-laden plume with buoyant fluid: In this case, both the particles and the fluid are buoyant, causing the plume to rise initially from the discharge point. However, as the upward speed falls below the particles’ settling speed, particles start to sediment out gradually from the plume.

Impact of ocean conditions: Ocean layers with different densities can either slow down or stop the plumes, influencing how far and in what direction particles and water from the plume spread.

The results of these experiments highlight the range of flow processes which may influence the initial dispersion and sedimentation of particles in deep sea mining plumes after they are released into the water.

Implications

A key priority for deep-sea mining operations should be to limit the environmental impact of the process on marine ecosystems and the sea floor beyond the mine.

  • Although highly idealised, the results of our experiments illustrate the difficulty of limiting environmental impact; particle plumes are influenced by various factors (e.g. currents, stratification, and sedimentation) and this complexity means that predicting and controlling the spread of mining plumes is challenging.
  • Our experiments demonstrate the potential for widespread sediment dispersal, meaning that mining impacts could extend far beyond the immediate site to affect a broader area of the marine ecosystem.
  • We have also thought about how our experiment results could apply to understanding ash spread in volcanic eruptions under the ocean.

Publications

Mingotti, N., and Woods, A.W. (2022). Dynamics of sediment-laden plumes in the ocean. Flow, Cambridge University Press.

Mingotti, N., & Woods, A. W. (2020). Stokes settling and particle-laden plumes: implications for deep-sea mining and volcanic eruption plumes. Phil. Trans. R. Soc. A.37820190532