Polymetallic nodules

Of all the mineral resources considered as potential targets for deep-sea mining, polymetallic nodules (also commonly called manganese nodules) are probably the most likely commodity to be developed into a commercial operation. As well as containing commerically attractive (though variable) levels of metals such as nickel, copper and cobalt, their occurrence on the seafloor surface presents a relatively straightforward engineering challenge in terms of their extraction when compared to some other metal deposits in the deep sea.

Formation

Polymetallic nodules are rounded accretions of manganese and iron hydroxides that cover vast areas of the seafloor, but are most abundant on abyssal plains at water depths of 4000-6500 metres. They form through the aggregation of layers of iron and manganese hydroxides around a central particle (such as a shell or small rock fragment), and range in size from a few millimeters to tens of centimeters. Growth of these nodules is extremely slow, at a rate of millimetres per million years, and they remain on the seafloor surface, often partially buried in a thin later of sediment. The composition of nodules varies with their environment of formation, but in additon to manganese and iron, they can contain nickel, copper and cobalt in commercially attractive concentrations as well as traces of other valuable metals such as molybdenum, zirconium and REEs. 

Distribution

The nodules of greatest commercial interest occur in the Clarion-Clipperton Zone in the equatorial Pacific Ocean (CCZ) and in the Central Indian Ocean Basin. In the CCZ, polymetallic nodules cover 9 million km² with typical concentrations of 15 kg m^-2. The nodules contain nickel, copper, and cobalt (around 2 - 3% of the nodule weight) as well as traces of other metals such as molybdenum, Rare Earth Elements and lithium, which are important to high-tech industries. The amount of copper contained in the CCZ nodules is estimated to be about 20% of that held in global land-based reserves.

Associated ecosystems

The nodules provide a substrate for a wide variety of suspension feeders and specialised nodule epi- and infauna, which are dependent on the hard substrate provided by the nodules. Sediment communities show significant differences in community structure across provinces. Recolonisation and restoration rates appear to be extremely slow but require greater experimental investigation. A greater appreciation of the spatial distributions of species relative to the size of direct (seabed disturbance, nodule clearance) and indirect (operational and discharge plumes) mining impacts and the patchiness of operations is required.

Extraction

The concept of polymetallic nodules as a serious commercial prospect emerged in the 1960's and 70's, leading to a rush to develop the technology to extract them. However, the industry never developed due to the cheap availability of nickel from more easily accessible land-based deposits. For current mining scenarios, the mining process is predicted to disturb about 120 km² of seabed per year per mining operation and represents a major environmental impact. However, owing to the presence of numerous volcanic hills with a relief of about 100m the extraction of nodules will be extremely patchy.

Current mining scenarios are based on a remotely-operated crawler 'harvesting' nodules from the seafloor, which are them pumped up to a production support vessel at the surface. A certain level of processing will take place aboard this vessel, with the ore then being shipped off to land for further refinement, and the particulate waste being discharged back to the seabed.