Where and How do Subsea Minerals Form?
Subsea mineral reserves are found throughout the world's oceans:
Source needed
Subsea mineral deposits occur because sediments, dead plants, animals and other nutrients are washed into the oceans. These decompose and release minerals (including metals) into the waters of the oceans. Most minerals do not dissolve in seawater, and so they precipitate out onto surfaces (e.g. seamounts, rocks, or pieces of debris) to form mineral-rich deposits that can be collected and refined into metals.
What are the Different Types of Subsea Minerals?
There are broadly three different types of subsea minerals; polymetallic nodules, polymetallic sulphides and cobalt-rich crusts:
Polymetallic Nodules
Polymetallic nodules are egg-shaped accumulations of minerals that form on the deep ocea seabeds.
They form around a nucleus (typically a rock fragment or shark's tooth) and grow slowly over millions of years.
Polymetallic nodules lie loose on the seabed, and can simply be picked up by a robot arm, or sucked up by pumps in a hydraulic collector.
Polymetallic Sulphides
Polymetallic sulphides occur at hydrothermal vent sites, where warm mineral-rich waters from underground hit the cold waters of the oceans. This causes metal sulphides to precipitate out, creating "heaps" of minerals.
Polymetallic sulphide deposits are typically hard, and cannot simply be picked up. They require mechanical extraction, for example in the form of drilling.
Cobalt-Rich Crusts
Cobalt-rich crusts are hard iron-manganese deposits that form from the precipitation of minerals in the seawater onto the surface of underwater mountains called seamounts.
They primarily comprise iron and manganese oxides, with around 1% cobalt.
Cobalt-rich crusts are thick and hard, and similarly require mechanical extraction.
How do we Collect Polymetallic Nodules?
Polymetallic nodules are the easiest type of subsea mineral to collect, as they simply lie on the seabed and can be picked up by underwater robots.
A number of different subsea collectors have been developed and tested that are able to pick up nodules from the seabed. They typically use robot arms to select individual nodules, or else they use hydraulic systems to suck up nodules from the seafloor.
Allseas' Collector
Impossible Metal's Collector
Loke's Collector
What Equipment is Required?
Subsea collectors are one part of the operation to collect polymetallic nodules from the seafloor. They require more equipment, including a Production Support Vessel, a riser lift system (to lift the nodules more than 4500m to the surface) and ore carriers to store the nodules
A. Production Support VesselB. Subsea collector
C. Subsea collector launch and recovery system
D. Riser
A Production Support Vessel (PSV) is a large surface vessel that is the core of a deep sea mining operation. The PSV is the base from which a subsea collector is launched, and it provides power, control and guidance to the subsea collector via a large umbilical cable.
Source needed
The subsea collector gathers nodules, which are fed to a Riser Lift System (RALS) which lifts them to the surface. Nodules are located very deep in the ocean, typically 4000+m below the surface, and so a large amount of energy is required to lift them this distance to the surface. RALS use water or air pumps to help pump the nodules to the PSV
Hidden Gem
Olympia
Nodules arrive at the PSV and are separated from the water. The dry nodules are then temporarily stored in large holds within the PSV, whilst the water (and some sediments) are returned back to the depths of the ocean. This discharge is the main source of sediment plumes, whose impact on local flora and fauna needs carefully understanding before operations can start.
Nodules are offload from the PSV to a bulk ore carrier, and are shipped to a refinery for refining into usable metals.
Why are Subsea Minerals Important?
Subsea mineral deposits contain a wide variety of metals that are critical to the modern economy, including copper, iron and manganese. They also contain metals that are needed for building batteries for electric vehicles, such as lithium, and cobalt. Some deposits also contain other valuable metals, such as titanium, or Rare Earth Elements (REEs) which are used in the manufacture of computer chips and mobile phones.
"Deep-ocean mineral deposits as a source of critical metals for high- and green-technology applications: Comparison with land-based resources", 2013, Hein et al
What Metals are in Polymetallic Nodules?
Polymetallic nodules vary in terms of their composition, but typically they're around 20+% metals ores. This is very high compared to most current mines on land, where ore concentrations are normally less than 1%, and more typically around 0.5%
Nodules contain large amounts of manganese, iron, copper and cobalt, as well as other metals. The average composition of nodules from 2 different major regions can be seen below:
