Sustainable extraction of critical  metals from deep-sea nodules

The global shift from fossil fuels to renewable energy is not merely a change in power sources; it is, fundamentally, a transition from fuel-intensive systems to material-intensive technologies. The electric vehicles (EVs), wind turbines, and grid storage solutions required to decarbonize our planet are voracious consumers of critical metals. By 2050, markets will demand approximately 60 million metric tons of copper, 10 million tons of nickel, and 1 million tons of cobalt annually. Yet, the supply chains underpinning this transition are dangerously fragile. As we strive to eliminate the carbon footprint of our energy consumption, we face a paradox: the extraction of the very metals needed to support the green transition is becoming increasingly energy-intensive, carbon-heavy, and environmentally destructive.

Our aim as metallurgists and materials scientists is to lay the foundations and provide the basic reserach to ensure that the greenhouse gas emissions, energy burden, and environmental toxicity of metal extraction do not outweigh the benefits of the fossil-free energy carriers they enable. To achieve this, we must look beyond declining terrestrial reserves and toward the deep-sea polymetallic nodules of the Clarion-Clipperton Zone (CCZ), unlocking their potential not through archaic, dirty smelting, but through Hydrogen Plasma Smelting Reduction (HPSR)—a fossil-free, single-step process that redefines metallurgical efficiency.

The current paradigm of metal extraction is hitting a geological wall. Critical elements like nickel, cobalt, and copper are sourced from rapidly declining terrestrial reserves. For instance, the average nickel content in laterite ores fluctuates between 0.5 to 2 wt%, meaning that to extract a single ton of nickel, industry must process more than 80 metric tons of ore. Copper mining faces similar degradation in ore quality; historically rich deposits are exhausted, leaving ores with grades as low as 0.6 wt%, generating around 200 tons of waste for every ton of copper produced.
These inefficiencies result in the annual generation of 4 to 5 billion tons of waste rock and tailings, often laden with toxic heavy metals. Furthermore, the processing of these ores—specifically the Rotary Kiln-Electric Arc Furnace (RK-EF) route for nickel—is exceptionally carbon-intensive, emitting up to 45 tons of carbon dioxide equivalent (CO2eq​) per ton of nickel.

In contrast, polymetallic nodules found on the abyssal plains of the Pacific Ocean present a geologically superior feedstock. These nodules contain high concentrations of manganese (27%), nickel (1-2%), copper (0.5-1%), and cobalt (0.1-0.2%) within a single deposit. Unlike land-based ores that require massive overburden removal and deforestation, these nodules sit on the sediment surface. Estimates suggest the CCZ alone holds 5,992 million tons of manganese and 274 million tons of nickel—resources that potentially surpass known land-based reserves. However, the sustainability of utilizing these resources hinges entirely on the extraction technology employed. If we apply conventional hydrometallurgical leaching or coal-based smelting to these nodules, we simply relocate environmental harm from land to sea.

We present a radical departure from conventional multi-step refining: the Hydrogen Plasma Smelting Reduction (HPSR) process. This method leverages green hydrogen and renewable energy to condense calcination, smelting, reduction, and refining into a single-step metal extraction.
Current commercial proposals, such as the TMC NORI-D project, rely on the conventional RK-EF route. This involves drying nodules, calcining them at 900°C with coal or natural gas to reduce oxides partially, and then smelting them in an electric arc furnace with more carbon. This approach is thermodynamically inefficient and chemically dirty, producing slag, impurities (sulfur, phosphorus), and massive CO2 emissions.
HPSR replaces carbonaceous reductants entirely. By using hydrogen plasma, we utilize a dual-force approach: the thermal energy of the plasma melts the oxides, while the chemical reactivity of the hydrogen species (atomic H, ions, and molecules) strips oxygen from the metal oxides. The only by-product of this reduction is water vapor (H2​O).

The scientific elegance of HPSR lies in its ability to break down the complex mineralogical structures of polymetallic nodules into an ionic liquid state, facilitating rapid reduction kinetics.
Upon melting in the furnace (at temperatures exceeding 1600°C), the intricate manganese-oxide matrix—which hosts the critical metals Ni, Co, and Cu within its crystal structure—dissociates. The system transforms into a molten pool of ions, primarily Cu1+, Ni2+, Fe2+, Co2+, and Mn2+, alongside oxygen anions (O2−). The reduction is driven by the removal of these oxygen anions at the arc-melt interface, where temperatures can reach 2000°C.
The system transforms into a molten pool of ions, primarily Cu1+, Ni2+, Fe2+, Co2+, and Mn2+, alongside oxygen anions (O2−). 
The reduction is driven by the removal of these oxygen anions at the arc-melt interface, where temperatures can reach 2000°C.

The mechanism proceeds as follows:
 Oxygen Extraction: Hydrogen plasma species interact with free oxygen (O2−) in the melt to form water vapor, which evaporates.
Electron Release: This reaction releases electrons (2e−) back into the melt.
Metallization: These electrons react with the metal cations (Mn+). Thermodynamic hierarchy dictates that metals with the lowest oxygen affinity reduce first. Consequently, Copper (Cu) precipitates first, followed sequentially by Nickel (Ni), Cobalt (Co), Iron (Fe), and finally Manganese (Mn).

This process creates a distinct phase separation. The reduced metals, driven by high surface tension and immiscibility with the oxide slag, coalesce into metallic droplets that sink to the bottom of the crucible to form a dense alloy nugget.

One of the most profound discoveries in our research is the behavior of copper within this system. We demonstrated that selective copper recovery is possible via a simple heat treatment requiring no acids or reducing agents.

When the nodules are melted under an inert argon atmosphere (without hydrogen), the system undergoes a unique internal redox reaction. The nodules are naturally rich in Manganese (IV) Oxide (MnO2​), a strong oxidizer, while copper exists as oxide. At elevated temperatures, MnO2​ thermally decomposes to MnO. As the melt cools below 1200°C, a "displacement reaction" becomes thermodynamically favorable: MnO acts as a reducing agent for Copper(I) Oxide (Cu2​O).

The reaction can be expressed as:
MnO+Cu2​O→2Cu+Mn3​O4​

This means the MnO essentially "sacrifices" itself, oxidizing to Mn3​O4​ (hausmannite), to force the copper to precipitate as pure metallic copper. This spontaneous precipitation allows for a processing strategy where copper—massively critical for global electrification—can be extracted first by simple melting, leaving a copper-depleted residue rich in Ni and Co for subsequent hydrogen reduction. This completely eliminates the need for the aggressive sulfuric acid leaching typical of hydrometallurgical routes.