In places near the edges of tectonic plates, hydrothermal vents form, spewing out underwater clouds of water, as well as many precious metals. Theses metals precipitate as the water cools, forming polymetallic nodules. Over the past century, there has been huge interest in collecting these nodules for profit, but so far, no venture has been deemed capable of making money. Will deep sea mining ever be profitable?
Polymetallic nodules are mostly composed of manganese, hence their other name, manganese nodules. Manganese is not very valuable, selling for $2.90/kg on average. However, the sheer abundance of manganese nodules in some places is enough to potentially make the mining of these nodules profitable. The nodules also contain smaller amounts of iron, silicon and aluminium, but these would probably not be profitable to mine. Nickel, copper, and cobalt due to their higher cost, are more profitable, and so would be a secondary focus of any mining operation. In addition to polymetallic nodules, other marine deposits, such as manganese crust and sulfide deposits, also exist.
Shallow water mining has existed for decades. Marine Diamond Corp was able to gather 1 million carats (200kg) of diamonds from the continental shelf of Namibia in the 1960s, and DeBeers currently mines diamonds in shallow waters near to South Africa. However, these mining operations occur less than 200m below the surface, and so the technological challenges of high pressure don't apply here.
High pressure is one of the greatest challenges faced by deep-sea mining operations. At 5km below the surface, the pressure is approximately 500atm, or 500 times the pressure at sea level. Very few submarines are capable of reaching these depths, so specialised submarines such as an Alvin-class deep-submergence vehicle are needed. In addition, Alvin takes a total of 2 hours to dive to 5km, and another 2 hours to resurface. This would significantly reduce the efficiency of any mining operation. This is complicated further by Alvin's inability to carry large heavy loads, as it was only designed to take relatively light samples of the seafloor and fauna.
For these reasons, any mining machine operating on the seafloor is likely to be a custom-built machine which trawls the seafloor, is unmanned, and sends the nodules it collects up to the surface without it. This would remove the inefficiency faced in repeatedly ascending and descending. A potential design involves the collected material being pumped up through tubes all the way to the surface. While this would require a lot of power and tubing, it may still be profitable.
One of the biggest problems with this design is the catastrophic effect that it would have on the ecosystem. Firstly, the collector would remove the benthic substrate, composed of decomposing organic material from the epipelagic zone (the region of the ocean between 0-200m). By removing the substrate, the collector would remove the food source for detritivores, more or less starving the ecosystem from the bottom up. Although some would survive due to new deposits formed after the collector moved through, the ecosystem would still be severely damaged. In addition, light pollution would confuse benthic life significantly, or at least those who still retain sight. Some species may gather around the seafloor collector, only to be gathered and killed. The machine may also hit a hydrothermal vent, causing significant damage to the ecosystem by destroying tube worms and killing crabs and fish. Damage to hydrothermal vents is very likely to happen, since the highest concentrations of manganese nodules are around hydrothermal vents. However, it is important to note that terrestrial mines cause extensive pollution to the surrounding area, and so underwater mines might be a slightly less harmful alternative.
Until 2019, the government of Papua New Guinea was strongly backing deep sea mining, and gave a license to operate in the Bismarck Sea to the mining company Nautilus Minerals. Nautilus Minerals finished developing their mining machines in 2012, which are to be remotely operated with a video feed. Nautilus Minerals planned to first send a small autonomous vehicle to the location. The vehicle is equipped with sonar and a magnetometer, enabling it to map the topography of the region and the location of metals ores. They then planned to send down another robot to collect ore samples, to judge if the area is worth mining.
After the first preliminary steps, Nautilus created three much larger robots to collect the nodules. The first, a 250-ton cutter, levels the seafloor, creating a level working surface for the following robots. This step disperses sediment and levels any chimneys in the area, destroying the ecosystem surrounding the chimney. This step is, however, necessary for mining, as otherwise the following robots would be unable to move across the floor, and could potentially fall into a vent or crevice.
The second robot is a bulk cutter, which crushes rock faces (containing ore) it encounters into loose pieces, which are then collected by the third robot, which pumps a slurry of ore and water up to a ship at the surface, which dries out the ore. The ore is then transferred to a nearby bulk ore carrier, which then transports it back to the mainland to be smelted and processed.
Unfortunately, in 2019, Nautilus Minerals went bankrupt, and its technology was subsumed by other companies. This lost the Papuan government an estimated $157 million, and since then the Papuan government has lost faith in deep sea mining, marking a change across Oceania, with governments either banning deep sea mining or refusing to grant licenses. Currently, the future of deep sea mining seems bleak, and it is unlikely that the field will progress significantly at least for a few decades, especially since the governments in charge of the major commercially viable areas in the Pacific have shown such strong opposition.
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