Manganese nodules

Manganese nodules are rough spheres of manganese and other minerals deposited by seawater on the seabed. These nodules, about the size of a potato, are being considered as a potential source of manganese, which is essential for steel production. Manganese also is used to make pigments, alkaline batteries, and fertilizers.

Manganese

Manganese nodules are vaguely spheroid rocks made of deposited sediments, primarily the eponymous manganese. These nodules also contain enough other metals so that they are referred to as polymetallic nodules. Nodules form through millions of years of deposition, and the highest concentrations are on the abyssal plains of the oceans, at about 4,000 to 6,000 meters, or 2.5 miles (mi) deep. Manganese nodules have been suggested as potential sources of manganese for industry.

Chemistry of Manganese Nodules

Uses for manganese nodules come from their chemical properties. Manganese is a transition metal, like iron or nickel, and has several common oxidation states, with the most stable being +2. Manganese forms from a variety of minerals, though the majority of the minerals in a nodule consists of manganese hydroxides.

Seawater, an important element in the formation of manganese nodules, is not pure dihydrogen monoxide. Instead, it includes many other dissolved substances, but mostly such minerals as sodium chloride (table salt). Water is a good solvent because it is a polar molecule. Water is a polar molecule not simply because of its bent shape, but also because of its electronegativity. The oxygen in water is more electronegative than is water’s hydrogen; thus, the electrons are closer to the oxygen. Oxygen is more electronegative because of various properties of atoms and electron orbitals; the atoms with eight electrons in their outermost orbits are the most stable. Atoms seek to have eight electrons, or to have none. Thus, hydrogen “gives” electrons to the oxygen. The electrons are, thus, closer to the oxygen than the hydrogen, giving it a net charge.

Limits exist to the amount of any substance that can be dissolved, however. The maximum amount of solute depends on properties such as temperature and pressure but varies for every material combination. Some substances increase in solubility with temperature and others decrease, all due to the binding characteristics of the compounds. The bonds of compounds are either more ionic or more covalent. As a metal, manganese compounds tend to be ionic and to dissolve in water. When the solubility of the water changes, either through cooling or through the addition of more solutes, the dissolved manganese precipitates out. It then crystallizes around a core, such as a fragment of a nodule, a shark’s tooth, microfossils, or basalt fragments.

Crystallization is the formation of crystals of a substance as they precipitate from solution. Crystals are ordered arrangements of atoms. They are organized by the bonding properties of the atoms in question. However, the precise pattern is still difficult to predict (computers and algorithms are leading to progress in this field, however). At any rate, continued deposition of the component of the crystal will lead to crystal growth.

Nodule Formation and Location

Nodules can be found in any body of water suitable for their growth, including some lakes, but most nodules are on the abyssal plains of the oceans. In particular, the northern regions of the Pacific equatorial seabed and the Atlantic have been found to have high concentrations. The Indian Ocean also appears to have concentrations of the nodules north of the equator.

These patterns in Pacific distribution seem to be a result of seafloor composition. North of the equator, the tropical Pacific is mostly siliceous in content, made of the noncarbonate portions of the skeletons of plankton. Because the sea floor here is deeper than the calcite compensation depth (calcite is calcium carbonate, which is what plankton shells are made of), the plankton dissolve. Calcium carbonate dissolves in water naturally, but because sufficient amounts exist in surface-level water, shells made of calcium carbonate are virtually insoluble. This explains why so many marine creatures make their shells of calcium carbonate.

At lower depths, calcium carbonate solubility increases dramatically. As a result the sea floor at lower depths is made not of calcium carbonate but of other materials, such as silicon, which does not dissolve. This effect leads to the creation of siliceous ooze. Calcareous sediments are formed where the seabed is above the calcite compensation level. The calcium compensation level depends upon many factors, but it is at about 42 to 500 meters (m), or 138 to 1,640 feet (ft), in the Pacific, except in the equatorial upwelling zone (where it is at 5,000 m, or 3 mi). Farther from the tropics, the sea floor tends to be made of red pelagic clay and has little biological material.

These differences in seafloor composition mean that the composition of nodules can be different, too. Where calcareous ooze, formed by sedimentation of calcareous shells, predominates, concentrations of nodules are lower. Equally, deposits in red clay regions have higher iron content than other deposits.

The sedimentation rate of siliceous ooze and pelagic clay regions is far faster than the rate of growth of nodules; therefore, the means by which nodules remain on the seabed and avoid getting buried are unknown. It has been suggested that deep-sea fauna might occasionally run into the nodules, pushing them over and thus preventing them from being buried. It also is possible that bottom currents push the nodules to the top or that buried nodules dissolve and are redeposited on the surface nodules.

The

The abyssal plain is the sea floor beneath most of the ocean, typically at a depth of between 3,000 and 6,000 m (1.8 to 3.7 mi). The plain covers more than one-half the earth’s surface and is among the flattest landscapes on the planet.

The plain also is one of the least explored surfaces on Earth. Despite the overall flatness, the plain also contains midocean ridges and oceanic trench subduction zones. These features are caused by plate subduction and crust formation.

In a phenomenon known as marine snow, organic detritus, such as dead organisms, fecal matter, land sediment, and inorganic sediment, continually fall from the upper levels of the ocean, making this detritus the main source of nutrients for the abyssal plain’s ecosystem; the detritus also provides much of the sediment for magnesium nodules. Once thought lifeless, the abyssal plain is biologically diverse. Many species are bottom-dwelling species that subsist on the marine snow. Despite recent discoveries, however, much remains to be learned about these organisms.

Experts fear that manganese-nodule mining could negatively affect the seafloor environment. Disruptions to the seabed could increase the toxicity of the water near the seabed. Another possible problem is the formation of sediment plumes. A sediment plume is caused when tailings from mining are dropped back into the water.

Deep Sea Mining

Deep-sea mining of manganese remains speculative. Modern prospecting methods tend to use remotely operated vehicles to collect samples from prospective mining areas. After a site has been found, a mining ship or platform is placed into position.

Two main mining methods are being considered: the continuous-line bucket system (CLB) and the hydraulic suction system. CLB is the primary method under consideration and is akin to a giant conveyer belt. It operates by using dredging buckets to pull material from the seabed. The material is processed on a platform and the waste materials, called tailings, are returned to the seabed in the dredging buckets. The other method uses hydraulic suction to pull the materials through a large tube. Another tube returns the waste material to the seabed.

The CLB system was put into use by a syndicate of thirty companies in the 1970s using an 8-kilometer (5-mi) cable launched from a former whaling vessel. The system worked but was prone to tangling.

The hydraulic system was tried by an American consortia in the Pacific Ocean. The system used a dredge on the sea floor attached through a tube to a surface object, such as a ship or a platform. This allowed it to reach an area of seabed. The initial tests ran into problems because the cables to the dredge could not be fully waterproofed; later dredges had problems that included muck and hurricanes. However, the system was demonstrated successfully in 1979.

Many current designs work on variants of this system. The fully operational system would have a central mining platform with a fleet of ships to supply and transport ore back to shore. To be of economic interest, however, the abundance of manganese must be 10 kilograms, or 22 pounds (lb) per square meter or more, with an average of 15 kg (33 lb) per square meter in several tenths of a square kilometer.

Sonar is used to map the sea floor for mining, too. Modern sonar systems can survey strips of more than 20 km (12.5 mi) at a time from the surface. This mapping can be supplemented with the use of arrays, which are towed above the seabed. Additionally, free-fall devices can take readings as they descend and return to the surface on their own. They even can take samples and photographs from the sea floor. Cable-operated apparatus also can perform this function.

Manganese Nodule Processing

The idea for processing manganese nodules was first proposed in the 1960s by John Mero in his landmark paper on the subject, “Mineral Resources of the Sea.” The paper received considerable attention, leading to several efforts to utilize the manganese resources.

Once on shore, several methods exist for refining. Some of the most popular methods include the cuprion process, sulfuric leaching, and smelting. The cuprion process, developed by Kennecott, an American mining company, involves grinding the nodules into slurry. The slurry is reduced by carbon monoxide in the presence of ammonia, in a low- temperature tank. The metals are made soluble and then are electrowinned, or separated by electrolysis. (Electrolysis is the process by which ions are pulled out of solution by an electric field to be deposited on a collector. Because atoms have different electrical properties, different voltages can pull out different materials.) Recovery of manganese from the remaining ferromanganese residue remains problematic for the mining industry.

Sulfuric leaching works by dissolving the nodules in sulfuric acid at high temperature and pressure. Copper, nickel, and cobalt are precipitated with hydrogen sulfide. The resultant materials are then refined. These products are electrowinned, and the ferromanganese residue is smelted after drying.

Current Efforts

Harvesting manganese nodules remains expensive, given available technology. Many countries are placing claims on regions of the sea floor, however. As of 2024, the pipeline-lifting mining system was the best option for harvesting manganese nodules. This systen had a deep-sea mining vehicle. However, according to Frontiers in Robotics in 2023, a new vehicle was proposed that, when developed, will be more efficient.

Principal Terms

abyssal plain: vast, flat underwater plains

crystal: a solid with a repeating structure

crystallization: the process by which crystals are formed when a substance comes out of solution

electronegative: a measure of how tightly an atom holds its electrons

electrowinning: a process by which metals are pulled from solution or a liquid

ion: an atom with a net charge through either electron addition or electron loss

ionic bond: a bond in which molecules are held together by an electrostatic bond created when the constituent atoms transfer electrons

ooze: seafloor sediments

solution: a mixture in which a solute is dissolved in a solvent

tailings: waste from mining operations

Bibliography

Chang, Raymond. Chemistry. Boston: McGraw-Hill Higher Education, 2007.

Cronan, D. S. Handbook of Marine Mineral Deposits. Boca Raton, Fla.: CRC Press, 2000.

Halfar, Jochen, and Rodney M. Fujita. Science 316, no. 5827 (2007): 987.

Koslow, J. A. The Silent Deep: The Discovery, Ecology, and Conservation of the Deep Sea. Chicago: University of Chicago Press, 2007.

Rona, Peter A. “Resources of the Seafloor.” Science 299, no. 5607, n.s. (2003): 673-674.

Volz, Jessica B. et al. "Alpha Radiation for Polymetallic Nodules and Potential Health Risks from Deep-Sea Mining." Scientific Reports, vol. 13, no. 7985, 17 May 2023, doi.org/10.1038/s41598-023-33971-w. Accessed 29 July 2024.

Xie, Yingchun, et al. "Research on Path Planning of Autonomous Manganese Nodule Mining Vehicle Based on Lifting Mining System." Frontiers in Robotics and AI, vol. 10, 27 June 2023, doi.org/10.3389%2Ffrobt.2023.1224115. Accessed 25 July 2024.