Why have the abundant mineral deposits or the so called manganese nodules of the deep oceans not been developed to provide mankind with an abundance of metals such as copper and nickel?

Manganese nodules are red-brown to black coloured polymetallic concretions composed predominantly of oxides of manganese (Mn) and iron (Fe), clay minerals and water. They can also contain traces of over 70 additional elements, the most abundant and commercially important of which are copper (Cu), nickel (Ni) and cobalt (Co) (Earney, 1990). Ranging in size from one to twenty centimetres, manganese nodules come in a variety of shapes from spherical and ellipsoidal to botryoidal and irregular (Barton, 1970; Kent, 1980; Earney, 1990; Hsü and Thiede, 1991; Thurman and Trujillo, 2004; Glasby, 2006). Although they have been known from freshwater deposits for hundreds of years, they were first recovered in the Kara Sea, Siberia in 1868, and more extensively on the voyage of the HMS Challenger in 1872, 160 miles southwest of the island of Ferro in the Canary Islands Group (Anon, 1979; Cronan, 1980; Earney, 1990; Thurman and Trujillo, 2004). They have since been discovered on the floor of all the Earth’s major oceans at depths of 3000 to 6000 metres (Barton, 1970; Earney, 1990), forming a carpet of varying density on top of the abyssal clays.
Manganese is an important element in the steel industry for making alloys; copper has a variety of uses including electrical wiring, pipes and making brass and bronze; nickel is used to make stainless steel; and cobalt is used as an alloy with iron to make strong magnets, steel tools, and jet engine parts (Barkenbus, 1979; Thurman and Trujillo, 2004). The co-existence of these three metals in one ore makes manganese nodules an exciting reliable economic resource; but this potential was not recognised until the late 1950s (Barton, 1970). As global mineral shortages were predicted, the publication of Mero’s over-optimistic study into the economics of manganese nodule mining in 1965 prompted widespread interest in exploration for and research into manganese nodules throughout the 1960s and 70s (Glasby, 2000). However, despite substantial investment and decades of research and innovation, the dreams of deep-ocean mining vessels tapping into this abundant source of Ni, Cu and Co has not materialised. What follows is a brief history of manganese nodule research and exploration, and the reasons for the failure to mine them.
How manganese nodules form has been an intriguing puzzle and is still not entirely understood. The minerals are precipitated concentrically around a nucleus which may be a piece of indurated sediment, volcanic rock, coral or the remains of an animal such as a whale’s ear bone, fish bone or shark’s tooth (Barton, 1970; Kent, 1980; Hsü and Thiede, 1991; Truman and Trujillo, 2004; Glasby, 2006). The nodules form at the sediment-water interface – resting half in and half out of the red clays of the deep ocean basins. The nodules lower surfaces are usually much more irregular in shape and an equatorial band may develop in spherical forms. Sedimentation rates must be extremely low (less than 5 mm per thousand years) so that the nodules do not become buried. This is aided by the bioturbation of the sediments by benthic organisms. Areas conducive to nodule growth also have strong bottom currents and are oxidising environments, with supplies of potential nuclei present (Earney, 1990; Cronan, 1992; Hsü and Thiede, 1991; Truman and Trujillo, 2004; Glasby 2006).
The major components, manganese dioxide (30% by weight) and iron oxide (20%), are thought to precipitate directly out of seawater (hydrogenetic deposition), aided by bacteria. The growth rate is extremely slow – only a few millimetres every million years – one of the slowest chemical reactions known (Earney, 1990; Truman and Trujillo, 2004; Glasby 2006). Manganese and iron oxides have high adsorption properties which suck in elements from the surrounding seawater and sediments, resulting in nodules enriched with several metals including nickel, copper, cobalt, zinc and molybdenum (Earney, 1990; Cronan, 1992; Glasby 2006). The concentration of the accessory metals Ni, Cu and Co is usually less than 1% but can be more than 3%, making them of economic interest (Thurman and Trujillo, 2004). Their concentrations are increased by digenetic processes which leach the metals from the underlying sediment pore waters enriched with elements from decomposed plankton and faecal matter, and as a result are more concentrated on the underside of the nodules (Earney, 1990; Glasby, 2006). The high porosity of the nodules (approximately 50% by volume) means the nodules also have a high water content (Anon 1979). The local conditions of the sediment and water column, therefore, play a large role in the relative enrichment of the manganese nodules with Ni, Cu and Co, and each individual nodule can vary considerably in composition on both local and regional scales (Glasby, 2006).
Manganese nodules are more abundant in waters deeper than 4000 metres and where plankton levels are high. When the Atlantic, Indian Ocean and Pacific Ocean are compared, the Pacific has the highest abundance of nodules, as it is the oldest ocean with areas of greatest depth, abundant plankton, strong bottom currents and low rates of sedimentation due to a lesser number of continental rivers discharging sediment into it (Earney, 1990; Glasby 2006). Areal density of nodules on the sea floor varies from 100 nodules per square metre to much greater concentrations (Truman and Trujillo, 2004). The areas of greatest concentration are the equatorial and central Pacific Basin, central Southern Pacific Basin, an east-west trending belt in the Southern Ocean corresponding to the Antarctic Bottom Water Flow, the northern sector of the Peru Basin, and the area around the Musicians Seamounts in the northeast Pacific Basin (Glasby, 2006)
The sea floors of the deep oceans are not flat plains but contain huge variations in topography. Some studies have found nodules concentrated on slopes, whereas others found that they collected in valleys (Anon, 1979 and references therein). The manganese nodules form an unusual two-dimensional ore body only a few centimetres thick, meaning the metals are spread over a very large area (Anon, 1979). The variation in nodule density and composition, and uneven topography makes mining the nodules very difficult. Mining feasibility and mapping studies were carried out from the 1960s using a variety of techniques including: echosounders, which penetrate the upper 50 metres of the sediment to survey topography and geology; bottom-towed side-scan sonars to give information on obstacles and nodule density; remote-controlled still cameras to photograph the sea floor; and retrieval of samples of the nodules using box cores, dredges and grab samplers (Anon, 1979; Earney, 1990).
Due to the high levels of plankton in the equatorial up-welling zone of the North Pacific, this area was found to have the most abundant nodules with economic concentrations of Ni, Cu and Co (~3% by weight). In particular the area bounded by the Clarion and Clipperton fracture zones (C-C FZ) was found to be especially rich, and as a result most research has concentrated on this area (Anon, 1979; Cameron, et al., 1980; Kunzendorf, 1986; Earney, 1990; Glasby, 2000, 2006). Initial estimates of the number of nodules in the North Pacific by Mero (1965) were in the region of over 1 trillion tonnes covering an area of approximately 6 million km2, and growing at a rate faster than could be exploited – an essentially unlimited resource (Glasby 2006). This spurred into action the numerous research and development programmes with over 200 exploratory cruises carried out during 1972-1982 by the United States of America, the Soviet Union, France and Germany alone (Glasby, 2000, 2006). Despite difficulties in predicting the number and grade of nodules on the sea floor and economic forecasting, full-scale mining was envisioned to be in operation by the mid 1980s (Anon, 1979). More recently, Morgan (2000) conservatively estimated 34 billion tonnes of non-renewable nodules existed in an area of 9 million km2 in the C-C FZ.
Potential mine sites were defined as areas where abundant nodules of a high grade were found in areas of moderate topography that were large enough to support a commercially viable mining operation (Anon, 1979). An economically viable mine site was considered to have nodules containing a minimum of 2.5% Ni+Cu+Co, an average abundance of nodules of more than 10 kg/m2 over an area of >6000 km2, and capable of producing 3 million tonnes of nodules per year for 20 years (Anon, 1979; Cameron, et al., 1980; Cronan, 1992; Glasby, 2006). Twenty to two-hundred mine sites are estimated for the Pacific Belt (Anon, 1979), though a more recent estimate by Lenoble (2004) suggested 3-10 sites producing 100-600 million tonnes of nodules over 20 years was more realistic given the current economic climate (Glasby, 2006).
During the 1970s and 80s several major consortia were set up by companies from the USA, Germany, France, the Netherlands, Japan, Canada and the UK to develop mining technology and systems, and investigate the possibility of commercial exploitation of the manganese nodules (Anon, 1979; Glasby, 2000, 2006). Problems that would have to be overcome if mining of the nodules was to be successful included: the great depth of water (up to 5 km); pitch black, corrosive conditions and high pressures at the seabed; and the location of the mining sites’ vast distances from land (1600-2400 km) (Anon, 1979; Kent, 1980; Kunzendorf, 1986). Ideas were drawn from the already established offshore drilling programmes (Anon, 1979).
The main questions can be summarised as, how were the fissile nodules to be collected from the sea bed from the abyssal clays which can have the properties of grease (Anon, 1979), how were the nodules to be transported to and stored on the mining ship, and how were the metals to be extracted from the nodules? These were answered by the development of three mining subsystems – collection, transport and processing. The methods of collection devised centred around passive dredging systems, or more active suction systems (Anon, 1979; Kent, 1980). The collecting system needed to be efficient, sucking up as little sediment as possible, as to make it economically viable large tonnages of nodules needed to be collected and processed (Anon, 1979).
The nodules were lifted to the mining vessel through a near vertical pipe by either air-lift or mechanical pumps; though a more ambitious Continuous Line Bucket system involving a series of buckets on a cable was tested in 1973 and later abandoned due to problems with tangled cables and inefficient nodule pick-up (Anon, 1979; Earney, 1990). The air-lift system was found to be 40-50% more energy intensive than mechanical pumps, but had the advantage of not requiring moving the mechanical parts at depth (Anon, 1979; Cameron, et al., 1980). A particularly ambitious French project looked at building a remote surface-to-seabed shuttle vehicle (Earney, 1990).
The sea-surface mining vessel required large storage areas for the nodules, as well as living accommodation for the crew, precise navigational equipment, excellent steering and handling, and a way to keep the pipeline still as motions of the ship affected the collector’s efficiency (Cameron, et al., 1980; Earney, 1990). Offshore drilling ships were found to be suitable for the job with some adaptations. In the late 1970s belief in the commercial viability of manganese nodule mining was encouraged by an American consortium developing a purpose built ship, the Hughes Glomar Explorer. Millions of dollars were invested in it, and ground-breaking technology developed, including state-of-the-art lifting equipment. It was later discovered to be a cover story for a secret CIA-funded operation to recover a sunken Russian nuclear submarine from the seafloor (Kent, 1980; Glasby, 2000, 2006).
The Ni, Cu and Co is distributed throughout the matrix of manganese oxide. This complex structure and the high level of impurities makes processing the nodules very difficult (Anon, 1979). Methods developed to extract the metals involve pyrometallurgy (smelting) and/or hydrometallurgy (leaching and solvent extraction) (Anon, 1979; Cameron, et al., 1980). In order to smelt the nodules the water must be removed, which, as the nodules contain 30-40% seawater, is a very costly operation, so hydrometallurgical methods were preferred (Anon, 1979). Both methods have a 90% recovery rate for Ni, and a 90% and 80% recovery rate for Cu with the hydrometallurgical and pyrometallurgical methods respectively (Cameron, et al., 1980).
In order for the mining operations to be economically viable, costs based on estimates of productivity, reliability and maintainability of the equipment, and initial investment and operating costs needed to be calculated. As discussed above, the inherent complexity of the nodules, mine sites and systems made predictions varied and often unreliable (Anon, 1979; Post, 1983). Processing was determined to be the most expensive subsystem needing 65% of the operating and 70% of the capital costs. Transport was the cheapest subsystem as the technology was already known, taking only 15% of the operating, and 10% of the capital costs. But in terms of technological risk and economic uncertainty the mining subsystem had the highest risk, followed by processing and transport subsystems (Cameron, et al., 1980). An economic model proposed by Flipse in the 1980s predicted an annual operating budget of $200 million, plus $175 million in working capital and $1000 million investment in equipment, vehicles and the processing plant, with anticipated annual revenues of $423 million (Earney, 1990).
Testing of many of the mining systems was carried out in 1977-79. The results, together with the optimistic economic predictions, showed it was, at least in theory technically possible to mine manganese nodules in the deep oceans. However, all the systems remain in the prototype stage and most research programmes have been put on hold since the mid 1980s (Earney, 1990). The reason is three-fold:
The first and major problem was economic. The vigorous research programmes had been motivated by predictions of world mineral shortages and, therefore, continued rises in metal prices (Glasby, 2000, 2006). Metal prices did rise until 1979 after which Ni and Cu prices fell significantly (prices in 1998 were 40 % less than at their peak in the 1970s) effectively rendering all nodule mining unprofitable (Glasby, 2006). The risks involved in investing in the mining operations and the enormous capital investment needed, had made attracting funding difficult. Only those consortia receiving government subsidies are still researching into deep-sea nodule mining (Earney, 1990).
The second major problem was political. In 1973, following interest generated by initial exploration of the manganese nodules in the 1960s by the USA, the question of who owns the rights to exploit deep sea nodules became a sticking point at the third United Nations conference on the Law of the Sea (UNCLOS III) (Cameron, et al., 1980; Post, 1983). Differences of opinion developed between the developed and the developing nations who were in favour of the resources being treated as the ‘common heritage of mankind’, giving them fair opportunity to participate in and benefit from the new industry. A governing body, the International Seabed Authority (ISA) was proposed to oversee the mineral resources outside the 200 nautical mile Exclusive Economic Zones, and divide up the assumed profitable mine sites equally between the mining consortia and the Enterprise representing the developing nations. Technology was also to be shared between the two (Cameron, et al., 1980; McKelvey, 1980; Glasby, 2000). Several developed countries including the USA and the UK, refused to sign the treaty which was passed in 1982, as it considerably lessened the economic feasibility (Glasby, 2000). The treaty was finally ratified in 1994 and the ISA set up (Glasby, 2000). By this stage most consortia had abandoned their research programmes. The only interest has been from France, Russia, India, Japan, China and Korea, who have registered claims to deep-sea mining areas in the Pacific and Indian Oceans under UNCLOS largely for strategic rather than commercial reasons (Glasby, 2006).
Finally, concerns about the environmental impact of deep-sea mining came to the forefront and were relatively unknown. Initial studies suggested little environmental problems associated with discharge of sediment and disturbance of the sea floor (Amos, et al., 1977; Mero, 1977), but more research is needed, especially taking into account processing as well as mining (Hsü and Thiede, 1991).
In conclusion, the high costs of and risks involved in developing and operating the deep-sea mining systems, together with the low metal prices, complicated political and legal situation regarding mineral exploitation in the deep oceans, environmental concerns, advances in terrestrial mining technology, and new finds of terrestrial mineral deposits have prevented the commercialisation of manganese nodule mining. Although limited research continues, manganese nodules are unlikely to be a source of Ni, Cu and Co in the near future. There is, however, interest in deep-sea mining of sulphide deposits and shallower-water Co-rich manganese crusts (Glasby, 2000, 2002), which may at least learn from the past failures and successes of the nodule mining research.