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Marine mining: birth of a new industry
Marine mining: birth of a new industry John Yates, Daniel Spagni, and Jim Keane The oceans are known to be a vast storehouse of natural resources, yet, except in the case of fisheries, those resources have up to now been exploited only in coastal areas and in the shallow part of the continental shelves. Until recently, little was known of the deep seabed which is now revealing some of its potential to us. The oceans are increasingly going to be able to provide alternative sources of raw materials in direct competition with land-based deposits.
Successful exploitation of the minerals from the sea does not depend only on technological advances. In many cases the development of these resources will be highly dependent on economic, legal, and institutional factors. This article reviews the major hard mineral resources identified to date, within the marine environment; the methods which have been researched and developed for their exploration and exploitation; and their commercial prospects (Table 1). Offshore oil and gas deposits are not considered except as an example of how land-based operations are likely to progress seawards, as the most accessible deposits become depleted. From a geological and legal point of view, the oceans are divided into two distinct domains: the continental shelves and the abyssal plains. Continental shelves are an extension of the continents beneath the sea, accounting for less than 10 per cent of ocean floor John Yates, M.Sc. Is a graduate in geology of Manchester University. His main research interests are in marine minerals, marine technology, and technology transfer. Daniel Spagni, Maitrise de Droit in Int. Law and EEC Law, Diploma ISTEC Graduated in law at the University of Aix-enProvence and subsequently did research in international law in France and the United Kingdom. His main research interests are in R&D management, marine policy, and marine resources management. Jim Keane, MSc. Is a graduate in economics of Loughborough University of Technology. His current research interests are in waste disposal at sea, marine exploitation, and oceanographic instrumentation. All three authors are attached to the Marine Resources Project of the Programme of Policy Research in Engineering, Science and Technology (PREST), University of Manchester.
EURO ARTICLE (see p.ii) Endeavour, New Series, Volume 10, No.1, OWO-9327/96 90.00 + .50 Pergamon Press. Printed in Great Britain
1986
and rarely extending beyond the 200metre isobath. In international law, the continental shelf is defined as ‘the natural prolongation of the land territory’ and consequently falls within the sovereignty of the coastal state in a similar fashion to the land territory. The abyssal plains have an average depth of 4000 me&es, the deepest part being the Challenger Deep north of New Guinea at 10 915 metres. The seafloor of the deep ocean is quite distinct from the shelves, being composed predominantly of theoleiitic basalt, covered by a layer of pelagic sediments of varying thickness. International law defines the deep seabed as an area beyond national jurisdiction but is unclear as to whether or not it is susceptible of national appropriation. Deposits
of the continental
shelf
Aggregates. These are deposits of sands, gravels, or shells. They are the product of hydrodynamic forces and occur both on land and on the continental shelves. Sand and gravel were among the first minerals to be recovered from the sea and the oldest ocean mining operations involved the recovery of sand and gravel from the North Sea and English Channel over 100 years ago [l]. Offshore aggregate mining has expanded dramatically in the last few years because of the progressive depletion of land deposits and stricter environmental controls. Aggregates are used primarily by the construction industry; they are a very low value commodity, and transport costs play a vital part in their exploitation. It is far cheaper to transport large volumes by sea than land, and so to some extent the increased cost of marine mining is offset by reduction in transportation overheads. To be economically viable, a typical marine aggregate deposit needs to be close to a centre of high demand, as the cost of aggregate can double on a journey of 50 km [2]. The prime markets are large coastal metropolitan areas. The deposits themselves need to be
homogeneous with little overburden so as to minimise processing. The main extraction technique used is dredging. In the past, dredging was carried out by grab and bucket dredgers, but now most dredging is done by sophisticated suction hopper vessels, often equipped with cutters. Owing to the limitation on the depth to which centrifugal pumps can be operated, few dredgers are capable of working beyond 30 metres water depth. However, with the development of new types of pumps such as those used for mining deep seabed minerals, aggregates will ultimately be exploited in deeper waters. The maximum depth achieved using jet assisted suction is 45 metres by anchor dredging [3]. Whether it will ever be economically viable to dredge aggregates beyond the continental shelf is more doubtful, in view of the high transport costs which would be involved. Placer deposits. Placers occur as a result of chemical and physical weathering forming concentrations of minerals which are transported by running water up to 15 km, with the larger, denser grains being found nearer to their source. They can be found in rivers, lakes, and at sea and contain a variety of valuable metals and gems which are the source of much industrial activity at present. Thailand has dredged for tin in offshore areas since 1907 [4]. During its lifetime, the Alaskan operation at Nome sands produced over $100 million of gold and the bauxite mine at the Gulf of Carpentaria in Australia has been one of the largest of its kind in the world since operations began in 1955. Japanese firms have mined for magnetite in iron sands in Tokyo Bay and the Philippines since the late 1960s [5]. Placer mining operations thus already have an historical perspective and their importance will increase over the next few years as technologies for extraction develop and land-based sources dwindle. Extraction usually involves the use of dredgers, either bucket-line or, in more
TABLE 1
CONTEMPORARY
STATUS
Mining
MINING
technology
Participant
countries
Type of deposit
Economically minerals
significant
Commercial
Aggregates
Sand, gravel, carbonate
calcium
Commercial exploitation
Dredging/suction
UK, US, Japan, Denmark, France, Netherlands etc
Placers
Chromite, platinum, etc.
Commercial exploitation
Dredging/suction
US, UK, South Africa, Indonesia, India, Australia, Thailand etc
Phosphorites
Phosphate
Resource assessment Commercial exploitation
Dredging/Suction
US, New Zealand, South Africa, Chile, Australia
Manganese nodules
Manganese, nickel, copper, cobalt
Resource assessment Pre-pilot phase
Bottom collector and hydraulic lift chantier Sous-Marin
Japan, France, US, UK, West Germany, India, Korea, Canada
Metalliferous muds
Silver, gold
Pilot phase
Vibration/suction
Saudi Arabia,
Marine polymetallic sulphides
Zinc, copper, silver, etc.
Resource assessment Basic research phase
Bottom collector and hydraulic lift, Vent capping
US, France, Japan, West Germany
Cobalt rich crusts
Manganese, cobalt, nickel, copper
Resource assessment Basic research phase
Continous system
US, Japan
Minerals seawater
Salt, magnesium, uranium, bromine, lithium, etc
Commercial exploitation, research
Solvents, soda, ion exchange resins
from
silver, gems, tin (P,O,)
zinc, copper cobalt,
recent times, suction dredgers. Future developments for deeper deposits will involve the use of drilling to locate and delineate reserves. Various types of operations are possible, and at present Thailand is planning to exploit tin deposits at depths between 40 and 50 metres, using drilling techniques supplied by an Indonesian company. Marine placer mineral deposits occur and are widely exploited throughout the world especially in Alaska, Florida, South Carolina, South Africa, South East Asia, Sri Lanka, and the United Kingdom. Their promixity to shore and relative ease of extraction will ensure the growth of this sector of marine mining. Phosphorites. The principal mineral of marine phosphorite is carbonate fluorapatite or francolite. Found as pellets, nodules, or sand on the continental shelf, the mineral develops in situ at depths of up to 1OOOm where deep, nutrient-rich waters upwell to produce a wealth of biological activity. Since their discovery during the famous Challenger Expedition (1873-76) significant deposits have been found at many offshore locations including South West Africa, Florida in the United States, and Chatham Rise off New Zealand [6]. At present, approximately 95 per cent of the phosphate rock mined on land is processed for fertiliser, and marine deposits will be used for a similar purpose. The principle commer-
status
OF MARINE
basic
cial advantage that certain marine deposits have over land-based deposits is their proximity to existing markets. Phosphorite is a low cost per unit commodity, making the transport element significant in the overall cost. Certain countries have both a high import bill for phosphate and proven offshore deposits. In New Zealand, detailed exploration has been undertaken of the Chatham Rise deposits, located in 400 metres of water. Potentially rich reserves (2&30 per cent P20s) have been discovered (100 million tonnes) and two German companies, Preussag and Salzgitter, appear ready to mine these deposits once environmental concerns are alleviated and a New Zealand company can be found who will partner the project. Phosphorite sands are currently being dredged at San Domingo, off the coast of Baja, Southern California, by the Rofomex company of Mexico. The latter started operations in 1982, following a study which estimate 1.1 billion tonnes of phosphorite to be present at the site, with a phosphate content of 4 to 5 per cent. Mining is undertaken using a dredge (figure 1) and the sand is pre-processed on a barge. Production began at 1.5 million tonnes per year and is to be increased to 4.5 million tonnes. The capital cost of the project is estimated at $US 100 million and should provide 15 per cent of Mexico’s phosphate needs. Australia, South Africa, Peru, Chile, the United States, and India have all undertaken a series
line bucket
Japan,
Sudan
US, UK
of preliminary studies of their marine phosphorite deposits. Minerals from seawater. The total volume of water in the ocean is approximately 1350 x lo6 km’ or - put another way - 318 million cubic miles. Within this vast body of water minerals of all types can be found in varying concentrations. At present, sodium chloride (salt), bromide, and magnesium are commercially extracted from seawater. Of the more dilute minerals in seawater, uranium has commanded the greatest interest, due primarily to its strategic value. Uranium is found in seawater as uranyl tricarbonate at a constant concentration of 0.003 mg per litre down to 400 metres depth. The total amount of uranium in seawater has been estimated at 4 X 10’ tonnes, while terrestrial resources appear to be 2.59 x lo6 tonnes [7]. Early attempts at extracting uranium from seawater concentrated on the use of solvents, which eventually proved too complex and costly a process. More recently, studies have been made on precipitating uranium through the addition of caustic soda (NaOH). This approach has proved very effective, with 90 per cent resource recovery rates achieved. However, problems of restoring the pH level of the seawater after extraction have added considerably to the cost and may prove prohibitive to future applications. Of promise for the future in this domain is the use of synthetic 45
TABLE 2 TYPICAL METAL CONTENT OF NODULES IN A POTENTIAL ‘FIRST GENERATION’ MINESITE AREA
Metal
%
Manganese Nickel Copper Cobalt Lead Molybdenum Vanadium Zinc
Figure 1
Rofomex
1. Phosphorite
mining
resins to achieve extraction through an ion exchange process. One significant breakthrough was recently achieved by a research team at the University of Salford, who developed a synthetic resin PHA (polyhydroxamic acid). A German company Uranersbergbau compared PHA granules with titanium oxide granules developed in Japan. The experiment took place in the Baltic, and PHA proved itself to have a stronger resistance to attrition and a better physical and chemical stability in the marine environment. Being cheaper to produce than titanium oxide, PHA’s only major drawback appears to be its ability to absorb other seawater minerals alongside uranium, making recycling and reapplication of the material difficult. The Salford team are currently investigating the possibility of weaving Orlon fibres with PHA fibres to produce a composite carpet which would be suspended in a continuous flow of seawater. Commercial prospects for the extraction of the more dilute minerals in seawater are very much dependent on technological advances. Because of its strategic importance uranium has been the catalyst of the research in this field. However, uranium extraction on a commercial basis seems distant. Even assuming 100 per cent resource recovery, an industrial operation would require the pumping of 2.2 million litres of seawater and the use of 12 000 cubic metres of PHA resin. The requirement for a continuous flow of such a large volume of water at a constant rate will also prove very costly. An MIT cost study, which included pumping costs, concluded that a uranium oxide seawater extraction operation would cost a 46
barge (source:
Rofomex).
minimum of $US 697 for each kilogram produced, which is many times that of present land-based mining operations [8]. One way in which mineral extraction from seawater might prove more commercially viable, is through offsetting some of the costs by coupling an extraction operation with other marine activities. The obvious operations to which extraction could be adjoined are desalination, tidal, or ocean thermal energy conversion (OTEC) plants which Viould provide the constant flow of seawater required in the extraction
Figure 2 One-tenth Mineral Co.).
scale prototype
30.00 1.3 1.1 0.2 0.09 0.05 0.05 0.04
process. To date, the largest desalination plant in the world has a daily capacity of only 800 000 m3 of water, which would provide a meagre 2.5 tonnes of uranium a year. Only the Japanese are actively engaged in a programme aimed at commercially extracting uranium from seawater. The Ministry of International Trade and Industry (MITI) has established an experimental plant at Shikoku which they hope will eventually produce 100 tonnes of uranium oxide per year. No doubt, if they are successful this will encourage other countries to review their R & D policies in this field and probably in the whole area of mineral extraction from seawater. Deposits
of the deep ocean
Manganese nodules. Manganese, or polymetallic, nodule deposits are to be found in the deepest parts of all the
manganese
nodule
miner (source:
Ocean
world’s oceans. Discovered over a century ago, they have attracted considerable commercial interest since the early 1960s as a potential source of metals, primarily nickel, copper, cobalt, and manganese (Table 2). Several international consortia of companies, as well as national projects have been conducting research and development programmes for more than ten years, aimed at designing recovery, transport, and processing systems which would allow economic production of some, or all, of the metals contained in nodules. Most of the surveys conducted to date have concentrated on the Pacific Ocean in the so-called Clarion-Clipperton Zone, midway between the coast of North America and the Hawaiian chain. In addition, the French have undertaken some work rather closer to Tahiti in an area showing titanium-rich nodules, while the Japanese have also had a wide sweep toward the Japanese ‘home’ islands. The sites claimed by the various consortia under the national deep sea mining legislations have all fallen in the Clarion-Clipperton Zone, where nodules have a combined nickel/ cobalt content of well above 2 per cent [91. Many uncertainties still remain as to nodule distribution elsewhere in the world. Nevertheless, the Indian government has, in the past few years, conducted a vigorous programme of oceanographic cruises in the Indian Ocean, where they have claimed major discoveries of rich nodule deposits. For a combination of technical and economic reasons, all the nodulemining systems which have been designed, patented, and proved at prototype stage, aim at recovering 3 million dry tonnes of nodules per year, from depths greater than 4.5 km. The mining operation itself entails collecting the nodules with a steerable bottom collector unit, and lifting them to a mineship through a pipestring by a hydraulic lift system, using either an airlift or conventional slurry pumps (figure 2). In France, the Association Francaise pour la Recherche et l’Exploitation des Nodules (AFERNOD) a grouping of private and public organisations - is developing an original system which uses underwater robots to collect the nodules on the seabed. The novelty of many of the technologies involves, the absence of an internationally agreed legal framework to govern mineral rights in the deep ocean, together with low metal prices caused by the world recession, have led to delays in planning commercial ventures and in developing the deep sea mining nodule technology. Japan, France, and India are the only remaining countries having a very active nodule programme. The
Japanese Large Scale Government Project started at the end of 1980 with a budget of 20 billion yen ($200 million) over nine years. The programme is carried out by an organisation comprising MITI, Metal Mining Agency (MMA) and 19 private companies mainly from the engineering and shipbuilding sectors. The National Research Institute for Pollution and Resources is currently operating a test facility which includes a 200 metre deep vertical water tank used for lifting natural and artificial nodules [lo]. The French nodule programme started in the early 1970s is spearheaded by AFERNOD. Nodule activity in France was reorganised in 1983 when a new organisation GEMONOD was created alongside AFERNOD to develop the technology necessary for the collection and processing of nodules. Having studied and tested all the existing collector devices, the AFERNOD group decided in the late 1970s to develop a radically new mining concept, the ‘Chantiere sous-marin’ (CSM). This required the development of a self-propelled underwater vehicle or shuttle capable of collecting several hundred tons of nodules at 6000 metres depth and lifting them to a surface mining platform [ll]. GEMONOD had already developed and tested several experimental devices and has recently concluded a collaboration agreement with the German Company Pressuag in order to build a full scale prototype. The Indian nodule exploration programme began in 1980 and was coordinated initially through the National Institute of Oceanography (NIO) in Goa. In 1982, the Department of Ocean Development was created and the All India Coordinated Project was set up under its auspices. The programme envisages an Indian nodule mining capability by the end of this decade with the investment of $100 m. Most of the funds have to date been directed at the exploration of the Indian Ocean, using both Indian survey vessels and the Farnella research vessel chartered from John Marr and Son Ltd in the United Kingdom. Metalliferous muds. Metalliferous sediments, in which metals are concentrated, are common occurrences along all actively spreading ocean ridges (regions of crustal separation where new seafloor is created from upwelling basaltic magma) and volcanic island arcs. The vast majority of these accumulations are too low grade to be considered worth mining. However, anomalous local conditions can occasionally give rise to more concentrated deposits. Such a phenomenon has been shown to exist in the Red Sea, where 18 brine pools and associated mud
Figure 3 Box cores of Red Sea metalliferous muds (source: Red Sea Commission).
Figure 4 Sedco 445. Drillship in the Red Sea (source: Red Sea Commission).
Figure 5 Black smoker on the East Pacific Rise (source: IFREMER).
47
deposits have been discovered since the 1960s; the largest of these, the Atlantis II Deep, has a surface area of 60 square kilometres. The Deep occurs at a depth of 2200 metres and acts as a trap for metalliferous sediments, produced by undersea volcanic activity and dense hot brines. The large deposits of multi-coloured muds, the consistency of soft toothpaste, are rich in various metals, especially copper, zinc, and silver (figure 3). In 1976, the Saudi Arabian and Sudanese governments began detailed investigations of these deposits aimed at assessing their commercial viability. They engaged the German company Preussag as mining contractor, and the French Bureau de Recherche Geologique et Mini&e (BRGM) as geological consultants. The pre-pilot mining operation was concluded in 1981, after five years of intensive research. The richness of the mineralisation was shown to vary from one area to another but analysis of the data showed that the Atlantis II deposits could produce 920 000 tons of zinc, 182 000 tons of copper, 1635 tons of silver, and 13 tons of gold. Assuming an overall mining and processing recovery of around 50 per cent, the proven reserves have been shown to be able to support the production of 60 000 tons per year of zinc, and associated metals in proportion, for 16 years [12]. An original mining method had to be devised to mine these sediments, which are partly consolidated and contain hard anhydrite formations at various depths and with varying lateral extension. Mining tests were conducted by the SEDCO 445 mining vessel (figure 4) with a specially adapted cuttersuction head, a large diameter pipe, and two submerged pump stations equipped with centrifugal pumps. A vertically vibrating screen disaggregated the consolidated muds and the mining action and pumping were assisted by high pressure seawater injection and dilution. Another priority research goal was to investigate the feasibility of concentrating the freshly mined sediments at sea. On land, such a process which extracts a mineral-rich concentrate from the slurry, and which is called ‘froth flotation’, is common. However, to carry out such a processing at a minesite 90 kilometres from shore it no simple task. Pre-pilot mining tests showed that the ultra-fine mud particles were more floatable than expected, and that the motion of the vessel did not seriously affect flotation plant performance. Enrichment ratios of 2:3 for zinc and recoveries of up to 70 per cent were achieved. Separation into single mineral concentrates is carried out onshore using a hydrometallurgical process based on pressure 48
chloride leaching, followed by solvent extraction and electrowinning. After more than eight years of exploration and research, the Atlantic II Deep has been proved to contain an orebody of commercial significance and the project has now moved into its pilot operation phase, to last five years; this, if successful, will be followed by commercial production.
Northern California and Oregon; these lie within the 200 mile exclusive economic zone of the United States and as such fall within American jurisdiction. Other countries such as Japan, France, Canada, and West Germany are also actively involved in the current systematic exploration of mid-ocean ridges especially in the Pacific.
Colbalt-rich manganese crusts. These An ac- are believed to form in similar way to tive metallogenic ocean floor hyd- manganese nodules, but at a much rothermal system was first discovered shallower depth (SO&l000 metres). in 1979 on the East Pacific Rise, off the They are widespread in the marine coast of Mexico. Since then, resear- environment wherever a suitable subschers have found similar phenomena at trate exists and there is little sediment around 40 other sites, predominantly in accumulation. Crusts have been found the Pacific and always in association on the flanks of various islands and in the with submarine volcanic activity. The seamounts, predominantly majority of discoveries have been along Pacific. The Hawaiian Islands deposits, the central axis of spreading centres, where crusts have been collected from where new oceanic crust is created ocean depths as shallow as 3 metres, from upwelling basaltic magma, but are the ones which have attracted most interest. Unlike nodules, which are deposits have also been found on intraloose deposits, crusts form as encrustaplate seamounts. The combined processes of volcan- tions on the basaltic substrate between ism, tectonic fracturing, and hydrotherand 1 and 7 cm thick. Two factors might explain the recent mal circulation together form massive deposits of sulphides at more than 2000 interest that these deposits have metres depth. Seawater percolates attracted in industrialised countries. down through fractures, becomes First, the crusts are very rich in cobalt, super-heated and leaches metals from often containing in excess of 1 per cent. the surrounding bedrock. When the By contrast, manganese nodules conextremely volatile, metal-rich solution, tain only about 0.25 per cent cobalt, ejected at temperatures of up to 35o”C, similar to many land-based deposits. mixes with cold alkaline seawater at 1 Second, cobalt is a highly strategic to 2°C it rapidly precipitates out the metal with important civilian and militmetals it contains, in the form of sul- ary applications. The existence of subphides. This precipitation is so rapid stantial deposits of crusts within the that it produces an effect known as jurisdiction of countries such as the ‘smoking’ with dense clouds of particu- United States is likely to be perceived late sulphides forming chimneys up to by their government and private inves20m high at the mouth of the hyd- tors as having a strategic advantage rothermal vents (figure 5). The chim- over the present African sources of neys themselves eventually collapse, cobalt (Zaire and Zambia). forming mounds of sulphides which The technology necessary to recover may be large enough to produce an cobalt-crust will be very similar to that economic deposit. Samples from va- proposed for mining manganese rious sites have contained up to 55 per nodules. In particular the Continuous cent zinc, 6.5 per cent copper, econo- Line Bucket (CLB) System which was mic concentrations of silver, and smal- tested in the 1960s by several companies, and abandoned in the case of ler amounts of other metals [13]. Though in many respects their gene- manganese nodules, could be used for sis is similar to the Red Sea metallifer- crust mining. The CLB system consists ous mud deposits, marine polymetallic of a continuous loop of polypropylene sulphides extend below the ocean floor or polyamide rope with a series of and would need to be fragmented be- attached dredge buckets. The viability fore extraction. Alternatively, it may of such a system is highly dependent on be possible to cap the hydrothermal the degree to which crusts are bonded vents and pump the superheated brine to the rock substrate. Some are quite emerging from the ‘smokers’ directly to weakly bonded, others appear to readithe surface; each htre of brine contains ly part at a thickness of 2 to 4 cm. A 40 between 0.6 and 6.5 grams of zinc. per cent recovery rate has been estiHowever, it is not certain that the mated to be possible for a commercial brine is concentrated enough to tap operation off Hawaii [14]. from a small area. Most of the interest in cobalt crust Despite the difficulties, some de- mining has been in the United States posits are attracting interest from com- but several Pacific nations could dispanies. This is particularly the case for cover such deposits within their jurisdeposits of sulphides found 100 km off diction. In Japan, the Prime Minister Marine polymetallic
sulphides.
has recently established a committee on cobalt rich crusts, chaired by Dr Okamura, to report on the potential of these deposits for the Japanese economy. Mankind is not well adapted to carrying out activities in the marine environment. Thus, despite the predictions of some observers, the future will not necessarily see an inexorable move to the oceans to exploit all their resources. Particular mining activities will be considered only when they show distinct advantages over their landbased alternatives. The development of new technologies, originating both in the marine industry and elsewhere, has led to a stage where marine mining, even in the deep seabed, is now possible. Although further prospecting will increase the terrestrial reserves, their identification with lower grade ores and high mining costs may make marine minerals more competitive. Political and strategic considerations may also play their part, as industrialised nations
are striving to reduce their dependence on the traditional Third World suppliers of raw materials.
References [ 1) Mero. .I. L. Marine Mining 1 243, 1978. (2) Glasby, G. P. Marine Mining, 3, 383, 1982. [3] Huton, A. H. ‘An Appraisal of the Marine Sand and Gravel Industry of the UK’. Thesis, University of Manchester. 1983. (41 Kulvanich. S. In ‘Offshore Mineral Resources’. 2nd International GERMINAL Seminar, Brest, Proceedings pp. 247-257, 1984. [5] Cameron, H. M. In ‘Indigeneous Raw Materials for Industry’, Royal Society, London, 57-61, 1983. [6] Phosphorite Nodules:- Fertilisers from the Sea, PREST. Marine Technology Brief no. 5, 1984. [7] Organisation for Economic Cooperation and Development, International Atomic Energy Agency. Uranium resources. production and demand, 1982.
[8] Best, F. R. and Driscoll, M. J. Prospects for the Recovery of Uranium from Seawater, MIT Energy Laboratory and Department of Nuclear Engineering, 1980. [9] Gibbons. M. et al. Manganese Nodule Mining: Issues and Perspectives, Prest. Marine Resources Project, 1980. [lo] Technocrat, 18. no. 1, p. 91. 1985. [ll] Marchal. P. In ‘Offshore Mineral Resources’. 2nd International GERMINAL Seminar, Brest Proceedings, pp. 509540, 1984. [12] Mustafa, Z. et al In ‘Offshore Mineral Resources’, 2nd International GERMINAL Seminar, Brest Proceedings pp. 509-540, 1984 131 Marine Technoloav Societv Journal Special Issue:-’ Polyme;allic sulphides, vol. 16, no. 3, 1982. 141 Clark, A. et al. Assessment of cobaltrich manganese crusts in the Hawaiian. Johnston and Palmyra Islands’ Exclusive Economic Zones. Mimeo 1983, East-West Resource Systems Institute, Honolulu, Hawaii.
49
Successful exploitation of the minerals from the sea does not depend only on technological advances. In many cases the development of these resources will be highly dependent on economic, legal, and institutional factors. This article reviews the major hard mineral resources identified to date, within the marine environment; the methods which have been researched and developed for their exploration and exploitation; and their commercial prospects (Table 1). Offshore oil and gas deposits are not considered except as an example of how land-based operations are likely to progress seawards, as the most accessible deposits become depleted. From a geological and legal point of view, the oceans are divided into two distinct domains: the continental shelves and the abyssal plains. Continental shelves are an extension of the continents beneath the sea, accounting for less than 10 per cent of ocean floor John Yates, M.Sc. Is a graduate in geology of Manchester University. His main research interests are in marine minerals, marine technology, and technology transfer. Daniel Spagni, Maitrise de Droit in Int. Law and EEC Law, Diploma ISTEC Graduated in law at the University of Aix-enProvence and subsequently did research in international law in France and the United Kingdom. His main research interests are in R&D management, marine policy, and marine resources management. Jim Keane, MSc. Is a graduate in economics of Loughborough University of Technology. His current research interests are in waste disposal at sea, marine exploitation, and oceanographic instrumentation. All three authors are attached to the Marine Resources Project of the Programme of Policy Research in Engineering, Science and Technology (PREST), University of Manchester.
EURO ARTICLE (see p.ii) Endeavour, New Series, Volume 10, No.1, OWO-9327/96 90.00 + .50 Pergamon Press. Printed in Great Britain
1986
and rarely extending beyond the 200metre isobath. In international law, the continental shelf is defined as ‘the natural prolongation of the land territory’ and consequently falls within the sovereignty of the coastal state in a similar fashion to the land territory. The abyssal plains have an average depth of 4000 me&es, the deepest part being the Challenger Deep north of New Guinea at 10 915 metres. The seafloor of the deep ocean is quite distinct from the shelves, being composed predominantly of theoleiitic basalt, covered by a layer of pelagic sediments of varying thickness. International law defines the deep seabed as an area beyond national jurisdiction but is unclear as to whether or not it is susceptible of national appropriation. Deposits
of the continental
shelf
Aggregates. These are deposits of sands, gravels, or shells. They are the product of hydrodynamic forces and occur both on land and on the continental shelves. Sand and gravel were among the first minerals to be recovered from the sea and the oldest ocean mining operations involved the recovery of sand and gravel from the North Sea and English Channel over 100 years ago [l]. Offshore aggregate mining has expanded dramatically in the last few years because of the progressive depletion of land deposits and stricter environmental controls. Aggregates are used primarily by the construction industry; they are a very low value commodity, and transport costs play a vital part in their exploitation. It is far cheaper to transport large volumes by sea than land, and so to some extent the increased cost of marine mining is offset by reduction in transportation overheads. To be economically viable, a typical marine aggregate deposit needs to be close to a centre of high demand, as the cost of aggregate can double on a journey of 50 km [2]. The prime markets are large coastal metropolitan areas. The deposits themselves need to be
homogeneous with little overburden so as to minimise processing. The main extraction technique used is dredging. In the past, dredging was carried out by grab and bucket dredgers, but now most dredging is done by sophisticated suction hopper vessels, often equipped with cutters. Owing to the limitation on the depth to which centrifugal pumps can be operated, few dredgers are capable of working beyond 30 metres water depth. However, with the development of new types of pumps such as those used for mining deep seabed minerals, aggregates will ultimately be exploited in deeper waters. The maximum depth achieved using jet assisted suction is 45 metres by anchor dredging [3]. Whether it will ever be economically viable to dredge aggregates beyond the continental shelf is more doubtful, in view of the high transport costs which would be involved. Placer deposits. Placers occur as a result of chemical and physical weathering forming concentrations of minerals which are transported by running water up to 15 km, with the larger, denser grains being found nearer to their source. They can be found in rivers, lakes, and at sea and contain a variety of valuable metals and gems which are the source of much industrial activity at present. Thailand has dredged for tin in offshore areas since 1907 [4]. During its lifetime, the Alaskan operation at Nome sands produced over $100 million of gold and the bauxite mine at the Gulf of Carpentaria in Australia has been one of the largest of its kind in the world since operations began in 1955. Japanese firms have mined for magnetite in iron sands in Tokyo Bay and the Philippines since the late 1960s [5]. Placer mining operations thus already have an historical perspective and their importance will increase over the next few years as technologies for extraction develop and land-based sources dwindle. Extraction usually involves the use of dredgers, either bucket-line or, in more
TABLE 1
CONTEMPORARY
STATUS
Mining
MINING
technology
Participant
countries
Type of deposit
Economically minerals
significant
Commercial
Aggregates
Sand, gravel, carbonate
calcium
Commercial exploitation
Dredging/suction
UK, US, Japan, Denmark, France, Netherlands etc
Placers
Chromite, platinum, etc.
Commercial exploitation
Dredging/suction
US, UK, South Africa, Indonesia, India, Australia, Thailand etc
Phosphorites
Phosphate
Resource assessment Commercial exploitation
Dredging/Suction
US, New Zealand, South Africa, Chile, Australia
Manganese nodules
Manganese, nickel, copper, cobalt
Resource assessment Pre-pilot phase
Bottom collector and hydraulic lift chantier Sous-Marin
Japan, France, US, UK, West Germany, India, Korea, Canada
Metalliferous muds
Silver, gold
Pilot phase
Vibration/suction
Saudi Arabia,
Marine polymetallic sulphides
Zinc, copper, silver, etc.
Resource assessment Basic research phase
Bottom collector and hydraulic lift, Vent capping
US, France, Japan, West Germany
Cobalt rich crusts
Manganese, cobalt, nickel, copper
Resource assessment Basic research phase
Continous system
US, Japan
Minerals seawater
Salt, magnesium, uranium, bromine, lithium, etc
Commercial exploitation, research
Solvents, soda, ion exchange resins
from
silver, gems, tin (P,O,)
zinc, copper cobalt,
recent times, suction dredgers. Future developments for deeper deposits will involve the use of drilling to locate and delineate reserves. Various types of operations are possible, and at present Thailand is planning to exploit tin deposits at depths between 40 and 50 metres, using drilling techniques supplied by an Indonesian company. Marine placer mineral deposits occur and are widely exploited throughout the world especially in Alaska, Florida, South Carolina, South Africa, South East Asia, Sri Lanka, and the United Kingdom. Their promixity to shore and relative ease of extraction will ensure the growth of this sector of marine mining. Phosphorites. The principal mineral of marine phosphorite is carbonate fluorapatite or francolite. Found as pellets, nodules, or sand on the continental shelf, the mineral develops in situ at depths of up to 1OOOm where deep, nutrient-rich waters upwell to produce a wealth of biological activity. Since their discovery during the famous Challenger Expedition (1873-76) significant deposits have been found at many offshore locations including South West Africa, Florida in the United States, and Chatham Rise off New Zealand [6]. At present, approximately 95 per cent of the phosphate rock mined on land is processed for fertiliser, and marine deposits will be used for a similar purpose. The principle commer-
status
OF MARINE
basic
cial advantage that certain marine deposits have over land-based deposits is their proximity to existing markets. Phosphorite is a low cost per unit commodity, making the transport element significant in the overall cost. Certain countries have both a high import bill for phosphate and proven offshore deposits. In New Zealand, detailed exploration has been undertaken of the Chatham Rise deposits, located in 400 metres of water. Potentially rich reserves (2&30 per cent P20s) have been discovered (100 million tonnes) and two German companies, Preussag and Salzgitter, appear ready to mine these deposits once environmental concerns are alleviated and a New Zealand company can be found who will partner the project. Phosphorite sands are currently being dredged at San Domingo, off the coast of Baja, Southern California, by the Rofomex company of Mexico. The latter started operations in 1982, following a study which estimate 1.1 billion tonnes of phosphorite to be present at the site, with a phosphate content of 4 to 5 per cent. Mining is undertaken using a dredge (figure 1) and the sand is pre-processed on a barge. Production began at 1.5 million tonnes per year and is to be increased to 4.5 million tonnes. The capital cost of the project is estimated at $US 100 million and should provide 15 per cent of Mexico’s phosphate needs. Australia, South Africa, Peru, Chile, the United States, and India have all undertaken a series
line bucket
Japan,
Sudan
US, UK
of preliminary studies of their marine phosphorite deposits. Minerals from seawater. The total volume of water in the ocean is approximately 1350 x lo6 km’ or - put another way - 318 million cubic miles. Within this vast body of water minerals of all types can be found in varying concentrations. At present, sodium chloride (salt), bromide, and magnesium are commercially extracted from seawater. Of the more dilute minerals in seawater, uranium has commanded the greatest interest, due primarily to its strategic value. Uranium is found in seawater as uranyl tricarbonate at a constant concentration of 0.003 mg per litre down to 400 metres depth. The total amount of uranium in seawater has been estimated at 4 X 10’ tonnes, while terrestrial resources appear to be 2.59 x lo6 tonnes [7]. Early attempts at extracting uranium from seawater concentrated on the use of solvents, which eventually proved too complex and costly a process. More recently, studies have been made on precipitating uranium through the addition of caustic soda (NaOH). This approach has proved very effective, with 90 per cent resource recovery rates achieved. However, problems of restoring the pH level of the seawater after extraction have added considerably to the cost and may prove prohibitive to future applications. Of promise for the future in this domain is the use of synthetic 45
TABLE 2 TYPICAL METAL CONTENT OF NODULES IN A POTENTIAL ‘FIRST GENERATION’ MINESITE AREA
Metal
%
Manganese Nickel Copper Cobalt Lead Molybdenum Vanadium Zinc
Figure 1
Rofomex
1. Phosphorite
mining
resins to achieve extraction through an ion exchange process. One significant breakthrough was recently achieved by a research team at the University of Salford, who developed a synthetic resin PHA (polyhydroxamic acid). A German company Uranersbergbau compared PHA granules with titanium oxide granules developed in Japan. The experiment took place in the Baltic, and PHA proved itself to have a stronger resistance to attrition and a better physical and chemical stability in the marine environment. Being cheaper to produce than titanium oxide, PHA’s only major drawback appears to be its ability to absorb other seawater minerals alongside uranium, making recycling and reapplication of the material difficult. The Salford team are currently investigating the possibility of weaving Orlon fibres with PHA fibres to produce a composite carpet which would be suspended in a continuous flow of seawater. Commercial prospects for the extraction of the more dilute minerals in seawater are very much dependent on technological advances. Because of its strategic importance uranium has been the catalyst of the research in this field. However, uranium extraction on a commercial basis seems distant. Even assuming 100 per cent resource recovery, an industrial operation would require the pumping of 2.2 million litres of seawater and the use of 12 000 cubic metres of PHA resin. The requirement for a continuous flow of such a large volume of water at a constant rate will also prove very costly. An MIT cost study, which included pumping costs, concluded that a uranium oxide seawater extraction operation would cost a 46
barge (source:
Rofomex).
minimum of $US 697 for each kilogram produced, which is many times that of present land-based mining operations [8]. One way in which mineral extraction from seawater might prove more commercially viable, is through offsetting some of the costs by coupling an extraction operation with other marine activities. The obvious operations to which extraction could be adjoined are desalination, tidal, or ocean thermal energy conversion (OTEC) plants which Viould provide the constant flow of seawater required in the extraction
Figure 2 One-tenth Mineral Co.).
scale prototype
30.00 1.3 1.1 0.2 0.09 0.05 0.05 0.04
process. To date, the largest desalination plant in the world has a daily capacity of only 800 000 m3 of water, which would provide a meagre 2.5 tonnes of uranium a year. Only the Japanese are actively engaged in a programme aimed at commercially extracting uranium from seawater. The Ministry of International Trade and Industry (MITI) has established an experimental plant at Shikoku which they hope will eventually produce 100 tonnes of uranium oxide per year. No doubt, if they are successful this will encourage other countries to review their R & D policies in this field and probably in the whole area of mineral extraction from seawater. Deposits
of the deep ocean
Manganese nodules. Manganese, or polymetallic, nodule deposits are to be found in the deepest parts of all the
manganese
nodule
miner (source:
Ocean
world’s oceans. Discovered over a century ago, they have attracted considerable commercial interest since the early 1960s as a potential source of metals, primarily nickel, copper, cobalt, and manganese (Table 2). Several international consortia of companies, as well as national projects have been conducting research and development programmes for more than ten years, aimed at designing recovery, transport, and processing systems which would allow economic production of some, or all, of the metals contained in nodules. Most of the surveys conducted to date have concentrated on the Pacific Ocean in the so-called Clarion-Clipperton Zone, midway between the coast of North America and the Hawaiian chain. In addition, the French have undertaken some work rather closer to Tahiti in an area showing titanium-rich nodules, while the Japanese have also had a wide sweep toward the Japanese ‘home’ islands. The sites claimed by the various consortia under the national deep sea mining legislations have all fallen in the Clarion-Clipperton Zone, where nodules have a combined nickel/ cobalt content of well above 2 per cent [91. Many uncertainties still remain as to nodule distribution elsewhere in the world. Nevertheless, the Indian government has, in the past few years, conducted a vigorous programme of oceanographic cruises in the Indian Ocean, where they have claimed major discoveries of rich nodule deposits. For a combination of technical and economic reasons, all the nodulemining systems which have been designed, patented, and proved at prototype stage, aim at recovering 3 million dry tonnes of nodules per year, from depths greater than 4.5 km. The mining operation itself entails collecting the nodules with a steerable bottom collector unit, and lifting them to a mineship through a pipestring by a hydraulic lift system, using either an airlift or conventional slurry pumps (figure 2). In France, the Association Francaise pour la Recherche et l’Exploitation des Nodules (AFERNOD) a grouping of private and public organisations - is developing an original system which uses underwater robots to collect the nodules on the seabed. The novelty of many of the technologies involves, the absence of an internationally agreed legal framework to govern mineral rights in the deep ocean, together with low metal prices caused by the world recession, have led to delays in planning commercial ventures and in developing the deep sea mining nodule technology. Japan, France, and India are the only remaining countries having a very active nodule programme. The
Japanese Large Scale Government Project started at the end of 1980 with a budget of 20 billion yen ($200 million) over nine years. The programme is carried out by an organisation comprising MITI, Metal Mining Agency (MMA) and 19 private companies mainly from the engineering and shipbuilding sectors. The National Research Institute for Pollution and Resources is currently operating a test facility which includes a 200 metre deep vertical water tank used for lifting natural and artificial nodules [lo]. The French nodule programme started in the early 1970s is spearheaded by AFERNOD. Nodule activity in France was reorganised in 1983 when a new organisation GEMONOD was created alongside AFERNOD to develop the technology necessary for the collection and processing of nodules. Having studied and tested all the existing collector devices, the AFERNOD group decided in the late 1970s to develop a radically new mining concept, the ‘Chantiere sous-marin’ (CSM). This required the development of a self-propelled underwater vehicle or shuttle capable of collecting several hundred tons of nodules at 6000 metres depth and lifting them to a surface mining platform [ll]. GEMONOD had already developed and tested several experimental devices and has recently concluded a collaboration agreement with the German Company Pressuag in order to build a full scale prototype. The Indian nodule exploration programme began in 1980 and was coordinated initially through the National Institute of Oceanography (NIO) in Goa. In 1982, the Department of Ocean Development was created and the All India Coordinated Project was set up under its auspices. The programme envisages an Indian nodule mining capability by the end of this decade with the investment of $100 m. Most of the funds have to date been directed at the exploration of the Indian Ocean, using both Indian survey vessels and the Farnella research vessel chartered from John Marr and Son Ltd in the United Kingdom. Metalliferous muds. Metalliferous sediments, in which metals are concentrated, are common occurrences along all actively spreading ocean ridges (regions of crustal separation where new seafloor is created from upwelling basaltic magma) and volcanic island arcs. The vast majority of these accumulations are too low grade to be considered worth mining. However, anomalous local conditions can occasionally give rise to more concentrated deposits. Such a phenomenon has been shown to exist in the Red Sea, where 18 brine pools and associated mud
Figure 3 Box cores of Red Sea metalliferous muds (source: Red Sea Commission).
Figure 4 Sedco 445. Drillship in the Red Sea (source: Red Sea Commission).
Figure 5 Black smoker on the East Pacific Rise (source: IFREMER).
47
deposits have been discovered since the 1960s; the largest of these, the Atlantis II Deep, has a surface area of 60 square kilometres. The Deep occurs at a depth of 2200 metres and acts as a trap for metalliferous sediments, produced by undersea volcanic activity and dense hot brines. The large deposits of multi-coloured muds, the consistency of soft toothpaste, are rich in various metals, especially copper, zinc, and silver (figure 3). In 1976, the Saudi Arabian and Sudanese governments began detailed investigations of these deposits aimed at assessing their commercial viability. They engaged the German company Preussag as mining contractor, and the French Bureau de Recherche Geologique et Mini&e (BRGM) as geological consultants. The pre-pilot mining operation was concluded in 1981, after five years of intensive research. The richness of the mineralisation was shown to vary from one area to another but analysis of the data showed that the Atlantis II deposits could produce 920 000 tons of zinc, 182 000 tons of copper, 1635 tons of silver, and 13 tons of gold. Assuming an overall mining and processing recovery of around 50 per cent, the proven reserves have been shown to be able to support the production of 60 000 tons per year of zinc, and associated metals in proportion, for 16 years [12]. An original mining method had to be devised to mine these sediments, which are partly consolidated and contain hard anhydrite formations at various depths and with varying lateral extension. Mining tests were conducted by the SEDCO 445 mining vessel (figure 4) with a specially adapted cuttersuction head, a large diameter pipe, and two submerged pump stations equipped with centrifugal pumps. A vertically vibrating screen disaggregated the consolidated muds and the mining action and pumping were assisted by high pressure seawater injection and dilution. Another priority research goal was to investigate the feasibility of concentrating the freshly mined sediments at sea. On land, such a process which extracts a mineral-rich concentrate from the slurry, and which is called ‘froth flotation’, is common. However, to carry out such a processing at a minesite 90 kilometres from shore it no simple task. Pre-pilot mining tests showed that the ultra-fine mud particles were more floatable than expected, and that the motion of the vessel did not seriously affect flotation plant performance. Enrichment ratios of 2:3 for zinc and recoveries of up to 70 per cent were achieved. Separation into single mineral concentrates is carried out onshore using a hydrometallurgical process based on pressure 48
chloride leaching, followed by solvent extraction and electrowinning. After more than eight years of exploration and research, the Atlantic II Deep has been proved to contain an orebody of commercial significance and the project has now moved into its pilot operation phase, to last five years; this, if successful, will be followed by commercial production.
Northern California and Oregon; these lie within the 200 mile exclusive economic zone of the United States and as such fall within American jurisdiction. Other countries such as Japan, France, Canada, and West Germany are also actively involved in the current systematic exploration of mid-ocean ridges especially in the Pacific.
Colbalt-rich manganese crusts. These An ac- are believed to form in similar way to tive metallogenic ocean floor hyd- manganese nodules, but at a much rothermal system was first discovered shallower depth (SO&l000 metres). in 1979 on the East Pacific Rise, off the They are widespread in the marine coast of Mexico. Since then, resear- environment wherever a suitable subschers have found similar phenomena at trate exists and there is little sediment around 40 other sites, predominantly in accumulation. Crusts have been found the Pacific and always in association on the flanks of various islands and in the with submarine volcanic activity. The seamounts, predominantly majority of discoveries have been along Pacific. The Hawaiian Islands deposits, the central axis of spreading centres, where crusts have been collected from where new oceanic crust is created ocean depths as shallow as 3 metres, from upwelling basaltic magma, but are the ones which have attracted most interest. Unlike nodules, which are deposits have also been found on intraloose deposits, crusts form as encrustaplate seamounts. The combined processes of volcan- tions on the basaltic substrate between ism, tectonic fracturing, and hydrotherand 1 and 7 cm thick. Two factors might explain the recent mal circulation together form massive deposits of sulphides at more than 2000 interest that these deposits have metres depth. Seawater percolates attracted in industrialised countries. down through fractures, becomes First, the crusts are very rich in cobalt, super-heated and leaches metals from often containing in excess of 1 per cent. the surrounding bedrock. When the By contrast, manganese nodules conextremely volatile, metal-rich solution, tain only about 0.25 per cent cobalt, ejected at temperatures of up to 35o”C, similar to many land-based deposits. mixes with cold alkaline seawater at 1 Second, cobalt is a highly strategic to 2°C it rapidly precipitates out the metal with important civilian and militmetals it contains, in the form of sul- ary applications. The existence of subphides. This precipitation is so rapid stantial deposits of crusts within the that it produces an effect known as jurisdiction of countries such as the ‘smoking’ with dense clouds of particu- United States is likely to be perceived late sulphides forming chimneys up to by their government and private inves20m high at the mouth of the hyd- tors as having a strategic advantage rothermal vents (figure 5). The chim- over the present African sources of neys themselves eventually collapse, cobalt (Zaire and Zambia). forming mounds of sulphides which The technology necessary to recover may be large enough to produce an cobalt-crust will be very similar to that economic deposit. Samples from va- proposed for mining manganese rious sites have contained up to 55 per nodules. In particular the Continuous cent zinc, 6.5 per cent copper, econo- Line Bucket (CLB) System which was mic concentrations of silver, and smal- tested in the 1960s by several companies, and abandoned in the case of ler amounts of other metals [13]. Though in many respects their gene- manganese nodules, could be used for sis is similar to the Red Sea metallifer- crust mining. The CLB system consists ous mud deposits, marine polymetallic of a continuous loop of polypropylene sulphides extend below the ocean floor or polyamide rope with a series of and would need to be fragmented be- attached dredge buckets. The viability fore extraction. Alternatively, it may of such a system is highly dependent on be possible to cap the hydrothermal the degree to which crusts are bonded vents and pump the superheated brine to the rock substrate. Some are quite emerging from the ‘smokers’ directly to weakly bonded, others appear to readithe surface; each htre of brine contains ly part at a thickness of 2 to 4 cm. A 40 between 0.6 and 6.5 grams of zinc. per cent recovery rate has been estiHowever, it is not certain that the mated to be possible for a commercial brine is concentrated enough to tap operation off Hawaii [14]. from a small area. Most of the interest in cobalt crust Despite the difficulties, some de- mining has been in the United States posits are attracting interest from com- but several Pacific nations could dispanies. This is particularly the case for cover such deposits within their jurisdeposits of sulphides found 100 km off diction. In Japan, the Prime Minister Marine polymetallic
sulphides.
has recently established a committee on cobalt rich crusts, chaired by Dr Okamura, to report on the potential of these deposits for the Japanese economy. Mankind is not well adapted to carrying out activities in the marine environment. Thus, despite the predictions of some observers, the future will not necessarily see an inexorable move to the oceans to exploit all their resources. Particular mining activities will be considered only when they show distinct advantages over their landbased alternatives. The development of new technologies, originating both in the marine industry and elsewhere, has led to a stage where marine mining, even in the deep seabed, is now possible. Although further prospecting will increase the terrestrial reserves, their identification with lower grade ores and high mining costs may make marine minerals more competitive. Political and strategic considerations may also play their part, as industrialised nations
are striving to reduce their dependence on the traditional Third World suppliers of raw materials.
References [ 1) Mero. .I. L. Marine Mining 1 243, 1978. (2) Glasby, G. P. Marine Mining, 3, 383, 1982. [3] Huton, A. H. ‘An Appraisal of the Marine Sand and Gravel Industry of the UK’. Thesis, University of Manchester. 1983. (41 Kulvanich. S. In ‘Offshore Mineral Resources’. 2nd International GERMINAL Seminar, Brest, Proceedings pp. 247-257, 1984. [5] Cameron, H. M. In ‘Indigeneous Raw Materials for Industry’, Royal Society, London, 57-61, 1983. [6] Phosphorite Nodules:- Fertilisers from the Sea, PREST. Marine Technology Brief no. 5, 1984. [7] Organisation for Economic Cooperation and Development, International Atomic Energy Agency. Uranium resources. production and demand, 1982.
[8] Best, F. R. and Driscoll, M. J. Prospects for the Recovery of Uranium from Seawater, MIT Energy Laboratory and Department of Nuclear Engineering, 1980. [9] Gibbons. M. et al. Manganese Nodule Mining: Issues and Perspectives, Prest. Marine Resources Project, 1980. [lo] Technocrat, 18. no. 1, p. 91. 1985. [ll] Marchal. P. In ‘Offshore Mineral Resources’. 2nd International GERMINAL Seminar, Brest Proceedings, pp. 509540, 1984. [12] Mustafa, Z. et al In ‘Offshore Mineral Resources’, 2nd International GERMINAL Seminar, Brest Proceedings pp. 509-540, 1984 131 Marine Technoloav Societv Journal Special Issue:-’ Polyme;allic sulphides, vol. 16, no. 3, 1982. 141 Clark, A. et al. Assessment of cobaltrich manganese crusts in the Hawaiian. Johnston and Palmyra Islands’ Exclusive Economic Zones. Mimeo 1983, East-West Resource Systems Institute, Honolulu, Hawaii.
49