II.- THE POLYMETALLIC NODULES
II.1.- History of the discovery of polymetallic nodules
The first iron-manganese submarine crusts were discovered in 1868 in the Kara Sea (ex-URSS).
From 1873 to 1876, during a trip around the world aboard the Oceanographic frigate "Challenger", many small blackish-brown balls, rich in Mn and Fe were collected. They were named "manganese nodules".
Around 1900, the American, Agassiz excavated nodules in almost all samples taken from the Eastern Pacific Ocean.
Nodules were also returned in dredges and corers from successive world ocean scientific.
It was not until 1957 that the American, John Méro managed to convince a few industrials of the economic interest of nodules and to lead then into exploring the central Pacific Ocean.
In the early sixties, manganese nodules were beginning to be regarded as a possible source of nickel, copper, cobalt and manganese.
The first companies are interested Kennecott and Newport Shipbuilding Company (1962) who start campaigns for sampling, while american academics (Fuerstenau, Arrhenius) worked on the geochemistry of nodules and their metallurgical processing.
Starting from 1965, new companies have begun to turn research in the fields of exploration, exploitation and processing of nodules, among these companies AFERNOD.
In 1972, the National Science Foundation launched a research program involving a dozen laboratories to study the origin of nodules.
In the Soviet Union, Bezrukov devotes a voluminous book to the nodules in the Pacific, but the industrial exploration began in 1982.
II.2.- Distribution of nodules in the oceans
The undertaken scientific and industrial provide a comprehensive picture of the distribution of nodules and to locate areas encompassed potential ore deposits.
Fe-Mn concretions were found everywhere on the seabed. The map published in 1969 by Mc Kelvey already revealed the presence of nodules in all oceans, it is even found in some lakes (USA, Canada, etc. ...). But not all are of interest because of the abundance of small nodules on the merits and / or low metal content (Mn, Ni, Cu, Co).
In 1973, Horn has shown the predominance of "East-West belt of the southern Pacific Northwest, between Clarion and Clipperton fractures, known thereafter as the" Horn area ". It is in this area that most mining groups have already worked.
Polymetallic nodules are in the form of small balls of brownish-black slightly flattened 5 to 10 centimeters in diameter lying in the deep seabed between 4,000 and 6,000 meters deep. Their wet density is close to 2 g/cm 3 ; moisture dry mass is 40% and porosity is 50%.
Size and shape vary widely (roughly spherical more or less ovoid), the nodules were classified as:
- mononodule: nodule simple spherical or ellipsoidal,
- polynodule: a single nodule on several nucleus,
- nodule composed of: multiple nodules welded.
In section, most nodules formed layers appear more or less concentric called "cortex" corresponding to the phases of growth around a core that can often be a microscopic debris from former nodule, a shark teeth or debris rock (basalt, limestone, etc. ...).
The layers are formed hydroxides of manganese and iron more or less crystallized. The better crystallized (todorokite, birnessite) are the richest in Mn, Ni and Cu, while the cryptocristalline structures (vernadite) are rich in Fe and Co.
Various studies have shown that the Ni and Cu are either adsorbed Mn oxides, or incorporated into their crystal lattice, replacing sites Mn ++ of the todorokite or birnessite.
The contents are a function of the crystallization. More nodules are crystallized, more content are high. However, the recovery of useful elements (Ni, Cu) seems easier when crystallization is incomplete.
The speed of growth of nodules is one of the slowest (around centimeter by a few million years). The age of nodules of the central Pacific Ocean is about 2 to 3 million years.
II.4.- Chemical composition
Chemical analysis showed that nodules consist of various chemical elements, including:
- manganese (29.40%),
- iron (6.00%),
- nickel (1.34%),
- copper (1.25%),
- cobalt (0.25%),
- titanium (0.60%),
- aluminum (2.90%),
- and sodium, magnesium, silica, zinc, hydrogen and oxygen (32.16%).
II.5.- Nodules origines
The problem of the genesis of nodules is far from resolved. Four origins were selected by Bonatti (1986):
- " hydrogenated" slow precipitation of metal from sea water, leading to concretions with equivalent levels in Fe and Mn content and a relatively strong Ni + Cu + Co,
- " hydrothermal" giving concretions generally rich in iron, low and Mn (Ni + Cu + Co),
- " diagenetic", giving remobilization of manganese in the sedimentary column and its precipitation to the water-sediment interface, nodules rich and poor in Mn and Fe (Ni + Cu + Co)
- " halmyrolitic", in which the source of metal components is the attack of the basaltic debris by seawater
These theories as "mineral" oppose so-called "biological "whose main supporters are Graham and Cooper (1959) who organism debris would be responsible for enrichment in Cu and Ni.
It is difficult to choose one or other of these theories. It appears that several processes are essential to the genesis of nodules:
- The low rate of sedimentation leaves ferromanganese concretions to grow before their burying can separate favorable conditions for their development,
- Metal hydroxides from living matter contain copper and nickel can be incorporated in manganese concretions,
- Manganese appears from the leaching of basalt and essentially hydrothermal phenomena observed along mid-ocean ridges,
- The concretion is favored by the biological activity of certain micro-organisms.
However, it is also very rapid formation of a deposit of ferro-manganese around shipwrecks of the First World War (Goldberg, 1958) or candles engine (Andrews, 1972).
This speed imposes some restrictions on the origin of the elements. Indeed, if the nodules are formed slowly, the origins "hydrogenated" or "diagenetic" may be taken into account. If on the contrary, they are formed quickly, it is necessary to find a source of metals other than sea water, in this case, the origins "hydrothermal" or be "halmyrolitic" are not removed.
Knowledge acquired so far on polymetallic nodules can classify them among the minerals of economic interest deserving special attention from mining companies.