<<Polymetallic nodules, also called manganese nodules, are rock concretions on the sea bottom formed of concentric layers of iron and manganese hydroxides around a core. Nodule growth is one of the slowest of all known geological phenomena
, on the order of a centimeter over several million years. As nodules can be found in vast quantities, and contain valuable metals, deposits have been identified as having economic interest. Polymetallic nodules are thought to have been a feature of the seas and oceans at least since the deep oceans oxidised in the Ediacaran period over 540 million years ago.
Most nodules are between 3 and 10 cm (1 and 4 in) in diameter, about the size of hen's eggs or potatoes. Their surface textures vary from smooth to rough. They frequently have botryoidal (mammilated or knobby) texture and vary from spherical in shape to typically oblate (flying saucer), sometimes prolate (American football), or are otherwise irregular. The bottom surface, buried in sediment, is generally rougher than the top due to a different type of growth.
Nodules lie on the seabed sediment, often partly or completely buried. They vary greatly in abundance, in some cases touching one another and covering more than 70% of the sea floor. The total amount of polymetallic nodules on the sea floor was estimated at 500 billion tons by Alan A. Archer of the London Geological Museum in 1981.
Polymetallic nodules were discovered in 1868 in the Kara Sea, in the Arctic Ocean of Siberia. During the scientific expeditions of HMS Challenger (1872–1876), they were found to occur in most oceans of the world. The largest deposits in terms of nodule abundance and metal concentration occur in the Clarion Clipperton Zone on vast abyssal plains in the deep ocean between 4,000 and 6,000 m. The International Seabed Authority estimates that the total amount of nodules in the Clarion Clipperton Zone exceeds 21 billions of tons (Bt), containing about 5.95 Bt of manganese, 0.27 Bt of nickel, 0.23 Bt of copper and 0.05 Bt of cobalt.
Those of greatest economic interest contain manganese (27–30%), nickel (1.25–1.5 %), copper (1–1.4 %) and cobalt (0.2–0.25 %). Other constituents include iron (6%), silicon (5%) and aluminium (3%), with lesser amounts of calcium, sodium, magnesium, potassium, titanium and barium, along with hydrogen and oxygen as well as water of crystallization and free water.
In the late seventies, two of the international joint ventures succeeded in collecting several hundred-ton quantities of manganese nodules from the abyssal plains of the eastern equatorial Pacific Ocean. Significant quantities of nickel (the primary target) as well as copper and cobalt were subsequently extracted from this "ore" using both pyrometallurgical and hydrometallurgical methods. In the course of these projects, a number of ancillary developments evolved, including the use of near-bottom towed side-scan sonar array to assay the nodule population density on the abyssal silt whilst simultaneously performing a sub-bottom profile with a derived, vertically oriented, low-frequency acoustic beam.
The technology and experience developed during the course of this project were never commercialized because the last two decades of the 20th century saw an excess of nickel production. Kennecott Copper had explored the potential profits in manganese nodule mining and found that it was not worth the cost. On top of the environmental issues and the fact that the profits had to be shared, there was no cheap way to get the manganese nodules off the sea floor.
In recent times, nickel and other metal supply has needed to turn to higher cost deposits in order to meet increased demand, and commercial interest in nodules has revived. The renewed interest in mining nodules has led to increased concern and scrutiny regarding possible environmental impacts. >>