Deep-sea nodules

Deep-Sea Manganese Nodules. A potentially important future source of manganese is the deep-sea nodules found over wide areas of... 

Table 7. Average Metal Content of Deep-Sea Nodules/ wt % Dry Basis ...

Cronan, D. S. "Deep Sea Nodules Distribution and Geochemistry", p. 11 Mero, J. L. "Economic Aspects of Nodule Mining," p. 327 in Marine Manganese Deposits, Glasby, G. P., ed. Elsevier, Amsterdam, 1977. 

Occurrence. In order of abundance in the earth s crustal rocks, it is the third within the transition elements (after Fe and Ti) and the 12th in the general order of all the elements. It occurs in several minerals such as primary deposits of silicates and as secondary deposits (commercially more important) of oxides and carbonates as pyrolusite, Mn02, hausmannite, Mn304, rhodochrosite, MnC03, etc. Large amounts of manganese are present in the deep sea nodules located over certain areas of the ocean floor. 

Its concentration in the earth s crust is estimated to he 0.095%. Its average concentration in seawater is 2p,g/L. Manganese also is found in large quantities in deep-sea nodules over the ocean floor at depths of 2.5 to 4 miles. The composition of some common manganese minerals is tabulated helow ... 

The nodules form in a layered structure around a nucleus, which may be almost any material on the ocean floor. Most deep-sea nodules tend to be spherical or oblate in form. Nodules may occur up to 25 centimeters in diameter, but they average about 5 centimeters. The deposits may occur as slabs or agglomerates, or as incrustations on rocks or as pavement in some areas. The nodules are disorderly ciystalline materials with layers of MnC>2 (mixed Mn -Mn44 oxides) alternating with Mn(OH>2 and Fe(OH)3. Excess iron appears as a mixture of goethite (Fe2C>3 H20) and lepidocrocite. 

Halbach P., Giovanoli R., and von Borstel D. (1982) Geochemical processes controlhng the relationship between Co, Mn, and Fe in early diagenetic deep-sea nodules. Earth Planet. Sci. Lett. 60, 226—236. 

Cobalt and Nickel Recovery. Cobalt and nickel are relatively valuable metals often found in complex ores such as laterites or deep sea nodules. The metals can only be extracted from these ores by hydrometallurgy. A proposed recovery scheme based on coupled transport is shown in Figure 9.29. The first membrane contains LIX 54, which produces a nickel and copper concentrate and a cobalt raffinate stream. The concentrate stream is then passed to a second Kelex 100 membrane, which produces a copper and nickel stream. The cobalt III raffinate stream is neutralized and reduced to cobalt II, which can then be concentrated by a LIX 51 membrane. 

Fortius purpose, we may therefore consider hydrogenous deep-sea nodules to be a baseline. When oxic diagenesis takes place, Mn, Ni and Cu are released into the sediment pore waters and are ultimately incorporated into the nodules. Ni + and Cu + substitute in the phyllomanganate lattice (see below). This explains the high Mn/Fe ratios and Ni+Cu contents of oxic nodules. When sub-oxic diagenesis takes place, Mtf+ is remobilized into the reducing sediments and... 

The hydrothermal Mn deposits are characterized by high Mn/Fe ratios and low contents of Cu, Ni, Zn, Co, Pb and detrital silicate minerals. They have growth rates exceeding 1,000 mm Ma in some cases, more than three orders of magnitude faster than that of hydrogenous deep-sea nodules and crusts. 

Fig. 11.25 Variation in the prices of Co, Ni and Cu in the United States from 1959-1998 expressed in constant 1992 dollars per kg (after Plunkert and Jones 1999). The prices of Co, Ni and Cu in 1999 in current dollar prices were Co US 21, Ni US 2 and Cu US 0.75. Co is therefore easily the most valuable metal in deep-sea nodules and Co-rich crusts after Pt.

Morgan (2000) considered Ni to be the primary metal of commercial interest in deep-sea nodules. Because deep-sea nodules occur at great depths in the oceans (greater than 4,000 m), they require sophisticated capital-intensive technology to mine and are in direct competition with land-based mineral resources. A decision to mine them must therefore ultimately be based on economic rather than technological considerations. 

For Co-rich manganese crusts, the crusts should contain >0.8% Co and be >40 mm thick to be considered economically viable. The crusts occur in shallower water depths (1,000-2,500 m) than deep-sea nodules (>4,000 m) and could be mined within national Exclusive Economic Zones (EEZs) and therefore imder national jurisdiction. Excellent accounts of the distribution and composition of Co-rich crusts in the world ocean have been presented by Andreev and Gramberg (1998) and Hein (2000) and several authors have attempted to assess of their economic potential (Wiltshire etal. 1999 Hein2000,2004 Wiltshire 2000). Andreev and Gramberg (2002) have estimated the total abundance of Co-rich crasts in the world ocean to be about 21-10%. 

The types of mining systems likely to be used for the recovery of deep-sea nodules and Co-rich Mn crusts are not well known. For nodules, Lenoble (2004) has described a system to mine the French area. This... 

Both Hein (2004) and Lenoble (2004) have considered the profitability of mining deep-sea nodules and Co-rich Mn crasts in some detail and concluded that mining of these deposits is not commercially viable at current world metal prices. Mining these deposits therefore remains a matter for the future. 

Cronan, D.S., 1977. Deep-sea nodule dishibution and chemishy. In Glasby, GP. (ed.). Marine manganese deposits. Elsevier, Amsterdam, pp. 11-44. 

Halhach, R, Scherhag, C., Hebisch, U. and Marchig, V., 1981. Geochemical and mineralogical control of different genetic types of deep-sea nodules from Pacific Ocean. Mineral Deposita, 16 59-84. 

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