Metal nodules

Mode of Source of Metals Nodule Growth Dominant Mn Oxide Mn ... 

Figure 4 Backscattered electron image of a highly altered Fe,Ni metal nodule in a chondrule in the Murchison CM2 carbonaceous chondrite. Thin veins of iron oxyhydroxides are present extending from the metal grain through the fine-grained rim, into the clastic matrix of the chondrite (reproduced by permission of University of Arizona and The Meteoritical Society from Meteorit. Planet. Sci, 2000, 35, 1300).

Silicon-bearing kamacite is a minor component of most aubrites, and contains 3.7-6.8 wt.% Ni and 0.1-2.4 wt.% Si. Metal nodules from several aubrites contain approximately chondritic abundances of refractory to moderately volatile siderophile elements (Casanova et al., 1993). 

Hassanzadeh J., Rubin A. E., and Wasson J. T. (1989) Large metal nodules in mesosiderites. Meteoritics 24 276-276. 

Extrusion of parent metal from the internal oxidation zone to form pure metal nodules on the surface. 

Figure 3.6 shows the surface and cross-section morphologies of the Ni-6Cr alloy after oxidation at 900°C under an oxygen partial pressure of 10 Pa for 2 min (Fig. 3.6a), 10" Pa for 40 hr (b) and 10 Pa for 40 hr (c). It can be seen that after both 2 min and 40 hr oxidation, oxide scales with small particles and big metal nodules appeared on the surfaces (Figs 3.6a and 3.6b) an internal oxidation zone can be seen in the cross-sectional image as well (Fig. 3.6c).

Because these oxidation experiments were performed with an oxygen partial pressure under which Ni and Co could not be oxidized, the oxides formed were only from oxidation of Cr, so the oxides were Cr203. As a special feature of the internal oxidation of dilute alloys, metal nodules formed on the surfaces, which were Ni and Co for Ni-Cr and Co-Cr alloys, respectively. The compositions and phases of these modules were confirmed by analyses using energy dispersive X-ray spectroscopy (EDS), X-ray diffraction and X-ray photoelectron energy spectroscopy [14,19]. 

Formation of pure solvent metal nodules during... 

Since a continuous oxide scale can form on the alloy surface before the formation of internal oxide precipitates, it is reasonable to suggest that the external oxide scale is another important factor in determining the formation of solvent metal nodules. When the internal oxide precipitation forms in the alloy substrate, stresses are generated, volume expansion occurs and the position of the alloy surface moves outward correspondingly. This volume expansion should be constrained by the oxide scale formed on the alloy... 

The formation of an external oxide scale is also an important factor in determining the formation of pure solvent metal nodules on the alloy surface during oxidation. The stresses generated by the internal oxide precipitation in the alloy are the driving force for the formation of these nodules. Under the constraint of the external oxide scale, solvent metal could be pushed out at the weak locations of the oxide scale to form nodules on the surface. 

Alloys with other useful properties can be obtained by using yttrium as an additive. The metal can be used as a deoxidizer for vanadium and other nonferrous metals. The metal has a low cross section for nuclear capture. 90Y, one of the isotopes of yttrium, exists in equilibrium with its parent 90Sr, a product of nuclear explosions. Yttrium has been considered for use as a nodulizer for producing nodular cast iron, in which the graphite forms compact nodules instead of the usual flakes. Such iron has increased ductility. 

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

Deep-sea manganese nodules represent a significant potential mineral resource. Whereas the principal constituent of these deposits is manganese, the primary interest has come from the associated metals that the nodules can also contain (see Ocean rawmaterials). For example, metals can range from 0.01—2.0% nickel, 0.01—2.0% copper, and 0.01—2.25% cobalt (12). Recovery is considered an economic potential in the northwestern equatorial Pacific, and to a lesser degree in the southern and western Pacific and Indian Oceans (13—18). 

Table 2. Examples of Metal Grades and Values for Crusts and Nodules ...

The calcium crowns can be sold as such for certain appHcations. However, further processing may be required, and the crowns can be reduced in size to pieces of about 25 cm or nodules of about 3 mm. They can also be melted under a protective atmosphere of argon and cast into billets or ingots. Calcium wire can be made by extmsion, and calcium turnings are produced as lathe cuttings from cast billets. Technologies have also been developed to manufacture calcium metal particulates and powders by atomization, comminution, and grinding processes. 

The most commonly observed effect of current flow is the development of alkaline conditions at the cathode. On bare metal this alkaline zone may exist only at the metal surface and may often reach pH values of 10 to 12. When the soil solution contains appreciable calcium or magnesium these cations usually form a layer of carbonate or hydroxide at the cathodic area. On coated lines the cations usually move to holidays or breaks in the coating. On failing asphalt or asphalt mastic type coatings, masses of precipitated calcium and magnesium often form nodules or tubercles several centimetres in diameter. 

This argument is confirmed by the study of CO pulse chemisorption by Biffis at al., mentioned above. In this piece of investigation, the authors prepared a 2% (w/w) palladium catalyst supported by Lewatit UCP 118, a macroreticular resin (nominal cld = 18 %) from Bayer. Its TEM characterization showed a remarkably heterogeneous distribution of the metal nanoclusters, which are apparently located close to the surface of the polymer nodules [62] (Figure 9). 

Another important consideration pertains to the metal content of the deposit. A deposit with a content of iron of about 20% can have little value as an iron ore since there are several deposits with 30-50% iron. Earlier, a copper ore with a minimum of 5% copper was regarded or accepted as a copper ore. However, today, thanks to advancements in technology, rocks with as little as 0.5% copper are mined and processed economically despite the fact that the price of copper, in comparison with those of some other metals, might be showing a downward trend. It is possible that in the future, other resources, which are not considered to be worth exploiting today (such as the manganese nodules or the clays), would become acceptable ores for manganese, copper, nickel, cobalt, and aluminum. 

Finally, it may be added that it is the terrestrial resources which have served and will continue to serve as practically the sole suppliers of metals and materials. Among the other resources only the manganese nodules extracted from the ocean bed have drawn serious attention the world over for exploring economic methods of processing. 

The combination of physical and chemical characteristics of nodules make impossible the application of methods of physical beneficiation such as flotation and magnetic separation to produce concentrates of valuable metals, and so chemical processing must be used. Their processing also tends to be much more energy-intensive, vis-a-vis that of conventional land-based ores. Deep-sea manganese nodules are quite unlike any terrestrial ores, both with respect to their physical characteristics and to their mineralogical and chemical compositions new processes are, therefore, required. 

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