15 
probably around 50 percent. The mine sweeping efficiency is a function 
of the characteristics of the mining equipment, navigational system, 
and operation procedures. Sweep efficiency may be around 45 to 65 per- 
cent. Two other factors to consider are unminable areas (about 15 to 25 
percent of the bottom appears to be too rough or otherwise physically 
unsuitable for mining) and cut-off grade (about 10 percent of the site 
is expected to contain nodules of too low grade to be mined). The most 
unfavorable combination of factors (accessibility, dredge efficiency, 
sweep efficiency, and cut-off grade may allow only 9 percent of the 
nodules in an area to be recovered. An optimistic combination is likely 
to allow 35 percent recovery and a completely ideal combination could 
yield 58 percent recovery. A working average for nodule recovery 
efficiency expected by industry is around 25 percent or higher. The 
production period for the venture to be profitable and the overall min- 
ing efficiency determine the minimum site size. Production periods of 
twenty years have generally been assumed. Greater efficiency would 
permit a smaller mine site to be commercial as would a shorter pro- 
duction period, if acceptable. 
Some experts believe that exploration for potential mine sites over a 
broad area may become more restricted or difficult if an international 
regime is established to regulate the seabed. However, most of the 
major groups involved in ferromanganese nodule mining investiga- 
tions have completed the exploration stage and are aware of a number 
of potential mining sites.” 
Nopute Minine TECHNOLOGY 
Once a mining site has been selected the next problem is to collect 
and elevate the nodules to the surface from depths of 3000 to 5000 
meters. Mining methods must be highly reliable and must recover 
volumes large enough to be economic. Three basic systems have been 
devised: (1) Air-lift, (2) hydraulic lift without air, and (3) mechani- 
eal lift such as the continuous line bucket (CLB) system. Each of these 
basic methods has individual variations. In addition to these continu- 
ous recovery methods, batch systems such as wireline dredging and 
buoyant hoppers have also been proposed.? However, while single- 
collector dredging is useful for obtaining large tonnage samples for 
evaluation, it is not considered an economic large-scale production sys- 
tem due to its high cost and low recovery rate when used at great 
depths.* 
ATR-LIFT PUMPING 
The air-lift pumping (ALP) or pneumatic lift method is techni- 
cally a three-phase flow: air, water, and nodules. Compressed air is 
injected into the pipe at various depths. The upward movement draws 
water, nodules, and surrounding sediment into the bottom end of the 
pipe. The nodules are literally vacuumed off the seabed and carried up 
2 Rothstein, A. J., and R. Kaufman. The approaching maturity of deep ocean mining— 
Be pace quickens. Offshore Technology Conference Preprints 1978, v. 1, 1973, pp. 323-— 
s Wenzel, J. C. Systems—development planning. In Ocean Engineering, Brahtz, J. F.. 
ed., Wiley, New York, 1968, p. 110. 
4United Nations, A/CONF.62/25, op. cit., p. 17. 
