Ch. 1— Summary, tssues, and Options * 19 



TECHNOLOGIES FOR MINING AND PROCESSING 

 MARINE MINERALS 



Existing or modified dredge mining sys- 

 tems could place many potential placer 

 deposits in the range of technical ex- 

 ploitability. 



From table-flat, heavy mineral sand placers de- 

 posited in shallow water to mounds and chimneys 

 of rock-like polymetallic sulfides at depths of over 

 a mile, marine minerals present a variety of chal- 

 lenges to the design, development, and operation 

 of marine mining systems. Development and cap- 

 ital costs for vessels and marine systems can be high. 

 Profitability of offshore mining ventures will hinge 

 on whether safe and efficient mining systems can 

 be built and operated at reasonable costs. With the 

 exception of conversions of onshore dredge min- 

 ing equipment for shallow, protected water offshore 

 and work done on deep seabed manganese nodule 

 mining systems, there has been little development 

 effort thus far. 



Dredge mining technology is used extensively for 

 harbor and channel dredging in coastal waters and 

 for onshore mining of phosphate rock and heavy 

 mineral sands. It has also been used for mining tin 

 in coastal waters in Asia and is currendy being used 

 in pilot mining of gold in State waters near Nome, 

 Alaska. 



In deeper waters subject to winds, waves, swells, 

 and currents, specially designed mining dredges 

 must be developed. High endurance dredges for 

 deep waters must be self-powered, seaworthy plat- 



forms with motion compensating systems and may 

 be equipped with onboard mineral processing plants 

 and storage capacity. Conceptual designs of such 

 equipment are being readied. The design of even 

 the most sophisticated dredge probably can be 

 achieved without major new technological break- 

 throughs. Cost will be the most important limit- 

 ing factor. 



The maximum practical operating depth for most 

 dredging systems is about 300 feet from the sur- 

 face of the water to the bottom of the excavation 

 on the seafloor. Airlift systems can be used on suc- 

 tion dredges to lift unconsolidated material from 

 much greater depths. Existing or modified dredge 

 mining systems could place many potential placer 

 deposits in the range of technical exploitability. 



Solution or borehole mining has been tested in 

 north Florida land-based phosphate rock deposits 

 as a means to reduce surface disturbance and envi- 

 ronmental impacts. The technique involves sink- 

 ing a shaft into the phosphorite deposit, jetting 

 water into the borehole, and pumping the result- 

 ing slurry to the surface. Although the technique 

 has not yet been tested under marine conditions, 

 some mining engineers speculate that it could have 

 potential for offshore phosphorite mining. 



Several preliminary mining systems have been 

 sketched out for recovering ferromanganese crusts 

 as well as for mining polymetallic sulfide deposits, 

 but little if any development work has proceeded 

 in either area. Collection and airlift recovery sys- 

 tems developed for deep seabed manganese nod- 

 ules may be adaptable to mining both crusts and 

 polymetallic sulfides. Too little is known about the 



DREDGES WHICH OPERATE HYDRAULICALLY 



DUSTPAN DREDOE 



SELF-PROPELLEO 

 HOPPER DREDGE 



HTDRAULJC 



PIPELINE 



CUTTERHEAD OREOOE 



DREDGES WHICH OPERATE MECHANICALLY 



DIPPER DREDGE 



BOCKET-LADOCR 

 DREDGE 



Dredge Technologies 



Dredge technologies are well developed and proven through years of experience. Adaptation of inshore dredge 

 mining systems for offshore use could make the technical exploitability of some heavy mineral placer deposits 

 possible if seabed mining is found to be economically competitive. 



Source: Office of Technology Assessment, 1987 



