Chapter 5 



Mining and At-Sea Processing Technologies 



INTRODUCTION 



Many factors influence whether a mineral de- 

 posit can be economically mined. Among the most 

 important are the extent and grade of a deposit; 

 the depth of water in which the deposit is located; 

 and ocean environment characteristics such as 

 wave, wind, current, tide, and storm conditions. 

 Offshore mineral deposits range from unconsoli- 

 dated sedimentary material (e.g., marine placers) 

 to consolidated material (e.g., cobalt- rich ferroman- 

 ganese crusts and massive sulfides). They may oc- 

 cur in a variety of forms, including beds, crusts, 

 nodules, and pavements and at all water depths. 

 Deposits may either lie at the surface of the seabed 

 or be buried below overburden. Some deposits may 

 be attached solidly to nonvaluable material (as are 

 cobalt-rich crusts), while others (gold) may lie atop 

 bedrock or at the surface of the seabed (manganese 

 nodules). The amount and grade of ore can vary 

 significantly by location. 



All of these variables affect the selection of a min- 

 ing system for a given deposit. Dredging is the most 

 widely used technology applicable to offshore min- 

 ing. Dredging consists of the various processes by 

 which large floating machines or dredges excavate 

 unconsolidated material from the ocean bottom, 

 raise it to the surface, and discharge it into a hop- 

 per, pipeline, or barge. Waste material excavated 

 with the ore may be returned to the water body af- 

 ter removal of valuable minerals. Dredging tech- 

 niques have long been applied to clearing sand and 

 silt from rivers, harbors, and ship channels. Ap- 

 plication of dredging to mining began over a cen- 

 tury ago in rivers draining the southern New 

 Zealand gold fields. Offshore, no minerals of any 

 type have been commercially dredged in waters 

 deeper than 300 feet, and very little dredge min- 

 ing has occurred in water deeper than 150 feet. Off- 

 shore dredging technology is currently used to re- 

 cover tin, diamonds, sea shells, and sand and gravel 

 at several locations around the world (table 5-1). 



Some of the problems of marine mining are com- 

 mon to all offshore deposits. Whether one consid- 

 ers mining placers or cobalt-rich ferromanganese 

 crusts, for instance, technology must be able to cope 

 with the effects of the ocean environment — storms, 

 waves, currents, tides, and winds. Other problems 

 are specific to a deposit or location (e.g., the pres- 

 ence of ice) and hence require technology specially 

 designed or adapted for that location. 



Just as many variables influence offshore mineral 

 processing. The processing scheme must be de- 

 signed to accommodate the composition and grade 

 of ore mined, the mineral product(s) to be recov- 

 ered, and the feed size of the material. Mineral 

 processing technology has a long history onshore. 

 Applications offshore differ in that technology must 

 be able to cope with the effects of vessel motion and 

 the use of seawater for processing. Technologies 

 currently applied to processing minerals at sea are 

 all mechanical operations and include dewatering, 

 sizing, and gravity separation. Processing at sea is 

 currently limited to the separation of the bulk of 

 the waste material from the useful minerals. This 

 may be all the processing required for such prod- 

 ucts as sand and gravel, diamonds, and gold; how- 

 ever, many other products, including, for example, 

 most heavy minerals, require further shore-based 

 processing. Chemical treatment, smelting, and 

 refining of metals have heretofore taken place on 

 shore, and, given the difficulty and expense of proc- 

 essing beyond the bulk concentrate stage at sea, are 

 likely to continue to be done on land in most cases. 



The degree to which processing at sea is under- 

 taken depends on economics as well as on the ca- 

 pabilities of technology. As with mining technol- 

 ogy, some processing technology is relatively well 

 developed (e.g., technology for extracting precious 

 metals or heavy minerals from a placer) while other 

 technology is unlikely to be refined for commer- 

 cial use in the absence of economic incentives. 



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