186 • Marine Minerals: Exploring Our New Ocean Frontier 



as gold, platinum, and diamonds, but all other 

 minerals would probably be taken ashore as bulk 

 concentrates to be further processed. Trade-offs 

 must be considered in evaluating whether to par- 

 tially process some minerals offshore. First, the cost 

 of transporting unbeneficiated ore to shore must 

 be weighed against the added costs and capital ex- 

 penses of putting a beneficiation plant offshore. 

 Transportation to shore of a smaller amount of high 

 grade concentrate may be more economical than 

 transporting a larger amount of lower grade ore to 

 shore for beneficiation and subsequent processing. 

 (This is also a standard problem on land when eval- 

 uating trade-offs between, for example, building 

 a smelter or investing in transportation to an ex- 

 isting smelter.) Second, it is generally thought to 

 be easier and more economical to discharge tailings 

 (waste materials) at sea than on land, but tailings 

 discharge may result in unacceptable environmental 

 impacts. Third, while seawater is an unlimited 

 source of water for use in many phases of process- 

 ing, its higher salinity could make processing more 

 difficult and concentrates could require additional 

 washing with fresh water. 



Important considerations in evaluating whether 

 to process minerals offshore may include the cost 

 of space aboard mining vessels and the sensitivity 

 of some processing steps to vessel motion. Space 

 is an important factor in the economics of a project. 

 Since larger platforms cost more, engineers must 

 consider the trade-offs between using a hull or plat- 

 form large and stable enough to contain additional 

 processing equipment, power, fuel, storage space, 

 and personnel and transporting unbeneficiated ore 

 to shore. Although litde experience is available, ves- 

 sel motion may make some processing steps diffi- 

 cult or impossible without motion compensation 

 equipment and may significantly reduce the effi- 

 ciency of recovering some minerals. Power require- 

 ments are also of major concern because all power 

 must be generated onboard, thus requiring both 

 additional space and costs. Personnel safety, the 

 availability of docking facilities, distance to refiner- 

 ies, and production rates may also influence proc- 

 essing decisions." 



"M.J. Cruickshank, "Marine Sand and Gravel Mining and Proc- 

 essing Technologies," Marine Mining, in press. 



Some basic development options include limit- 

 ing the motion of the platform (e.g., by using a 

 semi-submersible); isolating the processing equip- 

 ment from platform motion (e.g., by mounting it 

 on gimbals); redesigning the processing equipment 

 to make it more efficient at sea; or simply accept- 

 ing lower grade concentrate by using existing and, 

 hence, less cosdy equipment. In the case of mineral 

 processing, an initial priority probably would be 

 to test existing processing equipment at sea to ob- 

 tain operating experience. 



The costs and efficiency of operating a process- 

 ing plant at sea are highly uncertain. For exam- 

 ple, motion compensation of specific sections of the 

 onboard plant or of major portions of the vessel is 

 expensive. For most minerals, further development 

 of technology will be needed to optimize offshore 

 mineral processing equipment and procedures. In 

 general, one would probably attempt to perform 

 the easy and relatively inexpensive processing steps 

 offshore, such as size separation and rough grav- 

 ity concentration, to reduce the bulk of material 

 to be transported, then complete the processing on 

 land. 



There are three broad categories of mineral proc- 

 essing technology: 



1. technology for unconsolidated deposits of 

 chemically inert minerals, 



2. technology for unconsolidated or semi- 

 consolidated deposits of chemically active 

 minerals, and 



3. technology for consolidated deposits of min- 

 erals requiring crushing and size reduction. 



Processing Unconsolidated Deposits of 

 Chemically Inert Minerals 



Chemically inert minerals include gold; plati- 

 num; tin oxide (cassiterite); titanium oxides (il- 

 menite, rutile, and leucoxene); zircon; monazite; 

 diamonds; and a few others. These occur in na- 

 ture as mineral grains in placers (see ch. 2) and are 

 often found mixed with clay, sand, and/or gravel 

 particles of various sizes. Since these minerals are 

 generally heavier than the silicate and other min- 

 erals with which they may be mixed, the use of me- 

 chanical ^gravity separation methods is important 

 in processing (figure 5-14). However, the initial step 



