174 • Marine Minerals: Exploring Our New Ocean Frontier 



Figure 5-5.— Trailing Suction IHopper Dredge 



Power plant Self-discharge system' Self-discharge pump 



Dredgepump 



"•— Trailing dragarm 



// Draghead 



Hopper dredges have been used mainly to clear and maintain navigational channels and harbor entrances and to replenish 

 sand-depleted beaches. A hopper dredge is currently being used to mine sand and gravel in the Ambrose Channel entrance 

 to New York Harbor. 



SOURCE: Dredge Technology Corp. 



between 35 and 100 feet, and 260 feet is consid- 

 ered the maximum achievable depth with currendy 

 available technology. For current specifications and 

 capacities, the capital costs of hopper dredges range 

 from $5 million to $50 million. 



Except for sand and gravel mining in Japan and 

 the North Sea, hopper dredges have not been used 

 extensively to recover minerals. However, hopper 

 dredges adapted for preliminary concentration 

 (beneficiation) of heavy minerals at sea, with over- 

 board rejection of waste solids and water, are likely 

 candidates for mining any sizable, thin, and loosely 

 consolidated deposits of economic heavy minerals 

 that might be found in water less than 165 feet deep. 



A stationary suction dredge, similar in princi- 

 ple to the anchored suction hopper dredge, has been 

 designed and extensively tested for mining the 

 metalliferous muds of the Red Sea.' Although the 



'M.J. Cruicl<shank, "Technology for the Exploration and Exploi- 

 tation of Marine Mineral Deposits, ' ' Non-Living Marine Resources 

 (New York, NY: United Nations, Oceans, Economics, and Technol- 

 ogy Branch), in press. 



dredge has not been used commercially, it success- 

 fully retrieved muds in 7,200 feet of water. 



Cutter Head Suction Dredges 



Mechanically driven cutting devices may be 

 mounted near the intake of some suction dredges 

 to break up compacted material such as clay, clayey 

 sands, or gravel. The two main types are cutter 

 heads and bucket wheels. 



Cutter head dredges are equipped with a special 

 cutter (figure 5-6) mounted at the end of the suc- 

 tion pipe. The cutter rotates slowly into the bot- 

 tom material as the dredging platform sweeps side- 

 ways, pulling against "swing lines" anchored on 

 either side. Cutter head dredges usually advance 

 by lifting and swinging about their spuds when in 

 shallow water. 



Cutter head dredges are in widespread use on 

 inland waterways for civil engineering and mining 

 projects.. Onshore, these dredges have been used 

 to mine heavy minerals, (e.g., Umenite, rutile, and 

 zircon) from ancient beaches and sand dunes in the 



