Ch. 5— Mining and At-Sea Processing Teclinologies • 169 



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Photo credit: M.J. Cruickshank. U.S. Geological Survey 



Dredge technology for offshore mining must be designed for rough water conditions. 



DREDGING UNCONSOLIDATED MATERIALS 



The dredge is the standard technology for ex- 

 cavating unconsolidated materials from the seafloor. 

 Compacted material or even hard bedrock also can 

 be removed by dredging, provided it has been bro- 

 ken in advance by explosives or by mechanical cut- 

 ting methods. Dredges are mounted on floating 

 platforms that support the excavating equipment. 

 Mining dredges may also have equipment on board 

 to handle and/or process ore. 



Three principal dredging techniques are: buck- 

 etline, suction, and grab (table 5-2). For bucket- 

 line and suction dredging, the material is continu- 

 ously removed from the seabed and lifted to the sea 

 surface. Grab dredges also lift material to the sur- 

 face, but in discrete, discontinuous quantities. 



Most existing mining dredges are designed to 

 operate in relatively protected waters. Dredge min- 

 ing offshore in open water occurs in only a few 



countries (Southwest Africa, United Kingdom, In- 

 donesia, Thailand). The Bima, a mining dredge 

 built for tin mining offshore Indonesia, is being 

 adapted at this time for gold mining offshore Nome, 

 Alaska. Little special equipment capable of min- 

 ing the U.S. Exclusive Economic Zone (EEZ) has 

 yet been built, although some feasibility studies and 

 tests have been conducted. 



Bucketline or Bucket Ladder Dredging 



The bucketline or bucket ladder dredge consists 

 of a series of heavy steel buckets connected in a 

 closed loop around a massive steel ladder (in the 

 manner of the chain on a chain saw) (figure 5-1). 

 The ladder is suspended from a floating platform. 

 For mining, the ladder is lowered until the buckets 

 scrape against the dredging face, where each bucket 

 is filled with ore as it moves forward. The buckets 



