Ch. 5— Mining and At-Sea Processing Technologies » 171 



traveling up the ladder lift the material to the plat- 

 form and discharge the ore into the processing 

 plant. 



The bucket ladder dredge is the most proven and 

 widely used technology for mining offshore tin plac- 

 ers in open water in Southeast Asia. Bucket ladder 

 dredges are widely used to mine onshore gold, plati- 

 num, diamonds, tin, and rutile placers in Malaysia, 

 Thailand, Brazil, Colombia, Sierra Leone, Ghana, 

 New Zealand, and Alaska. Bucket ladder dredge 

 technology is still the best method to "clean" bed- 

 rock, which is particularly important for the recov- 

 ery from placer deposits of heavy, high-unit-value 

 minerals like gold and platinum. These dredges 

 have buckets ranging in size from 1 to 30 cubic feet. 

 The deepest digging bucket line dredges are de- 

 signed to dig up to 164 feet below the surface. 



Prices of bucket ladder dredges (including proc- 

 essing plants) for mining onshore vary with dredge 

 capacity (bucket size) and with dredging depth. A 

 small bucket dredge (with 3-cubic-foot buckets) may 

 sell for approximately $1.5 million (free on board 

 plant). Such a dredge can mine 60,000 to 80,000 

 cubic yards of ore per month at depths of 30 to 40 

 feet below the hull. The cost of larger onshore min- 

 ing bucket dredges (with buckets as large as 30 cu- 

 bic feet) and capacities up to 1 million cubic yards 

 per month may reach $10 million to $20 million, 

 depending on digging depth and other variables. 



The per-cubic-yard capital and operating costs 

 of larger dredges are lower than those of smaller 

 dredges (figure 5-2). Offshore bucket ladder dredges 

 cost more than onshore dredges because they must 

 be more self-contained. They must be buUt to carry 

 a powerplant, fuel, supplies, and mined ore. The 

 hull also must be larger and heavier to withstand 

 waves and to meet marine insurance specifications. 

 In 1979, the capital cost of the 30-cubic-foot Bima 

 was about $33 million. Approximately 10 bucket 

 dredges configured for offshore use are currently 

 mining tin in Indonesia in water depths of 100 to 

 165 feet at distances of 20 to 30 miles offshore. 



Despite their versatility, offshore uses of bucket 

 ladder dredges are limited. Much of the EEZ 

 around the United States is subject to waves and 

 ocean swells that could make bucket ladder dredg- 

 ing difficult. To ensure that the lower end of the 

 ladder maintains constant thrust against the cut- 



Figure 5-2.— Capital and Operating Costs 

 for Bucket Ladder Mining Dredges 



Dredges for use offshore w/ould cost nnore to build and 

 operate than the estimates illustrated here, since they would 

 have to be self-contained and contain a power plant, fuel, 

 supplies, and mined ore. They would also have to be capable 

 of withstanding waves and high winds. 



SOURCE: Adapted from M.J. Richardson and E.E. Norton, "Technologies for 

 Dredge Mining Minerals of the Exclusive Economic Zone," contractor 

 report prepared for the Office of Technology Assessment, August 1986. 



ting face, motion compensation systems must be 

 installed. These systems are large hydraulic and air 

 cylinders that act like springs to allow the end of 

 the ladder to remain in the same place while the 

 hull pitches and heaves in swells (figure 5-3). Other 

 limitations of current dredges include the high wear 

 rate of the excavating components (e.g., buckets, 

 pins, rollers, and tumblers) and the lack of mobil- 

 ity. Offshore bucket dredges are not self-propelled 

 and must be towed when changing locations. For 

 long tows across rough water, the ladder makes the 

 vessel unseaworthy and makes towing impractical. 

 The bucket dredge Bima was actually carried on 

 a submersible lift barge from Indonesia to Alaska. 

 In designing offshore dredges, especially those 

 working in rough water, careful attention must be 

 given to seaworthiness of the hull. 



Most bucket ladder dredges are now built out- 

 side the United States, although the capability and 

 know-how still exist in this country. Except for the 

 motion compensation systems installed on offshore 

 dredges, bucket ladder dredge technology has re- 

 mained essentially static, and there have been only 

 minor gains in dredging depth in the last 50 years. 



