198 • Marine Minerals: Exploring Our New Ocean Frontier 



sand mining scenario, the effect of vessel motion 

 on these devices needs to be evaluated. It is assumed 

 that 30 to 50 percent of the dredged ore will be kept 

 on the vessel and that the tailings will be continu- 

 ously discharged by pipe back to the seafloor. 



There are no at-sea processing plants of this type 

 in operation. Additional investigation is needed to 

 evaluate the feasibility and to determine the capi- 

 tal and operating costs of this system, but it is as- 

 sumed that the development engineering required 

 will not entail major costs. 



Mining and At-Sea Processing Cycle. — In- 

 creased suction capacity plus a shorter distance to 

 dockside and less elaborate processing at sea en- 

 able the dredge to deliver 5,000 tons of enriched 

 ore to shore per day (rather than every third day 

 as in the case of the titanium sands scenario). Un- 

 der normal operating conditions, the dredge is as- 

 sumed to take about 3 hours to fill to capacity. The 

 vessel then steams an average distance of 75 miles 

 for offloading at a shore facility. Transit time is esti- 

 mated to average about 8 hours, offloading time 

 less than 5 hours; hence, the vessel would be able 

 to make one round trip per day. At dockside the 

 dredge would be offloaded using either a dry scraper 

 or its own pumps. Pumped transfer decreases off- 

 loading time. If this method is used, the ore is 

 pumped into a dewatering bin and from there trans- 

 ported by conveyor belt to a stockpile. It is assumed 

 that the mining and processing system can be de- 

 signed so that mining and processing at sea can take 

 place 300 days a year. This would leave 65 days 



for downtime due to bad weather or sea conditions, 

 for drydocking and maintenance, and for other un- 

 foreseen events. Under these assumptions, 1.5 mil- 

 lion tons of chromite-rich concentrate are delivered 

 yearly to the offloading plant onshore. 



Capital and Operating Costs. — Capital and 

 operating costs (table 5-5) were estimated for min- 

 ing, at-sea processing, transportation, and offload- 

 ing at a shoreside facility, but not for subsequent 

 processing on land. Capital costs amount to approx- 

 imately $57 million for an operation that uses all 

 new equipment developed for the project and built 

 in the United States. These include a dredge ($40 

 million), shipboard primary beneficiation plant ($5 

 million), shoreside facility (about $5 million), and 

 design, engineering, and management ($7 million). 

 Annual operating costs are estimated to be approx- 

 imately $20 million; this figure includes costs to 

 operate the dredge and shore facility and general 

 and administrative expenses. 



Based on the above figures and assumptions, the 

 cost of delivering enriched chromite sand to a shore- 

 based facOity was calculated. In terms of dollars per 

 ton of beneficiated ore, the range is between $12.50 

 and $22. The lower cost assumes the use of a sec- 

 ondhand dredge. The higher cost includes a 20 per- 

 cent internal rate of return which is assumed to be 

 a realistic goal in view of the uncertainties (espe- 

 cially operating time) that surround the project. (If 

 the yearly operating time were reduced to 150 days, 

 the costs of delivering concentrates would double 

 to between $25 and $44 per ton). 



Table 5-5.— Offshore Chromite Sands Mining Scenario: 

 Capital and Operating Cost Estimates 



Millions of dollars 



Capital costs: 



Suction hopper dredge $40.0 



Shipboard primary beneficiation plant 5.0 



Shoreside facility 5.0 



Engineering procurement and management (15%) .... 7.5 



Total $57.5 



New equipment 



(excluding profit 



and risk) 



Annual operating costs: 



Dredge $17 



Shore facility 2 



General and administrative 3 



Annual total ' $22 



SOURCE: Office of Technology Assessment, 1987. 



Used equipment 



(excluding profit 



and risk) 



$15 



2 



2 



$19 



