Ch. 5— Mining and At-Sea Processing Tectinologies • 209 



• cycloning to reduce undersize material (e.g., 

 clays), and 



• flotation to reject silicates. 



Rejected material is returned to the sea floor. The 

 assumption that flotation can be adapted to ship- 

 board operation requires verification by develop- 

 ment and testing studies, the costs of which are pro- 

 vided for under the capital cost estimates below. 

 The use of an existing (and, therefore, already 

 capitalized) onshore calcining plant near Moorhead 

 City, North Carolina, some 80 miles north of the 

 mine site, is also assumed. 



Assuming that P2O5 makes up 12.4 percent (by 

 weight) of the Frying Pan unit and 6.3 percent of 

 the overburden (both of which are mined), the 

 mined feed to the at-sea processing plant contains 

 11.2 percent P2O5 by weight. A total of 6.9 mil- 

 lion short tons of ore are mined each year at the 

 dredging rate of 2,000 cubic yards per hour, yield- 

 ing a shipboard concentrate of about 1.7 million 

 tons for feed to the calcining plant onshore. This 

 yield assumes that shipboard ore flotation upgrades 

 the P2O5 content to 30 percent. 



Mining and At-Sea Processing Cycle. — It is 



estimated that six barges, each with a capacity to 

 carry 6,550 cubic yards of beneficiated ore, and two 



tugs will be required to conduct efficient and nearly 

 continuous loading while the mining vessel is on 

 station. The time required to load three barges, 

 transit to shore, unload, and return to the mining 

 vessel is expected to be 3 days. The mining vessel 

 is assumed to operate 82 percent of the time, or 

 300 days per year. 



Capital and Operating Costs. — The capital and 

 operating cost estimates (table 5-8) are based on 

 the assumption that new equipment is provided to 

 supply beneficiated ore to an existing shore-based 

 calcining plant. The capital costs of this shore-based 

 plant are not included in the following estimates 

 that may vary by as much as a factor of 2 or more. 



Estimated annual operating costs are $20 per 

 short ton. The estimated costs do not include cap- 

 ital recovery or the profit and risk components that 

 would be required to attract commercial investors 

 to an untried venture. Capital recovery alone over 

 20 years for a $71 million loan at a 9 percent in- 

 terest rate would add an additional $8 per short ton 

 of product. The current market price of compara- 

 ble phosphate rock is about $21 per short ton. 

 Hence, the potential for mining phosphorite in 

 Onslow Bay would not be immediately attractive 

 to commercial investors. 



Table 5-8.— Offshore Phosphorite Mining, Onslow Bay, North Carolina: 

 Capital and Operating Cost Estimates 



Millions of dollars 



Capital costs: 



Detailed exploration, metallurgical testing and feasibility studies $ 4 



Mining and beneficiation vessel 41 



Transportation to shore (tugs and barges witfi capacity to deliver 20,000 



cubic yards every 3 days) 16 



Loading, unloading, and storage installations 10 



Total capital costs 



(million $/year) 

 Operating costs: 



Mining $ 9 $ 5. 00 



Processing to 66% bone pfiosphate of lime 



(BPL) offshore 12 7.00 



Transport and handling 5 3.00 



Calcining to 68 percent BPL (31 percent phosphorous 



pentoxide) onshore 8 5.00 



Total operating costs $34 $20.00 



$71 



($/ton product) 



SOURCE: Office of Tecfinology Assessment, 1987. 



