Ch. 5— Mining and At-Sea Processing Technologies • 207 



comes technically and economically feasible. While 

 there is no existing facility within a reasonable dis- 

 tance of the Savannah Deposit, phosphorite ore lo- 

 cated off the coast of North Carolina (Onslow Bay) 

 potentially could be processed at the existing on- 

 shore facility near Moorhead City. 



The following scenario, developed by OTA, ex- 

 amines the feasibility of mining the Onslow Bay 

 deposits, of using onboard flotation to upgrade the 

 ore to 66 percent BPL, and of using the existing 

 facility at Moorhead City for calcining to 68 per- 

 cent BPL. 



Onslow Bay, North Carolina 



Location. — A high-grade offshore phosphorite 

 resource is described by Riggs^" and others on the 

 continental shelf adjacent to North Carolina. The 

 resource is located at the southern end of Onslow 

 Bay 20 to 30 miles southeast of Cape Fear (figure 

 5-21). A Federal/State task force was established 

 in 1986 to investigate the future of marine mining 

 offshore North Carolina. The task force has hired 

 Development Planning & Research Associates to 

 study the feasibility of mining Onslow Bay phos- 

 phorite; however, no private companies have ex- 

 pressed an immediate interest in mining offshore 

 phosphorite in this area. 



The Miocene Pungo Formation is a major sedi- 

 mentary phosphorite unit underlying the north- 

 central coastal plain of North Carolina. It is mined 

 extensively onshore. The seaward extension of the 

 Pungo Formation under Onslow Bay has been stud- 

 ied using seismic profiling and vibracore sampling 

 methods. The site selected for this scenario is where 

 the Frying Pan Phosphate Unit of the Pungo For- 

 mation outcrops offshore in a band 1 to 2 ^ miles 

 wide and about 18 miles long. 



Operational and Geological Characteris- 

 tics. — The site is characterized by open ocean con- 

 ditions consisting of wind waves from the north- 

 east and long period swell. Winds gusting above 

 30 knots occur less than 15 percent of the time. Cur- 

 rents are less than 1 knot and tidal influence is neg- 

 ligible. Hurricanes and associated wave conditions 

 occur on an average of 10 days per year. 



The phosphorite formation consists of fine, 

 muddy sands covering an area of 45 square miles. 

 Overburden consists of loose, fine, sandy sediment 

 varying in thickness from to 8 feet. Underneath, 

 the phosphorite sand has a thickness between 1 and 

 10 feet. Water depth averages about 80 feet; hence, 

 the total mining depth is not expected to exceed 

 98 feet. The overburden contains an average of 6.3 

 percent P2O5. The phosphorite unit contains be- 

 tween 4.8 and 22.9 percent P2O5, with an average 

 of 12.4 percent by weight.^' Laboratory analysis 

 of phosphate concentrates indicates the presence of 

 no other valuable minerals. 



Mining Technology. — A trailing suction dredge 

 with an onboard beneficiation plant is selected as 

 the most appropriate technology for the water depth 

 and geological characteristics of the deposit. It is 

 assumed that the phosphorite unit and overburden 

 are sufficiently unconsolidated to be mined by suc- 

 tion dredging methods without the need for a cut- 

 ter head. Only water jets and passive mechanical 

 teeth are used. The dredge and plant are housed 

 in a specially designed ship-configured hull. The 

 vessel is not a self-unloading hopper dredge and 

 has only a small storage capacity on board. The 

 beneficiated ore is discharged onto barges or small 

 ore carriers which are continuously in attendance 

 behind the mining vessel and which shuttle back 

 and forth to the unloading point near the shore 

 processing plant. Dredging capacity is about 2,000 

 cubic yards per hour; 75 percent dredging efficiency 

 is assumed. The suction head is kept on the seabed 

 by a suction arm that compensates for the motion 

 of the vessel in ocean swell. The vessel is self- 

 propelled, dredges underway, and is equipped with 

 precision position-keeping instrumentation. 



The above configuration is preferred to hopper 

 dredging because either a very large single hopper 

 dredge or several smaller hopper dredges would be 

 needed to meet the mining production re- 

 quirements. 



Processing Technology. — At-sea processing is 

 assumed to consist of: 



• conventional mechanical disintegration and 

 screening to eliminate oversize material, 



™S.R. Riggs, et al., "Geologic Framework of Phosphate Resources 

 in Onslow Bay, North Carolina Continental Shelf," Economic Ge- 

 ology, vol. 80 (1985), p. 735. 



'Ibid. 



