Ch. 5— Mining and At-Sea Processing Technologies • 205 



ern coast of Georgia was examined in some detail 

 in a 1979 study by Zellars-Williams, Inc., for the 

 Department of the Interior. To illustrate the tech- 

 nical and economic feasibility of offshore phos- 

 phorite mining, OTA has drawn heavily from Zel- 

 lars-Williams work. 



The Zellars-Williams study considers a 30- 

 square-mile area located about 12 miles offshore 

 Tybee Island, Georgia, not far from the South 

 Carolina border and in the same general area con- 

 sidered in the titanium scenario (figure 5-17). Only 

 scattered, widely spaced samples have been taken 

 in the vicinity, and none within the scenario area 

 itself. These samples and some seismic data sug- 

 gest the occurrence of a shallow phosphorite deposit 

 in the area, but much more sampling is required 

 to fully evaluate the deposit. The mine site is at- 

 tractive for several reasons: 



• water depths are uniform over the entire block, 

 with a mean depth of 42 feet; 



• the area is free of shipwrecks, artificial fish- 

 ing reefs, natural reefs, rock, and hard bottom; 



• the area is close to the Savannah Harbor en- 

 trance but not within shipping lanes for traf- 

 fic entering the harbor; and 



• an onshore plant site is available with an ade- 

 quate supply of river water for process use, in- 

 cluding washing of sea salts. 



Operational and Geological Characteristics. — 



Average windspeed during the year at the site is 

 about 7 miles per hour with peaks each month up 

 to 38 miles per hour. Winter surface winds are 

 chiefly out of the west, while in summer north and 

 east winds alternate with those from the west. Se- 

 vere tropical storms affect the area about once every 

 10 years and usually occur between June and mid- 

 October. The most severe wave conditions result 

 from strong fall and winter winds from the north 

 and west, but the proposed mining site is sheltered 

 by land from these directions. Waves of 12 feet or 

 more occur about 2 . 5 percent of the year while 4- 

 foot waves occur 57 percent of the year. The max- 

 imum spring tidal range is about 8 feet. Current 

 speeds are low, about 3 to 4 miles per day. Heavy 

 fog is common along the coast, and Savannah ex- 

 periences an average of 44 foggy days a year. 



Phosphorite ore occurring as pebbles and sand 

 at the mine site is part of what is known as the 



Savannah Deposit. The site straddles the crest of 

 the north-south trending Beaufort Arch, which sug- 

 gests that the top of the phosphatic matrix will be 

 closest to sea level in this area. The ore body lies 

 beneath 4 feet of overburden. It is assumed that 

 the ore body is of constant thickness over a reason- 

 ably large area and that the mine site contains 150 

 million short tons of phosphorite. The average 

 grade of the ore is assumed to be 11.2 percent phos- 

 phorous pentoxide (P2O5). 



Mining Technology. — An ocean-going cutter 

 suction dredge with an onboard beneficiation plant 

 is selected for mining. The dredge is equipped with 

 a 125-foot cutter ladder, enabling it to dredge to 

 a maximum depth of 100 feet below the water sur- 

 face, more than enough to reach all of the mine site 

 deposit. The dredge first removes the sandy over- 

 burden in a mine cut and places it away from the 

 cut or in a mined-out area. Phosphate matrix then 

 is loosened by the rotating cutter, sucked through 

 the suction pipe, and brought onboard the dredge. 

 The dredge is designed to mine approximately 

 2,500 cubic yards of phosphate matrix per hour. 

 It is estimated that approximately 450 acres of phos- 

 phate matrix are mined each year. Mining cuts are 

 1 mile long and 800 feet wide. 



Processing Technology. — Onboard processing 

 consists of simple mechanical disaggregation of the 

 matrix followed by size reduction. Oversize mate- 

 rial is screened with trommels and rejected. Under- 

 size material (mainly clays) is removed using 

 cyclones. The undersize material is flocculated 

 (thickened to a consistency suitable for disposal) and 

 pumped to the sea bottom. 



On shore, the sand size material is subjected to 

 further washing and sizing. Tailings and clays are 

 returned to the mine site for placement over the 

 flocculated clays. Phosphate is concentrated to 66 

 percent bone phosphate of lime (BPL) by a con- 

 ventional flotation sequence. The wet flotation con- 

 centrate is then blended and calcined to 68 percent 

 BPL (approximately 30 percent P2O5). 



It is assumed that, initially, 2.5 million short tons 

 per year of phosphate rock are produced. Eventu- 

 ally, the amount produced would increase to the 

 optimum rate of 3.5 million tons. It is also assumed 

 that only 4 cubic yards of ore would need to be 

 dredged per ton of final product. 



