Ch. 5— Mining and At-Sea Processing Technologies • 183 



to the surface probably would be similar to those 

 already designed for mining manganese nodules. 



Crusts form thin coatings on the surface of vari- 

 ous types of nonvaluable substrates. A principal 

 problem in designing a crust mining system will 

 be to separate crust from substrate in order to min- 

 imize dilution of the ore. The thickness and conti- 

 nuity of the crust (which are often highly variable), 

 the nature of its bonding to substrate, and the effi- 

 ciency of the cutting device used will affect how 

 much substrate is collected. The more substrate col- 

 lected, the lower the ore grade and the greater the 

 costs of transportation, processing, and waste dis- 

 posal. The principal alternatives are to separate 

 crust from unwanted substrate on the seabed (and 

 thus avoid lifting substrate to the surface) or to sep- 

 arate crust and substrate on the mining vessel. 

 Complete separation on the seabed of ore from 



waste material would be preferable (if at all feasi- 

 ble), but costs to do so may be prohibitively high. 

 It is more likely that only a small amount of the 

 necessary separation will take place on the seabed 

 and that most of the separation will take place on 

 the mining vessel or onshore. 



The mining system assumed in the EIS mining 

 scenario employs a controllable, bottom-crawling 

 tracked vehicle attached to a mining ship by a hy- 

 draulic lift system and electrical umbilical cord. 

 However, before mining concepts can be signifi- 

 cantly refined, more information will be required 

 about the physical characteristics of the crusts. 

 More data on the microtopography of crusts and 

 substrate are an especially important requirement 

 for the design of the key element of the mining sys- 

 tem, a crust fragmenting device. 



SOLUTION/BOREHOLE MINING 



Solution or borehole mining has much in com- 

 mon with drilling for oil and gas; in fact, much of 

 the technology for this mining method is borrowed 

 from the oil and gas industry. Both terms refer to 

 the mining of rock material from underground de- 

 posits by pumping water or a leaching solution 

 down wells into contact with the deposit and remov- 

 ing the slurry or brine thus created. Because the 

 mining process is accomplished through a drill hole, 

 this method is applicable for recovering some types 

 of ore without first removing overburden. 



The Frasch process, used since 1960 to mine sul- 

 fur from salt dome deposits in the Gulf of Mexico, 

 is the only current application of solution mining 

 offshore (figure 5-13). From an offshore drilling 

 platform, superheated water and compressed air are 

 pumped into the sulfur deposit. The hot water melts 

 the sulfur, and liquid sulfur, water, and air are 

 forced to the surface for collection.' 



Borehole mining has been considered for recov- 

 ery of both onshore and offshore phosphates. The 

 U.S. Bureau of Mines has tested a prototype bore- 

 hole mining tool onshore. For mining, the tool is 



lowered into a predrilled, steel-cased borehole to 

 the ore. A rotating water jet on the tool disintegrates 

 the phosphate matrix while a jet pump at the lower 

 end of the tool pumps the resulting slurry to the 

 surface. The slurry is then transported to a benefici- 

 ation plant by pipeline. The resulting cavity is back- 

 filled with sand to prevent subsidence. 



Results of economic feasibility studies of using 

 the borehole mining technique onshore show that, 

 where the thickness of the overburden is greater 

 than 150 feet, borehole mining may be more eco- 

 nomical than conventional surface mining systems.^ 

 An elaborate platform would be required for min- 

 ing offshore deposits, so capital costs are expected 

 to be higher than for onshore deposits. Borehole 

 mining of phosphate appears to be less destructive 

 to the environment than conventional phosphate 

 mining techniques and, if used offshore, would 

 probably not require backfilling of cavities. 



Solution mining also has been mentioned as a 

 possible technique for mining offshore massive sul- 

 fides. Significant drawbacks include the application 

 of chemical reagents capable of leaching these sul- 



'D.E. Morse, '"SuMut" Mineral Facts and Problems— 1985 Edi- 

 tion, Bulletin 675 (Washington, DC: U.S. Bureau of Mines, 1986), 

 p. 785. 



°J.A. Hrabik and D.J. Godesky, "Economic Evaluation of Bore- 

 hole and Conventional Mining Systems in Phosphate Deposits," Bu- 

 reau of Mines Information Circular 8929, 1983. 



