Borehole mining as a viable concept for mining deep mineral resources has historically been long on promise but short on results. Application of this technology has been of particular interest in the east coast deep middle miocene phosphate deposits. These deposits are potentially enormous in scope and underlie sections of the coastal plain and the near offshore continental shelf.
The landslide portion of these deposits are generally too deep to mine using open pit methods, forcing development of alternative mining methods. A significant proportion of the offshore resources appear to occur at depths below the practical limits of cutter suction dredges and even exceed the limits of bucket ladder dredges. In most cases these deposits are covered by overburden that has to be removed and redeposited before the mineral section can be mined. The depth, thickness, composition of overburden, and environmental sensitivity of a particular location greatly impact mining system selection. It is probable that conditions will exist on portions of the continental shelf that will eliminate dredge mining as a choice for extracting a particular deposit. In these cases new processes such as borehole mining must be developed or the mineral resource will remain undeveloped.
A successful borehole mining test program for extraction of these deep phosphate resources was conducted in northeast Florida in 1984-1985. Based on the results of this test program, plus subsequent laboratory testing, and engineering studies a prototype borehole mining system was designed in 1986. The design contained several innovations that appeared to offer higher productivity and lower power consumption than the test unit actually used. Economic restraints, at that time, prevented the actual construction and testing of this unit.
Extracting mineral values through a borehole is a very old process. Oil wells, brine solution mining, frasch sulphur recovery, etc. are only a few of the borehole recovery systems that have a major economic impact on our society. The extraction of mineral values in slurry form through a borehole has, as yet, not been as successful.
Some of the early work in borehole slurry mining started in the 1920's. Periodically new attempts have been made to develop this technology as specific requirements occured. Unfortunately, no major commercial projects have been developed as a result of this work.
The deep middle miocene phosphate resources generally occur from North Carolina to south Florida. Active test efforts were made to extract these phosphates on shore in North Carolina and Northeast Florida through a borehole during the later part of the 1960's. The cyclic nature of the phosphate industry, unfortunately, always seemed to start a down trend at times most inappropriate for ongoing research programs.
During the 1970's the United states Bureau of Mines (USBM) took an active role in the development of this technology. Test work was conducted on coal, oil sands, uranium bearing sandstone, etc, with varying degrees of success.