Print8.4.4: Borehole Mining Method
One hole is drilled into the deposit for the purpose of pumping the solvent into the deposit, and one is drilled at some distance from the other to recover the solute. The solvent flows between the two holes, creating a cavern where the ore was dissolved. For some minerals, such as common salt (NaCl) or trona, water is the solvent. For other minerals, a chemical solvent is required. Interestingly, sometimes these caverns are of greater value than the extracted mineral! In the Louisiana salt domes, for example, the caverns are used to store crude oil and natural gas. Often, multiple holes are drilled within the area of interest. Occasionally, only one hole is drilled, but the borehole is divided into an inner and outer annulus so that the solvent is forced down one, and the solute is retrieved through the other. This is the case for the Frasch process for recovering sulfur. The Frasch sulfur process uses steam to melt and dissolve the sulfur in the water, which is then forced back up the borehole; or, in other cases, the melted sulfur is allowed to drain and collected in subterranean pools where it is then pumped to the surface.