Solution mining

(mining engineering) The extraction of soluble minerals from subsurface strata by injection of fluids, and the controlled removal of mineral-laden solutions.

The extraction of the valuable components from a mineral deposit using an aqueous leaching solution. In its original sense, solution mining refers to evaporite mining, the dissolution of soluble rock material such as salt by using borehole wells to pump water into the deposit and remove the resulting saturated brine. In its current usage, solution mining also includes ore leaching, the in-place (in-situ) leaching of valuable metal components from an orebody, and the mine-site procedures of heap leaching and dump leaching. Often included is the Frasch process of using superheated water to melt sulfur in its deep deposits and recover the molten sulfur in borehole wells. See also Ore and mineral deposits.

Evaporites represent a broad class of water-soluble minerals (salts). Commercially important evaporites include halite (sodium chloride), sylvite and silvinite (potash), bischofite (magnesium chloride), nahcolite (sodium bicarbonate), trona (raw soda ash), and langbeinite and carnallite [both sources of potash and magnesia (magnesium oxide)]. See also Saline evaporites.

Solution mining involves injecting a solvent into the pay zone of the deposit through a cased borehole. For evaporites, the solvent is hot water, which forms brine as the soluble minerals dissolve. The brine is brought to the surface via the casing system in the same or another borehole and sent to a processing facility for recovery by the controlled crystallization of the desired product, followed by dewatering and drying. Some minerals may require additional purification steps, such as the flotation of potash crystals. The depleted brine is chemically reconditioned and injected back into the deposit. Thus, solution mining creates minimal surface disturbance and little waste, compared to conventional mining. See also Well.

Although solution mining is simple in principle, there are several key issues for successful operation. One is to maintain close control over the solvent parameters, such as pH, oxidation potential, temperature, and pressure. Another issue is isolation of the solution mining zone from the surrounding geologic structures. This requires effective well completion techniques that are compatible with the brine. Once the well is drilled and cased, the casing must be cemented into the formation. This prevents brine from migrating along the annular space outside the casing and contaminating adjacent aquifers. See also Electrochemical series; Oil and gas well completion.

Leaching large stockpiles and mine waste dumps (ore heaps) annually accounts for about one-third of both new copper and gold production in the United States, the world's second largest source of both commodities. Ore leaching is an important contributor to the total world production of these commodities, along with silver. Ore leaching's very low unit processing cost, combined with low-cost earthmoving, allows profitable treatment of huge tonnages of low-grade material that could not otherwise be exploited.

Ore containing rock must first be fragmented to allow chemical leaching solutions to percolate through it. The metal-bearing minerals are liberated using an aqueous wetting fluid that penetrates and saturates the microfractures within the ore particles by capillary action. The ore minerals, originally deposited geochemically from hydrothermal solutions, are typically located in these micropores. Once dissolved, the metals are removed from the ore particles by diffusion through the solution-filled micropores and swept out of the ore heap by the flowing leachate. In effect, nature's ore deposition process has been reversed at a quicker pace. Nevertheless, commercial leaching times are measured in weeks, months, and even years. Leachates, or “pregnant liquors,” are processed in surface plants to recover the metal and regenerate the leachant for reuse in the ore heap in a closed-loop system. See also Hydrometallurgy; Leaching.

Modern sulfur mining dates from the invention of the Frasch process in the late nineteenth century. H. Frasch accomplished in-place sulfur mining by using superheated water. Frasch mining begins by drilling wells into the sulfur deposit. Steel tubing (casing) is run into the drill hole to case off the barren overlying formation, and it is cemented in at the top of the sulfur deposit. Within this casing, three concentric strings of pipe are set within the sulfur deposit. Superheated water at 325°F (163°C) can be pumped down the annulus between the two strings of pipe to leave the casing string through the perforations above the packer, and to circulate in the sulfur deposit. Native sulfur melts at 275°F (135°C) in a cone-shaped area of influence around the well, and because liquid sulfur is denser than water, it settles to the bottom of the well.

The pressure from the heated water forces the molten sulfur into the lower set of perforations and into the inner string of casing. The molten sulfur rises to its hydrostatic head. Compressed air pumped through this inner tubing expands when it leaves the tubing, thus “jetting” the sulfur to the well collar, where it is collected in a tank (sulfur pan).

A mining technique whereby low-grade ores are injected with a solvent. The solvent leaches out the metal in solution which is then pumped to the surface. The metal is then extracted from the solution.