Well-Field Mechanics for In-Situ Mining
Over the last several decades in-situ leach mining has become increasingly popular. Conventional mining produces tailings, runoff, and considerable land disturbance—all requiring significant rehabilitation. With in-situ leach mining methods (see diagram below), disturbance is reduced because only multiple boreholes are drilled for recovery. Rehabilitation is much simpler, consequently, the number of in-situ mining operations is steadily increasing. In-situ leach mining can be done to extract a variety of different products including potash, uranium, copper, gold and silver.
Extraction can be accomplished using one of two methods; the well-to-well recovery method discussed here, or stope leaching, a process where broken, low-grade ore is leached from caved areas within former underground mine workings.The first large-scale implementation of the well-to-well recovery method for copper was completed in 1994 at the San Manuel mine in Arizona. Most in-situ mining projects are operated at sites with small ore deposits, lower-grade ore deposits, or where ore bodies are deep.
Distance from drinking water supplies or environmentally sensitive ecosystems is also considered. Sites with large deposits can be divided into several smaller sections that are mined one at a time, allowing the operator to optimize the process.
Basic to an in-situ mining operation is a thorough understanding of the site's hydrogeology, particularly the degree to which fluid movement can be predicted and controlled. The hydrogeology of each site is specific; in-situ mining may not be practical for every ore deposit. In determining feasibility, important questions to be answered include: Is the ore body deposited in the saturated zone with sufficient available drawdown? Do the upper and lower confining units of the aquifer provide enough vertical confinement for the lixiviant (leaching solution)? Is the formation's hydraulic conductivity high enough for wells to achieve reasonable well productivity and injectivity?
Well productivity and injectivity are directly proportional to the values of formation transmissivity. Hydraulic conductivity, defined as transmissivity divided by aquifer thickness, is generally used as the measure for well productivity and injectivity. In a typical well-field setting, in order to maintain a minimum well flow of 10 to 25 gallons per minute, a hydraulic conductivity of one foot per day would be considered the minimum value suitable for in-situ mining.
The storage coefficient, a ratio of the water pumped to the volume of cone of depression, is important when estimating the radius of influence of pumping and injection. Typically, unconfined and confined aquifers have storage coefficients ranging from 10 to 25 percent and from 0.001 to 0.1 percent, respectively.
Once hydrogeologic feasibility of ISR has been determined, engineers can increase the economic feasibility of the operation and minimize the associated environmental effects by addressing three major aspects. Recovery process design influences how efficiently the target ore mineral is removed; well-field design optimizes resource recovery and containment; and monitoring programs provide baseline data and detect potential leakage from the site.