Securing access to commodities is one of the top list items of industrialized countries’ agendas. With eco-sustainability issues and global changes, modern mining companies are now going to more zero emissions and green eco-responsible extraction technologies to harvest the precious underground metals/commodities and foster economic growth of our unsatiated advanced-technological industry.
The mining industry is often associated with dirty activity involving huge tailings/wastes of barren rocks disposed at the surface of the Earth, excavations of the ground endangering workers, damages on building surfaces, roads, and irreversible environmental impacts [1]. This collective-mind conventional old picture might be changed soon by the past decade’s advances in in-situ leaching (ISL) technologies that are now better mastered by the use of 3D computer modeling technology and chemical simulation, making ISL mining technologies a possible substitute to conventional mining in many cases [2] (Figure 1, above).
First implemented in Wyoming in the 1950s for uranium recovery, in-situ leaching (ISL) or in-situ recovery (ISR) involves injecting chemicals (typically sulfuric acid or ammonium carbonate) called “lixiviants” into porous geological formations that host the ore body (i.e. deposit), recovering the metal/commodities by dissolving them, pumping through production wells the pregnant solution to the surface where metal/commodities are recovered in processing plants, and regenerating the chemical solution (sometimes using biotechnology) for reinjection in wells.
Compared to traditional mining, this technology leaves the ore in the ground and extracts only the metals/commodities of interest, suppressing the huge mining wastes. This technology requires favorable geological conditions (i.e., impermeable layers on top and below the ore body) (Figure 2) to avoid the dispersion of the lixiviant in neighboring aquifers. In the case of uranium deposits, regeneration of exploited ore deposits is thought to be possible after less than 30 years for naturally re-confining possible toxic substances initially associated with the ore bodies but mobilized during exploitation. Groundwater contamination is the critical aspect requiring reagent management during ISL operations. The environmental regulation in many countries is sometimes a limiting factor in the use of ISL as they require that the water quality in the aquifer be restored to its pre-mining use.
The ISL technology (Solvay process) is widely used to extract water-soluble salts, including sodium chloride (halite, NaCl), sulfate (Na₂SO₄), trisodium hydrogendicarbonate dihydrate (trona, Na2CO3.2NaHCO3.3H2O)), bicarbonate (nahcolite, NaHCO3), potash (sylvinite, KCl and carnallite, KMgCl3·6(H2O)), and boron, and is often used for ore deposits that are too deep to be exploited by conventional underground mining [3].
Most uranium mining in the United States, Australia, Kazakhstan, and Uzbekistan is now exploited by ISL. With 46 percent of the annual world production, Kazakhstan is the world’s leading country in uranium mining [4]. In 2021, Kazakhstan extracted about 21,800 tons of U by in-situ leaching (ISL) mining [5]. The capacity of ISL mining of uranium is now superior to that of conventional uranium mines, reaching 57 percent of the world’s production in 2019.
ISL has been successfully developed over the past 20 years for other commodities such as copper, gold, nickel, scandium, rhenium, rare earth elements, yttrium, selenium, molybdenum, and vanadium ([2][3]). As a historical curiosity, the Chinese were probably the first to use solution mining to produce copper by 907 A.D/, and perhaps as early as 177 B.C. ([3], [6], [7]) In the 1970s, ISR was introduced for copper. It is mostly used as low-cost heap-leaching technology on ground ore and then recovered from solution by solvent extraction electrowinning (SX-EW) or by chemical precipitation [3]. There were several successful natural tests and mines such as recently in the Kupferschiefer underground copper mines in the Lubin region (Poland) within the BioMore European Research project ([8]-[10]), the oxidizing properties of the reagent solution was regenerated using bacteria (Biomining).
A recent paper published in the review Minerals [4] had investigated the key chemical parameters and 3D computer modeling for optimizing uranium extraction on a hexagonal grid of wells. Further progress is needed to fully understand the complex mechanisms involved in the dissolution processes underground. However, these recent results show that an improvement of about 20 percent in recovery and mining time can be expected by better chemical modeling. Other commodities are under study such as copper ([8],[9]) and gold [11].
Rare earth elements (REE), rhenium, scandium, selenium, yttrium, molybdenum, and vanadium were also mined in pilot tests as byproducts of uranium extraction but are often limited in practical uses because radioactive particles are often physisorbed on the metal surface ([12] [13]). ISR of copper, gold, nickel, rare earth elements (REE), and scandium has been successfully developed over these last recent years.
With the increasing demand for commodities and rare metals used in advanced space technology, investigations had begun in exploring and exploiting outer space; space agencies have recently renewed their interest in space mining, including ISL biomining [14], and in situ resource utilization (ISRU) [15].
In terms of environment, ISR technology extracts ore preserving existing natural conditions with minimal disturbance. In contrast to open-pit mining and underground, the volumes of hydro-metallurgical effluents and mine tailings are smaller. The critical aspect requiring management during an ISR operations is the possible contamination of groundwater by ISR reagents.
Valuable economic aspects of ISL benefits should also be accounted for. ISL involves lower Capex costs for mining development, processing plant, and infrastructures. A lower capital cost is necessary to start ISL production, allowing a modular increase in production and capacity. The Capex, Opex, and common cut-off grades for ISL differ according to commodities but are lower compared to conventional open pit or underground mining approaches, HS Today writes.
