Application of solvent extraction method for lithium recovery from shale gas-produced water

Abstract

Shale gas-produced water is a high saline wastewater generated during the development of shale gas. The shale gas development technique is hydraulic fracturing, and this process involves a massive water injection into the shale layer. After the hydraulic fracturing, the mixture of injected water and shale formation water flows back to the ground. This is shale gas-produced water, and it contains a relatively high concentration of lithium originating from the clay mineral of shale rock. Due to the increasing demand for lithium, recently, several studies have focused on seawater, whose lithium content is 0.17 mg/L, as a new source of lithium. The produced water in the Marcellus shale area contains about 95 mg/L of lithium

thus, if selective lithium recovery from produced water is possible, it will be much more efficient than recovery from seawater. Therefore, this study was conducted to examine the applicability of the solvent extraction method for lithium recovery from diluted shale gas-produced water. The total dissolved solids (TDS) level of shale gas-produced water is very high, up to 200,000 mg/L, and the concentration of competitive cations with lithium in extraction is also high. Therefore, multi-stage solvent extraction has been suggested to reduce the effect of competitive cations and to improve the selectivity of lithium. In the first-stage, the divalent cations were removed using di(2-ethylhexyl) phosphoric acid (D2EHPA) in kerosene. In the second-stage, tri-butyl phosphate (TBP) was applied as a synergistic additive with D2EHPA for selective lithium recovery. After the first-stage solvent extraction with 50X diluted shale gas-produced water, about 97.9% of magnesium ions were removed, and all other divalent cations, including calcium, strontium, and barium were removed. In the second-stage extraction for lithium recovery, the aqueous solution obtained after an appropriate number of first-stage extractions was applied. Finally, the highest extraction efficiency of lithium ion after the two-stage extraction was 26.46%. Almost all of the divalent cations were removed from the first-stage extraction, and D2EHPA has a higher affinity with lithium ion compared with sodium ion, so the selectivity of lithium was very high. In conclusion, the multi-stage solvent extraction method can be applied for highly selective lithium recovery.