Sustainable Ultrasound-Assisted Extraction and Recovery of Rare Earth Elements from Oil and Gas Drill Cuttings

Drill cuttings (DC) are mainly composed of fragmented rocks that are produced during the drilling of oil and natural gas wells. The rocks and minerals present in DC are sources of several chemical elements, including economically valuable elements, such as rare earth elements (REEs). In this paper the sustainability and advantages of the ultrasound-assisted extraction were applied for the extraction and recovery of REEs from DC. To determine the optimal extraction conditions, a full factorial and Doehlert matrix design was employed. The DC were characterized by XRD and EDXRF, and the REEs concentrations were determined by ICP-MS. The REEs recovery was performed by precipitation with oxalic acid. The analyzed DC samples were primarily composed of aluminosilicate minerals, the α-quartz, calcite, albite, and muscovite being the most common phases. The EDXRF analysis confirms the predominance of silicate and aluminosilicate rock-forming minerals and accounts for the significant content of Fe, Ca, Na, and Mg. The optimal extraction condition was achieved using HNO₃ at 7.0 mol L^-1 as extractor and ultrasound-assisted extraction at 80 °C for 60 min. The efficiency of extraction in the DC analyzed was high (for La and Ce, >82%), and after precipitation, good recoveries were achieved, especially for La and Ce (>90%). Thus, the proposed ultrasound-assisted extraction process proved to be suitable for the recovery of these elements from DC, contributing to sustainable development and the circular economy.