Ion adsorption clay (IAC) deposits accounted for approximately 35 per cent of China’s total rare earth element (REE) production as of 2009. They are the dominant global source of heavy rare earth elements (HREE). Commercial In situ Recovery (ISR) of REE from IAC deposits has been demonstrated outside China, at MCRE Resources’ Gerik Mine in Malaysia since 2023. The Gerik operation is reported to use Chinese-transferred technology and the same ammonium sulfate lixiviant as Chinese operations. A cohort of ex-China projects is now advancing through feasibility level studies. These include a greenfield wellfield ISR development in Brazil using an alternative lixiviant, and heap leach developments in Brazil and South Australia. This paper compares these technologies on a techno-economic basis. This paper compares REE ISR with the mature uranium ISR industry. Both use injection and recovery wellfield architecture, but the two processes diverge in host geology, lixiviant chemistry, hydrogeological containment and post-operational remediation. Uranium ISR targets confined and permeable sandstone aquifers. It uses strong oxidising acid or alkaline lixiviants, within regulatory frameworks that require aquifer restoration toward pre-mining groundwater quality. Reagent consumption ranges from approximately 8–80 kg acid per kg U depending on host geology, with wellfield solution recycle rates running at above 90 per cent. REE ISR instead operates through mild cation exchange within unconfined, near-surface saprolite profiles, using dilute salt solutions at materially lower reagent intensity. These saprolite deposits are shallower and lower-pressure. They present a distinct risk and regulatory paradigm, for which limited ex-China commercial precedent currently exists. Lixiviant selection is a critical environmental and permitting differentiator. Ammonium sulfate, the predominant Chinese reagent, carries a documented legacy of ammonia-nitrogen contamination. It is also an established driver of clay swelling and slope instability. Magnesium sulfate, the reagent selected for the Ema Project (Brazilian Critical Minerals, Amazonas, Brazil), presents a significantly lower nitrogen-contamination risk. Heap leach is compared here to ISR, as several commercial REE development companies have selected this extraction technique. In 2011 China domestically banned heap leaching of ion adsorption clay on environmental grounds, shifting extraction towards ISR. Serra Verde’s Pela Ema operation (Brazil) and Australian Rare Earths Limited Koppamurra Project (South Australia) are two examples of REE heap leaching currently in development outside of China. This paper weighs heap leach’s lower capital intensity and simpler process control against its conventional surface mining and materials-handling exposure, relative to ISR. Capital intensity and operating cost benchmarking is presented using a normalised flow sheet boundary framework at the mixed rare earth carbonate (MREC) product gate. Bankable Feasibility Study (BFS)-level estimates for the Brazilian Critical Minerals Ema Project are compared alongside published feasibility data for several comparable REE projects: Meteoric Resources’ Caldeira, Australian Rare Earths’ Koppamurra, Mkango Resources’ Songwe Hill, Hastings Technology Metals’ Yangibana, Pensana’s Longonjo, and Defense Metals’ Wicheeda (Defense Metals Corp, 2025; Hastings Technology Metals Limited, 2026; Mkango Resources Ltd, 2026; Meteoric Resources NL, 2026).