DFS Summary and Project Economics

Rhyolite Ridge Project Feasibility Study and Project Economics

A Rhyolite Ridge Definitive Feasibility Study (DFS) was completed in April 2020. The DFS placed the Project as a potential global leader for the economic production of lithium carbonate, lithium hydroxide and boric acid.

In October 2025 Ioneer published a technical report summary which serves as an update to the 2020 DFS.

Subsequently, Ioneer announced it had achieved material increases in lithium and boron production by decreasing leach retention time from three (3) days to two (2) days, and to one and a half (1.5) days. Throughout 2025, Ioneer has focused on increasing lithium yield (the quantity of lithium carbonate produced per tonne of sulphuric acid consumed) and optimising reagent efficiency.

For the first 25 years of operations, lithium carbonate equivalent (LCE) production at Rhyolite Ridge is now estimated to be 24,500 tonnes, and annual boric acid production of 135,500 tonnes. The estimates for 1.5-day leach retention represent annual production increases of approximately 30% for LCE tonnes and 15% boric acid tonnes, compared to 3-day leach retention.

These updated findings position Ioneer, on a LCE basis, in the lowest cost quartile for lithium production globally with an estimated all-in sustaining cost (AISC) per LCE tonne of U.S. $4,628 – net of expected boric acid revenue – in the first 25 years. The unique mineralogy at Rhyolite Ridge, including co-production of boron, allows for the Project to remain globally competitive in various lithium pricing environments.

 

The Updated DFS confirms that the Project has a stable overall operating cost structure for the production of lithium carbonate and battery-grade lithium hydroxide due to the scale and reliability of its boric acid credit. The extensive bench and pilot scale tests conducted by Fluor, Kemetco Research and Kappes Cassiday, with support from Veolia and FLSmidth, has proven highly successful with excellent recoveries, the innovative use of proven processing technologies, and the production of high-quality, high-purity lithium and boron products.

Ioneer and its engineering partner, Fluor, have refined Project plans over the past four years and updates now include an Association for the Advancement of Cost Engineering (AACE) Class 2 capital cost estimate (-10%, +15%) with approximately 70% of the Project’s engineering complete. As a result of this and other engineering work including RAM analysis and detailed engineering design, Ioneer has adopted a more conservative approach to plant availability, equipment downtime and maintenance scheduling.

The updated Ore Reserve is now estimated at 266 million metric tons, containing 2,039 kt of Lithium Carbonate Equivalent (LCE) and 7,761 kt of Boric Acid Equivalent (BAE).

The table below outlines key project metrics originally published in Ioneer’s announcement on
October 29, 2025 – “Further Leach Optimisation Enhances Project Economics”.

 

Footnotes

^[A] AISC incorporates all C1 cash costs, sustaining capex and estimated interest on DOE Loan

^[B] C1 cost includes raw materials, labor, utilities, maintenance materials, supplies, outside services and overburden storage costs

^[C] The DOE Office of Energy Dominance loan is comprised of $968 million in principal and $28 million in capitalised interest and has conditions to first draw. See Company Announcement titled, “Rhyolite Ridge Lithium-Boron Project Closes US$996 Million DOE Loan Guarantee” dated January 17, 2025.