Ark Mines Tests Show Up to 99% Rare Earth Recovery at Sandy Mitchell Project

ANSTO testwork delivers up to 98-99% rare earth extraction from Sandy Mitchell

Rare earth extraction rates above 98% sound impressive, but the detail behind them matters. In initial scoping ANSTO testwork, Ark Mines Ltd (ASX: AHK) reported light rare earth (LRE) extraction of 98% to 99% from milled monazite concentrate from its Sandy Mitchell Rare Earth Elements and Heavy Minerals Project in North Queensland.

The tests were completed by the Australian Nuclear Science and Technology Organisation (ANSTO) and announced on 8 October 2026. Acid bake and water leach of milled concentrate delivered the LRE result, with dysprosium extraction up to 99%.

Caustic conversion removed 97% of phosphate, and a following hydrochloric acid leach delivered approximately 98% LRE extraction. In plain terms, the concentrate responded to conventional processing methods, which is a useful technical step towards a mixed rare earth carbonate (MREC), an intermediate product containing the rare earths.

Ben Emery, Managing Director

“…Achieving light rare earth extractions of up to 99.6% indicates our high-grade monazite concentrate responds to conventional processing. Additionally, the fact that it is amenable to both the acid bake and caustic conversion routes gives us flexibility as we move towards producing an MREC…”

The announcement’s highlights cite 98% to 99%, while the results table and the quote above show up to ~99.6% at the higher acid dose.

What the scoping tests showed

Concentrate quality

The concentrate assays 48.7% TREO (total rare earth oxide), or 50.2% TREO+Y including yttrium. Automated mineralogy (QEMSCAN) showed 83% monazite and 0.5% xenotime, with zircon (11.5%) the major gangue (unwanted) mineral.

Liberation, meaning how well the target mineral is freed from other minerals, was strong. 93% of monazite reported to the 90% to 100% liberation band.

The sample was a combination of four rare earth products made by IHC Mining in Brisbane from a heavy mineral concentrate produced by Currumbin Minerals. The feed was a composite of retained one-metre drilling samples from within the Sandy Mitchell Mineral Resource.

Acid bake and water leach results

Three scoping tests were run. On milled concentrate (P80 of 43μm), LRE extraction rose from 98% at 1,100kg/t sulphuric acid to ~99.6% at 1,500kg/t, with dysprosium at ~99%.

Unmilled concentrate (P80 of 170μm) returned 95% LRE extraction under otherwise identical conditions, consistent with the acid-monazite reaction becoming kinetically limited at coarser particle sizes. For you, that says grinding the material finer matters to the result.

Source: ANSTO Progress Note 1, 18 September 2026. LRE = La, Ce, Pr, Nd. Acid additions are rounded; actual additions were 1,074kg/t, 1,512kg/t and 1,506kg/t. Caustic + HCl leach LRE extraction is at pH 2-3.

Caustic conversion route

The scoping test used ground concentrate at ~150°C for 3h, followed by a hydrochloric acid leach at 50°C. Key outcomes:

• 97% phosphate removal, the measure of conversion efficiency
• Rare earths remained in the solids
• Maximum LRE extraction of approximately 98% at pH 2-3

Why two processing routes matter for investors

Monazite is a rare earth phosphate mineral, and it must be “cracked” to release the rare earths. Acid bake uses sulphuric acid followed by a water leach, while caustic cracking uses sodium hydroxide to separate the phosphate from the rare earths.

Higher extraction means more of the contained rare earths are recovered from each tonne of concentrate processed. Because the concentrate suited both routes, Ark has flexibility as Sandy Mitchell moves through key study phases, in the words of management.

Sandy Mitchell hosts a basket of heavy minerals:

1. Monazite
2. Xenotime
3. Zircon
4. Garnet
5. Titanium minerals such as rutile and ilmenite

Muscovite and biotite are noted as potential additional products.

ANSTO is a leading Australian Government-owned research organisation with more than 50 years’ experience in minerals processing and extractive metallurgy.

Next steps and what the results do not show

A large-scale acid bake test is underway to generate leach liquor. Further treatment of that liquor is planned to remove impurities ahead of MREC precipitation. The announcement gave no timelines.

Before reading too much into the numbers, note the limits the company itself sets out:

• Results are scoping-level and have not been optimised
• They come from a single composite concentrate sample
• Recovery of monazite from ore to concentrate is not part of this program
• Results cannot be extrapolated to any specific area of the project
• Thorium and uranium are present at levels that require management under radiation safety regulations

Taken together, the ANSTO results are a positive technical step in the Sandy Mitchell development pathway. They are early, scoping-level data on one sample, and you should weigh them as that rather than as evidence of production or project economics.

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