On 8 October 2026, Ark Mines Ltd (ASX:AHK) reported initial results from a scoping metallurgical program completed by the Australian Nuclear Science and Technology Organisation (ANSTO) on monazite concentrate from its Sandy Mitchell Rare Earth Elements and Heavy Minerals Project in North Queensland. The headline result was light rare earth extraction of up to 98% to 99% from the concentrate.

The concentrate tested

The tested concentrate assayed 48.7% total rare earth oxide (TREO), or 50.2% total rare earths plus yttrium. It was made by combining four rare earth products generated by IHC Mining in Brisbane from a heavy mineral concentrate produced by Currumbin Minerals, using a composite bulk sample of retained one-metre resource drilling samples from within the Sandy Mitchell Mineral Resource.

Automated mineralogy showed the concentrate comprises 83% monazite and 0.5% xenotime, with zircon at 11.5% as the main gangue mineral. The monazite is well liberated, with 93% in the 90% to 100% liberation band.

Acid bake and water leach results

Three scoping acid bake and water leach tests were completed. On milled concentrate, light rare earth (La, Ce, Pr, Nd) extraction was 98% at 1,100 kg/t sulphuric acid and about 99.6% at 1,500 kg/t, with dysprosium extraction up to about 99%. Unmilled concentrate returned 95% light rare earth extraction at the higher acid dose.

Yttrium extraction ranged from 93% to 97% across the tests. ANSTO’s work is reported in a progress note dated 18 September 2026.

Caustic conversion route

In a caustic conversion and hydrochloric acid leach test, phosphate removal was 97% and light rare earth extraction was approximately 98% at pH 2 to 3. Ark said the monazite concentrate is amenable to both the acid bake and caustic conversion routes, with the aim of producing a mixed rare earth carbonate (MREC).

Limits of the testwork

The release states that the results are scoping-level tests on a monazite concentrate and that the recovery of monazite from ore to concentrate is not part of the program. A large-scale acid bake test is under way to generate leach liquor for Downstream processing, and the company said its focus now moves to impurity removal ahead of producing a rare earth product.

Sector backdrop

Monazite is a rare earth phosphate mineral, and processing routes such as acid bake and caustic conversion have been used in the industry for decades. Extraction percentages at the leaching stage are an early indicator, and overall project recovery also depends on Upstream concentration, impurity removal and product precipitation.

Rare earth developers in Australia commonly use ANSTO and similar laboratories for scoping metallurgical work, with larger pilot-scale work following if results support it.

Risks shareholders should weigh

The testwork was done on a concentrate produced in advance from drilling samples, not on run-of-mine ore. The company says recovery of monazite from ore to concentrate is not part of the program, so overall recovery has not been demonstrated.

Results are from a small number of scoping tests. Unmilled material gave lower extraction, and the highest figures required a high acid dose of 1,500 kg/t, which affects reagent consumption and cost. Impurity removal and downstream product quality are still to be tested.

Radioactive elements and waste handling, approvals, funding and rare earth prices are further factors for any project at this stage. Sandy Mitchell is also at a study stage, and the release does not provide economics.

What’s next

Ark said a large-scale acid bake test is under way to generate leach liquor, and that the focus is shifting to impurity removal before precipitating a rare earth product. Shareholders may wish to watch for the larger-scale results, MREC product data and further study phases for Sandy Mitchell.