London-listed Rainbow Rare Earths has recorded positive results from the ongoing phased test work programme at its Phalaborwa project, in South Africa.

The test work is being conducted in conjunction with Australia-based critical and strategic metals processing expert ANSTO Minerals and US-based processing technology developer K-Technologies (K-Tech).

The results of the test work are enabling Rainbow to develop an economic rare earths extraction flowsheet as part of the feasibility study for the project.

Results, to date, have provided the company with additional optimisation opportunities to explore, which can reduce both operating and capital costs for the project.

The next phase of the test work programme is now under way and includes a number of trade-off and project optimisation studies.

Key findings include strong recoveries and optimisation opportunities.

The test work at ANSTO has confirmed that a simple acid leaching process is expected to allow 65% to 70% of the rare earths contained in the Phalaborwa gypsum stacks to be recovered in solution, with an average 66% leach recovery reported from preliminary metallurgical variability test work.

The K-Tech purification and separation desktop study has confirmed the ability to deliver separated rare earths with over 99% purity oxides from the leach solution. Phalaborwa will be unique in producing separated neodymium and praseodymium oxide, dysprosium oxide and terbium oxide on site.

This will allow the full value of the rare earths to be realised – a 47% increase in revenue over the expected sales price for a mixed rare earth carbonate.

Trade-off studies have started at both ANSTO and K-Tech to determine the optimal method to maximise the grade of rare earth elements in the leach solution prior to the final K-Tech separation step, while managing the build-up of impurities present in the gypsum stacks that could impact overall rare earth recovery.

Increasing the leach solution grade by recycling the leach solution will significantly reduce both operating and capital costs.

Moreover, the test work has indicated reduced capital and operating costs, with flexibility in terms of project development.

The phosphogypsum will be hydraulically reclaimed from the stacks and pumped into the processing facility, reducing the cost per tonne compared with traditional hard-rock mining.

Nano filtration will successfully recycle over 60% of the sulphuric acid required to recover the rare earths to the leach solution, reducing operating and capital costs and minimising leach solution flows into the downstream K-Tech separation circuit.

Potential has been identified for phased development, providing versatility.

Sulphuric acid remains the lixiviant (leaching liquid) of choice following this test work, owing to the low cost and availability of the reagent in the local area, and the simpler materials of construction requirements.

With a slight elevation to the temperature of the sulphuric acid leach solution, recovery is optimised with a 12-hour leach residence period, providing significant operating and capital cost savings compared to the initial 24-hour leach residence period originally envisaged at atmospheric temperature.

The test work also shows environmental benefits.

Water neutralisation test work has confirmed the ability to treat the existing water from the stacks and reuse it in a closed circuit as plant process water, thereby reducing the substantial legacy issue of acid water from historic work and reducing overall freshwater use in the flowsheet.

Very low levels of radioactivity have been confirmed within the gypsum extracted, significantly below the International Atomic Energy Agency guidelines, therefore exempting the project from regulation pertaining to radioactivity.