Having started up its mixed rare earth carbonate (MREC) demonstration plant late in May, ASX-listed Viridis Mining and Minerals has achieved steady-state continuous production of MREC, with July's mixed rare earth oxide (MREO) recoveries having exceeded 80%.

The plant has been materially outperforming the metallurgical recoveries adopted in the prefeasibility study (PFS) of the Colossus project, in Brazil, for which the demonstration plant is an important derisking tool, Viridis confirms.

The average July recoveries of 79% MREO and 64% total rare earth oxide compare with PFS assumptions of 76% and 57%, respectively, which shows the potential for meaningful project economics improvements as these enhanced recoveries are incorporated into a definitive feasibility study (DFS).

The demonstration plant replicates the flowsheet of a proposed commercial facility and is designed to process 100 kg an hour of clay feed.

Viridis points out that further demonstration plant optimisation works are ongoing, with several process enhancements and operating initiatives yet to be implemented. These will further improve the plant's metallurgical recoveries.

"The demonstration plant has operated reliably on a continuous 24-hour basis, leveraging fully automated programmable logic controller control systems, integrated water and reagent recycle circuits and an on-site laboratory to optimise feed blends, ore types, pH operating windows and other key process parameters influencing rare earth recoveries," Viridis explains.

The company is also progressing product qualification work with its strategic offtake partner Solvay in France.

Viridis has been sourcing run-of-mine feed for its demonstration plant from the Northern Concessions, which underpin the early years of the company's flagship Colossus mine plan. The demonstration plant is already one of the largest known semi-industrial, continuous operation plants outside of China.

"Validation of the process flowsheet under continuous commercial-style operation has confirmed the robustness and repeatability of MREC production, enabling us to finalise key process design criteria and several capital and operating cost decisions for the DFS," says Viridis MD Rafael Moreno, adding that the company will advance towards a final investment decision on Colossus in the coming months.