Rainbow Rare Earths' latest inferred mineral resource for the project was 38.3 million metric tons, larger than the original 35Mt expected from the gypsum stacks.

Neodymium and Praseodymium oxide represent 29.1% of the total contained rare earth oxides, with an in-situ value of US$95/t gypsum, said the company.

"The Maiden Mineral Resource Estimate for Phalaborwa marks an important milestone for Rainbow, underpinning the opportunity for the rapid development of a low capital and operating cost, high-value processing facility in South Africa," said George Bennett, chief executive.

He added that the fact that the rare earths are present in a chemical rather than mineral form in the gypsum stacks removes the need, as with the majority of global rare earth projects, to firstly produce a mineral concentrate before undertaking a complex and energy intensive 'cracking' process. 

"The initial Phalaborwa process will deliver a high value cracked rare earth carbonate and studies are ongoing to investigate whether the valuable rare earth metals can be delivered as separated rare earth oxides for direct sale to industrial users.  This gives us confidence that Phalaborwa will become the cornerstone of Rainbow's business in the near term".

Bennett added that recently announced metallurgical test work results confirmed that the gypsum stacks are amenable to simple, direct leaching with low-cost sulphuric acid at ambient temperature and pressure.