Can't speak to those mines in specifics, but copper heap leaches...

Can't speak to those mines in specifics, but copper heap leaches (typically 10 g/L sulphuric or pH 1) are a good analogue to RDM's current pH 1 test work. For copper, Fe³⁺ is an issue but is removed by neutralising to pH 3 with limestone (similar to REE). From there, Fe²⁺ and Al³⁺ aren't an issue as Cu can be recovered over them with solvent extraction (not the same as REE). For uranium, U can be selectively recovered from either acidic or alkaline leaching with ion exchange, and over Fe and Al.

So REE has a unique challenge in that there is currently no direct extraction method that has been developed to extract REE over Fe/Al (like solvent or ion exchange for Cu and U). The main method is sequential precipitation - this is a good paper that discusses it https://www.sciencedirect.com/science/article/abs/pii/S0304386X20301134. The only ASX co to release any hard data on impurity removal in clay processing is VTM - https://hotcopper.com.au/posts/70739725/single, they were able to make a half decent MREC from pH 1 leaching but they had about half the head grade of RDM and about a quarter of the REO leach extraction.

Broadly. there are two impacts from Al and Fe leaching with the REO:
1. Need to add an alkali to remove the Al and Fe (OPEX). I account for this in my economic models using MgO as the alkali and associated pricing (as well as accounting for utilisation %, purity of the alkali etc). IMO this isn't a huge issue for RDM as can be seen in the results of my economic models, although lower OPEX will make the economics even stronger.
2. Some REO are lost when you remove Al and Fe. I assume 10% downstream losses in my economic models to account for this, but this is an unknown that we will hopefully find out about this quarter from the met testwork. Could be better, could be worse.