In certain unique geographical regions, such as areas of southern China, there has occurred an interesting rock weathering and chemical phenomenon that creates REE-rich clay material. There are two major components to this phenomenon:
- Silicate rocks transforming into a special type of clay called “ion-exchange clay”, “ion-adsorption clay” (IAC), or “IX clay”.
- Decomposition of the contained trace primary REE bearing minerals without the reforming of secondary REE minerals. This leads to dissolved mobile REEs.
As these dissolved REE migrated through the IX clay, they eventually adsorb into a narrow somewhat horizontal layer or band, and increasing in concentration as they do so. The REE can be re-extracted, this process is called ion exchange.
The following steps outline this production technique:
- Mine the narrow band of IX clay
- In a large vessel, make a water slurry of the IX clay
- Add the appropriate amounts of ammonium sulfate or equivalent
- Allow sufficient time/agitation for the ammonium cation to replace the REEs adsorbed in the clay
- Separate the REE containing liquid from the now REE-barren clay
- Add the appropriate amount of oxalic acid or equivalent to the REE containing liquid
- Allow sufficient time to precipitate the REE oxalate
- Filter off the REE oxalate
- Fire the REE oxalate (at ~600° C) to the hydrated oxide
- Sell product to SX plants as feed
Although the IX production technique is very simple, this route to REE production was only developed about 30-40 years ago by the Chinese, and is a great credit to them.
Further, a few points to note when discussing potential IX resources:
- The standard REE assay procedures do not apply, as they can overestimate values due to measuring all the contained REE, which may additionally exist in non-weathered minerals. One must use ammonium sulfate IX leaches, as developed for production processes, to obtain the true extractible REE grade.
- Although the total land surface area of IX material in China is very large, the grade is very low: approximately 0.2 wt% or less.
- Large amounts of land disturbances and potential environmental degradation can occur.
- The REE distribution in the product is dependent upon regional geology, with several products with different distributions readily available within China. Essentially all current heavy REE production is from these sources.