I can't seem to find the post which had about 20 links i once posted. ahhh found it. Post #:58007264 was on ITM forums when providing a rundown to people there. That's a good overview and explains the precipitation process.
""According to Bradbury and Baeyens17 as well as Piasecki and Sverjensky,18 for acidic and near‐ neutral conditions (pH < 6.5‐6.8), most of the surface‐adsorbed lanthanides occur as simple or hydrated cations such as “clay‐REE” or “clay‐REE(H2O)n” species derived from straightforward cation‐exchange reversible reactions at the permanent negative charge sites on the clays (physisorption); for pH > 7 the prevalent form is the hydrolyzed “clay‐O‐REE2+” species derived from permanent complexation reactions at the amphoteric surface hydroxyl groups (chemisorption) or soluble carbonate/bicarbonate complexes.
Due to various weathering conditions (i.e. nature of host rocks, water and soil pH, temperature, pressure, redox conditions) there are three main categories of REE present in the ion‐adsorption clays, as described by Chi and Tian8 :
1. Colloid phase: REE deposited as insoluble oxides or hydroxides or as part of colloidal polymeric organometallic compounds. These species have low occurrence in ores at the slightly acidic natural conditions and can be recovered only by acid leach.
2. Exchangeable phase: REE occur as soluble free cations/hydrated cations or part of positively‐charged complexes in solution adsorbed species on clays. These species account for 60‐90% of total content of rare earths in ores and can be recovered by ion‐exchange leaching with monovalent salts.
3. Mineral phase: REE part of solid fine particles with same mineral matrix as the host rocks (REE part of the crystal lattice). This phase usually accounts for the balance from the ion‐exchangeable phase towards the TREE content and can be recovered only by aggressive conditions (alkaline bake and acid leach)."
Number 2 is where you need to be. All 3 can be present in the clay. The fact it is kaolinite or halloysite doesn't preclude that it all or even some will be in the exchangeable or ionic phase. Post #:54369497
The extraction of RE elements from the ore body is an ion exchange process. With the injection of the leaching solution, RE cations absorbed on the ore body will be replaced by more active cations in the solution seepage process. The main chemical reaction is shown below.
The exchange between the RE cations and more active cations in the leaching solution follows the law of equivalent exchange, in which the equal amount of charges is exchanged. This process starts with the diffraction of cations in the leaching solution through the diffusion layer on to the ore particle surface. RE cations will then be replaced and eventually be extracted from the mother solution.
In this equation, RE3+ is the RE cations, SEn+ is cations with relative active chemical properties (i.e. NH4+, Na+, Mg2+, Al3+).
The key to how it recovers it not just how much you recovery it's what the concentration of the solution is.
If attempted to leach 1L cup of coffee (lets call it 900ml of water and 100ml of coffee equivalent) this is like a REO deposit at 10% TREO. and i recovered 90% it means that i recovered 90% of the 100ml of coffee. i.e. 90ml.
if that cup of coffee still contains say 800ml of water and it contains 90ml of coffee its 90/800 = 11.2%. So sure recovered 90% of the coffee but it's still a weak solution. When ionic clays are ionic in nature your recoveries with NaCl, or ammonium sulphate aren't usually that great. maybe 30-40% but it's the fact that the solution post leach is 90%+ concentration. aka, you end up with 90ml of coffee in 100ml of liquid. This is a 90% TREO equivalent and this is where you get the value."
That's probably 10% of what i've shared so people will need to dig more or use keyword specific searches for older posts here and i'm sure you'll find something that relates.
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