Was just about to post an explanation myself (after looking over the final Caralue assays) when I saw@setfire2thehive posted, buy I'll add 2ic's 2c.
Assume REE is present as either ionically bonded to clays (elemental form), colloidal particles precipitated from ground fluids (homogeneous non-crystalline) or crystalline minerals (sometimes primary rock, sometimes secondary from ground water). Ionic bonds are easiest to break,so salt and a small change in pH conditions cause the RE elements to dissolve back into fluids from where they originally came (weathering dissolution by ground water originally). Non-crystalline colloidal bonds next easiest to break, so drop pH more acidic some or all colloidal RE's dissolve into leach fluid. Most crystalline RE minerals need more acidic fluid to dissolve, some minerals like monazite need very acidic 750deg C conditions, some carbonates might still drop out at pH=1... it's a lucky dip.
It seems reasonable to assume that most of the ionic bonded RE dissolves at pH=4 and only little bit of colloidal, none of the crystalline minerals. Drop pH and rest of the ionic dissolves, most of the colloidal and maybe some crystalline minerals. Keep lifting acidity and eventually nearly all the RE dissolves with most of the sample... Makuutu recoveries with ammonia salt (best salt results) show between 30-40% basket average of RE is ionically bonded with maybe some contribution form colloidal minerals. Most of the ~45% lift in recoveries with pH dropping to 1 is probably associated with colloidal minerals and some ionic bons that needed lower pH to dissolve. Un-recovered RE likely hosted by crystalline minerals and more resistant colloidal particles.
It's almost certain there are some ionically bonded RE in all the current RE prospects, as there is some colloidal and some crystalline minerals. There's no point getting to caught up in what is what or the definition of what deposit is ionically hosted or otherwise, beyond the RESULTS. The recoveries are all that count, in conjunction with how much it costs to reach those recoveries. Unfortunately ASX companies like to keep as much data hidden as possible, very few I see give markets the range of tests ANSTO conducted or the acid consumptions figures etc. Fair to assume when a company does get good, economic results they will release it in flashing lights with all the data you like.
Keep in mind pH is a log scale, so each single number change is 10 times the acidity. pH=3 is 10 time more acidic than 4... pH=1 is thus 1000 times more acidic than pH=4 (natural kaolinite clay pH is between 4-5 generally). This is why I shake my head when some companies proudly show off recoveries with 10% HCl (ie MEK) with a pH of -0.44 = 44,000 times more acidic than pH=4 ... shareholder ignorance really is an explorer's greatest advantage.
ITM's early recovery tests are not very good and distinctly worse than IXR at Makuutu. No definition of exactly 'low' or 'moderate' acid means. MRD released recoveries using 'weak aqua regia' with great enthusiasm, at least they stated in the very fine print it was 1 mol H2SO4 and 1 mol HCl (pH=0... whih isn't 'weak' in the context of realistic REE leaching from clays or the fact it would strip your skin off).
As stated by SF2TH, anything less than pH=1 will become very expensive very quickly. Acid consumption is the other huge variable. No good having good recoveries at pH=2 if you have to keep pouring 500kg of acid into every tonne of ore.
Add to My Watchlist
What is My Watchlist?
