That's easy...
In general, metallurgical recoveries almost ALWAYS go up from the PEA assumptions... and that only makes sense, too... because at the PEA stage you haven't yet done any of the work in proving out the specific functions of those methods in separations that you are having tailored to the deposit in later work... and as you do that work to fit the rocks you have into a specific process designed to work just for them to extract more... it obviously should work better. It generally does.
In the specific example: Not at all rocket science to expect to see separations originally "estimated" at 89% and 50% respectively (89% for gold and 50% for silver) later proving out "better" at 91% and 54%... after they've done the work trying to improve the recovery. But, that 2% improvement... still isn't much to write home about ? Does it even make a difference in their bottom line ?
But, what about that 89% to 91% change in recovery factor being touted as "better" ? That is still NOT close to being a particularly good number for gold ? It means that for every 10 ounce of gold they mine... they're going to throw one away ? Yikes. So, the practice as you've presented it, there, appears to be a particular type of scam... one intended to distract you with the "huge" 2% "improvement" in extraction... to deflect your attention from what are actually rather poor values in recovery being reported.
That metallurgical recoveries tend to go up... still doesn't alter the fact that far and away the most common result of the transition from PEA to PFS and BFS... is for "shrinkage" to happen... on the bottom line...
And, I note, you haven't actually addressed what DID happen in the specific instance... in the changes made, altering the economic expectations of that project... as they moved from PEA to BFS level studies ?
So... there's also an obvious error... and a bait and switch there... in what you've posted... as the metallurgical recoveries aren't "the PEA numbers" that matter most... or "the BFS numbers" that matter most... but are just one number that goes into computing the bottom line numbers that matter. The key element... (and one which is vastly LESS of an issue in gold and silver than it is in graphite)... is that as you DO all that additional work in separations (which you expect will result in showing better results in extraction after more work is done)... what does it COST to do that work... relative to the value being added ? Was their reported 2% improvement in metallurgical recovery at their BFS level... an indication of a project that had a BFS that came out 2% ahead of prior economic projections ?
And, what we're seeing at ZEN... ALREADY... is those same numbers in aggregate aren't "improving"... the way you thought they had in the example you used...
And, they WON'T be improving... because of the nature of the errors in the assumptions ZEN has already touted... as others have already pointed out.
If ZEN were claiming they'd only get 15% extraction... thus over-estimating likely process losses if the "standard" was typically much higher... then, they'd have a built in expectation of "doing better" in a more realistic analysis done later...
But, ZEN has instead gone the other way... grossly over-estimating yields and values early in the process... not under-estimating them... while grossly under-estimating more than one aspect of how important and difficult separations and processing would be, and how much they'd be likely to cost...
What WILL HAVE TO happen... as a closer look than in the PEA level effort has you look at bringing those PEA level errors into focus... so that the pumpers can't ignore them they way they're trying to now... by claiming to not understand the issues ?
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