coming from a Coal background myself, that is infact an extremely hard question to answer:
Yield will vary greatly mostly varying on:
-Washability - Specifically, the amount of low density coal versus medium density coal, and medium density reject vs high density reject. Typically, the type of coal can give you a good idea, but you also need to know the type of reject you are dealing with, clays, high sulfar rocks etc. -Breakability - Both of the rock and coal (coal breaking away from rock) and the breakages of coal (into finer size particles) whilst being washed. If you have a low breakability , it means you can wash more in your course circuit and get higher yield. -Capital expenditure - the more you are willing to spend on washing, the more yield you can get. Usually the OPEX will increase also, but not as much (additional staff and pumping, but most equipment in a washplant costs nothing to run e.g. screens, sumps, cyclones)
The way that my company generates an initial yield for scoping studies is by location, coal type and slim cores.
For instance, most coals in the bowen basin are surrounded by similar material, so even if the coal quality is worse, the clays are the same density etc.
Coal type will give a clue to washability (even if not definitive), so we can use a coal washability from another mine of similar coal type and location.
It's fairly accurate, usually within 10% of what the final calculations will reveal, but a lot of this information would be hard to find if you dont have access to previous coal mines washability, core samples, breakability, coal type etc.
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