I wouldn't have a clue what could be a bottle neck but it could be many different parts.
Only guessing (not intelligent answer) that it could be the Thickener Tank or more piping required or larger pumps etc
Not unusual for to have more than one thickener tank. An alumina plant has dozens of them for example.
It would be reasonable to assume that the design has excess capacity in that it can be added to, not necessarily the current plant has excess capacity (not for 18,000 tpa) if you get my drift.
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