And while I'm no expert about solution potassium chemistry I do...

And while I'm no expert about solution potassium chemistry I do know enough about it to know it's bloody difficult and that even relatively minor changes to the rates of concentration, the concentration of other ions such as magnesium, NaCl and SO4—as exist in virtually all natural brines—, can make a massive difference to the salts that precipitate out. Now I understand that APC have an engineering contract that guarantees SO4 as an output no-one should underestimate how difficult that will be. It, at the very least, means the need to dynamically monitor the salt concentrations in the evaporation ponds.