I've done some more research trying to get a better understanding of what the EPA means by "salt dross" in paragraph 1.1.3 and how this might differ from the material that the MHM facility is capable of processing.
Not that I'm a chemist, but from some googling it seems that salt dross is a reference to "black dross", which MHM can process.
Black dross is formed during the secondary refining process (i.e. the recycling of aluminium) which uses relatively large amounts of Chloride salt fluxes. That explains the high concentration of potassium chloride and sodium chloride. These are some of the key contaminants that the EPA is interested in.
In other words, despite Alcoa's claims that the Moolap landfill is "closed" (="sealed"?), the EPA seems to have its sights set on it to ensure that it isn't leaking chemicals.
One resource I found online suggests that "Although salt cakes are sealed to prevent from leaching, the potential for leaks exists and in fact does occur which harms the environment."
This resource also shows that dross is eventually turned into salt cake. And in fact, the two seem to be intertwined concepts, because dross may be trapped within fragments of metal inside the cake.
Are there any chemists out there that are able to back up my conclusions before the markets open tomorrow??
It seems very likely that I'm right:
Have a read of page 1 of this PDF it explains the recycling process and the relationship between dross and salt cake:
https://www.wpi.edu/Pubs/ETD/Available/etd-010612-155135/unrestricted/cdai.pdf
(20min delay)