what are the risks?

We know that a successfully operating processing plant will almost completely de-risk this business.
What are the chances of this NOT happening?

There could be:-
1) Process failure.
Very unlikely, as the process has already been proven successful on a smaller scale pilot plant.

2) Delays.
Unlikely, as recent announcement confirms that commissioning is on track for October

3) Global Recession.
As the business involves recycling of previously valueless aluminium waste products into saleable commodities, and prevents their on-going landfill costs - as long as aluminium smelting continues somewhere on the planet, this business will be recession-proof.

The upside of MHM has already been described by other posters.
IMO, this company represents one of the best risk/reward plays currently available.