The first phase is expected to cost $5 billion.
Emal will become "the world's largest single site aluminium smelter" when it reaches its full production capacity of 1.4 million tonnes a year, it said.
Dubal, which is owned by the Dubai government, is the Middle East's largest aluminium producer with a capacity of 761,000 tonnes a year.
In August 2006, it acquired a 25 per cent stake in Canada's Global Alumina Corp for $200 million.
Mubadala owns a majority stake in Dolphin Energy, which is building a regional network to export gas via a submerged pipeline from Qatar to Abu Dhabi, and then on to Dubai, Oman and eventually Pakistan.