The moon has one interesting feature, and it's not colonization.
Aluminum has about the same concentration there as on Earth, but the gravity is significantly lower.
Iron has a slightly higher concentration than aluminum.
A railgun can achieve lunar orbital speed (2.4km/sec).
We have the technology. General Dynamics has a gun that can shoot at 2.55 km/sec.
This technology is more commonly known as a mass driver.
The thought is that a mining operation could use the 14-day light cycle to orbit refined metal or construction components. Since very little propellant would be necessary, a lot of material is attainable. Metal is the heaviest and therefore costliest material to move out of a gravity well.
Proposals like this show a profound misunderstanding of space flight costs. The two principal costs in space flight are the costs of making the space flight hardware, and the cost of maintaining and managing the vast ground-based infrastructure of a space flight program. Launch costs are relatively unimportant, and the focus on launch and orbital velocity changes is completely misplaced.
Currently, with SpaceX, we are at point where we can project $1000/lb launch costs. At that price point, space exploration would be essentially unchanged in its cost structure if launches were free. Any type of aerospace hardware costs several thousand dollars a pound to build. Look at an undemanding commercial system like the Boeing Dreamliner. Here you have a competitive marketplace, well proven technologies and designs, a benign operating environment, and the cost the plane is $1000/lb. Any spaceflight hardware costs an order of magnitude (or more) more than this. The SpaceX Dragon capsule for example weighs 7000 lb, and is expected to have a unit cost around $140 million, of $20,000/lb.
The aluminum on the moon would be extremely expensive aluminum, considering the cost of the fully automated factory that would have to be designed from scratch, built on Earth, launched to the Moon, and installed there. Yet, even if the aluminum produced there were free, it would do little to reduce the real costs of spaceflight.