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The initial concept has been around since the early 1900's, and in the 1970's they figured out how to do it. Nobody wanted to foot the bill to get the project going, even though it would more than pay for itself in the long term.

For anyone interested, the "Space Resources and Space Settlements (1977 study)" online paper from the National Space Society website has lots of detail on this in chapter 4.
http://www.nss.org/settlement/nasa/spaceres/index.html

BTW, in reference to the above argument, asteroids have Phosphorus, so problem solved