MELBOURNE (Reuters) -U.S. aluminium producer Alcoa Corp on Monday made a non-binding, conditional buyout proposal for its Australian joint venture partner Alumina, valuing the company at $2.2 billion in an all-stock offer. Alumina shareholders will also get exposure to the finished product aluminium in a bigger company. “To a very large degree it simplifies the corporate structure,” said portfolio manager Simon Mahwhinny of Allan Gray, adding that for that reason it offers the same or more upside for both sets of shareholders.