"Where did the figure of 30 years for the life of the mine some from??"

It came from the raising of annual production. They raised the production to make the mine appear more profitable per annum. This is an admission that with lower production they would be paying a large amount of the profits for the interest and capital from the mine construction.

Interest payments over a long time are a killer of many a venture.

Real production rate would have to be lower because there is only so much that the north American market can absorb without bringing the price down.