15% is generally considered to be the cut off grade used for rock phosphate and it is certainly economic at current prices which is why for example Joseph Gutnik's Legend operation in QLD is now looking to fund a massive $800m capex in order to mine their deposit (near Incitec Pivot's one) which was previously uneconomic back when prices were low.
Same with Minemakers who have a massive resource which has now become valuable with the increase in price.
7% though is a very low grade and would be expensive to mine. The other factor that comes in to play with rock phosphate is the size of the deposit. Its simply not feasible to make money without economies of scale that come with a large deposit. (
I have a bunch of data and research on world wide phosphate deposits and operations so let me know if you want any more info.
cheers
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