by EnergyUnlimited » Wed 12 Dec 2007, 05:38:30
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')So how would you "extract" it from a rock consisting of 98% of CaCO3 and 2% Ca3 (PO4)2 ?
Probably by limeburning as part of cement production, if you ever had to use ore grades that low. Which we wont this side of a thousand years.
Given that we reduce hundreds of millions of tons limestone a year, thousands of times that of phosphates, I'm not terribly worried about the miniscule expense phosphorus extraction will cost.
It seems that you don't know much about chemical process which can be used in phosphates extraction.
In lime burning you will convert calcium carbonate into oxide, then into hydroxide upon water treatment.
Phosphates would still stay with this oxide/hydroxide and any separation would require expensive "wet" chemistry.
Essentially your phosphate would end up as lime impurity, not separable without an expensive and resources intensive process.
(NB it would not be possible to use such product as efficient fertilizer
directly. Presence of too much calcium is reducing bioavailabilty of phosphorus).
In chemical industry apatite rock Ca3(PO4)2*CaClF is treated with moderately concentrated sulfuric acid.
This results in water phase containing phosphoric acid and calcium sulfate solid together with hydrochloric and hydrofluoric acids are produced as by products.
Hydrochloric and hydrofluoric acids are distilled off, calcium sulfate filtered off and phosphoric acid is left in liquid phase.
Phosphoric acid is then converted to fertilizer
Any calcium carbonate present would have to be converted into sulfate first, before desirable reaction takes place.
So if you have 2% of phosphate (in molar terms), your load of sulfuric acid would have to increase about 50 fold.
So you will soon face a problem with sulfuric acid supply, if you try.
Another method of phosphorus separation rely on endothermic reduction of apatite by carbon in presence of sand according to process:
2Ca3(PO4)2+6SiO2+10C= 6CaSiO3 + 10CO + P4 (at 1800*C)
P4 (white phosphorus) distills off, then is burned to P2O5 and this is treated with water to produce phosphoric acid finally converted to fertilizer.
You may note that
any calcium carbonate present would have to be converted into silicate (CaSiO3) for the process to work.
With 98% of CaCO3 it really look hopeless.
This would entail heating
massive amounts of rock with a lot of sand and some coke to produce a
tiny bit of phosphorus.
Expensive & extremely inefficient energy hog.
NB. CaSiO3 by product is a useless one.
So the question is, if phosphorus fertilizer is to be 50 times more expensive, will farmers be able to afford it?
And if they do, how much food will cost?