Uranium from Phosphates

December 2009

• Rock phosphate deposits contain some 22 million tonnes of uranium, which may be extracted as a by-product of making fertilisers.
• Some 20-30,000 tonnes of uranium has already been obtained from these rock phosphate deposits, but the process became uneconomic in the 1990s.
• Higher uranium prices and process refinements have changed the economic situation.

In addition to the 5.5 million tonnes of uranium in known recoverable resources, there are substantial amounts comprising what is known as "unconventional resources".  Chief among these is rock phosphate, or phosphorite.

About 20% of US uranium came from central Florida's phosphate deposits to the mid 1990s, as a by-product, but it then became uneconomic.  From 1981 to 1992 US production averaged just over 1000 tU per year, then fell way sharply and finished in 1998.  With higher uranium prices today the resource is being examined again, as is another lower-grade one in Morocco.  Plans for Florida extend only to 400 tU/yr at this stage.

Phosphate rock production for fertiliser in 2007, million tonnes (main countries)

World phosphate rock production was expected to increase by 28% from 2007 to 2012.  This includes restoring Mexico's production, which largely ceased in 2001 due to storm damage.
Source: USGS 2007 Minerals Yearbook. 

Phosphate rock (phosphorite) is a marine sedimentary rock which contains 18-40% P₂O₅, as well as some uranium and all its decay products, often 70 to 200 ppmU, and sometimes up to 800 ppm.  The main mineral in the phosphate rock is apatite, and most commonly, fluorapatite - Ca₅(PO₄)₃F or Ca₁₀(PO₄)₆(F,OH)₂.  This is insoluble, so cannot directly be used as a fertiliser (unless in very acid soils) so must first be processed.  This is normally in a wet process phosphoric acid (WPA) plant where it is first dissolved in sulphuric acid.  About 2-4% fluorine is usually present. There are about 400 plants using this wet process worldwide.

When phosphate rock is treated with sulphuric acid in sub stoichiometric quantity, normal superphosphate is formed.  If more sulphuric acid is added, a mixture of phosphoric acid and gypsum (calcium sulphate) is obtained.  After the gypsum is filtered out, the resultant phosphoric acid can be treated to recover uranium.  

The basic reaction is:

Ca₃(PO₄)₂ + 3H₂SO₄ + 6H₂O ==> 2H₃PO₄ + 3CaSO₄.2H₂O  - exothermic

An improved higher-temperature process produces hemihydrate: CaSO₄.1/2H₂O

Fluorides need to be controlled as gases and in effluents (HF, fluorosilicic acid) and about half the fluorine reports with the gypsum. 

The uranium is normally recovered form the phosphoric acid (bearing about 28% P₂O₅) by some form of solvent extraction.  A refinement of the old processes - PhosEnergy - was announced in 2009, offering uranium recovery costs of $25-30 /lb U₃O₈, compared with historical costs of double this.  It is expected to be in commercial use in 2011.*

* The PhosEnergy process is being developed by Uranium Equities Limited through a US registered company, Urtek LLC.  Cameco has secured rights to earn up to a 63% interest in the technology.

After this gypsum precipitation stage, triple superphosphate is obtained by reacting the phosphoric acid with further phosphate rock.  Otherwise, various ammonium phosphate fertilizers can be produced by reacting the phosphoric acid with ammonia.

In the USA eight plants for the recovery of uranium from phosphoric acid have been built and operated since the 1970s (six in Florida, two in Louisiana).  Plants have also been built in Canada, Spain, Belgium, Israel, and Taiwan. 
Brazil is planning a new plant with uranium as co-product with phosphate from 0.08%U ore in igneous rock.  Morocco has by far the largest known resources of uranium in phosphate rock.

The potential amount of uranium able to be recovered from WPA phosphoric acid streams is over 7000 tonnes U per year.  The economic benefit will be both in the value of the uranium and in reduced regulatory demands on disposal of low-level radioactive wastes arising from the WPA process.  Estimated production costs will put the new process in the lowest quartile of new uranium production.

Sources:

Guzman, ETR et al., 1995, Uranium in Phosphate Rock and Derivatives
WISE 2009, Uranium recovery from phosphates (web)