Question: WHen was the Uranium Hexafluoride project started?
About the article "Manhattan Project", section "Centrifuges".

Candidate passages:
During the Manhattan Project the need arose to find volatile compounds of uranium suitable for use in the diffusion separation of uranium isotopes.
Uranium borohydride is, after uranium hexafluoride, the most volatile compound of uranium known with a vapor pressure of at 60 °C.
Uranium borohydride was discovered by Hermann Irving Schlesinger and Herbert C. Brown, who also discovered sodium borohydride.
Uranium hexafluoride is very corrosive, which presented serious handling difficulties, leading to serious consideration of the borohydride.
However by the time the synthesis method was finalized, the uranium hexafluoride related problems were already solved.
Borohydrides are nonideal ligands for isotope separations since boron occurs naturally with two abundant isotopes, 10B (20%) and 11B (80%).
Enrico Fermi's purported comment when he observed the neutron cross section for boron—"My God!
It's as big as the side of a barn!
"—not only gave a name to the unit of cross section (the barn); it also put an end to using uranium borohydride in the diffusion process. 
Gaseous diffusion is a technology used to produce enriched uranium by forcing gaseous uranium hexafluoride (UF6) through semipermeable membranes.
This produces a slight separation between the molecules containing uranium-235 (235U) and uranium-238 (238U).
By use of a large cascade of many stages, high separations can be achieved.
It was the first process to be developed that was capable of producing enriched uranium in industrially useful quantities.
Gaseous diffusion was devised by Francis Simon and Nicholas Kurti at the Clarendon Laboratory in 1940, tasked by the MAUD Committee with finding a method for separating uranium-235 from uranium-238 in order to produce a bomb for the British Tube Alloys project.
The prototype gaseous diffusion equipment itself was manufactured by Metropolitan-Vickers (MetroVick) at Trafford Park, Manchester, at a cost of £150,000 for four units.
This work was later transferred to the United States when the Tube Alloys project became subsumed by the later Manhattan Project. 
The plant is located near Gore, Oklahoma, close to the Illinois River and Interstate 40.
The plant started construction in 1968 and began operation in 1970.
It converted yellowcake uranium into uranium hexafluoride.
In 1987 it started converting depleted uranium hexafluoride into depleted uranium tetrafluoride.
The plant ceased operation in 1993.
The plant was operated under Kerr-McGee Nuclear Corporation.
In 1983 KMNC split into Quivira Mining Corporation and Sequoyah Fuels Corporation.
The latter was given control of the plant.
In 1988 Sequoyah Fuels Corporation was sold to General Atomics.
In 2008 a company named International Isotopes said it would buy equipment and intellectual property from the Sequoyah Fuels Corp plant.
The equipment would be used in a new location.
It would be used for converting depleted uranium hexafluoride to depleted uranium tetrafluoride. 
Uranium sulfate (U(SO4)2) is a water soluble salt of uranium.
It is a very toxic compound and should not be ingested.
Uranium sulfate minerals commonly are widespread around uranium bearing mine sites, where they usually form during the evaporation of acid sulfate-rich mine tailings which have been leached by oxygen-bearing waters.
Uranium sulfate is a transitional compound in the production of Uranium hexafluoride.
It was also used to fuel aqueous Homogeneous Reactors. 
In nature, uranium is found as uranium-238 (99.2742%) and uranium-235 (0.7204%).
Isotope separation concentrates (enriches) the fissionable uranium-235 for nuclear weapons and most nuclear power plants, except for gas cooled reactors and pressurised heavy water reactors.
Most neutrons released by a fissioning atom of uranium-235 must impact other uranium-235 atoms to sustain the nuclear chain reaction.
The concentration and amount of uranium-235 needed to achieve this is called a 'critical mass'.
To be considered 'enriched', the uranium-235 fraction should be between 3% and 5%.
This process produces huge quantities of uranium that is depleted of uranium-235 and with a correspondingly increased fraction of uranium-238, called depleted uranium or 'DU'.
To be considered 'depleted', the uranium-235 isotope concentration should be no more than 0.3%.
The price of uranium has risen since 2001, so enrichment tailings containing more than 0.35% uranium-235 are being considered for re-enrichment, driving the price of depleted uranium hexafluoride above $130 per kilogram in July 2007 from $5 in 2001.
The gas centrifuge process, where gaseous uranium hexafluoride () is separated by the difference in molecular weight between 235UF6 and 238UF6 using high-speed centrifuges, is the cheapest and leading enrichment process.
The gaseous diffusion process had been the leading method for enrichment and was used in the Manhattan Project.
In this process, uranium hexafluoride is repeatedly diffused through a silver-zinc membrane, and the different isotopes of uranium are separated by diffusion rate (since uranium 238 is heavier it diffuses slightly slower than uranium-235).
The molecular laser isotope separation method employs a laser beam of precise energy to sever the bond between uranium-235 and fluorine.
This leaves uranium-238 bonded to fluorine and allows uranium-235 metal to precipitate from the solution.
An alternative laser method of enrichment is known as atomic vapor laser isotope separation (AVLIS) and employs visible tunable lasers such as dye lasers.
Another method used is liquid thermal diffusion.