Uranium tetrachloride
Encyclopedia
Uranium tetrachloride is compound of uranium
in oxidation state
+4. It was used in the electromagnetic isotope separation
(EMIS) process of uranium enrichment. It is one of the main starting materials for organouranium chemistry.
in organic solvents.
shows the uranium to be surrounded by eight chlorine atoms, four at 264 pm and the other four at 287pm. The molecule UCl4 is a Lewis acid
and dissolves in solvents that can act as non-protic Lewis bases.
Dissolution in protic solvents is more complicated. When UCl4 is added to water the uranium aqua ion
is formed.
The aqua ion [U(H2O)x]4+, (x is 8 or 9) is strongly hydrolyzed.
The pKa
for this reaction is ca. 1.6, so hydrolysis is absent only in solutions of acid strength 1 mol dm-3 or stronger (pH < 0). Further hydrolysis occurs at pH > 3. Weak chloro complexes of the aqua ion may be formed. Published estimates of the log K value for the formation of [UCl]3+(aq) vary from -0.5 to +3 because of difficulty in dealing with simultaneous hydrolysis.
With alcohols, partial solvolysis
may occur.
Uranium tetralchloride dissolves in non-protic solvents such as tetrahydrofuran
, acetonitrile
, dimethyl formamide etc. that can act as Lewis bases. Solvates of formula UCl4Lx are formed which may be isolated. The solvent must be completely free of dissolved water, or hydrolysis will occur, with the solvent, S, picking up the released proton.
The solvent molecules may be replaced by other ligand in a reaction such as
The solvent is not shown, just as when complexes of other metal ions are formed in aqueous solution.
Solutions of UCl4 are susceptible to oxidation by air, resulting in the production of complexes of the uranyl
ion.
with pure uranium dioxide
UO2 at 370°C. It has been used as feed in the electromagnetic isotope separation
(EMIS) process of uranium enrichment. Beginning in 1944, the Oak Ridge Y-12 Plant
converted UO3 to UCl4 feed for the for Ernest O. Lawrence’s Alpha Calutrons
. Its major benefit being the uranium tetrachloride used in the calutrons is not as corrosive as the uranium hexafluoride
used in most other enrichment technologies This process was abandoned in the 1950s. In the 1980s, however, Iraq
unexpectedly revived this option as part of its nuclear weapons program. In the enrichment process, uranium tetrachloride is ionized into a uranium plasma
.
The uranium ions are then accelerated and passed through a strong magnetic field
. After traveling along half of a circle the beam is split into a region nearer the outside wall which is depleted and a region nearer the inside wall which is enriched in U-235. The large amounts of energy required in maintaining the strong magnetic fields as well as the low recovery rates of the uranium feed material and slower more inconvenient facility operation make this an unlikely choice for large scale enrichment plants.
Work is being done in the use of molten uranium chloride-alkali chloride mixtures as reactor fuels in molten salt reactor
s. Uranium tetrachloride melts dissolved in a lithium chloride
-potassium chloride
eutectic have also been explored as a means to recover actinides from irradiated nuclear fuel
s through pyrochemical nuclear reprocessing
.
Uranium
Uranium is a silvery-white metallic chemical element in the actinide series of the periodic table, with atomic number 92. It is assigned the chemical symbol U. A uranium atom has 92 protons and 92 electrons, of which 6 are valence electrons...
in oxidation state
Oxidation state
In chemistry, the oxidation state is an indicator of the degree of oxidation of an atom in a chemical compound. The formal oxidation state is the hypothetical charge that an atom would have if all bonds to atoms of different elements were 100% ionic. Oxidation states are typically represented by...
+4. It was used in the electromagnetic isotope separation
Isotope separation
Isotope separation is the process of concentrating specific isotopes of a chemical element by removing other isotopes, for example separating natural uranium into enriched uranium and depleted uranium. This is a crucial process in the manufacture of uranium fuel for nuclear power stations, and is...
(EMIS) process of uranium enrichment. It is one of the main starting materials for organouranium chemistry.
Synthesis
Uranium tetrachloride is synthesised generally by the reaction of Uranium trioxide (UO3) and hexachloropropene. Solvent UCl4 adducts can be formed by a simpler reaction of UI4 with Hydrogen ChlorideHydrogen chloride
The compound hydrogen chloride has the formula HCl. At room temperature, it is a colorless gas, which forms white fumes of hydrochloric acid upon contact with atmospheric humidity. Hydrogen chloride gas and hydrochloric acid are important in technology and industry...
in organic solvents.
Chemical properties
Uranium tetrachloride is a hygroscopic, dark green solid, which sublimes in a high vacuum at ca. 500°C. The crystal structureCrystal structure
In mineralogy and crystallography, crystal structure is a unique arrangement of atoms or molecules in a crystalline liquid or solid. A crystal structure is composed of a pattern, a set of atoms arranged in a particular way, and a lattice exhibiting long-range order and symmetry...
shows the uranium to be surrounded by eight chlorine atoms, four at 264 pm and the other four at 287pm. The molecule UCl4 is a Lewis acid
Lewis acid
]The term Lewis acid refers to a definition of acid published by Gilbert N. Lewis in 1923, specifically: An acid substance is one which can employ a lone pair from another molecule in completing the stable group of one of its own atoms. Thus, H+ is a Lewis acid, since it can accept a lone pair,...
and dissolves in solvents that can act as non-protic Lewis bases.
Dissolution in protic solvents is more complicated. When UCl4 is added to water the uranium aqua ion
Metal ions in aqueous solution
A metal ion in aqueous solution is a cation, dissolved in water, of chemical formula [Mn]z+. The solvation number, n, determined by a variety of experimental methods is 4 for Li+ and Be2+ and 6 for elements in rows 3 and 4 of the periodic table. Lanthanide and actinide aqua ions have solvation...
is formed.
- UCl4 + xH2O → [U(H2O)x]4+ + 4Cl-
The aqua ion [U(H2O)x]4+, (x is 8 or 9) is strongly hydrolyzed.
- [U(H2O)x]4+ [U(H2O)x-1(OH)]3+ + H+
The pKa
Acid dissociation constant
An acid dissociation constant, Ka, is a quantitative measure of the strength of an acid in solution. It is the equilibrium constant for a chemical reaction known as dissociation in the context of acid-base reactions...
for this reaction is ca. 1.6, so hydrolysis is absent only in solutions of acid strength 1 mol dm-3 or stronger (pH < 0). Further hydrolysis occurs at pH > 3. Weak chloro complexes of the aqua ion may be formed. Published estimates of the log K value for the formation of [UCl]3+(aq) vary from -0.5 to +3 because of difficulty in dealing with simultaneous hydrolysis.
With alcohols, partial solvolysis
Solvolysis
Solvolysis is a special type of nucleophilic substitution or elimination where the nucleophile is a solvent molecule. For certain nucleophiles, there are specific terms for the type of solvolysis reaction...
may occur.
- UCl4 + xROH UCl4-x(OR)x + xH+
Uranium tetralchloride dissolves in non-protic solvents such as tetrahydrofuran
Tetrahydrofuran
Tetrahydrofuran is a colorless, water-miscible organic liquid with low viscosity at standard temperature and pressure. This heterocyclic compound has the chemical formula 4O. As one of the most polar ethers with a wide liquid range, it is a useful solvent. Its main use, however, is as a precursor...
, acetonitrile
Acetonitrile
Acetonitrile is the chemical compound with formula . This colourless liquid is the simplest organic nitrile. It is produced mainly as a byproduct of acrylonitrile manufacture...
, dimethyl formamide etc. that can act as Lewis bases. Solvates of formula UCl4Lx are formed which may be isolated. The solvent must be completely free of dissolved water, or hydrolysis will occur, with the solvent, S, picking up the released proton.
- UCl4 +H2O + S UCl3(OH) + SH+ +Cl-
The solvent molecules may be replaced by other ligand in a reaction such as
- UCl4 + 2Cl- → [UCl6]2-.
The solvent is not shown, just as when complexes of other metal ions are formed in aqueous solution.
Solutions of UCl4 are susceptible to oxidation by air, resulting in the production of complexes of the uranyl
Uranyl
The uranyl ion is an oxycation of uranium in the oxidation state +6, with the chemical formula [UO2]2+. It has a linear structure with short U-O bonds, indicative of the presence of multiple bonds between uranium and oxygen. Four or more ligands are bound to the uranyl ion in an equatorial plane...
ion.
Applications
Uranium tetrachloride is produced commercially by the reaction of carbon tetrachlorideCarbon tetrachloride
Carbon tetrachloride, also known by many other names is the organic compound with the formula CCl4. It was formerly widely used in fire extinguishers, as a precursor to refrigerants, and as a cleaning agent...
with pure uranium dioxide
Uranium dioxide
Uranium dioxide or uranium oxide , also known as urania or uranous oxide, is an oxide of uranium, and is a black, radioactive, crystalline powder that naturally occurs in the mineral uraninite. It is used in nuclear fuel rods in nuclear reactors. A mixture of uranium and plutonium dioxides is used...
UO2 at 370°C. It has been used as feed in the electromagnetic isotope separation
Isotope separation
Isotope separation is the process of concentrating specific isotopes of a chemical element by removing other isotopes, for example separating natural uranium into enriched uranium and depleted uranium. This is a crucial process in the manufacture of uranium fuel for nuclear power stations, and is...
(EMIS) process of uranium enrichment. Beginning in 1944, the Oak Ridge Y-12 Plant
Y-12 National Security Complex
The Y-12 National Security Complex is a United States Department of Energy National Nuclear Security Administration facility located in Oak Ridge, Tennessee, near the Oak Ridge National Laboratory...
converted UO3 to UCl4 feed for the for Ernest O. Lawrence’s Alpha Calutrons
Calutron
A calutron is a mass spectrometer used for separating the isotopes of uranium. It was developed by Ernest O. Lawrence during the Manhattan Project and was similar to the cyclotron invented by Lawrence. Its name is a concatenation of Cal. U.-tron, in tribute to the University of California,...
. Its major benefit being the uranium tetrachloride used in the calutrons is not as corrosive as the uranium hexafluoride
Uranium hexafluoride
Uranium hexafluoride , referred to as "hex" in the nuclear industry, is a compound used in the uranium enrichment process that produces fuel for nuclear reactors and nuclear weapons. It forms solid grey crystals at standard temperature and pressure , is highly toxic, reacts violently with water...
used in most other enrichment technologies This process was abandoned in the 1950s. In the 1980s, however, Iraq
Iraq
Iraq ; officially the Republic of Iraq is a country in Western Asia spanning most of the northwestern end of the Zagros mountain range, the eastern part of the Syrian Desert and the northern part of the Arabian Desert....
unexpectedly revived this option as part of its nuclear weapons program. In the enrichment process, uranium tetrachloride is ionized into a uranium plasma
Plasma (physics)
In physics and chemistry, plasma is a state of matter similar to gas in which a certain portion of the particles are ionized. Heating a gas may ionize its molecules or atoms , thus turning it into a plasma, which contains charged particles: positive ions and negative electrons or ions...
.
The uranium ions are then accelerated and passed through a strong magnetic field
Magnetic field
A magnetic field is a mathematical description of the magnetic influence of electric currents and magnetic materials. The magnetic field at any given point is specified by both a direction and a magnitude ; as such it is a vector field.Technically, a magnetic field is a pseudo vector;...
. After traveling along half of a circle the beam is split into a region nearer the outside wall which is depleted and a region nearer the inside wall which is enriched in U-235. The large amounts of energy required in maintaining the strong magnetic fields as well as the low recovery rates of the uranium feed material and slower more inconvenient facility operation make this an unlikely choice for large scale enrichment plants.
Work is being done in the use of molten uranium chloride-alkali chloride mixtures as reactor fuels in molten salt reactor
Molten salt reactor
A molten salt reactor is a type of nuclear fission reactor in which the primary coolant, or even the fuel itself is a molten salt mixture...
s. Uranium tetrachloride melts dissolved in a lithium chloride
Lithium chloride
Lithium chloride is a chemical compound with the formula LiCl. The salt is a typical ionic compound, although the small size of the Li+ ion gives rise to properties not seen for other alkali metal chlorides, such as extraordinary solubility in polar solvents and its hygroscopic...
-potassium chloride
Potassium chloride
The chemical compound potassium chloride is a metal halide salt composed of potassium and chlorine. In its pure state, it is odorless and has a white or colorless vitreous crystal appearance, with a crystal structure that cleaves easily in three directions. Potassium chloride crystals are...
eutectic have also been explored as a means to recover actinides from irradiated nuclear fuel
Nuclear fuel
Nuclear fuel is a material that can be 'consumed' by fission or fusion to derive nuclear energy. Nuclear fuels are the most dense sources of energy available...
s through pyrochemical nuclear reprocessing
Nuclear reprocessing
Nuclear reprocessing technology was developed to chemically separate and recover fissionable plutonium from irradiated nuclear fuel. Reprocessing serves multiple purposes, whose relative importance has changed over time. Originally reprocessing was used solely to extract plutonium for producing...
.