- Lithium is best known today as an ingredient of lithium ion batteries.
- Li-7 as a hydroxide is important in controlling the chemistry of PWR cooling systems.
- Li-7 is a key component of fluoride coolant in molten salt reactors.
- Li-6 is a source of tritium for nuclear fusion, through low-energy nuclear fission.
Lithium has two stable isotopes Li-6 & Li-7, the latter being 92.5% in nature (hence relative atomic mass of natural lithium of 6.94).
Nuclear industry use: Li-7
Lithium-7 has two important uses in nuclear power today and tomorrow due to its relative transparency to neutrons. As hydroxide it is necessary in small quantities for safe operation in pressurised water reactor (PWR) cooling systems as a pH stabilizer, to reduce corrosion in the primary circuit. As a fluoride, it is also expected to come into much greater demand for molten salt reactors (MSRs). However, for both purposes it must be very pure Li-7, otherwise tritium is formed by neutron capture (see later section).
The 99.95% Li-7 hydroxide is used in nuclear power engineering as an additive in PWR primary coolant, at about 2.2 ppm, for maintaining water chemistry, counteracting the corrosive effects of boric acid (used as neutron absorber) and minimizing corrosion in steam generators of PWRs. It is also used in the manufacture of chemical reagents for nuclear power engineering, and as a basic component for preparation of nuclear grade ion-exchange membranes which are used in PWR coolant water treatment facilities.
Lithium-7’s very low neutron cross-section (0.045 barns) makes it invaluable for nuclear power uses. There is concern in the USA about supplies of Li-7, and in December 2013 the Nuclear Energy Institute said that the critically important Li-7 supply situation highlighted the importance of monitoring all aspects of the nuclear supply chain. About 400 kg/yr Li-7 is required for US PWR reactors.
As a fluoride, Li-7 is used in the lithium fluoride (LiF) and lithium-beryllium fluorides (FLiBe) which comprise the coolant salt in most molten salt reactors (MSRs) now the focus of intensive development. FLiBe has about 14% lithium, so even higher levels of purity are required – 99.995% Li-7. In most cases the coolant salt also has the fuel dissolved in it. Such fluoride salts have very low vapour pressure even at red heat, carry more heat than the same volume of water, have good heat transfer properties, have low neutron absorbtion, are not damaged by radiation, do not react violently with air or water, and some are inert to some common structural metals.
LiF is exceptionally stable chemically, and the LiF-BeF2 mix (‘FLiBe’) is eutectic (at 459°C it has a lower melting point than either ingredient – LiF is about 500°C). FLiBe is favoured in MSR primary cooling, and when uncontaminated has a low corrosion effect. The three nuclides (Li-7, Be, F) are among the few to have low enough thermal neutron capture cross-sections not to interfere with fission reactions. FLiNaK (LiF-NaF-KF) is also eutectic and solidifies at 454°C. It has a higher neutron cross-section than FLiBe or LiF but can be used intermediate cooling loops, without the toxic beryllium.
Non-nuclear lithium uses
Lithium is widely used in lithium-ion batteries, including those for electric cars, either as natural lithium or with an enhanced proportion of Li-6 which improves performance, utilizing chemically-pure tails from enriching Li-7.
What is claimed to be the world’s largest lithium ion battery factory was opened in 2011 at Novosibirsk. It is owned by Liotech, a 50-50 joint venture between the Russian Nanotechnologies Corporation (RUSNANO) and the Chinese holding company Thunder Sky Ltd. While using Chinese feed initially, it aims to use only Russian raw materials by 2015, and apparently this will be depleted lithium tails with elevated proportion of Li-6 from lithium enrichment activities at Novsibirsk (see below).
Sources of lithium and Li-7
Lithium is not a scarce metal. In keeping with its name, lithium occurs in a number of minerals found in acid igneous rocks such as granite and pegmatites, spodumene and petalite being the most common source minerals. Due to its solubility as an ion it is present in ocean water and is commonly obtained from brines and clays (hectorite). At a conservative average 20 ppm in the Earth’s crust, lithium is the 25th most abundant element. Lithium carbonate prices are stable at about $4700 per tonne, with demand increasing about 12% per year.
According to estimates by the United States Geological Survey (USGS), which have been modified by Geoscience Australia for Australia’s resources, known world lithium resources in 2012 totalled about 13.5 million tonnes. Chile holds approximately 7.5 Mt, or about 56% of the total world resources, followed by China with 3.5 Mt (about 26%), Australia with 1.5 Mt (11.4%), and Argentina with 0.85 Mt (6.3%). World production in 2012 was about 37,000 tonnes. Chile was the leading producer with 13,000 t, closely followed by Australia, then China. Chile recovers the lithium from brine pools, Australia from mines. Considerable over-supply is anticipated to 2020 by TRU Group.
Lithium demand today is about 28,000 tonnes per year, about one third for batteries and one quarter for glass manufacture. A range of minor applications, including nuclear power, accounts for small shares of demand, including that specifically for lithium-7. In 2013 the US Department of Energy planned to set aside 200 kg of lithium-7 in reserve, and is funding research on production methods. World demand for Li-7 in PWR cooling systems is about one tonne per year, including about 400 kg annually for 65 US PWRs (Russia uses a different pH control process). When MSRs are built, several tonnes of pure Li-7 will be required in each (about 20 t/50m3 LiF with 5 tonnes Li-7, for each GWe by one account, 150-400 tonnes FLiBe with 21-56 tonnes Li-7 by another). Demand for Li-7 could readily reach 250 t per year with the kind of construction program envisaged by some, though it forms part of the capital set-up, not a consumable.
The USA became the prime producer of lithium from the late 1950s to the mid-1980s, by which time the stockpile was about 42,000 tonnes of lithium hydroxide. Lithium enrichment (to Li-6) has created a large US inventory of both tailings depleted in Li-6 (at Portsmouth, Ohio and K-25 site at Oak Ridge, Tenn) and unprocessed lithium. Most of this, notably Li-7, was then sold on the open market.* Production of lithium-7 had ceased in the USA in 1963, partly because of environmental and OHS concerns with mercury which was used in its enrichment.
* The US government is reported to have only 1.3 tonnes of contaminated or partly enriched Li-7.
Today the only sources of Li-7 (enriched from natural lithium) are Russia and China, though the latter is reported to be buying from Russia now. Production of lithium-7 at least in Russia and possibly China is as a by-product of enriching lithium-6 to produce tritium for thermonuclear weapons.
TVEL’s Novosibirsk Chemical Concentrates Plant (NCCP) in Siberia is the largest supplier of Li-7 hydroxide monohydrate (with purity up to 99.95%), meeting up to 80% of the world’s requirements. In June 2014, NCCP signed a three-year contract for supply of lithium-7 of 99.99% purity to China. From 2015 NCCP plans to produce this ultra high purity Li-7 as a new development.
Li-7 hydroxide monohydrate produced by NCCP accounts for over 70% of Li-7 world consumption (by isotopic composition). Equipment modernization carried out in 2013 make it possible to double the volume of Li-7 output there.
Properties of lithium, different isotopes
Lithium* easily ionizes to Li+, and LiOH forms readily. Lithium is the only stable light element which can produce net energy through fission (albeit only 4.8 MeV for Li-6, compared with about 200 MeV for uranium).
* Atomic number 3, melts 180.5°C, boils 1330°C. It is the lightest metal and the least dense solid element (about half as dense as water). It is highly reactive and flammable, like other alkali metals.
Lithium-6 has a very high neutron cross-section (940 barns) and so readily fissions to yield tritium and helium. It has been the main source of tritium for both thermonuclear weapons and future controlled fusion. Natural lithium is enriched in Li-6 for this purpose, leaving tails enriched beyond the natural 92.5% in Li-7.
|The fission of lithium to helium (and tritium) by Cockroft and Walton in 1932 was the first artificial fission reaction, in this case induced by proton bombardment:
Li-7 + proton ==> 2He-4 + 17 MeV
The more significant reaction today is:
Li-6 + neutron ==> He-4 + H-3 (tritium) + 4.8 MeV
The main US source of tritium since 2003 apart from deactivated weapons has been special burnable absorber rods containing lithium in TVA’s Watts Bar 1 PWR. Supplies need to be replenished due to tritium’s half-life being 12 years, so decaying at about 5% pa.
Lithium isotope separation
Isotope separation of Li-6 and Li-7 can be achieved chemically, using the column exchange (Colex) separation process, and also with laser processes on metal vapour, or crown-ether separation.
With mercury-based separation, Li-6 has a greater affinity to mercury than its more common partner. When a lithium-mercury amalgam is mixed with lithium hydroxide, the lithium-6 concentrates in the amalgam and the lithium-7 in the hydroxide. A counter-flow of amalgam and hydroxide passes through cascades followed by separation of the lithium-6 from the amalgam. Today this is undertaken only in Russia and China, though it was greatly used in the USA earlier, with major environmental impact. A lot of mercury is required – 11,000 tonnes was used in the USA – a significant amount being lost to “wastes, spills and evaporation”. Further use of the Colex process is banned in the USA.
At NCCP, lithium-7 hydroxide monohydrate is produced by electrolysis of lithium chloride using mercury solutions. After electrolysis, Li-7 hydroxide monohydrate solutions undergo further operations: purification, crystallization, centrifugation, drying, sieving and magnetic separation. The resulting product is in the form of white crystals.
Atomic vapour laser isotope separation (AVLIS) appears appropriate for smaller quantities to serve the needs of PWRs. At about 700°C Li-7 is photo-ionised selectively, giving highly-effective separation in an electromagnetic field, sufficient in one pass for PWR use. Several features of AVLIS mean that pure Li-6 is not produced.
Crown-ether enrichment using mixer-settler system appears best for larger-scale production such as envisaged for MSR fluorides. Certain ethers have ring properties that make them inclined to bond more with specific isotopes than others, and either resin columns or a water-insoluble solvent containing the crown ethers is added to an aqueous mixture of lithium, and the Li-6 concentrates in the solvent phase for removal. Concentration factors of 1.05 are comparable with the Colex process. Two crown ethers appear economically attractive: benzo-15-crown-5 and dicyclohexano-18-crown-6.
NCCP website www.nccp.ru/en
Anderson, E.R. TRU Group 2011, Shocking future battering the lithium industry through 2020
Nuclear Energy Institute, Industry Watching Supply of Lithium-7 for US PWRs (9 December 2013)
Tim Ault et al, Lithium Isotope Enrichment: Feasible Domestic Enrichment Alternatives, University of California, Berkeley, Report UCBTH-12-005 (5 May 2012)