Toward New Technologies for the Production of Lithium
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Lithium Overview Toward New Technologies for the Production of Lithium Georges J. Kipouros and Donald R. Sadoway The lightest of all metals, lithium is used Table II. Metals Production and Pricing Statistics in a variety of applications, including the production of organolithium compounds, as Fe Al Mg Ti Li an alloying addition to aluminum and mag- Production (t/y, 106) 752 21 0.35 0.0525 0.0125 nesium, and as the anode in rechargeable Price ($/kg) 0.45 1.50 3.96 19.82 89.43 lithium ion batteries. All of the world’s pri- Sales ($, 109) 338 31.5 1.39 1.04 1.12 mary lithium is produced by molten salt electrolysis. This article reviews the current sium the imminent possibility that its prices? Table III shows that aluminum is technology for lithium extraction and as- market distribution may be radically al- more abundant in the earth’s crust than sesses the prospects for change. tered by rapid growth in one applica- either iron or magnesium, yet steel is, by 9 INTRODUCTION tion: automotive die casting in the case far, the cheapest metal. So relative abun- of magnesium and anodes for recharge- dance does not explain pricing. How- With a density about half that of wa- able batteries in the case of lithium. It is ever, there is some correspondence be- ter, lithium (from the Greek “lithos,” curious that in the present lithium-ion tween the prices of steel and aluminum which means “stone”) boasts the lowest and the free energies of formation of density of all solids, a mere 0.53 g/cm3 at their oxides.10 Magnesium, titanium, and 20°C. Its electronic configuration is Lithium shares with lithium do not fit this pattern, largely 1s2␣2s1; its atomic number is 3; and its magnesium the due to the much smaller tonnages in- atomic mass is 6.941 g/mol. Typical of volved. As tonnages rise, making more alkali metals, lithium’s crystal structure imminent possibility efficient use of capital, prices of these is body-centered cubic. The element re- that its market latter three metals can be expected to acts with water (and moisture in the air) fall. Nicholson’s review of the pricing to produce the hydroxide, LiOH, and distribution may be history of lithium shows that from 1964 hydrogen gas. Unlike the heavier alka- to 1996, the price of this metal rose 23 lis, lithium does not react violently with radically altered by percent in constant dollars.7 However, oxygen; indeed, the metal is stable in dry rapid growth in one the effects of the drop in price of lithium air when the dew point is maintained carbonate in South America and the below –38°C. application: growing demand for lithium in recharge- Lithium is not a structural metal; in automotive die able batteries have yet to be felt by the the majority of its applications, it func- market. tions as a chemical reagent. Table I com- casting in the case LITHIUM METALLURGY pares the properties of lithium with those of steel and aluminum, the dominant of magnesium and With regard to the metallurgy of structural metals, and with those of mag- anodes for lithium,11–14 the metal is found in nature nesium and titanium, both considered as hard rock ore and as brine. The strat- too pricey for mainstream structural rechargeable egy for reducing these resources to metal applications.1 Table II reports world pro- batteries in the involves conversion to carbonate, then duction figures for these five metals.2–4 to chloride, followed by molten salt elec- Even though the world production of case of lithium. trolysis. Spodumene (LiAlSi2O6) is one lithium amounts to only 12,500 tonnes example of a hard rock resource. Con- per year, the high unit price of the metal battery,6 we find compounds of lithium version to carbonate involves either al- translates into annual sales of approxi- and cobalt, two of the four metals (the kaline fusion and carbonation or acid mately $1␣ billion and, thus, puts the value other two are gold and magnesium) for roasting, calcination, and carbonation. of the world lithium industry at par with which prices have risen over the past The double salt KLiSO4 is one example that of titanium and magnesium. Harben 30 years (after adjustments for inflation).7 of a brine resource. Conversion to car- and Edwards have reported upon trends In response, considerable effort is being bonate involves purification (removal of in the global lithium industry, which directed at displacing cobalt-bearing potassium) and precipitation. Potential they characterize as being in a state of compounds.8 methods to win lithium from pegmatites, rapid flux.5 Lithium shares with magne- Is there a scientific basis for metal natural brines, and clays have been dis- cussed.15–17 The carbonate is then con- Table I. Properties of Selected Metals verted to the chloride with the action of a chlorinating agent such as hydrogen Fe Al Mg Ti Li chloride. Table IV gives the salient fea- Melting Point (°C) 1,535 660 650 1,660 180 ° tures of the electrolytic reduction pro- Boiling Point ( C) 2,860 2,518 1,093 3,287 1,347 cess. The cell has a central cathode of Density (g/cm3) 7.87 2.70 1.74 4.54 0.53 E (GPa) 211 71 45 120 5 mild steel on which lithium is produced in liquid form. Opposing graphite plates 24 JOM • May 1998 serve as the anodes on which chlorine evolves. A bell-shaped structure posi- Table IV. The Electrolytic Production of Lithium tioned above the cathode collects the Electrolyte LiCl–KCl eutectic Feed LiCl rising liquid metal and prevents it from ° reacting with the chlorine gas which also Temperature 400–460 C Anode Carbon rises. Anhydrous lithium chloride is the Cathode Mild Steel cell feed. Potassium chloride is the sol- → ° Overall Cell Reaction LiCl (l) Li (l) + 1/2 Cl2 (g) for which E 427°C = 3.6 V vent and supporting electrolyte because – → – Anode Cl 1/2 Cl2 (g) + e it alone, among the common alkali and Cathode Li+ + e– → Li (l) alkaline-earth chlorides, has a decom- Current Density 2 A/cm2 position potential that is more extreme Energy Consumption 35 kWh/kg than that of lithium chloride. Put an- other way, electrolysis of a melt com- Table V. Reactions for the Production of Lithium by Thermochemical Reduction prising LiCl-NaCl will produce lithium metal containing a substantial amount Carbothermic Reduction Li O + C → 2 Li (g) + CO (lithia feed, carbon reductant) of sodium, which is highly undesirable 2 6 LiOH + 2 C → 2 Li (g) + 2 Li CO + 3 H (LiOH feed, carbon reductant) if lithium is to be used in battery applica- 2 3 2 3 LiOH + FeC → 3 Li (g) + Fe + 3/2 H + CO + CO (LiOH feed, iron carbide reductant) tions. The electrolysis temperature is 2 2 2 governed by the physical chemistry of Metallothermic Reduction of the Oxide → the molten salt. The eutectic point of 2 Li2O + 2 CaO + Si 4 Li (g) + Ca2SiO4 (c.f., Pidgeon process) 3 Li O + 2 Al → 6 Li (g) + Al O LiCl-KCl is 350°C and 42 mole percent 2 2 3 KCl.18 At 400°C, the liquid range extends Metallothermic Reduction of the Hydroxide from about 35–45 mole percent KCl. At 2 LiOH + 2 Mg → 2 Li (g) + 2 MgO + H → 2 this temperature lithium metal is mol- 2 LiOH + Al Li (g) + LiAlO2 ten, but its vapor pressure is acceptably Electrolytic-Calciothermic Reduction of the Oxide low. Operational data for the electroly- Cathode Ca2+ + 2 e– → Ca → sis cells may be found elsewhere in the Ca + Li2O CaO + 2 Li 14 2– → – literature. Anode O + 1/2 C 1/2 CO2 + 2 e What are the challenges and opportu- or 2– → – nities for lithium extraction? Certainly, O 1/2 O2 + 2 e there is the need for new electrode mate- rials. Avoiding the use of carbon would an amalgam intermediate by the elec- of feedstocks and carbothermic reduc- make the process environmentally trolysis of an aqueous solution of lithium tants. Metallothermic reduction is also a friendlier and might even enable the hydroxide. Lithium metal is then pro- possible route to metal. Also shown are design of a long-lived bipolar cell. One duced by electrorefining the amalgam at reactions for the reduction of lithium of the authors has enunciated criteria for 225°C in a cell containing an electrolyte oxide by silicon and aluminum. Some the selection of materials that can serve of molten LiI-CsI eutectic.22 experimental data taken under Pidgeon- like conditions have been reported.28 as inert anodes in Hall-Héroult cells pro- PARADIGM SHIFTS ducing aluminum.19 Metal alloys con- Table V continues with reactions for the sisting of a base metal (or metals) plus What about radical innovation (i.e., reduction of lithium hydroxide by mag- aluminum have been identified as the totally different reduction chemistry)? nesium and aluminum. Calcium can also most promising materials.20 Perhaps it is Here is a short list of candidates, none of reduce lithium oxide to metal. A novel time to apply these concepts to lithium which is in commercial use: electrolysis concept that combines electrolysis and electrolysis. Installing cathodes made of of carbonate feed, carbothermic reduc- metallothermic reduction is given in the a material that is wetted by molten tion, metallothermic reduction, electro- table as well. The process involves in- lithium would afford new opportunities lytic-calciothermic reduction, and elec- situ generation of calcium from a bath of in cell design.