Alkali Metals Production (Li, Na, K)

Transcription

1 1. Introduction Lithium (Li: atomic number 3), sodium (Na:11), and potassium (K:19) are very reactive, monovalent alkali metals that occupy the Group 1A position on the periodic table. These metals find extensive applications in many industries, both as pure metal as well as compounds. Application in compound forms is significant for all alkali metals, as some of the most important commercial chemicals contain these metals. Lithium is used as a stearate in lubricating greases, as oxide in ceramics and porcelain, as halides in welding fluxes and as salts in medical treatment. Recent applications of lithium metal include batteries and energy storage devices. Aluminum lithium alloys have found extensive use in aerospace applications. Sodium is primarily used in the synthesis of antiknock agent for automotive gasoline. Other uses include reductant for zirconium, titanium, and potassium metal production and as a reductant of oils to fatty alcohols for detergent applications. Its compounds, particularly the chloride and hydroxide are important industrial chemicals. Potassium in the form of chloride is mainly used in fertilizers. The primary production method of sodium and lithium is by molten salt electrolysis while potassium is produced using sodium in a metallothermic reduction process. Lithium cell uses a salt mixture of lithium chloride and potassium chloride whereas sodium cell (Downs cell) uses a mixture of sodium chloride and calcium chloride. The occurrence, metallurgical extraction methods, chemical properties, and prominent applications, including those of the compounds of alkali metals lithium, sodium, and potassium have been described. Alkali metals are excellent reductants for other metal compounds, since alkali metals form some of the most stable compounds. Thus, a hybrid process has been outlined where the alkali metal is electrochemically produced and used as a pyrochemical reductant in situ. In this scheme, alkali metal serves as a nonconsumable reducing metal. 2. Lithium 2.1 Sources of Lithium Lithium in nature, is primarily obtained from lithium-bearing pegmatites and brines. The lithium found within the pegmatite formations is in the mineral forms of spodumene, petalite, lepidolite, and amblygonite. These pegmatites are found as veins within or associated with granites and feldspar and are generally the primary sources for extraction of lithium. The mineral, spodumene (Li 2 OAl 2 O 3 4SiO 2 ), is of primary interest for the commercial production of lithium in North America due to its availability and high potential yield of lithium oxide (Li 2 O). Large reserves of spodumene can be found in North Carolina in the United States and in Quebec and Manitoba in Canada. Spodumene theoretically contains 8.03% of Li 2 O. Amblygonite, (LiAl(PO 4 )F, OH) contains 10.1% Li 2 O, and is mostly available in Europe, Africa, and South America for commercial extraction. Substantial amounts of petalite (Li 2 O Al 2 O 3 8SiO 2 : 4.09% Li 2 O) and lepidolite (K 2 (LiAl) 5 6 (Si 6 7 Al 2 1 O 20 ): 4.89% Li 2 O) are also found in Rhodesia, Africa. The other nonmineral source of lithium is brines. The concentration of lithium found in seawater is B0.1 ppm. It is also prevalent in other mineral waters such as Searles Lake in California. 2.2 Extraction and Purification of Lithium Flotation is primary used for mineral extraction from lithium ores. In this process, lithium ores are concentrated with respect to lithium oxide from 1 3% Li 2 O to 4 6% Li 2 O through heavy medium separation using dense nonaqueous liquids in a froth flotation process. Silicate ores are most widely processed using the flotation method, those products are subsequently chemically cleaned by an acid or alkaline process. In the acid cleaning process, the concentrated spodumene ore is placed in a kiln and heated to elevated temperatures between C and C. This process changes the naturally occurring alphaspodumene into beta-spodumene, which can be more readily attacked by the acid. The beta-spodumene is further cooled and ball-milled. This powder is then roasted in a second kiln under an excess of sulfuric acid at a temperature between 200 1C and 250 1C. The following reaction occurs at this stage: Li 2 OAl 2 O 3 4SiO 2 þ H 2 SO 4 -H 2 OAl 2 O 3 4SiO 2 þ Li 2 SO 4 at C ð1þ Once this reaction has taken place, the kiln is then leached with water. This yields a lithium sulfate product to be treated with sodium carbonate to convert it into lithium carbonate. Hydrochloric acid can then be used to react with the lithium carbonate to form lithium chloride. Li 2 SO 4 þ Na 2 CO 3 -Li 2 CO 3 þ Na 2 SO 4 Li 2 CO 3 þ 2HCl-2LiCl þ H 2 CO 3 ð2þ ð3þ In the alkaline cleaning process, either a spodumene or a lepidolite concentrated ore is ground and calcined with a mixture of 3.5 parts limestone to 1 part lithium. This is done at a temperature between 900 1C and C. In this process, the kiln is then hot-leached with water and the product is lithium hydroxide, which can be converted to lithium chloride using 1

2 hydrochloric acid: Li 2 OAl 2 O 3 4SiO 2 þ CaO -CaOAl 2 O 3 4SiO 2 þ Li 2 O at C ð4þ Li 2 O þ H 2 O þ 2HCl-2LiCl þ 2H 2 O ð5þ Lithium chloride is thus the source for electrolytic extraction of lithium. Metallic lithium can be obtained by the electrolysis of a melt, comprising of an equal mixture of lithium chloride and potassium chloride. A schematic diagram of this cell can be seen in Fig. 1 (Freitas 2000). Lithium chloride is fed into the cell, which is operated at a temperature between 400 1C and 420 1C. The voltage across the cell of molten lithium chloride and potassium chloride, is typically between 8 V and 9 V with a current consumption of 40 kwh kg 1 of lithium that is produced. A steel cathode with cast iron collectors and a graphite anode is employed. Tables 1 and 2 show that lithium has a lower density than the electrolytes. Therefore, the metal floats on top. The collector is helpful in the recovery of the metal. The electrical and ionic conductivities in the cell and the fluidity of the electrolyte, are critical parameters that control the material and energy balance of the process. Electrolytic lithium is refined by remelting, when the insolubles either float to the surface or sink to the bottom of the melt pot. Potassium is only slightly miscible in lithium. The remelting step produces lithium metal with less than 100 ppm of potassium. 2.3 Applications of Lithium Table 1 lists some of the physicochemical properties of the alkali metals. Its low density makes it a very useful alloying agent. The major industrial use of lithium is for lubricating greases in the form of lithium stearate. Lithium-based greases provide high temperature and water resistance as well as having good low temperature properties. A relatively new, but very popular application of lithium is for lightweight alkaline batteries in which the anode is comprised of lithium. Metallic lithium is also added to certain types of glasses and ceramics, as a flux to lower melting and sintering temperatures, as well as to lower the coefficient of expansion in the finished product (Fishwick 1974). Lithium can also be added to electrolytic cells in the production of aluminum to increase yield and reduce fluorine. Lithium also finds many uses in inorganic compounds, particularly in the form of chlorides and fluorides. Table 3 lists the prominent applications (Roskill 1979). Figure 1 Illustration of a cell for electrolytic production of lithium (Freitas 2000). 3. Sodium 3.1 Sources of Sodium As one of the most abundant minerals on earth, sodium is most commonly derived from natural salts. Table 1 Physicochemical properties of lithium, sodium, and potassium. Properties Lithium Sodium Potassium At. weight (At. no) 6.94 (3) (11) 39.1 (19) Melting point, 1C Boiling point, 1C Density, g cm Thermal conductivity, cal/(s.cm.1c) Specific heat (250 1C), cal g Heat of fusion, cal g Electronegativity, Pauling s Atomic volume, W/D

3 Table 2 Physicochemical properties of relevant metal chlorides. Properties LiCl NaCl KCl CaCl 2 Melting point, 1C Boiling point, 1C (sub) 1935 Density, g cm Mol. wt Std. free energy of formation (25 1C), kj mol Std. dissociation potential, V Elec. conductivity, O 1 cm Table 3 Applications of lithium compounds. Compound Application Lithium acetate Organic synthesis. Lithium aluminate Flux in highly refractory porcelain enamels. Lithium aluminum Preparation of vitamins, steroids, and Ziegler catalysts. Solvent drying and generation hydride of hydrogen. Lithium tetraborate Ceramics. Lithium bromide Reconstitution of brines, catalyst and dehydro-halogenating agent, swelling agent for proteins. Lithium carbonate Enamels, glasses, glazes, and ceramic specialties. Extractive metallurgy of aluminum and uranium. Pharmaceutical. Lithium chloride Preparation of lithium metal, brazing fluxes, electrolytes of low-temperature dry cell batteries, fire extinguishing solutions. Lithium chromate brine Corrosion inhibitor for lithium chloride and bromide brines, industrial battery additive, paint remover. Lithium fluoride Strong flux for enamels, glasses and glazes, welding and brazing fluxes, electrowinning of aluminum, heat sink material. Lithium hydride Amide and double hydrides of lithium, catalyst in polymerization reactions, hydrogen generation, radiation shielding, high energy fuels, fuel cells, heat sink, silane gas production. Lithium hydroxide Multi-purpose lubricating greases, additive to the KOH electrolyte of alkaline batteries, anhydrous lithium hydroxide used as a CO 2 absorbent in air purification systems of submarines and space capsules, alkaline reagent for corrosion inhibition in steam boilers, component of copper electroplating baths, absorption of CO 2 in air conditioning systems. Lithium manganite Bonding agent in ceramic bonded grinding wheels. Lithium molybdate Ceramics, corrosion inhibitor, metal surface treatment. Lithium nitrate Ammonia absorbent in refrigeration systems, oxidizing agent, flame colorant in fireworks. Lithium oxide Carbon dioxide absorption. Lithium peroxide Atmosphere regenerant, chemical oxidant. Lithium perchlorate Electrolyte constituent for lithium batteries, solid propellant mixtures for rockets, oxygen source. Lithium silicate Vehicle for water-based protective coatings, concrete sealant, adhesive, binder for welding rod coatings. Lithium sulphate High strength glass, photographic developer composition, tracer ingredient in chemical products. The important sodium salts are sodium chloride (rock salt), sodium carbonate (soda), sodium sulfate (thenardite), sodium nitrate (Chile salt peter), and sodium borate (borax). These sodium salts are commonly found in seawater, salt lakes, alkaline lakes, and mineral springs. Sodium chloride is the typical starting material for the manufacture and extraction of sodium metal. Deposits of sodium 3

4 chloride in the United States are also in the form of salt domes and are found in Virginia, West Virginia, Texas, and Louisiana. Since these salts were deposited many years ago by pre-existing seas and lakes, most of the deposits are located underground. Therefore, these salts are typically flushed out of underground mines by solution mining with water in the form of brines. 3.2 Extraction and Purification Current commercial practice for manufacturing of metallic sodium is based almost entirely on the reduction of sodium chloride. The most important technique for reducing this salt is by electrolytic reduction using the Downs process. Direct electrolysis of sodium hydroxide by the Castner process produced sodium prior to the advent of the Downs process. Carbothermic reduction of soda ash is also practiced as shown below: Na 2 CO 3 þ 2C-2Na þ 3CO ð6þ Equation (6) is associated with an enthalpy change of DH 298 ¼ 231 kcal g 1 mol 1. Chalk is often added to keep the charge material pasty and to prevent the separation of the molten sodium carbonate from carbon on heating in the thermochemical reduction system. In industry today, the most common way of producing metallic sodium is through the use of a Downs cell. A Downs cell is comprised of a large steel vessel that is lined with refractory firebricks and is operated at 590 1C. The cell has a large, vertical, central graphite anode descending from the bottom of the cell that is surrounded by cylindrical steel cathodes. This creates an annular electrolysis zone. Figure 2 shows a schematic illustration of this cell (Freitas 2000). A eutectic mixture of NaCl and CaCl 2 is used as the electrolyte. When this cell is activated, chlorine gas is produced and must be removed from the cell. As the product of sodium metal is lighter than the bath material, it floats to the surface of the bath where it is collected by an inverted collector ring and forced up the collector tube. The resulting sodium that is collected in the reservoir is 99.8% pure. The primary limitation of the Downs process is the contamination of the product by calcium, which has a solubility of 4 5 wt.%. At the cell operating temperature. Holding the sodium at just above its melting point, lowers calcium solubility to B100 ppm. Calcium contamination restricts the use of sodium as a reactor coolant due to its high affinity for oxygen and nitrogen. Electrochemical reactions are similar to that of a lithium cell where chlorine evolves at the anode and the metal deposits at the cathode (Foust 1979). Figure 2 Illustration of a cell for electrolytic production of sodium (Freitas 2000). 3.3 Applications of Sodium The greatest single demand for sodium in the world today is for the synthesis of tetraethyllead (Sittig 1956). This is used as an automotive gasoline additive to prevent engine knocking. This single application accounts for 60% of the metallic sodium that is manufactured. In this process, a sodium lead intermetallic alloy is reacted with an ethyl chloride that yields tetraethyllead by the following reaction: 4PbNa þ 4C 2 H 5 Cl-ðC 2 H 5 Þ 4 Pb þ 3Pb þ 4NaCl ð7þ The lead that is not used in the reaction is recycled back into the process. Another major use of sodium is in the reduction of animal and vegetable oils into long-chain fatty alcohols. These alcohols are then used in the production of detergents and soaps. A third major consumer of sodium is the Hunter s process for the reduction of titanium and zirconium halides to their respective metals. The process of reducing these halides with sodium has become more and more popular due to the cost of magnesium (Kroll process) over the cost of sodium. The high thermodynamic stability of sodium chloride with respect to the titanium or zirconium chloride makes it a useful reductant. Metallic sodium also finds many uses in inorganic compounds and solutions such as sodium hydride, sodium amide, and sodium cyanide. Sodium rock salt can also be used for curing fish and meat in the packing and food preparation industry. Minor applications of sodium are found in a heat treating medium, high temperature reactor kettles, fractional 4

5 condensers for metallic vapors, bus bars, sodium vapor lamps, photocells, modified aluminum alloys, and the hardening of bearing metals. 4. Potassium 4.1 Sources of Potassium The most important potassium compound in industry is potassium chloride (KCl). Potassium chloride is also found as potash, which is used as a fertilizer. Potassium is abundant in seawater, as well as in mineral form in the Earth s crust. Seawater contains 380 ppm potassium and the Earth s crust contains 2.6% combined potassium (Dalrymple and Lanphere 1969). Some of the important minerals of potassium are sylvite (KCl), carnallite (MgCl 2 KCl), and polyhalite (2CaSO 4 K 2 SO 4 2H 2 O). From these minerals, some important compounds can be extracted, such as potassium hydroxide (KOH), potassium carbonate (K 2 CO 3 ), and potassium nitrate (KNO 3 ). 4.2 Extraction and Purification Potassium chloride (KCl) is the most important potassium source, due to its wide use in fertilizers. Production of potassium by electrolysis, like sodium and lithium is grossly inefficient due to back diffusion of potassium in the salt that causes short-circuiting in the cell and the formation of metal fog. Salt mixtures with halides that are more stable than KCl have not been found, thus a low melting eutectic cannot be employed (Mausteller et al. 1967). Potassium can only be formed using thermochemical techniques where sodium is used as the reductant. In this process, commercial molten potassium chloride is continuously fed to a packed distillation column where it is further heated (Fig. 3). The molten chloride then encounters sodium vapors that are flowing up through the column produced by a gasfired reboiler (Freitas 2000). The resulting products of this interaction are sodium chloride and potassium metal at equilibrium: Na þ KCl-NaCl þ K; DGð871 1CÞ ¼3 kcal mol 1 ð8þ Equation (8) indicates that formation of pure potassium is not favorable. However, vaporization and condensation of volatile potassium drives the reaction to the right. Variation in condenser temperature and a sluggish removal of potassium can result in the production of NaK or potassium with sodium content of 1 wt.%. Sodium can be lowered to below 50 ppm by distillation in a multiplate tower. The sodium chloride that is produced must be continuously removed from the system. Figure 3 Illustration of a cell for electrolytic production of potassium (Freitas 2000). 5

6 4.3 Applications Most of the potassium that is processed goes into fertilizers in the form of potassium chloride. Potassium chloride is also used in the production of other potassium compounds and solutions such as potassium hydroxide. Potash also accounts for a lot of the potassium that is consumed for agricultural purposes. Finally, potassium can also be found in some explosives and as a coloring agent in fireworks. 5. Hybrid Processing Lithium and sodium are very reactive metals that are produced by electrolytic means. Potassium is produced by sodium reduction. These metals find applications in industry, particularly in the nuclear industry, as a reductant for other metal compounds since the alkali metals form some of the most thermodynamically stable compounds. This fact allows the development of a hybrid process where the electrolytically produced metal can be used in situ to reduce another metal compound. The general chemical scheme is given as follows: 4LiCl-4Li þ þ 4Cl 4Cl -2Cl 2 m þ 4e 4Li þ þ 4e -4Li TiCl 4 þ 4Li-Ti þ 4LiCl TiCl 4 -Ti þ 2Cl 2 ð9aþ ð9bþ ð9cþ ð9dþ ð10þ Titanium tetrachloride has been used as an example of a metal compound. The equations indicate that lithium is a nonconsumable reductant in this process where titanium metal can be produced along with anodically discharged chlorine gas. This type of electrochemical/chemical hybrid reaction schemes is being extensively researched. Some of the requirements for using such a process include insolubility of titanium and titanium chloride in the salt, lower thermodynamic stability of titanium chloride and lack of desire for intermetallic formation. In such a process, the metal compound can be continuously fed into the cell as lithium is being electrolytically generated. 6. Summary Lithium, sodium, and potassium are reactive alkali metals that are produced from their chlorides. Due to the high thermodynamic stability of the chlorides, electrolytic reduction at lower temperatures is commercially used by employing a eutectic salt mixture. Potassium chloride is pyrochemically reduced by sodium, where potassium must be removed constantly from the reaction site to enable the reduction. These alkali metals find extensive applications in industry, mostly in the compound form and as a reducing agent and a coolant in the metallic form. These metals must be handled with care due to their strong affinity for oxygen and moisture. Bibliography Dalrymple G B, Lanphere M A 1969 Potassium Argon Dating: Principles, Techniques and Applications to Geochronology. W H Freeman and Company, San Francisco, CA Fishwick J H 1974 Applications of Lithium in Ceramics. Cahners Publishing Company, Boston, MA Foust O J (ed.) 1979 Sodium-NaK Engineering Handbook. Gordon and Breach, New York, Vol. 5, Chap. 1, pp Freitas D M 2000 McGraw-Hill Science and Technology Encyclopedia. McGraw-Hill, New York Mausteller J W, Tepper F, Rodgers S J 1967 Alkali Metal Handling and Systems Operating Techniques. Gordon and Breach, New York, pp. 4 7 Roskill 1979 The Economics of Lithium, 3rd edn. Roskill Information Services, London, UK Sittig M 1956 Sodium: Its Manufacture, Properties, and Uses. Reinhold Publishing Corporation, New York B. Mishra Published by Elsevier Science Ltd Encyclopedia of Materials: Science and Technology ISBN: pp