ELECTROCHEMISTRY of RARE METALS* COLIN G. FINK Division of Electrochemistry, Columbia University, New York City
A brief accuunt i s given of the electric furnace a d electrolytu cell developments i n the jield of the rare metals. Many of these metals, although used i n industry today in comparatively small amwnts, nevertheless, perform a semice tkat is frequently most fundamental. Thus for example, modern industry and ci&lization considers rare metals such as cesium, barium, tungsten, and cobalt well nigh indisfiensable. Three of the major modern industries are fundamentally responsible for the commercial production of many of the rare metals only recently considered "imfiossible" or most dificult to isolate or to obtain i n quantity: the electrolytic copper and zinc industries; the automotive industry; and the wcuum tube industry.
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that electrochemistry contributed fundamentally in meeting the most exacting demands of the various twentieth century industries. With the aid of the high temperatures attainable only in the electric furnace, with the clean and convenient methods of ,heating by electric induction, and the efficient and completely controllable reduction and oxidation by electrolysis a t relatively low temperatures, it has been possible to evolve products heretofore not possible by non-electrical methods. Generally applicable, too, in the development of the various new electrochemical metals and alloys, has been the observation that as soon as the demand for raw materials from which certain rare metals are derived becomes sufficiently insistent, chemists. geologists, miners, prospectors, and explorers from all over the world respond and often, sooner than one would expect, locate deposits of commercial importance. The classical case of thorium is typical. The rare minerals thorite and orangite of Scandinavia (there is also a small percentage of thorium found in the lava of Vesuvius) soon gave way to the relatively abundant deposits of the monazite sands of Brazil.
URING the last fifteen or twenty years great strides have been made in the commercial development and utilization of a large number of a group of so-called "minor metals" heretofore classed as museum specimens or "curiosities." This development in rare metallurgy has gone hand in hand with developments in other fields, notably in the field of THE ALKALI METALS vacuum technology, comprising the manufacture of electric lamps, radio tubes, photoelectric cells, mercury Lithium is more abundant in the earth's crust than arc rectifiers, and other glass-enclosed apparatus and either zinc or lead or tin. I t occurs chiefly as comdevices. Next to vacuum technology's demands on the plex silicates such as spodumene and lepidolite. The metallurgist have been the demands of the building metal is produced upon electrolysis of a fused bath and transportation industries for metals and alloys (450%) of lithium chloride and potassium chloride that are stronger and lighter, that are more resistant to (about 1t o 1 in proportion). The cell is similar to that fatigue and to corrosion, that are cheaper and better used for fused NaC1. Lithium chloride, which is very than any metals and alloys ever produced in the past. deliquescent, is an important salt in the air-conditioning To appreciate the radical changes that have come about industry. This industry accordingly assures the elecin the building and transportation industries we might trometallnrgist of an ample supply of lithium chloride. recall that it is less than a generation ago that airplanes Metallic lithium is very active chemically and on this were constructed largely of wood and that stone was account it is a valuable scavenger. Its advantage over considered an essential element in building construc- other scavengers is the low melting point of the prodtion. ucts formed and the ease with which they rise to the To recount in detail the many interesting develop- surface of the molten steel or other metal being treated. ments in the electrochemistry of those rare metals that A remarkable lithium alloy is scleron (83 Al; 12 Zn; have contributed so largely to many of the present-day 2 Cu; 0.75 Mn; 0.5 Fe; 0.5 Si; and 0.1 per cent. Li). enterprises, such as cinematography or television, would The introduction of but 0.1 per cent. lithium imparts lead far beyond the scope and purpose of the present physical properties to this aluminum-zinc alloy that commtmication. Accordingly we shall confine our- approach those of steel. Lithium combines with hyselves to the selection of a number of typical cases. drogen to form the solid hydride LiH. Upon fusing In general, it is interesting and gratifying to record this (680°C.) and electrolyzing, lithium goes to the * Presented at the Tenth International Congress of Chemistry, cathode and hydrogen to the anode. Potasium.-As is well known, about ninety-five Rome. Italy. M a y 14-21. 1938.
to ninety-seven per cent. of the potash salts produced in the world are used as fertilizer. The remaining three to five per cent. represents an annual production of about one hundred million kilograms of KzO which is converted largely into chemicals such as chromates, manganates, bromides, and so forth. Only a small fraction is converted into potassium metal. The iron cell used for this latter purpose is charged with a mixture of about equal parts of KC1 and KF which melts a t about 600°C., that is, about 70' below theboiling point of metallic potassium. Following the pioneer experiments of Elster and Geitel, elaborate investigations have been undertaken at several laboratories, leading to the perfection of the potassium photoelectric cell. And although this cell is not as sensitive as either the rubidium or the cesium photoelectric cell, in particular in the red end of the spectrum, nevertheless the potassium cell is relatively cheap and has found many applications outside of the cinematographic and television industries.
METALS OR THE SECOND GROUP
Radical changes have taken place in recent years in the production of the metals magnesium, calcium, strontium, and barium. Although the method comprising the electrolysis of the fused chlorides of magnesium, calcium, and so forth, is still employed a t various factories throughout the world, a goodly proportion of the output of these metals is a t present made in the electric furnace rather than in the electrolytic cell. One of the reducing agents commonly employed is carbon. The success of the process is primarily dependent upon the completeness with which the reverse reaction is prevented: MgO
+C
=
Mg
+ CO
To accomplish this, the carbon monoxide is quickly removed with a stream of cold hydrogen. As will be appreciated, the reaction, which requires a temperature of about 2200°C., is carried out in a closed electric furnace. Magnesium metal, which boils at 1090°C., leaves the furnace as a gas and with the aid of underRubidium and Cesium.--These metals are so very cooled hydrogen its temperature is suddenly lowered to active chemically that they are preferably not produced about 175°C. until the moment they are needed. Both metals are Aside from the large-scale industrial development of used in photoelectric cells. In fact, without these magnesium and its alloys, such as Dow metal, in the metals the "talkies" and television would not he ad- automotive, aeronautic, and machine industries, very vanced as far as they are today. Rubidium is oh- important applications of all of the metals of the magtained from carnallite (KCl.MgCh6H20) in which it nesium group are based on their characteristic "getter" occurs as a valuable impurity. The presence of ru- or scavenger properties. Calcium is a pioneer in this bidium is easily established with the aid of the spectro- field and seriously competes with magnesium, the scope. Cesium occurs in the mineral pollucite, a com- ideal scavenger for nickel, besides being one of the plex cesium-sodium-aluminum silicate from which it earliest getters for radio tubes. It was calcium metal may be recovered by treatment with concentrated that contributed so largely to the success of Ramsay sulfuric acid and leaching with hot water. It can be and Rayleigh's isolation of argon. Calcium as well as separated from rubidium by converting both to the the other metals of the group will combine readily chlorides and then precipitating the cesium as the with every gas except the monatomic ones. Today insoluble complex CsCIShCls. Pollucite has been calcium is used on a very large scale as a scavenger found in a number of localities, notably in Maine and in for the removal of impurities from molten ferrous and South Dakota. The pollucite of the Island of Elha is non-ferrous metals; it is also used for the removal of very much like that found in Maine. Ore containing bismuth from molten lead. fifteen per cent. CspO is now quoted in New York a t One of the common getters used in several vacuum fifteen dollars a pound (thirty-three dollars per kg.) tube works in America is an alloy of twenty parts of which is much lower than the price a few years ago. barium, twenty of strontium, and sixty of magnesium. Various methods of producing cesium metal have A small pellet of this or similar alloy is placed in the been tried out. The usual procedure today is to attach radio tube and, after exhaustion and sealing off, the a miniature nickel crucible (4 mm. in diameter X 3 mm. alloy is volatilized with the aid of a high-frequency coil deep) to the glass stem of the photoelectric tube and to placed over the tube. A goodly percentage of the residplace into this crucible a small pellet composed of a ual gases is converted into solid alkali earth metal mixture of cesium chromate, chromic oxide, aluminum compounds, and the remaining gas in the tube amounts powder, and zirconium powder. We resort to the to a mere ninety million molecules per cubic centimeter, Goldschmidt reaction on a very small scale; The high a sufficiently small number for good tube performance. Of very special interest are the thermionic properties frequency induction coil is placed over the glass bulb, the contents of the nickel crucible become heated, and of barium. Barium is the very "heart" of the automatic Usually a barium-oxide-coated cesium and chromium metal are produced. The telephone service. chromium oxide plus aluminum reaction serves pri- filament is used. This same filament has contributed marily as a local source of heat to ensure the complete very largely to the success of the modern sodium and volatilization of the cesium metal produced. The mercury vapor lamps. In the sodium lamps there is a cesium metal (2 mg. per cell) is finally deposited on an small amount of neon. Upon closing the switch the barium ionizes the neon, the electric discharge through oxidized silver plate which serves as anode.
the neon is established, the sodium globules become heated and vaporize, and thereafter most of the current is carried by the sodium vapor. A very small amount of barium metal (0.09 per cent.) alloyed with the nickel used as electrode in automobile spark plugs assures a steady and uniform spark performance. Furthermore, barium compounds introduced into the electric arc furnace result in a quieter operation of the arcs. Barium metal sells a t five dollars per pound in New York. Calcium metal is quoted a t seventy-five cents a pound when bought in ton lots. Bwy1lium.-Beryllium is included in the second group of the periodic system. This element usually occurs in nature as the mineral beryl. In the early development of the beryllium metal industry, about fifteen years ago, costly and complicated extraction methods were resorted to, involving high temperatures and fusions. Recently, as a result of investigations carried out a t the writer's laboratory and elsewhere, a relatively simple method has been devised for liberating the beryllium from its mineral. The new process is based upon the fractional distillation of the various elements present in the ore, after the conversion of these into the respective chlorides. Beryllium metal is electrolytically produced from a fused salt bath. In Germany the fluoride bath is commonly used, whereas in America we prefer the fused chloride bath. Since almost the entire production of beryllium metal nowadays enters the copper alloy field, it has now become customary to use amolten copper cathode in the bottom of the fused salt bath into which the beryllium is deposited until its concentration reaches approximately thirteen per cent. by volume or about three per cent. by weight. The rapid growth of the industrial applications of the 2.25 per cent. beryllium-copper alloy has been most phenomenal. It is the fmt metal that has been able to compete with spring steel. Copper-beryllium not only has a fatigue resistance about equal to that of steel, but, furthermore, it is highly resistant to corrosion and, finally, i t is a fair conductor of electricity. Accordingly, when under certain exposed conditions the life of a steel spring is limited by fracture initiated by corrosion, frequently occurring a t a mere point, the life of a beryllium-copper spring is extended indefinitely due to its high resistance to corrosion. Millions of copper-beryllium springs are now in daily use withstanding millions of repeated flexures. Even the intricate works of watches are made of this alloy today which makes them proof against stray magnetic currents. The copper-beryllium alloy is fabricated into a wide variety of articles: knives, hammers, chisels (that will cut low carbon steel), spatulas, coiled and flat springs, and many types of wire and strip parts, for various devices, that are subject to repeated flexures. METALS OF THE THIRD GROUP
The important rare metals in the third group are gallium, indium, and thallium. Little research has
been done on the three other rare metals of this group, scandium, yttrium, and lanthanum, and although lanthanum is used to a small extent as ferro-lanthanum serving as the "flint" in place of ferro-cerium in certain types of cigaret lighters, little has been accomplished in finding a well-defined field of general application for these three metals. On the other hand, cousiderable study has, of late, been devoted to gallium, indium, and thallium, due mainly to the fact that these three metals are now available in relatively large quantities. The outstanding metallurgical developments in the copper and zinc industries during the last twentyfive years are largely responsible for the recovery in quantity of an entire group of rare elements formerly almost entirely lost in process. Gallium.-Small amounts of gallium occur in many of the zinc ores and bauxites, often associated with indium and germanium. The present supplies of gallium are derived largely from the copper ores of Mansfeld, Germany. Here gallium accumulates in the aluminum phosphate residues. In the final stage of separation from molybdenum, tin, and other metals, a strong alkaline (NaOH) solution is electrolyzed and gallium precipitated a t the cathode. A similar electrolytic refining step is employed in the recovery of gallium from leady zinc ore residues. Gallium metal has a relatively low melting point (29.5"C.). In the molten condition it adheres strongly to glass. When used in radio tubes i t emits electrons freely. It does not oxidize readily and on account of its high boiling point it does not volatilize appreciably even a t red heat. Electrochemists interested in rare metals will find gallium an interesting subject for research. The market price of gallium is about one dollar per gram. Indium.-Indium has, since 1932, been available in "commercial" quantities, that is, in quantities that permit more thorough investigation than formerly. The usual raw materials are zinc residues. Upon careful addition of an ammoniacal sodium carbonate solution to a dilute nitric acid solution of various impurities contained in the zinc blende, cadmium, zinc, copper, and nickel remain in solution and indium precipitates as the hydrate, In(OH)%. Indium metal plates out easily from an alkaline cyanide bath. Indium melts a t 155OC. and alloys readily with gallium, gold, lead, tin, thallium, cadmium, bismuth, and other metals. Investigations on indium are in progress a t half a dozen laboratories in America, including our own, and results so far are particularly promising in the alloy and electroplating fields. In New York indium now costs approximately one dollar per gram. Thallium,-Thallium occurs in zinc and in lead ores. It enters the market today chiefly as salts used for the extermination of rodents. Electrochemical studies of the metal have been most interesting and have revealed that good, smooth, adherent deposits of thallium metal are obtained from acid perchlorate baths. Similarly, alloys of lead and thallium are deposited from a perchlorate solution of the two metals. The alloys obtained are among the most corrosion-resistant
alloys known. Alloys containing from twenty-five to sixty per cent. of thallium and the balance lead are among the most insoluble anodes for mineral and acid solutions. METALS OF THE FOURTH GROUP
Germanium, like gallium and indium, is a by-product of the zinc industry. The residues of the zinc retorts in the Joplin district frequently average 0.25 per cent. germanium. Separation of germanium from other metals in these residues is readily brought about by means of chlorine, the tetrachloride of germanium, GeCl,, being comparatively volatile. In the electrolytic zinc industry germanium is an objectionable impurity; concentrations as low as 1 mg./Ge per litre of electrolyte result in serious losses in current efficiency. Germanium metal is very resistant to strong alkali solutions. I t is also resistant to dilute sulfuric acid and concentrated hydrochloric acid. Now that germanium is more easily procured, the corrosion-resistant qualities of the metal merit further study. Zirconium.-To see dozens of spools with thousands of meters of zirconium wire and to see beautiful bright sheets of zirconium metal, emphasizes the recent remarkable progress made in the metallurgy of this rare element. The selling price of the metal is less than seven dollars per pound. Most of the zirconium metal produced today is as an alloy of iron and silicon. The alloy is made in the electric furnace, with silicon as the reducing agent. It serves as a valuable scavenger in the cast iron and steel industries, eliminating oxygen, nitrogen, and sulfur. Zirconium metal is usually made by interacting zirconium chloride, ZrCL, with magnesium or sodium. Zirconium metal wire is used in radio tubes as are likewise the nickel-zirconium alloy wires. Zirconium sheet and zirconium-nickel alloy sheet are made into spinneret cups for rayon manufacture. Zirconium metal is very resistant to chemical attack. Hydroflnoric acid is the only acid that will dissolve the metal readily. The metal can be electroplated from baths containing sodium and zirconium sulfate. METALS OF THE RIPPA GROUP
Vamdium.-The rare metals of this group are: vanadium, columbium, tantalum and bismuth. There is no vanadium metal made as such, but large quantities of vanadium are produced as ferro-vanadium in the electric furnace. Formerly ferro-vanadium was made by the Goldschmidt reaction with finely divided aluminum metal used as reducing agent for the iron vanadate. The high cost of aluminum as a reducing agent and other factors led to the substitution of silicon in place of aluminum. Ferro-vanadium is now made in the electric arc furnace, with silicon nsed as reducing agent. Columbium.-Within the space of five years columbium has passed from the rating as a very rare and precious metal to one of everyday industrial application. In the study of the intergranular corrosion of
stainless steels, notably the 18/8 variety (eighteen per cent. Cr; eight per cent. Ni), it was found that the addition of small amounts of columbium effectively counteracted this corrosion. Immediately the world was searched for a commercial supply of columbium ore. A promising field is the Nigerian tin mines of Africa. The output has now reached about six bundred tons of columbite ore per year. The ore is converted to ferro-columbium in the electric arc furnace using silicon as a reducing agent. The ferro-columbium (fifty per cent. Cb) output is now in excess of one thousand tons per annum. In the production of metallic columbium we have found that we can deposit the metal on the high-speed rotating cathode from a sulfate-oxalate bath. We have also found that in place of the costly and cumbersome extraction of columbium from its mineral by fluoride fusion we can easily extract columbium, as well as tantalum, by treating the finely ground ore with an aqueous solution of oxalic and hydrofluoric acids. Tantalum.-The metal is now used extensively in chemical engineering due to its outstanding chemical and physical properties. It is highly resistant to corrosion; it can be hammered and drawn and welded. Stills and kettles are lined with tantalum sheet. It is nsed in making spinnerets for rayon; grids and plates for radio tubes; and cblorine-resistant needle valves. Seamless tantalum tubing is available in a wide range of sizes down to the small-bore tubing used in hypodermic needles. Tantalum carbide is very hard and competes with tungsten carbide. Bismuth.-The lead industries of America have, during recent years, been promoting new fields of application for this metal. It sells today in New York a t one dollar per pound and is used in low-fusing alloys for a wide variety of purposes. The Betts electrolytic refining process for lead was originally particularly attractive because it did recover the bismuth. The process has been in operation for over thirty years and is still considered one of the best. A relatively new development is the separation of bismuth from molten lead by the addition of calcium metal. Bismuth is readily electroplated from the perchlorate bath. Similarly, alloys of lead and bismuth are deposited from a perchlorate solution of the two metals. METALS OF THE SIXTH GROUP
Molybdenum is widely used both as metal and in steel alloys. The world's annual production of molybdenum has reached 90,000 metric tons so that rightly molybdenum should no longer be classed as a rare metal. Ferro-molybdenum is made in the electric furnace, and i t is in this form that it is added to steel. Thiuwalled, high-strength, chromium-molybdenum steel tubing has made modern high-speed airplanes possible. Molybdenum added to steel improves its toughness and fatigue resistance. Added to cast iron molybdenum eliminates porosity and increases the high temperature strength.
Molybdenum metal as such is used extensively for parts of radio tubes. Tungsten.-Tungsten is no longer considered a rare metal. Its many steel alloys for magnets and highspeed cutting are too well known to need further comment here. We have been particularly interested in metallic tungsten. Of the two methods of producing ductile tungsten which we discovered over thirty years ago, the one by hot working and the other by special heat treatment, we recommend the latter as eventually becoming the preferred method. We have converted ordinary brittle tungsten into soft malleable metal by carefully eliminating the last traces of oxygen by heat treatment in hydrogen containing a small percentage of hydrocarbon. Tungsten metal and tungsten alloys are electroplated from alkaline solutions. The deposit is not as resistant to wear and corrosion as chromium. Uranium.-Uranium metal is produced by the electrolysis of the fused potassium uranium fluoride (KUFs). The metal is malleable and ductile. Large quantities of uranium mineral are available as a forced by-product of the radium industry. In spite of several distinctive properties of uranium metal, little use has so far been found. Its X-ray efficiency is about onethird greater than that for tuugsten. Selenium and Tellurium.-Selenium and tellurium are both derived from the anode slimes of the electrolytic copper refineries. Although only a few years ago both metalloids were largely discarded and wasted, today they are carefully recovered. The outstanding development in selenium is its application in the new photoelectric cells of the cuprous oxide type. These cells are not based on the old principle of change in ohmic resistance with illumination but on the electronic emissivity of a thin film of selenium when exposed to light. Tellurium has become an important reagent in the purification of electrolytic zinc solutions. The addition of tellurium to the electrolyte assures a more complete removal of objectionable traces of cobalt. Of special interest, too, is the use of tellurium vapor in certain tube lamps. Ionized tellurium vapor radiates light similar to daylight with a continuous spectrum. The envelope is of quartz and the discharge through the tellurium vapor is started as in the case of the sodium lamp, with a little neon. Electrodeposits, a centimeter thick, of selenium and tellurium can be obtained from the fluoride-sulfate bath. Small percentages of selenium added to steel make it free machining. As in other cases, the selenium is added as a ferro-alloy. Similar effects on steel are obtained upon the addition of tellurium. Added to lead in small percentages tellurium makes lead more resistant to corrosion. Both selenium and tellurium are now available a t two dollars per pound.
METALS OF THE SEVENTH GROUP
Manganese.-Manganese metal of very high purity is now being electrolytically recovered from low-grade manganese ores. The electrolyte is an acid ammonium sulfate solution. The method promises to become applicable to many manganese ore deposits heretofore considered worthless or uneconomical. Rhenium.-Rhenium is a rare metal associated with stibnite and is also present in certain electrolytic copper refinery slimes. The metal may be readily deposited from acid sulfate solutions. We are still engaged in finding a wider applicatiou for rhenium metal nlntp r----. METALS OF THE EIGHTH GROUP
Cobalt.-Aside from the platinum metals we include cobalt as a rare metal of the eighth group. It is rare when compared with iron and nickel of this group. However, cobalt is a metal of very wide commercial application. The electric furnace produces many tons of the alloy stellite composed of chromium, tungsten, and cobalt, one of the very best alloys for cutting tools ever developed. Another recently developed alloy is the strongly magnetic aluminum-nickel-cobalt-iron alloy commonly termed Alnico. Its magnetic qualities are far superior to those of the old four per cent. tungsten steel. Cobalt is the best cement for tungsten carbide. Cobalt occurs in Canada as smaltite associated with silver; in Katanga and Rhodesia associated with copper minerals and in Australia associated with sphalerite. Cobalt is readily electroplated from the acid sulfate bath. Very little nickel as such is plated in America today. The so-called "bright nickel" deposits are usually an alloy deposit of nickel and cobalt. A deposit composed of one part of cobalt and two of nickel is the whitest deposit known. Even silver looks yellowish in comparison,due to an unavoidable tarnish filmon the silver. Finally of interest is the insoluble cobalt silicide anode which is more insoluble than platinum in a solution containing all three mineral acids, sulfuric, nitric, and hydrochloric. Rhodium.-Rhodium is the most interesting of the platinum group metals from an electrochemical point of view. Millions of articles are, nowadays, rhodium plated, including intricately designed silver jewelry and large three-foot reflectors used for the beacons to guide the air pilots a t night. The metal has a high reflectivity, is relatively hard and very resistant to corrosion. The sulfate plating bath is very stable and has been employed in our laboratories for years. The electrolytic nickel industry supplies a major part of the platinum metals today. One of these, palladium, deserves further intensive study. Comparatively large quantities of this metal are available.
