560
INDUSTRIAL AND ENGINEERING CHEMISTRY
Ductile Tantalum
reduction of these pure compounds of tantalum, which pracess involves the use of vacuum furnaces specidly constructed for the purpose.
By Clarence W. Balke FANSTEEL PRODUCTS Co., NORTH CHICAGO, I~L.
CHEMICALPROPERTIES
B
ECAUSE of its many unique properties, and particularly because of its great resistance to chemical corrosion, the metal t'antalum should soon become of considerable importance to t'he chemical and allied industries as a material for equipment construction. The extent to which this metal will be ultimately used in these connections mill, of course, depend in large measure on the price at' which it can eventua,lly be produced. Tantalum was known in its compounds for a. hundred years before any process 'was devised to produce it in the pure metallic form. In 1801 Hatchett discovered the oxide of the new met,al in a black mineral which he obtained from the British Museum, and which he named "columbium." A year later a similar discovery was made by Eckeberg while working with certain minerals obtained from Sweden, and he named the metal whose compounds he discovered "tantalum." Many distinguished chemists &died these elements. In 1866 Marignac developed his classical method for their separation depending upon the difference in solubility of their double fluorides with potassium. Impure Corms of the metal were produced in 1824 by Berzelius and in 1902 by Moisson. In 1903 yon Bolten developed a, process for the production of tantalum of sufficient purity to make it possible to produce drawn filament wire for incandescent lamps, and millions of these lamps were produced between the years 1905 a.nd 1911, when this material as a filament wire was replaced by tungsten. PREPARATIOK OF TANTALUM COMPOUNDS As a preliminary step to the preparation of metallic tantalum, it is necessary to produce pure compounds of this metal. Finely pulverized tarit.alite, which is the most important of tantalum-bearing minerals and which should contain at least 60 per cent of the oxide, is fused with potassium hydroxide. This operation converts the tantalum and columbium present in t'he ore int.0 soluble tantalates and columbates. Potassium hydroxide is used instead of sodium hydroxide, owing to the fact that the potassium salts are soluble in solutions containing free alkali, whereas the sodium Density ..................................
The most characteristic chemical property of tantalum is its unusual resistance to chemical corrosion. It is not attacked by hydrochloric or nitric acids or by aqua regia, either hot or cold. It is not attacked by dilute sulfuric acid a t ordinary or more elev?ted temperatures, but appears to be slowly attacked by boiling, concentrated suIfuric acid. Solutions of caustic alkalies do not attack the metal. Hydrofluoric acid seems to be the only chemical agent which will attack it, and in the case of very pure metal and very pure hydrofluoric acid the action is very slow. A mixture of hydrofluoric and nitric acids mill attack the metal with avidity, causing it to go into solution as tantalum fluoride. If tantalum is heated in the air, the surface becomes blue a t a temperature of about 400" C., and at a somewhat higher temperature, nearly black. Above a dull red heat the white oxide is produced and the metal gradually burns. This metal combines vith avidity with hydrogen, oxygen, or nitrogen. It will take up 740 times its own volume of hydrogen, producing a very coarse-grained, brittle product. Tantalum containing dissolved gases will be harder than the pure metal, and if their quantity is appreciable the metal may even be brittle-so all annealing or heating operations with tantalum must be carried out in a vacuum. Tantalum burns readily when heated in chlorine gm, producing the volatile pentachloride. Solutions of chlorine, however, are without any action on the metal. Tantalum is not affected by any of the chemicals or antiseptics used in dentistry o r surgery. PHYBICAL PROPERTIES
It has been possible to produce metallic tantalum of an exceedingly high degree of purity, and in commercial quantities with a purity of at least 99.5 per cent. The pure, worked material resembles platinum very much in color and appearance. Its melting point may at present be taken as 2850' C. The specific gravity of the worked metal is 16.6. The pure metal is characterized by toughness and by its great ductility and malleability.
Tungsten
......................
er sq. i n . , ....................... 490,000 Compressibility per kg. per sq. c m . , . . . . . . . . . . . . . . . . . . . 0.28 X lo-' Young's modulus of elasticity, kg. per sq. m m . . . . . . . . . . . 42,200 Melting point, C . . . . . . . . . . . . . . . . . . Boiling point C . .................... Specific heat 'cal. per p;. pe' degree.. ..... Linear coeffiAient of expansion per degree.. . . . . . . . . . . . . . 10 -6 Thermal conductivity in cal. per c c . , . . . . . . . . . . . . . . . . . . . 0.35 Temperature coefficient of expansion., . . . . . . . . . . . . . . . . . 0.0051 Electrical resistance microhm per cc. a t 15' C. annealed, . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
I
V d . 16, No. 6
Tantalum
Molybdenum
Platinum
Copper
16.6 10.9
10.2 8.8 260,000
21.4
8.80
8 84
62,000 38,000 0.76 X 10-6
120,000 70,000
ids3 ' '
L30,OOO 1.50 X 10-6 19,000 2770
0.47 X 10-8
......
7.9 10-8 I . 130 3.00335
2550 3617 0.072 ij.15 X 10-8 0.346 0.005
1755 3907 0,0323 8.84 X 10-6 0,1664 0.0039
0.7198 0.00393
......
22,000 1452 0.1084 13 X 10-6 0.140 0.0066
14.6
4.8
9.97
1.87
6.4
....
I1.0366
salts are practically insoluble in such solutions. The fusion is dissolved in water and after filtration it is treated with sufficient hydrofluoric acid to neutralize all the alkali present. This converts the tantalum and columbium into the double fluorides of these elements with potassium. The potassiumtantalum double fluoride is difficultly soluble and immediately precipitates, whereas all the columbium remains in solution. This precipitated potassium-tantalum double fluoride can then be purified to any desired extent by recrystallization. From this material any of the other compounds of tantalum may be produced. The oxide is easily obtained by precipitating tantalic acid from a solution of this salt, washing the precipitate, and igniting the acid to the oxide. The production cf pure metallic tantalum in the coherent form involves the
{Hard 54,000 Annealed 42,000
h3ckel
x
......
......
.
2310 0.0936
...... ......
It has been found possible to reduce a bar of tantalum about 3/8 in. in diameter to wire of only a few mile in diameter without intermediate heating to the annealing or equiaxing temperature, although evidences have been observed that the material is subject to strain-hardening, resembling the more common metals, such as silver or copper, in this respect. Copper and silver may be worked cold, but these metals become quite rapidly strain-hardened so that a further reduction makes it necessary to anneal the metal. Tantalum may be worked cold to a remarkable extent without annealing. The tensile strength of drawn tantalum wire may reach 130,000 lbs., being considerably more than that of harddrawn copper, nickel, or platinum, but being considerably less than that of molybdenum or tungsten.
INDUSTRIAL AND ENGINEE&IiVZ: CHEMISTRY
June, 1923
The h e a r emEicient of expansion is considerably more than that of m~olybdenumor tungsten, and only slightly less than that of platinum. For this reason it has been found possibEe to suaaessfdy seal tantalum into glass. The electricd nesistanoe is quite high, being about eight times that (of copper m d about three times that of tungsten. The foregoing table givesthe physical properties of tantalum as far as they .are known a t present, and also those of a number of other met& in eomparison.
USES
OF
TA~TALLJM
In considering the possible uses for this metal, we must take into account its high melting point, its resistance to cliemical corrosion, amd its tendency to absorb all of the common gases. We must also remember that me are limited by its rebtivdy low temperature of oxidation. Tantalum seems t o be a very d?esirable metal for the manufacture of certain dental instruments and dental spatulas, and undoubtedly for other dental and surgical tools or instruments. The metal is not ahtacked by any of the antiseptics or chemicals used, and can be readily sterilized by heat. A surface film of hard mnterial, about as hard as agate, can be produced on the metal by proper heat treatment. It will probably be found possible to harden the material throughout, thus combining all the advantages of tempered steel with absolute chemical inertness. It has been suggested for use in the manufacture of pens and analytical weights. Its use in chemical laboratories and in the chemical industries as containers, parts of pumps, and other equipment will undoubtedly depend upon the cost a t which the metal can ultimately be produced. Tantalum is suitable for cathodes in electrochemical analysis. In some respects it is more suitable than platinum. For instance, zinc can be plated directly upon the tantalum, as it does not alloy with the metal. Gold or platinum can be deposited upon the metal, as they can be removed by aqua regia without attacking the electrode. Undoubtedly, tantalum in the form of sheet, wire, or ribbon will find application in the manufacture of radio sending and receiving tubes. The property of absorbing gases would seem to make the metal its own “getter” in vacuum tubes, and would tend to maintain the high vacuum required, particularly in the sending tubes. It would seem that some part of the lamp made of tantalum could be so constructed that a t all times a portion of the metal mould be a t the proper temperature to absorb gas, and therefore tend to maintain a vacuum equilibrium within the bulb. As already mentioned, drawn tantalum wire was used for several years for the manufacture of incandescent lamps. While tungsten has replaced tantalum for this purpose, it still seems that there might be certain types of lamps in which tantalum would be preferable to tungsten, par?icularly those in which it is difficult to place a sufficient length of very fine tungsten wire to produce the resistance required on the ordinary circuit. The resistance of tantalum heing so much higher, either a shorter wire or a mire of larger diameter could be employed. The following table gives some data with reference to carbon, tantalum, and tungsten filament lamps: Watts per Hefner Candle MATERIAL Power Carbon 3.50 Tantalum 1.60 Tungsten 1 15
Temperature Hefner Candle of Ratio of Hot Power per Sq. Incandescence t o Cold Mm. of Surface “C Resistance 0 154 1800 0 50 0 307 1700 6 07 0.441 2150 12 1 2
The table shows that the candle power per watt in the tantaliini lamp is 220 per cent of that in the carbon lamp, and that in the tungsten lamp it is about 300 per cent of that
561
of the carbon lamp. The candle power per square millimeter of surface is in about the same ratio. Tantalum has interesting possibilities on account of its property of acting as an electrolytic valve. If two plates of bright tantalum metal are placed in an electrolyte and the two plates connected to an electric battery, there is an instantaneous flow of current similar to that between plates of the better-known metals. Within a few seconds the current flow will drop to a very small comparative value, provided the battery voltage is not too high. The order of the current flow will become that of 1 milliampere and less with impressed direct current voltage up to 75 volts, when sulfuric arid of the strength ordinarily used for storage batteries is the electrolyte. This drop in current flow will be accompanied by the formation of a film, presumably of tantalum oxide, on the tantalum anode. This film often shows beautiful iridescent colors. If a tantalum plate and a lead plate are placed in an electrolyte and a source of alternating current of the usual commercial frequency is connected to the tantalum and lead plates, the current flow in one direction will be almost entirely shut off and a pulsating direct current will be obtained. In such a set-up this flow of current is accompanied by electrolytic action, with evolution of hydrogen gas at the tantalum and oxygen at the lend. The action of the tantalum, therefore, is such that electrons are permitted to flow from the tantalum to release hydrogen ions,, but are prevented from passing from oxygen ions into the tantalum. The current derived from this apparatus may be utilized for charging storage batteries, for the electrodeposition of metals, and for various other electrochemical actions requiring a direct current. It is possible, by using two tantalum electrodes in a single cell, to so rectify the current that both half waves of alternating current pass in the same direction. This current may be smoothed out, by a suitable series of inductances and capacities, to give what is practically a constant direct current. The efficiency of tantalum as ,z valve with respect to leakage of the current varies with the impressed voltage, the electrolyte, current density, etc. Owing to the fact that tantalum is very inert toward the chemical action of solutions, there is a wide choice of electrolytes, and the life of the tantalum appears practically unlimited. When operating as a rectifier a cell such as described above shows a temperature rise. Investigation has led to the conclusion that this temperature rise is due to a high electrical resistance at or near the surface of the tantalum. d n idea of the amount of tantalum required can be given by the statement that where charging a 6- to 8-volt storage battery a t an indicated rate of 3 amperes, a tantalum electrode of very thin sheet and 2 in. square gives very satisfactory results. I t is possible to get practically the same indicating charging rat6 with a much smaller piece. For example, a jrire 2 or 3 in. long and 0.010 in. in diameter will give a similar reading on an ammeter. In the case of the sheet electrode the action is accompaiiied by the normal gas evolution of electrolytic weight, while in caqe of the wire there is considerable sparking or arcing around the tantalum. An oscillograph study of the wire electrode under the foregoing conditions shows that there are very short time periods when there is considerable leakage of current. For a charging set-up with a 6- to 8-volt battery the energy efficiency is approximately 33l/3 per cent, which compares favorably with rectifiers of the hot and cold electrode type and the mechanically vibrating rectifiers. The tantalum battery-charging rectifier is noiseless in operation, has no moving parts, and requires only one at-
562
INDUSTRIAL A N D ENGINEERING CXZMIXTRY
tention, which it has in common with the storage battery itself-that of distilled water being added to replace evaporated and decomposed water of the electrolyte. In addition to functioning directly as a rectifier for obtaining continuous current, apparatus built along similar principles may be used for electrolytic condensers and detectors, and possibly lightning arresters. Among other metals which have this property of valve action more or less in common with tantalum are especially magnesium and aluminium. However, owing to the ready susceptibility of both these metals to chemical corrosion, they have not proved very suitable as sources of direct current. Condensers and lightning arresters for high-potential transmission lines are commercially used with aluminium plates.
Concrete in the Construction of Chemical-Manufacturing Facilities
TABLEI-RATING
TYPEOP COLUMN Reinforced concrete
Reinforced concrete
Structural steel
PORTLAND CEMENTASSOCIATION, CHICAGO, ILL.
T
1 Tentative Standard Specifications for Floors of the American Concrete Institute may be had on request from the Portland Cement Association. 2 Copy of full report may be obtained from the Underwriters' Laboratories, the National Fire Protection Association, 70 William St., New York City, or the Bureau of Standards, Washington, D. C.
COLUMNS
ON
BASISOF FIRETESTS
RATING No. FIREPROOFING Hrs . 1 Limestone or calcareous aggregate with 2-in. covering over reinforcing steel 8 Embedded in limestone or calcareous gravel concrete with a minimum cover of 4 in. on wire mesh 8 3 Same as No. 2 but minimum covering 3 in. 0 Same as No. 2 but minimum covering 2 in. 4 Trap-rock aggregate with 2 in. covering over reinforcing steel 5 Embedded in trap-rock concrete with a minimum covering of 4 in. on wire mesh 5 Same as No. 6 but with minimum covering of 3 in. 4 Same as No. 6 but with minimum covering of 2 in. 3 9 Protected by common brick on side 5 10 Same as No. 9 but with common brick on side and end 1 '11 Embedded in granite sandstone or hard-coal cinder corkrete with &inimum cover of 4 in. on wire mesh 5 12 Same as iVo. 11 but with minimum cover of 3 in. 31/a Same as No. 11 but with minimum cover of 2 in. 2l/z Protected by solid gypsum block 4 in. thick 31/2 Same as No. 14 b u t 3 in. thick 21/2 Same as No. 14 but 2 in. thick 11/2 Partly protected having both the interior of t h e column and the exteior reentrant spaces filled with concrete 2 to 31/2 Embedded in siliceous gravel concrete with a minimum cover of 4 in. over wire mesh 21/a Same as No. 18 but with 2 in. cover 1 Protected b y hollow tile and concrete filling 2 t o 21/2 Protected by portland-cement plaster (10 per cent hydrated lime) on ribbed metal lath 2 Filled and protected by 2-in. thickness of trap rock, granite, or hardcoal cinder concrete or hollow tile and porous semi-fire clay 2 Protected with two layers of portland-cement laster (10 per cent h drated lime? on metal lath each in. thick 1'/n Same as No. 23 b u t with 1 in. layer of plaster a/, Solid section protected by hollow tile 1 to 11/2 Partly protected having exterior reentrant spaces filled with concrete 1/a t o S / r
i2
Structural steel
By A. C. Irwin and Fred W. Ashton
HE USE of concrete in facilities a t chemical-manufacturing plants may be divided into the following general headings-buildings proper, and tanks, containers, etc. BUILDINGS The methods of design of ordinary concrete buildings are matters of more or less common knowledge to structural engineers and a discussion of them would be out of place here. There are, however, some qualities which seem to pertain with especial importance to buildings used for chemical-manufacturing purposes. Among these may be mentioned fireproofness, inherent sanitary qualities, and resistance to wear or attack from chemicals. Floors should be able to resist wear from foot and truck traffic, be easy to clean, and not be disintegrated by chemicals which may come into contact with them. A well-made and cured concrete floor1 will leave little to be desired in its ability to resist wear and the ease with which it may be cleaned, but the question of the action of chemicals on it may need separate attention. I n selecting a floor for a chemical-manufacturing plant, the particular use to which the floor will be subjected should be considered. If the floor is continually wet or alternately wet and dry, material should be chosen which will not be affected by these conditions. Strength needed to carry heavy vats, furnaces, etc., and spans required to give unobstructed space may be decisive as to the type of structure. Ease of cleaning may be of great importance, and certain chemical processes may require special covering or treatment. Reinforced concrete girder spans of over 125 f t . have been used in buildings. Floor loads as high as 2000 lbs. per sq. ft. have been carried by concrete floors. If special floor coverings are necessary, concrete is admirably adapted to form a proper base or surface bond for covering or treatment. The best evidence of the fire-resistive qualities of various building materials or combinations of them was obtained in the extensive fire tests on building columns conducted at the Underwriters' Laboratories, Chicago.
OF
Vol. 15, No. 6
Round cast iron
4
75
Structural steel
Table I gives some rating periods assigned to the materials and 'combinations of materials involved in the tests. It will be seen that the columns having the highest rating consist of reinforced concrete columns with 2-in. covering over the steel reinforcement and structural steel columns with 4-in. covering of concrete over the steel. It is also worthy of note that the aggregate used in making the concrete has a decided effect on its fire-resistive ability. Thus, quartz aggregate has a tendency to spa11 off and expose fresh concrete or the reinforcement to the heat. Limestone or calcareous aggregate, on the other hand, calcines on the surface exposed to the heat, but the process of calcination is retarded or wholly inhibited by the protection to the underlying concrete afforded by the surface calcination. This protective coating has a counterpart in the case of many chemicals in contact with concrete. Certain acids form chemical compounds with the cement, which act as protective coatings and inhibit or greatly decrease further chemical action.
CONTAINERS AND TANKS I n general, the hydration of cement results in a number of compounds, among which are hydrated tricalcium aluminate, calcium hydroxide in hexagonal crystals, and noncrystalline (amorphous) hydrated calcium silicate. There are also formed other compounds of magnesia, iron, and alkalies. On contact with air a skin coat of calcium carbonate is formed on the surface of concrete through the action of carbon dioxide on the calcium hydroxide. With these compounds the chem-
