INDUSTRIAL A N D ENGINEERING CHEMISTRY
634
widely used in Japan, especially for use in inspections and sometimes in commercial transactions. I n order to compensate for the loss due to oxidation, 3 per cent of tin and 5 per cent of zinc should be added to the percentages of the metals shown in this table.
Table VII-Average
.
of Miura’s Charcoal Meter w i t h Mohs’ Scale of Hardness M O H S ’ SCALE M I U R A ’ S CHARCOAL OF HARDNESS METER T~IC 1 Under 1 Gypsum 2 4 Calcite 3 13 Fluorite 4 18
Table VI-Comparison
Vol. 23, No. 6
Hardness of Charcoal by Miura’s Charcoal Meter
S P E C I E S OF U’OOD
HARDNESS
WHITE CHARCOAL
Qucrcus ilcx. L . war. Phyllireoides Pranch. Qucrcus stenophylla M a k . Qucrcus acuta Thunb. Quercus glauca Thunb. Q U C ~ C U Sglandulifera Blume. Pasania crrpidafa Ocrst. Acer japonicurn, Thunb. v a r . Tygicum G s . el. Schw. Mallofus japonicus Mucll. Arg.
20.0 17.0 18.0 18.0 12.0
11.0 9.5
1.0
BLACK CHARCOAL
Quercus acutissima Carr. Quercus glanduliftra Blume. Ace? piclum Thunb. Castanea publinervis Schneid. Pinus densiflora S.el 2.
14.0 8.0 2.0
1.0
Under 1.0
Preparation of Metal Powders by Electrolysis of Fused Salts 111-Tantalum’ F. H. Driggs and W. C. Lilliendahl WESTINGHOUSE LAMPC O M P A N Y ,
HIS ,paper represents a
BLOOMFIELD,
N. J.
Electrolysis of fused salts has been successfully apt a n t a l u m b y s o m e other plied to the production of tantalum metal of a high means and offer a direct r e continuation of the indegree of purity. The method consists of electrolyzing v e s t i g a t io n dealing duction of a tantalum comtantalum oxide dissolved in potassium fluotantalate pound to pure m e t a l . The with the preparation of the in the presence of other alkali halides. most feasible compound for rare refractory metals by elecThe effect of fluorides and chlorides upon the charthis purpose appeared to be t r o m e t a l l u r g i c a l methods. acter of the deposited metal has been investigated. the double fluoride of potasThe successful preparation of Pressing, sintering, , and degasifying the metal by sium and tantalum, KzTaF,. uranium (1) and thorium (2) heat treatment in vacuo are necessary to obtain ductile The deposition of uranium by electrolysis strengthened metal. and thorium metal from baths the belief that this method The use of this method for obtaining a tantalum plate containing only a small permight have a r a t h e r wide on base metals has been investigated and some results centage of the halide of the a p p l i c a t i o n to metals of a are illustrated. metarmade it seem probable similar nature, such as tanthat a tantalum bath could be talum. The preparation of metallic tantalum by the electrolysis operated in which the proportion of potassium tantalum fluoride of fused salts has been limited, according to the literature, to was very low and thus avoid excessivevolatilization of the salt. The nature of the deposited metal obtained by electrolysis the purification of tantalum. A patent issued to Weintraub (4) describes the purification of tantalum by electrolysis of is especially important in the case of tantalum, because of the fused potassium fluotantalate using an impure tantalum anode subsequent purification, pressing, and heat-treatment steps. and depositing pure tantalum a t the cathode. A tantalum If a dense, coherent deposit is obtained, it must be ground to a powder in order that bars of suitable size may be pressed oxide container was recommended for the fused salt. While the above method might conceivably yield tantalum from the material. If, on the other hand, a very fine powder metal, several factors prevent its practical application. Some is obtained during the electrolysis, the difficulty of washing of the principal objections are: (1) It requires a previously and mechanical removal of insoluble impurities becomes too reduced tantalum metal as one of the raw materials; (2) great. A powder of approximately 200 to 400 mesh seemed the solvent action of potassium fluotantalate upon tantalum the most desirable from all standpoints, and in the following oxide is too pronounced to allow its use as a container; (3) experiments the bath composition and current densities were the high volatility of fused potassium fluotantalate would investigated to obtain a powder approaching this range of involve the use of large quantities of this salt owing to the particle size. high losses from the bath with constant operation; (4) a Preliminary Tests coherent mass of tantalum metal obtained by electrolysis of fused baths is not sufficiently free from combined or abThe first test runs were made in a graphite crucible using sorbed gases to be considered pure enough for subsequent cold- a carbon anode. At the end of the run the salt was dissolved working. out and the metal powder recovered from the sides and bottom I n order that the production of metallic tantalum by of the crucible. When prepared in this manner, the metal electrolysis of fused salts might be carried out on a practical always contained particles of graphite which were difficult scale, all of these objections would have to be avoided. to separate, However, the type of metal powder obtained Previous experience with other metals made it almost certain from various bath mixtures could be determined with a fair that the use of a tantalum anode was not necessary to main- degree of accuracy. tain the concentration of tantalum in a fused bath, since this It was later found that nickel could be substituted for could be accomplished by adding a halide compound of tan- graphite, as it was not attacked by the bath and little if talum to the bath as the tantalum was deposited out. This any alloying took place with the tantalum metal. This would avoid the necessity of a preliminary reduction of made it possible for a number of test runs to be made with a very simple apparatus consisting of a nickel crucible sup1 Received March 20, 1931.
T
INDUSTRIAL, A N D ENGINEERING CHEMISTRY
June, 1931
ported by an iron ring for making electrical contact and heated by means of a gas flame. A carbon rod extended into the center of the molten salt in the crucible and served as the anode. A summary of the results obtained with baths of varying composition is given in Table I. The yields of metal are not total yields, but are the weights of metal powder obtained after the washing and acid treatments. Thus any colloidal or extremely h e metal was lost during the purification and, since this is unsuitable for the preparation of ductile metal, it need not be considered from the standpoint, of useful yields.
as well as potassium chloride and fluoride, in the baths are the same, the only difference being that a portion of the tantalum in run 1s consists of tantalum oxide while all the tantalum in run 45 is in the form of fluotantalate. The particle size of the two powders was about the same, so that very little loss was incurred during washing, but the yield was diminished from 59 per cent in 1s to 10 per cent in 4s. This substantiates the indications in the previous runs that the presence of tantalum oxide increases the current efficiency of the bath.
Effect of Chloride
Ai marked increase in the particle size and current efficiency was noted in run 15, which could only be ascribed to the presence of chloride in the bath. I n order t o investigate this point in detail, a series of runs including lS, 25, and 3s was made in which the proportions of fluoride and chloride were varied between 0 and 100 per cent. The decided increase in the coarseness of the metal obtained by the addition of potassium chloride to the bath probably accounts for a large part of the increase in current yield of useful metal. I n Figure 1 the particle-size distribution of the purified metal obtained from the three baths is given.
Effect of Tantalum Oxide
The use of a straight halide bath usually resulted in the appearance of the familiar “anode effect,” and it was with the purpose of eliminating this that tantalum oxide was added. The effect of the presence of tantalum oxide in the bath was very pronounced in this respect. If a small amount of oxide was added when the anode effect was present, it disappeared almost immediately, only to reappear in a short time. This seemed to indicate that the action was almost identical to that existing in the electrolysis of aluminum oxide in cryolite, where the anode effect appears when the proportion of aluminum oxide in the cryolite bath falls below a critical value. It also suggested the possibility of the electrolytic decomposition of tantalum oxide rather than potassium fluotantalate, the latter playing the role of solvent for the tantalum oxide in the same manner that cryolite dissolves alumina. An investigation of the results of the trial runs listed in Table I indicates that the presence of tantalum oxide is desirable from the standpoint of yield of metal as well as elimination of the anode effect. The use of high percentages of fluotantalate does not increase the yield, as run 1 shows. The metal deposited from this bath was extremely fine and could not be separated mechanically. A distinct improvement was noted in runs 3,10, and 12, where the bath contained tantalum oxide. The powder was coarser and the yields were materially improved. A comparison of runs 1s and 4s illustrates the effect of tantalum oxide upon current efficiency. It will he noted that the concentrations of tantalum, Table I-Results RUN 1
BATHCOMPOSITIONCURRENTTIME Grams Houri A mP KzTaF7 200 15 2 KF 200
635
Preparation of Tantalum Metal
h bath composition corresponding to 1s in Table I seemed the most satisfactory on account of its lower melting point. A series of test runs was therefore made with this bath in the improved electrolytic apparatus used in the preparation of thorium (2). A graphite crucible capable of holding 1000 grams of fused salts served as the anode, and the tantalum was deposited upon a molybdenum or nickel cathode suspended in the center of the crucible. The deposited metal adhered sufficiently well to the cathode to allow of its withdrawal while the bath was molten. A new cathode was then inserted and the electrolysis continued. The complete data for four deposits are given in Table 11. The amount of oxide added to the bath during successive runs was governed by the weight of tantalum metal deposited from the bath, since it seemed evident that practically all of the metal was obtained by decomposition of the tantalum oxide and not the potassium fluotantalate. If, however,
of Preliminary Experiments on Deposition of T a n t a l u m CATHODE
ANODB
Graphite crucible
Carbon
CATHODE CURRENT CURRENT WEIGHT DENSITY EBFICIENCY OF MBTAL Grams AmP./dm.’ % Trace 10
...
Very finely divided
3
KxTaF: TazOs
400 80
25
4
Graphite crucible
Carbon
17
26
10
K*TaF? Tat06
300 10
25
3
Nickel crucible
Carbon
40
4
12
KzTaFl KF TazOs
200 200
26
4
Nickel crucible
Carbon
41
11
15
High percentage of fines
Nickel crucihle
Carbon
23
51
21
Coarse
36 3.8
50 50 10
15
High percentage of very fine powder High percentage of fines
100 40
1s
KC1 KF KtTaF7 Tar06
100
10
1
Nickel crucible
Carbon
16
59
7.99
Coarse
2s
KF KaTaF7 TazOs
150 25 6
10
1
Nickel crucible
I/d-in, carbon
16
22
2.97
High percentage o! fines
3s
KCI KzTaF7 Tal03
150
10
1
Nickel crucible
‘,’a
in. carbon
16
74
9.98
Coarse
4s
KCI KF KtTaFl
100 40 35
10
1
Nickel crucible
‘/,-in, carbon
16
10.68
1.44
Coarse
40
25 6
25 6
INDUSTIIIAI, AiVIi K N GI;VEERING CIIE'MISTRY
636
t.ho aiiode efleut appeared, iidditiooal oxiilc was aililcil i i r i t i f it had been eiimiiintcd. The rate of addition of tire oilier salts wac govwned by tlieir losses due to vihtilizatiori from the fused batli. It will be noted that t,lie total amount of flur)tant.nlate added is not equivaleiit to the weiglit of metal ihtained, thus oflcririg additional evidence that tlie t:rritaluiu is obtained by decomposition of tlie oxide. Tlie deluwit on the cat.iiode consisi.od of about equal proportio~isof mctai and wlidifird salt. K l i m the salt vas dissdveii iii water, the
P
P-i,dC
&=e
D,r,r,d"f,m
Fiilure I--Parficle-Slze Distribution of l'sntalunr Powder Obfained from Baths of \arying Chloride a n d Fluoride Content
Vol. 23, No.6
Tlie jrresence of absorbed or coriiijiiied gases, hor\ww, made it brittle if the rnetal po\der was pressed arid sintered witliiiiit taking any precnritions conrerning degasification in vwwi. Tlie metal powdcr was pressed into bars approximately :30 nim. sgoarc and 15 cm. in length in a hydraulic prf%ssiiiider :i pressore of 12,000 to 14,000 kg. per sq. cm. 1 lie Isus were tlreii degasifitxi in vactro using either the glass heat-treating laittle devised i n this laboratmy (8) or an allmctal vacmiin bot,tle which on account of its more rugged cliaraeter was more suitable since tantalum nrust lie heated wry close to its melting point for complete degasifieation. 'The evoliition of gas froin the tantaliirn metal becomes qirite marked when heated above 1000" C. arid continues until t l i e hnr has been heated to its melting point. I'ractically all of the gas may be renioucd, liowever, if the bar is heated with a ciirreirt e m e d i n g to 00-95 per cent of that required to fuse it and held at this point until the preasure inside tlie bottle has drop,ped to apiiroaiiirutely 0.001 mm. l h m our present knowledge 110 definite stateinelits can be made concerning the equilibrium between tantalum and the gases which are evolved. If definite eompoi~iids,such as hydrides or nitrides, are present, i t might be expected that complete degasificatim cnulil be obtained by liolding the bar at a temperatiire which corresponded, for esample, to a decomposition pressure of 2&60 microns, provided the pressure in the bottle was oiairitainetl below this figure. On the other hand, the difficulty of reirioviiig the last traces of gas, unless the metal is heated very close to its melting point, indicates the prohability of a solid solution of iiitrogeii or hydrogen in the metal.
,.
metal was left as a fairly coarse powder, which was freed from the small amount of insoluble basic tantalum fluoride by grinding and wasliiiig with water. The met& was then ground t,o pass a 25O-mesh screen and boiled with nitric and hydmoliloric acids to remove any traces of base metals. l'ahle Il--nepoaidon of l'sntnlum from Bath Onrre8pondieg to IS B A T HC O ~ i r o s i i i o a Auoso D U K L N C ELSCTKOI.YSIS G?
