Process for producing transition metal powders by electrolysis in melted salt baths

The invention relates to a process for producing transition metal powders by electrolysis in melted salt baths. This process is characterized in that electrolysis is performed in such a way that the deposition voltage of the transition metal is 0.1 to 0.4V below that of the alkali metal or alkaline earth metal which it is the easiest to reduce. It is used in the production of powders having dimensions between a few fractions of a micron and approximately 200 microns from metals belonging to groups IVb, Vb and VIb of the periodic classification of metals, such as e.g. titanium, zirconium and hafnium.

The present invention relates to the production of transition metal powders 
by the electrolysis of their halides in melted salt baths. 
1. Transition metals refer to any metal belonging to columns IVb, Vb, VIb 
of the periodic classification of elements. 
2. Powder is understood to mean a finely divided solid substance having 
grains with a size between a few fractions of a micron and approximately 
200 microns. 
In connection with expensive metals, such as transition metals, there is a 
considerable interest in applying powder metallurgy shaping methods, due 
to the considerable material economies resulting therefrom. The main 
difficulty encountered in this connection is the producing of powders with 
a suitable quality. 
Reference is made to the following among the presently used processes: 
from solid metals: 
1. the process involving hydrogenation, grinding and dehydrogenation, 
2. processes involving electron beam or arc melting and centrifugal 
atomization; 
from an oxide or a salt: 
the process involving reduction by hydrogen at a very high temperature. 
Generally, these processes require large, complex and costly installations. 
In addition, they do not always lead to suitable powders, either from the 
purity standpoint, or from the standpoint of grain size or grain shape. 
The process according to the present invention comprises electrolysis of a 
halide of the metal, particularly its chloride, dissolved in a bath of 
melted salts based on alkali metal or alkaline earth halides, performed 
under special conditions. Electrolytic processes, which are known for 
these metals, lead to deposits of excellent quality from the purity 
standpoint and which are in the form of more or less solid or dendritic 
crystals, which can be directly used for melting purposes, but which are 
unsuitable for powder metallurgy. 
It has been proposed to obtain more highly divided forms by greatly 
increasing the current densities on the deposition cathodes, but under 
these conditions there is a very poor or even non-existent adhesion of the 
metal to the cathodes. The products obtained become detached and are 
dispersed in the bath, where they are prejudicial to the electrolysis 
operation and are difficult to recover. 
The Applicant found that it was possible to obviate this disadvantage using 
conventional current densities (0.3 to 1.0 A/cm.sup.2) and obtain 
sufficiently adhesive pulverulent deposits to permit extraction with 
cathodes. 
The process is characterized in that electrolysis is obtained in such a way 
that the deposition voltage of the metal to be obtained in powder form is 
0.1 to 4.0 V and preferably 0.2 to 0.3 V below that of the alkali metal or 
alkaline earth metal which is the easiest to reduce. 
It is known that the deposition potential E of a metal from the solution of 
one of its salts is given by the NERNST law: 
##EQU1## 
in which E.sub.0 is the normal potential, R the constant of perfect gases, 
T the temperature in degrees K., n the number of electrons exchanged, F 
the FARADAY number and a the activity of the ions of the metal in the 
solution. 
Thus, there are clearly two ways of modifying E, either by acting on a, 
i.e. on the concentration, or by acting on E.sub.0 by modifying the 
complexing state of the ions. 
The research carried out for realizing the invention was carried out in a 
cell comprising a metal tank containing the molten bath and a metal cover 
ensuring the sealing of the system and having a number of openings, inter 
alia for the tight, insulated passage of the anode and cathode devices 
immersed in the bath, the supply of the bath with the halide of the metal 
to be produced and the extraction of the halogen formed through the anode.

The following examples illustrate the application of the process according 
to the two embodiments described hereinbefore. 
EXAMPLE 1 
This example relates to titanium. In this case, the anode device also has a 
diaphragm subdividing the bath into two compartments, namely an anode 
compartment only containing traces of titanium in solution and a cathode 
department in which the dissolved titanium content is kept constant as a 
result of a continuous supply means. 
The bath is constituted by an equimolecular mixture of potassium and sodium 
chlorides melted at 750.degree. C. Titanium tetrachloride is the halide 
introduced. Under conventional electrolysis conditions, the titanium 
content dissolved in the bath is 4%. 
With an initial cathode current density of 1.0 A/cm.sup.2 the titanium 
deposition voltage measured by plotting the voltage/current curve is 2.15 
V and that of the alkali which is the most difficult to reduce, i.e. in 
the present case sodium is 3.2 V. 
The deposits collected on the cathode are in the form of well crystallized 
dendrites which can reach several centimeters and comply with the 
following analysis in ppm: 
__________________________________________________________________________ 
O Al Fe Cu Mn Si Sn V Y Mo remainder Ti 
__________________________________________________________________________ 
380 
&lt;50 
77 &lt;20 
&lt;50 
&lt;100 
&lt;100 
&lt;50 &lt;50 
&lt;10 
__________________________________________________________________________ 
The electrical efficiency exceeds 90%. 
On reducing the titanium content in the cathode compartment to 0.1%, under 
the same current density conditions, the titanium deposition voltage 
becomes 2.9 V and that of the alkali remains equal to 3.2 V. On the 
cathode is collected a type of grey felt constituted by intermixed fine 
dendrites, which after washing with water give a powder which almost 
entirely passes through the 100 micron mesh size screen and which complies 
with the following analysis in ppm: 
__________________________________________________________________________ 
O Al Fe Cu Mn Si Sn V Y Mo remainder Ti 
__________________________________________________________________________ 
700 
&lt;50 
130 
&lt;20 
95 &lt;100 
&lt;100 
&lt;50 &lt;50 
&lt;10 
__________________________________________________________________________ 
The electrical efficiency exceeds 85%. 
EXAMPLE 2 
This example related to hafnium. 
Using the same cell as in example 1, but without an anode diaphragm, but 
still with the equimolecular NaCl/KCl bath, the halide introduced being on 
this occasion hafnium tetrachloride in a quantity of 25% and under normal 
electrolysis conditions, i.e. with a current density of 1.0 A/cm.sup.2, 
the hafnium deposition voltage is 2.2 V and deposits are obtained in the 
form of relatively solid dendrites (cauliflower appearance) with an 
electrical efficiency exceeding 95%. The analysis of these deposits gives 
the following results in ppm: 
__________________________________________________________________________ 
C N O Al 
B Cr 
Cu Fe Mn Si Ti V W remainder Hf 
__________________________________________________________________________ 
&lt;10 
&lt;10 
250 
39 
2.4 
27 
&lt;10 
&lt;20 
36 &lt;25 
&lt;10 
&lt;10 
&lt;15 
__________________________________________________________________________ 
If F-ions are introduced into the bath by adding e.g. sodium fluoride in 
such a way that the fluorine:hafnium molecular ratio is equal to 12, under 
the same electrolysis conditions the hafnium deposition voltage passes to 
2.9 V and, after washing the deposit, a powder is obtained which 
substantially entirely passes through the 200 micron mesh size screen and 
complies with the following analysis in ppm: 
__________________________________________________________________________ 
C N O Al 
B Cr 
Cu 
Fe Mn Si Ti V W remainder Hf 
__________________________________________________________________________ 
12 
&lt;10 
290 
68 
2.7 
20 
11 
&lt;20 
16 &lt;25 
&lt;10 
&lt;10 
&lt;10 
__________________________________________________________________________ 
It should be noted that on this occasion the fluorine:hafnium ratio is 
equal to 12, but that with other metals values of this ratio between 3 and 
20 can be used. The best results are obtained in the range of values 
between 6 and 12.