Chemistry: electrical and wave energy – Processes and products
Patent
1987-08-20
1988-09-13
Tufariello, T. M.
Chemistry: electrical and wave energy
Processes and products
204 64T, C25D 100, C25D 326
Patent
active
047707500
DESCRIPTION:
BRIEF SUMMARY
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:
1. the process involving hydrogenation, grinding and dehydrogenation,
2. processes involving electron beam or arc melting and centrifugal atomization;
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 continu
REFERENCES:
patent: 2951021 (1960-08-01), Di Pietro
Pechiney
Tufariello T. M.
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