Chemistry: electrical and wave energy – Processes and products
Patent
1985-11-06
1987-06-30
Williams, Howard S.
Chemistry: electrical and wave energy
Processes and products
204290F, C25D 100
Patent
active
046768777
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a process for production of fine and ultrafine zinc powders by electrolysis in a basic medium.
2. Technological Background
The electrolysis of zinc in a caustic medium has been studied for several decades, but its industrial applications have been rare.
Such electrolysis presents some non-negligible advantages:
1. overvoltage of the hydrogen in caustic medium less than in acid medium;
2. production of a very fragile deposit of zinc, eliminating the high costs of scraping of the electrodes necessary in acid medium; and
3. fewer problems of corrosion than in acid medium, allowing lower investment.
The deposit of zinc which is obtained in alkaline medium is slightly coherent, very slightly adherent and furnishes zinc powders hence, for the usual of zinc, the necessity of fusion and of ingoting of the powders is created. In this spirit, the production of zinc in fine particles may be considered as a drawback because high radioactivity leads to an easier oxidation and therefore to a lower yield of fusion (85.90% against 97.99% by the conventional processes).
This phenomenon probably explains the little success of electrolysis in caustic medium.
However, the existence of a large market of zinc powders and dust in the chemistry of catalysts, fillers in general and more particularly for the anti-corrosion paints industry, has guided the study of this type of electrolysis during recent decades.
The zinc powders presently marketed are obtained by thermal process, by fusion and vaporization in vacuo of ingots of zinc. They furnish a fairly wide range of production, from coarse zinc powder to fine and ultra-fine zinc powders.
The powders and dusts obtained by thermal process have the following characteristics: d.sub.90 =6 to 17.5.mu.
On the other hand, the powders obtained by alkaline electrolysis have the following characteristics:
The essential differences therefore reside in the higher specific surface area for the alkaline powders, their much coarser granulometry and their very different shapes, namely:
spherical for the powders by thermal process; and
planar for the powders obtained by alkaline electrolysis.
Thus, the powders obtained by alkaline electrolysis must be ground in order to approach the granulometric standards of the powders obtained by thermal process. However, grinding does not make it possible to obtain powders having a d50=20.mu. (compared to d50=7.5.mu. on average for the powders obtained by thermal process).
Hence the research has been conducted, intended to reduce the d50 of the powders obtained by electrolysis in alkaline medium.
The tests were carried out on the basis of the following parameters:
Electroplating obeys two processes:
The size of the grains obtained depends essentially on the ratio of the speeds of these two processes. If the speed of nucleation is higher than the speed of growth of the crystal, the deposit will be fine, and vice versa. The crystals will grow quickly and will therefore be coarse if the metal ions arriving at the solid-liquid interface are numerous. On the other hand, the centers of nucleation will be abundant and the crystals will grow slowly and will therefore be fine, if the metal ions arriving at the solid-liquid interface are rare.
Hence, in essence, the concentration of the metal ions near the interface, therefore in the NERNST layer, will determine the granulometry of the deposit. On these theoretical bases, the earlier studies have concerned the monitoring of the process of diffusion and therefore the following parameters:
All these techniques are interested in the electrolyte and in the faradic current, but none studies the electrode.
Now, electrolysis causes relationship to intervene between the current, the electrolyte and the electrodes. In the studies on alkaline electrolysis, the study of the cathode has always been seen from the standpoint of corrosion and it has been directed to the search for a corrosion-resistant material.
The essential conclusions of this st
REFERENCES:
patent: 1397008 (1921-11-01), McGall
patent: 1846697 (1932-02-01), Stout et al.
patent: 2285762 (1942-06-01), Tuwiner et al.
patent: 3208920 (1965-09-01), Crew
patent: 3326783 (1967-06-01), Winter
patent: 3397125 (1968-08-01), Tapley
"Advances in Electrochemistry" Electrochemical Engineering, vol. 2, Dec. 1962, pp. 53-65 article entitled Formation of Powdered Metal Deposits.
Chemical Abstract No. 84:142603g, Electrochemistry, vol. 84, 1976, p. 491.
Chemical Abstract No. 98:151794M, Electrochemistry, vol. 98, 1983, p. 523.
Castillo Jean-Michel
Ferlay Serge
Jankowski Christian
Societe de Promotion des Procedes Hydro-Metallurgiques
Williams Howard S.
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