US1968490A - Method of making pulverulent chromium - Google Patents

Description

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Patented July 31, 1934 v UNITED STATES PATENT OFFICE METHOD OF MAKING PULVEEULENT CHROMIUM Leo D. Jensen, Chicago, 111.

No Drawing. Application November 27, 1931, Serial No. 577,707

7 Claims.

, acter, said deposit generally forming curly scales,

collecting said deposit, washing and drying it and comminuting; all as more fully hereinafter set forth and as claimed.

Because of the great hardness and strength of the metal chromium, production of fine powdered or granular chromium has offered great technical difiiculties. Because of the high heat of oxidation of chromium, it is not practical to reduce it from its oxid by gas currents at a temperature below the melting point, as is done with various other metals. Metallic chromium formed by the aluminothermic process is extremely hard and cannot be economically comminuted or granulated by ordinary grinding machinery to give a powdered product. In granulated chromium so made, the proportion of ments is usually great. As a matter of fact, in attempts of. this character there was sometimes more wear on the crushing machinery than on the chromium. Electrically deposited chromium producedby the ordinary method is also of extreme hardness and is in thin films cohering to the metal used as the cathode. Sometimes, arborescent deposits, or trees", occur in ordinary plating methods, but these are also extremely hard and it is not practical to pulverize them to form pure, finely divided chromium.

an the electroplating processes for the deposition of chromium now in use employ a solution of @1103 as a bath, this solution containing minor amounts of sulfate and of trivalent chromium. Under proper conditions, very thin, extremely hard deposits can be obtained cohering to the metal plated; a fact which is responsible for the extensive use cl chromium plating today. In this plating operation, it is known that better emciency can be attained at temperatures cl 25 6. or below than at higher temperatures. In these plating processes, the current densities are ordinarily such. that there is a formation or hy drogen simultaneously with the chromium; but conditions are, oi course, so chosen that this iormation of gas does not interfere with the plathis.

I have found that by using high current densities, densities high. enough to cause a copious evolution of hydrogen, in lieu of forming a coherent continuous coating, I produce chromium as non-coherent deposits which can be readily resteel, etc., from the grinding ele-g moved. Advantageously, the current density is high enough to produce curly, flaky non-adherent deposits, cracking and breaking away from the cathode. I am of the impression that as the layer on the cathode thickens, the tension of the last formed layer cracks and curls the underlying layers. Chromium formed in this way, after removal from the bath, washing and drying, is of frangible character and can be readily comminuted to a powder as fine as may be desired,

without great wear on the crushing apparatus. Whether this frangibility is due to the formation of small independent crystals or is due to inclusion of hydrogen in the metal, I am not aware.

It may be due simply to unrelieved tensions in the layer.

The lower the temperature of the bath, the better is the current efliciency and the quicker is the formation of frangible chromium deposits.

It is found in practice that there is a lower (5 limit of current density varying with different cathode materials. At less densities, the curling deposit is not formed. Cathodes of aluminum or aluminum alloy are particularly eiiicient for the present purposes. quired for the production of detachable deposits. Lead or other unattackable anodes may be employed.' Current may be applied until a given bath is exhausted or the bath may be replenished from time to time by additions of ClOs 5 in well known manner. Ordinarily, however, I do not use the same bath without interruption for any length of time; this being partly because there is apt to be an accumulation of detached chromium which is slowly dissolved by the bath go with consequent waste.

In general the electrolysis is run under conditime so as to form curling, readily frangible flakes carried by the cathode, the cathode being removed before the flakes become thick enough cm to render crushing too expensive.

in a specific embodiment of the present invention, a bath was made by dissolving commercial (3103 to form a solution containing 400 grams per liter. This was acidified by addition of ordi- 1W nary commercial concentrated sulfuric. acid until the total H2804 content of the bath was 4 grams per liter. In this particular work, an aluminum cathode with about 500 square inches surface area was used, being placed in close proximity to lead anodes. The bath was made cold and kept at a temperature below 25 C.'by cooling coils. A current of 2,000 amperes was passed between the electrodes for an hour when a curly non-coherent deposit was formed. At this time, no

A certain time is rean occurring in the bath was also collected. The

' venient crushing device.

chromium was washed and dried and then crushed to a powder. In this particular work a ball mill was used.

Chromium made in the described manner is frangible enough to permit the-use'oi any con- The limiting current density varies somewhat with the cathodematerial and with the temperature, but using aluminum cathodes and a temperature below 25 C., it is of the order of 2 amperes per square inch.

The temperature or the bath is not specially important except as it controls efllciency. Good flaked deposits can be obtained at temperatures as high as 48 C. on nickel, aluminum and aluminum alloys.

Qther chromium plating baths than solutions of CrO; can be used; but the latter are the best at present known.

As a rule, the flaked chromium is washed and dried prior to\ comminuting, but it is sometimes convenient to wet grind the flakes to form a paste or pulp and afterwards dry this to produce the desired pulverulent chromium.

The present method may be applied to making granular chromium or pulverulent chromium of any mesh desired. The flake' chromium is sometimes utilized as such.

The product produced according to my invention has many uses, for example, in the manutacture oi. inks, paints, coatings, alloys, etc.

It is to be understood that the invention is not limited to any specific set oi. conditions described above, but may be more or less materially modifled without departure from the spirit and scope of the same as called for in the following claims.

What I claim is:

1. A process of producing metallic chromium as a flne powder which comprises electrolyzing a chromium plating bath containing a high concentrationoi' ClOs with a cathode made of a metal-not attacked by the bath, a high current density on said cathode with temperature and other conditions producing a copious evolution of hydrogen gas and a deposition of chromium upon the cathode in non-adherent brittle flakes, detaching said flakes irom the cathode and grinding the flakes to a powder.

2. The process of claim 1 wherein the electrodeposition is efl'ected on an aluminum cathode.

3. The process oi. claim 1 wherein a current density above 2 amperes per square inch of cathode is employed.

4. In the process 01' claim 1, maintenance of the plating bath at a temperature not above C.

5. In the production of pulverulent or granular chromium, the process which comprises electrolyzing a bath 01' C103 at a low temperature and at a high cathode current density giving metallic chromium in detachable flakes at the cathode with a copious evolution of hydrogen gas, washing and drying the detached flakes and comminuting to the flneness desired.

6. In the production 01' readily crushable chromium, the process which comprises electrolyzing a bath of CrO: with an aluminum cathode at a temperature not above 48 C. and at a sumciently high cathode current density to give metallic chromium in detachable flakes with a com-- ous evolution of hydrogen.

7. In the production of readily crushable chromium, the process which comprises electrolyzing a bath of CrOa with an aluminum cathode at a temperature not above 48 C. and at a sufllciently high cathode current density to give metallic chromium in detachable flakes with a copious evolution 01' hydrogen, and washing and drying the detached flakes.

LEO D. JENSEN.

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