US3367767A - Method of making nickel powder - Google Patents

Description

United States Patent Ofilice 3,367,757 Patented Feb. 6, 1968 3,367,767 METHOD OF MAKING NICKEL POWDER DeWitt Henry West, Port Eynon, and David Myers Llewelyn, Clydach, Swansea, Wales, assignors to The International Nickel Company, Inc., New York, N.Y., a corporation of Delaware N Drawing. Filed Aug. 16, 1965, Ser. No. 480,106 Claims priority, application Great Britain, Aug. 19, 1964, 33,907/ 64 5 Claims. (Cl. 75--.5)

ABSTRACT OF THE DISCLOSURE Carbonyl nickel powder with low carbon contents is produced by decomposing nickel carbonyl mixed with ammonia and in the substantial absence of water vapor in a decomposer having inner surfaces of nitrided steel.

This invention relates to carbonyl nickel powder, that is to say powder made by the thermal decomposition of nickel carbonyl vapour in the hot free space of a decomposer.

The production of carbonyl nickel powder in this way has been carried out on an industrial scale for many years, and it is well-established that according to the conditions of temperature, the concentration of carbonyl, and the presence or absence of diluent gases, e.g. carbon monoxide, the power produced may assume one of two forms. These are the so-called A carbonyl nickel powder, which consists of discrete particles with an irregular spiky surface, and the so-called B powder, which consists of agglomerates of interlocking filaments or chains of interconnected (aggregated) particles which again are individually irregular. Type B powder has a low bulk density and has a microscopic appearance of small spongy flakes. The size of the aggregates of particles making up the chains can vary widely.

Whatever its physical form, the powder product from the decomposer ordinarily contains a small amount, e.g. from 0.05 to 0.08% by weight, of carbon. Most of the carbon is present in the nickel particles, either chemically combined with the nickel or as graphite. However, the product is almost always contaminated with a very small proportion, generally less than 0.01% by weight, of particles of very high carbon content, e.g. 50% or more. For many uses of the powder the presence of these carbonaceous particles is most undesirable. Thus when the powder is sintered in thin layers the carbon particles burn away leaving holes. When the thin layer is a protective coating on steel the protection afforded is thereby reduced, and in the case of a porous product such as a fuel cell electrode, the pore distribution is rendered nonuniform and they must therefore be removed from the powder before it is used. To the extent that the carbonaceous particles are larger than the nickel particles this can be done by screening, but it is not always possible to eliminate all of them in this way.

Although attempts have been made to avoid the formation of carbonaceous particles during the production of carbonyl nickel powder and to obtain a product of lower carbon content, none, so far as we are aware, was entirely successful when carried into practice commercially on an industrial scale.

It has now been discovered that the formation of carbonaceous particles can be prevented or reduced by a special conditioning pretreatment of the steel inner surfaces of the decomposer, and that the carbon content of the particles of the nickel powder can be lowered by the use of special additions to the decomposer.

It is an object of the invention to produce carbonyl nickel powder substantially free from carbonaceous particles, for use in powder metallurgy.

Another object of the invention is to produce carbonyl nickel powder of which the particles have a lower carbon content.

Other objects and advantages will become apparent from the following description.

The present invention is based on the discovery that the carbonaceous particles are formed by the thermal decomposition of carbon monoxide at the surfaces of the steel walls and internal fittings of the decomposer, and that their formation can be wholly or partly prevented by nitriding these surfaces.

We further find that both the formation of the carbonaceous particles-and the incorporation of carbon into the nickel particles in other forms is promoted by water vapour. Based on these discoveries, the invention consists in producing nickel powder by decomposing nickel carbonyl vapour in the substantial absence of water vapour in the hot free space of a decomposer whose steel inner surface have been nitrided.

The nitriding of the steel decomposer surfaces may conveniently be performed by beating them in contact with ammonia gas, e.g. by leading ammonia gas into or through the decomposer while the walls and other internal surfaces are heated to a high enough temperature for a surface layer of iron nitride to be formed by reaction with the ammonia. A suitable temperature is 500 C., and the heating should be continued for at least 1 hour, e.g. for 3 hours. The effectiveness of a nitriding treatment may be assessed by simultaneously treating a test-piece of steel of similar composition in the decomposer and then exposing it at 500 C. to carbon monoxide containing small amounts of nickel carbonyl vapour, for two hours. If no black deposit of carbon is formed on the steel surface, the treatment is eifective.

While it is preferred to perform the nitriding as a separate pre-treatment before introducing the carbonyl into the decomposer, it may if desired be done by passing ammonia into the decomposer while a previous batch of nickel powder is being made and the decomposer walls are therefore heated. It will be appreciated that the full benefit of this treatment in preventing the formation of carbonaceous particles will not be obtained until the decomposer walls have been fully nitrided. In whichever way the treatment is performed, the internal surfaces of the decomposer must of course be initially clean.

Once the nitriding treatment is complete, it is still advantageous, according to a further feature of the invention, to mix a small proportion of ammonia with the nickel carbonyl being decomposed. This both serves to maintain the nitrided surface layer on the steel surfaces and also reduces the content of combined carbon in the powder formed. We believe that the latter efliect is due to inhibition of the decomposition of carbon monoxide on the newly-formed surfaces of the nickel powder particles to form carbon that is normally incorporated into them. This effect is however counteracted] by water vapour, and care should be taken that this is substantially absent.

A further slight reduction in the combined carbon content of the powder results if a small proportion of oxygen, e.g. from 0.01 to 0.1% by volume of the gas feed, is introduced during the decomposition as well as the ammonia. The ratio of ammonia to oxygen is preferably 4:3 by volume. Whether or not oxygen is introduced, the proportion of ammonia required is very small, as little as 0.01% by volume of the gas feed to the decomposer generally being effective. To minimise contamination of the eflluent gases and avoid the formation of significant Before the series of tests began, the internal surfaces of the steel decomposer walls were clean and free from nitride films. The results in the table show that when no addition of ammonia or oxygen was made, the carbon amounts of water vapour by the reaction of ammonia 5 content of the powder formed was 0.057% and the nitrowith oxygen, the proportion of ammonia preferably does gen content was negligible (Expt. 1). This powder connot exceed 0.1% by volume. The presence of oxygen protained a small proportion of black carbonaceous particles. motes nitriding of the powder owing to the formation of The walls of the decomposer were then nitrided by introactive nitrogen by reaction between the ammonia and ducing 5 litres/hour (0.25% by volume) of ammonia gas oxygen, and the nitrogen content of the product is thereinto the decomposer feed for 3 hours while nickel carbonyl fore increased. In any event the nitrogen content of the was decomposed as before, and further runs (Expts. 2-16) powder must be less than 0.01%, since with 0.01% or were then carried out without any additions, with ammore nitrogen the powder particles become spherical, and monia additions and with additions of both ammonia and the ammonia concentration must therefore not exceed oxygen. Care was taken that all the added gases were dry. 0.2% by volume when oxygen is present. In all of Expts. 2 to 16 according to the invention the The invention is applicable to the production of nickel powder product was free from black carbonaceous partipowder by decomposing nickel carbonyl vapour not only cles. The carbon contents of the powders made with amin concentrated form but also when it is diluted with carmonia additions were lower than those made under similar bon monoxide, which may be present in excess. It can be conditions without ammonia after the decomposer walls used to make both type A and type B powder. The form 20 had been nitrided, and those made in the presence of both of the powder particles is not affected by the presence of ammonia and oxygen are lower still. The somewhat higher ammonia alone in the decomposer, but when both arncarbon contents of the powders made in Expts. 3 to 5 are monia and oxygen are used and the nitrogen content apdue to the lower carbonyl concentration. proaches 0.01% by weight, the irregularities on the sur- The powders made using ammonia and Oxygen had face of the Pflfticles are smoothed out and they tend to higher nitrogen contents, and particles of No. 5, which become spherlcal. contained 0.009% nitrogen, were almost spherical, with The operating conditions under which A and B type roughed surfaces. nickel powders are obtained in a carbonyl decomposer are The powder produced in all the experiments was of now well understood. In general, conditions in which the Type A. That made in accordance with the invention disrate of nucleation is high lead to the formation of type solved completely in hydrochloric acid to a clear solution B powder, while a low rate of nucleation leads to the forof nickel chloride, whereas the product from Expt. 1 left mation of A type powder. A high rate of nucleation is an insoluble scum of carbon. favoured y high temperature, high Carbonyl cohcehtl'a' Although the present invention has been described in lion and high throughput, While a 10W nucleation rate is conjunction with preferred embodiments, it is to be underfavohred y low temperaturs, 10W carhohy1 Concentration stood that modifications and variations may be resorted to aRC1 10W throughput Thesfi Same considerations apply without departing from the spirit and scope of the inven- When the decomposition is Carried out in the Presence of tion, as those skilled in the art will readily understand. ammonia. It should b n t however, that increasing the Such modifications and variations are considered to be temperature increases the tendency for carbonaceous par- 40 within the purview and Scope f the invention and holes to form and for carbon to be introduced 1nto the pended 1 nickel particles. Preferably, therefore, the temperature We claim; used does not exceed about or 1. A process in which non spherical nickel powder sub- The features of the process are illustrated by the restantially free from particles of carbon is produced by the sults of a series of experiments in which nickel carbonyl decomposition of nickel carbonyl vapour in the substanvapour diluted with about 11 times its volume of carbon tial absence of water vapour in the hot free space of a monoxide (nickel carbonyl concentration 69% by decomposer having nitrided steel inner surfaces. volume) was fed into an externally-heated 10-inch diam- 2. A process according to claim 1 in which ammonia eter decomposer having mild steel walls at a total gas flow is admixed with the nickel carbonyl vapour. rate of 2000 litres/hour through an inlet at the top. Dry 3. A process according to claim 1 carried out in a deammonia gas and oxygen were injected into the inlet gas composer, the walls of which have been nitrided during stream in the amounts indicated in the table. The internal the decomposition of nickel carbonyl in the presence of temperature, measured at a point half-way between the ammonia in a previous operation. Wall and the axis and one-quarter of the distance down 4. A process for producing non-spherical nickel powder from the top, was 290 C. in all the tests, and the wall substantially free from particles of carbon which comtemperature, measured by a thermocouple strapped to the prises decomposing nickel carbonyl vapor in the substaninside of the heated wall, was about 400 C. tial absence of water vapor in the hot free space of a TABLE E Feed rate (litres/hr.) Concentration (percent by volume) Properties of Powder Product Nickel Fisher Bulk Chemical Composition NH3 0; Carbonyl NH3 02 Value, Density,

microns glIDS./CG. Percent C Percent N 3 9 4. 47 2. 47 0. 057 0. 001 7 3.66 1. 99 0. 030 0. 001 7 0. 4 4. 2 2. 42 0. 035 0. 002 7 0. 0s 4. 36 2. a9 0. 035 0. 002 7 0. 2 6. 20 2. 87 0. 033 0. 009 9 0. 1 5. 02 a. 02 0. 025 0. 003 9 0. 05 6. 20 2.82 0. 029 0. 004 9 0. 025 6.26 2. 74 0.028 0. 005 0 0. 0125 5.83 2. 73 0. 02s 0. 004 9 0. 0125 4. 5s 2. 41 0. 022 0. 004 9 0. 025 0. 075 4. 2s 2. 1s 0. 020 0. 004 9 0. 05 0. 075 5.10 2. 36 0. 025 0.004 9 0. 1 0. 075 4. 86 2. 25 0. 019 0. 000 0 0. 1 0. 0375 5. 5s 2. 92 0.020 0. 000 0 0.1 0. 010 5. 42 2. 48 0. 020 0.004 0 0.1 0. 0005 5. 02 2. 91 0. 010 0. 003

5 6 decomposer having nitrided steel inner surfaces and ad- References Cited mixing ammonia and oxygen with the nickel carbonyl UNITED STATES PATENTS vapor, said ammonia being added in amounts from about 0.01% to 0.2% by volume of the gas feed and said oxygen 1759661 5/1930 Muller et a1 75 0'56 being added in amounts of from about 0.01% to 0.1% by 5 HYLAND BIZOT, Primary Examiner. volume of the gas feed.

5. A process as described in claim 4 wherein the ratio DAVID RECK Exammer of ammonia to oxygen is about 4 to 3 by volume. W. STALLARD, Assistant Examiner.