US3397057A - Method for producing flowable metal powders - Google Patents

Description

United States Patent 3,397,057 METHOD FOR PRODUCING FLOWABLE METAL POWDERS John H. Harrington, Warwick, and Arnold L. Prill, Suffern, N.Y., assignors to The International Nickel Company, Inc., N ew York, N.Y., a corporation of Delaware No Drawing. Filed Sept. 26, 1966, Ser. No. 581,768

17 Claims. (Cl. 75-213) ABSTRACT OF THE DISCLOSURE Fine metal powders such as carbonyl nickel powder having poor flow properties are converted into agglomerates having improved flow properties by tumbling the powders in a balling liquid such as water to form separate ball agglomerates, drying the agglomerates and sintering the agglomerates in a protective atmosphere at a temperature at which substantial sintering occurs within the agglomerates but below that at which substantial sintering occurs between agglomerates so as to produce substantially spherical, flowable agglomerates having a particle size in the range of about 20 to about 1,000 microns.

The present invention solves a problem which has existed in the art for seventy years.

Metal powders produced by the thermal decomposition of metal carbonyls, e.g., nickel and iron powders so produced, have been available since the days of Ludwig Mond. Such metal powders are valuable metallurgical raw materials because of their high purity. The powders have good pressability, provide compacts having high green strength and green compacts made thereof can be sintered to high density at relatively low temperatures as compared to coarser nickel powders produced by other methods. However, the physical nature of these powders is such that they do not flow readily. This lack of flowability prevents utilization of carbonyl metal powders in conventional pressing equipment employed in the powder metallurgy art, despite the other marked advantages of these powders. The as-produced powders not only fail to fill dies in automatic pressing equipment, but, also, because of their fineness, quickly cause scoring and seizing of the punch within the die.

Carbonyl iron and nickel powders have average particle sizes of less than 10 microns and, more usually, less than microns. Carbonyl iron powders generally tend to be spherical in shape and their poor fiowability thus is principally due to the fine particle size thereof. Carbonyl nickel powders tend to be irregular in shape, with spiky projections extending from more or less spherical bodies and, in some grades, with an overall fibrous appearance. These physical attributes further contribute to poor flowability to the extent that a pile of the powder can be parted with a knife and the parted face removed leaving a substantially vertical face.

It will be appreciated that the fiowability of a particular powder is an empirical factor. Measurements of relative flow-abilities of various powders can be made by timing the passage of a measured quantity of powder through a funnel having an orifice with a standard size at the bottom. However, as pointed out by W.D. Jones in his work Fundamental Principles of Powder Metallurgy, 1960, at p. 973, so many factors influence the manner in which powders flow that present-day fiowability testing procedures are not completely satisfactory. The important test of flowability, in practice, is the rate at which powder can be made to flow from a hopper to a die in which it is to be compressed.

Flowable fine metal powders are usually produced industrially by three general techniques: (1) the metal or 3,397,057 Patented Aug. 13, 1968 alloy is produced as a relatively brittle ingot which is then crushed and pulverized; (2) the metal or alloy in nonflowing fine powder form is sintered to a cake which is then crushed; (3) molten metal or alloy is atomized by spraying a liquid stream thereof into a fluid medium. All of these techniques are characterized by commercial objections, including loss of material as unusable fines which must be reprocessed at extra cost, introduction of undesired impurities, including oxides, other compounds, tramp elements, etc., and by the fact that the product does not always have satisfactory flowability. Accordingly, despite many attempts in the art, no commercial method is presently available for increasing the flowability of essentially nonfiowable pure metal powders while still retaining the high purity, good pressability. and good sinterability thereof.

We have now discovered a process for greatly improving the flow properties of fine metal powders having poor flow characteristics while at the same time preserving the purity and other desirable metallurgical properties of the powders.

It is an object of the present invention to provide a method for improving the flow characteristics of fine metal powders having poor flow characteristics.

It is a further object of the invention to provide a practical means for improving the flow characteristics of fine metal powders.

It is another object of the invention to provide a method for agglomerating fine metal powders to improve the fiow characteristics thereof without detrimentally affecting the other important metallurgical characteristics thereof.

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

Generally speaking, the present invention is directed to a process for improving the flowability of fine metal powders having high purity and poor flowability comprising balling the metal powder with a liquid such as water, then drying and sintering a bed formed from the resulting agglomerates in a protective atmosphere to a temperature range in which substantial sintering occurs within the agglomerates but below that at which substantial sintering occurs between agglomerates to produce substantially spherical, flowable agglomerates.

The invention is particularly applicable to the treatment of fine metal powders of high purity having a particle size not exceeding about 10 microns and bulk densi ties of about 0.5 to about 3.5 grams per cubic centimeter (gm./cc.). Metal powders such as carbonyl iron, carbonyl nickel and carbonyl cobalt powders are particularly amenable to treatment in accordance with the invention to provide free-flowing particles having a particle size of at least about 20 microns to about 1,000 microns, e.g., about 20 to about microns, while still retaining the high purity and other desirable metallurgical qualities in these materials. Electrolytic copper and iron powders and powders obtained by the reduction of halides, e.g., the chlorides of nickel, copper, molybdenum, tungsten, etc., :are also amenable to treatment in accordance with the invention.

It is to be appreciated that the wet agglomerates, which may contain, for example, about 5% to about 30% water, by weight, have low strength and it is important that they should be transferred immediately, or at least after the lapse of only a short time during which loss of water is prevented, to the sintering and drying operations with only minimal handling. This can be readily accomplished, for example, by loading the wet agglomerates directly from the balling operation to a continuous belt communicating with the drying and sintering furnace. Any other means whereby the wet agglomerates are formed into a substantially fixed, quiescent or static bed during drying and sintering may be employed.

The balling operation may be conducted in a balling disc or drum, a vibrating table, vibrating screen, etc., equipped with a liquid fog or spray feed, or in a rotary twin-cone blender equipped with a rotating liquid spray bar located near the juncture of the cones or in any other convenient type of balling equipment known to those skilled in the art. Spray drying may also be employed for agglomerating. We prefer the aforementioned twin-cone liquid-solids blender having a rotating liquid spray bar adapted to introduce liquid in the form of a spray or fog under substantial velocity and pressure into the tumbling powder. Blenders of this general type are described, for example, in US. Patents Nos. 2,890,027 and 2,915,300 and patents mentioned therein. Bailing is conducted by tumbling the powder under conditions such that components of rolling and of compression are imparted thereto while the balling liquid is introduced into the tumbling powder. It is advantageous from the control standpoint to introduce the balling liquid in the form of fine droplets as a spray or fog into the dry powder while the powder is in motion in the balling apparatus. Difficulties are encountered in attempting to moisten the metal powder before balling and control of agglomerate size in balling is uncertain, particularly from the standpoint of size uniformity.

The green strength of the agglomerates can be increased to permit more handling and some motion of the agglomerates during drying and sintering by incorporating a soluble or dispersible organic heat-decomposable binder in the liquid employed for wetting the powder.

Thus, binders such as methyl cellulose, starch, gums, polyacrylamides, dextrines, etc., can be incorporated. While water, e.g., demineralized water, is preferred from the standpoints of operating ease and of maintaining product purity, volatile organic solvents, including carbon tetrachloride, trichlorethane, ethyl and methyl alcohol, etc., can be employed in the balling operation and solvent-soluble binders such as parafiin, stearic acid, waxes, ethylcellulose, etc., can be dissolved therein. In balling, density and strength of the agglomerates are increased by continuing the balling operation for some time after ball formation is initiated.

As noted previously, it is preferred to employe demineralized water itself as the liquid medium in the balling operation. The water can be removed from the agglomerates during the drying and sintering operations without any detrimental impurities such as carbon, etc., being retained in the final agglomerates. In this way, the other desirable metallurgical characteristics of the original powder, including good pressability, sinterability at low temperatures and purity are retained. It is important that the agglomerates not be disturbed during the interval after drying and before sintering since they then have low strength. It is found that even when minor amounts of material such as methyl cellulose are employed as binders for the purpose of strengthening water-Wetted agglomerates a carbon residue may result therefrom in the subsequent heat hardening or sintering operation and such residues may be undesirable in certain instances. The presence of water during sintering, e.g., a wet hydrogen atmosphere at 1700 F., promotes the decarburization of the nickel, iron or other metal powder. Since carbonyl powders do contain canbon, sintering in wet hydrogen permits purification by decarburizing and deoxidizing the powders during the sintering under reducing conditions in the presence of water. Carbon contents as low as 0.005% and lower, e.g., 0.001%, can readily be obtained in this manner.

The sintering operation is conducted in a protective atmosphere which may be, for example, hydrogen, cracked ammonia, partially-combusted natural gas, argon, etc. The essential requirement of the atmosphere in the heat hardening or sintering operation is that it prevents oxidation of the metal powder agglomerates being sintered. In the case of agglomerates made of nickel, iron or cobalt powder, the sintering operation is conducted at a temperature not exceeding about two-thirds of the melting point of the metal as measured in degrees Fahrenheit. In sintering nickel powder, a temperature of about 1000 F. to about 1730 F., e.g. ,about 1200 F. to about 1500 F., for a time between a few seconds up to several minutes, e.g., up to 15 minutes, depending upon temperature is satisfactory. It is surprisingly found that the initial agglomerates substantially retain their size and shape as a result of the sintering operation but that there is little adhesion between the agglomerates. Any caking or interagglomerate adhesion is readily removed by light mechanical treatment with only minimal loss of material in the form of fine dust. It is found that when sintering of nickel powder agglomerates is conducted at temperatures below about 1000 F., e.g., 800 F., an unduly high proportion of fine material is obtained whereas at a temperature of about 1500 F. only a small amount of fine material resulted. At temperatures exceeding two-thirds of the metal melting point in degrees Fahrenheit, interagglomerate bonding becomes predominant, control of agglomerate size is lost and the product becomes tough and ductile with loss of the desired pressability and relatively low temperature sinterability greatly desired for powder metallurgical operations. Again, with reference to carbonyl nickel powder agglomerates, a sintering temperature in the range of about 1200 F. to 1300 F. provides sintered agglomerates which do not break down on handling yet have the desired pressability and sinterability for pressing and sintering, direct powder rolling and other powder metallurgical operations. A sintering temperature of about 1500 F. to 1700 F. provides tough, free-flowing agglomerates useful in other applications such as tubular welding electrodes, seed material in fluid bed carbonyl decomposers, etc. Similar results are obtained with other metal powders such as carbonyl iron and carbonyl cobalt powders by sintering at temperatures comprising equivalent proportions of the melting points in degrees Fahrenheit for these metals.

In a further advantageous aspect of the invention, it is found that pulverizing the sintered agglomerates, for example, in a hammer mill, provides a further improvement in fiowability, i.e., reduction in flow time in a fiowmeter, an increase in apparent density and an improved capacity to be directly rolled to strip having higher apparent density. As applied to fine carbonyl nickel powders, wateragglomerated balls sintered in hydrogen at temperatures of about 1400 F. to about 1730 F. are particularly suitable for pulverization to improve the flow properties thereof. Carbonyl nickel powder agglomerates sintered at temperatures above 1730 F. cannot readily be pulverized. Carbonyl iron powder agglomerates and codeposited 50% iron-50% nickel carbonyl powder agglomerates sintered at temperatures circa I700 F. for 10 minutes in hydrogen displayed improved fiow rate and increased apparent density after pulverization.

In order to give those skilled in the art a better appreciation of the advantages of the invention, the following illustrative examples are given:

Example I About 2,000 grams of nickel powder produced by the decomposition of nickel carbonyl and having an average particle size in the range of about 3 to 5 microns with an apparent density in the range of about 1.6 to 2.1 gm./ cc. were water agglomerated in a liquid-solids blender of the twin-cone type equipped with a high speed liquid spray feed bar. About 350 milliliters of water were employed in the operation and the powders were simultaneously tumbled and blended during the wetting to achieve agglomeration. Portions of the wetted agglomerates were placed in a metal boat and were dried and sintered at 1150 F., 1200 F. and 1250 F. in a furnace having a hydrogen atmosphere for about 5 minutes. In each case, free-flowing agglomerates having substantial strength were obtained with only a minimum amount of fines. The agglomerated powders were compacted and sintered to evaluate response in conventional powder metallurgy processing. As compared to the original powder, there was only a slight decrease in strength and ductility of the resulting sintered compacts but it was found there was an offsetting beneficial reduction in the amount of shrinkage which occurred during sintering. A portion of the agglomerates sintered at 1200 F. in hydrogen had a fiow rate of about 50 seconds as determined in the Hall Flowmeter described in A.S.T.M. Standard B-213 whereas the initial powder would not pass through the flowmeter. The material had an apparent density of about 1.7 gm./ cc. and was directly cold roll-compacted to strip having a density about 60% of theoretical with no lamination in the rollcompacted strip being evident. Another portion of the agglomerates sintered in hydrogen at 1500 for minutes had a flow rate of 52.75 seconds in the aforementioned Hall Flowmeter and an apparent density of about 1.7 gm./ cc. This material was pulverized in a hammer mill whereupon the flow rate was increased to 30 seconds and the apparent density was increased to 2.7 gm./cc. It was directly cold roll-compacted to strip having a density about 80% of theoretical with no lamination in the rollcompacted strip being evident. Another portion of the agglomerates sintered in hydrogen at 1700 F. for 5 minutes had a How rate of 45 seconds and an apparent density of about 1.9 gm./-cc. All of the sintered agglomerates had reduced surface area and gas content as compared to the original powder.

Example II Two types of carbonyl iron powder having, respectively, an average particle size of about 5 microns and about 6 microns and an apparent density of about 3 gm./cc. and about 3 gm./ cc. were agglomerated with water in the manner described in conjunction with Example I and were sintered in hydrogen at a temperature of about 1250 F. for about 5 minutes. The fiow rate of the resulting material was about 46.9 seconds whereas the initial powder would not pass through the fiowmeter. The apparent density of the resulting material was about 2.12 gm./ cc.

Example III An electrodeposited copper powder having an average particle size of about 8 microns and an apparent density of about 2.3 gm./cc. was agglomerated with water in the manner described in Example I using about 230 milliliters of Water for a charge weight of 1,370 grams of powder. Portions of the resulting agglomerated powder were sintered at 895 F. and 1300 F. for about 10 minutes. The sintered agglomerates had a flow rate of 48.9 seconds whereas the initial powder would not pass through the flowmeter.

The invention also contemplates agglomerated metal powders, especially carbonyl nickel, cobalt and iron powders and mixtures and alloys thereof, having a particle size of at least about 20 microns and up to about 1,000 microns, having good flowability, e.g., a flow rate of about 25 to about 50 seconds in the standard Hall Flowmeter described in A.S.T.M. Standard B-213, and having an apparent density of about 1.5 to about 3 or 4 grams per cubic centimeter. With particular reference to fine carbonyl nickel powders, agglomerates having an apparent density in the range of about 1.7 to about 2.7 grams per cubic centimeter are readily provided. A further characteristic of the agglomerates is that they are compactible and may readily be hot or cold roll-compacted directly to strip without lamination of the strip. The agglomerates have an irregular particle outline.

In contrast to the results achieved in accordance with this invention, portions of the same carbonyl nickel powder described in conjunction with Example I were sintered in hydrogen to form cakes. A temperature in the range of 1700 F. to 2000 F. was required to produce sintered cakes from the loosely packed powder. The cakes were quite tough and ductile. When the cakes were mechanically crushed it was found that the resulting crushed powder aggregates were irregularly shaped and they exhibited poor flow characteristics. Again, when carbonyl nickel powder of the same type as that described in conjunction with Example I is sintered in hydrogen without agglomeration at a temperature of about 1200 F. and the resulting sintered material is crushed, the powder obtained has a size distribution similar to that of the initial powder and is not improved in flow rate. However, material processed with agglomeration in accordance with the invention and sintered at 1200 F. is a free-flowing agglomerated powder. The foregoing confirms that the invention alfords a method for producing free-flowing metal powder starting with materials such as fine carbony l nickel powder having a poor flow characteristic wherein only a low energy input is required. This feature provides economy in carrying out the process of the invention.

It will be appreciated that not only can powders of a single metal be agglomerated in accordance with the invention to provide free-flowing powder agglomerates but that also carbonyl codeposited iron-nickel powders, alloyed powders, coated powders and mixtures of initial single metal powders can be treated so as to produce powder agglomerates containing controlled proportions of the initial metals. Thus, agglomerates containing nickel and iron; nickel and cobalt; nickel, iron and cobalt; nickel and copper, etc., can readily be produced in accordance with the invention and these materials can be employed directly to produce alloy articles by pressing and sintering in accordance with conventional powder metallurgy techniques.

Furthermore, the process provided in accordance with the invention can be employed to provide coated powders. Thus, nickel coatings can be produced upon other powders such as chromium, graphite and copper by agglomerating and sintering in accordance with the invention. It will be appreciated that when such a procedure is employed, the particles to be coated may be of major particle sizes, e.g., about 15 to about microns, and these can be coated with fine particle size powders by wet agglomeration and sintering as described hereinbefore. The invention is also applicable to the production of free-flowing material for use in dispersionhardening systems. Thus, the 'agglomerating liquid may contain therein a salt which is heat-decomposable to a stable oxide such as thoria, alumina, etc., to provide thorough wetting of the initial powder with a salt such as, for example, thorium nitrate. During the sintering operation as described hereinbefore, the salt decomposes to provide finely dispersed material intimately mixed throughout the hardened agglomerates. Oxides, carbides, nitrides, silicides and other dispersants can be introduced into nickel and other metal powders in this manner by wet agglomeration with a solution of an appropriate decomposable salt.

Although the present invention has been described in conjunction with preferred embodiments, it is to be understood that modifications and variations may be resorted to without departing from the spirit and scope of the invention, as those skilled in the art will readily understand. Such modifications and variations are considered to be within the purview and scope of the invention and appended claims.

We claim:

1. The process for producing free-flowing metal powder agglomerates from a fine metal powder having an average particle size not exceeding about 10 microns and having poor flow properties which comprises tumbling said powder while spraying a balling liquid thereon to form separate ball agglomerates to said powder containing up to about 30% by weight of said liquid, quiescently drying said agglomerates to remove said liquid and thereafter sintering the resultant agglomerates in a protective atmosphere at a temperature at which sintering occurs within said agglomerates but below that at which substantial sintering occurs between agglomerates to produce freeflowing metal powder agglomerates having an average particle size of about 20 to 1,000 microns.

2. The process according to claim 1 wherein the metal powder is a fine metal powder from the group consisting of nickel, cobalt, iron, copper, tungsten and molybdenum, and alloys and mixtures thereof.

3. The process according to claim 1 wherein the metal powder is a metal carbonyl powder.

4. The process according to claim 2 wherein the sintering is conducted at a temperature not exceeding about two-thirds of the metal melting point in degrees Fahrenheit.

5. The process according to claim 1 wherein the powder is fine carbonyl nickel powder.

6. The process according to claim 1 wherein the powder is fine carbonyl iron powder.

7. The process according to claim 1 wherein the powder is fine carbonyl cobalt powder.

8. The process according to claim 1 wherein the balling liquid is water.

9. The process according to claim 2 wherein the sintered agglomerates are pulverized to improve the flow rate thereof.

10. The process according to claim 2 wherein the metal pow-der is a codeposited nickel-iron carbonyl powder, the sintering temperature is of the order of 1700 F. and the sintered agglomerates are pulverized to improve the flow rate thereof.

11. The process according to claim 1 wherein the balling liquid contains a heat-decomposable organic binder.

12. The process according to claim 5 wherein the sinterin-g temperature is about 1000 F. to 1730 F.

13. The process according to claim 5 wherein the sintering temperature is about 1400 F. to 1730 F. and the sintered agglomerates are pulverized to improve the flow rate thereof.

14. The process according to claim 6 wherein the sintering temperature is of the order of about 1700 F. and

the sintered agglomerates are pulverized to improve the flow rate thereof.

15. The process for producing free-flowing metal powder agglomerates from a fine metal pow-der from the group consisting of nickel, cobalt, iron, copper, tungsten and molybdenum and alloys or mixtures thereof having an average particle size not exceeding about 10 microns and having poor flow properties which comprises tumbling said powder while spraying a balling liquid consisting essentially of water thereon to form separate ball agglomerates of said powder containing about 5% to about 30% by weight of water, quiescently drying said agglomerates and thereafter sintering the resultant agglomerates in contact with each other in a protective atmosphere at a temperature not exceeding about two-thirds of the melting point of said metal in degrees Fahrenheit to produce free flowing metal powder agglomerates having an average particle size of about 20 to about 1,000 microns.

16. The process in accordance with claim 15 wherein the metal powder is carbonyl nickel powder, the balling liquid is demineralized water and the sintering temperature is about 1000 F. to about 1730 F.

17. The process according to claim 16 wherein the sintering temperature is about 1400 F. to about 1730 F. and the sintered powder is thereafter pulverized.

References Cited UNITED STATES PATENTS 2. 179,960 ll/l959 Schwarzkopf 2l3 XR 2,853,767 9/1958 Burkharnmer 75211 X 2,857,270 10/1958 Brundin 75213 3,001,871 9/1961 Thien-Chi 75-213 X FOREIGN PATENTS 689,349 3/1953 Great Britain.

727,807 4/1955 Great Britain.

818,191 8/1959 Great Britain.

930,003 6/ 1963 Great Britain.

CARL D. QUARFORTH, Primary Examiner.

A. J. STEINER, Assistant Examiner.