US2721135A - Method of producing metallic powders - Google Patents

Description

United States Patent METHOD OF PRODUCING METALLIC POWDERS Joseph J. Wimberly, College Park, Ga., assignor to Tennessee Corporation, New York, N. Y., a corporation of New York No Drawing. Application October 29, 1952,

Serial No. 317,573

4 Claims. (Cl. 75-.5)

This invention relates to the production of powdered metals, for example, powdered metallic iron, for use in powder metallurgy. Y

Powder-metallurgy processes for producing various types of metallic articles and products are now well known. The metal powders heretofore used have been produced by several different methods, including electrolysis, spraying or atomization, reduction of metal oxides, and other chemical methods. In general, metallic articles are manufactured from such powders by compressing the powder to the desired shape in a suitable mold and then subjecting it to an appropriate heat treatment. Such procedures are employed for a variety of purposes such as the production of machine parts and bearings, the manufacture of various articles from special alloys, and many others.

For best results in powder-metallurgy processes, the mesh classification, particle size distribution, uniformity and apparent density or apparent specific gravity of the powder must be appropriately controlled within well defined limits. Accordingly powders produced by the methods mentioned above are usually pulverized, screened and blended to assure that they have the desired properties before usev For example, an iron powder to be acceptable for most purposes must conform to the following requirements:

1. It must have an apparent specific gravity between 2.2 and 2.5.

2. It must have a flow rate of 40 seconds or less, as determined by the time required for 50 grams to pass through a Hall flowmeter.

3. All of the material must be less than 100 mesh in particle size, and a maximum of 60% may be less than 325 mesh. Within these limits, the particle size distribution or gradient may vary somewhat, but must be approximately of the order of one third greater than 200 mesh, one third greater than 325 mesh, and one third less than 325 mesh.

Oftentimes it is very difficult to meet such requirements, particularly in the case of metal powders obtained by reduction of metal oxides by means of gaseous or solid reducing agents. Even when ground to the permissible limit from the standpoint of particle size distribution, for example, the apparent specific gravity of the reduced and ground powder may still be less than the acceptable minimum and the powder therefore relatively worthless for the purposes of the present invention. In other cases the apparent specific gravity may fall within prescribed acceptable limits, but the powder may have an unacceptable particle size distribution; for example, it may contain too large a percentage of small particles.

The present invention provides improved methods of handling and treating such metal oxides, in particular prior to the reduction thereof, so as to minimize and in many cases to eliminate entirely the difiiculties mentioned above and to facilitate the production of metal powders meeting the specifications prescribed for powder metallurgy.

2,7Zl,l35 Patented Oct. 18, 1955 The invention has as a further object to make it possible to predetermine the characteristics of the ultimate metal powder by adjustment of the nature and properties of the metal oxide prior to reduction.

A still further object is to provide an improved method for treating pulverulent metal oxides having a particle size gradient such that heretofore they have been considered unsuitable for the manufacture of metal powders, Whereby such oxides can be reduced and ground to provide powders completely satisfying the accepted specifications for powder metallurgy.

Another object is to provide an improved metal powder capable of producing metallic articles of increased tensile strength as compared with metal powders heretofore used.

Still another object is to facilitate the reduction of the oxide.

I have found that the foregoing objects can be achieved by sintering the metallic oxide and then crushing and/ or grinding the sintered material to the desired mesh size before reduction. Sinter particles, even of sizes down to one micron or less, are characterized by a porous cellular structure resembling that of coke or a sponge. I

have observed that when such oxidic sinter particles are reduced at a temperature below the fusion point, the resulting metallic particles have the same cell-like structure and approximately the same size and irregular shape as the original sinter particles.

Thus by first sintering the oxidic material from which the powder is to be made, and by other known techniques such as screening, crushing and grinding, a particular oxidic sinter can be produced having approximately the particle size distribution desired in the final powder, and particle size will be approximately maintained during reduction so that further grinding or the like after reduction produces a metal powder having about the same particle size distribution as the sinter prior to reduction. This can be accomplished even with oxidic materials so finely divided that they have not been useful for powder metallurgy heretofore, since the necessary particle size enlargement is one of the results of sintering.

Any suitable sintering method and apparatus can be employed, preferably the well known down-draft Dwight- Lloyd or Greenawalt sintering machines in which a mixture of the oxidic material with fuel and water is burned on a down-draft sintering pan. Small particles of sinter can be screened out and used in the next batch of sinter. The oxidic sinter is then crushed and/or ground in any suitable manner to produce a particulate sinter having a mesh classification and particle size distribution within the limits desired in the final powder and then reduced by means of solid carbon or a gaseous reducing agent such as hydrogen, carbon monoxide, and mixtures of the same. Since some agglomeration takes place during reduction, the reduced material is then reground or milled until it once more has the desired predetermined particle size distribution as explained above.

Moreover, it has been found that the porous cellular structure of the sinter particles greatly facilitates the complete reduction of the oxide in a short time. Another advantage is that the reduced particles have the same irregular shapes and sizes as the sinter particles and the resulting interlocking qualities increase the tensile strength of the pressed parts made from the powder.

Also the sintering step burns out impurities such as sulfur; therefore the starting material may be a sulfide since it will be converted to an oxide in the sintering operation.

Accordingly it will be understood that the invention is applicable to any metal-bearing material which, after sintering, comprises an oxide capable of being reduced without volatilizing the metal, including metallic ores, mill scales, etc. Its advantages are well illustrated by its apin a typical case:

plication to a magnetite ore containing excess silica impurity as follows:

In order to reduce the silica content of the ore to below 0.2 of one per cent by magnetic separation, it is necessary to grind the ore to the following mesh sizes:

7 Per cent 100 mesh 200 mesh Trace 325 mesh 10.0 325 mesh 90.0

This material can be reduced to metallic iron and ground or milled to bring its apparent specific gravity within the required limits, but the size gradient is not ac- Accordingly the ground ore after separation of silica was sintered by burning a mixture of ore, coke, and water on a down-draft sintering pan similar in operation to a Dwight-Lloyd or Greenawalt sintering machine which are familiar to those versed in the art of sintering. The sintered ore varied. from large lumps, 6" or more across, down to fine particles. Any particles under 4 mesh were screened out and put into the next batch of sinter.

The sinter was then crushed down to A1" size or smaller, and then ground to the desired size gradient. For ex-' ample, the following particle size gradient was obtained Per cent On -l00 mesh 0 On 200 mesh 43.4 On 325 mesh 19.0 Thru 325 mesh- 37.6

This ground sintered ore was reduced and then milled or ground until the apparent specific gravity was within the desired range. This gave an iron powder with the follow- It is evident that the above characteristics are well within the accepted limits and in fact the reduced powder was highly satisfactory, whereas the powder obtained by reduction of the ore without sintering was not acceptable.

It will be noted that the particle size gradient of the reduced powder was approximately the same'as'that of the Thus by pregrinding a ground sinter'before reduction. sintered material to a particular size gradient, the corresponding properties of the resulting reduced and reground material can be predetermined and controlled. This etfect and result are further illustrated by the following examples:

A screen analysis on' a ground sintered ore showed the particle size gradient to be:

Per cent 7 On 100 mesh"; 0

On 200 mesh 32.2 On 325 mm.- 21.6 Thru 325 mesh 46.2

When'this ore was reduced and milled, an iron powder 7 with the following characteristics was obtained;

. 4 Apparent specific gravity 2.04 Flow rate;; seconds 38.8

Particle size gradient:

On 100 mesh percent 0 On 200 mesh d0 23.6 On 325 mesh do 27.6 Thru 325 mesh do 48.8

It will be seen that although near the limit with respect to particle size gradient, this powder still was not acceptable because of its low specific gravity. The relatively large percentage of very small particles in the ground sinter was practically duplicated in the iron powder, and

prevented further grinding in an efiort to improve the apparent specific gravity.

Another illustration of this effect was noted in the case of a coarse ground sintered ore having a particle size gradient as follows:

Per cent On 100 mesh 0 On 200 mesh 52.8 011 325 mesh 18.2 Thru 325 mesh 29.0

When the reduced material was milled to Within the desired specific gravity range the results were:

Apparent specific gravity 2.44 Flow rate seconds 28.8

Particle size gradient:

On 100 mesh per cent 0 On 200 mesh do 35.0 On 325 mesh do 29.6 Thru 325 mesh do 35.4

It will be seen that this material is just acceptable as to its apparent specific gravity and that a slightly coarser ground sinter would have produced a powder that was too heavy.

Thus by sintering and pregrinding the sinter before reduction, it is possible to regulate approximately the physical. properties that the resulting metallic powder will have. By sintering, moreover, it is possible to effect an initial size enlargement of excessively finely ground ores and like materials, so that materials heretofore considered unsuitable for powder metallurgy can be prepared for such pregrinding and reduction to metal powders of high quality. At the same time reduction of the oxide is facilitated; burnable impurities such as sulfur are removed; homogeneous metallic particles of porous or cellular structure are obtained, thus providing a uniform material so that the powder'metallurgy process can be controlled withpredictable and reproducible results; and

1. In the production of metal powder for use in powder V metallurgy from a finely divided metal-bearing material which when'reduced to metal forms a powder of particle size smaller than desired for such use, the method which comprises the steps of sintering said metal-bearing material to a metallic oxide with concomitant particle size enlargement, then comminuting the sinter to the particle size distribution desired for said metal powder, then reducing the oxide, and finally breaking up agglomerates formed during the reduction step.

2. The method of preparing a metal powder for use in powder metallurgy from an oxidic ore containing silicalike impurities which comprises comminuting the ore to separate said impurities with concomitant reduction of particle size below that desired in said powder, then sintering said comminuted material to a metallic oxide with concomitant particle size enlargement, comminuting the sinter to a particle size distribution Within the limits required for the metal powder, then reducing the comminuted sinter, and recomminuting the reduced material to a final particle size distribution within said limits.

3. The method of preparing an iron powder for use in powder metallurgy from an iron oxide ore containing silica which comprises finely dividing the ore to detach silica from the ore particles with concomitant reduction of particle size below that desired in said powder, then separating and sintering said comminuted ore with concomitant particle size enlargement, comminuting the sinter to a particle size distribution within the limits required for the metal powder, then subjecting the comminuted sinter to heat in the presence of a reducing agent, and recomminuting the reduced material to less than 100 mesh and at least 40% greater than 325 mesh.

4. The method defined in claim 3, applied to magnetite ore.

References Cited in the file of this patent UNITED STATES PATENTS

Claims (1)

1. IN THE PRODUCTION OF METAL POWDER FOR USE IN POWDER METALLURGY FROM A FINELY DIVIDED METAL-BEARING MATERIAL WHICH WHEN REDUCED TO METAL FORMS A POWDER OF PARTICLE SIZE SMALLER THAN DESIRED FOR SUCH USE, THE METHOD WHICH COMPRISES THE STEPS OF SINTERING SAID METAL-BEARING MATERIAL TO A METALLIC OXIDE WITH CONCOMITANT PARTICLE SIZE ENLARGEMENT, THEN COMMINUTING THE SINTER TO THE PARTICLE SIZE DISTRIBUTION DESIRED FOR SAID METAL POWDER, THEN REDUCING THE OXIDE, AND FINALLY BREAKING UP AGGLOMERATES FORMED DURING THE REDUCTION STEP.