US3765866A - Production of copper and copper oxide powder for powder metallurgy - Google Patents

Abstract

Copper base (including pure copper and copper alloy) or copper oxide powder is produced by atomizing a sulfurized copper base melt in the presence of oxygen to produce particles which are hollow as a result of sulfur dioxide formation therewithin, oxidizing the hollow particles in the presence of oxygen in order to cause weakening at the grain boundaries as a result of more sulfur dioxide formation therein, and crushing in order to make available the copper base particles. If desired, the oxidizing step, when carried for a longer time, results in oxidation of copper as well as sulfur and the crushing step results in copper oxide particles.

Description

United States Patent 1 1 1 1 ,866 Nayar Oct. 16, 1973 1 PRODUCTION OF COPPER AND COPPER 3,293,006 12/1966 Bartz 7s/o.5 R

OXIDE POWDER FOR POWDER METALLURGY Primary ExaminerW. W. Stallard [75] Inventor: Harbhajan S. Nayar, Waltham, Attorney-Morse, Altman & Oates Mass.

[73] Assignee: Contemporary Research llnc.,

Natick, Mass. [57] ABSTRACT [22] Filed: 1971 Copper base (including pure copper and copper alloy) [21] Appl. No.: 79,355 or copper oxide powder is produced by atomizing a sulfurized copper base melt in the presence of oxygen Related Apphcanon Data to produce particles which are hollow as a result of Division of 1 1 Sept- 9, 1963, sulfur dioxide formation therewithin,- oxidizing the hollow particles in the presence of oxygen in order to cause weakening at the grain boundaries as a result of [52] US. Cl 75/05 B more Sulfur dioxide formation therein, and crushing in [51] Int. Cl 1322f 9/00 Order to make available the copper base particles If [58] Field of Search 75/0.5 B desired, the idi i step, when carried for a longer time, results in oxidation of copper as well as sulfur (56] References C'ted and the crushing step results in copper oxide particles.

UNITED STATES PATENTS 2,870,485 1/1959 Jones 75/O.5 B 2 Claims, 8 Drawing Figures COPPER OXIDE HOLLOW PARTIALLY .OXIDIZED Cu POWDER- DARK LINES REPRESENT c11 0 AREA WITHIN DARK LINE IS Cu (50X) PAIENTEDncI 16 I975 V 3.765866 CRUSHER W REDUCING CHAMBER FIG. IA FIG- IB FIG- lC NVENTOR 3, MW BY Mo m f 0 1104 ATTORNEYS PATENTEDUBHBIBIS 3.766666 SHEET 2 BF 2 GU20 LAYER PARTIALLY DESULFURIZED Cu cRoss-sEcT "CRUDE" MIc TRUCTURE- EU TIC Cu +o+s DER NET K OF -cu 0- S FIG. 20 I 'F|G.2b,

Cu OXIDE (IOOXT 7 FIG. 2d PARTIALLY OXIDIZED Cu POWDER-- I DARK LINEs REPRESENT Cu O v AREA WITHIN DARK LINE IS Cu (50x) FIG. 26

4/ INVENTOR, y a g/l q m BY 6/ ATTORNEYS Cu TICLES AFTER CR NG.COVERED WITH PRODUCTION OF COPPER AND COPPER OXIDE POWDER FOR POWDER METALLURGY RELATED APPLICATION The present application is a division of Ser. No. 758,449, filed Sept. 9, 1968, and now U.S. Pat. No. 3,551,136.

BACKGROUND AND SUMMARY OF THE INVENTION The present invention relates to powder metallurgy and, more particularly, to the production of copper base (including pure copper and copper alloy) powder of the type which may be blended, compacted, and sintered in the production of mechanical parts andthe like. For example, bronze self lubricating bearings contain by total weight approximately 90 percent copper powder and percent tin powder. Also, high strength iron-base powder metallurgy parts contain by total weight from 1 to 25 percent copper powder and a remainder of iron powder. The present invention also relates to the production of fine copper oxide powder.

Desirable characteristics of copper base powder for powder metallurgy application are as follows. Powder metallurgy parts require low cost copper powder. Such low cost must result from production utilizing less power, fewer production steps and less human attention than currently is used. The composition preferably is fairly pure although in some applications, minor amounts of iron, manganese, carbon, etc. are desirable but not necessary. The composition must be such as to have highly compressible powder at room temperature.

Preferred dimensions range between 5 and 250 mi-' crons in diameter, the shape being irregular but equiaxial. Preferably the surface is rough in order to provide low apparent density and high green strength.

A variety of previous processes have been employed in the production of copper powder. These processes include atomization of copper melt, hydrogen reduction of copper mill scale, electrolysis and leaching. The atomization process, which is most typical, involves the following steps. A stream of first grade, scrap copper melt is atomized into small solid spherical copper particles by an impinging high pressure air stream projected from a nozzle attached to an air compressor. The typical air pressure required is about 250 pounds per square inch. The resulting particle size depends upon melt temperature, melt viscosity, melt surface tension and air pressure. Usually the apparent density of such solid spherical particles is greater than 4.5 grams per cubic centimeter. The resulting particle size range is very wide so that the particles must be screened into four or five mesh fractions for further treatment. The various mesh fractions, under heat, are oxidized separately in air or oxygen. Mesh fractions containing the larger particles generally require longer oxidizing times and higher temperatures than mesh fractions containing the smaller particles. Since the fully oxidized copper base powder is very brittle, it can be crushed into smaller irregular equiaxed copper oxide particles. Finally, after sizing, the crushed oxidized particles are reduced with hydrogen to provide pure and relatively fine copper particles suitable for powder metallurgy parts. Since a large proportion of allcopper used ultimately is reclaimed as scrap, it is desired that existing atomization processes be improved and made more economical. As is apparent, the elimination of one or more of the foregoing steps in an atomization process and/or the reduction in cost of one or more of the foregoing steps will reduce the overall copper powder production cost.

The primary object-of the present invention is to provide an improved process, for producing'copper base or copper oxide powder, comprising the steps of first sulfurizing the copper base melt, next atomizing by any standard technique the sulfurized copper melt in oxygen to produce hollow droplets by sulfur dioxide formation, next solidifying the hollow droplets to form hollow shells and weakening the grain boundaries of the shells by sulfur dioxide and copper oxide formation at the grain boundaries, then crushing the weakened shells to form particles and finally reducing the particles with hydrogen as desired. Under certain conditions, weak grain boundaries can be produced during solidification to form hollow shells, thus eliminating the subsequent steps and thus directly crushing the solid hollow shells.

Other objects of the present invention will in part be obvious and will in part appear hereinafter.

The invention accordingly comprises the processes.

characterized by the steps and relationships which are set forth in the accompanying disclosure, the scope of which will be indicated in the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS For a fuller understanding of the nature and objects of the present invention, reference is made to the folv lowing detailed description taken in connection with the accompanying drawings, wherein:

FIG. 1 is a series of flow diagrams of processes illustrating the various steps of the present invention; and

FIG. 2 illustrates physical details of copper containing particles undergoing a process of the present invention.

DETAILED DESCRIPTION FIG. 1 represents flow diagram illustrating the various steps and the various combinations of steps of the process of the present invention. It is clear from the FIG. 1 that various combinations of the steps provide slightly different processes to produce the powder metallurgy grade copper powder and copper oxide powder. Variations of the process of FIG. 1 are shown in FIGS.

' 1A, 1B, 1C and 1D, wherein analogous steps are designated by analogous numerals and appropriate letters. The main and common step in FIGS. 1A, 1B, 1C and 1D is the atomization of a sulfurized copper melt 10 by impinging a molten stream from the melt with an air stream 13 under pressure, giving hollow particles of low apparent density and high specific surface area due to the formation of SO within the particles.

FIG. 1A illustrates partially desulfurized hot hollow particles 11 being fed directly into a vertical tube 12 with a constant temperature zone of approximately 1,000 1050C. As the hollow particles pass through the zone, an exothermic reaction between copper oxide and copper sulphide at grain boundaries occurs by which apparent density is further decreased due to escape of S0 formed at grain boundaries and the grain boundaries of the particles are weakened by this further desulfurization. The particles then pass through a horizontal belt furnace 14 where preferential oxidation occurs at the grain boundaries. The hollow but partially oxidized particles are fed into a crusher 16. The

crushed and partially oxidized particles are fed through a fluidized chamber 18 or a normal horizontal belt furnace where they are reduced by hydrogen. The final product is a copper powder 19 suitable for powder metallurgy parts. It has been found that by controlling melt sulfur content, melt temperature and atomization conditions, a variety of different hollow particles can be achieved with selected shapes, sizes, apparent densities and microstructures. In turn, the size, shape and apparent density of the final product can be controlled. An alternatively possible specific particle shape is characterized by relatively flat minute platelet. In the foregoing process, sulfur intentionally is added to produce hollow particles of low apparent density, high specific surface area, and a unique composition. The particular element, sulfur, is selected because it can be removed easily during subsequent treatment. The process of adding sulfur is such as to produce particles of desired shape, size, microstructure, composition, wall thickness, etc.

Generally, the melt to be atomized ranges in temperature between the liquidus temperature of the copper sulfur alloy and 2600F and contains 0.05 to 3 percent sulfur with a remainder of copper, minor proportions of other alloying materials optionally being present. The air stream impinging on the molten copper stream ranges in pressure between and 500 pounds per square inch. During atomization, the hollow particles formed by oxidation of sulfur to sulfur dioxide, acquire very rough interior surfaces. The high pressure sulfur dioxide gas formed in each molten droplet inflates the droplet to a hollow shell, which in most cases bursts and the thus formed sulfur dioxide escapes. A particle in this crude form is shown schematically in FIG. 2A. The outer surface of the particle has a thin copper oxide layer. The material beneath this copper oxide layer is essentially partially desulfurized copper. The microstructure of this partially desulfurized copper is shown in FIG. 23 as being composed of a eutectic network of Cu, Cu S and Cu O distributed along the grain boundaries.

The melt sulfur content, melt temperature, air pressure and other atomization variables are controlled to give the following. The specific oxygen to sulfur ratio is in the range of 0.221 to 20.0:1 but preferably between 0.5:1 to 5:1. This particular ratio ensures removal of the sulfur by its combination with the oxygen that is available within the particle during subsequent heating treatment. The apparent density of the atomized particles preferably is below 3 grams per cubic centimeter and for best results below 2 grams per cubic centimeter. By virtue of its interior surface irregularity and hollowness, the surface area of the particle is large in relation to its mass, i.e. is large compared to the surface of a normal solid particle of like material. Preferably all or most, say at least 70 percent of the Cu S and Cu O is at the grain boundaries in the eutectic network. Preferably the grain size of the particle before crushing is approximately equal to the final size of the particle desired. The wall thickness of the hollow particle is very small, preferably less than 250 microns thick. The hollow particle size and the wall thickness are quite uniform from particle to particle.

As indicated above and shown in FIG. 2B, the grain boundaries are composed of a eutectic network of Cu, Cu O and Cu S. The melting points of Cu, Cu-Cu O eutectic and Cu-Cu S eutectic respectively are 1,083,

1,065 and l,067C.' It is to be noted that the melting points of Cu-Cu O eutectic and Cu-Cu S eutectic are lower than the melting point of pure Cu. It is also to be noted that the following reaction is exothermic:

If particles with this type of microstructure are heated at any temperature between some threshold temperature and 1,050C, the above reaction will occur. Optionally this reaction can be conducted in a vacuum. Since the above reaction is exothermic and if it is once triggered, the heat of reaction will raise the temperature at the grain boundaries to some temperature above that within the grain. If the particles are heated to say I,O50C, the temperature in consequence is raised within the grain boundaries to approximately 1,070C, thereby causing melting along grain boundaries. The sulfur dioxide erupts within the grain boundaries and, as it tries to escape, weakens the grain boundaries. At this point the escape of SO further decreases the apparent density of the hollow particles by as much as 25 percent.

FIG. 1B is similar to FIG. 1A except that furnace I4 is eliminated. FIG. 1C is similar to FIG. 1A except that vertical tube furnace 12 is eliminated. FIG. 1D is similar to FIG. 1A except the furnace l4 and the vertical tube furnace 12 are eliminated.

In the step of oxidizing, the hollow particles are heated in closed furnace 14, which is filled with air at 400-800C. In an alternative furnace, the ends are open to air. If the hollow particles are left in the furnace until the grain boundaries are completely oxidized, the resulting structure as shown in FIG. 20 is weak and brittle at the grain boundaries and the particles can be removed before the essentially pure copper composition of each grain is changed.

With respect to the crushing step, the hollow particles with weak and brittle grain boundaries now can be easily crushed into much smaller particles using a suitable crushing device. The resulting crushed particles, as shown in FIG. 2d, are essentially equiaxed with rough and irregular surfaces. The crushed particles are much smaller than the atomized hollow particles. The interior of each crushed particle essentially is pure copper whereas the surface is covered with a thin copper oxide layer.

With respect to reduction of the crushed particles, the oxygen can be removed by reduction with hydrogen in a fluidized bed or in a conventional reduction furnace. This latter reduction step can be completely eliminated if annealing is properly controlled, i.e., if annealing is completed in such a way that all oxygen and sulfur is eliminated at the grain boundaries during heating before crushing.

EXAMPLE I The following example will further illustrate the present invention. A quantity of copper having 0.5 percent sulfur by total weight was melted in a crucible at 2,200F. The crucible had a A; inch hole at its bottom. A molten stream of metal flowing out of the hole was imapac ted with compressed air at a pressure of pounds per square inch and a flow rate of 100 cubic feet per minute at room temperature and noramal atmospheric pressure. The resulting particles, which were hollow, had an apparent density of approximately 2.5 grams per cubic centimeter. The wall thickness of the particles typically was between 0.004 inch and 0.010 inch. The outside diameter of these particles varied between 0.010 inch and 0.10 inch. The inside surface of these particles was very rough and composed of ridges and valleys" giving a very high specific surface area. The solid portion of the particles had a eutectic network of copper, copper oxide, and copper sulphide with oxygen to sulfur ratio of about 4 to 1. These particles were heated in air for one-quarter hour at 800C in a stainless steel rotating container. The particles were cooled, crushed and reduced at 450C for one-half hour under hydrogen to give fine copper powder suitable for powder metallurgy parts.

Copper oxide particles also can be produced by the above process. For this purpose, the high specific surface particles produced by the atomization step described above are oxidized in a horizontal furnace. These hollow particles are oxidized in air at a temperature of between 600 and l,050C for a time sufficient to oxidize the particles completely. This oxidation proceeds in two distinctly different'ways. First, the exterior and interior surfaces are oxidized and the resulting copp r oxide-copper interfaces, one from outside the particle and the other from inside the particle, move toward each other. Secondly, Cu S and Cu O at the grain boundaries combine to produce sulfur dioxide, which escapes to the atmosphere via the grain boundaries. These grain boundaries become more and more open so that oxygen can travel or difiuse through thegrain boundaries, thereby greatly enhancing the oxidation rate. This enhanced oxidation rate reduces the total time needed for completely oxidizing a given weight of copper. Finally the hollow oxidized copper shells are crushed as desired in any suitable crushing equipment.

EXAMPLE 2 The following example illustrates a production of fine copper powder in accordance with present invention.

The process of Example 1 was repeated except the hol-.

low particles were instantly heated at 1,050C for about 1 minute. This caused weakening of the grain boundaries due to formation of S and its escape via the totally melted grain boundaries. The particles then were heated in air at 800C for 5 minutes to oxidize the erupted grain boundaries completely. The particles were cooled, crushed and reduced under hydrogen for one half hour at 450C to give fine powder metallurgy grade copper powder.

EXAMPLE 3 Same process as in Example 1, except that the sulfur content was 2 percent by total weight, the melt temperature was 2,400F and the air pressure was 200 pounds per square inch. The resulting particles had an apparent density of 2 grams per cubic centimeter with thinner walls and a thicker eutectic network as compared to that in Example 1. The particles were crushed and heated under hydrogen to remove excess oxygen and sulfur.

EXAMPLE 4 The following example illustrates a production of copper oxide particles in accordance with the present invention. Example 1 was repeated except that the hollow particles here were heated in the air at 1,050C for 5-7 minutes in order to achieve complete oxidation. The particles then were crushed to provide fine copper oxide powder.

In the foregoing examples, the melt was composed of essentially pure copper. it is to be understood that generally the present invention is applicable to a copper base melt containing copper as its characteristic ingredient (at least 50 percent by total weight), from 0.2 to 2 percent sulfur, and a remainder of an alloying metal, such as tin, nickel and zinc, having a heat of formation and a negative free energy of formation of its oxide and its sulfide, which are sufficiently low with respect to those of SO that appreciable quantities of SO are formed sufficiently quickly to effect the S0 puffing herein described.

The present invention thus provides an efficacious processfor producing copper and copper oxide powder in an unprecedently inexpensive way. Since certain changes may be made in the disclosure hereof without departing from the scope of the present invention, it is intended that all matter contained in the foregoing description or shown in the accompanying drawing be interpreted in an illustrative and not in a limiting sense.

What is claimed is:

l. A mass of low apparent density copper base particles with high specific surface area, equiaxed particle shape, high compactability and in the 5 to 300 micron size range, said copper base particles being characterized by an open hollow shell having an exterior surface with a thin copper oxide layer, an interior surface that is rough, and a composition containing as its characteristic ingredients sulfur, oxygen and a remainder of copper, there being from 0.05 to 3 percent sulfur, the specific oxygen to sulfur ratio ranging between 0.2:1 and 20.0:1, said composition containing a eutectic network of Cu, Cu S and Cu O, the apparent density of the atomized particles ranging below 3 grams per cubic centimeter, at least percent of said Cu S and 01 0 being at grain boundaries in said eutectic network.

2. The mass of low apparent density copper base particles of claim 1 wherein the wall thickness of said hollow shell characteristically is less than 250 microns thick.

Claims (1)

1. 2. The mass of low apparent density copper base particles of claim 1 wherein the wall thickness of said hollow shell characteristically is less than 250 microns thick.

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