Method For The Electrodeposition Of Copper Powder

Abstract

Metallic powder, e.g., copper powder, is deposited on a series of disc-shaped cathodes as they turn through an electrolytic solution of the metal. The cathodes, preferably of titanium, are partially immersed in a bath of electrolyte contained in an electrolytic cell tank. Insoluble anodes, preferably of platinized titanium, are disposed in the tank in interleaved arrangement with the cathodes. Powder is continuously deposited on the cathodes and continuously removed by doctor blades, preferably of plastic, mounted adjacent the cathodes above the electrolyte level of the cell. In the production of copper powder, the cell operates at a much higher temperature, current density, and acid concentration--and with a much lower copper ion concentration in the electrolyte--than is typical for the electrodeposition of copper in electrolytic cells.

Description

BACKGROUND OF THE INVENTION

Field

The present invention relates to the electrodeposition of metal powders and provides an improved apparatus for that purpose. In particular, it concerns the electrowinning of copper in the form of high- purity copper powder and provides a specific method for producing such a powder in the apparatus of this invention.

State of the Art

Various methods and apparatus for the electrodeposition of metallic powders are known. Certain of the processes and apparatus involve the deposition of metallic powder on a movable or continuous cathode. U.S. Pat. No. 1,736,857, for example, discloses and claims apparatus involving an endless cathode in the form of a band which continuously passes between anodes through a trough containing electrolyte. U.S. Pat. No. 2,810,682 discloses and claims a process whereby silver powder is produced from a soluble silver anode. The anode dissolves in the electrolyte and powder is formed on a disc-shaped cathode rotating slowly through the electrolyte. Deposited powder is removed as the rotating cathode surfaces pass between a pair of metallic doctor blades. The powder settles to the bottom of the electrolyte bath and is periodically recovered by filtering the electrolyte. U.S. Pat. No. 1,959,376 discloses a process and U.S. Pat No. 2,053,222 discloses an apparatus for producing copper powder. According to these patents, a series of disc-shaped copper cathodes is mounted in an electrolytic cell tank such that each cathode is partially immersed in the electrolyte bath contained therein. Soluble copper anodes are suspended in the electrolyte bath on each side of each cathode. The cathodes are rotated as a current is applied across the electrodes. Copper deposited on the rotating cathode surfaces is removed as powder by doctor blades mounted above the electrolyte surface.

The aforedescribed prior art processes each involves the dissolution of relatively impure metal in the electrolyte contained in an electrolytic cell tank and the continuous redeposition of this metal as powder on a moving cathode. A difficulty common to these processes is the entrapment of impurities present in the electrolyte as the result of dissolving impure metal in the cell tank. This difficulty is particularly significant because the cathode deposits are necessarily of a nature which contains a considerable volume of voids wherein the impurities may be entrapped. Moreover, after soluble anodes dissolve to a predetermined extent, typically 70 to 90 percent, they must be replaced and scrapped. Otherwise, the anodes weaken and fail structurally, resulting in electrical shorts and damage to cell walls.

SUMMARY OF THE INVENTION

The present invention provides a process whereby high-purity copper powder is produced by deposition on rotating cathodes. The process involves the rigid control of operating variables; in particular, temperature and composition of the electrolyte, current density, and plating time. This invention further provides a novel electrolytic cell, useful for the production of copper powder in accordance with the claimed process, as well as for the production of other metal powders by analogous processes.

The apparatus and process of this invention are particularly useful for the production of copper powder from electrolytes formed by leaching impure copper-bearing materials such as precipitate copper.

According to this invention, copper powder is produced by flowing an electrolyte through a tank in which are suspended insoluble anodes and titanium cathodes alternating in parallel, interleaved arrangement. The anodes are stationary and the cathodes are movable, preferably as rotating discs. A higher-than-ordinary current density is maintained across the electrodes; because of the carefully controlled electrolyte temperature and composition taught by this invention, current densities of about 200 to about 280 amperes per square foot at the anode and about 300 to about 400 amperes per square foot at the cathode are practical. To obtain adequate circulation, it is essential that the anodes be insoluble so that the anode reaction results in the evolution of oxygen gas. The required vigorous circulation is induced by the evolution of oxygen from the anode surfaces without additional agitation. The preferred anode surface is of platinum, platinum-coated titanium being presently considered the best material of construction. Because the electrolyte becomes extremely corrosive as it dissolves oxygen, the cathodes are constructed of titanium to avoid pitting of the plating surfaces.

The temperature of the electrolyte is maintained above at least about 130.degree. F., preferably at about 140.degree. F. At lower temperatures there is insufficient mobility of the copper ions in the electrolyte to maintain an appropriate concentration at the cathode-solution interface. Temperatures above about 150.degree. F. result in the generation of a serious acid mist above the cell and are rarely employed, although it is recognized that higher temperatures are advantageous provided control of the acid mist is economically justified.

Proper control of the chemical composition of the electrolyte is an important aspect of this invention. The copper ion level is maintained low, i.e., about 1.2 to about 1.5 percent by weight, based on the total weight of the electrolyte. The spent electrolyte leaving the cell is recycled through a leaching step and back to the cell tank. The sulfuric acid level is maintained at about 16 to about 18 percent by weight on the aforestated basis, thereby facilitating regeneration of the electrolyte by leaching. The high acid content also results in low cell resistance, thereby permitting effective deposition of copper at a lower voltage. The acid content of the electrolyte should be sufficiently high to result in a voltage drop of less than about 5 volts across the cell.

The deposition of copper powder in accordance with this invention is accomplished by maintaining the process conditions hereinbefore described in an electrolytic cell of special construction, including a continuously moving titanium cathode surface and insoluble anodes. Although the electrolytic cell of this invention is particularly useful for producing copper powder it may be employed for the production of other metal powders by following procedures similar to those specifically described in connection with the production of copper powder.

In the deposition of copper powder, it has been found that growth time is an important factor influencing the particle size distribution of the copper powder recovered. Longer plating times result in wider distributions including higher percentages of coarse fractions. Although the plating time may be selected to produce the product desired in a particular instance, the product most desired for powder metallurgy applications, i.e., substantially all -100 mesh and including about 40 to about 70 percent by weight -325 mesh, is produced with plating times of about 2 to about 4 minutes.

The apparatus of the present invention includes a tank constructed to maintain a predetermined, desired electrolyte depth therein; a plurality of insoluble anodes, preferably of platinized titanium, mounted vertically in the tank to immerse the surface area of the anodes in the electrolyte contained in the tank; a plurality of disc-shaped titanium cathodes suspended in the tank in alternating, parallel interleaved arrangement with the insoluble anodes and mounted to rotate about their axes such that as they are rotated in operation, progressive wedge-shaped segments of the plating surface areas of the cathodes traverse a circular path whereby they cyclically dip beneath the electrolyte surface level of the tank, traverse the tank past an adjacent anode, and then traverse a circular arc above the electrolyte surface level of the tank to again dip beneath the electrolyte surface level of the tank; the necessary bus bars and other electrical equipment required to provide a current between the anodes and cathodes; and scraping means, e.g. doctor blades, mounted adjacent the cathodes and above the electrolyte surface in position to scrape the plating surfaces of the cathodes as they rotate about their axes. Desirably the scraping means are associated with means for collecting any deposits removed from the cathodes by the scraping means.

DESCRIPTION OF THE DRAWINGS

In the drawings, which illustrate the best mode presently contemplated for practicing the invention:

FIG. 1 is a side elevation of the novel electrolytic cell of this invention;

FIG. 2, a sectional view taken along the line 2--2 of FIG. 1; and

FIG. 3, a sectional view taken along the line 3--3 of FIG. 2.

DESCRIPTION OF THE ILLUSTRATED EMBODIMENTS

The illustrated apparatus comprises an electrolytic tank 11 in which are disposed a plurality of anodes 12 and a plurality of cathodes 13 in alternating, parallel, interleaved arrangement. The cathodes 13 are titanium discs mounted on a round copper shaft 14 (FIG. 2). The entire series of cathodes is mounted on a reduced section 14a of the shaft, the individual cathodes being separated by cylindrical titanium spacers 15 which fit over the shaft between adjacent cathode discs. A similar spacer 16 extends from the cathode disc 13-1 at one end of the series of cathodes to a copper washer 17 which fits snugly against a bearing block 18. One end section 14b of the shaft, which is larger in diameter than the remainder of the shaft, is journaled in bearing block 18, being held in position by washer 17. The bearing block 18 is of copper and serves as a commutator for electrical connection to the power supply (not shown). The journaled connection is lubricated with graphite. Another spacer 19 extends from the cathode disc 13--2 at the other end of the series of cathodes toward a plastic bearing block 20 in which the opposite end section 14c of the shaft is journaled. The spacers and cathodes are fastened into position by a titanium washer 21 and a threaded titanium nut 22. The end section 14c of the shaft is appropriately reduced in diameter to accommodate the nut 22.

At the end of the copper shaft 14 opposite the commutator is a hub 23 and sprocket 24 arrangement for driving the shaft. A plastic spacer 25 fits between the bearing block 20 and the hub 23 to prevent the shaft from sliding. The hub is electrically insulated from the sprocket by plastic insulators 26. The sprocket is driven (FIG. 1) by a variable-speed reduction motor 27 and chain 28. The entire apparatus is mounted in a structural frame 29 which is insulated from the shaft by the plastic bearing block 20 and a plastic support block 30 between the bearing block 18 and the structural frame 29.

The cathodes are vertically suspended from the shaft so that they are immersed in the electrolyte to a depth of about one third of their diameters. The anodes are constructed of insoluble sheet metal, preferably platinized titanium and are suspended in the tank by plastic rods 31 and 32 (FIG. 3). The rods are held in place by plastic brackets 33 anchored to the tank wall 11a as illustrated by FIG. 2. The anodes are spaced along the support rods by cylindrical plastic spacers 34. Spacing of the bottoms of the anodes is maintained by plastic rods 35 and 36 (FIG. 3) and cylindrical plastic spacers 37 as illustrated by FIG. 2. Each anode 12 is welded at one end to a titanium rod 38 to which electrical connection is made by means of a copper lug 39 and cable 40. Power is supplied to the electrodes in conventional fashion by a rectifier (not shown).

In operation, copper-bearing solution is metered from a head tank (not shown) to the cell tank through a manifold port 41 (FIG. 1). Spent electrolyte overflows through a port 42 (FIG. 3) into a surge chamber 43 to maintain a preselected electrolyte level in the cell tank 11. The spent electrolyte is continuously withdrawn from the surge chamber 43 through a pipe 44 for recycle through a leaching step (not shown) and back to the cell tank 11. In this fashion, the desired metal ion, e.g., copper ion, concentration is maintained in the electrolyte solution in the cell tank.

As the electrolyte solution is circulated through the tank, cathodes 13 are rotated by shaft 14 thereby submerging progressive segments of the plating surfaces 13a of the cathodes in the electrolyte contained in the tank. Simultaneously, a current is applied between the anodes and cathodes to effect the deposition of metal powder 45 on the submerged portions of the plating surfaces of the cathodes. As the cathode discs continue to rotate, the metal deposited on a particular segment of the cathode surface is lifted up from the electrolyte and moved through an arc above the electrolyte. Plastic doctor blades 46 are mounted above the electrolyte surface and adjacent the cathodes to continuously scrape the metal deposit from progressive segments of the cathode surfaces as the cathodes rotate. As illustrated, the doctor blades contact the plating surfaces of the cathodes just above the electrolyte surface on the side of the cell at which the cathodes enter the electrolyte. After the metal deposit 45 is removed from the plating surfaces of the cathodes by the doctor blades, the scraped segments 13a' are again submerged beneath the surface of the electrolyte to receive additional metal deposit. The metal accumulating on the doctor blades 46 is removed by water jets 47 from nozzles 48 in a manifold tube 49 mounted to direct a water spray against both the cathode surfaces and the scraper blades. The manifold tube 49 is structurally supported by a pipe (not shown) which supplies it with water. The metal powder is discharged by the doctor blades 46 into a surge chamber 50 and is flushed out of the surge chamber through an outlet 51 for collection.

As an example of the apparatus and method claimed by the present invention, an electrolytic cell of the general configuration illustrated by the drawing is constructed and operated as follows: Two 36-inch diameter titanium discs, 0.180 inch thick, are mounted on a section of a round, copper shaft, having a diameter of 11/4 inches. The cathode discs are spaced as illustrated by hollow titanium cylinders with outside diameter of 2 inches to hold them in interleaved relation with three platinized titanium anodes. The anodes are constructed of 0.125-inch titanium sheet having a 50 microinch layer of platinum on both sides and have surface areas (including both sides) of about 5 square feet. The anodes are connected to the power source through copper lugs. Power is supplied by a selenium rectifier having a 2,000 ampere capacity. The cell tank is of stainless steel and holds about 35 gallons of electrolyte at a depth such that the anodes are completely submerged whereas the cathodes are immersed about 12 inches, measured upwardly from the circumference along the vertical radius.

Current is applied sufficient to provide a current density of about 250 amperes per square foot at the anodes and about 330 amperes per square foot at the cathodes. The electrolyte is a sulfuric acid leach solution of copper and its temperature in the tank is maintained at about 140.degree. F. The flow rate of electrolyte through the cell is controlled to maintain a copper ion level of between about 1.2 and 1.5 percent by weight, varying between about 0.2 and about 0.25 gallons per minute. The sulfuric acid level in the electrolyte is maintained at between about 16 and about 18 percent by weight so that the electrical potential drop across the cell is maintained below about 4.8 volts. The speed of rotation of the cathodes is adjusted to submerge each point on the circumference of each cathode for a period of between about 2 to about 4 minutes, during each rotation, i.e., to provide a plating time of about 2 to about 4 minutes. Spent electrolyte is circulated through a tank wherein it is regenerated by the addition of appropriate quantities of acid and by contact with copper precipitate, i.e., finely divided impure copper powder recovered from copper-bearing mine water by precipitation on iron. The regenerated electrolyte is recycled to the electrolytic cell tank.

Although the present invention has been described with reference to details of certain specific embodiments it is not intended thereby to limit the scope of the claims except insofar as the details are recited therein. Many modifications within the legitimate scope of the invention will be suggested to those skilled in the art by the present disclosure.

Claims

We claim:

1. A method for electrowinning copper values and recovering them in powder form, comprising

introducing an electrolyte solution containing copper values into an electrolytic cell, said cell having mounted therein at least one rotating, disc-shaped titanium cathode and at least one insoluble anode;

applying a current across the electrodes while maintaining a current density above about 200 amperes per square foot of anode area, and a sufficiently high electrolyte concentration to maintain an electrical potential drop of about 4.8 volts across the cell;

rotating the cathode in said electrolyte during the deposition of copper powder thereon; and

recovering the copper powder from the cathode surface.

2. A method according to claim 1, wherein the electrolyte in the cell tank is a sulfuric acid solution of copper containing between about 1.2 and about 1.5 percent by weight copper.

3. A method according to claim 2, wherein the temperature of the electrolyte is maintained between about 130.degree. and about 150.degree. F.

4. A method according to claim 3, wherein the electrolyte contains between about 16 and 18 percent by weight sulfuric acid.

5. A method according to claim 1, wherein the electrolyte is circulated from the electrolytic cell, through a leaching step wherein additional metal values are dissolved, and back to the electrolytic cell.

6. A method according to claim 5, wherein the electrolyte in the cell tank is a sulfuric acid solution controlled to contain between about 1.2 and about 1.5 percent by weight copper and between about 16 and about 18 percent by weight sulfuric acid and is held between about 130.degree. to about 150.degree. F.

7. The method of claim 6, wherein the temperature of the electrolyte in the cell tank is maintained at about 140.degree. F.

8. The method of claim 6, wherein the rate of rotation of the cathodes is controlled to provide a plating time of about 2 to about 4 minutes.