US2058259A - Electrodeposition of metal - Google Patents

Description

Patented Oct. 20, 1936 UNITED STATES PATENT OFFICE.

2,058,259 v v ELECTRODEPOSITION 0F METAL No Drawing. Application January 29, 1934, Serial No. 708,912

12 Claims.

This invention relates to the electrodeposition of zinc upon cathodes or starting sheets and has for an object the provision of certain improvements in methods for such electrolytic deposition. More particularly, the invention contemplates the provision of a method for producing uniform deposits of electrolytic zinc. upon zinc starting sheets or zinc cathodes.

Heretofore, in processes for the production of electrolytic zinc, aluminum cathodes have usually been employed. The use of aluminum cathodes or starting sheets involves considerable expense and causes operating difficulties under certain conditions. Aluminum cathodes have a high first cost due to the fact that specially refined metal must be employed in their manufacture. Ordinary commercial aluminum is not satisfactory for cathode use due to its higher tendency to corrosion in the presence of electrolytes, and even high grade aluminum is corroded by the electrolytes commonly employed, especially above the solution line. Hence aluminum cathodes have comparatively short lives. Still greater difiiculties in the use of aluminum cathodes result from the fact that the deposited zinc must be stripped from the cathodes at frequent intervals, entailing a high expenditure for labor, and requiring frequent polishing of thecathodes in order to prevent the formation of a bond between zinc and aluminum which is difficult to break.

In the electrolytic recovery of metals from solution, the more economicalsystem is to employ a cathode of the same metal that is being deposited. Such practice has been followed for many.

years in the electrolytic deposition of copper, where thin starting sheets are first formed on greased copper blanks. After stripping, the starting sheets are hung in the so-called commercial sections of the tank-house andadditional copper is deposited upon them until a relatively thick sheet is formed. As compared to processes where the entire metal production must be stripped from blanks or cathodes, this system results in savings in materials and labor.

Recently, efforts have been made to apply this practice to the electrolytic recovery of zinc. Thin zinc starting sheets are formed upon aluminum blanks of the old type. The starting sheets are stripped, and hung in other electrolytic cells where they accumulate heavy deposits of zinc. :I'hey are then withdrawn and sent to the meltingfurnaces. In this way it is sought to achieve the benefit which accrues to the use of the same practice in copper metallur y. namely, lower stripping costs per unit of metal produced, at

the same time, lower costs for aluminum cathodes. In practice, an additional benefit accrues in that the current efficiency when using zinc starting sheets is considerably higher than when aluminum cathodes are employed The superi- 5 ority of this process in the metallurgy of zinc is great enough to enable it to supersede the older practice of total deposition upon aluminum cathodes, provided that certain problems arising out of the character of zinc starting sheets are solved. It is to the solution of these problems that the present invention is directed.

The problems referred to, while also arising to some extent in the use of other cathodes to receive deposits of electrolytic zinc, are greatly augmented when zinc starting sheets are used and particularly when zinc starting sheets formed on aluminum blanks are used. In the formation of such sheets by electrolytic deposition upon aluminum blanks, some contamination of the 0 sheets with aluminum and impurities contained in the blanks almost invariably occurs. The contamination is greater on the face of the starting sheet in contact with the aluminum, but, since the sheet is relatively thin, such contamination usually pervades the whole sheet to some degree. A second source of contamination of the starting sheets is the stripping operation. A certain amount of handling is unavoidable, and, as a result, the sheets become contaminated through contact with gloves, tools and surfaces upon which they are laid. Furthermore, in the stripping operation, the starting sheets are inevitably flexed. Flexure induces strains in certain portions of the sheets and not in others with the 35 result that local differences in potential between strained and unstrained portions may arise when the starting sheet is subjected to the flow of electric current in the second stage of zinc deposition. Lastly, the thin character of the starting sheet, together with the inevitable sharp edges which are formed when the sheet is stripped from the aluminum blank gives rise to a variationin current density, especially at the edges of the sheet.

When the starting sheets are placed in the electrolytic cell to form cathodes for receiving additional deposits of zinc the above-described factors combine to produce exceedingly irregular deposition of zinc upon the' surfaces of the cathodes. Re-solution of zinc proceeds at some places while 50 in other zones an excessive development of excrescences or trees may occur. If deposition is allowed to proceed under these conditions, the re-solution of metal, especially at the solution line, may cause the sheets to rupture and fall to 55 the bottom of the tank, or the sprouting excrescences may close the gap between anode and cathode. In either case a short circuit is caused". It has been found that this irregularity of deposition is aggravated by the presence of impurities ,upon the starting sheets, by strains re-- sulting from fiexure and by the sharp points and edges on the sheets. The development of excrescences is most pronounced upon the edges of the sheets due to the high cin'rent density at these points. It is probable that when the sheets are first immersed in the electrolyte some etching occurs before deposition begins so that the points at which current density is increased are more pronounced.

As a result of our investigations, we have devised a method for overcoming the effects of contamination and flexing of electrolytically deposited zinc starting sheets and efiectively preventing rupturing of the sheets at the solution line. We have also devised a method for effectively preventing short-circuiting due to the growth of trees or excrescences. In order to prevent rupturing of the sheets due to re-solution at the solution line, we change the relative positions of the solution leveland the cathode or starting sheet after commencement of the electrolytic operation, and after a film of zinc of suit* .able thickness has been deposited, in such a manner as to present a thickerand more corrosionresistant section of electrodeposited zinc at the solution level. In order to prevent or inhibit the growth of trees or excrescences and thereby pre- .vent short-circuiting, we coat the edges of the portion of the starting sheet or cathode to be submerged with a thin layer of a substance which is resistant to the corrosive action of the electrolyte but permeable by zinc ions. When shortcircuiting due to the growth of trees or excrescences is not likely to occur, coating of the edge portions of the starting sheets or cathodes may be dispensed with, and deposition may be carried out effectively merely by altering the relative positions of the solution level and the starting sheet or cathode to maintain a section of suitable thickness and quality at the solution line. By coating the edges of the portion of the cathode to be submerged with a thin layer of a substance which is resistant to the corrosive action of the electrolyte but permeable by zinc ions, and by altering the relative positions of the solution level and the cathode after the commencement of deposition, it is possible to deposit a substantially uniform and heavy layer of electrolytic zinc upon the starting sheet or cathode even when relatively impure electrolytes and an electrolytically deposited starting sheet having rough edges are employed.

According to a preferred complete process of our invention, the side and bottom edges of the starting sheet to be employed are coated with a thin layer of rubber cement or some similar material which forms a film capable of withstanding attack by the electrolyte but pervious to migrating zinc ions. Rubber, bakelite varnish and other similar plastic derivatives possess the desired properties. When the coating is dry or when suitable films have been formed where desired, the sheets, attached to suitable carrying bars, are hung in the electrolytic cells, and electrolytic deposition is begun. At the end of a suitable period of time, say twenty-four hours, the solution level with respect to the sheets is lowered a suitable distance to present surfaces of freshly deposited zinc at and a short distance above the solution level. This may be accomplished by installing a temporary dam in the outlet of the cell at the beginning of the operation and subsequently removing it in whole or in part.' The same result may be obtained by raising the oathodes, as, for example by raising the bus-bars or by placing copper shims between the bus-bars and the starting sheet carrying bars at the points where they rest on the bus-bars. The former method of varying the height of solution with respect to the cathode is preferable, provided that the height of the cell is sumcient to permit the damming back of solution, because less labor is involved. In any, case, a thickened portion of the cathode or starting sheet is presented at the new solution line, sufiicient to resist the combined attack of the atmosphere and acid spray for a considerable period. In practice, while operating under a current density of thirty amperes per square foot, it has been found that the crosssectional area of the cathode at the new solution line at the end of about twenty-four hours is three times as large as it was originally, and ample to support the weight. of zinc precipitated a following three to five day period of deposi- During the three to five day period of deposition following the lowering of the solution level, deposition proceeds in a regular manner. Short circuits due to falling of cathodes or the growth of excrescences are practically never encoun tered. The completed cathode is uniform in thickness. The presence of the permeable film at the edges of the sheet prevents the growth of tree-like structures and allows the zinc to build up in a relatively smooth deposit only slightly thicker than that formedon the surface. The effect of the permeable film is probably to reduce the current density at the points where it is applied by covering the sharp angles and points from which the growth of excrescences begins.

No pitting of importance has been observed at and strained during the course of its production and preparation for use, and it is, therefore, readily attacked by the acid electrolyte and the acid spray and atmosphere at the solution line. During the period of deposition prior to alteration of the relative positions of the starting sheet and the solution level, the submerged portion of the starting sheet becomes coated with a film of, pure, uncontaminated and unstrained zinc which is more resistant to corrosive action than the starting sheet. Therefore, when the relative positions of the solution level and the starting sheet are altered, the starting sheet is provided at and a short distance above the solution level with a protective coating which is more resistant than the starting sheet to the combined action of the acid spray and atmosphere.

If desired or warranted by the conditions of operation the solution level may be lowered more than once, each time exposing a new and thicker cross-section of deposited zinc-to the action of the electrolyte. The relative positions of the cathode and the solution level may be changed any suitable amount and any suitable number of times during the course of an operation. We have obtained satisfactory results by making only one change of about a half inch or less during a complete operation and, also, by making changes of about one-eighth inch or slightly more every day during the course of an operation.

In treating corrosive solutions, we have obtained highly satisfactory results by providing the starting sheets with protective coatings of non-metallic materials or organic plastic materials such, for example, as rubber at and adjacent to the initial solution line, in accordance with the teachings of the United States patent to Ralston and Fowler No. 1,925,339, prior to the commencement of the electrolytic operation. According to this modified procedure, we apply a thin coating of acid resistant material, such as rubber, which is permeable by zinc ions to the starting sheet at and adjacent to the initial solution line and preferably extending 'a short distance above and a short distance below the solution line. The thus prepared starting sheet is placed in an electrolytic cell and deposition of zinc thereon is commenced. When a film of metallic zinc of suitable thickness has been deposited on the submerged portion of the starting sheet, the relative positions of the solution level and the starting sheet may be altered to present a film .or coating of the freshly deposited zinc at and a short distance above the solution line. The

relative positions of the solution level and the' starting sheet or cathode may be altered one or more times, and the edges of the starting sheet may be coated with protective material or not, as desired or required. The film or coating of protective material applied to the starting sheet prior to the commencement of the electrolytic operation effectively inhibits corrosion of the starting sheet'at the solution line during the period required for the deposition of a film of passive zinc capable of effectively resisting corrosion when the relative positions of the solution level and starting sheet are altered subsequently. The thickness of the initial protective film and the nature of the material employed need only be such that the film is capable of effectively inhibiting corrosion until a suitable film or passive zinc has been formed by deposition.

We claim:-

1. A method for promoting uniformity in the electrolytic deposition of zinc from solution upon a zinc cathode which comprises coating the edges of the portion of the cathode to be submerged prior to electrolytic deposition with a film of material which is resistant to attack by the solution but permeable by metallic ions, thereby to inhibit the growth of excrescences during electrolysis, and altering the relative positions of the solution level and the cathode one or more times during electrolytic deposition to cause deposition on the cathode at the solution level of a layer of zinc more passive than that previously exposed to the electrolyte, the thickness of the film of solution-resistant material being such as to permit the passage of zinc ions therethrough without permitting the growth of objectionable tree-like structures on the edges of the cathode during the electrolytic deposition.

2. In a method for the electrolytic deposition of zinc upon an electrolytically deposited zinc starting sheet suspended in an electrolyte, the

' improvement which comprises progressively lowtrolytic deposition to cause deposition on the starting sheet at the solution level of a layer of zinc more passive than that previously exposed to the electrolyte.

. 3. The method of recovering metallic zinc from a solution of a zinc salt which comprises forming a sheet of zinc metal by electrolytic deposition on an aluminum cathode, stripping the sheet from the aluminum cathode, partially immersing the sheet in the solution in an electrolytic cell, passing electric current through the electrolytic cell to cause zinc metal to deposit on the sheet, and altering the relative positions of the sheet and the solution level one or more times after commencement of the deposition to cause deposition on the cathode at the solution level of a layer of zinc more passive than that previously exposed to the electrolyte.

4. The method of recovering metallic zinc from a solution of a zinc salt which comprises forming a sheet of zinc metal, partially immersing the sheet in the solution in an electrolytic cell, passing electric current through the'electrolytic cell to cause zinc metal to deposit on the sheet, andaltering the relative positions of the sheet and the solution level two or more times after commencement of the deposition to cause deposition on the cathode at the solution level of a layer of zinc more passive than that pre-' sheet, and lowering the solution level one or more times after commencement of the deposition to cause deposition on the cathode at the solution level of a layer of zinc more passive than that previously exposed to the electrolyte.

6. The method of recovering metallic zinc from a solution of a zinc salt which comprises forming a sheet of zinc metal, partially immersing the sheet in the solution in an electrolytic cell, passing electric current through the electrolytic cell to cause zinc metal to deposit on the sheet, and raising the sheet relatively to the solution level one or more times after commencement of the deposition to cause deposition on the cathode at the solution level of a layer of zinc more passive than that previously exposed to the electrolyte.

'7. The method of recovering metallic zinc from a solution of a. zinc salt which comprises forming a sheet of zinc metal by electrolytic deposition on a blank of dissimilar metal, stripping the zinc sheet from the blank of dissimilar metal, partially immersing the zinc sheet in the solution in an electrolytic cell, through the electrolytic cell to cause zinc metal to deposit on the sheet, and altering the relative positions of the sheet and the solution level one or more times after commencement of the deposition to present one or more sections of the sheetcoated with freshly deposited metallic zinc at the solution level.

8. The method of recovering metallic zinc from a solution of a. zinc salt which comprises forming a sheet of zinc metal by electrolytic deposition on a blank of dissimilar metal, stripping the zinc sheet from the blank of dissimilar metal, partially immersing the zinc sheet in the solution in an electrolytic cell, passing electric current through the electrolytic cell to cause zinc metal to deposit on the sheet, and altering the relative passing electric current positions of the sheet and the solution level one or more times after commencement of the deposition to cause deposition on the cathode at the solution level of a layer of zinc more passive than that previously exposed to the electrolyte, deposition of zinc on the sheet being carried out for a period of about three to five days.

9. A method of electrolytically depositing zinc upon a zinc cathode, which comprises coating the edges of the cathode with a film of non-metallic material which is resistant to attack by the electrolyte employed but permeable by zinc ions, the thickness of the film being such as to permit the passage of zinc ions therethrough without permitting the growth of objectionable treeiike structures on the edges of the cathode during the electrolytic deposition, immersing said cathode in the electrolyte in an electrolytic cell and passing electric current through the electrolytic cell to cause deposition of a permanent coating of zinc on said cathode, including the coated edges thereof.

10. A method of electrolytically depositing zinc upon a zinc cathode, which comprises coating the edges of the cathode with a thin film of rubber, the thickness of the film being such as to permit the passage of zinc ions therethrough without permitting the growth of objectionable tree-like structures on the edges of the cathode during the electrolytic deposition, immersing said cathode in the electrolyte in an electrolytic cell,

and passing electric current through the electrolytic cell, to cause deposition of a permanent coating of zinc on said cathode, including the coated edges thereof.

11. A method of electrolytically depositing zinc upon a zinc starting sheet, which comprises coating the edges of said starting sheet with a. thin film containing a phenol derivative, the thickness of the film being such as to permit the passage of .zinc ions therethrough without permitting the electrolytic cell, passing electric current through the electrolytic cell to cause zinc to deposit on said sheet, and altering the relative positions of the sheet and the solution level one or more times after the commencement of the deposition to cause deposition on said starting sheet at the solution level of a layer of zinc more passive than that previously exposed to the electrolyte.

OLIVER C. RALSTON. WILLIAM JOHN UREN.