US3394060A

US3394060A - Process for electrolytically regenerating ferric chloride etching solutions

Info

Publication number: US3394060A
Application number: US434030A
Authority: US
Inventors: Ellwood S Douglas
Original assignee: Individual
Current assignee: Individual
Priority date: 1965-02-19
Filing date: 1965-02-19
Publication date: 1968-07-23
Anticipated expiration: 1985-07-23

Description

y 23, 1963 E. s, DOUGLAS 3,394,060

PROCESS FOR ELECTROLYTICALLIY REGENERATING FERRIC CHLORIDE ETCHING SOLUTIONS Filed Feb. 19, 1965 I /4a ..-I

I I I 2* I I I P 3 I i I/4 5 Regenerative I c I Solvent I Heat Extraction Exchanger Used ReservaIr c m tor Etching I Solution ,7 I 32 I 2/ I Copper Meta a, 15 I q 2 5 I 24 I Q IntermedIate I EIQCIIOIYIIC "L Concentrated I Cell c 1 SoIuIion I I 5 Reservoir l k L I r26 I I Electric Power I A II I I II I 25 7) I Organic EIChIIIQ s t I q p Extraction I L Contactor 27- I I l 5 28 L I Acid Vapor I l Condenser I I r" I l I 2.9 Ether I 4/ I Condenser :9 II 35 I I I I O G 'I Boner 3 8|ReservoIr 37 I Reqenerated I 8 I I 9 L l Etching I I I- 33 I i300 r Solution I 38- Hem f" I I Reservoir I I I Molten J I L I -J+\--"-' FeCIa 37 4Hem I 20 .4 y Contactor eCl I l r 4 lOb I00 Ellwood S. Douglas INVENTOR.

United States Patent 3,394,060 PROCESS FOR ELECTROLYTICALLY REGEN- ERATING FERRIC CHLORIDE ETCHING SOLUTIONS Ellwood S. Douglas, 1429 Oregon Blvd, Berkeley, Calif. 947% Filed Feb. 19, 1965, Ser. No. 434,030 6 Claims. (Cl. 204--94) This invention relates generally to etching solutions and more particularly to a process for regenerating spent ferric chloride solutions utilized when etching metal such as copper, nickel, etc.

In the manufacture of printed circuits, for example, copper is chemically removed by dissolving it in an aqueous solution of an oxidizing agent. One of the more desirable etching solutions used to etch copper is an aqueous solution of ferric chloride. The ferric chloride solution is generally supplied and used as an aqueous solution at a concentration of about 3.4 molar or about a density of 42 Baum. The ferric chloride is converted to ferrous chloride during the etching reaction as can be expressed by the equation:

As the concentration of chlorides of the etched metal in the solution increases, and the concentration of ferric chloride decreases, the time required to etch a given amount of metal greatly increases and, it becomes uneconomical to continue using the etching solution.

The relatively expensive ferric chloride is generally purchased as a 42 *Baum solution, although a comparable solution may be made up by utilizing ferric chloride crystals. This etching solution is generally used until the time required to etch a given amount of metal has doubled over that of fresh solution. The solution at that point would contain about FeCl FeCl 1.9M CuCl 1.0M

It may therefore be seen that where large quantities of spent etching solution are involved, it is not economical to discard the spent solution due to its inherent corrosiveness and its content of valuable metallic salts. Accordingly, attempts have been made to economically recover the metallic salts and regenerate the etching solution to a substantially fresh condition sothat it may be reused in the etching process, however, the prior attempts to accomplish this economically have not been entirely successful.

It is therefore an object of this invention to set forth an improved process for regenerating spent ferric chloride etching solution.

It is another object of this invention to set forth a process for economically recovering the valuable etched metal from a spent ferric chloride etching solution.

It is still another object of this invention to set forth an improved process for regenerating a spent ferric chloride etching solution by a novel organic solvent extractionconcentration-electrolytic process.

A further object of this invention is to set forth a process for continuously regenerating a solution used for the chemical etching of metals.

3,394,060 Patented July 23, 1968 iCe For purposes of illustration, the instant process will be (a) ZFeCl +Cu=CuCl ZFeCl The process for the regeneration of ferric chloride and the recovery of copper from the spent etching solution may be broadly described as a solvent extraction-electrolytic process. The ferric chloride remaining in the spent etching solution is removed by solvent extraction leaving FeCl and CuCl in solution. This solution is electrolyzed in a cell with a polarized cathode. The half reactions are at the cathode:

At low cathode currents only half reaction (c) occurs. When the current is increased beyond a value corresponding to reduction of all Fe+++ ions which diffuse to the cathode, voltage increases, and half reaction (d) occurs. Similarly, when current exceeds that given by reduction of all Fe+++ and all Cu++ ions which diffuse to the cathode, half reaction (e) begins. Likewise, when the current exceeds the value corresponding to the reduction of all Fe+++ to Fe++, and all Cu++ to Cu, voltage again increases causing half reaction (f). Half reaction (0) is undesirable; it is the reverse of half reaction (g) which occurs at the anode. Current which flows due to half reaction (c) represents a waste of power. One of the reasons for solvent extraction of FeCl is to maintain the Fe+++ concentration appreciably lower than the Cu++ concentration. Most of the current then participates in the useful half reactions (d) and (e), and very little is wasted in half reaction (c). The current due to reaction (c) is limited by the diifusion of Fe+++ to the cathode surface and diffusion is approximately proportional to concentration. Evolution of H by half reaction (h) is also undesirable. The evolution of H in this manner wastes power and may be hazardous. Under controlled conditions, however, the evolution of H produced by half reaction (h) may be monitored for the automatic control of the current density to assure completion of half reaction (e).

It is desirable that all Cu++ ions diffusing to the cathode and reduced to Cu+ are subsequently reduced to Cu. If Cu" ions escape from the vicinity of the cathode, they may encounter the anode and be oxidized back to Cu++ by half reaction (h). This again represents a waste of power. The desired reaction at the anode is (g). Alternate or simultaneous solvent extraction is used to remove Fe+++ as it is produced by half reaction (g). This minimizes the wasteful half reaction (c) as explained earlier.

The solvent extraction of the feric chloride is carried out at low pH. The distribution of Fe+++ between equal volumes of water and organic solvent such as ethyl ether or ethyl acetate, for example, in equilibrium contact is a function of R concentration in the water solution. At 0.1 molar H most of the Fe is in the water solution: at 3 molar H about half is in each phase. At 6 molar H 99% of the Pe+++ is in the organic solvent. It is therefor necessary to maintain high acidity in order to remove Fe effectively by organic solvent extraction, so that the electrolytic reduction of Cu+ to Cu can be efiiciently accomplished. If the organic solvent used is one which boils at a lower temperature than water, it may be boiled by contact with the hot water, and the solvent vapor condensed for reuse. The ferric chloride extracted is accordingly left in the hot water. This mixture of ferric chloride and hydrochloric acid in water gives up its hydrochloric acid upon boiling. Upon additional boiling, fenric chloride hydrolyzes to Fe(OH) giving up still more HCl as vapor. If the amount of heat applied is controlled by monitoring the pH to leave a slight excess of acid, and water added to give a density of 42 Baum (39% FeCl the resulting solution will be identical to fresh ferric chloride etching solution.

The HCl vapor given off is returned to the solution being reclaimed Where it is needed to augment the extraction of more Fe+++.

In order to keep the electrolytic cell to a reasonable size, and assure economical operation, it is necessary to electrolyze solutions of high FeC1 -CuCl -HCl concentration. To prevent dilution, as little Water as possible should be in the input or some means of removing water must be provided. The requirement of high H+ concentration for effective solvent extraction of the Fe+++ also demands control of dilution. Dehydration of the input results in hydrolysis and the loss of some HCl. A solid input of spent solution would therefore have to be gathered after dehydration and handled as a solid, which is more difficult than handling a liquid. These reasons make it more desirable to remove water from the electrolyte than to dehydrate the input. This can be done by boiling. It should be done at the point where the system has the lowest acidity, since HCl will also boil out. Water and HCl form a maximum boiling azeotr-ope of HCl, 80% Water. This is approximately 6 molar HCl solution. Solutions Weaker in HCl boil off water and approach the azeotropic concentration. Some HCl vapor escapes under these conditions: Boiling in a fractionating still with a reflux of pure water will allow concentration from 3 molar to nearly 6 molar with an output of steam at the top which contains little HCl.

The figure is a flow sheet of the instant process for regenerating spent ferric chloride etching solution, and recovering the etched metal contained therein.

The system shown schematically in the figure utilizes ethyl ether to extract ferric chloride from the aqueous solution, however, it is to be understood that many oxygen containing organic compounds could be utilized in place of ethyl ether in a system of this nature. Alternative solvents would include such compounds as ethyl acetate, isopropyl ether, n-butyl acetate, higher alcohols, aldehydes, ketones, etc., for example. For a better understanding of the instant process illustrative flow rates and flow compositions are given although it is to be understood that they do not necessarily represent optimum values.

Spent ferric chloride etching solution is withdrawn from the etching equipment through line 13 and is accumulated in a spent etching solution reservoir 1. The composition of the spent solution is assumed to be as follows:

The spent solution is withdrawn from the reservoir 1 by means of flow 14, and mixed with a return flow 22, which is from a succeeding stage of the system, as will be described later, and enters a primary solvent extraction contactor 2. Approximate flow rates elsewhere in the system are based upon a flow of approximately one gallon per unit time at 14.

The composition of flow 22 is about:

FeCl 0.1M FeCl 2.0M CuCl 1.0M HCl 5.0M

and is about three gallons.

Therefore about four gallons of aqueous solution of the composition:

FeCl 0375M FeCl 2.0M cuci 1.0M HCl 3.75M

F6C13 FeCl 2.0M cucl 1.0M HCl 3.4M

The aqueous flow 15 enters a regenerative heat exchanger 3 at about 25 C. and emerges as How 16 at about C. A vapor trap 16a is placed in flow 16 to prevent the loss of ether vapor due to ether dissolved on entrained in flow 15. The trapped vapor is fed through flow 16b to an ether condenser 9, the complete function of which will be described later.

The fiow 16 is fed into a fractionating column 4. The fractionating column receives one gallon of distilled water reflux at the top as represented by flow 19. The column exhausts steam as flow 20. The steam is equivalent to two gallons of water, giving a net removal of one gallon of water at the top. Flow 20 contains less than 0.1M HCl. A part of flow 14 as indicated by flow 14a may be used in place of water at flow 19 to absorb HCl gas from the steam in the fractionating column. Heat input flow 18 operates the fractionating column and represents the major use of fuel in the system. Flow 20, as indicated by the flow shown as a broken line 20a, may be used to provide the heat, and water flow required by an ether boiler 8, the complete function of which will be described later.

The flow 17 leaving the fractionating column 4 is about three gallons of an aqueous solution of the following composition:

FeCl 0.15M F6012 2.66M cuci 1.33M HCI 5.0M

The flow 17 enters the regenerative heat exchanger 3 at about C. and leaves the heat exchanger 3 as flow 21 at about 30 C. The flow 21 is fed to an intermediate solution regeneration reservoir 5 whose contents have a composition the same as flow 22 previously described. A portion of the solution reservoir 5 is recycled through flow 22 to the primary extractor 2 as previously described.

Flow 23, of about thirty gallons, removes solution from the intermediate reservoir 5 and passes it through an electrolytic cell 6 where (u is reduced to copper and deposited out at the cathode and Fe++ is oxidized to 5 Fe+++ at the anode. The electrolytic cell 6 is of conventional design and includes cathodes of suitable mate rial upon which copper may be accumulated and removed from the cell in the form of a foil, for example. As stated previously, the current input to the cell is such that the desirable half reactions:

at the cathode are:

(e) Cu++e-=Cu at the anode:

The high flow rate through the electrolytic cell 6 pro vides sufiicient agitation for the electrolyte.

A secondary organic solvent extraction is used simultaneously to remove Fe+++ as it is produced by the half reaction (-g) in the electrolytic cell 6. This minimizes the wasteful half reaction (c) Fe++++e- =Fe++ as explained earlier.

Accordingly, a portion of the electrolyte from cell 6 is returned to the intermediate reservoir 5 by flow 24 and a portion of the electrolyte is fed by flow 25 to the secondary organic solvent extraction contactor 7 Where Fe+++ is removed by ether. The ether for extractors 2 and 7 is supplied by an ether condenser and reservoir 9. Additional organic solvent may be added to the reservoir 9 to replace solvent lost. The aqueous phase from the extractor 7 is returned by flow 26 to the intermediate reservoir 5 and the ether laden with FeCl HCl-H O from the extractor 7 is fed by flow 27 to the ether boiler 8. The ether boiler 8, also receives the ether phase, through flow 28, from the primary organic solvent extractor 2. The ether laden wtih FeCl -HCl-H o returns by

flows

27 and 28 to the ether boiler 8-, where the ether is evaporated. The ether vapor passes by flow '30 back to the ether condenser 9 where it is condensed for reuse in the extractors 2 and 7.

Water flow 29 and heat input 33 are added to ether boiler 8 to evaporate the ether. As shown by the flow 20a (in broken lines) the water and heat required by the ether boiler 8 may be derived from the output of the fractionating column 4. The heat flow 33 is needed to evaporate the ether, and the water flow 29 to prevent the residue Fec-l -HCl-nH O from precipitating solid FeCl The aqueous solution represented by flow 34 contains FeCl and HCl. Flow 34 is fed to a boiler 10 which is heated by a heat input 39 which evaporates H and HCl so that flow 37 has a higher FeCl concentration than desired in the final regenerated ferric chloride solution. The HCl and H 0 vapor flow 35 from the boiler 10 is liquified in condenser 12 and returned by flow 36 to the intermediate reservoir 5.

Flow 37 is fed to a regenerated etching solution reservoir 11 where distilled water is added to adjust the re generated solution to 42 Baum (3.4M). The regenerated ferric chloride solution is then Withdrawn by flow 38, as required, to the etching equipment.

The ether may also be evaporated by contacting the

flows

27 and 28 which are ether laden with FeCl HCl -H O with molten FeC1 withdrawing the FeCl enriched molten FeCl and subsequently diluting the FeCl with water to substantially the same concentration as fresh etching solution. When utilizing molten FeCl to evaporate the ether and HCl the ether boiler 8 and boiler 10 would be eliminated and a molten FeC1 contactor 10a would be substituted in their place. Molten FeCl at about l50-175 C. would be fed into the contactor 10a by flow 10b. The

flows

27 and 28 would be fed to contactor 10a by the broken line flow 27a. The ether and acid vapors are fed to the

condensers

9 and 12 by the broken line flows 30a and 35a. The enriched molten FeCl is fed from the contactor 10a to the regenerated etching solution reservoir 11 by the broken line flow 37a where distilled water is added to adjust the regenerated solution to 42 Baurn (3.4M).

It may, therefore, be seen that a highly efficient process has been provided for the economical regeneration of the etching solution and recovery of the valuable etched metallic salts contained therein, thereby great-1y reducing the cost of the etching solution required. The instant solvent, extraction-concentration-electrolytic process provides an economy of operation not possible heretofore.

From the foregoing illustration and specific example it will be apparent that I have provided a new and improved process for regenerating spent ferric chloride etching solution. Accordingly I believe that my invention should not be necessarily limited to the specific example set forth, but rather should be determined by the claims appended below.

What is claimed as new is as follows:

1. In the process of etching a metal with an aqueous ferric chloride etching solution in which process a portion of the ferric chloride is reduce-d to ferrous chloride and the metal being etched is oxidized to a metallic chloride, the improved process of recovering the metal etched and regenerating the spent etching solution to a substantially fresh condition, comprising the steps of acidifying the spent solution, making a first extraction of residual fenric chloride with the aid of an organic solvent selected from the group consisting of etyhl ether, ethyl acetate, isopropyl ether, n-:butyl acetate, higher alcohols, aldehydes and ketones, concentrating the remaining aqueous solution laden with ferrous chloride, metallic chloride of the metal being etched and acid; electrolytically oxidizing the ferrous chloride to ferric chloride and reducing the metallic chloride of the metal being etched to elemental metal while simultaneously making a second extraction of ferric chlo ride so .produced, evaporating the organic solvent from the organic solvent phase of said first and second extraction steps, boiling off excess acid thereby producing a regenerated etching solution.

2. The process of claim 1 wherein the organic solvent and acid vapors are liquified for reuse in the process.

3. In the process of etching a metal with a ferric chloride etching solution in which process a portion of the ferric chloride is reduced to ferrous chloride, the improvement comprising the steps of regenerating the spent etching solution by first extracting substantially all of the residual ferric chloride from said spent etching solution with an organic solvent selected from the group consisting of ethyl ether, ethyl acetate, isopropyl ether, nbutyl acetate, higher alcohols, aldehydes and ketones in the presence of sufiicient hydrochloric acid to assure substantially complete extraction of the residual ferric chloride; partially dehydrating the remaining aqueous solution containing ferrous chloride and the chloride of the metal being etched; subjecting said ferrous chloride and said metal chloride to electrolysis in the presence of hydrochloric acid to oxidize the ferrous chloride to ferric chloride and reduce the ecthed metallic chloride to recover the elemental metal while simultaneously extracting ferric chloride so produced thereby regenerating the etc-hing solution.

4. In the process of electrolytically regenerating a spent aqueous ferric chloride etching solution comprising ferric chloride, ferrous chloride, and a metallic chloride of the metal etched, the improvement comprising the steps of extracting residual ferric chloride from the spent etching solution and simultaneously extracting the ferric chloride electrolytically produced with the aid of an organic solvent and separating said organic solvent to produce a ferric chloride etching solution of substantially the same composition as fresh etching solution.

5. The process of claim 4 wherein the organic solvent is boiled off by contact with hot water, thereby precluding the necessity of collecting the ferric chloride as a solid.

6. In the process of electrolytically regenerating a spent aqueous ferric chloride etching solution comprising ferric chloride, ferrous chloride, and a metallic chloride of the metal etched, the improvement comprising the steps of extracting residual ferric chloride from the spent etching solution and simultaneously extracting the ferric chloride electrolytically produced with the aid of an organic solvent, boiling off said organic solvent by contact With molten ferric chloride and diluting the residual solution with water to produce a ferric chloride etching solution of substantially the same composition as fresh etching solution.

References Cited UNITED STATES PATENTS 1,197,556 9/1916 Slater 20494 5 1,451,333 4/1923 Eustis et a1 2041l3 2,828,194 3/1958 Hopkins et a1 252-79.2 2,964,453 12/1960 Garn et al 20494 JOHN H. MACK, Primary Examiner.

10 H. M. FLOURNOY, Assistant Examiner.

Claims

1. IN THE PROCESS OF ETCHING A METAL WITH AN AQUEOUS FERRIC CHLORIDE ETCHING SOLUTION IN WHICH PROCESS A PORTION OF THE FERRIC CHLORIDE IS REDUCED TO FERROUS CHLORIDE AND THE METAL BEING ETCHED IS OXIDIZED TO A METALLIC CHLORIDE, THE IMPROVED PROCESS OF RECOVERING THE METAL ETCHED AND REGENERATING THE SPENT ETCHING SOLUTION TO A SUBSTANTIALLY FRESH CONDITION, COMPRISING THE STEPS OF ACIDIFYING THE SPENT SOLUTION, MAKING A FIRST EXTRACTION OF RESIDUAL FERRIC CHLORIDE WITH THE AID OF AN ORGANIC SOLVENT SELECTED FROM THE GROUP CONSISTING OF ETHYL ETHER, ETHYL ACETATE, ISOPROPYL ETHER, N-BUTYL ACETATE, HIGHER ALCOHOLS, ALDEHYDES AND KETONES, CONCENTRATING THE REMAINING AQUEOUS SOLUTION LADEN WITH FERROUS CHLORIDE, METALLIC CHLORIDE OF THE METAL BEING ETCHED AND ACID; ELECTROLYTICALLY OXIDIZING THE FERROUS CHLORIDE TO FERRIC CHLORIDE AND REDUCING THE METALLIC CHLORIDE OF THE METAL BEING ETCHED TO ELEMENTAL METAL WHILE SIMULTANEOUSLY MAKING A SECOND EXTRACTION OF FERRIC CHLORIDE SO PRODUCED, EVAPORATING THE ORGANIC SOLVENT FROM THE ORGANIC SOLVENT PHASE OF SAID FIRST AND SECOND EXTRACTION STEPS, BOILING OF EXCESS ACID THEREBY PRODUCING A REGENERATED ETCHING SOLUTION.