US 3615957 A
Description (OCR text may contain errors)
 Inventor Eftimios Konstantouros Munich, Germany  App]. No. 627,942  Filed Apr. 3, 1967  Patented Oct. 26, 1971  Assignee Siemens Aktiengesellschaft Munich, Germany  Priority Apr. 4, 1966  Germany  S 103016  METHOD FOR ETCHING METALLIC COPPER WITH CHROMOSULFURIC ACID AND REGENERATING THE ETCI-IIN G SOLUTIONS AS WELL AS RECOVERING THE CORRODED COPPER 9 Claims, 1 Drawing Fig.
 US. Cl 156/19, 156/18, 204/97, 204/106, 204/141  Int. Cl ..C0lg 37/02, C22d 1/16, C23f1/0O  Field of Search 156/18, 19; 204/97,106,140,141
 References Cited UNITED STATES PATENTS 665,783 1/1901 Hess 204/97 Primary Examiner-Allen B. Curtis Att0rneyHill, Sherman, Meroni, Gross & Simpson ABSTRACT: Process for the etching of metallic copper with chromosulfuric etching solution, in which the etching and regenerating of the solution are simultaneously effected but spatially separated from each other, the etching solution being circulated between the etching vessel and the anode space of an electrolytic cell provided with a diaphragm, in the anode space of which chromium-III-ions produced in the etching operation are oxidized anodically and half of the sulfuric acid consumed in the etching is reformed, and when copper-Il-ions in the circulated etching solution reaches a specified concentration, crystallizing out the copper sulfate and utilizing a solution thereof in the cathode space of the electroyltic cell to separate out metallic copper at the cathode and reproduce the second half of the sulfuric acid consumed in the etching process.
METHOD FOR ETCHING METALLIC COPPER WITH CHROMOSULFURIC AClD AN!) REGENERATING THE ETQHKNG SQLUTKONS AS WELL AS RECOVERING THE CORRODED COPPER in recent years chromosulfuric acids are often used in the production of printed circuits. As is well known, insulating plates with holes and provided with electrical conductors are inserted into the electric structural elements which, for example, are electrically connected with the conductors by dip soldering. The production of such printed circuits is, for example, so carried out that, starting from an insulating plate which is coated with metal, usually copper, the copper layer is coated with an etch-resistant layer, consisting, for example, of a photo lacquer, that the pattern of the desired conductors is covered by the lacquer (positive method) and subsequently the plate is immersed in an etching bath, e.g., an etching solution of copper chloride. Another known method, which is particularly advantageous since it leads to reinforced conductors, consists, for example, in so coating the insulating plate, which is covered by the copper layer, with a covering layer, e.g., lacquer, that the places to be corroded are coated with lacquer (negative method). Hence the places corresponding to the pattern of the desired conductors as are in this case free from the coating layer. These places are galvanically reinforced by immersing the entire plate in a galvanizing bath. Tin, in particular, has for several reasons favorably stood the test for galvanic reinforcement. Subsequently, the coating layer is removed and the exposed places of the copper layer are corroded by simply dipping the plate in an etching solution. If the metallic layer applied galvanically on the conductors consists of a precious metal, e.g., gold, the undesired parts of the base metal layer can be corroded by using the known copper-llchloride etching solution. However, as gold is very expensive, other methods are used whereby the conductors are reinforced with other metals, e.g., with tin. In this case, however, the copper chloride etching solution, which otherwise is very advantageous in itself, cannot be used for corroding the primary layer of copper, since tin is corroded by the etching solution of copper chloride, as well as by the known etching solution of iron chloride. For such cases etching solutions of chromosulfuric acid have proven to be particularly suitable. Against these etching solutions not only tin and, of course, gold, but a whole series of other metals, e.g., lead, silver, and possibly nickel, prove to be chemically resistent and suitable for galvanically reinforcing the conductors of printed circuits and, in comparison with gold, very inexpensive. Likewise, rohdium is not corroded by chromosulfuric acid.
The application on a large technical scale of chromosulfuric etching solution, however, has so far encountered various difficulties. chromosulfuric acid is very expensive and to date no method has been found by means of which used chromosulfuric etching solution enriched with copper can be regenerated, i.e., can be made reusable. The spent chromosulfuric etching solution, however, cannot be thrown into the sewer without further treatment, for chromosulfuric acid is one of the most dangerous pollutants in sewers. According to the laws of some countries, for the regulation of water conservation effective as of 3/ l/ 1959, the chromium concentration must be under 1 mg. per liter, care must be taken that any chromium ions still present in the spent chromosulfuric etching solution is reduced to trivalent chromium by means of a reducing agent, e.g., sodium hydrogensulfite and this is precipitated as chromic hydroxide in neutralization. For this purpose, therefore, a whole series of chemicals would have to be wasted. However, in this type of treatment of the spent chromosulfuric acid etching solution, not only the valuable chromium would go to waste, but at the same time also the corroded copper, apart from the additional expenditures required by such treatment.
Consequently, the basic problem of the invention is to develop method for the etching of metallic copper, as well as the composition of a superior chromosulfuric etching solution, by means of which the etching of metallic copper, in particular of printed circuits, is economically feasible. ln addition, a
method for the regeneration of the chromosulfuric etching solutions shall be provided whereby the ions used in the etching are reconstructed. In so doing, the copper corroded from the insulating plates shall be recovered. Such method shall be economically feasible and, beyond that, also capable of being easily practiced on a large technical scale.
According to the invention, a method is disclosed for the etching of metallic copper with a chromosulfuric acid etching solution, particularly for the production of printed circuits, whereby an etching solution is used, which is prepared from about 1.5 to 2.0 Mol chromium-Vl-oxide and 1.5 to 2.5 Mol sulfuric acid per 1 liter. Translated into grams, such etching solution would contain approximately to 200 grams chromium-VI-oxide and approximately 84 to 140 ml. concentrated sulfuric acid per 1 liter. The preferred etching solution is prepared from approximately 1.7 Mol chromium-Vl-oxidc and approximately 1.9 Mol sulfuric acid per l liter, or translated into grams, approximately g. chromium-Vl-oxide and 105 ml. concentrated sulfuric acid per liter. In order to increase the etching velocity hydrochloric acid can be added to such etching solution, preferably in a concentration of 0.05 to 0.1 Mol per liter of etching solution The etching velocity is increased by this addition by approximately 50 percent. As a result of the increased etching velocity through the addition of the hydrochloric acid the etching action at the edges is simultaneously improved.
In the etching of copper with chromosulfuric acid solution the following main reaction takes place:
2 H CrO,+3 CuH-6 H 80, Cr (SO,),+3 CuSO,+8 H,0 Thus, hexavalent chromium is reduced to trivalent chromium. At the same time hydrogen ions (sulfuric acid) are consumed. The reaction can also be formulated as follows:
Cr O,"+l4 H +3 Cu 2 Cr +3 Cu* +7 H 0 Hence, while the copper is being etched, the etching solution is reduced in Cr O-," or HCrO, ions and hydrogen ions which are capable of etching. A consequence thereof, of course, is the weakening of the etching force of the solution. At the same time the etching solution is enriched in Cu-ll-ions. Therefore, without simultaneous regeneration of the etching solution, undertaken in the course of the etching, i.e., reformation of the ions capable of etching, the etching velocity of the solution diminishes continuously until finally a point is reached where the etching velocity becomes prohibitively long. At this point, however, only about 30 percent of the ions of the etching solution capable of etching are consumed.
The invention, therefore, provides that the etching solution after a certain etching time, preferably, however, simultaneously with the etching, is electrolytically regenerated at the anode of an electrolytic cell, regeneration meaning that the ions capable of etching are reformed. According to the invention, after a certain time, preferably during the etching, the chromium-lll-ions formed during the etching are oxidized into chromium-Vl-ions at the anode of an electrolytic cell and thereby hydrogen ions are reproduced. For a complete restoration of the etching solution, however, the etched copper must be removed at least partially from the solution. Exhaustive research forming the foundation of the invention has shown that it is not possible to effect a simple reversal of the etching reaction in an electrolytic cell and, therefore, the reaction:
Cr,(SO +3 CuSO +8 H,O 2 H,Cr0,+3 Cu+6 H,SO 4 or 2 Cr *+3 Cu *+7 H 0 Cr,O "+l4 H +Cu with electrolytic precipitation of the copper at the cathode and oxidation of the chromium-lll-ions at the anode of the electrolytic cell cannot take place. As can be seen from the comparison of the normal potentials of the oxidation-reduction of the oxidation-reduction systems E, for the reactions possible at the the copper precipitation cannot take place before the reduction of the hexavalent chromium at the cathode present in the etching solution. Even practically spent chromosulfuric etching solutions, i.e. which are no longer capable of etching, still contain, naturally, a certain percentage of chromium=Vl=ins. In addition, chromium-Vl-ions are continuously formed at the anode of the electrolytic cell.
in the electrolytic treatment of a chromosulfuric etching solution enriched with copper, e.g. with copper cathode and lead anode, the trivalent chromium therefore is oxidized to hexavalent chromium at the anode, but at the cathode the reformed hexavalent chromium is reduced to trivalent chromium so that in the final result no change takes place in the composition of the bath. Therefore, due to the normal potentials, an electrolytic regeneration of a spent chromosulfuric etching solution with simultaneous precipitation of the etched copper at the cathode is without prospects. Even when the cathodic current density is strongly increased, no copper precipitation is possible at the cathode. Likewise, a diaphragm arranged between the cathode and anode space, which is supposed to make possible the precipitation of the copper after the complete reduction of the hexavalent chromium to trivalent chromium, is not successful.
Reference is also made to the text in Galvanotechnick 55, 1964, No. l pages 42 and 44, Regeneration of metal from etchings.
To avoid these difficulties, in accordance with the invention, the regeneration of the etching solution is carried out in the anode space of an electrolytic cell provided with a dividing diaphragm, and the precipitation of the copper is effected in the cathode space of such electrolytic cell after separation in the copper from the etching solution as copper sulfate, The chromium-llI-ions produced during the etching are thereby anodically oxidized to hexavalent chromium in the anode space, and likewise the spent hydrogen ions are formed in the anode space (half in the form of sulfuric acid, and half in the form of free hydrogen ions which move through the diaphragm into the cathode space and, with the sulfate ions which are becoming free in the cathode space from the copper sulfate form there the other half of the sulfuric acid spent in the etching.) The gain of copper occurs hereby in that the copper is crystallized from the chromosulfuric acid etching solution as copper sulfate, such crystallization preferably being undertaken when the concentration of the copper in the etching solution amounts about 50 to 60 grams per liter, with the crystallized copper sulfate, possibly after purification, being dissolved in water and such solution conducted to the cathode space of the electrolytic cell. At the same time the second half of the sulfuric acid spent in the etching is reformed in the cathode space during the copper precipitation.
As already explained, the regeneration of the etching solution can be undertaken at particular time intervals. Preferably, however, it takes place simultaneously with the etching.
When the etching solution is regenerated simultaneously with the etching, the invention provides that etching and regeneration are carried out spatially separated and that the etching solution is circulated between the etching vessel and the anode space of the electrolytic cell provided with the diaphragm.
During the electrolysis the watery copper sulfate solution will be contained in the cathode space, from which copper sulfate solution the copper is separated at the cathode, or sulfuric acid solution.
At the anode of the electrolytic cell the following reaction takes place:
Cr (S0 +8 H 0 2 H Cr 0,+3 H SO,+6 H +6 e' or 2 Cr* *+7 H 0 Cr tl,+l4 H +6 e while the reaction taking place at the cathode can be formulated as follows:
3 Cu SO +6 e 3 Cu+3 S0 Hence, the complete reversal of the etching reaction can be obtained from the sum of reactions: 3 Cu SOfl-Cr, (800 4-8 H 0 2 H, Cr 0 +6 H SO +3 Cu" or 2 Cr *+3 Cu +7 H30 Cr, 0,+l4 l-l*+3 Cu" Half of the sulfuric acid spent in the etching is hereby reformed at the anode. At the same time hydrogen ions are formed in the anode space. These ions move through the diaphragm to the cathode space and compensate the sulfate ions which are there being released. Half the sulfuric acid spent in the etching is thus formed at the anode, and the other half is formed in the cathode space through the free hydrogen ions released in the anode space and the sulfate ions released in the cathode space through the copper separation.
if, for example, 3 gram atoms copper are dissolved in the chromosulfuric acid, 6 Val hydrogen ions in the etching solution are replaced by copper-ll-ions, i.e. the etching solution is reduced by 3 Mol sulfuric acid by etching 3 gram atoms copper. In the regeneration 1% Mol sulfuric acid is reformed in the anode space of the electrolytic cell. in addition, 3 Val hydrogen atoms are thereby created in the anode space. These 3 Val hydrogen ions move through the diaphragm into the cathode space and there form with the corresponding quantity of sulfate ions the remaining 1% Mol sulfuric acid.
Etching and regeneration of the etching solution can be carried out successively according to the invention. However, it is particularly advantageous to carry out the individual steps simultaneously. The etching solution can be conducted into the circulation continuously or intermittently between etching vessel and anode space of the electrolytic cell. Through the simultaneous etching and regeneration, i.e. constant reformation of the spent ions which are capable of etching the particular advantage is attained that the etching velocity is maintained practically constant.
As already repeatedly explained, however, not only chromium-Vl-ions and hydrogen ions are consumed in the etching, but the etching solution is also enriched by the etched copper in the form of copper-ll-ions. In order not to impair the etching process, the invention provides that the copper obtained in the etching is crystallized out of the etching solution as copper sulfate CuSO 5 H 0, when the concentration of the etching solution in copper-ll-ions has reached a certain value. It is advisable to undertake the crystallization when the etching solution contains about 50 to 60 grams per liter of copper-ll-ions. The crystallization can, for example, take place by evaporating the etching solution and letting it remain at room or lower temperature or by cooling the etching solution. The crystallized copper sulfate can perhaps be purified by recrystallizing and dissolving in water. This solution will be conducted to the cathode space of the electrolytic cell where the copper is separated in metallic form at the cathode. At the same time sulfuric acid is formed in the cathode space.
Hence, according to the invention, at the start of the etching process a fresh etching solution is prepared which contains chromium-Vl-oxide and sulfuric acid. This etching solution is preferably conducted in the circulation between the etching vessel and the anode space of the electrolytic cell, perhaps over a supply vessel. Likewise, the cathode space of the same electrolytic cell is filled with a watery copper sulfate solution. Apart from these solutions, no chemicals are necessary in the etching and regeneration process in accordance with the invention. In the anode space chromium-Vl-ions are continuously reproduced as well as half of the sulfuric acid spent in the etching. The other part of the sulfuric acid is formed in the cathode space. Likewise, the etched copper is not lost, being recovered in the metallic form at the cathode from copper sulfate after crystallization out of copper sulfate from the copper-rich etching solution. Furthermore, there are no waste water problem, as the collected wash water can, for example, be added to the mother lye of the copper sulfate crystallization or in the case of copper sulfate crystallization obtained through cooling, can be concentrated. if the regeneration is simultaneously performed during the etching, such as is particularly provided in accordance with the invention, the etching velocities remain ractically constant. The term "etching velocity" is hereby understood to designate the relationship between the weight loss of the etched body, e.g. of the circuit plate, and the time of etching. The etching times, e.g.
for printed circuits, which possess a copper coating of about 35 p. m. are about 2% when the temperature of the etching solution is maintained between 40 and 50 C.
A preferred embodiment is described in detail in order to explain the invention.
An etching solution consisting of about L5 to 2.0 'Mol chromium-Vloxide and 1.5 to 2.5 Mol sulfuric acid per liter is disposed in an etching vessel and maintained in circulation between the etching vessel and the anode space of an electrolytic cell provided with a diaphragm. Disposed in the cathode space is a watery copper sulfate or sulfuric acid solution. Between anode and cathode a direct electric current of about 4 to 6 volts is applied. In the etching vessel the etching of the copper, e.g. of printed "circuits is effected with chromium-VI-ions being converted into chromium-III-ions and at the same time hydrogen ions (sulfuric acid) are consumed. In the anode space of the electrolytic cell chromium-VI-ions are reformed as well as the hydrogen ions spent in the etching, or half of the spent sulfuric acid. Half of the formed hydrogen ions move, however, through the diaphragm to the cathode space. Therefore, with increased etching the composition of the etching solution changes in spite of simultaneous regeneration, whereby the etching solution becomes increasingly poorer in hydrogen ions and richer in copper.
Etching and regeneration are carried out until the copper content in the etching solution has increased to about 50 to 60 grams per liter of etching solution, at which time the etching and regeneration are interrupted. The etching solution is conducted into a vessel in which the copper sulfate crystallization is effected. For example, the solution is evaporated to onefourth of the volume at the start and then it is left alone. Subsequently, the separated copper sulfate is freed from the mother lye through decanting and washed. The mother lye of the copper sulfate crystallization as well as at least a part of the cathode space fluid, from which copper is separated, are combined and are again available as etching solution. All that has to be done is to add or to remove water. The composition of this constituted etching solution, however, no longer corresponds to the composition of the etching solution at the start, for this etching solution already contains a part of the copper-II-ions. The portion of these copper ions corresponds to the saturation concentration of the concentrated or cooled etching solution in CuSO,. 5 H O as the case may be, at the temperature at which the crystallization has been undertaken, increased by the copper-II-ions perhaps still present in the cathode space fluid. By way of example, the etching solution now corresponds to a composition of about 1.5 to 2.0 Mol chromium-VI-oxide, 1.1 to 2.] Mol sulfuric acid and about 0.4 M01 copper sulfate per liter etching solution or the composition of 1.7 Mol chromium-VI-oxide, 1.5 Mol sulfuric acid and 0.4 Mol copper sulfate.
It is advisable in carrying out the etching and regenerating process in accordance with the invention to proceed in such a way that the etching and perhaps the regeneration can be further continued when the etching solution, which is heavily enriched in copper, is subjected to the treatment for the copper sulfate crystallization. Therefore, at the beginning of the process the portion of the etching solution to be produced will be so selected that it corresponds to triple the quantity of the etching solution to be used in each case in the etching vessel. For the sake of expediency, the copper sulfate solution to be used in the cathode space will be correspondingly tripled. One-third of the produced etching solution is hereby used in the first part of the working cycle for etching and for simultaneous regeneration and is circulated between the etching vessel and the anode space of the diaphragmed electrolytic cell. Likewise only one-third of the produced copper sulfate solution is in the cathodespace.
When this solution for the crystallization of the copper sulfate isconcentrated (second part of the working cycle), etching is effected with the second one-third of the produced etching solution. In the third part of the working cycle (purification of the copper sulfate solution), the third one-third of the prepared etching solution is utilized for etching the copper. In the meantime, the copper sulfate has precipitated from the first one-third, for instance, in the third part of the working cycle this is released from the mother liquor, washed, dissolved in water and can be conducted as copper sulfate solution to the cathode space of the electrolytic cell at the start of the next working cycle for the separation of the copper. A fresh etching solution is in each case prepared from the mother lye of the copper sulfate crystallization and from the cathode space fluid which is poor in copper, perhaps with the addition or elimination of water.
This process, in which simultaneously during the etching regeneration and copper separation are carried out, whereby etching, regeneration and gaining of copper occur side by side, is particularly economical and suitable for factory operation. In addition, the following advantages are achieved, which are particularly valuable for operation on a large technical scale: The etching velocity remains constant within narrow limits. No additional chemicals are necessary, once the etching solution and the fluid for the cathode space have been prepared. Even fresh water is not required if water, derived from the concentration of the etching solution, and possibly of the copper-poor cathode space fluid, as well as if need be of the used wash water, following evaporation and condensation, is utilized for washing and preparation of the cathode space fluid. Consequently no waste water problem arises.
Likewise, the etched copper is simultaneously recovered as pure electrolytic copper.
For the sake of expediency, the formulas for the etching solution and the cathode space fluid are so selected that each part of the working cycle corresponds to one working day. This practice is best suited for operation on a large technical scale. Thus, the quantity of copper to be etched per day and the portion of the etching fluid to be used such working day are, for the sake of expediency, so selected that after the day's operation the copper contents of the etching solution is increased by about 50 to 60 grams per liter.
on the second day this etching fluid is concentrated and is left standing over night for the crystallization of the copper sulfate.
On the third day the crystallized copper sulfate is freed from the mother liquor and washed. The washing water as well as the rinsing water which has collected on the second day when rinsing the etched circuits, can be mixed with the solution formed from the mother lye of the supper sulfate crystallization as well as the copper-poor but sulfuric acid-rich cathode space fluid of the second day. The etching solution for the next day is formed from the mother liquor and the cathode space fluid.
For further explanation of the invention, reference is made to the FIGURE of the drawing, which schematically represents, in block form, the preferred process of etching, regeneration and copper separation which run side by side, described in the example of the etching of printed circuits. In this case the individual circulations are represented by different schematic designations. The circulation for the etching vessel and anode space of the regeneration equipment is indicated by dash-dot-dash lines. The circulation for the depletion of the copper from the etching solution and the formation of fresh etching solution is indicated by dash-dash lines, and the partial process cycle of the copper separation from the crystallized copper sulfate is indicated by dash-dot-dot lines.
The conduction of chromosulfuric acid etching solution into the etching vessel 2 is schematically represented by arrow 19 and the introduction of the plate 1 made of insulating material, and coated with copper is similarly indicated. For clarity the copper to remain on the circuit plates is indicated by Cu and the copper to be etched by Cu The etching solution is maintained in circulation during the etching. It is conducted over line 20 to the anode space 54 of an electrolytic cell 5, which is provided with a diaphragm 53, and leaves the anode space 54 by line 2!. It then enters chamber 73 of -a supply vessel 7 and arrives again at etching vessel 2 over the line 19. During this circulation the copper in the etching vessel is etched, whereby chromium-Vl-ions are reduced to chromium-lll-ions and sulfuric acid is consumed according to the equation:
2 H CR +3 Cu+6 l-ll SO Cr (S0,) +3 Cu SO.,+ 8 H O in the anode space 54 the chromium-lll-ions are again oxidized to chromium-Vl-ions and half of the sulfuric acid consumed in the etching is reformed according to the equation: Cr (SO ),+8 ,0 2 H, Cr O +3 l-l SO +6 H +6 e The hydrogen ions produced simultaneously in the anode space 54 move through the diaphragm 53 to the cathode 52.
in the cathode space 55 the following reaction takes place: 2 Cu S0 +6 e 3 Cu"+3 S0,,
From the sulfate ions which have become free and the hydrogen ions which have moved through the diaphragm 53, the other half of the sulfuric acid consumed in the etching is formed.
When the first working day is finished and the copper concentration in the etching solution has risen to about 50 to 60 grams per liter, this etching solution is conducted to the evaporation vessel 8 over line 25. The solution in the vessel 8 is evaporated therein on the second day and is left standing over night. During the second day the etching solution from chamber 72 of the supply vessel 7 is utilized (which is simultaneously regenerated, during the etching, in the anode space 54). On the third day the crystallized copper sulfate CuSO, H O, schematically indicated by 10, is purified by recrystallization (chamber I1) and the purified copper sulfate is dissolved in water (chamber 12). This solution is conducted to the cathode space 55 of the electrolytic cell 5 over the line 34. Here, on the fourth day the copper, at cathode 52, is separated from this solution, which is gained at Cu,,6 atthe cathode 52. Following the separation of the copper, the cathode space fluid, which may be diluted with water or concentrated by evaporation, is conducted to chamber 73 of the supply vessel over line 26, together with the mother liquor of the copper sulfate crystallization.
The printed circuits are rinsed after they leave the etching vessel 2, leaving the working cycle as Cu 4. The rinse water from chamber 3 may be brought over line 37 to the copperpoor cathode space fluid 13 (and therewith to the chamber 73 for preparation of the new etching solution). If necessary or desirable, the fluid 13 can be concentrated. The evaporated water is conducted over line 38 and collected in a container 9, as is the water resulting from the concentration of the enriched etching solution, which is effected in crystallizer 8, and conducted to the container 9 over line 27. Thus, there is achieved a process, by way of example, for the rinsing of the etched plates, schematically represented by the arrow 36 or for the preparation of the copper sulfate solution 12, schematically indicated by the arrow 33, as well as for the purification of the crystallized copper sulfate 11, schematically represented by the arrow 30.
Changes may be made within the scope and spirit of the appended claims which define what is believed to be new and desired to have protected by Letters Patent.
1. A method of simultaneously etching copper and regenerating the etching solution which comprises the steps of etching copper with a chromosulfuric etching solution in an etching vessel, circulating the etching solution between the etching vessel and the anode compartment of an electrolytic cell provided with a diaphragm defining anode and cathode compartments whereby in said anode compartment Cr ions are oxidized to Cr ions and one-half the sulfuric acid utilized in the etching operation is regenerated, periodically crystallizing out hydrated copper sulfate from the etching solution at the time when the increasing Cu concentration in the etching solution reaches a point whereby the etching rate is materially slowed, redissolving said hydrated copper sulfate, adding said dissolved copper sulfate to the cathode compartment whereupon metallic copper is produced and the remaining one-half of the sulfuric acid utilized in the etching operation is regenerated.
2. The method of claim 1 wherein said crystallization 1S effected at a point when the concentration of the Cu ions in the etching solution reaches 5060 grams per liter expressed in terms of copper sulfate.
3. The method of claim 1 wherein said etching solution is divided into three parts, one part comprising a relatively fresh etching solution which is being simultaneously regenerated during the etching operation, the second part comprising a Cu rich solution and a third part comprising an etching solution derived from the cathode compartment after Cu precipitation.
4. .A process according to claim 1, wherein an etching solution is used comprising about [.5 to 2.0 Mol chromium-V1- oxide and 1.5 to 2.5 Mol sulfuric acid per liter.
5. A process according to claim 4, wherein the etching solution comprises approximately 1.7 Mol chromium-Vl-oxide and approximately 1 .9 Mol sulfuric acid per liter.
6. A process according to claim 1, wherein there is added to the etching solution about 0.05 to 0.1 Mol sulfuric acid per liter.
7. A process according to claim 1, wherein the etching operation is effected at temperature between approximately 40 to 50 C.
8. A process according to claim 1, comprising the step of combining the mother liquor of the copper sulfate crystallization and the cathode fluid remaining after the separation of the copper, and controlling the water content thereof as necessary, to reconstitute an etching solution.
9. A process according to claim 1, in which the process is utilized to etch undesired portions of copper on printed circuit boards retaining other portions of copper thereon, comprising the further steps of washing the etched boards in water, mixing the used wash water with the copper-poor solution from the cathode space, concentrating the same and utilizing water derived therefrom, and from the crystallization process, to provide a water supply available for use in the washing operation and as may otherwise be required in the practice of the process.