US3637467A

US3637467A - Metal reclamation process and apparatus

Info

Publication number: US3637467A
Application number: US35472A
Authority: US
Inventors: Donald Dean Spatz
Original assignee: Osmonics Inc
Current assignee: Suez WTS Solutions USA Inc
Priority date: 1970-05-07
Filing date: 1970-05-07
Publication date: 1972-01-25
Anticipated expiration: 1989-01-25

Abstract

A method of reclaiming metal salts from an electroplating process which utilizes exposing the rinse solution, under pressure, to one side of a semipermeable membrane for separation into a concentrate and a permeate. The concentrate and permeate are then recycled back into the electroplating system.

Description

1111mm States Patent 1151 3,637,467 Spatz [4 1 Jan. 25, 1972 54] METAL RECLAMATION PROCESS AND 2,865,823 12/1958 Harris, etal. ..204/23s APPARATUS 3,074,863 1/1963 Jasionowski ....204/237 3,542,651 11/1970 Yagishita ..2o4/237 [72] lnventor: Donald Dean Spatz, Minnetonka, Minn.

[73] Assignee: Osmonics, Inc., Minneapolis, Minn.

[22] Filed: May 7, 1970 [2]] Appl. No.: 35,472

[52] US. Cl. ..204/l4 R, 204/198, 204/239 [51] Int. Cl. ..C23b 5/00, 865g 49/00, BOlk 3/00 [58] Field of Search ..204/237-239, 23, 204/14 R [56] References Cited UNITED STATES PATENTS 2,088,498 7/l937 Tull ..204/239 Primary Examiner-John H. Mack Assistant ExaminerThomas Tufariello Attorney-Frederick E. Lange, William C. Babcock and Eugene L. Johnson [5 7] ABSTRACT A method of reclaiming metal salts from an electroplating process which utilizes exposing the rinse solution, under pressure, to one side of a semipermeable membrane for separation into a concentrate and a permeate. The concentrate and permeate are then recycled back into the electroplating system.

11 Claims, 4 Drawing Figures PARTS T0 i W PRIMARY SECONDARY T PLATED as PLATED PLATE TANK w 11 RINSE TANK 12 RINSE TANK PARTS 912 GRAMS mat-(c10 026 GRAMS KAu (cm) 0002 GRAMS mu (c111 PER LITER PER LITER PER LITER t 241 10 19 ION -20 EXCHANGE FILTER CONCENTRATE -27 BGALLONS /HOUR 2.5 GRAMS KAMCNl /LITER RINSE SOL/UTION MAKEUP 3O GALLONS HOUR 0,26 GRAMS K20 (010 /LITER WATER 3| PSI, 18

REVERSE OSMOSIS UNIT \PERMEATE 27 GALLDNiE/IKOU/R TER PRESSURE AND 0033 GRAMS KAu N2 1.1 FLOW CONTROL l l; 595.1

PATENTEU JANZS 23. 2

PARTS TO RIMARY S EFOIIJRY PLATED as. PLATED TANK 11 RINSE TANK 12 RINSE TANK PARTS 5H2 creams Mum) 0,26 GRAMS KAu (c 0.002 GRAMS KAu(CN) PER LITER PER LITER PER LITER Z2 19 low EXCHANGE FILTER PUMP CONCENTRATE -27 3 GALLONS HOUR 26 2.3 GRAMS KAu(CN) /L|TER IO s1 RINSE SOLUTION p so GALLONS/HOUR 0.26 GRAMS ggug cm /LITER\ WATER REVERSE 1 OSMOSIS UNIT PERMEATE 27 GALLONS/HOUR PRESSURE AND 0.033 GRAMS KAu (cm /LITER FLOW CONTROL S 21 59514 SEMI-PERMEABLE i MEMBRANE} PRESSURE M C E ATED LESS CONCENTRATED ORE 8 HON SOLUTION 19 CONCENTRATE Y s FLOW OF L RINSE SOLUTION 51 WATER FLOW 20 IIIIIIIIIIIIIIIIIII THE 2 PRESSURE & i SEMI-PERMEABLE MEMBRANE) LE 85 CONCENTRATED SOLUTION MORE CONCENTRATED SOLUTION 1 WATER FLOW PERMEATE I NVENTOR.

DEA/V SP4 rz METAL RECLAMATION PROCESS AND APPARATUS BACKGROUND OF THE INVENTION My invention relates generally to a method for reclaiming metal salts from a metal salt solution, and more specifically, to a method for reclaiming metal salt from an electroplating system and for continuously reusing the reclaimed metal salt and its separated solvent in the electroplating system.

In an electroplating system, the general object is to deposit a thin coating of some metal, through electrolytic deposition, onto an object for the purpose of rendering a lustrous or noncorrosive finish on that object. To accomplish this, the object is immersed into a solution (plating bath) of the salt of the metal which is to be plated and is employed as the cathode. The anode of the system may be of the same metal which is to be coated or may be a conductor whichis chemically unaffected by the electroplating reaction. A low-voltage current is then passed through the solution, electrolyzing and plating the cathodic object with the metal in the solution. When the coating on the object is of the desired thickness, the object is removed and passed to a rinse tank or series of rinse tanks where the residual plating solution is removed from the plated objects. After several rinses, an accumulation of the plating solution occurs in the rinse tank. However, the rinse solutions, even in their most concentrated form, are quite diluted in comparison to the concentrated solution of the plating bath and thus are not capable of being used, in their present concentration, as makeup for the plating bath. The various rinse solutions must also be kept below a certain level of concentration in order to maintain an effective rinse operation and to prevent spotting or other deterioration in the quality of the plated object.

This necessarily poses a problem as to what should be done with the rinse solutions which have reached a concentration where they are no longer effective as a rinse.When the plating metal is not a precious metal, the most economical disposal is by discharging them into rivers and streams or by ponding them. However, when the plating metal is a precious metal, e.g., gold, silver, etc., it is most economical to recover the precious metal from the rinse solutions and reuse it. Aside from purely economical considerations, the reasons for recovery and reuse of nonprecious metals is strong, especially in light of the current attitudes regarding pollution and conservation and preservation of natural resources.

In the past, distillation and ion exchange processes have been used to reclaim plating salts from the rinse solutions. Recovery by these processes, however, has been quite expensive. Until recently, the only salts reclaimed were the precious metal salts. Consequently, the high cost of recovery via distillation and ion exchange could be somewhat justified. However, with the new awareness of the effects of the metal salts on the environment, all plating salts, both precious and otherwise, will have to be reclaimed. Thus, it will be very important to use the most economical means for reclaiming the metal salts from the rinse solution.

SUMMARY OF THE INVENTION In contrast to prior methods, the present invention includes an economical and labor-saving method for reclaiming the plating salts from an electroplating rinse solution so that the reclaimed salt may be reused without further refinement or processing in the plating bath, and so that the relatively high purity solvent may be reused in the rinse operation. More specifically, the present invention utilizes a reverse osmosis process to separate the metal salts from the metal salt rinse solution. This is accomplished by exposing the rinse solution to a semipermeable membrane, under pressure, so that virtually only the solvent (water) of the solution passes through the membrane. The reverse osmosis membrane thus separates the rinse solution into two solutions of different concentrations.

Once the rinse solution has been separated into a solution of relatively high metal salt concentration (concentrate) and into a solution of relatively low metal salt concentration (permeate), the two solutions are recycled respectively into the plating bath and into the rinse tank for reuse. Thus, none of the metal salts, with the exception of those which are plated or those which are lost through accident or spillage, ever leave the electroplating system.

Also, the present invention provides for a very efficient reclamation and recycling system. The process of reverse osmosis operates most efficiently at high concentrations. This is not true, for example, with ion exchange where operation and separation is most efficient at very low concentrations. In the present system, the rinsing procedure operates satisfactorily even though the rinse water is not l00.percent pure, although, as noted previously, the rinse water must be dilute enough to prevent spotting or deterioration of the plated article. Because of this, the reverse osmosis purification process can be operated efficiently and with a minimum amount of energy and cost.

Accordingly, it is an object of the present invention to provide an improved method for reclaiming metal salts from an electroplating system so that the reclaimed salts may be reused in the electroplating process.

It is also an object of the present invention to provide an improved electroplating metal reclamation system in which none of the plating metal, with the exception of that coated to the object, ever leaves the electroplating system. To accomplish this, a portion of the separated rinse solution (the concentrate) is recycled into the plating bath and the other portion (the permeate) is recycled into the rinse operation.

A further object of the present invention is to provide a more efficient recovery and recycling process by considering the various concentrations of the rinse water when the rinse operation will and will not operate sufficiently.

A further object of the present invention is to provide an improved method for metal salt reclamation and recycling in an electroplating system where the system is continuous and requires little or no maintenance.

These and other objects and advantages of the present invention will become apparent with reference to the drawings, the description of the preferred embodiment, and the appended claims.

DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic flowsheet diagram of the process of the present invention indicating the major items of equipment and the operating conditions for each step in the process.

FIG. 2 is a diagram showing the principle of osmosis where the flow of water is from the less concentrated solution to the more concentrated solution.

FIG. 3 is a diagram showing the principle of reverse osmosis where the flow of water is from the more concentrated solution to the less concentrated solution.

FIG. 4 is a partial sectional view of a portion of the reverse osmosis membrane showing the flow of the rinse solution over the surface of the membrane and the separation of the feed solution into a concentrate and a permeate.

DESCRIPTION OF THE PREFERRED EMBODIMENT and secondary rinse tanks, respectively, all common elements of the well-known process of electroplating. The process of electroplating involves coating an article with a thin layer of metal through electrolytic deposition. The principal steps involved in the process can best be understood with general reference to FIG. 1.

First of all, the article (not shown) to be plated is immersed into the plating tank 10 which contains a plating bath composed of a solution of the salt of the metal to be plated. A number of different metals, as well as a number of different salts of the same metal may be used in the plating bath, however, for purposes of the preferred system, a solution of potassium cyanoaurate, KAu(CN) will be used. A typical concentration for a satisfactory plating solution of KAu(CN) is 9.12 grams per liter. With this solution, the plating metal is gold. After the article to be plated is immersed into the plating tank 10, the article is electrically connected to a source of direct current voltage as the cathode terminal in the electrolytic solution. The other terminal, the anode, may be of the same metal be plated, or may be some chemically unaffected conductor. A low-voltage current is then passed through the solution thereby electrolyzing and plating the cathodic article.

When the article is plated with the desired thickness of metal, the article is removed from the plating tank 10, and passed to the primary rinse tank 11 where residual plating solution is rinsed from the article. The rinse tank 11 may be a spray rinse, cascade rinse, stagnant rinse, or other suitable means of rinsing the excess plating solution from the plated article. As can be seen from FIG. 1, the concentration of plating salts in the primary rinse tank 11 (0.26 grams (KAu(CN) per liter), is much less than the concentration of plating salts in the plating tank (9.12 grams (KAu(CN per liter). The purpose of the primary rinse tank 11 is to remove as much of the excess plating solution from the plated article as possible. Consequently, the rinse solution eventually attains a high enough concentration to prevent it from being effective as a rinse solution. Then, the rinse solution must be changed or purified. This will be discussed in more detail later.

After sufficient rinsing in the primary rinse tank 11, the plated article passes to the secondary rinse tank 12 for the removal of trace salts. This secondary rinse tank 12 is especially useful when rinsing plated articles with numerous crevices where salt will deposit and at least two rinses are required to remove the salt. As shown, the secondary rinse solution 24 flows from the rinse tank 12 into an ordinary ion exchange unit 22 which removes the salts from the rinse solution 24. The purified extract 24 is then recycled back into the rinse tank 12 and reused as a rinse solution. The ion exchange unit 22 can be used relatively economically here because the rinse solution 22 is quite dilute (0.002 grams (KAu(CN) per liter). Thus, only a small quantity of salt need be removed from the solution 22 in order to purify it for reuse. The plating of the article, upon its removal from the secondary rinse tank 12, is complete.

It should also be noted that the secondary rinse tank is not necessary in many plating operations. For example, when the plated article is relatively void of crevices, a single rinse tank 11 may be sufficient. On the other hand, the article may be of such a nature that several rinse tanks, substantially identical to tank 11, are necessary to properly rinse the plated article.

Attention will now be directed to the reclamation of the metal salts from the rinse solution 19 of the primary rinse tank 11, and the recycling of the resultant solutions, 20 and 21 back into the electroplating process. As seen in FIG. 1, the rinse solution 19 (0.26 grams of KAu(CN) per liter) flows from the primary rinse tank 11 and through a mechanical filter or screen 14 in order to remove any large foreign particles. A typical filter for this purpose is a 50-micron filter. From here the rinse solution 19 passes into a pump 15 where it is pressurized and fed to the reverse osmosis unit 16. A typical pump for use in the preferred system is a multistage centrifugal pump. One such pump is a Goulds MB 5400-85. The solution 19 in the preferred system leaves the pump 15, and enters the reverse osmosis unit 16 at a rate of 30 gallons per hour, under a pressure of about 315 pounds per square inch, and with a concentration of 0.26 grams (KAu(CN) per liter.

The operation of the reverse osmosis unit 16, which is commonly known, can generally be understood with a brief description of the phenomenon of osmosis, and with reference to FIGS. 2, 3 and 4. Briefly, osmosis takes place when a solvent (water in this case) passes from a less concentrated solution through a semipermeable membrane to a more concentrated solution. In FIG. 2, a certain amount of potential energy exists between the two solutions (less concentrated and more concentrated) on either side of the semipermeable membrane 31. As a result, solvent will flow from the less concentrated solution to the more concentrated solution until the system is in equilibrium. Accordingly, the level of the more concentrated solution will rise (shown by broken line 32), and the level of the less concentrated solution will drop (shown by broken line 34). When equilibrium is reached, the vertical distance h, between the more concentrated solution level and the less concentrated solution level will be the head which is equal to the osmotic pressure between the two solutions.

When pressure is applied to the more concentrated solution as shown in FIG. 3, the flow of solvent through the membrane 55 will be reversed. If the pressure applied is equal to the osmotic pressure of the solution, equilibrium will be reached when the levels of the two solutions are equal. However, if a pressure greater than the osmotic pressure is applied, the solvent will continue to flow through the membrane 35 into the less concentrated solution. This is the process known as reverse osmosis. The rate of solvent flow through the membrane 35 in reverse osmosis is a function of the pressure which is applied to the more concentrated solution, the osmotic pressure between the two solutions, and the area of the membrane 35 being pressurized.

The application of the process of reverse osmosis to the reclamation of metal salts in an electroplating process is shown in FIG. 4. There, the flow of the rinse solution 19 enters the reverse osmosis unit 16 (FIG. 1) under pressure and flows over the surface of the semipermeable membrane 28 and the porous membrane support 29. Since the solution in contact with the membrane 28 is under pressure, part of the solvent will pass through the membrane 28 as permeate 21, having a sufficiently low concentration of metal salts (0.033 grams (KAu(CN per liter). Likewise, the solution above the membrane will become more concentrated, leaving the reverse osmosis unit 16 (FIG. 1) as concentrate 21 and having a substantially greater concentration of metal salt (2.3 grams (KAu(CN) per liter). A typical reverse osmosis unit which can be used in the preferred system is an OSMO l0043-SS, manufactured by Osmonics, Inc.

Referring again to FIG. 1, it can be seen that the permeate 21 leaving the reverse osmosis unit 16 is recycled back into the primary rinse tank 11 to be reused as a rinse solution. The permeate 21 for the preferred system flows at a rate of 27 gallons per hour, with a concentration of 0.033 grams of KAu(CN) per liter, and under a pressure of 5 pounds per square inch. Since the rinse solution 19 flows from the rinse tank 11 at a rate of 30 gallons per hour, makeup water 26 at a rate of 3 gallons per hour must be added to the permeate flow 21 in order to produce a resultant flow 27 which will allow continuous operation of the primary rinse tank 11. With the addition of the makeup water 26, the KAu(CN) concentration of the flow 27 is further diluted from the concentration of the permeate 21. The approximate concentration of the flow 27 actually being recycled into the tank 11 is 0.0297 grams of KAu(CN) per liter.

As the flow of concentrate 20 leaves the reverse osmosis unit 16, it is subjected to a flow and pressure controller 18 which allows the concentrate on one side of the controller to be equal to the pressure in the unit 16 (315 p.s.i.) while the pressure on the other side is sufficiently low to allow the concentrate to be recycled back into the plating tank 10. In the present system the concentrate 20 is recycled into the plating tank 10 at a rate of 3 gallons per hour, with a KAu(CN) concentration of 2.3 grams per liter, and under a pressure of 10 pounds per square inch.

In order to maintain the solution of the plating tank 10 at a sufiiciently high concentration, metal salt crystals or high concentrate solution of the same are added to replace the metal which is plated out onto the article. Since evaporation from the plating tank 10 is normally greater than 3 gallons per hour, the 3 gallon per hour flow of the recycled concentrate 20 is usually insufficient to maintain the volume of solution in the plating tank at a constant level. Consequently, the makeup concentrate, which is added to maintain the concentration level also includes enough solvent to maintain the volume of solution in the plating tank at a constant level.

As can be seen, the entire system, under ideal conditions, operates continuously. The concentration in the primary rinse tank 11 is kept at a constant level since the amount of l(Au(CN) salt entering the rinse tank 11 from the recycled permeate 21 and from the plated article passing from the plating tank is substantially equal to the amount of KAu(CN) leaving the rinse tank 11 as primary rinse solution 19. Likewise, the volume of liquid in the tank 11 remains constant since the liquid entering the system via the recycled permeate and makeup water 27 and the liquid entering along with the plated article is equal to the amount of liquid leaving the rinse tank 11 via the rinse solution 19 and via evaporation.

Also, a material balance over the reverse osmosis unit 16 shows that the material and volume flowing into the unit via the rinse solution 19 is equal to the material and volume flowing out of the unit as concentrate 20 and permeate 21.

When the system is operating continuously, the concentration of the primary rinse solution is about 0.26 grams of KAu(CN per liter. This concentration is low enough to permit an effective rinse in the primary rinse tank 11 as well as to prevent spotting or deterioration on the surface of the plated article. Since the rinse solution does not have to be completely purified for effective rinsing of the plated articles, the process of reverse osmosis can be used very economically. For example, in the range of purification with which the present system is concerned (0.26 grams per liter to 0.0297 grams per liter), the reverse osmosis unit operates with a minimum amount of energy and at a relatively modest pressure (315 p.s.i.). To further purify the permeate 21 would require much more energy and substantially greater pressures (1,200-1 .500 p.s.i.).

The above-described invention may be embodied in other specific forms without departing from the spirit or essential characteristics thereof. The present embodiments are therefore to be considered in all respects as illustrative and not restrictive, the scope of the invention being indicated by the appended claims and the equivalents embraced therein.

What is claimed is:

1. In an electroplating process in which articles to be plated are sequentially immersed in a plate solution of a relatively high metal salt concentration, and then a rinse solution of a relatively low metal salt concentration, the improvement comprising:

placing the rinse solution in contact with one side of a semipermeable membrane under a pressure greater than the osmotic pressure of said rinse solution, wherein said rinse solution is separated into a concentrate of greater metal salt concentration than said rinse solution and a permeate of less metal salt concentration than said rinse solution;

combining makeup solvent with said permeate to form a permeate recycle solution so that the volume flow of said permeate recycle solution is equal to the volume of said rinse solution placed in contact with the semipermeable membrane; and

introducing said permeate recycle solution into the rinsing tank for reuse as a rinse solution.

2. The improved electroplating process of claim 1 wherein the placing of the rinse solution in contact with a semipermeable membrane under pressure is a reverse osmosis process.

3. The improved electroplating process of claim 1 wherein said makeup solvent comprises water.

4. The improved electroplating process of claim 1 wherein the introduction of permeate recycle solution into the rinse tank maintains the rinse solution at a sufi'rciently low metal salt concentration so that it is effective as a rinse solution.

5. The improved electroplating process of claim 1, and exposing said rinse solution to a filter means for removing impurities from said rinse solution prior to contact with the semipermeable membrane.

6. The improved electroplating process of claim 1, and recycling said concentrate into said plating tank for reuse as a plating solution.

7. The improved electroplating process of claim 3 wherein the recycling of said concentrate into said plating tank includes:

reducing the pressure of said concentrate below the pressure at which it is in contact with said membrane to a level compatible with that in the plating tank,

combining makeup solution with said concentrate to form a concentrate recycle solution having a metal salt concentration such that the recycling of said concentrate recycle solution into the plating tank replaces the metal salt and the solvent leaving the plating tank, and

introducing the concentrate recycle solution into the plating tank for reuse as a plate solution.

8. The improved electroplating process of claim 1 wherein the placing of the rinse solution in contact with a semipermeable membrane under pressure includes pressurizing said rinse solution prior to placement into contact with the semipermeable membrane.

9. The improved electroplating process of claim 8 wherein the rinse solution is placed in contact with the semipermeable membrane at a pressure of about 300-400 pounds per square inch.

10. An improved electroplating apparatus for plating an article with a thin coating of metal comprising:

a plating tank adapted to contain plate solution of a relatively high metal salt concentration;

a rinsing tank adapted to contain a rinse solution of a relatively low metal salt concentration, and having an inlet and an outlet for introduction and exit of rinse solution;

means for sequentially immersing the article to be plated into the plate solution and then into the rinse solution;

a pump means for pressurizing the rinse solution which exits from the rinse tank through the rinse tank outlet;

a reverse osmosis unit for separating the pressurized rinse solution into two component solutions, a concentrate having a greater metal salt concentration than the rinse solution and a permeate having a lesser metal salt concentration than the rinse solution;

means for combining the permeate with a makeup solution having a lesser metal salt concentration than the permeate; and

conduction means for directing the rinse solution exiting from the outlet of the rinse tank to the pump means between the pump means and the reverse osmosis unit, between the reverse osmosis unit to the means for combining the permeate with makeup solution and from the means for combining the permeate with the makeup solution to the inlet of the rinse tank.

11. The improved electroplating apparatus of claim 10 wherein the plating tank has an inlet and a pressure control for reducing the pressure of the concentrate after leaving the reverse osmosis unit; and

means for directing the concentrate from the reverse osmosis unit to the pressure control and for directing the concentrate from the pressure control to the inlet of the plating tank.

Claims

2. The improved electroplating process of claim 1 wherein the placing of the rinse solution in contact with a semipermeable membrane under pressure is a reverse osmosis process.
3. The improved electroplating process of claim 1 wherein said makeup solvent comprises water.
4. The improved electroplating process of claim 1 wherein the introduction of permeate recycle solution into the rinse tank maintains the rinse solution at a sufficiently low metal salt concentration so that it is effective as a rinse solution.
5. The improved electroplating process of claim 1, and exposing said rinse solution to a filter means for removing impurities from said rinse solution prior to contact with the semipermeable membrane.
6. The improved electroplating process of claim 1, and recycling said concentrate into said plating tank for reuse as a plating solution.
7. The improved electroplating process of claim 3 wherein the recycling of said concentrate into said plating tank includes: reducing the pressure of said concentrate below the pressure at which it is in contact with said membrane to a level compatible with that in the plating tank, combining makeup solution with said concentrate to form a concentrate recycle solution having a metal salt concentration such that the recycling of said concentrate recycle solution into the plating tank replaces the metal salt and the solvent leaving the plating tank, and introducing the concentrate recycle solution into the plating tank for reuse as a plate solution.
8. The improved electroplating process of claim 1 wherein the placing of the rinse solution in contact with a semipermeable membrane under pressure includes pressurizing said rinse solution prior to placement into contact with the semipermeable membrane.
9. The improved electroplating process of claim 8 wherein the rinse solution is placed in contact with the semipermeable membrane at a pressure of about 300-400 pounds per square inch.
10. An improved electroplating apparatus for plating an article with a thin coating of metal comprising: a plating tank adapted to contain plate solution of a relatively high metal salt concentration; a rinsing tank adapted to contain a rinse solution of a relatively low metal salt concentration, and having an inlet and an outlet for introduction and exit of rinse solution; means for sequentially immersing the article to be plated into the plate solution and then into the rinse solution; a pump means for pressurizing the rinse solution which exits from the rinse tank through the rinse tank outlet; a reverse osmosis unit for separating the pressurized rinse solution into two component solutions, a concentrate having a greater metal salt concentration than the rinse solution and a permeate having a lesser metal salt concentration than the rinse solution; means for combining the permeate with a makeup solution having a lesser metal salt concentration than the permeate; and conduction means for directing the rinse solution exiting from the outlet of the rinse tank to the pump means between the pump means and the reverse osmosis unit, between the reverse osmosis unit to the means for combining the permeate with makeup solution and from the means for combining the permeate with the makeup solution to the inlet of the rinse tank.
11. The improved electroplating apparatus of claim 10 wherein the plating tank has an inlet and a pressure control for reducing the pressure of the concentrate after leaving the reverse osmosis unit; and means for directing the concentrate from the reverse osmosis unit to the pressure control and for directing the concentrate from the pressure control to the inlet of the plating tank.