Method and apparatus for electroplating

[57] ABSTRACT

This invention concerns an improved method and apparatus for electroplating in which rinse solutions and fumes from electroplating operations are treated to recover values contained therein, and to simultaneously adjust the concentration and the temperature of the electroplating bath. Fumes and spray from the electroplating and rinsing operations are contacted with controlled amounts of a process solution comprising overflow rinse water mixed with electroplating bath solution. The liquid-vapor contacting operation is controlled to evaporate water at a rate substantially equivalent to the rate of addition of rinse water to the process so that a water balance is maintained throughout the system, and to cool the process solution. The process is preferably conducted in apparatus comprising an electroplating tank, a rinse tank, a fume exhaust system, a heat exchanger and a liquid-vapor contacting means.

Claims, 1 Drawing Figure

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STEAM INLET

PATENTED MAY 9 1972

3,661,732

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ATTORNEYS

EMERGING STRIP

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3,661,732

METHOD AND APPARATUS FOR ELECTROPLATING

The present invention pertains to the art of electroplating and more particularly to a process and apparatus for treating fumes and rinse water to recover values contained therein and to control the concentration and temperature of the electroplating bath solution.

Electroplating from aqueous solutions is accomplished, generally speaking, by passing an electric current through an electroplating bath solution to deposit the plating material on the workpiece. In order to control the quality of the plated product, it is often necessary to maintain the temperature of the bath within a narrow temperature range, generally at somewhat elevated temperatures. The passage of electric current through the bath, however, tends to raise the bath temperature and to generate fumes from it. Further, the electroplating reaction usually forms gases at the electrodes. For example, in the electroplating of chromium from a chromic acid solution, large volumes of hydrogen and oxygen are given off. These gases bubble up through the bath and a spray of the plating solution is produced. In a continuous plating operation such as, for example, the plating of steel strip for use in the fabrication of containers, an endless strip is introduced into and removed from the electroplating bath by means of rollers which conduct the strip through the bath and through any required rinse baths. The splashing which results from the workpiece entering and leaving the electroplating bath also tends to form a spray of the bath solution.

It is thus seen that there are three main sources of fumes from the electroplating bath; (1) the heating of the bath by the imposed electric current, (2) the evolution of gases at the electrodes, and (3) the splashing of the workpiece entering and leaving the bath. The vapors and spray escaping from the electroplating bath contain chemical values which can represent serious loss of expensive reagents. Moreover, the vapors and spray can also create unpleasant working conditions and health hazards since some of the vaporized values may be noxious and/or toxic.

When the workpiece emerges from the electroplating bath, a significant amount of electroplating solution is carried out on it. This carry-out of solution not only represents another serious loss of chemical values from the electroplating bath but the solution must be removed from the workpiece lest succeeding equipment be damaged, the product be detrimentally contaminated or the health of workers handling the material be adversely affected by the noxious and/or toxic residues of the electroplating solution. The usual method of removing the solution is by a water-rinsing step following the electroplating step.

Aside from the economic penalty of losing electrolytic values in the rinse water, disposal of the rinse water contaminated with electroplating solution would create a severe water pollution problem if left untreated. In some instances, the water pollution problem was sought to be minimized by using large quantities of water for the rinsing operation. This latter procedure merely obscures the water pollution problem and does not avoid it. It has also been suggested to separately treat the rinse water, either by chemical or physical means, to recover the values dissolved therein. These suggestions have the common disadvantage of requiring separate processing and apparatus to recover values contained in and to de-contaminate the rinse water, which can substantially increase the cost of the electroplating operation.

One method of recovering the values in the rinse water and avoiding the pollution problem is to recycle the rinse water to the electroplating bath. However, this method dilutes the bath by an amount equivalent to the amount of rinse water introduced and a means must be found to remove an equivalent amount of water from the system in order to maintain control of the electrolyte concentration.

Another problem encountered in some electroplating operations is the control of the temperature of the electroplating bath. As previously stated, it is often necessary to control the temperature of the electroplating bath within a narrow temperature range. For example, it has been found that in chromium plating steel strip, it is advantageous to electroplate chromium within about 3° F. of a set point typically lying between about 105° F. to about 120° F. in order to insure that the chromium plate possesses those properties that are required for forming containers. Because chromium electroplating baths display high resistivities, the plating operation generates large quantities of heat by passage of electrical current through chromic acid solutions and the temperature of the bath is then increased to undesirable levels. The heat generated by the electroplating operation must be removed to maintain the proper electrolyte temperature. This has been done by providing the electroplating bath itself with heat exchangers or by withdrawing the solution from the electroplating bath, cooling it in separate equipment and returning it to the bath. Neither of these approaches is entirely satisfactory since they both increase the cost of the overall process by requiring additional cooling equipment and a separate operation.

From the foregoing it is seen that four ancillary or sub-functions are necessary for efficiently carrying out electroplating operations, and these are (1) control of fumes generated from the operations, (2) control of rinse water from the operations, (3) control of water dilution of the electrolyte in the bath, and (4) control of temperature of the electroplating bath. Although attempts have been made to overcome the foregoing problems and other disadvantages, none, insofar as I am aware, has been entirely successful when carried into practice commercially on an industrial scale. This, I believe, has been largely due to the fact that no previous method, insofar as I am aware, has attempted to solve the four basic problems hereinabove referred to in a single, simple operation. Accordingly, it is an object of the present invention to provide a process for recovering values contained in and for purifying fumes emanating from an electroplating operation.

Another object of the invention is to provide a process for recovering carried-out chemical values in rinse water.

It is a further object of the invention to provide a process for simultaneously controlling the temperature of an electroplating bath while recovering values contained in fumes and rinse water emanating from the electroplating process.

The invention further contemplates providing an apparatus for recovering values contained in fumes emanating from the electroplating operations.

Still another object of the invention is to provide an apparatus for recovering chemical values from rinse water which values were carried over from the electroplating operation.

It is also an object of the invention to provide apparatus for controlling the temperature of an electroplating bath while simultaneously recovering values contained in fumes from the electroplating operation and from rinse water.

Other object and advantages will become apparent from the following description taken in conjunction with the accompanying FIGURE which schematically depicts a preferred embodiment of the apparatus employed for continuously electroplating steel strip with chromium and recovering values contained in fumes from the electroplating operation and from the rinse water.

Generally speaking, the present invention contemplates a process for recovering values contained in fumes emanating from electroplating operations including rinsing operations, while simultaneously controlling the temperature and water balance of the electroplating bath. The process comprises contacting the fumes with at least a portion of the process solution to dissolve therein values contained in the fumes and to evaporate water therefrom. The term “process solution” as used herein includes, but is not necessarily limited to, the electroplating bath solution and/or the rinse water. The process solution, after being contacted with the fumes and having given up water to the fumes and absorbed values therefrom, is added to the electroplating solution.

The process and apparatus in accordance with the present invention can be employed generally in any electroplating operation which generates fumes which must be purified or

3,661,732 treated to recover components therefrom. However, in order to give those skilled in the art a better understanding of the invention and its advantages, the following description will be made in reference to electroplating chromium on steel strip on a continuous basis. Referring now to the FIGURE, the apparatus in accordance with the present invention comprises generally a conventional electroplating tank A, a rinse tank B, a concentrator C and gas hoods D for electroplating tank A and rinse tank B. Electroplating tank A comprises a number of individual plating cells 11, each forming an independent electroplating cell. The number of individual cells required will depend upon the plating requirements. Steel strip 12 is guided through cells 11 by rolls 14 and 16. Parallel plates 15 are insoluble plating anodes charged positively by DC generators, not shown in the drawing. (It will be recognized by those skilled in the art that many items such as generators, electrical connectors, controls, etc., necessary or useful to carry out the electroplating process have been omitted from the FIGURE and descriptions thereof for the sake of simplicity and clarity, since their use is well known to those skilled in the art.) The anodes 15 are typically made of lead containing small quantities of alloying materials such an antimony, tin and/or silver. Rolls 14 and contact rolls 16 are positioned so as to guide the steel strip 12 through anodes 15. Steel strip 12 is negatively charged by contact rolls 16 which are connected to the negative lead of the DC generator, so that the steel strip 12 becomes cathodic between the anodes 15 and conductor rolls 16.

When the desired thickness of chromium has been plated on the steel strip, it is then transferred to rinse tank B to rinse residual plating solution therefrom. The rinse tank B comprises a number of distinct compartments 20 which form a series of rinsing stages arranged for multi-stage counterflow rinsing. That is, rinse water is passed through successive compartments counter to the direction of movement of the strip. Walls 21 and 22 form the individual compartments 20. Different levels of rinse water are established in the individual compartments 20 by overflow pipes 32α, 32&amp;, and conduit 32 so that the first rinse stage is the most shallow with each succeeding stage being somewhat deeper, thereby establishing the countercurrent flow of rinse water. Steel strip 12 is guided into and out of individual rinsing compartments 20 by rolls 23 and is wiped by rolls 24 upon emerging from each of the rinsing stages. Upon emerging from the final rinsing stage, plated steel strip 12 passes through the last pair of wiping rolls 24 and is then sprayed with fresh water issuing from spray nozzles 25. The plated steel strip, after being sprayed with fresh water from nozzles 25, is then wiped with final wiping rolls 26 before being conveyed through passage 27 out of gas hood D and then through dryers 28. After drying, the strip can be coiled or treated in any other appropriate manner. Although the FIGURE in the foregoing description discloses four rinsing stages, three immersions and one spray, it will be appreciated that any other suitable number of stages can be employed. It will be appreciated by those skilled in the art that increasing the number of rinsing stages decreases the amount of fresh rinse water required to be added (usually by means of a spray rinse) to attain an equivalent strip cleanliness.

Plating solution from electroplating tank A and overflow rinse water from rinse tank B are passed through conduits 31 and 32, respectively, to conduit 33 which conveys the combined solutions to the concentrator C. The FIGURE shows the solutions from plating tank A flowing by gravity via overflow pipes 31α, 31&amp;, 31c, and 31d through conduit 31. The solution from rinsing tank B is shown flowing by gravity through conduit 32. The solutions are admixed in conduit 33 and then flow by gravity into the concentrator C. Furthermore, the FIGURE shows the rate of flow of solutions from tanks A and B as being controlled by the heights of the solutions in those compartments from which the solutions are being withdrawn. It will be appreciated by those skilled in the art that other means can be employed for withdrawing solution from tanks A and B and for conveying the withdrawn solutions to the concentrator C.

The FIGURE shows rinse water passing through a cation exchanger 34 before being combined with electroplating solution from tank A. The cation exchange process and the desirability of providing the same will be described in greater detail 5 hereinafter in conjunction with the more detailed description of the process.

Concentrator C may be any suitable means for effecting intimate liquid-vapor contact between the fumes obtained from tanks A and B and the process solution. In the specific em10 bodiment shown, concentrator C is preferably a conventional packed tower that is constructed of any material that is corrosion resistant to the plating solution being treated. For example, in the case of chrome plating with chromic acid, the tower j can be constructed of stainless steel or titanium or polyvinylchloride or glass lined carbon steel. The concentrator C comprises generally a lower chamber 40 forming a reservoir for combined plating solution and rinse water, a packed bed 41, a second packed bed 42, conduit 43 for passing fumes col20 lected by gas hood D into packed bed 41, damper 44 for controlling the flow of supplemental air into packed bed 41, blower 45 for exhausting gases that have passed through packed bed 41 and packed bed 42, pump 46 for conveying solution from reservoir 40 to packed bed 41 and means 47 for 25 distributing the solution uniformly over the bed. The lower chamber 40 can be constructed of any suitable material, such as carbon or graphite blocks or any other material which is resistant to chemical attack by the plating solution. The packed bed 41 is retained in position above the reservoir by screen or 30 multi-perforate plate 48 so that gases introduced via conduit 43 can readily pass upwardly therethrough and so that solution introduced into the bed 41 by distributor means 47 can flow downwardly through the bed 41 and through plate 48 into reservoir 40. In short, packed bed 41 serves to effect inti35 mate liquid-vapor contact between the gases flowing upwardly and the liquid trickling downwardly through it. Packed bed 42, which is supported on retainer screen 49 and may consist of materials similar to those of packed bed 41 or may be a blanket type packing, serves to remove residual spray en40 trained in the gases issuing from the packed bed 41. The entrained liquid so removed by packed bed 41 trickles downwardly through retainer screen 49 and ultimately back onto and through packed bed 41. Packed beds 41 and 42 can be packed with Raschig rings, Lessing rings, Berl saddles, In⁴⁵ talox saddles, Tellerettes, Pall rings, or the equivalent which are made of corrosion resistant materials and which provide a large surface area to facilitate vapor-liquid contact. In operation, fumes collected by gas hood D from plating tank A and ₅θ rinsing tank B are drawn through packed bed 41 by blower 45 and any values contained in the fumes are recovered in packed bed 41 by the scrubbing action provided by the solution passing therethrough. In addition to collecting values contained in the fiimes emanating from tanks A and B, the con55 centrator C also serves to evaporate water in an amount substantially equivalent to the amount of fresh water issuing from spray nozzles 25 and ultimately withdrawn via conduit 32. The gases drawn through packed bed 41 by blower 45 evaporate substantial amounts of water, but the volume of these gases 60 may be insufficient to evaporate water in amounts substantially equivalent to that introduced by spray nozzles 25. Therefore, an inlet 51 for supplemental air is employed to pass additional amounts of air through packed bed 41 to evaporate the desired amount of water. The amount of additional air passed 65 through packed bed 41 by inlet 51 can be controlled by damper 44. The rate of evaporation of water from the plating solution may be controlled by measuring the level of liquid in reservoir 40. If concentrator C is evaporating water from the system at a different rate from that at which water is being in70 traduced into the system via spray nozzles 25, the level of liquid in reservoir 40 will rise or fall accordingly, since tanks A and B are held at a constant volume of liquid. Accordingly, by monitoring the level of liquid in reservoir 40, the operation of concentrator C can be automatically adjusted by means well 75 known in the art to maintain the rate of evaporation substan3,661,732

6 tially equal to the rate of rinse water addition. The liquid level in reservoir 40 can also be used to help in controlling the concentration of electrolyte in the plating solution. By reducing the set point of a liquid level controller (not shown) in reservoir 40 to below the existing liquid level, additional water will 5 be evaporated which will tend to increase the electrolyte concentration. When the solution level in reservoir 40 reaches a designated minimum so that no additional net depletions of water from the system should be made, replacement chromic acid and other necessary constituents can be added to the ¹⁰system by being introduced into reservoir 40. Make-up water or a decreased evaporation rate, or a combination of the two can likewise be used to make a net addition of water to the system.

The electrolyte solution in reservoir 40 is circulated by pump 52 to tank A to replace the solution withdrawn from tank A via conduit 31. An independent-flow loop provides circulation through concentrator C of the solution from reservoir 40, by means of pump 46, thence through heat exchanger 54 -q to distributors 47, which distribute the solution over the surface of packed bed 41. The solution trickles through packed bed 41 wherein it comes into intimate gas-liquid contact with the fumes (and supplemental air) passing upwardly through packed bed 41. An amount of water, controlled as hereinbe- 25 fore described, is evaporated from the solution which is thereby concentrated to the desired degree and cooled while chemical values in the gas stream condense into the downwardly flowing solution. Heat exchanger 54 heats the solution supplied to the distributors 47 so that the solution 30 withdrawn from reservoir 40 will be at the desired temperature for introduction into tank A. In the specific embodiment described, the cooling engendered by evaporating sufficient water to maintain the water balance removes more heat than is generated in the electroplating step and additional heat 35 must be supplied to the system. The required heat is supplied by condensing steam in heat exchanger 54. The condensate or a portion thereof is passed via of line 55 to rinse sprayers 25 where it serves as the spray rinse water. A surge tank 57 is provided to receive excess condensate which may be formed dur- 40 ing periods of high heat input by heat exchanger 54, as when the electroplating operation is temporarily shutdown and it is desired to maintain the rest of the system in operation. Surge tank 57 also serves as a reservoir from which condensate is passed by pump 59 to rinse sprayers 25. 45

In general, it is seen that in accordance with the working of my invention, the electroplating solution is cooled by giving up heat of vaporization to the water evaporating in the concentrator in a single operation which simultaneously controls the water balance of the electroplating solution, recovers values from and purifies the fumes generated by the electroplating process and recycles, rather than discharging to waste, rinse water contaminated by the electroplating solution. The total amount of gases and fumes collected from the ^5 electroplating operation is supplemented by sufficient air to evaporate the required amount of water. The temperature of the electroplating bath can be controlled by adding thereto, controlled amounts of the cooled solution obtained from the gas-liquid contacting step, e.g., from reservoir 40, and if βθ necessary, passing the cooled solution in heat exchange, e.g., in heat exchanger 54 in the FIGURE, to further cool or to warm the solution as may be required in a specific case.

The plating solution, of course, is constantly being depleted of plating values, e.g., chromic acid, due to the plating reac- 65 tion; additional reagents along with other necessary or useful constituents can be added to the system at any convenient point. In the specific embodiment shown in the FIGURE, a dard bath” contains about 250 grams per liter (gpl) CrO₃ and 2.5 gpl SO₄. Even more advantageously, a silicofluoride type catalyst may be added to a standard bath in excess amounts to provide a self-regulating bath since silicofluoride catalysts display only limited solubilities and the required amount dissolves only as needed. Most advantageously, hydrofluoric acid is added in amounts to provide a chromic acid to fluoride ratio between about 50:1 and 250:1.

When the chromium deposit has reached the desired thickness, the plated steel strip passes to rinsing tank B and electroplating solution clinging thereto is rinsed from the strip. The rinsing operation is advantageously conducted on a countercurrent basis to provide more efficient rinsing and to provide an overflow rinse solution which is relatively concentrated in values carried over by the steel strip.

An advantageous specific embodiment of the present invention is the passage of the overflow rinse solution through a cation exchanger to remove trivalent chromium which is formed by the plating current from the hexavalent chromium required for plating. Trivalent chromium increases the resistivity of the electroplating solution causing the electroplating solution to be unduly heated to undesirable temperatures. An additional advantage of passing the overflow rinse solution through the cation exchanger is that any iron dissolved during the electroplating operation is also removed by the cation exchanger. Cation exchanger 34 can advantageously be a tower packed with a cation exchange resin, such as esters of sulfonic acids. A specific example is Dowes-50WX12, which is the trademark given to a sulfonic acid resin manufactured by the Dow Chemical Company. Generally, the high electrical currents used in electroplating cause certain electrolytes to undergo chemical changes to form undesirable substances which must be removed from the electroplating bath.

It will be appreciated by those skilled in the art, that other ion exchange processes can be employed. For example, liquidliquid techniques are also effective in lowering the trivalent chromium content. An example of a liquid ion exchanger is diononylnaphthalene sulfonic acid dissolved in heptane.

It will be further appreciated that various other chemical separation and/or conversion techniques can be used in conjunction with a system as described herein to purify the electrolytes or to enhance the electroplating operation without departing from the spirit and scope of this invention.

After ion exchange treatment, the overflow rinse solution can be treated directly to evaporate an amount of water substantially equivalent to the amount of water added during rinsing. Thus, the treated rinse water from the cation exchanger could be sent directly to a concentrator which evaporates water at a rate substantially equivalent to the amount of water added during spray rinsing, while employing in the concentrator a circulating solution of substantially the same composition as the electroplating bath with a portion of the circulating solution being added to the electroplating bath. The amount of circulating solution added to the electroplating bath would be substantially equivalent to the amount of plating solution carried over to the rinse tank B by the plated steel strip. However, in the preferred embodiment of the invention illustrated by the FIGURE, the ion-exchanged rinse water is advantageously combined with controlled amounts of the plating solution from tank B which combined solution is then fed to the concentrator.

For the purpose of giving those skilled in the art a better understanding of the invention and/or a better appreciation of the advantages of the invention, the following illustrative example is given:

convenient point for additions would be to the solution contained in reservoir 40. For example, in plating steel with 70 chromium, catalytic acid radicals are needed in the electroplating tank A to insure constant and soluble plating conditions. For example, sulphate ions may be employed in amount sufficient to maintain a chromate (CrO₃) to sulphate (SO₄) ratio of between about 50:1 and 250:1. For example, a “stan- 75

EXAMPLE

A carbon steel strip having a width of 42 inches and traveling at a speed of 1,500 feet per minute, enters and is immersed in a self-regulating aqueous plating bath containing about 250 gpl CrO₃, about 2.5 gpl SO₄ and 2.5 gpl of a fluoride catalyst in apparatus similar to that schematically represented in the

3,661,732

FIGURE. The plating bath is maintained at a temperature of about 110° F., and electrical energy is supplied to the anodes immersed in the bath and to the cathodic steel strip through conductor rolls. Strip is plated in this bath and emerges directly into a multiple stage rinsing tank and carries with it about three gallons per minute of the plating solution. Spray rinse water at a temperature of 150° F. is employed at a rate of 30 gallons per minute as a final rinse. Plating solution at a rate of 2,000 gallons per minute is combined with the overflow rinse water to provide a solution at a temperature of about 113° F. The combined solution is transferred to the reservoir of a packed tower, which reservoir contains 10,000 gallons of solution at a temperature of 110° F. A packed bed is sprayed with 2,000 gallons per minute of solution from reservoir 40 at a temperature of 125° F„ while 35,000 standard cubic feet per minute of fume exhaust collected by gas hoods D are drawn through the packed bed along with 30,000 standard cubic feet per minute additional air by blower 45. Gases after passing through packed bed 42 to collect any mist generated in packed bed 41 or in the operation thereof are exhausted to the atmosphere at a temperature of 125° F. Operation of concentrator C evaporates 30 gallons of water per minute which is substantially equivalent to the amount of fresh water added by spray rinsing. The temperature of the electroplating solution is maintained at a temperature of 110° F. by adding the 2,000 gallons per minute of the solution emerging from packing 41 into reservoir 40 at a temperature of 107° F. to the 2,000 gallons per minute of plating solution arriving at reservoir 40 at a temperature of 113° F.

The foregoing example shows that by practice of the process in accordance with the present invention, values contained in fumes emanating from both the plating tank A and rinse tank B and values contained in rinse water overflowing rinse tank B can be recovered, the temperature of the plating bath can be maintained, and recycled, all in one simple operation.

Although the present invention has largely been described in conjunction with preferred embodiments, it is to be understood that modification and variations may be resorted to without departing from the spirit and scope of the invention, as those skilled in the art will readily understand. Such modifications and variations are considered to be within the purview and scope of the invention.