GB2313605A - Application of ultrasonic wave energy to electrolytic cell to reduce fume emission - Google Patents

Abstract

The emission of fumes from an electroplating bath or other electrolytic cell is reduced by the application of ultrasonic wave energy to a zone immediately above the electrolyte, thereby disrupting bubbles gathering on the surface of the electrolyte before they are allowed to increase significantly in size, thereby reducing the energy imparted to droplets formed when the bubbles burst. The ultrasonic wave energy is produced by ultrasonic transducers which are configured in the vertical plane in one or more compartments 15 provided at the external side walls 11 of a plating bath 10.

Description

Title: "Electroplating and like processes" Description of the invention This invention relates to electroplating and like processes in which gas bubbles are generated in a liquid bath in which electrodes are subjected to an electrochemical process.

In plating of hard chrome, for example, a solution containing hexavelant chrome salts in sulphuric acid is used with a lead anode and steel cathode (the workpiece). In decorative chrome plating, a solution containing trivalent chromium salts is used often with a carbon anode and a steel or nickel plated steel cathode.

The purpose is to deposit a coating of metallic chromium on the workpiece in order to enhance physical characteristics such as wear resistance, corrosion and appearance.

Electrochemical processes produce gases which are emitted at the electrodes. In electroplating hexavelant chromium, hydrogen is released at the cathode and forms bubbles that become detached from the cathode and rise to the surface of the bath liquid. The bubbles accumulate on the surface of the liquid in the bath and grow in size as more gases are emitted until they burst and generate a mist containing chromium or other salts. The liberation of this mist, in the case of chromium plating baths, has been associated with deleterious health effects in process operators such as cancer of the mouth, nose and throat. In order to control such risks, it is good practice to install air extraction equipment above the plating bath so that the emitted mists are ducted away from the workplace. Additionally, the use of fume suppressants to the bath is also a recommended practice.

The application of liquid-based ultrasound to plating bath solutions has been shown to be beneficial in improving the plating process. For example, Namgoong and Chun (Thin Solid Films v120, 152, 1984) arranged ultrasonic transducers beneath the plating bath so that the sound waves passed through the liquid medium, and showed that ultrasound accelerated metal deposition, microhardness and microstructure of the chromium deposit, at least in part by encouraging release of bubbles from the cathode whilst still very small, i.e. as micro-bubbles.

The application of sound waves to a liquid medium generates a series of rarefaction and compression waves which results in, particularly at ultrasonic frequencies, the formation of cavitation bubbles. The collapse of these bubbles in succeeding compression cycles generates exceedingly high energies equivalent to local temperatures around 5000 OC and pressure around 1000 atmospheres.

The application of such energy to the liquid phase in an electroplating bath can be very beneficial for the reasons given above.

In accordance with the present invention ultrasonic waves are applied both to the liquid in the plating bath and to the air immediately above the liquid in the plating bath. Not only does this achieve the benefits disclosed by Namgoong and Chun, but additionally major improvements in the control of chrome mists and the like are achievable. Bubbles collecting on the surface of the liquid are disrupted by the air-borne soundwaves before growing in size, as is the case in the absence of air-borne soundwaves, with consequent reduction in mist generation to a very significant extent.

It is standard practice in chrome plating installations to incorporate an air sparge device within the bath. Compressed air is injected through nozzles or diffusers located in a grid at the bottom of the bath. The purpose is to encourage mixing and thereby accelerate the electrochemical processes. However, as El Sharif et al (Proceedings of AEST Annual Technical Conference 1993 p451) has shown, the configuration and control of air flow rate is important if reproducible plating quality is to be attained.

According to the invention excellent mixing and high plating quality can be achieved in the complete absence of air sparge by using ultrasound as described below.

The present invention firstly resides in a method of depositing metal on a workpiece by electroplating from an aqueous solution containing a salt of such metal, and applying ultrasonic wave energy to the solution and to a zone immediately above the solution in an electroplating bath.

The invention further resides in an electroplating bath comprising a tank for receiving an aqueous solution of a salt of an appropriate metal for electroplating, and means for applying ultrasonic wave energy to the liquid within the tank from at least one side thereof, and means for applying ultrasonic wave energy to a zone immediately above the solution in the tank.

In a preferred arrangement ultrasonic transducers are arranged at one or more lateral sides of the tank, externally or internally thereof, so as to extend from a position substantially adjacent to the bottom of the tank to a position above the working level of liquid within the tank. The transducers may be housed in an enclosure which is disposed within a compartment outside the tank, the compartment containing a liquid wave transmission medium, in contact with the tank, or contained in an additional tank or tanks submerged within the plating tank.

The preferred configuration of the plating bath is shown in Figure 1 and, diagrammatically in more detail, in Figure 2.

As shown in Figure 1, the plating bath comprises a rectangular tank 10 with side walls 11 and bottom wall 12. In use, the tank is filled, up to a normal working level 13, with a plating solution 14.

A plurality of electromechanical ultrasonic transducers (not shown) are configured in the vertical plane in one or more compartments 15 provided at the external side walls 11 of the tank 10. The transducers are housed in box-like containers 16 located in the respective compartments 15 in such a way that the upper end 18 is above the level 13 of the plating solution 14 in the tank 10. The transducer boxes 16 are submerged in a liquid, typically water, to provide acoustic coupling with the adjacent side wall 11 of the tank, and thence to the plating solution within the tank.

The location of the transducer box above the level of the plating liquid permits the production of air-borne (as well as liquid borne) ultrasound. The passage of sound waves across the surface of the plating solution explodes gas bubbles as they reach the surface 13 before they have grown in size. This is effective in reducing the generation of chrome mists which are a potential hazard to plant operators.

Experimental Examples In the following examples, steel cathodes were used. These were subjected to standard procedure for cleaning and surface preparation before each run, consisting of: (a) dipping into a warm solution of soap for five minutes; (b) dipping into anodic cleaning solution for 5 minutes with 15 amps current and 2.6. volts; (c) washing in clean cold water jet; (d) dipping in dilute hydrochloric acid for 5 minutes; (e) washing in clean cold water jet; (f) placing in plating bath with reverse polarity and run at 15 amps per dm for 2 minutes.

A filter was installed at a fixed position some 76 mm above the surface of the plating solution and air was abstracted through it at a controlled rate in accordance with Health & Safety Executive Standard MDHS/52/2 (ISBN 0 717 6034166). At the conclusion of the test, the filter paper was removed, digested in concentrated nitric acid and made up to a standard volume. This solution was analysed by Inductively Coupled Plasma Spectrometery and by Atomic Absorption Spectrophotometry and the results expressed as mg Cr per cubic metre of air abstracted.

EXAMPLE 1 A cathode prepared in this way was immersed in a plating bath containing the equivalent of 250 g/litre of CrO3 and 2.50 g/litre of HOST4 at a temperature of 500C together with a lead anode and plated using a current density of 28 amps per dm2 and a voltage of 3.2 volts for a duration of 1 hour without the application of ultrasound. The plating efficiency was found to be 18.8%, the hardness (as determined by the readings with a Vickers Micro Hardness Testing Machine) was found to be 951 and the mist concentration was 900 mg Cr per cubic metre of air abstracted.

EXAMPLE 2 Experiment 1 was repeated with ultrasound (at a frequency of 28 kilocycles per second) applied to the base of the tank, but not to the air above the solution, and the plating efficiency was found to be 21.1% and the hardness 1035 VHN. The mist concentration was 212 mg Cr per cubic metre of air abstracted.

EXAMPLE 3 Experiment 1 was repeated with ultrasound applied to the sides of the plating bath in the manner shown in Figure 1 in such a way as to sonicate both the liquid and the air above the bath. The plating efficiency was found to be 21.5% and the hardness 1045 VHN, i.e. substantially similar to the values achieved in Example 2, but the mist concentration was 6.41 mg Cr per cubic metre of air abstracted, representing a very significant reduction as compared with Example 2.

The application of ultrasound to enhance liquid phase reactions is well known. The cited literature describes improvements obtained from the sonication of chrome plating solutions by placing the source of the ultrasound (a transducer) beneath the workpiece so that sound waves are generated vertically and pass through liquid phases before contacting the cathode.

In accordance with this invention these benefits are retained by sonicating the cathode from transducers placed vertically so that the sound waves pass horizontally through liquid before contacting the cathode, and additional benefits are obtained from the arrangement of the transducers in such a way that they also generate air-borne sound waves which are arranged to travel horizontally close to the surface of the plating bath. Moreover, the plating process no longer requires the inclusion of an air sparge system, since ultrasound supplies a much superior form of mixing. The conventional air sparging process is difficult to control, results in heat loss from the system and accelerates the carryover of chromium mist particles thereby increasing health risks to operators.

Where air extraction systems are used to control this risk, the air needs to be scrubbed prior to discharge and this produces a waste liquor containing chromium salts which have to be precipitated from the solution prior to discharge. This produces a sludge residue which is a hazardous waste and needs to be collected and disposed of in accordance with strict regulations. In short, air sparging contributes to a chemical loss from the system which the use of ultrasound avoids.

Whilst the invention is particularly applicable to chromium plating processes, it will be appreciated that similar problems may arise in the plating of other metals or in other process that give rise to the generation of gases evolved as bubbles at the surface of the treatment liquid in a tank, and that the invention may be applied with similar advantage to such other processes, and accordingly the invention further resides in a method of electrolysis in which bubbles are formed at one or more electrodes immersed in an electrolyte, wherein ultrasonic wave energy is applied to a zone immediately above the electrolyte to disrupt bubbles gathering on the surface of the electrolyte, and in an electrolytic cell comprising a tank for receiving an electrolyte, and means for applying ultrasonic wave energy to a zone immediately above the electrolyte in the cell.

The features disclosed in the foregoing description, or the accompanying drawing, expressed in their specific forms or in terms of a means for performing the disclosed function, or a method or process for attaining the disclosed result, may, separately or in any combination of such features, be utilised for realising the invention in diverse forms thereof.

Claims (12)

CLAIMS:

1. A method of depositing metal on a workpiece by electroplating from an aqueous solution containing a salt of such metal, and applying ultrasonic wave energy to the solution and to a zone immediately above the solution in an electroplating bath.

2. An electroplating bath comprising a tank for receiving an aqueous solution of a salt of an appropriate metal for electroplating, and means for applying ultrasonic wave energy to the liquid within the tank from at least one side thereof, and means for applying ultrasonic wave energy to a zone immediately above the solution in the tank.

3. An electroplating bath according to Claim 2 wherein said means for applying ultrasonic wave energy comprising ultrasonic transducers arrange at one or more lateral sides of the tank so as to extend from a position substantially adjacent to the bottom of the tank to a position above the working level of liquid within the tank.

4. An electroplating bath according to Claim 3 wherein the transducers are housed in an enclosure which is disposed within a compartment outside the tank and in contact with the tank, the compartment containing a liquid wave transmission medium.

5. An electroplating bath according to Claim 3 wherein the transducers are housed in an enclosure contained in an additional tank or tanks submerged within the plating tank.

6. A method of electrolysis in which bubbles are formed at one or more electrodes immersed in an electrolyte, wherein ultrasonic wave energy is applied to a zone immediately above the electrolyte to disrupt bubbles gathering on the surface of the electrolyte.

7. A method according to Claim 6 wherein ultrasonic wave energy is additionally applied to the electrolyte.

8. An electrolytic cell comprising a tank for receiving an electrolyte and means for applying ultrasonic wave energy to a zone immediately above the electrolyte in the cell.

9. An electrolytic cell according to Claim 8 wherein means are additionally provided for applying ultrasonic wave energy to the electrolyte within the cell.

10. A method of reducing the emission of fumes from the surface of the electrolyte in an electrolytic cell characterised by applying ultrasonic wave energy to a zone immediately above the electrolyte to disrupt bubbles gathering on the surface of the electrolyte.

11. An electrolytic cell or electroplating bath substantially as hereinbefore described with reference to and as shown in the accompanying drawings.

12. Any novel feature or novel combination of features disclosed herein and/or as shown in the accompanying drawing.