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Figure 2: Schematic Diagram (side view) of a rinse system comprising dragin and dragout tanks, followed by counterflow rinsing. The assumption is that nickel plate is a final layer.
Nickel losses refer to any nickel, in whatever form, that leaves the plating department by any route other than as a deposit on a component. Nickel that is reused or sold as scrap is not considered to be "lost;" however, a reduction in the quantity of nickel taking this route should still be considered a savings. Loss routes include effluent outflow, atmospheric and landfill.
Nickel losses in liquid form are the most important, since they are the most common. They represent the greatest financial loss and have the greatest impact on the environment. A major percentage of nickel electrolyte lost will eventually end up in a landfill, be precipitated in the treatment plant and filtered or settled out before the plant effluent becomes outflow from the site.
Preventable losses represent the opportunity to achieve the greatest financial return, since the savings may be frequently made with little or no capital expenditure. The primary requirement is a strong and consistent management commitment to good housekeeping practices.
An electrolyte has two costs: 1) the initial cost of labor, chemicals and factory resources to make up the bath and 2) the additional cost of treatment and disposal.
A normal nickel plating bath, with adequate anode area, will remain more or less in balance, i.e., the nickel deposited will be replaced by anode dissolution. Any additions of nickel salts may be considered as replacement of electrolyte losses.
Calculating the annual amount spent on nickel salts will indicate the savings available. The calculation should also include the cost of labor and plant resources in making the additions and an estimate of disposal costs. Reclamation of lost electrolyte is a poor substitute for prevention of loss, since this requires labor and plant resources.
Dragout losses may be defined as any electrolyte that is removed from the plating bath by components or fixtures. It may be subdivided into that electrolyte that clings to the surface of the components and fixtures (surface dragout) and that removed by the inability of the electrolyte to drain away (carryover).
Surface dragout is relatively small, generally accepted to be 0.1 liter/m2 for fixtured, flat components, with an adequate drainage period. Carryover from cup-shaped components could be as much as one-hundred times that quantity.
Fixtures must be given an adequate drain period while still suspended over the process tank. Obviously this will be greater for a barrel, which should be rotated while suspended over the plating bath. Barrel design will also help drainage. The perforations should be as large as is practical with regard to component size. This will also help the deposition process by allowing good electrolyte movement. Allowing a barrel to drain properly over the nickel tank will pay several times over for the small increase in process time.
Consider the initial overall cost of making up two or three liters of nickel solution, add the cost of the effluent treatment chemicals required to dispose of it and the additional cost of precipitate removal from the site. Multiply that by the number of barrels processed per annum. Obviously this cost will vary from plant to plant, but the result will always be much higher than expected.
While fixtured work also benefits from adequate drainage, it is possible for workpieces to become passive if allowed to flash dry after immersion in a hot nickel bath. One possibility might be mist sprays that rinse the emerging fixtures while still over the plating bath.
Carry over. In an ideal world, the components will have been designed for plating, promoting an even deposit thickness and adequate drainage, unfortunately, this is rarely so. In an "in-house" finishing shop, it is essential that the person responsible for plating be given the opportunity to comment on a new design at the design stage so that there is minimal carry over.
Other operational losses. Electrolyte is frequently, but less noticeably, lost to effluent by planned maintenance operations in the plant. For example:
- Electrolyte is pumped to storage for routine maintenance. Sufficient drainage time for anode bags and air or solution pipework is imperative. It is essential that all of the electrolyte is removed; the last few inches of solution could represent more nickel lost at this time than during a whole week of plating. New tanks should have a sloping bottom to box sumps so that drainage is complete.
- When a filter is taken off line for servicing all the electrolyte must be removed and the filter washed through before servicing. If not, a considerable quantity will again be lostâ€”either as effluent or as solid waste. See Figure 1 for a diagram of a system that will allow washing of the filter system before maintenance.
- When organic material contaminates the electrolyte, the necessary carbon or carbon + permanganate treatment could result in the loss of up to 20% of the electrolyte volume.
- Finished components exhibit surface roughness caused by dissolving dropped work or a damaged anode bag. The operation of pumping out and filtering back after cleaning will cost dearly.
Operational losses are best avoided by good housekeeping and training. Operatives may not be aware of the cost, both financial and environmental, of their actions. While a great deal of thought may be applied to the design of a plant as a production unit, there is less attention paid to the peripheral maintenance activities. Good process engineering design and good housekeeping will ensure that as little electrolyte as possible is lost during maintenance.
Accidental losses may include the following:
- Tanks overflow because a hose was left unattended.
- Electrolyte sucked into the extraction ducting because the level was too high.
- Pumps and filters leak because of poor reassembly or seal failure.
- Drain valves left open.
- Heating or cooling coils leak because they were made of an inappropriate material.
- Plastic tanks melt because a heater control failed.
- A barrel lid comes loose and dumps the whole load of components into the bath.
- Incorrect fixturing of components causes them to fall into electrolyte.
- An electrical short circuit causes recti- fier fire.
- Electrolytes contaminated because the line was operated when the rinse tanks were empty or not running.
- Electrolytes contaminated by badly maintained fixtures, damaged anode bags or filter media.
- Plant operated with process tanks at incorrect temperatures.
Incorrect quantities of chemical makeup added or wrong chemicals added.
Water is an expensive commodity that you pay for on the way in and on the way out. The quantity of water that must be used will vary with every plant, dependant on the nature of the operation. Frequently, excess water is used to dilute the final outflow. This is a false economy.
The following suggestions must be considered in the light of operational needs and existing plant design, and the most suitable options selected. The first consideration is whether or not the nickel layer is to have a final topcoat. If it does, then one of the primary considerations is to maintain a chemically active surface to promote adhesion of the next deposit. This may preclude the adoption of some suggestions because they may cause passivity of the surface.
Static rinses. The most obvious concept of a static rinse is that the nickel electrolyte from it will be fed back to the plating bath in one way or another. The simplest scheme is to use the rinse to replace evaporation losses, although this may lead to an increase in metal concentration.
If there is space, water in the static rinse should circulate back to the rinse prior to nickel plating. Fixtures and components will drag in a dilute electrolyte to balance the dragout. If the dragout and dragin rinses are joined by a low volume circulation system, the dragin will increase in nickel content and help maintain equilibrium. However, dilute nickel solutions have been known to cause passivity, particularly when stray DC current is possible.
Agitating static rinses is frequently neglected, but the improvement in rinsing (and reduction in nickel carried over), by removing the boundary layer of electrolyte, is considerable. Nickel tanks that run at low temperatures may not evaporate sufficiently for the dragout to be fed back. In this case, there are several methods of concentrating the dragout before return.
If, for special process considerations, it is not acceptable to return dragout material to the process tank, then the dragout may be treated to remove the nickel or concentrated for disposal or batch treatment.
Running rinses that follow nickel plating should operate as countercurrent, including the final hot rinse, if used. There is no reason why this hot water should not be fed back to previous rinses. It will increase the temperature of the components and reduce the thermal load on the final rinse heating system. Any additional water input to the final cold rinse should be controlled by a conductivity meter and all the rinses should be agitated for best efficiency. Two cold rinses and a final hot rinse, operated in this manner, will use a surprisingly small quantity of fresh water, while maintaining an excellent standard of rinsing.
Losses to Atmosphere
It is unlikely that a cold, still, nickel plating bath would generate any detectable nickelbearing mist. Given that deposition efficiency of a nickel electrolyte is about 9698%, (and therefore there is very little gassing from electrolysis), the application of DC current for deposition is unlikely to increase the presence of nickel in the immediate atmosphere by any significant quantity.
Add a gently rotating barrel to the scene, and there would be no significant change. It is questionable whether a whole department of cold, Watts nickel, barrel lines would create an appreciable quantity of nickel in the atmosphere. So what creates the problem?
Raise the solution temperature to 60Â°C and molecular activity increases significantly. In this situation, there could be nickel aerosol in the immediate vicinity of the electrolyte surface. Add the violent air agitation found in many plating shops, and the likelihood of nickel in the atmosphere is certain. Turn on the excessive extraction, and you have a scenario for substantial nickel losses to the atmosphere and as nickel salts from the aerosol mist, as they rapidly crystallize in the extraction ducting. Include a fume scrubber to remove nickel from the extracted air, and we now have a worstcase scenario. Nickel is lost to atmosphere, to effluent (from the extract scrubber) and as a solid (when the salts have to be chiseled out of blocked extraction ducting).
Electrolyte heating. There is no doubt that both the rate and the quality of nickel deposition benefit from the use of a warm electrolyte. The cost of bringing a bath to temperature and then maintaining that temperature is high. Savings can be made by determining the lowest temperature at which the operation is efficient and using a thermostatic control to accurately maintain that temperature. (An additional overtemperature cutout is very inexpensive insurance.) By reducing the bath temperature to an operational minimum, you also reduce molecular activity to an operational minimum, preventing unnecessary aerosol mist over the electrolyte and requiring less work to remove it.
Electrolyte agitation. Agitation is necessary to present fresh electrolyte to the anode and cathode surfaces and maintain the efficiency of the process. It also prevents the growth of gas bubbles at the cathode face that can cause "gas pitting" and subsequent rejects. This only applies to rack-plated components, since the tumbling action of barrel plating renders any other method of electrolyte movement unnecessary. Movement of the cathode (workpiece) through the electrolyte will often suffice for the cathode, but does not have the benefit of providing fresh electrolyte at the anode face, resulting in undesirable pH changes in that area.
Traditionally, airflow has been the popular method of agitating a nickel plating bath. Plant operators seem to think that it is better to have as much agitation as possible, as long as it doesnâ€™t actually remove the components from the fixtures!
There are several disadvantages to this method, one being the cost and quality of compressed air. General factory compressed air supplies are rarely suitable for use in a plating bath because many supplies are intended to operate machinery and have oil included for machine lubrication.
If there is an oil separator in the agitation air supply line that ever needs to be emptied, there is a real danger of organic contamination. If there is not an oil separator in the line, it would be sensible to fit one, if only to check the air quality.
Oil in a nickel electrolyte will give dark deposits in the lower current density areas, will often adversely affect the organic additions to a bright nickel bath, will increase the need for carbon treatments and, will result in considerable losses of nickel salts to effluent and landfill. Carbon treatments will also remove expensive organic additives along with the oil contamination.
A dedicated, low-pressure, dry-air supply is an expensive proposition, but the cost may be offset by the reduction in rejects and reduced electrolyte maintenance. The cost of air agitation and its impact upon air quality in the immediate vicinity of the bath may convince one that airflow agitation is not the best method.
Alternatives. There can be no immediate change to a plant that has air agitation. The best that can be done is to ensure that the air is as clean as possible, and that the agitation pipework is in good order, capable of supplying air in the correct volume and in the required places.
A replacement for air agitation is a system that uses the existing electrolyte returned from filtration and passes through piping to a series of eductors. There are no gases involved, and the electrolyte movement, while generally more than sufficient, is less likely to create additional aerosol effect.
Inevitable losses. If, having completed all of the plant improvements designed to reduce aerosol mist, the levels of nickel detected in the immediate vicinity of the plating bath are still out of compliance, the fume must be extracted. The extract must also be suitable for discharge to atmosphere.
Any warm plant air removed must be replaced with cold air from outside, which must then be heated. Excessive airflow across the surface of the electrolyte may actually reduce pressure locally and promote the release of aerosol, which defeats the objective. The aim is to create sufficient air movement across the surface of the electrolyte to remove any mist.
If the air volume is considerable, (for a long tank), or should the width of the tank be greater than three feet, it may be worth considering the provision of a low pressure air input at the opposite side of the tank to the extract, i.e., a pushpull system.
A welldesigned extraction system, together with a welldesigned agitation system, may make the use and expense of a fume scrubber unnecessary. The environmental impact upon the surrounding area will be reduced, as will losses.
Additional options. There is a possible addition to this system, depending on the type of plant in use and the components being processed. A cover for the tank may allow extraction to be reduced considerably. Obviously, if the process times are short, and the lid must be frequently lifted, then there is no point in having it. Where covers are used it is important to ensure that the enclosed copper busbars are not attacked by fume, allowing copper contamination of the nickel electrolyte.
This article puts forward some general process engineering recommendations, in the hope that they will prompt more plantspecific ideas from the people who are responsible for the efficient operation of nickel plating facility. Few suggestions are original, although some are the result of hardwon experience.
The intention has been to indicate that plant improvements that will reduce losses to the environment do not always have to cost money. Indeed, they can be demonstrated to save money on many occasions; after all, the materials that are being lost to the environment have already been purchased, and their disposal will carry an additional cost burden. It is essential that labor and plant utilities costs must also be included in the considerations; plant maintenance is frequently a premiumtime activity. Avoid lossesâ€” protect your profitability and the environment.
Paper reprinted with permission from the Nickel Institute, Toronto, Ontario, Canada, www.nickelinstitute.org.blog comments powered by Disqus