We associate cleaning parts, clarifying process solutions, and purification with the process of filtration.
Stephen RudyMany of us rise daily to enjoy our first cup of fresh-brewed coffee, which has passed through a filter. Perhaps a filter is running to condition the air we breathe. We start the car engine and efficiently operate oil and gas filters to make the trip to a destination possible.
These are a few examples, and many more exemplify how essential filtration is to our daily lives. For our metal finishing concern, filtration is a process that separates particles and fluid from a mixture using a filter medium.
Let us consider some general facts regarding the importance of filtration in metal finishing. Proper filtration influences optimum performance and quality part processing.
Water conservation, minimizing drag-out losses, closed looping, and easing the burden on waste treatment all place a critical burden on plating solutions. This, in turn, contributes to introducing contaminants from various process waters. Recycled or reclaimed waters may contain organic soils and dissolved metals as sources of inadvertent contamination. These are some contributing factors that make filtration so important. It is very common to filter plating solutions. It is especially critical to continually introduce fresh solution (catholyte) to parts being plated and avoid the occurrence of deposit pitting.
Filtration should be conducted correctly and maintained on a working schedule. Routine maintenance of the plating bath should include analysis, hull cell, and continuous filtration. Minimizing contaminants in a plating bath greatly affects meeting deposit specifications, salt spray, wear resistance tests, and anticipated field service life of finished parts. In this way, the quality of the deposit is maintained.
Some general considerations for filtering plating and other solutions are listed.
Powder Carbon and Granular Carbon.Carbon. Granular and powdered carbon are equally effective. Both forms of carbon are used for continuous filtration. Powdered carbon accelerates purification due to its greater surface-to-volume ratio, which is why it is preferred for batch purification treatments. Granular carbon is much less dusty as an alternative to powdered carbon. Commercially activated carbon grades that have been chemically treated with special agents to improve adsorption activity and sequester some heavy metals are available. 
Diatomaceous Earth and Wood Pulp.Diatomaceous Earth. These are generally forms of clay that can be used as filtering media. Recommendations apply to nickel, acid zinc, and acid copper plating solutions. They are commonly referred to as filter aids. In some purification applications, particular organic contaminants may not be as susceptible to adsorption to carbon. These filter aids offer additional binding sites to improve the removal of such tougher organic contaminants. They also, in combination with carbon, help to remove very fine particulates. Wood pulp, also called alpha-cellulose, is another filter aid recommended for alkaline and cyanide-based plating solutions.- Filter Cartridges. These are normally pleated, having different pores. It benefits the plater by removing particles down to one micron in diameter. This is especially critical to remove nickel particles in electroless nickel systems, thereby preventing plate out, achieving critical solution movement, and consistent bath temperature. Removing particles improves solution clarity. Deposit roughness and pitting are avoided. The unit may be a simple, single cartridge filter. It could also be a multi-chambered system with a carbon-containing bag or cartridge inserted. Pleated or wound sleeves and discs may also be used. Specialized pads and sheets are charged into filter canisters and tank side filters to continually remove accumulated oils and grease from operating cleaners. Filtering cleaners not only extend solution service life but also help significantly to achieve satisfactory parts cleaning. Filtration in the final waste treatment steps removes any trace floating floc that may flow in the discharge to the sewer pipe. It is an excellent application for final water polishing in waste treatment. Carbon Cartridges Filtration of Cleaners
Multiple Disk Filter with Slurry Tank.Multiple Disks or Sheets. A large, effective surface area can be achieved by precoating multiple disks, sheets, or sleeves with alternating layers of filter aid or diatomaceous earth and carbon. The plating solution is continually pumped through the aggregate layers to maximize particle removal and adsorption of organic contaminants. Disks can be precoated with the filter aid of choice to approximately 2 ounces per ft2 of disk or filter surface area. Usually, one half to three ounces of carbon per ft2 of filter is sufficient. Filter disks can be precoated from a slurry tank in the progression of filter aid followed by carbon, with the balance of filter aid. Equipment manufacturers offer specific operating instructions for their specialized filtration units to obtain maximum operating performance. Avoiding too aggressive carbon filtration is important, as critical bath additives, such as organic wetting agents, would be significantly depleted.- Balancing Activity. Preferred solution turnover is factored with required purification to obtain the optimum balance and effectiveness. A rule of thumb (general purposes) for nickel and other plating solutions is continuous 1-2 solution turnovers through the carbon-packed filter. The requirement to replace carbon in the disk chambers or precoat filter may be determined by monitoring the rise in unit pressure of the filter versus the decrease in return flow from the filter discharge. Filter manufacturers install a pressure gauge with servicing recommendations based on measured pressure readings. Don’t ignore these visual maintenance aids.
Plating baths will always be exposed to organic contamination in continuous operation. This may, for example, occur by drag in of cleaners, oils & grease, thermal & electrolytic breakdown of bath additives, and the effects of closed looping. Effective filtration will minimize drag in organic contaminants and in-tank breakdown products, in conjunction with quality post-treatment rinsing. An effective filtration system incorporates the following progression:
Pump plating solution from tank through a filter chamber → Continue through a carbon chamber → Return to the plating tank.
This has proven most effective since carbon is an excellent medium for removing specific contaminants. Over the years, hard-to-handle powdered carbon has been supplanted by granular material in most filtration applications. This does not dispel the fact that powdered carbon provides a tremendous surface-to-volume ratio for effective purification. The filtration system may employ carbon-filled cartridges for baths up to a few hundred gallons. Larger precoated, enclosed disk units are more effective on baths approaching 1,000 gallons and above.
An optimum granular, activated carbon type for most plating bath filtration and purification purposes consists of +10 mesh, 5% maximum, and -40 mesh, 0.5% maximum.
The material's total surface area is between 1,000 and 1,100 square meters/gram.
Precoat media can also be used when coating plates or disks with carbon. Diatomaceous earth is recommended for common plating solutions such as nickel, acid copper, tin, and zinc. Alpha cellulose is recommended for the alkaline and cyanide plating baths.
It has been found that high solution flow rates using coarse or granular carbon maximize the soil holding capacity. This is because a small quantity of fine particles are retained in the media. Upon each subsequent solution pass, more fine particles are held, forming a precoat that seals the coarse surface and increases the capacity to remove and hold fine particles.
Carbon cartridges are a convenient and clean system that maintains adequate solution purification. The cartridges are commercially available in lengths 10 to 30 inches, fitting most filter chambers. Carbon is prepacked with a binder, and the cylinder is sealed. Maintenance consists of simply replacing the used cartridge with a fresh one. There is no mess or personal handling of carbon.
Recently, reports based on critical research have confirmed that activated carbon is beneficial in helping remove PFOS / PFAS from contaminated waters. Filtration is essential to operating metal finishing systems. Various equipment types achieve the desired filtration and purification requirements. As long as metal finishing utilizes liquid process systems, filtration, with the application of carbon, will help maintain high purity processes, developing the desired characteristics of finished parts.
Stephen F. Rudy, CEF, is president of Chem Analytic and has written extensively about the finishing industry. Visit www.chemanalytic.com or call him at 917-604-5001.
