Recovery/Recycling Methods for Platers
Because of increasing costs for disposal of wastewater contaminants, and the public and political pressure on industry to reduce toxic chemical emissions into the environment, those electroplaters and metal finishers who respond to this challenge by seriously evaluating and implementing recycling and recovery opportunities will have a great competitive edge into the 21st century.
This article attempts to explore the major technologies in use and commercially available to the electroplater and metal finisher, although there is not enough space to cover all available and emerging technologies.
First Things First
Electroplaters and metal finishers will agree that the most important process material both in terms of quantity and quality is water. Many a recovery recycling technology has failed because of the lack of attention paid to the incoming water supply. Since many recovery technologies recover the bath process dragout from the rinses, the contaminants found in well water and surface waters can build up to sufficient levels that they could cause a failure of bath chemistry, resulting in the disposal and makeup of expensive process solutions.
Before considering the implementation of recovery methods, metal finishers and electroplaters must investigate the quality of their incoming water. Water to be used for bath and recovery rinse makeup may have to be treated to a very high quality.
The two most common technologies used for treatment of tap water are ion exchange and reverse osmosis. If one is a high water user and the incoming water has total dissolved solids over 500 ppm, it may be more economically advantageous to use reverse osmosis alone or as a "roughing" filter in front of ion exchange. How these technologies work is discussed later.
Figure 1 shows the common process flow schematics when using ion exchange and reverse osmosis for incoming water.
Another key action when preparing for recovery and recycling is to determine the least possible rinse water flow rate that can be achieved without adversely affecting product quality (For how to calculate this, see Kushner, first reference under Resource Materials at end of this article).
The most effective way of reducing wastewater flow is to convert parallel rinses to counterflow rinses, and, if at all possible, add additional counterflow rinses. If counter-flow rinsing cannot be made by gravity, this can be easily overcome by using pumps or air lifts. Figure 2 shows the difference between parallel rinsing and counterflow rinsing. Parallel rinsing will use roughly "N" times the amount of water that a counterflow rinsing arrangement uses, where "N" equals the number of rinse tanks.
In order to maximize the cost effectiveness and efficiency of any recycling technology, all steps must be taken to reduce the dragout of process solutions into succeeding rinse tanks. Techniques that can be used to reduce dragout include:
There are a number of technologies that metal finishers and electroplaters can utilize. In fact, through a combination of technologies, it is possible that a facility can accomplish a "zero" wastewater discharge to sewer or stream, and generate only a small volume of concentrated liquid and/or solid waste.
By utilizing the following technologies, metal finishers can practice cost-effective and environmentally friendly resource recovery in several ways:
Following are some technologies that have been proven effective.
Evaporation is the most common, simplest, and, in most cases, cost-effective form of recovery. Evaporation can provide total or partial recovery.
The most common use is the "natural" evaporation from the process tank. As shown in Figure 3, dragout solution recovered in the following rinses is placed back into the process tank at a rate equal to evaporation. If the evaporation is sufficiently high, and there is room to add a number of recovery rinse tanks, a closed-loop system can be implemented. Even in an open loop system, recovery rate of over 90 pct can be obtained. In fact, there are open-loop recovery systems where recovery is high enough to allow the rinse water to be discharged to public sewers without further treatment.
In evaluating the effectiveness of evaporative recovery, we use a similar equation as with counterflow rinsing discussed earlier. Instead of a rinse ratio, a recycle ratio (R) is used.
R = E/D where E is the evaporation rate (gal/hr) and D is the dragout rate (gal/hr).
In order to determine the constituent concentration in the Nth recovery tank, the following equation is used:
Cn = Cp/(1+R+R2...+Rn) recovery rinses.
Through trial and error, these two equations can be used to estimate the amount of evaporation needed.
The following equation is used to estimate percent recovery:
[1 - (1/1+R+R2...Rn]) × 100 pct = percent recovery.
For low-temperature baths, a finisher may want to investigate the use of a dragin/dragout system where a workpiece drags the recovered solution back into the process tank (Figure 4). By using a dragin/dragout system, the recycle ratio increases since the dragout rate is added to the evaporation rate in the above equation.
Atmospheric and Vacuum Evaporators. If "natural" evaporation is insufficient to provide recovery desired, finishers can then turn to atmospheric and vacuum evaporators.
Atmospheric evaporators use wet surfaces, forced air, and heated solution to cause evaporation. Figure 5 illustrates an atmospheric evaporator on a closed-loop system. Atmospheric evaporators are also used extensively on open-loop systems. In addition, if process solution temperature is insufficient for atmospheric evaporation, the atmospheric evaporator could be attached to an off-line heated tank. In this scenario, the counterflow from the first rinse flows to this off-line heated tank. Solution in this tank is recirculated through the atmospheric evaporator to the desired concentration, and transferred to the process tank periodically.
Atmospheric evaporators are used on a wide variety of process solutions including cleaners, nickel plating, chromium plating, cyanide plating, and zinc plating.
While the capital costs of atmospheric evaporating systems are relatively low, they do have high operating costs for energy. It takes approximately 9,000 Btus to evaporate one gallon of water.
Vacuum evaporation lowers atmospheric pressure, thus reducing water's boiling point. In the system shown in Figure 6, a vacuum pump draws rinse water that contains dragged-out process chemicals from selected rinse tanks through the system. One of the drawbacks for vacuum evaporation is its high capital and operating costs. However, it has found a niche in evaporating process solutions that have heat sensitive compounds.
Ion exchange technology has become very popular in the metal finishing industry. Due to major advances in resin development and technology, ion exchange is able to process a wide variety of chemicals and even recover selective chemicals.
In the ion exchange process (Figure 7), columns are packed with resin beads which provide a large surface area for the cation and anion sites. Cationic resins exchange hydrogen ions (H+) for positive charged ions such as nickel, copper, sodium, and cadmium. Anionic resins exchange hydroxyl ions (OH-) for negatively charged sulfates, chlorides, and chromates. Because of the limited on-line time of one exchanger, two exchangers are usually used in parallel. The first exchanger operates, while the second is on standby.
Regeneration of ion exchange columns depends on the type of column. If the column is cationic, acid is used. Here acid flows through the bed exchanging its hydrogen ions for the metallic ions. Anionic columns are regenerated in a similar fashion using sodium hydroxide. Here, the sodium hydroxide flows through the bed exchanging its hydroxyl ions for the negatively charged ions.
Depending on the conditions, the regenerant solution can be directly returned to the process tank or must undergo further reclamation or treatment.
Some facilities using ion exchange regenerate their own columns while others ship them out or have someone come into the plant to do it.
A slight variation in the ion exchange technology is the use of chelating resins which behave similar to chelators such as EDTA. Chelating resins can hold certain transitional metal ions (such as Copper +2, Iron +3,) very tightly but do not hold tightly lighter metals such as sodium, magnesium, and calcium. Because of this characteristic, chelating resins can be used to recover low concentrations of valuable or toxic metals from solutions that contain a high level of unwanted background salts.
A second variation to ion exchange is a resin that can adsorb various chemicals present in solutions to varying degrees. Acid retardation is the most used adsorption technology for the recovery of acid (Figure 8). In this process the waste acid is passed through a bed of special resin which has the ability to hold onto strong mineral acids such as sulfuric, hydrochloric, and nitric but not to metal salts. In order to regenerate the resin, clean water is passed through the resin bed so that the acid is desorbed from the resin.
Another variation of the ion exchange process is the affinity that biological materials such as plant cell walls and micro-organisms have for certain metal ions. The biological ion exchange resin is used like a conventional ion exchange resin to remove metals such as copper, iron, aluminum, cobalt, nickel, chromium, cadmium, lead, zinc, silver, gold, and platinum. Unlike conventional ion exchange systems, calcium, magnesium, sodium, and potassium ions do not interfere with the binding of metal ions.
Diffusion dialysis is a new technology for the recovery of mineral acids. Used acid flows past one side of an anion membrane that allows acid to pass through to the other side where fresh DI water is passing by the membrane. The recovered acid goes back to process bath while rejected metals can go to waste treatment or another recovery technology (Figure 9).
Electrowinning processes can remove copper, cyanide cadmium, cyanide zinc, brass, tin-lead, gold, silver, and nickel from plating wastes. In order for electrowinning to operate efficiently, a high concentration of metals is needed. Therefore, electrowinning is typically used on dragout recovery solutions or combined with ion exchange.
Essentially, electrowinning is an electroplating process whereby a direct current is applied and the metal ions plate out onto a cathode. The anodes and cathodes are usually made of an inert material. Cathodes are commonly made of polished stainless steel or carbon fibers, while the anodes are usually made of non-consumable materials such as platinized titanium, ruthenized titanium, lead, or graphite.
Once the cathode's capacity is reached, it is removed from the recovery cell and the recovered metal is stripped. It may be reused on site or sold for scrap. Depending on the type of electrolytic recovery system, and stripping process, the recovered metal can take the form of flakes, sheets, slurry or concentrated solution.
Often electrowinning is combined with ion exchange to recover metals (Figure 10). Ion exchange processes a large volume of dilute rinse water and concentrates the metals. The regenerant flow is pumped to the recovery cell. Electrowinning can recover 90 to 95 pct of the available metals.
Electrowinning has proven itself to be a cost-effective and technologically sound alternative for the recovery of metals from wastes and rinse water. Electrowinning concentrates the heavy metals to their smallest volume as a metallic solid which can be used in the plating tank or sold as scrap metal. This is important when one considers the complexities and liabilities of hazardous waste regulations.
Electrodialysis (ED) is a proven recovery technology that uses membranes and direct electrical current to concentrate and separate ionic contaminants found in rinse water. In ED the rinse water is passed through a series of alternately placed cation- and anion-permeable membranes across which there is an electropotential charge (Figure 11). The cationic exchange membranes allow only the positively charged ions such as nickel, copper, and zinc to pass through. Anionic exchange membranes allow only negatively charged ions such as sulfate, chloride or cyanide to pass through. Because of the alternating arrangement and electropotential, a concentrated stream and purified stream are produced. The concentrated solution is returned to the plating tank; the purified stream to the rinse tank.
Typically, electrodialysis systems are connected to the recovery rinse tank(s) after the plating tank, as is shown in Figure 12. The reason for this is that ED units require turbulent flow and remove only 20-30 pct of the metals in solution at each pass. Therefore the ED process is not suitable for a once-flow-through arrangement.
A variation of the electrodialysis process that is quite popular for processes using chromic acid is an electrolytic purification process that uses a semi-permeable membrane surrounding a cathode or anode. If one is trying to recover chromic acid from rinse water, the rinse water is pumped through a unit that has a semi-permeable membrane surrounding the cathode (positive charge). The semi-permeable membranes allow the negatively charged chromate ions to be collected around the anode.
In order to purify chromic acid solutions, the solution is passed through a unit where the anode is surrounded by a semi-permeable membrane. Here, positively charged contaminants such as trivalent chromium, aluminum, and iron are concentrated around a negatively charged anode and periodically removed.
ED technology has been successfully applied to a number of acid-, alkaline- and cyanide-based plating solutions, but has been most successful in recovering chro-mates, tin, nickel, gold, and silver. Also ED has been effective as a "roughing" treatment before reverse osmosis or ion exchange for water with a very high total dissolved solids content.
Reverse osmosis (RO) is a process that separates water from larger-molecular-weight compounds. RO accomplishes this separation by forcing the waste stream under high pressure against a semi-permeable membrane. This high pressure overcomes the natural osmotic pressure forcing the water to flow from a more concentrated solution to a less concentrated solution. Depending upon the chemicals to be recovered by RO, feed pressures can range from 100 to 600 PSI.
RO splits the flow into two streams: concentrate and permeate. For the vast majority of processes, the RO unit is used with recovery rinse tanks where the concentrate is returned to the plating tank while the permeate is recirculated back through the rinse tanks. Figure 13 shows a typical RO installation. Recovery rates for typical finishing operations are approximately 95 pct or higher.
RO systems have been used to recover chromium, copper, zinc, brass, cadmium, tin, palladium, and other precious metals but the most common application for RO technology is in the recovery of acid nickel.
Ultrafiltration (UF) is very similar to RO. It uses a semi-permeable membrane to separate, under pressure, molecular and/or colloidal materials dissolved or suspended in a liquid. However, the membrane size for ultrafiltration is approximately 20 angstroms as compared to 5 angstroms for most RO membranes, and its feed pressures are not nearly as high.
The most common use for ultrafiltration for recovery is the removal of oil from process solutions, particularly alkaline cleaners. This extends solution life, minimizing chemical costs and generation of wastewater treatment sludges. Figure 14 shows a typical UF installation on an alkaline cleaner tank. UF allows most, if not all, of the detergents and other chemicals in the cleaner solution to pass through the membrane for return to the process.
Off-Site Reclamation. Over the last ten years, numerous facilities around North America have been constructed to recover heavy metals from various waste streams. These facilities not only use some of the recovery technologies described above but others that are cost effective on a large scale such as pyrometallurgy (high temperature thermal reduction), hydrometallurgy (selective metal precipitation), crystallization, and electrochemical heavy metal removal.
Because of the liability regarding land disposal of hazardous waste, off-site recovery opportunities should be explored. One can contact the state hazardous/solid waste regulatory agency to obtain the names of organizations within your region that offer reclamation services.
Conclusion. Waste generated by metal finishers and electroplaters, especially those containing process solution chemicals such as metals, cyanides, acids, caustics, and organic additives, are really indicators of process inefficiencies. The technologies described above are tools to improve the efficiency of these operations, making metal finishing services more valuable and competitive in today's ever-changing marketplace.
The secondary benefits are:
The two keys in implementing recovery/recycling is first to take a forward looking and proactive approach. Waiting for a law to be passed will likely cause you to rush into the process, making hasty decisions. Second, it is important to conduct a comprehensive evaluation of the entire facility.
It is never too late to consider and implement recovery and recycling technologies and you need not do it alone. Begin discussing these technologies with equipment suppliers. Attend seminars. PFD
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