Metal Finishing Industry
Pollution Prevention in Rinsing
Rinsing follows cleaning, plating, and stripping operations. Adequate rinsing is a critical step within the plating process. Rinsing prepares a part for subsequent finishing operations, stops the chemical reaction, and prevents cross contamination of subsequent plating tanks. Poor rinsing can result in staining, spotting, blistering, or peeling of the workpiece. Therefore, rinsing must be effective and efficient. Alternative rinsing practices succeed only if they are properly designed, operated, and maintained. In some cases, the only practical means of preventing or reducing pollution is by improving, modifying, or installing recovery/reuse technologies to the rinsing process (Pinkerton 1984).
Most of the hazardous waste in a metal finishing operation comes from the wastewater generated by rinsing operations. Two general strategies to reduce waste from rinsing operations are preventing rinse contamination, and recovering and recycling materials from the rinsing process. Facilities should evaluate alternative rinsing practices prior to investigating recovery technologies. Nevertheless, facilities might need to use a combination of the two strategies for an effective rinsing system that complies with the regulations.
The goals of alternative rinsing practices are two-fold: (1) to control the dragout of solution from process baths into the rinsewater and (2) to minimize water consumption. These two goals have a significant effect on the amount of waste, mainly sludge, generated by waste treatment systems. The amount of wastewater sludge generated is directly proportional to the amount of metal, organic, and other bath constituents in the rinsewater. Therefore, any technique for reducing dragout also will reduce sludge generation (EPA 1992).
Dragout, the bath solution that is carried out of the process bath and into succeeding tanks, is the primary source of contamination in rinsewater. Reducing dragout can be the single most effective way to reduce waste and conserve water in rinsing operations. Figure 8 illustrates typical generation of dragout.
Reducing dragout extends the life of the process baths and reduces sludge generation. The rate of dragout varies considerably among different parts and processes. For instance, barrel plating commonly carries 10 times more solution into the rinsing process than a typical rack plating operation (Ford 1994). Several factors contribute to dragout including workpiece size and shape, bath viscosity and chemical concentration, surface tension, and temperature of the process solution.
Most dragout reduction methods are inexpensive to implement and, in most cases, have short payback periods. Savings are mainly in the area of reduced use of plating and processing chemicals. Additional savings, often many times the cost of the pollution prevention project, include decreased operating costs of pollution control systems. Many of the methods to reduce dragout require only the cost to properly train employees with no capital expenditures. For example, removing workpiece racks at a slower rate or allowing the rack to drain over the process tank for a longer time does not require capital outlays, but the method does require a conscientious, properly trained operator. Such procedures should not significantly affect production and should result in reducing process chemical purchases, water and sewer use fees, treatment chemical purchases, and sludge handling costs (Cushnie 1994).
Measuring dragout allows facilities to determine the extent of the problem and to monitor the effectiveness of reduction techniques. Facilities can use several methods to effectively monitor dragout rates. Some facilities use a tensiometer to measure surface tension. A tensiometer measures the force necessary to lift a metal wire ring off the surface of a liquid. The cost for this tool is approximately $2,000. A second method for determining surface tension is a stalagmometer. While stalagmometers are much less expensive than tensiometers, they are more difficult to use. For instance, plating solution tends to dissolve the ink marks on the meter that are used to calculate surface tension.
Facilities also can use a conductivity meter to determine dragout rates. Using conductivity measurements to generate information on rinsing can greatly reduce analytical fees and eliminate the lag time between sampling and results since samples do not need to be sent to a lab. Most plating facilities have combination pH/conductivity meters that can be used for this purpose or they can purchase a portable unit for $200 to $300 (Cushnie 1994).
Platers can reduce dragout using a variety of techniques that involve a combination of employee retraining and relatively simple technology. These methods include:
These techniques are described in detail in the following sections.
Workpiece Withdrawal and Drain Rates
The speed at which workpieces are removed from the process bath can have a substantial impact on dragout volume. The more slowly a workpiece is removed from the bath, the thinner the film of process solution is on the workpiece, and the less solution is dragged into rinse tanks. The effect is so significant that many experts believe that most of the time allowed for draining should instead be used for withdrawing the workpiece. A recent case study demonstrated that a drain time of 10 seconds reduced dragout by 40 percent compared to the industry average of 3 seconds (IAMS 1995).
Facilities can control drain times by posting them on tanks as a reminder to employees on manual lines or by building delays into automated process lines. Smooth, gradual removal of parts from the solution is the preferred method. A bar or rail above the process tank can ensure adequate drain time prior to rinsing. If platers use drip bars, employees can work on more than one rack during an operation. In rotation plating, an operator removes a rack from a plating bath and lets it drain above the process tank while other racks are handled. Increased drain time, though, can have some negative effects such as drying, which is especially problematic with hot cleaners because it can cause spotting on the workpiece (Cushnie 1994).
Bath Concentration and Temperature
Lowering the viscosity of the bath can reduce dragout. Facilities can lower the viscosity of a plating solution in two ways: (1) reducing the chemical concentration of the process bath or (2) increasing the temperature of the process bath. For further information on this option, refer to the general pollution prevention section on plating baths in Pollution Prevention in the Plating Process.
The placement of workpieces on racks can have a significant impact on the chemicals carried into the rinse tanks. Positioning pieces so that solution drains freely without being trapped in grooves or cavities reduces dragout. Positioning workpieces so that they face downward also can improve drainage efficiency. However, proper placement must take into account both proper plating and rinsing. For example, a saucer-shaped object placed upside down would drain well, but the plating solution would not entirely coat the inside of the cup because of entrapped gas bubbles. Therefore, an angled position is the most efficient. This placement allows for proper plating and efficient draining. Proper racking also can reduce surface tension and improve draining. The following are some suggestions for properly orienting and positioning workpieces (EPA 1992):
If a workpiece is designed so that it does not drain easily, facilities can work with their designers or, in the case of job shops, their customers to see if modifications are possible. For example, a plater asked his customer whether he could drill four holes in the workpiece to improve drainage. The customer agreed and the pollution prevention technique was successfully implemented (IAMS 1995).
Design and Maintenance of Racks
Improving the design of racks, baskets, or barrels can reduce the amount of dragout. If equipment is not properly maintained, it can increase contamination both in terms of increased dragout and contamination from residue on racks. These contaminants include rust and salt deposits that form on racks, barrels, and baskets. Keeping racks clean can reduce contamination of process baths and rinsewaters (Ford 1994).
Metal finishing operators can use drainboards to collect dragout and drippage when transferring racks from one tank to the next. Boards should be mounted so that they cover the entire space between the two tanks, allowing the solution to drain back into the previous bath. This method prevents chemical solutions from dripping onto the floor. Figure 9 presents the typical set up of a drainboard. Many operators prefer removable drainboards because they permit access to plumbing and pumps. Drainboards should be made of a compatible material such as polyvinylchloride (PVC). Use of drainboards is a cost-effective technique for reducing chemical consumption and rinsewater contamination (IAMS 1994).
Dragout Tanks (Dead or Static Rinse Tanks)
Dragout tanks are essentially rinse tanks that operate without a continuous flow of feed water. The workpiece is placed in the dragout tank before the standard rinsing operation. Dragout tanks are used primarily with process baths that operate at an elevated temperature.
Chemical concentrations in the dragout tank increase as the operator passes the work through the tank. Because dragout tanks do not have feed water flow to agitate the rinsewater, air agitation often is used to enhance rinsing. Eventually, the chemical concentration of the dragout tank solution will increase so that it can replenish the process bath. Adding the dragout solution back to the process bath compensates for evaporative losses that occur because of high evaporation rates (EPA 1992).
The cost of a dragout tank depends on the size of the tank. Since these tanks are not used as flow-through tanks, they can be installed without plumbing. Typically, dragout solutions are added back to the process bath manually. However, automation is more efficient as it maintains the best concentration in the dragout tank (EPA 1992).
Rinsing Over the Plating Tank
If the process tank has a high evaporation rate, workpieces can be rinsed directly over the process solution, returning water and chemicals directly to the process tank. This form of rinsing requires a high evaporation rate so that the work can be done without splashing solution onto the equipment. Rinsing often is practiced over electroless plating tanks because they have no buss bars or rectifiers that can be splashed. However, operators can rinse over other plating tanks if they are careful (EPA 1992).
Metal platers use air knives to blow air across the surface of workpieces as they are withdrawn from process or rinse solutions, physically pushing liquid off of workpieces. This technique returns solution directly to the process bath, reclaiming dragout and reducing the amount of rinsewater required to clean the workpiece, which enhances the drying process. In some applications, however, this rapid-dry method can cause poor bonding, spotting, and staining (APPU 1995).
Specific Techniques for Reducing Dragout in Barrel Plating Operations
In barrel plating, floor spills are less likely to occur since automation typically moves the barrel from one tank to the next. However, barrels potentially create more dragout since they hold more solution. In addition, although the barrels are perforated, complete drainage can be difficult. As with rack plating, extending drip times reduces dragout.
In order to reduce dragout, the correct barrel must be used for the parts being plated. For example, if the average part is 2 inches in diameter and the barrel contains holes that are too small (less than ½ inch) drainage can be too slow, resulting in significant dragout. In general, using a barrel with the largest holes possible minimizes dragout. Operators should make sure that barrel holes are not plugged. If they become plugged, they should be cleaned or redrilled and deburred to ensure maximum drainage. The most important technique for reducing dragout in barrel plating is proper hole size and rotation of the barrel in the upright position (Gallerani 1996).
Angled withdrawal from the plating solution also can result in reduced dragout. However, no producer/vendor of angled barrel technology has been found for large, horizontal double-hung barrels. Nevertheless, one of the following modifications of existing barrels might prove useful in minimizing dragout:
Prior to implementing an angled or sloped barrel technique, platers should check the design of all tanks to ensure proper clearance for the modified system (IAMS 1995).
This section presents alternatives to traditional rinsing techniques. Two strategies for reducing water use are improving the efficiency of the rinsing operation and controlling the flow of water to the rinsing operations. Contact time and agitation influence the effectiveness of the rinsing operations.
Platers can use several methods to improve rinsing efficiency. These methods include finding the optimal amount of contact time and the correct level of agitation.
Contact time refers to the length of time workpieces are in the tank. For a given workpiece and tank size, the efficacy of rinsing varies with contact time. Production rate, however, varies inversely with contact time. Through experimentation, the operator should find the contact time that satisfies production requirements while providing the highest rinsing efficiency (IAMS 1995).
Rinses that are agitated reduce the required amount of contact time and improve the efficiency of the rinsing process. Rinsewater can be agitated by pumping either air or water into the rinse tank. Air bubbles create the best turbulence for removing chemical process solution from the workpiece surface. However, misting as the air bubbles break the surface can cause air emissions problems (Cushnie 1994).
A finishing shop can use many methods to agitate rinse tanks. In manual plating, the operator lifts and lowers the workpiece in the rinse tank, creating turbulence. In other tanks, the most effective form of agitation involves a propeller-type agitator, but this method requires extra room to prevent parts from touching the agitator blades. Good agitation also can be obtained with the use of a low-pressure blower. The following is a list of other effective agitation methods:
Air spargers, water pumps, or agitators can be installed in existing rinse tanks. Installation of an air sparger with a blower costs approximately $200 to $325 for a 50 gallon tank. Air blowers can reduce costs because they eliminate the need for air cleaners and filters to remove oils in compressed air systems. An in-tank pump for forced water agitation can be purchased for $200 to $1,000 depending on the flow rate desired. Selection of the optimum method of agitation entails balancing capital and operating costs against revenues from increased production rates and decreased water use (IAMS 1995).
The following sections present rinsing methods that use less water and increase the efficiency of the rinsing operations.
Countercurrent rinsing uses sequential rinse tanks in which the water flows in the opposite direction of the work flow (dirtiest to cleanest). Fresh water is added only to the final rinse station and is conveyed, normally by gravity overflow, to the previous rinse tank. Wastewater exits the system from the first rinse tank. Figure 10 illustrates a three-stage countercurrent rinse system. In some cases, the water contained in the first rinse can be used as makeup water for the process bath (see discussion of water quality in Common Pollution Prevention Practices). Many shops have used this technique successfully to minimize water consumption. The amount of water conserved will depend on the number of tanks installed for countercurrent rinsing. In some cases, countercurrent rinsing can achieve 95 percent reductions in rinse flow if the facility uses three rinse tanks; 90 percent is possible with two tanks (Hunt 1988).
Limitations governing the use of countercurrent rinsing include:
Limited shop floor space can present a significant problem for the electroplater. However, careful review of the shop often can reveal opportunities for added rinse stations. The following list presents some of the ways a shop can make room for countercurrent rinsing:
Static Rinsing (Recovery Rinsing)
If direct countercurrent rinsewater overflow to the process tank is not possible, the first rinse tank after a process bath can be a static rinse that builds up a concentration of dragin. Static rinse tanks used with low-temperature processes can be used as pre-dip or post-dip rinses to recover dragout (as much as 80 percent). Periodically, the accumulation in this bath should be concentrated enough for reuse/recycling into the process bath (EPA 1992).
Multistage Static Rinsing
Multistage static rinsing uses multiple dead tanks rather than a system where the water flows from one rinse tank to the other. This process often is used in cadmium plating to keep the metal from entering the waste treatment system. Solution from the first rinse tank can be used to replenish the process bath. However, the solution might need treatment prior to reuse such as filtration to remove contaminants.
Warm rinsing is effective particularly in the case of alkaline solutions such as cleaners and cyanide plating baths. Alkaline solutions tend to freeze onto parts when immersed in cold water, making effective cleaning difficult. Warm rinses reduce freezing rate and increase rinsing effectiveness (Ford 1994).
Reactive rinsing uses less water and saves chemicals. Most cleaning lines use an alkaline cleaner followed by an acidic pickle. Taking advantage of the chemical nature of the pickle liquors and alkaline cleaner, reactive rinsing feeds the water from the acid pickle to the alkaline rinse. This step neutralizes the cleaner and also prevents alkaline material from being dragged into the acid, prolonging the life of the pickle solution. Reactive rinsing cuts water use in half, and, in some cases, enables the plater to plumb more than two rinses in a series. However, acidic water should never be fed into a rinse that contains cyanide solution (Hunt 1988).
In the example in Figure 11, nickel rinsewater is recycled back to the acid dip rinse tank, allowing nickel plating solution dragged out of the process bath to be dragged back into the bath. Such a modification will not harm the rinse step and will allow the fresh water feed to the acid rinse to be turned off. The acid rinsewater then can be recycled to the alkaline cleaner rinse tank. Advantages include allowing the feed water to be shut off, improving the rinsing efficiency by neutralizing the dragged-in alkaline solution, and prolonging the life of the acid rinse bath as the rinsewater dragin already will be partially neutralized. This concept can be taken one step further and the rinsewater can be recycled among process lines (Hunt 1988).
The reuse of rinsewater cannot be indiscriminate. Facilities must avoid contaminating the process baths and reducing the plating quality (e.g., pitting). However, following careful evaluation, reactive rinsing can produce significant water and chemical savings (Gallerani 1990).
Spray or Fog Rinsing
Installation of fixed or movable rinse spray nozzles over the process tank can replace separate rinse tanks. Overspray is returned to the process tank, resulting in reduced dragout. This spray or fog rinsing can be used for either rack or barrel plating (IAMS 1995).
Spray rinsing uses between 10 to 25 percent less water than dip rinsing. However, this method is not always applicable to metal finishing because the spray rinse might not reach all of the parts of the workpiece. The effectiveness of spray rinsing depends upon part geometry and complexity. Spray rinsing compares favorably with single-dip rinses, but is not as effective as countercurrent rinsing. To address this problem, spray rinsing can be combined with immersion rinsing. In this technique, the workpiece is spray rinsed over the process tank as soon as the part is removed from the process solution. The part then is submerged in an immersion tank. As a result, the spray rinse removes much of the dragout, returning it to the process bath before the workpiece is placed in the dip rinse tank. This allows facilities to use lower water flow rates and reduce dragout (EPA 1992).
Platers also can use spray or fog rinse systems above heated baths to recover dragout solutions. Spray rinsing washes process solutions through impact and diffusion forces and can reduce water use by 75 percent. If an operator can adjust the spray rinse flow rate to equal the evaporation loss rate, the spray rinse solution can be used to replenish the process bath. Purified water should be used for the spray systems to reduce the possibility of contamination entering the bath. Fog rinsing uses water and air pressure to reduce concentration of dragout films. This method is most useful in finishing simple pieces (Cushnie 1994).
An important concept in rinsewater conservation is to use only as much water as you need. Many electroplaters use far more water than they need. Reducing water flow also can make certain recovery technologies (such as reverse osmosis) economically feasible because of the linear relationship between cost and flow rate. In fact, most of these technologies will only be cost effective if water flow rates are reduced to the lowest possible levels.
Typically, a rinse tank uses a wide-open, unrestricted water feed. Installing flow restrictors or conductivity cells can result in a significant reduction in water use. The operator should first determine the current flow rate and then the optimum flow rate at which the facility should be functioning.
Platers should size rinsewater flow requirements according to contaminant loading (process dragout) and required final rinsewater purity or concentration. In a multiple rinse setup, the flow requirements should be exponentially lower than those in static tanks. Operators can determine the required rinsewater flow for a single station rinse using the following equation:
D x Cp = F x Cr
D = Dragin rinse or process dragout
Cp = Concentration of dragin
F = Rinsewater flow
Cr = Concentration of the rinse (Gallerani 1990)
Once platers have determined the appropriate flow rate, they can use several methods to control flow rates in their facilities including flow restrictors, conductivity cells, and pH meters.
A number of simple methods are available to restrict water flow and conserve water including flow restrictors and conductivity controllers.
Flow restrictors limit the volume of rinsewater flowing through a rinse system by limiting the volume of water that can enter the rinse system. This method will maintain a constant flow of fresh water to the rinse process. Since most small- and medium-sized metal finishers operate batch process operations, pressure-activated flow control devices as foot-pedal-activated valves or timers can be helpful to ensure that water is not left on after completion of the rinse operation (EPA 1992).
Conductivity Cells/pH Meters
Platers can use conductivity controllers in place of flow restrictors on a rinse system where dragin is highly variable or where monitoring the bath for extreme conditions (over or under concentration) is desirable. Figure 12 shows a typical application of conductivity cells. These devices control water flow through a rinse system by means of a conductivity sensor that measures the level of ions in the rinsewater. When the ion level reaches a preset minimum, the sensor activates a valve that shuts off the flow of fresh water into therinse system. When the concentration builds to a preset maximum level, the sensor opens a valve to resume the flow of fresh water. These meters can alert production line staff to imbalances in rinsewater concentration so that they can be replenished on an as-needed rather than a continuous basis. These systems are relatively expensive and require a good deal of maintenance (Gallerani 1990). If these systems are not maintained consistently, water use actually can increase. If the solenoid valve becomes clogged, it will remain open, allowing water to flow freely. A conductivity meter equipped with the necessary solenoid valve can cost approximately $700 to $4,000 per system.
Water conservation and metal recovery techniques have become an integral component of pollution prevention programs for platers. Many techniques to recover water, metals, or acids that have contaminated rinsewater are available. In some cases, the technologies merely recover materials so that they can be sent off site for easier disposal. In other cases, the technology can return the material back to the process line for reuse.
Rinsewater can be recycled in either a closed- or open-loop system. In a closed-loop system, the treated effluent is returned to the rinse system. Recycling rinsewater can reduce water use and the volume of water discharged to the wastewater treatment system significantly. Closed-loop systems discharge a small amount of waste. An open-loop system allows the treated effluent to be reused in the rinse system, but the final rinse is fed by fresh water to ensure high-quality rinsing. Therefore, some treated effluent will continue to be discharged to the sanitary sewer (EPA 1995). These two systems are shown in Figure 13.
To improve the economic feasibility of closed- or open-loop systems, platers should first implement rinsewater efficiency techniques. In the past, material recovery from metal finishing was not considered economical. However, effluent pretreatment regulations and treatment and disposal costs are now a significant financial factor. As a result, metal finishers now might find reusing rinsewater as well as recovering metals and metal salts from spent process baths and rinsewater are cost effective.
After rinse solutions have become too contaminated for their original purpose, they can be used in other rinse processes. For example, effluent from a rinse system following an acid cleaning bath sometimes can be reused as influent to a rinse system following an alkaline cleaning bath (reactive rinsing). If both rinse systems require the same flow rate, 50 percent less rinsewater could be used to operate the system.
Facilities can manage the captured dragout solution from rinsewater recovery in three ways: (1) recycling solution back into the process, (2) on-site recovery, and (3) shipment off site for disposal or recovery. The choice will depend upon the type of process bath, composition of the dragout, and the cost of the technique.
Platers must understand the chemical properties of a wastestream to assess the potential for reusing the waste as a raw material. Although the properties of process bath or rinsewater solutions might make them unacceptable for their original use, the waste material might still be valuable for other applications. A common reuse option in multiple-use rinses is using rinsewater from one process as rinsewater in another. For example, rinsewater from a cleaning rinse can be reused in a plating rinse line. The primary cost associated with rinsewater reuse is replumbing. Depending on the design of the rinse system, firms also might need to purchase storage tanks and pumps.
Typically, operators dump spent acid or alkaline solutions when contaminants exceed an acceptable level. However, these solutions might remain sufficiently acidic or alkaline to act as pH adjusters. For example, alkaline solutions can be used to adjust the pH in a precipitation tank while acid solutions can be used in chromium reduction treatment. The operator must ensure that the spent solutions are compatible. For example, because spent cleaners often contain high concentrations of metals, they should not be used for final pH adjustments. Facilities should check with chemical suppliers to determine whether they have reclamation services for plating baths. Be aware that some states classify reclamation as treatment under the RCRA program, requiring compliance with additional regulatory requirements (in some cases, an abbreviated treatment license).
Every year, the plating industry pours millions of dollars down the drain in valuable metals. Closed-loop systems reduce rinsewater volumes and facilitate the recovery of metal salts for reuse in plating baths, using separation processes such as evaporation, ion exchange, reverse osmosis, electrolysis, and electrodialysis. An industry consultant recently estimated that it would be economically feasible to recover 80 to 90 percent of copper, 30 to 40 percent of zinc, 90 to 95 percent of nickel, and 70 to 75 percent of chromium presently disposed of as sludge (Gallerani 1990). Recovered metals can be reused in several ways:
The savings achieved through metal recovery are site-specific. Factors that determine whether metal recovery is economically justified include:
Table 19 highlights different technologies that can be used for chemical recovery, metal recovery, and chemical solution maintenance.
Table 20 provides an overview of technologies for recovering metals, plating solutions, and water.
A number of rinsewater recovery technologies are available to platers. Many platers already use these systems. The recovery systems include various types of electrolytic recovery and evaporators.
Electrolytic metals recovery (EMR) is used to recover the metallic content of rinsewater. EMR, one of the most common methods of recovering metal from finishing operations, is capable of recovering 90 to 95 percent of the available metals in gold, silver, tin, copper, zinc, solder alloy, and cadmium plating operations (Bennati and McLay 1983). The basic unit of this technology is an electrolytic cell with two electrodes (an anode and a cathode) placed in the solution. Ions in the solution move toward the charged electrode. The dissolved metal ions are reduced and deposited on the cathode. The material that is deposited onto the cathode is removed either by mechanical or chemical means and then is sent off site for refining, recycling, or disposal (Cushnie 1995). Table 20 provides a summary of the metals and their potential for successfully applying electrowinning. The table also includes an indication of the use of EMR for certain groups of metals.
As shown in Table 21, some metals are not particularly suited to EMR. The only common metal salt that cannot use electrowinning is chromium. This technology can recover nickel, but it requires close monitoring of the pH. Platers also can use electrowinning in electroless plating operations. However, this application is not as straightforward because of the presence of chelated metals, reducing agents, and stabilizers (Cushnie 1994). The most common applications of EMR include acid copper plating, cyanide cadmium plating, cyanide zinc plating, and cyanide copper plating (Freeman 1994).
Electrolytic recovery is most effective when metal concentrations are high. Platers can take the residual metals and sell them or recycle them in the plating process. Because plating becomes inefficient at low metal-ion concentrations, it alone is not suitable for producing wastewater that complies with discharge regulations. EMR can be an effective reclaim/recycle method with lower capital costs in conjunction with another technique such as ion exchange (EPA 1995).
Metal finishers also can use EMR for spent plating bath solutions, recovered spills, discharge from static rinse tanks, and regeneration solutions from ion exchangers. Firms generally use EMR for reducing the amount of inexpensive regulated metals and cyanide that they discharge to treatment systems or for recovering expensive metals, both common and precious. In either case, companies use EMR for gross metal recovery from concentrated solutions such as dragout rinses or ion-exchange regenerant (Cushnie 1994). Figure 14 illustrates the EMR process.
Several basic design features, which are well known to the electroplating industry, are used in electrolytic recovery:
Two electrolytic recovery methods are conventional metal cathode (electrowinning or dummy plating) and high surface-area cathode (HSAC). Conventional electrowinning involves the placement of a cathode and an anode in the rinse solution. As the current passes between the cathode and the anode, metallic ions deposit onto the cathode, generating a solid metallic slab that can be reclaimed or used as an anode in an electroplating tank. Electroplaters can make their electrowinning units by closely spacing parallel rows of anodes and cathodes in a plating tank and circulating rinse solutions through the tank (Cushnie 1994).
In HSAC, the operator pumps the metal-containing solution through a carbon fiber cathode or conductive foam polymer, which is used as the plating surface. To recover the metals, the carbon fiber cathode assembly is removed and placed in an electrorefiner, which reverses the current and allows the metal to plate onto a stainless steel starter sheet. These systems recover a wide variety of metals and regenerate many types of solutions. Platers use HSAC recovery mainly with dilute solutions such as rinsewater effluent.
The types of cathodes used in electrowinning can be classified into three categories in order of increasing surface area: (1) flat plate; (2) expanded metal, wire mesh, or reticulate plate; and (3) porous or woven carbon and graphite plate. Platers use flat plates for applications of gross metal recovery from concentrated solutions including expanded metal, wire mesh or reticulate plates, and porous or woven types for recovering metals with lower concentrations. Facilities also use cathodes to recover metals from spent process baths prior to wastewater treatment (Cushnie 1994).
Restrictions on Applications
Strong oxidizing substances, such as nitric acid or fluoroboric acid, generally are not feasible options for electrowinning primarily because of the short life of the anodes in such environments. Hydrochloric acid or other compounds containing the chloride ion also might not be suitable because of the generation of chlorine gas at the anodes. However, ventilation can control gas formation (EPA 1995).
In general, capital cost for electrolytic recovery equipment is low. A unit equipped with a 100-ampere rectifier can cost between $8,000 and $15,000 depending on the type of anodes and cathodes. Such a unit can remove up to 500 grams of metal per day from a dragout tank (EPA 1995).
Electricity, electrode replacement, and maintenance costs are the most significant operating costs. Electricity costs per unit mass of metal recovered vary with the concentration of metal in the electrolyte. A low concentration of metal ions leads to lower efficiency and higher energy costs. Anodes require replacement every 1 to 5 years depending on the nature of the electrolytes being electrowinned. The cost of anodes varies widely, from $600 to more than $3,000 per square meter for platinum-coated titanium types, although some anodes rarely require replacement. For example, flat plate steel cathodes can be reused after being scraped free of metal deposits. Wire mesh and reticulate cathodes usually are rated to hold more than 1 kilogram of metal and generally cost less than $100 per square meter. The labor costs for operating and maintaining an electrowinning unit are generally low. Besides daily checks for electrical settings and overall operation, many installations require little scheduled maintenance (EPA 1995).
Evaporation is widely used by platers to recover a variety of plating bath chemicals. This technology separates water from dissolved solids such as heavy metals. Evaporators create additional room in a process bath so that dragout can be returned to the process tank. They also can concentrate rinsewater so that less volume goes back to the process tank. Evaporators often return recovered dragout to the process tank in higher concentrations than that of the original process solution. This technology is used most often in decorative chromium, nickel, and copper cyanide plating, although it is not limited to these applications (Freeman 1995).
Evaporators are most economical when the amount of water is small and the product concentration is high or when natural atmospheric evaporation can be used. For instance, evaporation is efficient with multistage countercurrent rinsing because the quantity of rinsewater to be processed is small. However, this energy-intensive technology is expensive when used for large volumes of water. Another problem with this technology is that when the water volume is high, sludge generation rates increase as the flow volume increases. Effective rinsing and reduced dragout, however, increases the effectiveness of evaporation (see Pollution Prevention in the Plating Process for more information). In cases where large volumes of water have low metal concentrations, ion exchange, reverse osmosis, or electrodialysis are more cost effective than evaporation. In some cases where water volume is high, even precipitation, settling, and resolubilization can be more efficient procedures (Veit 1989).
Evaporators should not be confused with drying devices, which produce a solid or semi-solid product. While both dryers and evaporators use volatilization, evaporators are designed to concentrate a solution to no greater than one-half to three-quarters solubility (Veit 1989).
Two types of evaporation systems are atmospheric and vacuum.
Atmospheric evaporators operate by spraying the dilute wastestreams over packing media, grids, or plates, and then blowing air from the facility to vaporize water. These units consist of a heater which preheats the rinsewater (most commonly, the process tank's heating system), a pump which transfers the fluid to the evaporation chamber, and the chamber which consists of fins or a packing surface to increase the surface area of the air-fluid interface. The source of air in these systems is important because the bath can absorb airborne impurities. Evaporation rates depend on the size of the chamber, the solution temperature, and the humidity of the air blown across the chamber. The most common units are designed for less than 150 gallons per hour. However, units are available in a large range of sizes (Cushnie 1994).
Applications and Restrictions
Metal finishers generally use atmospheric evaporators on a variety of plating processes including nickel, chrome, and acid zinc plating. Figure 15 illustrates the application of atmospheric evaporators to high- and moderate-temperature rinse systems. Atmospheric evaporators commonly are applied to a heated process bath to increase the evaporation rate and to make room in process tanks for water return in a countercurrent rinse system. The system directs rinsewater from the system to an off-line tank where it circulates through the evaporator. Operators then return the concentrated fluid to the process tank. Ambient temperature baths require a similar configuration, but operators must circulate some fluid to the off-line tank and evaporator to make room in the process tank. Evaporators are most efficient when used in plating baths that are already heated between 49 and 65 degrees Celsius (Cushnie 1994).
Atmospheric evaporators are not appropriate for process fluids or additives (e.g., brighteners) that degrade with heat or solutions degraded by aeration such as cyanide or tin plating baths. A major disadvantage of atmospheric evaporators is their inability to evaporate on days when air humidity levels reach 80 to 90 percent unless a heating system is installed. Another disadvantage of atmospheric evaporation is that all dragout, including bath contaminants, is returned to the process tank, increasing the potential for contamination of the process solution. This problem can be minimized if deionized water is used. If evaporators are used with cyanide solutions, the rate of carbonate buildup will increase because of carbon dioxide adsorption from the entrained air and thermal breakdown of cyanide (Freeman 1994).
Capital costs of evaporators vary. A typical atmospheric evaporator that processes 40 to 75 liters per hour costs less than $10,000. Installation costs can be high depending on plumbing and duct modification requirements. Operating costs (e.g., electricity and labor) average $0.25 to $0.35 per gallon. Many companies prefer atmospheric evaporators to other types of evaporators because they are relatively inexpensive (EPA 1995).
Vacuum evaporators are closed systems that use one or more vacuum chambers to reduce the boiling point of water to volatilize water from the wastestream. In practice, platers pump preheated fluid into the vacuum chamber where it quickly vaporizes. These units do not require large air volumes and generally produce distilled water as a byproduct. A number of different designs are available. They differ in how the vacuum is achieved (i.e., eductor or vacuum pump) and how much energy is used (i.e., single effect or double effect). These systems take advantage of the depression of the boiling point of water as air pressure decreases. The higher the vacuum, the lower the boiling point for water. By lowering the boiling point, vacuum evaporation protects some of the ingredients in the processing solution from degrading.
The four types of vacuum evaporators include:
Applications and Restrictions
Metal finishers typically use vacuum evaporators in those applications in which atmospheric evaporators are not suitable. Operating expenses favor vacuum evaporators when feed rates are 190 to 265 liters per hour. These systems offer major advantages when configured to trap condensate for reuse in rinsing operations (EPA 1995). The primary advantages are:
Capital costs for vacuum evaporators range from $125,000 to $175,000. Operating costs are lower than atmospheric evaporators, averaging $0.05 to $0.12 per gallon (EPA 1995).
Metal finishers use membrane filtration to remove suspended solids, oils, and other impurities from wastewater as well as to recover/recycle process solution. The membranes separate suspended or dissolved solids by applying pressure to one side of the membrane. Water and low molecular-weight compounds flow through the pores while larger molecules and suspended solids flow across the membrane and become part of the concentrate. In membrane filtration systems, wastewater flows parallel to the membrane surface. This cross flow allows high filtration rates to be maintained continuously (RI DEM 1994). Membrane flow is illustrated in Figure 16. Platers moving toward zero discharge or total recycling should consider these systems as a means to achieve that goal.
Several different membrane filtration technologies are available including microfiltration, ultrafiltration, and nanofiltration. These technologies differ in the size of the membrane's pores and the amount of pressure that is applied to the wastestream. Table 22 presents the differences in the membrane processes.
Many industries use membrane technology for filtration. Membrane materials can be organic (e.g., polypropylene, polyethylene, polyester, polyacrylonitrile, and polysulfone) or inorganic (e.g., carbon fiber or ceramics). The choice of membrane depends upon pH, temperature, and specific application (Ieronimo 1995).
In recent years, membranes have become the preferred method of liquid/solid separation because of the consistent permeate (filtrate) quality achieved and lower pretreatment chemistry requirements. The membrane technologies used most commonly by metal finishing shops are microfiltration and ultrafiltration. However, platers use other membranes in specific applications (Ieronimo 1995).
In general, microfiltration applications work best for metal finishing shops that have large amounts of oils in the wastestream. Ultrafiltration applications are best for facilities with mixed wastes containing emulsified oils from aqueous cleaners. Metal finishers use other membranes in specific waste minimization activities including acid recycling (i.e., electrodialysis) or recycling wastewater (i.e., reverse osmosis). Nanofiltration membranes are becoming popular for recycling systems as well and some membrane suppliers offer them for polishing treated water for recycling (IAMS 1995).
Platers should conduct a pilot test of any membrane system to avoid problems with flow (flux) rate deterioration or compatibility with trace constituents such as solvents or silicones. Manufacturers' warranties vary and many do not guarantee that effluent limits will be met (Ieronimo 1995).
Depending upon the application, membrane systems require periodic flushing and cleaning. Some require little maintenance while other applications where a higher concentration of materials that could foul the membrane is present require additional maintenance. In all applications, the concentrate generated by the filtration system must be managed in one of three ways: (1) companies can use the solution in another application, (2) they can discharge the solution to the sewer, or (3) they can hire a licensed hauler to remove it (Ieronimo 1995).
The capital cost of a membrane system depends on the processing rate and the type of membrane material used. Cost can vary from $4,000 for a 50 gallon-per-day system to more than $100,000 for a 50,000 gallon-per-day system. Typical annual operating costs, which include maintenance, replacement membranes, and electricity, are 10 percent of the initial investment (EPA 1995).
Microfiltration is a relatively new technology for the removal of oil and grease from aqueous and semi-aqueous degreasing baths. Captive shops and non-plating facilities such as metal fabricators and painters currently use microfiltration. Microfiltration separates emulsified oils and suspended solids from cleaning solutions in the process bath, extending the life of the solution. Microfiltration also can remove cleaning solution dragout from rinsewater lines (Cushnie 1994).
To remove large particulates, platers typically filter the feed stream entering the microfiltration unit with conventional methods (e.g., cartridge filters). Facilities use various holding tank designs to trap or skim floating oils, allowing heavier solids to settle. Operators then pump fluid into the membrane compartment of the unit. The membrane separates the remaining oils and grease while water, solvent, and cleaning bath constituents pass through. Figure 17 illustrates a microfiltration system.
Two common configurations for microfiltration are dead-end filtration and cross-flow filtration. In dead-end filtration units, flows are similar to those in laboratory Buchner funnels, while in cross-flow filtration units, flows are tangential to the filter surface. Filters used in these systems can be either membranes with pore sizes smaller than the diameter of the suspended solids or depth filters with pore sizes larger than the particle size, but that can still trap particles in interstices. Cross-flow filtration is used predominantly in metal finishing because of its self-cleaning ability, low pressure requirements, and high permeate fluxes. The membranes can be polymeric or ceramic materials. Polymeric membranes have service lives of 2 to 4 years while ceramic membranes can last 10 years. Despite a cost that is twice that of polymeric membranes, ceramic membranes are becoming more popular because of their high temperature and chemical resistance. All microfiltration systems require periodic cleaning to remove deposits on the surface and unplug membrane pores. Cleaning usually is accomplished by circulating acid (for inorganic scales), detergents (for colloids emulsions), alkali (for biological materials), or solvents (for organics) through the microfiltration membrane (Freeman 1995).
The equipment selected for microfiltration should have a simple mechanical configuration that is physically sturdy and compact. The unit should be constructed of materials that can withstand high alkalinity and temperatures and that can tolerate temperature fluctuations. It also should be impenetrable to soils and metal shavings. Selection of the membrane and designation of pressure, retentate flow rate, and concentration of oil in the influent are the most important factors in determining the appropriate microfiltration system (Ieronimo 1995).
Applications and Restrictions
Microfiltration is used in the recovery of caustic aqueous cleaners. As caustic cleaning solution is used, it accumulates dirt, grease, grime, free and emulsified oils, and metal particulates. With use, caustic cleaners lose their ability to remove contaminants. Rather than dumping the cleaning bath, it can be sent to a microfiltration unit for regeneration. Not all cleaners are good candidates for microfiltration and a facility might need to change its cleaning chemistry to use microfiltration. For example, high silicate cleaners that accumulate metal ions can foul membranes. Because these membranes do not remove dissolved ions such as aluminum or copper, bath life remains limited (EPA 1995). Microfiltration also can be used to polish wastewater after hydroxide precipitation (Freeman 1995).
The cost of microfiltration systems varies depending on the size of the machine. Systems can range from $15,000 to $20,000 for a 1,000 liters-per-day unit to $25,000 to $35,000 for a 5,000 liters-per-day unit. Installation costs are usually 10 to 30 percent of the equipment cost.
Operating costs include membrane replacement, labor, and energy. The lifespan of a membrane depends upon the application. Some facilities might need to change the membrane every few years while other facilities can expect the membrane to function properly for more than 10 years. Companies can save money by reducing or eliminating replacement of spent cleaners and neutralization chemicals (EPA 1995).
Ultrafiltration (UF) membranes have smaller pores than microfiltration membranes with pore sizes of 0.0025 to 0.01 microns. The layout of a typical ultrafiltration recycling system is depicted in Figure 18. As shown, the operator pumps spent process water from a process tank to a holding/settling tank. If the spent process solution has a high solids content, the rinsewater first passes through a prefiltration unit (e.g., bag filter) before being pumped to a holding tank. From the holding tank, the ultrafiltration system recirculates and concentrates the process solution, providing a steady stream of clean fluid for reuse. The system then sends a stream of clean fluid to the holding tank for the operator to draw on as necessary. Typically, ultrafiltration systems use higher pressure than microfiltration systems (60 to 80 pounds per square inch) (RI DEM 1994).
Ultrafiltration membranes are tubular, hollow fiber, and spiral wound. Platers generally use tubular membranes in small flow, high-solids loading applications. The construction of tubular membranes allows easy cleaning, making them excellent applications where the operator expects severe fouling (RI DEM 1994).
The hollow fiber design consists of a membrane wound into a hollow cylinder. The expected solids loading governs the size of the cylinder that is needed for a specific application. Platers usually use spiral-wound membranes for high-volume applications. The spiral membrane consists of a rolled flat membrane that is netted together with specially designed spacer material. Spiral membranes cannot be mechanically cleaned and usually are reserved for applications where total suspended solids loading is low or has been reduced by prefiltration (RI DEM 1994).
Reverse osmosis (RO) is a pressure-driven membrane filtration process. In RO, a semi-permeable membrane permits the passage of purified water under pressure, but does not allow the passage of larger molecular-weight components. Water that passes through the membrane usually is recycled as rinsewater. Water that is rejected by the membrane (i.e., water containing dissolved solids) is returned directly to the process tank. Reverse osmosis is capable of removing up to 98 percent of dissolved solids, 99 percent of organics, and 99 percent of bacteria. Figure 19 illustrates a typical RO system. Reverse osmosis is a good component of a low- or zero-discharge configuration. The equipment, however, tends to be more expensive and less effective at recycling rinsewater than other technologies such as ion exchange (EPA 1995).
Reverse osmosis is especially suited for closing the loop on plating operations and sending concentrate back to the plating bath. Firms apply RO to a variety of processes including brass cyanide, copper cyanide, copper sulfate, nickel, silver cyanide, non-cyanide alkaline zinc, and zinc cyanide plating. Recovery of dragout from acid nickel process bath rinses is the most common RO application. Reverse osmosis also is used to purify tap water, recover plating chemicals from rinsewater, and polish wastewater effluent. Although RO recovers a concentrated dragout solution, some materials (e.g., boric acid) cannot be fully recovered (Freeman 1995). Reverse osmosis generally is not suitable for applications that have a highly concentrated oxidative solution such as chromic acid, nitric acid, and peroxy-sulfuric etchant. Also, the membranes will not completely reject many non-ionized organic compounds. Therefore, activated carbon treatment typically is required before the rinsewater solution can be returned to the rinse system, which can be costly (Cushnie 1994).
Facilities must carefully consider the membrane used in RO. The membrane must be specifically matched with the process chemicals. For instance, polyamide membranes work best on zinc chloride and watts nickel baths, while polyetheramide membranes work best with chromic acid and acid copper solutions (EPA 1995).
Although similar to other filtration technologies, RO is different in that:
The membranes in RO are unable to withstand pH extremes and long-term pressures. Feed concentrations can reach saturation, precipitate on the membrane, and cause the membrane to fail. Precipitation of contaminants must be avoided or RO will fail. Feed stream concentrations must be kept low by adding a pre-filtering system to the RO unit, usually an ultrafiltration unit (Warheit 1988). Reverse osmosis membranes also can be damaged by some incoming materials (e.g., iron and manganese).
Another concern is the potential for a reject rate of more than 50 percent of incoming flow depending on the characteristics of the influent and membrane porosity. Such a high rejection rate can be difficult to handle in a metal finishing operation unless the firm is using RO to generate deionized water where the disposal of rejected flow is not expensive. In a waste application, platers must treat discharge of concentrate, increasing the cost of the system and limiting the use of RO to wastewater recycling applications.
A typical application for process recovery using RO is nickel plating as shown in Figure 20. Because RO is such a delicate process, any change in bath chemistry can affect the operation of the RO unit.
While widely used in other industrial applications such as desalinization, RO is not used frequently as a recovery technology in metal finishing. The limited number of baths in which firms have successfully applied RO and the availability of competing technologies might be reasons. Other technologies that are available at much lower costs, such as atmospheric evaporation, often are more attractive options for metal finishers (Cushnie 1994).
Since flux rates vary from application to application, and customization and special engineering can be necessary, cost estimates based simply on flow or flux rates are approximate. Reverse osmosis units can cost $50,000 to $75,000 for flow rates of 75 liters per minute with cost as high as $300,000 for flow rates of 800 liters per minute. Operating costs include labor, energy, and membrane cleaning and replacement (EPA 1995).
Reverse Osmosis in Specific Baths
Table 23 provides information on specific metals used with RO.
Many metal finishers are familiar with ion exchange technology. This versatile technology has been used for decades and can be a major component of a low- or zero-discharge configuration. The most common applications in plating include:
The ion exchange process replaces somewhat harmless ions located in a resin with ions of concern (i.e., plating chemicals). The system is a molecular process where metal ions in solution are removed by a chemical substitution reaction with the ions in a resin bed. Resins are normally contained in vessels referred to as columns, rinsewater is passed through a series of resin beds that selectively remove both cations and anions. As rinsewater passes through the resin bed, the resin bed exchanges ions with organic compounds in the rinsewater. Figure 21 presents two typical configurations of ion exchange for bath maintenance.
Basically, ion exchange processes are either anionic or cationic. Anion resins exchange hydroxyl ions for negatively charged ions such as chromates, sulfates, and cyanides. Cation resins exchange hydrogen ions for positively charged ions such as nickel, copper, and sodium. An example of ion exchange is shown in Figure 22. Ion exchange systems typically operate in cycles consisting of the following four steps (Cushnie 1994):
Metals held in the solution are recovered by cleaning the resin with an acid or alkaline solution. Operators can electrowin metals from the resin regeneration solution while the water treated by ion exchange can be returned to the rinse system for reuse (Cushnie 1994). Figure 23 presents a typical ion exchange configuration for chemical recovery applications.
Ion exchange can selectively remove contaminants from a wastestream. In recycling applications, however, contaminants are not recovered along with the desired materials. Close control of the influent is important with low pHs reduce the capacity of the resin and high pHs tend to clog the resin with solids. One disadvantage of these systems is that no method exists to monitor the saturation of the resin. However, physical indicators such as reduced effluent quality can signify when the resin bed is saturated. Typically, facilities clean the cylinders on a time-based schedule (Freeman 1995). The recovery of chemicals from the resin columns generates significant volumes of regenerant and wash solutions, which can add to the wastewater treatment load (IAMS 1995).
Ion exchange should only be applied to relatively dilute streams and is best used in association with other conventional dragout recovery practices. Ion exchange systems are less delicate than RO systems, however, operators must filter the water to protect the resin, removing oil, grease, and dirt. In addition, certain other metals can foul the resin, requiring a special procedure to remove the foulant (Hunt 1988). In some applications, the solution generated from ion exchange (i.e., regenerant) is returned directly to the process tank. In most cases, however, regenerant is electrowinned or goes to traditional waste treatment systems (EPA 1995).
Optimizing Ion Exchange System Performance
A properly designed system will operate at maximum efficiency. Conducting treatability testing of specific wastestreams to ensure proper resin selection and sizing of the system is critical to the overall success. Treatability testing also will ensure that the system is not undersized or oversized and that interferences are not present that will render the resin ineffective. Other items that facilities should consider include:
Capital costs depend on the volume of flow and the level of automation. The components of an ion exchange system are relatively inexpensive and, depending on the application, can cost from $100 to $400 per cubic foot. Installation costs can be quite high. Platers can purchase and install small manual units, applied to flows of 20 liters per minute or less for $15,000 or less. A fully automatic 75 liters-per-minute unit with an integrated electrowinning unit costs approximately $75,000 installed (EPA 1995).
Operation and maintenance costs are generally low for ion exchange. A major expense is resin replacement. Resin should last 3 years or more, however, in certain applications (e.g., chrome) it can have a shorter lifespan. Resin costs from $7 to $22 per liter. Labor costs depend on the level of automation included with the unit and can cost from more than $1 per 1,000 liters for manual or undersized installations to less than $0.25 per 1,000 liters for fully automatic systems. Upstream components such as sand, polypropylene, and carbon filters also contribute to operationing costs (EPA 1995).
Two types of membrane dialysis systems are electrodialysis and diffusion dialysis. These systems are becoming increasingly popular for chemical solution recovery especially because they are more efficient and less expensive than other recovery technologies for reclaiming acid. They also can remove metals and recycle water in plating or anodizing shops (EPA 1995).
Platers commonly use electrodialysis to reclaim nickel and gold from plating rinsewaters. Figure 24 presents a flow schematic for a nickel plating line before and after installation of electrodialysis. This process uses both anion- and cation-charged selective membranes between a set of non-corrosive electrodes. As the plater recirculates contaminated rinsewaters between the charged surfaces, salts containing the metals are retained and returned to the plating tank. Rinsewater is reused in the dirtiest rinse or dragout tank. Separation is accomplished by applying a direct current across a stack of selective membranes. The membranes are stacked in alternating cation/anion stacks. Each stack is separated by a spacer through which solutions are allowed to flow (Cushnie 1994).
Figure 24. Example of Process Flow of a Nickel Plating Line Before and After Installation of Electrodialysis (EPA 1995)
When the solution passes through a cation-selective membrane, cations pass through and anions are trapped. As the solution continues to migrate, it will encounter an anion-selective membrane that will not allow the cations to pass. In this way, the wastestream is diluted of both anions and cations. The solution, which is returned to process tank, can be 10 times more concentrated than the feed stream, but usually is not as concentrated as the process bath (Cushnie 1994).
Applications and Restrictions
For electrodialysis to offer any advantages over competing technologies, the process fluid must tolerate the direct return of the concentrate. Because the returned solution is usually less concentrated than the bath itself, and because of the evaporative nature of the process, only heated fluids are candidates for this process (EPA 1995). One advantage of electrodialysis is its ability to selectively retard the recovery of certain organic materials, especially nickel, that build up in some baths. In so doing, electrodialysis can reduce the frequency of bath purification (Cushnie 1994). Most applications for electrodialysis are nickel-related although manufacturers have used this technology in copper cyanide, cadmium cyanide, and zinc phosphate applications (EPA 1995).
Capital costs are related to membrane surface area or to feed flow volume and characterization. Vendors customize most units for a particular application. In general, electrodialysis is more expensive than other recovery technologies. Units range in price from $75,000 to several hundred thousand dollars depending on the capacity of the unit. Operating and maintenance costs include energy, labor, and membrane replacement (EPA 1995). One vendor estimated that operating costs are $0.78 per gallon of acid feed. Primarily, these costs are incurred from operation and maintenance, labor, energy, deionized water, and membrane replacement (Cushnie 1994).
Diffusion dialysis is an ion exchange membrane technology used for the recovery of acids contaminated with metals from pickling, anodizing, stripping, etching, or passivation baths. This technology is commonly used in finishing facilities in Europe and Japan, but not in the United States. Companies use diffusion dialysis to purify some acid baths that are contaminated by metals. This technology can separate mineral acids and metals such as copper, chrome, nickel, iron, and aluminum so that acid can be reused. Recovery rates in some instances are as high as 95 percent for acid solutions and 60 to 90 percent for metal contaminants (Cushnie 1994). Currently, this technology is popular with anodizers that generate large amounts of waste sulfuric acid (EPA 1995).
The efficiency of a membrane to concentrate dilute acids in solution depends on the surface area available and the type of acid. Diffusion dialysis separates acids from metal contaminants via an acid concentration gradient that is placed between two solution compartments. These compartments are divided by an anion exchange membrane. Water is metered through one side of an anionic membrane, causing the acid to migrate to one side and the metals to stay on the other. Purified acid is sentback to the process tank and contaminant-laden spent acid and metals are sent to the metal recovery or waste treatment system. This technology does not use pressure or charge to move material across the membrane as do other membrane technologies. Movement is caused by the different acid concentrations on either side of the membrane (Cushnie 1994).
Capital costs for diffusion dialysis systems start at $18,000 for a 50 gallons-per-day system (EPA 1995).
Acid sorption is an acid purification technology used on a variety of acid solutions including pickling or sulfuric acid anodizing baths. A bed of alkaline anion exchange resin separates the acid from the metal ions. The acid is taken up by the resin while the metal ions pass through the membrane. The acid then is desorbed from the resin by water. This technology is rarely used by the plating industry (Cushnie 1994).
Figure 25 shows the steps in the acid sorption process. First, spent acid is pumped upward through the resin bed. A metal-rich, mildly acidic solution passes through the resin bed and is collected at the top of the bed. Second, water is pumped downward through the bed and desorbs the acid from the resin. The purified acid solution is collected at the bottom of the bed (EPA 1995).
This technology can recover approximately 80 percent of the free acid remaining in a spent solution. Facilities can purify the acid solutions in a batch mode, but using the technology in a continuous mode can produce a steady metal concentration in the concentrate. The capacity of a system is determined by the size of the resin bed and usually is expressed in terms of the mass of metal removed from the acid solution. Equipment capacities range from 100 grams per hour to several thousand per hour. Typically, vendors size a unit to remove metal near or above the rate at which metal is being introduced (EPA 1995).
Applications and Restrictions
Many plating shops with acid solutions could use acid sorption technology. Heated solutions and those containing oxidizers have to be cooled and filtered respectively prior to purification. Platers generally send the process byproduct (i.e., metal-rich solution) to the treatment system, but some electrowin the solution for metals recovery. In addition to anodizing and pickling baths, companies can apply acid sorption to non-chromic acid copper, brass etch, bright dips, nitric acid strippers, aluminum bright dips, and cation ion exchange regenerant. Chromates, concentrated acids, and some hydrochloric acid processes are not good candidates for this technology (EPA 1995).
Capital costs of acid sorption range from $30,000 to $40,000 for capacities under 200 grams per hour to more than $100,000 for capacities of 1 kilogram per hour. Little data is available on operating costs (EPA 1995).
Ion transfer generally is restricted to chromic acid plating baths, etches, and anodizing baths. As with the other chromic acid purification technologies, this technology selectively removes cations from chromic acid process fluids. Designs range from low-cost in-tank small porous pots to large multi-cell automated units with unified rectifiers and transfer pumps (EPA 1995).
Figure 26 presents a typical ion transfer arrangement. Ion transfer units consist of one or several membrane compartments that separate the cathode from the anode of an electrolytic cell. The membrane is usually a porous ceramic pot that contains the cathode. The anode surrounds the pot. The membrane also can be constructed of a polyfluorocarbon material and the catholyte compartment can be reinforced with polyethylene. The anode is in direct contact with the process fluid while the cathode is separated from the process fluid by the membrane. Equipment can be in-tank or external. Small in-tank units often use a process rectifier and operate only while parts are being plated. Operators must remove these units when the rectifier is switched off because the membrane will leak cations back into the process tank. Automated ion transfer units include a system that replenishes the catholyte with fresh fluid at regular intervals (Cushnie 1994).
Vendors determine the cation removal rates by the membrane area, the amperage applied to the cell, and the concentration of cations in the process fluid. Small units remove 10 to 50 cations per day while a multi-cell unit can remove up to 1,000 grams per day. Generally, removal rates fall sharply when the concentration of cations is below 3 grams per liter in the process fluid. Units usually are sized to remove cations at a rate near or somewhat faster than the introduction rate (EPA 1995).
Applications and Restrictions
Because of relatively low cation removal rates, ion transfer is best suited to maintaining relatively clean baths rather than attempting to clean highly concentrated ones. Tramp metal concentrations of 4 grams per liter can be achieved with this technology. Achieving lower concentrations, if possible, will result in higher energy costs and an increase in the volume of waste catholyte. The waste catholyte can contain some chromium, which is lost during catholyte changes (EPA 1995).
Companies have applied ion transfer to aluminum and other cation removal operations such as chromic acid etch or anodizing solutions, although such applications are rare. In etch solutions, the introduction rate is quite high and a multi-cell external unit is required (EPA 1995).
In-tank ceramic pot styles that operate with an off-the-tank rectifier can be purchased for less than $1,000. External units with 400 grams-per-liter removal capacities cost $30,000 or more depending on automation and instrumentation. Operating expenses include labor, electricity, and membrane or pot replacement. Membranes can last for several years. However, the pots can be broken during cleaning and handling. Manual systems require frequent catholyte changes and operators generally clean the pot during these changes. Sludge buildup in the catholyte requires frequent cleaning. Extending the bath life, and thereby reducing chemical use and waste generation, can produce significant savings (EPA 1995).
Membrane electrolysis is one of the newer technologies for recovery in metal finishing. Membrane electrolysis units consist of a tank containing an anode and a cathode compartment separated by a selective membrane(s) and a power source. Similar to ion exchange, the resins in membrane electrolysis are ion specific. Depending upon the membrane, they allow the passage of only negative or positive ions. The use of ion-specific membranes rather than ceramic pots or polyfluorocarbon materials differentiates this technology from ion transfer or other non-ion permeable technologies (Cushnie 1994).
The primary function of membrane electrolysis, when applied as a bath maintenance technology, is to lower or maintain acceptable levels of contaminates in plating, anodizing, etching, stripping, and other metal finishing solutions. For the plating industry, membrane electrolysis is most applicable to the maintenance of chromic acid solutions including hard chromium and decorative chromium plating, chromic acid etching, chromic acid anodizing, and chromic acid stripping. Other potential applications include sulfuric and nitric acid and sodium hydroxide-based solutions (e.g., pickling, etching, stripping, and rust removal solutions), chromate conversion coating, and sodium dichromate deoxidizer (Cushnie 1994). Figure 27 illustrates a common configuration of membrane electrolysis.
Costs for this technology are based on the removal of capacity of the unit and can range from $10,000 to $300,000. On average, however, the systems cost between $25,000 and $100,000. Installation costs are approximately 5 to 20 percent of the equipment costs. The main operating cost is labor. Other costs include electricity, cathodes, anodes, catholyte, and membranes.
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