PLATING AND SURFACE FINISHING
AMERICAN ELECTROPLATERS AND SURFACE FINISHERS SOCIETY
12644 Research Pkwy.
Orlando, FL 32826-3298
Escalating energy costs, new control technology, and regulations governing wastewater and hazardous materials have impacted the design and application of evaDorative systems for recovering and recycling plating solutions. The outcome has been the development of small, efficient, automatic systems that employ partial recovery and offer an improved economic picture.
Since 1949, when evaporative systems were used to recover chromic acid, several design changes have been effected and applications have been expanded to include practically every type of plating solution. At first, recovery systems were purchased on the basis of a payback in two to three years by virtue of the value of recovered solution less operating costs. Later, evaporative recovery was implemented as a pollution-abatement tool. Even though the Environmental Protection Agency was not yet in existence, certain communities had imposed discharge limitations on specific toxic materialsóprimarily cyanide and hexavalent chromiumóand some companies found they needed only evaporative recovery to solve the pollution problem.
These systems, however, had a relatively large evaporative capacity to provide a sufficient amount of distilled water to satisfy the entire rinsing requirements of the plating bath in concern. This mode of rinsing became known as closed-loop recovery because there is no discharge to the sewer from the rinse tanks serving the plating tank. All of the water for rinsing is provided by the recovery system and all of the rinse tanks counterflow back to the recovery system. Closed-loop recovery is illustrated schematically in Fig. 1.
In days past, large flows of rinsewater were required because of the limited number of counterflow rinse tanks available on plating lines. Multiple counterflow rinses were not common because water was inexpensive and dilution provided a solution to pollution. The volume of water required for adequate rinsing is determined by five factors: dragout volume; dragout concentration; rinsing efficiency; concentration of plating chemicals on the work surface after the final rinse; and the number of counterflow rinse tanks.
Figure 2 plots the dilution factor vs. the rinse ratio for one, two, three and four counterflow rinse tanks. Using this graph, the required flow of rinsewater can be determined when the number of counterflowrinsetanks,n,thedilutionfactor,cb/cn (chemical/ metal concentration in the plating bath divided by the maximum allowable concentration in the final rinse), and the dragout rate are known. For this graph, the rinsing efficiency, which is the concentration of contaminants in the rinse tank divided by the concentra tion of lragout from the rinse tank, has been chosen as 1.0. For example: If the dilution factor is 10,000 and the number of counterflow rinse tanks is three, the required flow of rinsewater is 21.5 (rinse ratio) times the dragout rate. This graph shows that a limited number of counterflow rinse tanks with closed-loop recovery requires a relatively large flow of rinsewater that equals the capacof the recovery system.
Up until the early 1970s, it was not unusual for companies to purchase a recovery system rated at an evaporative capacity of 1500 to 2300 L/hr (400 to 600 gal/hr). These large systems were economically viable primarily because the cost of energy in the form of steam was just $0.80 per million BTUs. Energy costs were reduced by double-effect evaporators, but with an attendant increase in the physical size and operational complexity of the system.
But, in 1974, with the advent of rapidly rising energy costs, there began a metamorphosis to downsize recovery systems in a manner that paralleled strategy on the part of U.S. auto manufacturers. Other factors that influenced this downsizing included the widespread use of waste-treatment systems and the use of multiple counterflow rinse tanks. The net result of these factors is that the optimum capacity of evaporative recovery systems can be decreased substantially.
Today, smaller systems have improved the economic posture of evaporative recovery. No longer is there a need to operate in a closed-loop manner and depend on the recovery system to supply all of the rinsewater. Instead, systems now can operate with open-loop rinsing, as depicted by the flow diagram in Fig. 3.
This approach allows the system to operate with the first one or two rinse tanks and recover 90 to 99 percent of the dragout from the plating tank. This range of recovery is verified in Fig. 2. The last one or two rinse tanks receive external water, as required, to achieve the desired rinsing results. The water leaving these final rinse tanks contains just 1 to 10 percent of the dragout from the bath and is sent to the waste-treatment system. Open-loop rinsing enables both recovery and waste treatment to complement the strengths and weaknesses of the other.
Recovery is most economical where there are low flow rates of water and high concentrations of contaminants. Conversely, treatment is more economical where there exist higher flow rates and lower concentrations of chemicals. Using a combined setup however, there will be a decrease in energy consumption by the recovery system, and the treatment operation will require less chemicals and generate lower volumes of sludge. Hence, the economic attractiveness of evaporative recovery has expanded beyond just the value of the recovered plating solution.
To appreciate the difference in the economic posture of open vs. closed-loop evaporative recovery, consider the following example: a chromium plating operation with a bath containing 240 g/L (32 oz/gal) of CrO3 and having a dragout rate of 20 L/hr (5.3 gal/hr), using three counterflow rinse tanks. The chromic acid concentration in the final rinse tank is assumed to be 4.8 mg/L (0.0006 oz/gal) cost.
Closed-Loop Recovery (Fig. 1) Example: Referring to Fig. 2, the required flow of rinsewater is determined using n = 3 and cb/cn = 240/0.0048 = 50,000. This provides essentially 100 percent recovery. The rinse ratio is approximately 40; therefore, the rinsewater flow rate is 40 x 20 L/hr, or 800 L/hr (210 gal/hr). The operating cost of an 800-L/hr recovery system is equal to about $2.50/kg ($1.1 4/lb) of recovered chromic acid. This figure does not include the savings of $1 /kg ($0.45/lb) of chromic acid from treatment chemicals avoided or the credit for eliminated sludge-disposal costs, which amount to $1.05/kg ($0.48/lb) of chromic acid.
Open-Loop Recovery (Fig. 3) Examle: With this type of setup (Fig. 2), the required flow rate of rinsewater can be determined using n = 1 and the 90-percent recovery line that represents a dilution factor of 10. The rinse ratio is 10 and the rinsewater flow rate is 10 x 20 L/hr, or 200 L/hr (53 gal/hr). As compared with the 800-L/hr closed-loop recovery system, the 200-L/hr open-loop system recovers 90 percent of the dragout at one-fourth of the operating cost. The open-loop approach requires 23.5 L/min (6.2 gal/min) of city water for rinsing in the final two tanks to achieve the dilution factor of 50,000. The 10 percent of the dragout not recovered normally requires treatment. The total economic benefits for open-loop, or partial, recovery for typical decorative chromium plating operations are as shown in Fig. 4.
The inclusion of costs for both treatment and sludge disposal also enhances the economic justification for evaporative recovery of other plating solutions. For instance although the value of cadmium cyanide solution is much less than that of chromic acid electrolytes, the recovery economics are just as great. The reason for this is the high cost of cyanide treatment, which is approximately $6.60/kg ($3/lb) for two-stage alkaline chlorination using sodium hypochlorite.
Regardless of the capacity, evaporative recovery involves a physical processóthe evaporation of water. Referring to Fig. 5, contaminated rinsewater overflowing the first rinse tank flows by gravity to a collection tank, commonly called the feed tank of the recovery system. From the feed tank, rinsewater is drawn into the recovery system by vacuum. The feed rate is governed by the evaporation rate. Inside the system, the contaminated rinsewater passes through the tubes of a rising-film thermosyphon reboiled Low-pressure steam is introduced on the shell side of the reboiled condenses on the outside of the tubes, and imparts its heat to the solution in the tubes.
The solution exits the reboiler and enters the separator as a mixture of vapor and liquid. The liquid falls to the lower section of the separator. The vapor is pulled by the vacuum pump through a de-entrainment device and into the condenser, where it condenses as distilled water. It is recycled back to the rinse tank.
The solution remaining in the separator passes through the reboiler repeatedly until it reaches the desired concentration. The system then shuts down automatically and discharges the recovered plating solution to a holding tank for transfer to the plating tank.
Evaporative recovery systems are available for the plating solutions shown below. In addition, pilot-plant studies with a trivalent chromium bath indicate that evaporation can be successfully adopted for this plating solution.
(Solution/Types or Comments)
A new, expanding market for evaporative recovery comprises concentrating aqueous solutions that must be hauled away for off-site disposal. Hauling and disposal costs generally are in the range of $0.15 to $0.25/L ($0.60 to $1 /gal). Evaporative recovery systems can reduce the volume of waste solution prior to disposal by reducing the water content. Volume reductions of 90 percent are not unusual and the operating cost of the recovery system is less than $0.02/ L ($0.08/gal). Obviously, the volume of the waste solution must be substantial in order to make its concentration economically beneficial.
The advent of small evaporative capacity levels has led not only to more compact systems (Fig. 5), but paved the way for automatic controls. One-button controls that automatically start the system in the proper sequence and have it operational within 5 min are available.* Sequencing involves the following: cooling water; initiating vacuum; controlling liquid levels; and monitoring the steam and concentration of solution. These functions monitored for proper values, and the controls will shut down the recovery system in the event of a significant deviation from a desired value. All of the sequencing and monitoring controls are achieved with conventional relay logic. A typical control panel is shown in Fig. 7 (not available).
A conventional control panel employs approximately 15 timedelay and mechanical relays. These can be replaced by one small microprocessor-based programmable controller. The price difference between the two types of controls is small. The hardware for the conventional relay controls is less expensive; however, the microprocessor has a lower installation cost because it requires less hard wiring. The microprocessor has no mechanical partsójust solid-state circuitryóand it simplifies troubleshooting.
Reliable and Cost Effective
Evaporative recovery systems have proven to be reliable, effective and economical in more than 30 years of field experience. Today's systems are compact, economically sound, and offer microprocessor-based controls. The economic benefits of such recovery systems include:
Evaporative systems recover valuable resources, conserve waste-treatment chemicals, and minimize the production of metal hydroxide sludge. Unlike chemical treatment, evaporative recovery provides users a return on investment and helps to lower plating costs.