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Pollution Prevention and Control Technologies for Plating Operations
Section 2 - General Waste Reduction Practices
2.5 RINSEWATER REDUCTION
2.5.3 Alternative Rinsing Configurations
2.5.3.1 Generating Rinsing Data
2.5.3.2 Counterflow Rinsing
2.5.3.3 Cascade, Reactive, and Dual Purpose Rinsing
2.5.3.4 Chemical Rinsing
2.5.3.5 Spray Rinsing
2.5.3.6 Combined Drag-Out Loss/Rinse Water Reduction Rinsing Arrangements
2.5.3.1 Generating Rinsing Data
To evaluate rinsing alternatives effectively, the plater needs drag-out and water use data, rinse quality criteria and equations that utilize these data and criteria. Of these items, the drag-out data are the most difficult and expensive to obtain. The high cost is due to a need for expensive analytical work. A well proven method of generating drag-out data, that is easy and inexpensive to apply, is discussed in this subsection. This method, which utilizes conductivity measurements, was originally described by Mohler (ref. 2).
In order to utilize this methodology, a conductivity meter is required. Using a conductivity meter greatly reduces the analytical costs of generating rinsing data. In a matter of hours a plater can generate a quantity of data equal to that generated by days of laboratory work. Also, unlike using laboratory analytical methods, with the conductivity meter approach there is almost no lag time associated between the taking of samples and generating of results. Most plating shops have combination pH/conductivity meters in their shops that can be used for this purpose. Alternatively, a portable unit of sufficient quality can be purchased for $200 to $300. Exhibits 2-17 and 2-18 present the conductivity values and rinsing criteria used in the following method.
1. Use conductivity measurements to determine the present concentration of a plating solution in a rinse tank (example: sulfuric acid pickle).
- Measure the conductivity of your tap water (or DI water if used): a = 100 µmho (micromhos)
- Measure the conductivity of the rinse water. If a multiple rinse tank arrangement is present (e.g., counterflow), make the conductivity measurement in the last rinse (i.e., least contaminated): b = 5,500 µmho
- In a container, add one ml of the pickle solution to one liter of rinse water and measure the conductivity: c = 6,250 µmho
- The increase in conductivity for one ml of pickle solution per one liter of rinse water is: c - b = 6,250 - 5,500 = 750
- The concentration of pickle solution in the rinse water is: e = (b - a)/(c - b) = 5,400/750 = 7.2 ml/l
- The equivalent sulfuric acid in the rinse is (see Exhibit 2-17, 6,300 µmho = 1,000 mg/l): 1,000 (c - a)/6,300 = 6,150,000/6,300 f = 976 mg/l equivalent H2SO4
- Compare this concentration to the criteria in Exhibit 2-18 to determine if the proper rinse flow rate is being used. If necessary, adjust the rinse flow and repeat the measurements until the rinse criteria is met.
2. Use conductivity measurements to determine the drag-out rate.
- First determine the rinse water flow rate. Record the number of racks processed per hour. Note the valve setting, turn off the flow, bail out six or more inches of water, set the flow and record the rate of rise of the water in inches per minute. From the number of racks processed and the rate of rise of the water calculate the flow as liters per rack (alternatively flow can be expressed in l/ft2 of wetted area, l/barrel, or another relevant unit of measure): g = 8.0 l/rack
- Second, measure the drag-in per rack. Turn off the flow, agitate the rinse, measure the conductivity, process a number of racks of work through the rinse, agitate the rinse, and measure the conductivity again. From the µmohs increase per rack calculate the drag-in ml/rack: h = 50 ml/rack
3. Use the equation presented in Section 2.5.3.2 to evaluate the effect of adding additional rinse tanks.
Electroplaters have long reduced water use by employing several rinse tanks connected in series. Fresh water flows into the rinse tank located farthest from the process tank and overflows, in turn, to the rinse tanks closer to the process tank (see Exhibit 2-13). This technique is termed counterflow (or countercurrent) rinsing because the work piece and the rinse water move in opposite directions. Over time, the first rinse becomes contaminated with drag-out and reaches a stable concentration which is lower than the process solution. The second rinse stabilizes at an even lower concentration which enables less rinse water to be used then if only one rinse tank were in place. The more counterflow rinse tanks (three-stage, four-stage, etc.), the lower the rinse rate needed for adequate removal of the process solution.
Counterflow rinsing systems are not without drawbacks. The negative aspects of counterflow rinsing include: (1) cost of additional rinse tanks; (2) loss of valuable production space; and (3) an increase in production time/labor.
The rinse rate needed for adequate cleaning is governed by an exponential equation that depends on the concentration of plating chemicals in the drag-out, the concentration of plating chemicals that can be tolerated in the final rinse tank before poor plating results, and the number of counterflow rinse tanks. The mathematical rinsing models are based on complete rinsing (i.e., removal of all drag-out from the part/fixture) and complete mixing (i.e., homogeneous rinse water). These conditions are not achieved or even approached unless there is sufficient residence time and agitation in the rinse tank. More typically, each added rinse stage reduces rinse water use by 50 percent.
The most commonly applied counterflow rinsing model follows:
Rc = (Ct/Cr)1/n where, Rc = rinsing ratio; Ct = concentration in the preceding tank; Cr = required concentration in the last rinse tank; n = number of rinse tanks
This model does not predict required rinse rates accurately when the value of Rc falls below 10. Also, complete rinsing will not be achieved unless there is sufficient residence time and agitation in the rinse tank.
The following example demonstrates the use of the counterflow rinsing equation. A typical Watts nickel plating solution contains 270,000 mg/l of total dissolved solids, and the selected final rinse concentration is 37 mg/l of dissolved solids. The ratio of Ct/Cr is 7,300 and approximately 7,300 gallons of rinse water are required for each gallon of process solution drag-in with a single-tank rinse system. By installing a two-stage rinse system, water requirements are reduced to 86 gallons of water per gallon of process solution drag-in (assumes 100% rinsing efficiency). The same degree of dilution is obtained in the final rinse, and the rinse water consumption is reduced by 99 percent. The mass flow of pollutants exiting the rinse system remains constant, however, the pollutants are much more concentrated with a two-stage rinse system than with a single-tank rinse system.
From the Users Survey results, 217 respondents (or 68.2%) employ counterflow rinsing. These respondents gave this method of water use reduction a success rating of 4.21, the highest for any water use reduction method.
2.5.3.3 Cascade, Reactive, and Dual Purpose Rinsing
Cascade rinsing refers to the practice of reusing rinse water multiple times in different rinse tanks for succeeding less critical rinsing. Reactive rinsing is similar, but it refers to cases where a chemical reaction takes place as a result of using the rinse water for multiple purposes. An example is reusing the rinse water following acid cleaning as rinse water following alkaline cleaning. In this case, the acid rinse water helps to remove the viscous alkaline film remaining on a part after alkaline cleaning.
Dual purpose rinsing refers to the practice of using the same rinse tank for rinsing following more than one process tank. It provides essentially the same results as cascade and reactive rinsing but uses a fewer number of rinse tanks. Often, the employment of dual purpose rinsing means transporting a dripping rack/part over a considerable distance. This can result in dripping onto floors and/or the accidental contamination of other tanks. An exhaustive evaluation of dual purpose rinsing is presented by Mohler (ref. 3). Mohler presents methods and guidelines for ascertaining the accumulated concentration of chemicals in counterflow, dual-purpose rinses in order to determine the feasibility and economics of this technique.
Typically, cascade and reactive rinsing are used for automatic plating machines whereas dual purpose rinsing is used for manual operations.
Use of any of these methods must closely consider the combined chemistry in the rinse tank to prevent undesirable reactions that may impact worker safety (e.g., hydrogen cyanide formation) or work quality (e.g., precipitation of solids).
Reactive or cascade rinsing is used by 76 (or 23.9%) of the survey respondents to reduce water usage. These respondents gave this method an average success rating of 3.79.
The technique of chemical rinsing has been used by the metal finishing industry for many years. One of its earliest applications was to eliminate staining from chromium solution, which is notoriously difficult to rinse. By simply making the first rinse after chromium plate a stagnant rinse containing sodium bisulfite, the drag-in of hexavalent chromium was converted to trivalent chromium. The rinsability of the workpiece in the second rinse was improved considerably by changing the chemical nature of the film on the workpiece in the stagnant rinse and by reducing film concentrations before attempting to rinse by diffusion. The same principles are frequently employed in "neutralizing" dips.
The application of chemical rinsing to plant effluent treatment, known in the industry as integrated waste treatment, has been described by Lancy and Pinner (ref. 305). Aside from the environmental benefits, this type of rinsing also prevents the majority of heavy metal solids formed in the chemical rinse from reaching the succeeding water rinses by removing these materials in an external settling vessel. Removal of these solids is accomplished by flowing the chemical rinse solution to a treatment reservoir. The overflow from the reservoir is pumped back to the rinse tanks, forming a complete closed-loop system. Integrated treatment gained some popularity in the 1970's, but is believed to be in little use today, mostly due to high maintenance requirements.
There were no questions in the Users Survey regarding chemical rinsing, nor did any respondents indicate that they currently use it or have used it in the past. Due to this lack of data, the presumed decline of integrated waste treatment use has not been verified.
Spray rinsing is employed in various manners to reduce drag-out losses and rinse water use. Spray rinsing over process tanks (Section 2.4.2.2) provides direct recovery of drag-out. Spray rinse tanks can be used as drag-out tanks, single rinses, or multiple rinses.
A common use of spray rinsing is to substitute a spray rinse tank for an overflow rinse tank. Depending on the part configuration, spray rinsing generally uses from one-eighth to one-fourth the amount of water that would be used for equivalent dip rinsing (ref. 305). Spray rinsing is most effective for flat-surfaced parts and is less effective with recessed and hidden surfaces.
Although less commonly used, a spray rinse can substitute for a drag-out tank. Several shops indicated that they consider this a very successful pollution prevention measure (e.g., PS 177, PS 242, PS 275). Using this method, one shop has reduced their discharge from 15,000 gpd to an average flow of 800 gpd (PS 242). PS 275 employs a fog spray rather than a common spray rinse (success rating of "4").
Combined spray and dip rinse tank designs are employed where the bottom portion of a rinse tank acts as a dip tank and the upper portion a spray rinse. A weir is located at approximately the middle of the tank which maintains the solution level in the tank. In operation, the rack is lowered into the dip rinse, raised above the solution level, and sprayed with fresh water. This combination rinse can be nearly as effective as a counterflow rinse, but takes up the floor space of one tank.
The design of spray rinses must consider the size and shape of the part. Spray nozzles are available in many sizes and spray patterns, and should be selected appropriately. Usually, the pressure in the waterline is sufficient to operate an effective spray rinse, however, higher spray velocities can be obtained by pumping.
A special application of the spray rinse is a patented unit (ref. 6) that contains five to seven progressively cleaner rinse solutions in separate compartments. The solutions are successively pumped (up to 20 gpm) to a spray rinse tank and drain back to the unit. During each cycle, only the water use in the first spray is discarded or processed for recovery. The subsequent sprays are collected for reuse in the following cycles. The advantage of this unit is that it provides the effect of multiple counterflow rinsing with use of a single rinse tank. The floor space requirement of the unit is 7.5 square feet (five stage rinse unit) or 11.0 square feet (seven stage rinse unit).
Although it is not widely used, spray rinsing is applicable to barrel plating operations as previously shown in Exhibit 2-11.
One shop reported that their platers are mistrustful of the efficiency of spray rinses. Whenever plating quality problems arise, the platers target the spray nozzels as the cause (PS 176). Another shop complained that spray rinses often clog and must be cleaned or replaced (PS 230).
The Users Survey showed a moderate to high usage rate of spray rinsing. Most frequently it was used as a water use reduction method and to a lesser degree for drag-out loss prevention (fog or spray rinse over process tank). As a water use reduction method 124 (or 39.0%) of the shops employed spray rinsing. These respondents gave spray rinsing an average success rating of 3.82.
2.5.3.6 Combined Drag-Out Loss/Rinse Water Reduction Rinsing Arrangements
Many plating shops combine drag-out tanks and overflow rinsing in the same rinse systems (see Exhibit 2-13). For example, a four rinse system could consist of two drag-out tanks connected in series and two free-flowing rinses connected in series (counterflow). Alternatively, the system could consist of three drag-out tanks in series and a single overflow rinse or a drag-in/drag-out arrangement and two counterflow rinses. Various rinsing configurations can also be combined with chemical recovery technologies, as discussed in Section 3. The optimal rinse configuration will depend on numerous factors including: (1) the evaporation rate in the plating tank, (2) the drag-out rate, (3) the rinse water quality requirement (final rinse), (4) process chemical costs, (5) alternative technology recovery costs, (6) water costs, and (7) wastewater treatment/sludge disposal costs.
In general, when more of the available rinse tanks are used as drag-out tanks, the process chemical and wastewater treatment operating costs are lowered and water use costs are increased. The reverse is true when more tanks are used for counterflow rinsing than for drag-out recovery tanks.
The optimal configuration can be determined through mathematical means which must be supported by data collection (i.e., drag-out and evaporation measurements, production rates, etc.) for producing accurate results. Examples of such analyses are presented in ref. 305 for chromium and nickel plating. As an alternative to using the rinsing equations to perform the calculations, a modeling program can be employed. A commercially available software program (ref. 317) permits an analysis considering up to five rinse stations and the use of supplemental evaporative recovery. This program permits the user to add recovery rinses, change tank volumes, experiment with process chemistries, add evaporators, and change workload to find the combination that makes the most environmental and economic sense (ref. 398).