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Pollution Prevention and Control Technologies for Plating Operations
Section 2 - General Waste Reduction Practices
2.4 DRAG-OUT REDUCTION
2.4.2 Drag-Out Reduction Techniques
2.4.2.3 Drag-Out Recovery and Return
2.4.2.3.1 Drip Tank
2.4.2.3.2 Drag-Out Tank
2.4.2.3.3 Drag-In/Drag-Out Rinsing
2.4.2.3 Drag-Out Recovery and Return
2.4.2.3.1 Drip Tank
A drip tank is an ordinary rinse tank that, instead of being filled with water, simply collects the drips from racked parts and barrels after plating and before rinsing. The drip tank is useful with work that involves continuous dripping over a period of time. Barrel plating, therefore, is a better candidate than rack plating for use of drip tanks. With barrel plating, the barrel should be rotated while it is suspended over the drip tank to ensure maximum drainage (see barrel rotation discussion in Section 2.4.2.2.1). When a sizable volume of solution has been collected in the drip tank, it can be returned to the plating bath. Because drag-out is not diluted with water when using a drip tank, this technique is especially applicable to lower temperature process solutions (ambient to 120F).
Using a drip tank will restrict the use of an additional rinse tank, when floor space is limited. As will be discussed, an additional rinse tank, used as a drag-out tank or in a counterflow arrangement, is usually much more beneficial than a drip tank since a drip tank only recovers the drag-out that freely flows off the part/rack. The determining factors are the volume of drag-out, part configuration (i.e., drainability) and the evaporation rate in the process tank.
A total of 21 (or 6.6%) of the survey respondents use drip tanks with automated plating equipment and 86 (or 27.0%) with manual plating lines. The success ratings for drip tanks were 3.24 (manual) and 3.40 (automatic), which are lower than the success ratings of drag-out tanks.
The drag-out tank is a rinse tank that initially is filled with pure water. As the plating line is operated, the drag-out rinse tank remains stagnant and its chemical concentration increases as more work is processed. Air agitation is often used to aid the rinsing process because there is no water flow within the tank to cause turbulence. The presence of a wetting agent is also helpful, according to Kushner (ref. 1). After a period of operation, the solution in the drag-out tank can be used to replenish the losses to the plating bath. If sufficient evaporation has taken place, a portion of the drag-out tank solution can be added directly to the plating bath (e.g., using a transfer pump). Evaporation usually will be sufficient with baths, such as chromium and nickel plating solutions, that are operated at elevated temperatures. Low-temperature baths, such as cadmium or zinc plating solutions, have minimum surface evaporation and often their temperature cannot be significantly increased without degrading heat-sensitive additives. Reportedly, new additives, which are not as readily degraded by heat, have been developed for many of these plating baths. These additives might make operation of the plating bath possible at higher temperatures, facilitating drag-out recovery. Usually the value of the recovered chemicals is much greater than the increased energy cost associated with operating the bath at a higher temperature (ref. 305).
Diagrams showing the use of drag-out tanks and other rinsing configurations discussed in this section are shown in Exhibit 2-13. Drag-out tanks can be combined with counterflow rinsing to provide both chemical recovery and flow reduction. Combinations of rinse configurations are discussed in Section 2.5.3.6.
As a rough estimate, drag-out recovery, which is also referred to as recuperative rinsing (ref. 13, 14), will reduce drag-out losses by 50 percent or more. The efficiency of the drag-out tank arrangement can be increased significantly by adding a second drag-out tank. Use of a two-stage drag-out system usually reduces drag-out losses by 70 percent or more. In some cases, multiple drag-out tanks (e.g., three to five tanks) can be used to completely close the loop and return essentially 100 percent of drag-out (ref. 305).
The potential benefits (i.e., percent recovery) of drag-out rinse tanks can be estimated using either derived or empirical equations, a discussion of which follows. Actual results will depend on local factors (e.g., part configuration, operating practices, humidity).
The drag-out rate and evaporation rate are the key parameters that determine what percentage of the drag-out can be recycled back to the process tank. Various mathematical formula have been used to estimate the recovery rate (ref. 1, 39, 301). Exhibit 2-14 presents estimates for common conditions that can be used in lieu of the more complex equations.
Various methods can be employed for estimating or measuring drag-out, as discussed in Section 2.4.1. Evaporation rates of process tanks depend mostly on the operating temperature of the bath and to a lesser degree on the intensity of solution agitation. Where there is little evaporation in a process tank (e.g., baths operated below 110F) there is little benefit from drag-out recovery tanks. PS 114 gave this method a rating of two for this reason. Various formulae and graphs are published for estimating evaporation (ref. 1, 39, 305). The following formulae are considered accurate, however, due to the relative ease of measuring evaporation rates, actual measurements should be used whenever possible.
Surface evaporation rates:
Still Tank: gal/hr/ft2 = e(0.03236T-7.20)
Agitated Tank: gal/hr/ft2 = e(0.02655T-5.95)
Where T = °F for process bath
(Source: ref. 316) The transfer of solution between drag-out tanks and the plating tank and the addition of fresh make-up water to the system can be accomplished in several ways. Ryder (ref. 16) recommends that transfers to the plating tank be accomplished using a small pump (magnetic drive, seal-less types) which is activated by a "dead-man" switch. The dead-man switch only permits solution transfer while the switch is depressed. If the operator leaves, the solution transfer automatically stops, which prevents catastrophic tank overflows. For adding make-up water, Ryder suggests using a level controlled valve (local float controlled) in the first rinse. When the solution level in the first rinse is lowered (i.e., after solution is transferred to the plating bath) the float switch is activated and fresh water is added to the final rinse. Ryder further suggests the use of a water control valve on the inlet water line for shut-off during non-operating periods.
With multiple rinse tank arrangements, the transfer of solution from rinse tank to rinse tank can be accomplished in the same manner as a flowing counterflow rinse system. These are discussed in Section 2.5.
The use of an automatic drag-out return system was described by Roy (ref. 4). In this system, chemical metering pumps were used to return drag-out from the rinse tank to the plating tank. The pumps were controlled by a level sensor in the plating tank. Roy also described a more complicated configuration where multiple drag-out tanks on different lines were connected to a "sump tank" using U-tubes. A level sensor in the sump tank controlled the water level in the drag-out tanks. As the drag-out return pumps drop the level in the sump tank, a level sensor turns on fresh water solenoids and the operating level is quickly restored.
It should be noted that although drag-out reduction can be a very effective means of pollution prevention, it may also present the plater with a new set of problems. In particular, by reducing drag-out, the plater reduces the purging of bath contaminants. The contaminants are contributed to process baths mainly by a breakdown of process chemicals and low concentration constituents in the fresh water (e.g., hardness). Other sources include: cross contamination due to transporting dripping racks over tanks, corrosion of bus bars, racks, anodes, tanks, etc., and airborne contaminants. Several shops identified contaminant buildup as a significant problem with drag-out reduction (PS 102, PS 124, PS 156). For example, PS 156 attempted to operate their shop on a near closed-loop basis. This facility, which now operates with an average flow of 16,800 gpd had reduced their flow, at one time, to 3,000 gpd. Their flow reduction efforts were partly discontinued after "contaminants entered the closed loop and caused plating problems." They also noted that their low volume discharge was contaminated with nickel and cyanide although these related processes were "closed loop." The presence of nickel and cyanide in their discharge may have resulted from cross contamination of baths caused by using the same racks for multiple lines. The presence of nickel and cyanide may also have been present when the flow rate was higher, but less noticeable due to dilution.
In some cases, drag-out reduction/recovery may result in serious degradation of the bath. To minimize the impact of contaminants, platers must do one or both of the following: (1) treat the raw rinse water prior to use with ion exchange and/or reverse osmosis technologies, (2) perform bath maintenance. Bath maintenance technologies are discussed in Section 4 rather than in this section. However, as a brief example of using bath maintenance with drag-out recovery, Exhibit 2-15 shows how one respondent has integrated electrolytic purification (dummying) into a closed-loop nickel rinsing configuration.
Another problem identified with drag-out tanks was the staining of parts by the drag-out rinse (PS 124 and PS 133). According to these shops, this was partly caused by drying of the solution on the part after drag-out rinsing.
The results of the Users Survey show that drag-out tanks are employed by 194 respondents (or 61.0% of all respondents) with manual plating operations and 62 (or 19.5% of all respondents) with automatic plating operations. The success ratings are 3.81 and 3.74, respectively.
2.4.2.3.3 Drag-In/Drag-Out Rinsing.
Drag-in/drag-out rinsing (also referred to as double-dipping) involves rinsing in the same solution before and after plating (see Exhibit 2-13). This can be achieved by using a single rinse tank or two hydraulically connected rinse tanks, usually located on opposite sides of the process tank. In the latter case, which is most applicable to automatic plating machines, the rinse water is recirculated between the two rinse tanks using a transfer pump to maintain equal concentrations of chemicals in the tanks.
The advantage of a drag-in/drag-out arrangement is that plating chemicals rather than pure rinse water are transferred into the process tank by incoming racks or barrels. This increases the recovery efficiency of the recovery rinse.
The drag-in/drag-out system finds application with plating baths that have a low to moderate evaporation rate and especially with baths that tend to increase in volume (i.e., equivalent to a negative evaporation rate). This condition, referred to as "solution growth" is common to zinc cyanide baths (e.g., PS 051), where the volume of drag-in (water from the preceding rinse) can be greater than the sum of drag-out and evaporation. The recycle ratio, which determines recovery efficiency, is calculated as the volume of recycled rinse plus the volume of drag-out divided by the volume of drag-out. The recycle ratio, therefore, is greater with a drag-in/drag-out system than a common recovery tank. If the evaporation rate is low, the difference between the recycle ratios for common recovery and drag-in/drag-out systems is significant. When evaporation ratios are high, the difference is less. Generally, the use of a drag-in/drag-out arrangement will increase the recovery rate by 25 to 40 percent (ref. 305). The use of atmospheric evaporators for eliminating solution growth, an alternative to the drag-in/drag-out system, is discussed in Section 3.2.
As with drag-out tanks, the drag-in/drag-out arrangement can result in bath contaminant buildup, as indicated by PS 124, who rated the success of this method as a "1."
The use of a drag-in tank adds an extra labor step to the recovery rinsing process. PS 173 indicated that although this method worked, only a few of their employees will take the time to use it.
Exhibit 2-15 shows a drag-in/drag-out arrangement used by PS 118. Rinse water is circulated between the drag-in and first drag-out tank by pumping from one to the other and having a gravity return by raising the height of one tank. They are able to operate on a closed-loop basis except when plating certain assembled tubular pieces that have excessive drag-out. A porous pot is used to prevent excessive contaminant buildup.
A substantial number of survey respondents use drag-in/drag-out rinsing configurations. A total of 66 (or 20.8%) of the respondents indicated that they use this method with manual operations and 34 (or 10.7%) use it with automatic operations. The success ratings for drag-in/drag-out rinsing are 3.39 (manual) and 3.82 (automatic).