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
188.8.131.52 Drag-Out Recovery and Return
184.108.40.206.1 Drip Tank
220.127.116.11.2 Drag-Out Tank
18.104.22.168.3 Drag-In/Drag-Out Rinsing
22.214.171.124 Drag-Out Recovery and Return
126.96.36.199.1 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 188.8.131.52.1).
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.
184.108.40.206.2 Drag-Out Tank
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
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 220.127.116.11.
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
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.
18.104.22.168.3 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).
Next Section|Main Table of Contents|Section 2