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MNTAP MINNESOTA TECHNICAL ASSISTANCE PROGRAM The Minnesota Technical Assistance Program is funded through a grant provided by the Waste Management Board. I

Reducing Chromium Losses from a Chromium Plating Bath

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Page 1: Reducing Chromium Losses from a Chromium Plating Bath

MNTAP MINNESOTA TECHNICAL ASSISTANCE PROGRAM

The Minnesota Technical Assistance Program is funded through a grant provided by the Waste Management Board. I

Page 2: Reducing Chromium Losses from a Chromium Plating Bath
Page 3: Reducing Chromium Losses from a Chromium Plating Bath

1 n 1 1 :1 :1 : I :I n :I I] 3 :I :I 11

Reducing Chromium Losses from a Chromium Plating Bath

1987 Summer Intern Report

By Diane Achman

Project Conducted at New Dimension Plating

Hutchinson, M N

Page 4: Reducing Chromium Losses from a Chromium Plating Bath

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Page 5: Reducing Chromium Losses from a Chromium Plating Bath

New Dimension Plating, Inc. of Hutchinson, Minnesota plates a variety of parts including motorcycle accessories, stove parts, exercise equipment parts and also does some custom plating. The company employs about 40 people and operates for 1 or 2 eight hour shifts each day depending on the work load. Reducing the chromium carried out of the plating bath is the focus of this report.

Because New Dimension Plating is a job shop, they plate parts that vary greatly in size and shape. This fact along with the fact that the plating is done manually are the main reasons that a significatnt amount of chromium is carried out from the plating tank on the racks each day. At the beginning of this project, the daily increase in chromium concentration in the stagnant rinse tank was determined to be about 1 oz/gal. As the tank contains 115 gallons, this loss added up t o about 7 lbs of chromium per day. On an average day 300-350 racks of parts are plated with chromium.

Options reviewed to reduce chromium drag-out included using a drip bar, installing an automatic hoist, or utilizing a spray rinsing system. Utilizing a spray rinsing system in a separate tank before the stagnant rinse was evaluated to be the best option.

In addition to reducing chromium dragout, options for returning chromium to the plating tank were investigated. The options reviewed included evaporation, reverse osmosis and use of ion exchange columns. Because of the relatively low volume of processing in this line, reverse osmosis and ion exchange were ruled out in part due to cost. Evaporation was selected as the preferred option although no units on the market were small enough for New Dimension Plating.

Once the preferred options were selected, an appropriately sized spray rinse system and evaporators were built on-site. The spray rinse system fitted with 6 nozzles is rectangular, fitting around the inside of a 115 gallon rinse tank. A drip bar allows the parts to be suspended in the spray rinse tank while they are sprayed with a fine mist of water for 5 seconds. After dripping, parts are rinsed in the stagnant rinse, and the overflow rinse which follow.

Two small evaporators have been installed, each with a 20 gallon capacity. One is used to reduce the volume of the plating tank, and the second one concentrates the stagnant rinse.

After one month of operation, test results show drag-out to the stagnant rinse has been reduced from 7 lb. of chromium per day to just over 1 lb. of chromium per day. The spray rinse volume averaging about 25 gallons per day is added back to the plating tank as is the stagnant rinse after concentration in the second evaporator.

Page 6: Reducing Chromium Losses from a Chromium Plating Bath

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Page 7: Reducing Chromium Losses from a Chromium Plating Bath

T a b l e of C o n t e n t s

PE11-e

I n t r o d u c t i o n . . . . . . . . . . . . . . . . . . . . . . . . . 1

O b j e c t i v e . . . . . . . . . . . . . . . . . . . . . W a s t e s u r v e y . . . . . . . . . . . . . . . . . . . .

E v a l u a t i o n of Chromium Loss t o S t a g n a n t R i n s e . O t h e r F a c t o r s E v a l u a t e d . . . . . . . . . . . .

Waste R e d u c t i o n O p t i o n s Development . . . . . . . . . R e d u c i n g Chromium D r a g o u t . . . . . . . . . . . R e c o v e r y of Chromium for R e t u r n t o P l a t i n g Tank

C o n t a m i n a n t Removal . . . . . . . . . . . . . . E v a l u a t i o n of O p t i o n s . . . . . . . . . . . . . . .

O p t i o n s f o r R e d u c i n g Chromium D r a g o u t . . . . . R e c o v e r y of Chromium for R e t u r n t o P l a t i n g Tank

C o n t a m i n a n t Removal . . . . . . . . . . . . . . Recommendation . . . . . . . . . . . . . . . .

I m p l e m e n t a t i o n . . . . . . . . . . . . . . . . . . .

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Resu l t s . . . . . . . . . . . . . . . . . . . . . . . . . 21

Servings E v a l u a t i o n . . . . . . . . . . . . . . . . . . . 23

Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . 25

A p p e n d i c e s

A p p e n d i x A . . . . . . . . . . . . . . . . . . . . . . . 2 6 A p p e n d i x B . . . . . . . . . . . . . . . . . . . . . . . 27 A p p e n d i x C . . . . . . . . . . . . . . . . . . . . . . . 2 8 A p p e n d i x D . . . . . . . . . . . . . . . . . . . . . . . 29 A p p e n d i x E . . . . . . . . . . . . . . . . . . . . . . . . 30 A p p e n d i x F . . . . . . . . . . . . . . . . . . . . . . . 3 1 A p p e n d i x G . . . . . . . . . . . . . . . . . . . . . . . 32

Page 8: Reducing Chromium Losses from a Chromium Plating Bath

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New Dimension Plating, Inc. is a small metal plating shop located

in Hutchinson, Minnesota. The company employs about fourty people and

operates for one or two eight hour shifts with an average of 315 racks

of chrome plating per eight hour day. They plate a variety of metals

including copper, nickel, gold, brass and chromium. (See Appendix A,

page 2 6 ) Chromium is the major metal plated and is usually the last

step in plating cycle. Most parts are copper plated and then nickel

plated in preparation for chrome plating. The main difference between

New Dimension Plating and other plating shops is the variety of parts

plated. As New Dimension Plating is a job shop, a wide range of parts

such as motorcycle accessories, stove parts, and custom items are

metal finished. The plating lines are manual, meaning employees dip

the racks into the tanks by hand. This fact along with the fact that

parts vary greatly in size and shape accounts for the significant

drag-out on the chromium plating line.

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Page 9: Reducing Chromium Losses from a Chromium Plating Bath

N I Dimension’s prim

OBJECTIVE

ry concern is to reduce chromium o ste and

still maintain a high level of quality in plating. The company is

currently meeting EPA and city requirements for wastewater discharges

but at a great expense. The objective of this project has three main

parts. The first was to quantify the amount of chromium lost as drag-

out from the plating tank. The second was to identify the particulars

of the process to pinpoint where the major losses of chromium occur and

explore alternative processes for these areas. Finally it became

necessary to identify options to reduce the amount of chromium that is

inevitably carried out of the plating tank. At this stage research

was conducted on different technologies available for recovering and

reusing the chromium in their plating process tank as opposed t o

recovering the metal to ship off-site for use elsewhere. With

information about different options, New Dimension Plating could decide

what management system best suits the company’s needs.

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Page 10: Reducing Chromium Losses from a Chromium Plating Bath

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HASTE S_U_R_VE_Y The chromium plating line at New Dimension Plating consists of a

470 gallon chromium tank, a 115 gallon stagnant rinse tank, and three

counterflow rinse tanks followed by a hot still rinse. (See Appendix

B, page 2 7 ) The plating tank contains chromium at a concentration of

32 ounces per gallon and a temperature of 115'F.

tank serves to "catch" a large portion of the chromium rinsed off the

parts with the idea that it can be added directly back to the plating

tank to replace water lost to evaporation. The countercurrent rinsed

have a continuous water flow which averaged 1.2 gallons per minute.

The last rinse in the countercurrent series, the rinse directly after

the stagnant rinse tank, drains to the plant's waste treatment system

which removes chromium from the wastewater. The current waste

treatment operation for removing chromium in the waste water begins by

reducing the hexavalent chromium to trivalent chromium with sodium

metabisulphite. This waste stream is then mixed with the waste

streams from other processes and the pH is adjusted to between 7 and 9

with sodium hydroxide. Sludge is generated in a filter press and then

dried in a sludge drier. An average of 5 barrels of dried sludge from

total wastewater treatment are produced each month.

The stagnant rinse

To evaluate the loss of chromium from the plating tank, the

stagnant rinse and flowing rinse tanks were emptied and replaced with

clean water at the beginning of this project. Due to the wide

variability of the shape of the parts which influences the amount of

chromic acid solution they drag-out, it is difficult to establish a

close correlation between surface area plated and increase in

concentration in the stagnant rinse tank. Although the parts plated

vary from small screws to large tubular parts, the number of racks

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Page 11: Reducing Chromium Losses from a Chromium Plating Bath

processed each day is fairly constant, the average being 315.

Evaluation of the stagnant rinsewater to determine chromium

losses was a primary emphasis of the waste survey. At the time of the

survey, the evaluation of the countercurrent rinses was limited to

flow rate considerations. The countercurrent rinses are continually

diluted s o they are maintained at low enough chromium concentrations

to provide good rinsing. Low chromium concentrations in the

counterflow rinses were difficult to measure by the titrimetric method

used at New Dimension Plating therefore it was decided to concentrate

first on reducing the drag-out to the stagnant rinse tank and evaluate

the rinses later if necessary. Lowering the concentration in the

stagnant rinse tank would reduce the amount of chromium dragged into

the countercurrent rinses.

EVALUATION OF CHROMIUM LOSS TO THE STATIC RINSE

The stagnant rinse concentration was monitored for two days and

the hourly increases were recorded. The results for the total daily

increases are graphed 'for one week in Figure 1 along with the result

of an additional sample tested about two weeks later. The data shows

that the daily increases of chromium in the stagnant rinse tank is

quite steady for the first few days, then it begins to level off.

As the concentration increase levels off, a significant amount of

chromium is no longer being saved in the stagnant rinse tank.

(Appendix C, page 28 provides the data used to construct Figure 1.)

The daily increase in a stagnant rinse concentration was calculated

to be approximately 1 oz/gal per day. This adds up to nearly 7 lbs. of

chromium (about 4 gallons of plating solution) that is being removed

from the plating tank by drag-out each day. Previous to emptying the

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Page 12: Reducing Chromium Losses from a Chromium Plating Bath

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Figure 1.

Equilibration o f Drag Out

0 1 2 3 4 5 6 7 8 9 10 1 1 12 13 14 15 16 17 18 19 20 21 22 23 24

Date o f June -- 2:45 p.m.

stagnant rinse tank it had not been changed for five months yet the

concentration of chromic acid was only 8 .4 ounces per gallon. This

revealed that an equilibrium had been reached and the amount of chromic

acid solution dragged out of the stagnant rinse tank equalled the volume

dragged-in.

Once the chromium increase of about 1 ounce per gallon per day in

the stagnant rinse was known, the volume of drag-out per rack could be

calculated. This was done by dividing the total number of racks

processed by the total number of gallons of solution dragged out that

day. The total volume dragged out was determined by calculating how

much of the 32 oz/gal chromium plating solution was dragged out t o give

the stagnant rinse concentration increased by 1 oz/yal and it holds 115

gallons of water, this means that 115 ounces of chromium or 3 . 6 gallons

5

Page 13: Reducing Chromium Losses from a Chromium Plating Bath

of plating solution were dragged into the stagnant rinse that day. As

the average number of racks plated per day was 315, the average drag-out

volume was calculated to be .012 gal/rack. This value compares well

with the experimental value of .013 gal/rack obtained by allowing a

plated rack to drip into an empty container for 2 minutes and then

measuring the volume that dripped off. The figure of .013 gal/rack is

the average of repeating this test three times.

Once the drag-out per rack was known, it became possible to

experiment on the amount of time needed for racks to stop dripping

plating solution. This would give insight as to whether drip bars would

be advantageous. Ten seconds was sufficient time for dripping to end,

and five seconds of dripping removed as much as 7 5 % of the carried out

solution (drag-out). This value was estimated by comparing the volume

that dripped off in 5 seconds as opposed to letting the rack drip for 5

minutes. But it was observed that a film of chromic acid coated the

parts even after extended draining periods. By rinsing the film off and

then measuring the concentration of the resulting solution, a value for

the ounces of chromic acid on the part was obtained. From this value it

was estimated that up to 50% of the chromic acid solution carried out

clings to the parts and does not drip off. This was important

information to be considered when procedural changes were studied.

OTHER FACTORS EVALUATED

Other areas included in the process of evaluating for the waste

survey were; flow rates of overflow rinses, the evaporation rate from

the plating tank, and possible contaminants. The flow rate of the

countercurrent rinses was determined by allowing the stream to fill a

container for one minute, then measuring the amount collected. The flow

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Page 14: Reducing Chromium Losses from a Chromium Plating Bath

rate varies greatly when an additional source of water is pumped into

the last rinse tank in the countercurrent series. This additional water

is pumped in once every ten minutes for a one minute interval. The

water flow is nearly tripled during this time. The flow rate was

calculated by incorporating both values proportionately and was

determined to be an average of 1.2 gal/min.

Rate of evaporation is one factor that limits the amount of drag-

out that can be added back to the plating tank. The evaporation rate is

relatively low because the plating bath is kept at only 115OF. Another

factor is the wetting agent that is added to the bath which creates a

foam blanket to control fumes thus inhibiting evaporation. An average

of 11 gallons evaporates each day from the plating tank which is 470

gallons and has a surface area of' 15 square feet.

Another factor that limits the quantity of drag-out that can be

added back to the plating tank is the effect of returning unwanted

contaminants to the plating bath. The contaminants are carried into the

plating bath from baths through which parts pass prior to the chromium

plating tank. The contaminant concentration in the plating tank has

remained low and relatively constant because as impurities are carried

into the bath, the impurities are also being dragged out with the

chromic acid solution. The impurities include metals such as nickel,

copper, zinc and iron and the chloride ion. Trivalent chromium, which

is produced as a secondary reaction in the plating tank, is also a

contaminant of concern. The monthly increases as determined by M&T

Chemicals, Inc. are listed in Appendix D. Controlling contaminant build

up will be an important limiting factor when considering methods f o r

returning the chromium to the plating tank.

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Page 15: Reducing Chromium Losses from a Chromium Plating Bath

WASTE RE_DUC_TION OllTIONS DEVELOPMENT

New Dimension Plating, Inc. was interested in first reducing the

chromium drag-out from the plating tank and then recovering the

chromium that had been carried out for return to the plating tank.

Non-return methods were not studied because New Dimension’s aim was to

return a concentrated solution back to the originating process rather

than recover metals or chemicals for use elsewhere. Information

gathered is described in three sections; options which would reduce

chromium drag-out, options which could be used to recover chromium

from drag-out solution and a third section is used to discuss options

for removing contaminants.

REDUCING CHROMIUM DRAG-OUT

Reducing the amount of plating solution carried out could be

accomplished at a low cost by simply increasing the time allowed for

the solution to drip off the parts. The racks used are made of solid

copper so they are quite heavy and difficult to hold up for even short

periods of time. For this reason the racks are usually not allowed to

drip thoroughly and large amounts of chromic acid solution are carried

out into the rinse baths. The three options that were evaluated to

reduce drag-out are installing a drip bar, using an automatic hoist,

or spray rinsing the parts before they are placed in the drag out

tank.

Drip bars could be used either above the plating tank or the

stagnant rinse tank. The two problems that must be considered are

possible staining of parts and loss of efficiency in production. If

the racks were hung on the drip bar and then accidently forgotten, the I1

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Page 16: Reducing Chromium Losses from a Chromium Plating Bath

chromic acid could stain the parts. Sometimes the stains can be wiped

off with a cloth, but other times only grinding out the stain will

remove it. The added step of hanging the racks on the drip bars and

waiting for them to drip thoroughly could be inconvenient and time

consuming.

An automatic hoist could help decrease drag-out by allowing the

parts to drip thoroughly, but may reduce production also. It may

actually be more work to use a hoist than to plate manually due to the

variability of the size of the parts. The layout of the shop would

also make it quite difficult for a hoist to be used efficiently.

Rapid Plating located in Sauk Rapids, Minnesota was visited to view

their use of automatic hoists on their plating lines. They plate

mainly zinc on industrial type parts and plate many parts of the same

type at one time. Because of the shapes of the parts and the set up

of their tanks, the hoists aided in their production.

A spray rinse tank could be used before the stagnant rinse tank

to keep the concentration of the stagnant rinse low. If a drip bar

were used above a spray rinse tank, the majority of drag-out could be

confined to one tank. The water would dilute the concentrated

solution clinging to the parts and the drip bar would allow the parts

to drip more completely. Also , the fine mist from the spray rinsing

would create a very humid atmosphere which would prevent the parts

from being stained because the solution would not dry on them. The

process could be very efficient if a timer were installed s o the

spraying would occur for a given number of seconds each time a button

was pushed. An automatic timer would allow the employees t o simply

hang the rack in the spray rinsing tank, push the button, and attend

to other duties while the parts are rinsed and allowed to drip off.

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Page 17: Reducing Chromium Losses from a Chromium Plating Bath

The length of time for spraying and number of spray nozzles needed to

provide adequate rinsing would have to be determined. The amount of

solution resulting from spray rinsing could be added directly back to

the plating tank each day if the amount produced did not exceed the

amount of evaporation from the tank.

RECOVERY OF CHROMIUM FOR RETURN TO THE PLATING TANK

Methods of recovering chromic acid solution can be divided into

two basic categories; those processes that deal with the diluted

rinse waters and those that work with concentrated solutions such as

the drag-out solution.

Reverse osmosis and ion exchange are the two options that were

considered for removing chromium from dilute rinse waters. In reverse

osmosis, water is forced out of solution through a selective membrane

thus concentrating the solution. It is called reverse osmosis because

a pressure greater than the osmotic pressure must be applied to a dilute

solution to force water through the membrane. The process yields a

moderately concentrated solution that can he added back to the plating

tank. However, it does not remove impurities so they must be removed

by another method. Water Technologies Inc. located in Mineapolis,

Minnesota offered a unit for New Dimension to use on a trial basis but

because the cost of the systems was out of the company's range, it was

decided that our time would better be spent pursuing other options.

Ion exchange uses specific resins to remove either cations or

anions or can have both incorporated on one unit. This method is

effective in collecting essentially all traces of cations and/or anions

from dilute rinse streams. Resins must be regenerated chemically when

they become ''loaded". This method was studied rather extensively

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Page 18: Reducing Chromium Losses from a Chromium Plating Bath

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because it can remove impurities as well as chromium from rinse water

with the use of different resins. Douglas America Inc. of Eden

Prairie, Minnesota was visited and the Douglas recycling system

evaluated to see if it would suit New Dimension’s needs. Their system

used two resins: a cation resin to remove the metals and an anion

resin to remove the non-metals, including the chromic acid anion from

the water. Indian Head Plating of Chippewa Falls, Wisconsin was also

visited to evaluate their ion exchange system, They plated hard

chromium as opposed to New Dimension’s decorative plating s o the system

differed slightly from what New Dimension would need. The system was

manufactured by Eco-Tec Limited located in Ontario, Canada. This

system used the same basic principles of ion exchange but differed in

design from the Douglas system.

Evaporation is the primary method used to recover chromium from

concentrated solutions. This method was studied extensively due to

its relatively low cost and ease of operation. It is a very

straightforward and reliable method. An evaporating unit can be

constructed with a few calculations to determine evaporation rate once

surface area, temperature, and power available are known. By adjusting

these three variables the efficiency of the evaporator can be

maximized.

The Surface Finishing Convention held in Chicago was attended and

representatives from Industrial Filter and Pump Manufacturing Company

were consulted on their Max-Evap evaporator. The design of the

evaporator enables it to evaporate approximately 8 to 10 gallons of

water from the chromium tank per hour when the tank is maintained at

llO°F to 115’F.

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Page 19: Reducing Chromium Losses from a Chromium Plating Bath

CONTAMINANT REMOVAL

When concentrated drag-out solution is returned to the plating

tank, impurities concentrated in the drag-out are also returned to the

tank. Therefore, procedures which could be used to remove impuritiies

had to be considered in evaluating reverse osmosis and evaporation as

options for the recovery of chromium. Two methods that could be used

to remove impurities were studied.

The first method is the use of electrolytic purification cells

manufactured by Cosmos Mineral Corporation of Camarillo, California.

The system is set up as a separate tank that is connected to the

plating tank. The unit incorporates the use of ceramic cells in

compartments with sets of anodes and cathodes to remove contaminating

metal cations from the plating solution. The size of tank and number

of cells required is based on the level of contaminants in the

solution and the size of the process tank. The chromic acid solution

is circulated from the process tank through the purification cell and

returned to the process tank. This system is capable of removing large

amounts of impurities.

The second method studied was removal of cation impurities by the

use of a mini CatNapper manufactured by Innova Technology, Inc. of

Clearwater, Florida. The mini CatNapper uses a specially designed

membrane in conjunction with electromigration principles to remove the

cations. It is actually just one cell from the larger CatNapper unit

which is commonly sold. The mini CatNapper modules are installed

directly into the existing plating bath. It is convenient f o r New

Dimension Plating that Innova offers a small unit to handle a low

contaminant level.

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Page 20: Reducing Chromium Losses from a Chromium Plating Bath

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In addition t o the metal impurities, the chloride ion is also a

contaminant, however it can be more easily removed. Chemical

suppliers for New Dimension Plating suggest dummying the tank with the

smallest cathode area possible and highest attainable voltage when the

chloride concentration becomes too high. Flew Dimension uses an old

rack with 3" wires connected radially from the center of the rack as a

dummy rack. Plating this dummy rack at a high voltage removes the

chloride ion as an impurity plated in the chromium metal.

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Page 21: Reducing Chromium Losses from a Chromium Plating Bath

rtYAliUATIQN BE OPTIQNS The process of evaluating options for reducing chromium waste

occurred in three steps. The first step was to review options which

would decrease the amount of chromium carried out from the plating

tank. After reducing the amount carried out as much as possible,

options to recover the chromium that does get carried out were

evaluated. Finally if chromium is recovered and added back to the

plating tank, the impurities will also be added back so options to

remove them were reviewed.

OPTIONS FOR REDUCING CHROMIUM DRAG-OUT

The installation of a drip bar is a very low cost way to help

decrease the amount of chromic acid solution carried out of the

plating tank. The major concern with this is the possibility of

staining if the racks are left to drip too long. One way to solve the

problem would be to install a drip bar in a spray rinse tank. By

using the spray rinse the volume of concentrated drag-out could be

greatly reduced. The size of nozzles and length of spraying time are

two variables that could be adjusted to control the amount of solution

produced so that it could be returned to the plating tank each day.

It was calculated that six fine nozzles in an empty tank the size of

the current stagnant rinse tank would produce approximately 20 gallons

of solution each 8 hour day if the spraying was pulsed for 5 seconds.

Each nozzle was experimentally determined to release 75 ml of water

when pulsed for 5 seconds. The expected concentration of the solution

was calculated to be 5 ounces per gallon by knowing an approximate

value of the amount of chromium drag-out per rack was and the amount

of water released during a 5 second pulse of spraying. If the

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Page 22: Reducing Chromium Losses from a Chromium Plating Bath

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evaporation from the chromium tank was increased by 10 gallons per

day, all of this solution could be returned to the plating tank each

day.

The estimated cost for making a spray rinsing systems is about

$200 since the current stagnant rinse tank would be used. The drip

bar can be made from a sturdy piece of pipe. The combination of spray

rinsing with the use of a drip bar was in fact pursued as discussed in

the implementation section.

The use of an automatic hoist was ruled out because it would be

inefficient with the way the tanks are arranged. A hoist would work

best when process tanks are large and arranged in a line in plating

order and when parts are very similar in type.

RECOVERY OF CHROMIUM FOR RETURN TO THE PLATING TANK

When using reverse osmosis to achieve a concentrated stream for

return to the plating bath the solution must be circulated through

the unit several times because there is insufficient loss through

natural evaporation from the plating tank to accomodate the direct

return of a large amount of solution. Another problem is the fact

that undesirable impurities are not removed through reverse osmosis s o

an additional method would be needed to purify the solution. The cost

of the reverse osmosis system manufactured by Water Technologies, Inc.

is approximately $30,000. Since the solution would probably need to

be purified and further concentrated following reverse osmosis, this

system would not be economical for New Dimension Plating, Inc.

The Douglas Recycling System sold by Douglas America, Inc.

utilized a series of ion exchange columns consisting of four

cartridges. The cartridges contain resins through which rinse water

a 3

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Page 23: Reducing Chromium Losses from a Chromium Plating Bath

is pumped upward from a reservoir at the base of the column.

Maintenance of the system is relatively simple. The bottom cartridge

11

ii fills up first and is removed for regeneration. The top cartridge

takes its place and a regenerated cartridge is added. Regeneration is

accomplished by running an acid through the cartridge to dissolve the

metal. The cation resin collects metal impurities and the anion resin 11

/ I collected the chromic acid anion. Options for the handling of

solution obtained when the resins are regenerated would need to be

considered.

The advantage of this system is that both functions are performed ll II l l II II

within the same unit, only in different cartridges, making it very

convenient. The estimated cost of the system is $30,000. The cost of

the ion exchange system manufactured b y Eco-Tech Limited is about

$60,000. The payback period would be a few years so it was decided

that perhaps the purchase of an ion exchange system could be a long ' I term goal. Because of the high cost for this system, it was not seen

as an option to pursue currently.

Evaporation as a method of recovery is very widely applicable and l l

the materials of construction are readily available. Two offsetting

/ I factors are relatively high energy consumption and the fact that

evaporation will not remove undesirable impurities. If a high

percentage of chromium drag-out is returned to the plating tank an

additional technique will be needed to remove accumulated metal

1 1

l l impurities.

Evaporation can produce a highly concentrated solution which can I1

be added directly to the plating tank or can be used to evaporate

water from the plating tank so additional solution can be returned to

the tank. The costs of the Mac-Evap evaporation built b y Industrial l l

16

Page 24: Reducing Chromium Losses from a Chromium Plating Bath

1

1 c. 1 3 3 3 I]

3 3

11

Filter and Pump Manufacturing Company is $4,000. It will evaporate up

to 10 gallons of water from the chromium plating tank per hour. This

would create extra space in the tank so more drag-out solution or spray

rinsing solution could be added back to it each day. The estimated

cost to build an evaporator, depending on size and design, is under

$500. The cost can be even lower if existing equipment, such as a

used tank, is used in its construction. The payback would be within 6

months to two years depending on the size of the evaporator and its

initial cost. This is quite reasonable for a company the size of New

Dimension Plating, and was selected as the option to pursue.

CONTAMINANT REMOVAL

The electropurification system manufactured by Cosmos Mineral

Corporation is capable of removing a much higher level of contaminants

than is necessary for New Dimension Plating. For this reason the mini

CatNapper suits their needs more efficiently. The fact that the mini

CatNapper module is installed directly into the plating bath makes it

very convenient t o use. Another positive feature is that the

maintenance of the mini CatNapper is minimal. The cost of one module

is $900. New Dimension would need one or two cells to purify their

plating bath. This is much lower in costs than any other purification

method researched. Once implementation of recovery options begins,

New Dimension Plating plans to conduct further testing on the rate of

increase of impurity concentration in the plating bath before making

any final decision on the purchase of a purification system.

' I

17

Page 25: Reducing Chromium Losses from a Chromium Plating Bath

RECOMMENDATION

In summary, the most economical and efficient way to reduce New

Dimension’s chromium loss seems to be a combination of three different

options. Incorporating the use of spray rinsing in conjunction with a

drip bar before the stagnant rinse is the first step. The rinse tank

directly following the spray rinse could be converted to a stagnant

rinse. An evaporator used to evaporate water from the plating tank

would allow all of the spray rinse solution and some of the stagnant

rinse water to be added back to it each day. An additional evaporator

may be needed to concentrate the stagnant rinse. A mini CatNapper

installed in the plating tank would continually purify the plating

bath.

l l

18 ll

Page 26: Reducing Chromium Losses from a Chromium Plating Bath

-------------- IMPLEMENTATION

New Dimension Plating, Inc. has made the following changes to

incorporate the recommendations made. The stagnant rinse tank was

converted into a spray rinse tank with a drip bar and the next tank in

line was changed to a stagnant rinse tank. This set up provided

adequate rinsing so that the next two smaller rinses could be combined

into one larger rinse tank. Figure 2 shows the new layout for the

rinse system. The flow rate was decreased from 1.2 gal/min to 1

gal/min. Two evaporators were installed, one above the plating tank

to evaporate directly from it and one behind the stagnant rinse tank

to concentrate its solution.

Evaporator Overflow to drain

Figure 2. Layout of Chromium Plating Line before and after Implementation of the Spray Rinse Tank

. Evaporator

Hot S t i l l 0 Overflow to drain

Stagnant Rinse

AFTER -

Hot Still c) 19

Page 27: Reducing Chromium Losses from a Chromium Plating Bath

The spray rinse ring and evaporators were

Plating. The spray rinse ring was constructed

built by New Dimension

to fit inside the tank

previously used as a stagnant rinse tank. It was constructed from

1/2" PVC piping and is secured 4" from the top of the tank. Three

nozzles were evenly spaced along each of the two long sides of the

ring. The nozzles were purchased at a farm implement store and are

usually used on equipment to apply fertilizer. Their spray angle is

120 which is very effective in covering all parts of a rack.

Testing had to be done to determine flow rate because they were rated

in gal/acre. Other sources of nozzles would be from companies that

sell fluid handling equipment. These would generally be rated in

gal/min. Appendix E, page 30 shows a picture of possible nozzles.

The main feature of these nozzles is that they are installed

vertically but spray horizontally. This is important so that the spray

rinsing apparatus is parallel with the tank so that the racks will not

catch on protruding objects when they are placed in the tank. A

solenoid valve and timer were connected to the spray rinse ring so

that when a button is pushed, spraying occurs for 5 seconds. A drip

bar rests on the top of the tank. It can be easily slid back so racks

can be placed in the tank, then slid to the middle so the racks can

hang on it while they are sprayed and allowed to drip. Using the drip

bar allows the parts to hang above the surface of the water.

The evaporators were constructed from used 45 gallon rinse tanks.

Their dimensions are approximately 24" x 18" x 12". They are sealed

but have access ports on the top in case problems occur. Each

evaporator is divided into 4 equal sections. An air blower is mounted

on the top of the first section to push air into the first chamber.

The second chamber is filled with 3/4" polyethylene balls and has a

Page 28: Reducing Chromium Losses from a Chromium Plating Bath

solution inlet at the top where the solution is pumped in and allowed to

run down through the balls. The air flows through the balls covered

with solution and goes into chamber 3 . There is a drain at the bottom

of this chamber to allow unevaporated solution to drain back into the

tank. The air must pass through baffles before it goes into chamber 4

where it is exhausted to the ventilation system. (See Appendix F, page

31 for diagram.) One evaporator is connected to the plating tank s o

n 1 1 1

the solution is already heated. The second evaporator evaporates 1 rinse water from the 45 gallon tank filled with stagnant rinse water.

The rinse water is heated llO°F using a water heater and pump to

circulate hot water through PVC coils immersed in the solution.

. J

._ I 1

RESULTS

n The spray rinse tank was experimentally determined to collect

approximately 2 . 8 gallons of solution per hour. This adds up to about

22 gallons per 8 hour day with an average of 315 racks plated. The

evaporator over the plating tank evaporates approximately 1 gal/hour.

Combined with the natural evaporation from the plating tank of .5

gal/hr, this adds up to about 36 gallons per day which allows all of

-1 3

the spray rinse solution and some stagnant rinse solution t o be

returned to the plating tank. 3 The drag-out solution from the stagnant rinse is concentrated in

the second evaporator before it is added t o the plating tank.

Once the spray rinse tank was functioning properly and

employees were comfortably using it, chromium analyses were conducted

to evaluate improvements. At the beginning of the project, the level

1

of chromium in the stagnant rinse increased at a rate of approximately J II 21

Page 29: Reducing Chromium Losses from a Chromium Plating Bath

concentration in the stagnant rinse increases at only . 02 oz/gal per

hour. These values were averages calculated for an 8 hour day. In

both cases the number of racks processes per day was close to 315.

Comparison of the data once the spray rinse was implemented

indicates that the dragout to the stagnant rinse tank is about one

sixth of the chromium previously lost to the stagnant rinse. At the

beginnning of the project about 7 lbs of chromium was carried into the

stagnant rinse daily. With the spray rinse operating, just over 1 lb

is carried out into the stagnant rinse. Appendix 0 , page 32 shows

increases in chromium concentration in the stagnant rinse tank after

the spray rinse was implemented.

At the time of this report, both evaporators had been operating

for four weeks. Evaporation rates from the plating tank have been as

predicted allowing about 25 gallons of spray rinse to be returned to

the plating tank every day. In additign to this, 10 gallons of

concentrated stagnant rinse is returned. Each day about 30 gallons of

stagnant rinse is added to the 45 gallon tank evaporator used for the

stagnant rinse. It is hoped that by removing this amount on a daily

basis, the stagnant rinse concentration can be kept below 2 oz/gal.

The additional solution needed to fill the plating tank is taken from

this evaporator. Since the evaporator for the stagnant rinse

evaporates about 1 gallon/hour, about 20 gallons will evaporate in a

day, leaving 10 gallons that can be returned to the plating tank.

Figure 3 provides a visual image of the process just described.

Both evaporators are exhausted through a common vent. There is

concern about the l o s s of chromium to the atmosphere, although it is

not a problem mentioned by manufacturers of evaporators. I1

22

Page 30: Reducing Chromium Losses from a Chromium Plating Bath

n n 1

n :I 1

1

3 3 ,1

J

Q

Figure 3 . Chromium Return and Evaporating Process

25 gallons transferred daily 30 gallons transferred daily 10 gallons transferred daily Approximately 35 gallons evaporated daily

PLATING YANK

I I

SAVINGS EVALUATION

Attempts were made to determine the savings achieved by the

implementations of the spray rinse system and the evaporators. The

reduction in chromium drag-out has resulted both in a decrease in

chromic acid use and in the use of chemicals needed to treat the

chromium in the wastewater.

An evaluation of savings has been difficult to formulate because

the volume of work increased, therefore shifts were lengthened and

production increased. An attempt was made to come up with a value

with these variables factored in.

I] 23

Page 31: Reducing Chromium Losses from a Chromium Plating Bath

M o n t h l y S a v i n g s :

c h g n ~ i y g u s e :

c o s t ( p r e v i o u s u s e ) - c o s t ( c u r r e n t u s e ) = $ s a v i n g s / m o n t h 2 .96 /1b (100 l b ) - $2 .96 /1b ( 2 0 l b ) = $238/month

gqgjyg M&lEligy&Eh&gg gs_e igsggcgingll

3.16 l b / l b C r = l b s used /mon th $ . 4 2 / l b ( 3 1 6 l b ) - $ . 4 2 / l b ( 6 3 . 2 l b ) = $ l l O / m o n t h

T o t a l m o n t h l y s a v i n g s : $348.00

I n a d d i t i o n t o t h e s e s a v i n g s , t h e r e w i l l b e a r e d u c t i o n i n t h e u s e of

s u l f u r i c a c i d n e e d e d t o l o w e r t h e p H o f h e x a v a l e n t chromium b e f o r e

r e d u c i n g i t a n d i n t h e u s e of sodium h y d r o x i d e used t o r a i s e t h e pH a t

t h e t i m e of p r e t r e a t m e n t b e f o r e f i n a l release t o t h e sewer. The

volume of s l u d g e w i l l a l s o b e r e d u c e d , b u t s i n c e t h e volume p r o d u c e d

by t h e chromium d r a g o u t w a s c a l c u l a t e d t o b e o n l y be tween 1 / 2 a n d 1

b a r r e l p e r month b e f o r e t h e p r o j e c t s t a r t e d , t h e s a v i n g s w i l l n o t b e

as s i g n i f i c a n t as s a v i n g s i n c h e m i c a l c o s t s f o r chromium a n d sod ium

m e t a b i s u l p h i t e .

A cost e v a l u a t i o n i n terms o f payback w a s n o t c o n d u c t e d b e c a u s e

b o t h t h e s p r a y r i n s i n g s y s t e m a n d t h e e v a p o r a t o r s were f a b r i c a t e d a t

New Dimens ion p l a t i n g . An e v a p o r a t o r c o m p a r a b l e i n s i z e and d e s i g n t o

t h e o n e b u i l t b y N e w Dimens ion P l a t i n g was n o t f o u n d on t h e m a r k e t .

A l s o no s p r a y r i n s i n g s y s t e m o f a n y k i n d w a s f o u n d .

ll I1 II ii ll ii I1 I/ I1 I1 /I ll II l l l l I1

24

Page 32: Reducing Chromium Losses from a Chromium Plating Bath

3 n

:1 1

1 3

11

SUMMARY

The changes made at New Dimension Plating, Inc. were possible

because of the involvement and cooperation of management and

employees. Because of the fact that New Dimension is a job shop,

there were always many variables to keep in mind when testing was

conducted. With the help of the employees, these variables were

controlled ae, much as possible t o collect consistent, reliable data.

From the data collected at the beginning of the project and then after

the implementations were made, it can be shown that a significant

amount of chromium is now being recovered and returned to the plating

t a n k . However, the project does not end here, as there will a lways be

room for further improvements.

- - - - - - - Diane Achman is a senior chemistry major at St. Cloud State University, St. Cloud, Minnesota.

J 25

Page 33: Reducing Chromium Losses from a Chromium Plating Bath

C h r o m i u m P Z a L i n g Process

I

I I I 1

m I

t 1 4 0 n

A l l racks cleaning line. D) Nickel plating Copper plating of steel and zinc die cast parts, Aluminum cleaning and precoat. 2) steel parts Zincate and copper strike. 3) steel parts

1) zinc and aluminum parts

E) Chrome plating (after nickel) Note: Blowup for (E) on following page

Page 34: Reducing Chromium Losses from a Chromium Plating Bath

lu U

r + Cr ,

Rinse l inse 45 ga1.45 gal.

6 0 Hot S t i l l 0

I

0 Rinse 115 gal. @

Stagnant Rinse 115 gal.

chrome 470 gal. 0

Tank Set Up

%munterflow rinsing i n tanks 3,4,5 Wmmiun plating tank 110-115 F :"xnium plating tank approximately 32 oz/gal

Page 35: Reducing Chromium Losses from a Chromium Plating Bath

Appendix C

Drag Out R e s u l t s

J u n e 1

7 : 00 . o o 8 : 1 0 .20 9: 30 . 3 8 10: 30 .54 11: 30 . 6 5 2 : 4 5 ( e n d of d a y ) 1 . 0 1 T o t a l # of r a c k s : 334

J u n e 2

7: 00 1 . 0 1 8 : 00 1 .34

10: 00 1.69 1 1 : o o 1 .87 12: 00 1 .94 1: 00 2 . 0 0 2:oo 2 .03

T o t a l # of r a c k s : 372

.........................................

9: 00 1 . 4 8

2:45 ( e n d of d a y ) 2 . 1 8

' I

J u n e 4

' 2 : 4 5 ( e n d o f d a y ) 3 . 6 8 T o t a l # of r a c k s : 257

ll

28

Page 36: Reducing Chromium Losses from a Chromium Plating Bath

n n I 1 1 n :-I 3 n 111

Appendix D

Contaminant Concentration

Dates

-- Imparity ---- Eel?: ss Des: sG AEEL 82 J w e 82

trace Trivalent -18 . 06 -

Iron .04 - . 0 7 . 0 5

.03 .05 Copper .08 -

e 007 Chloride . 009 a 003 -

a 13 e 19 Nickel .26 -

.01 . 0 2 Zinc -

* Concentration units: ounces/gallon

* Analysis done b y M&T Chemicals Inc. of New Jersey. Their midwest representatives are Industrial Chemical and Equipment Company of Minneapolis.

I

2 9

Page 37: Reducing Chromium Losses from a Chromium Plating Bath

Appendix E

b d u d No.

Spray Nozzles

THREAD CONNECTION 5PSIG 20PSlG

,145 ,168

,239 ,274

.32 .37

.083 ,095

278 ,314

,120 .135

,252. ,285

25PSlG

,192

,307 .41

,108

,360

.I51

,321

- EFLEC TlON ,NGLE - 650

68O

680

710

215-1 118"male NPT

215-2 1/8"male NPT

215.12 118"male NPT

215.14 114" 28 straight

215-15 1/4"28 straight

215.16 114" 28 straight

215.28 1/8"male NPT

215.29 118 male NPT

215.30 1/8"male NPT

215.65 118'' male NPT

with O-ring groovl

with O-ring groovc

with O-ring groow

Suitable for - Liquid Spray Characteristic - Flat Material - PFA

I s* CAPACITY

% 39 1

146

19

05

163

075

153

.19

,092

,050 - Model No. A

215.2 .59" 215-14 97" 215-15 .55" 215-16 9 7 "

,215.28 59'' 215-65 59''

DIMENSIONS

"O-ring Groove

- ID. 0 2 Gland Depth .400" 598 ' ' ,020''

(Use AS-012 0-Ring where required.)

0 PSlG

,125

.195

.2 7

,068

,233

,099

,208

-

.27

,122

,067 - 0

.28"

.70"

.28"

.70"

.28"

.28"

.32 1 .l1 1 .41

,145 ,168 ,193

.078 ,102

OPSIG 35PSlG

,208 .225

,333 .349 .45 .48

,118 .125

,383 ,417

,167 ,179

,349 372

.45 .48

,208 .225

.114 . .120

Hex Model No. A

60PSlG 10PSlG

.242 1140

.380 250 .51 150°

.133 210

.463 goo

,192 460

,400 34O

.51 1500

,242 1160

.128

0 Hex

518" 215-1 .88" .27" 518" 511 6" 215-12 .97" .27" 518" 5/16" ~ 2 1 5 - 2 9 .9i'#* .27" 518" 5/16" *215-30 .88,# .27" 518" 518" 518"

t- A---l

Y e

'(when applicable)

rY ANGLE at PSlG OPSlG 30PSlG

1230 1350

350 410 1730 173O

410 490

990 1020

670 740

500 560

1730 1730

1330 1420

go' 120

LO P S l C

1400

430 177O

550

1040

79Q

610

-

1770

1450

1 60 -

I

j/

l l /I /I II 11 ll I1 li

ll

11 11

II '(when applicabli

I1 I 1

li Deflection

Angle lJ I

Hex

1' 1986 Galtek Corporation 30

Page 38: Reducing Chromium Losses from a Chromium Plating Bath

A f :

31

Page 39: Reducing Chromium Losses from a Chromium Plating Bath

Appendix G

ii /I II ij I; ! ! I1