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Historical Articles

February, 1952 issue of Plating

Recovery of Chromic Acid from Plating Operations*

F. R. KELLER, Plant Manager; C. C. CUPPS Engineer; and R. E. SHAW, Plating Supervisor, Standard Steel Spring Company, Newton Falls, Ohio

*Presented at 24th Annual Meeting of the Federation of Sewage and Industrial Wastes Associations, Washington, D. C., October 8-11, 1951. Reprinted with permission Sewage and Industrial Wastes, February, 1952.

Many authors have pointed out that the design of an industrial waste-disposal plant starts with a reappraisal of plant layout and processes.

The present paper describes a case in point. At the Standard Steel Spring Company’s Newton Falls, Ohio plant, installation of a recovery system has already resulted in the reduction of 55 per cent in the amount of chromic acid lost by dragout. Not only has this materially reduced the load on the treatment plant and saves some $1,650 monthly on ferrous sulfate used for treatment; but has paid off in reduced chromic acid consumption at the rate of $2,500 monthly. The recovery system cost approximately $10,000 to install, and operating costs are about $500 per month. Even though the recovery plant has not been operating consistently at its designed efficiency, it had paid for itself three months after installation.

Not every chromium plating operation would yield such spectacular savings from a similar installation, but wherever dragout represents a substantial proportion of the chromic acid consumption, the situation will be very similar. It is suggested that many operators would be surprised if they were to calculate the amount of chromic acid corresponding to the chromium deposited during a month and compare this figure with actual chromic acid consumption.

At Newton Falls, where automobile bumpers are plated automatically, the dragout is quite heavy, owing to the cup formed at the bottom when a bumper is hung vertically. Each bumper carries with it from 1/4 to 1 pint (120-470 ml) of solution, which represents a considerable fraction of the daily usage of chromic acid. The following percentages represent the use of chromic acid with a double recovery tank before any further recovery system was installed:

Chromic acid actually used to deposit chromium 5.7%
Miscellaneous losses including those in fume exhaust (estimated) 20.0%
Dragout 74.3%
Total 100.0%

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Fig. 1. Flow sheet of recovery system showing designed flow rates and equilibrium concentration of chromic acid

 

The successful use of a recovery tank depends upon an appreciable quantity of the dragout liquor being returned to and used in the plating or process tank. Normally the only use for dragout liquors in a plating tank is to replace loss of solution by evaporation, because the dragin is equal to the dragout on the work being processed. Decorative-chromium plating tanks, however, are usually operated at 110-125° F, which results in very little evaporation.

To improve the operation of the recovery tanks at Newton Falls, the amount of solution continuously taken from these tanks was increased materially; first, by reducing the dragin to the plating tank and, second, by installing an evaporating unit to reduce the volume of the dragout solution before adding it to the plating tank.

The diagram of the installation (Fig. 1) shows the two rinses (tanks 4 and 5) preceding the plating tank and the three cold rinses (tanks 7, 8 and 9) and final hot rinse (tank 10) following it. This normal rinsing cycle has been altered as follows:

(1) Tank 4 is operated normally, i. e., full and overflowing to the sewer.
(2) Tank 5 is operated empty. Blow-off fixtures (Fig. 2) remove by a blast of air the bulk of water carried in the bumpers.
(3) Tank 7 is also operated empty. In it the bumpers are first rinsed by a spray, which is arranged, mechanically, to operate intermittently, i. e., when the carrier is in proper position. The diluted dragout now carried by the bars is then blown off by air jets as in tank 5.
(4) Tank 8 is operated full, but not overflowing. The sprays for tank 7 are fed from tank 8 by means of a small centrifugal pump, and only enough fresh water is added to tank 8 to replenish this spray water and the normal dragout. Thus the only portion of the chromic acid dragout which finds its way to waste is that dragged out of tank 8. The primary object then is to keep the concentration in this tank to a minimum.

Referring again to Fig. 1, it will be seen that the efflux from tank 7, containing the bulk of the chromic acid dragout, is pumped through a flow meter and heat exchanger to the concentrator. A portion of the concentrate is recycled and the rest returned to the plating tank substantially at plating bath concentration. It will also be seen that the chief work by the concentrator is to evaporate the water used by the sprays in tank 7. Provided that both blow-offs are equally efficient so that the dragin from tank 5 equals the dragout from tank 7 and that the evaporated solution has the approximate composition of the plating bath, there is always room for it in the plating tank.

It may be observed that it would be very desirable to blow off the bulk of the concentrated solution over the plating tank itself, but for mechanical reasons that was not practical.

The design of the evaporator system presents a major problem owing to the corrosive nature of hot chromic acid. In the past, attempts have been made to concentrate chromic acid dragout in open steel or lead-lined tanks, using lead steam coils. Invariably these have wound up on the scrap heap because of corrosion. Without some method of providing enough room in the plating tank for the return, it has often been necessary to concentrate it to a sirupy liquor which was difficult to handle and very corrosive when hot, even to lead. These operations were usually performed batchwise, and because even dilute chromic acid at boiling temperatures attacks lead, especially at the air-liquid junction, this attack occurred all down the side-walls as the liquid level fell during evaporation. In addition, the lead coils became heavily encrusted, which interfered with the heat transfer.

In the present design, a Maurice Knight concentrator (Fig. 3) is used in which air is introduced below a bed of berl saddles, while the hot (but not boiling) liquid is trickled down from the top. The air picks up moisture and carries it out at the top, passing first through another thin layer of smaller saddles to remove entrained chromic acid.

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Fig. 2. Diagram of spray and blowoff in Tank 7.

 

At first it was thought that it would be satisfactory to operate the heat exchanger with steel tubes, which would be expendable but cheap. The loss of tubes and consequent shut-down, however, proved excessive, and a Pyrex glass heat exchanger is now being used. Low temperature pipe lines are iron, and all pumps are Duriron. One interesting feature is the use of Teflon envelopes surrounding the gasketing materials at the flanged joints of the Pyrex glass assembly. A Teflon envelope is use, shaped like a flattened inner tube but open on the outside diameter. This permits the insertion of a split gasket of any convenient material, which is then effectively insulated from the action of the solution.

It has already been observed that the function of the concentrator is to evaporate the water which is added to tank 8 and used for the sprays in tank 7. Since the ultimate loss of chromic acid is controlled by the equilibrium concentration in tank 8, it is essential to keep this concentration at a minimum. This is done by making the flow of fresh water and spray equal to the evaporation capacity of the concentrator. One can calculate what savings in chromic acid are possible for a given size evaporator, and thus choose the most economical size for the job. Due allowance must be made for the savings in disposal-plant installation and operating costs.

Without going into detail, the recovery system was designed on the basis of the data shown in Table I and Fig. 1.

According to the data in Table I, the dragout from the plating tank, 60 lb of chromic acid per hour, would be a total loss if no recovery of any kind were used. By actual measurement at different times, it was determined that the double-rinse recovery tanks were saving 1/2 of this dragout, or 30 lb chromic acid per hour. The use of the evaporator and the combination spray-and-blowoff arrangement has reduced this loss again by over 50 per cent, corresponding to a total saving of at least 45 lb of chromic acid per hour. Recent checks have shown that the loss may be as low as 3 lb of chromic acid per hour using the latter method of operation.

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Fig. 3. Diagram of Knight Concentrator for chromic acid recovery.

 

Other means of recovering the dragout have been proposed, such as vacuum evaporation and ion exchangers. All of them, including the one described here, have the advantage that the waste-disposal problem is handled right at the source by reducing the waste concentration before the waste is being treated.

ACKNOWLEDGMENT
The authors wish to thank Dr. A. Kenneth Graham, of Graham, Crowley and Associates, Inc., for his many suggestions and help in this undertaking.

 

 

 

TABLE I. DESIGN DATA
 
Transfer or Dragout gal/hr
CrO3 Conc., oz/gal
CrO3 Dragout or transfer lb/hr
Dragout from plating tank 6
Dragout from rinse tank 8—loss
Water feed to rinse tank 8
Dragout from empty tank 7
Vapor from concentrator 6
Return to plating tank 6
20
20
85.4
85.4
64.2
21.2
48
6.1
0
11.1
0
44.6
60
0.8
0
59.2
0
59.2



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