d LITERATURE CITED
Ayres, J. A, IND.ENG.Cmnf., 43, 1526 (1951). Kunin, Robert, and McGarvey, F., Anal. Chem., 26, 104 (1954). Eunin, Robert, and McGarvey, F., IND.ENG.CHEM-,46, 118 (1954). ,
I
Eunin, Robert, and Myers, Robert J., “Ion Exchange Resins,” John Wiley & Sons, New York, 1950. (6) Nachod, Frederick C., “Ion Exchange,” dcadeinio Press, New York, 1949.
o
n Exchange
(6) National Bureau of Standards Handbook 52 (Supt. Documents Washington 25, D. C.), March 20, 1953. (7) Rodger, UT.A , , and Fineman, P., A-ucleonics, 9, 50 (December,
---*,.
1a‘;i 1
(8)
Rodger, W. A , , Fineman, P., and Swope, H. Gladys, “Disposal of Radioactive Wastes at Arnonne National Laboratorv.” Proceedings of t h e Eighth 1nd;strial Waste Conference,”Purdue University, May 4-6, 1953; Purdue Engineering Bull. 38, 474 (January 1954).
RECBIVEDfor review September 2 , 1954.
ACCEPTEDNovember 3, 1954.
Treatment of Chromic Acid Wastes EVALUATION OF METHODS R. F. LEDFORD iiND J. C. HESLER Industrial Filter & Pump M f g . Co., Chicago, 111.
Chromium plating lines and other related operations discharge appreciable quantities of chromic acid to the subsequent water rinses that remove plating solution film from the treated part. The amounts of chromium discharged are frequently large enough to warrant recovery or to require treatment o f the rinse waters to comply with antipollution laws. Chromic acid-copper stripping solutions must be treated to keep copper content low in order to maintain effcctive production rates, but the high cost of solution replacement often requires that the bath be kept in service after the stripping rate has decreased appreciably. Cation exchange treatment of these impure chromic acid solutions to remove metallic impurities, followed by reconcentration to the desired strength, makes recovery procedures financially attractive. Actual case histories illustrate how economic evaluation of ion exchange led to the selection of chromic acid recovery over conventional methods of waste disposal.
T
HE metal plating industry utilizes many chemical baths
that require frequent replenishment to replace “drag-out” losses and to maintain required purity of the bath itself. Drag out is removed from the plated parts by still or flowing rinses that then become contaminated with the plating solutions. Where these solutions contain compounds toxic to fish and plant life or to sewage plant organisms, treatment to destroy or convert them to a harmless form often must be practiced. Chromic acid and cyanide plating baths are typical t,oxic baths. Obviously, the cost of rendering these wastes innocuous is extremely important; the recent development of stable ion exchange resins capable of withstanding chemical attack has provided a means of turning former waste disposal problems to profitable recovery operations. This paper illustrates with actual case histories how evaluation of ion exchange treatment for recovery of chromic acid from several wastes led to its selection over conventional chemical treatment methods. The method permits re-use of recovered acid and eliminates its disposal cost as a waste material. CHROMIUM PLATE STILL-DIP RECLAMATION
A manufacturer expanding plating facilities was advised by local authorities that rinse water from chromium plating operations would require correction before approval of the expanded plant would be granted; the plating baths in use were the conventional type containing chromic and sulfuric acids in the amounts reported in Table I. A number of these plating baths, ranging from 800 gallons to 1500 gallons in size, were to be installed for chromium plating on both die cast and steel parts; January 1955
all plating was to be done over previous copper and nickel base deposits.
TABLE I. DATAO N CHRONIUM PLATIKG OPERATIOXS Chromic acid d a t i n g b a t h s Number of b a t h s CrOa concn., oz./gal. HzSOa concn., oz./gal. Rinse waste water estimated Total flow, gal./min. Daily volume, gal. Average CrOs concn., p.p.m.
5 40.0 0.40 400
500 000 ~
150
The expected total rinse water wastr flow based on data obtained from existing equipment was estimated a t 400 gallons per minute; rinse water analysis sampled during 20-hour operation showed chromic acid contamination ranging from 50 p p.m to more than 200 p.p m., mith a daily average of about 150 p.p.m. Since this degree of sewer contamination would not be permitted in the new plant, a program to reduce chromic acid in the rinses was initiated. Prior t o installation of additional equipment, lLsave”rinse or reclaim rinse tanks mere employed 011 some of the existing lines where drag out of chromium was excessive. These solutions were allowed to build to approximately 4 ounces CrOa per gallon and were returned to the plating tanks whenever possible, as permitted by evaporation and drag-out losses. However, it was estimated that return of all 6ave rinses to the proposed and existing tanks would not he feasible since the nature of the work required that there be sufficient drag out to maintain heavy metal contaminants below a level that would not adversely affect plating.
INDUSTRIAL AND ENGINEERING CHEMISTRY
83
I n one method of treatment chromic acid reduction equipment is installed to use either sulfur dioxide or sodium sulfite as the reducing agent, folloned by neutralization with lime, and separation of sludge containing insoluble hydrous chromic oxide in conventional clarifying equipment. The cost of this equipment was computed a t approximately tL0,000 plus installation costs for a continuous plant operating a t 400 gallons per minute. This figure did not include equipment for denatering the sludge produced, since space was available for a small sludge lagoon. The calculated chemical cost of treatment based on the expected average contamination of 100 to 150 p.p.m. chromic acid nhen using sulfur rli3uide for destruction is given in Tahle 11.
T.IRI.E11. ~ ~ T I M . I B ? . COST': E~) FOR REWVIXG CHROMICJf I ~ I S J1T.A'PER E BY REOCCT1O.U A S D P R E C I P I T A T I O X
FROM
(Initial cost of plant, exclusive of installation, S40,000) Chemicals reqriired/1000 gal. 3.0lb. 60' B$ HzSO4 at 80.02/lh. 1.4 lh. Sot (liquid) a t Y O . l O / I b . 4.0 lb. C a (OH)? ( t r c h . ) a t BO.Ol/lb. Chemical cost/1000 gal.
s
Clieiiiical oost/day (d00,(100,aalions, 20-hr. day) Plant labor/day, 4 hr. a t 82.00 T o t a l chemical and labor costs/day
5120 00 8.00 $128.00
0 00
0.14 0 01
m
TABLE 111. STILL-DIPTANK CONPOSITIOS (2000-Gallon caparitv, ? O - h o u ~operation) ~ Hexavalent rhroiiiiuni, CrOr Sulfates, HzSOd
Oz./Gal. 2 1 0 21 P.p.m. CaCOs Equiv. A06
30 35 1148 138
The initial cost of this plant together with excessive chemical costs and the problem of disposing of the sludge produced led to tests on the existing equipment, utilizing a battery of reclaim rinse tanks following each chromium plating operation to determine whether the bulk of the solution adhering to the plated articles could be removed; each plating tank was followed by two dip tanks in series with pump and piping connections enabling the operator t o discharge the first into a separate holding tank, transfer the contents of the second tank into the first, and refill the second with fresh or distilled water. By operating in the countercurrent fashion, the combined ffowing rinse effluent following the still rinses contained less than 15 p.p.m. of chromic acid, which, on dilution with other plant rinse waters, resulted in a degree of contamination that would not exceed the limit imposed by city authorities. I n the series operation the chromic acid content of the first tank built t o a level where drag out from the second to the floming rinse tank became excessive; tests indicated that the contents of the first tank should be discharged when the chromic acid concentration reached 2 t o 3 ounces per gallon. Daily discharge of the first tank (20-hour operation day) resulted in a still rinse chromic acid concentration of approximately 2 ounces per gallon without measurable drag out through the second tank t o the sewer. It was estimated that the use of two reclaim rinscs a ould result in isolating the chromic acid drag out in approximately 6000 gallons of solution The problem of disposing of the still dip was then considered; a typical analyfiis of the first bath after a one day run showed that heavy metal ions were piesent (Table 111). Re-use of a11 the recovered chromium in the pIating tanks in diIute form was not possible because of volume limitations, and because the chromium baths are adversely affected by a build84
up of metallic impurities produced by attack on tlie metals being plated. For example, in excess of certain specified limits, iron, trivalent chromium, and copper all cause a "cloud" on the plated finish and result in lo&ered cathode efficiency. Zinc and cadmium cause whitening as contrasted ~ i t hbrilliance in the plated surfaces. I n normal operation these contaminants have no practical effect since drag-out lossea maintairi them a t harniless levels. However, return of the entire still rinse t o the bath to recover chromium also returns these impurities, with tlie result that they build up t o objectionable levels. Actual 011eration of this proposed method on a month trial basis using a temporary evaporation unit to concentrate the still rinse ,soon resulted in plated work of poorer quality than that obtained in previous operations where drag out minimized contamination The use OF a cation exchanger operated in the acid cycalr, tn remove or reduce these troublesome ions was then evaluated Work done by others ( 1 , 3,6),including actual plant scale operdtions, has shown that chromic acid can be freed of aluminum (aluminum anodizing baths) or metallic contaminants (chroniium plating baths) by the use of the sulfonated polystyrene tylie resins in the acid cycle. Excessive degradation of the resin docnot occur as long as the chromic acid content does not exceed approximately 100 grams per liter. Preliminary laboratory work with sinal1 columns a t the contaminant levels established the capacity of the exchange material a t approximately 40 t o 45 ounces of metallic impurities as calcium carbonate per cubic foot a t an acid regeneration level of approximately 25 pounds of 66" B6 sulfuric acid. Tests involving recovery of the last portion of acid applied to the resin, witli its re-use in a subsequent regeneration, resulted in an avrrage acid consumption of approximately 15 pounds per cubic foot of rtlsin. Capacity was based on test work employing contaminated chromic acids as shown in Table 111. Additional work indicated that variations in the chromic acid concentration and in the contaminant load, but especially in the former, result in some\?hat diKerent capacities, so that each individual case must kie evaluated t o obtain actual capacity under the load conditions. Leakage of contaminants is determined by preferential sorption of one ion over another and by relative concentrations. In this particular application it was possible to utilize almost the full capacity of the resin eince some leakage of ions in realizing maximum capacity does no harm so long as the tolerance of the plating bath for those ions is not exceeded. This mode of operation resulted in the most econoniical utilization of the exchanger. In the actual design of the treating unit finally evolved, the still rinses from each of the plating lines xa9 combined in a single holding tank from which the exchanger was supplied. Rased on the ronPiderations outlined, the unil was sizcd to hold approui-
T.4RI.E
ITr. ESTIMATED
COSTS
I O N EXCHAXGE AND EV.4PORATIOS FOR CHROMIUM PLATE REECLIIIM R I N S E RECOVERY
(Baaed on one coniplete cycle/day) Expense C'ost of sulfuric acid regenerant ,525 lb. tieo BB HzSOa a t $0.02/lb. Resin replacement based on estd. life of 300 cycles/year, 3 5 cii. ft. resin a t $24.00 Ion exchanger depreciation based on amortization over a 10year period, 300 cycles/year Evaporator depreciation based on amortiaation over IO-year period 300 cycles/year Steam' cost approximately 2600 lb./hr. for 20 hr. (75 lb./sq. inch e w e ) at 90.75/1000 Ib. Total direct labor exchanger a n d evaporator. 5 inan lir. a t SZ.OO/hr. Total daily cost Credit Y;iliie of rhromic acid recovered daily a s 300 gal. 40 oz./gal. aulution, 750 lb. a t S0.30/1b. Va!ue of condensate returned t o rrclaiiii rinse tank, approximatoly 5700 gal. a t S O . l O / l O O O gal.
INDUSTRIAL AND ENGINEERING CHEMISTRY
810 50
2.80 2.50
7.00 39.00
-Ut.OO S71.80
W2aR. 00
,57
Vol. 47, No. 1
d mately 35 cubic feet of resin with adequate freeboard; the general appearance of the complete unit is shown in Figure 1 . Piping and fittings are saran-lined &eel; valves are manual Saunders-patent type, glass or saran-lined, with Teflon diaphragms. Both regenerant tanks and exchanger tank are lined with plastic sheet to resist sulfuric and chromic acid attack.
o
n Exchange
In the event that the chromium plating baths slowly build up contaminants affecting plating, the ion exchanger and evaporator combination may be used for their purification and reconcentration. While the baths will require dilutions to approximately 100 grams CrOa per liter before treatment to avoid accelerated attack on the resin, work done by others as well as actual field experience indicates that such purification methods are economi*cally sound. CHROMIC ACID-COPPER STRIPPING SOLUTION
Figure 1. Chromic Acid Recovery Ion Exchanger
The purified diluted chromic acid containing nearly 2 ounces CrOj per gallon and approximately 6000 t o 6200 gallons in volume
One of the most desirable solutions in stripping electrodeposited copper from ferrous base metals is by means of chromic acid. This method of chemically removing copper is selected when B maximum rate of tjtrippirig is required with the least etching of the base metal. One diffjculty with this treatment, hoivever, is that efficiency of the chromic acid solution is influenced by the amount of dissolved copper in solution. Figure 3 illustrates the progressive reduction in the speed of stripping with a corresponding increase in copper concentration The increased time required to strip commercial thicknesses of copper films as the dissolved copper concentration increases results in decreased production rates, and consequently such baths are frequently replaced in part or whole.
could not be returned directly to the plating tanks without concentration except to make up evaporation losses; a vacuum evaporator capable of reducing this volume of dilute chromium solution to 300 gallons of concentrated liquor ( 4 ) was therefore included in the cost figures; the flow diagram is shown in Figure 2. The estimated costs of operation and expected gain in return of chromic acid are shown in Table IV. The cost of cooling water, approximately 150 gallons per minute, is not included since it is planned to UEB this water in various rinsing operations throughout the plant where approximately 4000 gallons per minute are required, Power consumption, based on intermittent use of pumps, has also been neglected. It was planned to add any additional make-up water over the volume of condensate returned to the reclaim rinse from the evaporator through the use of existing deionization equipment. COPPLCR CONC€N7Rd7/ON /# 8 A 7 4 O U M L .
I
Figure 3.
Figure 2.
Chromic Acid Recovery System Flow Diagram
Based on comparison of disposal costs with these recovery costs and the overwhelming advantages estimated, the latter method of treatment has been selected, and the treating plant will be in operation in a ehort time. In addition to the direct savings of chromium, it is expected that other advantages will result-namely, improved plating efficiency, easier solution control, constant operating conditions, fcwer rejects: and lower operating costs.
January 1955
Chromic Acid Copper Stripping Bath
Until an economical ion exchange method of purification WM developed for selectively removing copper contaminant, it waa necessary to dbpose of the entire solution when the copper concentration in the chromic acid solution became so great that further stripping was impractically slow. Obviously, dumping this valuable material is a n expensive and unsatisfactory solution to the problem, especially where chemical reduction of chromium is required to meet antipollution laws. Barreling of the waste is a solution, but it SLSO is expensive since it is becoming increasingly difficult to find suitable dumping sites within reasonable distances of the plant. The ion exchange purification system was suggested to a gear manufacturing plant forced to discontinue discharging chromic acid into the sewer of a large midwestern city. Copper plate was applied to part of each gear to permit subsequent selective hardening of the teeth, followed by stripping of the copper protective coating before other operations. The need t o maintain production rates required frequent replacement or modification of the bath. The alternate t o the suggested method was barreling and hauling to a dump since destruction costs rrere considered prohibitive. Tests made on freshly prepared chromic acid revealed that the stripping rate began to decrease below desired production rates
INDUSTRIAL AND ENGINEERING CHEMISTRY
8s
when the copper concentration exceeded about 2.5 ounces per gallon; this condition occurred after about 4 days of operation, a t which time the composition of the bath was approximately that shown in Table V.
TABLE v. CHROMIC- % C I D - ~ O P P E R
STRIPPIXG BATH COXPOSITION
(4 Day8 service)
Approximate volume, gal. Hexavalent chromium, CrOs oz./gal. Sulfuric Acid, H2SO1 oz.:gal. Copper, Cu, p.p.m. CaCos Iron, Fe, p,p.m. CaCOz Trivalent Chromium, p.p.m. CaCOs Total cationic impurities build-up, p.p.m. CaC03 Calculated build-uplday, p.p.m. CaCOs
1,000 40 G 17,500 1,033 13,125 31,660 7,915
Although in previous operations the chromic acid stripping bath was not replaced or modified until the copper concentration exceeded 5 t o 6 ounces per gallon because of the cost of t,he bat,h, with ion exchange a maximum limit of about 2.5 ounces of copper per gallon could he set in order t o keep production a t a uniformly high level. Without, ion exchange t,reatment the rate of copper build-up decreased as the stripping rate dropped. The bath was replaced completely every 10 working days, or a portion was withdrawn daily and replaced with fresh solut,ion to maintain copper a t a level dictated by a compromise b e b e e n the effective stripping rate and the cost of chromic acid. The volume of the stripping bath in question was approximately 1000 gallons. Calculations indicated that if approximately one fourth of the hath, 250 gallons, were treat,ed daily for removal of impurities, the desired copper level would not, he exceeded. Dilution of t'he 40-ounce bath (300 grams per liter) with water in the ratio of 1:2 n-as necessary so that a final volume of approximat,ely 750 gallons of diluted chromic acid (100 grams per liter) was selected for sizing the ion exchanger. The cation load from 250 gallons of concent,rated solution v a s estimated a t 1055 ounces, as calcium carbonate, of iron, copper, and trivalent; chromium in the rat>iosgiven. Test work in tshe laboratory on this solution established a practical capacity of approximately 18 ounces of mixed metals (as CaC03) per cubic foot when employing an acid dosage of 25 pounds of 66" RB sulfuric acid per foot. About 10 pounds of acid were reclaimed, so that t,he average consumption was 15 pounds per foot. An ion exchanger of the type described, holding approximately 60 cubic €eet of resin, was rccomrnended together with an evaporator of suitable size. To avoid unnecessary dilution of the solution by water in the exchanger, a sight port was provided to permit control of the liquor at approximat'ely bed level. Dilution or "sweetening on and off" water was estimated from laboratory a-orlc a t approximately 300 gallons, requiring a continuous evaporator capacity to concentrate a total of 1000 t o 1100 gallons of purified chromium solution in 20 hours. Estimated operating costs and recovery are given in Table VI. I n operation of the syst,em an extra 250 gallons of chromic acid was considered so that the bath level could he maintained at, t,he time a portion was removed for t>reatment..
86
Cooling water for t,he evaporator is not included since it n-ill be used in subsequent rinsing operations. The minor cost of electric power for intermittent pump operation has also been excluded. Resin replacement has not been included in the daily costs since no complete data have been collected yet. Tupublished results by ot,hers (9)working wit,h a field unit treating chroniic acid plating baths at' concentrations of 100 to 117 grams per liter hake ePtablished that after more than 100 cycles of operation during 2 years the capacit,y has not decreased, and only small changes in physical characteristics have occurred. These workers estimate an effective life of 300 t o 400 cycles for the resin. At an estimated life of only 200 cycles the cost of resin replarement will be approximately $7.00 per day. On the basis of the recovery made possible by use of this system, the manufacturer in question has decided to proceed with the installation.
TABLE
S'I. EGTIXATED COST FOR
COPPER S T R I P CHROlcrIC A C I D
RECOVERY SYSTEM
(Rased on one complete cycle/day) Expense Sulfuric acid regenerant, 900 lb. a t S0.02 Ion exchanger depreciation based on a 10-year amortization period, 300 cycleslyear Evaporator depreciation based on 10-year life, 300 cycles/ ear Steam cost estimate, 9000 lb. (75 l b . / q . inch gage) a t $0.755000 lb. Total direct labor, 5 hr. a t S2.00 Daily costs, exclusive of resin replacement
S18 00 3.00 3.33 ~
6.75 10 00 ____ S41 .08
Credit Cost of replacing approsimatcly 250 gal. chromio aoid in stripping bath, 40 oe. CrOs, 6 0 8 . HzSO4;gal.. t o maintain desired copper level when ion eschange is not utilized 625 Ib. CrOa a t S0.30 $ 1 8 7 , BO 94 lb. HzSOa a t 50.02 1.88 Net recovery of copper from spent regenerant acid b y electrodeposition, 23 lh. Cu a t $0.23 5.30 Gross recovery/day 8 s 8 Net recovery/day 8153 60
SIJJMMARY
Two instances have heen cited mhhrre the application of ion exchangers for recovery of chromic acid in rinse waters will result in considerable savings ovcr methods of disposal. This method of mmte treatment not only prevents sewer contaniination, but decreases operating coste, and in some instances alga results in profitahle reclamation of valuable rhi omic acid. LITERATURE CITED
(1) Costa, R. I,.. ISD. EXG.CHEX..42, 308 (1950). (2) G i l b e r t , L., Rock Island =tr.senal, Rock Island, Ill., u n p u b lished data, 1954. ( 3 ) Gilbert, L., LIorrison. IT. 'S., Iiahler, F. IS.,S S t h B m . Pim. Tech. Sessions Ana. EZcct~opZate~a'Soc., p. 31 (1952). ( 4 ) h-eben, E. IT.,and Swanton, IT.F., Plating, 38, 457 (1991). ( 5 ) Stromquist, D. > I . , a n d Reents, -4.C,., I'roc. 6th Ann. I r ~ d . Waste Conf.Ser., 76, IS1 (1951).
RECEIVED for rel-iew March 29, 1984.
INDUSTRIAL AND ENGINEERING CHEMISTRY
ACCEPTEDlu'oveinber 15, 1934.
Vol. 41,No. 1
