Evaluation of Electrochemical Recovery Of Cadmium at a Metal Finishing Plant

By D.T. Vachon, W. Bissett. B.A Calver and G.C. Dickson

A prototype electrochemical reactor was installed at a jobshop to recover cadmlum and destroy cyanide from plating rlnsewater. Nlnety-eight percent recovery of projected cadmlum losses and 93 percent cyanide destructlon were achieved. Although payback was not reallzed In terms of metal recovery alone, installing the new system was more economical than upgrading the existlng waste-treatment facility. The quality of effluent discharged to the municipal sewer was also improved significantly. Monitoring Identified the need for improved rlnslng procedures and additional cyanide destruction capaclty.

n today’s society, the metal finisher must pay his share of the cost to ensure a clean environment. Expenses include: the capital investment for wastewater treatment I equipment; operating costs, which are continuous in

termsof personnel and chemica1s;and sludge disposal, the cost of which is ever-increasing as access to existing sites becomes more restrictive.

Environmental agencies at all gwernment levels are requiring more rigorous adherence to regulations. Toxic levels of metals discharged to receiving waters-either directly or via municipal sewage treatment plants-are of

significant concern. Another serious problem is the creasingly stringent restrictions on metal levels in slu@ generated by municipal treatment plantsand disposed Ofi*

landfills. And then there is the problem of disposing liqu’: sludges-and those from metal finishing shops-in landflls where groundwater contamination may take place.

Faced with the dilemma of enforcement action and the inevitability of increasing costs for conventional treatme‘’ with no return on investment, many metal finishers aR looking at alternatives to recover and recycle metals an: solutions inside their plants.

Case Background Discharges from X-Pert Metal Finishing Ltd., Burlingto‘ Ontario, Canada, toa municipal sewer system are regulate’ under bylaws established by the regional municipalltY Halton. Limitsfor specific parameters are given in Table‘ The company was having trouble meeting limitsforseve’ of these parameters with its existing facilities. Also, Sludt from Halton’s Burlington sewage treatment plant dld ”‘ meet Ontario’s cadmium limit for agricultural utilization

X-Pert is a medium-sized jobshop that plates a varietyd items such as fasteners, stampings and screw math" parts. In the 1400-m2 plant, mechanical plating electroplating are carried out. Zinc, cadmium, copPeraF’

t:n are platedl barrel methot1 tiomare outlii shift basis sii manual hoist dyeing and WI

RI nsewaterr convent iona II combined wai tank in whict+ using a contrcc by lime and ((

lation in the c. effluent is CEl underflow is press. Appra produced daill tlon is not prai

During the 11, concerned ab) and sludge dii lties by the mui imp rove men t:: were installec: Cooling wateii !o reduce the

Water usac! analyzed. The! .educe flows tion. Asignific inidragout tal dragout conil cadmium 3nd

Electrocherr In late 1980, a installed. The chemical rea carbon-fiber f as it was initit

The electrc bop circuit v h e . The elec cathode and i oxide. The nu needs of the taming two r

was electro-o tamed at a electrolyte ir chlorine, whi cyanide destr To remove

With a high cy "nit along wit cadmium or electricity, on Only salt lost

The reacto “-w stage fl 3-hr Strippinc The woto t ypf ‘ecovery tanh

aqueous pha:

68 PLATING AND SURFACE FIN1

!m is the in. els in sludge lisposed of in Dosing liquid ;-in landfills )lace. :tion and the la1 treatment finishers are ? metals and

Burlington .re regulated nicipality 01 n in Table 1 s for several 91~0, sludge lant did not itilization. 1 a variety 01 !w machine ilating and copper and

E FINISHING

tin are Plated on steel and aluminum using both rack and barrel methods. Typical production volumes and opera- tlOnSare outlined in Table 2. The plant operates on a three- shift basis six days a week. All plating operations use

hoist lines. Post-plate finishes include chromating, dyeing and waxing.

msewaters and other waste streams are handled in a treatment system installed in 1973. The

;ombined Wastewaters (180 m3/day) flow to an in-ground lank in which metals are precipitated by pH adjustment Jslng a controlled addition of waste pickle liquor followed ,y lime and caustic. A polyelectrolyte enhances floccu- ation in the clarifier, which has a diameter Of 3 m. Clarifier !!fluent is discharged to the municipal sewer. The ,nderflOw is dewatered using a plate-and-frame filter ~ress. Approximately 400 L of dewatered sludge is !reduced daily for disposal to a landfill. Cyanide destruc-

is not practiced in the conventional treatment system. During the late seventies, company management became

oncerned about the increasing cost of metals, chemicals nd sludge disposal and the vigorous enforcement activ- les by the municipality. In response, several environmental nprovements were made. Flow controllersand drip shields #ere installed and countercurrent rinsing was instituted. ;ooling water was diverted from the waste-treatment area 0 reduce the hydraulic loading on the clarifier. Water usage and contaminant concentrations were

nalyzed. The results indicated areas where the plant could sduce flows and pinpointed sources of metal contamina- on. Asignificant modification was the inclusion of a drag- idragout tank used before and after plating to limit ragout concentration.' The tank reduced losses of admium and cyanide to post-plating rinses by 50 percent.

lectrochemical Reactor ilate 1980,a prototype reactorforremoving cadmium was lstalled. The reactor works on the principle of electro- iemical recovery of metals on a large surface area of irbon-fiber electrodes. Figure 1 is a sketch of the reactor i it was initially incorporated in the plating line at X-Pert. The electrochemical reactor was operated in a closed- op circuit with a recovery tank installed on the plating ie. The electrolytic cell module contains a carbon-fiber ithode and an anode of titanium coated with a rare earth :ide. The number of modules per reactor is specific for the beds of the plating shop. X-Pert needed a system con- ining two modules. Cadmium was removed from the ueous phase by deposition on the cathode while cyanide IS electro-oxidized at the anode. Sodium chloride main- ined at a concentration of 80 s/L was used as the ec!roly!e ir! the system and provided the source of llorine, which also acted as an oxidant for enhanced ianide destruction. To remove cadmium from the cathode, plating solution ith a high cyanide concentration was pumped through the rit along with oxygen, which oxidizes cadmium, allowing ldmium oxide to dissolve in solution. Other than ectricity, only oxygen was consumed in the system. The Ily salt lost from the process solution was by dragout. The reactor operated on a 19-hr cycle comprised of a 1-hr stage for metal removaVcyanide destruction and a hr stripping stage for metal recovery from the reactor. le prototype unit had a total capacityof 700 L (500 Lin the covery tank, 67 L in the reactor, remainder in the filters

?IL 1986

Sludge d (10 dewatering 8 lanefllli

I Ig. I-lnstallatlon of electrochemical reactor on cadmlum plating line.

f Product Flow

t Wafer Flow

* Sam!% PO,",%

110 dewatering B laodllll)

g. 2-Modifled electrochemical reactor system.

69

and plumbing). System modifications provided an additional 2000 Lof capacity with the inclusion of a holding tank and a cyanide-destruct module (Fig. 2).

Flow through the reactor was set at 175 Umin to provide a retention time qf 3% sec in the electrolytic cell. Dilute plating solutior: was carried into the recovery tank at volumesof 1 .Stir8 Ubarrel, and it was here that the highest potential for recovery of cadmium and cyanide was found. Concentrations of cadmium and cyanide in the process tank werd maintained at low levels by the electrochemical reactor. Only small amounts of the metal or cyanide were carried over into the subsequent rinse tanks and ultimately to the treatment facility.

Typical performance data for the electrochemical reactor during a day’s plating activity is presented in Fig. 3. These data represent sampling of the recovery tank prior to and immediately following a barrel rinse. Twenty-one ba[rels of nuts with varying sizes and threads were processed during a 12-hr period by two different platers. The 21 cycles created by the batch processing are evident in Fig. 3. In most cases, the metal concentration in the solution returned to a low equilibrium level before the next barrel was rinsed.

Figure 4 represents a single cycle and shows a typical reductidn curve for cadmium during a 10-min period immediatelyfollowing a barrel rinse. Within 2 min, the base concentration was regained in the recovery tank solution.

Environment Canada’s Wastewater Technology Centre, Burlington, evaluated the cadmium-recovery system after installation. The government agency had followed the system’s development and was interested in seeing how it would operate in practice.

Experimental Program Several sampling campaigns were conducted after the prototype reactor became operational in the spring of 1981. The objective of the program was to evaluate the operation of the electrochemical reactor on the cadmium plating line to determine metal recovery efficiencies and cyanide destruction rates. Both grab and composite samples were collected at the locations shown in Fig. 1. Composite samples (continuous-flow types collected during the entire test period) were taken from the recovery tank, final rinse tanks, and influent and effluent streams to the clarifier. From the other sites, grab samples were periodically acquired throughout each sampling campaign. Analyses were performed for cyanide, cadmium and other heavy metals. Details of the four tests conducted to assess each stage of development follow.

70

A 16-hr test was conducted on September 15, 1981,f assess the prototype reactor’s performance. The objecbv was to simulate a controlled “typical” production run. P regular 30-min intervals, 2 L of plating solution was add8 to the recovery tank to simulate dragout losses. Samplini took place during the entire 16-hr metal-recovery stage

The following day, the reactor’s performance wa evaluated while operating on an actual productioi schedule, but only the final 4% hr of the 16-hr recover stage was monitored. Following the assessment of dat from these first two tests, system modifications werefoun necessary. These included the installation of a holding tan and a cyanide-destruct unit (Fig. 2) and alterations to thl operators’ rinsing techniques. The cyanide-destruct un was basically a mini (or postelectrochemical) reactor tha provided more surface area and an extended retention tlm for increased cyanide destruction.

Following these changes and the replacement of thi prototype reactor with a production model, two m0n production runs were conducted, but plating took placeii each for only 4 and 5 hr, respectively. Therefore, sample could only be taken during these periods.

The system was tested again on March 4, 1982, withou using the cyanide-destruct module. This permitted 8’

assessment of cyanide destruction with the benefit ofth1 extended retention time in the system. The monitoringwa

CONCENTRATIONS I N PROCESS TANK

< 700 i CYANIDE h

o 1 2 3 4 5 6 7 8 9 1 0 i l .

I TIME (h)

Flg. 3-Typlcai performance of electrochemlcal reactor.

PLATING AND SURFACE FINISHIN‘

conducted a Monitoring c provided an1 performance

Results and During the !! mium remow 3). The meas was substani tation of 99 PIC was probabll nificant V O ~ U I I

r eccw, Y soltc Prior to thct

to the rinsinc dipped briefll transported Percent of til centrations v i March 4, eac In the recov dilution, ther. min.

AS shown the stage at contaminant barrel was re the dragout c

1986

)er 15, 198 ?.The obje, juction ruI tion was ac sses. Sam1 :overy stas formance al produc 16-hr reco ssment of ons were fc f a holding terations tc e-destruct :al) reactor d retention

cement of del, two n ig took plac *efore, Sam

1, 1982, wit1 i permittec e benefit oi monitoring

TANK ~-

A a 9 10 11

FACE FINISH

conducted during the reactor’s first 4 hr of operation. Monitoring during 5% hr of operation on March 15, 1982, provided an evaluation of the electrochemical reactor’s performance with all operational system modifications.

ResUitS and Discussion During the September 15 simulated run, complete cad- mium removal was observed at a rate of 1.86 g/min (Table 3). The measured recoveryof 89 percent on September 16 was substantially lower than the manufacturer’s expec- tation of 99 percent cadmium removal. The lower efficiency was probably due to poor rinsing, which allowed a sig- nificant Volume of plating electrolyte to pass through the recovery solution (unremoved from the barrel).

Prior to the March 4 run, significant changes were made o the rinsing procedure. Previously, each barrel had been hpped briefly once or twice into the recovery tank, then ransported immediately to the rinse tanks. Although 90 wcent of the cadmium was removed, significant con- :entrations were carried over into subsequent rinses. On darch 4, each barrel was slowly dipped three or four times n the recovery tank to ensure adequate mixing and Itlution, then allowed to remain submerged for as long as5 nin.

As shown in Fig. 4, the system reached equilibrium, i.e., l e stage at which there are only low concentrations of ontaminants, after just about 2 min. Therefore, when the arrel was removed, the amount of cadmium or cyanide in le dragout carried to the final rinsewater was significantly

300

250

200 J

r, ’ 150 ‘ 3 )

100

50

0

Barrel Entry

I I I I I

0 2 4 6 8 1 0 1 2 t (min)

.4-Typical reductlon curve for cadmium.

IL 1986 71

lower than under previous conditions. Enhanced cadmium recovery (96 to 98 percent) was observed following the system modifications and the improvement in rinsing.

During the simulated and the September 16 production runs, the cyanide destruction rate was much lower than the loading to the system (Table 4). This resulted in an increasing concentration of cyanide in the recovery tank. The reactor was designed for a specific cadmium loading and was handling the cadmium satisfactorily but could not cope with the high cyanide loading. This would directly affect cyanide concentrations in the final effluent by carryover to the final rinsewater.

The system modifications that were specifically designed to enhance cyanide destruction reduced the potential for cyanide carryover to the final rinsewater and downstream discharge. With the cyanide-destruct unit operating and with the extended retention time, effective cyanide destruction occurred on March 15 when the cyanide loading was high. Because of a lower cyanide loading, a high destruction rate occurred on March 14 as well. Although the highest cyanide destruction rate (5.34 g/min) was observed on March 15, concentrations in the process tankand final rinse werestill greaterthan 1000and about24 mg/L, respectively. However, when combined with in- house waste streams, the result was an effluent discharge of 1 rng/L cyanide to the municipal sewer.

Other Benefits An additional benefit of adding the reactor was increased bath stability. The bath was filtered each time the plating solution was used to strip cadmium from the reactor. The filters removed oily wastesand kept :he bath clean. Roijiiiie recharging of the plating solution (accomplished when the reactor was stripped) also dampened the sinusoidal swinging of the cadmium concentrations encountered when it was manually recharged. The end result was a cleaner, more stable plating solution resulting in a higher- quality product.

Relatively little floorspace is necessary. The recovery tank must be installed in the plating line. However, all related equipment (reactor, filter, pumps, etc.) can be positioned away from the plating line and on mezzanine areas.

The electrochemical reactor possibly could be used as an economical means of disposing contaminated orexcess

72

plating solutions. At X-Pert, 20 L of excess solution \ disposed of by this method.

Data on final effluent quality provided by the Hal Region (Fig. 5 ) show that there was a significant impro ment upon introducing the electrochemical reactor. . average cadmium concentration in the discharge Y

reduced from 4.0 to 1.0 mg/L. Although peaks of h cadmium concentration continued, the majority of anaiy showed less than the local limit of 2 mg/L. Priorto install the reactor, X-Pert had an average compliance rate of percent. With the reactor, the compliance rate increaw more than 70 percent.

It is suspected that the high concentrations noted in

effluent were due to precipitated cadmium from OH

sources escaping from the clarifier as suspended soli X-Pert is taking steps to improve the operation of clarifier system.

Economic Considerations If all metal finishing waste were treated by ConVentlO technology and no electrochemical reactor existed capital expenditure of $40,000 would be required to Ins

an alkaline chlorination facility at X-Pert. To upgrade' existing treatment facility to handle the cadmium load1

CLARIFIER EFFLUENT 100

t PROBABILITY C X L--- ~h~~ valu- Ind~=~~.'

/ g. 5-Final effluent quallty of treatment faclllty.

'rom the plating Region bylaws, ii 38 needed to inn

Table 5 sumrr 'actlity. The figull n d an annual I1 expenditures am mount to $25,:: and operating ccc for adeq uate treE

With the electrr levels are reduc: cdditional requil mcessary. Thus, ~ I Y those assocc gue to the electr 3fld operation of Ss50,050. The e 6 yearly cost si Wading the ex: "Jt Include the cc 'om sources in t

h m a r y and ( ' :'ter rinsing pro1 .ian!d+beatiiici .lemicai reactol admiurn dragou 'a medium-size! 'dltional benefit h e reactor wa %aded convet 'eded for effect1 the plant disc1

'"Per operation 'ntrol the cadn- @ h r s a n d dips. 'Ould be tonsil Ilution.

"1 1986

solution was

)y the Halton cant improve. I reactor. The i sc ha rge was leaks of high ityof analyses or to installing nce rate of 38 e increased to

IS noted in the n from other iended solids ?ration of the

conventiona or existed, a Jired to install 3 upgrade the mium loading

:ACE FINISHIN

from the plating line and ensure compliance with the Halton Region bylaws, a capital investment of about $50,000 would be needed to improve the precipitation facilities.

Table 5 summarizes the costs of upgrading the existing hcility.Thefiguresare based onadragout rateof 5 Ubarrel and an annual line production of 13,200 barrels. Capital expenditures amortized at 14 percent interest for 5 years amount to $25,200 annually and supplemental chemical and operating costs of $44,10O/year would be anticipated faradequate treatment-a total of $69,300 each 12 months.

With the electrochemical reactor, cadmium and cyanide levels are reduced to acceptable concentrations and no additional requirements for downstream treatment are necessary. Thus, the costs illustrated in Table 6 represent mly those associated with the consumption of reagents due to the electrochemical process, as well as installation and operation of the reactor. The annual cost of this system 1s$50,050. The electrochemical reactor therefore provides a yearly cost saving of almost $20,000 compared with upgrading the existing treatment facility. These figures do not include the cost of treating cadmium-laden wastewater ':om sources in the shop other than the plating baths.

Summary and Conclusions *!ter rinsing p:ocedtires were improved ana a secondary cyanide-destruct system installed, a prototype electro- chemical reactor system recovered 98 percent of the cadmium dragout and destroyed 93 percent of the cyanide ata medium-sized jobshop. Improved bath stability was an additional benefit.

The reactor was deemed an economic alternative to the upgraded conventional treatment facility that would be needed for effective control of cadmium and cyanide ions in the plant discharge. However, it was recognized that Proper operation of the treatment system was required to control the cadmium levels from other sources such as cleanersand dips. Electrochemical recoveryat each source should be considered a significant part of the overall sohtion.

APRIL 1986

Postscript About a year after writing the original version of this report, the automated cadmium-recovery system failed. Although many attempts were made to correct the problems, no automated unit could be operated with complete success. Finally, X-Pert removed the cadmium-recovery system. We believe that the system cannot be operated in the automatic mode. However, adequate manually operated units' exist.

Acknowledgments This paper is based on a presentation made in 1984 at the EPNAESF 5th Conference on Pollution Control. It is also an expanded version of a paper submitted to the Water Pollution Research Journal of Canada. We acknowledge the analytical services of Environment Canada's Waste- water Technology Centre.

Reference 1. P. Agathoklis, Plat. and Surf. Fin., 68, 16 (Oct. 1981).

*The cadmium recovery system described in this report was manufactured by HSA Systems Inc., Rexdale, Ontario, Canada. HSA no longerexists. The technology is now being handled by Metal Removal Systems, Farmingdale,

Vachon sirsen

About the Authors

Dlckson Calver

Derek Vachon is a physical scientist In the Physicai/Chemical Processes Section at Environment Canada, Wastewater Technology Centre, P.O. Box 5050, Burlington, Ontarlo L7R 4A6 Canada. His responsibilities include developing and demonstrating wastewater treatment processes for the municipal and Industriel sectors. Mr. Vachon has worked with Environment Canada since obtaining his BS degree in chemistry in 1970 from Bishop's University.

Environmental Protection Service, Ottawa, Canada. Since assuming this position in 1980, Mr. Bissett has been involved in many projects with the metal finishing industry In Canada. He is a member of the AESF Ottawa Branch.

Brian A. Calver, CEF, is vice president of X-Pert Metal Finlshlng Ltd., Burlington, Ontario. He has worked for the company for 13 years and has been involved with plating since 1966. Mr. Caiver is a member of the AESF Toronto Branch and a past president of that branch.

Processing Inc., Misslssauga, Ontario, Canada. Previously, Mr. Dickson worked for HSA Reactors Ltd., where he held various management positions. Before that, he was a research officer in the Metallurgy Dept. at imperial College of Science and Technology, London, England. Mr. Dickson received his BS in chemistry from imperial College.

D. Wayne Blssett Is chic+! s! the Chemical industries Div.,

G.C. Dickson is vice president of R&D at Hitec Ore

73