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Copper Recovery and Cyanide Destructionwith a Plating Barrel Cathode and a Packed-Bed AnodeReport-Research Project78

By C.-D.Zhouand D.-T. Chin

A batch electrochemical cell, consisting of a platingbarrel cathode and a packed-bed anode, was used torecover copper from a waste acid copper sulfatesolution and simultaneously to recover copper anddestroy cyanide ina waste copper cyanide solution. Thecyanide destruction experiments were carried out withand without the addition of NaCI. The concentrations oftotal cyanide, free cyanide and copper were measuredas a function of electrolysis time at various solutiontemperatures, cell currents, barrel rotation speeds,barrel loadings, and barrel immersion levels. The totalcyanide concentration was reduced from580ppm toless than 10 ppm. The average energy consumption was80 to 340 kWhlkg of total cyanide destroyed, dependingupon the operating conditions. A cost analysis indicatesthat the present electrolytic method is more costeffective than the conventional alkaline chlorinationtreatment for waste cyanide solutions. It also offers acost advantage over a commercial carbon fiber electro-lytic process because of a lower capital investment.M tal cyanide compounds are extensively used in theelectroplating and metal finishing industries. The proc-ess wastewater is toxic and disposal causes a lossofvaluable materials. Cyanide wastes must be adequatelytreatedbefore being discharged from the process plant.The conventional method for cyanide treatment is chlorina-ti or^'-^By reacting with chlorine gas or sodium hypochlorite inalkalinesolutions, cyanide is converted nto CNO, CO,, N andNH The cost of chlorination is high; it has the drawback thatcomplex cyanides and strong cyanide solutions cannot beadequately treated. Incinerationof cyanide waste and destruc-tion of cyanide by thermal hydrolysis are alternate methods.The cost of incineration, including equipment, maintenanceand operation, is also high. Although the cost of thermalhydrolysis is in the rangeof 10to35percent of the cost of con-ventional chlorination the method does not permitthe recovery of heavy metal ions from waste plating solutions.

Electrochemical destruction of cyanide isa promising proc-ess.The method s capable of simultaneously recovering metaland destroying complex cyanides in waste plating solutions.Electrochemical oxidation of cyanide was first reported byClevenger and Hall in 1913. ' Since then, numerous reportshave been published on this subject.8-12 he electrochemicalmethod can be accomplished by two different techniques: Thefirst is based on electrodepositionof metal ions at the cathodeand oxidation of cyanideto cyanate, carbon dioxide and nitro-gen gases at the an~de.'~-'~or a waste copper cyanidesolution, the cathodic and anodic reactions may be describedby the following equations:

June 1993

P 04219

Cathode reactions:Cu(CN);- +e- --f Cu+3 CN-2H,O +2e- -+H +2 OH-

(EO=-1.09 V) (1 1(2)(E"=-0.83V) (3)Cu(CN); +e- --f Cu+2 CN- (E"=-0. 43V)

Anode reactions:CN- +2 OH- -+CNO- +HO +2 e-CNO- +2 OH- -+CO, +lhN +H,O +3e- (EO=-0.76V)Cu(CN)B+6OH-+

(EO=-0.97V) (4)(5)

Cu++3CNO-+3H,O + 6 e- ( Eo=-0.69V) (6)(E"=0. 40V) (7)OH-+ O, +2 H,O +4 e-where E" is the standard reduction electrode potential vs.SHE at 25 "C.

The second technique is based on an in-situ liberation ofchlorine by electrolysis of a cyanide waste to which sodiumchloride has been added.'"" This technique is suitable fortreating dilute electroplating rinsewater with a cyanide concen-tration less than 500 ppm. For a waste copper cyanide solution,the cathodic reactions are described by Reactions(1)-(3).Theanodic reactions include Reactions (4)-(7) and the followingadditional reactions:

2CI-+CI, +2e-CI, +2 OH-+CI-+CIO- +HO(E"=1. 36V) (8)(9)2 CN-+5 CIO-+ 2 OH-+5C-+N +2 COl-+ HO (10)For dilute plating waste and rinsewater, three-dimensional

electrodes, such as a packed and a fluidized bed$are often used. This type of electrode has a large surface areaand a high reaction rate per unit cell volume. A high masstransfer rate is neededto increase current efficiency for metaldeposition reactions in dilute wastewater. For this aspect, atumbling bed, such asaplating barrel, wouldbe a good choice,where the relative movementofelectrode particles with respectto the electrolyte improves mass transfer and current efficiencyfor metal deposition reactions. Several patents describe theuse of tumbling bed electrodes to treat Oehrused a barrel plater as the anode for cyanide oxidation?' Tisondescribed a bipolar tumbling bed electrochemical cell to re-cover copper from a dilute copper sulfate solution.srmA com-parison between the packed bed and tumbling bed electrodesfor metal recovery wasalsodiscussed byTison.34The tumblingbed has more uniform current and metal deposition distribu-tions than those of the packed bed. The tumbling bed also offersa higher mass transfer rate than the packed bed because of themovement of particles. In a plating shop, an existing barrelplater may be used, and little nvestment is needed for in-housewaste treatments.

This study examines the feasibilityof using an electroplatingbarrel cathode and a packed-bed anodeto recover metals and

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cyanide and copper at the beginning of electrolysis were 520ppm, 112ppm, and317 ppm, respectively.

destroy cyanide simultaneously in waste plating solutions. Theeffects of temperature, cell current, barrel rotation speed, barrelloading, and barrel immersion level were studied with wastecopper sulfate and copper cyanide solutions.- Exper imental ProcedureTest SolutionsBatch cell experiments were carried outto recover copper froman acid copper sulfate solution and simultaneously to recovercopper and destroy cyanide in a copper cyanide solution, withand without the addition of NaCI. Table 1 lists the composition,volume and temperature of test solutions used in the experiments.

For the acid copper sulfate solution, 10 liters of an aqueoussolution containing0.1MCuSO, and 1 MHSO, at20 to27 "Cwere used. The copper ion concentration in the solution wasequivalentto 6350 ppm, as shown in Table 1.

For the copper cyanide solution without NaCI, 8 to 11 liters ofan aqueous solution containing 0.0056 M CuCN, 0.017 MNaCN and0.2 M NaOH were used. The solution compositioncorresponded to an initial concentration of 585 ppm of totalcyanide, 119ppm of free cyanide, and356ppm of copper. Thetests were carried out in the temperature rangeof 25 to65 "C.

For the copper cyanide solution with the addition of NaCI,1.35 liters of an aqueous solution containing0.005 M CuCN,0.015 M NaCN, 0.1 M NaOH and0.2 to 0.6 M NaCl at25 "Cwere used. The initial concentrations of total cyanide, free

r

IZ - b JElectrolyte outAnode Current

OuterContainer

Packed Bed Anode Drain

Cell Set-upThe electrochemical cell used in the tests of acid copper sulfatesolution and copper cyanide solution without NaCl is shownschematically in Fig. 1. The cell consisted of rectangularPlexiglasTMnner and outer containers. The total volume of thecell was approximately12liters.A variable speed plating barrel,13cm in diameter by 15cm long,a oaded with copper shot wasplaced n the inner container as the cathode, to recover copperfrom the waste plating solutions. The diameter of the coppershot was 0.3 to 0.5cm for the treatment of acid copper sulfatesolutions, and0.1 to 0.3 cm for copper cyanide solution. Twostainless steel balls2.5cm in diameter, coated with a thin layerof gold, were used as the dangler contacts n the plating barrel.A packed bed 13x 13x 10cm (wlh), located on the bottom ofthe inner container, was used as the anode. Forthe acid coppersulfate solution, the anode bed was packed with lead shot0.2cm in diameter and a lead plate was usedasthe anode currentcollector. For the copper cyanide solution, the anode bed waspacked with steel nails0.2 cm in diameter by 3.8 cm in lengthand a stainless steel screen was used as the anode currentcollector. The test solution was recirculated between the outletof the anode bed and the top of the plating barrel with a meteringpump.b The solution flowed through mesh openings in thebarrel walls to the interiorof the plating barrel, where copperions in the solution were cathodically deposited on copper shot,according to Reactions (1)-(2). The catholyte exited thescreened barrel walls by gravity and flowed into the anode bed,where cyanide ion was oxidizedtonontoxic chemical species,accordingto Reactions(4)-(6). The anolyte exited the packedbed by gravity and was recirculated by the metering pumpthrough an overflow port on the outer container wall of the cell.The direction of solution flow is shown by the arrows in Fig. 1.The solution temperature in the cell was controlled by a quartzheater and a thermistor probe connected to an exterior tem-perature controller. Table 2 summarizes the experimentalconditions of cell set-up.

Forthecoppercyanide solution with addition of NaCI, a smallcell similar to that of Fig. 1was used. The total volume of thesmall cell was approximately 6.5 liters. A miniature platingbarrel6.4 cm in diameter by10cm long,c oaded with coppershot, 0.1 to 0.3 cm in diameter, was used as the cathode. Thedangler contact for the cathode bed was a copper cylinder0.6cm in diameter by 1.2 cm in length. The anodic packed bedbelow the plating barrel was 13x 10x 9cm (wlh). twas packedwith graphite pellets0.3cm in diameter by 0.5 cm in thickness.A piece of graphite felt, 1.25 cm thick, was used as the anodecurrent collector. The experimental condition of cell set-up isshown in the third column of Table 2.Test ProceduresFor each run, a known amount of copper shot was placed n theplating barrel, which was then put into the cell filled with the testsolution. The recirculation pump was turned on and the platingbarrel beganto rotate at a constant speed. After the solutiontemperature had been raised to the desired level by thetemperature controller, a constant current from a DC powersupply" was appliedtothe cell. The cell voltage and solution pHwere monitored during the electrolysis. A small amount of

Model 46A, Sterling Systems , Streamwood , IL.Masterflex US, ole-Parmer Instrument Co.. Chicago, IL .Snap-On Barrel, Singleton Co., Cleveland, OH.Model 62748, Hewlett-Packard, Albany, NY.ig. I-Schematic of cell set-up

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NaOH was periodically addedtothe cell to keep the solution pHabove 11 during the treatmentof copper cyanide solution. Forthe copper sulfate solution, a run was terminated when copperion concentration was below 10 ppm. For the copper cyanidesolutions, a run was terminated when the total cyanide concen-tration dropped below 10 ppm.For the acid copper sulfate solution, 10 mL of sample solutionwere taken from the cell every onetotwo hours, and copper ionconcentration was analyzed with a copper-ion-selective elec-trode and a double-junction Ag/AgCI reference electrode filledwith 10-percent KNO, solution.

For the copper cyanide solution, 10 mL of sample solutionwere taken every oneto two hours during the electrolysis. Freecyanide concentrationwas measuredwith a cyanide-ion-selec-tive electrode and a double junction Ag/AgCI reference elec-trode. Afterwards, the sample was treated with 50 mL of 6 MH,SO, in a distillation device. The gaseous HCN liberated wasabsorbed in a glass flask containing 100 mL of 1.25M NaOHsolution. The total cyanide absorbed in the NaOH solution wasdeterminedwith the cyanide-ion-selectiveelectrode. The sampleleft in the distillation flask was used to determine copper ionconcentration with the copper-ion-selectiveelectrode. For thecop-per cyanide solution containing NaCI, a small amount of AgNO,solution was added to precipitate CI- before the copper-ion-selective electrode was usedto measure copper concentration.Results and DiscussionAcid Copper Sulfate SolutionAll the runs with acid copper sulfate solution were carriedoutin10 liters of a solution containing 0.1M CuSO, and 1MH,SO,June 1993

at a barrel rotation speed of 22 rpm, a barrel immersion evel of75 percent of barrel diameter, and a solution recirculation rateof 3.1 mUsec. The runs were made at room temperature;however, the solution temperature was raised by the cellcurrent from 20 to 27 "C during the run. The experiments werecarriedoutat cell currents of 6, 9, 12 and 15 A overa range ofbarrel loadingo 25 to 75percent of barrel volume.

Table 3 summarizes the results of copper recovery from theacid copper sulfate solution at various operating conditions.The average cathode current efficiency and energy consump-tion per kilogram of copper recovered were based on thereduction of cupric ion to copper and a copper concentrationchange in the solution from 6350 ppm to 10 ppm.

Copper sul fate solution ('20-27 'CBarrels p e d =22rpmBarrel oadlng =25%

9 A 6 A

I I ......... . . . . . . . . . , . . . . . . . . . , . . .......,...*...,5 10 15 20Time, hr

Fig. 2-Copper ion conc entration vs. t im e fo r t reatment of a waste coppssul fate solut ion.

71

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Figure 2 shows the changes n copper ion concentration withrespect to electrolysis time at cell currents of 6,9, 12 and 15 A.The data were obtained at a barrel loading of 25 percent ofbarrel volume. The rate of copper recovery increased withincreasing cell current. Low cell current offered high currentefficiency and low energy consumption. Table 3 shows that theaverage cathode current efficiency for copper deposition reac-tion decreased from 39 percent at a low cell current of 6 A, to29 percent at a high cell current of 15 A. The average energyconsumption per kilogram of copper deposited in the barrelincreased from 13 kWh at 6 A to 30 kWh at 15 A. Theinstantaneous cathodic current efficiency and energy con-sumption per kilogram of copper recovered were calculatedfrom the slope of the concentration vs. time curve by thefollowing equations:

where n is the number of electrons transferred n the electrodereaction, Vis the solution volume in iters,I s the cell current (A),C is the concentration of copper ion in mol/L, is the electrolysistime in hr, E,, is the measured anode-to-cathodecell voltage(V), M is the molecular weight of copper in @mol,and dddt isthe slope of the concentration vs. time curve in moVUhr.

The instantaneous cathodic current efficiency and energyconsumption at cell currents of 6 A and15A are plotted againstcopper ion concentration in Fig. 3. The runs with a 6-A cellcurrent had higher current efficiency and lower energy con-sumption than those with cell current of 15A. When the copperion concentration dropped below 200 ppm, the current effi-ciency became less than 10percent and energy consumptionbecame greater than 100 kWh/kg Cu. At low copper ionconcentrations, a large fraction of cell current was used forhydrogen ion reduction and a large amount of energy wasneededtorecovercopper. The results indicate that the presentelectrochemical method is economical for the treatment ofwaste copper sulfate solution when copper ion concentration isabove 200 ppm.

Figure 4 shows the instantaneous current efficiency andenergy consumption for two barrel loadings of 25 and 75percent of barrel volume at a cell current of 15A. Barrel oading72

Copper ion concentration, ppmFig. 3-Electric energy consumption and current efficien cy vs. cop per ionconcentration for treatment of a wast e cop per sulfate solution.of 75 percent produced higher current efficiency and lowerenergy consumption than 25 percent barrel loading. The im-provement n current efficiency with high barrel oading was theresult of increased cathode surface area available for copperdeposition.Copper Cyanide Solution without NaClThe experiments for the treatment of copper cyanide solutionwere carried out with 8 to 11 liters of a solution containing0.0056 M CuCN, 0.017 M NaCN and 0.2 M NaOH in thetemperature range of 25 to 65 "C, ata recirculation rate of 0.8mUsec. To determine the effect of operating variables, four cellcurrents of 3,6,9 and 12 A, two barrel rotation speeds of 5and10rpm, three barrel oadings of 25,50 and 75 percentofbarrelvolume, and three barrel immersion levels of 50, 75 and 100percent were used.

Table 4 summarizes the results for copper recovery andcyanide destruction or various operating conditions. The aver-age cyanide current efficiency and energy consumption in thetable were based on Reaction 4) and a change of total cyanideconcentration from580ppmto 10ppm.

Figure 5 shows the total cyanide, free cyanide and copperconcentrations vs. electrolysis time for a run with a cell currentof6A and a barrel speedof 10rpm at 25 "C. The barrel loadingwas 50 percent of barrel volume and the barrel mmersion evelwas 75 percent of barrel diameter. During the first four hr ofelectrolysis, there was a large drop in total cyanide concentra-tion, while free cyanide concentration changed little. The small

100Copper sulfate solutio n (20-27 C )Barrel speed E 22 rpmCell current I 15 A Barrel oading

40

20

100 lo00 10000Copper ion concentr ation, ppm

Fig. 4--Electric energy consumption and current efficienc y vs. copper ionconcentration lor treatment of a waste copper sulfate solufion with two barrelloadings.

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E2d

Fig. 7-Free cyan ide conc entration vs. time for treatment of a w aste coppercyanide solution at three solution tem peratures.and energy consumption values were calculated from theslope of the total cyanide vs. time curves, using equations(11) and (12), a value of n equal to 2, and molecular weightof CN-radical of 26 g/mol. The results indicate that astemperature increased, energy consumption was reducedand current efficiency for cyanide oxidation was improved.The improvement in cyanide current efficiency was espe-cially large in the total cyanide concentration regionof100to400 ppm.At acell temperature of 25 "C, the current eff ciencyfor cyanide oxidation decreased sharply in the total cyanideconcentration range of 100to200 ppm.At 65 "C, the regimeof a sharp decrease in cyanide current efficiency changed to70 to 100 ppm of total cyanide. When the total cyanideconcentrationbecame less than 70 ppm, the improvementin current efficiency by raising solution temperature wasnot obvious.

The results indicate that the present electrochemicalmethod s economical or the treatment of copper cyanidesolution when the total cyanide concentration is above100 ppm, where the current efficiency for cyanide oxida-tion was greater than 18 percent and energy consumptionwas less than 200 kWh/kg of cyanide destroyed at asolution temperature of 65 "C.

C . ~

Copper cyanlde solut ionCel current=6 ABarrel speed =10 rpmBarrel oading =50%Barrel mmerslon=75 %

2. Effect of Cell Current.The rates of cyanide destruction andcopper recovery increased with increasingcell current; how-ever, the current efficiency was lower and more energy wasneeded at higher cell current. Figure 10 shows the energy

1100Copper cyanlde soluti onCell current =6 A

loo00. 25 'c Barrel speed =10 rpm

* . * 0lo00O0 100Total cyanide concentration, ppm JJg. 9-lectric energy consum ption and current efficiency vs. total cyanideconc entration for treatment of a waste cop per cyanid e solution at two solutiontempera ures.

loo0 Copper cyanide solutionCell current =6 ABarrelspeed =10 rpmBarrel loading =50 %Barrel mmersion=7 5%Recirculation ate=0.8 mUsec

.0 Time, hr

Fig. &Cop per concentration vs. t ime for treatment of a waste copper cyanidesolution at three solution temperatures.consumption and cyanide current efficiency vs. total cyanideconcentration curves for two cell currents of 3and 12A at asolution temperature of 25 "C. The data were obtained at abarrel rotation speed of 10rpm, barrel loading of50percentof barrel volume, and barrel mmersion of 75 percent of barreldiameter.

At a cell current of 12 A , the cyanide current efficiencybecame less than 10 percent and energy consumptionwas greater than400 kWh/kg CN when the total cyanideconcentrationdropped below 120 ppm. At a cell current of3A , the cyanide current efficiency was 18 percent and theenergy consumption was 110 kWh/kg CN at a total cya-nide concentration of 120 ppm. The results indicate thatthe present electrochemical method is more efficient at alow cell current of 3 A than at 12A .

3. Effect of Barrel Imm ersion, Speed and Loading.The effectsof barrel immersion level, rotation speed, and loading weresmall. Accordingto the present experimental results shownin Table 4, the optimal barrel settings were: loading of 50percent of barrel volume, immersion of 50 percent of diam-eter, and a rotation speed of 10 rpm.Table 4shows that the immersion level had no significant

effect on the current efficiency for copper deposition andcyanide oxidation. A partially immersed barrel at 50 percent ofdiameter was slightly better than a fully immersed barrel inreplenishing solution within the barrel bed.

IO000 Copper cyanide solution (2 5 "c) 1wBarrel s p e d =10 rpmRecirculation=0.8 mUsec

Total cyanide concentration. ppmFig. 70--Electric energy consu mptio n and current efficiency vs . total cyanideconcentration or treatment of a cop percy anide solution at cell currents of 3an d12A.

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The effect of barrel rotation speed oncyanide oxidation is shown inTable 4. Aspeed of 10 rpm offered higher cyanidecurrent efficiency and a lower energyconsumption than at 5 rpm. This is be-

--cause the mass transfer rate at 10 rpmwas higher than at5 rpm.Table 4shows that a barrel loading of

50percent of volume offered higher cya-nide current efficiency and lower energyconsumption than at loadings of25per-cent and 75 percent of volume. The re-sults agreed with the operating guidelinediscussed by Hignett, who pointed outthat the optimal load level for a normalbarrel platingoperation should be around60 percent of barrel volume.35The im-provement from 25to50percent loadingresulted from increased cathode surfacearea available for copper deposition.When the rate of cathodic copper depo-sition increased, more free cyanide ionswere released, and the anodic currentefficiency was improved. There are tworeasons for the better performance of a50-percent loading over that of a 75-percent loading. The first reason is bedmovement; particle motion with 50-per-cent loading was faster and offered ahigher mass transfer rate than with a 75-percent loading. The second reason is cur-rent distribution. According to Geissmanand Carlson, the current distribution in aplating barrel became less uniform withincreasing barrel oading.% n a barrel with75-percent loading, a large fraction of cop-per shot was relatively inaccessible tocopper deposition reaction because ofnon-uniform current distribution.Copper CyanideSolutionwithAddition of NaClIn the presence of CI- in a waste coppercyanide solution, CIO- ons may be gen-

erated by anodic oxidation of chlorideions at a graphite anode, in agreementwith Reactions (8) and(9).The CIO onssubsequently reacted with CN- ions toproduce non-toxic N gas and C0 ionsvia Reaction (10). As a result, the de-struction of cyanide in an electrochemi-cal process may be accelerated by addi-tion of NaCltoa waste plating solution. Inthe present study, several experimentswere carried out with the addition of NaClto 1.35 liters of a solution containing0.005MCuCN, 0.015MNaCN and 0.1MNaOH at 25 "C. The concentration ofNaCl in the resulting solution, at the be-ginning of electrolysis, ranged from0 to0.6M.The experiments were carried outin a small cell with a miniature platingbarrel 6.4 cm in diameter by 10 cm longat a cell current of 3 A. A barrel rotationspeed of 9 rpm, loading of 50 percent,immersion level of 50 percent and asolution recirculation rate of 0.8 mUsecwere used. Table5 summarizes the ex-perimental conditions and results.

Figures 11 and 12 show the total cya-nide and copper concentration vs. elec-trolysis time curves, respectively, or fourNaCl concentrationsof 0,0.2,0.4and 0.6M.As expected, the rates of total cyanidedestruction and copper recovery in-creased with increasing NaCl concentra-tion. Figure 13 shows the instantaneousenergy consumption and cyanide currentefficiencyvs . total cyanide concentrationcurves without NaCl and with 0.6MNaCI.Without NaCI, the current efficiency wasless than 10 percent when the total cya-nide concentration dropped below 180ppm. With the addition of NaCltothe testsolution, CIO ions were produced at theanode during electrolysis. The subse-quent reaction between the CIO- ons andC N n bulk solution appearedto improve

the current efficiency for cyanide oxida-tion in the overall electrolysis. For the runwith 0.6 M NaCI, only when the totalcyanide concentration dropped below 85ppm, did the apparent current efficiencybecome less than 10 percent. The re-sults indicate that the addition of NaClimproves the apparent current eff ciencyfor cyanide oxidation and reduces theenergy consumption per kilogram ofcyanide destroyed.CostAnalysisA cost analysis was performed for an in-house cyanide treatment of a waste plat-ing solution containing580ppm of totalcyanide, using the present electrolyticprocess with the addition of 0.6MNaCI.Itwas assumed thata spare plating bar-rel would be used as the cathode, andthat the packed bed anode would beassembled by packing graphite pelletson the bottom of an electrolytic cell. Thecapital investment, consisting of a 700-liter cell tank, a recirculation pump, pipes,filters, rectifier, and installation was am-ortized over a 10-year period, using astraight line depreciation method and azero interest rate. The electrolytic cellwould be operated 20 hr/day and 300dayslyr, with a total of 490 kg of cyanidedestroyed annually.

Based on the results of the presentexperimental studies, various operatingcosts were estimated. The estimated op-erating and capital nvestmentcosts werethen pro-rated for one kilogram of cya-nide destroyed by the waste treatmentprocess. The results are listed n the thirdcolumn of Table 6.For comparison, theoperating and capital investmenvkg ofcyanide destroyed using the conventionalchlorination method3and a carbon fiberelectrolyticcelPare also isted on the firstand second columns ofTable 6.Al l costsin Table 6 were adjusted to 1992 U.S.dollars, using the cost information andprice indexes listed in Refs. 37and 38.

Capital investment for the chlorina-tion and carbon fiber electrolytic meth-ods was also amortized over a 10-yearoperating period, using the straight linedepreciation method and zero interestrate. The cost of chlorination amountsto $71.81/kg of cyanide destroyed, be-cause of the hi'gh operating costs forchemicals, labor, and disposal of solidsludges. The electrolytic treatment ismore cost effective at $14.54/kg CN,using carbon fiber electrodes, and$12.52/kg CN, using a plating barrelcathode and packed,bed anode. Theoperating cost of the present electro-lytic process is slightly higher than that

J une 1993 75

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Operating CostsNaOH ($O.62/kg)l3'1NaOCl ($O.l2/L)PlLime ($49.5/metric ton)NaCl ($0.1 1/kg)@qElectricity($O.O4/kWh)Anode (replaced every 2 YE.)Labor & Maintenance ($lO/hr)Filter Cartridges (24honth)Sludge TransportationPId($0.19Imetric ton-km)Sludge Disposal ($253/ton) PIdSubtotal

Copper cyanide solution ( with addition o f NaCl)Cell current =3Al o00

\. Barrel loadlng=50% \. '\. Barrel Immersion =50% \ '-.4M Na2.2M NaC'l.1.1. .0 1 2 3 4 5 6

Reclrculation r ate I 0.8 mUWC \0.6M NaCl

Capital CostsEqualizationTank 3I dCyanide Oxidation Unit131dPrecipitation ReactorPIFlocculator/Clarifier14Sludge Holding Tanks I3ldFilter Press I3ldElectrochemicalCellSubtotalTotal

ChlorlnationDla

Consumption(Per kg CN)orrota1 nvestmentcost

8 kg57 L0.008 metric ton

0.84metric ton-km0.036 metric ton

$20,000$31,000$28,000$21 Ooo$3,500$1 1,000

4.966.840.40

42.000.169.1063.461.50'2.30 I2.001.500.250.80 I8.3571.81

I Time. hr

ble 6nation and Electrochemlcal7

ET Using Carbon F i b e F .Consumption(Per kg CN)orTotal Investmentcost

0.18 kg

2.65 kg20 kWh

0.14 piece

$129,000

Fig. I I-Totalcyanide concentrationsvs . time for treatment ofa coppercyanidesolution with four NaCl concentrations.

76

0.1 1

0.290.802.004.001.20

8.40

6.14'6.1414.54

?atment ET)n 1992dollars)

ET Us ing P lating Barrel'(present study)Consumption(per kg CN)orTotal Investmentcost

5.0 kg100 kWh

0.50 piece

$12,100

0.554.000.27e4.001.20

10.02

2.502.5012.52

of the carbon fiber electrolytic method. This is a result of alow initial cyanide concentration of 580 ppm and a highelectrical requirement of 100 kWh/kg CN used in the costestimate of the present electrolytic process.

In the carbon fiber electrolytic method,21a waste cadmiumcyanide solution containing 1,000to 4,500 ppm of total cyanideand an electric energy consumption of 20 kWh/kg CN de-stroyed were used in the analysis. The present electrolyticmethod has a lower capital investment cost, which results in anet saving of $2.00/kg of cyanide destroyed over the carbonfiber electrolytic method. The cost figures in Table 6 do notinclude any credits resulting from metal recovery at the cath-ode. The present electrolytic treatment method would becomemore cost-effective if a more concentrated waste cyanidesolution were used for the treatment.FindingsFor an acid copper sulfate solution, copper ion concentrationwas reduced from 6350 ppm to less than 10 ppm with anaverage cathode current efficiency of 29 to 40 percent and anenergy consumption of 13 to 33 kWh/kg of copper recovered.

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Copper cyanlde solut ion ( with addition of NaCI)Cell current =3A1wO

Temperature=25 CBarrel loadin g=50%Barrel Immersion =50%

0 1 2 3 4 5 6Tim.hr

1woO 100Copper cyanlde solutlon (with NaCI). Cell current L 3A. Temperature=25 OCz Barrel speed =9 rpm 80o i

g 100

101

-0)

Barrel loading I 50% \B a w d Immersion=50%

010 100 lo00Total cyanlde concentratlon. ppm I

Fig. ISElectric energy consumption and current efficiency for cyanide oxida-tion vs. total cyanide concentration for treatment of a copper cyanide solutionwitbout NaCl and with 0.6M NaCl.

Fig. 12 4o pp er concentration vs. time for treatment of a copper cyanidesolution with four NaCl concentrations.

For a copper cyanide solution, the total cyanide concentrationwas reduced from 580 ppm to below 10 ppm, free cyanide from120 ppmto 1 ppm, and copper from 356 ppmto 1 ppm, with anaverage cyanide current efficiency of 9 to 23 percent and anaverage energy consumption of 80 to 340 kWh/kg of cyanidedestroyed, depending upon the operating conditions. Thecath-ode and anode current efficiencies increased and energy con-sumption decreased with: (1) increasing solution temperature;(2)decreasing cell current; (3) increasing NaCl concentrations.

The optimal barrel settings were found to be: Rotation speedof 10 rpm; loading of 50 percent of barrel volume; and animmersion level of 50 percent of barrel diameter.

A cost analysis indicates that the present electrolytic methodis more cost effective than theconventionalchlorination method,and has a small cost advantage over acommercial carbon fiberelectrolytic process, because of a lower capital investment.

AcknowledgmentThe authors wish to thank the project supervisor, Dr. PeterBratin of ECI Technology, for his assistance and discussions nthe course of this study.

References1. Ohio River Valley Sanitation Commission, Methods for

Treating Metal-Finishing Wastes, Cincinnati, OH (1953).2. M.R. Watson, Pollution Control in Metal Finishing, pp.

147-178, Noyes Data Corp., Park Ridge, NJ (1973).3. S.A.K. Palmer, M.A. Breton,T.J . Nunn0andD.M. Sullivan,

Metal Cyanide Containing Wastes Treatment Technolo-gies, pp. 602-624, Noyes Data Corp., Park Ridge, NJ(1988).

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49 (Nov., 1990).

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(1975).

Society Meeting, Seattle, WA, May 21-26, 1978.

(Sept., 1984).

June 1993 77

7/30/2019 38792

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C . IR STRIPPINGCOLUMNS:@ P.V.C. Poly-Pro, F.R.P.,or

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P.V.C. Poly-Pro,F.R.P., orHorizontal or vertical designContinuous scrubbingsolution recirculationNon-clogging mass packing

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36. W.C. Geissman and R.A. Carlson, Proc. AES Annual37. Chemical Marketing Report,pp. 27-31, August 31,1992.38. Chemical & Engineering News, 7 0 (26), 48 (1992).39. AESAR 1992-1 993 Catalog, AESAWJ ohnson Matthey,

Tech. Conf.,p. 153 (1952).

Ward Hill, MA (1992).

About the AuthorsC.-D. Zhou is a doctoral candidate in chemical engineering atClarkson University, Potsdam,NY 13699-5705. She receivedBS and MS degrees in chemical engineering from the EastChina Universityof Chemical Technology.

Dr. Der-Tau Chin is professor of chemical engineeringatClarkson University. He has more than 20 years researchexperience in electroplating, corrosion, electrochemical en-ergy conversion, and industrial electrolytic processes. Priorto joining Clarkson, he was a senior research engineer in theElectrochemistry Department of General Motors ResearchLaboratories. Prof. Chin holds a PhD from the University ofPennsylvania. He s the author or co-author of more than110technical papers.

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