J. P. Wiaux, J. J. Dietrich

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1 EFFECTIVE ELECTROLYTIC RECOVERY OF HEAVY METALS FROM DILUTE EFFLUENTS Authors: J. P. Wiaux, J. J. Dietrich Over the past several years, it has become increasingly difficult and costly to dispose of sludges that contain heavy metals. Therefore, there has been a major effort to either reduce or eliminate the sludge. In contrast to precipitation methods which produce larg6 vol umes of toxic sludge, and ion-exchange systems which produce elutants with concentrated heavy metals electrolysis a1 lows the effluent to be treated at the source to recover the metal ions as metal. Electrochemical metal recovery techniques have been 1 imi ted by low electrochemical efficiencies associated with recovering actual ions in parts per mi 11 ion (ppm) concentrations from solutions treated. Progress in electrolyzer design in conjunction with new cathode materials has dramati cal ly increased the effi ci ency of electrolytic metal recovery (1, 2 ). One of these electrolyzers, utilizing reticulate metal cathodes, was designed to efficiently remove heavy metals to very low ppm levels. The system consi sts of a1 ternating foam metal cathodes and precious metal catalyzed titanium anodes (Figure 1). DSA" ANOOt- 1,.- ROW fllter CATHODE -\ BUS RETICULATE - CATHODE ANODE BUS W/ CONNECTOR CLIPS /" CATHODE CONNECTOR -/ FIG. 1. RETEC-SO CEU

2 .../._.*." ""-.- This titanium anode, which i s stable in acidic and basic media, allows operation without contamination while the reticulate cathode a1 lows efficient metal rec ue to the large surface area that is about 10 times the geometric dow area. The expanded view of the cathode i s shown in Figure 2.

3 To better understand the critical electrolysis parameters, it is necessary to look at some basic mathematics. For the recovery of a metal from an industrial plating effluent, the following relationship appl ies (3): C e o A x V Rf where: V = Vo t Pa t Pc t Ir, and Ce: Energy consumed expressed in KHhIkg; A: Theoretical quantity of current necessary to deposit 1 metal in per kah; V: Cell voltaae: Vo: Minimum voitage required to deposit the metal; Pa: Anode polarization: Pc: Cathode polarization; Ir: Ohmic drop in the electrolyte; Rf: Faraday yield of the process; Figure 3 shows the key parameters. kg of 'h FIG. 3 KEY CELL PARAMETERS

4 As a first approximation, it is estimated that for a given effluent, the terms Vo, Pa and Ir are reasonably constant during the course of the electrolysis and that the energy consumed can be written as: The terms Pc and Rf vary during the course of the electrolysis. At high concentrations, the reaction at the cathode i s governed by the deposition of metal. As the electrolysis proceeds and the metal concentration - decreases, the voltage increases and hydrogen evolution begins as shown in the following two equations. M+* + Ze' M H2 + 20H- 2H20 + 2e- - Because of the competition between metal deposition and hydrogen evolution, it i s certain that the nature of the effluent and the geometry of the electrolyzer play an important role in the Faradic efficiency of the overall electrolysis. Electrolyzers with plate cathodes give inferior yields as the metal concentration goes below 0.5 gm/l. This implies an unfavorable energy consumption and the necessity to have a cathode material with a large surface area to satisfy conditions of low metal concentration. A list of the precious and common metals that can be recovered electrochemically i s shown in Table 1. Conspicuously missing from this list are iron and chromium. These metals cannot be recovered in systems that do not have separators between the anode and the cathode. In addition, the presence of iron and chromium can hinder or preclude the recovery of the c,etals listed in Table 1. TABLE 1: ADDllCatfOnS for the Electrolvtic Recovery of Metals Precious Metals Au, Ag, Rh, Pd, Pt... Plating baths and etch baths Au-Cu, Au-N1 a1 loys Au from Co Separation of: Cu from Pd, Au from Co... Ag photographic fix baths Common Metals cu Cd Ni Sn Sn-Pb Zn Separation Acid and ammoniacal etches of H202 and (NH412S208 Acid and Cyanide Watts, Wood and sulfamate Acid Acid Cyanide Cd from Ni and Co Cu from Cd and Zn

6 Because of oxidation at the anode, some of the bath constituents may need replacement to maintain the desired concentrations. The overall result is a longer bath lifetime and efficient recovery of the metal (Table 2). TABLE 2: Recovery of Acidic Coppe r (CuSO4 - H2S04 - ph = 1) Electrolysis pa rs: Voltage: 2.ov Current: 15 A Electrode Surface: 28 dm2 (3.1 ft2) Electrode Type: Ni-(20 pores per inch) Efficiency (3.0 gr to 0.8 ppm) 35% Vol ume 30 1 (7.9 gal) Ti me (Hours) Copper Concen t rat i on (ma/l) Ti me (Hours) Copper Concentration (mq/l) o 0.8

7 Recoverv of Come r and Destruct ion of Cvanide: Although not aggressively marketed for treat1 ng cyanide, a precious metal catalyzed titanium anode in an ETS unit is capable of efficiently destroying cyanide present in plating solutions. Under the conditions listed in Table 3, the efficiency shown in Table 4 has been obtained. The changing concentration with time has been plotted in Figure 6. At low concentrations, cyanide destruction plateaus as the reaction becomes very inefficient. TABLE 3: El ectrol vsi s Paramete rs for a Come r Cyanide Bath. Volume: F1 ow: Mode: Current : Voltage: Electrodes: Anodes : Agi tat i on : (291 gal.) 850 l/h (225 gal/hr) Recyc 1 e 600 A 3.0 V 50 Ni-(20 pores per inch) 51 DSA Ai r TABLE 4: Treatment of a CopDe r Cvanide Bath: Analvtical Results(*> Ti me in Hours Copper (moll) X Efficiency i Cyan i de (mall1 X Efficiency (*) For the two processes, an exchange of one electron has been chosen in making Faraday yield calculations.

8 CYANIDE COPPER FIG. 6, TIME (HOURS) SIS OF A COPPER CYANIDE 7 - *7- remove the last traces

9 PLATING BATH PRIMARY SECONDARY ION RINSE RINSE EXCHANGE. I ELECTROLYTIC 1 I I REGENERANT. _. - I I TREATMENT r I TANK J I 1 DISPOSAL FIG. 7 TREATMENT OF ION LXCHANGE REGENERANT Treatment of Effluent before Ion-Exchanue: For solutions with high concentrations of metal and moderate volumes, it is desirable to utilize ETS to efficiently remove the metal to the low ppm levels and then to polish, if necessary, with ion-exchange (Figure 8). This method is desirable where the discharge regulations are extremely stringent or recovery of a prec;ous metal i s involved. The regenerant from the ion-exchange column can then be recycled through the ETS unit. ELECTROLVIC / REGENERANT ( TREATMENT TANK J DISPOSAL PLATING BATH - PRIMARY SECONDARY ION I RINSE.I * RINSE EXCHANGE - 1 FIG. 8 TREATMENT OF EFFLUENT BEFORE ION EXCHANGE

10 The economics of electrolytic metal recovery wi 11 vary depending on the scrap value of the metal recovered and the various costs associated with sludge treatment, handling and disposal. Table 5 gives the basis for calculations involving nickel and copper for a plating facility in Switzerland. The methodology i s valid for the United States but the numbers would have to be changed. The data are summarized in Table 6. TABLE 5: Economic Studies of Metal Recovery by Electrolysis lnickel and Come r Cases) Items Capital : RETEC-50 Rectifier 10V-750A ' Total Depreciation over 5 years Metal recovery capacity Cathode costs Number to recover 4000 kg/of meta; Total Annual Cost Cathode Capacity Electrolytic Copper Value Electrolytic Nickel Value Total Annual Recycle Annual Recycle Value Operating Expense Electric Power 4,000 kg (8,800 lbs)/year 15.-FrS ($lo)/pi ece kg (8.8 lbs)/cathode 1.O FrS/kg ($0.30/lb) 10.0 FtS/kg ($3.03/1b) 4000 kg (8,800 lbs) Copper: Nickel : Cost in Frs/Year ($111.5 FrS) 34, ($22 670) ( $7.670) 45, ($30,000) ( $6,07O)/year ( $1 0,000) year Copper 750 A x 2.0 V x QQQ 8,333 x 0.15 ($0.10) = 1,250.-($833) kwh x FrS/kWh Nickel 750 A x 6.0 V x QQQ 25,000 x 0.15 ($0.10) = 3,750.- ($2,500) kwh x FrS/kWh Labor; 4 h/week (30 FrS ($20)/h times 50 weeks) S1 udge disposal cost 6, ($4,000) At 10% metal content (8167)m ton For 4000 kg of metal, 40 tons of sludge disposed of annual ly 1Ol0O0.-($6,670)/year Precipitation, storage, s 1 udge hand1 i ng 100.-($67) /m ton 4,000.-($2,670) /year

11 TABLE 6: Economic Summary of the Electrolytic Recovery of Metals (case of Ni and of Cu in FrS Der Year of ODeration). Annual ExDense CoDDer Nickel Depreciation 9,100- -( $6,067) * 9,100 -($6,0671 Cathodes 15,000. -( $1 0,000) 1 5,000. -( $10,000) El ectri ci ty 1,250.-($833) 3,750. -($2,500) Labor 6,000. -( $4,000) ( $4,000) TOTAL 31,350.-( $20,900) 33,850. -($22,567) Operating Expense: FrS/Kg ($/l b) 7.85 ($2.38) 8.50 ($2.58) Annual Savi nas Recovered Metal 4,000. -( $2,667) 40,000. -( $26,667 Sludge Treatment 4,000. -( $2,667) 4,000 -( $2,667) Di sposal ($6,666) ($6,666) TOTAL 18,000. -( $1 2,000) 54,000. -( $36,000) Savings: FrS/kg ($/lb) 4.50 ($1.36) ($4.09) $1 =I 1.5 FrS.

12 It can be clearly seen in this particular case that electrolytic recovery is the economic choice for nickel but sludge disposal is more favorable for copper. As can be seen from the tables, the numbers can be dramatically a1 tered depending on the recovery value of the metal and the disposal cost of the sludge. Utilizing the Swiss data, it is possible to draw the chart shown in Figure 9. The validity of the chart does not change when one converts from Swiss Francs per kilogram into dollars per pound. FIG. 9. ELECTROLYTIC RECOVERY OF METAL: PRICE OF METAL AND ELECTROLYSIS COST

13 Developments in the structure of the electrodes and in the design of electrolyzers have led to the appearance in the marketplace of a number of grow'lng applications in the area of dilute effluent treatments. As sludge disposal becomes more difficult and more costly, there i s increasing incentive to employ electrolytic recove y systems. In addition to the reduction or elimination of sludge, the electrolytic systems allow the user to recover the waste as metal and, thus eliminate the ongoing liability associated with sludge. It is interest ng to note that there i s no one simplistic solution to waste treatment. The waste treatment problem can best be solved by a judicious choice of treatment systems which meet the environmental restraints and give the user the best economic return. The ultimate goal i s to obtain technical, political, economic and environmental harmony, 4091 P

14 Bi bl iocrraphy (1) Sioda. et al. Flowthrough Porous Electrodes. Chemical Engineering. Feb 21 pp (1983) (2) Konicek M.G., Platek G. Reti cul ated Electrodes Cell Removes Heavy Metal s from Ri nse Waters. New Mater1 a1 s and Processes. Vol. 2. pp (1983) (3) a) Doniat D. Procede Electrochimique de Recuperation. No Fev. (1979). b> Coeuret F.et Storck A. Elements de Genie Electrochimique. Ed. Lavoisler Tech. & Doc. Paris ISBN Trai tements de Surface.

-- Recovery of metals in their noble form;

-- Recovery of metals in their noble form; \ INDUSTRIAL ELECTROLYSIS APPLICATIONS IN THF RECOVERY AND RECYCLING OF MET ALS* Author: J. P. Wlaux 2f3 7-r P3 IC Abstract The electrolytic recovery of metals is finding more and more justification in