-- Recovery of metals in their noble form;

Transcription

1 \ 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 the context of protecting the environment. The principle reasons that are stimulating the uti 1 ization of electrochemical techniques in the shops of the electroplaters and the circuit board fabricators are the following: -- Reduce the volume of toxic waste by eliminating the heavy metal hydroxide sludges; -- Recovery of metals in their noble form; -- Possibility of recycling the products accordlng to the quantlty and the purity of the recovered metal. Thts article describes the utilization of electrolysis to recover metals present in rinse water and waste water from surface treatment shops. It is shown that electrolysis may be utilized not only for cathodic but also for anodic reactions. The simultaneous destruction of cyanides at the anode in a system where metal i s being recovered at the cathode adds a positive value to the effectiveness of this process. DescriDtion of the Electrolysis Ce 11 The electrolysis system for this study uti 1 ized RETEC cell s fabricated by ELTECH Systems Corporation-Geneva Branch, and commercialized in - Switzerland by the companies Titalyse S.A. and Erne AG. A RETEC 50 cell 1s- shown in Figure 1. It contains 51 titanium (type DSA@P> anodes. These anodes permit the cell to operate in both acidic and basic media. Their specificity for the evolution of chlorine i s well known, but they also support very well the evolution of oxygen in acid media or the oxidation of complexing agents such as cyanides in basic media. Eiectroiyte flow in the cell is perpendicular to the active surface of the electrodes. The cathodes which can be utilized in these electrolyzers are of several types: metal sheets, perforated sheets, and reticulated metal cathodes (RME). The latter offer a major advantage by having a high surface area as compared to the geometric surface. DSA i s a registered trademark of DST S.A. * Translated from French by J. J. Dietrich

2 CATHOOECONNECTOR FIGURE 1. RETEC-50 CELL ' AIR SPARGER It is the high surface area which permits these electrolyzers to be effective over a large range of concentration which extends from several grams/liter to a fractlon of a milligram per liter. These electrolysis cells are modular. They can contain a variable number of electrodes which is determined by the quantity of metal to be recovered and the method of utilization: closed circuit or in line. Table 1 summarizes the recovery capacities of different metals for the RETEC systems that are commercially available.!xlum RETEC-6 gr/hr RETEC-15 gr/hr w!ooatsoa cu 119 Cd 210 CO 1 la N1 109 Zn 122 Pb 386 Sn 221 Au 7 36 Ag 402 Pd RETEC-20 grlhr SOOA RETEC-50 grlhr 75oA RETEC-LO grlhr loooa

3 I Pest ructio n of Cvanides a nd the Recoverv of Copprt r The recovery of metal contained in the copper cyanide bath was achieved using a RETEC 50 cell. The electrolysis parameters are described in Tab1 e 2. The study was made on the recovery,of metal in parallel to the destruction of cyanides. The total change in concentration of copper and of cyanide is given in Table 3. The average electrolysis efficiency was greater than 10% for the recovery of metal in a concentration range of 2 g/l to 0.2 milligrams per liter, while the cyanide was reduced more than 20 to 40% over the same concentration range (see Figure 2). V0luIRI: (291 gal.) Flow: 850 llh (225 gallhr) Uode: Recyc 1 e Current: 600 A Voltage: 3.0 V Electrodes: 50 Mi420 pores per inch) Anodes: 51 ou Agi tati on: Air Time Copper Copper Cyanide Cyanide InHoutsm ZEfClclencvu LWl&xY lo I (9 For the two processes. an exchange of one electron ha: been chosen in laaklng Faraday yield calculations. The low yield of copper is probably due to a partial re-oxidation of copper one to copper two on the anodes. We see that the reduction yield of copper in sulphuric acid media i s very high; the phenomonen of monovalent copper ion oxidation was absent in acid medium.

4 2.2 h s" WWLK x \\ 5 E lo4j 1.2 v F t I a nue (HOURS) FIG. 2. ELECTROLYSIS OF A COPPER CYANIDE RINSE BATH USING A RETEC 50 Recoverv of Acid Comer Acid copper solutions are, along with silver and cadmium solutions, those which electrolyze with the best yields, even with low metal concentrations. The parameters of this experiment are given in Table 4. Electmlyr! 5 parametsrs: Voltage: 2.0v ' Curren t : Electrode Surface: 15 A 28 dm2 (3.1 ft2) Electrode Type: Etfici ency N1420 pores per Inch) (3.0 gr to 0.8 ppm) 35% Vo 1 ume 30 1 (7.9 gal) Time (Hours) Copper Concentration (mall) I I Time (Hours) Copper Concentration (mall) o

5 This shows that the copper recovery rate is much superior to that observed in the case of cyanide solutions: a yield of 35% was observed in the case described. With RETEC 50 and RETEC 6 cells, yields approaching 50% have been obtained over identical concentration ranges. i An economic study of copper recovery by electrolysis is presented in Tables 5 and 6. Up to 75% of the metal recovery cost Is recovered through the added value gained from the elimination of sludge and the resale of the electrolytic metal. TABLE % Econaric Studios of Mota1 Rrcovrry by Elrctrolysls LMck@l and ) r r r Q - u.enk Capital : RETEC-50 Recti ff et 10V-750A Total Deprecfatlon over 5 yoars Metal recovery capaci ty Cathode costs Number to recovrr 4000 kg/of mrtrl Total Annual Cost Cathode Capacity Electrolytic Copper Value Electrolytic Nickel Valur Total Annual Recyclr Annual Recycle Valur Operating Elprnsr Electric Power kg (8.800 lbrllyear 15.-FrS ($10) Iptrcr loo0 4 kg (8.8 1bt)lcathodr I.O FrSlkg ($0.30/lb) 10.0 FrSIkg (S3.0311b) 4000 kg (8.800 lbs) Copprr: Nicktl: Cost in FrrlYllr ($1-1.5 FrS) ($22,670) 11.5M).- ( $7,670) ($30.000) ( S6.070)lyrar 15,OOO.-( $10,000) lyrar ($2.670)lyrar 40.OO0.-($26,670)/year Copprr zip A I Zey x 8333 x 0.15 ($0.10) - 1,250.-($833) kmh x FrSlkHh Nickel 750 A X 6.0 Y X 4ppp X 0.15 ($0.10) I 3,750.- ($2.500) wlh x FrSIW Labor: 4 hlwerk (30 FrS (S2O)lh tims 50 works) Sludgr disposal cost 6,000.- ($4,000) At 102 mota1 contrnt (f167)m ton For 4000 kg of notal. 40 tons of sludgo df spoitd of annually Preci pl tation, storagr, sludgo handling ($67) In ton 10,000. -( $6.670) lyrar 4,000.-($2.670)lyear

6 IAklLli Economic Sunmary of tho Elrctrolytic Rrcovcry of Mrtal$ Cu In FrS oor Year af Oarntfan). Depreciation Cathodes El ectrtci ty Labor TOTAL 31,350.-(S20.900) ($22.567) Operating Expense: FrSIKg (Sllb) 7.85 ($2.38) 8.50 ($2.58) " Recovered Metal 4,OW.-(S2.667) 4O.OOO. -(f26,667) Sludge Treatment 4,OW.-($2,667) 4,OW. 4S2.667) 01 sposal 1o ( $6.666) (56.666) Savings: FrSlkg (Sllb) 4.50 ($1.36) ($4.09) ' $1-1.5 FrS. The Recoverv of Nickel Electrolytic recovery of nickel from spent baths or rinse baths is easier and more economical compared to the elimination of nickel as a hydroxi de s 1 udge. Tables 5 and 6 contain the basic assumptions used to make the economic calculations of electrolytic recovery. In the case studled, a metal recovery cost close to 8.5 FrS/kg (approximately $2.50/lb) is obtalned for a recycle volume of 4,000 kg (8,800 lbs.) per year. Thts Is less than 1/2 of the metal value as compared to the commercial price of the pure metal. The electrolysis results of a spent nickel sulfamate bath or a static rinse bath are given in Figures 3a and 3b.

7 ,. I ( ' x RETEC - 50 C.15 - VOLUME: 1000 LITERS PUMP RATE: 2000 L/HR. AMPS: w 1 0 0' e 0.4 y 0.2 o f I I I I 1 I I I TIME (HOURS) -- FIG. 3a. ELECTROLYTIC RECOVERY OF NICKEL FROM A WATTS RINSE BATH n e 3.5 x E RETEC - 50 C 15 - VOLUME: 1000 LITERS PUMP RATE: 2000 L/HR. AMPS: 750 TIME (HOURS) FIG. 3b. ELECTROLYTIC RECOVERY.OF NICKEL FROM A SULFAMATE RINSE BATH.

8 Jhe Oest ruction of Etch Bat hg An interesting example of the utilization of electrolysis is i ts applicatlon in the recovery of metals from oxidizing baths such as solutions of hydrogen peroxide sulfuric acid or ammonium persul fate-sul furl c acid. In the first stage, the strong oxidizing agent in the bath is destroyed by the action of bisulfite (in the form of Na2S 05.7H20). The reaction is slow but complete. In both cases t f e formation of sulfuric acid makes it possible to recycle the bath. I NaHS03 -+ NaHS04 + H20 (NH4)2S205 + NaHS03 + H20-+NaHS04 + (NHq)2S04 + H2SO4 When the reactions are complete, the electrolysis of the etch bath can be run under normal conditions for a sulfuric acid copper bath. Conclusions This series of results shows that electrolysis can be utilized for the recovery of certain metals from industrial waste water. Table 7 contains a list of the most current applications. Precious Metals Au. Ag, Rh, Pd, Pt... Plating baths and etch baths Au-CU. AU-Ni alloy^ Au from Co Separatlon of: Cu from Pd, Au from Co... Ag photographic fix baths C m n Metals CU Cd Ni Sn Sn-Pb In Separation Acfd and amnlacal etches Of H2OZ and (NH412S208 Acfd and Cyaplde Watts. Waod and sulfamate Acid Acid Cyanldr and acid Cd from 111 and Co Cu from Cd and Zn

9 By controlling certain chemical parameters which favor the deposition of metal especially at low concentrations, it 1s possible to use modern electrochemical systems to reach very low metal concentrations with effi ci ent electrochemical yields. For the metal to have value, in the majorlty of cases, it must be recovered in a dense and stable form. In recovering the metal at its source in its most noble state, electrolysis contributes to the evolution of techniques which favor the recycle of metal and the protection of the environment (Figure 4). METAL TYPE FIG. 4. ELECTROLYTIC RECOVERY OF METAL: PRICE OF METAL AND ELECTROLYSIS COST -