Final Report on Removal of Lead from Printed Circuit Board Scrap by Electrolysis-Chemical Precipitation Method for Control Data Corporationand Minnesota Waste Management Board by I. Iwasaki B.L. Pozzo A.S. Malicsi June 28, 1988 MINERAL RESOURCES RESEARCH CENTER DEPARTMENT OF CIVIL AND MINERAL ENGINEERING UNIVERSITY OF MININESOTA
REMOVAL OF LEAD FROM PRINTED CIRCUIT BOARD SCRAP BY ELECTROLYSIS-CHEMICAL PRECIPITATION METHOD TABLE OF CONTENTS Page Summary 1 Introduction 2 Electrodissolution in a Sulfuric Acid Medium Electrochemical Measurements Pilot-Scale Electrodissolution in a Sulfuric Acid Medium Electrodissolution in a Caustic Soda Medium Electrochemical Measurements Pilot-Scale Electrodissolution in an Alkaline Medium 2 3 4 8 8 9 Delamination and Separation of Scrap Boards 11 Delamination by Grinding or by Roasting 11 Separation of Copper and Glass Fiber Sheets 12 SEM Observations of Solder Coatings 15 Printed Circuit Board Scrap 15 Copper Sheets Delaminated by Roasting 16 Conclusions References Appendix 16 18 20
REMOVAL OF LEAD FROM PRINTED CIRCUIT BOARD SCRAP BY ELECTROLYSIS - CHEMICAL PRECIPITATION METHOD Summary Printed circuit board scrap with external solder coating is classified as a hazardous waste by the EP Toxicity test because of lead ion release, and is shipped out of state to landfill sites at high cost. The removal of. lead, therefore, would allow the scrap to be reclassified as a non-hazardous waste. The scrap analyzes as high as 45% copper, and together with the stripped lead and tin, these metals, if separated, would not only defray the high cost of diposal, but also may profitably be sold in the open market. The present project demonstrated the technical feasibility of removing lead and tin selectively from copper by electrodissolution in a dilute caustic soda solution. A pilotscale unit was constructed and operated satisfactorily. The power consumption was estimated at 24 kwh per short ton. The removed lead precipitated while tin remained in solution. The two metals, therefore, can be recovered separately. The scrap may be delaminated by roasting at 325 to 250 C. Although electrodissolution readily removed lead from the solder coating on the roasted copper sheets, it did not remove tin. Tin-free delaminated copper products may be produced, therefore, by electrodissolution of the solder coating from the scrap boards first followed by delamination through roasting and gravity separation/flotation..
Introduction Printed circuit boards consist of laminated copper sheets (typically 5 oz of copper/sq ft) and glass fiber boards with external coating by solder (40% lead, 60% tin) of 0.0005 inch in thickness for etch resist and solderability. In their manufacture, substantial amounts of the laminated boards become scrap which, according to one estimate, amounts to half a million to a million lbs per year in the Twin Cities area. Because of the lead content in the solder layer, printed circuit board scrap is classified as a hazardous waste. Therefore, the removal of lead would allow the scrap to be reclassified as non-hazardous waste. The standard electrode potentials of lead, tin and copper are, respectively, -0.13, -0.14 and +0.34 volts, which suggest that the solder coating may be electrolytically dissolved and separated from copper. Delamination of the lead- and tin-free product may render the scrap more desirable raw material to be sold to copper smelters. The present investigation was undertaken to establish the following four areas: (1) The electrochemical measurements on metals relevant to the selective removal of solder coatings from copper sheets by anodic dissolution in a sulfuric acid medium. (2) Development of a reactor that allows electrolytic dissolution of the solder coatings in a pilot-scale unit. (3) Technical feasibility of electrolytic treatment in a dilute caustic soda medium, thereby preventing the dissolution of the reactor material. (4) Delamination and separation of copper sheets from glass. fiber boards by grinding or by roasting. Electrodissolution in a Sulfuric Acid Medium An investigation was initiated to study the electrolytic dissolution behaviors of metals relevant to the scrap and reactor materials in a dilute sulfuric acid medium in 2
order to define the optimum voltage range and then to test the basic findings in a pilot setup consisting of a stainless steel trommel screen basket partially submerged in a dilute sulfuric acid bath. Electrochemical Measurements An electrochemical study was carried out to investigate the anodic behavior of lead, tin, copper and lead-tin solder alloy in one molar sulfuric acid solution and to evaluate the withstanding capacity to corrosion of anode materials such as stainless steel (SS), mild steel (MS) and titanium (Ti) in the same acid medium. Electrodes were made of metal samples mounted in a lucite tubing of 0.625 cm ID with 3M Scotch-Weld structural adhesive. The surface of the electrodes exposed to the solution (about 0.16 cm²) was carefully, cleaned and polished prior to their use on a metallurgical polishing wheel with alumina powder (0.5pm) as the abrasive material. The electrical contact with the external circuit was made by placing a few mercury drops to connect with a copper wire. The polarization curves were determined for each electrode by applying a variable potential ranging from -0.9 to 1.2 V (vs SCE) at a scan rate of 0.3 mv/sec between the metal electrode and a saturated calomel electrode (SCE) via a slat bridge containing saturated ammonium nitrate solution. The applied potentials and the currents generated between the, working electrode and two graphite counter electrodes were recorded as potential vs log current plots. The measurements were made with an EG & G Corrosion Measurement System, Model 350 A. Sodium sulfate was added to the solution as a supporting electrolyte at a concentration of 0.25 M. Results are presented in Figure 1. As it can be observed, lead passivated quite readily and the transpassive region for this electrode was not reached within the applied potential range. However, the passivation current increased by almost one order of magnitude in the potential range of 0.2 to 0.8 V (vs SCE). This would be an indication 3
of a gradual change in the characteristic of the passivating film. Tin also developed some passivation, even though the passivation current was several orders of magnitude higher than that developed by the lead electrode. The lead-tin solder alloy showed a behavior quite similar to that of tin except for the passivation current which was approximately one order of magnitude smaller than the same current in the &in electrode. The anodic behavior developed by copper almost coincided with that of tin. However, the rest potential of the copper electrode was about 0.4 V more cathodic than the rest potential developed by the tin, lead and solder electrodes, which, in turn, were very similar among them (about -0.55 V vs SCE). The polarization curves of the potential anode materials are given in Figure 2. Austenitic stainless steel showed passivation in the potential range of -0.3 to 1.0 V (vs SCE). Higher anodic potentials drove the stainless steel electrode to a transpassive regime. Mild steel also showed a characteristic passivation pattern. However, its passivation current was almost two orders of magnitude larger than the same current for the stainless steel. Titanium behaved as the noblest among the anode materials tested and its passivating current was lower than that of stainless steel. Pilot-Scale Electrodissolution in a Sulfuric Acid Medium A series of pilot-scale electrodissolution tests were carried out in an electrolytic cell to investigate the dissolution -behavior of the solder coating on printed circuit board scrap in 1 M sulfuric acid. A rotating trommel-screen basket made of stainless steel was used as the anode and a semi-cylindrical shaped sheet of the same material was used as the cathode in the electrolytic cell. The anode was rotated with an electrical motor at 40 rpm. The electrode setup and the electrolytic solution were all inside a rectangular container made of Fiberglass of about six-gallon capacity. A Nobatron DCR 40-35 potentiostat was used as the power source and the electrical connection with the rotating anode was made by 4
using a carbon sliding contact. A schematic view of the cell setup is presented in Figure 3. Three gallons (11.3 lb) of a solution made of sulfuric acid and sodium sulfate at 1 M and 0.25 M concentrations, respectively, was used as the electrolyte and samples of printed circuit board scrap with a bulk weight of 500 g were charged into the basket anode. The charges were made of rectangular chips of printed circuit board scrap of approximately two inches (5.1 cm) long by one inch (2.5 cm) wide. Some tests were also run with a 200 g charge. Voltages of 0.1, 0.5 1, 2 and 4 V were applied between the anode and the cathode. The anode potential against a saturated calomel electrode (SCE) was monitored throughout the tests as well as the cell current, ph and bath temperature. Sampling of the electrolyte solution and of the electrolyzed scrap chips were made every 15 or 30 min. At the end of each test, the precipitate was separated by filtration and the metal deposited on the cathode was redissolved by reversing the cell potential for about 10 minutes. All the liquid samples were analyzed for lead, tin, copper, iron, chromium and nickel contents. Drillings from the samples of scrap chips as well as the precipitates were also analyzed for lead, tin and copper, Lead, copper, iron, nickel and chromium were analyzed by atomic absorption (AA) spectrophotometry and tin was analyzed by inductively coupled plasma (ICP) emission spectroscopy.. Initially, a series of blank tests was carried out without any charge of circuit board scrap at different cell potentials in an attempt to evaluate the rate of dissolution of stainless steel anode and the significance of electrolyte contamination by dissolved chromium and nickel. The results of the blank tests are presented in Figures 4, 5, 6 and
Anode (trommsl-screen, basket) Saypling Hole C a t h o d e Electrolyte Solution Schematic Representation of Electrolytic Cell
160 0 20 I I I I I I I I 40 60 80 100 120 140 160 Time (min) Figure 6. Dissolution of Cr and Ni from the ASS Anode at 2 and 4V Cell as a Function of Time. Electrolyte: 2S0 4 + IM 0.25M H 2 Na 4 SO
As seen in Figures 4 and 5, the anode dissolved very little for cell potentials of 0.5 and 1 V, suggesting that passivation occurred on the stainless steel anode at these cell potentials. On the other hand, as Figures 6 and 7 show, the dissolution rates of stainless steel at cell potentials of 2 and 4 V were substantially higher, indicating that the anode was working in the transpassive region at these cell potentials. The observations should be carefully taken in account, since the build-up of chromium and nickel in the electrolyte solution will not be desirable. A series of the electrodissolution tests was performed to study the effects of cell voltage, anode material and the amount of scrap charge on the removal of solder coating as a function of time. From visual observations of the final products, some preliminary conclusions could be drawn on the effectiveness of acid electrolysis in dissolving the solder coating from the scrap. It was observed, for example,- that the dissolution was poor when the scrap was electrolyzed at cell potentials below 1 V even after 5 or 6 hours of electrolysis. Conversely, for cell potentials of 1 V and higher, the solder coating virtually disappeared after 3 to 4 hours of electrolysis. The liquid as well as the drilling samples from the electrodissolution tests are presented in Tables 1 to 10 in the Appendix, in which the metal contents in the scrap, percentages of metal extraction, and the metal concentrations in solutions are shown as a function of time under relevant conditions. Anode potential versus a saturated calomel electrode and cell current are also listed as a function of electrolysis time, as well as the composition and the amount of precipitates at the end of each test. Lead and tin contents in the scrap as a function of electrolysis time and cell voltage are plotted in Figures 8 and 9. As seen in Figure 8, the rate of dissolution of lead was higher for a cell voltage of 1 V than for lower voltages (0.1 and 0.5 V). Preliminary testing (Table 2 in the Appendix) indicated that at a cell voltage of 4 V, the lead and tin dissolution was better than at a cell voltage of 1 V. The dissolution of 6
Cell Voltage 0.1 v 2 0.5 V 1 v, 1 60 180 Time (min) 240 Figure 9. Tin Content of Printed Circuit Scrap as a Function of Electrolysis Time at Different Cell Voltages. Electrolyte: 1M H 2 S0 4 + 0.25M Na 2 S0 4
lead and tin did not improve appreciably at a cell voltage of 2 V (Table 3in the Appendix) as compared to a cell voltage of 1 V. As reported in the previous section, the dissolution of the austenitic stainless steel anode was quite pronounced at cell voltages of 2 and 4 V (Figure 6). High chromium and nickel contamination of the electrolyte can then be expected by working at these cell voltages. At a cell voltage of 1 V, the anode dissolution appeared to be relatively low (Figure 4), but, increasing contamination by recycling of the electrolyte solution may eventually become a problem in the acidic electrolysis of printed circuit board scrap. According to the polarization curves presented in Figure 2, the mild steel electrode showed some passivation in acid solutions in the potential range of 0.4 to 1.2 V (vs SCE). A few tests were carried out with a mild steel anode of a similar shape to the stainless steel reactor (Figure 3). The cell voltage was fixed at 1 and 1.5 V in such a way that the anode potential was in the passive range for mild steel (0.7 and 0.9 V vs SCE, respectively). The results presented in Figures 8 and 9 and in Tables 7, 8 and 10 in the Appendix clearly indicate that even though the problem of chromium and nickel contamination of the electrolyte was eliminated by using a mild steel anode, the dissolution of iron was too high and the extraction of lead and tin too low for this approach to be economically viable. It is apparent from Figures 8 and 9 that the anodic dissolution of lead was not as rapid as that of tin in the acid electrolyte. These dissolution tendencies were consistent with a severer passivation developed by lead as compared to tin when exposed to anodic polarization (Figure 1). The low concentrations of lead in the electrolyte solutions and the relatively high proportions of lead in the precipitates (Tables 5, 6, 7 and 8 in the Appendix) indicate
passivating unless it is covered with a passivating film of a precipitated lead salt. In sulfuric acid solutions, lead sulfate would serve this purpose. Electrodissolution in a Caustic Soda Medium Since lead sulfate will not form in an alkaline solution, it was felt that lead will anodically dissolve more readily in a sodium hydroxide solution than in an acid solution. In addition, since mild steel is strongly passivated in an alkaline ph, an anode made of this material could be successfully used, thereby avoiding the undesirable chromium and nickel contamination associated with a stainless steel anode. Electrochemical Measurements The anodic behaviors of lead, tin, copper, solder alloy and mild steel in one molar sodium hydroxide solution were investigated. Polarization curves were determined on electrodes made of metal samples as described in a previous section. Scan rate, potential range and general procedure were similar to those reported previously. Results are presented in Figures 10 and 11. Figure 10 shows that both mild steel and copper electrodes passivated almost within the same potential range. The passivation chemistry of copper appears to be quite complex since the current changed from anodic to cathodic and then to anodic again as the potential was increased in the anodic direction from -0.5 to -0.3 V. Tin also passivated after going through a region of oscillating current (Figure 11). Apparently, this type of behavior is characteristic of tin in alkaline solutions as observed by a number of investigators. 2,3,4 The oscillation of current are thought to be the manifestation of competing mechanisms between chemical and electrochemical reactions of dissolution and passivation processes. The passivation of lead was notably less pronounced than that of tin except for a narrow potential range (0.5 to 0.9 V vs SCE), in which lead was strongly passivated before entering the transpassive regime (over 0.9 V 8
vs SCE). Pilot-Scale Electrodissolution in an Alkaline Medium A series of tests was carried out to investigate the dissolution behavior of the solder coating on printed circuit board scrap in 1M sodium hydroxide solution. The same cell set-up and the test procedure as described in a previous section were followed for these tests. The anode was made of mild steel and the cell voltages of 0.5, 0.7, 1.0, 1.5, 2.0 and 2.3 V were applied between the anode and the cathode. The results are presented in Tables 11 to 18 in the Appendix, and plotted in Figures 12, 13 and 14. It is observed, in general, that lead and tin dissolved quite readily in the alkaline environment (Figures 12 and 13), while the dissolution of the mild steel anode was negligible, as indicated in the columns corresponding to the iron concentration in the electrolyte solutions in all the tables. The rate of dissolution of lead was, in general, lower than that of tin, particularly at the cell voltages of 1 V and lower. For instance, after 120 minutes of reaction time at the cell voltages of 1 and 0.7 V, the tin extraction was near completion, while only 20 and 40% of lead were extracted at the respective cell voltages. This suggests the possibility of selective dissolution. However, this is in apparent contradiction with what can be expected from the higher passivation tendency of tin vis-a-vis lead, according to the polarization curves in Figure 1-l. It might be speculated that any passivating film which eventually develops on tin is quite unstable so that the simple rubbing action involved in the continuous contact among the printed circuit scrap chips inside the rotating anode can remove the coating thereby favoring dissolution. It is also observed (Tables 11 to 18 in the Appendix) that most of the dissolved lead did not remain in solution and appeared in the precipitates. According to Burbank, lead under anodic polarization in alkaline solutions of ph over 9.4 corrodes as divalent ion, which by accumulation and oxidation, develops a loose film of PbO². 9
Tin, on the other hand, remained almost totally in solution except for a relatively small amount in the precipitates, presumably in the form of metallic tin. 5 Linley reported a process for recovering tin from tin plates based on the anodic dissolution in an alkaline solution and simultaneous deposition on the cathode of a spongy metallic tin which can be easily removed. According to Linley, a good deposition efficiency can be achieved only at near boiling temperature. To investigate the effect of the amount of scrap charge and of the rotating speed of the basket anode on the dissolution efficiency of lead and tin, a few tests were carried out by doubling both variables (from 500 to 1000 g and from 40 to 80 rpm, respectively) at a cell voltage of 2 V. As shown in Figure 14, the rates of dissolution improved for both metals at higher rotating speed. More contact time of the charge with each other at the higher speed may account for the increased efficiency. The percent extraction fur both 1000g and 500g charges were similar (about 75% for lead and nearly 100% for tin) after 90 minutes of electrolysis. This is equivalent in saying that after 90 minutes of reaction time, the amount of dissolved metal increased with the amount of the charge. In an attempt to estimate the power consumption in the dissolution of the solder coating, the cell current data along with the lead and tin analyses were plotted against time, and the area under the cell current curve until the lead content of the scrap became nil was determined. A typical example of the plots is shown in Figure 15. In this figure, the area to 120 min. was determined to be 12.6 Ah. At the ceil voltage of 2.0V, the power consumption will be 25.1 VAh for a feed amount of 1000 g scrap, or 22.8 kwh/short ton. The power consumption for the other tests were estimated in a similar manner and the results are summarized in Table 19.
Table 19. Power Consumption in the Removal of Solder Coating from Printed Circuit Board Scrap. Cell Voltage Feed Wt Drum Area Power Consumption Test No. V 8 RPM A h VAh kwh/t 10 2.0 500 40 6.5 13.0 23.6 11 2.0 1000 40 12.6 25.1 22.8 12 1.75 500 40 4.1 7.2 13.1 13 2.0 500 80 7.8 15.6 28.3 14 0.7 500 40 30.0 21.0 38.1 15 2.3 500 40 4.7 10.8 19.6 In the table, the power consumption averages 24 kwh/short ton. It is interesting to note in Figure 15 that the cell current became vanishingiy low after the solder coating was removed, which suggests that very little copper dissolves by the electrolytic treatment. De-lamination and Separation of Scrap Boards In the foregoing tests, the scrap boards were anodically electroiyzed to remove the solder coating. The results in a caustic soda solution indicated that both lead and tin were selectively dissolved with a minimum loss of copper from the scrap boards. The removal of the solder coating would render the scrap nonhazardous due to the absence of lead. For recycling as a scrap raw material to copper smelters, however, it becomes desireable to de-laminate copper from glass fiber sheets. Some attempts were made, therefore, to de-laminate the glass fiber sheets from copper sheets and separate them, and to dissolve the solder coating on the de-laminated copper sheets by electrolysis. Delamination by Grinding or by Roasting Initially, a scrap board sample, cut to about half an inch pieces, were ground in a laboratory stainless steel rod mill, 7 inches in diameter and 9 inches long. Even after hours of grinding, the scrap board chips remained intact and only the corners were 11
rounded. A few attempts were made to grind the scrap board after cooling in liquid nitrogen without success. It became apparent that the bonds between the laminated sheets were so strong that the boards could not be de-laminated by grinding. A few preliminary roasting tests indicated that the destruction of the organic material by heat rendered the copper and fiber glass sheets to come apart. Accordingly, a series of tests was performed to ascertain the effect of roasting temperature and the time at temperature on the de-lamination behaviors of the boards. The results are shown photographically in Figure 16. The boards started to show some sign of de-lamination from 250 C, but the optimum temperature range for complete de-lamination was 325 to 35O C and the time at temperature of I5 to 30 minutes. When a 135 g batch of scrap board chips was roasted in a muffle furnace at 35O C for 15 minutes, the weight loss was 18.7%. By peeling the copper sheets away from the glass fiber sheets and separating them manually, the copper sheets weighed 55% and the glass fiber sheets 45% of the roasted product. It would mean that the copper content of the scrap board sample before roasting was 45%. For the liberation of the copper from the fiber glass sheets, a manually operated shredder, patterned after a nut mill, was constructed of aluminum. In general, the copper sheets were liberated from the fiber glass sheets reasonably well, but the copper sheets were torn into smaller pieces unsuitable for the electrolysis tests. Further developmental work on the liberation of roasted products is necessary. Perhaps a highspeed hammer mill may work more satisfactorily. During roasting, pungent odor, presumably of bromine, was released. Lowering the temperature was found to decrease the odor substantially, but the use of a scrubber on the exhaust gas would be necessary for the elimination of the odor. Separation of Copper and Glass Fiber Sheets For the separation of the copper sheets from the fiber glass boards, either gravity 12
separation or flotation may be used. The specific gravity of the fiber glass sheets roasted at 35O C for 15 minutes was determined to be 2.73. Since the specific gravity of metallic copper is 8.92, gravity separation should be feasible. The choice of a suitable equipment, however, requires pilot-scale tests because of the flaky nature of the products. Perhaps a hydraulic classifier, for example, a Dorr or an Akins classifier, may be applicable. Another approach that was found to be effective was flotation. A liberated mixture, consisting of 55% by weight of copper sheets and 45% fiber glass sheets, was pulped in a Fagergren laboratory flotation cell, conditioned with 1.6 lb of pine oil per ton and floated for 3 minutes. Essentially complete separation of the two products was achieved by this procedure. However, a part of the copper sheets was seen to plug up the inside of the rotor and some of the glass fiber sheets were also trapped. Perhaps, a flotation cell with an external blower for air supply, for example, a Galligher cell, may alleviate such a difficulty. Pilot-Scale Electra-dissolution of Delaminated Copper in an Alkaline Medium It was demonstrated earlier that solder coating could be, more or less, selectively dissolved in 1 M sodium hydroxide solution. In these tests, an anode basket made of mild steel was used satisfactorily. Hence, an electro-dissolution -test was performed by charging 200 g of the delaminated copper sheets in the mild steel basket, and an attempt was made to apply a cell voltage of 2.0V. When the applied voltage exceeded about lv, a vigorous generation of gas bubbles was noted in the electrolysis cell, and the applied voltage could not be increased any further. Nevertheless, the electrolysis test was started since it was shown earlier to be possible to dissolve lead and tin at a cell voltage of 1V. Even after 3 hours of operation, however, the solder coating remained on the surfaces of the copper sheets. This is contrary to the results of Test 9, in which both lead and tin were 13
removed after 3 hours. Therefore, a test with the operating conditions identical to Test 9 using the scrap board chips were repeated. After 4 hours of operation, again no solder coating was dissolved. A cursory test was then performed on the printed circuit board scrap material in 1 M sodium hydroxide at a cell voltage of 2V with a stainless steel basket, which was used earlier with a sulfuric acid electrolyte. After two hours of operation, the dissolution of solder coating was evident. The results are shown in Table 19 in the Appendix. It was surmised that, after a few months of standing in air, the mild steel basket may have changed its nature in such a way that the oxygen over-voltage was affected, or the electrical contacts between the basket by the oxide coating and the d&solution behavior of the scrap board chips became adversely affected. Prior to the electro-dissolution tests on de-laminated copper sheets, a test was carried out on printed circuit board scrap in 1 M sodium hydroxide using the stainless steel basket. The purpose of the test was to confirm if the previous test results could be duplicated. The experimental conditions of Test 15 were used. After having established that the electrolysis cell was operating satisfactorily, two tests were performed with de-laminated copper sheets. Since the copper sheets were rather bulky, only 200 g could be accommodated in the basket in order to have active tumbling action inside the basket. In one test, the ceil voltage was fixed in the range of 1.3 to 1.5V and in the other at 2.3V. The former showed no sign of gas bubble generation, while in the latter test, a substantial generation of gas bubbles was observed. In these tests the white metal coating remained on the copper surfaces even after 4.5 hours of operation. The results are presented in Tables 21 and 22 in the Appendix and plotted in Figure 17. It is evident that roasting at 35O C rendered tin more difficultly soluble by the electrolytic treatment.
SEM Observations of Solder Coatings In Figures I2 and 13, the solder coating was completely removed in less than 3 hours when the cell voltage was maintained above 0.7V. Furthermore, the dissolution rates of tin was noted to be appreciably faster than those of lead during the electrolysis. When the copper sheets de-laminated by roasting were electrolyzed, however, the white metal coating remained on copper sheets even after 5 hours of electrolysis. An attempt was made, therefore, to observe the dissolution behavior of solder coating under a scanning electron microscope (SEM), JEOL Model JSM 84011. The changes in the composition of the solder coating was examined qualitatively by use of the energy dispersive x-ray spectroscopy (EDS). Printed Circuit Board Scrap An electrolysis test was repeated under the conditions of Test No. 15 and a sample chip was removed from the reactor every 30 minutes. The SEM photomicrographs of the scrap raw material is shown in Figure 18. The solder consists of an eutectic composition of 40% Pb, and 60% Sn, and the two metals have relatively low solubilities in each other in the solid state. Upon solidification, therefore, the solder separates into two metals. The bright areas (Figure 18a) are lead with minor amounts of tin as shown in the EDS spectrum (Figure 18c). The dark area (Figure 18b) is essentially pure tin as seen in Figure 18d. Figure 19 shows the SEM photomicrographs of the printed circuit board scrap surfaces as a function of electrolysis time, and Figure 20 presents the EDS spectra of the corresponding surfaces. It is interesting to note that even after as short as 30 minutes of electrolysis, the solder coating became free of lead. The surfaces appeared essentially the same to 2 hours. After 3 hours, numerous spots of copper (dark areas) began to appear, and after 4 hours only small spots of tin (bright areas) are left on the predominantly copper surface. After 5 hours, the copper surface was completely free of 15
d Figure 18. Back-scattered Electron Images of Solder Coating on the Printed Circuit Board Scrap. The White Area (a) is Lead and the Dark Area (b) is Tin
Figure 19. Back-scattered Electron Images of the Printed Circuit Board Scrap Surfaces as a Function of Electrolysis Time (a) 0 h; (b) 0.5 h; (c) 2 h; (d) 3 h; (e) 4 h; (f) 5 h
Figure 20. EDS Spectra of the Printed Circuit Board Scrap Surfaces as a Function of Electrolysis Time (a) 0 h; (b) 0.5 h; (c) 2 h (d) 3 h; (e) 4 h; (f) 5 h
tin. The foregoing SEM observations on the preferential dissolution of lead from the solder coating is diametrically opposite to the results obtained with the mild steel basket. Copper Sheets De-laminated by Roasting As shown in Figure 21, the white metal coating on the copper sheets de-laminated by roasting remained even after 5 hours of electrolysis. Hence, the roasted copper sheets before electrolysis and after 5 hours of electrolysis were examined under SEM and the results are shown in Figure 21. The surface of a roasted copper sheet before electrolysis show typical bright and dark patterns of Pb-Sn eutectic mixture (Figure 21a). The white metal coating on the copper sheets after 5 hours of electrolysis was found to be tin, free of lead. Figure 21b and c show the surface texture at two magnifications. The strong peaks of copper and the fact that the tin did not dissolve even after 5 hours of electrolysis may suggest that the tin remaining on the surface might have been alloyed with copper. Conclusions The results of this investigation lead to the following conclusions. 1. The anodic polarization curves of lead, tin, copper and lead-tin solder alloy in a dilute sulfuric acid medium showed that these metals can be readily electrolyzed with very little selectivity in their dissolution behaviors. 2. A pilot-scale rotating trommel-screen basket reactor was constructed and tested successfully for the electrodissolution of solder coating from printed circuit board scrap chips. 3. When 1M sulfuric acid was used as an electrolyte, the solder coating was removed rapidly when a cell voltage of over 2 V was applied. However, the stainless steel anode was also electrolyzed, and the electrolyte became heavily 16
Figure 21. Back-scattered Electron Images of Copper Sheets Delaminated by Roasting Tin Coating after 5 hours of Electrolysis EDS Spectra of the Tin Coating
contaminated with nickel and chromium ions. 4. A mild steel anode in 1 M sulfuric acid dissolved too rapidly to be of practical interest for the present application. 5. The anodic polarization curves in a dilute caustic soda solution showed that mild steel and copper passivated, while lead dissolved readily. The dissolution behavior of tin showed a complex behavior upon anodic polarization. 6. In pilot-scale electrodissolution tests in 1 M caustic soda, both lead and tin dissolved selectively from copper.the mild steel reactor remained virtually unaffected. Contrary to the anodic polarization curves, tin dissolved more rapidly than lead. 7. The mild steel reactor after a few months standing became ineffective due presumably to the formation of a heavy rust coating. 8. When a stainless steel reactor was used, lead dissolved more rapidly than tin, in agreement with the anodic polarization curves. 9. In the caustic soda solution, dissolved lead precipitated while tin remained in solution. 10. An increased rotating speed of the reactor basket improved the dissolution rate. 11. The percent extraction of lead and tin remained nearly the same with an increased amount of the scrap charge. 12. Power consumption in the removal of lead from printed circuit board scrap in 1 M caustic soda averaged 24 kwh per short ton. 13. Printed circuit board scrap chips could not be delaminated by grinding in a rod mill. 14. Printed circuit board scrap chips could be delaminated by roasting at 325 to 35O C for 15 to 30 minutes.during roasting, pungent odor, presumably of 17
bromine, was released which necessitates the scrubbing of the exhaust gas. 15. The separation of the delaminated copper and glass fiber sheets may be achieved by gravity separation or by flotation. 16. With the roasted and delaminated copper sheets, the electrolysis in 1 M caustic soda removed lead readily, but tin could not be removed even after 4.5 hours. 17. Lead- and tin-free delaminated copper scrap may be produced by electrodissolution of the solder coating followed by delamination through roasting and gravity separation/flotation. References 1. Burbank, Jeanne, The Anodic Oxides of Lead, Journal of the Electrochemical Society, May 1959, (369-376). 2. Stirrup, B.N. and Hampson, N.A., Anodic Passivation of Tin in Sodium Hydroxide Solutions, J. Electroanal. Cehm., 67 (1976), 45-46. 3. Hamson, N.A., Anodic Behavior of Tin in Potassium Hydroxide Solution, Br. Corrosion J., Vol. 3 (1968), 1-6. 4. Lieht, S. and Manassen, J., Thin Film Chalcogenide/Aqueous Polysulfide Photoelectrochemcal Solar Cells with In-situ Tin Storage, J. Electrochem. Sot. (May 1987), 1064-70. 5. Linley, B.D., Recycling of Tin and Tinplate, First International Tinplate Conference, London (1976), 427-440.
APPENDIX
TABLE 1. EXTRACTION OF LEAD AND TIN FROM PRINTED CIRCUIT SCRAP AFTER 180 MINUTES OF ELECTROLYSIS IN 1M H O, SOLUTION AS A FUNCTION OF CELL VOLTAGE. 2 4 Anode Cell Anode Potential % of Pb % of Sn Material Voltage, V V (vs SCE) Dissolved Dissolved MS 1.5 0.7 12 10 ASS 1.0 0.4 -> 0.5 40 100 ASS 0.5 0.2 -> 0.3 <l0 50 ASS 0.1-0.3 l0 50
TABLE 2. PRELIMINARY TEST 2 ELECTROLYTE: 1 M H SO + 0.25 M Na SO CELL VOLTAGE: 4 V 2 4 2 4 ANODE MATERIAL: AUSTENITIC STAINLESS STEEL ----===================o================================================== Sampling Cell Metal concentration in time current solution (ppm) (min) A Pb Sn cu 0-5 23 6.36 282 8.53 30 22 8.36 482 183 60 22 8.61 253 150 22 365 219 180 22 5.75 373 243 Redissolved* - - 3.23 100 120.6 Composition of Precipitates, % Pb Sn Cu 10.90 13.30 46.3 Total amount = 43 g ----------------------------------------------------------------------- * Electrolyte replaced with a fresh solution before redissolution
TABLE 3. PRELIMINARY TEST 1 ELECTROLYTE: 1 M H SO + 0.25 M Na SO CELL VOLTAGE: 2 V 2 4 2 4 ANODE MATERIAL: AUSTENITIC STAINLESS STEEL =========================-r============================================================== Sampling Cell Metal concentration in Metal content from time current solution (ppm) drilled samples, % (min) A Pb Sn Cu Pb Sn Cu 0-5 5 3.34 26.6 1.24 1.32 2.27 23.12 30 4 14.2 92 0.90 60 4 22.3 119 2.30-90 3 28.1 159 1.69-120 3 52.4 180 2.49 150 3 38.3 132 2.84 180 2.5 30.6 105 6.73 0.94 0.67 Composition of Precipitates, % Pb Sn Cu 13.4-4.91
TABLE 4. Test 1 ELECTROLYTE: 1 M H SO + 0.25 M Na SO CELL VOLTAGE: 0.5 V 2 4 2 4 ANODE MATERIAL: AUSTENITIC STAINLESS STEEL = = = = = = = = = = = = = = = = = = 3 = 3 = - 1 = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = Sampling Anode Metal concentration in Metal content from time potential solution (ppm) drilled samples, 96 (min) V (vs SCE) Pb Sn Cu Fe Cr Ni Pb Sn Cu 0-0.3 6.78 13 7.53 0.83 0.08 0.03 1.10 1.31 38.1 30 " 6.37 76 3.53 0.74 0.1 0.05 1.36 1.19-60 " 6.88 134 2.79 0.62 0.12 0.05 1.62 1.64-90 " 7.52 183 2.68 0.67 0.11 0.02 -. 1.26 1.41-120 " 7.78 235 1.97 0.58 0.13 0.03 1.38 1.25-150 -0.25 8.20 281 1.94 0.76 0.14 0.04 1.24 0.89-180 -0.25 8.06 307 1.91 0.87 0.13 0.03 1.20-42.7
TABLE 5. TEST 2. ELECTROLYTE: 1 M H SO + 0.25 M Na SO CELL VOLTAGE: 0.5 2 V 4 2 4 ANODE MATERIAL: AUSTENITE STAINLESS STEEL ========================================================================================= sampling Anode Cell Metal concentration in Metal content from time potential current solution (ppm) drilled samples, % (min) V (vs SCE) A Pb Sn Cu Fe Cr Ni Pb Sn Cu 0 40.2 0.98 1.81 0.16 0.20 1.25 1.31 42.2 30 60 " " 0.3 5.84 0.15 5.86 98 1.14 124 1.50 2.08 0.27 3.62 0.45 0.21 1.25 1.22-0.31 1.46 0.90-90 " 157 1.41 3.31 0.59 0.34 1.22 0.94-12O 0.2 " 5.98 1.99 1.35 3.97 0.71 0.39 - - - 150 " 234 1.21 4.39 0.51 0.45 - - - 180 0.08 " 6.62 269 5.89 4.20 0.60 0.52 1.14 0.43 37.9 dissolved* 2.22 < 1 1.37 Composition of Precipitate, % Pb Sn 25.6 0.36 Cu 17.3 Fe Cr Ni - - - Total amount = 2.24 g Electrolyte replaced with a fresh solution before redissolution