RECOVERING GOLD FROM SCRAP ELECTRONIC SOLDERS BY DROSSING by E. F. Ferrell1 ABSTRACT Gold was recovered from scrap electronic 60-40 (tin-lead) solders by drossing with aluminum or zinc at elevated temperatures. The gold, together with other metallic impurities, collected in the dross phase which could then be separated from the molten solder by skimming or filtering. Drossing upgraded the gold concentration by a factor of 10. Further upgrading was achieved by a combination of steps including an oxidation roast and either a combined cyanidation- amalgamation treatment or an aqua regia leach. The gold was then recovered in highly concentrated form from the aqua regia or cyanide solutions by cementation and from the amalgam by volatilization or dissolution of the mercury. The solder was purified by reaction with NaOH to remove dissolved aluminum or with SnCl2 to remove dissolved zinc. INTRODUCTION For several years, the Bureau of Mines has been actively involved in developing techniques for recovering and recycling metals and alloys from industrial scrap, including waste materials generated by the electronics industry. Among the latter materials is tin-lead solder that has become contaminated with gold and other impurities to a point where it no longer has the physical properties requisite for use in fabricating circuit boards or other electronic components. These solders are nominally 60-40 tin-lead mixtures containing 50 to 175 oz gold per ton, and other major impurities such as Ag, Sb, Cu, and Fe. Recognizing the need for processes to recover gold and reclaim the solder in relatively pure form, studies were conducted at the Reno Metallurgy Research Center to evaluate a molten salt electrolytic recovery technique (3-4). The present studies were conducted to determine the applicability of a simple phase-separation method. Metallurgist. Underlined numbers in parentheses refer to items in the list of references at the end of this report. The objective of the phase-separation study was to develop a procedure similar to the Parkes process for softening lead. In the Parkes process, zinc metal is added to molten silver-bearing lead metal. The zinc combines with the silver to form the high-melting, insoluble compound Ag2 Zn, which rises to the surface and is skimmed off as dross (1). Application of the separable phase principle to contaminated solders involved the addition of aluminum or zinc metal to molten solder. results in formation of high-melting, low-density intermetallic compounds such as Al Au which float to the surface along with any undissolved additive metal, such as aluminum, yielding a dross easily removed by skimming or molten metal filtration. Included in the investigation was an evaluation of techniques for recovering gold from the resulting dross. MATERIALS, EQUIPMENT, AND PROCEDURE FOR DROSSING Electronic solders used in this study were nominal 60-40 (tin-lead) solders that had become contaminated during the wave soldering of printed circuit boards to a degree that they would no longer form acceptable bonds. Although there is no such thing as a typical scrap electronic solder, the solders used in this study were fairly representative and contained 50 to 60 oz gold per ton of solder together with appreciable quantities of copper and silver and lesser amounts of Fe, Ni, Bi, Si, and Sb. Analyses of two asreceived solders are noted in table 1. Ag A1 Au Bi The recovery of gold from electronic solders by phase separation requires a density difference between the solder and the drossing agent. Scrap electronic solders had specific gravities from 8.45 to 8.85 g/cm3, while the specific gravities of aluminum and zinc are 2.70 and 7.14 g/cm3, respectively. Other specific gravities of interest, together with melting point data, are given in table 2. Quantities of scrap solder ranging from 200 to 4,000 g were melted in open clay and clay-graphite crucibles in a conventional pot furnace. Bath temperatures were held at 550° C for aluminum-treated solders, and at 350° C for zinc-treated solders. Wires of the additive metal were wrapped around a carbon stirring rod and immersed in the molten solder bath. technique provided for greater surface contact between the two phases. Agitation of the bath during cooling helped phase disengagement. The bath temperature was then lowered to 200° to 250° C, and the dross was removed from the solder by ordinary skimming methods or by filtering in equipment similar to that reported by Ruppert and Sullivan (5). This Skimming was the simpler of the two methods, requiring only a rake or perforated ladle to remove the dross from the solder. Filtration required more elaborate equipment but provided better separation of the dross from the molten solder. Briefly, the filtering unit consisted of a cylindrical upper chamber into which the treated solder and dross were poured, and a cylindrical graphite-lined lower chamber that received the filtrate. The filtering medium consisted of a double thickness of glass cloth supported on a perforated stainless steel plate held between the upper and lower chambers. The lower chamber was connected to a mechanical pump which sustained a vacuum of 25 inches of mercury. The filtration apparatus was contained within a vertical tube furnace which was maintained at 200° to 225° C. Solder samples for analysis were taken from the molten bath by dipping out a 6- to 12-g portion with a graphite cup or by pouring a 1/8- by 3/4-inch slab into a copper mold. Dipped samples were used to determine gold and silver by fire assay, and poured samples were used to determine other metals by X-ray fluorescence, atomic absorption, and spectrographic analysis. Analyses were carried out on the as-received solder, solder after dross removal, and solder after final purification. Dross products and subsequent gold products were analyzed by fire assay. RESULTS AND DISCUSSION Initial experiments on aluminum drossing focused on the effect of temperature on the two-phase operation. A bath temperature of 550° C was decided upon because at lower temperatures solder could not break through the Al2O3 film on solid aluminum and means of injecting molten aluminum would have to be devised. Bath temperatures much higher than 550° C would require bath protection to prevent excessive oxidation of the solder. Oxide coating on the aluminum inhibited its reaction with the molten solder; however, once this film was broken the solder reacted rapidly with the aluminum metal. Small amounts of Pb0 or Sno that formed when the solder was heated were reduced by molten aluminum or zinc. The effect on gold recovery of adding different ratios of aluminum-togold and using a different number of contacts of aluminum-to-gold was studied. As noted in table 3, experiment A was conducted by adding aluminum in three successive increments to the solder. The first contact removed only 57 pct of the gold, while the second contact brought the recovery figure to 92 pct. After a third contact, gold recovery was essentially complete. In the first step of experiment B, more aluminum was used than had been used in all three steps of experiment A, but only 74 pct of the gold reported to the dross phase. Increasing the molar ratio A1:Au to 9.0, however, removed essentially all of the gold from the solder. The third experiment utilized a very high molar ratio Al: Au in a single contact. Gold recovery was 98.2 pct. It is evident that multiple contacts are more effective for removing gold from electronic solders, but that a very high percentage of the gold can be removed from the solder in one step if an excess of aluminum is used. In addition to gold, the aluminum also removed significant amounts of Sb, Cu, Fe, and Ni from the solder. Aluminum was not effective for silver removal. Zinc drossing was conducted at 350° C as opposed to the 550° C necessary for aluminum drossing. Zinc was as effective as aluminum for removing gold from solder and also removed Ag, Cu, Fe, and Ni, but not Sb. The use of zinc, however, imposed distinct disadvantages. Approximately eight times as much zinc on a molar basis as aluminum was needed to completely remove gold from the solder. Zinc dross did not become as viscous as aluminum dross as the bath temperature was lowered, thus complicating phase separation. In addition, solder dissolved larger quantities of zinc than aluminum, making it more difficult to repurify the solder to an acceptable electronic grade. In general, aluminum drossing is preferred over zinc drossing except for solders containing substantial quantities of silver. Separation of Aluminum Dross From Treated Solders The gold-laden aluminum dross was removed from the solders by skimming or by filtration. Skimming required multiple stirring -skimming operations for complete removal of the dross, whereas filtration removed all of the dross from the solder in one pass. Table 4 presents data for skimming and filtration operations. The dross had such a low viscosity at 350° and 450° C that it could not be effectively removed by skimming. Skimming removes considerably more material than does filtering, and the total amount removed increased with the number of skimmings and with the amount of aluminum added. Filtration temperature was also an |