Catalytic Selective Recovery of Metals from E-Waste

Abstract

The new process is proposed for selective recovery of metal ions from leachates of electronic wastes.1,2 Hydrogenation of E-waste leachates can result in reduction and deposition of metal ions (Eq.1) if cell potential described by Eq. (2) is positive. For instance, Cu2+ and Ag+ ions with standard potentials of reduction to elemental metals of 0.34 V and 0.799 V (vs. SHE) (respectively) can be theoretically reduced by H2 gas at any pH≥0 at temperature of 25°C and hydrogen pressure of 1 bar. The Ni2+ ions can be removed at the same conditions if the pH value in treated solution is above pH5.2.Unfortunately, the kinetics of Cu2+ ions hydrogenation at ambient temperature is very slow and the process proceeds at reasonable rates only at high temperature (i.e., >150°C). The same is true for Ag+ ions.It was shown by us that highly selective recovery of Cu2+ and Ag+ ions by hydrogenation is possible at ambient temperature and hydrogen pressure of 1 atm using the Pt catalyst supported on activated carbon.Figure 1 shows the results of batch mode experiments aimed at separation of silver ions from the AgNO3 solution (1 liter, pH0 = 4.5). Hydrogenation with Pt/AC catalyst (1% wt. Pt in granular activated carbon) resulted in complete reduction of Ag+ ions and precipitation of elemental Ag metal on the Pt/AC. The kinetics of the process was almost the same in 0-4.5 pH range.Using the catalytic hydrogenation process, the copper ions were selectively separated from a mixture of Cu2+, Ni2+, Cd2+, Fe3+, Ca2+, Mg2+ and Zn2+ ions (100 mg/l each).2 The process was studied at varied pHs (0.5, 1,5 and 4.8) and varied loading of Pt in activated carbon (0.1 to 2.5 %wt.). Figure 2 shows the Pt/AC particles before and after catalytic hydrogenation of CuCl2 solution. As it is shown in Figure 2B hydrogenation resulted in formation of Cu deposits in Pt/AC particles.After recovery from the E-waste leachate the metals are released from the Pt/AC particles into a brine solution by oxidation with air oxygen. This process described by Eq. (3) is thermodynamically favorable if the corresponding Nernst potential (Eq. (4)) is positive. Aeration of Pt/AC particles loaded with Cu and Ag crystals (formed in corresponding hydrogenation operations) resulted in oxidation of both metals and release of Cu2+ and Ag+ ions into the H2SO4 solutions. For example, it is shown in Figure 1 that recirculation of H2SO4 (0.5 M) through the 1.0%Pt/AC granules loaded with Ag metal at N2 atmosphere did not result in release of Ag+ ions. However, once the air was supplied into the system the silver metal was successfully dissolved.The new catalytic process is proposed for the recovery of copper, silver, and probably other metals (e.g., bismuth and nickel) from the leachates of printed circuit boards, Ag2O-Zn batteries, silicon solar cells and other types of E-wastes.[1] Gendel, Y., Amikam, G., Nativ, P., 2020. Capacitive-Faradaic or Pseudocapacitive-Faradaic Fuel Cells. US Provisional Patent Application 62/983,689.[2] Darom, Y., Amikam, G., Ruck, E.B., Gendel, Y., (In Press) Capacitive-faradaic fuel cells (CFFCs) for selective separation of copper(II) ions from water and wastewater, Chemical Engineering Journal. Figure 1