The leaching of Cu in ammoniacal solutions has proven an efficient method to recover Cu from waste printed circuit boards (WPCBs) that has used by many researchers over the last two decades. This study investigates the feasibility of a counter-current leaching circuit that would be coupled with an electrowinning (EW) cell. To accomplish this objective, the paper is divided into three parts. In Part 1, a leaching kinetic framework is developed from a set of experiments that were designed and conducted using end-of-life waste RAM chips as feed sources and Cu(II)-ammoniacal solution as the lixiviant. Various processing parameters, such as particle size, stirring rates, initial Cu(II) concentrations, and temperatures, were evaluated for their effects on the Cu recovery and the leaching rate. It was found that the particle size and initial Cu(II) concentration were the two most important factors in Cu leaching. Using a 1.2 mm particle size diameter and 40 g/L of initial Cu(II) concentration, a maximum Cu recovery of 96% was achieved. The Zhuravlev changing-concentration model was selected to develop the empirically fitted kinetic coefficients. In Part 2, kinetic data were adapted into a leaching function suitable for continuously stirred tank reactors. This was achieved via using the coefficients from the Zhuravlev model and adapting them to the Jander constant concentration model for use in the counter-current circuit model. Part 3 details the development of a counter-current circuit model based on the relevant kinetic model, and the circuit performance was modeled to provide a tool that would allow the exploration of maximum copper recovery whilst minimizing the Cu(II) reporting to electrowinning. A 4-stage counter-current circuit was modeled incorporating a feed of 35 g/L of Cu(II), achieving a 4.12 g/L Cu(II) output with 93% copper recovery.

Oxidative thiosulfate leaching using Cu(II)-NH3 has been explored for both mining and recycling applications as a promising method for Au extraction. This study seeks to understand the dissolution behavior of Au from waste RAM chips using a Cu(II)-NH3-S2O3 solution. In the course of this work, bimodal leaching and Au loss were observed in a manner that we have not identified in the literature. Identification of the existence of a specific Au-Ni-Cu lamellar structure in the gold fingers from RAM chips by scanning electron microscopy and energy dispersive X-ray spectroscopy (SEM-EDS) revealed the possibility of interference between Au recovery and the existence of Cu and Ni. During leaching, the co-extraction of Ni was found to predict a negative impact on the Au recovery, as a result of chemical interactions from the Au-Ni-Cu interlayer. Decopperization as a pretreatment was found necessary to remove the pre-existing Cu and promote Au leaching. As part of the study parameters, such as Cu(II) concentration, aeration rates, thiosulfate and ammonia concentrations, particle sizes, and temperatures, were investigated. A satisfactory Au recovery of 98% was achieved using 50 mM Cu(II), 120 mL/min aeration rate, 0.5 M (NH3)2S2O3, and 0.75 M NH4OH (i.e., AT/AH ratio of 0.67) for 4 h residence time at room temperature (25 °C). However, there were several high recoveries prior to Au loss from the lixiviant. It was revealed that the main cause of lower Au recovery was due to a precipitation or cementation reaction that included a sulfur species formation. Because of the bimodal leaching, a composite response comprised of the time to Au loss and maximum recovery was developed, termed leaching proclivity, to facilitate statistical analysis. Furthermore, this study explores the interactions between Au-Ni-Cu and provides suggestions for improving Au thiosulfate leaching under the interference of co-existing metals from waste PCB materials.

Due to the rapid development of electronic devices and their shortened lifespans, waste electrical and electronic equipment (WEEE), or E-waste, is regarded as one of the most fast-growing wastes. Among the categories of E-waste, waste printed circuit boards (WPCBs) are considered the most complex waste materials, owing to their various constitutes, such as plastics, capacitors, wiring, and metal plating. To date, a variety of processing technologies have been developed and studied. However, due to the heterogeneous nature of WPCBs, a thorough study on both material characterization and physical separation was needed to provide a better understanding in material handling, as well as to prepare a suitable feedstock prior to the downstream chemical process. In the present study, integrated size and density separations were performed to understand the liberation of contained metals, particularly Cu and Au, from the plastic substrates. The separation performance was evaluated by the elemental concentration, distribution, and enrichment ratio of valuable metals in different size and density fractions. Further, SEM-EDS on the density separation products was carried out to characterize the surface morphology, elemental mapping, and quantified elemental contents. Moreover, thermo-gravimetric properties of waste PCBs were investigated by TGA, in order to understand the effect of temperature on volatile and combustible fractions during the thermal processing.

Leaching Cu by ammoniacal lixiviants has been known and practiced for over 100 years in various processes yet adoption of this interesting and potentially green chemistry has not been widespread. Potential benefits of this chemistry are non-acid leaching, lower electrowinning power consumption, and closed loop oxidizer regeneration. Owing to the oxidation states of Cu in ammoniacal solutions, this work seeks to provide additional insights into the use and control of a Cu-ammoniacal lixiviant utilizing Cu(II) as an oxidizer on Cu(0) from the end-of-life (EoL) printed circuit boards (PCBs). In leaching, a Cu(I)-rich ammine solution with minimized Cu(II) was investigated. The relationship of Cu recovery, residence time and Cu(II) consumption was studied, for the purpose of evaluating the feasibility of direct electrowinning coupled with leaching. To provide data for the design of Cu leaching for its recycling from E-waste, several factors were studied, including the particle size, stirring speed, initial Cu(II) concentrations and temperature. The experimental data in batch leaching tests was further adopted for the kinetic modeling. In kinetic modeling, several models were examined, and the three exhibiting superior fits, i.e. the product layer diffusion model, the changing concentration model and the mixed diffusion model, were studied in a greater details to provide understanding of the reaction mechanism.

Bastnaesite, monazite and xenotime are rare earth minerals (REMs) that are typical sources for rare earth elements (REEs). To advance the understanding of their leaching and precipitation behavior in different hydrometallurgical processes, Eh-pH diagrams were constructed and modified using the HSC 9.9 software. The aqueous stability of rare earth elements in H2O and acid leaching systems, i.e., the REE-Ligands-H2O systems, were depicted and studied based on the Eh-pH diagrams. This study considers the most relevant lixiviants, their resulting equilibrium states and the importance in the hydrometallurgical recovery of rare earth elements (REMs). A literature review was performed summarizing relevant Eh-pH diagrams and associated thermodynamic data. Shifting stability regions for REEs were discovered with additions of acid ligands and a narrow stability region for soluble REE-(SO4/Cl/NO3) complexes under highly acidic conditions. As such, the recovery of REEs can be enhanced by adjusting pH and Eh values. In addition, the Eh-pH diagrams of the major contaminants (i.e., Fe, Ca and Al) in leaching systems were studied. The resulting Eh-pH diagrams provide possible insights into potential passivation on the particle surfaces due to the formation of an insoluble product layer.

An environmentally friendly organic biosorbent was fabricated using hay by succinylation. Metallic cation adsorption tests were performed using synthetic nickel(II) and cadmium(II) solutions to simulate heavy-metal recovery from aqueous solution. The adsorption efficiency was greater than 98% for both cadmium and nickel ions when the biosorbent concentration was 5.0 g/L and the initial metal concentrations were 50 mg/L. The surface of the biosorbent was characterized using Fourier transform infrared spectroscopy to investigate the changes in the surface functional groups. The functional groups changed according to the surface treatment, resulting in an effective biosorbent. The kinetics of the metals adsorption revealed that the reactions are pseudo-second order, and the adsorption isotherm well followed the Langmuir model. The maximum adsorption capacities predicted by the Langmuir model were 75.19 mg/g and 57.77 mg/g for cadmium and nickel, respectively. The fabricated biosorbent was regenerated using NaCl multiple times, with 2.1% for Cd and 4.0% for Ni in adsorption capacity after three regeneration cycles. The proposed biosorbent can be a good alternative to resin or other chemical adsorbents for heavy-metal recovery in metallurgical processing or municipal water treatment.

Wheat straw was chemically modified by alkaline treatment followed by succinylation and sodium carbonate treatment for the use of cadmium and nickel adsorbent. The adsorption efficiency was calculated in different conditions. The experimental parameters were solution pH, adsorbent dosage, contact time and initial metal concentrations. The biosorbent was analysed by FTIR (Fourier-Transform Infrared Spectroscopy) to investigate the changes of surface functional group. The maximum adsorption percentages of cadmium (>99%) and nickel (>98%) were achieved using 5 g/L adsorbent with 50 mg/L initial metal concentrations. The adsorption kinetic study showed that the reaction was pseudo-second order kinetics and the adsorption isotherm was well fit by the Langmuir adsorption model. The dynamic column study indicates that modified adsorbent has a better adsorption capacity towards cadmium than nickel.

In this study, the dissolution kinetics of smithsonite as an alternative zinc source in trichloroacetic acid (TCA) solution was investigated. TCA can be derived from industrial waste acid in the pharmaceutical, biotechnology and chemical fields. Results showed that the dissolution kinetics conformed to the shrinking core model controlled by surface chemical reaction. The apparent activation energy of the reaction was calculated as 47.61 kJ/mol, indicating the obvious effect of temperature on the reaction rate. The reaction kinetic equation associated with the main influencing factors was eventually established as 1– (1–x)–1/3 = [0.0002 (C)0.384 (P)0.969 exp(–5726T)] t. The high reaction speed of smithsonite in TCA solution shows that TCA has a dissolution effect on zinc oxide ores; thus, it can be employed as an advantageous organic leaching reagent environmentally friendly. In addition, the experimental parameters obtained provide basic data and reference for the leaching of other carbonate minerals of copper, zinc, lead and cobalt, among others, in an organic acid system.