Kinetics and Modeling of Counter-Current Leaching of Waste Random-Access Memory Chips in a Cu-NH3-SO4 System Utilizing Cu(II) as an Oxidizer

Abstract

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.

Keywords: ammoniacal solution; circuit boards; copper leaching; diffusion-controlled; kinetic modeling; recycling.

Figure 1

Schematic illustration of the coupled leaching-EW circuit in the Cu(II)-NH₃-SO₄ system. Adapted from references [7,16].

Figure 2

Schematic illustration of the mechanism and kinetic considerations in a Cu-ammoniacal leaching system. Adapted from reference [28].

Figure 3

The effect of the stirring rate (rpm) on Cu recovery in ammoniacal leaching. (S/L ratio: 50 g/L, (NH₃)₂SO₄: 1 M, NH₄OH: 4 M, Cu(II): 40 g/L, particle size: −2 mm, temp. 18 °C).

Figure 4

The effect of the particle size (top size, diameter, mm) on Cu recovery in ammoniacal leaching (S/L ratio: 50 g/L, (NH₃)₂SO₄: 1 M, NH₄OH: 4 M, Cu(II): 40 g/L, stirring: 750 rpm, temp.: 18 °C).

Figure 5

The effect of the initial Cu(II) concentration (g/L) on Cu recovery in ammoniacal leaching (S/L ratio: 50 g/L, (NH₃)₂SO₄: 1 M, NH₄OH: 4 M, particle size: −2 mm, stirring: 750 rpm, temp.: 18 °C).

Figure 6

The effect of the temperature (°C) on Cu recovery in ammoniacal leaching (S/L ratio: 50 g/L, (NH₃)₂SO₄: 1 M, NH₄OH: 4 M, Cu(II): 40 g/L, particle size: −2 mm, stirring: 750 rpm).

Figure 7

Plot of ${({\left(1-\alpha \right)}^{-\frac{1}{3}}-1)}^2$ vs. time under various particle sizes (diameter, mm); the data correspond to those in Figure 4.

Figure 8

Plot of ${({\left(1-\alpha \right)}^{-\frac{1}{3}}-1)}^2$ vs. time under various initial Cu(II) concentrations (g/L); the data correspond to those in Figure 5.

Figure 9

Plot of ${({\left(1-\alpha \right)}^{-\frac{1}{3}}-1)}^2$ vs. time under various temperatures (°C) (a), and the Arrhenius plot for Cu ammoniacal leaching (b); the data correspond to those in Figure 6.

Figure 10

Estimation of the reaction order: plot of $ln\left[\frac{d}{dt}\left\{{({\left(1-\alpha \right)}^{-\frac{1}{3}}-1)}^2\right\}\right]\mathrm{v}\mathrm{s}.\ln \left[\left(\frac{1}{R^2}\right)/\left(\mathrm{m}\mathrm{m}\right)\right]$ (a), and plot of $ln\left[\frac{d}{dt}\left\{{({\left(1-\alpha \right)}^{-\frac{1}{3}}-1)}^2\right\}\right]\mathrm{v}\mathrm{s}.\ln \left[c\left({Cu}^{2+}\right)/(\mathrm{m}\mathrm{o}\mathrm{l}/\mathrm{L})\right]$ (b).

Figure 11

Comparison of the model-predicted reacted fraction with that of the experimental results, using Equation (15).

Figure 12

Graphic illustration of the designed CCL showing inputs, outputs, sum of square error (SSE) minimization, and the calculation of the virtual time in and out of tanks.

Figure 13

The out-of-tank Cu(II) concentration (g/L), estimated via the developed model.

Figure 14

The predicted reaction fraction (α) in each leaching stage under various initial Cu(II) concentrations, according to the developed model.

Figure 15

The accumulated reacted fraction (α) in the CCL circuit, predicted via the developed model.