A robotic platform for high-throughput electrochemical analysis of chalcopyrite leaching

Abstract

Cu extraction from chalcopyrite ores is typically a slow process that involves aggressive chemical reagents with significant environmental impact. Ionic liquids (IL) have been proposed as a potentially more benign solution, but the sheer number of IL variants complicates the search for the most efficient solvent systems. Here, we present an automated electrochemical platform that allows for screening of 180 and more leaching samples in parallel with minimal solvent consumption. In a proof-of-concept study, we screen 25 samples with different IL and water contents, and find two orders of magnitude difference in leaching performance within this array. The best performing system is then applied in a tank leaching configuration, with real-time electrochemical monitoring of Cu evolution in solution. All electrochemical data is found to be in excellent agreement with off-line ICP-AES data.

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Introduction

The scope of industrial, material and electronic applications of copper make for an extremely valuable economic commodity, with extraction-demand pressures being heightened in recent years.^1–3

Efforts to decrease reliance on traditional, high-consumption pyrometallurgy from flotation concentrates (∼30% Cu) have focused on enhancing hydrometallurgic beneficiation of sulfide ores,⁴ with the principal copper ore chalcopyrite (CuFeS_2(s), ∼70% global reserves⁵) receiving primary attention.

The industry is also increasingly challenged with processing lower-grade chalcopyrite ores,⁶ driving the development of new solution-based processes, applicable to both (bio-)heap and tank leaching.⁷ Conventional acid-oxidant chemical leaching systems, including the commonly employed Fe₂(SO₄)₃–H₂SO_4(aq) system, are established and cheap (see recent reviews^6,8). However, the overall leaching process is slow at moderate temperatures, whilst also ultimately limited by surface passivation⁹ or other kinetic effects.^10,11 Hence, there is still significant potential towards improving the leaching performance.⁶

Accordingly, improved Cu extraction from CuFeS_2(s) has been achieved, e.g. by using alternative acid-oxidant combinations, microorganisms, ultra-fine grinding, elevated temperatures and pressures.¹² However, the benefit of such methods is offset by increased cost and energy consumption, incompatibility with existing workflows or more generally higher environmental impact.

In the search for alternative lixiviant systems, ionic liquids (ILs) have emerged as an interesting alternative. ILs have well-documented benefits as solvents in synthetic chemistry,¹³ and it is conceivable that their chemical structure could be tailored in such a way to avoid the formation of kinetic barriers and maintain a high Cu extraction efficiency over time. To this end, notable dissolution enhancement, compared to ferric H₂SO₄-based lixiviants, has been reported for first generation alkylimidazolium hydrogen sulfate ionic liquids towards CuFeS_2(s). Maximum Cu recovery of 86.8% from a Cu concentrate (∼70% CuFeS_2(s)) by 100% [C₄C₁im][HSO₄] (bmim·HSO₄; pH ∼ −1 [ESI†]) presented a ∼60% enhancement compared to 1 M H₂SO_4(aq) benchmark solutions (pH −0.3), both containing excess Fe₂(SO₄)₃ oxidant.¹⁴ However, as noted above, these cited enhancements have not been rigorously normalised for medium pH, such that there may be no IL-related Cu extraction enhancement. A subsequent kinetic study of [C₄C₁im][HSO₄]_(aq) leaching applied to chalcopyrite ore (∼20% Cu), focused on the effect of temperature and agitation, measuring an Arrhenius-type activation energy from chalcopyrite dissolution of 69.4 kJ mol⁻¹.¹⁵

While such studies are a promising first step, further improvement is highly likely, given the structural diversity of ILs. However, the complex nature of the dissolution process renders ‘ab initio’ rational design of optimal lixiviant systems out of reach. On the other hand, screening a large number of ILs or lixiviant compositions simultaneously and at a small scale appears to be a more realistic option. Such a combinatorial approach would furthermore allow for the optimisation of the leaching performance over several generations of leaches, where the best performing lixiviants could then be upscaled for more in-depth studies. Similar strategies have yielded excellent results in protein design and other areas.¹⁶

Here, we demonstrate that such a combinatorial methodology is indeed a powerful tool for screening and improving the Cu extraction performance of IL-based lixiviants, Fig. 1. Similar robotic electrochemical workstations have been applied in other contexts before, namely for automated combinatorial electrochemistry in large sample arrays for microelectrode studies^17,18 and in endothelial cell NO_x excretion screening.¹⁹

Fig. 1 To-scale (overhead) technical schematic of the automated robotic platform, marking key features. Overlying, is an exemplar two-variable lixiviant screening result, highlighting regions of enhanced and moderate Cu leaching from CuFeS_2(s).

Our platform allows for the screening of up to 180‡ samples at the same time, with electrochemical, in situ monitoring of Cu extraction from CuFeS_2(s). We found that the detection capabilities of cupric ion-selective electrodes (ISE) were insufficient to detect Cu²⁺ in solution in the presence of ILs at the initial stages of the leach ([Cu] < ∼1 mM – see ESI Fig. S4†), but that Cu electrodeposition/anodic stripping improved the detection limit by approximately one order of magnitude, based on the conditions used here, see below. Comparison with ICP-AES as a standard ex situ method, yielded a 1 : 1 correspondence with the electrodeposition/anodic stripping results. After calibration and testing with model samples, we have applied the robotic screening platform to an array of 25 leaching samples in a proof-of-concept study. We find a 100-fold variation of the leaching performance between the best and worst performing lixiviants within the array. Subsequently, we examined the best-performing sample via electrochemical monitoring in a 120 mL scale tank leaching reactor over approximately 6 days.

Results and discussion

Atomic emission spectroscopy (ICP-AES) is a standard quantification method for metal ions in solution. However, ex situ solution sampling can be time-consuming and disruptive to leaching processes. Additionally, careful calibration for matrix effects may be required in complex solution environments, which extends to IL variations.^20,21 On the other hand, our remit demands a technique allowing for high throughput automated study, alongside real-time monitoring of [Cu²⁺] in diverse solution environments at all stages of the leaching process. In this study, ICP-AES was thus used only as an independent benchmark for electrochemically derived [Cu] measurements.

Since Cu ion-selective electrodes (ISE) appeared to be a facile and straightforward real-time detection approach for Cu²⁺ in solution, we first tested Cu ISE suitability for the task at hand (see ESI for full experimental details inc. Fig. S4†). Using a commercial ISE sensor (Cole-Palmer, Cupric Combination ISE), flat-line indistinguishable sensor response was obtained for [CuSO₄]_(aq) < 10⁻⁶ mol dm⁻³ and ∼10⁻⁴ mol dm⁻³ in H₂SO_4(aq) and IL_(aq) media, respectively. Above the respective lower [Cu²⁺] detection limit, all studied calibration plots exhibit near ideal Nernstian potential dependence of 29.6 mV per [Cu²⁺] decade (75 mM H₂SO₄ = 26.9 mV; 450 mM [C₄Him][HSO₄] = 30.6 mV; 450 mM NH₄·HSO₄ = 31.7 mV). Thus, in strongly Cu-coordinating IL_(aq) media lower cupric detection limits are deemed unsuitable for the present purpose of monitoring ambient IL_(aq) leaching on timescales of <2 days. However, as we show below, electrodeposition combined with anodic stripping of copper (ASV) indeed enables real-time quantification of [Cu²⁺] in solution, with sufficient sensitivity even in the presence of ILs.

In light of our aforementioned automation objectives, a powerful robotic electrochemical platform has been built (Fig. 1). For our ASV studies, the instrument is fitted with a Pt-disc working electrode, assembled into a glass fused probe construct (d_WE = 1 mm, Pt CE, Ag/AgCl RE). The fabricated probe is docked at the labelled ‘electrode mount’. Motorised probe positioning and potentiostat functions are programmatically controlled via USB 2.0 serial port connectivity. Further details can be found below and in ESI.†

Initial testing of the platform setup included determining the geometric factor for several fabricated electrodes in certified KCl_(aq) conductivity standards (Sigma Aldrich), followed by accurate measurement of CuSO_4(aq) solution conductivities with <6% error (1–50 mmol dm⁻³).²² Automated data acquisition for various IL_(aq) ASV calibration plots (see ESI – Fig. S8†) provided some ASV specific platform validation, however more complex electrochemical study was desirable, as described next.

Fe³⁺ ions are a common additive oxidant for acid-sulfate chemical leaching (1) and are also regenerated through the oxidation of leached Fe²⁺(3).^6,8 In a similar fashion to previous wastewater studies,²³ the influence of [Fe³⁺] on [Cu²⁺] ASV was investigated. A conventional ASV approach was used, in which the stripping process occurs in fresh electrolytic media (0.5 mol dm⁻³ H₂SO_4(aq)), avoiding potential interference from electroactive species within the analyte-bearing solution (i.e. a ‘2-vial’ ASV configuration). An array of 50 samples containing 1–10 mmol dm⁻³ [CuSO₄]_(aq) and 0–45 mmol dm⁻³ [Fe₂(SO₄)₃]_(aq) (combinations indicated in Fig. 2), were prepared in 75 mmol dm⁻³ H₂SO_4(aq) electrolyte (pH 1.3 ± 0.05) and sequentially subjected to the described ‘2-vial’ ASV procedure. ICP-AES was later employed to confirm [Cu] and [Fe] (Fig. 2a and ESI†).

CuFeS₂ + 4Fe³⁺ → Cu²⁺ + 5Fe²⁺ + 2S⁰ (1)

CuFeS₂ + 4H⁺ + O₂ → Cu²⁺ + Fe²⁺ + 2S⁰ + 2H₂O (2)

Fe²⁺ + [ox] → Fe³⁺ + [ox]⁻ (3)

Fig. 2 (a) ICP-AES confirmation of [CuSO₄]_(aq) across a 50 sample array. Samples also contain the indicated concentrations of [Fe₂(SO₄)₃]_(aq) (see ESI†) in background 75 mM H₂SO_4(aq). (b) ASV Cu_(s) stripping charge data obtained as a result of electrodeposition within standard aqueous solution containing indicated [Cu²⁺] and [Fe³⁺], with constant 75 mM H₂SO_4(aq) background electrolyte.

In the absence of ferric ions, Cu_(s) stripping data (Fig. 2b) indicates that Cu_(s) electrodeposition is increased by 517 nC mM⁻¹ [Cu²⁺] s⁻¹. The addition of reducible ferric ions impedes cupric electrodeposition by 83 nC mM⁻¹ [Fe³⁺] s⁻¹ at constant [Cu²⁺]. In cases where [Fe³⁺] is comparable to or greater than [Cu²⁺], parasitic ferric-ferrous reduction accounts for 60–100% of total reduction currents (see ESI†), leading to gross under-estimation of [Cu²⁺] via ASV when compared to ICP-AES reference values. Independent Cu ICP-AES sampling is essential to highlight any such ASV [Cu] measurement deviations – however, accepted reaction dynamics (1)–(3) should regulate [Fe³⁺] to negligible levels for CuFeS_2(s) dissolution in the absence of Fe₂(SO₄)₃ oxidant addition.

For our lixiviant systems of interest, adaptation to a simplified one pot (‘1-vial’) ASV procedure proved beneficial from numerous perspectives. Cupric electrodeposition and Cu_(s) stripping can be performed back-to-back within the sample vial, whilst significantly reducing the standard deviation of ASV repeats leading to 30–70% reduction in fitting standard errors (Table 2 and ESI Fig. S8b†). Additionally, ‘1-vial’ ASV requires significantly fewer probe positioning steps, thereby minimising the combined duration of probe motion to <7% of the overall automation cycle, in turn maximising sample throughput. Hereafter, ASV experiments have been performed with the aforementioned in situ simplifications, unless otherwise stated.

Having established that ASV is capable of monitoring [Cu²⁺] with sufficient sensitivity in the presence of IL_(aq), we then moved on to demonstrate array-based monitoring of leaching performance, as a precursor to large-scale IL_(aq) screening experiments.

In order to investigate the sample-to-sample variability, 10 identical samples (4 mL; 0.45 mol dm⁻³ NH₄·HSO_4(aq); cf. Experimental) were leached for a total of 216 h. ASV and ICP-AES [Cu] measurements were obtained after 72, 120 and 216 h (Table 1, Fig. 3a). A linear fit of these two independent [Cu] measures yields a strong correlation showing <10% discrepancy (m = 1.10 ± 0.14; R² = 0.994 – Fig. 3b). For rigour, each Cu_(s) stripping charge datapoint was normalised by its corresponding ICP-determined [Cu], producing an average ‘molar’ Cu_(s) stripping charge of 59 ± 5 mC mol dm⁻³ with low variance (rel. σ² = 0.04%; see ESI Fig. S16†), in excellent agreement with expectations from Table 2 (cf. 56.6 ± 0.6 mC mol dm⁻³). A [Cu] : [Fe] extraction ratio of unity is found, with all average ICP-AES values falling inside (low) measurement uncertainty ranges. Ultimately, sample-to-sample variation was found to be small, with relative variances in the range of 0.7–2.1%, providing adequate scope to distinguish between lixiviant performance and inter-sample variability within a screening assay, as exploited further below.

Fig. 3 (a) ASV determined [Cu] at 72 (○), 120 () and 216 h (△) for 10 samples leached under equivalent conditions (450 mmol dm⁻³ NH₄·HSO_4(aq); 4 mL leachate; room temperature; 100 mg CuFeS_2(s) 32 ≤ x ≤ 75 μm). (b) [Cu] quantification correlation plot, for all [Cu] measurements made independently using ASV and ICP-AES at equivalent leach duration.

Table 1 Analysis of sample-to-sample variation for 10 samples leached under equivalent conditions (0.45 mol dm⁻³ NH₄·HSO_4(aq); 4 mL leachate; room temperature; 100 mg CuFeS_2(s) 32 ≤ x ≤ 75 μm)

t, h	[M]av ± σ (rel. σ^2), mmol dm^−3
	ICP-AES [Fe]	ICP-AES [Cu]	ASV [Cu]
72	0.89 ± 0.08 (0.78%)	0.85 ± 0.08 (0.78%)	0.99 ± 0.08 (0.71%)
120	1.07 ± 0.09 (0.70%)	1.05 ± 0.09 (0.77%)	1.15 ± 0.16 (2.10%)
216	1.91 ± 0.20 (1.99%)	1.91 ± 0.19 (1.92%)	2.03 ± 0.21 (2.11%)

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Table 2 Cu_(s) stripping calibration parameters, in a range of leachate mimetic acidic media. Conventional 2-vial ASV is simplified to a 1-vial procedure using back-to-back electrodeposition and stripping cycles within the cupric analyte-containing leachate/standard solution

Medium	pH (±0.05)	ASV Mode	m, μC mM^−1 (σ)	c, μC (σ)	R ^2
75 mM H2SO4(aq)	1.3	2-vial	120 (±2.0)	49.3 (±8.2)	0.997
		1-vial	70.6 (±1.4)	−11.3 (±9.5)	0.996
450 mM [C4Him][HSO4](aq)	1.2	2-vial	59.0 (±2.0)	71.9 (±9.2)	0.990
		1-vial	64.9 (±1.1)	−4.5 (±5.3)	0.997
450 mM NH4·HSO4(aq)	0.9	2-vial	87.6 (±2.2)	84.0 (±12.9)	0.994
		1-vial	56.6 (±0.6)	−4.5 (±3.1)	0.999

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Subsequently, a two-variable IL_(aq) lixiviant screening assay was undertaken, as a proof-of-concept experiment towards larger arrays. 25 [NH₄·HSO₄]-based lixiviant samples were leached at room temperature for 264 h. [NH₄·HSO₄]_(aq) (3 mL; 100–1800 mM) and CuFeS_2(s) mass (37.5–600 mg) were varied logically across the two-dimensions of a square 5 × 5 sample array with ICP-AES (Fig. 4a) and ASV (Fig. 4b) [Cu] sampling after 264 h of ambient chemical leaching. ICP-determined [Fe] measurements fall within ±5% of ICP-[Cu] values, averaging 101.3 ± 1.7% of extracted [Cu] levels.

Fig. 4 (a) ICP-AES determined [Cu] for an array of 25 samples after 264 h ambient leaching, with the [NH₄·HSO₄]_(aq) and CuFeS_2(s) mass indicated. (b) Equivalent [Cu] measurements made using ASV after the same leach duration (264 h). A failed reading within the dataset is marked X.

Strictly speaking, ASV Cu_(s) stripping charge calibration parameters apply only at one single [IL]_(aq). However, for the concentration range used in this experiment, we found that the variation is in fact relatively small (Table 2). For simplicity, we used a single set of calibration parameters for all samples, namely those obtained for 450 mM [NH₄·HSO₄]_(aq) – Table 2. This decision is justified by retaining a strong correlation between [Cu] measures (m = 1.02 ± 0.04; R² = 0.963 – see ESI Fig. S21†).

Crucially, equivalent regions of darkened ‘hotspot’ lixiviant performance are highlighted in each panel of Fig. 4. Within the 25 sample array, 2 orders of magnitude difference in leaching performance are observed between the best performing (450 mmol dm⁻³; 600 mg) and poorest performing combinations (1800 mmol dm⁻³; 75 mg). Broad variation in leaching performance is also reflected by relative variances of 70–320%; a minimum of 35-fold larger than rel. σ² values for 10 equivalently leached [NH₄·HSO₄] samples (cf. Table 1). Thus, we have established confidence limits for distinguishing lixiviant performance from intersample variability, which operate on different magnitude scales.

Notably, these results suggest a non-trivial optimal [NH₄·HSO₄]_(aq) in the vicinity of 450 mmol dm⁻³. Since the pH is lowered as [IL]_(aq) increases, factors other than proton consumption (cf. eqn (2)) must play an important role during the leaching process in [NH₄·HSO₄]_(aq). Dutrizac²⁴ found that high [SO₄]²⁻_(aq) contributed to reduced CuFeS_2(s) dissolution rates. Therefore dissociation of the acidic [HSO₄]⁻ anion (pK_a ∼ 1.99) to form high quantities of [SO₄]²⁻ may potentially impose limits to [IL]_(aq) for such lixiviant systems. We further explore this 450 mmol dm⁻³ [NH₄·HSO₄]_(aq) as a potential lead system in larger scale tank leaching experiments below. Overall, whilst the array size is limited in this proof-of-concept experiment, results do suggest that further, potentially significant performance enhancement may be achieved with a more comprehensive screening effort.

Proceeding to scale-up this ‘hotspot’ performance system, a two-neck round bottomed flask was used for a 120 mL scale, 6 day leaching study with automated [Cu] sensing. Freshly milled CuFeS_2(s) (3 g; 32 ≤ x ≤ 75 μm) was leached at room temperature in [NH₄·HSO₄]_(aq) (450 mmol dm⁻³; 120 mL; 40 mL g⁻¹), while stirred at a constant rate of ∼120 rpm. Stirring was intermittently stopped (marked * – Fig. 5a), providing extended periods of unstirred ASV for ease of calibration. A second equivalent experiment was conducted using 450 mmol dm⁻³ [C₄Him][HSO₄]_(aq). Our electrode system was pre-conditioned§ and inserted as a static probe, with a programmed electrochemical schedule set to ascertain ASV response at 2 h intervals, for a total leach duration of 140 h.

Fig. 5 (a) Real-time Cu_(s) stripping data recorded by a static in situ ASV probe throughout ambient CuFeS_2(s) leaching (3 g; 38 ≤ x ≤ 75 μm) in stirred and unstirred (*) 0.45 mol dm⁻³ NH₄·HSO_4(aq) lixiviant (120 mL). Data is normalised for 30 s electrodeposition (300 s , 120 s □, 60 s , 30 s ○, 30 s [unstirred] ). (b) ICP-AES [Cu] sampling (solid markers), with respect to independent electrochemically-derived [Cu] measurements, acquired by the static in situ sensor, while monitoring unstirred leaching in 0.45 mol dm⁻³ NH₄·HSO_4(aq) () and [C₄Him][HSO₄]_(aq) ().

Beginning with 300 s, electrodeposition duration was adjusted to maintain Cu_(s) stripping charges within a calibrated linear range (<1.5 mC). Calibration irregularities (plateaus and high standard deviations) have previously been observed above 2 mC, the origins of which are unclear and are under investigation (see ESI – Fig. S9†).

Fig. 5a displays the acquired Cu_(s) stripping data from fast leaching 450 mmol dm⁻³ [NH₄·HSO₄]_(aq), when normalised for 30 s electro-deposition – see ESI for analogous [C₄Him][HSO₄]_(aq) data. A regular slope (∼10 μC h⁻¹) indicates a near-linear Cu extraction profile (/●, Fig. 5b), with excellent [Cu] leaching sensitivity below 5 h leaching time. Ambient cupric ion leaching in 450 mmol dm⁻³ [NH₄·HSO₄]_(aq), occurs at a rate of 1.37 mM day⁻¹ (1.01% Cu extraction per day), exhibiting no indication of kinetic retardation over 6 days. In one key comparative study, Ahmadi et al.²⁵ compare unaided Fe₂(SO₄)₃–H₂SO₄ chemical leaching (pH 1.8, 35 °C, 300 rpm) to enhancements attained through ORP control and microbial action. They discuss parabolic Cu extraction profiles found for unaided chemical leaching, with clear plateaus forming inside 1–5 days and resulting in extraction plateaus at <15% Cu, which persist to over 30 days. In our studies, leaching in 450 mM NH₄·HSO_4(aq) reaches 5.9% in 6 days at 25 °C with 120 rpm stirring. The continuation of linear Cu extraction in promising IL_(aq) systems is the subject of future studies. [Fe] extraction was determined as 98.9 ± 2.5% of extracted [Cu] at all ICP-AES sampling points for NH₄·HSO₄ leaching, in agreement with generally accepted acid-sulfate CuFeS_2(s) dissolution schemes (1)–(3), producing a Cu : Fe extraction ratio of unity.^6,8

Despite using CuFeS_2(s) from the same batch and equal [IL]_(aq), comparative leaching in 450 mmol dm⁻³ [C₄Him][HSO₄]_(aq) (□/■, Fig. 5b) produces a parabolic, kinetically slow, Cu extraction profile. Interestingly, and in stark contrast to NH₄·HSO₄, an initial period with very little leaching is observed below 50 h, after which point, familiar electrochemical response and [Cu] tracking is resumed (see ESI – Fig. S30a†). This further exemplifies the value of our continuous automated approach to leached [Cu] monitoring and the leach-specific insights that can be extracted from reconstruction of a time-dependent extraction profile. Extracted [Fe] levels were found to be significantly higher than that of [Cu], averaging 132.6 ± 1.4% of corresponding [Cu] (see ESI – Fig. S30b†).

Aqua regia-based digestion of the milled, unleached CuFeS_2(s) starting material, confirmed the expected Cu : Fe metal ratio of unity – see below. Differing solution pH of 450 mmol dm⁻³ [NH₄·HSO₄]_(aq) (0.9 ± 0.05) and [C₄Him][HSO₄]_(aq) (1.2 ± 0.05) may go some way in explaining the difference in leaching performance. However, pH alone cannot explain the presence/absence of induction periods or linear/parabolic Cu extraction behaviour for equivalent CuFeS_2(s) starting material – further detailed study is required.

Notably, there are currently few published IL-CuFeS₂ studies. Existing studies are disparate and difficult to compare; varying in CuFeS_2(s) source, leach temperature and focused mainly on [C₄C₁im][HSO₄].^14,15 Therefore, there is a significant lack of available data for ambient IL_(aq) leaching of CuFeS_2(s), for comparison. However, our Cu leaching studies consistently reveal that [C₄Him][HSO₄]_(aq) outperforms [C₄C₁im][HSO₄]_(aq) by up to 200% (e.g. see ESI – Fig. S31a†), earmarking the former as the superior Cu lixiviant despite bearing a higher pH at equivalent [IL]_(aq) (see ESI – Fig. S31b†). Once again, more detailed insight is needed regarding the pH dependence of IL_(aq) leaching, as discussed above (cf. also Fig. 4). Furthermore the focal IL_(aq) system presented herein, NH₄·HSO_4(aq), vastly outperforms [C₄Him][HSO₄]_(aq) by up to 400% over equivalent leach durations (Fig. 5b).

Overall, our automated platform for data acquisition has proven effective in addressing several challenges existing within the field of acid-sulfate hydrometallurgy. Indication of promising IL_(aq) systems amongst wide-ranging leaching performances within a modest-scale screening experiment has paved the way for large array screening of unstudied IL_(aq) systems, which can utilise assessed sample-to-sample variability to define confidence limits. Furthermore, we have presented a new in situ approach to automated CuFeS_2(s) leach monitoring. The tool is applicable across diverse [IL]_(aq) systems, minimising reliance on laborious ex situ ICP-AES sampling, and allows full reconstruction of Cu extraction profiles – where leaching dynamics can be clearly observed. There are thus significant prospects in employing this approach to even larger scale studies in pursuit of next generation Cu lixiviant systems.

Conclusion

The setup of an automated electrochemical platform has been described, offering potentially high-throughput, low-volume screening capabilities, with proven applicability to IL_(aq) systems and CuFeS_2(s) hydrometallurgy.

Screening has been characterized through parallel leaching of ten equivalent samples in 450 mmol dm⁻³ [NH₄·HSO₄]_(aq), revealing sample-to-sample variation of 0.7–2.1% (rel. σ²). Subsequent screening within a modest 5 × 5 sample array returned lixiviant performances ranging over 2 orders of magnitude, at least 35-fold larger than measured sample-to-sample variability. The presence of an ‘optimum’ leaching performance at [NH₄·HSO₄]_(aq) = 450 mmol dm⁻³ was unexpected and is incompatible with purely pH dependent leach dynamics. This lixiviant composition was then further explored in up-scaled ambient leach experiments and displayed linear extraction dynamics over 6 days of continuous [Cu] sensing. This in situ electrochemical monitoring of leached [Cu] proved effective for reconstructing full extraction profiles for two IL_(aq) systems, with high time resolution. Differentiation of the two 450 mmol dm⁻³ [IL] systems was straightforward, through clear differences in extraction rates, the shape of the extraction profiles (i.e. linear/parabolic). Additional, potentially mechanistically relevant features were uncovered, including a 50 h dormant period for CuFeS_2(s) leaching in [C₄Him][HSO₄]_(aq).

Work to-date suggests that some promising IL_(aq) lixiviant systems, such as NH₄·HSO_4(aq), may not suffer from the same surface passivation effects as conventional ferric-acid-sulfate media,^6,8 although longer duration studies with focus on other key variables (E_h, constant T etc.) are required. Moving forward, we will employ the tools introduced herein, in an iterative approach to large scale IL_(aq) screening and extended electrochemical monitoring of lead systems for up-scaled studies. A broad unexplored IL chemical space awaits.

Experimental

Materials

All chemicals are used as received unless otherwise stated. All standard solutions were prepared from standard aqueous salt solutions of copper sulfate (anhydrous, 99.99% trace metal basis, Sigma Aldrich), ferric sulfate (pentahydrate, 97%, Acros Organics) and potassium chloride (99.99%, VWR International), using ultra-pure water (Purite Select Fusion 160). The same purified water source was used to create leachate solutions of 75 mM H₂SO_4(aq) (95–98%, Sigma Aldrich), 450 mM NH₄·HSO_4(aq) (99.99% trace metals basis, Sigma Aldrich) and 450 mM [C₄Him][HSO₄]_(aq), the latter of which was synthesised using a previously published method developed by colleagues.²⁶

CuFeS_2(s) preparation and compositional analysis

Freshly milled and dry-sieved CuFeS_2(s) is stored under purified Ar_(g) (Alfa-Aesar; 100.0 mg ± 1%; 38 ≤ x ≤ 75 μm). The powdered sample can be completely digested in aquaregia (24 h; 3 HCl_(aq): 1 HNO_3(aq) wt%; 20 mg mL⁻¹), yielding 93.3% ± 2.8% of the theoretical [Cu] maximum, judged over 9 ICP-AES samples taken from 3 separate mineral digestions. Measured [Fe] concentrations are 96.9% ± 2.8% of the theoretical maximum. SEM/EDS surface analysis (plus commercial certification) confirms expected CuFeS_2(s) stoichiometry in unleached samples – alongside detection of silicates and other trace metallic elements (Mn, Zn, Ni, Mg etc.).

Platform design

A commercial milling platform (Heiz CNC Technik High-Z S-400 T) provided the basis for platform development (Fig. 1). Four stepper motors (1600 step per rev, Nanotec) are wired appropriately to commercial driver boards (Easydriver) and digital output ports (DO) of a microprocessor board (ATmega328, Arduino UNO). The microcontroller is interfaced with a graphical programming package (VISA Instrument Control Palette, NI LabView) using USB-delivered custom-designed firmware. Fig. 1 shows an overhead scaled technical diagram of the platform, indicating the electrode probe mount and sample holder (204 vial wells). At first use, probe 3D positioning is zero-referenced at X (0,0,0), from which positive (referenced) coordinate changes define the current probe positioning (+x,+y,+z), as tracked by firmware coding. Fixed cartesian (x,y) vial locations are stored within the graphical programming suite and retrieved for motor operation as necessary.

Full potentiostatic functionality is accessed through a manufacturer designed dynamic link library (.dll – Compactstat, Ivium Technologies) interfaced with the graphical programming suite. All operations are sequenced back-to-back for custom automation design, with phase completion and triggering managed by monitoring appropriately constructed instrument status signals.

Electrode probe preparation

A glass-encased double Pt disk electrode system is fabricated through the glass-blowing of soda glass tubes (d_out = 5 mm, d_in = 3.2 mm, VWR International) under a hydrogen flame to encase two high purity Pt wires (99.99%, d = 1 mm, Goodfellow). Disk electrodes are revealed using SiC paper (180/320/800 grit, Struers), with further fine-polishing prior to each use (LaboPol-6, Struers) using a range of alumina nanoparticle suspensions (200/100/50 nm AP-A, Struers). Following polishing, the electrode probe is thoroughly rinsed with distilled water and electrochemically cleaned using high potential cyclic voltammetry (500 mM H₂SO_4(aq); −0.4–1.9 V vs. Ag/AgCl; 100 mV s⁻¹; 20 cyc).

Anodic stripping voltammetry (ASV)

Anodic stripping Voltammetry (ASV) is conducted in a 3-electrode configuration (vs. Ag/AgCl, I.J. Cambria). Sample changes are punctuated by electrochemical Pt-cleansing (500 mM H₂SO_4(aq); −0.4–1.9 V vs. Ag/AgCl; 100 mV s⁻¹; 10 cyc). Unless otherwise stated, electrodeposition (120 s; −500 mV vs. Ag/AgCl) and positive polarity cyclic voltammetry (“Cu_(s) stripping”; −0.3–1.5 V, 50 mV s⁻¹, 4 cyc) occur back-to-back, within the cupric analyte containing vial (‘1-vial’), with consistent parameter choices. Analytical justification for simplification from a two solution process (‘2-vial’) to a single vial (‘1-vial’) approach is summarised in Table 2, showing lower error in calibration parameters. First repeat ASV data, obtained from the ‘1-vial’ electrodeposition and stripping procedure has been discarded throughout due to first cycle electrode preconditioning, leading to unpredictable data with high associated error (see ESI†).

Calibration plots for unstirred ASV can be rapidly generated utilising the electrochemical platform and [CuSO₄]_(aq) solutions of known concentration. Consequently, stirring was switched off intermittently during in situ Cu sensing experiments, providing sections of reference data for rapid production of electrochemical [Cu] measurements. ICP-AES sampling provided regular [Cu] and [Fe] reference points for comparison of ASV [Cu] measurements.

In an effort to quantify the detection limit for Cu stripping under the present conditions, we divide the standard error of the intercept in Table 2 by the sensitivity, and obtain values between 0.11 and 0.31 mM, depending on the solution medium. We take this is as an estimate for the minimum stripping charge that we can detect in the present experimental configuration.

Inductively-coupled plasma atomic emission spectroscopy (ICP-AES)

Lixiviant/standard solutions are filtered (200 nm porous, Acrodisc Supor) to provide an end-point to the leaching process through removal of CuFeS_2(s) and other particulate matter, before dilution onto a calibrated metal ion concentration range with 2 mol dm⁻³ HNO_3(aq). ICP-AES [Cu] and [Fe] measurements were made using non-interfering emission lines (Cu: 224.7 nm; Fe: 238.2 nm; Thermo Scientific iCAP 7600). Six calibration standard solutions (0 μM, 6 μM, 30 μM, 60 μM, 120 μM, 480 μM), each containing 2 M HNO_3(aq), were prepared from 6 mM [CuSO₄]_(aq) and [Fe₂(SO₄)₃]_(aq) stock solutions.

Further experimental details

The conditions and experimental process used for assessment of the leaching of 10 equivalent samples were 4 mL 0.45 mol dm⁻³ NH₄·HSO_4(aq); room temperature; 100 mg CuFeS_2(s) 32 ≤ x ≤ 75 μm). At leach durations of 72, 120 and 216 h, ASV leachate characterisations were followed by immediate ICP-AES sampling (1 mL filtered solution removed with fresh lixiviant replacement).

Acknowledgements

This work was funded by the Rio Tinto Advanced Leaching Project at Imperial College, London. Special thanks go to Simon Turner for his technical advice and practical expertise.

Notes and references

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Footnotes

† Electronic supplementary information (ESI) available: General information, detailed overview of ASV protocol, extended platform validation, raw ASV electrodeposition and stripping data, in situ Cu sensor setup, ICP-AES [Fe] extraction data. See DOI: 10.1039/c5gc02306h

‡ Maximum of 180 samples is limited by our vial holder design and could be increased to ∼250.

§ Prior to insertion, the electrode is pre-conditioned using five back-to-back electrodeposition/stripping cycles in 0.45 M IL_(aq) containing 10 mM CuSO_4(aq).

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