Hydrometallurgical Approach for Leaching of Metals from Copper Rich Side Stream Originating from Base Metal Production

Abstract

Pyrometallurgical metal production results in side streams, such as dusts and slags, which are carriers of metals, though commonly containing lower metal concentrations compared to the main process stream. In order to improve the circular economy of metals, selective leaching of copper from an intermediate raw material originating from primary base metal production plant was investigated. The raw material investigated was rich in Cu (12.5%), Ni (2.6%), Zn (1.6%), and Fe (23.6%) with the particle size D₈₀ of 124 µm. The main compounds present were nickel ferrite (NiFe₂O₄), fayalite (Fe₂SiO₄), cuprite (Cu₂O), and metallic copper. Leaching was studied in 16 different solutions. The results revealed that copper phases could be dissolved with high yield (>90%) and selectivity towards nickel (Cu/Ni > 7) already at room temperature with the following solutions: 0.5 M HCl, 1.5 M HCl, 4 M NaOH, and 2 M HNO₃. A concentration of 4 M NaOH provided a superior selectivity between Cu/Ni (340) and Cu/Zn (51). In addition, 1–2 M HNO₃ and 0.5 M HCl solutions were shown to result in high Pb dissolution (>98%). Consequently, 0.5 M HCl leaching is suggested to provide a low temperature, low chemical consumption method for selective copper removal from the investigated side stream, resulting in PLS (pregnant leach solution) which is a rich in Cu and lead free residue, also rich in Ni and Fe.

1. Introduction

The growth in metal production has resulted in a gradual decrease in metal grades of ore deposits. Therefore, new technologies and flow-sheets are needed for the more efficient utilization of ore processing tailings, metallurgical slags, flue dusts, etc. In the base metal production, various solid side-streams are generated, such as slags, dusts, and leach residues. Inherently, these side-streams contain valuable base metals.

Thermodynamics determines the distributions of metals between metal and slag in high temperature processing [1,2,3]. In addition, kinetics and physical entrainment cause metal traces ending up to the slag in different steps of the production. About 60% of the world’s copper and 50% of world sulphidic nickel production comes from plants using flash smelting furnace (FSF) technologies [4]. The main advantages of the FSF processes are high sulfur recovery, flexibility to feed materials and the efficient energy utilization [5]. The subsequent converting takes place in two sequential steps:

(a)

The FeS elimination or slag making stage

2FeS_(s) + 3O_2(g) + 2SiO_2(s) = 2FeO·SiO_2(s) + 2SO_2(g)

(1)

(b)

The copper making stage

Cu₂S_(s) + 2O_2(g) = 2Cu (s) + 2SO_2(g)

(2)

As the process throughputs are generally high [6,7,8] the slags of the primary production can present a valuable secondary raw materials for metal recovery in future.

The composition of slags in base metal processing vary depending on the process and raw material. Copper flash smelting furnace slag generally consist of 30–50% Fe, 30–40% SiO₂, 1–10% Al₂O₃, 1–16% CaO and 0.2–1.2% of Cu [9]. Copper is mainly entrapped in the slag as chalcocite and metallic copper, as well as trace copper oxide [10]. The converter slag is usually characterized by 20–25% SiO₂, 40–45% Fe, and 5% Cu. The slags of anode furnace differs from the converter slags due its very high copper content, containing typically above 50 wt. % CuO_x, 30–35 wt. % FeO, 5–15 wt. % SiO₂, and minor amounts of As, Sb, and Pb [11,12]. Nickel flash smelting furnace slag has been reported to contain 8.7% Fe₂SiO₄, 10% Fe₃O₄, 20.5% SiO₂, 3.1% Al₂O₃, 1.3% MgO, and 1.1% CaO [13]. Generally, the slag former used is SiO₂.

Industrial smelting and converting slags are cleaned before discarding them. In most cases an electric furnace settling or reduction is used, but some copper smelters use milling and slag flotation.

In the literature, new methods for slag cleaning have been studied for eliminating trace element or cutting their internal circulations in the smelter. Thus, the impurity levels in the slags and anode copper will be lowered. Roasting of the converter slag with ferric sulphate and selective sulphation roasting are the documented pyrometallurgical methods used for the recovery of nickel, copper and zinc [14,15]. Also, pyro-hydrometallurgical methods involving acid roasting or thermal decomposition followed by water leaching have been suggested [16,17,18]. Various hydrometallurgical methods have been developed using lixiviants such as acids, bases, and salts for base metal extraction. Atmospheric leaching of different slag fractions has been studied in H₂SO₄, FeSO₄, (NH₄)₂SO₄, FeS₂, NaCl, and FeCl₂ media [19,20,21,22,23]. In addition, pressure leaching of copper slag containing 4.03% Cu, 0.48% Co, and 1.98% Ni at 130 °C have resulted in significant recoveries of Cu, Co, and Ni, amounting to 90% [24]. Leaching with aqueous sulfur dioxide has also proven effective in recovering 77% Co and 35% Ni from a nickel smelter slag [25].

The current study was undertaken to investigate the dissolution behaviour of selected metals, from the Cu, Ni, Fe, and Zn rich intermediate of base metal production. The focus was to dissolve copper selectively in order to produce PLS rich in copper and a residue with Fe and Ni, applicable for recovery of metals. The lixiviants used in the present study were 0.5–0.5 M HCl, 0.5–3.06 M H₂SO₄, 1–2 M HNO₃, 0.5 M NaCl + 0.1 M CuCl_2, 4.5 M NaCl + 0.5 M CuCl₂, 4.5 M NaCl + 0.1 M CuCl₂, and 4 M NaOH.

2. Materials and Methods

Characterization studies by Scanning Electron Microscopy (SEM), X-ray diffraction (XRD), and Particle Size Distribution (PSD) were conducted to determine the morphology, mineralogical composition, and elemental distribution of the raw material.

2.1. The Raw Material

Chemical analysis of the raw material was performed by employing microwave-assisted digestion in aqua regia (ETHOS Touch Control, Milestone Microwave Laboratory Systems, Sorisole, Italy), as aqua regia is one of the strongest and effective solvent used for metal digestion [26],

Table 1. Chemical analysis of metals of interest in raw material investigated.

Element	Concentration [wt. %]
Cu	12.5
Fe	23.6
Ni	2.6
Al	0.5
Cr	0.1
Zn	1.6
Pb	0.1
As	0.1

. The solution analyses were conducted by ICP-OES (Inductively Coupled Plasma Optical Emission Spectroscopy, Perkin Elmer Optima 7100 DV, Waltham, MA, USA) by Milomatic Oy.

The particle size of the crushed intermediate raw material was analyzed by a Mastersizer 2000 laser diffraction particle size analyzer with a Scirocco 2000 Dry Powder Feeder, both manufactured by Malvern Instruments (UK). Dispersion pressure was varied from 2.0 to 3.0 bar, vibration feed rate was 50% and measurement time was varied from 12 to 30 s. Fraunhofer diffraction model was used as an optical model. The particle size distribution of the homogenized raw material is demonstrated in the volume versus particle size diagram, Figure 1. The size distribution was observed to extend from 1.4 μm to 1905 μm. The cumulative particle size distribution revealed D₈₀ value of 123 µm. The mean particle size D₁₀ = 13 µm, the surface weighted mean was D₃₂ = 25 μm, and the volume weighted mean D₄₃ = 114 μm.

An X’Pert PRO-PAN Analytical X-ray diffractometer, operating at an anode current of 40 mA at 45 kV with a Cukɑ, by Rietveld refinement method [27] using HighScore Plus software (PANalytical), performed mineralogical analysis of the sample. Fixed Divergence Slit (FDS) 1/2° was fitted in the incident beam path to control the equatorial divergence of the incident beam and fixed incident beam. A copper mask of 15 mm was fitted in the incident beam path to control the axial width of the incident beam. Fixed Anti-Scatter Slit (FASS) 1° was used to reduce background signal. The XRD analysis of the raw material by Rietveld refinement suggested a composition of 52.2 wt. % NiFe₂O₄, 25.0 wt. % Fe₂SiO₄ (fayalite), 20.5 wt. % of Cu₂O (cuprite), and 2.3 wt. % of metallic Cu, Figure 2.

SEM-EDS analysis for two raw material samples was performed with a LEO 1450 VP (Carl Zeiss, Oberkochen, Germany) scanning electron microscope (SEM) and a X-MAX-50 mm² energy dispersive X-ray spectrometer (EDS) with INCA Software (Oxford Instruments, Abingdon, UK). Tungsten filament was used as a cathode and the acceleration voltage used was 15 kV.

The raw material samples were cast in epoxy and treated in vacuum, for eliminating gas bubbles attached into the particles, and prepared for SEM-EDS examination polished sections using standard wet methods. It can be discerned from Figure 3 that a larger particle of size around 500 µm is encompassed by smaller particles of particle size ranging 2–50 µm in the raw material. Three phases could be observed, one of the larger particles and two phases in the smaller particles. The average weight percentages of the elements detected in spectrum 1–14 in Figure 3 are presented in

Table 2. SEM-EDS point analysis of the particles presented in Figure 3.

[wt. %]	Spectra #1–5	Spectra #6–9, 14	Spectra #10–12
Cu	88.6	2.2	3.2
Fe	1.9	52.4	3.2
Ni	-	14.7	1.1
O	8.8	23.8	45.8
Si	0.6	0.6	32.3
Na	-	-	0.9
Mg	-	1.0	3.7
Al	-	1.7	3.6
K	-	-	1.9
Ca	-	-	0.7
Ti	-	0.8	0.4
Cr	-	1.9	-
Zn	-	2.0	-
Pb	-	-	4.3

. The lightest color in the back scattered electron (BSE) image corresponds to the phase of the larger particle. It consisted of an average of 88.6 wt. % Cu, 1.9 wt. % Fe, 8.8 wt. % O, and 0.6 wt. % Si (Spectra 1–5), suggesting the presence of Cu and Cu₂O (cuprite), as analyzed oxygen eventually is trace from a surface contamination. The light-gray areas in spectra 6, 9, and 12 correspond to an average of 2.2 wt. % Cu, 52.4 wt. % Fe, 14.7 wt. % Ni, 23.8 wt. % O, and 0.6 wt. % Si (Spectra 6–9, 14), indicating the three main phases, namely Fe₂SiO₄, possibly NiFe₂O₄ and Cu₂O. Nevertheless, the dark-gray region represented by Spectra 10–12, consisted of an average of 3.2 wt. % Cu, 3.2 wt. % Fe, 1.1 wt. % Ni, 45.8 wt. % O, and 32.3 wt. % Si, corresponding to the presence of almost pure SiO₂. Spectra 13 corresponds to epoxy, where samples were casted.

2.2. Leaching Experiments

In order to investigate the extraction without external heating, leaching was conducted at ambient temperature (25 °C) for 48 h in several solutions (

Table 3. Solutions used in the leaching tests.

Solution	Concentrations	Chemicals	Manufacturer (Grade)
HCl	0.5 M	HCl 37%	EMPARTA ACS (for analysis)
	1.5 M
	2.5 M
	3.0 M
	5 M
H2SO4	0.51 M	H2SO4 95–97%	EMSURE ISO (for analysis)
	1.22 M
	1.93 M
	2.65 M
	3.06 M
HNO3	1 M	HNO3 65%	EMSURE (for analysis)
CuCl2, pH 1	0.5 M NaCl + 0.1 M CuCl2	CuCl2·2H2O	VWR Chemicals (technical)
	4.5 M NaCl + 0.5 M CuCl2
	4.5 M NaCl + 0.1 M CuCl2
NaOH	4 M	NaOH	SIGMA-ALDRICH (technical)

). Leaching experiments were conducted in an Erlenmeyer flasks and the solutions were mixed by an IKA RO 10 Multi Station Digital Magnetic Stirrer at 300 RPM. The used S/L ratio was 0.025 (5 g solids/200 mL solution). To evaluate the leaching efficiency of Ni, Zn, Cr, Pb, Cu, Fe, and Al, the solution was filtered after the leaching step and the filtrate was analysed by AAS (atomic absorption spectrophotometer), using a Varian AA240 (Varian, Palo Alto, CA, USA), and ICP-OES [28,29].

No external oxidation by gas bubbling was used in the experiments. Redox potential was measured by a Fluke 115 True RMS Multimeter using platinum wire and Saturated Calomel Electrode (SCE). Mettler Toledo Seven (Easy pH meter) was used for pH measurements, except in the NaOH solutions, where Hanna Instruments Edge pH meter was employed.

3. Results and Discussion

Leaching was performed on the raw material to get an insight into the dissolution phenomena related to Cu, Ni, Zn, and Fe in various lixiviants. Also leaching of trace metals, such as Cr, Pb, and Al, was explored. The aim was to find a selective, low temperature, and low chemical consumption leaching procedure for copper present in the raw material. Furthermore, the target was to leave nickel in the leach residue in the leaching stage.

Table 4. Extraction of investigated metals from the raw material (%).

Solution	Ni	Cu	Fe	Zn	Cr	Pb	Al
0.5 M HCl	10	*	55	40	20	98	39
1.5 M HCl	18	*	78	48	45	93	56
2.5 M HCl	43	95	81	64	67	97	69
3 M HCl	97	72	54	66	84	99	71
5 M HCl	96	86	74	92	55	97	79
0.5 M H2SO4	35	70	53	63	84	21	51
1.22 M H2SO4	64	77	60	76	56	23	63
1.93 M H2SO4	77	81	82	80	62	23	69
2.65 M H2SO4	86	71	78	86	57	23	65
3.0 M H2SO4	81	65	62	86	56	17	65
4.5 M NaCl + 0.5 M Cu2+ pH 1	1	5	-	1	-	65	7
4.5 M NaCl + 0.1 M Cu2+ pH 1	1	3	-	1	-	62	5
0.5 M NaCl + 0.1 M Cu2+ pH 1	3	61	-	27	-	53	5
1 M HNO3	3	79	-	30	-	98	16
2 M HNO3	4	93	-	30	-	*	13
4 M NaOH	0.3	*	-	2	-	59	22

* Full leaching.

presents the metal yields to the solution in all 16 investigated media. The corresponding redox potentials, as well as pHs before and after the experiment are presented in Figure 4. It can be seen that there is some variety in the recovery percentage—this is most likely attributed to the heterogeneous nature of the investigated raw material with big particle size and wide particle size range combined with small solid/liquid ratio in the leaching experiments. This leads in to some variation in the representativeness of each sample, thus also resulting some error in the recovery calculations.

3.1. Leaching of Copper

Table 4 shows that copper was dissolved well into most lixiviants investigated. The highest extraction of Cu was achieved with 1.5 M HCl. Also 4 M NaOH, 0.5 M HCl, 2.5 M HCl, and 2 M HNO₃ resulted in yields higher than 93%, and 1 M HNO₃,1.22 M H₂SO₄, and 1.93 M H₂SO₄ showed >75% extraction. The chloride leaching experiments (0.1 and 0.5 M of copper (II) as oxidant along with 4.5 M NaCl) showed only minor Cu dissolution (≤5%), most likely due to a final pH close to 3 (see Figure 4), indicating copper precipitation as atacamite [30]. Sulfuric acid concentration increase was shown to increase Cu extraction up to 80% at 1.93 M, however at higher concentrations the extraction was decreased, being 65% at 3.0 M H₂SO₄. The extraction efficiency of copper was found to be comparatively lower in H₂SO₄ than in HCl and HNO₃ medium (Table 4). Habashi et al. [31] have suggested that since HCl and HNO₃ generate 1 mole of H⁺ ions when dissolved in water, they produce similar dissolution efficiency compared to H₂SO₄, which produces 2 moles of H⁺ ions. Also, the extraction efficiency of Cu was higher in 2 M HNO₃ than in 1 M HNO₃ (Table 4) as the oxidizing potential of NO₃⁻ ions has been reported to increase with increase in solution acidity [32].

In chloride media, it is suggested that cuprous chloride complexes CuCl₃²⁻ and CuCl₄³⁻ will be produced sequentially from CuCl₂⁻ with chloride concentration above 1 M [33]. Chloride ions complexes can stabilize Cu(I) ions thereby increasing copper solubility. The complexation also increases the redox potential of Cu(II)/Cu(I) thereby enhancing the oxidative power of the solution. Copper is also known to be dissolvable at high pHs such as in 4 M NaOH media. The pH values measured in NaOH leaching (Figure 4) suggest the prevailing species as Cu(OH)₃⁻ [34].

The suggested reactions acid/basic leaching reactions in HCl (3), sulfuric acid (4), and basic NaOH (5) for Cu₂O, are presented below with their standard Gibbs energies of the reactions at 25 °C from HSC Chemistry database [35]:

Cu₂O + 8HCl_(a) = 2CuCl₄³⁻_(a) + 6H⁺_(a) + H₂O_(a), ∆G° = −121.28 kcal/mol

(3)

Cu₂O + H₂SO_4(l) = Cu + CuSO_4(ia) + H₂O_(a), ∆G° = −14.71 kcal/mol

(4)

Cu₂O + 4NaOH_(a) + H₂O_(a) = 2Cu(OH)₃⁻_(a) + 4Na⁺_(a) + 2e⁻, ∆G° = −28.13 kcal/mol

(5)

The species (a), (ia) and (l) refers to aqueous, neutral aqueous and liquid phase.

3.2. Ni Leaching and Selectivity between Copper and Nickel

According to the mineralogy, the prevailing nickel phase in the raw material investigated is nickel ferrite NiFe₂O₄. Ferrites are known to be refractory in leaching. This is confirmed by the results which showed that the maximum Ni extraction (97%) was observed in aggressive concentrated leaching media (3–5 M HCl). The suggested leaching reactions in HCl are presented in (6) and (7). From the speciation diagram of nickel containing NiCl₂ and HCl [36], most nickel is suggested to exist as Ni²⁺ up to 5 M HCl. However, the concentration of NiCl⁺ gradually increases with increases in HCl. Nickel dissolution did not show any selectivity versus iron in any of the leaching media investigated. This is due to the dominating Ni phase NiFe₂O₄ resulting in a simultaneous Ni and Fe dissolution. Also, in the absence of neutralization, no back precipitation was observed.

NiFe₂O₄ + 8HCl_(a) = Ni⁺²_(a) + 2FeCl⁺²_(a) + 4H₂O_(a) + 6Cl⁻_(a), ∆G° = −127.78 kcal/mol

(6)

NiFe₂O₄ + 8HCl_(a) = NiCl⁺_(a) + 2FeCl₂⁺_(a) + 4H₂O_(a) + 3Cl⁻_(a), ∆G° = −161.17 kcal/mol

(7)

The current study aims to selectively dissolve Cu versus nickel. Figure 5 presents the dissolved Cu/Ni ratio in solution with eight of the most selective lixiviants. It can be seen that the highest selectivity was achieved with 4 M NaOH (w(Cu):w(Ni) = 340 in solution). Also 1 and 2 M HNO₃ (w(Cu)/w(Ni) = 26 and 23) provided excellent selectivity as well as 0.5 M HCl solution (w(Cu)/w(Ni) = 10).

The lowest Ni dissolution (≤4%) was observed with 4 M NaOH, 1–2 M HNO₃, and all the investigated NaCl solutions. Dilute HCl (0.5 M) dissolved only 10% of nickel.

3.3. Leaching of Iron

Iron originated from the two main minerals of the raw material, NiFe₂O₄ and Fe₂SiO₄. Most notable extractions of Fe (81–82%) was observed in 1.93 M H₂SO₄ and 2.5 M HCl. Furthermore, the Fe extraction was high (>50%) in all hydrochloric and sulphuric acid media. Generally, no selectivity between iron and nickel or copper was found. However, minor selectivity between Cu and Fe was observed in 0.5 M and 1.5 M HCl (Cu/Fe = 1.9 and 1.5, respectively).

In chloride media, Fe(III) forms FeCl²⁺ and Fe³⁺ at lower Cl⁻ concentrations, whereas FeCl₂⁺ is formed at higher chloride concentration [37,38]. The suggested reactions for NiFe₂O₄ are presented earlier in (6) and (7), in addition fayalite is suggested to leach according to reactions (8) and (9):

Fe₂SiO₄ + 2H₂SO_{4 (l)} = 2FeSO_{4 (ia)} + H₄SiO_4(a), ∆G° = −51.67 kcal/mol

(8)

H₄SiO₄ (colloid) = SiO₂·2H₂O

(9)

It is clear that iron dissolution is strongly related to the solution pH. At pHs < 2 iron is known to remain soluble [39]. This can be taken as an advantage in the leaching, as pH adjustment can significantly improve the selectivity between Cu and Fe.

3.4. Leaching of Zinc

The maximum extraction of Zn (90%) was achieved in 5 M HCl. Several researchers [40,41] have reported 90% recovery of Zn in leaching of zinc ferrite in the concentration range of 0.5–6 M HCl. When Zn(II) is dissolved into chloride media, it is known to form complex such as ZnCl₃⁻ [42]. Decrease in the Zn yield from 86% to 40% was noticed in the following order of the lixiviants:

2.65 M H₂SO₄ > 3.0 M H₂SO₄ > 1.93 M H₂SO₄ > 1.23 M H₂SO₄ > 3 M HCl > 2.5 M HCl > 0.5 M H₂SO₄ > 0.5 M HCl

However, Zn extraction was ≤30% in 1–2 M HNO₃ and 0.5 M NaCl + 0.1 M Cu²⁺. The alkaline leaching of zinc ferrite (4 M NaOH) was shown to result in lower Zn extraction compared to HCl and H₂SO₄. Zn extraction was found to be 2% in 4 M NaOH. Thus, zinc ferrite fraction of magnetite was not decomposed even in strong alkaline media. The suggested reactions for Zn dissolution in HCl (10), and sulfuric acid (11) media are:

ZnFe₂O₄ + 8HCl_(a) = ZnCl₃⁻_(a) + 2FeCl₂⁺_(a) + 4H₂O_(l) + 3Cl⁻_(a), ∆G° = −124.77 kcal/mol

(10)

ZnFe₂O₄ + H₂SO_4(l) = ZnSO_{4 (a)} + Fe₂O₃ + H₂O_(l), ∆G° = −26.98 kcal/mol

(11)

3.5. Leaching of Cr, Pb, and Al

The dissolution of minor elements, such as Cr, Pb, and Al, was observed in the investigated leaching solutions. Figure 6 displays a complete 3D schematic representation of the elements of Al, Pb, and Cr and shows that generally aluminum had a high solubility into sulfuric acid and HCl media. Lead had high tendency towards HCl, NaOH, HNO₃, and chloride leaching, but it was dissolved only slightly into sulfuric acid media, evidently due to the low solubility of lead sulfate.

It was also observed from the above graph (Figure 6) that the maximum Cr yield was 83% in 3 M HCl. Furthermore, the yield of Cr decreased from 66 to 20% in presence of lixiviants in the following order: 2.5 M HCl > 1.93 M H₂SO₄ > 0.5 M H₂SO₄ > 2.65 M H₂SO₄ > 3.06 M H₂SO₄ > 1.22 M H₂SO₄ > 5 M HCl > 1.5 M HCl.

Highest aluminum extraction (>70%) was observed in 3 and 5 M HCl solutions. Additionally, all sulphuric acid solutions and HCl solutions ≥1.5 M resulted in higher than 50% leaching. Yield percentage of Pb in all the investigated HCl solutions was high (93–99%), as Pb dissolves forming chloride complexes PbCl⁺ and PbCl₂ at low Cl⁻ concentrations, and forms PbCl³⁻ and PbCl₄²⁻ at higher concentrations [43].

3.6. A Comparison of the Used Lixiviants

Table 5. The best lixiviants selected for Cu, Fe, Pb, and Zn extraction.

Solution	Cu Extraction (%)	Cu/Ni Selectivity	Cu/Zn Selectivity	Cu/Fe Selectivity	Pb Extraction (%)	Zn Extraction (%)
1.5 M HCl	*	7	3	1.5	93	48
0.5 M HCl	*	10	3	1.9	98	40
4 M NaOH	*	340	51	-	59	2
0.5 M NaCl, 0.1 M Cu2+	61	20	2	-	53	27
2 M HNO3	93	23	3	-	*	30
1 M HNO3	79	26	3	-	98	30

* Full leaching.

shows a comparison of the best copper lixiviants and the selectivities of copper dissolution for nickel and iron. It can be seen that 4 M NaOH has the highest selectivity in copper extraction. In addition, it has the highest selectivity for Zn as well. The goal of mild, selective, and low temperature leaching of copper is best approached by the 0.5 M HCl media, which was able to dissolve all copper with only 10% nickel extraction, selectivity between copper and nickel concentrations in the solution being 10. Furthermore, 0.5 M HCl was the media showing the highest selectivity towards Fe (Cu/Fe = 1.9) without pH adjustment.

The calculated composition of the residue after 0.5 M HCl leaching suggests the leach residue composition being 23 mg/g Ni, 106 mg /g Fe, 9 mg/g Zn, 2 mg/g Al, 1.9 mg/g Cu, 0.9 mg/g Cr, and 0.02 mg/g Pb. The advantage of 0.5 M HCl is that it could dissolve also almost all Pb (98%).

4. Conclusions

Along the principles of circular economy, the recovery of metals from industrial side streams, waste, and intermediate fractions is of increasing importance. In the current study, the leaching phenomena and selective leaching of copper was investigated from an intermediate raw material originating from base metal production, with a mineralogy of 52.2% NiFe₂O₄, 25.0% Fe₂SiO₄ (fayalite), 20.5% of Cu₂O (cuprite), and 2.3% of metallic Cu. In the raw material, the large particles were shown to consist mainly of Cu₂O and elemental Cu.

Copper present in the raw material was shown to be easily dissolvable, over 98% Cu could be dissolved with 0.5 M, 1.5 M HCl, and 4 M NaOH. In addition, 0.5 M HCl was shown to provide selectivity towards Ni, with the Cu/Ni concentration ratio in solution being 10. Alkaline leaching in 4 M NaOH resulted in the highest selectivity for copper leaching, with the ratio of dissolved elements of Cu/Ni = 340 and Cu/Zn = 51. Also 1 and 2 M HNO₃ provided high selectivity for copper dissolution with a Cu/Ni ratio of 26 and 23, respectively. Aluminum showed high dissolution into sulfuric acid and hydrochloric acid media, the highest Al extraction being 79% in 5 M HCl whereas lead dissolution was strong in HCl, chloride, NaOH, and HNO₃. The highest extraction for Ni was obtained in 5 M HCl.

The results indicate that from the 16 investigated leaching media, hydrochloric acid leaching (0.5 M HCl) presents the lowest concentration solution matrix for selective and high copper extraction, even at room temperature.