Novel treatment for mixed copper ores: Leaching ammonia – Precipitation – Flotation (L.A.P.F.)

Minerals Engineering 149 (2020) 106242

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Novel treatment for mixed copper ores: Leaching ammonia – Precipitation – Flotation (L.A.P.F.)

T

Víctor Conejerosa, Kevin Pérezb, Ricardo I. Jeldresc, Jonathan Castillod, Pía Hernándezc, ⁎ Norman Toroa,e, a

Departamento de Ingeniería Metalúrgica y Minas, Universidad Católica del Norte, Av. Angamos 610, Antofagasta, Chile Faculty of Engineering and Architecture, Universidad Arturo Prat, Almirante Juan José Latorre 2901, Antofagasta 1244260, Chile c Departamento de Ingeniería Química y Procesos de Minerales, Universidad de Antofagasta, Avda. Angamos 601, Antofagasta, Chile d Departamento de Ingeniería en Metalurgia, Universidad de Atacama, Av. Copayapu 485, Copiapó, Chile e Department of Mining, Geological and Cartographic Department, Universidad Politécnica de Cartagena, Paseo Alfonso Xlll N◦52, Cartagena, Spain b

A R T I C LE I N FO

A B S T R A C T

Keywords: Mineral processing Copper dissolution Sulfide recovery, mixed copper ore

Mixed copper ores are challenging to process, where the treatments are expensive, resulting in low recoveries, with products that are not commercially viable. The proposed research aims a sustainable method to recover copper from a mixture of oxides and sulfides minerals. The procedure involves three stages: (i) leaching with aqueous ammonia solutions at room temperature and varied pH, ammonia concentration, solid concentration, and particle size; (ii) PLS precipitation in copper sulfides by the addition of sulfur (or other copper sulfates) and SO2 (or additional sulfites or bisulfites); (iii) froth flotation to recover the value sulfide, considering a set of collectors and frothers. The outcomes obtained in this work were promising, achieving copper extraction of 70% in the leaching stage, precipitation of copper sulfides that surrounds 90%, and 75% of recovery by flotation.

1. Introduction Chile provides 5.5 million tons of copper per year, which represents 27% of the worldwide stock (U.S. Geological Survey, 2018). Still, COCHILCO (2018) predicted that Chilean production would grow 28.3% over the next ten years, giving 7.06 Mt of fine copper in 2029. Concentrate production is expected to represent 88.4%, while hydrometallurgical methods would treat the remaining 11.6%. The low prognostication for this latter is sustained by the constant depletion of copper oxides (leachable resources), but the perplexity is that many leaching facilities will eventually be left unused. Consequently, an auxiliary posture is being adopted that seeks to extend the leaching processes to copper sulfides, aiming to prolong the lifetime of the current plants. Nevertheless, to guarantee sustainability, it is essential to implement new technological approaches that can complement the conventional designs, which by themselves are characterized by low recovery rates, high supplies consumption and high cost of reagents. Oxide and/or mixed ore flotation are separated into two distinct treatments: (i) direct flotation and (ii) sulfating flotation (controlled by potential sulfidation). The collectors for direct flotation are fatty acids (oleic acids, sodium oleate), fatty amines, petroleum sulfonate, and hydroxyl acids (Deng and Chen, 1991; Li et al., 2015). Such reagents

⁎

are typically restricted by their high cost, lack of selectivity, and even low performance (Lee et al., 1998). Otherwise, the sulfidation flotation requires agents (like NaSH, Na2S, and H2S) that can sulfur the copper oxides, altering its surface chemistry by forming species of “synthetic covellite” (see Eq. (1)). This procedure is the most employed for flotation of copper oxides (Yin et al., 2019), but the concerns arise in controlling the pulp's potential (Corin et al., 2017; HUA et al., 2018; Lee et al., 2009) and management of reagent concentration (Zhou and Chander, 1993). A sub-optimal amount leads to a low copper recovery, while overdose is associated with a depression of the valuable mineral due to an over sulfuration (HUA et al., 2018; Lee et al., 2009). Most studies of oxide sulfidation are based on malachite (Liu et al., 2019; Park et al., 2016; Shen et al., 2019; Wu et al., 2017) since it is the most abundant copper oxide worldwide (Mindat, 2019). The processes involve diverse reagents like sodium sulfide (Na2S) (Feng et al., 2017; Liu et al., 2019), sodium hydrosulfide (NaSH) (Corin et al., 2017), hydrogen sulfide (H2S), ammonium chloride (Wang et al., 2009), ammonium hydroxide, ammonium carbonate (Bingöl et al., 2005; Zhao et al., 2017), and ammonium sulfide ((NH4)2S), as well as collectors like ethylenediamine (Feng et al., 2018), tert-butylsalicylaldoxime (Li et al., 2019), and alkyl hydroxamate (Lee et al., 1998). While sulfides (NaSH and Na2S) provide a better performance, they involve more significant

Corresponding author at: Departamento de Ingeniería Metalúrgica y Minas, Universidad Católica del Norte, Av. Angamos 610, Antofagasta, Chile. E-mail address: [email protected] (N. Toro).

https://doi.org/10.1016/j.mineng.2020.106242 Received 25 July 2019; Received in revised form 24 December 2019; Accepted 30 January 2020 0892-6875/ © 2020 Elsevier Ltd. All rights reserved.

Minerals Engineering 149 (2020) 106242

V. Conejeros, et al.

concerns for the industry in terms of pollution and toxicity (Lee et al., 2009).

2CuFeS2 + 8NH3 + +5H2 + 4S + Fe2O3 + 4OH−

+ 16NH3 + 4H2O 4CuFeS2 (NH3)42+ + 8S + 2Fe2O3 + 8OH−

1.1. Oxide sulfidation reactions

CuCO3*Cu(OH)2 + 2HS− = 2CuS + 2H2O + CO3−2

Cu2Cl(OH)3 + 8NH3 = 2Cu(NH3)4

+

+ 2OH

−

−

+ 3OH

+ Cl

(11)

2HSO4−

+ Fe2+ (12)

2+

+ CuFeS2 + 2SO2 + 4H2O → Cu2S + 6H + +

Cu2+ + CuS + SO2 + 2H2O → Cu2S + 3H+ + HSO4−

(13)

2. Methodology The trials were conducted with mixed ores from Mantos Blancos, with 0.95% copper grade, and 40% of soluble copper (approx.). Preliminary assays sought to define the best-operating conditions, starting with the leaching of ammonia at varied pH, particle size, and concentrations of ammonium and chloride. In the precipitation tests, the effect of sulfur and sulfur dioxide at distinct levels of ammonium was investigated. Ultimately, the experiments were performed for the selection of reagents (collectors and frothers), and their respective dosages. The leaching of ammonia was done in stirred reactors of 1 L, considering 500 mL of solution and timing classified according to the step: (i) 60 min of leaching, (ii) 60 min of precipitation in the same reactor with SO2 bubbling, and (iii) 6 min of final flotation. The next tests were executed:

• The material, obtained from Mantos Blancos, with a size of 20% • • • • •

(3)

2+

(9)

Cu(NH3)4+2 + SO2 + S + 4H2O = CuS + 4NH4+ + SO4−2 3Cu

(2)

−

4Cu (7)

2Cu(NH3)4+2 + 2SO2 + S + 4H2O = Cu2S + 8NH4+ + 2SO4−2(10)

1.2. Leaching reactions in ammonia systems

CO32−

=

1.3. Precipitation reactions

Researches on ammonia leaching of sulfides or mixed ore (D’yachenko and Kraidenko, 2010; Dutrizac, 1981) have stated that high temperatures and pressure are needed, as well as dissolved oxygen to enhance the system diffusion. Applying ammonium chloride at temperatures among 27 °C and 327 °C (without the addition of oxygen), the Gibbs free energy is 230 and 101 kJ/mol, respectively (D’yachenko and Kraidenko, 2010), so the dissolution of copper from sulfides is unlikely to occur and, consequently, the copper sulfides will remain unchanged.

2Cu(NH3)42+

5O2

The process is based on the reaction of the tetraamminecopper(II) ion, which is the main soluble copper compound at pH 9 and 10 (Radmehr et al., 2013). This ion reacts with elemental sulfur and sulfur dioxide (see Eqs. (10) and (11)), forming copper sulfide as a precipitate. The copper obtained has a composition that ranges from CuS (covellite) to Cu2S (chalcocite), and may be efficiently recovered by flotation. The precipitation is viable at room temperature, and the obtained copper sulfide has microcrystalline characteristics, with an average size of 30 µm. The absorption of SO2 is suitable due to the basic pH (pH > 8), which lessens contamination and equipment design concerns. Moreover, the ammonia can be recovered from the solution by adding lime (Morales et al., 2009).

bonate and small amounts of sulfate in the leaching solution.

NH3 + H2O = NH4− + OH−

+

2CuS + 8NH3 + 2H2O + O2 = 2Cu(NH3)42+ + 2S + 4OH−

• It is more selective to copper concerning acid leaching and allows keeping the gangue unaltered; • Prevents the corrosion of the equipment by excluding the application of acids; • Metals like iron and manganese are not soluble in ammonia; • Ammonia prevents the solubility of calcium in the presence of car-

NH4OH = NH3 + H2O

2Cu(NH3)42+ (6)

=

2Cu2S + 8NH3 + 2H2O + O2 = 2Cu(NH3)42+ + 2CuS + 4OH− (8)

(1)

Sulfidation is a recurring technique, although it is not much useful for mixed copper ores (composed of mixtures of sulfides and oxides), knowing that copper sulfides may depress by ions of sulfide (S2−) and hydrosulfides (HS−). In this sense, an attractive matter has been looking for selective collectors that work for both oxides and sulfides. This strategy came up more than three decades ago, considering hydroxamate (Fuerstenau and Pradip, 1984) and later octyl hydroxamate (Hanson and Fuerstenau, 1991). While promising outcomes have emerged (Hanson and Fuerstenau, 1991; Hope et al., 2012; Lee et al., 2009; Marion et al., 2017; Parker et al., 2012), the problems remain about low copper hydrophobicity, poor recovery in copper oxides (e.g. malachite), and high costs in offering a commercially viable product. An interesting process to treat mixed copper sulfides-oxide is the leaching-precipitation-flotation (LPF) sequence. The treatment begins with an acid leaching amidst a pH 1–2, followed by precipitation at pH 3–3.5, which is fixed with lime to avoid unnecessary iron consumption. Finally, the copper precipitate (like copper cement) and copper sulfides (without leaching) are floated in an acid medium at pH 4–4.5 (ITGE, 1991). The economic and environmental concerns derive principally from the high temperatures required to leach (50–77 °C). This work aims a novel approach, analogous to LPF, but with specific differences that include leaching with ammonia instead of acid, keeping alkaline pH (pH 8–9); copper precipitation using S and SO2 (at room temperature); and then the precipitate and sulfides minerals (leaching residue) are recovered by flotation. Several studies have been conducted on the treatment of oxides in ammonia media (Bingöl et al., 2005; Wang et al., 2009; Zhao et al., 2017), sulfides (HUA et al., 2018) and complex mixed copper ores (D’yachenko and Kraidenko, 2010), but ammonia medium provides advantages (Dutrizac, 1981; Radmehr et al., 2013) such as:

Cu2(OH)2CO3 + 8NH3 =

7H2O

(4) (5) 2

+65# Tyler, which determines a size equivalent to 50% −200# Tyler. It was leached with NH3 (added has hydroxide) 0.5 M and 2 M at pH 9, which was adjusted with lime. Experiences with and without NaCl (3 M); the NaCl simulates the effect of atacamite or the use of seawater. It was precipitated with SO2 and S (100% excess). The treated pulp was floated, both directly and separating the solid by filtration and drying. The precipitate was washed and then floated with SF-323 (collector, 57 g/ton) and MIBC (frother, 55 g/ton), considering 5 min of conditioning and 6 min of flotation. The experiments were carried out in a 3 L Denver cell, keeping the pH 10–11. The froth was scraped every 10 s.

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• Flotation tests were performed with 40 wt% of solids concentration,

80

for two particle size: 20% +65 # Tyler (50% −200 # Tyler) and with a size of 100% −200 #. In both cases, the combination of reagents was: 1. SF-323 (47 g/t) and MIBC (55 g/t). 2. SF-323 (10, 40, 55 g/t), SF-114 (20, 30, 55 g/t) and MIBC (40, 10, 55 g/t). 3. S-7156 (10, 40, 55 g/t), SF114 (20, 30, 55 g/t) and MIBC (40, 10, 55 g/t). 4. AERO 7156 (20 g/t), SF 114 (30 g/t) and MIBC (55 g/t). Test to select frothers agents were performed with 40 wt% of solids concentration, and size 20% +65 # Tyler (50% −200 # Tyler). Working with the best mixture of collectors (20 g/t of SF-323, 30 g/t of SF-114), the frother combinations were: 1. DF-250. 2. DF-250 and MIBC (1:1; 1:2; 2:1 in proportion completing 55 g/t in each case).

Cu extraction [%]

70

•

30 30 30 30 40 40 40 40

15 30 15 30 15 30 15 30

20 20 40 40 20 20 40 40

55 55 55 55 55 55 55 55

20

30

40

50

60

80

Cu extraction [%]

70 60 50 A B C D E F G H

40 30 20 10 0 0

10

20

30

40

50

60

Time [min] Fig. 2. The effect of solid content, pH and ammonia concentration in a grain size 27% +100 # Ty on ammonia leaching. A: Solid content 30 wt%, 1 M, pH 8; B: Solid content 50 wt%, 1 M, pH 8; C: Solid content 30 wt%, 2 M, pH 8; D: Solid content 50 wt%, 2 M, pH 8; E: Solid content 30 wt%, 1 M, pH 9; F: Solid content 50 wt%, 1 M, pH 9; G: Solid content 30 wt%, 2 M, pH 9; H: Solid content 50 wt%, 2 M, pH 9.

significantly, while at pH > 10 this becomes autonomous of the concentration. Then the pH should be kept in an operating range between pH 9–10. The behaviors presented in Figs. 1 and 2 are similar for the trends produced by the three analyzed variables, where the latter is for larger particles (27% +#100 and 50% −#200). The copper dissolution rate was higher under ideal conditions (30% Solid content, 2 M NH3 and pH 9), but the other curves displayed distinctive performance compared to the smaller grains (15% +#100 and 50% −#200); then no absolute response appears according to the grain size. The tests with the coarser material (27% +#100 and 50% −#200) continued for the operational facility. The principal minerals after leaching were copper sulfides and gangue (quartz and magnetite). In smaller proportions, appeared bornite and copper oxides that still possess the value metal. Agreeable with past researches (D’yachenko and Kraidenko, 2010), the copper sulfides did not experience the dissolution in an ammonia environment at room temperature, so the ore found in the residue was taken along with the mineral formed in the precipitation to the next stage of flotation. Fig. 3 outlines the increase of copper precipitation in the form of

Reactive doses (g/t)

1 2 3 4 5 6 7 8

10

Fig. 1. The effect of solid content, pH and ammonia concentration in a grain sizes of 15% +100 # Ty on ammonia leaching. A: Solid content 30 wt%, 1 M, pH 8; B: Solid content 50 wt%, 1 M, pH 8; C: Solid content 30 wt%, 2 M, pH 8; D: Solid content 50 wt%, 2 M, pH 8; E: Solid content 30 wt%, 1 M, pH 9; F: Solid content 50 wt%, 1 M, pH 9; G: Solid content 30 wt%, 2 M, pH 9; H: Solid content 50 wt%, 2 M, pH 9.

Table 1 Parameters for the flotation test.

MIBC

20

Time [min]

In the current work, the copper extraction was examined based on the copper oxides in the starting sample (0.38% soluble copper), and the copper precipitate based on the maximum extraction rate. Subsequently, the recovery rate in froth flotation was measured depending on the initial copper concentration (0.95%). Fig. 1 shows that the most critical variable was the solid concentration in the leaching solution, wherein the copper extraction rate increased by 23% (for similar pH values and NH3 level). This coincides with several investigations, which showed that the lower the S/L ratio, the higher the kinetics of copper dissolution. The more significant contact of the leaching solution with the mineral is an essential aspect for the operation. Another influential variable is the concentration of NH3. Dutrizac (Dutrizac, 1981) employed hydroxide and carbonate to evaluate the level of ammonia between 0.1 M and 2 M, reporting that the kinetics of dissolution grew while the ammonia increased, and the effect was most noticeable at low concentrations (0.1–0.6 M). As can be seen in Fig. 1, under an increase of NH3 from 1 to 2 M, the copper extraction rate grew by 22%. The pH had the least impact, but the outcomes suggest that this should also be controlled. At pH 9–10, most of the ammonia is like free NH3 and not NH4+, which enhances the kinetics of the copper dissolution (Dutrizac, 1981). At pH < 7 the amount of NH3 diminishes

SF-114

30

0

3. Results and discussions

SF-323

A B C D E F G H

40

0

(a) Leaching: Concentration of NH3: 1 and 2 M, pH: 8 and 9, Solid concentration: 30 and 50 wt%, grain size: 15 and 27% +#100; (b) Precipitation: Concentration of NH3: 1 and 2 M; addition of excess sulfur: 50, 100, 150, and 200%; addition of excess sulfur dioxide: 50, 100, 150 and 200%; (c) Flotation: described in Table 1.

Solid content (%)

50

10

The following conditions were selected for analysis in the final tests:

Test

60

3

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V. Conejeros, et al.

Cu precipitation [%]

90

•

80 A B C D E F G H

70

60

50 0

50

100

150

•

200

concentration, 30 wt% of solids, and a leaching time of 60 min. This gave a maximum extraction rate of 75%. The leaching of ammonia began at pH 8, and then was fixed to pH 9 with lime. Then, the precipitation runs spontaneously in the sulfurization time, considering a total ammonia concentration of 1 M, 100% excess of SO2, 200% excess of S, and 30 wt% of solids. Soluble copper diminished under the operational conditions from 0.38 to 0.035%, so attaining sulfurizing the ore, it was possible to proceed to the next flotation stage. The better performance was achieved with SF-323 (15 g/t), SF-114 (40 g/t), and MIBC (55 g/t), giving a grade of copper concentrate of 12.9%, and 80% of recovery.

Leaching and precipitation efficiencies were not significantly affected by the addition of 3 M of NaCl since the treatment of minerals like atacamite, and the use of seawater does not offer major challenges.

Excess of SO2 [%] Fig. 3. Leaching-precipitation tests with pH 9, 30% Solid content and a grain size of 27% +#100. A: 2 M, 50% excess of S; B: 1 M, 50% excess of S; C: 2 M, 100% excess of S; D: 1 M, 100% excess of S; E: 2 M, 150% excess of S; F: 1 M, 150% excess of S; G: 2 M, 200% excess of S; H: 1 M, 200% excess of S.

CRediT authorship contribution statement Víctor Conejeros: Validation. Kevin Pérez: Data curation, Writing - original draft. Ricardo I. Jeldres: Writing - review & editing. Jonathan Castillo: Investigation. Pía Hernández: Investigation. Norman Toro: Project administration.

100

Cu recovery [%]

80

Declaration of Competing Interest 60

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

40

Acknowledgments

20

R.I.J thanks Centro CRHIAM Project Conicyt/Fondap/15130015. 0 1

2

3

4

5

6

7

References

8

Test

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Fig. 4. Total Cu recovery rate in the flotation stage.

sulfides as a function of the excess of SO2 and the ammonia concentration used in the leaching solution. The tests were done at pH 9, 30 wt% of solids concentration, and fixed time (30 min). The precipitate of copper raised as both the ammonia concentration and the excess of SO2 increased. Up to 91% of precipitation was achieved, being significant since the process was performed at room temperature, while the original LPF requires demanding temperatures, between 55 and 77 °C (ITGE, 1991). Approximately 0.2 (g/L) of copper remain in solution, and the ammonia can be regenerated and recirculated with lime (Morales et al., 2009). Fig. 4 shows the copper recovery by evaluating the reagents SF-114 (Sodium isobutyl xanthate) and SF-323 Isopropyl ethyl thiocarbamate) and varying its dosage slightly. The highest copper recovery was achieved in the test 7, in which the mineral floated with 40 wt% (solids concentration) and 15 g/t of SF-323 (collector), 40 g/t of SF-114 (collector), and 55 g/t of MIBC (frother). The nature of the collectors justifies that SF-114 is useful for oxidized minerals with the previous sulfidation, while SF-323 is more selective for chalcocite and covellite than xanthate (typically used for chalcopyrite). 4. Conclusions The present work analyzed operational variables in LAPF treatment and reports the following:

• The best ammonia leaching conditions were: pH 9, 2 M of ammonia 4

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