Leaching of Chalcopyrite in Acidified Nitrate Using Seawater-Based Media

The leaching of copper from industrial copper ore with 4.8 wt % chalcopyrite by acidified nitrate with seawater based media was investigated. Water quality (pure water and seawater), temperature (25–70 ◦C), reagent concentration, and nitrate type (sodium and potassium) were studied variables. Leaching conditions were: 100 g ore/1 L solution; P80 of 62.5 μm; 400 rpm and leaching time, varying between 3 and 7 days. Nitrates in sulfuric acid are known to be good oxidants for sulfide ores. This study showed that up to 80 wt % copper could be extracted at 45 ◦C in 7 days. In the absence of nitrate, under the same leaching conditions, only a 28 wt % copper extraction was achievable. The extraction rate increased to 97.2 wt % when leach temperature was increased to 70 ◦C in nitrate-chloride-acid media. The presence of chloride ions also increased the copper extraction rate. The copper extraction achieved in seawater systems were higher than in water systems under the same leaching conditions (increased by an average of 18 wt %). This effect can be attributable to the contribution of chloride that increases proton (H+) activity.


Introduction
Given that copper oxide minerals will probably be depleted in the near future and hydrometallurgical plants could then be left unused, hydrometallurgical methods to process copper sulfide represent an alternative use for solvent extraction and electro-winning plants.
There are large reserves of nitrate (caliche) in Northern Chile [19][20][21].Sodium nitrate is obtained by caliche leaching with water and then crystallizing the leaching solution by cooling or evaporation.In the case of evaporation, discarded salts (tailings) from the solar pond still contains significant amount (4.6 wt % NaNO 3 ) of nitrate salts [22].Such tailings can be used as oxidants in chalcopyrite leaching.Habashi [23] proposed the use of nitric acid (HNO 3 ) and indicated that in sulfide ore leaching nitrate ions act as an oxidant by forming NO or NO 2 gases and oxygen, from the decomposition of HNO 3 .Other researchers [24,25] related the better performance of HNO 3 as an oxidant due to its high redox potential.In a system of CuFeS 2 , HNO 3 and H 2 SO 4 , the reaction products are Cu 2+ , Fe 3+ , S 0 and NO gas.This gas (NO) can be captured and oxidized with air and water to form HNO 3 .At higher temperatures, sodium nitrate converts to nitric acid in presence of sulfuric acid (Equations ( 1)-( 2); Kazakov et al. [26]): NaNO 3 + H 2 SO 4 = HNO 3 + NaHSO 4 (1) There are several leaching studies [27][28][29][30][31] that use nitrogen species (e.g., NO 3 − , nitrate, NO 2 − , nitrite, HNO 3 , nitric acid, or HNO 2 , nitrous acid, and NO 2 , nitrogen dioxide) as they increase the oxidization capacity, and enhance kinetics and/or dissolution of ores.Nitrogen species catalyzed (NSC) process uses nitric and sulfuric acid to leach sulfide ores at high temperature and pressure.This process has been successfully used in the mining industry for pressure leaching of copper sulfides [32].
The main advantage of this process is faster reaction rates.Tsogtkhankhai et al. [33] studied leaching copper concentrates using nitric acid, and determined the effects of nitric acid strength, liquid solid ratio, time and temperature on the copper extraction rate.Depending on the activation energy, temperature has a slight effect on the rate of copper extraction from chalcopyrite.The outset of leaching is controlled by an external diffusion regime.As time passes, the dissolution reaction is controlled by an internal diffusion regime due to the formation of a film around the particles.The same authors [34] determined optimal sulfide dissolution conditions for the Cu-Fe-S-N-O-H system in nitric acid at different temperatures (20 to 100 • C) using thermodynamic diagrams.They found that the best conditions were: high initial oxidizing potentials (high concentration of nitric acid), pH lower than 1 and high temperature.Arias [35] patented a hydrometallurgical process of copper sulfide heap leaching using H 2 SO 4 and NaNO 3 at pH ≤ 2, with a sulfide ore/nitrate ratio (by weight) of 1:1.Sodium nitrate was mixed with sulfuric acid and water.Prater and Queneau [36] patented a process for dissolving sulfide ores (copper, iron, cobalt, nickel, silver) using nitric acid which is continuously added during leaching.Iron (in the form of hydrogen jarosite) and sulfur compounds were precipitated; these products are highly filterable and are subsequently removed.It is mentioned that the formation of jarosites, which is followed by drop of pH, may cause losses of valuable ions from solution as a result of co-precipitation [37].
Leaching of copper ores in sulfuric acid with sodium nitrate/nitrite solutions resulted in copper extraction rates of 80-99 wt % [28,29].Vračar et al. [30] leached Cu 2 S at different temperatures, NaNO 3 and H 2 SO 4 concentrations, stirring speeds and solid-liquid ratios.The authors determined that Cu 2 S was leached in two stages, forming CuS, followed by elemental S, according to X-ray diffraction (XRD) analysis of leaching residues.The activation energy for the process was 60 kJ•mol −1 .Ore leaching was a first order reaction and a second order reaction with respect to NaNO 3 and H 2 SO 4 concentrations, respectively.Sokić et al. [28] leached a chalcopyrite concentrate and studied the effect of different temperatures, particle sizes, concentrations of NaNO 3 and H 2 SO 4 and stirring speeds.The authors proposed the following reactions to explain the dissolution of chalcopyrite in acid-nitrate media (Equations (3)-( 5)).These reactions are thermodynamically viable at 25 • C and atmospheric pressure due to the negative value of the Gibbs energy: Minerals 2018, 8, 238 3 of 16 The authors found that the modeled kinetics of the reaction is dominated by a chemical surface reaction, followed by diffusion in the product layer with an activation energy of 83 kJ•mol −1 .The residues produced were unreacted chalcopyrite and elemental sulfur.Gok and Anderson [29] leached chalcopyrite using an acidic solution containing a nitrite salt (Equation ( 6)).They obtained a 5 wt % increase in copper extraction rate by using nitrite instead of nitrate.The overall copper extraction rate was positively affected by the system temperature and the newly formed elemental sulfur in the residue.
This study considers the use of nitrate instead of nitrite because Chile has an industry based in nitrate production.Moreover, nitrite is more expensive than nitrate.
Shiers et al. [38] carried out leaching tests using nitrate as an oxidant in chalcopyrite concentrate and ore at 50 • C. The authors determined the effect of nitrate, ferric nitrate, ferric chloride and ferric sulfate on copper extraction.A mixed chloride-nitrate system was favorable for extracting copper from chalcopyrite while the presence of ferric chloride was also beneficial.
However, the use of nitrate as an oxidant in an industrial process could be a problem in the solvent extraction stage (SX) due to the degradation of oxime-type extractants by nitration (nitration is the degradation of organic compounds caused by the presence of high nitrate concentration in pregnant leaching solution, PLS) [39][40][41].Nowadays, chemical industries have developed nitration resistant reagents to solve this problem [42][43][44].
In this study, seawater and nitrate salts (NaNO 3 , KNO 3 ) were used to investigate the acid dissolution of commercial copper ore that contains 4.8 wt % chalcopyrite (1.6 wt % Cu).The effect of different physical and chemical conditions and variables were studied, including temperature, sulfuric acid and nitrate concentrations, nitrate sources, leaching time and water quality.

Ore Sample
In this study, an ore from the Atacama Region, Chile was used.Table 1 provides the mineralogical and chemical composition of the ore sample.The mineral characterization was carried out by semi-quantitative X-ray diffraction (Siemens/Bruker, Semi-QXRD, model D5000, Germany).The mineralogical data show that chalcopyrite is the only copper mineral present in the ore.Chemical analysis was performed by inductively coupled plasma atomic emission spectroscopy (ICP-AES, ICPE-9000, Shimadzu, Tokyo, Japan).The chemical analysis by atomic absorption spectrometry (AAS, Perkin-Elmer 2380, Perkin Elmer, Wellesley, MA, USA) measured the copper grade as 1.6 wt %.The particle size of the sample (P 80 ) in all tests was 62.5 µm, which was determined using a Microtrac Particle Analyzer (Microtrac S3500, Microtrac, Montgomeryville, PA, USA).

Reagents
Analytical grade H 2 SO 4 , NaNO 3 , KNO 3 and NaCl were used in all leaching tests.Distilled water (referred to as "water" in the text) and seawater, obtained from San Jorge Bay, Antofagasta, Chile, were used to prepare dilute sulfuric acid.The seawater was filtered using quartz sand (50 µm) and a polyethylene membrane (1 µm).The seawater composition is shown in Table 2.One test used a synthetic saline solution composed of distilled water and 35 g•L −1 NaCl.That value was chosen because the salinity of seawater is 3.5%, so all salinity of seawater was considered as NaCl.

Experimental Procedure
The leaching tests used 2 L jacketed glass reactors equipped with lids to prevent evaporative loss (the reactors are not hermetically sealed).Temperature was controlled by a thermostatic bath.Stirring was mechanical at 400 rpm, which provided stable suspension.The solid/liquid ratio used for all tests was 100 g ore in 1 L of solution.1 L of leach solution was prepared by adding sodium nitrate (solid) and sulfuric acid to seawater or water.The tests were performed separately using both seawater and water.The stirring was periodically interrupted to collect 10 mL samples of supernatant solution from the reactors to analyze the copper content using AAS method.The copper extraction over time was calculated by dividing the copper concentration in the solution at time t and the initial copper concentration (1.6 wt %).The evaporation loss and the volume of samples removed during the tests are considered in calculating copper extraction (loss of mass).The redox potential (ORP) and pH were monitored during the leaching time.ORP was measured with a redox electrode (Ag/AgCl reference electrode with 3.5 M KCl as electrolyte) and pH was measured with a glass membrane electrode calibrated to buffers at pH 1 and pH 4. At the end of the leaching time, the pulp was filtered and washed with distilled water and dried at 60 • C. The leaching residues were characterized (Section 2.4).The total copper extracted from solid residues were compared with the leached copper extractions with an average standard deviation of ±2%.
In test 14, a synthetic saline solution was used in the preparation of a H 2 SO 4 (1 M) + NaNO 3 (1 M) leach liquor.The objective of the test was to determine whether NaCl is the only component of seawater that aids higher copper extraction.

Characterization of Ore Residues
The solid residues were characterized by AAS, optical microscopy with reflected light, scanning electron microscopy (SEM-EDX, JEOL 6260 LV, Tokyo, Japan) and semi-quantitative XRD (QXRD) methods.

Effect of Sodium Nitrate and Concentrations
Figure 1 shows copper extraction from the leach liquors using H 2 SO 4 (1 M) with and without NaNO 3 in seawater and water-based media (tests 1 and 2 in Table 3).
It can be seen that the addition of NaNO 3 (1 M) to systems with H 2 SO 4 (1 M), increased the copper extraction from 27.9 to 80.2% in seawater-based media and from 14.8 to 63.9% in water-based media.This improvement was related to the oxidizing potential of nitrate ions.The use of seawater  It can be seen that the addition of NaNO3 (1 M) to systems with H2SO4 (1 M), increased the copper extraction from 27.9 to 80.2% in seawater-based media and from 14.8 to 63.9% in water-based media.This improvement was related to the oxidizing potential of nitrate ions.The use of seawater clearly had a positive effect on these two sets of tests, yielding higher copper extraction rates than with water.
Copper extraction in seawater reached its maximum at about 96 h (4 days).
Figure 2a shows that a concentration of 0.5 M of sodium nitrate in seawater was sufficient to achieve a reasonably good level of copper extraction (77.3%) in 3 days.For the water system, a much higher concentration of nitrate (1 M) was needed to extract only 60.9% of copper in the same period (Figure 2b).This indicates that nitrate performs better with chloride ions during chalcopyrite leaching.Figure 2a shows that a concentration of 0.5 M of sodium nitrate in seawater was sufficient to achieve a reasonably good level of copper extraction (77.3%) in 3 days.For the water system, a much higher concentration of nitrate (1 M) was needed to extract only 60.9% of copper in the same period (Figure 2b).This indicates that nitrate performs better with chloride ions during chalcopyrite leaching.It can be seen that the addition of NaNO3 (1 M) to systems with H2SO4 (1 M), increased the copper extraction from 27.9 to 80.2% in seawater-based media and from 14.8 to 63.9% in water-based media.This improvement was related to the oxidizing potential of nitrate ions.The use of seawater clearly had a positive effect on these two sets of tests, yielding higher copper extraction rates than with water.Copper extraction in seawater reached its maximum at about 96 h (4 days).
Figure 2a shows that a concentration of 0.5 M of sodium nitrate in seawater was sufficient to achieve a reasonably good level of copper extraction (77.3%) in 3 days.For the water system, a much higher concentration of nitrate (1 M) was needed to extract only 60.9% of copper in the same period (Figure 2b).This indicates that nitrate performs better with chloride ions during chalcopyrite leaching.

Effect of the Nitrate Source
Figure 3 shows the effect of the type of nitrate.Similar copper extraction rates were obtained when NaNO3 or KNO3 was used.Copper extraction reached 60% in seawater systems and 48% in water systems (0.5 M H2SO4) in 5 days.Thus, the type of nitrate (NaNO3 or KNO3) does not affect copper extraction, which is consistent with Shiers et al. [38].

Effect of the Nitrate Source
Figure 3 shows the effect of the type of nitrate.Similar copper extraction rates were obtained when NaNO 3 or KNO 3 was used.Copper extraction reached 60% in seawater systems and 48% in water systems (0.5 M H 2 SO 4 ) in 5 days.Thus, the type of nitrate (NaNO 3 or KNO 3 ) does not affect copper extraction, which is consistent with Shiers et al. [38].

Effect of the Nitrate Source
Figure 3 shows the effect of the type of nitrate.Similar copper extraction rates were obtained when NaNO3 or KNO3 was used.Copper extraction reached 60% in seawater systems and 48% in water systems (0.5 M H2SO4) in 5 days.Thus, the type of nitrate (NaNO3 or KNO3) does not affect copper extraction, which is consistent with Shiers et al. [38].

Effect of Sulfuric Acid Concentration
Figure 4a,b show the influence of sulfuric acid concentration.The highest copper extraction rate was obtained at higher concentrations of acid in seawater and water systems.The results indicate that the oxidation power of nitrate ions increases with increased in sulfuric acid concentrations.This observation is in good agreement with the findings of Sokić et al. [28].In this study, copper extraction increased by 50% and 45% when the sulfuric acid concentration increased from 0.25 to 1.0 M in seawater and water systems, respectively.

Effect of Sulfuric Acid Concentration
Figure 4a,b show the influence of sulfuric acid concentration.The highest copper extraction rate was obtained at higher concentrations of acid in seawater and water systems.The results indicate that the oxidation power of nitrate ions increases with increased in sulfuric acid concentrations.This observation is in good agreement with the findings of Sokić et al. [28].In this study, copper extraction increased by 50% and 45% when the sulfuric acid concentration increased from 0.25 to 1.0 M in seawater and water systems, respectively.As Figure 5 shows, copper extraction using 0.5 M of H2SO4 in a seawater-based media is similar to that with 1 M of H2SO4 in a water-based media.The same behavior is observed in the kinetic curves  As Figure 5 shows, copper extraction using 0.5 M of H 2 SO 4 in a seawater-based media is similar to that with 1 M of H 2 SO 4 in a water-based media.The same behavior is observed in the kinetic curves of copper extraction using 0.25 M of H 2 SO 4 in seawater and 0.5 M of H 2 SO 4 in water.This confirms that chloride ions from seawater increase the acid activity of the systems.Puvvada et al. [45] and Senanayake [46] obtained similar results.These authors indicated that the presence of salts such as NaCl, CaCl 2 or MgCl 2 increase acid activity.This shows that a lower acid concentration can be used when seawater is present in the system. of copper extraction using 0.25 M of H2SO4 in seawater and 0.5 M of H2SO4 in water.This confirms that chloride ions from seawater increase the acid activity of the systems.Puvvada et al. [45] and Senanayake [46] obtained similar results.These authors indicated that the presence of salts such as NaCl, CaCl2 or MgCl2 increase acid activity.This shows that a lower acid concentration can be used when seawater is present in the system.

Redox Potential and pH
Figure 6 shows the copper extraction rates and redox potential values obtained during the tests at different concentrations of reagents (1 and 0.5 M of H2SO4 and NaNO3).Because of the high acid concentrations (1 and 0.5 M) used in the tests, all the pH values were < 1.The redox potential (ORP) ranged from 742 mV to 793 mV in seawater and from 739 mV to 789 mV in water when 1 M of sulfuric acid and sodium nitrate were used.When 0.5 M was used, the range of redox potential was 701 mV to 738 mV in seawater and 696 mV to 729 mV in water.In both cases, the redox potential was higher in the seawater system than in water system.This indicates that copper leaching is more effective at higher redox potentials and in a strongly acid medium with seawater.

Redox Potential and pH
Figure 6 shows the copper extraction rates and redox potential values obtained during the tests at different concentrations of reagents (1 and 0.5 M of H 2 SO 4 and NaNO 3 ).Because of the high acid concentrations (1 and 0.5 M) used in the tests, all the pH values were < 1.The redox potential (ORP) ranged from 742 mV to 793 mV in seawater and from 739 mV to 789 mV in water when 1 M of sulfuric acid and sodium nitrate were used.When 0.5 M was used, the range of redox potential was 701 mV to 738 mV in seawater and 696 mV to 729 mV in water.In both cases, the redox potential was higher in the seawater system than in water system.This indicates that copper leaching is more effective at higher redox potentials and in a strongly acid medium with seawater.The highest copper extraction rate of 97.2% was obtained at 70 °C after 3 days of leaching in a seawater media.This high rate of copper extraction was due to the presence of chloride in solution, higher acid and nitrate content, and higher temperature.The results obtained of the effect of the temperature are in agreement with the results reported in the literature [28,30].The highest copper extraction rate of 97.2% was obtained at 70 • C after 3 days of leaching in a seawater media.This high rate of copper extraction was due to the presence of chloride in solution, higher acid and nitrate content, and higher temperature.The results obtained of the effect of the temperature are in agreement with the results reported in the literature [28,30].
Copper extraction was higher in the seawater-based media than in the water-based media independent of temperature.The increase in temperature from 25 to 70 • C resulted in an increase in the copper extraction rate from ≈31% to ≈97% when seawater was used.Copper extraction was higher in the seawater-based media than in the water-based media independent of temperature.The increase in temperature from 25 to 70 °C resulted in an increase in the copper extraction rate from ≈31% to ≈97% when seawater was used.

Effect of Synthetic Saline Solution
Figure 8 compares the kinetic curves of copper extraction in seawater and a synthetic saline solution.The kinetics curves in Figure 8 show that the two extraction rates are similar.The small variations may be related to the presence of SO4 2− ions in the seawater.Other ions in seawater, such as calcium, potassium, magnesium, do not contribute significantly to leaching under the conditions in this work.Figure 8 shows that chloride ions, present in both solvents, helped in the leaching process.

Effect of Synthetic Saline Solution
Figure 8 compares the kinetic curves of copper extraction in seawater and a synthetic saline solution.The kinetics curves in Figure 8 show that the two extraction rates are similar.The small variations may be related to the presence of SO 4 2− ions in the seawater.Other ions in seawater, such as calcium, potassium, magnesium, do not contribute significantly to leaching under the conditions in this work.Figure 8 shows that chloride ions, present in both solvents, helped in the leaching process.

Characterization of Ore and Residue Samples
The residue characterization (test 11) confirms that chalcopyrite was significantly leached in the seawater media.Covellite was found in small quantities, 1% and 3%, in the solid residues of the leaching test in the seawater and water media, respectively (Figure 9).It is supposed that covellite in the solid residue is formed as an intermediate product during chalcopyrite dissolution.

Characterization of Ore and Residue Samples
The residue characterization (test 11) confirms that chalcopyrite was significantly leached in the seawater media.Covellite was found in small quantities, 1% and 3%, in the solid residues of the leaching test in the seawater and water media, respectively (Figure 9).It is supposed that covellite in the solid residue is formed as an intermediate product during chalcopyrite dissolution.
The formation of elemental sulfur as a product of chalcopyrite leaching in both seawater and water-based media was confirmed by SEM-EDS analysis (red region, Figure 10).

Characterization of Ore and Residue Samples
The residue characterization (test 11) confirms that chalcopyrite was significantly leached in the seawater media.Covellite was found in small quantities, 1% and 3%, in the solid residues of the leaching test in the seawater and water media, respectively (Figure 9).It is supposed that covellite in the solid residue is formed as an intermediate product during chalcopyrite dissolution.The formation of elemental sulfur as a product of chalcopyrite leaching in both seawater and water-based media was confirmed by SEM-EDS analysis (red region, Figure 10).

Thermodynamic and Chemical Reactions
Equations ( 7)-( 10) are chemical reactions proposed for the system CuFeS2-NO3 − -H + -Cl − .In the literature, the system CuFeS2-NO3 − -H + was analyzed and respective chemical reactions were provided [28,47], but these studies did not include the presence of chloride ions.

Figure 1 .
Figure 1.Copper extraction in the leach liquors using H 2 SO 4 (1 M) + NaNO 3 (0 or 1 M) in seawater and water based media (at 45 • C and 7 days).

Figure 2 .
Figure 2. Effect of sodium nitrate concentration on copper extraction using H 2 SO 4 (1 M), 3 days of leaching and 45 • C: (a) in seawater and (b) in water.

Figure 2 .
Figure 2. Effect of sodium nitrate concentration on copper extraction using H2SO4 (1 M), 3 days of leaching and 45 °C: (a) in seawater and (b) in water.

Figure 4 .
Figure 4. Effect of sulfuric acid concentration on copper extraction using NaNO3 (1 M), 3 days of leaching and 45 °C: (a) seawater and (b) water.

Figure 4 .
Figure 4. Effect of sulfuric acid concentration on copper extraction using NaNO 3 (1 M), 3 days of leaching and 45 • C: (a) seawater and (b) water.

3. 5 .
Figure7a,b show the effect of temperature.The highest copper extraction rate of 97.2% was obtained at 70 °C after 3 days of leaching in a seawater media.This high rate of copper extraction was due to the presence of chloride in solution, higher acid and nitrate content, and higher temperature.The results obtained of the effect of the temperature are in agreement with the results reported in the literature[28,30].

3. 5 .
Figure7a,b show the effect of temperature.The highest copper extraction rate of 97.2% was obtained at 70 • C after 3 days of leaching in a seawater media.This high rate of copper extraction was due to the presence of chloride in solution, higher acid and nitrate content, and higher temperature.The results obtained of the effect of the temperature are in agreement with the results reported in the literature[28,30].Copper extraction was higher in the seawater-based media than in the water-based media independent of temperature.The increase in temperature from 25 to 70 • C resulted in an increase in the copper extraction rate from ≈31% to ≈97% when seawater was used.

Figure 9 .
Figure 9. Optical microscope images of solid residue after leaching from (a) seawater, and (b) water system.

Figure 10 .
Figure 10.Results of scanning electron microscopy (SEM) analysis carried out using the solid residue.The ore sample was leached in: (a) seawater medium and (b) water medium.The false red color shows the sulfur presence.

Table 2 .
Composition of seawater obtained from San Jorge Bay, Antofagasta, Chile.

Table 3 .
Experimental conditions used in chalcopyrite leaching tests * 0.5 M as KNO 3 .
had a positive effect on these two sets of tests, yielding higher copper extraction rates than with water.Copper extraction in seawater reached its maximum at about 96 h (4 days). clearly