**1. Introduction**

Chilean mining is facing a shortage of water resources, depletion of copper oxide ores and those containing secondary sulfide copper. This depletion will leave chalcopyrite ores as the main source of copper for future plants. To assure sustainable production alternative extractive methods must be developed.

Chalcopyrite (CuFeS2) is the most important copper sulfide mineral, representing approximately 70% of the world's known copper reserves [1]. Chalcopyrite is normally associated with pyrite (FeS2), bornite (Cu5FeS4), sphalerite (ZnS), chalcocite (Cu2S), covellite (CuS), enargite (Cu3AsS4) or molybdenite (MoS2). The extractive metallurgy of chalcopyrite is based largely on a traditional route involving comminution and flotation, smelting and electrorefining, representing around the 85% of the copper production in Chile [2]. Environmental, financial and technical disadvantages of these processes have drawn attention to hydrometallurgy methods as an alternative. The main problem for treating chalcopyrite by a hydrometallurgy route is its refractoriness. This refractoriness is due to the formation of a passivating layer on the surface of chalcopyrite that inhibits the contact of mineral with oxidizing agents, reducing the dissolution rate. It is well known that the dissolution of chalcopyrite is a potential-dependent reaction and many studies have been carried out to elucidate the relationship between the solution potential and this passivating layer [2,3].

Several studies have been carried out in order to increase the dissolution rate of chalcopyrite, including: (i) leaching with strong oxidizing agents [4], (ii) leaching under high pressure and temperature [5], (iii) bacterial leaching [6], (iv) chloride media [2,3,7] and nitrate/nitrite media [8].

There is a considerable body of published information on the dissolution of chalcopyrite in sulfate and chloride media. Most of this literature reports that the dissolution of chalcopyrite is more effective in chloride media due to the greater reactivity of sulfide minerals in this medium. Seawater is then presented as an alternative source of chloride ions [9–13].

Sodium nitrate in an acid medium provides an option for possible leaching of many sulfide minerals, including chalcopyrite, at an acceptable kinetic rate due to the high proton activity created by the presence of sodium nitrate, and a strong oxidizing agent [14]. Sodium nitrate has been used as an oxidizing agent for more than 130 years [15]. However, these applications involve high temperatures and high levels of pressure. The main advantages of using nitrate ions in acidic leaching are: (i) nitrate is a strong oxidizing agent to decompose sulfides (Reaction (1)), (ii) the gases produced (NOx) can easily be scrubbed, and (iii) scrubbed NaNO3 can be recycled [8,16,17]:

$$\rm NO\_3^- + 4H^+ + 3e^- \rightarrow NO\_{(g)} + 2H\_2O \qquad \qquad E^\circ = 0.96\ V \tag{1}$$

Several authors have investigated the possible leaching reactions by considering select elements, ions, solid phases, and gas products [16,18–20]. Soki´c et al. [16] proposed the most thermodynamically favorable overall reaction (Reaction (2)) for chalcopyrite dissolution in the CuFeS2-NaNO3-H2SO4-H2O system:

$$\rm C\_6F\_6S\_2 + 4N\_aNO\_3 + 4H\_2SO\_4 \rightarrow C\_6SO\_4 + F\_cSO\_4 + 2N\_{a2}S\_{O4} + 4NO\_{2(g)} + 2S^\circ + 4H\_2O \tag{2}$$

Reaction (2) is accompanied by several secondary reactions shown by Reactions (3) to (8). Oxidative leaching of a chalcopyrite concentrate in an acid medium using nitrate as an oxidizing agent generates elemental sulfur [16,18,21]. The solution creates a strong oxidizing environment, as shown in Reaction (5):

$$FeSO\_4 + \frac{1}{3}NaNO\_3 + \frac{2}{3}H\_2SO\_4 \rightarrow \frac{1}{2}Fe\_2(SO\_4)\_3 + \frac{1}{6}Na\_2SO\_4 + \frac{1}{3}NO + \frac{2}{3}H\_2O\tag{3}$$

$$\text{CuFeS}\_2 + 2\text{Fe}\_2(\text{SO}\_4)\_3 \to \text{CuSO}\_4 + 5\text{FeSO}\_4 + 2\text{S}^\circ \tag{4}$$

$$2\text{NO}\_{2(g)} + \text{H}\_2\text{O} \to 2\text{HNO}\_3 + \text{NO}\_{(g)}\tag{5}$$

$$\text{NO}\_{(g)} + 2\text{NaNO}\_{3} + \text{H}\_{2}\text{SO}\_{4} \rightarrow 3\text{NO}\_{2(g)} + \text{Na}\_{2}\text{SO}\_{4} + \text{H}\_{2}\text{O} \tag{6}$$

Sulfur, produced in reactions (2) or/and (4) is then oxidized to sulfate in a sodium nitrate medium, according to reactions (7) and (8). At room temperature, anhydrous sodium sulfate crystallizes [22,23], as shown in Reaction (7):

$$\rm{S}^{\circ} + 2NaNO\_3 \rightarrow Na\_2SO\_4 + 2NO\_{(g)}\tag{7}$$

$$\rm{S}^{\circ} + \rm{3NO}\_{2(g)} + H\_2O \to H\_2SO\_4 + \rm{3NO}\_{(g)}\tag{8}$$

The thermodynamic feasibility of these reactions at atmospheric condition at 298 K and 318 K was estimated from standard Gibbs free energy calculations based on HSC Chemistry software V9 as shown in Table 1 (calculated data are based on the thermodynamic values of ΔG◦, ΔS◦, and ΔH◦, of the chemical elements and their compounds). These calculations show that almost all reactions predict negative Gibbs energy values, which clearly indicates they are all thermodynamically favorable under the given conditions, except for Reaction (5). Reaction (2) is more favorable than Reaction (4) under the same conditions.

According to the reactions shown previously the leaching of chalcopyrite in acidic sodium nitrate solution and at high temperature produces notable amounts of gaseous nitrogen oxide gases or NOx. These gases are a source of serious environmental problems, therefore, their treatment is mandatory.


**Table 1.** Standard Gibbs free energies for the CuFeS2-H2SO4-NaNO3-H2O system at 298 K and 318 K (reactions taken Soki´c et al. [16] and values corrected using HSC 9.0).

Suchak and Joshi [24] stated that scrubbing NOx gases is a complex process controlled by mass transfer limitations and involving bulk gas, gas film, interface, liquid film and bulk liquid mechanisms. Other factors that have been reported to affect the overall scrubbing rate and selectivity are temperature, pressure, the composition of NOx gas, and the partial pressures of oxygen and water in the gas phase [25–30]. In the aerated leaching process, the nitrate ion is reduced to nitrogen oxide gas, NO(g), then the NO(g) is oxidized rapidly (in less than 0.1 s) in the gas phase to nitrogen dioxide, NO2(g). Sodium nitrate (NaNO3) and sodium nitrite (NaNO2) can then be regenerated by bubbling nitrous oxide gas in a dilute solution of sodium hydroxide (NaOH), which can then be crystallized and reused in the leaching stage.

The NOx gas released from an acidic leaching contains several nitrogen oxides, mainly NO, NO2, N2O3 and N2O4. The most important reactions are showed from reactions (9)–(11):

$$2NO\_{(g)} + O\_{2(g)} \to 2NO\_{2(g)}\tag{9}$$

$$2NO\_{2(g)} \to N\_2O\_{4(g)}\tag{10}$$

$$NO\_{(\mathfrak{g})} + NO\_{2(\mathfrak{g})} \to N\_2O\_{3(\mathfrak{g})}\tag{11}$$

When NOx is in contact with an alkaline solution of NaOH, it forms nitrite and nitrate ions. The liquid phase reactions are presented from reaction (12) to (14). The nitrogen oxides (NO2, N2O3, and N2O4) are scrubbed in the liquid film phase [31] but the selectivity is poor which leads to the formation of a mixture of nitrite and nitrate ions [29]. According to Suchak et al. [29] chemical reactions (12) to (14) show that NaNO2 is more selective than NaNO3 and NO2(g) captured in the aqueous solution is generally produced through disproportionation reactions:

$$2NO\_{2(g)} + 2NaOH\_{(aq)} \to NaNO\_{3(aq)} + NaNO\_{2(aq)} + H\_2O\_{(aq)}\tag{12}$$

$$\mathrm{NaO\_{4(g)}} + 2\mathrm{NaOH\_{(aq)}} \to \mathrm{NaNO\_{3(aq)}} + \mathrm{NaNO\_{2(aq)}} + \mathrm{H\_2O\_{(aq)}}\tag{13}$$

$$2NaO\_{3(g)} + 2NaOH\_{(aq)} \to 2NaNO\_{2(aq)} + H\_2O\_{(aq)}\tag{14}$$

The oxidation of NO, the formation in the gas phase of N2O3 and N2O4, and the scrubbing of NO2, N2O3 and N2O4 in the liquid phase are all exothermic reactions. Thus, their equilibria shift to the right as temperature decreases. The gas dispersion rate has a significant effect on kinetics, and smaller bubbles provide a better contact with the alkaline scrubbing solution. Table 2 shows several reactions and related enthalpies calculated using HSC Chemistry Software V.9.0 (Outokumpu Research Oy, Helsinki, Finland).

The most effective methods to scrub NOx are using urea + nitric acid [32], using ammonium bisulfate [33] and using sodium hydroxide as the scrubbing solution.

This paper proposes a feasible process to leach chalcopyrite concentrate with a mixture of sodium nitrate and diluted sulfuric acid dissolved in seawater as a source of chloride ions at atmospheric pressure and moderate temperature (≤45 ◦C). Due to its low cost the process includes a scrubbing stage using NaOH to capture NOx gases that could enhance the formation of NaNOx salt (or a mixture of NaNO3 and NaNO2), the NaNOx (oxidants) can be recovered by crystallization and reused again as the leaching process oxidants.


**Table 2.** Thermodynamic data for gas and liquid phase reactions (reactions taken Suchak et al. [15] and values corrected using HSC V.9.0).
