**4. Conclusions**

The leaching efficiencies of metal ions (As, Cu, Zn, Fe, and Pb) and the recovery of gold from the refractory gold concentrate were investigated using a microwave system. As the acid concentration increased, the metal ion leaching increased, but the leach residues for the low nitric acid concentration (> 0.5 M) consisted mainly of untreated pyrite. The S/Fe ratio determines the surface of the leach residue, which affects the efficiency of passive layer removal. S/Fe ratio in nitric acid leaching with a reaction time of 10 min reached 4.34 at 0.1 M and 4.20 at 0.5 M. It was found that a relatively lower proportion of iron was consumed, and the pyrite in the refractory gold concentrate did not significantly dissolve during nitric acid leaching at > 0.5 M.

In the refractory gold concentrate leaching experiments, nitric acid leaching at high-temperature could limit the decomposition of sulfide minerals, due to the passive layer in the refractory gold concentrate. During the leaching process, As, Cu, Fe, and Zn, leaching increased with time, due to the oxidation of insoluble sulfides to soluble sulfate phases. Initially, the As leaching rate was slow, but steadily increased with increased nitric acid concentration. For the refractory gold concentrate, the As, Cu, Fe, and Zn leaching increased from 7.28% to 84.0% (As), 20.8% to 85.8% (Cu), 9.19% to 98.7% (Fe), and 2.46% to 14.9% (Zn), upon increasing the HNO3 concentration from 0.1M to 1.0M (80 ◦C, S/L ration 5, leaching time 15 min). As the temperature increased from 80 to 120 ◦C, the Cu leaching rate constant increased from 0.18 to 0.36 min−<sup>1</sup> and the Fe leaching rate constant increased from 0.21 to 0.66 min−1. However, Pb leaching decreased at > 80 ◦C, due to complex lead and passivation, by increased elemental sulfur formation from the high-temperature oxidation.

Microwave-assisted leaching experiments for gold recovery were conducted for the refractory gold concentrate. More extreme reaction conditions, such as the increase in nitric acid concentration from 1.0 to 5.0 M, facilitated the decomposition of passivation species derived from metal ion dissolution and the liberation of gangue minerals from the sulfide surface. From the comparison between the XRD patterns of the refractory gold concentrate and the leach residues after leaching with different nitric acid concentrations, it can be concluded that pyrite in the sulfide minerals can be destroyed. The SEM-EDS analyses of the leach residue showed that dissolution and decomposition of pyrite in the complex sulfide concentrate leave many vacant areas and microstructures, which can effectively liberate encapsulated gold and improve the recovery of gold.

The recovery rate of gold in the leach residue was improved with microwave-assisted leaching and the gold recovery was about 132.55 g/t after 20 min of the leaching experiment (nitric acid at 2.0 M), according to fire assays. The effect of the increase in nitric acid concentration was consistent with increased exposure to reactive sulfide minerals, which could effectively liberate, encapsulate and improve the gold recovery rate.

The current rapid decline in high-grade gold ores has made the mineral processing industry increasingly reliant on complex and refractory gold ores. At present, several approaches have been employed for mineral processing, including metallurgy and hydro-metallurgy. The leaching process is a method used in hydrometallurgy, which is used to leach base and precious metals from source materials. Particularly, percolation leaching methods, such as Heap leaching, dump leaching, bioleaching and in situ leaching, have been very effective in extracting metals from low grade ores, which could not otherwise be economically extracted. However, the challenge of this facility has been the handling of waste and control of environmental pollution caused by toxic leakages from heaps [26]. In addition, the formation of complex or refractory substances has limited its application. The main reason for this is that the complex or refractory ores require more processing and the various approaches used for gold recovery are challenging to perform in aqueous solutions, owing to surface passivation.

Gold-bearing ores contain pyrite, chalcopyrite, arsenopyrite, and galena, which interfere with gold recovery and lower its efficiency. Therefore, studying the effects of relational minerals on gold is important. Among the alternative processes, the use of microwave additives followed by leaching is important for the recovery of precious metals from complex or refractory minerals. To increase the recovery of gold, the microwave-assisted leaching process may be used as a process to address the formation of complex or refractory sulfide minerals.

**Supplementary Materials:** The following are available online at http://www.mdpi.com/2075-4701/10/5/571/s1, Table S1: The summary of leaching parameters for As, Cu, Zn, and Fe leaching using Equation (2). The leaching conditions were a reaction time of 15 min, reaction temperature of 80 ◦C, and HNO3 concentration of 0.1, 0.5, or 1.0 M. Table S2: The summary of leaching parameter for As, Cu, Zn, and Fe leaching, using Equation (2). The leaching conditions were a reaction temperature between 80 and 120 ◦C, HNO3 concentration of 1.0 M, and reaction time of 15 min.

**Author Contributions:** H.K.: Experiment, Investigation; E.M.: Formal analysis, Visualization; O.P.: Formal analysis; N.C.: Funding acquisition, Conceptualization; C.P.: Project administration, Conceptualization; K.C.: Writing-original draft, Writing-review & editing. All authors have read and agreed to the published version of the manuscript.

**Funding:** This study was supported by the Korea Ministry of Environment (MOE), South Korea, as an Advanced Industrial Technology Development Project (No. 2016000140010).

**Conflicts of Interest:** The authors declare no conflicts of interest.
