Integrated Eco-sustainable System for Alluvial Gold Leaching-Recovery

An Integrated system for alluvial gold leaching-recovery was design to leach gold from alluvial ore as alternative to mercury and cyanide leaching. NaClO is obtained by NaCl 1 % in-situ electrolysis at pH 2 and used as leachate solution. Gold leaching optimization is achieved using a rotary drum reactor fed with the leaching solution, the process takes 6 hours and 95 % of gold recovery is obtained, the remnant gold from the alluvial ore is treated in an stationary reactor with NaClO 200 ppm at pH 2; reaching 99.6 % recovery of the total alluvial gold. This leaching-recovery alluvial system has the potential to replaced mercury and cyanide leaching process.


Introduction
Artisanal gold (alluvial gold) production in Peru is mainly represented by the departments of Puno, Arequipa, Piura and Madre de Dios. In such places, artisanal gold extraction commonly uses mercury for its amalgamation and subsequent elimination by heating, which leads to mercury being found not only in the air, but also in water and soil. 8 High use of mercury in Madre de Dios by formal and informal artisanal mining, is affecting the natural reserves and is causing serious damage to inhabitants health. 9 It is estimated that around 30,000 artisanal miners are in Madre de Dios. 8 In recent decades, research has been conducted to find alternative chemical substances to avoid mercury and / or cyanide used for gold extraction. Thioure 10 , thiocyanate 11 , ammonia 12 and others 13 can be used as an alternatives to cyanide for gold leaching. Additionally, chlorine is a powerful oxidant used for gold leachate during the second half of the century; however, it was replace for cyanidation process due to the low cost. The environment needs an eco-friendly alternative to gold leaching, for this reason, chlorine and hypochlorite are being used an interesting non-polluting proposal 1,14,15 .
Publications of recent years, make reference to the use of chloride-hypochlorite for gold leaching in minerals containing pyrite obtaining AuCl4 − 2-4, 16 . Hypochlorite produced by electrolysis of sodium chloride has been used to leach gold from the mercury amalgam 5,6 .
In this research work an integrated system is designed for the leaching of gold in alluvial ores using sodium hypochlorite in sodium chloride at pH 2 as a leaching solution.

Preparation of the sample
The alluvial ore sample was obtained from the "La Familia" Mining Concession of the lower Puquiri, belonging to Madre de Dios department, Peru. The sample was homogenized by roll and quartering, from which two portions were taken at random, mixed and separated into fractions and storage. The alluvial gold was separated from the alluvial ore sample using a gravimetric table, obtaining alluvial sand as a remnant of the process.

Characterization of the alluvial ore sample
The mineralogical characterization of the alluvial sand sample was made using a Nikon Trinocular Stereomicroscope SMZ-745T. Additionally, a semiquantitative mineralogical analysis of all present minerals (crystalline phases) was performed by X-ray diffraction with a D8 Advance Tube Co-diffractometer (38kV, 25mA): KAlfa1: 6930.48 eV KAlfa2: 1.7891 Å, Filter: Kbeta: Ni.
The alluvial gold separated by gravimetry was isolated by the Fire Assay technique 17 and quantified with an atomic absorption spectrometer using a Perkin Elmer -Analyst 200 Atomic Absorption Spectrometer with a hollow cathode lamp of Au at λ = 242.8 nm. For the analysis of the remaining gold in the alluvial sand, 100 g of sample was weighed, a solid / liquid dilution (S/L) was made in a proportion of 5 to 2. Following this, 300 ppm of NaCN was added and the pH was adjusted to 11 with NaOH 4 mol L -1 . The solution was stirred for 2 hours and then left to stand for 3 days at room temperature and analyzed by atomic absorption spectrophotometry.

Gold corrosion studies by cyclic voltammetry
For electrochemical studies, a commercial gold electrode of 0.08 cm 2 area was used, which was previously treated with concentrated HNO3 for 5 minutes. Then, it was washed with distilled water, and immersed in a H2SO4/H2O2 solution 1:1 v/v, finally it was washed with distilled water.
The commercial gold corrosion studies were performed in a three electrode electrochemical cell. A commercial gold electrode was used as the working electrode, a high surface area graphite electrode was used as the auxiliary electrode, and Ag / AgCl as a reference electrode saturated in KCl 3 mol L -1 . The measurements were made using a Potentiostat-Galvanostat PalmSens. The Na2SO4 0.1 mol L -1 was used as supporting electrolyte, the solution was deoxygenated by N2 bubbling. The measurements were made by cyclic voltammetry in a potential window from 0 to +1.5 V vs Ag / AgCl, at a scanning speed of 100 mV s-1. During the measurements, successive additions of NaCl were made from 1,000 ppm to 10,000 ppm. After this, the effect of the pH was evaluated (pH: 2, 3, 4, 5), adjusting the pH with concentrated HCl. Additionally, the effect of hypochlorite in the leaching solution was studied, 10,000 ppm NaCl was used as the supporting electrolyte, 40,000 ppm of NaClO was added and it was evaluated at different pH values (2,3 and 4).

Integrated system for alluvial gold leaching-recovery
The integrated system for alluvial gold leaching-recovery consists of an electrochemical filter-press cell, which generates chlorine in the form of sodium hypochlorite in situ from sodium chloride electrolysis at pH 2. The electrolyzed solution enters to the rotary drum reactor to leach the alluvial gold, time rotation programmed was 6 hours, after the set time, the leached solution is filtered and sent to the next stage in which it proceeds to gold precipitation by adding dilute ferrous sulfate in a stoichiometric ratio of 2:1 with respect to gold. After 1 hour of resting solution, the metal is separated by filtering and dried it for storage.
In an internal stage of the lixiviation process, the alluvial sand coming from the rotary drum reactor is separated from the leachate, and it is taken to a stationary leaching tank in which it will finish recovering the remaining unleaded gold in the drum, after 24 hours the leachate of the tank stationary arrives to leach the total gold contained in the sand, the solution obtained in this stage can be used to feed the rotary drum reactor electrolyte and process a new load of sand for leaching. A type H cell with cationic membrane separation was used for in situ sodium hypochlorite generation, commercial Ru-Ir / Ti electrode was used as anode and sodium chloride 3% was used as anolyte. The efficiency was determined by the iodometric method and neutralized with sodium thiosulfite. To determine the efficiency, 40 mL of 2 % NaCl was added to the anodic compartment once the oxidation process was finished. Aliquots of 15 mL of a total of 100 mL were taken. It was placed in a flask and 5 mL of glacial acetic acid and 1 g of KI(s) were added. Following this, a titration was performed with Na2S2O3 0.01 N, previously standardized with K2Cr2O7(ac) 0.1 N.
The theoretical production of active chlorine under the same conditions was determined with the following equation: The general equation for electrolysis time setting is:

Eq. 2
Once, the electrolyte is produced in the press-type electrochemical cell, it enters to the rotary drum reactor containing the alluvial ore sample, 1 % NaCl and water, the pH was adjusted to 2 with phosphoric acid. To control the consumption of hypochlorite, the oxidation-reduction potential (ORP) instrument is used. The remaining gold, whose kinetics is slower due mainly to the existence of passivating components in the alluvial ore, it is transferred to a stationary leaching vessel, mixed with a solution of 200 ppm of sodium hypochlorite at pH 2 during 24 hours.
The leached gold was recovered by precipitation using ferrous sulphate at pH 2, with a 3:1 Fe / Au ratio, which represents 200 % of the stoichiometric value.

Alluvial gold and alluvial ore leaching tests
For the alluvial ore leaching tests, the reactor was loaded with 1 L of sodium hypochlorite solution produced in the electrochemical cell, 500.56 mg of alluvial gold was added in 500 g of alluvial sand and the rotating operation was started. During the process samples of the rotary drum reactor were taken every 30 minutes to observe the evolution of the oxidation reduction potential (ORP) of the process. The amount of leached gold is determined by atomic absorption spectrometry. Additionally, an alluvial gold leaching test previously separated from the alluvial sand was carried out, in order to know the influence of the sand on the leaching rate. For this experiment 500 mg of alluvial gold was used.

Physicochemical characterization of the alluvial ore
Alluvial ore mineralogical characterization by Trinocular Stereoscopic Loupe is observed in Figure 2B, the mineralogical characteristics of the possible components are observed in Table 1.  The results of the mineralogical analysis by X-ray diffraction, expressed as relative mass percentage (g / g) of the alluvial sand sample are presented in Table 2. Where it can be seen that the grit contains quartz, zircon, ilmenite, hematite and pyrrhotite. ppm of Au was found and 0.02% of the residual gold present in the alluvial sand. Figure 3 shows the cyclic voltammetries for commercial gold, using Na2SO4 0.1 mol L -1 as supporting electrolyte, aliquots of NaCl were added to vary the concentration in the range of 1 g L -1 to 10 g/L -1 . According to the Pourbaix diagram, the [AuCl4]is the most stable species of soluble gold chloride at high Clconcentrations in the pH range of 0-8 18 .

Gold corrosion studies by cyclic voltammetry
The curves of the Figure 3a show the typical behavior for gold corrosion, the corrosion peak is observed at approximately +1.2 V, it is due to the formation of the Au 3+ ion mainly in its complex form [AuCl4] -. In addition, it is observed that the corrosion current increases with the of NaCl concentration increment. in the presence of chloride ions, aqueous chlorine is formed. All chlorine species are powerful oxidants but HClO is the most effective. Therefore, the pH must be maintained in a range of stability of HClO 20 . Figure 3b shows that at pH 2 the corrosion current highest value is obtained. Figure 3c shows gold cyclic voltammetries in NaCl 10 g L -1 / NaClO 40 g L -1 at different pH values (4,3,2). It can be seen that the current corrosion increases when pH is increased.

Integrated system for alluvial gold leaching-recovery
The integrated system for gold leaching works with chlorine generated in situ. In this work a commercial Ru-Ir / Ti electrode is used for chlorine generation. The efficiency of the commercial Ru-Ir / Ti electrode is obtained by the iodometry method, using type H cell. The results are shown in Table 3. According to the working conditions, the theoretical production of active chlorine is 661.2 ppm; experimentally a production of  The unleached gold is treated in a stationary alluvial mineral leaching container, it takes 24 hours to leach 100% of the gold with a solution of NaClO 200 ppm at pH 2. The potential was controlled by the oxidation-reduction potential (ORP) instrument, at a pH 2 the potential is maintained at a value higher than 1000 mV, the decrease in value is interpreted as the total consumption of the NaClO and the need to add a second dose to complete the leaching. The variation of hypochlorite consumption can be associated to the nature of the sand and clays that accompany the gold, so the need to control the pH and ORP to regulate it throughout the process must be considered.

Alluvial gold and alluvial ore leaching tests
The results of the oxidation reduction potential (ORP) evolution during alluvial gold leaching in the rotary drum can be seen in figure 5. In the absence of NaClO a potential of 539 mV is obtained (time 0), the potential is rises to 1129 mV when NaClO is added and continues stable during 4.50 hours of rotation. Figure 4 shows the evolution curve. In the case of alluvial ore throughout the process, the ORP remained at values above 1000 mV, ensuring the necessary amount of hypochlorite for leaching. Comparing the alluvial gold with the alluvial ore (Figure 4), it can be seen that it is necessary more time in contact of the leaching solution with the sample containing the alluvial sand in comparison to the alluvial gold. The instrument of ORP and pH meter are used to control the precipitation process at < 320 mV and pH 1.2.
The reaction involved in the process is: and the stoichiometric ratio is: 1.41 g FeSO4.7H2O / 1 g Au However, the leached solution still contains a remnant of hypochlorous acid evidenced by the high ORP value. In this case the iron sulphate acts as a reducer according to the following reaction.

Conclusion
The importance of recovery the total gold amount from alluvial ore without using mercury or cyanide is a necessity for miner artisanal and to decrease the impact environment. As an alternative, NaClO can be used as leachate, NaClO 10 000 ppm in NaCl 1 % at pH 2 is the optimal concentration for commercial gold according to corrosion studies by cyclic voltammetry. For the remnant gold in the alluvial ore, an integrated system for alluvial gold leaching-recovery was used to achieve 99.6 % of Au leaching at the end of the process. The proposed system could be used as an environmental friendly alternative to gold cyanidation for extracting Au from low-grade ore.

Acknowledgements
The authors gratefully acknowledge the financial support granted by contract 209-2015

Declarations a) Availability of data and materials
The datasets during and/or analysed during the current study available from the corresponding author on reasonable request.