Effective Gold Recovery from Near-Surface Oxide Zone Using Reductive Microwave Roasting and Magnetic Separation

: High content of gold in near-surface oxide zones above the gold ore deposit could be recovered using cyanidation. However, restricting the use of cyanide in mines has made it difﬁcult to recover gold within the oxide zone. In this study, we investigated an application of the reductive microwave roasting and magnetic separation (RMR-MS) process for the effective gold recovery from ores in a near-surface oxide zone. Ore samples obtained from the near-surface oxide zone in Moisan Gold Mine (Haenam, South Korea) were used in RMR-MS tests for the recovery of iron and gold. The effect of the RMR process on the recovery of iron and gold was evaluated by given various conditions of the microwave irradiation as well as the dosages of reductant and additive. The microwave roasting resulted in a chemical reduction of non-magnetic iron oxide minerals (hematite) to magnetite minerals, such as magnetite and maghemite. This mineral phase change could induce the effective separation of iron minerals from the gangue minerals by magnetic separation process. The increased iron recovery was directly proportional to the gold recovery due to the coexistence of gold with iron minerals. The RMR-MS process could be a promising method for gold recovery from the ores in near-surface oxide zones.


Introduction
Gossans (or iron cap) are highly oxidized and weathered rock, usually associated with the upper and exposed parts of an ore deposit or mineralized vein, and so it was an important guide to find the buried gold ore deposits in the 19th and 20th centuries. Gold is captured during the formation of Fe-oxide, either internally or where concentric structures are produced [1]. In the oxide zone, refractory gold is found together with hematite as coatings or internal cements in goethite, saprolite, laterite, Al-hydroxide, and ilmenite.
The recovery of precious metals such as gold from refractory ores has received considerable attention [2][3][4][5][6]. To achieve a satisfactory recovery, a pretreatment stage is required to break down or at least to modify the matrix and release the precious metals before applying any conventional treatment [7]. A suitable pre-treatment process, such as roasting [8], pressure oxidation [9], pretreated by aqua regia digestion and analyzed using atomic absorption spectrophotometry (AAS) using a Shimadzu AA-7000 (Shimadzu, Kyoto, Japan) from Japan. The major element composition was determined by X-ray fluorescence (XRF) using a Rigaku ZSX Primus II (Rigaku, Tokyo, Japan) from Japan.

Reductive Microwave Roasting (RMR)-Magnetic Separation (MS) Process
The overall processes of RMR-MS are shown in Figure 1. pretreated by aqua regia digestion and analyzed using atomic absorption spectrophotometry (AAS) using a Shimadzu AA-7000 (Shimadzu, Kyoto, Japan) from Japan. The major element composition was determined by X-ray fluorescence (XRF) using a Rigaku ZSX Primus II (Rigaku, Tokyo, Japan) from Japan.

Reductive Microwave Roasting (RMR)-Magnetic Separation (MS) process
The overall processes of RMR-MS are shown in Figure 1.

RMR Process
RMR processing was conducted as a pre-processing to separate the Fe-oxides from the auriferous ore in the near-surface oxide zone. The microwave roasting process, especially controlling irradiation time, could be used to control the mineral phase change of ore sample [29]. For the reduction of hematite, various reducing agents, such as activated carbon, sodium carbonate and charcoal, have been used in the reductive roasting process [30][31][32]. In this study, the pulverized ore sample was mixed with activated carbon and sodium carbonate in a closed crucible (7.5 cm diameter by 25 cm height). Here, the activated carbon was added to the ore sample as a reducing agent. The ore mixture was irradiated with a microwave using a microwave generator. The microwave generator was a batch-type device operating at 2450 MHz with adjustable power of 1-6 kW. The microwave irradiation test was conducted under different conditions of reducing agent (Exps. A and B), microwave power (Exps. C) and irradiation time (Exps. D). Details of the different conditions during the experiment are given in Table 1.

RMR Process
RMR processing was conducted as a pre-processing to separate the Fe-oxides from the auriferous ore in the near-surface oxide zone. The microwave roasting process, especially controlling irradiation time, could be used to control the mineral phase change of ore sample [29]. For the reduction of hematite, various reducing agents, such as activated carbon, sodium carbonate and charcoal, have been used in the reductive roasting process [30][31][32]. In this study, the pulverized ore sample was mixed with activated carbon and sodium carbonate in a closed crucible (7.5 cm diameter by 25 cm height). Here, the activated carbon was added to the ore sample as a reducing agent. The ore mixture was irradiated with a microwave using a microwave generator. The microwave generator was a batch-type device operating at 2450 MHz with adjustable power of 1-6 kW. The microwave irradiation test was conducted under different conditions of reducing agent (Exps. A and B), microwave power (Exps. C) and irradiation time (Exps. D). Details of the different conditions during the experiment are given in Table 1. After the processed sample was cooled sufficiently in ambient conditions, an aggregated ore mixture was obtained. Polished thin sections were prepared using a part of the aggregated samples. The RMR-processed sample was ground using a ball mill with a 10:1 material ratio at a rotational speed 300 r/min for 15 min. Part of the powdered sample was pretreated by aqua regia digestion and analyzed using AAS to determine the chemical content of the RMR-processed samples.

MS Process
Magnetic separation was conducted for the RMR-processed samples after milling the aggregate particles. For a comparison, magnetic separation was also conducted for the raw sample without RMR treatment. The magnetic separation was carried out using a Davis tube magnetic separator (XCGS-73, Changsha Research Institute of Mining and Metallurgy Co., Changsha, China) for 20 min with a magnetic intensity of 4500 G. The weights of the magnetic concentrate and non-magnetic tailings were measured to calculate magnetic fraction.
The chemical composition and iron grade of the magnetic concentrate and non-magnetic tailings were determined through aqua regia digestion and AAS. In order to study the effect of MS process on the magnetic property obtained by the transformation of hematite, the magnetic hysteresis and magnetic intensity were analyzed using Vibration Sample magnetometer (VSM, BHV-50HTI, Riken, Keiki, Japan).

Characteristics of Ore in the Near-Surface Oxide Zone
The major element composition and mineralogical characteristics of the ore sample are presented in Table 2. The ore contains high concentrations of precious metals; 6.4 mg/kg of gold and 35.6 mg/kg of silver, as well as deleterious elements, such as 911.6 mg/kg of As. The composition of the total iron in the ore sample accounted for 28.6 wt. %. Hematite (Fe 2 O 3 ) is the most abundant Fe-oxide mineral and accounts for 64% of the total iron content in the sample. The high concentrations of SiO 2 and Al 2 O 3 were related to the occurrence of quartz and muscovite in the ore sample from the near-surface oxide zone ( Figure 2). Table 2. XRF and AAS analysis for the ore sample from near-surface oxide zone in Moisan Gold deposit (Haenam, South Korea). The contents of the target elements such as Au, Ag, As and total Fe were analyzed using AAS. The mineral compositions were determined XRF analysis. After the processed sample was cooled sufficiently in ambient conditions, an aggregated ore mixture was obtained. Polished thin sections were prepared using a part of the aggregated samples. The RMR-processed sample was ground using a ball mill with a 10:1 material ratio at a rotational speed 300 r/min for 15 min. Part of the powdered sample was pretreated by aqua regia digestion and analyzed using AAS to determine the chemical content of the RMR-processed samples.

MS Process
Magnetic separation was conducted for the RMR-processed samples after milling the aggregate particles. For a comparison, magnetic separation was also conducted for the raw sample without RMR treatment. The magnetic separation was carried out using a Davis tube magnetic separator (XCGS-73, Changsha Research Institute of Mining and Metallurgy Co., Changsha, China) for 20 min with a magnetic intensity of 4500 G. The weights of the magnetic concentrate and non-magnetic tailings were measured to calculate magnetic fraction.
The chemical composition and iron grade of the magnetic concentrate and non-magnetic tailings were determined through aqua regia digestion and AAS. In order to study the effect of MS process on the magnetic property obtained by the transformation of hematite, the magnetic hysteresis and magnetic intensity were analyzed using Vibration Sample magnetometer (VSM, BHV-50HTI, Riken. Keiki, Japan).

Characteristics of Ore in the Near-Surface Oxide Zone
The major element composition and mineralogical characteristics of the ore sample are presented in Table 2. The ore contains high concentrations of precious metals; 6.4 mg/kg of gold and 35.6 mg/kg of silver, as well as deleterious elements, such as 911.6 mg/kg of As. The composition of the total iron in the ore sample accounted for 28.6 wt. %. Hematite (Fe2O3) is the most abundant Feoxide mineral and accounts for 64% of the total iron content in the sample. The high concentrations of SiO2 and Al2O3 were related to the occurrence of quartz and muscovite in the ore sample from the near-surface oxide zone ( Figure 2). Table 2. XRF and AAS analysis for the ore sample from near-surface oxide zone in Moisan Gold deposit (Haenam, South Korea). The contents of the target elements such as Au, Ag, As and total Fe were analyzed using AAS. The mineral compositions were determined XRF analysis.   Polarized light microscopy identified spherical hematite particles associated with cubic pyrite crystals (Figure 3a,b) that formed through the leaching of pyrite by acidic water (i.e., supergene fluid). The acidic water with Fe in solution also formed fine veins and penetrated cubic spaces where it deposited the spherical hematite particles (Figure 3c). The dissolved iron also formed concentric hematite structures within the veins (Figure 3d), which is commonly observed in the oxide zone [33]. Polarized light microscopy identified spherical hematite particles associated with cubic pyrite crystals (Figure 3a,b) that formed through the leaching of pyrite by acidic water (i.e., supergene fluid). The acidic water with Fe in solution also formed fine veins and penetrated cubic spaces where it deposited the spherical hematite particles (Figure 3c). The dissolved iron also formed concentric hematite structures within the veins (Figure 3d), which is commonly observed in the oxide zone [33].

RMR-MS for Recovery of Au Concentrate from Oxide Ore
The raw sample had sufficiently high levels of gold and silver, but a high content of hematite in the Fe-oxide reduced the effectiveness of gold recovery processes, such as flotation. The RMR-MS process was tested as a method for the efficient separation of Fe-oxides from the auriferous ore in the near-surface oxide zone. The effect of the RMR-MS process on the composition of the concentrate was evaluated by comparing with the compositions of the raw and MS-processed samples (Table 3). Here, the RMR was conducted on 50-g of pulverized ore using 75 g of activated carbon and 10 g of sodium carbonate. Microwave irradiation took place at 5 kW for 10 min. RMR-MS-processed ore sample showed increased contents of Au, Ag and Fe. The content of Au increased from 6.4 to 10.8 mg/kg and Ag content from 35.6 to 43.2 mg/kg. The total iron content increased from 28.6 to 41.9 wt. % but the content of Fe2O3 decreased from 26.2% to 2.7%. This indicates that Fe2O3 reduced and changed into magnetic minerals during the microwave roasting process and that the magnetic fraction of the RMR treated ore was successfully separated by the MS process. The ratios of Au/total Fe and Ag/total Fe shows that Au and Ag migrated with the Fe-oxide minerals during the MS process. This suggests that successful separation of Fe-oxides could be an effective method for increasing gold recovery. In addition, as content decreased from 911.6 to 10.5 mg/kg during RMR-MS process by evaporation of As2O3. The noticeable elimination of the penalty element could increase the value of the ore concentrate. The raw sample and magnetic fraction showed similar ratios of Au/Fe2O3 and Ag/Fe2O3. This supports the concept that gold and silver in hematite are effectively recovered by the magnetic separation after mineral phase change of hematite to magnetic minerals. Table 3. Composition of Au, Ag, As, Fe and hematite (Fe2O3) in the raw ore sample and RMR-MS processed samples. a RMR was conducted on 50 g of pulverized ore using 75 g of activated carbon and 10 g of sodium carbonate. Microwave irradiation took place at 5 kW for 10 min.

RMR-MS for Recovery of Au Concentrate from Oxide Ore
The raw sample had sufficiently high levels of gold and silver, but a high content of hematite in the Fe-oxide reduced the effectiveness of gold recovery processes, such as flotation. The RMR-MS process was tested as a method for the efficient separation of Fe-oxides from the auriferous ore in the near-surface oxide zone. The effect of the RMR-MS process on the composition of the concentrate was evaluated by comparing with the compositions of the raw and MS-processed samples (Table 3). Here, the RMR was conducted on 50-g of pulverized ore using 75 g of activated carbon and 10 g of sodium carbonate. Microwave irradiation took place at 5 kW for 10 min. RMR-MS-processed ore sample showed increased contents of Au, Ag and Fe. The content of Au increased from 6.4 to 10.8 mg/kg and Ag content from 35.6 to 43.2 mg/kg. The total iron content increased from 28.6 to 41.9 wt. % but the content of Fe 2 O 3 decreased from 26.2% to 2.7%. This indicates that Fe 2 O 3 reduced and changed into magnetic minerals during the microwave roasting process and that the magnetic fraction of the RMR treated ore was successfully separated by the MS process. The ratios of Au/total Fe and Ag/total Fe shows that Au and Ag migrated with the Fe-oxide minerals during the MS process. This suggests that successful separation of Fe-oxides could be an effective method for increasing gold recovery. In addition, as content decreased from 911.6 to 10.5 mg/kg during RMR-MS process by evaporation of As 2 O 3 . The noticeable elimination of the penalty element could increase the value of the ore concentrate. The raw sample and magnetic fraction showed similar ratios of Au/Fe 2 O 3 and Ag/Fe 2 O 3 . This supports the concept that gold and silver in hematite are effectively recovered by the magnetic separation after mineral phase change of hematite to magnetic minerals.
The mineral phase change was observed in the image of reflected light microphotograph for the RMR-MS-processed sample. Figure 4a shows a core-rim structure with hematite along the boundary and magnetite making up the core of the mineral. This structure could be generated through a number of processes: (1) the mineralogical transformation of Fe-oxide minerals from the interior site by energy focusing; (2) by the uniform distribution of temperature through the mineral with lower stresses at the grain boundaries [22]; or (3) during the microwave sintering of pyrite by the dissolution re-precipitation mechanism [3]. The mineral phase change was observed in the image of reflected light microphotograph for the RMR-MS-processed sample. Figure 4a shows a core-rim structure with hematite along the boundary and magnetite making up the core of the mineral. This structure could be generated through a number of processes: (1) the mineralogical transformation of Fe-oxide minerals from the interior site by energy focusing; (2) by the uniform distribution of temperature through the mineral with lower stresses at the grain boundaries [22]; or (3) during the microwave sintering of pyrite by the dissolution re-precipitation mechanism [3]. The XRD pattern for the magnetic fraction after RMR shows the generation of magnetic minerals such as maghemite (γ-Fe2O3), which has ferromagnetic properties, and magnetite (Figure 3b). Maghemite can be produced along with magnetite by the reduction of hematite at high temperature, with oxidation upon contact with air during cooling [1]. The increase in the magnetization of the roasted sample is illustrated by comparing the magnetic hysteresis loops of the raw and roasted samples ( Figure 5). The curves show that the saturation magnetization significantly increased from 2.74 to 26.91 emu/g after reduction roasting. The reduction of hematite using electric furnaces reported an increase in saturation magnetization intensities (Ms) from 9 to 37 emu/g by the complete conversion of hematite to magnetite [34]. Good magnetic separation could be achieved after roasting since the residual magnetism (Mr) increased from 0.5 to 3.9 emu/g [34].  The XRD pattern for the magnetic fraction after RMR shows the generation of magnetic minerals such as maghemite (γ-Fe 2 O 3 ), which has ferromagnetic properties, and magnetite ( Figure 3b). Maghemite can be produced along with magnetite by the reduction of hematite at high temperature, with oxidation upon contact with air during cooling [1]. The increase in the magnetization of the roasted sample is illustrated by comparing the magnetic hysteresis loops of the raw and roasted samples ( Figure 5). The curves show that the saturation magnetization significantly increased from 2.74 to 26.91 emu/g after reduction roasting. The reduction of hematite using electric furnaces reported an increase in saturation magnetization intensities (Ms) from 9 to 37 emu/g by the complete conversion of hematite to magnetite [34]. Good magnetic separation could be achieved after roasting since the residual magnetism (Mr) increased from 0.5 to 3.9 emu/g [34]. The mineral phase change was observed in the image of reflected light microphotograph for the RMR-MS-processed sample. Figure 4a shows a core-rim structure with hematite along the boundary and magnetite making up the core of the mineral. This structure could be generated through a number of processes: (1) the mineralogical transformation of Fe-oxide minerals from the interior site by energy focusing; (2) by the uniform distribution of temperature through the mineral with lower stresses at the grain boundaries [22]; or (3) during the microwave sintering of pyrite by the dissolution re-precipitation mechanism [3]. The XRD pattern for the magnetic fraction after RMR shows the generation of magnetic minerals such as maghemite (γ-Fe2O3), which has ferromagnetic properties, and magnetite (Figure 3b). Maghemite can be produced along with magnetite by the reduction of hematite at high temperature, with oxidation upon contact with air during cooling [1]. The increase in the magnetization of the roasted sample is illustrated by comparing the magnetic hysteresis loops of the raw and roasted samples ( Figure 5). The curves show that the saturation magnetization significantly increased from 2.74 to 26.91 emu/g after reduction roasting. The reduction of hematite using electric furnaces reported an increase in saturation magnetization intensities (Ms) from 9 to 37 emu/g by the complete conversion of hematite to magnetite [34]. Good magnetic separation could be achieved after roasting since the residual magnetism (Mr) increased from 0.5 to 3.9 emu/g [34].

Effects of RMR Conditions on Iron Recovery
The effective recovery of gold from the near-surface oxide zone using RMR and magnetic separation (MS), requires the efficient reduction of hematite. The RMR-MS tests were conducted under different conditions of reducing agents (contents of activated carbon and sodium carbonate) and microwave operation (microwave power and irradiation time), to optimize the RMR process for gold recovery. In the following, the effects of the operation conditions on the recovery of iron minerals coexisting with gold were investigated to better understand the gold recovery. Figure 6 shows the results of RMR experiments that were conducted using different concentrations of activated carbon and sodium carbonate. Hematite was not sufficiently reduced through microwave irradiation without activated carbon, which clearly illustrates that a reducing agent, such as activated carbon is essential for the reduction of hematite by microwave roasting (Figure 6a). The results show an increase in grade and recovery of iron associated with an increase in the concentration of activated carbon, but a rapid decrease in both indices when the carbon-to-ore ratio is less than 1.5. This may be explained by the incomplete reduction of hematite when the activated carbon is insufficient, and an over-reduction of hematite to ferrous oxide when activated carbon is excessive. This illustrates the important effect of the ratio of activated carbon-to-iron-oxide minerals on iron recovery. The optimal ratio of activated carbon-to-ore was determined to be 1.5. At this ratio, the activated carbon acted as a microwave susceptor, as well as a reducing agent [4,35]. When activated carbon is irradiated with microwaves, CO 2 and CO gases are generated sequentially by heat (Equation (1)) and by Boudouard's reaction (Equation (2)), respectively [25,36]. The generated CO can reduce hematite to magnetite by the reaction in Equation (3) [37].

Effects of RMR Conditions on Iron Recovery
The effective recovery of gold from the near-surface oxide zone using RMR and magnetic separation (MS), requires the efficient reduction of hematite. The RMR-MS tests were conducted under different conditions of reducing agents (contents of activated carbon and sodium carbonate) and microwave operation (microwave power and irradiation time), to optimize the RMR process for gold recovery. In the following, the effects of the operation conditions on the recovery of iron minerals coexisting with gold were investigated to better understand the gold recovery. Figure 6 shows the results of RMR experiments that were conducted using different concentrations of activated carbon and sodium carbonate. Hematite was not sufficiently reduced through microwave irradiation without activated carbon, which clearly illustrates that a reducing agent, such as activated carbon is essential for the reduction of hematite by microwave roasting (Figure 6a). The results show an increase in grade and recovery of iron associated with an increase in the concentration of activated carbon, but a rapid decrease in both indices when the carbon-to-ore ratio is less than 1.5. This may be explained by the incomplete reduction of hematite when the activated carbon is insufficient, and an over-reduction of hematite to ferrous oxide when activated carbon is excessive. This illustrates the important effect of the ratio of activated carbon-to-iron-oxide minerals on iron recovery. The optimal ratio of activated carbon-to-ore was determined to be 1.5. At this ratio, the activated carbon acted as a microwave susceptor, as well as a reducing agent [4,35]. When activated carbon is irradiated with microwaves, CO2 and CO gases are generated sequentially by heat (Equation (1)) and by Boudouard's reaction (Equation (2)), respectively [25,36]. The generated CO can reduce hematite to magnetite by the reaction in Equation (3) [37].  Na 2 CO 3 was added along with the activated carbon to increase the reduction of ore during the RMR process [38]. In our study, different amounts of Na 2 CO 3 were added for microwave roasting. Figure 6b shows the grade and recovery of iron as a function of the Na 2 CO 3 dosage. The maximum recovery of iron increased up to 77% by adding 20% of sodium carbonate and an additional supply of CO 2 , which is essential for the reductive roasting of hematite (Equation 2). Na 2 CO 3 melts at 874 • C and produces CO 2 ( Na 2 CO 3 → Na 2 O + CO 2 ). Therefore, the production of CO 2 gas by the addition of Na 2 CO 3 facilitated the reduction of hematite to magnetite [39,40]. This, in turn, improves the efficiency of magnetic separation [5,41,42]. Figure 6a,b shows reduced iron recovery at high levels of activated carbon and sodium carbonate. This can be explained by the reduction process of hematite. According to the principle of Baykova, reduction of Fe-oxides, such as hematite, takes place in three steps: [43]. Due to the non-magnetic properties of FeO, the generation of FeO by high levels of reduction inhibits the efficient recovery of iron. Figure 6c shows the increase of gold content by increasing dosage of activated carbon, but the addition of sodium carbonate did not show any significant change in gold content (Figure 6d).

Effect of Microwave Power and Irradiation Time
The microwave process was optimized through different operation conditions of the microwaves in the RMR for ore from the near-surface oxide zone. Figure 7a,c shows that both grade and recovery of iron increased rapidly when the microwave power increased from 3 to 5 kW, but decreased when the microwave power increased to 6 kW. In Figure 7b,d, the grade and recovery of iron and gold increased with increasing irradiation time, but prolonged irradiation inhibited the efficient recovery of iron and gold. This could be due to over-reduced hematite to weakly magnetic ferrous oxide, such as FeO. In this RMR condition, 5 kW of microwave power and 10 min of irradiation are the optimum conditions for the effective recovery of iron and gold. Na2CO3 was added along with the activated carbon to increase the reduction of ore during the RMR process [38]. In our study, different amounts of Na2CO3 were added for microwave roasting. Figure 6b shows the grade and recovery of iron as a function of the Na2CO3 dosage. The maximum recovery of iron increased up to 77% by adding 20% of sodium carbonate and an additional supply of CO2, which is essential for the reductive roasting of hematite (Equation 2). Na2CO3 melts at 874 °C and produces CO2 (Na CO → Na O CO ). Therefore, the production of CO2 gas by the addition of Na2CO3 facilitated the reduction of hematite to magnetite [39,40]. This, in turn, improves the efficiency of magnetic separation [5,41,42]. Figure 6a,b shows reduced iron recovery at high levels of activated carbon and sodium carbonate. This can be explained by the reduction process of hematite. According to the principle of Baykova, reduction of Fe-oxides, such as hematite, takes place in three steps: Fe2O3 → Fe3O4 → FeO → Fe [43]. Due to the non-magnetic properties of FeO, the generation of FeO by high levels of reduction inhibits the efficient recovery of iron. Figure 6c shows the increase of gold content by increasing dosage of activated carbon, but the addition of sodium carbonate did not show any significant change in gold content (Figure 6d).

Effect of Microwave Power and Irradiation Time
The microwave process was optimized through different operation conditions of the microwaves in the RMR for ore from the near-surface oxide zone. Figure 7a,c shows that both grade and recovery of iron increased rapidly when the microwave power increased from 3 to 5 kW, but decreased when the microwave power increased to 6 kW. In Figure 7b,d, the grade and recovery of iron and gold increased with increasing irradiation time, but prolonged irradiation inhibited the efficient recovery of iron and gold. This could be due to over-reduced hematite to weakly magnetic ferrous oxide, such as FeO. In this RMR condition, 5 kW of microwave power and 10 min of irradiation are the optimum conditions for the effective recovery of iron and gold.

Au Concentration during RMR-MS
RMR-MS-processed samples showed an increase in iron grade and gold contents. The enhancements of the gold content and iron grade, which were obtained through RMR-MS test under various conditions of RMR, are shown together in Figure 8. The result shows that the gold content increase is closely related to the increase of the iron grade. This is because gold and silver show the same behaviors with iron at the gossan and the oxidation zone [44]. This suggests that the separation of iron-bearing minerals obtained through reduction of oxide minerals is applicable as an effective method for gold recovery from the ores in near-surface oxide zone.

Au Concentration during RMR-MS
RMR-MS-processed samples showed an increase in iron grade and gold contents. The enhancements of the gold content and iron grade, which were obtained through RMR-MS test under various conditions of RMR, are shown together in Figure 8. The result shows that the gold content increase is closely related to the increase of the iron grade. This is because gold and silver show the same behaviors with iron at the gossan and the oxidation zone [44]. This suggests that the separation of iron-bearing minerals obtained through reduction of oxide minerals is applicable as an effective method for gold recovery from the ores in near-surface oxide zone.

Conclusions
This research focused on the recovery of high-grade Au associated with a near-surface oxide zone through RMR-MS. The main mineral phases in the ore sample from the near-surface oxide zone are hematite, quartz, and muscovite. Microwave irradiation enhanced the recovery of iron through magnetic separation by transforming hematite to magnetic iron minerals, such as magnetite and maghemite. Gold and silver that coexist with hematite are also recovered efficiently by this operation. The microwave treatment also removed arsenic, a deleterious element. Reductive microwave roasting needs to be optimized because the over-reduction of hematite could generate weakly magnetic ferrous minerals, which inhibit the recovery of iron. Efficient iron recovery (which could also be an indicator of Au recovery), is influenced by the microwave process conditions, as well as the reducing agents, such as activated carbon and sodium carbonate. The RMR-MS process is a promising method to extract gold from a near-surface oxide zone. This process allows for most of the gold in a near-surface oxide zone to be extracted during mining.