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Enzymes application in mineral processing is a novel alternative to oxidize carbonaceous matter in double refractory gold ores (DRGOs) and reduce Au(CN) 2 - adsorption during cyanidation. In this work, contrasting flotation concentrates from DRGOs with different Au(CN) 2 - affinities were sequentially treated to improve Au cyanidation. Sequential pre-treatment comprised sulfide decomposition and carbonaceous matter degradation through laccase-mediator system (LMS). In addition, carbonaceous matter was characterized from various aspects using Raman spectroscopy, 3D-fluorescence spectrometry, preg-robbing test, and gas chromatography-mass spectrometry (GC-MS). The characterization of the original ores showed that Au(CN) 2 - adsorption densities were positively correlated with the relative intensities of defective carbon to graphitic carbon obtained in Raman spectra. Degradation of carbonaceous matter through LMS was inhibited by the presence of biopolymers formed during extensive bio-oxidation, thus reconsidering the appropriate sulfide decomposition step. When FeCl 3 leaching to decompose sulfides proceeded enzymatic treatment, mass spectra of extracted intermediates from carbonaceous matter suggest the presence of alcohols, fatty acids and aliphatic hydrocarbon moieties, mainly on the most polar solvent fractions. This suggests that enzymatic degradation went through the indirect disturbance of the aromatic planes, making the surface more hydrophilic due to the inclusion of oxygen-containing functional groups, thus improving Au extraction. Finally, the current findings provide a general panorama for selecting the appropriate pre-treatments to maximize gold extraction on DRGOs when using LMS. [Display omitted] • Raman spectroscopy and Au(CN 2)- adsorption test predict carbon preg-robbing ability. • Mass-spectra similar to benzene aromatic moieties were observed in the high pre-robbing gold concentrate. • Biopolymers produced after bio-oxidation interferes with carbonaceous matter enzymatic degradation. • FeCl 3 leaching before enzymatic treatment is recommended for high preg-robbing gold concentrates. • GC-MS suggested carbonaceous matter surface is more hydrophilic after laccase-mediator treatment. [ABSTRACT FROM AUTHOR]

• Naphthalene sulphonate, as a blanking agent, improved gold extraction by CIL. • Surface negativity of the ore increased after naphthalene sulphonate adsorption. • Ore pre-conditioning resulted in similar NS loading under acidic and alkaline conditions. • NS adsorption increased gold extraction from 57% to 80% in the presence of Ca(OH) 2 - • Naphthalene sulphonate concentration greater than 4.48 mM negatively impacted gold extraction. Naphthalene sulphonate was used as a blanking agent to treat a gold ore with an 87% preg-robbing index with the aim of improving the gold recovery during carbon-in-leach (CIL). The adsorption of naphthalene sulfonate, an anionic surfactant, was significantly higher under acid conditions: 0.51 µmol/g at pH 2.4 and 0.24 µmol/g at pH 11.3. However, conditioning the ore at the expected pH for 30 mins prior to adsorption led to a similar loading of naphthalene sulfonate regardless of the initial solution pH. The gold recovery of the ore was improved from 57% to 80% during CIL, after the ore was treated with 4.48 mM naphthalene sulfonate at pH 11.3 adjusted by 4.36 mM Ca(OH) 2. The main factor, that was likely responsible for the improved gold recovery, was the passivation of the adsorption sites on the carbonaceous matter in the ore. The impact of electrostatic repulsion of Au(CN) 2 - was negligible due to relatively unchanged surface potential before and after naphthalene sulphonate adsorption. [ABSTRACT FROM AUTHOR]

• Crude enzyme from white-rot fungus was applied to double refractory gold ore. • Crude enzyme includes lignin peroxidase and Mn peroxidase. • 92% of gold recovery was achieved by thermophilic acidophile and crude enzyme. • QEMSCAN analysis directly revealed the contribution of the enzyme treatment. Double refractory gold ore was sequentially pretreated to oxidize sulfides by thermophilic archaeon Acidianus brierleyi and then to decompose carbonaceous matters using the cell-free spent medium (CFSM) from white-rot fungus Phanerochaete chrysosporium. The pretreatment by A. brierleyi significantly improved the gold recovery from 25% to 77%. Additionally, the crude lignin-degrading enzymes in the CFSM converted the carbonaceous matters into more easily degradable substances, which were removed by alkaline washing, leading to a final gold recovery of 92%. These mineralogical alterations were confirmed by differential thermogravimetric analysis and quantitative evaluation of minerals with scanning electron microscopy. Based on the results, gold grains were mostly liberated after bio-oxidation of sulfides, and in following CFSM treatment, large particles of carbonaceous aluminosilicate were formed from the aggregation of clay minerals, gold grains and with partially decomposed carbonaceous matters acting as binders. [ABSTRACT FROM AUTHOR]

Highlights • Thermal degradation of a Japanese oil sand was performed using TG-DTA. • Effects of heating rates on thermal degradation were studied. • Three regions of thermal degradation were identified. • Kinetic parameters were calculated. Abstract Thermal degradation characteristics of a Japanese oil sand at different heating rates (10, 20, and 30 °C/min), and 30 ml/min air flow rate have been investigated. The kinetic parameters have been calculated based on three stages of weight loss and/or the conversion of the sample. These include, stage 1 (SI): volatilization of moisture content and the light hydrocarbon (20–227 °C), stage 2 (SII): combustion of heavy hydrocarbon (227–527 °C), and stage 3 (SIII): oxidative decomposition of carbonaceous organic matter (502–877 °C). The results showed that the rate of change of the oil sand conversion with time d α dt was affected by the heating rate. The time taken by the system to reach 0.99 conversion was observed as 85, 50, and 35 min at the heating rates of 10, 20, and 30 °C/min, respectively. The frequency factor, A , at SI was between 0.09 and 0.54 min−1, while the activation energy, E a , was 11.2–12.5 KJmol−1 (the percentage weight loss, W t , was 0–3.6 %w/w; and the conversion, α, was 0–0.2.). At SII, the values of A and E a were 2.1–5.5 min−1 and 17.6–19 KJmol−1, respectively (W t = 3.1–15.88 %w/w; α = 0.17–0.86.). The value of A at SIII was 5.5E11–1.1E13 min−1, while E a was 160–200 KJmol−1 (W t = 15.33–17.99 %w/w; and α = 0.84–0.99). [ABSTRACT FROM AUTHOR]

Carbonaceous matter in refractory gold ore is known to be one of the primary causes of gold recovery loss. Model experiments were conducted to simulate the bio-modification of carbonaceous matter using powdered activated carbon (PAC) as a surrogate and cell-free spent medium (CFSM) of Phanerochaete chrysosporium . The CFSM was used because of the lignin peroxidase and manganese peroxidase secreted by the microbe during its incubation. In the present work, an investigation was conducted to determine the physical and chemical alterations in PAC after enzymatic treatment and its effect on Au(CN) 2 − uptake. Characterization of the solid residues of PAC by 13 C NMR and N 2 adsorption after bio-modification revealed that the treatment had decomposed poly-aromatic carbons into aliphatic carbons and also reduced the specific surface area from 1430 m 2 /g to 697 m 2 /g in 14 days. As a result, Au(CN) 2 − uptake decreased from 100% (0.048 mmol/g) to 43% within 12 h primarily due to the enzyme treatment and adsorption of CFSM components. It further decreased to 26% due to surface passivation by bio-chemicals derived from CFSM and/or decomposed aliphatic hydrocarbons from aromatic carbons between 7 days and 14 days. These findings may contribute to efforts to decrease preg-robbing in hydrometallurgical processing of refractory gold ores. [ABSTRACT FROM AUTHOR]

Cysteine (CSH: C 3 H 7 NO 2 S) was used to leach gold and silver from sulfidic gold ore with a grade of 29 g/t Au and 28 g/t Ag. <20% of the gold was enclosed in sulfides/silicates, and over 70% of the silver was Ag 2 O. The gold extracted by 0.5 M cysteine at 70 °C was 42% in 24 h. When leaching was conducted at an initial pH of 11.5, the leachate was very alkaline, pH 12.5–13.5, and likely caused the partial decomposition of the gold-bearing sulfides, improving Au extraction. The addition of 5:1–2:1 M ratio of cysteine: Na 2 SO 3 significantly improved gold extraction to 71% - 90%, respectively. The sulfite most likely mediated the conversion of cystine (CSSC: C 6 H 12 N 2 O 4 S 2) back to cysteine. Finally, the presence of pyrite, arsenopyrite, and carbonaceous matter in the ore did not significantly impact gold extraction under the current experimental conditions. This finding might change if the undesirable cysteine oxidation could be mitigated, allowing for a lower lixiviant dosage to be used for leaching tests. • Cysteine leaching was conducted using sulfidic ore with 20% enclosed Au in sulfides and silicates and > 70% Ag as Ag 2 O. • 90% Au and 20% Ag were recovered from the ore using 0.5 M cysteine, 70 °C within 24 h. • Gold dissolution occurred when both the thiol and amine groups were deprotonated. • The leachate was highly alkaline, resulting in the partial pyrite decomposition. [ABSTRACT FROM AUTHOR]

Several efforts are made to find various green alternatives for carbonaceous matter (CM) degradation in carbonaceous gold ores. The deleterious effect of CM is observed during cyanidation, where CM adsorbs Au(CN) 2 - behaving like activated carbon, an effect called pre-robbing. Thus, biotechnological approaches like enzyme degradation of CM to reduce Au(CN) 2 - adsorption are currently being studied. In the present work, the potential of CM degradation and gold preg-robbing reduction by laccase-mediator system (LMS) oxidation was evaluated using powder activated carbon (PAC) as CM surrogate material through model experiments. Based upon gas chromatography-mass spectrometry (GC-MS) analysis of extracted by-products from bio-treated PAC, LMS treatment after seven days physically and chemically altered the surface of PAC by the reduction of specific surface area and pore volume, the disruption of aromatic moieties into aliphatic compounds, and the formation of oxygen-containing functional groups. Subsequently, under the studied condition, the adsorption capacity of Au(CN) 2 - on the surface-degraded PAC was dramatically decreased from 46 μmol/g to 7.36 μmol/g. The present findings provide a new outlook of an alternative pre-treatment process of carbonaceous gold ore in mineral processing. [Display omitted] • PAC characteristics were highly modified by LMS after 7 days. • GC-MS revealed biotransformation of aromatic to aliphatic compounds. • LMS promoted the formation of oxygen-containing groups, mostly C O. • Specific surface area and pore volume was reduced by LMS. • Au(CN) 2 − adsorption decreased 84% after LMS and water rinse. [ABSTRACT FROM AUTHOR]

• Silver occurred as electrum, hessite (Ag 2 Te) and other Ag-bearing minerals. • Lignin-degrading enzymes were applied in a carbonaceous Au-Ag bearing concentrate. • ∼100% of silver was recovered after sequential treatment including enzyme process. Sequential treatment was applied to carbonaceous Au-Ag-bearing ore concentrate to maximize the Au and Ag recovery. In the preliminary test, the present carbonaceous ore had well liberated and exposed type of gold grains, which are not refractory, but included mainly three types of Ag presented as electrum, hessite (Ag 2 Te) and Ag-bearing other minerals. Au recovery was ∼100% without any treatment, meanwhile Ag recovery was only 33.3%. The sequential treatment comprises two oxidation steps: (a) mixed culture of iron- and sulfur-oxidizing microorganisms at pH 1.2, followed by (b) cell-free spent medium (CFSM) at pH 4.0 from a white rot-fungus, Phanerochaete chrysosporium, which includes lignin-degrading enzymes. As a result, Ag recovery was 55.5% after the first step and greatly improved to ∼100%, including the dissolved Ag+ concentration in the first step of acid treatment. Although the acidophilic iron-oxidizing microorganisms were inhibited by dissolved Ag+ and Cd2+ ions, the strong acidic conditions dissolved hessite and Ag-bearing oxide minerals. However, the remaining carbonaceous matter acted to sorb Ag(CN) 2 − in cyanidation, causing the recovery loss. In the next step the lignin-degrading enzymes degraded carbonaceous matter in the ore. This step is necessary to avoid the adsorption of Ag(CN) 2 − on graphitic carbonaceous matter, leading a mostly perfect recovery of the remaining Ag in the solid residues, without necessity of alkaline washing. The sequential treatment including enzymatic lignin-degrading process was also effective in carbonaceous silver ore avoiding the emission of air pollutants. [ABSTRACT FROM AUTHOR]

The pretreatment of carbonaceous material in double refractory gold ores (DRGO) is necessary to decrease preg-robbing of gold and maximize gold recovery. DRGO contains of carbonaceous matter and gold grains encapsulated in sulfide minerals, which typically results in very poor gold recovery. However, there is growing interest in DRGO because some estimates show that it makes up about a third of the total available gold for production by mining. This can be achieved by chemical and biological techniques, however, the chemical techniques like flotation, surface passivation and chemical oxidation have received more extensive study and either have to be retooled or modified to be applied to the carbonaceous matter in the DRGO. In comparison, the biological techniques are relatively unknown with significant gaps in the knowledge about the bio-treatment mechanism, byproducts and avenues for scaling up like bioreactor design. This study reviews the enzymatic pretreatment of DRGO to facilitate gold recovery and minimize reagent consumption. It focuses on the potential for application of oxidative enzymes like lignin peroxidase, manganese peroxidase and laccase to pretreat carbonaceous matter in DRGO with or without an additional step of sulfide oxidation and addresses characterization of byproducts of the enzymatic decomposition. Further, potential bioreactor configurations for the enzymatic decomposition without direct contact of ore with microorganisms are considered, both in terms of understanding the mechanisms within the pretreatment and in terms of application. • Sequential biotreatment is a promising biohydrometallurgical process of DRGO. • Graphitic degree of carbonaceous matter affects the reactivity. • Lignin-degrading enzymes can be applied to DRGO to enhance gold recovery. • Optimal pH and temperature of lignolytic and thermophilic bio-oxidative reactions differ • Fungal cultivation should be separated from enzymatic pretreatment of ore. [ABSTRACT FROM AUTHOR]

Natural carbonaceous matter aided the bioleaching of Cu from chalcopyrite concentrates. The oxidative dissolution of chalcopyrite was enhanced more significantly by anthracite than carbonaceous matter in double refractory gold ore (DRGO). This was achieved through Galvanic interactions between the chalcopyrite and natural carbonaceous matter. Measurement of impedance verified that the electro-resistance is smaller in anthracite, which has a greater graphitic degree than carbonaceous matter in DRGO. The electron shuttle between chalcopyrite and the Fe3+ /Fe2+ redox couple was facilitated not only by the amounts of carbonaceous matter but also the degree of graphitization of carbonaceous matter. A higher graphitization degree increased the electron conductivity of the carbonaceous matter to help mediate Cu bioleaching while avoiding direct contact of thermophile cells with refractory copper sulfides. Unlabelled Image • Natural carbonaceous matter aided bioleaching of Cu from chalcopyrite. • Anthracite was more electron conductive than carbonaceous matter in DRGO. • Measurement of impedance verified the relation of graphitic degree with galvanic effect. • Higher graphitization facilitated electron shuttle between chalcopyrite and Fe3+. [ABSTRACT FROM AUTHOR]

Sulfides and carbonaceous matter in double refractory gold ore (DRGO) were bio-treated sequentially using an iron-oxidizing archaeon Acidianus brierleyi followed by lignin peroxidase-dominating crude enzymes released from the white-rot fungus Phanerochaete chrysosporium to significantly improve gold recovery from 24% to 92%. Transformation of the carbonaceous matter in the sequential bio-treatment was interpreted with Quantitative Evaluation of Materials by Scanning Electron Microscopy (QEMSCAN), Raman spectroscopy and three-dimensional fluorescence spectrometry. Firstly, microbiological sulfide oxidation did not affect carbonaceous matter but decreased the arsenic content in the solid residue, facilitating the following enzymatic reaction. Next, the crude enzymes predominantly decomposed the defect-bearing graphitic carbon into humic-like substances. The humic-like substances were not completely soluble under pH 4 but were instead retained in the solid residue as a part of a newly formed carbonaceous aluminosilicate (C–Si–Al) phase. Due to a wide p K a range of humic-like substances, it is proposed that at pH 4, electrostatic interaction between humic substances and illite, with and without heavy metals, might have enabled the agglomeration of fine aluminosilicate particles. Some gold grains trapped in C–Si–Al agglomerates were released by the dissolution of humic-like substances in 1 M NaOH, resulting in a further increase in gold recovery of approximately 15%. Image 1 • Carbonaceous matter was associated with illite in the as-received sample. • Crude enzymes converted the carbonaceous matter into humic-like substances. • Humic-like substances aided in the formation of large aluminosilicate aggregates. [ABSTRACT FROM AUTHOR]

The sequential bio-treatment of refractory carbonaceous gold ore is a promising solution to recover gold effectively by environmentally friendly technology, which includes bio-oxidation of sulfide and biodegradation of carbonaceous matter by lignin-degrading enzymes. There are several drawbacks in enzyme treatment using cell-free spent medium (CFSM), including lignin peroxidase and manganese peroxidase from Phanerochaete chrysosporium, in particular the poor stability of enzyme activities. In the present work, laccase-mediator system (LMS) was applied for the degradation of carbonaceous matter in real gold ore to improve the efficiency of gold extraction as well as handling. The LMS was intended to be a great alternative process of CFSM with utilizing purified laccase in the presence of 1-hydroxybenzotriazole as a mediator. The application of LMS provided several advantages including not only greater stability, greater efficiency to degrade carbonaceous matter, better handling, much saving the treatment time, but also wider availability in laccase. In addition, replacing bio-oxidation with ferric chloride leaching as the dissolution path of sulfides facilitated avoiding the formation of jarosite and saving the required time. The gold recovery by cyanidation was improved from 41.5 ± 0.3% for the starting material to 81.3 ± 3.9% (n = 2) for the solid residues after the modified sequential pretreatment. This is correspondent to 86.3% of gold recovery for the extractable maximum gold excluding the enclosed gold in acid-insoluble silicates. The improved process involving LMS can be proposed with valuable advantages to fit a sustainable metallurgical technology of gold ores. [Display omitted] • Laccase-mediator system (LMS) was applied to real DRGO at the first time. • TG-DTA, Raman, and 3D fluorescence spectrometry supported the efficiency in LMS. • Gold recovery was improved to 86% after LMS treatment. • LMS could be established as an alternative of degradation of carbonaceous matters. • Ferric chloride leaching reduced sulfides without formation of jarosite. [ABSTRACT FROM AUTHOR]

[Display omitted] • Three gold ores with 2.70, 7.08 and 14.09 wt% Fe were leached with TU and ETU. • TU performed better than ETU for gold leaching from ore with the lowest Fe content. • ETU had a comparable gold extraction to cyanide for the most sulfidic gold ore. • Dissolved Au(TU) 2 + complex began decomposing after 12 hrs of leaching the sulfidic gold ores. • The addition of reductant improved Au extraction by TU from the most sulfidic gold ore. Environmentally friendly ligands, alternatives to cyanide, are desired to extract gold for the sustainable hydrometallurgy of gold. Leaching characteristics of gold were examined using thiourea (TU) and ethylene thiourea (ETU) as ligands from three types of gold ores with different Fe contents from 2.70 to 14.09 wt% under the acidic condition. The Au recovery by TU leaching reached the extractable maximum with the lowest Fe-bearing gold ore. This type of gold ore is suitable for TU/ETU leaching. The highest Fe-bearing gold ore was the most difficult to extract Au in ETU/TU leaching. There are at least two detrimental factors in TU leaching of Fe-rich gold ores, that is (i) the oxidative decomposition of TU, and (ii) complexation of TU with Fe3+, which both cause to deduce the complexation of TU with Au+. However, adding Na 2 SO 3 improved the Au extraction from such an ore to reduce Fe3+ to Fe2+. ETU facilitated the formation of more stable complexes with Au than TU against the coexisting Fe3+. This finding is useful when considering the cyanide-free leaching of different types of gold ores. [ABSTRACT FROM AUTHOR]