Recovery of Gold from Ore with Potassium Ferrocyanide Solution Under UV Light

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Article Recovery of Gold from Ore with Potassium Ferrocyanide Solution under UV Light

Ziyuan Liu 1 , Jue Kou 1,*, Yi Xing 2 and Chunbao Sun 1

1 School of Civil and Resource Engineering, University of Science and Technology, Beijing 100083, China; [email protected] (Z.L.); [email protected] (C.S.) 2 School of Energy and Environmental Engineering, University of Science and Technology, Beijing 100083, China; [email protected] * Correspondence: [email protected]

Abstract: In this study, potassium ferrocyanide, a nontoxic cyanide precursor in dark and diffuse reﬂection environment, was applied as reagent for the leaching of gold. The free cyanide ions could gradually release from potassium ferrocyanide solution under the ultraviolet light. Orthogonal leaching experiments were performed in gold ore to analyze the effect of solution pH, potassium ferrocyanide dosage, and temperature in a potassium ferrocyanide solution system under UV light. Response surface methodology (RSM) was applied to explore the role of potassium ferrocyanide in gold leaching; optimized results showed that the gold recovery reached 67.74% in a high-alkaline environment at a 12.6 pH, 3.8 kg/t potassium ferrocyanide dosage, 62 ◦C, and irradiance of 10 mW·cm−2. The gold leaching kinetics were monitored by quartz crystal microbalance with dissipation (QCM-D) of potassium ferrocyanide solution. The results indicate that the gold extraction process could be divided into two stages: adsorption and leaching, and a rigid adsorption layer formed on the reaction surface. Furthermore, X-ray photoelectron spectroscopy (XPS) analysis of the gold sensor surface after leaching reaction showed that –C≡N appears on the gold sensor surface, and the gold is Citation: Liu, Z.; Kou, J.; Xing, Y.; oxidized to form AuCN complexes. Sun, C. Recovery of Gold from Ore with Potassium Ferrocyanide Keywords: gold leaching; potassium ferrocyanide; UV light; QCM-D; AuCN complexes Solution under UV Light. Minerals 2021, 11, 387. https://doi.org/ 10.3390/min11040387 1. Introduction Academic Editor: Brajendra Mishra The hydrometallurgy is a main method of gold extraction. The leaching process as an important part of hydrometallurgy can be classiﬁed into three species depending on Received: 1 March 2021 the different leaching conditions: under acid phase (pH < 3) with chlorine, thiourea [1], Accepted: 2 April 2021 Published: 5 April 2021 ferric chloride, and thiocyanate [2]; under neutral phase (pH within 5–9) with halogens [3] and thiosulfate [4]; and under the alkaline phase (pH > 10) with cyanide [5], sodium

Publisher’s Note: MDPI stays neutral sulfide, ammonia [6], ammonia-cyanide, nitriles, and sulfur. Among them, thiosulfate with regard to jurisdictional claims in leaching is the most potential method of gold leaching due to the low toxicity and high published maps and institutional affil- efficiency. The U.S. Bureau of Mines uses statistical experimental methods to determine iations. the feasibility of thiosulfate leaching [7]. Up to 62% gold extraction rate can be obtained from the thiosulfate leaching of low-grade carbonaceous ore, and it has been successfully applied at Goldstrike mine, Nevada, of Barrick Gold Corp [8]. In addition, glycine leaching has the advantages of a simple process, stability in a wide Eh-pH range, easy recovery and nontoxicity, which attracted the interest of researchers. A new study shows the recovery of Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. gold from oxidized ore in glycine alkaline solution is 85.1% under the condition of strong This article is an open access article oxidation of potassium permanganate [9]. However, the basic phase cyanide is still the most distributed under the terms and widely used in industrial applications because of its mature technology, high selectivity, conditions of the Creative Commons handy operation, and stable complexation to gold [5]. Meanwhile, disadvantages of cyanide 2+ 2− Attribution (CC BY) license (https:// leaching include long leaching time and poor adaptability towards Cu and S [7]. creativecommons.org/licenses/by/ Cyanide can be deadly in high concentrations, posing a serious health threat to a wide 4.0/). range of ecological entities [10]. With this limitation in mind, a nontoxic cyanide precursor,

Minerals 2021, 11, 387. https://doi.org/10.3390/min11040387 https://www.mdpi.com/journal/minerals Minerals 2021, 11, 387 2 of 15

potassium ferrocyanide (PF), has been investigated. It could release free cyanide ions under the ultraviolet light but have the high stability of ferrous cyanide complex in diffuse light or in the darkness (acute toxicity test, LD50 = 1600–3200 mg/kg) [11]. PF solid is a product of the coal chemical industry, and it is considered a nontoxic strong metal-cyanide complex, widely used in the pharmaceutical, chemical, and food industries [12,13]. Ferrous cyanogen ions have ionic activity in solutions. Already in 1952 [14], the stability of PF solution had been tested, and the results showed that it could decompose free cyanide ions under ultraviolet light, but a reverse reaction occurred after interrupting the illumination. The photodissociation of PF includes photo-oxidation and photo-aquation for ferrous hexacyanide in aqueous solution [15]. At present, photocatalytic treatment of ferrous cyanide complex in wastewater is one of the motivations for the utilization of PF. LA Betancourt-Buitrago [16] described a method of complexed cyanide recovery from synthetic mining wastewater by means of anoxic photocatalytic treatment using TiO2 and scavengers. It demonstrated that the photocatalytic decomposition of the ferricyanide was carried out by the photoreduction of the metal complex. Sergio Hanela [17] introduced a treatment system by UV ozone and modiﬁed zeolite to remove ferricyanide complex from wastewater, which achieved effective removal rate of iron (55%) and cyanide (68%). The above are the applications of photocatalytics in PF solution to eliminate cyanide, but there have been few reports of applying dissociation of cyanide ions for the recovery of gold from gold ore. Recently, Weida D. Chen [18] demonstrated that a cumulative light sensor made of gold, which was a photochemical device to convert light signals into electrical signals and convert photons into free cyanide ions in the presence of PF solution. It inspired us to apply free cyanide ions released by PF solution to extract gold from gold ore. It could make reutilization of ferrous cyanide complex in wastewater and avoid the highly toxic dangers of cyanide in transport and preservation. Although the price of PF (1800 $/ton) is slightly higher than that of cyanide (1600 $/ton), PF solid has greater advantages in terms of transportation cost and safety cost due to the strong toxicity of cyanide. This paper provides a novel hydrometallurgical method for gold extraction with PF solution irradiated by UV light.

2. Experimental 2.1. Materials and Reagents Gold concentrates from the ﬂotation process at SQS (ShuangQiShan) Mining Industry, Quanzhou City, Fujian Province, China, were used as test samples for the experiments. The main chemical components of the test sample are shown in Table1. The main valuable metals are gold and silver, with a grade of 56.78 g/t and 38.8 g/t, respectively. The X-ray powder diffraction (XRD) analysis was performed on a Rigaku-RA high power rotating anode X-ray diffractometer with Cu-Kα radiation (40 kV, 100 mA) at a scanning rate of ◦ ◦ ◦ 10 min, from 10 to 90 . The main mineral components are pyrite (FeS2), quartz (SiO2), muscovite (KAl2(AlSi3O10)(OH)2), and gismondine (CaAl2Si2O8·4(H2O)), as shown in Figure1.

Table 1. Main chemical components of the test sample (%).

Au * Ag * Fe Cu As Zn 56.78 38.8 26.03 0.11 0.082 0.044

Pb S CaO MgO Al2O3 SiO2 0.10 26.64 1.27 0.95 6.69 33.51 * Unit is g/t. Minerals 2021, 11, 387 3 of 15

Minerals 2021, 11, x FOR PEER REVIEW 3 of 15 Minerals 2021, 11, x FOR PEER REVIEW 3 of 15 Minerals 2021, 11, x FOR PEER REVIEW 3 of 15 A-Pyrite 10,00010,000 A A-Pyrite A A-PyriteB-Quartz 10,000 B A B-Quartz B B-QuartzC-Muscovite 8,000 B C-Muscovite 8000 C-MuscoviteD-GismondineD-Gismondine 8,000 D-Gismondine A 60006,000 A 6,000 A

4,000 A

Intensity 4000

Intensity A A 4,000 A A A Intensity A A 2,000 B AD 2000 B AD A A A D A A A 2,000 D B D D B D D D B D C A CD B D D B DA A B C C D D B A 0 D D B D D B 0 C C D B 0 1010 20 20 30 30 40 40 50 50 60 60 70 70 10 20 30 40 50 60 70 22θθ/(°)/(°) 2θ/(°)

FigureFigure 1.1. XRDXRD patternspatterns ofof thethe sample.sample. Figure 1. XRD patterns of the sample. AllAll experimentsexperiments werewere performedperformed withwith analyticalanalytical gradegrade reagents reagents and and deionized deionized water. water. TheThe solidAllsolid experiments PF PF trihydrate trihydrate were (K (K Fe(CN)4performedFe(CN)·6·3H3H with2O,O, ≥≥ analytical99.5%)and99.5%)and grade sodiumsodium reagents hydroxidehydroxide and deionized (NaOH, (NaOH,≥ ≥ water.99.5%)99.5%) The solid PF trihydrate (K44Fe(CN)66·3H22O, ≥99.5%)and sodium hydroxide (NaOH, ≥99.5%)

Thewerewere solid purchasedpurchased PF trihydrate fromfrom XiLong XiLong(K4Fe(CN) SCIENTIFICSCIENTIFIC6·3H2O, ≥99.5%)and Co Co.,Co.,., Ltd Ltd (Shantou(Shantousodium hydroxide city, city, GuangdongGuangdong (NaOH, province,province, ≥99.5%) China). The chemical structure of PF is shown in Figure2. The PF solution was prepared wereChina).China). purchased TheThe chemicalchemical from structureXiLongstructure SCIENTIFIC ofof PFPF isis shownshown Co., inin Ltd FigureFigure (Shantou 2.2. TheThe city, PFPF solutionGuangdongsolution waswas province, preparedprepared by dissolving a known number of particles into deionized water, and its solubility curve is China).byby dissolvingdissolving The chemical aa knownknown structure numbernumber of ofof PF particlesparticles is shown intointo in deionized deionizedFigure 2. The water,water, PF andsolutionand itsits solubilitysolubility was prepared curvecurve shown in Figure3. The pH of the leaching system was adjusted by using 0.1 mol/L NaOH byisis shownshowndissolving inin FigureFigure a known 3.3. TheThenumber pHpH ofof theparticlesthe leachingleaching into system systemdeionized waswas water, adjustedadjusted and bybyits usingsolubilityusing 0.10.1 mol/Lcurvemol/L solution and measured by using a HQ30d-phc10103 (Hach Company, Loveland, CO, USA) isNaOHNaOH shown solutionsolution in Figure andand 3. measuredmeasured The pH ofbyby the usingusing leaching aa HQ30d-phc10103HQ30d-phc10103 system was adjusted (Hach(Hach Company,Company, by using Loveland,0.1Loveland, mol/L portable pH meter. NaOHCO,CO, USA)USA) solution portableportable and pH pHmeasured meter.meter. by using a HQ30d-phc10103 (Hach Company, Loveland, CO, USA) portable pH meter.

Figure 2. Chemical structure of PF. Figure 2. Chemical structure of PF. Figure 2. Chemical structure of PF. 100100 100 9090 90 80 K4Fe(CN)6 80 K4Fe(CN)6 80 K Fe(CN) 7070 4 6 70 6060 6050 50 50 4040 40 3030 30 2020 20 1010 Solubility (g per 100g water) Solubility Solubility (g per 100g water) Solubility 100 Solubility (g per 100g water) Solubility 0 0 102030405060708090100 0 0 102030405060708090100 0 102030405060708090100 TemperatureTemperature (℃)(℃) Temperature (℃) FigureFigure 3.3. SolubilitySolubility ofof PFPF vs.vs. temperaturetemperature [19][19].. Figure 3. SolubilitySolubility of of PF PF vs. vs. temperature temperature [19] [19]..

Minerals 20212021,, 11,, x 387 FOR PEER REVIEW 4 4of of 15 15 Minerals 2021, 11, x FOR PEER REVIEW 4 of 15 2.2. Light source 2.2. Light source The photochemical system BL-GHX produced by BILON Co., Ltd (Shanghai, China) was2.2.The Lightshown photochemical Source in Figure 4. system Illumination BL-GHX source produced was provided by BILON by Co., a UVA Ltd (Shanghai,high pressure China) mer- was shown in Figure 4. Illumination source was provided by a UVA high pressure mercury lampThe photochemical with a peak emission system at BL-GHX 365 nm. produced The spectral by BILONdistribution Co., Ltdand (Shanghai, relative intensity China) cury lamp with a peak emission at 365 nm. The spectral distribution and relative intensity ofwas the shown high-pressure in Figure mercury4. Illumination lamp were source shown was providedin Table 2. by For a UVA the photochemical high pressure mercuryreaction of the high-pressure mercury lamp were shown in Table 2. For the photochemical reaction experiments,lamp with a peaka 100 emissionmL quartz at test 365 tube nm. The(diameter spectral of distribution30 mm, height and of relative190 mm) intensity was used of experiments, a 100 mL quartz test tube (diameter of 30 mm, height of 190 mm) was used asthe the high-pressure photoreactor, mercury which was lamp placed were shown5 cm under in Table the2 .lamp. For the In photochemicaladdition, the system reaction is as the photoreactor, which was placed 5 cm under the lamp. In addition, the system is equippedexperiments, with a a 100 condensation mL quartz testcycle tube to cool (diameter the ultraviolet of 30 mm, light height source of 190 and mm) adjust was the used re- equippedactionas the temperature. photoreactor, with a condensation The which irradiation was cycle placed to intensity cool 5 cmthe of underultraviolet incident the lamp. lightlight enteringsource In addition, and PF adjustsolution the system the is read- is actionjustedequipped temperature. by changing with a condensationThe the irradiationcurrent of cycle theintensity high-pressure to cool of theincident ultraviolet mercury light entering lightlampsource in PFthe solution reactor. and adjust isBefore ad- the justedthereaction experiments, by changing temperature. the the irradiancecurrent The irradiation of correspondingthe high-pressure intensity to of themercury incident current lightlamp value enteringin theof systemreactor. PF solution was Before cor-is therectedadjusted experiments, quantitatively by changing the irradiance by the using current UVA-365corresponding of the high-pressure radiom toeter; the thecurrent mercury calibration value lamp ofresults in system the reactor. are was shown Beforecor- in rectedFigurethe experiments, quantitatively 5. the by irradiance using UVA-365 corresponding radiom toeter; the the current calibration value of results system are was shown corrected in Figurequantitatively 5. by using UVA-365 radiometer; the calibration results are shown in Figure5.

FigureFigure 4. 4. DiagramDiagram of of photochemical photochemical reaction reaction device. device. Figure 4. Diagram of photochemical reaction device. TableTable 2. Spectral distribution and relative in intensitytensity of high-pressure mercury lamp. Table 2. Spectral distribution and relative intensity of high-pressure mercury lamp. Wavelength/nmWavelength/nm 250 313 250 365 313 365 400 400 510 510 620 620 720 720 Wavelength/nm 250 313 365 400 510 620 720 RelativeRelative intensity/% intensity/% 20 85 20 100 85 100 30 30 20 20 40 40 80 80 Relative intensity/% 20 85 100 30 20 40 80

25 (Measured value) 25 (Measured (Linear fit value) of measured value) (Linear fit of measured value) 20

− 2 20 -2 15 15

10 10

Irradiance/mW·cm 5 Irradiance/mW·cm 0 0 024681012 024681012 Current/A Current/A FigureFigure 5. 5. IrradianceIrradiance vs. vs. current current value value of of device. device. Figure 5. Irradiance vs. current value of device. 2.3. Gold Ore Leaching Experimental Design The gold leaching effect of the PF solution was investigated by a magnetic stirring system. The leaching tests were carried out in a 100 mL quartz test tube and the stirring

Minerals 2021, 11, 387 5 of 15

frequency was set at 400 rpm. For each individual leaching test, 30 g of sample was leached for 24 h at a constant solid/liquid ratio of 1:3. In order to determine the optimal process parameters of gold leaching in PF solution under ultraviolet light, analysis experiments with three-factors and three-levels were designed using response surface methodology (RSM). All tests were carried out under ultraviolet irradiance of 10 mW·cm−2 high pressure mercury lamp. The results were analyzed using the Design Expert software. The experimental factors included initial pH (X1), PF dosage (X2), and leaching temperature ◦ (X3). As shown in Table3, the ranges of X 1,X2, and X3 were 11–13, 1–5 kg/t, and 45–75 C, respectively, with central values of 12, 3 kg/t, and 60 ◦C, respectively. To avoid systematic errors, the sequence of experiments was set randomly. The obtained results were calculated by a quadratic equation to evaluate the coefﬁcient of determination (R2), and statistically analyzed by analysis of variance (ANOVA).

Table 3. Selected ranges for coded and actual values of the independent variables.

◦ Levels X1: pH X2: Dosage (kg/t) X3: Temperature ( C) −1 11 1 45 0 12 3 60 1 13 5 75

2.4. Quartz Crystal Microbalance with Dissipation (QCM-D) QCM-D is a real-time measuring instrument for surface reaction analysis. The main element of the QCM-D is a piezoelectric AT-cut quartz crystal between the metal film electrodes [20,21]. The quartz crystal produces mechanical deformation if an electric field is applied in the direction of the crystal polarization. Therefore, alternating currents can cause mechanical vibration on the quartz crystal, and crystal resonance occurs when the current frequency is the same as the crystal natural frequency [21]. The resonant frequency (∆f ) of the quartz crystal varies with the mass (∆m) of the metal film electrode [22]. Through measuring changes in energy dissipation (∆D), the surface property changes of quartz crystal become known. The Sauerbrey equation (Equation (1)) can describe the relationship between ∆m and ∆f [23]. ρqtq C∆ f ∆m = − ∆ f = − (1) f0n n

where ρq is the density of the quartz crystal used as sensor; tq is the thickness of the quartz crystal; and n is the harmonic number, which is equal to 1, 3, 5, 7 ... (when n = 1, f 0 = 5 MHz)[24]. The seventh overtone data was used in the plots for this study because it presented the lowest signal-to-noise ratio. The resonant frequency of the quartz crystal resonator changes linearly with the surface mass of the quartz crystal, and it decreases during the leaching process. The relationship is applicable to the corrosion process on the surface of coated quartz resonators [25]. Its mass sensitivity is 0.9 ng/cm2 in water [26]. The QCM-D measurement equipment was the Q-Sense E4 system (Biolin Scientific, Gothenburg, Sweden), shown in Figure6. This system consists of three main parts: the chamber platform, which holds the sensor crystal modules; the electronics unit, which controls the sensor; and the software for data analysis [27]. The AT-Cut quartz crystal sensor was prepared with gold electrodes of 100 nm thickness as coat for the quartz crystals and a 5 nm chromium coat placed in the intermediate for enhanced adhesiveness. The fundamental frequency of the quartz crystal sensor was 5 MHz, as the f 0 value mentioned above. Before the test, the sensors were cleaned with deionized water and dried with nitrogen. Limited by equipment conditions, the temperature was set at 30 ◦C which is the highest value of temperature-controlled chamber, and the flow rate of the leaching reagent was set to 50 µL/min in all experiments, unless otherwise specified. Minerals 2021, 11, x FOR PEER REVIEW 6 of 15

Minerals 2021, 11, 387 6 of 15

Minerals 2021, 11, x FOR PEER REVIEW 6 of 15

Figure 6. QCM-D instrumental setup.

The AT-Cut quartz crystal sensor was prepared with gold electrodes of 100 nm thickness as coat for the quartz crystals and a 5 nm chromium coat placed in the intermediate for enhanced adhesiveness. The fundamental frequency of the quartz crystal sensor was 5 MHz, as the f0 value mentioned above. Before the test, the sensors were cleaned with deionized water and dried with nitrogen. Limited by equipment conditions, the temperature was set at 30 °C which is the highest value of temperature-controlled chamber, and the flow rate of the leaching reagent was set to 50 μL/min in all experiments, unless otherwise specified.

3. Results and Discussion FigureFigure 6. QCM-D6. QCM-D instrumental instrumental setup. setup. 3.1. Decomposition of PF Solution Under UV Light 3. Results and Discussion 3.1. DecompositionInThe order AT-Cut to ofinvestigate PF quartz Solution crystal underthe dependence UV sensor Light was of prepar free cyanideed with iongold decomposition electrodes of 100 on nmthe thick-con- centrationnessIn as order coat of to PFfor investigate and the quartzirradiation the dependencecrystals time, and the of a free fi5rst nm cyanide experiment chromium ion decomposition was coat carried placed on outin the the by intermediatecomparing theconcentrationfor different enhanced of concentration PF adhesiveness. and irradiation of time, ThPFe thesolutionfundamental ﬁrst experiment at 0–100 frequency was mmol/L carried of outwithin th bye quartz comparing 1 h. crystalThe results sensor were was the different concentration of PF solution at 0–100 mmol/L within 1 h. The results were shown5 MHz, in asFigure the f7.0 value In addition, mentioned the effects above. of Before irradiation the te intensityst, the sensors on PF dissociationwere cleaned were with observedshown in Figure by changing7. In addition, the the current effects ofas irradiation shown in intensity Figure on 8. PF In dissociation the assay were part, silver nitrate observeddeionized by changingwater and the currentdried with as shown nitrogen. in Figure Li8mited. In the by assay equipment part, silver conditions, nitrate the temper- titrationtitrationature was was was selectedset selected at 30 to °C determine to which determine free is the cyanide free highest cyan concentrations valueide concentrations of intemperature-controlled aqueous solutions. in aqueous solutions. chamber, and the flow rate of the leaching reagent was set to 50 μL/min in all experiments, unless oth-

erwise300 specified. 5 min 250 200 ]

15min 1 − 3. Results250 and Discussion 30min 150

/mg·L 3.1. Decomposition of PF Solution 45min Under- UV100 Light 50 60min [CN C 200 1 In order to investigate the dependence0 of free cyanide ion decomposition on the con- − centration of PF and irradiation time, the fi012345rst experiment was carried out by comparing − C [K Fe(CN) /mmol·L 1] the different150 concentration of PF solution at 0–1004 6mmol/L within 1 h. The results were ]/mg·L

shown- in Figure 7. In addition, the effects of irradiation intensity on PF dissociation were observed100 by changing the current as shown in Figure 8. In the assay part, silver nitrate titration was selected to determine free cyanide concentrations in aqueous solutions. C[CN

50 300 5 min 250 200 ]

15min 1

0 − 250 30min 150

/mg·L 0 20406080100 45min - 100 50 60min [CN C 200 −1 1 0 − C[K4Fe(CN)6 ] /mmol·L 012345 − C [K Fe(CN) /mmol·L 1] Figure 7.150Number of free cyanide ions release in various concentrations4 of PF6 at time stages (0–60 min) under ultraviolet irradiance of 10 mW·cm−2 high pressure mercury lamp (buffered at pH 12). ]/mg·L

- 100

C[CN 50

0 20406080100 −1 C[K4Fe(CN)6 ] /mmol·L

Minerals 2021, 11, x FOR PEER REVIEW 7 of 15

Figure 7. Number of free cyanide ions release in various concentrations of PF at time stages (0–60 min) under ultraviolet irradiance of 10 Mw·cm−2 high pressure mercury lamp (buffered at pH 12).

For better comparison the effects of different reagent concentrations on cyanide ion release comprehensively, 10 mL solution were extracted every 15 min to detect its free cyanide ion concentration. The concentration of the free cyanide ion in solution is zero without ultraviolet irradiation, which is consistent with previous reports that the solution of PF is stable under dark or diffuse reflectance conditions. At different time intervals, PF undergoes a sequential photolysis reaction under ultraviolet irradiation for a long time, gradually releasing free cyanogen ions. A novel phenomenon is that the free cyanide ion has saturation value in the range of 1–5 mmol/L and decreases when exceeding the concentration range. In order to determine the maximum concentration of free cyanide ions released from the solution, the interval between the concentration ranges was narrowed, as shown in Figure 7. We found that the release concentration of free cyanide ion in solution is the highest when the concentration of PF is 2.5 mmol/L at different time intervals, Minerals 2021, 11, 387 which shows that there is saturated PF concentration of free cyanide 7ion of 15 decomposition under UV light. At this point, the conversion rate of cyanide ions is 65.28%.

−2 2 mW·cm 300 − 6 mW·cm 2 − 10 mW·cm 2 − 250 14 mW·cm 2 − 18 mW·cm 2 ] 1 − 200 /mg·L - 150 CN [ C 100

1.0 1.5 2.0 2.5 3.0 3.5 4.0 −1 C [K Fe(CN) /mmol·L ] 4 6

Figure 8. Five different irradiation intensities which were assessed, all with the same reaction time of Figure 8. Five different irradiation intensities which were assessed, all with the same reaction time 30 min (buffered at pH 12). of 30 min (buffered at pH 12). For better comparison the effects of different reagent concentrations on cyanide ion releaseThe comprehensively, effect of illumination 10 mL solution intensity were on extracted the release every 15of minfree to cyanide detect its ions free from PF solu- tioncyanide was ion investigated. concentration. The The concentrationinitial solution of the of free different cyanide concentration ion in solution is reacted zero for 30 min without ultraviolet irradiation, which is consistent with previous reports that the solution −2 underof PF is ultraviolet stable under light dark orin diffuse the irradiance reﬂectance range conditions. of 2–18 At different mW·cm time. The intervals, determination PF of free cyanideundergoes ions a sequential in the solution photolysis was reaction shown under in ultravioletFigure 8. irradiationThe release for of a longcyanide time, ions increases undergradually every releasing concentration free cyanogen condition ions. A novelwith phenomenonthe increase is in that illumination the free cyanide power, which is dueion has to saturationthe increase value of in photon the range release of 1–5 mmol/Lunder high and decreases power conditions when exceeding promoting the more de- compositionconcentration range. of PF In molecules. order to determine In addition, the maximum with concentrationthe increase of of free irradiance, cyanide ions the gradient of released from the solution, the interval between the concentration ranges was narrowed, as theshown increase in Figure of7 free. We foundcyanide that ions the release in solution concentration slowed of down free cyanide gradually, ion in solution but the concentration ofis thethe highest maximum when therelease concentration of free ofcyanide PF is 2.5 ion mmol/L did not at different change time with intervals, the change which of irradiance. shows that there is saturated PF concentration of free cyanide ion decomposition under 3.2.UV light.Box-Behnken At this point, Design the conversionand Response rate ofSurface cyanide Methodology ions is 65.28%. The effect of illumination intensity on the release of free cyanide ions from PF solution was investigated.The influence The of initial initial solution pH, ofPF different dosage, concentration leaching temperature reacted for 30 and min underany two-way inter- actionsultraviolet between light in them the irradiance on the rangeextraction of 2–18 rate mW of·cm gold−2. Theunder determination ultraviolet of light free were investi- gatedcyanide using ions in a themultiple solution linear was shown regression in Figure model.8. The release The experimental of cyanide ions increasesscheme was run for 17 under every concentration condition with the increase in illumination power, which is due combinations, and the results are shown in Table 4. All experiments were performed in to the increase of photon release under high power conditions promoting more decomposition of PF molecules. In addition, with the increase of irradiance, the gradient of the increase of free cyanide ions in solution slowed down gradually, but the concentration of the maximum release of free cyanide ion did not change with the change of irradiance.

3.2. Box-Behnken Design and Response Surface Methodology

The inﬂuence of initial pH, PF dosage, leaching temperature and any two-way interactions between them on the extraction rate of gold under ultraviolet light were investigated using a multiple linear regression model. The experimental scheme was run for 17 combinations, and the results are shown in Table4. All experiments were performed in triplicate. Minerals 2021, 11, 387 8 of 15

Fitting of the experimental data by regression analysis led to a second-order polynomial equation model (Equation (2)), as follows:

2 2 2 Y = 64.21 + 5.17X1 + 6.58X2 + 8.54X3 + 1.72X1X2 + 0.088X1X3 + 1.23X2X3 − 4.36X1 − 13.26X2 − 7.14X3 (2)

where Y was the predicted gold extraction rate; and X1,X2, and X3 were the code values of pH, PF dosage, and leaching temperature, respectively.

Table 4. Independent variables and their levels for the Box-Behnken design.

Factors Response Run ◦ X1: pH X2: Dosage (kg/t) X3: Temperature ( C) Y: Au Extraction (%) 1 11 5 60 44.17 2 12 1 45 29.38 3 12 3 60 63.19 4 13 3 45 47.69 5 12 3 60 62.49 6 13 3 75 64.42 7 12 5 75 60.73 8 11 3 75 57.56 9 12 5 45 40.65 10 12 3 60 65.13 11 12 3 60 64.95 12 13 1 60 45.58 13 11 3 45 41.18 14 12 1 75 44.52 15 11 1 60 35.01 16 13 5 60 61.61 17 12 3 60 65.30

Table5 shows the significance of the equation fitting, which was tested by ANOVA. The model yielded a coefficient of determination R2 of 0.9851 and the adjusted determination coefficient was 0.9660, which indicates that the model presented high significance. The F-value obtained from the F-test was 51.48, which implies that the model was ade- quate. The P-value was used to check the significance of the variables, and it reflected the interaction strength between each independent variable. The smaller the P-value, the more significant the corresponding variable [28]. The lack of fit F-value of 5.67 implies that the lack of fit was insignificant and the correlation between the variable and process response was sufficient.

Table 5. ANOVA of the ﬁtted models for gold extraction.

Degree of Adjusted Mean P Value Source Sum of Squares F Value Freedom Square Probability > F Model 2289.29 9 254.37 51.48 <0.0001 X1 214.04 1 214.04 43.32 0.0003 X2 346.77 1 346.77 70.19 <0.0001 X3 583.62 1 583.62 118.13 <0.0001 X1X2 11.8 1 11.80 2.39 0.1662 X1X3 0.031 1 0.031 0.0062 0.9394 X2X3 6.1 1 6.10 1.23 0.3032 2 X1 80.17 1 80.17 16.23 0.005 2 X2 739.88 1 739.88 149.76 <0.0001 2 X3 214.41 1 214.41 43.40 0.0003 Lack of Fit 28 3 9.33 5.67 0.0634 R2 = 98.51%, R2 (adj) = 96.60%; signiﬁcant at 95% conﬁdence degree (p < 0.05). Minerals 2021, 11, 387 9 of 15

The F-value was used to examine the statistical signiﬁcance, and the model was evaluated for a P-value with a 95% conﬁdence level. In the model, the values of P > F

Minerals 2021, 11, x FOR PEER REVIEW(<0.0001) indicated that the model terms were significant and the chance of model F-value9 of 15 noise was 0.01% (<5%). As shown in Table5, the P-values for X 2 and X3 were significantly X22 739.88 lower than 5%, indicating 1 that the PF dosage 739.88 and temperature149.76 were more significant<0.0001 for 2 X3 214.41 the gold extraction 1 rate than the other variables. 214.41 In addition, the 43.40 P-value for X0.00031 was 0.0003, Lack of Fit 28 which suggested that3 the interaction strength 9.33 to the gold extraction 5.67 was strong. 0.0634 These three 2 2 R = 98.51%,components R (adj) (X= 96.60%;1,X2, and significant X3) played at 95% a significantconfidence degree role in (P the < 0.05). gold extraction. The interactions between these three components and their optimal level for gold The interactions between these three components and their optimal level for gold extraction were further analyzed by RSM. The three-dimensional response surface pre- extraction were further analyzed by RSM. The three-dimensional response surface presented by the Design-Expert 8.0 describes the interaction between the dual experimental sented by the Design-Expert 8.0 describes the interaction between the dual experimental factors and the extraction rate, whereas the other factor was held at the central level, as factors and the extraction rate, whereas the other factor was held at the central level, as shown in Figure9. All plots show relative peaks within the range of variables. The optimal shown in Figure 9. All plots show relative peaks within the range of variables. The optimal extraction rate was determined based on the economic considerations and operational extraction rate was determined based on the economic considerations and operational dif- difficulty. Optimized results of RSM showed that, at a pH of 12.6, 3.8 kg/t PF dosage, and ficulty. Optimized results of RSM showed that, at a pH of 12.6, 3.8 kg/t PF dosage, and 62 62 ◦C, the optimal extraction of 67.74% was achieved. There was no clear extraction rate im- °C, the optimal extraction of 67.74% was achieved. There was no clear extraction rate im- provement when the pH and leaching temperature continuously increased. However, the provement when the pH and leaching temperature continuously increased. However, the reagent dosage had an optimal range, and the leaching effect deteriorates when the dosage reagentis excessive. dosage At had this an point, optimal the amount range, and of PF the can leaching be converted effect deteriorates to a molar concentration when the dos- of age2.64 is mmol/L, excessive. which At this is consistentpoint, the amount with the of release PF can law be converted of cyanide to ion a molar under concentration UV light. of 2.64 mmol/L, which is consistent with the release law of cyanide ion under UV light. Y: Au extraction(%) 5.00 70 50 (a) 60 4.00 50

40 60 3.00 5 30

X2: Dosage(kg/t) 2.00 Y:Au extraction(%)

50 5.00 13.00 40 4.00 12.50 1.00 3.00 12.00 11.00 11.50 12.00 12.50 13.00 X2: Dosage(kg/t) 2.00 11.50 X1: pH 1.00 11.00 X1: pH Y: Au extraction(%) (b) 75.00 70

60 69.00

50 63.00 40 5 30 60 57.00 20 Y: Au extraction(%) extraction(%) Au Y: X3: Temperature(℃) 51.00 50 75.00 13.00 40 50 69.00 12.50 45.00 63.00 12.00 57.00 11.00 11.50 12.00 12.50 13.00 X3: Temperature(℃) 51.00 11.50 X1: pH 45.00 11.00 X1: pH Figure 9. Cont.

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Minerals 2021, 11, x FOR PEER REVIEW 10 of 15

Y: Au extraction(%) (c) 75.00 70

60 69.00 50

40 63.00 5 30 57.00 20 60 Y: Auextraction(%) X3: Temperature(℃) 51.00 50 40 75.00 5.00 69.00 4.00 45.00 63.00 3.00 1.00 2.00 3.00 4.00 5.00 57.00 X3: Temperature(℃) 51.00 2.00 X2: Dosage(kg/t) 45.00 1.00 X2: Dosage(kg/t)

Figure 9. Response surface plots, corresponding contour plots, and interactions between pH (X1), dosage (X2), and Figure 9. Response surface plots, corresponding contour plots, and interactions between pH (X1), dosage (X2), and tem- temperature (X ) for Au extraction under ultraviolet radiation of 10 mW·cm−2 high pressure mercury lamp. (a) pH vs. perature (X3) for3 Au extraction under ultraviolet radiation of 10 mW·cm−2 high pressure mercury lamp. (a) pH vs. dosage dosage(b) pH (vs.b) pHtemperature( vs. temperature(c) dosagec) dosagevs. temperature vs. temperature.

InIn orderorder toto verifyverify thethe reliabilityreliability ofof RSMRSM optimization,optimization, threethree setssets ofof parallelparallel leachingleaching teststests werewere conductedconducted underunder thethe aboveabove optimaloptimal conditions.conditions. ComparisonComparison resultsresults ofof thethe testtest and predicted values are shown in Table6. The gold leaching extraction was 67.01%, 67.47% and predicted values are shown in Table 6. The gold leaching extraction was 67.01%, and 67.33% respectively. The average gold leaching extraction is 67.27%, which shows 67.47% and 67.33% respectively. The average gold leaching extraction is 67.27%, which that the experimental value is basically consistent with the predicted value and the error shows that the experimental value is basically consistent with the predicted value and the is only 0.69%. Additionally, the residual amount of cyanide ion in the pregnant solution error is only 0.69%. Additionally, the residual amount of cyanide ion in the pregnant so- was determined. As shown in the Table6, the mean value of the free cyanide ion in the lution was determined. As shown in the Table 6, the mean value of the free cyanide ion in PF pregnant solution was 18.5 mg/L, which was lower than the free cyanide ion in the the PF pregnant solution was 18.5 mg/L, which was lower than the free cyanide ion in the cyanide pregnant solution when the gold extraction was similar. The general regulatory cyanide pregnant solution when the gold extraction was similar. The general regulatory threshold for cyanide emissions from mineral processing operation is 0.2 mg/L [29], and threshold for cyanide emissions from mineral processing operation is 0.2 mg/L [29], and therefore the PF leaching solution still requires treatment before discharge. therefore the PF leaching solution still requires treatment before discharge. Table 6. Comparison of gold extraction predicted value and test value. Table 6. Comparison of gold extraction predicted value and test value. Dosage Temperature Cyanide Ion Gold Extraction Reagent pHDosage Temperature Cyanide Ion Gold Extraction Reagent pH (kg/t) (◦C) (mg/L) (Test Value %) (kg/t) (℃) (mg/L) (Test Value %) PFPF 12.6 12.63.8 3.8 62 62 19.1 19.1 67.01 67.01 PF 12.6 3.8 62 17.9 67.47 PF 12.6 3.8 62 17.9 67.47 PF 12.6 3.8 62 18.4 67.33 PF 12.6 3.8 62 18.4 67.33 Cyanide 12.6 2 62 84.9 68.02 Cyanide 12.6 2 62 84.9 68.02

3.3.3.3. KineticKinetic ProcessProcess ofof GoldGold LeachingLeaching onon AuAu SensorSensor withwith PFPF byby QCM-DQCM-D TheThe kineticskinetics ofof goldgold leachingleaching forfor differentdifferent concentrationsconcentrations ofof PFPF solutionsolution underunder UVUV irradiation of 10 mW·cm−2 on the Au sensor were determined by QCM-D. The real-time irradiation of 10 mW·cm−2 on the Au sensor were determined by QCM-D. The real-time experimental data of frequency shift over time were obtained from the seventh overtone experimental data of frequency shift over time were obtained from the seventh overtone (resonance frequency = 35 MHz). All operations ensure that the baseline before the test (resonance frequency = 35 MHz). All operations ensure that the baseline before the test was less than 1 Hz within 5 min. The frequency shift (∆f ) and dissipation shift (∆D) of the was less than 1 Hz within 5 min. The frequency shift (△f) and dissipation shift (△D) of the leaching process in PF solution are shown in Figure 10. Arrow 1 represents the injection leaching process in PF solution are shown in Figure 10. Arrow 1 represents the injection of PF solution into the system, and arrow 2 indicates the entrance of background solution of PF solution into the system, and arrow 2 indicates the entrance of background solution (deionized water) into the system after the leaching rate stabilized. (deionized water) into the system after the leaching rate stabilized. According to Figure 10, the injection of PF solution caused a decrease in ∆f and a sharp increase in △D, indicating that there are substances adsorbed on the surface of the sensor in stage b. This is due to AuCN and Au-hydroxide species form and precipitate on surfaces when there is a deficiency in free cyanide. Upon continuous injection of PF solution, ∆f reached its lowest point and then began to rise. The increase of △f at a gradually increasing rate suggests that the cyanide ions in the solution increased with the extension of irradiation time in leaching stage c. Meanwhile, △D increased to an equilibrium state, indicating that the reaction between the leaching solution and Au had little effect on the

Minerals 2021, 11, x FOR PEER REVIEW 11 of 15

surface viscoelasticity, and that behavior can be interpreted as rigid adsorption. When the background solution was pumped into the system, △f exhibited a slightly faster upward trend and ultimately reached an equilibrium at a nonzero value, indicating that irreversi- Minerals 2021, 11, 387 ble reactions had occurred between PF and gold. In contrast, △D declined nearly11 to of its 15 initial state, indicating that the surface properties of the sensor did not change significantly after this experiment.

2.0 10 a b c d e 8 1.5 2 6 1.0 )

4 6 0.5 − 1 (10 (Hz)

f 2 D △ 0 0.0 △

−2 −0.5 −4 −1.0 −10 0 10 20 30 40 50 60 70 80 90 Time (min)

Figure 10. Frequency shift (∆△ff)) andand dissipationdissipation shiftshift ((∆△DD)) vs. vs. leaching leaching time time at at 1 1 mmol/L mmol/L PF, PF, pH pH = = 12. 12.a, b, a, c,b, d c, and d and e represent e represent the the initial, initial, adsorption, adsorption, leaching, leaching, desorption desorption and and ﬂushing flushing stages. stages. 1 1 and and 2 2indicate indicate PF PF solution solution and and deionized deionized water water enter enter the the system, system, respectively. respectively.

3.4. SurfaceAccording Product to Figure Composition 10, the Analysis injection by of XPS PF solution caused a decrease in ∆f and a sharp increaseTo study in ∆D ,the indicating elemental that composition there are substances and structural adsorbed morphology on the surface of the of surface the sensor prod- in ucts,stage the b. ThisXPS iswide due energy to AuCN spectrum and Au-hydroxide of the clean and species agent-treated form and sensors precipitate were on obtained. surfaces Thewhen results there were is a deficiencyused to illustrate in free the cyanide. chemical Upon composition continuous of the injection Au sensor of PF surface. solution, As shown∆f reached in Figure its lowest 11, the point surface and of then the beganclean tosensor rise. contains The increase mainly of Au∆f 4atf and a gradually a small amountincreasing of rateC 1s suggestsand O 1s that, thus the confirming cyanide ions that in the the gold solution was increasedsuccessfully with coated the extension onto the surfaceof irradiation of the sensor time in without leaching any stage other c. Meanwhile,impure elements.∆D increased After leaching, to an equilibrium the relative state,con- tentindicating of C 1s thatand theO 1s reaction increased between substantially, the leaching and a solutionnew element, and Au N 1 hads, appeared. little effect This on new the elementsurface viscoelasticity,indicated that andC 1s that, O behavior1s, and N can 1s were be interpreted deposited as on rigid the adsorption.surface of the When sensor the background solution was pumped into the system, ∆f exhibited a slightly faster upward after the leaching process, resulting in the formation of some complexes. To further deter- trend and ultimately reached an equilibrium at a nonzero value, indicating that irreversible mine the chemical morphology of the elements, the bonding properties of each element reactions had occurred between PF and gold. In contrast, ∆D declined nearly to its initial were investigated. Therefore, the Au, C, N, and O in the surface products were scanned state, indicating that the surface properties of the sensor did not change significantly after at narrow energies, and the curve fitting program of the Thermo Avantage software was this experiment. used to fit the obtained spectral lines. 3.4. Surface Product Composition Analysis by XPS To study the elemental composition and structural morphology of the surface products, the XPS wide energy spectrum of the clean and agent-treated sensors were obtained. The results were used to illustrate the chemical composition of the Au sensor surface. As shown in Figure 11, the surface of the clean sensor contains mainly Au 4f and a small amount of C 1s and O 1s, thus confirming that the gold was successfully coated onto the surface of the sensor without any other impure elements. After leaching, the relative content of C 1s and O 1s increased substantially, and a new element, N 1s, appeared. This new element indicated that C 1s, O 1s, and N 1s were deposited on the surface of the sensor after the leaching process, resulting in the formation of some complexes. To further determine the chemical morphology of the elements, the bonding properties of each element were investigated. Therefore, the Au, C, N, and O in the surface products were scanned at narrow energies, and the curve fitting program of the Thermo Avantage software was used to fit the obtained spectral lines. Minerals 2021, 11, 387 12 of 15

Minerals 2021, 11, x FOR PEER REVIEW 12 of 15 Minerals 2021, 11, x FOR PEER REVIEW 12 of 15 7 5 5 f f

(b)Dissolution surface 3 d d 5 7 5 f f

(b)Dissolution surface 3 d d s Au 4 Au 4 s Au 4 4 Au Au 4 s Au 4 Au 4 s N 1 Au 4 4 Au O 1 Au 4 N 1 C 1s O 1 C 1s 5 7 f f

(a)Initial surface 5 3 d 7 5 f f d

(a)Initial surface 5 3 d Au 4 4 Au Au 4 4 Au d Au 4 Au 4 4 Au Au 4 4 Au Au 4 Au 4 Au 4 s s s O 1 s O 1 C 1 C 1 700 600 500 400 300 200 100 0 700Binding 600 Energy 500 (eV) 400 300 200 100 0 Binding Energy (eV) Figure 11. XPS wide energy spectrum ofof initialinitial andand dissolutiondissolution surfaces.surfaces. Figure 11. XPS wide energy spectrum of initial and dissolution surfaces. The narrow sweep curve ofof AuAu onon thethe initialinitial andand dissolutiondissolution reactionreaction surfacessurfaces areare shown in in Figure Figure 12a. 12a.The The The narrow binding binding sweep energies energies curve of the ofof thetwoAu two onmain the main peaks initia peaks arel and approximately are dissolution approximately reaction 83.90 surfaces are 83.90eV, which eV, which is consistentshown is consistent in with Figure the with 12a.binding the The binding energiesbinding energies energiesof the of 4f the7 of peak 4thef 7 peakfromtwo main fromthe Handbook peaks the Handbook are approximatelyof X- 83.90 ofray X-ray Photoelectron PhotoelectroneV, Spectroscopy which Spectroscopy is consistent [30]. The [ 30with]. obtained The the obtainedbinding results energies results confirmed conﬁrmed of the that 4f 7there pea thatk was therefrom a large the was Handbook of X- aamount large amount of Au onra ofy the AuPhotoelectron surface on the surfaceof the Spectroscopy sensor. of the sensor.In addition, [30]. In The addition, theobtained characteristic the results characteristic confirmed peak of peakthe that dis- of there was a large thesolution dissolution surfaceamount surface shifted shiftedoftowards Au on towards thethe highsurfa the energyce high of the energydirection sensor. direction comparedIn addition, compared to thethe characteristicinitial to the surface. initial peak of the dis- surface.This result This indicates resultsolution indicates that surfacethe charge that shiftedthe density charge towa around densityrds theAu around atoms high Au energydecreased atoms direction decreased after the compared reaction, after the to the initial reaction,and positively and positively chargedsurface. ThisAu charged emerged result Au ind emergedto the surface to the of surface the sensor. of the sensor. 1-Initial surface 1-Initial surface (b) C 1s 4f 7/2 (a) Au 4f 2-Dissolution surface 1-Initial surface 2-Dissolution surface1-Initial surface (b) C 1s 2 4f 7/2 (a) Au 4f 2-Dissolution surface 2-Dissolution surface 4f 5/2 1 2 4f 5/2 1 2 2 Intensity Intensity Intensity Intensity

1 1 90 88 86 84 82 292 290 288 286 284 282 280 90 88 86 84 82 292 290 288 286 284 282 280 Binding energy (eV) Binding energy (eV) Binding energy (eV) Binding energy (eV) Dissolution surface (c) N 1s 1-Initial surface (d) O 1s Dissolution surface 2-Dissolution(c) N 1 ssurface 1-Initial surface (d) O 1s 2-Dissolution surface 2 2

1 Intensity Intensity 1 Intensity Intensity

406 404 402 400 398 396 536 534 532 530 528 526 406 404 402 400 398 396 536 534 532 530 528 526 Binding energy (eV) Binding energy (eV) Binding energy (eV) Binding energy (eV) Figure 12. High-resolution XPS spectrum of Au 4f (a), C 1s (b), N 1s (c), and O 1s (d). Figure 12.FigureHigh-resolution 12. High-resolution XPS spectrum XPS spectrum of Au 4f of(a Au), C 4 1fs (a(b),), C N 1s 1 s(b(c),), N and 1s ( Oc), 1 ands (d). O 1s (d). The high resolution XPS spectra of C 1s on the surface of original and reagent-treated sensors are shown inThe Figure high resolution12b. According XPS spectra to the graph,of C 1s onthe the main surface peak of of original C 1s was and at reagent-treated sensors are shown in Figure 12b. According to the graph, the main peak of C 1s was at

Minerals 2021, 11, 387 13 of 15

The high resolution XPS spectra of C 1s on the surface of original and reagent-treated sensors are shown in Figure 12b. According to the graph, the main peak of C 1s was at 284.80 eV, which demonstrates that the C on the surface approximates the characteristic peak of the nonbound state C due to trace hydrocarbon contamination on the surface of the sensor [31]. The spectra exhibited signiﬁcant asymmetric characteristics, and the C 1s peak on the dissolution surface shifted 0.12 eV towards the high binding energy direction, thus indicating the existence of other carbon-containing species of C 1s [32]. Peak ﬁtting was then performed to analyze the existent forms of C 1s. On the original surface, peaks were recorded at 284.84 and 286.70 eV in the XPS spectrum, and they correspond to the traces of organic pollutants pre-adsorbed on the sensor’s surface. Compared to the C 1s peak of the original surface, the decomposition of the C 1s peak on the dissolution surface consists of three peaks. In addition to the C peak at 284.96 and 286.78 eV, another peak with binding energy greater than the C-C bond appeared at 288.21 eV, corresponding to –C≡N from complexes formed during the leaching process [33]. As shown in Figure 12 c, the absence of an N 1s characteristic peak on the initial surface indicates that the detection of N was not affected by the environment. On the dissolution surface, the characteristic peak of N 1s was 399.96 eV, which coincides with the binding energy of the –C≡N bond [34]. Considering the XPS analysis of C and Au, it can be inferred that N exists in the form of –C≡N and in complexes with Au on the dissolution surface. As shown by the O 1s spectra in Figure 12d, the binding energy of O 1s was 531.94 eV, which coincides with the standard peak of adsorbed oxygen (Oads) [35]. It is generally believed that the Oads is O2 or -OH when the binding energy is approximately 532 eV. Ac- cording to the graph, the relative content of O 1s on the dissolution surface clearly increased, which indicates the presence of -OH and its existence likely in the form of Au(OH)n.

4. Conclusions (1) Under the UV irradiation, PF solution could dissociate and release free cyanide ions little by little. Additionally, the amount of free cyanide ions gradually increased with the increase of irradiation time and intensity. However, there has been a saturation value of the release of free cyanide ions at the concentration. The maximum release amount corresponds to a PF concentration of 2.5 mmol/L, and the conversion rate of cyanide ions is 65.28% after irradiation for 60 min at the irradiance of 10 mW·cm−2. (2) The leaching experiments analyzed by response surface methodology showed a 67.74% extraction of gold at 62 ◦C, pH of 12.6, and 3.8 kg/t dosage of reagent under the UV light of 10 mW·cm−2. The leaching kinetics of the PF solution monitored by QCM-D showed that the release of cyanide ions is a process of gradual release. The leaching reaction in the solution consists of two processes: adsorption and leaching. (3) The XPS analysis showed that there was –C≡N on the dissolution reaction surface, and that gold was oxidized to form AuCN complexes. It can be assumed that the free cyanide ion was released from the solution of PF during the leaching reaction, and it formed AuCN and Au (OH)n.

Author Contributions: Z.L. wrote the manuscript, with review from J.K., Y.X., and C.S. All authors have read and aggreed to the published version of the manuscript. Funding: This work was supported by the National Natural Science Foundation of China (grant number: 51974016). Data Availability Statement: Not applicable. Acknowledgments: The authors would like to thank the Analytical and Testing Center of the Uni- versity of Science and Technology Beijing, which supplied the facilities to perform the measurements. Conﬂicts of Interest: The authors declare no conﬂict of interest. Minerals 2021, 11, 387 14 of 15

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