Comprehensive Recovery Technology for Te, Au, and Ag from a Telluride-Type Refractory Gold Mine

minerals

Article Comprehensive Recovery Technology for Te, Au, and Ag from a Telluride-Type Refractory Gold Mine

Wei Yang 1,2,*, Gang Wang 1,2,*, Qian Wang 1,2, Ping Dong 1, Huan Cao 1 and Kai Zhang 1

1 School of Resources Engineering, Xi’an University of Architecture and Technology, Xi’an 710055, China; [email protected] (Q.W.); [email protected] (P.D.); [email protected] (H.C.); [email protected] (K.Z.) 2 Key Laboratory of Gold and Resources in Shaanxi Province, Xi’an 710055, China * Correspondence: [email protected] (W.Y.); [email protected] (G.W.)

Received: 22 August 2019; Accepted: 26 September 2019; Published: 30 September 2019

Abstract: While extracting gold and silver from telluride-type gold deposits, it is beneficial to develop a comprehensive recovery technology for tellurium. In this paper, we report process mineralogy based on the backward processing technology and the low comprehensive utilization rate of typical telluride-type gold deposits in Xiaoqinling, China. The findings show that tellurium, gold, and silver are the most valuable elements in the ore fissures and gangue minerals and are encapsulated in metallic sulfur ore in the form of altaite, hessite, calaverite, antamokite and natural gold. The flotation method was innovatively applied in this study to comprehensively recover Te, Au and Ag. The results show that when the ore particle size was 0.074 mm (70%), the flotation pulp density was 33%, − the pulp pH was 8, and the combined collector (isoamyl xanthate + ethyl thiocarbamate (1:1)) was 120 g/t, in the process involving one rough flotation step, two cleaning flotations and two scavenging flotations as well as a continuous 8 d industrial test, the recovery degree was stable and the average grades of Te, Au, and Ag were 241.61, 90.30, and 92.74 g/t with 95.42%, 97.28%, and 94.65% recovery rates, respectively; thus, excellent recovery degrees were obtained. Compared with the original flotation process, the recovery rates of Te, Au, and Ag were increased by 19.91%, 6.93%, and 5.67%, which boosted the effective enrichment of all valuable elements in the telluride-type gold mine and achieved technological progress.

Keywords: telluride-type gold mine; ﬂotation; tellurium; comprehensive recovery

1. Introduction Rare elements have received increasing attention in recent years as they play important roles in modern high-tech industries and national defense [1–4]. However, the average abundance of tellurium in the crust is only 0.005 ppm [5], and there are no independent deposits of it. At present, tellurium is mainly extracted from tellurium-rich copper and lead smelting slag [6–9]. These processes are not only complex and time-consuming, but also result in great losses of tellurium during the recovery process. There are a large number of telluride-type gold deposits in Xiaoqinling, China, which content of tellurium is relatively high [10,11]. To recover this kind of ore, the direct recovery of tellurium from ore by flotation can both reduce the loss of tellurium and improve the utilization rate of ore, which is of great significance. Tellurium-bearing gold deposits are an important type of gold ore. Among the 19 known types of industrial gold minerals, tellurium-bearing gold minerals account for 11 of them [12]. So far, gold and silver are mainly recovered from telluride-type gold deposits, but there has been no report of tellurium recovery from such sources. Even more problematic is that, for telluride-type gold deposits, the existing flotation system for recovering Au and Ag mostly applies xanthate as the collector and CaO as the regulator, which can only recover tellurium closely related to Au and Ag.

Minerals 2019, 9, 597; doi:10.3390/min9100597 www.mdpi.com/journal/minerals Minerals 2019, 9, 597 2 of 12

The flotation process of tellurobismuthite is similar to that of pyrites [13]. Inhibitors are often used to inhibit pyrite [14], so as to promote the recovery of tellurobismuthite. However, such a reagent system can result in the loss of gold and silver closely related to pyrite. Moreover, the brittleness and sliming of telluride [15–17] will further increase the difficulty of Te recovery. Therefore, it is of great value to develop a flotation process for recovering Te from telluride-type gold ores, while taking into account the recovery of Au and Ag. Aiming at decreasing the difficulty of comprehensively utilizing a telluride-type gold deposit in the Xiaoqinling, China, single factor condition test as well as open-circuit and closed-circuit tests were conducted by the flotation method on the basis of process mineralogy. Once the optimum process conditions were set, industrial tests of 300 t/d throughputs were completed, and excellent efficiency was obtained, which, in turn, led to a comprehensive recovery of Te, Au and Ag from telluride-type gold deposits. This research provides a feasible method of extracting valuable elements from telluride-type gold deposits.

2. Experimental Section

2.1. Experimental Sample and Experimental Reagents Telluride-type gold mine samples used in the flotation tests were obtained from Xiaoqinling, China. In the test, raw samples, after they were crushed to less than 3 mm in a roller crusher, were finely ground to 0.074 mm (60–80%) with a ball mill before flotation. Butyl xanthate (industrial grade), − sodium isoamyl xanthate (industrial grade), ammonium dibutyl dithiophosphate (industrial grade) and ethyl thiocarbamate (industrial grade) were used as collectors. Butyl xanthine and sodium isoamyl xanthate are commonly used as sulfide ore collectors; ammonium dibutyl dithiophosphate can improve the recovery of refractory ores, and ethyl thiocarbamate is often used in the flotation of galena. Sodium hydroxide (analytical grade) was used as the pH modifier, and terpineol (industrial grade) was used as the frother.

2.2. Experimental Methods Bench-scale flotation experiments were performed in a 1.5-L XFD (Nanchang Haifeng Mining Equipment Co, Ltd., Nanchang, China) flotation cell. Ore sample (500 g) was added to an XMQ (Nanchang Haifeng Mining Equipment Co, Ltd., Nanchang, China) ball mill with a pulp density of 60 wt. %, and the samples were placed in a flotation cell after they were ground, and the amount of solid minerals was 33 wt. %. When the flotation machine was open-stirred for 3 min while pH was adjusted, a collector and the frother were added to the pulp. The dosage of the collector and the foaming reagent in the scavenging flotation was half of the dosage of the roughing reagent. A diagram of the single factor test is shown in Figure1. After the test, the collected products were filtered, dried and weighed; the concentrations of Te, Au, and Ag in the samples were determined, and the recovery rate was calculated. The recovery rate amount was then calculated by using the expression:

ε = βγ/β1 (1) where ε is the degree of recovery for a speciﬁc element (%); β is the grade of the element in the product (g/t); γ is the yield of the product (%) and β1 is the grade of feed minerals (g/t). Minerals 2018, 8, x FOR PEER REVIEW 2 of 12 to Au and Ag. The flotation process of tellurobismuthite is similar to that of pyrites [13]. Inhibitors are often used to inhibit pyrite [14], so as to promote the recovery of tellurobismuthite. However, such a reagent system can result in the loss of gold and silver closely related to pyrite. Moreover, the brittleness and sliming of telluride [15–17] will further increase the difficulty of Te recovery. Therefore, it is of great value to develop a flotation process for recovering Te from telluride-type gold ores, while taking into account the recovery of Au and Ag. Aiming at decreasing the difficulty of comprehensively utilizing a telluride-type gold deposit in the Xiaoqinling, China, single factor condition test as well as open-circuit and closed-circuit tests were conducted by the flotation method on the basis of process mineralogy. Once the optimum process conditions were set, industrial tests of 300 t/d throughputs were completed, and excellent efficiency was obtained, which, in turn, led to a comprehensive recovery of Te, Au and Ag from telluride-type gold deposits. This research provides a feasible method of extracting valuable elements from telluride-type gold deposits.

2. Experimental Section

2.1. Experimental Sample and Experimental Reagents Telluride-type gold mine samples used in the flotation tests were obtained from Xiaoqinling, China. In the test, raw samples, after they were crushed to less than 3 mm in a roller crusher, were finely ground to −0.074 mm (60–80%) with a ball mill before flotation. Butyl xanthate (industrial grade), sodium isoamyl xanthate (industrial grade), ammonium dibutyl dithiophosphate (industrial grade) and ethyl thiocarbamate (industrial grade) were used as collectors. Butyl xanthine and sodium isoamyl xanthate are commonly used as sulfide ore collectors; ammonium dibutyl dithiophosphate can improve the recovery of refractory ores, and ethyl thiocarbamate is often used in the flotation of galena. Sodium hydroxide (analytical grade) was used as the pH modifier, and terpineol (industrial grade) was used as the frother.

2.2. Experimental Methods Bench-scale flotation experiments were performed in a 1.5 -L XFD (Nanchang Haifeng Mining Equipment Co, Ltd., Nanchang, China) flotation cell. Ore sample (500 g) was added to an XMQ (Nanchang Haifeng Mining Equipment Co, Ltd., Nanchang, China) ball mill with a pulp density of 60 wt. %, and the samples were placed in a flotation cell after they were ground, and the amount of solid minerals was 33 wt. %. When the flotation machine was open-stirred for 3 min while pH was adjusted, a collector and the frother were added to the pulp. The dosage of the collector and the Mineralsfoaming2019 reagent, 9, 597 in the scavenging flotation was half of the dosage of the roughing reagent.3 of 12A diagram of the single factor test is shown in Figure 1.

Figure 1. Chart of the single factor condition test.

2.3. Analytical Method With the help of an OPTON mineral liberation analyzer (MLA, Beijing Opton Optical Technology Co., Ltd., Beijing, China) and an APOLLO X-ray energy dispersive spectrometer (Denver, CO, USA), the mineral composition, particle size and ore association relation of the telluride-type gold mine experimental sample were determined, and the MLA operating voltage is 25 kV during the test. With the help of an S4 Pioneer wavelength dispersive X-ray Fluorescence (XRF) spectrometer (Bruker, Germany), the mineral elements and content of the mine experimental sample were determined, and the test sample needs to be in a dry state, the weight is above 3 g, and the fineness is –0.074 mm. With the help of a MM-7C polarizing microscope (Shanghai Wanheng Precision Instrument Co, Ltd., Shanghai, China), the ore association relation of the mine experimental sample was determined. With the help of a PerkinElmer NexION 300X inductively coupled plasma mass spectrometry (ICP-MS) ((Boston, MA, USA), the content of elements of the mine experimental sample were determined, and the test process RF power is 1300 W and the auxiliary gas is 0.7 L/min. With the help of an Ultima IV X-ray diffraction X-ray diffractometer (XRD) (Rigaku Corporation, Tokyo, Japan), fitted with a copper tube (copper Kα radiation). The mineral compositions of the mine experimental sample were determined, and the data were analyzed with software (jade 6.5) and the mineral content was calculated. The test sample weight is above 3 g, and the fineness is 0.074 mm, the test process is set to 5–90 and the − ◦ scanning rate is 5◦/min.

3. Results and Discussion

3.1. Mineralogical Analysis With the help of a MLA, the telluride-type gold mine experimental sample was analyzed, and the results are shown in Table1. The results of observation and analysis under a polarizing microscope are shown in Figure2. With the help of XRF and ICP-MS, the chemical multi-element analysis of the telluride-type gold mine experimental sample was determined and the results are shown in Table2. A mineralogical study showed the following: (1) there were 11 ore minerals in the sample, five of which are Au, Ag, and Te minerals, namely natural gold, antamokite, calaverite, hessite, and altaite, which are major target minerals; (2) the other metal sulfide minerals were pyrite, chalcopyrite, bornite, galena, blende, and pyrrhotite; (3) Au, Ag, and Te mainly exist in ore fissures and gangue minerals and are encapsulated in pyrite and other metal sulfide minerals; (4) hessite and altaite are closely related to galena and natural gold is closely related to quartz. This occurrence state is beneficial for the recovery of Au, Ag, and Te. The particle size of Au, Ag, and Te minerals is small, all below 0.090 mm. The particles of antamokite and altaite are relatively coarse, while the particle sizes of natural gold, calaverite, and Minerals 2019, 9, 597 4 of 12 Minerals 2018, 8, x FOR PEER REVIEW 4 of 12 hessite all haveTable particle 1. Mineral sizes composition, below 0.038 chemical mm. formula, The particle and dissemination size of pyrite, size galena of the and raw chalcopyriteore. is larger thanMineral 0.016 Composition mm, and some of themChemical have reached Formula 1–2 mm, which isDissemination better sufficient Size for (mm) grinding dissociation.Natural The goldgold ore is primary deposit andAu contains polymetallic sulfide<0.038 with a quartz vein, which is easilyCalaverite recovered by flotation technology.AuTe Simultaneously,2 the grades of tellurium,<0.038 gold, and silver are 14.80,Hessite 5.30, and 5.70 g/t respectively, accordingAg2Te to multi-element analysis.<0.038 These are the main recovered elements.Altaite The concentrations of thePbTe other elements are too low to have0.008 a– recovery0.09 value. Antamokite Ag3AuTe2 0.006–0.09 TablePyrite 1. Mineral composition, chemicalFeS formula,2 and dissemination size of the0.048 raw–2 ore. Galena PbS 0.037–2 ChalcopyriteMineral Composition ChemicalCuFeS Formula2 Dissemination Size0.016 (mm)–1 BorniteNatural goldCu Au5FeS4 <0.038 - SphaleriteCalaverite AuTeZnS2 <0.038 - PyrrhotiteHessite AgFe21Te-xS <0.038 - QuartzAltaite PbTeSiO2 0.008–0.09>0.005 Antamokite Ag3AuTe2 0.006–0.09 PyriteTable 2. Percentage concentrations FeS2 of multi-element minerals0.048–2. Galena PbS 0.037–2 Component ChalcopyriteSiO2 TiO2 Al CuFeS2O3 2 TFe Mn 0.016–1MgO CaO K2O Content (%) Bornite74.92 0.28 Cu6.385FeS 4 2.35 0.076 1.06- 1.71 3.76 Sphalerite ZnS - Component Na2O Cu Pb Zn S Au* Ag* Te * Pyrrhotite Fe S- Content (%) 0.84 0.013 0.11-x 0.03 2.76 5.30 5.70 14.80 Quartz SiO2 >0.005 “*” means that the unit is g/t.

a b

40μm 40μm

c d

10μm 10μm

Figure 2. 2. OreOre association: association (a ): Long(a) Long strips strips and round and grainsround of grains ﬁne-grained of fine altaite-grained and altaite galena and co-existing galena andco-exist intersperseding and interspersed with chalcopyrit; with chalcopyrit; (b) long strips (b) long and strips round and granular round ﬁne-grainedgranular fine- altaitegrained encased altaite inencased galena; in (galena;c) a long (c) strip a long of strip natural of natural gold enclosed gold enclosed in late in quartz; late quartz (d) circular; (d) circular grain grain of natural of natural gold enclosedgold enclosed in late quartz.1cm in late quartz. (Py—pyrite;1cm Gal—galena;(Py—pyrite; Te—altaite; Gal—galena; Ccp—chalcopyrite; Te—altaite; Ccp Au—natural—chalcopyrite; gold; Q—quartz).Au—natural gold; Q—quartz).

Minerals 2019, 9, 597 5 of 12

Table 2. Percentage concentrations of multi-element minerals.

Component SiO2 TiO2 Al2O3 TFe Mn MgO CaO K2O Content (%) 74.92 0.28 6.38 2.35 0.076 1.06 1.71 3.76

Component Na2O Cu Pb Zn S Au* Ag* Te * Content (%) 0.84 0.013 0.1 0.03 2.76 5.30 5.70 14.80 “*” means that the unit is g/t. Minerals 2018, 8, x FOR PEER REVIEW 5 of 12

3.2.3.2. Study on Flotation EEfficiencyﬃciency

3.2.1.3.2.1. InfluenceInfluence of Ore Particle Size onon thethe RecoveryRecovery ofof Te,Te, Au,Au, andand AgAg FigureFigure3 3 shows shows thethe results of the the effect e ffect of of ore ore part particleicle size size on on the the recovery recovery of Te, ofTe, Au, Au, and and Ag. Ag. As Asshown shown in Figure in Figure 3, 3with, with the the increase increase in in ore ore particle particle size, size, the the recovery recovery of of Au, Au, Ag, and Te firstfirst increasesincreases andand then decreases. The The recovery recovery of of Au, Au, Ag, Ag, and and Te Te reaches reaches 92.35, 92.35, 94.32, 94.32, and and 92.09%, 92.09%, respectively, respectively, at ata particle a particle size size of of−0.0740.074 mm mm (70%). (70%). A Acontinuous continuous increase increase in in ore ore particle particle size size will will result in aa − significantsignificant decrease in the recovery recovery of of all all minerals, minerals, an andd this this is is due due to to the the fact fact that that telluride telluride is isliable liable to tobe beover-ground, over-ground, resulting resulting in slimming in slimming and and making making it difficult it difficult for forflotation flotation to occur. to occur. Therefore, Therefore, the theoptimum optimum ore oreparticle particle size size is − is0.0740.074 mm mm (70%). (70%). − 220 100

200

180 95 160

140 90 120 Grade (g/t) 100 Au Grade Au Recovery (%) Recovery Ag Grade Ag Recovery 85 80 Te Grade Te Recovery

60 80 60 65 70 75 80 Content of -0.074mm (%) FigureFigure 3.3. Flotation recovery and grade of Te, Au,Au, andand AgAg asas aa functionfunction ofof thethe oreore particleparticle sizesize valuevalue (pulp(pulp density:density: 33%;33%; pulppulp pHpH value:value: 8; butyl xanthate: 120 g/t). g/t).

3.2.2.3.2.2. EEffectffect of Pulp pHpH onon thethe RecoveryRecovery ofof Te,Te, Au,Au, andand AgAg TheThe eeffectffect ofof pulppulp pHpH on on the the recovery recovery of of Te, Te, Au, Au, and and Ag Ag shown shown in in Figure Figure4. As4. As shown shown in Figurein Figure4, pH4, pH has has the the highest highest effect effect on the on recovery the recovery of Au, of but Au has, but little has eff littleect on effect the recovery on the recovery of Ag. The of recoveryAg. The raterecovery of Au rate first of increases Au first andincreases then decreasesand then decrea with theses rise with in the pH rise value, in pH reaching value, a reaching maximum a maximum of 93.91% whenof 93.91% pH =when8. pH=8.

220 100

200

180 95 160 (%) 140 90 y 120 Au Grade Au Recovery

Grade (g/t) Grade Ag Grade Ag Recovery Recover 100 85 Te Grade Te Recovery 80

60 80 7.0 7.5 8.0 8.5 9.0 pH Figure 4. Flotation recovery and grade of Te, Au, and Ag as a function of pulp pH value (content of −0.074 mm (70%); pulp density: 33%; butyl xanthate: 120 g/t).

Minerals 2018, 8, x FOR PEER REVIEW 5 of 12

3.2. Study on Flotation Efficiency

3.2.1. Influence of Ore Particle Size on the Recovery of Te, Au, and Ag Figure 3 shows the results of the effect of ore particle size on the recovery of Te, Au, and Ag. As shown in Figure 3, with the increase in ore particle size, the recovery of Au, Ag, and Te first increases and then decreases. The recovery of Au, Ag, and Te reaches 92.35, 94.32, and 92.09%, respectively, at a particle size of −0.074 mm (70%). A continuous increase in ore particle size will result in a significant decrease in the recovery of all minerals, and this is due to the fact that telluride is liable to be over-ground, resulting in slimming and making it difficult for flotation to occur. Therefore, the optimum ore particle size is −0.074 mm (70%).

220 100

200

180 95 160

140 90 120 Grade (g/t) 100 Au Grade Au Recovery (%) Recovery Ag Grade Ag Recovery 85 80 Te Grade Te Recovery

60 80 60 65 70 75 80 Content of -0.074mm (%) Figure 3. Flotation recovery and grade of Te, Au, and Ag as a function of the ore particle size value (pulp density: 33%; pulp pH value: 8; butyl xanthate: 120 g/t).

3.2.2. Effect of Pulp pH on the Recovery of Te, Au, and Ag The effect of pulp pH on the recovery of Te, Au, and Ag shown in Figure 4. As shown in Figure 4, pH has the highest effect on the recovery of Au, but has little effect on the recovery of Ag. The Mineralsrecovery2019 rate, 9, 597of Au first increases and then decreases with the rise in pH value, reaching a maximum6 of 12 of 93.91% when pH=8.

220 100

200

180 95 160 (%) 140 90 y 120 Au Grade Au Recovery

Grade (g/t) Grade Ag Grade Ag Recovery Recover 100 85 Te Grade Te Recovery 80

60 80 7.0 7.5 8.0 8.5 9.0 pH Figure 4. Flotation recovery and grade of Te, Au, and Ag as a function of pulp pH value (content of −0.0740.074 mm mm (70%); (70%); pulp pulp density: density: 33%; 33%; butyl butyl xanthate: xanthate: 120 120 g/t). g/t). − Moreover, 95.14% Ag is obtained at this point, and 92.96% Te is recovered when pH = 7.5. Since Au and Ag are the most valuable elements, and since the recovery rate of Te at pH 8 is similar to that at pH 7.5, a pH value of 8 was selected for the slurry as the optimal pH for subsequent experiments.

3.2.3. Effect of Collector Types on the Recovery of Te, Au, and Ag The effect of collector type on the recovery of Te, Au and Ag is shown in Table3. The results in the table are the mean values after three tests. The products are concentrates. It is shown in Table3 that, when ammonium dibutyl dithiophosphate was used as a collector, the concentrate yield was the highest, reaching 14.18%; the grades of Te, Au, and Ag were 94.73, 29.62, and 37.88 g/t, but the recovery rates were only 90.88, 80.32, and 94.57%. The low grade and recovery rates indicate that ammonium dibutyl dithiophosphate has a strong collecting performance but poor selectivity, and it is not suitable for telluride-type gold ore. Compared with butyl xanthate, sodium isoamyl xanthate has a better flotation effect. Te, Au, and Ag were relatively low in grade, but the recovery rate was obviously high. This is because the collection ability and selectivity of xanthate are closely related to the length of the hydrocarbon chain in its molecule [18–20]. When the number of carbon atoms is small (C1–C6), the longer the carbon chain is, the stronger the collection performance of xanthate will be, but the selectivity of xanthate is the opposite—the longer the carbon chain is, the worse the selectivity will be. Compared with sodium isoamyl xanthate and ethyl thiocarbamate, isoamyl xanthate has a higher recovery of Au, while ethyl thiocarbamate has a higher recovery of Ag and Te, which is because hessite and altaite are closely related to galena. As an effective collector of galena, ethyl thiocarbamate [21] can significantly improve the recovery of Ag and Te, but it is not suitable to recover gold because it is not closely related to galena. When sodium isoamyl xanthate and ethyl thiocarbamate are used as a combined collector, all valuable elements in this type of ore can be recovered. The recovery of Te, Au, and Ag is better than with a single reagent. The grades of Te, Au, and Ag that can be reached are 190.93, 68.34, and 73.45 g/t, and the recovery rates are 95.08%, 95.03%, and 94.98%, respectively. Minerals 2019, 9, 597 7 of 12

Table 3. Flotation recovery and grade of Te, Au, and Ag of collector type value.

Grade (g/t) Recovery (%) Collector Type Yield (%) Au Ag Te Au Ag Te Butyl xanthate 6.91 72.27 76.70 195.5 93.88 93.28 91.95 Sodium isoamyl xanthate 7.27 70.28 72.03 190.31 95.33 94.64 92.73 Ammonium dibutyl 14.18 29.62 37.88 94.73 80.32 94.57 90.88 dithiophosphate Ethyl thiocarbamate 7.45 53.39 70.03 186.65 71.66 95.27 95.31 Sodium isoamyl xanthate and 7.37 68.34 73.45 190.93 95.03 94.98 95.08 ethyl thiocarbamate (1:1) (Test conditions: content of 0.074 mm (70%); pulp density: 33%; pulp pH: 8; amount of collector: 120 g/t). −

3.2.4. Effect of Collector Dosage on the Recovery of Te, Au, and Ag The effect of collector dosage on the recovery of Te, Au, and Ag is shown in Figure5. As can be seen, the recovery rate of Te, Au, and Ag first increases and then tends to be stable with the rise of in collector dosage. When the dosage of the collector is 120 g/t, the recovery rates of Te, Au and Ag are 92.43%, 95.43%, and 96.69%. When the dosage of the collector is 150 g/t, the recovery rate decreases slightly, so the suitable dosage of the combined collector (sodium isoamyl xanthate + ethyl Mineralsthio-carbamate 2018, 8, x FOR (1:1)) PEER is 120REVIEW g/t. 7 of 12

220 100

200

180 95

160

140 90

120 Au Grade Au Recovery Grade (g/t) Grade

100 Ag Grade Ag Recovery 85 Recovery (%) Te Grade Te Recovery 80

60 80 20 40 60 80 100 120 140 160 Collector dosage (g/t) Figure 5.5. FlotationFlotation recovery recovery and and grade grade of Te,of Te, Au, Au, and Agand as Ag a function as a function of collector of collector dosage valuedosage (content value (contentof 0.074 of mm −0.074 (70%); mm pulp (70%); density: pulp33%; density: pulp 33%; pH: 8;pulp combined pH: 8; collector:combinedsodium collector: isoamyl sodium xanthate isoamyl+ − xanthateethyl thiocarbamate + ethyl thiocarbamate (1:1)). (1:1)).

3.2.5. Effect of Slurry Pulp Density on the Recovery of Te, Au and Ag 3.2.5. Effect of Slurry Pulp Density on the Recovery of Te, Au and Ag The effect effect of slurry concentration on the recovery of Te, Au,Au, andand AgAg isis shownshown in Figure6 6.. ItIt cancan be seen in in Figure Figure 6 that the grades grades of of Te Te and and Ag Ag first first increase increase and and then then decrease decrease with with the the increase increase in pulpin pulp density, density, and and reach reach the the highest highest when when the the pu pulplp density density is is 33%; 33%; however, however, the the grade grade of of Au gradually increases.increases. WhenWhen the the concentration concentration of theof pulpthe pulp is too is low, too the low, probability the probability of collision of collision between betweenbubbles andbubbles minerals and minerals will be reduced, will be reduced, which is notwhich conducive is not cond to theucive flotation to the offlotation the target of the minerals, target minerals,and the consumption and the consumption of the reagents of the is increased;reagents is when increased; the concentration when the concentration of the slurry of reaches the slurry 33%, reachesthe recovery 33%, ratesthe recovery of Au, Ag, rates and of Te Au, are 95.84%,Ag, and 97.10%, Te are and95.84%, 93.32%, 97.10%, slightly and higher 93.32%, than slightly the recovery higher thanrate whenthe recovery the density rate iswhen 35%, the so density the flotation is 35%, pulp so densitythe flotation was finallypulp density set to 33%. was finally set to 33%.

220 100

200

180 95

160

140 90

120 Au Grade Au Recovery Grade (g/t) Grade Ag Grade Ag Recovery 100 85 Recovery (%) Te Grade Te Recovery 80

60 80 24 26 28 30 32 34 36 Pulp concentration (%)

Figure 6. Flotation recovery and grade of Te, Au, and Ag as a function of pulp concentration value (content of −0.074 mm (70%); pulp pH: 8; combined collector: sodium isoamyl xanthate + ethyl thiocarbamate (1:1) 120 g/t).

3.2.6. The Closed-Circuit Flotation Test A closed-circuit test was carried out on the basis of the conditions of the above tests. The process chart is shown in Figure 7, and the test results are shown in Table 4. It is shown in Table 4 that, through a closed-circuit flotation test with one rough flotation step, two cleaning flotations and two scavenging flotation processes, Te, Au, and Ag concentrates with grades of 242.95, 89.30, and

Minerals 2018, 8, x FOR PEER REVIEW 7 of 12

220 100

200

180 95

160

140 90

120 Au Grade Au Recovery Grade (g/t) Grade

100 Ag Grade Ag Recovery 85 Recovery (%) Te Grade Te Recovery 80

60 80 20 40 60 80 100 120 140 160 Collector dosage (g/t) Figure 5. Flotation recovery and grade of Te, Au, and Ag as a function of collector dosage value (content of −0.074 mm (70%); pulp density: 33%; pulp pH: 8; combined collector: sodium isoamyl xanthate + ethyl thiocarbamate (1:1)).

3.2.5. Effect of Slurry Pulp Density on the Recovery of Te, Au and Ag The effect of slurry concentration on the recovery of Te, Au, and Ag is shown in Figure 6. It can be seen in Figure 6 that the grades of Te and Ag first increase and then decrease with the increase in pulp density, and reach the highest when the pulp density is 33%; however, the grade of Au gradually increases. When the concentration of the pulp is too low, the probability of collision between bubbles and minerals will be reduced, which is not conducive to the flotation of the target minerals, and the consumption of the reagents is increased; when the concentration of the slurry

Mineralsreaches2019 33%,, 9, 597the recovery rates of Au, Ag, and Te are 95.84%, 97.10%, and 93.32%, slightly higher8 of 12 than the recovery rate when the density is 35%, so the flotation pulp density was finally set to 33%.

220 100

200

180 95

160

140 90

120 Au Grade Au Recovery Grade (g/t) Grade Ag Grade Ag Recovery 100 85 Recovery (%) Te Grade Te Recovery 80

60 80 24 26 28 30 32 34 36 Pulp concentration (%)

Figure 6. Flotation recovery and grade of Te, Au, and Ag as a functionfunction of pulp concentration value (content of −0.0740.074 mm (70%); pulp pH: 8; combined collector: sodium isoamyl xanthate + ethylethyl thio- thio- − carbamate (1:1) 120 gg/t)./t). 3.2.6. The Closed-Circuit Flotation Test 3.2.6. The Closed-Circuit Flotation Test A closed-circuit test was carried out on the basis of the conditions of the above tests. The process A closed-circuit test was carried out on the basis of the conditions of the above tests. The chart is shown in Figure7, and the test results are shown in Table4. It is shown in Table4 that, through process chart is shown in Figure 7, and the test results are shown in Table 4. It is shown in Table 4 a closed-circuit flotation test with one rough flotation step, two cleaning flotations and two scavenging that, through a closed-circuit flotation test with one rough flotation step, two cleaning flotations and flotationMinerals processes, 2018, 8, x Te,FOR PEER Au, REVIEW and Ag concentrates with grades of 242.95, 89.30, and 93.008 of g 12/t can be two scavenging flotation processes, Te, Au, and Ag concentrates with grades of 242.95, 89.30, and obtained, and the recovery rates are 95.54%, 97.18%, and 94.96% respectively. The target minerals were 93.00 g/t can be obtained, and the recovery rates are 95.54%, 97.18%, and 94.96% respectively. The basically recovered. target minerals were basically recovered.

FigureFigure 7. Chart7. Chart of of a a closed-circuitclosed-circuit flotation ﬂotation test. test.

Table 4. Results of a closed-circuit flotation test.

Grade (g/t) Recovery (%) Product Yield (%) Au Ag Te Au Ag Te Concentrates 5.82 89.30 93.00 242.95 97.18 94.96 95.54 Final tailings 94.18 0.16 0.31 0.70 2.82 5.04 4.46 Feed minerals 100.00 5.30 5.70 14.80 100.00 100.00 100.00

3.3. Industrial Tests On the basis of the above tests, continuous industrial tests of a 300 t/d concentrator in Xiaoqinling in China were conducted for eight days. Figure 8a presents the actual process operating conditions and production indicators obtained at the production site of a concentrator in Xiaoqinling. Figure 8b presents the industrial test results after the optimization of technological conditions in the study. In Figure 8a, the average grades of Te, Au, and Ag were 195.61, 83.90, and 88.98g/t, and the recovery rates were 75.44%, 90.35%, and 89.07%. After adopting the new process, in Figure 8b, the average grades of Te, Au, and Ag were 241.61, 90.02, and 92.74 g/t, and the recovery rates were 95.35%, 97.28% and 94.74%, the recovery rates of Te, Au, and Ag increased by 19.91%, 6.93% and 5.67%. Thus, excellent flotation efficiency was obtained, and comprehensive recovery of Te, Au, and Ag in a telluride-type gold mine was achieved.

Minerals 2019, 9, 597 9 of 12

Table 4. Results of a closed-circuit ﬂotation test.

3.3. Industrial Tests On the basis of the above tests, continuous industrial tests of a 300 t/d concentrator in Xiaoqinling in China were conducted for eight days. Figure8a presents the actual process operating conditions and production indicators obtained at the production site of a concentrator in Xiaoqinling. Figure8b presents the industrial test results after the optimization of technological conditions in the study. In Figure8a, the average grades of Te, Au, and Ag were 195.61, 83.90, and 88.98g /t, and the recovery rates were 75.44%, 90.35%, and 89.07%. After adopting the new process, in Figure8b, the average grades of Te, Au, and Ag were 241.61, 90.02, and 92.74 g/t, and the recovery rates were 95.35%, 97.28% and 94.74%, the recovery rates of Te, Au, and Ag increased by 19.91%, 6.93% and 5.67%. Thus, excellent ﬂotation eﬃciency was obtained, and comprehensive recovery of Te, Au, and Ag in a telluride-type goldMinerals mine 2018 was, 8, x FOR achieved. PEER REVIEW 9 of 12

a 100 250 b 250 100

200 200 Te Grade Te Recovery 80 Au Grade Au Recovery 80 Ag Grade 150 150 Ag Recovery Te Grade Te Recovery

Grade (g/t) Grade (g/t) Grade Au Grade Au Recovery Recovery (%) Recovery 100 Ag Grade Ag Recovery (%) Recovery 100 60 60

50 50 012345678 012345678 Time (d) Time (d) Figure 8. (a) Production data obtained from the operation sitesite ofof aa 300300 tt/d/d concentratorconcentrator inin Xiaoqinling:Xiaoqinling: content ofof −0.0740.074 mm mm (66%); (66%); CaO CaO adjusts adjusts pulp pulp pH: 9;pH: collector: 9; collector: sodium sodium isoamyl isoamyl xanthate xanthate 90 g/t; roughing 90 g/t; − pulproughing density: pulp 33%; density: (b)The 33%; 300 t /d(b Industrial)The 300 testt/d resultsIndustrial after test optimization results after of process optimization conditions: of process content ofconditions:0.074 mm content (70%); of NaOH −0.074 adjusts mm pulp(70%); pH: NaOH 8; combined adjusts collector:pulp pH sodium: 8; combined isoamyl collector: xanthate sodium+ ethyl − thiocarbamateisoamyl xanthate (1:1) + ethyl 120 g /thiocarbamatt; pulp density:e (1:1) 33%. 120 g/t; pulp density: 33%. 3.4. Flotation Product Analysis 3.4. Flotation Product Analysis The flotation concentrates and tailings crystal phase composition in the industrial tests was The flotation concentrates and tailings crystal phase composition in the industrial tests was determined by XRD, whose detailed processes and methods have been previously described by determined by XRD, whose detailed processes and methods have been previously described by Han Han [22]. The data were analyzed with software (jade 6.5, Modern Dispersions, Convent, LA, USA) to [22]. The data were analyzed with software (jade 6.5, Modern Dispersions, Convent, LA, USA) to calculate the content of minerals in the ore. The results of concentrate analysis are shown in Figure9, calculate the content of minerals in the ore. The results of concentrate analysis are shown in Figure 9, Table5 and tailings analysis is shown in Figure 10, Table6. Table 5 and tailings analysis is shown in Figure 10, Table 6. It is shown in Figure9 and Table5 that pyrite is the main mineral in flotation concentrate, with a It is shown in Figure 9 and Table 5 that pyrite is the main mineral in flotation concentrate, with high content of 84.34%. It also contains a small amount of galena and pyrrhotite, but the gangue mineral a high content of 84.34%. It also contains a small amount of galena and pyrrhotite, but the gangue content is small. As shown in Figure 10 and Table6, gangue minerals such as quartz, muscovite and mineral content is small. As shown in Figure 10 and Table 6, gangue minerals such as quartz, potassium feldspar are the main types of flotation tailings. Comparing the analysis results of flotation muscovite and potassium feldspar are the main types of flotation tailings. Comparing the analysis concentrate and flotation tailings, it can be inferred that the minerals that can be comprehensively results of flotation concentrate and flotation tailings, it can be inferred that the minerals that can be recovered and utilized, as well as the main objective minerals, have mostly been enriched in flotation comprehensively recovered and utilized, as well as the main objective minerals, have mostly been concentrate, and the flotation effect is satisfactory. enriched in flotation concentrate, and the flotation effect is satisfactory.

Figure 9. XRD detection and analysis of concentrate composition.

Minerals 2018, 8, x FOR PEER REVIEW 9 of 12

a 100 250 b 250 100

200 200 Te Grade Te Recovery 80 Au Grade Au Recovery 80 Ag Grade 150 150 Ag Recovery Te Grade Te Recovery

Grade (g/t) Grade (g/t) Grade Au Grade Au Recovery Recovery (%) Recovery 100 Ag Grade Ag Recovery (%) Recovery 100 60 60

50 50 012345678 012345678 Time (d) Time (d) Figure 8. (a) Production data obtained from the operation site of a 300 t/d concentrator in Xiaoqinling: content of −0.074 mm (66%); CaO adjusts pulp pH: 9; collector: sodium isoamyl xanthate 90 g/t; roughing pulp density: 33%; (b)The 300 t/d Industrial test results after optimization of process conditions: content of −0.074 mm (70%); NaOH adjusts pulp pH: 8; combined collector: sodium isoamyl xanthate + ethyl thiocarbamate (1:1) 120 g/t; pulp density: 33%.

3.4. Flotation Product Analysis The flotation concentrates and tailings crystal phase composition in the industrial tests was determined by XRD, whose detailed processes and methods have been previously described by Han [22]. The data were analyzed with software (jade 6.5, Modern Dispersions, Convent, LA, USA) to calculate the content of minerals in the ore. The results of concentrate analysis are shown in Figure 9, Table 5 and tailings analysis is shown in Figure 10, Table 6. It is shown in Figure 9 and Table 5 that pyrite is the main mineral in flotation concentrate, with a high content of 84.34%. It also contains a small amount of galena and pyrrhotite, but the gangue mineral content is small. As shown in Figure 10 and Table 6, gangue minerals such as quartz, muscovite and potassium feldspar are the main types of flotation tailings. Comparing the analysis results of flotation concentrate and flotation tailings, it can be inferred that the minerals that can be comprehensively recovered and utilized, as well as the main objective minerals, have mostly been

Mineralsenriched2019 in, 9, flotation 597 concentrate, and the flotation effect is satisfactory. 10 of 12

Minerals 2018, 8, x FOR PEER REVIEW 10 of 12 Figure 9. XRD detection and analysis of concentrate composition. Figure 9. XRD detection and analysis of concentrate composition. Table 5. Mineral composition content of concentrate. Table 5. Mineral composition content of concentrate. Mineral Composition Chemical Formula Content (%) Mineral Composition Chemical Formula Content (%) Pyrite FeS2 84.34 GalenaPyrite FeS2 PbS 84.34 1.20 PyrrhotiteGalena PbSFe1 − xS 1.20 1.45 Pyrrhotite Fe1 xS 1.45 Muscovite (K,Na)(Al,Fe)− 2(Si,Al)4O10(OH)2 13.01 Muscovite (K,Na)(Al,Fe)2(Si,Al)4O10(OH)2 13.01

Figure 10. XRD detection and analysis of the tailings composition. Figure 10. XRD detection and analysis of the tailings composition.

Table 6. Mineral composition content of tailings.

Mineral Composition Chemical Formula Content (%) Quartz SiO2 24.87 Muscovite (K,Na)(Al,Fe)2(Si,Al)4O10(OH)2 30.94 Plagioclase (Na,Ca)(Al1.02Si2.98O8) 10.82 Calcite (Mg0.03Ca0.97)(CO3) 6.38 Potash feldspar KAl2(Si3Al)O10(OH,F)2 26.98

4. Conclusions There were five kinds of Au, Ag, and Te minerals in the original ore: natural gold, antamokite, calaverite, hessite and altaite, which exist in ore fissures and gangue minerals and are encapsulated in metal sulfide minerals; the size of the Au, Ag, and Te minerals was small the particle size of the natural gold, calaverite and hessite are below 0.038 mm. The ore type was primary gold ore containing polymetallic sulfide with a quartz vein. The comprehensive recovery of telluride-type gold ores is suitable for low alkalinity condition, and ethyl sulfur nitrogen is suitable as a collector for hessite and altaite, both of which are more closely related to galena. Xanthate collectors work well for collecting Au and Ag. When a primary gold ore containing polymetallic sulfide with quartz vein was processed under the new technology, the average grades of the Te, Au and Ag were 241.61, 90.30, and 92.74 g/t in the flotation concentrate, and the recovery rates were 95.42%, 97.28%, and 94.65% respectively, so excellent technical indicators were obtained. Compared with the original flotation process, the

Minerals 2019, 9, 597 11 of 12

Table 6. Mineral composition content of tailings.

Mineral Composition Chemical Formula Content (%)

Quartz SiO2 24.87 Muscovite (K,Na)(Al,Fe)2(Si,Al)4O10(OH)2 30.94 Plagioclase (Na,Ca)(Al1.02Si2.98O8) 10.82 Calcite (Mg0.03Ca0.97)(CO3) 6.38 Potash feldspar KAl2(Si3Al)O10(OH,F)2 26.98

4. Conclusions There were five kinds of Au, Ag, and Te minerals in the original ore: natural gold, antamokite, calaverite, hessite and altaite, which exist in ore fissures and gangue minerals and are encapsulated in metal sulfide minerals; the size of the Au, Ag, and Te minerals was small the particle size of the natural gold, calaverite and hessite are below 0.038 mm. The ore type was primary gold ore containing polymetallic sulfide with a quartz vein. The comprehensive recovery of telluride-type gold ores is suitable for low alkalinity condition, and ethyl sulfur nitrogen is suitable as a collector for hessite and altaite, both of which are more closely related to galena. Xanthate collectors work well for collecting Au and Ag. When a primary gold ore containing polymetallic sulfide with quartz vein was processed under the new technology, the average grades of the Te, Au and Ag were 241.61, 90.30, and 92.74 g/t in the flotation concentrate, and the recovery rates were 95.42%, 97.28%, and 94.65% respectively, so excellent technical indicators were obtained. Compared with the original flotation process, the recovery rates of the Te, Au, and Ag increased by 19.91%, 6.93%, and 5.67%. The comprehensive recovery of Te, Au and Ag in telluride-type gold mines can thus be achieved.

Author Contributions: W.Y. and G.W. conceived of and designed the experiments; W.Y. prepared the samples and performed the experiments; W.Y., G.W., Q.W., and H.C., analyzed the data; W.Y., G.W., Q.W., K.Z., and P.D. contributed to the writing and revising of the paper. Funding: We acknowledge the financial support provided by the National Natural Science Foundation of China (Grant No. 51474169), the Shaanxi Provincial Institute of Industrial Science and Technology (Grant No. 2016GY-154), the Key Laboratory of Gold and Resources in Shaanxi Province, Xi’an, China (-710055), the Education Department of Shaanxi Province Key Laboratory Project (Grant No. 18JS061) and the Key Science and Technology Program of Shaanxi Province, China (Grant No. 2018GY-080). Conflicts of Interest: The authors declare no conflict of interest.

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