Selective Separation of Chalcopyrite from Galena Using a Green Reagent Scheme

Article Selective Separation of Chalcopyrite from Galena Using a Green Reagent Scheme

Kaile Zhao 1,2,3, Chao Ma 1,4, Guohua Gu 1,* and Zhiyong Gao 1,*

1 School of Minerals Processing and Bio-Engineering, Central South University, Changsha 410083, China; [email protected] (K.Z.); [email protected] (C.M.) 2 State Key Laboratory of Mineral Processing, Beijing 100162, China 3 Institute of Multipurpose Utilization of Mineral Resources, Chinese Academy of Geological Sciences, Chengdu 610041, China 4 Hunan Research Academy of Environmental Sciences, Changsha 410004, China * Correspondence: [email protected] (G.G.); [email protected] (Z.G.)

Abstract: The study of the depression effect of non-toxic depressants on the ﬂotation separation of chalcopyrite from galena is of great importance for both industrial applications and theoretical

research. The mixed depressant (DFinal) of four common inhibitors—sodium carboxymethyl cellulose, sodium silicate, sodium sulﬁte, and zinc sulfate—exhibited high selectivity during the separation of chalcopyrite from galena. Flotation tests on an industrial copper–lead bulk concentrate showed that using this depressant mixture can achieve highly efﬁcient separation of chalcopyrite from galena at the natural pH of the pulp. Copper and lead concentrates were produced at grades of 21.88% (Cu) and 75.53% (Pb), with recoveries of 89.07% (Cu) and 98.26% (Pb). This showed a similar performance

of DFinal with dichromate, which is a depressant that is widely used in industry, but without the

environmental risks or the need for pH control. Zeta potential and Fourier transform infrared (FT-IR) results showed that interaction between the surface of the chalcopyrite and the mixed depressant

Citation: Zhao, K.; Ma, C.; Gu, G.; was prevented by pre-treatment with a composite thiophosphate collector (CSU11), while the mixed Gao, Z. Selective Separation of depressant could expel/replace the composite thiophosphate on the surface of galena by chemical Chalcopyrite from Galena Using a adsorption, depressing its flotation. This is the reason why this non-toxic depressant achieved the Green Reagent Scheme. Minerals 2021, selective depression of galena from chalcopyrite, leading to efficient flotation separation. 11, 796. https://doi.org/10.3390/ min11080796 Keywords: chalcopyrite; galena; non-toxic depressants; selective separation

Academic Editor: Cyril O’Connor

Received: 7 June 2021 1. Introduction Accepted: 20 July 2021 Copper and lead are non-ferrous metals widely used in modern industry. They are Published: 22 July 2021 mainly extracted from chalcopyrite (CuFeS2) and galena (PbS), which are often associated with each other [1–3]. Due to the similar flotation behavior of chalcopyrite and galena, their Publisher’s Note: MDPI stays neutral separation has been always one of the difficult problems in the processing of copper-lead– with regard to jurisdictional claims in zinc sulfide ores [4–8]. The commonlyused collectors such as xanthate and dithiophosphate published maps and institutional affil- iations. show poor selectivity between chalcopyrite and galena, so it is necessary to add a depressant to achieve separation [9–12]. There are two traditional technical schemes for the separation of chalcopyrite and galena. One uses dichromate to depress galena and float chalcopyrite, and the other uses cyanide to depress chalcopyrite and float galena [13–15]. However, because Cr6+ ions and cyanide pose serious environmental problems, flotation Copyright: © 2021 by the authors. processes without cyanide and Cr6+ are gradually replacing the traditional flotation pro- Licensee MDPI, Basel, Switzerland. cesses. As a result, the development of non-toxic and environmentally friendly depressants This article is an open access article has become the main research direction [16–18]. These depressants can be divided into distributed under the terms and conditions of the Creative Commons inorganic depressants, such as sulfite [19–21], polyphosphate [22], sodium sulfide [23], and Attribution (CC BY) license (https:// lime, and organic depressants such as dextrin [24], sodium humate [25], and alginate [2]. creativecommons.org/licenses/by/ In recent years, organic depressants have been widely used depending on their en- 4.0/). vironmental friendliness, availability, and selectivity [16,26–28]. It has been reported that

Minerals 2021, 11, 796. https://doi.org/10.3390/min11080796 https://www.mdpi.com/journal/minerals Minerals 2021, 11, 796 2 of 13

sodium carboxymethyl cellulose (CMC) is an effective depressant for galena, and it can be used to separate chalcopyrite from galena due to the carboxyl substituent groups along the cellulose chain [2,17,24,29,30]. However, it also negatively impacts the floatability of chalcopyrite, leading to decreased chalcopyrite recovery. As an inorganic depressant, sodium silicate can also be used to separate chalcopyrite from galena [8]. However, on its own, it is not effective in flotation processes. Sodium sulfite and zinc sulfate constitute a commonly used combined depressant for separating galena from sphalerite in the flotation of sulfide ores [31]. Sodium sulfite itself is a multi- functional reagent with various uses in the industrial flotation of sulfide minerals and in hydrometallurgy [21]. The zinc hydroxide precipitate is the species from the zinc sulfate solution which inhibits the flotation of the sphalerite [32]. In this study, the non-toxic, environmentally friendly mixture of sodium carboxymethyl cellulose (CMC) (an organic depressant) and sodium silicate, sodium sulfite, and zinc sulfate (all inorganic depressants) was used as a depressant for the separation of chalcopyrite from galena. For the first time, a combination of an organic depressant and inorganic depressants was used in a chalcopyrite–galena flotation system as a clean and efficient depressant of galena. High-purity specimens of chalcopyrite and galena and an industrial copper– lead bulk concentrate were selected as test samples. The effect of the mixed depressant was studied through laboratory flotation experiments on the high-purity, single-mineral samples and samples from an industrial copper–lead bulk concentrate. The interaction mechanisms between the reagents and the minerals were studied using zeta potential and Fourier transform infrared spectrum (FT-IR) measurements. Finally, a green flotation and reagent system for the separation of chalcopyrite from galena was established.

2. Materials and Methods 2.1. Samples and Reagents High-purity samples of chalcopyrite and galena were obtained from Beijing Shuiyuan Shanchang mineral specimen Co. LTD. Using chemical assays and X-ray diffraction (XRD, Figure1a,b; The Netherlands, Almelo, Malvern Panalytical X Pert pro), the purities of chalcopyrite and galena were found to be 94.92% (32.86% Cu, 29.03% Fe, 33.17% S) and 97.67% (84.58% Pb, 13.09% S), respectively. The samples were dry ground and screened to produce −74 + 38 µm and −38 µm samples for the flotation tests and the interaction mechanism measurements (zeta-potential and FT-IR), respectively. The ground minerals were all sealed in the jar and put into a vacuum tank to avoid the oxidation of the sulfides surface. The commercial copper–lead bulk concentrate for the locked-cycle batch flotation tests was obtained from the Nanan Copper–lead–zinc Mine, Guangxi province, China. The chemical assays are shown in Table1. Table2 displays the chemical phase of copper and lead in the copper–lead bulk concentrate, and Table3 displays its mineral composition.

Table 1. Chemical analysis of the industrial copper–lead bulk concentrate (wt %).

Element Cu Pb Zn S Fe As Al2O3 SiO2 CaO MgO Ag Grade 3.53 65.84 2.11 16.63 5.32 0.021 0.41 2.89 1.66 0.73 752.5 (g/t)

Table 2. Chemical phase of copper and lead in the copper–lead bulk concentrate (wt %).

Chemical Phase of Primary Copper Secondary Copper Conjunction Copper Copper Sulfide Sulfide Free Copper Oxide Oxide Total wt % 3.26 0.078 0.16 0.032 3.53 % of Total Cu 92.35 2.21 4.53 0.91 100.00 Chemical Phase of Lead Lead Sulfide Lead Oxide Plumbojarosite Total wt % 64.81 0.62 0.41 65.84 % of Total Pb 98.44 0.94 0.62 100.00 Minerals 2021, 11, 796 3 of 14

Table 2. Chemical phase of copper and lead in the copper–lead bulk concentrate (wt %).

Chemical Phase of Primary Copper Secondary Copper Free Copper Conjunction Copper Total Copper Sulfide Sulfide Oxide Oxide wt % 3.26 0.078 0.16 0.032 3.53 % of Total Cu 92.35 2.21 4.53 0.91 100.00 Chemical Phase of Lead Sulfide Lead Oxide Plumbojarosite Total Lead wt % 64.81 0.62 0.41 65.84 % of Total Pb 98.44 0.94 0.62 100.00

Table 3. The mineral composition of the copper–lead bulk concentrate (wt %).

Mineral wt % Mineral wt % Galena 75.1 Tetrahedrite 1.8 Chalcopyrite 9.4 Quartz 4.3 Sphalerite 3.2 Calcite 0.7 Pyrite 2.2 Feldspar 0.7 Chalcocite 2.2 Chlorite 0.4

In the experiments, the reagent CSU11 (a composite thiophosphate) was used as a dual-function reagent, acting as both collector and frother. In addition, sodium carboxymethyl cellulose (CMC, whose structure is illustrated in Figure 1c) [33], sodium silicate (Na2O2.7SiO2, SS), sodium sulfite (Na2SO3), zinc sulfate (ZnSO4), and potassium dichromate (K2Cr2O7) were used as depressants. All reagents used in the micro-flotation tests and mechanism measurements were of analytical (AR) grade. The reagents used in the batch flotation tests were tech grade. Deionized water was used for the micro-flotation Minerals 2021, 11, 796 tests, zeta potential, and FT-IR measurements. Tap water was used in the batch flotation3 of 13 tests.

G C （a） Chalcopyrite-C Galena-G （b）

G G G G G C C G C C C C C C C C C

10 20 30 40 50 60 70 80 90 10 20 30 40 50 60 70 80 90 2 Theta (°) 2 Theta (°)

OH CH2OH

O O OH OH O O

CH2OCH2COONa OH n

(c)

FigureFigure 1. 1. XRDXRD pattern pattern of of ( (aa)) chalcopyrite chalcopyrite and and ( (bb)) galena; galena; ( (cc)) the the structure structure of of CMC. CMC.

Table 3. The mineral composition of the copper–lead bulk concentrate (wt %).

Mineral wt % Mineral wt % Galena 75.1 Tetrahedrite 1.8 Chalcopyrite 9.4 Quartz 4.3 Sphalerite 3.2 Calcite 0.7 Pyrite 2.2 Feldspar 0.7 Chalcocite 2.2 Chlorite 0.4

In the experiments, the reagent CSU11 (a composite thiophosphate) was used as a dual-function reagent, acting as both collector and frother. In addition, sodium carboxymethyl cellulose (CMC, whose structure is illustrated in Figure1c) [ 33], sodium silicate (Na2O2.7SiO2, SS), sodium sulfite (Na2SO3), zinc sulfate (ZnSO4), and potassium dichromate (K2Cr2O7) were used as depressants. All reagents used in the micro-flotation tests and mechanism measurements were of analytical (AR) grade. The reagents used in the batch flotation tests were tech grade. Deionized water was used for the micro-flotation tests, zeta potential, and FT-IR measurements. Tap water was used in the batch flotation tests.

2.2. Flotation Experiments The micro-flotation experiments were carried out in an XFG−40 mL flotation machine (Figure2; XFG II, Jilin Exploration Machinary Plant, Jilin, China). The impeller speed of the flotation machine was set to 1800 rpm. During each experiment, 2.0 g of sample was placed in a Plexiglass cell, which was then filled with 35 mL of distilled water. After agitating the pulp for 1 min, the collector (conditioning time = 2 min) and the depressant (conditioning time = 2 min) were added in sequence. Then, flotation was performed for 5 min. The concentrate (floated by the air bubbles) and tailings were weighed separately after filtration and drying, and the recovery was calculated based on the weight of the products. Three micro-flotation experiments were performed for each set of conditions, and the average results are reported. Minerals 2021, 11, 796 4 of 14

2.2. Flotation Experiments The micro-flotation experiments were carried out in an XFG−40 mL flotation machine (Figure 2; XFGII, Jilin Exploration Machinary Plant, Jilin, China). The impeller speed of the flotation machine was set to 1800 rpm. During each experiment, 2.0 g of sample was placed in a Plexiglass cell, which was then filled with 35 mL of distilled water. After agitating the pulp for 1 min, the collector (conditioning time = 2 min) and the depressant (conditioning time = 2 min) were added in sequence. Then, flotation was performed for 5 min. The concentrate (floated by the air bubbles) and tailings were Minerals 2021, 11, 796 weighed separately after filtration and drying, and the recovery was calculated based4 of 13on the weight of the products. Three micro-flotation experiments were performed for each set of conditions, and the average results are reported.

FigureFigure 2. 2.XFG XFG−−4040 mLmL ﬂotationflotation machinemachine withwith thethe ﬂotationflotation cell,cell, damdam board,board, andand scraper.scraper.

TheThe batchbatch flotationflotation teststests forfor thethe industrialindustrial copper–leadcopper–lead bulkbulk concentrateconcentrate werewere per-per- formedformed in in an an XFD-63 XFD-63 flotation flotation cell, cell, using using a 1000 a 1000 mL mL cell cell for thefor rougherthe rougher and scavengerand scavenger and aand 500 a mL 500 cell mL for cell the for cleaner. the cleaner. The impeller The impeller speed speed was againwas again set to set 1800 to rpm.1800 rpm. During During the flotationthe flotation tests, tests, the reagents the reagents were addedwere added in sequence in sequence to the to flotation the flotation cell at 2cell min at intervals,2 min in- andtervals, then and flotation then wasflotation performed was performed for 5 min. for The 5 finalmin. productsThe final ofproducts the concentrates of the concen- and tailingstrates and were tailings filtered, were dried, filtered, weighed, dried, and weighed analyzed, and for analyzed Cu and for Pb, Cu respectively. and Pb, respec- The flowsheettively. The of flowsheet the batch locked-cycle of the batch tests locked is- showncycle tests in Figure is shown3. The in experimental Figure 3. The system experi- usedmental the system non-toxic used depressant the non-toxic mixture depressant of CMC mixture + SS +sodium of CMC sulfite + SS + +sodium zinc sulfate, sulfite while + zinc thesulfate, baseline while comparison the baseline system comparison used potassium system bichromate used potassium as the depressant.bichromate To as ensure the de- apressant. high degree To ensure of confidence a high degree in the of batch confidence flotation in the tests, batch the flotation calculated tests, feed the grade calculated was comparedfeed grade to was the headcompared assay. to If thethe calculatedhead assay. Cu If feed the gradecalculated did notCu fallfeed within grade the did range not fall of 3.53within± 0.02%, the range then of the 3.5 concentrates3 ± 0.02%, then and the tailings concentrates were re-assayed, and tailings and ifwe there newre-assayed, calculated and Cuif the feed new grade calculated remained Cu outsidefeed grade this remained range, then outside those this results range, were then discarded, those results and thatwere groupdiscarded, of flotation and that tests group was of repeated. flotation tests was repeated.

2.3. Zeta Potential Tests Zeta potential tests on chalcopyrite and galena were carried out using a Coulter DELSA440S II (Beckman Coulter & Company, Brea, CA, USA). The −38 µm fractions were

ﬁrst hand milled to about −5 µm using an agate mortar. Then, 2.0 g of sample was added to 40 mL of deionized water with or without the prescribed amount of reagent, and the suspension was ultrasonicated for 3 min and then magnetically stirred for 10 min. Then, the pH was adjusted by adding HCl or NaOH. The pH value was measured using a PHS-3C type pH meter. Then, the zeta potential of each sample was measured separately. The test results were found to be within 3 mV after at least three tests under each experimental condition, and the average values are reported. Minerals 2021, 11, 796 5 of 13 Minerals 2021, 11, 796 5 of 14

FigureFigure 3.3. Flowsheet ofof thethe batchbatch locked-cyclelocked-cycle tests.tests.

2.4.2.3. FT-IRZeta Potential Tests Tests TheZeta following potential procedure tests on chalcopyrite was used for and the FT-IR galena spectra were measurements.carried out using The a − Coulter38 µm fractionsDELSA440S were II first(Beckman hand milled Coulter to & about Company−5 µm, Brea, using CA, an agate USA).mortar. The −38 The μm spectra fractions of thewere solids first werehand measuredmilled to about against −5 aμm KBr using background an agate spectrum.mortar. Then, For 2.0 each g of experimental sample was condition,added to 40 2.0 mL g of of sample deionized was water added with into or 30 without mL aqueous the prescribed solution with amount or without of reagent, the prescribedand the suspension amount of was reagent, ultrasonicated followed by for ultrasonication 3 min and then for magnetically 5 min. Then, stirred the solution for 10 wasmin. magnetically Then, the pH stirred was adjusted for 40 min by and adding allowed HCl to or settle NaOH. for anotherThe pH 40 value min. was After measured filtering andusing vacuum a PHS drying,-3C type the pH Fourier meter. transform Then, the infrared zeta potential (FT-IR) spectrumof each sample was recorded was measured using a Nicoletseparately. FT-IR-740 The test spectrometer results were (ThemoScientific, found to be within Waltham, 3 mV after MA, at USA). least three tests under each experimental condition, and the average values are reported. 3. Results and Discussion 3.1. Micro-Flotation Test Results 2.4. FT-IR Tests Figure4 shows the effect of the concentration of the single SS and CMC, and their The following procedure was used for the FT-IR spectra measurements. The −38 μm depressant mixtures on chalcopyrite and galena flotation at natural pH. During the tests, fractions were first hand milled to about −5 μm using an agate mortar. The spectra of the the pH was observed in the range of pH 7–7.5. solids were measured against a KBr background spectrum. For each experimental con- As shown in Figure4a, single CMC and SS all showed some depression on the flotation dition, 2.0 g of sample was added into 30 mL aqueous solution with or without the pre- of chalcopyrite and galena. However, they either inhibited both the two minerals slightly scribed amount of reagent, followed by ultrasonication for 5 min. Then, the solution was (CMC) or inhibited both of them stronger (SS); i.e., the selectivities of SS and CMC are magnetically stirred for 40 min and allowed to settle for another 40 min. After filtering all poor. and vacuum drying, the Fourier transform infrared (FT-IR) spectrum was recorded using Two depressant mixtures were used—SM1 (with a 50:1 mass ratio of SS and CMC) a Nicolet FT-IR-740 spectrometer (ThemoScientific, Waltham, MA, USA). and SM2 (with a 20:1 mass ratio). Although both SM1 and SM2 significantly reduced the floatability of galena, it was reduced more by SM1 than SM2. Meanwhile, SM1 significantly 3. Results and Discussion depressed galena, and it had a small effect on chalcopyrite. The best separation results were observed3.1. Micro when-Flotation the SM1Test Results concentration was approximately 200 mg/L. At this concentration, the recoveriesFigure 4 shows of chalcopyrite the effect andof the galena concentration were 85.5% of andthe single 8.4%, respectively.SS and CMC, Moreover, and their compareddepressant with mixture singles on CMC chalcopyrite and SS, their and mixtures galena fl SM1otation and at SM2 natural showed pH. betterDuring selectivity, the tests, andthe pH the was optimal observed depressant in the concentrationrange of pH 7 became–7.5. lower than that of SS. FromAs shown Figure in4 Figureb, it can 4a, be single found CMC that and the SS depression all showed effect some of depression SM1 varied on signifi-the flo- cantlytation of with cha concentration.lcopyrite and galena. Figure 5However, shows the they effect either of the inhibited combined both use the of two SM1 minerals (fixed 200slightly mg/L), (CMC sodium) or inhibited sulfite, and both zinc of sulfate them on stronger chalcopyrite (SS); i.e., and the galena selectivit flotationies atof natural SS and pHCMC (pH are 7–7.5 all poor.). As shown in Figure5a, the addition of sodium sulfite to SM1 increased the difference in floatability between chalcopyrite and galena, and at the same time, it also had an activation effect on the flotation of chalcopyrite. When zinc sulfate was added to SM1 and sodium sulfite, the floatability difference between the two minerals further enlarged (Figure5b). The mixed depressant (D Final) with a mass ratio of 1:2:2 (SM1: sodium

Minerals 2021, 11, 796 6 of 13

sulfite: zinc sulfate = 1:2:2) achieved the best flotation performance. Chalcopyrite had a very favorable recovery of 99.6%, and the recovery of galena was 0.7%, indicating that a Minerals 2021, 11, 796 highly efficient separation of chalcopyrite from galena could be achieved at natural pH 6 of 14

using this depressant mixture.

CMC dosage (mg/L) 0 2 4 6 8 10 100 (a) 90

80 Chalcopyrite+SS Galena+SS 70 Chalcopyrite+CMC

Galena+CMC

60 Recovery Recovery (%) 50

pH 7–7.5 40 CSU11 50 mg/L

30 0 50 100 150 200 250 300 350 SS dosage (mg/L)

100 (b) 90

60 Chalcopyrite+ SM1

Galena+ SM1 50 Chalcopyrite+ SM2

40 Galena+ SM2 Recovery (%) Recovery

20 pH 7–7.5 10 CSU11 concentration 50 mg/L

0 0 50 100 150 200 250 Depressant dosage (mg/L)

Figure 4. Effect of (a) SS/CMC and (b) their mixtures SM1/SM2 on single mineral flotation behavior. Figure 4. Effect of (a) SS/CMC and (b) their mixtures SM1/SM2 on single mineral flotation behav- Figure6 shows the effect of mixed depressant (SM1: sodium sulfite: zinc sulfate = 1:2:2, ior. DFinal) concentration on chalcopyrite and galena flotation at natural pH (pH 7–7.5). The mixed depressant DFinal selectively depressed galena. The effect was significant: galena recoveryTwo rapidly depressant decreased withmixtures increasing were depressant used—SM1 concentration (with a until50:1 900mass mg/L ratio (at of SS and CMC) whichand theSM2 recovery (with ofa 20:1 galena mass was ratio 3.3%),). withAlthough little effect both on SM1 chalcopyrite and SM2 flotation significantly (the reduced the recoveryfloatability of chalcopyrite of galena, was it 93%). was As reduced the concentration more by of SM1 mixed than depressant SM2. DMeanFinal waswhile, SM1 signifi- increased further, the recovery of chalcopyrite decreased to some degree. cantly depressed galena, and it had a small effect on chalcopyrite. The best separation results were observed when the SM1 concentration was approximately 200 mg/L. At this concentration, the recoveries of chalcopyrite and galena were 85.5% and 8.4%, respectively. Moreover, compared with single CMC and SS, their mixtures SM1 and SM2 showed better selectivity, and the optimal depressant concentration became lower than that of SS. From Figure 4b, it can be found that the depression effect of SM1 varied significantly with concentration. Figure 5 shows the effect of the combined use of SM1 (fixed 200 mg/L), sodium sulfite, and zinc sulfate on chalcopyrite and galena flotation at natural pH (pH 7–7.5). As shown in Figure 5a, the addition of sodium sulfite to SM1 increased the difference in floatability between chalcopyrite and galena, and at the same time, it also had an activation effect on the flotation of chalcopyrite. When zinc sulfate was added to SM1 and sodium sulfite, the floatability difference between the two minerals further enlarged (Figure 5b). The mixed depressant (DFinal) with a mass ratio of 1:2:2 (SM1: sodium sulfite: zinc sulfate = 1:2:2) achieved the best flotation performance. Chal- copyrite had a very favorable recovery of 99.6%, and the recovery of galena was 0.7%,

Minerals 2021, 11, 796 7 of 14

indicating that a highly efficient separation of chalcopyrite from galena could be achieved at natural pH using this depressant mixture. Minerals 2021, 11, 796 7 of 13

100

70 (a) 60 Chalcopyrite

50 Galena

40 pH 7–7.5 Recovery Recovery (%) 30 SM1 200 mg/L 20 CSU11 50 mg/L 10

0 100 200 300 400 Na SO dosage (mg/L) 2 3

100

80 (b) 70

60 Calcopyrite Galena

40 pH 7–7.5 Recovery Recovery (%) 30 SM1 200 mg/L+Na SO 400mg/L 2 3 20 CSU11 50 mg/L

0 0 100 200 300 400 ZnSO dosage (mg/L) 4

Minerals 2021, 11, 796 Figure 5. (a) Effect of Na2SO3 dosage on single mineral ﬂotation behavior; (b) Effect of ZnSO4 dosage8 of 14

Figureon single 5. (a mineral) Effect of ﬂotation Na2SO behavior.3 dosage on single mineral flotation behavior; (b) Effect of ZnSO4 dosage on single mineral flotation behavior.

F100igure 6 shows the effect of mixed depressant (SM1: sodium sulfite: zinc sulfate = 1:2:2, DFinal) concentration on chalcopyrite and galena flotation at natural pH (pH 7–7.5).

The mixed80 depressant DFinal selectively depressed galena. The effect was significant: galena recovery rapidly decreased with increasing depressant concentration until 900 mg/L (at which the recovery of galena was 3.3%) ,Chalcopyrite with little effect on chalcopyrite flotation (the 60 Galena recovery of chalcopyrite was 93%). As the concentration of mixed depressant DFinal was

pH 7–7.5 increased further, the recovery of chalcopyrite decreased to some degree. 40

Recovery (%) Recovery CSU11 50 mg/L 20

0 300 600 900 1200 1500 Depressant dosage (mg/L)

FigureFigure 6. 6.Effect Effect of of depressant depressant mixture mixture D DFinalFinal concentration on single mineral flotationflotation behavior.behavior. 3.2. Batch Flotation Test Results 3.2. Batch Flotation Test Results Based on the results of the micro-flotation tests and the subsequent flotation condi- Based on the results of the micro-flotation tests and the subsequent flotation condition tests, batch locked-cycle testing was carried out using SM1 + sodium sulfite + zinc tion tests, batch locked-cycle testing was carried out using SM1 + sodium sulfite + zinc sulfate (mass ratio of 1:2:2) as a mixed depressant (DFinal) at natural pH (experimental system). The comparison test was conducted using the industry-standard depressant potassium dichromate. The closed flotation circuit included one roughing, one scavenging, and three cleaning steps (shown in Figure 3). The flotation conditions are shown in Table 4. The best flotation result achieved is shown in Table 5. Concentrate grades of 22.8% Cu and 75.34% Pb at recoveries of 90% Cu and 98.49% Pb were achieved using the industry standard depressant potassium dichromate. Concentrate grades of 21.88% Cu and 75.53% Pb at recoveries of 89.07% Cu and 98.26% Pb were achieved using the depressant mixture DFinal. This matched the performance of the industry standard depressant but without the environmental risks or the need for pH control. This illustrates that the 1:2:2 depressant mixture SM1 + sodium sulfite + zinc sulfate has great potential as an effective, non-toxic depressant for industrial application in the flotation of copper–lead bulk concentrate.

Table 4. Flotation conditions of the batch locked-cycle tests.

Locked-Cycle Test Reagent Concentrations (g/t) Experimental System Flotation Process Comparison System Non-Toxic Depressant Potassium Dichromate (SM1: Sodium Sulfite: Zinc Sulfate = 1:2:2) Rougher CSU11 40, DFinal 850 CSU11 40, potassium dichromate 100 Scavenger CSU11 15 CSU11 15 1st cleaner DFinal 400 Potassium dichromate 50 2nd cleaner DFinal 260 Potassium dichromate 20 3rd cleaner DFinal 100 Potassium dichromate 10

Table 5. Results of the batch locked-cycle tests.

Grades (%) Recovery (%) Flotation System Products Ratio (wt %) Cu Pb Cu Pb Experimental system Cu concentrate 14.36 21.88 7.96 89.07 1.74 (Non-toxic depressant, DFinal) Pb concentrate 85.64 0.45 75.53 10.93 98.26

Minerals 2021, 11, 796 8 of 13

sulfate (mass ratio of 1:2:2) as a mixed depressant (DFinal) at natural pH (experimental system). The comparison test was conducted using the industry-standard depressant potassium dichromate. The closed flotation circuit included one roughing, one scavenging, and three cleaning steps (shown in Figure3). The flotation conditions are shown in Table4. The best flotation result achieved is shown in Table5. Concentrate grades of 22.8% Cu and 75.34% Pb at recoveries of 90% Cu and 98.49% Pb were achieved using the industry standard depressant potassium dichromate. Concentrate grades of 21.88% Cu and 75.53% Pb at recoveries of 89.07% Cu and 98.26% Pb were achieved using the depressant mixture DFinal. This matched the performance of the industry standard depressant but without the environmental risks or the need for pH control. This illustrates that the 1:2:2 depressant mixture SM1 + sodium sulfite + zinc sulfate has great potential as an effective, non-toxic depressant for industrial application in the flotation of copper–lead bulk concentrate.

Table 4. Flotation conditions of the batch locked-cycle tests.

Locked-Cycle Test Reagent Concentrations (g/t) Experimental System Flotation Process Comparison System Non-Toxic Depressant Potassium Dichromate (SM1: Sodium Sulﬁte: Zinc Sulfate = 1:2:2)

Rougher CSU11 40, DFinal 850 CSU11 40, potassium dichromate 100 Scavenger CSU11 15 CSU11 15 1st cleaner DFinal 400 Potassium dichromate 50 2nd cleaner DFinal 260 Potassium dichromate 20 3rd cleaner DFinal 100 Potassium dichromate 10

Table 5. Results of the batch locked-cycle tests.

Grades (%) Recovery (%) Flotation System Products Ratio (wt %) Cu Pb Cu Pb Cu concentrate 14.36 21.88 7.96 89.07 1.74 Experimental system Pb concentrate 85.64 0.45 75.53 10.93 98.26 (Non-toxic depressant, D ) Final Feed 100.00 3.53 65.83 100.00 100.00 Cu concentrate 13.93 22.80 7.12 90.00 1.51 Comparison system Pb concentrate 86.07 0.41 75.34 10.00 98.49 (Potassium dichromate) Feed 100.00 3.52 65.84 100.00 100.00

3.3. Selective Adsorption Mechanism of the Mixed Depressant Zeta potential and FT-IR measurements were conducted to investigate the selective depression mechanism of SM1 (the 50:1 mixing mass ratio of SS and CMC, the key part of the mixed depressant DFinal) in the separation of galena from chalcopyrite. Figure7 shows the effect of the concentration of collector CSU11 on the zeta potentials of chalcopyrite and galena at pH 7.0. Figure8 shows the effect of the concentration of the depressant SM1 on the zeta potentials of chalcopyrite and galena, also at pH 7.0. Before interacting with any reagent, zeta potentials of −24.3 mV and −32.2 mV were obtained for chalcopyrite and galena, respectively. These values are consistent with the previous work of Chen [2]. After the two minerals were treated with anionic CSU11, the zeta potentials decreased, indicating that CSU11 had adsorbed onto the bare mineral surface [34]. As the concentration of collector CSU11 or the depressant SM1 increased, the zeta potential of the chalcopyrite with pre-adsorbed CSU11 (labeled “chalcopyrite + CSU11 + SM1”) remained nearly unchanged, implying that the interaction between the surface of the chalcopyrite and SM1 had been prevented by the CSU11 pre-treatment. However, a remarkable decrease in the zeta potential of galena with pre-adsorbed CSU11 (labeled “galena + CSU11 + SM1”) was observed as the concentration of CSU11 or SM1 increased, the drop indicated Minerals 2021, 11, 796 9 of 14

Feed 100.00 3.53 65.83 100.00 100.00 Cu concentrate 13.93 22.80 7.12 90.00 1.51 Comparison system Pb concentrate 86.07 0.41 75.34 10.00 98.49 (Potassium dichromate) Feed 100.00 3.52 65.84 100.00 100.00

3.3. Selective Adsorption Mechanism of the Mixed Depressant Zeta potential and FT-IR measurements were conducted to investigate the selective depression mechanism of SM1 (the 50:1 mixing mass ratio of SS and CMC, the key part of the mixed depressant DFinal) in the separation of galena from chalcopyrite. Figure 7 shows the effect of the concentration of collector CSU11 on the zeta potentials of chalcopyrite and galena at pH 7.0. Figure 8 shows the effect of the concentration of the depressant SM1 on the zeta potentials of chalcopyrite and galena, also at pH 7.0. Be- fore interacting with any reagent, zeta potentials of −24.3 mV and −32.2 mV were obtained for chalcopyrite and galena, respectively. These values are consistent with the previous work of Chen [2]. After the two minerals were treated with anionic CSU11, the zeta potentials decreased, indicating that CSU11 had adsorbed onto the bare mineral surface [34]. As the concentration of collector CSU11 or the depressant SM1 increased, the zeta potential of the chalcopyrite with pre-adsorbed CSU11 (labeled “chalcopyrite + CSU11 + SM1”) remained nearly unchanged, implying that the interaction between the surface of the chalcopyrite and SM1 had been prevented by the CSU11 pre-treatment. However, a remarkable decrease in the zeta potential of galena with pre-adsorbed CSU11 (labeled “galena + CSU11 + SM1”) was observed as the concentration of CSU11 or SM1 increased, the drop indicated that the anionic SM1 still strongly interacted with the surface of the galena with CSU11 pre-treatment (“galena + CSU11”). These results illustrate that SM1 can preferentially adsorb onto the surface of “galena + CSU11”, while the addition of SM1 has negligible effect on the adsorption of CSU11 onto the surface of the chalcopyrite. In other words, SM1 can expel/replace or cover the adsorbed CSU11 on the galena surface, but fails this on chalcopyrite surface.

Minerals 2021, 11, 796 The difference in SM1 adsorption onto the surfaces of “chalcopyrite + CSU11” and 9“ga- of 13 lena + CSU11” explains the high copper recovery and strong depression of galena when using the “SM1 + sodium sulfite + zinc sulfate” (DFinal) reagent scheme. The results are of great importance in explaining the selective depression effect of SM1 on galena and in definingthat the anionicthe depression SM1 still mechanism strongly interacted of SM1 in with the separation the surface process. of the galena These with result CSU11s are consistentpre-treatment with (“galena the flotation + CSU11”). experiments.

-20

-25 Chalcopyrite+ CSU11 Chalcopyrite+ CSU11+SM1 -30 Galena+ CSU11

) Galena+ CSU11+SM1

mV -35

( pH 7.0

SM1 200 mg/L -40

-45 Zeta potential potential Zeta -50

-55

-60 0 100 200 300 Collector dosage (mg/L) Minerals 2021, 11, 796 10 of 14 Figure 7. Effect of collector CSU11 concentration on the zeta potentials of mineral samples.

Figure 7. Effect of collector CSU11 concentration on the zeta potentials of mineral samples. -20

-25 Chalcopyrite+ CSU11+SM1 Galena+ CSU11+SM1 -30

) -35

mV ( -40

-45 pH 7.0

-50 CSU11 50 mg/L Zeta Zeta potential -55

-60

-65 0 200 400 600 800 1000 Depressant dosage (mg/L) Figure 8.8. Effect of SM1 concentration onon thethe zetazeta potentialspotentials ofof thethe mineralmineral samples.samples.

TheseFigure results 9 shows illustrate the FT- thatIR spectra SM1 can of preferentiallythe collector CSU11 adsorb and onto the the depressant surface of “galenaSM1. In +the CSU11”, FT-IR spectrum while the of addition CSU11, the of SM1bands has at negligible3411 cm−1 and effect 3225 on cm the−1 adsorptionwere attributed of CSU11 to the ontostretching the surface vibration of the of chalcopyrite.-NH [35]. The In bands other at words, 2560 cm SM1−1 an cand expel/replace1039 cm−1 corresponded or cover the to adsorbed-SH and P CSU11-O-C stretching on the galena vibrations, surface, respectively. but fails this onThe chalcopyrite results confirmed surface. the The main difference func- intional SM1 groups adsorption of the onto CSU11 the surfacesmolecule of(-NH, “chalcopyrite -SH, and +P- CSU11”O-C). In andthe “galenaFT-IR spectrum + CSU11” of explainsSM1, the thecharacteristic high copper bands recovery at 3411 and cm strong−1, 2920 depression cm−1, 140 of5 cm galena−1, and when 1027 using cm−1 correspond the “SM1 + sodiumto the stretching sulfite +zinc vibrations sulfate” of (D –FinalOH,) C reagent-H, COO scheme.-, and Si The-O, results respectively are of great [25,36, importance37]. in explaining the selective depression effect of SM1 on galena and in defining the depression mechanism of SM1 in the separation process. These results are consistent with the flotation

experiments. N－ H Figure9 3411showsN－ theH FT-IR spectra of the collector CSU11 and the depressant SM1. In 3225 － SH 1500 −1 −1 the FT-IR spectrum of CSU11,2560 the bandsN－ H at 3411 cm and 3225 cm were attributed to the stretching vibration of -NH [35]. The bands at 2560 cm−1 and 1039 cm−1 corresponded to 2912 1420 2961 － CH 1039 -SH and P-O-C stretching vibrations,2 respectively.N－ H The results conﬁrmed the main functional － CH 3 P－ O－ C 688 groups of the CSU11 molecule CSU11 (-NH, -SH, and P-O-C).P=S In the FT-IR spectrum of SM1, the − − − − characteristic bands at 3411 cm SM11, 2920 cm 1, 1405 cm 1, and 1027 cm 1 correspond to the stretching vibrations of –OH, C-H, COO-, and Si-O, respectively [25,36,37]. Reflectance - － OH － COO 3411 1405 2920 C－ H 1643 1125 1027 C－ O Si－ O

4000 3500 3000 2500 2000 1500 1000 500 -1 Wavenumber(cm ) Figure 9. FT-IR spectra of the collector CSU11 and the depressant SM1 (CSU11 concentration 50 mg/L, SM1 concentration 200 mg/L).

Figures 10 and 11 show the FT-IR spectra of chalcopyrite and galena after being treated with the collector CSU11 and the depressant SM1. After the two minerals were treated with CSU11, new bands appeared at 3410 cm−1, 3347 cm−1, and 1039 cm−1 corresponding to -NH, -NH, and P-O-C stretching vibrations, respectively [2,38]. This suggests that CSU11 had strongly adsorbed onto the mineral surfaces.

Minerals 2021, 11, 796 10 of 14

-20

-25 Chalcopyrite+ CSU11+SM1 Galena+ CSU11+SM1 -30

) -35

mV ( -40

-45 pH 7.0

-50 CSU11 50 mg/L Zeta Zeta potential -55

-60

-65 0 200 400 600 800 1000 Depressant dosage (mg/L) Figure 8. Effect of SM1 concentration on the zeta potentials of the mineral samples.

Figure 9 shows the FT-IR spectra of the collector CSU11 and the depressant SM1. In the FT-IR spectrum of CSU11, the bands at 3411 cm−1 and 3225 cm−1 were attributed to the stretching vibration of -NH [35]. The bands at 2560 cm−1 and 1039 cm−1 corresponded to -SH and P-O-C stretching vibrations, respectively. The results confirmed the main func- Minerals 2021, 11, 796 tional groups of the CSU11 molecule (-NH, -SH, and P-O-C). In the FT-IR spectrum10 of 13 of SM1, the characteristic bands at 3411 cm−1, 2920 cm−1, 1405 cm−1, and 1027 cm−1 correspond to the stretching vibrations of –OH, C-H, COO-, and Si-O, respectively [25,36,37].

N－ H 3411 N－ H 3225 － SH 1500 2560 N－ H 2912 1420 2961 － CH 1039 2 N－ H － CH 3 P－ O－ C 688 CSU11 P=S SM1

Reflectance - － OH － COO 3411 1405 2920 C－ H 1643 1125 1027 C－ O Si－ O

4000 3500 3000 2500 2000 1500 1000 500 -1 Wavenumber(cm ) FigureFigure 9. 9.FT-IR FT-IR spectra spectra of theof the collector collector CSU11 CSU11 and theand depressant the depressant SM1 (CSU11 SM1 (CSU11 concentration concentration 50 mg/L, 50 SM1mg/L, concentration SM1 concentration 200 mg/L). 200 mg/L).

FiguresFigures 10 10 and and 11 1show1 show the FT-IR the FT spectra-IR spectra of chalcopyrite of chalcopyrite and galena and after galena being after treated being withtreated the collectorwith the CSU11collector and CSU11 the depressant and the SM1.depressant After theSM1. two After minerals the two were minerals treated withwere CSU11,treated newwith bands CSU11, appeared new bands at 3410 appeared cm−1, 3347at 3410 cm −cm1,−1 and, 3347 1039 cm cm−1, −an1 correspondingd 1039 cm−1 corre- to Minerals 2021, 11, 796 -NH,sponding -NH, and to -NH, P-O-C -NH stretching, and P- vibrations,O-C stretching respectively vibrations, [2,38 respectively]. This suggests [2,38 that]. This CSU1111 sug-of 14

hadgests strongly that CSU11 adsorbed had strongly onto the adsorbed mineral surfaces. onto the mineral surfaces.

Chalcopyrite+ CSU11 Chalcopyrite+ CSU11+SM1

N－ H 3343 612 P－ S 609 2960 2911 1486 － CH 1442 1039 3410 － CH 2 3 N－ H N－ H P－ O－ C

N－ H Reflectance 3324 1036 689

2968 2914 3416 1503 1436

4000 3500 3000 2500 2000 1500 1000 500 Wavenumber (cm-1)

FigureFigure 10. 10.FT-IR FT-IR spectra spectra of of chalcopyrite chalcopyrite treated treated with with the the collector collector CSU11 CSU11 and and the the depressant depressant SM1 SM1 at at naturalnatural pH pH (CSU11 (CSU11 concentration concentration 50 50 mg/L, mg/L, SM1 SM1 concentration concentration 200 200 mg/L). mg/L).

In the FT-IR spectrum of chalcopyrite with pre-adsorbed CSU11 (labeled “chalcopyrite + CSU11 + SM1”), Galena+ CSU11 no new absorption bands that could be attributed to the functional Galena+ CSU11+SM1 groups of SM1 were observed, indicating that there was no SM1 adsorption. However, in the FT-IR spectrum of galena with pre-adsorbed CSU11 (labeled “galena + CSU11 + SM1”), 3347 −1 −1 −1 −1 new bands emergedN－ H at 3442 cm , 2920 cm , 1405 cm , and 1009 cm , corresponding 2960— to -OH, C-H,－ CH COO2911, and－ CH Si-O stretching vibrations,1039 respectively [39]. The new bands 3 2 P－ O－ C correspond to the stretching vibrations1527 of1500 the polar groups in the SM1 molecule and clearly demonstrate the intense chemisorption of SM1 onto the surface of galena with CSU11

pre-treatment,Reflectance which was consistent with in the results of the zeta potential measurements. 3442 － OH － OH 2920 699 － OH Si－ O C－ H 1332 1009 1689 1405 1155 － COO C－ O 1527 1500 4000 3500 3000 2500 2000 1500 1000 500 -1 Wavenumber (cm )

Figure 11. FT-IR spectra of galena treated with the collector CSU11 and the depressant SM1 at natural pH (CSU11 concentration 50 mg/L, SM1 concentration 200 mg/L).

In the FT-IR spectrum of chalcopyrite with pre-adsorbed CSU11 (labeled “chalcopyrite + CSU11 + SM1”), no new absorption bands that could be attributed to the functional groups of SM1 were observed, indicating that there was no SM1 adsorption. However, in the FT-IR spectrum of galena with pre-adsorbed CSU11 (labeled “galena + CSU11 + SM1”), new bands emerged at 3442 cm−1, 2920 cm−1, 1405 cm−1, and 1009 cm−1, corresponding to -OH, C-H, COO—, and Si-O stretching vibrations, respectively [39]. The new bands correspond to the stretching vibrations of the polar groups in the SM1 molecule and clearly demonstrate the intense chemisorption of SM1 onto the surface of galena with CSU11 pre-treatment, which was consistent with in the results of the zeta potential measurements. Based on the flotation results and the zeta potential and FT-IR measurements, it can be concluded that SM1 selectively adsorbs onto the surface of galena pre-treated with CSU11. A possible adsorption model of the SM1 and CSU11 onto the mineral surfaces is simplified in Figure 12. The collector CSU11 can adsorb onto the bare mineral surfaces and make them hydrophobic. The depressant SM1 has almost no interaction with chalcopyrite. The interaction between the surface of the chalcopyrite and SM1 is prevented by the CSU11 pre-treatment, while SM1 can cover the CSU11 on the surface of galena by

Minerals 2021, 11, 796 11 of 14

Chalcopyrite+ CSU11 Chalcopyrite+ CSU11+SM1

N－ H 3343 612 P－ S 609 2960 2911 1486 － CH 1442 1039 3410 － CH 2 3 N－ H N－ H P－ O－ C

N－ H Reflectance 3324 1036 689

2968 2914 3416 1503 1436

4000 3500 3000 2500 2000 1500 1000 500 Wavenumber (cm-1) Minerals 2021, 11, 796 11 of 13 Figure 10. FT-IR spectra of chalcopyrite treated with the collector CSU11 and the depressant SM1 at natural pH (CSU11 concentration 50 mg/L, SM1 concentration 200 mg/L).

Galena+ CSU11 Galena+ CSU11+SM1

3347 N－ H 2960 － CH 2911－ CH 1039 3 2 P－ O－ C

1527 1500

Reflectance 3442 － OH － OH 2920 699 － OH Si－ O C－ H 1332 1009 1689 1405 1155 － COO C－ O 1527 1500 4000 3500 3000 2500 2000 1500 1000 500 -1 Wavenumber (cm )

FigureFigure 11. 11.FT-IR FT-IR spectra spectra of galenaof galena treated treated with with thecollector the collector CSU11 CSU11 and the an depressantd the depressant SM1 at natural SM1 at pHnatural (CSU11 pH concentration(CSU11 concentration 50 mg/L, 50 SM1 mg/L, concentration SM1 concentration 200 mg/L). 200 mg/L).

BasedIn the onFT the-IR flotationspectrum results of chalcopyrite and the zeta with potential pre-adsorbed and FT-IR CSU11 measurements, (labeled “chalco- it can bepyrite concluded + CSU11 that + SM1” SM1) selectively, no new absorption adsorbs onto bands the that surface could of be galena attributed pre-treated to the withfunc- CSU11.tional groups A possible of SM1 adsorption were observed, model of the indicating SM1 and that CSU11 there onto was the no mineral SM1 adsorption. surfaces is simplifiedHowever, inin Figurethe FT 12-IR. Thespectrum collector of CSU11galena canwith adsorb pre-adsorbed onto the bareCSU11 mineral (labeled surfaces “galena and + Minerals 2021, 11, 796 make them hydrophobic. The depressant SM1 has almost no interaction with chalcopyrite.12 of 14 CSU11 + SM1”), new bands emerged at 3442 cm−1, 2920 cm−1, 1405 cm−1, and 1009 cm−1, The interaction between the surface of the chalcopyrite and SM1 is prevented by the corresponding to -OH, C-H, COO—, and Si-O stretching vibrations, respectively [39]. The CSU11 pre-treatment, while SM1 can cover the CSU11 on the surface of galena by chemical new bands correspond to the stretching vibrations of the polar groups in the SM1 mole- adsorption, making it hydrophilic. SM1 can expel/replace or cover the adsorbed CSU11 culechemical and clearly adsorption, demonstrate making the it hydrophilic. intense chemisorption SM1 can expel/repl of SM1 ontoace orthe cover surface the of adsorbed galena on the galena surface, but it fails this on chalcopyrite surface. This is the reason why withCSU11 CSU11 on the pre galena-treatment, surface, which but was it fails consistent this on withchalcopyrite in the results surface. of theThis zeta is the potential reason the non-toxic depressant has a strong depression effect on galena but does not affect the measurements.why the non-toxic depressant has a strong depression effect on galena but does not affect flotation of chalcopyrite. the flotationBased on of the chalcopyrite. flotation results and the zeta potential and FT-IR measurements, it can be concluded that SM1 selectively adsorbs onto the surface of galena pre-treated with CSU11. A possible adsorption model of the SM1 and CSU11 onto the mineral surfaces is simplified in Figure 12. The collector CSU11 can adsorb onto the bare mineral surfaces and make them hydrophobic. The depressant SM1 has almost no interaction with chalcopyrite. The interaction between the surface of the chalcopyrite and SM1 is prevented by the CSU11 pre-treatment, while SM1 can cover the CSU11 on the surface of galena by

FigureFigure 12. 12.Schematic Schematic diagram diagram of of the the adsorption adsorption model model of of the the collector collector CSU11 CSU11 and and the the depressant depressant SM1SM1 onto onto the the mineral mineral surfaces. surfaces.

4.4. Conclusions Conclusions

TheThe mixed mixed depressant depressant (D (DFinalFinal)) ofof sodium sodium carboxymethyl carboxymethyl cellulose, cellulose, sodium sodium silicate, silicate, sodiumsodium sulﬁte, sulfite and, and zinc zinc sulfate sulfate exhibit exhibit high hig efﬁciencyh efficiency for for selectively selectively separating separating chalcopy- chalco- ritepyrite from from galena. galena. The The suggested suggested mechanism mechanism is that is that the interactionthe interaction between between the surfacethe surface of theof chalcopyritethe chalcopyrite surface surface and and the the 50:1 50:1 depressant depressant mixture mixture of of sodium sodium silicate silicate and and sodium sodium carboxymethylcarboxymethyl cellulose cellulose (SM1) (SM1) has has been been prevented prevented by by pre-treatment pre-treatment with with the the composite composite thiophosphatethiophosphate CSU11. CSU11. However, However, SM1 SM1 can can favorably favorably adsorb adsorb onto onto the the surface surface of of galena galena afterafter pre-treatment pre-treatment withwith CSU11CSU11 by chemisorption. Based Based on on the the key key component component SM1, SM1, the 1:2:2 depressant mixture of SM1 + sodium sulfite + zinc sulfate shows great potential as an effective, non-toxic depressant for industrial application in separating galena from chalcopyrite in the flotation of copper–lead bulk concentrate.

Author Contributions: Methodology, G.G.; formal analysis, Z.G. and G.G.; investigation, K.Z. and C.M.; writing—original draft preparation, K.Z. and C.M.; writing—review and editing, Z.G. All authors have read and agreed to the published version of the manuscript. Funding: The authors acknowledge the support of the Key Program for International S & T Coop- eration Projects of China (2021YFE0106800), the National Natural Science Foundation of China (No. 52074358, U2067201), the Science Fund for Distinguished Young Scholars of Hunan Province (2020JJ2044), the Young Elite Scientists Sponsorship Program of Hunan province, China (2018RS3011), the Open Foundation of State Key Laboratory of Mineral Processing (BGRIMM-KJSKL-2021-20), and the Geological Survey project of the Geological Survey of China (DD20211235). Conflicts of Interest: The authors declare no conflict of interest.

References 1. Hayes, R.A.; Ralston, J. The collectorless flotation and separation of sulphide minerals by Eh control. Int. J. Miner. Process. 1988, 23, 55–84. 2. Chen, W.; Chen, T.; Bu, X.Z.; Chen, F.F.; Ding, Y.H.; Zhang, C.H.; Deng, S.; Song, Y.H. The selective flotation of chalcopyrite against galena using alginate as a depressant. Miner. Eng. 2019, 141, 105848. 3. Lotter, N.O.; Bradshaw, D.J.; Barnes, A.R. Classification of the Major Copper Sulphides into semiconductor types, and associated flotation characteristics. Miner. Eng. 2016, 96, 177–184. 4. Bu, Y.J.; Hu, Y.H.; Sun, W.; Gao, Z.Y.; Liu, R.Q. Fundamental Flotation Behaviors of Chalcopyrite and Galena Using O-Isopropyl-N-Ethyl Thionocarbamate as a Collector. Minerals 2018, 8, 115.

Minerals 2021, 11, 796 12 of 13

the 1:2:2 depressant mixture of SM1 + sodium sulﬁte + zinc sulfate shows great potential as an effective, non-toxic depressant for industrial application in separating galena from chalcopyrite in the ﬂotation of copper–lead bulk concentrate.

Author Contributions: Methodology, G.G.; formal analysis, Z.G. and G.G.; investigation, K.Z. and C.M.; writing—original draft preparation, K.Z. and C.M.; writing—review and editing, Z.G. All authors have read and agreed to the published version of the manuscript. Funding: The authors acknowledge the support of the Key Program for International S & T Co- operation Projects of China (2021YFE0106800), the National Natural Science Foundation of China (No. 52074358, U2067201), the Science Fund for Distinguished Young Scholars of Hunan Province (2020JJ2044), the Young Elite Scientists Sponsorship Program of Hunan province, China (2018RS3011), the Open Foundation of State Key Laboratory of Mineral Processing (BGRIMM-KJSKL-2021-20), and the Geological Survey project of the Geological Survey of China (DD20211235). Conﬂicts of Interest: The authors declare no conﬂict of interest.

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