Fundamental Flotation Behaviors of Chalcopyrite and Galena Using O-Isopropyl-N-Ethyl Thionocarbamate As a Collector

minerals

Article Fundamental Flotation Behaviors of Chalcopyrite and Galena Using O-Isopropyl-N-Ethyl Thionocarbamate as a Collector

Yongjie Bu ID , Yuehua Hu *, Wei Sun *, Zhiyong Gao ID and Runqing Liu

School of Mineral Processing and Bioengineering, Central South University, Changsha 410083, China; [email protected] (Y.B.); [email protected] (Z.G.); [email protected] (R.L.) * Correspondence: [email protected] (Y.H.); [email protected] (W.S.); Tel.: +86-731-8830-482 (Y.H.); +86-0731-8883-6873 (W.S.)

Received: 31 January 2018; Accepted: 12 March 2018; Published: 13 March 2018

Abstract: Copper and lead are two important and widely used metals in industry. Chalcopyrite (CuFeS2) is associated with galena (PbS) in ore, and it has been a research hotspot in separating galena from chalcopyrite by flotation. In this study, the flotation behaviors of chalcopyrite and galena were studied through flotation tests, adsorption measurements, solution chemistry calculation, Fourier transform infrared spectroscopy (FTIR) and molecular dynamics (MD) simulations. The results show that the floatability of chalcopyrite is better than that of galena in the presence of O-isopropyl-N-ethyl thionocarbamate (IPETC), and the recovery difference between chalcopyrite and galena is about 20% when IPETC is 7 × 10−4 mol/L at pH 9.5, while the floatability difference between the two minerals is significant. Competitive adsorption of OH− and IPETC on mineral surfaces leads to lower floatability of galena than that of chalcopyrite. IPETC is able to remove the hydration layer on mineral surfaces and then adsorb on active sites. The floatability of minerals is enhanced with the increase of their hydrophobicity. This study provides a reference to separate galena from chalcopyrite.

Keywords: chalcopyrite; galena; IPETC; ﬂotation

1. Introduction Copper and lead are two of the most widely used metals in industry and daily life; their applications include building construction, various metal alloys, heat and electricity, and machine manufacturing [1,2]. Chalcopyrite and galena are the major sources of copper and lead; normally, they are associated with each other [3]. With the growing utilization of these resources, the ores with high grade and large dissemination size are on the decrease, and more attention must be paid to the utilization of low-grade and complicatedly disseminated ores [4–6]. In the flotation of polymetallic ores, chalcopyrite and galena are often achieved with zinc/iron sulfides being depressed, followed by the separation of chalcopyrite/galena and zinc/iron sulfides. Due to the special properties and complex composition of copper-lead sulfide ore, effective flotation separation of chalcopyrite from galena is a focus of research and a difficult point in mineral processing [7]. Chalcopyrite depression with galena flotation and galena depression with chalcopyrite flotation are two conventional methods in copper/lead separation. In the separation process of chalcopyrite and galena, it is rather important to choose an effective collector [8]. A selective collector will make the separation more economical. Enlarging and using the floatability of the two minerals will be a good basis and start in flotation [9]. Qin conducted a great deal of research on the flotation separation of copper/lead concentrate. Collector O-isopropyl-N-ethyl thionocarbamate (IPETC) and ammonium dibutyl dithiophosphate (ADTP) were investigated. Compared with IPETC, ADTP is not selective

Minerals 2018, 8, 115; doi:10.3390/min8030115 www.mdpi.com/journal/minerals Minerals 2018, 8, 115 2 of 10 against galena; the problem is that the collectors are not selective enough and the mechanism of IPETC and mineral is not studied [10]. IPETC is a kind of non-ionizing collector of oily liquid, and slightly soluble in water; it is regarded as having different forms in different pH environments [11]. Nagarajt considered the molecular structure of IPETC to contain a C=S group in neutral and acidic environments, and a SH group in an alkaline environment. The inversion is proposed in Equation (1):

C3H7OC(=S)NHC2H5 C3H7OC(SH)=NC2H5 (1)

Stability in solution, being selective for chalcopyrite, lower operating pH, and higher economic performance make IPETC the best choice for the separation of chalcopyrite and galena [12]. Buckley believed that IPETC adsorbed on the mineral surface as metal thionocarbamate was formed, but he was not sure whether the mechanism was dissolution-precipitation or specific adsorption, as the pKa of IPETC is high and no experiment could support it [13]. Woods and Hope investigated the reaction of kinds of copper sulfide ores with IPETC by SERS spectra. IPETC is considered as an acid HIPETC, interacting with Cu or Fe on the mineral surface. While in mineral flotation, water had a great influence on the reaction of the reagent and the minerals. As is well known, the adsorption of IPETC on the mineral surface is not strong in the flotation of chalcopyrite and galena, and after time, the concentration of the mineral flotation capacity decreased. A large number of studies have been conducted with focus on the adsorption of IPETC and minerals [14,15], but there is no well-recognized interaction mechanism. Wei [16] and Wang [17] studied the adsorption by FTIR spectra, and they obtained different results. Wei claimed specific adsorption occurred where new peaks appeared, but Wang insisted that nonspecific adsorption was the reaction mechanism of the reagent and mineral. As previous studies were not well and deep enough, inconsistent conclusions were obtained; this research aims at conducting a systematic study and reveals the reaction mechanism. Thus, many electrochemical studies on these two minerals have been conducted, but the mechanism remains unclear [18], and this study is carried out from another point of view. If IPETC is helpful for the flotation separation of chalcopyrite and galena, the copper and lead shortage of China will be alleviated; this study also helps to fundamentally understand how collectors and OH− work on the surface of minerals. This paper investigates the effect of collector IPETC and pH on the flotation performance of chalcopyrite and galena, and a pure mineral test is performed. Adsorption quantity determination, FTIR spectra, and Materials Studio simulation are chosen to further study the reaction. Finally, the reaction mechanism of the collector and mineral, and the effect of pH on the adsorption of the collector are proposed.

2. Experimental

2.1. Materials and Reagents Pure mineral chalcopyrite and galena were supplied by Daye Mine of Hubei Province and Mengzi Mine of Yunnan Province, respectively. After manually removing impurities, the mineral was crushed to −3 mm particles with a steel hammer followed by further milling to −74 µm for ﬂotation, and −38 µm for FTIR spectra and adsorption tests. The samples were stored in sealed glass bottles that kept in a vacuum oven. The mineral particles were washed several times with deionized water in ultrasonic cleaning machine before test. The chemical analysis indicated that the purity of chalcopyrite was 94.94% and that of galena was 97.47%. The collector IPETC was of industrial grade and obtained from Zhuzhou Reagent Limited Company (Zhuzhou, China). Other chemicals used in the tests were analytically pure. Distilled water was used in the experiments. Solutions of hydrochloric acid and sodium hydroxide were used to adjust pH. Minerals 2018, 8, x FOR PEER REVIEW 3 of 9

2.2. Flotation Tests The pure mineral flotation tests were performed by a laboratory XFD type flotation machine Minerals 2018, 8, 115 3 of 10 (Prospecting Machinery Factory, Changchun, China) at the speed of 1650 r/min. Two gram pure mineral together with 35 mL deionized water were added into the 40 mL flotation cell. After the pH 2.2. Flotation Tests was adjusted with NaOH or HCl aqueous solution for 2 min, the reagents were added successively. The conditioningThe pure time mineral was flotation2 min, 3 testsmin, were and performed1 min, respectively. by a laboratory The flotation XFD type process flotation was machine conducted for 4 min.(Prospecting Foam products Machinery and Factory, cell mineral Changchun, went China) through at the filtration, speed of 1650dring, r/min. and weifging Two gram to pure calculate mineral together with 35 mL deionized water were added into the 40 mL flotation cell. After the pH the recovery. was adjusted with NaOH or HCl aqueous solution for 2 min, the reagents were added successively. The conditioning time was 2 min, 3 min, and 1 min, respectively. The flotation process was conducted 2.3. Adsorptionfor 4 min. Tests Foam products and cell mineral went through filtration, dring, and weifging to calculate Thethe adsorption recovery. of IPETC onto chalcopyrite and galena was measured by ultraviolet (UV-1810) spectrometer2.3. Adsorption (Beijing Tests Purkinje General Instrument Co., Ltd., Beijing, China). The pH range from 4 to 13 was determined by measuring the adsorption at wavelength 246 nm [19], as shown in Figure 1. The adsorption of IPETC onto chalcopyrite and galena was measured by ultraviolet (UV-1810) Two gramspectrometer pure mineral (Beijing of Purkinje −38 µm General was added Instrument into Co., 40 Ltd.,mL deionized Beijing, China). water, The then pH range NaOH from or 4 toHCl was added 13to wasadjust determined pH. After by measuringthe desired the reagents adsorption were at wavelength added in order 246 nm and [19], the as shownconditioning in Figure time1. was identicalTwo with gram flotation pure mineral test. Then, of −38 theµm pulp was was added cent intorifuged 40 mL for deionized 10 min using water, a then high-speed NaOH or refrigerated HCl centrifugewas addedat 9000 to adjustr/min. pH. Later After the desiredsupernatant reagents was were filtered, added in the order solution and the conditioningafter adsorption time was measuredwas identicalby the withUV spectrometer, flotation test. Then, and thethe pulp adso wasrption centrifuged of IPETC for 10 on min the using mineral a high-speed surface was refrigerated centrifuge at 9000 r/min. Later the supernatant was filtered, the solution after adsorption calculated from the difference in the concentration of initial and residual ITEPC in the solution as the was measured by the UV spectrometer, and the adsorption of IPETC on the mineral surface was followingcalculated equation: from the difference in the concentration of initial and residual ITEPC in the solution as the following equation: Γ = 0 − ×⁄ Γ = ((C0 − C) × V)/M where Γ is adsorption quantity, C0 is the initial concentration of the solution, C is the residual where Γ is adsorption quantity, C0 is the initial concentration of the solution, C is the residual concentrationconcentration of the of solution, the solution, V isV theis the volume volume of of solution, and andM Mis theis the mass mass of mineral. of mineral.

1.0

0.8

0.6

0.4 Absorbance

0.2

0.0 220 240 260 280 300 λ/nm

FigureFigure 1. 1. UVUV spectroscopy spectroscopy of of IPETC. IPETC.

2.4. FTIR2.4. Measurements FTIR Measurements FTIR spectrumFTIR spectrum tests tests were were carried carried out out using using aa NexusNexus 670 670 FTIR FTIR spectroscopy spectroscopy (Thermo (Thermo Electron Electron Corporation, Waltham, MA, USA). The mineral (1.0 g) was ﬁnely ground to −2 µm in an agate mortar. Corporation, Waltham, MA, USA). The mineral (1.0 g) was finely ground to −2 µm in an agate mortar. Ore sample and desired reagents were added into a beaker added with 25 mL of deionized water at Ore samplethe corresponding and desired pH.reagents The suspension were added was into stirred a beaker for 60 min.added The with compounds 25 mL of were deionized washed water with at the correspondingdeionized pH. water The three suspension times, ﬁltered, was and stirred vacuum for dried,60 min. then The loaded compounds into the FTIR were spectroscopy washed with for test. deionized water three times, filtered, and vacuum dried, then loaded into the FTIR spectroscopy for test.

2.5. Molecular Dynamics (MD) Simulations The simulations were performed in the framework of Molecular Dynamics by the Material Studio version 5.0, and pcff force field was adopted throughout the simulation. The experimental lattice optimization of chalcopyrite and galena are a = 5.289 Å, b = 5.289 Å, c = 10.423 Å, α = 90°, β = 90°,

Minerals 2018, 8, x FOR PEER REVIEW 4 of 9

γ = 90°; a = 5. 9362 Å, b = 5.9362 Å, c = 5.9362 Å, α = 90°, β = 90°, γ = 90°, respectively. IPETC was modeled with MS software, and the atomic charges of IPETC were derived from the DFT calculation. The chalcopyrite and galena surface were built by cleaving the structure along (1 1 2) and (1 1 0) plane. MineralsThe mineral2018, 8, 115 crystal structure and IPETC molecule were optimized using the CASTEP4 of 10 module and DMol3 module, respectively. Discover module was employed to calculate adsorption energies. NVT ensemble2.5. Molecular was Dynamics employed, (MD) the Simulations temperature was fixed at 298.15 K and was controlled by the Nose method. EwaldThe simulations method werewas performedemployed in to the calculat frameworke Van of Molecular der Waals Dynamics and Coulomb by the Material interactions. Studio 2 ns simulationversion was 5.0, conducted. and pcff force The ﬁeld interaction was adopted ener throughoutgy could be the calculated simulation. as The Equation experimental (2): lattice optimization of chalcopyrite and galena are a = 5.289 Å, b = 5.289 Å, c = 10.423 Å, α = 90◦, β = 90◦, − γ = 90◦; a = 5. 9362 Å, b = 5.9362∆ Å,Ec==E 5.9362Total Å,EαSurface= 90◦+, βE=Reagent 90◦, γ = 90◦, respectively. IPETC was (2) modeled with MS software, and the atomic charges of IPETC were derived from the DFT calculation. where ∆E is interaction energy, ETotal is the total energy of the optimized reagent-mineral complex, The chalcopyrite and galena surface were built by cleaving the structure along (1 1 2) and (1 1 0) ESurface is the energy of the optimized mineral surface, and EReagent is the energy of the optimized plane. The mineral crystal structure and IPETC molecule were optimized using the CASTEP module reagent.and The DMol3 smaller module, values respectively. of ∆E, Discoverthe more module available was employedfor the interactions to calculate adsorption between energies. minerals and reagents.NVT ensemble was employed, the temperature was ﬁxed at 298.15 K and was controlled by the Nose method. Ewald method was employed to calculate Van der Waals and Coulomb interactions. 2 ns 3. Resultssimulation and Discussion was conducted. The interaction energy could be calculated as Equation (2): 3.1. Flotation Test ∆E = ETotal − ESurface + EReagent (2)

Thewhere flotation∆E is interaction behaviors energy, of chalcopyrite ETotal is the total and energy galena of the were optimized investigated. reagent-mineral Figure complex, 2 shows ESurface flotation recoveriesis the of energy chalcopyrite of the optimized and galena mineral as a surface, function and of E ReagentpH withis the collector energy ofIPETC the optimized (8 × 10−4 reagent.mol/L). It can The smaller values of ∆E, the more available for the interactions between minerals and reagents. be seen that with the increase of pH from 4 to 13, the flotation recoveries of the two minerals increase at the beginning3. Results and and Discussion then decrease; the recovery of galena is always lower than chalcopyrite, and the recovery of galena is affected more in alkaline pH pulp. IPETC is selective, and pH has influence on 3.1. Flotation Test the flotation capacity of IPETC; that is, OH− has an effect on the reaction of IPETC and mineral. The resultsThe flotation of pure behaviors mineral flotation of chalcopyrite tests show and galena that IPETC were investigated. has excellent Figure performance2 shows flotation in separating × −4 galena recoveriesfrom chalcopyrite of chalcopyrite at a andpH galena9.5. Figure as a function 3 show ofs pHthe withrecovery-IPETC collector IPETC dosage (8 10 curvesmol/L). of chalcopyrite It can be seen that with the increase of pH from 4 to 13, the flotation recoveries of the two minerals increase −4 −4 and galena,at the beginningand it can and be thenseen decrease; that with the the recovery increase of galena of IPETC is always dosage lower from than chalcopyrite,2 × 10 mol/L and to the 12 × 10 mol/L, recoverythe flotation of galena recoveries is affected of morethe two in alkaline mineral pH increase, pulp. IPETC with is the selective, recovery and pHof galena has influence growing on faster, especiallythe flotationas dosage capacity over of7 IPETC;× 10−4 mol/L. that is, OH− has an effect on the reaction of IPETC and mineral.

100

40 CuFeS2 PbS Flotation recovery/% 20

0 34567891011121314 pH

−4 FigureFigure 2. Recoveries 2. Recoveries of ofchalcopyrite chalcopyrite andgalena galena as as afunction a function of pH of(IPETC: pH (IPETC: 8 × 10 −84 ×mol/L). 10 mol/L).

3.2. AdsorptionThe resultsStudies of pure mineral ﬂotation tests show that IPETC has excellent performance in separating galena from chalcopyrite at a pH 9.5. Figure3 shows the recovery-IPETC dosage curves of chalcopyrite The adsorption density of IPETC on the surfaces of both chalcopyrite and galena as a function and galena, and it can be seen that with the increase of IPETC dosage from 2 × 10−4 mol/L to of pH was investigated. Figure 4 shows the adsorption density of IPETC (7 × 10−4 mol/L) onto chalcopyrite and galena. It can be seen that with the increase of pH from 4 to 13, the adsorption density of IPETC onto both two minerals increases at the beginning and then decreases, and the adsorption density onto galena is always lower than that onto chalcopyrite. The adsorption density

Minerals 2018, 8, x FOR PEER REVIEW 5 of 9 Minerals 2018, 8, 115 5 of 10 of IPETC onto both minerals decreases rapidly when pH is greater than 10. The larger the adsorption density12 onto× 10 −the4 mol/L, minerals the ﬂotation is, the recoverieshigher recovery of the two of mineral mineral increase, will withbe. OH the recovery− has an of effect galena on the adsorptiongrowing of IPETC, faster, especially especially as dosagewhen overthe pH 7 × is10 −high,4 mol/L. and OH− can inhibit IPETC adsorption clearly.

100

Minerals 2018, 8, x FOR PEER REVIEW70 5 of 9

60 of IPETC onto both minerals decreases rapidly when pH is greater than 10. The larger the adsorption 50 density onto the minerals is, the higher recovery of mineral will be. OH− has an effect on the 40 CuFeS adsorption of IPETC, especially when the pH is high, and OH2 − can inhibit IPETC adsorption clearly. PbS 30 Flotation recovery/% Flotation 10020

1090

800 02468101214 70 -4 Concentration of IPETC/(10 mol/L) 60 FigureFigure 3. Recoveries 3. Recoveries of chalcopyrite of chalcopyrite and and galena as as a a function function of dosageof dosage (pH =(pH 9.5). = 9.5). 50

1640 CuFeS 3.2. Adsorption Studies 2 PbS 30 The adsorption density14 of IPETC on the surfaces of both chalcopyrite and galena as a function Flotation recovery/% Flotation − of pH was investigated.20 Figure4 shows the adsorption density of IPETC (7 × 10 4 mol/L) onto 12

chalcopyrite and galena.mol/g) It10 can be seen that with the increase of pH from 4 to 13, the adsorption density of IPETC onto both two-7 minerals increases at the beginning and then decreases, and the adsorption 100 density onto galena is always02468101214 lower than that onto chalcopyrite. The adsorption density of IPETC onto -4 both minerals decreases rapidly8 whenConcentration pH is greater of thanIPETC/(10 10. Themol/L) larger the adsorption density onto the minerals is, the higher recovery of mineral will be. OH− has an effect on the adsorption of IPETC, especiallyFigure when 3. the Recoveries pH is6 high, of andchalcopyrite OH− can and inhibit galena IPETC as adsorptiona function of clearly. dosage (pH = 9.5). CuFeS 4 2 16 PbS

Adsorption density(10 Adsorption 2 14

0 1234567891011121314 mol/g)

-7 pH 10 Figure 4. Effect of pH on IPETC adsorptionon chalcopyrite and galena surfaces (IPETC: 7 × 10−4 mol/L). 8

HIPETC is the protonated6 (H+) and molecular form of IPETC− (A−) monobasic acid, thus HIPETC can be seen as HA. For an acid HA in solution, a dissociation CuFeS reaction occurs as follows: 4 2 PbS +

Adsorption density(10 Adsorption HA ⇌ H + A (3) 2 K = HA⁄ HA (4) 0 34567891011121314 ⁄ pH − pKa = logpH A HA (5) FigureIn flotation, 4.Figure Effect the 4. of pHreactionEffect on IPETC of of pH the adsorptionon on collector IPETC adsorptionon and chal mineralcopyrite chalcopyrite canand begalena judged and surfaces galena through surfaces(IPETC: the (IPETC:7equations × 10−4 mol/L). above −4 if pH < pKa7. ×The10 mainmol/L). reaction of the minerals and the collector is a molecular adsorbent. The pKa of IPETCHIPETC is about is the11.52, protonated and if the (H pH+) and of molecularthe pulp in form flotation of IPETC is too− (A high,−) monobasic the adsorption acid, thus mode HIPETC will change,can be seen and asthe HA. adsorption For an acid density HA inwill solution decrease., a dissociation reaction occurs as follows: According to the solubility product theory of Taggart [20], the smaller the solubility product HA ⇌ H+ + A (3) value of metal and collector, the stronger the product hydrophobicity is, indicating that the strong collectivity capacity for minerals. The adsorptionK = H ofA chalcopyrite⁄ HA and galena with IPETC, and (4)the solution chemistry calculation can be given as follows: pH − pKa = log A ⁄ HA (5) Pb + 2A ⇌ PbA , K = PbA = 18.00 (6) In flotation, the reaction of the collector and, mineral can be judged through the equations above

if pH < pKa. The main reaction of the minerals and the collector is a molecular adsorbent. The pKa of IPETC is about 11.52, and if the pH of the pulp in flotation is too high, the adsorption mode will change, and the adsorption density will decrease. According to the solubility product theory of Taggart [20], the smaller the solubility product value of metal and collector, the stronger the product hydrophobicity is, indicating that the strong collectivity capacity for minerals. The adsorption of chalcopyrite and galena with IPETC, and the solution chemistry calculation can be given as follows: Pb + 2A ⇌ PbA, K, = Pb A = 18.00 (6)

Minerals 2018, 8, 115 6 of 10

HIPETC is the protonated (H+) and molecular form of IPETC− (A−) monobasic acid, thus HIPETC can be seen as HA. For an acid HA in solution, a dissociation reaction occurs as follows:

+ − HA H + A (3)

+ − Ka = H A /[HA] (4) − pH − pKa = log A /[HA] (5) In ﬂotation, the reaction of the collector and mineral can be judged through the equations above if pH < pKa. The main reaction of the minerals and the collector is a molecular adsorbent. The pKa of IPETC is about 11.52, and if the pH of the pulp in ﬂotation is too high, the adsorption mode will change, and the adsorption density will decrease. According to the solubility product theory of Taggart [20], the smaller the solubility product value of metal and collector, the stronger the product hydrophobicity is, indicating that the strong collectivity capacity for minerals. The adsorption of chalcopyrite and galena with IPETC, and the solution chemistry calculation can be given as follows: h i 2+ + − = 2+ −2 = Pb 2A PbA2,Ksp,PbA2 Pb A 18.00 (6) h i 2+ + − = 2+ −2 = Cu 2A CuA2,Ksp,PbA2 Cu A 26.2 (7)

h i 2 Pb2+ + 2OH− Pb(OH) ,K = Pb2+ OH− = 15.1 (8) 2 sp,Pb(OH)2

h i 2 Cu2+ + 2OH− Cu(OH) ,K = Cu2+ OH− = 18.32 (9) 2 sp,Cu(OH)2 The competitive adsorption of IPETC and OH− can be given as follows:

− − PbA2 + 2OH Pb(OH)2 + 2A (10)

− − CuA2 + 2OH Cu(OH)2 + 2A (11) 2 2 K0 = A− /OH− = K /K = 10−18.0/10−15.1 = 10−2.9 (12) Pb sp,PbA2 sp,Pb(OH)2 2 2 K0 = A− /OH− = K /K = 10−26.2/10−18.32 = 10−7.88 (13) Cu sp,CuA2 sp,Cu(OH)2 q 0 − − −1.45 KPb = K Pb = A / OH = 10 (14) q 0 − − −3.94 KCu = K Cu = A / OH = 10 (15) IPETC is a kind of non-ionizing collector of oily liquid and slightly soluble in water, and in this system the dissociation constant of IPETC is 10−2:

OH− = A−/K (16)

The critical pH value of IPETC to chalcopyrite and galena is 12.39 and 10.89, which agrees well with the ﬂotation test and adsorption results. The solubility product value of copper and IPETC is smaller than that of lead, so the adsorption on chalcopyrite is always higher. At low pH, the dissociation reaction is depressed; with the increase of OH−, the dissociation reaction is promoted, while adsorption increase with pH; exceeding the critical pH the adsorption decreases. With IPETC as a collector, galena is more sensitive to the variation of pH, and the competitive adsorption of IPETC and OH− on galena surface occurs at lower pH. Minerals 2018, 8, 115 7 of 10

3.3. FTIR Study FTIR spectra of chalcopyrite, galena and both minerals conditioned with IPETC (7 × 10−4 mol/L) at pH 9.5 and 13 are given in Figures5 and6. The spectrum obtained at pH 9.5 shows characteristic absorption bands of –C=N (stretching vibration) at 1267 cm−1 appearing both on chalcopyrite and galena surfaces [21]. The asymmetrical bending vibration of –CH3 and scissor rocking vibration of −1 −1 –CH2 appear on chalcopyrite at 2850 and 2920 cm [22], and on galena at 2854 and 2925 cm [23]. Comparing with the two peaks of IPETC at 2877 and 2978 cm−1 [24], redshift occurs signiﬁcantly on both minerals. The bands shift on chalcopyrite is 5 cm−1 more than galena, and the bond energy of IPETC and chalcopyrite is greater. The peak –C=N indicates that IPETC mainly exists as C3H7OC (SH)=NC2H5 in an alkaline environment. These peaks mean IPETC adsorbs onto both the surface of chalcopyrite and galena. The spectrum obtained at pH 13 shows there are no peaks or shift appearing Mineralson two 2018 minerals,, 8, x FOR PEER indicating REVIEW that the adsorption of IPETC is dominated by nonspeciﬁc adsorption. 7 of 9 Minerals 2018, 8, x FOR PEER REVIEW 7 of 9

chalcopyrite+IPETC(pH=13） chalcopyrite+IPETC(pH=13）

chalcopyrite+IPETC(pH=9.5） chalcopyrite+IPETC(pH=9.5）

2919.9 2850.32

2919.9 2850.32 1626.9

chalcopyrite 1626.9 chalcopyrite

3500 3000 2500 2000 1500 1000 500 3500 3000 2500 2000 1500 1000 500 Wavenumber/cm-1 Wavenumber/cm-1 FigureFigure 5.5. InfraredInfrared spectra spectra of of chalcopyrite chalcopyritewith with IPETC IPETC at at different different pH. pH.

galena+IPETC(pH=13） galena+IPETC(pH=13）

galena+IPETC(pH=9.5） galena+IPETC(pH=9.5）

2925.53 2854.18

galena

galena 1627.65 1627.65

3500 3000 2500 2000 1500 1000 500 3500 3000 2500 2000 1500 1000 500 Wavenumber/cm-1 Wavenumber/cm-1 Figure 6. Infrared spectra of galena with IPETC at different pH. FigureFigure 6. 6. InfraredInfrared spectra spectra of of galena withwith IPETC IPETC at at different different pH. pH.

(a) (b)

Figure 7. The crystal structure of galena (a) and chalcopyrite (b).

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chalcopyrite+IPETC(pH=13）

chalcopyrite+IPETC(pH=9.5）

2919.9 2850.32

chalcopyrite 1626.9

3500 3000 2500 2000 1500 1000 500 Wavenumber/cm-1 Figure 5. Infrared spectra of chalcopyrite with IPETC at different pH.

galena+IPETC(pH=13）

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galena+IPETC(pH=9.5）

3.4. MD Simulations

2925.53 2854.18 MD simulation is a useful tool to describe the specificity of collector with mineral. The crystal structures of chalcopyrite and galena are shown in Figure7, respectively. IPETC interacts with atom − like Cu, Pb on the surfacegalena of mineral mainly. The interaction energies of IPETC, H2O, and OH with chalcopyrite (112) and galena (100) surfaces are shown1627.65 in Table1. The interaction energies of reagent and minerals are all negative [25,26]. The interaction energies of IPETC with chalcopyrite and galena are −29.7839 kJ/mol and −47.3861 kJ/mol, respectively. The value is not large enough and just − slightly more than the interaction energies of H2O and OH with minerals. IPETC is able to overcome the hydration layer3500 on the mineral 3000 surface 2500 and the 2000 floatability 1500 is improved; 1000 when 500 the pH increases, -1 the OH− concentration increases, the interactionWavenumber/cm energies of more OH− are greater, and the floatability − of the minerals decreasesFigure due6. Infrared to the competitive spectra of galena adsorption with IPETC of IPETC at different and OH pH..

(a) (b)

FigureFigure 7. 7.TheThe crystal crystal structure structure of galenagalena ( a()a and) and chalcopyrite chalcopyrite (b). (b).

Table 1. The interaction energy (kJ/mol) of reagent and minerals.

Reagent Mineral − IPETC H2O OH Chalcopyrite −29.7338 −5.8816 −4.0217 Galena −47.3861 −11.3905 −7.5328

4. Conclusions Results of flotation tests show that the selective collection of IPETC for chalcopyrite is better than that for galena. According to adsorption studies, OH− has an unfavorable effect on the adsorption of IPETC at high pulp pH. Solution chemistry calculations show the critical pH of IPETC for chalcopyrite and galena is 12.39 and 10.89, respectively. The floatability differences of the two minerals will become more significant through adjusting the pulp pH. The FTIR spectra show the appearance of IPETC on the surface of chalcopyrite and galena at pH 9.5, –C=N, –CH2 and –CH3 peaks are observed; but at pH 13, no new peak occurs on the surface of both minerals, so the adsorption is changed from specific adsorption to nonspecific adsorption. MD simulations show the interaction energies of IPETC with chalcopyrite and galena are, respectively, −29.7839 kJ/mol and −47.3861 kJ/mol, and the value is − bigger than the interaction energies of H2O and OH with minerals. IPETC is able to overcome the hydration layer on the surface of minerals and the floatability is then improved. The results of FTIR, adsorption, and MD simulations are in good agreement with the flotation tests. Minerals 2018, 8, 115 9 of 10

Acknowledgments: Financial support for this work was provided by the National 111 Project (B14031), supported by Collaborative Innovation Center for Clean and Efficient utilization of Strategic Metal Mineral Resources; Natural Science Foundation of China (51374247) and Independent Exploration Innovation Fund of Central South University (2014zzts058). Author Contributions: Yongjie Bu and Yuehua Hu conceived and designed the experiments; Yongjie Bu performed the experiments; Wei Sun and Zhiyong Gao analyzed the data; Yuehua Hu, Wei Sun, Runqing Liu contributed reagents/materials/analysis tools; Yongjie Bu and Zhiyong Gao wrote the paper. Conflicts of Interest: The authors declare no conflict of interest.

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