Utilization of Sodium Hexametaphosphate for Separating Scheelite from Calcite and Fluorite Using an Anionic–Nonionic Collector

minerals

Article Utilization of Sodium Hexametaphosphate for Separating Scheelite from Calcite and Fluorite Using an Anionic–Nonionic Collector

Jianhua Kang 1,2, Yuehua Hu 1,2,*, Wei Sun 1,2,*, Zhiyong Gao 1,2 and Runqing Liu 1,2 1 School of Minerals Processing and Bioengineering, Central South University, Changsha 410083, China; [email protected] (J.K.); [email protected] (Z.G.); [email protected] (R.L.) 2 Key Laboratory of Hunan Province for Clean and Eﬃcient Utilisation of Strategic Calcium-containing Mineral Resources, Central South University, Changsha 410083, China * Correspondence: [email protected] (Y.H.); [email protected] (W.S.); Tel.: +86-0731-88830482 (W.S. & Y.H.) Received: 24 September 2019; Accepted: 11 November 2019; Published: 14 November 2019

Abstract: This study presents a highly selective reagent system that utilizes sodium hexametaphosphate (SHMP) to improve the separation of scheelite from calcite and fluorite using an anionic–nonionic collector. The recoveries of calcite and fluorite decreased to 20% as the SHMP dose exceeded 6 10 6 mol/L, whereas that of scheelite remained at 85%. The interaction mechanisms × − of minerals with SHMP were investigated through equilibrium speciation, Zeta potential, Fourier transform infrared spectrometry, and X-ray photoelectron spectroscopy analyses. SHMP exists as hydrogen phosphate anion in the aqueous solution with a pH of 7–12. Moreover, it may be adsorbed intensively on the positively charged surfaces of calcite and fluorite via electrostatic force or chelation with calcium ion to impede further adsorption of the assembled collector. By comparison, the adsorption of SHMP is feeble on the scheelite surface because of its negative charge. The roughing grade of low-grade scheelite ore is substantially improved from 0.74% to 1.65% compared with that in the contrast test in the absence of SHMP.

Keywords: scheelite; ﬂotation; sodium hexametaphosphate; selective depressant

1. Introduction Tungsten is widely used in the industrial production of high-performance alloys and electro-optical materials because of its excellent properties [1,2]. According to the 2016 report in the United States Geological Survey [3], the mine production of tungsten worldwide was 87,000 tons in 2015, of which 71,000 tons were supplied by China. Moreover, the demand for tungsten is predicted to steadily grow in the future. Therefore, promoting the efficient utilization of tungsten, a nonrenewable resource, is an important pathway to satisfy the ever-increasing demand for tungsten. The natural sources of tungsten mainly include scheelite (CaWO4) and wolframite ((Fe, Mn)WO4). Wolframite was overexploited in the last century and gradually exhausted. Highly efficient mining and processing of scheelite have been the focus of research over the last decades [4,5]. Scheelite is a calcium-bearing mineral and is generally associated with calcite, wollastonite, and fluorite. Their uniform active site of calcium results in the analogical reaction model with anionic collectors, which exerts a major effect on the separation of scheelite from these gangue minerals [6]. Therefore, the development and utilization of highly selective collectors and depressants are effective approaches to amplify the floatability difference of scheelite with gangue minerals. Sodium oleate (NaOL) is an emblematic anionic collector and exhibits excellent collecting performance for scheelite flotation by linking carboxyl groups with calcium ions through oxygen atoms

Minerals 2019, 9, 705; doi:10.3390/min9110705 www.mdpi.com/journal/minerals Minerals 2019, 9, 705 2 of 16 to generate calcium oleate [7]. However, NaOL has poor selectivity and adaptability for complex ores associated with calcium-bearing gangue minerals. Consequently, research on mixed collectors has been conducted to improve the selectivity of fatty acids. Dodecylamine (DDA) is a classical cationic collector that adsorbs on scheelite surface through the electrostatic force attributed to the cationic + species of RNH3 [8].The anionic–cationic mixed collector composed of sodium oleate and DDA exerts greater collecting ability and higher selectivity than a single collector for scheelite flotation [9,10]. The anionic–nonionic mixed collector composed of sodium oleate and polyoxyethylene ether exhibits better adaptability at low temperatures than a single sodium oleate [11,12]. Nonionic surfactants, such as polyoxyethylene ether and fatty acid amide, have excellent synergistic effect with other surfactants, owing to their outstanding abilities of emulsification, solubilization, and dispersion [13,14]. Oleamide is a typical nonionic surfactant and a potential reagent to improve the collective performance of sodium oleate as a synergistic agent. Organic depressants, such as etidronic acid [15], citric acid [16], tannic acid [17], and carboxymethyl cellulose [18], exhibit excellent performance in depressing calcite-bearing gangue minerals. Acidified sodium silicate composed of sulfuric acid and sodium silicate is used to depress calcite in scheelite flotation [10,19]. Al–Na2SiO3 polymer, which is a mixture of aluminum sulphate and sodium silicate, exerts better selectivity than a single sodium silicate for the scheelite flotation from calcite [20]. Phosphate is an inorganic depressant which is diffusely used for scheelite flotation in laboratory research and industrial applications [21,22]. Sodium hexametaphosphate (SHMP) is a typical phosphate polymer with excellent chelating and dispersing abilities and can thus be applied to depress gangue minerals. This study presents a novel anionic–nonionic assembled collector to improve the selectivity of the collector. SHMP was applied to facilitate the depression of calcite and fluorite, thereby establishing a highly selective reagent system for scheelite flotation. The flotation performance of scheelite, calcite, and fluorite and their interaction mechanism with SHMP were investigated by equilibrium speciation, Zeta potential, Fourier transform infrared (FTIR) spectrometry, and X-ray photoelectron spectroscopy (XPS) analyses. The performances of the assembled collector and SHMP on scheelite ore flotation were demonstrated through roughing experiments.

2. Materials and Methods

2.1. Materials and Reagents Pure scheelite, calcite, and fluorite were obtained from the Shizhuyuan mine in Chenzhou, China. They are natural mineral crystals collected by picking by hand, ball milling with a ceramic pot, and screening via standard sieves of 200 mesh and 400 mesh. The particle sizes of the pure minerals ranged from 0.037–0.074 mm. The purities of scheelite, calcite, and fluorite reached 96.08%, 98.36%, and 99.31%, respectively, based on chemical analysis. The smooth curves of X-ray diffraction (XRD) with regular peaks presented in Figure1 indicate that the minerals had good crystallization. In addition, the absence of impurity peaks on the curves confirmed their high purities. The raw ore used for scheelite roughing was collected from a plant in Luanchuan, China. The results of X-ray fluorescence (XRF) analysis, listed in Table1, indicate that the main valuable element was W. The impurity elements contained Ca, Si, and Fe. Figure2 illustrates the XRD pattern of the scheelite raw ore. The characteristic peaks of scheelite did not obviously appear on the curve because of its low grade. The main gangue minerals included garnet, quartz, amphibole, feldspar, calcite, and fluorite. The grades of scheelite (WO3), calcite, and fluorite were 0.11%, 9.68%, and 8.56%, respectively, in raw ore. The raw ore was screened with a standard sieve of 200 mesh, and the particle size distribution in 0.074 mm was 78.69%. − The distribution rate of scheelite in this size fraction was 92.46%. The main reagents contained sodium oleate, oleamide, SHMP, and sodium carbonate, which were analytically pure. They were purchased from Tianjin Kemiou Chemical Reagent Co., Ltd., Tianjin, China. Minerals 2019, 9, x FOR PEER REVIEW 3 of 16

Table 1. X-ray fluorescence (XRF) analysis results of the scheelite raw ore.

MineralsElements2019, 9, 705 O F Na Mg Al Si P S K Ca Mn Fe Mo W3 of 16 Minerals 2019, 9, x FOR PEER REVIEW 3 of 16 Contents (%) 35.25 4.46 0.51 1.81 2.45 18.34 0.69 0.81 1.64 21.91 1.08 10.94 0.02 0.09

Table 1. X-ray fluorescence (XRF) analysis results of the scheelite raw ore.

Elements O F Na Mg Al Si P S K Ca Mn Fe Mo W Contents (%) 35.25 4.46 0.51 1.81 2.45 18.34 0.69 0.81 1.64 21.91 1.08 10.94 0.02 0.09

Figure 1. X-ray diffcration (XRD) patterns of scheelite, calcite, and fluorite pure minerals. Figure 1. X-ray diffcration (XRD) patterns of scheelite, calcite, and fluorite pure minerals. Table 1. X-ray fluorescence (XRF) analysis results of the scheelite raw ore.

Elements O F Na Mg Al Si P S K Ca Mn Fe Mo W ContentsFigure (%) 1.35.25 X-ray4.46 diffcration 0.51 (XRD) 1.81 patterns 2.45 18.34 of scheelit0.69e, 0.81calcite, 1.64 and fluo21.91rite pure1.08 minerals.10.94 0.02 0.09

Figure 2. XRD pattern of the scheelite raw ore.

2.2. Experimental Procedures

Ⅱ A flotation machine (XFGFigure, Jilin 2. XRDProspectingXRD pattern pattern of ofMachinery the the scheelite scheelite Plant, raw ore. Changchun, China) was used to conduct the flotation tests of a single mineral. The volume of the flotation cell was 40 mL, and the 2.2. Experimental Procedures 2.2.speed Experimental of impeller Procedures was 1650 rpm. The mineral suspension consisted of minerals (2 g) and deionized water.A flotationThe pH machinewas adjusted (XFG IIby, Jilin adding Prospecting sodium Machinerycarbonate Plant,or hydrochloric Changchun, acid China) solutions was used to the to A flotation machine (XFGⅡ, Jilin Prospecting Machinery Plant, Changchun, China) was used to conductrequested the values. flotation The tests anionic–nonionic of a single mineral. assembled The volume collector of the was flotation composed cell was of 40 sodium mL, and oleate the speed and conduct the flotation tests of a single mineral. The volume of the flotation cell was 40 mL, and the ofoleamide impeller with was a 1650mole rpm.ratio Theof 2:1. mineral The froth suspension and tailing consisted were filtrated, of minerals dried, (2 and g) and weighed deionized to obtain water. a speed of impeller was 1650 rpm. The mineral suspension consisted of minerals (2 g) and deionized Therecovery pH was according adjusted to Equation by adding (1). sodium The flotation carbonate procedure or hydrochloric of mixed acidminerals solutions was similar to the requested to that of water. The pH was adjusted by adding sodium carbonate or hydrochloric acid solutions to the values.the single The mineral anionic–nonionic flotation experiments. assembled The collector mass wasratio composed of scheelite, of calcite, sodium and oleate fluorite and oleamidein mixed requested values. The anionic–nonionic assembled collector was composed of sodium oleate and withminerals a mole was ratio 1:1:1. of 2:1.The Thegrades froth of and scheelite tailing (WO were3), filtrated, calcite, dried,and fluorite and weighed were 26.85%, to obtain 33.33%, a recovery and oleamide with a mole ratio of 2:1. The froth and tailing were filtrated, dried, and weighed to obtain a according33.33%, respectively. to Equation Figure (1). The 3 describes flotation the procedure reagent ofsystem mixed and minerals the process was similar of scheelite to that roughing. of the single The recovery according to Equation (1). The flotation procedure of mixed minerals was similar to that of mineral flotation experiments. The mass ratio of scheelite, calcite, and fluorite in mixed minerals was the single mineral flotation experiments. The mass ratio of scheelite, calcite, and fluorite in mixed 1:1:1. The grades of scheelite (WO3), calcite, and fluorite were 26.85%, 33.33%, and 33.33%, respectively. minerals was 1:1:1. The grades of scheelite (WO3), calcite, and fluorite were 26.85%, 33.33%, and 33.33%, respectively. Figure 3 describes the reagent system and the process of scheelite roughing. The

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Figure3 describes the reagent system and the process of scheelite roughing. The WO 3 grades of froth and tailing in the mixed mineral (1:1:1) ﬂotation and scheelite roughing were measured by XRF. The scheelite recovery was quantiﬁed based on Equation (2). Minerals 2019, 9, x FOR PEER REVIEW 4 of 16 M1 WO3 grades of froth and tailing in theR = mixed mineral (1:1:1)100 flotation and scheelite roughing were (1) measured by XRF. The scheelite recovery wasM 1quantified+ M2 × based on Equation (2). M R =×M 1 β 100 (1) = 1+ R MM12· 100 (2) M β + M2 θ × 1 · M ⋅β · R =×1 100 where R is the recovery (%), M (g) and M (g), denote⋅+βθ the ⋅ weights of froth and tailing,(2) and (%) and 1 2 MM12 β θ (%) refer to theirwhere grades, R is the respectively. recovery (%), M1 (g) and M2 (g), denote the weights of froth and tailing, and β (%) and θ (%) refer to their grades, respectively.

Figure 3. FigureThe reagent3. The reagent system system andand proc processess of scheelite of scheelite roughing. roughing.

2.3. Equilibrium2.3. Speciation Equilibrium ModellingSpeciation Modelling The pC–pH diagram, namely the negative logarithm of ion concentrations with varying pH, was The pC–pHbuilt diagram, based on the namelyprotonation the reactions. negative The pHlogarithm solution is a key of factor ion that concentrations notably influences withthe varying pH, hydrolytic equilibrium of SHMP, which was simplified and established by employing sodium was built based on the protonation reactions. The pH solution is a key factor that notably influences phosphate (Na3PO4) as an example. The calculation of hydrolytic equilibrium at diverse pH values the hydrolyticwas equilibrium conducted with of SHMP, Visual MINTEQ which was3.1 (Versi simplifiedon 3.1, Environmental and established Protection by Agency, employing sodium Washington, WA, USA). phosphate (Na3PO4) as an example. The calculation of hydrolytic equilibrium at diverse pH values was conducted2.4. with Zeta VisualPotential Tests MINTEQ 3.1 (Version 3.1, Environmental Protection Agency, Washington, WA, USA). The Zeta potentials of scheelite, calcite, and fluorite before and after reacting with SHMP and the assembled collector were measured using a Nano-Z Zeta potential analyzer (Malvern 2.4. Zeta PotentialInstruments, Tests Malvern, UK). The mineral slurry was composed of minerals (0.04 g) and 40 mL of KNO3 (0.01 mol/L) solution as an electrolyte. SHMP and the assembled collector were added to the The Zeta potentialssuspension in of order scheelite, and the calcite,pH was adjusted and fluorite to the desired before value and using after sodium reacting carbonate with or SHMP and the hydrochloric acid solutions. The mixture was stirred for 10 min for sufficient reaction. After free assembled collectorsettling werefor 5 min, measured the supernatant using was taken a Nano-Z for Zeta potential Zeta potential tests. analyzer (Malvern Instruments, Malvern, UK). The mineral slurry was composed of minerals (0.04 g) and 40 mL of KNO (0.01 mol/L) 2.5. FTIR Tests 3 solution as an electrolyte. SHMP and the assembled collector were added to the suspension in order The mineral pulp consisted of minerals (0.5 g) and deionized water. SHMP and the assembled and the pH wascollector adjusted were toadded the in desired order and value the mixture using was sodium stirred for carbonateadequate reaction or hydrochloric for 30 min. The acid solutions. The mixture wasreacted stirred minerals for 10were min filtered for suwithffi cientmicropor reaction.ous membrane, After washed, free settling and air dried for 5 at min, room the supernatant temperature for FTIR analysis (Nicolet iS50, Thermo, Waltham, MA, USA). was taken for Zeta potential tests. 2.6. XPS Tests 2.5. FTIR Tests XPS tests were conducted using an ESCALAB 250Xi spectrometer (Thermo Fisher Scientific, Waltham, MA, USA) with Al Kα as the sputtering source at 12 kV and 6 mA. The mineral samples The mineralwere pulp prepared consisted by mixing ofminerals minerals with SHMP (0.5 in g) deionized and deionized water. The mixture water. was SHMP stirred for and 30 the assembled collector were added in order and the mixture was stirred for adequate reaction for 30 min. The reacted minerals were filtered with microporous membrane, washed, and air dried at room temperature for FTIR analysis (Nicolet iS50, Thermo, Waltham, MA, USA).

2.6. XPS Tests XPS tests were conducted using an ESCALAB 250Xi spectrometer (Thermo Fisher Scientific, Waltham, MA, USA) with Al Kα as the sputtering source at 12 kV and 6 mA. The mineral samples were prepared by mixing minerals with SHMP in deionized water. The mixture was stirred for 30 min and filtrated through a microfiltration membrane. The filtration residue was washed and dried in natural Minerals 2019, 9, 705 5 of 16 wind for XPS tests. The test results were fitted using Thermo Avantage 5.96 (Version 5.96, Thermo Fisher Scientific, Waltham, MA, USA).

3. Results and Discussions

3.1. Flotation Experiments of Pure Mineral Sodium oleate possesses an excellent ability to collect scheelite but poor selectivity to float calcite and fluorite because of their same calcium action sites. The anionic–nonionic assembled collector, namely, sodium oleate mixed with oleamide at a mole ratio of 2:1, was created to improve the selectivity of the collector. SHMP was used to further enhance the floatability difference of scheelite with calcite and fluorite. The flotation performances of a single mineral at diverse pH levels are described in Figure4. The scheelite recovery increased with the rise of pH and reached its maximum (about 85%) at a pH of 10. The calcite recovery maintained at 70–75% at the pH range of 7–10 and increased to 95% at the pH of 12. Whereas, the fluorite recovery decreased from 87% to 65% with the increase of pH from 7 to 12. Compared with the recovery variations of scheelite, calcite, and fluorite, the floatability difference of scheelite with calcite and fluorite reached the maximum at a pH of 10. The recovery of scheelite reached 86.6%, while those of calcite and fluorite were 74.4% and 69.8%, respectively. Figure5 reveals the flotation behaviors of scheelite, calcite, and fluorite at different SHMP doses. Unlike the results of previous studies that used a single sodium oleate as the collector [2,23], the floatability of calcite and fluorite was significantly reduced, but that of scheelite remained the same. The recovery of scheelite reached 85.68% without using SHMP, whereas those of calcite and fluorite were only 73.26% and 71.58%, respectively. As the SHMP dose increased to 10 10 6 mol/L, the recoveries of calcite and × − fluorite were gradually reduced to 20%, whereas that of scheelite remained above 85%. These results indicate that SHMP exerted a preeminent ability to depress calcite and fluorite, but it had no effect Minerals 2019, 9, x FOR PEER REVIEW 6 of 16 on scheelite.

Figure 4. Flotation performances of single mineral at diverse pH levels when applying the Figure 4. Flotation performances of single mineral at diverse pH levels when applying the assembled assembled collector. collector. Considering the excellent performance of SHMP on single mineral flotation, it was utilized in the mixed mineral flotation to verify its ability to enhance the enrichment of scheelite. Figure6 presents the flotation performance of scheelite in mixed minerals with diverse SHMP doses. With the absence of SHMP, the WO3 grade of concentrate was 28.16% with a recovery of 79.86%. With increasing SHMP doses, the WO3 grades gradually increased to the peak and then rapidly decreased. When the SHMP dose was 3 10 6 mol/L, the WO grade was improved from 28.16% to 32.11% with × − 3 an acceptable recovery of 69.18%. The results demonstrate that SHMP can effectively enhance the enrichment of scheelite in the flotation of mixed minerals with an appropriate dose. However, calcite

Figure 4. Flotation performances of a single mineral with diverse SHMP doses when applying the assembled collector at a pH of 10.

Figure 5. Flotation performance of scheelite in artificial mixed minerals with diverse sodium hexametaphosphate (SHMP) doses when applying the assembled collector at a pH of 10.

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Minerals 2019, 9, 705 6 of 16 and fluorite cannot be completely depressed in the mixed mineral flotation, which results in a lower grade of concentrate than pure scheelite (WO 80.56%). When the SHMP dose exceeded 4 10 6 mol/L, 3 × − the scheelite flotation deteriorated, and consequently, the grade and recovery sharply decreased. Compared to the results in single mineral flotation, scheelite was also depressed to a certain extent in the mixed mineral flotation. The possible reasons that result in the different behaviors of scheelite flotation may be attributed to the influence of dissolved ions. In the single mineral flotation, scheelite was not affected by the presence of impurity ions because the ions in the solution were dissolved from itself. However, in the mixed mineral flotation, scheelite, calcite, and fluorite dissolved and released calcium ions, tungstate ions, carbonate ions, and fluorine ions in the aqueous solution. These ions dissolved and deposited continually on the three minerals surfaces until reaching an equilibrium of dissolution and precipitation. During this process, the precipitations were transformed into each other Figure 4. Flotation performances of single mineral at diverse pH levels when applying the assembled on the surfaces of three minerals. The precipitations of scheelite, calcite, and fluorite may exist on each collector. mineralFigure surface 4. Flotation at the performances same time, which of single causes mineral the surfaceat diverse properties pH levels ofwhen scheelite applying to vary, the assembled thus resulting in thecollector. different flotation behavior from that of the single mineral flotation.

Figure 4. Flotation performances of a single mineral with diverse SHMP doses when applying the assembledFigure 5. Flotation collector performancesat a pH of 10. of a single mineral with diverse SHMP doses when applying the Figure 4. Flotation performances of a single mineral with diverse SHMP doses when applying the assembled collector at a pH of 10. assembled collector at a pH of 10.

Figure 6. Flotation performance of scheelite in artiﬁcial mixed minerals with diverse sodium Figure 5. Flotation performance of scheelite in artificial mixed minerals with diverse sodium hexametaphosphate (SHMP) doses when applying the assembled collector at a pH of 10. hexametaphosphate (SHMP) doses when applying the assembled collector at a pH of 10. Figure 5. Flotation performance of scheelite in artificial mixed minerals with diverse sodium hexametaphosphate (SHMP) doses when applying the assembled collector at a pH of 10.

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3.2. Equilibrium Speciation Analysis 3.2. Equilibrium Speciation Analysis SHMP is a hexamer of sodium metaphosphate and will degrade into lower phosphates [24], SHMP is a hexamer of sodium metaphosphate and will degrade into lower phosphates [24], such such as Na3PO4, Na2HPO4 and NaH2PO4, in the aqueous solution [25,26]. In the mineral flotation, as Na3PO4, Na2HPO4 and NaH2PO4, in the aqueous solution [25,26]. In the mineral flotation, the active – 2– the active ingredients of SHMP were hydrogen phosphates, namely H2PO4 and HPO4 , which adsorb ingredients of SHMP were hydrogen phosphates, namely H2PO4– and HPO42–, which adsorb on the on the minerals surfaces via electrostatic force or chelation with metal ions [27]. The equilibrium minerals surfaces via electrostatic force or chelation with metal ions [27]. The equilibrium speciation speciation analysis of SHMP was simplified and established with sodium phosphate (Na3PO4) by analysis of SHMP was simplified and established with sodium phosphate (Na3PO4) by protonation protonation reactions and material balance. Figure7 illustrates the pC–pH diagram of trisodium reactions and material balance. Figure 7 illustrates the pC–pH diagram of trisodium phosphate (3 × –6 phosphate (3 10 mol/L) solution. H3PO4 is the dominant species when pH is below 2, whereas the 10–6 mol/L) solution. H3PO4 is the dominant species when pH is below 2, whereas the dominant × – 2– dominant components turn into H2PO4 and HPO4 at pH ranges of 2–7 and 7–12, respectively. When components turn into H2PO4– and HPO42– at pH ranges of 2–7 and 7–12, respectively. When pH 3– pH exceeds 12, PO4 becomes the predominant species. The appropriate pH range for the flotation of exceeds 12, PO43– becomes the predominant species. The appropriate pH range for the flotation of 2– scheelite is 10. Thus, the active species to depress calcite and fluorite is HPO4 , which is a negatively scheelite is 10. Thus, the active species to depress calcite and fluorite is HPO42–, which is a negatively chargedcharged hydrogen hydrogen phosphate anion.

–6 Figure 7. pC–pH diagram of 3 10 mol/L Na3PO4 solution. Figure 6. pC–pH diagram of 3 ×× 10–6 mol/L Na3PO4 solution. 3.3. Zeta Potential Analysis 3.3. Zeta Potential Analysis Zeta potential is an important factor to characterize the surface electrokinetic properties of minerals. FigureZeta8 presents potential the is Zeta an important potentials offactor scheelite, to charac calcite,terize and the fluorite surface with electrokinetic the absence andproperties presence of minerals.of SHMP Figure and the 8 presents assembled the collector. Zeta potentials The Zeta of sc potentialsheelite, calcite, of raw and scheelite fluorite were with negative the absence at a and pH presence of SHMP and the assembled collector. The Zeta potentials of raw2 scheelite were negative at range of 7–11 because the localized ions on scheelite surface were WO4 − [18]. With the presence of aSHMP, pH range the Zetaof 7–11 potentials because ofthe scheelite localized negatively ions on scheelite shifted tosurface some were extent. WO With42− [18]. the presenceWith the ofpresence SHMP ofand SHMP, the assembled the Zeta collector,potentials the of Zetascheelite potentials negative ofly scheelite shiftedhad to some a significant extent. furtherWith the negative presence shift. of SHMPThe Zeta and potentials the assembled of calcite collector, and fluorite the Zeta were potentials positive of at scheelite a pH range had ofa significant 7–10 due tofurther their localizednegative shift.ions ofThe Ca 2Zeta+ [2] potentials and gradually of calcite decreased and tofluorite negatives were when positive the pHat a exceeded pH range 10, of which 7–10 indicatesdue to their that 2+ localizedthe isoelectric ions pointof Ca (IEP) [2] ofand calcite gradually and fluorite decreased was 10–11to negatives [8,28]. With when the the presence pH exceeded of SHMP, 10, the which Zeta indicatespotentials that of calcite the isoelectric and fluorite point had (IEP) significant of calcite negative and fluorite shifts was that 10–11 were greater[8,28]. With than thatthe presence of scheelite. of SHMP,With the the presence Zeta potentials of SHMP of and calcite the and assembled fluorite collector, had significant the Zeta negative potentials shifts of calcitethat were and greater fluorite than had thatno obvious of scheelite. further With negative the presence shift. of These SHMP results and indicatethe assembled that the collector, adsorption the Ze ofta SHMP potentials on calcite of calcite and andfluorite fluorite surfaces had no was obvious stronger further than negative that on scheelite shift. These surface. results The indicate negatively that the charged adsorption SHMP of maySHMP be onadsorbed calcite onand the fluorite positively surfaces charged was surfaces stronger of than calcite that and on fluorite scheelite by surface. electrostatic The forcenegatively and hinder charged the SHMPfurther may adsorption be adsorbed of the on assembled the positively collector. charged Whereas, surfaces the of calcite adsorption and fluorite of SHMP by electrostatic on the negatively force andcharged hinder surface the further of scheelite adsorption was weak, of the thus assemble havingd less collector. effect on Whereas, the adsorption the adsorption of assembled of SHMP collector. on the negatively charged surface of scheelite was weak, thus having less effect on the adsorption of assembled collector.

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Figure 8. Zeta potentials of scheelite (a), calcite (b), and ﬂuorite (c) at the diﬀerent pH with the absence Figure 8. Zeta potentials of scheelite (a), calcite (b), and fluorite (c) at the different pH with the absence and presence of SHMP and the assembled collector. and presence of SHMP and the assembled collector.

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3.4. FTIR Analysis The adsorption mechanism of SHMP on the mineral surfaces was demonstrated by comparing the changes in the FTIR spectra curves, which are presented in Figure9 before and after the reaction with SHMP and the assembled collector. The FTIR spectra of SHMP are consistent with those reported 1 1 by previous studies [27,29]. The main characteristic peaks were located at 3444 cm− , 1646 cm− , 1 1 1 1 1 1 1283 cm− , 1097 cm− , 879 cm− , 519 cm− , and 472 cm− . The peak at 1283 cm− refers to the 1 1 asymmetric stretching vibration of the P–O group [24]. The peaks at 1097 cm− and 472 cm− were due to the antisymmetric stretching vibration and symmetric variable angular vibration of the P–O 1 groups, respectively. The peak at 879 cm− represents the symmetrical stretching vibration of the 1 P–O–P group [30,31]. As shown in Figure9b, the main characteristic peaks of scheelite at 809 cm − and 1 441 cm− represent the stretching and bending vibration of the W–O bonds, respectively [6]. After reacting with SHMP, no obvious new peak was observed on the scheelite spectrum. After reacting 1 1 1 1 with the assembled collector, new peaks emerged at 2920 cm− , 2850 cm− , 1646 cm− , 1542 cm− , –1 1 –1 and 1458 cm . The peaks at 2920 cm− and 2850 cm represent the stretching vibration of methylene 1 –1 and methyl groups [32] in the sodium oleate or oleamide. The peaks at 1542 cm− and 1458 cm refer to the asymmetric stretching and symmetrical vibration of the carboxyl [33] in the sodium oleate. The peak at 1646 cm–1 represents the characteristic peak of acylamino. After reacting with SHMP and the assembled collector, the location and strength of the characteristic peaks of sodium oleate and oleamide exhibited no obvious change. The changes in FTIR spectra demonstrate that SHMP was hardly adsorbed on scheelite surface, which had less influence on the further adsorption of the assembled collector. 1 1 As presented in Figure9c, the primary characteristic peaks of calcite at 1429 cm − , 876 cm− , and 712 cm–1 refer to the asymmetric stretching vibration and deformation vibration of carbonate 2– (CO3 ) groups [19,34]. After reacting with SHMP, no obvious new peak appeared on the spectrum curve of calcite. After reacting with the assembled collector, the characteristic peaks of methylene 1 –1 and methyl groups at 2923 cm− and 2854 cm , respectively, emerged on the calcite spectrum curve. After reacting with SHMP and the assembled collector, the intensities of methyl and methylene absorption peaks considerably weakened. This behavior demonstrates that SHMP impeded the further adsorption of the assembled collector on the calcite surface and reduced its adsorption amount. As exhibited in Figure9d, the variations in fluorite spectra were similar to those of calcite after reacting with SHMP and the assembled collector. The characteristic peaks of SHMP did not emerge on the 1 1 fluorite spectra, whereas the characteristic peaks of the assembled collector at 2924 cm− , 2853 cm− , 1 1 –1 1646 cm− , 1541 cm− , and 1461 cm were present. The characteristic peak intensities of the assembled collector weakened after the sequential reaction with SHMP and assembled collector. Combined with the results of Zeta potential and FTIR analyses, the absence of SHMP characteristic peaks and the presence of assembled collector characteristic peaks on the minerals surfaces indicates that SHMP was likely to be adsorbed on the surfaces of scheelite, calcite, and fluorite via electrostatic force, whereas the assembled collector was adsorbed on these minerals surfaces by chemical adsorption. When the solution pH was 10, the dominant component on scheelite surface was the negatively charged tungstate species, whereas the localized ions on the surfaces of calcite and fluorite were the positively charged calcium species [18]. By contrast, SHMP was more likely to be adsorbed on the surfaces of calcite and fluorite through electrostatic force, thereby occupying the active sites and impeding the further adsorption of the assembled collector. Nevertheless, SHMP was feebly adsorbed on the scheelite surface because of its negative charge, thus exhibiting less effect on scheelite flotation. Minerals 2019, 9, 705 10 of 16 Minerals 2019, 9, x FOR PEER REVIEW 10 of 16

Figure 9. Cont.

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Figure 9. Fourier-transform infrared (FTIR) spectra of SHMP (a) and scheelite (b), calcite (c), and Figure 9. Fourier-transform infrared (FTIR) spectra of SHMP (a) and scheelite (b), calcite (c), and fluorite (d) before and after reacting with different reagents. fluorite (d) before and after reacting with different reagents. 3.5. XPS Analysis 3.5. XPS Analysis XPS analysis is an efficient approach to examine the adsorption mode and model of minerals with reagents.XPS analysis Table is2 anpresents efficient the approach changes to in examine atomic contentsthe adsorption of various mode elements and model on of the minerals surfaces ofwith scheelite, reagents. calcite, Table and 2 presents fluorite the after changes the reaction in atomic with contents SHMP. of various The carbon elements on theon the surfaces surfaces of of raw scheelite, calcite, and fluorite after the reaction with SHMP. The carbon on the surfaces of raw fluorite fluorite and scheelite may result in the pollution of CO2 in the air due to sample preparation in the and scheelite may result in the pollution of CO2 in the air due to sample preparation in the wild. The wild. The contents of calcium, tungsten, and oxygen on scheelite surface were reduced by 0.37%, contents of calcium, tungsten, and oxygen on scheelite surface were reduced by −0.37%, −0.83%,− and 0.83%, and 1.47%, respectively, whereas that of phosphorus was augmented by 0.51%. The calcium − −1.47%, respectively,− whereas that of phosphorus was augmented by 0.51%. The calcium and carbon and carbon contents on calcite surface were reduced by 3.13% and 2.23%, respectively, whereas contents on calcite surface were reduced by −3.13% and −2.23%,− respectively,− whereas those of oxygen thoseand of phosphorus oxygen and increased phosphorus by 3.45% increased and 1.91%, by 3.45% respectively. and 1.91%, The respectively. calcium and The fluorine calcium contents and fluorine on contents on fluorite surface were reduced by 4.78% and 8.41%, respectively, but those of oxygen fluorite surface were reduced by −4.78% and− −8.41%, −respectively, but those of oxygen and andphosphorus phosphorus increased increased by by6.72% 6.72% and and 2.13%, 2.13%, respectively. respectively. The The variations variations in inthe the atomic atomic content content variationvariation of eachof each element element indicate indicate that that the the adsorption adsorption strength strength of of SHMP SHMP on on these these mineral mineral surfaces surfaces was diverse.was diverse. In contrast In contrast with the with variations the variations in calcium in calcium and phosphorus and phosphorus contents, contents, the decrease the decrease in calcium in andcalcium the increase and the in increase phosphorus in phosphorus on the surfaces on the of surfac calcitees andof calcite fluorite and were fluorite larger were than larger that inthan scheelite. that Thesein scheelite. results indicateThese results that theindicate adsorption that the amounts adsorption of SHMPamounts on of the SHMP surfaces on the of surfaces calcite and of calcite fluorite wereand greater, fluorite thereby were greater, covering thereby their covering original their surfaces original and surfaces reducing and the reducing relative the content relative of content elements. SHMPof elements. was weakly SHMP adsorbed was weakly on scheelite adsorbed surface on sche andelite it had surface minimal and it impact had minimal on the atomic impact content on the of eachatomic element. content of each element.

TableTable 2. Variations1. Variations in atomicin atomic contents contents of the of elementsthe elements on mineral on mineral surfaces surfaces after after thereaction the reaction with with SHMP. SHMP. Atomic Contents Changes (%) Elements Atomic Contents Changes (%) Elements Scheelite Calcite Fluorite Scheelite Calcite Fluorite C 2.16 2.23 4.34 C 2.16 −−2.23 4.34 O 1.47 3.45 6.72 − W O 0.83−1.47 3.45 - 6.72 - − F--W −0.83 - - 8.41 − Ca F 0.37 - 3.13- −8.414.78 − − − PCa 0.51−0.37 1.91−3.13 −4.78 2.13 P 0.51 1.91 2.13 The Ca2p XPS spectra of scheelite, calcite, and ﬂuorite before and after the reaction with SHMP are shown in Figure 10. The peaks of Ca2p1 and Ca2p3 on raw scheelite surface, located at 350.51 eV and 346.94 eV, respectively, are consistent with those reported in the literature [27,35], and they have a Minerals 2019, 9, x FOR PEER REVIEW 12 of 16

The Ca2p XPS spectra of scheelite, calcite, and fluorite before and after the reaction with SHMP are shown in Figure 10. The peaks of Ca2p1 and Ca2p3 on raw scheelite surface, located at 350.51 eV and 346.94 eV, respectively, are consistent with those reported in the literature [27,35], and they have a slight shift after the interaction with SHMP. The peaks of Ca2p1 and Ca2p3 of calcite were located at 350.56 eV and 346.99 eV and distinctly moved to 350.69 eV and 347.13 eV, respectively. The Ca2p spectrum of raw fluorite presented two characteristic peaks located at 351.37 eV and 347.79 eV, which are similar to those reported in the literature [36,37]. After the reaction with SHMP, the peaks of Ca2p1 and Ca2p3 remarkably shifted to 351.82 eV and 348.25 eV, respectively. Compared with the binding energy shifts of Ca2p after reaction with SHMP enumerated in Table 3, the shifts of Ca2p1 and Ca2p3 peaks on scheelite surface were only 0.03 eV and 0 eV, respectively, whereas those on calcite surface reached 0.13 eV and 0.14 eV, respectively. The shifts of Ca2p1 and Ca2p3 peaks on fluoriteMinerals surface2019, 9 ,reached 705 0.45 eV and 0.46 eV, respectively. The binding energy shifts of Ca2p12 of 16prove that SHMP was likely to be adsorbed on the surfaces of calcite and fluorite via chelation with calcium species, whereas the adsorption of SHMP was feeble because the localized ion on scheelite surface is slight shift after the interaction with SHMP. The peaks of Ca2p1 and Ca2p3 of calcite were located a tungstate species. at 350.56 eV and 346.99 eV and distinctly moved to 350.69 eV and 347.13 eV, respectively. The Ca2p spectrum of raw fluorite presented two characteristic peaks located at 351.37 eV and 347.79 eV, which Table 2. Binding energies and their shifts of Ca2p1 and Ca2p3 of minerals before and after the are similar to those reported in the literature [36,37]. After the reaction with SHMP, the peaks of Ca2p1 andinteraction Ca2p3 remarkablywith SHMP. shifted to 351.82 eV and 348.25 eV, respectively. Compared with the binding energyMinerals shifts of Ca2p after reactionScheelite with SHMP enumerated Calcite in Table 3, the shifts of Ca2p1 Fluorite and Ca2p3 peaks on scheelite surface were only 0.03 eV and 0 eV, respectively, whereas those on calcite surface Elements Ca2p1 Ca2p3 Ca2p1 Ca2p3 Ca2p1 Ca2p3 reached 0.13 eV and 0.14 eV, respectively. The shifts of Ca2p1 and Ca2p3 peaks on fluorite surface Before (eV) 350.51 346.94 350.56 346.99 351.37 347.79 reached 0.45 eV and 0.46 eV, respectively. The binding energy shifts of Ca2p prove that SHMP was likelyAfter to (eV) be adsorbed on 350.54 the surfaces of346.94 calcite and fluorite350.69 via chelation347.13 with calcium351.82 species, whereas348.25 theShifts adsorption (eV) of SHMP0.03 was feeble because0 the localized0.13 ion on scheelite0.14 surface is0.45 a tungstate species.0.46

Figure 10. Cont. Minerals 2019, 9, 705 13 of 16 Minerals 2019, 9, x FOR PEER REVIEW 13 of 16

Figure 10. Ca2p X-ray photoelectron spectroscopy (XPS) of scheelite (a), calcite (b), and fluorite (c) Figurebefore 10. andCa2p after X-ray the reactionphotoelectron with SHMP. spectroscopy (XPS) of scheelite (a), calcite (b), and fluorite (c) before and after the reaction with SHMP. Table 3. Binding energies and their shifts of Ca2p1 and Ca2p3 of minerals before and after the interaction 3.6. Low-Gradewith SHMP. Scheelite Ore Flotation Tests Minerals Scheelite Calcite Fluorite The tungsten ore, obtained from Luanchuan, was a representative skarn scheelite. The WO3 grade wasElements only 0.1%. The Ca2p1 raw ore underwent Ca2p3 roughing Ca2p1 with a Ca2p3 fatty acid collector Ca2p1 and then Ca2p3 underwent Before (eV) 350.51 346.94 350.56 346.99 351.37 347.79 heatingAfter cleaning (eV) by applying 350.54 the Petrov 346.94 method. 350.69 However, 347.13the associated 351.82 calcite and 348.25fluorite were also enrichedShifts (eV) during the 0.03 roughing stage 0 due to 0.13 the poor selectivity 0.14 of the 0.45 collector. Owing 0.46 to the outstanding ability of SHMP to depress calcite and fluorite, it was applied to promote the scheelite roughing3.6. Low-Grade efficiency. Scheelite Figure Ore 11 Flotation presents Tests the roughing performance of scheelite with varying SHMP doses applied to the assembled collector. In the blank test, namely, in the absence of SHMP, the The tungsten ore, obtained from Luanchuan, was a representative skarn scheelite. The WO3 roughinggrade wasWO only3 grade 0.1%. of Thescheelite raw ore was underwent only 0.74% roughing with with a considerable a fatty acid collector recovery and of then 84.66%. underwent When the SHMPheating dose cleaning was increased by applying to 15 the g/t, Petrov the method. roughing However, grade theincreased associated to calcite1.65%, and while fluorite the wererecovery decreasedalso enriched to 72.83%. during The the roughing roughing grade stage was due increase to the poord by selectivity two-fold ofcompared the collector. with Owing that in to the the blank test. outstandingThe results abilityprove ofthat SHMP SHMP to depressis an effective calcite and depressant fluorite, itto was depress applied the to gangue promote minerals the scheelite at a low dose,roughing thereby eimprovingfficiency. Figure the scheelite 11 presents roughing the roughing efficiency. performance of scheelite with varying SHMP doses applied to the assembled collector. In the blank test, namely, in the absence of SHMP,the roughing WO3 grade of scheelite was only 0.74% with a considerable recovery of 84.66%. When the SHMP dose was increased to 15 g/t, the roughing grade increased to 1.65%, while the recovery decreased to 72.83%. The roughing grade was increased by two-fold compared with that in the blank test. The results prove that SHMP is an effective depressant to depress the gangue minerals at a low dose, thereby improving the scheelite roughing efficiency.

Figure 11. Scheelite roughing performance with diverse SHMP doses when applying the assembled collector.

Minerals 2019, 9, x FOR PEER REVIEW 13 of 16

Figure 10. Ca2p X-ray photoelectron spectroscopy (XPS) of scheelite (a), calcite (b), and fluorite (c) before and after the reaction with SHMP.

3.6. Low-Grade Scheelite Ore Flotation Tests

The tungsten ore, obtained from Luanchuan, was a representative skarn scheelite. The WO3 grade was only 0.1%. The raw ore underwent roughing with a fatty acid collector and then underwent heating cleaning by applying the Petrov method. However, the associated calcite and fluorite were also enriched during the roughing stage due to the poor selectivity of the collector. Owing to the outstanding ability of SHMP to depress calcite and fluorite, it was applied to promote the scheelite roughing efficiency. Figure 11 presents the roughing performance of scheelite with varying SHMP doses applied to the assembled collector. In the blank test, namely, in the absence of SHMP, the roughing WO3 grade of scheelite was only 0.74% with a considerable recovery of 84.66%. When the SHMP dose was increased to 15 g/t, the roughing grade increased to 1.65%, while the recovery decreased to 72.83%. The roughing grade was increased by two-fold compared with that in the blank test. The results prove that SHMP is an effective depressant to depress the gangue minerals at a low Minerals 2019, 9, 705 14 of 16 dose, thereby improving the scheelite roughing efficiency.

FigureFigure 11. Scheelite roughingroughing performance performanc withe with diverse diverse SHMP SHMP doses doses when when applying applying the assembled the assembled collector. collector. 4. Conclusions A highly selective reagent system that utilizes SHMP as depressant and sodium oleate mixed with

oleamide as the collector to enhance the enrichment of scheelite was presented. The flotation behaviors of scheelite, calcite, and fluorite were examined through flotation tests. The reaction mechanism of minerals with SHMP was performed by equilibrium speciation, Zeta potential, FTIR, and XPS analyses. Finally, the highly selective reagent system was applied to improve the roughing performance of low-grade scheelite ore. The following conclusions were drawn. (1) SHMP exhibited outstanding performance to depress calcite and fluorite, thus facilitating the flotation enrichment of scheelite. 2– 2– (2) SHMP degraded into lower phosphates and hydrolyses into HPO4 at a pH of 10. HPO4 may be adsorbed on the positively charged surfaces of calcite and fluorite via electrostatic force or chelation with calcium species, whereas it was hardly adsorbed on scheelite surface because of its negative charge. (3) The scheelite roughing grade was considerably improved from 0.74% to 1.65% in the presence of SHMP. The application of SHMP presented a practical case to depress gangue minerals, thereby promoting the roughing efficiency of scheelite.

Author Contributions: Y.H., W.S., Z.G., and R.L. conceived and designed the experiments; J.K. performed the experiments, analyzed the date and wrote the paper. Funding: The research was supported by Natural Science Foundation of China (Nos. 51374247 and 51634009), Innovation Driven Plan of Central South University (No. 2015CX005), Key Laboratory of Hunan Province for Clean and Efficient Utilisation of Strategic Calcium-containing Mineral Resources (No. 2018TP1002), the National 111 Project (No. B14034) and Collaborative Innovation Center for Clean and Efficient Utilisation of Strategic Metal Mineral Resources. Conflicts of Interest: The authors declare no conflict of interest.

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