Selective Flotation of Pyrite from Galena Using Chitosan with Diﬀerent Molecular Weights

Article Selective Flotation of Pyrite from Galena Using Chitosan with Diﬀerent Molecular Weights

Wanjia Zhang 1,2 , Wei Sun 1,2, Yuehua Hu 1,2, Jian Cao 1,2,3,4,* and Zhiyong Gao 1,2,*

1 School of Minerals Processing and Bioengineering, Central South University, Changsha 410083, China 2 Key Laboratory of Hunan Province for Clean and Eﬃcient Utilization of Strategic Calcium Containing − Mineral Resources, Central South University, Changsha 410083, China 3 Guangdong Institute of Resources Comprehensive Utilization, Guangzhou 510651, China 4 Guangdong Provincial Key Laboratory of Development and Comprehensive Utilization of Mineral Resources, Guangzhou 510651, China * Correspondence: [email protected] (J.C.); [email protected] (Z.G.); Tel.: +86-198-0749-8656 (J.C.); +86-135-7489-2751 (Z.G.) Received: 29 July 2019; Accepted: 6 September 2019; Published: 12 September 2019

Abstract: Pyrite is a major gangue mineral associated with galena and other valuable minerals, and it is necessary to selectively remove pyrite to upgrade the lead concentrate by froth flotation. In this study, the flotation experiments of a single mineral and mixed minerals were performed using chitosan with different molecular weights (MW = 2 3, 3 6, 10 and 100 kDa) as a depressant, − − ethyl xanthate as a collector, and terpineol as a frother, in a bid to testify the separation of pyrite from galena. Flotation results showed that the selective flotation of pyrite from galena can be achieved under the preferred reagent scheme, i.e., 400 g/t chitosan (10 kDa), 1600 g/t ethyl xanthate, and 100 g/t terpineol, while chitosan with other molecular weights cannot. Furthermore, the results of the zeta potential and contact angle measurements revealed that chitosan (10 kDa) has a strong adsorption on galena yet a very weak adsorption on pyrite at the dosage of 400 g/t. This study showed that chitosan (10 kDa) has great potential in the industrial flotation separation of pyrite from lead concentrates.

Keywords: chitosan; molecular weight; selective depression; reverse ﬂotation; pyrite; galena

1. Introduction

Pyrite (FeS2) is the main gangue associated with valuable sulfide ores, such as galena, chalcopyrite, sphalerite, and others [1–5]. Pyrite is usually depressed during the flotation of sulfide minerals [6] by the use of a depressant [4,7], pulp-environment management [8,9], and grinding-environment changes [10,11]. Although these techniques were proven to be effective, the loss of valuable minerals in tailings [12,13] and the doping of pyrite in concentrates [14] still cannot be ignored. According to the principle of “float less and depress more”, it is more cost-effective to reversely float pyrite while depressing lead concentrates, in a bid to upgrade lead concentrates containing pyrite impurities [15]. Our previous studies have also confirmed that reverse flotation is an effective method to improve the grade of desired mineral/metal in flotation concentrate, and the development of a new reagents scheme [16–18] is the key to achieve this [19,20]. Therefore, in this study,we explored an efficient and highly selective galena depressant and a pyrite-removal strategy to upgrade lead concentrates. Polysaccharide polymers [21–24] are a hot topic in mineral processing due to their efficiency and degradability as the depressants for the flotation of sulfide ores [25–27]. In recent years, there were many reports on the relations between the depression effect of macromolecular depressants and molecular weight (MW), i.e., low molecular weight (LMW: 1 kDa MW < 50 kDa), medium molecular weight (MMW: 50 kDa ≤ MW < 500 kDa), and high molecular weight (HMW: 500 kDa MW < 1000 kDa) [28–31]. These studies ≤ ≤

Minerals 2019, 9, 549; doi:10.3390/min9090549 www.mdpi.com/journal/minerals Minerals 2018, 8, x FOR PEER REVIEW 2 of 11 many reports on the relations between the depression effect of macromolecular depressants and molecular weight (MW), i.e., low molecular weight (LMW: 1 kDa ≤ MW < 50 kDa), medium molecular

Mineralsweight2019 (MMW:, 9, 549 50 kDa ≤ MW < 500 kDa), and high molecular weight (HMW: 500 kDa ≤ MW <2 1000 of 11 kDa) [28–31]. These studies showed that MMW-macromolecule depressants can selectively depress the mixed-mineral systems through competitive adsorption. However, the selective depression effect showedof MMW-macromolecule that MMW-macromolecule depressants depressants is still not satisfactory, can selectively especially depress for the polymetallic mixed-mineral sulfide systems ores. throughIn this competitive study, the adsorption. depression However, effect of the chitosan selective with depression different effect molecular of MMW-macromolecule weights (ULMW depressants(ultralow molecular is still not weight) satisfactory, = 2−3, especially 3−6 kDa, for LMW polymetallic = 10 kDa, sulfide and ores.MMW = 100 kDa) on galena and pyriteIn were this study, investigated, the depression and a novel effect ofreagent chitosan mode with was diff erentused molecularto evaluate weights the flotation (ULMW behavior (ultralow of molecularthe single mineral. weight) The= 2 separation3, 3 6 kDa, performance LMW = 10 of kDa, this novel and MMW reagent= mode100 kDa) was testified on galena by andthe mixed- pyrite − − weremineral investigated, flotation andexperiments, a novel reagent and the mode mechanism was used of to evaluatethe selective the flotation separation behavior was ofinvestigated the single mineral.through zeta The separationpotential measurements performance ofand this contact novel angle reagent tests. mode was testified by the mixed-mineral flotation experiments, and the mechanism of the selective separation was investigated through zeta potential2. Materials measurements and Methods and contact angle tests.

2.2.1. Materials Materials and and MethodsReagents

2.1. MaterialsThe pure and pyrite Reagents and galena crystal samples were obtained from the Huangshaping Mine, Hunan, China. Firstly, the commonly exposed cleavage surfaces were carefully selected and cleaved The pure pyrite and galena crystal samples were obtained from the Huangshaping Mine, to collect samples for contact angle tests. The selected pyrite and galena crystals were respectively Hunan, China. Firstly, the commonly exposed cleavage surfaces were carefully selected and cleaved embedded in the resin, and the required surfaces remained exposed for further grinding and to collect samples for contact angle tests. The selected pyrite and galena crystals were respectively polishing. In a bid to obtain a flat surface, the desired exposed surfaces of pyrite and galena were embedded in the resin, and the required surfaces remained exposed for further grinding and polishing. successively ground by sandpapers at a roughness of 90, 38, 18, 6.5, and 2.5 µm. The ground surfaces In a bid to obtain a flat surface, the desired exposed surfaces of pyrite and galena were successively were successively polished with an alumina powder (1.0, 0.5, and 0.05 µm) solution on a polishing ground by sandpapers at a roughness of 90, 38, 18, 6.5, and 2.5 µm. The ground surfaces were cloth [32]. successively polished with an alumina powder (1.0, 0.5, and 0.05 µm) solution on a polishing cloth [32]. Pure pyrite and galena crystal samples were freshly ground in a ceramic ball mill (Tencan Pure pyrite and galena crystal samples were freshly ground in a ceramic ball mill (Tencan Powder Powder Co., Ltd., Changsha, China) and screened for required flotation samples with a size of −74 Co., Ltd., Changsha, China) and screened for required flotation samples with a size of 74 + 37 µm. +37 µm. Samples stored within 1–3 days after preparation were further ground to −5 µm− in an agate Samples stored within 1–3 days after preparation were further ground to 5 µm in an agate mortar mortar and used for X-ray powder diffraction and zeta potential measurements.− As shown in Figure and used for X-ray powder diffraction and zeta potential measurements. As shown in Figure1, 1, the X-ray powder diffraction spectra of the pyrite and galena samples confirmed that both the the X-ray powder diffraction spectra of the pyrite and galena samples confirmed that both the pyrite pyrite and galena samples were sufficiently pure. and galena samples were sufficiently pure.

Figure 1. Cont.

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Figure 1. XRD spectrums of (a) pyrite and (b) galena particle samples.

Figure 1. XRD spectrums of (a) pyrite and (b) galena particle samples. Figure 1. XRD spectrums of (a) pyrite and (b) galena particle samples. The ethyl xanthateFigure (90%, 1. XRD Macklin spectrums Chemical of (a) pyrite Reag andent (Co.,b) galena Ltd., particle Changsha, samples. China) was used as a collectorThe [33],ethyl ethyl the xanthate xanthate terpineol (90%, (90%, (90%, Macklin Macklin Tianjin Chemical Kermil Chemical Chem Reag Reagenticalent ReagentCo., Co., Ltd., Ltd., Centre, Changsha, Changsha, Tianjin, China) China) China) was as wasused a frother used as a collector[34],as a collectorandThe [33], ethylthe thechitosan [33xanthate terpineol], the (Bomei terpineol(90%, (90%, MacklinBiotechnology Tianjin (90%, Chemical Kermil Tianjin Co., Chem Kermil Reag Ltd.,icalent Chemical ReagentHefei, Co., Ltd., China) Centre, Reagent Changsha, (2–3, Tianjin, Centre, 3–6, China) China) 10 Tianjin, and was as 100 useda frother China) kDa, as a [34],chitosanascollector a frother and >99%, [33],the [34 ],chitosanthe water-soluble, and terpineol the chitosan(Bomei (90%, deacetylation Biotechnology (Bomei Tianjin Biotechnology Kermil rate Co., Chem>85%) Ltd., Co.,ical as Hefei, Ltd.,depressants.Reagent Hefei, China) Centre, China) The (2–3, Tianjin, colors (2–3, 3–6, 3–6,and China)10 10 andmorphologies and as 100 100a frother kDa, kDa, chitosanof[34], chitosan and >>99%, 99%,the with chitosanwater-soluble, water-soluble, different (Bomei molecular deacetylation Biotechnology weights rateare Co.,shown >>85%)85%) Ltd., inas as Figuredepressants. Hefei, 2,China) and Thethe The (2–3, structural colors colors 3–6, and 10 unit morphologies and of 100chitosan kDa, ofischitosan shownchitosan >99%,in with Figure water-soluble,different diﬀ 3.erent When molecular the deacetylation pulp weights pH was arerate not shown >85%) equal in as to Figure depressants. 8, the2 2,, pulp andand the the conditionThe structuralstructural colors was and unitunit adjusted morphologies ofof chitosanchitosan with isHClof shown chitosan and NaOH inin FigurewithFigure stock different3 .3. When solutionsWhen molecular the the pulp (0.1 pulp mol/L). pH weights pH was was All not are notexpe equal shown equalriments to in 8,to Figure the 8,were the pulp 2, carriedpulp conditionand conditionthe out structural in was deionized was adjusted unit adjusted (DI)of with chitosan water with HCl HClwithandis shown NaOH anda resistivity NaOH in stock Figure stock solutionsof over3. solutions When 18 (0.1 M theΩ mol(0.1 × pulp cm./ L).mol/L). AllpH experimentswas All expenot equalriments were to were8, carried the carried pulp out incondition out deionized in deionized was (DI) adjusted water(DI) water withwith withaHCl resistivity aand resistivity NaOH of over stockof 18over Msolutions Ω18 Mcm.Ω (0.1× cm. mol/L). All experiments were carried out in deionized (DI) water with a resistivity of over 18× MΩ × cm.

Figure 2. The morphologies of chitosan with different molecular weights: (a) 2−3 kDa chitosan; (b) Figure 2. The morphologies of chitosan with diﬀerent molecular weights: (a) 2 3 kDa chitosan; Figure3−6 kDa 2. chitosan; The morphologies (c) 10 kDa chitosan;of chitosan (d )with 100 kDadifferent chitosan. molecular weights: (a) 2−3 kDa− chitosan; (b) (b) 3 6 kDa chitosan; (c) 10 kDa chitosan; (d) 100 kDa chitosan. 3Figure−6 kDa− 2.chitosan; The morphologies (c) 10 kDa chitosan; of chitosan (d) with100 kDa different chitosan. molecular weights: (a) 2−3 kDa chitosan; (b) 3−6 kDa chitosan; (c) 10 kDa chitosan; (d) 100 kDa chitosan.

Figure 3. The structural unit of chitosan. Figure 3. The structural unit of chitosan.

2.2. Flotation Experiments of SingleFigure Mineral 3. The and structural Mixed Minerals unit of chitosan. 2.2. Flotation Experiments of SingleFigure Mineral 3. Theand structuralMixed Minerals unit of chitosan. The flotation experiments were carried out in an XFG-type flotation machine [19,35–38] at a spindle 2.2. FlotationThe flotation Experiments experiments of Single were Mineral carried and Mixedout in Minerals an XFG-type flotation machine [19,35–38] at a speed2.2. Flotation of 1850 Experiments rpm. In each of Single flotation Mineral experiment, and Mixed a single-mineral Minerals sample (2.00 g) or mixed-mineral spindlesampleThe (1.00speed flotation g pyriteof 1850 experiments plus rpm. 1.00 In geach galena)were flotation carried was added expeout riment,in to an the XFG-type flotation a single-mineral cell flotation (with sample 40machine mL DI(2.00 water)[19,35–38] g) or to mixed- obtain at a spindlemineralthe mineralThe speedsample flotation suspension. of (1.00 1850 experiments g rpm.pyrite If necessary, In plus each were 1.00 flotation HCl carriedg galena) or NaOH expe out wasriment, in stock added an XFG-type solutiona single-mineralto the flotat (0.1 flotation molion/ L)cellsample machine was (with added (2.00 40 [19,35–38] mL g) to or adjustDI mixed- water) at the a pulpmineraltospindle obtain pH. samplespeed the Then, mineral of (1.00 chitosan 1850 gsuspension. pyriterpm. (2 In3, plus 3each 6, If1.00 necessary, 10,flotation g and galena) 100 expe HCl kDa),wasriment, or added NaOH ethyl a to xanthate,single-mineral stock the flotat solution andion terpineolcell sample(0.1 (with mol/L) (2.00 with40 wasmL g) the DI oradded desired water)mixed- to − − toadjustdosagemineral obtain the were samplethe pulp mineral added pH. (1.00 Then, in suspension.g sequence, pyrite chitosan plus andIf 1.00 (2necessary,− floated3, g 3 galena)−6, 10, products HCl andwas or 100 added NaOH were kDa), to collected stock theethyl flotat solution xanthate, forion 3 min.cell(0.1 and (with mol/L) Finally, terpineol 40 was mL the addedDI flotationwith water) the to adjustconcentratesto obtain the thepulp and mineral pH. tailings Then, suspension. were chitosan filtered, If (2 necessary,−3, dried, 3−6, 10, and HCl and weighed or 100 NaOH kDa), for stock recoveryethyl solution xanthate, calculation. (0.1 and mol/L) terpineol At the was same withadded time, the to adjust the pulp pH. Then, chitosan (2−3, 3−6, 10, and 100 kDa), ethyl xanthate, and terpineol with the

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Fe and Pb grades of ﬂotation concentrates and tailings were assayed. Each ﬂotation experiment was repeated three times to obtain an average value.

2.3. Zeta Potential Measurements The zeta potential measurement is a common electrochemical test used to detect the reaction process on mineral surfaces and adsorption behavior of ﬂotation reagents [39]. Zeta potential measurements were conducted at 20 ◦C, using a Zetasizer Nano series (Malvern, United Kingdom). The mineral suspension was prepared by adding a pyrite or galena sample (20 mg, 5 µm) to a KCl electrolyte − solution (0.01 mol/L, 40 mL) in a beaker (100 mL), at a given pH value and chitosan dosage. The mineral suspension was then used for zeta potential measurements after magnetically stirring for 15 min and standing for 5 min. Each zeta potential was tested six times, and the average value was obtained.

2.4. Contact Angle Tests The contact angle test is a common method to detect the hydrophobic changes of mineral surfaces in the presence or absence of flotation reagents [40]. The wettability of the prepared pyrite and galena samples in the presence or absence of chitosan with different molecular weights at a dosage of 400 g/t was measured at pH 8. The prepared pyrite or galena sample was first immersed in a beaker (150 mL) containing a given solution (100 mL) and stirred at 20 ◦C for 30 min. Then, the sample was taken out, gently washed with DI water, and vacuum dried at 50 ◦C for 10 min, followed by measuring contact angles via the water droplet method [41]. In each contact angle test, droplets of 3.5 µL DI water were added, and 25 35 s later, when a stable state was reached, the contact angles were calculated based − on the shape of the droplets (Windrop++ Software). Under each condition, contact angle tests were carried out three times, to take the average value. After each test, the sample surface was repeatedly washed with DI water and alcohol. Finally, the sample was polished using a 0.05 µm alumina powder solution for the next test.

3. Results

3.1. Single-Mineral Flotation Experiments Results Using different molecular weights of chitosan as depressants, ethyl xanthate as a collector, and terpineol as a frother, the flotation behavior of pyrite and galena was evaluated by the single-mineral flotation, with results shown in Figure4. At a certain dosage, with the increase of the chitosan molecular weight, the flotation recoveries of pyrite and galena decrease gradually. The flotation recoveries of pyrite and galena vary slightly with chitosan (2 3 and 3 6 kDa), yet greatly with chitosan (10 and 100 kDa). − − In particular, the difference in flotation recovery between pyrite and galena using chitosan (10 kDa) is much greater than that using chitosan (100 kDa). Furthermore, in Figure4, the arrows point to the optimum molecular weight (10 kDa) and dosage (400 g/t) of chitosan. At a certain molecular weight of chitosan, recoveries of pyrite and galena decrease gradually with the increase of dosage. Notably, when the chitosan (10 kDa) dosage is increased to 400 g/t, galena recovery drops significantly to 10.29%, whereas the pyrite recovery decreases slightly to 75.02%. Hence, using 400 g/t of chitosan (10 kDa), 1600 g/t of ethyl xanthate, and 100 g/t of terpineol, the maximum recovery difference (64.73%) between pyrite and galena can be achieved. This is the preferred reagent scheme for subsequent experiments. MineralsMinerals2019 2018,,9 8,, 549x FOR PEER REVIEW 55 of 1111

Figure 4. Pyrite and galena recovery results of single-mineral flotation experiments using chitosan Figure 4. Pyrite and galena recovery results of single-mineral flotation experiments using chitosan with different dosages and molecular weights as a depressant, 1600 g/t ethyl xanthate as a collector, with different dosages and molecular weights as a depressant, 1600 g/t ethyl xanthate as a collector, and 100 g/t terpineol as a frother at pH 8. and 100 g/t terpineol as a frother at pH 8. 3.2. Mixed-Mineral Flotation Experiments Results 3.2. Mixed-Mineral Flotation Experiments Results To further evaluate the flotation separation performance of chitosan with different molecular weights,To further flotation evaluate experiments the flotation of mixed separation pyrite/galena performance minerals wereof chitosan carried with out underdifferent the molecular preferred reagentweights, scheme flotation (400 experiments g/t of chitosan of mixed with pyrite/gal differentena molecular minerals weights,were carried 1600 out g/ tunder of ethyl the xanthate,preferred andreagent 100 gscheme/t of terpineol), (400 g/t of with chitosan the results with showndifferent in molecular Table1. weights, 1600 g/t of ethyl xanthate, and 100 g/t of terpineol), with the results shown in Table 1. Table 1. The results of mixed-mineral flotation experiments using 400 g/t chitosan with different molecular weightsTable 1. as The a depressant, results of 1600mixed-mineral g/t ethyl xanthate flotation as a experi collector,ments and using 100 g /t400 terpineol g/t chitosan as a frother with atdifferent pH 8. molecular weights as a depressant, 1600 g/t ethyl xanthate as a collector, and 100 g/t terpineol as a frotherChitosan at pH with 8. Different Recovery (%) Grade (%) Flotation Product Weight (g) Molecular Weights (kDa) Fe Pb Fe Pb Chitosan with Recovery (%) Grade (%) Iron concentrate 1.68 81.83 85.72 22.78 44.28 Different Molecular- Flotation Product Weight (g) Lead concentrate 0.32 18.17Fe 14.28Pb 17.22Fe 55.72Pb Weights (kDa) Iron concentrate 1.59 78.91 79.65 22.98 43.11 2–3 IronLead concentrate concentrate 1.68 0.41 81.83 21.09 85.72 20.35 23.5022.78 42.1444.28 - Iron concentrate 1.49 75.57 73.39 23.43 42.28 3–6 Lead concentrate 0.32 18.17 14.28 17.22 55.72 IronLead concentrate concentrate 1.59 0.51 78.91 24.43 79.65 26.61 22.1022.98 44.7443.11 2–3 Iron concentrate 0.86 74.04 11.98 39.75 11.95 10 Lead concentrate 0.41 21.09 20.35 23.50 42.14 Lead concentrate 1.14 25.96 88.02 10.52 66.27 Iron concentrate 1.49 75.57 73.39 23.43 42.28 3–6 Iron concentrate 0.52 40.32 11.54 35.90 19.10 100 Lead concentrate 0.51 24.43 26.61 22.10 44.74 Lead concentrate 1.48 59.68 88.46 18.60 51.25 Iron concentrate 0.86 74.04 11.98 39.75 11.95 10- Mixed minerals - - - 23.09 42.91 Lead concentrate 1.14 25.96 88.02 10.52 66.27 Note: The values in parentheses denote deviations. Iron concentrate 0.52 40.32 11.54 35.90 19.10 Table1 shows100 that, when using chitosan with a MW of 2 3 kDa, the metal grades in iron concentrates Lead concentrate 1.48− 59.68 88.46 18.60 51.25 and lead concentrates are comparable (22.98% Fe and 43.11% Pb in iron concentrates, and 23.50% Fe and 42.14% Pb- in lead concentrates).Mixed Analogously,minerals chitosan- with a MW- of 3 - 6 kDa23.09 also shows42.91 inferior − selectivity (23.43% Fe and 42.28%Note: Pb The in ironvalues concentrates, in parentheses and denote 22.1% deviations Fe and 44.74%. Pb in lead concentrates).

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Table 1 shows that, when using chitosan with a MW of 2−3 kDa, the metal grades in iron concentrates and lead concentrates are comparable (22.98% Fe and 43.11% Pb in iron concentrates, Mineralsand 23.50%2019, 9 ,Fe 549 and 42.14% Pb in lead concentrates). Analogously, chitosan with a MW of 3−66 ofkDa 11 also shows inferior selectivity (23.43% Fe and 42.28% Pb in iron concentrates, and 22.1% Fe and 44.74% Pb in lead concentrates). These results demonstrate that chitosans with a low molecular These results demonstrate that chitosans with a low molecular weight (2 3 and 3 6 kDa) fail to perform weight (2−3 and 3−6 kDa) fail to perform depression selectivity. In contrast,− when− chitosan (10 and depression100 kDa) is selectivity.used, significant In contrast, improvements when chitosan of the (10 Fe and grade 100 in kDa) ironis concentrates used, significant and the improvements Pb grade in oflead the concentrates Fe grade in ironare obtained. concentrates In particular, and the Pb when grade chitosan in lead concentrates (10 kDa) is areused, obtained. the Fe grade In particular, in iron whenconcentrates chitosan and (10 the kDa) Pb is grade used, thein lead Fe grade concentrates in iron concentrates stand at 39.75% and theand Pb 66.27%, grade inrespectively, lead concentrates which standare higher at 39.75% than and those 66.27%, of 35.90% respectively, and 51.25% which when are higher chitosan than those(100 kDa) of 35.90% is used. and 51.25%In addition, when with chitosan 400 (100g/t of kDa) chitosan is used. (10 In kDa) addition, as a withdepressant, 400 g/t ofthe chitosan maximum (10 kDa) recovery as a depressant, difference thebetween maximum Fe and recovery Pb in differencepyrite or lead between concentrates Fe and Pb is in62.60%, pyrite compared or lead concentrates with that isof 62.60%,28.78% comparedwith 400 g/t with of chitosan that of 28.78% (100 kDa). with 400 g/t of chitosan (100 kDa). 3.3. Zeta Potential Measurements Results 3.3. Zeta Potential Measurements Results The zeta potential measurement is conducted to reveal the flotation mechanism [42–44]. Zeta The zeta potential measurement is conducted to reveal the flotation mechanism [42–44]. potentials of pyrite and galena in the presence or absence of 400 g/t of chitosan with different Zeta potentials of pyrite and galena in the presence or absence of 400 g/t of chitosan with different molecular weights were measured under pH 2−12. As shown in Figure 5, the isoelectric points (IEP) molecular weights were measured under pH 2 12. As shown in Figure5, the isoelectric points (IEP) of bare pyrite and galena occurred at about pH− 3, which is in line with previous reports [45–47]. of bare pyrite and galena occurred at about pH 3, which is in line with previous reports [45–47]. As can be seen from Figure 5, compared with zeta potential curves of bare pyrite and galena, the As can be seen from Figure5, compared with zeta potential curves of bare pyrite and galena, curves of both pyrite and galena with chitosan (2−3 and 3−6 kDa) present a very weak negative shift the curves of both pyrite and galena with chitosan (2 3 and 3 6 kDa) present a very weak negative at pH 4−12. Furthermore, in the presence of chitosan (100− kDa),− the zeta potential curves of both pyrite shift at pH 4 12. Furthermore, in the presence of chitosan (100 kDa), the zeta potential curves of both and galena demonstrate− a strong negative shift at pH 4−12. However, in the presence of chitosan (10 pyrite and galena demonstrate a strong negative shift at pH 4 12. However, in the presence of chitosan kDa), the zeta potential curve of galena shows a strong negative− shift, while that of pyrite presents a (10 kDa), the zeta potential curve of galena shows a strong negative shift, while that of pyrite presents slight one. a slight one. At a dosage of 400 g/t and pH 8, and in the presence of chitosan with different molecular weights, At a dosage of 400 g/t and pH 8, and in the presence of chitosan with different molecular weights, the zeta potential changes of pyrite and galena are presented in Table 2. Compared with that of pyrite the zeta potential changes of pyrite and galena are presented in Table2. Compared with that of pyrite and galena, the zeta potential values in the presence of chitosan (2−3 and 3−6 kDa) exhibit a slight and galena, the zeta potential values in the presence of chitosan (2 3 and 3 6 kDa) exhibit a slight negative decrease. In addition, in the presence of chitosan (100 kDa),− the −zeta potential values of negative decrease. In addition, in the presence of chitosan (100 kDa), the zeta potential values of pyrite pyrite and galena are significantly reduced by 11.3 and 22.2 mV, respectively. However, in the and galena are significantly reduced by 11.3 and 22.2 mV, respectively. However, in the presence presence of chitosan (10 kDa), the zeta potential value of galena drops greatly by 21.3 mV at pH 8, of chitosan (10 kDa), the zeta potential value of galena drops greatly by 21.3 mV at pH 8, while that while that of pyrite exhibits a negligible diminution by 4.0 mV. of pyrite exhibits a negligible diminution by 4.0 mV.

Figure 5. Cont.

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FigureFigure 5.5. TheThe eeffectffect ofof thethe additionaddition ofof chitosanchitosan withwith didifffferenterent molecularmolecular weightsweights (400(400 gg/t)/t) onon thethe zetazeta potentialpotential ofof ((aa)) pyritepyrite and and ( b(b)) galena. galena. Table 2. Zeta potential changes (mV) of pyrite and galena in the presence of chitosan with different molecularTable 2. Zeta weights potential (400 gchanges/t) at pH (mV) 8. of pyrite and galena in the presence of chitosan with different molecular weights (400 g/t) at pH 8. Chitosan with Different Molecular Weights (kDa) System Chitosan with Different Molecular Weights (kDa) System 2–3 3–6 10 100 2–3 3–6 10 100 Pyrite + Chitosan 1.8 2.0 4.0 11.3 Pyrite + Chitosan −−1.8 −2.0− −4.0− −−11.3 Galena + Chitosan 2.9 8.0 21.3 22.2 Galena + Chitosan −−2.9 −8.0− −21.3− −−22.2

3.4.3.4. ContactContact AngleAngle TestsTests ResultsResults EstablishingEstablishing thethe contactcontact angleangle isis aa goodgood methodmethod forfor determiningdetermining thethe hydrophobicityhydrophobicity ofof aa mineralmineral surfacesurface andand measuringmeasuring aa mineral’smineral’s floatabilityfloatability [[48–5148–51].]. ToTo furtherfurther exploreexplore thethe mechanismmechanism ofof flotationflotation separation,separation, the the contact contact angles angles of of pyrite pyrite and and galena galena surfaces surfaces in the in presencethe presence or absence or absence of chitosan of chitosan with differentwith different molecular molecular weights weights (400 g/t), (400 at pH g/t), 8, wereat pH tested. 8, were As tested. shown inAs Figure shown6, thein Figure contact 6, angle the valuescontact ofangle pyrite values and galenaof pyrite conditioned and galena with conditioned DI water stand with at DI 58.8 water and 69.3stand◦, respectively. at 58.8 and Compared69.3°, respectively. with the resultsCompared conditioned with the with results DI water,conditioned the contact with DI angles water, of pyritethe contact and galenaangles inof thepyrite presence and galena of chitosan in the (2 3 and 3 6 kDa) decrease marginally by 4.5 and 1.7 (chitosan (2 3 kDa)) or 5.7 and 3.4 (chitosan presence− of− chitosan (2−3 and 3−6 kDa) decrease marginally◦ by 4.5− and 1.7° (chitosan (2−◦3 kDa)) or (3–65.7 and kDa)), 3.4° respectively. (chitosan (3–6 The kDa)), contact respectively. angles of pyriteThe contact and galena angles in of the pyrite presence and galena of chitosan in the (100 presence kDa) decreaseof chitosan significantly (100 kDa) decrease by 16.1 and significantly 32.1◦, respectively. by 16.1 and However, 32.1°, respectively. in the presence However, of chitosan in the (10presence kDa), theof chitosan contact angle (10 kDa), of galena the reducescontact sharplyangle of by galena 30.6◦, whilereduces that sharply of pyrite by decreases 30.6°, while slightly that by 9.5of ◦pyrite. decreases slightly by 9.5°.

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Figure 6. The effect of the addition of chitosan with different molecular weights (400 g/t) on the contact anglesFigure of6. (Thea) pyrite effect andof the (b addition) galena. of chitosan with different molecular weights (400 g/t) on the contact angles of (a) pyrite and (b) galena. 4. Discussion 4. Discussion The single-mineral flotation results demonstrated that, under the proposed reagent scheme (400 ofThe g/t single-mineral chitosan with di flotationfferent molecular results demonstrat weights, 1600ed that, g/t ofunder ethyl the xanthate, proposed and reagent 100 g/t ofscheme terpineol), (400 theof g/t depression chitosan ewithffect different of chitosan molecular could be weights, significantly 1600 improved g/t of ethyl with xanthate, an increase and 100 in molecular g/t of terpineol), weight fromthe depression 2 3 to 100 kDaeffect [52 of]. Chitosanchitosan could (2 3 kDa) be significantly and chitosan improved (3 6 kDa) possesswith anno increase selective indepression molecular − − − eweightffect due from to their 2−3 weakto 100 depression kDa [52]. abilities Chitosan for (2 both−3 kDa) pyrite and and chitosan galena. However, (3−6 kDa) both possess chitosan no (selective10 kDa) anddepression chitosan effect (100 kDa)due haveto their selective weak depressiondepression on abi galena,lities for and both the formerpyrite hasanda galena. much better However, selectivity, both whichchitosan leads (10 tokDa) a bigger and chitosan recovery (100 diff kDa)erence have between selective pyrite depression and galena. on galena, This result and the means former that has the a separationmuch better performance selectivity, iswhich better leads by using to a bigger this reagent recovery scheme. difference The results between of mixed-mineralpyrite and galena. flotation This furtherresult means confirm that that the chitosan separation (2 3 kDa)performance and chitosan is be (3tter6 by kDa) using have this no reagent separation scheme. effect, The whereas results both of − − chitosanmixed-mineral (10 kDa) flotation and chitosan further (100 confi kDa)rm that have chitosan a separation (2−3 kDa) effect. and The chitosan chitosan (3 (100−6 kDa) kDa) have obtains no aseparation limited separation effect, whereas performance both chitosan [29,53], (10 while kDa) chitosan and chitosan (10 kDa) (100 has kDa) a much have better a separation performance, effect. obtainingThe chitosan a high-grade (100 kDa) leadobtains concentrate a limited withseparation a Pb grade performance of 66.27% [29,53], and a Pbwhile recovery chitosan of 88.02%(10 kDa) from has thea much feed better with aperformance, Pb grade of 42.91%.obtaining The a high-grade flotation e ffleadect concentrate is better than with other a Pb depressants grade of 66.27% previously and a reportedPb recovery [11, of12 ,88.02%29]. from the feed with a Pb grade of 42.91%. The flotation effect is better than other depressantsHuang etpreviously al [29]. previously reported [11,12,29]. used chitosan to depress pyrite and float galena. In their study, a reagentHuang scheme et al [29]. of 1.25previously10 4 usedmol /chitosanL of ethyl to xanthatedepress pyrite (EX), and chitosan float galena. (100–300 In kDa,their study, 75–85% a × − deacetylation),reagent scheme and of pH 1.25 4 was × 10 employed,−4 mol/L whichof ethyl is dixanthatefferent from (EX), the chitosan scheme here(100–300 (e.g., 5.5kDa,10 75%–85%4 mol/L × − ofdeacetylation), EX, chitosan and (100 pH kDa, 4 was 85–98% employed, deacetylation), which is different and pH 8).from In the addition, scheme the here samples (e.g., 5.5 used × 10 for−4 mol/L their flotationof EX, chitosan experiments (100 kDa, were 85%–98% sourced deacetylation), differently from and those pH 8). used In addition, in this study. the samples It can beused concluded for their thatflotation the chitosan experiments with were different sourced molecular differently weights from and those deacetylation used in this levels study. at It a can diff erentbe concluded pH can lead that tothe distinctly chitosan diwithfferent different flotation molecular performances. weights and deacetylation levels at a different pH can lead to distinctlyThe values different of the flotation zeta potential performances. and contact angle on the surfaces of both pyrite and galena change slightlyThe in values the presence of the ofzeta chitosan potential (2 3 and kDa) contact or chitosan angle (3 on6 kDa),the surfaces indicating of bo a weakth pyrite adsorption and galena and, − − aschange a result, slightly a slight in depressionthe presence for of both chitosan minerals. (2− In3 kDa) the presence or chitosan of chitosan (3−6 kDa), (100 kDa),indicating the opposite a weak isadsorption the case. Therefore,and, as a result, chitosans a slight (2 3, depression 3 6, and 100 for kDa)both exhibitminerals. no In adsorption the presence selectivity of chitosan on pyrite (100 − − andkDa), galena, the opposite and, therefore, is the case. the selectiveTherefore, separation chitosans cannot(2−3, 3− be6, achieved.and 100 kDa) However, exhibit in no the adsorption presence ofselectivity chitosan on (10 pyrite kDa) and and galena, at pH and, 8, thetherefore, zeta potential the selective (21.3 separation mV) and canno contactt be angle achieved. (30.6 However,◦) on the galenain the presence surface decrease of chitosan significantly, (10 kDa) and while at pH the 8, zeta the potential zeta potential (4.0 mV) (21.3 and mV) contact and contact angle (9.5angle◦) on(30.6°) the pyriteon the surfacegalena surface only show decrease a slight significantly, decrease, indicating while the zeta that potential chitosan (10(4.0 kDa) mV) isand selectively contact angle adsorbed (9.5°) on the pyrite surface only show a slight decrease, indicating that chitosan (10 kDa) is selectively

Minerals 2019, 9, 549 9 of 11 on galena rather than pyrite. Therefore, the preferred reagent scheme (400 g/t of chitosan (10 kDa), 1600 g/t of ethyl xanthate, and 100 g/t of terpineol) can be an eﬀective pyrite-removal strategy to upgrade lead concentrates.

5. Conclusions In this work, the flotation separation performance of chitosan with different molecular weights on pyrite and galena was studied. A novel reagent scheme, i.e., 400 g/t of chitosan (10 kDa) as a depression, 1600 g/t of ethyl xanthate as a collector, and 100 g/t of terpineol as a frother, can achieve the reverse flotation separation of pyrite from galena at pH 8. Zeta potential and contact angle measurements revealed a stronger chitosan (10 kDa) adsorption on the galena surface than on the pyrite surface. Thus, chitosan (10 kDa) has a good industrial application potential to separate pyrite from galena to upgrade lead concentrates.

Author Contributions: Methodology, Z.G.; formal analysis, Y.H. and W.S.; investigation, W.Z. and J.C.; Writing—Original-draft preparation, W.Z. and J.C.; Writing—Review and editing, Z.G. Funding: This research was funded by the National Natural Science Foundation of China (51774328 and 51404300), the Young Elite Scientists Sponsorship Program by CAST (2017QNRC001), the Young Elite Scientists Sponsorship Program by Hunan province of China (2018RS3011), the Natural Science Foundation of Hunan Province of China (2018JJ2520), the Open-End Fund for the Valuable and Precision Instruments of Central South University (CSUZC201806), the Open Fund of Guangdong Provincial Key Laboratory of Development and Comprehensive Utilization of Mineral Resources (2017B030314046), and the National 111 Project (B14034). Acknowledgments: The authors would like to thank the editors and the anonymous reviewers for their helpful and constructive comments. Conﬂicts of Interest: The authors declare no conﬂict of interest.

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