Hemimorphite Flotation with 1-Hydroxydodecylidene-1,1-Diphosphonic Acid and Its Mechanism

minerals

Article Hemimorphite Flotation with 1-Hydroxydodecylidene-1,1-diphosphonic Acid and Its Mechanism

Wen Tan, Guangyi Liu *, Jingqin Qin and Hongli Fan

College of Chemistry and Chemical Engineering, Central South University, Changsha 410083, China; [email protected] (W.T.); [email protected] (J.Q.); [email protected] (H.F.) * Correspondence: [email protected]; Tel.: +86-731-8887-9616

Received: 8 December 2017; Accepted: 15 January 2018; Published: 24 January 2018

Abstract: 1-hydroxydodecylidene-1,1-diphosphonic acid (HDDPA) was prepared and first applied in flotation of hemimorphite. HDDPA exhibited superior flotation performances for recovery of hemimorphite in comparison with lauric acid, and it also possessed good selectivity against quartz flotation under pH 7.0–11.0. Contact angle results revealed that HDDPA preferred to attach on hemimorphite rather than quartz and promoted the hydrophobicity of hemimorphite surfaces. In the presence of HDDPA anions, the zeta potential of hemimorphite particles shifted to more negative value even if hemimorphite was negatively charged, inferring a strong chemisorption of hemimorphite to HDDPA. The Fourier transform infrared (FTIR) recommended that HDDPA might anchor on hemimorphite surfaces through bonding the oxygen atoms of its P(=O)–O groups with surface Zn(II) atoms. X-ray photoelectron spectroscopy (XPS) gave additional evidence that the Zn(II)-HDDPA surface complexes were formed on hemimorphite.

Keywords: 1-hydroxydodecylidene-1,1-diphosphonic acid; hemimorphite; ﬂotation; hydrophobic mechanism.

1. Introduction The adsorption of surfactants on to material particles can modify their surface properties such as surface energy, surface charge and hydrophobicity. These changes are prerequisites to froth flotation, which is the most principal process for enrichment and separation of value minerals from their ores [1–5]. In froth flotation, a collector (one kind of surfactant) selectively attaches to a target mineral using its minerophilic group, and then its hydrophobic group attaches to air/nitrogen bubbles which delivers the mineral particles to the slurry interface [6,7]. In general, zinc oxide minerals such as hemimorphite (Zn4(Si2O7)(OH)2·H2O), smithsonite (ZnCO3), willemite (Zn2SiO4) and hydrozincite (Zn5(CO3)2(OH)6) exhibit good hydrophilicity and unsatisfactory floatability. According to the predominant zinc oxide minerals, the ore varieties of zinc oxide are divided into hemimorphite, smithsonite and willemite + hemimorphite, and quartz is of the main gangue mineral for the three basic ore types [8]. Zinc oxide ores are much more difficult to be recovered by froth flotation than zinc sulfide ores which are becoming depleted due to continuous exploitation [9,10]. Zinc oxide minerals are commonly floated by amine collectors after presulfidization with sulfurizing agents like Na2S, NaHS or NH4HS [11]. Xanthates and the mixture of amines and xanthates also exhibit suitable performance for beneficiation of zinc oxide minerals [12,13]. However, it is tough to accurately control the dose of sulfidizing agents, which has a significant influence on presulfidization effects, pulp pH and then on zinc flotation recovery [14,15]. The application of fatty acid collectors can wipe out the presulfidization process [13]. It also faces flaws such as high operational costs and weak selectivity for gangue

Minerals 2018, 8, 38; doi:10.3390/min8020038 www.mdpi.com/journal/minerals Minerals 2018, 8, 38 2 of 13 minerals [14]. Mercaptan surfactants might be selective collectors for flotation of zinc oxide minerals. They also suffer from high consumption and foul odor [13,16]. Chelating collectors, such as salicylaldoxime, 8-hydroxyquinoline and amino-thio-phenol (ATP), can exert chelating action towards Zn atoms and possess good selectivity to beneficiate zinc oxide minerals [13,17]. Therefore, the flotation of zinc oxide minerals by chelating agents might be a promising approach [18,19]. Because of their favorable chelating nature for Zn, Cu, Sn, Ti and Cd ions, phosphorous acid surfactants have been selected as flotation collectors for recovery of metal oxide minerals [20–23]. Styryl phosphonic acid (SPA) has been recommended as a selective collector for enrichment of the finely disseminated rutile and cassiterite [24,25]. α-Hydroxyl octyl phosphinic acid (HPA) exhibited good flotation response to ilmenite, cassiterite or malachite [26–29]. 1-Hydroxyalkylidene-1,1-diphosphonic acid (HADPA), as an extensively investigated chelating agent for extraction of metal ions, is able to form stable metal chelation with zinc, manganese, copper, lead, calcium, nickel or cobalt. Among these complexes, its zinc complexes might be the most stable [30,31]. HADPA surfactants have been chosen as collectors for flotation of phosphate, fluorite, tin, tungsten and oxidized sulfide ores [32,33]. The flotation reagent Flotol-7,9, mainly containing HADPA with 7–9 carbon atoms, was used as a commercial collector by Kotlyarevsky et al. [32], they found that Flotol-7,9 was very effective for fluorite flotation from silicate-fluorite ores. However, the fatty acid collectors did not achieve good separation efficiency in flotation of these ores. Additionally, Flotol-7,9 exhibited better selectivity for cassiterite flotation than p-tolylarsonic acid, SPA, alkyl hydroxamic acids with 7–9 carbon atoms and tetrasodium salt of N-octadecyl-N-sulphosuccinoyl aspartic acid [32]. Flotol-7,9 can selectively separate apatite from brazilite-apatite-magnetite ores by froth flotation and formed calcium acid orthophosphates on apatite surfaces [32,34]. Pradip and Chaudhari [35] considered HADPA possessed strong chelating properties toward tungsten and exhibited better selectivity for flotation of wolframite than sodium oleate. Chen et al. [36] thought that 1-hydroxyoctylidene-1,1-diphosphonic acid was an effective collector for niobite flotation with good selectivity, and it chemisorbed on to niobite surfaces through its –P=O and –P–O– group. Zheng et al. [37] also found that 1-hydroxyoctylidene-1,1-diphosphonic acid chemisorbed on to the synthesized fersmite surfaces through its –P=O and –P–O– group. Nevertheless, HADPA has been hardly reported as a collector for flotation recovery of zinc oxide minerals in the published literature. For the excellent affinity to zinc ions [30,31], a novel HADPA surfactant, 1-hydroxydodecylidene- 1,1-diphosphonic acid (HDDPA) was selected as a collector for hemimorphite flotation. The flotation response of HDDPA to hemimorphite and quartz was investigated by micro-flotation experiments on single and artificially mixed minerals. After that, zeta potential, contact angle, Fourier transform infrared (FTIR) and X-ray photoelectron spectroscopy (XPS) were adopted to evaluate the hydrophobic mechanism of HDDPA on hemimorphite/water interface.

2. Materials and Methods

2.1. Materials HDDPA was prepared and purified in our laboratory, and characterized by and 1H and 31P NMR. Other chemicals used in this research were analytical grade (AR) from commercial suppliers. Distilled H2O was used in this research unless elsewise asserted. The X-ray diffraction (XRD) pattern and X-ray fluorescence (XRF) of hemimorphite and quartz samples were separately listed in Figure1 and Table1, which show that the purity of the two minerals was over 98%. The fraction with particle size ranging from +37 to −74 µm was employed for the micro-flotation tests and that below 5 µm was selected for the zeta potential, FTIR and XPS measurements. The specific surface area of the +37 to −74 µm portion was determined as 0.76 for hemimorphite and 0.12 m2·g−1 for quartz via the Brunauer-Emmett-Teller method on the Nova1000 analyzer (Quantachrome Instru., Boynton Beach, FL, USA). Minerals 2018, 8, 38 3 of 13 Minerals 2018, 7, x FOR PEER REVIEW 3 of 14

a b ★Quartz

■ Hemimorphite Intensity(cps) Intensity(cps)

10 20 30 40 50 60 70 80 10 20 30 40 50 60 70 80 Two-Theta(deg) Two-Theta(deg)

Figure 1. X-rayX-ray Diffraction (XRD) of hemimorphite ( a) and quartz ( b) samples.

TableTable 1. The element content of hemimorphite and quartz by X-ray ﬂuorescencefluorescence (XRF) (wt %).

CompositionComposition ZnZn O O Si Si Mg Mg Ca Ca Cu Cu P P Cl Cl S S FeFe PbPb AlAl HemimorphiteHemimorphite 57.5357.53 31.0031.00 10.92 10.92 0.218 0.218 0.115 0.115 0.08 0.08 0.04 0.04 0.01 0.01 0.020.02 0.010.01 0.020.02 - - QuartzQuartz - - 56.2056.20 43.60 43.60 0.06 0.06 0.04 0.04 ------0.020.02 -- 0.070.07

2.2. Micro-Flotation Tests Micro-flotationMicro-flotation tests were carried out in an improved Hallimond tube. For For each experiment, 180 mL water and 2 g of mineral mineral samples samples were were placed into a a 400-mL 400-mL beaker. beaker. After After regulating regulating the pulp pH to a designed value by dilute NaOH or HCl so solutions,lutions, collector (HDDPA or lauric acid), frother MIBC (methyl isobutyl carbinol), and extra H 2OO were were put put in in to obtain a 220 mL suspension where − − MIBC concentration waswas 1.51.5 ×× 10−4 4mol·Lmol·L−1. 1The. The pulp pulp was was stirred stirred for for 3 3min min and and its its pH pH values values were detected. Later, Later, the suspension was siphoned into the Hallimond tube and the micro-flotationmicro-flotation was −1 performed for 3 minmin underunder 0.20.2 LL·min·min −1 ofof N N22. .The The froth froth or or tailings tailings products products were were filtered, filtered, dried dried and weighted. TheThe recoveryrecovery was was computed computed according accordin to theg to products the products obtained asobtained listed inas Equation listed (1)in Equationfor pure mineral, (1) for pure Equation mineral, (2) for Equation hemimorphite (2) for andhemimorphite Equation (3) and for Equation quartz in the(3) for artificially quartz mixedin the artificiallyminerals, respectively. mixed minerals, respectively. m ε = 1 × 100% (1) m m+1m ε=1 2× 100% (1) m+m12 m1 × β1 εHemimorphite = × 100% (2) m1 ×mβ1×+β m2 × β2 ε =×11 100% Hemimorphite ×× (2) mm−11mβ +m× β 22β ε = 1 1 1 × 100% (3) Quartz + − × β − × β m1 m2 m-mm1 ×β1 m2 2 ε =11 1 × 100% Quartz ×× (3) where ε is the recovery of hemimorphitem+m-m or121122 quartz,β m-m1 andβ m2 are the weight of froth products and tailings (g), β1 and β2 are the hemimorphite content of the froth products and tailings (%, w/w), where ε is the recovery of hemimorphite or quartz, m1 and m2 are the weight of froth products respectively. The presented recovery was the average value of two independent flotation tests. and tailings (g), β1 and β2 are the hemimorphite content of the froth products and tailings (%, w/w), respectively.2.3. Zeta Potential The presented Measurement recovery was the average value of two independent flotation tests.

2.3. ZetaThe Potential zeta potential Measurement of hemimorphite or quartz in the presence or absence of HDDPA was measured −2 −1 in 1 × 10 mol·L KNO3 by using the ZetaPlus zeta potential analyzer (Brookhaven, NY, USA). The zeta potential of hemimorphite or quartz in the presence or absence of HDDPA was In each test, 50 mg of hemimorphite or quartz samples were introduced into 40 mL KNO3 solutions. measured in 1 × 10−2 mol·L−1 KNO3 by using the ZetaPlus zeta potential analyzer (Brookhaven, NY, After adjusting the suspension pH with dilute HNO3 or KOH solutions, extra KNO3 solutions withUSA). or In without each test, HDDPA 50 mg were of he addedmimorphite to obtain or quartz the 50 samples mL slurry. were The introduced suspension into with 40 ormL without KNO3 solutions.2 × 10−4 mol After·L− 1adjustingHDDPA the was suspension agitated for pH 10 with min, dilute then, HNO its zeta-potential3 or KOH solutions, and pH valuesextra KNO were3 solutionsmeasured. with The or reported without zeta HDDPA potential were was added the average to obtain value the of50 six mL independent slurry. The measurementssuspension with with or −4 −1 without±5 mV variation.2 × 10 mol·L HDDPA was agitated for 10 min, then, its zeta-potential and pH values were measured. The reported zeta potential was the average value of six independent measurements with ±5 mV variation.

Minerals 2018, 8, 38 4 of 13

2.4. Contact Angle Measurements The contact angles of hemimorphite or quartz surfaces before and after HDDPA modiﬁcation were measured by using the water droplet captive method on the JC2000C contact angle instrument (Zhongchen Digital, Shanghai, China). The chunk samples were successively wet-polished by hand with SiC abrasive papers, and 1 and 0.05 µm Al2O3 powder suspensions. After cleaning several times with distilled H2O under ultrasound, the mineral samples were dried by pure N2 and applied for contact angle measurements. The results reported were the average value of ﬁve measurements.

2.5. FTIR Measurement When 2 × 10−4 mol·L−1 HDDPA solutions were mixed with 2 × 10−4 mol·L−1 Zn2+ aqueous solutions with a volume ratio of 1:8, a white precipitate emerged. The precipitates in the mixture were ﬁltrated, rinsed four times with distilled H2O, and then desiccated in a vacuum oven. 0.5 g of 5 µm hemimorphite particles were put into 100 mL of 5 × 10−4 mol·L−1 HDDPA, which was agitated for 4 h at 298 ± 1 K. After ﬁltrating, washing thrice with H2O and drying, the hemimorphite particles adsorbed HDDPA were applied for FTIR detection on the 740 FTIR instrument (Nicolet, Glendale, WI, USA) through KBr disks.

2.6. XPS Measurement The XPS of HDDPA, hemimorphite before and after HDDPA adsorption, and Zn-HDDPA precipitates were measured on the ESCALAB 250Xi (Thermo Scientiﬁc, Waltham, MA, USA) using Al Kα X-ray source conducted at 200 W with 20 eV pass energy. The vacuum pressure was between 10−9 to 10−8 Torr and the takeoff angle was set at 45◦.

3. Results

3.1. Preparation of HDDPA HDDPA was prepared according to the method reported by Kieczykowski et al. [38]. Lauric acid (20 g, 0.1 mol), phosphorus acid (8.2 g, 0.1 mol) and methanesulfonic acid (20 mL) were put in a three-necked bottle equipped with a reflux condenser and a CaCl2-tube. After heating the mixture ◦ to 65 C, PCl3 (13 mL, 0.146 mol) was introduced dropwise and the reactive mixture was agitated ◦ at 70 C for 18 h. Then, the mixture was cooled, 20 mL CH3CH2OH (EtOH) was added to separate methanesulfonic acid (with a near 80% recovery), and 50 mL H2O was put into the mixture which was agitated under reflux for 5 h. After cooling the solution to 20 ◦C, the pH was adjusted to around 1.0 with NaOH to precipitate HDDPA which was filtrated, washed with EtOH and collected as white solid with 97% yield. 1 Characterization of HDDPA: H-NMR (500 MHz, D2O/NaOD, Bruker AvanceIII, Bruker, Uster, Switzerland), δ (ppm): 1.83–1.71 (m, 2H), 1.45 (m, 2H), 1.19 (s, 16H), 0.77 (t, 3H). 31P NMR(202 MHz, −1 D2O/NaOD), δ (ppm): 19.01. FTIR (KBr), v (cm ): 3540, 3446, 3228, 2960, 2920, 2850, 2240, 1470, 1290, 1160, 1060, 922, 721, 667, 596, 544.

3.2. Micro-Flotation Results The effect of pH or collector concentration on the flotation recovery of hemimorphite or quartz is presented in Figure2. The results in Figure2a,c indicated that under pH 7.0–11.0, HDDPA achieved higher flotation recovery of hemimorphite and exhibited better flotation selectivity against quartz than lauric acid. The results in Figure2b,d showed that the flotation recovery of hemimorphite increased with an increase of collector dose, and 1 × 10−4 mol·L−1 HDDPA recovered near 85.2% hemimorphite or 29.4% quartz, while the recoveries of hemimorphite and quartz were 72.3% and 51.6% by using 1 × 10−4 mol/L lauric acid, respectively. Minerals 2018, 8, 38 5 of 13 Minerals 2018, 7, x FOR PEER REVIEW 5 of 14

Minerals 2018100, 7, x FOR PEER REVIEW 100 5 of 14 a b 10080 10080 a b 80 Hemimorphite 8060 60 Hemimorphite Quartz Quartz Hemimorphite 60 6040 40 Hemimorphite Quartz Quartz Recovery(%)

Recovery(%) 4020 4020 Recovery(%) Recovery(%) 200 200 0 5 10 15 20 6789101112 -1 pH Initial concentration (10-5mol· L ) 0 0 0 5 10 15 20 100 6789101112 100 -5 -1 pH Initial concentration (10 mol· L ) c d 10080 10080 c Hemimorphite Quartz d 80 8060 60 Hemimorphite

Quartz 6040 6040 Recovery(%)

Recovery(%) Hemimorphite 4020 4020 Quartz Recovery(%) Recovery(%) Hemimorphite 200 200 Quartz 0 5 10 15 567891011 -5 -1 Initial concentration (10 mol· L ) pH 0 0 0 5 10 15 567891011 -5 -1 pH Initial concentration (10 mol· L −) 4 −1 FigureFigure 2. Flotation 2. Flotation recovery recovery of hemimorphiteof hemimorphite and and quartz quartz as asa afunction function of pH (under 1 1 ×× 1010−4 mol·Lmol −1 ·L HDDPAHDDPA (a) or (a lauric) or lauric acid acid (c)) (c and)) and initial initial concentration concentration of of HDDPA HDDPA (b ()b or) or lauric lauric acid acid (d) (under (d) (under pH 8.5). pH 8.5). Figure 2. Flotation recovery of hemimorphite and quartz as a function of pH (under 1 × 10−4 mol·L−1 HDDPA (a) or lauric acid (c)) and initial concentration of HDDPA (b) or lauric acid (d) (under pH 8.5). 3.3. Flotation3.3. Flotation of the of Artificiallythe Artificially Mixed Mixed Minerals Minerals As3.3. shownFlotationAs shown above,of the above, Artificially HDDPA HDDPA Mixed floated floated Minerals out out more more hemimorphitehemimorphite than than quartz. quartz. To Tofurther further explore explore HDDPA’s flotation selectivity, flotation separation tests for the artificially mixed minerals (with a 1:1 HDDPA’sAs flotation shown selectivity,above, HDDPA flotation floated separation out more testshemi formorphite the artificially than quartz. mixed To mineralsfurther explore (with a 1:1 weight ratio of hemimorphite to quartz) were carried out and the experimental findings were weightHDDPA’s ratio of flotation hemimorphite selectivity, to quartz) flotation were separation carried tests out for and the the artificially experimental mixed findings minerals were(with presenteda 1:1 presented in Figure 3. It could be seen from Figure 3a that the preferred pH values for the flotation weight ratio of hemimorphite to quartz) were carried out and the experimental findings were in Figurerecovery3. It couldof hemimorphite be seen from from Figure its mixture3a that with the quartz preferred were 8.0–10.5 pH values with for 1.0 the× 10 flotation−4 mol·L−1 HDDPA. recovery of presented in Figure 3. It could be seen from Figure 3a that the preferred pH−4 values for−1 the flotation hemimorphiteAt pH around from 9.1, its HDDPA mixture floated with quartz out 76.1% were hemimorphite 8.0–10.5 with as 1.0 well× as10 32.3%mol quartz,·L HDDPA. which was At pH recovery of hemimorphite from its mixture with quartz were 8.0–10.5 with 1.0 × 10−4 mol·L−1 HDDPA. aroundslightly 9.1, HDDPA lower than floated the outflotation 76.1% recovery hemimorphite of 84.6% as for well hemimorphite as 32.3% quartz, or 33.6% which forwas quartz slightly in the lower At pH around 9.1, HDDPA floated out 76.1% hemimorphite as well as 32.3% quartz, which was than theindividually flotation pure recovery mineral of flotation 84.6% for tests hemimorphite as listed in Figure or 33.6% 2a. The for results quartz in Figure in the 3b individually showed that pure slightly lower than the flotation recovery of 84.6% for hemimorphite or 33.6% for quartz in the mineralwith flotation increase testsof HDDPA as listed dosage, in Figure the flotation2a. The reco resultsveries in of Figure hemimorphite3b showed increased that with and increasethose of of individually pure mineral flotation tests as listed in Figure 2a. The results in Figure 3b showed that quartz increased very slightly. At 2.0 × 10−4 mol·L−1 HDDPA, the recovery of hemimorphite or quartz HDDPAwith dosage,increase theof HDDPA flotation dosage, recoveries the flotation of hemimorphite recoveries of increased hemimorphite and increased those of quartzand those increased of reached 83.3% or 34.2%,−4 respectively.−1 The flotation results of the artificially mixed minerals veryquartz slightly. increased At 2.0 ×very10 slightly.mol· LAt 2.0HDDPA, × 10−4 mol·L the−1 recovery HDDPA, ofthe hemimorphite recovery of hemimorphite or quartzreached or quartz 83.3% recommended that an efficient flotation separation of hemimorphite from quartz could be reached or 34.2%,reached respectively. 83.3% or 34.2%, The flotation respectively. results The of theflotation artificially results mixed of the minerals artificially recommended mixed minerals that an by using HDDPA as a collector. efficientrecommended flotation separationthat an efficient of hemimorphite flotation separation from of quartz hemimorphite could befrom reached quartz bycould using be reached HDDPA as a collector.by using HDDPA100 as a collector. 100 a b 100 80 10080 a b 80 60 8060 Hemimorphite Hemimorphite Quartz

Quartz 60 40 6040 Hemimorphite Hemimorphite Quartz

Quartz Recovery(%)

Recovery(%) 40 40 20 20 Recovery(%) Recovery(%) 20 20 0 0 6 7 8 9 10 11 12 0 5 10 15 20 25 -5 -1 pH Initial concentration (10 mol· L ) 0 0 6 7 8 9 10 11 12 0 5 10 15 20 25 -5 -1 pH Initial concentration (10 mol· L )

Figure 3. Flotation separation of artiﬁcially mixed minerals under different (a) pH (under 1.0 × 10−4 mol·L−1 HDDPA) and (b) initial concentration of HDDPA (at pH 8.5). Minerals 2018, 7, x FOR PEER REVIEW 6 of 14

Figure 3. Flotation separation of artificially mixed minerals under different (a) pH (under 1.0 × 10−4 mol·L−1 HDDPA) and (b) initial concentration of HDDPA (at pH 8.5).

3.4.Minerals Contact 2018 , Angle7, x FOR Results PEER REVIEW 6 of 14

ContactFigure 3.angle Flotation is an separation important of parameterartificially mixedfor eval mineralsuation underof the different wettability (a) pHof mineral(under surfaces, which1.0 is ×the 10− 4most mol·L intuitive−1 HDDPA) sign and (ofb) mineralinitial concentration surface hydrophobicity of HDDPA (at pH [39]. 8.5). The contactMinerals angles2018, 8, 38 of hemimorphite or quartz surfaces before and after 5 × 10−5 mol·L6 of 13 −1 HDDPA treatment3.4. Contact are Angle shown Results in Figure 4. It demonstrated that the contact angle of the clean hemimorphite surfacesContact was3.4. 42.0angle Contact ± is1.3°. Angle an importantAfter Results HDDPA parameter treatment for eval ofuation 2 min of and the 5wettability min, hemimorphite of mineral surfaces, contact angle increasedwhich is theto most67.8Contact intuitive± 1.4° angle andsign is an importantof88.1 mineral ± 1.5°, parameter surface respectively. forhydrophobicity evaluation However, of the [39]. wettability the offresh mineral quartz surfaces, surface was hydrophilicThe contactwhich and isanglesits the contact most of intuitivehemimorphite angle sign was of mineralmaintained or quartz surface surfaces hydrophobicityat about before 14° [and 39before]. after and 5 × 10after−5 mol·L HDDPA−1 HDDPA treatment. −5 −1 Thistreatment difference are shown Theillustrated contact in Figure angles that of 4.HDDPA hemimorphite It demonstrated preferred or quartz that surfacesto adsorbthe contact before on and hemimorphiteangle after 5of× the10 cleanmol rather·L hemimorphiteHDDPA than on quartz. surfaces wastreatment 42.0 ± are1.3°. shown After in HDDPA Figure4. It treatment demonstrated of that2 min the contactand 5 min, angle ofhemimorphite the clean hemimorphite contact angle surfaces was 42.0 ± 1.3◦. After HDDPA treatment of 2 min and 5 min, hemimorphite contact angle increased to 67.8 ± 1.4° and 88.1 ± 1.5°, respectively. However, the fresh quartz surface was increased to 67.8 ± 1.4◦ and 88.1 ± 1.5◦, respectively. However, the fresh quartz surface was hydrophilic hydrophilicand and its its contact contact angle angle was maintained was maintained at about 14 at◦ before about and 14° after before HDDPA and treatment. after HDDPA This difference treatment. This differenceillustrated illustrated that HDDPA that HDDPA preferred topreferred adsorb on to hemimorphite adsorb on ratherhemimorphite than on quartz. rather than on quartz.

Figure 4. Contact angles of hemimorphite or quartz surfaces as a function of treatment time at 5 × 10−5 mol·L−1 HDDPA.

TheFigure contact 4. ContactFigure angle 4. angles Contactof hemimorphite of angles hemimorphite of hemimorphite or quartzor quartz or quartz as su a rfacessurfacesfunction as as a a of function pH under of of treatment treatment 5 × 10 time− 5time atmol·L at − 1 HDDPA −5 −1 treatment5 × 10 for−5 mol·L 5 5min×−110 HDDPA. is molshown·L HDDPA. in Figure 5. It elucidated that the contact angles of hemimorphite were slightly larger at pH 7.0–9.8 than under acidic or strong alkaline conditions,−5 while− 1the pH values had The contactThe angle contact of anglehemimorphite of hemimorphite or quartz or quartz as as a afunction function of of pH pH under under 5 × 105 × 10mol−5 ·mol·LL HDDPA−1 HDDPA an insignificanttreatment effect for on 5 min the is contact shown in angle Figure5 of. It quartz elucidated under that the pH contact 6.0–11.0. angles The of hemimorphite contact angle were results were treatment for 5 min is shown in Figure 5. It elucidated that the contact angles of hemimorphite were further confirmedslightly larger by at pHthe 7.0–9.8 selective than under hydropho acidic or strongbization alkaline of conditions, HDDPA while to the pHhemimorphite, values had which slightly larger at pH 7.0–9.8 than under acidic or strong alkaline conditions, while the pH values had correspondedan insignificantto its effectselective on the contactflotation angle ofto quartz hemi undermorphite pH 6.0–11.0. against The contact quartz angle results as werepresented in an insignificantfurther effect confirmed on the by contact the selective angle hydrophobization of quartz under of HDDPA pH 6.0–11.0. to hemimorphite, The contact which angle corresponded results were Figures 2a,b and 3. further confirmedto its selective by flotationthe selective to hemimorphite hydropho againstbization quartz as presentedof HDDPA in Figure to 2a,bhemimorphite, and Figure3. which corresponded to its selective flotation to hemimorphite against quartz as presented in Figures 2a,b and 3. 100 )

o 80 100(

) 60 Hemimorphite

o 80 (

Quartz

6040 Hemimorphite

Quartz

40 angles Contact 20

Contact angles Contact 20 0 567891011 0 pH 567891011 Figure 5. ContactFigure 5. anglesContact anglesof hemimorphite of hemimorphite porH or quartz quartzsurfaces surfaces as aas function a function of pH under of pH under −5 −1 −5 5 ×−110 mol·L HDDPA. 5Figure × 10 mol·L5. Contact HDDPA. angles of hemimorphite or quartz surfaces as a function of pH under 5 × 10−53.5. mol·L Zeta−1 PotentialHDDPA. Results 3.5. Zeta Potential Results The zeta potential of hemimorphite particles as a function of pH in the absence or presence of 3.5. Zeta Potential− Results4 −1 The zeta1 × potential10 mol·L of HDDPAhemimorphite is shown inparticles Figure6a. as It elucidateda function that of the pH isoelectric in the pointabsence (IEP) or of presence of hemimorphite particles occurred at pH ~6.0, which was consistent with the previous reported value of 1 × 10The−4 mol·L zeta potential−1 HDDPA of hemimorphite is shown in particlesFigure 6a.as aIt function elucidated of pH that in the the absence isoelectric or presence point of(IEP) of pH 6.0 [40]. In the presence of HDDPA, the zeta potential of hemimorphite particles transferred to hemimorphite1 × 10−4 mol·L particles−1 HDDPA occurred is shown at inpH Figure ~ 6.0, whic6a. Ith elucidated was consistent that the with isoelectric the previous point reported (IEP) of value ofhemimorphite pH 6.0 [40]. In particles the presence occurred of atHDDPA, pH ~ 6.0, the whic zetah was potential consistent of hemimorphite with the previous particles reported transferred value to moreof pH negative 6.0 [40]. values,In the presence and the of IEP HDDPA, shifted the to pH~5zeta potential.1, which of implied hemimorphite that HDDPA particles species transferred might to attach more negative values, and the IEP shifted to pH~5.1, which implied that HDDPA species might attach on to the positively charged sites of hemimorphite particles to lessen their zeta potential. on to the positively charged sites of hemimorphite particles to lessen their zeta potential.

Minerals 2018, 8, 38 7 of 13

more negative values, and the IEP shifted to pH~5.1, which implied that HDDPA species might attach Mineralson to2018 the, 7 positively, x FOR PEER charged REVIEW sites of hemimorphite particles to lessen their zeta potential. 7 of 14

40 20 30 a 10 b Hemimorphite 20 Hemimorphite+HDDPA 0 10 -10 Quartz 0 -20 Quartz+HDDPA -10 -30 -20 -40 -30 -50

-40 Zeta potential(mV) -60 Zeta potential (mv) -50 -70 4567891011 024681012 pH pH

FigureFigure 6. Zeta 6. Zeta potential potential of ofhemimorphite hemimorphite (a (a) )or or quartz quartz (b)) asas aa functionfunction of of pH pH in in the the absence absence and and presencepresence of 1 of × 110×−410 mol·L−4 mol−1 ·HDDPA.L−1 HDDPA.

−4 −1 TheThe zeta zeta potential potential values values of of quartz quartz particles withwith or or without without of 1of× 110 ×− 104 mol mol·L·L−1 HDDPA HDDPA are are shownshown in Figure in Figure 6b.6 b.The The IEP IEP of of quartz quartz particles particles occurredoccurred at at pH pH 2.0, 2.0, similar similar to to what what was was previously previously measuredmeasured at pH at pH 2.2 2.2 [41]. [41 ].In Inthe the presence presence of of HDDPA, HDDPA, thethe zetazeta potentialpotential of of quartz quartz particles particles barely barely changed,changed, which which implied implied that that HDDPA HDDPA species species hardly adsorbedadsorbed on on to to quartz quartz surfaces. surfaces.

3.6. 3.6.FTIR FTIR Analyses Analyses 2+ TheThe FTIR FTIR spectra spectra of of HDDPAHDDPA and and its precipitatesits precipitates with Znwithare Zn presented2+ are presented in Figure 7ina. ForFigure HDDPA, 7a. For the characteristic peaks at around 3540 and 3228 cm−1 were attributed to its O–H vibrations for HDDPA, the characteristic peaks at around 3540 and 3228 cm−1 were attributed to its O–H vibrations intramolecular and intermolecular hydrogen bonds [42]. The vibrations of C–H bonds appeared at for intramolecular and intermolecular hydrogen bonds [42]. The vibrations of C–H bonds appeared about 2960, 2920, 2850 and 1470 cm−1. The bands at around 2240 cm−1 were assigned to the stretching at about 2960, 2920, 2850 and 1470 cm−1. The bands at around 2240 cm−1 were assigned to the stretching vibration of PO–H [43]. The absorption peaks at 1060 and 922 cm−1 represented the symmetric −1 vibrationand asymmetric of PO–H stretching[43]. The absorption vibrations of peaks –P–O– at bonds 1060 inand the 922 –P(=O)OH cm represented groups and the those symmetric at around and asymmetric1290 and stretching 1160 cm−1 vibrationswere related of to –P–O– the P=O bonds and in the the P=O –P(=O)OH in condition groups of hydrogen and those bond at [around37,42,44 ],1290 and respectively.1160 cm−1 Afterwere reactionrelated with to the Zn2+ P=O, the –P–O–and the absorption P=O in peak condition at 922 cm of− 1hydrogenfor HDDPA bond dramatically [37,42,44], respectively.weakened andAfter shifted reaction to 960 with cm− 1Zn, and2+, thatthe at–P–O– 1060 cmabsorption−1 moved topeak 1080 at cm −9221 in cm the−1 Zn-HDDPA for HDDPA dramaticallyprecipitates, weakened suggesting and the shifted formation to 960 of thecm− P–O–Zn1, and that bond at 1060 [45]. Additionally,cm−1 moved to the 1080 peaks cm at−1 aroundin the Zn- HDDPA1160, 1290,precipitates, 2240, 3228 suggesting and 3540 cmthe− 1formationfor HDDPA of almostthe P–O–Zn disappeared. bond [45]. These Additionally, changes suggested the peaks that at aroundthe P=O1160, and 1290, –P–OH 2240, groups 3228 might and react3540 with cm Zn−1 for2+ and HDDPA the previously almost existeddisappeared. O–H and These P=O bondschanges suggestedmight bethat broken the P=O (P=O and was –P–O changedH groups to P–O), might which react was with consistent Zn2+ and with the previous previously reports existed that the O–H and oxygenP=O bonds atoms might in both be of broken P=O and (P=O –P–OH was groups changed could to combine P–O), which with zinc was atoms consistent [36,37,46 with,47]. previous reports thatThe FTIRthe oxygen spectra ofatoms hemimorphite in both of before P=O and and after –P–OH HDDPA groups adsorption could are combine illustrated with in Figure zinc 7atomsb. Compared with clean mineral particles, the appearance of the –C–H peaks at 2920, 2850 and 1470 cm−1 [36,37,46,47]. implied the adsorption of HDDPA on hemimorphite surfaces. Other characteristic bands at around The FTIR spectra of hemimorphite before and after HDDPA adsorption are illustrated in 1080 and 960 cm−1 due to the P–O–Zn bonds might be covered by the strong and wide adsorption Figurebands 7b. ofCompared hemimorphite. with Additionally,clean mineral the particles, bands near the 1635appearance and 3446 of cm the−1 as–C–H presented peaks inat Figure2920, 72850 −1 andwere 1470 assigned cm implied to the O–Hthe adsorption bending and of stretching HDDPA vibrations on hemimorphite of water molecules. surfaces. Other characteristic bands at around 1080 and 960 cm−1 due to the P–O–Zn bonds might be covered by the strong and wide adsorption bands of hemimorphite. Additionally, the bands near 1635 and 3446 cm−1 as presented in Figure 7 were assigned to the O–H bending and stretching vibrations of water molecules.

Minerals 2018, 8, 38 8 of 13 Minerals 2018, 7, x FOR PEER REVIEW 8 of 14 a

Zn-HDDPA 1470 1635 1080 960 2960 HDDPA 2850 3446 2920 2240 1290 3228 1470 2850 3446 3540 2960 922 2920 1160 1060

3500 3000 2500 2000 1500 1000 500 Wavenumber(cm-1) Hemimorphite + HDDPA b 1470 2850 2920 1635 3446 1090

Hemimorphite 933 1635 3446 1090 933 3500 3000 2500 2000 1500 1000 500 Wavenumber(cm-1)

Figure 7. Fourier transform infrared (FTIR) spectra of HDDPA and Zn-HDDPA precipitate (a), Figure 7. Fourier transform infrared (FTIR) spectra of HDDPA and Zn-HDDPA precipitate (a), hemimorphite before and after HDDPA adsorbed (b). hemimorphite before and after HDDPA adsorbed (b). 3.7. XPS Analyses 3.7. XPS Analyses 3.7.1. Survey XPS 3.7.1. SurveyThe XPS survey scan XPS of HDDPA, Zn-HDDPA precipitates, hemimorphite before and after HDDPA treatment are displayed in Figure 8, and the atomic concentrations for elements C 1s, O 1s, The survey scan XPS of HDDPA, Zn-HDDPA precipitates, hemimorphite before and after HDDPA P 2p, Si 2p and Zn 2p are presented in Table 2. The results elucidated that the atomic proportion of treatmentcarbon, are displayedoxygen and inphosphorus Figure8, andin HDDPA the atomic was in concentrations accordance with forits molecular elements composition C 1s, O 1s, of P 2p, Si 2p and ZnC12H 2p28P2 areO7. presentedFor the Zn-HDDPA in Table 2precipitate,. The results the elucidatedatomic ratio thatof phosphorus the atomic to proportionzinc was 1.17:1, of carbon, oxygen andinferring phosphorus that the configuration in HDDPA of was the inZn-HDDPA accordance precipitate with its might molecular be (HDDPA)Zn composition2 [46,47]. of CAfter12H 28P2O7. For the Zn-HDDPAHDDPA treatment, precipitate, the atomic the concentrations atomic ratio of of ca phosphorusrbon and phosphorus to zinc on was hemimorphite 1.17:1, inferring surfaces that the conﬁgurationincreased of and the those Zn-HDDPA of oxygen, precipitate silicon and mightzinc decreased, be (HDDPA)Zn confirming [the46 ,adsorption47]. After of HDDPA HDDPA treatment,on hemimorphite. 2 the atomic concentrations of carbon and phosphorus on hemimorphite surfaces increased and those of oxygen, silicon and zinc decreased, conﬁrming the adsorption of HDDPA on hemimorphite.

Table 2. Atomic concentration of elements as determined by X-ray photoelectron spectroscopy (XPS).

Atomic Concentration/% Species C 1s O 1s Si 2p P 2p Zn 2p HDDPA 59.24 30.69 - 9.87 - Zn-HDDPA precipitate 57.11 27.49 - 8.31 7.09 Hemimorphite 9.79 54.02 12.46 - 23.73 Hemimorphite after HDDPA treatment 39.12 41.74 5.51 3.44 10.19 Minerals 2018, 8, 38 9 of 13 Minerals 2018, 7, x FOR PEER REVIEW 9 of 14

O 1s C 1s P 2p a

Zn 2p3/2 Zn 2p1/2 b

c Si 2p

Zn 2p1/2 Zn 2p3/2 O 1s d C 1s P 2p

1200 1000 800 600 400 200 0 Binding Energye(eV) Figure 8. The X-ray photoelectron spectroscopy (XPS) survey scan of HDDPA (a) and Zn-HDDPA Figure 8. The X-ray photoelectron spectroscopy (XPS) survey scan of HDDPA (a) and Zn-HDDPA precipitates (b), hemimorphite without (c) or with (d) HDDPA treatment. precipitates (b), hemimorphite without (c) or with (d) HDDPA treatment. Table 2. Atomic concentration of elements as determined by X-ray photoelectron spectroscopy (XPS). 3.7.2. High-Resolution XPS Atomic Concentration/% Species The high-resolution XPS of Zn 2p and P 2pC and1s for O 1s HDDPA, Si 2p P Zn-HDDPA 2p Zn 2p precipitates, hemimorphite with or withoutHDDPA HDDPA treatment are59.24 shown 30.69 in Figure- 9.9.87 The results- in Figure9a illustrated that the Zn 2pZn-HDDPA3/2 XPS peak precipitate of hemimorphite 57.11 appeared 27.49 at around - 1022.14 8.31 7.09 eV [48] and that of Hemimorphite 9.79 54.02 12.46 - 23.73 the Zn-HDDPA precipitate emerged on 1022.68 eV [49]. After HDDPA treatment, the Zn 2p3/2 XPS bands of hemimorphiteHemimorphite were composedafter HDDPA of twotreatment components 39.12 at 41.74 around 5.51 1022.12 3.44 and 1022.7610.19 eV. The low binding energy peak was assigned to the inner-layer Zn(II) species of hemimorphite interface [48,49], 3.7.2. High-Resolution XPS the high binding energy peak might be due to the Zn(II) surface species which combined with HDDPA. This inferredThe thathigh-resolution hemimorphite XPS might of Zn chemosorb 2p and P HDDPA2p and onfor itsHDDPA, surfaces Zn-HDDPA [29]. precipitates, Mineralshemimorphite 2018, 7, x FOR with PEER or REVIEW without HDDPA treatment are shown in Figure 9. The results in Figure10 of 9a14 illustrated that the Zn 2p3/2 XPS peak of hemimorphite appeared at around 1022.14 eV [48] and that b of the Zn-HDDPA 1022.68 precipitate eV emerged on 1022.68a eV [49]. After HDDPA treatment, the Zn 2p3/2 XPS bands of hemimorphite were composed of two components at around134.15 eV 1022.12 and 1022.76 eV. The HDDPA low binding energy peak was assigned to the inner-layer Zn(II) species of hemimorphite interface [48,49], the high binding energy peak HDDPA-Znmight be due to the Zn(II) surface species which combined with HDDPA. This inferred that hemimorphite might chemosorb HDDPA on its surfaces [29]. Figure 9b illustrated that the P 2p XPS bands occurred at around 134.15 eV for HDDPA, 133.70 eV 133.70 eV for the Zn-HDDPA precipitateHemimorphite+HDDPA and 133.46 eV for HDDPA adsorbed on hemimorphite surfaces, which elucidated that after HDDPA reaction with Zn ions or adsorption on hemimorphite 1022.76 eV 1022.12 eV HDDPA-Zn surfaces, the P 2p binding energy decreased, suggesting that HDDPA might chemisorb on to hemimorphite surfaces to increase the electron density around the P atom [50]. The increased binding energy for Zn 2p3/2 and the reduced for P 2p confirmed that the Zn(II)-HDDPA surface complexes

Hemimorphite + HDDPA might be formed on hemimorphite1022.14 [46,47]. eV 133.46 eV

Hemimorphite

1028 1026 1024 1022 1020 1018 1016 137 136 135 134 133 132 131 130 Binding Energy(eV) Binding Energy(eV) Figure 9. High-resolution XPS of Zn 2p3/2 (a) and P 2p (b). Figure 9. High-resolution XPS of Zn 2p3/2 (a) and P 2p (b). 4. Discussion The micro-flotation findings showed that HDDPA displayed superior flotation performance to hemimorphite in comparison with lauric acid and good selectivity against quartz flotation under pH 7.0–11.0. The contact angle results displayed that HDDPA preferred to adsorb on to hemimorphite rather than quartz and promoted the hydrophobicity of hemimorphite surfaces. The pKa1, pKa2, pKa3 and pKa4 (Ka is defined as an acid dissociation constant) of 1-hydroxyoctylidene-1,1-diphosphonic acid were 1.56, 3.00, 7.03 and 11.01 [37], respectively. This means that HDDPA mainly existed in aqueous solutions as HDDPA at pH ≤ 1.5, HDDPA− at pH 1.5– 3.0, HDDPA2− at pH 3.0–7.0, HDDPA3− at pH 7.0–11.0 or HDDPA4− at pH ≥ 11.0. The IEP of hemimorphite particles occurred at pH around 6.0. Under pH > 6.0, HDDPA mainly exists as its negatively charged species and hemimorphite particles might be negatively charged. The zeta potential results indicated that HDDPA species did adsorb on hemimorphite at pH > 6.0, implying that HDDPA might chemisorb on to hemimorphite surfaces, which can conquer the electrostatic repulsion interaction between them. The FTIR recommended that HDDPA might anchor on hemimorphite surfaces through bonding the oxygen atoms in its P(=O)–O group with surface Zn(II) species [48,49]. The P 2p and Zn 2p3/2 XPS gave clear evidence that HDDPA anchored on hemimorphite surfaces by chemisorption. Thus, Zn(II)-HDDPA surface complexes might be formed on hemimorphite [46,47], which hydrophobized hemimorphite particles to be floated out in froth flotation. Hemimorphite (Zn4(Si2O7)(OH)2·H2O) might dissolve in water according to Equation 4 [51]. Medusa was adopted to evaluate the concentration of Zn or Si species from hemimorphite as a function of pH; the plot is presented in Figure 10. It demonstrated that in acidic solutions, Zn2+ ions dominate in water, and under pH > 11.5, Zn species mainly exists as Zn(OH)2, Zn(OH)3− and ZnSiO3.

Zn4(Si2O7)(OH)2·H2O(s) ⇌ 2SiO32− + 4Zn2+ + 4OH− (4)

Minerals 2018, 8, 38 10 of 13

Figure9b illustrated that the P 2p XPS bands occurred at around 134.15 eV for HDDPA, 133.70 eV for the Zn-HDDPA precipitate and 133.46 eV for HDDPA adsorbed on hemimorphite surfaces, which elucidated that after HDDPA reaction with Zn ions or adsorption on hemimorphite surfaces, the P 2p binding energy decreased, suggesting that HDDPA might chemisorb on to hemimorphite surfaces to increase the electron density around the P atom [50]. The increased binding energy for Zn 2p3/2 and the reduced for P 2p conﬁrmed that the Zn(II)-HDDPA surface complexes might be formed on hemimorphite [46,47].

4. Discussion The micro-flotation findings showed that HDDPA displayed superior flotation performance to hemimorphite in comparison with lauric acid and good selectivity against quartz flotation under pH 7.0–11.0. The contact angle results displayed that HDDPA preferred to adsorb on to hemimorphite rather than quartz and promoted the hydrophobicity of hemimorphite surfaces. The pKa1, pKa2, pKa3 and pKa4 (Ka is defined as an acid dissociation constant) of 1-hydroxyoctylidene-1,1-diphosphonic acid were 1.56, 3.00, 7.03 and 11.01 [37], respectively. This means that HDDPA mainly existed in aqueous solutions as HDDPA at pH ≤ 1.5, HDDPA− at pH 1.5–3.0, HDDPA2− at pH 3.0–7.0, HDDPA3− at pH 7.0–11.0 or HDDPA4− at pH ≥ 11.0. The IEP of hemimorphite particles occurred at pH around 6.0. Under pH > 6.0, HDDPA mainly exists as its negatively charged species and hemimorphite particles might be negatively charged. The zeta potential results indicated that HDDPA species did adsorb on hemimorphite at pH > 6.0, implying that HDDPA might chemisorb on to hemimorphite surfaces, which can conquer the electrostatic repulsion interaction between them. The FTIR recommended that HDDPA might anchor on hemimorphite surfaces through bonding the oxygen atoms in its P(=O)–O group with surface Zn(II) species [48,49]. The P 2p and Zn 2p3/2 XPS gave clear evidence that HDDPA anchored on hemimorphite surfaces by chemisorption. Thus, Zn(II)-HDDPA surface complexes might be formed on hemimorphite [46,47], which hydrophobized hemimorphite particles to be floated out in froth flotation. Hemimorphite (Zn4(Si2O7)(OH)2·H2O) might dissolve in water according to Equation 4 [51]. Medusa was adopted to evaluate the concentration of Zn or Si species from hemimorphite as a function of pH; the plot is presented in Figure 10. It demonstrated that in acidic solutions, Zn2+ ions dominate − in water, and under pH > 11.5, Zn species mainly exists as Zn(OH)2, Zn(OH)3 and ZnSiO3.

2− 2+ − Zn4(Si2O7)(OH)2·H2O(s) 2SiO3 + 4Zn + 4OH (4) Minerals 2018, 7, x FOR PEER REVIEW 11 of 14

[Zn2+] = 4 ×10-6 mol·L-1 [Si(OH) ] = 2 ×10-6 mol·L-1 TOT 4 TOT OH− -2

H+ -4 − 2+ ZnSiO (c) Zn(OH)3 Zn 3 − ZnOH+ SiO(OH)3 Si(OH)4 Log C -6 Zn(OH)2 − SiO2(OH)22 − Zn(OH)42 -8

4681012 pH

FigureFigure 10. 10.Zn Zn or or SiSi speciesspecies of of hemimorphite hemimorphite in inwater. water.

Under pH 7.0–11.0, HDDPA mainly exists as HDDPA3− species in aqueous solutions, Zn(OH)2 and ZnSiO3 might be the main Zn species on hemimorphite surfaces, and some ZnOH+ species would be adsorbed on to hemimorphite surfaces. Therefore, HDDPA3− might react with Zn(II) species of hemimorphite surfaces to form Zn(II)-HDDPA surface complexes. In acidic conditions, the dissolved Zn2+ ions would combine with HDDPA2− ions to reduce HDDPA2− adsorption towards hemimorphite, resulting in a descended flotation recovery of hemimorphite. Under pH > 11.0, the adsorption of Zn(OH)3− or SiO(OH)3− on to hemimorphite surfaces would increase, and HDDPA4− adsorption to hemimorphite might face strong electrostatic repulsion between them, which also rendered a rapidly reduced flotation recovery of hemimorphite.

5. Conclusions In this paper, 1-hydroxydodecylidene-1,1-diphosphonic acid (HDDPA) was prepared and introduced as a collector for hemimorphite flotation. Based on the experimental findings and analyses, we could draw the following conclusions: HDDPA exhibited good performances for flotation recovery of hemimorphite in comparison with lauric acid, and it also possessed good selectivity against quartz flotation under pH 7.0–11.0. Contact angle results revealed that HDDPA selectively adsorbed on to hemimorphite rather than quartz and dramatically improved hemimorphite hydrophobicity. In the presence of HDDPA anions, the zeta potential of hemimorphite particles moved to more negative values, inferring a strong chemisorption of HDDPA on to hemimorphite. The FTIR recommended that HDDPA anchored on hemimorphite surfaces through bonding the oxygen atoms of its P(=O)–O groups with surface Zn(II) species. XPS gave additional evidence that Zn(II)-HDDPA surface complexes were formed on hemimorphite.

Author Contributions: W.T. and G.L. conceived and designed the experiments; W.T. performed the experiments; W.T. and J.Q. analyzed the data; H.F. contributed reagents/materials/analysis tools; W.T. and G.L. wrote the paper.

Conflicts of Interest: The authors declare no conflict of interest.

Minerals 2018, 8, 38 11 of 13

3− Under pH 7.0–11.0, HDDPA mainly exists as HDDPA species in aqueous solutions, Zn(OH)2 + and ZnSiO3 might be the main Zn species on hemimorphite surfaces, and some ZnOH species would be adsorbed on to hemimorphite surfaces. Therefore, HDDPA3− might react with Zn(II) species of hemimorphite surfaces to form Zn(II)-HDDPA surface complexes. In acidic conditions, the dissolved Zn2+ ions would combine with HDDPA2− ions to reduce HDDPA2− adsorption towards hemimorphite, resulting in a descended ﬂotation recovery of hemimorphite. Under pH > 11.0, the adsorption of − − 4− Zn(OH)3 or SiO(OH)3 on to hemimorphite surfaces would increase, and HDDPA adsorption to hemimorphite might face strong electrostatic repulsion between them, which also rendered a rapidly reduced ﬂotation recovery of hemimorphite.

5. Conclusions In this paper, 1-hydroxydodecylidene-1,1-diphosphonic acid (HDDPA) was prepared and introduced as a collector for hemimorphite flotation. Based on the experimental findings and analyses, we could draw the following conclusions: HDDPA exhibited good performances for flotation recovery of hemimorphite in comparison with lauric acid, and it also possessed good selectivity against quartz flotation under pH 7.0–11.0. Contact angle results revealed that HDDPA selectively adsorbed on to hemimorphite rather than quartz and dramatically improved hemimorphite hydrophobicity. In the presence of HDDPA anions, the zeta potential of hemimorphite particles moved to more negative values, inferring a strong chemisorption of HDDPA on to hemimorphite. The FTIR recommended that HDDPA anchored on hemimorphite surfaces through bonding the oxygen atoms of its P(=O)–O groups with surface Zn(II) species. XPS gave additional evidence that Zn(II)-HDDPA surface complexes were formed on hemimorphite.

Acknowledgments: The authors would like to thank the National Natural Science Foundation of China (51474253), the National Basic Research Program of China (973 program) (2014CB643403), and the National High Technology Research and Development Program of China (863 program). Author Contributions: W.T. and G.L. conceived and designed the experiments; W.T. performed the experiments; W.T. and J.Q. analyzed the data; H.F. contributed reagents/materials/analysis tools; W.T. and G.L. wrote the paper. Conﬂicts of Interest: The authors declare no conﬂict of interest.

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