minerals Article
Effect of Sodium Pyrophosphate on the Reverse Flotation of Dolomite from Apatite Yanfei Chen, Qiming Feng, Guofan Zhang *, Dezhi Liu and Runzhe Liu School of Minerals Processing and Bioengineering, Central South University, Changsha 410083, China;
[email protected] (Y.C.);
[email protected] (Q.F.);
[email protected] (D.L.);
[email protected] (R.L.) * Correspondence:
[email protected]; Tel: +86-731-88-830-913
Received: 4 June 2018; Accepted: 26 June 2018; Published: 29 June 2018
Abstract: In this study, the effect of sodium pyrophosphate (NaPP) on the separation of apatite from dolomite by flotation was systematically investigated. Flotation results revealed that NaPP could selectively depress the flotation of apatite, thus realizing the separation of apatite from dolomite. Further, the selective depression mechanism of NaPP was studied through zeta potential measurements, contact angle measurements, and X-ray photoelectron spectroscopy (XPS) analysis. The results demonstrated that the adsorption of sodium oleate (NaOL) onto apatite surface was depressed by the preferential interaction of NaPP with active Ca sites. For dolomite, while the presence of NaPP hindered the interaction of NaOL with active Ca sites, it appeared to enhance the reactivity with active Mg sites. Thus, the adsorption of NaOL onto dolomite surface was hardly influenced. In this way, the separation of apatite from dolomite was achieved. Keywords: sodium pyrophosphate; flotation; depression; apatite; dolomite
1. Introduction Phosphorous, as one of the most common elements on earth and essential elements in organisms, is widely used in the production of fertilizers, detergents, pharmaceuticals, fluxes, cement, and many other industrial processes [1–5]. Nevertheless, with the rapid development of China’s economy, the production of phosphorous cannot meet the requirement of the country [6]. Hence, the efficient exploitation of phosphate ore resource becomes more and more important. Moreover, growing demands for phosphorous have motivated the development of new technologies to concentrate phosphates from low-grade ores [7]. China has the second largest reserve of phosphate ore. However, vast majority of these resources are complex low-grade ores [8,9]. In these ores, the main valuable mineral, apatite is usually associated with gangue minerals, such as dolomite. Reverse flotation can be applied for phosphate ores that have high dolomite content [9–12]. However, there are still considerable difficulties in separating apatite from dolomite by means of reverse flotation. Furthermore, for fine-grained dissemination and complex minerals composition, apatite is often intergrown with dolomite. On the other hand, as calcium-bearing minerals, apatite and dolomite have similar surface properties. Additionally, dissolved components from apatite and dolomite will hydrolyze, precipitate, and adsorb onto the minerals [13–15]. All of these factors determine the interfacial properties of the minerals and make it more difficult to separate apatite from dolomite. For these reasons, to separate apatite from dolomite effectively, research has focused on developing selective flotation agent. As known, fatty acids are most widely used collectors in the flotation of apatite and dolomite, and the development of novel flotation collectors has been an area of research interest. Despite successful and economic recovery of apatite via reverse flotation, the selectivity of using fatty acids and their derivatives as collectors is still not satisfactory due to their similar surface reactivity of calcium-bearing minerals such as apatite and dolomite [10,16–18]. For effective Minerals 2018, 8, 278; doi:10.3390/min8070278
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selectivity of using fatty acids and their derivatives as collectors is still not satisfactory due to their similar surface reactivity of calcium-bearing minerals such as apatite and dolomite [10,16–18]. For effective utilization of phosphate ore resource, it is important to selectively separate apatite from utilization of phosphate ore resource, it is important to selectively separate apatite from dolomite. dolomite. an Therefore, effectiveisdepressant isimproving essential for floatability differences Therefore, effectivean depressant essential for theimproving floatability the differences between apatite between apatite and dolomite. and dolomite. Due to its its extensive extensive sources sources and and low low unit unit cost, cost, H H22SO SO44 has depressant in in Due to has been been widely widely used used as as aa depressant apatite flotation. flotation. However, large dosages dosages of of H H22SO apatite However, large SO44 will will lead lead to to the the corrosion corrosion of of flotation flotation equipment, equipment, and the of acidic In addition, addition, the the production production of of sediment sediment (CaSO (CaSO44)) could and the production production of acidic wastewater. wastewater. In could cause cause pipeline blocking, blocking, which which cannot cannot be be appropriately appropriately resolved resolved as as yet yet [8,14,19]. [8,14,19]. Thus, Thus, the the utilization utilization of of pipeline H 2 SO 4 to separate apatite from dolomite remains a problem. H2 SO4 to separate apatite from dolomite remains a problem. Sodium pyrophosphate, known knownas ascondensed condensedphosphate, phosphate,isisformed formed repeated condensation Sodium pyrophosphate, byby repeated condensation of of tetrahedral [PO 4units. ] units. It is usually used as water softener, emulsifier, and chelating agent. For the tetrahedral [PO ] It is usually used as water softener, emulsifier, and chelating agent. For the 4 presence of of chelating chelating group, group, the the addition addition of of sodium sodium pyrophosphate pyrophosphate may may promote promote the the formation formation of of presence metal ion-pyrophosphate, preventing its reaction with collector [20]. It was demonstrated that metal ion-pyrophosphate, preventing its reaction with collector [20]. It was demonstrated that sodium sodium pyrophosphate couldthe reduce theeffect adverse effect of serpentine on theofflotation of pentlandite pyrophosphate could reduce adverse of serpentine on the flotation pentlandite by shifting by shifting the slime surface charge [21]. In addition, sodium pyrophosphate has also been used in the slime surface charge [21]. In addition, sodium pyrophosphate has also been used in scheelite scheelite flotation [22,23]. However, there few reports the utilization of sodium pyrophosphate flotation [22,23]. However, there are feware reports on theon utilization of sodium pyrophosphate as a as a depressant in apatite flotation. depressant in apatite flotation. In this thispaper, paper, sodium pyrophosphate (NaPP) was introduced as a flotation depressant to In sodium pyrophosphate (NaPP) was introduced as a flotation depressant to selectively selectively separate apatite from dolomite. Micro-flotation tests to were performed to depression reveal the separate apatite from dolomite. Micro-flotation tests were performed reveal the selective selective depression of apatite by NaPP. In addition, the underlying mechanism was investigated of apatite by NaPP. In addition, the underlying mechanism was investigated through zeta potential and throughangle zeta measurements, potential and contact angle measurements, and XPS analyses. contact and XPS analyses. 2. Experiments 2.1. Materials 2.1. Materials Chemical Group Co., Ltd. located in The sample sample of of apatite apatitewas wasobtained obtainedfrom fromYunnan YunnanPhosphate Phosphate Chemical Group Co., Ltd. located Yunnan, China. The oreore was dry-ground inin a lab-scale ball 74 ++ 38 μm µm in Yunnan, China. The was dry-ground a lab-scale ballmill milland andthen thensieved sievedtotoget get−−74 fraction for the flotation tests. In addition, a part of −2 μm) µm) was also obtained for the of fine fine fraction fraction ((−2 zeta potential measurements. X-ray diffraction (XRD) analyses (Figure 1) revealed that the purity of apatite sample was considerably high with a bit of quartz. quartz. 650
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Figure 1. X-ray diffraction (XRD) pattern of the apatite sample.
The sample of dolomite was The sample of dolomite was obtained obtained from from Changsha Changsha Ore Ore Powder Powder Factory Factory located located in in Hunan, Hunan, China. The ore was dry-ground in a lab-scale ball mill and then sieved to get to get –74 China. The ore was dry-ground in a lab-scale ball mill and then sieved to get to get −74 ++ 38 38 μm µm fraction for the flotation tests. In addition, a part of fine fraction (−2 μm) was also obtained for the fraction for the flotation tests. In addition, a part of fine fraction (−2 µm) was also obtained for the
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zeta zeta potential potential measurements. measurements. X-ray diffraction (XRD) analyses (Figure 2) revealed that that the the purity purity of of dolomite sample was very high. dolomite sample was very high. Sodium Sodium pyrophosphate pyrophosphate (NaPP) used as the depressant in this this study study was was bought bought from from Tianjin Tianjin Yongda YongdaChemical Chemical Reagent Reagent Development Development Center. Center. Sodium Sodium oleate oleate (NaOL) (NaOL) bought bought from from Tianjin Tianjin Kermil Kermil Chemical Reagents Development Centre was used as a collector. HCl and NaOH obtained Chemical Reagents Development was used as a collector. and NaOH obtained from from Aladdin Aladdin Reagent Reagent Co. Co. Ltd. Ltd. (Shanghai, China) China) were were used used to to adjust adjust the the pH pH value. value. All All the the reagents reagents used used in this study were of analytical grade. Deionized water was used for all the tests. in this study were of analytical grade. Deionized water was used for all the tests. 9000
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Figure 2. XRD pattern of the dolomite sample. Figure 2. XRD pattern of the dolomite sample.
2.2. Micro-Flotation Experiments 2.2. Micro-Flotation Experiments Micro-flotation experiments were carried out in an XFG flotation machine (Exploring machinery Micro-flotation experiments were carried out in an XFG flotation machine (Exploring machinery Plant, Changchun, China) equipped with a 40-mL cell at 2000 rpm agitation speed. For single mineral Plant, Changchun, China) equipped with a 40-mL cell at 2000 rpm agitation speed. For single mineral tests, 2.0 g of pure mineral was placed in the flotation cell with 35 mL deionized water and then tests, 2.0 g of pure mineral was placed in the flotation cell with 35 mL deionized water and then conditioned for 1 min. NaPP and NaOL were successively added and stirred for 3 min, respectively. conditioned for 1 min. NaPP and NaOL were successively added and stirred for 3 min, respectively. The flotation process lasted 3 min for each test. Following this, the flotation recoveries were calculated The flotation process lasted 3 min for each test. Following this, the flotation recoveries were calculated based on the weights of concentrates and tailings. based on the weights of concentrates and tailings. For artificial mixed minerals flotation experiments, the mass ratio of apatite and dolomite For artificial mixed minerals flotation experiments, the mass ratio of apatite and dolomite mineral mineral was 1:1 for binary mixture. The flotation process was the same as single mineral tests. After was 1:1 for binary mixture. The flotation process was the same as single mineral tests. After the the flotation process, the concentrates and tailings were assayed for P and Ca. The recovery of apatite flotation process, the concentrates and tailings were assayed for P and Ca. The recovery of apatite was was calculated based on P2O5 contents of concentrates and tailings. calculated based on P2 O5 contents of concentrates and tailings. 2.3. Zeta Zeta Potential Potential Measurements Measurements 2.3. Zeta potential potential measurements measurements were were carried carried out out using using aa Zeta Zeta Plus Plus Zeta Zeta Potential Potential Meter Meter (Bruker, Zeta (Bruker, −3 mol/L. Small amounts of Karlsruhe, Germany). KNO 3 was used to maintain the ionic strength at 10 − 3 Karlsruhe, Germany). KNO3 was used to maintain the ionic strength at 10 mol/L. Small amounts of sample below below − −22 μm for 10 sample µm were were added added to to desired desired amounts amounts of of solution solution and and magnetically magnetically stirred stirred for 10 min, min, and the the pH pH was was adjusted adjusted using using HCl HCl or or NaOH. NaOH. The The zeta zeta potential potential of of samples samples was was then then measured measured three three and times using usingaaZeta ZetaPlus PlusZeta ZetaPotential Potential Meter (Bruker, Karlsruhe, Germany). average value times Meter (Bruker, Karlsruhe, Germany). TheThe average value and and the the standard deviation of zeta potential were respectively calculated. standard deviation of zeta potential were respectively calculated. 2.4. Contact Contact Angle Angle Measurements Measurements 2.4. The contact contact angle angle measurements measurements were were performed performed with with sessile sessile drop drop method method using using aa Digidrop Digidrop The goniometer (GBX, Isere, France). The crystals of apatite and dolomite were embedded in resin and goniometer (GBX, Isere, France). The crystals of apatite and dolomite were embedded in resin and then then polished with 500 grit, 1000 grit and 4000 grit alumina sandpapers, successively. For the polished with 500 grit, 1000 grit and 4000 grit alumina sandpapers, successively. For the measurements measurements of minerals in NaPP, the absence of NaPP,sample the prepared sample was in a desired of minerals in the absence of the prepared was immersed in a immersed desired concentration concentration NaOL solution for 15 min. For the measurements of minerals in the presence of NaPP, NaOL solution for 15 min. For the measurements of minerals in the presence of NaPP, the prepared the prepared sample was firstly immersed in NaPP solution for 15 min and then in NaOL solution for another 15 min. Next, the sample was washed with deionized water and then air dried. A water
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sample was firstly immersed in NaPP solution for 15 min and then in NaOL solution for another droplet (about 2 mm in diameter) was introduced onto the sample surface, and then the contact angle 15 min. Next, the sample was washed with deionized water and then air dried. A water droplet (about results were analyzed by computer software. 2 mm in diameter) was introduced onto the sample surface, and then the contact angle results were analyzed by computer software. 2.5. XPS Analysis 2.5. XPS The Analysis change of surface chemical composition of mineral samples (apatite and dolomite) pretreated with different reagents was determined by X-ray photoelectron spectroscopy (XPS). To The change of surface chemical composition of mineral samples (apatite and dolomite) pretreated prevent extra surface change, the samples were stored in a vacuum drier under the temperature of with different reagents was determined by X-ray photoelectron spectroscopy (XPS). To prevent 25 °C. The XPS measurements were performed on a X-ray photoelectron spectrometer (PHI5000, extra surface change, the samples were stored in a vacuum drier under the temperature of 25 ◦ C. ULVAC-PHI, Chigasaki, Japan). Firstly, chemical components of the samples were identified by The XPS measurements were performed on a X-ray photoelectron spectrometer (PHI5000, ULVAC-PHI, survey scan. Then, high-resolution scans were conducted focusing on certain elements. Sample Chigasaki, Japan). Firstly, chemical components of the samples were identified by survey scan. Then, charging was compensated by taking the C1s peak of background hydrocarbon at 284.8 eV as an high-resolution scans were conducted focusing on certain elements. Sample charging was compensated internal standard. by taking the C1s peak of background hydrocarbon at 284.8 eV as an internal standard.
3. 3. Results Results and and Discussion Discussion 3.1. Micro-flotation Micro-flotation Experiments Figure 33shows results of apatite andand dolomite as a function of NaOL dosage. It can showsthe theflotation flotation results of apatite dolomite as a function of NaOL dosage. be seen 3 that3 the recoveries of two minerals increased with thethe increase of It can befrom seen Figure from Figure thatflotation the flotation recoveries of two minerals increased with increase NaOL dosage. The recoveries ofofapatite of NaOL dosage. The recoveries apatiteand anddolomite dolomiteatat60 60mg/L mg/Ldosage dosageNaOL NaOLwere were 80.78% 80.78% and 93.6%, 93.6%, respectively. Meanwhile, Meanwhile, the the flotation flotation recovery recovery of of dolomite dolomite was was higher higher than than that that of of apatite, apatite, which implied that NaOL had a better collecting collecting ability to to dolomite. dolomite. Therefore, Therefore, for the better better flotation flotation performance of dolomite, reverse flotation is a proper proper method method to to separate separate apatite apatite from from dolomite. dolomite. 100
Apatite Dolomite Flotation Recovery (%)
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Figure 3. 3. Effect Effect of of sodium sodium oleate oleate (NaOL) (NaOL) dosage dosage on the flotation of apatite and dolomite. Figure
The effectof ofNaPP NaPPdosage dosage flotation of apatite dolomite is shown in Figure Our The effect onon thethe flotation of apatite and and dolomite is shown in Figure 4. Our 4. results results with single mineral flotation show that while NaPP was an effective depressant for apatite, it with single mineral flotation show that while NaPP was an effective depressant for apatite, it had had minimal on dolomite flotation. At adosage NaPPofdosage of 100 mg/L, nearly minimal effecteffect on dolomite flotation. At a NaPP 100 mg/L, apatite wasapatite nearlywas completely completely depressed while that of dolomite decreased from 93.8% to 84%. depressed while that of dolomite decreased from 93.8% to 84%. The distinctdifference differenceof of flotation recovery makes it possible to separate apatite from The distinct thethe flotation recovery makes it possible to separate apatite from dolomite dolomite using as the depressant. Therefore, the flotation the artificial minerals using NaPP as NaPP the depressant. Therefore, the flotation tests ontests the on artificial mixedmixed minerals were were performed using NaPP as depressant. The flotation results are presented in Table 1. performed using NaPP as depressant. The flotation results are presented in Table 1. As As can can be be seen seen from from Table Table 11 that, that, the the flotation flotation recovery recovery of of apatite apatite was wasimproved improvedfrom from39.4% 39.4%to to 96.5% in the presence of NaPP, while the recovery of MgO slightly increased from 11.3% to 16.3%. 96.5% in the presence of NaPP, while the recovery of MgO slightly increased from 11.3% to 16.3%. Further, Further, the the addition addition of of NaPP NaPP also also resulted resulted in in aa sharp sharp increase increase of of P P22O O55 grade gradeof of concentrate concentratefrom from 26.4% 26.4% to to 34.1% 34.1% and and aa decrease decrease of of MgO MgO grade grade of of concentrate concentrate from from 3.8% 3.8% to to 2.9%. 2.9%. The Theartificial artificialmixed mixed minerals separationreported reportedhere here suggests that NaPP be used for separating dolomite from minerals separation suggests that NaPP can can be used for separating dolomite from apatite. apatite. However, the availability of utility of this approach has to be demonstrated with “real” complex ores such as those described in the Section 1.
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However, the availability of utility of this approach has to be demonstrated with “real” complex ores 5 of 11 5 of 11 1.
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Figure Figure 4. Effect of of sodium pyrophosphate (NaPP) dosage on the flotation of apatite and dolomite. Figure 4. 4. Effect Effect of sodium sodium pyrophosphate pyrophosphate (NaPP) (NaPP) dosage dosage on on the the flotation flotation of of apatite apatite and and dolomite. dolomite. Table 1. Artificial mixed minerals flotation results (NaOL: 60 mg/L). Table Table1. 1.Artificial Artificialmixed mixedminerals mineralsflotation flotationresults results(NaOL: (NaOL:60 60mg/L). mg/L).
NaPP NaPP (mg/L) (mg/L)
Production Production NaPP (mg/L)
Production
Concentrate 29.4 Concentrate 29.4 Concentrate Tailing 70.6 Tailing 70.6 Tailing 0Feed 100.0 Feed Feed100.0 Concentrate 55.7 ConcentrateConcentrate 55.7 Tailing 44.3 Tailing 44.3 Tailing 100 Feed Feed100.0 Feed 100.0
00
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Grade (%) Grade (%)(%) Grade P 2 O 5 Yield (%) P2O5 P O MgO MgO MgO 2 5 26.4 3.8 26.4 3.8 29.4 16.9 26.4 3.8 12.4 16.9 12.4 70.6 16.9 12.4 19.7 9.9 9.9 100.0 19.7 19.7 9.9 34.1 2.9 34.1 2.9 55.7 34.1 2.9 1.5 18.6 18.6 44.3 1.5 1.5 18.6 9.9 100.0 19.7 9.9 19.7 19.7 9.9
Yield Yield (%) (%)
Recovery (%) PP22O 5 MgO OMgO 5 MgO P2 O5 39.4 11.3 39.4 11.3 39.4 60.611.3 88.7 60.6 88.7 60.6 100.088.7 100.0 100.0 100.0100.0 100.0 96.5 16.3 96.5 16.3 96.5 16.3 3.5 83.7 3.5 83.7 3.5 83.7 100.0 100.0100.0 100.0 100.0 100.0 Recovery (%) Recovery (%)
3.2. 3.2. Zeta Zeta Potential Potential Measurements Measurements 3.2. Measurements To reveal the underlying the zeta potential To reveal reveal the the underlying underlying depression depression mechanism mechanism of of NaPP, NaPP, the the zeta zeta potential potential measurements measurements of of To depression mechanism NaPP, measurements of apatite and dolomite under different reagent conditions were performed as a function of pH, and the apatite and and dolomite dolomite under under different different reagent reagent conditions conditions were were performed performed as as aa function function of of pH, pH, and and the the apatite results 5a,b. results are are shown shown in in Figure Figure 5a,b. 5a,b. results are shown in Figure 30 30 20 20
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Figure of Figure 5. 5. Effect Effect of different different reagents reagents on on the the zeta zeta potential potential of of (a) (a) apatite apatite and and (b) (b) dolomite. dolomite. Figure 5. Effect of different reagents on the zeta potential of (a) apatite and (b) dolomite.
Comparing Comparing the the data data in in Figure Figure 5a,b, 5a,b, there there were were obvious obvious potential potential differences differences which which resulted resulted in in Comparing the data in Figure 5a,b, there were obvious potential differences which resulted in the the difference in the flotation performance of the two minerals. For apatite, it was positively the difference in the flotation performance of the two minerals. For apatite, it was positively charged charged difference in the flotation performance of the two minerals. For apatite, it was positively charged in in in the the pH pH range range of of 3–6, 3–6, which which matched matched well well with with other other literature. literature. The The isoelectric isoelectric point point (IEP) (IEP) of of dolomite dolomite was was near near aa pH pH of of 6.5, 6.5, which which was was also also in in agreement agreement with with the the previous previous study study [15,24]. [15,24]. With With the the addition addition of of NaPP, NaPP, zeta zeta potentials potentials of of two two minerals minerals sharply sharply dropped dropped over over the the entire entire pH pH range, probably due to the adsorption of dissolved NaPP components which were negatively range, probably due to the adsorption of dissolved NaPP components which were negatively charged. charged. Similar Similar shifts shifts of of zeta zeta potentials potentials of of both both minerals minerals revealed revealed that that NaPP NaPP interacted interacted intensively intensively with two mineral surfaces. with two mineral surfaces.
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the pH range of 3–6, which matched well with other literature. The isoelectric point (IEP) of dolomite was near a pH of 6.5, which was also in agreement with the previous study [15,24]. With the addition of NaPP, zeta potentials of two minerals sharply dropped over the entire pH Minerals 2018, 8, x FOR PEER REVIEW 6 of 11 range, probably due to the adsorption of dissolved NaPP components which were negatively charged. Similar shifts of zeta potentials of both minerals revealed that NaPP interacted intensively with two When NaOL was applied after the addition of NaPP, apatite gave zeta potential which was about mineral surfaces. the same value obtained for apatite in addition the presence of NaPP alone. the zeta potential of When NaOL was applied after the of NaPP, apatite gaveHowever, zeta potential which was about dolomite is more negative compared with that using NaPP alone. The zeta potential results illustrated the same value obtained for apatite in the presence of NaPP alone. However, the zeta potential of that the pre-treatment of NaPP prior to NaOL notNaPP prevent the interaction of NaOL with illustrated dolomite, dolomite is more negative compared with thatdid using alone. The zeta potential results but hindered the adsorption of NaOl onto apatite surface. The zeta potential measurements results that the pre-treatment of NaPP prior to NaOL did not prevent the interaction of NaOL with dolomite, provided a preliminary understanding of the depressant effect NaPP. but hindered the adsorption of NaOl onto apatite surface. Theof zeta potential measurements results provided a preliminary understanding of the depressant effect of NaPP. 3.3. Contact Angle Measurements 3.3. Contact Angle Measurements The advancing contact angle of the two minerals before and after interaction with NaPP as a function of NaOL dosage measured to reveal thebefore changes surface wettability the two The advancing contactwas angle of the two minerals andofafter interaction withof NaPP as a minerals. can be seen was from Figure 6,tosurface hydrophobicity of both mineralsofinthe the absence of function ofAs NaOL dosage measured reveal the changes of surface wettability two minerals. NaPP was dramatically improved with the increase of NaOL dosage, which indicated the increasing As can be seen from Figure 6, surface hydrophobicity of both minerals in the absence of NaPP was adsorption ofimproved NaOL onto thethe minerals Indosage, addition, the contact angle values of apatite and dramatically with increasesurface. of NaOL which indicated the increasing adsorption dolomite werethe in minerals accord with the flotation recoveries presented in Figure 3. After interaction of NaOL onto surface. In addition, the contact angle values of apatite and dolomite with were NaPP, the contact angle of apatite was significantly decreased. However, as for dolomite, the in accord with the flotation recoveries presented in Figure 3. After interaction with NaPP, the contact insignificant change of contact angle showed little influence of NaPP on its surface hydrophobicity. angle of apatite was significantly decreased. However, as for dolomite, the insignificant change of The different of little NaPPinfluence on the surface hydrophobicity apatite and dolomite revealed that contact angle effects showed of NaPP on its surface of hydrophobicity. The different effects NaPP could restrict the adsorption of NaOL onto apatite while hardly affect NaOL adsorption onto of NaPP on the surface hydrophobicity of apatite and dolomite revealed that NaPP could restrict dolomite. Therefore, the onto decrease of NaOL adsorption apatite resulted in the depression of its the adsorption of NaOL apatite while hardly affectonto NaOL adsorption onto dolomite. Therefore, flotation performance. the decrease of NaOL adsorption onto apatite resulted in the depression of its flotation performance. 80
Apatite Apatite+NaPP Dolomite Dolomite+NaPP
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60 50 40 30 20 10 0
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NaOl dosage(mg/L)
Figure 6. Contact Contactangle angleofofthe the minerals before interaction as a function ofdosage. NaOL Figure 6. minerals before afterafter interaction withwith NaPPNaPP as a function of NaOL dosage.
3.4. XPS Analysis 3.4. XPS Analysis To further investigate the interaction mechanism between NaPP and the two minerals (apatite and To further investigate the interaction mechanism betweenand NaPP and the two minerals (apatite dolomite), XPS analyses of apatite and dolomite in the absence presence of NaPP were conducted, and dolomite), XPS analyses of apatite and dolomite in the absence and presence of NaPP were and the fitted results are shown Figure 7. With reference to data from United States National Institute conducted, and the fitted results are shown Figure 7. With reference to data from United States of Standards and Technology (NIST), the binding energy of 133.39 eV and 133.80 eV corresponds to National of Standards and Technology (NIST), the binding energy of 133.39 eV and 133.80 Ca5 (PO4)Institute 3 F and Ca2 P2 O7 , respectively. As can be seen from Figure 7a, only the peak of Ca5 (PO4)3 F was eV corresponds to Ca 5(PO4)3F and Ca2P2O7, respectively. As can be seen from Figure 7a, only the peak observed in the absence of NaPP, whereas the P2p peaks could be decomposed into two P2p-P2p3/2 of Ca 5(PO4)3F was observed in the absence of NaPP, whereas the P2p peaks could be decomposed doublets, indicating the existence of Ca5 (PO4)3 F and Ca2 P2 O7 in the presence of NaPP. Furthermore, into two P2p-P2p3/2 doublets, indicating the existence of Ca5(PO4)3F and Ca2P2O7 in the presence of NaPP. Furthermore, after adding NaPP, the P2p atomic concentration increased from 10.84% to 11.69%, which also showed NaPP successfully chemisorbed onto the apatite surface, and thus depressed the flotation of apatite. As for dolomite, the Ca2p and Mg1s peaks with and without NaPP are also fitted to confirm the chemical information of surface species. It could be seen from Figure 7b, the Ca2p peak of single dolomite appeared at 347.00 eV, in which the peak of 347.00 eV was assigned to CaCO3. After the
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after adding NaPP, the P2p atomic concentration increased from 10.84% to 11.69%, which also showed NaPP successfully chemisorbed onto the apatite surface, and thus depressed the flotation of apatite. As for dolomite, the Ca2p and Mg1s peaks with and without NaPP are also fitted to confirm the chemical information of surface species. It could be seen from Figure 7b, the Ca2p peak of single 2018, dolomite appeared at 347.00 eV, in which the peak of 347.00 eV was assigned to CaCO Minerals 8, x FOR PEER REVIEW 7 of 113 . After the addition of NaPP, a new Ca2p3/2 peak for Ca2 P2 O7 was observed and the characteristic P was also dolomite surface.the However, the Mg1s binding energy didin not change in the detected ondetected dolomiteon surface. However, Mg1s binding energy did not change the presence of presence NaPP.changes All theseoffered changesgood offered good evidence thatadsorbed NaPP adsorbed onto dolomite surface, NaPP. Allofthese evidence that NaPP onto dolomite surface, and and selectively reacted active Ca sites rather Mg sites. selectively reacted withwith active Ca sites rather thanthan Mg sites. 133.39eV Ca5(PO4)3F
Apatite P2p
Dolomite Ca2p
(a)
347.00eV CaCO3
350.69
Counts (s)
Counts (s)
(b)
Apatite+NaPP P2p
133.39eV Ca5(PO4)3F
347.56 Ca2P2O7
Dolomite+NaPP Ca2p
350.96
133.80eV Ca2P2O7
347.00eV CaCO3
140
138
136
134
132
130
128
354
Binding energy (eV)
352
350
348
346
344
Binding energy (eV)
Counts(s)
Dolomite Mg1s
1303.73eV MgO
1303.73eV MgO
Dolomite+NaPP Mg1s
1307
1306
(c)
1305
1304
1303
1302
1301
1300
Binding energy (eV)
Figure Figure 7. 7. Fitting Fitting of of X-ray X-ray photoelectron photoelectron spectroscopy spectroscopy (XPS) (XPS) spectra spectra of of (a) (a) P2p P2p of of apatite, apatite, (b) (b) Ca2p Ca2p of of dolomite and (c) Mg1s of dolomite before and after treatment with NaPP. dolomite and (c) Mg1s of dolomite before and after treatment with NaPP.
In addition, to obtain more detailed information about the selective depression of NaPP on In addition, to obtain more detailed information about the selective depression of NaPP on apatite apatite flotation, broad scan XPS analyses of apatite and dolomite under different reagent conditions flotation, broad scan XPS analyses of apatite and dolomite under different reagent conditions were also were also employed, and the results are shown in Figure 8 and Table 2. Generally, an obvious shift employed, and the results are shown in Figure 8 and Table 2. Generally, an obvious shift of binding of binding energy indicates variations of chemical environment. From Table 2 we can see that after energy indicates variations of chemical environment. From Table 2 we can see that after apatite was apatite was treated with NaOL, the binding energy of Ca2p decreased by 0.51 eV, which revealed a treated with NaOL, the binding energy of Ca2p decreased by 0.51 eV, which revealed a chemical chemical interaction between apatite and NaOL. For the chemical adsorption of NaOL onto apatite interaction between apatite and NaOL. For the chemical adsorption of NaOL onto apatite surface, surface, apatite was hydrophobic. After apatite was treated with both NaPP and NaOL, the binding apatite was hydrophobic. After apatite was treated with both NaPP and NaOL, the binding energy of energy of Ca2p shifted by 0.05 eV, which was much smaller than that treated with single NaOL, Ca2p shifted by 0.05 eV, which was much smaller than that treated with single NaOL, indicating that indicating that the interaction between apatite and NaOL was obviously suppressed by NaPP. the interaction between apatite and NaOL was obviously suppressed by NaPP. Meanwhile, according Meanwhile, according to Figure 7 we deduced that active Ca sites strongly interacted with both NaPP to Figure 7 we deduced that active Ca sites strongly interacted with both NaPP and NaOL. When and NaOL. When NaPP was added before NaOL, NaPP chemisorbed onto apatite surface and thus restricted the reaction of Ca with NaOL. Thus, the flotation of apatite was depressed by NaPP. In the case of dolomite, after treatment with NaOL, the binding energy of Ca2p and Mg1s shifted by −0.63 eV and −0.29 eV, respectively. These changes suggested that both Ca and Mg were active sites for chemical reaction with NaOL [5,25]. Moreover, the shift of Ca2p binding energy of dolomite was larger than that of apatite, revealing that the interaction between Ca of dolomite and NaOL was
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NaPP was added before NaOL, NaPP chemisorbed onto apatite surface and thus restricted the reaction of Ca with NaOL. Thus, the flotation of apatite was depressed by NaPP. In the case of dolomite, after treatment with NaOL, the binding energy of Ca2p and Mg1s shifted by −0.63 eV and −0.29 eV, respectively. These changes suggested that both Ca and Mg were active sites for chemical reaction with NaOL [5,25]. Moreover, the shift of Ca2p binding energy of dolomite was larger than that of apatite, revealing that the interaction between Ca of dolomite and NaOL was more intensive, which was in accordance with the flotation results of Figure 3 and the contact Minerals 2018, 8, x FOR PEER REVIEW6. Interestingly, after dolomite was treated with both NaPP and NaOL, 8 of 11 angle measurements of Figure the binding energy of Ca2p and Mg1s changed by −0.07 eV and −0.56 eV, respectively. These results demonstrated thatfor fordolomite dolomite both Mg were sitesreacted that reacted withThe NaOL. The demonstrated that both Ca Ca andand Mg were activeactive sites that with NaOL. presence presence of NaPP restricted the interaction between active Ca sites and NaOL, but enhanced the of NaPP restricted the interaction between active Ca sites and NaOL, but enhanced the interaction interaction sitesThus, and Mg NaOL. Thus, Mg becameactive predominant activeadsorption. sites for NaOL between Mgbetween sites andMg NaOL. became predominant sites for NaOL Thus, adsorption. Thus, changes not affectofthe adsorption of NaOL and maintained the flotation these changes didthese not affect the did adsorption NaOL and maintained the flotation performance of performance of dolomite with the addition of NaPP. dolomite with the addition of NaPP. O1s
Apatite
(a) Ca2p F1s
P2p
C1s O1s
Counts (s)
Apatite+NaOL
Ca2p F1s
P2p
C1s
O1s Apatite+NaPP+NaOL Ca2p F1s
P2p
C1s
1400
1200
1000
800
600
400
200
0
Binding energy (eV) O1s
Dolomite
(b)
Mg1s Ca2p C1s O1s
Counts (s)
Dolomite+NaOL
Mg1s Ca2p C1s
O1s Dolomite+NaPP+NaOL Mg1s Ca2p
1400
1200
1000
800
600
400
C1s
200
0
Binding energy (eV)
Figure Figure 8. 8. Broad Broad scan scan XPS XPS spectra spectra of of (a) (a)apatite apatiteand and(b) (b)dolomite dolomitebefore before and and after after treatment treatment with with NaPP NaPP or NaPP + NaOL. or NaPP + NaOL. Table 2. Binding energy of elements on the minerals surface under different reagent conditions. Sample Apatite Apatite + NaOL
Binding Energy (eV) Ca Mg 347.54 347.03 -
Chemical Shift (eV) Ca Mg −0.51 -
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Table 2. Binding energy of elements on the minerals surface under different reagent conditions.
Sample Apatite Apatite + NaOL Apatite + NaPP + NaOL Dolomite Dolomite + NaOL Dolomite + NaPP + NaOL
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Binding Energy (eV)
Chemical Shift (eV)
Ca
Mg
Ca
Mg
347.54 347.03 347.49 347.14 346.51 347.07
1303.79 1303.50 1303.23
−0.51 −0.05 −0.63 −0.07
−0.29 −0.56
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3.5. Depression Depression Mechanism Mechanism of of NaPP 3.5. NaPP Based on on the the flotation flotation results, results, zeta zeta potential potential and and contact contact angle angle measurements, measurements, and and XPS XPS analysis, analysis, Based the possible mechanism about NaPP depression is proposed in Figure 9. the possible mechanism about NaPP depression is proposed in Figure 9. For apatite, apatite, Ca Caprovided providedthe theonly only active sites which interacted NaPP and NaOL. For active sites which interacted withwith bothboth NaPP and NaOL. This This led to the competitive adsorption of NaPP and NaOL onto apatite surface. The pre-adsorption of led to the competitive adsorption of NaPP and NaOL onto apatite surface. The pre-adsorption of hydrophilic NaPP occupied active Ca sites, restricting the adsorption of NaOL onto apatite surface hydrophilic NaPP occupied active Ca sites, restricting the adsorption of NaOL onto apatite surface and thus in apatite. However, dolomite dolomite has has both both active active Ca Ca and and Mg Mg sites sites for for and thus producing producing hydrophilia hydrophilia in apatite. However, chemical reaction with NaOL. Thus, while NaPP interfered with the interaction of active Ca sites chemical reaction with NaOL. Thus, while NaPP interfered with the interaction of active Ca sites with with NaOL, it improved reactivity of the sites withcollector. collector.Thus, Thus,the the interaction interaction differences differences NaOL, it improved the the reactivity of the MgMg sites with resulted in resulted in the the selective selective depression depression of of apatite apatite by by NaPP. NaPP.
Figure 9. Schematic diagram of potential depression mechanism of NaPP. Figure 9. Schematic diagram of potential depression mechanism of NaPP.
4. Conclusions 4. Conclusions This study systematically investigated the effect of sodium pyrophosphate on the selective This study systematically investigated the effect of sodium pyrophosphate on the selective reverse reverse flotation of apatite from dolomite. NaPP showed selective depression of apatite, thus flotation of apatite from dolomite. NaPP showed selective depression of apatite, thus realizing the realizing the preferential flotation separation of apatite from dolomite. Based on the results of zeta preferential flotation separation of apatite from dolomite. Based on the results of zeta potential and potential and contact angle measurements, and XPS analyses, it was concluded that NaPP occupied contact angle measurements, and XPS analyses, it was concluded that NaPP occupied active Ca sites active Ca sites and hindered the adsorption of NaOL onto apatite surface. As for dolomite, although and hindered the adsorption of NaOL onto apatite surface. As for dolomite, although the presence of the presence of NaPP interfered with the interaction between active Ca sites and NaOL, it improves NaPP interfered with the interaction between active Ca sites and NaOL, it improves the reactivity of the reactivity of active Mg sites with NaOL. Thus, the flotation of dolomite was slightly influenced active Mg sites with NaOL. Thus, the flotation of dolomite was slightly influenced by NaPP. In this by NaPP. In this way, the separation of apatite from dolomite was achieved. However, the limitation way, the separation of apatite from dolomite was achieved. However, the limitation of the work is that of the work is that the test was carried out on pure minerals. The focus of the following work is to confirm the results with dolomitic phosphate ores. Author Contributions: Y.C. and G.Z. conceived and designed the experiments; Y.C. prepared the samples and performed the experiments; Q.F. and G.Z. contributed reagents/materials/analysis tools; Y.C. analyzed the data; Y.C., R.L., and D.L., wrote and revised the paper.
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the test was carried out on pure minerals. The focus of the following work is to confirm the results with dolomitic phosphate ores. Author Contributions: Y.C. and G.Z. conceived and designed the experiments; Y.C. prepared the samples and performed the experiments; Q.F. and G.Z. contributed reagents/materials/analysis tools; Y.C. analyzed the data; Y.C., R.L., and D.L., wrote and revised the paper. Funding: This research received no external funding. Acknowledgments: This work was supported by the National Basic Research Program of China (2014CB643402). Conflicts of Interest: The authors declare no conflicts of interest.
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