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Magnesite-dolomite separation using potassium cetyl phosphate as a novel flotation collector and related surface chemistry
Applied Surface Science 508 (2020) 145191
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Full Length Article
Magnesite-dolomite separation using potassium cetyl phosphate as a novel flotation collector and related surface chemistry ⁎
Yuan Tanga,b, Wanzhong Yina, , Sadan Kelebekb, a b
T
⁎
School of Resources and Civil Engineering, Northeastern University, Shenyang 110819, Liaoning, China The Robert M. Buchan Department of Mining, Queen’s University, Kingston, Ontario K7L 3N6, Canada
A R T I C LE I N FO
A B S T R A C T
Keywords: Potassium cetyl phosphate Sodium silicate Flotation Magnesite Dolomite
To investigate the separation effect of magnesite from dolomite using potassium cetyl phosphate (PCP) as a collector, flotation experiments were carried out. In this study, the results obtained from the single minerals experiments showed an excellent selectivity with high recoveries for magnesite using PCP as the collector in contrast to the case with dolomite. The separation effect of the binary mixtures at pulp pH 9.0 was also investigated. The results confirmed that the PCP is an appropriate collector for the flotation separation of magnesite from dolomite. Surface chemical aspects of the separation mechanism were investigated through measurements of zeta potential, contact angle, and X-ray photoelectron spectroscopy analysis. The results support the flotation selectivity observed and are consistent with chemisorption of PCP on magnesite in the presence of sodium silicate that was used as a modifier to stabilize hydrophilicity of dolomite and remove its adverse effect on the recovery of magnesite.
1. Introduction Magnesite (MgCO3) and dolomite (MgCa(CO3)2) are important mineral commodities, which are widely utilized in the agriculture, chemical, construction, environmental, metallurgical and other industries [1]. However, in natural magnesite ore deposits, magnesite often coexists with dolomite [2]. With the increasing market demand, for the production of high-quality magnesite, highly selective separation of impurities such as dolomite is essential in order to reduce the calcium content [3–5]. This means that the impurity minerals coexisting in the ore deposit (i.e., gangue minerals) need to be efficiently depressed since froth flotation is the most effective industrial method used for the separation of magnesite from other carbonates [6,7]. The flotation separation is primarily based on the physicochemical differences between the surfaces of valuable and associated minerals. Dolomite often causes significant difficulties in the separation process since it has a crystal structure that is similar to that of magnesite and both have the same Mg-active sites on their dominant cleavage surfaces [8,9]. As the salt-type minerals, magnesite and dolomite are sparingly soluble and give lattice ions, which lead to similar surface chemistries in the process of environments [9,10]. Hence, various surfactants are used to improve the difference in surface properties. Clear-cut separation of minerals requires selective adsorption of collectors on the mineral surface to enhance its surface hydrophobicity, which is conducive ⁎
to increasing the floatability of minerals [11,12]. The commonly used cationic and anionic collectors show strong collecting ability for magnesite and dolomite as well as other associated minerals [13–15]. The main problem for those collectors, obviously, is the low level of selectivity in the absence of modifiers. Consequently, there is a need that has been recognized among researchers to identify a more selective collector action on such minerals. To achieve more effective separation, mixed collectors have been used to exhibit enhanced recovery and selectivity for different kinds of minerals, and most studies have focused on the combination of traditional collectors, as reported recently [15–17]. In addition to the mixed collectors, a series of novel surfactants have been reported to be effective for some oxide and silicate minerals. For example, Weng et al., reported a quaternary ammonium surfactant for reverse flotation of silicates [18]. An improved collecting ability has been demonstrated for quartz using a Gemini surfactant (containing two quaternary ammonium groups) as a collector [19,20]. However, most studies found so far are limited to the cationic collectors, which have been reported to be effective in reverse flotation applications [20,21]. In addition, some surfactants with phosphorous-bearing functional groups have been reported to exhibit enhanced response as collectors for a variety of minerals, such as rare-earth minerals [22,23], oxidized minerals [23,25], and dolomite as a carbonate mineral [25]. Based on the infrared spectrum analysis, the eP]O group has been shown to
Corresponding authors. E-mail addresses: [email protected] (W. Yin), [email protected] (S. Kelebek).
https://doi.org/10.1016/j.apsusc.2019.145191 Received 19 October 2019; Received in revised form 19 November 2019; Accepted 24 December 2019 Available online 28 December 2019 0169-4332/ © 2019 Elsevier B.V. All rights reserved.
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was set at 60 mg/L as the desired concentration level in the current experimental work to be reported. Analytically pure sodium silicate (Na2SiO3·9H2O) was supplied by Fisher Scientific Company, USA. It was used at concentrations up to 800 mg/L. The slurry pH level was adjusted with hydrochloric acid (HCl) and sodium carbonate (Na2CO3) solution. Deionized water with a resistivity of above 18 MΩ · cm used in all experiments was sourced from a laboratory ultrapure water system (Thermo-Fisher Scientific, Ontario, Canada).
exhibit a strong reaction with the cationic sites on various minerals involving chemisorption [23–27]. More specifically, dodecyl phosphate was reported to play an active role in the flotation of dolomite from magnesite, with both cases involving chemisorption [25]. Although the results indicate that the phosphate group connected to lauryl chain might be an effective functional group for recovery of dolomite, there has been no study reported for direct flotation of magnesite using potassium cetyl phosphate (PCP) as an anionic collector with a longer hydrocarbon. In a previous communication, we reported some results on selective flotation of magnesite from calcite using PCP as a novel anionic collector [28]. As a continuation, we extended our investigation into its effect on the separation of magnesite from dolomite. A variety of experiments were conducted to characterize the PCP flotation of magnesite and to be able to explain selective separation of magnesite from dolomite as a gangue mineral. Experimental investigations to deduce the adsorption mechanism of PCP in this flotation system included measurements of zeta potentials and contact angles as well as X-ray photoelectron spectroscopy (XPS) analysis. It is anticipated that the findings of this research will provide a fundamental understanding of the flotation chemistry of carbonate minerals using alkyl phosphate as an anionic collector while generating interest in possible application of longer chain phosphates as selective surfactants for mineral separations.
2.2. Flotation tests The flotation tests for single minerals and mixed binary minerals with PCP as the collector were carried out with a vertical micro-flotation cell of 25 cm using the general procedure reported previously [29]. Typically, a 1.0 g sample (75–150 μm) was added to the cell with the deionized water and then conditioned for 2 min by magnetic stirring bar at a speed of 1100 rpm. The pH modifier was then introduced, which was followed by conditioning with depressant for 2 min time intervals. Subsequently, the collector solution was added and conditioned for another 2 min. The collection of the froth product was continued for 3 min, and concentrates and tailings were filtered, dried and weighed for estimation of recovery. The flowsheet of single magnesite flotation is presented in Fig. 2. The average recovery plotted represents three repeat tests. For the separation case from binary mixtures (0.5 g magnesite and 0.5 g dolomite), the flotation products were assayed for Mg and Ca by using an atomic absorption spectrometer (AAS, iCE3000).
2. Materials and methods
2.3. Zeta potential measurements
2.1. Materials and reagents
The zeta potential measurements were carried out using a Zeta Potential Finder Analyzer (Matec Applied Science, USA) at room temperature (25 ± 2 ℃) to determine the relationship between the collector action and the surface charge of particles [30,31]. The Ludox TM suspension (10 wt%, with a standard zeta potential of −38 mV) was used to calibrate the analyzer. For each measurement, a dilute mineral suspension was prepared by adding 1.2 g of mineral to 200 ml deionized water, mixing constantly for 15 min. Reagents were added in the same order and the same time intervals as in flotation tests. The pH values were adjusted by adding 1% HCl or 3% Na2CO3 solutions to the slurry. Each measurement was repeated three times, and the average values were reported as the final results.
The magnesite sample, obtained from Refratechnik group (British Columbia, Canada) was used in this research. In addition, pure dolomite blocks were acquired from Ward’s Science (New York, USA). Hand-picked specimens from each mineral were crushed and dry ground to 75-150 μm, 45-75 μm, and 25-45μm for the micro-flotation tests. Both minerals were further ground to less than 5 µm in an agate mortar were used for zeta potential measurements. Analysis of the magnesite and dolomite samples based on the carbon content that was determined using a dual Carbon-Sulphur analyzer (CS-2000, ELTRA GmbH) indicated a high purity for each mineral at 98.4% and 96.4%, respectively. Fig. 1 shows results of the X-ray powder diffraction analysis of each mineral taken using a laboratory diffractometer (MPDDY2094, PANalytical B.V.), which confirms the high level purity of these samples. Potassium cetyl phosphate was acquired from the research laboratories of Colonial Chemical Inc., (South Pittsburg, Pennsylvania, USA). Its aqueous solutions ranging from 20 to 120 mg/L were used during the initial series of exploratory tests. Subsequently, its concentration
Fig. 2. Flowchart of magnesite flotation test procedure.
Fig. 1. XRD patters of magnesite (top) and dolomite (bottom) samples used. 2
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2.4. Contact angle measurements Contact angle measurements were carried out with both minerals in the absence and presence of PCP to examine the level of hydrophobicity with respect to pH variation. The method of measurement was based on the self-attachment of captive bubbles onto polished mineral specimens, which has been used in flotation research with and without some modification [32–34]. Pertinent features of the contact angle machine used in the present work can be found elsewhere [32]. The procedure involved checking the cleanliness of the polished surfaces of magnesite and dolomite by initial bubble contact, which was zero to ensure the surface hydrophilicity in the absence of the collector solution. The bubble size was selected to be in the range of 1.5 ± 0.2 mm in diameter from a glass capillary, which was controlled horizontally and vertically in a transparent cell, and a light source was placed behind the cell to illuminate the point of contact between the bubble and magnesite surface. The captive bubbles of air with a diameter of 1.5–2 mm was introduced at the tip of the glass capillary tube (1.0 mm diameter) were allowed to attach to the magnesite surface submerged in the solution. All measurements were carried out at 25 ± 2 ℃. Each measurement was repeated with up to five bubbles, and all bubbles attached were gently tapped from the bottom of the cell and kept in contact with the surface for 15–25 min before the mean value of the angle was reported. The contact angles obtained this way have been presumed to approach their equilibrium values. More details of the apparatus and measurement steps can be found in the literature cited above.
Fig. 3. The effect of added dolomite content on magnesite flotation at pH 9.0 in the presence of PCP as the collector [60 mg/L].
magnesite and dolomite particles in the presence of each other. Such a behavior observed in the PCP flotation resembles that which has been recently reported in dodecylamine flotation used for the separation of magnesite [3]. This particular case involved the recovery of quartz in the reverse flotation mode to achieve separation of magnesite. Mineralmineral interactions such as these can influence the selectivity of separation in the industrial application of flotation processes. There have been theoretical studies carried out in relation to flotation that can also be helpful to explain such observations [34-38]. One relevant approach makes use of DLVO (Derjaguin–Landau–Verwey–Overbeek) theory on the basis of differences in estimated interaction energies, which can promote particle aggregations during the conditioning and flotation stages [3,34]. A similar phenomenon is believed to have happened in the current system involving direct flotation of magnesite in the presence of dolomite. The PCP flotation of magnesite appears to be more sensitive to such interactions since the particle size of dolomite as the hydrophilic entity is much coarser (i.e., +25 μm) than that used in reverse flotation of quartz referred to earlier (i.e., particle size representing slimes at −5μm). However, the presence of some slime-like particles in the current system is not ruled out as it is possible that much finer particles that may have been generated during conditioning at 1100 rpm. It is also possible that the transfer of soluble species involving Ca (II) might also have played a role in the observed change of the flotation behaviour of magnesite, a point which has been corroborated by other researches [4–6]. It was shown that the dissolved Ca (II) species from calcite can affect surface chemistry of apatite [4]. Similarly, Ca(II) species dissolved from dolomite could precipitate on the magnesite surface, leading to changes in surface properties of magnesite [5]. Obviously, to induce favourable conditions to separate magnesite from dolomite efficiently, a modifying agent is required to achieve the selective recovery of magnesite. Sodium silicate is one of the most common modifiers successfully used for various flotation systems [39,40] and was chosen for this purpose. Fig. 4 shows the results of single mineral flotation experiments as a function of sodium silicate concentration, respectively. It can be seen that the magnesite recovery can be maintained at a high (90%+) level in the presence of sodium silicate, which represents a substantial improvement in its behaviour compared to the case in Fig. 3 that indicated a transition to severely non-floatable conditions in the absence of sodium silicate. It is interesting to note that an increase in sodium silicate concentration up to 800 mg/L shows no detrimental effect on the flotation of magnesite whereas dolomite can remain completely depressed in the same concentration range of sodium silicate used. Experiments carried out with the binary mixture of these minerals
2.5. XPS measurements X-ray photoelectron spectroscopy (XPS) measurements were conducted to assess the changes in the surface chemistry of magnesite and dolomite before and after treatment with the reagents used. XPS data were collected using a Kratos Axis Nova spectrometer equipped with an Al X-ray source (AlKα radiation at 1486.69 eV). Binding energies were referred to the C1s peak at 285 eV, and high-resolution spectra were recorded at a pass energy of 20 eV. In each test, 1.0 g samples with particle size less than 5 µm were conditioned with or without the reagents added in the same order as in micro-flotation experiments. Sample suspension was stirred for 30 min, then rinsed with deionized water for three times, dried in a high-vacuum oven at 40℃ for at least 48 h. 3. Results and discussion 3.1. Flotation behaviour A series of initial flotation experiments carried out with magnesite as a single mineral indicated that a quite high level of recoveries was achievable at low PCP concentrations in a wide range of pH values. In the subsequent tests, the concentration of this collector was kept at 60 mg/L as the desired dosage level to obtain reproducible recovery levels for magnesite. Under the same experimental conditions, the flotation activity for dolomite was very low. It was immediately noticeable that PCP might be used as a potential collector to achieve a highly selective separation between magnesite and dolomite, which has not been reported in any publications in the literature. However, the subsequent experiments carried out as a follow up indicated that individual behaviour of magnesite was observed to be adversely affected by the presence of dolomite in the same flotation environment. The results obtained with the binary mixtures of magnesite and dolomite are shown in Fig. 3. As can be noted, magnesite flotation recoveries remarkably decrease in the presence of a small percentage of dolomite. Furthermore, it was found that this adverse effect is particle size-dependent. The finer the size of the dolomite in the mixture, the sharper the decrease in the recovery of magnesite. These observations suggest that strong interactions occur between the 3
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Fig. 6. The variation of zeta potentials of magnesite and dolomite in deionized water as a function of pH adjusted with hydrochloric acid and sodium carbonate solutions.
Fig. 4. The effect of sodium silicate on flotation of magnesite and dolomite used as single minerals [PCP concentration = 60 mg/L, pH = 9.0].
experiments, the pH values lower than 4.5 were not explored due to the tendency of these carbonate minerals to dissolve under acidic conditions. The isoelectric point (IEP) values of magnesite and dolomite were determined to be in the vicinity of pH 7.5 and 6.4, respectively, which are in agreement in the range of previously reported results [6,41].For these minerals, the zeta potential values at pH 9.0, which is of interest from the flotation point of view, are −112.8 mV and −148.2 mV, respectively. The zeta potential measurements of magnesite and dolomite as a function of sodium silicate dosage at pH 9.0 in the absence and presence of PCP were also carried out, and the results are shown in Fig. 7. It can be seen that the zeta potential of magnesite in the presence of 200 mg/L sodium silicate became more negative by about 40.0 mV in reference to its absence, which was about −113 mV (Fig. 6). The difference can be attributable to the electrostatic adsorption of SiO (OH)3−, which is known to be the primary species of sodium silicate on magnesite surfaces [38,39]. In addition, the presence of the PCP surfactant causes a significant increase in the negative zeta potential by 64.7 mV, which is related to the adsorption of PCP on magnesite surface. As an anionic surfactant, PCP can be initially attracted to magnesium sites representing locally positive charges on the magnesite surface thus giving rise to the development of a more negatively charged surface on its surface in the presence of hydroxy silicate species.
Fig. 5. The effect of sodium silicate concentration on flotation of magnesite and dolomite in binary mixtures [PCP concentration = 60 mg/L, pH = 9.0].
indicated a greater sensitivity to the concentration of sodium silicate. The concentration range of silicate had to be reduced significantly to a low level below 200 mg/L to see some desirable results in the separation of magnesite from dolomite. Fig. 5 shows that the selective depression of dolomite is possible although the detrimental effect of dolomite on the flotation of magnesite has not been eliminated. As may be noted, magnesite floatability was adversely affected by increasing concentration of sodium silicate, with its recovery decreasing from about 94% at 80 mg/L to nearly 64% at 180 mg/L. However, the depression effect on floatability of dolomite is significantly greater with its recovery dropping nearly by 50% within the same range of silicate concentration. The variation of magnesite concentrate grade with its recovery is expected. These results suggest that relative effects of sodium silicate as a modifier on the floatability of these two minerals can be exploited for their separation using PCP as the collector. However, the dosage of each reagent needs to be optimized for improved separation to be attempted in a larger scale flotation investigation.
3.2. Zeta potential behaviour Fig. 6 shows the effect of pH level on zeta potentials of the two minerals in deionized water modified by the pH regulators, namely dilute solutions of hydrochloric acid and sodium carbonate. As may be noted, dolomite develops a more negative zeta potential profile compared to magnesite in the pH range from 4.5 to 11.5. In these
Fig. 7. The variation of zeta potentials of magnesite and dolomite as a function of sodium silicate concentrations in the absence and presence of PCP at pH 9.0 [PCP concentration = 60 mg/L]. 4
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Fig. 8. Variation of bubble contact angles in the absence and presence of PCP as a function of pH level with respect to use of sodium silicate [where applicable, PCP concentration = 60 mg/L, Sodium silicate concentration = 200 mg/L].
In contrast, a more significant level of adsorption of SiO(OH)3− species on the dolomite surface, is evident due to a much greater increase in the negative zeta potential of dolomite from −148.2 mV to −223.9 mV in the presence of 200 mg/L sodium silicate. However, no remarkable changes occurred with the addition of PCP as a manifestation of its poor surface affinity towards dolomite. It is believed that these results provide some additional insight into the selectivity observed in this flotation separation system. 3.3. Wettability behavior Contact angle behaviour as an expression of wettability can provide an additional understanding of the separation of minerals including those under current investigation [8,42,43]. Surface cleanliness before any measurements is an important prerequisite in these studies. Results of the captive bubble contact angle measurements for magnesite and dolomite before and after the treatment with sodium silicate and PCP are shown as a function of pH in Fig. 8. The contact angles of air bubbles on magnesite and dolomite surfaces in the absence of any reagents were zero in the several pH levels tested, thus confirming strong hydrophilicity of both mineral surfaces in deionized water. The measurements carried out after the conditioning stage in the presence of sodium silicate are also zero, as reported as a function of its concentrations in Fig. 8. In contrast, the contact angles measured after conditioning the magnesite specimen with PCP were consistently in the measurable range. An average contact angle of 36° was obtained at pH 9.0 treated with the 60 mg/L PCP concentration in the presence of 200 mg/L sodium silicate. The emergence of a hydrophobic condition on magnesite was quite distinct with the establishment of a stable three-phase contact throughout the experimentations. Under experimental conditions covering the pH range from 5 to 10.5, the contact angles did not exhibit a significant variation. In contrast, no bubbles have been captured by the dolomite surface even after conditioning with PCP. These differences in wettability correlate well with the results of the flotation behaviour discussed earlier. Thus, it is clear that PCP as a novel collector is capable of inducing selective hydrophobicity on magnesite giving rise to its flotation separation from dolomite as a common impurity (gangue) mineral.
Fig. 9. XPS patterns of magnesite (a) and dolomite (b) obtained in the absence and presence of sodium silicate and PCP during conditioning period.
surface chemistry of minerals, with the availability of a large database [44]. The XPS results for magnesite and dolomite in the absence and presence of sodium silicate and PCP were presented in Fig. 9. Detailed values of binding energy and offset of each element in the composition of magnesite and dolomite before and after treatment with sodium silicate and PCP are shown in Table 1. As can be noted from Fig. 9a, the treatment of magnesite with sodium silicate resulted in the development of no apparent peak that can be attributed to a Si-bearing surface entity on magnesite. However, when this mineral was treated with PCP as the collector in the presence of sodium silicate, a new peak that is related to the formation of a PTable 1 Changes of electron binding energy on magnesite and dolomite surfaces upon interactions with the reagents used in this study. Mineral
3.4. XPS studies The nature of the adsorption of PCP in the flotation system under study was investigated using XPS as an additional tool for analysis of 5
Atomic orbital
Binding energy /eV Magnesite
Mineral + Sodium silicate
Mineral + Sodium silicate + PCP
Magnesite
C1s O1s Mg2p
285.00 530.68 49.53
285.00 530.69 49.54
285.22 530.90 49.20
Dolomite
C1s O1s Mg2p Ca2p
285.00 530.68 49.53 346.68
285.41 531.21 49.85 347.08
285.44 531.19 49.96 347.09
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anion on magnesite. (3) Based on the results of XPS studies, the treatment of magnesite with PCP causes the development of new peaks, marking the formation of a P-bearing surface product on this mineral through strong chemical interactions. The dolomite surface does not have this feature when treated under similar conditions. However, it appears that by comparison, the dolomite surface has a more specific and strong interaction with silicate species from the addition of sodium silicate, which is marked by changes of electron binding energy of the atomic orbitals involving Ca2p, Mg2p, C1s and O1s bands.
bearing surface product has become noticeable. This can be interpreted as evidence of a chemisorption mechanism operating on the magnesite surface. In contrast, the conditioning of dolomite with sodium silicate, resulted in peak formation representing a Si-bearing surface product (see Fig. 9b). It is also important to note that there is no peak formation representing a P-bearing surface product on the dolomite surface after conditioning with PCP collector in the same way as in the case of magnesite. Such a difference suggests that there is an insufficient chemical affinity between PCP and the dolomite surface. These findings are consistent with the results of zeta potential measurements. Changes of electron binding energy on magnesite and dolomite surfaces before and after treatment with sodium silicate and PCP are shown in Table 1. Note that the binding energy for pure magnesite (Mg2p) is 49.53 eV, which shifts to 49.20 eV after treatment with PCP collector even in the presence of sodium silicate. Apart from Mg2p, the position of O1s band also has marked shifts when treated with the collector. Thus, it can be concluded that such changes of electron binding energy on magnesite surface are attributable to the chemical adsorption of PCP on Mg sites [44]. Another notable difference in the XPS patterns is related to the action of sodium silicate in this flotation system. There is a significant Ca2p band offset (0.40 eV) caused by treatment of dolomite surface with sodium silicate. In addition, the Mg2p, C1s and O1s bands also show a certain shift by 0.32, 0.41 and 0.53 eV, respectively. These characteristic shifts indicate that sodium silicate has a strong adsorption effect on dolomite surface. In contrast, it has a very weak adsorption behaviour on magnesite. More importantly, no further changes seem to have occurred after the treatment of dolomite with PCP as the collector. These findings suggest that much stronger adsorption of sodium silicate on the dolomite surfaces could be an additional cause for effective and consistent depression behaviour, which is connected to a lack of subsequent interactions with PCP. Therefore, the selective separation of dolomite in the presence of sodium silicate has been successfully accomplished. Further research is in progress to gather more details on additional aspects of the PCP flotation system for the selective separation of magnesite.
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4. Summary and conclusions A novel anionic surfactant, PCP, has been identified and successfully used to separate and concentrate magnesite from dolomite. Some fundamental aspects of surface chemistry of the two minerals have been investigated to characterize this flotation system through variations of zeta potentials and bubble contact angles, as well as XPS analysis. With the advantage of providing selective separation and concentration, PCP is likely to have a considerable number of potential applications in the flotation of calcium-bearing magnesite ores. An area of further research can be on the effect of this collector on the separation of magnesite from calcite and the interfacial phenomenon associated with such a separation system. While research work on the magnesite-dolomite separation is in progress and the following points can conclusively be made at this time: (1) Based on the results of flotation experiments with magnesite and dolomite as single minerals and with their 1:1 mixture, it is clear that PCP as a collector has a stronger collecting ability and much higher selectivity for magnesite than dolomite. As a depressant, sodium silicate has a selective depression effect on dolomite, thus significantly contributing to the flotation selectivity between magnesite and dolomite. (2) Based on the results of zeta potential and contact angle measurements, PCP is readily adsorbed on the magnesite surface, which causes more significant changes in the zeta potential behavior while inducing a significant level of hydrophobicity, in contrast to the cases with dolomite. These results from each type of experiments represent evidence for stable adsorption of the PCP monovalent 6
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133–139. [34] J. Yao, H. Han, Y. Hou, E. Gong, W. Yin, A method of calculating the interaction energy between particles in minerals flotation, Math. Probl. Eng. 2016 (2016). [35] R.H. Yoon, L. Mao, Application of extended DLVO theory, IV: derivation of flotation rate equation from first principles, J. Colloid Interf. Sci. 181 (1996) 613–626. [36] Z. Gao, W. Sun, Y. Hu, X. Liu, Surface energies and appearances of commonly exposed surfaces of scheelite crystal, T. Nonferr. Metal. Soc. 23 (2013) 2147–2152. [37] M. Zareei, H. Yoozbashizadeh, H.R.M. Hosseini, Investigating the effects of pH, surfactant and ionic strength on the stability of alumina/water nanofluids using DLVO theory, J. Therm. Anal. Calorim. 135 (2019) 1185–1196. [38] J. Wang, Z. Gao, Y. Gao, Y. Hu, W. Sun, Flotation separation of scheelite from calcite using mixed cationic/anionic collectors, Miner. Eng. 98 (2016) 261–263. [39] Z. Gao, D. Bai, W. Sun, X. Cao, Y. Hu, Selective flotation of scheelite from calcite and fluorite using a collector mixture, Miner. Eng. 72 (2015) 23–26. [40] G.W. Qi, A. Parentich, L.H. Little, L.J. Warren, Selective flotation of apatite from iron oxides, Int. J. Min. Process. 34 (1992) 83–102. [41] G.N. Anastassakis, A study on the separation of magnesite fines by magnetic carrier methods, Colloids Surf. A 149 (1999) 585–659. [42] J. Skvarla, S. Kmet, The Relationship between the wettability and aggregation of ultrafine minerals in Methanol-Water Mixtures, J. Adhes. 38 (1992) 19–30. [43] S. Duzyol, A. Ozkan, Role of hydrophobicity and surface tension on shear flocculation and oil agglomeration of magnesite, Sep. Purif. Technol. 72 (2010) 7–12. [44] A.V. Naumkin, A. Kraut-Vass, S.W. Gaarenstroom, C.J. Powell, NIST X-ray Photoelectron Spectroscopy Database, Version 4.1 (National Institute of Standards and Technology, Gaithersburg, 2012). http://srdata.nist.gov/xps/.
hydroxy-2-methyl-2-octenyl) phosphonic acid to cassiterite, T. Nonferr. Metal. Soc. 26 (2016) 2469–2478. G. Chen, D. Tao, Reverse flotation of magnesite by dodecyl phosphate from dolomite in the presence of sodium silicate, Sep. Sci. Technol. 39 (2004) 377–390. G. Zhou, J. Luo, Mechanism of flotation using mono-alkyl ester phosphoric acid for bastnaesite, J. Chin. Rare Earth Soc. 8 (1990) 261–264. G. Zhou, J. Luo, Mechanism of flotation using citric acid for separating monazite from bastnaesite, Nonferrous Met. 41 (1989) 33–40. Y. Tang, W. Yin, S. Kelebek, Selective flotation of magnesite from calcite using potassium cetyl phosphate as a collector in the presence of sodium silicate, Miner. Eng. 146 (2020) 106154, 1-4. doi: 10.1016/j.mineng.2019.106154. C.E. Gibson, R. Hansuld, S. Kelebek, M. Aghamirian, Behaviour of ilmenite as a gangue mineral in the benzohydroxamic flotation of a complex pyrochlore-bearing ore, Miner. Eng. 109 (2017) 98–108. W. Liu, X. Wang, Z. Wang, J.D. Miller, Flotation chemistry features in bastnaesite flotation with potassium lauryl phosphate, Miner. Eng. 85 (2016) 17–22. X. Zhang, H. Du, X. Wang, J.D. Miller, Surface chemistry considerations in the flotation of rare-earth and other semi-soluble salt minerals, Miner. Metall. Process. 30 (2013) 24–37. Ş. Kelebek, J.C. Donini, G.W. Smith, J.A. Finch, Wetting characteristics of hydrophobic minerals in relation to the surface tension of aqueous methanol solutions, J. Chem. Soc., Faraday Trans. 1 85 (1) (1989) 91, https://doi.org/10.1039/ f19898500091. J. Xue, P. Shi, L. Zhu, J. Ding, Q. Chen, Q. Wang, A modified captive bubble method for determining advancing and receding contact angles, Appl. Surf. Sci. 296 (2014)
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Full Length Article
Magnesite-dolomite separation using potassium cetyl phosphate as a novel flotation collector and related surface chemistry ⁎
Yuan Tanga,b, Wanzhong Yina, , Sadan Kelebekb, a b
T
⁎
School of Resources and Civil Engineering, Northeastern University, Shenyang 110819, Liaoning, China The Robert M. Buchan Department of Mining, Queen’s University, Kingston, Ontario K7L 3N6, Canada
A R T I C LE I N FO
A B S T R A C T
Keywords: Potassium cetyl phosphate Sodium silicate Flotation Magnesite Dolomite
To investigate the separation effect of magnesite from dolomite using potassium cetyl phosphate (PCP) as a collector, flotation experiments were carried out. In this study, the results obtained from the single minerals experiments showed an excellent selectivity with high recoveries for magnesite using PCP as the collector in contrast to the case with dolomite. The separation effect of the binary mixtures at pulp pH 9.0 was also investigated. The results confirmed that the PCP is an appropriate collector for the flotation separation of magnesite from dolomite. Surface chemical aspects of the separation mechanism were investigated through measurements of zeta potential, contact angle, and X-ray photoelectron spectroscopy analysis. The results support the flotation selectivity observed and are consistent with chemisorption of PCP on magnesite in the presence of sodium silicate that was used as a modifier to stabilize hydrophilicity of dolomite and remove its adverse effect on the recovery of magnesite.
1. Introduction Magnesite (MgCO3) and dolomite (MgCa(CO3)2) are important mineral commodities, which are widely utilized in the agriculture, chemical, construction, environmental, metallurgical and other industries [1]. However, in natural magnesite ore deposits, magnesite often coexists with dolomite [2]. With the increasing market demand, for the production of high-quality magnesite, highly selective separation of impurities such as dolomite is essential in order to reduce the calcium content [3–5]. This means that the impurity minerals coexisting in the ore deposit (i.e., gangue minerals) need to be efficiently depressed since froth flotation is the most effective industrial method used for the separation of magnesite from other carbonates [6,7]. The flotation separation is primarily based on the physicochemical differences between the surfaces of valuable and associated minerals. Dolomite often causes significant difficulties in the separation process since it has a crystal structure that is similar to that of magnesite and both have the same Mg-active sites on their dominant cleavage surfaces [8,9]. As the salt-type minerals, magnesite and dolomite are sparingly soluble and give lattice ions, which lead to similar surface chemistries in the process of environments [9,10]. Hence, various surfactants are used to improve the difference in surface properties. Clear-cut separation of minerals requires selective adsorption of collectors on the mineral surface to enhance its surface hydrophobicity, which is conducive ⁎
to increasing the floatability of minerals [11,12]. The commonly used cationic and anionic collectors show strong collecting ability for magnesite and dolomite as well as other associated minerals [13–15]. The main problem for those collectors, obviously, is the low level of selectivity in the absence of modifiers. Consequently, there is a need that has been recognized among researchers to identify a more selective collector action on such minerals. To achieve more effective separation, mixed collectors have been used to exhibit enhanced recovery and selectivity for different kinds of minerals, and most studies have focused on the combination of traditional collectors, as reported recently [15–17]. In addition to the mixed collectors, a series of novel surfactants have been reported to be effective for some oxide and silicate minerals. For example, Weng et al., reported a quaternary ammonium surfactant for reverse flotation of silicates [18]. An improved collecting ability has been demonstrated for quartz using a Gemini surfactant (containing two quaternary ammonium groups) as a collector [19,20]. However, most studies found so far are limited to the cationic collectors, which have been reported to be effective in reverse flotation applications [20,21]. In addition, some surfactants with phosphorous-bearing functional groups have been reported to exhibit enhanced response as collectors for a variety of minerals, such as rare-earth minerals [22,23], oxidized minerals [23,25], and dolomite as a carbonate mineral [25]. Based on the infrared spectrum analysis, the eP]O group has been shown to
Corresponding authors. E-mail addresses: [email protected] (W. Yin), [email protected] (S. Kelebek).
https://doi.org/10.1016/j.apsusc.2019.145191 Received 19 October 2019; Received in revised form 19 November 2019; Accepted 24 December 2019 Available online 28 December 2019 0169-4332/ © 2019 Elsevier B.V. All rights reserved.
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was set at 60 mg/L as the desired concentration level in the current experimental work to be reported. Analytically pure sodium silicate (Na2SiO3·9H2O) was supplied by Fisher Scientific Company, USA. It was used at concentrations up to 800 mg/L. The slurry pH level was adjusted with hydrochloric acid (HCl) and sodium carbonate (Na2CO3) solution. Deionized water with a resistivity of above 18 MΩ · cm used in all experiments was sourced from a laboratory ultrapure water system (Thermo-Fisher Scientific, Ontario, Canada).
exhibit a strong reaction with the cationic sites on various minerals involving chemisorption [23–27]. More specifically, dodecyl phosphate was reported to play an active role in the flotation of dolomite from magnesite, with both cases involving chemisorption [25]. Although the results indicate that the phosphate group connected to lauryl chain might be an effective functional group for recovery of dolomite, there has been no study reported for direct flotation of magnesite using potassium cetyl phosphate (PCP) as an anionic collector with a longer hydrocarbon. In a previous communication, we reported some results on selective flotation of magnesite from calcite using PCP as a novel anionic collector [28]. As a continuation, we extended our investigation into its effect on the separation of magnesite from dolomite. A variety of experiments were conducted to characterize the PCP flotation of magnesite and to be able to explain selective separation of magnesite from dolomite as a gangue mineral. Experimental investigations to deduce the adsorption mechanism of PCP in this flotation system included measurements of zeta potentials and contact angles as well as X-ray photoelectron spectroscopy (XPS) analysis. It is anticipated that the findings of this research will provide a fundamental understanding of the flotation chemistry of carbonate minerals using alkyl phosphate as an anionic collector while generating interest in possible application of longer chain phosphates as selective surfactants for mineral separations.
2.2. Flotation tests The flotation tests for single minerals and mixed binary minerals with PCP as the collector were carried out with a vertical micro-flotation cell of 25 cm using the general procedure reported previously [29]. Typically, a 1.0 g sample (75–150 μm) was added to the cell with the deionized water and then conditioned for 2 min by magnetic stirring bar at a speed of 1100 rpm. The pH modifier was then introduced, which was followed by conditioning with depressant for 2 min time intervals. Subsequently, the collector solution was added and conditioned for another 2 min. The collection of the froth product was continued for 3 min, and concentrates and tailings were filtered, dried and weighed for estimation of recovery. The flowsheet of single magnesite flotation is presented in Fig. 2. The average recovery plotted represents three repeat tests. For the separation case from binary mixtures (0.5 g magnesite and 0.5 g dolomite), the flotation products were assayed for Mg and Ca by using an atomic absorption spectrometer (AAS, iCE3000).
2. Materials and methods
2.3. Zeta potential measurements
2.1. Materials and reagents
The zeta potential measurements were carried out using a Zeta Potential Finder Analyzer (Matec Applied Science, USA) at room temperature (25 ± 2 ℃) to determine the relationship between the collector action and the surface charge of particles [30,31]. The Ludox TM suspension (10 wt%, with a standard zeta potential of −38 mV) was used to calibrate the analyzer. For each measurement, a dilute mineral suspension was prepared by adding 1.2 g of mineral to 200 ml deionized water, mixing constantly for 15 min. Reagents were added in the same order and the same time intervals as in flotation tests. The pH values were adjusted by adding 1% HCl or 3% Na2CO3 solutions to the slurry. Each measurement was repeated three times, and the average values were reported as the final results.
The magnesite sample, obtained from Refratechnik group (British Columbia, Canada) was used in this research. In addition, pure dolomite blocks were acquired from Ward’s Science (New York, USA). Hand-picked specimens from each mineral were crushed and dry ground to 75-150 μm, 45-75 μm, and 25-45μm for the micro-flotation tests. Both minerals were further ground to less than 5 µm in an agate mortar were used for zeta potential measurements. Analysis of the magnesite and dolomite samples based on the carbon content that was determined using a dual Carbon-Sulphur analyzer (CS-2000, ELTRA GmbH) indicated a high purity for each mineral at 98.4% and 96.4%, respectively. Fig. 1 shows results of the X-ray powder diffraction analysis of each mineral taken using a laboratory diffractometer (MPDDY2094, PANalytical B.V.), which confirms the high level purity of these samples. Potassium cetyl phosphate was acquired from the research laboratories of Colonial Chemical Inc., (South Pittsburg, Pennsylvania, USA). Its aqueous solutions ranging from 20 to 120 mg/L were used during the initial series of exploratory tests. Subsequently, its concentration
Fig. 2. Flowchart of magnesite flotation test procedure.
Fig. 1. XRD patters of magnesite (top) and dolomite (bottom) samples used. 2
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2.4. Contact angle measurements Contact angle measurements were carried out with both minerals in the absence and presence of PCP to examine the level of hydrophobicity with respect to pH variation. The method of measurement was based on the self-attachment of captive bubbles onto polished mineral specimens, which has been used in flotation research with and without some modification [32–34]. Pertinent features of the contact angle machine used in the present work can be found elsewhere [32]. The procedure involved checking the cleanliness of the polished surfaces of magnesite and dolomite by initial bubble contact, which was zero to ensure the surface hydrophilicity in the absence of the collector solution. The bubble size was selected to be in the range of 1.5 ± 0.2 mm in diameter from a glass capillary, which was controlled horizontally and vertically in a transparent cell, and a light source was placed behind the cell to illuminate the point of contact between the bubble and magnesite surface. The captive bubbles of air with a diameter of 1.5–2 mm was introduced at the tip of the glass capillary tube (1.0 mm diameter) were allowed to attach to the magnesite surface submerged in the solution. All measurements were carried out at 25 ± 2 ℃. Each measurement was repeated with up to five bubbles, and all bubbles attached were gently tapped from the bottom of the cell and kept in contact with the surface for 15–25 min before the mean value of the angle was reported. The contact angles obtained this way have been presumed to approach their equilibrium values. More details of the apparatus and measurement steps can be found in the literature cited above.
Fig. 3. The effect of added dolomite content on magnesite flotation at pH 9.0 in the presence of PCP as the collector [60 mg/L].
magnesite and dolomite particles in the presence of each other. Such a behavior observed in the PCP flotation resembles that which has been recently reported in dodecylamine flotation used for the separation of magnesite [3]. This particular case involved the recovery of quartz in the reverse flotation mode to achieve separation of magnesite. Mineralmineral interactions such as these can influence the selectivity of separation in the industrial application of flotation processes. There have been theoretical studies carried out in relation to flotation that can also be helpful to explain such observations [34-38]. One relevant approach makes use of DLVO (Derjaguin–Landau–Verwey–Overbeek) theory on the basis of differences in estimated interaction energies, which can promote particle aggregations during the conditioning and flotation stages [3,34]. A similar phenomenon is believed to have happened in the current system involving direct flotation of magnesite in the presence of dolomite. The PCP flotation of magnesite appears to be more sensitive to such interactions since the particle size of dolomite as the hydrophilic entity is much coarser (i.e., +25 μm) than that used in reverse flotation of quartz referred to earlier (i.e., particle size representing slimes at −5μm). However, the presence of some slime-like particles in the current system is not ruled out as it is possible that much finer particles that may have been generated during conditioning at 1100 rpm. It is also possible that the transfer of soluble species involving Ca (II) might also have played a role in the observed change of the flotation behaviour of magnesite, a point which has been corroborated by other researches [4–6]. It was shown that the dissolved Ca (II) species from calcite can affect surface chemistry of apatite [4]. Similarly, Ca(II) species dissolved from dolomite could precipitate on the magnesite surface, leading to changes in surface properties of magnesite [5]. Obviously, to induce favourable conditions to separate magnesite from dolomite efficiently, a modifying agent is required to achieve the selective recovery of magnesite. Sodium silicate is one of the most common modifiers successfully used for various flotation systems [39,40] and was chosen for this purpose. Fig. 4 shows the results of single mineral flotation experiments as a function of sodium silicate concentration, respectively. It can be seen that the magnesite recovery can be maintained at a high (90%+) level in the presence of sodium silicate, which represents a substantial improvement in its behaviour compared to the case in Fig. 3 that indicated a transition to severely non-floatable conditions in the absence of sodium silicate. It is interesting to note that an increase in sodium silicate concentration up to 800 mg/L shows no detrimental effect on the flotation of magnesite whereas dolomite can remain completely depressed in the same concentration range of sodium silicate used. Experiments carried out with the binary mixture of these minerals
2.5. XPS measurements X-ray photoelectron spectroscopy (XPS) measurements were conducted to assess the changes in the surface chemistry of magnesite and dolomite before and after treatment with the reagents used. XPS data were collected using a Kratos Axis Nova spectrometer equipped with an Al X-ray source (AlKα radiation at 1486.69 eV). Binding energies were referred to the C1s peak at 285 eV, and high-resolution spectra were recorded at a pass energy of 20 eV. In each test, 1.0 g samples with particle size less than 5 µm were conditioned with or without the reagents added in the same order as in micro-flotation experiments. Sample suspension was stirred for 30 min, then rinsed with deionized water for three times, dried in a high-vacuum oven at 40℃ for at least 48 h. 3. Results and discussion 3.1. Flotation behaviour A series of initial flotation experiments carried out with magnesite as a single mineral indicated that a quite high level of recoveries was achievable at low PCP concentrations in a wide range of pH values. In the subsequent tests, the concentration of this collector was kept at 60 mg/L as the desired dosage level to obtain reproducible recovery levels for magnesite. Under the same experimental conditions, the flotation activity for dolomite was very low. It was immediately noticeable that PCP might be used as a potential collector to achieve a highly selective separation between magnesite and dolomite, which has not been reported in any publications in the literature. However, the subsequent experiments carried out as a follow up indicated that individual behaviour of magnesite was observed to be adversely affected by the presence of dolomite in the same flotation environment. The results obtained with the binary mixtures of magnesite and dolomite are shown in Fig. 3. As can be noted, magnesite flotation recoveries remarkably decrease in the presence of a small percentage of dolomite. Furthermore, it was found that this adverse effect is particle size-dependent. The finer the size of the dolomite in the mixture, the sharper the decrease in the recovery of magnesite. These observations suggest that strong interactions occur between the 3
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Fig. 6. The variation of zeta potentials of magnesite and dolomite in deionized water as a function of pH adjusted with hydrochloric acid and sodium carbonate solutions.
Fig. 4. The effect of sodium silicate on flotation of magnesite and dolomite used as single minerals [PCP concentration = 60 mg/L, pH = 9.0].
experiments, the pH values lower than 4.5 were not explored due to the tendency of these carbonate minerals to dissolve under acidic conditions. The isoelectric point (IEP) values of magnesite and dolomite were determined to be in the vicinity of pH 7.5 and 6.4, respectively, which are in agreement in the range of previously reported results [6,41].For these minerals, the zeta potential values at pH 9.0, which is of interest from the flotation point of view, are −112.8 mV and −148.2 mV, respectively. The zeta potential measurements of magnesite and dolomite as a function of sodium silicate dosage at pH 9.0 in the absence and presence of PCP were also carried out, and the results are shown in Fig. 7. It can be seen that the zeta potential of magnesite in the presence of 200 mg/L sodium silicate became more negative by about 40.0 mV in reference to its absence, which was about −113 mV (Fig. 6). The difference can be attributable to the electrostatic adsorption of SiO (OH)3−, which is known to be the primary species of sodium silicate on magnesite surfaces [38,39]. In addition, the presence of the PCP surfactant causes a significant increase in the negative zeta potential by 64.7 mV, which is related to the adsorption of PCP on magnesite surface. As an anionic surfactant, PCP can be initially attracted to magnesium sites representing locally positive charges on the magnesite surface thus giving rise to the development of a more negatively charged surface on its surface in the presence of hydroxy silicate species.
Fig. 5. The effect of sodium silicate concentration on flotation of magnesite and dolomite in binary mixtures [PCP concentration = 60 mg/L, pH = 9.0].
indicated a greater sensitivity to the concentration of sodium silicate. The concentration range of silicate had to be reduced significantly to a low level below 200 mg/L to see some desirable results in the separation of magnesite from dolomite. Fig. 5 shows that the selective depression of dolomite is possible although the detrimental effect of dolomite on the flotation of magnesite has not been eliminated. As may be noted, magnesite floatability was adversely affected by increasing concentration of sodium silicate, with its recovery decreasing from about 94% at 80 mg/L to nearly 64% at 180 mg/L. However, the depression effect on floatability of dolomite is significantly greater with its recovery dropping nearly by 50% within the same range of silicate concentration. The variation of magnesite concentrate grade with its recovery is expected. These results suggest that relative effects of sodium silicate as a modifier on the floatability of these two minerals can be exploited for their separation using PCP as the collector. However, the dosage of each reagent needs to be optimized for improved separation to be attempted in a larger scale flotation investigation.
3.2. Zeta potential behaviour Fig. 6 shows the effect of pH level on zeta potentials of the two minerals in deionized water modified by the pH regulators, namely dilute solutions of hydrochloric acid and sodium carbonate. As may be noted, dolomite develops a more negative zeta potential profile compared to magnesite in the pH range from 4.5 to 11.5. In these
Fig. 7. The variation of zeta potentials of magnesite and dolomite as a function of sodium silicate concentrations in the absence and presence of PCP at pH 9.0 [PCP concentration = 60 mg/L]. 4
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Fig. 8. Variation of bubble contact angles in the absence and presence of PCP as a function of pH level with respect to use of sodium silicate [where applicable, PCP concentration = 60 mg/L, Sodium silicate concentration = 200 mg/L].
In contrast, a more significant level of adsorption of SiO(OH)3− species on the dolomite surface, is evident due to a much greater increase in the negative zeta potential of dolomite from −148.2 mV to −223.9 mV in the presence of 200 mg/L sodium silicate. However, no remarkable changes occurred with the addition of PCP as a manifestation of its poor surface affinity towards dolomite. It is believed that these results provide some additional insight into the selectivity observed in this flotation separation system. 3.3. Wettability behavior Contact angle behaviour as an expression of wettability can provide an additional understanding of the separation of minerals including those under current investigation [8,42,43]. Surface cleanliness before any measurements is an important prerequisite in these studies. Results of the captive bubble contact angle measurements for magnesite and dolomite before and after the treatment with sodium silicate and PCP are shown as a function of pH in Fig. 8. The contact angles of air bubbles on magnesite and dolomite surfaces in the absence of any reagents were zero in the several pH levels tested, thus confirming strong hydrophilicity of both mineral surfaces in deionized water. The measurements carried out after the conditioning stage in the presence of sodium silicate are also zero, as reported as a function of its concentrations in Fig. 8. In contrast, the contact angles measured after conditioning the magnesite specimen with PCP were consistently in the measurable range. An average contact angle of 36° was obtained at pH 9.0 treated with the 60 mg/L PCP concentration in the presence of 200 mg/L sodium silicate. The emergence of a hydrophobic condition on magnesite was quite distinct with the establishment of a stable three-phase contact throughout the experimentations. Under experimental conditions covering the pH range from 5 to 10.5, the contact angles did not exhibit a significant variation. In contrast, no bubbles have been captured by the dolomite surface even after conditioning with PCP. These differences in wettability correlate well with the results of the flotation behaviour discussed earlier. Thus, it is clear that PCP as a novel collector is capable of inducing selective hydrophobicity on magnesite giving rise to its flotation separation from dolomite as a common impurity (gangue) mineral.
Fig. 9. XPS patterns of magnesite (a) and dolomite (b) obtained in the absence and presence of sodium silicate and PCP during conditioning period.
surface chemistry of minerals, with the availability of a large database [44]. The XPS results for magnesite and dolomite in the absence and presence of sodium silicate and PCP were presented in Fig. 9. Detailed values of binding energy and offset of each element in the composition of magnesite and dolomite before and after treatment with sodium silicate and PCP are shown in Table 1. As can be noted from Fig. 9a, the treatment of magnesite with sodium silicate resulted in the development of no apparent peak that can be attributed to a Si-bearing surface entity on magnesite. However, when this mineral was treated with PCP as the collector in the presence of sodium silicate, a new peak that is related to the formation of a PTable 1 Changes of electron binding energy on magnesite and dolomite surfaces upon interactions with the reagents used in this study. Mineral
3.4. XPS studies The nature of the adsorption of PCP in the flotation system under study was investigated using XPS as an additional tool for analysis of 5
Atomic orbital
Binding energy /eV Magnesite
Mineral + Sodium silicate
Mineral + Sodium silicate + PCP
Magnesite
C1s O1s Mg2p
285.00 530.68 49.53
285.00 530.69 49.54
285.22 530.90 49.20
Dolomite
C1s O1s Mg2p Ca2p
285.00 530.68 49.53 346.68
285.41 531.21 49.85 347.08
285.44 531.19 49.96 347.09
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anion on magnesite. (3) Based on the results of XPS studies, the treatment of magnesite with PCP causes the development of new peaks, marking the formation of a P-bearing surface product on this mineral through strong chemical interactions. The dolomite surface does not have this feature when treated under similar conditions. However, it appears that by comparison, the dolomite surface has a more specific and strong interaction with silicate species from the addition of sodium silicate, which is marked by changes of electron binding energy of the atomic orbitals involving Ca2p, Mg2p, C1s and O1s bands.
bearing surface product has become noticeable. This can be interpreted as evidence of a chemisorption mechanism operating on the magnesite surface. In contrast, the conditioning of dolomite with sodium silicate, resulted in peak formation representing a Si-bearing surface product (see Fig. 9b). It is also important to note that there is no peak formation representing a P-bearing surface product on the dolomite surface after conditioning with PCP collector in the same way as in the case of magnesite. Such a difference suggests that there is an insufficient chemical affinity between PCP and the dolomite surface. These findings are consistent with the results of zeta potential measurements. Changes of electron binding energy on magnesite and dolomite surfaces before and after treatment with sodium silicate and PCP are shown in Table 1. Note that the binding energy for pure magnesite (Mg2p) is 49.53 eV, which shifts to 49.20 eV after treatment with PCP collector even in the presence of sodium silicate. Apart from Mg2p, the position of O1s band also has marked shifts when treated with the collector. Thus, it can be concluded that such changes of electron binding energy on magnesite surface are attributable to the chemical adsorption of PCP on Mg sites [44]. Another notable difference in the XPS patterns is related to the action of sodium silicate in this flotation system. There is a significant Ca2p band offset (0.40 eV) caused by treatment of dolomite surface with sodium silicate. In addition, the Mg2p, C1s and O1s bands also show a certain shift by 0.32, 0.41 and 0.53 eV, respectively. These characteristic shifts indicate that sodium silicate has a strong adsorption effect on dolomite surface. In contrast, it has a very weak adsorption behaviour on magnesite. More importantly, no further changes seem to have occurred after the treatment of dolomite with PCP as the collector. These findings suggest that much stronger adsorption of sodium silicate on the dolomite surfaces could be an additional cause for effective and consistent depression behaviour, which is connected to a lack of subsequent interactions with PCP. Therefore, the selective separation of dolomite in the presence of sodium silicate has been successfully accomplished. Further research is in progress to gather more details on additional aspects of the PCP flotation system for the selective separation of magnesite.
Acknowledgements This work is financially supported by the National Natural Science Foundation of China (51874072) and also through Yuan’s fellowship provided by the China Scholarship Council (No. 201806080082) to support part of his PhD studies as a visiting research fellow at Queen’s University, in Canada. Some of the research expenses has been covered through a discovery grant from NSERC, Canada. References [1] J.E. Kogel, N.C. Trivedi, J.M. Barker, S.T. Krukowski, Industrial Minerals and Rocks – Commodities, Markets, and Uses, 7th ed. SME, CO, USA, 2006. Retrieved from https://app.knovel.com /hotlink/toc/id: kpIMRCMUE1/industrial-minerals-rocks/ industrial-minerals-rocks. [2] W.A. Deer, R.A. Howie, J. Zussman, An Introduction to the Rock-forming Minerals, third ed., The Mineralogical Society, London, U.K., 2013. [3] J. Yao, W. Yin, E. Gong, Depressing effect of fine hydrophilic particles on magnesite reverse flotation, Int. J. Miner. Process. 149 (2016) 84–93. [4] J.O. Amankonah, P. Somasundaran, Effects of dissolved mineral species on the electrokinetic behavior of calcite and apatite, Colloids Surf. 15 (1985) 335–353. [5] N. Luo, D. Wei, Y. Shen, C. Han, C. Zhang, Elimination of the adverse effect of calcium ion on the flotation separation of magnesite from dolomite, Minerals 7 (2017) 150. [6] H. Zhang, W. Liu, C. Han, H. Hao, Effects of monohydric alcohols on the flotation of magnesite and dolomite by sodium oleate, J. Mol. Liq. 249 (2018) 1060–1067. [7] O. Kangal, A.A. Sirkeci, A. Güney, Flotation behaviour of huntite (Mg3Ca(CO3)4) with anionic collectors, Int. J. Miner. Process. 75 (2005) 31–39. [8] N. Gence, Wetting behavior of magnesite and dolomite surfaces, Appl. Surf. Sci. 252 (2006) 3744–3750. [9] K.A. Matis, G.P. Gallios, Anionic flotation of magnesium carbonates by modifiers, Int. J. Miner. Process. 25 (1989) 261–274. [10] G. Nermin, O. Nurgul, pH dependence of electrokinetic behavior of dolomite and magnesite in aqueous electrolyte solutions, Appl. Surf. Sci. 252 (2006) 8057–8061. [11] E. Forbes, Shear, selective and temperature responsive flocculation: a comparison of fine particle flotation techniques, Int. J. Miner. Process. 99 (1–4) (2011) 1–10. [12] H. Zhang, W. Liu, C. Han, D. Wei, Intensify dodecylamine adsorption on magnesite and dolomite surfaces by monohydric alcohols, Appl. Surf. Sci. 444 (2018) 729–738. [13] I. Florek, The effects of radiation pretreatment on the floatability of magnesite and siderite, Miner. Eng. 8 (1995) 329–331. [14] P.R.G. Brando, G.W. Poling, Anionic flotation of magnesite, Can. Metall. Q. 3 (1982) 211–220. [15] D. Wonyen, V. Kromah, B. Gibson, S. Nah, S. Chelgani, A review of flotation separation of Mg carbonates (Dolomite and Magnesite), Minerals 8 (2018) 354. [16] J. Penfold, E. Staples, P. Cummins, I. Tucker, L. Thompson, R.K. Thomas, E.A. Simister, J.R. Lu, Adsorption ofmixed cationic-non-ionic surfactants at the air/ water interface, J. Chem. Soc. Faraday Trans. 92 (1996) 1773–1779. [17] L. Xu, Y. Hu, J. Tian, H. Wu, L. Wang, Y. Yang, Z. Wang, Synergistic effect of mixed cat ionic/anionic collectors on flotation and adsorption of muscovite, Colloids Surf. A Physicochem. Eng. Asp. 492 (2016) 181–189. [18] X. Weng, G. Mei, T. Zhao, Y. Zhu, Utilization of novel ester-containing quaternary ammonium surfactant as cationic collector for iron ore flotation, Sep. Purif. Technol. 103 (2013) 187–194. [19] Z. Huang, H. Zhong, S. Wang, L. Xia, W. Zou, G. Liu, Investigations on reverse cationic flotation of iron ore by using a Gemini surfactant: ethane-1, 2-bis (dimethyl-dodecyl-ammonium bromide), Chem. Eng. J. 257 (2014) 218–228. [20] L. Xia, H. Zhong, G. Liu, Z. Huang, Q. Chang, Flotation separation of the aluminosilicates from diaspore by a Gemini cationic collector, Int. J. Miner. Process. 92 (2009) 74–83. [21] A.M. Vieira, A.E. Peres, The effect of amine type, pH, and size range in the flotation of quartz, Miner. Eng. 20 (2007) 1008–1013. [22] W. Liu, L.W. McDonald IV, X. Wang, J.D. Miller, Bastnaesite flotation chemistry issues associated with alkyl phosphate collectors, Miner. Eng. 127 (2018) 286–295. [23] F. Li, H. Zhong, G. Zhao, S. Wang, G. Liu, Flotation performances and adsorption mechanism of α-hydroxyoctyl phosphinic acid to cassiterite, Appl. Surf. Sci. 353 (2015) 856–864. [24] X. Tan, F. He, Y. Shang, W. Yin, Flotation behavior and adsorption mechanism of (1-
4. Summary and conclusions A novel anionic surfactant, PCP, has been identified and successfully used to separate and concentrate magnesite from dolomite. Some fundamental aspects of surface chemistry of the two minerals have been investigated to characterize this flotation system through variations of zeta potentials and bubble contact angles, as well as XPS analysis. With the advantage of providing selective separation and concentration, PCP is likely to have a considerable number of potential applications in the flotation of calcium-bearing magnesite ores. An area of further research can be on the effect of this collector on the separation of magnesite from calcite and the interfacial phenomenon associated with such a separation system. While research work on the magnesite-dolomite separation is in progress and the following points can conclusively be made at this time: (1) Based on the results of flotation experiments with magnesite and dolomite as single minerals and with their 1:1 mixture, it is clear that PCP as a collector has a stronger collecting ability and much higher selectivity for magnesite than dolomite. As a depressant, sodium silicate has a selective depression effect on dolomite, thus significantly contributing to the flotation selectivity between magnesite and dolomite. (2) Based on the results of zeta potential and contact angle measurements, PCP is readily adsorbed on the magnesite surface, which causes more significant changes in the zeta potential behavior while inducing a significant level of hydrophobicity, in contrast to the cases with dolomite. These results from each type of experiments represent evidence for stable adsorption of the PCP monovalent 6
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