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Study on the mechanism and application of a novel collector-complexes in cassiterite flotation
Accepted Manuscript Title: Study on the mechanism and application of a novel collector-complexes in cassiterite flotation Author: Mengjie Tian Yuehua Hu Wei Sun Runqing Liu PII: DOI: Reference:
S0927-7757(17)30185-1 http://dx.doi.org/doi:10.1016/j.colsurfa.2017.02.051 COLSUA 21406
To appear in:
Colloids and Surfaces A: Physicochem. Eng. Aspects
Received date: Revised date: Accepted date:
15-10-2016 19-2-2017 20-2-2017
Please cite this article as: M. Tian, Y. Hu, W. Sun, R. Liu, Study on the mechanism and application of a novel collector-complexes in cassiterite flotation, Colloids and Surfaces A: Physicochemical and Engineering Aspects (2017), http://dx.doi.org/10.1016/j.colsurfa.2017.02.051 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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*Graphical Abstract (for review)
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*Highlights (for review)
Highlights 1. The complexes formed by lead(Ⅱ) and hydroxides with BHA are regarded as a novel collector. 2. The superior collecting ability of Pb2+/Pb(OH)+-BHA complexes is observed.
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3. The complexes absorb on the mineral surface as a result of electrostatic attraction
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and chemical adsorption.
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*Manuscript
Study on the mechanism and application of a novel collector-complexes in cassiterite flotation Mengjie Tian, Yuehua Hu, Wei Sun*, Runqing Liu**
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School of Minerals Processing and Bioengineering, Central South University,
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Changsha 410083, China *
Corresponding author
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E-mail address: [email protected]；[email protected]
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Abstract
Benzohydroxamic acid (BHA) has been widely used as the collector for
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cassiterite flotation, whereas low collecting ability of BHA need to be improved. It is
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shown through flotation that BHA exhibits weak collecting capacity to cassiterite even in the case of using lead nitrate as the activator, but the superior floatation
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performances of lead(Ⅱ)-BHA complexes is observed. The adsorption isotherms of BHA onto cassiterite reveal a higher adsorption density around pH 9 using lead(Ⅱ)-BHA complexes as the collector. The complexation of lead(Ⅱ) by BHA has
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been studied by differential pulse voltammetry. In weak alkaline solution, [Pb(HL)]+ and [Pb(OH)(HL)] species with significantly collecting power are well predominant. Zeta potential studies demonstrate a stronger affinity to the mineral surface of Pb2+/Pb(OH)+-BHA complexes. XPS and FTIR studies indicate that chemisorption with the similar formation of Sn-O-Pb bond is the main interaction between the two kinds of collector and cassiterite. Keywords: cassiterite; flotation; BHA; lead ion; complexes
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Introduction The gravity separation process is a main beneficiation approach for recovery of cassiterite mineral. Only 50-70% fine cassiterite particles could be recovered through
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gravity separation technique[1]. As an alternative technique, flotation has been proved to be a more attractive method for recovery of fine cassiterite particle [2; 3]. Previous
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investigations have found that fine cassiterite particles could be effectively floated out
suffer a low selectivity against gangue minerals.
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by using oleic acid or alkyl sulfate as the collector [4; 5]. However, these collectors
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Commonly, hydroxamate collectors exhibit superior selectivity in the flotation
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process [6; 7; 8]. BHA could easily perform the flotation separation of cassiterite from quartz and calcite compared with other hydroxamic acids [9; 10; 11]. During flotation,
ed
metal ions are usually used as activators to improve the collecting power of BHA [12; 13]. Lead nitrate is used as the activator in the flotation with hydroxamic acid [14; 15].
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BHA is one of chelating reagents which are a particular class of complexing reagents consisting of large organic molecules capable of bonding to the metal ion via two or
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more functional groups, with the formation of one or more rings. It has been reported that hydroxamic acid can chelate with metal ions and metal hydroxyl ion to form five-member ring complexes [16]. As a result, research of coordination of BHA with lead ions can help to improve its collecting ability to cassiterite, and study the activation mechanism of lead nitrate and the design of a novel collector. The paper focuses on the effect of lead (Ⅱ) on the adsorption capacity of BHA and the discussion of the novel kind of collector——Pb2+/Pb(OH)+-BHA complexes.
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To elucidate the adsorption mechanisms of two kinds of collector——BHA and Pb2+/Pb(OH)+-BHA complexes on cassiterite, adsorption measurements, infrared and X-ray photoelectron spectroscopy (XPS) studies are employed. Simultaneously,
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determination of the stability constants of dissolved lead (II)–BHA system in using differential pulse voltammetry is performed.
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2. Experimental
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2.1. Materials and reagents
Hand-picked natural pure samples of cassiterite are obtained from Yunnan
an
province in China. Mineralogical and X-ray diffraction data confirms that the
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cassiterite is high 99% purity. The samples are ground in a pottery ball mill and the -75+38μm fractions are used in flotation and adsorption tests. The -38μm samples are
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further ground in an agate mortar to obtain particles of size less than 5μm for Fourier transform infrared (FTIR) spectroscopy, zeta potential, and the X-ray photoelectron
ce pt
spectroscopy (XPS) measurements.
Samples of benzohydroxamic acids (BHA) with 98% and lead nitrate with 99%
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purity are obtained from Tokyo Chemical Industry and Tianjin Kermil Chemical Reagents Development Centre. The novel kind of collector (Pb2+/Pb(OH)+-BHA complexes) is freshly prepared by mixing given mass of lead nitrate and BHA at fixed pH value in the water. 2.2. Flotation tests Flotation tests for single mineral (2 g) were conducted using a XFG flotation machine with a volume of 40 dm3 and a spindle speed of 1600 r/min.(a) Using lead
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nitrate and BHA as the activator and the collector respectively, the reagents were added in the following order: (Ⅰ) HCl or NaOH conditioning for 3 min; (Ⅱ) lead(Ⅱ) ions conditioning for 3min, followed by BHA collector period for 5 min; (Ⅲ) 25mg/L
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pine oil as the frother conditioning for 1 min. (b)Using Pb2+/Pb(OH)+-BHA complexes as the collector, given mass of lead nitrate and BHA at fixed pH value is
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mixed in the water to form the complexes accordingly at first. In the flotation process,
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pH modifier is firstly added and conditioned for 3 min to adjust the pulp pH. The novel collector (complexes) is then added with a conditioning time of 3 min. Frother
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is lastly added and conditioned for 1 min. The total flotation time is 6 min and the
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concentrate is collected by manually scraping similarly. Both the concentrate collected and the tailing remained in the cell were dried and weighed for calculating
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flotation recovery. Repeat tests were conducted throughout this study and the standard deviation was less than 2% recovery.
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2.3. Measurement of BHA adsorption amount 2.0 g of mineral sample is added to 40 ml of aqueous solution with the desired
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pH value and concentration of lead(Ⅱ) ions and BHA. The suspension is agitated at 25℃for 10 min. After that, the conditioned pulp is centrifuged at 9000 rpm for 20 min, and the equilibrated solution is immediately gathered to analyze the concentration of the collector by total organic carbon analyzer (TOC-L CPH/CN, Shimadzu Corporation). The surface area of ground sample, as determined by B.E.T. method, is 0.231m2/g. The adsorption amount of the collector is calculated by the following equation:
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where
is the amount (mg/m3) of collector adsorbed on mineral surface；C0 and
C is the concentration (mg/L) of the initial collector solution and the
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collector-equilibrated solution respectively; V is the volume (L) of the solution and is the amount of the particles per sample; A is the mineral specific surface area.
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2.4. Voltammetric measurements
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Electrochemical measurements are performed in the solution of potassium nitrate, with ionic strength 0.10 mol/L. All the measurements are carried out in a glass cell at
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25℃, and the solutions are purged with nitrogen for at least 30 min prior to each
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experiment. The nitrogen atmosphere is maintained thereafter. Each titration experiment is repeated at least three times.
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In all cases, a three-electrode system consisting of a gold electrode as the working electrode with the diameter of two mm, a platinum wire counter electrode
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and an Ag/AgCl, KCl (3 mol/L) reference electrode is used. Electrochemical techniques used are differential pulse voltammetry (DPV) applied under the selected
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conditions of pulse amplitude (a) = 25 mV, potential step increment (Einc) = 2 mV, time between the pulses (tint) = 0.2 s, and pulse duration (tp) = 0.05 s. 2.5. Zeta potential measurement The zeta potential of minerals treated and untreated by the flotation reagents is measured by a JS94H micro electrophoresis instrument (Shanghai Zhongchen Digital Technic Apparatus Co., China). The measurement is carried out at 25℃. A 16mg sample is placed in a 200 ml beaker with 80ml aqueous solution containing 10mM
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KNO3 as the supporting electrolyte, and the mineral suspension is conditioned with reagents over the pH range of 2-12. All the samples are thoroughly equilibrated and an average zeta potential value of at least three individual measurements is recorded.
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2.6. FTIR measurements Fourier transform infrared (FTIR) spectra are recorded on a Nicolet 6700
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spectrometer at room temperature in the range from 4000 to 500 cm-1. Spectra of the
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solids are taken with KBr pellets. 2.7. XPS (X-ray photoelectron spectroscopy) measurements
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The XPS spectra of cassiterite powders untreated and treated by (a) lead(Ⅱ)ions
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as the activator and BHA as the collector, and (b) the Pb2+/Pb(OH)+-BHA complexes as the novel collector are recorded with a K-Alpha 1063 (Thermo Fisher Scientific)
ed
spectrometer, respectively. The instrument uses Al Kα as sputtering source at 12 kV and 6 mA, with pressure in the analytical chamber at 1.0 10-7 Pa. A value of 284.8 eV
ce pt
is adopted as the standard C (1s) binding energy. 3. Results and discussion
3.1. Flotation performance of single minerals
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Fig.1 shows the effect of pH on the floatability of cassiterite. The recovery of
cassiterite increases from 6.82% to a maximum 48.7% with an increase of pH from 4 to 6, then nearly levels out for a narrow pH range 6-8，and decreases sharply with further pH increase to pH 12 in the presence of lead nitrate (Figure.1a). Figure.1b follows the same trend with 1a, but the maximum recovery is achieved under a pH value of 8-9 not 6-8. The suitable pH value of flotation is 7 and 9 for BHA and Pb2+/Pb(OH)+-BHA complexes, respectively, to obtain maximum recovery of
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cassiterite. 60
40
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30
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20 a: Pb+BHA b:Pb/Pb(OH)-BHA complexes
10 0
4
6
8
10
12
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pH
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Recovery (%)
50
Fig. 1. Recovery of cassiterite as a function of pH (lead nitrate: 5mg/L; BHA: 10mg/L)
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Fig.2a1 and 2b1 show a monotonic increase in recovery of cassiterite with BHA concentration. Compared with BHA as the collector, the complexes formed by lead(Ⅱ)
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and BHA could obtain higher recovery and float out more than 95% cassiterite with 80mg/L BHA. At Figure 2b2, there is a steep increase in the recovery 18.81% to 89.3%
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from concentration of BHA 10mg/L to 40mg/L followed by a marginal increase up to 120mg/L. However, with an increase of BHA concentration from 10mg/L to 40mg/L,
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the recovery of cassiterite has a limited increase from 35.2% to 60.92% (Figure 2a2). In order to obtain more than 90% recovery, it needs to consume 100mg/L BHA (a) compared with 40mg/L (b). The recovery of cassiterite shows a rapid increase with an increase in Pb(NO3)2 concentration up to 5 mg/L and 15mg/L at Fig.3a1 and 3b1 respectively, and thereafter declines slowly. Fig 3b2 portrays a steep increase in recovery from 15.39% to 84% with an increase in Pb (NO3)2 concentration from 0 to 5 mg/L. It is noteworthy that the result of Fig 3b2 shows the higher recovery of more
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than 10% compared with Fig 3a2. The results indicate that Pb2+/Pb(OH)+-BHA complexes exhibit superior floatation performances to cassiterite. 100
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40
a1
20
b1 a2
cr
60
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Recovery (%)
80
b2
0
20
40
60
80
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0
100
120
Concentration of BHA(mg/L) Fig.2. Recovery of cassiterite as a function of BHA concentration (a1, b1: 5mg/L lead nitrate; a2, b2:
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25mg/L lead nitrate. a: BHA + Pb, at pH 7; b: Pb2+/Pb(OH)+-BHA complexes, at pH 9)
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100
60
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Recovery (%)
80
40
a1 b1
20
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a2
0
0
b2 10
20
30
Concentration of lead nitrate (mg/L)
Fig.3. Recovery of cassiterite as a function of lead nitrate concentration (a1, b1:10mg/L BHA; a2, b2:40mg/L BHA. a: BHA+Pb, at pH 7; b: Pb2+/Pb(OH)+-BHA complexes, at pH 9)
Page 10 of 25
3.2. Adsorption Studies
300
150
4
6
8
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0
Pb+BHA Pb/Pb(OH)-BHA complexes
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450
10
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Adsorption (10-3mg/m2)
600
12
pH
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Fig.4. Adsorption of BHA onto cassiterite as a function of pH (lead nitrate: 25mg/L; BHA: 40mg/L)
The results of adsorption of BHA onto cassiterite as a function of pH are present
ed
at Fig.4. It is observed that adsorption density of BHA steadily increases with an increase in pH from 4 to 9 and thereafter decreases with an increase in pH when using
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Pb2+/Pb(OH)+-BHA complexes as the collector. While adding lead nitrate and BHA in sequence, there is an increase in the adsorption density with an increase in pH from 4
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to 7, and then remain stable (7-8) followed by a decrease up to pH 11. It is interesting to note that region of higher adsorption density is observed in the pH range 7-8 when using lead nitrate as the activator and BHA as the collector, respectively. Nevertheless, the highest adsorption density is obtained at pH 9 when using Pb2+/Pb(OH)+-BHA complexes as the collector. The result is consistent with the floatation tests and attributed to the superior collecting ability of Pb2+/Pb(OH)+-BHA complexes for cassiterite.
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1000
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800
600
400 20
30
40
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Experimental isotherm(a) Langmuir adsorption isotherm(a) Experimental isotherm(b) Langmuir adsorption isotherm(b)
50
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-3 2 Adsorption(10 mg/m )
1200
60
Equilibrium concentration (mg/L)
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Fig. 5. Langmuir isotherm models fits for adsorption of BHA (a: BHA+Pb, at pH 7; b: Pb2+/Pb(OH)+-BHA complexes, at pH 9)
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The Langmuir adsorption isotherm [17] is a typical adsorption isotherm. Fig.5 shows the amounts of BHA adsorbed on cassiterite as a function of equilibrium
ed
concentration. The order of the differences between the amounts adsorbed on cassiterite for (a) and (b) is: (b) > (a). The adsorption mechanisms of BHA on
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cassiterite is further studied by fitting the adsorption data to Langmuir adsorption isotherm. The Fig.5a and 5b show nearly-identical degrees of fitting, and the
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Langmuir model provides the best fit for BHA adsorption. The result is attributed to the weak strength of the lateral hydrophobic associations between surfactants as a result of the molar structure of BHA with short non-polar hydrocarbon chain (benzene). 3.3. Voltammetric measurements Different chelating agents with the restricted collecting ability have been applied in floatation process when using lead nitrate as the activator. The research is carried
Page 12 of 25
out by electrochemical techniques since these have been used successfully in complexation studies of several heavy metals, including Pb(Ⅱ), with chelating agent[18; 19; 20]. In the research, the electrochemical measurements of the redox
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system Pb(Ⅱ)-BHA are performed in a model solution of potassium nitrate, at ionic strength=0.1mol/L and 25±1℃. Dissolved Pb(Ⅱ) concentration is 10-5mol/L. To the
composition
and
stability
of
the
complexes
formed,
the
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determine
constant while the concentration of BHA is varied.
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Pb(Ⅱ)-BHA-OH- systems are studied in series of experiments in which pH is kept
E1/2VS.Ag/AgCl(mV)
F0(106）
1
0.661
0.194
271
3.562
2
1.322
0.207
284
10.070
4
2.644
0.224
301
37.003
6
3.966
8
5.288
0.233
310
78.259
0.240
317
134.685
10
6.610
0.246
323
206.281
15
9.915
0.256
333
451.639
20
13.220
0.263
340
791.809
9
12
M
L[FREE](10-3mol/L)
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L[TOL](10-3mol/L)
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Table 1. Electrochemical experiments data for Pb2+-BHA-OH- systems (pH=9)
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A1=1; B1= (2.47±0.06) ×10 ; C1= (4.34±0.01) × 10
Table 2. Electrochemical experiments data for Pb 2+-BHA-OH- systems (pH=10) L[TOL](10-3mol/L)
L[FREE](10-3mol/L)
1
0.951
3
F0(106）
0.214
291
15.855
2.854
0.232
309
70.599
3.805
0.237
314
104.221
6
5.707
0.245
322
194.339
8
7.608
0.252
329
339.561
10
9.512
0.257
334
496.894
15
14.268
0.266
343
997.83
20
19.024
0.273
350
1721.368
Ac
4
E1/2VS.Ag/AgCl(mV)
10
A2=1; B2= (1.24±0.10) ×10 ; C2= (4.09±0.06) ×10
12
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1.60E+009
1 2 3 4
8.00E+008
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F0
1.20E+009
0.00E+000 0.000
0.004
0.008
0.012
0.016
0.020
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L[FREE](mol/L)
cr
4.00E+008
Fig. 6. F0 as a function of BHA concentration presence of 0.1mol/L potassium nitrate
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（pH=9:1-experimental, 3-regression line; pH=10:2-experimental, 4-regression line)
Fig. 6 shows a typical set of DPV obtained in the titration of a lead(Ⅱ) ions
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solution with BHA. As the table shows, the increment in ligand concentration
ed
produces a shift of the peak potential toward more negative values. The basis for
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interpreting the DPV data is well-known by the De Ford-Hume relation [21]:
where E stands for a half-wave, or a peak potential, whichever is appropriate, and α0 stands for the fraction of non-ligated lead. The ratio of the diffusion currents of
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a ligated and a non-ligated lead is omitted. The final mathematical model can be written as:
The BHA dissociation constant
where H2L
is C7H7NO2 is used for consequent calculation of free BHA concentration. The complex formation equilibrium of the Pb2+ with BHA in dilute aqueous solution is assumed to be established by the consecutive two-step reactions:
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Computations of stability constants, under the assumption of a series of mononuclear complexes ([M][HL]n; n=1,2) estimated by voltammetry, yield the
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values of formation constants as follows: logβ1= 9.30±0.01, logβ2 = 12.64±0.01 calculated from Table.1 and line 3 at Fig.6; logβ1= 10.09±0.04, logβ2 = 12.61±0.01
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calculated from Table.2 and line 4 at Fig.6. There is a sharp difference of logβ1 at pH
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9 and 10 respectively. It illustrates that the effect of lead hydroxide(Ⅰ) is not negligible and the presence of Pb(OH)(HL) gives rise to this result. Analysis reveals
ed
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the presence of the complexes listed as follows, together with their stability constants.
In weak alkaline media, [Pb(HL)]+ and [Pb(OH)(HL)] species with significantly
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collecting power are well predominant. The strong electrostatic interaction between [Pb(HL)]+ and the mineral surface with negative charge plays an important role in
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flotation process and results in higher recovery of cassiterite. 3.4. Zeta-potential studies Fig.7A gives zeta-potential of cassiterite in the absence and presence of lead
nitrate and BHA under different pH conditions. The iso-electric point (IEP) of cassiterite in the control experiment is approximately pH 3.0. The IEP of the cassiterite is nearly similar in the presence of 25mg/L lead nitrate, but an increase in zeta potential is distinctly observed with pH ranging from 4 to 12. At low pH values,
Page 15 of 25
lead nitrate in solution is primarily present as Pb2+ and adsorbed on cassiterite through electrostatic forces. A negative shift in the cassiterite is observed when the pH is above 9.5 because of the adsorption of Pb(OH)3−. In the pH range between 5.5 and 7,
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the maximum shift of zeta potential occurs because a large amount of hydrolysed lead species PbOH+ is adsorbed on cassiterite surface via specific adsorption. The zeta
cr
potential in BHA when used alone appears to be similar to distilled water because of
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low adsorption of BHA on the cassiterite surface.
Generally, minerals are activated by metal ions through the increasing adsorption
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capacity of the collector during flotation. In this research, the effect of lead (Ⅱ) ions
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on the zeta potential of cassiterite as a function of pH is studied (Fig.7B). The measurements are used to determine the adsorption capacity of collector on cassiterite
ed
by measuring the shifts in zeta potential of cassiterite in the absence and presence of BHA. The negative shift of cassiterite is due to the adsorption of anionic BHA. This
ce pt
shift reflects the adsorption capacity of BHA as a function of pH. The maximum variation of cassiterite occurs in the pH range between 5.5 and 7.0 where the specific
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adsorption of a large amount of PbOH+ on cassiterite also occurred (Fig.7B), suggesting a maximum adsorption capacity of BHA promoted by PbOH+. The result also confirms that the activating entity of lead nitrate is PbOH+ in cassiterite flotation. It is worth noting that the zeta potential is visibly more positive when adding Pb2+/Pb(OH)+-BHA complexes than adding lead nitrate and BHA in sequence in the pH range between 6.0 and 9.0 (Fig.7C), suggesting that the adsorption capacity of PbOH+ is significantly enhanced. Actually, that is due to a stronger affinity to the
Page 16 of 25
mineral surface of Pb2+/Pb(OH)+-BHA complexes ([Pb(HL)]+).
(A)
0
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distilled water lead nitrate BHA
cr
-20
-40
2
4
6
8
M
0
12
(B)
lead nitrate Pb+BHA
ed
-20
ce pt
Zeta potential (mV)
10
an
pH
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Zeta potential (mV)
20
-40
4
6
8
10
12
pH
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2
Page 17 of 25
20
0
Pb+BHA Pb/Pb(OH)-BHA complexes 2
4
6
8 pH
cr
-40
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-20
10
12
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Zeta potential(mV)
(C)
Fig.7 Zeta potential of cassiterite as a function of pH (lead nitrate (Pb):25mg/L; BHA:40mg/L))
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3.5. FTIR analysis
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Infrared spectra of BHA, and cassiterite untreated and treated by lead nitrate and BHA are present in Fig.8 and 9. The BHA exhibits two adsorption bands at 3030 and
ed
2760 cm-1, which are associated with N-H and O-H stretching respectively. The adsorption band at 3299 cm-1 shapes ‘w’ resulting from partly overlapping of N-H and
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O-H peaks, and is a characteristic of hydroxamic acid (–NHOH group). Compared with the spectrum of cassiterite, that of the (a) and (b) has the following remarkable
Ac
changes: disappearance of O-H stretching adsorption band due to the possible formation of O-Metal bond; The adsorption band of N-H shifts from 3030 to 2984 cm-1. In addition, the adsorption band at 2984 cm-1 when adding Pb2+/Pb(OH)+-BHA complexes is much stronger than adding lead nitrate and BHA in sequence, but there is no difference between the shapes. It suggests the adsorption form is similar（Fig.11） and the intensity on the mineral surface is different.
Page 18 of 25
ip t
2760
4000
3200
2400
1600
800
us
Wavenumber(cm-1)
cr
3030 3299
an
Fig.8. FTIR spectra of BHA
M
2984
b:Pb/Pb(OH)-BHA a:Pb+BHA cassiterite
ce pt
ed
2984
4000
3200
2400
1600
800
Wavenumber(cm-1)
Ac
Fig.9. FTIR spectra of cassiterite untreated and treated by lead nitrate and BHA
3.5. XPS results
Table 3. Atom percentages for various samples as determined by XPS (a: Pb+BHA; b: Pb/Pb(OH)-BHA) Sample
C (%)
O (%)
N (%)
Sn (%)
Pb (%)
Total (%)
a
15.94
61.22
0.41
20.45
1.98
100.00
b
20.12
56.31
1.09
19.96
2.52
100.00
Table 3 shows the larger percentage of Pb and C on the chosen area of mineral surfaces treated by Pb2+/Pb(OH)+-BHA complexes, which reveals that Pb2+ and BHA form the complexes at first and then complexes with the superior collecting ability
Page 19 of 25
absorb on the surface. The Sn3d XPS of cassiterite is present in Fig.10. The results illustrate that the Sn3d XPS band of cassiterite is composed of two components at 486.66 eV and 495.06 eV. After treatment (a), the Sn3d XPS bands slightly shifts to
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around 486.54 eV and 494.96 eV, respectively. The finding indicates that the adsorption of Pb(OH)+ onto cassiterite changes the chemical circumstance of Sn
cr
atoms, implying the appearance of Pb-O-Sn bond because of the smaller value of
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Sn3d. And for (b), the Sn3d peaks emerges in the same position, which reveals the
an
same adsorption form on the mineral surface.
Cassiterite Sn3d3/2495.06
M
80000
ce pt
40000
484
488
492
496
Binding energy (eV)
Ac
0
Cassiterite Sn3d5/2486.66
ed
Counts/s
120000
Page 20 of 25
(a)
Cassiterite Sn3d3/2494.96
Cassiterite Sn3d5/2486.54
80000
ip t
Counts/s
120000
488
492
496
us
0 484
cr
40000
Binding energy (eV)
an
40000
ce pt
20000
0 484
Cassiterite Sn3d5/2486.54
ed
Counts/s
Cassiterite Sn3d3/2494.96
M
(b) 60000
488
492
496
Binding energy (eV)
(b)
Ac
Fig.10. Sn3d XPS of cassiterit untreated and treated by Pb+BHA (a) and Pb/Pb(OH)-BHA complexes
Page 21 of 25
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Fig.11. The chemisorption model of BHA (lead(Ⅱ) ions as the activator) and Pb2+/Pb(OH)+-BHA complexes adsorbed on cassiterite surfaces
an
4. Conclusions
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Flotation and adsorption studies indicate that the complexes formed by lead (Ⅱ) ions and BHA have a stronger collecting ability for cassiterite than using lead (Ⅱ) ions
ed
as the activator and BHA as the collector, respectively. At optimum flotation pH value around 9, it is found that [Pb(BHA)]+ and Pb(BHA)OH are predominant as the
ce pt
fractions of total lead(II) ions in the system by voltammetric measurements. The results of zeta potential deduce a stronger affinity to the mineral surface of complexes
Ac
in weak basic solution. XPS and FTIR studies provide evidence in support of chemical interaction between caasiterite and Pb2+/Pb(OH)+-BHA complexes, and the similar adsorption form of the complexes on mineral surface with using lead (Ⅱ) ions and BHA as the activator and the collector, respectively. Acknowledgments This work was supported financially by Innovation Driven Plan of Central South University（No. 2015CX005）, the National 111 Project (No. B14034), Collaborative
Page 22 of 25
Innovation Center for Clean and Efficient utilization of Strategic Metal Mineral Resources, Sublimation scholars distinguished professor of Central South University
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and the National science and Technology Support Project of China (2015BAB14B02).
Page 23 of 25
References: [1] S. Bulatovic, and E.D. Silvio, Process development for impurity removal from a tin gravity concentrate. Minerals Engineering 13 (2000) 871-879. [2] Y. Zhou, X. Tong, S. Song, X. Wang, and Z. Deng, Beneficiation of Cassiterite Fines from a Tin Tailing Slime by Froth Flotation. Separation Science & Technology (2014). [3] X. Weng, G. Mei, T. Zhao, and Y. Zhu, Utilization of novel ester-containing quaternary ammonium surfactant as cationic collector for iron ore flotation. Separation and Purification
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Technology 103 (2013) 187-194.
[4] B. Janczuk, The contribution of double layers on the free energy of interactions in the cassiterite-SDS solution system. Journal of Physical Chemistry 93 (2002) 6596-6601.
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[5] O. Ofor, and C. Nwoko, Oleate Flotation of a Nigerian Baryte: The Relation between Flotation
Recovery and Adsorption Density at Varying Oleate Concentrations, pH, and Temperatures. Journal of
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Colloid & Interface Science 186 (1997) 225-33.
[6] A.N. Buckley, J.A. Denman, and G.A. Hope, The Adsorption of n-Octanohydroxamate Collector on Cu and Fe Oxide Minerals Investigated by Static Secondary Ion Mass Spectrometry. Minerals 2
an
(2012) 493-515.
[7] O. Ec., L.J.F. Dj., N. Kb., and S. Leroy, METAL COMPLEXES OF SALICYLHYDROXAMIC ACID AND O-ACETYLSALICYLHYDROXAMIC ACID. Inorganica Chimica Acta 266 (1997) 117-120.
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[8] T. Sreenivas, and N.P.H. Padmanabhan, Surface chemistry and flotation of cassiterite with alkyl hydroxamates. Colloids and Surfaces A: Physicochemical and Engineering Aspects 205 (2002) 47-59. [9] X.Q. Wu, and J.G. Zhu, Selective flotation of cassiterite with benzohydroxamic acid. Minerals
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Engineering 19 (2006) 1410-1417.
[10] W. Sun, K.E. Li-Fang, and L. Sun, Study of the application and mechanism of benzohydroxamic 1367-1382.
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acid in the flotation of cassiterite. Journal of China University of Mining & Technology 10 (2013) págs. [11] A. Jordens, C. Marion, T. Grammatikopoulos, B. Hart, and K.E. Waters, Beneficiation of the Nechalacho rare earth deposit: Flotation response using benzohydroxamic acid. Minerals Engineering (2016).
[12] C.G. Zhong, Y.D. Gao, X.Y. Qiu, and Q.M. Feng, The Effects of Metal Ions on Wolframite Flotation with Benzohydroxamic Acid. China Tungsten Industry (2013).
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[13] B. Liu, X. Wang, H. Du, J. Liu, and S. Zheng, The surface features of lead activation in amyl xanthate flotation of quartz. International Journal of Mineral Processing (2016). [14] H. Li, S. Mu, X. Weng, Y. Zhao, and S. Song, Rutile flotation with Pb2+ ions as activator: Adsorption of Pb2+ at rutile/water interface. Colloids and Surfaces A: Physicochemical and Engineering Aspects 506 (2016) 431-437. [15] C. Liu, Q. Feng, G. Zhang, W. Ma, and Q. Meng, Effects of lead ions on the flotation of hemimorphite using sodium oleate. Minerals Engineering (2016). [16] A.M. Marabini, M. Ciriachi, P. Plescia, and M. Barbaro, Chelating reagents for flotation. Minerals Engineering 20 (2007) 1014-1025. [17] D. Chaturvedi, Langmuir, I. (1918). The adsorption of gases on plane surfaces of glass, mica and platinum. J. Am. Chem. Soc., 40, 1361-1403.. [18] M.A. Kamyabi, F. Soleymani-Bonoti, and S. Zakavi, Voltammetric determination of stability
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constants of lead complexes with diallyl disulfide, dimethyl disulfide, and diallyl sulfide. Chinese Chemical Letters 27 (2016) 71-76. [19] V. Cuculi, I. Pi Eta, and M. Branica, Voltammetric determination of stability constants of iron(III) –glycine complexes in water solution. Journal of Electroanalytical Chemistry 583 (2005) 140-147. [20] J.I. Kigunda, Voltammetric determination of stability constants and speciation of copper in salt-water lake (Bogoria) in Kenya. Copper//Saline waters--Kenya//Voltammetry. (2012).
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[21] D.R. Crow, and D.R. Crow, Polarography of Metal Complexes, Academic Press, London, 1969.
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S0927-7757(17)30185-1 http://dx.doi.org/doi:10.1016/j.colsurfa.2017.02.051 COLSUA 21406
To appear in:
Colloids and Surfaces A: Physicochem. Eng. Aspects
Received date: Revised date: Accepted date:
15-10-2016 19-2-2017 20-2-2017
Please cite this article as: M. Tian, Y. Hu, W. Sun, R. Liu, Study on the mechanism and application of a novel collector-complexes in cassiterite flotation, Colloids and Surfaces A: Physicochemical and Engineering Aspects (2017), http://dx.doi.org/10.1016/j.colsurfa.2017.02.051 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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*Graphical Abstract (for review)
Page 1 of 25
*Highlights (for review)
Highlights 1. The complexes formed by lead(Ⅱ) and hydroxides with BHA are regarded as a novel collector. 2. The superior collecting ability of Pb2+/Pb(OH)+-BHA complexes is observed.
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3. The complexes absorb on the mineral surface as a result of electrostatic attraction
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and chemical adsorption.
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*Manuscript
Study on the mechanism and application of a novel collector-complexes in cassiterite flotation Mengjie Tian, Yuehua Hu, Wei Sun*, Runqing Liu**
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School of Minerals Processing and Bioengineering, Central South University,
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Changsha 410083, China *
Corresponding author
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E-mail address: [email protected]；[email protected]
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Abstract
Benzohydroxamic acid (BHA) has been widely used as the collector for
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cassiterite flotation, whereas low collecting ability of BHA need to be improved. It is
ed
shown through flotation that BHA exhibits weak collecting capacity to cassiterite even in the case of using lead nitrate as the activator, but the superior floatation
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performances of lead(Ⅱ)-BHA complexes is observed. The adsorption isotherms of BHA onto cassiterite reveal a higher adsorption density around pH 9 using lead(Ⅱ)-BHA complexes as the collector. The complexation of lead(Ⅱ) by BHA has
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been studied by differential pulse voltammetry. In weak alkaline solution, [Pb(HL)]+ and [Pb(OH)(HL)] species with significantly collecting power are well predominant. Zeta potential studies demonstrate a stronger affinity to the mineral surface of Pb2+/Pb(OH)+-BHA complexes. XPS and FTIR studies indicate that chemisorption with the similar formation of Sn-O-Pb bond is the main interaction between the two kinds of collector and cassiterite. Keywords: cassiterite; flotation; BHA; lead ion; complexes
Page 3 of 25
Introduction The gravity separation process is a main beneficiation approach for recovery of cassiterite mineral. Only 50-70% fine cassiterite particles could be recovered through
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gravity separation technique[1]. As an alternative technique, flotation has been proved to be a more attractive method for recovery of fine cassiterite particle [2; 3]. Previous
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investigations have found that fine cassiterite particles could be effectively floated out
suffer a low selectivity against gangue minerals.
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by using oleic acid or alkyl sulfate as the collector [4; 5]. However, these collectors
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Commonly, hydroxamate collectors exhibit superior selectivity in the flotation
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process [6; 7; 8]. BHA could easily perform the flotation separation of cassiterite from quartz and calcite compared with other hydroxamic acids [9; 10; 11]. During flotation,
ed
metal ions are usually used as activators to improve the collecting power of BHA [12; 13]. Lead nitrate is used as the activator in the flotation with hydroxamic acid [14; 15].
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BHA is one of chelating reagents which are a particular class of complexing reagents consisting of large organic molecules capable of bonding to the metal ion via two or
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more functional groups, with the formation of one or more rings. It has been reported that hydroxamic acid can chelate with metal ions and metal hydroxyl ion to form five-member ring complexes [16]. As a result, research of coordination of BHA with lead ions can help to improve its collecting ability to cassiterite, and study the activation mechanism of lead nitrate and the design of a novel collector. The paper focuses on the effect of lead (Ⅱ) on the adsorption capacity of BHA and the discussion of the novel kind of collector——Pb2+/Pb(OH)+-BHA complexes.
Page 4 of 25
To elucidate the adsorption mechanisms of two kinds of collector——BHA and Pb2+/Pb(OH)+-BHA complexes on cassiterite, adsorption measurements, infrared and X-ray photoelectron spectroscopy (XPS) studies are employed. Simultaneously,
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determination of the stability constants of dissolved lead (II)–BHA system in using differential pulse voltammetry is performed.
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2. Experimental
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2.1. Materials and reagents
Hand-picked natural pure samples of cassiterite are obtained from Yunnan
an
province in China. Mineralogical and X-ray diffraction data confirms that the
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cassiterite is high 99% purity. The samples are ground in a pottery ball mill and the -75+38μm fractions are used in flotation and adsorption tests. The -38μm samples are
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further ground in an agate mortar to obtain particles of size less than 5μm for Fourier transform infrared (FTIR) spectroscopy, zeta potential, and the X-ray photoelectron
ce pt
spectroscopy (XPS) measurements.
Samples of benzohydroxamic acids (BHA) with 98% and lead nitrate with 99%
Ac
purity are obtained from Tokyo Chemical Industry and Tianjin Kermil Chemical Reagents Development Centre. The novel kind of collector (Pb2+/Pb(OH)+-BHA complexes) is freshly prepared by mixing given mass of lead nitrate and BHA at fixed pH value in the water. 2.2. Flotation tests Flotation tests for single mineral (2 g) were conducted using a XFG flotation machine with a volume of 40 dm3 and a spindle speed of 1600 r/min.(a) Using lead
Page 5 of 25
nitrate and BHA as the activator and the collector respectively, the reagents were added in the following order: (Ⅰ) HCl or NaOH conditioning for 3 min; (Ⅱ) lead(Ⅱ) ions conditioning for 3min, followed by BHA collector period for 5 min; (Ⅲ) 25mg/L
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pine oil as the frother conditioning for 1 min. (b)Using Pb2+/Pb(OH)+-BHA complexes as the collector, given mass of lead nitrate and BHA at fixed pH value is
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mixed in the water to form the complexes accordingly at first. In the flotation process,
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pH modifier is firstly added and conditioned for 3 min to adjust the pulp pH. The novel collector (complexes) is then added with a conditioning time of 3 min. Frother
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is lastly added and conditioned for 1 min. The total flotation time is 6 min and the
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concentrate is collected by manually scraping similarly. Both the concentrate collected and the tailing remained in the cell were dried and weighed for calculating
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flotation recovery. Repeat tests were conducted throughout this study and the standard deviation was less than 2% recovery.
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2.3. Measurement of BHA adsorption amount 2.0 g of mineral sample is added to 40 ml of aqueous solution with the desired
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pH value and concentration of lead(Ⅱ) ions and BHA. The suspension is agitated at 25℃for 10 min. After that, the conditioned pulp is centrifuged at 9000 rpm for 20 min, and the equilibrated solution is immediately gathered to analyze the concentration of the collector by total organic carbon analyzer (TOC-L CPH/CN, Shimadzu Corporation). The surface area of ground sample, as determined by B.E.T. method, is 0.231m2/g. The adsorption amount of the collector is calculated by the following equation:
Page 6 of 25
where
is the amount (mg/m3) of collector adsorbed on mineral surface；C0 and
C is the concentration (mg/L) of the initial collector solution and the
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collector-equilibrated solution respectively; V is the volume (L) of the solution and is the amount of the particles per sample; A is the mineral specific surface area.
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2.4. Voltammetric measurements
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Electrochemical measurements are performed in the solution of potassium nitrate, with ionic strength 0.10 mol/L. All the measurements are carried out in a glass cell at
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25℃, and the solutions are purged with nitrogen for at least 30 min prior to each
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experiment. The nitrogen atmosphere is maintained thereafter. Each titration experiment is repeated at least three times.
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In all cases, a three-electrode system consisting of a gold electrode as the working electrode with the diameter of two mm, a platinum wire counter electrode
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and an Ag/AgCl, KCl (3 mol/L) reference electrode is used. Electrochemical techniques used are differential pulse voltammetry (DPV) applied under the selected
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conditions of pulse amplitude (a) = 25 mV, potential step increment (Einc) = 2 mV, time between the pulses (tint) = 0.2 s, and pulse duration (tp) = 0.05 s. 2.5. Zeta potential measurement The zeta potential of minerals treated and untreated by the flotation reagents is measured by a JS94H micro electrophoresis instrument (Shanghai Zhongchen Digital Technic Apparatus Co., China). The measurement is carried out at 25℃. A 16mg sample is placed in a 200 ml beaker with 80ml aqueous solution containing 10mM
Page 7 of 25
KNO3 as the supporting electrolyte, and the mineral suspension is conditioned with reagents over the pH range of 2-12. All the samples are thoroughly equilibrated and an average zeta potential value of at least three individual measurements is recorded.
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2.6. FTIR measurements Fourier transform infrared (FTIR) spectra are recorded on a Nicolet 6700
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spectrometer at room temperature in the range from 4000 to 500 cm-1. Spectra of the
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solids are taken with KBr pellets. 2.7. XPS (X-ray photoelectron spectroscopy) measurements
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The XPS spectra of cassiterite powders untreated and treated by (a) lead(Ⅱ)ions
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as the activator and BHA as the collector, and (b) the Pb2+/Pb(OH)+-BHA complexes as the novel collector are recorded with a K-Alpha 1063 (Thermo Fisher Scientific)
ed
spectrometer, respectively. The instrument uses Al Kα as sputtering source at 12 kV and 6 mA, with pressure in the analytical chamber at 1.0 10-7 Pa. A value of 284.8 eV
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is adopted as the standard C (1s) binding energy. 3. Results and discussion
3.1. Flotation performance of single minerals
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Fig.1 shows the effect of pH on the floatability of cassiterite. The recovery of
cassiterite increases from 6.82% to a maximum 48.7% with an increase of pH from 4 to 6, then nearly levels out for a narrow pH range 6-8，and decreases sharply with further pH increase to pH 12 in the presence of lead nitrate (Figure.1a). Figure.1b follows the same trend with 1a, but the maximum recovery is achieved under a pH value of 8-9 not 6-8. The suitable pH value of flotation is 7 and 9 for BHA and Pb2+/Pb(OH)+-BHA complexes, respectively, to obtain maximum recovery of
Page 8 of 25
cassiterite. 60
40
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30
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20 a: Pb+BHA b:Pb/Pb(OH)-BHA complexes
10 0
4
6
8
10
12
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pH
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Recovery (%)
50
Fig. 1. Recovery of cassiterite as a function of pH (lead nitrate: 5mg/L; BHA: 10mg/L)
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Fig.2a1 and 2b1 show a monotonic increase in recovery of cassiterite with BHA concentration. Compared with BHA as the collector, the complexes formed by lead(Ⅱ)
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and BHA could obtain higher recovery and float out more than 95% cassiterite with 80mg/L BHA. At Figure 2b2, there is a steep increase in the recovery 18.81% to 89.3%
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from concentration of BHA 10mg/L to 40mg/L followed by a marginal increase up to 120mg/L. However, with an increase of BHA concentration from 10mg/L to 40mg/L,
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the recovery of cassiterite has a limited increase from 35.2% to 60.92% (Figure 2a2). In order to obtain more than 90% recovery, it needs to consume 100mg/L BHA (a) compared with 40mg/L (b). The recovery of cassiterite shows a rapid increase with an increase in Pb(NO3)2 concentration up to 5 mg/L and 15mg/L at Fig.3a1 and 3b1 respectively, and thereafter declines slowly. Fig 3b2 portrays a steep increase in recovery from 15.39% to 84% with an increase in Pb (NO3)2 concentration from 0 to 5 mg/L. It is noteworthy that the result of Fig 3b2 shows the higher recovery of more
Page 9 of 25
than 10% compared with Fig 3a2. The results indicate that Pb2+/Pb(OH)+-BHA complexes exhibit superior floatation performances to cassiterite. 100
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40
a1
20
b1 a2
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60
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Recovery (%)
80
b2
0
20
40
60
80
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0
100
120
Concentration of BHA(mg/L) Fig.2. Recovery of cassiterite as a function of BHA concentration (a1, b1: 5mg/L lead nitrate; a2, b2:
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25mg/L lead nitrate. a: BHA + Pb, at pH 7; b: Pb2+/Pb(OH)+-BHA complexes, at pH 9)
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100
60
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Recovery (%)
80
40
a1 b1
20
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a2
0
0
b2 10
20
30
Concentration of lead nitrate (mg/L)
Fig.3. Recovery of cassiterite as a function of lead nitrate concentration (a1, b1:10mg/L BHA; a2, b2:40mg/L BHA. a: BHA+Pb, at pH 7; b: Pb2+/Pb(OH)+-BHA complexes, at pH 9)
Page 10 of 25
3.2. Adsorption Studies
300
150
4
6
8
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0
Pb+BHA Pb/Pb(OH)-BHA complexes
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450
10
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Adsorption (10-3mg/m2)
600
12
pH
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Fig.4. Adsorption of BHA onto cassiterite as a function of pH (lead nitrate: 25mg/L; BHA: 40mg/L)
The results of adsorption of BHA onto cassiterite as a function of pH are present
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at Fig.4. It is observed that adsorption density of BHA steadily increases with an increase in pH from 4 to 9 and thereafter decreases with an increase in pH when using
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Pb2+/Pb(OH)+-BHA complexes as the collector. While adding lead nitrate and BHA in sequence, there is an increase in the adsorption density with an increase in pH from 4
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to 7, and then remain stable (7-8) followed by a decrease up to pH 11. It is interesting to note that region of higher adsorption density is observed in the pH range 7-8 when using lead nitrate as the activator and BHA as the collector, respectively. Nevertheless, the highest adsorption density is obtained at pH 9 when using Pb2+/Pb(OH)+-BHA complexes as the collector. The result is consistent with the floatation tests and attributed to the superior collecting ability of Pb2+/Pb(OH)+-BHA complexes for cassiterite.
Page 11 of 25
1000
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800
600
400 20
30
40
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Experimental isotherm(a) Langmuir adsorption isotherm(a) Experimental isotherm(b) Langmuir adsorption isotherm(b)
50
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-3 2 Adsorption(10 mg/m )
1200
60
Equilibrium concentration (mg/L)
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Fig. 5. Langmuir isotherm models fits for adsorption of BHA (a: BHA+Pb, at pH 7; b: Pb2+/Pb(OH)+-BHA complexes, at pH 9)
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The Langmuir adsorption isotherm [17] is a typical adsorption isotherm. Fig.5 shows the amounts of BHA adsorbed on cassiterite as a function of equilibrium
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concentration. The order of the differences between the amounts adsorbed on cassiterite for (a) and (b) is: (b) > (a). The adsorption mechanisms of BHA on
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cassiterite is further studied by fitting the adsorption data to Langmuir adsorption isotherm. The Fig.5a and 5b show nearly-identical degrees of fitting, and the
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Langmuir model provides the best fit for BHA adsorption. The result is attributed to the weak strength of the lateral hydrophobic associations between surfactants as a result of the molar structure of BHA with short non-polar hydrocarbon chain (benzene). 3.3. Voltammetric measurements Different chelating agents with the restricted collecting ability have been applied in floatation process when using lead nitrate as the activator. The research is carried
Page 12 of 25
out by electrochemical techniques since these have been used successfully in complexation studies of several heavy metals, including Pb(Ⅱ), with chelating agent[18; 19; 20]. In the research, the electrochemical measurements of the redox
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system Pb(Ⅱ)-BHA are performed in a model solution of potassium nitrate, at ionic strength=0.1mol/L and 25±1℃. Dissolved Pb(Ⅱ) concentration is 10-5mol/L. To the
composition
and
stability
of
the
complexes
formed,
the
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determine
constant while the concentration of BHA is varied.
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Pb(Ⅱ)-BHA-OH- systems are studied in series of experiments in which pH is kept
E1/2VS.Ag/AgCl(mV)
F0(106）
1
0.661
0.194
271
3.562
2
1.322
0.207
284
10.070
4
2.644
0.224
301
37.003
6
3.966
8
5.288
0.233
310
78.259
0.240
317
134.685
10
6.610
0.246
323
206.281
15
9.915
0.256
333
451.639
20
13.220
0.263
340
791.809
9
12
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L[FREE](10-3mol/L)
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L[TOL](10-3mol/L)
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Table 1. Electrochemical experiments data for Pb2+-BHA-OH- systems (pH=9)
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A1=1; B1= (2.47±0.06) ×10 ; C1= (4.34±0.01) × 10
Table 2. Electrochemical experiments data for Pb 2+-BHA-OH- systems (pH=10) L[TOL](10-3mol/L)
L[FREE](10-3mol/L)
1
0.951
3
F0(106）
0.214
291
15.855
2.854
0.232
309
70.599
3.805
0.237
314
104.221
6
5.707
0.245
322
194.339
8
7.608
0.252
329
339.561
10
9.512
0.257
334
496.894
15
14.268
0.266
343
997.83
20
19.024
0.273
350
1721.368
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4
E1/2VS.Ag/AgCl(mV)
10
A2=1; B2= (1.24±0.10) ×10 ; C2= (4.09±0.06) ×10
12
Page 13 of 25
1.60E+009
1 2 3 4
8.00E+008
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F0
1.20E+009
0.00E+000 0.000
0.004
0.008
0.012
0.016
0.020
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L[FREE](mol/L)
cr
4.00E+008
Fig. 6. F0 as a function of BHA concentration presence of 0.1mol/L potassium nitrate
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（pH=9:1-experimental, 3-regression line; pH=10:2-experimental, 4-regression line)
Fig. 6 shows a typical set of DPV obtained in the titration of a lead(Ⅱ) ions
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solution with BHA. As the table shows, the increment in ligand concentration
ed
produces a shift of the peak potential toward more negative values. The basis for
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interpreting the DPV data is well-known by the De Ford-Hume relation [21]:
where E stands for a half-wave, or a peak potential, whichever is appropriate, and α0 stands for the fraction of non-ligated lead. The ratio of the diffusion currents of
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a ligated and a non-ligated lead is omitted. The final mathematical model can be written as:
The BHA dissociation constant
where H2L
is C7H7NO2 is used for consequent calculation of free BHA concentration. The complex formation equilibrium of the Pb2+ with BHA in dilute aqueous solution is assumed to be established by the consecutive two-step reactions:
Page 14 of 25
Computations of stability constants, under the assumption of a series of mononuclear complexes ([M][HL]n; n=1,2) estimated by voltammetry, yield the
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values of formation constants as follows: logβ1= 9.30±0.01, logβ2 = 12.64±0.01 calculated from Table.1 and line 3 at Fig.6; logβ1= 10.09±0.04, logβ2 = 12.61±0.01
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calculated from Table.2 and line 4 at Fig.6. There is a sharp difference of logβ1 at pH
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9 and 10 respectively. It illustrates that the effect of lead hydroxide(Ⅰ) is not negligible and the presence of Pb(OH)(HL) gives rise to this result. Analysis reveals
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the presence of the complexes listed as follows, together with their stability constants.
In weak alkaline media, [Pb(HL)]+ and [Pb(OH)(HL)] species with significantly
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collecting power are well predominant. The strong electrostatic interaction between [Pb(HL)]+ and the mineral surface with negative charge plays an important role in
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flotation process and results in higher recovery of cassiterite. 3.4. Zeta-potential studies Fig.7A gives zeta-potential of cassiterite in the absence and presence of lead
nitrate and BHA under different pH conditions. The iso-electric point (IEP) of cassiterite in the control experiment is approximately pH 3.0. The IEP of the cassiterite is nearly similar in the presence of 25mg/L lead nitrate, but an increase in zeta potential is distinctly observed with pH ranging from 4 to 12. At low pH values,
Page 15 of 25
lead nitrate in solution is primarily present as Pb2+ and adsorbed on cassiterite through electrostatic forces. A negative shift in the cassiterite is observed when the pH is above 9.5 because of the adsorption of Pb(OH)3−. In the pH range between 5.5 and 7,
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the maximum shift of zeta potential occurs because a large amount of hydrolysed lead species PbOH+ is adsorbed on cassiterite surface via specific adsorption. The zeta
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potential in BHA when used alone appears to be similar to distilled water because of
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low adsorption of BHA on the cassiterite surface.
Generally, minerals are activated by metal ions through the increasing adsorption
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capacity of the collector during flotation. In this research, the effect of lead (Ⅱ) ions
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on the zeta potential of cassiterite as a function of pH is studied (Fig.7B). The measurements are used to determine the adsorption capacity of collector on cassiterite
ed
by measuring the shifts in zeta potential of cassiterite in the absence and presence of BHA. The negative shift of cassiterite is due to the adsorption of anionic BHA. This
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shift reflects the adsorption capacity of BHA as a function of pH. The maximum variation of cassiterite occurs in the pH range between 5.5 and 7.0 where the specific
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adsorption of a large amount of PbOH+ on cassiterite also occurred (Fig.7B), suggesting a maximum adsorption capacity of BHA promoted by PbOH+. The result also confirms that the activating entity of lead nitrate is PbOH+ in cassiterite flotation. It is worth noting that the zeta potential is visibly more positive when adding Pb2+/Pb(OH)+-BHA complexes than adding lead nitrate and BHA in sequence in the pH range between 6.0 and 9.0 (Fig.7C), suggesting that the adsorption capacity of PbOH+ is significantly enhanced. Actually, that is due to a stronger affinity to the
Page 16 of 25
mineral surface of Pb2+/Pb(OH)+-BHA complexes ([Pb(HL)]+).
(A)
0
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distilled water lead nitrate BHA
cr
-20
-40
2
4
6
8
M
0
12
(B)
lead nitrate Pb+BHA
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-20
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Zeta potential (mV)
10
an
pH
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Zeta potential (mV)
20
-40
4
6
8
10
12
pH
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2
Page 17 of 25
20
0
Pb+BHA Pb/Pb(OH)-BHA complexes 2
4
6
8 pH
cr
-40
ip t
-20
10
12
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Zeta potential(mV)
(C)
Fig.7 Zeta potential of cassiterite as a function of pH (lead nitrate (Pb):25mg/L; BHA:40mg/L))
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3.5. FTIR analysis
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Infrared spectra of BHA, and cassiterite untreated and treated by lead nitrate and BHA are present in Fig.8 and 9. The BHA exhibits two adsorption bands at 3030 and
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2760 cm-1, which are associated with N-H and O-H stretching respectively. The adsorption band at 3299 cm-1 shapes ‘w’ resulting from partly overlapping of N-H and
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O-H peaks, and is a characteristic of hydroxamic acid (–NHOH group). Compared with the spectrum of cassiterite, that of the (a) and (b) has the following remarkable
Ac
changes: disappearance of O-H stretching adsorption band due to the possible formation of O-Metal bond; The adsorption band of N-H shifts from 3030 to 2984 cm-1. In addition, the adsorption band at 2984 cm-1 when adding Pb2+/Pb(OH)+-BHA complexes is much stronger than adding lead nitrate and BHA in sequence, but there is no difference between the shapes. It suggests the adsorption form is similar（Fig.11） and the intensity on the mineral surface is different.
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ip t
2760
4000
3200
2400
1600
800
us
Wavenumber(cm-1)
cr
3030 3299
an
Fig.8. FTIR spectra of BHA
M
2984
b:Pb/Pb(OH)-BHA a:Pb+BHA cassiterite
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ed
2984
4000
3200
2400
1600
800
Wavenumber(cm-1)
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Fig.9. FTIR spectra of cassiterite untreated and treated by lead nitrate and BHA
3.5. XPS results
Table 3. Atom percentages for various samples as determined by XPS (a: Pb+BHA; b: Pb/Pb(OH)-BHA) Sample
C (%)
O (%)
N (%)
Sn (%)
Pb (%)
Total (%)
a
15.94
61.22
0.41
20.45
1.98
100.00
b
20.12
56.31
1.09
19.96
2.52
100.00
Table 3 shows the larger percentage of Pb and C on the chosen area of mineral surfaces treated by Pb2+/Pb(OH)+-BHA complexes, which reveals that Pb2+ and BHA form the complexes at first and then complexes with the superior collecting ability
Page 19 of 25
absorb on the surface. The Sn3d XPS of cassiterite is present in Fig.10. The results illustrate that the Sn3d XPS band of cassiterite is composed of two components at 486.66 eV and 495.06 eV. After treatment (a), the Sn3d XPS bands slightly shifts to
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around 486.54 eV and 494.96 eV, respectively. The finding indicates that the adsorption of Pb(OH)+ onto cassiterite changes the chemical circumstance of Sn
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atoms, implying the appearance of Pb-O-Sn bond because of the smaller value of
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Sn3d. And for (b), the Sn3d peaks emerges in the same position, which reveals the
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same adsorption form on the mineral surface.
Cassiterite Sn3d3/2495.06
M
80000
ce pt
40000
484
488
492
496
Binding energy (eV)
Ac
0
Cassiterite Sn3d5/2486.66
ed
Counts/s
120000
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(a)
Cassiterite Sn3d3/2494.96
Cassiterite Sn3d5/2486.54
80000
ip t
Counts/s
120000
488
492
496
us
0 484
cr
40000
Binding energy (eV)
an
40000
ce pt
20000
0 484
Cassiterite Sn3d5/2486.54
ed
Counts/s
Cassiterite Sn3d3/2494.96
M
(b) 60000
488
492
496
Binding energy (eV)
(b)
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Fig.10. Sn3d XPS of cassiterit untreated and treated by Pb+BHA (a) and Pb/Pb(OH)-BHA complexes
Page 21 of 25
ip t cr us
Fig.11. The chemisorption model of BHA (lead(Ⅱ) ions as the activator) and Pb2+/Pb(OH)+-BHA complexes adsorbed on cassiterite surfaces
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4. Conclusions
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Flotation and adsorption studies indicate that the complexes formed by lead (Ⅱ) ions and BHA have a stronger collecting ability for cassiterite than using lead (Ⅱ) ions
ed
as the activator and BHA as the collector, respectively. At optimum flotation pH value around 9, it is found that [Pb(BHA)]+ and Pb(BHA)OH are predominant as the
ce pt
fractions of total lead(II) ions in the system by voltammetric measurements. The results of zeta potential deduce a stronger affinity to the mineral surface of complexes
Ac
in weak basic solution. XPS and FTIR studies provide evidence in support of chemical interaction between caasiterite and Pb2+/Pb(OH)+-BHA complexes, and the similar adsorption form of the complexes on mineral surface with using lead (Ⅱ) ions and BHA as the activator and the collector, respectively. Acknowledgments This work was supported financially by Innovation Driven Plan of Central South University（No. 2015CX005）, the National 111 Project (No. B14034), Collaborative
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Innovation Center for Clean and Efficient utilization of Strategic Metal Mineral Resources, Sublimation scholars distinguished professor of Central South University
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ce pt
ed
M
an
us
cr
ip t
and the National science and Technology Support Project of China (2015BAB14B02).
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ip t
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cr
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[13] B. Liu, X. Wang, H. Du, J. Liu, and S. Zheng, The surface features of lead activation in amyl xanthate flotation of quartz. International Journal of Mineral Processing (2016). [14] H. Li, S. Mu, X. Weng, Y. Zhao, and S. Song, Rutile flotation with Pb2+ ions as activator: Adsorption of Pb2+ at rutile/water interface. Colloids and Surfaces A: Physicochemical and Engineering Aspects 506 (2016) 431-437. [15] C. Liu, Q. Feng, G. Zhang, W. Ma, and Q. Meng, Effects of lead ions on the flotation of hemimorphite using sodium oleate. Minerals Engineering (2016). [16] A.M. Marabini, M. Ciriachi, P. Plescia, and M. Barbaro, Chelating reagents for flotation. Minerals Engineering 20 (2007) 1014-1025. [17] D. Chaturvedi, Langmuir, I. (1918). The adsorption of gases on plane surfaces of glass, mica and platinum. J. Am. Chem. Soc., 40, 1361-1403.. [18] M.A. Kamyabi, F. Soleymani-Bonoti, and S. Zakavi, Voltammetric determination of stability
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constants of lead complexes with diallyl disulfide, dimethyl disulfide, and diallyl sulfide. Chinese Chemical Letters 27 (2016) 71-76. [19] V. Cuculi, I. Pi Eta, and M. Branica, Voltammetric determination of stability constants of iron(III) –glycine complexes in water solution. Journal of Electroanalytical Chemistry 583 (2005) 140-147. [20] J.I. Kigunda, Voltammetric determination of stability constants and speciation of copper in salt-water lake (Bogoria) in Kenya. Copper//Saline waters--Kenya//Voltammetry. (2012).
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[21] D.R. Crow, and D.R. Crow, Polarography of Metal Complexes, Academic Press, London, 1969.
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