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The flotation separation of pyrite from serpentine using lemon yellow as selective depressant
Colloids and Surfaces A 581 (2019) 123823
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The flotation separation of pyrite from serpentine using lemon yellow as selective depressant
T
⁎
Dezhi Liua,b, Guofan Zhanga,b, , Ganghong Huanga,b, Yawen Gaoa,b, Mengtao Wanga,b a
School of Minerals Processing and Bioengineering, Central South University, Changsha 410083, China Key Laboratory of Hunan Province for Clean and Efficient Utilization of Strategic Calcium-containing Mineral Resources, Central South University, Changsha 410083, China b
G R A P H I C A L A B S T R A C T
With the addition of LY, the electrostatic attraction between pyrite and serpentine was converted to electrostatic repulsion. Therefore, the hetero-coagulation was broken, and the pyrite floatability was regained.
A R T I C LE I N FO
A B S T R A C T
Keywords: Serpentine Pyrite Flotation separation Lemon yellow Electrostatic force
To limit the adverse effect of serpentine on the flotation of pyrite, lemon yellow (LY) was used as a potential depressant for serpentine, and the depression effects and mechanisms were systematically investigated in this paper. Micro-flotation results revealed that the addition of LY could efficiently limit the detrimental effect of serpentine on pyrite flotation with a maximum increase of pyrite recovery from 14% to 96% at pH 9.0. X-ray photoelectron spectroscopy analysis indicated that LY adsorbed on serpentine surface through physical interaction, and electrostatic interaction was considered as the main driving force. Zeta potential results demonstrated that the serpentine surface charge changed from positive to negative after interacting with LY. Thus, the electrostatic attraction between pyrite and serpentine was converted to electrostatic repulsion. Adsorption measurements showed that the adsorption density of potassium butyl xanthate (PBX) on pyrite surface regained after the addition of LY in the presence of serpentine, and thus the pyrite floatability was restored.
1. Introduction Serpentine is a stratified magnesium rich phyllosilicate that often
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associates with metal sulfide ore deposits as the gangue mineral [1–3], flotation is the commonly used method for collecting these sulfide minerals and other non-ferrous minerals [4]. As the studies reported
Corresponding author at: School of Mineral Processing and Bioengineering, Central South University, Changsha 410083, China. E-mail address: [email protected] (G. Zhang).
https://doi.org/10.1016/j.colsurfa.2019.123823 Received 25 June 2019; Received in revised form 15 August 2019; Accepted 18 August 2019 Available online 19 August 2019 0927-7757/ © 2019 Published by Elsevier B.V.
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China, respectively. The samples were crushed to −1 mm using a laboratory roll crusher and grounded in an agate mortar to the designed fraction. Size fractions of -150 + 74 μm pyrite and −10 μm serpentine were collected for flotation tests, and XPS tests. The size fractions of pyrite and serpentine used in zeta potential measurements were −2 μm. Besides, the −38 μm pyrite with a BET surface area of 3.16 m2/ g and −10 μm serpentine were used of adsorption measurements. Fig. 2 shows the XRD patterns of pyrite and serpentine used in this work, and it can be seen that the purities of the two minerals was very high. Potassium butyl xanthate (PBX, Macklin Biochemical., Ltd., Shanghai, China) and methyl isobutyl carbinol (MIBC, Tianzhuo Flotation Reagent Co., Ltd., Jian, Jiangxi, China) were used as collector and frother in this study, respectively. LY (with purity > 99.0%) and sodium hexametaphosphate (SHMP) used in this study were purchased from Guangfu Fine Chemical Research Institute, Tianjin, China. Sodium hydroxide (NaOH) and hydrochloric acid (HCl) were used as pH regulators for all experiments, and both of them were obtained from Zhuzhou Flotation Reagent Co., Ltd., Hunan Province of China. All reagents used in this study were of analytical grade. Distilled water with a resistivity of 18.2 MΩ.cm at 25 ℃ was adopted for all the experiments. FTIR study of LY was performed and the result was exhibited in Fig. 3. As shown in Fig. 3, the peak at 3479.9 cm−1 is attributed to the stretching vibration of eOH. The peaks at 1641.5 cm−1, 1486.3 cm−1, and 1415.5 cm−1 are caused by the vibration of the framework of the benzene ring in LY structure, and the peaks at 650 – 900 cm−1 represent the out-of-plane deformation vibration of the C–H of the benzene ring. Besides, the peak at 1351.9 cm-1 is corresponding to the characteristic peak of the CeN attached to the benzene ring, and the stretching vibration of N]N appears at 1567.9 cm−1 [23].
previously, in the flotation process of sulfide ores, serpentine can form the slimes coating on the surface of objective minerals through electrostatic attraction at weak alkaline conditions (around pH 8.5) [5–7]. The hydrophilic serpentine slimes can reduce the adsorption of collector and decrease the hydrophobicity of sulfide minerals [8,9]. In addition, the serpentine entrained in flotation concentrate will also lower the grade of valuable minerals and cause problems in further smelting process [10,11]. To address the problem of the flotation separation of sulfides from serpentine and other Mg-bearing silicate minerals, polysaccharide depressants such as carboxymethyl cellulose (CMC), N-carboxymethyl chitosan (N-CMC), guar gum have been used to disperse slime particles of serpentine from sulfide surfaces [6,12–14]. However, the poor solubility of the depressants above normally results in large solution volume additions [3,15]. Sodium hexametaphosphate (SHMP) is also reported as an effective depressant in the pyrite-serpentine system [16]. But SHMP is easy to decompose to sodium phosphate (Na3PO4), thus decreasing the depression effect of SHMP on serpentine [17]. As common coagulants used in wastewater treatments [18], aluminum sulfate (Al2(SO4)3) and aluminum potassium sulfate dodecahydrate (KAl(SO4)2·12H2O) have also been reported to have the potential to limit the adverse effect of fine serpentine particles on sulfides flotation due to the coagulation of Al(OH)3(s) hydrolyzed by Al3+ [19,20]. However, there are few studies on the utilization of Al2(SO4)3 and KAl (SO4)2·12H2O in industrial flotation of sulfides. Besides, the aluminum hydroxyl formed in solutions has an inevitable depression effect on sulfides flotation [21]. Therefore, a new depressant that can depress serpentine efficiently is urgent to be studied on sulfide flotation. Lemon yellow (LY) (C16H9N4O9S2Na3) is known as tartrazine, a synthetic azo acid dye, such as textiles, leather, foodstuffs and cosmetics [22]. The main composition of LY is 1-(4-sulfonate phenyl)-4-(4sulfonate phenyl azo)-5-pyrazolone -3- trisodium carboxylate, and its molecular structure is shown in Fig. 1. Due to the two -SO3Na and one −COONa groups in LY molecular structure (see Fig. 1), the LY is electronegative in aqueous solution. Besides, the -SO3− and −COOmight be able to interact with Mg2+ on serpentine surface. Therefore, LY has the potential to act as a depressant for serpentine in the flotation of pyrite. However, there is little literature referring to using LY in flotation field, especially in the separation of pyrite from serpentine. In this research, LY was introduced as a flotation modifier to separate pyrite from serpentine with potassium butyl xanthate (PBX) as collector. Flotation results showed its remarkable depression effect on serpentine, and the depression mechanism was analyzed through zeta potential measurements, adsorption measurements, and XPS studies.
2.2. Experiments 2.2.1. Micro-flotation tests Flotation of single minerals was performed with a laboratory mechanical agitation flotation machine (XFGCⅡ, Jilin Exploration Machinery Plant, China) [24]. For each test, 2 g pyrite which was treated by 5 min ultrasonic pretreatment and 0.2 g serpentine (if needed) samples were mixed with 35 mL distilled water in a 40 mL plexiglass cell. The pulp pH was adjusted by adding NaOH and HCl stock solutions, then LY and collector were added sequentially to the pulp, and the conditioning time for each reagent was 3 min. The frother MIBC was then added into the suspension with 1 min conditioning time before the beginning of flotation, and the flotation time was 3 min. Following this, both the concentrate and tailing were collected, filtered, dried, then weighed, and the mass distributions between two products were used to calculate the flotation recovery. Each micro-flotation test was repeated three times, the average value was adopted as the flotation results shown in Fig. 4 and 5, and the standard deviation was calculated and presented as an error bar.
2. Experimental 2.1. Samples and reagents The pure mineral samples of pyrite and serpentine were obtained from Donghai, Jiangsu Province, and Yunfu, Guangdong Province of
2.2.2. Zeta potential measurements Zeta potential measurements on pyrite and serpentine with and without the addition of LY were performed on a Coulter Delsa 440sx Zeta Potential Analyzer (Malvern, Instruments Ltd, United Kingdom). The suspension for measurement was prepared by dispersing 40 mg mineral samples into 40 mL of the KNO3 background electrolyte solution with a concentration of 1×10−3 mol/L. The pH was adjusted to 9.0 and then LY (if needed) was added and conditioned for 10 min to allow the system to equilibrate. After 5 min of settling, the supernatant liquid was collected for measurements. Each measurement was repeated three times, the average value was exhibited and the standard deviation was calculated and reported as error bar shown in Fig. 6. 2.2.3. Adsorption measurements The adsorption of PBX on pyrite and serpentine was investigated on a UV-2001 ultraviolet spectrophotometer (Rayleigh, Beijing, China)
Fig. 1. Molecular structure of LY. 2
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Fig. 2. XRD of pyrite (a) and serpentine (b) samples.
Fig. 3. FTIR spectrum of LY.
Fig. 5. Effect of depressant concentration on the flotation recovery of pyrite under different conditions ([PBX] = 7 mg/L, [MIBC] = 20.44 mg/L, pH = 9.0).
Fig. 4. Effect of pulp pH on the flotation recovery of pyrite under different conditions ([PBX] = 7 mg/L, [MIBC] = 20.44 mg/L, [LY] =50 mg/L).
Fig. 6. Zeta potentials of pyrite and serpentine as a function of pH under different conditions ([LY] =50 mg/L).
with the absorbance at 300 nm. For each test, 1 g pure pyrite was added into 100 mL distilled water in a 250 mL Erlenmeyer flask. The suspension was stirred evenly after adjusting pH to the desired value. Then LY and PBX were added according to the reagent condition and conditioning time. After that, the suspension was centrifuged and filtered,
and the filtered liquor was collected for adsorption measurements. The PBX concentration in supernatant was measured and calculated based on the standard curve of PBX which was obtained by different concentration of PBX, and the adsorbed amount of PBX on mineral surface was calculated based on the difference between the PBX initial 3
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concentration and residual concentration in the solution. 2.2.4. X-ray photoelectron spectroscopy (XPS) X-ray photoelectron spectroscopy was utilized to determine the chemical compositions of mineral samples. For each test, 1 g serpentine was added into LY solution in a 40 ml beaker to prepare the slurry. The slurry pH and the LY concentration were adjusted to the desired value according to the flotation conditions, and then the slurry was magnetically stirred for 5 min. The slurry was then centrifuged and the solid particles were rinsed 3 times by the distilled water. After which, the samples were dried in a vacuum drying oven. The XPS tests were performed on a K-Alpha+ X-ray Photoelectron Spectroscopy (Thermo Fisher Scientific, USA) with monochromatic Al Kα (hμ = 1486.6 eV). The vacuum in the analyzer chamber was approximately 2 × 10−8 mbar, and the energy scale was calibrated using the C1 s peak of background hydrocarbon at 284.8 eV as an internal standard. 3. Results and discussions Fig. 7. Adsorption density of PBX on pyrite and serpentine as a function of PBX initial dosage([Serpentine] = 2 g/L, [LY] =50 mg/L, pH = 9.0).
3.1. Micro-flotation results Flotation studies of single minerals were performed first to investigate the effects of pulp pH, LY concentration, SHMP concentration on the flotation of pyrite-serpentine system. And the results are shown in Figs. 4 and 5. Fig. 4 shows the flotation recovery of pyrite as a function of pulp pH with and without serpentine particles. It is evident that high recoveries (98%) of pyrite was obtained in the absence of serpentine at pH 3.0-9.0. When the pulp pH increased, the flotation recovery of pyrite decreased due to the formation of Fe(OH)3 species on pyrite surface [25]. Compared to the flotation of bare pyrite, the flotation recovery of pyrite decreased significantly with increasing pH from 3.0 to 11.0 when 0.2 g/ L serpentine was added prior to the addition of collector, and a maximum decrease was obtained with the pyrite recovery of 14% at pH 9.0, which was similar to the observations of Zhang et al and Feng et al [12,26]. It can also be seen from Fig. 4 that the flotation recovery of pyrite was restored with the addition of LY. At pH 9.0, the recovery of pyrite depressed by serpentine increased from 14% to 96% with the addition of 50 mg/L LY, which indicated that the use of LY could efficiently eliminate the adverse effect of serpentine on pyrite flotation. The effects of LY and SHMP concentration on the flotation performance of pyrite at pH 9.0 in the presence of serpentine are exhibited in Fig. 5. It can be seen that with the increase of LY or SHMP concentration, the flotation recovery of pyrite in the presence of LY presented rising trends as same as using SHMP. Pyrite recovery reached the maximum value (around 96%) when 50 mg/L LY was added, and the recovery remained stabilized when LY concentration was higher than 50 mg/L. However, when SHMP was used as the depressant, a maximum increment of pyrite recovery could be obtained from 14% to 85% with the SHMP concentration of 87.5 mg/L. The results showed that the LY possessed a better depression effect on serpentine compared with SHMP.
surface charge of serpentine significantly, which suggested that LY could interact with serpentine surface. After that, the positive charge of serpentine surface was converted to negative at the whole pH tested. It can also be seen from Fig. 6 that the addition of LY had a slight influence on pyrite surface charge. After the addition of LY with negative charge, the electrostatic repulsion appeared between the added counterion and original counterion in the diffusion layer, which compressed the original counterion into the tight layer and made the diffusion layer thinner, lowering the magnitude of the zeta potential of pyrite [28]. However, the surface charge of pyrite remained in the negative area in the presence of LY. Following this, both of pyrite and serpentine surfaces were negatively charged at pH 9.0, and thus the electrostatic attraction between the two minerals was changed to electrostatic repulsion, and the hetero-coagulation between serpentine and pyrite was broken.
3.2. Zeta potential results
3.4. XPS spectroscopic analysis
Zeta potential measurements were performed on pyrite and serpentine in the absence and presence of LY. The results are shown in Fig. 6. It can be seen that the IEP (isoelectric point) of native serpentine was at pH 11.2, and the surface charge of serpentine was positive below pH 11.2, which was consistent to the previous study [3]. However, the surface of pyrite was negatively charged in the whole pH tested without reagent. Under neutral or weak alkaline conditions, where flotation of sulfide ores normally performed, the positively charged fine serpentine particles would attach on the negative pyrite surface through electrostatic attraction, and thus resulting in the hydrophilicity of pyrite [27]. As is also shown in Fig. 6, the addition of 50 mg/L LY decreased the
In order to get a better understanding of the interaction mechanism of LY and serpentine, the surface characteristics of serpentine samples before and after the treatment of LY were analyzed using XPS. Results of the surface analysis are exhibited in Table 1. The high-resolution spectra of N 1s, S 2p and Mg 1 s of serpentine are shown in Fig. 8. As shown in Fig. 8 (a), the N 1s peak of serpentine was extremely weak that can be considered as background noise, indicating that there was no nitrogen on the serpentine surface without the pretreatment of LY. After the treatment of LY, a new peak around 400.30 eV appeared in N 1s spectrum. The new peak can be deconvoluted into two peaks, the two peaks at 400.74 eV and 399.79 eV were originated from NeH and
3.3. Adsorption results Adsorption measurements were conducted to detect the adsorption density of PBX on mineral surfaces under different conditions. Fig. 7 shows the adsorption density of PBX on two mineral surfaces with different initial dosages at pH 9.0. The results show that PBX did not adsorb on serpentine surface, which was consistent with the earlier observations [20]. However, the presence of serpentine decreased the adsorption density of PBX on pyrite surface significantly, which indicated that hetero-coagulation occurred on pyrite surface and the adsorption of PBX on pyrite was prevented. It can also be seen from Fig. 7 that the adsorption density of PBX on pyrite surface was regained after adding 50 mg/L LY, suggesting that the serpentine slimes on pyrite surface were removed.
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Table 1 XPS characterization of samples. Sample
Serpentine Serpentine + LY
Mole fraction/% or (Binding energy/eV) C 1s
O 1s
Mg 1 s
Si 2p
N 1s
S 2p
8.82 (284.80) 9.82 (284.80)
57.41 (531.36) 56.92 (531.46)
21.20 (1303.21) 19.39 (1303.29)
12.57 (102.50) 12.65 (102.60)
/ 0.83 (400.30)
/ 0.39 (168.35)
initial serpentine, S 2p and N 1s adsorption on serpentine surface was found with an increase of C 1s content, illustrating that LY adsorbed on serpentine surface. Figs. 8 (e) and (f) show that the Mg 1 s spectrum of serpentine
C]NeC/CeN, respectively [29]. It can also be seen from S 2p spectra exhibited in Figs. 8 (c) and (d) that a new peak was observed at around 168.35 eV after LY treatment, which was corresponding to the eSO3− group [30,31]. It can also be seen from Table 1 that compared with
Fig. 8. High-resolution spectrum of (a, b) N1s, (c, d) S2p, and (e, f) Mg1s of serpentine and serpentine treated with LY. 5
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4. Conclusions
treated with and without LY. With reference to the date from United States National Institute of Standards and Technology (NIST), the peak at around 1303.21 eV in Fig. 8 (e) and 1303.29 eV in Fig. 8 (f) was assigned to Mg 1 s. However, it can also be seen from Figs. 8 (e) and (f) that the binding energy of Mg 1 s has little shift (0.08 eV) at the serpentine surface after the treatment of LY, indicating that the adsorption of LY on serpentine surface was through physical interaction. In the molecule structure of LY (see Fig. 1), there are two -SO3Na and one −COONa groups. Hence a number of sodium atoms deprotonated from LY molecule in solution, and negative charge formed in solution by ionized equilibrium. Because of the negative charge of LY in aqueous solution, it may be adsorbed on the positive charge minerals surface through electrostatic interaction. As shown in Fig. 6, pyrite surface is negatively charged while serpentine surface is positively charged at pH 9.0. The addition of LY decreases the zeta potential of serpentine significantly but has little effect on that of pyrite. Therefore, the adsorption of LY on serpentine surface occurred. Combining with the results of XPS tests that the little shift of the Mg 1 s and Si 2p binding energies of serpentine in the absence and presence of LY, we therefore believe that the adsorption of LY on serpentine surface is physical, and electrostatic interaction is considered as the main driving force. Based on the abovementioned analyses, the schematic illustration of the effect of LY on the separation of pyrite from serpentine at pH 9.0 is listed in Fig. 9. As shown in Fig. 9, due to the positive charge of serpentine and negative charge of pyrite in aqueous solution, the serpentine slimes coating was formed on the surface of pyrite by electrostatic attraction in the absence of LY [19,32], and then it prevented the adsorption of PBX on pyrite surface. Thus, pyrite was finally depressed. However, the positive charge of serpentine surface was reversed to negative one after the treatment of LY. Therefore, the electrostatic attraction between pyrite and serpentine might be changed to electrostatic repulsion, resulting in the elimination of the hetero-coagulation between the two minerals, and the flotation performance of pyrite was restored.
The utilization of LY as a potential depressant to selectively separate pyrite from fine serpentine was systematically investigated in this paper. Based on the results of micro-flotation tests, zeta potential measurements, adsorption measurements and XPS measurements above, conclusions could be reached as follows. Fine serpentine can depress the floatability of pyrite significantly due to the serpentine slimes when using PBX as the collector. However, the floatability of pyrite was restored through using LY as the depressant. According to the zeta potential measurements and XPS measurements, LY can adsorb on serpentine surface by electrostatic interaction, and decreased the zeta potential of serpentine significantly but had a negligible influence on that of pyrite. At pH 9.0, both pyrite and serpentine were negatively charged; the electrostatic attraction between pyrite and serpentine was converted to electrostatic repulsion; and the hetero-coagulation between two minerals was broken. Adsorption measurements also showed that the presence of serpentine slimes prevented the adsorption of PBX on pyrite surface. However, the adsorption density of PBX on pyrite surface was regained with the addition of LY, thereby achieving good flotation improvement. Therefore, LY is likely to be a reagent of great significance in the separation of pyrite-serpentine system. Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgments The authors acknowledge the support of the Major State Basic Research Development Program of China (973 program) (2014CB643402).
Fig. 9. Schematic illustration of LY on the separation of pyrite from serpentine. 6
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The flotation separation of pyrite from serpentine using lemon yellow as selective depressant
T
⁎
Dezhi Liua,b, Guofan Zhanga,b, , Ganghong Huanga,b, Yawen Gaoa,b, Mengtao Wanga,b a
School of Minerals Processing and Bioengineering, Central South University, Changsha 410083, China Key Laboratory of Hunan Province for Clean and Efficient Utilization of Strategic Calcium-containing Mineral Resources, Central South University, Changsha 410083, China b
G R A P H I C A L A B S T R A C T
With the addition of LY, the electrostatic attraction between pyrite and serpentine was converted to electrostatic repulsion. Therefore, the hetero-coagulation was broken, and the pyrite floatability was regained.
A R T I C LE I N FO
A B S T R A C T
Keywords: Serpentine Pyrite Flotation separation Lemon yellow Electrostatic force
To limit the adverse effect of serpentine on the flotation of pyrite, lemon yellow (LY) was used as a potential depressant for serpentine, and the depression effects and mechanisms were systematically investigated in this paper. Micro-flotation results revealed that the addition of LY could efficiently limit the detrimental effect of serpentine on pyrite flotation with a maximum increase of pyrite recovery from 14% to 96% at pH 9.0. X-ray photoelectron spectroscopy analysis indicated that LY adsorbed on serpentine surface through physical interaction, and electrostatic interaction was considered as the main driving force. Zeta potential results demonstrated that the serpentine surface charge changed from positive to negative after interacting with LY. Thus, the electrostatic attraction between pyrite and serpentine was converted to electrostatic repulsion. Adsorption measurements showed that the adsorption density of potassium butyl xanthate (PBX) on pyrite surface regained after the addition of LY in the presence of serpentine, and thus the pyrite floatability was restored.
1. Introduction Serpentine is a stratified magnesium rich phyllosilicate that often
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associates with metal sulfide ore deposits as the gangue mineral [1–3], flotation is the commonly used method for collecting these sulfide minerals and other non-ferrous minerals [4]. As the studies reported
Corresponding author at: School of Mineral Processing and Bioengineering, Central South University, Changsha 410083, China. E-mail address: [email protected] (G. Zhang).
https://doi.org/10.1016/j.colsurfa.2019.123823 Received 25 June 2019; Received in revised form 15 August 2019; Accepted 18 August 2019 Available online 19 August 2019 0927-7757/ © 2019 Published by Elsevier B.V.
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China, respectively. The samples were crushed to −1 mm using a laboratory roll crusher and grounded in an agate mortar to the designed fraction. Size fractions of -150 + 74 μm pyrite and −10 μm serpentine were collected for flotation tests, and XPS tests. The size fractions of pyrite and serpentine used in zeta potential measurements were −2 μm. Besides, the −38 μm pyrite with a BET surface area of 3.16 m2/ g and −10 μm serpentine were used of adsorption measurements. Fig. 2 shows the XRD patterns of pyrite and serpentine used in this work, and it can be seen that the purities of the two minerals was very high. Potassium butyl xanthate (PBX, Macklin Biochemical., Ltd., Shanghai, China) and methyl isobutyl carbinol (MIBC, Tianzhuo Flotation Reagent Co., Ltd., Jian, Jiangxi, China) were used as collector and frother in this study, respectively. LY (with purity > 99.0%) and sodium hexametaphosphate (SHMP) used in this study were purchased from Guangfu Fine Chemical Research Institute, Tianjin, China. Sodium hydroxide (NaOH) and hydrochloric acid (HCl) were used as pH regulators for all experiments, and both of them were obtained from Zhuzhou Flotation Reagent Co., Ltd., Hunan Province of China. All reagents used in this study were of analytical grade. Distilled water with a resistivity of 18.2 MΩ.cm at 25 ℃ was adopted for all the experiments. FTIR study of LY was performed and the result was exhibited in Fig. 3. As shown in Fig. 3, the peak at 3479.9 cm−1 is attributed to the stretching vibration of eOH. The peaks at 1641.5 cm−1, 1486.3 cm−1, and 1415.5 cm−1 are caused by the vibration of the framework of the benzene ring in LY structure, and the peaks at 650 – 900 cm−1 represent the out-of-plane deformation vibration of the C–H of the benzene ring. Besides, the peak at 1351.9 cm-1 is corresponding to the characteristic peak of the CeN attached to the benzene ring, and the stretching vibration of N]N appears at 1567.9 cm−1 [23].
previously, in the flotation process of sulfide ores, serpentine can form the slimes coating on the surface of objective minerals through electrostatic attraction at weak alkaline conditions (around pH 8.5) [5–7]. The hydrophilic serpentine slimes can reduce the adsorption of collector and decrease the hydrophobicity of sulfide minerals [8,9]. In addition, the serpentine entrained in flotation concentrate will also lower the grade of valuable minerals and cause problems in further smelting process [10,11]. To address the problem of the flotation separation of sulfides from serpentine and other Mg-bearing silicate minerals, polysaccharide depressants such as carboxymethyl cellulose (CMC), N-carboxymethyl chitosan (N-CMC), guar gum have been used to disperse slime particles of serpentine from sulfide surfaces [6,12–14]. However, the poor solubility of the depressants above normally results in large solution volume additions [3,15]. Sodium hexametaphosphate (SHMP) is also reported as an effective depressant in the pyrite-serpentine system [16]. But SHMP is easy to decompose to sodium phosphate (Na3PO4), thus decreasing the depression effect of SHMP on serpentine [17]. As common coagulants used in wastewater treatments [18], aluminum sulfate (Al2(SO4)3) and aluminum potassium sulfate dodecahydrate (KAl(SO4)2·12H2O) have also been reported to have the potential to limit the adverse effect of fine serpentine particles on sulfides flotation due to the coagulation of Al(OH)3(s) hydrolyzed by Al3+ [19,20]. However, there are few studies on the utilization of Al2(SO4)3 and KAl (SO4)2·12H2O in industrial flotation of sulfides. Besides, the aluminum hydroxyl formed in solutions has an inevitable depression effect on sulfides flotation [21]. Therefore, a new depressant that can depress serpentine efficiently is urgent to be studied on sulfide flotation. Lemon yellow (LY) (C16H9N4O9S2Na3) is known as tartrazine, a synthetic azo acid dye, such as textiles, leather, foodstuffs and cosmetics [22]. The main composition of LY is 1-(4-sulfonate phenyl)-4-(4sulfonate phenyl azo)-5-pyrazolone -3- trisodium carboxylate, and its molecular structure is shown in Fig. 1. Due to the two -SO3Na and one −COONa groups in LY molecular structure (see Fig. 1), the LY is electronegative in aqueous solution. Besides, the -SO3− and −COOmight be able to interact with Mg2+ on serpentine surface. Therefore, LY has the potential to act as a depressant for serpentine in the flotation of pyrite. However, there is little literature referring to using LY in flotation field, especially in the separation of pyrite from serpentine. In this research, LY was introduced as a flotation modifier to separate pyrite from serpentine with potassium butyl xanthate (PBX) as collector. Flotation results showed its remarkable depression effect on serpentine, and the depression mechanism was analyzed through zeta potential measurements, adsorption measurements, and XPS studies.
2.2. Experiments 2.2.1. Micro-flotation tests Flotation of single minerals was performed with a laboratory mechanical agitation flotation machine (XFGCⅡ, Jilin Exploration Machinery Plant, China) [24]. For each test, 2 g pyrite which was treated by 5 min ultrasonic pretreatment and 0.2 g serpentine (if needed) samples were mixed with 35 mL distilled water in a 40 mL plexiglass cell. The pulp pH was adjusted by adding NaOH and HCl stock solutions, then LY and collector were added sequentially to the pulp, and the conditioning time for each reagent was 3 min. The frother MIBC was then added into the suspension with 1 min conditioning time before the beginning of flotation, and the flotation time was 3 min. Following this, both the concentrate and tailing were collected, filtered, dried, then weighed, and the mass distributions between two products were used to calculate the flotation recovery. Each micro-flotation test was repeated three times, the average value was adopted as the flotation results shown in Fig. 4 and 5, and the standard deviation was calculated and presented as an error bar.
2. Experimental 2.1. Samples and reagents The pure mineral samples of pyrite and serpentine were obtained from Donghai, Jiangsu Province, and Yunfu, Guangdong Province of
2.2.2. Zeta potential measurements Zeta potential measurements on pyrite and serpentine with and without the addition of LY were performed on a Coulter Delsa 440sx Zeta Potential Analyzer (Malvern, Instruments Ltd, United Kingdom). The suspension for measurement was prepared by dispersing 40 mg mineral samples into 40 mL of the KNO3 background electrolyte solution with a concentration of 1×10−3 mol/L. The pH was adjusted to 9.0 and then LY (if needed) was added and conditioned for 10 min to allow the system to equilibrate. After 5 min of settling, the supernatant liquid was collected for measurements. Each measurement was repeated three times, the average value was exhibited and the standard deviation was calculated and reported as error bar shown in Fig. 6. 2.2.3. Adsorption measurements The adsorption of PBX on pyrite and serpentine was investigated on a UV-2001 ultraviolet spectrophotometer (Rayleigh, Beijing, China)
Fig. 1. Molecular structure of LY. 2
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Fig. 2. XRD of pyrite (a) and serpentine (b) samples.
Fig. 3. FTIR spectrum of LY.
Fig. 5. Effect of depressant concentration on the flotation recovery of pyrite under different conditions ([PBX] = 7 mg/L, [MIBC] = 20.44 mg/L, pH = 9.0).
Fig. 4. Effect of pulp pH on the flotation recovery of pyrite under different conditions ([PBX] = 7 mg/L, [MIBC] = 20.44 mg/L, [LY] =50 mg/L).
Fig. 6. Zeta potentials of pyrite and serpentine as a function of pH under different conditions ([LY] =50 mg/L).
with the absorbance at 300 nm. For each test, 1 g pure pyrite was added into 100 mL distilled water in a 250 mL Erlenmeyer flask. The suspension was stirred evenly after adjusting pH to the desired value. Then LY and PBX were added according to the reagent condition and conditioning time. After that, the suspension was centrifuged and filtered,
and the filtered liquor was collected for adsorption measurements. The PBX concentration in supernatant was measured and calculated based on the standard curve of PBX which was obtained by different concentration of PBX, and the adsorbed amount of PBX on mineral surface was calculated based on the difference between the PBX initial 3
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concentration and residual concentration in the solution. 2.2.4. X-ray photoelectron spectroscopy (XPS) X-ray photoelectron spectroscopy was utilized to determine the chemical compositions of mineral samples. For each test, 1 g serpentine was added into LY solution in a 40 ml beaker to prepare the slurry. The slurry pH and the LY concentration were adjusted to the desired value according to the flotation conditions, and then the slurry was magnetically stirred for 5 min. The slurry was then centrifuged and the solid particles were rinsed 3 times by the distilled water. After which, the samples were dried in a vacuum drying oven. The XPS tests were performed on a K-Alpha+ X-ray Photoelectron Spectroscopy (Thermo Fisher Scientific, USA) with monochromatic Al Kα (hμ = 1486.6 eV). The vacuum in the analyzer chamber was approximately 2 × 10−8 mbar, and the energy scale was calibrated using the C1 s peak of background hydrocarbon at 284.8 eV as an internal standard. 3. Results and discussions Fig. 7. Adsorption density of PBX on pyrite and serpentine as a function of PBX initial dosage([Serpentine] = 2 g/L, [LY] =50 mg/L, pH = 9.0).
3.1. Micro-flotation results Flotation studies of single minerals were performed first to investigate the effects of pulp pH, LY concentration, SHMP concentration on the flotation of pyrite-serpentine system. And the results are shown in Figs. 4 and 5. Fig. 4 shows the flotation recovery of pyrite as a function of pulp pH with and without serpentine particles. It is evident that high recoveries (98%) of pyrite was obtained in the absence of serpentine at pH 3.0-9.0. When the pulp pH increased, the flotation recovery of pyrite decreased due to the formation of Fe(OH)3 species on pyrite surface [25]. Compared to the flotation of bare pyrite, the flotation recovery of pyrite decreased significantly with increasing pH from 3.0 to 11.0 when 0.2 g/ L serpentine was added prior to the addition of collector, and a maximum decrease was obtained with the pyrite recovery of 14% at pH 9.0, which was similar to the observations of Zhang et al and Feng et al [12,26]. It can also be seen from Fig. 4 that the flotation recovery of pyrite was restored with the addition of LY. At pH 9.0, the recovery of pyrite depressed by serpentine increased from 14% to 96% with the addition of 50 mg/L LY, which indicated that the use of LY could efficiently eliminate the adverse effect of serpentine on pyrite flotation. The effects of LY and SHMP concentration on the flotation performance of pyrite at pH 9.0 in the presence of serpentine are exhibited in Fig. 5. It can be seen that with the increase of LY or SHMP concentration, the flotation recovery of pyrite in the presence of LY presented rising trends as same as using SHMP. Pyrite recovery reached the maximum value (around 96%) when 50 mg/L LY was added, and the recovery remained stabilized when LY concentration was higher than 50 mg/L. However, when SHMP was used as the depressant, a maximum increment of pyrite recovery could be obtained from 14% to 85% with the SHMP concentration of 87.5 mg/L. The results showed that the LY possessed a better depression effect on serpentine compared with SHMP.
surface charge of serpentine significantly, which suggested that LY could interact with serpentine surface. After that, the positive charge of serpentine surface was converted to negative at the whole pH tested. It can also be seen from Fig. 6 that the addition of LY had a slight influence on pyrite surface charge. After the addition of LY with negative charge, the electrostatic repulsion appeared between the added counterion and original counterion in the diffusion layer, which compressed the original counterion into the tight layer and made the diffusion layer thinner, lowering the magnitude of the zeta potential of pyrite [28]. However, the surface charge of pyrite remained in the negative area in the presence of LY. Following this, both of pyrite and serpentine surfaces were negatively charged at pH 9.0, and thus the electrostatic attraction between the two minerals was changed to electrostatic repulsion, and the hetero-coagulation between serpentine and pyrite was broken.
3.2. Zeta potential results
3.4. XPS spectroscopic analysis
Zeta potential measurements were performed on pyrite and serpentine in the absence and presence of LY. The results are shown in Fig. 6. It can be seen that the IEP (isoelectric point) of native serpentine was at pH 11.2, and the surface charge of serpentine was positive below pH 11.2, which was consistent to the previous study [3]. However, the surface of pyrite was negatively charged in the whole pH tested without reagent. Under neutral or weak alkaline conditions, where flotation of sulfide ores normally performed, the positively charged fine serpentine particles would attach on the negative pyrite surface through electrostatic attraction, and thus resulting in the hydrophilicity of pyrite [27]. As is also shown in Fig. 6, the addition of 50 mg/L LY decreased the
In order to get a better understanding of the interaction mechanism of LY and serpentine, the surface characteristics of serpentine samples before and after the treatment of LY were analyzed using XPS. Results of the surface analysis are exhibited in Table 1. The high-resolution spectra of N 1s, S 2p and Mg 1 s of serpentine are shown in Fig. 8. As shown in Fig. 8 (a), the N 1s peak of serpentine was extremely weak that can be considered as background noise, indicating that there was no nitrogen on the serpentine surface without the pretreatment of LY. After the treatment of LY, a new peak around 400.30 eV appeared in N 1s spectrum. The new peak can be deconvoluted into two peaks, the two peaks at 400.74 eV and 399.79 eV were originated from NeH and
3.3. Adsorption results Adsorption measurements were conducted to detect the adsorption density of PBX on mineral surfaces under different conditions. Fig. 7 shows the adsorption density of PBX on two mineral surfaces with different initial dosages at pH 9.0. The results show that PBX did not adsorb on serpentine surface, which was consistent with the earlier observations [20]. However, the presence of serpentine decreased the adsorption density of PBX on pyrite surface significantly, which indicated that hetero-coagulation occurred on pyrite surface and the adsorption of PBX on pyrite was prevented. It can also be seen from Fig. 7 that the adsorption density of PBX on pyrite surface was regained after adding 50 mg/L LY, suggesting that the serpentine slimes on pyrite surface were removed.
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Table 1 XPS characterization of samples. Sample
Serpentine Serpentine + LY
Mole fraction/% or (Binding energy/eV) C 1s
O 1s
Mg 1 s
Si 2p
N 1s
S 2p
8.82 (284.80) 9.82 (284.80)
57.41 (531.36) 56.92 (531.46)
21.20 (1303.21) 19.39 (1303.29)
12.57 (102.50) 12.65 (102.60)
/ 0.83 (400.30)
/ 0.39 (168.35)
initial serpentine, S 2p and N 1s adsorption on serpentine surface was found with an increase of C 1s content, illustrating that LY adsorbed on serpentine surface. Figs. 8 (e) and (f) show that the Mg 1 s spectrum of serpentine
C]NeC/CeN, respectively [29]. It can also be seen from S 2p spectra exhibited in Figs. 8 (c) and (d) that a new peak was observed at around 168.35 eV after LY treatment, which was corresponding to the eSO3− group [30,31]. It can also be seen from Table 1 that compared with
Fig. 8. High-resolution spectrum of (a, b) N1s, (c, d) S2p, and (e, f) Mg1s of serpentine and serpentine treated with LY. 5
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4. Conclusions
treated with and without LY. With reference to the date from United States National Institute of Standards and Technology (NIST), the peak at around 1303.21 eV in Fig. 8 (e) and 1303.29 eV in Fig. 8 (f) was assigned to Mg 1 s. However, it can also be seen from Figs. 8 (e) and (f) that the binding energy of Mg 1 s has little shift (0.08 eV) at the serpentine surface after the treatment of LY, indicating that the adsorption of LY on serpentine surface was through physical interaction. In the molecule structure of LY (see Fig. 1), there are two -SO3Na and one −COONa groups. Hence a number of sodium atoms deprotonated from LY molecule in solution, and negative charge formed in solution by ionized equilibrium. Because of the negative charge of LY in aqueous solution, it may be adsorbed on the positive charge minerals surface through electrostatic interaction. As shown in Fig. 6, pyrite surface is negatively charged while serpentine surface is positively charged at pH 9.0. The addition of LY decreases the zeta potential of serpentine significantly but has little effect on that of pyrite. Therefore, the adsorption of LY on serpentine surface occurred. Combining with the results of XPS tests that the little shift of the Mg 1 s and Si 2p binding energies of serpentine in the absence and presence of LY, we therefore believe that the adsorption of LY on serpentine surface is physical, and electrostatic interaction is considered as the main driving force. Based on the abovementioned analyses, the schematic illustration of the effect of LY on the separation of pyrite from serpentine at pH 9.0 is listed in Fig. 9. As shown in Fig. 9, due to the positive charge of serpentine and negative charge of pyrite in aqueous solution, the serpentine slimes coating was formed on the surface of pyrite by electrostatic attraction in the absence of LY [19,32], and then it prevented the adsorption of PBX on pyrite surface. Thus, pyrite was finally depressed. However, the positive charge of serpentine surface was reversed to negative one after the treatment of LY. Therefore, the electrostatic attraction between pyrite and serpentine might be changed to electrostatic repulsion, resulting in the elimination of the hetero-coagulation between the two minerals, and the flotation performance of pyrite was restored.
The utilization of LY as a potential depressant to selectively separate pyrite from fine serpentine was systematically investigated in this paper. Based on the results of micro-flotation tests, zeta potential measurements, adsorption measurements and XPS measurements above, conclusions could be reached as follows. Fine serpentine can depress the floatability of pyrite significantly due to the serpentine slimes when using PBX as the collector. However, the floatability of pyrite was restored through using LY as the depressant. According to the zeta potential measurements and XPS measurements, LY can adsorb on serpentine surface by electrostatic interaction, and decreased the zeta potential of serpentine significantly but had a negligible influence on that of pyrite. At pH 9.0, both pyrite and serpentine were negatively charged; the electrostatic attraction between pyrite and serpentine was converted to electrostatic repulsion; and the hetero-coagulation between two minerals was broken. Adsorption measurements also showed that the presence of serpentine slimes prevented the adsorption of PBX on pyrite surface. However, the adsorption density of PBX on pyrite surface was regained with the addition of LY, thereby achieving good flotation improvement. Therefore, LY is likely to be a reagent of great significance in the separation of pyrite-serpentine system. Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgments The authors acknowledge the support of the Major State Basic Research Development Program of China (973 program) (2014CB643402).
Fig. 9. Schematic illustration of LY on the separation of pyrite from serpentine. 6
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