Interaction mechanism of miscible DDA–Kerosene and fine quartz and its effect on the reverse flotation of magnetic separation concentrate

Minerals Engineering 65 (2014) 41–50

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Interaction mechanism of miscible DDA–Kerosene and fine quartz and its effect on the reverse flotation of magnetic separation concentrate An Liu, Min-qiang Fan ⇑, Pan-pan Fan College of Mining Engineering, Taiyuan University of Technology, Taiyuan 030024, Shanxi, China

a r t i c l e

i n f o

Article history: Received 11 December 2013 Revised 24 March 2014 Accepted 7 May 2014

Keywords: Quartz DDA–Kerosene Adsorption Agglomeration Magnetic separation concentrate

a b s t r a c t In this study, a modification of oil assisted flotation processes of quartz particles has been proposed, which is based on introduction of miscible Dodecylamine (DDA)-Kerosene as collector with DDA cationic surfactant coated on kerosene to the hydrophilic quartz particles in the pulp. The property of miscible DDA–Kerosene emulsion was investigated. Due to the adsorption of DDA at kerosene/water interface, a smaller and uniform kerosene emulsion formed. Addition of cationic surfactant to the kerosene emulsion changed the zeta potential value from negative to positive, which resulted in enhancing the adhesion of the oil droplets to negatively charged quartz. The results showed that agglomeration and flotation process can be realized simultaneously with DDA–Kerosene. The agglomeration of fine quartz minerals in the presence of miscible DDA–Kerosene led to the formation of very large compact agglomerates resulting in increasing hydrophobicity of the particles and inducing a higher probability of collision and adhesion to air bubble. Experimental data indicated that miscible DDA–Kerosene had better selectivity and stronger collectability to quartz than DDA–HCl, which can be used as an efficient collector in the reverse flotation of magnetic separation concentrate of TISCO. At the same DDA dosage (60 g/t), separation efficiency got to 18.53% when using DDA–HCl as collector; while a better result was obtained with DDA–Kerosene, the efficiency of separation reached 59.07% which was identical with 120 g/t DDA–HCl. Ó 2014 Elsevier Ltd. All rights reserved.

1. Introduction In the recent years, the demand for high-quality magnetite in the iron and steel industries has increased significantly. Major global reserves of iron ores are associated with formations of magnetite–hematite ores of metamorphogenic origin, which are represented by fine-grained magnetite–hematite ores with Fe content at 25–40% and rich ores with Fe content at 50–70% (Glembotsky and Bekhtle, 1964). The most effective technologies for the beneficiation of iron quartzite are wet magnetic separation in low intensity magnetic field and reverse flotation of quartz. Removing quartz from iron ore is of great importance because it represents one of the most common gangue minerals in iron ores. In the reverse flotation of quartz, finely disseminated magnetite ore characterized by embedding with quartz must be ground to 38 lm fraction up to 50–60% to reach the required liberation degree for achieving concentrate grades within the specifications. Also a large amount of slime is produced during mineral regrinding processing. These fine particles would cause numerous problems in concentration stage and other treatment stages. First, ⇑ Corresponding author. Tel.: +86 351 6014776. E-mail address: [email protected] (M.-q. Fan). http://dx.doi.org/10.1016/j.mineng.2014.05.008 0892-6875/Ó 2014 Elsevier Ltd. All rights reserved.

fine quartz particles exhibit much higher specific surface area than coarse particles, so the collector consumption required for a certain coverage degree is much larger for fine particles than coarse particles (Vieira and Peres, 2007); heterocoagulation may occur, the slime may adsorb on the useful mineral surface changing the surface properties. On the other hand, froth flotation poorly responds to fine mineral particles due to the small mass of the particles and thus low probability of collision and adhesion of particles to air bubble, leading to a low flotation rate (Arbiter, 1979; Fuerstenaua et al., 1979). It is well accepted that floc-flotation, in which mineral fines are floated as hydrophobic agglomerates, is an effective process to improve the beneficiation of valuable minerals in fine particle size (House and Veal, 1992; Coleman et al., 1995; Song and Trass, 1997; Yin et al., 2011; Pascoe and Doherty, 1997; Song et al., 2001). It has been long known that hydrophobic flocculation can be greatly enhanced by the addition of non-polar oil (Song et al., 2012; Song et al., 1999). In the conventional oil agglomeration process, the oil drops interact with the particles previously hydrophobized with a collector and then spreads over the surface of the minerals as oil layer following the formation of agglomerates (Song et al., 1999). This process features a large amount of fuel oil addition to induce the hydrophobic flocculation of desired mineral fines and

A. Liu et al. / Minerals Engineering 65 (2014) 41–50

to strengthen the adhesion of flocs to air bubbles (Laskowski and Yu, 2000). It has been shown, that the quantity of bridging liquid required for sufficient agglomeration of calcite was 140 kg/t (Cebeci and Sönmez, 2004), of molybdenite was 13.8 kg/t (Fu et al., 2012), and of coal was 128 kg/t (Cebeci et al., 2002), respectively. Recently, numerous researchers have concerned with the oil agglomeration flotation of fine sulfide mineral for the purification of coal from sulfide minerals and coal ash (Rodrigues and Matiolo, 2007; Laskowski and Yu, 2000; Mehrotra et al., 1983), agglomeration of oxide mineral such as calcite and hematite (Cebeci and Sönmez, 2004; Sadowski and Polowczyk, 2004) and recovery of fine sized gold (Sen et al., 2010; Moses and Petersen, 2000). It has been shown, fuel oil used as auxiliary collector in the conventional flotation process has been investigated by many researchers, and the technique has been utilized in the mineral industry for the purification of several minerals such as zinc ores and phosphate ores flotation (Sis, 2001; Sis and Chander, 2003; Pereira, 2003; Araujo et al., 2005; Glembotsky, 1963). To sum up, most of the oil agglomeration studies and oil assist flotation studies have been performed with natural hydrophobic particles as well as previously hydrophobized particles. The interaction mechanism of oil and particles is mainly due to hydrophobic interaction. Hydrophobization of mineral surface is the premise of deep interaction of oil and mineral. Jianshan iron (Loufan, Shanxi Province, China) ore bodies occur in Yuanjiacun formation Luliang group and the main crystalline phases are magnetite, quartz and a small amount of grunerite. The iron ore belongs to magnetite–quartz and magnetite–grunerite type deposit containing 35.55% Fe and 41.06–48.36% SiO2 with characteristics of fine dissemination size range of 0.02–0.10 mm (Fan and Niu, 1996). At present, the process consisting of three staged grinding, five staged low intensity magnetic separation and anionic reverse flotation was adopted for the beneficiation of the Jianshan iron ore. Anionic reverse flotation process needs to heat in the routine flotation and add lime as activator. Compared with the anionic reverse flotation process, cationic reverse floatation technology had overcome the shortage of heating in the routine floatation, and avoided the calcium ion contamination on subsequent transport and water treatment process. Flotation feed was the fifth staged low intensity magnetic separation concentrate containing 64.13% Fe and 9.35% SiO2, particle size range of 0.038 mm was more than 60%. Collector consumption would increase for fine particles. In order to reduce the DDA consumption meanwhile retain its collecting ability, a modification of oil assist flotation process was proposed in this work, which was based on introduction of miscible DDA–Kerosene as collector with cationic surfactant coated on kerosene to the hydrophilic quartz particles in the pulp. Distinguished from oil agglomerate flotation, a small amount of kerosene was added in the pulp premixed with the collector to form a miscible system reagent, and the agglomeration and flotation process was conducted simultaneously. The objective of this paper was to obtain understandings of the oil assist flotation of fine quartz with miscible DDA–Kerosene in order to develop a new reagent regime for the beneficiation of finely disseminated quartz from magnetite. The effect of kerosene in the flotation was especially investigated. 2. Materials and methods

+74 lm). The volume median diameter (d50) of the fine quartz and coarse quartz was 27.6 and 126.4 lm respectively. The real ore sample was a wet magnetic separation concentrate of the primary iron ore concentration containing 64.13% Fe and 9.35% SiO2 from Jianshan iron ore processing plant of Taiyuan Iron & Steel Corporation (TISCO). The iron ore sample was analyzed by using a Shimadzu Labx XRD-6000 diffractometer with Cu Ka radiation and Ni for filter. Fig. 1 illustrated the X-ray diffraction pattern of the sample. The XRD analysis demonstrated that the main mineral contents of the ore sample were magnetite (Fe4O3) and quartz (SiO2). The size analysis of the sample was presented in Table 1. The mineral distribution was strongly affected by the particle size, and the total iron content increased with the decrease of grain size and silica content decreased with the decrease of grain size. Magnetite mainly distributed in the 0.038 mm fraction, and quartz mainly distributed in the 0.045–0.074 mm and 0.038 mm fraction. The 0.038 mm fraction was 63.79% by weight and had high iron content and low silica content, since it was represented mainly by liberated grains of Fe-oxides. The +0.045 mm fraction comprised locked particles of magnetite, as well as aggregates of gangue, which was evidenced by the high content of SiO2, and especially in the +0.074 mm fraction the silica content was up to 43.92%. The analytically grade DDA used in this study was obtained from Tianjin Guangfu Chemical Research Institute, reagent grade kerosene regarded as oily solvent was supplied by Aladdin Reagent. Hydrochloric acid (HCl) and sodium hydroxide (NaOH) were used as pH regulators and Methyl Isobutyl Carbinol (MIBC) was used as frother. HCl, NaOH and MIBC used in this study were of analytical grade obtained from Aladdin Reagent. DDA 100%neutralized with HCl regarded as DDA–HCl, the solution concentration was 2% in weight. Different concentrations of DDA were dissolved in the kerosene and stirred with a magnetic stirrer at

400

Magnetite 300

Intensity / cps

42

Quartz

200

100

0 20

40

50

60

70

80

2-Theta/deg. Fig. 1. XRD patterns of the magnetic separation concentrate sample.

Table 1 Result of size analyses of magnetic concentrate. Fraction/mm

Weight (%)

2.1. Samples and reagents A pure quartz (99%) sample from Lingshou (Hebei Province, PR China) was carefully comminuted to avoid contamination yielding two fractions designated as fine quartz (mainly in the size range 74 lm) and coarse quartz (mainly in the size range 150 lm to

30

+0.074 0.045–0.074 0.038–0.045 0.038 Total (calc.)

2.36 18.45 15.40 63.79 100

Grade (%)

Distribution degree (%)

Fe

SiO2

Fe

SiO2

39.03 57.09 65.03 67.07 64.25

43.92 18.37 8.87 5.45 9.27

1.43 16.39 15.59 66.59 100

11.18 36.57 14.74 37.51 100

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A. Liu et al. / Minerals Engineering 65 (2014) 41–50

2.2. Methods 2.2.1. DDA–Kerosene binary system miscibility tests Miscibility of DDA and kerosene was determined by means of turbid-metric analysis under certain composition and temperature (Wu, 2003). Preparing various weight ratios of DDA–Kerosene samples at 50 °C, gradually cooling the environmental temperature, when the homogeneous solution became turbid or layered state, resulting in a significant absorption rise, the temperature was DDA–Kerosene’s critical solution temperature. 2.2.2. Flotation tests Flotation tests of pure quartz have been carried out in a XFG-1.5IV flotation machine with 1.5 L cell. Stirring speed has been set at 1500 rev/min, with pulp density at 20%, and the temperature of pulp was 25 °C. When DDA–HCl used as collector conditioning time was 2 min. When DDA or DDA–Kerosene used as collector, conditioning time was 2 min; MIBC should be added as frother as a result of poor foaming property of pure DDA, the dosage was 50 g/t, and agitation time with MIBC was 15 s. When DDA and kerosene used as collector, kerosene was added after DDA; DDA conditioning time was 2 min, and kerosene conditioning time was 2 min; MIBC was added as frother, the dosage was 50 g/t, and agitation time was 15 s. The water used in flotation tests was tap water and the value of pH was regulated by addition of NaOH and HCl solution in the tap water. It is noted that we did not measure the pulp pH after minerals were added in the water; the pH was referred to water pH in all the tests. For real ore sample magnetic separation concentrate the flotation procedure was the same as single mineral whereas with pulp density at 30%. 2.2.3. Observation of hydrophobic agglomerates and oil emulsion An optical microscope equipped with a digital camera was used to observe hydrophobic agglomerates in aqueous suspensions and oil droplets in emulsions. The samples were prepared by dropping quartz suspension or kerosene emulsion on a glass plate, followed by putting a thin glass slide on the drop to fix the suspension or emulsion. The oil emulsion was prepared by mixing kerosene or DDA–Kerosene with water in the concentration of 1 wt.%, followed by agitation treatment for 10 min at the speed of 1500 rev/min. The agglomeration of quartz fines was prepared in a flotation cell with the predetermined collector concentration. During the observation, some of agglomerates and oil droplets were photographed. 2.2.4. Zeta potential measurement The oil droplets at 0.3 wt.% of concentration in solutions were prepared with a high speed agitator. The agitation time lasted for about 10 min at 1500 rev/min in the presence and in the absence of DDA to obtain oil emulsion. About 0.5 ml of the prepared oil droplet emulsion was used to fill the electrophoresis cell. The zeta-potentials of oil emulsion were measured using a JS94H micro-electrophoresis apparatus of Powereach. Through the laser-illuminating and video-viewing system, the movement of the droplets in the upper stationary layer in the cell was traced, six times for each direction by alternating positive/negative electrode potentials. The captured images were then analyzed with the built-in software. The distribution histogram of electrophoretic mobility and corresponding average values were determined and converted to zeta potential values. In this study, the ambient temperature was maintained at 25 °C. The results presented in this

paper were the average of three independent measurements with a typical variation of ±2 mV. A suspension of quartz with 0.5 wt.% of solid concentration was prepared in 1 mM KNO3 supporting electrolyte solution and conditioned at 300 rev/min for 15 min in the presence of predetermined collector concentration and pH at 25 °C. The zeta-potentials of quartz were measured using a JS94H zeta plus apparatus of Powereach. The quartz samples were ground to 5 lm. NaOH and HCl were used for pH control. 2.2.5. Mineral particle size analysis and oil droplets size analysis The size distribution of the raw quartz ore, flotation froth concentrate and oil droplets were determined by using a S3500 particle size analyzer of Microtrac. The oil emulsion was prepared by mixing kerosene or DDA–Kerosene with water in the concentration of 1 wt.%, followed by agitation treatment for 10 min at the speed of 1500 rev/min. The froth concentrate was dried after the flotation. The raw quartz ore and flotation froth concentrate was the dry powder in the test. 2.2.6. Efficiency of separation Efficiency of separation reflects a composite index of separation performance. In this paper, it was calculated by the following:

E¼

cðb  aÞbm  100% aðbm  aÞ

ð1Þ

where a is the grade of raw ore, b is iron concentrate grade, and c is the iron concentrate yield, bm is the theoretical content of pure mineral, the bm of pure magnetite is 72.41%. 3. Result and discussion 3.1. Miscibility studies on the DDA–Kerosene binary system The miscibility of DDA–Kerosene mixture collector was investigated; the results were depicted in Fig. 2. As shown in Fig. 2, the upper critical solution temperature of DDA and kerosene was about 34 °C; above the upper critical solution temperature DDA and kerosene were fully miscible in any proportion without addition of cosolvent. This temperature existed because the greater thermal motion overcomes any potential energy advantage to molecules of one type being close together. Known from the solubility phase diagram, DDA can be dissolved in kerosene usually with two

35 34

C-miscible area

A-miscible area

33

Temperature/

40 °C until a homogeneous sample was obtained; the binary system collector was miscible DDA–Kerosene. The water used in flotation tests was tap water, and its pH was 7.6. The water used in other experiments was distilled first, and then treated by passing resin beds and a 0.2 lm filter.

32

B-immiscible area

31

Critical solution temperature

30 29 28 0.0

0.2

0.4

0.6

0.8

Weight fraction of DDA, wDDA Fig. 2. Critical solution temperature of DDA and kerosene.

1.0

44

A. Liu et al. / Minerals Engineering 65 (2014) 41–50

threshold values Wl and Wh (Wl < Wh) at certain temperature, when DDA content was smaller than Wl or greater than Wh in the area A or C, DDA and kerosene were miscible to form a clear solution, while DDA content was within the threshold volume in the area B, DDA and kerosene would stratify. The composition of the two phases at equilibrium varied with the temperature (Atkins and Paula, 2006; Wang and Zhou, 2001). To ensure the DDA–Kerosene was fully miscible, the mixture was placing in the 40 °C water bath to form a clear solution.

the collecting ability of DDA–Kerosene was weaker than DDA–HCl, with 40 g/t DDA–HCl the quartz recovery was close to 90%, while only 70% with DDA–Kerosene which was approximated to 25 g/t DDA–HCl effect. For the fine quartz, the collecting ability of DDA–Kerosene was fairly with DDA–HCl, and the curve’s trends were the same for DDA–HCl and DDA–Kerosene with the same reagent dosage. It is well known that non-polar oil cannot collect natural hydrophilic minerals; quartz cannot float with pure kerosene. While in the DDA–Kerosene collector DDA content was 50%, it means that 50% DDA and 50% kerosene can achieve 100% DDA effect for fine quartz. On the contrary, for coarse quartz, 50% DDA and 50% kerosene can achieve about 65% DDA effect (it was deduced from the result of that 80 g/t DDA–Kerosene was equal to 50 g/t DDA–HCl effect, 60 g/t DDA–Kerosene was equal to 40 g/t DDA–HCl effect, 40 g/t DDA–Kerosene was equal to 25 g/t DDA–HCl). It demonstrated that the addition of kerosene has improved the flotation performance of both fine quartz and coarse quartz; while the recovery increment of fine quartz was more than coarse quartz. Synergistic effect of DDA–Kerosene was found to fine quartz compared with coarse quartz. It indicated DDA partly replaced by kerosene is feasible in the fine quartz flotation. In order to further understand the size composition of flotation concentrate with DDA–HCl and DDA–Kerosene, the volume particle size distribution of quartz feed and froth concentrate was determined. The flotation condition was with 40 g/t reagent at natural pH. The size distribution analysis of the minerals showed that the volume median diameter (d50) of fine quartz and coarse quartz was 27.6 lm and 126.4 lm respectively. As shown in Fig. 4, the average size of fine quartz in flotation froth concentrate using DDA–Kerosene as collector was 18.0 lm, while the average particle size with DDA–HCl was more than 25 lm, which was close to the raw fine quartz. Therefore, it can be inferred that the DDA– Kerosene could preferentially recover the ultrafine quartz particles, but DDA–HCl was not able to do it. Similar results were observed between the d50 of coarse quartz with DDA–Kerosene and DDA– HCl, especially the volume median diameter of coarse quartz froth concentrate in the presence of DDA–Kerosene was far lower than raw ore. This may be a result of inadequate reagents adsorption on quartz surface, as the synergism of DDA and kerosene was not obvious to coarse quartz, and the collect ability of DDA–Kerosene was less than DDA–HCl to coarse quartz, fine and medium quartz would selectively float under this condition. The agglomerates of quartz fines, which were prepared from the flotation cell with the stirring speed of 1500 rev/min for 5 min and

3.2. Flotation study on fine quartz and coarse quartz using DDA–HCl and DDA–Kerosene Fig. 3 illustrated the recovery of fine and coarse quartz achieved by varying the concentration of DDA–HCl and DDA–Kerosene (WDDA = 50%, the weight fraction of DDA was 50% in miscible DDA–Kerosene) respectively. Experiments were performed at 25 °C at natural pH. The results shown in Fig. 3 indicated the flotation performance of coarse quartz was superior to fine quartz; the recovery of coarse quartz was higher than fine quartz consistently under the same reagent dosage. This is because fine quartz exhibits much higher specific surface, for achieving the same result, so the collector consumption required for a certain coverage degree is much larger for fine particles. The effect of DDA–Kerosene dosage on recovery of different quartz was various. For the coarse quartz, 100

Recovery (%)

90

80

70

Coarse quartz+DDA-HCl 60

Coarse quartz+DDA-Kerosene Fine quartz+DDA-HCl

50

Fine quartz+DDA-Kerosene 40 20

30

40

50

60

70

80

Reagent dosage (g/t) Fig. 3. Effect of DDA–HCl and DDA–Kerosene to the recovery of fine and coarse quartz.

18

12

120

Fine quartz (a) Fine quartz + DDA-Kerosene (b) Fine quartz + DDA-HCl (c) Coarse quartz (d) Coarse quartz + DDA-Kerosene (e) Coarse quartz + DDA-HCl (f)

d50 (microns)

Volume (%)

15

9

90 60 30 0

6

a

b

c

d

e

f

Average particle diameter of the feed and product

3 0 1

10

100

1000

10000

Particle size, microns Fig. 4. Size distribution of raw quartz and froth concentrate with DDA–Kerosene and DDA–HCl.

100000

A. Liu et al. / Minerals Engineering 65 (2014) 41–50

Fig. 5. Optical microscopy image of quartz fines in the presence of DDA–HCl and DDA–Kerosene.

45

Fig. 6. Optical microscopy image of pure kerosene, DDA–Kerosene and DDA.

3.3. Property of DDA–Kerosene emulsion 40 g/t reagent at natural pH, has been observed through an optical microscope. The optical image of quartz agglomerates formed in the presence of DDA–HCl and DDA–Kerosene was given in Fig. 5. It is shown in the image that the raw fine quartz was dispersive granule, particle size of about 30 lm. Conditioned with DDA–HCl a small chain of agglomerates occurred, the agglomerates dimensions about 30  80 lm, the agglomerates distribution was still a very loose. Compared with the agglomerates formed in the presence of DDA–HCl, the quartz agglomerates with DDA–Kerosene were observed very compact. The agglomerate was much larger, about 150  300 lm in the dimension. It looked like to be consisted of several small compact agglomerates. Because of the presence of larger aggregation with DDA–Kerosene, the increase in flotation response was probably due to the increased hydrophobicity of the particles and a higher probability of collision and adhesion to air bubble. It can be imagined that there was a tiny contact interface when a small kerosene droplet adheres to coarse quartz as a result of relatively larger size compared with fine quartz. Thus, the tiny contact interface of coarse particle and kerosene was not sufficient for a stable attachment in turbulent conditions. The main cause of poor flotation of coarse particle with DDA–Kerosene can be related to the rupture of oil–quartz agglomerates due to the tiny contact interface and shear effects in the flotation cell.

The property of DDA–Kerosene emulsion and pure kerosene has been studied. In this study, two kinds of kerosene emulsions, miscible DDA–Kerosene and pure kerosene emulsion that were prepared by using high speed stirrer were used as collector or collector extender in order to understand the roles of DDA in the DDA–Kerosene emulsion and the property of DDA–Kerosene. The oil emulsion was prepared by mixing kerosene or DDA– Kerosene with water in the concentration of 1 wt.%, followed by agitation treatment for 10 min at the speed of 1500 rev/min. Fig. 6 shows the optical microscopy images of the two kerosene emulsions. The DDA–Kerosene produced a much better kerosene emulsion than the pure kerosene. The droplet size of DDA–Kerosene was much smaller and uniform in size. The conclusion was supported by the size distribution of the kerosene emulsion (Fig. 7). The size distribution analysis of the emulsion showed that the volume median diameter (d50) of DDA–Kerosene and pure kerosene was 3.6 lm and 4.8 lm respectively. More than 30% pure kerosene droplets were of a size greater than 10 lm, 20% of which were more than 40 lm in size. While 90% DDA–Kerosene droplets were less than 10 lm in size. Due to the DDA adsorption on the oil/water interface, lowering surface free energy, a uniform and smaller emulsion formed. The decrease of oil droplets size corresponded to the increase of their specific surface area. Therefore, in the same

20

100

16

80

12

60

DDA-Kerosene Kerosene DDA-Kerosene Kerosene

8

40

20

4

0

0.1

1

10

Cumulative frequency,%

A. Liu et al. / Minerals Engineering 65 (2014) 41–50

Volume (%)

46

0 1000

100

Droplet size, microns Fig. 7. Size distribution of DDA–Kerosene and pure kerosene emulsion.

(WDDA = 20%), at pH about 9.0 for 50% DDA–Kerosene and pH approximately 9.5 for 80% DDA–Kerosene respectively. The results have shown that the increase of DDA concentration in DDA– Kerosene emulsion resulted in a shift of the zeta potential of oil droplets towards more positive one. It was illustrated in the species distribution diagram of DDA as a function of pH at the total bulk concentration of 4.3  105 mol/L, which was correspond to the real flotation process of 40 g/t DDA (Fig. 9). The solution chemistry equilibria of DDA are as follows (Somasundaran and Wang, 2006):

S ¼ 104:69

RNH2ðsÞ ¼ RNH2ðaqÞ

RNHþ3 ¼ RNH2ðaqÞ þ Hþ

ð2Þ

K a ¼ ½Hþ ½RNH2ðaqÞ =½RNHþ3  ¼ 1010:63 ð3Þ 2

2RNHþ3 ¼ ðRNH3 Þ2þ 2

2:08 þ K d ¼ ½ðRNH3 Þ2þ 2 =½RNH3  ¼ 10

RNHþ3 þ RNH2ðaqÞ ¼ ½RNHþ3  RNH2ðaqÞ 

ð4Þ

K im

¼ ½RNHþ3  RNH2ðaqÞ =½RNHþ3 ½RNH2ðaqÞ  dosage of kerosene, this resulted in a better availability of oil to the minerals’ surfaces, resulting in larger agglomerates formation and well flotation response. As a result of the small solubility of pure DDA, the optical microscopy image of DDA emulsion was also carried out. Different from kerosene emulsion, pure DDA emulsion was in the shape of floc, oil droplets shape was not detectable. To understand the activity of DDA–Kerosene droplets, electrokinetics of DDA–Kerosene emulsion was measured. In order to simulate the real flotation process, the kerosene emulsion first prepared at the low kerosene water ratio (0.01 wt.%) as required for zeta potential measurement, in this situation, it was difficult to detect the scarce oil droplets. The proper experimental condition was chosen to at 0.3 wt.% of concentration in solutions after many experiments; zeta potentials of kerosene droplets were measured, and the results were shown in Fig. 8. In the absence of DDA, kerosene emulsion exhibited negatively charged from pH = 3.0 to pH = 12.0. The iso-electric point (IEP) of kerosene was about pH 2.5 which was slightly higher than that for pure hydrocarbon oil droplets (Liu et al., 2002), indicating a relatively complex nature of the kerosene as anticipated. In the presence of DDA, the zeta potential of kerosene emulsion became positive, suggesting the adsorption of DDA molecules at kerosene/water interface. Fig. 8 showed that in DDA–Kerosene emulsion, kerosene emulsion became positively charged over a wide pH range with an IEP at pH around 8.0 for 20% DDA–Kerosene

¼ 103:12

DDA molecules were mostly protonated at pH < 10 with the dominant species being positively charged amine groups. The observed zeta potential reversal of kerosene emulsions at pH 8.0–10.0 was due to the adsorption of protonated cationic amine on kerosene droplets. Unfortunately, it was impossible to accurately measure the zeta potential of DDA–Kerosene at low concentration in our laboratory. The results still showed that DDA can adsorb on kerosene/water interface to change its surface property. 3.4. Flotation study on fine quartz using DDA–HCl and DDA–Kerosene Fig. 10 showed the effect of pulp pH on the recovery of fine quartz in the presence of DDA–Kerosene, compared with those without oil. The DDA dosage was 40 g/t. It is noted that the flotation performance was strongly dependent on pulp pH value, which may be as a result of that the adsorption of amine cations on the mineral and kerosene droplet, surface charges greatly rely on pH within a non-extreme range of pH. As illustrated in Fig. 10, the curve with DDA–Kerosene addition was almost consistent with DDA–HCl, having the maximum recovery in the range of pH 4.0–9.5. The quartz recovery was greatly enhanced due to kerosene

-4

+

RNH3

RNH 2(s) RNH 2(aq)

-5 -6

-40

-9

DDA-Kerosene (WDDA=80%)

-60

DDA-Kerosene (WDDA=50%)

-10

DDA-Kerosene (WDDA=20%)

-80

4

0

2

4

6

3

+

8

10

12

14

pH

Kerosene

2

RN H

-8

.R NH

-20

-7

2

0

+ 2+

(RNH3 )2

RN H

logC

20

2(a q)

40

Zeta potential (mv)

ð5Þ

6

8

10

12

-5

C(DDA) =4.3×10 mol/L

pH Fig. 8. Zeta potential of DDA–Kerosene emulsion and kerosene emulsion.

Fig. 9. Species distribution diagram of DDA as function of pH (DDA total conc. = 4.3  105 mol/L).

47

A. Liu et al. / Minerals Engineering 65 (2014) 41–50

90

Recovery (%)

80 70 60 50 40

Fine quartz+DDA-HCl

30

Fine quartz+DDA-Kerosene

20 10

2

4

6

8

10

12

pH Fig. 10. Effect of pH to the recovery of quartz with DDA–HCl and DDA–Kerosene (WDDA = 50%).

addition, leading to an increase in recovery of over 20% at pH < 9.5. At pH > 10.0, the synergistic effect of DDA and kerosene disappeared, the recovery with DDA–Kerosene was almost identical with DDA–HCl. It is well known that the adsorption was controlled primarily by electrostatic interactions at low concentrations in physisorbing systems. According to Novich and Ring (1985), in the lower concentration range single alkylamine ion adsorbed on the quartz surface by electrostatic effect and the adsorption density increased linearly with equilibrium concentration; at the equilibrium concentration of approximately 0.1 mM (neutral pH) the surface hemimicelle formed with the flattened adsorption curves, which then abruptly steepened at an equilibrium concentration closed to the CMC and continued to rise linearly with the increase of concentration; multilayer adsorption occurred at high surfactant concentrations. It means that in our study at low DDA concentrations (0–100 g/t), the initial concentration of DDA was less than 1.08  104 mol/L; it primarily was electrostatic double layer interaction that induced the attractive interaction between the fine quartz particles and the collector. As depicted in Fig. 9, the neutral molecule RNH2 precipitates in the pH = 11 and the ionic forms RNH+3 and (RNH3)2+ 2 dominate in the range of pH = 2.0–9.0. Again, the ion-molecular complex exhibits a maximum at pH = 10.5. At pH > 11, the primary species are RNH2 molecule and RNH2 precipitation, and the active collecting species RNH+3, (RNH3)2+ and RNH2RNH+3 have very obvious 2 decreasing trend. In this condition, the quartz particles surface reaches the maximum amount of negative charge, but there is not enough amine cationic RNH+3 for the adsorption on quartz surface. At the same time, the nitrogen lone pair electrons of the RNH2 will be mutually exclusive with negatively charged quartz, make it hard for the adsorption of DDA. At the pH range of 9.0–11.0, the RNH+3 and (RNH3)2+ 2 species begin to decrease, while the RNH2 molecule increases sharply and the RNH2RNH+3 dimmer exhibits a maximum at pH = 10.5. In theory, the coadsorption of the molecular and ionic forms of the collector yields a maximum in the hydrophobicity or floatability of the mineral because of the formation of a compact adsorption layer as a result of the reduction of repulsion between polar headgroups at this pH. In fact, the sample of fine quartz nearly floats in 80–90% at the adsorption density of about 0.25–0.35 monolayer (Crawford and Ralston, 1988; Chibowski and Holysz, 1985). The advantage of reducing repulsion is not obviously under this condition. The quantity of the active collecting species positively charged amine groups is the key factor in the flotation process. Overall, the quartz recovery decreased with the

increase of the pH as a result of the reduction of RNH+3 and (RNH3)2+ 2 species at the pH range of 9.0–11.0. When at the pH range of 4.0–9.0, ionized RNH+3 prevails over the molecule species, there are enough amine cationic RNH+3 for adsorption. Flotation performances respond well in this pH region. With further decrease of the pH, even though the amine cationic RNH+3 is plenty in pulp, but the negative charge of quartz is significantly reduced, and the cationic amine RNH+3 adsorption capacity substantially decreases (Somasundaran and Wang, 2006). Fig. 11 illustrated the zeta potential of quartz particles as the function of pH in aqueous solutions in the presence of DDA–HCl and DDA–Kerosene. The IEP of quartz was at pH 2.7, which agrees well with values reported by other researchers (Jada et al., 2006; Tschapek and Wasowski, 1986). As the cationic collector was introduced into pulp, the zeta potential vs. pH curve moved upward and the negative potential was greatly reduced. This observation suggested the adsorption of DDA cations on the electrical double layers of quartz. The results have shown that the increase of DDA concentration resulted in a shift of the zeta potential of quartz towards more positive one. The negative potential of quartz reduced more using DDA–HCl as collector than DDA–Kerosene under the same dosage (when the DDA–HCl dosage was 100 g/t the DDA concentration was 1.08  104 mol/L, and the DDA–Kerosene (WDDA = 50%) dosage was 100 g/t the DDA concentration was 5.4  105 mol/L). This is because under the same dosage the content of DDA in the DDA– Kerosene is the half as DDA–HCl. The active positively charged species of DDA–HCl prevailed over DDA–Kerosene. It has been shown, even quartz completely floated at the concentration of 1.0  104 mol/L, a sharp decrease of negative potential occurred, the surface potential remained negative; until the concentration was 1.0  103 mol/L, the second collector layer was formed because of chain-chain interaction and the quartz surface potential became positive (Fuerstenaua and Pradip, 2005). This means quartz surface potential remains negative within the effective flotation reagent dosage range.

3.5. Probable interaction mechanism of quartz and DDA–Kerosene A probable interaction mechanism of fine quartz and DDA–Kerosene was proposed according to the surface property of DDA–Kerosene emulsion, morphological of fine quartz concentrate and variation of surface potential of quartz. Fig. 12 showed the schematic representation of the interaction of quartz and DDA–Kerosene in the pulp. When DDA–Kerosene was added in

30 0

Zeta potential (mv)

100

-30 -60

Quartz -4 Quartz + 1.0×10 mol/L DDA-HCl -5 Quartz + 5.0×10 mol/L DDA-Kerosene

-90

-3

Quartz + 1.0×10 mol/L DDA-HCl -120 2

4

6

8

10

12

pH Fig. 11. Zeta potential of quartz in the presence of DDA–HCl and DDA–Kerosene (the concentration is referred to DDA concentration).

A. Liu et al. / Minerals Engineering 65 (2014) 41–50

the pulp, a portion of DDA would adsorb on the kerosene/water surface as emulsifier to reduce the interfacial tension and change its electrokinetic potential, and another portion would adsorb on the quartz/water surface to reduce negative potential due to the electrostatic attraction. DDA hydrocarbon chain was inserted into the oil droplets inside, while the polar head was at oil–water interface towards water phase; addition of cationic surfactant to the kerosene emulsion changed the zeta potential value from negative to positive, which resulted in a strong electrostatic attraction between minerals particles and kerosene droplets and enhancing their adhesion. In general, non-polar oil does not adhere on a hydrophilic surface. In our study, a weak hydrophobicity occurs due to a very sparse amine ion adsorption on particle surfaces; however the modified kerosene interacts with negatively charged quartz by electrostatic attraction forces via surface reactive DDA cations promoting aggregation by bridging the fine quartz particles. In oil assist flotation process, kerosene was added after DDA and interacted with previously hydrophobized quartz. The interaction mechanism of kerosene and quartz was mainly due to hydrophobic interaction. To a great extent, the hydrophobization degree of quartz decided the interaction of kerosene and quartz. Since it has been established that DDA–Kerosene droplets are positively charged up to pH 9.5 (Fig. 8), while the IEP of quartz is around pH = 2.7, it appears that the electrostatic double layer interaction mechanism between positively charged DDA head groups at the kerosene/water interface and negatively charged quartz surfaces facilitates the adsorption of DDA cations and subsequent adhesion of the kerosene droplets on negatively charged quartz surfaces in the range of pH = 3.0–9.5. At pH above 9.5, a significant decrease in recovery is attributed to the reduction of the active collecting species positively charged amine groups (Fig. 10).

was consistent 40 g/t. Experiments were performed at 25 °C at natural pH. Fig. 13 showed towards both addition manners, the synergistic effect of DDA and kerosene can be obtained with optimal weight ratio of 1:1. While there was less increase in recovery as a function of kerosene concentration added after DDA. The most probable reason was the competitive adsorption of DDA between quartz and kerosene. In the flotation test, kerosene was added after DDA, DDA conditioning time was 2 min, and kerosene conditioning time was 2 min. It is well known that DDA primarily adsorb on the surface of quartz and it could reach the adsorption equilibrium quickly. A competitive adsorption between quartz and kerosene would happen when the kerosene added after collector. DDA would preferentially adsorb on the quartz surface, there would be insufficient DDA for kerosene, the modified kerosene amount decreased significantly. Hydrophobization degree of quartz decided the interaction of pure kerosene and quartz; unmodified kerosene would not adhere on a weak hydrophobic surface. It

95 90

Recovery (%)

48

85 80 75

Fine quartz+DDA-Kerosene Fine quartz+DDA+Kerosene

70

3.6. Addition manner of kerosene used as auxiliary collector 0

Results of total quartz recovery using DDA–Kerosene and pure DDA (kerosene added after DDA) as collector, with different amounts of kerosene, were shown in Fig. 13. The DDA dosage

20

40

60

80

Kerosene dosage (g/t) Fig. 13. Effect of addition manner of kerosene to the recovery of fine quartz.

Fig. 12. Schematic representation of the interaction of quartz and DDA (a), DDA–Kerosene (b) and DDA and kerosene (c).

49

A. Liu et al. / Minerals Engineering 65 (2014) 41–50 Table 2 Result of flotation of the magnetic concentrate with DDA–Kerosene (WDDA = 0.5) and DDA–HCl at natural pH. Reagent dosage (g/t)

Products

Yields (%)

Fe

SiO2

Fe

SiO2

60 g/t DDA–HCl

Froth Concentrate Total (calc.)

9.70 90.30 100.00

50.61 65.74 64.27

26.99 7.26 9.17

7.64 92.36 100.00

28.56 71.44 100.00

18.53

120 g/t DDA–Kerosene

Froth Concentrate Total (calc.)

25.67 74.33 100.00

47.42 69.98 64.18

31.16 1.72 9.28

18.96 81.04 100.00

86.19 13.81 100.00

59.07

120 g/t DDA–HCl

Froth Concentrate Total (calc.)

26.61 73.39 100.00

48.26 69.79 64.06

30.07 1.97 9.44

20.04 79.96 100.00

84.68 15.32 100.00

56.92

70 g/t DDA–HCl

Froth Concentrate Total (calc.)

13.75 86.25 100.00

49.35 66.59 64.22

29.15 5.96 9.15

10.57 89.43 100.00

43.82 56.18 100.00

28.14

140 g/t DDA–Kerosene

Froth Concentrate Total (calc.)

26.75 73.25 100.00

48.34 69.81 64.07

29.96 1.95 9.44

20.19 79.81 100.00

84.90 15.10 100.00

56.99

140 g/t DDA–HCl

Froth Concentrate Total (calc.)

29.84 70.16 100.00

50.94 69.88 64.22

26.57 2.51 9.69

23.67 76.33 100.00

81.85 18.15 100.00

54.67

80 g/t DDA–HCl

Froth Concentrate Total (calc.)

18.85 81.15 100.00

49.61 67.55 64.17

28.30 4.90 9.31

14.57 85.43 100.00

57.32 42.68 100.00

37.57

160 g/t DDA–Kerosene

Froth Concentrate Total (calc.)

33.66 66.34 100.00

54.48 68.84 64.01

21.95 3.21 9.52

28.65 71.35 100.00

77.62 22.38 100.00

43.15

160 g/t DDA–HCl

Froth Concentrate Total (calc.)

36.44 65.36 100.00

55.13 68.96 63.92

21.10 3.06 9.63

31.44 68.56 100.00

79.84 20.16 100.00

42.74

indicated greater synergistic effect would obtain in the case of kerosene used as auxiliary collector in the form of miscible DDA–Kerosene. 4. Flotation study on magnetic concentrate DDA–Kerosene exhibited an obvious advantage of collecting fine quartz, and the effect of collector concentration on efficiency of separation using DDA–Kerosene and DDA–HCl as collectors in the reverse flotation of magnetic separation concentrate was shown in Table 2. Experiments were performed at 25 °C at natural pH. The advantage of DDA–Kerosene was well beyond DDA–HCl. It was clear that the Fe grade of the concentrate obtained by DDA– Kerosene was higher than that by DDA–HCl and SiO2 content was lower. When the concentration of DDA–HCl was at 60 g/t, efficiency of separation got to 18.53%, while with a dosage of 120 g/t DDA–Kerosene, the efficiency of separation reached 59.07% which was identical with 120 g/t DDA–HCl. However, DDA content in miscible DDA–Kerosene was only the half as DDA–HCl. It was possible that kerosene partially substituted for DDA in the reverse flotation of magnetic separation concentrate. It indicated that DDA–Kerosene had better selectivity and stronger collectability to silicates than DDA–HCl, which can be used as an efficient collector in the reverse flotation of magnetic separation concentrate.

Grade (%)

Recovery (%)

Efficiency (%)

hydrophobicity of the fine quartz particles and induced a higher probability of collision and adhesion to air bubble. DDA–Kerosene can change the zeta potential of oil droplets from negative to positive with a significant shift of the IEP being observed and produce a much better kerosene emulsion than the pure kerosene of smaller and uniform size due to the adsorption of DDA at kerosene/water interface. In miscible DDA–Kerosene collector system, the probable interaction mechanism of quartz and DDA–Kerosene was expounded as follows. When the miscible DDA–Kerosene was added in pulp, DDA hydrocarbon chain was inserted into the oil droplets inside, and the polar head was at oil–water interface towards water phase. Addition of cationic surfactant to the kerosene emulsion changed the zeta potential value from negative to positive; the modified kerosene interacts with quartz by electrostatic forces via surface active DDA cations promoting aggregation by bridging the fine quartz particles. The synergistic effect of DDA and kerosene can be obtained with optimal weight ratio of 1:1. When the kerosene added after DDA, a competitive adsorption between quartz and kerosene of DDA would happen, resulting in a decrease in the recovery due to the reduction of reactive kerosene. Miscible DDA–Kerosene has better selectivity and stronger collectability to silicates than DDA–HCl, which can be used as an efficient collector in the reverse flotation of magnetic separation concentrate.

5. Conclusions References The experimental data showed that the proposed modification of oil assisted flotation process allowed agglomeration of hydrophilic fine quartz particles at lower oil consumption. The experimental results have demonstrated that the larger compact agglomerates formed by DDA–Kerosene collector increased

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