silica-bearing minerals

Minerals Engineering 81 (2015) 135–141

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Preliminary results on selective surface magnetization and separation of alumina-/silica-bearing minerals Xiaobin Li, Xiujiao Huang, Tiangui Qi ⇑, Qiusheng Zhou, Yilin Wang, Zhihong Peng, Guihua Liu School of Metallurgy and Environment, Central South University, Changsha 410083, Hunan, China

a r t i c l e

i n f o

Article history: Received 26 April 2015 Revised 25 July 2015 Accepted 5 August 2015 Available online 8 August 2015 Keywords: Alumina-/silica-bearing minerals Selective surface magnetization Zeta potential Magnetic separation Magnetite

a b s t r a c t The efficient and economical separation of alumina/silica minerals is critical to alumina production. In this work, selective surface magnetization was adopted to separate alumina-/silica-bearing minerals. The zeta potentials of boehmite, quartz, and magnetite with and without 100 ppm sodium fluoride (NaF) were measured. Results show that NaF could extend the favorable pH range to 7.4–8.5 for the selective magnetic coating of boehmite. X-ray diffraction and scanning electron microscopy analyses indicate that synthesized magnetite particles are preferentially adsorbed onto boehmite in the artificial bauxite of boehmite and quartz, and then aggregate to form a dense magnetic layer during surface magnetization. The magnetic hysteresis loop determined by Vibrating Sample Magnetometer further confirms that the concentrate from magnetic separation contains magnetic substances and the concentrate is weakly magnetic. Based on the separation results for artificial bauxite, experiments were conducted on the selective surface magnetization of gibbsite–boehmite mixed bauxite and low-temperature Bayer red mud. The results demonstrate that alumina-/silica-bearing minerals could also be separated to a certain extent. Selective surface magnetization exhibits the potential to separate alumina/silica minerals for practical applications and thus deserves further study. Ó 2015 Elsevier Ltd. All rights reserved.

1. Introduction The demand for high-grade gibbsitic bauxite in China has continually increased in recent years with the establishment of alumina refineries that employ the low-temperature Bayer process to refine imported bauxite. This situation has led to the importation of a considerable amount of boehmite–gibbsite mixed bauxite. Considering that the leaching temperature is approximately 100 °C for gibbsite and over 200 °C for boehmite, a twostage digestion process, in which gibbsite is initially leached at a low temperature and then the resultant residue is treated at an elevated temperature, is proposed to reduce the required hightemperature process, and thus, save energy. Unleachable boehmite and silica-bearing minerals, including quartz, are mainly present in low-temperature residue. Separating unreacted silica-bearing minerals before the high-temperature digestion stage can effectively reduce soda consumption and alumina loss caused by silicabearing minerals. Therefore, studying how boehmite can be separated from silica-bearing minerals is necessary.

⇑ Corresponding author. E-mail address: [email protected] (T. Qi). http://dx.doi.org/10.1016/j.mineng.2015.08.002 0892-6875/Ó 2015 Elsevier Ltd. All rights reserved.

However, traditional separation methods, such as flotation and roasting predesilication, are energy- and cost-intensive, have low flexibility, and pose environmental risks (Vasan et al., 2001). In the selective grinding method, the alumina-to-silica mass ratio (A/S) of the concentrate is too low for the Bayer process (Wang et al., 2011). Therefore, identifying an economical and efficient method to separate alumina-/silica-bearing minerals remains significant. The magnetic separation of weak or non-magnetic minerals by selective surface magnetization has been investigated in the fields of rare metal recovery (Bheemalingeswara, 1995), iron mineral separation (Georgios, 1999; Prakash et al., 1999; Wu et al., 2011, 2012), and sewage treatment (Fang et al., 2010; Audrey et al., 2012; Mandel et al., 2013) because of its simplicity and effectiveness. The principle of this process involves the selective adsorption of magnetic materials onto the surface of a weak or non-magnetic mineral by adjusting the superficial physicochemical properties of the minerals in the slurry. The magnetic separation technique can then be applied to recover coated grains. Parsonage (1988) pointed out that normal magnetic separation would be effective if a 0.01–0.1% magnetite covering was present on the surface of the non-magnetic mineral. As for the separation of alumina-/ silica-bearing minerals through selective surface magnetization, few studies have been reported. Hwang et al. (1982) studied the

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selective surface magnetization for gibbsite/quartz separation, in which the gibbsite was selectively connected to pre-synthesized magnetite particles by flocculants and separated from quartz by magnetic separation. Aiming to separate alumina-/silica-bearing minerals, the present work explored the feasibility of selective surface magnetization by in-situ co-precipitated magnetic material on target minerals. A mixture of boehmite and quartz was used as artificial bauxite while gibbsite–boehmite mixed bauxite and lowtemperature Bayer red mud were used as starting materials. 2. Experiment 2.1. Materials Boehmite, with a purity of P99% by weight, was prepared through the hydrothermal reaction of aluminum trihydroxide (analytic grade) at 270 °C for 6.5 h. Analytic grade quartz was provided by Simopharm Chemical Reagent Co., Ltd. (China). Magnetite, with a purity of P99% by weight, was synthesized through alkaline coprecipitation from an aqueous stoichiometric mixture of ferric chloride (analytic grade), ferrous sulfate (analytic grade), and concentrated ammonium hydroxide (analytic grade) at 85 °C for 15 min. The artificial bauxite with different A/S values comprised a mixture of boehmite and quartz with 90% of the particles measuring
10 lm and 100% measuring
15 lm (Mastersizer 2000, Malvern Instruments, UK). The gibbsite–boehmite mixed bauxite was obtained from Australia. The low-temperature Bayer red mud was the residue of the mixed bauxite leached at 110 °C for 30 min in a sodium aluminate solution with an ak (molar ratio of Na2O to Al2O3) of 3.0 and a caustic soda concentration (q(Na2O)) of 169.81 g/L. During selective surface magnetization, ammonium hydroxide, ferric chloride, and ferrous sulfate were used to synthesize the magnetic agent, whereas sodium fluoride (NaF, analytical grade) functioned as a regulator to adjust the surface potential of minerals. 2.2. Experiments For the surface magnetization, a solution consisting of 5 g bauxite/red mud, 100 mL pure water, and 2 mL 0.5 wt.% NaF was placed in a 150 mL steel bomb and stirred for 5 min. Quantitative ammonium hydroxide, ferric chloride, and ferrous sulfate, of which mole ratio of Fe3+ to Fe2+ ([Fe3+]/[Fe2+]) was 1.5, were then added into the bomb. The sealed bomb was immersed in glycerol and rotated for 30 min at 85 °C. The resultant was cooled and then separated by a magnetic separator (SSC, U50, China) at a magnetic intensity (Hs) of 0.175 T. The obtained concentrate and tailing were dried separately for analyses. The zeta potentials of quartz, synthesized boehmite, and magnetite with a particle size of
5 lm were determined using a potentiometric analyzer (MPT-2, Malvern Instruments, UK) in a diluted aqueous suspension of 0.05 wt.% solids, in which 10
3 M KNO3 was used as a supporting electrolyte and either a solution of HCl or NaOH was used as a pH regulator. The chemical compositions of the samples were analyzed either by chemical titration or by using an inductively coupled plasma emission spectrometer (Intrepid II XSP, USA). The phases were identified by XRD (TTRIII, Rigaku Corporation, Japan) using Cu Ka radiation with a scan range 2h of 5–75° at a scan rate of 10°/min. Meanwhile, surface microscopic morphology was observed through SEM (JSM6360LV, JEOL, Japan). In addition, the magnetic hysteresis loop of the concentrate was determined using a Vibrating Sample

Magnetometer (VSM, HH-15, Nanjing NanDa Instrument Plant, China). 3. Results and discussion 3.1. Influences of pH and NaF on the zeta potentials of different minerals The surface potential of a mineral is the main factor that influences the selective deposition of magnetite on objective minerals (Parsonage, 1984). To understand electrostatic interactions among boehmite, quartz, and magnetite particles better and to obtain a favorable pH range for the selective surface magnetization of magnetite on boehmite, the zeta potentials of boehmite, quartz, and magnetite were measured at different pH levels. The X-ray diffraction (XRD) patterns of the boehmite, quartz and magnetite are shown in Fig. 1, and the results for influence of pH and the concentration of NaF (q(NaF)) on the zeta potentials are presented in Fig. 2. Fig. 2A shows that the surface potentials of boehmite and magnetite decrease as pH increases. Under an acidic condition, boehmite and magnetite are positively charged because of the superficial O ions of the minerals that absorb H+. When pH is above the point-of-zero charge (pzc) of the mineral, the metal ions (Al, Fe) on the surface of the minerals absorb OH
; absorption becomes stronger as pH rises, which leads to a more negative surface charge. For quartz, aside from being more negative with a higher pH, surface potential is considerably more negative than those of boehmite and magnetite within a pH range of 6–12, with only the supporting electrolyte present in the solution. Stable and neutral meta-acid Si(OH) forms readily on the surface of fine quartz particles through hydrolysis, and is liable to react with OH
to form an anion group Si(O
), which results in a more negative charge on the surface than those of boehmite and magnetite (Zhang et al., 2003). Considering that some additives could change surface potential by influencing the electric double layer of the mineral surface, the surface potential of the minerals was measured with an additional 100 ppm NaF added to the suspension. Comparing Fig. 2A with Fig. 2B, the surface charges of quartz, boehmite, and magnetite are more positive than their charges without NaF; this shift is more obvious in high pH regions. Adding NaF may cause a ligand exchange between F
and M–O bonds (M represents Al, Si, and Fe) to form M–F bonds. Consequently, the adsorption of H+ onto the stern layer is enhanced because of the lone pair electrons of F
, and thus, the surface potential of the minerals is increased. Fig. 2B shows that adding 100 ppm NaF increases the pzc of boehmite and magnetite from pH 7.2 and 6.8 to 8.5 and 7.4, respectively, which indicates electrostatic attraction between magnetite and quartz and electrostatic repulsion between magnetite and boehmite when pH < 7.4. The interactions are reversed when 7.4 < pH < 8.5. However, the interactions among magnetite, quartz, and boehmite are all repulsive when pH > 8.5. Appropriately adjusting the pH of the suspension can make the interaction between boehmite and magnetite particles attractive and that between quartz and magnetite particles repulsive, which indicates that the selective magnetic coating of magnetite on boehmite can be achieved by regulating the pH of the suspension. In addition, adding NaF can extend the favorable pH range of the selective magnetic coating. 3.2. Surface magnetization of artificial bauxites Selective surface magnetization, which is known for its simplicity and low cost (Petcharoen and Sirivat, 2012), is achieved by coating the surface of boehmite with magnetite through the reaction of

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B

(B)

Q

Q-Quartz

Intensity/A.U

B-Boehmite

Intensity/A.U

(A)

B

B Q B B

10

20

30

40

B

B

50

B

BB

B

Q Q Q Q Q

B

60

10

70

20

30

40

Q

Q Q

50

60

Q

Q

70

2θ/°

2θ/°

(C)

Intensity/A.U

M

M-Magnetite

M

M M M

M

10

20

M

30

M

40

50

60

70

2θ/° Fig. 1. XRD patterns of (A) boehmite, (B) quartz, and (C) magnetite.

ferrous and ferric ions in an alkaline suspension according to the following equation:

Fe2þ þ 2Fe3þ þ 8OH
¼ Fe3 O4 þ 4H2 O: Given that OH
is consumed during the magnetite formation process, ammonia is employed to provide OH
and maintain the pH of the suspension within the optimal range of 7.4–8.5, as determined by previous zeta potential measurements. According to the results of the exploratory experiment, an initial amount of 1.25 wt. % ammonia in the suspension (w(NH3H2O)) is appropriate for selective surface magnetization. Hence, subsequent selective surface magnetization tests were conducted by adding 1.25 wt.% ammonia. Fig. 3 illustrates that the separation efficiency of artificial bauxite is influenced by the initial Fe2+ concentration (C(Fe2+)), and that the optimal initial Fe2+ concentration is 0.025 mol/L with respect to the difference in A/S values between the concentrate and the tailing. When the initial Fe2+ concentration is less than 0.025 mol/L, Al2O3 recovery and concentrate yield rise rapidly with increasing concentration, and then remain nearly constant with further increases in concentration. Correspondingly, the A/S of the concentrate increases sharply and then decreases slightly, whereas that of the tailing decreases rapidly and then gradually approaches approximately 0.05. With the increase in initial Fe2+ concentration, magnetite may preferentially coat the surface of boehmite, which enhances the magnetism of boehmite particles continuously until the boehmite particles are coated to a certain extent. Afterward, extra magnetite is likely to be deposited on quartz particles.

Thus, Al2O3 recovery and concentrate yield increase, but the A/S of the concentrate decreases. Further tests were performed to verify the separation efficiency of artificial bauxites with different A/S values through surface magnetization with 0.025 mol/L Fe2+. The results are presented in Fig. 4. With the increase in A/S of the artificial bauxite from 1.0 to 3.0, concentrate yield gradually increases from 74.91% to 86.67%, and Al2O3 recovery increases from 96.07% to 97.94%. Meanwhile, the A/S of the concentrate rises from 1.96 to 6.95, whereas that of the tailing remains less than 0.1. The results suggest that boehmite and quartz in artificial bauxite can be separated by magnetic separation after selective surface magnetization and the separation becomes more effective with higher A/S ratios in the starting mixture. 3.3. Analysis of the magnetized layer The XRD patterns of the concentrate and the tailing obtained from artificial bauxite with an A/S of 1.5 at the initial Fe2+ of 0.025 mol/L, along with the untreated artificial bauxite, are shown in Fig. 5A. A portion of Fig. 5A is magnified in Fig. 5B to identify phase composition. The comparison of the XRD patterns shows that boehmite and quartz are enriched in the concentrate and the tailing, respectively, whereas a small amount of magnetite is observed only in the concentrate. The susceptibility of the concentrate was further measured using VSM and the magnetic hysteresis loop was obtained as Fig. 6. This figure shows that the concentrate displays magnetic hysteresis together with some other magnetic characteristics such as remnant magnetism Mr of 4.95 emu/g,

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40

100

7

(A)

Quartz Magnetite Boehmite

20

6

80

60

3

%

A/S of the concentrate A/S of the tailing Al2O3 recovery Concentrate yield

4

0

A/S

Zeta-potential/mV

5

40

-20 2

20 1

-40

0

0

1.0 -60

6

7

8

9

10

11

2.0

2.5

3.0

A/S of the artificial bauxite

12

pH

Fig. 4. Influence of the A/S of artificial bauxite on separation efficiency [q(NaF) = 100 ppm, w(NH3H2O) = 1.25 wt.%, C(Fe2+) = 0.025 mol/L, [Fe3+]/[Fe2+] = 1.5, temperature = 85 °C, duration = 30 min, and Hs = 0.175 T].

40

(B)

Quartz Magnetite Boehmite

20

*

(A)

.*

ο Boehmite Quartz Magnetite

Intensity/A.U

0

-20

*

(c) ο

*

(b) -40 6

7

8

9

10

11

12

ο

pH Fig. 2. Zeta potentials of boehmite, quartz, and magnetite in water with 10
3 M KNO3 as the supporting electrolyte, (A) without NaF and (B) with 100 ppm NaF.

(a)

*

10

20

* ** *** * * * * *

.

*ο * ο

ο

Zeta-potential/mV

1.5

ο

*

* ο* * * ο*

30

40

*

50

*

60

70

2θ/°

*

(B)

100 3.75

*

80 3.00

Quartz Magnetite

2.25 A/S of the concentrate A/S of the tailing Al2O3 recovery Concentrate yield

1.50

40

Intensity/A.U

* 60

%

A/S

.*

ο Boehmite

(c)

(b)

0.75

20

0.00

0

ο

.

*

ο

* 0.010

0.015

0.020

0.025

Initial concentration of Fe2+/(mol/L)

*

(a)

0.030 32

34

36

*

38

* 40

42

2θ/° 2+

Fig. 3. Influence of initial Fe concentration on the separation efficiency for artificial bauxite with an A/S of 1.5 [q(NaF) = 100 ppm, w(NH3H2O) = 1.25 wt.%, [Fe3+]/[Fe2+] = 1.5, temperature = 85 °C, duration = 30 min, and Hs = 0.175 T].

Fig. 5. XRD patterns of (a) artificial bauxite with an A/S of 1.5, (b) the concentrate, and (c) the tailing.

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20

Magnetization intensity, B/(emu/g)

15 10 Mr=4.95 emu/g

5 0 -5 -10

Hc=88.19 Oe -15 -20 -6000

-4000

-2000

0

2000

4000

6000

Magnetic field intensity, H/Oe Fig. 6. The magnetic hysteresis loop of the concentrate.

coercivity Hc of 88.19 Oe, and a certain magnetic permeability l (the ratio of magnetization intensity B to magnetic field intensity H). These characteristics further confirm that the concentrate

(A)

contains magnetic substances, while the concentrate is weakly magnetic due to the low saturation mass magnetization (rs) of 14.26 emu/g. The surface microscopic morphology of the synthesized boehmite, the concentrate with an A/S of 6.95, and the tailing with an A/S of 0.049 are presented in Fig. 7. The surface of the synthesized boehmite is smooth before surface magnetization (Fig. 7A), the particles of the concentrate are densely coated by a layer of uniform magnetite (Fig. 7B), and the particles of the tailing are sparsely coated by tiny particles (Fig. 7C). These results provide further evidence for the selective coating of magnetite on mineral surfaces. When the sizes of the magnetite particles are less than 6–7 lm, the magnetic field gradients (gradH) of their surface (within 0.5 lm) rise sharply, which leads to a significant increase in the magnetic force (f = vHgradH) acting on the magnetic particles. This phenomenon is known as the high gradient effect of the fine-grained magnetic medium (Zhang et al., 1986). The mean particle diameter of magnetite is within 1 lm and the aggregation between magnetite particles is tight and compact (Fig. 7B), which indicates that the selective coating of magnetite may also be caused by a high gradient effect. In summary, the selective magnetic coating process appears to include two steps: (1) the selective adsorption of magnetite particles onto the surface of boehmite by electrostatic attraction and (2) the aggregation of magnetite

(B)

1µm

1µm

(C)

1µm Fig. 7. SEM images of (A) synthesized boehmite, (B) the concentrate, and (C) the tailing.

Table 1 Separation efficiency of alumina/silica minerals in mixed bauxite and Bayer red mud. Starting materials

Samples

Al2O3/wt.%

A/S

Al2O3 recovery/%

Concentrate yield/%

Gibbsite–boehmite mixed bauxite

Mixed bauxite Concentrate Tailing

55.15 58.54 43.84

SiO2/wt.% 8.77 6.24 9.46

6.29 9.38 4.63

– 51.87 –

– 44.66 –

Bayer red mud

Red mud Concentrate Tailing

46.11 48.48 38.03

20.06 13.27 27.90

2.30 3.65 1.36

– 65.26 –

– 59.57 –

q(NaF) = 100 ppm, w(NH3H2O) = 1.25 wt.%, C(Fe2+) = 0.025 mol/L, [Fe3+]/[Fe2+] = 1.5, temperature = 85 °C, duration = 30 min, Hs = 0.175 T.

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particles by the high gradient effect. Obviously, the selective magnetization in this work is realized by electrostatic attraction and high gradient effect rather than selective flocculation reported in Hwang’s work (Hwang et al., 1982). 3.4. Preliminary investigation on the selective surface magnetization of mixed bauxite and red mud

Intensity/A.U

To verify the feasibility of selective surface magnetization for practical applications, experiments were conducted using gibbsite–boehmite mixed bauxite and low-temperature Bayer red mud as starting materials under the following conditions: q(NaF) = 100 ppm, w(NH3H2O) = 1.25 wt.%, C(Fe2+) = 0.025 mol/L, [Fe3+]/[Fe2+] = 1.5, temperature = 85 °C, duration = 30 min, and Hs = 0.0175 T. The results listed in Table 1 show that the A/S of the concentrates of mixed bauxite increases from 6.29 to 9.38, and concentrate yield and Al2O3 recovery reach 44.66% and 51.87%, respectively. For Bayer red mud, the A/S of the concentrates rises from 2.30 to 3.65, and concentrate yield and Al2O3

Fig. 10. Back-scattered electron image (A) and EDS image (B) of low-temperature Bayer red mud.

10

20

30

40

50

60

70

Fig. 8. XRD patterns of (a) mixed bauxite, (b) the concentrate, and (c) the tailing.

10

20

30

40

50

60

70

Fig. 9. XRD patterns of (a) low-temperature Bayer red mud, (b) the concentrate, and (c) the tailing.

recovery are 59.57% and 65.26%, respectively. These results indicate that alumina/silica minerals in mixed bauxite and red mud can be separated to a certain extent by selective surface magnetization. The XRD patterns of the corresponding samples (Figs. 8 and 9) further demonstrate that gibbsite, boehmite, hematite, kaolinite, anatase, and quartz are present in the mixed bauxite, and that the alumina-containing minerals (gibbsite, boehmite, and kaolinite) are present in both the concentrate and the tailing, whereas nearly all of the quartz remains in the tailing. For red mud, the distribution feature of the alumina-/silica-bearing minerals in both the concentrate and the tailing (Fig. 9) is similar to that of the mixed bauxite. Compared with the separation efficiency of artificial bauxite, the separation of alumina/silica minerals either in the mixed bauxite or in the low-temperature Bayer red mud is less effective. This result may be attributed to the following reasons. (1) As shown in the energy-dispersive X-ray spectroscopy (EDS) image of the Bayer red mud (Fig. 10), alumina-/silica-bearing minerals always coexist closely in both the mixed bauxite and the Bayer red mud, which makes separating them into individual minerals using conventional grinding methods difficult. (2) In both the mixed bauxite and the red mud, a certain amount of kaolinite (Al2O32SiO22H2O), in which alumina/silica cannot be separated using physical methods, is present. (3) The complexity of the mineral composition of the mixed bauxite and the red mud probably

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influences the selective adsorption of magnetite. Therefore, future studies can focus on regulating the surface properties of minerals and efficient separation methods for practical applications. 4. Conclusion The adsorption of magnetite particles on alumina-containing minerals is possible in the presence of NaF within a weakly alkaline pH range. When artificial bauxite is used, alumina-/silica-bearing minerals can be separated by selective surface magnetization followed by magnetic separation. The separation of alumina-/silicabearing minerals in mixed bauxite or Bayer red mud can also be achieved to a certain extent. The deposition of magnetite on alumina-containing minerals is achieved by controlling the surface potential of the particles involved. NaF can increase the zeta potentials of boehmite, quartz, and magnetite, as well as extend the favorable pH range from 6.8– 7.2 to 7.4–8.5 for selective magnetic coating. Newly formed magnetite particles, with an average particle size of less than 1 lm, are preferentially adsorbed onto aluminacontaining minerals, and form a dense magnetic layer. The concentrate obtained from magnetic separation exhibits the typical characteristics of magnetite with a saturation mass magnetization (rs) of 14.26 emu/g. Acknowledgments The authors would like to acknowledge the financial support from the National Natural Science Foundation of China (Grant No. 51374239).

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